WO2025036923A1 - Production of an isocyanate by way of a gas phase reaction in several gas phase reaction arrangements operated in parallel - Google Patents

Abstract

The present invention relates to the production of an isocyanate by reacting a primary amine with phosgene by way of a gas phase reaction in several gas phase reaction arrangements connected in parallel. Excess phosgene not reacted in the gas phase reaction is returned, as a return flow, to a phosgene gas generation device to form a phosgene gas stream and a state parameter of the formed phosgene gas steam is set by means of a setting parameter of the return flow. This concept enables stable reaction conditions of the gas phase reaction, in particular a stable stoichiometric phosgene excess in the gas phase reaction arrangements, to be ensured.

Description

Production of an isocyanate by means of a gas phase reaction in several gas phase reaction arrangements operated in parallel

The present invention relates to a process for producing an isocyanate by reacting a primary amine with phosgene by means of a gas phase reaction. The present invention also relates to a plant for carrying out the said process, a computer system for controlling the stoichiometric phosgene excess, a computer program product and a computer-implemented process for setting the stoichiometric phosgene excess. Specifically, the present invention is concerned with the production of an isocyanate in several gas phase reaction arrangements connected in parallel. In this case, excess phosgene that has not reacted in the gas phase reaction is recycled as a recycle stream into a phosgene gas generation device to form a phosgene gas stream and a state parameter of the phosgene gas stream formed is adjusted via an adjustment parameter of the recycle stream. With this concept, stable reaction conditions of the gas phase reaction, in particular a stable stoichiometric phosgene excess in the gas phase reaction arrangements, can be ensured.

Isocyanates, especially diisocyanates such as toluene diisocyanate, are produced on an industrial scale and are mainly used as starting materials for the production of flexible polyurethane foams, in particular for the production of upholstery and mattresses, but also as insulation or protective materials. One possibility for the production of isocyanates that has long been described in the state of the art is the reaction of primary amines with phosgene in a gas phase reaction. This process is characterized by the fact that under the selected reaction conditions at least the reaction components primary amine and phosgene are gaseous. Production by means of a gas phase reaction has numerous advantages over the classic liquid phase production processes, such as increased reaction yields, simpler processing to the desired isocyanate, lower investment costs for plants with the said technology, as well as energy and raw material savings and lower emissions of Carbon dioxide, nitrogen oxides and other environmentally harmful gases.

However, the gas phase reaction also presents manufacturers with particular challenges. The isocyanates produced are not thermally stable at the reaction temperatures of up to 500 °C required for the gas phase reaction and form undesirable by-products such as isocyanurates, biurets, allophanates, carbodiimides or ureas, both through thermal decomposition and through further reactions. As a result of the formation of by-products, solids accumulate at various points in the production plant during ongoing production, e.g. on the walls of gas phase reactors or in mixing nozzles for injecting liquids or gases, and increasingly block them. It is therefore of great interest to ensure that production runs smoothly to reduce the formation of such by-products as much as possible. For this purpose, it is currently preferred in the prior art, for example, to cool the gaseous reaction product mixture formed in the gas phase reaction to temperatures below 150 °C by bringing it into contact with quenching liquids that are inert under the reaction conditions, with as complete condensation of the isocyanate formed as possible, in order to prevent decomposition and subsequent reactions. For example, WO 2018/224529 Al relates to a process for producing isocyanates by phosgenating primary amines in the gas phase, wherein a crude gaseous reaction product mixture obtained from the gas phase reaction is cooled and partially condensed by bringing it into contact with at least one stream of a quenching liquid. WO 2018/224529 Al describes the use of a stoichiometric excess of phosgene based on primary amine groups, which is advantageous for the gas phase reaction. The possibility is also mentioned of freeing a gaseous stream obtained after scrubbing from excess phosgene used in excess of stoichiometric amounts, e.g. B. by means of a cold trap, or by absorption in an inert solvent.

WO 2022/106716 A1 describes a process for preparing isocyanates and/or polyisocyanates by reacting the corresponding amines with phosgene, comprising the steps of a) feeding a first reactant stream containing the amine in gaseous or liquid form into a reactor, b) feeding a second reactant stream containing the phosgene in gaseous form into the reactor, c) mixing and reacting the first and second reactant streams in the reactor to obtain a reaction mixture, d) cooling the reaction mixture by indirect heat transfer or by quenching with cooled reaction product to obtain a liquid reaction product, e) separating the cooled reaction mixture into a liquid reaction product containing isocyanate and a gaseous reaction product, f) purifying the liquid reaction product to obtain liquid isocyanate, and h) separating the gaseous reaction product into a first gaseous product stream essentially containing HCl and a second liquid or gaseous product stream essentially containing phosgene, wherein no liquid solvent is used in steps a) to f) and h).

WO 2017/055311 A1 describes a continuously operated process for the production of isocyanates by reacting the corresponding amines with phosgene, in which, when the desired production capacity is changed, starting from an initial state with a certain production capacity and ending in a final state with a different production capacity in the transition period between the initial and final state

- the instantaneous phosgene excess at the time of the transition period at which the change in the amine mass flow is started is at least as large as, and preferably larger than, the phosgene excess during the period of production with the production capacity in the initial state before the start of the transition period, and that

- the average phosgene surplus during the transition period is greater than the phosgene surplus during the period of production with the initial production capacity before the start of the transition period.

WO 2011/003532 A1 describes a process for preparing isocyanates by reacting primary amines with phosgene in stoichiometric excess in the gas phase, in which a) the amine is reacted with phosgene in a reactor above the boiling point of the amine, a liquid stream comprising the isocyanate and a gas stream comprising HCl and phosgene being obtained, and b) the gas stream obtained in step a) comprising HCl and Phosgene is first separated into a gas stream containing HCl and a liquid stream containing phosgene, and c) the liquid stream containing phosgene obtained in step b) is then at least partially converted into a gaseous stream containing phosgene, and d) the gaseous stream containing phosgene obtained in step c) is recycled to the reaction in step a), wherein e) the pressure of the gaseous stream containing phosgene obtained in step c) is higher than the pressure of the liquid stream containing phosgene obtained in step b).

Despite known possibilities to reduce the formation of by-products, the processes known in the prior art still have the disadvantage that decomposition and by-products in the form of residues, in particular solid residues, can never be completely avoided, in particular not their deposition on the reactor walls of commonly used gas-phase reactors. This so-called "caking" can suddenly break off from the reactor walls, thereby causing a drastic drop in pressure in the gas phase reactor used. Amine gas generation devices used to generate the amine reaction gases used in the gas phase reaction in particular react very sensitively to such a drop in pressure, and a very brief, violent discharge of amine reaction gas into the gas phase reactor occurs. If several gas phase reactors are operated in parallel and supplied with gaseous phosgene from a common phosgene gas generation system via a distribution device, a drop in pressure in one of the gas phase reactors also affects the other gas phase reactors via the distribution device. This creates unstable operating and reaction conditions, which in turn promote the formation of by-products and further destabilize the production process. For example, a previously precisely set very short residence time of the gaseous reactants in a reaction zone of the gas phase reactor (the gas phase reaction is very fast and exothermic) before a reaction is terminated by Cooling in a quench zone can be impaired. Both too short and too long residence times can lead to a greatly increased formation of by-products. The desired stoichiometric excess of phosgene can also be impaired. The most precise possible adherence to a previously defined stoichiometric Phosgene excess, i.e. ensuring a defined, previously determined molar ratio of phosgene to primary amino groups, whereby this molar ratio is greater than 1 (phosgene is used in excess of stoichiometric amounts in relation to primary amino groups), has proven to be of great importance for a trouble-free reaction.

The object of the invention is therefore to develop a process and a plant for carrying out the process for producing an isocyanate by reacting a primary amine with phosgene by means of a gas phase reaction, with which the described disadvantages of the processes and plants used in the prior art are avoided or at least reduced. Furthermore, a suitable computer-implemented control system should be provided for this purpose. The process proposed according to the invention or the plant for carrying out the process should be able to be carried out with greater operational stability compared to processes and plants known from the prior art, in particular with regard to maintaining a desired stoichiometric excess of phosgene as consistently as possible in all gas phase reactors. In particular, pressure drops caused by breaking off solid residues of by-products and subsequent products deposited on the walls of gas phase reactors should be able to be compensated in order to ensure more stable reaction conditions of the gas phase reaction compared to known processes and plants, in particular with regard to a constant stoichiometric excess of phosgene in all gas phase reactors.

To solve this problem, the invention provides the following:

In a first aspect, the present invention relates to a process for producing an isocyanate by reacting a primary amine with phosgene by means of a gas phase reaction in a plant comprising m gas phase reaction arrangements (100-1, 100-2, ..., 100-m), where m is a natural number in the range from 2 to 6. Said process comprises at least the following steps:

(i) generating m amine reaction gas streams (Al, A-2, ..., Am), each comprising a primary amine; (ii) generating a phosgene gas stream (P) comprising phosgene at a pressure (pP) in a phosgene gas generating device (104) (= device for generating gaseous phosgene from liquid phosgene and phosgene solutions) and dividing the phosgene gas stream (P) comprising phosgene into m phosgene reaction gas streams (Pl, P-2, ... Pm) comprising phosgene;

(iii) combining one of the amine reaction gas streams (A-l, A-2, ..., A-m) and one of the m phosgene-comprising phosgene reaction gas streams (P-l, P-2, ... P-m) in each case in one of the gas phase reaction arrangements (100-1, 100-2, ..., 100-m), wherein (at least) the m phosgene reaction gas streams (P-l, P-2, ... P-m) pass through one of m phosgene flow control valves (PV-1, PV-2, ..., PV-m) to adjust the stoichiometric phosgene excess before entering the m gas phase reaction arrangements (100-1, 100-2, ..., 100-m), and carrying out the gas phase reaction to obtain m isocyanate-comprising isocyanate crude product fluid streams (IC-1, IC-2, ..., IC-m) and m residual fluid streams (R-l, R-2, ..., R-m), wherein the m residual fluid streams (R-l, R-2, ..., R-m) each comprise excess phosgene not converted in the gas phase reaction;

(iv) recycling the unreacted excess phosgene to the phosgene gas generating device (104), namely by

(iv-1) optionally, combining the m residual fluid streams (R-l, R-2, ..., R-m) into n residual fluid streams (R-l, R-2, ... R-n), where n is a natural number in the range from 1 to m-1 (and in particular 1),

(iv-2) introducing each of the residual fluid streams into a respective phosgene absorption device (103-1, 103-2, ... 103-m or 103-n), each having a phosgene absorption device collection area (115-1, 115-2, ... 115-m or 115-n), wherein an absorbent fluid stream (AbsM-1, AbsM-2, ..., AbsM-m or AbsM-n), each containing an absorbent (AbsM), is also introduced into each of the phosgene absorption devices (103-1, 103-2, ... 103-m or 103-n), with absorption of the excess phosgene not converted in the gas phase reaction by the absorbent (ie by the absorption of the phosgene [= absorbate] into the free volume of the absorbent [= absorbent]), wherein in each phosgene absorption device collection area (115-1, 115-2, ... 115-m respectively. 115-n) collects a recycle medium (Recycle-1, Recycle-2, Recycle-m or Recycle-n) comprising absorbed phosgene and the absorption medium, from which, if necessary after combining several recycle media (Recycle-1, Recycle-2, ..., Recycle-m or Recycle-n), a (single) recycle stream (Recycle) comprising phosgene is fed, and

(iv-3) introducing the recycle stream (Return) at a flow rate (f-Return) into the phosgene gas generation device (104) with desorption of the phosgene to be recycled from the absorption medium (ie with the transfer of phosgene dissolved in the absorption medium into the gas phase) to obtain desorbed (= transferred into the gas phase) phosgene and a liquid desorption medium (DesM) (= the liquid phase largely freed from phosgene, the chemical composition of which is essentially the same as that of the absorption medium (AbsM) used in (iv-2)), wherein the phosgene gas stream (P) generated in the phosgene gas generation device (104) comprises the desorbed phosgene and a phosgene fresh fluid stream (Fresh) additionally introduced into the phosgene gas generation device (104) at a flow rate (f-Fresh), and wherein to ensure the stoichiometric phosgene excess in each of the m Gas phase reaction arrangements (100-1, 100-2, ..., 100-m)) a state parameter (X) of the phosgene gas stream (P) is set (= regulated) via a setting parameter (Y), wherein (at least) the pressure (pP) serves as the state parameter (X) and wherein the setting parameter (Y) comprises at least one setting parameter (Yl) of the recirculation stream (Rück), namely such that in the case of a detected change in (at least) a pressure (pP-1, pP-2, ... pPm) of the m phosgene reaction gas streams (Pl, P-2, ... Pm) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m), the flow rate (f-Rück) of the recirculation stream (Rück) is used as the setting parameter (Yl). In a second aspect, the present invention relates to a computer system for controlling the stoichiometric excess of phosgene in the process according to the invention for producing an isocyanate, comprising: an interface unit which is configured to read in (periodically or continuously) data which are suitable for determining a change in (at least) one pressure (pP-1, pP-2, ... pPm) of the m phosgene reaction gas streams (Pl, P-2, ... Pm) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m), and a processor which is configured to calculate an adjusted setpoint value (Y)SOLL for the setting parameter (Y) (ie at least for (Yl), optionally also for (Y-2)) using the read-in data when a change in the pressure (pP-1, pP-2, ... pPm) is detected, and to assign the adjusted setpoint value (Y)SOLL to a processor which is in communicative connection with the processor. actuator for manipulating the setting parameter (Y) in order to counteract the detected change in pressure (pP-1, pP-2, ... pPm) (and thus the change in the stoichiometric phosgene excess).

In a third aspect, the present invention relates to a plant for producing an isocyanate by reacting a primary amine with phosgene in a stoichiometric excess of phosgene by means of a gas phase reaction, wherein the plant is designed to carry out the process according to the invention and further comprises the computer system according to the invention.

