Stability-related tests on visbreaking residues obtained at increasing severity

Stability-related tests on visbreaking residues obtained at increasing severity Tiziana

Zerlia

and Giacomo

Pinelli

Stazione Sperimentale per i Combustibili, (Received 2 1 September 7992)

20097

San Donato

Milanese,

Italy

Xylene-equivalent (XE) and sediment by hot filtration (‘Sediment’) are often used to characterize instability of petroleum thermal residues. Nine visbreaking (VB) residues obtained at increasing severity from the same feedstock were studied to explain the significance of these characteristics in the light of the chemical changes induced by the VB process. The results indicate two different severity-related relations between XE (and Sediment) and chemical composition, at low and high severity: under mild process conditions the instability (XE) increases as the asphaltene content decreases, whereas the opposite trend is found at high severity. These opposing trends can be explained in the light of the VB chemistry and shed light on the different mechanisms governing the peptization state of asphaltenes under mild and severe process conditions. Sediment, constant at low severity, starts to increase only when condensation reactions involving asphaltenes occur. Thus Sediment, which unlike XE cannot be adjusted with a suitable diluent fuel, plays a key role in evaluation of the stability of thermal residues: high Sediment, in the absence of inorganic or coke-like material, would be concomitant with storage problems, whatever the XE level.

(Keywords: visbreaking; specific gravity; viscosity)

Visbreaking (VB) is considered as a low-cost and mild thermal conversion process for petroleum, often used to increase the yield of distillates at the expense of residuum. In general the process economics improve as visbreaking severity is increased. However, an increase in severity produces progressive changes in chemical composition and in particular modifies the peptization state of the asphaltenes, increasing the instability of the residue. Experience has shown that conventional fuel oil properties (for instance specific gravity, carbon residue, asphaltene content, viscosity) are inadequate to predict instability and/or stability problems, i.e. storage incompatibility on blending with other fuels. Refineries therefore use certain empirical test procedures to predict the stability of residues. However, the choice of suitable tests and of appropriate test values to ensure stability of products are generally defined by individual refineries on the basis of experience acquired with their own products. Thus work is still in progress, even though it dates back some 50 years’ and although research has improved the significance of the empirical tests2s3. The xylene-equivalent (XE) and sediment by hot filtration (‘Sediment’) are two of the characteristics in use. When the severity of the VB process increases, parallel increases in XE and in Sediment are expected. Sediment is defined4 as a measure of ‘types of suspended material which may eventually deposit’. Thus it would be a measure of storage stability. XE, which is the percentage of xylene in a mixture of xylene and paraffinic solvent (generally n-heptane or iso-octane) ‘which just fails to cause flocculation of the colloidal asphaltene system when examined with the fuel oil in a given ratio”, is a measure of the ‘peptization state of asphaltene’. Thus it would be 001~2361/93/08/1109%06 c 1993 Butterworth-Heinemann

Ltd.

related to the miscibility characteristics of the residue, i.e. to the aromatic character required of a diluent to prevent asphaltene flocculation. To account for the significance of the above stability tests (XE and Sediment) in the light of the chemical changes induced by the VB process, nine VB residues obtained at increasing severity were examined in the present work.

EXPERIMENTAL Residues

and characterization

The nine VB residues examined were obtained from the same soaker-type VB unit processing the same feedstock (a straight-run residue) at increasing severity. Ultraviolet spectra of the residues were obtained on samples dissolved in CHCl, at a concentration of - 30 mg l- I. A rapid spectral data-processing system (details of which are given elsewhere5) provided the following parameters: area = total area between 250 and 450 nm areas in the ranges R 2uw Rl”” and ASuv=percentage 2563 18, 3 18-344 and 344-450 nm respectively K,, = R2uv/UL, + &J a, = area/c, where c is the sample concentration

(mg l- ‘)

Molecular weights were determined in toluene by vapour pressure osmometry. Standard methods were used for determining asphaltenes (IP-143) and toluene-insolubles (ASTM D893) and for separating saturates, aromatics and resins (ASTM D2007).

