High-Temperature Pyrolysis of Propylene-Propane Mixtures Ken K. Robinson' and Eric Weger Washingfon Vniversity, St. Louis, X o .
Propylene-propane mixtures were pyrolyzed at temperatures near 1 100°C and 1 msec reaction time in a quartz annular flow reactor to promote the formation of butene-1. Propylene selectivity fell from 55y0 a t 3% propylene conversion to about 14% a t soy0 conversion. The highest butene-1 yield achieved was 5 moles of butene-1 per 100 moles of propylene f e d a t 45y0 propylene conversion. Reaction kinetics for propane and propylene decomposition and butene-1 formation were studied in a nonisothermal flow reactor operated differentially in which the nonisothermal temperature profile was held constant from one set of experiments to the next. The hot reactor effluent was rapidly quenched to help improve the selectivity to butene- 1, b y suppressing secondary decomposition reactions. Decomposition kinetics of propylene and propane were found to b e first order over a long range of propylene and propane concentrations under copyrolysis reaction conditions. At low propane concentrations, propylene decomposition appeared to b e accelerated b y the addition of propane. Butene-1 formation was directly proportional to propylene concentration and independent of propane.
T h e r m a l cracking of hydrocarbons takes place by a hydrocarbon ru1)turiiig a t its weakest, molecular bond and, in most cases, yields lower-molecular-weight compounds. Recent studies (Scott, 1967) have demonstrated that under eactioii conditions higher-molecular-weight 111-drocarbons will also be formed. This novel thermal reforming system is reported to form these new products by means of free radical combiliation. This reaction may be represented by the following general equation in which two like or unlike free radicals, R1' and Rz.,collide with each other to form the stable molecule
RiR2
RI.
+ R?.
+
RiR2
(1)
Free radicals are very reactive (reaction rate constant is known to be quite large), but are normally present in estreniely l o a concentrations. This causes the rate of radical combination (eq 1) to be small under conventional pj.rolj-sis conditions. However, Scott found in his high-temperature pyrolysis studies that' significant amounts of radical conibinatioii products were formed. For example, the copyrolysis of ethylene-acetolie, I)ropyleiie-l)ropaae, and toluene-propane mistures produced propylene, butene-1, and ethylbenzene, respectively, wit,li the methyl radical playing a n important role in these thermal reforniing tems. The specific reaction conditiolis (characterized by temperatures around 1000°C and reaction times of the order of milliseconds) are believed to promote the reaction by causing high coilcentrations of free radicals. =ilso. by rapidly quenching the hot reactor effluent, one can achieve a higher equilibrium yield of the higher molecular weight compounds. The objective of the research being reported in this paper was to obtain quantitat,iveinformation on tlie factors governing the formation of these compounds under copyrolysis conditions. The particular reactants chosen consisted of proLIonsanto Company, 800 S . Lindbergh Blvd., St. L o ~ i a 110. , 63166. T o whom correspondence ,should be sent. 198 Ind. Eng. Chem. Fundam., Vol. 10, No. 2, 1971
pyleiie-propane mistures and thp product of interest was butene-1 (methyl-allyl radical combination). Particular attention w i s paid to the high-temperature (pyrolysis) zone and the low-temperature (reforming) zone of the esperimeiital reactor. d necessary ausiliary part of tlie n-ork was determination of the decomposition kinetic.? of propylene and propane. Experimental Section
The annular flow reactor (see Figure 1) was constructed from t,wo concentric quartz tubes (1 X 3 mm inner, 4 X 6 mm outer) a i d was jacketed by a 2-in. carbon jacket' (inductively heated by a high-frequency generator), -1quench leg was connected immediately below the hot pyrolysis zone where the hot gases leaving the reactor could be cooled with nitrogen or methane. The temperature in the pyrolj was measured with a movable ~~latiiiuiii-~~latiiiuiii-lO~ rhodium thermocou1)le situated inside an asial tliermowell. Corrections to the t'liermocouple readings were made for induction and radiation effects. The temperature a t the queiicliing junction n-as measured \vit,li a stainless steel sheathed chroniel-alumel thermocouple. It was inserted through a l/lc-iii. Swngelok fitting with the tip estending into the quench zone. A finely divided metal rule was soldered to a bracket on the far elid so that t,he tip of the thermocouple could be accurately positioned. The reactor was jacketed by two large concentric tubes, 6IVs in. long, which had cooling water circulating through the annulus between theni. The inner tube was constructed from quartz and was 2Ij2 in. in diameter while the outer tube was Pyres and 31j2 in. in diameter. Stainless steel plates fitted with silicone rubber gaskets held tlie concentric tubes in place. .1 33/4-in. diameter induction coil n-as constructed from 1/'4-in.copper tubing and heated the carbon jacket with power supplied by a Lepel Model T25-3, 25-kW genenitor with a frequency range between 180 and 450 kc. The complete reaction system is illustrated in Figure 2 .
