Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a method and apparatus
for producing potable water from sea water. More particularly,
this invention relates to a method and apparatus for
emergency use in lifeboats.
In a survival situation at sea procurement of 1
sufficient drinking water to enable survival for an extended
period of time is of paramount importance~ It has been ;
estimated that each person aboard a lifeboat or other
survival craft requires a minimum of one ~uart of resh
water per day if dehydration i5 to be a~oided. For a
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2~-man boat this means storage of something in excess of
50 pounds of water, occupying about 1 cubic foot of space for
.
each day of survival. For any reasonably lengthy survival
time, therefore, stored water presents problems
I of weight and space utilization in a lifeboat or on an -~
inflatable raft. The initial water supply may be supplemented
by catching rain water but of course this is not a predictable
`' or certain source and in any case is not available in areas
;l of low or negligible rainfall. Evaporation techniques, based
20 on solar stills are generally of limited effectiveness, the
equipment required is bulky, relatively slow, extremely ;~ `
difficult to disassemble in adverse weather conditions, and
of course only available for use under favourable weather
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conditions.
` As an alternative to the foregoing, it has been
~' suggested that devices operating upon the principle of xeverse
"Z osmosis should be employed, and several devices in which sea
~ water is pumped, under pressures of the order of 1000~1500 psi,
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through a semi-permeable membrane have been described in
the literature. One such device is described in "Manual
Sea Water Demineralizer Development" - Cosmodyne Corporation,
June, 1973, a report distributed by the National Technical
In~ormation Service of the United States Department of
Commerce under AD-775,634. In such devices, manual pumps,
with or without energy-recovery cycles, are provided so that
survivors can pressurize a reverse osmosis vessel and produce
sufficient potable water for survival needs. At the pressures
r~quired for reverse osmosis of sea water the pumps are
necessarily of a fairly sophisticated nature and hence
relatively expensive. Further, increased complexity usually -
leads to increased maintenance requirements,and in the
survival situation it is unlikely that any maintenance or ~ ;
repairs at all can be carried out.
It is an object of the present invention, therefore,
to provide a device for the production of potable water from
sea water which avoids the use of manual pumps to provide the
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pressure necessary to carry out a reverse osmosis process
of sea water.
Another object of this invention is to provide a novel
method for the production of potable water by reverse
~ osmosis from sea water.
i Thus, by one aspect of this invention there is pro-
vided an apparatus for the production of potable water from
sea water adapted to be lowered to depths of at least
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130 fathoms below the surface of the sea and comprising (a) potable water
collecting means including: (i) a support member having a semipermeable
membrane secured thereto and adapted to permit substantially potable water
to permeate therethrough, at pressures corresponding to said dep~hs below
the surface, into potable water collecting chamber defined by said support
member and said semipermeable membrane; and (ii) outlet means to recover
said substantially potable water from said potable water collecting chamber; ;;~ -
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~ (b) potable water storage means including: (i) inlet means for receiving ~
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said potable water from said co]lecting means; (ii) outlet means to recover
said potable water; and (iii) pressure relief valve means adapted to relieve ~` ~
air pressure therein and thereby maintain an internal pressure substantially ~ ;
;~ equal to ambient external pressures during recovery of said apparatus from said
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~ depths; and (c) pressure relief and check valve means defining a one-way flow ~` ~
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~!~ path for said potable water from said collecting means to said storage means,
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to prevent any backpressure, created during raising of the apparatus from said
I depths, from reaching and damaging the semi-permeable membrane.
Ry another aspect of this invention there is provided a method for ~ -
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producing potable water from sea water comprising lowering a reverse osmosis ~ ;
module to a depth of at least 130 fathoms below the surface of the sea, there-
by creating an external pressure on a semipermeable membrane in said module ~`
sufficient to cause substantially potable water to permeate therethrough into
~` a closed storage vessel, raising said module and simultaneously reducing ~1
pressure in said storage vessel to maintain an internal pressure substantially P
equal to ambient external pressure, and removing potable water from said i `
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~ storage vessel.
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In order to eliminate the manual pumps of the prior art, an alter~
native reliable source of pressure to drive the reverse osmosis process must `
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be provided. Clearly the one source of pressure which is always available
in survival situations is the sea itself, provided only that it is deep
enough. However, in shallow seas or on the continental shelf
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extended surviv~ pexiods before rescue are most unlikely,
Thus, if a reverse osmosis unit is lowered into the ocean
depths, a sufficient pressure differential between one side
of a semi-permeable membrane and the other may be developed
so that reverse osmosis can be made to occur. Potable water
passes through the membrane and is collected within. The
osmosis unit can then he retrieved and the potable water
collected. Calculations show that at pressures of approx-
imately 24 atmospheres or 350 psi reverse osmosis o sea ;.
water can be effected. 24 atmospheres corresponds to a depth
of approximately 130 fathoms (780 ft). ~
The invention will now be described in more detai~ -
with reference to the following examples and the drawings in `
which: ~
Fig. 1 is a front elevation o~ a first embodiment `
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of the invention;
Fig. 2 is a sectional view of part of the first
embodiment of the apparatus of the invention;
Fig. 3 is a sectional view of a second embodiment
20 of the apparatus of the in~ention; and
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Fig. 4 is a graph relating salinity with immersion
at selected depths for different periods o~ time.
