Method for preparing isocyanate

Abstract

The present invention provides a method for preparing an isocyanate. The preparation of an isocyanate by using the method of the present invention facilitates cost reduction. The method comprises the following steps: (1) subjecting an amine compound, a solvent stream I, and a phosgene solution to a cold phosgenation reaction to obtain a first mixed solution; (2) feeding the first mixed solution into a thermal reactor for heating to carry out a hot phosgenation reaction; (3) removing hydrogen chloride and phosgene from a second mixed solution to obtain a third mixed solution and a second gas mixture containing hydrogen chloride and phosgene; washing and absorbing a first gas mixture and the second gas mixture with a solvent to obtain a phosgene solution; feeding the phosgene solution into the cold reactor in step (1); and (4) subjecting the third mixed solution to a solvent removal treatment to obtain a crude isocyanate product. In addition, the method is controlled to be carried out under the conditions of A1=5 to 55 and A2=-15 to 50.

Description

A method for preparing isocyanates Technical Field

[0001] This invention relates to the field of isocyanate preparation technology, and more specifically to a method for preparing isocyanates that reduces costs. Background Technology

[0002] Isocyanates, as organic reaction intermediates, are widely used in various industries such as industry, agriculture, construction, automobiles, and insulation because they can be further synthesized into materials such as polyisocyanates, polyurethanes, polyureas, and spandex. Currently, the mainstream industrial method for synthesizing isocyanates is the phosgenation method, which involves mixing the amine corresponding to the target product with an inert solvent and then reacting it with a phosgene solution to obtain a phosgenation reaction solution. The phosgenation reaction solution contains isocyanate products, inert solvents, residual phosgene, and the reaction product hydrogen chloride. In a subsequent separation process, phosgene, hydrogen chloride, and solvent are removed sequentially to finally obtain the isocyanate product.

[0003] The core of this method lies in dividing the reaction process into two stages: cold and hot. The principle is as follows:

[0004] Cold reaction stage:

[0005] RNH₂ + COCl₂ → RNHCOCl (carbamoyl chloride) + HCl

[0006] RNH2 + HCl → RNH2·HCl (amino hydrochloride)

[0007] Thermal reaction stage:

[0008] RNHCOCl→RNCO+HCl

[0009] RNH2·HCl + COCl2 → RNCO + 3HCl

[0010] The cold reaction stage is relatively fast and the reaction is completed quickly; however, the hot reaction stage is characterized by the following side reactions:

[0011] RNH2·HCl→RNH2+HCl

[0012] RNH2 + RNCO → RNHCONHR (urea)

[0013] The presence of urea, a byproduct, can catalyze product polymerization, leading to equipment and pipeline blockage and affecting isocyanate yield. Therefore, its formation must be strictly controlled. Current technologies generally achieve high product selectivity by using a significantly excess phosgene to reduce urea formation. However, this excess phosgene needs to be recycled. Hydrogen chloride in the phosgene recovered through condensation and absorption methods cannot be effectively removed. Excessive hydrogen chloride content in the recycled phosgene leads to the formation of amine hydrochloride during amine phosgenation. Phosgenation of amine hydrochloride requires a long reaction residence time and a high excess phosgene, and it is prone to reacting with isocyanates to produce urea, severely impacting the quality of the phosgenation reaction solution and product yield. Furthermore, a high excess phosgene rate means higher energy consumption for recycling.

[0014] Chinese patent application CN104892458A discloses a method for preparing isocyanates by reactive distillation, which describes that the phosgenation reaction is carried out in a distillation column, and excess phosgene and hydrogen chloride are separated by an external distillation column. This method effectively reduces the hydrogen chloride content in the circulating phosgene solution, thereby reducing the formation of amino hydrochloride and the probability of urea formation. However, it consumes a large amount of steam and cooling energy, and hydrogen chloride is difficult to separate from phosgene under high pressure, resulting in high process operating costs.

[0015] There are no existing solutions that take a holistic approach to the process, considering how to integrate and optimize the isocyanate preparation process to achieve good control over process operation and maintenance costs and maximize profits.

