Crude isocyanate separation method and automatic control method thereof

Abstract

The invention relates to a crude isocyanate separation method and an automatic control method thereof, and the separation method comprises the following steps: treating crude isocyanate through a rectifying tower to obtain a polyisocyanate product and an MDI crude product; wherein the rectifying tower comprises a tower kettle, a tower top and a tower body connected with the tower kettle and the tower top; the crude isocyanate is fed into the rectifying tower from the tower kettle and the tower body. According to the separation method of the crude isocyanate in one embodiment, the rectifying tower is fed through the tower kettle and the tower body at the same time, the content of 2, 4-MDI in the polyisocyanate product extracted from the tower kettle can be controlled, and then the activity of the polyisocyanate product is controlled.

Description

Technical Field

[0001] The invention relates to the separation of crude isocyanate, and in particular to a polyisocyanate product capable of being separated to obtain a high-activity product. Background Art

[0002] Polyisocyanate, also known as "polymeric MDI", has the chemical name polyphenyl polymethylene polyisocyanate, which is a mixture of components with the following molecular structure expression:

[0003]

[0004] Polyisocyanate is the main raw material for the production of polyurethane foam plastics, and is mainly used in refrigerators, spraying, pipe insulation, panels, solar energy and other industries. The activity of polyisocyanate is a key factor affecting the viscosity change rate in the production process of coatings, adhesives, polyurethane foams, etc. and the stability of the later products. There are many factors that affect the activity of polyisocyanates, among which the isomer content of 2,4-diphenylmethane diisocyanate (2,4-MDI) mainly affects the maturation rate of downstream applications of polyisocyanates and the stability of product size. When the content of 2,4-MDI isomers in polyisocyanates is high or low, the downstream reaction activity and maturation rate will be high or low, resulting in fluctuations in production process and product quality, especially may cause serious shrinkage of product size. Therefore, the content of 2,4-MDI isomers in polyisocyanate products needs to be strictly controlled.

[0005] At present, the production technology of isocyanate mainly uses aniline and formaldehyde to generate polyamines by condensation reaction under the catalytic action of Lewis acid, and then generates photochemical liquid by cold and hot two-step reaction with polyamine and phosgene in solvent, and then obtains crude isocyanate by removing solvent from the photochemical liquid through a series of post-treatment. Crude isocyanate can obtain polyisocyanate (polymerization MDI) and diphenylmethane diisocyanate (MDI) through rectification separation. However, a large amount of chlorinated substances will be produced in the above process, and some chlorinated substances such as acyl chlorides, adducts of isocyanates and hydrogen chloride are easily decomposed to generate substances such as hydrogen chloride (collectively referred to as "heat-sensitive chlorine" in the text) under high temperature conditions, and these chlorinated substances will eventually be brought into the crude isocyanate.

[0006] After continuous research by the inventor, it was found that the content of 2,4-MDI in crude isocyanate is usually in the range of 3-30wt% under the existing process, which is affected by the process parameters such as the ratio of the upstream condensation reaction. This causes the content of 2,4-MDI in polyisocyanate to fluctuate greatly in the range of 0.5-20wt%, seriously affecting the stability of product quality. The existing technology mainly reduces the fluctuation of the 2,4-MDI content of polyisocyanate as much as possible by adjusting the upstream process parameters and controlling the fluctuation range of the 2,4-MDI content in the crude isocyanate, but this method will seriously affect the flexibility of production. For example, when the demand for high 2,4-MDI content products is high, it is necessary to control the 2,4-MDI content in the crude isocyanate to be high. At the same time, if you want to control the content of polyisocyanate 2,4-MDI in the crude isocyanate separation process, it is bound to cause fluctuations in the content of heat-sensitive chlorine in the crude diphenylmethane diisocyanate (crude MDI), thereby affecting the activity of downstream products. In addition, under the existing process, the free chlorine of the diphenylmethane diisocyanate product usually fluctuates between 0.1 and 10 ppm, and the product activity fluctuates greatly, affecting the downstream production stability and product performance. The existing technology does not mention the existence of the above problems, or provide corresponding solutions. Summary of the invention

[0007] In order to overcome at least one defect of the above-mentioned prior art, in the first aspect, an embodiment of the present invention provides a method for separating crude isocyanate, comprising treating the crude isocyanate through a distillation tower to obtain a polyisocyanate product and a crude MDI product; wherein the distillation tower comprises a tower bottom, a tower top and a tower body connecting the tower bottom and the tower top; the crude isocyanate is fed into the distillation tower from the tower bottom and the tower body.

[0008] In a second aspect, an embodiment of the present invention provides an automatic control method for the above-mentioned crude isocyanate separation method, comprising the following steps:

[0009] (1) Sampling and analyzing the crude MDI product discharged from the top of the distillation tower;

[0010] (2) After sampling is completed, the initial feed ratio A1 of crude isocyanate in the tower body is recorded, and the initial time t1 is recorded;

[0011] (3) obtaining the heat-sensitive chlorine content in the crude MDI product obtained by sampling and analysis, and adjusting the temperature of the reflux material at the top of the tower according to Formula 1 based on the heat-sensitive chlorine content;

[0012] (4) When the difference between the current time t2 and the initial time t1 is greater than the sampling interval, sampling is performed again to repeat the operations of steps (1) to (3).

[0013] A crude isocyanate separation method according to one embodiment of the present invention can control the content of 2,4-MDI in the polyisocyanate product extracted from the bottom of the tower by simultaneously feeding the distillation tower through the bottom of the tower and the tower body, thereby achieving control of the activity of the polyisocyanate product. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The accompanying drawings are only used to illustrate specific embodiments and are not to be considered as limiting the present invention.

[0015] Figure 1 This is a schematic structural diagram of a crude isocyanate separation device according to an embodiment of the present invention;

[0016] Figure 2 A flow chart of an automatic control method for separating crude isocyanate according to an embodiment of the present invention;

[0017] The following are the descriptions of the reference numerals:

[0018] 10. Distillation tower; 20. Reboiler; 30. Condenser; 40. Preheater; 50. Online near-infrared analyzer; 60. MDI crude sampler; 70. First feed pipeline; 71. First flowmeter 72. First regulating valve; 80. Second feed pipeline; 81. Second flowmeter; 82. Second regulating valve; 91. Thermometer; 92. Third flowmeter; 93. Third regulating valve; 100. Logic controller. DETAILED DESCRIPTION

[0019] Typical embodiments that embody the features and advantages of the present invention will be described in detail in the following description. It should be understood that the present invention can have various changes in different embodiments without departing from the scope of the present invention, and the descriptions therein are essentially used as illustrations rather than to limit the present invention.

[0020] One embodiment of the present invention provides a method for separating crude isocyanate, comprising treating the crude isocyanate through a distillation tower to obtain a polyisocyanate product and a crude diphenylmethane diisocyanate (crude MDI); wherein the distillation tower comprises a tower bottom, a tower top and a tower body connecting the tower bottom and the tower top; and the crude isocyanate is fed into the distillation tower from the tower bottom and the tower body.

[0021] The inventors found that as the market demand for products with high 2,4-MDI content changes, the 2,4-MDI content in crude isocyanate can be adjusted by adjusting the upstream process parameters. At this time, since the separation capacity of the distillation tower remains unchanged, the 2,4-MDI content in the produced polyisocyanate product will also fluctuate greatly, affecting its downstream application. The effect of common operations such as adjusting process parameters such as tower bottom temperature and reflux ratio is very limited, and additional operations such as viscosity adjustment may be required later. The inventors further found that in the separation process of crude isocyanate, the activity of the polyisocyanate product can be controlled by feeding the tower bottom and the tower body at the same time.

[0022] In one embodiment, 1-99 wt% of the crude isocyanate is fed from the bottom of the tower, and 1-99 wt% (i.e., the feed ratio of the tower body) of the crude isocyanate is fed from the tower body. Further, 30-70 wt% of the crude isocyanate is fed from the bottom of the tower, and 30-70 wt% of the crude isocyanate is fed from the tower body.

