DESALINATION 203
evolved in the single-effect evaporator, is lost in the con-
denser. A better heat recuperation would be obtained if the
heat, released by the condensing vapor, is not rejected in a
condenser, but is used to heat the brine of a second evapora-
tor and so on.
This leads to the concept of multiple-effect distillation,
where the vapors from one effect are used as the heat source
of the next effect, as long as the difference in temperature
between the condensing vapor and the solution is high
enough to act as the driving force in the evaporation pro-
cess, each effect being at progressively lower temperature
and pressure. Vapor condensing because of lower boiling
temperature, in each effect, produces fresh water as distil-
late, whereas the vapor from the final effect is condensed by
a circulating seawater cooling stream.
Theoretically, an additional kg of distillate would be
obtained in each consecutive effect for the same kg of
steam initially introduced into the first effect and the plant
gain output ratio would be equal to the number of effects in
operation. However, this is not true in practice. Part of the
condensation heat to be recovered is lost to the atmosphere,
in design features and in the differences of temperature used
as the plant’s driving force.
Multiple-effect distillation process uses evaporators
which are modified successors of evaporators that have been
used in sugar and other process industries for more than 100
years and have been in use for seawater distillation about
90 years. The latter were originally built for shipboard use,
the main requirements being for compactness, simplicity in
operation and reliability. In land based industrial evapora-
tor plants the requirements are mainly directed to the cost
of product water with emphasis on cheaper materials of
construction, high boiling temperatures, efficient descaling
methods and the use of the cheapest type of evaporator.
Previously multiple-effect distillation was second in
importance of the distillation process, as medium capacity
plants but day hardly is applied. Worldwide capacity of ME
plants for units producing more than 100 m^3 /d of fresh water,
is only 765,143 m^3 /day or 4.1% of total world capacity.^5Long Tube Vertical Evaporator, LTVE Long tube evapo-
rators consist of a series of long tubes arranged vertically
inside the evaporator shell. Seawater feed may be from the
top or from the bottom, called respectively falling or rising
film LTV evaporators.
In the falling film evaporator seawater is introduced at
the top and the incoming seawater flows across an upper
tube plate and is equally distributed to the tubes, and flows
downward by gravity as a thin film. The principal advantage
of the VTE process is that high heat-transfer can be achieved,
which considerably reduces the required heat-transfer sur-
face area. This forward feed is the usual method of feeding a
multiple-effect-evaporator.
The VTE rising film is similar to falling film evaporator
except that seawater is introduced at the bottom of the first
effect, thus reducing the overall pumping requirements. Heat
transfer in the VTE evaporators is increased by using fluted
tubes, which enhance heat transfer.Steam condenses outside the tubes, forming also a thin
film of distillate. Surface tension forces are created, which
are inversely proportional to the flute radius. This causes the
condensate film to drain from the crests to the grooves, so
that a very thin condensate layer is remaining on the crests,
which promotes heat transfer.
The flow sheet of a typical multiple-effect vertical tube
distillation plant is shown in Figure 5. In each effect is adapted
a feed heater C which uses the product vapors as heating
medium in the form of distilled water or vapor condensate.
Vapors produced in the first effect condense outside the tubes
of effect 2 and the brine is pumped from each effect to the top
of the next. The average efficiency K of each effect is usu-
ally between 0.85 and 0.95. Concerning N effects in a LTE
system, as in Figure 5, the GOR is given by the equation:GOR K(1 K) N /(1 K) (3)Thus when K 0.95 and the number of effects N is 15,
GOR 10 kg/kg. Doubling the effects to 30, the GOR is
only 14.9, and it will attain a maximum of 19.0 for an infi-
nite number of effects, when K 0.95.
There are some economic limitations increasing the
number of effects. The investment costs and consequently the
fixed charges are increasing almost linearly with the number
of effects. The costs of steam and water fall off rapidly at
first, but the savings diminish progressively. The total cost
of operating an evaporator leads to an optimum number of
effects, at the point where the sum of fixed costs and the cost
of utilities shows a minimum. The most probable number of
effects will be between 10 and 20.The Multiple-Effect Horizontal-Tube Evaporator The
(MEHT) type of evaporator operates on the same principle
as the VTE evaporator, but the steam condenses on the inside
of the horizontal tubes imparting its latent heat of conden-
sation to the brine, which cascades and evaporates over the
outside of the tubes. The brine falls to the next effect by
gravity and the vapors formed in one effect are used in the
next effect. The horizontal-tube evaporator eliminates the
pumps required for each effect of the VTE brine circulation,
by an arrangement in which the effects are stacked vertically
on the top of each other. This compact arrangement of the
ME evaporators, called also multiple-effect stack (MES), is
constructed in low capacity units and though there are many
advantages, it accounts for only 1% of the world capacity.
In Figure 6 a typical HTE evaporator is presented.^19 Multiple-
effect-horizontal-tube evaporators are suitable to operate with
solar energy plants.VAPOR COMPRESSION (VC)Vapor compression (VC) distillation takes advantage of
the latent heat of the vapors produced in the process. Vapor
produced by evaporation from a salt solution is superheated
because of the boiling point elevation of the solution andC004_001_r03.indd 203C004_001_r03.indd 203 11/18/2005 10:18:54 AM11/18/2005 10:18:54 AM