[0016] Summary of the Invention

[0017] This invention provides a method for preparing isocyanates. The method of this invention facilitates the control of process operation and maintenance costs, and helps to reduce costs.

[0018] One embodiment of the present invention provides a method for preparing isocyanate, comprising the following steps:

[0019] To achieve its objective, the present invention provides the following technical solution:

[0020] This invention provides a method for preparing isocyanates, the method comprising the following steps:

[0021] (1) The amine compound, solvent stream I and phosgene solution are fed into a cold reactor to carry out a cold phosgene reaction to obtain the first mixture;

[0022] In step (1), the mass ratio of the solvent in solvent stream I to the amine compound is a, the mass ratio of pure phosgene in the phosgene solution to the amine compound is b, and the cold phosgene reaction is carried out at a temperature of c℃ and a pressure of d MPaG.

[0023] (2) The first mixture is fed into a thermal reactor and heated to carry out a thermo-phosgene reaction to obtain a second mixture containing isocyanate and a first gas mixture containing hydrogen chloride and phosgene.

[0024] In step (2), the thermophotogasification reaction is carried out at a temperature of e℃ and a pressure of g MPaG, and the residence time of the material in the thermophotogasification reaction system in the thermal reactor is f hours.

[0025] (3) The second mixture is subjected to treatment to remove hydrogen chloride and phosgene at a temperature of h℃ and a pressure of i MPaG to obtain a third mixture and a second gas mixture containing hydrogen chloride and phosgene.

[0026] The first gas mixture and the second gas mixture are washed and absorbed with a solvent to obtain the phosgene solution at a temperature of j℃ and the washed hydrogen chloride; the phosgene solution is then fed into the cold reactor of step (1);

[0027] (4) The third mixture is subjected to solvent removal treatment at a temperature of k℃ and a pressure of p MPaG to obtain crude isocyanate;

[0028] Meanwhile, the method is controlled to operate under the conditions of A1 = 5 to 55 and A2 = -15 to 50; A1 is the cost characteristic index, which is dimensionless; A2 is the operational characteristic index, which is dimensionless; the calculation formulas for A1 and A2 are as follows:

[0029] A1 = 0.928 × a 5 +0.239×b 2 -0.033×c-0.214×d 2 +0.022×e+0.061×f 2 +5.78×g 2 +0.0135×h+85×i 2 -0.032×j+0.027×k+99.17×p 2

[0030] A2 = 0.485 × a 2 +0.852×b 2 -0.042×c-0.83×d 2 -0.052×e+0.192×f 2 -3.016×g 2 -0.026×h-153×i 2 +0.097×j-0.011×k+10.09×p 2 .

[0031] Further, in step (1), the mass ratio a of the solvent in the solvent stream I to the amine compound is 0.5 to 5, the mass ratio b of the pure phosgene in the phosgene solution to the amine compound is 1 to 10, the temperature c of the cold phosgene reaction is 50-180℃, and the pressure d is 0.1-5MPaG.

[0032] Further, in step (2), the temperature e℃ of the thermophotogasification reaction is 50-200℃, the pressure g MPaG is 0.1-1MPaG, and the residence time f hours of the material in the thermophotogasification reaction system in the thermal reactor is 0.5-12 hours.

[0033] Further, in step (3), the temperature h°C for removing hydrogen chloride and phosgene is 100-200°C, and the pressure i MPaG is -0.05 MPaG to 0.20 MPaG; the temperature j°C of the phosgene solution obtained by washing and absorbing the first gas mixture and the second gas mixture with a solvent is -50°C to 150°C.

[0034] Further, in step (4), the temperature k℃ for solvent removal is 100-250℃ and the pressure p MPaG is -0.1MPaG to 0.1MPaG.

[0035] Preferably, A1 is 10 to 20, and A2 is -10 to 10.

[0036] In some embodiments, the amine compound includes one or more of diphenylmethane diamine, polymethylene polyphenyl polyamine, diaminotoluene, isophorone diamine, hexamethylenediamine, cyclohexanediamine, p-phenylenediamine, and naphthyldiamine.

