Problems with PSA hydrogen production adsorbents?
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Current situation: The PSA hydrogen production unit in our plant was originally designed to produce 50,000 M3/h, but the current output is 43,000 M3/h; if the production volume increases, the CO content in the hydrogen product will exceed the specified limits. The adsorbent has now been in use for 13 years; its designed lifespan is 10 years, and it is a adsorbent from German company Linde. Question 1: There is a leak within the heat exchanger where the medium-pressure gas and the feedstock exchange heat; the feedstock leaks into the medium-pressure gas, resulting in an increase in C4–C7 hydrocarbons in the crude hydrogen. Upon entering the PSA unit, these hydrocarbons are adsorbed by the adsorbent, thereby affecting its activity. We analyzed the crude hydrogen entering the PSA, and found that the concentration of C4 hydrocarbons was 0.2% (with no other hydrocarbons present). However, this has not been analyzed before, so there is no comparison available; it’s unclear whether this value can be used to determine whether there is leakage in the heat exchanger used for heat exchange between the intermediate-pressure gas and the feedstock. Please help us determine this. Are there any other companies that have requirements regarding the hydrocarbon content in the crude hydrogen entering their PSA units? What is the normal hydrocarbon content in crude hydrogen entering an PSA unit during normal operation? Question 2: After C4–C7 hydrocarbons are adsorbed by the adsorbent, can they be desorbed under normal operating conditions? Why?
It’s the effect of components above C4; it’s possible to get the product after operating for a long time under normal conditions, but it’s better to perform vacuum regeneration before using it again. By the way, it seems you asked this question a few days ago
Moderator, my first question hasn’t been answered. “Are there any other units that have requirements regarding the hydrocarbon content in the crude hydrogen fed into the PSA? What is the typical hydrocarbon content in the crude hydrogen entering the PSA under normal operating conditions? Could our situation be due to a leak in the heat exchanger earlier in the process? ”That’s the key; only then can we determine whether it’s regeneration or treatment Moderator, you mentioned that vacuum regeneration can be used to remove components with molecular weights above C4 – is there any evidence for this? For example, arethere isotherm curves showing the adsorption of components above C4 by molecular sieves, or any application examples from other petrochemical facilities?
By the way, the moderator said I asked this question before, but not on this exact topic; if it’s similar, then it’s only somewhat related. Previously, the question was about how to regenerate it; this time, it is about whether the deactivation of the adsorbent is caused by internal leakage in the heat exchanger. If you want to regenerate it, the first thing you should know is how to inactivate it, right? Also, we are currently using product hydrogen purging for continuous production, and the effect of product hydrogen purging is such; could vacuum pumping achieve a similar regenerating effect? This post was last edited by lionangelwkp on 2007-12-9 20:30]
Under normal conditions, there should be no components above C2 in the PSA feed; if such components are present, it indicates a leak in the heat exchanger between the medium-pressure gas and the feed gas. The purification of PSA adsorbents can be achieved through thermal nitrogen regeneration. In typical hydrogen production plants, there is a nitrogen line at the inlet of the feed gas heater, and another line leading to the PSA unit at the outlet; this setup can be utilized to heat the nitrogen and thereby regenerate the adsorbents.
““There will be no components above C2 in the PSA feed” – what evidence does the moderator have for this? I haven’t seen any indication that the reaction is 100% complete after the raw materials have been transformed; could you please provide details on where this information can be found?
The PSA inlet gas is the output gas from the medium-temperature reactor; under normal production conditions, it should not contain C4 hydrocarbons. The analysis you conducted shows their presence, which may indicate a leak in the heat exchanger. But what is your feed gas composed of? It can’t be just C4 hydrocarbons, right? Additionally, if there is indeed a leak, the adsorption capacity of the PSA unit will be affected, as high-molecular-weight hydrocarbons are theoretically difficult to desorb. Your adsorbent has been in use for 13 years, although its designed lifespan is 10 years – I think it’s time to replace it.//
Our raw materials include coking dry gas, hydrogenation dry gas, and light naphtha. I think it was Jinling Petrochemical; once the adsorbent reached the end of its useful life, it was regenerated using hot nitrogen so that it could be used again. This post was last edited by lionangelwkp on 2007-12-10 20:03]
Based on my experience, internal leakage in the equipment is possible; it should be checked. The proportion of components above C4 is approaching 0.2%, which is a relatively high value; the existing Linde molecular sieves are struggling to handle this situation. After one cycle of thermal nitrogen regeneration, it is necessary to check the degree of pulverization. Generally speaking, it’s been 10 years, and considering the use of the adsorption tower, I recommend replacing the molecular sieve. The price now is much lower than it was 10 years ago.
Everyone talks about internal leakage, but when making a decision in production, there needs to be evidence. Production cannot stop; check to make sure there are no leaks before restarting – losses occur as a result of chain reactions. So, doubting internal leakage is not acceptable; what matters is having evidence.
First, it is necessary to check for internal leaks. Hydrogen-rich dry gas, coking dry gas, and light naphtha contain little carbon, so the analysis results may be inaccurate; analyzing the composition of the desorbed gas further will help determine whether there is an internal leak. After 10 years of use, the adsorption capacity of PSA molecular sieves declines due to the absorption of water and other organic substances. Their activity can be restored to 90% through thermal nitrogen regeneration; therefore, it is recommended to carry out such regeneration once, as washing alone does not achieve the desired effect