Process for the separation of crude isocyanates and automatic control method thereof

Abstract

The present application relates to a kind of separation methods of crude isocyanate and its automatic control method, the separation method includes that crude isocyanate is handled by rectifying column, obtains polyisocyanate product and MDI crude product;Wherein, the rectifying column includes tower kettle, tower top and the tower body connecting the tower kettle, the tower top;The crude isocyanate is fed from the tower kettle and the tower body to the rectifying column.This embodiment of the present application, the polyisocyanate product in the tower kettle is extracted, and the control of 2,4-MDI content in polyisocyanate product can be realized by the rectifying column feeding from tower kettle and tower body, and then the control of polyisocyanate product activity is realized.

Description

Technical Field

[0001] This invention relates to the separation of crude isocyanates, and more particularly to a method for separating polyisocyanates with high product activity. Background Technology

[0002] Polyisocyanate, also known as "polymeric MDI", chemically named polyphenyl polymethylene polyisocyanate, is a mixture of components containing the following molecular structure expression:

[0003]

[0004] Polyisocyanate is a primary raw material for the production of polyurethane foam, mainly used in industries such as refrigerators, spraying, pipe insulation, boards, and solar energy. The reactivity of polyisocyanate is a key factor affecting the viscosity change rate and subsequent product stability during the production of coatings, adhesives, and polyurethane foam. Many factors influence the reactivity of polyisocyanate, among which the content of the 2,4-diphenylmethane diisocyanate (2,4-MDI) isomer primarily affects the curing rate of downstream applications and the dimensional stability of finished products. When the 2,4-MDI isomer content in polyisocyanate is high or low, the downstream reactivity and curing rate will be excessively high or low, leading to fluctuations in production processes and product quality, particularly potentially causing severe dimensional shrinkage in finished products. Therefore, the 2,4-MDI isomer content in polyisocyanate products needs to be strictly controlled.

[0005] Currently, the main production process of isocyanate involves the condensation reaction of aniline and formaldehyde under the catalysis of Lewis acid to generate polyamines. The polyamines are then reacted with phosgene in a solvent through a two-step cold and hot reaction to generate a photochemical liquid. This photochemical liquid is then subjected to a series of post-treatment processes to remove the solvent, yielding crude isocyanate. The crude isocyanate can be separated by distillation to obtain polyisocyanate (polymeric MDI) and diphenylmethane diisocyanate (MDI). However, this process generates a large amount of chlorine-containing substances. Some of these chlorine-containing substances, such as acyl chlorides and adducts of isocyanate and hydrogen chloride, easily decompose under high temperatures to generate hydrogen chloride and other substances (collectively referred to as "thermosensitive chlorine" in this text). These chlorine-containing substances are ultimately carried into the crude isocyanate.

[0006] Through continuous research, the inventors have discovered that, influenced by upstream condensation reaction ratios and other process parameters, the 2,4-MDI content in crude isocyanate under existing processes typically ranges from 3% to 30 wt%. This results in significant fluctuations in the 2,4-MDI content of polyisocyanate within the range of 0.5% to 20 wt%, severely impacting product quality stability. Current technologies primarily aim to minimize fluctuations in the 2,4-MDI content of polyisocyanate by adjusting upstream process parameters to control the range of fluctuations. However, this method severely restricts production flexibility. For example, when there is a high demand for products with high 2,4-MDI content, it is necessary to control the 2,4-MDI content in the crude isocyanate to a higher level. Meanwhile, controlling the 2,4-MDI content of polyisocyanate during the separation process of crude isocyanate inevitably leads to fluctuations in the thermosensitive chlorine content of crude diphenylmethane diisocyanate (crude MDI), thus affecting the activity of downstream products. Furthermore, under existing processes, the free chlorine content of diphenylmethane diisocyanate typically fluctuates between 0.1 and 10 ppm, resulting in significant fluctuations in product activity and impacting the stability of downstream production and the performance of finished products. Existing technologies do not address these issues or provide corresponding solutions. Summary of the Invention

[0007] To overcome at least one of the defects of the prior art, in a first aspect, one embodiment of the present invention provides a method for separating crude isocyanate, comprising processing the crude isocyanate through a distillation column to obtain polyisocyanate product and crude MDI product; wherein, the distillation column includes a reboiler, a top, and a column body connecting the reboiler and the top; the crude isocyanate is fed into the distillation column from the reboiler and the column body.

[0008] Secondly, one embodiment of the present invention provides an automatic control method for the above-described separation method of crude isocyanate, comprising the following steps:

[0009] (1) Samples of crude MDI discharged from the top of the distillation column were analyzed.

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

[0011] (3) Obtain the thermosensitive chlorine content in the crude MDI obtained from the sampling analysis, and adjust the temperature of the reflux material at the top of the tower according to Formula 1 based on the thermosensitive chlorine content;

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

[0013] One embodiment of the present invention provides a method for separating crude isocyanate by simultaneously feeding the distillation column into the bottom and the column body. This method enables control of the 2,4-MDI content in the polyisocyanate product collected from the bottom, thereby achieving control of the activity of the polyisocyanate product. Attached Figure Description

[0014] The accompanying drawings are for illustrative purposes only and are not intended to limit the scope of the invention. Wherein:

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

[0016] Figure 2 A flowchart illustrating an automatic control method for crude isocyanate separation according to one embodiment of the present invention;

[0017] The annotations in the attached figures are explained as follows:

[0018] 10. Distillation column; 20. Reboiler; 30. Condenser; 40. Preheater; 50. Online near-infrared analyzer; 60. MDI crude sampler; 70. First feed line; 71. First flow meter; 72. First regulating valve; 80. Second feed line; 81. Second flow meter; 82. Second regulating valve; 91. Thermometer; 92. Third flow meter; 93. Third regulating valve; 100. Logic controller. Detailed Implementation

[0019] Typical embodiments embodying 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 variations in different embodiments without departing from the scope of the present invention, and the description herein is for illustrative purposes only and not intended to limit the present invention.