[0023] In one embodiment, the crude isocyanate includes 0-5wt% of 2,2-MDI, 3-30wt% of 2,4-MDI, 30-80wt% of 4,4-MDI, 5-30wt% of polyphenyl polymethylene polyisocyanate containing three benzene rings, 0.5-20wt% of polyphenyl polymethylene polyisocyanate containing four benzene rings, and 5-30wt% of polyphenyl polymethylene polyisocyanate containing more than five benzene rings.

[0024] In one embodiment, the crude isocyanate includes 0-3 wt % of 2,2-MDI, 5-20 wt % of 2,4-MDI, 40-70 wt % of 4,4-MDI, 10-25 wt % of polyphenyl polymethylene polyisocyanate containing three benzene rings, 1.5-15 wt % of polyphenyl polymethylene polyisocyanate containing four benzene rings, and 10-25 wt % of polyphenyl polymethylene polyisocyanate containing more than five benzene rings.

[0025] In one embodiment, the content of 2,2-MDI in the crude isocyanate can be 0.1wt%, 0.4wt%, 0.5wt%, 1wt%, 1.2wt%, 1.5wt%, 2wt%, 2.5wt%, 4wt%, 4.5wt%; the content of 2,4-MDI in the crude isocyanate can be 5.9wt%, 6wt%, 8wt%, 9wt%, 10wt%, 12wt%, 15wt%, 18wt%, 22wt%, 23wt%, 25wt%; the content of 4,4-MDI in the crude isocyanate can be 33.5wt%, 34wt%, 35wt%, 36wt%, 45wt%, 50wt%, 55wt%, 60wt%, 65wt%, 66wt%; polyphenyl polymethylene polyisocyanate containing three benzene rings is A mixture of multiple isomers, whose content in crude isocyanate can be 7wt%, 8wt%, 12wt%, 14wt%, 15wt%, 18wt%, 20wt%, 24wt%, 25wt%; polyphenyl polymethylene polyisocyanate containing four benzene rings is a mixture of multiple isomers, and its content in crude isocyanate can be 2wt%, 4.5wt%, 4.7wt%, 5wt%, 6wt%, 7wt%, 8wt%, 10wt%, 12wt%, 18wt%, 19wt%; polyphenyl polymethylene polyisocyanate containing more than five benzene rings is a mixture of multiple isomers, and its content in crude isocyanate can be 6wt%, 7wt%, 12wt%, 15wt%, 16wt%, 18wt%, 20wt%, 22wt%, 23wt%.

[0026] In one embodiment, the crude isocyanate further includes residual solvent (eg, chlorobenzene) and chlorinated MDI.

[0027] In one embodiment, the crude isocyanate may contain 1000 to 10000 ppm of heat-sensitive chlorine; further, the crude isocyanate may contain 2000 to 5000 ppm of heat-sensitive chlorine; wherein the heat-sensitive chlorine is a chlorine-containing substance (such as an adduct of acyl chloride, isocyanate and hydrogen chloride, etc.) that is easily decomposed to generate hydrogen chloride under high temperature conditions, and the content of the heat-sensitive chlorine is obtained by the following formula:

[0028] Thermosensitive chlorine content = total chlorine content - free chlorine content - chlorine content of solvent - chlorine content of chlorinated MDI

[0029] The free chlorine is chloride ions, which are generated by the reaction of HCl and / or acyl chloride dissolved in organic matter with water. The solvent may be chlorobenzene.

[0030] Reference Figure 1As shown, an embodiment of the present invention provides a crude isocyanate separation device, including a distillation tower 10, a reboiler 20, a condenser 30, and a preheater 40; wherein the distillation tower 10 includes a tower bottom, a tower top, and a tower body connecting the tower bottom and the tower top, and the reboiler 20 is connected to the tower bottom of the distillation tower 10; the condenser 30 is connected to the tower top of the distillation tower 10, and is used to condense the gas discharged from the tower top; the preheater 40 is arranged between the condenser 30 and the tower top, and is used to heat the condensed liquid and reflux it to the distillation tower 10.

[0031] In one embodiment, a reboiler 20 connected to the bottom of the tower is provided outside the distillation tower 10, and part of the crude isocyanate is fed from the bottom of the tower to the distillation tower 10 through the reboiler 20. In other words, part of the crude isocyanate (e.g., 1 to 99 wt%) is fed to the distillation tower 10 through the reboiler inlet located at the bottom of the tower.

[0032] In one embodiment, the distillation tower 10 has a theoretical plate number of 1 to 30, further 5 to 20, for example 6, 7, 8, 10, 11, 12, 13, 15, 18, 19, 22, 23, 24, 25.

[0033] In one embodiment, 1 to 8 layers of packing are provided in the distillation tower 10, further 1 to 5 layers of packing, further 2 to 5 layers of packing, for example 2 layers of packing.

[0034] In one embodiment, the crude isocyanate fed from the tower body enters the distillation tower 10 through a packing layer (or referred to as “packing layer feed”).

[0035] In one embodiment, a liquid distributor is provided on one or more layers of packing, and further, a liquid distributor is provided above each layer of packing, and the crude isocyanate (e.g., 1 to 99 wt%) fed from the tower body is fed into the distillation tower 10 through one or more liquid distributors.

[0036] In one embodiment, the liquid distributor is located directly above each layer of packing and is in direct contact with the packing.

[0037] In one embodiment, crude isocyanate is fed to the distillation column 10 via a reboiler 20 and one or more liquid distributors.

[0038] In one embodiment, two or three layers of packing are provided in the distillation tower 10, and the crude isocyanate is fed into the distillation tower 10 through the reboiler 20 (or the reboiler inlet) and the liquid distributor above the second layer of packing.

[0039] In one embodiment, a first feed pipeline 70 and a second feed pipeline 80 are provided outside the distillation tower 10, the first feed pipeline 70 is connected to the reboiler 20, and the second feed pipeline 80 is connected to the tower body, and can further be connected to the liquid distributor of the packing layer. A first flow control meter 71 and a first regulating valve 72 are provided on the first feed pipeline 70, and a second flow control meter 81 and a second regulating valve 82 are provided on the second feed pipeline 80. Further, the first regulating valve 72 and the second regulating valve 82 are both flow regulating valves.

[0040] In one embodiment, the number of theoretical plates between the feed position of the tower body and the bottom of the tower can be 5 to 20, for example 6, 7, 8, 9, 10, 11, 12, 15, 16, 17, 18, 19.

[0041] In one embodiment, the number of theoretical plates between the feed position of the tower body (or the position of the liquid distributor for feeding) and the bottom of the tower is less than 15, and the mass content of 2,4-MDI is less than 15% based on the total mass of the crude isocyanate, then 10-60wt% (e.g., 20wt%, 25wt%, 30wt%, 35wt%, 40wt%, 45wt%, 50wt%, 55wt%) of the crude isocyanate is fed through the liquid distributor of the packing layer, and the remaining (40-90wt%, e.g., 45wt%, 50wt%, 55wt%, 60wt%, 65wt%, 70wt%, 75wt%, 80wt%, 85wt%) of the crude isocyanate is fed through the reboiler.

[0042] In one embodiment, the number of theoretical plates between the feed position of the tower body (or the position of the liquid distributor for feeding) and the bottom of the tower is less than 15, and based on the total mass of the crude isocyanate, the mass content of 2,4-MDI is above 15%, then 70-99wt% (e.g., 75wt%, 80wt%, 85wt%, 90wt%, 95wt%) of the crude isocyanate is fed through the liquid distributor of the packing layer, and the remaining (1-30wt%, e.g., 2wt%, 5wt%, 8wt%, 10wt%, 15wt%, 20wt%, 25wt%) of the crude isocyanate is fed through the reboiler.

[0043] In one embodiment, the number of theoretical plates between the feed position of the tower body (or the liquid distributor for feeding) and the tower bottom is 15 or more, and based on the total mass of the crude isocyanate, the mass content of 2,4-MDI is less than 15%, then 1-30wt% (e.g., 2wt%, 5wt%, 8wt%, 10wt%, 15wt%, 20wt%, 25wt%) of the crude isocyanate is fed through the liquid distributor of the packing layer, and the remaining (70-99wt%, e.g., 75wt%, 80wt%, 85wt%, 90wt%, 95wt%) of the crude isocyanate is fed through the reboiler 20.

