HK1120028B - Process for preparing acetone cyanohydrin and its conversion products by controlled cooling - Google Patents

Description

Method for producing acetone cyanohydrin and subsequent products thereof by targeted cooling

Technical Field

The present invention relates generally to a process for preparing acetone cyanohydrin, to a process for preparing alkyl methacrylates, to a process for preparing methacrylic acid, to an apparatus for preparing alkyl methacrylates, to a process for preparing polymers based at least in part on alkyl methacrylates, to the use of the alkyl methacrylates obtained by the process according to the invention in chemical products and to chemical products based on the alkyl methacrylates obtained by the process according to the invention.

Background

Acetone cyanohydrin is an important starting component in the preparation of methacrylic acid and alkyl methacrylates. The two compounds are monomers of great significance for a range of bulk (masssen) plastics and, as the use of these bulk plastics continues to increase, the demand for these monomers increases and, consequently, the demand for acetone cyanohydrin as a starting product also increases. Acetone cyanohydrin is obtained, for example, from hydrocyanic acid and acetone or also from acetone and hydrocyanic acid salt. Hydrocyanic acid or a hydrocyanic acid salt is a significantly more toxic compound than acetone, and must be handled with greater care and therefore at great technical safety costs. The reaction regime in the preparation of acetone cyanohydrin is particularly important in order to keep the safety costs to a commercially acceptable limit. In addition to the general meaning of obtaining high yields as little as possible at the starting material concentration, the safety meaning here is such that, together with acetone cyanohydrin as the reaction product, as little as possible of hydrocyanic acid as the reaction starting material is present in the acetone cyanohydrin treatment for further processing.

This safety aspect is achieved in the prior art, for example, by EP 1371632 a1, by a process for preparing acetone cyanohydrin in which a metal cyanide composition is used. This is disadvantageous, in particular, due to the intermediate stage of the metal cyanide, when the hydrocyanic acid produced can be fed directly into a plant close to the acetone cyanohydrin plant, since the plant for producing hydrocyanic acid and the plant for producing acetone cyanohydrin are located on the same site.

There is therefore a general need to at least partially or completely eliminate the disadvantages arising from the prior art.

Disclosure of Invention

It is also an object of the present invention to keep the safety costs as low as possible by means of a reaction which is as complete as possible, in particular as noted in the purification of acetone cyanohydrin.

It is also an object of the present invention to provide a process for preparing acetone cyanohydrin which forms as few by-products as possible.

The invention also aims to improve the production of methacrylic acid or alkyl methacrylates in general, so that polymers based thereon are obtained at lower cost.

A contribution to solving at least one of the above objects is made by the subject matter of the forming claims, dependent claims dependent on which describe preferred solutions of the invention.

Accordingly, the present invention relates to a process for the preparation of acetone cyanohydrin comprising the steps of:

A. contacting acetone and hydrocyanic acid in a reactor to form a reaction mixture, wherein the reaction mixture is recycled and acetone cyanohydrin is obtained;

B. cooling at least a portion, preferably at least 10% by weight, preferably at least 50% by weight, particularly preferably at least 70% by weight, based in each case on the reaction mixture, of the reaction mixture by flowing through a cooling zone of a cooler comprising one cooling element or at least two cooling elements;

C. discharging at least a portion of the acetone cyanohydrin produced from the reactor,

wherein the volume of the cooling zone of the cooler is greater than the volume of the cooling element or at least two cooling elements of the cooler, based on the total internal volume of the cooler.

The circulation of the reaction mixture can preferably be carried out in a so-called loop reactor, in which the reaction mixture is preferably pumped into the loop via a pipe system. The flow of the reaction mixture through the cooling zone can be effected, on the one hand, in such a way that the cooling zone directly forms one of the loop reactorsAnd (4) partial. In a further variant, however, it is possible to provide a cooling zone in addition to the circulation of the loop reactor, into which cooling zone a portion of the reaction mixture circulating in the loop reactor is passed parallel to the loop reactor. The reaction mixture is generally present at from about 30 to about 700m³H, preferably about 100 to 500m³H, particularly preferably from about 150 to about 400m³The volume flow per hour is circulated. The flow of the reaction mixture through the cooling zone can be carried out on the one hand at the same (as described above) speed range. It may furthermore be preferred that a defined amount of the reaction mixture flows through the cooling zone about 10 to 50 times, preferably about 20 to 40 times, particularly preferably about 25 to 35 times. The reaction mixture reaches a temperature of about 0 to about 50 ℃, preferably about 20 to 45 ℃, particularly preferably about 30 to 40 ℃ in the cooling zone. The aim was to keep the reaction mixture as constant as possible. It is furthermore preferred that the pressure at least in the cooling zone is from about 0.1 to about 10bar, preferably from about 0.5 to about 5bar, particularly preferably from about 0.9 to 1.5 bar. It is also preferred that the pH at least in the cooling zone is from about 5 to about 9, preferably from 6 to about 8, particularly preferably from about 6.5 to about 7.5.

The cooling zone can generally have any configuration known and deemed suitable to the skilled person. The cooling zone of the cooler therefore often has one or at least two cooling elements, which ensure a heat transfer between the cooled medium (here the reaction mixture) and the cooling medium located in the cooling element or elements. The volume determined by the manufacturer of the cooler is often referred to as the total internal volume of the cooler. The volume often refers to the volume of the area in which heat transfer occurs. Thus, in most cases, the tubes leading to and from the cooler are not part of the total internal volume of the cooler, since they do not perform a cooling function. The volume of the cooling zone is usually determined by subtracting the volume of the cooling element arranged in the cooling zone or of a plurality of cooling elements arranged there from the total internal volume of the cooler. The volume of the cooling element is usually greater than the volume of the coolant contained by the cooling element, since the components of the cooling element are also taken into account when determining the volume of the cooling element. According to the invention, it may be preferred that the volume of the cooling zone is at least about 1.01 times, preferably at least about 1.1 times, particularly preferably at least about 1.5 times as large as the volume of the cooling element or elements of the cooler.

It is furthermore preferred in the process according to the invention that at least a part of the reaction mixture flows, at least during cooling, in a cooler flow direction which is different from the main flow direction. The main flow direction is obtained as the axial direction between the entry point of the reaction mixture into the cooler and the exit point of the reaction mixture out of the cooler. According to the invention, it may be preferred that, during cooling, at least about 5%, preferably at least about 20%, preferably at least about 40%, of the reaction mixture flows in the cooler in a direction of flow of the cooler which is different from the direction of the main flow. Furthermore, it is possible according to the invention for at least one partial flow, preferably at least 10% by volume, preferably at least 20% by volume, particularly preferably at least 50% by volume, of the reaction mixture fed into the cooling zone to be conveyed past by bypassing the outer wall delimiting the cooling zone. This sideways transport is preferably effected by means of a circulating flow which preferably extends helically from the inlet of the reaction mixture in the cooling zone to the outlet of the reaction mixture from the cooling zone. This flow situation can be visualized in real time, for example by adding a dye such as Fuxin on-line, or by simulation with a suitable computer program and mathematical model.

It is also preferred according to the invention that the cooler flow direction is formed by deflecting the reaction mixture. In this case, it is also preferred that the deflection of the reaction mixture is effected by one or at least two deflection elements which are arranged in the cooler or are connected to the cooler. The deflector can be arranged generally over the entire area of the cooler or can be connected to the cooler. It may also be preferred that the deflector is mounted primarily in the region of the cooler where the reaction mixture enters the cooler at a relatively high flow rate. The density of the deflector therefore decreases progressively from the inlet zone of the reaction mixture towards the outlet zone of the reaction mixture from the cooler.

In principle, all devices known and suitable to the skilled person can be considered as deflection elements. Particularly preferred deflection means according to the invention are jet elements, stirring elements or flow-dividing elements or a combination of at least two of these.

All devices known to the skilled worker for spraying reaction mixtures are conceivable as spraying elements. The device can distribute the reaction mixture in the cooling zone as a passive nozzle on the one hand by means of the reaction mixture pressure and as an active nozzle on the other hand by means of gas jets or the like. All stirrers known to the skilled worker and considered suitable, such as paddle or propeller stirrers, can likewise be considered as stirring elements. As flow-blocking element, in principle all devices known to the skilled person and suitable for this purpose are also conceivable. In this case, in particular, flat internal webs such as plates, for example, plates known from static mixers for mixing and distribution, are used. Preferably, the deflector is arranged in the cooling zone. The flow-blocking element can thus be mounted in particular both on the wall of the cooling zone and on the wall of the cooling element or on the walls of both.

As cooling elements, all cooling elements generally known and deemed suitable by the skilled person can be considered. The cooling element is preferably an elongated hollow body through which a coolant can flow. The cooling element may have a rod-like or plate-like configuration. Thus, tube bundles or plate coolers, or a combination of both, can be considered, with tube bundle coolers being particularly preferred here. In the case of tube bundle coolers, it is also preferred that the deflection elements forming the flow barrier elements are located on the outer wall of the cooler, into which the cooling zone is entered.

Furthermore, in the process according to the invention, the residence time of the reaction mixture in the cooler is preferably selected such that as few by-products as possible are formed with the highest possible conversion of acetone cyanohydrin. The residence time of the reaction mixture in the cooler is therefore from about 0.1 to about 2 hours, preferably from about 0.2 to about 1.5 hours, particularly preferably from about 0.3 to about 1 hour.

The invention also relates to a method for producing alkyl methacrylates, comprising the following steps:

a. preparing acetone cyanohydrin according to the method of the invention;

b. contacting acetone cyanohydrin with inorganic acid to obtain methacrylamide;

c. contacting methacrylamide with an alcohol to obtain an alkyl methacrylate;

d. optionally purifying the alkyl methacrylate.

