influence of residence time and catalyst regeneration on the pyrolysis–zeolite catalysis of oil...

Journal of Analytical and Applied Pyrolysis60 (2001) 187–203

Influence of residence time and catalystregeneration on the pyrolysis–zeolite catalysis

of oil shale

Paul T. Williams *, Hafeez M. Chishti 1

Department of Fuel and Energy, The Uni�ersity of Leeds, Leeds LS2 9JT, UK

Received 21 April 2000; accepted 20 October 2000

Abstract

Oil shale from the Kark region of Pakistan has been pyrolysed in a fixed bed batch reactorand the properties of the derived shale oil determined. The reactor system was then modifiedto incorporate a second reactor where the derived vapours from oil shale pyrolysis werepassed directly to the second reactor containing zeolite ZSM-5 catalyst. The influence of theprocess parameters of vapour residence time (VRT) over the catalyst and the regeneration ofthe catalyst were examined. The yield and composition of the derived gases before and aftercatalysis were determined. In addition, the yield and composition of the derived oil in termsof total nitrogen and sulphur content and the content of aromatic hydrocarbons in the oilswas investigated. The results showed that the yield of oil after catalysis was reduced with aconsequent higher yield of gases and formation of coke on the catalyst. The main gases fromthe pyrolysis of oil shales were CO2, CO, H2, CH4, C2H4, C2H6 and C3H6, C3H8 and minorconcentrations of other hydrocarbon gases. The main role of catalysis was to convert thelong chain alkanes and alkenes in the oil to lower molecular weight, short chain, alkylsubstituted and iso species and high concentrations of aromatic hydrocarbons. Totalnitrogen and sulphur contents in the oils were markedly reduced after catalysis. Thisreduction was reflected in the reduced concentration of nitrogen and sulphur containingaromatic hydrocarbons. The influence of longer VRTs was to increase the formation ofaromatic hydrocarbons, reduce the nitrogen, and sulphur compounds in the oils. Theinfluence of catalyst regeneration, involving five regenerations was not significant on the yieldand composition of the derived catalytically upgraded oils. © 2001 Elsevier Science B.V. Allrights reserved.

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* Corresponding author. Tel.: +44 1132 332 504; fax: +44 1132 440 572.E-mail address: [email protected] (P.T. Williams).1 On leave from the Institute of Geology, Punjab University, Lahore, Pakistan.

0165-2370/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S0165 -2370 (00 )00198 -4

188 P.T. Williams, H.M. Chishti / J. Anal. Appl. Pyrolysis 60 (2001) 187–203

Keywords: Oil shale; Zeolite; Catalysis

1. Introduction

Oil shales represent a long range potential source of liquid hydrocarbons as analternative to petroleum. Although there are very large, reserves of oil shale theirwidespread development has not yet been realised due in part to the processingcosts of oil shale retorting and the relatively high concentrations of nitrogen andsulphur in the derived shale oil [1,2]. It is essential to remove nitrogen and sulphurfrom the oils if transport grade fuels are to be produced due to the legislativerequirements related to NOx and SOx emissions when such derived fuels arecombusted in automotive engines. The process of nitrogen and sulphur removalfrom shale oils has almost exclusively concentrated on high pressure catalytichydrotreatment since this is the conventional route used to treat heavy crudepetroleum oils and refinery residues in the petroleum industry [3–6]. Furtherrefining of the oils in the petroleum industry involves low-pressure zeolite catalyticcracking to produce refined products [7].

In a recent paper, the authors reported on the coupling of the pyrolysis stage ofprocessing and the nitrogen and sulphur removal/catalytic upgrading stage ofprocessing [8]. The pyrolysis and on-line zeolite catalysis of oil shale produced anupgraded, low nitrogen and sulphur oil with a high aromatic content and reducedheavier end. The influence of catalyst temperature between 400°C and 550°C on theyield and composition of the derived oils and gases was reported. In this paper, wereport on the role of vapour residence time (VRT) of pyrolysis vapours over thecatalyst and the influence of the number of catalyst regenerations on the yield andcomposition of the derived oils from the same two-stage pyrolysis/catalysis of oilshales experimental system. Pyrolysis of the oil shales was also undertaken in theabsence of catalysis to compare with the two-stage pyrolysis/catalysis results.