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

[0021] The inventors discovered that, with changes in market demand for products with high 2,4-MDI content, the 2,4-MDI content in crude isocyanate can be adjusted by modifying upstream process parameters. However, since the separation capacity of the distillation column remains constant, the 2,4-MDI content in the produced polyisocyanate product will fluctuate significantly, affecting its downstream applications. Common adjustments to process parameters such as column bottom temperature and reflux ratio have very limited effects and may require additional operations such as viscosity adjustment. The inventors further discovered that, during the separation process of crude isocyanate, simultaneous feeding into both the column bottom and the column body allows for control of the polyisocyanate product's activity.

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

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

[0024] In one embodiment, 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 five or more benzene rings.

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

[0026] In one embodiment, the crude isocyanate also includes residual solvent (e.g., chlorobenzene) and chloro-MDI.

[0027] In one embodiment, the crude isocyanate may contain 1000–10000 ppm of thermosensitive chlorine; further, the crude isocyanate may contain 2000–5000 ppm of thermosensitive chlorine; wherein, the thermosensitive chlorine is a chlorine-containing substance (e.g., an adduct of acyl chloride, isocyanate, and hydrogen chloride) that readily decomposes to form hydrogen chloride under high-temperature conditions, and the content of the thermosensitive 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] Free chlorine is chloride ions, which originate from the reaction of HCl and / or acyl chlorides dissolved in organic matter with water to generate chloride ions. The solvent can be chlorobenzene.

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

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

[0032] In one embodiment, the number of theoretical plates of the distillation column 10 is 1 to 30, more specifically 5 to 20, for example 6, 7, 8, 10, 11, 12, 13, 15, 18, 19, 22, 23, 24, 25.

[0033] In one embodiment, the distillation column 10 is provided with 1 to 8 layers of packing, further 1 to 5 layers of packing, and even further 2 to 5 layers of packing, for example 2 layers of packing.

[0034] In one embodiment, crude isocyanate fed from the column body enters the distillation column 10 through a packing layer (or "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, through which crude isocyanate (e.g., 1-99 wt%) fed from the column body is fed into the distillation column 10.

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

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

[0038] In one embodiment, the distillation column 10 is provided with two or three layers of packing, and crude isocyanate is fed into the distillation column 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 line 70 and a second feed line 80 are provided outside the distillation column 10. The first feed line 70 is connected to the reboiler 20, and the second feed line 80 is connected to the column body, or more specifically, to the liquid distributor of the packing layer. A first flow meter 71 and a first regulating valve 72 are provided on the first feed line 70, and a second flow meter 81 and a second regulating valve 82 are provided on the second feed line 80. Further, both the first regulating valve 72 and the second regulating valve 82 are flow regulating valves.

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

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

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

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

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

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

[0046] In one embodiment, in the separation process of the distillation column, 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, or 265°C.

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

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

[0049] In one embodiment, after the gas discharged from the top of the column 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 exiting the condenser 30; among them, part of the condensate is returned to the distillation column 10, and the remaining condensate is discharged through the discharge pipeline of crude MDI.

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

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

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

[0053] In one embodiment, the crude MDI contains thermosensitive chlorine, the content of which is equal to the total chlorine content minus the free chlorine content minus the chlorine content of the solvent minus the chlorine content of the chlorinated MDI. The solvent may be, for example, chlorobenzene.

[0054] The inventors unexpectedly discovered that, while maintaining a constant reflux flow rate at the top of the distillation column, adjusting the temperature of the reflux entering the column at the top of the column can effectively regulate the thermosensitive chlorine content in the crude MDI product collected at the top of the column. Furthermore, because the reflux flow rate at the top of the column is constant, the distribution effect of the distributor can always be maintained at the optimal level, which can effectively 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 column, and polyisocyanate product is obtained by collecting the material discharged from the bottom of the column. Further, an online infrared analyzer 50 is installed on the discharge line of the polyisocyanate product outside the bottom of the column for real-time online analysis of the 2,4-MDI isomer content in the polyisocyanate product discharged from the bottom of the column. An MDI crude sampler 60 is installed on the discharge line of the crude MDI outside the condenser 30 for offline timed sampling of the crude MDI discharged from the top of the column and measurement of the thermosensitive chlorine content in the sample.

[0056] In one embodiment, the sampling time interval for crude MDI is 4 to 168 hours, and more specifically 24 to 48 hours, such as 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 collected from the top of the column is sampled periodically, and its thermosensitive chlorine content m1 is measured. The temperature of the reflux material at the top of the column is adjusted according to the thermosensitive chlorine content, and the temperature adjustment value ΔT1 (in °C) is:

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

[0059] Where ΔT1 = T1 - T0, T0 is the temperature of the reflux material at the top of the column before adjustment, and T1 is the temperature of the reflux material at the top of the column after adjustment. If ΔT1 is positive, it means that the temperature of the reflux material should be increased; otherwise, if ΔT1 is negative, it means that the temperature of the reflux material should be decreased.

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

[0061] In one implementation, m1*10 6 The value range can be 180 to 330, and further can be 200 to 290, such as 210, 220, 230, 240, 250, 260, 270, 280, 300.

