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EH/OCT 2008/CHE450/471/481

UNIVERSITI TEKNOLOGI MARA

FINAL EXAMINATION

COURSE

COURSE CODE

EXAMINATION

TIME

CHEMICAL PROCESS PRINCIPLESI

CHE450/471/481

OCTOBER 2008

: HOURS

INSTRUCTIONS TOCANDIDATES

1.

This question paper consistsoffive 5)questions.

Answer ALL questionsinthe Answer Booklet. Start each answer ona newpage.

.

3.

4.

Do notbring any material into the examination room unless permission is given by the

invigilator.

Please checktomake sure that this examination pack consistsof:

i) theQuestion Paper

ii) anAnswer Booklet- providedby theFaculty

iii) anAppendicesforChemical Process Principles Booklet- providedby theFaculty

iv) aone-page appendix

DO NOT TURN

T IS

P GE

UNTIL

YOU RE TOLD TO

O SO

This examination pape r consists

o

5 printed pages

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QUESTION 1

A

a) A virial equ ation of state expresses the quantity of PVIRT (also known as z,

compressibility factor) as a power series in the inverse of specific volume:

PV

A

B C D

= Z = 1 r-

r

RT

A

A

2

A

3

KI

V V V

where P = absolute pressure, N/m

2

A

V = specific volume,

m

3

/mol

T = tem perature, K

R = gas constant, m

3

Pa/ (mo l -K)

B,C and D are constants

i) W hat are the units for B, C and D respectively?

(4 marks)

ii) W hat is the new expression of com pressibility factor (power series), if the

volume is expressed in terms of American Engineering System of units?

(5 marks)

b) The comp ressibility factor, z can also be determined by using a generalized

compressibility chart (refer to Appendix 1). Using the chart, calculate the diameter

(in cm) of the pipe transporting a process stream flowing at 120 kg/hr of a gas

mixture containing 10 wt hydrogen and balance

1-butene

(C

4

H

8

). The stream

pressure is 70 atm gauge pressure, the tempe rature is 90C and the velocity is 3.5

m/s.

Data of critical tempera ture, T

c

and Critical pressure, P

c

:

T

c

(H

2

)=33.33 K P

c

(H

2

) = 12.8 atm

T

c

(1-butene) = 41 9.6 K P

c

(i-butene) = 39.7 atm

(Relative atomic mass: C = 12, H = 1)

(16 marks)

c) A mixture is 10 mo le ethyl alcohol (C

2

H

5

OH), 75 mo le ethyl acetate (C

4

H

8

0

2

),

and 15 mo le acetic acid(CH

3

COO H). Calculate:

i) The mass fractions of each com pound .

(3 marks)

ii) The average molecular weight of the mixture.

(1 m ark)

iii) The mass of a sample (in kg) containing 25 kmol of ethyl acetate.

(1 m ark)

(Relative atom ic mass: C = 12, H = 1 , O = 16 )

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QUESTION 2

a) In a power plant, wet wood chips (F) containing 70 wt woo d chips and balance

water is being dried in a continuous steady state drier to yield a product (P)

containing 95 wt wood chips with the rest being water. Evaporation water (W) is

released from the drier to the surrounding at a rate of 4 kg/m in.

i) Draw and label a complete flow chart for the process.

(1 m ark)

ii) Calculate the streams F and P in kg/h.

(4 marks)

b) An oil (CH

x

)

n

is burned in a furnace. A n analysis of the flue gas yielded the following

dry-basis composition:

C 0

2

7.6

CO 3.3

0

2

7.3

N

2

81 .8

(Relative atomic m ass: C = 1 2, H = 1 , O = 1 6, N = 14)

i) Ca lculate the percen tage of excess air.

(7 marks)

ii) Calculate the stack gas analysis on wet basis.

(4 marks)

iii) Ca lculate the atom ic ratio (x) of hydrogen to carbon in oil.

