Page 305 - Distillation theory

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8.3 Thermodinamically Improved and Thermally Integrated Separation 279

N
2

3

Figure 8.6. Double column for air separation. 1, rectifying section
2 of high pressure; 2, stripping section of low pressure; 3, rectifying
O 2 section of low pressure.

1
N ,O 2
2

8.3.2. Thermally Integrated Separation Flowsheets
Another way consists of organization of heat exchange between condenser of one
column and reboiler of another column. This way decreases the amount of heat
received from heat carriers and/or given to refrigerants. The application of this
method usually requires the use of different pressures in the columns. For the
purpose that the temperature of condenser of one column would be higher than
the temperature of reboiler of the second column, the pressure in the first column
should be higher than in the second one. This method is used also at separation of
binary mixtures, for example, in low-temperature units of air separation (Fig. 8.6)
(Baldus et al., 1983). In this case the first column, working at higher pressure, is
used as prefractionator, products of which are concentrated nitrogen and oxygen,
and the second column serves to obtain nitrogen and oxygen of the required purity.
Reboiler is absent in the first column because its feeding is brought in gas phase;
in the second column, condenser is absent because distillate of the first column is
used there as a reflux. Thermodynamic efficiency of units of air separation is very
high – 18% (Haselden, 1958).
At multicomponent distillation change of pressures in the columns to ensure
heat exchange between their condensers and reboilers leads to the increase of total
difference of temperatures between the hottest reboiler and the coldest condenser.
This increase is limited by temperatures of available heat carriers and refrigerants
and in a number of cases also by thermolability of the mixture.
The application of thermally integrated separation flowsheets can give consid-
erable economy of energy expenditures, but runs across the mentioned limitation.
Besides that, usage of this flowsheet usually leads to the increase of capital costs
for apparatuses and for the control system.

8.3.3. The Heat Pump
One more method to decrease 5 consists of the application of the so-called “heat
pump” of mechanical or absorption type (Fonyo & Benko, 1998). Heat, given in