Multi-effect boiling systems from an energy viewpoint

All desalting systems consume energy, either thermal or mechanical or both. In the search for energy-efficient desalting systems, it is clear that the reverse osmosis (RO) desalting system is more efficient than the widely used multi-stage flash (MSF) desalting system. For seawater in the Gulf area, as an example, RO consumes about 5 kWh/m3 of mechanical work, while MSF desalting units consume about 4 kWh/m3 pumping energy, besides thermal energy. The thermal energy input to large MSF units is in the range of 250-300 kJ/kg, and is usually in the form of slightly superheated steam extracted from steam turbines at about 2-3 bar. The equivalent work of this thermal energy is in the range of 17 kWh/m3. The conventional multi-effect boiling (MEB) desalting system uses about half of the MSF pumping energy, and almost the same amount of thermal energy used by the MSF, if both have the same gain ratio. However, a recent trend of using low-temperature MEB (LTME) allows the use of low temperature (in the range of 70°C) steam as heat source, and consequently of low exergy and low equivalent work. This can bring the LTME consumed equivalent mechanical energy close to that consumed by the efficient RO system. The conventional multieffect desalting system was revisited to show that thermal energy input to LTME is low, and can bring the equivalent mechanical energy, and thus the consumed fuel energy, to low values close to those of a seawater RO desalting system.