Page 67 - Applied Process Design For Chemical And Petrochemical Plants Volume II

56 Applied Process Design for Chemical and Petrochemical Plants

Vapor (b) Calculate relative volatility, ai, referenced to
Final Press. Final =Pi heavy key component, at top and bottom temper-
Component Bi @95”Pi Xi = PiWl atures, and determine geometric average =.
(c) Calculate minimum theoretical plates at total
C2H6 0.378 750 0.0066 4.8
C3H8 10.0 174 0.175 30.4 reflux by Fenske’s equation (8-32)
n-W10 26.4 47 0.464 21.8 Use Gilliiand correlation to determine actual
iC4H10 20.2 67 0.354 23.7 reflux ratio, using an estimated number of actual
56.97 80.7 plates, and a minimum reflux ratio from:
psia

Therefore the final temperature should be close to
95”F, because 80.7 psia compares satisfactorily with the
operating pressure of 80 psia. Calculate:

Total mols of bottoms remaining at end 56.9 mols liquid L/D
Internal (L/V) = -
L/D + 1
Total mols vaporized = 100 - 56.9 = 43.1
2. Set up table: Keep XlD values constant
Liquid composition mol fraction is given in column
“Final xi,” and corresponds to the actual mols Bi, noting “Assumed “XI”
that there are the required 10 mols of propane in the bot- values (L/V) (1 -L/V) (xln -XI”) A B
toms under these conditions. x (Bottoms) 8 8 0 0 8

x2 8 8 a 0 8
Batch Distillation With Fractionation Trays-Constant x3 8 a 8 8
Overhead Product Composition, Multicomponent and 8 8 8 8 0
Binary x (Feed) 8 8 0 8
A=(I-L/V) (xlD-XI*)‘
The method of Bogart [4] is useful in this case. The
basic relation is:

*Assume ”XI” values of bottoms compositions of
light key for approximate equal increments from
final bottoms to initial feed charge. Calculate L/V
Application may be (1) to determine a column diame- values corresponding to the assumed “XI” values by
ter and number of plates or (2) to take an existing column inserting the various “XI” values in the Fenske equa-
and assume an operating reflux for the fured trays and tion for minimum reflux ratio of l-(d) . The “XI= val-
determine the time to separate a desired cut or product. ues replace the X1B of this relation as the various
assumptions are calculated. The actual (L/D) are cal-
where 0 = time from start when given L/V will produce culated as in l-(d) keeping the minimum number of
constant overhead composition, X~D trays constant. CompIete the table values.
BT~ = mols total batch charge to still
V = total mols per hour vapor overhead
X~D = mol fraction light key component in overhead
product
X~B = mol faction light key component in original
charge The total area, ZA, under the curve may be
obtained in several ways; the rectangular or trape-
Suggested procedure for situation (2) above: using exist- zoidal rules are generally quite satisfactory. The area
ing column concerned is between the original feed and the final
bottoms composition for the particular component.
1. Calculate minimum number of plates and minimum 4. Time required for a batch
reflux ratio
(a) For multicomponent mixture, select key compo-
nents, light and heavy (8 - 104)