Hindawi Publishing CorporationISRN Chemical EngineeringVolume 2013 Article ID 152893 8 pageshttpdxdoiorg1011552013152893
Research ArticleDehydrogenation of MethylcyclohexaneParametric Sensitivity of the Power Law Kinetics
Muhammad R Usman12 David L Cresswell1 and Arthur A Garforth1
1 School of Chemical Engineering and Analytical Science The University of Manchester Manchester M60 1QD UK2 Institute of Chemical Engineering and Technology University of the Punjab New Campus Lahore-54590 Pakistan
Correspondence should be addressed to Muhammad R Usman mrusmanicetpuedupk
Received 4 June 2013 Accepted 5 July 2013
Academic Editors D Cazorla-Amoros G DrsquoErrico F Lefebvre and E A OrsquoRear
Copyright copy 2013 Muhammad R Usman et al This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited
For heterogeneous catalytic reactions the empirical power lawmodel is a valuable tool that explains variation in the kinetic behaviorwith changes in operating conditions and therefore aids in the development of an appropriate and robust kinetic model In thepresent work experiments are performed on 10 wtPtAl
2O3catalyst over awide range of experimental conditions and parametric
sensitivity of the power law model to the kinetics of the dehydrogenation of methylcyclohexane is studied Power law parameterssuch as order of the reaction activation energy and kinetic rate constants are found dependent upon the operating conditionsWith H
2in the feed both apparent order of the reaction and apparent activation energy generally increase with an increase in
pressure The results suggest a kinetic model which involves nonlinear dependence of rate on the partial pressure of hydrogen andadsorption kinetics of toluene or some intermediate
1 Introduction
Dehydrogenation of a cycloalkane such as methylcyclohex-ane (MCH) is an important model reaction in reforming ofnaphtha [1] A typical reformer feedstock has 20ndash60 volnaphthenes [2] which principally undergo dehydrogenationto aromatics The dehydrogenation of MCH is an essentialreaction in themethylcyclohexane-toluene-hydrogen (MTH)system for the safe and economical storage and utilization ofhydrogen [3] Moreover it can be a valuable model reactionin the refining of naphthenic-based heavy crude oils [4] Forheterogeneous catalytic reactions the empirical power lawmodel is an important tool in providing the variation in thekinetic behavior to elaborate the insight of the kinetics of areaction and therefore helps in guiding towards the develop-ment of an appropriate and robust kinetic model Kinetics ofthe MCH dehydrogenation over supported Pt catalysts hasbeen studied by a number of researchers [1 4ndash21] Howeverthe variation in kinetic behavior and kinetic parameters(power law index rate constant and activation energy) of thedehydrogenation reaction with variation in operating condi-tions is rarely studied [19 22] Both Jossens and Petersen [22]
and Alhumaidan et al [19] carried out experiments in thepresence of hydrogen and analyzed the data with initial ratemethod The present study on the other hand is designed toconduct a more rigorous and comprehensive kinetic investi-gation including experiments without hydrogen in the feedover 10 wt Pt120574-Al
2O3catalyst Based on the power law
model kinetic analysis is carried out in which the effect of theoperating conditions on the kinetic parameters of the dehy-drogenation is studied
2 Experimental
The experimental setup is shown in Figure 1 The dehydro-genation reactor was a fixed bed reactor made of 102 cm IDstainless tube 201 g of 10 wt Pt120574-Al
2O3catalyst of the size
minus710 + 425120583m were loaded in the reactor The reactor tubewas placed within a three zone tubular furnace in order tomaintain the required temperature 120574-Al
2O3of Alfa-Aesar
(Johnson-Matthey) was used as a catalyst support The sup-port had a pore volume of 058m3g median pore diameterof 69 A and BET surface area of 208m2g Chloroplatinic
2 ISRN Chemical Engineering
Table 1 Groups formation for the experimental rate data obtained for the dehydrogenation of MCH over a 10 wt Pt120574-Al2O3 catalyst
acid H2PtCl6sdot6H2O of Sigma-Aldrich was used as platinum
source to impregnate the Pt metal over the 120574-Al2O3support
The catalyst was calcined and then reduced in situ Calci-nation was carried out in air while hydrogen was used toreduce the catalyst The total operating pressure and reactorwall temperature were varied in the range of 1013 to 90bar and 6142 to 6532 K respectively Molal space time (feedflowrate) was studied between 311 times 104 and 1244 times 104 ssdotg-catmol-MCH Feed compositional effect was studied byintroducing hydrogen and nitrogen gas H
2to MCH and N
2
toMCHmolar ratios were studied between 0ndash84 and 0ndash105respectively Four experimental runs were carried out dailyThe first experimental run was initiated with 025mLminMCHwhichwas followed respectively by 0125mLmin and05mLmin Each of the previous runs lasted for 90minThe fourth and the last run (for 45min only) was the repeat
of the first run (025mLmin) and facilitated in inspectingthe short-term deactivation that had taken place during thecourse of the dehydrogenation reaction The products of thedehydrogenation were analyzed in Varian 3400 gas chro-matograph containing 100m nonpolar capillary column and(BP-5 5 phenyl and 95 dimethylpolysiloxane) equippedwith flame ionization detector (FID)The details of the exper-imental setup and experimental procedure may be foundelsewhere [4]
3 Data Analyses Technique andBasic Equations
31 Grouping of Experimental Data The experimental dataobtained over the 10 wt Pt120574-Al
2O3catalyst is grouped
according to the feed composition pressure and tempera-ture Table 1 shows the manner in which the experimentaldata is groupedThe data collection in groups is carried out inorder to highlight trends in the parameter values to observethe effects of feed composition on a given fit for a specifiedpressure and to signify the effects of pressure itself
32 Basic Equations The dehydrogenation reaction wasfound highly selective with toluene as the onlymajor productThe kinetic analysis was therefore performed for the princi-pal reaction (1) only
MCH 999445999468 Toluene + 3H2 (1)
The power law kinetics of the following form containingthe effect of short-term deactivation is employed [4]
As mentioned in Section 2 the last run was the repeat ofthe first run (025mLmin) and upon examination shows adecrease in the final conversion of methylcyclohexane overthe intervening period To account for such reversible deac-tivation time online deactivation constant 119896
119889 is introduced
in (2)
ISRN Chemical Engineering 3
The value of the equilibrium constant 119870 was experimen-tally determined by Schildhauer et al [23] and is given by
119870 = 3600 sdot exp(minus217650119877
(1
119879minus1
650)) (3)
with 119870 in bar3 119877 in Jsdotmolminus1sdotKminus1 and 119879 in KThe rate constant 119896 is assumed to follow the Arrhenius
temperature dependency and is rearranged in terms of thereference temperature 119879
119903 as shown next in (4)
119896 = 119896119903sdot exp(119861 sdot (1 minus
119879119903
119879)) (4)
The introduction of 119879119903facilitates in regression of the data
and avoids correlation among the parameters [24] The ref-erence temperature the central temperature of all the tem-peratures is taken as 6172 K
The term 119861 in (4) is called the dimensionless activationenergy and is given by the expression
119861 =119864
119877 sdot 119879119903
(5)
The average temperature 119879 as shown in Figure 2 is cal-culated using the following formula assuming a parabolictemperature distribution in the radial direction [4 19]
119879 =1
119873sdot
119894=119873
sum
119894=1
(119879119911119894+ 119879119908
2) (6)
where 119879119911119894
is the measured temperature at the 119894th axialposition at the centerline in the catalyst bed 119879
119908is the reactor
wall temperature and 119873 is the number of axial temperaturemeasurements
The following differential equation is fitted against theexperimental data and four kinetic variables 119899 119896
119903 119861 and 119896
119889
are collectively estimated
119889119883
119889119884= (minus119903) times 10
5
= 119896 times 105sdot (119901A minus
119901B sdot 1199013
C119870
)
119899
sdot (1 minus 119896119889sdot 119905119889)
(7)
where
119884 = 10minus5times119882
1198651198600
(8)
33 Regression Procedure The regression of the kinetic datais carried out using a FORTRAN code and the followingobjective function the sum of squares of the errors (SSE) isminimized
SSE =119894=119873
sum
119894=1
(119883119894obs minus 119883119894mod)
2
(9)
where119883119894obs is the 119894th measured or observed value of conver-
sion 119883119894mod is the corresponding value calculated from (7)
and119873 is the total number of data points
Catalyst bed
WFA0 = 622 times 104 smiddotg-catmol MCH
Catalyst bed
4 smiddotg-catmol MCH
400
380
360
340
320
300
280
Tem
pera
ture
(∘C)
minus4 minus2 0 2 4 6 8Axial position in the reactor (cm)
Flow direction(downflow)
TwTz T
Tw = 380∘C
H2MCH = 84
Tave
p = 1013 bar
Figure 2 A typical set of temperature profiles in the dehydrogenat-ing reactor119879
119908= reactor wall temperature119879
119911=measured centerline
temperatures in the reactor 119879ave = local average temperaturesbetween the previous two values and 119879 = overall average catalystbed temperature
4 Results and Discussion
Table 2 provides the parameter values and overall statisticsobtained during the regression of the data in the individualgroups On the other hand Figure 3 provides a relationshipfor the selected groups between the measured values of con-version119883obs and model values119883mod to visualize the good-ness of the fit A graphical version of the effect of pressure andcomposition on the kinetic parameters is shown in Figures 45 and 6
41 1013 Bar Pressure It is obvious that each individual groupis relatively better fitted at 1013 bar by the power law modelgiving SSE values always less than 000989 and the corre-sponding values of Adj(1198772) not less than 0983
Inspecting orders of the reaction for the individual groupsfitted it is apparent that the order of the reaction decreasesfrom group 11 to group 21 and then remains virtually thesame for group 31 and group 41 This observation is crucialin explaining the fact that the concentration of hydrogen andnot the MCH concentration in the feed is responsible forchange in the order of the reaction The highest value of theorder of the reaction is the result for group 11 in whichthe concentration of hydrogen is the maximum that is893mol in the feed A comparison of the results for group21 and group 41 allows a direct assessment of the effects ofreplacing H
2with N
2in the feed while maintaining a con-
stant partial pressure of MCH Calculating initial rates at 119879 =119879119903= 6172K the initial rate of the reaction decreases from
80 times 10minus5 to 409 times 10minus5molsdotg-catminus1sdotsminus1 on replacing H2by
N2 This observation should not be a result of irreversible
loss of activity since the periodic activity test showed nolong-term activity loss These observations suggest that H
2 a
product of the reaction appears to act as a promoter and has
4 ISRN Chemical Engineering
Table 2 Results of the power law regression for the data in individual groups
Figure 3 Scatter diagrams for the power law model relating observed and model values of conversions at 1013 bar
ISRN Chemical Engineering 5
0 2 4 6 8 10
n
04
06
08
10
12
14
16
p (bar)
Group 1Group 2Group 4
Figure 4 Effect of pressure on the order of the reaction for thepower law kinetics
0 2 4 6 8 10
E(k
Jmol
)
140
120
100
80
60
40
20
p (bar)
Group 1Group 2Group 4
Figure 5 Effect of pressure on the activation energy of the reactionfor the power law kinetics
a positive effect towards the kinetics of the reaction at least atatmospheric pressure The observation may be explained onthe basis of relatively higher toluene inhibition under the con-ditionswhen no hydrogen is in the feedThis same promotionin the presence of H
2and toluene inhibition at atmospheric
pressure was also observed by other researchers in the field[21 22 25] A reaction order close to unity for group 11suggests a low MCH coverage However a decrease in theorder of the reaction with decreasing H
2in the feed suggests
an increase in coverage of MCH Combining the previousstatements it is concluded that the presence of H
2may be
helpful in replacing the strongly adsorbed products specieswhich otherwise cover the active surface The activationenergy parameter 119861 remains more or less the same and theapparent activation energy lies within 534 to 588 kJmol
p (bar)0 2 4 6 8 10
8
6
4
2
0
(minusr)
0times10
5(m
olg
-catmiddots)
Group 1Group 2Group 4
Figure 6 Effect of pressure on the initial rate of the reaction for thepower law kinetics
A wide range of apparent activation energies is reported inthe literature The values previously given tend to be towardsthe bottom of the range The deactivation rate constant 119896
119889
is always significant confirming the importance of includingthe short-term deactivation
42 50 Bar Pressure Also at 119901 = 50 bar individual groupsare fitted very well It is apparent for group 15 and group 25 inTable 2 that 119899 gt 1 This seems unlikely on physical groundsHowever the manifestation of an apparent order 119899 gt 1 canbe reconciled with strong chemisorption of one or both of thereaction products toluene or hydrogen or of reaction inter-mediates such as methylcyclohexenes or methylcyclohexa-dienes The former intermediate methylcyclohexene wasobserved in low concentrations in the condensate corre-sponding to intermediate levels of conversion of methylcy-clohexane Methylcyclohexadienes on the other hand havenever been observedThis is not to say however that they arenot formed on the catalyst surface It is obvious that theapparent order 119899 is significantly greater at 50 bar than at1013 bar when hydrogen is in the feed Langmuir-Hin-shelwood-Hougen-Watson (LHHW) postulates on the otherhand predict a decreasing apparent order 119899 with increasingpressure consistent with an increasing surface coverage ofmethylcyclohexane
The values of the activation energy are considerablyhigher for group 15 and group 25 This shows that pressurehas a significant effect on the activation energy of the reactionwhen H
2is present in the feed Comparing group 25 and
group 45 in contrast to 1013 bar results no promotion ofhydrogen is observedThe initial rate of reaction (minus119903
0) times 105
is reduced by up to an order of magnitude on increasing thepressure from 1013 bar to 50 bar which suggests strongproduct retarding effects On average the value of thedeactivation constant 119896
119889 is less than that for 1013 bar and
especially for group 15 (highest hydrogen feed concentration)
6 ISRN Chemical Engineering
where the 95 confidence interval involves zero suggestingthe parameter 119896
119889becomes insignificant
43 90 Bar Pressure In all the cases at 90 bar an extremelygood individual group fit of the data is found with Adj(1198772) gt099 Similar to the results at 1013 bar and 50 bar the powerlaw model parameters appear to be group dependent Thevalue of 119899 remains virtually the same in group 19 group 29and group 49 a slight increase however is noticeable in thegroups containing H
