Mechanistic Studies of Methane Pyrolysis at Low Pressures - ACS

4 Mechanistic Studies of Methane Pyrolysis at Low Pressures K. D. WILLIAMSON and H. G. DAVIS

Downloaded by DICLE UNIV on November 11, 2014 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0032.ch004

Union Carbide Corp., South Charleston, W.Va. 25303

Introduction In recent years, most workers have adopted an abridged v e r s i o n o f the R i c e - H e r z f e l d mechanism(1-5) f o r ethane p y r o l y s i s H

C

>

2 CH *

Δ b C

V

+ C

H

2 6

C H 2

1)

(Initiation)

ô

5

H. + C H 2

5

2 CgHg-

>

C H

4

+ C

H

>

C H

>

H

>

C H

2

2

2 5'| + H.

4

)

^(Propagation) 3)

+ C H . J

2

2

4

5

(Termination) 5a)

1 0

or C H 2

4

+ C H 2

5b)

6

Other p o s s i b l e t e r m i n a t i o n r e a c t i o n s , such as ( 6 ) , (7), o r (8) are o f minor importance because the C

H

V *

+

+

C

C

H

H

2 5'

2 5'

C

H

C

H

6 )

> 3 8 > 2 6 or C H 2

2 CH

3

> C H 2

7

4

+ H

2

6

a

)

7b) 8)

concentrations of hydrogen atom and methyl r a d i c a l are small compared to the concentration of e t h y l r a d i c a l , at common temperatures and pressures. Termination reactions f o r reactions (9) and (10) are ruled out also because they require three-body c o l l i s i o n s

51

In Industrial and Laboratory Pyrolyses; Albright, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

52

INDUSTRIAL AND LABORATORY PYROLYSES

2 Η· and H- + CH «


3

>

H

>

CH

9)

2

4

10)

to s t a b i l i z e the newly-formed molecules, and are im probable on that account. The o v e r - a l l r e a c t i o n becomes more complicated than i n d i c a t e d by r e a c t i o n s ( l ) - ( 5 ) before much p y r o l y s i s has occurred. Data on the r a t e s of the primary r e a c t i o n s (obtained photochemically and other wise) show that r e a c t i o n s ( 3 R ) and ( 4 R ) , the reverse of ( 3 ) and ( 4 ) , must become s i g n i f i c a n t very quick l y (2). These r e a c t i o n s o f f e r the s i m p l e s t e x p l a n a t i o n of the s e l f - i n h i b i t i o n of the o v e r - a l l r e a c t i o n which develops when only a few per cent decomposition of ethane has occurred. Moreover, the primary product, ethylene, r e a c t s with the free r a d i c a l s of the system, p r i n c i p a l l y e t h y l , because of i t s r e l a t i v e abundance, to form molecules i n the C 3 - C 4 range, and e v e n t u a l l y molecules of even higher molecular weight. A d d i t i o n a l l y , the butane which accumulates as a c h a i n termination product, pyrolyzes approximately four times as f a s t as i t s parent ethane—making only s l i g h t s t o i chiometric changes, but f a s t obscuring the mechanistic c l u e s a s s o c i a t e d with minor products. Obviously, as p y r o l y s i s of the ethane proceeds, the o v e r - a l l p i c t u r e becomes ever more complicated. Any present hope of understanding the mechanism must be based on s a t i s f a c t o r y knowledge of the i n i t i a l phenomena. I d e a l l y , data would be taken at e s s e n t i a l l y zero decomposition of the ethane. However, t h i s i s extremely d i f f i c u l t e x p e r i m e n t a l l y , e s p e c i a l l y at the higher temperatures of commercial i n t e r e s t . The necessary p r a c t i c a l compromise i s to take data at a few per cent decomposition, where product compositions, r e a c t i o n times, e t c . , are f i n i t e — t h e n e x t r a p o l a t e or c o r r e c t back to zero composition. That i s the course followed here. The primary purpose of the present work was to check some phenomena which f o l l o w from the p r o j e c t e d mechanism. Two p r i n c i p a l e f f e c t s to be looked f o r were: ( 1 ) An increase i n the s p e c i f i c r a t e of the o v e r - a l l p y r o l y s i s as the p a r t i a l pressure of ethane i s lowered, and (2) an i n d i c a t i o n that e t h y l com b i n a t i o n was not the e x c l u s i v e r e a c t i o n f o r chain termination at very low pressure. These phenomena are deduced from a steady-state treatment of r e a c t i o n s ( 1 ) ( 5 ) , w i t h the back r e a c t i o n s ( 3 R ) and ( 4 R ) i n c l u d e d . This treatment gives f o r the o v e r - a l l r a t e of ethane


4.

