DESALINATION 219
MESH SPACERREVERSE OSMOSIS
PERMEATE MEMBRANE
CARRIERPERMEATEFEED WATERFEED
WATERBRINE SEALPRODUCT TUBEPERMEATECONCENTRATEFIGURE 19 Reverse osmosis spiral wound arrangement, made from flat sheet, rectangular shaped membranes. A TORAY Industries,
Inc. arrangement. Between the membranes is inserted a porous material, usually a dacron tricot, impregnated with a resin, and both are
sealed together, forming the so called “sandwich” on three sides only, the fourth side joined to a slotted collection tube. More sealed
membranes connected to the same tube compose the module. A spacer, mesh screen is inserted between the sandwiches and they are
rolled to form the spiral wound module.The vacuum flash vapor absorption system (VFVA) uti-
lizes heat transfer by direct contact by means of an absorption
refrigeration machine. For the above reasons, freezing pro-
cesses found no commercial application. Worldwide there is
only one unit producing 210 m^3 /d.Dual Purpose PlantsThe economics of desalination plants can be considerably
affected, and as a consequence the cost of product water,
when electric-power generation is combined with a desalt-
ing plant in dual operation. This is especially true in loca-
tions where power demand and water requirements are in
favorable relation, independent of the method applied in the
desalting process. This system has been used for many years
in various chemical plants, in which both electricity and low
pressure steam are required for processing. This combination
results in lower costs for both power and water than would
be achieved by separate plants for the production of each.
The main part of the steam enthalpy used in distillation
is the heat of condensation. On the other hand, turbines in
a power plant use efficiently high temperature and pressure
steam. The principle of a dual-purpose plant is to use high
temperature steam for power production and low pressure
steam leaving the turbine for the brine heater of the desalting
plant. Thermodynamic considerations define dual- purpose
plant profit when the ratio of water-to-power demand is
between certain limits.
Electricity cannot be stored and its production has to
meet the actual power demand. Water can be stored, up tothe existing facilities, and this gives to the dual purpose plant
a certain degree of flexibility in operation. The ratio of the
water output to the power output is an important parameter
in the economic analysis of dual-purpose plants, as the two
products are not necessarily consumed by a single market.
To obtain this requirement the size and characteristics of
each component of the dual-purpose plant must be selected
in such a way, taking the power and water demand curves in
consideration, as to arrive at the optimum cost. It should be
noted that power and water demand may present daily and
seasonal variations.
The ratio of the water output to the power output is an
important parameter in the economic analysis of dual purpose
plants. As the two products are not necessarily consumed by
a single market, it is important to consider what product ratio
can be marketed and how the costs of the two products are
affected by varying the product ratio.
Many of the MSF desalination plants in the Arabian
peninsula are dual-purpose plants producing electricity and
desalinated water.Water ReuseThe heavy pollution of river, lake or other natural land waters
and the restricted sources of water in some places over the
world led to the application of desalination methods to treat
industrial or municipal effluents for reuse, either for drink-
ing and household purposes or as process water.
The great variety of the raw water to be treated by a desal-
ination process for reuse affects not only the selection of theC004_001_r03.indd 219C004_001_r03.indd 219 11/18/2005 10:19:02 AM11/18/2005 10:19:02 AM