HK1063048A - Process for the purification of toluenediisocyanate incorporating a dividing-wall distillation column for the final purification - Google Patents

Description

Method for purifying toluene diisocyanate by dividing wall type distillation column for final purification

The present invention relates to an improvement in the recovery and purification of p-Toluene Diisocyanate (TDI) using a dividing wall column to ultimately purify the TDI product. The process of the present invention benefits from the ability to achieve higher TDI purity.

The invention relates to a process in which toluenediamine is reacted with phosgene in the liquid phase in the presence of a solvent solution, or in which toluenediamine is reacted directly with phosgene in the gas phase and quenched with a solvent after the reaction; excess phosgene is then partially or completely removed from the resulting reaction mixture and the crude phosgenated distillation feed is fed to a fractionation unit, wherein the solvent and optionally residues have been removed. The crude TDI feed was then fed to a dividing wall distillation column from which four fractions were recovered.

1) The gaseous low boilers and the solvent-rich product, preferably condensable species, are recovered therefrom and returned to the process for the removal of phosgenation, residues or solvents.

2) The product enriched in low boilers is preferably returned to the phosgenation, residue removal or solvent removal process or recovered as a separate product gas.

3) The high boiler rich bottoms product is preferably fed to a residue removal system for further recovery of volatiles.

4) An isocyanate product stream.

The invention belongs to the technical field of a method for purifying a Toluene Diisocyanate (TDI) mixture. The TDI mixture is typically prepared by reacting toluene with nitric acid to produce Dinitrotoluene (DNT), hydrogenating the Dinitrotoluene (DNT) to produce Toluenediamine (TDA), and reacting the Toluenediamine (TDA) with phosgene to produce Toluene Diisocyanate (TDI). Toluene Diisocyanate (TDI) is a commercially available material that is particularly useful in the manufacture of polyurethane, polyurea, and polyisocyanurate polymers, especially foamed polymers.

DE-A1-3736988 discloses that organic mono-or polyisocyanates are prepared continuously by reacting the corresponding mono-or polyamines in inert solvents with phosgene, likewise in inert solvents, at temperatures below 150 ℃. The amine and phosgene solutions are combined and passed through one or more reaction columns, the bottom of which is connected to the top in succession and which have at least 10 separate reaction chambers separated by perforated plates, the holes in which preferably have a maximum diameter of 20 mm.

EP-A1-570799 discloses the production of aromatic diisocyanates by reaction of diamines with phosgene. The phosgene and the diamine have an average contact time of 0.5 to 5 seconds when the temperature is higher than the boiling temperature of the diamine. The mixture was passed continuously through a 200-600 ℃ column reaction space to allow sufficient reaction and avoid back mixing. The gaseous mixture is then cooled to condense the diisocyanate and the temperature is maintained above the decomposition temperature of the carbamate corresponding to the diamine used. The uncondensed diisocyanate is washed off with an inert solvent, which is recovered by distillation.

The Handbook of polyurethanes (Polyurethane Handbook) (Oertel, eds., Handbook of polyurethanes (Polyurethane Handbook), Munich, Germany: Hanser Press, 1985, pages 62-73) describes the state of the art phosgenation and distillation processes for the production of toluene diisocyanate. In the distillation process, the solvent is completely removed from the crude TDI mixture as an overhead product from the solvent column, which is returned to the phosgenation process or to the excess phosgene recovery. The remaining crude isocyanate bottoms stream from the solvent column is sent to a pre-flasher (pre-flash) where two products are obtained: an isocyanate-rich overhead product and a residue-rich bottoms stream, the latter being fed to the residue removal unit. In the residue removal unit, the volatile substances are then removed from the above-mentioned residue-rich stream and condensed. The condensed volatiles from the residue and the overhead stream from the pre-evaporator are then combined and fed to the isocyanate column. In the isocyanate column, the product isocyanate is recovered as a top stream, while the bottom stream, which is rich in high boilers, is returned to the pre-evaporator. The fact that complete solvent removal (complete solvent removal) is carried out in a solvent column limits the process. Since the yield of TDI is known to be negatively affected by high temperatures, complete solvent removal must be performed at relatively low pressures to keep the cell temperature (temperature) low enough to avoid yield loss, thus requiring larger cells. Furthermore, the isocyanate and residues are left in the heated zone for a long time, which leads to a higher rate of residue formation. Finally, condensing the overhead stream from the pre-evaporator before passing it to the isocyanate column is energy inefficient.

