Biomedical Engineering Reference
In-Depth Information
polyphosphates.are.the.most.economical,.while.effectively.retarding.scale.formation.and.
offering. corrosion. protection.
10
. Organophosphonates. are. suitable. for. a. wider. range. of.
operating.pH.and.temperature.conditions.than.polyphosphates.
10
.The.main.consequence.
of.all.pretreatment.processes.is.that.they.all.involve.increased.energy.consumption.and.
material.costs,.regardless.of.the.speciic.desalination.methodology.employed..There.is.also.
growing.concern.that.the.rejection.of.these.pretreatment.chemicals.poses.environmental.
threats.due.to.their.toxicity..The.subsequent.treatment,.if.required,.to.remove.the.antiscal-
ants.also.consumes.energy.
19.2.3 Common Separation Methods
About.88%.of.the.available.desalination.production.capacity.employs.either.reverse.osmosis.
(RO).or.multistage.lash.(MSF).distillation.processes.for.freshwater.production.from.either.
brackish.water.or.seawater.
11
.Other.main.technologies.include.multieffect.distillation.(MED),.
vapor. compression. (VC),. and. electrodialysis. (ED).
11
. The. primary. energy. requirement. for.
multistage.lash,.multieffect.distillation,.and.vapor.compression.is.in.the.form.of.thermal.
energy,. while. reverse. osmosis. requires. primarily. mechanical. energy,. and. electrodialysis.
requires.primarily.electrical.energy..Other.methods,.such.as.solar.distillation,.freezing,.gas.
hydrate. processes,. membrane. distillation,. capacitive. deionization,. and. ion. exchange,. are.
used.for.desalination,.but.current.technology.levels.for.these.processes.ind.limited.use.and.
are.not.commercially.viable.on.a.worldwide.scale.for.widespread.implementation.
19.2.3.1 Multistage Flash (MSF) Distillation
The.MSF.process.is.well.suited.for.highly.saline.or.contaminated.waters
12
.because.lashing.
of.water.vapor.from.the.top.of.brine.pools.allows.for.minimal.scale.formation,.as.the.pre-
cipitates.resulting.from.evaporation.form.in.liquid.rather.than.the.critical.surfaces.of.heat.
transfer.
7
.The.desalination.procedure.begins.by.heating.incoming.seawater.by.condensing.
steam.contained.in.a.set.of.tubes.running.through.the.brine.heater..The.MSF.process.ele-
ments.are.depicted.schematically.in.Figure 19.2..To.reduce.energy.costs,.MSF.plants.are.
Steam ejector
(Vacuum pump)
Ejector steam
Cooling water
discharge
Brine Heater
Condensate
discharge
Saline
feed
Steam in
Fresh
water
Brine
discharge
Brine
Brine
Brine
Brine
Condensate
back to boiler
FIGURE 19.2
A.block.diagram.schematic.showing.the.basic.elements.of.a.multistage.lash.(MSF).distillation.process..(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;.
Spiegler,.K.S.,.
Salt water puriication
,.New.York:.John.Wiley.&.Sons,.1962.)