2. Experimental section

The details of the experimental equipment and analytical methodologies havebeen described in a previous related paper [8]. Therefore, only a brief outline will bepresented here.

2.1. Oil shale

The oil shale used was the Kark oil shale of Eocene age from the Kohat basinarea of northern Pakistan and has been described in detail before [9]. The catalystwas of the high acidity, shape selective, zeolite ZSM-5 type and consisted of 2 mmdiameter spheres. The catalyst pore size was a mean of 5.5 A� and the surface areawas 300 m2g−1.

189P.T. Williams, H.M. Chishti / J. Anal. Appl. Pyrolysis 60 (2001) 187–203

2.2. Experimental

The reactor used for pyrolysis was a 200 cm3 fixed bed gas purged type heated byan electric ring furnace. The weight of oil shale used throughout the experimentswas 50 g and grain size 0.5–1 mm. The heating programme was a fixed heating rateof 10°C min−1 to the final pyrolysis temperature of 520°C and held at the finaltemperature for 1 h. The liquid oil phase was trapped in a series of cold traps andthe non-condensed gases were sampled and analysed off-line using packed columngas chromatography. Fig. 1 shows a schematic diagram of the pyrolysis reactor.The two-stage pyrolysis/catalysis reactor incorporated a second batch reactorcontaining the zeolite catalyst which was attached directly to the pyrolysis reactorso that the pyrolysis gases generated were passed directly over the fixed catalystbed. For the pyrolysis/catalysis experiments, the oil shale was heated at fixedpyrolysis conditions of 10°Cmin−1 to 520°C and the catalyst temperature was450°C. The effluent from the reactor was passed to the condensation systemdescribed for the pyrolysis reactor to trap the derived oils and subsequent analysisof gases. Coke formation on the catalyst was determined by the difference in mass

Fig. 1. Schematic diagram of the pyrolysis reactor.


Fig. 2. Schematic diagram of the pyrolysis–catalysis reactor.

before and after catalytic regeneration. The VRT of pyrolysis vapours over thecatalyst of 16.6, 10.9, 6.1 and 3.0 s and the number of catalyst regenerations usingfresh catalyst and catalysts after 1, 3 and 5 regenerations were investigated. Theregeneration took the form of heating the used catalyst in a furnace at a tempera-ture of 550°C in the presence of air for a period of 8 h. Fig. 2 shows a schematicdiagram of the pyrolysis/catalysis reactor.

2.3. Analytical procedures

The non-condensable gases from the pyrolysis and pyrolysis/catalysis experimentswere analysed for C4 hydrocarbons, CO2, CO, N2, H2, CH4, and O2 by packedcolumn gas chromatography. In this work, the gas yield was calculated from thetotal individual gas concentrations rather than ‘by difference’.

The derived oils from pyrolysis and pyrolysis/catalysis were analysed for their


molecular weight distribution using size exclusion chromatography. The totalnitrogen and sulphur content of the oils was carried out using a Perkin–Elmerelemental analyser.

It has been shown that the majority of nitrogen and sulphur present in shale oilsis contained in the aromatic fraction of the oils [2,3,10]. Therefore, the aromaticfraction of the oils was isolated using chemical class fractionation using liquidcolumn chromatography. Isolation was followed by analysis of the fractions usingcapillary gas chromatography with mass spectrometry and also with a variety ofselective detectors. The liquid column chromatography used sequential elution ofthe column with pentane, benzene, ethyl acetate and methanol to produce chemicalclass fractionation in terms of increasing polarity, namely, aliphatic, aromatic,ester and polar fractions.