The invention also relates to a method for producing methacrylic acid, comprising the following steps:

α) preparation of acetone cyanohydrin according to the process of the invention;

beta) contacting acetone cyanohydrin with inorganic acid to obtain methacrylamide;

gamma) methacrylamide reacts with water to form methacrylic acid.

The invention further relates to a device for producing alkyl methacrylate, comprising, in fluid-conducting connection:

the invention furthermore relates to a device for producing alkyl methacrylates, comprising, in fluid-conducting connection:

-the plant elements for the preparation of acetone cyanohydrin, followed by;

-the elements of the plant for the preparation of methacrylamide, followed by;

-a piece of equipment for the preparation of alkyl methacrylate, optionally followed by;

-a piece of equipment for purifying alkyl methacrylate, optionally followed by;

-a polymerized plant element, optionally followed by;

-a piece of equipment to be put in order,

wherein the plant components for the production of acetone cyanohydrin comprise a loop reactor with a cooler comprising a cooling zone through which acetone cyanohydrin can flow and a cooling element. A fluid-conductive connection is defined herein as a connection that can conduct gases, liquids, and gas-liquid-mixtures or other flowable substances. The relationship with the cooler or cooling zone and the preferred configuration of the cooling element or elements is referred to herein with respect to the practice thereof.

Furthermore, it is also preferred according to the invention that the process according to the invention for preparing alkyl methacrylates is carried out in the apparatus according to the invention.

The invention also relates to a process for preparing an at least partially alkyl methacrylate-based polymer, comprising the steps of:

A1) preparing an alkyl methacrylate according to the process of the invention;

A2) polymerizing an alkyl methacrylate and optionally a comonomer;

A3) the alkyl methacrylate is treated.

All comonomers known and deemed suitable by the skilled worker can be considered as comonomers, with free-radically polymerizable monomers being particularly preferred here. Mention may in particular be made of styrene, butyl acrylate or acrylonitrile, methyl acrylate or ethyl acrylate. The polymerization may be carried out by solution polymerization, bead polymerization, emulsion polymerization, or suspension polymerization, or bulk polymerization. The polymer is treated, for example, by precipitating the polymeric material containing the solvent in an insoluble agent that acts as a precipitating agent for the polymer. Thus, for example, a polymeric substance containing acetone as a solvent and polymethyl methacrylate is precipitated in a precipitant consisting of methanol and water, separated from the precipitant and dried.

The invention further relates to the use of the purest alkyl methacrylates obtained by the inventive method as preferred chemical products in fibers, films, paints, molding compositions, molded bodies, paper auxiliaries, leather auxiliaries, flocculants and floor additives.

The invention furthermore relates to fibers, films, lacquers, moulding compositions, mouldings, paper auxiliaries, leather auxiliaries, flocculants and drilling additives based on the pure methacrylic esters obtained by the process according to the invention as preferred chemical products.

The following explains the different process elements and apparatus parts, which can in principle be combined with the invention either individually or as an aggregate consisting of two or more of the described process elements. It is optionally advantageous for the process elements described in the context of the present invention to be combined with the invention in such a way that they are integrated into the process for preparing methacrylic esters as a whole or into the process for preparing methacrylic acid. It has nevertheless been shown that many of the advantageous results can be achieved when the subject matter of the present invention is used in other fields per se or in combination with only a part of the method elements described herein.

Preparation of acetone cyanohydrin

In the process elements, the generally known methods are followed (see, for example, Ullmanns)der technischen Chemie, 4 th edition, volume 7) for the preparation of acetone cyanohydrin. Acetone and hydrocyanic acid are frequently used as reaction partners in this case. The reaction is exothermic. In order to counteract decomposition of the acetone cyanohydrin formed in this reaction, the heat of reaction is generally removed by suitable means. The reaction can in principle be carried out as a batch process or as a continuous process, if a continuous operation is preferred, the reaction is frequently carried out in a correspondingly arranged loop reactor.

The main characteristic of the operating mode for producing the desired product in high yields is often that, with sufficient reaction time, the reaction product is cooled and the reaction equilibrium is shifted toward the reaction product. Furthermore, it is advantageous for the overall yield that the reaction products are often mixed with corresponding stabilizers in order to prevent decomposition to the starting materials in the subsequent workup.

The mixing of the reaction partners acetone and hydrocyanic acid can in principle be carried out in essentially any manner. The type of mixing depends in particular on whether a discontinuous mode of operation, for example in a batch reactor, or a continuous mode of operation, for example in a loop reactor, is selected.

It is advantageous in principle for the acetone to be fed to the reaction via a storage vessel equipped with a washing column. Thus, for example, an exhaust gas line which conveys off-gas containing acetone and hydrocyanic acid can pass through the storage vessel. In a scrubber connected to the storage vessel, the off-gas escaping from the storage vessel can be scrubbed with acetone, whereby hydrocyanic acid is removed from the off-gas and returned to the process. For this purpose, for example, a portion of the acetone quantity fed into the reaction from the storage vessel is passed in divided streams through a cooler, preferably a brine cooler, to the top of the washing column, thus achieving the desired result.

Depending on the scale of the amount of end product to be produced, it is advantageous if acetone is fed to the reaction from more than just one storage vessel. Here, two or more storage vessels may each be provided with a respective scrubber. In most cases, however, it is sufficient to equip only one of the storage vessels with a corresponding scrubber. In this case, however, it is often expedient for the corresponding lines carrying the offgas (which can carry acetone and hydrocyanic acid) to pass through the vessel or through the scrubber.

The temperature of the acetone in the storage vessel can in principle be in essentially any range, as long as the acetone is present in liquid form at the respective temperature. However, the temperature in the storage vessel is advantageously from about 0 to about 20 ℃.

The acetone used for washing in the washing column is cooled to a temperature of about 0 to about 10 ℃ by means of a corresponding cooler, for example a plate cooler, with brine. Therefore, the temperature of acetone at the inlet of the washing column is preferably, for example, about 2 to about 6 ℃.

The hydrocyanic acid required for the reaction can be fed to the reactor in liquid or gaseous form. This can be, for example, raw gas from the BMA process or from the Andrussow process.

The hydrogen cyanide can be liquefied, for example, by using a corresponding cooled brine. Instead of liquefied hydrocyanic acid, coke oven gas may also be used. Thus, for example, coke oven gas containing hydrogen cyanide is continuously washed, after washing with potash, with acetone containing 10% water by convection, and the reaction to form acetone cyanohydrin can be carried out in the presence of a basic catalyst in two gas washing columns connected one after the other.

In another embodiment, a gas mixture comprising hydrogen cyanide and an inert gas, in particular crude gas from BMA-or from Andrussow-process, is reacted with acetone in a gas-liquid-reactor in the presence of a basic catalyst and acetone cyanohydrin.

In the process described here, preference is given to using BMA crude gas or Andrussow crude gas. The gas mixture obtained from the above conventional process for preparing hydrogen cyanide can be used as such or after pickling. Crude gas from a BMA process, in which predominantly hydrocyanic acid and hydrogen are formed from methane and ammonia, typically contains 22.9% by volume HCN, 71.8% by volume H₂2.5% by volume NH₃1.1% by volume N₂1.7 vol.% CH₄. In the known Andrussow process, hydrocyanic acid and water are formed from methane and ammonia and atmospheric oxygen. When oxygen is used as the source of oxygen, the raw gas of the Andrussow process typically contains 8 vol% HCN, 22 vol% H₂O, 46.5 vol.% N₂15% by volume H₂5% by volume CO, 2.5% by volume NH₃And 0.5% by volume each of CH₄And CO₂。

When using a non-acid-washed raw gas from the BMA-or Andrussow-process, the ammonia contained in the raw gas often acts as a catalyst for the reaction. Since the ammonia contained in the raw gas often exceeds the amount required as a catalyst, which leads to a too high loss of sulfuric acid used for stabilization, such raw gas is often subjected to an acid wash in order to remove ammonia therefrom. However, when using such acid-washed crude gas, a suitable basic catalyst must be added to the reactor in catalytic amounts. In this connection, in principle, known inorganic or organic basic compounds act as catalysts.

The hydrogen cyanide is fed continuously into the loop reactor in the form of a gas or liquid or a gas mixture containing hydrogen cyanide and acetone in a continuous mode of operation. The loop reactor comprises at least one possibility for adding acetone, or two or more of these possibilities, at least one possibility for adding liquid or gaseous hydrocyanic acid, or two or more of these possibilities, and at least one possibility for adding catalyst.

In principle any basic compound, such as ammonia, caustic soda solution or caustic potash solution, is suitable as catalyst, which can catalyze the reaction of acetone and hydrocyanic acid to form acetone cyanohydrin. However, it has been shown that it is advantageous when organic catalysts, in particular amines, are used as catalysts. Suitable are, for example, primary or tertiary amines, such as diethylamine, dipropylamine, triethylamine, tri-n-propylamine, and the like.

The loop reactor which can be used in the process elements also has at least one pump or two or more pumps and at least one mixing device or two or more such mixing devices.

In principle, all pumps which are suitable for ensuring the circulation of the reaction mixture in the loop reactor are suitable as pumps.

Mixing devices with movable elements and so-called static mixers, in which immovable flow resistances are provided, are suitable as mixing devices. Where static mixers are used, it is suitable, for example, that under operating conditions, an operating shift of at least about 10, for example at least about 15 or at least about 20bar is permitted without substantial limitation of functional capacity. The respective mixers may be formed of plastic or metal. Such as PVC, PP; HDPE, PVDF, PFA or PTFE are suitable as plastics. The metal mixer may be formed of, for example, a nickel alloy, zirconium, titanium, or the like. For example a rectangular mixer is equally suitable.

