Desalination device and process

• US 4,545,862 A
• Filed: 03/05/1982
• Issued: 10/08/1985
• Est. Priority Date: 03/17/1981
• Status: Expired due to Fees

• Current Assignee: W. L. Gore & Associates (W.L. Gore & Associates)
• Original Assignee: W. L. Gore & Associates (W.L. Gore & Associates)
• Inventors: Gore, Robert W., Gore, David W., Gore, Wilbert L.
• Primary Examiner(s): BASCOMB JR, WILBUR
• Application Number:
  US06/354,140
• Time in Patent Office: 1,313 Days
• Field of Search: 203/10, 203/11, 203/21-27, 203/86, 203/99, 203/100, 203/DIG. 22, 203/DIG. 16, 203/DIG. 1, 203/DIG. 17, 202/180, 202/202, 202/266, 202/270, 202/172, 202/173, 202/175, 202/265, 159/DIG. 27, 210/321.5, 210/486, 210/346, 210/640, 55/11, 55/16, 55/158
• US Class Current: 203/10
• CPC Class Codes: B01D 61/3641 comprising multiple membrane distillation steps
• CPC Class Codes: Y10S 159/27 Micropores
• CPC Class Codes: Y10S 203/23 Methanol

Abstract

A membrane distillation module is provided with a thin, flexible microporous membrane positioned against an impermeable condensor sheet that is stiffer in bending than the membrane. Distillate vapor diffuses through the membrane from the hot feed flowing rapidly past the membrane, and condenses and collects between, and in intimate contact with, the membrane and the condensor sheet. The condensed distillate is stripped by forces exerted by the hot feed acting on the distillate across the membrane, and flows toward a distillate outlet located in the downstream direction of the hot feed flow. Cold feed flows past the condensor sheet in the counter direction to that of the hot feed for absorbing the latent heat of condensation, and is subsequently additionally heated and introduced into the hot feed channel. Expanded microporous polytetrafluoroethylene (PTFE) is the preferred membrane material, and a spiral-wound assembly is the preferred configuration.

Claims

1. Apparatus for the continuous distillation of a feed of a hot aqueous liquid containing a low volatility solute such as salt comprising:

a thermally conductive, water vapor impermeable sheet forming one longitudinal wall of a distillate collecting chamber;

a hydrophobic microporous membrane forming an opposing longitudinal wall of said chamber;

means for flowing the hot aqueous feed past, and in intimate contact with, the surface of said microporous membrane opposite the distillate collecting chamber, water vapor from the hot feed diffusing through said microporous membrane into said distillate collecting chamber;

means for cooling the surface of said impermeable sheet opposite the distillate collecting chamber, the cooled sheet causing condensation of the diffused water vapor in the collecting chamber, the condensed distillate being in intimate contact with both said sheet and said membrane; and

means for stripping condensed distillate from said distillate collecting chamber, said stripping means including

(a) means using forces exerted by the hot flowing feed acting across said microporous membrane on the distillate sandwiched between said porous membrane and said impermeable sheet for causing distillate flow in the direction of hot feed flow, wherein said distillate forcing means includes means for stiffening each impermeable sheet against deformation in the direction normal to its surface relative to the respective microporous membrane, said more stiff impermeable sheet and said less stiff microporous membrane being positioned in surface-contacting relationship when no liquid distillate is present in said collecting chamber, said less stiff microporous membrane being locally deformable away from said more stiff impermeable sheet during distillation operation by forces induced by the flowing hot feed to allow passage of liquid distillate, and

(b) means for draining the distillate from the collecting chamber along the direction of hot feed flow.

2. Apparatus as in claim 1 wherein said cooling means includes means for contacting said impermeable sheet on the surface opposite the collecting chamber with cold aqueous feed at low temperature relative to the hot feed flowing past said microporous membrane, the apparatus further including means for flowing said cold feed past said impermeable membrane opposite surface in the opposite direction from the hot feed flow past said microporous membrane.

3. Apparatus as in claim 2 including heater means operatively connected to said cold feed flow means and said hot feed flow means for receiving the cold feed after it has cooled said impermeable membrane, further heating said received cold feed to the temperature of the hot feed flowing past said microporous membrane, and introducing said heated feed for flow past said microporous membrane.

10. Apparatus as in claim 1, 4 or 5 wherein said stiffening means is an impermeable sheet thickness dimension and material selected such that said sheet is stiffer in bending than said microporous membrane.

