Info

1.9426 = Rm + 1

Interpolation gives B = 1.3647.

Interpolation gives B = 1.3647.

total-reflux calculation. As noted earlier, the xD values should be those at minimum reflux. This inconsistency may reduce the accuracy of the Underwood method, but to be useful a shortcut method must be fast, and it has not been shown that a more rigorous estimation of xD values results in an overall improvement in accuracy. The calculated Rm is 0.9426. The actual reflux assumed is obtained from the specified maximum top vapor rate of 0.022 (kg-mol)/s [175 (lb-mol)/h] and the calculated D of 49.2 (from the Fenske equation).

The Rm = 0.9426, R = 2.557, and N = 10 values are now used with the Gilliland correlation in Fig. 13-41 or Eq. (13-30) to check the initially assumed value of 6.0 for Nm. Equation (13-30) gives Nm = 6.95, which differs considerably from the assumed value.

Repetition of the calculations with Nm = 7.0 gives R = 2.519, Rm = 0.9782, and a calculated check value of Nm = 6.85, which is close enough. The final-product compositions and the a values used are as follows:

Component

a.

DXd

BXb

%D

Xb

C3

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