Examples of separation in single-entry separators

Example 2.2.2 In the flash expansion desalination process (also called the flash vaporization process), cold sea water, heated to a temperature of 121 °C at the corresponding saturation pressure in a preheater, is allowed to enter a flash chamber where the temperature is 38 °C and the pressure is much lower at the corresponding saturation pressure. This produces some pure water vapor and a concentrated brine. In practical processes, the vapor mass flow rate produced can be at most 20% (Silver, 1966) of the feed brine mass flow rate. If the brine feed has 3.5 wt% salt, describe the separation achieved with the separation factors, extent of separation, distribution ratio and equilibrium ratio for two vapor flow rates expressed as a percentage of the feed brine mass flow rate (a) 20 wt%, (b) 10 wt%. 

Solution Basis 100 g/s of feed brine flow rate = w,/, species 1 is water, species 2 is NaCI, so 

The index f, as per expression (2.2.14) is such that it can have a maximum value of 1 and a minimum value of 0. A variety of conditions can lead to the zero value the reader can easily verify this. An aspect of f that should be noted is that it is invariant to a permutation in the subscripts i or j in Yy. 

Since f combines the separation factor as well as the component cut, it is a composite index. Another such composite index would be the distribution ratio fe i defined as 

Obviously, a flash desalinator is an asymmetric single-entry separator. We have 

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