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Thermodynamics: Dehydrogenation of

Lower Alkanes Desired products:

100 = 1 bar

p Lower alkenes

p = 1 bar

(mol%)

(mol%) 80 ↔

iso-butane iso-butene + H

2

conversion

conversion 60 ↔

propane propene + H

2

40 Favourable

Alkane

Alkane conditions??

20 ↔

ethane ethene + H

2

0 high T

700 800 900 1000 1100 1200 low p

Temperature (K)

Temperature (K)

T

U Delft Mechanism • •

Initiation: H C-CH H C + H C

3 3 3 3 •

Propagation: H C + H C-CH CH + H C-CH

3 3 3 4 3 2

• •

H C=CH + H

H C-CH 2 2

3 2 •

H + H C-CH H + H C-CH

3 3 2 3 2

• etc.

H C-CH

3 2

• •

H + H H

Termination: 2

• •

+ H C H C=CH + CH

H C-CH 3 2 2 4

3 2

etc.

T

U Delft Thermal Cracking of Higher Alkanes

Dehydrogenation

C H C H

7 16 7 14

Primary

cracking Cracked products

C H C H C H C H

+

3 6 3 8 4 8 4 6

Secondary Cycloadditions

cracking and dehydrogenations

CH C H C H

+ 2 Coke

4 2 4 2 4

not too high:

Optimal selectivity decrease

conversion?? (secondary reactions)

T

U Delft Kinetics

100 n-pentane

n-butane

(1/s) isobutane

10 propane

k ethane

constant 1

rate C-C-C-C-C C-C

Reaction 0.1 C-C-C

0.01

800 900 1000 1100 1200

Temperature (K)

Optimal temperature??

T

U Delft Isothermal Plug-Flow Reactor

X dX

=

V F −

0

A r A −

0 1 X

⋅ = ⋅ ⋅

Volumetric k C k C

ϕ ⋅ ε

+

C A A 1 X

0 0

v A

flow rate ε

Assume = 0

  X 1 1

V dX

 

τ = = = ⋅ ln

  ( )

ϕ ⋅ − −

  1 1

k X k X

0

v 0

τ = 0.5 s

E.g., 1 1

= ⋅ = T ?

ln 1 . 83

k τ −

= 0.6

X 1 X

Ethane = 1100

T K

Propane 1060

From figure Isobutane 1040

1030

n-Butane

T

U Delft 1010

n-Pentane

Starting Points

• High heat input

• Low HC pressure

• High temperature

• Low pressure ∼

• Short residence time ( 0 .5s) • Reactor?

• Efficient quench • T-profile?

• Low pressure drop

• Plug-flow reactor

T

T

U Delft Steam Cracker

Stack

Naphtha (360 K)

Dilution steam Convection section

870 K

700 K 1120 K

high pressure

steam Radiation section

T

U Delft Composition of Typical Naphtha

Feedstock

Component Analysis ASTM D 86 distillation

wt. % of total vol. % K

1)

C , C 306

8.0 bp

3 4 I

C 5 320

22.4

5

C 10 325

19.9

6

C 20 334

17.2

7

C 30 344

12.4

8

C 40 355

11.5

9

C - C 50 369

8.6

10 15

Total 60 381

100.0 70 397

n-Alkanes 80 418

30.5

iso-Alkanes 90 460

39.9 2)

Naphthenes 17.7 480

bp

f

Aromatics 11.9

Total 100.0

1) 2)

Initial boiling point; Final boiling point

T

U Delft

Examples of Cracking Yields of Ethane

and Naphtha

Ethane Ethane Ethane Naphtha Naphtha Naphtha

A B C D E F

3 3 3)

) )

Conversion, wt% 53.9 94.19 92.04

69.0 69.3 81.47

1053 1073 1073

Outlet temperature, K 1073 1106 1107

Steam ratio, 0.50 0.50 0.75

kg steam/ kg feed 0.30 0.30 0.45

Yield, wt% 0.54 0.72 0.71

Hydrogen 3.35 4.21 4.27 11.98 15.26 14.21

Methane 3.08 6.21 5.64 0.09 0.25 0.28

Ethyne 0.14 0.32 0.38 19.46 23.52 24.00

Ethene 42.50 50.10 51.45 3.97 3.95 3.40

Ethane 46.00 30.93 30.60

41)

H 0.31 0.64 0.68

C 0.01 0.02 0.02

3

Propene 16.15 16.15 15.50

1.41 1.67 1.55

Propane 0.56 0.50 0.45

0.16 0.22 0.20

Butadiene 3.73 3.95 4.28

0.89 1.41 1.47

Butenes 7.93 5.44 6.05

0.23 0.24 0.23

Butanes 2.63 1.37 1.65

0.33 0.25 0.24

2)

- 478 K

C 30.19 26.04 25.78

1.82 3.94 3.57

5

Fuel oil 2.46 3.40 2.95

0.08 0.48 0.38

Total 100.00 100.00 100.00

100.00 100.00 100.00

T

U Delft Calculated Process Parameters

Ethane Ethane Ethane Naphtha Naphtha Naphtha

A B C D E F

Time in coil, s 0.737 0.704 0.627 0.537 0.519 0.432

Time to quench, s 0.050 0.046 0.041 0.030 0.027 0.022

HC partial pressure

Coil outlet (bar) 1.54 1.57 1.41 1.04 1.10 0.89

Quench point (bar) 1.49 1.52 1.36 1.01 1.06 0.85

Temperature, K

Tube wall, max. 1213 1268 1270 1174 1219 1223

Gas inlet 922 922 922 863 863 863

T

U Delft

Temperature Profile in Steam Cracker

1150

Temperature Ethane

1100

K 1050

1000 Naphtha

950 Gas oil

900

850

800 0 20 40 60 80 100

Distance from inlet (% of coil length)

Reactors far from isothermal

T

U Delft


PAGINE

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PESO

212.96 KB

AUTORE

Atreyu

PUBBLICATO

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DETTAGLI
Corso di laurea: Corso di laurea in chimica industriale e tecnologie del packaging
SSD:
Università: Parma - Unipr
A.A.: 2007-2008

I contenuti di questa pagina costituiscono rielaborazioni personali del Publisher Atreyu di informazioni apprese con la frequenza delle lezioni di Chimica industriale II e studio autonomo di eventuali libri di riferimento in preparazione dell'esame finale o della tesi. Non devono intendersi come materiale ufficiale dell'università Parma - Unipr o del prof Moggi Pietro.

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