analyze the met'hane composit,ion. A Perkin-Elmer gas sampling valve was used to introduce a reproducible sample size (0.8 ml) into the chroinatograph. Tlie component peak areas were measured with a Ilisc iiit'egrator and then corrected with t'heir corresponding thermal respoiise factors. Standard gas blends of k n o w composition were used to calibrate the chromatograph and deterniine the thermal response factors. The product analysis was combined with feed and quench rates to yield reactant conversion and ,qelectivity. The calculat,ion of propylene coiiver,:ioii iii the presence of propane was not straightforward since propylene is one of the niajor thermal decomposition products of propaiie. In particular, the amount of propylene available for reaction was defined as the sum of t,he propylene fed plus the propylene produced from propane. X set of p y r o l ~ experiments was conducted on propane separately t o est,ablish a relationship between propane conversion and it5 selectivity to propylene. The formula given below was found to adequately correlate conversion and selectivit,y and is in general agreement u-itli recent work on propane pyrolj (Uenson, 1967; Cryiies and Albright,, 1968).
U
Figure 1. 1. 2.
3. 4. 5. 6. 7. 8. 9.
10. 11. 12.
13. 14.
Experimental reactor
% C3H6 selectivity
Feed leg Thermowell Outer wall Inner jacket woll Outer jocket wall Carbon heating element Purge tube Discharge leg Thermocouple Quench leg Support rod Induction coil Boron nitride disks Support plate
=
38.
- 0.5817, C3Hs coiiversion]
Instrument grade propane (99.57, purity) and C1' grade propylene (99.0% purity) were used as feedqtock. Tlie reaction kiiiet,ics were determined by a special esperiineiital technique (Tonell and Martin, 1961) in wliicli the reactor is operated differentially with the noni~otlicrrnal temperature lrofile held constant. Power lair rate exliressioiis were substituted into a differential rate equation and iiitcgrated (subject to differential reactor operation) to yield the m a q a M a l i c e equation
Nitrogen carrier gas plus propylene and propane were each metered separately from gas cylinders through Brooks Sho-Rate rotameters. Cylinder nitrogen and building air were connected t'o a three-way solenoid operated valve so t h a t carbon deposits could be conveniently burned out of the react'or after each set, of runs. The gases were combined a t a mixing tee and then passed through a drying t'ube packed with molecular sieve (5A) t o remove t'race quantities of water. The gaseous feed was then passed rapidly through the reactor and quenched. Pressure drop across the reactor was measured with a pressure gage on the feed side and a mercury manometer on the discharge leg. After being quenched, the warm reaction gas mixture was furt'her cooled in a small shell and tube heat exchanger and then analyzed chromatographically. The reactor effluent, sampled with a six-way valve, was measured with a Rockwell gas meter. Quantitative product analysis was carried out on a PerkinElmer 154-B gas chromatograph (T.C. detector). The coniponents analyzed for were methane, et,hane, ethylene, propane, propylene, propadiene, acetylene, butene-1 isobutylene, cis-butene-2, and 1,3-butadiene. The chromatograph was operated isothermally a t 89OC and required two injections on different columns t'o completely analyze for all components. For the major hydrocarbon analysis, a 5 ft x 1/8 in. stainless steel column packed with SE-30 (silicone gum rubber) on 60-80 mesh F-20 activated alumina was used. It, separated all components except methane and nitrogen. -4second sample a-as subsequently injected into a 5 ft x Ijgin. stainless steel column packed with 60-80 mesh molecular sieve (SA$) to ~
By holding teml)ertiture constant from one set of csperiments to t,he nest, the term.: in the brackets rcniain coiistant and the equation can be eqxewed as
