WO2000059612A1

WO2000059612A1 - Seawater pressure-driven desalinization apparatus and method with gravity-driven brine return

Info

Publication number: WO2000059612A1
Application number: PCT/US1999/026025
Authority: WO
Inventors: Kenneth R. Bosley
Original assignee: Bosley Kenneth R
Priority date: 1999-04-07
Filing date: 1999-11-30
Publication date: 2000-10-12
Also published as: AU1812800A; US20020125190A1; US6656352B2; EP1214137B1; US20040108272A1; US6800201B2; US6348148B1; DE69924411D1; EP1214137A4; ATE291482T1; DE69924411T2; EP1214137A1

Abstract

An apparatus and method of removing salt from seawater to produce potable freshwater. A large metal cylinder (12), with open top and bottom ends, is anchored to the sea floor offshore. Several pressure hulls (15, 16) are attached to the side of the cylinder. The interior of each pressure hull is maintained at about one atmosphere, but the hulls are submerged at a depth at which the ambient water pressure is several atmospheres. Within each pressure hull there are several reverse osmosis devices ('RODs') (20). Check valves (30) allow seawater to pass from outside the hulls into the RODs. Due to the pressure differential, freshwater passes through the membranes (22) of the RODs by reverse osmosis, and is pumped out of the pressure hulls to a storage facility (36) onshore.

Description

SEAWATER PRESSURE-DRIVEN DESALINIZATION APPARATUS AND METHOD WITH GRAVITY-DRIVEN BRINE RETURN

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

The present invention relates to a reverse osmosis

method of removing the salt from water in the ocean or

inland bodies of salt water, using the pressure of the

seawater itself, and the force of gravity.

2. DESCRIPTION OF THE PRIOR ART

Due to the shortage of freshwater in the

southwestern United States and other arid parts of the

world, there have been numerous inventions for

desalinating sea water, by reverse osmosis,

distillation, and other means. However, desalinization

remains an expensive process . The concentrated brine

produced as a by-product of desalinization can itself

contribute to pollution of the environment in onshore facilities. The production of electricity or other

forms of energy consumed in desalinization can also

contribute to pollution of the air, water and land.

U.S. Patent No. 4,335,576, issued on June 22,

1982, to Harold H. Hopfe, discloses a device for

producing freshwater from seawater which floats on the

surface of the sea. It derives the energy for

desalinization from the motion of the waves on the

surface of the water. Movement of the water on the

surface causes reaction plates to move, and the

movement is ultimately transmitted to pistons that move

in cylinders to exert pressure on seawater to force

reverse osmosis.

U.S. Patent No. 4,452,969, issued on June 5, 1984,

to Fernand Lopez, discloses a reverse osmosis apparatus

for producing freshwater from seawater, which is

designed to be temporarily submerged, as on a fishing

line. U.S. Patent No. 4,770,775, issued on September

13, 1988, to Fernand Lopez, discloses another apparatus

for the production of freshwater from seawater, which

is also designed to be temporarily submerged, and has a chamber that expands as freshwater is produced. Both of

these apparatuses use the pressure of the seawater

itself to force reverse osmosis .

U.S. Patent No. 5,167,786, issued on December 1,

1992, to William J. Eberle, discloses a wave power

collection apparatus, which is anchored in the sea

floor, and in one embodiment desalinates seawater by

reverse osmosis. The movement of floats is used in

that embodiment to turn a generator which produces

electricity to power pumps that force seawater through

a membrane in a reverse osmosis unit.

U.S. Patent No. 5,229,005, issued on July 20,

1993, to Yu-Si Fok and Sushil K. Gupta, discloses a

process for the desalinization of seawater, by lowering

reverse osmosis devices into the ocean by means of

lines attached to pulleys, and raising them again by

the same means to remove the freshwater produced. The

pressure of the seawater itself is used to force

reverse osmosis of the seawater across a membrane to

produce freshwater. U.S. Patent No. 5,366,635, issued on November 22,

1994, to Larry 0. Watkins, discloses a desalinization

apparatus and means in which a separator is placed on

the sea floor, and the pressure at the sea floor is

used to force seawater through a membrane to form

freshwater by reverse osmosis, which is then pumped

out .