[0037] In some embodiments, the solvent in the solvent stream I in step (1) and the solvent used for washing and absorption in step (3) respectively include one or more of chlorobenzene, dichlorobenzene, trichlorobenzene, toluene, xylene, benzene, and diethyl isophthalate, preferably chlorobenzene and / or o-dichlorobenzene;

[0038] Preferably, the solvent in the solvent stream I in step (1) is the same as the solvent used for the washing and absorption in step (3).

[0039] In some embodiments, in step (3), the phosgene solution obtained by washing and absorption has a phosgene mass percentage content of 50-90%.

[0040] In some embodiments, the cold reactor in step (1) is a jet reactor or a high-shear reactor;

[0041] In some embodiments, the thermal reactor in step (2) is a batch reactor or a tower reactor;

[0042] In some embodiments, the treatment to remove hydrogen chloride and phosgene described in step (3) or the solvent removal treatment described in step (4) are carried out in a distillation column.

[0043] The technical solution provided by this invention has the following beneficial effects:

[0044] The isocyanate preparation method provided by this invention improves the operational flexibility of the isocyanate production process by controlling the characteristic indices A1 and A2. It has advantages such as simple process, convenient operation, energy saving, and low cost, which is conducive to the control of process operation and maintenance costs. Attached Figure Description

[0045] Figure 1 is a schematic diagram of the process system for isocyanates in some embodiments. Detailed Implementation

[0046] To facilitate understanding of the present invention, the following description, in conjunction with embodiments, will further illustrate the invention. It should be understood that the following embodiments are merely for a better understanding of the invention and do not imply that the invention is limited to these embodiments.

[0047] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The term "and / or" as used herein may include any and all combinations of one or more of the associated listed items. The terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0048] This invention provides a method for preparing isocyanates, the method comprising the following steps:

[0049] (1) The amine compound, solvent stream I and phosgene solution are fed into a cold reactor to carry out a cold phosgene reaction to obtain the first mixture;

[0050] In step (1), the mass ratio of the solvent in solvent stream I to the amine compound is a, the mass ratio of pure phosgene in the phosgene solution to the amine compound is b, and the cold phosgene reaction is carried out at a temperature of c℃ and a pressure of d MPaG.

[0051] (2) The first mixture is fed into a thermal reactor and heated to carry out a thermo-phosgene reaction to obtain a second mixture containing isocyanate and a first gas mixture containing hydrogen chloride and phosgene.

[0052] In step (2), the thermophotogasification reaction is carried out at a temperature of e℃ and a pressure of g MPaG, and the residence time of the material in the thermophotogasification reaction system in the thermal reactor is f hours.

[0053] (3) The second mixture is subjected to treatment to remove hydrogen chloride and phosgene at a temperature of h℃ and a pressure of i MPaG to obtain a third mixture and a second gas mixture containing hydrogen chloride and phosgene.

[0054] The first gas mixture and the second gas mixture are washed and absorbed with a solvent to obtain the phosgene solution at a temperature of j℃ and the washed hydrogen chloride; the phosgene solution is then fed into the cold reactor of step (1);

[0055] (4) The third mixture is subjected to solvent removal treatment at a temperature of k℃ and a pressure of p MPaG to obtain crude isocyanate;

[0056] Meanwhile, the method is controlled to operate under the conditions of A1 = 5 to 55 and A2 = -15 to 50; A1 is the cost characteristic index, which is dimensionless; A2 is the operational characteristic index, which is dimensionless; the calculation formulas for A1 and A2 are as follows:

[0057] A1 = 0.928 × a 2 +0.239×b 2 -0.033×c-0.214×d 2 +0.022×e+0.061×f 2 +5.78×g 2 +0.0135×h+85×i 2 -0.032×j+0.027×k+99.17×p 2

[0058] A2 = 0.485 × a 2 +0.852×b 2 -0.042×c-0.83×d 2 -0.052×e+0.192×f 2 -3.016×g 2 -0.026×h-153×i 2 +0.097×j-0.011×k+10.09×p 2 .

