f~I,

Indian Journal of Chemical TechnologyVol. 1, November 1994, pp. 319-324

Kinetics of hydrodesulphurization of thiophene in benzene over acobalt-molybdenum catalyst

Mohd Kamila, S S Alam" & S K Saraf"

"Institute of Petroleum Studies and Chemical Engineering, AJigarh Muslim University, AJigarh 202 002, India"Department of Chemical Engineering, Aligarh Muslim University, Aligarh 202 002, India

'Envirotech Consultants {P}Ltd., A-239, Okhla Industrial Area, Phase-I, New Delhi 110 020, India

Received 25 October 1993; accepted 22 April 1994

The kinetics of the hydrodesulphurization of thiophene dissolved in benzene have been studiedover a commercial cobalt molybdenum catalyst in a fixed bed reactor at atmospheric pressure andtemperatures of 320-390°C. Partial pressures of reactant species were varied in the range of2.12 x 10-3-5.03 x 10-3 and 0.682-0.886 atm for thiophene and H2, respectively. Rate equations ofthe Langmuir-Hinshelwood type have been compared with the rate data using a non-linear least-squares regression technique.

Hydrodesulphurization (HDS) is an importantcommercial process for removal of organo-sul-phur compounds from petroleum products. In thisprocess, thiophenic compounds are often used asthe model compounds because of their lesserreactivity. The reaction generally proceeds on var-ious HDS catalysts as!":C4H4S+3H2-C4Hs+H2S (1)C4Bs + H2 -C4HlO (2)

The first step of the overall reaction, reaction(1), proceeds' through butadiene intermediate. Thesecond step, reaction (2), is not rapid comparedwith the first step so that the C4 product obtainedconsists of butane and a mixture of the n-buteneisomers.

A variety of catalysts has been employed forthe reaction, including oxides of vanadium",chromium-P-!", oxides or sulphides of molybde-num? and the so called cobalt molybdate catal-yst5.9.1l-14.Cobalt-molybdenum is an importantcommercial catalyst for hydrodesulphurization.

Despite the commercial importance of thio-phene hydrogenolysis, studies of the kinetics ofthis reaction have been limited. The physico-chemical characterization of the catalyst and itsproperties have already been reviewed+'<". Themajority of the kinetic studies were carried outunder conditions far from those practised in in-dustrial operation and relatively over a narrowrange of variables. The "resent paper aims at a ri-gorous kinetic study of thiophene hydrodesulphuri-zation over commercial cobalt-molybdenum cata-lyst at atmospheric pressure and commerciallyused temperatures of 320-390°C.

Experimental ProcedureFig. 1 shows the schematic diagram of the ex-

perimental apparatus used in the present study. Itconsisted of flow arrangement for benzene con-taining thiophene, a vapourizer, hydrogen gasthrough a silica gel column and rotameter, a tubepreheater and reactor placed in a molten salt bathand product analysis system.

The tube preheater and 10 mm i.d. reactorwere heated by a molten salt bath, containing aneutectic mixture.of NaN03 and KN03• The reac-tor was positioned vertically for all runs" and thecatalyst was supported on a stainless steel screen(BSS 100 mesh) which was silver soldered intothe reactor. A laboratory stirrer was provided toagitate the molten salt so that the temperature isuniform throughout. The temperature of the bathwas regulated by a temperature controller within± 2°C. The preheated mixture entered the reactor

1 From ~ cyt~2 From HI cyinder3 Slhco ge'I cDknlnto RO\a~tt1'5 liqutd storoge' klnk6 Capillary flcJw'meter7 VaporiZPT8 Prf>hfoot.r9~""() Cotolyst ~d11 T'-mocoupto p-cbe12_t..-o San bothIioCooioI'tS Gos ctwomotograph

(i)

<D

Fig. I-Schematic diagram of experimental apparatus

320

zene; at the HDS reaction conditions benzene wasdemonstrated to be unreactive. The amount ofsecondary products encountered was always in-significant and did not influence the calculation ofthiophene conversion. The conversion of thio-phene was calculated as moles of thiophene react-ed per hour to the moles of thiophene fed perhour. Because of the relatively small amounts ofthiophene reacting and the large amount of excesshydrogen and benzene present, volumetricchanges due to the reaction stoichiometry couldbe neglected.

