Membranes - Principles of Chemical Separations with Environmental Applications - page 264

Table 9.6 Classes of VOCs based on volatility [14].

Pervaporation

Henry's Law separation

coefficient Solubility factor with Ease of

(atm / mole in water silicone rubber separation by

Class frac.) (wt%) membranes pervaporation Examples

Highly volatile > 200 < 0.5 > 1,000 Very good TCE, toluene

Moderately 0.5 to 200 0.5-infinite 20 to 1,000 Good to Methylene chloride,

volatile moderate acetone, butanol

Non-volatile < 2 Infinite < 20 Poor Ethanol, methanol

Example 9.6

Zeolite membranes are inorganic with uniform nanoporous structures. One important
application is pervaporation to separate organic-water feed mixtures. A Ge-ZSM-5
membrane was used to separate 5 wt% organics in water mixtures. The results
(Table 9.7) were obtained at 303 K with a membrane thickness = 30 µm. The

activity coefficient ( γ ) was calculated using the Wilson equation, Equation (3.6).

9.10 Factors that reduce membrane performance

The performance of membranes often decreases over time due to effects such as fouling
and concentration polarization. This is seen as a decrease in flux (Figure 9.15). This per-
formance decline is a major concern for filtration processes, but less so for gas separation
processes.
Concentration polarization is a reversible increase in the concentration of retained
solutes at the membrane feed interface. During operation, this effect will achieve a steady-
state value and will disappear when the process is turned off.
Fouling is the (ir)reversible deposition of retained components on the membrane feed
surface. These deposits can be biofilms, organic components, and/or inorganic salts. Ac-
cumulation tends to increase with time and the effect does not disappear when the process
is turned off. There are some approaches to reducing the effect of fouling that will be
described shortly. In Figure 9.16 we separate the flux decline in Figure 9.15 into these two
effects. We can derive an equation to estimate the increase in retained solute concentration
at the feed membrane surface due to concentration polarization.