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

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

[0064] Where ΔT2 = T2 - T0, T0 is the temperature of the reflux material at the top of the column before adjustment, and T2 is the temperature of the reflux material at the top of the column after adjustment. If ΔT2 is positive, it means that the temperature of the reflux material should be increased; conversely, if ΔT2 is negative, it means that the temperature of the reflux material should be decreased. The change in the feed ratio of the column body is the change in the percentage of crude isocyanate fed from the column body to the total mass of crude isocyanate entering the distillation column, that is, the difference between the real-time feed ratio A2 and the initial feed ratio A1. The column body feed rate and the column bottom feed rate are obtained by flow meters, and then the real-time column body feed ratio A2 is calculated according to the formula "column body feed rate / (column bottom feed rate + column body feed rate)".

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

[0066] In one implementation, Formulas 1 and 2 above only indicate that the 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 adjustments of ΔT1 and ΔT2 do not affect each other and can be performed independently or simultaneously.

[0068] In one embodiment, a logic controller 100 is installed in the crude isocyanate separation device to control the content of 2,4-MDI isomers in the polyisocyanate product and the content of thermosensitive chlorine in the crude MDI product using the two formulas mentioned above. Specifically, the content of 2,4-MDI isomers in the polyisocyanate product collected from the bottom of the column is first set. Then, the logic controller automatically adjusts the feed ratio of the column body (i.e., the feed ratio of the packing layer) until the requirements are met. At the same time, the thermosensitive chlorine analysis results of the crude MDI product and the changes in the feed ratio of the column body will trigger the logic controller to adjust the temperature of the reflux material at the top of the column to ensure the stability of the thermosensitive chlorine content in the crude MDI product.

[0069] In one implementation, after completing the offline sampling and analysis of the thermosensitive chlorine content in crude MDI 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 input of the next offline sampling result. After each offline sampling, an alarm is activated for the feed ratio of the tower body. When the cumulative change in the feed ratio of the tower body exceeds the cumulative alarm value Δm%, an alarm is triggered. At this time, the temperature of the reflux material at the top of the tower is adjusted according to Formula 2. After the adjustment is completed, the alarm program for the feed ratio is restarted until the next sampling begins.

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

[0071] Aniline and formaldehyde undergo a condensation reaction under the catalysis of Lewis acid to generate polyamines;

[0072] Polyamines and phosgene are reacted with each other in a solvent under cold and hot conditions to obtain a photochemical liquid; and

[0073] The photochemical solution was post-treated to remove residual phosgene and solvent, yielding crude isocyanate.

[0074] In one embodiment, the raw materials and process parameters involved in the preparation process of crude isocyanate can be those 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 condensation reaction temperature can be 40–100°C; the mass ratio of phosgene to polyamine reaction can be 1–5, the cold reaction temperature can be 60–120°C, and the hot reaction temperature can be 100–150°C; the post-treatment process includes dephosgene and desolventization treatment, the dephosgene treatment temperature can be 110–170°C, and the pressure can be 10–50 kPaG; the desolventization treatment temperature can be 130–220°C, and the pressure can be 10–60 kPaA.

[0076] In one embodiment, the polyisocyanate product obtained by separation contains 2 to 5 wt% 2,4-MDI, more specifically 3 to 4 wt%.

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

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

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

[0080] In one embodiment, the content of thermosensitive chlorine in the separated crude MDI is 180–330 ppm, and more specifically, 200–290 ppm. The MDI product can be regulated by controlling the content of thermosensitive chlorine in the crude MDI product collected from the top of the column.

[0081] In one embodiment, the crude MDI obtained by separation includes 0.1–5 wt% 2,2-MDI, 4–15 wt% 2,4-MDI, 80–95 wt% 4,4-MDI, and 180–330 ppm thermosensitive chlorine.

[0082] In one embodiment, the crude MDI obtained by separation includes 0.5–3 wt% 2,2-MDI, 8–12 wt% 2,4-MDI, 85–90 wt% 4,4-MDI, and 180–330 ppm thermosensitive chlorine.

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

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

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

[0086] One embodiment of the present invention provides an automatic control method for the separation of crude isocyanate as described above, which uses a distillation column to separate the crude isocyanate. The control method includes the following steps:

[0087] (1) Samples of crude MDI discharged from the top of the distillation column were analyzed.

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

[0089] (3) Obtain the thermosensitive chlorine content (m1*10) in the crude MDI obtained from sampling analysis. 6 (ppm), and adjust the temperature of the reflux material at the top of the tower according to the aforementioned formula 1 based on this content;

[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 and the above steps (1) to (3) are repeated.

[0091] In one implementation, after the above step (2) is completed, the control program for 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 aforementioned formula 2, and the current time is recorded as the initial time t1. The value of the current feed ratio A2 of the tower body is used as the initial feed ratio A1 for the next adjustment to start calculating the difference again. When the difference between the 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] Conversely, if the difference between the current feed ratio A2 and the initial feed ratio A1 does not exceed the cumulative alarm value, then it is not necessary 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 to 50%, and further, Δm% can be 2%, 3%, 4%, 5%, 6%, 8%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, or 45%.

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

[0095] In one implementation, the above-described automated control method can be achieved by inputting the corresponding instructions into an existing application.

[0096] In one embodiment, the thermosensitive chlorine content in the crude MDI is detected offline. After obtaining the test results, the results can be input into the program, and then the temperature of the reflux material at the top of the tower can be automatically adjusted according to the results using Formula 1.

