Propylene from Paraffin Pyrolysis Philip D. Paceyl and J. Howard Purnell* Department of Chemistry, Cniversity College of Swansea, Swansea, Wales, C . K .
The pyrolysis of ethane and of n-butane in a flow reactor in conditions (10-400 mm and 600-720°C) not far removed from those of industrual steam cracking has been conducted. Product yields have been compared with those predicted b y a computer program based upon mechanisms and individual reaction step rate constants established previously in investigations with static reactors operated in the much lower temperature range 400-600°C. Excellent agreement between prediction and experiment was found for the absolute rates of product formation in the high-temperaiure ethane pyrolyses and for product ratios in the hightemperature n-butane pyrolyses. This validates the mechanisms, the rate data employed, and the view that the relatively technically easier, low-temperature studies can provide most of the information required for industrial purposes. Consideration of low-temperature mechanisms suggested that the copyrolysis of ethylene and ethane or n-butane should increase propylene yields substantially on account of conversion reaction C2H4 + 1-C3H7* + C3H6 H and C2Hj. sequences CH3. C2H4 1-C4H9* + 2-CdH9. CaH6 C H I . . This proposal has been fully substantiated b y experiment. It is shown that, in enhancing propylene yields in these alkane pyrolyses, ethylene can act as a homogeneous catalyst.
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D u r i n g the past 10 years most of the outstanding problems relating to paraffin pyrolyses at low temperature (ca. 4 5 s 600°C) have been resolved by detailed study of the decompositions of C2-Cj compounds. Thus, there is now no doubt that the reactions proceed exclusively via radical chain processes of the Rice-Herzfeld type. Further, substantial discrepancies between rate data from different laboratories, even for the earliest moments of reaction, have been traced to the effects of (i) traces of oxygen, (ii) the nature of reaction vessel surfaces, and (iii) the surface/volume ratio ( S / V ) of reaction vessels. The significance of these may be judged from the findings of hIartin, et al. (1962), and Niclause, et al. (1965, 1967), that the decompositions of propane, isobutane, and isopentane a t about 500°C can be almost totally suppressed in PbO-coated vessels of high S / V in the presence of a trivial amount of oxygen. These observations and others have led to the adoption of consistent technique and, in consequence, quite excellent agreement is now often found between the experimental data published by different laboratories. This reproducibility and the comprehensive nature of the recent data have allowed the important features of homogeneous pyrolyses to be clearly recognized, understood, and agreed. Only in the matter of the absolute value of Arrhenius parameters of some individual reaction steps and in the detailed interpretation of some relatively minor reactions does any controversy still exist. As pointed out in a recent review (Leathard and Purnell, 1970), the existing body of knowledge about C2-C5 paraffin pyrolyses certainly allows prediction of the mechanism of any higher paraffin pyrolysis, even one which has not been studied experimentally. If, alternatively, the reaction of any paraffin has been studied, quantitative prediction of product yields, in reaction conditions far removed from those used in the original study, can be made with some confidence. It should thus be possible to predict optimum conditions for the production of individual products and to determine the extent to
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which their relative yields might be controlled. This proposal attains importance because: (a) the major olefinic products of decomposition of all but a few highly branched paraffins are ethylene and propylene, both commercially important; (b) published work on paraffin pyrolyses is largely restricted to the temperature and pressure ranges 400-600°C and 10-500 mm. Thus, the main temperature and pressure regimes of probable industrial interest are not very well documented in the scientific literature. The aim of this work is to test two proposals which stem from the above: (i) that low-temperature studies carried out in static systems provide information which can be used accurately to predict events in pyrolyses carried out in flow reactors a t ca. 600-800°C; (ii) that means are available to manipulate ethylene/propylene ratios so as, effectively, to convert ethylene to propylene or vice versa. The procedure adopted has been first, to devise, on the basis of results and interpretation of low-temperature studies, computer programs to calculate product yields, secondly, to check these against the lowtemperature data, thirdly, to predict product yields for high-temperature reactions, and, finally, to check the validity of the predictions by experirnentq with a flow reactor system. The Pyrolysis of Ethane
K e first consider the pyrolysis of C2&, since it is well documented and is mechanistically the simplest of the systems which have been studied in greatest detail. The mechanism for the homogeneous reaction, above 650"C, is now generally accepted as Initiation
C2H6
---f
2CH3.
(1)
Present address, Department of Chemistry, Dalhousie University, Halifax, Novia Scotia, Canada. Ind. Eng. Chem. Fundam., Vol. 1 1 , No. 2, 1972
233
Table 1. Arrhenius Parameters for Rate Constants Used for the Computer Simulation of Ethane Pyrolysis. Law temperature
Log A,
0.04
r E E
Reactian
'3
I* 2. 3b 4. 5
r
L
5
0.02 5 6 4 *C
1000
500
0
TI M E . s e c
Figure 1 . The formation of methane during the initial stages of the pyrolysis of ethane at 564°C. Circles represent experimental points obtained at five reactant pressures in a static reactor system by Quinn (1 963a,b); lines represent data calculated by the computer program using the rate constants derived by Quinn
C,H,
FORMED.mm Hg
Figure 2. Rates of formation of n-butane and propylene as a function of extent of reaction (measured by the pressure of ethylene formed) during the pyrolysis of ethane a t 698°C and 1 18 mm total pressure. Dashed line represents the rate of propylene formation predicted by the computer program using the rate constants reported by Lin and Back (1 966a,b) (see Table I). Solid line shows the predicted decrease in the rate of butane formation as the reactant becomes depleted. The squares (butane) and circles (propylene) represent data measured with the flow reactor system described here
Propylene formation
+ CzH4 +C3Hs + CHa.
