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KINETIC AND EQUILIBRIUM STUDIESOF BENZENE HYDROGENATIONIN BATCH RECYCLE REACTOR

P A U L F . K O R B A C H ' A N D W A R R E N E . S T E W A R TDepartment of Chemical Engineering, University of Tl'zsconsin, .Madison, Wis.

Rate and equilibrium data are reported for vapor-phase hydrogenation of benzene over a pulverizedplatinum-alumina catalyst. The measurements were made with a fi xe d-be d diffe rential reac tor in a batchrecycle system, and illus trate the possibilities of this unusual experimental technique. The reaction condi-tions studied range from 500" o 600" F., 2- to 14-atm. total pressure, 6 to 12 moles of Hz charged permole of benzene, and 0 to 99.9% conversion. The results do not indicate a definite reaction order, but theinitial ra te varies appr oxi mate ly as the cube of the total pressure. No side reactions were observed.

HE hydrogenation of benzene to cyclohexane is a reactionTo f cont inuing impor tance for k ine tic inves t iga t ion . T hereac t ion is indus tr ial ly im por ta n t , k ine t ical ly complica ted , a ndsubject to s ignif icant heat an d mass trans port effects . T hepresent paper provides new kine t ic and equi l ib r ium da ta forthis reaction. Of grea te r im por tance , however , is the experi-mental technique developed in this research (3 ) . Th e t e c h -nique is presented he re in some d etai l, as it app ears promisingfor the study of man y types of f ixed-bed reactions.

Kinetic s tudies of rapid solid-catalyzed reactions are gen-erall y diff icult. In the conventional one-pass differential re-ac tor , opera t ion a t convers ions la rge enough for accura temeasurement may lead to gross uncertainties in the tempera-ture or cornposition at t he catalyst par ticle surfaces ( 6 ) . \Yhileit is possible to reduce these uncertainties by use of low-activity catalyst. high mass velocities, or a series of catalyst bedswith intermediate heat exchange, each of these methods raisesneiv difficulties . Anoth er diff iculty encountered with one-passreactors is that large variations in feed rate or catalyst chargeare usually necessary to study the change in reaction rate withreacti on cond ition s; this also app1:es to recycle reactors whenopera ted a t s teady s ta te .The above diff iculties \vere avoided in the present s tudy byusing a batch recycle reaction system (Figure 1) . Th e highf low ve loc i ty through the reac tor , and the small ca ta lys tparticle s ize, minimized heat and mass transfer diff iculties .Th e ba tchwise opera t ion a l lo\ved rap id de te rm ina t ion of r a teand eq ui l ib r ium da ta , f rom m easurements of the compos i tion ofthe reaction mixture as a function of time. A thousandfoldrange of reaction rates was measured readily o n a s ingle catalystc h a r g e . Xo cont inuous feed sys tem was required . Th e smallfeed consum ption m inimized the dang er of cumulativ e poison-ing of the ca ta lys t . Accu ra te de te rmina t ions of reaction selec-tivity \\'ere m ade , which \vould not h ave been possible in a one-pass differential reacto r .

Since the composition in a batch reaction system varies withtime. the results obtained ma y depend on the transient responseof the catalyst to changes in fluid compo sition. Th e presentsystem, however , \vas operated \vith a very small catalyst-re-ac tan t r a t io , JO give a small rate of composition change and aclose app roac h to steady-state catalyst performance. Resultsgiven later indicate th at this objective was achieved.Th e present paper is devoted to the exper im enta l technique

1 Present address, Humble Oil and Refining Co., Baytown, Tex.

and resul ts .a re s t i ll unde r s tudy , and a re not d iscussed here .

Th e ques t ions of mechanism and ra t e equa t ion s

ReactantsSpectrom eter-grade benzene from Distillation ProductIndus tries was used . Chrom atograph ic ana lysis ind ica ted thecomposition to be 99.31 mole 76 benzene, 0.69 mole % cyclo-hex ane; this analysis was used in all calculations.Elec tro ly t ic hydrogen f rom Sa t iona l Cylinder Gas Co. wasused . This gas conta ined about 0.5 mole 70 oxygen andsome \cater vapor. To remove these impurities the gas waspassed through a bed of p la t inum and pa l lad ium ca ta lys ts ,and then through a bed of activated alumina, before enter ingthe reaction system.

