Pyrolysis of petroleum asphaltene in tetralin

Pyrolysis of petroleum Mahmood

F. Al-Samarraie

asphaltene

and William

in tetralin

Steedman*

Military Technical College, PO Box 478, Baghdad, Iraq *Department of Chemistry, Heriot- Watt University, Edinburgh (Received 24 September 1984; revised 25 October 7984)

EH74 4AS,

UK

of a petroleum asphaltene in tetralin donor solvent at 450°C has been examined, and the products characterized by elemental, molecular weight, n.m.r. and g.c.-m.s. analysis. Degradation to mainly lower molecular weight products takes place, the residual asphaltene having a smaller average cluster size, higher aromaticity and a heteroatom content resistant to further reaction. b-bond scission is important in the early stages of reaction. The pyrolysis

(Keywords: petroleum; petroleum asphaltene; pyrolysis; tetralin)

Petroleum asphaltenes, whose principal characteristics have been described1v2, are important in high temperature thermal and catalytic processes. They are generally undesirable, acting for example as coke precursors3 or, because of their heteroatom and metal atom content, as catalyst poisoners. Asphaltenes are composed mainly of hydrocarbon structures and would be expected to degrade thermally at > ~35&4oo”C. In practice, thermal temperatures activity commences at considerably lower temperatures for a variety of asphaltenes from different sources, probably because of weak links such as open chain carbon-sulphur bonds in the structure. Deep seated degradation does not occur, however, until a temperature of ~400°C is reached. These events have been demonstrated for, for example, Athabasca tar sand asphaltenes4 and Venezuelan Boscan asphaltene?. The latter study gives a clear and coherent description of thermally induced changes from volatiles evolution and fusion reactions below 300°C to the completion of coking at 550°C. In recent years numerous studies of the coal liquefaction process have shown that if coal liquids are pyrolysed, usually in the temperature range 37545o”C, in a hydrogen rich environment such as that provided by a hydrogen donor solvent, the coking reaction can be suppressed and the (0 +N + S) content greatly reduced. Very few studies of petroleum asphaltenes in donor solvents have been reported. The hydropyrolysis of the atypical, high sulphur, Athabasca tar sand asphaltenes with the model hydrogen donor solvent tetralin in the low temperature range 195-390°C has been examined and interpreted as a process dominated by sulphur*arbon bond cleavage6. Sulphur and oxygen loss amounted to =40”/, of the total and, as would be expected at these temperatures, coke formation was negligible. More recently, petroleum asphaltenes from a Volga crude have been the subject of hydrogenation experiments using high pressures of hydrogen gas and a catalyst, along with decalin solvent’. In order to make clearer the nature of donor solvent-asphaltene interactions a study has been carried out of the pyrolysis of a petroleum asphaltene in tetralin in an inert atmosphere. The results are reported below.

OOM-2361/85/070941~3%3.00 0 1985 Butterworth & Co. (Publishers) Ltd

EXPERIMENTAL The asphaltene used in this work was extracted from a Venezuelan bitumen (50 penetration) and donated by Esso Petroleum Co., Ltd, Abingdon, UK. The isolation procedure was carried out according to the IP method 143/78. The asphaltene yield using n-heptane as precipitant was lOwt%. Solvents were distilled before use. Acridine, carbazole, benzothiophene and dibenzothiophene were used as supplied. Hydropyrolyses were carried out in stainless steel tubes as described previously’, the charge being asphaltene (1.5g) and freshly distilled tetralin (5g). The pyrolysis temperature was 450°C. The fractions isolated after pyrolysis and work-up were designated maltenes (heptane soluble), asphaltenes (toluene soluble), carbenes (pyridine soluble) and coke (pyridine insoluble). Elemental analyses were obtained from Butterworth Laboratories, Teddington, UK. Molecular weights were obtained from the University of Strathclyde, and were determined by vapour pressure osmometry (v.p.0.) by extrapolating the v.p.0. data to infinite dilution. Tetrahydrofuran was used as solvent. G.c.-m.s. analysis was carried out by Dr R. G. S. Ritchie at the University of Calgary, using an OV 101 capillary column coupled to a VG micromass 707OF with data station. N.m.r. spectra were recorded on a Bruker WP200 instrument, with deuteriochloroform solvent. RESULTS

