Multiple effect evaporation - Vapour compression desalination processes

A performance analysis is presented for the vapour compression parallel fee d multiple effect evaporation water desalination system. The systems includ e mechanical (MVC) and thermal (TVC) vapour compression, The system models take into account the dependence of the stream physical properties on tempe rature and salinity, thermodynamic losses, temperature depression in the va pour stream caused by pressure losses and non-condensable gases, flashing w ithin the effects, and the presence of flashing boxes. The analysis is perf ormed as a function of the brine distribution configuration (parallel or pa rallel/cross flow), the top brine temperature, the temperature of the brine blowdown, and the temperature difference of the compressed vapour condensa te and the brine blowdown. The analysis is focused on variations in the par ameters that control the product cost, which includes the specific heat tra nsfer area, the thermal performance ratio, the specific power consumption, the conversion ratio, and the specific flow rate of the cooling water. Resu lts show consistent behaviour with industrial practice, where the thermal p erformance ratio of the TVC system decreases at higher top brine temperatur es, while the specific power consumption of the MVC systems decreases at hi gher temperatures. Also, the specific heat transfer area for all configurat ions decreases at higher operating temperatures. The conversion ratio is fo und to depend on the brine flow configuration and to be independent of the vapour compression mode. For the parallel flow configuration, the conversio n ratio decreases with the increase of the operating temperature. On the ot her hand, the conversion ratio for the parallel/cross flow system decreases with the increase of the brine blowdown temperature. Predictions of both m odels show good agreement with field data.