Spiral electrodeionization device with uniform operating characteristics

Abstract

EDI apparatus for demineralizing a liquid flow is assembled in a housing having a cylindrical shape, and includes two metal electrodes, and one or more leafs, each leaf comprising a pair of selectively ion-permeable membranes arranged parallel to each other and spaced apart by spacing elements that allow liquid to flow in the interstitial space between membranes, thus forming an arrangement of dilute and concentrate cells in a desired flow configuration. Spacing elements between membranes, as well as between leaves, can be formed of inert polymer material, ion exchange beads, ion exchange fibers, a combination of two or more these elements, or a porous media incorporating one or more of such elements as an intrinsic part. An inner or central electrode and an outer or perimeter electrode establish a generally uniform and radially-oriented electrical or ionic current between the inner and the outer electrodes, across the helical flow spaces defined by the membrane / spacer windings. One or both electrodes may include a pocket, and the adjacent flow cells lie parallel to the electrode and free of shadowing and field inhomogeneity around a full circumference of the electrode. Flow paths within the helical cells are defined by barrier seals, which may form a path-lengthening maze, while unfilled cell regions may disperse or collect flow within a cell and define pressure gradients promote directional flows. Impermeable barriers between membranes further prevent the feed and concentrate flows from mixing. In various embodiments, seals along or between portions of the flow path may define a multi-stage device, may define separate feed and / or concentrate flows for different stages, and / or may direct the feed and concentrate flows along preferred directions which may be co-current, counter-current or cross-current with respect to each other within the apparatus.

Description

REFERENCE TO RELATED APPLICATIONS [0001] This application claims the benefit of earlier-filed PCT International Application US2004 / 034909 filed Oct. 20, 2004, which claimed priority of U.S. Provisional Application Ser. No. 60 / 512,661 filed Oct. 20, 2003.BACKGROUND [0002] The present invention relates to methods and devices for demineralizing fluids, and relates to filtration or treatment cartridges or modules, having a generally cylindrical aspect and constructed with plies of spirally wound selectively-permeable membranes. It particularly relates to electrodialysis and electrodeionization devices, wherein the membranes possess ionic selectivity, and the device includes electrodes for inducing transport of species across the membranes by ionic conduction. [0003] In general, electrodialysis (ED) and so-called electrodeionization (EDI) devices operate by providing a structure that arranges flow channels such that a flow of a feed fluid that is to be treated is channeled between two io...

AI Technical Summary

Benefits of technology

[0022] A preferred electrode is formed as a conductive sheet wound in a volute circumscribing more than one full turn, with an axially-extending strip-like gap or opening defined in an overlap region between the radially inner and the radially outer edges of the volute. The opening accommodates passage of the end of a leaf along a tangential path parallel to the electrode surface into a sealed pocket of the electrode (e.g., between the inner surface and the edge of the outer surface of the volute), while fluid communication with the inter-membrane flow space and connection to a fluid port or manifold may be effected through the electrode. The leaf entering or exiting the electrode pocket approaches at a tangential angle, and may therefore wrap closely parallel to the electrode surface, providing an exceptionally uniform current distribution around the electrode, free of the shadowing and inhomogeneities that occur with prior art constructions that employ clamping, membrane doubling or other irregular fixing or termination structures at the electrode. The construction also avoids introducing brine manifold shorting or back diffusion that may, to some extent, plague prior art constructions. A membrane may also be sealed or attached to the electrode in the overlap region, simplifying fabrication of the spiral-rolled cell structure.

[0027] Flow may also be segregated on the concentrate side by oriented strands of a mesh spacer, or by bands of impermeable material placed on the spacer in an oriented pattern. In one preferred embodiment of this aspect of the invention, the dilute flow follows a path parallel to the axis of the cylindrical winding, and the concentrate flow is directed such that it remains in or flows parallel to a plane perpendicular to the axis. Thus, as bivalent metals such as calcium, followed by ions such as sulfate or carbonate, are successively removed from the axial dilute flow and enter the concentrate cells, each of the removed impurities remains in a band within the cell as it flows toward the cell outlet, and does not mix with the other removed species. The segregated flow effectively prevents scale from forming. In another or further embodiment of this aspect of the invention, the dilute cells may have a banded filling wherein the resin in a band at a stage along the feed-product flow path is of a type selected to enhance removal of the species (for example, of, to capture scale forming metallic ions) in that stage, or to selectively block capture or transmembrane passage of a species (for example, sulfate or carbonate) until a later position where it may encounter a resin selected to promote capture. Such banded resin filling thus sharpens the separation of the different potentially scale-forming species that may be present in the feed.

[0028] In accordance with another aspect of the invention, structures normalize current density to promote uniform and effective demineralization. A distribution of dilute inlet and / or outlet passages may define a radially varying flow distribution tailored to the prevailing electrical current density.

Problems solved by technology

However, it is not entirely clear from the '528 patent or from the commercial product description why bead replacement is deemed necessary.

It is also possible, however, that the device described in the '528 patent was prone to scaling as a result of the minerals (such as calcium and silica) present in the local waters and the nature of fluid flows and electrical fields within the device, and that resin replacement was necessary in that particular context.

However, the rate or flow distribution and other factors governing all these effects are such that conditions of high concentration of specific ions, extreme pH, or flow stagnation may all arise in use, and certain combinations of these conditions may pose control problems, impair the efficiency or degree of treatment, or risk introducing irreversible membrane damage and / or localized occurrences of resin or membrane scaling within the device.

Some heterogeneous exchange membranes may swell by twenty percent, and wetted beds of exchange beads also increase their volume and may exert high pressure if unduly confined.

Such swelling may impair the flow impedance, or may affect the integrity of membranes or structural elements.

For spiral constructions, the dimensional instability of membrane and bead media, together with the local slippage introduced by winding at different radii, and the relatively large length of individual membranes, raise additional potential problems of membrane spacing or support, stress, shrinking or buckling, and cracking.

Within this general picture, various problems or perceived problems or design constraints may arise.

It is apparent that a small cell thickness introduces hydraulic flow limitations that will vary greatly as a function of exchange bead size and feed fluid viscosity; theoretical or empirical modeling done with water would not necessarily apply to systems for treating common alimentary fluids.

These current variations and resulting potentials may profoundly alter the intended operating performance.

In addition, in spiral devices, current density increases inversely with radial position, raising further control or operational difficulties.

Moreover, fluids such as alimentary or fermentation fluids are notoriously prone to fouling—both functional fouling of exchange bead surfaces and functionality, and physical blockage of flow through the exchange beds.

Fluidized exchange beds have been employed to address the latter problem, but this approach cannot be employed with the exchange bead filling of EDI devices, because it is inconsistent with the requirement of direct contact between exchange beads and the constricted space existing between the exchange membranes.

For such reasons, the fabrication and operation of EDI demineralization devices remain rather complex and costly, and each particular construction may have its own limitations or drawbacks.

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