Published September 1991
Ammonia and carbon dioxide react under high pressure to form ammonium carbamate, which is then converted to urea by elimination of a molecule of water. While the formation of ammonium carbamate is rapid and essentially complete under the reaction conditions used in commercial plants, the dehydration of ammonium carbamate to urea is relatively slow and the conversion is limited by equilibrium considerations. The unconverted ammonium carbamate must be separated from the urea solution and recycled to the process. Generally, this is accomplished by decomposing the unreacted ammonium carbamate to ammonia and carbon dioxide, recombining them at a lower temperature to form ammonium carbamate, then returning the ammonium carbamate to the reactor.
Heat must be supplied to the ammonium carbamate solution to cause it to decompose to ammonia and carbon dioxide. On the other hand, heat is generated by the formation of ammonium carbamate. Urea Technologies Inc. (UTI) licenses a Heat Recycle Urea (HRU) process that reuses the generated heat directly for decomposing the ammonium carbamate, rather than for generating steam.
Also, approximately 46% of the carbon dioxide feed is introduced to the process at a lower pressure in the HRU process; this low pressure carbon dioxide is combined with ammonia to form ammonium carbamate and is pumped to the process, thereby reducing compression costs. Other special features of the HRU process include a special reactor design, a high ammonia-to-carbon dioxide feed ratio, and a high per pass conversion of carbon dioxide (ca.76%).
In this review, we present a preliminary process design and economics for UTl's HRU process. The economics are compared to those for Montecatini's Isobaric Double Recycle (IDR) process. Our estimates show that the HRU process requires less capital investment and results in a product value which is lower than that for the IDR process in a 1,500 ton/day facility.