Chemistry: electrical and wave energy – Apparatus – Coating – forming or etching by sputtering
Patent
1986-02-18
1988-06-14
Niebling, John F.
Chemistry: electrical and wave energy
Apparatus
Coating, forming or etching by sputtering
2041823, 21032175, B01D 1302, B01D 1300
Patent
active
047509830
DESCRIPTION:
BRIEF SUMMARY
A fluid separation cell, for instance for reverse osmosis, ultrafiltration, dialysis or, particularly, electrodialysis comprises a stack of shallow chambers, each chamber being defined on its opposing faces by a membrane that separates it from the next chamber and around its sides by a spacer assembly. A different fluid flows across each adjacent chamber with transfer of impurities across the separating membrane from the fluid (the diluate) in one chamber to the fluid (the concentrate) in the adjacent chamber. For instance the diluate may be water contaminated with salts and that is to be purified by passage through one chamber whilst the concentrate may be brackish, sea or other water that is to receive impurities across the membrane from the diluate in the adjacent chamber.
Passages for the supply and withdrawal of diluate to one set of chambers and concentrate to another set of chambers must be provided through the spacer assembly and the membranes and so the spacer assembly and the membranes contain appropriate co-operating supply and withdrawal conduits. Each spacer has an open centre that will define the area of the fluid chamber and conduit passages for supply and withdrawal of fluid.
Suitable transfer means must be provided to permit the transfer of fluid from an appropriate supply or withdrawal conduit into a chamber. These transfer means take the form of slots, holes or other passages extending between the conduits and the open centre of a chamber.
A serious problem with fluid separation cells of this type is that if there is leakage of fluid containing impurities (the concentrate) into fluid that has been or is being purified (the diluate) the degree of purification that is obtainable will be greatly reduced. It is therefore essential to minimise or eliminate unwanted leakage between adjacent chambers and between the supply or withdrawal conduits and the chambers, except through the desired transfer means. The spacers and the membranes must therefore stack together in such a manner as to prevent leakage. Unfortunately this has not previously been easily achieved and as a result it has seriously restricted the value of processes such as electrodialysis. One fundamental difficulty is that if a groove or other transfer means extends in the surface of a spacer between a supply or withdrawal passage and the open centre there will be a serious tendency for the adjacent membrane and adjacent spacer assembly to bow into that groove, thus permitting leakage.
A particular problem arises in dialysis, especially electrodialysis because of the need to maintain a constant high flow rate and a flow rate that is uniform across each chamber. If the flow rate is not uniform the concentration of impurity ions in the areas of the concentrate cell where the flow rate is low will become excessive, and this will lead to precipitation of impurity salts within the shallow chamber. Once precipitation occurs to any significant extent the cell tends to become inoperable.
A simple type of spacer assembly that has been tested in electrodialysis cells is a uniform mesh of plastics material extending throughout the cell, thus separating the membranes throughout their area, optionally with a rigid flat bar on each side of the mesh around it edges with the intention that the flexible membrane would form a tight seal with the bar. However this is not very effective, the assembly of the spacers is difficult and expensive and leakage remains a problem.
In U.S. Pat. No. 3,256,174 a single spacer is used in each chamber and the spacer frame has holes drilled from the supply passages to the open centre or has grooves formed in one layer of the spacer and covered with an overlying laminate. However this system is expensive and requires a thick spacer which results in less efficient separation within the chamber. Also the fluids are not supplied and withdrawn uniformly to the entire width of each chamber and so precipitation in the concentrate chamber is a danger.
In GB No. 921094 and FR No. 1281549 there is again a single spacer provid
REFERENCES:
patent: 3459310 (1969-08-01), Edwards
patent: 3703466 (1972-11-01), LaForest et al.
patent: 4124478 (1978-11-01), Tsien et al.
patent: 4339324 (1982-07-01), Hass
Foster Paul A.
Stolberg Errol M.
Niebling John F.
Starsiak Jr. John S.
The Permutit Company Limited
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