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Model for the coal hydrogenation process and its applications
Model for the coal hydrogenation and its applications Michio
Shibaoka
and Sammy
process
Heng
CSIRO Division of Fossil Fuels, PO Box 136, North Ryde, NS W 2 113, Australia (Received 4 March 1983; revised 14 June 1983)
A coal hydrogenation model has been formulated which incorporates both chemical and microscopic experimental data. In this generalized model, carbonization and hydrogenation areviewed asconcurrent processes in the liquefaction of coal. Insufficient hydrogen availability, rapid heating rates and long reaction times at elevated temperatures can promote carbonization reactions. The model describes in detail the reaction pathways involved in the hydrogenation of both inertinite and vitrinite. When vitrinite is hydrogenated in the presence of a hydrogen donor solvent, a plastic material called vitroplast is formed. The vitroplast is either converted to liquid and gaseous products when hydrogen availability is high or becomes mesophase and then semicoke when hydrogen availability is low. Even under favourable hydrogenation conditions, the major reaction pathway in the hydrogenation of inertinite is one of initial mild carbonization followed by hydrogenation. It is evident that the difference in hydrogenation behaviour between vitrinite and inertinite is due, in part, to the ability of the hydrogen donor solvent to penetrate vitrinite but not inertinite particles. The hydrogenation model is useful for explaining various phenomena that occur during hydrogenation, such as the formation of mesophase and semicoke, and the blockage of reactors and preheaters. (Keywords: coal; hydrogenation; models)
Essentially coal hydrogenation is a chemical process. However, it is greatly influenced by the physical attributes of the reactants. At the early stages of the coal hydrogenation process the physical state of coal particles is one of the most important factors affecting its chemistry. Furthermore, changes in the physical state of the coal particles, which can be observed microscopically, provide important information on the coal hydrogenation process. As coal is usually highly heterogeneous at the microscopic level, individual coal particles, or different parts of single coal particles, are expected to show significantly different hydrogenation behaviour. Furthermore, the reactions which occur in the core of a solid coal particle may be quite different from those which occur at its margins in immediate contact with solvent, gas and catalyst. Most chemical techniques do not detect this microscopic differentiation in the hydrogenation process, and only the overall chemical transformations are determined. Therefore it is essential to combine the results from both chemical and microscopic investigations to derive a correct understanding of the hydrogenation process. The hydrogenation model developed in this Paper is based on the results of recent investigations into the hydrogenation behaviour of the vitrinite and inertinite macerals of Australian bituminous coals. These coals are mainly of Permian and Triassic ages and therefore differ from the common Carboniferous coals of the Northern Hemisphere in several respects, especially their generally higher content of low-reflectance inertinite mace&. Investigations in this laboratory have shown that the low-reflectance inertinite macerals, particularly semifusinite, can make significant contributions to the yields of liquid and gaseous products though with somewhat 0016-2361/84/020174-04$3.00 @ 1984 Butterworth & Co. (Publishers)
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greater difficulty than the accompanying vitrinite. High reflectance inertinite in Australian coals is as unconvertible as that of Northern Hemisphere Carboniferous coals. The model presented in this Paper sets out the different behaviour patterns of the vitrinite and inertinite macerals and explains them in terms of the interaction between two concurrent processes, carbonization and hydrogenation192. In this Paper carbonized material derived directly from unhydrogenated coal is defined as primary semi-coke whereas that derived from mesophase and partiallyhydrogenated anisotropic inertinite is defined as secondary semi-coke l*’ . Primary semi-coke derived from vitrinite is either isotropic or exhibits a line-grained anisotropic texture, depending on the rank of the original coal. Primary semi-coke derived from inertinite is usually isotropic while secondary semi-coke usually exhibits a medium- to coarse-grained anisotropic mosaic texture. Many semi-cokes, which are of intermediate form, in terms of genesis and optical properties, also occur. HYDROGENATION
MODEL
The model describing the overall coal hydrogenation process using hydrogen donor solvents is illustrated in Figure 1.
