Introduction to Optical Chemical Processes - Pressurized Production Process

As previously mentioned, the reaction between organic amines and phosgene to produce organic isocyanates generally proceeds in two steps under normal pressure:

R-NH2+COCl→R-NHCOOCl+HCl      (1)

R-NHCOCl→R-NCO+HCl                   (2)

From reaction (1), it can be seen that the volume of the gas mixture of phosgene and HCl does not change, and increasing the pressure does not adversely affect the reaction from left to right. However, the situation in reaction (2) is different; only under normal pressure or vacuum conditions, when the generated HCl gas is promptly removed, does it favor the decomposition of carbamoyl chloride into isocyanate.

However, contrary to this, whether it is reaction (1) or reaction (2), when the operating pressure is between 1.0 to 5.0 MPa, a higher yield of isocyanates will be obtained. In theory, significantly increasing the pressure would cause the reverse reaction of (2), hindering the formation of isocyanates. The main reason why the high-pressure method can increase the yield is possibly that the solubility of phosgene at high temperatures increases under pressure.

Other characteristics of the pressurized method are as follows:

1. Under high pressure, the issue of separating excess phosgene from hydrogen chloride is reduced. Because phosgene is easily condensed and recovered under high pressure, the recovery equipment is smaller, and the produced hydrogen chloride contains little to no phosgene, making the operation safer.

2. Under high pressure, in the second step reaction, a higher concentration of isocyanates can be achieved in the reaction material, improving distillation efficiency.

3. Low boiling point solvents can be used as diluents, allowing isocyanates to be separated by simple distillation, thus saving energy.

The flow chart of the pressurized production of isocyanates is shown below. This process achieves better yields. The phosgene used must be in excess, generally 100% or more of the theoretical amount (150%-350%); the reaction temperature is controlled between 80-115°C; and the pressure is 1.0-5.0 MPa.

Pressurized Production Process of Organic Isocyanates

The organic amine solution enters the reaction cycle zone through a pipeline, and phosgene also enters the liquid circulation pump 1 and separator 2 through the pipeline and balancing tank 3 in proportion (preferably a high-speed circulating reaction in a reaction zone composed of a horizontal conical decanter, with a circulation rate 20-30 times the total volume of materials added per unit time. The functions of reaction rings 1, 2, 3, and 4 can also be replaced by a Venturi device). The mixture from the first step reaction exits the separator and then enters the middle of the high-pressure phosgene tower 4 through a pipeline. The operating pressure of the tower is above 1.0 MPa, the bottom temperature is 145°C, and the top temperature is 80°C. The HCl gas produced by the reaction is discharged after secondary freezing and phosgene recovery, and the generated isocyanates, solvents, and phosgene are sent to the middle of the atmospheric pressure tower 8 for further processing through the pressure relief valve 6. The reaction products are output to the degassing and distillation section through pipelines.

For example, if the total volume of the reaction system is 15 liters, a 12.75% toluene diamine solution in isobutyl isobutyrate is continuously fed at a rate of 94 kg/h, and the liquid phosgene is fed at a rate of 48.6 kg/h, which is 150% of the theoretical amount of phosgene. The liquid circulation amount in the system is 20 times the total amount of liquid entering per unit time, the temperature in the reaction zone is maintained at 110°C, and the pressure is 1.5 MPa.

The reaction mixture flows out from the reaction zone to the high-pressure phosgene tower 4 through a pipeline at a rate of 142.6 kg/h. The tower is 1.5 m high, with an inner diameter of 35 cm, and is packed with Raschig rings. The temperature in the tower is maintained by heating with 33 kg/h of steam through heater 7, keeping the bottom temperature at 145°C and the top temperature at 82°C. The pressure in the tower is 1.5 MPa. The HCl produced by the reaction (containing 2.5 mol of phosgene per 100 mol of HCl) is discharged after phosgene recovery by a 20°C condenser 5 and a -20°C condenser 5. The composition of the reaction materials discharged from the bottom of tower 4 is as follows:

Toluene diisocyanate: 16.5 kg/h

Isobutyl isobutyrate: 82 kg/h

Phosgene: 20.2 kg/h

Tower 8 operates at atmospheric pressure, with a bottom temperature of 158°C and a top temperature of about 70°C. The toluene diisocyanate and solvent are discharged through pipelines, and then the pure diisocyanate is obtained through general processes, with a yield of 96.6%.