|
Distillation is
one of mankind’s earliest forms of separating fresh water from a
salt-water solution. When salt water is boiled, the dissolved
salt remains behind as the fresh water vapor is boiled away. In
a distillation process, water is first boiled and then the
steam, or water vapor, is cooled. This cooling condenses the
steam into water again (See the figure). Thus, distillation
involves adding heat energy to salt water in order to vaporize
the water and then removing the heat energy from the steam to
condense it into fresh water.
In nature, this basic process is responsible for the hydrologic
cycle. The sun causes water to evaporate from surface sources
such as lakes, oceans, and streams. The water vapor eventually
comes in contact with cooler air, where it re-condenses to form
dew or rain. This process can be imitated artificially, and more
rapidly than in nature, using alternative sources of heating and
cooling.
When water is heated, its temperature increases until the
boiling point is reached. While water is boiling, the
steam and the boiling water are at the same temperature.
However, raising water to its boiling point is not enough to
cause it to boil. More heat must be added to change the water
into steam. The amount of heat required to change water at its
boiling point into steam at the same temperature is called heat
of vaporization of water. The heat of vaporization is of major
importance in distillation. The amount of heat required to
vaporize water into steam is approximately five times greater
than the heat needed to raise water from its freezing point to
its boiling point (at ordinary sea-level atmospheric pressure
(14.7 psi) water boils at 100oc).
Distillation is a two-step process involving both evaporation
and condensation, heat must be added in one step and removed in
the other. If these two steps were accomplished independently,
the process would be inefficient and costly. In all the
distillation processes, the steam is condensed by transferring
heat from the steam to salt water as part of the heat source
required to convert more water into steam. In this way some of
the heat energy used in one step is recovered and used in the
other step.
The conversion of saline water
into fresh potable water and water for
industrial purposes has been practiced worldwide over the last
forty to fifty years of the twentieth century. It is a technique
of providing and augmenting freshwater supplies in areas
deficient therein, such as arid regions close to the ocean, or
to other saline water bodies.
(To read more about the conversion of saline water, subscribe
DESWARE Online)
Encyclopedia of Desalination and Water
Resources (DEWARE) was first released on CDROM in 2000. In
2002 the CDROM version was discontinued and a new edition was
launched on the Internet at www.desware.net. Since then DESWARE
has been continually updated and augmented with additional
content and the process is still continuing.
There is no print version of DESWARE so far. But now there are
plans to release it in e-book format with the possibility to
print-on-demand. |
 |
|
The global water
crisis
Water has always been
earth's most valuable resource. All ecosystems and every field
of human activity depend on water. The world's supply of fresh
water is running out. Already one person in five has no access
to safe drinking water. The amount of water in the world is
limited. The human race, and the other species which share the
planet, cannot expect an infinite supply. 97.5% of the total
global stock of water is saline and only 2.5% is fresh water.
Approximately 70% of this global freshwater stock is locked up
in polar icecaps and a major part of the remaining 30% lies in
remote underground aquifers.
Population growth and the increasing need for fresh water for
industrial, agricultural uses and municipal indicate there will
be no letup in the increasing demand for water in the years to
come. These factors account for the concern over water shortages
that exist now in some areas of the country and over the more
serious shortages that are projected for the near future.
See:
Desalination and the Continuity of Human Civilization)
1- Dead vegetation in
drought-stricken area, Senegal. 2- Children have a special
relationship with water
(Source: UNESCO Photo bank)
Free Articles
 Timelines - Desalination Technology
Small Scale Desalination
Global Production of Desalination Water
Emerging Technologies
Sustaining Water
Why
Use Renewable Energy for Desalination
Energy Requirements for Desalination Processes
Humanity's Need for Energy
Economics of Water and
Energy Technology
|
Encyclopedia of
Desalination and Water Resources - Contents
History, Development and Management
Physical, Chemical and Biological Properties of Water
The Desalination Site and Civil Works
Water Treatment
Common Fundamentals and Unit Operations in Thermal Desalination
Thermal Desalination Processes
Membrane Processes
Renewable Energy Systems and Desalination
Ancillary Equipment
Process Instrumentation, Control and Automation
Material Selection and Corrosion
Plant Operation, Maintenance and Management
Environmental Aspects
Thermal Power Plants and Co-generation Planning
Water Sciences and Technology Resources
DESWARE
is a subset of the Encyclopedia of Water Sciences, Engineering and
Technology Resources, which is part of the UNESCO Encyclopedia of
Life Support Systems (EOLSS)
If you are interested in the broad area of Water Sciences,
Engineering and Technology, please visit http://www.eolss.net
Special Special Bulk Offers For Institutional / Public Libraries:
e-Books (without DRM restriction)
|