The feed stock used in the present study wasbenzene (BD.H) containing thiophene (99%,Switzerland). Hydrogen and nitrogen were ob-tained from Indian Oxygen Limited having puritymore than 99%. The catalyst was obtained fromUnited Catalysts India Limited, C20-6 (3.5%CoO - 14.0%, Mo03 - Alumina) having surfacearea as 209 m2/g, total pore volume as 0.30crnVg and density as 660 kg/m ', respectively.

The details of the equipment and proceduresare given else where!".

I\

INDIAN J. CHEM. TECHNOL.. NOVEMBER 1994

from near the top of the bed and flowed down-ward across the catalyst bed. The reactor inletand outlet temperatures were measured with thehelp of two thermocouple probes. The reactoroutlet was connected to a water cooled condenserfrom where the products were sent to a gas chro-matograph for analysis.

The material of construction for the entire set-up was stainless steel grade 316. The thermo-couples used in the probes were made of chrom-el-alumel wires. They were connected to a digitaltemperature indicator through a selector switch.

The benzene containing thiophene was fed intothe vaporiser by a capillary flow meter. In orderto maintain the constant flow rate of liquid feedin spite of variations of level in the tank andchanges in the pressure during vaporisation andreaction, a nitrogen pressure was applied over theliquid level in the storage tank. The flow rate ofdry hydrogen gas. was .controlled by a needlevalve and measured by a rotameter. The flow ofhydrogen gas was then adjusted at a rate corre-sponding to the desired hydrogen to hydrocarbonratio. The reactants were mixed and preheatedbefore entering the reactor. The preheater andreactor were immersed in an electrically heatedmolten salt bath. The catalyst was reduced at350°C for 2 h first in hydrogen and then withfeed for 1.5 h. Prior to starting an actual run, thegaseous feed was by-passed the reactor until thedesired feed ratio with respect to percentage thio-phene in benzene had been established. After thesteady state was attained, readings of reactantflow rates, temperatures at inlet and exit of thereactor were recorded. Similar runs were taken byvarying the flow rates of reactant streams and H2to hydrocarbon ratios at three reaction tempera-trues. During a run, the product stream was ana-lyzed at approximately 20 rnin intervals. Bothfeed and product streams were analyzed by aNUCON-5700 gas chromatograph, using thethermal conductivity cell detector. Two columnswere used at 80°C and an inlet pressure of 4 kg!em". Column 1 contained 12 wt% squalane onchromosorb W while column 2 had n-octane onporasil C. Column 1 separated H2S, thiophenewhile column 2 analyzed H2S, n-C4HIO, cis andtransbutene- 2.

The fraction of thiophene reacted was calculat-ed from the difference in ratio of thiophene to in-ternal standard observed between feed and pro-duct streams. In general, benzene mixed with thethiophene was used as the internal standard. Thedistribution of products and their respectiveyields were also determined relative to the ben-

Analysis of Kinetic DataThe experimentally determined reaction rates

were correlated using both power-law and semi-empirical mechanistic forms of rate expressions.

Power law model-The basis for estimatingreaction order with respect to a particular compo-nent is provided by the general power-law modelfor a reversible reaction:

... (3)

Only the first term is applicable when the reac-tion is irreversible. Under the conditions used inthe present investigation, the hydrodesulphuriza-tion reaction is assumed to be irreversible 1.6.

Therefore, the power-law model suggests that theHDS rate should depend upon the partial pres-sures of thiophene and hydrogen over the entirerange of conversion and may be expressed as:rT= kP~P~ ... (4)The values of k, a and b were estimated for theexperimental data using a digital computer VAX11/780.