WILLIAMSON AND DAVIS

Ethane Pyrolysis at Low Pressures

pyrolysis*:

53

-1/2

Η

d I n &2 β1 dt

(

k l

/k )

1 / 2

5

C

H

k (1- |) [ 2

C H 2

6

or H + C H 2

2

4


4.



55

s i n c e the products of these chain terminations could not be d i s t i n g u i s h e d from other major amounts o f these species i n the r e a c t i o n . E v e n t u a l l y , of course, i f the chain t e r m i n a t i o n steps change from (5) to ( 6 ) , e t c . , the o v e r - a l l r a t e expression and the expressions f o r r a d i c a l con c e n t r a t i o n s w i l l change. For chain t e r m i n a t i o n dominated by ( 7 ) , CH · remains the same as p r e v i o u s l y , but


3

H

[ ]o

=

k„k \^ξ) C

and

H

d lni 2 J -jz

1/2 (

k

k

n

o

t a

/^l 3 4 1 J \ 7 /

=4—τ-

f

f

e

c

t

e

d

b

y

[ 2 (J > C

H

2

(a true f i r s t - o r d e r expression).

Thus, a t r a n s i t i o n from e t h y l - e t h y l t e r m i n a t i o n t o ethyl-hydrogen atom t e r m i n a t i o n would e x h i b i t a l i n i n g - o u t o f the o v e r - a l l decomposition r a t e a t some maximum l e v e l . These various p o i n t s w i l l be checked against the experimental data reported here. Experimental* Work The experimental work reported here was per formed i n d i f f e r e n t quartz flow r e a c t o r s , having surface-volume r a t i o s from 1 t o 20. D i f f e r e n t r e a c t o r s were used f o r two r e a s o n s — f i r s t , t o de monstrate that r e s u l t s were r e l a t i v e l y i n s e n s i t i v e to the surface-volume r a t i o , and other features r e l a t i n g t o a p a r t i c u l a r r e a c t o r ; second, t o permit maintenance of proper flow r a t i o s , f o r d e s i r e d r e a c t i o n times, without appreciable pressure drop. Flow r e a c t o r s were r e q u i r e d because of the r e l a t i v e l y high temperatures used, and the short r e a c t i o n times r e q u i r e d f o r low conversions. V i r t u a l l y a l l l o n g i t u d i n a l temperature gradient was removed from two annular r e a c t o r s used (B and C) by mounting them w i t h i n f l u i d i z e d beds, then making f i n a l adjustment on segment heaters along the r e a c t o r s . This type o f r e a c t o r i s i l l u s t r a t e d i n Figure 1. Gradients were minimized along the 1" tubular r e a c t o r (A) by mounting i t w i t h i n a massive copper block. Reactors were seasoned by passing ethane through them f o r s e v e r a l hours p r i o r t o a run. At the r e l a t i v e l y high temperatures used here, surface r e a c t i o n s should be l e s s important than f o r some r e ported i n the l i t e r a t u r e . However, as discussed


56

INDUSTRIAL AND LABORATORY PYROLYSIS (REACTOR DETAIL NOT TO SCALE)

THERMOWELL REACTOR EXIT DISENGAGING ZONE FLUIOIZEO SAND MANOMETER PURGE


ANNULAR SPACE FOR REACTANT GAS FLOW

PLATINUM HEATER WINDINGS

MOORE FLOW REGULATOR GAS CYLINDER

QUARTZ FRIT REACTOR INLET NITROGEN STREAM FOR FLUIDIZATION Figure 1.

Schematic of apparatus for pyrolysis of hydrocarbons

l a t e r , the p o s s i b i l i t y of s u r f a c e r e a c t i o n s must be considered at the lower p a r t i a l pressures of ethane explored i n the present work. Gas f l o w i n g out of the r e a c t o r was immediately quenched by a small quartz water condenser j u s t beyond the r e a c t i o n zone. Beyond the condenser was a sampling p o i n t , f o r c o l l e c t i n g samples f o r gas chromatography. A l l components i n the a n a l y s i s were checked q u a n t i t a t i v e l y against standard samples. I n a d d i t i o n , some analyses were performed by mass spectrometer, as a check against the GC. Complete atom balances were made on each run. These atom balances provided the most q u a n t i t a t i v e b a s i s f o r c a l c u l a t i n g the expansion of gas due to r e a c t i o n , a f a c t o r r e q u i r e d f o r c a l c u l a t i n g accurate residence times.