A second prior art process is described in Industrie Amomatenchemie (Franck H. -G., and Stadelhofer J., Industrie Amomathemie Berlin, Germany: Springer Verlag, 1987, page 253). In this process, the crude TDI-solvent mixture is fed to a two-stage pre-evaporation step to yield a low boiling overhead vapor product and a solvent-free residue-rich bottoms product which is fed to a residue removal unit. In the removal of the residue, volatile substances are then removed from the residue-rich stream and condensed. The overhead product from the pre-evaporator is fed to the solvent column. In the solvent column, the solvent is completely removed as overhead product, and the solvent is returned to the phosgenation unit or excess phosgene recovery unit. The remaining crude isocyanate bottoms stream from the solvent column is fed to the isocyanate column together with the condensed volatile materials from the residue removal unit. In the isocyanate column, the isocyanate product is recovered as an overhead stream, while the bottom stream, which is rich in high boilers (polymeric isocyanate and Hydrolysable Chloride Compounds (HCC), as well as other non-volatiles), is returned to the pre-evaporator. The process is also limited by the fact that complete solvent removal (complete solvantremoval) must be carried out in one solvent column. As described in the polyurethane handbook (polyurethane handbook), complete solvent removal must be performed at relatively low pressures to keep the cell temperature low enough to avoid yield loss, thus requiring larger columns. However, this process reduces the residence time of the isocyanate and residues in the heating zone compared to the above process, which may result in a lower rate of residue formation. Furthermore, the process is energy-efficient since the gas stream fed back to the isocyanate column does not have to be recondensed.

PERP Report for TDI/MDI (Chemsystems, Process) from chemical System for TDI/MDI (chemSystems)EvaluationResearch Planning TDI/MDI 98/99S8, Tarrytown, N.Y., USA: chem Systems, 1999, pages 27-32) that fractionation of crude TDI distillation feed products can be achieved in the following manner. In general, the liquid product from the dephosgenation stage is fed to a pre-evaporator, in which a residue-rich liquid-phase product is produced as bottom product and a gas-phase product, comprising predominantly solvent and isocyanate, as top product. The bottom product from the pre-evaporation step is subjected to a step of removing volatile compounds from the reaction residue (residue removal). The volatile constituents separated off in the residue removal stage and the gas-phase product from the pre-evaporator are passed to a solvent column, where an initial separation of isocyanate from the solvent and removal of any remaining phosgene is effected. The resulting product is a phosgene-rich overhead product, a relative intermediate productA pure solvent stream and an isocyanate-rich bottoms product. The phosgene stream is then returned to the de-phosgenation step or the excess phosgene recovery step. The solvent product is then used in the phosgenation step and in the excess phosgene recovery step. The bottom isocyanate-rich product is then sent to a second solvent removal column where the remaining solvent is recovered. The overhead solvent product from this step, when relatively pure, can be used in the phosgenation step or excess phosgene recovery step, or can be returned to the main solvent recovery step. The final solvent-free bottom isocyanate product is sent to an isocyanate column, yielding an isocyanate overhead product and residue and a bottom gas stream rich in Hydrolysable Chloride Compounds (HCC) which is returned to the pre-evaporation stage or residue removal stage. This process, like the one described in industrille aromatic chemie, achieves a reduction in the residence time of the isocyanate and residues in the heating zone, compared to the process described in the Handbook of polyurethanes (Polyurethane Handbook), which may lead to a lower rate of residue formation. Furthermore, similar to the process described in industrille aromatic chemie, the process will be more energy efficient than the process disclosed in the polyurethane handbook, since the gas stream fed back to the isocyanate column does not have to be recondensed. It has the additional advantage that the solvent removal is achieved in two steps. Since the solvent has a boiling point lower than that of the isocyanate, most of the solvent can be removed at a higher pressure, thus reducing the cost of removing the solvent. Furthermore, the use of two solvent removal steps increases the flexibility of the operation. However, the presence of a third column increases the complexity of the process.

