Material efficiency and fabrication of membrane elements
Membrane elements that use multiple membrane leaves may have a limited total active membrane area due to an increased diameter at the ends of the element. Membrane leaves may comprise a permeate carrier positioned between one or more membrane sheets. Adhesive may be used to seal one or more edges of the membrane leaf. The membrane sheets, permeate carrier and the adhesive contribute to the thickness of the edges of the membrane leaf and the diameter at the ends of the element. A reduced thickness of the edges of the permeate carrier may reduce the diameter at the ends of an element. Another permeate carrier sheet may also be used that is distanced from at least one edge of the membrane sheet so the permeate carrier sheet does not contribute towards the increased diameter at the ends of the element.
1. A method of making a membrane leaf for a spiral wound membrane module comprising the steps of,
a) placing a permeate carrier between two membrane sheets, the permeate carrier comprising a central portion and one or more borders along one or both side edges of the central portion, wherein the permeate carrier has a front side and a back side and the one or more borders are thinner than the central portion;
b) applying an adhesive to the borders on the front side of the permeate carrier; and,
c) compressing the membrane sheets together;
wherein the adhesive is applied to the border only outside of one or more transitions between the one or more borders and the central portion of the permeate carrier.
2. The method of claim 1 wherein the step of compressing the membrane sheets together comprises winding the membrane sheets and permeate carrier around a central tube.
3. The method of claim 1 wherein the permeate carrier comprises a woven sheet.
4. The method of claim 1 wherein the permeate carrier comprises a tricot fabric and wherein the front side of the permeate carrier is the wale side of the tricot fabric.
5. The method of claim 4 wherein the one or more borders are formed by one or more of a) knitting the tricot fabric with thinner yarn in the machine direction in the one or more borders relative to the yarn in the central portion, b) knitting the tricot fabric without wales in the one or more borders, c) knitting the tricot fabric with more deformable yarn in the one or more borders relative to the yarn in the central portion and applying pressure to the one or more borders, and d) knitting the tricot fabric with fewer wales per inch in the one or more borders relative to the central portion and applying pressure to the one or more borders.
6. A method of making a membrane leaf for a spiral wound membrane module comprising the steps of,
a) placing a permeate carrier between two membrane sheets, the permeate carrier comprising a central portion and one or more borders along one or both side edges of the central portion, wherein the permeate carrier has a front side and a back side and the one or more borders are thinner than the central portion;
b) applying an adhesive to the borders on the front side of the permeate carrier; and,
c) compressing the membrane sheets together;
wherein the permeate carrier comprises a tricot fabric and wherein the front side of the permeate carrier is the wale side of the tricot fabric; and
wherein the one or more borders are formed by applying pressure to the tricot fabric.
7. A method of making a membrane leaf for a spiral wound membrane module comprising the steps of,
a) placing a permeate carrier between two membrane sheets, the permeate carrier comprising a central portion and one or more borders along one or both side edges of the central portion, wherein the permeate carrier has a front side and a back side and the one or more borders are thinner than the central portion;
b) applying an adhesive to the borders on the front side of the permeate carrier; and,
c) compressing the membrane sheets together;
wherein the one or more borders are 2-5 mil thinner than the central portion.