The addition of the catalyst to the loop reactor is preferably carried out after the pump and before the mixing elements present in the loop reactor. The catalyst is used in the reaction, for example, in such an amount that the overall reaction is carried out at a pH of at most 8, in particular at most about 7.5 or about 7. It may be preferred that the pH fluctuates in the range of about 6.5 to about 7.5, for example about 6.8 to about 7.2, during the reaction.

In the process, instead of adding the catalyst to the loop reactor after the pump and before the mixing device, the catalyst can also be fed to the loop reactor together with the acetone. In this case, it is advantageous to ensure that the acetone and the catalyst are mixed accordingly before being fed into the loop reactor. The respective mixing can be carried out, for example, by using a mixer with movable parts or by using a static mixer.

In the process, when a continuous mode of operation is selected as the mode of operation in the loop reactor, it is then appropriate to check the state of the reaction mixture by means of a fixed-point or continuous analysis. This has the advantage that optionally also rapid reactions to changes in the state of the reaction mixture are possible. Furthermore, it is possible, for example, to meter the reaction partners as accurately as possible in order to minimize yield losses.

The corresponding analysis can be carried out, for example, by taking a sample in the reactor loop. Suitable analytical methods are, for example, pH measurements, measurement of thermal effects or measurement of the composition of the reaction mixture by means of suitable spectroscopy.

In particular with regard to conversion control, quality and safety, it has often proven appropriate to determine the conversion of the reaction mixture by means of the heat evolved from the reaction mixture and to compare this with the heat theoretically liberated.

With a suitable choice of the loop reactor, it is in principle possible to carry out the actual reaction in a pipe system installed in the loop reactor. However, since the reaction is exothermic, care should be taken to cool it sufficiently or to remove the heat of reaction sufficiently in order to avoid loss of yield. It has often proven advantageous to carry out the reaction in a heat exchanger, preferably a tube bundle heat exchanger. The capacity of the respective heat exchanger can be chosen differently, depending on the amount of product to be produced. For large industrial processes, volumes of about 10 to about40m³The heat exchanger of (3) is particularly suitable. The tube bundle heat exchanger preferably used is a heat exchanger having a tube bundle through which a liquid flows in a hood through which the liquid flows. Depending on the diameter of the tubes, the packing density, etc., the heat transfer between the two liquids can be adjusted accordingly. In the process, the reaction can in principle be carried out in such a way that the reaction mixture runs through the heat exchanger in the tube bundle itself and the reaction takes place within the tube bundle, heat being removed from the tube bundle into the hood liquid.

However, it has also proven possible and in most cases makes sense to feed the reaction mixture through the hood of the heat exchanger, while the liquid used for cooling is circulated within the tube bundle. In this connection, it has proven advantageous in many cases for the reaction mixture to be distributed in the hood via flow resistances, preferably deflector plates, so that a higher residence time and better homogeneous mixing are achieved.

Depending on the reactor design, the ratio of hood volume to tube bundle volume can be in the range from about 10: 1 to about 1: 10, with hood volumes being preferably greater than tube bundle volume (based on tube volume).

The hot discharge from the reactor is adjusted with a corresponding coolant, for example water, in such a way that the reaction temperature in the channels is from about 25 to about 45 deg.c, in particular from about 30 to about 38 deg.c, in particular from about 33 to about 35 deg.c.

The product was continuously withdrawn from the loop reactor. The product has a temperature within the reaction temperature range described above, for example a temperature of about 35 ℃. The product is cooled by means of one or more heat exchangers, in particular by means of one or more plate heat exchangers. Here, for example, brine cooling can be used. The temperature of the cooled product should be about 0 to 10 deg.C, especially 1 to about 5 deg.C. The product is preferably transferred to a storage container with a buffer function. Furthermore, the product in the storage vessel can be further cooled, for example by continuously withdrawing a partial stream from the storage vessel into a suitable heat exchanger, for example into a plate heat exchanger, or kept at a suitable storage temperature. It is entirely possible for post-reactions to take place in the storage vessel.

The product can in principle be returned to the storage vessel in any manner. In some cases, however, it has proven advantageous to return the product to the storage vessel via a system of one or more nozzles in such a way that a correspondingly homogeneous mixing of the stored product takes place in the storage vessel.

In addition, the product is continuously introduced from the storage vessel into the stabilization vessel. There, the product is reacted with a suitable acid, for example with H₂SO₄And (4) mixing. At this point the catalyst is deactivated and the pH of the reaction mixture is adjusted to a value of from about 1 to about 3, especially about 2. Suitable acids are, in particular, sulfuric acid, for example containing from about 90 to about 105%, in particular from about 93 to about 98%, of H₂SO₄Sulfuric acid (c).

The stabilized product is removed from the stabilization vessel and passed to the purification stage. In this case, a portion of the withdrawn, stabilized product can be returned, for example, to the stabilization vessel in such a way that a sufficiently homogeneous mixing of the vessel is ensured by means of a system consisting of one or more nozzles.

ACH treatment

In a further process element which can be used in connection with the present invention, acetone cyanohydrin which is obtained in a preceding stage, for example from the reaction of acetone with hydrocyanic acid, is subjected to a distillation treatment. The stabilized crude acetone cyanohydrin is passed through a corresponding column to remove the low-boiling components. Suitable distillation methods may, for example, use only one column. However, it is likewise possible to use a combination of two or more distillation columns in the corresponding purification of the crude acetone cyanohydrin, in combination with a falling-film evaporator. Furthermore, it is also possible to combine two or more falling-film evaporators or two or more distillation columns with one another.

The crude acetone cyanohydrin may typically be distilled from storage at a temperature of from about 0 to about 15 c, for example from about 5 to about 10 c. In principle, the crude acetone cyanohydrin can be fed directly into the column. In individual cases, however, it has proven suitable for the cooled crude acetone cyanohydrin to first receive a portion of the heat of the product which has been purified by distillation via a heat exchanger. Thus, in another embodiment of the process described herein, the crude acetone cyanohydrin is heated to a temperature of about 60 to 80 ℃ via a heat exchanger.

The distillative purification of acetone cyanohydrin is carried out by means of one distillation column, preferably with more than 10 trays or by means of a cascade of two or more correspondingly suitable distillation columns. The bottom of the column is preferably heated with steam. It has proven advantageous that the bottom temperature does not exceed a temperature of 140 c, good yields and good purification being achieved when the bottom temperature is not more than about 130 c or not more than about 110 c. The temperature data relate here to the wall temperature of the column bottom.

Crude acetone cyanohydrin is added into the column body at the upper third of the column. The distillation is preferably carried out at low pressures, with good results, for example at pressures of from about 50 to about 900mbar, in particular from about 50 to about 250mbar, and from 50 to about 150 mbar.

Gaseous impurities, in particular acetone and hydrocyanic acid, are removed at the top of the column and the gaseous substances separated off are cooled by means of a heat exchanger or a cascade of two or more heat exchangers. Preference is given here to using brine cooling at temperatures of from about 0 to about 10 ℃. The gaseous contents of the residual steam (Brueden) are now provided with an opportunity to condense. The first condensation stage can be carried out, for example, at atmospheric pressure. However, it is also possible and in individual cases advantageous to carry out the first condensation stage at low pressure, preferably at the pressure prevailing during the distillation. The condensate is reintroduced into the cooled receiving vessel and collected therein at a temperature of from about 0 to about 15 c, especially from about 5 to about 10 c.

The gaseous compounds which are not condensed in the first condensation step are removed from the sub-pressure chamber by means of a vacuum pump. In this case, any vacuum pump can in principle be used. However, it has proven advantageous in many cases to use a vacuum pump which, owing to its type of construction, does not introduce liquid impurities into the gas stream. Thus, for example, a dry-running vacuum pump is preferably used here.

The gas stream escaping at the pressure side of the pump is passed to another heat exchanger which is preferably cooled with brine at a temperature of about 0 to about 15 c. Here, the condensed content is likewise collected in a collection vessel which has received the condensate obtained under vacuum conditions. The condensation on the pressure side of the vacuum pump can be carried out, for example, by means of a heat exchanger, but can also be carried out by means of a cascade of two or more series of parallel-mounted heat exchangers. The gaseous material remaining after this condensation step is discharged and directed to any other application, such as a thermal application.

The collected condensate can likewise optionally be reused. Although it has proved most advantageous from an economic point of view to return the condensate to the reaction for the preparation of acetone cyanohydrin. This is preferably done at one or more locations where access to the loop reactor is possible. The condensate can in principle have any composition, as long as the preparation of acetone cyanohydrin is not impeded. In most cases, however, the condensate consists predominantly of acetone and hydrocyanic acid, for example in a molar ratio of from about 2: 1 to about 1: 2, frequently in a ratio of about 1: 1.

The acetone cyanohydrin obtained from the bottom of the distillation column is first cooled by passing it through a first heat exchanger to a temperature of about 40 to about 80 c by passing cold crude acetone cyanohydrin. The acetone cyanohydrin is then cooled to a temperature of about 30 to about 35 ℃ by at least one other heat exchanger, optionally with intermediate storage.

Amidation

In a further process element, as is frequently provided for the preparation of methacrylic acid or esters of methacrylic acid, acetone cyanohydrin is hydrolyzed. In this case, methacrylamide is formed as a product after a series of reactions at different temperature stages.

The reaction is carried out in a manner known to the skilled worker by reaction between concentrated sulfuric acid and acetone cyanohydrin. The reaction is exothermic and the heat of reaction is therefore removed from the system in an advantageous manner.

Here, the reaction may also be carried out in a batch process or a continuous process. The latter has proved to be advantageous in most cases. If the reaction is carried out as a continuous process, the use of a loop reactor has proven to be suitable. The reaction can be carried out, for example, in only one loop reactor. Advantageously, however, the reaction is carried out in a cascade of two or more loop reactors.