[0044] In one embodiment, the number of theoretical plates between the feed position of the tower body (or the liquid distributor for feeding) and the tower bottom is 15 or more, and based on the total mass of the crude isocyanate, the mass content of 2,4-MDI is above 15%, then 40-90wt% (e.g., 45wt%, 50wt%, 55wt%, 60wt%, 65wt%, 70wt%, 75wt%, 80wt%, 85wt%) of the crude isocyanate is fed through the liquid distributor of the packing layer, and the remaining (10-60wt%, e.g., 15wt%, 20wt%, 25wt%, 30wt%, 35wt%, 40wt%, 45wt%, 50wt%, 55wt%) of the crude isocyanate is fed through the reboiler.

[0045] In one embodiment, the feed temperature of crude isocyanate is 150-270°C, and can further be 170-230°C, for example, 180°C, 190°C, 200°C, 210°C, 220°C, 250°C, 253°C, 255°C.

[0046] In one embodiment, in the separation process of the distillation tower, the bottom temperature is 170-270°C, and can further be 190-230°C, for example, 180°C, 195°C, 200°C, 205°C, 210°C, 215°C, 220°C, 225°C, 260°C, 264°C, 265°C.

[0047] In one embodiment, in the separation process of the distillation tower, the absolute pressure at the top of the tower is controlled at 0.1-2.5 kPa, and can further be 0.4-0.8 kPa, for example, 0.5 kPa, 0.6 kPa, 0.7 kPa, 1.0 kPa, 1.5 kPa, 2.0 kPa.

[0048] In one embodiment, in the separation process of the distillation tower, the temperature of the reflux material at the top of the tower can be 40-200°C, further 70-180°C, for example 80°C, 100°C, 120°C, 140°C, 150°C, 160°C, 170°C.

[0049] In one embodiment, after the gas discharged from the top of the tower enters the condenser 30, most of the gas is condensed into liquid, and a small amount of non-condensable gas is sent to the vacuum unit system after being discharged from the condenser 30; wherein, part of the condensate is refluxed into the distillation tower 10, and the remaining condensate is discharged through the discharge pipeline of the MDI crude product.

[0050] In one embodiment, a thermometer 91, a third flowmeter 92 and a third regulating valve 93 are provided outside the preheater 40, and the third regulating valve 93 can be a flow regulating valve; wherein, the third flowmeter 92 and the third regulating valve 93 are used to monitor and control the amount of heating medium (heat transfer oil or high-temperature material) entering the preheater 40, and the coordinated use of the thermometer 91, the third flowmeter 92 and the third regulating valve 93 can realize the control of the temperature of the reflux material at the top of the tower.

[0051] In one embodiment, in the separation process of the distillation tower 10, the flow rate of the reflux material at the top of the tower is maintained constant at 4 to 20 t / h, further at 10 to 15 t / h, for example, 5 t / h, 11 t / h, 12 t / h, 13 t / h, 14 t / h, and 18 t / h.

[0052] In one embodiment, the withdrawal mass of the bottom liquid can be 40-95% of the mass of the crude isocyanate feed (i.e., the withdrawal ratio of the bottom liquid), and further 60-80%, for example, 47%, 50%, 65%, 70%, 75%, 90%; the temperature of the top withdrawal condenser can be 40-100°C, and further 60-80°C, for example, 50°C, 65°C, 70°C, 75°C, 85°C, 90°C.

[0053] In one embodiment, the crude MDI contains heat-sensitive chlorine, and the content of heat-sensitive chlorine = total chlorine content - free chlorine content - chlorine content of the solvent - chlorine content of chlorinated MDI, and the solvent may be chlorobenzene, for example.

[0054] The inventors unexpectedly found that, while maintaining a constant reflux rate at the top of the distillation tower, by adjusting the temperature of the top reflux into the tower, a better adjustment effect can be achieved for the heat-sensitive chlorine content in the crude MDI product produced from the top of the tower; and since the top reflux rate is constant, the distribution effect of the distributor can always be maintained at an optimal level, which can well solve the problem of packing coking caused by the adjustment of process parameters such as the reflux ratio.

[0055] In one embodiment, crude MDI is obtained by collecting the material discharged from the top of the tower, and the material discharged from the bottom of the tower is collected to obtain the polyisocyanate product. Further, outside the bottom of the tower, an online infrared analyzer 50 is provided on the discharge pipeline of the polyisocyanate product for online real-time analysis of the content of 2,4-MDI isomers in the polyisocyanate product discharged from the bottom of the tower. Outside the condenser 30, an MDI crude product sampler 60 is provided on the discharge pipeline of the crude MDI product to perform offline timed sampling of the crude MDI product discharged from the top of the tower and measure the content of heat-sensitive chlorine in the sample.

[0056] In one embodiment, the time interval for sampling the crude MDI product is 4 to 168 hours, and can further be 24 to 48 hours, for example, 25 hours, 28 hours, 30 hours, 32 hours, 35 hours, 38 hours, 40 hours, 42 hours, 45 hours, 50 hours, 60 hours, 80 hours, 90 hours, 100 hours, 120 hours, and 150 hours.

[0057] In one embodiment, the crude MDI product taken from the top of the tower is sampled at regular intervals and its heat-sensitive chlorine content m1 is measured, and the temperature of the reflux material at the top of the tower is adjusted according to the heat-sensitive chlorine content. The temperature adjustment value ΔT1 (in ° C) is:

[0058] ΔT1=80-10.716*(m1*10 6 ) 0.3653 (Formula 1)

[0059] Among them, ΔT1=T1-T0, T0 is the temperature of the reflux material at the top of the tower before adjustment, T1 is the temperature of the reflux material at the top of the tower after adjustment. If ΔT1 is a positive value, it means that the temperature of the reflux material is to be increased; conversely, if ΔT1 is a negative value, it means that the temperature of the reflux material is to be reduced.

[0060] In one embodiment, the heat-sensitive chlorine content of the crude MDI product may be 180 to 330 ppm, i.e., m1*10 6 The value range can be 180 to 330.

[0061] In one embodiment, m1*10 6 The value range of can be 180-330, and further can be 200-290, for example, 210, 220, 230, 240, 250, 260, 270, 280, 300.

[0062] In one embodiment, before and after the crude MDI product is sampled, when the feed ratio change of the tower body (i.e., the feed ratio change of the packing layer or the feed ratio change of the liquid distributor) exceeds Δm%, the temperature of the reflux material at the top of the tower is adjusted according to the following formula, and the temperature adjustment value ΔT2 (in ° C) is:

[0063] ΔT2 = -2* change in feed ratio of the tower body (Formula 2)

[0064] Among them, ΔT2=T2-T0, T0 is the temperature of the reflux material at the top of the tower before adjustment, T2 is the temperature of the reflux material at the top of the tower after adjustment, if ΔT2 is a positive value, it means that the temperature of the reflux material is to be increased, on the contrary, if ΔT2 is a negative value, it means that the temperature of the reflux material is to be reduced; the feed ratio change of the tower body is the percentage change of the crude isocyanate fed from the tower body to the total mass of the crude isocyanate entering the distillation tower, that is, the difference between the real-time feed ratio A2 and the initial feed ratio A1. Among them, the tower body feed amount and the tower bottom feed amount are obtained respectively by the flow meter, and then the real-time tower body feed ratio A2 is calculated according to the formula "tower body feed amount / (tower bottom feed amount+tower body feed amount)".

[0065] In one embodiment, the feed proportion of the tower body can be 1-99wt%, and can further be 30-70wt%, for example, 10wt%, 11wt%, 12wt%, 15wt%, 16wt%, 17wt%, 20wt%, 40wt%, 43wt%, 45wt%, 50wt%, 55wt%, 60wt%, 65wt%, 68wt%, 69wt%, 80wt%, 85wt%, 88wt%, 90wt%, 91wt%, 92wt%, 95wt%.