British Patent No. 2,068,774, published on August

19, 1981, to Jose Luis Ramo Mesple, discloses an

apparatus for desalinating water by reverse osmosis in

cells located deep underground, utilizing the pressure

resulting from the water being deep underground.

The present invention is distinguishable from the

prior art cited, in that only it takes advantage of the

fact that the concentrated brine produced as a by¬

product of reverse osmosis desalinization is heavier

than seawater to reduce the energy consumed in

desalinization. None of the above inventions and

patents, taken either singly or in combination, will be

seen to describe the present invention as claimed. SUMMARY OF THE INVENTION

The present invention is an apparatus and method

of removing salt from seawater to produce potable

freshwater. A large metal cylinder, with open top and

bottom ends, is anchored to the floor of the ocean (or

inland sea) offshore. Several pressure hulls are

attached to the side of the cylinder. The interior of

each pressure hull is maintained at about one

atmosphere of pressure, but the hulls are submerged at

a depth at which the ambient water pressure is several

atmospheres. Within each pressure hull there are

several reverse osmosis devices ("RODs"), each

containing a membrane that will allow water molecules,

but not sodium and chlorine ions, to pass through.

Check valves allow sea water to pass from outside the

hulls into the RODs. Due to the pressure differential,

water molecules pass through the membranes by reverse

osmosis, while salt is left behind, and freshwater is

pumped out of the pressure hulls to a storage facility

on shore (or where ever else it is needed) . Initially, the seawater remaining on the other side of the

membrane, which has a greatly increased concentration

of salt due to water passing through the membrane, is

pumped into the cylinder. (The water with an increased

concentration of salt is hereinafter referred to as

"brine".) After an initial surge, the level of the

brine in the cylinder will eventually reach

equilibrium, at a height below the height of the

seawater outside the cylinder, due to the greater

weight of the brine compared to unconcentrated

seawater. After equilibrium is reached, the pumps for

the brine can be turned off, as gravity will cause it

to flow down from the pressure hulls to the surface of

the brine in the cylinder. This will reduce the energy

needed to desalinate seawater. (It will still be

necessary to pump out the freshwater.)

Accordingly, it is a principal object of the

invention to provide a means for reducing the energy

required to desalinate seawater. Conventional

desalinization plants, located on or near the seashore,

require four pumping processes: first, pumping the seawater to the plant; second, pumping to raise the

pressure high enough for the RODs to operate; third,

pumping the brine back out to sea; and fourth, pumping

the freshwater to a reservoir or a treatment facility

for further purification, and ultimately to the

consumer. The present invention eliminates all but the

fourth pumping process. While prior inventions of

offshore desalinization apparatus, as in U.S. Patent

No. 5,366,635 to Watkins, will also eliminate the first

and second processes, only the instant invention will

also eliminate the third process of pumping out the

brine, without requiring that energy be expended in

raising the RODs, as in U.S. Patent No. 4,452,969 to

Lopez and U.S. Patent No. 5,229,005 to Fok et al .

It is second object of the invention to provide a

means for reducing the need for using expensive real

estate on or near the oceanfront for desalinization

facilities. As no oceanfront or near-oceanfront

property is used exclusively for the process, most real

estate costs associated with desalinization plants can

be avoided. Some offshore site leasing may be required, but this cost should be much lower than for

offshore sites involved in petroleum or mineral

extraction.

It is a third object of the invention to provide a

means for making the expansion of desalinization

facilities easier and less expensive. As no dry land

is used, and each platform must have a clear navigation

zone around it (as most jurisdictions require by law) ,

sufficient space for attaching additional pressure

hulls to the cylinder will be available and facility

expansion considerably eased. The expansion of a

facility is limited only by the number of pressure

hulls that can be fitted onto the cylinder at

appropriate depths, rather than allowances made by a

zoning commission with many other constituents to

satisfy, as may the case with a land-based

desalinization facility.

It is a fourth object of the invention is to

provide a means for reducing the cost of desalinizing

seawater by centralizing maintenance facilities, as the

pressure hulls can be removed and taken to a central facility for maintenance, rather than the on-site

maintenance required by conventional shore-based

desalinization plants.