This equation is then easily traiihfornied to :i linear forin by taking the logarithm. The reaction orders foi, the prol)ane, prol)ylene>and butene1 rate equations may he detei,miiied 11). a least-squares a n a l p i s on the linearized form of eq. 3. For this study, however, preliminary data analysis revealed that the rate equations for the components in question were a fuiictioii of only one variable. Thus the reaction orders were determined directly by plotting conversion or formation rates 2's. mole fraction on logarithmic coordinates and calculating the slope. Results
Propane and propylene were firht pyrolyzed beparately to establish general t,rends in the product distribution. S o butene-1 was formed when propaiie m 1))-rolyzed b:. itself and only very small amounts when propylene was pyrolyzed separately. For the latter case, t,lie butene-1 yield (moles of butene-1 formed per mole of propylene fed) was less thaii 1,/2yo for 10% conversion of propylene. However, when proliane and propylene were copyrolyzed, butene-1 form a t ion ' was increased significantly. Alq)rosimately, a sixfold increase in propylene selectivity (nioles of butene-1 formed per mole of prop)-lene decomposed) resulted when propane (a methyl free radical generator) was added t o the propylene. Of course, Ind. Eng. Chem. Fundom., Vol. 10,
No. 2, 1971
199
Figure 2.
Experimental reaction system
1. 2.
Three-way valve Rotameter 3. Temperature recorder 4. Drying tube 5. Thermocouple positioner 6. Pyrolysis reactor 7. Manometer 0. Sampling valve 9. Chromatographic recorder 10. Gas chromatograph 11. Temperature controller 12. Gas meter 13. Induction generator
in addition to butene-1, substantial quantities of methane, ethylene, and hydrogen were formed. The base operating conditions selected are given in Table I. Systematic variations were then made in these conditions as the study proceeded. A typical product distribution is given iii Table 11. Almost ten times as much methane and ethylene were formed as the desired product butene-1. This general trend in the product distribution was observed for all experimental runs and points out that butene-1 is definitely not one of the major products (typical of a long-chain process in which major products are formed in chain propagation steps). It was observed that butene-1 and 1,3-butadiene were always present in equimolar quantities. The selectivity and yield of propylene to butene-1 has been plotted a s a function of propylene conversion in Figure 3. Butene-1 is formed quite selectively a t low propylene conversions and then exponentially decreases with increasing conversion. More specifically the selectivity goes from 55% at 3% propylene conversion to around 14% at 29% conversion. Reaction parameters which include temperature, flow rate, and concentration were varied to produce different values of propylene conversion in Figure 3. The corresponding plot of butene-1 yield (moles of butene-1 formed per mole of propylene fed) increases from zero for no propylene converted t o about 5% at 35% propylene conversion. The reaction order for hydrocarbon reforming is directly related to the type of initiation and termination steps taking place in the free radical reaction mechanism. Therefore, it was considered necessary to determine the kinetics of decomposition of propylene and propane in the present study. 200
Ind. Eng. Chem. Fundam., Vol. 10, No. 2, 1971
The chain-terminating reaction involving the methyl and allyl radicals which are generated in these decompositions will, of course, form butene-1. The decomposition kinetics of both propane and propylene were determined to be first order over a reasonably wide concentration range. For a given temperature profile, the molar conversion rates of the two hydrocarbons were plotted against average mole fraction (Figure 4) and correlated by a family of straight lines having a slope of one. More specific details of the experiments are given in Table 111. The first-order propylene kinetics breaks down a t low propane concentrations where propane appears t'o promote propylene decomposition. Runs no. 20 and 21 in Table IV illustrate that the addition of small amounts of propane leads to much higher propylene conversions. When propylene is pyrolyzed separately, t,he propylene molecule must generate its Table 1. Base Conditions Reactor volume 0.20 cc Residence time 1 msec Reaction temperature 1000-1200°c: (at hottest point) Quench zone 350-450°C temperature Quench gas Methane and iiitrogeii Feed Composition: Nitrogen 70 5 mole % Propane 14 75 mole % Propylene 14 75 mole yo Reynolds number, R e 311 (in annulus)