2 The same observation is observed at
50 bar however with a greater variation The order of thereaction 119899 gt 10 can be described in the same way asexplained previously The activation energy is found to bequite high greater than 85 kJmol in all the individual grouplistings Comparing group 29 and group 49 an increasedinitial rate and lower activation energy are observed for group49 Similarly a low value of the deactivation constant (119896
119889)
is observed when hydrogen is in the feed which confirmsthe beneficial effects of hydrogen in maintaining the catalystactivity A high value of the deactivation constant (119896
119889) in the
absence of hydrogen may be explained on the basis of theformation of unsaturated intermediates which act as cokeprecursors
The previous discussion reveals that at each pressureparameters are found to be group dependent This is sum-marized graphically in Figures 4 to 6 When H
2is in the feed
both apparent order of the reaction and apparent activationenergy have increased values at high pressures Actually forhigher concentrations of hydrogen in the feed these passthrough a maximum and then fall back while for lowerhydrogen feed concentrations the values increase and thenremain almost same An increased value of activation energyprovides the clue of some strongly adsorbed components thatrequire higher activation energies of desorption to desorbinto the gas phase A higher order of the reaction greater thanunity at increased pressure somewhat confirms this hypoth-esis The initial rates of reaction at 119879 = 119879
119903= 6172K tend to
decrease with pressure for all the groups This kind of behav-ior is less common though it is compatible with an LHHWdual-site kinetic model The effect of hydrogen at higherpressures (say) 90 bar seems to be vanishing as values ofparameters somehow approach each other Comparing group2 at different pressures it is observed that at low pressures apromotion ofH
2is observed asmentioned earlier however at
increased pressure values in fact H2adversely affects the rate
whichmay bemanifested that reverse reaction is important athigher pressures than at 1013 bar or the excess H
2in the feed
may be involved in the associative adsorption of some prod-ucts species which otherwise are gaseous products Anotherpossibility that at high pressures hydrogenmay be competingfor the active adsorption sites and therefore lowering therates of the reactionmaynot be ruled outThis effect of hydro-gen was observed to be more pronounced at 50 bar than at90 bar
In the overall discussion it may be summarized that thepartial pressures of hydrogen and MCH and adsorption ofMCH are found to be important contributors at atmosphericpressure while partial pressures of H
2and adsorption of
hydrogen and the other major product (toluene) are found
important at higher pressures with adsorption of hydro-gen less pronounced This suggests a kinetic equation thatincludes the effects of partial pressure of MCH and hydrogenand adsorption kinetics of MCH hydrogen and toluene Aswith pressures the initial rates are decreased nonlinearly soneed is there for a term in the denominator that constitutesproduct of square or cube of the partial pressure of hydrogen(nonlinear dependence of hydrogen partial pressure) andsome parameter representing adsorption kinetics of at leastone of the major products other than hydrogen that istoluene The best-fit kinetic rate model (based on Langmuir-Hinshelwood-Hougen-Watson single-site kinetics with lossof first hydrogen as the rate controlling step) of the overallexperimental data as carried out in our previous study isshown in (10) [4] Equation (10) clearly shows the strongnonlinear dependence of hydrogen presence and adsorptionkinetics of toluene on the rate of the reaction
Using power law kinetics the limited data in individualgroups is fitted remarkably well and at each pressure param-eters are found to be group dependent With H
2in the feed
both apparent order of the reaction and apparent activationenergy generally increase with pressure At atmosphericpressure with H
2in the feed a promotion in the rate of the
dehydrogenation reaction is observed However at increasedpressures H
2adversely affects the rate The results suggest a
kinetic model which involves nonlinear dependence of rateon the partial pressure of hydrogen and adsorption kinetics oftoluene or some intermediate
Nomenclature
119861 Dimensionless activation energy119864 Activation energy Jmol1198651198600 Initial molar flow rate of MCH mols
Δℎ1015840 Lumped heat of adsorption Jmol
119896 Rate constant for the MCH dehydrogenation reactionmolsdotkgminus1sdotsminus1sdotbarminus1 for (2) and molsdotkgminus1sdotsminus1 for (10)
119896119903 Rate constant at the reference temperature
molsdotkgminus1sdotsminus1sdotbarminus119899119870 Equilibrium constant of MCH dehydrogenation
reaction Pa3119870A Adsorption equilibrium constant for
methylcyclohexane Paminus1119870B Adsorption equilibrium constant for toluene Paminus1
ISRN Chemical Engineering 7
1198701015840 Lumped equilibrium constant that is the ratio of
119870B to the product of surface equilibrium constantsof the dehydrogenation of methylcyclohexene tomethylcyclohexadiene and methylcyclohexadieneto toluene in the single-site surface reactionmechanism Paminus3
1198701015840
119903 Lumped adsorption equilibrium constant defined
at 119879119903 Paminus3
119899 Order of the reaction119873 Number of data points119901 Pressure Pa119901A Partial pressure of methylcyclohexane Pa119901B Partial pressure of toluene Pa119901C Partial pressure of hydrogen Pa(minus119903) Rate of the dehydrogenation reaction molsdotkgminus1sdotsminus1(minus119903)0 Initial rate of the dehydrogenation reactionmolsdotkgminus1sdotsminus1
119877 Universal gas constant Jsdotmolminus1sdotKminus1119905119889 Online reaction deactivation time s
119879 Temperature K119879ave Local average temperature defined in Figure 2 K119879119903 Reference temperature K
119879119908 Reactor wall temperature K
119879119911 Temperature at any position in the axial direction
K119882 Weight of catalyst kg119883 Conversion (fractional conversion) of MCH119883mod Model or calculated conversion of MCH119883obs Observed or measured conversion of MCH119910A0 Initial mole fraction of MCH in the vapor phase119910C0 Initial mole fraction of hydrogen in the vapor
phase1199101198680 Initial mole fraction of inert in the vapor phase
Acknowledgment
Muhammad R Usmanwould like to acknowledge theHigherEducation Commission of Pakistan for funding the research
References
[1] K Jothimurugesan S Bhatia and R D Srivastava ldquoKineticsof dehydrogenation of methylcyclohexane over a platinum-rhe-nium-aluminium catalyst in the presence of added hydrogenrdquoIndustrial amp Engineering Chemistry Fundamentals vol 24 no4 pp 433ndash438 1985
[2] J H Gary and G E Handwerk Petroleum Refining Technologyand Economics Marcel Dekker New York NY USA 4th edi-tion 2001
[3] M R Usman and D L Cresswell ldquoOptions for on-board use ofhydrogen based on the Methylcyclohexane-Toluene-Hydrogensystemrdquo International Journal of Green Energy vol 10 pp 177ndash189 2013
[4] M Usman D Cresswell and A Garforth ldquoDetailed reactionkinetics for the dehydrogenation of methylcyclohexane over Ptcatalystrdquo Industrial and Engineering Chemistry Research vol 51no 1 pp 158ndash170 2012
[5] J H Sinfelt H Hurwitz and R A Shulman ldquoKinetics ofmethylcyclohexane dehydrogenation over Pt-Al
2O3rdquo The Jour-
nal of Physical Chemistry vol 64 no 10 pp 1559ndash1562 1960[6] A W Ritchie and A C Nixon ldquoDehydrogenation of methylcy-
clohexane over a platinum-alumina catalyst in absence of addedhydrogenrdquo IndustrialampEngineeringChemistry Product Researchand Development vol 5 no 1 pp 59ndash64 1966
[7] A Corma R Cid and A Lopez Agudo ldquoCatalyst decay in thekinetics of methylcyclohexane dehydrogenation over Pt-NaYzeoliterdquoThe Canadian Journal of Chemical Engineering vol 57no 5 pp 638ndash642 1979
[8] A Touzani D Klvana and G Belanger ldquoDehydrogenation ofmethylcyclohexane on the industrial catalyst kinetic studyrdquoStudies in Surface Science and Catalysis vol 19 pp 357ndash3641984
[9] M A Pacheco and E E Petersen ldquoReaction kinetics of methyl-cyclohexane dehydrogenation over a sulfided Pt +ReAl
2O3
reforming catalystrdquo Journal of Catalysis vol 96 no 2 pp 507ndash516 1985
[10] P A van Trimpont G B Marin and G F Froment ldquoKineticsofmethylcyclohexane dehydrogenation on sulfided commercialplatinumalumina and platinum-rheniumalumina catalystsrdquoIndustrial amp Engineering Chemistry Fundamentals vol 25 no 4pp 544ndash553 1986
[11] A K Pal M Bhowmick and R D Srivastava ldquoDeactivationkinetics of platinum-rhenium re-forming catalyst accompany-ing the dehydrogenation of methylcyclohexanerdquo Industrial ampEngineering Chemistry Process Design and Development vol 25no 1 pp 236ndash241 1986
[12] J Chaouki A Touzani D Klvana J P Bournonville andG Belanger ldquoDeshydrogenation du Methylcyclohexane sur leCatalyseur Industriel Pt-SnA
2O3rdquo Revue de lrsquoInstitut Francais
du Petrole vol 43 no 6 pp 873ndash881 1988[13] M El-Sawi F A Infortuna P G Lignola A Parmaliana F
Frusteri and N Giordano ldquoParameter estimation in the kineticmodel of methylcyclohexane dehydrogenation on a Pt-Al
2O3
catalyst by sequential experiment designrdquo The Chemical Engi-neering Journal vol 42 no 3 pp 137ndash144 1989
[14] M-R Chai and K Kawakami ldquoKinetic model and simulationfor catalyst deactivation during dehydrogenation of methylcy-clohexane over commercial Pt- PtRe- and presulfided PtRe-Al2O3catalystsrdquo Journal of Chemical Technology and Biotechnol-
ogy vol 51 no 3 pp 335ndash345 1991[15] R H Manser Sonderer Methylcyclohexane dehydrogenation
kinetics reactor design and simulation for a hydrogen poweredvehicle [PhD thesis] Swiss Federal Institute of Technology1992
[16] G Maria A Marin C Wyss et al ldquoModelling and scaleup ofthe kinetics with deactivation of methylcyclohexane dehydro-genation for hydrogen energy storagerdquo Chemical EngineeringScience vol 51 no 11 pp 2891ndash2896 1996
[17] J H Sinfelt ldquoThe turnover frequency of methylcyclohexanedehydrogenation to toluene on a Pt reforming catalystrdquo Journalof Molecular Catalysis A vol 163 no 1-2 pp 123ndash128 2000
[18] D E Tsakiris Catalytic production of hydrogen from liquid or-ganic hydride [PhD thesis]TheUniversity ofManchester 2007
[19] FAlhumaidanDCresswell andAGarforth ldquoKineticmodel ofthe dehydrogenation of methylcyclohexane over monometallicand bimetallic Pt catalystsrdquo Industrial and Engineering Chem-istry Research vol 50 no 5 pp 2509ndash2522 2011
8 ISRN Chemical Engineering
[20] M R Usman ldquoMethylcyclohexane dehydrogenation over com-mercial 03 Wt PtAl
2O3catalystrdquo Proceedings of the Pakistan
Academy of Sciences vol 48 no 1 pp 13ndash17 2011[21] M R Usman R Aslam and F Alotaibi ldquoHydrogen storage
in a recyclable organic hydride kinetic modeling of methylcy-clohexane dehydrogenation over 10 wt Pt120579-Al
2O3rdquo Energy
Sources A vol 33 no 24 pp 2264ndash2271 2011[22] LW Jossens and E E Petersen ldquoFouling of a platinum reform-
ing catalyst accompanying the dehydrogenation of methyl cy-clohexanerdquo Journal of Catalysis vol 73 no 2 pp 377ndash386 1982
[23] T H Schildhauer E Newson and S Muller ldquoThe equilibriumconstant for the methylcyclohexane-toluene systemrdquo Journal ofCatalysis vol 198 no 2 pp 355ndash358 2001
[24] J R Kittrell ldquoMathematical modeling of chemical reactionsrdquoAdvances in Chemical Engineering vol 8 pp 97ndash183 1970
[25] J C Rohrer and J H Sinfelt ldquoInteraction of hydrocarbons withPt-Al2O3in the presence of hydrogen and heliumrdquoThe Journal
of Physical Chemistry vol 66 no 6 pp 1193ndash1194 1962
acid H2PtCl6sdot6H2O of Sigma-Aldrich was used as platinum
source to impregnate the Pt metal over the 120574-Al2O3support
The catalyst was calcined and then reduced in situ Calci-nation was carried out in air while hydrogen was used toreduce the catalyst The total operating pressure and reactorwall temperature were varied in the range of 1013 to 90bar and 6142 to 6532 K respectively Molal space time (feedflowrate) was studied between 311 times 104 and 1244 times 104 ssdotg-catmol-MCH Feed compositional effect was studied byintroducing hydrogen and nitrogen gas H
2to MCH and N
2
toMCHmolar ratios were studied between 0ndash84 and 0ndash105respectively Four experimental runs were carried out dailyThe first experimental run was initiated with 025mLminMCHwhichwas followed respectively by 0125mLmin and05mLmin Each of the previous runs lasted for 90minThe fourth and the last run (for 45min only) was the repeat
of the first run (025mLmin) and facilitated in inspectingthe short-term deactivation that had taken place during thecourse of the dehydrogenation reaction The products of thedehydrogenation were analyzed in Varian 3400 gas chro-matograph containing 100m nonpolar capillary column and(BP-5 5 phenyl and 95 dimethylpolysiloxane) equippedwith flame ionization detector (FID)The details of the exper-imental setup and experimental procedure may be foundelsewhere [4]
3 Data Analyses Technique andBasic Equations
31 Grouping of Experimental Data The experimental dataobtained over the 10 wt Pt120574-Al
2O3catalyst is grouped
according to the feed composition pressure and tempera-ture Table 1 shows the manner in which the experimentaldata is groupedThe data collection in groups is carried out inorder to highlight trends in the parameter values to observethe effects of feed composition on a given fit for a specifiedpressure and to signify the effects of pressure itself
32 Basic Equations The dehydrogenation reaction wasfound highly selective with toluene as the onlymajor productThe kinetic analysis was therefore performed for the princi-pal reaction (1) only
MCH 999445999468 Toluene + 3H2 (1)
The power law kinetics of the following form containingthe effect of short-term deactivation is employed [4]
As mentioned in Section 2 the last run was the repeat ofthe first run (025mLmin) and upon examination shows adecrease in the final conversion of methylcyclohexane overthe intervening period To account for such reversible deac-tivation time online deactivation constant 119896
119889 is introduced
in (2)
ISRN Chemical Engineering 3
The value of the equilibrium constant 119870 was experimen-tally determined by Schildhauer et al [23] and is given by
119870 = 3600 sdot exp(minus217650119877
(1
119879minus1
650)) (3)
with 119870 in bar3 119877 in Jsdotmolminus1sdotKminus1 and 119879 in KThe rate constant 119896 is assumed to follow the Arrhenius
temperature dependency and is rearranged in terms of thereference temperature 119879
119903 as shown next in (4)
119896 = 119896119903sdot exp(119861 sdot (1 minus
119879119903
119879)) (4)
The introduction of 119879119903facilitates in regression of the data
and avoids correlation among the parameters [24] The ref-erence temperature the central temperature of all the tem-peratures is taken as 6172 K
The term 119861 in (4) is called the dimensionless activationenergy and is given by the expression
119861 =119864