[0059] Further, in step (1), the mass ratio a of the solvent in the solvent stream I to the amine compound is 0.5 to 5 (e.g., 1, 2, 3, 4, etc.), the mass ratio b of the pure phosgene in the phosgene solution to the amine compound is 1 to 10 (e.g., 2, 4, 6, 8, etc.), the temperature c℃ of the cold phosgene reaction is 50-180℃ (e.g., 70℃, 90℃, 110℃, 130℃, 150℃, 170℃, etc.), and the pressure d MPaG is 0.1-5 MPaG (e.g., 1 MPaG, 2 MPaG, 3 MPaG, 4 MPaG, etc.).

[0060] In step (2), the temperature e℃ of the thermophotogasification reaction is 50-200℃ (e.g., 80℃, 100℃, 120℃, 140℃, 160℃, 180℃, etc.), the pressure g MPaG is 0.1-1MPaG, and the residence time f hours of the material in the thermophotogasification reaction system in the thermal reactor is 0.5-12 hours.

[0061] In step (3), the temperature h°C for removing hydrogen chloride and phosgene is 100-200°C (e.g., 120°C, 140°C, 160°C, 180°C, etc.), and the pressure i MPaG is -0.05 MPaG to 0.20 MPaG (e.g., 0 MPaG, 0.05 MPaG, 0.10 MPaG, 0.15 MPaG, etc.); the temperature j°C of the phosgene solution obtained by washing and absorbing the first gas mixture and the second gas mixture with a solvent is -50°C to 150°C (e.g., -30°C, 0°C, 30°C, 60°C, 90°C, 120°C, etc.).

[0062] In step (4), the temperature k℃ for solvent removal is 100-250℃ (e.g., 120℃, 140℃, 160℃, 180℃, 200℃, 220℃, 240℃, etc.), and the pressure p MPaG is -0.1MPaG to 0.1MPaG (e.g., -0.1MPaG, 0MPaG, 0.1MPaG, etc.).

[0063] The method for preparing isocyanates according to the present invention is carried out under the above-mentioned process conditions. By adjusting the above-mentioned process parameters a, b, c, d, e, f, g, h, i, j, k, and p, and making the cost characteristic index A1 within the range of 5 to 55 and the operating characteristic index A2 within the range of -15 to 50, the preparation of isocyanates can be carried out effectively, thereby reducing operating and maintenance costs, while also ensuring relatively good process stability.

[0064] In this invention, A1 is 5 to 55, for example 5, 10, 12, 14, 16, 18, 20, 25, 30, 35, 40, 45, 50, 55, etc., preferably 10 to 20; A2 is -15 to 50, for example -15, -10, -6, -2, 2, 6, 8, 10, 15, 20, 25, 30, 35, 40, 45, 50, etc., preferably -10 to 10.

[0065] In some embodiments, the amine compound includes one or more of diaminodiphenylmethane, polymethylene polyphenyl polyamine, diaminotoluene, isophorone diamine, hexamethylenediamine, cyclohexanediamine, p-phenylenediamine, and naphthyldiamine.

[0066] In some embodiments, the solvent in solvent stream I in step (1) and the solvent used for washing and absorption in step (3) respectively include one or more of chlorobenzene, dichlorobenzene, trichlorobenzene, toluene, xylene, benzene, and diethyl isophthalate, preferably chlorobenzene and / or o-dichlorobenzene. Preferably, the solvent in solvent stream I in step (1) and the solvent used for washing and absorption in step (3) are the same.

[0067] In one embodiment, an amine compound, a phosgene solution, and solvent stream I are mixed and subjected to a phosgenation reaction to obtain a second mixture containing isocyanate and a first gaseous mixture containing phosgene and hydrogen chloride. The solvent in the phosgene solution can be the same as the solvent from solvent stream I in the phosgenation reaction system.

