Multiple effect evaporation - Vapour compression desalination processes

A performance analysis is presented for the vapour compression parallel feed multiple effect evaporation water desalination system. The systems include mechanical (MVC) and thermal (TVC) vapour compression, The system models take into account the dependence of the stream physical properties on temperature and salinity, thermodynamic losses, temperature depression in the vapour stream caused by pressure losses and non-condensable gases, flashing within the effects, and the presence of flashing boxes. The analysis is performed as a function of the brine distribution configuration (parallel or parallel/cross flow), the top brine temperature, the temperature of the brine blowdown, and the temperature difference of the compressed vapour condensate and the brine blowdown. The analysis is focused on variations in the parameters that control the product cost, which includes the specific heat transfer area, the thermal performance ratio, the specific power consumption, the conversion ratio, and the specific flow rate of the cooling water. Results show consistent behaviour with industrial practice, where the thermal performance ratio of the TVC system decreases at higher top brine temperatures, while the specific power consumption of the MVC systems decreases at higher temperatures. Also, the specific heat transfer area for all configurations decreases at higher operating temperatures. The conversion ratio is found to depend on the brine flow configuration and to be independent of the vapour compression mode. For the parallel flow configuration, the conversion ratio decreases with the increase of the operating temperature. On the other hand, the conversion ratio for the parallel/cross flow system decreases with the increase of the brine blowdown temperature. Predictions of both models show good agreement with field data.