Membranes - Principles of Chemical Separations with Environmental Applications - page 263

10 5

10 4

mem > 1

TCE
Toluene
Methylene
chloride
1,1,2-trichloroethane

Pervaporation
separation
factor
( pervap )

10 3

Butyl acetate
Ethyl acetate
MEK

Acetone

10 2

mem < 1

10 1

10 1 10 2 10 3 10 4 10 5

Evaporation separation factor ( evap )

Figure 9.14 Pervaporation separation factor, β pervap , as a function of the VOC

evaporation separation factor, β evap . Data obtained with laboratory-scale spiral-
wound modules containing a silicone rubber membrane [14].

This equation shows that the separation is equal to the product of the separation due
to evaporation of the liquid as well as separation by selective permeation through the
membrane.
An example where β mem is significant is the removal of water from an azeotropic mixture

of water and alcohol. By definition,

β evap for an azeotropic mixture is 1. Thus, the 200- to
500-fold separation achieved is due entirely to the selectivity of the membrane.
In the separation of VOCs from dilute aqueous solutions, β evap is often large and con-

trolling relative to β mem . Some data showing measured pervaporation separation factors

for dilute aqueous VOC solutions are shown in Figure 9.14, in which β pervap is plotted

against β evap obtained from Henry's Law data. Acetone has a pervaporation factor of

approximately 150. In this case, β mem is 15. As the VOCs become more

hydrophobic, the total separation factor increases, but the contribution of the membrane to
the total separation decreases (data points relative to 45 ◦ line). For example,

β evap is 10 and

β pervap of 500
for butyl acetate is almost all due to the evaporation step, since this point is very close to
the 45 ◦ line. The total separation factor for TCE is 4,000, while

β mem is only 0.3. The de-
crease in membrane selectivity is unexpected because silicone rubber is hydrophobic. The
selectivity for VOCs from water should increase as the VOCs become more hydrophobic.
The reason given is concentration polarization.
Table 9.6 summarizes the relative ease of VOC separation using silicone rubber mem-
branes in pervaporation mode. As the Henry's Law coefficient decreases, pervapora-
tion becomes less selective and less competitive to other approaches, such as steam
stripping.