[0068] In some embodiments, in step (3), the phosgene solution obtained by the washing and absorption process has a phosgene mass percentage of 50-90%, such as 60%, 70%, 80%, etc. In step (3), the first gas mixture and the second gas mixture can be washed and absorbed with a solvent in a phosgene washing and absorption tower, and the resulting phosgene solution is sent to the cold reactor in step (1), that is, the phosgene solution in the cold reactor is provided by the phosgene washing and absorption tower; wherein, the phosgene washing and absorption tower can be a plate tower, a packed tower, etc.

[0069] In some embodiments, the cold reactor in step (1) can be a jet reactor or a high-shear reactor; the hot reactor in step (2) can be a batch reactor or a tower reactor. The treatment to remove hydrogen chloride and phosgene in step (3) or the solvent removal treatment in step (4) can be carried out in a distillation column; specifically, the treatment to remove hydrogen chloride and phosgene in step (3) can be carried out in a dephosgene tower, such as a plate tower or a packed tower; the solvent removal treatment in step (4) can be carried out in a solvent removal tower, such as a plate tower or a packed tower.

[0070] The inventors obtained two characteristic indices, A1 and A2, by studying the coupling effect of parameters related to production costs and operating cycles in the isocyanate production process. By adjusting the process parameters, the characteristic indices can be controlled within a certain range, which can ensure low operating costs. Moreover, while ensuring low operating costs, relatively good process operation stability can also be taken into account, thereby maximizing profits.

[0071] The isocyanate preparation method provided by this invention improves the operational flexibility of the isocyanate production process by controlling the characteristic indices A1 and A2, and has the advantages of simple process, convenient operation, energy saving and low cost.

[0072] The technical solution of the present invention will be further illustrated by the following embodiments, but it should not be construed as the present invention being limited to these embodiments.

[0073] Where specific experimental steps or conditions are not specified in the examples, the corresponding conventional experimental steps or conditions in this technical field can be followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0074] The schematic diagram of the process system for isocyanate preparation in the following examples is shown in Figure 1. The process system includes a cold reactor 1 (using a jet reactor), a hot reactor 2 (using a batch reactor), a phosgene removal tower 3 (a plate tower), a solvent removal tower 4 (a plate tower), and a phosgene scrubbing and absorption tower 5 (a plate tower).

[0075] In the embodiments and comparative examples, when the system has been running for a long period of time and the system becomes severely scaled and clogged, resulting in reduced heat exchange and transport efficiency and failing to meet normal operating requirements, it is necessary to shut down the system for cleaning and maintenance. In all embodiments and comparative examples, the time other than the cumulative maintenance time during the three-year operation period is considered as stable operating time.

[0076] Operating and maintenance costs include utility costs during operation and equipment replacement, repair, and labor costs during maintenance. Specifically, average operating and maintenance cost = operating cost + maintenance cost, where:

[0077] Operating cost = Three years of utility costs (including steam, electricity, circulating water, chilled water, etc.) / Three years of crude isocyanate production, in yuan / t.

[0078] In all embodiments and comparative examples, the unit prices of steam, electricity, circulating water, and chilled water are calculated according to a unified standard in the calculation of operation and maintenance costs.

[0079] Maintenance cost = all maintenance expenses within three years / crude isocyanate production within three years, in yuan / t.

[0080] Example 1

[0081] (1) Diphenylmethane diamine (amine compound) with a flow rate of 30 t / h and chlorobenzene (solvent stream I) with a flow rate of 48 t / h are mixed and then introduced into cold reactor 1. At the same time, phosgene solution is introduced into cold reactor 1. The flow rate of phosgene solution is 100 t / h and the mass percentage of phosgene in phosgene solution is 90%. Diphenylmethane diamine and phosgene undergo cold phosgenation reaction in cold reactor 1 to obtain the first mixture.

[0082] (2) The first mixture is then passed into the thermal reactor 2 for a thermo-phosgene reaction to obtain a second mixture containing isocyanate and a first gas mixture containing phosgene and hydrogen chloride.