[0097] An automatic control method for crude isocyanate separation according to one embodiment of the present invention includes a feed ratio program for the column body and a sequential control program for controlling the thermosensitive chlorine content in the crude MDI product collected from the top of the column. The feed ratio program for the column body is a single-loop control of the 2,4-MDI content in the polyisocyanate product collected from the bottom of the column. Specifically, after setting the 2,4-MDI content in the polyisocyanate product, the system adjusts the feed ratio for the column body. When the 2,4-MDI content in the polyisocyanate product is higher than the set value (e.g., 2-5 wt%), the feed ratio for the column body is increased; conversely, it is decreased, until the 2,4-MDI content in the polyisocyanate product reaches the set value. This control logic continuously cycles in a single loop and is unaffected by other control parameters. Furthermore, the above-mentioned control of the feed ratio for the column body can be implemented using a PID control loop.

[0098] One embodiment of the present invention discloses a method for separating crude isocyanate. By adjusting the feed location and feed ratio of the distillation column, the 2,4-MDI content in the polyisocyanate product collected from the bottom of the column can be controlled. Furthermore, by adjusting the temperature of the reflux material at the top of the column, the thermosensitive chlorine content of the crude MDI product collected at the top of the column can be controlled, thereby obtaining a free chlorine-stable MDI product downstream. Therefore, by controlling the feed location, feed ratio, and reflux material temperature at the top of the distillation column, the stable control of the activity of both the polyisocyanate and MDI products can be achieved simultaneously.

[0099] The crude isocyanate separation method of one embodiment of the present invention can simultaneously achieve precise control of the activity of polyisocyanate products and diphenylmethane diisocyanate products, is convenient to operate, and has a rapid response; moreover, the reflux flow rate at the top of the distillation column is kept constant in this method, which can effectively avoid coking of the packing and extend the operating cycle of the device.

[0100] The crude isocyanate separation method of one embodiment of the present invention can realize the automated control of the activity of polyisocyanate products and MDI products by using a feed ratio program for the tower body and a sequential control program for controlling the thermosensitive chlorine content in the crude MDI product taken out from the top of the tower. It has the advantages of convenient operation and rapid response.

[0101] The method for separating crude isocyanate according to one embodiment of the present invention is simple in process, requires little technical modification to existing equipment, and involves little investment.

[0102] The following description, in conjunction with the accompanying drawings and specific embodiments, further illustrates a method for separating crude isocyanate according to one embodiment of the present invention. In this document, the formula for calculating the thermosensitive chlorine content is: Thermosensitive chlorine content = Total chlorine content - Free chlorine content - Chlorine content in chlorobenzene - Chlorine content in chloromethyl MDI. The testing methods for the total chlorine content, free chlorine content, chlorine content in chlorobenzene, chlorine content in chloromethyl MDI, and the content of each component of the isocyanate are as follows:

[0103] 1. Total chlorine content test

[0104] For total chlorine analysis using an XRF instrument, take a 5g sample, perform an XRF test for 30 seconds, and read the value according to the mark.

[0105] 2. Free chlorine test

[0106] Free chlorine was analyzed using ion chromatography. 5g of isocyanate sample, 5g of dichloromethane, and 5g of water were mixed thoroughly, and the supernatant was collected for ion chromatography analysis. The mobile phase for ion chromatography was 5 mmol / L Na₂CO₃ solution, and the chromatographic column was an A7 anion exchange column. The column temperature was 40℃, and the flow rate was 1 mL / min. The peak position at 6.7 min in the chromatogram represents the concentration of chlorine. - The quantity can be determined based on the reading of the marking.

[0107] 3. Chlorine content test for chlorobenzene and chloromethyl MDI

[0108] Isocyanate samples were directly analyzed by gas chromatography (GC), with an injection volume of 1 μL. A weakly polar column was used. The initial temperature of the column oven was 150 °C for 0.5 min; the temperature was then programmed to increase to 250 °C at 5 °C / min and held for 7 min; finally, the temperature was programmed to increase to 280 °C at 10 °C / min and held for 2 min. An ECD detector was used. The characteristic peaks at 1.9 min and 18.2 / 19.2 / 21.1 min in the chromatogram represent chlorobenzene and chloro-MDI, respectively. The mass percentages of chlorobenzene and chloro-MDI were obtained using the internal standard method, and the chlorine content could then be calculated based on the molecular weight of the substances.

[0109] 4. Testing of component content in isocyanates

[0110] The content of 2,2-MDI, 2,4-MDI, 4,4-MDI, and tricyclic, tetracyclic, pentacyclic, and higher-order substances in isocyanate samples was determined by combining GPC and gas chromatography analysis results. First, GPC analysis was used to determine the content of bicyclic, tricyclic, tetracyclic, pentacyclic, and higher-order substances in the isocyanate sample. The bicyclic content is the sum of the contents of 2,2-MDI, 2,4-MDI, and 4,4-MDI. Then, their actual contents were calculated based on the ratios of 2,2-MDI, 2,4-MDI, and 4,4-MDI obtained from gas chromatography analysis. The specific testing methods for GPC and gas chromatography are as follows:

[0111] GPC Analysis: Isocyanate samples were analyzed by gel permeation chromatography (GPC) with an injection volume of 20 μL. The chromatographic column type was a Styragel HR column, multiple columns in series, the column oven temperature was 40℃, the analysis time was 40 min, and the detector type was a differential refractive index detector (DRC). The characteristic peaks at 20.7 min, 21.4 min, 22.2 min, and 23.9 min in the chromatogram represented five or more rings, four rings, three rings, and two rings, respectively. The mass percentage of each ring was obtained using the external standard method.

