Page 61 - Applied Process Design For Chemical And Petrochemical Plants Volume II

50 Applied Process Design for Chemical and Petrochemical Plants

A trial-and-error calculation is necessary to solve for W
until a value is found from the In Wi/W equation above
that matches the XD which represents the required
overhead distillate composition. By material balance:

V= L + D, and R= L/D
V/D = L/D + 1 = R+ 1

D = V/(R + 1)
W =Wi - Dl3 = Wi -Ve/(R+ 1) (8 - 94)

Mol Fraction Component A in the liquid, x
[ T ’)I Figure 8-35. Batch distillation: constant retlux ratio after McCabe-Thiele
diagram. Revisedadapted and used by permission, Schweiizer, PA.
[
Handbook of Separation Techniques for Chemical Engineers, McGraw-
e=(R+l) 2 - (8 - 96) Hill Book Co. (1979); also reprinted by special permission, Chem. Eng.
Jan. 23 (1 961), p. 134., 0 1961 by McGraw-Hill, Inc., New York.

(8 - 97)
/xD -xwo\
w=w, (8 - 99)
\ XD-Xw
Referring to Figure 835 the constant internal reflux where W = mols in still/bottoms at any time
ratio, L/V is shown for several selected values of reflux W, = mols in still/bottoms at initial charge time
ratios [131]. Only one at a time can be used for actual
operation. Starting at the intersection of the diagonal line This mode of batch rectification requires the continuous
(distillate composition), step off the theoretical plates. For adjustment of the reflux to the column in order to achieve
example, from Figure 835 at constant reflux, using oper- a steady overhead distillate composition. Starting with a
ating line No. 1, starting at XD = 0.95, for one theoretical kettle obviously rich in the more volatile component, a rel-
plate, the bottoms composition in component A would be atively low reflux ratio will be required to achieve the spec-
approximately xw = 0.885; then going down one more ified overhead distillate composition. With time, the reflux
plate at the same L/V, a second theoretical plate yields a ratio must be continuously increased to maintain a fixed
bottoms of XB or xw = 0.83, still yielding XD = 0.95. If the overhead composition. Ultimately, a practical maximum
L/V for the operating line No. 4 is used (same slope as reflux is reached and the operation normally would be
line No. l), then the expected performance would be XD stopped to avoid distillate contamination.
= 0.60, and after one theoretical plate, the bottoms would At constant molal overflow: The time required for the
be 0.41 at the same reflux ratio as the first case; and for XD distillation only,
= 0.60 and two theoretical plates, xw = 0.31.

where D = relates to distillate
i = relates to initial condition
W = relates to bottom or pot liquors
does not include charging the kettle, shutdown, cleaning,
Batch with Variable Reflux Rate Rectification with Fixed etc.
Number Theoretical Plates in Column, Constant To determine the column (with trays) diameter, an
Overhead Composition approach [130] is to (1) assume 8 hours; (2) solve for V,
lb/hr vapor up the column at selected, calculated, or
Overall material balance at time 8 [130,131]: assumed temperature and pressure; (3) calculate column
diameter using an assumed reasonable vapor velocity for
(8 - 98) the type of column internals (see section in this volume on
‘‘Mechanical Designs for Tray Performance”).