(4 marks)

QUESTION 3

a) Propane (C

3

H

8

) at 27C and 2 bar enters a heat exchanger at a flow rate of 50,000

m

3

/h. Saturated steam at 3 bar enters the heat exchanger to heat up the propane to

98C at co nstant pressure. The steam exits the hea t exchanger as condensed liquid

at 27C. The specific enthalpies of propane at 27C and 98C are 1048 kJ/kg and

131 0 kJ/kg, respectively. Atmosph eric pressure is approximately at 1.0 bar.

i) Calculate the amou nt of energy (in kW) required to heat up propane from

27C to 98C.

(5 marks)

ii) Assu ming no heat loss during the process, calculate the mass flow rate in

kg/s of steam required to heat up the ethane .

(5 marks)

(Gas constant, R = 0.08314 Lb ar/m olK ; relative atomic weights: C = 12 , H = 1,

0 = 16)

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b) Figure

below shows a schematic diagram for making fresh water from sea water by

freezing. The pre-chilled sea water is sprayed into a vacuum at a low pressure. The

cooling required to freeze some of the feed sea water comes from evaporation of a

fraction of the water entering the chamber. The c oncentration of the brine stream, B,

is 4.8 salt. The pure salt-free water vapor is com pressed and fed to a melter at a

higher pressure where the heat of condensation of the vapor is removed through the

heat of fusion of the ice which contains no salt. As a result, pure cold water and

concentrated chilled brine (6.9 ) leave the process as products.

i) Determine the flow rates of streams A, B, C, D and W if the feed is 1000 kg

per hour.

(8 marks)

ii) Determ ine the ratio of pure chilled wate r with respe ct to water contained in

chilled sea w ater

feed.

(2 marks)

(Hint: Compressor unit changes only the pressure of water vapor w ithout altering the

mass of water vapor)

Pure water vapor, A

Com pressor

1000 kg/h

Chilled sea water feed

3.45 NaCi

Pure fresh

chilled water,

W

Chilled brine

6.9 NaCI

Figure Production of fresh water from sea w ater

QUESTION 4

In a gas production plant, 80 kg of

C 0

2

at 25C is filled into a 7 m

3

tank. Using the Soave-

Redlich-Kwong (SRK) equation of state, calculate the gas pressure in the tank. Also

compare and comment your results if you use the ideal equation of state for your

calculations.

SRK equation of state:

P =

RT

a a

V-b V V + b)

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where P, R and V are the system pressure, gas constant and specific volume

respectively.The parameters a, b, and

a

are empirical functions of the critical temperature

(T

c

),

pressure (P

c

), the Pitzer acentric factor () and the system temperature (T). The

parameters can be estimated from the following correlations where T

r

is the reduced

temperature:

, R T

C

J

a

b

a

T

r

m

= 0.42747

x c

RT

~ 0.08664

c

Pc

= [\ m l-jT

r

)

2

= T /T

c

= 0.4 850 8+1 .55 1 7 1co -0.1561a)

2

Molecular weight of C 0

2

= 44, critical temperature, T

c

= 304.2 K, P

c

= 72 .9 atm and co =

0.225.

(10 m arks)

QUESTION

5

Ethylene oxide is produced by the catalytic oxidation of ethylene:

2C

2

H

4

+ 0

2

-> 2C

2

H

4

0

An undesired com peting reaction is the combustion of ethylene:

C

2

H

4

+ 3 0

2

- > 2 C 0

2

+ 2 H

2

0

The feed to the reactor contains 2.5 moles ethylene per mole of oxygen. The single pass

conversion of ethylene is 2 1 and for every 100 moles of ethylene consumed in the

reactor, 90 moles of ethylene oxide emerges in the reactor products. A m ultiple unit

process is used to separate the products: ethylene and oxygen are recycled to the reactor,

ethylene oxide is sold as product and carbon dioxide and water are discarded. Draw and

label the flowchart of the reaction and by using extent of reaction me thod, calculate:

a) the molar flow rates of ethylene and oxygen in the freshfeed.

(8 marks)

b) the production rate of ethylene oxide

(6 marks)

c) The overall conversion of ethylene .

(6 marks)

END OF QUESTION PAPER

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APPENDIX 1

EH/OCT 2008/CHE450/471/481

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