A circulating solvent for preparing MDI by phosgenation and a purification method thereof

Abstract

The present invention discloses a circulating solvent for preparing MDI by phosgenation and a purification method thereof. By controlling the mass content of halogenated hydrocarbon substances in the circulating solvent, the product quality can be improved and the product color number can be reduced. The method of combining a bulkhead tower with a multi-effect distillation can achieve efficient separation of halogenated hydrocarbon substances and solvents, greatly improve product quality, and can be directly recycled. The method combines the degassing and desolventizing systems in the refining process of crude isocyanate, saves the solvent purification unit, realizes complete energy coupling, and has a simple and reliable process, which can greatly reduce investment and operating costs.

Description

Technical Field

[0001] The invention belongs to the field of isocyanate, and particularly relates to a circulating solvent for preparing MDI by phosgenation and a purification method thereof. Technical Background

[0002] Isocyanate is one of the most important raw materials in the polyurethane material building-up process, and it is widely used in polyurethane foam, rubber, fiber, coating, adhesive, synthetic leather, etc. Most of the production methods of industrial isocyanate currently adopt phosgenation, adopt organic primary amine and phosgene to react in an inert solvent, and then prepare and generate through a series of aftertreatment and separation process. In the process of producing diphenylmethane diisocyanate (MDI) by liquid phase phosgene process, for improving reaction quality, reduce the generation of side reaction, in phosgene and diphenylmethane diamine (MDA), a large amount of inert solvents can be added simultaneously, through a series of product purification and solvent refining process, solvent recycling is used, in this process, impurities may be brought into the circulating solvent and participate in the phosgenation reaction, affecting reaction and product quality, which is extremely undesirable to see.

[0003] Patent US3410888 discloses a method for separating aromatic diisocyanate from a reaction mixture, wherein the isocyanate has two benzene rings and the isocyanate functional groups are connected to carbon atoms of different benzene rings. The solvent and the isocyanate are separated by distillation. During the process of removing the solvent, the isocyanate product and the intermediate colored components are also removed and separated together, and then the partially separated mixture of isocyanate and solvent is returned as feed to the solvent removal process or sent to a separate evaporation or fractionation process to concentrate the isocyanate. The disadvantage of this method is that part of the isocyanate separated during the solvent removal process must be re-distilled and purified, resulting in a waste of energy consumption. At the same time, the removed solvent may contain isocyanate, which is circulated and mixed with amines to form urea, causing abnormal blockage of the system or a decrease in product quality.

[0004] Patent CN 101302174A discloses a method for producing isocyanates, wherein an amine and phosgene are reacted in a solvent to produce the corresponding isocyanate, and then the solvent is recovered by distillation and recycled. The patent mentions that reducing the content of phosgene and isocyanate in the solvent is beneficial to improving product quality, but does not restrict other substances in the solvent that affect the reaction, such as halogenated hydrocarbons.

[0005] Patent CN 114380714A discloses a circulating solvent and impurity removal method in phosgenation reaction production. The present invention mixes the solvent and amine in advance in the solvent reactor, and then performs solid-liquid separation to ensure that the total amount of amine-consuming substances in the circulating solvent is reasonably controlled, thereby reducing the generation of solid by-products in the phosgenation reaction. The present invention focuses on substances in the solvent that can react with amines, and reduces the impact on the reaction by consuming raw material amines, resulting in a waste of raw materials and an increase in operating costs. At the same time, the method does not take into account the impact of substances that do not react with amines on the system.

[0006] The above patents mainly focus on the purification of isocyanate and some impurities in the solvent during its preparation process for description and protection. The processes described are all conventional processes, and the influence of halogenated hydrocarbon impurities in the solvent on the reaction quality is not recognized. In the actual MDI production and preparation process, halogenated hydrocarbon substances in the solvent will lead to poor quality of phosgenation reaction and even darken the color of the product. Therefore, it is necessary to develop a new circulating solvent for phosgenation preparation of MDI and its purification method, so as to separate and purify the circulating solvent from the reaction mixture in a more efficient and low-cost way and reduce the content of halogenated hydrocarbons in the solvent. Summary of the invention

[0007] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a circulating solvent for preparing MDI by phosgenation and a purification method thereof, which can greatly reduce the content of halogenated hydrocarbons in the circulating solvent, and at the same time can greatly simplify the process flow and reduce the operating cost.

