Thank you for joining us for "Pressure Swing Adsorption: A Ubiquitous Gas Separation Technology".
Our speaker is Dr. James Ritter of the University of South Carolina.
Once you get the selectivity there's different ways or concepts based on the adsorption
isotherm that show how PSA, in effect, works. And what you want to achieve at the end of the
day is a change in the loading. You don't have to go to zero loading. So I'm showing a change
in the loading just as a Δq here, in moles/kg. It's a very modest change somewhere on the
isotherm, and I am showing two physical adsorption isotherms at two different
temperatures, T1 and T2. And because it is phys-adsorption, there is less adsorption at
higher temperatures; that's how we regenerate. So T2 > T1. And of course if we change
temperature at constant pressure, that's essentially called temperature swing adsorption,
and you can achieve this change in loading. Or if we change pressure at constant
temperature and we just follow this isotherm down, we can get the same change in loading
and that is essentially the principle of pressure swing adsorption. And of course you can do
both; it's a TSA/PSA type process. That is practiced. I know of some select processes that
do it, namely some work we've been doing with NASA does a pressure and thermal swing
regeneration on their system. They pull a vacuum and heat it. The problem with doing
both is, in general, especially for large beds, a temperature swing adsorption process has
very long heating and cooling times which implies large beds. At the end of the day,
your bed that's taking the feed has to be able to handle all the feed while your other beds are
being regenerated and being put back on line to take the feed. For a thermal swing process that
can be on the order of 10, 20, 30, 40 minutes, even an hour or so or more to heat and cool
the beds. And with pressure swing, we can change pressure in a matter of half a minute.
In very large beds, 20 seconds. So you can blow these beds down. You've got to be careful.
You can hit sonic velocities and things like that. And there's mufflers and they can be very loud,
but you can do it, depending on the size. So because you can change pressure much
quicker than temperature, and achieve the same separation, or the same change in
loading, if you will, pressure swing adsorption tends to have smaller beds. But of course these
are case specific. I do know of some instances where pressure swing adsorption has
supplanted temperature swing adsorption in industrial separations. In other cases you
definitely have to go with TSA, and the best thing to do is run simulations and look at the
type of separation you are getting once have a good isotherm. That's sort of the basic concept
of how this works. Now I would like to go over this slide. This is a Skarstrom cycle that was
patented right around 1960. Many people give Skarstrom from the old Esso which is Exxon,
credit for inventing PSA. There are some who would argue that it invented earlier or
simultaneously in France. It doesn't matter; he has one of the first patents that conclusively
shows, in my opinion, how PSA works. We are simply drying air in a twin bed system.
We have wet air going in at 40 psig and 1 standard cubic foot per minute.
We take that air and we pass it through Bed A. And notice we are producing dry air at a half a
standard cubic feet per minute, at roughly the feed pressure. So that's coming through Bed A.
And if we had indicating silica gel here that goes from blue to pink while the water is being removed,
we would see a pink wave moving up through this bed. And eventually it would be very bright
pink here, and very diffuse or blue here, and you would see the mass transfer zone develop as
you are flowing through this bed. And eventually you've got to stop feeding the bed because
water will break through at this end. So while Bed A is being fed, Bed B is actually being
regenerated at low pressure. This is psig so they usually blow the bed down to atmospheric
pressure, so this is 0 psig I would assume here. And we are purging the bed. Notice we are
feeding 1 scfm and we are only removing 0.5 as product, so half of your feed gas is being used
to purge the bed counter-currently to push the water wave back in the bed and you would
actually see the pink wave moving back. And of course now you are producing wet air out at
0.5 cfm. After a specified cycle time, and these beds might run 20 minutes, they'll switch
pressure very quickly. I'll show you an example of a typical cycle schedule.
They have the pressurized feed blow down and purge. After say 20 minutes they'll switch their
roles very quickly. This bed will blow down; this bed will pressurize. In fact you can pressurize
with light product. Just close the bottom valve and pressurize it. And once it is pressurized
you switch the solenoid valve, and this bed becomes the feed bed and this bed goes into
a regeneration mode. So that's the essence of how any PSA system works, to be honest.
And I like this second graph a lot because it shows September 1956, from September 12th
to September 17th, so over a 5 day period. And I like the fact that he started with about 3000 ppms
of water coming out of his product stream. So he is starting with silica gel or activated alumina
which is probably in these beds. I don't know if it says it ... "Mobilbeads" is what they are saying.
It could be a silica gel or activated alumina; you would have to read the patent. But he starts
where the bed is already contaminated with water, and what he shows over a period of five days
is he can dry that stream up, and actually dry it down to 1 ppm. I think if he started with pristine
dry silica gel, and just ran the bed for five days, and showed just 1 ppm or less coming out,
people wouldn't have believed it would work. I'm guessing that this is why he did it this way. It is
very conclusive. And of course, in principle, this will run forever until something terrible happens
to the absorbent, it wears out, some contaminant in the feed gets on it. But you can
cycle these beds for five years or more and it will produce very dry air with 1 ppm moisture in it.
And if you had the indicating silica gel turning from blue to pink, you would have seen
those waves pushed back in the bed and you would see the pulsing of the pink and blue
waves going back and forth. It would be a nice demonstration. It reaches a periodic state
after a number of days; it could be hours. It's usually a number of cycles, 10, 15, 20, maybe
50 or 100 cycles. It will reach a nice periodic behavior, and you design for that. And that's a
function of a lot of parameters, including bed size, feed flow rate, feed concentration, cycle
time, your purge-to-feed ratio, which in here is like 50%, which is pretty high. That's on the
absolute amount. Typically 10% or so is more common, but that depends on the value of the gas,
and whether you're at all interested in your recovery. Right? The recovery here is only 50%.
That's pretty bad. So that's not a great example. But anyway that's the basic concept.
I would call this a 2-bed 4-step PSA cycle.