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Heat Transfer
This article presents an in-depth analysis of thermal and membrane desalination economics and outlines a simple procedure for calculating the unit product cost for various desalination processes.
CURRENTLY OVER 12,500 INDUSTRIAL-- scale desalination units, with an average production rate of 22.8 million m3/d, are operating worldwide. Continuous progress in desalination technology makes it a prime, if not the only, candidate for alleviating severe water shortages across the globe. Moreover, desalination costs are competitive with the operation and maintenance costs of long-distance water transport systems.
Desalination costs have been continuously decreasing over the years as a result of advances in system design and operating experience, and the associated reductions in specific unit size and specific power consumption. This article outlines various economic parameters used in calculating the unit product cost and briefly discusses trends in desalination costs over the years.
Desalination processes
The most widely used desalination processes are membrane separation via reverse osmosis (RO), and three types of thermal separation - multistage flash desalination (MSF), multiple-effect evaporation, with thermal vapor compression (MEE-TVC) and without (MEE), and mechanical vapor compression (MVC). The MSF and RO processes dominate the market for both brackish water and seawater desalination, with a total share of more than 90%.
Thermal processes. All three types of thermal desalination systems are equipped with condenser tube bundles. In MSF (Figure 1), these are used to preheat the brine recycle stream. The tube bundles in MEE and MVC (Figures 2 and 3) function as condensers/evaporators, where the heating steam condenses inside the tubes and vapor is formed outside the tubes. The MEE and MVC systems are divided into evaporating effects, while MSF systems are divided into flashing stages.
All of the systems employ a number of large pumping units, including pumps for seawater intake, distillate product, brine blowdown and chemical dosing. The MSF and MEE systems have additional pumps for the cooling seawater. In addition, MSF has pumps for brine recycle.
In MSF and MEE, steam extracted from low- and medium-pressure turbine lines provides the heat necessary for flashing or evaporation. In MSF, the heating steam is routed to the brine beater; in MEE, the heating steam is routed to the first evaporating effect.
The MSF process operates with a top...