In a fourth aspect, the present invention relates to a computer program product which, when loaded into a memory unit of a computing unit of the computer system according to the invention and executed by the processor, detects changes in (at least) one pressure (pP-1, pP-2, ... pPm) of the m phosgene reaction gas streams (Pl, P-2, ... Pm) (periodically or continuously) after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m) in the method according to the invention and, in the event of a detected change in the pressure (pP-1, pP-2, ... pPm) adjusts the setpoint (Y)SETPOINT of the setting parameter (Y) (ie at least of the setting parameter (Yl), if necessary also of the setting parameter (Y-2)) (ie calculates a new setpoint (Y)SETPOINT) in order to counteract the change in pressure (pP-1, pP-2, ... pPm) (and thus the change in the stoichiometric phosgene excess).

The attached drawings show:

FIG. 1 shows an overview (without control devices) of a possible design of the plant for producing an isocyanate using the example of m = n = 2;

FIG. 2 shows a more detailed representation of a production line;

FIG. 3 shows an overview of a possible design of the plant for producing an isocyanate with m = 6 and n = 2; and

FIG. 4 shows a possible design of the plant for producing an isocyanate using the example m = n = 2 including control equipment.

The guided fluid flows shown in the drawings are mainly guided via pipes. The arrows shown in the drawings indicate the main flow direction of the guided fluid flows. The symbols PP and WT generally stand for one or more intermediate pumps PP for guiding liquids, or intermediate heat exchangers WT, through which the fluid flows shown are guided.

In the context of the present invention, a "gas phase reaction arrangement (100-1, 100-2, ... 100-m)" is understood to mean an arrangement which is designed to combine a phosgene reaction gas stream comprising phosgene and an amine reaction gas stream comprising primary amine during a gas phase reaction of a primary amine with phosgene.

In the context of the present invention, a "phosgene gas generation device (104)" is understood to mean a technical device for providing the phosgene gas stream (P) which is fed to the gas phase phosgenation. In the phosgene gas generation device (104), the phosgene fresh fluid stream (Fresh) and the phosgene to be recycled The phosgene gas stream (P) is generated from a recycle stream (return). This is to be distinguished from a fresh phosgene generation device (109 in FIG. 1) in which phosgene is produced (chemically synthesized).

In the context of the present invention, a "phosgene absorption device (103-1, 103-2, ... 103-m or 103-n)" is understood to mean a technical device for absorbing the excess phosgene not converted in the gas phase reaction by an absorption medium (i.e. by taking up the phosgene [= absorbate] in the free volume of the absorption medium [= absorbent]). The "phosgene absorption device (103-1, 103-2, ... 103-m or 103-n)" has a "phosgene absorption device collection region (115-1, 115-2, ... 115-m or 115-n)". This is a region in which the absorption medium collects with the phosgene absorbed therein; this composition of absorption medium and phosgene absorbed therein (= phosgene solution in the absorption medium) represents the "recycle medium (recycle 1, recycle 2, ..., recycle m or recycle n)". A simple example of a phosgene absorption device is an absorption column in which a liquid absorbent, for example ortho-dichlorobenzene, and gaseous phosgene are brought into contact (preferably in countercurrent), whereby the recycle medium (recycle) (solution of phosgene in ortho-dichlorobenzene) is formed and is collected in the phosgene absorption device collection region (which in the case of an absorption column is the column bottom).

In the terminology of the present invention, a "state parameter (X) of the phosgene gas stream (P)" refers to a physical and/or chemical parameter characteristic of the phosgene gas stream (P), i.e. a parameter that describes the state of the phosgene gas stream in physical and/or chemical terms. A state parameter can therefore be a physical or chemical parameter, for example a density, temperature, flow rate, concentration, volume or pressure of a system under consideration. In the context of the present invention, at least the pressure (p-P) of the phosgene gas stream (P) is considered to be such a state parameter.

In the terminology of the present invention, a "setting parameter (Y)" refers to a parameter which influences the previously defined state parameter (X), by means of which the state parameter (X) can be set to a desired value. In principle, any setting parameter can be used. Parameters can be viewed by means of which the physical state of a system under consideration can be adjusted. Within the scope of the present invention, at least the feedback current (Feedback) is used to adjust the state parameter (X), specifically in the form of its flow rate (f-Feedback) as the adjustment parameter (Yl).

According to the invention, the "pressure (pP-1, pP-2, ... pPm) of the m phosgene reaction gas streams (Pl, P-2, ... Pm) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m)" is examined for changes. Such a change can be determined directly, for example, by continuously or periodically measuring the pressure. The principle of a pressure measurement using known pressure measuring devices is basically known to the person skilled in the art. A phosgene pressure measuring device can, for example, be an electronic, mechatronic or mechanical pressure gauge or a pressure transmitter. An electronic pressure gauge can be based on a pressure sensor that converts the measured pressure into an electronic signal. The advantage of electronic pressure gauges is their highly dynamic behavior under low material stress and a correspondingly high load resistance and long-term stability. A mechanical pressure gauge can include the integration of electronic components at the measuring location. The measured pressure can be displayed on site, but the mechanical pressure gauge can also emit an electrical signal or contain an electrical switching function. The measured value can advantageously be read reliably on site with mechanical pressure gauges even if the power supply fails or the measurement signal is disturbed. By combining mechanical measuring devices with different signals and switches, a wide range of mechatronic pressure gauges can be created. A mechanical pressure gauge can include elastic measuring elements that deform under the influence of pressure. The measuring system of the mechanical pressure gauge can include a capsule, plate or Bourdon tube. Mechanical pressure gauges can be advantageous due to their robustness and ease of use. Alternatively, the phosgene pressure gauge can be a pressure transmitter. A pressure transmitter can be a pressure gauge that enables pressure measurement under difficult measuring conditions. Difficult measuring conditions can include, for example, corrosive, highly viscous or fibrous media, very high temperatures, poorly located measuring points, hygiene regulations or even toxic or environmentally harmful measuring media. The selection of a suitable measuring device is a routine task for the specialist and can also be carried out with the help of technical regulations, for example.

It is also possible to indirectly detect a change in pressure (p-P-l, p-P-2, ... p-P-m) by observing the effects that a change in pressure has on other devices. In particular, the position of the phosgene flow control valves (PV-1, PV-2, ..., PV-m) is a suitable indicator of pressure (p-P-l, p-P-2, ... p-P-m). If, for example, a phosgene flow control valve opens further and further or has to be opened, this is a sure sign of a pressure increase in the gas phase reaction arrangement downstream (the "back pressure" in the gas phase reaction arrangement increases, which must be compensated by a greater degree of opening of the phosgene flow control valve). If a change is detected, "the flow rate (f-return) of the recirculation flow (return) is used as the setting parameter (Y-l)". In the described example of a pressure increase, the flow rate (f-back) is increased to ensure a sufficiently high stoichiometric excess of phosgene. If the pressure drops (for example due to solid deposits suddenly coming loose), the flow rate (f-back) is reduced to avoid an excessively large stoichiometric excess of phosgene. For example, it can be provided that, when (at least) one of the phosgene flow control valves (PV-1, PV-2, ..., PV-m) is opened to a degree of 70% or more, preferably 80% or more, the inventive setting (control) of the state parameter (X) of the phosgene gas stream (P) is carried out via the setting parameter (Y). The percentage "opening degree" of a valve (the phosgene flow control valve as well as any other valve) describes its setting relative to the fully closed state. The opening degree of a fully closed valve is therefore 0% and that of a fully open valve is 100%.

Phosgene is used in the context of the present invention in a molar ratio of phosgene to primary amino groups of greater than 1, ie phosgene is used in a stoichiometric excess. This so-called "stoichiometric phosgene excess" is given in % of theory. Theoretically, 1 mole of phosgene reacts with 1 mole of primary amino groups. A phosgene excess of x % compared to primary amino groups therefore corresponds to a molar ratio n(phosgene)/n(primary amino groups) i + — i + —

(n = amount of substance) of — , for example — = 1.1 with 10% i + — stoichiometric excess of phosgene or for example — = 1.4 with

40% stoichiometric excess of phosgene. A target value (PÜSOLL) is specified for the stoichiometric excess of phosgene (which must be maintained equally in each of the m gas phase reaction arrangements (100-1, 100-2, ..., 100-m).

The "flow rate (f-return)" of the return flow (return) is understood to mean the quantity flow (specified as volume, quantity of substance or mass flow; also called flow rate or flow rate) of the return flow. The flow rate of the return flow can be measured using one or more return measuring devices. The principle of measuring a flow rate using known measuring devices is basically known to the person skilled in the art. A return measuring device can be a flow sensor or flow indicator, whereby the flow measuring device is suitable for continuously measuring a medium flowing through the flow sensor or flow indicator, such as a liquid, gas or vapor quantity of the medium flowing through. A flow sensor can be designed as a time measuring device (measuring a flow rate over a specified period of time). Flow sensors can comprise a primary media connection device, a measuring transducer and a transmitter. A measuring transducer can detect the liquid flowing through the primary device. A transmitter can then generate an output signal from the raw signal of the measuring transducer. A flow sensor can be, for example, a paddle wheel flow sensor, a turbine, a Rotaflow measuring device or a magnetic inductive sensor. Flow rates can also be determined using imaging techniques, e.g. MRI (Magnetic Resonance Imaging).

First, a brief summary of various possible embodiments of the invention follows:

In a first embodiment of the process for producing an isocyanate, which is compatible with all other embodiments except the one described below second embodiment, step (i) is carried out in m amine gas generating devices (106-1, 106-2, ..., 106-m).

In a second embodiment of the process for producing an isocyanate, which can be combined with all other embodiments except the first embodiment described above, in step (i), an amine reaction gas stream (A) is first generated in a (single) amine gas generating device (106), which is then divided into the m amine reaction gas streams (A-1, A-2, ..., A-m).

In a third embodiment of the process for producing an isocyanate, which can be combined with all other embodiments, each of the m gas phase reaction arrangements (100-1, 100-2, ..., 100-m) has a reaction device (100a-l, 100a-2, ..., 100a-m) and one of the

The reaction device has a separation device (100b-l, 100b-2, ..., 100b-m) arranged downstream of the process.

In a fourth embodiment of the process for producing an isocyanate, which is a special embodiment of the third embodiment, the reaction device (100a-l, 100a-2, ..., 100a-m) comprises a

Mixing zone (101-1, 101-2, ..., 101-m) and a reaction zone (102-1, 102-2, ..., 102-m) downstream of the mixing zone, wherein in the mixing zone the phosgene reaction gas stream (P-l, P-2, ..., P-m) and the

Amine reaction gas stream (A-l, A-2, ..., A-m) are mixed, and wherein in the reaction zone (102-1, 102-2, ..., 102-m) the gas phase reaction of the primary amine (contained in the respective amine reaction gas stream (A-l, A-2, ..., A-m)) with phosgene (contained in the respective phosgene reaction gas stream (P-l, P-2, ..., P-m)) takes place to obtain a reaction product mixture (RP-1, RP-2, ... RP-m).

In a fifth embodiment of the process for producing an isocyanate, which is a special embodiment of the third and fourth embodiments, the separation device (100b-l, 100b-2, ..., 100b-m) comprises a quench zone (107-1, 107-2, ..., 107-m) and a phase separation zone (108-1, 108-2, ..., 108-m) arranged downstream of the quench zone in terms of process technology, wherein in the quench zone the reaction product mixture (RP-1, RP-2, ..., RP-m) is cooled by bringing it into contact with a quench liquid to obtain a quench mixture, and wherein in the phase separation zone (108-1, 108-2, 108-m) a phase separation of the resulting quench mixture into the isocyanate crude product fluid stream (IC-1, IC-2, IC-m) and an additional crude residual fluid stream comprising isocyanate (R-crude-1, R-crude-2, R-crude-m) takes place.

In a sixth embodiment of the process for producing an isocyanate, which is a special embodiment of the fifth embodiment, the crude residual fluid stream (R-raw-1, R-raw-2, ..., R-raw-m) comprising additional isocyanate obtained from the phase separation zone (108-1, 108-2, ..., 108-m) is fed to a washing device (105-1, 105-2, ..., 105-m) arranged downstream of the phase separation zone in terms of process technology, in which the additional isocyanate is washed out of the crude residual fluid stream (R-raw-1, R-raw-2, ..., R-raw-m) with a washing liquid to obtain the residual fluid stream (R-l, R-2, ..., R-m).

In a seventh embodiment of the process for producing an isocyanate, which is a special embodiment of the sixth embodiment, a first partial stream of the liquid desorption medium (DesM) is returned to the phosgene gas generation device (104) via a first heat exchanger, the first partial stream being indirectly heated in the first heat exchanger by a heat transfer medium, and a second partial stream of the liquid desorption medium (DesM) is passed through a second heat exchanger, the second partial stream indirectly heating the phosgene reaction gas stream (P) in the second heat exchanger and the second partial stream of the liquid desorption medium (DesM) cooled in the process being used as a component of the washout liquid.

In an eighth embodiment of the process for producing an isocyanate, which can be combined with all other embodiments and is in particular a special embodiment of the seventh embodiment, the phosgene gas generation device (104) has an introduction region (111) for introducing the recycle stream (return) and a phosgene gas generation device collection region (110), wherein the desorbed phosgene and the phosgene obtained from the phosgene fresh fluid stream (fresh) collect in the phosgene gas generation device collection region (110) to form the phosgene reaction gas stream (P). In a ninth embodiment of the process for producing an isocyanate, which is a special embodiment of the eighth embodiment, a desorption column is used as the phosgene gas generating device (104), which has a sump (112) in which the liquid desorption medium (DesM) collects.

