subject: Multiple Effect Evaporation Systems For Water Conservation [print this page] Multiple Effect Evaporation Systems For Water Conservation
They find their use in concentration of liquids through multiple or repeated use of higher temperature vapours originating from a single point application of live steam which is generally dry and saturated. Norbert Rillieux is credited with a patent for the first triple effect evaporator which was installed in a Louisiana sugar factory in 1843. It is incredible that Rillieux had such an advanced plant working over 100 years ago.
Simply put, this concept theoretically means from a unit mass of low pressure steam, one can evaporate almost the same amount of vapour from a solution; the vapour emanating from the solution is used to further evaporate the solution in another vessel and so on infinitely. Practically, this is not possible due to changes in the physical properties of the solution that gets progressively concentrated leading to lower heat transfer rates. Thus, a triple effect evaporation system (that is, three evaporators in series connected to one another) can generate about 2.5 to 2.7 kilograms of vapour from one kilogram of steam. In other words, the solution gets concentrated in the same proportion, more or less.
In the current world scenario of depleting ground water sources, it is inevitable that water is conserved to the fullest extent. In spite of so many regulations, in almost all countries one can find people throwing away water wastefully, never bothering to reduce unwanted and unnecessary consumption or to think of recycling. The industries are the worst culprits in this regard. Many process and manufacturing industries consume gallons and gallons of water and let out much more waste water than permissible. The quantity let out is many times more because the erring industries 'dilute' the effluents before letting out to show that they comply with environmental regulations.
Environmental protection authorities are becoming increasingly intolerant, now-a-days, against polluting industries, thanks to the intervention of courts of law and environmental activists. However, a lot needs to be done, especially, by the erring industrialists themselves. Some of the progressive and responsible ones have seen the utility and benefit of pollution control and water conservation by installing multiple effect evaporators.
Evaporators are simple to construct and do not cost a great deal of money. They are rugged and robust. They are available in various designs like short-tube evaporators, long-tube evaporators, natural circulation evaporators, forced circulation evaporators, climbing film evaporators, falling film evaporators etc. Several engineers have contributed to our wealth of knowledge of these systems, notable being Robert, Kestner, Rillieux etc.
The material of construction used varies from cast steel to even titanium. The choice depends upon the properties and nature of the liquid to be concentrated. A question arises - is the concentrated syrup not more dangerous the dilute effluent? The answer would be that by means of multiple evaporation, we are seeking water recycling and re-use. The concentrated syrup should be dried into powder form or it should be burnt (if it is carbonaceous) in boilers like it is being done with spent wash generated from ethanol distilleries in China and India.
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