WO2014186226A1

WO2014186226A1 - Electrochemical separation device and method for using the same

Info

Publication number: WO2014186226A1
Application number: PCT/US2014/037498
Authority: WO
Inventors: Chengqian Zhang; Hai Yang; Rihua Xiong; Zhenjiang Yu
Original assignee: General Electric Company
Priority date: 2013-05-15
Filing date: 2014-05-09
Publication date: 2014-11-20
Also published as: CN104147933A; TW201503951A

Abstract

An electrochemical separation device includes a pair of electrodes and at least a dilution spacer between the electrodes. The dilution spacer provides a dilute compartment having an inlet for a solution to be diluted to enter the dilute compartment, an outlet for a diluted solution to exit the dilute compartment, and a dilute flow path connecting the inlet and outlet. The dilute flow path has a width gradually decreased substantially along a flow direction from the inlet to the outlet, or in some cases, the dilute flow path includes longitudinal sections and turning sections that join the longitudinal sections to form a winding series flow path, wherein the longitudinal sections have a width gradually decreased along a flow direction from the inlet to the outlet.

Description

ELECTROCHEMICAL SEPARATION DEVICE AND METHOD FOR USING

THE SAME

BACKGROUND

[0001] Embodiments of the present invention relate generally to electrochemical separation devices and methods for using the electrochemical separation devices.

[0002] Electrochemical technologies such as electrodialysis, electrodialysis reversal and super-capacitor desalination are widely applied in separation processes such as solution desalination. Taking electrodialysis as an example for illustration, in an electrochemical separation process, when the feedstock (e.g., a solution to be processed) flows in concentrate compartments and dilute compartments of the separation device, the solute ions in the solution directionally migrate under a DC electric field. There is a stagnant layer between ion exchange membranes and the solution. When the working current increases to a certain extent, the ions in the solution cannot timely reach the surface of the membrane, at which ion concentration tends to zero. As a result, a large number of water in the stagnant layer is dissociated into H+ and OH- to load a charge, which is called as concentration polarization. Then the current density reaches its limiting value, which is called as the limiting current. Once polarization occurs, pH-dependent scale is produced at the surface of the membrane and resistance of the membrane significantly increases, which causes the increase of energy consumption and membrane damage. As such, both the quality of the product water and the working life of the electrodialysis stack may be reduced. In order to prevent from polarization, it is needed to limit the working current of the electrodialysis stack and make sure that it does not exceed the limiting current. However, with a limited working current, it is hard to achieve a high salt removal by one single electrodialysis stack. In current electrodialysis processes, multiple stacks connected in series are used. For example, in order to achieve a salt removal of about 85%, electrodialysis device and system including three or more series-connected stacks are used. The use of the multiple stacks increases the cost of the electrochemical separation device. As such, the electrochemical separation process may lose its cost advantage in compare with reverse osmosis desalination processes.

[0003] If the limiting current of a single stack is increased, the desalination capability of the single stack can be increased, and thus the number of the stacks needed by the desalination process can be reduced. For example, two series- connected membrane stacks may achieve the same desalination capability as original three series-connected stacks. As such, the cost of the electrochemical separation process can be significantly reduced.

BRIEF DESCRIPTION

[0004] In accordance with an embodiment disclosed herein, an electrochemical separation device includes a pair of electrodes and at least a dilution spacer between the electrodes. The dilution spacer provides a dilute compartment including an inlet for a solution to be diluted to enter the dilute compartment, an outlet for a diluted solution to exit the dilute compartment, and a dilute flow path connecting the inlet and outlet. The dilute flow path has a width gradually decreased substantially along a flow direction from the inlet to the outlet.

[0005] In accordance with another embodiment disclosed herein, an electrochemical separation device includes a pair of electrodes and at least a dilution spacer between the electrodes. The dilution spacer provides a dilute compartment comprising an inlet for a solution to be diluted to enter the dilute compartment, an outlet for a diluted solution to exit the dilute compartment, and a dilute flow path connecting the inlet and outlet. The dilute flow path includes longitudinal sections and turning sections that join the longitudinal sections to form a winding series flow path, wherein the longitudinal sections have a width gradually decreased along a flow direction from the inlet to the outlet.

