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Mathematical model of intensive pyrolysis of ethane based on reaction mechanisms
Intensive pyrolysis of ethane
215
8. C. A. OLAH, Carbonium Ions 2, Ed. G. A. Olah, 1971 9. A. A. TABATSKAYA, I. A. SHTABEL' and V. A. 80KOLENKO, Zh. organ, khimii 18,
1818, 1977 10. L. E. T~gH~I, I. A. SHTABEL', T. N. BAZUYEVA. A. A. TABATSKAYA and V. A. $OlgOLENXO, Mater. nauch..prakt, konf. molodykh uchenykh (Proceedings of a
Scientific-Practical Conference of Young Scientists). Tomsk, 1975
Petrol. Chem. U.S.S.R. Vol. 19, pp. 215-223. © Pergamem Press Ltd. 1980. Printed in Poland
0031-6458/79/1101-02.15807.5010
1KATlq~.MATICAL MODEL OF INTENSIVE PYROLYSIS OF ETHANE BASED ON REACTION MECHANISMS* V. V. KAFAROV, YE. A. FEIGII~', M. G. BELOSTOTSKIIancl N. P. VASH~raI~X I). I. Mendeleyev Moscow Chemico-technologicalInstitute All-Union Scientific Design Institute of Petroleum
(Received 22 Sptember 1978) ~,~A~v. pyrolysis is one of the most important sources of ethylene. Empirical mathematical models are used for practical calculations related to reaction coils in pyrolytic ovens, in which the chemistry of pyrolysis is described b y several gross reactions. The use of these models normally involves a fairly narrow range of variation of process l~rameters, which hinders calculations and theoretical study. On the other hand, it is known that models developed using this process mechanism are fully adequate and may be used for fore-
casting [1]. An approach was previously developed to describe conversion kinetics in relation to the conversion of the major component [2], in order to simulate pyrolysis of hydrocarbons using the reaction mechanism developed. Equations of the rate of decomposition of comlconents of the reaction mixture in this case incorpbrate relative rate constants of decomposition of hydrocarbons [2, 3]. Using the approach mentioned a mathematical model was developed in this stud)- for reactions of industrial p)Tolysis of ethane and ethane fractions. As a result of extensive research main reactions of the radical-chain mechanism of pyrolysis of ethane and ma~.ncomponents of industrial ethane fractions-propane and propylene, were established. The p)~rolytic mechanism of ethane incorporates primary reactions to form largely ethylene and hydrogen and Secondary reactions to form methane, propylene, but ad~.ene and other products. * Neftokhimiya 19, No. 6, 869-875, 1979.
216
V.V. Ka~,~mov et al.
According to former studies [4, 5], primary reactions of ethane decomposition result in the following: C2H6 ~ C H ; t C H ; (D CH~+C~I-Ie ~ CH~+C2H; (II) C~H; -, C~H,+H" ([II) H'+C2He --' H2 ,-I-C~H; C2H; +CH.; -* Calls 2C~H; --* CdH10
(IV) (V) (VII
Reactions (III) and (IV) are tb,e most significant among reactions (I)-(VI) as it was established that ethane decomposition is a long-chain reaction [4-8]. Reactions (I)-(VI) describe the process at low pressures and high temperatures. In industrial kilns ethane pyrolysis begLrm at 620-680 ° and at partial ethane pressures of 0.1-0-2 ~lPa. Under these conditions part of C,.,H~ radicals is reduced b y the reaction C~H; ÷
RH
-* C2H~
÷ R"
(VII)
Both ethane and propane (when present in the raw material mixtm'e) and ethylene m a y function as R H . The reaction with ethane is not normally taken into account since from a stoiehiometric point of view it does not alter product concentration, but is significant from a ldnetic point of view since it influences process rate as a whole. Reactions (I)-(VII) give a fair description of etlo,ane pyrolysis up to conversions of 40%. For the description of the process to conversion values of 50-65°/0 carried out in industry, it is essential to consider secondary reactions, the mechanism of which was est~flflished in much less detail. It was shown for these reactions that the main source of secondary products is ethylene decomposition [9]. Since the content of each of the secondary products of C 3 -C: 4 is under 1-o-o/,o, when simulating conversions of these hvd.rocarbons ~ , (in spite of the complexity of secondary reactions) we can be confined to calculating rates of formation of these products from ethylene and of overall decomposition. The mathematical model of p)n'olysis of ethane incorporates equations of conversion of ethane and products of decomposition. According to former studies [2, 3], tbr a reaction coil kinetic equations m a y be described in relation to a variable u, wlfieh characterizes the degree of conversion of the key component and is relate([ to coil length l according to the equation
i
where dr is the diameter of the reactor, P , T--pressure and temperature, N--gas constant, n~--molar consumptions of reaction mixture components, [R~]--radical concentrations and k~ is the rate constant of H atom abstrac-
Intensive pyrolysis of ethane
217
tion by reactions R~+C.,H~ -. C~H;+R~H. The equation of ethane decomposition is described in the form d n C,H, -
-
du
Jr-
5;~C,H,,
--
(2)
where J is the proportion of ethyl radicals decomposed:
J= l~ln+ kvn [R H]-- l'm/kvn + [RH]"
(3)
The value of k~/lcv~ is determined in n-butane pyrolysis [10] km/kvn =exp(8.6--109,000/RT) (the relevant activation energy of 90,710 kJ/mole was TABLE
1. CO~IPOSITION OF PYROLYTIC PRODI:CTS ( ° 0 w t . ) 1-~ REACTI01~S OF DIFFERENT SCALES
Scale of the l reactor
Ethane conver*
T a t the outlet,
Pressure IDilution at the [ with outlet, i steam, !ra >: 10-' I kg/k~
sion, %
¢C
60.0
824
9.8
0.0
Pilot [8]
59'1
Indtmtrial
59.87
840 835
10.8 18-6
0-4 0'4
Laboratory
i ,
i
I
I I ! !
