MVR Evaporation

MVR stands for Mechanical Vapor Recompression and describes a thermal process that typically uses an electric motor driven single stage centrifugal compressor as a heat pump.

To vaporize (1) kg of water approximately 570 kcal are needed (heat of evaporation). Almost always in industrial applications, this heat is provided in the form of steam which is condensed on the outside of the tubes in a heat exchanger. This steam then generates the same amount of water vapor from a liquid such as sea water on the inside of the tubes. These water vapors are very pure, even if evaporated from dirty solutions or salt water, as long as these solutions do not contain any components with boiling temperatures lower than the one of water.

To effect the heat transfer for this evaporation process, the steam that heats the heat exchanger needs to be at a higher temperature and pressure than the vapor being generated. However, since the vapor contains about the same amount of heat as released by the condensed steam, it actually can be used as the heating medium, provided it's pressure and temperature are at the required higher level. This is done with the MVR compressor that aspires all the vapor generated and delivers it to the heating side of the heat exchanger. There it condenses by releasing its latent heat and produces a very clean condensate which for the purpose of water purification constitutes the desired product, while the impurities in the evaporator feed will be more concentrated and discharged, for example as brine in the case of sea water desalination. A graphical explanation is found below:

For sea water desalination MVR evaporation is similar in energy efficiency to R.O. at approximately 10 to 15 Watts / liter while R.O. uses less energy for brackish water and for water of even lesser salinity.

MVR makes water (distillate) at a very high degree of purity which actually is not needed for most of the general uses. Since MVR is very capital cost intensive and of some mechanical complexity, R.O. will be the preferred method for most applications. MVR will be used however in cases where R.O. membranes are not resistant to certain components in the feed (for example chlorine) or if certain components render the membranes inefficient or non-functional (oils, fats, etc.) or in the rare cases where the osmotic pressure is too high.

Also, safe drinking water can be produced as a byproduct from MVR operation when it is being used as a concentrator for certain food and feed products such as milk, fruit juices, coffee extract, stillage, etc.

The Aguayuda system is a modular and very compact system that can be manufactured at relatively low cost with an energy efficiency of approximately 15 Watts / liter and a production rate of maximum 75,000 liters / day. Large systems are available, custom designed.