CA2009251A1

CA2009251A1 - Distillation separation membrane and method for membrane distillation separation

Info

Publication number: CA2009251A1
Application number: CA002009251A
Authority: CA
Inventors: Wang Ying
Original assignee: Wang Ying; World Industrial Membrane Corporation
Current assignee: WORLD INDUSTRIAL MEMBRANE Corp
Priority date: 1989-02-03
Filing date: 1990-02-02
Publication date: 1990-08-03
Also published as: GB2229379A; GB8902437D0; GB9002412D0

Abstract

ABSTRACT OF THE DISCLOSURE

A distillation separation membrane for use in desalination of sea water or brine is provided. The membrane has a hydrophobic separation coating on the surface of a porous support member. A method is provided wherein such a coating is formed dynamically on the porous support member by subjecting the surface of the porous support member to a membrane forming component for a period of time sufficient to form a separation coating capable of distillation separation of salt from water. A method of desalinating sea water or brine by means of membrane distillation is provided using a separation membrane having an electro-conductive porous support member and supplying electrical current to the porous support member to heat the brine or sea water in contact with the membrane and cause vaporization of the brine or sea water, the vapour thus created passing through the distillation separation membrane for subsequent liquefication.

Description

2~

FIELD OF 'l'lilS INVEN$ION
This invention relates to method and apparatus for use in membrane distillation separation particularly, [but not exclusively,] for desalination of sea water or brine.

BACRGROUND OF THE INVENTION
The principle of membrane separation is known.
In one previous method of membrane distillation separation, heated sea water or brine is passed along one side of a capillary membrane and fresh water coolant is passed along the opposite side of the capillary membrane. In this method, water permeates through the membrane and mixes with the fresh water coolant on ~he oppo ite side of the membrane. The membrane prevents migration of the salts contained in the sea water or brine across the membrane.
In such prior membrane distillation desalination processes, all of the sea water or brine must be heated to a specified temperature even though only a portion of the heated water permeates through the membrane. Accordingly, the energy used in heating the balance of the sea water or brine is lost giving rise to a source of energy inefficiency. Furthermore, there is a need to improve the rate of permeation across the membrane.

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SUMMARY OF THE INVENTION
A distillation separation membrane is provided for use in the desalination of sea water or brine. The distillation separation membrane comprises: a porous support member of a material selected from the group of polymers consisting of polyvinyl chloride, polypropylene, nylon, polystyrene, polycarbonate and polysulphone; and a hydrophobic separation coating within the interstices of substantially all of the pores of said porous support member, which are adjacent the surface of said porous support member, said separation coating having been formed dynamically by subjecting the surface of said porous support member to a membrane forming component for a period of time sufficient to form a separation coating capable of distillation separation of salt from water.
A method is provided of forming a distillation separation membrane which method comprises the dynamic formation, in situ, of a thin film within the pores of or on a porous support, of a hydrophobic separation membrane by passing suitable membrane forming components together across the support surface and causing the components to react together at said surface to form said membrane.
A method is also provided for the desalination of sea water using a membrane as set out above and including the step of heating said sea water thereby causing vaporization of said sea water, the vapour thus created 3~32~L

passing through said distillation separation membrane for subsequent liquefication.

The invention also provides, in a method of desalination of sea water by means of membrane distillation, the use of an electro-conductive separation membrane having an electro-conductive porous support member and supplying electric current to said porous support member to heat said sea water in contact with said distillation separation membrane thereby causing vaporization of said sea water, the vapour thus created passing through said distillation separation membrane for subsequent liquefication.

BRI13F Dl~SCRIPTION OF THE DRAl~INGS
Fig. l is a diagrammatic view of a known distillation separation module to which the various aspects of the present invention may be advantageously applied, Fig. 2 is a diagrammatic view of a distillation separation module in accordance with the present invention;
Fig. 3 is a diagrammatic layout of a system for membrane formation in accordance with the present invention;
Figs. 4A to 4C illustrate various arrangements of membrane modules in accordance with the present invention.

Figs. 5A to 5C show further assemblies of membrane modules according to the present invention.

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Fi~. 6 is a perspective view of a membrane module according to the present invention;
Fig. 7 is a perspective view of a membrane separation distillation module assembly according to the present;
Fig. 8 is a further perspective view of a membrane core according to the present invention;
Fig. 9 is a diagrammatic view of a membrane separation distillation assembly according to the present invention;
Fig. lO is a further diagrammatic view of a membrane core module assembly according to the present invention;
Fig. ll is a diagrammatic illustration of a membrane distillation separation apparatus according to the present invention and incorporating various aspects of the present invention;
Fig. 12 is a sectional view of an apparatus for membrane distillation according to the present invention;
Fig. 13 is a schematic view of a membrane distillation desalination apparatu~ according to the present invention;
Fig. 14 is a schematic view of a membrane distillation desalination system incorporating membrane distillation desalination apparatus according to the present invention;

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Fig. 15 is a schematic view of a further membrane distillation desalination system incorporating membrane distillation desalination apparatus according to the present invention; and, Fig. 16 is a schematic view of a membrane distillation desalination plant incorporating membrane distillation desalination apparatus according to the present invention.

