【Urea Knowledge Q&A】One Question per Day 2010.03.12
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What determines the pressure for medium-pressure decomposition and absorption? Please hide the answer. In the medium-pressure decomposition and recovery system, there are processes of decomposition, as well as absorption and condensation. Decomposition requires low pressure to achieve high efficiency, while absorption and condensation require higher pressure to ensure efficient gas absorption and condensation; thus, the selection of pressure in a medium-pressure system represents a balanced trade-off between these opposing requirements. The pressure is determined based on the cooling water temperature of the ammonia condenser; to convert gaseous ammonia into liquid ammonia, according to the principles of single-component gas-liquid equilibrium, the condensation temperature and pressure must be set at or above the vapor equilibrium point. In a single-component gas-liquid equilibrium system, there is 1 degree of freedom – once the condensation temperature is determined, the condensation pressure is also fixed. The condensation temperature of ammonia is influenced by the temperature of the condensate water, which in turn is determined by various factors such as climate and water quality. Therefore, in the ammonia condenser, once the condensation temperature of ammonia is determined, the condensation pressure is also fixed; and once the condensation pressure of ammonia is determined, the lowest pressure in the medium-pressure decomposition and recovery system is also established. The selection of pressure also needs to take into account the composition of the ammonium methoxide solution after absorption in the medium-pressure absorption tower, in order to meet the system’s water balance requirements; at the same time, the decomposition rate of ammonium methoxide during medium-pressure decomposition and the evaporation rate of ammonia must also be considered. Therefore, the selection of the medium-pressure cycle pressure: (1) it must first be subject to the condensation of ammonia. (2) The components and temperature of the medium-pressure absorption liquid need to be considered comprehensively. (3) Consider the medium-pressure decomposition temperature and decomposition rate. (4) Consider the partial pressure of the inert gas. In summary, the medium-pressure cycle pressure is set at 17 kg/cm2.
Based on the properties of the gas being analyzed and the temperature
It is a unity of contradictions: the pressure required for decomposition is low, and the decomposition efficiency is high. Absorption and condensation require high pressure for efficient absorption and condensation of gases. Therefore, its selection must first take into account the condensation of ammonia; secondly, the decomposition temperature and rate need to be considered. Additionally, the composition and temperature of the medium-pressure absorption liquid are important factors, as well as the partial pressure of inert gases. It is generally controlled at 1.7 Mpa
The last edit to this post was made by Luo Hu 15185080221 on 2010-3-14 at 13:22. The amount of unreacted material resulting from medium-pressure decomposition accounts for 90% of the total unreacted material; thus, the decomposition rate of methylammonium is approximately 88%, while the overall ammonia evaporation rate is about 89%. Then CO2 is absorbed in the medium-pressure absorption tower, and excess ammonia is condensed in the ammonia condenser; it is subsequently pumped into the synthesis tower by a high-pressure ammonia pump and a first-stage ammonium methanate pump. The pressure selection for the medium-pressure decomposition and absorption system should take the following aspects into consideration: 1. The equilibrium concentration of the decomposition reaction ； 2. Equilibrium concentration of CO2 absorption ； 3. The equilibrium pressure required for ammonia condensation at a certain temperature ； 4. The effect of inert gas content on the partial pressures of various components.