CatalystA commerc ia l p la t inum-a lumina ca ta lys t , S inc la ir -BakerRD-150, was used . Per t inent proper t ies of this catalyst are:

BE T ni t rogen a rea approxim ate ly 500 sq . mete r s per gram, bulkdensity 0.79 gram per cc.: pore volume 0.56 cc. per gram,pla t inum content 0 .6 weight yc. Th e ca ta lys t charge (0 .250gram) used for the main par t of the s tudy was ground to 45to 50 mesh in a nitrogen-purged dr y box, transported to thereactor in a sealed vessel, and inser ted into the reactor in astream of dry n i t rogen .Apparatus

T h e flow plan of the reaction system is shown in Figure 1.Deta i ls a re g iven by Korbach (3 ) .

T he system consisted essentially of a small f ixed-bed reactorand two var iab le-volume p i s holders , with a pu mp cont inuous lyc ircu lat ing the react ion mixture through them . Th e reac torwas operated at 500' to 600' F. , an d the balan ce of the systemwas main ta in ed a t 400 ' F. or above to keep the reac t ion mixturefully vaporized. Th e variable-volume vessels, circulationpu mp , and pressure contro l le r were mounted in a vapor ches t,hea ted by Te tra l in vapor condens ing a t 402 ' F:Th e reac tor she l l was an 18- inch length of 1; ?-inch Schedule40 stainless s teel 304 pipe. T he catalyst was mo unte d in ascreen-bottomed annular basket which f itted closely to theaxia l thermowell and the tube \\ .a ll . Th e ca ta lys t bed had aninside diameter of 0.14 inch, an outside diameter of 0.54 inch,a n d a d e p th of a b o u t 0 .1 in c h . Th e re a c to r tu be a n d p r e h e a tcoil were mounted in a sand bath f luidized with preheated air ,Lvhich proved to be a simple and effective thermostat.Tw o variable-volume vessels ivere provided (see Figure 1)to allow operation at a s teady total pressure du rin g the reaction.Th e volumes were contro l led by m eans of a d i aphra gm pres-sure sbvitch, pump, and solenoid valve, which regulated theam oun t of hyd rauli c oil in the space arou nd the bellows. In-

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REACTORn s o n dbath heater,w i t h pr e-h e a l coil

1 I JHYDRAULIC BACKUP SYSTEM

Figure 1 . Flow plan of reaction unit

side each bello\vs a hollclw f iller was mounted to allow greaterf lexibility in ope ratio n. \\ 'i th the filler volume excluded fromthe system, the operation of the bellows allowed a 337 , shrink-age in system volume (from 8.6 liters to 5.75 liters) ; with the f illervolume inc luded , a 1970 shrinkage could be obtained (from14.8 liters to 11.9 5 liters) .Protection of the bellows against un bala nced pressures proved

diff icult. T he f irst met hod used was to install rup ture disksbe t \veen the hydraul ic oil and the d ischarge of the gas recyclepu m p; th is metho d \vas d iscont inued a f te r several p rematuredisk failures and consequent oil contamination of the reactionsys tem. Th e second meth od was to l imit the t r ave l of thebellows by means of level-sensing switches on the external oilreservoir ; this metho d w.as used in most of the runs report edhere. T h e level-sensing meth od worked w ell but was not in-fa l l ib le , an d the work repor ted here was te rmina ted by rup tureof th e bellows.

Gas c ircu la t ion was provided by a d iaphra gm pu m p (Pres -s u r e P r o d u c t s M o d e l 2705) with a s ta in less s tee l d iaphragmand V iton-A gaske ts an d sea ls . This pum p gave a c ircu la t ionra te of about 0 .65 cu . foo t per m inute a t suc t ion condi t ions,which was sufficient to e:jtablish a well-stir red system w ithin 1or 2 minutes f rom the s ta r t o f each ru n . Th e r a p id c i rc u la t io na lso minimized tem pera ture a nd compos i t ion grad ien ts in thecatalyst bed.Op e r a t i o n

Before each run the gas space in the uni t was purged thor -oughly with hydrogen , and the be l lows were pos i t ioned . T h euni t was then f i l led with hydrogen to the des ired opera t ingpressure , the c ircu la t ion p um p w as s ta r ted , the pressure-contro l

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a 4115 0.1154116 0.1120 4041 0.1160'4117 0.112A 4118 0.112

