Why is excess ammonia used in the synthesis reaction of urea?
Thread Content
In the synthesis reaction of urea, the raw material gases to generate urea are CO2 and NH3. This reaction is a reversible reaction. In order to make the reaction proceed in the direction of the forward reaction, excess NH3 has been used in production control. However, the market price of CO2 is cheaper than NH3. Why not use excess CO2 to control the reaction in the direction of the forward reaction. I hope you can discuss it and I will continue to think about it so that I can share it with you!!
For the synthesis of urea, when the temperature is 170~190°C, the molar ratio of ammonia to carbon dioxide is 2.0, and the pressure is high enough to keep the reactants in a liquid state, the conversion rate of methylammonium into urea (calculated as CO2) is 50%； The reaction rate increases with the increase of temperature. When the temperature remains unchanged, the conversion rate increases with the increase of pressure. After the conversion rate reaches a certain value, the pressure continues to increase and there is no obvious increase. At this time, almost all the reaction mixture exists in a liquid state. Increasing the molar ratio of ammonia to carbon dioxide can increase the conversion rate of carbon dioxide and reduce the conversion rate of ammonia. In the actual production process, since ammonia is easier to recover than carbon dioxide, excess ammonia is used. Generally, the molar ratio of ammonia to carbon dioxide is ≥3.
1. The presence of excess ammonia can increase the urea synthesis conversion rate. Every time the NH3/CO2 ratio increases by 0.2, the conversion rate increases by 1.0% to 1.5%. 2. The presence of excess ammonia is conducive to the generation of methane ammonium, allowing all carbon dioxide to react into methane ammonium, maintaining the most appropriate reaction temperature, the highest equilibrium conversion rate, and the least corrosion rate of the reaction medium on the urea synthesis tower material. 3. Excess ammonia can combine with the water generated by the reaction to reduce the concentration of reactant water, thereby increasing the urea formation reaction. 4. Excess ammonia can inhibit side reactions in the urea tower (generating biuret and hydrolysis of urea) to increase the urea conversion rate. Can the above 4 advantages be achieved or better achieved by using excess carbon dioxide? I think it's hard for us to answer. 5. If excess gaseous carbon dioxide is used, it will inevitably result in more gas phases for urea synthesis. It remains to be discussed how to improve the urea synthesis conversion rate and the corrosion rate of the medium to the urea synthesis tower materials. 6. There are other reasons for using excessive ammonia: Is the process of carbon dioxide recovery more complicated than that of ammonia recovery and is the feasibility of industrialization of recovery technology?
In addition, in terms of system control, the pressure of the high-pressure system with excess NH3 is easy to control, and the pressure fluctuation is gentle when the N/C ratio changes. If there is excess CO2, let alone the product quality will not be qualified, even the system pressure cannot be controlled.