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LECO 600 and LECO SC 132 analysers were used for C-H-N and S determinations respectively. Stability tests Sediment by hot filtration was determined according to AM-S 77-065, and XE according to Broom4. Iso-octane was preferred to the ‘Oliensis solvent’ as suggested by Butlin6. XE indicates the percentage of xylene in the solvent (iso-octaneexylene) which gives no nuclear ring when a drop of the sample-solvent mixture is placed on a filter paper (sample/solvent ratio 2:lO v/v). Since it is known that the reproducibility of the XE test is rather poor4 (f 10 XE units), the following procedure was adopted. A rough value of XE was obtained from an initial examination of the sample-solvent mixture, using solvents of progressively increasing xylene content in 10% increments. The procedure was then repeated in the XE range encompassing the rough XE point, using solvents of increasing xylene content in 1% increments. For the VB residues examined, a repeatability (same operator, same experimental conditions) of f 1 XE units was obtained. Data program The PLS algorithm included in the SIMCA/MACUP chemometric software package7 for IBM-compatible computers was used to derive the models presented here. In SIMCA, the model dimensions, i.e. the number of principal components of the model, are derived by an iterative process using a cross-validation method*. To ensure significative model dimensions, this iterative procedure was stopped when the standard error of prediction (SEP)9, i.e. the equivalent of one standard deviation of the predicted error, was found to be comparable with the typical error found in the experimental determination of the parameters concerned (XE and Sediment, used as dependent variables in developing the PLS models). (SEP = (c e,/N)0.5, where each ei represents the difference between the experimental and the predicted value for the ith sample of the PLS model.) The models presented in the paper are ‘refined’ models, i.e. they were obtained by the following procedure: the weight of each variable, obtained after a first run of the PLS program, was multiplied by the modelling (or explanatory) power lo-l2 of the pertinent variable. Thus the ‘corrected’ weights (which take into account the relative relevance of variables) were obtained. Then the PLS program was run again (with the ‘corrected’ weights) and the ‘refined’ model was obtained. RESULTS

AND

DISCUSSION

XE and Sediment values of the nine VB residues are reported in Table 1. The increase in severity is consistent with the XE increase from sample 1 to 9. Initial screening of the samples was performed by U.V. spectrometry, which provides rapid qualitative evaluation of petroleum products5. The U.V. parameters reported in Table 1 were obtained from the spectra of the samples according to the procedure developed in ref. 5. The meaning of these parameters is briefly summarized as follows. The R zuVparameter is mainly related to the U.V. absorption of aromatics and resins, while Rluv and ASuv mainly concern the absorption of asphaltenic-type compounds; S,, is a measure of the ratio of the ‘light’ to the ‘heavy’ aromatic fractions of a petroleum product.

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Table 1

U.V. parameters

and stability

data of visbreaking

residues

Column:

1

2

3

4

5

6

Sample

L

R,,,

L,

As,,

a,

;:I

7 Sediment W)

1 2 3 4 5 6 7 8 9

2.0 2.0 2.0 2.0 2.2 2.3 2.2 2.2 2.0

66.1 66.7 66.8 68.2 68.7 69.4 69.1 68.4 68.4

12.6 12.6 12.6 12.5 13.0 12.9 12.8 12.9 13.5

20.6 20.5 20.3 19.4 18.3 17.7 18.1 18.8 20.4

2.47 2.43 2.33 2.22 2.69 2.71 2.66 2.92 2.99

44 46 48 50 62 64 66 68 72

0.02 0.02 0.02 0.01 0.06 0.06 0.08 0.10 0.14

The a, parameter is related to the density: the heavier the product, the higher is a,. Thus it can be expected that the U.V. parameters, related to the composition, allow an initial estimate to be made of the chemical differences between the samples. A visual inspection of the U.V. parameters reported in Table 1 shows some difference between two sets of samples: l-4 and 5-9. The mean values of Rzuv are higher for samples 5-9 than for 1-4, while (RI,, + As,,) is higher for samples l-4 than for 5-9. S,, is constant for samples l-4. Finally, a, is lower for samples l-4 than for 5-9. These differences suggest that samples l-4, having a higher content of asphaltenic-type material (related to R luv and As,,) and a lower density (related to a,) than samples 5-9, contain more saturates. Samples l-4 will be indicated in the following as set L (low XE and Sediment values), and samples 5-9 as set H (high XE and Sediment values). Since the higher are XE and Sediment the greater is the instability, it can be immediately observed that the conclusion drawn from the U.V. data does not agree well with the predicted stability of petroleum products: because of the characteristics of asphaltenes, which are soluble in aromatic but insoluble in paraffinic solvents13, a higher asphaltenes and saturates content could be expected to result in a lower stability. To investigate this in more detail and to derive a stability-related U.V. trend within each sample set (L and H), the method of partial-least-squares (PLS) modelling in latent variables’2,‘4~‘5, already used to predict properties of petroleum products from U.V.data16’17, was utilized. The PLS method provides a mathematical model that accounts for the relation between one or several dependent parameters (y-data block) and a number of potential explanatory parameters (x-data block) characterizing a set of similar samples. Moreover, it allows an immediate picture of the relation between the parameters concerned to be derived and provides a more rigorous criterion (cross-validation*) than a subjective evaluation. Here the U.V. parameters l-5 (Table I) are used as the x-block and XE as dependent variable (Y). (Sediment data are treated later, since they exhibit a constant value in set L.) The application of PLS modelling to set L (for u.v.-XE data, columns l-5 and 6 of Table I respectively) gives a one-principal-component (PC) model explaining - 80% of the y-variance (SEP= 1.4 XE units). The same procedure applied to set H gives a l-PC model which accounts for -66% of the y-variance (SEP= 1.1). Figure I shows the ‘variable loadings’ within each class model, which indicate how much each variable is ‘loaded