In a tenth embodiment of the process for preparing an isocyanate, which can be combined with all other embodiments, an absorption column is used as the phosgene absorption device (103-1, 103-2, ... 103-m or 103-n), wherein the phosgene absorption device collection region (115-1, 115-2, ... 115-m or 115-n) is arranged in a bottom of the absorption column.

In an eleventh embodiment of the process for producing an isocyanate, which can be combined with all other embodiments, the setting parameter (Y) additionally comprises a setting parameter (Y-2) of the phosgene fresh fluid stream (Fresh), wherein the state parameter (X) is set (= regulated) in such a way that when a change is detected in (at least) one pressure (p-P-1, p-P-2, ... p-P-m) of the m phosgene reaction gas streams (P-l, P-2, ... P-m) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m), first the flow rate (f-return) of the recycle stream (return) is used as the setting parameter (Y-l) and then the flow rate (f-fresh) of the introduced phosgene fresh fluid stream (Fresh) is used as the setting parameter (Y-2).

In a twelfth embodiment of the process for producing an isocyanate, which is a particular embodiment of the eleventh embodiment, the flow rate (f-Return) of the recycle stream (Return) is adjusted by setting a first adjustment parameter (ZI) of the absorbed phosgene comprised in the recycle medium and a second adjustment parameter (Z2) of the absorbent comprised in the recycle medium.

In a thirteenth embodiment of the process for producing an isocyanate, which is a particular embodiment of the twelfth embodiment, the flow rate (f-Return) of the recycle stream (Return) is adjusted by setting a phosgene mass (mP-1, mP-2, ..., mPm or mPn) of the absorbed phosgene contained in the recycle medium (Return-1, Return-2, ..., Return-m or Return-n) as a first adjustment parameter (Zl-1, Zl-2, ..., Zl-n or Zl-m) and an absorption medium mass (m-AbsM-1, m-AbsM-2, ... m-AbsM-m or m-AbsM-n) of the absorption medium contained in the return medium (Return-1, Return-2, ..., Return-m or Return-n) as a second adjustment parameter (Z2-1, Z2-2, ..., Z2-n or Z2-m).

In a fourteenth embodiment of the process for producing an isocyanate, which is a particular embodiment of the thirteenth embodiment, the phosgene mass (m-P-1, m-P-2, ..., m-P-m or m-P-n) is adjusted by manipulating the flow rate (f-fresh) of the phosgene fresh fluid stream (fresh), wherein the absorbent mass (m-AbsM-1, m-AbsM-2, ... m-AbsM-m or m-AbsM-n) is adjusted by manipulating a flow rate (f-AbsM-1, f-AbsM-2, ..., f-AbsM-m or f-AbsM-n) of the absorbent fluid stream (AbsM-1, AbsM-2, ..., AbsM-m or AbsM-n).

In a fifteenth embodiment of the process for producing an isocyanate, which is a further particular embodiment of the thirteenth embodiment, the recycle medium (recycle-1, recycle-2, ..., recycle-m or recycle-n) comprises the absorbed phosgene and non-phosgene components, wherein the non-phosgene components comprise the absorption medium, wherein the phosgene mass (m-P-1, m-P-2, ..., m-P-m or m-P-n) is determined in each case by multiplying a filling level (L-recycle-1, L-recycle-2, ..., L-recycle-m or L-recycle-n) of the recycle medium in the phosgene absorption device collection region (115-1, 115-2, ... 115-m or 115-n) and a concentration (x-P-1, x-P-2, ..., x-P-m or x-P-n) of the absorbed phosgene in the recycle medium (Recycle-1, Recycle-2, ..., Recycle-m or Recycle-n) is calculated, and wherein the absorption medium mass (m-AbsM-1, m-AbsM-2, ... m-AbsM-m or m-AbsM-n) is calculated by multiplying the filling level (L-recycle-1, L-recycle-2, ..., L-recycle-m or L-recycle-n) of the recycle medium (Recycle-1, Recycle-2, ..., L-recycle-m or L-recycle-n) in the phosgene absorption device collection area (115-1, 115-2, ... 115-m or 115-n) and a non-phosgene component concentration (l-(x-P-l), l-(x-P-2), l-(x- P-m) or l-(x-P-n)] of the non-phosgene components in the return medium (return-1, return-2, ..., return-m or return-n) is calculated.

In a sixteenth embodiment of the process for producing an isocyanate, which can be combined with all other embodiments,

- the absorbent (AbsM) selected from decahydronaphthalene, toluene, xylene, chlorobenzene, para-dichlorobenzene, orü7?o-dichlorobenzene, chlorotoluene, Chloronaphthalene or mixtures thereof; preferably the absorbent is orü7?o-dichlorobenzene, and/or

- the primary amine is selected from toluenediamine, diphenylmethanediamine, xylylenediamine, 1,5-pentanediamine, 1,6-hexamethylenediamine, isophoronediamine, diaminodicyclohexylmethane or mixtures thereof; preferably the primary amine is toluenediamine.

In a seventeenth embodiment of the process for preparing an isocyanate, which can be combined with all other embodiments, a target value (PÜSOLL) in the range from 10% of theory to 400% of theory, preferably from 20% of theory to 150% of theory, is set for the stoichiometric phosgene excess.

In a first embodiment of the method for controlling the stoichiometric phosgene excess, which can be combined with all other embodiments, a self-learning algorithm is implemented on the processor for automatically comparing changes in (at least) one pressure (p-P-1, p-P-2, ... p-P-m) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m) with the stoichiometric phosgene excess and, in response thereto, for automatically setting and adjusting the setting parameter (Y), wherein the algorithm is configured to optimize the process for producing an isocyanate by adjusting the setting parameter (Y) (i.e. to counteract deviations from the stoichiometric phosgene excess more quickly).

The embodiments briefly described above and further possible embodiments of the invention are explained in more detail below. All embodiments described above and the further embodiments of the invention described below can be combined with one another as desired, even across category boundaries, unless the context clearly indicates the opposite to the person skilled in the art or unless something else is expressly stated. For example, all embodiments or features mentioned in connection with the proposed process for producing an isocyanate can also be used without further ado. Represent designs or features of the proposed plant for producing an isocyanate, and vice versa.

For the sake of simplicity, for those devices of which two or more are or can be provided according to the invention (i.e., for example, the gas phase reaction arrangements (100-1, 100-2, ..., 100-m) or the amine gas generation devices (106-1, 106-2, ..., 106-m)), only the respective main reference symbol (i.e., for example, "100" or "106", without the suffixed numbering) is used in the following text. This is done solely for reasons of linguistic simplification and is not intended to imply that there is only one such device (if this is even possible within the scope of the invention), unless this is clear from the context (as, for example, in the description of the embodiment using a single amine gas generation device 106 for all m gas phase reaction arrangements).

The process according to the invention and the plant according to the invention designed to carry it out serve to produce an isocyanate. The isocyanate is obtained by reacting a primary amine with phosgene by means of a gas phase reaction. In connection with the present invention, in principle any primary amine that can be evaporated without decomposition can be reacted. For example, the primary amine can be a primary diamine. The primary amine can be an isomer mixture that includes several isomeric forms of a primary amine. If the primary amine can exist in different isomeric forms, the isomer mixture can in principle include all possible isomer distributions. The term gas phase reaction is understood to mean that during the reaction of the primary amine with phosgene to produce the isocyanate, at least the reaction components primary amine, phosgene and isocyanate are present in a gaseous state.

In order to enable the gas phase reaction, in the first process step (i) of the process according to the invention, the m amine reaction gas streams (Al, A-2, ..., Am), each comprising a primary amine, are first generated (see also FIG. 1). An amine reaction gas stream is understood to mean a gas stream which comprises the primary amine in the gaseous state. An amine reaction gas stream is generated in an amine gas generation device (106). It is possible to use a separate amine gas generation device (106-1, 106-2, ..., 106-m) for each of the amine reaction gas streams (Al, A-2, ..., Am) to be provided. This is shown in FIG. 1. It is also possible to first generate an amine reaction gas stream (A) in a (single) amine gas generation device (106), which is then divided into the m amine reaction gas streams (Al, A-2, ..., Am). The first variant simplifies the control of the system, the second variant requires less equipment. What is more favorable in a specific case depends on the exact circumstances of the individual case. According to the invention, a plant for carrying out the method according to the invention comprises several gas phase reaction arrangements and one or more amine gas generating devices. Preferably, the plant comprises several amine gas generating devices, with each gas phase reaction arrangement being assigned one amine gas generating device. In other words, the plant preferably comprises one amine gas generating device per gas phase reaction arrangement. Each of the several amine gas generating devices can be arranged in the one gas phase reaction arrangement associated with it in terms of process technology. Particularly preferably, each of the several amine gas generating devices is part of the one gas phase reaction arrangement associated with it in terms of process technology.

An amine gas generation device can in principle be understood as any device suitable for generating an amine reaction gas stream. Methods for generating an amine reaction gas stream are known to the person skilled in the art. The conversion of the primary amine into the gas phase can in principle take place in all amine gas generation devices known from the prior art. For example, the amine gas generation device can be an evaporation apparatus such as a falling film evaporator. The amine gas generation device can for example be an evaporation apparatus in which a small working content is passed over a falling film evaporator with a high circulation capacity. For example, evaporation apparatus can be used in which a small working content is circulated over at least one micro heat exchanger or micro evaporator. Such use of appropriate heat exchangers for the evaporation of amines is disclosed for example in EP 1754698. To minimize the thermal load on the amine, the evaporation process can be supported by feeding in an inert gas such as nitrogen, helium or argon, regardless of the precise design of the evaporation apparatus. The evaporation of the primary amine and, if necessary, its superheating, in particular to a temperature in the range from 225 °C to 430 °C, preferably 250 °C to 420 °C, particularly preferably 250 °C to 400 °C, can be carried out in several stages in order to avoid unevaporated droplets in the amine reaction gas stream generated. Multi-stage evaporation and superheating steps can be carried out, with droplet separators being installed between the evaporation and superheating systems and/or the evaporation apparatus also having the function of a droplet separator. Suitable droplet separators are known to the person skilled in the art. After leaving the last superheater in the direction of flow, the gaseous amine, which has been preheated to its target temperature, can then be introduced into the gas phase reaction arrangement. Regardless of the detailed design of the amine feed, the risk of renewed droplet formation can be counteracted by technical measures such as sufficient insulation to avoid radiation losses.

In a preferred embodiment, the primary amine is selected from the group consisting of toluenediamine, diphenylmethanediamine, xylylenediamine, 1,5-pentanediamine, 1,6-hexamethylenediamine, isophoronediamine, diaminodicyclohexylmethane, and mixtures thereof. If the primary amine can be present in various isomeric forms, all isomer distributions are included. In principle, mixtures of several primary amines can also be reacted. The primary amine is preferably toluenediamine. Toluenediamine, which is preferably used, comprises in particular 78% by mass to 82% by mass of 2,4-toluenediamine and 18% by mass to 22% by mass of 2,6-toluenediamine, based on the total mass of the 2,4- and 2,6-toluenediamine isomers. Based on the total mass of the toluenediamine, the 2,4- and 2,6-toluenediamine isomers preferably make up from 95% by mass to 100% by mass, particularly preferably from 98% by mass to 100% by mass. The content of toluenediamine isomers with amine groups in the 7?o position relative to one another in the toluenediamine to be used is very particularly preferably less than 0.2% by mass, based on the total mass of the toluenediamine to be used. Toluenediamine. Processes for providing toluenediamine with the above requirements are known to the person skilled in the art.

Scheme: Chemical structures of 2,4-toluenediamine (left) and 2,6-toluenediamine (right).

In the second process step (ii) of the process according to the invention, the phosgene gas stream (P) comprising phosgene is first generated in a phosgene gas generation device (104) and then divided into the m phosgene reaction gas streams (P-1, P-2, ... P-m) comprising phosgene. For this purpose, splitting devices (not shown as separate devices in FIG. 1) are provided. Such a splitting device is designed to split the phosgene gas stream (P) into several phosgene reaction gas streams (P-1, P-2, P-m). One of the phosgene reaction gas streams (P-1, P-2, P-m) is fed to each gas phase reaction arrangement.

A phosgene (reaction) gas stream is understood to be a gas stream which comprises phosgene in the gaseous state. A phosgene gas generation device (104) is understood to be a device which is designed to generate a phosgene gas stream from liquid or dissolved phosgene. For example, phosgene present in the phosgene gas generation device can be heated to a temperature in the range from 200 °C to 350 °C, preferably 250 °C to 325 °C, particularly preferably 250 °C to 300 °C, for generating the phosgene gas stream and optionally diluted with an inert gas such as nitrogen, helium or argon. The phosgene gas stream (P) leaving a phosgene gas generation device (104) contains portions of freshly produced phosgene (fresh) and portions of recycled phosgene (recycled). The freshly produced phosgene comes from a fresh phosgene production facility (109), in which phosgene is synthesized from chlorine and carbon monoxide in a manner known per se.

According to a preferred embodiment of the method according to the invention, which is also shown in FIG. 1, it is provided that a phosgene gas generating device (104) is used which has an introduction area (111) for Introduction of the recycle stream (return) into the phosgene gas generation device (104) and a phosgene gas generation device collection area (110). Desorbed phosgene and the phosgene obtained from the phosgene fresh fluid stream (fresh) collect in the phosgene gas generation device collection area (110) to form the phosgene gas stream (P). The pressure of the phosgene gas stream formed (ie the phosgene gas stream which is formed from desorbed phosgene and phosgene obtained from the phosgene fresh fluid stream) is set as a state parameter (X) via the flow rate of the recycle stream upon introduction into the introduction area as a setting parameter (Yl). An introduction region (111) can be understood to mean a region which is designed to receive the recycle stream (return) and to introduce phosgene to be recycled contained in the recycle stream (return) into the phosgene gas generation device for desorption from the absorption medium. The introduction region can be, for example, an inlet to the phosgene gas generation device (104).