[0112] Gas chromatography analysis: Isocyanate samples were analyzed by gas chromatography with an injection volume of 1 μL. A weakly polar column was used. The initial temperature of the column oven was 150℃, held for 0.5 min; the temperature was programmed to increase to 250℃ at 5℃ / min, held for 7 min; and then programmed to increase to 280℃ at 10℃ / min, held for 2 min. An ECD detector was used. The characteristic peaks at 12.1 min, 13.5 min, and 14.7 min 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 were obtained using the internal standard method.

[0113] Preparation Example 1

[0114] (1) Aniline, formaldehyde and hydrochloric acid are mixed in a mass ratio of 1:0.33:0.40 and reacted at a temperature of 80℃.

[0115] (2) The mixture obtained in step (1) is subjected to vacuum to remove aniline and water to obtain a diphenylmethane diamine / polyamine mixture; then, the solvent chlorobenzene is mixed with the diphenylmethane diamine / polyamine mixture in a static mixer at a mass ratio of 4:1 to form a mixed solution.

[0116] (3) The phosgene and the mixed solution from step (2) are mixed in a dynamic mixer at a mass ratio of phosgene: diphenylmethane diamine / polyamine = 4:1. The mixed material is subjected to a cold and hot phosgene reaction. The cold reaction temperature is 65°C and the pressure is 270 kPaG. 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 dephosgene tower to remove phosgene and hydrogen chloride at a temperature of 120°C and a pressure of 45 kPaG. After passing through a desolventizing tower at a temperature of 140°C and a pressure of 50 kPaA to remove chlorobenzene, crude isocyanate A is obtained.

[0118] Analysis revealed that crude isocyanate A contained: 0.4 wt% 2,2-MDI, 5.9 wt% 2,4-MDI, 65.5 wt% 4,4-MDI, 7.4 wt% polyphenyl polymethylene polyisocyanate containing three benzene rings, 4.7 wt% polyphenyl polymethylene polyisocyanate containing four benzene rings, 16 wt% polyphenyl polymethylene polyisocyanate containing five or more benzene rings, and 6486 ppm thermosensitive chlorine.

[0119] Preparation Example 2

[0120] (1) Aniline, formaldehyde and hydrochloric acid are mixed in a mass ratio of 1:0.68:0.22 and reacted at a reaction temperature of 70℃.

[0121] (2) The mixture obtained in step (1) is subjected to vacuum to remove aniline and water to obtain a diphenylmethane diamine / polyamine mixture; then, the solvent chlorobenzene is mixed with the diphenylmethane diamine / polyamine mixture in a static mixer at a mass ratio of 4:1 to form a mixed solution.

[0122] (3) The phosgene and the mixed solution from step (2) are mixed in a dynamic mixer at a mass ratio of phosgene: diphenylmethane diamine / polyamine = 3:1. The mixed material is subjected to a cold and hot phosgene reaction. The cold reaction temperature is 85°C and the pressure is 270 kPaG. 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 dephosgene tower to remove phosgene and hydrogen chloride at a temperature of 150°C and a pressure of 35 kPaG. After passing through a desolventizing tower at a temperature of 180°C and a pressure of 30 kPaA to remove chlorobenzene, crude isocyanate B is obtained.

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

[0125] Preparation Example 3

[0126] (1) Aniline, formaldehyde and hydrochloric acid are mixed in a mass ratio of 1:0.45:0.12 and reacted at a reaction temperature of 50℃.

[0127] (2) The mixture obtained in step (1) is subjected to vacuum to remove aniline and water to obtain a diphenylmethane diamine / polyamine mixture; then, the solvent chlorobenzene is mixed with the diphenylmethane diamine / polyamine mixture in a static mixer at a mass ratio of 4:1 to form a mixed solution.

[0128] (3) The phosgene and the mixed solution from step (2) are mixed in a dynamic mixer at a mass ratio of phosgene: diphenylmethane diamine / polyamine = 2:1. The mixed material is subjected to a cold and hot phosgene reaction. The cold reaction temperature is 115℃ and the pressure is 270kPaG. The hot reactor temperature is 140℃ and the pressure is 270kPaG.

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

[0130] The crude isocyanate C was found to contain: 4.1 wt% 2,2-MDI, 22.3 wt% 2,4-MDI, 35.5 wt% 4,4-MDI, 24.4 wt% polyphenyl polymethylene polyisocyanate containing three benzene rings, 6.8 wt% polyphenyl polymethylene polyisocyanate containing four benzene rings, 6.8 wt% polyphenyl polymethylene polyisocyanate containing five or more benzene rings, and 2909 ppm thermosensitive chlorine.

[0131] Example 1

[0132] S1: A portion of the crude isocyanate A prepared in Preparation Example 1 is fed into distillation column A. Distillation column A has 7 theoretical plates and two large packing layers. A liquid distributor is installed above each large packing layer. The feed position in the column is above the liquid distributor of the second large packing layer (counting from top to bottom). The number of theoretical plates between the feed position and the bottom of the column is 7. The feed ratio program in the column is started to control the 2,4-MDI content in the polyisocyanate product taken from the bottom of the column. At the same time, the program to control the thermosensitive chlorine content 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 24 hours, and the cumulative alarm value Δm% of the feed ratio in the column is 5%.

[0133] S2: Distillation column A is set to feed simultaneously to the liquid distributor and the reboiler, with the crude isocyanate feed temperature set at 251℃, the reboiler liquid percentage at 47%, the top reflux flow rate at 15t / h, and the top condenser temperature at 40℃. Ultimately, the distillation column remains stable under the following process conditions: a liquid distributor feed percentage (i.e., feed percentage to the column body) of 11%, a reboiler temperature of 264℃, a top absolute pressure of 2.2kPa, and a top reflux material temperature of 44℃. The content of 2,4-MDI in the polyisocyanate product collected from the reboiler was detected to be 3.1wt%, and the content of thermosensitive chlorine in the crude MDI product was 271ppm. The free chlorine content of the MDI product obtained after distillation of the crude MDI was 1.4ppm.

