Multiple-effect distillation

Consider special multiple-effect distillation schemes if the D/B ratio is either very high or very low, or if the relative volatihty is low. 

A variety of desalting technologies has been developed over the last 40 years. Based on their commercial success, they can be classified into major (viz., multistage flash distillation, MSFD multiple-effect distillation, MED vapor compression, VC ED RO) and minor (i.e., freezing, membrane distillation solar humidification) processes. 

The saving of energy, without taking any regard of capital cost, indicates that about 40% saving by way of the extraction and the recompression routes and about 25% for multiple effect distillation. 

Figure 14. Energy requirement and cost of various desalination technologies. Data for multi-stage flash (MSF), multiple effect distillation (MED), reverse osmosis (RO), and two-stage RO (brine concentrator) taken from [57]. More recent data reportedfor a high flux membrane taken from [58].

Understand several approaches to designing energy-efficient distillation trains, including the adjustment of tower pressure, multiple-effect distillation, and heat pumping, vapor recompression, and reboiler flashing. 

Be able to adjust the pressure in distillation colunms to achieve heat integration and to consider the usage of multiple-effect distillation and the usage of compression to achieve designs that are more profitable. 

The thermal technologies consist of multiple-stage-flash (MSF) evaporation, multiple-effect distillation and vapour compression, while the membrane technologies are reverse osmosis (RO) and electro-dialysis (ED). MSF and RO are the most frequently used techniques, and together account for 87% of the world wide desalination activity (Meindersma et al., 2006). 

El-Nashar, Ali M. Multiple Effect Distillation of Seawater Using Solar Energy. New York Nova Science, 2008. Tells the story of the Abu Dhabi, United Arab Emirates, desalinization plant that uses solar energy to distill seawater. 

Energy recovery in a distillation column means, practically speaking, recovaing or reusing heat contained in the column produa streams, whether they are liquid or vapor. A number of schemes have appeared in the literature. The two chief ones involve (1) multiple effect distillation, analogous to multiple effect evaporation, and (2) vapor recompression. But, regardless of the scheme, there are five design factors that must be considered ... 

Let us now look at three types of multiple-effect distillation. 

Figure 10.2.4. (a) Multiple-effect distillation with two columns and feed to both columns, (b) Cryogenic air separation in a Linde doublecolumn device forward feed of liquefied feed air to the bottom column. 

FIGURE 14.1 Global market share of the technologies applied for desalination. MSF, multistage flash MED, multiple effect distillation ED, electrodialysis RO, reverse osmosis. (Data from T. Mezher et al.. Desalination, 266, 263, 2011.)... 

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