Abstract:
In one embodiment, a membrane stack is described for separating dialysate and diffusate. The membrane stack comprises a plurality of membrane sheets and a plurality of spacers interspersed between the plurality of membrane sheets. Each spacer includes a gasket bordering a screen diagonally woven. These membrane sheets and spacers are affixed together by form a collective unit.

Description:
1. FIELD  
         [0001]    The invention relates to the field of spacer technology for diffusion dialysis and electrodialysis.  
         2. GENERAL BACKGROUND  
         [0002]    Currently, a conventional diffusion dialysis apparatus features a multiplicity of alternating ion selective, anion or cation selective membranes. This apparatus was apparently first described by K. Meyers and W. Strauss in 1940 (Ihelv. Chim. Acta 23 (1940) 795-800). However, the membranes used were poorly ion selective. The discovery of ion exchange (IX) membranes (U.S. Pat. No. Re.24,865), which have high ion perm-selectivity, low electrical resistance and excellent stability led rapidly to diffusion dialysis (DD) systems using such membranes (U.S. Pat. No. 2,636,852) and to the increased usage of DD systems for purification and recovery of acids in metal working industries.  
           [0003]    During the last forty years, several thousand DD systems have been installed on a worldwide basis. However, a number of disadvantages are evident in conventional DD systems that can be overcome by the invention as described below. For example, the temperature expansion properties for a multiple element spacer are one cause of leakage in the DD systems.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]    The features and advantages of the invention will become apparent from the following detailed description of the invention in which:  
         [0005]    [0005]FIG. 1 is an exemplary embodiment of a system employing the membrane stack.  
         [0006]    [0006]FIG. 2 is an exemplary embodiment of the ion exchange conducted by the membrane stack of FIG. 1.  
         [0007]    [0007]FIG. 3 is an exemplary schematic diagram of an embodiment of a membrane stack.  
         [0008]    [0008]FIG. 4 is an exemplary diagram illustrating a transport mechanism associated with the membrane sheet of the membrane stack of FIG. 3.  
         [0009]    [0009]FIG. 5 is an exemplary diagram illustrating a spacer of the membrane stack of FIG. 3.  
     
    
     DETAILED DESCRIPTION  
       [0010]    Herein, an exemplary embodiment of the invention relates to a spacer for a membrane stack and a technique for utilizing the membrane stack. The membrane stack may be utilized for diffusion dialysis (DD) or electrodialysis (ED). The embodiment described herein is not exclusive; rather, it merely provides a thorough understanding of the present invention. Also, well-known elements are not set forth in detail in order to avoid unnecessarily obscuring the invention.  
         [0011]    In the following description, certain terminology is used to describe features of the invention. For example, a “spacer” is generally defined as a device that provides a generally defined distance between two adjacent membrane sheets for liquid to flow or move therebetween. A “membrane sheet” is generally defined as a thin section of material that allows chemicals of a certain chemical composition to permeate from one side to another, while other chemical compositions are precluded from passing through the material.  
         [0012]    Advantages associated with the membrane stack described herein are numerous. For instance, the membrane stack provides enhanced performance by improved mass transfer, purification efficiency, and/or stack sealing properties. When employed in a system, the membrane stack provides more efficiently remove monvalent ions of one sign from liquids in preference to divalent ions of the opposite charge sign.  
         [0013]    Referring to FIG. 1, an exemplary embodiment of a system employing a membrane stack in accordance with the invention is shown. The system  100  includes a cell  110  separated into two compartments  110 A and  110 B by a membrane  120 . Used acid  130  (and perhaps at least one conjugate base  131  such as SO 4   2−  or Cl −  as shown in FIG. 2) is provided to the first compartment  110 A from a first head tank  140 . Aqueous solution, such as de-ionized water  150 , is provided to the second compartment  110 B from a second head tank  160 . The used acid  130  migrates through the membrane  120  (from the first compartment  110 A to the second compartment  110 B) while other aqueous solution (e.g., the de-ionized water  150  and resultant water after chemical reaction) flows through the second compartment  100 B and absorbs acid that migrated from compartment  110 A. The recovered acid  170  (referred to herein as “diffusate”) is collected in a process tank while dialysate (waste)  180  is provided for waste treatment or metals recovery.  
         [0014]    Referring now to FIG. 3, an exemplary embodiment of the membrane stack  120  comprises a plurality (N) of membrane sheets  200   1 - 200   N , which are alternatively separated by a spacer  210   1 - 210   N+1 . Normally, the stack  120  is physically stabilized using two end plates and a hydraulic clamping unit, which do not impede the flow (not shown) and configured for optimal flow density (1/h m2). For this embodiment, besides membrane sheets  200   1 - 200   N  and spacers  210   1 - 210   N+1 , no other components (such as O-rings) are required to assemble the membrane stack  120  between the clamping unit. Other means for attachment of the sheets  200   1 - 200   N  and spacer  210   1 - 210   N+1  may include any mechanism for applying pressure to opposite ends of the stack  120 .  