[0083] (3) The second mixture is passed into the dephosgene tower 3 to remove hydrogen chloride and phosgene, and a third mixture is obtained in the bottom of the tower and a second gas mixture containing hydrogen chloride and phosgene is obtained. The second gas mixture is discharged from the top of the tower and enters the phosgene scrubbing and absorption tower 5 together with the first gas mixture. In the phosgene scrubbing and absorption tower 5, the solvent chlorobenzene is passed into the first and second gas mixtures to scrub and absorb the phosgene, and a phosgene solution is obtained. Hydrogen chloride is collected from the top of the tower. The phosgene solution is sent to the cold reactor 1 in step (1) to participate in the cold phosgene reaction.

[0084] (4) Pass the third mixture into solvent removal tower 4 to remove chlorobenzene, collect the solution in the bottom of the tower to obtain crude isocyanate; discharge the removed chlorobenzene from the top of the tower.

[0085] In step (1), the mass ratio a of solvent chlorobenzene to amine compounds in solvent stream I is 1.6, the mass ratio b of pure phosgene to amine compounds in phosgene solution is 3, the temperature of cold phosgene reaction is 100℃ (i.e., c = 100), and the pressure of cold phosgene reaction is 3 MPaG (i.e., d = 3).

[0086] In step (2), the temperature of the thermophotogasification reaction is 160℃ (i.e., e = 160), the residence time of the material in the thermophotogasification reaction system in the reaction vessel is 4 hours (i.e., f = 4), and the pressure of the thermophotogasification reaction is 0.6 MPaG (i.e., g = 0.6).

[0087] In step (3), the temperature condition for removing hydrogen chloride and phosgene is 135℃ (i.e., h = 135), the pressure condition for removing hydrogen chloride and phosgene is 0.02 MPaG (i.e., i = 0.02), and the temperature of the phosgene solution obtained by solvent washing and absorption of the first gas mixture and the second gas mixture in the phosgene washing and absorption tower 5 is 80℃ (i.e., j = 80).

[0088] In step (4), the temperature condition for removing chlorobenzene solvent in solvent removal tower 4 is 200℃ (i.e., k = 200), and the pressure condition for removing chlorobenzene solvent is -0.07 MPaG (i.e., p = -0.07).

[0089] Calculated using the aforementioned formula, the cost characteristic index A1 is 11.06, and the operating characteristic index A2 is -7.81. The above process has a cumulative maintenance time of 68 days over three years of operation, with an average operating and maintenance cost of 1572 yuan / t (crude isocyanate).

[0090] Examples 2-6

[0091] The process was carried out in accordance with Example 1, except that the process parameters a, b, c, d, e, f, g, h, i, j, k, and p were adjusted to control A1 and A2. Relevant data from different examples are shown in Table 1. Unless otherwise specified, all other aspects were carried out in accordance with Example 1.

[0092] Table 1

[0093] Note: If the descriptions of the parameters in the embodiments do not match those in Table 1, the data in Table 1 shall prevail.

[0094] Comparative Example 1

[0095] (1) Diphenylmethane diamine (amine compound) with a flow rate of 30 t / h and chlorobenzene (solvent stream I) with a flow rate of 45 t / h are mixed and then introduced into cold reactor 1. At the same time, phosgene solution is introduced into cold reactor 1. The flow rate of phosgene solution is 64 t / h and the mass percentage of phosgene in phosgene solution is 95%. Diphenylmethane diamine and phosgene react in cold reactor 1 to obtain the first mixture.

[0096] (2) The first mixture is then passed into the thermal reactor 2 for a thermo-phosgene reaction to obtain a second mixture containing isocyanate and a first gas mixture containing phosgene and hydrogen chloride.

[0097] (3) The second mixture is passed into the dephosgene tower 3 to remove hydrogen chloride and phosgene, and a third mixture is obtained in the bottom of the tower and a second gas mixture containing hydrogen chloride and phosgene is obtained. The second gas mixture is discharged from the top of the tower and enters the phosgene scrubbing and absorption tower 5 together with the first gas mixture. In the phosgene scrubbing and absorption tower 5, the solvent chlorobenzene is passed into the first and second gas mixtures to scrub and absorb the phosgene, and a phosgene solution is obtained. Hydrogen chloride is collected from the top of the tower. The phosgene solution is sent to the cold reactor 1 in step (1) to participate in the cold phosgene reaction.