119877 sdot 119879119903
(5)
The average temperature 119879 as shown in Figure 2 is cal-culated using the following formula assuming a parabolictemperature distribution in the radial direction [4 19]
119879 =1
119873sdot
119894=119873
sum
119894=1
(119879119911119894+ 119879119908
2) (6)
where 119879119911119894
is the measured temperature at the 119894th axialposition at the centerline in the catalyst bed 119879
119908is the reactor
wall temperature and 119873 is the number of axial temperaturemeasurements
The following differential equation is fitted against theexperimental data and four kinetic variables 119899 119896
119903 119861 and 119896
119889
are collectively estimated
119889119883
119889119884= (minus119903) times 10
5
= 119896 times 105sdot (119901A minus
119901B sdot 1199013
C119870
)
119899
sdot (1 minus 119896119889sdot 119905119889)
(7)
where
119884 = 10minus5times119882
1198651198600
(8)
33 Regression Procedure The regression of the kinetic datais carried out using a FORTRAN code and the followingobjective function the sum of squares of the errors (SSE) isminimized
SSE =119894=119873
sum
119894=1
(119883119894obs minus 119883119894mod)
2
(9)
where119883119894obs is the 119894th measured or observed value of conver-
sion 119883119894mod is the corresponding value calculated from (7)
and119873 is the total number of data points
Catalyst bed
WFA0 = 622 times 104 smiddotg-catmol MCH
Catalyst bed
4 smiddotg-catmol MCH
400
380
360
340
320
300
280
Tem
pera
ture
(∘C)
minus4 minus2 0 2 4 6 8Axial position in the reactor (cm)
Flow direction(downflow)
TwTz T
Tw = 380∘C
H2MCH = 84
Tave
p = 1013 bar
Figure 2 A typical set of temperature profiles in the dehydrogenat-ing reactor119879
119908= reactor wall temperature119879
119911=measured centerline
temperatures in the reactor 119879ave = local average temperaturesbetween the previous two values and 119879 = overall average catalystbed temperature
4 Results and Discussion
Table 2 provides the parameter values and overall statisticsobtained during the regression of the data in the individualgroups On the other hand Figure 3 provides a relationshipfor the selected groups between the measured values of con-version119883obs and model values119883mod to visualize the good-ness of the fit A graphical version of the effect of pressure andcomposition on the kinetic parameters is shown in Figures 45 and 6
41 1013 Bar Pressure It is obvious that each individual groupis relatively better fitted at 1013 bar by the power law modelgiving SSE values always less than 000989 and the corre-sponding values of Adj(1198772) not less than 0983
Inspecting orders of the reaction for the individual groupsfitted it is apparent that the order of the reaction decreasesfrom group 11 to group 21 and then remains virtually thesame for group 31 and group 41 This observation is crucialin explaining the fact that the concentration of hydrogen andnot the MCH concentration in the feed is responsible forchange in the order of the reaction The highest value of theorder of the reaction is the result for group 11 in whichthe concentration of hydrogen is the maximum that is893mol in the feed A comparison of the results for group21 and group 41 allows a direct assessment of the effects ofreplacing H
2with N
2in the feed while maintaining a con-
stant partial pressure of MCH Calculating initial rates at 119879 =119879119903= 6172K the initial rate of the reaction decreases from
80 times 10minus5 to 409 times 10minus5molsdotg-catminus1sdotsminus1 on replacing H2by
N2 This observation should not be a result of irreversible
loss of activity since the periodic activity test showed nolong-term activity loss These observations suggest that H
2 a
product of the reaction appears to act as a promoter and has
4 ISRN Chemical Engineering
Table 2 Results of the power law regression for the data in individual groups
Figure 3 Scatter diagrams for the power law model relating observed and model values of conversions at 1013 bar
ISRN Chemical Engineering 5
0 2 4 6 8 10
n
04
06
08
10
12
14
16
p (bar)
Group 1Group 2Group 4
Figure 4 Effect of pressure on the order of the reaction for thepower law kinetics
0 2 4 6 8 10
E(k
Jmol
)
140
120
100
80
60
40
20
p (bar)
Group 1Group 2Group 4
Figure 5 Effect of pressure on the activation energy of the reactionfor the power law kinetics
a positive effect towards the kinetics of the reaction at least atatmospheric pressure The observation may be explained onthe basis of relatively higher toluene inhibition under the con-ditionswhen no hydrogen is in the feedThis same promotionin the presence of H
2and toluene inhibition at atmospheric
pressure was also observed by other researchers in the field[21 22 25] A reaction order close to unity for group 11suggests a low MCH coverage However a decrease in theorder of the reaction with decreasing H
2in the feed suggests
an increase in coverage of MCH Combining the previousstatements it is concluded that the presence of H
2may be
helpful in replacing the strongly adsorbed products specieswhich otherwise cover the active surface The activationenergy parameter 119861 remains more or less the same and theapparent activation energy lies within 534 to 588 kJmol
p (bar)0 2 4 6 8 10
8
6
4
2
0
(minusr)
0times10
5(m
olg
-catmiddots)
Group 1Group 2Group 4
Figure 6 Effect of pressure on the initial rate of the reaction for thepower law kinetics
A wide range of apparent activation energies is reported inthe literature The values previously given tend to be towardsthe bottom of the range The deactivation rate constant 119896
119889
is always significant confirming the importance of includingthe short-term deactivation
42 50 Bar Pressure Also at 119901 = 50 bar individual groupsare fitted very well It is apparent for group 15 and group 25 inTable 2 that 119899 gt 1 This seems unlikely on physical groundsHowever the manifestation of an apparent order 119899 gt 1 canbe reconciled with strong chemisorption of one or both of thereaction products toluene or hydrogen or of reaction inter-mediates such as methylcyclohexenes or methylcyclohexa-dienes The former intermediate methylcyclohexene wasobserved in low concentrations in the condensate corre-sponding to intermediate levels of conversion of methylcy-clohexane Methylcyclohexadienes on the other hand havenever been observedThis is not to say however that they arenot formed on the catalyst surface It is obvious that theapparent order 119899 is significantly greater at 50 bar than at1013 bar when hydrogen is in the feed Langmuir-Hin-shelwood-Hougen-Watson (LHHW) postulates on the otherhand predict a decreasing apparent order 119899 with increasingpressure consistent with an increasing surface coverage ofmethylcyclohexane
The values of the activation energy are considerablyhigher for group 15 and group 25 This shows that pressurehas a significant effect on the activation energy of the reactionwhen H
2is present in the feed Comparing group 25 and
group 45 in contrast to 1013 bar results no promotion ofhydrogen is observedThe initial rate of reaction (minus119903
0) times 105
is reduced by up to an order of magnitude on increasing thepressure from 1013 bar to 50 bar which suggests strongproduct retarding effects On average the value of thedeactivation constant 119896
119889 is less than that for 1013 bar and
especially for group 15 (highest hydrogen feed concentration)
6 ISRN Chemical Engineering
where the 95 confidence interval involves zero suggestingthe parameter 119896
119889becomes insignificant
43 90 Bar Pressure In all the cases at 90 bar an extremelygood individual group fit of the data is found with Adj(1198772) gt099 Similar to the results at 1013 bar and 50 bar the powerlaw model parameters appear to be group dependent Thevalue of 119899 remains virtually the same in group 19 group 29and group 49 a slight increase however is noticeable in thegroups containing H
2 The same observation is observed at
50 bar however with a greater variation The order of thereaction 119899 gt 10 can be described in the same way asexplained previously The activation energy is found to bequite high greater than 85 kJmol in all the individual grouplistings Comparing group 29 and group 49 an increasedinitial rate and lower activation energy are observed for group49 Similarly a low value of the deactivation constant (119896
119889)
is observed when hydrogen is in the feed which confirmsthe beneficial effects of hydrogen in maintaining the catalystactivity A high value of the deactivation constant (119896
119889) in the
absence of hydrogen may be explained on the basis of theformation of unsaturated intermediates which act as cokeprecursors
The previous discussion reveals that at each pressureparameters are found to be group dependent This is sum-marized graphically in Figures 4 to 6 When H
2is in the feed
both apparent order of the reaction and apparent activationenergy have increased values at high pressures Actually forhigher concentrations of hydrogen in the feed these passthrough a maximum and then fall back while for lowerhydrogen feed concentrations the values increase and thenremain almost same An increased value of activation energyprovides the clue of some strongly adsorbed components thatrequire higher activation energies of desorption to desorbinto the gas phase A higher order of the reaction greater thanunity at increased pressure somewhat confirms this hypoth-esis The initial rates of reaction at 119879 = 119879
119903= 6172K tend to
decrease with pressure for all the groups This kind of behav-ior is less common though it is compatible with an LHHWdual-site kinetic model The effect of hydrogen at higherpressures (say) 90 bar seems to be vanishing as values ofparameters somehow approach each other Comparing group2 at different pressures it is observed that at low pressures apromotion ofH
2is observed asmentioned earlier however at
increased pressure values in fact H2adversely affects the rate
whichmay bemanifested that reverse reaction is important athigher pressures than at 1013 bar or the excess H
2in the feed
may be involved in the associative adsorption of some prod-ucts species which otherwise are gaseous products Anotherpossibility that at high pressures hydrogenmay be competingfor the active adsorption sites and therefore lowering therates of the reactionmaynot be ruled outThis effect of hydro-gen was observed to be more pronounced at 50 bar than at90 bar
In the overall discussion it may be summarized that thepartial pressures of hydrogen and MCH and adsorption ofMCH are found to be important contributors at atmosphericpressure while partial pressures of H
2and adsorption of
hydrogen and the other major product (toluene) are found
important at higher pressures with adsorption of hydro-gen less pronounced This suggests a kinetic equation thatincludes the effects of partial pressure of MCH and hydrogenand adsorption kinetics of MCH hydrogen and toluene Aswith pressures the initial rates are decreased nonlinearly soneed is there for a term in the denominator that constitutesproduct of square or cube of the partial pressure of hydrogen(nonlinear dependence of hydrogen partial pressure) andsome parameter representing adsorption kinetics of at leastone of the major products other than hydrogen that istoluene The best-fit kinetic rate model (based on Langmuir-Hinshelwood-Hougen-Watson single-site kinetics with lossof first hydrogen as the rate controlling step) of the overallexperimental data as carried out in our previous study isshown in (10) [4] Equation (10) clearly shows the strongnonlinear dependence of hydrogen presence and adsorptionkinetics of toluene on the rate of the reaction
Using power law kinetics the limited data in individualgroups is fitted remarkably well and at each pressure param-eters are found to be group dependent With H
2in the feed
both apparent order of the reaction and apparent activationenergy generally increase with pressure At atmosphericpressure with H
2in the feed a promotion in the rate of the
dehydrogenation reaction is observed However at increasedpressures H
2adversely affects the rate The results suggest a
kinetic model which involves nonlinear dependence of rateon the partial pressure of hydrogen and adsorption kinetics oftoluene or some intermediate
Nomenclature
119861 Dimensionless activation energy119864 Activation energy Jmol1198651198600 Initial molar flow rate of MCH mols
Δℎ1015840 Lumped heat of adsorption Jmol
119896 Rate constant for the MCH dehydrogenation reactionmolsdotkgminus1sdotsminus1sdotbarminus1 for (2) and molsdotkgminus1sdotsminus1 for (10)
119896119903 Rate constant at the reference temperature
molsdotkgminus1sdotsminus1sdotbarminus119899119870 Equilibrium constant of MCH dehydrogenation
reaction Pa3119870A Adsorption equilibrium constant for
methylcyclohexane Paminus1119870B Adsorption equilibrium constant for toluene Paminus1
ISRN Chemical Engineering 7
1198701015840 Lumped equilibrium constant that is the ratio of
119870B to the product of surface equilibrium constantsof the dehydrogenation of methylcyclohexene tomethylcyclohexadiene and methylcyclohexadieneto toluene in the single-site surface reactionmechanism Paminus3
1198701015840
119903 Lumped adsorption equilibrium constant defined
at 119879119903 Paminus3
119899 Order of the reaction119873 Number of data points119901 Pressure Pa119901A Partial pressure of methylcyclohexane Pa119901B Partial pressure of toluene Pa119901C Partial pressure of hydrogen Pa(minus119903) Rate of the dehydrogenation reaction molsdotkgminus1sdotsminus1(minus119903)0 Initial rate of the dehydrogenation reactionmolsdotkgminus1sdotsminus1
119877 Universal gas constant Jsdotmolminus1sdotKminus1119905119889 Online reaction deactivation time s
119879 Temperature K119879ave Local average temperature defined in Figure 2 K119879119903 Reference temperature K
119879119908 Reactor wall temperature K
119879119911 Temperature at any position in the axial direction
K119882 Weight of catalyst kg119883 Conversion (fractional conversion) of MCH119883mod Model or calculated conversion of MCH119883obs Observed or measured conversion of MCH119910A0 Initial mole fraction of MCH in the vapor phase119910C0 Initial mole fraction of hydrogen in the vapor
phase1199101198680 Initial mole fraction of inert in the vapor phase
Acknowledgment
Muhammad R Usmanwould like to acknowledge theHigherEducation Commission of Pakistan for funding the research
References
[1] K Jothimurugesan S Bhatia and R D Srivastava ldquoKineticsof dehydrogenation of methylcyclohexane over a platinum-rhe-nium-aluminium catalyst in the presence of added hydrogenrdquoIndustrial amp Engineering Chemistry Fundamentals vol 24 no4 pp 433ndash438 1985
[2] J H Gary and G E Handwerk Petroleum Refining Technologyand Economics Marcel Dekker New York NY USA 4th edi-tion 2001
[3] M R Usman and D L Cresswell ldquoOptions for on-board use ofhydrogen based on the Methylcyclohexane-Toluene-Hydrogensystemrdquo International Journal of Green Energy vol 10 pp 177ndash189 2013
[4] M Usman D Cresswell and A Garforth ldquoDetailed reactionkinetics for the dehydrogenation of methylcyclohexane over Ptcatalystrdquo Industrial and Engineering Chemistry Research vol 51no 1 pp 158ndash170 2012