It is a fifth object of the invention to reduce

pollution of the shoreline from the release of

concentrated brine by desalinization plants.

Conventional onshore desalinization facilities pump

their brine out to sea through a bottom-laid pipeline,

which releases the brine on or near the ocean floor.

Releasing the brine near the ocean floor increases the

area affected by the brine's toxicity. Existing

methods to reduce the toxic effects add to the cost of

desalinization through greater plant infrastructural

requirements or reduced process efficiency. The

present invention allows an offshore desalinization

facility to release its brine into mid-water, where

mixing with the ocean current is more efficient, with

fewer effects upon bottom-dwelling flora and fauna.

Because the facility can be located offshore, ocean

currents and tidal action will thoroughly mix the brine

back into the surrounding seawater, and the overall impact of increased salinity from the brine release

could be infinitesimal as little as two or three

kilometers down-current.

It is an object of the invention to provide

improved elements and arrangements thereof in an

apparatus for the purposes described which is

inexpensive, dependable and fully effective in

accomplishing its intended purposes.

These and other objects of the present invention

will become readily apparent upon further review of the

following specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic environmental front

elevational view of the first preferred embodiment of

the invention.

Fig. 2 is a schematic environmental front

elevational view of the second preferred embodiment of

the invention. Fig. 3 is a schematic environmental front

elevational view of the third preferred embodiment of

the invention.

Fig. 4 is a schematic cross-sectional view of one

of the pressure hulls of the first type that may be

used in any of the preferred embodiments of the

invention.

Fig. 5 is a schematic cross-sectional view of one

of the pressure hulls of the second type that may be

used in any of the preferred embodiments of the

invention.

Similar reference characters denote corresponding

features consistently throughout the attached drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is an apparatus and method

of removing salt from seawater to produce potable fresh

water. It may be used in either the oceans or in

inland bodies of salt water. Fig. 1 is a schematic environmental front

elevational view of the first preferred embodiment of

the invention. A large metal cylinder 10, with an open

top end 12 and an bottom opening 14, rests on platform

13 which is anchored to the floor A of the ocean B (or

inland sea) offshore. (Alternatively, a tube or

channel of a different shape and/or with a plurality of

top openings and/or a plurality of bottom openings may

be used. Also, the apparatus can be supported by

flotation devices, or by cables attached to ships,

rather than resting on the sea floor.) Bottom pressure

hulls 15 are removably attached to the side of the

cylinder, just above the equilibrium level C of brine

in the cylinder. When greater production capacity is

needed, upper pressure hulls 16 are added.

Fig. 2 is a schematic environmental front

elevational view of the second preferred embodiment of

the invention, which is the same as the first preferred

embodiment, except that the bottom of the cylinder

rests directly on the sea floor. Fig. 3 is a schematic environmental front

elevational view of the third preferred embodiment of

the invention, which is the

same as the second preferred embodiment, except that

brine pipe 35 projects over a cliff E in the sea floor.

This embodiment may be used in areas such as the Red

Sea where the submarine topography makes it possible.

Sending the brine over a submarine cliff will make

possible more efficient mixing of the brine with the

sea water.

Fig. 4 is a schematic cross-sectional view of one

of the pressure hulls of the first type that may be

used in any of the preferred embodiments of the

invention. The fresh water enclosure 18 in the

interior of each pressure hull is maintained at a

pressure below that of the ambient seawater, preferably

at one atmosphere of pressure, but the hulls are

preferably submerged at a depth at which the ambient

water pressure is several atmospheres. Within each

pressure hull there are several reverse osmosis devices

20 ("RODs"), each having a selectively permeable membrane 22 surrounding a brine enclosure 24. The

membrane allows water molecules, but not sodium and

chlorine ions, to pass through. (Other substances may

also be filtered out of the seawater, depending on the

characteristics of the membrane.) The pressure hulls

have an external skin 26 which is impermeable to water.

Seawater conduits 28, having check valves 30, pass

through the external skin and the membranes, to allow

seawater to pass from outside the hulls into the RODs.