[0006] In accordance with yet another embodiment disclosed herein, a solution treatment method is provided. An electrochemical separation device including a pair of electrodes and at least a dilution spacer between the electrodes is provided. The dilution spacer provides a dilute compartment including an inlet for a solution to be diluted to enter the dilute compartment, an outlet for a diluted solution to exit the dilute compartment, and a dilute flow path connecting the inlet and outlet. The dilute flow path has a width gradually decreased substantially along a flow direction from the inlet to the outlet. A solution is fed to the inlet of the dilute compartment and caused to flow along the dilute flow path towards the outlet of the dilute compartment, during which anions and cations in the solution move towards anode and cathode of the electrodes, respectively. A diluted solution is collected from the outlet of the dilute compartments.

[0007] In accordance with yet another embodiment disclosed herein, a solution treatment method is provided. An electrochemical separation device including a pair of electrodes and at least a dilution spacer between the electrodes is provided. The dilution spacer provides a dilute compartment including an inlet for a solution to be diluted to enter the dilute compartment, an outlet for a diluted solution to exit the dilute compartment, and a dilute flow path connecting the inlet and outlet. The dilute flow path includes longitudinal sections and turning sections that join the longitudinal sections to form a winding series flow path, wherein the longitudinal sections have a width gradually decreased along a flow direction from the inlet to the outlet. A solution is fed to the inlet of the dilute compartment and caused to flow along the dilute flow path towards the outlet of the dilute compartment, during which anions and cations in the solution move towards anode and cathode of the electrodes, respectively. A diluted solution is collected from the outlet of the dilute compartments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

[0009] FIG. 1 illustrates a schematic diagram of an electrochemical separation device according to one embodiment of the present invention. [0010] FIG. 2 illustrates the anion and cation migration mode in concentrate compartments and dilute compartments of the electrochemical separation device of FIG. 1.

[0011] FIG. 3 A illustrates a dilution spacer that can be used in the electrochemical separation device according to one embodiment of the present invention.

[0012] FIGs. 3B and 3C illustrate two types of concentration spacers that can be used cooperatively with the dilution spacer of FIG. 3 A, respectively.

[0013] FIG. 4A illustrates another dilution spacer.

[0014] FIG. 4B illustrates a concentration spacer that can be used cooperatively with the dilution spacer of FIG. 4A, respectively.

[0015] FIG. 5A and 5B illustrates yet another dilution spacer

[0016] FIG. 5B illustrates a concentration spacer that can be used cooperatively with the dilution spacer of FIG. 5A, respectively.

[0017] FIG. 6 illustrates alignment of the inlets/outlets and manifold holes on the dilution spacer and concentration spacer of FIGs. 4A and 4B when they are used in an electrochemical separation device.

[0018] FIG. 7 illustrates a schematic diagram of another electrochemical separation device according to one embodiment of the present invention.

[0019] FIG. 8 illustrates voltage-current curves resulted from processes that use an electrochemical separation device with a dilute flow path of a uniform width and an electrochemical separation device with a dilute flow path of a gradually decreased width, respectively.

DETAILED DESCRIPTION [0020] Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as "about," is not to be limited to the precise value specified. In certain embodiments, the term "about" means plus or minus ten percent (10%) of a value. For example, "about 100" would refer to any number between 90 and 110. Additionally, when using an expression of "about a first value - a second value," the about is intended to modify both values. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value or values.

[0021] Any numerical values recited herein include all values from the lower value to the upper value in increments of one unit provided that there is a separation of at least 2 units between any lower value and any higher value. As an example, if it is stated that the dosage of a component or a value of a process variable such as, for example, temperature, pressure, time and the like is, for example, from 1 to 90, preferably from 20 to 80, more preferably from 30 to 70, it is intended that values such as 15 to 85, 22 to 68, 43 to 51, 30 to 32 etc. are expressly enumerated in this specification. For values which are less than one, one unit is considered to be 0.0001, 0.001, 0.01 or 0.1 as appropriate. These are only examples of what is specifically intended and all possible combinations of numerical values between the lowest value and the highest value enumerated are to be considered to be expressly stated in this application in a similar manner.