[5] J H Sinfelt H Hurwitz and R A Shulman ldquoKinetics ofmethylcyclohexane dehydrogenation over Pt-Al
2O3rdquo The Jour-
nal of Physical Chemistry vol 64 no 10 pp 1559ndash1562 1960[6] A W Ritchie and A C Nixon ldquoDehydrogenation of methylcy-
clohexane over a platinum-alumina catalyst in absence of addedhydrogenrdquo IndustrialampEngineeringChemistry Product Researchand Development vol 5 no 1 pp 59ndash64 1966
[7] A Corma R Cid and A Lopez Agudo ldquoCatalyst decay in thekinetics of methylcyclohexane dehydrogenation over Pt-NaYzeoliterdquoThe Canadian Journal of Chemical Engineering vol 57no 5 pp 638ndash642 1979
[8] A Touzani D Klvana and G Belanger ldquoDehydrogenation ofmethylcyclohexane on the industrial catalyst kinetic studyrdquoStudies in Surface Science and Catalysis vol 19 pp 357ndash3641984
[9] M A Pacheco and E E Petersen ldquoReaction kinetics of methyl-cyclohexane dehydrogenation over a sulfided Pt +ReAl
2O3
reforming catalystrdquo Journal of Catalysis vol 96 no 2 pp 507ndash516 1985
[10] P A van Trimpont G B Marin and G F Froment ldquoKineticsofmethylcyclohexane dehydrogenation on sulfided commercialplatinumalumina and platinum-rheniumalumina catalystsrdquoIndustrial amp Engineering Chemistry Fundamentals vol 25 no 4pp 544ndash553 1986
[11] A K Pal M Bhowmick and R D Srivastava ldquoDeactivationkinetics of platinum-rhenium re-forming catalyst accompany-ing the dehydrogenation of methylcyclohexanerdquo Industrial ampEngineering Chemistry Process Design and Development vol 25no 1 pp 236ndash241 1986
[12] J Chaouki A Touzani D Klvana J P Bournonville andG Belanger ldquoDeshydrogenation du Methylcyclohexane sur leCatalyseur Industriel Pt-SnA
2O3rdquo Revue de lrsquoInstitut Francais
du Petrole vol 43 no 6 pp 873ndash881 1988[13] M El-Sawi F A Infortuna P G Lignola A Parmaliana F
Frusteri and N Giordano ldquoParameter estimation in the kineticmodel of methylcyclohexane dehydrogenation on a Pt-Al
2O3
catalyst by sequential experiment designrdquo The Chemical Engi-neering Journal vol 42 no 3 pp 137ndash144 1989
[14] M-R Chai and K Kawakami ldquoKinetic model and simulationfor catalyst deactivation during dehydrogenation of methylcy-clohexane over commercial Pt- PtRe- and presulfided PtRe-Al2O3catalystsrdquo Journal of Chemical Technology and Biotechnol-
ogy vol 51 no 3 pp 335ndash345 1991[15] R H Manser Sonderer Methylcyclohexane dehydrogenation
kinetics reactor design and simulation for a hydrogen poweredvehicle [PhD thesis] Swiss Federal Institute of Technology1992
[16] G Maria A Marin C Wyss et al ldquoModelling and scaleup ofthe kinetics with deactivation of methylcyclohexane dehydro-genation for hydrogen energy storagerdquo Chemical EngineeringScience vol 51 no 11 pp 2891ndash2896 1996
[17] J H Sinfelt ldquoThe turnover frequency of methylcyclohexanedehydrogenation to toluene on a Pt reforming catalystrdquo Journalof Molecular Catalysis A vol 163 no 1-2 pp 123ndash128 2000
[18] D E Tsakiris Catalytic production of hydrogen from liquid or-ganic hydride [PhD thesis]TheUniversity ofManchester 2007
[19] FAlhumaidanDCresswell andAGarforth ldquoKineticmodel ofthe dehydrogenation of methylcyclohexane over monometallicand bimetallic Pt catalystsrdquo Industrial and Engineering Chem-istry Research vol 50 no 5 pp 2509ndash2522 2011
8 ISRN Chemical Engineering
[20] M R Usman ldquoMethylcyclohexane dehydrogenation over com-mercial 03 Wt PtAl
2O3catalystrdquo Proceedings of the Pakistan
Academy of Sciences vol 48 no 1 pp 13ndash17 2011[21] M R Usman R Aslam and F Alotaibi ldquoHydrogen storage
in a recyclable organic hydride kinetic modeling of methylcy-clohexane dehydrogenation over 10 wt Pt120579-Al
2O3rdquo Energy
Sources A vol 33 no 24 pp 2264ndash2271 2011[22] LW Jossens and E E Petersen ldquoFouling of a platinum reform-
ing catalyst accompanying the dehydrogenation of methyl cy-clohexanerdquo Journal of Catalysis vol 73 no 2 pp 377ndash386 1982
[23] T H Schildhauer E Newson and S Muller ldquoThe equilibriumconstant for the methylcyclohexane-toluene systemrdquo Journal ofCatalysis vol 198 no 2 pp 355ndash358 2001
[24] J R Kittrell ldquoMathematical modeling of chemical reactionsrdquoAdvances in Chemical Engineering vol 8 pp 97ndash183 1970
[25] J C Rohrer and J H Sinfelt ldquoInteraction of hydrocarbons withPt-Al2O3in the presence of hydrogen and heliumrdquoThe Journal
of Physical Chemistry vol 66 no 6 pp 1193ndash1194 1962
The value of the equilibrium constant 119870 was experimen-tally determined by Schildhauer et al [23] and is given by
119870 = 3600 sdot exp(minus217650119877
(1
119879minus1
650)) (3)
with 119870 in bar3 119877 in Jsdotmolminus1sdotKminus1 and 119879 in KThe rate constant 119896 is assumed to follow the Arrhenius
temperature dependency and is rearranged in terms of thereference temperature 119879
119903 as shown next in (4)
119896 = 119896119903sdot exp(119861 sdot (1 minus
119879119903
119879)) (4)
The introduction of 119879119903facilitates in regression of the data
and avoids correlation among the parameters [24] The ref-erence temperature the central temperature of all the tem-peratures is taken as 6172 K
The term 119861 in (4) is called the dimensionless activationenergy and is given by the expression
119861 =119864
119877 sdot 119879119903
(5)
The average temperature 119879 as shown in Figure 2 is cal-culated using the following formula assuming a parabolictemperature distribution in the radial direction [4 19]
119879 =1
119873sdot
119894=119873
sum
119894=1
(119879119911119894+ 119879119908
2) (6)
where 119879119911119894
is the measured temperature at the 119894th axialposition at the centerline in the catalyst bed 119879
119908is the reactor
wall temperature and 119873 is the number of axial temperaturemeasurements
The following differential equation is fitted against theexperimental data and four kinetic variables 119899 119896
119903 119861 and 119896
119889
are collectively estimated
119889119883
119889119884= (minus119903) times 10
5
= 119896 times 105sdot (119901A minus
119901B sdot 1199013
C119870
)
119899
sdot (1 minus 119896119889sdot 119905119889)
(7)
where
119884 = 10minus5times119882
1198651198600
(8)
33 Regression Procedure The regression of the kinetic datais carried out using a FORTRAN code and the followingobjective function the sum of squares of the errors (SSE) isminimized
SSE =119894=119873
sum
119894=1
(119883119894obs minus 119883119894mod)
2
(9)
where119883119894obs is the 119894th measured or observed value of conver-
sion 119883119894mod is the corresponding value calculated from (7)
and119873 is the total number of data points
Catalyst bed
WFA0 = 622 times 104 smiddotg-catmol MCH
Catalyst bed
4 smiddotg-catmol MCH
400
380
360
340
320
300
280
Tem
pera
ture
(∘C)
minus4 minus2 0 2 4 6 8Axial position in the reactor (cm)
Flow direction(downflow)
TwTz T
Tw = 380∘C
H2MCH = 84
Tave
p = 1013 bar
Figure 2 A typical set of temperature profiles in the dehydrogenat-ing reactor119879
119908= reactor wall temperature119879
119911=measured centerline
temperatures in the reactor 119879ave = local average temperaturesbetween the previous two values and 119879 = overall average catalystbed temperature
4 Results and Discussion
Table 2 provides the parameter values and overall statisticsobtained during the regression of the data in the individualgroups On the other hand Figure 3 provides a relationshipfor the selected groups between the measured values of con-version119883obs and model values119883mod to visualize the good-ness of the fit A graphical version of the effect of pressure andcomposition on the kinetic parameters is shown in Figures 45 and 6
41 1013 Bar Pressure It is obvious that each individual groupis relatively better fitted at 1013 bar by the power law modelgiving SSE values always less than 000989 and the corre-sponding values of Adj(1198772) not less than 0983
Inspecting orders of the reaction for the individual groupsfitted it is apparent that the order of the reaction decreasesfrom group 11 to group 21 and then remains virtually thesame for group 31 and group 41 This observation is crucialin explaining the fact that the concentration of hydrogen andnot the MCH concentration in the feed is responsible forchange in the order of the reaction The highest value of theorder of the reaction is the result for group 11 in whichthe concentration of hydrogen is the maximum that is893mol in the feed A comparison of the results for group21 and group 41 allows a direct assessment of the effects ofreplacing H
2with N
2in the feed while maintaining a con-
stant partial pressure of MCH Calculating initial rates at 119879 =119879119903= 6172K the initial rate of the reaction decreases from
80 times 10minus5 to 409 times 10minus5molsdotg-catminus1sdotsminus1 on replacing H2by
N2 This observation should not be a result of irreversible
loss of activity since the periodic activity test showed nolong-term activity loss These observations suggest that H
2 a
product of the reaction appears to act as a promoter and has
4 ISRN Chemical Engineering
Table 2 Results of the power law regression for the data in individual groups
Figure 3 Scatter diagrams for the power law model relating observed and model values of conversions at 1013 bar
ISRN Chemical Engineering 5
0 2 4 6 8 10
n
04
06
08
10
12
14
16
p (bar)
Group 1Group 2Group 4
Figure 4 Effect of pressure on the order of the reaction for thepower law kinetics
0 2 4 6 8 10
E(k
Jmol
)
140
120
100
80
60
40
20
p (bar)
Group 1Group 2Group 4
Figure 5 Effect of pressure on the activation energy of the reactionfor the power law kinetics
a positive effect towards the kinetics of the reaction at least atatmospheric pressure The observation may be explained onthe basis of relatively higher toluene inhibition under the con-ditionswhen no hydrogen is in the feedThis same promotionin the presence of H
2and toluene inhibition at atmospheric
pressure was also observed by other researchers in the field[21 22 25] A reaction order close to unity for group 11suggests a low MCH coverage However a decrease in theorder of the reaction with decreasing H
2in the feed suggests
an increase in coverage of MCH Combining the previousstatements it is concluded that the presence of H
2may be
helpful in replacing the strongly adsorbed products specieswhich otherwise cover the active surface The activationenergy parameter 119861 remains more or less the same and theapparent activation energy lies within 534 to 588 kJmol
p (bar)0 2 4 6 8 10
8
6
4
2
0
(minusr)
0times10
5(m
olg
-catmiddots)
Group 1Group 2Group 4
Figure 6 Effect of pressure on the initial rate of the reaction for thepower law kinetics
A wide range of apparent activation energies is reported inthe literature The values previously given tend to be towardsthe bottom of the range The deactivation rate constant 119896
119889
is always significant confirming the importance of includingthe short-term deactivation
42 50 Bar Pressure Also at 119901 = 50 bar individual groupsare fitted very well It is apparent for group 15 and group 25 inTable 2 that 119899 gt 1 This seems unlikely on physical groundsHowever the manifestation of an apparent order 119899 gt 1 canbe reconciled with strong chemisorption of one or both of thereaction products toluene or hydrogen or of reaction inter-mediates such as methylcyclohexenes or methylcyclohexa-dienes The former intermediate methylcyclohexene wasobserved in low concentrations in the condensate corre-sponding to intermediate levels of conversion of methylcy-clohexane Methylcyclohexadienes on the other hand havenever been observedThis is not to say however that they arenot formed on the catalyst surface It is obvious that theapparent order 119899 is significantly greater at 50 bar than at1013 bar when hydrogen is in the feed Langmuir-Hin-shelwood-Hougen-Watson (LHHW) postulates on the otherhand predict a decreasing apparent order 119899 with increasingpressure consistent with an increasing surface coverage ofmethylcyclohexane
The values of the activation energy are considerablyhigher for group 15 and group 25 This shows that pressurehas a significant effect on the activation energy of the reactionwhen H
2is present in the feed Comparing group 25 and
group 45 in contrast to 1013 bar results no promotion ofhydrogen is observedThe initial rate of reaction (minus119903
0) times 105
is reduced by up to an order of magnitude on increasing thepressure from 1013 bar to 50 bar which suggests strongproduct retarding effects On average the value of thedeactivation constant 119896
119889 is less than that for 1013 bar and
especially for group 15 (highest hydrogen feed concentration)
6 ISRN Chemical Engineering
where the 95 confidence interval involves zero suggestingthe parameter 119896
119889becomes insignificant
43 90 Bar Pressure In all the cases at 90 bar an extremelygood individual group fit of the data is found with Adj(1198772) gt099 Similar to the results at 1013 bar and 50 bar the powerlaw model parameters appear to be group dependent Thevalue of 119899 remains virtually the same in group 19 group 29and group 49 a slight increase however is noticeable in thegroups containing H
2 The same observation is observed at
50 bar however with a greater variation The order of thereaction 119899 gt 10 can be described in the same way asexplained previously The activation energy is found to bequite high greater than 85 kJmol in all the individual grouplistings Comparing group 29 and group 49 an increasedinitial rate and lower activation energy are observed for group49 Similarly a low value of the deactivation constant (119896
119889)
is observed when hydrogen is in the feed which confirmsthe beneficial effects of hydrogen in maintaining the catalystactivity A high value of the deactivation constant (119896
119889) in the
absence of hydrogen may be explained on the basis of theformation of unsaturated intermediates which act as cokeprecursors
The previous discussion reveals that at each pressureparameters are found to be group dependent This is sum-marized graphically in Figures 4 to 6 When H
2is in the feed
both apparent order of the reaction and apparent activationenergy have increased values at high pressures Actually forhigher concentrations of hydrogen in the feed these passthrough a maximum and then fall back while for lowerhydrogen feed concentrations the values increase and thenremain almost same An increased value of activation energyprovides the clue of some strongly adsorbed components thatrequire higher activation energies of desorption to desorbinto the gas phase A higher order of the reaction greater thanunity at increased pressure somewhat confirms this hypoth-esis The initial rates of reaction at 119879 = 119879
119903= 6172K tend to
decrease with pressure for all the groups This kind of behav-ior is less common though it is compatible with an LHHWdual-site kinetic model The effect of hydrogen at higherpressures (say) 90 bar seems to be vanishing as values ofparameters somehow approach each other Comparing group2 at different pressures it is observed that at low pressures apromotion ofH
2is observed asmentioned earlier however at
increased pressure values in fact H2adversely affects the rate
whichmay bemanifested that reverse reaction is important athigher pressures than at 1013 bar or the excess H
