Page 98 - Separation process principles 2

Exercises 63

Section 2.3 2.12 Toluene can be hydrodealkylated to benzene, but the conver-
sion per pass through the reactor is only about 70%. Consequently,
2.9 A binary ideal-gas mixture of A and B undergoes an isother-
mal, isobaric separation at To, the infinite surroundings tempera- the toluene must be recovered and recycled. Typical conditions
for the feed to a commercial distillation unit are 100°F, 20 psia,
ture. Starting with Eq. (4), Table 2.1, derive an equation for the
415 lbmoVh of benzene, and 131 Ibmolih of toluene. Based on the
minimum work of separation, Wmin, in terms of mole fractions of
property constants below, and assuming that the ideal gas, ideal
the feed and the two products. Use your equation to prepare a plot
liquid solution model of Table 2.4 applies at this low pressure, prove
of the dimensionless group, Wm,,/RTonF, as a function of mole frac-
that the mixture is a liquid and estimate VL and p~ in American engi-
tion of A in the feed for:
neering units.
(a) A perfect separation
Property constants for (2-38) and (2-39), where in all cases, Tis
(b) A separation with SFA = 0.98, SFB = 0.02 in K, are
(c) A separation with SRA = 9.0 and SRB = $
Benzene Toluene
(d) A separation with SF = 0.95 for A and SPAIB = 361
How sensitive is Wmin to product purities? Does Wmin depend on the
particular separation operation used?
Prove, by calculus, that the largest value of Wmin occurs for a
feed with equimolar quantities of A and B.
2.10 The separation of isopentane from n-pentane by distillation
is difficult (approximately 100 trays are required), but is commonly
practiced in industry. Using the extended Antoine vapor pressure
equation, (2-39), with the constants below and in conjunction with
Raoult's law, calculate relative volatilities for the isopentanel
n-pentane system and compare the values on a plot with the
following smoothed experimental values [J. Chem. Eng. Data, 8, Section 2.4
504 (1963)l:
2.13 Measured conditions for the bottoms from a depropanizer
distillation unit in a small refinery are given below. Using the data
Temperature, OF ai~~,,,~~
in Figure 2.3 and assuming an ideal liquid solution (volume of mix-
ing = 0), compute the liquid density in lb/ft3, lbtgal, lbibbl(42 gal),
and kg/m3.
Phase Condition Liquid
Temperature, OF 229
Pressure, psia 282
Flow rates, lbmollh:
What do you conclude about the applicability of Raoult's law in
c3 2.2
this temperature range for this binary system?
iC4 171.1
Vapor pressure constants for (2-39) with vapor pressure in kPa
nC4 226.6
and Tin K are
iC5 28.1
nCs 17.5
2.14 Isopropanol, containing 13 wt% water, can he dehydrated to
obtain almost pure isopropanol at a 90% recovery by azeotropic
distillation with benzene. When condensed, the overhead vapor
from the column splits into two immiscible liquid phases. Use the
relations in Table 2.4 with data in Perry's Handbook and the oper-
2.11 Operating conditions at the top of a vacuum distillation col- ating conditions below to compute the rate of heat transfer in Btuih
umn for the separation of ethylbenzene from styrene are given below, and kJ/h for the condenser.
where the overhead vapor is condensed in an air-cooled condenser to
Organic-
give subcooled reflux and distillate. Using the property constants in
Water-Rich Rich
Example 2.3, estimate the heat transfer rate (duty) for the condenser
Overhead Phase Phase
in kT/h, assuming an ideal gas and ideal gas and liquid solutions.
Phase Vapor Liquid Liquid
Overhead Temperature, "C 76 40 40
Vapor Reflux Distillate Pressure, bar 1.4 1.4 1.4
Flow rate, kgih:
Phase condition Vapor Liquid Liquid
Isopropanol 6,800 5,870 930
Temperature, K 33 1 325 325
Water 2,350 1,790 560
Pressure, kPa 6.69 6.40 6.40
Benzene 24,600 30 24,570
Component flow
rates, kgih: 2.15 A hydrocarbon vapor-liquid mixture at 250°F and 500 psia
Ethylbenzene 77,500 66,960 10,540 contains NZ, H2S, COz, and all the normal paraffins from methane
Styrene 2,500 2,160 340 to heptane. Use Figure 2.8 to estimate the K-value of each