If raw brine as feed and electrical power as energy source are available a vapor recompression process will be the logical choice. Improving the Mechanical Vapour Recompression technology in Europe, EVATHERM used a single stage compressor and Titanium tubes for the first time. This technology has now become industry standard.
An evaporation plant with Mechanical Vapor Recompression (MVR) works like an “open heat-pump” (Carnot process), where the vapors are recompressed up to the pressure level of the heating steam. The driving energy for the evaporation process results from the isentropic enthalphy increase of the vapor steam. The vapor condensate and the purge stream as well are used to preheat the feed nearly to the operation temperature of the unit. Due to this intensive heat recovery the make-up steam consumption is nearly zero, only for the start up a small heat quantity is required.
The compressor can be a radial turbo compressor type or industrial blowers (fans) connected serial. The compressors are volumetric rotating machines, which means they work with constant volumetric flow rates on a given speed. Subsequently the steam mass flow varies depending on the suction pressure of the compressor.
The main characteristics of a MVR system are summarized below:
- almost no steam consumption
- no cooling water requirement
- flexible operation
It also has to be mentioned that the turn-down ration of an MVR system is limited to about 35% because of the surge protection system of the compressors and the minimum suction pressure. The thermodynamical design of such a unit has to be done very carefully in order to operate the compressor on its highest efficiency. Very important is the design of the crystallizer as well. The so called “thermical short circuit” should be minimized or entirely be avoided; good experiences have been made with the EVATHERM MSCPR crystallizer with radial inlet nozzle.