[0022] Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art to which this invention belongs. The terms "first," "second," and the like, as used herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. Also, the terms "a" and "an" do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items.

[0023] As illustrated in FIG. l, embodiments of the present invention provide an electrochemical separation device 100, which includes a pair of electrodes (cathode 102 and anode 104) and a membrance stack 106 between the electrodes. The membrane stack 106 includes alternately stacked anion-exchange membranes (anion permeable membranes) 108, cation-exchange membranes (cation-permeable membranes) 1 10 and spacers between the anion-exchange membranes and cation- exchange membranes. The spacers include concentration spacers 112 and dilution spacers 114, which are alternately inter-arranged between the ion-exchange membranes, to make the ion-exchange membranes spaced from each other, and so as to provide alternate concentrate compartments 116 and dilute compartments 118, as the passages of concentration and dilution, respectively.

[0024] As illustrated in FIG. 2, a solution or fluid (hereinafter referred to as solution in general) to be processed may be fed into the concentrate compartments 116 and dilute compartments 118 after voltage polarity is reversed on the electrodes. As the solution flows in the concentrate compartments 1 16 and dilute compartments 118, the cations in the solution migrate towards the cathode 102 while the anions in the solution migrate towards the anode 104 under the electric field. It is a cation- exchange membrane 110 at a side of the dilute compartment 118 where is adjacent to the cathode 102, and it is an anion-exchange membrane 108 at a side of the dilute compartment 1 18 where is adjacent to the cathode 102. The anions in the solution flowing in the dilute compartments 118 pass through the anion-exchange membranes 108 and move into the solution flowing in the concentrate compartments 116 adjacent to the anion-exchange membranes 108, whereas the cations in the solution flowing in the dilute compartments 1 18 permeate the cation-exchange membranes 110 and move into the solution flowing in the concentrate compartments 116 adjacent to the cation- exchange membranes 1 10. As the solution flows, the ions in the solution flowing in the dilute compartments 118 continually move into the adjacent concentrate compartments 1 16, and the ion concentration of the solution flowing in the dilute compartments 1 18 continually decreases, such that a diluted solution (dilute) can be obtained. The ion concentration of the solution flowing in the concentrate compartments 1 16 increases and a concentrated solution (concentrate) is resulted and discharged from the concentrate compartments 116. [0025] The spacers may be a sheet-like object of a substantially uniform thickness over the entire sheet, and it may have a grad or mesh structure, which allows solutions to flow there-through and provides the flow paths, i.e., the concentrate flow path and/or dilute flow path. The spacers may be made from materials such as plastic. Materials suitable for fabricating the spacers include but are not limited to polypropylene (PP) and polyvinylidene fluoride (PVDF). In some embodiments, the thickness of the spacers ranges from about 0.5mm to about 2.0mm.

[0026] In the embodiments, through the particular design of the dilution spacers 1 14, for example, having the dilute flow path gradually decrease in its width along a flow direction of the solution, the ion concentration in the solution flowing in the dilute compartments 1 18 may be maintained stable or even may gradually increase along the flow direction of the solution, rather than to decrease as the solution flows.

[0027] FIG. 3 A illustrates a dilution spacer 212 that can be used in the membrane stack of the electrochemical separation device 100. In one embodiment, the dilution spacer 212 is a sheet-like object of a substantially uniform thickness over the entire sheet, such as a plastic sheet. The dilution spacer 212 includes pipe holes 216 and 218, which function as the inlet and outlet of the dilute compartment, respectively, and a grad or mesh structure which connects the inlet 216 and the outlet 218 and provides the dilute flow path 220. The width of the dilute flow path 220 gradually decreases along a flow direction of the solution starting from the solution inlet 216 to the solution outlet 218 (except some parts such as the joint portions that connect the path 220 with the inlet 216 or outlet 218).