[0066] In one implementation, the above formulas 1 and 2 only indicate that the numerical values ​​of the parameters on both sides of the equal sign are equal, and do not reflect the relationship between the units of the parameters on both sides of the equal sign.

[0067] In one implementation, the adjustment of ΔT1 and ΔT2 does not affect each other, and they can be adjusted independently or simultaneously.

[0068] In one embodiment, a logic controller 100 is provided in the separation device of crude isocyanate, which is used to realize the control of the content of 2,4-MDI isomers in the polyisocyanate product and the content of heat-sensitive chlorine in the crude MDI product through the above two formulas. Specifically, the content of 2,4-MDI isomers in the polyisocyanate product extracted from the bottom of the tower is first set, and then the logic controller will automatically adjust the feed ratio of the tower body (i.e., the feed ratio of the packing layer) until the requirements are met. At the same time, the analysis results of the heat-sensitive chlorine of the crude MDI product and the change in the feed ratio of the tower body will trigger the logic controller to adjust the temperature of the top reflux material to ensure the stability of the heat-sensitive chlorine content in the crude MDI product.

[0069] In one embodiment, after completing the offline sampling and analysis of the thermosensitive chlorine content in the crude MDI product and inputting the analysis results into the system, the logic controller adjusts the temperature of the reflux material at the top of the tower according to Formula 1, and waits for the next offline sampling result input after the adjustment; after each offline sampling, the alarm of the feed ratio of the tower body is started, and when the cumulative change of the feed ratio of the tower body exceeds the cumulative alarm value Δm%, the alarm is triggered. At this time, the temperature of the reflux material at the top of the tower is adjusted according to Formula 2, and the alarm program of the feed ratio is restarted after the adjustment is completed until the next sampling starts.

[0070] In one embodiment, the preparation process of crude isocyanate comprises the following steps:

[0071] Aniline and formaldehyde are condensed under the catalysis of Lewis acid to form polyamines;

[0072] The polyamine and phosgene are subjected to cold or hot reaction in a solvent to obtain a photochemical liquid; and

[0073] The photochemical liquid is post-treated to remove residual phosgene and solvent to obtain crude isocyanate.

[0074] In one embodiment, the raw materials, process parameters, etc. involved in the preparation process of crude isocyanate may be the raw materials and process parameters disclosed in the prior art.

[0075] In one embodiment, in the preparation process of crude isocyanate, the Lewis acid can be hydrochloric acid, the mass ratio of formaldehyde to aniline can be 0.3-0.8, the mass ratio of hydrochloric acid to aniline can be 0.1-0.5, and the temperature of the condensation reaction can be 40-100°C; the mass ratio of phosgene to polyamine reaction can be 1-5, the temperature of the cold reaction can be 60-120°C, and the temperature of the hot reaction can be 100-150°C; the post-treatment process includes dephosgene and desolventization treatment, the temperature of the dephosgene treatment can be 110-170°C, and the pressure can be 10-50kPaG; the temperature of the desolventization treatment can be 130-220°C, and the pressure can be 10-60kPaA.

[0076] In one embodiment, the content of 2,4-MDI in the separated polyisocyanate product is 2 to 5 wt %, further 3 to 4 wt %.

[0077] In one embodiment, the separated polyisocyanate product includes 2 to 5 wt% of 2,4-MDI, 0 to 5 wt% of 2,2-MDI, 5 to 75 wt% of 4,4-MDI, 10 to 50 wt% of polyphenyl polymethylene polyisocyanate containing three benzene rings, 5 to 30 wt% of polyphenyl polymethylene polyisocyanate containing four benzene rings, and 5 to 50 wt% of polyphenyl polymethylene polyisocyanate containing more than five benzene rings.

[0078] In one embodiment, the separated polyisocyanate product includes 2 to 5 wt% of 2,4-MDI, 0 to 1 wt% of 2,2-MDI, 10 to 50 wt% of 4,4-MDI, 15 to 40 wt% of polyphenyl polymethylene polyisocyanate containing three benzene rings, 5 to 20 wt% of polyphenyl polymethylene polyisocyanate containing four benzene rings, and 10 to 40 wt% of polyphenyl polymethylene polyisocyanate containing more than five benzene rings.

[0079] In one embodiment, the separated MDI crude product is treated by distillation or crystallization to obtain a diphenylmethane diisocyanate product (MDI product).

[0080] In one embodiment, the content of heat-sensitive chlorine in the separated crude MDI product is 180 to 330 ppm, and further can be 200 to 290 ppm. The MDI product can be regulated by controlling the content of heat-sensitive chlorine in the crude MDI product taken from the top of the tower.

[0081] In one embodiment, the separated crude MDI includes 0.1-5 wt % of 2,2-MDI, 4-15 wt % of 2,4-MDI, 80-95 wt % of 4,4-MDI and 180-330 ppm of heat-sensitive chlorine.

[0082] In one embodiment, the separated crude MDI includes 0.5-3 wt % of 2,2-MDI, 8-12 wt % of 2,4-MDI, 85-90 wt % of 4,4-MDI and 180-330 ppm of heat-sensitive chlorine.

[0083] In one embodiment, the free chlorine content of the MDI product is 0.8 to 1.8 ppm, further 1.0 to 1.5 ppm.

[0084] In one embodiment, the MDI product includes 0.5-2 wt% 2,4-MDI, 98-99.5 wt% 4,4-MDI, 0-500 ppm 2,2-MDI, and 0.8-1.8 ppm free chlorine.

[0085] In one embodiment, the MDI product includes 0.8-1.5 wt% 2,4-MDI, 98.5-99.2 wt% 4,4-MDI, 0-50 ppm 2,2-MDI, and 0.8-1.8 ppm free chlorine.

[0086] One embodiment of the present invention provides an automatic control method for separating the crude isocyanate, wherein the crude isocyanate is separated by a distillation tower. The control method comprises the following steps:

[0087] (1) Sampling and analyzing the crude MDI product discharged from the top of the distillation tower;

[0088] (2) After sampling is completed, record the initial feed ratio A1 of the tower body and the initial time t1;

[0089] (3) Obtain the content of heat-sensitive chlorine in the crude MDI product obtained by sampling and analysis (m1*10 6 ppm), and according to the content, the temperature of the reflux material at the top of the tower is adjusted according to the aforementioned formula 1;

[0090] (4) When the difference between the current time t2 and the initial time t1 is greater than the sampling interval, sampling is performed again to repeat the above steps (1) to (3).

[0091] In one embodiment, after the above step (2) is completed, the control program of the feed ratio of the liquid distributor is started, that is, when the difference between the current feed ratio of the tower body (or the real-time feed ratio of the tower body) A2 and the initial feed ratio A1 is greater than the set cumulative alarm value Δm%, the temperature of the reflux material at the top of the tower is adjusted according to the above formula 2, and the current time is recorded as the initial time t1, and the value of the current feed ratio A2 of the tower body is used as the initial feed ratio A1 of the next regulation to recalculate the difference, and when the difference between the subsequent current time t2 and the initial time t1 is greater than the sampling interval time, sampling is performed again to repeat the above monitoring operation;

[0092] On the contrary, if the difference between the current feed proportion A2 of the tower body and the initial feed proportion A1 does not exceed the cumulative alarm value, there is no need to adjust the temperature of the reflux material at the top of the tower according to Formula 2.

[0093] In one embodiment, the cumulative alarm value Δm% is 1-50%, and further, Δm% can be 2%, 3%, 4%, 5%, 6%, 8%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, or 45%.

[0094] In one embodiment, the 2,4-MDI content in the polyisocyanate product is controlled by a feed ratio program of a separate column.

[0095] In one embodiment, the above-mentioned automatic control method can be implemented by inputting corresponding instructions into an existing application program.

[0096] In one embodiment, the heat-sensitive chlorine content in the crude MDI product is detected offline. After the test result is obtained, the result can be input into the program, and then the temperature of the reflux material at the top of the tower is automatically adjusted according to the result through Formula 1.