BRIEF DESCRIPTION OF PREFERRED RMRODIMENTS OF TH~ INVENTION
Referring to the drawings, Figure diagrammatically shows a known membrane distillation separation ~odule which is generally identified by reference 20. The module 20 has a tubular membrane 22 mounted within a generally co-axial outer casing 24 provided with end caps 26. In use, elevated temperature sea water is passed along the interior of the tubular membrane 22. The term "elevated temperature" is used in this specification in a relative sense to indicate that the feedwater has a higher temperature than the coolant water referred to below. Vapour from the sea water passes through the walls of the tubular membrane 22 and mingles with fresh water coolant flowing in the annular space defined between the exterior of the tubular membrane 6 and the interior of the outer casing 24.
Referring to Figure 2, a membrane distillation separation module according to the present invention is 7 2(~ 51 generally identified by reference 28. The module 28 has a porous support tube 30 running generally centrally there-along. The porous support tube 30 has a separation membrane incorporated therewith by means of a dynamic membrane formation method which is described in more detail below. The porous support tube 30 is supported within a heat conductive, thin-walled and waterproof exchange tube 32, to leave a concentric gap 34. The assembly of the porous support tube 30 and the exchange tube 32 is mounted within an outer casing 36 which is provided with end caps 38. An annular passage 40 is defined between the exterior of the exchange tube 32 and the interior of the outer casing 36.
In use, elevated temperature sea water is arranged to flow through the porous support tube 30 while relatively cold sea w~ter is arranged to flow in the passage 40 between the exchange tube 32 and the outer casing 36. The elevated temperature sea water within the porous support tube 30 is subjected to heat to cause the sea water to vaporize; the water vapour or steam thus formed passes through the wall of tube 30 into the concentric gap 34 where it is cooled and liquified by contact with the inner surface of the heat conductive exchange tube 32 whose outer surface is in con~act with the cold sea water.
The efficiency of the prior art device and the device of the present invention may be increased by - 8 - ~ 2S~

rendering the tubular membrane (reference 22 in Figure 1) and the porous support tube (reference 30 in Figure 2) electro-conductive and by applying an electric current therealong. This results in a much more localized heating of the sea water with a consequent energy saving.
Furthermore, by replacing the tubular membrane of the prior art device with a porous support carryin~ a thin dynamically formed membrane, as shown by reference 30 in Figure 3, the flow rate of the permeate through the membrane may be increased significantly.
The formation of the dynamically formed membrane and the structure of the membrane modules will now be discussed in more detail. Referring to Figure 3, the production of the membrane is shown schematically.
Membrane formation liquids are placed in a container 42.
Suitable liquids include, for example, silico organic alcohol polymers, organic siloxane, soluble emulsions, or soluble fluorinated ~polytetrafluoroethylene) polymeric emulsions. A suitable indicator agent is also added.
When using the invention for membrane distillation desalination, sodium chloride may be used in a small amount as the indicator agent. A solvent will be present which may be water or an organic solvent or a mixture o~ organic solvents or water. Also present in the container 42 are substances to cause the deposition of the membrane. In the case of silico organic-alcohol polymers the membrane depositing substance is preferably an acidic substance 9 x~ zs whilst in the case of emulsions an emulsion-breaking agent is present.
The membrane is formed within a membrane module 44. To form the membrane, a pump 46 is activated to feed liquid from the container 42 through a heater 48 to the membrane module 44. At the same time, coolants from a container 50 are pumped by means of a coolant pump 52 through the ~embrane module 44.
Initially, because there is little or no membrane present within the pore structure of the porous support tubes in the membrane module 44, the indicator agent of the membrane forming liquid will be able to flow through the porous support tubes (30 in Figure 2) of the membrane module 44 and may be detected for example by conductometric means. Gradually however, membrane formation will occur within the pores of the porous support tube 30 and the flow of indicator agent through the walls of the support body will gradually diminish. When the level of transported indicator agent falls to a pre-determined minimum value, the pumps 46 and 52 and the heater 48 are switched off and the membrane module 44 is removed for post-processing.
Such post-processing generally incudes curing by physical treatment which may include air, room te~perature air or heating in an oven. Conditions which have been found to be satisfactory for membrane formation are summarized in the following chart:

1 0 ~ ;2~ 3~32~o Conditions for Membrane formation a. Feed Formula Membrane formulation materials - 1 to 5000 PPM.
Indicator agents - ratio of molecular no. and membrane materials is 1/16 to 1 Additives 0 to 1 molar Solvent 90 to 100%.

b. Process Conditions Temperature of feed 30 to 99 deg. C
Time required 20 to 460 minutes Pressure 0.01 to 5 kg/cm Flow Speed 5 to 100 cm/min (Feed flow inside capillary tube membrane) Coolant Temperature 5 to 80 deg. lower than that of feed temperature c. Post Treatment Processinq Air-blow temperature for membrane lB0 deg. C
Timing for heating ~or not heating) membrane 0.5 to 24 hours.

d Properties of Resultant Membrane .