[0028] As used herein, the path width refers to a size of the path in a direction substantially perpendicular to the thickness of the spacer and the flow direction of the solution flowing in the path. In the embodiments as described herein, the joint portions that connect the flow path with the solution inlet or outlet may have a width smaller than the other part, but as it accounts for only a small part of the entire path length, it does not influence the general decrease situation/tendency of the path width substantially along the flow direction. To be specific, "the path width gradually decreases substantially along the flow direction" does not require the path width to continually decrease along the flow direction throughout the entire path. Rather, it means that, the width as for the main part of the flow path, for example, more than 50% of the entire path length, decreases along the flow direction of the solution. In a specific embodiment, the width as for more than 60%, or preferably more than 70%, 80% or 90% of the entire path length, decreases along the flow direction of the solution. Similarly, the term "gradually decrease" as used herein does not require the width to continually decrease. Rather, it allows for some small/slight fluctuation without changing the overall decrease tendency. For example, in some embodiments, the edges of path 220 may be jagged or corrugated other than straight. In such situations, along the flow direction, the path width may slightly grow and diminish in a micro-scope as the jagged or corrugate edge rises and falls.

[0029] Different from the dilute flow path, the concentrate flow path is not required to have its width gradually decrease along the solution flow direction. The concentrate flow path corresponding to the dilute flow path may have a width identical to or not identical to that of the dilute flow path. For example, in an embodiment, as illustrated in FIG. 3B, the concentration spacer 214 accordingly used with the dilution spacer 212 includes pipe holes 226 and 228 that function as the inlet and outlet of the concentrate compartment, respectively, and a concentrate flow path 230 of a grad or mesh structure, that connects the inlet 226 and outlet 228. The concentrate flow path 230 has a width identical to that of the dilute flow path 220, which has a decrease tendency along the solution flow direction from the inlet 226 to the outlet 228. In another embodiment, as illustrated in FIG. 3C, the concentration spacer 244 accordingly used with the dilution spacer 212 includes holes 246 and 248 that function as the inlet and outlet of the concentrate compartment, respectively, and a concentrate flow path 250 of a grad or mesh structure, that connects the inlet 246 and outlet 248. The width of the concentrate flow path 250 does not identify with that of the dilute flow path 220, and it remains about the same in the solution flow direction.

[0030] Moreover, the dilution spacer 212 as illustrated in FIG. 3 A further includes a pair of manifold holes 222 and 224 corresponding to the inlet 226 (246) and outlet 228 (248) in the concentration spacer 214, respectively. The manifold holes 222 and 224 are not in fluid communication with the dilute flow path 220 in the dilution spacer 212, but once in assembly as a stack, will be in fluid communication with the inlet 226 (246) and outlet 228 (248) in the concentration spacer 214, respectively. The concentration spacer 214 includes a pair of manifold holes 232 and 234 corresponding to the inlet 216 and outlet 218 in the dilution spacer 212, respectively. The manifold holes 232 and 234 are not in fluid communication with the concentrate flow path 230 in the concentration spacer 214, but once in assembly as a membrane stack, will be in fluid communication with the inlet 216 and outlet 218 in the dilution spacer 212, respectively. Similarly, the concentration spacer 244 also includes a pair of manifold holes 252 and 254 corresponding to the inlet 216 and outlet 218 in the dilution spacer 212, respectively. The manifold holes 252 and 254 are not in fluid communication with the concentrate flow path 250 in the concentration spacer 244, but once in assembly as a membrane stack, will be in fluid communication with the inlet 216 and outlet 218 in the dilution spacer 212, respectively.

[0031] The dilute flow path may be of different suitable shapes if only its width gradually decrease substantially along the flow direction. For example, it may be a winding series flow path that is bended or detours in various ways. As illustrated in FIG. 4A, the dilute flow path provided by the grad or mesh structure of the dilution spacer 312 is a winding series flow path formed by joining a plurality of longitudinal path sections in series by turning path sections (e.g., bent portions). The dilution spacer 312 includes holes 316 and 318 that function as the inlet and outlet of the dilute compartment, respectively, and a dilute flow path 320 that connects the inlet 316 and outlet 318. The dilute flow path 320 includes two longitudinal sections 323 and 325 that are separated by a rib 321, and a turning section 327 joining the two longitudinal sections 323 and 325.