[0097] An automatic control method for separating crude isocyanates according to an embodiment of the present invention includes a feed ratio program for a tower body, and a sequential control program for controlling the content of heat-sensitive chlorine in the crude MDI product extracted from the top of the tower; wherein the feed ratio program for the tower body is a single loop for controlling the content of 2,4-MDI in the polyisocyanate product extracted from the bottom of the tower, that is, after setting the content of 2,4-MDI in the polyisocyanate product, the system will adjust the feed ratio of the tower body, and when the content of 2,4-MDI in the polyisocyanate product is higher than the set value (e.g., 2 to 5 wt%), the feed ratio of the tower body is increased, and vice versa, the feed ratio of the tower body is reduced until the content of 2,4-MDI in the polyisocyanate product reaches the set value, and this control logic is always carried out in a single loop cycle and is not affected by other control parameters. Further, the above-mentioned control of the feed ratio of the tower body can be achieved through a PID control loop.

[0098] The method for separating crude isocyanate according to one embodiment of the present invention can control the content of 2,4-MDI in the polyisocyanate product extracted from the bottom of the tower by adjusting the feed position and feed ratio of the distillation tower accordingly; in addition, by adjusting the temperature of the reflux material at the top of the tower, the heat-sensitive chlorine content of the crude MDI product extracted from the top of the tower can be controlled, so that a free chlorine-stabilized MDI product can be obtained downstream. Therefore, by controlling the feed position, feed ratio and temperature of the reflux material at the top of the distillation tower, the stable control of the activity of the polyisocyanate product and the MDI product can be achieved simultaneously.

[0099] A method for separating crude isocyanate according to one embodiment of the present invention can simultaneously achieve precise control of the activities of a polyisocyanate product and a diphenylmethane diisocyanate product, and is convenient to operate and quick to respond. In this method, the reflux flow rate at the top of the distillation tower is maintained constant, which can effectively avoid coking of the filler and extend the operating cycle of the device.

[0100] A method for separating crude isocyanate according to one embodiment of the present invention can realize automatic control of the activity of polyisocyanate products and MDI products through a feed ratio program of a tower body and a sequential control program for controlling the content of heat-sensitive chlorine in a crude MDI product taken out from a tower top, and has the advantages of convenient operation and rapid response.

[0101] The crude isocyanate separation method of one embodiment of the present invention has a simple process, small workload for technical transformation of existing devices, and small investment.

[0102] Hereinafter, the separation method of crude isocyanate according to one embodiment of the present invention is further described in conjunction with the accompanying drawings and specific examples. Herein, the calculation formula of the heat-sensitive chlorine content is: heat-sensitive chlorine content = total chlorine content - free chlorine content - chlorine content in chlorobenzene - chlorine content in chloro-MDI. Among them, the test methods for the total chlorine content, free chlorine content, chlorine content in chlorobenzene, chlorine content in chloro-MDI and the content of each component of isocyanate are as follows:

[0103] 1. Total chlorine content test

[0104] Use XRF instrument to analyze total chlorine, take 5g sample, test with XRF for 30s, and read according to the marked line.

[0105] 2. Free chlorine test

[0106] Use ion chromatography to analyze free chlorine. Take 5g of isocyanate sample, 5g of dichloromethane, and 5g of water, mix them evenly, and take the supernatant for ion chromatography analysis. The mobile phase of the ion chromatography is 5mmol / L Na2CO3 solution, the chromatographic column is A7 anion chromatography column, the column temperature is 40℃, the flow rate is 1mL / min, and the peak position at 6.7min in the chromatogram is Cl - , just quantify according to the marking line reading.

[0107] 3. Chlorine content test of chlorobenzene and chlorinated MDI

[0108] Take the isocyanate sample and directly perform gas chromatography analysis with an injection volume of 1μl. The chromatographic column type is a weak polar chromatographic column, the initial temperature of the column oven is 150℃, maintained for 0.5min; the temperature is programmed to 250℃ at 5℃ / min and maintained for 7min; the temperature is programmed to 280℃ at 10℃ / min and maintained for 2min; the detector type is an ECD detector. The characteristic peaks at 1.9min and 18.2 / 19.2 / 21.1min in the chromatogram represent chlorobenzene and chloro-MDI, respectively. The mass percentage of chlorobenzene and chloro-MDI can be obtained by the internal standard method, and the content of chlorine in it can be converted according to the molecular weight of the substance.

[0109] 4. Test of component content in isocyanate

[0110] The content of 2,2-MDI, 2,4-MDI, 4,4-MDI and three-ring, four-ring, five-ring and above substances in isocyanate samples is determined by combining the analysis results of GPC and gas chromatography. First, the content of two-ring, three-ring, four-ring, five-ring and above substances in isocyanate samples is analyzed by GPC, where the two-ring content is the sum of the contents of 2,2-MDI, 2,4-MDI and 4,4-MDI. Then, according to the ratio of 2,2-MDI, 2,4-MDI and 4,4-MDI obtained by gas chromatography, their actual content is calculated. The specific test methods of GPC and gas chromatography are as follows:

[0111] GPC analysis: Take isocyanate samples for gel permeation chromatography analysis, with an injection volume of 20μl. The chromatographic column type is Styragel HR chromatographic column, with multiple columns connected in series, the column oven temperature is 40°C, the analysis time is 40min, and the detector type is a differential refractive index detector. The characteristic peaks at 20.7min, 21.4min, 22.2min, and 23.9min in the chromatogram represent five rings and above, four rings, three rings, and two rings, respectively. The mass percentage of each ring can be obtained by the external standard method.

[0112] Gas chromatography analysis: Take the isocyanate sample for gas chromatography injection analysis, and the injection volume is 1μl. The chromatographic column type is a weak polar chromatographic column, the initial temperature of the column oven is 150℃, and it is maintained for 0.5min; the temperature is raised to 250℃ at 5℃ / min and maintained for 7min; the temperature is raised to 280℃ at 10℃ / min and maintained for 2min; the detector type is an ECD detector. The characteristic peaks at 12.1min, 13.5min, and 14.7min in the chromatogram represent 2,2-MDI, 2,4-MDI, and 4,4-MDI, respectively. The mass percentages of 2,2-MDI, 2,4-MDI, and 4,4-MDI can be obtained by the internal standard method.

[0113] Preparation Example 1

[0114] (1) Aniline, formaldehyde and hydrochloric acid were mixed in a mass ratio of 1:0.33:0.40 for reaction at a reaction temperature of 80°C.

[0115] (2) removing aniline and water from the mixture obtained by the reaction in step (1) under vacuum to obtain a diphenylmethanediamine / polyamine mixture; then, mixing the solvent chlorobenzene with the diphenylmethanediamine / polyamine mixture in a mass ratio of 4:1 in a static mixer to form a mixed solution.

[0116] (3) Phosgene and the mixed solution of step (2) are mixed in a dynamic mixer at a mass ratio of phosgene: diphenylmethanediamine / polyamine = 4:1, and the mixed materials are subjected to cold and hot phosgenation reactions, wherein the cold reaction temperature is 65° C. and the pressure is 270 kPaG, and the hot reactor temperature is 110° C. and the pressure is 270 kPaG.

[0117] (4) The hot reaction liquid obtained in step (3) is passed into a phosgene removal tower at a temperature of 120° C. and a pressure of 45 kPaG to remove phosgene and hydrogen chloride, and then passed through a desolventizing tower at a temperature of 140° C. and a pressure of 50 kPaA to remove chlorobenzene to obtain crude isocyanate A.

[0118] Analysis shows that crude isocyanate A includes: 0.4wt% of 2,2-MDI, 5.9wt% of 2,4-MDI, 65.5wt% of 4,4-MDI, 7.4wt% of polyphenylpolymethylene polyisocyanate containing three benzene rings, 4.7wt% of polyphenylpolymethylene polyisocyanate containing four benzene rings, 16wt% of polyphenylpolymethylene polyisocyanate containing more than five benzene rings and 6486ppm of thermal-sensitive chlorine.

[0119] Preparation Example 2

[0120] (1) Aniline, formaldehyde and hydrochloric acid were mixed in a mass ratio of 1:0.68:0.22 for reaction at a reaction temperature of 70°C.

