Process of and apparatus for producing urea

摘要

In a process for producing urea from carbon dioxide and ammonia in which urea is synthesized and subsequently separated from the crude synthesis effluent, the conditions of temperature and pressure are such that the pressure difference between the effluent of the urea synthesis and the subsequent distillation-condensation separation stage is utilized to recycle the separated unconverted materials to the urea synthesis. Liquid carbon dioxide and liquid ammonia (obtained as described in Specification 974,412 from vessels 5 and 9 pass through a heat exchanger 10 to an autoclave 11 under a pressure of 270 atmospheres where urea is obtained together with water, ammonia, carbon dioxide and ammonium carbamate. The product mixture passes through heat exchanger 10 and a distributor 16 having a movable member 17 which alternately directs the mixture to one or other of the outer ends A and D of cylinders 18 having pistons joined by a rod 19, the pressure on the pistons causing separated ammonia, carbon dioxide and ammonium carbamate to be pumped from a condenser 22 through valves e and f back into autoclave 11. At the end of the stroke member 17 changes its position and distributer 16 directs the mixture to the other cylinder e.g. to end D instead of to end A, the mixture previously collected e.g. in end A, being returned through distributer 16 to a heater 13. Mixture passes from heater 13 to a separator 20 where liquid rich in urea separates, the vapour passing down a pipe to a condenser 22 where water is condensed and ammonia and carbon dioxide are separated to be recycled to autoclave 11. Liquid rich in urea from separator 20 passes through a heater 23 to another separator 24 and condenser 26, further purified and pumped to condenser 22. Urea solution is concentrated in an apparatus 28, separated from ammonia in a separator 15 and granulated in a tower 33.