Salt recovery 99 to 99.8~

Permeate productivity 45 to 100 litre /day m 11- 2C~ S~

The porous support tube 30 should have a pore size of from .02-1 micron. Such porous support tubes 30 are commercially available manufactured from nylon, poly-propylene, cellulose acetate and cellulose triacetate.
Examples of commercially available support tubes include those manufactured by Micaon Separations Inc. and sold under the trade mark Calyx Capsules. Alternatively, porous support tubes 30 may be manufactured from polysulfone, polyvinyl chloride, polyvinyl and polyvinyl alcohol. Such support tubes may be manufactured using known techniques such as described in "Membrane Science and Technology" Wang Ying 1986 Second China.
Referring now to Figures 4A-4C, arrangements are shown wherein a plurality of the porous support tubes 30 having separation membranes on them are bundled together to form membrane modules of different geometric configurations. The membrane modules are identified generally by a reference 53. The membrane modules 53 illustrated in Figure 4A-4C have one or more porous support tubes 30 within a single support tube casing 51. In these arrangements, it is preferable that the difference between the external diameter of the support tube 30 or the bundle of support tubes 30 and the internal diameter of the support tube 51 be relatively small so as to maximize the efficiency of heat exchange.

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A~ shown in Figure 4B, the support tube casing does not have to be a round tube as shown in Figure 4A, instead, flat walled tubing 54 may be employed. The support tube casing 51 or 54 may be of plastics material or thin corrosion-proof metal. As also shown in figure 4B, the individual support tubes 30 may be secured together by means of fine fibres 56 for subsequent insertion into a support tube support casing. Alternatively, a sheet of assembled support tubes 30 (Figure 4C) may be rolled up between sheets of plastic film 70 about a central tube 62.
A sheet of net-like material 60 is placed on the sheet of plastic film 70 on one of the faces of ~he film opposite the support tubes 30 and rolled up with the film and tube assembly. The net-like material 60 is included to provide a conduit for cooling water adjacent the plastic film 70.
The central tube 62 i5 provided with openings 63 through the sides thereof to receive permeate from between the sheets of plastic film 70 which then collects within the central tube 62.
As may be seen from Figures 5A through 5C, the support tube casings 51 containing bundles of support tubes 30, may themselves be assembled within the outer casing 36 in a variety of ways. Figures 5A, 5B and 5C correspond respectively to the arrangements shown in Figures 4A, 4B
and 4C. The diameter of the outer casing 36 shown in Figures 5A-5C would typically be at least one inch and may be greater than eight inches. The assembling of the - 13 - ~ ~a support tube bundles of Figures 5A through 5C is discussed in further detail below.
As shown in Figure 6, the assembly of support tubes within their respective tube support casings 51 may be secured together by providing bonding structures such as 64. Such bonding structures may be made of epoxy resin. Each assembled bundle of tube support casings 51 with porous support tubes 30 contained therein is hereinafter referred to as a membrane core 66. To provide such a membrane core 66 with suitable rigidity, a stiffening member such as indicated by reference 68 may be located therein.
To assemble a membrane core of the type shown in Figuxe SC, a tube arrangement as illustrated in Figure 4C
may be employed. The individual support tubes 30 having membranes formed thereon as described above, are placed, in parallel between two thin plastic membrane sheets 70 as shown in Figure 4C. The length of the membrane sheets 70 should be such that the sheet extends lO centimetres or more beyond each end of the support tubes 30. Along the sides of the plastic sheets 70, a silica gel mixture is disposed in a one inch to 2 inch thick layer. The thickness of the silica gel layer should be greater than the diameter of the porous support tubes 30. As the plastic sheets are coiled, bonding agents envelop each porous support tube 30 so as to bond the support tubes 30 and the plastic sheeting 70 together. As the silica gel - 14 - 2 ~r~ 2 5 extends beyond the ends of the support tubes 30, coiling of the plastic sheets 70 and support tubes 30 will seal the ends of the support tubes. The ends of the rolled up bundle of support tubes however, may be cropped in order to reopen the support tubes to permit access to the interior of the support tubes. Alternatively, a sheet of plastic may be used which is narrower than the length of the support tubes so that upon rolling, the ends of the tubes will protrude beyond the silica gel mixture which acts as a bonding agent. Such an arrangement is shown in Figure 8 where the bonding agent is designated by reference 74.
As shown in Figure 8, the ends of the porous support tubes 30 may be encased in a further bonding agent indicated by reference 72. The bonding agent may be epoxy or, if it is desired to make electrical connection to the porous support tubes 30 (which will be discussed in more detail below), a mixture of epoxy and graphite or other suitable conductive material may be used.
Figure 7 shows a membrane core made up of tube arrangements as illustrated in Figure 4A. The porous support tube casings 51 containing porous support tubes 30 are arranged in a generally parallel cylindrical arrangement. The porous support tubes 30 extend beyond the ends of the support tube casings 51. The ends of the support tubes casing 51 are bonded together with a suitable bonding agent at 74. The ends of the porous support tubes 30 are bonded together by a further bonding agent at 72.

