June 29, 2010

Mechanical Vapor Re-Compression

You may have seen the term Mechanical vapor compression or re-compression. It is basically the compression of vapors. The main difference is the general compression is used for gases not for vapors while this term is used for compression of vapors.

You know the difference in VAPOR & GAS...I am not going to cover it here.

The term is derived mainly from the compression of steam evaporated from the evaporators. It seems that the need got generated from the large scale desalination plants where very large amount of water is to be evaporated economically. So some brilliant chemical / mechanical engineer (I don't know him) thought about the use of some compression device so that evaporated steam it self can be used for heating purpose. It can be visualized from the diagram below.

The short description of the process is as below.

1. Unheated wastewater is fed to the liquid vapor separation tank (1) and is heated to a boil by the electric heating element (6).

2. Recirculation pump begins circulating heated water through the main heat exchanger (2) and back into the separation tank (1).

3.Steam flows up through the mist pad (8) to inlet side of vapor compressor (4).

4.Compressed steam from vapor compressor (4) is forced through the steam side of main heat exchanger (3) giving up latent heat to the counter flowing water from the separation tank (1).

5.High temperature distillate flows out of the main heat exchanger (3) to feedstock heat exchanger (7) giving up sensible heat to feedstock wastewater.

6.Upon reaching steady state, cold feedstock wastewater is fed at a constant rate through the feedstock heat exchanger (7) to raise temperature before feeding to recirculation loop.

7.Concentrate is periodically discharged based on temperature, conductivity or time from separation tank (1) to concentrate tank (9) through residue pump (10).

Benefits of MVR
The beauty of MVR is that by putting in a fraction of energy into comprssor, we can recover entire latent heat of steam. Thus it can produce a COP of >15. Though it has got many limitations but the economy ratio of 15 (Simply understand it this way - that one MVR evaporator is equal to 15 Multi Effect Evaporators running on successive levels.)

I will try to show one practical example with calculations in the next post as & when I get some more time from my schedule.

Work On it NOW....

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