Thank you for joining us for this informative American Institute of Chemical Engineers
webinar, "Pressure Swing Adsorption: A Ubiquitous Gas Separation Technology."
Our speaker is Dr. James Ritter. James Ritter is Distinguished Professsor in the Department
of Chemical Engineering at the University of South Carolina.
Thanks, Pam, for the introduction. So I'm going to start with how Pressure Swing Absorption
works, and it's a very rudimentary argument of what you need and the fundamentals behind it.
I don't go into a lot of detail. Of course you need a very porous material. This is a picture of an
aluminosilicate A-type zeolite, a certain structure. The crystals you are looking at might
be 2-5 microns on a face, and within that structure you will have apertures or pore openings
on the order of 5 angstroms or so in this structure. So there will be about 3-4 thousand
pinholes in every single dimension on the surface of this crystal that would look something
like this. This is sort of the atomic view. You can see the tiny little opening here in the center.
That's about 5 angstroms for this particular structure. In the X-type zeolite that may be
7 or 8 angstroms. If you strip away the silicon, oxygen and aluminum molecules you can see
the cage where a lot of adsorption takes place, and there's many cations.
This is just a typical example of a porous material. What's interesting about separations,
or gas separations anyway, is you need pores on the order of the size of the molecule, a little
bit bigger, or they don't work very well. You really need the interactions with the walls to cause
strong enough forces to retain the material by phys-adsorption, but weak enough forces that
you can regenerate easily with a drop in concentration or a drop in pressure, namely
pressure swing. Sometimes you might have to use thermal swing. So a lot of larger pore
materials, like some of the aluminum phosphate 15 angstrom type pore materials, really haven't
caught on as far as gas separation techniques because the pores are too big, even though
they are micropores, which are defined less than 20 angstroms. So you need some small pores.
This just gives you an idea of the X-type cage where this opening now is about 7 or 8 angstroms.
There is a number of cations in here. The silico- aluminum structure has a net negative
charge so it's a natural ion exchanger, and you can change this opening. The difference
between a 3A, 4A and 5A zeolite is really the cation that's in there, and there are several
cations within each cage. I forget offhand but, you know, like a dozen.
There's various places, and it is different in the A-type structure than the X-type structure.
To just get the idea, if you need a porous material, you can't pack cut these kinds of
crystals into a bed. These are micron sized particles, and the same thing with dusty carbon
materials or dusty silica gel materials. You really have to pelletize them into something that is
more manageable. Now you are in the millimeter range. These pellets may be
3-4 mm long and 2-3 mm in diameter. They could be spheres, 3 mm in diameter, and it
varies all over the gamut. You can get carbon materials that are chunks of small pieces
of charcoal, anywhere from fairly small, you know, fractions of a millimeter, to several
millimeters in size, and you pack those into a column. Once you get that packed into a column,
now the fundamentals show up. You have to have some selectivity between the gases you
are trying to separate on a particular adsorbent. I am showing here adsorption isotherms for
oxygen and nitrogen on LiX zeolite. This is a partially lithium exchanged X-type zeolite.
These really are the isotherms. I've removed a point that's here at 25°, and it is showing the amount
of these gases adsorbed per kilogram of adsorbent as a function of pressure in kPa up to about
7 atm, 6 atm or so. And you can see the significant difference, the uptake in nitrogen
for this material is 3-4 times greater than oxygen, which means that if you expose
air to this adsorbent in a very specific manner, you can produce oxygen as the light gas while
nitrogen is being adsorbed. And, of course, once the material is more or less saturated at
the feed conditions, you have to regenerate, and we are going to get into that, and that is what
this talk is about, but you have to have this selectivity. It's just like a relative volatility in
distillation. On the order of two or more you might have something feasible. Can you do this
separation with very small selectivities? Yes, but it becomes problematic and very energy
intensive, and it's going to have to be something that's of very high value, like an isotope.
We've actually done simulations and there is work in the literature where selectivities of 1.05
or 1.1 are good enough to enrich an isotope, but certainly you are not going to get high purity.
It's just a slight enrichment process, but it does work, in principle.