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Flash Desalination Processes
DOI 10.1615/hedhme.a.000384
3.22 FLASH EVAPORATION
3.22.2 Flash desalination processes
H. El-Dessouky and H. Ettouney
On an industrial scale the flash desalination process has a multistage configuration for flash evaporation. The main elements of the process include the brine heater and the flashing stages. Other process units include the feed intake, brine disposal lines, deaerator, chemical dosing, pumping station, vacuum ejectors, and the control room. The most common industrial process is the multistage flashing process with brine circulation (MSF). On the other hand, the once through multistage flashing process (MSF-OT) is found on a very limited scale. Also, it should be noted that the single stage flashing system (SSF) combined with a brine heater is not used on industrial scale because of its low performance. However, discussion of the SSF process and development of a mathematical model for it are necessary to have better understanding of the more complex processes found in the multistage flashing systems.
The most characteristic feature of the flash desalination process is that vapor formation takes place within the liquid bulk instead of the surface of hot tubes. The hot brine is allowed to flow freely and flash in a series of chambers; this feature keeps the hot and concentrated brine from the inside or outside surfaces of heating tubes. This is a major advantage over the original and simple concept of thermal evaporation, where submerged tubes of heating steam are used to perform fresh water evaporation. The performance of submerged tubes was far from satisfactory, where salt scale is formed progressively on the outside surface of the tubes. The formed scale has a low thermal conductivity and acts as an insulating layer between the heating steam and the boiling seawater. Consequently, the evaporation rate is drastically reduced and cleaning becomes essential to restore the process efficiency.
Earlier designs of the flash desalination process were developed on commercial scale in the early 1950’s. Silver (1970) modified these earlier designs and devised the MSF process, where the number of flashing stages was much larger than the thermal performance ratio. The development optimized the system heat transfer area and the number of flashing stages. Since the establishment of the process in the late fifties, a huge amount of field experience has been accumulating in process technology, system design, construction practice, operation, and maintenance. Developments addressed several operational problems that include scale formation, foaming, fouling, and corrosion. The experience gained has led to use of inexpensive construction material capable of withstanding the harsh conditions at high seawater salinity. The MSF system does not include moving parts, other than conventional pumps. Construction of the MSF plants is simple and contains a small number of connections, which limits leakage problems and simplifies maintenance work. It is believed that the MSF system has remained the optimum choice for large capacity plants for desalination of seawater for the following reasons:
- The conservative nature of the desalination owner.
- The product is a strategic life-supporting element.
- Extensive experience in construction and operation.
- Process reliability.
- Limited experience, small database, and unknown risks with new technologies.
Since inception, several developments have been achieved in system design and operation. These developments include the following:
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