In fractionation, it is sometimes desirable to separate a multi-component feed stream into multiple streams, with the product stream containing various fractions of the desired component. If one feed stream has two product streams, separation can be achieved by withdrawing the distillate and the bottoms product. Further separation can be achieved by repeating the above process of two product streams for either the distillate or the bottoms stream. However, the addition of additional columns requires a corresponding number of reboilers and condensers. Therefore, additional operating expenses are required when the condensation and reboiling processes are repeated. Numerous references can be found in the prior art which discuss efforts to reduce capital and operating costs when separating several fractions from a multi-component feed stream. The oldest and well known PETLYUK system has given the basis for the lowest energy consumption (Agrawal, R and Fidkowski, A, makes Thermally coupled distillation Columns Thermodynamically Always More Efficient for ternary distillation (Are Thermally coupled distillation Columns Always thermodynamic More Efficient for Ternary distillation. In this configuration, the feed is split into two streams using one vapor stream from the stripping section of the main column and another divided stream from the rectifying section of the main column. The resulting vapor and liquid streams leaving the prefractionator are richer in light and heavy components, respectively. These two semi-processed streams are then returned to the main column. One advantage of this configuration is that it allows the main fractionation column to enhance the purity of the side draw. Meanwhile, the main fractionating tower can also ensure that the stripping section and the rectifying section have better quality feedback. The combined effect is that the vapor/liquid stream cycle can be effectively utilized to produce three product streams.

US-A-2,471,134 discloses A modified Petyluk process which proposes to combine the prefractionating column and the main column into one fractionation unit by installing A baffle vertically along the central portion of the column. This column is equipped with an overhead condenser and a bottoms reboiler.

The dividing wall distillation column described in US-A-2,471,134 is A vertical column fractionation column equipped with A reboiler and A condenser, which is divided into four separate column sections by the use of A central partition in the middle section of the column. These sections have a common bottom section (stripping section) and top section (rectifying section) of the column, while the pre-fractionation section and the main fractionation section, which are located in the middle section of the column, are separated by a dividing wall. The multi-component mixture is fed to a prefractionation section, the overhead product is withdrawn from a common rectification section and the bottoms product is withdrawn from a common stripping section, with the intermediate product stream being withdrawn as a by-product from the main fractionation section. One significant advantage of using a dividing wall fractionation column is that a side draw can be obtained from the dividing wall fractionation column at a lower concentration and lower boiling impurities than the by-products obtained from a simple side wall product column.

The dividing wall type distillation tower can effectively overcome the hydraulic limitation of a PETLYUK system. At the same time, the capital expenditure is reduced since it has only one common shell. The dividing wall distillation column disclosed in US-A-2,471,134 can be used in many processes.

In the present invention, the final purification of the TDI product during the distillation of TDI using such a dividing wall distillation column can greatly improve the purity of TDI by reducing the amount of low boiling impurities and reducing energy consumption and capital cost as compared to a standard side draw column (sidedraw column) to obtain the same product quality. Examples of such impurities include solvents, solvent impurities, aromatic monoisocyanates, chlorinated aromatic monoisocyanates, and the like.

The invention relates to a method for purifying toluene diisocyanate from a crude distillation raw material with a phosgene content of less than 2 wt%, comprising

a) Fractionating a crude distillation feed having a phosgene content of less than 2% by weight to remove solvent, optionally removing reaction residues, to produce a crude toluene diisocyanate feed having a solvent content of less than 20% by weight, and

b) dividing the crude toluene diisocyanate having a solvent content of less than 20% by weight in a dividing wall distillation column into four product fractions P1-P4, wherein

P1 is a gas stream rich in gas-phase low boilers and solvent,

p2 is a product rich in low-boiling agent and solvent,

p3 is a high boiler-rich bottom product containing toluene diisocyanate,

p4 is a toluene diisocyanate product stream with a low content of low boilers, high boilers and reaction residues.

