【Urea Knowledge Q&A】One Question per Day 2010.03.17

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What are the methods used in the production process to increase the carbon dioxide conversion rate in the synthesis tower by adjusting the water-to-carbon ratio in the absorbent solution of the absorption tower? The water-to-carbon ratio in the medium-pressure absorption tower affects the water-to-carbon ratio in the feed to the synthesis tower; therefore, a lower water-to-carbon ratio in the absorbent solution is beneficial for increasing the carbon dioxide conversion rate in the synthesis tower. When the water-to-carbon ratio in the absorption solution increases, there are two adjustment methods to choose from, depending on the actual operating conditions at that time. The first adjustment method is as follows: when the contents of ammonia and carbon dioxide in the feed to the synthesis tower remain constant, if the water-to-carbon ratio in the absorption solution increases, then the water-to-carbon ratio in the feed to the synthesis tower also increases, which leads to a decrease in the conversion rate of carbon dioxide and an increase in the amount of unreacted material that can be recovered. If the original concentration in the absorption solution is to be maintained, it is necessary to increase the amount of water used as the absorbent; this results in an increase in the amount of water circulating in the system, as well as an increase in the amount of ammonium methoxide solution that returns to the synthesis tower. This shortens the residence time of the materials within the synthesis tower, further reducing the conversion rate. When the conversion rate drops to a certain level, the system enters a vicious cycle; at this point, the only way to adjust the system and reach a new equilibrium is by reducing the amount of unreacted material recovered and discharging the excess medium-pressure absorption liquid outside the system. The second method is: as the water-to-carbon ratio in the absorption liquid increases, the carbon dioxide conversion rate in the synthesis tower declines and the amount of unreacted material rises. If the amount of water used as absorbent is not increased, then, where possible, by raising the concentration of the medium-pressure absorption liquid and reducing the water-to-carbon ratio of the methammonium solution, it is also possible to restore the conversion rate in the synthesis tower and bring the system to a new equilibrium. When changing the water-carbon ratio of the medium-pressure absorption solution, it is necessary to take into account the water-carbon ratio of the feed to the synthesis tower. As the water-carbon ratio in the ammonium methoxide solution decreases, its melting point rises and its unsaturation level drops; as a result, ammonium methoxide crystals precipitate out of the solution. At the same time, the carbon dioxide content in the gas phase increases, leading to poor absorption efficiency. Therefore, when considering the medium-pressure recovery solution, it is necessary to take into account both the carbon dioxide conversion rate in the synthesis tower and the carbon dioxide absorption rate in the medium-pressure absorption tower. In the production of urea in medium-sized urea plants, the water-to-carbon ratio in the medium-pressure absorption liquid is 1.8–2.0 (on a molecular basis). Ignoring the amount of hydrolysis, the relationship between the water-to-carbon ratio in the medium-pressure absorption liquid and that in the feed to the synthesis tower can be expressed by the following formula: M = m(1 – xCO2), where m represents the water/CO2 ratio (on a molecular basis) in the ammonium methoxide solution; xCO2 represents the CO2 conversion rate in the synthesis tower; and M represents the water/CO2 ratio in the feed to the synthesis tower. It can be seen from this formula that, when the CO2 conversion rate xCO2 remains constant, an increase in the water-to-carbon ratio in the ammonium methoxide solution, i.e., an increase in m, leads to an increase in the water-to-carbon ratio M in the feed to the synthesis tower. Conversely, when m remains constant, an increase in xCO2 results in a decrease in M (i.e., a reduction in the amount of liquid fed to the pump). Please hide the answer