Life Support and Zeolites

[Businfu]

Totally ridiculous and esoteric issue here… I’ve recently been tasked with a fascinating (and difficult) side project at work, and I figure there are lots smart people on these forums with a good understanding of life support systems, so here I am asking for answers/input.

The main question I have is, can electricity be used to induce desorption of gas on a zeolite? Ie. could an induced voltage or an applied current be used instead of (or to augment) a pressure swing.

We are working on developing a proof of concept for a novel O2 concentration device. These things exist already; they typically use Li-subsituted Zeolites on a rapid pressure swing cycle to adsorb N2 out of ambient air and create a stream of relatively concentrated oxygen. Portable models can generate about 3 liters per minute of 90% O2. This patent http://www.google.com.sv/patents/EP2485794A2?cl=en describes the current devices fairly well. We are planning on building a similar device but utilizing a different Zeolite, Cu+ Substituted ZSM-5, which has demonstrated great N2 adsorption characteristics, but has the added benefit of not using spendy Lithium. Adsorption isotherms and some other data on these Zeolites are here:

http://pubs.acs.org/doi/pdf/10.1021/jz100467w

http://pubs.rsc.org/en/content/articlelanding/2001/cp/b009942m/unauth#!divAbstract

So back to my main question. I’m wondering if there is a way to cause the N2 to desorb off the zeolite without using a pressure swing. The N2 adsorbs due to ligand formation with the Cu+ in the zeolite framework; could it be desorbed using a voltage swing or a current? I’ve found literature that mentions electrical swing adsorption for CO2 scrubbers, but the electrical swing only works by creating heat, so it’s really a thermal swing. If anyone has a good working knowledge of this kind of thing (it is used for CO2 scrubbing and gas management on the ISS after all!) it would be greatly appreciated.

Some background: I work in a medical research lab. The Gates Foundation has certain problem areas in healthcare that they fund research in. They focus on developing countries, particularly in Africa. One of the biggest healthcare problems in rural setting is maintaining supplies of O2. They’ve promised to fund (and I mean 6-7 figures kind of fund) further research into novel technology to cheaply and renewably generate O2 without using much power. Thus, my boss gave me this as a ridiculously ambitious side project.

[KSK]

Bit of a stab in the dark here but would laser or (probably cheaper) microwave desorption work?

[Businfu]

It's an interesting thought... do you have any theoretical reason why it could? That might help develop an experiment. I'm currently planing to sandwich zeolite between capacitor plates to examine whether or not the applied field will change adsorbence. The main issue I see with working with microwaves (or lasers) is cost and complexity.

[KSK]

Mainly because the first thing that came to mind when you mentioned desorption was MALDI. I figured that using lasers would be expensive (as you said) so wondered about microwave desorption instead.

Not even sure if microwave desorption is a thing but my, kind of ad-hoc, theoretical justification would be that microwaves should provide enough energy to desorb the N2 without heating your zeolite (which you seemed to be looking for alternatives to). Microwave spectroscopy is all about rotational quantum states, so it would make sense to me that microwaves should be absorbed more by the N2 molecules (which are free to rotate) than the zeolite framework (which isn't).

Edit - from a quick online search it seems that Microwave Swing Adsorption is a thing so might actually be worth reading up on more seriously. Who knew!

[SciMan]

Why use zeolite adsorption?

I work at my dad's auto repair shop, and we have a bunch of tool catalogs that we get for free from various tool suppliers.

In one of those catalogs, I saw a device that uses a "molecular sieve" to provide a relatively pure nitrogen stream from a compressed air line.

The nitrogen stream produced by the device is used to inflate tires (with a claim of better fuel economy, which is probably marginal at best).

My understanding is that the device works by letting only nitrogen thru the pores in the sieve, and discarding the other atmospheric components.

Assuming that the device operates as I understand it, couldn't it just discard the nitrogen instead, and keep the other component, which is now mostly oxygen?

Of course, I could be describing a pressure swing adsorption device without knowing it, as I am not familiar with that technology.

Edit:

After some searching, it appears that the molecular sieve media in the nitrogen concentrator device I described has a pore size of 10 angstroms, and the type used for oxygen generator use has a pore diameter of 5 angstroms (with a special variant for medical oxygen generation use).

Edited July 11, 2015 by SciMan

[Businfu]

KSK: Thanks for the input. It's an interesting prospect, and you are right there is quite a bit of literature out there. Most of what I came up with focuses on systems designed to pull volatile organics out of air, but that doesn't mean the concept couldn't be applied. Since you seem to have a solid background in physics I'll pose another question. I know microwaves themselves aren't very high energy, but how much energy does it take to make microwaves? Brief googling yielded that a home microwave oven runs on something crazy like 1200 watts. I don't have a good grasp of this kind of hardware.

SciMan: Zeolite is a molecular sieve! It's like the first one listed on the wikipedia page you linked. I would assume that the nitrogen generator you describe actually uses Zeolite as well, or possibly active carbon. The whole issue is 'discarding the other components'. Molecular sieves adsorb certain molecules into pores, but once everything is saturated, no more can be adsorbed. A pressure swing adsorbtion device removes the unwanted stuff by lowering the pressure which causes unwanted gas (N2 in my case, O2 in the automobile case) to vent out.

Interestingly though, I found this article about capillary condensation of N2 on multiwall carbon nanotubes. http://pubs.acs.org/doi/abs/10.1021/jp973319n. Unfortunately I don't have access to that journal at my institution. Can anyone else view it? I got a crazy idea about building a device that would use the capillary action in these guys to wick N2 out of a stream of air... but I have no idea if that's even how it works!

[SciMan]

Here I thought the molecular sieve actually let the molecules of gas actually pass thru it, but blocked anything larger from passing.

Something like how a Reverse Osmosis filter works (only water can get thru the membrane).

Thanks for correcting my misconception.

[Businfu]

Here I thought the molecular sieve actually let the molecules of gas actually pass thru it, but blocked anything larger from passing.

Something like how a Reverse Osmosis filter works (only water can get thru the membrane).

Thanks for correcting my misconception.

Well that kind of thing does exist. For example dialysis tubing has pores designed to block cells and large molecules (read proteins) but allow small molecules like water and gas to pass through. The issue is that diatomic gasses like nitrogen and oxygen are too small and similarly sized to filter in this way. Briefly poking around on the webz I found that the bond length for N2 is 1.098A and in O2 it is 1.207A. I'd guess the molecules themselves are very similar in size.

Because of this, you need some way of selectively attracting one and not the other, physically blocking is not enough.

[KSK]

I'm not a physicist I'm afraid - just a chemist with a decent general science background and a good poker face.

So I don't have much of a feel for power requirements either, although this website, seems to have some decent numbers and your 1200 watts looks about right. Not a problem in a lab or a hospital or anywhere you can plug them into a wall, maybe not as useful in the kinds of environments you might be looking at using them in.

Doing some quick Internet digging, I found this prototype air purifier which uses microwave-thermal desorption to regenerate the various beds. For CO2 and water, incident microwave power was reported as 300W, so maybe double that for required wall plug power to drive the thing. That's getting better but 600W is still quite a lot