DESALINATION 211
sulfonic acid derivatives and various esters of polyalkylene
glycols. Polyphosphates act as sequestrants for calcium and
magnesium ions. Lignin sulfonic acid, starch, tannin, etc. act
as dispersants in coating surfaces such that scale adherence
and crystal growth are inhibited. Polyalkylene-glycols are
surface active agents which tend to retard foaming of the
seawater.
The use of polyphosphate based additives is limited to
temperatures below 90C (190F). Above this temperature,
polyphosphates undergo chemical changes which restrict
their effectiveness as antiscaling agents. Other additives are
low molecular weight polyacrylic acid or 10% ethyl acrylate-
acrylic acid copolymer. An optimum polymer concentration
of about 2 mg/L was observed to be most effective.Materials of Construction: CorrosionThe selection of suitable metals to construct desalination
plants is of prime importance as the use of inadequate mate-
rials may lead to shutdowns, increased replacement and
maintenance costs and affect the overall economics of the
plant.
The optimization of a distillation process has as a first
object the minimization of the amounts of thermal or mechan-
ical energy and of the amount of equipment and, hence, the
amounts of materials used. Quite often, in such optimization
studies, as energy goes down equipment goes up, and vice
versa. Thus the most economical balance must be struck.
Often this is at a point where the energy cost and the capital
cost of the equipment are about equal.
Of the billion dollars per year to be spent for plants,
materials for the equipment might be regarded as at least
50%, with costs of engineering, fabrication, transportation,
installation, etc. accounting for the rest. Of materials used
in equipment by far the largest amount will be for metals,
particularly those metals which are least corroded by sea-
water. These are not the most abundant or least expensive.
The specification of these metals for equipment—those
which are suitable for withstanding the corrosion and other
deleterious effects of this service—are of greatest impor-
tance. Their fabrication into sheets, tubes, shapes, and then
finished vessels and parts and accessories will be a great test
of the skill of the metallurgist, the chemical engineer, and the
mechanical engineer.Membrane ProcessesMembrane processes are classified in two main categories:- Methods which separate salts from the water, in their
ionic form, called ionic processes. Electrodialysis
and ion-exchange are the separation processes for
desalination. - Methods which separate water from a salt solution.
Reverse osmosis, nanofiltration, ultrafiltration and
microfiltration are the main processes of this type
of separation which are applied to desalination or
to water purification.
Ionic Processes Common salt, and other salts as well as
acids and bases, are ionized in solution into the positive
(for example, sodium ions) and the negative (for example,
chlorine ions). Whereas in distillation processes the amount
and kind of salts dissolved in the raw feed water are of no
importance to the process and do not affect the economics,
in all ionic processes the amount of dissolved salts is of pri-
mary importance. In electrodialysis, the amount of salts to
be removed affects the consumption of electrical energy and
in ion-exchange affect the amount and the cost of regenerant.
Thus, ionic processes are much too expensive to use with
the higher salt concentration of seawater, as compared with
brackish, river or estuary water. In general ionic processes
depend on specially developed polymeric resins in the form
of membrane in the case of electrodialysis and as granules
for ion exchange.Ion Exchange The chemical system for removal of the ions
of salt is called ion exchange and has long been used in the
reverse process in which, for example, the sodium ion of salt
is used to replace calcium ions to “soften” hard water. In
this case the “bed,” of ion-exchanging material consists of
granules of resin which have the property of removing posi-
tive or metal ions from a circulating aqueous solution by dis-
placing them by a chemical bond with the resin of the bed.
This replaces them with ions which are loosely bound in
the molecular structure of the resin. When the positive ions
of the resin are completely replaced, an aqueous solution,
relatively concentrated in the other positive ion, is cycled,
and the process is reversed. Other beds of resin have the
properties of exchanging negative ions.
Similarly the sodium ion of a salt solution such as sea-
water passing through a suitable first bed may be interchanged
with a hydrogen ion from the resin to leave the sodium ion
in a loose combination with the resin and a resulting hydro-
chloric acid in the effluent solution. This solution is then
passed through a second resin bed, in which a hydroxyl ion
interchanges with the chlorine ion to give a resin chloride
combination and an equivalent amount of chemically formed
water which is added to the aqueous stream.
When the two resin beds have interchanged all of their
respective hydrogen and hydroxyl ions, their activity ceases
and they must be generated. This is done by stopping the
flow and adjusting valves to take the beds off-stream and to
pass through the first bed a dilute solution of sulfuric acid
and then to pass through the second bed a dilute solution
of caustic soda. The hydrogen ions of the sulphuric acid
displace the sodium ions on the resin of the first bed to give
sodium sulphate in its effluent which is run to waste and
the hydroxyl ions of the caustic soda displace the chlorine
ions on the second resin to give sodium chloride (salt) in the
effluent of the second bed, which also runs to waste. Again,
the ions on the beds are completely interchanged, the beds
are thus generated and the controlling valves are adjusted,
with those allowing the sequential flow of the original sea-
water being opened to allow the process to be repeated.
This process thus required both sulphuric acid and caus-
tic soda in amounts which are chemically equivalent to theC004_001_r03.indd 211C004_001_r03.indd 211 11/18/2005 10:18:58 AM11/18/2005 10:18:58 AM