When vitrinite is heated in tetralin, highly-gelilied vitrinite macerals (e.g. collinite) gradually become plastic (2+3 in Figure I), while poorly-gelitied vitrinite macerals (e.g. telinite) remain as fibrous materia13. The plastic material is called vitroplast4 while the fibrous material is called vitrofibre. The lower molecular weight compounds formed during vitroplast hydrogenation form liquid and gaseous products (34). As the vitrolibre is usually line,
Model for the coal hydrogenation process: M. Shibaoka and S. Heng 12 (Primory
. ,#5 ._-
( Secondary semicokel
Temperoture (or time) * Model for the coal hydrogenation process with solid; -. -. -, mesophasa; hydrogen donor solvent. -, liquid and gas. Numbers are referred to in ., plastic; -, text
most of it is dissolved and then hydrogenated to liquid and gas relatively quickly. Vitrofibre hydrogenation is not shown in Figure I to avoid complicating the diagram. During the formation of vitroplast (2-3) and vitrofibre and their conversion to liquid and gaseous products (3+4) the rate of carbonization can exceed that of hydrogenation, particularly if the level of hydrogen availability falls’ or if the heating rate is excessively rapid. As a result, mesophase is formed from the vitroplast, and further heating under hydrogen-deficient conditions will bring about the formation of secondary semi-coke (3-5). If hydrogen deficiency occurs at point 2, repolymerization and solid-state molecular reorganization commence and accelerate suddenly when the temperature reaches certain levels (6) to yield primary semi-coke (2&12). Under conditions of good hydrogen availability the hydrogenation and dissolution of very line particles of the low-reflectance inertinite and peripheral parts of larger inertinite particles (6-+7) can occur but at a higher temperature (6) than that for vitrinite. The hydrogenation of high-reflectance inertinite and core parts of the large low-reflectance inertinite particles (849) takes place at higher temperature and is preceded by a slight carbonization reaction (6-+8)6. Under favourable hydrogenation conditions a significantly large part of the low-reflectance inertinite is converted to liquid and gaseous products’ (9+10) while the rest becomes secondary semi-coke (9+11). However, under hydrogen-deficient conditions where the rate of carbonization exceeds the rate of hydrogenation, inertinite becomes primary semi-coke (6 and 8-12). At every bifurcation beyond point 2 in the hydrogenation model (i.e. 3,6,8 and 9), the course of the process depends on both hydrogen availability and the rate of thermal bond-breaking in the coal. EXPERIMENTAL DEVELOPMENT MODEL
BASIS UNDERLYING OF HYDROGENATION
of uitroplast and vitrofibre (2-3 in Figure 1) When small blocks of coal are treated with organic solvents (including tetralin) at their boiling points, exten-
Formation
sive swelling of the vitrinite occurs’. Brenner (1981)’ reported that substantial swelling of vitrinite occurs on exposure to n-propylamine vapour at room temperature. The swelling of vitrinite can be reversed by desorption of the solvents, although in some cases residual deformation of the vitrinite layers may be observed8v9. The extent of swelling of the vitrinite depends on coal rank and the properties of the solvent u~ed~-‘~. The swelling of vitrinite is closely related to changes in its pore structure. The porosity is increased by solvents at elevated temperatures giving the residual coal sample a much larger total surface area than that of the parent coal”. When the vitrinite from a high volatile bituminous coal was treated with tetralin at 335-34O”C, collinite became slightly plastic and large vesicles were formed. Telinite and telinite/collinite were partially dissolved, leaving a network of coaly material which consisted mainly of cell wall-derived material (vitrofibre). It is reasonable to assume that a considerable amount of tetralin is present within the vitrinite particles, filling both the network and submicroscopic pores. While thermal fragmentation of the coal matrix at these temperatures may not be extensive, should this occur and active sites be formed, then they would be rapidly stabilized by the hydrogen from the tetralin within the vitrinite, and consequently the development of plasticity in the vitrinite would be enhanced. The tetralin within the vitrinite particles is most useful at higher temperatures (>4OO”C) when extensive thermal fragmentation of the vitrinite molecular structure begins to take place and stabilization of active sites becomes important 3*14.Thus, the influence of particle size in the hydrogenation of vitrinite must be reduced14v15 when the hydrogen donor solvent can penetrate, react with and disperse the coal substance. Vitroplast is essentially a mixture of asphaltenes and preasphaltenes with inclusions of tetralin and/or naphthalene. Further hydrogenation of vitroplast will depend largely on the diffusion of fresh tetralin into the vitroplast or on the extent of the dissolution of the vitroplast in the tetralin. If reaction temperature is lowered before most of the vitroplast material dissolved in solvent is hydrogenated to oil and gas, this material is precipitated as secondary vitroplast3s14.
