Desalination Processes

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Desalination can be achieved using a number of techniques. Industrial desalination technologies either use phase change or involve semipermeable membranes to separate the solvent or some solutes. Therefore, desalination techniques may be classified into the following categories: phase change or thermal processes and membrane or single-phase processes.

All processes require a chemical pre-treatment of raw seawater to avoid scaling, foaming, corrosion, biological growth, and fouling and also require a chemical post-treatment.

In Table 8.1, the most important technologies in use are listed. In the phase change or thermal processes, the distillation of seawater is achieved by utilizing a thermal energy source. The thermal energy may be obtained from a conventional

Table 8.1 Desalination Processes

Phase change processes

Membrane processes

1. Multi-stage flash (MSF)

1. Reverse osmosis (RO)

2. Multiple effect boiling (MEB)

RO without energy recovery

3. Vapor compression (VC)

RO with energy recovery (ER-RO)

4. Freezing

2. Electrodialysis (ED)

5. Humidification-dehumidification

6. Solar stills

Conventional stills

Special stills

Cascaded-type solar stills

Wick-type stills

Multiple-wick-type stills

fossil fuel source, nuclear energy, or a non-conventional solar energy source or geothermal energy. In the membrane processes, electricity is used for either driving high-pressure pumps or ionization of salts contained in the seawater.

Commercial desalination processes based on thermal energy are multi-stage flash (MSF) distillation, multiple-effect boiling (MEB), and vapor compression (VC), which could be thermal vapor compression (TVC) or mechanical vapor compression (MVC). MSF and MEB processes consist of a set of stages at successively decreasing temperature and pressure. The MSF process is based on the generation of vapor from seawater or brine due to a sudden pressure reduction when seawater enters an evacuated chamber. The process is repeated stage by stage at successively decreasing pressure. This process requires an external steam supply, normally at a temperature around 100°C. The maximum temperature is limited by the salt concentration to avoid scaling, and this maximum limits the performance of the process. In MEB, vapors are generated through the absorption of thermal energy by the seawater. The steam generated in one stage or effect can heat the salt solution in the next stage because the next stage is at lower temperature and pressure. The performance of the MEB and MSF processes is proportional to the number of stages or effects. MEB plants normally use an external steam supply at a temperature of about 70°C. In TVC and MVC, after the initial vapor is generated from the saline solution, it is thermally or mechanically compressed to generate additional production. More details about these processes are given in Section 8.4.

Not only distillation processes but also freezing and humidification-dehumidification processes involve phase change. The conversion of saline water to freshwater by freezing has always existed in nature and has been known to humankind for thousands of years. In desalination of water by freezing, freshwater is removed and leaves behind a concentrated brine. It is a separation process related to the solid-liquid phase change phenomenon. When the temperature of saline water is reduced to its freezing point, which is a function of salinity, ice crystals of pure water are formed within the salt solution. These ice crystals can be mechanically separated from the concentrated solution, washed, and re-melted to obtain pure water. Therefore the basic energy input for this method is for the refrigeration system (Tleimat, 1980). The humidification-dehumidification method also uses a refrigeration system, but the principle of operation is different. The humidification-dehumidification process is based on the fact that air can be mixed with large quantities of water vapor. Additionally, the vapor-carrying capability of air increases with temperature (Parekh et al., 2003). In this process, seawater is added into an air stream to increase its humidity. Then this humid air is directed to a cool coil, on the surface of which water vapor contained in the air is condensed and collected as freshwater. These processes, however, exhibit technical problems that limit their industrial development. Because these technologies have not yet industrially matured, they are not described in this chapter.

The other category of industrial desalination processes involves not phase change but membranes. These are reverse osmosis (RO) and electrodialysis (ED). The first one requires electricity or shaft power to drive the pump that increases the pressure of the saline solution to that required. The required pressure depends on the salt concentration of the resource of saline solution, and it is normally around 70 bar for seawater desalination.

ED also requires electricity for the ionization of water, which is cleaned by using suitable membranes located at the two oppositively charged electrodes. Both RO and ED are used for brackish water desalination, but only RO competes with distillation processes in seawater desalination. The dominant processes are MSF and RO, which account for 44% and 42% of worldwide capacity, respectively (Garcia-Rodriguez, 2003). The MSF process represents more than 93% of the thermal process production, whereas the RO process represents more than 88% of membrane processes production (El-Dessouky and Ettouney, 2000). The membrane processes are described in more detail in Section 8.4.

Solar energy can be used for seawater desalination by producing either the thermal energy required to drive the phase change processes or the electricity required to drive the membrane processes. Solar desalination systems are thus classified into two categories: direct and indirect collection systems. As their name implies, direct collection systems use solar energy to produce distillate directly in the solar collector, whereas in indirect collection systems, two subsystems are employed (one for solar energy collection and one for desalination). Conventional desalination systems are similar to solar energy systems, since the same type of equipment is applied. The prime difference is that, in the former, either a conventional boiler is used to provide the required heat or public mains electricity is used to provide the required electric power; whereas in the latter, solar energy is applied. The most promising and applicable renewable energy system (RES) desalination combinations are shown in Table 8.2. These are obtained from a survey conducted under a European research project (THERMIE Program, 1998).

Over the last two decades, numerous desalination systems utilizing renewable energy have been constructed. Almost all of these systems have been built

Table 8.2 RES Desalination Combinations

RES technology

Feedwater salinity

Desalination technology

Solar thermal

Seawater Seawater

Multiple-effect boiling (MEB) Multi-stage flash (MSF)

Photovoltaics

Seawater Brackish water Brackish water

Reverse osmosis (RO) Reverse osmosis (RO) Electrodialysis (ED)

Wind energy

Seawater Brackish water Seawater

Reverse osmosis (RO) Reverse osmosis (RO) Mechanical vapor compression (MVC)

Geothermal

Seawater

Multiple-effect boiling (MEB)

as research or demonstration projects and are consequently of a small capacity. It is not known how many of these plants still exist, but it is likely that only some remain in operation. The lessons learned, hopefully, have been passed on and are reflected in the plants currently being built and tested. A list of installed desalination plants operated with renewable energy sources is given by Tzen and Morris (2003).

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Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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Responses

  • Muhammad
    What phenomena during brine freeze desalination?
    8 years ago

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