2in the feed
may be involved in the associative adsorption of some prod-ucts species which otherwise are gaseous products Anotherpossibility that at high pressures hydrogenmay be competingfor the active adsorption sites and therefore lowering therates of the reactionmaynot be ruled outThis effect of hydro-gen was observed to be more pronounced at 50 bar than at90 bar
In the overall discussion it may be summarized that thepartial pressures of hydrogen and MCH and adsorption ofMCH are found to be important contributors at atmosphericpressure while partial pressures of H
2and adsorption of
hydrogen and the other major product (toluene) are found
important at higher pressures with adsorption of hydro-gen less pronounced This suggests a kinetic equation thatincludes the effects of partial pressure of MCH and hydrogenand adsorption kinetics of MCH hydrogen and toluene Aswith pressures the initial rates are decreased nonlinearly soneed is there for a term in the denominator that constitutesproduct of square or cube of the partial pressure of hydrogen(nonlinear dependence of hydrogen partial pressure) andsome parameter representing adsorption kinetics of at leastone of the major products other than hydrogen that istoluene The best-fit kinetic rate model (based on Langmuir-Hinshelwood-Hougen-Watson single-site kinetics with lossof first hydrogen as the rate controlling step) of the overallexperimental data as carried out in our previous study isshown in (10) [4] Equation (10) clearly shows the strongnonlinear dependence of hydrogen presence and adsorptionkinetics of toluene on the rate of the reaction
Using power law kinetics the limited data in individualgroups is fitted remarkably well and at each pressure param-eters are found to be group dependent With H
2in the feed
both apparent order of the reaction and apparent activationenergy generally increase with pressure At atmosphericpressure with H
2in the feed a promotion in the rate of the
dehydrogenation reaction is observed However at increasedpressures H
2adversely affects the rate The results suggest a
kinetic model which involves nonlinear dependence of rateon the partial pressure of hydrogen and adsorption kinetics oftoluene or some intermediate
Nomenclature
119861 Dimensionless activation energy119864 Activation energy Jmol1198651198600 Initial molar flow rate of MCH mols
Δℎ1015840 Lumped heat of adsorption Jmol
119896 Rate constant for the MCH dehydrogenation reactionmolsdotkgminus1sdotsminus1sdotbarminus1 for (2) and molsdotkgminus1sdotsminus1 for (10)
119896119903 Rate constant at the reference temperature
molsdotkgminus1sdotsminus1sdotbarminus119899119870 Equilibrium constant of MCH dehydrogenation
reaction Pa3119870A Adsorption equilibrium constant for
methylcyclohexane Paminus1119870B Adsorption equilibrium constant for toluene Paminus1
ISRN Chemical Engineering 7
1198701015840 Lumped equilibrium constant that is the ratio of
119870B to the product of surface equilibrium constantsof the dehydrogenation of methylcyclohexene tomethylcyclohexadiene and methylcyclohexadieneto toluene in the single-site surface reactionmechanism Paminus3
1198701015840
119903 Lumped adsorption equilibrium constant defined
at 119879119903 Paminus3
119899 Order of the reaction119873 Number of data points119901 Pressure Pa119901A Partial pressure of methylcyclohexane Pa119901B Partial pressure of toluene Pa119901C Partial pressure of hydrogen Pa(minus119903) Rate of the dehydrogenation reaction molsdotkgminus1sdotsminus1(minus119903)0 Initial rate of the dehydrogenation reactionmolsdotkgminus1sdotsminus1
119877 Universal gas constant Jsdotmolminus1sdotKminus1119905119889 Online reaction deactivation time s
119879 Temperature K119879ave Local average temperature defined in Figure 2 K119879119903 Reference temperature K
119879119908 Reactor wall temperature K
119879119911 Temperature at any position in the axial direction
K119882 Weight of catalyst kg119883 Conversion (fractional conversion) of MCH119883mod Model or calculated conversion of MCH119883obs Observed or measured conversion of MCH119910A0 Initial mole fraction of MCH in the vapor phase119910C0 Initial mole fraction of hydrogen in the vapor
phase1199101198680 Initial mole fraction of inert in the vapor phase
Acknowledgment
Muhammad R Usmanwould like to acknowledge theHigherEducation Commission of Pakistan for funding the research
References
[1] K Jothimurugesan S Bhatia and R D Srivastava ldquoKineticsof dehydrogenation of methylcyclohexane over a platinum-rhe-nium-aluminium catalyst in the presence of added hydrogenrdquoIndustrial amp Engineering Chemistry Fundamentals vol 24 no4 pp 433ndash438 1985
[2] J H Gary and G E Handwerk Petroleum Refining Technologyand Economics Marcel Dekker New York NY USA 4th edi-tion 2001
[3] M R Usman and D L Cresswell ldquoOptions for on-board use ofhydrogen based on the Methylcyclohexane-Toluene-Hydrogensystemrdquo International Journal of Green Energy vol 10 pp 177ndash189 2013
[4] M Usman D Cresswell and A Garforth ldquoDetailed reactionkinetics for the dehydrogenation of methylcyclohexane over Ptcatalystrdquo Industrial and Engineering Chemistry Research vol 51no 1 pp 158ndash170 2012
[5] J H Sinfelt H Hurwitz and R A Shulman ldquoKinetics ofmethylcyclohexane dehydrogenation over Pt-Al
2O3rdquo The Jour-
nal of Physical Chemistry vol 64 no 10 pp 1559ndash1562 1960[6] A W Ritchie and A C Nixon ldquoDehydrogenation of methylcy-
clohexane over a platinum-alumina catalyst in absence of addedhydrogenrdquo IndustrialampEngineeringChemistry Product Researchand Development vol 5 no 1 pp 59ndash64 1966
[7] A Corma R Cid and A Lopez Agudo ldquoCatalyst decay in thekinetics of methylcyclohexane dehydrogenation over Pt-NaYzeoliterdquoThe Canadian Journal of Chemical Engineering vol 57no 5 pp 638ndash642 1979
[8] A Touzani D Klvana and G Belanger ldquoDehydrogenation ofmethylcyclohexane on the industrial catalyst kinetic studyrdquoStudies in Surface Science and Catalysis vol 19 pp 357ndash3641984
[9] M A Pacheco and E E Petersen ldquoReaction kinetics of methyl-cyclohexane dehydrogenation over a sulfided Pt +ReAl
2O3
reforming catalystrdquo Journal of Catalysis vol 96 no 2 pp 507ndash516 1985
[10] P A van Trimpont G B Marin and G F Froment ldquoKineticsofmethylcyclohexane dehydrogenation on sulfided commercialplatinumalumina and platinum-rheniumalumina catalystsrdquoIndustrial amp Engineering Chemistry Fundamentals vol 25 no 4pp 544ndash553 1986
[11] A K Pal M Bhowmick and R D Srivastava ldquoDeactivationkinetics of platinum-rhenium re-forming catalyst accompany-ing the dehydrogenation of methylcyclohexanerdquo Industrial ampEngineering Chemistry Process Design and Development vol 25no 1 pp 236ndash241 1986
[12] J Chaouki A Touzani D Klvana J P Bournonville andG Belanger ldquoDeshydrogenation du Methylcyclohexane sur leCatalyseur Industriel Pt-SnA
2O3rdquo Revue de lrsquoInstitut Francais
du Petrole vol 43 no 6 pp 873ndash881 1988[13] M El-Sawi F A Infortuna P G Lignola A Parmaliana F
Frusteri and N Giordano ldquoParameter estimation in the kineticmodel of methylcyclohexane dehydrogenation on a Pt-Al
2O3
catalyst by sequential experiment designrdquo The Chemical Engi-neering Journal vol 42 no 3 pp 137ndash144 1989
[14] M-R Chai and K Kawakami ldquoKinetic model and simulationfor catalyst deactivation during dehydrogenation of methylcy-clohexane over commercial Pt- PtRe- and presulfided PtRe-Al2O3catalystsrdquo Journal of Chemical Technology and Biotechnol-
ogy vol 51 no 3 pp 335ndash345 1991[15] R H Manser Sonderer Methylcyclohexane dehydrogenation
kinetics reactor design and simulation for a hydrogen poweredvehicle [PhD thesis] Swiss Federal Institute of Technology1992
[16] G Maria A Marin C Wyss et al ldquoModelling and scaleup ofthe kinetics with deactivation of methylcyclohexane dehydro-genation for hydrogen energy storagerdquo Chemical EngineeringScience vol 51 no 11 pp 2891ndash2896 1996
[17] J H Sinfelt ldquoThe turnover frequency of methylcyclohexanedehydrogenation to toluene on a Pt reforming catalystrdquo Journalof Molecular Catalysis A vol 163 no 1-2 pp 123ndash128 2000
[18] D E Tsakiris Catalytic production of hydrogen from liquid or-ganic hydride [PhD thesis]TheUniversity ofManchester 2007
[19] FAlhumaidanDCresswell andAGarforth ldquoKineticmodel ofthe dehydrogenation of methylcyclohexane over monometallicand bimetallic Pt catalystsrdquo Industrial and Engineering Chem-istry Research vol 50 no 5 pp 2509ndash2522 2011
8 ISRN Chemical Engineering
[20] M R Usman ldquoMethylcyclohexane dehydrogenation over com-mercial 03 Wt PtAl
2O3catalystrdquo Proceedings of the Pakistan
Academy of Sciences vol 48 no 1 pp 13ndash17 2011[21] M R Usman R Aslam and F Alotaibi ldquoHydrogen storage
in a recyclable organic hydride kinetic modeling of methylcy-clohexane dehydrogenation over 10 wt Pt120579-Al
2O3rdquo Energy
Sources A vol 33 no 24 pp 2264ndash2271 2011[22] LW Jossens and E E Petersen ldquoFouling of a platinum reform-
ing catalyst accompanying the dehydrogenation of methyl cy-clohexanerdquo Journal of Catalysis vol 73 no 2 pp 377ndash386 1982
[23] T H Schildhauer E Newson and S Muller ldquoThe equilibriumconstant for the methylcyclohexane-toluene systemrdquo Journal ofCatalysis vol 198 no 2 pp 355ndash358 2001
[24] J R Kittrell ldquoMathematical modeling of chemical reactionsrdquoAdvances in Chemical Engineering vol 8 pp 97ndash183 1970
[25] J C Rohrer and J H Sinfelt ldquoInteraction of hydrocarbons withPt-Al2O3in the presence of hydrogen and heliumrdquoThe Journal
of Physical Chemistry vol 66 no 6 pp 1193ndash1194 1962
Figure 3 Scatter diagrams for the power law model relating observed and model values of conversions at 1013 bar
ISRN Chemical Engineering 5
0 2 4 6 8 10
n
04
06
08
10
12
14
16
p (bar)
Group 1Group 2Group 4
Figure 4 Effect of pressure on the order of the reaction for thepower law kinetics
0 2 4 6 8 10
E(k
Jmol
)
140
120
100
80
60
40
20
p (bar)
Group 1Group 2Group 4
Figure 5 Effect of pressure on the activation energy of the reactionfor the power law kinetics
a positive effect towards the kinetics of the reaction at least atatmospheric pressure The observation may be explained onthe basis of relatively higher toluene inhibition under the con-ditionswhen no hydrogen is in the feedThis same promotionin the presence of H
2and toluene inhibition at atmospheric
pressure was also observed by other researchers in the field[21 22 25] A reaction order close to unity for group 11suggests a low MCH coverage However a decrease in theorder of the reaction with decreasing H
2in the feed suggests
an increase in coverage of MCH Combining the previousstatements it is concluded that the presence of H
2may be
helpful in replacing the strongly adsorbed products specieswhich otherwise cover the active surface The activationenergy parameter 119861 remains more or less the same and theapparent activation energy lies within 534 to 588 kJmol
p (bar)0 2 4 6 8 10
8
6
4
2
0
(minusr)
0times10
5(m
olg
-catmiddots)
Group 1Group 2Group 4
Figure 6 Effect of pressure on the initial rate of the reaction for thepower law kinetics
A wide range of apparent activation energies is reported inthe literature The values previously given tend to be towardsthe bottom of the range The deactivation rate constant 119896
119889
is always significant confirming the importance of includingthe short-term deactivation
42 50 Bar Pressure Also at 119901 = 50 bar individual groupsare fitted very well It is apparent for group 15 and group 25 inTable 2 that 119899 gt 1 This seems unlikely on physical groundsHowever the manifestation of an apparent order 119899 gt 1 canbe reconciled with strong chemisorption of one or both of thereaction products toluene or hydrogen or of reaction inter-mediates such as methylcyclohexenes or methylcyclohexa-dienes The former intermediate methylcyclohexene wasobserved in low concentrations in the condensate corre-sponding to intermediate levels of conversion of methylcy-clohexane Methylcyclohexadienes on the other hand havenever been observedThis is not to say however that they arenot formed on the catalyst surface It is obvious that theapparent order 119899 is significantly greater at 50 bar than at1013 bar when hydrogen is in the feed Langmuir-Hin-shelwood-Hougen-Watson (LHHW) postulates on the otherhand predict a decreasing apparent order 119899 with increasingpressure consistent with an increasing surface coverage ofmethylcyclohexane
The values of the activation energy are considerablyhigher for group 15 and group 25 This shows that pressurehas a significant effect on the activation energy of the reactionwhen H
2is present in the feed Comparing group 25 and
group 45 in contrast to 1013 bar results no promotion ofhydrogen is observedThe initial rate of reaction (minus119903
0) times 105
is reduced by up to an order of magnitude on increasing thepressure from 1013 bar to 50 bar which suggests strongproduct retarding effects On average the value of thedeactivation constant 119896
119889 is less than that for 1013 bar and
especially for group 15 (highest hydrogen feed concentration)
6 ISRN Chemical Engineering
where the 95 confidence interval involves zero suggestingthe parameter 119896
119889becomes insignificant
43 90 Bar Pressure In all the cases at 90 bar an extremelygood individual group fit of the data is found with Adj(1198772) gt099 Similar to the results at 1013 bar and 50 bar the powerlaw model parameters appear to be group dependent Thevalue of 119899 remains virtually the same in group 19 group 29and group 49 a slight increase however is noticeable in thegroups containing H
2 The same observation is observed at
50 bar however with a greater variation The order of thereaction 119899 gt 10 can be described in the same way asexplained previously The activation energy is found to bequite high greater than 85 kJmol in all the individual grouplistings Comparing group 29 and group 49 an increasedinitial rate and lower activation energy are observed for group49 Similarly a low value of the deactivation constant (119896
119889)
is observed when hydrogen is in the feed which confirmsthe beneficial effects of hydrogen in maintaining the catalystactivity A high value of the deactivation constant (119896
119889) in the
absence of hydrogen may be explained on the basis of theformation of unsaturated intermediates which act as cokeprecursors
The previous discussion reveals that at each pressureparameters are found to be group dependent This is sum-marized graphically in Figures 4 to 6 When H
2is in the feed
both apparent order of the reaction and apparent activationenergy have increased values at high pressures Actually forhigher concentrations of hydrogen in the feed these passthrough a maximum and then fall back while for lowerhydrogen feed concentrations the values increase and thenremain almost same An increased value of activation energyprovides the clue of some strongly adsorbed components thatrequire higher activation energies of desorption to desorbinto the gas phase A higher order of the reaction greater thanunity at increased pressure somewhat confirms this hypoth-esis The initial rates of reaction at 119879 = 119879
119903= 6172K tend to
