1. Mechanical Vapor Compression
2. Reverse Osmosis
3. A Comparative Analysis of other Desalination Processes 4.1. A brief description of various Desalination Processes 4.2.1. Distillation Processes
22.214.171.124. Multistage Flash Distillation
126.96.36.199. Multieffect Distillation
188.8.131.52. Vapor Compression
4.2.2. Membrane Processes
184.108.40.206. Reverse Osmosis
4.2.3. Solar Distillation/Humidification
4.2. Process Selection
4. A Derivation of the Rate of depletion of Limestone used to make water portable 5.3. Derivation
5.4. Sample Calculation
A Comparative Analysis of other Desalination Processes
In the following chapter we compare and contrast various types of Desalination Processes. There are many methods of desalination currently in practice today. They vary in efficiency, cost of installment, purity of the product, geographical requirements and a variety of other factors which will be further highlighted in the following pages.
3.1. A brief description of various Desalination Processes:
The following section gives an introduction as well as the principle, working, chemical treatments required and advantages & disadvantages of the various desalination processes. 3.1.1. Distillation Processes:
220.127.116.11. Multistage Flash Distillation
Single stage flash evaporators have been used since the early nineteen hundreds when they were used in the Alberger salt process to obtain salt from brine. Multistage flash evaporators however, were first installed about 55 years ago and were usually of small capacity with low thermal efficiencies. In 1956, a four stage, four unit, 9460 m3/d flash plant was installed in Kuwait. By avoiding separate shells for each stage, a great improvement was made in the economics of evaporators. Today, Multi-stage flash distillation plants produce over 85 percent of all desalinated water in the world, despite the fact that Reverse Osmosis plants are the more numerous. The patent for the MSF process was filed by Mr. Silver. It was described as a plant employing flash distillation in which the number of stages is more than twice the performance ratio (pounds of distillate produced/1,000 BTU of heat input) which in some cases was about 3 times the actual value. MSF solved some of the basic problems of the desalination process such as scale formation, which when combined with the ability of these plants to be built in large capacities resulted in these plants being the largest source of desalinated water in the world. By 1984, 67.6% of all desalination plants (6,075,000 m3/day) were operating on the MSF principle. The unit size has also increased as much as 100 fold since 1972.
Multistage flash processes work on the concept that vapors can be produced from any liquid which is at its boiling point by lowering the pressure. This is due to the fact that reducing the pressure decreases the boiling point of water.
The plant has a series of spaces called stages, each containing a heat exchanger and a condensate collector. The series has one cold end and one hot end while the stages in between have intermediate temperatures. The pressure in each stage corresponds to the pressure required to boil water at the respective temperatures. Beyond the hot end there exists a container called the brine heater. When the plant is operating in steady state, feed water at the cold inlet temperature flows through the heat exchangers in the stages and is heated. When it reaches the brine heater it already has nearly the maximum temperature. In the heater, an amount of additional heat is added. After the heater, the water flows through valves back into the stages which have ever lower pressure and temperature. As it flows back through the stages the water is now called...