[0134] Example 2

[0135] S1: A portion of the crude isocyanate B prepared in Preparation Example 2 is fed into distillation column B. Distillation column B has 13 theoretical plates and three layers of packing. A liquid distributor is installed above each layer of packing. The feed position in the column is above the liquid distributor of the second layer of packing (counting from top to bottom). The number of theoretical plates between the feed position and the bottom of the column is 10. The feed ratio program of the column body is started to control the content of 2,4-MDI in the polyisocyanate product taken from the bottom of the column. At the same time, the program to control the thermosensitive chlorine content in the crude MDI product is started. In the program, the target value of 2,4-MDI content in the polyisocyanate product is set to 4.0%, the sampling interval of crude MDI product is 48 hours, and the cumulative alarm value Δm% of the feed ratio in the column body is 10%.

[0136] S2: Distillation column B is set to feed simultaneously to the liquid distributor and the reboiler, with the crude isocyanate feed temperature set at 160℃, the reboiler liquid percentage at 93%, the top reflux flow rate at 12 t / h, and the top condenser temperature at 40℃. Ultimately, the distillation column remains stable under the following process conditions: a liquid distributor feed percentage (i.e., feed percentage to the column body) of 43%, a reboiler temperature of 183℃, a top absolute pressure of 0.35 kPa, and a top reflux material temperature of 92℃. The content of 2,4-MDI in the polyisocyanate product collected from the reboiler was detected to be 4.3 wt%, and the content of thermosensitive chlorine in the crude MDI product was 259 ppm. The free chlorine content of the MDI product obtained after distillation of the crude MDI was 1.3 ppm.

[0137] Example 3

[0138] S1: A portion of the crude isocyanate C prepared in Preparation Example 3 is fed into distillation column C. Distillation column C has 25 theoretical plates and 5 packing layers. A liquid distributor is installed above each packing layer. The feed position in the column is above the liquid distributor of the 3rd packing layer (counting from top to bottom). The number of theoretical plates between the feed position and the bottom of the column is 18. The feed ratio program in the column is started to control the 2,4-MDI content in the polyisocyanate product taken from the bottom of the column. At the same time, the program to control the thermosensitive chlorine content 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 48 hours, and the cumulative alarm value Δm% of the feed ratio in the column is 20%.

[0139] S2: Distillation column C is set to feed simultaneously to the liquid distributor and the reboiler, with the crude isocyanate feed temperature set at 200℃, the reboiler liquid percentage at 64%, the top reflux flow rate at 10t / h, and the top condenser temperature at 40℃. Ultimately, the distillation column remains stable under the following process conditions: a liquid distributor feed percentage (i.e., feed percentage to the column body) of 69%, a reboiler temperature of 225℃, a top absolute pressure of 1.0kPa, and a top reflux material temperature of 145℃. The content of 2,4-MDI in the polyisocyanate product collected from the reboiler was detected to be 3.9wt%, and the content of thermosensitive chlorine in the crude MDI product was 234ppm. The free chlorine content of the MDI product obtained after distillation of the crude MDI was 1.1ppm.

[0140] Example 4

[0141] S1: A portion of the crude isocyanate B prepared in Preparation Example 2 is fed into distillation column C. Distillation column C has 25 theoretical plates and 5 packing layers. A liquid distributor is installed above each packing layer. The feed position in the column is above the liquid distributor of the 3rd packing layer (counting from top to bottom). The number of theoretical plates between the feed position and the bottom of the column is 18. The feed ratio program in the column is started to control the 2,4-MDI content in the polyisocyanate product taken from the bottom of the column. At the same time, the program to control the thermosensitive chlorine content 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 24 hours, and the cumulative alarm value Δm% of the feed ratio in the column is 20%.

[0142] S2: Distillation column C is set to feed simultaneously to the liquid distributor and the reboiler, with the crude isocyanate feed temperature set at 205℃, the reboiler liquid percentage at 93%, the top reflux flow rate at 12 t / h, and the top condenser temperature at 40℃. Ultimately, the distillation column remains stable under the following process conditions: a liquid distributor feed percentage (i.e., feed percentage to the column body) of 16%, a reboiler temperature of 219℃, a top absolute pressure of 0.9 kPa, and a top reflux material temperature of 95℃. The content of 2,4-MDI in the polyisocyanate product collected from the reboiler was detected to be 4.1 wt%, and the content of thermosensitive chlorine in the crude MDI product was 216 ppm. The free chlorine content of the MDI product obtained after distillation of the crude MDI was 1.0 ppm.

[0143] Example 5

[0144] S1: A portion of the crude isocyanate C prepared in Preparation Example 3 is fed into distillation column B. Distillation column B has 13 theoretical plates and three packing layers. A liquid distributor is installed above each packing layer. The feed position in the column is above the liquid distributor of the second packing layer (counting from top to bottom). The number of theoretical plates between the feed position and the bottom of the column is 10. The feed ratio program in the column is started to control the 2,4-MDI content in the polyisocyanate product taken from the bottom of the column. At the same time, the program to control the thermosensitive chlorine content 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 48 hours, and the cumulative alarm value Δm% of the feed ratio in the column is 10%.