H~ CH,C:~,%~, C',H,ic,~,ic,m,c,~,~c,n,c,~,o !
I
[
Ci ÷
2"54
[8] [6]
3.,1!...9,
4~.7 iag.o
1~5, ;2 0.9o I -
-
1"85 1"6
derived in a previous study [11]). In pyrolysis of RH hydrocarbons, which differ from n-butane, the value derived should be classified as the relative rate constant of decomposition of RH in relation to n-butane [3]. Then, exp (8.6-- 109,000/RT)/]~RH.,c~rlo [RH] a J----exp (8.6-- 109,000/RT)./kR~/c,H,o [RH] .-61 a + 1"
(4)
Equations of conversion of pyrolytic products of,ethane take the form
where v¢,t are the stoichiometric coefficients (selectivity) of the formation of product n~ during decomposition; n~, )[:iadn l=j are the relative rate constants o f decomposition of hydrocarbons. Since ratio (4) incorporates relative rate constants of decomposition in relation to n-butane, constants l:~ and kj axe also considered in relation to n-butane. The numeration of components in the model is as follows: 1 - - H 2, 2--CH4, 3--C:Ho., 4--C2H~, 5--O2H5, 6--Carte, 7--Calla, 8--C4Ha, 9--C4Hs, 10--Call10 and 11--C6:-. It follows from equation (2) that u values are related to the degree of conversion of ethane x; at lfigh temperatures (T>750 °) a>>l, which gives
-218
V.V. K~AROV e~ aL
x = 1--exp (--u). Since in ethane pyrolysis this condition holds good over a larger part of the reaction coil, the degree of conversion x is the main technical :parameter which affects product composition (Table 1). According to (2), (4) and (5), product )fields are generally determined by the degree of conversion, temperature and pressm~ and are independent of the type and scale of the reactor, which is confirmed by results shown in Table 1. Relative rate constants are determined by the molecular structure of hydrocarbons and therefore parameters of model (2)-(5) are invariant not only to the type and scale of the reactor, but also to the composition of the reaction mixture and reaction conditions [3]. For low molecular weight olefms values of/¢i, vl,~ are not, strictly speaking, constant since conversions of these hydrocarbons conform to more complex equations than those incorporated in ttle model obtained [3]. For ethane pyrolysis, however, it may be assumed that these values show a slight variation during the reaction. Relative rate constants of decomposition of saturated hydrocarbons were obtained previously [3]. To determine the relative rate constant of the decomposition of ethylene, results were used, which had been obtained in stladies of laboratory and pilot-scale pyrolysis of ethane at 750-820 °. Experimental results were analysed using a linear equation
dnc,m du =nc,m--knc,H,, k=kc,~jc,m, having the solution e-u __ e-ku
o
(6)
As a result it was found that k=0.29, i.e. in ethane pyrolysis the effective rate constant of the decomposition of ethane is higher by a factor of 3.4 than the value for ethylene. Figure 1 shows calculated and experimental values of ethylene yield. Parameters vl,~ may be calculated in accordance with the Rais theory [13], or determined experimentally. Since for pyrolysis of ethane and ethane fractions ethane, propane, propylene and ethylene are the main components that undergo decomposition, values of vl,~ are calculated only for the hydrocarbons indicated. For the main products of pyrolytic decomposition--hydrogen and ethylene--v~.~-=vs.4=5. The selectivity of formation of hydrogen, methane, ethylene and propylene is close to 0-5 for propane [13]. It is known that during pyrolysis of propylene in ethane it breaks down to the extent of 92-95% to methane and ethylene [7,14]. As no similar results are available in the literature for ethylene, stoichiometric coefficients of decomposition~ of ethylene *'4,~are regarded as u n s o w n parameters of the model. Equations of obtaining acetylene, ~ ropylene, butylene and butadiene are of the same type
dn~ -du=V~, ~Icdn~--k~nt (i----3,6, 8, 9).
(7)
Intensive pyrolysis of ethane
219
'To determine vj,~ and k~, the most coml~lete laboratory results were used [8]. I n eqn. (7) the v~lue of k4 is -known and n4 is a function of u, according to (6). 'Therefore, parameters *'4,~,k~ may be determined for each product separately. With low ethane conversions the solution of eqn. (7) is indel~endent of ki n~-
Y4~ i J¢4 X2 •
(8)
2
Equation (8) made it possible to determine v4,~ according to the initial .~ection of curves showing product yield according to conversion. Parameters ks were determined by integration of eqn. (7) as part of system (~) up to high .degrees of conversion. Figure 2 shows the dependence of the calculated curve of _:propylene ~ e l d on parameter /Q. , lO'~rno/e/mo/eini~iolethane i
I
mole/mole inifial efh~e
0"8I o.s
:o.o
8
j_
0.2 0"! i
0
]
I
0.2 0.~ 0.6 Ethane conversion Fro. 1
t
I
O.8
0
O.Z~ O'8 E.thone conve~ion ~'~o. 2
:Fro. 1. Calculated and experimental yield of ethylene in ethane pyrolysis. Experimental results of studies: ©--[6], []--[7], A--[8], O--[12]. P I o . 2. Dependence of the calculated curve of propylene yield on parameter kl. k=: 1--0.6; 2--1.0; 3--1.2; continuouslines--calculation; O--experimental results [8].
An analysis of experimental curves of hydrogen yield [6, 8] indicates that ~rith high ethane conversions hydrogen is consumed. This effect was con.sidered in the model in the form of a term concerning the decomposition of hydrogen with effective relative rate constant kl. It is known that with empirical simulation of industrial p~-rolysis of ~tha~e considerable difficulties arise in describing methane formation [15].