AND DISCUSSION

The yields of the various fractions obtained in a series of experiments in which the reaction temperature was 450°C and reaction time was varied from 20 to 300 min are shown in Table 1. The material unaccounted for and tabulated as ‘loss’ is ascribed mainly to gaseous products, which were not determined. The main feature of the product distribution is that asphaltenes were rapidly converted to maltenes and volatiles in the early stages of reaction, with only minor quantities of carbenes and coke being formed. At z 1 h reaction time the formation of maltenes and volatiles slowed considerably, and over extended reaction times

FUEL, 1985, Vol 64, July

941

Pyrolysis Table 1

of petroleum Product

Reaction

asphaltene:

M. F. Al-Samarraie

yields (wt%)

time (min)

20 40 60 80 100 120 150 180 240 300

Maltenes

Asphaltenes

Carbenes

Coke

Loss

18.7 17.3 21.3 23.3 29.3 30.0 22.7 26.7 30.7 26.0

56.0 54.0 27.3 15.3 9.3 8.7 16.7 14.0 14.0 14.0

2.1 5.7 6.3 7.3 5.6 10.7 10.5 12.7 13.3 5.1

6.0 6.2 12.4 13.7 18.0 13.5 4.1 4.9 7.7 11.3

17.2 16.8 32.7 40.4 37.7 37.1 46.0 41.7 34.3 43.0

approached a limiting value of 25-30% for maltenes. Concomitant increases in carbene and coke formation were recorded. These data indicate that petroleum asphaltenes are converted in part to maltenes and volatile products in the presence of a donor solvent alone. The molecular weight data (Table2) indicate a depolymerization of the asphaltene structure, followed by hydrogen abstraction from the donor solvent to give stable products. The yields in Table 1 show that recombination of primary fragments to higher molecular weight products is largely suppressed in the presence of the donor solvent, the yield of coke in particular being very much less than that reported for the pyrolysis of asphaltenes5. Proton magnetic resonance analysis proved very useful in identifying the main structrual changes occurring. The parameters, obtained by Speight’s method’, are listed in Table3. The H, (hydrogen aromaticity) and L (carbon aromaticity) figures indicate an increasingly aromatic character in the residual asphaltenes, which is particularly marked in the first hour. The fall in the average number of aromatic rings, R,, per asphaltene molecule is interpreted as being due to the breaking of labile bridges between polycondensed aromatic clusters in the original asphaltene. The final average cluster size of z 6-7 rings is seen from Table3 to be a limiting value and points to a Table 3

‘H n.m.r. analysis

of residual

Reaction

time (mm)

Reaction

0 20 40 60 80 100 180 300

Elemental

analyses

Table 2

Variation

Reaction

time (min)

time (min)

0 20 40 80 120 180 300

temperature

of residual

molecular

weight

Molecular

weight

2750 1550 730 610 450 420 400 490 520 450 490

structure resistant to further hydropyrolysis under the conditions used. The rapid increase in H,, the benzylic hydrogen parameter, indicated that C-C (or C-heteroatom) bonds /I to an aromatic ring are especially labile, resulting in the formation of resonance stabilized benzylic species. It is clear from these results that /?-bond scission is a dominant process in the early stages of degradation. Later reactions must involve stronger bonds, such as bonds a to the aromatic ring or aromatic C-H bonds. It is interesting to note in this context that the bond dissociation energy of the CC /?-bond in ethyl benzene is z 293 kJ mol- ‘, and

(“C)

H,

H,

fa

%

0.13 0.18 0.24 0.35 0.34 0.43 0.49 0.46

0.12 0.24 0.20 0.22 0.20 0.24 0.24 0.19

0.62 0.66 0.72 0.74 0.77 0.77 0.81 0.82

43 21.5 12.3 7.6 8.0 5.1 1.5 7.5

asphaltenes composition

(wt”/,)

C

H

N

0”

s

84.0 80.4 85.2 81.3 86.4 87.9 86.4

6.8 6.1 5.9 5.8 5.7 5.6 5.2

1.8 2.4 3.3 2.4 2.9 3.5 2.9

3.7 7.9 2.1 0.9 2.4 0.5 2.4

3.7 3.2 3.5 3.6 2.6 2.5 3.1

a By diff.

