Mechanical Vapor Re-compression System

Distillation is one of a common method of separation of solvent from a mixture in a process industry. As we know in distillation process water or other solvents are separated with addition heat to a the temperature of boiling point of solvent. Like in waste water treatment process water is evaporated though evaporator with the help of steam. Heat requirement for evaporation is high because latent heat needs to be added for phase change of solvent.

Therefore for increasing the efficiency in evaporation industries are widely adopting multiple effect evaporator. In MEE latent heat can be minimized by reusing it 3-4 times by differentiating evaporation temperature.

Multiple effect evaporators are proven and reliable method of reducing thermal energy by 60-70% with marginal increase in power cost. For further reduction of thermal energy mechanical vapor recompressor (MVR) system can be one of the best option, Where complete latent heat of evaporation can be used in the same system.

So let us understand Mechanical vapour recompression system :-

Mechanical vapor recompression (MVR) is an energy recovery process where energy is added to low-pressure vapor by compressing it to higher pressure. The result is a smaller volume of vapor at a higher temperature and pressure, which can be used to do useful work. Typically, the compressed vapor can be used to heat the mother liquor to produce the low pressure vapor.

Sketch of Mechanical Vapor Recompression system

In MVR system evaporated vapors are compressed with the help of external energy say Compressor / Blower / Fan. The pressure of vapor is increased, with the increase in the vapor pressure condensation temperature of the vapor will increase. The higher pressure vapor is used to further evaporate solvent from the mixture by exchanging its latent heat. In MVR system complete latent heat of vapor is utilized. The only energy required for increasing the vapor pressure. 

MVR has potential to reduce thermal energy cost by more than 90% with marginal increase or same power as consumed in 3-5 Stage. Considering 3-5 Deg C of boiling point elevation with the increase in concentration level required to be achieved. 

As in MVR separated vapors are compressed with the help of Fan/Blower/Compressor. This compressed vapor elevate condensation temperature of Vapor and then allow to phase change (condensation) in the heat exchanger by further evaporating solvent from the mixture.

Mechanical vapor re-compression system based on Compressor

Power requirement is depended on the concentration level and boiling point elevation (BPE) of the solvent. If BPE of solvent is high (above 1 bar) Compressor is required to operate Mechanical vapor re-compression system. Therefore, Mechanical vapor re-compression system with higher compression ratio shall not be economical to operate because of higher power in compression,

Blower is generally used for pressure rise from .1 to 1 bar. Lower the BPE lower the cost of evaporation. We may also use Simple fan if the BPE of solvent is lower. 

Mechanical vapor re-compression system based on Fan

Selection of Fan / Blower / Compressor depend on the the BPE required to operate Mechanical vapor compressor system.

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