Desalter water

Equipment and Economics A veiy large electrodialysis plant would produce 500 /s of desalted water. A rather typical plant was built in 1993 to process 4700 mVday (54.4 /s). Capital costs for this plant, running on low-salinity brackish feed were 1,210,000 for all the process equipment, including pumps, membranes, instrumentation, and so on. Building and site preparation cost an additional 600,000. The building footprint is 300 itt. For plants above a threshold level of about 40 m Vday, process-equipment costs usually scale at around the 0.7 power, not too different from other process eqiiip-ment. On this basis, process equipment (excluding the ouilding) for a 2000 mVday plant would have a 1993 predicted cost of 665,000. 

Desalting in Israel, initiated in the mid-sixties to solve the regional potable water shortage, would have to be enhanced in the future to supply an additional amout of 100-200 million cubic meters per year of desalted water required for the country s development. 

A peculiar method of desalting water is the osmionic demineralization developed by G. W. Murphy (702,103). In a multicell are circulated side by side a very concentrated salt solution, a moderately concentrated salt solution and a dilute salt solution to be desalted. The arrangement is now such that only Na+ and Cl- ions can move from the brine to the moderately concentrated salt solution, when simultaneously also Na+ and Cl- ions can move from the dilute salt solution to this moderately concentrated solution. In this manner the dilute solution is desalted. 

Depending on the enrichment term (E0) of the membrane, the modulus can be larger or smaller than 1.0. For reverse osmosis E0 is less than 1.0, and the concentration polarization modulus is normally between 1.1 and 1.5 that is, the concentration of salt at the membrane surface is 1.1 to 1.5 times larger than it would be in the absence of concentration polarization. The salt leakage through the membrane and the osmotic pressure that must be overcome to produce a flow of water are increased proportionately. Fortunately, modem reverse osmosis membranes are extremely selective and permeable, and can still produce useful desalted water under these conditions. In other membrane processes, such as pervaporation or ultrafiltration, the concentration polarization modulus may be as large as 5 to 10 or as small as 0.2 to 0.1, and may seriously affect the performance of the membrane. 

Equation (4.9) shows that concentration polarization increases exponentially as the total volume flow Jv through the membrane increases. This is one of the reasons why modem spiral-wound reverse osmosis membrane modules are operated at low pressures. Modem membranes have two to five times the water permeability, at equivalent salt selectivities, of the first-generation cellulose acetate reverse osmosis membranes. If membrane modules containing these new membranes were operated at the same pressures as early cellulose acetate modules, two to five times the desalted water throughput could be achieved with the same... 

Figure 5.19 Skid-mounted reverse osmosis plant able to produce 700 000 gal/day of desalted water. Courtesy of Christ Water Technology Group...

It is important to note that you should consider removing all free water before attempting to size the electrostatic water separation section. The dL factor calculated is only for this electrostatic section. You should therefore make a good estimate of how much water will pass into the electrostatic section and input this value in the Prod + Desalting Water, BPD input block. Inputting all of your production water will seriously err sizing results. Free water should be removed in free water knockout (KO) tanks or vessels upstream. True and needed electrostatic treater sizing may thereby be determined. 

All buffers and solutions were prepared in desalted water with a conductivity below 18.2 mS and a total organic content below 2 ppb (parts per billion) using a ELGA PURELAB Ultra system (ELGA Lab Water, Celle, Germany). 

Osmosis involves the transfer, by a concentration gradient, of a solvent through a membrane into a mixture of solute and solvent. The membrane is almost non-permeable to the solute. In reverse osmosis, transport of solvent in the opposite direction is effected by imposing a pressure, higher than the osmotic pressure, on the feed side. Using a non-porous membrane, reverse osmosis successfully desalts water. 

Desalted water passes through the membranes on both sides of the porous product water carrier. 

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