[0121] (2) removing aniline and water from the mixture obtained by the reaction in step (1) under vacuum to obtain a diphenylmethanediamine / polyamine mixture; then, mixing the solvent chlorobenzene with the diphenylmethanediamine / polyamine mixture in a mass ratio of 4:1 in a static mixer to form a mixed solution.

[0122] (3) Phosgene and the mixed solution of step (2) are mixed in a dynamic mixer at a mass ratio of phosgene: diphenylmethanediamine / polyamine = 3:1, and the mixed materials are subjected to cold and hot phosgenation reactions, wherein the cold reaction temperature is 85° C. and the pressure is 270 kPaG, and the hot reactor temperature is 120° C. and the pressure is 270 kPaG.

[0123] (4) The hot reaction liquid obtained in step (3) is passed into a phosgene removal tower at a temperature of 150° C. and a pressure of 35 kPaG to remove phosgene and hydrogen chloride, and then passed through a desolventizing tower at a temperature of 180° C. and a pressure of 30 kPaA to remove chlorobenzene to obtain crude isocyanate B.

[0124] The crude isocyanate B was measured to include: 1.1 wt% of 2,2-MDI, 9.2 wt% of 2,4-MDI, 33.9 wt% of 4,4-MDI, 14.6 wt% of polyphenyl polymethylene polyisocyanate containing three benzene rings, 18.3 wt% of polyphenyl polymethylene polyisocyanate containing four benzene rings, 22.8 wt% of polyphenyl polymethylene polyisocyanate containing more than five benzene rings, and 8870 ppm of thermally sensitive chlorine.

[0125] Preparation Example 3

[0126] (1) Aniline, formaldehyde and hydrochloric acid were mixed in a mass ratio of 1:0.45:0.12 for reaction at a reaction temperature of 50°C.

[0127] (2) removing aniline and water from the mixture obtained by the reaction in step (1) under vacuum to obtain a diphenylmethanediamine / polyamine mixture; then, mixing the solvent chlorobenzene with the diphenylmethanediamine / polyamine mixture in a mass ratio of 4:1 in a static mixer to form a mixed solution.

[0128] (3) Phosgene and the mixed solution of step (2) are mixed in a dynamic mixer at a mass ratio of phosgene: diphenylmethane diamine / polyamine = 2:1, and the mixed materials are subjected to cold and hot phosgenation reactions, wherein the cold reaction temperature is 115° C. and the pressure is 270 kPaG, and the hot reactor temperature is 140° C. and the pressure is 270 kPaG.

[0129] (4) The hot reaction liquid obtained in step (3) is passed into a phosgene removal tower at a temperature of 165° C. and a pressure of 15 kPaG to remove phosgene and hydrogen chloride, and then passed through a desolventizing tower at a temperature of 215° C. and a pressure of 20 kPaA to remove chlorobenzene to obtain crude isocyanate C.

[0130] The crude isocyanate C was measured to include: 4.1 wt% of 2,2-MDI, 22.3 wt% of 2,4-MDI, 35.5 wt% of 4,4-MDI, 24.4 wt% of polyphenyl polymethylene polyisocyanate containing three benzene rings, 6.8 wt% of polyphenyl polymethylene polyisocyanate containing four benzene rings, 6.8 wt% of polyphenyl polymethylene polyisocyanate containing more than five benzene rings, and 2909 ppm of thermally sensitive chlorine.

[0131] Example 1

[0132] S1: Feed part of the crude isocyanate A prepared in Preparation Example 1 into a distillation tower A. The theoretical number of plates of distillation tower A is 7, and the filler is 2 large layers. A liquid distributor is arranged above each large layer of filler. The feed position of the tower body is the liquid distributor above the filler of the second large layer (counted from top to bottom). The theoretical number of plates between the feed position and the bottom of the tower is 7. The feed proportion program of the tower body is started to control the content of 2,4-MDI in the polyisocyanate product extracted from the bottom of the tower. At the same time, the program for controlling the content of heat-sensitive chlorine in the crude MDI product is started. In the program, the target value of the 2,4-MDI content in the polyisocyanate product is set to 3.0%, the sampling interval of the crude MDI product is 24h, and the cumulative alarm value Δm% of the feed proportion of the tower body is 5%.

[0133] S2: Set the liquid distributor and the bottom of the distillation tower A to feed at the same time, set the feed temperature of crude isocyanate to 251°C, the bottom liquid extraction ratio to 47%, the top reflux flow rate to 15t / h, and the top condenser temperature to 40°C; Finally, the distillation tower remains stable under the process conditions of 11% feed ratio of the liquid distributor (i.e., the feed ratio of the tower body), 264°C bottom temperature, 2.2kPa top absolute pressure, and 44°C top reflux material temperature. It was detected that the content of 2,4-MDI in the polyisocyanate product extracted from the bottom of the tower was 3.1wt%, and the content of heat-sensitive chlorine in the crude MDI product was 271ppm; the free chlorine content of the MDI product obtained after the crude MDI product was distilled was 1.4ppm.

[0134] Example 2

[0135] S1: Feed part of the crude isocyanate B prepared in Preparation Example 2 into a distillation tower B. The theoretical number of plates of distillation tower B is 13, and the filler is 3 large layers. A liquid distributor is arranged above each large layer of filler. The feed position of the tower body is the liquid distributor above the filler of the second large layer (counted from top to bottom). The theoretical number of plates between the feed position and the bottom of the tower is 10. The feed proportion program of the liquid tower body is started to control the content of 2,4-MDI in the polyisocyanate product extracted from the bottom of the tower. At the same time, the program for controlling the content of heat-sensitive chlorine in the crude MDI product is started. In the program, the target value of the 2,4-MDI content in the polyisocyanate product is set to 4.0%, the sampling interval of the crude MDI product is 48h, and the cumulative alarm value Δm% of the feed proportion of the tower body is 10%.

[0136] S2: Set the liquid distributor and the bottom of the distillation tower B to feed at the same time, set the feed temperature of crude isocyanate to 160°C, the bottom liquid extraction ratio to 93%, the top reflux flow rate to 12t / h, and the top condenser temperature to 40°C; Finally, the distillation tower remains stable under the process conditions of 43% feed ratio of the liquid distributor (i.e., the feed ratio of the tower body), 183°C bottom temperature, 0.35kPa top absolute pressure, and 92°C top reflux material temperature. It was detected that the content of 2,4-MDI in the polyisocyanate product extracted from the bottom of the tower was 4.3wt%, and the content of heat-sensitive chlorine in the crude MDI product was 259ppm; the free chlorine content of the MDI product obtained after the crude MDI product was distilled was 1.3ppm.

[0137] Example 3

[0138] S1: Feed part of the crude isocyanate C prepared in Preparation Example 3 into a distillation tower C. The theoretical number of plates of the distillation tower C is 25, and the filler is 5 large layers. A liquid distributor is arranged above each large layer of filler. The feed position of the tower body is the liquid distributor above the filler of the third large layer (counted from top to bottom). The theoretical number of plates between the feed position and the bottom of the tower is 18. The feed proportion program of the tower body is started to control the content of 2,4-MDI in the polyisocyanate product extracted from the bottom of the tower. At the same time, the program for controlling the content of heat-sensitive chlorine in the crude MDI product is started. In the program, the target value of the 2,4-MDI content in the polyisocyanate product is set to 4.0%, the sampling interval of the crude MDI product is 48h, and the cumulative alarm value Δm% of the feed proportion of the tower body is 20%.

[0139] S2: Set the liquid distributor and the bottom of the distillation tower C to feed at the same time, and set the feed temperature of crude isocyanate to 200°C, the bottom liquid extraction ratio to 64%, the top reflux flow rate to 10t / h, and the top condenser temperature to 40°C; Finally, the distillation tower remains stable under the process conditions of 69% feed ratio of the liquid distributor (i.e., the feed ratio of the tower body), 225°C bottom temperature, 1.0kPa top absolute pressure, and 145°C top reflux material temperature. It was detected that the content of 2,4-MDI in the polyisocyanate product extracted from the bottom of the tower was 3.9wt%, and the content of heat-sensitive chlorine in the crude MDI product was 234ppm; the free chlorine content of the MDI product obtained after the crude MDI product was distilled was 1.1ppm.

