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By Tomer Efrat and Hadar Goshen I n a time when gas prices are plummeting across the globe, traditional methods employed in many industries must be reexamined from a techno-economic standpoint. One such example is the water desalination industry, and more specifically, the mechanical vapor compression (MVC) application. Conventionally, an MVC unit utilizes an electric motor to drive its compressor, which is the central component in this desalination method, accounting for 80 to 90 percent of the plant's overall power consumption. The compressor induces the temperature difference required for the evaporation/condensation process, in which seawater is converted to high-quality distillate. The electric motor driving the compressor can be replaced with a gas engine, thus dramatically reducing the operation costs. The gas engine (which is standard industrial equipment) utilizes all types of gas — from petroleum gas to natural gas — in an inner-combustion chamber, to facilitate the mechanical drive to the compressor. The waste heat from the gas engine can be further utilized in the MVC unit to reduce the heat transfer area, which in turn reduces the capital costs of the unit, making the investment all the more worthwhile. These modifications can be done in new plants or as a retrofit to existing plants. In this article, we review the economic implications and the advantages of the innovative solution of a gas-driven MVC unit. Natural Gas In The U.S. Natural gas has served a growing role in the U.S. economy in the last decade, second only to petroleum as the primary source of energy and the primary source of energy when it comes to power generation. 1 Since 2009, the U.S. has been considered the world's largest producer of natural gas, increasing the natural gas availability for local markets. As a result, the U.S. power and industrial markets benefit from some of the lowest natural gas prices in the world. These low prices, together with other benefits of using the natural gas as an energy source, such as its environmental aspects, encouraged a rapid growth in the use of natural gas for a growing number of industries. As a result, the natural gas consumption in the local U.S. market has increased by about 25 percent in the last decade, 2 during which period the U.S. natural gas reserves have climbed by 49 percent. These figures alone indicate the large potential that still exists for the growth of natural gas consumption in the U.S., encouraging developers to introduce more innovative ways to utilize natural gas. MVC Process Description The MVC process is an evaporation-condensation distillation process utilizing a centrifugal compressor to generate the motive energy for distillation. The feed water enters heat exchangers where it is heated by the discharged distilled water and brine, thus recovering process heat. Next, the feed water enters an auxiliary deaerator/ condenser in order to remove non- condensable gases (NCG) from it. The heated and deaerated feed water then flows to the evaporator through spray nozzles, forming continuous, thin water films over the horizontal tubes of the evaporator. Since the suction of the compressor provides a pressure lower than the equilibrium pressure of the brine film on the tubes, part of the brine flashes into vapor. The vapor generated passes through a set of deflectors, louver demister, and mesh demister to remove droplet carryover and maintain the purity of the distillate. The vapor is then compressed by the compressor and discharged into the tubes of the evaporator at a pressure that is now slightly higher than the liquid-vapor equilibrium pressure. The vapor inside the tubes condenses, transferring its latent heat of condensation across the walls of the tubes to the brine flowing on the outside, thus providing the required heat to initially raise the temperature of the brine to its liquid-vapor equilibrium temperature, and then to evaporate part of the brine. The newly created vapor is then drawn out by the compressor. The condensed vapor from the evaporator is collected and pumped out as distillate. The brine and distillate are rejected out of the evaporator by pumps, and on the way out they 24 wateronline.com n Water Innovations Simple Steps To Drive Down The Cost Of Desal Using natural gas to power the mechanical vapor compression unit — the "heart" of conventional desalination and its most energy- intensive process — may prove to be an economic game-changer. The electric motor driving the compressor can be replaced with a gas engine, thus dramatically reducing the operation costs.