The check valves enhance the efficiency of the process,

by preventing brine from returning directly to the

surrounding seawater by the same route . The space

between the external skin and the other contents of the

pressure hulls forms the fresh water enclosure 18. Due

to the pressure difference, water molecules pass

through the membranes by reverse osmosis, and

desalinated water is pumped out of the pressure hulls

through the freshwater conduit 32 and (referring back

to Fig. 1) pipeline 34 to an onshore storage facility

36 (or where ever else it is needed) . The freshwater

conduit should be external to the cylinder, as concentrated brine is highly corrosive. Freshwater

pumps (not shown in the drawings) may be located in the

storage facility, the pipeline, the cylinder, and/or

elsewhere. The pumping out of the freshwater maintains

the pressure difference across the membrane, so that

reverse osmosis can continue. The desalinated water

may undergo further purification at a local water

treatment plant .

Fig. 5 is a schematic cross-sectional view of one

of the pressure hulls of the second type that may be

used in any of the preferred embodiments of the

invention. It differs from the first type in having a

dry interior 42, which is kept dry by an air vent 44 to

the atmosphere above the surface D through which any

moisture evaporates . The RODs are enclosed by water

proof surfaces 46. Freshwater is drained from the RODs

by freshwater pipes 48 which are connected to the fresh

water conduit 32.

Initially, the seawater remaining on the other

side of the membranes ("brine"), which has a greatly

increased concentration of salt due to water passing through the membranes, is pumped into the cylinder by

the brine pumps 38 through the brine conduits 40 (see

Fig. 4) . At least one of the brine pumps is preferably

located in each pressure hull, as shown, but other

locations are possible. The pumping out of the brine

maintains a pressure difference across the seawater

conduits, causing seawater to continue to flow into the

reverse osmosis devices. After an initial surge, the

level of the brine C in the cylinder will eventually

reach equilibrium at an elevation below the sea level D

outside the cylinder (see Fig. 1) , due to the greater

weight of the brine compared to unconcentrated

seawater. After equilibrium is reached the pumps for

the brine can be turned off, as gravity will cause it

to flow down from the hulls to the surface of the brine

in the cylinder. This will reduce the energy needed to

desalinate seawater. (It will still be necessary to

pump out the freshwater.) The lower pressure hulls 15

should be attached to the cylinder first, as the

pressure difference will be greatest just above the

brine level C. (While the pressure difference would be greater at lower depths, gravity will not cause brine

to flow out of the pressure hulls if they are below the

brine's surface.) When greater capacity is needed, the

upper hulls 16 should be added, desalinization will not

be as efficient in them, as the pressure difference

will be lower.

The earth's gravity will cause the brine in the

tube to flow out of the bottom opening until the weight

of the brine in the tube equals the weight of an

equivalent column of water in the sea outside the tube.

As brine continually flows into the tube when the

invention is in operation, the weight of the brine in

the tube will continue to be heavier than that an

equivalent column of seawater outside, and brine will

continue to flow out. If there were no currents in the

sea, the salinity of the sea in the immediate area

around the tube could eventually rise to almost the

degree of salinity in the tube (though not to complete

equality, due to diffusion of salt through the

seawater) . This would cause the level of brine in the

tube to rise to almost the level of the sea outside the tube, and it would be necessary to reactivate the brine

pumps for desalinization to continue. (This might

actually happen in inland bodies of salt water, which

lack drainage to the oceans, if desalinization were

carried out on a massive scale over a long period of

time.) Thus, the present invention derives its energy

savings, not out of nothing, as would a perpetual

motion machine, but from the force of the earth's

gravity, from ocean currents and interlayer mixing that

are driven by electromagnetic radiation produced by

nuclear reactions in the sun, and from diffusion made

possible by random movements of molecules and ions in

the seawater that are also driven by heat from the sun.

It is to be understood that the present invention

is not limited to the embodiments described above, but

encompasses any and all embodiments within the scope of

the following claims. Although the method of the

invention is primarily intended as a means for

desalinating seawater, it can also be used to remove a

purified solvent from any solution, where a solution

with a higher concentration of solute is denser than a solution with a lower concentration of solute. An

apparatus similar to the preferred embodiment, but

smaller in size, may be useful in chemistry

laboratories and chemical processing plants.