In the process, suitable loop reactors have one or more points of addition of acetone cyanohydrin, one or more points of addition of concentrated sulfuric acid, one or more gas separators, one or more heat exchangers and one or more mixers, and often also a pump as a conveyor.

As mentioned above, the hydrolysis of acetone cyanohydrin with sulfuric acid to methacrylamide is exothermic. However, the heat of reaction generated in the reaction must be removed from the system at least as far as possible, since the yield decreases with increasing temperature in the reaction. Although in principle a rapid and complete removal of the heat of reaction can be achieved with a corresponding heat exchanger. However, too intensive a cooling of the mixture is also disadvantageous, since sufficient heat transfer is required for the corresponding exchange in the heat exchanger. Since the viscosity of the mixture increases significantly as the temperature decreases, on the one hand, circulation or flow through the loop reactor becomes difficult and, on the other hand, sufficient removal of the reaction energy from the system is no longer ensured.

In addition, too low a temperature in the reaction mixture can lead to crystallization of the contents of the reaction mixture on a heat exchanger. The heat transfer is thus worsened, which shows a clear yield reduction. Furthermore, the loop reactor cannot be supplied with an optimal amount of reactants, thus compromising the process efficiency as a whole.

In one process variant, a portion, preferably from about two-thirds to about three-quarters, of the volume flow from the acetone cyanohydrin stream is fed to the first loop reactor. The first loop reactor preferably has one or more heat exchangers, one or more pumps, one or more mixing elements and one or more gas separators. First loop of flowThe circulating flow of the loop reactor is preferably about 50-650 m³Preferably 100 to 500 m/h³A further preferred range is from about 150 to 450m³H is used as the reference value. In at least one further loop reactor following the first loop reactor, the recycle stream is preferably from about 40 to 650m³Preferably 50 to 500 m/h³A further preferred range is from about 60 to 350m³H is used as the reference value. In addition, the temperature difference of the heat exchanger is preferably about 1 to 20 ℃, particularly preferably about 2 to 7 ℃.

Acetone cyanohydrin can in principle be fed to the loop reactor at any point. However, it has proven advantageous to add the components in a mixing element, for example in a mixer with movable parts or a static mixer, or at a point of good homogeneous mixing. The addition of sulfuric acid is advantageously carried out before the addition of acetone cyanohydrin. However, it is also possible to feed sulfuric acid into the loop reactor at any point in addition.

The ratio of the reactants in the loop reactor is controlled such that sulfuric acid is present in excess. The excess of sulfuric acid, in terms of mole ratio of contents, is about 1.8: 1 to about 3: 1 in the first loop reactor and about 1.3: 1 to about 2: 1 in the last loop reactor.

In individual cases it has proven advantageous to carry out the reaction in the loop reactor with this excess sulfuric acid. Here, sulfuric acid can be used, for example, as solvent and the viscosity of the reaction mixture is kept low, so that a high heat of reaction removal and a low temperature of the reaction mixture can be ensured. This leads to significant yield advantages. The temperature of the reaction mixture is about 90 to about 120 ℃.

Heat removal is ensured by one or more heat exchangers in the loop reactor. It has proven advantageous here for the heat exchanger to have suitable sensors for adjusting the cooling capacity in order to prevent the reaction mixture from being cooled too intensively for the reasons mentioned above. It is therefore advantageous, for example, to measure the heat transfer in the heat exchanger or heat exchangers in a point-like or continuous manner, in order to adapt the cooling capacity of the heat exchanger to this. This can be achieved, for example, by the coolant itself. Likewise, the reaction mixture can be heated accordingly by correspondingly varying the addition of the reaction partners and by generating more reaction heat. A combination of the two possibilities is also conceivable. The loop reactor should furthermore have at least one gas separator. The continuously formed product is removed from the loop reactor via a gas separator. On the other hand, gases formed in the reaction are therefore pumped away from the reaction chamber. Carbon monoxide is mainly formed as a gas. The product withdrawn from the loop reactor is preferably transferred to a second loop reactor. In this second loop reactor, the reaction mixture containing sulfuric acid and methacrylamide, as obtained in the first loop reactor by reaction, is reacted with a retained acetone cyanohydrin stream. Here, excess sulfuric acid from the first loop reactor, or at least a portion of the excess sulfuric acid, reacts with acetone cyanohydrin to further form methacrylamide. The advantage of carrying out the reaction in two or more loop reactors is that the pumpability of the reaction mixture, and thus the heat transfer and ultimately the yield, are improved due to the excess of sulfuric acid in the first loop reactor. In the second loop reactor, in turn, at least one mixing element, at least one heat exchanger and at least one gas separator are installed. The reaction temperature in the second loop reactor is likewise from about 90 to about 120 ℃.

The problems of pumpability of the reaction mixture, heat transfer and reaction temperature as low as possible occur in each of the other loop reactors as in the first. Advantageously, the second loop reactor therefore also has a heat exchanger, the cooling capacity of which can be adjusted by means of corresponding sensors.

The addition of acetone cyanohydrin is also carried out in suitable mixing elements, preferably in static mixers or at a point of good homogeneous mixing.

The product is removed from the separator, in particular the gas separator, of the second loop reactor and heated to a temperature of about 130 to about 180 ℃ for the reaction to completion and for the formation of methacrylamide.

The heating is preferably carried out such that the maximum temperature is reached in as short a time as possible, for example in a time of from about 1 minute to about 30 minutes, in particular in a time of from about 2 to about 8 minutes or from about 3 to about 5 minutes. Heating can in principle be carried out in any apparatus to reach such a temperature in such a short time. The energy can be input, for example, by means of electrical energy or by means of steam in a conventional manner. However, energy can also be supplied by electromagnetic radiation, for example by microwaves.

In many cases it has proven advantageous if the heating step is carried out in a heat exchanger with a two-stage or more tube spiral arrangement, which preferably is present in an at least doubled, oppositely directed arrangement. Here, the reaction mixture is rapidly heated to a temperature of about 130 to 180 ℃.

The heat exchanger may for example be combined with one or more gas separators. Thus, for example, the reaction mixture can be passed through a gas separator into a heat exchanger after leaving the first tube spiral. In this case, for example, gaseous constituents formed during the reaction can be separated off from the reaction mixture. It is likewise possible to treat the reaction mixture after it leaves the second spiral with a gas separator. It may furthermore prove advantageous to treat the reaction mixture with a gas separator at two locations, i.e. after leaving the first tube spiral and after leaving the second tube spiral.

The amide solution thus obtained generally has a temperature of more than 100 c, generally a temperature of about 130 to 180 c.

The gaseous compounds produced in the amidation can in principle be removed in any way or fed to further processing. However, it is advantageous in individual cases if the respective gases are collected in the transport duct in such a way that they can be subjected to a steam-admission impact either continuously or optionally with pressure, for example with steam pressure, so that they can be transported further.

Esterification

Another step, which represents a process element, which can be used in connection with the process of the present invention, is the hydrolysis of methacrylamide to methacrylic acid, while esterifying it to methacrylate esters. The reaction may be carried out in one or more heated boilers, for example heated by steam. In many cases, it has proven advantageous to carry out the esterification in at least two successive boilers, but it can also be carried out, for example, in three or four or more successive boilers. Here, the methacrylamide solution is fed into the boiler or into a first boiler of a boiler cascade comprising two or more boilers.

It is often preferred to carry out the corresponding esterification reaction with a cascade of two or more boilers. The following therefore relates only to this solution.

In the process described herein, for example, an amide solution, such as that obtained from the amidation reaction described herein, may be fed into the first boiler. For example, the boiler is heated with steam. The amide solution added generally has a relatively high temperature, for example a temperature of about 100 to about 180 ℃, corresponding substantially to the temperature of the amide solution exiting from the amidation reaction described above. In addition, alcohol that can be used for esterification is added to the boiler.

In principle, any straight-chain or branched, saturated or unsaturated alcohol having from 1 to about 4 carbon atoms is suitable for this purpose, methanol being particularly preferred. Likewise, these alcohols can be used together with methacrylates, especially in the case of transesterification.

The boiler is furthermore supplied with water such that the concentration of water present in the boiler is generally in the range of about 13 to about 26 wt.%, in particular about 18 to about 20 wt.%.

The amounts of amide solution and alcohol are adjusted so that the total molar ratio of amide to alcohol is from about 1: 1.4 to about 1: 1.6. The alcohol can be distributed in the boiler cascade in such a way that the molar ratio in the first reactor is from about 1: 1.1 to about 1: 1.4 and the molar ratio in the subsequent reaction stages, based on the total amide flow, is adjusted from about 1: 0.05 to about 1: 0.3. The alcohol added to the esterification can consist of "fresh alcohol" as well as the alcohol of the recycle stream of the treatment stage and, if necessary, of the recycle stream of the process downstream of the production system.

The supply of water to the first boiler can in principle be carried out by passing water from any source into the boiler, provided that this water does not adversely affect the contents of the esterification reaction or of the subsequent process stages. For example, VE water or well water can be introduced into the boiler. However, it is likewise possible to introduce into the boiler a mixture of water and organic compounds, such as are produced in the purification of methacrylic acid or methacrylic esters. In a preferred embodiment of the method described here, a mixture of water and these organic compounds is supplied to the boiler at least in portions.

When a cascade of two or more boilers is used in the esterification reaction, the gaseous substances formed, in particular the methacrylic acid esters, can in principle be drawn off separately from each boiler and directed to purification. In many cases, however, it has proven advantageous to feed the gaseous products from the first boiler first into the second reaction boiler without the gaseous compounds from the first boiler being directed directly to purification in the case of a cascade of two or more boilers. This has the advantage that costly, device-based defoaming of the strongly formed foam that is customary in the first boiler does not have to be carried out. In the case of the cascade connection of gaseous substances from the first boiler in the second boiler, the foam formed in the first boiler and optionally entrained therewith tends to appear in the reaction chamber of the second boiler. Since there is usually significantly less foam formation, defoaming with a device is not necessary.