11. Apparatus as in claim 1, 4 or 5 wherein said stiffening means includes means for constraining the respective edges of each impermeable sheet and associated microporous membrane such that each microporous membrane has less tension relative to the associated impermeable sheet and means for maintaining during operation a difference in static pressure across each impermeable sheet for tensioning the impermeable sheet relative to the associated microporous membrane, said relatively tensioned sheet thereby becoming stiffer against bending than the associated membrane.

12. Apparatus as in claim 1, 4 or 5 wherein a pressure gradient is maintained in said hot feed flow such that static pressure decreases along the direction of hot feed flow.

13. Apparatus as in claim 1, 4 or 5 wherein means are provided in the stream of hot feed flowing past each membrane to provide turbulence and mixing.

14. Apparatus as in claim 13 wherein hot feed flowing means includes opposing wall means defining in part a hot feed flow chamber together with said membrane, wherein said mixing means includes an element for spacing said membrane away from said opposing wall means.

19. Apparatus as in claim 1, 4, 5 or 15 wherein each microporous membrane is expanded polytetrafluoroethylene having a void content of from about 80% to about 90% and a thickness from about 0.001 inches to about 0.005 inches.

20. Apparatus as in claim 19 wherein each impermeable sheet is polyethylene sheet having a thickness from about 0.001 inches to about 0.005 inches.

4. Energy efficient apparatus for the continuous distillation of a feed of hot aqueous liquid containing a low volatility solute such as salt comprising:

a thermally conductive, water vapor impermeable sheet forming one longitudinal wall of a distillation collecting chamber;

a hydrophobic microporous membrane forming an opposing longitudinal wall of said chamber;

means for rapidly flowing the hot aqueous feed past, and in intimate contact with, the surface of said microporous membrane opposite the distillate collecting chamber, water vapor from the hot feed diffusing through said microporous membrane into said distillate collecting chamber;

means for cooling the surface of said impermeable sheet opposite the distillate collecting chamber, the cooled sheet causing condensation of the diffused water vapor in the collecting chamber, the condensed distillate film being in intimate contact with both said sheet and said membrane, said cooling means including

(a) means for flowing past said impermeable sheet cold aqueous feed at a temperature lower than that of the hot feed flowing past said membrane, the direction of flow of said cold feed being counter to that of the hot feed, and

(b) means for recovering the latent heat of the condensing distillate absorbed by said cold feed; and

means for stripping condensed distillate from said distillate collecting chamber, said stripping means including

(a) means using forces exerted by the rapidly flowing hot flowing feed acting across said microporous membrane on the distillate sandwiched between said porous membrane and said impermeable sheet for causing distillate flow in the direction of hot feed flow, wherein said distillate forcing means includes means for stiffening each impermeable sheet against deformation in the direction normal to its surface relative to the respective microporous membrane, said more stiff impermeable sheet and said less stiff microporous membrane being positioned in surface-contacting relationship when no liquid distillate is present in said collecting chamber, said less stiff microporous membrane being locally deformable away from said more stiff impermeable sheet during distillation operation by forces induced by the flowing hot feed to allow passage of liquid distillate, and

(b) means for draining the distillate from the collecting chamber along the direction of hot feed flow, said stripping means preventing thick layers of distillate from forming in said distillate collecting chamber and allowing said distillate to give up its sensible heat to said cold feed.

9. Apparatus as in claim 4, 7, or 8 further including heater means for receiving the cold feed after it has cooled the impermeable sheets, heating the cold feed to the temperature of the feed flowing past the microporous membranes, and introducing the heated feed for subsequent flow past said microporous membranes, said heater means being operatively connected to said cold feed flow means and said hot feed flow means.

