18-04-2012, 05:11 PM
DESALINATION BY DISTILLATION
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INTRODUCTION
Distillation is the oldest and most commonly used method of desalination.
The world's first land-based desalination plant, a multiple-effect distillation (MED) process plant that had a capacity of 60 m3/day, was installed on Curaçao, Netherlands Antilles, in 1928.
Commercial development of seawater distillation units took place in the late 1950s.
Initially industrial evaporators (such as sugar concentrators) were used for distillation process.
Implementation of Modern Processes such as multistage-flash (MSF), MED, and vapor-compression (VC) have led to the widespread use of distillation to desalinate seawater.
VAPOUR COMPRESSION
The vapor-compression process uses mechanical energy rather than direct heat as a source of thermal energy.
Water vapor is drawn from the evaporation chamber by a compressor and except in the first stage it is condensed on the outsides of tubes in the same chambers(fig2)
The heat of condensation is used to evaporate a film of saline water applied to the insides of the tubes within the evaporation chambers.
units are usually built with capacities of less than 100 m'/day and are often used at resorts and industrial sites.
MEMBRANE DISTILLATION
Membrane distillation is a relatively new process,The process works by using a specialized membrane which will pass water vapor but not liquid water.
Membrane is placed over a moving stream of warm water, and as the water vapor passes through the membrane it is condensed on a second surface which is at a lower temperature than that of the feed-water.
ADVANTAGES
Distillation offers significant savings in operational and maintenance costs compared with other desalination technologies.
In most cases, distillation does not require the addition of chemicals or water softening agents to pre-treat feed water.
Low temperature distillation plants are energy-efficient and cost-effective to operate.
Many plants are fully automated and require a limited number of personnel to operate.