220
et al.
V.V.K.~_Rov
In this model to determine parameters of the equation of methane formation it was considered that, according to results previously obtained [16], methane is formed b y reactions (VIII) in addition to propylene C:H~~ C:H 4 --> C,H; ~ m C3H~÷ CH;(CHd)
(VIII)
Part of methyl radicals added to ethylene also forms propylene: CH~÷C,H4 -~. C3I=[; k~CaH,÷H"
(IX)
Therefore, methane yield during the decomposition of ethylene yd., should be lower than propylene field Vd.~. With an increase in temperature, in view of the higher relative role of CH; radicals [17] and the higher activation energy 2. PAI:tAMETE:~S t'/i
TABLE
J
Components of model i 1
2
0,2
i :
4
--
5 6
0.955 --
0.05J ;
7
0.5
0.5
3
0-2
1.0
:
~
4
5
6
7
s
--
--
0.35
--
---
0.955 1.0
-.
-.
0-065 . .
--
0.5
--
.
0,5
9
0.t2
.
0.14
.
!
i 0.12
0.01J !
. .
11
i
--
--
--
.
lo
.
]
---
i
--
.Vote. The value of 5 was calculated using formula (4).
vd.6>vd.z i s i n c r e a s e d . As regards reaction (VIII) it may b e noted that [9] in pyrolysis of ethane-tracer ethylene mixtltres the process is conrfimed to be the reaction of propylene formation and not methane formation. This is, apparently, due to the fact that as a result of the addition of C2H~ radicals to tracer ethylene and of isomerization of C,H; radicals [16] two types of radical are formed: (!*--C*--C--C and C--C'*--C--C, during the decomposition of which radioactivity is localized in propytene. Methyl radicals (methane) are used up at a ]ower rate in the presence of olefins, compared with H atoms (molecalar hydrogen) therefore, bearing in mind the high rate of formation of methane from ethylene, 1)ropylene and k,x,
the
inequality
TABLE 3. No. o f ¢xpenmant
Scale ()f the r~ttetor
E t h a n e i ReaeComposition of the conver- '1 tioli raw material mixture sl0a, %1 t e m p . , ~C
I 2 3
L a b o r a t o r y [7] Same Indtmtrial [6]
('.H,
35.0 14.0 60.0
4
Industrial [18]
C z H , + 5 0 wt. % C=H,
60'0
C~Hq ('~:H~ + 9 vot. % C~H,
774 775 835
CALCULATION
OF THE
CO,~IPOSlTI01~"
Material balance Pressure', If, 10 ', Pa , exp. calc. 9.~
Io4,5
t'H~
--
3'71
C,H=
exp.:calc.:exp, ieate.
i ~,dr,6 I [
1~'6
!
3"97 b
J
!
1.0 I -- ~0.03 ",.5 ~ 0'3 0"08
3.3~ o-21 o-15
22"8
~'ote. The material balance in the experiments was given a~ fol|ows: No. 1 -- % vol., No. 2 -- mole/100 mole etho f ' R , + C H ~ ; 0'8 and 0.5, respectively - same for C~Hg--C~PI~,.
Intensive
221
pyrolysis of ethane
propane, the rate of decomposition of metlmne has not been taken into account. It m a y be pointed out under these conditions that the dependence of methane yield in the range of high ethane conversions takes the form (l--x) .
nCH,~--ln
Parameters ]~ of the model, in order of numeration of components, are as follows: kt
l
2
0.1
0.0
3
4
0-002 0-058
5
6
7
8
9
l0
11
0"2
1'2
0.49
0"2
2.2
0'7
0"1
mole/molein#/a/
efhane
2.
Values of vs~ are shown in Table
[C;],/O' [CH.],IO
,~ole/mole i.#ial e~hane / t.
[.,] [C,H.]
t
---
z
Io
'~
0.4
0-6 ~ 0"2 0"2 ~
O
~0 80 Lenq~n of fhe reacfor , m
20
:FIo. 3. P r o d u c t
yields of ethane
l--H=;
model:
pyrolysis
80
in a n i n d u s t r i a l r e a c t o r , c a l c u l a t e d 5--ethane conversion.
uRing a
2--C=H4; 3--C5+; 4 - - C H , ;
It is known that propylene is the main unsaturated component of ethane fractions. The value of k 0 (Table 2) was derived for the case of pyrolysis of pure ethane. It is therefore essential to evaluate the variation of ks according OF P Y R O L ~ r I C PRODUCTS OF ETHAI~E o f decomposition
C,H. exp.
calc.
CzH,
C,H, exp.
talc.
calc. I exp.
0.2
0.8
24"6 24.68 48.1 48.1 29.6 80"0 64-4 65"1 48"68 48"19 89"27 39'4
i
60.9
! 0.9
62-8
C,H.
exp.
8.7 1.07
cale.
i
0"08 ! 0-09 4,0 ' [ 0-09 1'15 0"21! 0"27
5.1 , -
I-
]
C,H.
C,H,
exp.
calc.
exp.
0"16
0"19
0'8
0"17
1.12
1.38
0.05 I 0"07 0,07 0.09 0-21 0.81
1"8
0"8
I 1"9
calc.
C,H, exp.