[0008] A circulating solvent for preparing MDI by phosgenation, wherein the mass content of halogenated hydrocarbon substances in the circulating solvent is 0.1-200 ppm, preferably 1-150 ppm, and more preferably 10-100 ppm.

[0009] In the present invention, the halogenated hydrocarbon substances refer to methane or ethylene substances substituted by chlorine and / or bromine, including carbon tetrachloride, trichlorobromomethane, dichlorobromomethane, tetrachloroethylene, dichlorobromoethylene, trichloroethylene, dibromoethylene and the like.

[0010] In the present invention, the circulating solvent is preferably one or more of chlorinated aromatics, dialkyl terephthalates, diethyl phthalate, toluene and xylene, more preferably one or more of chlorobenzene, dichlorobenzene, toluene and xylene, and further preferably chlorobenzene.

[0011] In the production and preparation process of MDI, a series of purification processes are required, including HCl removal, phosgene removal, solvent removal and other processes. The removed solvent usually contains light component impurities such as HCl, phosgene, and water, which need to be further refined by the solvent. Through continuous research, the inventors found that halogenated hydrocarbon substances also exist in the circulating solvent. The halogenated hydrocarbon substances have a boiling point similar to that of the solvent, are difficult to remove by ordinary refining processes, and can exist stably in the solvent. As the solvent circulates, they participate in the phosgenation reaction and affect the reaction quality.

[0012] Another object of the present invention is to provide a method for purifying a circulating solvent for preparing MDI by phosgenation, the method comprising the following steps:

[0013] a. Phosgenation reaction: diphenylmethanediamine (MDA) and phosgene are subjected to phosgenation reaction in an inert solvent to generate a photochemical reaction liquid;

[0014] b. Separation by next-door separation: The reaction liquid is separated and purified in the next-door tower, the light components containing phosgene and HCl are extracted from the top of the tower, the gas phase solvent is extracted from the tower as the extraction solvent, and the concentrated liquid in the bottom of the tower is sent to the solvent removal unit;

[0015] c. Solvent removal: The concentrated liquid from the bottom of the tower in step b is further distilled under reduced pressure in a solvent removal tower, and the condensate from the top of the solvent removal tower is used as the condensation solvent.

[0016] The circulating solvent includes the produced solvent and the condensed solvent.

[0017] The produced solvent from step b can be used as a heat source for the reboiler in the bottom of the solvent removal tower, and after heat exchange, it is combined with the condensed solvent at the top of the solvent removal tower as a circulating solvent.

[0018] In a preferred embodiment of the present invention, the inert solvent in step a is preferably one or more of chlorinated aromatics, dialkyl terephthalates, diethyl phthalate, toluene and xylene, more preferably one or more of chlorobenzene, dichlorobenzene, toluene and xylene, further preferably chlorobenzene.

[0019] In a preferred embodiment of the present invention, the inert solvent in step a is recycled, and the mass content of halogenated hydrocarbons in the recycled solvent is 0.1-200 ppm, preferably 1-150 ppm, and more preferably 10-100 ppm.

[0020] In the present invention, the halogenated hydrocarbon substances refer to methane or ethylene substances substituted by chlorine and / or bromine, including carbon tetrachloride, trichlorobromomethane, dichlorobromomethane, tetrachloroethylene, dichlorobromoethylene, trichloroethylene, dibromoethylene and the like.

[0021] In some preferred embodiments of the present invention, the mass ratio of MDA and inert solvent added in step a is 1:(1-8), preferably 1:(2-5).

[0022] In some preferred embodiments of the present invention, the mass ratio of MDA and phosgene added in step a is 1:(1-15), preferably 1:(2-5).

[0023] In some preferred embodiments of the present invention, the phosgenation reaction in step a is divided into two steps, cold and hot reactions, wherein the cold reaction temperature is 60-120°C, the pressure is 0.2-3.0 MPaA, and the reaction time is within 5 minutes; the hot reaction temperature is 90-150°C, the pressure is 0.2-0.6 MPaA, and the reaction residence time is 0.5-3 hours.

[0024] In some preferred embodiments of the present invention, the phosgene concentration in the photochemical reaction solution in step a is 0.1-10%, and the solvent concentration is 30-90%.

[0025] In some preferred embodiments of the present invention, a vertical partition is arranged in the middle of the next-door tower in step b, and the tower type is a plate tower, a packed tower or a combination thereof. To reduce the clogging of the tower tray, the tower tray is preferably a fixed valve type.

