performance study of certain commercial catalysts

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PERFORMANCE STUDY OF CERTAIN COMMERCIAL

CATALYSTS IN HYDRODESULFURIZATION OF DIESEL

OILSV. Ramesh Kumar

a, K. S. Balaraman

b, V. S. Ramachandra Rao

c& M. S. Ananth

c

aUniversity College of Technology, Osmania University, Hyderabad, 500 007, India

bResearch and Development Centre, Madras Refineries Limited, Manali, Chennai, 600

068, Indiac

Department of Chemical Engineering, Indian Institute of Technology, Madras, Chenna

600 036, India

Version of record first published: 14 Feb 2007.

To cite this article: V. Ramesh Kumar , K. S. Balaraman , V. S. Ramachandra Rao & M. S. Ananth (2001): PERFORMANCESTUDY OF CERTAIN COMMERCIAL CATALYSTS IN HYDRODESULFURIZATION OF DIESEL OILS, Petroleum Science and

Technology, 19:9-10, 1029-1038

To link to this article: http://dx.doi.org/10.1081/LFT-100108292

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PERFORMANCE STUDY OFCERTAIN COMMERCIAL CATALYSTS

IN HYDRODESULFURIZATION

OF DIESEL OILS

V. Ramesh Kumar,1,* K. S. Balaraman,2

V. S. Ramachandra Rao,1 and M. S. Ananth1

1Department of Chemical Engineering, Indian Institute

of Technology, Madras, Chennai 600 036, India2Research and Development Centre, Madras Refineries

Limited, Manali, Chennai 600 068, India

ABSTRACT

Hydrodesulfurization experiments have been carried out in

a cocurrent down-flow trickle-bed reactor over two

commercially available high activity Co-Mo/Al2O3 catalysts

(A & B) using diesel oil as feed stock. Similar drying and

presulfiding procedures are followed for both the catalysts.

The efficiency of these catalysts is evaluated based on catalyst

activity and compared at typical operating conditions. It isfound that the catalyst-A exhibits higher activity than the

catalyst-B.

1029

Copyright & 2001 by Marcel Dekker, Inc. www.dekker.com

PETROLEUM SCIENCE AND TECHNOLOGY, 19(9&10), 10291038 (2001)

*Corresponding author. Current address: University College of Technology,

Osmania University, Hyderabad 500 007, India.


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INTRODUCTION

Catalytic hydrodesulfurization is widely used in petroleum refineries to

reduce sulfur content from the feedstocks. Because of stringent environmen-

tal regulations and demand for high quality products, this process is gaining

significant attention throughout the world. Trickle-bed reactors are usually

employed for carrying out hydrodesulfurization reactions. These reactors

are often preferred because of their simple and stable mode of operation,

ease of control and flexibility of application to a wide range of feedstocks.

It is common practice to test the catalysts and evaluate process con-

ditions through laboratory experiments. These experiments help in under-

standing the catalyst efficiency and chemical kinetics and enable us in

predicting the behaviour of industrial reactor. The problems associatedwith laboratory reactors, for example, axial dispersion and partial wetting

are discussed by de Bruijn, 1976; Van Klinken and Van Dongen, 1980;

Carruthers and Dicamillo, 1988; Sie, 1991; Tsamatsoulis and

Papayannakos, 1994. Laboratory as well as pilot scale trickle-bed reactors

deviate from plug flow due to axial dispersion, channeling effects and poor

catalyst wetting. Several authors (Mears, 1971; Doraiswamy and Tajbl,

1974; Satterfield, 1975) have proposed different criteria, in order to ensure

the kinetic data free from transport, channeling and wall effects. Mears

criterion (Mears, 1971) is useful in determining the axial dispersion or

back mixing. If the L/dp (L is the length of the reactor and dp is the diameter

of the particle) ratio is greater than 350, axial dispersion or back mixing is

minimum. Doraiswamy and Tajbl (1974) suggested that if the radial aspect

ratio dt/dp (dt is the diameter of the reactor tube and dp is the particle

diameter) is greater than 4, it can be assumed that the liquid distribution

is good enough and there is no adverse channeling and heat transfer effects

at the reactor wall. Incomplete wetting of the catalyst bed prevails in the

operation of laboratory trickle-bed reactors. By diluting the catalyst bed

with an equal volume of inert material, the wetting efficiency can be

increased. The inert fines dispersed among the catalyst particles provide

more solidsolid contact points and areas over which the liquid flows

(Aldahhan et al, 1995).

