Study on the Expansion and Renovation Plan for the Evaporation System of Lutianhua’s 1620 t/d Urea Plant – Li Yong, Master’s Thesis, Sichuan University, 2003.

China now has hundreds of urea production units, of which around 30 are large-scale facilities with a production capacity of 500 kt/a or more. The total output of urea accounts for 60% of the total nitrogen fertilizer production. With China’s accession to the WTO and the continuous rise in prices of raw materials such as natural gas, competition in the urea industry is becoming increasingly fierce. Faced with these significant market challenges and pressures, it is urgent to address the bottlenecks in urea production units and increase their production capacity in order to reduce the consumption per unit of product. This article focuses on the analysis and design of the evaporation system, which represents a bottleneck in 500 kt/a urea production units, as part of the efforts to expand their capacity. Through a detailed analysis of the thermosensitive properties of urea solutions during the evaporation process, the mechanisms, reaction rates, and conditions for side reactions such as the formation of biuret, urea hydrolysis, and urea decomposition were elucidated. It was found that the amount of by-products generated increases with the evaporation temperature and the residence time of the urea solution. Taking into account these thermosensitive properties and the characteristics of various types of evaporation heaters, a rising-film evaporator with high heat transfer coefficients and short residence times was chosen as the heater for evaporating the urea solution. An analysis of the equilibrium phase diagram of the Ur-HO system showed that it is impossible to concentrate the urea solution from a low concentration to molten urea in just one step, as this would require passing through the crystallization zone of the saturated solution. This approach also does not meet the requirements for operating conditions necessary to minimize the impact of side reactions. Therefore, a two-stage vacuum evaporation process was adopted for the evaporation of urea solution. Based on the equilibrium phase diagram of the Ur-HO system, appropriate process parameters for the two-stage vacuum evaporation were determined: for the first stage, the temperature was 130°C, the pressure was 32 kPa (absolute, or 240 mmHg), and the urea concentration was 95% (by mass); for the second stage, the temperature was 140°C, the pressure was 3.4 kPa (absolute, or 25.5 mmHg), and the urea concentration was 99.7% (by mass). A thorough analysis of the heat transfer process in the rising-film heater led to the development of models for calculating the heat load on the vapor-liquid phases, the heat transfer coefficient of the steam condensate film, the heat transfer coefficient of the urea solution film, and the calculation methods for the evaporation heater. By comparing the calculated values based on the original design capacity of 1620 t/d with the actual capacity of the facility, the reliability of the mathematical models was verified. These models were then used to determine the modification plans for the evaporation heater, with a target capacity of 2000 t/d.