1 C'+
csic. exp. ca]c,
Ol01 I 0-01 I 0.05
o.5
-- , -1"9 1.6
T o.8 e-s
a~e, ~o. 8 - % wt., ~'o. 4 - % wt., excludlng ethane and propane; 22.S =rod U~) -- experimental and c~dcul=ted v=Im
222
V.V.K.trJmov
eta/.
to propylene concentration in the raw material. The dependence of pax&meters. k~ on the composition of ethane fractions for other components is less significant. Experiments on pyrolysis of ethane ,~ud propylene mixtures [7] indicatea th e variation of the ratio of effective rate constants 7 = kC3He/]cc2~e according to propylene content: ~ is equal to 5 for mixtures of C2H~+I°/o C~H6 and 7 is 3 for mixtures of C2H6+9~0 C~H6, These values are in agreement with t he relative rate constant of propylene in relation to ethane previously derived ( ~ = k ' = 6 ) . Bearing in mind all these data the dependence of parameter /¢~ on the content of propylene (°~o vol.) in the ethane fraction m a y be approximated 0.2[C~H8] + 1.2 ]Q~--0.45 [C.~H6]+ 1 The ad eq ua c y of the model was verified using results of laboratory a n d industrial experiments. Results of calculation are sho~ax in Table 3. Figure 3 shows an example of calculating curves of product yield for dat a of an industrial reactor [6]. F o r calculationa related to tim reaction in laboratory experiments t h e equations were integrated to • given ethane conversion. For industrial conditions integration was carried out together with eqn. (1). SUMMARY' 1. A mathematical model was developed based on the radical-chain reaction mechanism for industrial pyrolysis of ethane and ethane fractions. 2. The adequacy of the model was verified using laboratory and industrial experimental data. REFERENCES I. V. V. KAFAROV, Met.ody kibernetiki v khimii i khimieheskoi tekhnologii (Methods of Cybernetics in Chemistry and Chemical Teehnolo~3"). Khimiya, Moscow, 1976 2. Ye. A. FEIGLN, V. V. KA~AROV, M. G. BELOSTOTSKII and S. N. VEKBITSKAYA, Neftekhimiya I8, 383, 1978 3. S. N. VER]MTSK.&YA, M. G. BELOSTOTSKII ~nd Ye. A. FEIGLN, ~eftekhimiya 18, 228, t978 4. M. C. LIN and M. H. BACK, C~nad. J. Chem. 44, 505. 2357, 1966 5. S. B. ZDONIK, E. J. GREEN and L. P. HAL]LEE, ~Ianufacturing Ethylene Petr. Publ. Co., Tulsa, Okl~, 1970 6. G. F. FROMENT, B. O. VAN de STEENE, P. VAN DAMME and S. NARAYANAN, Ind. Eng. Chem., Proc. Des, Dev. 15, 495, i976 7. H. G. DAVIS andK. D. W~.LIAMSON, Proc. 5-th World Petr. Cong., Sect. IV, 1960 8. V. ILLES and K. VCELTHER, Magyar Kemikusok Lapja 26, 587, 1971 9. A. M. BRODSKII, R. A. KALINENKO, K. P. LAVROVSKII and L. V. SItEVEL'KOVA, Kinetika i kataliz 5, 49, 1964 10. M. MURATA, N. TAKADA and Sh. SAITO, J. Chem. Eng. Japan 4, 286, 1974
Auto-oxidation of isomeric n-octynee
223
11. Yu. P. YAMPOL'$~n end N. S. NAMETKIN, Neftekhirm'ya 15, 338, 1975 12. L FERKARA, G. BARBAGAIJ~ and U. FI~.T~,, Ls Chimica e l'Xndustria. Milan 58, 1138, 1971 13. F. O. RAIS and K. O. RA~, Svobocln~e alifaticheskiye radikaly (Free Aliphatic Radicals). ONTI, Leningrad, 1937 14. R. A. ]~kLINENKO, K. P. L A V R O V ~ , L. V. SHEVEL'KOVA, G. BAKH and Z. NOVAK, Neftekhhniya 9, 542, 1969 15. Ire. A. FEIGIN, Kand. dis., INKhS, U.S.S.R. Academy of Sciences, Moscow, '1966 16. P. D. PACEY and J. H. PURNELL, Industr. Engng. Chem. Fund. 11, 233, 1972 17. Yu. P. YAMPOL'Sk'T[, V. M. RYDIN and Ye. I. MOLODYIgH, Dold. AN SSSR 213, 394, 1973 18. R. G. MINET and J. D. HAMMOND, Oil and Gas J. 78, 31, 80, 1975
Petrol. Chem.U.S.S.R.Vol.19, pp. 223-230, © PergamonPre~ Ltd. 1980.Printedin Poland
AUTO-0XIDATION
0031...6458/7911101-02~$07.50]0
OF ISOMERIC n-OCTYNES*
V. PRITZKOW, R. RADEGLIYA and V. SHMIDT-REI~!~'ER ~rl
Schorlcmmer Technological Institute, .Merseburg, German Democratic Republic (Received 20 October 1978)
AVTO-OXIDATIO~ of acetylene hydrocarbons was systematically studied b y K. I. Ivanov and A. I. Chirko [1-4]. F. R. Ma~ and I). I. van Sickle carried out independent investigations related to their work on auto-oxidation of olefins and were concerned with auto-oxidation of hex-l-x~ne and hex-2-)nae [5]. All these studies are similar in their conclusions: that acetylene hydrocarbons chiefly undergo attack at the O - - H bonds which are in the ~-position to the C = C ternary bond a n d that corresponding hydroperoxides are formed as primar T products of auto-oxidation; these m a y decompose to acetylene ketones and acetylene alcohols. No oxidizing attack has so far been observed at the O----C ternary bond which ought to give oxirenes and products of further conversion. Furthermore, isomerization of the C--C ternary bond has not so far been clearly confirmed. This paper gives a description of results of investigating oxidation of n-oet~nes. EXPERIMENTAL
Pure n-oct~mes were oxidized ill a closed apparatus at 85°/101.3 k P a while stirring vigorously. Oxygen consumption was measured volumetrically. With an oxygen consumption of 25-27 mmole/100 mmole octyne the reaction * Neftekhimiya 19, l~o. 6, 885-891, 1979.
