Newsletter and Technical Publications
of Alternative Technologies for Freshwater Augumentation
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PART B. TECHNOLOGY PROFILES
2.2 Desalination by Distillation
Distillation is the oldest and most commonly used method of
desalination. The world's first land-based desalination plant, a
multiple-effect distillation (MED) process plant that had a capacity of 60
m3/day, was installed on Curaçao, Netherlands Antilles,
in 1928. Further commercial development of land-based seawater
distillation units took place in the late 1950s, and initially relied on
the technology developed for industrial evaporators (such as sugar
concentrators) and for the shipboard distillation plants which were built
during World War II. The multistage-flash (MSF), MED, and
vapor-compression (VC) processes have led to the widespread use of
distillation to desalinate seawater.
Distillation is a phase separation method whereby saline water is heated
to produce water vapor, which is then condensed to produce freshwater. The
various distillation processes used to produce potable water, including
MSF, MED, VC, and waste-heat evaporators, all generally operate on the
principle of reducing the vapor pressure of water within the unit to
permit boiling to occur at lower temperatures, without the use of
additional heat. Distillation units routinely use designs that conserve as
much thermal energy as possible by interchanging the heat of condensation
and heat of vaporization within the units. The major energy requirement in
the distillation process thus becomes providing the heat for vaporization
to the feedwater.
Figure 18 shows a simplified schematic of a multistage-flash unit. The
incoming seawater passes through the heating stage(s) and is heated
further in the heat recovery sections of each subsequent stage. After
passing through the last heat recovery section, and before entering the
first stage where flash-boiling (or flashing) occurs, the feedwater is
further heated in the brine heater using externally supplied steam. This
raises the feedwater to its highest temperature, after which it is passed
through the various stages where flashing takes place. The vapor pressure
in each of these stages is controlled so that the heated brine enters each
chamber at the proper temperature and pressure (each lower than the
preceding stage) to cause instantaneous and violent boiling/evaporation.
The freshwater is formed by condensation of the water vapor, which is
collected at each stage and passed on from stage to stage in parallel with
the brine. At each stage, the product water is also flash-boiled so that
it can be cooled and the surplus heat recovered for preheating the
Because of the large amount of flashing brine required in an MSF plant,
a portion (50% to 75%) of the brine from the last stage is often mixed
with the incoming feedwater, recirculated through the heat recovery
sections of the brine heater, and flashed again through all of the
subsequent stages. A facility of this type is often referred to as a "brine
recycle" plant. This mode of operation reduces the amount of
water-conditioning chemicals that must be added, and can significantly
affect operating costs. On the other hand, it increases the salinity of
the brine at the product end of the plant, raises the boiling point, and
increases the danger of corrosion and scaling in the plant. In order to
maintain a proper brine density in the system, a portion of the
concentrated brine from the last stage is discharged to the ocean. The
discharge flow rate is controlled by the brine concentration at the last
In multiple-effect units steam is condensed on one side of a tube wall
while saline water is evaporated on the other side (in a manner similar to
the VC process shown in Figure 19). The energy used for evaporation is the
heat of condensation of the steam. Usually there is a series of
condensation-evaporation processes taking place (each being an "effect").
The saline water is usually applied to the tubes in the form of a thin
film so that it will evaporate easily.
Figure 18: Simplified Schematic of a Multistage Flash
(MSF) Distillation Plant.
Source: O.K. Buros, et.
al., The USAID Desalination Manual. Englewood, N.J., U.S.A., IDEA
Although this is an older technology than the MSF process described
above, it has not been extensively utilized for water production. However,
a new type of low-temperature, horizontal-tube MED process has been
successfully developed and used in the Caribbean. These plants appear to
be very rugged, easy to operate, and economical, since they can be made of
aluminum or other low-cost materials.
The vapor-compression process uses mechanical energy rather than direct
heat as a source of thermal energy. Water vapor is drawn from the
evaporation chamber by a compressor and except in the first stage is
condensed on the outsides of tubes in the same chambers, as is shown in
Figure 19. The heat of condensation is used to evaporate a film of saline
water applied to the insides of the tubes within the evaporation chambers.
These units are usually built with capacities of less than 100 m3/day
and are often used at resorts and industrial sites.
Figure 19: Simplified Schematic of a Vapor Compression
Source: O.K. Buros, et al., The
USAID Desalination Manual. Englewood, New Jersey, U.S.A., IDEA
Membrane distillation is a relatively new process, having been
introduced commercially only in the last few years. The process works by
using a specialized membrane which will pass water vapor but not liquid
water. This membrane is placed over a moving stream of warm water, and as
the water vapor passes through the membrane it is condensed on a second
surface which is at a lower temperature than that of the feedwater.
Most of the large distillation units in the world are dual-purpose
facilities. Specifically, they derive their source of thermal energy from
steam that has been used for other purposes, usually for power generation.
