The mixture of gases contains about 5vol.% of HCl, 25-30vol.% of hydrogen (H2), and the rest is water steam (65-70vol.%). I want to separate hydrogen to use it afterwards for power generation (i. e. in a fuel cell). I think one of the recommendations is using certain types of membranes, any ideas?
The mixture of gases is coming from the exothermic reaction between Al(OH)3 and HCl (mainly), carried out during the preparation of PAC (aluminium polychloride) and ACH (aluminium hydroxide).
You may use special types of molecular sieves which traps/ adsorbs HCl and water molecules (as their sizes are bigger than H2 molecule) and leaves H2 molecules.
I am assuming this as lab scale model.
Hello,
I've just an idea that may not need membranes. Maybe you can just condensate the water vapour and the HCl will dissolve in it forming hydrochloric acid that you may reuse in in your exothermic reaction. According to literature solubility of HCl in water at 20°C is some orders of magnitude larger than hydrogen: 720 gHCl/L compared to about 2mg/L.
The question regarding the idea is your process conditions (pressure and temperature) as well as the material that you use.
If you try this idea, I like to hear whether or not it works.
Good luck with your research,
André.
Thanks a lot for your answers, in fact is a medium scale process. It has been calculated per annum the amount of H2 produced is around 30 Tons, that is why we want to recover it. Thus the moelcular sieves might be a good idea but it sounds expensive, not sure if there are any medium-big scale sieves used so far in the industry.
I have considered condensation before, as will take out some HCl together with the water (indeed due to the HCl solubility), then might be suitable to recover HCl and then further use H2 in a fuel cell or so. I will just need to double check if still some HCl will remain in the gaseous mixture.
Dear Paula,
you certainly need to check it, but if you have a completely vapour free gas with traces of HCl left you can adsorb the residual HCl in a second stage either on a solid substrate or in a liquid that specifically absorbs HCl.
Especially regarding absorption in water you should take into account the high absorption enthalpy of HCl in water, since it is exothermic, if I recall correctly. I am quite sure you will find some information regarding HCl absorption in water if you check some chemical/thermal engineering textbook. Also you may use process simulators such as Aspen or ChemCAD to check for general applicability of the idea.
Regards,
André
Dear Andre,
Thanks a lot for your help, I will definitely do that, although the main aim is to separate effectively the hydrogen to use it in further processes (e.g. fuel cells), due to the considerable amounts produced.
Will let you know if works.
Regards
Paula
I have not looked into the separation barriers for the process you mentioned, but as a rule of thumb, always start separation for the component that is lowest in the amount. As you mention, steam is in larger quantity in your mixture dont start to separate it first. if you do not find any suitable way of separating H2 and HCl directly, then of course try the steam option.
If you use the condensing route, as Andre says you can remove any traces of remaining HCl with another step. Something as simple as a lime slurry or a solution of sodium hydroxide will do it. Then you need to dry the hydrogen.
Of course you could simply treat the whole gas with a lime slurry or sodium hydroxide solution and you would be rid of the HCl as well.
Peters idea for removal of residual HCl should definetely work and it is quite simple. But I actually wouldn't treat he entire stream in such a way. If you condense the water and subsequently remove a preferably large amount of the HCl, you might be able to reuse it in the reaction. If you do a neutralization treatment with lime or sodium hydroxide you are left with a chloride solution that you need to treat further.
suitable permeable membranes or suitable gas absorbers for each gas will help separating
A membrane separation approach is an option depending on the operating conditions. Another alternative is an absorber, using a packed bed with water, or a weak NaOH solution as the absorbent. As long as the incoming water is cool enough, and you have a means to remove the heat of absorption due to HCL, you should be able to get a fairly clean H2 stream overhead, and some aqueous HCl solution if water is the absorbent. This approach can be evaluated using ChemCad as mentioned earlier or Aspen Plus, or Pro II. Key issue for the separation approach may be the capital cost of a separation column vs a membrane separation unit. There may be other contributing factors that may affect the decision; things like operating costs, and the reliability of membrane separation vs absorption.
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