JPS6126987B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an industrially advantageous method for continuously producing isocyanate.

More specifically, under pressure of 3 to 7 kg/cm ² gauge pressure, the intermediate product carbamyl chloride and amine hydrochloride are decomposed into isocyanates at a temperature of 60 to 100°C so that 30 to 70% of the intermediate products are present. The first stage reaction is carried out in a tank reactor while maintaining the temperature, and then it is led to the second stage reactor where the temperature is raised to 120-160°C to complete the reaction and convert it to isocyanate. This method is characterized by a relatively short reaction time, the ability to obtain isocyanate in good yield even at high reaction concentrations, and the ability to mass-produce using a small-capacity phosgenation reactor. can be carried out industrially and advantageously.

Recently, organic isocyanates, particularly aromatic polyisocyanates such as tolylene diisocyanate (hereinafter referred to as TDI) and diphenylmethane diisocyanate (hereinafter referred to as MDI), It is produced in large quantities by phosgenation in an inert organic solvent. Products manufactured in such large quantities are of great importance industrially, as even small economic improvements can yield great benefits if implemented industrially.

It is known to produce this compound by reacting a primary amine (hereinafter referred to as amine) or a salt thereof with phosgene, and numerous documents have proposed this method.

However, many of these methods have a yield of 90%.
Even when the reaction concentration is below or the yield is high, the concentration of the reaction is extremely low, or the reaction time is extremely long, which are disadvantageous and are not yet satisfactory for industrial implementation.

Most of the conventional methods, for example in the case of TDI, involve reacting liquid phosgene or an inert solvent solution of phosgene with an inert solvent solution of amine at a low temperature of 30 to 40°C or less, as shown in the following formula, to form an amino group. into carbamyl chloride or amine hydrochloride form,
Next, it is generally heated to 100 to 180°C to convert carbamyl chloride into isocyanate, and phosgene is blown into the reactor to convert unreacted amine hydrochloride, which is a by-product during the reaction process, into isocyanate.

Various reaction methods have been proposed for this method, such as a tank type, a column type, or a circulation line, and various methods have been proposed, including changing the molar ratio of amine and phosgene, using a catalyst, or phosgenation under high pressure.

In this reaction, the reaction between the amine of formula (1) and phosgene is very rapid even at low temperatures, and amine hydrochloride is produced as a by-product during the reaction process. The formation of carbamyl chloride (formula 2) by the reaction between this amine hydrochloride and phosgene is relatively slow and hardly progresses at low temperatures. A method has been adopted in which the carbamyl chloride of formula (3) is decomposed and converted into isocyanate.

In this case, carbamyl chloride starts a decomposition reaction relatively easily when heated and is almost completely decomposed into isocyanate at temperatures above about 120°C, but the rate of phosgenation of unreacted amine hydrochloride is relatively slow. When the reaction temperature is raised to speed up this reaction, the isocyanate produced by the decomposition reacts with the amine hydrochloride, producing undesirable urea compounds, which are further phosgenated or polycondensed with the isocyanate, resulting in complex Various side reactions occurred, leading to the by-product of tar-like substances, which was thought to be a major cause of the decrease in yield.