57

TABLE I PYROLYSIS OF ETHANE AT ATMOSPHERIC PRESSURE Temp., °C Reaction Time (seconds)

2. 52

4.9

H


C

0.088

4

H

2 2 C H 2

4

C

2 6

C

H

H

C

3 8

C

H

4

2

C

8

H

Chains i n i t i a t e d *

1.,66

1.88

21.1

31.9

0.33

0.91

2.06

0.004

0.007

0.033

45. 6 4.71 0.,107

21.4

31.9

44.0

95. 0

88. 1

77.8

66.2

50. ,3

0.14

0.35

0..66

0.003

0.020

0. 051

0.013

0.060

0.149

0..390

0.019

0.043

0.054

0.,072

0.004

0.005

0.010

0,.038

0.,028

0.,074

0.128

0.149

0,.144

1..24

1.32

1.09

1.33

1,.20

0.,003

H

4 10

768

11. 6

~ 4 8 C

11. 5

0.033

H

4 6 1-C H

2.17

744

4.9 0,,005

3 6

2.37

723

Moles/100 Moles of Ethane Fed

Products 2 CH

698

673

Chains terminated

•Estimated number of chains i n i t i a t e d divided by estimated number of chains ended by ethyl combination and disproportionation. See text.

Scope o f Data, Experiments covered the temper atur^nFâTngënS^--775°C, absolute pressures from 0.13-1.67 atm., and steam d i l u t i o n s up t o 100:1 molar. Reaction times were from 0.5-2.5 seconds, depending on other c o n d i t i o n s , t o give a range of conversions from 0.2-75%. For present purposes, the data a t low conversions are most p e r t i n e n t . Sample data f o r one set of runs i s shown i n Table I. C h a r a c t e r i s t i c s o f the various sets of data used f o r t h i s paper are given i n Table I I . Discussion The f i r s t p r e d i c t i o n o f the model to be checked i s v a r i a t i o n o f the i n i t i a l r a t e o f disappearance of ethane with pressure. As mentioned e a r l i e r , the data were taken at a p p r e c i a b l e decomposition o f ethane,


58

INDUSTRIAL A N D L A B O R A T O R Y PYROLYSES

TABLE II DATA USED IN THIS STUDY PYROLYSIS OF ETHANE AT CONDITIONS SHOWN

Reactor


A A A A A A A A Β C

Pressure (atm.) 1 1 1 1.67 1.67 1 1 1 1 0.13

A = Quartz reactor 1" I.D., ml vol.

Steam D i l u t i o n 0:1 0.64:1 8:1 0:1 0.77:1 9:1 100:1 100:1 0:1 0:1

Nominal Reaction Time (sec.) 2.5 2.5 2.5 2.5 2.5 1.0 1.0 2.5 0.5 0.5

90 cm long i n copper block, 147

Β = Quartz annular reactor 1 mm annulus, 60 cm long i n f l u i d bed, 38 ml. v o l . C = Quartz annular reactor 1 mm annulus, 60 cm long i n f l u i d bed, 16 ml vol.

and must be c o r r e c t e d back t o the values a t "zero" decomposition f o r v a l i d comparison. The method of achieving t h i s w i l l be described. From expression (A) the r a t i o of the r a t e con s t a n t a t time zero t o that a t time t i s derived as

where the values with s u b s c r i p t t are those e x i s t i n g at time t. R can be s e t up as a f u n c t i o n of the f r a c t i o n of ethane decomposed, with one a d j u s t a b l e parameter, k 4 / k 3 R . This parameter can be evaluated by matching R with r e a l data f o r ethane decomposition at constant temperature and v a r i a b l e time. This has


4.