The pentane fraction was analysed for aliphatic compounds using capillarycolumn gas chromatography with flame ionisation detection. The benzene fractionwas analysed for aromatic compounds including nitrogen and sulphur containingspecies. Aromatic nitrogen compounds were determined using capillary column gaschromatography with mass spectrometry (g.c./m.s.) and also using gas chromatog-raphy with alkali salt, nitrogen selective detection. Aromatic sulphur compoundswere identified with the aid of the g.c./m.s., and the characteristic mass numbers ofsulphur containing species and also using capillary column gas chromatographywith flame photometric sulphur selective detection. Aromatic hydrocarbons con-taining no nitrogen or sulphur were analysed by capillary g.c./m.s. Extensive usesof retention indices were also used for identification throughout the analyticalwork. The ethyl acetate and methanol fractions could not be analysed using theavailable instrumentation.

3. Results and discussion

3.1. Product yield and gas composition

Table 1 shows the product yield and gas composition from the pyrolysis of oilshale at 520°C and also the pyrolysis of oil shale with zeolite catalytic upgrading inrelation to the pyrolysis VRT over the catalyst. The oil yield in the absence of thecatalyst was 14.6 wt.% representing a significant yield of oil. The main gasesconsisted of hydrogen, carbon monoxide, carbon dioxide, methane, ethane, ethene,propane, propene, with lower concentrations of iso-butane, butane and butene.

Table 1 shows that the influence of the zeolite catalyst was to significantlyreduce the yield of oil, there was a consequent increase in the production of gasesand formation of coke on the catalyst. Water production was slightly decreased inthe presence of the catalyst. The spent shale left after pyrolysis remained fairlyconstant, since the pyrolysis was undertaken at identical conditions for the catalyst


experiments. As the residence time of the pyrolysis gases over the catalyst wasreduced, the influence of the catalyst was thereby reduced, and an increase in theyield of oil, coupled with a reduction in gas and water yields and coke formationon the catalyst was apparent. The formation of carbonaceous coke on the catalystis an inevitable disadvantage of catalytic upgrading and under the conditions offixed bed, catalysis used in this work represents a significant loss of organicmaterial. The formation of coke is related to the interaction of the catalyst with thehigh concentrations of asphaltenes, aromatic compounds and alkenes found in theshale oil [7].

Table 1 shows that there was a marked increase in gases in the presence of thezeolite catalyst. The gas increase was due mainly to increases in carbon dioxide andthe hydrocarbon gases. Both alkane and alkene gases were increased in the presenceof the catalyst compared to the uncatalysed pyrolysis. As the VRT over the catalyst

Table 1Influence of VRT on the pyrolysis/catalysis of oil shale (wt.%)

No catalystProduct VRT (s)

10.9 6.1 3.016.6

Oil 6.614.6 6.7 7.75.48.710.210.1Gases 12.25.3

3.8Coke 3.7– 3.24.69.2 8.0Water 7.6 7.6 6.5

71.271.270.1 71.2Spent shale 71.899.5 99.3Mass closure 97.3102.0 99.4

Non-hydrocarbon gasesHydrogen 0.12 0.16 0.16 0.190.08

0.490.340.26Carbon monoxide 0.47 0.653.574.02Carbon dioxide 4.112.09 4.74

4.65 4.41Total 2.43 5.20 4.75

Hydrocarbon gasesMethane 0.92 0.75 0.71 0.640.42

0.680.34 0.55 0.56 0.58Ethane0.23 1.26Ethene 1.04 0.92 0.89

Propane 1.270.17 1.54 0.621.230.06Propene 0.861.191.221.46

0.360.13 0.27Isobutane 0.25 0.090.11 0.13Butane 0.050.13 0.17

0.220.03 0.07 0.05 0.02Butene5.27 5.04 3.76Total 1.48 6.60

Sum of Alkanes (C1–C4) 1.992.882.953.671.192.322.93 2.160.32Sum of Alkenes (C1–C4) 1.73