[0026] In some preferred embodiments of the present invention, in step b, the partition wall in the middle of the dividing wall tower divides the dividing wall tower into a pre-separation section and a side line extraction section, the lower part of the partition wall is a common stripping section, and the upper part of the partition wall is a common rectification section;

[0027] Preferably, the height of the middle partition is 2-12m, preferably 4-8m; the height of the common stripping section is 1-8m, preferably 2-5m;

[0028] Preferably, the bottom of the middle partition is provided with a radially adjustable rotating adjustment device to control the cross-sectional area ratio of the bottom of the pre-separation section and the side line production section to be 1:(0.5-2), more preferably 1:(0.8-1.8), thereby controlling the gas flow rate from bottom to top through the pre-separation section and the side line production section to achieve a reasonable distribution of the gas phase.

[0029] Preferably, the pre-separation section has a reflux flow regulating device to control the ratio of reflux flow to feed flow to be 1:(5-30) to prevent the heavy components from being carried out from the gas phase and achieve effective separation of light and heavy components.

[0030] In some preferred embodiments of the present invention, after the reaction solution is separated and purified by the bulkhead tower, the halogenated hydrocarbon substances are mostly taken out from the light components of the gas phase at the top of the tower. In some preferred embodiments of the present invention, the produced solvent in the tower in step b is produced in the gas phase, the mass content of the halogenated hydrocarbon substances in the produced solvent is 0.1-300ppm, and the mass flow rate of the produced solvent accounts for 30%-60% of the total solvent flow rate.

[0031] In some preferred embodiments of the present invention, the temperature of the next-wall tower bottom in step b is 170-210° C. and the pressure is 0.1-0.3 MPaA.

[0032] In some preferred embodiments of the present invention, the solvent removal tower in step c is a plate tower, a packed tower or a combination thereof, and the solvent is recovered by condensation at the top of the tower.

[0033] In some preferred embodiments of the present invention, the temperature of the solvent removal tower bottom in step c is 120-180° C., preferably 130-170° C.; the pressure is 2-50 KPaA, preferably 5-20 KPaA.

[0034] In some preferred embodiments of the present invention, the condensation temperature at the top of the solvent removal tower in step c is 40-90°C, preferably 60-80°C; the content of halogenated hydrocarbons in the top condensation solvent is 0.1-50 ppm (mass ratio).

[0035] In some preferred embodiments of the present invention, the produced solvent in the tower in step b is used as a heat source for the reboiler of the solvent removal tower in step c, and the outlet temperature of the reboiler is controlled to be 140-180° C. After heat exchange, the produced solvent is combined with the condensed solvent at the top of the solvent removal tower as a circulating solvent, and the circulating solvent is returned to step a for phosgenation reaction.

[0036] A vertical partition is set in the middle of the next-tower to divide the next-tower into the pre-separation side and the main tower side. The partition can make a single tower realize the functions of two towers and avoid the back-mixing of intermediate components in the two-tower process. The material is pre-separated on the feed side of the partition and further distilled and separated on the other side, which reduces the mixing effect near the feed section, improves thermodynamic efficiency, greatly reduces energy consumption, and realizes the function of separating multiple components in a single tower. At the same time, the use of the next-tower technology can save a distillation tower and ancillary equipment, including condensers, reboilers, reflux tanks, reflux pumps, etc., reducing equipment investment and floor space.

[0037] Compared with the prior art, the present invention has the following positive effects:

[0038] (1) The present invention can achieve efficient separation of halogenated hydrocarbons and solvents, and the obtained circulating solvent has high purity. The content of halogenated hydrocarbons can be reduced to 0.1-200 ppm (mass ratio), the product quality is greatly improved, and it can be directly recycled, saving the solvent purification unit, and the process is simple and reliable.

[0039] (2) Compared with conventional separation and distillation processes, the use of a dividing wall tower to achieve the separation effect of multiple towers greatly reduces equipment investment and floor space, and the gas phase solvent extracted from the dividing wall tower is used as the heat source of the crude product purification tower reboiler, achieving complete energy coupling and significantly reducing operating costs. At the same time, the dividing wall tower distillation technology and multi-effect distillation technology are combined to simplify production equipment and process flow, reducing energy consumption in the production process. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 It is a process flow chart of purifying the circulating solvent in the MDI preparation process of the present invention.