Mechanistic kinetic models-The power-lawkinetic model is useful in establishing reaction or-ders only under limiting conditions. It does notgive much information about the mechanism ofthe reaction. Several rate equations were, there-fore, derived based on Langrniur-Hinshelwood

KAMIL et al: KINETICS OF HYDRODESULPHURIZATION 321

mechanisms and examined for precise matchingwith the kinetic data.

It is established that the surface reaction in-volved in HDS reactions may be rate controll-ing 1.3.4.6. Thus, on the basis of surface reaction asthe controlling step several models were postulat-ed and the respective kinetic rate expressions de-veloped. Since the rate is a complex function ofthe partial pressures of reactants and products,various forms of Langmiur-Hinshelwood type ofrate equations were used to describe the kineticsof the hydrodesulphurization reaction. In the rateequation terms for the adsorption of thiopheneand hydrogen sulphide were included as denom-inators in the rate equation. The terms were notincluded in the rate equation for the adsorption ofbutane and hydrogen, since previous work',('·'osuggests that butane have only a negligible effecton the reaction rate. Hydrogen was not includedbecause hydrogen was in excess and the variationof the hydrogen partial pressure from run to runwas small to allow a meaningful hydrogen adsorp-tion coefficient to be calculated. However, it isfound that the hydrodesulphurization of thio-phene can be best analyzed by assuming competi-tive adsorption of thiophene and hydrogen onsingle type of catalyst site with surface reactionbetween adsorbed thiophene and adsorbed undis-sociated hydrogen as the rate controllingstep3.4.6.13. For reaction (1), the general form ofthe rate equation was thus taken to be

'T= kP~'P~H/(l + KTPT+ KH,sPH,S t" ... (5)

However, the Langmiur-Hinshelwood mechanism,in general, will imply that nT' nH and no shallhave only zero or integer values. Therefore, someot the models with different combinations of nT'nH , a: rd no are summarized in Table 1.

In order to find out the rate equations whichdescribe the experimental data best, the rate datawere fitted into the above rate equations using anon-linear least-squares regression analysis. Thecomputer package MINMIT FOR was used forthe parameter estimation in VAX 11/780 compu-

Table 1- Langmiur-Hinshelwood rate models tested

Model No. nT nH nD

1

234

56

1

1o121

1

22334

ter. This program estimated the parameters, K byminimizing the residual sum of squares:

... (6)

The results of non-linear analysis is summarizedin Table 2. For the discrimination among variousmodels, the following criteria were used:(i) The estimated rate constants and the para-

meters which could be identified in tenns ofLangmuir-Hirrshelwood models as adsorptionequilibrium constants should be positive.

(ii) A plot of the logarithm of the rate constantversus reciprocal absolute temperature (Ar-rhenius plot) should be linear with a negativeslope.

(iii) A plot of the logarithm of each adsorptionequilibrium constant versus reciprocal abso-lute temperature (van't Hoff plot) should belinear with a positive slope, except whenchemisorption is endothermic.

(iv) Problems of discriminating between two ormore models which satisfy the above threefundamental criteria are solved by consider-ing the correlation of the data and analysis ofresiduals for each model.

ModelNo.