Therefore, various improved methods have been proposed in order to carry out the phosgenation of this amine hydrochloride at as low a temperature as possible to obtain isocyanate in good yield. For example, in Japanese Patent Publication No. 39-14664, in the first stage, the reaction of formula (1) is first carried out near room temperature, and in the second stage, the reaction of formula (2) is carried out at 60 to 80°C for 4 to 8 hours. A method is described in which, after complete conversion to carbamyl chloride by reaction, the carbamyl chloride of formula (3) is decomposed into isocyanate in the third stage to obtain isocyanate in good yield. However, this method requires a very long reaction time and requires a large-capacity reactor because the reaction concentration cannot be very high.
It is not industrially satisfactory when mass producing TDI or MDI. On the other hand, methods that allow mass production using a relatively small-capacity reactor include, for example, Japanese Patent Publication No. 10774 (1974);
-17381, and Special Publication No. 51-6126. In Japanese Patent Publication No. 35-10774, a solution of isocyanate is recirculated around a closed loop at a temperature higher than that at which carbamyl chloride decomposes, and an amine is charged therein to carry out the reaction all at once. , a method is described in which the by-product hydrogen chloride and excess phosgene are removed at a low temperature at a point, and a portion of the isocyanate solution is then recovered. In this method, amine and phosgene are brought into contact with each other in a short period of time under a vortex state with a high Reynozzle number, and the corresponding amine can be converted into isocyanate at once. It is possible to continuously produce organic isocyanates with high yields, and considerable results have been obtained. However, in this method, as the reaction concentration increases, in addition to the reaction between amine and phosgene, the undesirable reaction between amine and isocyanate also increases, leading to side reactions of non-volatile tar-like substances, and reducing the yield. There was a drawback that the reaction concentration could not be increased, such as causing a decrease in the reaction concentration. Further, Japanese Patent Publication No. 17381/1973 deals with the catalytic reaction between an amine and an excess amount of phosgene. An intermediate is formed with isocyanate, and before the reaction in the first stage (100-110℃) is completed, the reactant is passed through another reaction system (150-170℃) (for a few seconds to 30 minutes) to form phosgene and hydrogen chloride. This method involves contacting a mixture of phosgene and amine in such a manner that the concentration of hydrogen chloride is greater than that formed by the complete reaction of phosgene and amine introduced into the first reaction system. In this method, it is described that controlling the residence time in the first stage and the ratio of phosgene and hydrogen chloride in the second stage to a constant ratio are important points for increasing the yield of isocyanate. There is. However, in industrial implementation, the control is complicated and the reaction concentration cannot be increased, so it cannot be said to be a fully satisfactory method. In addition, in Japanese Patent Publication No. 51-6126, phosgene and amines were heated at a temperature of 40 to 120°C and
The resulting reaction mixture carbamyl chloride was reacted at a gauge pressure of ~50Kg/ ^cm2 and
A process for continuously producing organic isocyanates by heating at a temperature of ~180° C. and a gauge pressure of at least 15 Kg/cm ² , embodiments of which include:
The phosgene-amine reaction is carried out with circulation of the reaction mixture in a tubular circuit containing a gas separator, and then a step of heating the completed carbamyl chloride is carried out under pressure in a distillation column, where the carbamyl chloride is Both the decomposition to isocyanates and the distillation of hydrogen chloride and phosgene take place; the phosgene-containing diluted isocyanate mixture removed from the base of the column is distilled at atmospheric pressure or under slight pressure, and the isocyanate solution is separated. It describes how to do it.

It is true that if the reaction is carried out under high pressure such as 10 to 50 atmospheres, the solubility of phosgene in the reaction solution increases significantly.
It has been known for a long time that the production reaction rate to carbamyl chloride is faster and that isocyanate can be obtained in good yield even at high reaction concentrations.

Furthermore, if the decomposition of carbamyl chloride in the second stage is carried out at high pressure, unreacted phosgene can be easily condensed and the problem of separating hydrogen chloride can certainly be facilitated. However, handling a large amount of a reaction solution containing a large excess of phosgene under such high pressure is extremely dangerous when carried out industrially, and special consideration is required from the viewpoint of safety.

In addition, carbamyl chloride is poorly soluble in solvents, and circulating such a highly concentrated slurry liquid in a tubular circuit under high pressure will significantly increase the degree of corrosion to metal materials. Steel cannot be used, and expensive materials are required, resulting in a significant increase in equipment costs.

Furthermore, increasing the concentration of the reaction not only increases tar-like byproducts but also significantly increases the viscosity of the carbamyl chloride slurry solution, causing blockage of the tubular circuit and making it difficult to circulate the reaction solution. become. In addition, in the pressurized one-step method that completes the phosgenation reaction in one step, such as the method of Japanese Patent Publication No. 51-6126, in the first step, the amine hydrochloride produced in the reaction process is converted to carbamyl chloride. If this reaction is not completed, the amine hydrochloride will cause blockage of the column during the decomposition reaction in the distillation column in the second stage, which will also cause a decrease in the yield of isocyanate. Sufficient residence time is required for completion.