215
8. C. A. OLAH, Carbonium Ions 2, Ed. G. A. Olah, 1971 9. A. A. TABATSKAYA, I. A. SHTABEL' and V. A. 80KOLENKO, Zh. organ, khimii 18,
1818, 1977 10. L. E. T~gH~I, I. A. SHTABEL', T. N. BAZUYEVA. A. A. TABATSKAYA and V. A. $OlgOLENXO, Mater. nauch..prakt, konf. molodykh uchenykh (Proceedings of a
Scientific-Practical Conference of Young Scientists). Tomsk, 1975
Petrol. Chem. U.S.S.R. Vol. 19, pp. 215-223. © Pergamem Press Ltd. 1980. Printed in Poland
0031-6458/79/1101-02.15807.5010
1KATlq~.MATICAL MODEL OF INTENSIVE PYROLYSIS OF ETHANE BASED ON REACTION MECHANISMS* V. V. KAFAROV, YE. A. FEIGII~', M. G. BELOSTOTSKIIancl N. P. VASH~raI~X I). I. Mendeleyev Moscow Chemico-technologicalInstitute All-Union Scientific Design Institute of Petroleum
(Received 22 Sptember 1978) ~,~A~v. pyrolysis is one of the most important sources of ethylene. Empirical mathematical models are used for practical calculations related to reaction coils in pyrolytic ovens, in which the chemistry of pyrolysis is described b y several gross reactions. The use of these models normally involves a fairly narrow range of variation of process l~rameters, which hinders calculations and theoretical study. On the other hand, it is known that models developed using this process mechanism are fully adequate and may be used for fore-
casting [1]. An approach was previously developed to describe conversion kinetics in relation to the conversion of the major component [2], in order to simulate pyrolysis of hydrocarbons using the reaction mechanism developed. Equations of the rate of decomposition of comlconents of the reaction mixture in this case incorpbrate relative rate constants of decomposition of hydrocarbons [2, 3]. Using the approach mentioned a mathematical model was developed in this stud)- for reactions of industrial p)Tolysis of ethane and ethane fractions. As a result of extensive research main reactions of the radical-chain mechanism of pyrolysis of ethane and ma~.ncomponents of industrial ethane fractions-propane and propylene, were established. The p)~rolytic mechanism of ethane incorporates primary reactions to form largely ethylene and hydrogen and Secondary reactions to form methane, propylene, but ad~.ene and other products. * Neftokhimiya 19, No. 6, 869-875, 1979.
216
V.V. Ka~,~mov et al.
According to former studies [4, 5], primary reactions of ethane decomposition result in the following: C2H6 ~ C H ; t C H ; (D CH~+C~I-Ie ~ CH~+C2H; (II) C~H; -, C~H,+H" ([II) H'+C2He --' H2 ,-I-C~H; C2H; +CH.; -* Calls 2C~H; --* CdH10
(IV) (V) (VII
Reactions (III) and (IV) are tb,e most significant among reactions (I)-(VI) as it was established that ethane decomposition is a long-chain reaction [4-8]. Reactions (I)-(VI) describe the process at low pressures and high temperatures. In industrial kilns ethane pyrolysis begLrm at 620-680 ° and at partial ethane pressures of 0.1-0-2 ~lPa. Under these conditions part of C,.,H~ radicals is reduced b y the reaction C~H; ÷
RH
-* C2H~
÷ R"
(VII)
Both ethane and propane (when present in the raw material mixtm'e) and ethylene m a y function as R H . The reaction with ethane is not normally taken into account since from a stoiehiometric point of view it does not alter product concentration, but is significant from a ldnetic point of view since it influences process rate as a whole. Reactions (I)-(VII) give a fair description of etlo,ane pyrolysis up to conversions of 40%. For the description of the process to conversion values of 50-65°/0 carried out in industry, it is essential to consider secondary reactions, the mechanism of which was est~flflished in much less detail. It was shown for these reactions that the main source of secondary products is ethylene decomposition [9]. Since the content of each of the secondary products of C 3 -C: 4 is under 1-o-o/,o, when simulating conversions of these hvd.rocarbons ~ , (in spite of the complexity of secondary reactions) we can be confined to calculating rates of formation of these products from ethylene and of overall decomposition. The mathematical model of p)n'olysis of ethane incorporates equations of conversion of ethane and products of decomposition. According to former studies [2, 3], tbr a reaction coil kinetic equations m a y be described in relation to a variable u, wlfieh characterizes the degree of conversion of the key component and is relate([ to coil length l according to the equation
i
where dr is the diameter of the reactor, P , T--pressure and temperature, N--gas constant, n~--molar consumptions of reaction mixture components, [R~]--radical concentrations and k~ is the rate constant of H atom abstrac-
Intensive pyrolysis of ethane
217
tion by reactions R~+C.,H~ -. C~H;+R~H. The equation of ethane decomposition is described in the form d n C,H, -
-
du
Jr-
5;~C,H,,
--
(2)
where J is the proportion of ethyl radicals decomposed:
J= l~ln+ kvn [R H]-- l'm/kvn + [RH]"
(3)
The value of k~/lcv~ is determined in n-butane pyrolysis [10] km/kvn =exp(8.6--109,000/RT) (the relevant activation energy of 90,710 kJ/mole was TABLE
1. CO~IPOSITION OF PYROLYTIC PRODI:CTS ( ° 0 w t . ) 1-~ REACTI01~S OF DIFFERENT SCALES
Scale of the l reactor
Ethane conver*
T a t the outlet,
Pressure IDilution at the [ with outlet, i steam, !ra >: 10-' I kg/k~
sion, %
¢C
60.0
824
9.8
0.0
Pilot [8]
59'1
Indtmtrial
59.87
840 835
10.8 18-6
0-4 0'4
Laboratory
i ,
i
I
I I ! !