[0140] Example 4

[0141] S1: Feed part of the crude isocyanate B prepared in Preparation Example 2 into a distillation tower C. The theoretical number of plates of the distillation tower C is 25, and the filler is 5 large layers. A liquid distributor is arranged above each large layer of filler. The feed position of the tower body is the liquid distributor above the filler of the third large layer (counted from top to bottom). The theoretical number of plates between the feed position and the bottom of the tower is 18. The feed proportion program of the tower body is started to control the content of 2,4-MDI in the polyisocyanate product extracted from the bottom of the tower. At the same time, the program for controlling the content of heat-sensitive chlorine in the crude MDI product is started. In the program, the target value of the 2,4-MDI content in the polyisocyanate product is set to 4.0%, the sampling interval of the crude MDI product is 24h, and the cumulative alarm value Δm% of the feed proportion of the tower body is 20%.

[0142] S2: Set the liquid distributor and the bottom of the distillation tower C to feed at the same time, set the feed temperature of crude isocyanate to 205°C, the bottom liquid extraction ratio to 93%, the top reflux flow rate to 12t / h, and the top condenser temperature to 40°C; Finally, the distillation tower remains stable under the process conditions of 16% feed ratio of the liquid distributor (i.e., the feed ratio of the tower body), 219°C bottom temperature, 0.9kPa top absolute pressure, and 95°C top reflux material temperature. It was detected that the content of 2,4-MDI in the polyisocyanate product extracted from the bottom of the tower was 4.1wt%, and the content of heat-sensitive chlorine in the crude MDI product was 216ppm; the free chlorine content of the MDI product obtained after the crude MDI product was distilled was 1.0ppm.

[0143] Example 5

[0144] S1: Feed part of the crude isocyanate C prepared in Preparation Example 3 into a distillation tower B. The theoretical number of plates of the distillation tower B is 13, and the filler is 3 large layers. A liquid distributor is arranged above each large layer of filler. The feed position of the tower body is the liquid distributor above the filler of the second large layer (counted from top to bottom). The theoretical number of plates between the feed position and the bottom of the tower is 10. The feed proportion program of the liquid tower body is started to control the content of 2,4-MDI in the polyisocyanate product extracted from the bottom of the tower. At the same time, the program for controlling the content of heat-sensitive chlorine in the crude MDI product is started. In the program, the target value of the 2,4-MDI content in the polyisocyanate product is set to 4.0%, the sampling interval of the crude MDI product is 48h, and the cumulative alarm value Δm% of the feed proportion of the tower body is 10%.

[0145] S2: Set the liquid distributor and the bottom of the distillation tower B to feed at the same time, set the feed temperature of crude isocyanate to 160°C, the bottom liquid extraction ratio to 64%, the top reflux flow rate to 15t / h, and the top condenser temperature to 40°C; Finally, the distillation tower remained stable under the process conditions of 91% feed ratio of the liquid distributor (i.e., the feed ratio of the tower body), 178°C bottom temperature, 0.35kPa top absolute pressure, and 42°C top reflux material temperature. It was detected that the content of 2,4-MDI in the polyisocyanate product extracted from the bottom of the tower was 4.2wt%, and the content of heat-sensitive chlorine in the crude MDI product was 266ppm; the free chlorine content of the MDI product obtained after the crude MDI product was distilled was 1.3ppm.

[0146] The crude isocyanate was separated according to a process similar to that in Example 1. The relevant data of Formulas 1 and 2 in the automatic control process are shown in Table 1 below.

[0147] Table 1

[0148]

[0149] According to the results in Table 1, it can be seen that the heat-sensitive chlorine content of the crude MDI product can be controlled within the range of 180 to 330 ppm by adjusting formulas 1 and 2, thereby achieving controllable activity of the MDI product.

[0150] Comparative Example 1

[0151] Part of the crude isocyanate A prepared in Preparation Example 1 was fed into a distillation tower A, the feed position was the tower bottom reboiler, the theoretical number of plates of the distillation tower A was 7, the filler was 2 large layers, and the crude isocyanate feed temperature was 251°C, the tower bottom temperature was 263°C, the absolute pressure at the top of the tower was 2.1 kPa, the bottom liquid extraction ratio was 47%, the top condenser temperature was 80°C, the top reflux into the tower temperature was 80°C, and the top reflux flow rate was 5 t / h. The separation was carried out under the process conditions to obtain a polyisocyanate product and a crude MDI product.

[0152] The content of 2,4-MDI in the polyisocyanate product was detected to be 3.6 wt %, the content of heat-sensitive chlorine in the crude MDI product was 1100 ppm, and the free chlorine content of the MDI product obtained after the crude MDI product was distilled was 5.7 ppm.

[0153] Comparative Example 2

[0154] Part of the crude isocyanate B prepared in Preparation Example 2 was fed into a distillation tower B, the feed position was the bottom reboiler, the theoretical plate number of the distillation tower B was 13, the filler was 3 large layers, and the crude isocyanate feed temperature was 160°C, the bottom temperature was 179°C, the top absolute pressure was 0.3 kPa, the bottom liquid extraction ratio was 93%, the top condenser temperature was 50°C, the top reflux into the tower temperature was 50°C, and the top reflux flow rate was 5 t / h. The polyisocyanate product and the crude MDI product were separated under the process conditions.

[0155] The content of 2,4-MDI in the polyisocyanate product was detected to be 8.3wt%, the content of heat-sensitive chlorine in the crude MDI product was 470ppm, and the free chlorine content of the MDI product obtained after the crude MDI product was distilled was 2.8ppm.

[0156] Comparative Example 3

[0157] Part of the crude isocyanate C prepared in Preparation Example 3 was fed into a distillation tower C, the feed position was the tower bottom reboiler, the theoretical plate number of the distillation tower C was 25, the filler was 5 large layers, and the crude isocyanate feed temperature was 200°C, the tower bottom temperature was 222°C, the top absolute pressure was 0.9 kPa, the bottom liquid extraction ratio was 64%, the top condenser temperature was 70°C, the top reflux into the tower temperature was 70°C, and the top reflux flow rate was 5 t / h. The process conditions were separated to obtain a polyisocyanate product and a crude MDI product.

[0158] The content of 2,4-MDI in the polyisocyanate product was detected to be 14.4 wt %, the content of heat-sensitive chlorine in the crude MDI product was 152 ppm, and the free chlorine content of the MDI product obtained after the crude MDI product was distilled was 0.6 ppm.

[0159] Comparative Example 4

[0160] Part of the crude isocyanate B prepared in Preparation Example 2 was fed into a distillation tower C, the feed position was a bottom reboiler, the theoretical plate number of the distillation tower C was 25, the filler was 5 large layers, and the crude isocyanate feed temperature was 205°C, the bottom temperature was 218°C, the top absolute pressure was 0.9 kPa, the bottom liquid extraction ratio was 93%, the top condenser temperature was 60°C, the top reflux into the tower temperature was 60°C, and the top reflux flow rate was 5 t / h. The polyisocyanate product and the crude MDI product were separated under the process conditions.

[0161] The content of 2,4-MDI in the polyisocyanate product was detected to be 6.5wt%, the content of heat-sensitive chlorine in the crude MDI product was 163ppm, and the free chlorine content of the MDI product obtained after the crude MDI product was distilled was 0.6ppm.

[0162] Comparative Example 5

[0163] Part of the crude isocyanate C prepared in Preparation Example 3 was fed into a distillation tower B, the feed position was the bottom reboiler, the theoretical plate number of the distillation tower B was 13, the filler was 3 large layers, and the crude isocyanate feed temperature was 160°C, the bottom temperature was 173°C, the top absolute pressure was 0.3 kPa, the bottom liquid extraction ratio was 64%, the top condenser temperature was 50°C, the top reflux temperature was 50°C, and the top reflux flow rate was 5 t / h. The polyisocyanate product and the crude MDI product were separated under the process conditions.