Several types of sulfur compounds are present in diesel fuels: mercap-

tants R-SH, sulfides R-S-R0, disulfides R-S-S-R0, polysulfides R-Sn-R0, thio-

phene, benzothiophene (BT), dibenzothiophene (DBT) and their alkyl

derivatives. The specification on diesel fuel for both technical and environ-mental reasons are getting more stringent. For example, the sulfur specifica-

tions for diesel fuel in the USA were reduced from 2000ppm to 500 ppm by

the Clean Air Act (CAA) as of October 1, 1993. Canada is adopting the

same US regulations and the European countries are limiting the sulfur level

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in diesel fuel to 500 ppm (maximum) by 1996 inline with EC directives

(Qabazard et al, 1995). In Japan, the maximum permissible level of

0.05 wt% S is implemented from 1997. The Bureau of Indian Standards

(BIS) proposes to reduce the sulfur content to 0.20 to 0.25 wt% by

2000 AD. The selection of active catalyst plays important role in sulfur

removal operation. The role of the hydrodesulfurization catalyst is to

activate C-S bond on the sulfur compounds and to hydrogenate the hydro-

carbons, from which sulfur has been removed. In general Co-Mo catalysts

have higher desulfurization activity. In this study, the main objective is to

evaluate the efficiency of certain commercial catalysts and compare.

EXPERIMENTAL

The flow diagram of the experimental unit used for the investigations

is shown in Figure 1. The unit consists of a cocurrent down-flow fixed bed

reactor of internal dia 28mm and length of 830 mm. The reactor is heated by

4 heating shells. Each shell is controlled independently by temperature con-

trollers to maintain essentially isothermal conditions within the reactor.

A 150 ml sample of catalyst diluted with an equal amount of inerts (carbo-

rundum particles 2 mm size) is loaded inside the reactor. The entrance and

exit effects are avoided using small inert beds at both the ends of the catalyst

bed. The properties of the diesel fuel used in the investigation is presented

in Table 1 and properties of the commercial catalysts are given in Table 2.

The catalysts are manufactured in the oxide form. Prior to use they are

sulfided in reducing environment. Presulfing is carried out to obtain high

and stable activity, when the catalyst is loaded in the reactor. The details of

sulfiding procedure carried out are given in the Appendix.

The hydrodesulfurization reactions are carried out under steady-state

operation at 3.0, 3.6, 4.2 and 4.8 MPa. Temperatures of 593, 613 and 633 K,

hydrogen to oil ratio of 200 vol/vol abd 3.24 LHSVs are made use of. The

following sets of experimental data are obtained using both the catalysts

separately. (i) The reaction temperature is varied at constant pressure

(3.6 MPa) and constant LHSVs (3.2 and 4 1/h) (ii) The total pressure

is varied at constant temperature (633 K) and constant LHSVs (3 and

4 1/h). The H2/oil ratio is maintained constant in all the experimental runs.

KINETICS

The interpretation of kinetic results of hydrotreating reactions in

trickle-bed reactors is a difficult task due to various complexities. The

HYDRODESULFURIZATION OF DIESEL OILS 1031
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Figure 1. Flow diagram of experimental setup.

Table 1. Properties of the Feedstock

Density ASTM D-1298 0.8548

Aniline point ASTM D-611 76.0

Viscosity at 40

C (Cst) ASTM D-445 5.12

Pour point (

C) ASTM D-97 3

Sulfur (wt%) Antek 1.65

Simulated distillation ASTM D-2887

IBP 205

10 wt% 270

30 wt% 288

50 wt% 304

70 wt% 329

90 wt% 347

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models reported earlier are generally based on several assumptions. The

models assume plug-flow pattern and isothermality of bed. The chemical

complexity of the reaction has been empirically taken into account by

assuming nonlinear kinetic laws of the power law form. The rate of reaction

including temperature and pressure terms is expressed as

dC

dt ko expE=RTPmCn 1

Since several compounds with different chemical structures and molecular

weights react simultaneously with different rates, the apparent reaction

resulting from several parallel reactions may behave like nth order reaction,

with n >1. Therefore for a given reaction the outlet concentration is calcu-

lated using Eq. (1) at different values of n between 1 and 2 and m between

0 and 1. For each value of n and m Arrhenius rate constant and activation

energy are determined using exponential regression analysis. From this pro-

cedure the best values of n and m corresponding kinetic parameters are

arrived at.

COMPARISON OF PERFORMANCE OF

COMMERCIAL CATALYSTS

Catalysts are generally compared with respect to activity, selectivity,

life, mechanical properties and costs. The commonly used measures of activ-

ity are (i) temperature required for a specified conversion (ii) conversion

Table 2. Typical Catalyst Properties

Catalyst A B

Chemical properties:

(wt% dry basis)

CoO 110 34

MoO3 < 25 10.2

Alumina Balance Balance

Physical properties:

Shape Quadralobe Quadralobe

extrudates extrudates

Size, mm 1.6 1.6

Bulk density, kg/m

3

550900 690Avg. Crush strength, kg/mm 3.2 1.1

Surface area, m2/gm 237 224

Pore volume, ml/gm 0.43 0.56

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achieved at desired temperatures (iii) space velocities required for a given

conversion at given temperature and (iv) overall reaction rate at given con-

ditions. In the present study, the catalysts comparison is made based on

catalyst activity using procedure (iv) assuming the ratio of catalyst activities

equal to the ratio of observed rates.

