In 1967 Stamicarbon revolutionized the urea process by the invention of the HP CO2 Stripper by Mr. Petrus JC Kaasenbrood. US patent 3,356,723 describes the invention of the HP CO2 stripper.
“In a continuous process for the preparation of urea wherein NH3 and CO2 are reacted at elevated temperature and pressure to continuously provide an ammonium carbamate melt, thereafter in an autoclave, said melt being converted into a urea solution containing ammonium carbamate and the carbamate being stripped from said solution by decomposing said carbamate into NH3 and CO2 by heat and expelling NH3, CO2 and H2O from said solution, the improvement which comprises continuously stripping the urea solution with CO2 in a stripping zone outside said autoclave and at a pressure of at least 50 atmospheres up to urea synthesis pressure wherein the urea solution flows downwardly along the inside of externally steam heated tubes in the stripping zone and CO2 stripping gas passes upwardly in said tubes in contact with said urea solution, the pressure of the heating steam being in the range of 15-30 atm., whereby NH3 and CO2 are expelled from said solution, condensing the resulting mixture of CO2 gas and gases expelled from said urea solution after addition of further NH3 and at a pressure of at least 50 atmospheres up to urea synthesis pressure to form a carbamate solution and returning the thus formed carbamate solution to said autoclave for further urea synthesis.”
In the HP CO2 stripper urea reactor effluent is thus treated counter currently with carbon dioxide in a stripper at high pressure in order to dissociate carbamate and at the same time enable easy condensation of the carbamate gasses without the addition of water. Preferably this is done at the same pressure as the reactor is operating. The condensation of strip gasses will produce steam leading to significant reduction in steam consumption to produce urea.
This patent from 1967 has revolutionized urea technology and nowadays all modern urea processes operate a HP stripper.
A HP CO2 stripper did lead to three main benefits:
1. the carbamate could be recycled at synthesis pressure so now extra water needed to be added to recycle the carbamate;
2. no medium pressure recirculation section was needed anymore and
3. with the condensation of strip gasses in the high pressure carbamate condenser low pressure steam could be produced, which could be used in the downstream sections leading to a reduction of the steam consumption of a urea plant of about a factor two.
This originally Chinese Technical Paper provides an overview of the causes why the efficiency of a CO2 stripper reduces and at the same time it provides practical solutions to recover the efficiency again.