How to improve the PSA hydrogen recovery rate
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I would appreciate the advice of experts: Our company’s PSA unit consists of ten towers. The feed gas is methanol synthesis off-gas in an amount of 12,000 cubic meters; through the PSA process, hydrogen with a purity of over 99% is obtained, and this hydrogen is combined with nitrogen supplied by air separation to produce ammonia. Currently, the carbon dioxide content in the feed gas is 14%, which is much higher than the specified value of 5%. The hydrogen recovery rate is around 60%, and it is no longer possible to extend the adsorption time due to concerns that carbon dioxide might penetrate the adsorbent. The following plan has been devised to increase production: first, remove the ten-tower molecular sieve to increase the amount of activated carbon. II. Add a carbon dioxide adsorption unit before the PSA. III. Add a membrane device after gas separation to recover hydrogen. Which option is more valuable? What are the disadvantages of the other options, or are there any better suggestions? I am not very familiar with membrane processes, and I would like to ask: for a situation where the gas flow rate is 5,000 cubic meters, the hydrogen content is around 25%, and the remaining gases are mainly nitrogen, carbon dioxide, carbon monoxide, and methane, with a pressure of 10 KPa, how much investment would be required to set up a membrane-based hydrogen extraction system? What devices are there? How to improve it
I think Option 2 is better. In Option 1, since the adsorption tower has already been determined, although it is possible to increase the amount of CO2 that can be adsorbed, it is not necessarily possible to significantly improve the hydrogen recovery rate ； In Option 3, the pressure of the gas being separated is too low; using a membrane for hydrogen extraction requires an additional compressor. The compression ratio of this compressor is too high, and the hydrogen content is only around 25%, meaning that most of the energy is wasted – resulting in inefficient use of energy.
Option 2 is equivalent to expanding the scale of PSA, using a two-stage approach
Extracting the molecular sieve and then filling it back in is an unacceptable approach. At the moment, CO2 levels have increased by 5%; the only concern is penetration. If there is no actual penetration, it is recommended to continue using it as is, as long as you closely monitor the hydrogen levels of the product. Once it is determined that the hydrogen produced does not meet the standards, options such as reducing the amount of hydrogen generated, ensuring its purity, or shortening the adsorption time to maintain hydrogen purity can be considered. If the purity still cannot be ensured using these two methods, the adsorbent can be replaced directly, this time by using more activated carbon. Of course, if it’s just to be prepared for any situation, Option 2 is recommended.