M.A. Darwish

The heat recovery thermal vapour-compression desalting system: A comparison with other thermal desalination processes

Technical factors affecting the choice of distillation system for desalting water are presented. In particular, the thermal vapour-compression process is compared with the predominant multi-stage flash (MSF) desalting system. It was shown that the conventional multi-effect (ME) system can produce desalted water at a lower cost than the MSF system when both are supplied with steam after its expansion in steam turbines. Mechanical or thermal vapour-compression desalting systems are more cost-effective when compared with directly boiler-operated MSF systems. Thermal analysis of the multi-effect thermo-vapour-compression system is presented with an example.

Thermovapor compression desalters: energy and availability — Analysis of single- and multi-effect systems

Sidem (France) has made some recent advancements in thermal vapor compression (TVC) systems [1,2] by installing four units, each having four effects, 1 migd in capacity with a gain ratio of 8, and another four units of 12 effects each, with 2 migd capacity per unit and a gain ratio close to 17. This increased the interest in the system particularly for plants of low and medium capacity. The new system is characterized by the following: (1) the compression of most of the generated vapor and its usage as a heating medium drastically reduce the primary heat source (boiler) and heat sink (i.e., cooling water and condenser), as compared to conventional single-effect desalting systems; (2) low energy consumption; (3) simple water pretreatment as compared with reverse osmosis (RO) systems, which are the main competitors of mechanical and thermal vapor compression systems in small and medium capacities; (4) low capital and operating costs; and (5) recently developed reliable thermocompressors. Since very little is known about the principles and design of the system, a parametric analysis, using the first and second laws of thermodynamics, was conducted for the TVC system components, e.g., steam ejector, evaporator, condenser, as well as the system as a whole. The analysis pinpoints the deficiencies in the system and the methods of overcoming these deficiencies.

Experimental and theoretical study of a humidification-dehumidification desalting system

Abstract A pilot humidification-dehumidification desalting (HDD) system has been built in Kuwait University. Solar heated saline water is used to humidify air in a direct contact heat exchanger (humidifying column), where air is forced upward while the hot saline water is falling downward in the humidifier. The heated and humidified air is passed to a dehumidifying column through cross connected ducts between the two columns. The water condensed from the air represents the desalted water. A theoretical design procedure for a moderate capacity humidification dehumidification system is presented. The experimental results obtained from the designed and built pilot HDD system are used to check the design data. Good agreement between the experimental and theoretical results was shown.

Thermal analysis of multi-stage flash desalting systems

Abstract In this paper a thermal analysis is made of the MSF desalting system in an attempt to arrive at a better quantitative assessment of design and operation parameters on MSF performance.

Energy consumption by multi-stage flash and reverse osmosis desalters

Abstract Kuwait and most of the Gulf countries, depend mainly on desalted water from the sea for satisfying their fresh water needs. These countries are using the multi-stage flash (MSF) desalting system, as the ‘work horse’ for their water production. This system is less efficient in energy consumption as compared to the reverse osmosis (RO) system. Moreover, large units based on the MSF system have to be combined with steam or gas turbines power plants for better utilization of steam supplied to the MSF units at moderately low temperature and pressure (as compared to steam produced by large steam generators). The value and the cost of the thermal energy supplied to the MSF desalting system depends on the method of supplying this energy. This steam can be supplied directly from a fuel operated boiler or heat recovery steam generator associated with a gas turbine. It can also be supplied from the exhaust of a steam back pressure turbine or bled from condensed extraction steam turbine at a pressure suitable for the desalting process. Any energy comparison should be based on simple criteria, either how much fuel energy is consumed to produce this energy or how much mechanical energy is needed per unit product. The energy consumed in the light of the practice used in most Gulf countries are discussed here. In this study, reference desalting and power plants are used for comparison purposes. This study shows that shifting from MSF desalting system to the RO system can save up to 66% of the fuel energy used to desalt seawater.