The present invention also relates to a process for the manufacture of toluene diisocyanate, said process comprising the steps of:

a) reacting toluenediamine with phosgene to obtain crude distillation raw material,

b) if the crude distillation feed from step a) contains more than 2% by weight or more phosgene, separating phosgene from the crude distillation feed from step a) to give a crude distillation feed having a phosgene content of less than 2% by weight,

c) fractionating a crude distillation feed having a phosgene content of less than 2% by weight to remove solvent, optionally removing reaction residues, to produce a crude toluene diisocyanate feed having a solvent content of less than 20% by weight, and

d) dividing the crude toluene diisocyanate having a solvent content of less than 20% by weight in a dividing wall distillation column into four product fractions P1-P4, wherein

P1 is a gas stream rich in gas-phase low boilers and solvent,

p2 is a product rich in low-boiling agent and solvent,

p3 is a high boiler-rich bottom product containing toluene diisocyanate,

p4 is a toluene diisocyanate product stream with a low content of low boilers, high boilers and reaction residues.

The phosgenation is carried out according to the state of the art. The toluene diamine is reacted with phosgene in the liquid phase when a solvent solution is present, or the toluene diamine is reacted directly with phosgene in the gas phase and quenched with a solvent after the reaction. The reaction mixture obtained preferably has a composition of 5 to 40% by weight of toluene diisocyanate, 1 to 2% by weight of hydrochloric acid, 1 to 5% by weight of phosgene, 0.1 to 2% by weight of high-boiling agent (polymeric isocyanate, Hydrolyzable Chloride Compound (HCC)), and the balance of solvent. In this case, a hydrolysable chloride compound is defined as a compound in which the chlorine present is "loosely" bound. Examples of these compounds are the following classes: ClCH₂C₆H₃(NCO)₂And (CH)₃NCOCl)CH₃C₆H₃(NCO)。

HydrolyzableChloride compoundThe content of compounds is generally determined by reacting the chlorine present in the sample with a hot aqueous-alcoholic solution to give HCl, followed by titration to determine the hydrolyzable natureChlorineIs determined by the concentration of (c). This value is typically reported as a weight percent of Hydrolyzable Chlorine (HC).

Chlorinated aromatic hydrocarbons are substances in which the chlorine is "tightly" bound. Examples of such compounds are the common o-dichlorobenzene and chlorobenzene solvents, and the corresponding compounds.

After the reaction, if the reaction mixture (crude distillation raw material) contains 2% by weight or more of phosgene, the resultant reaction mixture is sent to a separation step. In this separation step, excess phosgene is at least partially removed, so that the crude distillation feed contains less than 2% by weight phosgene. The separation of phosgene can be achieved in a number of different ways or by using these methods in combination. Examples of such processes are simple gas/liquid flash separation (with or without the need to raise the temperature or lower the pressure), gas stripping, distillation, etc.

The resulting crude distillation feed having a phosgene content of less than 2% by weight is then fed to a distillation column or system of distillation columns where the solvent and optionally reaction residues are removed from the crude distillation feed having a phosgene content of less than 2% by weight, thereby producing a crude toluene diisocyanate feed having a solvent content of less than 20% by weight. The distillation column(s) used to remove the solvent and optionally the reaction residue may be conventional distillation columns or divided wall distillation columns. It is preferred to use a conventional distillation column of the prior art.

The crude toluene diisocyanate feed obtained, having a solvent content of less than 20% by weight, is then fed to a dividing wall distillation column and divided into four product fractions P1 to P4.

The product fraction P1 is a product rich in vapor-phase low boiling agent and solvent, and contains 20 to 99% by weight of condensable substances (i.e., solvent, low boiling agent and TDI), and the balance of non-condensable gases, i.e., air, hydrogen chloride, etc. The condensable fraction of the product may contain solvent, low boilers and TDI. The condensable materials are preferably recovered and returned to the phosgenation, residue removal or solvent removal step.

The product fraction P2 is a product rich in low boilers and solvent, which is preferably then returned to the phosgenation, residue removal or solvent removal step or recovered as a separate product stream. The product fraction P2 may contain solvent, low boilers and TDI.

Product fraction P3 is a high boiler rich bottoms product that is preferably sent to a residue removal system for further recovery of volatiles. Fraction P3 preferably contains 0.5 to 15% by weight of high boilers (polymeric isocyanates, Hydrolyzable Chloride Compounds (HCC) and other nonvolatiles) and the remainder is predominantly toluene diisocyanate.

