Principle of Multiple Effect Evaporator (MEE):
Multiple Effect Evaporator (MEE) Utilizes the vapor produced from one effect (stage) to heat the next effect, thus reusing energy and improving overall efficiency.
Construction:
Effects (Stages): Multiple evaporators connected in series.
Heat Exchangers: In each effect where the liquid is heated.
Condensers: To condense the vapor from each effect.
Feed and Product Tanks: For the initial liquid and concentrated output.
Working:
The feed liquid enters the first effect, where it is heated by steam and partially evaporated.
The vapor from the first effect is used as the heating medium for the second effect.
This process continues through multiple effects, with each subsequent effect operating at a lower pressure and temperature.
The final product is a concentrated liquid, and the vapor is condensed.
Uses:
Concentrating solutions in the sugar, desalination, pulp and paper, and chemical industries.
Merits:
High energy efficiency due to vapor reuse.
Lower operating costs compared to single-effect evaporators.
Effective for large-scale continuous operations.
Demerits:
Complex design and operation.
High initial investment.
Maintenance can be challenging due to multiple stages.
Economy of Multiple Effect Evaporator
Economy:
Defined as the amount of steam required to evaporate a given amount of water.
Steam Economy = kg of water evaporated / kg of steam used.
In a single-effect evaporator, the steam economy is approximately 1.
In multiple effect evaporators, the steam economy improves significantly, often reaching values of 3-6, meaning one kilogram of steam can evaporate 3-6 kilograms of water.
Factors Affecting Economy:
Number of Effects: More effects lead to better economy but increase complexity and cost.
Temperature Difference: Smaller temperature differences between effects reduce heat loss.
Heat Transfer Area: Larger areas improve heat transfer efficiency.
Feed Flow Configuration: Forward feed, backward feed, and mixed feed affect the overall efficiency and economy.