Graded spacers in spiral wound elements

Abstract

Embodiments of the present invention provide elements that are beneficial for use in fluid filtration. Embodiments provide elements that have variable feed spacer height, variable permeate spacer height, or both. The variable height allows flow properties to be matched to fluid volume as the filtration occurs.

We claim: 
     
         1 . An element for use in fluid filtration, comprising:
 (a) a permeate spacer element, permeable to a fluid;   (b) a membrane sheet disposed next to a first surface of the permeate spacer element;   (c) a permeable sheet, permeable to a fluid, disposed next to a second surface, opposite the first surface, of the permeate spacer element; and   (d) a feed-reject spacer element mounted on a surface of the membrane sheet and separated from the permeate spacer element by the membrane sheet, or mounted on the permeable sheet and separated from the permeate spacer element by the permeable sheet;   (e) wherein, the permeate spacer element, permeable sheet, membrane sheet, and feed-reject spacer element are rolled around a central collection tube into a spiral wound element, and the permeate spacer element provides a permeate volume between the permeable sheet and the membrane sheet, and the feed-reject spacer element provides a feed-reject volume between the permeable sheet and the membrane sheet;   (f) wherein the central collection tube defines a tube permeate volume in the interior of the tube that is in communication with the exterior of the tube through one or more permeate openings;   (g) wherein the feed-reject spacer element provides a separation that increases from a first separation proximal the permeate openings to a second separation, greater than the first separation, distal from the permeate openings.   
     
     
         2 . The element of  claim 1 , wherein the feed-reject spacer element comprises an extruded or woven mesh with a varying thickness. 
     
     
         3 . The element of  claim 1 , wherein the feed-reject spacer element comprises two or more layers of extruded mesh, woven mesh, or porous material, stacked to provide the varying thickness. 
     
     
         4 . The element of  claim 1 , wherein the feed-reject spacer element comprises features deposited on the surface of the membrane sheet facing away from the permeate volume, or the surface of the permeable sheet facing away from the permeate volume, or both, where the features project from the surface on which they are deposited by a height that decreases from a first height distal from permeate openings to a second height, lesser than the first height, proximal to the permeate openings. 
     
     
         5 . The element of  claim 1 , wherein the permeate spacer element comprises an extruded or woven mesh with a varying thickness. 
     
     
         6 . The element of  claim 1 , wherein the central collection tube defines a tube reject volume in the interior of the tube, separated from the tube permeate volume, wherein the tube reject volume is in communication with the exterior of the tube through one or more reject openings.