What phase is the ethylene? How can you tell?

ChE 201 – ChE Fundamentals

Final Exam Review Problems

1. Natural gas composed of 60. mol% methane, 30. mol% ethane, and 10. mol% propane at 25 C and 500. psia is fed at a rate of 50. m3/hr into a boiler. In the boiler it is blended with enough air to provide 20% excess oxygen and combusted. The exhaust gas is passed through several air pollution control unit operations before being released through a smokestack into the atmosphere at a temperature of 400 C. Determine the flow rate (in m3/hr) of exhaust gas through the smokestack. {Answer: 79,400 m3/hr}

2. Determine the density of steam at 200 bar and 400 °C.

a) Use the ideal gas law {Answer: 64.3 g/l}

b) Use the compressibility factor. {Answer: 101 g/l}

c) How can you easily verify your number? How well did the models work?

3. Low density polyethylene (LDPE) is produced in a long tubular reactor under harsh conditions (T = 200 C and 1800 bar). Ethylene is pressurized, mixed with catalyst, and pumped into the reactor where (LDPE) is produced. The reactor itself is 0.5 m in diameter and 1800 m in length. Hint: the following equations may help you.

and

Where is the mass flow rate in kg/s, is the volumetric flow rate in m3/s,  is the fluid density in kg/m3, u is the average fluid velocity in m/s, and A is the cross sectional area in m2.

a) What phase is the ethylene? How can you tell? {Answer: supercritical}

b) If the flow rate of ethylene at the beginning of the reaction (i.e. when conversion is 0) in the reactor is 2.0 m/s, determine the mass flow rate in kg/s. {Answer: 192 kg/s}

4- One method of producing propylene is the dehydrogenation of propane (C3H8) to propylene (C3H6) according to the reaction:

A schematic of the reaction is shown below. The fresh feed (Stream 1) has a total flow rate of 100. mol/min and is composed of 96 mol% propane and 4.0 mol% propylene. The single pass conversion of propane across the reactor is 40% and all of the hydrogen is separated in the flash drum. Stream 5 exiting the flash drum is fed to a distillation column where 90% of the propylene is separated. The product (Stream 6) exiting the distillation column is composed of 99 mol% propylene and 1.0 mol% propane. The second stream exiting the column (Stream 7) is recycled back into the process.

a) Why is there no purge stream for this process? (5 points) {no impurities or excess reactants}

b) What is the overall conversion of propane? (15 pts) {99%}

c) What is the molar flow rate of propane in Stream 2? (10 pts) {238 mol/min}

d) What is the molar flow rate of propylene in Stream 7? (10 pts) {11mol/min}

e) What is the recycle to fresh feed ratio (Stream 7 / Stream 1)? (15pts) {1.5}

f) Suggest 2 possible reasons why the single pass conversion of propane is relatively low. (5 points) {slow kinetics, reaction reaches equilibrium}

Stream 2

Reactor

Flash

drum

Column

96% Propane

1.0% Propane

99% Propylene

Reactor

Flash

drum

Column

100. mol/min

4.0% Propylene

Stream 1

100.% Hydrogen

Stream 3

Stream 5

Stream 4

Stream 7

Stream 6

5-

V

&

2

6

3

8

3

H

H

C

H

C

+

®

r

×

=

V

m

&

&

A

u

V

×

=

&

m

&

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