MATHEMATICAL APPENDIX

In the following discussion, let:

P_d = Process depth

P_e = Process efficiency (1.0 = 100% efficiency)

d_f = Density of fresh water (999.97 kilograms per cubic

meter at four degrees Celcius)

d_s = Density of sea water (1,025 kilograms per cubic

meter at four degrees Celcius)

d_b = Density of brine (variable dependant upon process

efficiency)

h_s = Height of a seawater column (in meters)

h_b = Height of a brine column (in meters)

For brine to flow out of the cylinder by the force

of gravity, the pressure of the brine column must exceed the pressure of the seawater at the same depth

as the cylinder's bottom opening:

(1) d_bh_b > d_sh_s

The brine density may be calculated from the

process efficiency and the densities of freshwater and

seawater as follows:

(2) d_b = d_f + [(d₈ - d_f)/(l - P_e)]

Alternatively, the brine's density will equal the

density of the freshwater plus the concentration of

Total Dissolved Solids ("TDS") in the remaining fluid.

The concentration of TDS in seawater is given by:

( 3 ) TDS_seawater = ( d_s - d_f )

Dividing equation (3) by 1 - P_e gives the

concentration factor. E.g., an 80% efficient process

(P_e = 0.80) will concentrate the TDS of 100 cubic

meters of seawater into 20 cubic meters of brine and

produce 80 cubic meters of freshwater:

(4) 1.00 - 0.80 = 0.20

with the TDS concentration of the brine being five

times that of seawater, as dividing by 0.20 is

equivalent to multiplying by five. Thus, the TDS load in the brine is given by:

(5) [(d_s - d_f)/(l - P_e)]

Adding the TDS load for the brine to the density

of freshwater gives the total density of the brine:

(6) d_b = d_f + [(d_s - d_f)/(l - P ]

which is identical to equation (2) .

The process depth may be determined by the desired

operating pressure for the desalinization process. If

a pressure of four atmospheres (4.05 bars or 58.8

pounds per square inch) is desired, then the process

must operate at a depth of forty meters below mean low

tide. For every increase or decrease of one atmosphere

(1.013 bars or 14.7 pounds per square inch) in the

process operating pressure the process depth must

change by approximately ten meters.

For any process depth and any process efficiency,

the range of allowable depths for the bottom opening of

the cylinder is given by the following inequality:

(7) h_b > P_d(d_s/d_b)/(l - d_s/d_b)

This inequality is derived as follows: We seek

the depth at which the pressure of the brine column exceeds that of the seawater column, i.e., when the

following inequality is satisfied:

(8) d_bh_b > d_sh_s

We know that the height of the seawater column is

the same or greater than the height of the brine column

plus the process depth:

(9) h_s > h_b + P_d

An inequality is preferred to an equation to allow

a safety factor for brine density fluctuations.

Substituting inequality (9) into inequality (8), we

get:

(10) d_bh_b > d_s(h_b + P_d)

Isolating the height of the brine column, we get:

h_b > d_s(h_b + P_d) /d_b = h_b(d_s/d_b) + P_d(d_s/d_b)

Subtracting h_b(d_s/d_b) from both sides of the inequality

gives :

h_b(l - d_s/d_b) > P_d(d_s/d_b)

Dividing both sides of the inequality by (1 - d_s/d_b)

gives :

(11) h_b > P_d(d_s/d_b)/(l - d_s/d_b)

which is the same as inequality (7) . E.g., for a process which is 80% efficient,

operating at a depth of 40 meters, we first use

equation (2) to determine the brine's density:

d_b = d_f + [(d_s - d_f)/(l - P_e)]

= 999.97 + (1,025.00 - 999.97)/(l - 0.80)

= 999.97 + (25.03/0.20)

= 999.97 + 125.15

= 1,125.12 kg/m³

Substituting the value of 1,125.12 kg/m³ into

inequality (7) gives:

h_b > P_d(d_a/d_b)/(l - d_s/d_b)

> 40 (1,025.00/1,125.12) / (1 - 1,025.00/1,125.12)

