1458
J. DE GRAAFAND HAROLD KTVART
1701. 67
A CAPA4CITY FLOTY REACTOR FOR DETERMINING T H E KINETICS OF HOMOGEXEOUS GAS PEASE REACTIOSS Organisch Chemisch Laboratoriuin o j the Rijksmiversitcit te Leiden, The Setherlands Received Decrrnber 3, 1962
A “capacity flow” or “steady state” reactor applicable t o study of the kinetics of high temperature liomogeneous gas phase reactions has been constructed. The anticipated range of applicability of this apparatus has been discussed. Data obtained in this fashion on the rates of pyrolysis of ethyl acetate have been presented and compared to values measured by other authors bv means of tube flow reactors. Some consideration has been given to the sources of error and the reliability of the activation parameters romputed from our measurement.
Introduction The methods and practices commonly applied in studying the kinetics of homogeneous gas phase reactions have recently been reviewed by Walters2 and Ma~coll.~ These authors have classified exi$ting knowledge in the categories of static and flow methods and have considered various applications in both of these categories together with their respective advantages. Evidently, in both methods two distinct measurements must be made in order to achieve a measure of the rate of reaction; the extent of reaction and the time of reaction, although in the flow methods the time measurements are effected on a space coordinate The particular advantages of the flov methods have been realized in tube-flow reactors in which a time invariant concentration gradient is established along the length of a tubular reaction chamber. These circumstances lend themselves very well for measurement of very rapid reaction rates in solution3 as well as in the gas phase. A type of continuous flow reactor first discussed by Denbigh and c o - ~ o r k e r s ,called ~ the “capacity flow7’or “steady state” reactor, has been experimentally elaborated by Hamniett and co-morkers.j The capacity flow reactor seems to have been applied previously only for homogeneous liquid phase kinetics. It differs in essence from the usual flow reactor3 in that the reactiofi space is a well stirred vessel instead of an unstirred tube. As in the tube reactor, the capacity flow reactor also approaches a steady state but here the composition is uniform and invariant throughout the reaction zone. We have undertaken to develop a capacity flow reactor designed for rate measurements 011 homogeneous gas phase reactions in the effort to realize one of the inherent advantages of the method, namely, for determinatidn of the iiistantafieous rate of reaction as a function of entirely constant reaction conditions. It has already been pointed out by Young and Hammett5 that varibtion of reaction conditions durilig the course of reaction in a static system or in a tube flow system may lead to (1) Author to whom inquiries should be addressed a t his permanent address, The tlniveisity of Delaware, Department of Chemistry, Newark, Delaware, The work reported here was carrled out while this author was National Science Foundation Senior Post-doctoral Fellow a n d Visiting Professor a t the University of Leiden, 1960-1961. (2) W. D. Walters in “Technique of Organic Chemistry,” Val. 8, Interscience Publishers, Inc., New York, N. Y., 1953,p. 231. (3) A. Macooll, ref. 2, Vol. 8, part I, 1961,p. 427. (4) R. Stead, F . M. Page, and IC. G.Denbigh, Dascussions Faraday Soc., 3 , 263 (1947); K. G. Denbigh and M. Hicks, Trans. Faraday Soc., 44, 479 (1948). ( 5 ) H. R . Young and L. P. Hammett, J . Am. Chem. Soc., 73,280 (1950); J. Raldick a n d L. P. Hammett, abid., 72, 283 (1950): >f J. Rand and L. P. llammett, zbzd,, ?a, 287 (1950). See also R.I,.Burnett, Dissertation, Columbia University, as reported in Dzssertatzon Abstv., 80, 4537 (1960).