942

asphaltene

0 20 40 60 80 100 120 150 180 240 300

Elemental Reaction

of residual

asphaltenes

450 450 450 450 450 450 450

Table 4

and W. Steedman

FUEL, 1985, Vol 64, July

Pyrolysis of petroleum asphaltene: M. F. Al-Samarraie

that of the C-C a-bond in toluene is % 389 kJ mol- ‘. The elemental analyses (Table 4) show that a significant proportion of the initial heteroatom content remains in the residual asphaltene. This must be present in structures which in the first instance are resistant to homolytic fission, and are most likely to be heteroaromatic rings imbedded in the aromatic clusters. This thesis was tested by subjecting model compounds to hydropyrolysis in tetralin. The results (7We.Y) demonstrate the considerable resistance of heterocyclic nitrogen and sulphur to degradation in donor solvents. Regarding sulphur, these findings confirm those of Ignasiak and Strausz6, who reported that while the more labile sulphur structures in Athabasca asphaltene degraded in tetralin at temperatures <4Oo”C, thiophenic nuclei were very resistant to attack. G.C.-m.s. analysis of the maltene fraction obtained after 4 h reaction at 450°C revealed the presence of tetralin,

Table 5

Pyrolysis

Model

of model compounds

in tetralin

Structure

Recovered

Benzothiophene

70

Dibenzothiophene

cJr$@

95

Acridine

m

9o

Carbazole

QI$Ql

93

ti ’ After 4 h, 450°C;

1:l wt ratio tetralin

to model

(wt%)

and W. Steedman

dihydronaphthalene, naphthalene, xylene, ethyl benzene, indane and methyl indanes, all of which were present in blank solvent runs. The hydropyrolysate also contained a series of n-alkanes up to C,, (n-heptadecane), branched alkanes and corresponding alkenes. Aromatics present methylated) alkylated (particularly included naphthalenes, biphenyls and bibenzyls. A number of these products could be readily ascribed to a /?-bond scission process, others to the thermal scrambling inevitable in such a complex pyrolysis system. No heteroatom species was observed in the maltene fraction. The above results can be summarized thus: 1. asphaltenes readily ‘depolymerize’ in tetralin ; 2. the residual asphaltene has a reduced average aromatic cluster size of % 6-7 rings, and is more aromatic than its precursor; 3. /?-bond scission is a dominant process in the early stages of reaction; and 4. the residual asphaltene retains a heteroatom content which is highly resistant to further treatment in tetralin. REFERENCES Girdler, R. B. Proc. Assoc. Asphalt Paving Technol. 1965, 34, 45 Corbett, L. W. and Petrossi, U. Ind. Eng. Chem. Prod. Res. Dec. 1978, 17, 342 Kemp, W., Steedman, W. and Stewart, D. in ‘Int. Symp. on Characterization of Heavy Crude Oils and Petroleum Residues’. Lyon, 1984, p. 433 Ritchie, R. G. S., Roche, R. S. and Steedman, W. Fuel 1979,58, 523 Cotte, E. A. and Calderon, J. L. Reo. Tee. Inteuep. 1981, 1, 109 Ignasiak, T. M. and Strausz, 0. P. Fuel 1978, 57, 617 Sebor, G., Reynoso, S., Hajek, M., Weisser, 0. and Mostecky, J. Coil. Czech. Chem. Comm. 1981, 46, 409 Grigson, S. J. W., Kemp, W., Ludgate, P. R. and Steedman, W. Fuel 1983,62, 695 Speight, J. G. Fuel 1971, 50, 102

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