In a further embodiment of the method according to the invention, it is provided that a desorption column is used as the phosgene gas generating device (104). A desorption column can be understood to mean a rectification column in which a phase equilibrium between desorption and absorption prevails, with the phase equilibrium being on the desorption side. In the desorption column, the phosgene to be recycled, which is contained in the recycle stream, is therefore (predominantly) desorbed from the absorbent to obtain desorbed phosgene. The desorption column can comprise at least one separation stage, preferably several separation stages. The desorption column can, for example, be a tray column with horizontal trays installed at regular intervals, with each tray corresponding to a separation stage. Trays can consist of circular plates with several passage openings for the gas flow, so-called bells, slots or valves. The trays can be installed horizontally in the tray column at regular vertical intervals, for example at vertical intervals of 0.3 to 1.0 m. In addition to the trays, the desorption column can also contain packings, particularly in the stripping section. The Desorption column has a sump (112) in which liquid desorption medium (DesM) collects.

For example, a desorption column can be used which is fed with the recycle stream via a desorption column inlet, the desorption column inlet being arranged in the introduction region (111) of the desorption column. The phosgene gas generation device collection region (110) is arranged at a top of the desorption column. The pressure of the phosgene gas stream (P) formed in the phosgene gas generation device collection region (110) arranged at the top of the desorption column can thus be set as a state parameter (X) via the flow rate of the recycle stream when introduced into the desorption column inlet as a setting parameter (Y-1). This is particularly the case when the stream of phosgene absorbed in the absorbent, which is introduced into the desorption column inlet, is directly evaporated (at least to a considerable extent).

In the third process step (iii), each of the phosgene reaction gas streams generated (two, Pl and P-2, are shown in FIG. 1 by way of example, i.e. m = 2) is combined with one of the amine reaction gas streams generated (Al and A-2 in FIG. 1) in one of the gas phase reaction arrangements. It is preferred that each of the m gas phase reaction arrangements has a phosgene gas inlet and a separate amine gas inlet, the respective phosgene reaction gas stream being introduced into the corresponding gas phase reaction arrangement through the phosgene gas inlet and the respective amine reaction gas stream being introduced through the separate amine gas inlet. To regulate the phosgene reaction gas stream (Pl, P-2, ..., Pm) introduced into the gas phase reaction arrangement, an adjustment means is provided which comprises a phosgene gas valve, the phosgene flow control valve (PV-1, PV-2, ..., PV-m), arranged in particular at the phosgene gas inlet. To regulate the amine reaction gas stream introduced into the gas phase reaction arrangement, an adjustment means can preferably be used which comprises an amine gas valve, i.e. an amine flow control valve (AV-1, AV-2, ..., AV-m), arranged in particular at the separate amine gas inlet. The actual adjustment of the amount of gaseous amine reaction gas stream required for a desired amine flow takes place by supplying energy to the amine gas generation device (106). In particular, the amine gas generation device, and accordingly also the amine reaction gas stream generated therein, which is introduced into the gas phase reaction arrangement through the amine gas inlet, reacts very sensitively to pressure drops in the gas phase reaction arrangement. As previously described, such pressure drops can be caused in particular by the breaking off of subsequent and by-products of the gas phase reaction adhering to the inner walls of the gas phase reaction arrangement in the form of solid residues. A separate introduction of the phosgene reaction gas stream and the amine reaction gas stream into each of the several gas phase reaction arrangements now results in the particular advantage that in the event of a pressure drop in the gas phase reaction arrangement, a disturbance of the amine gas generation device or the generated amine reaction gas stream has no effect on the generated phosgene reaction gas stream or its introduction into the gas phase reaction arrangement. In other words, the plant according to the invention comprising the adjusting agent according to the invention is able to compensate for the described pressure drops in the gas phase reaction arrangement (e.g. a gas phase reactor), also by the separate introduction of the phosgene and amine reaction gas stream.

The combination of the phosgene and amine reaction gas streams is carried out while maintaining a stoichiometric excess of phosgene with respect to the amino groups of the primary amine to be reacted. A preferred molar ratio of phosgene to primary amino groups in normal operation (i.e. as soon as the desired flow rates of phosgene reaction gas stream and amine reaction gas stream are reached after starting up a gas phase reaction arrangement) of 1.1:1 (corresponding to a stoichiometric phosgene excess of 10% of theory) to 5.0:1 (corresponding to a stoichiometric phosgene excess of 400% of theory), particularly preferably of 1.2:1 (corresponding to a stoichiometric phosgene excess of 20% of theory) to 2.5:1 (corresponding to a stoichiometric phosgene excess of 150% of theory) can be set as the setpoint PÜSOLL. When starting up the plant, only phosgene is added first and then the amine. In the start-up phase, the excess of phosgene is therefore initially "infinite" and can reach very high values even after the first addition of the amine, such as 1900% of theory (corresponding to a molar ratio of phosgene to primary amino groups of 20:1). The invention allows the stoichiometric excess of phosgene to be adjusted so that it is as constant as possible, i.e. fluctuates as little as possible (the phosgene excess has a significant influence on the reaction temperature and thus on the reaction rate; the residence time also changes depending on the phosgene excess).

The gas phase reaction arrangement comprises in particular a gas phase reactor. The gas phase reactor can be, for example, a tubular reactor, in particular a vertically arranged tubular reactor with a cylindrical, conical or cylindrical-conical shape.

The gas phase reaction of the primary amine with phosgene to form the isocyanate then takes place in the gas phase reaction arrangement, with an isocyanate crude product fluid stream IC (IC-1 and IC-2 in FIG. 1) and a residual fluid stream R (R-1 and R-2 in FIG. 1) being obtained from each of the m gas phase reaction arrangements. The isocyanate crude product fluid stream IC obtained in each case comprises isocyanate produced. The residual fluid stream R in turn comprises unreacted, excess phosgene in the gas phase reaction. The method according to the invention basically provides for unreacted, excess phosgene to be returned to the phosgene gas generation device (104) and thus made available for the generation of the phosgene-comprising phosgene gas stream (P) and the m phosgene reaction gas streams. In other words, the stoichiometric phosgene excess is cyclized.

The return of the unreacted excess phosgene contained in the residual fluid stream to the phosgene gas generation device (104) in the fourth process step (iv) optionally initially comprises, in a first sub-step (iv-1), combining the m residual fluid streams (R-1, R-2, ..., R-m) to form n residual fluid streams (R-1, R-2, ... R-n), where n is a natural number in the range from 1 to m-1 ("m minus one") and in particular 1. This optional step is not shown in FIG. 1, but may nevertheless be technically and economically expedient, depending on the circumstances of the individual case.

In a second sub-step (iv-2), each of the m or n residual fluid streams is then introduced into a respective phosgene absorption device (103-1, 103-2, ... 103-m or 103-n). Each phosgene absorption device has a phosgene absorption device collection region (115-1, 115-2, ... 115-m or 115-n). To carry out the absorption, an absorption medium fluid stream (AbsM-1, AbsM-2, ..., AbsM-m or AbsM-n), each containing an absorption medium (AbsM), is also introduced into each of the phosgene absorption devices (103-1, 103-2, ..., AbsM-m or AbsM-n), each containing an absorption medium (AbsM). In each phosgene absorption device, the excess phosgene not converted in the gas phase reaction is absorbed by the absorption medium to form phosgene to be recycled. The excess phosgene not converted is present in a gas stream together with the hydrogen chloride gas formed as a co-product and is washed out of this gas stream in the phosgene absorption device. The recycle stream (recycle) is then obtained from the phosgene absorption device, which comprises phosgene to be recycled. In principle, an absorption medium can be understood as any inert solvent that is suitable for absorbing phosgene. An inert solvent can be understood as a solvent that is at least inert towards the primary amine, phosgene and the isocyanate produced. In other words, the inert solvent is not involved in the gas phase reaction. The inert solvent is preferably an organic solvent. The organic solvent can comprise, for example, aliphatic hydrocarbons, aromatic hydrocarbons with or without halogen substitution, and mixtures thereof. Absorption of phosgene by the absorption medium is understood as the absorption of phosgene into the free volume of the absorption medium that is in a different phase than phosgene. For example, phosgene that is in a gaseous state can form a gaseous phase in the phosgene absorption device, while absorption medium that is in a liquid state forms a liquid - i.e. a different - phase in the phosgene absorption device. During absorption of the phosgene by the absorption medium, the gaseous phosgene is absorbed predominantly or completely into the free volume of the liquid absorption medium. A particularly preferred absorption medium is ortho-dichlorobenzene. According to one embodiment of the process according to the invention, it can be provided that an absorption column is used as a phosgene absorption device (103-1, 103-2, ... 103-m or 103-n). An absorption column (also called an absorber) is understood to mean a process engineering apparatus. understand in which the residual fluid stream is brought into contact with the absorption medium fluid stream comprising the absorption medium in order to absorb phosgene from the residual fluid stream in the absorption medium. In the absorption column, a phase equilibrium between desorption and absorption can prevail, with the phase equilibrium in the absorption column being on the absorption side. In the absorption column, therefore, the unreacted, excess phosgene contained in the residual fluid stream is (predominantly) absorbed by the absorption medium to obtain phosgene for recycling. For this purpose, the absorption column can have one or more of the following six absorption column regions: a bottom, a gas inlet, a contact section, an absorption medium feed, a droplet separator, and a head. The absorption column can have a sump as a phosgene absorption device collection area (115-1, 115-2, ... 115-m or 115-n), the recycle medium collecting in the sump, the recycle medium being withdrawn from the sump and fed as a recycle stream to the phosgene gas generation device, which serves as a phosgene desorption device. The absorption column can also have a gas inlet, the residual fluid stream being fed into the gas inlet, after which the interior is uniformly supplied with the recycle stream by suitable flow guidance (e.g. by internals). In addition, the absorption column can have a contact section in which the absorption takes place. The absorption column can also have an absorption agent feed, in which the absorption agent fluid stream is fed, after which the interior is uniformly supplied with the absorption agent by suitable flow guidance (e.g. by internals). In addition, the absorption column can have a droplet separator and/or an absorption column head, with a hydrogen chloride stream freed from unreacted, excess phosgene leaving the absorption column at the absorption column head. The absorption column can be a packed or tray column, a vortex washer, a rotary washer, a venturi washer, an immersion washer or a spray washer. The absorption column is preferably a packed column. A packed column can be understood as a column which is loosely layered with packing is filled. Filling elements can consist of a material that includes stainless steel.

Irrespective of the precise design of the phosgene absorption device (103), it is preferred that the absorption medium (AbsM) is selected from the group consisting of decahydronaphthalene, toluene, xylene, chlorobenzene, para-dichlorobenzene, ortho-dichlorobenzene, chlorotoluene, chloronaphthalene, and mixtures thereof. The absorption medium is preferably chlorobenzene or ortho-dichlorobenzene, particularly preferably ortho-dichlorobenzene.

The recycle stream (recycle) obtained from the phosgene absorption device (103) and comprising phosgene to be recycled is introduced into the phosgene gas generation device (104) at a flow rate (f-recycle) in the third sub-step (iv-3), in which desorption of the phosgene to be recycled from the absorption medium takes place to obtain desorbed phosgene. In the context of the present invention, the term desorption describes the release of gaseous phosgene from the phosgene absorbed in the absorption medium (dissolved in the absorption medium) obtained in the phosgene absorption device. This can be done by simply expelling gaseous phosgene by reducing the pressure and/or by increasing the temperature, but also by distillation or rectification with a liquid return of condensed phosgene. The pressure during desorption should in particular be above the pressure at the inlet to the m gas phase reaction arrangements.

In addition, as already mentioned, a fresh phosgene fluid stream (Fresh) is introduced into the phosgene gas generation device (104). The phosgene gas stream (P) generated in the phosgene gas generation device is thus replaced by the desorbed phosgene and the phosgene from the additionally introduced into the phosgene gas generation device.

Fresh fluid stream (fresh) obtained phosgene. The phosgene gas generation device can be a rectification column. A rectification column can be, for example, a tray column or a packed column. Rectification is understood to be a thermal separation process which is an extension of distillation or a series of many distillation steps. The main advantages of rectification are that the plant can be operated continuously and that the separation effect is many times greater than that of distillation, since the steam is mixed several times in countercurrent with the liquid. one behind the other. A rectification column therefore operates more efficiently in terms of energy, is less technically complex and takes up less space than a series of single distillations. The contact surface between the vapor and liquid phases in the rectification column is provided, for example, by internals (e.g. bubble cap trays, random packings, packing), with a mixture with a changed concentration condensing from the vapor mixture at each of these additional contact surfaces in accordance with the phase equilibrium, while a mixture with a higher concentration of the more volatile component evaporates from the liquid phase as a result of the heat of condensation released. The temperatures on the separation stages of a rectification column can be influenced at a given pressure by the ratio of liquid to vapor (F/D ratio). The F/D ratio in rectification columns is set by the reflux ratio at the top of the column. If a liquid mixture is evaporated in the rectification column, the concentrations of the individual substances in the gas and liquid phase are determined by the temperature and pressure of the rectification column.

According to a further embodiment of the method according to the invention, gas phase reaction arrangements (100) can be provided which each have a reaction device (100a) and a separation device (100b) arranged downstream of the reaction device in terms of process technology. This is shown in FIG. 2, whereby for reasons of clarity only one of the at least two gas phase reaction arrangements (100) according to the invention is shown, namely (100-1). In the reaction device (100a-1), the respective phosgene reaction gas stream and the respective amine reaction gas stream are combined, whereby the gas phase reaction of the primary amine with phosgene takes place to obtain a reaction product mixture. In the separation device (100b-l), the reaction product mixture obtained from the reaction device (100a-l) is again cooled by contacting it with a quench liquid to obtain a quench mixture, and a phase separation of the resulting quench mixture into the isocyanate crude product fluid stream and an additional crude residual fluid stream comprising isocyanate is carried out. In a preferred embodiment of the invention, which is also shown in FIG. 2, reaction devices (100a-1, 100a-2, ..., 100a-m) are provided for this purpose, each of which comprises a mixing zone (101-1, 101-2, ..., 101-m) and a reaction zone (102-1, 102-2, ..., 102-m) downstream of the mixing zone in terms of process technology. In the mixing zone, the corresponding phosgene reaction gas stream and the corresponding amine reaction gas stream are mixed. In the reaction zone, the gas phase reaction of the primary amine with phosgene takes place to obtain the reaction product mixture (RP-1, RP-2, ... RP-m). The reaction product mixture comprises at least produced isocyanate.

