Page 200 - Distillation theory

P1: FCH/FFX P2: FCH/FFX QC: FCH/FFX T1: FCH
0521820928c06 CB644-Petlyuk-v1 June 11, 2004 20:17

174 Distillation Trajectories in Infinite Complex Columns and Complexes

the bottom section in Fig. 6.2. In the main and intermediate condensers, it is
proportional to the hatched area in the top section. Under the assumptions men-
tioned, the values opt T int and opt T int will correspond to the minimum of these
con reb
areas.
Minimum values of the parameters (L/V) r and (V/L) s in the feed cross-section
of the column and compositions at trays above and below this cross-section x f −1
and x f at adiabatic and nonadiabatic distillation remain the same. The stationary
points S r and S s also coincide, but at parts of reversible distillation trajectories be-
tween column ends and stationary points S r and S s the additional stationary points
int
N r int and N ,correspondingtothepointsofintermediateinputsandoutputofheat
s
(Fig. 6.2a), appear.
Therefore,theconceptualcalculationofinfinitecolumnwithintermediateinput
and/or output of heat consists in two stages: (1) calculation of minimum reflux
int
int
mode for adiabatic column, and (2) determination of opt T , opt Q int , opt T ,
con con reb
and opt Q int (“pinch method”).
reb
Figure 6.2 shows the results of such calculation at the example of direct separa-
tion of ideal three-component mixtures. However, this approach can also be easily
used in the most general case for any kinds of mixtures, including azeotropic ones,
at any component numbers and for any splits.

6.3. Distillation Trajectories and Minimum Reflux Mode in Two-Feed
Columns with Nonsharp Separation in Intermediate Sections

Columns with several inputs of feed are used in a number of different cases: (1)
when flows with the same set of components but different compositions come
to the unit; (2) when the raw materials are gradually warmed and are put into
the column in several flows different in temperature, or when they are gradually
evaporated or condensed, and after separation into liquid and vapor phases they
are put into the column in several flows, different in temperature, composition,
and phase state (units of petroleum refining, units of productions of ethylene and
propylene); (3) when an absorbent is used for separation (units having absorbers
or fractioning absorbers); and (4) when an entrainer that is put into the column
of extractive distillation in a separate flow is used for separation.
Gradual heating and evaporation is used in the case of separation of mixtures
with a wide interval of boiling, when heat is put in at a lower, and cold is put in at
a higher temperature, compared with their input in the reboiler and condenser.
This allows for a decrease of total energy consumption in separation.
Absorption is used in the case of extraction of liquid components from the gas
phase, when the usage of distillation is unprofitable because of the necessity for
too low temperatures in condensers.
Extractive distillation is used to increase the relative volatility of components
being separated of nonideal mixtures and to separate azeotropic mixtures that
cannot be separated by means of simple distillation.
Columns with several inputs of feed have one or several intermediate sec-
tions, located between these inputs of feed. To calculate minimum reflux mode