Product fraction P4 is the isocyanate product stream. Fraction P4 preferably contains less than 200ppm by weight of solvents and/or chlorinated aromatic hydrocarbons (in total), less than 100ppm by weight of Hydrolyzable Chlorine (HC), less than 40ppm by weight of acidic substances, and a concentration of toluene diisocyanate of at least 99.5% by weight.

Fractional distillation using a dividing wall column can be successfully used to purify the partially to completely phosgenated TDI reaction product obtained by reacting toluene diamine and phosgene in the presence of a solvent solution or in the gas phase with phosgene and quenching with a solvent after the reaction. The distillation feed obtained contained phosgene and other low-boiling constituents, solvent, toluene diisocyanate, hydrolyzable chloride compounds and high-boiling residues. Thus, this stream is fractionated to remove the solvent and optionally the reaction residues to give pure TDI, which is then sent to a dividing wall TDI purification column. The four products from the dividing wall column are the low boiler and inert gas rich product which is preferably recovered from condensable materials and returned to the phosgene, residue removal or solvent removal step, the low boiler rich liquid product which is then preferably returned to the phosgene, residue removal or solvent removal step or recovered as a separate product stream, the high boiler rich bottom product which is preferably sent to the residue removal system for further recovery of volatiles, and the isocyanate product stream. The solvent used may be any suitable solvent, preferably o-dichloro-benzene, p-dichlorobenzene, chlorobenzene, toluene, benzene, nitrobenzene, anisole, xylene or any mixture thereof. Depending on the reaction conditions, different concentrations of TDI were obtained in the crude distillation product.

The final purification of the crude toluene diisocyanate feed having a solvent content of less than 20% by weight is carried out in a dividing wall distillation column as shown in FIG. 1. The dividing wall distillation column is equipped with at least one reboiler and one condenser. The reboiler may be of any standard type commonly used in the chemical industry, including partial falling film evaporators, forced circulation evaporators, large capacity boiling (kettle) evaporators, natural circulation evaporators, and the like. The condenser may be of any standard type commonly used in the chemical industry, including cocurrent and countercurrent (reflux condenser). The tower may be fitted with any material-carrying internals commonly used in the chemical industry. Including sieve plates, valve trays, fixed valve trays, and structural or arbitrary distillation packages.

The invention is described in more detail below with reference to the attached drawing, in which

FIG. 1 shows the structure of a dividing wall distillation column used in the TDI mixture purification step.

Figure 1 shows a dividing wall distillation column 1 equipped with a reboiler 2, a condenser 3, a partition 4 and material handling internals 5.

The dividing wall distillation column 1 is divided into four distinct operating zones, namely a prefractionation zone for the feed, a stripping zone containing the high boiler product P3, a main fractionation zone containing the isocyanate product P4, and a rectification zone containing the vapor phase low boiler product P1 and enriched with the vapor phase low boiler and solvent product P2. The prefractionation and main fractionation zones are arranged side-by-side on either side of a dividing wall fractionation column 1 and are divided into two zones by a partition 4.

Prefractionation zone

The crude distillation feed A is passed to a prefractionating section where it is divided into two streams, a liquid TDI stream B rich in residue and Hydrolysable Chloride Compounds (HCC) and a vapor stream C rich in low boiling agent. This separation is carried out by means of two flows, one liquid D and the other vapor E. The liquid gas flow D containing the low-boiling agent and TDI enters the pre-fractionating area from the rectifying area. The vapor stream containing TDI and HCC passes from the stripping zone to the pre-fractionation zone.

Stripping zone

Liquid product B from the pre-fractionation zone and liquid product F containing TDI and HCC from the main fractionation zone enter the upper portion of the stripping zone. The intermediate components are separated from the heavy components in stream R by steam G generated by reboiler 2. The resulting residue-rich liquid high boiler leaves as bottom product stream P3. The column is designed with a certain operating pressure, so the temperature in the reboiler is preferably in the range of 140 ℃ and 190 ℃. The TDI-rich vapor streams E and H are sent to a pre-fractionation zone and a main fractionation zone, respectively. The distribution of the vapor streams to the pre-fractionation region and the main fractionation region is made by the inherent pressure drop of the various column sections.