[0041] Among them, 1 is a mixed solution of MDA and solvent, 2 is phosgene feed, 3 is a mixer, 4 is a reactor, 5 is phosgenation reaction liquid, 6 is a dividing wall tower, 7 is a rotary adjustment mechanism, 8 is a produced solvent, 9 is a concentrated liquid, 10 is a dividing wall tower reboiler, 11 is a solvent removal tower, 12 is MDI, 13 is a solvent removal tower reboiler, 14 is a condensed solvent, and 15 is a circulating solvent. DETAILED DESCRIPTION

[0042] The present invention is further described below by means of specific examples. The examples described in the present invention are only used to illustrate the present invention and do not limit the scope of the present invention.

[0043] The main sources of raw materials involved in the embodiments and comparative examples are as follows:

[0044] Phosgene: produced by the MDI unit in Yantai Wanhua Industrial Park, industrial products;

[0045] MDI (diphenylmethanediamine): produced by the MDI unit in Yantai Wanhua Industrial Park, industrial product;

[0046] Chlorobenzene: Produced by TDI unit in Yantai Wanhua Industrial Park, industrial product;

[0047] The sample analysis methods involved in the embodiments and comparative examples are as follows:

[0048] Analysis method of halogenated hydrocarbons in solvents:

[0049] After diluting the solvent by 100 times volume with methanol, the total amount of stable halogenated hydrocarbons was determined by gas chromatography and ECD detector;

[0050] Gas chromatography conditions: the column box temperature was maintained at 35°C for 8 min, then increased to 100°C at a rate of 5°C / min, and then increased to 200°C at a rate of 10°C / min and maintained for 5 min; column flow rate: 1.5 ml / min; inlet temperature: 220°C; detector temperature: 320°C; split ratio: 5:1; tail gas flow rate: 60 ml / min.

[0051] L color analysis of MDI: measured by the L, a, and b values ​​of the CIE colorimetric system well known to those skilled in the art.

[0052] [Example 1]

[0053] Phosgenation reaction: 10t / h MDA and 10t / h chlorobenzene are mixed in advance and reacted with 10t / h phosgene to generate phosgenation reaction liquid. The phosgenation reaction is divided into two steps, cold and hot, in which the cold reaction temperature is 60℃, the pressure is 0.2MPaA, and the reaction time is 2min; the hot reaction temperature is 90℃, the pressure is 0.2MpaA, and the reaction residence time is 0.5h. The phosgene concentration in the generated reaction liquid is 10%.

[0054] Separation by next door: The reaction liquid is sent to the next door tower for separation and purification. A vertically arranged middle partition is set in the middle of the next door tower, which divides the next door tower into the feed pre-separation side and the side line extraction side. The lower part of the partition is the common stripping section, and the upper part of the partition is the common rectification section. The bottom of the middle partition is equipped with a radially adjustable rotary adjustment device, and the top is equipped with a reflux adjustment device. After the reaction liquid is separated and purified by the next door tower, light components such as phosgene, HCl and halogenated hydrocarbons are extracted from the top of the tower, and the gas phase solvent chlorobenzene is extracted from the tower. The concentrated liquid in the tower bottom is sent to the solvent removal unit. The tower tray of the next door tower adopts a solid valve type, the middle partition height is 2m, the lower common stripping section height is 1m, the upper common rectification section height is 1m, the feed pre-separation side and the side line extraction side cross-sectional area ratio is 1:0.5, the pre-separation section reflux flow and feed flow mass ratio is 1:5, the next door tower bottom temperature is 170℃, and the tower pressure is 0.1MPaA.

[0055] Solvent removal: The concentrated liquid in the bottom of the next-door tower is sent to the solvent removal tower for further vacuum distillation to obtain MDI. The temperature of the bottom of the solvent removal tower is 140°C and the pressure is 2KPaA. The gaseous chlorobenzene produced from the side line of the next-door tower is used as the heat source of the reboiler in the bottom of the solvent removal tower. The outlet temperature of the reboiler is controlled at 150°C. The produced chlorobenzene is combined with the condensed chlorobenzene at the top of the solvent removal tower as the circulating solvent after heat exchange.

[0056] [Example 2]

[0057] Phosgenation reaction: 10t / h MDA and 80t / h chlorobenzene are mixed in advance and reacted with 150t / h phosgene to generate phosgenation reaction liquid. The phosgenation reaction is divided into two steps, cold and hot, in which the cold reaction temperature is 120℃, the pressure is 3MPaA, and the reaction time is 4min; the hot reaction temperature is 150℃, the pressure is 0.6MpaA, and the reaction residence time is 3h. The phosgene concentration in the generated reaction liquid is 0.1%.

