Environmental Engineering Reference
In-Depth Information
Saline
feed
Brine feed
Vacuum
Vacuum
Vacuum
Steam in
Vapor
Vapor
Condensate
back to boiler
Brine
discharge
Brine
discharge
Brine
discharge
Freshwater
Condensate
FIGURE 27.4
Block diagram schematic showing the basic elements of an MED process. (Based on Trieb, F., ed.
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.)
sugar cane juice or salt through evaporation [24]. MED systems were employed for solu-
tions at lower temperatures than MSF, and thus have found use for processes that have
left-over steam heat below 100°C. Today many dedicated MED systems are used for water
desalination alone. It has been estimated in the literature that the MED process offers
superior thermal performance to the MSF processes; however, the scaling problems within
the plants were noted to be higher [22,23]. Current systems resolve some of the scaling and
corrosion issues by operating at a maximum brine temperature of approximately 70°C
and, in some cases, systems employ a maximum brine temperature of 55°C allowing for
utilization of low-grade waste heat [22]. The MED process begins by distributing (usually
by spraying) the preheated saline feedwater onto the heat exchange surface in a thin ilm
to encourage boiling and evaporation of water [24] through a surface area enhancement
for improved heat transfer. The vapor phase of the water is then condensed in a lower-
pressure chamber, much like the MSF design [22], while simultaneously creating vapor
to be fed into the next chamber as shown in Figure 27.4. The MED process continues for
8-16 effects or cycles, depending on plant design [24]. It is not uncommon for MED plants
to be integrated with additional heat inputs between stages, usually by thermal vapor
compression (TVC) or mechanical vapor compression (MVC) [22]. Energy consumption in
MED plants incorporated with the cold end of a steam cycle require about 145-390 kJ/kg
in process steam withdrawn from a steam turbine and 5.4-9 kJ/l electricity for control and
pumping processes [22]. Owing to the relatively low operating temperatures of the MED
process, a large amount of surface area is required by the system, thus requiring large
areas for MED production facilities [24]. The integration of MED plants with TVC reduces
the surface area and the number of effects needed per plant capacity [24].
27.2.3.3 Vapor Compression (VC) Processes
By themselves, VC processes (both thermal and mechanical) offer simple, consistent oper-
ation that are usually deployed in small- to medium-scale desalination units. They are
often used for applications such as resorts, industries, or drilling sites with a lack of direct
access to freshwater supply [22]. Figure 27.5 shows a schematic representation of the unit
