Question

Describe entirely in your own words the operational principles of Pressure Swing Adsorption (PSA). Focus in particular on the description of the phases defining a characteristic operation cycle of a PSA unit.

Answer 1

P1 Adsorbent Loading A B 91 P2<P1 D 92 C Y3 Yı Y2 Mole fraction

The above figure shows the equillibrium relation between the adsorbate loading and the mole fraction of solute in the fluid at two different pressures, $P_{1}$ and

P

In the pressure swing adsorption (PSA) process, adsorption occurs at high pressure and desorption at los pressure.

There are mainly four phases in PSA ()

A, B, C, D

(i) First the feed fluid at total pressure of $P_{1}$ and
Ti
with a mole concentration $y_{1}$ is brought into contact with the adsorbent. So now the solute get's absorbed on the adsorbent and achieves a loading of $q_{1}$ . This conditions is represented by point on the figure.

We should also understand the fact that, $y_{1}=\frac{p_{1}}{P_{1}}$

Where, $p_{1}$ is the partial pressure of the solute.

(ii) In the second step the pressure is lowered to

P

Here the loading is still same $q_{1}$ and the partial pressure of the solute is still  $p_{1}$, So, the equillibrium mole fraction associated with this will increase, $y_{2}=\frac{p_{1}}{P_{2}}$ , that means more solute will be soluble in the solvent fluid. Thus there will exist a driving force for desorption of solute/adsorbate into which the concentration of solute will be less than $y_{2}$

Now this condition is represented by point on the figure.

(iii)

Now the purge fluid is passed across the bed to sweep away the desorbed fluid and the equillibrium now shifts to the point .

As the purge fluid takes away the solute/adsorbate now the adsorbate loading will be decreased from $q_{1}$ to $q_{2}$ .

(iv)

Now again to prepare the adsorbent bed for adsorption, the pressure is increased to $P_{1}$ . The bed is now at conditions represented by point .

This will decrease the equillibrium molar concentration, which will favour in adsorbing the solute from the incoming fluid and is ready for next cycle.

The differential loading $q_{1}-q_{2}$ is the maximum loading that can be achieved for pressure swing cycle operating between a feed containing a molar concentration $y_{1}$ and product containing $y_{3}$ of the adsorbate.