J. A. Schufle New Mexico Institute of Mining and Technology Socorro
II
Deionized Water by Electrodialysis
A
great deal of attention is being given to the use of elcctrodialysis in the preparation of potable vater from brackish water because of the interest being shown in t,his area by the U. S. Department of the Interior, Office of Saline Water. Not much attention has been given, however, to the use of electrodialysis in the preparation of deionized water, equivalent in quality to distilled water. Yet this is the area in which the met,hod potentially is most efficient. If we wish to prepare distilled water, the amount of energy required is practically independent of the salt content. of t,he original water. Thus it costs about the same to distill sea water as it does to distill pure Rocky Mountain spring water. The energy cost for converting water to steam is calculated to be: E = 540 cd/g X 1 g/cmQ 3785 emVgd. X 1000 gal. X 1.16 x 10-6 kw hr/cal = 2400 kw hr/lOOO gal.
Even if we recover part of this energy by using the incoming stream t.o cool the condenser, it is estimated that the minimum energy requirements might still he several hundred kilowatt hours per thousand gallons. To arrive at t,he basic minimum energy cost for separating water and salt in a saline solution, it is helpful to consider the separation to be carried out by a process which we might call reverse osmosis. Suppose we have a very large tank (1,000,000 gal. capacity) of sea water, and suppose this tank has one wall consisting of an osmotic membrane. If the tank were also equipped with an imaginary piston which could sweep through a volume of 1000 gal., then we could theoretically exert enough pressure on the piston to cause water to pass out of the tank through the osmotic membrane. This pressure ~ o u l have d to be slightly greater than the osmotic pressure of the saline solution. The osmotic pressure of sea water is about 335 psi. We may then calculate the work done in causing the piston to sweep through a volume of 1000 gal. at 335 psi:
osmotic pressure and the lower the basic minimum energy required to separate solute and water. However, in order to take advantage of this basic minimum energy we must use some method of separation other than distillation. Methods have been developed using membranes through which salt can be caused to move under the impulse of an electric field. I n 1940, Kurt H. Meyer' described the effects of passing an electric current across selective membranes. His suggestions for both the single three-compartment cell and the multiple-cell electrodialysis apparatus are the basis of most of the equipment in use today. The development of such equipment was delayed until the 1950's because suitable selective membranes were not available. Such ion-exchange membrane materials are now commercially available a t a cost of a few dollars per square foot. The Cells in Use Starting in 1957, we built several electrodialysis cells of the type described by Meyer using Permutit cation and anion exchange membranes No. 3142 and No. 3148. It occurred to us that, since the basic minimum energy requirement in such an electrodialysis cell decreases with t,he salt concentration, such a cell might be very efficient for the production of deionized water. We designed a cell (Fig. 1) to produce deionized water a t the same rate as the usual laboratory still, about 2 gal./hr. Thirty compartments were provided, each about 1cm thick and 156 cm2in internal cross sectional area. One can calculate from average ionic mobilities at the voltage gradients used (about 1
Meyer, Kurt H., Helv. Chim. Acta, 23, 795-800 (1940).
w = 335 psi X 1000 gals. X 68,947 dynes/cmz/psi X 3785 cm3/ gal. X 2.78 X I O - l 4 kw hr/erg = 2.4 kw hr/1000 gal.
This hasic minimum energy requirement for removing the wat,er from the salt in sea water is thus roughly a tenth of 1% of the energy required to convert water into steam aud about of the energy required in a multiple effect distillation process. Many years ago van't Hoff pointed out that the osmotic pressure, r, is proportional to the concentrat,ion of the solute, C:
lvo
a =
CRT
Thus the lower the salt concentration the lower the Presented before the Division of Industrial and Engineering Chemistry at the 137th Meeting of the American Chemical Society, Cleveland, Ohio, April 1960.