The second boiler, which is installed after the first boiler, absorbs the overflow of the first boiler on the one hand and, on the other hand, supplies it with the substances formed in the gaseous state in the first boiler or present in the first boiler. The second and in some cases subsequent boilers are also supplied with methanol. Preferably, the amount of methanol from boiler to boiler is reduced by at least 10%, based in each case on the preceding boiler. The water concentration in the second boiler, as well as in the other boilers, may be different from that in the first boiler, although the concentration difference is usually small.

The residual steam formed in the second boiler is discharged from the boiler and conducted to the bottom of the distillation column.

When esterification is carried out with a cascade of three or more boilers, the overflow of the second boiler is then forwarded to a third boiler, and the overflow of the third boiler is optionally forwarded to a fourth boiler, respectively. The other boilers are also steam heated. The temperature of the boiler 3 and optionally the boiler 4 is preferably adjusted to a temperature of about 120 c to about 140 c.

The residual steam escaping from the boiler is conducted into the distillation column, preferably in the lower region of the distillation column. The residual vapor comprises an azeotrope formed by the carrier-vapor, the methacrylate and the alcohol, and has a temperature of about 60 to about 120 c depending on the alcohol used, for example about 70 to about 90 c when methanol is used. In the distillation column, the methacrylic acid esters are separated in the gaseous state from the vapor components boiling at higher temperatures. The high boiling fractions (mainly methacrylic acid, hydroxyisobutyrate and water) were returned to the first reaction boiler. The methacrylate formed is taken off at the top of the column and cooled by means of a heat exchanger or a cascade of two or more heat exchangers. In individual cases it has proven suitable to cool the methacrylic acid esters by means of at least two heat exchangers, wherein the first heat exchanger is condensed with water and cooled to a temperature of from about 60 to about 30 ℃ and the second, brine-cooled heat exchanger is cooled to from about 5 to about 15 ℃. A partial stream of the water-cooled condensate is fed as reflux to the column, thereby effecting concentration control of the column. It is equally possible, however, to cool the methacrylate formed by means of a cascade of more than two heat exchangers. In this case, for example, the cooling can be effected first by two successively connected, water-cooled heat exchangers and then by a corresponding brine-cooled heat exchanger.

Thus, for example, in the process described here, the gaseous methacrylic acid esters formed can be cooled by means of a first heat exchanger with water cooling. The condensed and uncondensed material is then reintroduced into the second heat exchanger where it is further condensed by water cooling. At this point, the gaseous substance can be transferred, for example, into a separate heat exchanger cooled with brine. The condensate from the brine cooled heat exchanger is then added to the distillate stream, while the remaining gaseous material can be used further or passed to purge. The methacrylate condensate from the second water-cooled heat exchanger is now cooled in a heat exchanger cooled with water or with brine to a temperature of less than 15 c, preferably from about 8 to about 12 c. This cooling step results in the formation of methacrylic acid esters having a distinctly lower formic acid content than would be the case without the corresponding cooling step. The cooled condensate is then forwarded to a phase separator. Here, the organic phase (methacrylate) is separated from the aqueous phase. The aqueous phase, which may contain, in addition to water, organic compounds from the distillation step, in particular alcohols, may in principle be used further. However, as mentioned above, it is preferred that the mixture consisting of water and organic compounds can be returned to the esterification process again by feeding it into the first reaction boiler.

The separated organic phase is sent to a scrubber. Where the methacrylate is washed with demineralized water. The separated aqueous phase, which contains a mixture of water and organic compounds, in particular alcohols, can in principle be used again at will. Although economically advantageous, this aqueous phase is returned again to the esterification step, for example by being conveyed to the first boiler.

Since methacrylic acid esters have a strong tendency to polymerize, it is advantageous in many cases to carefully esterify methacrylic acid to prevent such polymerization.

When methacrylic acid or methacrylic esters on the one hand have a small flow rate, polymerization frequently occurs in the apparatus for preparing methacrylic acid or methacrylic esters, so that local calming zones are formed, in which the contact between methacrylic acid or methacrylic ester and the polymerization initiator can be adjusted for a longer time, which initiator then causes polymerization.

In order to avoid corresponding polymerization behavior, it is advantageous to optimize the streams such that, on the one hand, the flow rate of methacrylate or methacrylic acid is so high at as many points in the system as possible, that the number of calming zones is minimized. It may also be advantageous to mix the methacrylic acid or methacrylate stream with suitable stabilizers in such a way that the polymerization is suppressed as far as possible.

For this purpose, in the process described here, the substance flows are in principle mixed with the stabilizer in such a way that as little polymerization as possible takes place in the system itself. For this purpose, the respective stabilizers are supplied, in particular, to the part of the plant in which the methacrylic acid or the methacrylic acid esters are present in high concentrations during or after the distillation.

Thus, for example, it has proven to be of interest to add the stabilizer to the methacrylate stream drawn off there at the top of the distillation column. Furthermore, it has proven advantageous to flush such equipment parts with a solution of the stabilizer in a methacrylate, wherein the methacrylic acid or methacrylate is circulated at a temperature of more than about 20 ℃, preferably at a temperature of about 20 to about 120 ℃. Thus, for example, a portion of the condensate produced in the heat exchanger is returned with a suitable stabilizer to the top of the distillation column in such a way that there is constantly sprayed on its inside stabilized methacrylic acid ester or stabilized methacrylic acid. This is preferably achieved so that a calming zone in which polymerization of methacrylic acid or methacrylic acid ester is feared may not be formed at the top of the column. It is likewise possible to correspondingly subject the heat exchanger itself to a steam impingement with a stabilizing solution of methacrylic acid or methacrylic acid esters in such a way that a calming zone is also not formed here.

Furthermore, in the process described here, it has proven advantageous for example for the carbon monoxide-containing offgas from a previously carried out process, in particular the amidation step, to be conducted through the esterification apparatus together with steam. In this way, a gaseous mixture of compounds which can be separated off as a solid or as a liquid is purified again. On the other hand, they are collected at a central location and can be fed into other applications or cleaned.

The MMA obtained in the esterification and subsequent preliminary purification or the methacrylic acid ester obtained or the methacrylic acid obtained is subsequently conveyed to further processing. Dilute sulfuric acid is produced from the esterification as a remaining residue, which can likewise be fed to other applications.

Preliminary purification of esters or acids

In the process described here, the subject matter of the invention can also be used in conjunction with a process for prepurifying methacrylic acid or methacrylic acid esters, as described in the subsequent process elements. Thus, the crude methacrylic acid or crude methacrylic acid esters are in principle further purified in order to obtain products which are as pure as possible. This purification, which constitutes a further process element, can be carried out, for example, in one stage. However, in most cases it has proven advantageous for this purification to comprise at least two stages, in which the low-boiling components of the product are removed in a first prepurification as described here. For this purpose, the crude methacrylic acid ester or crude methacrylic acid is first of all passed to a distillation column in which low-boiling components and water can be separated off. For this purpose, the crude methacrylic acid ester is passed into a distillation column, the addition taking place approximately in the upper half of the column. The bottom of the column is heated with steam, for example, in such a way that a wall temperature of about 50 to about 120 ℃ is achieved. Purification was performed under vacuum. In the case of esters, the pressure inside the column is preferably from about 100 to about 600 mbar. In the case of acids, the pressure inside the column is preferably from about 40 to about 300 mbar.

The low-boiling components are taken off at the top of the column. These components are, for example, ethers, acetone and methyl formate, among others. The residual steam is then condensed by one or more heat exchangers. In this case, it has proven expedient in individual cases, for example, to first carry out the condensation by means of two heat exchangers which are connected in series and cooled with water. But it is equally possible to use only one heat exchanger at this location. In order to increase the flow rate and to prevent the formation of stationary phases, the heat exchanger is preferably operated in a vertical state, in which case as complete a wetting as possible is preferably achieved. If a water-cooled heat exchanger or a plurality of water-cooled heat exchangers are connected downstream, a brine-cooled heat exchanger is possible, but a cascade of two or more brine-cooled heat exchangers can also be connected downstream. In the heat exchanger cascade, the residual steam is condensed, the stabilizer is provided, and, for example, passed into a phase separator. Since the residual steam may also contain water, the aqueous phase produced in some cases is removed or fed to other applications. Other applications refer, for example, to return to the esterification reaction, for example, to the esterification reaction described above. In this case, the aqueous phase is preferably returned to the first esterification boiler.

The separated organic phase is fed to the top of the column as reflux. A portion of the organic phase can be reused at the top of the spray exchanger and overhead. Since the separated organic phase is the phase mixed with the stabilizer, on the one hand, the formation of calming zones can be effectively prevented. On the other hand, the presence of the stabilizer can further suppress the tendency of the separated residual steam to polymerize.

The condensate stream obtained from the heat exchanger is preferably also mixed with demineralised water in such a way that a sufficient separation can be achieved in the phase separator.

The gaseous compounds remaining in the heat exchanger cascade after condensation can preferably be condensed by means of a steam ejector as a vacuum pump, again through one or more further heat exchangers. In this case, it has proven advantageous from an economic point of view if, in such a postcondensation, not only the gaseous substances from the prepurification are condensed. Thus, for example, other gaseous substances, such as those which result from the main purification of the methacrylic acid esters, can be passed into this postcondensation. The advantage of this process is, for example, that a portion of the methacrylic acid esters which have not been condensed in the main purification stage can thus be transferred in the prepurification again via the phase separator into the purification column. Thus, for example, it is ensured that the yield can be maximized and that as little losses as possible of the methacrylate occur. Furthermore, the composition of the offgas leaving the heat exchanger, in particular the content of low boilers, can also be adjusted by suitable selection of the design and operation of the further heat exchanger.

