Multi-effect Distillation Process Research Articles

Pre-project study of a nuclear desalination demonstration plant for Tan-Tan, Morocco

A pre-project study of a nuclear desalination demonstration plant has been jointly carried out by Morocco and China for Tan-Tan, Morocco under the support of the International Atomic Energy Agency. The plant will produce 8000 m 3 per day of potable water using a 10 MWth nuclear heating reactor developed by China coupled to a high-temperature multi-effect distillation process. This paper presents the outline of the Tan-Tan project, including the main features of the reactor and desalination plant, siting and environmental impact assessment, and economic evaluation.

Coupling of nuclear heating reactor with desalination process

A seawater desalination plant using a nuclear heating reactor (NHR) coupled with the multi-effect distillation (MED) process was developed by the Institute of Nuclear Energy Technology, Tsinghua University, China. The seawater desalination plant was designed to supply potable water demand to some coastal location or island where both fresh water and energy sources are severely lacking. The NHR design possesses intrinsic and passive safety features, which was demonstrated by the NHR-5 experiences. The intermediate circuit and steam circuit were designed as the safety barriers between the NHR and MED desalination systems. With the power range of the heating reactor within 10 to 200 MWt, the desalination plant could provide 8,000 to 160,000 m 3/d of potable water of appropriate quality. The design concept and parameters, safety features, and comparative investigation of coupling schemes are presented in the paper.

Thermal hydraulic performance of a seawater desalination system coupled with a nuclear heating reactor

A physical and mathematical model for the multi-effect distillation process with vertical tube evaporators (VTE—MED) coupled with a nuclear heating reactor (NHR) was described. Results of analysis on thermal hydraulic performances of the NHR—VTE—MED system are provided. Two types of temperature difference distribution schemes (equal temperature difference and in-section equal heat transfer area) were adopted for the thermal hydraulic performance analysis. Heat transfer coefficient, heat transfer area, and production of potable water in every effect were analyzed. The analysis shows that the GOR in the NHR—VTE—MED system for the two types of temperature difference distribution schemes studied can reach 20.

Potential of solar desalination in Israel and in its close vicinity

Recent studies dealing with the potential of Renewable Energy Sources (RES) for desalination along the Mediterranean Coast and in the Middle East choose to use RES to generate electricity first, and then use this electricity to power desalination. The present work eliminates the phase of electricity generation by using solar thermal energy directly for distillation by evaporation. Saving the thermal to mechanical conversion losses allows the proposed Multi Effect Distillation (MED) process to compete economically with Reverse Osmosis (RO) process of significantly lower energy consumption. The new opportunity to revive direct thermal evaporation, arises from a new collector technology developed by Solel Solar Systems, that is coupled to the familiar MED process modified by IDE Technologies to match the solar steam characteristics. Solel has applied its unique technique of selective coating to demonstrate a very efficient solar radiation collection, achieving high temperatures with relatively low installation costs. While the generated steam is not sufficient for efficient generation of electrical power to be used for RO — its quality far exceeds the minimum necessary for the existing methods of steam powered desalination by evaporation. Our analysis shows that a combination of a large number of effects of evaporation, together with high pressure saturated steam available for recycling, yields a dramatic improvement in the production rate of water desalination, accompanied by relatively modest increase in the desalination installation cost. For the specific case analyzed in detail, replacing the commonly used low temperature MED with the new combination, increases the Economic Ratio (ER) from 7 to 16 — a factor of 2.3, while the installation costs grew by only 60%. This implies that the distillate production costs are significantly lower with the new proposed combination. The cost increase is mainly due to the higher costs for the expansion of the desalination system, with relatively low additional costs for producing the high temperature solar steam. A basic assumption, drawn from economic considerations and technological constraints, is that the desalination system would operate continuously, while the solar system, which is limited to daytime operation, would feed a steam generator combined with a storage tank. Therefore utilization of solar energy requires either large and expensive heat storage capacity or fossil fuel backup — a hybrid plant. The effects of storage and fuel cost are presented. The paper refers to three levels of desalination capacity: 1) A small 1000m3/d plant, typical for plants serving small settlements or industries at rural locations, isolated from fresh water and grid power sources. Applying the present model distillate cost for such a solar powered plant along the Red Sea coast is about $1.2/m3 for solar-only plant with large capacity steam storage, and $1.1/m3 for hybrid plant using $0.18/kg diesel oil when solar-steam is not available. Where brackish water are available for mixing, these costs decline approximately by 30%; 2) The medium size 10,000m3/d plant is of the scale actually required for the town of Eilat. Here the Solar-MED plant would produce distillate at $0.92/m3 and by blending with brackish water available on site, the cost would decline to $0.74/m3; 3) On the other extreme we evaluated a large 100,000m3/d plant, on the scale of a national water supply plant. Here the distillate cost is about $0.69/m3 for hybrid plant (including land cost) at an available site close to the southern end of Israel's Mediterranean shore. These preliminary results suggest a competitive distillate cost as compared to grid-powered RO, when electricity cost is about ¢6.5/kWh. To conclude: Solar-powered desalination is conceivable in Israel at a reasonable cost, and has even broader economic potential along the Red Sea and similar sites.

