Water. desalination + reuse

November/December 2012

Water. Desalination + reuse

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TECHNOLOGY TECHNOLOGY Understanding the potential of low temperature distillation system for desalination _________ Espen Mansfeldt, CEO, Watersolutions AG ___ Editor���s Note: In October 2012, Watersolutions AG introduced the Watersolutions LTD system, a patented thermal process for desalination based on the principle of low temperature distillation. The company���s vision is to offer a new option for desalination ��� one that is simple to install, yet robust and highly efficient with low running and maintenance costs. Layout of a large two-stage LTD plant. In today���s desalInatIon industry, there is a great drive towards lowering energy consumption and increasing energy efficiency. The Low Temperature Distillation (LTD) system, just introduced by Watersolutions AG of Switzerland, is designed to achieve those goals. The LTD system condenses water at low temperature and pressure, using lowgrade waste-heat (50-110��C) derived from thermal processes. The term ���waste heat��� is more an economic descriptor than a technical one. Most thermal processes generate excess heat that, in a warm climate, has no or very limited use. Typically, power plants will use approximately 40% of the energy to produce electricity; the rest has to be discharged. This figure varies: a single-cycle plant uses around 35%; combined-cycle 45%, nuclear 45%, concentrated solar power 20%. In addition, cooling processes (eg, district cooling), incineration, industrial processes (eg, cement, fertilizers etc), mining and geothermal sources provide large amounts of waste heat. In most societies, there is a strong correlation between electricity usage and water needs. Thus from electricity production alone, enough waste heat would be available to desalinate the water required. Current thermal desalination processes (multi-effect distillation (MED) and multistage flash) require steam, albeit at relatively low temperatures, to evaporate water. This reduces the output of a powerplant in terms of the overall electricity production. Typically, the overall electricity equivalent consumption would be 5.8 ��� 11.7 kWh/m3 (source: IDA, MIT presentation 2009). Reverse osmosis (RO) plants do not require steam, but typically use 3.5-4 kWh of electricity per cubic meter (m3) of water produced. An LTD plant does not require any steam, only low grade waste heat available either from cooling water or air (exhaust). The source of this heat is irrelevant to the process; all sources mentioned above could be used. The waste heat needs only to be available in sufficient quantities (6-30 MW or more) and with an ideal gradient of 20 Kelvin between the heat source and recooling source. As low as < 10 Kelvin per stage is possible; the pilot plant at El Gouna in Egypt (see below) operated as low as 3 K. Typically, an LTD plant would use between 0.8 ��� 1.5 kWh (up to 3 with many stages and difficult re-cooling) of electricity per m3 of very clean water (< 10 ppm of dissolved solids) produced. A COsT-EffECTivE sOLuTiON Costs associated with a desalination plant fall into two main categories ��� investment capital costs (Capex) and operating costs (Opex). The investment costs for an LTD plant are competitive for two reasons. First, the investment cost itself is comparable with RO modules, and | 34 | Desalination & Water Reuse | November-December 2012 significant cost-saving occurs when several LTD modules are combined to produce larger volumes. It is also expected that, as more plants are installed, there will be significant economies of scale. Second, an LTD plant has a very high conversion ratio, requiring about 1.5 m3 of seawater to produce 1 m3 of very clean water. (The corresponding number for RO is 2.5-3 m3.) This means that the peripheral investments (water intake, filtration, pretreatment etc), which can cost as much as the plant itself, are much reduced, especially if RO brine is used as feed. Apart from depreciation, the main operating cost is the cost of energy ��� electricity and steam. Since the LTD system utilizes waste heat that is free, the relatively low electricity usage has a big effect on the overall costs. This, combined with less chemical usage (eg, antiscalants) and less maintenance, means that the operating costs are half or less compared to other systems. Typically, the low electricity consumption of an LTD plant makes up some 75-80% of the overall operating costs (excluding depreciation), the rest being manpower, chemicals and parts. Also included in OPEX of desalination plants are the maintenance and replacement costs of parts such as membranes, which become worn over time. With LTD, which has no membranes and no interior pipe bundles, the requirement for maintenance is very low. ��� One should note that the cost is dependent on the final specification of the plant and varies according to: the amount and temperature of waste heat; the sources of heat extraction