Page 86 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 75

8. Calculate internal vapor and liquid flows.
(LJD)min = assumed

Solve for (L,/V,) min

(8- 143)

L, - (number) (I7,) = (Lr/D)mk(D)
D is known
Calculate V, and L, from above.
In stripping section: Because the second term of the denominator is
Solve directly for L, usually negligible when the light key in the bottoms is
very small; less than 0.1 mol fraction, this term is
L, = L,. + qF often omitted.

Solve for Vs:

(8- 144)
lvote that these calculations are made for the light
key, 1; heavy key, h; and all components heavier than
Calculate L,/B the heavy key, H. For split key systems, the calcula-
9. Evaluate pinch compositions at the assumed temper- tions are made for all components heavier than the
atures of Step 7. If this temperature does not give a light key.
balance, other temperatures should be assumed and 10. Calculate mol fraction ratio:
a balance sought as indicated below. Either of the fol- (a) Stripping pinch
lowing balances can be used, depending upon the
light key
convenience of the designer: r=
Rectifying: ps heavy key
(b) Rectifying pinch

light key
rpr =
heavy key
(c) p = rpshpr
11. Calculate for each component in pinch.
Rectifying: apply only to components lighter than
light key, i = L’

(ai - 1) ai
When the heavy key in the overhead is very small,
less than 0.1 mol fraction, the last term of the denom- ai
inator can be omitted.
Read from Figure 848 value of C,i for each com-
ponent.
Calculate for each component:
(Cni) kPr)
Kote that the calculations are only made for the
heavy key, h; light key, I; and all components lighter Sum these values:
than it, L’. If there are split keys, the calculation is to
include all components lighter than the heavy key. 2,Gi xip
Stripping pinch compositions: i=L