CL 351 Homework # 6
Due date:
a.
The minimum number of equilibrium stages.
b.
The minimum reflux and reboil
ratios.
c.
The minimum vapour flows
(relative to feed flow) in each column section, and the corresponding energy
requirements.
d.
The number of equilibrium stages in each column section, and the energy requirements at a reflux ratio 60%
larger than the minimum.
e.
Make a plot of reflux ratio Vs.
number of plates. Use Matlab program to produce data; start with a high reflux
ratio, gradually reduce it and make sure you have enough data to generate a
smooth curve.
2.
Consider the distillation of a mixture containing
methanol and 1,4-dioxane in a column operating at 1
atmosphere. The feed contains 3,000
lb/hr of methanol and 7,000 lb/hr of dioxane (Mol. Wt.
= 88 g/gmol) as a saturated liquid mixture, also at 1 atm
pressure. The VLE data are tabulated
below. It is desired to recover 98% of
the methanol, which should have a purity of 95 mol%. For a reflux ratio of 2.5, plot the liquid
composition, vapour composition and temperature
profiles in the column.
T, oC |
101.1 |
94.9 |
89.6 |
82.2 |
76.9 |
73.1 |
70.7 |
68.8 |
67.5 |
66.2 |
65.1 |
64.7 |
64.5 |
x (MeOH) |
0 |
0.05 |
0.1 |
0.2 |
0.3 |
0.4 |
0.5 |
0.6 |
0.7 |
0.8 |
0.9 |
0.95 |
1 |
y (MeOH) |
0 |
0.21 |
0.37 |
0.54 |
0.66 |
0.74 |
0.78 |
0.82 |
0.85 |
0.89 |
0.95 |
0.97 |
1 |
1.
A 600 lbmol/hr mixture of
40 mol% propane and 60 mol% propylene is to be separated to produce a
distillate that is 99% pure in propane and a bottoms product 95% pure in
propylene. The mixture is available at
70oF and 1 atm pressure. To what pressure
the mixture must be compressed in order to use cooling water as cooling medium
in the condenser. The water can be
supplied at 25oC and the overhead from the column is 20oC
above the cooling water temperature.
Refer to Wankat’s book for equations to
compute K values for propane and propylene.
2.
An equimolar saturated
liquid feed containing ethanol and water is being separated in a distillation
column with ten equilibrium stages to produce a distillate with 80 mol% ethanol
and bottoms with 1 mol% ethanol. What is
the required reflux ratio? To play it
safe, operator runs the column so that distillate is 82 mol% pure in ethanol. At what reflux ratio is the column being
operated? What is the percentage rise in
the steam consumption in the reboiler for this reflux
ratio? In order to drive home the point
that raising product composition, even by small percentage, may rise the steam consumption dramatically, make a plot of
steam consumption Vs. purity as the purity is changed in steps of 0.01 from 0.8
to 0.85. Saturated steam at 120oC
is available for the reboiler. (enthalpy
of steam = 39641.4 kJ/kmol, enthalpy of saturated
water at 100oC = 40624.2 kJ/kmol). Use Matlab program
to solve this problem.
3.
Ethylbenzene(1), ortho-xylene(2), p-xylene(3) and m-xylene(4) having
same molecular formula are produced in naphtha reforming units in oil
refineries. The xylene
isomers are intermediates in the manufacture of polyesters, films, fibers and, ethylbenzene is raw material for styrene and poly styrene
production. The boiling points of four
compounds are quite close; 136.2, 144.4, 138.3, and 139.1 degrees Celsius in
the order stated above and their mole fractions in the feed are 0.25, 0.16, 0.14
and 0.45. The relative volatilities of ethylbenzene, para and meta xylenes with respect to ortho xylene are 1.235, 1.170,
1.147, respectively. Ortho xylene and sometimes ethylbenzene
are separated by distillation; relative volatility between para
and meta isomers is too low to warrant distillation as
a means of separating them.
The mixture can be separated in two distillation schemes as
shown in the Figures below. In Figure a,
ortho xylene is separated
in the first column and then ethylbenzene is
distilled off in the second column. Bottoms
purity in the first column is 95 mol% o-xylene
and its fractional reovery is 99.9%. Distillate purity in the second column is
99.7% ethylbenzene and its fractional recovery is
90.3%. Treat 1, 3 and 4 as one compound
in the first column and take relative volatility of this compound with respect
to ortho xylene as
1.147. In the second column, group 2, 3
into one compound and approximate its relative volatility with respect to ethylbenzene as 1.056.
In Figure b, ethylbenzene is
distilled off in the first column and then ortho xylene is removed in the second column. Consider same purities and fractional
recoveries of ethylbenzene and o-xylene
as in Figure a. For the first column in
Figure b, group 2, 3, 4 as one compound whose relative volatility with respect
to 1 is 1.056. In the second column,
treat 2, 3 as one mixture with its relative volatility with respect to 4 as
1.147.
Which configuration do you recommend? Make comparison of minimum vapour rate and minimum number of stages for your
recommendation. Use Matlab
programs to solve this problem.