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Figures 9 and 10 show membrane separation distribution modules of the type generally illustrated in Figure 2. Similar components are indicated by li~e references. The module 28 shown in Figure 9 corresponds to the tube arrangement shown in figure 7 whereas the module 28 shown in Figure 10 corresponds to the tube arrangement of Figuré 8.
Referring to Figure 9, the membrane GOre 66 is placed within an outer casing 36 so that the tube support casings 51 extend the length of outer casing 36. The membrane core 66 is fluidly sealed to the casing 36 by a suitable seal, such as an O-ring, at 75 which extends between the interior of the casing 36 and the bonding agent 74.
The outer casing 36 is provided with tubular extensiQn portions 37 at either end. The ends of the porous support tubes 30 extending from the bonding agent 74 are received within the extension portions 37. The outer ends of the extension portions 37 are provided with end caps 26. Suitable seals such as an O-ring 77 provide a seal between the interior portion of the extension 37, the bonding agent 72 at the ends of the tubes 30 and the end caps 26.
The operation of the membrane separation distillation module 28 in Figure 9 will now be described.
Elevated temperature water is introduced as shown by the arrow at reference 98, flows along the length of the - 16 ~ 5~

support tubes 30 and is discharged as shown at reference 100 through the opposite end cap 26. Cold water is . introduced as shown by the arrow at reference 102, flows around the exterior of the tube support casings 51 and discharges from the outer casing 36 as shown by the arrow at reference 104. The water vapour which permeates through the porous support tubes will cond~nse within the tube support casings 51 and exit from the tube support casings between the two bands of bonding agent 72 and 74. This condensed fresh water permeate will then be discharged from the membrane distillation separation distillation apparatus 28 as shown by the arrow at reference 106.
The membrane separation distillation module of Figure 10 will now be described in more detail. The outer casing 36 contains that portion of the membrane core 66 of Figure 8 which is covered by the sheet of plastic film 70.
As discussed above in reference to Figure 9, the outer casing 36 is provided with extensions 37 at either end. O-ring seals 79 seal between the outer casing 36 and the extension 37 at either end of the outer casing 36 but do not seal between the plastic sheet 70 and the interior of the casing 36. The support tubes 30 which extend beyond the ends of the plastic sheet 70 are contained wi~hin the extensions 37. An O-ring 77 seals between the bonding agent 72 at the ends of the support tubes 30 and the interior of the extensions 37. The O-ring 77 also - 17 - ~9~

provides a seal between end caps 26 and the outer ends of the extensions 37.
In use, elevated temperature sea water is admitted at reference 98 through the left hand end cap, flows through the interior of the porous support tubes 30 and is discharged through the opposite end cap at lO0.
Cold water is admitted through the ri~ht hand extension 37 at reference 102, flows between the plastic sheet 70 and the outer casing 36 to be discharged through the left hand extension 37 at reference 104. As discussed above in relation to reference 4C, a net-like material 60 is coiled up within the plastic sheet 70 and support tube bundle 58 to permit the cold water to circulate over a substantial portion of the area of the plastic sheets 70. Elevated temperature sea water permeating through the porous support tubes 30 will condense between the porous support tubes 30 and the plastic sheets 70. The permeate will eventually collect within the central tube 62 through the openings 63 in Figure 4C and be discharged through the ends of the central tube 62 which extend through both of the end caps 26 as indicated by references 106 in Figure lO.
Figure ll shows a further embodiment of a membrane separation distribution module 28 according to the present invention. The embodiment of Figure ll differs from the embodiments of Figures 9 and lO insofar as the regions of elevated temperature and cold water flow are Z~ 5~

concerned. The embodiment of Figure 11 uses a singleporous support tube 30 rather than a tube bundle. The porous support tube 30 has a hydrophobic membrane 80 formed on its exterior and interior surfaces. This membrane is both electro-conductive and solar heat absorptive. Within the porous support tube 30, there is a thin wall tube 82 which is both waterproof and heat conductive. The porous support tube 30 and thin wall tube 82 are generally concentrically disposed within an outer casing 36.
In use, elevated temperature sea water introduced at reference 98, passes along the m~mbrane 80 on the porous support tube 30 and exits at reference 100. A coolant, such as cold sea water, is passed within the thin wall tube 82, entering at reference 102 and exiting at reference 104.
In this embodiment, permeate passing through the membrane 80 will collect between the thin wall tube 82 and the support tube 30 and be discharged at reference 106~ A
connector 108 is provided through the outer casing 36 to enable the elevated temperature water 104 being discharged from the membrane separation distillation module 28 to be further utilized, for example, to be fed into a further module.
The support tubes 30 in the embodiment of Figure 11 are preferably electro-conductive and are connected to a source of electrical power 84. Such electrical connection may be made by having an electro-conductive layer of bonding agent, for example epoxy containing graphite, which acts as a conductor between the power source 84 and the porous support tubes 30A
Furthermore, it is preferable that the outer casing 36 be transmittive to solar radiation, that the porous support tube be solar radiation absorptive and that the module be arranged so that the support tubes 30 may receive solar radiation. The effect of the solar radiation is to heat up the incoming sea water whilst the effect of the electric current supplied to the porous support tube is to cause vaporization of sea water in contact therewith.
The formation of electro-conductive and hydrophobic membranes will now be discussed. A porous suppor~ tube may be formed by means such as extrusion, for example, from a plastic material to which has been added suitable organic and inorganic additives (such as graphite, crushed carbon, polyethylene glycol and/or dodecane sulphite) to obtain a porosity of 3Q to 50% and pore diameters within the porous support tube of from 0.2 to 1 microns. The support tube diameter preferably ranges from 2-6 mm. with a wall thickness of from 10-1,000 microns. A
typical formulation of the porous support tube is:
polypropylene 25-75~ by weight;
graphite or crushed carbon 5-75~ by weight;
polyethylene glycol 10-30~ by weight;
dodecane sodium sulphate 5~25~ by weight and phthaly dibutyl ester 5 20~ by weight.