[0145] S2: Distillation column B is set to feed simultaneously to the liquid distributor and the reboiler, with the crude isocyanate feed temperature set at 160℃, the reboiler liquid percentage at 64%, the top reflux flow rate at 15t / h, and the top condenser temperature at 40℃. Ultimately, the distillation column remains stable under the following process conditions: a liquid distributor feed percentage (i.e., feed percentage to the column body) of 91%, a reboiler temperature of 178℃, a top absolute pressure of 0.35kPa, and a top reflux material temperature of 42℃. The content of 2,4-MDI in the polyisocyanate product collected from the reboiler was detected to be 4.2wt%, and the content of thermosensitive chlorine in the crude MDI product was 266ppm. The free chlorine content of the MDI product obtained after distillation of the crude MDI was 1.3ppm.

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

[0147] Table 1

[0148]

[0149] As can be seen from the results in Table 1, by adjusting Formulas 1 and 2, the thermosensitive chlorine content of crude MDI can be controlled within the range of 180–330 ppm, thereby achieving controllable activity of the MDI product.

[0150] Comparative Example 1

[0151] A portion of the crude isocyanate A prepared in Preparation Example 1 was fed into distillation column A. The feed location was the reboiler at the bottom of the column. Distillation column A had 7 theoretical plates and 2 large packing layers. Separation was carried out under the following process conditions: crude isocyanate feed temperature of 251°C, bottom temperature of 263°C, absolute pressure at the top of the column of 2.1 kPa, bottom liquid recovery rate of 47%, condenser temperature at the top of the column of 80°C, reflux inlet temperature at the top of the column of 80°C, and reflux flow rate at the top of the column of 5 t / h. Polyisocyanate product and crude MDI were obtained.

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

[0153] Comparative Example 2

[0154] Part of the crude isocyanate B prepared in Preparation Example 2 was fed into distillation column B. The feed location was the reboiler at the bottom of the column. Distillation column B had 13 theoretical plates and 3 packing layers. The separation was carried out under the following process conditions: crude isocyanate feed temperature of 160°C, bottom temperature of 179°C, absolute pressure at the top of the column of 0.3 kPa, bottom liquid recovery rate of 93%, condenser temperature at the top of the column of 50°C, reflux inlet temperature at the top of the column of 50°C, and reflux flow rate at the top of the column of 5 t / h. Polyisocyanate product and crude MDI were obtained.

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

[0156] Comparative Example 3

[0157] Part of the crude isocyanate C prepared in Preparation Example 3 was fed into distillation column C. The feed location was the reboiler at the bottom of the column. The theoretical number of plates in distillation column C was 25, and the packing consisted of 5 layers. Separation was carried out under the following process conditions: crude isocyanate feed temperature of 200°C, bottom temperature of 222°C, absolute pressure at the top of the column of 0.9 kPa, bottom liquid recovery rate of 64%, condenser temperature at the top of the column of 70°C, reflux inlet temperature at the top of the column of 70°C, and reflux flow rate at the top of the column of 5 t / h. Polyisocyanate product and crude MDI were obtained.

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

[0159] Comparative Example 4

[0160] Part of the crude isocyanate B prepared in Preparation Example 2 was fed into distillation column C. The feed location was the reboiler at the bottom of the column. Distillation column C had 25 theoretical plates and 5 packing layers. Separation was carried out under the following process conditions: crude isocyanate feed temperature of 205°C, bottom temperature of 218°C, absolute pressure at the top of the column of 0.9 kPa, bottom liquid recovery rate of 93%, condenser temperature at the top of the column of 60°C, reflux inlet temperature at the top of the column of 60°C, and reflux flow rate at the top of the column of 5 t / h. Polyisocyanate product and crude MDI were obtained.

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

[0162] Comparative Example 5

[0163] Part of the crude isocyanate C prepared in Preparation Example 3 was fed into distillation column B. The feed location was the reboiler at the bottom of the column. Distillation column B had 13 theoretical plates and 3 packing layers. The separation was carried out under the following process conditions: crude isocyanate feed temperature of 160°C, bottom temperature of 173°C, absolute pressure at the top of the column of 0.3 kPa, bottom liquid recovery rate of 64%, condenser temperature at the top of the column of 50°C, reflux inlet temperature at the top of the column of 50°C, and reflux flow rate at the top of the column of 5 t / h. Polyisocyanate product and crude MDI were obtained.

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

[0165] The crude isocyanate A prepared in Preparation Example 1 contained 5.9 wt% 2,4-MDI, while the crude isocyanate C prepared in Preparation Example 3 contained 22.3 wt% 2,4-MDI. A comparison shows a significant difference in the 2,4-MDI content between the two crude isocyanates. Examples 1, 3, and 5 used crude isocyanate A, crude isocyanate C, and crude isocyanate C, respectively, as raw materials for separation treatment. The results showed that the 2,4-MDI content of the polyisocyanate products obtained in the three examples remained in the range of 2–5 wt%, and the thermosensitive chlorine content of the crude MDI products remained in the range of 180–330 ppm. Therefore, the separation method of the present invention can be used for the efficient separation of crude isocyanates with significantly different compositions.

[0166] The separation methods used in Comparative Examples 1 to 5 only involved feeding the material into the bottom of a tower. According to the test results, Comparative Examples 1 to 5 could not simultaneously achieve effective control of the 2,4-MDI content in the polyisocyanate product and the thermosensitive chlorine content in the crude MDI product. Either the 2,4-MDI content in the polyisocyanate product was too high (above 2-5 wt%), or the thermosensitive chlorine content in the crude MDI product was too high / too low (above / below 180-330 ppm), or both deviated from the required range.

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

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

Claims

1. A method for separating crude isocyanate, comprising processing the crude isocyanate through a distillation column to obtain polyisocyanate product and crude MDI product; wherein, The distillation column includes a reboiler, a top, and a column body connecting the reboiler and the top; the crude isocyanate is fed into the distillation column from the reboiler and the column body.

