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In all CO2 stripping processes, ammonia and carbon dioxide are fed directly to the synthesis section.
In a first fast exothermic reaction between ammonia and carbon dioxide, ammonium carbamate is formed. The second reaction is the endothermic dehydration of carbamate to urea and water. Optimum process conditions of approx. 145 bar and 180°C and a nitrogen / carbon (N/C) ratio of 3 are maintained.
The dehydration of ammonium carbamate is an equilibrium reaction. For this reason, the reaction mixture from the reactor is subjected to high pressure stripping, using carbon dioxide as the stripping agent to remove non-reacted ammonia and carbon dioxide.
The stripper gases are sent to the pool condenser, where they are condensed in a pool of liquid together with fresh ammonia and overhead vapors. The pool condenser is basically a horizontal vessel with a submerged U-tube bundle. Low-pressure steam is generated within the tubes of the heat exchanger. The pool condenser combines the function of the conventional falling film-type carbamate condenser with part of the urea reactor function. Thus, the reactor volume can be decreased by about 30-40% compared with the conventional process. This is the particular advantage for high-capacity single-line plants since the reactor is one of the heaviest items of equipment.
Exhaust gases, which are separated from the liquid at the reactor top, are scrubbed in a high pressure and a low-pressure scrubber, thus minimizing ammonia emissions.
In view of the low ammonia and carbon dioxide concentrations in the stripped urea solution, a separate high pressure ammonia recycling stage is not necessary. The ammonia and carbon dioxide still contained in the urea solution discharged by the stripper are recovered in the low pressure recirculation stage.
The urea solution leaving the recirculation section is further concentrated in the evaporation section from approx. 74% to 96% or 98.5% by weight, depending on the requirements of the downstream fluid bed granulation process. Evaporation takes place in a vacuum section to minimize biuret formation.
The entire process condensate from the evaporation section is treated in the desorption and hydrolysis section. Ammonia and carbon dioxide are stripped off in a stacked two-desorber column whereas small amounts of urea are dissociated in the hydrolysis column.
The purity of the treated condensate allows it to be used as make-up water for the cooling water cycle. Furthermore, it may even be used as boiler feedwater after polishing. Consequently, the Stamicarbon urea process has no waste water effluent.