20 2~.~''3~

The presence of the graphite or crushed carbon will provide the poro-.s support tube with the necessary de~ree of electro-conductivity, thermal conductivity and solar absorptiveness for use in the present invention.
To produce the membrane within the walls of the support tube, it is preferred to use the method set out above in the description of Figure 3 using the following formula:

1. Silico-organic alcohol (or siloxane emulsion)liquid solution 1-5000 PPM
Colloidal graphite 0.5-1250 PPM
Polyvinyl Alcohol 0.5-100 PPM

2. Silico-organic alcohol (or siloxane emulsion) liquid solution 1-5000 PPM
Intrinsic conductive polymeric polyelectrolyte solution 1-2000 PPM
Polyvinyl Alcohol 0.5-100 PPM

A membrane so formed will be hydrophobic in nature. The conductivity of the membrane will not increase even when the membrane is in contact with sea water because of this hydrophobicity. When using the membrane in distillation desalination, the sea water salts will not generally be in contact with the membrane because there will be a layer of saturated water vapour therebetween.

Since this water vapour layer contains little if any salt, it is relatively non-conductive.
The electrically conductive and thermally -conductive hydrophobic, porous, membrane containing support 5 so formed possesses the following characteristics Volume resistivity (~) 102 _ 105 Solar absorptivity (~) 0.7 - 0,9 Surface Tension at 20C. (dyne/cm) 20 - 30 Membrane Thickness (microns) 0.1 - 0.4 Voltage Applied (v) 5 - 35 Salt Recovery (~) 99 - 99.8 Permeate productivity (litres/day/MZ) 45 - 100 A suitable module according to Figure ll has been 15 produced using components having the following dimensions:
Internal diameter of membrane tube (mm) 2-6 Thickness of membrane tube (microns) 10~1000 External diameter of coolant tube (mm) 1.8-5.8 Thickness of coolant tube (microns) 2.5-6.5 Internal diameter of the external shell tube (mm) 2.5-6.5 Wall thickness of the external shell (microns) lOO 1000 - 2 2 - 2~d~ 25~L

Overall length of the external shell tube (m) 0.4-20 The entire membrane separation apparatus would typically combine a series of membrane separation distribution modules 28 having all of the respective flow channels of feeds, coolant water and permeate of each module fluidly connected. In order to increase the length of the apparatus, a plaiting method may be used wherein fibre lines are used to plait the individual modules 28 together in a generally parallel arrangement in a single or a pair of lines. Such a combination is referred to as a "membrane stack" and is generally identified by reference 86 in Figure 12. The stacks may include 5 to 5,000 or more individual modules 28. The respective reference numbers indicating flow of elevated temperature water, coolant water and permeate are the same as used in Figure 11.
The membrane stack 86 is provided with end caps 88 at either end. The end caps 88 may be sealed to the ends of the modules 28 by an elastic silica gel after insertion of the ends of the modules into the end caps.
Membrane stacks such as 88 may be placed on oblique ground or racks which are coated with solar reflection paints. The degree of obliqueness of the ground or racks should be selected so as to correspond to the latitude of the location in order to maximize the amount of solar heat absorbed.

23 - Z~`925~

The flow of permeates may be considerably increased during the membrane distillation desalination process by creating a partial vacuum in the space adjacent the fresh water side of the membrane. ln the embodiment of Figure 11 this would be the space between the thin walled tube 82 and the porous support tube 30. It has been found that such reduced pressure tends to bring down the boiling point at the contact region between the membrane 80 and the sea water and produce more saturated vapours. Moreover, such a reduced pressure enables the water vapour or steam which has penetrated the membrane 80 to leave the pores in the porous support tube 30 so that the steam does not condense inside the pores and block the heat exchange between the cold air and the steam. It is believed that this method has not been previously used in the process of membrane distillation desalination and is therefore regarded as another novel characteristic of the present invention.
A preferred methocl is illustrated in Figure 13 which shows the use of a venturi vacuum pump. A venturi vacuum pump 89 is fluidly connected to the permeate discharge openinq of the membrane separation distillation module 28 by conduit 91. The venturi vacuum pump 89 has a venturi tube 90. Permeate which has collected in a tank 92 is pumped by a pump 94 through the venturi tube 90. An area of reduced pressure is thus created adjacent a venturi within the venturi tube 90. The conduit 91 is connected to - 24 - ~ 51.