0.67 1.85Ethene/Ethane 1.90 1.64 1.530.35 1.38Propene/Propane 0.970.960.950.23 1.29Butene/Butane 0.58 0.38 0.420.27 0.79Alkenes/Alkanes 0.860.750.80


was shortened, the influence of the catalyst was reduced and the production of, inparticular, carbon dioxide and the hydrocarbon gases were reduced. Also shown inTable 1 are the ratios of the C2–C4 alkanes and alkenes. The alkene/alkane ratiosall showed a significant increase after catalysis. Alkene/alkane gas ratios particu-larly ethene/ethane ratios in the evolved gases from oil shale pyrolysis have beenused to indicate the degree of oil cracking reactions [9,11]. Higher alkene/alkaneratios indicating a higher degree of cracking. It has also been shown that the oilvapour cracking reactions, which produce increased ethene/ethane ratios, are linkedto reduce yields of oil [12]. The alkene/alkane ratios of Table 1 show a clearincrease after catalysis showing that indeed cracking reactions are as expectedoccurring. The reduced oil yield is also apparent. In addition, the ratios ofethene/ethane and butene/butane show a clear reduction as the VRT over thecatalyst is reduced reflecting a decreased time for the cracking reactions to occurand consequently a higher oil yield. There is also a reduced formation of coke onthe catalyst. However, the relationship of the propene/propane ratio and totalC2–C4 alkene/alkane ratios does not show such a clear relationship.

Table 2 shows the product yield and gas composition for the pyrolysis/catalysisof oil shale in relation to the number of catalyst regenerations. Also shown are theresults for pyrolysis of oil shale in the absence of the catalyst, discussed earlier, forcomparison. It might be expected that fresh catalyst would be more active thanregenerated catalyst and after each subsequent regeneration the effectiveness of thecatalyst in cracking the oil may be reduced. This is shown by the fresh catalysthaving a higher conversion of the pyrolysis vapours to gases and a higher cokeformation on the catalyst compared to catalyst which had been regenerated anumber of times. This was also reflected in consequent lower oil yields. As before,the yield of all the gases and particularly carbon dioxide and the alkane and alkenegases dominate the gas produced after catalysis. The ethene/ethane ratios, which forthe VRT experiments showed a clear link between the degree of catalyst cracking,in this case showed an increase with the number of catalyst regenerations. Freshcatalyst would be expected to be the most effective in cracking the oil and thereforea higher ethene/ethane ratio would be expected compared to a catalyst which hadbeen regenerated several times. However, the opposite trend is shown in Table 2.Similarly, the propene/propane ratios also show no clear trend. Overall, theinfluence of the number of catalyst regenerations appeared not to be significant forthe range of regenerations investigated. However, the range of 0–5 regenerations isvery limited and industrial scale catalytic cracking may involve orders of magnitudehigher numbers of regenerations.

3.2. Elemental composition of the oils

Table 3 shows the elemental composition of the uncatalysed pyrolysis oil and thepyrolysis/catalysis oils in relation to VRT and the number of catalyst regenerations.Initial nitrogen and sulphur contents in the pyrolysis shale oil were 0.4 and 1.5wt.%, respectively. After catalysis, the oils showed significant reductions in nitrogenand sulphur levels. There was evidence that with shorter residence times of the


Table 2Influence of the number of catalyst regenerations on the pyrolysis/catalysis of oil shale (wt.%)

No catalystProduct Number of regenerations

0 1 3 5

6.7 6.514.6 6.66.1Oil5.3 13.4 11.7 11.1 11.1Gases

4.9 3.95.0 3.8–Coke7.6 7.6Water 7.69.2 8.6

70.2 70.069.7 70.3Spent shale 70.1Mass closure 101.8 99.1 99.1 99.599.5

Non-hydrocarbon gases0.17 0.16Hydrogen 0.160.08 0.210.68 0.620.95 0.51Carbon monoxide 0.264.75 4.51Carbon dioxide 4.102.09 5.505.60 5.28 4.776.672.43Total

Hydrocarbon gases0.69 0.710.82 0.75Methane 0.42

0.34 1.07 0.88 0.59 0.57Ethane1.28 1.041.47 1.050.23Ethene1.22 1.10Propane 1.120.17 1.261.17 1.121.29 1.10Propene 0.060.25 0.26 0.26Isobutane 0.13 0.300.14 0.130.16 0.110.13Butane0.08 0.05 0.06Butene 0.03 0.125.71 5.00 5.026.48Total 1.48

