i. Comment this special condition from the thermodynamic point of view. Does it justify the elevated temperature in the reactor? Hint. Evaluate the equilibrium conversion for reactions 1-2, e.g. for the temperature and pressure conditions at the inlet of

CHEMICAL ENGINEERING 3G04

DAY CLASS

DURATION OF EXAMINATION: 2 Hours

MCMASTER UNIVERSITY FINAL EXAMINATION

BENOIT CHACHUAT

A pril 17, 2010

THIS EXAMINATION PAPER INCLUDES 7 PAGES AND 3 QUESTIONS

₍₊

A 4TH BONUS QUESTION). YOU ARE RESPONSIBLE FOR ENSURING THAT YOUR COPY OF THE PAPER IS COMPLETE. BRING ANY DISCREPANCY TO THE ATTENTION OF YOUR INVIGILATOR.

Special I nstructions:

• O pen course notes, any calculator, and a ruler are allowed.

• Books as well as copies of tests, assignments and tutorials are prohibited.

• P artial credit will be given. Show intermediate calculations and derivations and explain your conclusions clearly.

• You must write on only one side of the page in the exam booklet. You need not copy data or figures given on this examination paper into your booklet. You may use pencil or pen, and you must present your answers in a clear and neat manner.

• All pages of this examination paper must be returned with your answers.

Q uestions l(a)-(e) 2(a)-(c) 3(a)-(c) 4 Total

G rade

___

_,14

___

/14

___

/12

___

/4 (Bonus)

___

/40

+

Bonus

Chemical Engineering 3G04 2

3 pts

1 pt

1 pt 1 pt

2.5 pts

1. Styrene is the monomer used to make polystyrene, which has a multitude of uses, the most common of which are in packaging and insulated Styrofoam beverage cups.

Problem Data:

Styrene (Sty,

C6HsC2H3)

is produced by the (catalytic) dehydrogenation of ethylbenzene

(EtB,

CeH5C2H5),

1

CeH5C2H5

�

₂

CeH5C2H3

+

H2.

This reaction may be equilibrium limited, and the equilibrium constant is given as:

In K = 15.5408

- 148

;

2·6,

with T in K, P in bar, and Xi denoting the molar fraction of species i. Moreover, the kinetic equations for the forward

(

ri

)

and reverse

(r2)

reactions are:

8

(

21708

)

r1 = 4.24 x 10 exp

-

RT XEtaP,

5

(

7804

)

2

r2

= 7.55 x 10 exp

RT XEtBXH2P ,

with R = 1.987 cal/molK, T in K, P in bar, and ri is in mol/m3 s.

Decomposition of ethylbenzene to benzene

(C6H6)

and ethylene

(C2:1:4),

and hydrodealkyla­ tion to give methane

(CH4)

and toluene

(C6HsCH3),

are unwanted side reactions:

The process flow diagram (PFD) is shown in the figure on page 3, and the corresponding flow summary table is given on pages 4-5.

Questions:

(a)

(b)

(c)

Give a brief description (about 10-15 lines) of the PFD of the styrene production unit: reaction, separation and feed preparation sections as well as recycles and heat integration. From the information given in the flow summary table, determine the following:

i. the single-pass conversion of ethylbenzene in reactors R-401a-e and R-402a-e, both

separately and together

ii. the overall conversion of ethylbenzene in the unit iii. the yield of styrene

Comment these values.

High temperature is a condition of special concern in the reactors R-40la-e and R-402a-e. i. Comment this special condition from the thermodynamic point of view. Does it

justify the elevated temperature in the reactor?

Hint. Evaluate the equilibrium conversion for reactions 1-2, e.g. for the temperature and pressure conditions at the inlet of R-402.

(") 0 ::::l !:!". ::::l i:: (I) a.. 0 ::::l ""U Ill ()ti _(I) .j::>. H-401 Steam Heater E-401 Feed Heater

E-402 R-402a-e E-403 Inter- Styrene Product heater Reactors Cooler

R-401a-e Styrene Reactors

E-404 E-405 V-401 C-401 P-401 NB T-401 E-406 E-407 P-402NB V-402 T-402 E-408 E-409 P-403 NB Product Product Three- Compres- Waste- Benzene Reboiler Condenser Reflux Reflux Styrene Reboiler Condenser Reflux

Column Pump

Cooler

A 9.