the area of reduced pressure. Using such an arrangement satisfactory results have been obtained by using a negative pressure of from .02 to 0.8 kg/cm2 and an elevated water temperature of less than 50C. The application of the vacuum should be gentle in order not to damage the membrane.
When using a membrane distillation separation apparatus according to the present invention in tropical areas, the difference in temperature between deep sea water (8' deep or more) and surface sea water may be utilized to reduce energy input required. The coolant may comprise deep cool sea water and the warmer surface sea water may be used to contact the membrane to perform the desalina~ion process more efficiently.
To further minimize energy consumption, during heating the rate of vaporization should be matched to the rate of steam penetration through the membrane.
Figure 14 sch~matically illustrates a membrane distillation separation and salt recovery system utilizing membrane distillation separation modules 28a and 28b of the type illustrated in Figure 11 arranged in menlbrane stacks according to Figure 12. A pump llG draws in surface sea water, in surface sea water, which has an elevated temperature, and pumps it through a filter 114 and into the first membrane separation distillation module 28a.
Pump 112 draws in cooler, subsurface sea water and pumps it through filter 118 into the coolant port of membrane - 25 - ~3~

distillation separation module 28a. The sea water is desalinated in the membrane distillation separation module 28a and permeate is withdrawn using a venturi-vacuum pump 89a with the permeate collecting in container 92a. Pump 94a is used to pump permeate through the venturi vacuum pump 89a to create the vacuum. Current from an electrical source 84 is applied across the ends of the porous support tubes within the module 28a to vaporize the elevated temperature water and cause it to permeate through the membrane layers on the porous support tubes.
A portion of the coolant water discharging from the membrane separation distillation module 28a is diverted through a heat exchanger 120 the function of which will be discussed in more detail below. The remainder of the coolant water is further utilized as coolant water through a second membrane separation distillation module 28b.
The elevated temperature sea water passing through the first membrane separation distillation module 28a is pumped by pump 118 through the elevated temperature sea water ports in a second membrane separation distillation module 28b. It will be appreciated that the concentration of salt in this elevated temperature water will have been increased as a result of fresh water permeate being removed in the first membrane separation distillation module 28a. The second membrane separation distillation module 28b removes further water from the concentrated elevated temperature sea water. The permeate - 26 - 2C,~ 2~

is drawn from the second membrane separation distillation module 28b by venturi vacuum pump 89b to collect in container 92b. Pump 94b is used to pump water from the container 92b through the second venturi vacuum pump 89b.
Concentrated elevated temperature sea water emanating from the second membrane separation distillation module 28b passes through the heat exchanger 120 where it is cooled by a portion of the coolant emanating from the first membrane separation distillation module 28a. This cooling of the concentrated sea water causes a portion of the salts to come out of solution. The cooled sea water and salt precipitate is pumped to a centrifuge 122 where they are centrifugally separated.
Figure 15 shows yet another membrane separation distillation and salt recovery system. In the system of Figure 15 a pump 110 draws sea water and pumps it through a filter 114. A portion of the sea water emanating from filter 114 is used as elevated temperature sea water. The balance of the sea water is used as coolant water.
Reference 124 indicates a solar heat exchanger which heats that portion of the water which is to pass throuqh the membrane separation distillation modules 28a and 28b. The water may be further heated, for example by electrical heaters 126. The heated water passes through a first membrane separation distillation modllle 28a which also receives coolant from filter 114. Permeate discharging from the first membran~ separation distillation module 28a - 27 - 2~`~3~

will itself have an elevated temperature. The permeate is therefore passed through a heat exchanger 128 where a portion of the heat is used to further heat elevated temperature sea water passing through the solar heat exchanger 124. A venturi vacuum pump 89a draws the permeate from the heat exchanger 128 in a manner analogous to those discussed above. Concentrated elevated temperature sea water passes from the first membrane separation distillation module 28a through a further heater 130 and into a second membrane separation distillation module 28b.
The second membrane separation distillation module 28b also receives coolant water from the filter 114. Permeate from the second membrane separation distillation module 28B is also passed through heat exchanger 128. Concentrated elevated temperature sea water from the second membrane separation distillation module 28b is cooled in a heat exchanger 120 to cause salt to come out of solution. The salt and the water are separated centrifugally by a centrifuge 122. Coolant water which has passed through the first and second membrane sepaxation distillation modules, 28a and 28b respectively, is used as a cooling source for the heat exchanger 12~.
The system illustrated in Figure 14 uses a membrane separation distillation module of the type illustrated in Figures 11 and 12 whereas the system illustrated in Figure 15 uses membrane separation 28 - 2~3~ 51.