In a further preferred embodiment of the invention, which is also shown in FIG. 2, such separation devices (100b-l, 100b-2, ..., 100b-m) are also provided, each of which comprises a quench zone (107-1, 107-2, ..., 107-m) and a phase separation zone (108-1, 108-2, ..., 108-m) arranged downstream of the quench zone in terms of process technology. In the quench zone, the reaction product mixture obtained from the reaction device is cooled by bringing it into contact with a quench liquid to obtain a quench mixture. The quench liquid can be introduced into the quench zone in the form of one or more quench streams. A quench stream can, for example, comprise a portion of the isocyanate crude product fluid stream, which is branched off from the gas phase reaction arrangement after the isocyanate crude product fluid stream has been obtained and is returned to the quench zone arranged in the gas phase reaction arrangement. The quench liquid can, for example, be injected into the quench zone (e.g. via a mixing nozzle). The quench mixture obtained comprises at least the quench liquid and the isocyanate produced. In the phase separation zone downstream in the process, the quench mixture obtained from the quench zone is then phase separated into the isocyanate crude product fluid stream and the crude residual fluid stream comprising additional isocyanate. The isocyanate crude product fluid stream comprises at least the isocyanate produced.

FIG. 2 also shows further preferred details of the invention relating to heat transfer processes from one process stream to another. The generated phosgene gas stream (P) is divided into Pl, P-2, ... Pm and fed from the phosgene gas generating device (104) via one (or more) Heat exchanger (WT2) and introduced into the mixing zone (101) via a phosgene gas valve (= phosgene flow control valve PV-1). In the mixing zone 101, the phosgene reaction gas stream Pl and the amine reaction gas stream Al are mixed. This results in a mixture of at least gaseous primary amine and gaseous phosgene, which is then passed through a reaction zone 102-1 arranged within the gas phase reaction arrangement. In the reaction zone 102-1, the gas phase reaction of the primary amine with phosgene takes place to produce isocyanate. This gives a gaseous reaction product mixture RP-1, which is passed into a quench zone 107-1, which is also arranged within the gas phase reaction arrangement. In the quench zone 107-1, the gaseous reaction product mixture RP-1 is brought into contact with a quench liquid and thus cooled. The quench liquid is introduced into the quench zone 107-1 in the form of two quench streams. The first quench stream corresponds to a portion of a liquid isocyanate crude product liquid stream IC-1 comprising produced isocyanate, which is branched off after being obtained from a phase separation zone (108-1) downstream of the quench zone 107-1 in terms of process technology and is pumped back into the quench zone 107-1 through a heat exchanger WT3 using a pump PP. The second quench stream in turn corresponds to a liquid washout stream Aus-1, which is obtained from a washout device 105-1 downstream of the phase separation zone 108-1 in terms of process technology. According to a further embodiment of the method according to the invention, it is namely provided that the additional crude residual fluid stream comprising isocyanate obtained from the phase separation zone is fed to a washout device downstream of the phase separation zone in terms of process technology. In the washing device, additional isocyanate is washed out of the raw residual fluid stream using a washing liquid while retaining the residual fluid stream. A washing liquid can basically be understood to mean any liquid that is suitable for washing out isocyanate from the raw residual fluid stream. The washing liquid can comprise parts of the absorbent. The washing liquid preferably comprises an inert solvent, and the washing liquid particularly preferably comprises a halogenated aromatic solvent such as chlorobenzene or dichlorobenzene (in particular ortho-dichlorobenzene). The two liquid quench streams are fed into the quench zone via a mixing nozzle

107-1. A quench mixture obtained from the quench process is then fed into a phase separation zone 108-1, which is also arranged within the gas phase reaction arrangement. In this zone, a phase separation of the quench mixture into a liquid phase and a gaseous phase is carried out. The liquid phase, which comprises at least the quench liquid and liquid isocyanate produced, collects at the bottom of the phase separation zone

108-1, and is discharged there through an outlet arranged at the bottom of the phase separation zone 108-1 as isocyanate crude product fluid stream IC-1. The gaseous phase is discharged from the phase separation zone 108-1 (e.g. via flaps for shutting off) and fed into the scrubbing device 105-1 as gaseous crude residual fluid stream R-crude-1. The gaseous crude residual fluid stream is scrubbed in the scrubbing device 105-1 with a scrubbing liquid which is composed of liquid desorption medium (DesM-2; see also the explanations below) taken from the heat exchanger WT2 and optionally (particularly when starting up the system) a branched-off part of a liquid absorbent fluid stream AbsM comprising an absorbent. Chlorobenzene or dichlorobenzene, in particular ortho-dichlorobenzene, is preferably suitable as the liquid absorbent. A gaseous residual fluid stream Rl is obtained from the scrubbing device 105-1, which comprises excess phosgene that was not converted in the gas phase reaction. The excess phosgene that was not converted is returned to the phosgene gas generation device 104. For this purpose, the residual fluid stream Rl is introduced via one (or more) heat exchangers WT4 into a region 114-1 of the phosgene absorption device 103-1 located above the liquid level in the sump, the phosgene absorption device 103-1 being an absorption column and the phosgene absorption device collection region 115-1 of the phosgene absorption device 103 being the sump of the absorption column. In addition, part of the liquid absorbent fluid stream AbsM-1 is introduced into the absorption column. In the absorption column, excess phosgene gas contained in the gaseous residual fluid stream Rl and not converted in the gas phase reaction is then absorbed by the liquid absorption medium. A liquid recycle medium collects in the bottom of the absorption column, which contains absorbed phosgene with a mass mP and the absorption medium with a mass m-AbsM. Hydrogen chloride gas (HCl-1) formed in the gas phase reaction is discharged through a gas outlet arranged at the top of the absorption column. The hydrogen chloride gas can optionally be converted into chlorine and hydrogen in chemical or electrochemical conversion processes. The chlorine obtained in this way can then be fed to the fresh phosgene generation device 109 to generate phosgene and converted there into fresh phosgene. The liquid recycle medium in the sump is discharged through a sump outlet arranged in the bottom of the absorption column and pumped back as a recycle stream via a heat exchanger WT5 by means of a pump into a phosgene gas generation device 104, which is a desorption column. For this purpose, the recycle stream is introduced into an introduction region 111, which is a desorption column feed. The recycle stream comprises at least phosgene to be returned, which has been absorbed by the absorbent. In the desorption column, the phosgene to be recycled is desorbed from the absorption medium to form desorbed phosgene gas. In addition, a cold, liquid phosgene fresh fluid stream Frisch is introduced into a phosgene gas generation device collection area 110, which is arranged at the top of the desorption column. The phosgene gas stream P is formed in the phosgene gas generation device collection area 110 from the desorbed phosgene gas and the phosgene gas evaporated from the phosgene fresh fluid stream Frisch, which is discharged through a gas outlet arranged at the top of the desorption column. In the bottom of the desorption column (112) a liquid desorption medium (DesM) is also formed, which essentially consists of the absorption medium used in the phosgene absorption device 103 (i.e. preferably chlorobenzene or dichlorobenzene, in particular ortho-dichlorobenzene) and which is discharged through a bottom outlet arranged in the bottom of the desorption column and is partially (DesM-2) fed by means of a pump PP into a heat exchanger WT2 arranged downstream of the desorption column 104. In the heat exchanger WT2 an indirect heat transfer from DesM-2 to P takes place, cooling the former and heating the latter. The phosgene gas stream P heated in this way is divided into Pl, P-2, ... Pm and passed via the phosgene flow control valve PV-1 into the mixing zone 101-1. This completes the phosgene cycle via which in the Excess phosgene not converted in the gas phase reaction is recycled and made available again for the generation of the phosgene reaction gas stream for the gas phase reaction. The cooled stream of the desorption medium DesM-2 is passed into the scrubbing device 105-1. Before entering the heat exchanger WT2, part of the liquid desorption medium (DesM-1) is branched off and returned to the bottom of the desorption column via a heat exchanger WT1. The heat exchanger WT1 serves as an evaporator of the phosgene gas generation device (desorption column) 104. It is preferred that a first partial flow (DesM-1 in FIG. 2) of the liquid desorption medium (DesM) accumulating in the phosgene gas generation device (104) is returned to the phosgene gas generation device (104) via a first heat exchanger (WT1 in FIG. 2), the first partial flow being indirectly heated in the first heat exchanger by a heat transfer medium, and a second partial flow (DesM-2 in FIG. 2) of the liquid desorption medium (DesM) being passed through a second heat exchanger (WT2 in FIG. 2), the second partial flow in the second heat exchanger indirectly heating the phosgene reaction gas flow (P) and the second partial flow of the liquid desorption medium (DesM) cooled in the process being used as a component of the washout fluid is used.

In a preferred embodiment of the invention, the gas phase reaction arrangements comprise gas phase reactors. Such a gas phase reactor preferably comprises both the mixing zone 101-1 and reaction zone 102-1 as well as the quenching zone 107-1 and phase separation zone 108-1. In other words, both the mixing and reaction zone and the quenching and phase separation zone are arranged in the gas phase reactor. The respective phosgene reaction gas stream and the respective amine reaction gas stream can be introduced separately from one another into the mixing zone of the gas phase reactor. This has the advantage that in the event of a pressure drop due to the susceptibility of the amine gas generation device to failure, short bursts of amine reaction gas introduced into the gas phase reactor cannot influence or impair the supply of the respective phosgene reaction gas stream into the gas phase reactor. If phosgene and amine reaction gas streams were introduced together, the short amine reaction gas bursts would be amplified many times over and would have a negative effect on the gas phase process. Such a disturbance or amplification of unavoidable disturbances can be advantageously prevented by the proposed separate introduction of the phosgene and amine reaction gas streams. The phosgene and amine reaction gas streams are then mixed in the mixing zone, forming a gaseous reaction mixture which comprises at least the reaction components primary amine and phosgene. The gaseous reaction mixture can then be passed through a reaction zone of the gas phase reactor, whereby the gas phase reaction of the primary amine with phosgene takes place. The gas phase reactor is preferably a tubular reactor. The tubular reactor preferably has a round (in particular circularly symmetrical) cross-section in the region of the mixing zone, reaction zone and quenching zone and is either cylindrical in shape or has sections of different cross-sections, so that the tubular reactor in the region of the mixing zone, reaction zone and quenching zone consists of cylinders of different diameters which are connected via conical transition pieces. The tubular reactor is preferably upright, with the phosgene reaction gas stream and the amine reaction gas stream (and the intermediate and end products formed from these) preferably passing through the tubular reactor from top to bottom. The mixing zone, reaction zone and quench zone can be arranged in this order from top to bottom in the tubular reactor. When the tubular reactor is arranged upright, the phase separation zone is preferably arranged below the quench zone of the tubular reactor. The phase separation zone of the tubular reactor can also have a round (in particular circularly symmetrical) cross-section.

FIG. 3 shows a system for carrying out the method according to the invention, carried out in six gas phase reaction arrangements 100-1, 100-2, 100-3, 100-4, 100-5, and 100-6 (m = 6). The system or the method carried out therewith basically comprises the features shown or described in FIG. 1 and FIG. 2. However, the system shown in FIG. 3 and the method carried out therewith differ from those shown in FIG. 1 and FIG. 2 essentially in that the gas phase reaction is carried out simultaneously in six gas phase reaction arrangements 100-1, 100-2, 100-3, 100-4, 100-5, and 100-6 (with a basically identical structure to the previously described gas phase reaction arrangements 100). For this purpose, the system comprises a first and a second production line, which are connected in parallel to one another. The first production line runs through a first Production arrangement 1000-1, which comprises the gas phase reaction arrangements 100-1, 100-2 and 100-3, a first washout column 105-1, a first absorption column 103-1, and a single desorption column 104. The second production line in turn runs via a second production arrangement 1000-2, which comprises the gas phase reaction arrangements 100-4, 100-5 and 100-6, a second washout column 105-2, a second absorption column 103-2, and the single desorption column 104. Thus, m = 6 and n = 2.

The parallel connection of the first and second production lines, including the parallel connection of the gas phase reaction arrangements 100-1, 100-2, 100-3 arranged in the first production line and the gas phase reaction arrangements 100-4, 100-5 and 100-6 arranged in the second production line, is implemented via several splitting devices (S) and combining devices (V) arranged in the system. Thus, several splitting devices S are arranged in the system, which split fluid flows passed through the respective splitting device or branch off part or several parts of the fluid flow passed through. In addition, several combining devices V are arranged in the system, which combine partial fluid flows passed into the respective combining device to form a combined fluid flow. The splitting devices S and the combining devices V are each shown as hatched circles in FIG. 3.

