86
The Last Resource
suggested that gold—plated heat exchange surfaces could be the
solution; copper pipes coated with a layer a few thousandths of an
inch thick of silver, gold, rhodium, palladium or platinum have
produced increases in condensation rates of 50 per cent or more.
The membrane processes also have their weaknesses. Most of the
membranes in electrodialysis consist of a base material such as
polyethylene or polystyrene which is treated chemically to develop
the necessary properties. Once more the chemicals, other than
sodium chloride, in the sea or brackish water play havoc with the
process: insoluble salts collect on the membranes and block them
or change their chemical composition. Whereas in distillation heat—
ing above certain threshold temperatures leads to scale formation,
in electrodialysis the interaction of the migrating ions causes the
trouble. Local reactions occur in the compartments where the ions
collect particularly at the surface of the membranes. As the mem—
branes become more resistant to the ow of ions through them—a
process known as polarization—more and more energy is needed to
push them along until eventually the process ‘grinds to a halt’.
Israeli scientists at the Negev Institute for Arid Zone Research
have developed one possible line of attack against polarization of
the membranes and its attendant deposition of insoluble chemicals;
they reverse the ow of electricity for brief periods. The reversed
pulses drive the main concentration of ions fractionally away from
the surface of the membranes. lt does not prevent their interaction,
but the scale now forms in the stream of brine and can be ushed
away. lnevitably such solutions reduce the efciency of the plant,
but savings in terms of membranes and the time lost during their
replacement can more than cover this.
So far, the choice in electrodialysis seems to rest between mem—
branes which are cheap to make but have a short life and those
which are expensive but last longer. Obviously a considerable payoff awaits the company which manages to produce membranes
combining efciency, long life and cheapness. One of the more
successful membranes so far is ‘Selemion’, developed and manu—
factured by the Asahi Glass Company of japan—as early as 1964
this company was turning out some I 5,000 pairs of membranes per
month. The Selemion membrane seems to prevent much of the
deposition of calcium sulphate on its surface in the compartments
of concentrated brine.
The membranes for reverse osmosis have not even reached this
The Last Resource
suggested that gold—plated heat exchange surfaces could be the
solution; copper pipes coated with a layer a few thousandths of an
inch thick of silver, gold, rhodium, palladium or platinum have
produced increases in condensation rates of 50 per cent or more.
The membrane processes also have their weaknesses. Most of the
membranes in electrodialysis consist of a base material such as
polyethylene or polystyrene which is treated chemically to develop
the necessary properties. Once more the chemicals, other than
sodium chloride, in the sea or brackish water play havoc with the
process: insoluble salts collect on the membranes and block them
or change their chemical composition. Whereas in distillation heat—
ing above certain threshold temperatures leads to scale formation,
in electrodialysis the interaction of the migrating ions causes the
trouble. Local reactions occur in the compartments where the ions
collect particularly at the surface of the membranes. As the mem—
branes become more resistant to the ow of ions through them—a
process known as polarization—more and more energy is needed to
push them along until eventually the process ‘grinds to a halt’.
Israeli scientists at the Negev Institute for Arid Zone Research
have developed one possible line of attack against polarization of
the membranes and its attendant deposition of insoluble chemicals;
they reverse the ow of electricity for brief periods. The reversed
pulses drive the main concentration of ions fractionally away from
the surface of the membranes. lt does not prevent their interaction,
but the scale now forms in the stream of brine and can be ushed
away. lnevitably such solutions reduce the efciency of the plant,
but savings in terms of membranes and the time lost during their
replacement can more than cover this.
So far, the choice in electrodialysis seems to rest between mem—
branes which are cheap to make but have a short life and those
which are expensive but last longer. Obviously a considerable payoff awaits the company which manages to produce membranes
combining efciency, long life and cheapness. One of the more
successful membranes so far is ‘Selemion’, developed and manu—
factured by the Asahi Glass Company of japan—as early as 1964
this company was turning out some I 5,000 pairs of membranes per
month. The Selemion membrane seems to prevent much of the
deposition of calcium sulphate on its surface in the compartments
of concentrated brine.
The membranes for reverse osmosis have not even reached this