Activity Calculation

The activities of the catalysts are expressed in terms of reaction rates

using experimentally evaluated reaction kinetics. The relative activities (RA)

are calculated from the reaction rates of the sample and the standard

catalyst using the following equations:

k LHSV

n 1

1

Pm1

Cn1sf

1

Cn1si

2

RAsample ksample

kstandardRAstandard 3

where k is the reaction rate constant, LHSV is liquid hourly space velocity,

P is reaction pressure, Csi is sulfur content in the product and Csf is the

sulfur content in feed.

RESULTS AND DISCUSSION

Using Mears criterion (Mears, 1971), the L/dp ratio is calculated for

both the commercial catalysts and found to be greater than 350. It shows

that there is no significant axial dispersion or back mixing. The radial aspect

ratio calculated shows that the liquid distribution is good and there is no

adverse channeling effects. It is found that 1.3 order dependence with respect

to sulfur content at m 0.8 gave the best fit data for catalyst-A. The reac-

tion orders m and n are found as 1.0 and 1.4 for catalyst-B. The kinetic

results for catalyst-A and catalyst-B are presented in Table 3. The experi-

mental and calculated results are shown in Figures 2 and 3.

The catalyst B is considered as the standard catalyst with a Relative

Activity (RA) of 100. The hydrodesulfurization rate of each catalyst at

613 K is used in the above equation to calculate RA. The results indicatethat the catalyst A shows 1.25 times higher activity than catalyst B.

A possible explanation for the superior performance of catalyst A is

due to its high molybdenum content. The catalyst activity is due to presence

of MoO4 molecules, which have the property of combining in pairs.

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Table 3. Kinetic Results

M n K E

Catalyst-A

HDS 0.8 1.3 1.40E05 12104.67

Catalyst-B

HDS 1.0 1.4 5.071E06 16808.20

Figure 2. Performance of Catalyst-A for sulfur removal. Comparison between

experimental and calculated values (m 0.8; n 1.3).

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In reducing atmosphere, two oxygen atoms are readily reduced by hydrogen

and split off, forming lattice defects which become active sites. The number

of active sites per unit surface of the catalyst can increase with increasing

Mo and Co loading (Qabazard et al., 1995). Thus the catalyst with high

molybdenum content can exhibit higher activity.

CONCLUSIONS

The hydrotreatment of diesel oil is investigated over two commercially

available catalysts (A and B) in a trickle-bed reactor at temperatures

Figure 3. Performance of Catalyst-B for sulfur removal. Comparison between

experimental and calculated values (m 1.0; n 1.4).

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593633 K, pressures 34.8 MPa and 3.24 LHSVs. The kinetic parameters

are evaluated for both the catalysts using power law approach. The catalysts

are compared in terms of their hydrodesulfurization activity. Catalyst-A is

found to be superior to catalyst-B.

APPENDIX

The sulfiding is done with 11.47 gms of DMDS per litre of kerosine.

Initially the catalyst bed is purged with 50 litre/h of N2 for 2 h at atmo-

spheric pressure and temperature. The temperature is increased to 643 K

at the rate of 50 K/h and maintained for 6 h. Then the temperature is gra-

dually reduced to 373K. At 373 K the reactor pressure is raised to 10 kg/cm2

at a H2 flow rate of 50 litre/h. Kerosine with DMDS at a rate of 250 ml/h is

introduced and maintained for 4 h. Then the following temperature pro-

gramming is maintained.

The temperature is increased to 433 K at a rate of 50 K/h and kept

for 16 h; thereafter the temperature is increased to 477 K at a rate of 50 K/h

and retained for 16 h and then increased to 533 K at the same rate and

maintained for 16 h; the temperature is again increased to 589K at a rate

of 50 K/h and held for 16 h.

ACKNOWLEDGMENTS

One of the authors (VRK) is grateful to Madras Refineries Limited,

Manali, Chennai-600 068, India for financial support.

REFERENCES

Aldahhan, M.H., Wu, Y. and Dudukovic, M.P. 1995. Ind. Eng. Chem. Res.

34: 741.

Caruthers, J.C. and Dicamillo, D.J. 1988. Appl. Catal. 43: 253.

Doraiswamy, L.K. and Tajbl, D.G. 1974. Catal. Rev. Sci. Eng. 10: 177.

De Bruijn, A. 1976. Testing of HDS Catalysts in Small Trickle Phase

Reactors. 6th International Congress on Catalysis., London, UK,

paper B34.Mears, D.E. 1971. The Role of Axial Dispersion in Trickle-Flow

Laboratory Reactors. Chem. Eng. Sci. 26: 1361.

Qabazard, H., Abu-Seedo, F., Stanislaus, A., Andari, M. and Absi-Halabi,

M. 1995. Fuel Sci. & Tech. Intl. 13(9): 1135.

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Satterfield, C.N. 1975. A.I.Ch.E. 21(2): 209.

Sie, S.T. 1991. Revue de Institut Francais du Petrole. 45: 501.

Tsamatsoulis, D. and Papayannakos, N. 1994. Chem. Eng. Sci. 49: 523.

Van Klinken, J. and Van Dongen, R.H. 1980. Chem. Eng. Sci. 35: 59.

Received June 15, 2000

Accepted May 7, 2001

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performance study of certain commercial catalysts

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