5. Apparatus for the distillation of a hot feed of an aqueous liquid containing a low volatility solute such as salt comprising:

first and second thermally conductive, water vapor impermeable sheets positioned in spaced side-by-side relationship;

first and second microporous membranes positioned in spaced side-by-side relationship between said impermeable sheets, each of said microporous membranes together with the proximate respective impermeable sheet forming opposing longitudinal walls of first and second distillate collection chambers;

means for flowing the hot feed between and in intimate contact with said first and second microporous membranes, water vapor from the hot feed diffusing through said membranes into the respective collecting chambers;

means for cooling said first and second impermeable sheets, said cooled sheets causing the diffused vapor to condense and collect in the respective collecting chambers, the condensed distillate being intimately in contact with the both the membrane and the sheet forming the respective collecting chamber; and

means for stripping the condensed distillate from said collecting chambers, said stripping means including

(a) means using forces exerted by the hot flowing feed acting across said first and second microporous membranes on the distillate sandwiched between each of said first and second microporous membranes and the respective one of said first and second impermeable sheets for causing distillate flow in the direction of the hot feed flow, wherein said distillate forcing means includes means for stiffening each impermeable sheet against deformation in the direction normal to its surface relative to the respective microporous membrane, said more stiff impermeable sheet and said less stiff microporous membrane being positioned in surface-contacting relationship when no liquid distillate is present in said collecting chamber, each of said less stiff microporous membranes being locally deformable away from said respective more stiff impermeable sheet during distillation operation by forces induced by the flowing hot feed to allow passage of liquid distillate, and

(b) means for draining the distillate from the collecting chambers along the direction of hot feed flow.

6. Apparatus as in claim 5 wherein said cooling means includes means for flowing cold feed past said impermeable sheets in a direction counter to the hot feed flow direction, the cold feed temperature being low relative to that of the hot feed flowing past the microporous membranes.

7. Apparatus as in claim 6 formed in a spiral having the spiral axis transverse to the hot feed flow direction, said first impermeable sheet overlapping said second impermeable sheet in said spiral, the overlapping adjacent portions of said first and said second impermeable sheets being spaced and defining the flow path for the cold feed.

8. Apparatus comprising a plurality of distillation units as defined by claim 6, said units arranged in parallel with adjacent impermeable sheets of adjacent units being spaced and defining the flow path for the cold feed between the respective units.

15. Apparatus for the distillation of a hot feed of an aqueous liquid containing a low volatility solute such as salt comprising:

first and second thermally conductive, water vapor impermeable sheets positioned in spaced side-by-side relationship;

first and second microporous membranes positioned in spaced side-by-side relationship between said impermeable sheets, each of said microporous membranes together with the proximate one of said impermeable sheets forming a respective distillate collecting chamber;

means for flowing the hot feed between and in intimate contact with both said first and second microporous membranes, water vapor from the hot feed diffusing through said membranes into the respective collecting chambers;

spacer means for separating said first and second microporous membranes and for promoting turbulence and mixing in said hot feed flowing therebetween;

means for cooling said first and second impermeable sheets, said cooled sheets causing the diffused vapor to condense and collect in the respective collecting chambers, the condensed distillate being intimately contacted by both the membrane and the sheet forming the respective collecting chamber; and

means for stripping the condensed distillate from said collecting chambers, said stripping means including

(a) means using forces exerted by the flowing hot feed acting across said first and second microporous membranes on the distillate sandwiched between each of said first and second microporous membranes and the respective one of said first and second impermeable sheets for causing distillate flow in the direction of the hot feed flow, wherein a pressure gradient is maintained in the hot feed flow between said membranes with the static pressure decreasing along the direction of hot feed flow, and

(b) means for draining the distillate from the collecting chamber along the direction of hot feed flow, wherein said spacer means includes a plurality of first ribs extending longitudinally in the feed flow direction; and

a plurality of second ribs extending transversely to the feed flow direction and attached to, and transversely spacing, said first ribs, said first ribs being sized and configured to provide the required separation, and said second ribs being sized and configured to provide turbulence and to mix the feed flowing past said second ribs.

18. Apparatus as in claim 15 wherein the portions of said first ribs contacting said microporous membranes are configured to prevent damage to said membranes.

16. Apparatus for the distillation of a hot feed of an aqueous liquid containing a low volatility solute such as salt comprising:

first and second thermally conductive, water vapor impermeable sheets positioned in spaced side-by-side relationship;

first and second microporous membranes positioned in spaced side-by-side relationship between said impermeable sheets, each of said microporous membranes together with the proximate one of said impermeable sheets forming a respective distillate collecting chamber;

means for flowing the hot feed between and in intimate contact with both said first and second microporous membranes, water vapor from the hot feed diffusing through said membranes into the respective collecting chambers;

spacer means for separating said first and second microporous membranes and for promoting turbulence and mixing in said hot feed flowing therebetween;

means for cooling said first and second impermeable sheets, said cooled sheets causing the diffused vapor to condense and collect in the respective collecting chambers, the condensed distillate being intimately contacted by both the membrane and the sheet forming the respective collecting chamber; and

means for stripping the condensed distillate from said collecting chambers, said stripping means including