been done f o r s e v e r a l s e t s o f data, not presented here, to give k4/k3R = 2 , a reasonable value i n terms of photochemical data (2). The f u n c t i o n R i s then used to generate an i n t e g r a l f u n c t i o n k/k , where k i s the v i r t u a l r a t e constant, which has been measured, and k i s the r a t e constant at zero time. Figure 2 gives a p l o t of t h i s f u n c t i o n . I f , f o r example, a r a t e constant i s c a l c u l a t e d f o r a run w i t h 30% decomposition of ethane, then that r a t e constant i s d i v i d e d by 0.7 5 to o b t a i n k . A n a l y s i s shows that k/k should be nearly l i n e a r u n t i l e q u i l i b r i u m i s c l o s e l y approached. Near e q u i l i b r i u m , the value of k/k drops s h a r p l y . The departure from l i n e a r i t y , o c c u r r i n g at d i f f e r e n t decompositions at d i f f e r e n t temperatures i s not shown here. For the r e l a t i v e l y low decompositions considered here, i t was not important. Rate constants, so adjusted, are shown as Arrhenius p l o t s i n Figure 3. The somewhat d i f f e r e n t slopes of the various l i n e s probably r e f l e c t some e r r o r i n the c o r r e c t i o n technique f o r runs at very high decomposition, and e r r o r s i n measuring decomposition f o r runs at very low decomposition. Another p o i n t of i n t e r e s t i s comparison of data from d i f f e r e n t r e a c t o r s . Rate constants measured i n the annular r e a c t o r (B) seem to run about 25% higher than those i n the r e a c t o r of one-inch diameter (A). This p o s s i b l y measures a d i f f e r e n c e i n r a t e of heat t r a n s f e r l e a d i n g to e r r o r s i n opposite d i r e c t i o n s i n the estimate of absolute temperature l e v e l . Despite these r e l a t i v e l y s m a l l d i s c r e p a n c i e s , the data f o r the d i f f e r e n t r e a c t o r s over a wide span of decomposition demonstrate the general v a l i d i t y of the experimental work. C l o s e r comparisons can be drawn, of course, between s e t s of runs done i n the same r e a c t o r than between s e t s done i n d i f f e r e n t r e a c t o r s . The r a t e constants f o r runs at one atmosphere, but 9:1 d i l u t i o n , are much higher than t h e i r counterp a r t s f o r u n d i l u t e d ethane at one atmosphere. F u r t h e r more, r a t e constants f o r 100:1 d i l u t i o n are s t i l l higher. The comparison between r a t e constants a t these three c o n d i t i o n s at 725°, and p r e d i c t i o n s from the model are shown below. Shown a l s o i s k f o r another set at 1.67 atmosphere pressure (no d i l u t i o n ) , which i s not i n c l u d e d on the Arrhenius p l o t . Q

Q

Q

Q


59

Q

Q


60

INDUSTRIAL

A N D L A B O R A T O R Y PYROLYSES

Ο ο *

CO ο *

οο

•

ο *


1 ο -I

ε

Où

ο

I •3 ο

!

*^ *δ?

Ο

ο ο

•8

Ι SP

i

ο

8

I Où

s

ε



Ethane Pyrolysis at Low

Pressures

61


4.

Figure 3.

Ethane decomposition rate constants for zero time. Comparison at different partial pressures of ethane.



62

TABLE III RATE CONSTANT FOR CgHg

> 2 CHg- AT 675°C

Conditions


Reactor

Pressure (atm.)

Β C

Residence Time (sec.) 2.52 2.,60 2,.48 2.,46 1..01 1..01 1..98 0..57

0:1 8::1 0:;1 0.77::1 9::1 100: :1 100: :1 0::1 0::1

1 1 1.67 1.67 1 1 1 1 0.13

A A A A A A A

(See

Dilution

0,.59

(corr) K

l

. , xl0*(sec ) A

1.65 1.59 1.63 1.77 1.91 2.42 2.47 3.25 1.61

Table I for identification of reactors.)

