One Component Transferred Material Balances

The basic expressions for material balances and their graphical interpretation were presented for any mass-transfer operation in Chap. 5. Here they are adapted to the problems of gas absorption and stripping. 

Similarly the liquid stream consists of L total mol/(area) (time), containing x mole fraction soluble gas, or mole ratio Xy and essentially nonvolatile solvent mol/(area) (time). 

Since the solvent gas and solvent liquid are essentially unchanged in quantity as they pass through the tower, it is convenient to express the material balance in terms of these. A solute balance about the lower part of the tower 

This is the equation of a straight line (the operating line) on X, Y coordinates, of slope Ls/G y which passes through (X, 7,). Substitution of X2 and Tj for X and y shows the line to pass through (X2, Y, as on Fig, 8,5o for an absorber. This line indicates the relation between the liquid and gas concentration at any level in the tower, as at point P. 

The qquilibrium-solubility data for the solute gas in the solvent liquid can also be plotted in terms of these concentration units on the same diagram, as curve MN, for example. Each point on this curve represents the gas concentration in equilibrium with the corresponding liquid at its local concentration and temperature. For an absorber (mass transfer from gas to liquid) the operating line always lies above the equilibrium-solubility curve, while for a stripper (mass transfer from liquid to gas) the line is always below, as in Fig. 8.5 . 

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