In order to improve these drawbacks, the present inventors have developed a method for continuously producing the corresponding isocyanate by reacting excess phosgene with an amine in the presence of an inert organic solvent. (1) 10Kg/cm ² gauge Phosgenation of amine and amine hydrochloride produced as a by-product during the reaction process in a tubular circuit while maintaining a temperature of 60 to 100°C at which the intermediate product carbamyl chloride decomposes 30 to 70% under pressure. (2) Next, this reaction mixture is transferred to a second stage reactor maintained at a temperature of 120 to 160°C and a pressure of 10 Kg/cm ² gauge pressure or less, where unreacted amine hydrochloric acid is removed. We have previously filed an application for a method for producing organic isocyanates using a two-step continuous pressurization process that completes the phosgenation of salt and the decomposition of carbamyl chloride. (Japanese Patent Application No. 56-51216) The object of the invention of the above-mentioned Japanese Patent Application No. 56-51216 is to
In the first stage reaction, the reaction is carried out in a tubular circulation circuit under the temperature and pressure of the above critical conditions, thereby increasing the solubility of phosgene and increasing the molar ratio of phosgene to amine equivalent by circulating the reaction solution. The reaction frequency is increased to maintain a fast reaction rate and suppress tar-like by-products, while maintaining a temperature at which 30 to 70% of the resulting carbamyl chloride decomposes. On the other hand, by keeping the isocyanate concentration lower than the actual concentration after the reaction is completed and reducing the chance of reaction between amine and isocyanate, even at high reaction concentrations, tar-like products are produced and it is preferable. The purpose of this method is to obtain isocyanate in a high yield, and to suppress the concentration of carbamyl chloride in the reaction solution to a relatively low level, so that the viscosity of the reaction solution is low and the circulation circuit can be easily operated.

However, when carried out in a tubular circuit in the first stage reaction, carbamyl chloride
Even if the reaction is carried out at a temperature that causes up to 70% decomposition, if the reaction liquid is circulated in large quantities in a tube under pressure and in a vortex state with a high Reynolds number, corrosion of the material in the circulation circuit can be avoided. At the same time, the power cost for circulating the reaction solution increases. The present inventors investigated a tank type reaction in order to solve these drawbacks of the tubular loop reaction, and found that it is not necessary to use a tubular loop type to circulate the reaction solution and increase the phosgene/amine molar ratio to carry out the reaction. However, in a reaction solution with a controlled carbamyl chloride content at a specific temperature, if the amine dispersion rate in the tank type reaction is increased and the amine dispersion effect is increased, the reaction will be the same as in the tubular loop reaction method. It was found that by-products can be suppressed and the reaction can be carried out with a high yield of isocyanate.

The method of the present invention is based on the above knowledge, and the first stage reaction is carried out in a tank type reactor instead of the tubular loop of the above-mentioned Japanese Patent Application No. 56-51216.
This is an improvement method.

Figure 1 shows the relationship between the temperature at each pressure and the solubility of phosgene in the reaction liquid phase (TDI equivalent concentration: 20% by weight in the solvent o-dichlorobenzene). Also figure-
2 is the concentration in terms of TDI in the first stage of the invention.
It is a diagram showing the relationship between the reaction temperature and the production concentration of TDI, carbamyl chloride, and amine hydrochloride in the reaction solution under 5 Kg/cm ² gauge pressure of 20%,
From these figures, each critical condition when implementing the method of the present invention can be understood.

From Figure 1, when the temperature is in the range of 60 to 100℃, 5
It was found that phosgene solubility increases significantly with a slight increase in pressure of about Kg/cm ² gauge pressure, and 3 Kg/cm
It was found that isocyanate can be obtained in high yield if the process is carried out at a cm ² gauge pressure or higher. Also, as can be seen from Figure 2, the pressure was increased to 5% in the first stage reaction.
Kg/cm ² Gauge pressure and reaction temperature 60-100℃
By maintaining the amount of carbamyl chloride present in the reaction solution within the range of 30% to 70%, it is possible to maintain the production equilibrium of TDI with some amine hydrochloride simultaneously produced, and under the same pressurized conditions. The viscosity of the reaction solution due to high concentration carbamyl chloride and amine hydrochloride will not increase significantly and the amine dispersion effect in the tank will not deteriorate, unlike when carried out at a low temperature of 50℃ or less. By maintaining this, the production rate of TDI can be suppressed to 30% or less, and by-products can be suppressed in a highly concentrated reaction solution. In addition, in the first stage reaction, the experimental results show that when the pressure is below 7 Kg/cm ² G, the effect on the composition ratio of carbamyl chloride, amine hydrochloride, and isocyanate is not as great as the effect of temperature.
It was also found that the pressure can be appropriately selected from 3 to 7 kg/cm ² G pressure depending on the amine concentration to be supplied, the dispersion rate of the amine, the residence time, etc.

