Environmental Engineering Reference
In-Depth Information
of the membranes by avoiding scaling and organic fouling. Scales usually consist of
compounds like calcium carbonate, calcium sulfate, silica, calcium phosphate, strontium
sulfate, barium sulfate, and calcium fluoride. Pretreatment can reduce the effects of scaling;
usually adjustment of pH or a softening process prior to RO will help. Metal-oxide fouling
is best avoided by filtration or aeration plus filtration prior to RO.
9.9.6
Dialysis
Dialysis is not used very often in environmental applications, but a brief discussion is
useful to compare it to other types of membrane processes. Dialysis separates solutes of
different ionic or molecular size in a solution. The driving force is a difference in solute
concentration across the membrane. The smaller ions and molecules will pass through the
membrane, but the bigger particles cannot make it through the pore openings.
On one side of the membrane is the solvent, and on the other is the solution to be
separated. The particles will pass from the solution side to the solvent side, in the direction
of decreasing solute concentration. In a batch dialysis process, the mass transfer of solute
passing through the membrane at a given time is:
d M
d t =
KA (
C )
,
(9.17)
where d M
/
d t
=
mass transferred per unit time
K
=
mass transfer coefficient
A
=
membrane area
C
=
difference in concentration of the solute passing through the membrane.
C will decrease with time in the case of a batch dialysis cell, which is not at steady
state. If the same type of membrane were run in a continuous process where the flow of
the solution is countercurrent to the solvent flow direction,
C would be constant. In most
applications, many of the cells are pressed together to make a stack, and all the cells are
run in parallel.
One real application of membrane dialysis is the recovery of sodium hydroxide from a
textile mill. The percent recovery was reported to be between 87.3% and 94.6%. However,
dialysis is limited to smaller flowrates since the mass transfer coefficient ( K ) is relatively
small. See Refs. [11,13] for additional information.
9.9.7
Electrodialysis
Electrodialysis involves the use of a selectively permeable membrane, but the driving
force is an electrical potential across the membrane. Electrodialysis is useful for separating
inorganic electrolytes from a solution, and can therefore be used to produce freshwater
from brackish water or seawater. Electrodialysis typically consists of many cells arranged
side by side, in a stack. Figure 9.12 illustrates a two-cell stack.
Search WWH ::




Custom Search