1 reply to this topic

[JeffRandlesGhost]

Hello all forgive me for taking up your time but I'm struggling to move forward with a UG design project pertaining to the purification of H2 and in particular final drying stage to remove excess moisture prior to storage. Any help would be greatly appreciated.

 

The aim is to prepare H2 of purity in excess of 99.999% through a gas drying operation. The overall project is to assess the feasibility of using renewables (onshore wind turbines) to generate enough H2 to power a small community via combustion in fuel cells. The stream entering the drying unit is composed of 580 kg/h of H2 and 65 kg/h H2O at approx 80 C and 30 bar. 

 

I have considered many different strategies such as PSA, TSA with the main drawback of loss of H2 during the purging of the regenerated column. In addition I assume condensing of the stream is a pre-requisite to reduce the around of H2O passing through the column to prevent quick saturation.

 

I've looked at condensing the stream by exploiting the Joule-Thompson effect, expanding the gas to allow cooling and thereby condensation of the water but reaching the required H2 purity seems implausible.

 

Are there any other technologies that you would recommend, taking into account the operating conditions specified above?

 

Once again, any help would be greatly appreciated.

 

D

 

 

[Art Montemayor]

David:

 

In trying to help you out, I’ve got some bad news and good news for you:

• You need to do some work on your Chemical Engineering basic process research and studies.
• In spite of ill-defined information and flawed planning on this assignment, you’ve come to the right place to get your problem resolved in the correct manner and with the logical engineering steps.  Our members can help you with their expertise and recommendations - but you have to feed them ALL the detailed facts and data.

Let’s get to the basic data and your starting plans and ideas first:

• Please help yourself first by helping our members understand accurately what your query is based on.  Is it a usual, homework assignment, a serious, end-of-term assignment, or something else?  Does it have a time limit or other constraints?  The reason for this is so our members can know beforehand how much time or effort is involved.  Some students come to our Forums at the 11^th hour expecting all the answers.
• Stop with the acronyms and badly defined wording.  Our members shouldn’t have to guess what it is that you are describing - or trying to describe.  UG means undergraduate  - or what?  PSA and TSA are known to me for all the work I’ve done on adsorption processes.  Is THAT what you are referring to?  If so, then state that you are basing your first attempts to dry hydrogen on an adsorption process - and what adsorbents you have in mind.  A flow diagram goes a long way in accurately defining your process.
• You are making a drastic and embarrassing mistake by wasting time and effort trying to subject hydrogen to the Joule-Thomson Effect.  Your instructor(s) will have a field day revealing to you that two gases are notoriously contradictory (as compared with almost all other gases) in NOT cooling when subjected to Joule-Thomson’s: hydrogen and helium.  So don’t waste time on Joule-Thomson.  Hydrogen and helium will cool upon expansion only if their initial temperatures are very low because the long-range forces in these gases are unusually weak.  This should be a lesson learned.
• Purifying Hydrogen to 99.999% vol. purity is not rare or strange.  I’ve done it with adsorption many years ago, when the present-day electronics research work was being done around the late 1970s.  Our analyzers at that time barely went up to that level of accuracy then.  So, you know from those facts that the process is not new or arcane.  However, once again, be specific and describe your process and basic data as an engineer would: you fail to state the basis of your purity.  Is it % volume or % mass ---- or % mole??  Are we supposed to guess?  I think you mean volume %.
• Are you proposing to generate hydrogen in electrolytic cells by using wind turbines to generate the necessary electricity?  What will be the total composition of the hydrogen exiting the cells - only hydrogen and water? --- and how does “combustion fit into all this?  A flow diagram would describe it better than your wording.
• If you are proposing adsorption, what type of cycles and beds’ configuration are you contemplating - and why?
• If indeed you are generating the hydrogen at electrolytic cell pressure, then you are being confronted with a very low pressure hydrogen source - low for an adsorption application.  If so, I believe a reciprocating compressor application is in front of your proposed  adsorption bed.  Does the reciprocating compressor present any potential contamination for the adsorption beds - like cylinder oil?
• You state that “The stream entering the drying unit is composed of 580 kg/h of H₂ and 65 kg/h H₂O at approx 80 C and 30 bar”.  Why are you subjecting the adsorption beds to such HOT (176 ^oF) gas?  Nobody would do that without first cooling the feed gas as low as possible.  How do you arrive at the water content in the hydrogen gas?  If you calculated it, submit your calculations to verify the dryer loading.
• Have you done your study and research on adsorption processes - like studying Kohl and Neilsen’s book, “Gas Purification”?  I highly recommend that you do this - if you haven’t done your basic study and research up front already.

These are just some comments to help you get “your act” together, before going forward on something that simply won’t work or will cause you grief and loss of time and effort.  I’m sure our members will jump into this thread and offer their suggestions and recommendations.  Be detailed, specific, accurate, and submit your work and effort up to this point for them to scrutinize and check out.  Calculations, flow diagrams, references, and other material will help them help you.

 

Do not be discouraged and take my comments as being negative.  They are meant to confront you (and other students) with the truth of how engineers think and react to real problems and how we attack and resolve them.  I can assure you that any comments, recommendations, and advice is meant only for your (and other students’) benefit and profit.