Membranes - Introduction to Carbon Capture and Sequestration - page 301

Question 7.3.1 Membrane area

Compare the approximate solution of the expression of the relative mem-
brane area with the full solution shown in Figure 7.3.2 .

driving force for transport across the membrane. This driving force is the
difference in partial pressure between the feed and permeate, or:

px − px ≥ 0

F CO , F P CO , P

2 2

Hence, the maximum concentration in the permeate is set by:

px

F CO , F

( )

2

x max =

CO , P

p

2

P

0.1. Hence,
to obtain a higher purity we need to increase the pressure of our feed.
Importantly, this is independent of the performance of our membrane.
Irrespective of the permeation or selectivity, we will never obtain a higher
purity than the limits set by the fl ue gas concentration and the pressure
difference.
This pressure ratio issue has important practical consequences. On
a laboratory scale it is easy to achieve any reasonable pressure range,
but our choices are very limited in coal-fi red power plant applications
considering we need to compress on the order of 500 m 3 gas per sec-
ond. This requires very special equipment, and to compress the gas
above 10 atm on the feed side or below 0.1 atm on the permeate side
would require most of the energy that is produced by the power plant! If
we assume that we are to use compressors that are readily available, we
have to work with a pressure ratio (the feed pressure divided by the per-
meate pressure) of 5 in order to ensure that most of the energy of our
power plant is not used for the compression or for pulling a vacuum. As
we have used this limit in our example, we can conclude that our single-
stage membrane separation will never give us the required purity.
The second question was related to the membrane area. Our calcula-
tions show that if we increase the selectivity while keeping the CO 2 per-
meation constant, we obtain the counterintuitive result that the required
area of our membrane increases. We would have expected to see that if

In our example we have used p F / p P = 5 atm and x CO 2 , F =