[0032] In some embodiments, the width of the dilute flow path 320 except joint portions where the dilute flow path 320 is connected to the inlet 316 and outlet 318, including the longitudinal section 323, turning section 327 and longitudinal section 325 gradually decreases along the flow direction. [0033] In some embodiments, the width of the dilute flow path 320 except the joint portions where the dilute flow path 320 is connected to the inlet 316 and outlet 318 and the turning section 327, including the two longitudinal sections 323 and 325, gradually decreases along the flow direction. The width of the turning section 327 may remain the same along the flow direction. If the dilute flow path includes more than one turning sections, the different turning sections may have the same or different widths. In a specific embodiment, the turning section 327 has a width that substantially the same along the flow direction, and the width of the longitudinal section 323 upstream the turning section 327 substantially equals to the width of the turning section 327 where it joins the turning section 327 while the width of the longitudinal section 325 downstream the turning section 327 substantially equals to the width of the turning section 327 where it joins the turning section 327. In a specific embodiment, the width of the turning section 327 is smaller than the width of the longitudinal sections 323 and 325 where they join the turning section 327, such that the solution flowing though the turning section can have a relatively high flow rate to prevent a flow dead corner. In particular, the width of the turning section 327 is smaller than the smallest width of the longitudinal sections 323 and 325.

[0034] Similarly, the concentrate flow path corresponding to the dilute flow path 320 may have a width identical to or not identical to that of the dilute flow path 320. In a specific embodiment, as illustrated in FIG. 4B, the concentration spacer 314 accordingly used with the dilution spacer 312 includes holes 326 and 328 that function as the inlet and outlet of the concentrate compartment, respectively, and a concentrate flow path 330 that connects the inlet 326 and outlet 328. The concentrate flow path 330 has a width identical to that of the dilute flow path 320.

[0035] Moreover, the dilution spacer 312 includes manifold holes 322 and 324 that are not in fluid communication with the dilute flow path 320 in the dilution spacer 312, but once in assembly as a membrane stack, will be in fluid communication with the inlet 326 and outlet 328 in the concentration spacer 314, respectively. Similarly, the concentration spacer 314 includes manifold holes 332 and 334 that are not in fluid communication with the concentrate flow path 330 in the concentration spacer 314, but once in assembly as a membrane stack, will be in fluid communication with the inlet 316 and outlet 318 in the dilution spacer 312, respectively.

[0036] There is no limit on the number of the longitudinal sections included in the winding series flow path, and it may be two or more longitudinal sections in a winding flow path. For example, as for the pair of dilution spacer 412 and concentration spacer 414 illustrated in FIG. 5A and FIG. 5B, respectively, the winding flow path is formed by joining three longitudinal sections in series, and it may have a width similar to that of the embodiment illustrated in FIG. 4A and FIG. 4B.

[0037] In the embodiments described above, the width of the dilute flow path and/or concentrate flow path may gradually decrease in a linear way or substantially in a linear way.

[0038] Taking the dilution spacer 312 and concentration spacer 314 as an example, the corresponding relation of the holes in the dilution spacer and concentration spacer of the membrane stack of the electrochemical separation device 100 will be described in detail hereafter. As illustrated in FIG. 6, the dilution spacers 312 and concentration spacers 314 are alternately stacked. For concision purpose, only one set of alternately stacked dilution and concentration spacers (two concentration spacers and one dilution spacer) is shown in FIG. 6, and other elements or structures of the membrane stack are omitted. However, it should be understood by one of skill in the art that the membrane stack may include one or more sets of stacked dilution and concentration spacers. As shown in FIG. 6, the inlets 326 in the concentration spacers 314 are aligned with the manifold hole 322 in the dilution spacer 312, the outlets 328 in the concentration spacers 314 are aligned with the manifold hole 324 in the dilution spacer 312, the manifold holes 332 in the concentration spacer 314 are aligned with the inlet 316 in the dilution spacer 312, and the manifold holes 334 in the concentration spacer 314 are aligned with the outlet 318 in the dilution spacer 312. The inlet(s)/outlet(s) and manifold hole(s) that are aligned with each other may be in fluid communication with a same pipe (not shown). In a specific embodiment, a pipe connecting the inlet 326 and the manifold holes 322 may be used to introduce a solution into the concentrate flow path in the concentration spacer 314, a pipe connecting the manifold holes 332 and the inlet 316 may be used to introduce a solution into the dilute flow path in the dilution spacer 312, a pipe connecting the outlets 328 and manifold hole 324 may be used to output the concentrate from the concentration path in the concentration spacer 314, a pipe connecting the manifold holes 334 and the outlet 318 may be used to output the diluted solution from the dilute flow path of the dilution spacer 312. The paths in both concentration spacer 314 and dilution spacer 312 may be fed with the solution to be diluted. Or the concentration spacer 314 may be fed with a solution different from the solution to be diluted.