Stability-related

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XE R2uv

-0.5 1

2

3 Variable

4

5

Column:

Sample

Principal 8

characteristics 9

MW

Asphaltene MW

I

I

I

I

I

I

6

1

2

3

4

5

6

Variable

number

number

Figure 1 Relevance (‘loadings’) of the U.Y. parameters on abscissa correspond to columns of Table I

Table 2

SUV

I

(columns

l-5, Tnbfe I)versus XE in the low-severity

(L) and high-severity

of visbreaking

residues

10

11

12

13

14

15 16 Atomic ratios

17

18

Saturates W)

Aromatics W)

Resins W)

Asphaltenes WI

H/C

N/C

OjCQ

SJC

Toluene-insolubles (“/I

(H) classes. Numbers

19 20 21 22 Atomic ratios of asphaltenes H/C

N/C

o/c

S/C

1

600

3500

28.9

40.0

18.9

8.1

1.50

0.004

0.009

0.014

0.01

1.06

0.017

0.011

0.024

2

600

3500

28.7

40.6

18.5

7.1

n.d.b

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

3

550

2650

29.4

42.0

16.7

7.2

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

4

560

2900

35.5

41.7

11.3

6.2

1.56

0.004

0.005

0.012

0.02

1.08

0.017

0.008

0.020

5

510

2150

26.6

35.0

28.6

7.7

1.45

0.004

0.005

0.019

0.06

1.03

0.013

0.058

0.039

6

500

2300

28.6

35.5

28.1

7.5

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

7

510

2300

24.7

37.5

28.6

7.7

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

8

530

2200

18.4

37.0

33.3

9.0

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

9

580

3150

22.6

31.0

33.3

10.9

1.42

0.003

0.004

0.025

0.06

0.98

0.010

0.099

0.050

“0 by difference b Not determined

into’ the class model”, i.e. from a practical viewpoint they can be considered as measures of the importance of the contribution of each variable in describing the model. Thus Figure I depicts the relative ‘intensity’ of the chemical changes in explaining the XE data for the L and H classes. Comparison of the results for the L and H classes provides more details about the U.V. trend: the relations between the U.V. variables and XE values are in opposite senses. In particular, as XE increases, As,,” and Rluv increase in the H class but decrease in the L class. (As expected, S,,, being constant, has no relevance in the L class.) This signifies that, while in the H class an increase in instability is related, as generally expected, to a rise in asphaltene content (or in the content of asphaltenic-type material), in the L class an increase in the instability is related to a fall in asphaltene content. Nevertheless, although these findings confirm the unexpected result mentioned earlier, they do not explain it. To obtain supporting experimental evidence for the

u.v.-derived result, samples 1-9 were separated into aromatics, resins, saturates and asphaltenes. The results are reported in Table 2 together with the molecular weights (MW) of the samples and asphaltenes. Visual inspection of these data provides an impression consistent with the finding of opposite trends from U.V. data: as XE increases (instability increases), in the L class, asphaltenes decrease, whereas in the H class, they increase. To derive a relation between chemical composition and stability data, the PLS method was also applied to the data in Table 2. Obviously parameters 8-13 of Table 2 are now taken as the x-variables block (potential explanatory parameters) and XE (column 6, Table 2) as the dependent variable. For the L class a l-PC model was obtained, with 94% of the y-variance explained (SEP=O.Z). The H class gave a l-PC model explaining -80% of the y-variance (SEP = 1.1). The variable loadings obtained from the PLS analysis are reported in Figure 2. The results, besides confirming the opposing trends