The present inventors have accumulated such basic data and have completed the method of the present invention. By selecting the temperature and pressure of the first stage reaction, the concentrations of carbamyl chloride, amine hydrochloride, and isocyanate can be controlled. By controlling the concentration of isocyanate and increasing the solubility of phosgene, the concentration of isocyanate in the reaction solution is maintained at approximately half of the concentration when it is finally converted to isocyanate. There are fewer opportunities for reaction, and isocyanate can be obtained in high yield even at high reaction concentrations.

The method of the present invention uses TDI as the isocyanate,
This method is particularly suitable for the production of MDI and is described below for the production of TDI or MDI from tolylene diamine (TDA) or diaminodiphenylmethane (MDA) using o-dichlorobenzene as a solvent. do.

In the process of the invention, the molar ratio of freshly fed phosgene to amine in the first tank reactor is at least 50%/stoichiometric excess, i.e. NH ₂
It is necessary that there be an excess of 1.5 moles or more, preferably 70 to 150%, of phosgene per group. The pressure inside the tank is 3Kg/cm ² G pressure to 7Kg/cm ² G pressure. The pressure is then easily regulated at the outlet for the by-product hydrogen chloride (off-gas), which is generally present together with the solvent and phosgene. Since the first stage of the process according to the invention is operated at relatively low temperatures and under pressure, most of the phosgene is recycled and the phosgene molar ratio to the actual amine equivalents is substantially greater than the above-mentioned molar ratio.

The method of the present invention is characterized in that it can be carried out without reducing the isocyanate yield even if the amine concentration in the first stage phosgenation reaction is quite high.For example, the amine concentration can be up to 50%, and the The concentration of isocyanate can be up to 30%, but usually, considering the yield of isocyanate, the amine concentration is 10 to 30%, preferably 10 to 25%, and the concentration of isocyanate at the end of the reaction is 10 to 30%. It is preferable to carry out within the range of 25%.

Further, the reaction temperature in the tank is in the range of 60 to 100°C, and the reaction liquid becomes a slurry liquid containing carbamyl chloride, amine hydrochloride, and isocyanate.
When using TDA or MDA, a temperature of 70 to 90°C is particularly preferred. The pressure is 3-7 Kg/cm ² gauge pressure. When the reaction temperature increases to 15% by weight or more, the effect on the yield by pressurizing phosgene increases rapidly, but a sufficient yield can be obtained at about 5 atm, and the yield can be increased to 7 Kg/cm ² gauge pressure or higher. Even if it is increased, the effect on yield will hardly change. Rather, it is undesirable in terms of safety when handling excess phosgene.

In the method of the present invention, the first stage phosgenation reaction is carried out under these reaction conditions in a pressure-resistant reaction tank equipped with a stirrer and a heat-insulating jacket. At this time, the dispersion rate of the amine solution fed from the amine dispersion tube is 10
The speed is maintained at m/sec or higher, preferably 10 to 30 m/sec. If the amine dispersion rate is slower than this, by-products such as tar-like nonvolatile residues will increase even if the reaction temperature is controlled. In addition, there is no need to create a special dispersion tube or a special design in the reaction tank to increase the dispersion speed to 30 m/sec or more, and even if the dispersion speed is made faster than this, the amount of by-products will remain the same and at least 10 m/sec. If the amine is fed at a flow rate of seconds or more, the amine will be dispersed substantially instantly, so a flow rate of this level or higher is sufficient.
The amine dispersion rate can be appropriately determined depending on the design such as the feed amount of the amine solution, the diameter of the dispersion tube, and the residence time of the reaction solution. When using TDA or MDA, a flow rate of about 20 m/sec is preferable, and a pressure pump is used to disperse and supply the water into a tank that is being rotated and stirred at about 350 RPM.

In the process of the present invention, the reaction in the second stage of phosgenation can be carried out by tank-type stirring or circulation stirring into the tank, but the need for stirring is not as great as in the first stage. It is sufficient to circulate the reaction solution with a pump. The temperature of the second stage reaction is in the range of 120-160℃,
Although the pressure is below 7 Kg/cm ² gauge pressure, it does not affect the yield as much as the pressure in the first stage.

