I would like to ask a senior expert about an issue with one of the streams in the LUCCHI low-temperature methanol washing process!
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At the bottom of the absorption tower, a methanol-rich liquid absorbs gases such as H2S, CO2, HCN, and NH3 from the shift gas; thereafter, this liquid from the tower bottom is flashed in a pre-washing flash tank before entering the middle section of the heat regenerator. There are a few questions: 1. What is the composition of the circulating gas that comes out of the flash tank? Is it mainly CO2? Then go to the recycled gas and add the reforming gas again for absorption in the absorber? Then the question is, isn’t CO2 a gas that needs to be removed? How did it get back into a loop again? 2. If what I said is incorrect, then it is CO and H2 that evaporate; in that case, it seems more logical for them to end up in the shift gas. Wouldn’t the product from the bottom of the flash tank then contain a large amount of CO2? Methanol vapor is used for stripping H2S in the thermal regenerator; so what happens to most of the CO and H2? 3. HCN and NH3 gases are absorbed during discharge, and they cannot be removed even in the thermal regenerator? I think what I said is incorrect. Please advise me, senior. I’m sorry if my explanation isn’t clear. Thank you!
1. The vaporized mixture mainly consists of 35% CO2, 30% CO, 30% H2, along with trace amounts of H2S and N2. CO2 is not removed here; it is eliminated in the vaporization section at the top of the reabsorption tower that follows. 2. The CO2 content in the material discharged from the bottom of the flash tank is around 30, along with small amounts of CO and H2S; these are removed through flashing in the reabsorption tower. 3. The methanol that has absorbed HCN and NH3 gases during the discharge process enters the thermal regeneration tower, where it is regenerated; ammonia-containing methanol is then discharged periodically to reduce the amount of ammonia in the system.
In the methanol washing unit, the solubility of hydrogen in methanol decreases as the temperature drops. By performing vacuum flashing of the by-methanol, the temperature of the by-methanol liquid is reduced on one hand, while the useful gas H2 is recovered on the other hand