Fresh Water for Arizona by Salt Replacement Desalination

Transcription

1 Fresh Water for Arizona by Salt Replacement Desalination Item Type text; Proceedings Authors Muller, Anthony B. Publisher Arizona-Nevada Academy of Science Journal Hydrology and Water Resources in Arizona and the Southwest Rights Copyright, where appropriate, is held by the author. Download date 04/05/ :36:19 Link to Item

2 FRESH WATER FOR ARIZONA BY SALT REPLACEMENT DESALINATION by Anthony B. Muller* INTRODUCTION The lack of fresh water to meet the requirements of Arizona's agriculture, municipalities, and industry has been a serious constraint to the development of the state. Several methods of water supply augmentation and conservation have traditionally been employed. The electrodialysis desalination facility used to augment the municipal water supply of Buckeye, Arizona, a plant constructed in the mid- 1960's, is leading the way to a modern approach of practical water supply augmentation. Conventional desalination processes have only been able to. aid in such municipal, and industrial, water demand since the marginal cost of agricultural water has been by far exceeded by the product water cost of these processes. Although large bodies of saline groundwater and river water are available in the state these sources have not lent themselves to such treatment because of the cost constraints. The process of salt replacement desalination proposed herein is believed to be able to produce vast quantities of fresh water by desalination which may be used in all aspects of water demand in Arizona, including agriculture. The ability of a method to convert saline to fresh water is not the sole fundamental objective of a practicable desalination process. It is also a fundamental objective for that fresh water to be obtained at the least possible cost to the producer. The elements of cost associated with a desalination process may be grouped under the broad headings of initial investment and subsequent operating costs. The differences in initial investment among various processes of comparable production generally become nbgligible for large -scale plants of long design life when considered on a cost per unit product basis. Fixed operating costs may be treated in a similar manner, leaving variable operating costs as the principal criterion of comparison of various desalination processes. The costs of the energy and /or the raw materials and chemicals required by the process are the major items making up the variable costs of operation (Water desalination: proposals for a costing procedure and related technical and economic considerations; 1965). The energy requirements of a desalination process may be divided into five basic parts (Moody and Kessler, 1971): 1. The minimum thermodynamic energy required to remove the solutes from solution. 2. The increase in energy requirement due to the establishment of concentration boundary layers at the phase separation interface, which occur to some extent in all desalination processes. 3. The excess energy required to produce a practical product flow rate above the infinitesimal rate at which the reversible separation minimum energy requirement is evaluated. *Department of Hydrology and Water Resources, University of Arizona, Tucson, Arizona

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4 separation characteristics across a semipermeable membrane, and the subsequent low- energy (and /or low -material consumption) separation of that replacer by virtue of its characteristics (Muller, 1974). This replacement process is based on the idea of producing agricultural nutrient solutions from saline water by replacing the salts by fertilizer materials (Moody and Kessler, 1971). For water to move from the saline solution, through a semipermeable membrane, into a replacer solution, the osmotic pressure of this solution must be greater than that of the saline solution. The minimum thermodynamic energy required to remove the replacer from solution is, thus, greater than that required to remove corresponding solutes from the saline solution. The difference in the osmotic pressure determines the rate at which the salts in solution are replaced by the replacer chemical in a system with fixed membrane characteristics. The increased osmotic pressure and the corresponding boundary layer effects must be more than compensated by the energy savings in the other aspects of the process energy requirements for the replacement to be justified. The replacer separation method is determined by the characteristics of the replacer chemical employed. Thus, any of a number of replacer -process combinations may be coupled to the actual replacement step in salt replacement desalination. This is demonstrated in Figure 1, where a number of such combinations which show promise as low- energy separation techniques are also presented. The description of a number of these processes is included in this paper. ULTRAFILTRATION To the first approximation, the osmotic pressure of a solution is inversely proportional to the molecular weight of the solute; that is, at equal weight concentrations high molecular weight solutes generally have lower osmotic pressures than do solutes of low molecular weight. Ultrafiltration is the term generally applied to the reverse osmosis separation of such macromolecules and colloids from solution, where the osmotic pressure is usually so small that simple filtration is the principal method of separation. This enables ultrafiltration processes to operate well under the pressures of conventional reverse osmosis and to employ membranes of much higher hydraulic conductivity. To take advantage of these features of ultrafiltration, a macromolecular replacer chemical must be employed which has sufficient osmotic pressure at reasonable concentrations to withdraw water across a membrane from the saline solution to be desalted. The replacer solution would, thus, have an osmotic pressure greater than that of the saline solution, and the low pressure advantage of ultrafiltration would be lost. The ultrafiltration of high osmotic pressure solutions may be accomplished at low pressures by using membranes of low rejection characteristics in a repeated stepwise filtration procedure. Replacer solution concentrated by filtration is returned to the replacement portion of the process to remove more water from the saline solution, making the system conservative with respect to material balance. Such a desalination system, using salt replacement and low -pressure step ultrafiltration, has been theoretically and experimentally examined by Muller (1974). Sucrose (342.5 molecular weight) was found to have sufficient osmotic pressure at low concentrations, and simultaneously have sufficient molecular dimensions to be a suitable replacer chemical. The acid- induced hydrolysis of sucrose was determined not to be significant in the ph range which occurs in the replacer system. The UM10 ultrafiltration membranes produced by Amicon Corporation (Lexington, Massachusetts) have a 10,000 mol. wt. cutoff level and, thus, meet the low rejection requirement for multiple ultrafiltration. This and other UM series membranes were experimentally examined at 2.72 atm (40 psi) 129

