DESALINATION 205
CO 2 O 2
to ejectorSea water30°C Sea40°C
Product water
Blow-down
40°C brine40°CVentEjector
Feed water 40°CCOO 22Sulfuric
acid40°C 40°CWE BOTF101°CHealing 120°C
steam
SCapacity
selectorFIGURE 6 Flow-diagram of a multiple-effect-horizontal tube
evaporator. The effects are vertically oriented, one on top of the
other. This arrangement is compact and is called multiple-effect-
stack type (MES) distillation equipment.^19 Preheated seawater
feed is sprayed, F, onto the outer surface O of the evaporator tubes
in the first effect at the top of the column, T, where a portion of
seawater is evaporated by the heating steam S. The remaining
seawater is collected at the bottom B, of the first effect and then
sprayed onto the outer surfaces of the second effect where another
portion of seawater is evaporated, being heated by the vapor gen-
erated in the first effect. The generated vapor is delivered through
a mist eliminator section, E. The vapor itself condenses into fresh
water in the side section W. The cycle is repeated in each succes-
sive effect up to the last one. Vapors generated in the last effect is
condensed in a heat rejection condenser C.Multi-Stage-Flash (MSF) DistillationWhen saline water is heated to a temperature slightly below
its boiling point at a given pressure and then introduced into a
chamber where a sufficiently lower pressure exists, explosive
boiling will occur. Bubbles are evolving from the whole mass
of the liquid and part of the water will evaporate until equilib-
rium with its vapor at the prevailing pressure is reached.
This evaporation lowers the temperature of the remain-
ing brine. The liquid may then be passed into another cham-
ber at an even lower pressure, where it flashes again to vapor.If a higher rate of saline water circulation is supplied, an
increased proportion of flash will occur. The increased flow
rate may be considered as a means of obtaining increased
evaporative yield in a system without increasing the evapo-
rating surface. It is, therefore, equivalent to diminishing the
evaporating surface.
Flashing of vapor requires a finite residence time of the
liquid in the evaporation chamber in order to achieve near
equilibrium conditions. For a given flow rate the residence
time is determined by the chamber length. Mass-transfer
rates in two-phase flow depend on the interfacial geometry
of the two phases and on the degree of turbulence. They
accordingly determine the residence time required and thus
the size of the flashing chamber.
On the other hand, the length of the flashing chamber
must be sufficient to achieve the required temperature rise
of the incoming seawater under the acceptable maximum
velocity inside the condensing tubes. As there are limita-
tions for both brine and feed-water flow rates, the width of
the flashing chamber becomes the important determinant for
increasing the plant size. Figure 9 presents a flow diagram of
a vacuum vapor compression unit.
MSF is the most widely applied distillation process,
especially for large units, and despite the thermodynamic
advantages of ME evaporation, all major plants installed
are of the MSF principle because of the simplicity and
reliability of the process. It accounts for 51.5% of the
world desalination capacity and 86.9% of total distillation
processes. The capacity of MSF plants capable of produc-
ing 100 m^3 /d per unit or more fresh water was, by the end
of 1993, about 9,640,000 m^3 /d, 51.5% and 8,960,000 m^3 /d,
or 71.7% of world desalination capacity for desalting plants
producing more than 4,000 m^3 /d unit fresh water.
Multi-stage-distillation process, as applied in large scale
desalting of seawater, may be considered as consisting of
three sections in handling heat: the heat input section, usu-
ally named brine heater, by condensing external steam; the
heat recovery section, in which the heat of the evaporation
is recovered in the condensers at the various stages; and the
heat rejection section, which maintains the thermodynamic
process by reducing temperature and pressure and accounts
for the last stages of the plant.
MSF distillation plants operate with recirculation part of
the brine. Recirculation can be applied so far as the concen-
tration of the scale-forming compounds does not reach, after
the evaporation, the critical point. It is a disadvantage of this
design that the brine concentration at the hottest stages of
the plant is much higher than the concentration of dissolved
solids in the seawater. The fact limits the maximum brine
temperature of the process.
Operating with this cycle arrangement, the maximum
operating temperature with acid injection is limited to 121C
(250F), with brine concentration 1.5 times. The number of
stages is usually limited by 2C flashdown per stage, because
of the low pressure differential available at the deep vacuum
conditions prevalent in the last stages.
The total number of stages is affected by the initial
and final brine temperatures, as well as by the necessaryC004_001_r03.indd 205C004_001_r03.indd 205 11/18/2005 10:18:55 AM11/18/2005 10:18:55 AM