CHE 441
__________________
LAST NAME, FIRST
Problem set #8
1. A heat exchanger with two tube passes has been recommended for cooling distilled water
from 33.9 to 29.4oC. The proposed unit contains 160 copper tubes, each with a 0.0191-m
outside diameter, 18 BWG, and 4.876-m length. The tubes are laid out on a 0.0238-m triangular
pitch within a 0.387-m inside diameter shell. Twenty-five percent cut segmental baffles, spaced
0.3 m apart, are located within the shell. Cooling water at a temperature of 24oC and a flow rate
of 2.0 m/s will be used in the tubes to provide the cooling. Under these conditions, the fouling
coefficients for the distilled water and the cooling water can be assumed to be 11,360 and 5680
W/m2K, respectively. The pressure drop on either the tube or the tube side may not exceed 69
kPa. Would the recommended design be adequate for cooling 22 kg/s of distilled water?
2. In a depropanizer described by King, a six-component feed at 96oC and 2170 kPa has the
following composition:
Component
Methane
Ethane
Propane
n-Butane
n-Pentane
n-Hexane
Feed, mol %
26.0
9.0
25.0
17.0
11.0
12.0
K value at 96oC
15.0
3.8
1.55
0.80
0.38
0.19
The feed is to be separated in a sieve tray distillation column with a recovery of 98.4 percent of
the propane in the distillate product and 98.2 percent of the n-butane in the bottom product. The
feed quality is 66 percent vapor. The column is equipped with a partial condenser. What are the
minimum number of stages and minimum reflux required for the separation? If a reflux ratio of
1.5 is selected, how many theoretical stages are required, and where is the feed location?
3. A valve tray tower has been designed to separate a mixture of 60 mol percent benzene and 40
mol percent toluene into an overhead product containing 96 mol percent benzene and a bottom
product containing 25 mol percent benzene. Calculations have shown that 6.1 theoretical stages
will be required to obtain the desired separation conducted essentially at atmospheric pressure.
The temperature at the top of the column is 82.8oC while at the bottom of the column it is
100.5oC. Assuming the reboiler acts as one theoretical stage, estimate the number of actual
trays required. To simplify the calculation, assume that mixtures of benzene and toluene may be
considered as ideal. At an average temperature of 91.6oC, the vapor pressure of pure benzene is
1070 mmHg, and the vapor pressure of pure toluene is 429 mmHg.
4. A benzene-toluene mixture consists of 40 mols benzene and 60 mols toluene. It is desired to
reduce the residual benzene concentration in a batch distillation column down to 10 percent.
What is the mol fraction of benzene in the receiver after the batch separation has been
completed? Assume that the relative volatility of the benzene-toluene mixture remains constant
at 2.90 throughout the separation process, vapor rate is considered to be constant, and column
holdup is negligible.