> 409.51 meters

Thus, the depth of the bottom opening in the

cylinder must be at least 409.51 + 40.00 = 449.51

meters below the surface of the surrounding seawater

(at mean low tide) . Adding a ten percent safety factor

(about 40.1 meters) increases this difference to

approximately 490 meters. We can check this result by

comparing the difference between the pressures of the

seawater column at 490 meters and the brine column at 450 meters (with the 10% safety factor) with the

difference between the pressures of the seawater column

at 450 meters and the brine column at 410 meters

(without the 10% safety factor) :

With the safety factor:

d_bh_b - d_sh_s = (l,125kg/m³) (450m) -

(l,025.00kg/m³) (490m)

= (506,304.0 - 502,250.0) kg/m²

= 4,054.0 kg/m²

Without the safety factor:

d_bh_b - d_sh_s = (l,125kg/m³) (410m) -

(1, 025.00kg/m³) (450m)

= (461,299.2 - 461,250.0) kg/m²

= 49.2 kg/m²

The fact that the difference is positive in both

cases ensures an outward flow of brine. These depths

and pressures are well within the state of the art in

off-shore platform construction and operation. The

desalinated water will need to be pumped out of the

pressure hulls with sufficient pressure to overcome the difference in hydraulic head, its own viscosity, and the friction in the pipeline through which it is

carried to shore. This is a proven engineering task

that is well with our current capabilities.

Claims

CLAIMSI claim:

1. An apparatus for desalinating seawater,

comprising :

at least one membrane through which water

molecules can flow, but through which sodium and

chlorine ions cannot flow;

at least one fresh water enclosure, within which

water that has been desalinated by passing through at

least one membrane, can be collected and separated from

salt water;

at least one brine enclosure, within which water

that has not passed through at least one membrane, and

has an increased concentration of salt, can be

collected and separated from water with a lower

concentration of salt;

a channel having at least one top opening and at

least one bottom opening; and

at least one brine conduit between at least one brine enclosure and the channel.

2. The apparatus for desalinating seawater

according to claim 1, wherein:

at least one membrane and at least one freshwater

enclosure are positioned far enough below the surface

of a body of salt water, that there is a sufficient

pressure difference across the membrane for water to be

desalinated by reverse osmosis and to collect in the

freshwater enclosure;

the top openings of the channel are positioned

above the surface of a body of water;

the bottom openings of the channel are positioned

below the surface of the body of water and below at

least one brine enclosure and at least one brine

conduit; and

at least one brine enclosure and at least one

brine conduit are positioned above a level at which

water having an increased concentration of salt will

reach equilibrium in the channel.

3. The apparatus for desalinating seawater

according to claim 2, further comprising:

at least one freshwater pump for removing

desalinated water from at least one freshwater

enclosure; and

at least one brine pump for removing water having

an increased concentration of salt from at least one

brine enclosure, until the level of water having an

increased concentration of salt has reached equilibrium

in the channel.

4. The apparatus for desalinating seawater

according to claim 3, further comprising:

at least one pressure hull, retained on the

channel, and containing at least one membrane, at least

one brine enclosure, and at least one freshwater

enclosure;

at least one seawater conduit, through which water

exterior to the pressure hull can flow into at least

one brine enclosure; and

at least one check valve in at least one seawater

conduit, that prevents water from flowing from the

brine enclosure back through the seawater conduit .

5. The apparatus for desalinating seawater

according to claim 4, wherein there are a plurality of

pressure hulls.

6. The apparatus for desalinating seawater

according to claim 5, wherein the channel has only one

top opening and only one bottom opening, and the

channel is impermeable to water except at the top

opening and the bottom opening .

7. The apparatus for desalinating seawater

according to claim 6, wherein each pressure hull has an

external skin that is impermeable to water.

8. The apparatus for desalinating seawater

according to claim 7, wherein each pressure hull has

a plurality of reverse osmosis devices, with each

of the reverse osmosis devices comprising one of the

brine enclosures surrounded by one of the membranes,

with a seawater conduit for each reverse osmosis device

passing through the external skin of the pressure hull

and the membrane .

