Water Online

July 2016

Water Innovations gives Water and Wastewater Engineers and end-users a venue to find project solutions and source valuable product information. We aim to educate the engineering and operations community on important issues and trends.

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exchange heat with the incoming feed water and then flow out. This process can be done in one effect, as well as with two or three subsequent effects. The MVC low-temperature process is suitable for industrial uses, 3 such as boiler feed water, flue gas desulfurization (FGD) make-up water, cooling-tower make-up water, and others. Driving The Compressor With A Natural Gas Engine As previously mentioned, the main innovative feature of this MVC method is driving the MVC unit with a gas engine. A gas engine, as opposed to an electric motor, is an internal combustion engine where the gas is mixed with air, compressed, and burned to yield mechanical power. This power can be converted to electricity (by coupling it to an electric generator) or used as a mechanical drive (not very different from the mechanism in a natural gas vehicle). In the case of MVC, it is not necessary to convert the power into electricity and then back to a mechanical drive. As a result, the conversion/ transmission-related losses are spared, as well as the equipment costs associated with them. As the gas engine rotates at relatively low speeds (1,400 to 1,800 rpm) and the compressor usually rotates at higher speeds (2,500 to 3,600 rpm), the gas engine is connected to the compressor via a gearbox, which increases its rotation speed. The dimensions of the gas engine are fairly similar to those of an electric motor, so it can still be installed on top of the unit. A gas infrastructure is required on-site. The required gas pressure at the inlet to the engine is between 1.5 and 50 pounds per square inch gage (psig), depending on the engine size. A pressure regulator can be an integral part of the system. The gas engine can consume natural gas, biogas, petroleum gas, coal seam gas, and other types of fuel gas. Heat Recovery The typical efficiency of a gas engine is about 40 percent, meaning that about 60 percent of the energy input will not be converted to mechanical energy. Instead, it will be lost as heat energy, where 40 percent is lost to the atmosphere (whether through the exhaust gases or by radiation), and the additional 20 percent is lost to the engine cooling systems (jacket water cooling or oil cooling). Therefore, when considering the use of a gas engine, utilizing its waste heat would be a smart move. In an MVC/gas-engine integration, there are several options for use of the waste heat: 1. Heating the feed water — As mentioned, the MVC incorporates two heat exchangers that recover the heat from the product and brine streams in order to preheat the feed water. In this innovative solution, the waste heat from the gas engine is utilized for the same purpose and contributes to minimizing the heat exchange areas, thus reducing cost. The waste heat can be recovered from the gas engine's oil cooling cycle or the jacket cooling water cycle. 2. Producing steam — The exhaust gases of the gas engine are released at a fairly high temperature of 400 to 500°C (750 to 930°F). This heat source can be utilized to produce steam by employing a heat recovery steam generator (HRSG). The source water for this steam would be the product water of the unit (or service water). The steam can be used to heat the MVC unit at startup, to assist a stripping process (if the feed water contains any constituents that should be removed prior to entry to the MVC unit), or sent to any other industrial processes, thus improving the heat rate of the plant. Carbon Dioxide Recovery An additional benefit from the exhaust gas stems from its chemical composition. The main emission product from natural gas combustion is carbon dioxide (CO 2 ), which can be harnessed to increase its value. When the designation of the distillate from the MVC unit is drinking water (rather than high-quality process water/boiler feed water), a post-treatment stage is necessary. Usually, the post-treatment process involves the dissolution of limestone (calcium carbonate [CaCO 3 ]) into the product water to regain hardness (calcium), which in turn decreases the corrosion potential of the water (calculated by Langelier saturation index [LSI]). When looking at the carbonate system, this dissolution takes place at low pH levels, which are achieved by dosing either acid or CO 2 prior to the limestone reaction chamber. In the case of using a gas engine, the CO 2 can be recovered from its exhaust gases and used in the post-treatment process, thus decreasing greenhouse gas (GHG) emissions, as well as reducing the operational expenses (OPEX) of the plant. Techno-Economical Evaluation A techno-economical evaluation was performed to assess the benefits of this system. In the presented case study, we considered an MVC-1000 unit, a fairly commonly used unit from IDE Technologies. This two-effect unit produces 1,000 m 3 (264 kgal)/day (or 264,000 gpd) of distillate, with a specific power consumption of about 11 kWh/m 3 (of which 9.6 kWh/ m 3 accounts for the compressor). The electricity price was taken as $0.1/kWh and natural gas price as $2.5/MMBtu (1 MMBtu = 1 million BTU [British thermal units]), which is a conservative value. From the wateronline.com n Water Innovations 25 Flow scheme of MVC (mechanical vapor compression) unit DESALINATION

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