Ammonia charging issues during the initial startup of the ammonia synthesis system
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How to understand the low water vapor pressure of ammonia water? Is ammonia easier to separate, or is water easier to separate from the process gas? Why? At the beginning of operation in our plant, a low-temperature ammonia cooler was not installed; therefore, there is no issue of water freezing. Is it still necessary to inject ammonia? ? ? This post was last edited by snowdfr on 2009-3-15 22:37.]
According to the rules of the \"colligative properties\" of solutions, in dilute solutions, the values of parameters such as the vapor pressure of the solvent, the freezing point depression, the boiling point elevation, and osmotic pressure are all related to the amount of solute present in the solution. Therefore, the partial pressure of water in ammonia solution decreases. The saturated vapor pressure of water decreases; in other words, the amount of water in the mixture under the same conditions reduces. A lower amount means it is easier to separate. Ammonia cooling is not used at the beginning of operation because it is a period of temperature rise during which no chemical water has been generated yet. Once chemical water is produced, ammonia cooling must be employed; the lower temperature helps to significantly reduce the moisture concentration in the gas entering the tower, thereby preventing repeated oxidation and reduction of the catalyst. Based on the analysis above, charging ammonia before heating and reduction is necessary.
It depends on the catalyst; generally, the system needs to be filled with ammonia for catalysts. However, for catalysts like A301, which are of the true low-temperature type, they exhibit significant activity at low temperatures (when the temperature at the hot spot reaches 330 degrees), and the synthesis reaction is quite pronounced, so there is no need to fill the system with ammonia. We use an A301 catalyst, and no ammonia needs to be charged for each reduction.
At first, the ammonia cooler was not used in order to achieve a faster temperature rise; it was only added when physical water was about to appear. We kept feeding ammonia throughout the process
Without adding ammonia, the temperature at the water cooling outlet is 30 degrees, the pressure is 4–5 MPa, and the water vapor content is 0.1%, which corresponds to 0.8 g/m3; whereas the controlled water vapor concentration at the outlet of the tower is only 3 g/m3, or 0.4%. Failing to use ammonia cooling will result in an excessively high concentration of water vapor in the circulating gas, affecting the quality of reduction. To prevent freezing, ammonia has to be added to lower the freezing point of the ammonia solution, thereby reducing the vapor pressure of water as well.
Reason for filling the system with ammonia before ammonia catalyst reduction
This issue has been discussed before: http://bbs.hcbbs.com/viewthread.php?tid=416590&highlight=%B3%E4%B0%B1 Please search first when creating a topic.