Swenson thermal vapor recompression (TVR) evaporators use a thermocompressor to reduce the steam required per pound of evaporation.
To reduce energy consumption, water vapor from an evaporator is entrained and compressed with high pressure steam in a thermocompressor so it can be condensed in the evaporator heat exchanger. The resultant pressure is intermediate to that of the motive steam and the water vapor. A thermocompressor is similar to a steam-jet air ejector used to maintain vacuum in an evaporator.
Only a portion of the vapor from an evaporator can be compressed in a thermocompressor with the remainder condensed in the next-effect heat exchanger or a condenser. A thermocompressor is normally used on a single-effect evaporator or on the first effect of a double- or triple-effect evaporator to reduce energy consumption. As with mechanical recompression, thermal recompression is more applicable to low boiling-point rise liquids and low to moderate differential temperatures in the heat exchanger to minimize the compression ratio.
To illustrate the energy effectiveness of a thermocompressor, compare the steam usage for a double-effect evaporator with that of a double-effect evaporator with a thermocompressor. Motive steam at 85 psig is utilized to compress first-effect vapor from 19″ Hg to 32″ Hg absolute. The forward-feed, double-effect evaporator is used to concentrate a weak sodium sulfate solution. A rising-film evaporator is used for the first effect, and a forced-circulation evaporator is required for the second effect where sodium sulfate crystals precipitate.
|Configuration||lb/hr of 85 psig Steam||lb evap./ lb Steam|
|Double Effect w/ Thermocompressor||8,540||2.90|
The double-effect evaporator with thermal recompression requires 33% less steam than the conventional double effect. In essence, the steam usage for the double effect with thermal recompression is comparable to that of a triple-effect evaporator. The main advantage of thermal recompression is improved steam economy for a moderate capital expenditure which is less than that for an additional effect.