Info

note: To convert pound-moles per minute to kilogram-moles per minute, multiply by 0.454; to convert pound-moles to kilogram-moles, multiply by 0.454.

[Am. Inst. Chem. Eng. J., 29, 1017 (1983)], the steep concentration and temperature fronts can be extremely sensitive to small changes in reflux ratio, boil-up rate, product recovery and purity, and feed rate and composition.

Consider azeotropic distillation to dehydrate ethanol with benzene. Initial steady-state conditions are as shown in Fig. 13-108. The overhead vapor is condensed and cooled to 298 K to form two liquid phases that are separated in the decanter. The organic-rich phase is returned to the top tray as reflux together with a portion of the water-rich phase and makeup benzene. The other portion of the water-rich phase is sent to a stripper to recover organic compounds. Ordinarily, vapor from that stripper is condensed and recycled to the decanter, but that coupling is ignored here.

Equations for the decanter are as follows if it is assumed that (1) there are constant holdups in the decanter of both phases in the same ratio as the ratio of the flow rates leaving the decanter, (2) there is a constant decanter temperature, and (3) the two liquid phases in the decanter are in physical equilibrium and each is perfectly mixed.

—— [Md(xi)d] = VNy,,N - L0(x,)0 - Lw(x)w (13-177)

where Md is the total molar holdup of both phases in the decanter and the total composition in the decanter is

Combination of Eqs. (13-177), (13-178), and (13-180) gives

These equations, together with those for the tower, constitute a so-called stiff system. They were solved by Prokopakis and Seider (op. cit.), following a prescribed disturbance, using an adaptive semi-implicit Runge-Kutta integration technique by which VN and the were obtained by integration of the equations for the tower. Then Eq. (13-181) was integrated to give (x,)d, which was used with Eqs.

Mole fraction of light component in liquid

Time, min

Bottoms

Stage 5

Stage 10

Stage 15

Distillate

Reflux

Boli-up

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