A strange phenomenon
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Our factory uses a full-circulation aqueous solution process for urea production; the exhaust gas exiting the scrubber is subjected to pressure absorption, and the exhaust gas after absorption contains approximately 5.0% ammonia. To further reduce the ammonia content in this gas, it was attempted during the technical upgrades to introduce this gas into the original tail suction tower; however, the results were unsatisfactory, as the ammonia content in the gas exiting the tail suction tower increased, sometimes reaching as high as 10%. I would like to ask fellow sailors, why is this?
What is the concentration of your absorbent solution? Is water or concentrated dilute ammonia being used?
The absorbing solution is still the original ammonium carbonate solution, with an ammonia concentration of about 5%
What is the outlet pressure of the absorbent solution? There may be a situation of absorption liquid flashing. To achieve good absorption results in the end, it is generally necessary to divide the absorption tower into upper and lower sections, with water being used for absorption in the upper section.
Then you need to compare the data from before the renovation with the data from after it – are there any changes in the exhaust gas data of the tail suction tower before and after the modification? It’s possible that the device has a low absorption capacity! Think about it: with the low-pressure load already high, it’s simply impossible to have the tail suction tower absorb inert gases as well! Unless it is changed to two sections for absorption, as mentioned above!
Before the modification, the tail suction tower was used to absorb the exhaust gas from the second stage; since the volume of this exhaust gas was small, no measurement was taken of the ammonia content in the exhaust gas after it passed through the tail suction tower. During the modification, it was mainly taken into account that a portion of the exhaust gas had already been pressurized and washed until its ammonia content dropped to 5%. If this 5% ammonia-containing gas is then washed at atmospheric pressure in a tail scrubber, wouldn’t even less ammonia remain? However, the operating conditions after the technical upgrades were not at all as expected; instead, there was an increase in ammonia levels.
For absorption, pressure and low temperature are favorable for it. If you reduce the pressure of the gas after pressure-assisted absorption, the concentration of the absorbent solution remains unchanged – how can absorption continue in such a situation? Furthermore, when the ammonia-containing inert gas comes into contact with the absorbent solution, it reduces the ammonia partial pressure at the liquid surface, causing some ammonia to transfer from the liquid phase to the gas phase; as a result, the ammonia content in the exhaust gases increases.
Affected so much by temperature and pressure, is such a technical upgrade really necessary?
Reply to 1# ty0ty00: The ammonia partial pressure in the gas phase may be lower than that in the liquid phase; coupled with the presence of inert gases, this causes the ammonia in the solution to be stripped into the gas phase. After our inert washing gas is absorbed in an absorption tower using an ammonium carbonate solution containing 4-5% ammonia at atmospheric pressure, the ammonia content in the gas exiting the tower is 10-13%