The phosgene gas stream P is thus divided into a first and a second production stream by a first splitting device S1, which is connected downstream of a single desorption column 104. The first production stream is fed into a first partial splitting device S2-1 and split there into three phosgene reaction gas streams P1, P-2 and P-3. The second production stream is in turn fed into a second partial splitting device S2-2 and split there into three phosgene reaction gas streams P-4, P-5 and P-6. Each of the gas phase reaction arrangements 100-1, 100-2, 100-3, 100-4, 100-5 and 100-6 is supplied with exactly one phosgene reaction gas stream P1, P-2, P-3, P-4, P-5, and P-6 (in this assignment), ie a first phosgene reaction gas stream P1 is supplied to a first gas phase reaction arrangement 100-1, a second phosgene reaction gas stream P-2 is supplied to a second gas phase reaction arrangement 100-2, etc. In addition, from the gas phase reaction arrangements 100-1, 100-2 and 100-3 arranged in the first production arrangement 1000-1,

Raw residual fluid streams R-raw-1, R-raw-2 and R-raw-3 are combined in a first partial combination device Vl-1 to form a first combined raw residual fluid stream which is fed into the first washout column 105-1. Raw residual fluid streams R-raw-4, R-raw-5 and R-raw-6 obtained from the gas phase reaction arrangements 100-4, 100-5 and 100-6 arranged in the second production arrangement 1000-2 are in turn combined in a second partial combination device Vl-2 to form a second combined raw residual fluid stream which is fed into the second washout column 105-2.

A first residual fluid stream R-1 is obtained from the first washout column 105-1 arranged in the first production line and is fed into the first absorption column 103-1. A second residual fluid stream R-2 is obtained from the second washout column 105-2 arranged in the second production line and is fed into the second absorption column 103-2. In addition, the absorption medium fluid stream AbsM comprising absorption medium is split in a third splitting device S3 into a first absorption medium partial fluid stream AbsM-1, which is fed to the first absorption column 103-1, and a second absorption medium partial fluid stream AbsM-2, which is fed to the second absorption column 103-2.

A first partial recycle stream Recycle-1 and a first hydrogen chloride stream HCl-1 are obtained from the first absorption column 103-1, and a second partial recycle stream Recycle-2 and a second hydrogen chloride stream HCl-2 are obtained from the second absorption column 103-2. The first partial recycle stream Recycle-1 and the second partial recycle stream Recycle-2 are combined to form a combined recycle stream (Recycle) in a second combining device V2 downstream of the absorption columns 103-1 and 103-2. The combined recycle stream (Recycle) is then introduced into the single desorption column 104. In addition, a single fresh phosgene fluid stream (Fresh), which is generated in a single fresh phosgene generation device 109, is introduced into the desorption column 104. In the desorption column, the phosgene gas stream P is again formed, which is discharged and fed to the first splitting device S1. The method according to the invention provides for a state parameter (X) of the

Phosgene reaction gas stream via a setting parameter (Y), namely at least via a setting parameter (Y-l) of the recycle stream (Return). The setting of the state parameter (X) of the

phosgene gas flow (P) via the setting parameter (Y-l) of the

The feedback current (return) can be adjusted by means of a

adjustment process. The adjustment means can comprise adjustment units that are connected to one another by signal technology. For example, the adjustment means can comprise a central control unit, an evaluation unit and a measuring unit as adjustment units. Signal technology connections can generally be wireless or wired connections. As part of the adjustment process, the pressure (p-P-1, p-P-2, ... p-P-m) of the m phosgene reaction gas flows (P-l, P-2, ... P-m) that exists after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m) can be measured at at least one phosgene measuring point within the system using the measuring unit of the adjustment means. Measurements of the pressure (p-P-1, p-P-2, ... p-P-m) or generally of a state of a system under consideration can generally be carried out in one or more measuring stages or with one or more measuring devices connected in series. Connecting several measuring devices in series (a redundant measurement) has the advantage that the measurement errors of all measurements are balanced out, i.e. a more accurate measurement can be made than with just a single-stage measuring process or just one measuring device. Measurements can generally be carried out directly or indirectly. For example, an indirect measurement can include the calculation of an indirectly measured quantity to be determined on the basis of a directly measured quantity ("soft sensor"). This is an advantage, for example, if the direct measurement of the quantity to be determined is not possible.

A change in at least one pressure (pP-1, pP-2, ... pPm) can also be determined indirectly by observing the effects that such a change has on other devices. In particular, the position of the phosgene flow control valves (PV-1, PV-2, ..., PV-m) is a suitable indicator for the pressure fp-P-1, pP-2, ... pPm. If the degree of opening of a phosgene flow control valve, for example, has to be increased continuously, this is a sure sign of a Pressure increase in the gas phase reaction arrangement downstream in the direction of flow: If the "back pressure" to be overcome in the gas phase reaction arrangement becomes ever greater, for example due to the formation of solid deposits, the degree of opening of the phosgene flow control valve must be increased further and further in order to compensate for this. Conversely, a sudden drop in pressure (e.g. due to the detachment and falling off of solid deposits) must be counteracted by reducing the degree of opening of the phosgene flow control valve. A necessary change in a phosgene flow control valve is therefore a reliable indication of a change in the pressure behind it (pP-1, pP-2, ... pPm). Such a pressure change is counteracted according to the invention by using the flow rate (f-back) of the return flow (back) as the setting parameter (Yl) of the return flow (back) for the state parameter (X), i.e. at least for the pressure (pP) of the phosgene gas flow (P). In the case of a (however determined) If at least one pressure (pP-1, pP-2, ... pPm) increases, the flow rate (f-back) and thus the pressure (pP) of the phosgene gas stream (P) is increased in order to ensure a sufficiently high stoichiometric phosgene excess. If at least one pressure (pP-1, pP-2, ... pPm) drops (however determined), the setting parameter (Y), i.e. at least the setting parameter (Yl), is adjusted. In the example case of the pressure drop, the flow rate (f-back) and thus the pressure (pP) of the phosgene gas stream (P) is reduced in order to avoid an unnecessarily high stoichiometric phosgene excess. Specifically, if a change in at least one pressure (pP-1, pP-2, ... pPm) is detected, a new setpoint (Y)SETPOINT is calculated for the setting parameter (Y) (ie at least for (Yl), if necessary also for (Y-2)) for the pressure (pP), so that at least the flow rate f-back, if necessary also the flow rate f-fresh, is adjusted.

For measuring the state parameter of the phosgene gas stream as the system under consideration at at least one phosgene measuring location, one or more phosgene measuring devices belonging to the measuring unit can be arranged at the phosgene measuring locations. From the measurement of the state parameter at at least one phosgene measuring location, state information, in particular the pressure (pP) at the at least one phosgene measuring location, can be obtained. The obtained state information can then be sent to a signaling device connected to the one or several phosgene measuring devices. The status information received can then be evaluated in the evaluation unit, in particular by means of the computer system according to the invention. The evaluation can then produce evaluated information on the basis of which an instruction for action is created by the evaluation unit and transmitted to the central control unit. For example, the evaluation can show that the status parameter does not correspond to a desired status parameter (in particular because the pressure (pP-1, pP-2, ..., pPm) present after passing through one of the phosgene flow control valves has changed) and therefore needs to be adjusted. In this case, an instruction for action can be created using the evaluation unit, which is transmitted to the central control unit and states that and how the status parameter of the phosgene gas flow needs to be adjusted. The central control unit can then be used to adjust (set) the setting parameter of the recirculation flow. For this purpose, at least one measurement of the state of the return fluid flow can be carried out using a measuring unit at at least one return measuring location according to the measuring principle previously explained for the state parameter of the phosgene gas flow. For this purpose, one or more return measuring devices belonging to the measuring unit can be used. The at least one measurement at the at least one return measuring location can provide setting information about the state of the return flow (setting parameter) at the at least one return measuring location, which is then transmitted to the evaluation unit. The received setting information can be evaluated by means of the evaluation unit. For example, the evaluation of the setting information can show that the state of the return flow at the at least one return measuring location (actual value) does not correspond to a desired state (setpoint value) of the return flow, wherein the desired state of the return flow is suitable for setting the state parameter of the phosgene gas flow, i.e. for setting a desired state of the phosgene gas flow. In this case, as previously explained for the state parameter, an instruction can be created using the evaluation unit, which can be transmitted to the central control unit. The central control unit can then evaluate the instruction received and set the desired state of the feedback current. By setting the desired By setting the state of the recirculation stream at the recirculation control location as a setting parameter, the state parameter of the phosgene gas stream is set. For example, via the setting process mentioned with the aid of the setting means mentioned, the setting of the pressure (pP) of the phosgene gas stream as a state parameter (X) can be achieved by setting a necessary flow rate of the recirculation stream as a setting parameter (Yl).

According to the invention, the setting of the state parameter (X) of the phosgene gas stream is a regulation of the state parameter (X) of the phosgene gas stream via the setting parameter (Y), wherein the setting parameter (Y) is at least one setting parameter (Y-1) of the recycle stream (return), namely its flow rate (f-return). The setting process is therefore a control process, and the setting means is a control means for carrying out the control process. The control or the control process and the control means for carrying out the control process are particularly suitable for ensuring stable reaction conditions of the gas phase reaction in the gas phase reaction arrangement, in particular with regard to keeping the stoichiometric phosgene excess constant.

A control of a state variable of a system under consideration can in principle be carried out at the same location at which a state variable of the system under consideration is also measured. In other words, control locations can in principle be arranged at the same locations as one or more measuring locations. To control a state parameter of the phosgene gas flow via a setting parameter of the return flow, a central control unit can be provided that controls one or more measuring devices and one or more controllers. A controller can in principle bring about a change in the state of a system under consideration, such as a guided fluid flow. The controller can, for example, be a flow rate controller that controls a flow rate of a guided fluid flow, or a level controller that controls a level of a medium in a device. For example, changes in the flow rate of a controlled fluid flow or changes in a medium level in a specific device controlled by a level controller can be implemented in the form of a ramp. In other words, the intensity of a flow rate or level change can be increased or decreased as slowly as possible. For example, by increasing the flow rate in Form of a ramp, the overflow of a guided liquid can be prevented in a device into which the liquid is fed at the regulated flow rate, whereby the device itself is not designed to compensate for a sudden increase in flow rate.

A controller can be a continuous linear controller, a non-linear controller or a discontinuous controller. A continuous linear controller can be a P controller, I controller, D element, PI controller, PD controller, PD2 element with conjugated complex zeros, PID controller, a state controller with state or output feedback, or a controller for multi-variable systems. A non-linear controller can be a fuzzy controller, an adaptive controller or an extreme value controller. A discontinuous controller can be a two-point controller, a three-point controller or a multi-point controller. A controller can basically be implemented as an analog controller, digital controller, compact controller, pneumatic controller or universal controller. The controller is preferably a universal controller. A universal controller can be understood as a controller that can be operated as a P, PI, PD or PID controller due to the flexible setting of its controller parameters (proportional coefficient, reset time and derivative time). A universal controller can have standardized input and output signals, so-called standard signals. The input signal can come from a measuring system and the output signal can act on an actuator, which means that devices from different manufacturers can be operated together in a control loop. The standard signal can be an electrical standard signal. The electrical standard signal is preferably a current signal. Current signals can be advantageous for long signal lines, as they are less susceptible to external interference fields and a voltage drop on the signal line has no effect on the current signal.

To regulate a pressure of the phosgene gas stream (P) via a flow rate of the recirculation stream, the central control unit can, for example, control one or more recirculation regulators and one or more phosgene pressure regulators. Preferably, the central control unit is a universal regulator which is connected to one or more measuring devices and one or more regulators by signal technology. Accordingly, a pressure of the phosgene reaction gas stream can be regulated via a universal regulator which is connected to one or more phosgene pressure measuring devices, phosgene pressure regulators, recirculation measuring devices and recirculation regulators. A phosgene pressure regulator can be used as such A controller can be understood as one which is suitable for regulating a pressure of the phosgene gas stream (P) at at least one phosgene control location. A recirculation controller can in turn be understood as one which is suitable for regulating a flow rate of the recirculation stream at at least one recirculation control location.

The process according to the invention or the plant for carrying it out is particularly suitable for generating a stable phosgene reaction gas stream in each of the gas phase reaction arrangements with a constantly adjustable state parameter X (the pressure p-P). In this way, stable reaction conditions of the gas phase reaction can be generated after introducing the phosgene reaction gas streams into the m gas phase reaction arrangements. As a result, the process according to the invention or the plant for carrying out the process can be operated with greater operational stability compared to known processes or plants, in particular with as constant a stoichiometric excess of phosgene as possible in each of the m gas phase reaction arrangements, which also results in higher product selectivity and space-time yield. These advantages also arise in particular from the combination of the setting according to the invention, the recycling of unreacted, excess phosgene and the separate introduction of phosgene and amine reaction gas streams into the gas phase reaction arrangement. Thus, with the plant and the method according to the invention, not only can the amounts of solid by-products and secondary product residues formed in the gas phase reaction be reduced overall in comparison with known methods and plants, but at the same time pressure drops resulting from the breaking off of fundamentally unavoidable residue amounts from the walls of the gas phase reaction arrangement can also be compensated.

According to the invention, each of the m or n phosgene absorption devices has a phosgene absorption device collection area (115-1, 115-2, ... 115-m or 115-n) in which a recycle medium collects. The recycle medium comprises both absorbed phosgene and the absorption medium. The recycle stream is fed from the recycle medium collected in the phosgene absorption device collection area. In one embodiment of the invention, a single (ie n = 1) downstream absorption device (103) is assigned to the m gas phase reaction arrangements, wherein the recycle stream (return) is fed from the one single absorption device (103).

According to an alternative embodiment, several, in particular m, absorption devices connected in parallel are connected downstream of the m gas phase reaction arrangements. In this case, one of the several absorption devices is assigned to one of the several gas phase reaction arrangements or to a plurality of the several gas phase reaction arrangements. The recycle stream is fed from the several absorption devices. In the case of several absorption devices, at least one combining device (not shown as a separate device in FIGS. 1 and 2) connected downstream of the absorption devices is provided. The combining device is designed to combine partial recycle streams obtained from the several absorption devices to form the recycle stream.

This means that a single downstream phosgene gas generation device is assigned to a single absorption device, or several absorption devices connected in parallel are assigned to a single downstream phosgene gas generation device.

