Patent ID: 8466315

Claim:
A urea granulation process with scrubbing system including several waste streams for removal of dust and ammonia from the off-gas of a urea granulation unit which comprises: a urea granulator; a granulator scrubber dust stage; a granulator scrubber acid stage; product coolers; a product cooler scrubber dust stage; an evaporation unit; and a condenser unit; the process comprising: sending a first stream of fresh air, running through a first sequence of process steps, into the urea granulator; whereby drawing off dust- and ammonia-laden air from the granulator and conveying it into a granulator scrubber dust stage followed by a granulator scrubber acid stage, in which scrubber acid stage the ammonia-laden air is contacted with an acid in liquid phase and ammonia is scrubbed from that air by the generation of an ammonium salt; sending a second stream of fresh air through a second sequence of process steps, to cool the product drawn off from the urea granulator; whereby the cooling is performed in product coolers; the air is heated and afterwards conveyed to a product cooler scrubber dust stage; drawing clean off-gas from the granulator scrubber acid stage and drawing clean off-gas from the product cooler scrubber dust stage, and releasing the clean off-gas into the atmosphere; wherein the scrubbing system is a complete closed system, whereby: ammonium salt solution stream from the granulator scrubber acid stage is fed into the product cooler scrubber dust stage, whereby the ammonia of the dust-laden air stream exiting the product coolers is removed; released liquor from the product cooler scrubber dust stage and released liquor from the granulator scrubber dust stage is sent to the evaporation unit; a vapour stream from the evaporation unit, which contains ammonia, is sent into the condenser unit which releases a liquid process condensate, the liquid process condensate being sent into the granulator scrubber acid stage; and a concentrated liquor stream from the evaporation unit, containing urea and ammonium salt, and a urea melt from a synthesis unit are separately conveyed into the urea granulator.