Introduction to the Optical Chemical Process – High-Temperature One-Step Process

General Process Description:

The reaction of organic primary amines with phosgene typically adopts a two-step method: low-temperature phosgenation and high-temperature phosgenation. The purpose of low-temperature phosgenation is to allow organic primary amines to form ammonium salts (hydrochlorides) and carbamoyl compounds at low temperatures (i.e., <70℃), reducing side reactions. However, the intermediate produced after low-temperature phosgenation is a yellow slurry with high viscosity, making it difficult to transport. Additionally, low-temperature phosgenation is an exothermic reaction that releases about 155 kJ/mol of heat. To maintain a low-temperature state, a large amount of chilled brine is needed to remove the reaction heat, ensuring the normal progress of low-temperature phosgenation. Therefore, the disadvantage of the two-step process is the high energy consumption and the need for metering pumps to transport the viscous liquid.

The high-temperature one-step phosgenation process eliminates the low-temperature phosgenation stage, increasing reaction speed and thereby shortening the reaction cycle. Generally, carbamoyl chloride is stable below 70℃ but decomposes into isocyanates and HCl above 80℃:

R-NHCOCl→R-NCO+HCl

Organic ammonium salts can also react directly with phosgene above 80℃ to form organic isocyanates:

R-NH2·HCl+COCl2→R-NCO)+3HCl

In this method, the reaction temperature must be controlled above 80℃, typically within the range of 120-200℃, to achieve a high yield. The required amount of phosgene is less than in conventional processes. If two series-connected reactors are used for phosgenation, the phosgene excess is minimal, about 5% to 20%.

This process can be continuously conducted in two acid-resistant steel reactors capable of heating. The reactors are equipped with impeller-type stirrers, an inlet for phosgene and amine solution, an outlet for removing the reaction mixture, and a reflux condenser to remove HCl and residual phosgene produced during the reaction.

Specific Process:

First, a 15% concentration of amine solution is preheated to 100℃ and fed simultaneously with phosgene into the first reactor. The reaction temperature is controlled between 120-160℃, with the phosgene amount matching the theoretical requirement for the organic amine. When the conversion rate reaches 75% to 88%, the material overflows into the second reactor, maintaining the same temperature and supplementing with phosgene in the amount of 5% to 20% of the theoretical requirement. In the second reactor, when the conversion rate reaches 93% to 96%, the reaction mixture is transferred to a collector and dried with nitrogen gas to remove HCl and phosgene.

The isocyanate produced by this method is of high purity. After vacuum distillation to remove a small amount of impurities and solvent, the isocyanate purity can reach over 99%. The initial distillate solvent containing a small amount of isocyanate is recycled into the first reactor, eliminating the need for separation and cleaning equipment. Excess phosgene separated from the two reactors can be reused after removing HCl.

Example Implementation:

In a 28.5L acid-resistant reactor with a reflux condenser and impeller stirrer, preheated to 160℃ with a saturated solution of ortho-dichlorobenzene phosgene, a 2,6-toluenediamine solution preheated to 100℃ is fed at 6g/min and phosgene at 9.6g/min. The reaction material overflows from the first reactor into the second, where phosgene is supplemented at 1.9g/min, maintaining a temperature of 160℃. After nitrogen purging, vacuum distillation yields 2,6-toluene diisocyanate with a yield of 94% (based on amine) and a purity of 99%.

Another industrial production process for isocyanates using the one-step high-temperature method is illustrated in the diagram below.

This process involves conducting the phosgenation reaction in pipelines at 150℃ with high-speed circulation. The phosgenation liquid undergoes high-temperature degassing to remove residual phosgene and HCl, with phosgene recovery done by pressurized ambient temperature absorption.

A 16% solution of toluenediamine and ortho-dichlorobenzene is stored in tank 1, fed by metering pump 3 into mixer 7. Gaseous phosgene and phosgene recovery solution (containing 25% phosgene) enter mixers 9 and 10, respectively, heated above 150℃ by heater 8, and quickly mixed with the diamine solution in a specially structured pipeline at 150℃ and 0.1MPa. The generated HCl gas is discharged from the top of separator 5, while other materials circulate at high speed by pump 6, with a Reynolds number above 10^6, and a circulation volume 50-100 times the amount of semi-finished TDI output. Phosgenation liquid exits the reaction zone into degassing tower 11 at 150-200℃ and 0.1MPa, with a residence time of 30 minutes to 1 hour. The mixed gas of phosgene, solvent, and HCl discharged from the top of the degassing tower is condensed in condenser 12, with 10% solvent refluxing to the reaction zone. The bottom of the degassing tower releases a toluene diisocyanate-ortho-dichlorobenzene solution with hydrolyzed chlorine reduced to 0.03%.

Uncondensed phosgene-HCl gas from condenser 12 is compressed to 0.5MPa by compressor 15, entering the bottom of phosgene absorption tower 16 at 20℃, with the material inside absorbing phosgene and releasing heat, raising the bottom temperature to 50℃. To maintain tower temperature, 10℃ cooling water circulates. The bottom absorption liquid containing 25% phosgene enters tank 17, while unabsorbed phosgene is recycled by freezing at 0-5℃, with discharged HCl containing only 10mg/kg phosgene, sent to the hydrochloric acid production section.