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residues: T. Zerlia and G. Pine/i 1 .o,

I

H class

L class

AsphalteneS

XE

XE

Aromatics

0 ‘I

Aromatics

-0.5

-0.5

L

I

I

I

I

I

I

I

8

9

10

11

12

13

6

Variable

components to columns

separated from VB residues of Tables I and 2

revealed by PLS of the U.V. parameters, provide more detailed information: an instability increase in the L class is consistent with a fall in both asphaltene and resin contents (parallel to the decrease in asphaltene MW). In the H class the opposite trend is observed. It can be seen that a change in asphaltene content plays a more marked role in the H class than in the L class. It is worth noting, however, that since U.V.data can be obtained in about half an hour, whereas a few days are required for the conventional separation procedure, the U.V. method actually provides a rapid tool for screening purposes. It is now possible to interpret the results in the light of general knowledge about VB chemistry’8-22. In the L class the decrease in asphaltene and resin contents (together with the fall in asphaltene MW) is consistent with cracking reactions involving asphaltenes and resins. The decomposition of these materials starts23,24 at -3O&350°C and is matched by the decrease in asphaltene MW 25. As the decrease in resin and asphaltene contents is paralleled by an increase in aromatics, it is reasonable to suppose that resins and asphaltenes crack to give small fragments composed of a few aromatic rings, in such a way that the core of the resins and asphaltenes is maintained. With increasing severity (H class) the increase in asphaltenes (in parallel with their MW as well as the MW of the residue) and in resins is consistent with the existence of condensation reactions which produce fresh and heavier asphaltenes and resins. Although the above considerations about the chemical changes and XE data are consistent with the general mechanism claimed to explain the VB process, they do not in fact immediately account for the opposing trends with respect to stability (XE) observed in the two classes.

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I 10

I 11

Variable

number

Figure 2 Relevance (‘loadings’) of main chemical L and H classes. Numbers on abscissa correspond

1112

I 8

8

and residue and asphaltene

I

I

I

12

13

6

number

MW versus XE within the

To account for this feature, further observations must be made. In general the stability of petroleum products (at least, that of virgin and straight-run products) is considered to be a consequence of the peptization state of the asphaltenes26,27, which in turn would be governed by the peptizing capacity of the resins. Two main mechanisms are proposed to explain the peptizing power of resins: cosolvency and ‘special interaction’27. The former is attributed to the increase in the ‘oil’ polarity (‘oil’= saturates plus aromatics) by resins which improve the ‘solubility’ of asphaltenes in the medium, and the latter to specific and selective interaction between resins and asphaltenes (probably involving hydrogen bonds28,2g) wh’ic h would be effective only between the original resins and asphaltenes (i.e. between resins and asphaltenes from the same crude). Now both mechanisms suggest that the instability should be related, roughly, to the asphaltenes/resins ratio (I,). Moreover, to account for the fact that in a thermal product the asphaltenes-resins equilibrium would be modified and the asphaltenes could directly interact with the ‘oil’ also, the instability should also be related to I, = [(asphaltenes + saturates)/(resins + aromatics)], on consideration of the different behaviour of asphaltenes in aromatic and paraffinic solvents and of the presumed peptizing power of resins. I, and I, roughly account for the decrease in stability (XE) within each class. In fact in the L class, 0.43
Stability-related

I1 10

a

0' 42

1

Y

1

I

I

1

I

46

50

54

58

62

66

70

;;; 1& - i’

'0 74

XE Figure 3 Reversal of the stability-related trend of the asphaltenes, their heteroatoms and Sediment from the L class (XE 44-50, samples 14) to the H class (XE 62-72, samples 5-9)

Thus it has to be concluded that in the L class some particular interaction exists between resins and asphaltenes which confers greater stability and a distinctive stability-related trend, compared with the H class. These findings shed light on the above-mentioned mechanism of action of resins on asphaltenes: the main peptizing capacity of resins must be attributed to the ‘specific features’ which only the original resins and asphaltenes still existing in the mild stages of VB (L class) can provide. The supposition that the above interaction with the is due to hydrogen bonds 28,29 is consistent decrease in heteroatom content of the asphaltenes. It can be seen from Figure 3 and Table 2 that (O/C),, (as well as (S/C),,) decreases from sample 1 to 4 (L class). Thus the decrease in asphaltene content, from which an increase in stability could be expected, provides less favourable conditions for peptization of the asphaltenes because of the progressive breakdown of the special linkages between resins and asphaltenes: although asphaltenes decrease, there is a parallel decrease in resins and above all in their peptizing power. The opposing heteroatom trends, paralleling the opposing asphaltene trends, in the two sets (Figure 3), suggests a severity-dependent evolution of asphaltenes from a state mainly governed by a true peptization mechanism to one influenced by the overall solubility properties of the medium (aromatics plus resins plus saturates). The reversal of the trend probably starts after the original linkages between resins and asphaltenes have been destroyed ~ a point roughly corresponding to the minimum heteroatom content. In the H class the XE-stability dependence would be governed by the solubility properties of the medium, which, besides being affected by the higher resins content, suffer from the more polar character of the asphaltenes (the heteroatom content increases from sample 5 to 9, Figure 3) as well as from their heavier character as indicated by the progressive increase in MW and by an accompanying decrease in H/C atomic ratio (Table 2).

