Economics on Intermediate Pressure Separation of CO₂Connected with Urea Process

Abstract

Although a higher purity of CO₂ gas brings about a lower cost of the urea process, the separation cost of CO₂ gas from NH₃ synthesis becomes higher. So there will be the optimum pressure of intermediate stage separation.
In this paper this economics is analyzed from point of view of utility and investment. The calculations were carried out as follows:
At first the flash equilibria at two stages were calculated at various intermediate pressure, and then the increase in cost was estimated for both CO₂ separation system and urea process. The calculated results showed that the optimum pressure was between 8 and 10 atm. At a lower pressure than 8 atm, the total cost is controlled by the increase in CO₂ separation cost because of very large amount of recycle gas and water. On the contrary, at a higher pressure than 10 atm, it is controlled by the increas in eurea production cost because of increase in amount of unconverted NH3 and CO₂.