3.18 2.79 2.81Sum of Alkanes (C1–C4) 1.19 3.612.53 2.21 2.212.88Sum of Alkenes (C1–C4) 0.32

1.45 1.76Ethene/Ethane 1.840.67 1.370.96 1.021.02 0.98Propene/Propane 0.350.61 0.35Butene/Butane 0.560.23 0.760.80 0.79 0.790.80Alkenes/Alkanes 0.27

pyrolysis vapours over the catalyst the catalyst was less effective in removing thenitrogen and sulphur. However, the number of catalyst regenerations did notsignificantly influence the nitrogen and sulphur content of the derived oils.

There are few data in the literature reporting the removal of nitrogen andsulphur from shale oils using zeolite catalysis. However, zeolite catalytic crackingof petroleum shows that nitrogen removal from petroleum oils is largely to formproducts on the coke and some formation of gaseous nitric oxide [12]. Theremoval of sulphur from petroleum oils is mostly through the production ofhydrogen sulphide gas and only a small faction forms products on the coke[13,14].


Table 3Elemental composition of pyrolysis and pyrolysis/catalysis oils in relation to VRT and number ofcatalyst regenerations

Element (wt.%)Process conditions

Nitrogen SulphurCarbon Hydrogen

Pyrolysis0.4 1.510.1520°C 79.2

Pyrolysis/CatalysisVRT

0.1 0.79.116.6 s 88.510.9 s 0.287.9 0.79.3

0.2 0.89.684.36.1 s9.882.7 0.2 0.93.0 s

Regenerations0.1 0.69.20 89.30.11 0.788.6 9.20.1 0.79.33 87.9

5 0.287.9 0.79.3

Fig. 3. Molecular weight range of the oils derived from the pyrolysis and pyrolysis/catalysis of oil shalein relation to VRT.


Fig. 4. Molecular weight range of the oils derived from the pyrolysis and pyrolysis/catalysis of oil shalein relation to the number of catalyst regenerations.

3.3. Molecular weight range of the oils

Fig. 3 shows the molecular weight range of the uncatalysed pyrolysis oil and thepyrolysis/catalysis oils in relation to the residence time of the pyrolysis vapours overthe catalyst. Only the uncatalysed oil and 16.6 s and 3.0 s VRT results are shownfor clarity. Fig. 4 shows the molecular weight range of the uncatalysed oil and thepyrolysis/catalysis oils in relation to the number of catalyst regenerations. Only theuncatalysed oil and 0 and 5 number of catalyst regeneration results are shown forclarity. The molecular weight range of the uncatalysed shale oil ranged from anominal 60 to over 2300 Da with a peak at �600 Da. After catalysis, there was asignificant decrease in the molecular weight range of the derived oils. The influenceof VRT showed a further reduction in the molecular weight range in relation tolonger residence times of the vapours over the catalyst. Extended reaction timesproducing more catalytic cracking with a consequent lowering of the molecularweight of the oils, such that at 16.6 s residence time, the molecular weight range ofthe pyrolysis/catalysis oil was from a nominal 60 to �800 Da and with a peak atabout 200 Da. The influence of the number of catalyst regenerations was nothowever, significant. Although there was a clear reduction in molecular weightdistribution after catalysis, the oils generated from the 0–5 number of regenerationswere almost identical.

3.4. Detailed composition of the oils

Table 4 shows the chemical class fractionation of the oil derived from theuncatalysed pyrolysis of oil shale and the pyrolysis/catalysis of oil shale in relation


to the residence time of the pyrolysis vapours over the catalyst and the number ofcatalyst regenerations. The pentane, benzene, ethyl acetate and methanol fractionsrepresenting, aliphatic, aromatic, ester and polar material respectively. The un-catalysed shale oil contained high concentrations of the more polar material.However, after catalysis the polar material fraction was markedly reduced with aconsequent increase in the aliphatic and aromatic material of the pentane andbenzene fractions. The influence of VRT was that, as the residence time of thepyrolysis vapours over the catalyst was reduced, there was less time for reactionand consequently a progressive increase in more polar material and reducedproduction of aliphatic and aromatic material. The number of catalyst regenera-tions had a marginal effect on the chemical class fractionation of the pyrolysis/catalysis oils.