[0058] Separation by next door: The reaction liquid is sent to the next door tower for separation and purification. A vertically arranged middle partition is set in the middle of the next door tower, which divides the next door tower into the feed pre-separation side and the side line extraction side. The lower part of the partition is the common stripping section, and the upper part of the partition is the common rectification section. The bottom of the middle partition is equipped with a radially adjustable rotary adjustment device, and the top is equipped with a reflux adjustment device. After the reaction liquid is separated and purified by the next door tower, light components such as phosgene, HCl and halogenated hydrocarbons are extracted from the top of the tower, and the gas phase solvent chlorobenzene is extracted from the tower. The concentrated liquid in the bottom of the tower is sent to the solvent removal unit. The tower tray of the next door tower adopts a solid valve type, the height of the middle partition is 12m, the height of the lower common stripping section is 8m, the height of the upper common rectification section is 8m, the cross-sectional area ratio of the feed pre-separation side and the side line extraction side is 1:2, the mass ratio of the reflux flow rate of the pre-separation section to the feed flow rate is 1:30, the temperature of the next door tower is 210℃, and the tower pressure is 0.3MPaA.

[0059] Solvent removal: The concentrated liquid in the bottom of the next-door tower is sent to the solvent removal tower for further vacuum distillation to obtain MDI. The temperature of the solvent removal tower bottom is 180°C and the pressure is 50KPaA. The gaseous chlorobenzene produced from the side line of the next-door tower is used as the heat source of the solvent removal tower bottom reboiler. The reboiler outlet temperature is controlled at 190°C. The produced chlorobenzene is combined with the condensed chlorobenzene at the top of the solvent removal tower as the circulating solvent after heat exchange.

[0060] [Example 3]

[0061] Phosgenation reaction: 10t / h MDA and 20t / h chlorobenzene are mixed in advance and reacted with 10t / h phosgene to generate phosgenation reaction liquid. The phosgenation reaction is divided into two steps, cold and hot, in which the cold reaction temperature is 70℃, the pressure is 0.5MPaA, and the reaction time is 2min; the hot reaction temperature is 100℃, the pressure is 0.3MpaA, and the reaction residence time is 1h. The phosgene concentration in the generated reaction liquid is 8%.

[0062] Separation by next door: The reaction liquid is sent to the next door tower for separation and purification. A vertically arranged middle partition is set in the middle of the next door tower, which divides the next door tower into the feed pre-separation side and the side line extraction side. The lower part of the partition is the common stripping section, and the upper part of the partition is the common rectification section. The bottom of the middle partition is equipped with a radially adjustable rotary adjustment device, and the top is equipped with a reflux adjustment device. After the reaction liquid is separated and purified by the next door tower, light components such as phosgene, HCl and halogenated hydrocarbons are extracted from the top of the tower, and the gas phase solvent chlorobenzene is extracted from the tower. The concentrated liquid in the tower bottom is sent to the solvent removal unit. The tower tray of the next door tower adopts a solid valve type, the middle partition height is 4m, the lower common stripping section height is 2m, the upper common rectification section height is 2m, the feed pre-separation side and the side line extraction side cross-sectional area ratio is 1:0.8, the pre-separation section reflux flow and feed flow mass ratio is 1:8, the next door tower bottom temperature is 180℃, and the tower pressure is 0.15MPaA.

[0063] Solvent removal: The concentrated liquid in the bottom of the next-door tower is sent to the solvent removal tower for further vacuum distillation to obtain MDI. The temperature of the bottom of the solvent removal tower is 150°C and the pressure is 5KPaA. The gaseous chlorobenzene produced from the side line of the next-door tower is used as the heat source of the reboiler in the bottom of the solvent removal tower. The outlet temperature of the reboiler is controlled at 160°C. The produced chlorobenzene is combined with the condensed chlorobenzene at the top of the solvent removal tower as the circulating solvent after heat exchange.

[0064] [Example 4]

[0065] Phosgenation reaction: 10t / h MDA and 50t / h chlorobenzene are mixed in advance and reacted with 50t / h phosgene to generate phosgenation reaction liquid. The phosgenation reaction is divided into two steps, cold and hot, in which the cold reaction temperature is 110℃, the pressure is 2.5MPaA, and the reaction time is 4min; the hot reaction temperature is 140℃, the pressure is 0.5MpaA, and the reaction residence time is 2h. The phosgene concentration in the generated reaction liquid is 1%.