0 I 1 ' 1 h ' 1 1 I I0 2 4 6 8 IO 12 14 16W/F, GM. CAT./(GM. MJHR.)Figure 2. Activity tests on stabilized catalyst charge

sys tem \vas ac t iva ted . and the reac tor was brought to opera t ingtem pera tu re . \ \ 'hen the des ired reac tor tempera ture wasreachvd. the hydrogen flo\v \vas directed through the chargepo t of vaporized b enzene (see Figure 1 ) to s ta r t the run .7 'he progress of the reac t ion v a s measured f requent ly dur in geach run, by chromatographic analysis of gas samples ventedf r o m th e u n i t . T he run s were cont inued . in mos t cases , un t i leithe r th e limit of the bellobvs travel \vas reached or the reactionhad prac t ica l ly r eached equi l ib r ium..4f te r each run the uni t was vented and purged thoroughly\v i th hy-J rogen , un t i l th e chrom atograph showed the ventedgas to be f ree of hydrocarbons . T he uni t was then pressuredto 85 p.s .i .g. with hydrogen and left s tanding overnight withthe reactor at 500' F. Th e f i r s tthree were deactivated, af ter short per iods of testing, by hy-draul ic o i l which en te red the sys tem via the rupture d isks .T he ruptu re d isks were then e l im ina ted an d the f ina l ca ta lystcharge \vas installed.T he f ina l cata lys t charge showed h igh but var iab le ac t iv i tyin the f irs t 15 runs, durin g which it was held at 500' F. I h eactivity was then stabilized by heating the catalyst f rom 500'to 900' F. over a 5-hour period, u nde r 90 -p.s . i.g . pressure in as ta t ic hydrogen a tmosph ere . No fur ther var ia tions in ac t iv itywere not iced unt i l run 41 21, when the test program was essen-t ia lly comp le ted . T he da ta r eported here were al l ob ta inedwith this s tabilized catalyst charge, except for the f irs t tworuns made with the second catalyst charge, which \teere at acomparable ac t iv i ty leve l .

I t is believed tha t the activity f luctuations observed with thefour th charge m ay have resu l ted f rom incomple te r educ t ion ofthe catalyst. Con ven tion ally, this catalyst is treated in thereac tor with hydrogen a t 800' F. or h igher to reduce the p la t-inum to meta l l ic form ; however in the present work , un t i l thes tab i liza t ion , the four th cha rge had not been exposed to hydro-gen above 600' F. \Vhatever the cause? he activity did becomestable when the catalyst \vas exposed to hydrogen at the normalreduc t ion tem pera ture of 900 ' F . Fur th er s tudy of these ac-tivity effects is warranted.

Four 0.25-gram charges of catalyst were tested.

Calcu la t ionsTh e amounts of hydrogen a nd benzene charged for each run

were de te rmined by volume and weight , r espec t ive ly . Th ecomple te inventory of the uni t a t each sampling t ime was ca l-cu la ted l rom the measured ra t io of the cyc lohexane and benzeneconcentrations, assuming a well-mixed system and correctiiigfor the mater ia l r emoved in the gas samples . So productso ther than cyc lohexane \vere found. Th e ma te r i a l b a la n c e schecked within i-576 for all runs reported here.

20si0w>z

15

8 105

'0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6W/F, GM. CAT./(GM. M JH R.)Figure 3.of feed

Catalyst response tests made b y vary ing amount

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Figure 4. Integral conversion data at 500" F.10080

82 0Pa5 40v)W

82000 2 4 6 8 10 12 14 16W/F, GM. CAT./(GM. M./HR.)

Figure 5 . Integral conversion data at 600" F.

T h e r a t e o f react ion, r , per uni t mass of catalyst was cal-cula ted f rom the re la t ion

r = f dx (1 )W dtwhich holds for a perfectly stirred batch system. H e r e T1 isthe mass of cata lys t , t is time, x is fractional conversion ofbenzene, and f i s the ins tantaneous mo lar in 'ventory of benzeneand cyclohexane in the sys tem. Th e de r iva t ive d x l d t , a t t h emea n of successive measur ed conversions, rvas approx im a ted byAx /& . Tests of 24-hour durat ion wi thout ca ta lys t showed nodetectable convers ion ; hence no correct ion for thermal ef fectswas needed.