tests on visbreaking residues:

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The original resins and asphaltenes also seem to protect against the condensation reactions which appear to start just after the specific interactions cease, as is suggested by the reversal of the trend in heteroatom content from the L class to the H class (Figure 3). It may be noted that, in the light of the above findings, the stability of the L class could be expressed by the I, ratio and that of the H class by the I, ratio, the values of which give a rough measure of instability over the whole range of samples. The problem of the relation between the chemical composition changes and Sediment can be faced in the same way as for XE. PLS modelling on the H set (x-variables: columns 8-13, Table 2; y-variable: column 7, Table I) shows that the dependence of Sediment on the chemical changes induced by VB with increasing severity is extremely similar to that for XE in class H. No attempt has been made to derive a PLS model for set L where Sediment is constant. Quite similar results to those for the u.v.-XE relation were obtained from the U.V. data for the H class. A l-PC model explaining -94% of the y-variance with SEP =O.Ol% Sediment content was obtained for the relation between Sediment and chemical composition, and a 1-PC model with 85 % Sediment variance explained and SEP = 0.01% for the u.v.Sediment relation. Thus the different Sediment trends (constant in the L class, related to the chemical changes in the H class, Figure 3) is consistent with the definition of Sediment. In fact, as already seen, in the L class the specific resin-asphaltene interactions are still present and provide well-peptized asphaltenes exhibiting no great tendency to form deposits. In the H class, as already stated, the progressive condensation reactions provide a decrease in the H/C atomic ratio of the asphaltenes (as well as the residue), accompanying an increase in their MW, which indicates a trend towards a coke-like structure, insoluble whatever the surrounding medium. This tendency is also consistent with the increase in toluene-insolubles (related to coke-like material) from the L to the H class (Table 2). The severity-dependence of Sediment is also consistent with Sediment data obtained on straight-run residues, for which values falling below the Sediment determination limit (0.01%) have been found3’.

CONCLUSIONS The chemical changes induced by the visbreaking process in residues derived from the same feedstock at increasing severity reveal two different relations between chemical composition and stability data (XE and Sediment) at low and high severity. These opposing severity-related trends shed light on the mechanism which governs the peptization state of asphaltenes: at mild severity the main mechanism results from specific interactions between the original resins and asphaltenes, which still exist under these mild VB conditions. An increase in severity first causes a progressive reduction in the asphaltenes heteroatom content involved in the specific interactions with resins, thus providing a progressive direct interaction between asphaltenes and the surrounding medium. It then produces ‘new’ asphaltenes by condensation reactions in such a way that the state of the asphaltenes (the residual now modified original and above all the freshly formed asphaltenes) mainly depends on the solubility

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characteristics of the overall medium, as well as on the increase in their polar character and their lower H/C atomic ratio. These observations add a further element to the evaluation of ‘stability problems’: it can be expected that residues where the strong peptizing power of the original resins is still present (i.e. at mild severity) would in most instances have potential stability problems. Residues with characteristics similar to those observed at high severity would show both instability and miscibility problems, the latter being probably more severe than for the mild-severity products. However, a caveat must be entered. It has been shown that the different XE-related trends with respect to the chemical changes at low and high severity strictly depend on the key role of the original resins and asphaltenes, which in turn depends on the feedstock. It is quite probable that different feedstocks, with different contents and characteristics of resins and asphaltenes, will give different XE values in spite of identical VB process conditions. This means that the absolute XE values found here cannot be directly used for a general classification of the quality of thermal residues. Nevertheless a general conclusion can be drawn: it has been observed that the increase in Sediment begins just after condensation reactions start. Thus Sediment values (20.02) would indicate, in the absence of inorganic and coked materials, that the asphaltenes tend towards a coke-like structure. Thus, although the XE data of unknown samples can be properly improved by suitable aromatic diluents, Sediment, which is just related to the coke-like structural characteristics of asphaltenes, cannot be adjusted. As a consequence, Sediment plays a key role in the stability evaluation of unknown samples, since residues exhibiting Sediment values k 0.02 indicate, whatever the XE value, intrinsic instability of the asphaltenes. Thus storage problems, and the higher the Sediment value the more severe the problems, will have to be taken into account.

