Principle, Construction, Working, Uses, Merits and Demerits of Multiple Effect Evaporator and Its Economy

Multiple Effect Evaporator

Principle

It is a type of heat transfer equipment that uses natural convection or forced convection for the transfer of heat. Steam is used in the evaporator to heat up a solution containing a desired product. When the solution is heated, the water in it is converted into vapour, which is condensed. The vaporized concentrated solution is then removed or fed into a second evaporator for a further concentration process. Any solution that is concentrated using a single evaporator is called a single effect evaporator system, and any solution that is concentrated using more than one evaporator is called a multiple effect evaporator system. Multiple-effect evaporators use vapor from one evaporator to feed vapor to the steam chest of another evaporator. The heat from the initial steam is reused in successive effects in such a system.

Construction

Three evaporators, i.e., triple effect evaporator, are used to construct the multiple-effect evaporator shown in Figures. Other aspects of the vertical tube evaporator's construction are unchanged. Using vapor from the first evaporator to heat the second evaporator is illustrated in Figure. As with the 2nd evaporator, the 3rd evaporator is also heated by the vapor from the 2nd evaporator vacuum pump. The last evaporator is connected to a vacuum pump.

Working

An evaporator with three effects (3-stages) is a tube-type forced circulation evaporator in which strong steam is used for the first effect to evaporate the solvent from the feed. In the second effect, the vaporized solvent is used to evaporate the feed at atmospheric pressure. In the third effect, the evaporation of the concentrating feed from the second stage is used to evaporate solvent from the second stage. The evaporated solvent from the third effect is finally condensed with cooling water on the other side of the steam condenser. In all three processes, condensate is collected in condensate receiving tanks. The pure solvent in this condensate can be reused in the subsequent process.

Uses

As the vapor pressure falls, the second effect of a multiple-effect evaporator lowers the boiling point. Multi-effect evaporation refers to this reusing of latent heat from vapor molecules.

Merits

• Operation on a large scale and in continuous mode is possible.
• As compared to single effects, it is highly economic.
• There can be attached about five evaporators.
• Utilizing input energy as efficiently as possible (steam economy)
• Requires less energy to run the entire system.
• Condensing load is reduced, resulting in reduced cooling water requirements.
• Controlling the flow of liquor.
• Easily collect and process foul condensate.

Demerits

It has the disadvantage that it requires a lot of headroom. In a falling film evaporator, the tubes have a greater pressure drop. Temperature sensitivities can be affected by hydrostatic heads located at the bottom of tubes.

Economy of Multiple Effect Evaporator

A multiple effect evaporator's efficiency and capacity are measured. A vaporizer's capacity is measured in kilograms of vaporized water per hour. According to this indicator, the amount of water vaporized is based on the total amount of steam used per kilogram of water vaporized. Approximately 0.8 (<1) is the steam economy of a single-effect evaporator. Compared to a single effect evaporator, the capacity of a n-effect evaporator is n times greater, and the economy is 0.8 n. The cost of pumping liquid from one effect to another is higher because of the interconnecting pipes, valves, and pumps necessary.

It is more economical to use a multiple effect evaporator than a single effect evaporator because the vapors generated in one effect will feed vapors in another effect, and so on. Evaporative capacity and steam consumption decrease as the efficiency of the process increases.

As far as energy costs are concerned, the multiple-effect evaporator may seem to provide the best results because heat is used over and over again and the evaporation is achieved for nothing in the second and subsequent effects. The heating economy comes at a cost, however.

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