Membranes - Principles of Chemical Separations with Environmental Applications - page 271

Flux

With
backflushing

Without
backflushing

Time

Figure 9.19 Schematic drawing of the flux vs time behavior in a given microfiltra-
tion process with and without backflushing [2]. Reproduced with kind permission
of Kluwer Academic Publishers.

permeation and prolong shelf life. An environmental example is geomembranes. They are
polymer sheets that are used as a liner for hazardous waste containment applications such as
landfills. High-density polyethylene (HDPE) is the current choice for landfills containing
a variety of waste products. HDPE is chemically compatible and resists degradation by
a wide range of chemicals. Geomembranes need to be mechanically stable, resistant to
degradation by a wide variety of mechanisms (chemical or biological attack, thermal
swings, etc.) and serve as a barrier to both gas and liquid intrusion.

9.13 Remember

Membrane processes separate the components of a gas or a liquid stream by taking advan-
tage of the fact that membrane materials are “selectively permeable.” A membrane will
allow certain species to pass, each at a different rate. There are various mechanisms that
control this rate. Compared to most other processes, membrane separations use energy
more efficiently, but have a relatively small mass flux.
Some important things to remember about membranes:
Membrane processes are UNIT OPERATIONS. Regardless of what chemicals are being
separated, the basic design principles for different types of membrane separations are
always similar.
Membrane processes are not reversible.
Although membranes will usually separate a substance from a solution containing
numerous substances with high efficiencies, fouling and scaling of the membranes can
occur when insoluble species are encountered. Some membranes are also susceptible to
biological attack.
Membrane separations can occur in a batch or a continuous process. Most applications
are continuous.
Each membrane process requires a driving force for separation to occur. For example,
dialysis requires a concentration difference, electrodialysis requires a difference in elec-
tric potential, and reverse osmosis requires a pressure difference.