decrease with pressure for all the groups This kind of behav-ior is less common though it is compatible with an LHHWdual-site kinetic model The effect of hydrogen at higherpressures (say) 90 bar seems to be vanishing as values ofparameters somehow approach each other Comparing group2 at different pressures it is observed that at low pressures apromotion ofH
2is observed asmentioned earlier however at
increased pressure values in fact H2adversely affects the rate
whichmay bemanifested that reverse reaction is important athigher pressures than at 1013 bar or the excess H
2in the feed
may be involved in the associative adsorption of some prod-ucts species which otherwise are gaseous products Anotherpossibility that at high pressures hydrogenmay be competingfor the active adsorption sites and therefore lowering therates of the reactionmaynot be ruled outThis effect of hydro-gen was observed to be more pronounced at 50 bar than at90 bar
In the overall discussion it may be summarized that thepartial pressures of hydrogen and MCH and adsorption ofMCH are found to be important contributors at atmosphericpressure while partial pressures of H
2and adsorption of
hydrogen and the other major product (toluene) are found
important at higher pressures with adsorption of hydro-gen less pronounced This suggests a kinetic equation thatincludes the effects of partial pressure of MCH and hydrogenand adsorption kinetics of MCH hydrogen and toluene Aswith pressures the initial rates are decreased nonlinearly soneed is there for a term in the denominator that constitutesproduct of square or cube of the partial pressure of hydrogen(nonlinear dependence of hydrogen partial pressure) andsome parameter representing adsorption kinetics of at leastone of the major products other than hydrogen that istoluene The best-fit kinetic rate model (based on Langmuir-Hinshelwood-Hougen-Watson single-site kinetics with lossof first hydrogen as the rate controlling step) of the overallexperimental data as carried out in our previous study isshown in (10) [4] Equation (10) clearly shows the strongnonlinear dependence of hydrogen presence and adsorptionkinetics of toluene on the rate of the reaction
Using power law kinetics the limited data in individualgroups is fitted remarkably well and at each pressure param-eters are found to be group dependent With H
2in the feed
both apparent order of the reaction and apparent activationenergy generally increase with pressure At atmosphericpressure with H
2in the feed a promotion in the rate of the
dehydrogenation reaction is observed However at increasedpressures H
2adversely affects the rate The results suggest a
kinetic model which involves nonlinear dependence of rateon the partial pressure of hydrogen and adsorption kinetics oftoluene or some intermediate
Nomenclature
119861 Dimensionless activation energy119864 Activation energy Jmol1198651198600 Initial molar flow rate of MCH mols
Δℎ1015840 Lumped heat of adsorption Jmol
119896 Rate constant for the MCH dehydrogenation reactionmolsdotkgminus1sdotsminus1sdotbarminus1 for (2) and molsdotkgminus1sdotsminus1 for (10)
119896119903 Rate constant at the reference temperature
molsdotkgminus1sdotsminus1sdotbarminus119899119870 Equilibrium constant of MCH dehydrogenation
reaction Pa3119870A Adsorption equilibrium constant for
methylcyclohexane Paminus1119870B Adsorption equilibrium constant for toluene Paminus1
ISRN Chemical Engineering 7
1198701015840 Lumped equilibrium constant that is the ratio of
119870B to the product of surface equilibrium constantsof the dehydrogenation of methylcyclohexene tomethylcyclohexadiene and methylcyclohexadieneto toluene in the single-site surface reactionmechanism Paminus3
1198701015840
119903 Lumped adsorption equilibrium constant defined
at 119879119903 Paminus3
119899 Order of the reaction119873 Number of data points119901 Pressure Pa119901A Partial pressure of methylcyclohexane Pa119901B Partial pressure of toluene Pa119901C Partial pressure of hydrogen Pa(minus119903) Rate of the dehydrogenation reaction molsdotkgminus1sdotsminus1(minus119903)0 Initial rate of the dehydrogenation reactionmolsdotkgminus1sdotsminus1
119877 Universal gas constant Jsdotmolminus1sdotKminus1119905119889 Online reaction deactivation time s
119879 Temperature K119879ave Local average temperature defined in Figure 2 K119879119903 Reference temperature K
119879119908 Reactor wall temperature K
119879119911 Temperature at any position in the axial direction
K119882 Weight of catalyst kg119883 Conversion (fractional conversion) of MCH119883mod Model or calculated conversion of MCH119883obs Observed or measured conversion of MCH119910A0 Initial mole fraction of MCH in the vapor phase119910C0 Initial mole fraction of hydrogen in the vapor
phase1199101198680 Initial mole fraction of inert in the vapor phase
Acknowledgment
Muhammad R Usmanwould like to acknowledge theHigherEducation Commission of Pakistan for funding the research
References
[1] K Jothimurugesan S Bhatia and R D Srivastava ldquoKineticsof dehydrogenation of methylcyclohexane over a platinum-rhe-nium-aluminium catalyst in the presence of added hydrogenrdquoIndustrial amp Engineering Chemistry Fundamentals vol 24 no4 pp 433ndash438 1985
[2] J H Gary and G E Handwerk Petroleum Refining Technologyand Economics Marcel Dekker New York NY USA 4th edi-tion 2001
[3] M R Usman and D L Cresswell ldquoOptions for on-board use ofhydrogen based on the Methylcyclohexane-Toluene-Hydrogensystemrdquo International Journal of Green Energy vol 10 pp 177ndash189 2013
[4] M Usman D Cresswell and A Garforth ldquoDetailed reactionkinetics for the dehydrogenation of methylcyclohexane over Ptcatalystrdquo Industrial and Engineering Chemistry Research vol 51no 1 pp 158ndash170 2012
[5] J H Sinfelt H Hurwitz and R A Shulman ldquoKinetics ofmethylcyclohexane dehydrogenation over Pt-Al
2O3rdquo The Jour-
nal of Physical Chemistry vol 64 no 10 pp 1559ndash1562 1960[6] A W Ritchie and A C Nixon ldquoDehydrogenation of methylcy-
clohexane over a platinum-alumina catalyst in absence of addedhydrogenrdquo IndustrialampEngineeringChemistry Product Researchand Development vol 5 no 1 pp 59ndash64 1966
[7] A Corma R Cid and A Lopez Agudo ldquoCatalyst decay in thekinetics of methylcyclohexane dehydrogenation over Pt-NaYzeoliterdquoThe Canadian Journal of Chemical Engineering vol 57no 5 pp 638ndash642 1979
[8] A Touzani D Klvana and G Belanger ldquoDehydrogenation ofmethylcyclohexane on the industrial catalyst kinetic studyrdquoStudies in Surface Science and Catalysis vol 19 pp 357ndash3641984
[9] M A Pacheco and E E Petersen ldquoReaction kinetics of methyl-cyclohexane dehydrogenation over a sulfided Pt +ReAl
2O3
reforming catalystrdquo Journal of Catalysis vol 96 no 2 pp 507ndash516 1985
[10] P A van Trimpont G B Marin and G F Froment ldquoKineticsofmethylcyclohexane dehydrogenation on sulfided commercialplatinumalumina and platinum-rheniumalumina catalystsrdquoIndustrial amp Engineering Chemistry Fundamentals vol 25 no 4pp 544ndash553 1986
[11] A K Pal M Bhowmick and R D Srivastava ldquoDeactivationkinetics of platinum-rhenium re-forming catalyst accompany-ing the dehydrogenation of methylcyclohexanerdquo Industrial ampEngineering Chemistry Process Design and Development vol 25no 1 pp 236ndash241 1986
[12] J Chaouki A Touzani D Klvana J P Bournonville andG Belanger ldquoDeshydrogenation du Methylcyclohexane sur leCatalyseur Industriel Pt-SnA
2O3rdquo Revue de lrsquoInstitut Francais
du Petrole vol 43 no 6 pp 873ndash881 1988[13] M El-Sawi F A Infortuna P G Lignola A Parmaliana F
Frusteri and N Giordano ldquoParameter estimation in the kineticmodel of methylcyclohexane dehydrogenation on a Pt-Al
2O3
catalyst by sequential experiment designrdquo The Chemical Engi-neering Journal vol 42 no 3 pp 137ndash144 1989
[14] M-R Chai and K Kawakami ldquoKinetic model and simulationfor catalyst deactivation during dehydrogenation of methylcy-clohexane over commercial Pt- PtRe- and presulfided PtRe-Al2O3catalystsrdquo Journal of Chemical Technology and Biotechnol-
ogy vol 51 no 3 pp 335ndash345 1991[15] R H Manser Sonderer Methylcyclohexane dehydrogenation
kinetics reactor design and simulation for a hydrogen poweredvehicle [PhD thesis] Swiss Federal Institute of Technology1992
[16] G Maria A Marin C Wyss et al ldquoModelling and scaleup ofthe kinetics with deactivation of methylcyclohexane dehydro-genation for hydrogen energy storagerdquo Chemical EngineeringScience vol 51 no 11 pp 2891ndash2896 1996
[17] J H Sinfelt ldquoThe turnover frequency of methylcyclohexanedehydrogenation to toluene on a Pt reforming catalystrdquo Journalof Molecular Catalysis A vol 163 no 1-2 pp 123ndash128 2000
[18] D E Tsakiris Catalytic production of hydrogen from liquid or-ganic hydride [PhD thesis]TheUniversity ofManchester 2007
[19] FAlhumaidanDCresswell andAGarforth ldquoKineticmodel ofthe dehydrogenation of methylcyclohexane over monometallicand bimetallic Pt catalystsrdquo Industrial and Engineering Chem-istry Research vol 50 no 5 pp 2509ndash2522 2011
8 ISRN Chemical Engineering
[20] M R Usman ldquoMethylcyclohexane dehydrogenation over com-mercial 03 Wt PtAl
2O3catalystrdquo Proceedings of the Pakistan
Academy of Sciences vol 48 no 1 pp 13ndash17 2011[21] M R Usman R Aslam and F Alotaibi ldquoHydrogen storage
in a recyclable organic hydride kinetic modeling of methylcy-clohexane dehydrogenation over 10 wt Pt120579-Al
2O3rdquo Energy
Sources A vol 33 no 24 pp 2264ndash2271 2011[22] LW Jossens and E E Petersen ldquoFouling of a platinum reform-
ing catalyst accompanying the dehydrogenation of methyl cy-clohexanerdquo Journal of Catalysis vol 73 no 2 pp 377ndash386 1982
[23] T H Schildhauer E Newson and S Muller ldquoThe equilibriumconstant for the methylcyclohexane-toluene systemrdquo Journal ofCatalysis vol 198 no 2 pp 355ndash358 2001
[24] J R Kittrell ldquoMathematical modeling of chemical reactionsrdquoAdvances in Chemical Engineering vol 8 pp 97ndash183 1970
[25] J C Rohrer and J H Sinfelt ldquoInteraction of hydrocarbons withPt-Al2O3in the presence of hydrogen and heliumrdquoThe Journal
of Physical Chemistry vol 66 no 6 pp 1193ndash1194 1962
Figure 4 Effect of pressure on the order of the reaction for thepower law kinetics
0 2 4 6 8 10
E(k
Jmol
)
140
120
100
80
60
40
20
p (bar)
Group 1Group 2Group 4
Figure 5 Effect of pressure on the activation energy of the reactionfor the power law kinetics
a positive effect towards the kinetics of the reaction at least atatmospheric pressure The observation may be explained onthe basis of relatively higher toluene inhibition under the con-ditionswhen no hydrogen is in the feedThis same promotionin the presence of H
2and toluene inhibition at atmospheric
pressure was also observed by other researchers in the field[21 22 25] A reaction order close to unity for group 11suggests a low MCH coverage However a decrease in theorder of the reaction with decreasing H
2in the feed suggests
an increase in coverage of MCH Combining the previousstatements it is concluded that the presence of H
2may be
helpful in replacing the strongly adsorbed products specieswhich otherwise cover the active surface The activationenergy parameter 119861 remains more or less the same and theapparent activation energy lies within 534 to 588 kJmol
p (bar)0 2 4 6 8 10
8
6
4
2
0
(minusr)
0times10
5(m
olg
-catmiddots)
Group 1Group 2Group 4
Figure 6 Effect of pressure on the initial rate of the reaction for thepower law kinetics
A wide range of apparent activation energies is reported inthe literature The values previously given tend to be towardsthe bottom of the range The deactivation rate constant 119896
119889
is always significant confirming the importance of includingthe short-term deactivation
42 50 Bar Pressure Also at 119901 = 50 bar individual groupsare fitted very well It is apparent for group 15 and group 25 inTable 2 that 119899 gt 1 This seems unlikely on physical groundsHowever the manifestation of an apparent order 119899 gt 1 canbe reconciled with strong chemisorption of one or both of thereaction products toluene or hydrogen or of reaction inter-mediates such as methylcyclohexenes or methylcyclohexa-dienes The former intermediate methylcyclohexene wasobserved in low concentrations in the condensate corre-sponding to intermediate levels of conversion of methylcy-clohexane Methylcyclohexadienes on the other hand havenever been observedThis is not to say however that they arenot formed on the catalyst surface It is obvious that theapparent order 119899 is significantly greater at 50 bar than at1013 bar when hydrogen is in the feed Langmuir-Hin-shelwood-Hougen-Watson (LHHW) postulates on the otherhand predict a decreasing apparent order 119899 with increasingpressure consistent with an increasing surface coverage ofmethylcyclohexane
The values of the activation energy are considerablyhigher for group 15 and group 25 This shows that pressurehas a significant effect on the activation energy of the reactionwhen H
2is present in the feed Comparing group 25 and
group 45 in contrast to 1013 bar results no promotion ofhydrogen is observedThe initial rate of reaction (minus119903
0) times 105
is reduced by up to an order of magnitude on increasing thepressure from 1013 bar to 50 bar which suggests strongproduct retarding effects On average the value of thedeactivation constant 119896
119889 is less than that for 1013 bar and
especially for group 15 (highest hydrogen feed concentration)
6 ISRN Chemical Engineering
where the 95 confidence interval involves zero suggestingthe parameter 119896
119889becomes insignificant
43 90 Bar Pressure In all the cases at 90 bar an extremelygood individual group fit of the data is found with Adj(1198772) gt099 Similar to the results at 1013 bar and 50 bar the powerlaw model parameters appear to be group dependent Thevalue of 119899 remains virtually the same in group 19 group 29and group 49 a slight increase however is noticeable in thegroups containing H
2 The same observation is observed at
50 bar however with a greater variation The order of thereaction 119899 gt 10 can be described in the same way asexplained previously The activation energy is found to bequite high greater than 85 kJmol in all the individual grouplistings Comparing group 29 and group 49 an increasedinitial rate and lower activation energy are observed for group49 Similarly a low value of the deactivation constant (119896
119889)
is observed when hydrogen is in the feed which confirmsthe beneficial effects of hydrogen in maintaining the catalystactivity A high value of the deactivation constant (119896
119889) in the
absence of hydrogen may be explained on the basis of theformation of unsaturated intermediates which act as cokeprecursors
The previous discussion reveals that at each pressureparameters are found to be group dependent This is sum-marized graphically in Figures 4 to 6 When H
2is in the feed
both apparent order of the reaction and apparent activationenergy have increased values at high pressures Actually forhigher concentrations of hydrogen in the feed these passthrough a maximum and then fall back while for lowerhydrogen feed concentrations the values increase and thenremain almost same An increased value of activation energyprovides the clue of some strongly adsorbed components thatrequire higher activation energies of desorption to desorbinto the gas phase A higher order of the reaction greater thanunity at increased pressure somewhat confirms this hypoth-esis The initial rates of reaction at 119879 = 119879