Energy consumptions and costs of different desalting systems

Abstract Energy consumptions and costs of desalting systems are among the main parameters affecting the choice of certain desalting system and desalted water final cost. The cost of the consumed energy depends, beside the performance of the plant, on the quality of energy, i.e., either mechanical or thermal, and if thermal at what temperature. The cost depends also on the way this energy is applied (e.g. thermal energy can be directly applied from a boiler or from cogeneration steam turbine, or from heat recovery steam generator of a gas turbine). This paper uses the concept of availability to bring different kinds of energy to a common basis of comparison. The way energy applied was considered in comparing the energy consumptions of different desalting systems, namely, multi stage flash MSF, multi effect boiling MEB, thermovapor compression TVC, mechanical vapor compression MVC and reverse osmosis RO desalting systems. Rational basis for costing thermal and electrical energy were used in comparing the energy cost of different types of exsisting desalting systems.

The multi-effect boiling desalting system and its comparison with the multi-stage flash system

Abstract The basic characteristics of the multi-effect boiling (MEB) desalination system are presented. Recent developments are considered, and a thermodynamic analysis and evaluation of the heat transfer areas required for the MEB system are given. In comparing the heat transfer areas, water pretreatment and salt concentration, pumping power requirement and stability of the MEB and multi-stage flash (MSF) systems, MEB seems to have definite advantages over MSF. However, in the absence of well-proven large capacity MEB units, and in view of the good experience with and reliability of MSF systems, large producers of desalted water favor continued installation of MSF systems.

Developments in the multi-stage flash desalting system

New developments in the design of the multi-stage flash (MSF) desalting system are outlined. The history of changing the flow sheet is given. The main design parameters as well as specifications of the equipment used are outlined.

Co-generation power desalting plants: new outlook with gas turbines

Abstract Kuwait is using only MSF desalting system coupled to steam turbines to desalt seawater. There is shortage in desalting capacity, and 180 MIGD desalting units have to be installed within 5 years. Meanwhile there are not enough operating steam turbines to supply steam to desalting units if MSF units are chosen. At the same time there are peak load gas turbines (GT) that are usually operated for very short times, only 16 h in the year 2001 as example. Also Kuwait is planning to have 400-MW total capacity GT just to be operated at peak load. These GT can be used to operate reverse osmosis (RO) desalting system, when not needed to carry load. Other desalting types can be added, beside the RO system, to produce water more efficiently, energy wise than the presently used method. Forming combined gas/steam cycles by adding steam turbines to these GT increases the installed power capacity, efficiency of power production, and ability to produce more water. This paper presents some variants of combining GT with desalting systems. These variants use standard, well-proven type and reliable equipment, besides being more energy efficient compared to MSF units combined with stearn turbines.

On electric power and desalted water production in Kuwait

Abstract The Ministry of Electricity and Water (MEW) in Kuwait issues annual reports showing the efforts made to satisfy the continuous increasing demands of power and desalted water, status of the operating plants, projects under construction, and future planning. Careful reading of the reports is required for better understanding of power and desalted water production economics, and better planning and utilization of the available resources. The published data in the 2000 MEW statistical books on power and desalted water reveal some of the main characteristics of the co-generation power desalting plants in Kuwait, and raise some concern and comments that are reported in this paper such as: (1) MEW future planning is coping very well with electric power demand, but can face real shortage of fresh water in the immediate near future; (2) Desalting seawater is done using the MSF desalting method. The MSF system is not energy efficient. It consumes about three times the equivalent energy consumed by reverse osmosis (RO), which only consumes mechanical (pumping) energy. (3) There is mismatching between power production (depends on load) and process heat required by the operating MSF desalting units. (4) Demands of electric power and desalted water are continuously increasing, as does the need for installing new power and desalting plants. There is real need to rationalize the public use of water and power. (5) Variation of electric power demands is significant due to power consumed by A/C units. (6) Operation of power plants is at low capacity most of the year, due to part-load operation, except for a few hours at peak demand in summer. This means inefficient use of fuel energy and existing equipment. (7) High cost of generating power and desalted water. (8) Low fuel cost estimation by MEW. These points are discussed in this paper. The paper also introduces a method to allocate fuel energy consumption between desalted water and electric power production and use it to estimate the cost of each product. It also discusses future forecasting for power and water needs, turbine unit size choice, and how to reduce power and desalted water consumption.