GEORGE W. MURPHY1 State University of New York College for Teachers, Albany, N. Y.
Osmionic Demineralization This method of purifying water is promising for arid regions-power
is
derived from concentration differences between two streams of saline water
SINCE
highly efficient synthetic permselective membranes were introduced recently, purification of saline waters by low cost electrodialysis ( I , 71, 72) has received widespread attention. However, the process requires electric power which is an important fraction of the over-all product cost. I n the method described here, membranes are used for demineralization and no electric power is needed, except that for pumping fluid streams. Power, derived from concentration differences between two streams of saline water, can be called osmotic and, because it demineralizes by driving ions through the membranes, the over-all process is called osmionic (3, 7). Since the work described here was completed, a British patent has appeared which describes a similar process ( 9 ) . Principle of the Osmionic Process
T o convert completely a saline water of vapor pressure p, to pure water of vapor pressure p", a minimum energy per mole of water must be supplied, equal to the free energy difference, AG = RT In (p"/p), between the pure and saline water. The various forms of energy used previously for this purpose are mechanical, thermal (here, AG is the work equivalent of thermal energy), and electrical. The difference in molar free energy between a concentrated brine of vapor pressure, p', and a saline water 'of AG = RT In (p'/p) is a potential or driving force which can be used in separating the constituents of saline water (Figure 1). Single-Effect Cell. A membrane pack consisting of two cation-(C) and two anion-(A) permeable membranes forms a cell of three closed compartments, which is immersed in a vessel of brine (Figure 1). Ions can pass only through the membranes or around the whole assembly. For discussion of principles, the membranes are presumed perfectly 1 Present address, Department of Chemistry, University of Oklahoma, Norman, Okla.
cells can produce a more economical permselective to cation or anion and improduct. I n brackish water demineralpervious to water. The dissolved salt ization, minimum membrane area is the is considered a_ completely dissociated objective, because single-effect cells can 1 to 1 electrolyte. The brine has a provide the desired extent of demineralsalt concentration substantially greater ization. To demineralize sea water than that of the saline water to be puriwith its higher salt content, three or four fied. effects would be needed as a minimum, Because of the osmotic force between even with unlimited membrane area; the brine and the saline water, cation tends to flow from the brine into SI, in practice, additional effects would be " provided to minimize membrane cost. and anion from the brine into S p . For Stagewise coupling of two or more singlethis to occur, gross neutrality for the effect cells (Figure 4) can be employed system requires simultaneous passage in place of multiple-effect cells to imof anions and cations around the asprove process efficiency. sembly from left to right and from right to left, respectively; anions from P to SI;and cations from P to S2. Experimental Compartments SIand SQbecome more Laboratory investigations of a singleconcentrated in salt and P less conceneffect cell in batchwise operation has trated. If the brine bath is large and yielded data which, when interpreted concentrations in SI and Sp are mainby the theory, can be used to establish tained constant by continuous flushing operating parameters for designing a with solution from an external source, continuous flow apparatus in any numthe concentration in P will be reduced ber of effects or stages. to a point dependent only on the brine Excluding the six bolting ears, the and S concentrations. Real membranes outside frame dimensions of the exhave imperfect selectivity, and this will perimental cell were 2 X 4 X l / 4 inch also be a factor determining equilibrium thick, forming a n inner rectangle of concentration. 11/2 >E: 3l/2 inches for exposed memThis demineralization principle is readily adapted to continuous flow operation (Figure 2). Multiple-Effect Cell. By using additional membrane pairs, the elementary driving force can be multiplied, which will increase rate and ultimate extent of demineralization. For example, in Figure 3, adding two series membrane pairs enclosing brine isolated from the outside supply triples the driving force for demineralizing P. The effect is analogous to series connection of dry cells in a battery circuit. I n large scale demineralization, multiplying the driving force can have two important objectives-ultimate or equi-. librium extent of demineralization is Figure 1. In this single-effect cell greater, and for a given degree of dewhich illustrates the osmionic principle, mineralization desired at a given output, C1 and CZ are cation- and AI and A2 the membrane area is less than for single are anion-selective membranes. S is effect. Because the major cost of desaline water being enriched with salt, mineralization is determined by the P is saline water being purified, and 6 amount of membrane area and assois brine ciated assembly costs, multiple-effect VOL. 50, NO. 8
AUGUST 1958
1 18 1
Dernin-eralized Water
Brine-
l
l
Figure 2. Single-effect cells can be adapted to continuous flow operation required in large scale demineralizers
Figure 3. Membrane pairs added to a single-effect cell increase driving force for demineralization somswhat like dry batteries added to an electrical circuit connected in series A. B.
Triple-effect cell General multiple-effect cell with an indefinite number of membrane pairs added to chamber P
To Brine Source Brine
TI -p"+Product Water
--g+Rejact
Stage
Stage
I
Feed
Reject
f Figure 4. Stagewise operation. Greater demineralization or reduced membrane area can b e obtained b y coupling two or more single-effect cells in series. Sfage I i s a large and stage II i s a small single-effect cell
brane (Figure 5). Earlier cells were constructed from all-Monel frames which later were partially replaced by Lucite. Thus, the cell interior could be observed during operation. Because the membranes (Amberplex, Rohm & Haas Co.) tended to swell and buckle after prolonged immersion and thus change the geometry of the various compartments, a central spacer in the form of a Lucite button, '/4 inch in both diameter and thickness, was cemented between each membrane pair.
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These buttons were also held against the exterior membranes by metal straps (Figure 5). During assembly, a gasket cement which dried to semistiff consistency was applied to the clamping area of the membranes. The membranes and the assembly were regularly tested for leakage. The low voltage developed (
