Mechanical Vapour Recompressor (MVR)

Mechanical Vapour Recompression (MVR) is an energy-efficient evaporation technology in which the secondary vapour generated during evaporation is mechanically recompressed by a high-speed centrifugal or positive-displacement compressor and returned as the heating medium to the same evaporator. By recycling the latent heat of evaporation rather than discarding it to a condenser, MVR systems achieve thermal efficiencies far beyond those of conventional multi-effect evaporators — making them particularly suited to applications where steam costs are high or where zero liquid discharge (ZLD) is a regulatory requirement.

In a conventional multi-effect evaporator (MEE), each additional effect recovers heat from the previous stage's vapour at a progressively lower pressure and temperature, typically achieving a steam economy (kg water evaporated per kg steam consumed) of 3–6. In contrast, an MVR system achieves steam economies equivalent to a 10–40 effect MEE using only electrical energy for the compressor, dramatically reducing operating costs in high-evaporation-rate applications. The trade-off is higher capital cost of the compressor versus lower ongoing steam and cooling water consumption.

MVR systems are most economically attractive when the temperature difference required across the heat transfer surface (boiling point elevation) is small — typically in clean or moderately fouling liquors where operating temperatures are moderate. For heavily scaling liquors (high CaSO₄, CaCO₃, or silica concentrations), forced circulation MVR evaporators with anti-scale provisions and regular chemical cleaning cycles are used. MVR is the enabling technology for economically viable Zero Liquid Discharge (ZLD) plants in industries such as power generation, chemicals, textiles, and pharmaceuticals.

Integration of MVR with a crystalliser in a ZLD train enables concentration of the evaporator concentrate to the point of crystallisation, producing a dry salt cake for landfill disposal or recovery. The combination of MVR evaporation (low energy cost, high throughput) with a Forced Circulation Crystalliser achieves the lowest operating cost path to zero liquid discharge for high-volume high-TDS streams.