Thus, the feedwater is heated in a boiler to a high energy level and
passed through a steam turbine before the steam is extracted for use at a
lower temperature to provide the heat required in the distillation plants.
At this point, the desalination then conforms to the processes described
Extent of Use
Since 1971, about 65 single-purpose service or experimental plants have
been installed in Latin America and the Caribbean, with capacities ranging
from 15 to 1 000 m3/day. In Mexico they supply freshwater to
fishing villages and/or tourist resorts in Baja California and in the
north-central and southeastern parts of the country. They also provide
freshwater to agricultural communities.
Desalination for municipal freshwater supply purposes started in Mexico
in the late 1960s, when the Federal Electricity Commission installed two
14 000 m3/day MSF distillation units in its Rosarito Power
Plant in the city of Tijuana in northwest Mexico. At that time, those
units were among the largest in the world. The Federal Electricity
Commission currently operates about 31 desalination plants to produce
high-quality boiler make-up water, and maintains the two dual-purpose
units in Tijuana. The Mexican Navy also installed some smaller solar
distillation plants to provide a supply of freshwater to some islands in
the Pacific Ocean. PEMEX, the national oil company of Mexico, operates
about 62 small seawater desalination plants for human freshwater
consumption on off-shore oil platforms or ships. These distillation units
are mainly VC, waste heat, submerged-tube evaporators, and RO plants.
The island of Curaçao, in the Netherlands Antilles, currently has
two distillation plants. One is for public water supply and the other is
used by the oil refinery, PEDEVESA. Both use the MSF process. The public
supply plant has a maximum design capacity of 47 000 m3/day
(although the average daily production is currently 41 000 m3/day),
which is higher than the estimated domestic water consumption of 35 000 m3/day.
Operation and Maintenance
Most plants are installed in isolated locations where construction is
troublesome and where the availability of fuel, chemicals, and spare parts
is limited. In these places, there is usually also a scarcity of qualified
personnel; therefore, people are often selected from the local communities
and trained to operate the plants. The operation of distillation plants
requires careful planning, well-trained operators, and adequate operation
and maintenance budgets to guarantee the supply of good quality water.
Except for an annual shut-down of 6 to 8 weeks for general inspection and
maintenance, the operation of desalination plants is usually continuous.
Maintenance and preventive maintenance work, for a MSF plant, consists of:
- Repairing damage (cracks) to the stainless steel liners in the
- Removing scale and marine growths in the tubes in all stages using
high pressure "hydrolaser" sprayers.
- Removing the vacuum system ejectors for cleaning, inspection, and
replacement as necessary; most parts have a lifetime of 3 to 4 years.
- Inspecting all pumps and motors, replacing bearings and bushings, and
renewing protective coatings on exposed parts (e.g., pumps must be
primed and painted before being installed).
Level of Involvement
The manufacturing capacity to produce MSF evaporators is available in
those places where power plant equipment is fabricated. Thus, many
countries in Latin America have the potential to manufacture locally the
equipment needed to develop desalination plants. Further, some local
manufacturers have signed licensing agreements with major foreign
desalination manufacturing firms as a result of governmental policies of
import substitution, in order to offer desalination equipment,
particularly MSF plants, to the electric-generating industry in the
In the Caribbean, desalination by distillation is being used primarily
in the private sector, especially in the tourist industry. Some industrial
concerns and power companies have incorporated distillation into their
operations as part of a dual process approach. Government participation
has been very limited. Future developments of this technology, which are
expected to reduce the cost of desalination plants, will be likely to
encourage greater government participation in the use of distillation in
the development of public water supply systems.
The production cost of water is a function of the type of distillation
process used, the plant capacity, the salinity in the feedwater (seawater
or brackish water), and the level of familiarity with the distillation
process that exists in the region. Table 8 shows a range of costs that
have been reported by different countries using this technology.
Production costs appear to increase in proportion to the capacity of the
plant. In many applications, distillation provides the best means of
achieving waters of high purity for industrial use: for volumes of less
than 4 000 m3/day, the VC process is likely to be most
effective; above that range, the MSF process will probably be preferable.
TABLE 8. Estimated Cost of Distillation Processes in Latin American
|| Distillation Process
|| Capital Cost ($)
||Operation and Maintenance Cost ($/year)
|| Energy Cost
| Production Cost
|| 1 612
|U.S. Virgin Is.
|| MED and VC
Effectiveness of the Technology
Desalination of seawater is a relatively expensive method of obtaining
freshwater. The MSF system has proved to be a very efficient system, when
properly maintained. It produces high quality product water (between 2 and
150 mg/l of total dissolved solids at the plant in Curaçao); TDS
contents of less than 10 mg/l have been reported from the VC plant in
Because the water is boiled, the risk of bacterial or pathogenic virus
contamination of the product water is minimal.
MSF plants have been extensively used in the Middle East, North Africa,
and the Caribbean. Although MED is an older technology than the MSF
process, having been used in sugar refineries, it has not been extensively
utilized for water production. However, the new low-temperature
horizontal-tube MED process has been successfully used in the Caribbean,
usually in units with capacities of less than 100 m3/d (25,000
gpd) installed at resorts and industrial sites.