119903= 6172K tend to
decrease with pressure for all the groups This kind of behav-ior is less common though it is compatible with an LHHWdual-site kinetic model The effect of hydrogen at higherpressures (say) 90 bar seems to be vanishing as values ofparameters somehow approach each other Comparing group2 at different pressures it is observed that at low pressures apromotion ofH
2is observed asmentioned earlier however at
increased pressure values in fact H2adversely affects the rate
whichmay bemanifested that reverse reaction is important athigher pressures than at 1013 bar or the excess H
2in the feed
may be involved in the associative adsorption of some prod-ucts species which otherwise are gaseous products Anotherpossibility that at high pressures hydrogenmay be competingfor the active adsorption sites and therefore lowering therates of the reactionmaynot be ruled outThis effect of hydro-gen was observed to be more pronounced at 50 bar than at90 bar
In the overall discussion it may be summarized that thepartial pressures of hydrogen and MCH and adsorption ofMCH are found to be important contributors at atmosphericpressure while partial pressures of H
2and adsorption of
hydrogen and the other major product (toluene) are found
important at higher pressures with adsorption of hydro-gen less pronounced This suggests a kinetic equation thatincludes the effects of partial pressure of MCH and hydrogenand adsorption kinetics of MCH hydrogen and toluene Aswith pressures the initial rates are decreased nonlinearly soneed is there for a term in the denominator that constitutesproduct of square or cube of the partial pressure of hydrogen(nonlinear dependence of hydrogen partial pressure) andsome parameter representing adsorption kinetics of at leastone of the major products other than hydrogen that istoluene The best-fit kinetic rate model (based on Langmuir-Hinshelwood-Hougen-Watson single-site kinetics with lossof first hydrogen as the rate controlling step) of the overallexperimental data as carried out in our previous study isshown in (10) [4] Equation (10) clearly shows the strongnonlinear dependence of hydrogen presence and adsorptionkinetics of toluene on the rate of the reaction
Using power law kinetics the limited data in individualgroups is fitted remarkably well and at each pressure param-eters are found to be group dependent With H
2in the feed
both apparent order of the reaction and apparent activationenergy generally increase with pressure At atmosphericpressure with H
2in the feed a promotion in the rate of the
dehydrogenation reaction is observed However at increasedpressures H
2adversely affects the rate The results suggest a
kinetic model which involves nonlinear dependence of rateon the partial pressure of hydrogen and adsorption kinetics oftoluene or some intermediate
Nomenclature
119861 Dimensionless activation energy119864 Activation energy Jmol1198651198600 Initial molar flow rate of MCH mols
Δℎ1015840 Lumped heat of adsorption Jmol
119896 Rate constant for the MCH dehydrogenation reactionmolsdotkgminus1sdotsminus1sdotbarminus1 for (2) and molsdotkgminus1sdotsminus1 for (10)
119896119903 Rate constant at the reference temperature
molsdotkgminus1sdotsminus1sdotbarminus119899119870 Equilibrium constant of MCH dehydrogenation
reaction Pa3119870A Adsorption equilibrium constant for
methylcyclohexane Paminus1119870B Adsorption equilibrium constant for toluene Paminus1
ISRN Chemical Engineering 7
1198701015840 Lumped equilibrium constant that is the ratio of
119870B to the product of surface equilibrium constantsof the dehydrogenation of methylcyclohexene tomethylcyclohexadiene and methylcyclohexadieneto toluene in the single-site surface reactionmechanism Paminus3
1198701015840
119903 Lumped adsorption equilibrium constant defined
at 119879119903 Paminus3
119899 Order of the reaction119873 Number of data points119901 Pressure Pa119901A Partial pressure of methylcyclohexane Pa119901B Partial pressure of toluene Pa119901C Partial pressure of hydrogen Pa(minus119903) Rate of the dehydrogenation reaction molsdotkgminus1sdotsminus1(minus119903)0 Initial rate of the dehydrogenation reactionmolsdotkgminus1sdotsminus1
119877 Universal gas constant Jsdotmolminus1sdotKminus1119905119889 Online reaction deactivation time s
119879 Temperature K119879ave Local average temperature defined in Figure 2 K119879119903 Reference temperature K
119879119908 Reactor wall temperature K
119879119911 Temperature at any position in the axial direction
K119882 Weight of catalyst kg119883 Conversion (fractional conversion) of MCH119883mod Model or calculated conversion of MCH119883obs Observed or measured conversion of MCH119910A0 Initial mole fraction of MCH in the vapor phase119910C0 Initial mole fraction of hydrogen in the vapor
phase1199101198680 Initial mole fraction of inert in the vapor phase
Acknowledgment
Muhammad R Usmanwould like to acknowledge theHigherEducation Commission of Pakistan for funding the research
References
[1] K Jothimurugesan S Bhatia and R D Srivastava ldquoKineticsof dehydrogenation of methylcyclohexane over a platinum-rhe-nium-aluminium catalyst in the presence of added hydrogenrdquoIndustrial amp Engineering Chemistry Fundamentals vol 24 no4 pp 433ndash438 1985
[2] J H Gary and G E Handwerk Petroleum Refining Technologyand Economics Marcel Dekker New York NY USA 4th edi-tion 2001
[3] M R Usman and D L Cresswell ldquoOptions for on-board use ofhydrogen based on the Methylcyclohexane-Toluene-Hydrogensystemrdquo International Journal of Green Energy vol 10 pp 177ndash189 2013
[4] M Usman D Cresswell and A Garforth ldquoDetailed reactionkinetics for the dehydrogenation of methylcyclohexane over Ptcatalystrdquo Industrial and Engineering Chemistry Research vol 51no 1 pp 158ndash170 2012
[5] J H Sinfelt H Hurwitz and R A Shulman ldquoKinetics ofmethylcyclohexane dehydrogenation over Pt-Al
2O3rdquo The Jour-
nal of Physical Chemistry vol 64 no 10 pp 1559ndash1562 1960[6] A W Ritchie and A C Nixon ldquoDehydrogenation of methylcy-
clohexane over a platinum-alumina catalyst in absence of addedhydrogenrdquo IndustrialampEngineeringChemistry Product Researchand Development vol 5 no 1 pp 59ndash64 1966
[7] A Corma R Cid and A Lopez Agudo ldquoCatalyst decay in thekinetics of methylcyclohexane dehydrogenation over Pt-NaYzeoliterdquoThe Canadian Journal of Chemical Engineering vol 57no 5 pp 638ndash642 1979
[8] A Touzani D Klvana and G Belanger ldquoDehydrogenation ofmethylcyclohexane on the industrial catalyst kinetic studyrdquoStudies in Surface Science and Catalysis vol 19 pp 357ndash3641984
[9] M A Pacheco and E E Petersen ldquoReaction kinetics of methyl-cyclohexane dehydrogenation over a sulfided Pt +ReAl
2O3
reforming catalystrdquo Journal of Catalysis vol 96 no 2 pp 507ndash516 1985
[10] P A van Trimpont G B Marin and G F Froment ldquoKineticsofmethylcyclohexane dehydrogenation on sulfided commercialplatinumalumina and platinum-rheniumalumina catalystsrdquoIndustrial amp Engineering Chemistry Fundamentals vol 25 no 4pp 544ndash553 1986
[11] A K Pal M Bhowmick and R D Srivastava ldquoDeactivationkinetics of platinum-rhenium re-forming catalyst accompany-ing the dehydrogenation of methylcyclohexanerdquo Industrial ampEngineering Chemistry Process Design and Development vol 25no 1 pp 236ndash241 1986
[12] J Chaouki A Touzani D Klvana J P Bournonville andG Belanger ldquoDeshydrogenation du Methylcyclohexane sur leCatalyseur Industriel Pt-SnA
2O3rdquo Revue de lrsquoInstitut Francais
du Petrole vol 43 no 6 pp 873ndash881 1988[13] M El-Sawi F A Infortuna P G Lignola A Parmaliana F
Frusteri and N Giordano ldquoParameter estimation in the kineticmodel of methylcyclohexane dehydrogenation on a Pt-Al
2O3
catalyst by sequential experiment designrdquo The Chemical Engi-neering Journal vol 42 no 3 pp 137ndash144 1989
[14] M-R Chai and K Kawakami ldquoKinetic model and simulationfor catalyst deactivation during dehydrogenation of methylcy-clohexane over commercial Pt- PtRe- and presulfided PtRe-Al2O3catalystsrdquo Journal of Chemical Technology and Biotechnol-
ogy vol 51 no 3 pp 335ndash345 1991[15] R H Manser Sonderer Methylcyclohexane dehydrogenation
kinetics reactor design and simulation for a hydrogen poweredvehicle [PhD thesis] Swiss Federal Institute of Technology1992
[16] G Maria A Marin C Wyss et al ldquoModelling and scaleup ofthe kinetics with deactivation of methylcyclohexane dehydro-genation for hydrogen energy storagerdquo Chemical EngineeringScience vol 51 no 11 pp 2891ndash2896 1996
[17] J H Sinfelt ldquoThe turnover frequency of methylcyclohexanedehydrogenation to toluene on a Pt reforming catalystrdquo Journalof Molecular Catalysis A vol 163 no 1-2 pp 123ndash128 2000
[18] D E Tsakiris Catalytic production of hydrogen from liquid or-ganic hydride [PhD thesis]TheUniversity ofManchester 2007
[19] FAlhumaidanDCresswell andAGarforth ldquoKineticmodel ofthe dehydrogenation of methylcyclohexane over monometallicand bimetallic Pt catalystsrdquo Industrial and Engineering Chem-istry Research vol 50 no 5 pp 2509ndash2522 2011
8 ISRN Chemical Engineering
[20] M R Usman ldquoMethylcyclohexane dehydrogenation over com-mercial 03 Wt PtAl
2O3catalystrdquo Proceedings of the Pakistan
Academy of Sciences vol 48 no 1 pp 13ndash17 2011[21] M R Usman R Aslam and F Alotaibi ldquoHydrogen storage
in a recyclable organic hydride kinetic modeling of methylcy-clohexane dehydrogenation over 10 wt Pt120579-Al
2O3rdquo Energy
Sources A vol 33 no 24 pp 2264ndash2271 2011[22] LW Jossens and E E Petersen ldquoFouling of a platinum reform-
ing catalyst accompanying the dehydrogenation of methyl cy-clohexanerdquo Journal of Catalysis vol 73 no 2 pp 377ndash386 1982
[23] T H Schildhauer E Newson and S Muller ldquoThe equilibriumconstant for the methylcyclohexane-toluene systemrdquo Journal ofCatalysis vol 198 no 2 pp 355ndash358 2001
[24] J R Kittrell ldquoMathematical modeling of chemical reactionsrdquoAdvances in Chemical Engineering vol 8 pp 97ndash183 1970
[25] J C Rohrer and J H Sinfelt ldquoInteraction of hydrocarbons withPt-Al2O3in the presence of hydrogen and heliumrdquoThe Journal
of Physical Chemistry vol 66 no 6 pp 1193ndash1194 1962
where the 95 confidence interval involves zero suggestingthe parameter 119896
119889becomes insignificant
43 90 Bar Pressure In all the cases at 90 bar an extremelygood individual group fit of the data is found with Adj(1198772) gt099 Similar to the results at 1013 bar and 50 bar the powerlaw model parameters appear to be group dependent Thevalue of 119899 remains virtually the same in group 19 group 29and group 49 a slight increase however is noticeable in thegroups containing H
2 The same observation is observed at
50 bar however with a greater variation The order of thereaction 119899 gt 10 can be described in the same way asexplained previously The activation energy is found to bequite high greater than 85 kJmol in all the individual grouplistings Comparing group 29 and group 49 an increasedinitial rate and lower activation energy are observed for group49 Similarly a low value of the deactivation constant (119896
119889)
is observed when hydrogen is in the feed which confirmsthe beneficial effects of hydrogen in maintaining the catalystactivity A high value of the deactivation constant (119896
119889) in the
absence of hydrogen may be explained on the basis of theformation of unsaturated intermediates which act as cokeprecursors
The previous discussion reveals that at each pressureparameters are found to be group dependent This is sum-marized graphically in Figures 4 to 6 When H
2is in the feed
both apparent order of the reaction and apparent activationenergy have increased values at high pressures Actually forhigher concentrations of hydrogen in the feed these passthrough a maximum and then fall back while for lowerhydrogen feed concentrations the values increase and thenremain almost same An increased value of activation energyprovides the clue of some strongly adsorbed components thatrequire higher activation energies of desorption to desorbinto the gas phase A higher order of the reaction greater thanunity at increased pressure somewhat confirms this hypoth-esis The initial rates of reaction at 119879 = 119879
119903= 6172K tend to
decrease with pressure for all the groups This kind of behav-ior is less common though it is compatible with an LHHWdual-site kinetic model The effect of hydrogen at higherpressures (say) 90 bar seems to be vanishing as values ofparameters somehow approach each other Comparing group2 at different pressures it is observed that at low pressures apromotion ofH
2is observed asmentioned earlier however at
increased pressure values in fact H2adversely affects the rate
whichmay bemanifested that reverse reaction is important athigher pressures than at 1013 bar or the excess H
2in the feed
may be involved in the associative adsorption of some prod-ucts species which otherwise are gaseous products Anotherpossibility that at high pressures hydrogenmay be competingfor the active adsorption sites and therefore lowering therates of the reactionmaynot be ruled outThis effect of hydro-gen was observed to be more pronounced at 50 bar than at90 bar
In the overall discussion it may be summarized that thepartial pressures of hydrogen and MCH and adsorption ofMCH are found to be important contributors at atmosphericpressure while partial pressures of H
2and adsorption of
hydrogen and the other major product (toluene) are found
important at higher pressures with adsorption of hydro-gen less pronounced This suggests a kinetic equation thatincludes the effects of partial pressure of MCH and hydrogenand adsorption kinetics of MCH hydrogen and toluene Aswith pressures the initial rates are decreased nonlinearly soneed is there for a term in the denominator that constitutesproduct of square or cube of the partial pressure of hydrogen(nonlinear dependence of hydrogen partial pressure) andsome parameter representing adsorption kinetics of at leastone of the major products other than hydrogen that istoluene The best-fit kinetic rate model (based on Langmuir-Hinshelwood-Hougen-Watson single-site kinetics with lossof first hydrogen as the rate controlling step) of the overallexperimental data as carried out in our previous study isshown in (10) [4] Equation (10) clearly shows the strongnonlinear dependence of hydrogen presence and adsorptionkinetics of toluene on the rate of the reaction
Using power law kinetics the limited data in individualgroups is fitted remarkably well and at each pressure param-eters are found to be group dependent With H
2in the feed
both apparent order of the reaction and apparent activationenergy generally increase with pressure At atmosphericpressure with H
2in the feed a promotion in the rate of the
dehydrogenation reaction is observed However at increasedpressures H
2adversely affects the rate The results suggest a
kinetic model which involves nonlinear dependence of rateon the partial pressure of hydrogen and adsorption kinetics oftoluene or some intermediate