         [0015]    Normally, each spacer (e.g., spacer  210   2 ) provides a defined distance between two adjacent membrane sheets  200   1  and  200   2  and a space for liquid to flow or move between membrane sheets  200   1  and  200   2 . Normally, each spacer  210   X  (“X”&gt;1) is responsible for optimized fluid distribution between the membrane sheets  200   1 - 200   N  and linear fluid velocity for optimized mass transfer. The mass transfer occurs between two liquids separated by a membrane sheet. As shown, spacer  210   2  is positioned flush against neighboring membrane sheets  200   1  and  200   2  for attachment therewith.  
         [0016]    Herein, a spacer (e.g., spacer  210   2 ) comprises a single gasket  400  and screen  410  operating as a single collective unit as shown in FIG. 5. This “single unit” spacer  210   2  enables simple stack assembly, provides better pressure distribution, and provides an optimized blend of flexibility and sealing capabilities for fluid separation performance. The optimized thickness of the spacer  210   2  varies for industrial applications and it is not only important for system performance but also provides appropriate mechanical stability and properties for the distance between foil (gasket) and woven material (screen). The particular gasket materials are selected to provide good mechanical and stability properties at the interface between the gasket  400  and screen material  410 . The optimized thickness of the spacer  210   2  may range from 0.1 to 1.2 millimeters and the particular materials of the gasket  400  may include, for example, a polymeric mixture with its base material made of PVC or polypropylene.  
         [0017]    As shown in FIGS. 3 and 5, each spacer (e.g., spacer  210   2 ) includes at least one diagonally woven screen  410  that provides optimized flow characteristics. In one embodiment, strings  420  forming the screen  410  are woven at selected angles to form trapezium shaped openings  430  and each possesses a thickness of approximately 0.25 millimeters. Herein, the selected angle ranges from forty degrees (40°) up to fifty-five degrees (55°). Of course, other angles may be used besides ninety degree (90°) as used in conventional, rectangular screens. This angled screen configuration provides higher performance, perhaps 50-65% better performance, than the conventional (rectangular) screens. This optimizes fluid distribution and flux for high ion separation efficiency.  
         [0018]    As further shown in FIG. 3, proximate to its first side, a first spacer  210   1  receives used acid  130  and alters the flow of the used acid  130  through both a first opening  220  and a second opening  221 . Upon encountering the next spacer  210   2 , the flow of the used acid  130  continues through openings  222  and  223 . However, upon encountering the following spacer  210   3 , the flow of the used acid  130  is altered to opening  224  to provide the used acid to a common outflow channel.  
         [0019]    Additionally, as further shown, the first spacer  210   1  receives de-ionized water  150 , which flows through a third opening  225 . Upon encountering the next spacer  210   2 , the flow of the de-ionized water  150  is routed through both the opening  228  and a fourth opening  226 . Upon encountering the next spacer  210   3 , the flow continues until a spacer  210   N+1  is reached. Upon encountering the spacer  210   N+1 , the flow of the de-ionized water  150  is altered to opening  227  to provide the de-ionized water to a common outflow channel for acid recovery.  
         [0020]    Referring to FIGS. 3 and 4, exemplary embodiments of the membrane stack  120  and the transport mechanism for a membrane sheet are shown. Separated by membrane sheets  200  that allow the migration of protons  300  and anions  310  through the membrane sheet  200  but not metal cations  320 , each spacer  210   X  comprises one in-flow and one outflow channel. Herein, every other membrane/spacer is designed in the same manner, with inflows and outflows connected to each other. Namely, the fluid or de-ionized water flows in and out of each spacer into an alternating fashion as shown. All inflows and outflows have a common inflow feed channel, and all outflows go into a common outflow channel, so all alternate connected spacers contribute liquid to one channel.  
         [0021]    For ED use, for example, the membrane stack and its operating conditions allow for high current mass transfer and performance. Moreover, small distances between membrane sheets  200   1 - 200   N  (&lt;0.5 mm) allow for improved ion transportation rate and low diffusion resistance. Optimized fluid dynamics and high flow velocity provide high ionic concentrations in cell compartments. The high ionic concentrations allow high diffusion rates through the membrane sheets  200   1 - 200   N , and thus high performance.  
         [0022]    In summary, optimized fluid dynamics (e.g., 45° screen orientation), optimized cell distance (high ionic conductance) and optimized cell flow rate (high mass transport rate) result in a enhanced stack design and performance, and significantly reduced leakage characteristics.  
         [0023]    While the invention has been described in terms of several embodiments, the invention should not limited to only those embodiments described, but can be practiced with modification and alteration within the spirit and scope of the appended claims. The description is thus to be regarded as illustrative instead of limiting.