CZHS. C2H5'
Inhibition
[
+ C2H4
l-CdH9.
1-CdHg. +2-C4Hg. 2-C&g. +C3Hs H.
+ CHa.
+ CzH4 +C2H5.
I
(6)
(7)
Reactions 1-5 represent the simple Rice-Herzfeld scheme corresponding to the reactions occurring a t zero extent of reaction. A steady-state treatment of the reactions as nritteii indicates that the rate of removal of ethane ( i e . , appearance of ethylene and hydrogen) should be half-order in ethane. I n fact, the reaction is normally a little over first-order in ethane, 234 Ind.
Eng. Chem. Fundam., Vol. 1 1 , No. 2, 1972
High temperature
sec-l or I. mole-' sec-'
E, kcal mole-'
Lag A, sec-' or I. mole-' sec-'
E , kcal mole-'
17.45
91.7 16.0 86 10.8 8.5 10.8 13.6 38 13.6 38 11.1 9.7 11.1 9.7 10.3 0 10.4 0 6 ... ... 9 . 5 (9.6) 19 7b1d ... ... 10.7 -0.8 a Unless otherwise specified, the lowtemperature values are taken from Quinn (1963a,b) and the high-temperature: values and both values for k3 from Lin and Back (1966a)b). High-pressure limiting values. c From Benson (1968). Calculated from the reverse rate arid the thermochemistry of Frey and Walsh (1969).
8.5
the actual order being slightly pressure and temperature dependent. This arises because, in the usual conditions of static equipment studies, the order of reaction 3 is around 1.5 (a result confirmed also by studies of n-butane pyrolysis) and this value will depend both on pressure and on temperature as expected of a unimolecular reaction in its fall-off region. The unimolecular reaction I also has a n order greater than unity around 600°C and pressures below about 100 mm. Allowance for this variation of kl and k3 must be made in predictive calculations. The rate constants of reactions 5a and 5b are temperature and pressure independent. As the ratio of their rates is 7: 1, we may use a combined rate constant k 5 = 1.15k5, (Terry and Futrell, 1967). Reaction 6 becomes significant only above about 640°C (Lin and Back, 1966a, b). Below this temperature, secondary propylene formation occurs through a more complex scheme (Quinn, 1963a,b), involving ethylene addition to 1-C4Hg* to yield l-CBH13. which successively isomerizes to 2-C6H13. and decomposes to C3H6 C2H4 C H 3 . .This reaction sequence is always slow and, having a much smaller activation energy than (6), cannot compete a t the higher temperatures. The inhibition step (7) is very significant a t lower temperatures and causes very marked fall-off in pyrolysis rate as reaction proceeds (Leathard, 1969). It should be noted that, since it is the reverse of reaction 3, i t must have a compatible pressure dependence of the second-order rate constant to that of the first-order constant of (3). This mechanism forms the basis for a computer program which, for given initial conditions and rate constants, has been used here t o predict the reacted gas composition as pyrolysis proceeds. The program is described in detail in Appendix A. An initial check of the computer program was made by comparing the methane yields observed experimentally by Quinn (1963a,b) with those calculated using, in the program, the rate constants originally derived by Quinn from his raw data (see the first columns of Table I ) . Figure 1 illustrates a n example of the findings. The points shown are those determined by Quinn for reaction of various initial pressures of ethane a t 564°C while the full lines represent data produced by the computer. The agreement is excellent, thus siniultaneously verifying the correctness of the program and the eelf-consistency of Quinn's derived rate constants. The ability of the program to reproduce so well the lowtemperature pyrolytic data having been established, it can
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I0
/
o /
/
0
0
/
\ \
\
1.8
2.0 2.2 2,4 2.6 LOG (PRESSURE),(mm Hg)
I
I
50
100
I
I
I
1.0
400 PRESSURE,(mm Hg)
200
Figure 3. Logarithmic order plot for the function of ([C3He]/ [CBH,]f) (the slope of plots such as Figure 2) a t various initial ethane pressures a t 696°C. The dashed line is a computer prediction using the value, log k6 = 9.6 - 19,000/2.3 RT; the circles represent data obtained in confirmatory experiments with the flow reactor.
now be used with some confidence to predict higher temperature yields. For this, the rate constants obtained by Lin and Back (1966a,b) from experiments a t somewhat higher temperatures than those used by Quinn and which include a more acceptable value of El have been used. These allo\T us to put values on k3 and k7 and also, k8 (see the final columns of Table I). For any particular temperature and pressure, values of k l , k 3 , and k7 have been chosen to maintain consistency with the pressure dependence observed by Lin and Back. Since each prediction made by the comput’erhas been checked by a flow reactor experimental study, it is appropriate to describe the react’or system briefly a t this point. Carefully degassed ethane (Matheson C P grade, 99% min) was passed through a n electrically heated quartz reactor a t subatmospheric pressures. Two reactors (40 cm long, internal diameters 7 and 17 mm) allowed choice of gas residence times within the range 0.2-10 sec. Temperatures ranged from 600 to 720’ and were measured by chroniel-alumel thermocouples situated in a a ell extending along the central axis. The effluent gas was analyzed by gas chromatography. This equipment and the procedures adopted are described in considerable detail elsewhere (Pacey and Purnell, 1972). Previous flow reactor pyrolyses of ethane in this temperature range have been reported, the most comprehensive being those of Davis and Wlliamson (1959). Most such studies were performed before the rapid progress in low-temperature pyrolysis studies of the past 10 years and so are of limited ut’ility. The experiments described here were planned in accordance with the systematics observed a t lower temperatures and were particularly concerned with the formation of propylene. To prevent carbon deposition on the reactor surface and to maintain similarity with the low-temperature studies, these experiments have been restricted to small extents of react’ion (