Table 2-Non-linear parameter estimation results

Temp. K KT K H,S SiK

593 0.303 56.50 134.00 2.37 x 10-11

623 0.519 237.00 247.00 1.56 x lO-h

663 0.595 122.00 599.00 2.51 x 10- 7

593 0.294 24.40 51.80 2.24 x 10-11

623 0.454 73.00 72.00 1.23 x 10 - 7

663 0.508 59.90 42.00 1.62 x 10-7

593 0.317 44.30 111.00 3.44 x 10-1.1

623 0.484 112.00 123.00 1.52xlO-b

663 0.541 78.00 219.00 1.68 x 10 - 7

593 0.292 19.10 26.50 4.58 x 10-11

623 0.429 37.60 48.40 1.72 x lO-h

663 0.490 36.40 76.50 3.00 x 10-7

593 0.279 3.970 7.78 2.54 x 10-111

623 0.424 20.60 29.10 1.85 x 10-10

663 0.569 15.80 58.70 3.63xlO-7

593 0.794 14.90 18.50 5.46 x 10-11

623 0.435 32.10 27.50 l.72xlO-h

663 0.567 21.70 56.40 3.41 x 10-7

2

3

4

5

6

322 INDIAN J. CHEM. TECHNOL., NOVEMBER 1994

From Table 2, it can be seen that the model 2showed the lowest value of Stand the constantsobtained there from showed a regular trend withtemperature. Therefore, the model 2 is consid-ered to be the most plausible one which repre-sents the data well over the whole range of var-iables studied. Thus, the final rate equation is:

rT= kPT PH/(l + KTPT + KH,s PH,S)2 ... (7)

Results and DiscussionThe power-law equation developed for the

HDS reaction is:rr= 0.00265 p~13p~83 ••• (8)

The comparison between the experimental andcalculated values using above equation has beenshown in Fig. 2. Eight out of ten data points arefound to be well within a maximum deviation of±4.65%.

The effect of partial pressure of thiophene, par-tial pressure of hydrogen and space velocity onthe rate of thiophene disappearance are illustratedin Figs 3,4 and 5, respectively. The reaction ratesas computed using the mechanistic model havebeen plotted as a function of partial pressure ofthiophene. The experimental data points have alsobeen placed on the same plots for comparison.The results are consistent with predictions andearlier reports':". At the constant partial pressureof thiophene, PT, the degree of hydrodesulphuri-zation increased with increase of temperature(Table 3).

8·0

"'>-0 7.58'"J:;

7·00E'"..•

6·55:'w

,t-

- 60

0-w

PH = O·729- O·886 aIm.P

T= 0·212,162_ 0.58g. 1(~2alm.

T = 623k

~v

~Ii 50"'-,-_--L __ -'-_---J~_-'-__ ..L..-_--'

5 0 55 6·0 6·5 70 7·5 eoEa~•.•mt'nlal• 'T a104 gmo(I h 9ealalysl

Fig. 2-Comparison between experimental and predicted ratesof HDS reaction

The values of reaction rate constant, k at threetemperatures have been presented on Arrhenius-type plot in Fig. 6. The three points are found tolie on a straight line with negative slope of 3.82.This may be represented by the following equa-tion:

k= 0.533 exp( - 3.821 RT) ... (9)The value of apparent activation energy, E is

found to be 3.82 kcal/mol. This shows that k isthe product of true rate constant and one or moreequilibrium constants. The value reported com-

t 1.0

0·9

~'lM"im.nlal- CoIculol~(Ea.7 )PH'0·729- 0.869 aIm.

T • 663k

OJO~--0~.2--04~--~~6~-~~-8-~1.0

Pr ,103.otm._

08.------------------------,

~ 0·7s~ 0·6~ 0·5

•

• E __ talCakuloWd(Ea.7 )

"H ·~729-0·"6 aIm.

T -623k

OI0~--~O~2~~O~4~--~~6~--~O~8--~1~PT ,103.alm._

Fig. 3 - Effect of partial pressure of thiophene on the reactionrate of thiophene HDS over cobalt-molybdenum catalyst

06,-----------------,

J:;

~ 06

'"

III [ap.,im.nlal

CalculaI.d ([a. 7 )PT = 0·212al02-O.254al(ialm.

T = 623k e

0.5L----~::-'-:------='"=----__:_!.0.7 0·8 0·9 10

PH. atm.---....