Volume 38, Number I,
January 196 1
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10 v/cm) that approximately 2 min. ~ o u l dhe required to deionize one compartment one cm thick. The 7vat.er mould remain about 15 minutes in t.he cell at. a flow rate of 2 gal./hr. The thicker cells were designed t.o hold a larger volume of water with the idea t,hat the cell might. act as its own storage tank and eliminate the necessity for an additional t,ank for storage. The cell constructed can indeed be operated in such a manner, as will be described later under operation a t slug
.\ typical plot of current passing through re11 as a function of time for several flow rates a t 450 v is hhown in Figure 2. The 150-ppm feed water filled the nhole cell when the voltage was applied a t time zero in each case. We can see that the depleted stream drops to the equilibrium value within 15 to 30 min. depending on the flow rate.
liil ppm Feed
I ill ml/min IIXI ml/min
: I ml/mln
Flon Flow Flow
Figure 3.
Water is brought into t,he cell at normal line pressure of approximately 75 psi. The Permut,it membrane is a fabric reinforced membrane and no difficulty has been experienced with rupturing of the unsupported membrane at these pressures. The inflowing stream of water, containing 15C-200 ppm total solids, is divided into two streams which flow through alternate cells in series. One stream becomes depleted, the other enriched. The enriched stream also carries an-ay the gas formed a t the electrodes, so that there are just two exit streams similar to an ordinary water still. The enriched stream may be discarded in the ordinary manner by which the effluent stream from a still is discarded. We use a stainless steel cathode and a carbon anode. For supplying the dc voltage we have constructed several transformer-isolated, silicon diode rectifiers supplying from 200 to 450 v dc at up to 200 ma across the cell which measures 40 cm betn.een electrodes (Fig. 1).
Perhaps a more significant record is that shown in Figure 3, where t.he resist,ivity of the depleted stream is plotted against t,ime for various flow rates at 250 v dc. Here two examples of slug flow are given, one in which depleted water was withdrawn a t a rate of 1 1/10 min., another at a rate of 1 1/15 min. The US Pharmacopoeia (11th Rev., 1936,p. 65) defines aqua destillata as water which "shall not contain more than 5 ppm total solids." If the dissolved solids are all ionic material, such xat,er should have a resist.ivity of about 85,000 ohm-cm. The straight horizontal line in Figure 3 represents this value of 85,000 ohm-cm resistivity, and thus the arbitrary standard we chose t,o call "distilled water." The current required by the cell operating a t 250 v dc and a flow rate of 150 ml/min. (2.4 gal/hr) is approximately 50 ma. This is a polver consumption of 12.5 watts. An electric still of the same rated capacity for producing distilled water uses 5000 watts or more. Furthermore a still wastes much energy because its high operatine tem~eraturecauses much heat to be lost to its surroundings, whereas the electrodialysis cell operates at room temperature. The performance of the cell over a prolonged period of operation is recorded in Figure 4. .it points a and 11, the cathode was removed and scoured clean. At point c. 2 N hydrochloric acid was introduced into the cell for 30 min. to remove scale throughout the whole cell. At this writing, the cell has been operating for another 150 hr since Figure 4 mas drawn, and the resistivwc ity of the depleted effluent is now in excess of 100,000 ohm-cm. Operation for 950 hr at 2 ~ a l . / h rflow rate
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has produced 1900 gallons of deinonized water. 3' , mce the feed water contained 150 ppm of solids, at least three pounds of salt have been transported through the membranes in less than one year's operation. The original set of membranes are still in use, and scale removal over the whole cell has been required only once in nearly 1000 hours of operation. Figure 1 shows the cell in operation. Summary
We believe that an elect,rodialysis cell can compete with a still for producing "distilled water" for the following reasons:
(1) The cell produces deionized water equivalent in quality to distilled water at a power consumption rate several hundred times less than that of a still of similar capacity. (2) The cell operates at room temperature. (3) The cell can act as its own storage tank for limited quantities. (4) Scale removal is less of a problem than for a still, since much of the salt removed from the water is carried away in the waste stream. Blueprints describing construction of the cell (Fig. 1) in detail and a wiring diagram for the power supply are availeble from the author on request.
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