Since water is added during the prepurification of the methacrylic acid esters, the water content in the esterification and the concentration of low-boiling components in the crude methyl methacrylate can be increased continuously as a whole. To avoid this, it is advantageous if a part of the water introduced into the system is preferably continuously discharged from the system. In principle, this removal can be carried out, for example, on a large scale, wherein, in the prepurification, water is passed into the system. The aqueous phase separated in the phase separator usually contains organic content. It is therefore advantageous to add this water to the scavenging form that takes advantage of this content of organic matter.

Thus, for example, it is advantageous for such organic-matter-laden water to be mixed into the combustion chamber in the sulfuric acid cracking process. Due to the oxidizable content, its combustion value can also be utilized at least in part. Furthermore, the removal of water laden with organic substances, which can be costly, is often avoided thereby.

Fine purification of methacrylic acid esters

For the fine purification of the methacrylic acid esters, the crude methacrylic acid esters which have been prepurified are redistilled. Here, the crude methacrylic acid ester is freed of its high-boiling components by means of a distillation column, so that pure methacrylic acid ester is obtained. For this purpose, the crude methacrylic acid ester is introduced into the distillation column, sometimes in its lower half, in a manner known to the skilled worker.

The distillation column may in principle correspond to any embodiment which the skilled person finds suitable. However, for the purity of the products obtained, it has proven advantageous in many cases to operate the distillation column with one or more packing layers which approximately correspond to the following predefined packing:

on the one hand, as little so-called "dead space" as possible should be formed in the column, just as in the other conduits through which the methacrylate flows. The dead space results in a relatively long residence time of the methacrylate, which is advantageous for its polymerization. This in turn leads to production interruptions and corresponding partial costs for purifying the incorporated polymer. In particular, dead spaces are formed both by design and by suitable operating modes of the column, which is always loaded with a sufficient quantity of liquid, so that continuous flushing of the column, in particular of the column internals such as packing layers, from four sides is achieved. The tower may thus be provided with spray means designed to spray the built-in web of the tower. Furthermore, the tower internals can be connected to one another in such a way that little or preferably no dead spaces are formed. For this purpose, the tower internals are connected to one another or to the tower via interrupted adhesive seams. Such a bond line has at least about 2, preferably at least about 5, particularly preferably at least about 10 interruptions over a bond line length of 1 m. The length of the interruption can be selected such that it represents at least about 10%, preferably at least about 20%, particularly preferably at least about 50%, but usually not more than 95%, of the length of the bond line. Another construction measure can be that less than about 50%, preferably less than about 25%, particularly preferably less than about 10%, of all sides, in particular of the internal webs of the column, extend horizontally in the column interior, in particular where contact with the methacrylate takes place. Thus, for example, the supports leading to the interior of the tower may be conical or have a slope. Furthermore, a measure can be taken to keep the amount of liquid methacrylate at the bottom of the column as small as possible during operation of the column, on the other hand to avoid overheating of this amount despite mild temperatures and large evaporation areas during evaporation. It can be advantageous here for the amount of liquid in the column bottom to be about 0.1 to 15%, preferably about 1 to 10%, based on the total amount of methacrylate in the column. The measures proposed in this paragraph can also be used in the distillation of methacrylic acid.

In the purification of methacrylic acid esters, the high-boiling components thereof are separated by distillation of the product. For this purpose, the bottom of the column is heated with steam. The temperature of the bottom of the column is preferably from about 50 to about 80 c, especially from about 60 to about 75 c, at a wall temperature of less than about 120 c.

The material produced at the bottom of the column is preferably discharged continuously and cooled via a heat exchanger or a cascade of more heat exchangers to a temperature of about 40 to about 80 ℃, preferably about 40 to about 60 ℃ and particularly preferably about 50 to 60 ℃.

The material, which contains predominantly methacrylate, hydroxyisobutyrate, methacrylic acid and stabilizer components, is then removed from the storage container or fed to another application, for example. In many cases, it has proven advantageous to return the material obtained at the bottom of the column to the esterification reaction. For example, the material from the bottom of the column is returned to the first esterification boiler. This has the advantage that the high boilers contained in the column bottoms are returned to the esterification reaction, in view of the most economical possible mode of operation and the highest possible yield.

The distillatively purified methacrylic acid ester is taken off overhead and cooled via a heat exchanger or a cascade of two or more heat exchangers. The residual steam heat can be removed here by a water-cooled heat exchanger or by a brine-cooled heat exchanger or by a combination of both. In individual cases it has proven to be suitable for the residual steam from the distillation column to be forwarded to two or more heat exchangers connected in parallel, which are operated by water cooling. The uncondensed fraction from the water-cooled heat exchanger can, for example, be conducted to one brine-cooled heat exchanger or to a cascade of two or more brine-cooled heat exchangers, which can be installed in series or in parallel. The condensate obtained from the heat exchanger is conducted into a collecting vessel and passed by means of a pump through another heat exchanger or a cascade of two or more further heat exchangers into a buffer vessel. The condensate stream is cooled down to a temperature of from about 0 to about 20 c, preferably from about 0 to about 15 c, particularly preferably from about 2 to 10 c, for example by means of a cascade of one or two water-cooled heat exchangers and one or two brine-cooled heat exchangers.

A partial stream is taken from the condensate stream and returned to the distillation column via the top of the column. The feeding of the condensate stream into the top of the column can in principle take place in any manner, for example via a distributor. Advantageously, however, a portion of the condensed stream is fed into, for example, a residual steam line at the upper part of the column top. It is also preferred that the stabilizer is introduced into the top of the column by this introduction.

A further partial flow of the condensate provided back into the column can, for example, be branched off before entering the residual steam line and directly into the column head. It is also preferred here that the stabilizer is introduced into the top of the column by this feed. The entry into the top of the column can be effected, for example, by spraying the condensate into the interior of the top of the column in such a way that no calming zone can form at the top of the column, in which the polymerization of the methacrylic acid esters takes place. It is also advantageous to add a stabilizer to the condensate stream returned to the column in order to prevent polymerization. This can be achieved, for example, by adding a corresponding amount of polymerization inhibitor as stabilizer to the condensate partial stream provided for the top of the spray tower. In this case, it has proven advantageous in individual cases for the condensate partial stream to be passed through a suitable mixing device, preferably a static mixer, after the addition of the stabilizer but before entry into the column top, in order to achieve a most homogeneous possible distribution of the stabilizer in the condensate partial stream.

The non-condensable gaseous substances produced in the purification process are, for example, passed into a purge.

The crude product present in the buffer vessel is maintained at a temperature of from about 0 to about 20 c, preferably from about 0 to about 15 c, particularly preferably from about 2 to 10 c, by means of a brine cooler.

In order to remove optional further impurities from the product and to obtain the purest alkyl methacrylate, the product may be subjected to an additional stage of adsorptive purification. It has proven suitable here, for example, for the pure product to be purified further completely or at least partially by means of molecular sieves. In particular acid impurities, in particular formic acid formed in the preparation process, can thus be removed from the product stream in a simple manner. In addition, it has also proven to be suitable in individual cases for the product stream, after passing through the adsorption purification stage, to also pass through one or more filters in order to remove solids optionally contained in the product.

The material flow generated in this treatment mainly contains polymerizable compounds. As has already been described several times, it has also proven advantageous in the case of the process described here to constantly flow the apparatus components in contact with the methacrylate in order to suppress the formation of calming zones. Thus, in another embodiment of the process described here, a partial stream of the methacrylic acid esters is taken off after the buffer vessel but before the adsorptive purification stage and is used to flush the top region of the same heat exchanger which absorbs the residual vapor from the distillation column.

The product obtained in the purification stage is then removed from the purification stage at a temperature of from about-5 to about 20 c, preferably from about 0 to about 15 c, particularly preferably from about 2 to 10 c.

Stripping of spent acid

In the process described here, it is expedient, for example, in a further process element, for the spent sulfuric acid produced in the process to be purified in order then to be returned to the process. In this case, for example, the stream containing the used sulfuric acid can be impinged with steam admission in a flotation vessel, for example as can be obtained from esterification. At least a part of the solids contained can be separated off at the surface of the liquid, and the separated solids can be recycled. The residual steam is then condensed, preferably by cooling with water in a heat exchanger, cooled and returned to the esterification reaction.

In individual cases it has proven advantageous to introduce the mixture into the heat exchanger in such a way that the mixture is sprayed onto the top of the heat exchanger in order to reduce corrosion in the heat exchanger and in order to further improve the cooling of the mixture consisting of water and organic compounds, as is obtained in the purification of the prepared methacrylic acid esters by washing in the esterification. In addition to reducing the effects of corrosion and cooling the acid in the heat exchanger, there is another advantage to doing so. The material from the esterification (mixture consisting of water and mainly methanol) is returned to the esterification process together with methacrylic acid and methacrylic acid esters also from the process. In the stripper, a mixture of acid and solids is obtained by flotation as described above. After separation, it is fed to any other application or cleaned. The resulting mixture can be burned, for example, in a cracking plant, to produce sulfuric acid again, and to recover a portion of the energy used in the process.

The non-condensable gaseous compounds produced during stripping are fed to any further use or are purged.

For reasons of operational safety, the apparatus described here for removing solids from the used acid and for returning the material of the esterification process to the same process can also be of double design, for example. Two or more flotation vessels can thus be used mixed in time. Since solids can be deposited in the vessels, it is advantageous to remove the solids when the flotation vessels are not in use.

