Page 306 - Distillation theory

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280 Synthesis of Separation Flowsheets

D D
a) b) D c)

F F F

B B B
Figure 8.7. (a) Heat pump with supplementary circulation, (b) the
overhead product as working medium for heat pump, and (c) the
bottom product as working medium for heat pump.

the condenser, and the mechanical work of the compressor for the bringing in of
heat into the reboiler are used in the flowsheets with a heat pump of mechanical
type. Here are many types of “heat pump”, some of which are shown in Fig. 8.7.
Heat can be passed from the condenser to the reboiler with the help of an
auxiliary contour, the working body of which can be steam or hot water (Fig.
8.7a). Vapor flow, going out from the column from above (Fig. 8.7b), or liquid,
going out from the column from below (Fig. 8.7c), are used as working body in the
auxiluary contour in other types of heat pump. Heat pumps of absorption type,
having the auxiliary system absorber-desorber, are more complicated.
The application of “heat pump” leads to additional capital and energy expen-
ditures. Therefore, total economy of expenditures on separation can be achieved
in such a way only if the difference (T reb − T con ) is not large (i.e., at separation
of a narrow-boiling mixture). Examples of application of “heat pump” in indus-
try are columns for separation of ethane and ethylene, propane and propylene,
ethylbenzene and styrene, and isobutane and butane.
In a number of cases, if bubble temperatures of the components of the mixture
under separation are very close to each other and the structure of their molecules is
different,itisprofitabletouseextractivedistillation,evenatseparationofzeotropic
mixtures, to decrease energy and capital expenditures (separation of mixtures of
hydrocarbons of different homologous rows). The economy of expenditures on
separation is being achieved at the expense of the fact that separation of one
narrow-boiling mixture is replaced by separation of two wide-boiling mixtures in
two column at extractive distillation.
While synthesizing separation flowsheets, it is necessary to consider the pos-
sibility of thermodynamic improvement and thermal integration. Therefore, for
each sequence, identified in the process of synthesis, it is necessary to realize pos-
sible thermodynamic improvements and thermal integration of the columns. The
estimation of expenditures on separation is made taking into consideration these
modifications, if these expenditures are smaller than for the sequence without
modifications under consideration. This estimation of expenditures is used while
comparing the sequence under consideration to all other possible sequences.
At such an approach, the total volume of calculations grows considerably. It
can become excessively big at big number of the sequences (big n, big number of