Page 104 - Distillation theory

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78 Trajectories of Thermodynamically Reversible Distillation

reflux: to determine approximately minimum reflux number (Koehler, Aguirre,
& Blass, 1991) and minimum entrainer rate (Knapp & Doherty, 1994).
Significance of reversible distillation theory consists in its application for anal-
ysis of evolution of trajectory bundles of real adiabatic distillation at any splits.
Numerous practical applications of this theory concern creation of optimum sep-
aration flowsheets; determination of optimum separation modes, which are close
to the mode of minimum reflux; and thermodynamic improvement of distillation
processes by means of optimum intermediate input and output of heat.

4.2. Essence of Reversible Distillation Process and Its Peculiarities

4.2.1. Essence of Reversible Distillation Process
The process, in which transformation of the system in direct and indirect direc-
tions is being accomplished through continuous series of equilibrium states, is
understood as reversible one. At the reversible process,

S = dQ/T (4.1)
Equation (4.1) concerns not only reversible distillation process, but also any
thermodynamically reversible process. For the distillation,

(4.2)
S dist = S F − S D − S B
dQ is equal to input or output of heat at temperature T in the reboiler, con-
denser, and in intermediate relatively to column height reboilers and condensers.
At the reversible process of distillation,

(4.3)
dQ/T = S F − S D − S B
Decrease of entropy of distillation products compared with entropy of feed is
written in the right side of Eq. (4.3), and increase of entropy of heat sources and
receivers is written to the left. The entropy of separation products is always below
that of feed, and the entropy of heat sources and receivers always increases during
the process of distillation because there is transmission of heat from the sources
with a higher temperature to the receivers with a lower temperature.
Total change of entropy in the incoming and outgoing flows of the column and
in the sources and receivers of heat should be equal to zero [Eq. (4.3)] in the case
of the thermodynamically reversible process of distillation.
In real processes of distillation, total change of entropy is always above zero
because of thermodynamic losses, and here lies the reason of nonreversibility:

dQ/T − S dist + S ir > 0 (4.4)

Growth of the entropy in real processes of distillation in view of nonreversibility
has a number of reasons: (1) nonequilibrium flows of liquid and vapor meet each
other at the plates of the column (it becomes especially apparent in the point