Fig. 4- Effect of partial pressure of hydrogen on the reactionrate of thiophene HDS over cobalt-molybdenum catalyst

KAMIL et al.: KINETICS OF HYDRODESULPHURIZATION 323

1·0

().t

0.1j.~..• 0·701

.t::.-l ().i01...~2.. 0.5or

040·2

• E•••••.imentalCaltulol.cl (E~ 7 )

••••• 0.729-0·115 aIm.

T'iUk

0.30 0.32

Fig. 5-Effect of space velocity on the reaction rate of thio-phene HDS over cobalt-molybdenum catalyst

Table 3-Effect of temperature on the rate of thiophene dis-appearance

Partial pressure of Temperature, KThiophene, PTx 10" _(atm)

0.2430.2890.503

593 623 663

0.614 x 103 0.622 X 103

0.572 X 103 0.625 X 103 0.629 X 103

0.659 X 103 0.687 X 103 0.672 X 103

pares well with the values reported earlier!", Fig.7 shows the van't Hoff plot of adsorption equilib-rium coefficients for the components thiopheneand hydrogen sulphide. The straight lines withpositive slopes may be well represented as:KT= 14.2 exp(3.56IRT) (10)KH,s = 12.0 exp(7.961 RT) (11)where KT and KH,s are so called adsorption equi-librium constants for thiophene and hydrogen sul-phide, respectively. The apparent heats of chemi-sorption for the two species, thiophene, E T andHydrogen sulphide, EH,s are - 3.56 and -7.96kcallmol, respectively. Both have negative valuesas required by theory.

ConclusionsThe kinetics of hydrodesulphurization of thio-

phene has been studied using cobalt molybdenumcatalyst over a rather narrow range of processconditions. Complex rate equations were found tobe necessary to describe the kinetic behaviour.The activation energy for hydrodesulphurizationreaction of thiophene obtained compares well

0·5

0·0

-0·5 k = 0·533 .xp(-3· 82/RT)

-1.0

-1.5

-2·0L.----~----...L..:-----:-'!::-:----:_!.1.48 1.54 1·60 1.66 1·72

m? K-1

T 'Fig. 6-Arrhenius plot: Thiophene HDS reaction rate coeffi-

cient

4r------------------,

HYDROGENSlLFIlE

10 2·0

Fig. 7-van't Hoff plots: Thiophene HDS adsorption rate coef-ficients

with those reported, The proposed kinetic modelis consistent with a reasonably plausible picture ofthe reaction mechanism, which can be derived byassuming that the catalyst surface has two types ofsites. Thiophene and hydrogen sulphide competewith each other for one type, but only hydrogenadsorbs on the second type, The inhibiting effectof thiophene is less obvious when rate of thio-phene HDS is piotted against the partial pressureof thiophene, The variation of the hydrogen par-tial pressure from run to run was too small to al-low a meaningful hydrog.en adsorption coefficient

324 INDIAN J. CHEM. TECHNOL., NOVEMBER 1994

to be calculated. So it can be said that the follow-ing mechanism with the surface reaction rate-con-trolling step is valid in the HDS of thiophene inbenzene over cobalt molybdenum catalyst.

Nomenclaturea, b = reaction order with respect to thiophene and hydrogen,

respectively.E = apparent activation energy, kcal!molE; = apparent heat of chemisorption of species iF, 'i' feed rate of species ik = reaction rate constant, kg mol!kg of catalyst h atrn-K = vector-of parameters to be estimatedK; = adsorption equilibrium constant of species i, atm - 1

n = number of chemical speciesnn = power of denominator in Langmuir-Hinshelwood rate

equations11; = reaction order with respect to species i in the numera-

tor of Langmuir-Hinshelwood rate equations1', = partial pressure of species i, atmr, = rate of disappearance of species i due to reaction, kg mol!

kg of catalyst hR = gas constant, kcal/rnol KSf = sum of squares of residualsT = absolute temperature, K

.SubscriptsB\ = benzeneT = thiopheneH, H, = hydrogen

S, H2S = hydrogen sulphide

Greek lettersu., aj = empirical orders of reaction

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