a very real kinetic deviation. Complex reactions are particularly susceptible to this possibility for inherent ei’ror in the traditional methods of gas phase kinetic measurements. Furthermore, mathematical analysis of the kinetics of complex reactions, ordinarily quite difficult when applied to static and tube flow reactor data, is carried out (as we shall see) much more simply by means of the capacity flow treatment of gas phase reactions. It will also be seen that several other factors, which often present difficulties in tube flow reactor measurement of homogeneous gas phase kinetics, are readily overcome in the capacity flow method. Thus, while the effect of change in volume during the reaction and the effect of diffusion must be reckoned with in the tube flow reactor, these variables are readily eliminated in the capacity flow experiment. In addition, due to the conventional difficulties in regulating for perfect temperature uniformity over the full length of the tube reactor,6 such “end effects” and longitudinal temperature gradients result in significant errors in rate measurements 011 high activation energy reactions. The stirred reactor condition ensures against such contingencies and renders errors due to irregularities in temperature control easily detectable. We have discussed below the elements of design of a capacity flow reactor and associated apparatus, which was constructed for measurement of homogeneous gas phase kinetics at temperatures up to 600’. We have indicated how this design may be applied for determination of reaction rates, reaction orders, and other parameters in complex reactions. We have also presented data obtained with this reactor in studies of a relatively simple reaction, the pyrolysis of ethyl acetate, in order to achieve a comparison with results reported in the literature on the same reaction and obtained by means of the conventional tube flow reactors. Experimental Desiqn Features of the Homogeneous Gas Phase Capacity Flow Reactor .-The accompanying sketrh represent8 the design of an npparatus whirh i9 presently being applied in a kinetic study of the high temperature chlorination of aromabic hydrocarbonp. Alternatively, however, provision can be readily made t o permit the use of any other liquid substrate than “aromatic” and any other gaseous reagent than the ‘‘c1.~” indicated. This complex reaction is merely used for illustration. The renctor chamber VI1 and the various flow control elements are all integrally mounted on a frame, which iq readily movable or secured in a desired position on rails mounted in the vertical plane above the center of the thermostatea73ath. I n i h e condition of measurement the renctur rhamber and the premix coilb (6) S. W. Benson, J . Chem. Phya., 32, 46 (1954).
wliicli le:d to the eritraiicc?port :it its very ljottom are ininiersed the ttiernioatated bath to the fiuid level. The left arm of the premix coil leads t,o the rewtor froin tlie preheatet-evaporator \', by nieaiis of which the liquid component (here labeled aroni:itic) is smoothly volatilized :ind :idmixed witli the nitrogen c:trric!r gas. Atiout a closer1 turns of 6-cni. arbor premix coils, f:tt)ricated from 4-inm. bore tubing, were c:tlculatcd to be of srifficient lerigtli when ininiersed in the Iuttli to bring the gas mixture to h t t h teriiperature and convey it directly to the eiit.rance port directly Ijeiieath tlie stirrer. 'Hie right :win preinis coilv nerc of the siinie coiistrriction arid served a siiniliir function in conveying the prelieated (in section \'I) mixture (JE ttic second re:tctioii component (liere lal)eled Cl,) :ind the c:irrier gas to tlie locus of niisirig of ttie rewtive cornpoiii
Ilellts.
Sonic attention h:id to be given to tlie diviriori of tlie carriw
4
g:~s ffow leiiding into the respect,ive :irnis of the premix coils.
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'I'liis h : d to he done to assure that the two stre:irris of gases have iie:trly the s:tnie flow velocities at the point of their conflueiicc, aiid tlwt there sliall Le little or no tendency for back diffusion of the one stre:tin jrito the premix coils conducting the other. One of the w':tys iri which this wiis acconiplislied is illustnited in Fig. 1. liere the t o t d llow of c:irrier g:~sW:LS nietered by the flowmeter cornplrx I, which consisted of :I rot:inieter equipped witli :L choice of t)y-p:Lssirig cnpillaries tliat permitted dteririg thc range of coritro1l:tble gas fkJW. l'lie flow of gas that issued f r o i n I wits divided between the two miis by :t coiiibirintiori of a fixed citpil1:try dif'fereritid niarioriieter-flo\~iiieter, shown as 111, : i d :i viuiable capillary device slwwii as IV. The latter consisteci of :i long liypoderinic syringe needle penetrating :I rieoprene diaphragm re:il and closely iitted to the inside diiirneter of a leiigtli of uiiifurni bore g1:iss tubing. The length of needle extending irito the gliiss sleeve W:M readily adjiisted and thus (witrolled the resktance to gas How into section V I and hence to tlie right arm preiiiis coils. AUtcrnativcly,when the flow rates required were large enougli to produce undesir:tt)le pressure ripstremi of tlic flowmeter cornplex I , the differential manoineter 111 had t,o be repluced by it varial)le capillary similar to IV. Froin miong the :ipparently innumerable coin1)in:rtions I)ossiblc with two such vui:t!)Ie c:qdl:iries a consttint totti1 resist:i!ice to gas flow w:\s o h i n e d which permitted a choice in the range 1: 5 to 5 : I in the division of carrier gas flow velocities in t.he two arnis. 