Referring firstly to Figs. ~ and 2, there is shown
;~ a cylindrical reverse osmosis module 10, having an outer
casing 1, a sea water inlet 2 at one end thereof, a sea water
outlet 2A at the other end thereof and a 1/3 psi NUPRO~
stainless steel pressure relief and che k valve 3 at the other
end thereof. Pxeferably, but not essentially, the module 10
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~05~899
is of the Enro UIIP 98 type, which is approximately 22" long
and 4" diameter, and provided with a spiral wrapped reverse
osmosis membrane 4 on a grid support 5, thus forming an
outer chamber 6 and an inner chamber 7 sealed therefrom. It
will, of course, be appreciated that any suitable reverse
osmosis membrane system may be employed. A 1000 cc stainless ste~
Whitney~ cylindrical storage vessel 8 is mounted on the ;~
top of the reverse osmosis modulel~ with any convenient -~
external mounting framework 9, and in fluid communication with ~ `
I0 the interior chamber 7 of the module 10via valve 3. A ~0 psi
air pressure relief valve 11 is provided at the top of ;~
vessel 8 and an outlet valve 12 for desalinated water at the -~
I bottom thereof. A lifting ring 13 is provided at the top of
;~ the framework 9 for attachment of a cable 14.
In operation, the entire apparatus, containing air at
atmospheric pressure and weighing about 70 lbs., is lowered ;
~ on cable l4, from a boat, to the desired depth, usually of `
`j~ thé order of 130-150 fathoms and held at that depth for a `~
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period of time of the order of 3-7 minutes and then retxieved.
` ~Lowering and raising takes about 4 minutes. When the apparatus
reaches the desired depth the external pressure in chamber 6
~ on the membrane 4 is sufficient to cause reverse osmosis
'! ~ therethrough and substantially pure water is orced to
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permeate th~ough the membrane into the inner chamber 7. As
potable water is produced, concentrated sea water escapes
through~outlet 2A in~order to avoid excessive salt concen-
,~ tration build-up in the outer chamber 6. Check valve 3 opens
',! :~ ~ and the water passes into storage vessel~8, compressing the
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air contained therein. Upon raising the apparatus, check
valve 3 closes thereby preventin~ the purified water from
returning to the module lO,and the pressure relief valve 11
opens to release sufficient air to maintain the pressure in
the storage tank at a desired level above that of the ~`
surroundings. Also, check valve 3 prevents pressurized air
in storage vessel 8 from applying back pressure on the
membrane 4, and osmosis taking place cluring recovery. Upon
recovery into the boat the desalinated water can be removed
by opening outlet valve 12 and the apparatus prepared for
another purification cycle. -
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I Example 1 ~.. . ..
A device as described hereinabove with reference to
Figures 1 and 2 was prepared and tested at various depths in
Haro Strait off Vancouver Island, B. C., Canada, for periods
of time varying between 1.5 and 7 minutes at depth. The
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natural salinity in thls area is about 2.7-3.0%. The depth ~
'~ of water was measured by means of a type 8 depth gauge ~;;
;l~ manufactured by Tsurami Saki Kosakusho Co. Ltd. and corrected
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`~` 20 for slope of cable. As seen from Fig. 4, which summarizes the
results, the relationship between salinity of water produced
and maximum depth of immersion appears to be linear when
the device was maintaine~ at maximum depth for 3 minutes and
raised and lowered at constant speed (approximately 4 minutes).
It was also found that by leaving the device at maximum depth
fox 7 minutes an improved, lower salinity, product was
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~ obtained.
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It will, of course, be appreciated that many modifi-
cations to the apparatus described hereinabove are possible
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and within the scope and purview of the present invention.
; For example, as the purity of the water produced improved
with depth the apparatus should be sixed so that as much
water as possible is collected at maximum depth, and as little
as possible during raising and lowering. For the same reason,
sea water inlet 2 and outlet 2A may be provided with a
pressure operated valve to accurately regulate water intake
to the desired pressure levels only. ;~
A simplified version of the device of Figures 1 and 2
is shown in Figure 3 and represents an inexpensive embodiment ;~
particularly suitable for use on small survival craft where
space is very limited and the amount of water required is
smaller. In Fig. 3, the reverse osmosis module and the water
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i storage vessel are combined in the form of a cylindrical
sintered porous vessel 20 with solid non-porous ends 21 and
22 and weighing only 4-6 lbs. in air and 2-4 lbs. in water
; when empty. Preferably, the vessel 20 is a sintered bronze
or stainless steel material and is conveniently 3" in diameter
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~ and 8" long. The porous walls of vessel 20 are covered with ~
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a semipermeable membrane 23, preferably cellulose diacetate- ;~
cellulose triacetate which is either cast or attached to the
outer surface, thereby forming a reverse osmosis module
similar to that of Figure 1 but without the casing 1. Non-
porous end 22 is provided with a 1/3 psi pressure relie
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valve 24~ an outlet valve 25, and a lifting ring 26. The
principle of operation is as beore in that as the unit is
lowered into the sea, the external pressure rises until, at
a pressure of about 24 atmospheres (130 fathoms depth) reverse
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osmosis takes place and desalinated water permeates through
the membrane 23 and porous walls 20 into the vessel. When
three-quarters full of water, the unit only weighs 6-8 lbs.
in air and 4-6 lbs. in water so ~hat raising and lowering
upon a thin steel wire, such as that used by sportsfishermen
presents little difficulty. Upon recovery, as the unit is
raised, the excess trapped air pressure in the vessel
escapes through relief valve 24 thereby preventing the internal
pressure from rising significantly above the surrounding
- 10 external pressure and forcing the membrane 23 off the surface
of the vessel 20. Upon recovery at the survival craft, the
potable water is removed via valve 25 and the unit is then
ready for return to the sea for further recovery.
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