[0066] Separation by bulkhead: The reaction liquid is sent to the bulkhead tower for separation and purification. A vertically arranged middle partition is set in the middle of the bulkhead tower, which divides the bulkhead tower into the feed pre-separation side and the side line extraction side. The lower part of the partition is a common stripping section, and the upper part of the partition is a common rectification section. The bottom of the middle partition is equipped with a radially adjustable rotary adjustment device, and the top is equipped with a reflux adjustment device. After the reaction liquid is separated and purified by the bulkhead tower, light components such as phosgene, HCl and halogenated hydrocarbons are extracted from the top of the tower, and the gas phase solvent chlorobenzene is extracted from the tower. The concentrated liquid in the bottom of the tower is sent to the solvent removal unit. The tower tray of the bulkhead tower adopts a solid valve type, with a middle partition height of 10m, a lower public stripping section height of 7m, an upper public rectification section height of 7m, a feed pre-separation side and a side line production side cross-sectional area ratio of 1:1.8, a pre-separation section reflux flow and a feed flow mass ratio of 1:25, a bulkhead tower kettle temperature of 200°C, a tower pressure of 0.25MPaA, and a chlorobenzene concentration in the kettle concentrate of 40%.

[0067] Solvent removal: The concentrated liquid in the bottom of the next-door tower is sent to the solvent removal tower for further vacuum distillation to obtain MDI. The temperature of the bottom of the solvent removal tower is 170°C and the pressure is 20KPaA. The gaseous chlorobenzene produced from the side line of the next-door tower is used as the heat source of the reboiler in the bottom of the solvent removal tower. The outlet temperature of the reboiler is controlled at 180°C. The produced chlorobenzene is combined with the condensed chlorobenzene at the top of the solvent removal tower as the circulating solvent after heat exchange.

[0068] [Example 5]

[0069] Phosgenation reaction: 10t / h MDA and 35t / h chlorobenzene are mixed in advance and reacted with 35t / h phosgene to generate phosgenation reaction liquid. The phosgenation reaction is divided into two steps: cold and hot reaction, in which the cold reaction temperature is 100℃, the pressure is 1.5MPaA, and the reaction time is 3min; the hot reaction temperature is 120℃, the pressure is 0.4MpaA, and the reaction residence time is 1.5h. The phosgene concentration in the generated reaction liquid is 3%.

[0070] Separation by bulkhead: The reaction liquid is sent to the bulkhead tower for separation and purification. A vertically arranged middle partition is set in the middle of the bulkhead tower, which divides the bulkhead tower into the feed pre-separation side and the side line extraction side. The lower part of the partition is a common stripping section, and the upper part of the partition is a common rectification section. The bottom of the middle partition is equipped with a radially adjustable rotary adjustment device, and the top is equipped with a reflux adjustment device. After the reaction liquid is separated and purified by the bulkhead tower, light components such as phosgene, HCl and halogenated hydrocarbons are extracted from the top of the tower, and the gas phase solvent chlorobenzene is extracted from the tower. The concentrated liquid in the bottom of the tower is sent to the solvent removal unit. The tower tray of the bulkhead tower adopts a solid valve type, with a middle partition height of 8m, a lower public stripping section height of 5m, an upper public rectification section height of 5m, a feed pre-separation side and a side line production side cross-sectional area ratio of 1:1.2, a pre-separation section reflux flow and a feed flow mass ratio of 1:15, a bulkhead tower kettle temperature of 190°C, a tower pressure of 0.2MPaA, and a chlorobenzene concentration in the kettle concentrate of 30%.

[0071] Solvent removal: The concentrated liquid in the bottom of the next-door tower is sent to the solvent removal tower for further vacuum distillation to obtain MDI. The temperature of the bottom of the solvent removal tower is 160°C and the pressure is 12KPaA. The gaseous chlorobenzene produced from the side line of the next-door tower is used as the heat source of the reboiler in the bottom of the solvent removal tower. The outlet temperature of the reboiler is controlled at 170°C. The produced chlorobenzene is combined with the condensed chlorobenzene at the top of the solvent removal tower as the circulating solvent after heat exchange.

[0072] [Comparative Example 1]

[0073] Phosgenation reaction: The phosgenation reaction process is the same as that in Example 3.

[0074] Removal of light components: The reaction liquid is sent to a conventional distillation tower for distillation to remove light components such as phosgene and HCl. The bottom temperature of the tower is 140°C and the pressure is 120KPaA.