Detailed analysis of the pentane fraction of the uncatalysed pyrolysis oil showedthat the aliphatic material consisted of mostly n-alkanes and 1-alkenes togetherwith lower concentrations of branched chain compounds ranging from carbonnumber C10 to C35. After catalysis the pentane fraction had a much reduced carbonnumber distribution, the majority of the aliphatic material having a carbon numberless than C15. In addition, the n-alkane and 1-alkene material had largely beenconverted to short chain, alkyl substituted and iso-aliphatic material. There hasbeen a large volume of literature related to the petroleum industry regarding thecracking of pure n-alkanes and 1-alkenes and mixtures of these straight chainhydrocarbons over zeolite catalysts [15–20]. The cracking of these compoundsproduces a marked reduction in the carbon number range of the products andincreased formation of aromatic material.

Table 4Chemical class fractionation of pyrolysis and pyrolysis/catalysis oils in relation to VRT and number ofcatalyst regenerations

Process conditions Chemical class fraction (wt.%)

BenzenePentane MethanolEthyl acetate

Pyrolysis8.8520°C 19.1 28.7 34.7

Pyrolysis/CatalysisVRT

26.4 42.616.6 s 13.8 1.22.115.941.410.9 s 24.8

23.5 34.66.1 s 21.7 3.433.4 24.2 4.13.0 s 21.2

Regenerations0 25.9 43.0 13.2 1.7

2.013.841.81 25.024.8 41.53 15.7 1.924.85 41.2 15.9 2.1


Table 5Influence of VRT on the nitrogen containing aromatic hydrocarbons in the oils from the pyrolysis andpyrolysis/catalysis of oil shale (ppm)

No catalystProduct VRT (s)

10.9 6.1 3.016.6

50 50125 55Pyridines 35Quinolines 370 460 500 7551115Indoles 100260 105 16090

80 90 15065Carbazole 220

Total 6901720 745 1120560

Table 5 shows the nitrogen containing aromatic hydrocarbons present in thebenzene fraction of the uncatalysed shale oil and the pyrolysis/catalysis shale oils inrelation to VRT. Table 6 shows the influence of the number of catalyst regenera-tions on the nitrogen containing aromatic compounds found in the benzene fractionof the oils. The nitrogen containing compounds in the oils were dominated bypyridines, quinolines, indoles and carbazoles. After catalysis there was a significantreduction in the nitrogen containing species, which was reflected in the reduction intotal nitrogen in the oils after catalysis, shown in Table 3. With shorter residencetimes of the pyrolysis vapours over the catalyst, the effectiveness of the catalyst inremoving nitrogen compounds was seen with a progressive increase in theirconcentration in the oils. However, the influence of the number of catalystregenerations was less significant. Although there was a noticeable increase in theconcentration of the nitrogen compounds as the number of regenerations was

Table 6Influence of number of catalyst regenerations on the nitrogen containing aromatic hydrocarbons in theoils from the pyrolysis and pyrolysis/catalysis of oil shale (ppm)


0 1 3 5

30125Pyridines 504035410395Quinolines 3001115 360

90 90Indoles 260 70 8060 70Carbazole 70220 50

4501720 535 595 620Total


Table 7Influence of VRT on the sulphur containing aromatic hydrocarbons in the oils from the pyrolysis andpyrolysis/catalysis of oil shale (ppm)

Product No catalyst VRT (s)

10.9 6.1 3.016.6

25 4065 55Benzothiophene 20Cn-Benzothiophenes 65 110 160 215245

30110 65 70 90Dibenzothiophene255 380 505215Cn-Dibenzothiophenes 595

Total 4551015 650 865330

increased, thereby reducing the effectiveness of the catalyst in nitrogen compoundremoval. The total nitrogen content of the uncatalysed oil shown in Table 3represented 4000 ppm, whilst those presented in Table 5 and Table 6 totalled muchlower. However, the benzene fraction contained some higher molecular weightunidentified material and in addition, the ethyl acetate and methanol fractions arealso likely to contain more polar nitrogen containing material.