T h e s t a t e of the f lu id was near ly uni form throughout theca t alys t bed a t any g iven tim e . Even a t t he unusua l ly h igh r e-act ion ra te of 1 O gram -m ole h r . -l gram - ' encoun te r ed i n run5022, the calcula ted te mp eratu re dif ferences (3 )were only 4"C .f rom bed inle t to out le t , 0.7 C . f rom the gas s t ream to the par-ticle s urface, and 1" C . ins ide the par t ic le . Th e correspondingmole fraction differences were less than 0.001 from bed inle t toout le t , and less tha n 0 .0002 f rom the gas s t ream to the par t ic lesu r f ace , fo r eac h of the three species. T he calculated effects ofdif fus ion ins ide the par t ic les were somewh at greate r , an ef fec-tiveness facto r of 0 .9 \vas estimated for run 5022 from diffusionand p ore s ize dis t r ibut ion data obta ined by Johnson (2 ) on thiscatalyst. Fo r the runs at loizer pressures, the differencesbet \veen gas-s t ream and internal par t ic le cond i t ions were evenless s igni f icant . Th e only ser ious problem in eva luat ion of re-act ion cond i t ions was the pressure dr op , which somet imesreached 107, of the reactor pressure . Th e react ion condi t ionslvere reported (3 ) n terms of the average s t ream pressure andtemperature in the reactor , and the gas composi t ion was re-po r t ed a s ana lyzed .

d 0.200se- .15aX\=.

Figure 6.ratio of 12.0Reaction rates at 500" F. and initial H2-benzene

Figure 7.ratio of 12.0Reaction rates at 600" F. and initial H2-benzene

ResultsIntegral -convers ion data obta ine d wi th the s tabi l ized cata lys t

T he abscissa of these plotsre plot ted in Figures 2, 3, 4, n d 5.is labeled as JP','F, where"=so ;to emphasize the s imi lar i ty between these plots and the corre-spond ing ones for isothermal plug-flow reactors ( 7 ) . Thi schoice of abscissa is also motivated by the fact that the localslope of the x us. Lb/F curve for any of the runs i s equal to thereact ion ra te? r? of Equation 1 at t ha t po in t i n t he run , Th eH, /HC atio sho\vn in each case is the in itial valu e for the ru n.

Activity test data on the s tabi l ized cata lys t are shown inFigures 2 an d 3. The reproducibi l i ty of the measurements :mad e a t in tervals between other ru ns in the progra m, is evident .Equal ly good agreem ent was found in repeated tes t ing at o the rcondi t ions . Th e cata lys t ac tiv i ty was essentially constant:except for a poss ible drop of about 5% between ru n 41 17 andthe la ter runs (4121> 5012, and 5011) sho\vn in Figure 3.Therefore , a l l resul ts in th is paper are presented as observed,wi thout ac t iv i ty corrections .

Tests of the transient response of the cata lys t to changesin gas composi t ion are shown in Figure 3 . T he four runsshown were a l l made on the same catalyst charge, xvithoperat ing condi t ions comparable except for the in i t ia l amountof react ion mix ture . In th is way the t ime required to a t ta ina given conversion \vas varied over a 2.5-fold range. rvith runs5012 and 5011 takin g the longest. T he results for x i ~ s . I7 ,F

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215 0.7

d 0.5\-a5i3 0.1e0.07q 0.2

U

- - .- .--PRESSURE, ATM;Figure 8. Reaction rates at zero comersion

in the four runs agree wel l , par t icular ly for runs 4121, 5012,an d 5011, which were mad e consecut ively . Therefore i t i sbelieved that , a t leas t u p to th is level of react ion ra te (0.2 g r a m -m ole h r . - l g r am - ) an d conve r sion (0 t o 3 0y0) , the p r e sen tdata may be used for s teady-s ta te reactor des ign.

Figures 4 and 5 give in tegral convers ion data for var iousoper at ing condi t ions . E,ach l ine represents a s ingle co nth uo usru n in the batch uni t , of 2 to 8 hour s du ra t i on . In view of th et ransient - response resul ts ment ioned abov e, these curves shouldalso represent th e Performance of i sothermal , s teady-s ta te , p lug-flow reactors lv i th cata lys t charge I17a nd benzene feed ra te F .

Figures 6 and 7 give react ion ra tes obta ined by di f ferencefrom success ive points of the unsmoothed integral -convers ionda ta . T h e large effect of pressure on the react ion ra te i s evi -den t . Th e da t a a r e sm osoth enough i n m os t cases t o al l ow easyest imat ion of in i t ia l react ion ra tes an d equi l ib r ium convers ions .Th e cha nge in shap e of { .he curves \v i th pressure indicates th atthe react ion does not have a def ini te order wi th respect tob e n z e n e .

