Multieffect distillation

A.djustingTrocess to Optimi AT. At first glance, there appear to be only three or four utiUty levels (temperatures), and these can be 50°C apart. Different ways to increase the options include using multieffect distillation, which spreads the AT across two or three towers using waste heat for reboil and recovering energy from the condenser. To make these options possible, the pressure in a column may have to be raised or lowered. 

Potable Water RO and NF both play a major role in providing potable water, defined either by the WHO criterion of <1000 ppm total dissolved solids (TDS) or the U.S. EPA limit of 500 ppm TDS. RO is most prominent in the Middle East and on islands where potable-water demand has outstripped natural supply. A plant awaiting startup at Al Jubail, Saudi Arabia produces over 1 mVs of fresh water (see Table 22-17). Small units are found on ships and boats. Seawater RO competes with multistage flash distillation (MSF) and multieffect distillation (MED) (see Sec. 13 Distillation ). It is too expensive to compete with conventional civil supply (canals, pipelines, w ls) in most locations. Low-pressure RO and NF compete with electrodialysis for the desalination of brackish water. The processes overlap economically, but they are sufficiently different so that the requirements of the application often favor one over the others. 

All commercial types of processes, with the exception of freezing, namely, distillation, reverse osmosis and electrodialysis, are being applied in the above units with various kinds of distillation processes being used for seawater desalting. Two of them, horizontal tube multieffect distillation and vapor compression units were developed and manufactured locally by the Israel Desalination Engineering Ltd. Recently, two small RO units with a combined capacity of approx. 100 cu. m/day were also used to desalt seawater. The main aim of these units is to test and demonstrate the feasibility of this new technology. 

Low temperature multieffect distillation, especially if solar or geothermal energy can be applied economically. 

Another new trend is called membrane distillation. This is based on open hydrophobic membranes that enable the passage of water vapor only. The product quality is expected to be better than RO since only water vapor may pass through the membrane. Vapor condensation is allowed on colder surfaces adjacent to the membranes or outside the membrane module, where vapors are pumped out. Another way is to condense the vapor in direct contact with a cold-water stream. The main problem using this technique is the need to evaporate the water. The energy demand for this is around 650kWh/m3. This enormous amount of energy may be reduced when energy reuse is possible, in a similar way to the multieffect distillation... 

Figure 10.6 Schematic view of a membrane distillation design based on multieffect distillation technology.

In this subsection we describe heat pumps, multieffect distillation of binary mixtures, synthesis of multicomponent distillation systems with heat integration, and multieffect distillation for thermally coupled configurations. 

FIG. 13-3 Compl ex distillation operations with two or more columns, (a) Multieffect distillation, (b) SRV distillation. 

Engehen, H.K. and S. Skogestad, Minimum energy diagrams for multieffect distillation arrangements. AIChE Journal, 2005, 51(6) 1714 1725. 

Agrawal, R., Multieffect Distillation for Thermally Coupled Configurations, MChE J., 46, 2211... 

B3. Multieffect distillation or column integration can be done with more than two columns. Use the basic ideas in Figures 11-5 and 11-6 to sketch as many ways of thermally connecting three columns as you can. 

This problem is concerned with balancing tbe two columns, 1 and 2, in the multieffect distillation process of Figure 10.2.4(a) by adjusting the two feed flow rates, and Wy2- Assume that the feed to both the columns is... 

A very significant fraction of desalination is achieved via thermal desalination processes such as multistage flash, multieffect distillation, etc. (Section 10.2.1) (primarily in older plants). Pretreatment processes include filtration, chlorination, deaeration and scale control via acid addition, antisealant treatment, etc. Post-treatment processes include building up the dissolved salt concentration to the level of 50 ppm via blending with brackish ground water or addition of lime, etc. (Howe, 1974), since distilled water is highly corrosive. 

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