The benzene fractions of the oils were also analysed for sulphur containingaromatic hydrocarbons. Table 7 and Table 8 show the influence of VRT over thecatalyst and the number of catalyst regenerations on these compounds in theuncatalysed oil compared to the pyrolysis/catalysis oils. The majority of sulphurcontaining aromatic hydrocarbons is benzothiophene and dibenzothiophene andtheir alkylated derivatives. The role of the catalyst was to reduce the concentrationof these compounds in the derived oils by significant levels. As the VRT over the

Table 8Influence of number of catalyst regenerations on the sulphur containing aromatic hydrocarbons in theoils from the pyrolysis and pyrolysis/catalysis of oil shale (ppm)


0 1 3 5

2065Benzothiophene 3025259095Cn-Benzothiophenes 100245 120

65 65Dibenzothiophene 110 55 60230 240Cn-Dibenzothiophenes 250595 210

3851015 335 425 435Total


Table 9Influence of VRT on the aromatic hydrocarbons in the oils from the pyrolysis and pyrolysis/catalysisof oil shale (mg g−1)

Product VRT (s)No catalyst

10.9 6.1 3.016.6

37.82 23.6033.58 15.891.331-Ring aromatic73.90 57.622-Ring aromatic 52.0232.55 79.46

4.94 3.905.43 3.893-Ring aromatic 2.811.73 2.28 2.07 1.64 1.904-Ring aromatic

118.73 86.7638.42 73.70120.75Total

catalyst was shortened, the catalyst was less effective in removing the sulphurcontaining compounds. The influence of the number of catalyst regenerations wasrelatively less significant, in some cases only reflecting a marginal influence on theextent of sulphur compound removal. As was the case for the nitrogen hydrocar-bons, the ethyl acetate and methanol fractions are likely to also contain sulphurspecies of higher polarity and molecular weight.

Table 9 shows the 1–4 ring aromatic hydrocarbons (non-hetero-atomic) presentin the benzene fraction of the uncatalysed shale oil and the pyrolysis/catalysis shaleoils in relation to VRT. Table 10 shows the influence of the number of catalystregenerations on the 1–4 ring aromatic compounds found in the benzene fractionof the oils.

The 1-ring compounds present were mainly benzene, toluene and alkylatedbenzenes, the 2-ring compounds were mainly naphthalene and its alkylated deriva-tives, 3-ring compounds included, phenanthrene, anthracene and their alklylatedcompounds and 4-ring compounds included pyrene and chrysene. The uncatalysedoil contained significant quantities of 2-ring hydrocarbons and lower concentrationsof 1-, 3- and 4-ring compounds. However, after catalysis there was a markedincrease in the 1-ring and 2-ring hydrocarbons present in the oils. The 3- and 4-ring

Table 10Influence of number of catalyst regenerations on the aromatic hydrocarbons in the oils from thepyrolysis and pyrolysis/catalysis of oil shale (mg g−1)


0 1 3 5

1.33 34.821-Ring aromatic 32.56 31.27 30.0832.55 72.932-Ring aromatic 75.4079.5085.332.81 5.783-Ring aromatic 5.38 5.20 4.94

2.072.182.262.434-Ring aromatic 1.73

110.02114.05119.70128.3638.42Total


hydrocarbons showed a smaller increase. The influence of VRT showed thatlonger residence times of pyrolysis vapours over the catalyst produce increasedformation of 1- and 2-ring hydrocarbons. Ono et al. [17] and Sirokman et al.[18] have shown a similar relationship between residence time and aromaticproduction for hexane and pentane over ZSM-5 zeolite catalyst respectively.Table 10 shows that the influence of the number of catalyst regenerations on theproduction of 1- and 2-ring hydrocarbons in the derived pyrolysis/catalysis oilswas small but significant. The increased number of regenerations reducing theeffectiveness of the catalyst.

