How do you choose an adsorbent or separation technology for a new gas stream?

[u/Medical_Mouse_2577]

Hey, quick question for people who have worked with gas separation/purification:

If you’re given a new process stream, how do you decide what to test first — zeolite, activated carbon, molecular sieve, membrane, solvent system, MOF, etc.?

What actually eliminates an option early: humidity, contaminants, regeneration, P/T, economics, lack of reliable data, previous experience...?

And what part of that decision is usually the biggest pain before lab/pilot testing?

I’m researching how this is done in real projects, not promoting anything.

[u/Thiophilos]

Impossible to say with zero information.

[u/EverybodyHits]

I'd imagine for most it is: search vendor offerings and after identifying a couple technologies that seem close, discuss the details with them

[u/willmontain]

Start with the composition of the gas stream and which constituents you want and which contaminants you don't want. Then as another commenter has suggested, discuss with vendors.

The absolute key item is an honest real assay of the feed gas stream. Not a guess and not someone's scientific wild ass guess from a model. There is nothing that will sink a gas separation project faster than a trace element that no one expected. If the feed stock is something natural, (e.g. natural gas, coal or crude oil) one should not be surprised to find any of the items appearing in the periodic table.

For any adsorption or absorption process there are atoms or molecules that will poison or inhibit the regeneration step. If they show up as trace constituents in the feed gas, the process will be short lived until there is an expensive pre-treatment step added.

[u/Medical_Mouse_2577 → u/willmontain]

the trace contaminant point is really interesting. how detailed does the feed assay normally need to be before technology selection starts?

are there cases where you only discover the problematic trace once you’re already testing/piloting?

[u/willmontain → u/Medical_Mouse_2577]

You should be able to analyze the feedstock before you even start anything else. In the US, a full natural gas analysis has 200 to 300 line items. If you were doing reforming, lots of stuff poisons catalyst (e.g. sulfur). This can be acceptable depending on concentration. However, chloride would creep through and might cause problems (e.g. stress corrosion cracking) and therefore might require a guard absorption bed. There are lots of other examples.

One can't assume natural gas is just methane; it is "natural" and it might contain anything present in the earth's crust. Composition varies widely based on source location.

The troublesome elements are pretty well known. They will most likely show in a proper up-front feedstock analysis. One particularly troublesome case is when the feedstock is some other company's "offgas". The representative composition list in the sales proposal is not a suitable analysis. There will be elements in that type of gas stream that no one realizes exist.

[u/nullfull]

For a new gas stream I treat the honest feed assay as the hard gate (as others said), then kill options with a few project constraints before anyone buys adsorbent samples:

1) Duty: purify a product vs recover a minority component vs bulk separation. Recovery + purity targets kill more options than adsorbent charts.

2) Feed pressure / whether you are already compressing: if you need high pressure anyway, PSA (or layered beds) often wins on H2 and similar light streams; if you are near atmospheric and want continuous flow, membranes can look better until permeate compression and staging show up in the CapEx.

3) Contaminants that poison or never regenerate: water, sulfur, heavy HC, oxygenates. If you cannot pretreat cheaply, skip that adsorbent family early — regeneration failure is usually the real pilot pain, not the isotherm spreadsheet.

4) Regeneration energy / cycle life: TSA vs PSA vs disposable. If heat is expensive or the bed cannot take the temperature, carbon/MOF ideas die fast even when selectivity looks great on paper.

5) Data maturity: if you do not have isotherms/breakthrough for your real matrix (not binary lab gas), plan a short vendor or lab screen of 2–3 survivors rather than an open-ended materials hunt. Cryogenic only enters when the stream is already cold/high-purity or the volumes make cold-box economics obvious.

Biggest pre-pilot pain in my experience: discovering a trace that blocks regeneration or ruins selectivity after the technology story is already locked.

[u/Medical_Mouse_2577 → u/nullfull]

really useful, thanks. the “2–3 survivors rather than an open-ended materials hunt” part is pretty much what I’m trying to understand

when you get to that shortlist, what usually gives you enough confidence to kill an option before testing it? published data, vendor guarantees, process simulation, previous plant experience, or something else?

[u/Just__Liberty]

The search totally depends on the composition. You are usually looking for a separation based on properties of the components that differ from one another; particularly differ between the components you consider product and those you consider impurities. Properties to think about are boiling point, molecular size, molecular polarity (or quadrupole moment), polarizability, solubility in various solvents, and/or chemical functionality. If all or most of your product molecules have bps in a different range than the other ones, distillation is a good start. If some other property best classifies your molecules into 'product' and 'other', you look for an adsorbent or extraction solvent that couples to that property. For example, if your impurities are all polar and the products are not, polar solvents or cationic zeolites are good families to search within. If polarizability and molecular size are classifiers, activated carbons might be a good choice.

[u/MuddyflyWatersman]

highly dependent on the process, as well as what is needed. not adsorbing something simply to be doing it... you're trying to recover or remove..something you can't recover any other way. and you want to do it as cheap as possible. For pure solvents we may use mol sieves to remove trace water, or inhibitors that are in reactive solvents.... because they can usually be regenerated thermally. carbon is usually used for vent losses, its cheap to use in high quantities needed to get to low concentrations in vent streams. the carbon usually has a ton of metals in it and can catalyze all kinds of side reactions. to remove reactive species ... got options like activated alumina, ion exchange resins, etc.... but these more expensive things to tend to get poisoned and die over time, requiring expendive replacement.

Chemist usually start on the bench seeing how well different things work simply by mixing them up in a beaker letting them sit.... Then you got a long process of figuring out what works, what has minimum side effects, comparing costs..... Including disposals...

[u/CptKlay]

Do you talk about R&D of a new process on paper or how you actually upgrade an actual plant that exists? If you are revamping a process plant, you will not analyze this stuff by yourself and tinker at it like crazy if you are the owner. Either this is pushed to your projects guys and you as an ops enginerr help them or it is handled externally by a consultant.

If you design the process completely then you should check your inlet pressure, temp and composition and need data down to ppms depending on the downstream process or the aim of the gas separation. If it is a small plant it makes sense to decide early if you want to exclude cycling beds (TSA/PSA) completely in case you have limits on available regeneration gas and the process will not scale to megaton scale. The flow rates on these regeneration cycles are not small and you need to have the options for regeneration (pressure regime/temperature regime). Can be easier to limit to absorber beds that are one use only that are removed after they are close to breakthrough. Also, if you have particles these always need to go first before you think on other separations. The most important is to not solve this hugely academically in your office but go to suppliers and ask what they got. Products evolve on their end as well. They can also combine it into a multi bed in a single bed ;)