This form of relationship may be written in terms of transfer coefficients, as discussed in Chapter 12, to give:

Relation of HTU to HETP

In a theoretical plate, the mole fraction of the more volatile component in the vapour will increase from y to ye, so that the total mass transfer is G'(ye — y). If the equilibrium curve may be considered linear over the height of column equivalent to a theoretical plate Zt, the logarithmic mean driving force may be used. Thus, referring to Figure 11.61:

= Hog {1 + \W ~ (mG'/L')] + |[1 - (mG'/L')]2 + • • •} (11.166)

Plate Distillation

If the operating and equilibrium lines are parallel, mG'/L' = 1, and:

Thus, the ratio of the height equivalent to a theoretical plate to the height of the transfer unit (Zt/HOG) may be greater or less than unity, according to whether the slope of the operating line is greater or less than that of the equilibrium curve.

Experimental determination of transfer units

There have been a number of reports presented by Furnas and Taylor(74), Duncan et al.(75) and Sawistowski and Smith(76) on the influence of flow parameters and physical properties on the value of the height of a transfer unit. Most of the work has been carried out in small laboratory columns and great care must be exercised if these data are applied to large diameter units. Some general indication of the values of HOG are given in Table 11.7, which gives values obtained by Furnas and Taylor(74) for experiments with

Table 11.7. Values of the height of the transfer unit HOG(74)


Depth of packing (m)

Liquid rate (kg/m2s)

Hog (m)

50 mm Raschig rings

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