t h e i r counterparts a t one atmosphere. The explan a t i o n f o r t h i s exception seems t o l i e i n the pressure s e n s i t i v i t y o f one o r more o f the r a t e constants w i t h i n the c l u s t e r C = ( k ] / k ) k 3 . A pressure e f f e c t on k5 would be i n the wrong d i r e c t i o n and i s u n l i k e l y i n any case, so k5 can not be the cause of the low k . Previous r e p o r t s (3,4a,6) have i n d i c a t e d that k^, f o r the cleavage o f ethane i n t o two methyls, becomes p r e s s u r e - s e n s i t i v e a t about 100 mm., the present pressure. However, data shown l a t e r i n t h i s paper i n d i c a t e no appreciable drop i n k i a t 100 mm., as compared w i t h i t s value a t higher pressure. Probably the p r i n c i p a l cause f o r the abnormally low ethane decomposition at 100 mm. i s the pressure s e n s i t i v i t y o f k^. T h i s has a l s o been documented p r e v i o u s l y (2-4) though i t i s hard to q u a n t i f y . Present evidence suggests that i n the r a t e de termining c l u s t e r ( k i / k 5 ) l / 2 kg [ C 2 H 5 J ~ l / 2 lowering the t o t a l pressure reduces k3 p r o p o r t i o n a t e l y more than i t increases [ C 2 H 5 J " / , thus leads to a net decrease i n the o v e r - a l l r a t e . E v i d e n t l y when the t o t a l pressure i s maintained at one atmosphere by s u b s t i t u t i n g steam f o r ethane, the dominant change i s i n [ C 2 H 6 J ' . * However, water molecules may not be q u i t e so e f f e c t i v e i n t r a n s f e r r i n g energy to ethane molecules as are other ethane molecules. This may account f o r the k s i n steam d i l u t i o n runs being somewhat l e s s than p r e d i c t e d . 1 / / 2

5

Q

?

1

1

2

2

f

Q




63

Constancy of I n i t i a t i o n Reaction. Despite the v a r i a t i o n i n the o v e r - a l l r a t e constant at a p a r t i c u l a r temperature of nearly an order of magnitude, depending on the pressure and d i l u t i o n , the r a t e constant f o r the i n i t i a t i o n r e a c t i o n i s remarkably constant. This i s i l l u s t r a t e d by Table I I I , which gives r a t e constants k i f o r 675°C. R e l a t i v e l y minor c o r r e c t i o n s f o r secondary methane were necessary at t h i s low temperature—the ethane decomposition never


Θ

Figure 4.

Rate constant for C H -» 2 CH . Variation with temperature. 2

6

S



64

exceeding 15%. From the lowest value to the highest, there i s about a f a c t o r of two d i f f e r e n c e i n k^. I t would appear that k i i s s l i g h t l y higher f o r the runs at 100:1 d i l u t i o n than f o r other runs i n the same r e a c t o r . The only other high value of k i i s f o r one atmosphere, no steam d i l u t i o n , i n an annular r e a c t o r . The low value of k i are i n e s p e c i a l l y good agreement w i t h the value c a l c u l a t e d from the equation given by L i n and Back (4a), derived from experiments i n the range 550-620°C: 1.50 χ 10~ s e c . " . I t i s evident that the major part of the de pendency of the o v e r - a l l r a t e constant on pressure i s not due to the pressure s e n s i t i v i t y of k^, i n the present range of study. The behavior of k^ at atmospheric pressure through a temperature range of over 200°C i s shown i n Figure 4. The slope of the Arrhenius p l o t , corresponds to an a c t i v a t i o n energy of 87 kcal./mole, c o n s i s t e n t w i t h the accepted s t r e n g t h at these tem peratures f o r the C-C bond s t r e n g t h i n ethane.


4

1

Chain I n i t i a t i o n and Chain Termination. V i a r e a c t i o n s ( l ) - ( 5 ) , a l l chains i n i t i a t e d are manifested by production of methane and a l l chains terminated by production of butane (or a l t e r n a t e l y , ethylene + ethane). Then, CH Chains i n i t i a t e d 4 , Chains terminated £ 4 1 0 c o n t r i b . from (5b)} Though the c o n t r i b u t i o n from (5b) cannot be determined i n p y r o l y s i s experiments, the r a t i o k5^/k5 has been determined repeatedly as about 0.15 (7). That f a c t o r i s used i n present c a l c u l a t i o n s . This p i c t u r e i s modified s l i g h t l y by the s m a l l amount of propane, e v i d e n t l y formed by r e a c t i o n (6). Each propane formed i n t h i s way denotes the stopping of one c h a i n . Thus, the moles of propane should be added to the denominator. (The formation of a molecule of propane consumes two r a d i c a l s , of course. However, the methyl r e a l l y i s i n t e r c e p t e d before i t r e a c t s v i a r e a c t i o n (2) to s t a r t a chain.) Secondary r e a c t i o n s almost immediately add to the amount of methane recovered and s u b t r a c t from the butane recovered. These secondary r e a c t i o n s must be c o r r e c t e d f o r , i n c o n s i d e r i n g chain i n i t i a t i o n and termination. Two p r i n c i p a l types of r e a c t i o n are s i g n i f i c a n t i n t h i s respect: (1) secondary r e a c t i o n s β

=

2

C

H

+

a


4.