H~ CH,C:~,%~, C',H,ic,~,ic,m,c,~,~c,n,c,~,o !
I
[
Ci ÷
2"54
[8] [6]
3.,1!...9,
4~.7 iag.o
1~5, ;2 0.9o I -
-
1"85 1"6
derived in a previous study [11]). In pyrolysis of RH hydrocarbons, which differ from n-butane, the value derived should be classified as the relative rate constant of decomposition of RH in relation to n-butane [3]. Then, exp (8.6-- 109,000/RT)/]~RH.,c~rlo [RH] a J----exp (8.6-- 109,000/RT)./kR~/c,H,o [RH] .-61 a + 1"
(4)
Equations of conversion of pyrolytic products of,ethane take the form
where v¢,t are the stoichiometric coefficients (selectivity) of the formation of product n~ during decomposition; n~, )[:iadn l=j are the relative rate constants o f decomposition of hydrocarbons. Since ratio (4) incorporates relative rate constants of decomposition in relation to n-butane, constants l:~ and kj axe also considered in relation to n-butane. The numeration of components in the model is as follows: 1 - - H 2, 2--CH4, 3--C:Ho., 4--C2H~, 5--O2H5, 6--Carte, 7--Calla, 8--C4Ha, 9--C4Hs, 10--Call10 and 11--C6:-. It follows from equation (2) that u values are related to the degree of conversion of ethane x; at lfigh temperatures (T>750 °) a>>l, which gives
-218
V.V. K~AROV e~ aL
x = 1--exp (--u). Since in ethane pyrolysis this condition holds good over a larger part of the reaction coil, the degree of conversion x is the main technical :parameter which affects product composition (Table 1). According to (2), (4) and (5), product )fields are generally determined by the degree of conversion, temperature and pressm~ and are independent of the type and scale of the reactor, which is confirmed by results shown in Table 1. Relative rate constants are determined by the molecular structure of hydrocarbons and therefore parameters of model (2)-(5) are invariant not only to the type and scale of the reactor, but also to the composition of the reaction mixture and reaction conditions [3]. For low molecular weight olefms values of/¢i, vl,~ are not, strictly speaking, constant since conversions of these hydrocarbons conform to more complex equations than those incorporated in ttle model obtained [3]. For ethane pyrolysis, however, it may be assumed that these values show a slight variation during the reaction. Relative rate constants of decomposition of saturated hydrocarbons were obtained previously [3]. To determine the relative rate constant of the decomposition of ethylene, results were used, which had been obtained in stladies of laboratory and pilot-scale pyrolysis of ethane at 750-820 °. Experimental results were analysed using a linear equation
dnc,m du =nc,m--knc,H,, k=kc,~jc,m, having the solution e-u __ e-ku
o
(6)
As a result it was found that k=0.29, i.e. in ethane pyrolysis the effective rate constant of the decomposition of ethane is higher by a factor of 3.4 than the value for ethylene. Figure 1 shows calculated and experimental values of ethylene yield. Parameters vl,~ may be calculated in accordance with the Rais theory [13], or determined experimentally. Since for pyrolysis of ethane and ethane fractions ethane, propane, propylene and ethylene are the main components that undergo decomposition, values of vl,~ are calculated only for the hydrocarbons indicated. For the main products of pyrolytic decomposition--hydrogen and ethylene--v~.~-=vs.4=5. The selectivity of formation of hydrogen, methane, ethylene and propylene is close to 0-5 for propane [13]. It is known that during pyrolysis of propylene in ethane it breaks down to the extent of 92-95% to methane and ethylene [7,14]. As no similar results are available in the literature for ethylene, stoichiometric coefficients of decomposition~ of ethylene *'4,~are regarded as u n s o w n parameters of the model. Equations of obtaining acetylene, ~ ropylene, butylene and butadiene are of the same type
dn~ -du=V~, ~Icdn~--k~nt (i----3,6, 8, 9).
(7)
Intensive pyrolysis of ethane
219
'To determine vj,~ and k~, the most coml~lete laboratory results were used [8]. I n eqn. (7) the v~lue of k4 is -known and n4 is a function of u, according to (6). 'Therefore, parameters *'4,~,k~ may be determined for each product separately. With low ethane conversions the solution of eqn. (7) is indel~endent of ki n~-
Y4~ i J¢4 X2 •
(8)
2
Equation (8) made it possible to determine v4,~ according to the initial .~ection of curves showing product yield according to conversion. Parameters ks were determined by integration of eqn. (7) as part of system (~) up to high .degrees of conversion. Figure 2 shows the dependence of the calculated curve of _:propylene ~ e l d on parameter /Q. , lO'~rno/e/mo/eini~iolethane i
I
mole/mole inifial efh~e
0"8I o.s
:o.o
8
j_
0.2 0"! i
0
]
I
0.2 0.~ 0.6 Ethane conversion Fro. 1
t
I
O.8
0
O.Z~ O'8 E.thone conve~ion ~'~o. 2
:Fro. 1. Calculated and experimental yield of ethylene in ethane pyrolysis. Experimental results of studies: ©--[6], []--[7], A--[8], O--[12]. P I o . 2. Dependence of the calculated curve of propylene yield on parameter kl. k=: 1--0.6; 2--1.0; 3--1.2; continuouslines--calculation; O--experimental results [8].
An analysis of experimental curves of hydrogen yield [6, 8] indicates that ~rith high ethane conversions hydrogen is consumed. This effect was con.sidered in the model in the form of a term concerning the decomposition of hydrogen with effective relative rate constant kl. It is known that with empirical simulation of industrial p~-rolysis of ~tha~e considerable difficulties arise in describing methane formation [15].
