Page 161 - Plant design and economics for chemical engineers

COMPUTER-AIDED DESIGN 135
(a) Find the flow rates for each of the product streams for each of these values of the
ethane mole fraction in the sales gas stream: 0.00, 0.01, 0.05.
(b) Find the value of the feed gas, the value of each of the product streams, and the
difference between product and feed values ($/hr) for each of the ethane mole
fractions in part (a).
Data:
Natural gas feed = $2.85/GJ of lower heating value (LHV)
Natural gas LHV,
feed Component Mel% kcal/gmol
Methane 83.0 191.8
Ethane 12.0 341.3
Propane 4.0 488.5
n-Butane 1.0 635.4
Products sales gas contains negligible propane and butane; value = $2.85/GJ (LHV).
Ethane product contains 1.0 mol% methane, 2.0 mol% propane, negligible butane;
value = $O.lB/kg.
Propane product contains negligible methane, all the n-butane and 2 mol% ethane;
value = $0.2O/kg.
2. A process has four streams with the characteristics given below. Devise a heat-
exchange network to maximize the annual savings as compared to no heat exchange.
Use a minimum approach temperature AT,, = WC.
Process T:,,* T CM, M*C,,
stream “C “C MJ/(h) (OC)

“Cold” to be
heated
A 60 160 27.4
C 116 260 22.0
“Hot” to be
cooled
B 160 93 31.7
D 250 138 38.0
Cost data: Heat-exchanger costs are to be taken from Fig. 15-13 of this text. Use a
design pressure of 150 psig for all exchangers except those using steam, for which a
design pressure above the operating (steam) pressure should be used. Neglect pump
and pumping costs.
Utilities: Cooling water, $0.15/1003 gal. (enters at 25”C, leaves at 50°C). Steam
(saturated) at 6200 kPa, $15/Mg; at 2850 kPa, $13.2/Mg; at 790 kPa, $ll/Mg; at 205
kPa, $7/Mg.
Annual cost of equipment, $/yr = (purchased cost, $)*(4.0)*(0.25)
Additional information: These estimated overall heat-transfer coefficients may be used
(values are in J/s m*K):
process stream to process stream-200
process stream to water-400
process stream to steam-550
Assume that all process streams have a heat capacity of 3.3 kJ/kg “C. Operation is
330 days per year, 24 h/day.