[0075] Crude product purification: The concentrated liquid in the distillation tower bottom is sent to the solvent removal tower for further vacuum distillation to obtain MDI. The gas phase components at the top of the tower are condensed and the solvent chlorobenzene is recycled. The temperature of the solvent removal tower bottom is 140℃ and the pressure is 2KPaA.

[0076] [Comparative Example 2]

[0077] Phosgenation reaction: The phosgenation reaction process is the same as that in Example 4.

[0078] Removal of light components: The reaction liquid is sent to a conventional distillation tower for distillation to remove light components such as phosgene and HCl. The bottom temperature of the tower is 140°C and the pressure is 120KPaA.

[0079] Crude product purification: The concentrated liquid in the distillation tower bottom is sent to the solvent removal tower for further vacuum distillation to obtain MDI. The gas phase components at the top of the tower are condensed and the solvent chlorobenzene is recycled. The temperature of the solvent removal tower bottom is 140℃ and the pressure is 2KPaA.

[0080] [Comparative Example 3]

[0081] Phosgenation reaction: The phosgenation reaction process is the same as that in Example 5.

[0082] Removal of light components: The reaction liquid is sent to a conventional distillation tower for distillation to remove light components such as phosgene and HCl. The bottom temperature of the tower is 140°C and the pressure is 120KPaA.

[0083] Crude product purification: The concentrated liquid in the distillation tower bottom is sent to the solvent removal tower for further vacuum distillation to obtain MDI. The gas phase components at the top of the tower are condensed and the solvent chlorobenzene is recycled. The temperature of the solvent removal tower bottom is 140℃ and the pressure is 2KPaA.

[0084] In the preparation processes of the above-mentioned embodiments and comparative examples, the contents of halogenated hydrocarbon substances in the extracted solvent, condensed solvent and circulating solvent were analyzed and tested, and the test results are shown in Table 1.

[0085] Table 1. Performance data of examples and comparative examples

[0086]

[0087] The above is only a preferred embodiment of the present invention. It should be pointed out that ordinary technicians in this field can make several improvements and supplements without departing from the method of the present invention. These improvements and supplements should also be regarded as the scope of protection of the present invention.

Claims

1. A method for purifying a circulating solvent for preparing MDI by phosgenation, the method comprising the following steps: a. Phosgenation reaction: diphenylmethanediamine and phosgene are subjected to phosgenation reaction in an inert solvent to generate a photochemical reaction liquid; b. Separation by next-door separation: The reaction liquid is separated and purified in the next-door tower, the light components containing phosgene and HCl are extracted from the top of the tower, the gas phase solvent is extracted from the tower as the extraction solvent, and the concentrated liquid in the bottom of the tower is sent to the solvent removal unit; c. Solvent removal: The concentrated liquid from the bottom of the tower in step b is further subjected to reduced pressure distillation in a solvent removal tower, and the condensate from the top of the solvent removal tower is used as the condensation solvent; In step a, the inert solvent is recycled, and the mass content of the halogenated hydrocarbon substances in the circulating solvent is 0.1-200 ppm; The circulating solvent includes the produced solvent and the condensed solvent.

2. The purification method according to claim 1, characterized in that The mass content of halogenated hydrocarbons in the circulating solvent is 1-150ppm.

3. The purification method according to claim 2, characterized in that The mass content of halogenated hydrocarbons in the circulating solvent is 10-100 ppm.

4. The purification method according to claim 1, characterized in that The produced solvent in step b is used as a heat source for the reboiler in the solvent removal tower kettle, and after heat exchange, is combined with the condensed solvent at the top of the solvent removal tower as a circulating solvent.

5. The purification method according to claim 1, characterized in that: In the step a, the inert solvent is selected from one or more of chloroaromatic hydrocarbons, dialkyl terephthalate, diethyl phthalate, toluene and xylene.

6. The purification method according to claim 5, characterized in that The inert solvent in step a is one or more of chlorobenzene, dichlorobenzene, toluene and xylene.

7. The purification method according to claim 6, characterized in that In the step a, the inert solvent is chlorobenzene.

8. The purification method according to claim 1, characterized in that: The halogenated hydrocarbon substances refer to methane or ethylene substances substituted by chlorine and / or bromine.

9. The purification method according to claim 8, characterized in that: The halogenated hydrocarbon substance is selected from carbon tetrachloride, trichlorobromomethane, dichlorobromomethane, tetrachloroethylene, dichlorobromoethylene, trichloroethylene, and dibromoethylene.

