Multi-Effect Evaporation: Review
Multi-effect evaporation systems are utilized by industries to desalinate seawater. A multi-effect evaporator works on the principle that lowering pressure would result in lower boiling temperature of water. The system involves the use of a series of water boiling containers arranged in a decreasing pressure manner (Sayyaadi & Saffari, 2010). The number of heating containers in a multi-effect evaporation system corresponds to the number of effects in that system. It is only the first container that requires an external source of energy to boil its water. Heating up the water in the first container causes it to evaporate and form steam which is used to heat up the water in the second container. This goes on until the water in the last container in the series is heated up as desired (Sayyaadi & Saffari, 2010).
On the other hand, multi-stage flash distillation (MSF) plant desalinates water using the countercurrent heat exchange principle (Shivayyanamath & Tewari, 2003). The stages in a multi-stage flash distillation plant are arranged in a series. Water is heated up as it flows through the stages. It achieves its almost maximum temperature by the time it gets to the brine heater at the high temperature end. The water is further heated up by the brine heater and flows back through the valves of the stages. Some of the heated water forms steam which condenses on the walls of the stages. The energy of the condensing water is used to heat up the water flowing from the cold end of the plant (Shivayyanamath & Tewari, 2003). If the number of effects in a multi-effect evaporation system is small, then it implies that there are substantial cost savings. This does not happen in multi-stage flash desalination plants (Nafey, Fath & Mabrouk, 2006). However, research shows that the small number of effects lead to low performance ratio and small amount of distillate produced (Sharaf, Nafey & García-Rodríguez, 2011). It would be prudent to save financial costs and produce a large amount of distillate in a multi-effect evaporation system. The end product of any desalination system is the clean distilled water converted from salt water. However, desalination plants have to be planned in such a manner that they yield maximum distillate while reducing the costs involved. The table below shows figures that could be involved in a multi-effect evaporation plant.
|Number of effects||Amount of steam (1kg of steam/1kg of water evaporated)||Running cost|
Figure 1. The table shows the relationships among the number of effects, the steam consumed and cost incurred in a MEE plant.
As the figure depicts, more number of effects require less energy and lower costs than a small number of effects. The design of a MEE plant should balance the number of effects and increase the amount of distillate. This could be achieved by the use of simulation procedures that could help to achieve numbers that are economically viable (Shivayyanamath & Tewari, 2003). For instance, it could require that a small number of effects be adopted to use high energy to produce a large amount of distillate. The economic value of the distillate produced could be compared to the cost of the quantity of energy used to boil water in the plant. If the economic value of the distillate is greater than the energy cost, then the design could be adopted (Marcovecchio, Mussati, Aguirre & Nicolas, 2005).
Marcovecchio, M. G., Mussati, S. F., Aguirre, P. A., & Nicolas, J. (2005). Optimization of hybrid desalination processes including multi stage flash and reverse osmosis systems. Desalination, 182(1), 111-122.
Nafey, A. S., Fath, H. E. S., & Mabrouk, A. A. (2006). Thermo-economic investigation of multi effect evaporation (MEE) and hybrid multi effect evaporation—multi stage flash (MEE-MSF) systems. Desalination, 201(1), 241-254.
Sayyaadi, H., & Saffari, A. (2010). Thermoeconomic optimization of multi effect distillation desalination systems. Applied Energy, 87(4), 1122-1133.
Sharaf, M. A., Nafey, A. S., & García-Rodríguez, L. (2011). Thermo-economic analysis of solar thermal power cycles assisted MED-VC (multi effect distillation-vapor compression) desalination processes. Energy, 36(5), 2753-2764.
Shivayyanamath, S., & Tewari, P. K. (2003). Simulation of start-up characteristics of multi-stage flash desalination plants. Desalination, 155(3), 277-286.