It is preferred that the setting parameter (Y) additionally comprises a setting parameter (Y-2) of the phosgene fresh fluid stream (Fresh), wherein the state parameter (X) is set (= regulated) such that when a change is detected in (at least) one pressure (pP-1, pP-2, ... pPm) of the m phosgene reaction gas streams (Pl, P-2, ... Pm) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m), first - as described above - the flow rate (f-return) of the return stream (return) is used as the setting parameter (Yl) and then the flow rate (f-fresh) of the introduced phosgene fresh fluid stream (Fresh) is used as the setting parameter (Y-2). In this case, the flow rate (f-back) of the recycle stream (back) is preferably adjusted by setting a first adjustment parameter (ZI) of the absorbed phosgene contained in the recycle medium and a second adjustment parameter (Z2) of the absorption medium contained in the recycle medium. An adjustment parameter is defined as a such parameters which are suitable for adjusting the physical state of a system under consideration. The first adjustment parameter can in principle be any possible state variable of the absorbed phosgene contained in the recycle medium. For example, the first adjustment parameter can be a density, temperature, mass, volume or pressure of the absorbed phosgene contained in the recycle medium. The second adjustment parameter of the absorption medium contained in the recycle medium can in turn be any possible state variable of the absorption medium contained in the recycle medium. For example, the second adjustment parameter can be a density, temperature, mass, volume or pressure of the absorption medium contained in the recycle medium. An embodiment is preferred in which the flow rate (f-return) of the recycle stream (return) is adjusted by setting a phosgene mass (mP-1, mP-2, ..., mPm or mPn) of the absorbed phosgene contained in the

Recycle medium (Recycle-1, Recycle-2, ..., Recycle-m or Recycle-n) respectively comprised absorbed phosgene as the first adjustment parameter (Zl-1, Zl-2, ..., Zl-n or Zl-m) and an absorption medium mass (m-AbsM-1, m-AbsM-2, ... m-AbsM-m or m-AbsM-n) of the absorption medium respectively comprised in the recycle medium (Recycle-1, Recycle-2, ..., Recycle-m or Recycle-n) as the second adjustment parameter (Z2-1, Z2-2, ..., Z2-n or Z2-m).

It can be provided that the phosgene mass (mP-1, mP-2, ..., mPm or mPn) is set by manipulating a flow rate (f-fresh) of the phosgene fresh fluid stream, and the absorption medium mass (m-AbsM-1, m-AbsM-2, ... m-AbsM-m or m-AbsM-n) is set by manipulating a flow rate of the absorption medium fluid stream (AbsM-1, AbsM-2, ..., AbsM-m or AbsM-n). It is also possible that the phosgene mass is calculated by multiplying a filling level of the return medium in the phosgene absorption device collection area and a concentration of the absorbed phosgene in the return medium. In addition, it can be provided that the absorption medium mass is calculated by multiplying the said filling level of the recycle medium in the phosgene absorption device collection area and a non-phosgene component concentration of non-phosgene components contained in the recycle medium. The recycle medium comprises both the absorbed phosgene and non-phosgene components. Non-phosgene components can be understood as all the The term "phosgene" refers to components contained in the recycle medium which are not phosgene. The non-phosgene components include in particular the absorption medium and in particular not the absorbed phosgene contained in the recycle medium.

As already mentioned, a further subject matter of the invention is a computer system for controlling the stoichiometric excess of phosgene in the process according to the invention for producing an isocyanate, comprising: an interface unit which is configured to read in (periodically or continuously) data which are suitable for determining a change in (at least) one pressure (p-P-1, p-P-2, ... p-P-m) of the m phosgene reaction gas streams (P-l, P-2, ... P-m) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m), and a processor which is configured to calculate an adjusted setpoint value (Y)SOLL for the setting parameter (Y) (i.e. at least for (Y-l), optionally also for (Y-2)) with the read-in data when a change in the pressure (p-P-1, p-P-2, ... p-P-m) is detected, and to adjust the adjusted setpoint value (Y)TARGET to be transmitted to an actuator in communicative connection with the processor for manipulating the setting parameter (Y) in order to counteract the detected change in pressure (p-P-1, p-P-2, ... p-P-m) (and thus the change in the stoichiometric phosgene excess).

The term "processor" as used in the terminology of the present invention corresponds to the common understanding in the art and can refer in particular to any logic circuit configured to perform basic operations of a computer or computer system and/or generally to a device configured to perform calculations or logical operations. In particular, the processor can be configured to process basic instructions that control the computer system. As an example, the processor can include at least one arithmetic logic unit (ALU), at least one floating point unit (FPU), such as a mathematical coprocessor or a numerical coprocessor, a plurality of registers, in particular registers used to provide operands to the ALU and to store operation results, and a memory, such as LI and L2 Cac/?e memory. In particular, the processor may be a multi-core processor. In particular, the processor may be or comprise a central processing unit (CPU). Additionally or alternatively, the processor may be or comprise a microprocessor, so that specifically the elements of the processor may be contained in a single integrated circuit (IC) chip. Additionally or alternatively, the processor may be or comprise one or more application specific integrated circuits (ASICs) and/or one or more field programmable gate arrays (FPGAs) or the like.

It is preferred that a self-learning algorithm is implemented on the processor for automatically comparing changes in (at least) one pressure (p-P-1, p-P-2, ... p-P-m) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m) with the stoichiometric phosgene excess and, in response thereto, for automatically setting and adjusting the setting parameter (Y), wherein the algorithm is configured to optimize the process for producing an isocyanate by adjusting the setting parameter (Y) (i.e. to counteract deviations from the stoichiometric phosgene excess more quickly).

A further subject of the present invention is a computer program product which, when loaded into a memory unit of a computing unit of the computer system according to claim 19 or 20 and executed by the processor, in a method according to one of claims 1 to 18, detects changes in (at least) one pressure (pP-1, pP-2, ... pPm) of the m phosgene reaction gas streams (Pl, P-2, ... Pm) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m) (periodically or continuously) and, when a change in the pressure (pP-1, pP-2, ... pPm) is detected, adjusts the setpoint value (Y)SOLL of the setting parameter (Y) (ie at least of the setting parameter (Yl), optionally also of the setting parameter (Y-2)) (ie calculates a new setpoint value (Y)SOLL) in order to correspond to the change in the pressure (pP-1, pP-2, ... pPm) (and thus the change in the stoichiometric excess of phosgene). The invention also relates to a computer-implemented method for controlling the stoichiometric excess of phosgene in the process according to the invention, comprising:

- receiving in real time data suitable for detecting a change in (at least) one pressure (p-P-1, p-P-2, ... p-P-m) of the m phosgene reaction gas streams (P-l, P-2, ... P-m) after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m), and

- when a change in pressure is detected (p-P-1, p-P-2, ... p-P-m):

Adjusting the setting parameter (Y) (i.e. at least the

setting parameter (Y-1), if necessary also the setting parameter (Y-2)) in order to counteract the detected change in pressure (p-P-1, p-P-2, ... p-P-m) (and thus the change in the stoichiometric phosgene excess).

The plant for producing an isocyanate by reacting a primary amine with phosgene with a stoichiometric excess of phosgene by means of a gas phase reaction, which is another subject of the present invention, has also already been mentioned several times and is designed to carry out the process according to the invention in it. The plant therefore comprises at least the devices described as necessary in connection with the process according to the invention, i.e. at least one, preferably m amine gas generation devices, a phosgene gas generation device, m gas phase reaction arrangements, m or n phosgene absorption devices and adjustment means for adjusting a state parameter (X) of the m phosgene reaction gas streams via an adjustment parameter (Y), wherein the adjustment parameter (Y) is at least one adjustment parameter (Y1) of the recycle stream and the adjustment means comprise at least the m phosgene flow control valves (PV-1, PV-2, ..., PV-m) and also the computer system according to the invention. Each amine gas generation device is designed to generate an amine reaction gas stream comprising primary amine. The phosgene gas generation device is designed to generate a phosgene gas stream and divide it into m phosgene reaction gas streams. The gas phase reaction arrangements are designed to combine the generated amine reaction gas streams and the generated phosgene reaction gas streams during the course of the gas phase reaction of the primary amine with phosgene, and for providing isocyanate crude product fluid streams and residual fluid streams obtained therefrom, comprising produced isocyanate, wherein the residual fluid streams comprise unreacted, excess phosgene in the gas phase reaction. Each phosgene absorption device is designed to receive an absorption medium fluid stream comprising the absorption medium and the residual fluid stream, further designed to absorb the unreacted, excess phosgene by the absorption medium to obtain phosgene to be recycled, and to provide a recycle stream comprising phosgene to be recycled. The phosgene gas generation device is designed to receive a phosgene fresh fluid stream and the recycle stream comprising phosgene to be recycled, and to desorb the phosgene to be recycled from the absorption medium to obtain desorbed phosgene. The phosgene gas stream is formed by the desorbed phosgene and phosgene obtained from the phosgene fresh fluid stream. The adjustment means for adjusting a state parameter (X) of the phosgene reaction gas stream via an adjustment parameter (Y) of the recycle stream is particularly suitable for ensuring stable reaction conditions of the gas phase reaction, in particular a stable stoichiometric phosgene excess, in the gas phase reaction arrangement.

Features, advantages and properties of the system according to the invention correspond to the features, advantages and properties of the method according to the invention, and vice versa.

FIG. 4 shows a system for carrying out the method according to the invention with the features shown and described in FIG. 1 and FIG. 2. FIG. 4 also shows an exemplary setting means arranged in the system for setting a pressure pP of the phosgene reaction gas stream P as a state parameter X via the flow rate f-return of the recycle stream Rück and the flow rate f-fresh of the fresh phosgene stream Frisch as setting parameters Y1 and Y-2. The control means comprises a central control unit, which is a universal controller UC, which is connected to an evaluation unit, which comprises the computer system according to the invention (not shown in FIG. 4), a measuring unit, which comprises several measuring devices and is connected to several controllers for signaling purposes. Measuring devices and controllers are shown as labeled circles in FIG. 4. Solid arrows show the flow or direction of fluid flows within the system, while dashed arrows show control loop information running between the universal controller UV and the individual measuring devices and controllers.

As shown in FIG. 4, the flow rate f-fresh of the fresh phosgene fluid stream Fresh is manipulated via a fresh flow rate controller FO. As a result, a phosgene mass m-P of the absorbed phosgene contained in the recycle medium is adjusted as a first adjustment parameter ZI. The phosgene mass m-P can be calculated by multiplying a fill level L-back of the recycle medium and a concentration x-P of the absorbed phosgene in the recycle medium. The fill level L-back depends on a first fill level L-back-1 of a first sump 115-1 of the first absorption column 103-1 and a second fill level L-back-2 of a second sump 115-2 of the second absorption column 103-2. The concentration x-P of the absorbed phosgene in the recycle medium depends in turn on the first concentration x-P-1 of the absorbed phosgene in the first recycle medium and on the second concentration x-P-2 of the absorbed phosgene in the second recycle medium. The flow rates of the recycle media Recycle-1 and Recycle-2 are manipulated by the flow rate controllers Fll and F21 assigned to them respectively. Al and A2 denote phosgene analyzers.

In addition, the flow rate of the first absorption medium partial fluid flow AbsM-1 is manipulated by means of a first flow rate controller F12 and the flow rate of the second absorption medium partial fluid flow AbsM-2 is manipulated by means of a second flow rate controller F22. As a result, an absorption medium mass m-AbsM of the absorption medium contained in the return medium is adjusted as a second adjustment parameter Z2. The absorption medium mass m-AbsM can in turn be calculated by multiplying the fill level L-back of the return medium by a non-phosgene component concentration l-(x-P)] of the non-phosgene components in the return medium.

The calculation is based on the objective that a difference A mP between an actual value (PV) and a target value (SP) of the phosgene mass mP in the return medium (RM) should be as small as possible, and the following calculation principles: mp — Trip py Trip P 0 (Equation 1),

PRM,PV ~ PRM,SP (equation 3 identical densities, temperature independent),

Am _P — A _back 0 p _RM x L _{back PV} x _{P PV} L _r ack,sp ^X P,SP (Equation 4),

If the actual value x-p-PV corresponds to the setpoint x-P-SP, then both an absorption medium control loop and a decoupled phosgene control loop act as level controllers. To do this, a first level controller LI, which is arranged at the sump 215-1 of the first absorption column 103-1, and a second level controller L2, which is arranged at the sump 115-2 of the second absorption column 103-2, move the levels L-back-1 and L-back-2 in a direction necessary for control. If it turns out that the levels L-backl and L-back-2 are too low, then they are increased, etc. If the actual value L-back-PV corresponds to the setpoint L-back-SP, then the absorption medium and phosgene control loops both act as concentration controllers, i.e. they move the concentrations x-P-1 and x-P-2 in a direction necessary for control.

By adjusting the phosgene mass mP as the first adjustment parameter ZI and the absorption agent mass m-AbsM as the second adjustment parameter Z2, the flow rate f-Back of the recycle stream Back is adjusted. The flow rate f-Back of the recycle stream Back is tapped by means of a recycle controller FC at the inlet of a single desorption column 104. By adjusting the flow rate f-Back of the recycle stream Back, the pressure pP of the in the single desorption column 104 formed

Phosgene reaction gas stream P is adjusted using the pressure regulator PO.

