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Desalination of Seawater and Brackish Water

To date, there are primarily two methods that have been used to desalinate seawater or brackish water, both of which are prohibitively expensive and energy intensive: Distillation (flashing) and Reverse osmosis (filtration).

Distillation (flashing).

People have turned saline or brackish water to steam in boilers or electric generation applications and thereafter condensed the steam to create distilled water (completely deionized) for drinking or industrial water. Flashing is prohibitively expensive because it is prohibitively energy intensive. In addition, given the capital and operating cost of the facilities that are typically used (e.g., electric generation processes), the alternative value of the facilities such as electric generation is generally far higher. In addition, flashing absolutely cannot avoid the latent heat of vaporization of water, which is reported in chemistry textbooks to be 40.7 kJ/mole. Given that there are 18.0 grams/mole of water, the latent heat of vaporization of water is 2,261.11 joules/gram of water. Conversion from joules to watt-hours occurs at the rate 1 joule = 0.000278 watt-hours. Thus, the latent heat of vaporization of water is 2,261.11 joules/gram * 0.000278 watt-hours/joule = 0.6286 watt-hours/gram of water. Given that there are 1000 grams of water in a liter, this implies a latent heat of vaporization of 628.6 watt-hours/liter of water. Given that there are 0.2642 U.S. gallons in one liter, the latent heat of vaporization of water can be expressed 2379 watt-hours/gallon = 2.379 MWh/1000 gallons of water. The present fair market price of electricity varies from $20-80/MWh from time of base to time of peak. Assuming an average price of $50/MWh (5 cents per kilowatt hour), the energy cost of flashing water if it were accomplished at 100 percent efficiency using electricity is $118.95/1000 gallons of water. (This calculation assumes that none of the heat of vaporization is recovered; all is lost. As such, it is a worst case scenario. But it can be worse, in reality. There is an "elevated boiling point" associated with saline water, so there is an additional heat required to get the salt water to the boiling point. The prospects are dim!)

If we express the latent heat of vaporization of water in Btu so that we can consider the price of thermal rather than electrical energy, we use the conversion factor 1 joule = 0.0009478134 Btus. Therefore, the latent heat of vaporization of water is 2.14311 Btu/gram of water. This means that we need 2143.11 Btu/liter of water or 8111.697 Btu/gal of water = 8111697 Btu/1000 gal = 8.112 MMBtu/1000 gal of water. The current market price of natural gas is $4-$8/MMBtu, meaning that if one flashed water at 80 percent efficiency (the approximate efficiency of a modern package boiler) using $5/MMBtu natural gas is $50.7/1000 gallons of water.

It is clear to see why thermal methods based on vaporization have virtually zero prospects for economic success. At present, reverse osmosis (discussed in the next section) argues that it can deliver drinking quality water at $2.00-$5.00/1000 gallons. It is instructive to ask what the implicit price of natural gas or of electricity would have to be to render vaporization economic at say $2.00/1000 gallons. The gas would have to be valued at a paltry $0.197/MMBtu or lower, and the price of electricity would have to be valued at a paltry $0.841/MWh. While natural gas in the Middle East at the wellhead might be valued as low as $0.20/MMBtu or so, the price of electricity will never be that low. It is simply too difficult, inefficient, and costly in terms of capital and operations to make it.

Reverse osmosis (filtration).

Reverse osmosis, abbreviated RO, is the quintessential "Rube Goldberg contraption". It involves creating a pressure differential across a permeable (and very expensive, ephemeral, and perishable) membrane. The required pressure is a direct function of the differential salt concentration across the membrane (brine-side versus purified-side) and the rate of production. Unfortunately, modern "high efficiency" RO membranes are sensitive to halides (e.g., chloride, fluoride, bromide) ions and deteriorate in the presence of halide ions! This is a most unfortunate happenstance! After all, what ions does seawater contain primarily? Halides! Chloride! Salt itself is sodium chloride! This renders salt water purification a particularly difficult and expensive objective for RO (which has several other problems in that application as well). RO works on the premise that the water is forced through the membrane by enough pressure to overcome the natural osmotic pressure, i.e. the pressure that is generated when water tries to go from the "clean" side to dilute the salty side. So theoretically, the lower the salt concentration, the lower the pressure to purify. However, RO cannot concentrate water more than approximately 3 for 1 without requiring a HUGE pressure in a single stage, or deploying a large number of stages in series. As the water becomes less saline as it proceeds through a series of RO stages, each successive stage costs the same as previous stages but is less efficacious. Because RO requires very high pressures, it requires pumping. Pumping is expensive and unreliable by its very nature (pump manufacturers often use pump curves ranging from 30-50% efficiency for pumps sized to applications typical of RO). Pumping and therefore RO requires large energy cost to generate high pressure, has a high operating and maintenance cost, and has low unit availability factor given the short life of membranes and the fact that they tend to scale, coat, and plug. RO plants must be shut down frequently to replace and maintain membranes. Photographs of expended membranes littering the beaches of developing countries attest to the disposal problems with such membranes as well as to the astronomical maintenance cost of such units. (In a case study in Israel in the late 1960s, over a 17-month period there were 216 "failed" membrane units that led to plant shutdowns. That corresponds to an average of 12 units per month or 1 unit shutdown every 2.4 days!) Littering expended membranes on the beach is not an acceptable or attractive solution in most venues. Burying them or burning them is problematic and expensive as well. Apparently expended or halide-damaged membranes have zero recycle value and simply must be thrown on the beach, burned, landfilled, or dealt with using a better disposal method.

Reticle Carbon Capacitive Deionization

Capacitive deionization using Reticle Carbon electrodes is potentially a salvation in the brackish water-seawater desalination industry. Reticle Carbon CDI can be conceived as a multistage serial process of "rougher" stages designed to remove gross quantities of salt from initial inbound brine solutions followed by "finisher" stages designed to purify water. Alternatively we conceive of Reticle Carbon capacitive deionization as a multistage seawater desalination process to enhance existing RO plants and thereafter Reticle capacitive deionization as a serial second stage "finishing stage" in a two or more stage, hybrid, RO-Reticle Carbon capacitive deionization process. Reticle Carbon will have very low operating and maintenance cost, including very low energy consumption, and the only waste created will be concentrated salt water, which can be disposed back into the ocean itself or reinjected into a deep, saline aquifer. In addition, Reticle Carbon works proportionately better for later stages in which rather pure water must be purified all the way to drinking water standard in a single step, e.g., 4000 parts per million TDS. This is most definitely NOT to imply that 4000 parts per million TDS is a standard or a limit. Reticle Carbon CDI can purify water beginning with any level of salinity. Rather, this degree of salinity is an estimate of TDS concentration that can realistically be achieved by a relatively few stages of RO but below which RO simply cannot easily go. Likewise, Reticle CDI would be significantly more efficient as a rougher stage feeding RO finishers, as well. We have proven Reticle Carbon CDI across a rather broad range of TDS ranges. The "optimization" problem hybrid desalination plants face is to determine the optimum level of TDS at which to make the "handoff" from the output of RO output to Reticle Carbon CDI input.

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