A@
IO
la, 7
-
- 8
.-ztd x
-
5,-
M - A
- 6 .o
&----
@&-
,xI
L
Table II. Typical Product Distribution at Base Operating Conditions Molen
%
- 4
b
L
Component
._ T
- / - - - -
Other information
Nit,rogen (by difference) 65 5 Total feed flow = 3350 scc/miii Hydrogen 0 . 8 C3Hs/CSH6/KZ = 1/1/4,7 Methane 2 . 6 Nitrogen quench = 8050 scc/~nin 0 . 7 Maximum temperature = 1030°C Ethane Ethylene 4 . 1 % Propylene coiiversion = 1 8 . 9 Propane 1 1 . 3 yo Propane conversioii = 2 4 . 1 Propyleiie 13. O % Selectivity to butene-1 = 1 3 , s A4cetylene 0 . 2 yo Yield to butene-1 = 2 . 6 Propadiene 1 . 2 Mass balance closure:b 1,3-T3utadieiie 0 . 3 (larbon balance = 101.3% Buteiie-1 0 . 4 Hydrogen balance = 99.3% I3 utei le-2 0.0 e Produrt analysis performed with gas chromatograph equipped with T.C. detector. Itelative accuracy is approximately +1.5yG. Bttlaiice made using a "no loss" basis; i.e., analysis was assumed t o measure all reactioii products.
o w i chain carrier species (via the initiation st'ep) before it will decompose at a n appreciable rate. On the other hand, when propaiie is added to the pyrolysis reaction system: one has ail additional source of free radicals (primarily methyl) which accelerate propylene deco~npositioiiby interaction in a typical propagation step. The observed promotional effect of propane on propylene decomposition might', therefore, be explained b y the fac,t that an additional source of chain carrier species allows the chain prop:igation steps to proceed independent of the propylene iiiit,iat,ioii step. This in turn leads t o a faster overall decomposition rate. Butene-1 is believed to be formed by the radical combination of the methyl and allyl radicals according to eq 4.
=
A exp( - A E / R T ]
(5)
With the exception of chain-terminating reactioiis (Le., radical combination) t,he rate constants have a finite activation energy and therefore depend on temperature. The activation energy for a radical combination reaction is com-
Be
- 2
0
b I
monly reported as zero indicating 110 dependence on temperature. I n this study the question of primary concerii was what factor or factors lead to increased yields of butene-1. It is first initructive to write the rate equation for butene-1 formation in terms of free radical concentratioiis.
The reaction conditions em1)loyed i i i this study were rather unconventional with temperatures iii excess of 1050°C: followed by rapid queiichirig being necessary to produce significant amoiiiits of butene-1. The rate coilstant for radical combination (as in eq 4) is independent of temperature since its activation energy is zei'o. Therefore, oiie might interlret the high butene-1 yields as being caiiscd by high coriceiitratioiis of methyl and allyl radicals. Gciierally speaking, an increase in temperature will 1)roduce a higher coiicrritrat~ion of free radicals since they are generated a t a faster rate in the chaiii-init,iat,ion step while the chain-terminatioii step re-
i
t
/
This is a chain-terminating reaction step in which free radicals collide with each other and combine to form a st'able molecule. I n a free radical mechanism, the rate constants for each step are commonly written with the familiar Arrhenius relatioiiship to show their dependency on temperature. k*
>
Propane Propane Proovlene
16-19
L-
0. 03
*
4
1
I
,
0. 05
'
'
0. I
Average Mole Fraction
Figure 4. Conversion rate of propane and propylene vs. average mole fraction Ind. Eng. Chem. Fundam., Vol. 10, No. 2, 1971
201
~
~~~
Table 111. Tabular Summary of Kinetic Runs
Konisothernial Temperature Profile Run
no.