9. The apparatus for desalinating seawater

according to claim 8, wherein each pressure hull has

one of the brine pumps, and internal brine conduits

through which water can flow from the brine enclosures

to the brine pump .

10. The apparatus for desalinating seawater

according to claim 9, wherein each pressure hull has

only one fresh water enclosure, which is formed by the

external skin, and occupies space between the external

skin and the reverse osmosis devices, the internal

brine conduits, and the brine pump, with a freshwater

conduit passing through the external skin through which

desalinated water can be pumped out.

11. The apparatus for desalinating seawater

according to claim 10, wherein the freshwater pump

maintains water pressure within the pressure hulls

lower than water pressure outside the pressure hulls.

12. The apparatus for desalinating seawater

according to claim 11, wherein the water pressure

within the pressure hulls is equivalent to pressure at

the surface of the sea.

13. The apparatus for desalinating seawater

according to claim 9, wherein each reverse osmosis

device is surrounded by a water proof surface connected

by a freshwater pipe to a freshwater conduit passing

through the external skin through which desalinated

water can be pumped out.

14. The apparatus for desalinating seawater

according to claim 13, wherein each pressure hull has

an interior between the external skin and the water

proof surfaces of the reverse osmosis devices, with the

interior connected to an air vent.

15. The apparatus for desalinating seawater

according to claim 9, wherein the pressure hulls are

removably retained on the channel, whereby they can be

temporarily removed for maintenance.

16. The apparatus for desalinating seawater

according to claim 15, wherein the channel is

cylindrical .

17. The apparatus for desalinating seawater

according to claim 16, wherein the desalinated water is

pumped out to a water treatment facility for further

purification .

18. The apparatus for desalinating seawater

according claim 1, wherein the apparatus is positioned

in a current so as to minimize environmental effects of

discharge of brine.

19. The apparatus for desalinating seawater

according to claim 1, wherein the channel has a bottom

end resting on the floor of the sea.

20. The apparatus for desalinating seawater

according to claim 1, wherein the apparatus is

supported by means of flotation.

21. The apparatus for desalinating seawater

according to claim 1, wherein the apparatus is

supported on an offshore platform.

22. The apparatus for desalinating seawater

according to claim 1, wherein the apparatus is adjacent

to a sudden drop in elevation of a sea floor, and

includes a means for conveying brine over the sudden

drop in elevation.

23. A method for desalinating seawater,

comprising the steps of:

allowing sea water to flow through a check valve

into a brine enclosure which is separated from a

freshwater enclosure by a membrane through which water

molecules, but not sodium and chlorine ions, can flow;

reverse osmosis of the seawater, by maintaining a

pressure differential across the membrane, by pumping

out desalinated water from the freshwater enclosure;

causing seawater to continue to flow into the

brine enclosure, by maintaining a pressure differential

across the check valve, by pumping out water having an

increased concentration of salt from the brine

enclosure into a channel with an open top above the

surface of the sea, and an open bottom below the

surface of the sea and below the brine enclosure; and

allowing the surface level of the water having an

increasing concentration of salt to reach equilibrium

in the channel below the surface of the sea and below

the brine enclosure, and then discontinuing the pumping

of water out of the brine enclosure, and allowing water to flow out of the brine enclosure into the channel by

the force of gravity.

24. A method for separating a lighter solvent

from a heavier solution, comprising the steps of:

allowing a solution to flow through a check valve

into a first enclosure which is separated from a second

enclosure by a membrane through which a solvent, but

not a solute, can flow;

reverse osmosis of the solution, by maintaining a

pressure differential across the membrane, by pumping

out purified solvent from the second enclosure;

causing the solution to continue to flow into the

first enclosure, by maintaining a pressure differential

across the check valve, by pumping out the solution

having an increased concentration of solute from the

first enclosure into a channel with an open top above a

surface of the solution, and an open bottom below the

surface of the solution and below the first enclosure;

and

allowing the surface level of the solution having

an increased concentration of solute to reach

equilibrium in the channel below the surface of the

solution and below the first enclosure, and then discontinuing the pumping of the solution out of the

first enclosure, and allowing the solution to flow out

of the first enclosure into the channel by the force of

gravity.