and recooling; the quality of the water; and the number of stages chosen by the client. And, as stated above, it supposes that the waste heat is free. The expected lifetime of an LTD plant is 25-30 years. This is due to the simplicity of the design, the materials used, and its ability to function at relatively low pressure. AppLiCATiONs Of LTD The LTD technology can either work as a standalone plant or alongside other technologies. In fact, it is an ideal complement to existing technologies. Generally, the LTD process is particularly suitable where the salt content is high, the price of electricity high, part load flexibility is required and/or where a minimum of maintenance is required. In addition, it can be used to treat problematic industrial wastewater from various sources such as produced water, mining, industrial waste etc. It can also accommodate variations in the plant load, running efficiently from 10-110% of plant design capacity. The process is self-adjusting, with the amount of water produced proportional to the amount of waste heat provided. LTD also works efficiently over a broad range of salinity. Because the process is very tolerant to the salinity of the feedwater, it can also handle brine concentrate from RO. As a result, retrofitting an existing RO plant with an LTD system would be an efficient way to increase the plant���s capacity. The brine from an LTD plant can be concentrated close to the saturation level of salt, thus making drying of salt and minerals easier and zero liquid discharge (ZLD) a real opportunity. (Please note that the LTD system is not on its own intended as a ZLD technology. Watersolutions is developing a different type of drying technology that has not yet been launched.) Another feature of the LTD system is that it is modular and scalable. The units are available in two sizes ��� a large module that produces 1,000-2,000 m3/d (depending on the amount of waste heat available and number of cascades) and a medium module with capacity of 500-1,000 m3/d. These units can be combined to scale up production as needed. COmpArisON wiTH mED The company is often asked about the differences between the LTD system and MED. In an MED system, the heat (steam) is brought into the evaporation chamber via extensive pipe bundles, often made from titanium to avoid corrosion, and the preheated feed water is sprayed onto these pipe bundles. The process is generally repeated over several stages, all involving the pipe bundles, and the final remaining steam is then condensed in the condenser where, again, pipe bundles are used to transfer the small amount of remaining heat to the feedwater. With the LTD process, the available waste heat is directly transferred to the feedwater via a simple plate heat-exchanger. It is a simpler, more effective and cheaper way to get the heat directly into the feedwater. The evaporation takes place directly in the evaporation chamber, where the warm feedwater is sprayed into the air. Millions of droplets form a very large surface, thus allowing more water to evaporate. In the evaporation chamber, a relatively small quantity of water (2-3 m3) is circulated very quickly ��� within 2-3 seconds ��� so that significant volumes of water evaporate. The process is self-adjusting so that brine is automatically extracted (conductivity sensor) and new feedwater added as required. The amount of water evaporated is almost linearly proportional to the amount of heat provided. Typically, the water in the evaporation cycle has a salinity of 80,000 ��� 100,000 ppm, but the system has been proven to work up to 200,000 ppm. While seawater generally has a salinity of some 35-40,000 ppm, the LTD system can easily handle much higher levels, and the process will self-adjust to the higher range mentioned above. For the condensation, the LTD process uses the simplest medium available ��� namely the cold distillate itself. The vapor from the evaporation chamber flows into the condenser where the cold condensate is finely sprayed to give a very large surface for the vapor to condense. This is not only a very inexpensive and efficient medium, but also avoids issues regarding cleaning, scaling etc. Again, a relatively small quantity of distillate is circulating fast, and the final distillate is extracted automatically. As with other thermal processes, several stages are possible. The process, as with MED, requires a reduced pressure, and non-condensable gases are extracted. The LTD plant consists of standard parts such as heat-exchangers, pumps, sensors (high quality from well-known suppliers), the unique evaporation chamber and the specially designed condenser. The control system is provided by Watersolutions AG. All the movable and sensitive elements have many years of usage in similar applications. fuLL-sCALE pLANT iN EL GOuNA, EGYpT The first full-scale plant was installed in El Gouna, Egypt, two years ago, following ��� November-December 2012 | Desalination & Water Reuse | 35 |

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