2. The separation method according to claim 1, wherein, 1-99 wt% of the crude isocyanate is fed from the bottom of the column, and 1-99 wt% of the crude isocyanate is fed from the body of the column; and / or, The crude isocyanate comprises 0–5 wt% 2,2-MDI, 3–30 wt% 2,4-MDI, 30–80 wt% 4,4-MDI, 5–30 wt% polyphenyl polymethylene polyisocyanate containing three benzene rings, 0.5–20 wt% polyphenyl polymethylene polyisocyanate containing four benzene rings, and 5–30 wt% polyphenyl polymethylene polyisocyanate containing five or more benzene rings; and / or, The theoretical plate number of the distillation column is 1 to 30; and / or, The distillation column is provided with 1 to 8 layers of packing, and a liquid distributor is provided on one or more layers of packing; and / or, A reboiler connected to the reboiler is provided outside the distillation column; and / or, The temperature of the reflux material at the top of the tower is 40–200°C; and / or, The temperature of the reboiler 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 column, and 30–70 wt% of the crude isocyanate is fed from the body of the column; and / or, The crude isocyanate comprises 0–3 wt% 2,2-MDI, 5–20 wt% 2,4-MDI, 40–70 wt% 4,4-MDI, 10–25 wt% polyphenylene polyisocyanate containing three benzene rings, 1.5–15 wt% polyphenylene polyisocyanate containing four benzene rings, and 10–25 wt% polyphenylene polyisocyanate containing five or more benzene rings; and / or, The theoretical plate number of the distillation column is 5 to 20; and / or, The distillation column is equipped with 2 to 5 layers of packing.

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

5. The separation method according to claim 4, wherein, The theoretical number of trays between the feed position in the column body and the reboiler is less than 15; based on the total mass of the crude isocyanate, the 2,4-MDI content is less than 15%, 10-60 wt% of the crude isocyanate is fed through the column body, and the remaining crude isocyanate is fed through the reboiler; or... The crude isocyanate, with a mass content of 2,4-MDI of 15% or more, is fed through the tower body at 70-99 wt%, while the remaining crude isocyanate is fed through the reboiler.

6. The separation method according to claim 4, wherein, The theoretical number of trays between the feed position in the column body and the reboiler is 15 or more; based on the total mass of the crude isocyanate, the 2,4-MDI mass content is less than 15%, 1-30 wt% of the crude isocyanate is fed through the column body, and the remaining crude isocyanate is fed through the reboiler; or... The crude isocyanate, comprising 40-90 wt% of 2,4-MDI and 45-90 wt%, is fed through the tower body, while the remaining crude isocyanate is fed through the reboiler.

7. The separation method according to claim 4, wherein, The crude MDI contains thermosensitive chlorine, and the content of the thermosensitive chlorine is equal to the total chlorine content, the free chlorine content, the chlorine content of the solvent, and the chlorine content of the chlorinated MDI. The crude MDI product collected from the top of the column is sampled periodically, and its thermosensitive chlorine content (m1) is measured. The temperature of the reflux material at the top of the column is adjusted according to the thermosensitive chlorine content. The temperature adjustment value is... T1 is: T1 = 80 - 10.716 * (m1 * 10 6 ) 0.3653 (Official 1).

8. The separation method according to claim 7, wherein, Before and after sampling the crude MDI, the change in the feed percentage in the tower exceeded the alarm threshold. When m%, the temperature of the reflux material at the top of the column is adjusted according to the following formula, where the temperature adjustment value is... T2 is: T2 = -2 * Change in the feed ratio of the tower body (Formula 2); The feed ratio of the column body is the percentage of crude isocyanate fed from the column body to the total mass of crude isocyanate entering the distillation column, and the change in the feed ratio of the column body is the difference between the real-time feed ratio A2 of the column body and the initial feed ratio A1. m% is 1–50%; and / or, The sampling time interval for the crude MDI is 4–168 h; and / or, m1*10 6 The value is 180–330; and / or, The free chlorine content refers to the chloride ion content.

9. An automatic control method for separating crude isocyanate according to any one of claims 1 to 8, comprising the following steps: (1) Samples of crude MDI discharged from the top of the distillation column were taken for analysis; (2) After sampling, record the initial feed ratio A1 of crude isocyanate in the tower body and record the initial time t1; (3) Obtain the thermosensitive chlorine content m1 in the crude MDI obtained from sampling analysis, and adjust the temperature of the reflux material at the top of the column according to Formula 1 based on the thermosensitive chlorine content. The temperature adjustment value is: T1; (4) When the difference between the current time t2 and the initial time t1 is greater than the sampling interval time, the sampling is performed again and the operations of steps (1) to (3) are repeated. in, Formula 1 is: T1 = 80 - 10.716 * (m1 * 10) 6 ) 0.3653 .

10. The method according to claim 9, wherein, The difference between the real-time feed ratio A2 and the initial feed ratio A1 in the tower body is greater than the alarm value. When m%, the temperature of the reflux material at the top of the column is adjusted according to Formula 2, and the adjustment value is: T2, and use the feed ratio A2 of the tower body at that time as the initial feed ratio A1 for the next adjustment to recalculate the difference; when the difference between the current time t2 and the initial time t1 is greater than the sampling interval, sampling is performed again; Formula 2 is: T2 = -2 * Change in the feed ratio of the tower body; The feed percentage of the column body is the percentage of crude isocyanate fed from the column body to the total mass of crude isocyanate entering the distillation column. The change in the feed percentage of the column body is the difference between the real-time feed percentage A2 of the column body and the initial feed percentage A1.

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