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note: To convert pound-moles per hour to kilogram-moles per second, multiply by 1.26 x 10_4.

note: To convert pound-moles per hour to kilogram-moles per second, multiply by 1.26 x 10_4.

Examination of interstage-composition results showed that maximum nC4 composition was achieved in the vapor leaving stage 12 rather than stage 13. Therefore, if the sidestream location were moved up one stage, a somewhat higher purity of nC4 could probably be achieved in that stream. Further improvement in purity of the sidestream as well as the other two products could be achieved by increasing the reflux rate and/or number of stages. Computed condenser and reboiler duties were 268,000 and 418,000 W (914,000 and 1,425,000 Btu/h) respectively.

SR Method for Absorption and Stripping As shown by Friday and Smith (op. cit.), when an attempt is made to apply the BP method to absorption, stripping, or distillation, in which the volatility range of the chemical components in the column is very wide, calculations of stage temperatures from Eq. (13-80) become very sensitive to liquid compositions. This generally causes very oscillatory excursions in temperature from iteration to iteration, making it impossible to obtain convergence. A very successful modification of the BP method for such cases is the sum-rates method, in which new stage temperatures are computed instead from the energy-balance equation. Interstage vapor rates are computed by material balance from new interstage liquid rates that are obtained by multiplying the previous interstage liquid rates by corresponding unnormalized liquid mole-fraction summations computed from the tridiagonal-matrix algorithm. The SR method proceeds by the following steps:

1. Specify N and all values of ztj, Fj, TFj, PFj, P., U, Wj and Q. For an adiabatic operation, all Q, are zero.

2. Provide initial guesses (k = 0) for values of all tear variables Tj and Vj. Temperature guesses are readily obtained by linear interpolation between estimates of the top- and bottom-stage temperatures, taking the top as that of the liquid feed to the top stage and the bottom as that of the vapor feed to the bottom stage. An estimate of the vapor-rate profile is readily obtained by assuming constant molal overflow working up from the bottom in using the specified vapor feed or feeds. Compute corresponding initial values of L, from Eq. (13-74).

3. Same as step 6 of the BP method.

4. Same as step 7 of the BP method.

5. Same as step 8 of the BP method.

6. Compute a new set of values of Lj from the sum-rates equation:

7. Compute a corresponding new set of Vj tear variables from the following total material balance, which is obtained by combining Eq. (13-74) with an overall material balance around the column:

8. Same as step 9 of the BP method.

9. Same as step 11 of the BP method.

10. Normalize values of y,j.

11. Compute a new set of values of the Tj tear variables by solving simultaneously the set of N energy-balance equations (13-72), which are nonlinear in the temperatures that determine the enthalpy values. When linearized by a Newton iterative procedure, a tridiagonal-matrix equation that is solved by the Thomas algorithm is obtained. If we set gj equal to Eq. (13-72), i.e., its residual, the linearized equations to be solved simultaneously are

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