1070-4272/01/7406-0933 $25.00 C 2001 MAIK [Nauka/Interperiodica]
Russian Journal of Applied Chemistry, Vol. 74, No. 6, 2001, pp. 933!938. Translated from Zhurnal Prikladnoi Khimii, Vol. 74, No. 6, 2001,
Original Russian Text Copyright + 2001 by Toikka, Aksenova, Kuznetsov.
OF SYSTEMS AND PROCESSES
Comparative Analysis of Open Evaporation and Pervaporation
in the Ternary System Water!Ethanol!Isopropanol
A. M. Toikka, E. L. Aksenova, and Yu. P. Kuznetsov
St. Petersburg State University, St. Petersburg, Russia
Institute of Macromolecular Compounds, Russian Academy of Sciences, St. Petersburg, Russia
Received April 24, 2000; in final form, February 2001
Abstract-A comparative analysis is made of pervaporation and equilibrium open evaporation. The possi-
bilities are considered of using data on the liquid3vapor equilibrium for analyzing the pervaporation process.
Some specific features of the structure of diagrams for pervaporation and open evaporation of a ternary system
are discussed for the example of the water3ethanol3isopropanol system.
The interest in membrane separation processes is
largely associated with technological issues. In par-
ticular, the efficiency of pervaporation3evaporation
through a membrane, compared with distillation, ini-
tiated quite a number of studies aimed to search for
highly selective high-throughput membranes. Theo-
retical investigations in this field include analysis of
the mechanisms of the process and its modeling .
At the same time, such traditional problem of thermo-
dynamics as the topology of diagrams has been little
studied. The present communication discusses some
aspects of the thermodynamic approach and makes
a comparative analysis of pervaporation and distilla-
tion in a ternary system.
Let us compare of the equilibrium open evapora-
tion and the evaporation through a membrane. As in
the case of equilibrium evaporation, the pervaporation
process is described by balance equations of the type
777 = x
(i=1, 2, ..., n),
are the concentrations (mole or
weight fraction) of i-th substance in the feed liq-
uid mixture and in the permeate (vapor); m
the amount of liquid phase (moles or kg); and n is
the number of substances.
Equations (1) allow calculation of the permeate
composition or verification of the relation between
the solution and permeate compositions and the
amount of evaporated solution.
The use of Eqs. (1) is limited to the following:
in most of experiments the solution composition is
maintained constant by means of the flow of the feed
mixture, and the system as a whole is in a steady state.
At constant composition, it is convenient to determine
the separation or enrichment factor and the flux at
a given solution concentration. At the same time, in
solving practical problems, e.g., substance purification
by pervaporation, information is necessary on com-
position changes in the retentate (liquid outflowing
from the membrane unit). A typical process of this
kind is the dehydration of organic solvents , which,
in contrast to conventional rectification, enables sep-
aration of azeotropic mixtures. According to , at
manufacture rates not exceeding 5000 l h
poration is undoubtedly preferable in economical re-
gard, too. In laboratory practice, changes in composi-
tion are possible when installations are used operat-
ing in a nonstationary mode, in the absence of feed
flow. To a shift of composition corresponds a trajec-
tory in an (n 3 1)-dimensional concentration space
described by equations of the type
(i = 1, 2, ..., n ! 1).
It should be noted that relations (1) and (2) are
merely mass balance equations and, strictly speaking,
are not thermodynamic.
Even though the theory of pervaporation relies
upon notions closely related to the theory of liquid3