AND DAVIS

WILLIAMSON


65

i n v o l v i n g the product, ethylene, and (2) decom p o s i t i o n of butane. The ethylene r e a c t i o n s are presumed t o take a form such as (8) C H . + C H 2

5

2

->

4

CHgCH CH CH CH CH


2

2

2

C H 2

4

3

+ CH . 3

In t h i s sequence, there i s no net g a i n o r l o s s o f r a d i c a l s — t h u s no chains should be counted as being i n i t i a t e d o r terminated. However, the methane generated here w i l l count as a chain s t a r t e d unless i t i s subtracted out. By the scheme o u t l i n e d here, the moles of excess methane equal the moles of propylene. T h i s w i l l not be s t r i c t l y true. For example, formation of 3-hexyl i n the i s o m e r i z a t i o n step would lead to other products; a l s o , small amounts of propylene can be formed by other routes such as l o s s of Η· from propyl or decomposition o f butene or butane. Moreover, as mentioned l a t e r , the propylene i t s e l f decomposes, and i t s l o s s must be c a l c u l a t e d . The l o s s of butane i s estimated from a knowledge of the r e l a t i v e r a t e s of decomposition of admixed butane and ethane. Over a wide range o f c o n d i t i o n s , the r a t e constant f o r decomposition of butane i s about four times that f o r ethane (11). I f the butane i s i n i t i a l l y generated from the ethane, then, a t f i r s t , the r a t e of butane generation w i l l g r e a t l y exceed the r a t e of i t s decomposition. Since the ethane concentration w i l l not change much during t h i s p e r i o d , the net accumulation o f butane w i l l be n e a r l y l i n e a r , as shown by Figure 5 . The amount of butane which does disappear from time 0 to time t can be approximated from the average c o n c e n t r a t i o n of butane during t h i s period. I f te oe where the s u b s c r i p t s b and e stand f o r butane and ethane, r e s p e c t i v e l y , then



Figure 5.

50 ETHANE

60 70 DECOMPOSED

0.5 S E C .

2 SEC. REACTION

REACTION

η-Butane yields in ethane pyrolysis. Effects of dilution (1 atm total pressure).

40 PERCENT

650-775·,

625-725·,

100:1 D I L . ,

TIME

TIME

TIME

TIME

TIME

REACTION

REACTION

REACTION

1 SEC.

2 SEC.

1 SEC.

650-750*,

675-775*,

100:1 D I L . ,

650-725%

9:1 D I L . , 8:1 D I L . ,


ο

I

ο

4.



67


4

and from the r e a c t i o n of ethane that has decomposed, the f r a c t i o n o f the average butane c o n c e n t r a t i o n that has decomposed can be c a l c u l a t e d . This scheme has not been used to c o r r e c t f o r butane d e f i c i e n c y a t l e v e l s of decomposition of ethane greater than 15%. At t h i s l e v e l , the c o n c e n t r a t i o n of butane i n the product must be m u l t i p l i e d by a f a c t o r of 1.24. The decomposition of the butane i s important, not only because of i t s own disappearance, but a l s o because i t s decomposition generates a d d i t i o n a l secondary methane. The butane, l i k e the methane, w i l l decompose p r i n c i p a l l y by r e a c t i o n with f r e e r a d i c a l s i n the system ( 9 ) ,