- Distillation offers significant savings in operational and
maintenance costs compared with other desalination technologies.
- In most cases, distillation does not require the addition of
chemicals or water softening agents to pretreat feedwater.
- Low temperature distillation plants are energy-efficient and
cost-effective to operate.
- Many plants are fully automated and require a limited number of
personnel to operate.
- Distillation has minimal environmental impacts, although brine
disposal must be considered in the plant design.
- The technology produces high-quality water, in some cases having less
than 10 mg/l of total dissolved solids.
- Distillation can be combined with other processes, such as using heat
energy from an electric-power generation plant.
- Some distillation processes are energy-intensive,
particularly the large-capacity plants.
- Disposal of the brine is a problem in many regions.
- The distillation process, particularly MSF distillation, is very
- Distillation requires a high level of technical knowledge to design
- The technology requires the use of chemical products, such as acids,
that need special handling.
Despite significant progress toward becoming more
energy-efficient and cost-effective, the level of community acceptance of
distillation technologies is still limited. Their use is mainly restricted
to resort hotels and high-value-added industries, and to the Caribbean
Further Development of the Technology
Research into the falling (or spray) film MED thermal desalination
process suggests that further development of distillation technologies can
produce product waters that are comparable in quality to those produced
with current MSF technologies and also offer additional advantages,
including lower pumping requirements, higher heat transfer rates, and
greatly reduced pressure differentials across the heat transfer surfaces.
These favorable comparisons also apply to a falling (or spray) film VC
design. Some additional considerations include:
- Lower operating temperatures (150 to 180oF)(66 to 82oC)
and vapor velocities, reducing system losses.
- Higher thermal efficiencies to reduce fuel and energy costs.
- Improved materials for evaporator heat transfer surfaces (aluminum
has two major benefits over other materials: a lower cost than
copper-nickel, with nearly triple the thermal conductivity and higher
operating temperatures, with an upper limit of 150oF (65oC)
for aluminum alloys containing approximately 2% magnesium).
- Improved coatings for use in shell construction (with aluminum
evaporator heat transfer surfaces, it is essential to prevent corrosion
caused by the proximity of other metal ions; the carbon steel shell must
be appropriately coated, and provision made for all supporting
structures to be protected).
- Improved piping material for use with low temperature distillation
techniques; piping should be of PVC, fiberglass, or other suitable
A further alternative and promising new concept for a dual purpose plant
has been the development of an evaporative condenser which is equipped
with dimpled flat plate elements that could greatly increase the
efficiency of this type of plant.
Roberto Espejo Guasp, Facultad de Ciencias, Universidad
Católica del Norte, Departamento de Física, Av. Angamos
0610, Casilla de Correo 1280, Antofagasta, Chile. Tel.
(56-55)24-1148/anexo 211-312-287. Fax (56-55)24-1756 / 24-1724. E-mail:
Carlos Plaza Bello and José García
Jara, Compañía Minera Michilla, S.A., Sucre 220 Of.
606, Antofagasta, Chile. Tel: (56-55) 25-1276. Fax: (56-55)26-7592.
Juan Pablo Vega W., Proyectos y Equipos, Torre Santa María,
Piso 24, Santiago, Chile. Tel. (56-2) 23-1543/ 23-16337. Fax
Theofilo Damien, Department of Agriculture, Husbandry
and Fisheries, 114-A Piedra Plat, Aruba. Tel. (297-8)58-102 / 56-473. Fax
Chris Winkel, Water and Soil Section, Department of
Agriculture, Animal Husbandry and Fisheries, Klein Kwartier # 33, Curaçao,
Netherlands Antilles. Tel. (599-9)37-6170. Fax (599-9)37-0723.
Henry H. Smith, Director, Water Resources Research,
Institute, University of the Virgin Islands, #2 John Brewers Bay, St.
Thomas, U.S. Virgin Islands 00802-9990. Tel. (809)693-1063. Fax
(809)693-1074. E-mail: firstname.lastname@example.org.
Buros, O.K., et al. 1982. The USAID Desalination Manual.
Englewood, N.J., IDEA Publications. (Reprint; originally USAID/CH2M Hill
Curaçao Eilandgebied. 1991. Alvalwaterstructuurplan
Samenvatting. Willemstad, Curaçao.
Desalination & Water Reuse. 1995. "Water Costs From the
Seawater Desalting Plants at the US Virgin Islands," 5(2), pp. 25-31.
Eisden, R.L. 1981. Awa i Electricidad pa Curaçao. Prome
edición. Willemstad, Curaçao.
UN. 1985. Non-Conventional Water Resources Use in Developing
Countries. Report on an Inter-Regional Seminar in Willemstad, Curaçao.
New York. (United Nations Report No. TCD/SEM.85/2-INT-84-R29)