Page 44 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 33

Operational Values of X Calculated via Equation 8-61 for a Example 8-10: Graphical Design for Binary Systems [59]
Range of Reflux Ratios*
--- The benzene-toluene example of Robinson and Gilliland
Rndm MdtiplierA [59] has been elaborated on and expanded after the
.-
__ - - - - - -. -
0.04 1.05 1.07 1.10 1.14 1.15 1.17 1.20 1.30 1.40 1.50 advanced distillation course of Holland [25], Figure 8-26.
-_ - - .- .- -
.-
1 0.010 0.024 0.034 0.048 0.063 0.070 0.092 0.091 0.130 0.167 0.200 It is desired to separate an equimolal mixture of ben-
3 0.015 0.036 0.050 0.070 0.092 0.101 0.132 0.130 0.184 0.231 0.273 zene and toluene into a top product containing 95 mol 76
5 0.016 0.040 0.055 0.077 0.102 0.111 0.145 0.143 0.200 0.950 0.294 benzene and a bottom product containing 95 mol %
10 - 0.018 0.043 0.060 0.083 0.110 0.120 0.157 0.154 0.214 0.267 0.313 toluene. The distillation is to be carried out at atmospher-
.-
Total ic pressure. Use a total condenser.
reflux 0.020 0.048 0.063 0.091 0.120 0.130 0.170 0.167 0.231 0.286 0.393
- __ -
*Used by permidon, SfcCmmick, J. E., C%emimlE~@~~g, 95 no. 13 (1988). all rights Ipserved. A. Calculate the minimum reflux ratio if the feed is liq-
v.
uid at its boiling point.
where A 6 parameter in correlating equation or multiplier E. Calculate the theoretical plates required if a reflux
on Rmin ratio (L/D) of 1.5 times the minimum is employed.
B = parameter in correlating equation
In - natural logarithm Feed = 50 mols benzene + 50 mols toluene
log = logarithm to the base 10 Overhead = 95% benzene
N = actual theoretical stages required for a given sepa- Bottoms = 95% toluene
ration
K- = minimum theoretical stages required for a given Material balance with respect to benzene:
separation +
R external reflux ratio for a given separation 0.50 (100) = (0.95) (D) 0.05B
50 = .95D + .05 (I00 - D)
Gin E minimum external ratio for a given separation 50 =c .95D + 5 - .05D
X = (R - Rmh)/(R + 1) 45 = .90D
Y = (N - Nmin)/(X + 1) D = 45/.90 = 50 mols overhead product
D = overhead product, mols
The following is a short approximation method for min- B = bottoms, mols
imum reflux ratios for multicomponent mixtures [98]:

(8 - 63) Step-wise Troys for Operating

where(xFLK)eff = xm/(xm+ xFHK)
n = number of components OperatJng Reflux LN - 0.65 ,653
hin = minimum reflux ratio b 0.8 (Rectifying Section)
x = liquid mol fraction
q = relative volatility of component i based on
heavy key
aLK = relative volatility of component, i, based on
light key.

subscripts:
avg = average
e?f = effective
F = feed
FHK = heavy key in feed
FLK = light key in feed
i = component 0. I
LK = light key
HK = heavy key '0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 ID
Pol Fraction Benzene in Liquid
Ester [94, 951 examines binary distillation systems
Figure 8-26. Equillbriurn curve; benzene-toluene for Example 8-1 0
with multiple feeds, one or more side products, one or (curve data only). Used by permission, Robinson, C. S. R. and
more points of heat removal or addition, and various Gilliland, E. R., Elemenis of Fractional Distillation, 4th Ed. McGraw-
combinations. Hill Book Co. (1 950), all rights reserved.