(a) means using forces exerted by the flowing hot feed acting across said first and second microporous membranes on the distillate sandwiched between each of said first and second microporous membranes and the respective one of said first and second impermeable sheets for causing distillate flow in the direction of the hot feed flow, wherein a pressure gradient is maintained in the hot feed flow between said membranes with the static pressure decreasing along the direction of hot feed flow, and

(b) means for draining the distillate from the collecting chamber along the direction of hot feed flow,

wherein said spacer means also contacts and separates said first and second impermeable sheets through the respective microporous membranes, said spacer means contacting said impermeable sheets intermittently in the transverse direction.

17. Apparatus as in claim 16 wherein said spacer means includes a plurality of first ribs extending longitudinally in the feed flow direction;

and a plurality of second ribs extending transversely to the feed flow direction and attached to, and transversely spacing, said first ribs, said first ribs being sized and configured to provide the required separation, and said second ribs being sized and configured to provide turbulence and to mix the feed flowing past said second ribs.

21. Method of continuously distilling a hot aqueous feed containing a low volatility solute such as salt comprising the steps of:

(a) flowing the hot feed past, and in intimate contact with, one side of a microporous membrane;

(b) diffusing water vapor through the membrane;

(c) condensing the diffused water vapor using an impermeable sheet positioned on the side of the microporous membrane opposite the hot feed, the condensing step including the steps of collecting the condensed vapor in the chamber defined in part by the microporous membrane and the impermeable sheet, and intimately contacting both the membrane and the sheet with the collected distillate;

(d) stripping the condensed and collected distillate from the chamber using forces exerted by the hot flowing feed flow acting across the microporous membrane on the distillate sandwiched between the membrane and the sheet, the forces causing distillate flow in the same direction as hot feed flow, said stripping step including the step of locally deforming the microporous membrane relative to the impermeable sheet and in a direction normal to the sheet to allow passage of the distillate in the direction of the hot feed flow; and

(e) draining the flowing distillate from the collecting chamber along the direction of hot feed flow.

22. Method as in claim 21 including the preliminary steps of stiffening the sheet relative to the membrane in respect to deformation in the direction normal to the sheet to provide deformation substantially in the membrane during distillate accumulation in the chamber, and positioning the sheet and the membrane in surface-contacting relationship when no liquid distillate is present in the collecting chamber.

23. Method as in claim 21 or 22 further including the step of maintaining a pressure gradient in the flowing feed, the static pressure decreasing along the direction of the feed flow.

24. Method of efficiently and continuously distilling a hot aqueous feed containing a low volatility solute such as salt comprising the steps of:

(a) rapidly flowing the hot feed past, and in intimate contact with, one side of a microporous membrane;

(b) diffusing water vapor through the membrane;

(c) condensing the diffused water vapor using a thermally conductive impermeable sheet positioned on the side of the microporous membrane opposite the hot feed, the condensing step including the steps of collecting the condensed distillate in the chamber defined in part by the microporous membrane and the impermeable sheet, and intimately contacting both the membrane and the sheet with the collected distillate;

(d) stripping the condensed and collected distillate from the chamber using forces exerted by the rapidly flowing hot feed flow acting across the microporous membrane on the distillate sandwiched between the membrane and the sheet, the forces causing distillate flow in the same direction as hot feed flow, said stripping step including the step of locally deforming the microporous membrane relative to the impermeable sheet and in a direction normal to the sheet to allow passage of the distillate in the direction of the hot feed flow, said stripping step maintaining the condensed distillate in relatively thin layers for efficient transfer of the sensible heat to the sheet;

(e) draining the flowing distillate from the collecting chamber along the direction of hot feed flow; and

(f) recovering the latent heat of condensation of the distillate, said latent heat recovering step including the steps of (1) cooling the impermeable sheet by flowing cold aqueous feed past the sheet in a direction opposite to the hot feed flow direction, the cold feed absorbing the latent heat of condensation transmitted through the heat conductive sheet, (2) additionally heating the cold feed after it has cooled the sheet and absorbed the latent heat, said additional heating step raising the temperature of the cold feed up to the temperature of the hot feed, and (3) introducing the heated cold feed for flow past the membrane.