Cooler Phase sor Separator

bfw E-404

cw E-405

water Toluene Pump Column

Pump Drum

P-404NB Benzene Toluene Pump

P-405 NB P-406 NB Styrene Recycle Pump Pump

...-.---<?3.'>---�

��---<�

I

.{a

C-401 Hydrogen

P-401 A/B lps

�---<98>---Wastewater (") ::r (I) 3 n Ill -m ::::l �-::::l (I) (I) _.... ::::l ()ti w "1J Cl ... 0 0 .j::>. (") ro VI VI "'T1

�

0 iii. (lq _...

(") 0 ::::l !:!". ::::l c: ro CL 0 ::::l ""'CJ ll>

O;J

01

Stream Number

Temperature (0C)

Pressure (kPa) Vapor mole fraction Total flow (kg/h} Total flow (kmol/h} Component flows

Water Ethylbenzene Styrene

Hydrogen .

Benzene Toluene Ethylene Methane

Stream Number

Temperature (0C)

Pressure (kPa} Vapor mole fraction Total flow (kg/h} Total flow (kmol/h} Component flows Water Ethylbenzene Styrene Hydrogen Benzene Toluene Ethylene Methane 1 16 20 0 19,417 183.6 0.0 180 0.0 0.0 1.8 1.8 0.0 0.0 7 800 565 1 44,120 8000 8000 0.0 0.0 0.0 0.0 0.0 0.0 0.0

2 3

116 225

200 180

0 1

54,890 54,890

517.8 517.8

0.0 0.0

512.7 512.7

1.2 1.2

0.0 0.0

1.8 1.8 2.13 2.13

0.0 0.0

0.0 0.0

"

8 '· 9

7g9 632

180 170

1 1

144,120 199,010

8000 8517.6

8000 8000

0.0 512.7

0.0 1.2

0.0 0.0

0.0 1.8

0.0 2.13

0.0 0.0

0.0 0.0

Stream Tables for Styrene Production Unit

4 5 6 Stream Number 13

159 800 800 Temperature (0C) 270

600 565 565 Pressure (kPa) 120 1 1 1 Vapor mole fraction 1

227,784 227,784 83,664 Total flow (kg/h} 199,010 12,644 12,644 4644 Total flow (kmol/h} 8662.7

Component flows

12,644 12,644 4644 Water 8000

0.0 0.0 0.0 Ethylbenzene 336.36 0.0 0.0 0.0 Styrene 120.67 0.0 0.0 0.0 Hydrogen 88.1 0.0 0.0 0.0 Benzene 27.5

0.0 0.0 0.0 Toluene 33.3

0.0 0:0 0.0 Ethylene 25.7

0.0 0.0 0.0 Methane 31.1

10 11 12 Stream Number 19

609 650 640 _{Temperature (0C) 197}

160 145 135 _{Pressure (kPa) 240}

1 1 1 _{Vapor mole fraction 1}

199,010 199,010 199,010 Total flow (kg/h} 2682

8614.7 8614.7 8662.7 Total flow (kmol/h) 216.3

Component flows

8000 8000 8000 _{Water 71.4}

399.1 399.1 336.36 _{Ethylbenzene 0.0}

86.8 86.8 120.67 Styrene 0.0

69.0 69.0 88.1 _{Hydrogen 88.l}

13.1 13.1 27.5 _{Benzene 0.0}

18.7 18.7 33.3 _{Toluene 0.0}

11.3 11.3 25.7 _{Ethylene 25.7}

16.6 16.6 31.1 _{Methane 31.1}

(continued)

14 15 16

180 65 65

105 90 75

1 O.Q25 1

199,010 199,010 2682 8662.7 8662.7 216.3

8000 8000 71.4

336.36 336.36 0.0

120.67 120.67 0.0

88.1 88.1 88.1

27.5 27.5 0.0

33.3 33.3 0.0 25.7 25.7 25.7

31.1 31.1 31.1

20 21 22

65 66 119.5

60 40 60

0 0 0

53,493 5548 47,905

517.8 63.9 453.9

0.0 0.0 0.0

336.36 3.36 333.0

120.67 0.1 120.53

0.0 0.0 0.0

27.5 27.5 0.0

33.3 32.9 0.33

0.0 0.0 0.0

0.0 0.0 0.0

17 18

65 65

75 75

0 0

53,493 142,715

517.8 7928.6

0.0 7928.6

336.36 . 0.0

120.67 0.0

0.0 0.0

27.5 0.0

33.3 ) _0.0

0.0 0.0

0 0.0 0.0

23 24

105 124.5

210 60

0 0

35,473 12,432 334.2 119.7

0.0 0.0

332.66 0.34

1.20 119.3

0.0 0.0

0.0 0.0

0.33 0.0

0.0 0.0

0.0 0.0

Chemical Engineering 3G04 5

2.5 pts

Stream Tables for Styrene Production Unit (cont'd)

