Biomedical Engineering Reference
In-Depth Information
Saline feed
Cation selective
membrane
Anion selective
membrane
+
+
+
-
-
DC current
positive pole
DC current
negative pole
-
+
+
+
+
+
-
-
-
+
-
-
-
-
+
+
-
-
-
+
+
+
+
-
+
-
-
+
+
+
-
-
-
Fresh water
-
+
+
-
-
+
+
-
-
-
+
+
+
+
-
-
-
Concentrated brine
FIGURE 19.6
Schematic. representation. of. a. bank. of. membranes. being. used. for. electrodialysis.. (Based. on. ref.. 12;. Trieb,. F.,.
Concentrating Solar Power for Water Desalination
,.ed..G.A.C..(DLR).and.I.o.T..Thermodynamics,.Federal.Ministry.
for.the.Environment,.Nature.Conservation.and.Nuclear.Energy,.Stuttgart,.Germany,.2007.)
selective.membranes.
14
.One.of.the.main.challenges.to.ED.is.the.high.energy.consumption.
rates.needed.to.obtain.high.lux.rates.of.desalted.water..High.lux.demands.higher.low.
rates,.which.are.determined.by.the.applied.DC.potential.at.the.electrodes..The.power.con-
sumption,.
P
,.for.a.given.salt.solution.of.resistance.
R
.varies.with.the.square.of.the.current,.
I
,.lowing.through.the.system..To.move.the.ions.across.the.solution.and.membranes,.a.DC.
potential.at.the.electrodes.has.to.be.added.to.move.ions.through.the.solution,
η
solution
,.across.
the.membrane,.η
membranes
,.and.over.the.potentials.needed.to.overcome.the.Faradaic.losses.at.
the.electrodes.(i.e.,.the.transformation.of.electrons.in.the.metal.to.ions.in.solution,.usually.
via.the.electrolysis.of.water),.η
overpotential
..The.power.consumed.can.be.expressed.as
P
=.
I
.(η
solution.
+.η
membranes
.+.η
overpotential
).
(19.1)
The. potentials. across. the. solution. and. membranes. (to. the. irst. order). are. simply. pro-
portional.to.the.current,.or.η
solution
=.
C
J
I.
However,.the.overpotentials.at.the.electrodes.can.
grow.exponentially.with.the.current,.such.that.η
overpotential.
=.
C
F
I
(n+1)
,
where
n
≥.1.and.
C
F
>>.
C
J
for. concentrated. salts.. Therefore,. the. power. consumption. for. desalination. by. ED. can.
increase.rapidly,.such.that
P
.=.
CJI
2
.+.
CFI
(
n
+2
).
(19.2)
so.that.when.either.high.luxes.are.desired.or.concentrated.solutions.are.being.desalted,.or.
both,.the.power.consumption.can.greatly.exceed.both.RO.or.thermal.methods..However,.
at. low. luxes. and. relatively. low. salt. concentrations. (<3,000. ppm),. ED. can. be. among. the.
most.eficient.methods.
Other.challenges.in.ED.are.similar.to.those.with.all.membrane.processes.and.relate.to.
membrane.fouling,.membrane.response.to.changing.pH.conditions.when.H
+
.and.OH
-
.ions.