'I'he smooth, oscillation-free flow of carrier gas wns assured by placing a 2-1. b:tllast bulb in the line before 1. .4 continuous H o w of liquid rerigeiit (iiromtitic) was delivered t u the cwporator V by iiie:tiie of :L niotor-driven syringe pump c:tpable of ti very large range of delivery rates; (I'nita 1, Rraun, RIelsungen, Germ:iny). However, the :Lctiievement of an oscillation-free Row and const:int c:)niposititrri or th. mixture of ciirricr gay and vo1:ttiliaed arom:itic require(l c:ireful adjustment of the temperature of the ev>ipor:itor. The. desired condition of steady How and composition of t,hc gas strc:ini from \' wus nionitorc.tl :ind juclgetl to :icc(!pt:iI)le on the h s i s of nic:isurenients with :L li;ith:ironietw ( therrii:il conductivity rriotJtlr),siniilar to that used :is :t g:rs-rlironiatogr:t~jli,y dcteCt(Jr element. The :ictrinl cv:lpor:it,or which fulfilled these rigorous deniands is pictured :is thc ripper part of section V in the figure. The (aromatic) liquid from the syringc p r i n i p entered the system through the innrr (part) of the concentric tuhes and discharged onto a saucer shaped hollow under which a v:tri:tc controiled heater elerncnt was mounted. 'l'he end of the concentric delivery tube ap1)ro:iched to within 1 rnm. of the 1ie:ttcd saucer. The citrrier g:ts which flowed in the outer annulus of the concentric tubes irnpingctd directly onto the hc:ited surface :inti rapidly conveyed the volatilized liquid iiito t,he surrounding, gl:tss-hclis packed void (which was heated by ttic tubc furn:ice as s h o w n ) :tnd thence to t lie left :mi1 premix coils ininiersed in the therniostat, fluid. l'he total void volume of \r :ind the premix coils was negligibly srn:\ll compared to the void volume of the stirred reactor VII. 'I'hc katharoinetric monitor showcd conclusively that the itle:tl condition for :ittaining a stemiy, constant composition flow lies in the use of an evaporator teniperaturc that is only slightly higher than the boiliiig temperttture of t,hc liquid t o be volatilized. The nisin reactor chamber VI1 wm fashioned from a spherically shaped Pyrex flask with four shallow, vertical pleats to iinprovo the inixirig action imparted by the glass paddle stirrer (shown). Thc void space in the neck of hhe flask above the :irtu:tl reactor volrimc was reduced to a negligible amount by use of :m evacuated length of tube connecting the truhure stirrer shaft to the stirrer paddle. This tube rotated with very small
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A R OMAT IC Fig. 1 . 4 c h e m a t i c reprcscntation of tlic rcwtor (see tt,st for legend). r1cnr:inw in the noclr or the re:wtor (ilwvc the level of thc! therniostat fluid). The clear:iricc, however, W:LS sufficient to :t!low UIIobstructed flow of th,' istoady stitte) g:is composition into the cold condenser flasks :it i.111. I n the present inst:mce the ncok of the re:tctor and the connecting exit tutw w ( w wr:ipped with hcating eleiiients and rrininkiincti :it about 'LOO". The stirrer seal dirwtly :il)ovc the neck h:td to be resistant t,o hot Clp, HCl, rind c1~loro:iromuticuyithout permitting nirtal in contact with the gas strcrini. A spring-l):tclccd 'l'eflon disk attiiched to the stirrer sh:ift arid rotuting tightly against :L fluorocarbon-gieased, ground surf:tce provided both the nccessary seding aption and chcmic:il and thermal inwtness. The truhore I w r i n g .for the stirrer shaft was silicone grcase lubricated and water cooled. The annular space between tlie reniova1)le stirrer bearing and the Teflon disk seal was kept iirider tt slight, positive pressure of pure, dry nitrogen bubbling through about L' cm. of sulfuric acid downstream. This construction ell'ectively p r o vented wuter and oxygen from dilrusing into the r e a d o n zone through the stirrer Y d . The rotation of the stirrer was readily regulated by a variable speed niotor also mounted on the rack and driving the shaft through IL flexible cable coupling. Considerable latitude was possible in the design of the traps VI11 in which the steady-state reaction product was received anti thereafter-analyzed. l'he two trap comtination shown in E'ig. 1 followed the suggestion of Huyten7 for tre:tking u p thr misting tendency that occurs with rapid cooling of the hot gases. Alternnt.ively, for very diflicrilt, c:tses, we were equipped to iise :t high voltrtge Cottrell precip;t:rtor. Thc theimostated I)nth consisted of :in open top stainless steel tank which w:ts insulated by at least IO cm. of inagnrsih-asbestos powder., A removable cover of thick 'I'ransite sheet, appropriately slotted t.0 acconirnodate the imincrsion heater, thermostat controls, apd the stirrcd reactor, could be put in place over the top of the bath. 'l'hc! thermostat fluid consisted of a mixture of KXO, (53'7;,),N:iNOi (40(!&), rindSaS0.4 ( i ( ; h ) called Hitec (I