Drawings

The foregoing will now be described in detail by way of non-limiting examples and figures. The figures are schematically represented as:

FIG. 1: a system of equipment for the preparation and treatment of methacrylic acid or methyl methacrylate,

FIG. 2: an apparatus for the preparation of acetone cyanohydrin,

FIG. 3: a treatment device of acetone cyanohydrin,

FIG. 4: an amidation device, a device for amidation,

FIG. 5: an esterification device is arranged in the esterification device,

FIG. 6: an apparatus for the pre-purification of esters,

FIG. 7: a device for the fine purification of the ester,

FIG. 8: as a heat exchanger that is part of a plant for the production of acetone cyanohydrin.

Detailed Description

FIG. 1 shows preferred elements of a plant system 1 for the production of methacrylic acid or methacrylic acid esters and their further processing products. The system of devices 1 has different devices, which are mostly connected in a fluid-conducting manner to one another, as elements of the system. The plant system includes acetone cyanohydrin production 20, followed by acetone cyanohydrin treatment 30, followed by amidation 40, followed by esterification/hydrolysis 50/50a), followed by treatment 60 of the ester or methacrylic acid, followed by purification 70, followed by the presence of the ester, mostly methyl methacrylate, or methacrylic acid. The resulting pure ester/pure acid may be passed to further processing equipment 80. As further processing units 80, in particular polymerization plants and reactors for other organic reactions can be considered. Polymethacrylates can be prepared in polymerization reactors and the pure monomers obtained here can be reacted to other organic compounds in reactors for organic reactions. After the further processing device or devices 80 is/are a finishing (Konfektionierung) 90. If the further processing products are polymers formed from methacrylic acid or methacrylic esters, in particular methyl methacrylate, they are further processed by suitable apparatus, for example extruders, blow molding machines, injection molding apparatus, spinning nozzles, etc., into fibers, molding compositions, in particular pellets, films, sheets, automobile parts and other molded bodies. In addition, the plant system 1 also includes a sulfuric acid plant 100 in most cases. In this case, in principle all sulfuric acid plants which the skilled worker finds suitable for this can be considered. See, for example, the chapter 4 of "Integrated polarization preservation and Control-draft reflection Document last average technologies for the manufacture of Large Volume organic Chemicals-Amonia acids and starters", p.89, obtained by the European Committee. The sulfuric acid plant 100 is connected to a series of other plants. Thus, concentrated sulfuric acid is supplied to the acetone cyanohydrin preparation 20 via the sulfuric acid conduit 2. Furthermore, a further sulfuric acid conduit 3 is formed between the sulfuric acid plant 100 and the amidation 40. Dilute sulfuric acid, also referred to as "spent acid", from esterification 50 (hydrolysis 50a) is forwarded to sulfuric acid plant 100 via conduit 4 or 5 for spent sulfuric acid. Dilute sulfuric acid may be treated in the sulfuric acid plant 100. The treatment with dilute sulfuric acid can be carried out, for example, as described in WO 02/23088A 1 or WO 02/23089A 1. Generally, the device is made of materials familiar to the skilled person and which are deemed suitable for the various requirements. Stainless steel is the most important here, which must have a particular acid resistance. Furthermore, the areas of the plant which are operated with sulfuric acid, in particular with concentrated sulfuric acid, are lined and protected with ceramic materials or plastics. Furthermore, the methacrylic acid obtained in the methacrylic acid apparatus 50a can be passed into the prepurification 60 via the methacrylic acid conduit 6. It has also proven suitable to incorporate stabilizers labeled "S" into acetone cyanohydrin preparation 20, amidation 40, esterification 50, hydrolysis 50a, prepurification 60, and final purification 70.

Acetone in an acetone container 21 and hydrocyanic acid in a hydrocyanic acid container 22 are supplied to an acetone cyanohydrin production 20 shown in fig. 2. The acetone container 21 has a washing column 23 which has one or more cooling elements 24 in its upper region. A series of waste gas conduits 25 from a plurality of different devices of the plant system 1 open into the scrubber 23. Acetone is fed into the loop reactor 26 via an acetone inlet line 27 and hydrocyanic acid via a hydrocyanic acid inlet line 28, downstream of the hydrocyanic acid inlet line 28 there being a pump 29, followed by a catalyst feed 210 and subsequently by a static mixer 211. A heat exchanger 212 is then connected, having a series of flow resistances 213 and at least one cooling conduit 214. In the loop reactor 26, the reaction mixture consisting of acetone, hydrocyanic acid and catalyst is operated in the circulation in considerable portions, indicated by the bold line. The reaction mixture emerges from the heat exchanger 212, runs through flow resistance along a cooling conduit 214, and a portion of the recycle stream is conducted into a further heat exchanger 215 which is connected to a collection vessel 216, in which a nozzle 217 is present as part of a cooling circuit 218 with a heat exchanger 219, so that the reaction product remains mobile on the one hand and cool on the other hand. The stabilization vessel 221 is connected via a discharge line 220 which is connected to the collection vessel 216, into which the sulfuric acid inlet line 222 opens and from which the crude acetone cyanohydrin emerges via a discharge line 223 which opens into the acetone cyanohydrin treatment 30.

In fig. 3, the discharge line 223 from the cyanohydrin preparation 20 opens into a heat exchanger 31, wherein the stream from the cyanohydrin preparation 20 is heated. The heat exchanger 31 is connected to a residual steam inlet 32 which opens into the upper, preferably top region of the column 33. The column 33 has a plurality of packing layers 34, most of which are designed as trays. In the lower region of the column 33, there is a column bottom 35, from which a column bottom outlet line 36 opens into the heat exchanger 31 and the stream running into the heat exchanger 31 via the outlet line 233 is heated. The heat exchanger 31 is connected to a pure product transfer 37, followed downstream by amidation 40. In the top region of the column 33, there is a top outlet line 38 which leads into a heat exchanger 39, to which a vacuum pump 310 is connected, which in turn leads into a heat exchanger 311. Both heat exchanger 39 and heat exchanger 311 are connected via conduits to a cooling vessel 312 which is connected to a return 313 which is connected to the loop reactor 26 in the acetone cyanohydrin preparation 20.

The amidation 40 shown in FIG. 4 first has an acetone cyanohydrin addition 41 and a sulfuric acid addition 42, which are both passed into a loop reactor 43. Acetone cyanohydrin which is connected to the acetone cyanohydrin treatment 30 is fed 41 into the circulation loop of the loop reactor 43 after the pump 44 and before the mixer 45. Sulfuric acid addition 42 is introduced before the pump 44. Downstream of the mixer 45 is a heat exchanger 46 which in turn leads into a gas separator 47 from which on the one hand a gas outlet line 48 and an inlet line 49 leading to a further loop reactor 410 leave. The further loop reactor 410 or a third can be constructed in comparison with the first loop reactor 43. From this further loop reactor 410, an inlet line 411 leads into a heat exchanger 412, followed by a gas separator 413, from which exit on the one hand a gas outlet line 414 and an amide conduit 415 leading to the esterification/saponification 50/MAS-plant 50 a.

Fig. 5 shows an esterification 50, in which a solvent conduit 51 for conveying water and organic solvent and an amide conduit 52 connected to the amidation 40 open into a boiler 53, which can be heated by means of boiler heating 54. Also, into boiler 53 is an alcohol conduit 55 shown in phantom. An alcohol conduit 55 leads into both the upper and lower portions of the boiler 53. The first boiler 53 is connected to a further boiler 53 'with further boiler heating 54' via a residual ester steam conduit 56, which is indicated with a dash-dot line. The further boiler 53' is also connected to an alcohol conduit 55 both in the lower and in the upper part. The upper region of the boiler 53' is connected to a residual ester stream conduit 56 which leads into the bottom 57 of a column 58. In addition, in the upper region of the boiler 53' there is a conduit 59 for dilute sulfuric acid. The boiler unit 510 contained in the ellipse drawn at the point is formed by the heatable boilers 53 and 54 as well as the alcohol conduit 55 and the ester offgas conduit 56. One, two or more such boiler units may be in cascade succession, wherein any one of the boiler units 510 is connected to the bottom 57 of the column 58 via the ester vapor residual conduit 56. Further, a high-boiling conduit 511 leads from the bottom 57 of the column 58 to the boiler 53, so that water and the organic solvent are fed again for esterification. In the upper region, preferably the top, of the column 58, a first heat exchanger 512 is connected via suitable conduits, followed by a further phase separator 513. A first stabilizer addition 514 (the stabilizer marked "S") and a further stabilizer addition 515 can be provided both at the top of the column 58 and in the first heat exchanger 512, in order to add inhibitors or stabilizers which prevent undesired polymerization. The further phase separator 513 is connected to a scrubber 516, from the lower part of which a solvent conduit 517 leaves, which leads via a heat exchanger 521 to the solvent conduit 51. The crude ester conduit exits from an upper region of scrubber 516 and passes to ester treatment 60. The spent acid conduit 59, which exits from the boiler 53' or the upper region of the boiler of the last boiler unit 510, passes into a flotation vessel 519 to separate off solids or components that are not soluble in the spent acid. From the flotation vessel 519, the waste acid outlet line 520 enters the sulfuric acid plant 100 and a low boiler residual steam line 522 for the low-boiling components is passed on for further processing and return to esterification.

The ester treatment represented in FIG. 6 is connected to esterification 50 via a crude ester conduit 61, wherein crude ester conduit 61 opens into an intermediate region of a vacuum distillation column 62. The tower 62 has a built-in tower web 63 and a tower bottom heater 64 mounted in the lower region of the tower 62. An ester outlet 65 is led out from the lower part of the column 62 (which is the bottom of the column), and is fed to an ester refining 70, and the crude ester from which low boiling substances have been removed is sent to the refining. In the upper region of the column 62, mostly at the top, a first heat exchanger 66 and a further heat exchanger or exchangers 67 are connected via a discharge line, followed by a phase separator 69. In phase separator 69, stream 68 and the mixture coming from heat exchanger 67 are divided into organic and aqueous components, a return 611 being connected in the upper region to phase separator 69, which returns to the upper region of column 62. In the lower region of the separator there is a water outlet line 610 which opens into the esterification 50 in order to add the separated water again to the esterification. The vacuum pump 613 is connected to the heat exchangers 66 and 67 via a vacuum conduit 612.