5 I/ I/ I/ I/ Ultrafiltration SALT REPLACEMENT I/ I/ ü I fl Osmotic Pressure Alteration Gas Solution lk 1\ Fixed Gel Separation I I Figure 1. Salt Replacement Desalination Processes. -- Any of a number of processes may be joined to the replacement step in a salt replacement method. Of these, ultrafiltration, osmotic pressure alteration, gas solution and fixed gel separation are discussed in this paper. 130

6 in ultrafiltration systems using replacer sucrose solutions with osmotic pressures above that of sea water. Fourteen filtrations at 2.72 atm were found to prepare fresh water from a 1.0 molal solution of sucrose which has an osmotic pressure of 26.5 atm, or 1.4 atm above that of sea water (Muller, 1974). This requires a work of 3.86 x 106 N -m /m3, or an amount comparable to that used in extreme low pressure ranges of reverse osmosis systems (Lonsdale et al., 1970). Further, since only pressures of 2.72 atm must be contained in the system, the initial capital investment for this equipment is far less than that required by the equipment which must contain pressures in the neighborhood of 100 atm in conventional reverse osmosis. The principal advantage to salt replacement and ultrafiltration over conventional reverse osmosis is in the reduction of variable operating costs: The Loeb -Sourirajan type cellulose acetate membranes used in reverse osmosis have hydraulic permeability coefficients (Lp) in the neighborhood of 1.5 x 10-5 cm3 /sec- atm -cm2 ( Lonsdale et al., 1970), while for tne UMii0 membrane L = 1.8 x 10-2 cm3 /sec -etm -cm2 (Muller, 1974). For evaluation, AP = 60 atm (Smith, Morton and K(ein, 1970) and Am = 25.1 atm were used as representative values for reverse osmosis, and AP = 2.72 atm and Am = 3.9 atm (Muller, 1974) for ultrafiltration. Using J = Lp (AP - An) [2] to approximate fresh water product rates, salt replacement and ultrafiltration show output rates 30 -fold above that of reverse osmosis while using approximotiy a total of half the applied pressure difference. Ultrafiltration, thus, greatîy reduces the energy required to produce comparable product flow rates. The lower pressures used in ultrafiltrat on offer a lower pressurization energy loss than in similar reverse osmosis systems. In general, salt replacement and ultrafiltration as a combined desalination process was shown to be able to produce fresh water at a lower initial investment and a much lower operating cost per unit volume of product water than comparable reverse osmosis installations (Muller, 1974). This process is still in the early experimental stages, but Gras demonstrated great potential as a 'large -scale economical desalination met''hod. OSMOTIC PRESSURE ALTERATION Im a salt replacement desalination procedure, the replacer chemical must exhibit high osmotic pressure characteristics in the replacement step while in the separation step low osmotic pressure and corresponding low removal energy would be optimal. Since a single replacer chemical cannot exhibit optimum characteristics for both steps in the process simultaneously, an alteration of the osmotic pressure characteristics of the solution within the system is indicated. Such an alteration requires energy in excess of the amount saved in the reduction of the minimum energy required for separation. This increase in energy requirement must again be more than compensated by the energy savings in the other aspects of the process for the alteration to be justified. An alteration of the molecular weight of the solutes in a solution brings about a corresponding alteration in osmotic pressure. Thus, if molecules of the replacer chemical could be persuaded to join in a one -to -one stoichiometric ratio, the osmotic pressure of the solution may be reduced by approximately one -half. Each subsequent molecule of replacer joining this conglomerate 131

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8 Sea Water \ Low Concentration Low Concentration Replacer I Altered Replacer SYNTHESIZING CONVERTER PUMP (High Concentra High Concentration Replacer Altered Replacer HYDROLYZING CONVERTER REPLACER CELL ULTRAFILTRATION CELL Ì Product Water Figure 2. Osmotic Pressure Alteration. -- The osmotic pressure of the replacer solution is reduced in an enzymatic synthesizing converter before fresh water is removed by low- pressure ultrafiltration. The concentrated replacer is then returned to its original form in an enzymatic or ph control hydrolyzing converter and is returned to the surface of the replacement membrane.