It is clear that zeolite catalysis produces a very aromatic oil, high in 1- and2-ring hydrocarbons. Thus indicating that the oils had undergone significantformation of aromatic compounds consistent with catalytic aromatisation reac-tions of alkanes and alkenes on zeolite catalysts. The clear reduction in the morepolar ethyl acetate and methanol fractions shown in Table 4 indicate that thecatalyst is also effective in cracking these more polar compounds resulting inpossibly aromatic hydrocarbon formation.

In a previous paper by the authors [8] the influence of catalyst temperature onthe pyrolysis/catalysis of oil shale was investigated, using the same experimentalsystem reported here. In that work, nitrogen and sulphur removal from the shaleoil were similar to that reported here, with increasing catalyst temperature be-tween 400°C and 550°C producing increased effectiveness of removal. Similarincreases in the 1- and 2-ring aromatic compounds were observed. Consequently,it may be concluded that of the process parameters investigated for the pyroly-sis/catalysis of oil shale to produce an upgraded oil, the important parameterswith respect to nitrogen and sulphur removal and aromatic production are theVRT over the catalyst and the catalyst temperature. However, over the limitedrange of 0–5 regenerations, the effectiveness of the catalyst was not significantlyaffected.

In addition to the catalytic reactions of the shale oil, the potential of merethermal cracking of the oil vapours should also be considered. Thermal crackingwould produce increased gas and reduced oil production; a shift to lower molec-ular weight ranges for the oil and increased alkene/alkane ratios as was foundhere in the presence of the zeolite catalyst. However, whilst it is inevitable thatthermal cracking reactions are occurring as the oil vapours pass through the hotbed of catalyst, there is also clear evidence for catalytic reactions occurring. Forexample, there is a marked decrease in oil yield and consequent increase in gasyield at a much higher level than would be expected with mere thermal cracking.Also, the nitrogen and sulphur contents are markedly reduced in the catalysedoil, if thermal cracking reactions were dominant, it would be expected that theheterocyclic compounds would be selectively concentrated in the oil and wouldshow an increase rather than a decrease. Further, zeolite catalysts are known topromote the formation of single ring and two ring aromatic compounds due totheir selective size and shape pore structure. The dramatic increase in single ringand two ring aromatic compounds shown in Table 9 and Table 10 confirm thatcatalytic reactions of the oil with the zeolite catalyst are indeed occurring.


4. Conclusions

Pyrolysis/catalysis of oil shales using a zeolite catalyst has shown that the processis effective in removing nitrogen and sulphur compounds from shale oils. Thequality of the oil is improved by reducing the higher molecular weight material,reducing the alkene content and increasing the aromatic content of the oils. The gasyield consisting mainly of carbon dioxide and alkane and alkene gases wassignificantly increased after catalysis compared to the uncatalysed pyrolysis. How-ever, a disadvantage of the pyrolysis/catalysis process is the significant formation ofcoke on the catalyst and the reduced yield of oil compared to pyrolysis with nocatalysis. The reductions in total nitrogen and sulphur contents in the oils werereflected in the reduced concentration of nitrogen and sulphur containing aromatichydrocarbons. The influence of longer VRTs was to increase the formation ofaromatic hydrocarbons and reduce the nitrogen and sulphur compounds. Theinfluence of catalyst regeneration, involving five regenerations was not significanton the yield and composition of the derived catalytically upgraded oils.

Acknowledgements

The authors would like to acknowledge the support of the University of Leedstechnical staff, Peter Thompson, Chris Brear, Ed Woodhouse and Rod Holt. Theaward of a Pakistan Government Scholarship to H. Chisti is also gratefullyacknowledged.

References

[1] P.L. Russell, Oil Shales of the World, Their Origin, Occurrence and Exploitation, Pergamon Press,Oxford, 1990.

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influence of residence time and catalyst regeneration on the pyrolysis–zeolite catalysis of oil...

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