Multi-effect distillation plants: state of the art

The multi-effect distillation (MED) process is the oldest process in desalination. References and patents have existed since 1840, more than 150 years ago. Vertical tubes, horizontal tubes and different types of submerged tubes have been commercialized and were used until 1960 when multi-stage flash (MSF) dominated the desalination market. MSF plants are presently the most widely used and are considered as reliable sources for the production of fresh water from the sea in Middle East countries, in general, and in the Gulf region, in particular. Development of MED in the last few years has brought this process to the point of competing technically and economically with the MSF process. Major features of the MED process are low primary energy consumption, low heat transfer are and high gain ratio. This paper discusses the general features of existing commercial MED plants and associated technical aspects related to steam, condensers, evaporators, pumps and capacity. It also discusses gain ratio, operating temperature, materials of construction, operation and maintenance of these plants, associated problems, and other available information. Attempts to identify areas of development are also presented.

Seawater desalination plant for Southern California

The paper describes Metropolitan's $30 million demonstration program to develop a 75 million-gallon-per-day (mgd) seawater desalination plant coupled to a combined cycle steam turbine coastal power generating plant that produces water at $600 per acre foot. The plant is a multi-effect distillation process incorporating a thirty-effect, vertically stacked evaporator in a concrete pressure vessel using vertical aluminum double-fluted bundles.

Adaptation of the multi-effect distillation (MED) process to yield high purity distillate for utilities, refineries and chemical industry

A unique design of the MED effects (evaporator/condensers) was developed in order to minimize energy losses and space requirements for optimal carry over separation properties as required to achieve high quality distillate. The design of this evaporator is characterized by very low thermal loads, by coaxial and very low velocity vapor flow and a very compact utilization of heat transfer areas and carry-over separator devices. This paper describes the properties of the MED evaporators, the results regarding purity of distillate and minimal energy requirements as achieved by MED commercial plants utilizing low temperature heat sources and by mechanical multi-effect vapor compression (incorporating the same evaporators) using unique water vapor compressors and utilizing mechanical energy. Several examples of recently commissioned plants of the two types (MED and MVC ME) operating to supply distillate at purities of 5 to 1 ppm will be presented including a detail of specific energy consumption and operational costs breakdown when using sea water for high purity distillation.

Multieffect distillation processes

Twenty-three different multieffect distillation systems are considered with detailed calculations for 19 variants and 2 single-column base cases. A new method for balancing columns by varying the feed rate of an additional feed stream is introduced. Schemes are compared on the basis of dimensionless energy use, separation volume per mole of feed, and heat-transfer area per mole of feed. For liquid feeds with excess less volatile component and relative volatility of 4.0 a multieffect system with a stripping column and a balanced fractionation column looked attractive. If energy savings are very important, a third effect (a balanced stripping column) looks attractive. For high-pressure vapor feeds with excess less volatile component a single column with the vapor feed as the energy source in the reboiler should be considered. High-concentration high-pressure vapor feeds can be processed with no energy cost in a high-pressure enriching column at the high pressure connected by forward feed to a low-pressure fractionating column. In all of these applications the use of multieffect distillation can lead to significant energy savings with modest increases in capital investment. Heuristics are presented to rapidly devise a feasible system.

