Optimized operation of the high-pressure system for urea production by carbon dioxide stripping method

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The main factors affecting the reaction equilibrium include temperature, N/C ratio, H/C ratio, and operating pressure; therefore, optimization should be carried out in these areas. 1. Raising the reaction temperature appropriately helps to increase the conversion rate of urea, but the temperature should be kept slightly below the equilibrium pressure of the system under operating pressure, otherwise the lining of the synthesis tower is prone to corrosion. 2. Increasing the ammonia-to-carbon ratio appropriately facilitates the shift toward product formation and improves the conversion rate of carbon dioxide. However, the optimal ammonia-to-carbon ratio should be that corresponding to the maximum urea production rate. In the carbon dioxide stripping process, a ratio of around 2.9 is most suitable; otherwise, an excessive ammonia-to-carbon ratio may disrupt the auto-ignition equilibrium, lead to a decrease in reaction temperature, and reduce the conversion rate. 3. When the water-to-carbon ratio is too high, the outlet liquid temperature in the high-methanol cooling unit and the synthesis tower increases, the synthesis pressure drops, and the conversion rate also decreases. Moreover, more methammonium is decomposed in the stripping tower, resulting in an increased steam consumption; accordingly, the load on the low-pressure recovery system rises. To control the water-to-carbon ratio, it is necessary to adjust the water content in the methammonium solution. Controlling this water content requires comprehensive adjustments to both the low-pressure circulation recovery system and the hydrolysis desorption system. Therefore, increasing the volume of the circulation recovery system and raising the concentration of the methammonium solution is an effective approach. 4. The urea synthesis reaction is a volume-reducing reaction; increasing the pressure appropriately facilitates this reaction, and the conversion rate of urea also increases with rising pressure. Once the conversion rate reaches a certain value, further increases in pressure are ineffective. Therefore, it is most appropriate to operate at a pressure that is about 2–3 Mpa higher than the equilibrium pressure. 5. The urea synthesis reaction refers to the dehydration of methylammonium to produce urea. The rate of dehydration of methylammonium increases as the temperature rises and the ammonia-to-carbon ratio increases. In order to ensure that this dehydration reaction proceeds to completion, it is necessary for the material to have sufficient residence time in the synthesis tower; increasing this residence time appropriately helps facilitate the production of urea.