In fig. 7, the ester outlet 65 from the ester treatment 60 leads into a distillation column 71. The distillation column comprises a plurality of built-in webs 72 and a column bottom heating 73 in the lower region of the distillation column 71. A pure ester vapor conduit 74 leads from the top region of the distillation column 71 into a first heat exchanger 75 followed by another heat exchanger(s) 76 which is connected to a vacuum pump 717. The outlet of the further heat exchanger 76 has a conduit from which, on the one hand, the ester return 77 passes into the upper region of the distillation column 71 or into the top thereof. The ester return 77 has a stabiliser dosing 79 installed in the ester return 77 before the mixer 78. On the other hand, a pure ester delivery line 710 exits from the conduit of the further heat exchanger 76. The lead-out pipe is connected in series with an additional heat exchanger 711 and a further heat exchanger 712. Next is molecular sieve vessel 713 with a layer of molecular sieve packing 714. Further purified by molecular sieves and the purest ester is forwarded to further processing equipment 80 via a purest ester outlet line connected to the molecular sieve vessel.

Fig. 8 represents a fragment from fig. 2 and is therefore described with reference to the drawing of fig. 2. In addition, the following applies: by means of the spray element 81, which is designed as a nozzle, the reaction mixture is fed into a cooling zone 85 which passes through and is bounded by a reaction mixture outlet 88. The cooling zone 85 has a volume symbolized by the area depicted by dots, excluding the volume of the plurality of cooling elements, symbolized by the area depicted by the hatched lines, through which the coolant flows via the coolant inlet 214 and the coolant outlet 82. It can be seen from fig. 8 that the volume of the cooling zone symbolized by the dotted area is larger than the volume of the cooling element symbolized by the hatched area. The deflecting elements or flow resistances 213, which are designed as flow baffles, can be arranged both on the cooler wall 89 surrounding the cooling zone 85 and on the cooling elements 83. By the arrangement and selection of the spray elements 81 and the deflection elements 213, it is achieved that the main flow direction 86, indicated by the dashed arrow, produces a deviating cooling flow direction 87, so that a good homogeneous mixing of the reaction mixture is obtained in the cooling zone 85.

Examples

In a reaction apparatus corresponding to FIG. 8 with a loop reactor 26, HCN and acetone were reacted at 10 ℃ in the presence of diethylamine as catalyst. The following table gives the reaction conditions.

Examples	Ratio of the volume of the cooling zone 85 to the outer volume of the cooling element 83	Reaction conditions	Residence time in heat exchanger 212	Conversion of ACH
					1.	1∶1.7	No flow resistance 213	0.2h	75％
2.	1∶1.7	Flow resistance 213	0.3	81％
					3.	1∶0.7	No flow resistance 213	0.5h	83％
4.	1∶0.7	Flow resistance 213	0.6	87％

List of tags

1 device system

2 sulfuric acid conduit

3 other sulfuric acid conduits

4 spent conduit-ester sulfate

5 spent sulfuric acid catheter-acid

6 methacrylic acid catheter

Preparation of 20 acetone cyanohydrin

30 acetone cyanohydrin treatment

40 amidation

50 esterification

50a hydrolysis

60 preliminary purification

70 fine purification

80 further processing apparatus

90 finishing

100 sulfuric acid plant

21 acetone container

22 hydrocyanic acid container

23 washing tower

24 cooling element

25 waste gas conduit

26-loop reactor

27 acetone inlet pipe

28 hydrocyanic acid introducing pipe

29 pump

210 catalyst introducing pipe

211 mixer

212 heat exchanger

213 flow resistance

214 cooling conduit

215 heat exchanger

216 collecting container

217 nozzle

218 cooling cycle

219 heat exchanger

220 delivery pipe

221 stabilizing vessel

222 sulfuric acid inlet pipe

223 delivery pipe

31 heat exchanger

32 residual steam leading-in pipe

33 Tower

34 layers of packing

35 tower bottom with heat exchanger

36 tower bottom delivery pipe

37 pure product transport

38 top delivery tube

39 heat exchanger

310 vacuum pump

311 heat exchanger

312 Cooling Container

313 return to

41 acetone cyanohydrin is added

42 sulfuric acid addition

43 Loop reactor

44 pump

45 mixer

46 heat exchanger

47 gas separator

48 gas delivery pipe

49 lead-in pipe

410 other loop reactor

411 introducing pipe

412 heat exchanger

413 gas separator

414 gas delivery pipe

415 amide catheter

51 solvent conduit

52 amide catheter

53 first boiler

54 first boiler heating

53' other boilers

54' other boiler heating

55 alcohol conduit

56 ester residual steam conduit

57 column bottom

58 towers

59 waste acid conduit

510 boiler unit

511 high-boiling point substance conduit

512 heat exchanger

513 phase separator

514 stabilizer addition

515 other stabilizers are added

516 extraction column

517 solvent conduit

518 crude ester catheter

519 flotation container

520 waste acid delivery pipe

521 heat exchanger

522 low-boiling-point substance residual steam conduit

61 crude ester introducing pipe

62 vacuum distillation column

63 built-in web of tower

64 bottoms heating

65 ester delivery pipe

66 heat exchanger

67 heat exchanger

68 introduction of water

69 phase separator

610 water delivery pipe

611 return

612 vacuum conduit

613 vacuum pump

71 distillation column

72 tower built-in web

73 column bottom heating

74 pure ester residual steam conduit

75 first heat exchanger

76 other heat exchangers

77 ester return

78 mixing device

79 stabilizer metering

710 pure ester delivery pipe

711 additional heat exchanger

712 other heat exchangers

713 molecular sieve container

714 molecular sieve packing layer

715 purest ester delivery pipe

716 high boiling point conduit

717 vacuum pump

81 jet element

82 coolant outlet

83 Cooling element

84 product outlet

85 cooling zone

86 main flow direction

87 direction of cooling flow

88 outlet for reaction mixture

89 cooler wall

Claims (20)

1. A process for the preparation of acetone cyanohydrin comprising the steps of:

A. contacting acetone and hydrocyanic acid in a reactor to form a reaction mixture, wherein the reaction mixture is recycled and acetone cyanohydrin is obtained;

B. cooling at least a portion of the reaction mixture by flowing through a cooling zone of a cooler, the cooler comprising one cooling element or at least two cooling elements;

C. discharging at least a portion of the acetone cyanohydrin produced from the reactor,

wherein the volume of the cooling zone of the cooler is greater than the volume of the cooling element or the at least two cooling elements of the cooler, based on the total internal volume of the cooler.

2. The process as claimed in claim 1, wherein a portion of the reaction mixture flows at least during cooling in a cooler flow direction which is different from the main flow direction.

3. The process of claim 2 wherein the cooler flow direction is established by deflecting the reaction mixture.

4. A method according to claim 3, wherein the folding is effected by a folding member provided in or connected to the cooler.

5. The method of claim 4, wherein the deflector is a jet member or a baffle member or both.

6. A method according to claim 4 or 5, wherein the deflector is provided in the cooling zone.

7. Method according to one of the preceding claims, wherein the cooling element is a hollow body of elongate shape through which a coolant can flow.

8. The method of claim 7, wherein the cooling element has a rod-like or plate-like configuration.

9. The method according to claim 7 or 8, wherein the cooling element is a tube bundle.

10. The process according to any of the preceding claims, wherein the residence time of the reaction mixture in the cooler is from 0.1 to 2 h.

11. A method for preparing an alkyl methacrylate, comprising the steps of:

a. preparation of acetone cyanohydrin according to the process of one of the preceding claims;

b. contacting acetone cyanohydrin with inorganic acid to obtain methacrylamide;

c. contacting methacrylamide with an alcohol to obtain an alkyl methacrylate;

d. optionally purifying the alkyl methacrylate.

12. A method for preparing methacrylic acid comprising the steps of:

α) preparation of acetone cyanohydrin according to a process of any one of claims 1 to 10;

beta) contacting acetone cyanohydrin with inorganic acid to obtain methacrylamide;

gamma) methacrylamide reacts with water to form methacrylic acid.

13. An apparatus for preparing alkyl methacrylate, comprising fluid-conducting interconnected:

-the plant elements for the preparation of acetone cyanohydrin, followed by;

-the elements of the plant for the preparation of methacrylamide, followed by;

-a piece of equipment for the preparation of alkyl methacrylate, optionally followed by;

-a piece of equipment for purifying the alkyl methacrylate, optionally followed by;

-polymerized equipment elements, optionally followed by;

-a piece of equipment to be put in order,

wherein the plant components for the production of acetone cyanohydrin comprise a loop reactor with a cooler comprising a cooling zone and one cooling element through which flow can pass, and the volume of the cooling zone of the cooler is greater than the volume of the cooling element or elements of the cooler, based on the total volume of the cooler.

14. The apparatus of claim 13, wherein the cooler has a deflector.

15. The apparatus of claim 14, wherein the deflector is a jet member or a baffle member.

16. Apparatus according to claim 13 or 14, wherein the deflector is provided in the cooling zone.

17. Apparatus according to any one of claims 13 to 16, wherein the cooling element is an elongate hollow body through which a coolant can flow.

18. The apparatus of claim 17, wherein the cooling element has a rod-like or plate-like cooling zone.

19. The apparatus of claim 17 or 18, wherein the cooling element is a tube bundle.

20. A process according to claim 11, wherein the process is carried out in an apparatus according to any one of claims 13 to 19.