[0098] (4) Pass the third mixture into solvent removal tower 4 to remove chlorobenzene, collect the solution in the bottom of the tower to obtain crude isocyanate; discharge the removed chlorobenzene from the top of the tower.

[0099] In step (1), the mass ratio a of solvent chlorobenzene to amine compounds in solvent stream I is 1.5, the mass ratio b of pure phosgene to amine compounds in phosgene solution is 2, the temperature of cold phosgene reaction is 160℃ (i.e., c = 160), and the pressure of cold phosgene reaction is 4 MPag (i.e., d = 4).

[0100] In step (2), the temperature of the thermophotogasification reaction is 120℃ (i.e., e = 120), the residence time of the material in the thermophotogasification reaction system in the reaction vessel is 2 hours (i.e., f = 2), and the pressure of the thermophotogasification reaction is 0.1 MPag (i.e., g = 0.1).

[0101] In step (3), the temperature condition for removing hydrogen chloride and phosgene is 115℃ (i.e., h = 115), the pressure condition for removing hydrogen chloride and phosgene is -0.07 MPag (i.e., i = -0.07), and the temperature of the phosgene solution obtained by solvent washing and absorption of the first gas mixture and the second gas mixture in the phosgene washing and absorption tower 5 is 100℃ (i.e., j = 100).

[0102] In step (4), the temperature condition for removing chlorobenzene solvent in solvent removal tower 4 is 220℃ (i.e., k = 220), and the pressure condition for removing chlorobenzene solvent is -0.1 MPag (i.e., p = -0.1).

[0103] Based on the aforementioned formula, the cost characteristic index A1 is 2.98, and the operating characteristic index A2 is -18.47. The cumulative maintenance time over three years and the average operating and maintenance costs over three years for the above process are shown in Table 2.

[0104] Comparative Examples 2-5

[0105] The process was carried out with reference to Comparative Example 1, except that the process parameters a, b, c, d, e, f, g, h, i, j, k, and p were adjusted to control A1 and A2. Relevant data for different comparative examples are shown in Table 2. Unless otherwise specified, all other cases refer to Comparative Example 1.

[0106] Table 2

[0107] Note: If there are any discrepancies between the descriptions of the parameters in the comparative figures and those in Table 2, the data in Table 2 shall prevail.

[0108] The results of the comparative examples and the comparative embodiments show that by controlling the cost characteristic index A1 and the operating characteristic index A2 within a reasonable range, operating and maintenance costs can be significantly reduced. Furthermore, relatively good process stability can be maintained simultaneously (e.g., during the three-year operation period, the downtime for maintenance is not excessively long, for example, the downtime for maintenance does not exceed 130 days, resulting in a relatively long cumulative stable operating time). This achieves a good balance between controlling low costs and maintaining process stability, maximizing profits. In contrast, the comparative examples show significantly higher operating and maintenance costs; although the downtime for maintenance is short in Comparative Examples 2 and 3, their operating and maintenance costs are substantially increased. The comparative examples fail to achieve low operating and maintenance costs and struggle to achieve a good balance between low cost and process stability.