Again, t h i s s e t of secondary r e a c t i o n s does not r e s u l t i n net formation or t e r m i n a t i o n of chains. However, the e x t r a methane so formed, must be s u b t r a c t e d from t o t a l methane, i n c a l c u l a t i n g the number of chains i n i t i a t e d . As with the secondary r e a c t i o n s i n v o l v i n g ethylene, a propylene tends t o be formed f o r each secondary methane. Insofar as t h i s remains unreacted, the propylene can be used t o c a l c u l a t e the secondary methane, from whichever source. Previous work has shown that propylene, i n excess ethane, decomposes much f a s t e r than does neat propylene (2). As a f i r s t approximation, the s p e c i f i c r a t e of propylene disappearance i n excess ethane can be taken as twice the s p e c i f i c r a t e of ethane d i s appearance. I f the average value f o r propylene conc e n t r a t i o n i s taken as h a l f the f i n a l value, then the t o t a l propylene formed during the r e a c t i o n can be c a l c u l a t e d by d i v i d i n g the propylene recovered by the f r a c t i o n ethane undecomposed. This c o r r e c t e d value f o r propylene should be approximately equal t o the secondary methane f o r as high as 15% decomposition of ethane. I t has been used f o r t h i s purpose, and w i t h i n t h i s range, w i t h the present data. Results of a p p l y i n g these g u i d e l i n e s to the c a l c u l a t i o n of chains i n i t i a t e d and chains terminated



0.132 0.132 0.132 0.132

1.67 1.67 1.67 1.67

Pressure (i m.)

0 0 0.64:1 0.64:1 8:1 0 0 0.77:1 0.77:1 9:1 9:1 100:1 100:1 100:1 100:1 0 0 0 0 0 0 0

Dilution (steam)

R e s i d e n c e Time (sec.)

2.52 2.37 2.28 2.15 2.60 2.48 2.21 2.46 2.32 1.06 1.01 1.06 1.01 2.25 1.98 0.57 0.55 0.51 0.63 0.59 0.51 0.58

Temperature °C

673 698 687 698 678 675 701 676 699 650 675 650 675 650 675 675 700 725 650 675 700 725

Chain

TERMINATION

1.18 1.49 1.29 1.37 1.39 1.09 1.28 1.38 1.55 1.40 1.47 6.32 11.30 10.50 9.70 3.10 1.40 1.30 5.00 8.30 5.20 3.40

Calculated Initiation/Chain

CALCULATED RATIOS OF CHAIN INITIATION TO CHAIN

TABLE IV


Termination


4.



69

are shown i n Table IV. These r e s u l t s show a rough balance between observed i n i t i a t i o n s and terminations for the experiments at one atmosphere u n d i l u t e d ethane, or up to 9:1 d i l u t i o n with steam. At 100:1 steam d i l u t i o n , only about 10% of the chain terminations can be accounted f o r ; at 100 mm. pressure of u n d i l u t e d ethane, only about 20% of the chain termination i s evident. The cause f o r t h i s apparent l a c k of terminators cannot be unambiguously i d e n t i f i e d from present r e s u l t s . The mechanism indeed p r e d i c t s that t h i s s i t u a t i o n w i l l occur, w i t h chains being terminated more and more by r e a c t i o n of H- and C2H5-, as the p a r t i a l pressure of ethane i s decreased. However, there i s no evidence of any intermediate stage, where r e a c t i o n (6), forming propane, assumes greater importance. I f we assume that termination i s by Η· + C2H5« (e.g., r e a c t i o n 7) and use the approximate r a d i c a l concentrations estimated e a r l i e r , we can c a l c u l a t e an order of magnitude value f o r the r e a c t i o n r a t e . At 1048K the second order r e a c t i o n r a t e constant needs to be roughly Ι Ο ^ ο η ^ ^ - Ι ^ ^ Ι ^ nearly two orders of magnitude l a r g e r than has been estimated by C a m i l l e r i , M a r s h a l l and P u r n e l l (12). Obviously, the homogeneous r e a c t i o n i s more important than has been suggested, o r termination i s by some other mechanism, such as a surface r e a c t i o n . C e r t a i n l y , the p o s s i b i l i t y of termination of chains by surface r e a c t i o n s cannot be disregarded. Several authors have discussed t h i s p o s s i b i l i t y f o r ethane p y r o l y s i s at lower temperatures (10). For example, chemisorption of hydrogen atom on a w a l l s i t e could be followed by r e a c t i o n by t h i s s i t e with an e t h y l r a d i c a l , forming ethane, which i s l a t e r desorbed. More work w i l l have to be done w i t h r e a c t o r s with d i f f e r e n t kinds of surfaces, with d i f f u s i o n paths of d i f f e r e n t length, and w i t h d i f f e r e n t surface/volume r a t i o s , before even the q u a l i t a t i v e aspects of the p o s s i b l e surface r e a c t i o n s are understood. o

r

Conclusions With modern instrumentation and a n a l y t i c a l techniques, much more can be done to confirm or r e j e c t mechanisms f o r gas p y r o l y s i s than was p o s s i b l e when theories of chain r e a c t i o n s were f i r s t propounded. The mechanism f o r ethane p y r o l y s i s devised by Rice and H e r z f e l d was based on l i m i t e d data on r a t e s and product formation at r e l a t i v e l y high decompositions of ethane, where both r a t e s and products were s t r o n g l y