distillation modules of the type illustrated in Figures 9 or 10.
Figure 16 is a schematic for a sea water desalination and salt production plant. Sea water is drawn into the plant at 132 by pump 134. Pump 134 passes the sea water through germicidal equipment 136. The germicidal equipment 136 inc~udes chlorine equipment 138 and a sodium hypochlorous evaporator 140. The germicidally treated water passes through a coarse filtration apparatus 142 and a precision filtration apparatus 144. The water then collects in a basin 146 from where it is drawn by pump 148 and passed on to a first membrane desalination distillation unit 150.
Fresh water leaves the first membrane desal-ination distillation unit 150 at reference 152, passes through heat exchangers 154 and 156, through venturi pump 158 to collect in basin 160. A pump 162 pumps water from the basin 160 through the venturi tube of the venturi pump 158. Pump 162 also pumps water from ~he basin 160 into a higher basin 164 which acts as a fresh water reservoir.
Coolant water from the first membrane separation distillation ~odule 150 is discharged at reference 166 ~t the top of Figure 16.
Concentrated sea water leaves the first membrane separation distillation module 150 at reference 168, is heated by heater 170, solar heater 172 and heat exchanger 174. The heated concentrated sea water then passes through s~

a further heater 176 into a second membrane separation distillation unit 178.
Fresh water permeate is drawn from the second membrane distillation separation module 178 as shown by arrow 180 and is passed through a heat exchanger 182 from where it passes on to heat exchanger 156 and through the venturi vacuum pump 158 to collect in the basin 160.
Coolant water is discharged from the second membrane distillation separation module 178 at reference 184.
Concentrated sea water from the second membrane separation desalination unit is passed through an evaporator 186 and the salt and bittern are further separated at centrifuge 188.
Equipment is also provided to control and monitor pressure, flow rate and water temperature. Typical locations for such equipment are indicated by references 5 which indicate temperature controllers, references 6 which indicate valves, references 7 which indicate pressure gauges and references 8 which indicate flow meters.
The present invention may be further illustrated by reference to the following examples.

Example 1 A dime~hoxy siloxane dynamically formed membrane was produced on a polysulfone porous support body. To do so, a polysulfone porous capillary tuhe was placed in a module in accordance with that illustrated in Figure 2.
The size of the pores in the poysulfone body was .25 microns, the interior diameter of the capillary tube was 2 millimetres and had a wall thickness of .12 millimetres.
The module was placed in a membrane forming system as illustrated in Figure 3. The formula for the dynamically formed membrane material was as follows:
Dimethylbutane methoxy siloxane emulsion - 50 PPM
Sodium chloride 20 PPM
Purified water 100~
The pump 46, heat exchanger 48, coolant pump 15 and coolant heat exchanger 96 were turned on for 30 minutes. At this time the feed temperature reached 60C., the coolant water was at 25C. and the permeate started to penetrate the porous support tube. The system was operated for 60 minutes, at which time a comparison of the conductivity of the permeate and that of the feed indicated a separation rate of 80~. 0.01 MCL siloxane was then added and the operation was continued for another 10 minutes. At this point the pump and heating were stopped, the module was blown dry and put into an oven to heat approximately 2 hours at 90C. The module was removed and cooled and put back into the system which was operated for an additional hour. At this point, the separation rate of the membrane was approximately 99%. It was found that the membrane thus produced when operated under conditions of - 31 - 2~ Z S~.

partial vacuum could give separation rates as high as 50 L/l:)M2 .
It is to be understood that what has been described are preferred embodiments of the present invention and that variations may be possible while staying within the spirit and scope of the present invention.

Claims

1. A distillation separation membrane for use in the desalination of sea water or brine comprising:
a porous support member of a material selected from the group of polymers consisting of polyvinyl chloride, polypropylene, nylon, polystyrene, polycarbonate and polysulfone; and a hydrophobic separation coating within the interstices of substantially all of the pores of said porous support member which are adjacent the surface of said porous support member, said separation coating having been formed dynamically by subjecting the surface of said porous support member to a membrane forming component for a period of time sufficient to form a separation coating capable of distillation separation of salt from water.

2. A membrane according to claim 1 wherein said porous support member is at least one of electro-conductive and heat-conductive.

3. A membrane according to claims 1 or 2 wherein said porous support member further is heat absorptive.

4. A method of desalinating sea water or brine by means of membrane distillation characterized by the use of a distillation separation membrane having a porous support member of a material selected from the group of polymers consisting of polyvinyl chloride, polypropylene, nylon, polystyrene, polycarbonate and polysulfone; and a hydrophobic separation coating within the interstices of substantially all of the pores of said porous support member, said separation coating having been formed dynamically by subjecting the surface of said porous support member to a membrane forming component for a period of time sufficient to form a separation coating capable of distillation separation of salt from water; said method including the step of heating said sea water thereby causing vaporization of said sea water, the vapour thus created passing through said distillation separation membrane for subsequent liquefication.

5. A method of desalinating sea water or brine by means of membrane distillation characterized by the use of a distillation separation membrane having an electro-conductive porous support member and supplying electrical current to said porous support member to heat said brine or sea water in contact with said distillation separation membrane thereby causing vaporization of said brine or sea water, the vapour thus created passing through said distillation separation membrane for subsequent liquefication.