Taking into account the recovery and recirculation of excess phosgene by combining the first and second stage offgases, the second stage
The stage pressure may be equal to or slightly lower than the first stage pressure. The pressure increase in the second stage may be achieved by introducing new phosgene, or the dissolved phosgene in the first stage reaction liquid introduced is sufficient.

In the second stage reaction, unreacted amine hydrochloride is phosgenated and carbamyl chloride is decomposed, but the main reaction is the decomposition of carbamyl chloride, which generates hydrogen chloride.

The residence time in the first and second stage reaction systems used in this method largely depends on the temperature used in each reaction system. A residence time of 30 minutes to 120 minutes in the temperature range of 60 to 100°C is sufficient in the first stage, and it is not necessary to complete the reaction in the first stage. The residence time of the second stage reaction must be sufficient to completely convert the intermediate amine hydrochloride produced in the first stage to isocyanate. Normally 120~160℃
It is carried out for about 10 minutes to 120 minutes at a temperature range of 10 to 120 minutes, and if necessary, phosgene is newly introduced. FIG. 3 is a flow sheet of a preferred example of carrying out the method of the present invention, and the present invention will be explained in more detail by way of examples according to FIG. Percentages are by weight unless otherwise indicated.

Example 1 In FIG. 3, the first stage tank type reaction system is a 50 pressure-resistant reaction tank 5 with a jacket, which has a stirrer 1 equipped with a stirring blade, an amine feed dispersion tube 2, a phosgene feed tube 3, and a new solvent feed tube 4. It is composed of

The second stage reaction system for simultaneously decomposing carbamyl chloride and converting amine hydrochloride into phosgenates is composed of an annular circuit having an external heating tube 7 and a gas-liquid separator 8.

The off-gas lines of the 1st and 2nd stages are discharged as the same line 10 and 11, the off-gas is cooled, and the condensate is returned to the reaction systems of the 1st and 2nd stages. After cooling and separation, the off-gas mainly consists of hydrogen chloride. It is equipped with a condenser and a condensate reservoir that are continuously discharged outside the system through a pressure control valve 9.

The first stage reactor in Figure 3 is a pressure-resistant reactor with an inner diameter of 350 mm made of rust-free steel, and the o-dichlorobenzene solution of phosgene and TDA mixed isomers is continuously stirred at a rotation speed of 350 RPM. It is introduced into a pressure-resistant reaction tank. Phosgene is 40.1Kg per hour from tube 3.
(0.4054 K mol) of TDA (2,4-, 2,6-,
An o-dichlorobenzene solution with a purity of 97.5%) was distributed and fed at a rate of 44 Kg (0.0902 Kmol of total amines) per hour using a pressure pump. Further, o-dichlorobenzene was supplied from conduit 4 at a rate of 29.7 kg/hour. At this time, the speed at which the TDA solution was blown out of the dispersion tube into the reaction tank was maintained at 20 m/sec.

The off-gas lines of the 1st and 2nd stage reaction systems are the same line, and the pressure is adjusted to 5.0 kg/kg by the pressure regulating valve 9.
^cm2 gauge pressure was maintained. The above amine solution is introduced from the dispersion section and instantly diffused and mixed.
A reaction between amine and phosgene occurs in the reaction tank.
C. resulting in an o-dichlorobenzene suspension containing about 10% TDI and about 10% carbamyl chloride and hydrochloride.

The by-produced hydrogen chloride was discharged through pipe 10, and the accompanying solvent and phosgene were returned to the first stage reaction system through pipe 12 from the condensate reservoir by a condenser.

The amount of liquid retained in the first stage reaction tank was approximately 49.5 kg, the residence time was maintained at 0.6 hours, and a portion of the reaction liquid was extracted in excess through the overflow pipe 6, and the second stage reactor was maintained at 150°C. It was fed to the lower part of the external heating tube 7 inside. The second-stage reactor is a circular circuit consisting of a heating tube with a capacity of 6 and a gas-liquid separator 8 with a capacity of 100 tanks, each having an external heating jacket. Hydrogen chloride gas generated by the decomposition of milk chloride and phosgene that vaporizes due to the solubility difference are rapidly released and flow toward the top of the heating tube, causing the reaction liquid in the second stage to self-circulate. Gas generated at the same time as the completion of the phosgenation reaction is separated in the gas-liquid separator 8 and discharged through a pipe 11 as an off-gas.