10. The purification method according to claim 1, characterized in that: In the step a, the mass ratio of diphenylmethanediamine and the inert solvent added is 1:(1-8).

11. The purification method according to claim 10, characterized in that: In the step a, the mass ratio of diphenylmethanediamine and the inert solvent added is 1:(2-5).

12. The purification method according to claim 1, characterized in that: In the step a, the mass ratio of diphenylmethanediamine to phosgene added is 1:(1-15).

13. The purification method according to claim 12, characterized in that: In the step a, the mass ratio of diphenylmethanediamine to phosgene added is 1:(2-5).

14. The purification method according to claim 1, characterized in that: The phosgenation reaction in step a is divided into two steps, cold and hot, wherein the cold reaction temperature is 60-120°C, the pressure is 0.2-3.0MPaA, and the reaction time is within 5min; the hot reaction temperature is 90-150°C, the pressure is 0.2-0.6MpaA, and the reaction residence time is 0.5-3h.

15. The purification method according to claim 1, characterized in that: In step a, the phosgene concentration in the photochemical reaction liquid is 0.1-10%, and the solvent concentration is 30-90%.

16. The purification method according to claim 1, characterized in that: In the step b, a vertical partition is arranged in the middle of the next-door tower, and the tower type is a plate tower, a packed tower or a combination thereof.

17. The purification method according to claim 16, characterized in that: The tower plate is of fixed valve type.

18. The purification method according to claim 16, characterized in that: In the step b, the partition in the middle of the dividing wall tower divides the dividing wall tower into a pre-separation section and a side line extraction section, the lower part of the partition is a common stripping section, and the upper part of the partition is a common rectification section.

19. The purification method according to claim 18, characterized in that: The height of the middle partition is 2-12m; the height of the common distillation section is 1-8m.

20. The purification method according to claim 19, characterized in that: The height of the middle partition is 4-8m; the height of the public distillation section is 2-5m.

21. The purification method according to claim 18, characterized in that: The bottom of the middle partition is provided with a radially adjustable rotary adjustment device to control the cross-sectional area ratio of the pre-separation section and the side line mining section to be 1:(0.5-2); the material flow ratio is 1:(5-30).

22. The purification method according to claim 21, characterized in that The bottom of the middle partition is equipped with a radially adjustable rotary adjustment device to control the cross-sectional area ratio of the pre-separation section and the side line mining section to be 1: (0.8-1.8)。 23. The purification method according to claim 1, characterized in that: After the reaction liquid is separated and purified by the dividing wall tower, most of the halogenated hydrocarbon substances are carried out from the light components of the gas phase at the top of the tower.

24. The purification method according to claim 1, characterized in that: In the step b, the solvent extracted from the tower is extracted in the gas phase, the mass content of halogenated hydrocarbons in the extracted solvent is 0.1-300 ppm, and the mass flow rate of the extracted solvent accounts for 30%-60% of the total solvent flow rate.

25. The purification method according to claim 1, characterized in that: In the step b, the temperature of the reactor of the next-door tower is 170-210° C., and the pressure is 0.1-0.3 MPaA.

26. The purification method according to claim 1, characterized in that: The solvent removal tower in step c is a plate tower, a packed tower or a combination thereof, and the solvent is recovered by condensation at the top of the tower.

27. The purification method according to claim 1, characterized in that In the step c, the temperature of the solvent removal tower kettle is 120-180° C. and the pressure is 2-50 KPaA.

28. The purification method according to claim 27, characterized in that In the step c, the temperature of the solvent removal tower kettle is 130-170° C. and the pressure is 5-20 KPaA.

29. The purification method according to claim 1, characterized in that: In the step c, the condensation temperature at the top of the solvent removal tower is 40-90° C.; the content of halogenated hydrocarbon substances in the condensation solvent at the top of the tower is 0.1-50 ppm.

30. The purification method according to claim 29, characterized in that The condensation temperature at the top of the solvent removal tower in step c is 60-80°C.

31. The purification method according to claim 1, characterized in that The solvent extracted from the tower in step b is used as a heat source for the reboiler of the solvent removal tower in step c, and the outlet temperature of the reboiler is controlled to be 140-180°C.

32. The purification method according to claim 1, characterized in that The produced solvent is combined with the condensed solvent at the top of the solvent removal tower as the circulating solvent after heat exchange, and the circulating solvent is returned to step a for phosgenation reaction.

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