1 2 3
4 5 6
7 8 9 10 11 12 13 14 1.5 16 17 18 19
Feed composition Max. C ~ H ~ / C ~ H S / N ~ temp, 'C
14.7/14.7/70.6 11.7/17.7/70.6 10.3/19.1/70.6 8.8/20.6/70.6 7.4/22.0/70.6 27.6/ 2 . 2 / 7 0 . 2 13.4/16.4/70.2 10.4/19.4/70.2 7.5/22.3/70.2 14.7/14.7/70,6 16.2/13.2/70.6 17.7/11.7/70.6 19.1/10.3/70.6 20.6/ 8.8/70.6 22.1,' 7 . 3 / 7 0 . 6 14,7/14,7/70,6 ll.i/17.i/70.6 10.3/19.1/70.6 8.8/20,6/70.6
1103 1103 1103 1103 1103 1030 1030 1030 1030 1075 1075 1075 1075 1075 1075 1075 1075 1075 1075
% C3H6 conversion
% C3Hs conversion
20.7 22.4 22.3 22.3 22.3
26.0 26.3 26.7 26.6 27.1 25.1 22.2 24.2 23.5 36.7 35.0 35.4 31.5 36.0 36.5 19.7 20.9 25.1 24.1
8.7 17.5 14.1 11.6 26,4 24.3 25.8 22.6 25.0 22.6 15.7 17.1 18.9 14,l
16-19
Rol
0.03
0.05
Propylene Mole Fraction
Figure 5. Butene-1 formation rate vs. propylene mole fraction
Table IV. Copyrolysis at l o w Propane Concentrations Run no.
20
Mole yo 111 feed Propylene Propane yo Propylene conversion yo Butene-1 yield
29 5 0 0 1 7 0 27
21
22
23
24
29 0 0 5
27 4
26 8 1 2
25 3 2 5
9 7 0 36
07 8 8 0 38
9 9 103 0 65 0 83
Since propane improves the selectivity of propylene to butene-1, one would expect some dependency of butene-1 rate on propane. As was mentioned earlier, the primary reason for pyrolyzing propane with propylene is to provide a source of methyl free radicals which can subsequently combine with allyl radicals to form butene-1. The methyl and allyl radicals can be generated from propane and propylene by the following chain-initiation steps
C3H8-% CH3. mains constant. High temperature is a n important feature in forming butene-1 and if it is coupled with rapid quenching, t'heii the product distribution will be shifted to favor but,ene-l and by-product formation (methane, ethylene, and hydrogen) will be suppressed. The above rate expression (eq 6) has litt'le utility since it is difficult to niexvure free radical concentrations b y coiiveiitional niialytical methods. Therefore, the major emphasis of this study was in determining the relationship between butene1 forniation and the hydrocarbon reactant concentrations. The rate of butene-1 formation is a fuiiction of the hydrocarbon concent'ration in the pyrolysis zone of the reactor (lvhich determines the steady-state concentration of free radicals) and the quench to feed ratio (which affects both the temperat'ure and coiiceiitration of free radicals preseiit' in t'he quench zone). In order to take into account the dilution effect, butene-1 forma tion rate was correlat'ed to the hydrocarbon concentration in the quench zone. Both propane and propylene feed concentrations were varied simultaneously and the rate of but'ene-1 formation was found to be only proportional to propylene concentrat'ion over a reasonably wide range of reaction conditions. The molar formation rate of butene-1 has been plotted as a function of propylene mole fraction in the quench zone for three sets of data (in Figure 5 ) and it is correlated b y three straight lines having a slope of approximately one. Thus, butene-1 formation displays the general kinetics shown in eq 7.
202
Ind. Eng. Chem. Fundam., Vol. 10, No. 2, 1971
+ C2Hs.