[0039] In a specific embodiment, when the electrochemical separation device is used to dilute the solution, the solution to be diluted may be fed into the flow path in the dilute compartment or concentrate compartment from the solution inlet. As the solution flows in the flow path from the inlet to the outlet, ions of the solution flowing in the dilute compartment pass through adjacent ion-exchange membranes and enter into the solution flowing in the concentrate compartment. As such, the ion concentration of the solution in the dilute compartment gradually decreases while the ion concentration of the solution in the concentrate compartment gradually increases. A diluted solution and concentrate may be obtained from the outlets of the dilute compartment and concentrate compartment, respectively.

[0040] Because ions of the solution in the dilute compartment gradually go into the solution in the concentrate compartment as the solution flows, total dissolved solids (TDS) in the solution in the dilute compartment are gradually reduced along the flow direction, making the total ion content in the solution in the dilute compartment gradually decrease. However, the ion concentration in the dilute compartment may not gradually decrease along the flow direction because the path width in the dilute compartment gradually decreases along the flow direction as described above. Rather, in some cases, the ion concentration in the dilute compartment may even gradually increase due to the gradually decreased flow path width. Moreover, as the path width gradually decreases along the flow direction, the flow rate of the solution flowing in the path may gradually increase along the flow direction. The limiting current of the electrochemical separation device is decided by the position with a lowest ionic flux (ion concentration*flow rate). As the ion concentration increases, the driving force for ion concentration diffusion in the membrane surface detention layer increases and the ion transfer rate in the membrane increases. Therefore the limiting current is expected to increase with the increase of the ion concentration. In addition, increase of the flow rate may also increase the limiting current. The increase of limiting current may significantly increase the desalination rate of the electrochemical separation device. Thus, the electrochemical separation devices as described herein can achieve significantly increased desalination rate in comparison with these with a uniform flow path width.

[0041] Besides the electrochemical separation device as illustrated in FIGs. 1 and 2, the dilution spacers as described above also can be used in any other electrochemical separation devices which need a dilution spacer. For example, in a super-capacitor desalination device 500 as shown in FIG. 7, a spacer 506 arranged between the cathode 502 and anode 504 may be a dilution spacer as described herein above.

Comparative example

[0042] In a comparative experiment, a conventional electrochemical separation device with a flow path of a uniform width and an electrochemical separation device with a flow path of a gradually decreased width, as illustrated in FIG. 4A and FIG. 4B are used to dilute a solution, respectively. Ion-exchange membranes and spacers used in the two types of electrochemical separation devices have substantially the same size. In the experiment, titanium electrodes and spacers of about 0.76 mm thick were used. 1% (by weight) sodium sulfate (Na₂S0₄) solution was used as the electrode solution. The solution to be diluted was a sodium chloride ( aCl) solution with electric conductivity of about 1200uS/cm and its flow rate at the inlet was about 0.2L/min. Voltage-current curves obtained from the experiment are shown in FIG. 8. As shown in the figure, the electrochemical separation device with a uniform path width has a limiting current density of about 1.8mA/cm², while the electrochemical separation device with a gradually decreased path width has a limiting current of about 2.1mA/cm², with an increase of about 15% in limiting current density.

[0043] The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing embodiments are therefore to be considered in all respects as illustrative rather than limiting on the invention described herein. The scope of embodiments of the invention is thus indicated by the appended claims rather than by the foregoing description, and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims

CLAIMS:

1. An electrochemical separation device comprising: a pair of electrodes; and at least a dilution spacer between the electrodes, the dilution spacer providing a diluting compartment comprising an inlet for a fluid to be diluted to enter the diluting compartment, an outlet for a diluted fluid to exit the diluting compartment, and a diluting path connecting the inlet and outlet, wherein the diluting path has a width progressively decreased substantially along a flow direction from the inlet to the outlet.

