Page 230 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN004E-182 June 8, 2001 18:16

554 Distillation

1. Fix the pressure of operation of the column. TABLE II Typical Operating Conditions in Distillation
2. Determine the number of theoretical stages necessary
Pressure Temperature
to achieve the required separation as a function of the (bars), ( C) Theoretical
◦
reflux ratio R. top Top Base stages
3. Estimate the optimum value for R.
Demethanizer 33 −94 −8 32
4. Relate the required number of theoretical stages to the
Deethanizer 28 −18 72 40
actual height of the column needed.
Ethane–ethylene
5. Determine the necessary column diameter.
splitter 21 −29 −45 80
6. Refine steps 1 to 5 to achieve an optimum design.
Propane–propylene
splitter 18 45 60 150
Thefollowingsectionsdealwithsteps1to5inmoredetail. Isobutane-n-butane
splitter 7 45 65 60
Deisohexanizer 1.6 55 120 60
A. Column Operating Pressure
Oxygen–nitrogen
The condensing temperature of the overhead vapor is re- separation 1.1 −194 −178 70
duced by lowering the column pressure. Very often, cool- Ethylbenzene–styrene
ing water is used for condensation, and typically it has a separator 0.06 55 115 85
temperature of ∼35 C. Consequently, the condensing va- Crude oil distillation 0.03 93 410 —
◦
por must have a temperature of not less than ∼50 C, and
◦
this sets the lower limit of the column operating pressure.
ing a slope R/(R + 1). The operating line for the lower
The boiling temperature of the bottoms product in-
section below the feed is drawn by joining the required
creases as the column pressure increases. Typically,
bottom composition to a point located by the intersection
medium-pressure steam, which has a temperature of
∼200 C, is used in the reboiler. of the upper section operating line and the q line. The
◦
When this steam is used for heating, the bottoms q line of Fig. 13 represents a liquid feed at its bubble
product cannot have a boiling temperature greater than point, but the slope of the q line differs for other thermal
∼185 C which sets an upper limit on the column operating conditions of the feed. The number of theoretical stages
◦
pressure. required is determined by stepping off between the oper-
ating lines and the equilibrium line, as shown in Fig. 13.
Other heating and cooling arrangements can be em-
Each step on the diagram represents a theoretical stage.
ployed, such as the use of a refrigerant in the condenser
For the example shown, only nine theoretical stages are
or higher pressure steam in the reboiler, but they increase
required, but usually many more are needed in industrial
costs and are avoided whenever possible. An additional
columns.
consideration that often limits the maximum tempera-
In practice, feeds rarely consist of only two compo-
ture of the bottoms product is polymerization and product
nents, and the McCabe–Thiele diagram cannot be used.
degradation at high temperatures (and therefore at high
pressures). Furthermore, at lower pressures the relative
volatility tends to increase so fewer theoretical stages are
required, but at the same time the column diameter tends
to increase.
As a result of these factors the distillation pressure
varies widely. Typically, the distillation pressure falls as
the molecular weight of the feed increases. Some typ-
ical operating pressures and temperatures are shown in
Table II.

B. Calculation of the Required Number
of Theoretical Stages
Figure 13 shows a McCabe–Thiele diagram, which can
be used when the mixture to be distilled consists of only
two components or can be represented by two compo-
nents. Starting at the required overhead product compo- FIGURE 13 McCabe–Thiele diagram for benzene and toluene
sition x D , an upper-section operating line is drawn hav- (top column pressure, 1.0 bar).