Stream No. 25 ·26 27 28 29

Ternperature(°C) 655 66 124.5

_-

65 105

Pressure (kPa) 565 200 200 200 210

Vapor mole fraction 1 0.0 0.0 0.0 0.0

Total flow (kg/h) 83,664 548 12,432 14,2715 35,473

Total flow (krnol/h) 4644 63.9 119.7 7928.6 334.2

Component flows

Water 4644 0.0 0.0 7928.6 0.0

Ethylbenzene 0.0 3.36 0.34 0.0 332.66

Styrene 0.0 0.1 119.3 0.0 1.20

Hydrogen 0.0 0.0 0.0 0.0 0.0

Benzene 0.0 27.5 0.0 0.0 0.0

Toluene 0.0 32.9 0.0 0.0 0.33

Ethylene 0.0 0.0 0.0 0.0 0.0

Methane 0.0 0.0 0.0 0.0 0.0

J

ii. Comment this special condition from the kinetics point of view. Does it justify the elevated temperature in the reactor?

Hint. Evaluate and compare the kinetic rates for reaction 1, both for the tempera­

ture and pressure conditions at the inlet of R-402 and for a lower temperature (e.g. 500°C).

1

pt (d) Discuss the reasons for using two sets of adiabatic packed beds in series (R-40la-e and

R-402a-e

)

with inter-heating. What are the advantages/drawbacks with respect to a

single set of packed beds?

2 pts (e) Based on qualitative reasoning, would it be advantageous to operate the reactors at higher pressure from the thermodynamic point of view? How about from the kinetics point of view?

2. Consider the fiowsheet shown in the figure on page 6. Units are identified with letters (from

A to

U),

whereas streams are numbered (from

1

to

31).

Questions:

6 pts (a) Partition and precedence order this fiowsheet using the algorithm by Sargent and

Westerberg. Start the algorithm at unit 0. Clearly indicate and comment all the intermediate steps.

5 pts (b) For the largest partition only, identify all of the (nonredundant) process loops. Then, determine a minimum tear set for that partition.

3

pts (c) Based on the tear set determined in Question 2b above, give the sequence of

calcula-tion for the corresponding partition.

() 0 ::I ...

5·

c: .. a. 0 ::I "U OJ

�

-.J

30

�---_15 ____ ----,-.. L ₁₄

(14---�

22 29 s

23

R

() :::r ..

3

�-Chemical Engineering 3G04 7

3. In a chemical plant, four streams need to be cooled and heated:

Stream T m

₍

oC

₎

T°ut

(

oC

)

mCp

₍

kW

_/

°C

₎

Hl 180 60 3

H2 150 30 1

Cl 30 135 2

C2 80 140 5

Questions:

6 pts

₍

a

₎

For a minimum temperature approach .6.Tmin = 10°C, construct the

temperature-interval and cascade diagrams, then find the minimum heating and cooling utili­

ties. What are the pinch temperatures?

3 pts

₍

b

₎

Determine the minimum number of heat exchangers above the pinch only.

3 pts

₍

c

₎

Design a feasible heat exchanger network

_(HEN)

to carry out the heat transfer for the

minimum utility requirements above the pinch only. Explain all of the intermediate

steps in your design.

4 pts 4. Bonus Question. Discuss

₍

in 10-15 lines

₎

an important lesson related to process equipment,

modeling and

_/

or simulation that you learned from either the boiler house visit or the seminar

on wastewater process simulators in 3G04.

Answers such as "Equipments in the boiler house are big" and "Wastewater process simulators are important" will receive zero points. You should cite a specific issue,

e.g., regarding refrigeration

_/

boiler equipment, simulation technology, modeling, etc.