[0164] The content of 2,4-MDI in the polyisocyanate product was detected to be 17.7 wt %, the content of heat-sensitive chlorine in the crude MDI product was 236 ppm, and the free chlorine content of the MDI product obtained after the crude MDI product was distilled was 1.1 ppm.

[0165] The content of 2,4-MDI in the crude isocyanate A obtained in Preparation Example 1 is 5.9wt%, and the content of 2,4-MDI in the crude isocyanate C obtained in Preparation Example 3 is 22.3wt%. By comparison, it can be seen that the content of 2,4-MDI in the two crude isocyanates is quite different. Examples 1, 3, and 5 are separated and processed with crude isocyanate A, crude isocyanate C, and crude isocyanate C as raw materials, respectively. The results show that the content of 2,4-MDI in the polyisocyanate products obtained in the three examples is maintained in the range of 2 to 5wt%, and the heat-sensitive chlorine in the crude MDI product is maintained in the range of 180 to 330ppm. Therefore, the separation method of the embodiment of the present invention can be used to efficiently separate crude isocyanates with large composition differences.

[0166] The separation method of Comparative Examples 1 to 5 is only to feed through the bottom of the tower. According to the test results, Comparative Examples 1 to 5 cannot simultaneously achieve effective control of the 2,4-MDI content of the polyisocyanate product and the heat-sensitive chlorine content of the MDI crude product, or the 2,4-MDI content of the polyisocyanate product is too high (higher than 2 to 5 wt%), or the heat-sensitive chlorine content of the MDI crude product is too high / too low (higher / lower than 180 to 330 ppm), or both deviate from the required range.

[0167] Unless otherwise defined, the terms used in the present invention have the meanings commonly understood by those skilled in the art.

[0168] The embodiments described in the present invention are for illustrative purposes only and are not intended to limit the scope of protection of the present invention. Those skilled in the art may make various other substitutions, changes and improvements within the scope of the present invention. Therefore, the present invention is not limited to the above embodiments, but is only limited by the claims.

Claims

1. A method for separating crude isocyanate, comprising treating the crude isocyanate through a distillation tower to obtain a polyisocyanate product and a crude MDI product; wherein: The distillation tower comprises a tower bottom, a tower top and a tower body connecting the tower bottom and the tower top; the crude isocyanate is fed into the distillation tower from the tower bottom and the tower body.

2. The separation method according to claim 1, wherein 1-99 wt% of the crude isocyanate is fed from the bottom of the tower, and 1-99 wt% of the crude isocyanate is fed from the tower body; and / or, The crude isocyanate comprises 0-5wt% of 2,2-MDI, 3-30wt% of 2,4-MDI, 30-80wt% of 4,4-MDI, 5-30wt% of polyphenyl polymethylene polyisocyanate containing three benzene rings, 0.5-20wt% of polyphenyl polymethylene polyisocyanate containing four benzene rings, 5-30wt% of polyphenyl polymethylene polyisocyanate containing more than five benzene rings; and / or, The number of theoretical plates of the distillation tower is 1 to 30; and / or, 1 to 8 layers of fillers are arranged in the distillation tower, and a liquid distributor is arranged on one or more layers of the fillers; and / or, A reboiler connected to the tower bottom is arranged outside the rectification tower; and / or, The temperature of the reflux material at the top of the tower is 40 to 200° C.; and / or, The temperature of the tower bottom is 170-270°C; and / or, The absolute pressure at the top of the tower is 0.1-2.5 kPa.

3. The separation method according to claim 2, wherein: 30-70 wt% of the crude isocyanate is fed from the bottom of the tower, and 30-70 wt% of the crude isocyanate is fed from the tower body; and / or, The crude isocyanate comprises 0-3 wt% of 2,2-MDI, 5-20 wt% of 2,4-MDI, 40-70 wt% of 4,4-MDI, 10-25 wt% of polyphenyl polymethylene polyisocyanate containing three benzene rings, 1.5-15 wt% of polyphenyl polymethylene polyisocyanate containing four benzene rings, and 10-25 wt% of polyphenyl polymethylene polyisocyanate containing more than five benzene rings; and / or, The number of theoretical plates of the distillation tower is 5 to 20; and / or, The distillation tower is provided with 2 to 5 layers of fillers.

4. The separation method according to claim 2, wherein: The crude isocyanate is fed to the distillation column through the reboiler and one or more liquid distributors; and / or, The temperature of the reflux material at the top of the tower is 70 to 180° C.; and / or, A liquid distributor is provided on each layer of the packing; and / or, The number of theoretical plates between the feed position of the tower body and the tower bottom is 5 to 20.

5. The separation method according to claim 4, wherein: The number of theoretical plates between the feeding position of the tower body and the tower bottom is less than 15; based on the total mass of the crude isocyanate, the mass content of 2,4-MDI is less than 15%, 10-60wt% of the crude isocyanate is fed through the tower body, and the remaining crude isocyanate is fed through the reboiler; or, The mass content of 2,4-MDI is above 15%, 70-99wt% of the crude isocyanate is fed through the tower body, and the remaining crude isocyanate is fed through the reboiler.

6. The separation method according to claim 4, wherein: The number of theoretical plates between the feeding position of the tower body and the tower bottom is 15 or more; based on the total mass of the crude isocyanate, the mass content of 2,4-MDI is less than 15%, 1 to 30 wt% of the crude isocyanate is fed through the tower body, and the remaining crude isocyanate is fed through the reboiler; or, The mass content of 2,4-MDI is above 15%, 40-90wt% of the crude isocyanate is fed through the tower body, and the remaining crude isocyanate is fed through the reboiler.

7. The separation method according to claim 4, wherein: The crude MDI contains heat-sensitive chlorine, and the content of the heat-sensitive chlorine = total chlorine content - free chlorine content - chlorine content of the solvent - chlorine content of chlorinated MDI; The crude MDI product taken from the tower top is sampled regularly and its heat-sensitive chlorine content m1 is measured, and the temperature of the reflux material at the tower top is adjusted according to the heat-sensitive chlorine content. The temperature adjustment value ΔT1 is: ΔT1=80-10.716*(m1*10 6 ) 0.3653 (Formula 1).

8. The separation method according to claim 7, wherein: Before and after sampling the crude MDI product, when the feed ratio of the tower body changes by more than the alarm value Δm%, the temperature of the reflux material at the top of the tower is adjusted according to the following formula, and the temperature adjustment value ΔT2 is: ΔT2 = -2*change in feed ratio of the tower body (Formula 2); The feed ratio of the tower body is the percentage of the crude isocyanate fed from the tower body to the total mass of the crude isocyanate entering the distillation tower, and the change of the feed ratio of the tower body is the difference between the real-time feed ratio A2 of the tower body and the initial feed ratio A1; Δm% is 1-50%; and / or, The time interval for sampling the crude MDI product is 4 to 168 hours; and / or, m1*10 6 The value of is 180 to 330; and / or, The free chlorine content is the content of chloride ions.

9. An automatic control method for the separation method of crude isocyanate according to any one of claims 1 to 8, comprising the following steps: (1) Sampling and analyzing the crude MDI product discharged from the top of the distillation tower; (2) After sampling is completed, the initial feed ratio A1 of crude isocyanate in the tower body is recorded, and the initial time t1 is recorded; (3) obtaining the heat-sensitive chlorine content in the crude MDI product obtained by sampling and analysis, and adjusting the temperature of the reflux material at the top of the tower according to Formula 1 based on the heat-sensitive chlorine content; (4) When the difference between the current time t2 and the initial time t1 is greater than the sampling interval, sampling is performed again to repeat the operations of steps (1) to (3).

10. The method according to claim 9, wherein: When the difference between the real-time feed ratio A2 of the tower body and the initial feed ratio A1 is greater than the alarm value, the temperature of the reflux material at the top of the tower is adjusted according to Formula 2, and the value of the feed ratio A2 of the tower body at that time is used as the initial feed ratio A1 for the next regulation to recalculate the difference; when the difference between the subsequent current time t2 and the initial time t1 is greater than the sampling interval, sampling is performed again.

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