[0109] It is readily understood that the above embodiments are merely illustrative examples for clear explanation and do not imply that the invention is limited thereto. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing isocyanate, characterized in that, The method includes the following steps: (1) The amine compound, solvent stream I and phosgene solution are fed into a cold reactor to carry out a cold phosgene reaction to obtain the first mixture; In step (1), the mass ratio of the solvent in solvent stream I to the amine compound is a, the mass ratio of pure phosgene in the phosgene solution to the amine compound is b, and the cold phosgene reaction is carried out at a temperature of c℃ and a pressure of d MPaG. (2) The first mixture is fed into a thermal reactor and heated to carry out a thermo-phosgene reaction to obtain a second mixture containing isocyanate and a first gas mixture containing hydrogen chloride and phosgene. In step (2), the thermo-photogasification reaction is carried out at a temperature of e℃ and a pressure of g MPaG, and the residence time of the material in the thermo-photogasification reaction system in the thermal reactor is f hours. (3) The second mixture is subjected to treatment to remove hydrogen chloride and phosgene at a temperature of h℃ and a pressure of iMPaG to obtain a third mixture and a second gas mixture containing hydrogen chloride and phosgene. The first gas mixture and the second gas mixture are washed and absorbed with a solvent to obtain the phosgene solution at a temperature of j℃ and the washed hydrogen chloride; the phosgene solution is then fed into the cold reactor of step (1); (4) The third mixture is subjected to solvent removal treatment at a temperature of k℃ and a pressure of p MPaG to obtain crude isocyanate; Meanwhile, the method is controlled to be carried out under the conditions of A1 = 5 to 55 and A2 = -15 to 50; A1 is the cost characteristic index, which is dimensionless; A2 is the operational characteristic index, which is dimensionless; the calculation formulas for A1 and A2 are as follows: A1=0.928×a 2 +0.239×b 2 -0.033×c-0.214×d 2 +0.022×e +0.061×f 2 +5.78×g 2 +0.0135×h+85×i 2 -0.032×j+0.027×k+99.17×p 2 A2=0.485×a 2 +0.852×b 2 -0.042×c-0.83×d 2 -0.052×e+ 0.192×f 2 -3.016×g 2 -0.026×h-153×i 2 +0.097×j-0.011×k+10.09×p 2 。 2. The method for preparing isocyanate according to claim 1, characterized in that, In step (1), the mass ratio a of the solvent in solvent stream I to the amine compound is 0.5 to 5, the mass ratio b of the pure phosgene in the phosgene solution to the amine compound is 1 to 10, the temperature c of the cold phosgene reaction is 50-180℃, and the pressure d is 0.1-5MPaG.

3. The method for preparing isocyanate according to claim 2, characterized in that, In step (2), the temperature e℃ of the thermophotogasification reaction is 50-200℃, the pressure g MPaG is 0.1-1MPaG, and the residence time f hours of the material in the thermophotogasification reaction system in the thermal reactor is 0.5-12 hours.

4. The method for preparing isocyanate according to claim 3, characterized in that, In step (3), the temperature h°C for removing hydrogen chloride and phosgene is 100-200°C, and the pressure iMPaG is -0.05MPaG to 0.20MPaG; the temperature j°C of the phosgene solution obtained by washing and absorbing the first gas mixture and the second gas mixture with a solvent is -50°C to 150°C.

5. The method for preparing isocyanate according to claim 4, characterized in that, In step (4), the temperature k℃ for solvent removal is 100-250℃ and the pressure p MPaG is -0.1MPaG to 0.1MPaG.

6. The method for preparing isocyanate according to any one of claims 1-5, characterized in that, A1 is 10 to 20, and A2 is -10 to 10.

7. The method for preparing isocyanate according to any one of claims 1-6, characterized in that, The amine compounds include one or more of diphenylmethane diamine, polymethylene polyphenyl polyamine, diaminotoluene, isophorone diamine, hexamethylenediamine, cyclohexanediamine, p-phenylenediamine, and naphthyldiamine.

8. The method for preparing isocyanate according to any one of claims 1-7, characterized in that, The solvent in solvent stream I in step (1) and the solvent used for washing and absorption in step (3) respectively include one or more of chlorobenzene, dichlorobenzene, trichlorobenzene, toluene, xylene, benzene, and diethyl isophthalate, preferably chlorobenzene and / or o-dichlorobenzene; Preferably, the solvent in the solvent stream I in step (1) is the same as the solvent used for the washing and absorption in step (3).

9. The method for preparing isocyanate according to any one of claims 1-8, characterized in that, In step (3), the phosgene solution obtained by washing and absorption has a phosgene mass percentage of 50-90%.

10. The method for preparing isocyanate according to any one of claims 1-9, characterized in that, The cold reactor mentioned in step (1) is a jet reactor or a high-shear reactor; And / or, the thermal reactor in step (2) is a batch reactor or a tower reactor; And / or, the removal of hydrogen chloride and phosgene as described in step (3) or the solvent removal as described in step (4) are carried out in a distillation column.

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