2. The device according to claim 1, further comprising a plurality of ion- permeable membranes and at least a concentration spacer providing a concentrating compartment comprising an inlet, an outlet, and a concentrating path connecting the inlet and outlet, wherein the dilution spacer and the concentration spacer are alternately inter-arranged between the ion-permeable membranes.

3. The device according to claim 2, wherein the concentrating path has a width substantially identical to that of the diluting path.

4. The device according to claim 2, wherein the concentrating path has a width not identical to that of the diluting path.

5. The device according to claim 4, wherein the width of the concentrating path is about the same along the flow direction.

6. The device according to claim 1 or 2, wherein the width of the diluting path decreases linearly or substantially linearly along the flow direction.

7. An electrochemical separation device comprising: a pair of electrodes; and at least a dilution spacer between the electrodes, the dilution spacer providing a diluting compartment comprising an inlet for a fluid to be diluted to enter the diluting compartment, an outlet for a diluted fluid to exit the diluting compartment, and a diluting path connecting the inlet and outlet, the diluting path comprising longitudinal sections and turning sections that joins the longitudinal sections to form a winding series flow path, wherein the longitudinal sections have a width progressively decreased along a flow direction from the inlet to the outlet.

8. The device according to claim 7, further comprising a plurality of ion- permeable membranes and at least a concentration spacer providing a concentrating compartment comprising an inlet, an outlet, and a concentrating path connecting the inlet and outlet, wherein the dilution spacer and the concentration spacer are alternately inter-arranged between the ion-permeable membranes.

9. The device according to claim 8, wherein the concentrating path has a width substantially identical to that of the diluting path.

10. The device according to claim 7 or 8, wherein the width of the longitudinal sections of the diluting path decreases linearly or substantially linearly along the flow direction.

11. The device according to claim 7 or 8, wherein each turning section has a width substantially the same with that of an upstream longitudinal section and a downstream longitudinal section where the upstream and downstream longitudinal sections join it.

12. The device according to claim 7 or 8, wherein all the turning sections have a same width that is no greater than a minimum width of the longitudinal sections.

13. The device according to claim 2 or 8, wherein the dilution spacer further comprises a pair of manifold holes aligned with the inlet and outlet of the concentrating compartment, respectively, and the concentration spacer further comprises a pair of manifold holes aligned with the inlet and outlet of the diluting compartment, respectively.

14. The device according to claim 1, 2, 7 or 8, wherein each of the spacers is a generally planar sheet having a substantially uniform thickness.

15. The device according to claim 14, wherein the thickness ranges from about 0.5 mm to about 2.0 mm.

16. A fluid treatment method comprising: providing an electrochemical separation device comprising a pair of electrodes and at least a dilution spacer between the electrodes, the dilution spacer providing a diluting compartment comprising an inlet for a fluid to be diluted to enter the diluting compartment, an outlet for a diluted fluid to exit the diluting compartment, and a diluting path connecting the inlet and outlet, wherein the diluting path has a width progressively decreased substantially along a flow direction from the inlet to the outlet; feeding a fluid to the inlet of the diluting compartment and causing it to flow along the diluting path towards the outlet of the diluting compartment, during which anions and cations in the fluid move towards anode and cathode of the electrodes, respectively; and collecting a dilute fluid from the outlet of the diluting compartments.

17. A fluid treatment method comprising: providing an electrochemical separation device comprising a pair of electrodes and at least a dilution spacer between the electrodes, the dilution spacer providing a diluting compartment comprising an inlet for a fluid to be diluted to enter the diluting compartment, an outlet for a diluted fluid to exit the diluting compartment, and a diluting path connecting the inlet and outlet, the diluting path comprising longitudinal sections and turning sections that joins the longitudinal sections to form a winding series flow path, wherein the longitudinal sections have a width progressively decreased along a flow direction from the inlet to the outlet; feeding a fluid to the inlet of the diluting compartment and causing it to flow along the diluting path towards the outlet of the diluting compartment, during which anions and cations in the fluid move towards anode and cathode of the electrodes, respectively; and collecting a dilute fluid from the outlet of the diluting compartments.