The non-equilibrium factor and the flashing evaporation rate inside the flash chamber of a multi-stage flash desalination plant

Abstract

A study was undertaken to measure the non-equilibrium factor and to correlate the flashing evaporation rate inside the flash chamber of a multi-stage flash (MSF) desalination plant. A computer code was developed to quantitatively simulate the MSF desalination plant operation and solve the mass, heat and salt balance equations. The simulator was tested against an MSF pilot plant with a four-stage heat recovery section and a two-stage heat rejection section, and then used to technically evaluate the flash chamber performance. For a constant top brine temperature of 112°C, the results indicated that (1) the non-equilibrium factor (1-β) varies between 0.24 to 0.66 (low values of flash chamber effectiveness, β). In order to reach thermodynamic equilibrium, the flashing evaporation process should be enhanced through increasing the brine superheat, flashing surface area, number of active nucleation sites, and brine residence time inside the flash chamber; (2) the average flashing heat flux ranges from 100 to 200 kW/m 2 and increases with the brine superheat and the stage pressure. Similar to the surface nucleate boiling, the flashing heat flux could be correlated as: Q″ = 0.055( T b( av) −T sat ) 3.