Environmental Engineering Reference
In-Depth Information
valves, so that the product cell becomes the brine cell and the brine cell becomes the product
cell. The salts are thus transferred in opposite directions across the membranes. Following
the reversal of polarity and flow, the product water is discharged to waste until the cells
and lines are flushed out and the desired water quality is restored. This process takes about
1-2 min. The reversal process aids in breaking up and flushing out scale, slimes, and other
deposits in the cells. Scaling in the electrode compartments is minimized due to the continuous
alternation of the environment from basic to acidic in the cells. Reversal of polarity, therefore,
eliminates the need to continuously add acid and or SHMP. Some cleaning of the membrane
stacks is still required, although with significantly reduced frequency than would be otherwise
necessary.
9.9.8
Pervaporation
The word pervaporation is a contraction of two words, permeation and evaporation. This
process is different from the other membrane processes in that there is a phase change as
the solute permeates across the membrane. Thus, both heat and mass transfer are important
aspects of the performance.
The feed is typically at atmospheric pressure and the permeate at high vacuum. As the
solute permeates across the membrane, there is a change from liquid to vapor phase. Thus,
both differences in permeation rates as well as heat of vaporization affect the productivity
and selectivity of the process. Pervaporation offers the possibility of separating liquid
solutions that are difficult to separate by distillation or other means.
The process is shown schematically in Figure 9.13. The feed liquid contacts one side
of a membrane, which selectively permeates one of the feed components. The permeate,
enriched in this component, is removed as a vapor from the opposite side of the membrane.
Purified
feed
Membrane
Liquid
feed
Permeate vapor
Condenser
Condensed
permeate
liquid
Figure 9.13 Schematic of the basic pervaporation process.
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