F igu re 8 sho\vs the effects of several variables on the initialreact ion ra te . A hundredfo ld increase in react ion ra te is ob-served in going f rom 2- t o 13-a tm . t o t a l p r e s su re . T h e d a t a a t500 F. show a compl icated pressure dependence which i sbelieved to be due t o a change i n t he dom inan t r eac t i onmechanism as the pressure is increased. T he reversal of thet em pera tu r e dependen ce ( and pe rhaps t he dependence onH2/HC ra t io) a t about 4-a tm. to ta l pressure a lso suggests akinet ica l ly compl icated react ion. Fur ther measure men ts wi llbe made to del ineate these ef fects before proposing a react ionra t e equa t i on .

Equ i l i b r i um da t a ob t a ined f rom the r eac t i on r a t e p lo t s a r egiven in Figure 9 . These fa l l between the corre la t ions of A P IProject 44 ( J ) and o f P r igog ine , O u te r , and H erbo (5)! ome-wha t c loser to the la t ter .C onc l us i ons

T h e ut il i ty of a batch recycle sys tem has been demo nst ra tedfor studies of a stable catalyst. Close con trol over the re-ac t ion cond i ti ons w as ob t a ined . Feed consum pt ion and a t t end -ant feed pur i f ica t ion problems \cere minimized E a c h r u nyie lded an integral -convers ion curve f rom zero to near equi -l ibr ium convers ion and react ion ra te data (by di f ferent ia t ion)ove r t he s am e r ange .

I O0 70 5

0 20 1

0 070 05

0 02001

0 0070 005 0 0039 588.50 0 0 4 5 5 8 6 0

A 0055 584.5

x I03T KFigure 9.3 Hz (9) + benzene (9)e yclohexane (9)Equilibrium constants for reaction

Th e da t a g iven he re a r e be li eved app l i cab l e t o s t eady- s t at ereactor desig n. Th is conclusion is based on the results inF igu re 3. which show no significant trend Lvith catalyst-feedrat io . In general , if ir reversible cata lys t ac t iv i ty changes canbe neglected, the react ion ra tes in a batch recycle u ni t w i l lapp roac h the s teady-s ta te fixed-bed cata lys t performance as thecata lyst - feed ra t io appro ache s zero. T h e resul ts a t h igh cata-lyst-feed ratios, on t he o the r hand . w i ll depend on t he t rans i en tresponse of the catalyst to changes i n t he r eac t ion env i ronm en t ;this region of opera t ion was avoided h ere bu t of fers in teres t ingpossibili t ies for future investigation.

T he hydrogenat ion of benzene i s k inet ica l ly compl ic ated.A dd i t i ona l expe r im en t s a r e needed t o de t e rm ine m ore com -p l et e ly t he depende nce of t he r a t e on p r e s su re and com pos i t i on ;this wi ll be done in the n ear future . now th at the reactor sys temhas been rebui l t .A c k n o w l e d g m e n t

J a m e s P. Hutchins gave valuable ass is tance in the develop-m e n t a n d o p e r a t i o n of the reaction sys tem. Fel lowship sup-por t was provided by the Visking Cor p. , Sincla i r Research. Inc . .an d W iscons in A lum ni R esea rch Founda t i on . C om put ingservices were f inanced by the Univers i ty Research Commit teean d t he N a t iona l Sc ience Foun da t i on . Sam ples o f RD-150cata lys t were suppl ied by Eng elhard Indust r ies , Inc . , an d bySinc la i r R esea rch , I nc .l i t e ra t u re C i t ed(1) Hougen, 0. A . , \Vatson, K. M. , Chemical Process Prin-ciples, Part 111, \Vi ley , New York, 1947.(2) Johnson, M. F. L., Sinclair Research, Inc. , Harvey, Il l . ,private communication, 1960.(3) Korbach, P. F. , Kinetics of the Vapor-Phase Hydrogenationof Benzene on a Platinum-Alumina Catalyst, Ph.D. thesis,University of LVisconsin, Madison, \Vis., 1962.(4) Natl . Bur. Std. Circ. (U . S.), bashington, D. C., API Re-search Project 44, Table 5y (Nov. 30. 1945) and 7y (April 30,10671/ ., .(5 ) Prigogine, I., Outer, P. , Herbo, CI., J . Phys. & Coiiozd Chem.(6) Yoshida, F. , Ramaswami, D., Hougen, 0. A , A.1.Ch.E. J .52, 321 (1948).8 , 5 (1962) .

R E C E I V E Dor review J u n e 3: 1963A C C EPTEDeptember 9, 1963V O L . 3 NO. 1 F E B R U A R Y 1 9 6 4 27