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10 FIXED GEL SEPARATION Plumb and Bridgman (1972) suggest that the ascent of water in plant xylem is controlled by a gradient of chemical activity of water rather than by hydrostatic tension. They suggest this activity gradient is constrained by filamentary monomolecular chains attached to the xylem cell walls which serve the same function as solutes but are held in a particular region of xylem and which resist displacement during hydrodynamic flow. If these filaments consist of ß -1,4- linked glucose residues of molecular weight 160, then the concentrations of such a filamentatious gel in the xylem solutions is about 1 weight percent per atmosphere of syneresis pressure (Plumb and Bridgman, 1972). This syneresic pressure would correspond to osmotic pressure in the presence of a membrane, since they have the same functional dependence to chemical activity. If a layer of such a gel designed to comprise 25 percent or more of a replacer solution is attached to a solid surface adjacent to an osmotic membrane, the system will withdraw water from sea water. The syneresis of gel may be accomplished very near the thermodynamic minimum energy since the filaments were never actually in solution. Such a process may be compared to the "ultrafiltration" of a molecule so large that no membrane is required. The pressure is applied directly to the gel, yielding the fresh water produce without having to accomodate the requirements of a membrane. Salt replacement and fixed gel separation offer a low- energy alternative to conventional direct salt removal desalination, but several technical considerations remain to be solved. The procedure for applying the gel to the solid substrate and the mechanics of effective continuous syneresis are two such considerations which merit further research. CONCLUSIONS Salt replacement desalination is a novel approach to minimizing the costs of saline water conversion. Salt replacement consists of the substitution of solutes in a solution to be desalted by a replacer chemical, and the low energy removal of that replacer chemical. This replacement is justified only when the increase in energy and material operating costs due to the replacement is more than compensated by saving in these factors in other aspects of the desalting process. Salt replacement makes available potential energy and material saving which are not directly applicable to conventional desalination methods. The ultrafiltration of macromolecular replacer chemicals with high flux membranes increases the produce yield rate and reduces corresponding energy requirement, with respect to reverse osmosis. The initial capital investment in such a system is also less than in reverse osmosis since no pressure constraining devices are required. The alteration of the osmotic pressure of the replacer solution within the desalination process is also a procedure by which salt replacement can take advantage of energy savings not available to conventional methods. The key to this approach is the utilization of an easily reversible reaction which synthesizes and breaks down a constituent that has a significant osmotic pressure difference between phases. The variable solubility of gasses and the osmotic pressures they generate in aqueous solutions may also be employed in salt replacement. Such processes are currently limited to the purification of low osmotic solutions. Finally, the unusual process of fixed gel syneresis shows potential as a low energy salt replacement type process, but still requires extensive investigation. 135

11 The general conclusion of this paper is that salt replacement offers a novel approach to the classical problem of preparing fresh, usable water from saline solutions or waters of impaired quality and, thus, merits serious consideration as an approach to the design of desalination processes. A salt replacement process is composed of the actual replacement step and any of a number of conjugate separation steps. Salt replacement desalination, thus, has a wide number of applications and variations. Based on the findings of the limited study of salt replacement desalination to date, it is concluded that such processes have so great a potential that research on these topics cannot be neglected. Such research may lead to a multitude of new practical low energy desalination methods. ACKNOWLEDGEMENT The author wishes to gratefully acknowledge the financial support of the Office of the Vice President for Research, University of Arizona. Without the assistance received through this office, the preliminary research on which this paper is based could never have been realized. REFERENCES CITED Barman, T. E Enzyme Handbook. New York: Springer -Verlag. Chun, M. J., R. H. F. Young and G. K. Anderson Wastewater effluents and surface runoff quality. Water Resources Research Center, Technical Report No. 63, University of Hawaii, Honolulu. Daniels, F., and R. A. Alberty Physical Chemistry (3rd edition). New York: John Wiley & Sons, Inc. Lonsdale, H. K., R. L. Riley, C. E. Milstead, L. D. LaGrange, A. S. Douglas and S. B. Sachs Office of Saline Water Research and Development Progress Report No Moody, C., and J. Kessler An initial investigation into the use of direct osmosis as a means for obtaining agricultural waters from brackish water. Unpublished paper, Department of Physics, University of Arizona, Tucson, June. Muller, A. B Desalination by salt replacement and ultrafiltration. Desalination Research Report No. 1, Department of Hydrology and Water Resources, University of Arizona, Tucson. Murphy, G. W Office of Saline Water Research and Development Progress Report No. 9. Plumb, R. C., and W. B. Bridgman Science, 176, p Smith, J. K., F. Morton and E. Klein Office of Saline Water Research and Development Progress Report No Water desalination: proposals for a costing procedure and related technical and economic considerations United Nations Publication No