220
et al.
V.V.K.~_Rov
In this model to determine parameters of the equation of methane formation it was considered that, according to results previously obtained [16], methane is formed b y reactions (VIII) in addition to propylene C:H~~ C:H 4 --> C,H; ~ m C3H~÷ CH;(CHd)
(VIII)
Part of methyl radicals added to ethylene also forms propylene: CH~÷C,H4 -~. C3I=[; k~CaH,÷H"
(IX)
Therefore, methane yield during the decomposition of ethylene yd., should be lower than propylene field Vd.~. With an increase in temperature, in view of the higher relative role of CH; radicals [17] and the higher activation energy 2. PAI:tAMETE:~S t'/i
TABLE
J
Components of model i 1
2
0,2
i :
4
--
5 6
0.955 --
0.05J ;
7
0.5
0.5
3
0-2
1.0
:
~
4
5
6
7
s
--
--
0.35
--
---
0.955 1.0
-.
-.
0-065 . .
--
0.5
--
.
0,5
9
0.t2
.
0.14
.
!
i 0.12
0.01J !
. .
11
i
--
--
--
.
lo
.
]
---
i
--
.Vote. The value of 5 was calculated using formula (4).
vd.6>vd.z i s i n c r e a s e d . As regards reaction (VIII) it may b e noted that [9] in pyrolysis of ethane-tracer ethylene mixtltres the process is conrfimed to be the reaction of propylene formation and not methane formation. This is, apparently, due to the fact that as a result of the addition of C2H~ radicals to tracer ethylene and of isomerization of C,H; radicals [16] two types of radical are formed: (!*--C*--C--C and C--C'*--C--C, during the decomposition of which radioactivity is localized in propytene. Methyl radicals (methane) are used up at a ]ower rate in the presence of olefins, compared with H atoms (molecalar hydrogen) therefore, bearing in mind the high rate of formation of methane from ethylene, 1)ropylene and k,x,
the
inequality
TABLE 3. No. o f ¢xpenmant
Scale ()f the r~ttetor
E t h a n e i ReaeComposition of the conver- '1 tioli raw material mixture sl0a, %1 t e m p . , ~C
I 2 3
L a b o r a t o r y [7] Same Indtmtrial [6]
('.H,
35.0 14.0 60.0
4
Industrial [18]
C z H , + 5 0 wt. % C=H,
60'0
C~Hq ('~:H~ + 9 vot. % C~H,
774 775 835
CALCULATION
OF THE
CO,~IPOSlTI01~"
Material balance Pressure', If, 10 ', Pa , exp. calc. 9.~
Io4,5
t'H~
--
3'71
C,H=
exp.:calc.:exp, ieate.
i ~,dr,6 I [
1~'6
!
3"97 b
J
!
1.0 I -- ~0.03 ",.5 ~ 0'3 0"08
3.3~ o-21 o-15
22"8
~'ote. The material balance in the experiments was given a~ fol|ows: No. 1 -- % vol., No. 2 -- mole/100 mole etho f ' R , + C H ~ ; 0'8 and 0.5, respectively - same for C~Hg--C~PI~,.
Intensive
221
pyrolysis of ethane
propane, the rate of decomposition of metlmne has not been taken into account. It m a y be pointed out under these conditions that the dependence of methane yield in the range of high ethane conversions takes the form (l--x) .
nCH,~--ln
Parameters ]~ of the model, in order of numeration of components, are as follows: kt
l
2
0.1
0.0
3
4
0-002 0-058
5
6
7
8
9
l0
11
0"2
1'2
0.49
0"2
2.2
0'7
0"1
mole/molein#/a/
efhane
2.
Values of vs~ are shown in Table
[C;],/O' [CH.],IO
,~ole/mole i.#ial e~hane / t.
[.,] [C,H.]
t
---
z
Io
'~
0.4
0-6 ~ 0"2 0"2 ~
O
~0 80 Lenq~n of fhe reacfor , m
20
:FIo. 3. P r o d u c t
yields of ethane
l--H=;
model:
pyrolysis
80
in a n i n d u s t r i a l r e a c t o r , c a l c u l a t e d 5--ethane conversion.
uRing a
2--C=H4; 3--C5+; 4 - - C H , ;
It is known that propylene is the main unsaturated component of ethane fractions. The value of k 0 (Table 2) was derived for the case of pyrolysis of pure ethane. It is therefore essential to evaluate the variation of ks according OF P Y R O L ~ r I C PRODUCTS OF ETHAI~E o f decomposition
C,H. exp.
calc.
CzH,
C,H, exp.
talc.
calc. I exp.
0.2
0.8
24"6 24.68 48.1 48.1 29.6 80"0 64-4 65"1 48"68 48"19 89"27 39'4
i
60.9
! 0.9
62-8
C,H.
exp.
8.7 1.07
cale.
i
0"08 ! 0-09 4,0 ' [ 0-09 1'15 0"21! 0"27
5.1 , -
I-
]
C,H.
C,H,
exp.
calc.
exp.
0"16
0"19
0'8
0"17
1.12
1.38
0.05 I 0"07 0,07 0.09 0-21 0.81
1"8
0"8
I 1"9
calc.
C,H, exp.
