Feed water arrangements in a multi-effect desalting system

A multi-effect boiling (MEB) desalting system with unit capacity up to 5 MIGD becomes a strong competitor to the multi-stage flash (MSF) desalting system due to its low specific energy consumption and the low temperature steam required to operate the system. A considerable number of units have been installed in the Arabian Gulf area recently. There are many flow sheet variants for the MEB desalting system. Each variant suits certain design objectives. The schemes of the feed water flow to the effects and implementation of feed water heaters and flash boxes are among the significant differences between these flow sheets that have major influences on the system performance and the adopted analysis to evaluate the system. This paper outlines the commonly used feed water arrangements in multi-effect desalting systems, e.g. forward, backward, parallel, and mixed feed. For each flow sheet arrangement, the thermodynamic analysis used is presented. These analyses determine the temperature and salinity profiles of the system, the amount of vapor generated by boiling and by flashing in each effect, the required heat transfer areas for the effects and feed heaters, the gain and recovery ratios, and cooling water to distillate ratio. The analytical results obtained in this work are compared with several practical multi-effect desalting systems of typical capacity and number of effects. This comparison illustrates the logic behind choosing each flow sheet arrangement.