Stagewise Contact

This may be a batch or a continuous operation. Refer to Fig. 10.12. The flowsheet shows the extraction stage. Feed of mass F (if batch) or F mass/tirae (if continuous) contains substances A and C at Xp weight fraction C. Tiis is contacted with mass 5i (or mass/time) of a solvent, principally B, containing weight fraction C, to give the equilibrium extract and raffinate R each measured in mass or mass/time. Solvent recovery then involves separate removal of solvent B from each product stream (not shown). 

The operation can be followed in either of the phase diagrams as shown, if the solvent is pure B = 0), it will be plotted at the B apex, but sometimes it has been recovered from a previous extraction and therefore contains a little A and C as well, as shown by the location of S. Adding 5 to F produces in the extraction stage a mixture A/j which, on settling, forms the equilibrium phases , and joined by the tie line through A/,. A total material balance is 

The quantities of extract and raffinate can be computed by the mixture rule, Eq. (10.1), or by the material balance for C  

Since two insoluble phases must form for an extraction operation, point Af, must lie within the heterogeneous liquid area, as shown. The mininimn amount of solvent is thus found by locating Af, at D, which would then provide an infinitesimal amount of extract at G, and the maximum amount of solvent is found by locating Af, at AT, which provides an infinitesimal amount of raffinate at L. Point L also represents the r finate with the lowest possible C concentration, and if a lower value were required, the recovered solvent S would have to have a lower C concentration. 

Computations for systems of two insoluble liquid pairs, or with a distributed solute which is a solid, are made in exactly the same manner, and Eqs. (10,4) to (10.8) all apply. 

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The internal devices may be gnoaped into two general categories tray-type and pecking-fype. The former provides a stagewise contacting mude whereas the latter provides a counlerearrem mode. In the present section the tray-type devices will be considered. 

Contacting Columns. A tall cylindrical column or tower can be filled with packing for continuous contact of two or more phases or fabricated with a number of trays at fixed distances apart for stagewise contact operation. Such columns, either alone or in series, are commonly specified for separations-in gas absorption, distillation, extraction, and humidification. 

The equilibrium curve is plotted in Fig. 7.3-5 from the terminal points ot each experimentally determined tie line on the triangular diagram.The use of Fig. 7.3-5 in determining either the number of theoretical stages needed (for stagewise contact) or the number of transfer units needed (for packed column contact) will be discussed shortly. 

Stagewise contact with controlled coalescence redispersion cycles Tray column Pulsed sieve column. Pulsed Mixer-Settler-cascade, Extraction tower with controlled cycle Scheibel column, ARD-Extractor, Leisibach column, Mixer-Settler cascade ... 

Stagewise contact. In this category are included those arrangements of equipment where solvent and mixture to be separated are intimately contacted, allowed to approach equilibrium, and separated. The operation may then be repeated and with a variety of flowsheets. 

In this chapter only stagewise contact with single solvents is considered. 

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