1 C'+
csic. exp. ca]c,
Ol01 I 0-01 I 0.05
o.5
-- , -1"9 1.6
T o.8 e-s
a~e, ~o. 8 - % wt., ~'o. 4 - % wt., excludlng ethane and propane; 22.S =rod U~) -- experimental and c~dcul=ted v=Im
222
V.V.K.trJmov
eta/.
to propylene concentration in the raw material. The dependence of pax&meters. k~ on the composition of ethane fractions for other components is less significant. Experiments on pyrolysis of ethane ,~ud propylene mixtures [7] indicatea th e variation of the ratio of effective rate constants 7 = kC3He/]cc2~e according to propylene content: ~ is equal to 5 for mixtures of C2H~+I°/o C~H6 and 7 is 3 for mixtures of C2H6+9~0 C~H6, These values are in agreement with t he relative rate constant of propylene in relation to ethane previously derived ( ~ = k ' = 6 ) . Bearing in mind all these data the dependence of parameter /¢~ on the content of propylene (°~o vol.) in the ethane fraction m a y be approximated 0.2[C~H8] + 1.2 ]Q~--0.45 [C.~H6]+ 1 The ad eq ua c y of the model was verified using results of laboratory a n d industrial experiments. Results of calculation are sho~ax in Table 3. Figure 3 shows an example of calculating curves of product yield for dat a of an industrial reactor [6]. F o r calculationa related to tim reaction in laboratory experiments t h e equations were integrated to • given ethane conversion. For industrial conditions integration was carried out together with eqn. (1). SUMMARY' 1. A mathematical model was developed based on the radical-chain reaction mechanism for industrial pyrolysis of ethane and ethane fractions. 2. The adequacy of the model was verified using laboratory and industrial experimental data. REFERENCES I. V. V. KAFAROV, Met.ody kibernetiki v khimii i khimieheskoi tekhnologii (Methods of Cybernetics in Chemistry and Chemical Teehnolo~3"). Khimiya, Moscow, 1976 2. Ye. A. FEIGLN, V. V. KA~AROV, M. G. BELOSTOTSKII and S. N. VEKBITSKAYA, Neftekhimiya I8, 383, 1978 3. S. N. VER]MTSK.&YA, M. G. BELOSTOTSKII ~nd Ye. A. FEIGLN, ~eftekhimiya 18, 228, t978 4. M. C. LIN and M. H. BACK, C~nad. J. Chem. 44, 505. 2357, 1966 5. S. B. ZDONIK, E. J. GREEN and L. P. HAL]LEE, ~Ianufacturing Ethylene Petr. Publ. Co., Tulsa, Okl~, 1970 6. G. F. FROMENT, B. O. VAN de STEENE, P. VAN DAMME and S. NARAYANAN, Ind. Eng. Chem., Proc. Des, Dev. 15, 495, i976 7. H. G. DAVIS andK. D. W~.LIAMSON, Proc. 5-th World Petr. Cong., Sect. IV, 1960 8. V. ILLES and K. VCELTHER, Magyar Kemikusok Lapja 26, 587, 1971 9. A. M. BRODSKII, R. A. KALINENKO, K. P. LAVROVSKII and L. V. SItEVEL'KOVA, Kinetika i kataliz 5, 49, 1964 10. M. MURATA, N. TAKADA and Sh. SAITO, J. Chem. Eng. Japan 4, 286, 1974
Auto-oxidation of isomeric n-octynee
223
11. Yu. P. YAMPOL'$~n end N. S. NAMETKIN, Neftekhirm'ya 15, 338, 1975 12. L FERKARA, G. BARBAGAIJ~ and U. FI~.T~,, Ls Chimica e l'Xndustria. Milan 58, 1138, 1971 13. F. O. RAIS and K. O. RA~, Svobocln~e alifaticheskiye radikaly (Free Aliphatic Radicals). ONTI, Leningrad, 1937 14. R. A. ]~kLINENKO, K. P. L A V R O V ~ , L. V. SHEVEL'KOVA, G. BAKH and Z. NOVAK, Neftekhhniya 9, 542, 1969 15. Ire. A. FEIGIN, Kand. dis., INKhS, U.S.S.R. Academy of Sciences, Moscow, '1966 16. P. D. PACEY and J. H. PURNELL, Industr. Engng. Chem. Fund. 11, 233, 1972 17. Yu. P. YAMPOL'Sk'T[, V. M. RYDIN and Ye. I. MOLODYIgH, Dold. AN SSSR 213, 394, 1973 18. R. G. MINET and J. D. HAMMOND, Oil and Gas J. 78, 31, 80, 1975
Petrol. Chem.U.S.S.R.Vol.19, pp. 223-230, © PergamonPre~ Ltd. 1980.Printedin Poland
AUTO-0XIDATION
0031...6458/7911101-02~$07.50]0
OF ISOMERIC n-OCTYNES*
V. PRITZKOW, R. RADEGLIYA and V. SHMIDT-REI~!~'ER ~rl
Schorlcmmer Technological Institute, .Merseburg, German Democratic Republic (Received 20 October 1978)
AVTO-OXIDATIO~ of acetylene hydrocarbons was systematically studied b y K. I. Ivanov and A. I. Chirko [1-4]. F. R. Ma~ and I). I. van Sickle carried out independent investigations related to their work on auto-oxidation of olefins and were concerned with auto-oxidation of hex-l-x~ne and hex-2-)nae [5]. All these studies are similar in their conclusions: that acetylene hydrocarbons chiefly undergo attack at the O - - H bonds which are in the ~-position to the C = C ternary bond a n d that corresponding hydroperoxides are formed as primar T products of auto-oxidation; these m a y decompose to acetylene ketones and acetylene alcohols. No oxidizing attack has so far been observed at the O----C ternary bond which ought to give oxirenes and products of further conversion. Furthermore, isomerization of the C--C ternary bond has not so far been clearly confirmed. This paper gives a description of results of investigating oxidation of n-oet~nes. EXPERIMENTAL
Pure n-oct~mes were oxidized ill a closed apparatus at 85°/101.3 k P a while stirring vigorously. Oxygen consumption was measured volumetrically. With an oxygen consumption of 25-27 mmole/100 mmole octyne the reaction * Neftekhimiya 19, l~o. 6, 885-891, 1979.