84
The Last Resource
the conversion process must be halted and the ion exchange material
regenerated. It is washed with chemicals which remove the unwanted
salt ions and replace them once more with hydrogen and hydroxyl
ones. This can be quite a costly process. Therefore, just as in dialysis
the amount of salt in solution is a critical factor—the more salt, the
more
often the ion exchange beds have to be regenerated. Not
surprisingly, apart from their use in life—raft survival kits and other
relatively small marine units, ion exchange systems are generally
restricted to the desalting of brackish waters.
Soloent extraction. If the chemistry of ion exchange seems compli—
cated, that of solvent extraction techniques should appear very
simple. All that is required is a solvent which takes up either salt or
water, but not both. Most of the materials tried have had an afnity
for water. The water is removed from the solvent which is used
again. So far, simplicity has not been much of a bonus. None of the
chemicals has been used on anything greater than a laboratory
scale; even then it has proved excessively expensive, largely because
_
few solvents have a great afnity for water and most of the solvent
materials have been very costly.
Hydrote formation. A hydrate is a crystallized substance formed
between water and certain ions. Liqueed propane gas, a hydrating
agent currently under investigation, forms a solid crystal with water
in the ratio of seventeen molecules of water to every one of the gas.
The hydrating agent is mixed with saline water in a reactor vessel
where it forms crystals composed solely of pure water and the agent
——only salt is left behind. After separation and washing, just as in
the freezing processes, the crystals are decomposed by melting,
to yield fresh water and the original hydrating agent. They divide
into two distinct layers, just like oil and water, and can easily be
separated. ln many ways the hydrate process resembles freeze
separation, but ultimately it depends upon the formation of chemical
bonds to separate the fresh water from the salt.
Problems golore
The fact that some desalting processes are available commercially
does not mean they have been perfected. Nothing could be less true:
the success of multi—stage ash distillation results largely from
compromises in design and working temperatures which avoid
The Last Resource
the conversion process must be halted and the ion exchange material
regenerated. It is washed with chemicals which remove the unwanted
salt ions and replace them once more with hydrogen and hydroxyl
ones. This can be quite a costly process. Therefore, just as in dialysis
the amount of salt in solution is a critical factor—the more salt, the
more
often the ion exchange beds have to be regenerated. Not
surprisingly, apart from their use in life—raft survival kits and other
relatively small marine units, ion exchange systems are generally
restricted to the desalting of brackish waters.
Soloent extraction. If the chemistry of ion exchange seems compli—
cated, that of solvent extraction techniques should appear very
simple. All that is required is a solvent which takes up either salt or
water, but not both. Most of the materials tried have had an afnity
for water. The water is removed from the solvent which is used
again. So far, simplicity has not been much of a bonus. None of the
chemicals has been used on anything greater than a laboratory
scale; even then it has proved excessively expensive, largely because
_
few solvents have a great afnity for water and most of the solvent
materials have been very costly.
Hydrote formation. A hydrate is a crystallized substance formed
between water and certain ions. Liqueed propane gas, a hydrating
agent currently under investigation, forms a solid crystal with water
in the ratio of seventeen molecules of water to every one of the gas.
The hydrating agent is mixed with saline water in a reactor vessel
where it forms crystals composed solely of pure water and the agent
——only salt is left behind. After separation and washing, just as in
the freezing processes, the crystals are decomposed by melting,
to yield fresh water and the original hydrating agent. They divide
into two distinct layers, just like oil and water, and can easily be
separated. ln many ways the hydrate process resembles freeze
separation, but ultimately it depends upon the formation of chemical
bonds to separate the fresh water from the salt.
Problems golore
The fact that some desalting processes are available commercially
does not mean they have been perfected. Nothing could be less true:
the success of multi—stage ash distillation results largely from
compromises in design and working temperatures which avoid