Rectification zone

The low boiler-rich vapor product C from the pre-fractionation zone and the product I from the main fractionation zone, both containing intermediates as well as low boilers, enter the rectification zone in the lower portion. The vapor product J from the rectification zone is sent to a condenser 3, and then part of the condensed product from the condenser is returned to the top of the rectification zone as reflux K, thus separating the light components from the intermediate components. The remainder of the condenser liquid product is withdrawn as a liquid product gas P2 rich in low boilers and solvent. The uncondensed vapor product from the condenser is the low boiler product stream P1. Internal reflux within the column produces a liquid stream. The liquid stream, which contains mainly low boilers and TDI, is divided into streams L and D, which are sent to a main fractionation zone and a pre-fractionation zone, respectively. The distribution ratio of these liquid gas streams is controlled to achieve the desired product quality. Product P2 may optionally be withdrawn as a side product from any stage of the rectification zone.

Main fractionation zone

The TDI-rich vapor product stream H from the stripping zone enters the main fractionation zone from the bottom. A portion of the liquid product L from the rectification zone enters the main fractionation zone from the top. Fractionation produces three products: the vaporous feed I to the rectification zone, the liquid feed F to the stripping zone and a side draw product P4 containing the desired quality of isocyanate product. Product P2 can optionally be withdrawn as a side draw from any stage in the main fractionation zone above the P4 product removal stage.

Claims (6)

1. A process for the purification of toluene diisocyanate from a crude distillation feed having a phosgene content of less than 2% by weight, characterized in that said process comprises

a) Fractionating a crude distillation feed having a phosgene content of less than 2% by weight to remove solvent, optionally removing reaction residues, to produce a crude toluene diisocyanate feed having a solvent content of less than 20% by weight, and

b) dividing the crude toluene diisocyanate having a solvent content of less than 20% by weight in a dividing wall distillation column into four product fractions P1-P4, wherein

P1 is a gas stream rich in gas-phase low boilers and solvent,

p2 is a product rich in low-boiling agent and solvent,

p3 is a high boiler-rich bottom product containing toluene diisocyanate,

p4 is a toluene diisocyanate product stream with a low content of low boilers, high boilers and reaction residues.

2. A process for the manufacture of toluene diisocyanate, characterized in that said process comprises the steps of:

a) reacting toluenediamine with phosgene to obtain crude distillation raw material,

b) if the crude distillation feed from step a) contains more than 2% by weight or more phosgene, separating phosgene from the crude distillation feed from step a) to give a crude distillation feed having a phosgene content of less than 2% by weight,

c) fractionating a crude distillation feed having a phosgene content of less than 2% by weight to remove solvent, optionally removing reaction residues, to produce a crude toluene diisocyanate feed having a solvent content of less than 20% by weight, and

d) dividing the crude toluene diisocyanate having a solvent content of less than 20% by weight in a dividing wall distillation column into four product fractions P1-P4, wherein

P1 is a gas stream rich in gas-phase low boilers and solvent,

p2 is a product rich in low-boiling agent and solvent,

p3 is a high boiler-rich bottom product containing toluene diisocyanate,

p4 is a toluene diisocyanate product stream with a low content of low boilers, high boilers and reaction residues.

3. The process as claimed in claim 1 or 2, wherein the product fraction P1 contains 20 to 99% by weight of solvent, low-boiling agent and toluene diisocyanate.

4. The process according to any one of claims 1 to 3, wherein the product fraction P2 contains solvents, low boilers and toluene diisocyanate.

5. The process as claimed in any of claims 1 to 4, characterized in that the product fraction P3 contains toluene diisocyanate and from 0.5 to 15% by weight of high-boiling agent.

6. The process according to any one of claims 1 to 5, wherein the toluene diisocyanate concentration in the product fraction P4 is at least 99.5% by weight and contains less than 200ppm by weight of solvent and/or chlorinated aromatic hydrocarbons, less than 100ppm by weight of hydrolyzable chlorine and less than 40ppm by weight of acidic substances.