Desalting - prospects for improvement in technology and reduction in costs

Desalting can already be considered as a reliable means of water production or water quality improvement. At the beginning of this decade, more than 2000 land-based desalting units having a combined capacity of more than 7 million cubic meters per day were operating or under construction. Many of these plants were reported to be operating reliably, demonstrating high plant availability factors. However, in most cases, the desalted water costs are still to high, especially for use in agriculture. In the past decade, desalting cost (in prevailing prices) increased, in some cases very drastically, despite improvement in technology. The main reasons for this were, obviously, the steep increase in energy costs and high rate of overall price inflation. In some cases, however, the cost of desalting was not increased in real prices, especially when using processes having relatively low specific energy consumptions. Desalting of brackish water even decreased in real prices due to impressive improvement in membrane processes, mainly reverse osmosis. For seawater desalting, recent developments and especially those now under development related to reverse osmosis membranes and efficient power recovery systems, are expected to reduce significantly the specific investment and overall operating costs. In the field of distillation, one of the more promising desalting options is that of low temperature multieffect distillation (MED) plants coupled to coal or nuclear power plants. A large prototype MED, using cheap alluminum tubing, has been recently built by Israel Desalination Engineering and is scheduled to start operation in the latter part of 1982. Various potential improvements of the MED process and optimized matching of system parameters with considered power plants will be analyzed. The two above discussed desalting options will be evaluated economically by assuming a common set of economic groundrules. The evaluation will be made for two stages of technological developments; current technology and projected technology. Recently made cost evaluations for large capacity plants, applying the two above-mentioned desalting options indicate specific investments in the range oa about $2/m 3 -year and unit water costs in the range of 60 to 70 cents/m 3, for current or near term technology and in the range of 40 to 60 cents/m 3 for projected technology. The two more important cost factors affecting competitiveness of these two options are unit energy and membrane replacement cost. A detailed analysis of these, as well as other cost factors, along with a breakdown of the cost effectiveness of the various projected technological improvements will be given and analyzed.

Present and future energy aspects in VC, MED single and dual purpose desalination plants

The specific energy consumption as well as investment for present Vapour Compression (VC) and Multi-Effect Distillation (MED) processes are demonstrated in this paper. Power consumption for coal dual-purpose stations is 6.5–7.4 kWh/m 3, while for nuclear stations it goes down to 5.1–6.1 kWh/m 3 Improvements in heat transfer and other future modifications may lead to even lower values, i.e., 5.5–6.4 kWh/m 3 for coal and 4.4–5.4 kWh/m 3 in case of nuclear dual-purpose stations. The cases of VC and single-purpose MED plants are discussed as well. Efficient VC units which use waste energy produce water with energy demand of 8 kWh/m 3. The role that capital costs play in the evaluation of plant expenses is discussed too. It is shown that in plants where aluminium heat transfer tubes are installed, the fraction of energy used for aluminium production out of the total investment will remain stable, due to improvements in aluminium manufacturing processes. It is recommended to develop enhanced heat transfer surfaces with inexpensive additional modifications. It is worthwhile to check always the extension of a VC process and prefer it to MED single-purpose plants up to 2 mgd enterprises.

Coupling of standard condensing nuclear power stations to horizontal aluminum tubes multieffect distillation plants

No large nuclear back-pressure turbines are available to day. Standard condensing nuclear turbines could operate continuously with a back-pressure of up to 7″ Hg, exhausting huge amounts of steam at 56°C - 640°c with a loss of electricity production of only 6%–10%. p]The horizontal aluminium tube multieffect distillation process developed by “Israel Desalination Engineering Ltd.” is very suitable for the use of such low-grade heat. A special flash-chamber loop constitutes a positive barrier against any possible contamination being carried over by the steam exhausted from the turbine to the desalination plant. The operation is designed to be flexible so that the power plant can be operated either in conjunction with the desalination plant, or as a single purpose plant. Flow sheets, heat and mass balances have been prepared for eight different combinations of plants. Only standard equipment is being used in the power plant. The desalination plant consists of 6 to 12 parallel double lines, each of them similar to a large prototype now being designed. Water production varies between 50 and 123 MGD and water cost between 90 and 137 ¢/1000 gallons. Costs are based on actual bids.