Nomenclature
119861 Dimensionless activation energy119864 Activation energy Jmol1198651198600 Initial molar flow rate of MCH mols
Δℎ1015840 Lumped heat of adsorption Jmol
119896 Rate constant for the MCH dehydrogenation reactionmolsdotkgminus1sdotsminus1sdotbarminus1 for (2) and molsdotkgminus1sdotsminus1 for (10)
119896119903 Rate constant at the reference temperature
molsdotkgminus1sdotsminus1sdotbarminus119899119870 Equilibrium constant of MCH dehydrogenation
reaction Pa3119870A Adsorption equilibrium constant for
methylcyclohexane Paminus1119870B Adsorption equilibrium constant for toluene Paminus1
ISRN Chemical Engineering 7
1198701015840 Lumped equilibrium constant that is the ratio of
119870B to the product of surface equilibrium constantsof the dehydrogenation of methylcyclohexene tomethylcyclohexadiene and methylcyclohexadieneto toluene in the single-site surface reactionmechanism Paminus3
1198701015840
119903 Lumped adsorption equilibrium constant defined
at 119879119903 Paminus3
119899 Order of the reaction119873 Number of data points119901 Pressure Pa119901A Partial pressure of methylcyclohexane Pa119901B Partial pressure of toluene Pa119901C Partial pressure of hydrogen Pa(minus119903) Rate of the dehydrogenation reaction molsdotkgminus1sdotsminus1(minus119903)0 Initial rate of the dehydrogenation reactionmolsdotkgminus1sdotsminus1
119877 Universal gas constant Jsdotmolminus1sdotKminus1119905119889 Online reaction deactivation time s
119879 Temperature K119879ave Local average temperature defined in Figure 2 K119879119903 Reference temperature K
119879119908 Reactor wall temperature K
119879119911 Temperature at any position in the axial direction
K119882 Weight of catalyst kg119883 Conversion (fractional conversion) of MCH119883mod Model or calculated conversion of MCH119883obs Observed or measured conversion of MCH119910A0 Initial mole fraction of MCH in the vapor phase119910C0 Initial mole fraction of hydrogen in the vapor
phase1199101198680 Initial mole fraction of inert in the vapor phase
Acknowledgment
Muhammad R Usmanwould like to acknowledge theHigherEducation Commission of Pakistan for funding the research
References
[1] K Jothimurugesan S Bhatia and R D Srivastava ldquoKineticsof dehydrogenation of methylcyclohexane over a platinum-rhe-nium-aluminium catalyst in the presence of added hydrogenrdquoIndustrial amp Engineering Chemistry Fundamentals vol 24 no4 pp 433ndash438 1985
[2] J H Gary and G E Handwerk Petroleum Refining Technologyand Economics Marcel Dekker New York NY USA 4th edi-tion 2001
[3] M R Usman and D L Cresswell ldquoOptions for on-board use ofhydrogen based on the Methylcyclohexane-Toluene-Hydrogensystemrdquo International Journal of Green Energy vol 10 pp 177ndash189 2013
[4] M Usman D Cresswell and A Garforth ldquoDetailed reactionkinetics for the dehydrogenation of methylcyclohexane over Ptcatalystrdquo Industrial and Engineering Chemistry Research vol 51no 1 pp 158ndash170 2012
[5] J H Sinfelt H Hurwitz and R A Shulman ldquoKinetics ofmethylcyclohexane dehydrogenation over Pt-Al
2O3rdquo The Jour-
nal of Physical Chemistry vol 64 no 10 pp 1559ndash1562 1960[6] A W Ritchie and A C Nixon ldquoDehydrogenation of methylcy-
clohexane over a platinum-alumina catalyst in absence of addedhydrogenrdquo IndustrialampEngineeringChemistry Product Researchand Development vol 5 no 1 pp 59ndash64 1966
[7] A Corma R Cid and A Lopez Agudo ldquoCatalyst decay in thekinetics of methylcyclohexane dehydrogenation over Pt-NaYzeoliterdquoThe Canadian Journal of Chemical Engineering vol 57no 5 pp 638ndash642 1979
[8] A Touzani D Klvana and G Belanger ldquoDehydrogenation ofmethylcyclohexane on the industrial catalyst kinetic studyrdquoStudies in Surface Science and Catalysis vol 19 pp 357ndash3641984
[9] M A Pacheco and E E Petersen ldquoReaction kinetics of methyl-cyclohexane dehydrogenation over a sulfided Pt +ReAl
2O3
reforming catalystrdquo Journal of Catalysis vol 96 no 2 pp 507ndash516 1985
[10] P A van Trimpont G B Marin and G F Froment ldquoKineticsofmethylcyclohexane dehydrogenation on sulfided commercialplatinumalumina and platinum-rheniumalumina catalystsrdquoIndustrial amp Engineering Chemistry Fundamentals vol 25 no 4pp 544ndash553 1986
[11] A K Pal M Bhowmick and R D Srivastava ldquoDeactivationkinetics of platinum-rhenium re-forming catalyst accompany-ing the dehydrogenation of methylcyclohexanerdquo Industrial ampEngineering Chemistry Process Design and Development vol 25no 1 pp 236ndash241 1986
[12] J Chaouki A Touzani D Klvana J P Bournonville andG Belanger ldquoDeshydrogenation du Methylcyclohexane sur leCatalyseur Industriel Pt-SnA
2O3rdquo Revue de lrsquoInstitut Francais
du Petrole vol 43 no 6 pp 873ndash881 1988[13] M El-Sawi F A Infortuna P G Lignola A Parmaliana F
Frusteri and N Giordano ldquoParameter estimation in the kineticmodel of methylcyclohexane dehydrogenation on a Pt-Al
2O3
catalyst by sequential experiment designrdquo The Chemical Engi-neering Journal vol 42 no 3 pp 137ndash144 1989
[14] M-R Chai and K Kawakami ldquoKinetic model and simulationfor catalyst deactivation during dehydrogenation of methylcy-clohexane over commercial Pt- PtRe- and presulfided PtRe-Al2O3catalystsrdquo Journal of Chemical Technology and Biotechnol-
ogy vol 51 no 3 pp 335ndash345 1991[15] R H Manser Sonderer Methylcyclohexane dehydrogenation
kinetics reactor design and simulation for a hydrogen poweredvehicle [PhD thesis] Swiss Federal Institute of Technology1992
[16] G Maria A Marin C Wyss et al ldquoModelling and scaleup ofthe kinetics with deactivation of methylcyclohexane dehydro-genation for hydrogen energy storagerdquo Chemical EngineeringScience vol 51 no 11 pp 2891ndash2896 1996
[17] J H Sinfelt ldquoThe turnover frequency of methylcyclohexanedehydrogenation to toluene on a Pt reforming catalystrdquo Journalof Molecular Catalysis A vol 163 no 1-2 pp 123ndash128 2000
[18] D E Tsakiris Catalytic production of hydrogen from liquid or-ganic hydride [PhD thesis]TheUniversity ofManchester 2007
[19] FAlhumaidanDCresswell andAGarforth ldquoKineticmodel ofthe dehydrogenation of methylcyclohexane over monometallicand bimetallic Pt catalystsrdquo Industrial and Engineering Chem-istry Research vol 50 no 5 pp 2509ndash2522 2011
8 ISRN Chemical Engineering
[20] M R Usman ldquoMethylcyclohexane dehydrogenation over com-mercial 03 Wt PtAl
2O3catalystrdquo Proceedings of the Pakistan
Academy of Sciences vol 48 no 1 pp 13ndash17 2011[21] M R Usman R Aslam and F Alotaibi ldquoHydrogen storage
in a recyclable organic hydride kinetic modeling of methylcy-clohexane dehydrogenation over 10 wt Pt120579-Al
2O3rdquo Energy
Sources A vol 33 no 24 pp 2264ndash2271 2011[22] LW Jossens and E E Petersen ldquoFouling of a platinum reform-
ing catalyst accompanying the dehydrogenation of methyl cy-clohexanerdquo Journal of Catalysis vol 73 no 2 pp 377ndash386 1982
[23] T H Schildhauer E Newson and S Muller ldquoThe equilibriumconstant for the methylcyclohexane-toluene systemrdquo Journal ofCatalysis vol 198 no 2 pp 355ndash358 2001
[24] J R Kittrell ldquoMathematical modeling of chemical reactionsrdquoAdvances in Chemical Engineering vol 8 pp 97ndash183 1970
[25] J C Rohrer and J H Sinfelt ldquoInteraction of hydrocarbons withPt-Al2O3in the presence of hydrogen and heliumrdquoThe Journal
of Physical Chemistry vol 66 no 6 pp 1193ndash1194 1962
1198701015840 Lumped equilibrium constant that is the ratio of
119870B to the product of surface equilibrium constantsof the dehydrogenation of methylcyclohexene tomethylcyclohexadiene and methylcyclohexadieneto toluene in the single-site surface reactionmechanism Paminus3
1198701015840
119903 Lumped adsorption equilibrium constant defined
at 119879119903 Paminus3
119899 Order of the reaction119873 Number of data points119901 Pressure Pa119901A Partial pressure of methylcyclohexane Pa119901B Partial pressure of toluene Pa119901C Partial pressure of hydrogen Pa(minus119903) Rate of the dehydrogenation reaction molsdotkgminus1sdotsminus1(minus119903)0 Initial rate of the dehydrogenation reactionmolsdotkgminus1sdotsminus1
119877 Universal gas constant Jsdotmolminus1sdotKminus1119905119889 Online reaction deactivation time s
119879 Temperature K119879ave Local average temperature defined in Figure 2 K119879119903 Reference temperature K
119879119908 Reactor wall temperature K
119879119911 Temperature at any position in the axial direction
K119882 Weight of catalyst kg119883 Conversion (fractional conversion) of MCH119883mod Model or calculated conversion of MCH119883obs Observed or measured conversion of MCH119910A0 Initial mole fraction of MCH in the vapor phase119910C0 Initial mole fraction of hydrogen in the vapor
phase1199101198680 Initial mole fraction of inert in the vapor phase
Acknowledgment
Muhammad R Usmanwould like to acknowledge theHigherEducation Commission of Pakistan for funding the research
References
[1] K Jothimurugesan S Bhatia and R D Srivastava ldquoKineticsof dehydrogenation of methylcyclohexane over a platinum-rhe-nium-aluminium catalyst in the presence of added hydrogenrdquoIndustrial amp Engineering Chemistry Fundamentals vol 24 no4 pp 433ndash438 1985
[2] J H Gary and G E Handwerk Petroleum Refining Technologyand Economics Marcel Dekker New York NY USA 4th edi-tion 2001
[3] M R Usman and D L Cresswell ldquoOptions for on-board use ofhydrogen based on the Methylcyclohexane-Toluene-Hydrogensystemrdquo International Journal of Green Energy vol 10 pp 177ndash189 2013
[4] M Usman D Cresswell and A Garforth ldquoDetailed reactionkinetics for the dehydrogenation of methylcyclohexane over Ptcatalystrdquo Industrial and Engineering Chemistry Research vol 51no 1 pp 158ndash170 2012
[5] J H Sinfelt H Hurwitz and R A Shulman ldquoKinetics ofmethylcyclohexane dehydrogenation over Pt-Al
2O3rdquo The Jour-
nal of Physical Chemistry vol 64 no 10 pp 1559ndash1562 1960[6] A W Ritchie and A C Nixon ldquoDehydrogenation of methylcy-
clohexane over a platinum-alumina catalyst in absence of addedhydrogenrdquo IndustrialampEngineeringChemistry Product Researchand Development vol 5 no 1 pp 59ndash64 1966
[7] A Corma R Cid and A Lopez Agudo ldquoCatalyst decay in thekinetics of methylcyclohexane dehydrogenation over Pt-NaYzeoliterdquoThe Canadian Journal of Chemical Engineering vol 57no 5 pp 638ndash642 1979
[8] A Touzani D Klvana and G Belanger ldquoDehydrogenation ofmethylcyclohexane on the industrial catalyst kinetic studyrdquoStudies in Surface Science and Catalysis vol 19 pp 357ndash3641984
[9] M A Pacheco and E E Petersen ldquoReaction kinetics of methyl-cyclohexane dehydrogenation over a sulfided Pt +ReAl
2O3
reforming catalystrdquo Journal of Catalysis vol 96 no 2 pp 507ndash516 1985
[10] P A van Trimpont G B Marin and G F Froment ldquoKineticsofmethylcyclohexane dehydrogenation on sulfided commercialplatinumalumina and platinum-rheniumalumina catalystsrdquoIndustrial amp Engineering Chemistry Fundamentals vol 25 no 4pp 544ndash553 1986
[11] A K Pal M Bhowmick and R D Srivastava ldquoDeactivationkinetics of platinum-rhenium re-forming catalyst accompany-ing the dehydrogenation of methylcyclohexanerdquo Industrial ampEngineering Chemistry Process Design and Development vol 25no 1 pp 236ndash241 1986
[12] J Chaouki A Touzani D Klvana J P Bournonville andG Belanger ldquoDeshydrogenation du Methylcyclohexane sur leCatalyseur Industriel Pt-SnA
2O3rdquo Revue de lrsquoInstitut Francais
du Petrole vol 43 no 6 pp 873ndash881 1988[13] M El-Sawi F A Infortuna P G Lignola A Parmaliana F
Frusteri and N Giordano ldquoParameter estimation in the kineticmodel of methylcyclohexane dehydrogenation on a Pt-Al
2O3
catalyst by sequential experiment designrdquo The Chemical Engi-neering Journal vol 42 no 3 pp 137ndash144 1989
[14] M-R Chai and K Kawakami ldquoKinetic model and simulationfor catalyst deactivation during dehydrogenation of methylcy-clohexane over commercial Pt- PtRe- and presulfided PtRe-Al2O3catalystsrdquo Journal of Chemical Technology and Biotechnol-
ogy vol 51 no 3 pp 335ndash345 1991[15] R H Manser Sonderer Methylcyclohexane dehydrogenation
kinetics reactor design and simulation for a hydrogen poweredvehicle [PhD thesis] Swiss Federal Institute of Technology1992
[16] G Maria A Marin C Wyss et al ldquoModelling and scaleup ofthe kinetics with deactivation of methylcyclohexane dehydro-genation for hydrogen energy storagerdquo Chemical EngineeringScience vol 51 no 11 pp 2891ndash2896 1996
[17] J H Sinfelt ldquoThe turnover frequency of methylcyclohexanedehydrogenation to toluene on a Pt reforming catalystrdquo Journalof Molecular Catalysis A vol 163 no 1-2 pp 123ndash128 2000
[18] D E Tsakiris Catalytic production of hydrogen from liquid or-ganic hydride [PhD thesis]TheUniversity ofManchester 2007
[19] FAlhumaidanDCresswell andAGarforth ldquoKineticmodel ofthe dehydrogenation of methylcyclohexane over monometallicand bimetallic Pt catalystsrdquo Industrial and Engineering Chem-istry Research vol 50 no 5 pp 2509ndash2522 2011
8 ISRN Chemical Engineering
[20] M R Usman ldquoMethylcyclohexane dehydrogenation over com-mercial 03 Wt PtAl
2O3catalystrdquo Proceedings of the Pakistan
Academy of Sciences vol 48 no 1 pp 13ndash17 2011[21] M R Usman R Aslam and F Alotaibi ldquoHydrogen storage
in a recyclable organic hydride kinetic modeling of methylcy-clohexane dehydrogenation over 10 wt Pt120579-Al
2O3rdquo Energy
Sources A vol 33 no 24 pp 2264ndash2271 2011[22] LW Jossens and E E Petersen ldquoFouling of a platinum reform-
ing catalyst accompanying the dehydrogenation of methyl cy-clohexanerdquo Journal of Catalysis vol 73 no 2 pp 377ndash386 1982
[23] T H Schildhauer E Newson and S Muller ldquoThe equilibriumconstant for the methylcyclohexane-toluene systemrdquo Journal ofCatalysis vol 198 no 2 pp 355ndash358 2001
[24] J R Kittrell ldquoMathematical modeling of chemical reactionsrdquoAdvances in Chemical Engineering vol 8 pp 97ndash183 1970
[25] J C Rohrer and J H Sinfelt ldquoInteraction of hydrocarbons withPt-Al2O3in the presence of hydrogen and heliumrdquoThe Journal
of Physical Chemistry vol 66 no 6 pp 1193ndash1194 1962
[20] M R Usman ldquoMethylcyclohexane dehydrogenation over com-mercial 03 Wt PtAl
2O3catalystrdquo Proceedings of the Pakistan
Academy of Sciences vol 48 no 1 pp 13ndash17 2011[21] M R Usman R Aslam and F Alotaibi ldquoHydrogen storage
in a recyclable organic hydride kinetic modeling of methylcy-clohexane dehydrogenation over 10 wt Pt120579-Al
2O3rdquo Energy
Sources A vol 33 no 24 pp 2264ndash2271 2011[22] LW Jossens and E E Petersen ldquoFouling of a platinum reform-
ing catalyst accompanying the dehydrogenation of methyl cy-clohexanerdquo Journal of Catalysis vol 73 no 2 pp 377ndash386 1982
[23] T H Schildhauer E Newson and S Muller ldquoThe equilibriumconstant for the methylcyclohexane-toluene systemrdquo Journal ofCatalysis vol 198 no 2 pp 355ndash358 2001
[24] J R Kittrell ldquoMathematical modeling of chemical reactionsrdquoAdvances in Chemical Engineering vol 8 pp 97ndash183 1970
[25] J C Rohrer and J H Sinfelt ldquoInteraction of hydrocarbons withPt-Al2O3in the presence of hydrogen and heliumrdquoThe Journal
of Physical Chemistry vol 66 no 6 pp 1193ndash1194 1962