Industriale (Minister0 Industria Commercio e Artigianato, Rome) for financial support. REFERENCES 1 2

3

4 5 6 I

8 9 10 11 12

13 14 15

16 17 18 19 20 21 22 23 24 25 26 21

ACKNOWLEDGEMENTS

28

The authors thank Cameli Petroli SpA (Mantova, Italy) for VB samples and Direzione Generale Produzione

29 30

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Martin, C. W. G. and Bailey, D. R. J. Inst. Pet. 1954, 40, 138 Anderson, R. P., Brinkman, D. W., Goetzinger, J. W. and Reynolds, J. W. In Proc. 2nd International Conference on Long-Term Storage Stabilities of Liquid Fuels (ed. L. L. Stavinoha), 1986, p, 25 Por, N., Brauch, R. and Brodsky, N. In Proc. 2nd International Conference on Long-Term Storage Stabilities of Liquid Fuels (ed. L. L. Stavinoha), 1986, p. 179 Broom, W. E. J. J. Inst. Pet. 1945, 31, 347 Zerlia, T., Pinelli, G., Zaghi, M. and Frignani, S. Fuel 1990, 69, 1381 Butlin, D. G. J. Inst. Pet. 1953, 36, 294 Wold, S. and Sjostrom, M. In ‘Chemometrics: Theory and Application’ (ed. B. R. Kowalski), Am Chem. Sot. Symp. Ser. 52, 1977, p. 243 Weld, S. Technometrics 1978, 20, 397 Haaland, D. M. Anal. Chem. 1988,60, 1208 Sharaf, M. A., Illmann, D. L. and Kowalsky, B. R. ‘Chemometrics’, Wiley, New York, 1986, p. 242 Weld, S. Pattern Recognition 1975, 8, 127 Weld, S., Albano, C., Dunn, W., Ebsensen, K., Hellberg, S., Johansson, E. and Sjostrom, M. In ‘Food Research and Data Analysis’ (ed. H. Martens and H. Russwurm Jr), Applied Science Publishers, London, 1983, p. 20 Mitchell, D. L. and Speight, J. G. Fuel 1973, 52, 149 Weld, H. In ‘Multivariate Analysis’ (ed. P. R. Krishnaiak), Academic Press, New York, 1966, p. 391 Wold, H. In ‘Systems under Indirect Observation’ (ed. K. G. Joreskog and H. Wold), Vol. 2, North Holland, Amsterdam, 1982, p. 1 Zerlia, T. and Pinelli, G. Fuel 1992, 71, 559 Zerlia, T. and Pinelli, G. Fuel (in press) Hus, M. Oil Gas J. 1981, (Apr. 13), 109 Gadda, L. Oil Gas J. 1982, (Oct. 18), 120 Favre, A. and Boulet, R. Rev. Inst. Fr. Pet. 1984, 39, 485 Favre, A., Boulet, R. and Behar, F. Rev. Inst. Fr. Pet. 1985,40,609 Le Page, J. F. and Davidson, M. Rev. Inst. Fr. Pet. 1986,41,131 Moschopedis, S. E., Parkash, S. and Speight, J. G. Fuel 1978, 57, 43 1 Moschopedis, S. E., Parkash, S. and Speight, J. G. Fuel 1980, 59, 64 Hall, G., Herron, S. P. Am. Chem. Sot. Div. Pet. Chem. Preprints 1975,54, 17 Tissot, B. Rev. Inst. Fr. Pet. 1981, 36, 1981 Speight, J. G., Wernick, D. L., Gould, K. A., Overfield, R. E., Rao, B. M. and Savage, D. W. Rea. Inst. Fr. Pet. 1985, 40, 58 Moschopedis, S. E., Fryer, J. F. and Speight, J. G. Fuel 1976, 55, 184 Moschopedis, S. E. and Speight, J. G. Fuel 1976, 55, 187 Zerlia, T. and Pinelli, G. Riu. Cornbust. 1988,42, 145