In most of all urea plants urea and water is separated in an evaporation or concentration section at vacuum pressures. The urea melt can have a concentration of 99.6 wt% when the urea melt is send to a prilling tower or a Sandvik Process Rotoformer or a somewhat lower concentration when a granulation technology is applied.
It is obvious that it is important to control the temperature of the urea melt just above the crystallization temperature in order to maintain it in liquid condition. On the other hand when the temperature is too high several negative side reactions in the urea melt do occur, which case higher biuret contents in the final product and higher ammonia and urea dust emissions from the finishing section.
Therefore typically urea melt lines are jacketed pipes and the pressure of the steam in the jacket is controlled by the required urea melt temperature. Urea melt pumps and 3-way valves (at the suction and discharge side of the urea melt pumps and just upstream the prilling bucket) are functioning is a challenging environment: handling a process liquid close to its crystallization temperature while at the same time a proper sealing is vital. The picture shows the sealing challenges of a traditional urea melt pump. Fortunately successful innovations are available, which realize a reliable solution.
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