Biomedical Engineering Reference
In-Depth Information
In. the. case. of. thermal. vapor. compression. (TVC). plants,. multiple. stages. are. used. due.
to. increased. eficiency,. with. designed. capacities. approaching. 36,000. m
3
/day.
12,14
. Vapor.
compression. systems. generate. the. heat. for. evaporation. by. compressing. feed. vapor. as.
the.heat.source.for.the.heat.exchanger.(i.e.,.changes.in.density),.as.opposed.to.mechani-
cally.produced.heat.steam,.as.in.MSF.and.MED.
13
.In.thermal.vapor.compression.units,.
steam.ejectors.are.used.for.vapor.compression,.whereas.in.mechanical.vapor.compres-
sion.units,.a.mechanical.compressor.is.used.
12
.Vapor.compression.is.often.coupled.with.
MED.systems,.which.increases.the.eficiency.of.the.system.while.raising.the.steam.pres-
sure.requirement.
12
19.2.3.4 Reverse Osmosis (RO)
In.contrast.to.the.thermal.processes.discussed.so.far,.another.class.of.water.desalination.
systems. arises. from. iltration. technologies. relying. on. the. use. of. polymeric. membranes..
Membrane.systems
15
.are.becoming.increasingly.popular.due.to.the.low.cost.of.polymeric.
membranes. and. relatively. high-energy. eficiencies.. The. water. industry. extensively. uses.
membrane. processes. for. pretreatment. iltration,. depending. on. particle. size. (e.g.,. micro-,.
nano-.or.ultrailtration)..Conceptual.treatment.of.ions.as.hard.spheres.allows.for.iltration.
to.work.with.ionic.salts,.similar.to.particles,.if.small.enough.pores.are.used.in.membranes..
This.model.led.to.the.development.of.reverse.osmosis.(RO).desalination..In.RO,.a.semiper-
meable.membrane.separates.the.feedwater.from.the.efluent.stream..RO.works.by.pressur-
izing.the.salinated.feedwater.to.a.pressure.higher.than.the.osmotic.pressure.of.the.solution,.
causing.the.semipermeable.membranes.of.the.system.to.reject.most.of.the.solute.(in.this.
case,.mostly.salts),.while.allowing.solvent.(freshwater).to.pass.through.
12
.A.schematic.of.
the.RO.process.is.shown.in.Figure 19.5..Since.osmotic.pressure.is.determined.by.the.salin-
ity.of.the.feedwater,.the.pressure.requirements.for.brackish.water.desalination.are.much.
lower. (15-25. bar). than. those. for. seawater. desalination. (54-80. bar).
14
. It. should. be. noted.
that.although.the.osmotic.pressure.of.seawater.is.approximately.25.bar,
16
.higher.operating.
pressures.are.required.to.achieve.practical.lows,.as.well.as.balance.the.increasing.salinity.
of. the. feedwater. and. overcome. concentration. polarization. across. the. membranes. as. the.
Concentrate
discharge
Saline
feed
Pretreatment
Fresh
water
FIGURE 19.5
A.simple.schematic.representation.of.the.RO.process.showing.the.essential.unit.processes..(Based.on.refs..12,.20;.
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.)
