94
T.Hopner
desalination plant, the boiling point is controlled by adjusting the atmospheric pressure of the water being boiled. The reduction of the boiling point is important in the
desalination process for two major reasons: multiple boiling and scale control. To significantly reduce the amount of energy needed for vaporisation, the distillation desalting
process usually uses multiple boiling in successive vessels, each operating at a lower
temperature and pressure. The process which accounts for the most desalting capacity
is multi-stage flash (MSF) distillation. Seawater is heated in a vessel called the Brine
Heater and flows into another vessel, called a Stage, where the ambient pressure is such
that the water will immediately boil. The sudden introduction of the heated water into
the chamber causes it to boil rapidly, almost exploding or flashing into steam. The
steam generated by flashing is converted to fresh water by being condensed on tubes of
heat exchangers that run through each stage. The tubes are cooled by the incoming
feed water going to the brine heater. This, in turn, warms up the feed water so that the
amount of thermal energy needed is reduced. MSF plants have been built commercially
since the 1950S. They are generally built in units of about 4000 to 30 000 m 3 day-I.
Multiple effect distillation (MED) also takes place in a series of vessels (effects) and
uses the principle of reducing the ambient pressure in the various effects. This permits
the seawater feed to undergo multiple boiling without supplying additional heat after
the first effect. MED plants are typically built in units of 2000 to 10000 m 3 day-I.
The vapor compression (VC) distillation process is generally used for small- and
medium-scale units. The heat for evaporating the water comes from the compression
of vapor rather than from the direct exchange of heat from steam production in a boiler.
VC units are usually built in the 20-2000-m 3 day-I range. They are often used for resorts, industries, and drilling sites where freshwater is not readily available.
9.5
Chemical Impacts
Chemical emissions into the sea are less well known than chemical emissions into the
atmosphere which are widely discussed in connection with power plants. They consist
a list of components:
• Corrosion products (metals)
• Antiscaling additives (polycarbonic acids, polyphosphates)
• Antifouling additives (mainly chlorine and hypochlorite)
• Halogenated organic compounds formed after chlorine addition
• Antifoaming additives
• Anticorrosion additives
• Oxygen scavngers (sodium sulfite)
• Acid
• The concentrate
• Heat
Additives may be discharged in the original form or as reaction products. Additives
may react among each other, an option not examined hitherto. Similarly, synergistic effects on the environment by the materials discharged have not been examined
so far.
T.Hopner
desalination plant, the boiling point is controlled by adjusting the atmospheric pressure of the water being boiled. The reduction of the boiling point is important in the
desalination process for two major reasons: multiple boiling and scale control. To significantly reduce the amount of energy needed for vaporisation, the distillation desalting
process usually uses multiple boiling in successive vessels, each operating at a lower
temperature and pressure. The process which accounts for the most desalting capacity
is multi-stage flash (MSF) distillation. Seawater is heated in a vessel called the Brine
Heater and flows into another vessel, called a Stage, where the ambient pressure is such
that the water will immediately boil. The sudden introduction of the heated water into
the chamber causes it to boil rapidly, almost exploding or flashing into steam. The
steam generated by flashing is converted to fresh water by being condensed on tubes of
heat exchangers that run through each stage. The tubes are cooled by the incoming
feed water going to the brine heater. This, in turn, warms up the feed water so that the
amount of thermal energy needed is reduced. MSF plants have been built commercially
since the 1950S. They are generally built in units of about 4000 to 30 000 m 3 day-I.
Multiple effect distillation (MED) also takes place in a series of vessels (effects) and
uses the principle of reducing the ambient pressure in the various effects. This permits
the seawater feed to undergo multiple boiling without supplying additional heat after
the first effect. MED plants are typically built in units of 2000 to 10000 m 3 day-I.
The vapor compression (VC) distillation process is generally used for small- and
medium-scale units. The heat for evaporating the water comes from the compression
of vapor rather than from the direct exchange of heat from steam production in a boiler.
VC units are usually built in the 20-2000-m 3 day-I range. They are often used for resorts, industries, and drilling sites where freshwater is not readily available.
9.5
Chemical Impacts
Chemical emissions into the sea are less well known than chemical emissions into the
atmosphere which are widely discussed in connection with power plants. They consist
a list of components:
• Corrosion products (metals)
• Antiscaling additives (polycarbonic acids, polyphosphates)
• Antifouling additives (mainly chlorine and hypochlorite)
• Halogenated organic compounds formed after chlorine addition
• Antifoaming additives
• Anticorrosion additives
• Oxygen scavngers (sodium sulfite)
• Acid
• The concentrate
• Heat
Additives may be discharged in the original form or as reaction products. Additives
may react among each other, an option not examined hitherto. Similarly, synergistic effects on the environment by the materials discharged have not been examined
so far.