6. A method according to claim 4 wherein a partial vacuum is applied to the downstream side of said distillation separation membrane to increase the flow of permeate across said membrane.

7. A method according to claim 5 wherein a partial vacuum is applied to the downstream side of said distillation separation membrane to increase the flow of permeate across said membrane.

8. A method according to claims 5, 6 or 7, wherein said distillation separation membrane is further solar heat absorptive and wherein said distillation separation membrane is arranged to be heated by solar radiation.

9. A method of forming a distillation separation membrane which method comprises the dynamic formation in situ within or on the pores of a porous support, of a hydrophobic separation membrane by passing suitable membrane-forming components together across the surface of said porous support and causing the components to react together at said surface to form said membrane.

10. A distillation separation membrane according to claim 1 wherein:
said porous support member has a porosity of from 30 to 50%, pore diameters of from 0.2 to 1 micron, a wall thickness of from 10 to 1000 microns and a diameter of from 2 to 6 mm;
said porous support member contains from about 25 to 75% by weight polypropylene, from 5 to 75% of a material selected from the group comprising graphite and crushed carbon, from 10 to 30% by weight polyethylene glycol, from 5 to 25% by weight dodecane sodium sulphate and from 5 to 20% by weight phthaly dibutyl ester.

11. A distillation separation membrane as claimed in claim 10 wherein said membrane forming component contains the following:
a member selected from the group consisting of silico-organic alcohol and siloxane emulsion liquid solutions in a concentration of from 1 to 5000 ppm;
colloidal graphite in a concentration of from 0.5 to 1,250 ppm; and, polyvinyl alcohol in a concentration of from 0.5 to 100 ppm.

12. A distillation separation membrane as claimed in claim 10 wherein said membrane forming component contains the following:

a member selected from the group comprising silico-organic alcohol and siloxane emulsion liquid solutions in a concentration of from 1 to 5000 ppm;
intrinsic conductive polymeric polyelectrolyte solution in a concentration of from 1 to 2000 ppm; and, polyvinyl alcohol in a concentration of from 0.5 to 100 ppm.

13. A distillation separation membrane according to claim 1 wherein:
said porous support member is made of polysulphone having a pore size of 0.25 microns, an interior diameter of 2 millimetres and a wall thickness of 0.12 millimetres;
said membrane forming component contains 50 ppm dimethylbutane methoxy siloxane emulsion, and 20 ppm sodium chloride in purified water;
said porous support member having been subjected to said membrane forming component for about 180 minutes at a feed temperature of 60°C followed by the addition of 0.01 MCL siloxane and further subjecting of said porous support member to said membrane forming component and siloxane for a further 10 minutes, followed by blowing dry and heating for about 2 hours at 90°C.

14. A method according to claims 6 or 7 wherein said partial vacuum is a negative pressure of 0.02 to 0.8 kg/cm2.

15. A method according to claims 6 or 7 wherein said distillation separation membrane is further solar heat absorptive and arranged to be heated by solar radiation and wherein said partial vacuum is a negative pressure of from 0.02 to 0.8 kg/cm2.

16. A method of forming a distillation separation membrane according to claim 9 wherein:
said porous support member has a porosity of from 30 to 50%, pore diameters of from 0.2 to 1 micron, a wall thickness of from 10 to 1,000 microns and a diameter of from 2 to 6 mm;
said porous support member contains from about 25 to 75% by weight polypropylene, from 5 to 75% of a material selected from the group comprising graphite and crushed carbon, from 10 to 30% by weight polyethylene glycol, from 5 to 25% by weight dodecane sodium sulphate and from 5 to 20% by weight phthaly dibutyl ester.

17. A method of forming a distillation separation membrane according to claim 16 wherein said membrane forming component contains the following:
a member selected from the group consisting of silico-organic alcohol and siloxane emulsion liquid solutions in a concentration of from 1 to 5000 ppm;
colloidal graphite in a concentration of from 0.5 to 1,250 ppm; and, polyvinyl alcohol in a concentration of from 0.5 to 100 ppm.

18. A method of forming a distillation separation membrane according to claim 16 wherein said membrane forming component contains the following: silico-organic alcohol and siloxane emulsion liquid solutions in a concentration of from 1 to 5000 ppm;
intrinsic conductive polymeric polyelectrolyte solution in a concentration of from 1 to 2000 ppm; and, polyvinyl alcohol in a concentration of from 0.5 to 100 ppm.

19. A method of forming a distillation separation membrane according to claim 9 wherein:
said porous support member is made of polysulphone having a pore size of 0.25 microns, an interior diameter of 2 millimetres and a wall thickness of 0.12 millimetres;
said membrane forming component contains 50 ppm dimethylbutane methoxy siloxane emulsion, and 20 ppm sodium chloride in purified water;

said membrane forming component is passed across the surface of said porous support member for about 180 minutes at a feed temperature of 60°C. followed by the addition of 0.01 MCL siloxane, said porous support member being further subject to said membrane forming component and siloxane for an additional 10 minutes, followed by blowing dry and heating for about 2 hours at 90°C.