Claims

patent claims 1. A process for producing an isocyanate by reacting a primary amine with phosgene in a stoichiometric excess of phosgene by means of a gas phase reaction in a plant comprising m gas phase reaction arrangements (100-1, 100-2, ..., 100-m), where m is a natural number in the range from 2 to 6, the process comprising: (i) generating m amine reaction gas streams (A-1, A-2, ..., A-m), each comprising a primary amine; (ii) generating a phosgene gas stream (P) comprising phosgene at a pressure (p-P) in a phosgene gas generating device (104) and dividing the phosgene gas stream (P) comprising phosgene into m phosgene reaction gas streams (P-1, P-2, ... P-m) comprising phosgene; (iii) Combining one of the amine reaction gas streams (A-1, A-2, ..., A-m) and one of the m phosgene-comprising phosgene reaction gas streams (P-l, P-2, ... P-m) in each case in one of the gas phase reaction arrangements (100-1, 100-2, ..., 100-m), wherein the m phosgene reaction gas streams (P-l, P-2, ... P-m) pass through one of m phosgene flow control valves (PV-1, PV-2, ..., PV-m) to adjust the stoichiometric phosgene excess before entering the m gas phase reaction arrangements (100-1, 100-2, ..., 100-m), and carrying out the gas phase reaction to obtain m isocyanate-comprising isocyanate crude product fluid streams (IC-1, IC-2, ..., IC-m) and m residual fluid streams (R-l, R-2, ..., R-m), wherein the m Residual fluid streams (R-l, R-2, ..., R-m) each comprise excess phosgene not converted in the gas phase reaction; (iv) recycling the unreacted excess phosgene to the phosgene gas generating device (104), namely by (iv-1) optionally, combining the m residual fluid streams (R-l, R-2, ..., R-m) into n residual fluid streams (R-l, R-2, ... R-n), where n is a natural number in the range from 1 to m-1, (iv-2) introducing each of the residual fluid streams into a respective phosgene absorption device (103-1, 103-2, ... 103-m or 103-n) each having a phosgene absorption device collection region (115-1, 115-2, ... 115-m or 115-n), wherein in each of the Phosgene absorption devices (103-1, 103-2, ... 103-m or 103-n) are also each fed with an absorption agent fluid stream (AbsM-1, AbsM-2, ..., AbsM-m or AbsM-n), each containing an absorption agent (AbsM), with the excess phosgene not converted in the gas phase reaction being absorbed by the absorption agent, wherein an absorbed phosgene and the absorption agent-comprising recycle medium (Return-1, Return-2, ..., Return-m or Return-n) collect in each phosgene absorption device collection area (115-1, 115-2, ... 115-m or 115-n), from which, if necessary after combining several return media (Return-1, Return-2, ..., Return-m or Return-n), a recycle stream (Return) comprising phosgene is fed, and (iv-3) introducing the recycle stream (Return) at a flow rate (f-Return) into the phosgene gas generation device (104) with desorption of the phosgene to be recycled from the absorption medium to obtain desorbed phosgene and a liquid desorption medium (DesM), wherein the phosgene gas stream (P) generated in the phosgene gas generation device (104) comprises the desorbed phosgene and a phosgene fresh fluid stream (Fresh) additionally introduced into the phosgene gas generation device (104) at a flow rate (f-Fresh), and wherein to ensure the stoichiometric phosgene excess in each of the m gas phase reaction arrangements (100-1, 100-2, ..., 100-m)) a state parameter (X) of the phosgene gas stream (P) is set via a setting parameter (Y), wherein the pressure (pP) is used as the state parameter (X) and wherein the setting parameter (Y) comprises at least one setting parameter (Yl) of the recirculation stream (Rück), namely such that when a change in a pressure (pP-1, pP-2, ... pPm) of the m phosgene reaction gas streams (Pl, P-2, ... Pm) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m) is detected, the flow rate (f-Rück) of the recirculation stream (Rück) is used as the setting parameter (Yl). 2. The method of claim 1, wherein step (i) is performed in m amine gas generating devices (106-1, 106-2, ..., 106-m). 3. The method according to claim 1, wherein in step (i) an amine reaction gas stream (A) is first generated in an amine gas generation device (106), which is then divided into the m amine reaction gas streams (A-1, A-2, ..., A-m). 4. A method according to any one of claims 1 to 3, wherein each of the m gas phase reaction arrangements (100-1, 100-2, ..., 100-m) comprises a reaction device (100a-l, 100a-2, ..., 100a-m) and one of the Reaction device has a separation device (100b-l, 100b-2, ..., 100b-m) arranged downstream of the process. 5. The method according to claim 4, wherein the reaction device (100a-l, 100a-2, ..., 100a-m) comprises a mixing zone (101-1, 101-2, ..., 101-m) and a reaction zone (102-1, 102-2, ..., 102-m) downstream of the mixing zone, wherein the phosgene reaction gas stream (P-l, P-2, ..., P-m) and the amine reaction gas stream (A-l, A-2, ..., A-m) are mixed in the mixing zone, and wherein the gas phase reaction of the primary amine with phosgene takes place in the reaction zone (102-1, 102-2, ..., 102-m) to obtain a reaction product mixture (RP-1, RP-2, ..., RP-m). 6. The method according to claim 4 or 5, wherein the separation device (100b-l, 100b-2, ..., 100b-m) comprises a quench zone (107-1, 107-2, ..., 107-m) and a phase separation zone (108-1, 108-2, ..., 108-m) arranged downstream of the quench zone, wherein in the quench zone the reaction product mixture (RP-1, RP-2, ..., RP-m) is cooled by bringing it into contact with a quench liquid to obtain a quench mixture, and wherein in the phase separation zone (108-1, 108-2, ..., 108-m) a phase separation of the quench mixture obtained into the isocyanate crude product fluid stream (IC-1, IC-2, ..., IC-m) and an additional crude residual fluid stream comprising isocyanate (R-raw-1, R-raw-2, ..., R-raw-m). 7. The method according to claim 6, wherein the crude residual fluid stream (R-raw-1, R-raw-2, ..., R-raw-m) comprising additional isocyanate obtained from the phase separation zone (108-1, 108-2, ..., 108-m) is fed to a washing device (105-1, 105-2, ..., 105-m) arranged downstream of the phase separation zone in terms of process technology, in which the additional isocyanate is washed out of the crude residual fluid stream (R-raw-1, R-raw-2, ..., R-raw-m) with a washing liquid to obtain the residual fluid stream (R-l, R-2, ..., R-m). 8. The method according to claim 7, wherein a first partial stream of the liquid desorption medium (DesM) is returned to the phosgene gas generation device (104) via a first heat exchanger, wherein the first partial stream is indirectly heated in the first heat exchanger by a heat transfer medium, and a second partial stream of the liquid desorption medium (DesM) is passed through a second heat exchanger, wherein the second partial stream in the second heat exchanger indirectly heats the phosgene reaction gas stream (P) and the thereby cooled second partial stream of the liquid desorption medium (DesM) is used as a component of the scrubbing liquid. 9. The method according to any one of claims 1 to 8, wherein the phosgene gas generating device (104) has an introduction region (111) for introducing the recycle stream (return) and a phosgene gas generating device collection region (110), wherein the desorbed phosgene and the phosgene obtained from the phosgene fresh fluid stream (fresh) collect in the phosgene gas generating device collection region (110) to form the phosgene reaction gas stream (P). 10. The method according to claim 9, wherein a desorption column is used as the phosgene gas generating device (104), which has a sump (112) in which the liquid desorption medium (DesM) collects. 11. Process according to one of claims 1 to 10, in which an absorption column is used as the phosgene absorption device (103-1, 103-2, ... 103-m or 103-n), the phosgene absorption device collection region (115-1, 115-2, ... 115-m or 115-n) being arranged in a bottom of the absorption column. 12. Method according to one of claims 1 to 11, in which the setting parameter (Y) additionally comprises a setting parameter (Y-2) of the phosgene fresh fluid stream (Fresh), wherein the state parameter (X) is set such that when a change in a pressure (p-P-1, p-P-2, ... p-P-m) of the m phosgene reaction gas streams (P-l, P-2, ... P-m) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m) is detected, first the flow rate (f-return) of the return stream (return) is used as the setting parameter (Y-l) and then the flow rate (f-fresh) of the introduced phosgene fresh fluid stream (Fresh) is used as the setting parameter (Y-2). 13. The method according to claim 12, wherein the flow rate (f-Rück) of the recycle stream (Rück) is adjusted by setting a first adjustment parameter (ZI) of the absorbed phosgene comprised in the recycle medium and a second adjustment parameter (Z2) of the absorbent comprised in the recycle medium. 14. The method according to claim 13, wherein the flow rate (f-Rück) of the recycle stream (Rück) is adjusted by setting a phosgene mass (mP-1, mP-2, ..., mPm or mPn) of the absorbed phosgene contained in the recycle medium (Rück-1, Rück-2, ..., Rück-m or Rück-n) as the first adjustment parameter (Zl-1, Zl-2, ..., Zl-n or Zl-m) and an absorbent mass (m-AbsM-1, m-AbsM-2, ... m-AbsM-m or m-AbsM-n) of the absorbent contained in the recycle medium (Rück-1, Rück-2, ..., Rück-m or Rück-n) as the second adjustment parameter (Z2-1, Z2-2, ..., Z2-n or Z2-m). 15. The method according to claim 14, wherein the phosgene mass (m-P-1, m-P-2, m-P-m or m-P-n) is adjusted by manipulating the flow rate (f-fresh) of the phosgene fresh fluid stream (fresh), and wherein the absorbent mass (m-AbsM-1, m-AbsM-2, ... m-AbsM-m or m-AbsM-n) is adjusted by manipulating a flow rate (f-AbsM-1, f-AbsM-2, ..., f-AbsM-m or f-AbsM-n) of the absorbent fluid stream (AbsM-1, AbsM-2, ..., AbsM-m or AbsM-n). 16. The method according to claim 14, wherein the recycle medium (Recycle-1, Recycle-2, ..., Recycle-m or Recycle-n) comprises the absorbed phosgene and non-phosgene components, wherein the non-phosgene components comprise the absorption medium, wherein the phosgene mass (m-P-1, m-P-2, ..., m-P-m or m-P-n) is determined in each case by multiplying a filling level (L-recycle-1, L-recycle-2, ..., L-recycle-m or L-recycle-n) of the recycle medium in the phosgene absorption device collection region (115-1, 115-2, ... 115-m or 115-n) and a concentration (x-P-1, x-P-2, ..., x-P-m or x-P-n) of the absorbed phosgene in the recycle medium (Recycle-1, Recycle-2, ..., Rück-m or Rück-n) is calculated, and wherein the absorption medium mass (m-AbsM-1, m-AbsM-2, ... m-AbsM-m or m-AbsM-n) is calculated in each case by multiplying the filling level (L-rück-1, L-rück-2, ..., L-rück-m or L-rück-n) of the recycle medium (Rück-1, Rück-2, ..., Rück-m or Rück-n) in the phosgene absorption device collection region (115-1, 115-2, ... 115-m or 115-n) and a non-phosgene component concentration of the non-phosgene components in the recycle medium (Rück-1, Rück-2, ..., Rück-m or Rück-n). 17. Method according to one of claims 1 to 16, in which - the absorption agent (AbsM) is selected from decahydronaphthalene, toluene, xylene, chlorobenzene, para-dichlorobenzene, orü7?o-dichlorobenzene, chlorotoluene, chloronaphthalene or mixtures thereof; preferably the absorption agent is ort7?o-dichlorobenzene, and/or - the primary amine is selected from toluenediamine, diphenylmethanediamine, xylylenediamine, 1,5-pentanediamine, 1,6-hexamethylenediamine, isophoronediamine, diaminodicyclo- hexylmethane or mixtures thereof; preferably the primary amine is toluenediamine. 18. Process according to one of claims 1 to 17, in which a target value in the range from 10% of theory to 400% of theory is set for the stoichiometric excess of phosgene. 19. Computer system for controlling the stoichiometric excess of phosgene in a process for producing an isocyanate according to one of claims 1 to 18, comprising: an interface unit configured to read in data suitable for determining a change in a pressure (p-P-1, p-P-2, ... p-P-m) of the m phosgene reaction gas streams (P-1, P-2, ... P-m) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m), and a processor configured to calculate an adjusted setpoint value (Y)SOLL for the setting parameter (Y) using the read-in data when a change in the pressure (p-P-1, p-P-2, ... p-P-m) is detected, and to transmit the adjusted setpoint value (Y)SOLL to an actuator in communicative connection with the processor for manipulating the setting parameter (Y) in order to to counteract the detected change in pressure (p-P-1, p-P-2, ... p-P-m). 20. Computer system according to claim 19, in which a self-learning algorithm is implemented on the processor for automatically comparing changes in a pressure (pP-1, pP-2, ... pPm) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m) with the stoichiometric phosgene excess and in response thereto for automatically setting and adjusting the setting parameter (Y), wherein the algorithm is configured to optimize the process for producing an isocyanate by adjusting the setting parameter (Y). 21. Plant for producing an isocyanate by reacting a primary amine with phosgene in the presence of a stoichiometric excess of phosgene by means of a gas phase reaction, wherein the plant is designed to carry out a process according to one of claims 1 to 18 and further comprises the computer system according to claim 19 or 20. 22. Computer program product which, when loaded into a memory unit of a computing unit of the computer system according to claim 19 or 20 and executed by the processor, in a method according to one of claims 1 to 18, detects changes in a pressure (p-P-1, p-P-2, ... p-P-m) of the m phosgene reaction gas streams (P-1, P-2, ... P-m) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m) and, when a change in the pressure (p-P-1, p-P-2, ... p-P-m) is detected, adapts the setpoint (Y)SOLL of the setting parameter (Y) in order to counteract the change in the pressure (p-P-1, p-P-2, ... p-P-m). 23. A computer-implemented method for controlling the stoichiometric excess of phosgene in a process for producing an isocyanate according to any one of claims 1 to 18, comprising: - receiving in real time data suitable for detecting a change in a pressure (p-P-1, p-P-2, ... p-P-m) of the m phosgene reaction gas streams (P-l, P-2, ... P-m) after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m), and - if a change in pressure is detected (p-P-1, p-P-2, ... p-P-m): adjust the setting parameter (Y) to counteract the change in pressure detected (p-P-1, p-P-2, ... p-P-m).