Page 74 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 63

From steam tables (saturated) at:

Top of tower, 175"F, vapor pressure water, psia = 6.8
Mol fraction water vapor at top of tower: 6.8/48 psia = 0.1416
Mol fraction hydrocarbon at top of tower: 1 - 0.1416 = 0.8584
Total mols mix HC vapor and water vapor (8-119)

775 + 830
at tower overhead : = 1,893.0 2. Equation of operating line in stripping section, light
0.8584
component
Mols of water vapor in tower overhead bn + 1 = Vm + B
1,893 - (775 + 850) = 268
Accumulator @ 48 psia & 135"F, water
vapor pressure: = 2.6 psia
Mol fraction water in accumulator vapor: = 2.6/48 = 0.0541
Mol fraction HC in accumulator vapor: = 1 - 0.0541 = 0.9459
[ 0.9459 ]
(775) (0.55) = 450*6
Total mols vapor leaving accumulator: = (8 - 120)
Mols water vapor leaving accumulator: = 430.6 - (773) (0.55)
= 24.35
Mols liquid water withdrawn from
accumulator: = 268 - 24.35 = 243.65
Mols liquid water collected on dehydrator where MB = hB - QB/B
tray and removed at that point up tower
above where reflux returns below this tray: = 777.7 - 268
(water vapor in tower overhead) = 309.7 mols/hr
Mols steam entering tower: = 14,000/18 = 777.7 mols/hr Mg=hw-- Q
w
Distillation With Heat Balance Hn = total molal enthalpy of vapor at conditions of plate

n, Hn = 2 Hni (Yni)
This type of evaluation of a distillation system involves a hn = total molal enthalpy of liquid at conditions of
material and heat balance around each tray. It is extreme- plate n, h, = Z hni (xni)
ly tedious to do by conventional means, and is now han- s = lb (or mols) steam per lb (or mol) bottoms
dled with computers. But even with this untiring worker, Hm = total molal enthalpy of vapor at plate m (below
the volume of calculations is large and requires a relative- feed)
ly long time. Only those special systems that defy a rea- N = mols residue or bottoms per unit time
sonable and apparently economical solution by other QB = heat added in still or bottoms
approaches are even considered for this type of solution.
The detailed method involves trial and error assumptions PonchonSavarit Method-Binary Mixtures
on both the material balance as well as the heat balance.
This graphical method allows solution of many distilla-
Unequal Molal Overflow tion systems which would require considerable work if
attempted by rigorous methods. Robinson and Gilliland
This is another way of expressing that the heat load have technical and descriptive details substantiating the
from tray to tray is varying in the column to such an extent method [8,59]. Figure 8-42 presents a summary of the use
as to make the usual simplifying assumption of equal of this method and appropriate interpretations. Scheiman
molal overflow invalid. The relations to follow do not [lo41 uses the Ponchon-Savarit diagrams to determine
include heats of mixing. In general they apply to most minimum reflux by heat balances. Campagne [216, 2171
hydrocarbon systems. suggests a detailed technique for using the Ponchon-
Savarit method with a computer simulation, which leads to
1. Equation of operating line in rectifjhg section, light designs not possible before. Many illustrations given in the
component [59] reference aid in understanding the technique.
The basic method allows the non-ideal heat effects of
Ln + 1 = vn - D the system to be considered as they affect the plate-teplate