70


a f f e c t e d by secondary r e a c t i o n s . The present work and other recent s t u d i e s look at the o v e r - a l l r e a c t i o n under circumstances r e l a t i v e l y f r e e of secondary e f f e c t s , with r a t i o n a l bases f o r c o r r e c t i n g f o r such i n c i p i e n t e f f e c t s . Even so, d e t a i l e d experimental data pose new questions, some of which are not r e a d i l y s o l u b l e by d i r e c t experimentation on the system i n v o l v e d . The u n c e r t a i n t i e s and ambiguities discussed i n t h i s r e p o r t might w e l l discourage any attempt to apply a fundamental approach to i n d u s t r i a l c r a c k i n g . However, some of the problems encountered here, though i n t e r e s t i n g , would be absent or attenuated i n i n d u s t r i a l c r a c k i n g furnaces. Pressure dependency of r a t e constants and chain termination by surface r e a c t i o n s should be q u i t e unimportant there. The number of fundamental f r e e - r a d i c a l r e a c t i o n s that must be considered i n high-conversion c r a c k i n g of mixed feeds i s , o f course, enormous. This problem becomes l e s s insurmountable as more r a t e data f o r the fundamental r e a c t i o n s becomes a v a i l a b l e , and computer technology progresses. An intermediate approach, applying fundamental concepts to s i m p l i f i e d models, has r e c e n t l y been reported as an e f f e c t i v e means o f handling complex c r a c k i n g systems (11). Abstract At p a r t i a l pressures near one atmosphere, ethane decomposes by a simple Rice-Herzfeld mechanism, with combination or disproportionation of e t h y l r a d i c a l s as the predominant chain-ending step. However, at a t o t a l pressure of 100 mm., or at a p a r t i a l pressure of 0.01 atm. another chain-ending step predominates. Unlike butane formed from e t h y l , the products of t h i s step cannot be distinguished a n a l y t i c a l l y from the major products of the reaction chain. It i s therefore believed to involve r e a c t i o n of H· and C2H5·, e i t h e r homogeneously or at the reactor w a l l . Quantitative rate and y i e l d data are given, as are methods of c o r r e c t i o n for secondary reactions and of extrapolation to zero reaction time.

Literature Cited (1) Rice, F. O., and Herzfeld, K.F., J . Amer. Chem Soc. 56,284(1934). (2) Davis, H. G., and Williamson, K. D., Proc. 5th World P e t r o l . Congr., V o l . IV, p. 37(1960). (3) Quinn, C. P . , Proc. Roy. Soc. London A275, 190(1963).


4. WILLIAMSON AND DAVIS

(4) (5) (6) (7)


(8) (9) (10) (11) (12)


71

Lin, M. C . , and Back, Μ. Η., (a) Can. J. Chem. 44, 505(1966); (b) ibid. 44, 2357(1966); (c) ibid 44, 2369(1966). Leathard, D. Α., and Purnell, J. Η., Rev. Phys. Chem. 1970, p. 202, 206, 209-15. Trenwith, Α. Β., (A) Trans. Faraday Soc. 59, 2543(1966); (b) ibid 60, 2452(1967). Terry, J. O., and Futrell, J. Η., Can. J. Chem. 45, 2327(1967). Quinn, C. P., Trans. Faraday Soc. 59, 2543 (1963). Pacey, P. D., and Purnell, J. Η., Int. J. Chem. Kinetics 4, 657(1972). Marshall, H. Μ., and Quinn, C. P., Trans. Faraday Soc. 61, 2671(1965). Davis, H. G . , and Farrell, T. J . , Ind. Engr. Chem. Process Des. and Develop. 12, 171(1973). Camilleri, P., Marshall, R. Μ., and Purnell, J. Η., J. C. S. Faraday I, 70, 1434(1974).


Mechanistic Studies of Methane Pyrolysis at Low Pressures - ACS

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