The amount of liquid remaining in the second stage reaction system is approximately 81 kg, and the residence time is maintained for 1 hour. A part of the reaction liquid is extracted due to the pressure difference, and after being depressurized in the flash tank 13, it is further degassed in the degassing tower. be done. The isocyanate solution of the product thus obtained was analyzed by distillation in the usual manner and found to have a TDI of 19.8% and a concentration of 1.1%.
It had a non-volatile residue of .

This is m-TDI with a purity of 94.7% based on tar ratio.
This corresponds to a theoretical yield of 9.71% in terms of m-TDA purity.

Comparative Example 1 Using the same apparatus as in Example 1, the same operation as in Example 1 was performed except that the blowing speed of the supplied TDA solution in the tank was set to 7 m/sec in the first stage reaction system. The resulting isocyanate solution was analyzed by distillation and contained 19.2% TDI and 1.7% non-volatile residue.

Comparative Example 2 Using the same equipment as in Example 1, the first stage reaction system was supplied at a temperature of 140°C and a pressure of 0.8 Kg/cm ² gauge pressure.
The same operation as in Example 1 was carried out, except that the blowing speed of the TDA solution in the tank was 20 m/sec, and the pressure of the second stage reaction system was 0.8 Kg/cm ² gauge pressure. Distillation analysis of the obtained isocyanate solution
Contained 19.0% TDI and 2.0% non-volatile residue.

This is m-TDI with a purity of 90.5% based on tar ratio.
This corresponds to a theoretical yield of 92.8% in terms of m-TDA purity.

Example 2 Using the same equipment as in Example 1,
Crude MDI was produced in exactly the same manner as in Example 1, except that crude MDA with a NH ₂ group content of 15.9% was used as a raw material instead of TDA.

The reaction solution was distilled to remove o-dichlorobenzene, and the distillate was analyzed and found to have an NCO content of 32.4%.
Crude MDI with a viscosity of 40 cps (25°C) and HC of 0.02% was obtained.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between the temperature at each pressure and the solubility of phosgene in the reaction liquid phase (a solution with a TDI equivalent concentration of 20% by weight in the solvent o-dichlorobenzene).
Figure 2 is a diagram showing the relationship between the reaction temperature and the concentration of TDI, carbamyl chloride, and amine hydrochloride produced in the reaction solution at a TDI equivalent reaction concentration of 20% by weight and a pressure of 5 kg/cm ² gauge pressure in the first stage reaction of the present invention. It is. FIG. 3 is a flow sheet of a preferred example of carrying out the method of the present invention, and includes: 5...tank reactor, 7... tubular heater, 8...
It is a tank type gas-liquid separator.

Claims (1)

[Claims] 1. A method for continuously producing a corresponding isocyanate by reacting excess phosgene with an organic primary amine in the presence of an inert organic solvent, comprising: (1) 3 to 7 kg/cm ² gauge pressure; Under pressure, temperature 60
Phosgenation of amine and amine hydrochloride produced as a by-product during the reaction process in a tank reactor in which the temperature is maintained at 100°C to 100°C and the feed flow rate of the amine solution dissolved in an inert organic solvent is maintained at 10 m/sec or more. and decompose the carbamyl chloride and amine hydrochloride into isocyanates such that 30 to 70% of the carbamyl chloride and amine hydrochloride are present in the reaction solution. It is characterized in that it is transferred to a second stage reactor maintained at a pressure of 3 to 7 kg/cm ² gauge pressure, where the phosgenation of unreacted amine hydrochloride and the decomposition of carbamyl chloride are completed. A continuous method for producing organic isocyanates using a two-stage pressurized method. 2. Claim 1, wherein the amount of phosgene supplied in the first stage reaction is 3.4 to 5 times the mole of the amine.
The method described in section. 3. The method according to claim 1, wherein the organic primary amine is tolylene diamine or diaminodiphenylmethane, and the corresponding organic isocyanate is tolylene diisocyanate or diphenyl isocyanate. 4. The method according to claim 1, wherein the inert organic solvent is o-dichlorobenzene. 5 Residence time of reaction solution in first stage 30~
The method according to claim 1, wherein the time is 120 minutes. 6. The method according to claim 1, wherein the feed flow rate of the amine solution is 10 to 30 m/sec. 7. The method according to claim 1, wherein the concentration of the amine solution to be fed is 10 to 25%. 8 The isocyanate concentration at the end of the second stage reaction is 10
25%.