Comparison of liquid ammonia and cascade refrigeration

Principle of two refrigeration modes
The refrigeration principle of low temperature cascade
Refrigeration unit according to the refrigeration cycle series is divided into single-stage refrigeration and cascade refrigeration and other form of refrigeration system, single-stage refrigeration system generally uses medium temperature refrigerant. In the operation of refrigeration evaporating temperature below 70 DEG C, refrigerant evaporation pressure is very low, increase the specific volume of steam, gas coefficient decreased, compressor inspiratory dyspnea, unit work efficiency greatly reduced. Therefore, the application of single stage refrigeration systems offer evaporation temperature than 70 deg.C cryogenic, the scale difficult to become bigger and, economy is very low.
Cascade refrigeration system is composed by the high and low temperature part and the low temperature part provide required cryogenic capability and the R13, R23, such as low temperature refrigerant, but this kind of refrigerant condensing temperature is very low, under the same pressure with cooling water to be condensed. Part of the high temperature and temperature refrigerant cycle, its role is for condensing temperature of refrigerant, the high and low temperature part shared a evaporative condenser and the "cascade" to application of two refrigerant complex cascade refrigeration cycle.

The requirements of the production process of reaction temperature of -50 deg.C, thus completely cryogenic unit can meet the technological requirements of large-scale cooling requirements.

Liquid nitrogen evaporative refrigeration
Directly through the liquid nitrogen can also achieve the purpose of cooling, but the following defects exist in the process of liquid nitrogen cooling:
In the cooling process of liquid nitrogen directly in contact with the materials, for the boiling point of liquid ammonia at atmospheric pressure - 196 deg.C, when the liquid distributor is ejected by the instantaneous temperature difference up to 160 deg.C ~-200 deg.C, reaction material local far deviate from the process temperature control, caused by strong cooling zone. The deviation of the partial process control points will cause the reaction process is not balanced, and the product quality will fall.:
Liquid nitrogen vaporization during the use of liquid nitrogen. In the form of gas discharged into the atmosphere, in the intense emission process will carry a portion of the material, resulting in increased product yield and the decrease of solvent consumption
Liquid nitrogen temperature and other materials added not easy at the same time. Because in the use of liquid nitrogen cooling process, due to the expansion of vaporization of liquid nitrogen. In the reaction vessel for positive pressure and the pressure is difficult to control, at this time to adding other materials involved in the reaction, you must first stop the liquid nitrogen cooling, it will cause reaction temperature interrupt control, process control deviation
The use of liquid nitrogen can not be recycled, resulting in the increase in production costs
The temperature drop process is unstable and the controllability is poor.. High operation requirements. The expansion ratio of liquid nitrogen is 600, the evaporation temperature is low, and the operation of the liquid nitrogen is easy to be caused by improper operation and the explosion, running, material solidification and blockage pipeline and even personal accident.

Compared with the cooling liquid nitrogen cryogenic unit
Low temperature cascade unit in semi synthetic use of anti production process is mainly to reduce and control the temperature of the material system to ensure in the different stages of the reaction temperature of the whole system effectively maintain within the scope of the requirements, drug synthesis process to meet the technological requirements on reaction temperature. Work, by a complex cascade refrigeration unit for the preparation of low temperature refrigerant, between the refrigerant and the reaction solution is not in direct contact with, and by indirect heat exchange to influence the reaction process. Only in this way can avoid refrigerant cause pollution to the materials. The refrigeration system in a real production process, the unit high, low temperature respectively R22, R23 refrigerant screw type compressor, units of the evaporation temperature is up to 80 deg.C output 70 deg.C ~-75 deg.C cryogenic capacity of more than 4 million kcal / hour.
The size of semi synthetic antibiotics output reached 1200 tons / year statistical analysis of the actual production process, cryogenic unit and liquid nitrogen cooling compared:
In terms of product yield and consumption, yield increased by 1.88%, production of solvent consumption decreased by 36.1%
Quality, product content increased by 0.43%, in product stability and other impurities and other indicators are also better than liquid nitrogen temperature
In investment and operating costs, although cryogenic unit and auxiliary cooling water device relatively large one-time investment, depreciation also nots allow to ignore, but liquid nitrogen and outsourcing cost is several times in cryogenic unit cooling way. In comparison, the liquid nitrogen cooling costs is 5.13 times of cryogenic cooling machine.
Through the above comparative analysis shows that in the actual production process, compared with traditional cooling liquid nitrogen cryogenic unit. From the aspects of product yield, solvent consumption, process control and product quality and investment, electric power costs have obvious advantages, it reduces the cost of the products, improve the market competitiveness. Cryogenic refrigeration unit is the preferred method for the construction of a large-scale cryogenic environment.
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Tags:cascade refrigeration,liquid nitrogen evaporative refrigeration