Page 231 - 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

Distillation 555

For multicomponent mixtures, the approach is to solve are used, but the simplest is an overall column efficiency
a complex system of matrix equations involving vapor E o for which
and liquid compositions, flow rates from each theoretical
stage, and temperature and pressure distributions through Actual trays = Theoretical stages/E o (11)
the column. This procedure, known as tray counting or For distillation, E o is typically in the range 0.5 to 0.9.
column simulation, usually gives the required reflux ratio The vertical spacing between trays ranges from 200 to
for specified product compositions and number of theo- 900 mm. In some trayed columns, an undesirable bubbly
retical stages. Several commercial computer programs are foam can form above the liquid–vapor mixture. Antifoam
available for tray counting. chemicals must be added to such columns or diameters
or tray spacings must be increased. Packed columns foam
less often than trayed columns.
C. Optimum Reflux Ratio
The required height of a packed column is determined
By using the procedures outlined in Section III.B, it is from:
possible to determine the number of theoretical stages re-
Packed height = Theoretical stages × HETP
quired to achieve the desired separation as a function of the
reflux ratio (Fig. 14). Two limits are apparent: the mini- where HETP is the height equivalent of a theoretical plate.
mum reflux ratio at which an infinite number of theoretical Note that the terms plate, stage, and tray tend to be used
stages is necessary and the minimum number of theoret- interchangeably. HETP varies with the packing size and
ical stages that would be needed as the reflux ratio tends is typically in the range of 250 to 800 mm.
toward infinity. (A column operating with no feed and no
product withdrawals operates at total reflux.) The opti-
E. Column Diameter
mum reflux ratio depends mainly on a balance between
the investment cost of extra stages, hence extra column The column diameter is sized to suit the maximum antic-
height, which results as R is reduced, and the operating ipated rates of vapor and liquid flow through the column.
cost of the heating medium used in the reboiler, which Usually, the diameter is determined primarily by the vapor
increases as R is increased. Generally, the optimum reflux flow rate, and a rough estimate can be obtained from:
ratio is about 1.2 to 1.5 times the minimum value.
D = 4.5Q V [ρ V /(ρ L − ρ V )] 0.25 (12)
0.5
D. Column Height where D is the column diameter in meters, Q V is the vapor
flow rate in cubic meters per second, and ρ V and ρ L are
The number of actual trays required in a column can be de- the vapor and liquid densities, respectively, in kilograms
termined from the calculated number of theoretical stages per cubic meter.
by invoking an efficiency. Various definitions of efficiency Columns operated at vapor and liquid flow rates
greater than those for which they were designed become
“flooded.” Unexpected foaming can also cause flood-
ing. In a flooded column, liquid cannot properly descend
against the upflowing vapor. Poor separation performance
results, the overhead condensation circuit fills with pro-
cess liquid, the reboiler is starved of process liquid, and
the column quickly becomes inoperable.

IV. APPLICATIONS OF DISTILLATION
INCLUDING ENERGYCONSIDERATIONS

A. Flash Distillation
In contrast to the description of distillation given earlier,
which dealt with multistage distillation, flash distillation
(Fig. 15) is carried out in a single stage. Liquid flows
continuously through a heater, across a valve, and into a
FIGURE 14 Theoretical stages versus reflux ratio (benzene–
toluene at 1.0 bar). x D , mole fraction benzene in overhead; x B , flashvessel.Byheatingtheliquidandreducingitspressure
mole fraction toluene in bottoms. across the valve, partial vaporization occurs in the flash