UNIVERSITY OF PRETORIA
DEPARTMENT OF CHEMICAL ENGINEERING
CPS 410 Process Synthesis 410
th
7 June 2012
EXAM
TIME : 120 minutes
TOTAL: {60 }
Answer all the questions
Open book, but no worked-out examples allowed
Programmable calculators may not be used.
QUESTION 1
Ethanol can be produced by the slightly exothermic reaction of ethylene with water:
CH2=CH2 + H2O  CH3CH2OH
ethylene + water  ethanol
The ethylene feed (100 kmol/h total at 25°C and 1 bara) consists of 99% ethylene and 1% methane. Water is
fed separately at a rate of 200 kmol/h. Conversion of ethylene is only 8% in the reactor operating at 70 bara
pressure, and at 320°C.
There is a second reaction operating at equilibrium:
2 CH3CH2OH  C2H5OC2H5 + H2O
Ethanol  diethyl ether + water
The diethyl ether (DEE) is recycled to extinction at a flow rate of 10 kmol/h.
(a)
Match sources and sinks by drawing a block flow diagram of the process showing recycle streams.
Assume that all reaction products can be separated, except methane from ethylene. Methane is
removed in a purge stream containing 10% methane in ethylene. The ethylene in the purge is not
recycled. Assume that excess water is also not recycled. Calculate the molar flow rates of each
component in the following streams:

The ethylene/methane purge stream

The ethylene/methane recycle stream

The reactor feed stream

Water and ethanol product streams (assuming they are pure)
(10)
(b)
Eliminate differences in composition by showing the separation steps required to get a product stream
consisting of 85% ethanol in water (molar). Assume that the reactor products are cooled to 40°C in a
flash drum, and the liquid portion contains no ethylene or methane. The vapour portion is washed with
30 kmol/h water in an absorber to perfectly separate methane and ethylene from DEE, ethanol and
water. The flash drum and absorber both operate at 70 bara.
Draw the separation system, showing the pressure of the distillation columns and temperatures of
bottoms and distillate products (where these are pure). Use heuristics to determine the order of
separation, and calculate relative volatilities at 50°C from the data in the table of vapour pressures.
Assume that the components act as ideal mixtures.
Vapour pressures for the higher-boiling components are given:
Vapour Pressures
(kPa)
35°C
50°C
79°C
100°C
DEE
101
164
372
620
Ethanol
13.5
29.1
103
222
Water
5.6
12.2
45
100
(10)
(c)
Eliminate differences in temperature, pressure and phase in the feed streams leading to the reactor
(excluding recycle streams). The boiling point of water at 70 bara is 286°C and the critical temperature
of ethylene is 10°C.
(5)
(d)
Draw a process flow diagram of the entire system, showing how you integrate tasks. Show temperature
and pressure and indicate the main components in streams where applicable. It is not necessary to give
an equipment list.
(12)
[37]
QUESTION 2
A mixture of components A, B, C, D and E must be separated into pure products. Use heuristics, and where
applicable, the method of Tedder and Rudd to determine the order of separation and type of distillation columns.
Vapour pressure at 100°C, kPa
Molar flow rate of feed, kmol/h
A
210
10
B
193
12
C
100
7
D
86
14
E
61
3
[15]
QUESTION 3
The attached ternary residue curve map of water, ethanol and phenol also shows the liquid-liquid equilibrium line
(dashed) at 50°C. A feed stream of 70% water, 20% ethanol and 10% phenol must be separated to make three
product streams: 1) 84% ethanol in water (no phenol), 2) pure water and 3) pure phenol.
Draw a simplified flow diagram showing how this can be achieved, and show the corresponding tie lines on the
diagram. Label mixed streams A, B, C etc. Show the direction of the residue curves, given that the boiling points
of water, ethanol and phenol are 100°C, 79°C and 92°C respectively, and the azeotropes are low-boiling
azeotropes. Show the area of possible bottoms compositions on the diagram.
[8]
TOTAL: {60}
Name ____________________________________________________ Student number: ______________
Ethanol
Water
Flow diagram:
Phenol