(8)
Two plausible explaiiatioiis can be given for the particular form of the kinetic rate expression for butene-1 formation (eq 7) and they are briefly the following: a. (Suggested by Eric J. Y. Scott.) Coiiceiitrat'ioii of the methyl radicals is insensitive to changes in propane concent,ratioii while allyl radical is strongly dependent on propylene concentration. The methyl radical, being very reactive, will not experience large concent'ration changes as propane concentration is increased, since it will readily abstract hydrogen from propane to form propyl radicals. Although t'he propyl radical will form some methyl radicals, it also decomposes to propylene and atomic hydrogen and undergoes disproportionation reactions (such a s with the methyl radical t o form methane and propylene). On the other hand, the allyl radical is much more stable and unreactive. Thus, t'he allyl radical concentration will continue to rise as propylene concentration is increased, producing a first-order dependency on propylene. b. At steady state, the rate of generation of allyl radicals in the chain-initiation step is equal to the rate a t which they are destroyed in a chain-terminating radical combination :eaction. Since the high-temperature pyrolysis reaction system is a long-chain process, the free radicals will be destroyed (viachain-terminating reactions) as fast as they are generated (viachain-initiation reactions). It should be poiiited out that allyl radicals are also formed by chain-t'ransfer reactions involving propylene with other radicals. Allyl radicals then regenerate atomic hydrogen by decomposing t o alleiie. Thus, the allyl radical is also involved in cyclic reaction steps or
inore qiecificall!. :Lclo.ml reactioii sequence. If it i h awuiiied that tlit :illy1 radical i.: priiii:ii,ily destroyed liy conihiiiing with the inetli~-li,:idic:il t o forni Iiutene-1 tlieii tlit i x t t of t,liis rractioii caii lie equated t o the :illy1 ratlicnl geiicr:itioii rate iii t h e followiiig Iiiaiiiier allyl genei~atioiirate = :i11>.1 destiuctioii i,ate ioia c1i:iiii-iiiitiatioii del)) (tia chniii-teriniiintioii stel))
k?[C,He] = k* [CH:,. ] [CH?=CHCH,. ]
(10)
It' c:iii be tlieii deduced that if tlie rate of butene-1 forinatioii equals the riglit side of eq 10, then the kinetic rnte erpressioii for butene-1 \voulti be a s in eq 7. Further diecussioii 011 niecliaiiistic iiii1)licatiotw may be fouiid iii Robiiison's latest work (Roliiiisoii. 1970). For extremely low pro1):iiic coiicentratioiih, buteiie-1 formitioii will esliibit a n-cik de1)eiidence on propane. This is to he exliecttd, ii:ituraIly, since prolxliie serves :I< a methyl free r:idical source. T1ii.q plieiiomenoii is illustrated by the data given earlier iii Table IT'. I t appears that propane plays :I dual role in ~)rol)ylene-l)ro~~aiie co~)yroI~ its 1)roinotioii:il effect on propyleiie decompo,qitioii. it also influences the butene-1 rate n h e n present in lo^ co1iceiitr:itions. A r:ite eqiintioii for butene-1 formntioii :i~)l)lic:ililc over :a u-idc raiige of reaction conditioiis ( i e . , iiiclusive of 110th low and iiitermediate 1)ropaiie concentrations) noiild iio doubt he iiiucli inore complicated tliaii eq 7. I t iz, iii fact. quite 1)ossilile that tlie pon-er law forin of the rate equation n.oultl not be adequate t o represeiit the reaction kinetics. Deteriniliation of kinetic. in a iionisotlieriiial reactor, however, iiiipoqes the restriction tlint, a power Inn. rate equatioii lie used &ice it ih easil!. trallqforii1ed t o a linear form b y taking the logaritlini. Hence, it \vas felt that the developnient of :I more definitive rate equatioii was beyond tlie scope of the study being reported.
The i.eactioii kiiietiw of butene-I foriiiatioii :iiid 1)rol):iiie and propyleiie decornl)o.+itioii have been establislietl for this 1iigIi-teiiii)elnture 1)j-rolJxis reactioii ,+y