Conversion of vitroplast to liquid and gas or mesophase and secondary semi-coke (3-4 or 5 in Figure 1)
The fate of the vitroplast is governed by the outcome of the competition between the polymerizationcarbonization and hydrogenation reactions14. If the heating rate is too rapid and/or the final temperature is too high (e.g. >47o”C), the rate of thermal repolymerization will exceed the rate of hydrogenation even when good hydrogen-donor solvent and catalysts are employed. Consequently, mesophase is often formed in the nucleus of massive vitroplast. Mesophase will form even if the vitroplast is maintained at relatively low temperatures (e.g. 410°C) for long periods of time without sufficient agitation. Essentially mesophase formation can occur at any stage of vitroplast hydrogenation (2+3)16. The conversion of vitroplast to liquid products can be promoted in several ways, such as by employing efficient hydrogen-donor solvents and catalysts, and by improving mass transfer with vigorous agitation.
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Model for the coal hydrogenation process: M. Shibaoka and S. Heng Hydrogenation of inertinite (6-7 and 6-8-9
Figure
1)
The macro- and microporosity of inertinite macerals have not yet been investigated in detail. However, results indicate that inertinite, even low-reflectance inertinite, does not undergo any obvious deformation in the presence of organic solvents at temperatures < 375°C 6. The degree of swelling of the inertinite in response to solvent treatment appears to be negligible when compared with that of vitrinite at the same temperature8. There are large cavities such as cell lumens in inertinite, but the population of micropores and the total surface area generated by these cavities is small when compared to the population of micropores in vitrinite and their total surface area. Therefore, even if tetralin can penetrate the inertinite particles, the quantity of tetralin likely to be retained within the inertinite will be relatively small compared to that in vitrinite. Due to the limited penetration of tetralin into inertinite, the hydrogenation reaction and, thus, dissolution of inertinite, is limited to the external surfaces of the inertinite particles6. In such situations, the rate of hydrogenation at an early stage of the reaction is dependent on the external surface area so that smaller and finely structured inertinite particles are usually converted earlier (67). Because of the limited penetration of tetralin into the inertinite particles, stabilization of active sites formed during fragmentation of the coal structure cannot occur to any significant extent, particularly in the cores of the inertinite particles. Thus, slight repolymerizationcarbonization is likely to occur. Unlike the hydrogenation of vitrinite, the inertinite particles do not become plastic at temperatures < 400°C. Instead, the inertinite undergoes solid-state molecular reorganization during the hydrogenation process, i.e., the limited mobility of the aromatic ring units at these temperatures only permits in-situ reorientation of the aromatic ring units which results in the development of weak local anisotropism. Temperatures >42o”C result in gradual hydrogenation of the inertinite particles, beginning mainly at the margins of the particles. Partly hydrogenated inertinite becomes slightly plastic and develops further anisotropism. The process involved in the development of anisotropism in inertinite during hydrogenation is different from that involved in the development of anisotropism due to mesophase formation in vitroplast. In inertinite, the aromatic ring systems do not have great mobility which permits them to stack together to form spherically shaped mesophase. The mesophase spheres in vitroplast are able to coalesce and migrate within the vitroplast while in inertinite such phenomena do not occur. In the process of mesophase development, the isotropic vitroplast and anisotropic mesophase exist side by side separated by a sharp boundary l’. In inertinite the boundary between anisotropic and isotropic regions is usually broad and poorly deIined6.
SnCl, or ZnClz by dipping the sample into a SnCl,- or ZnCl,-ethanol paste. When thermal decomposition of vitrinite begins, active sites thus formed are stabilized by hydrogen, and plasticoa12 (equivalent to vitroplast in a solvent system) is formed. The aromatic ring systems in the plasticoal are randomly orientated and their mobility is enhanced by smaller aliphatic fragments and gas molecules. As with vitroplast, the plastic&’ is converted to either mesophase or liquid and gas, depending on hydrogen availability. In contrast, the penetration of metal halides, and probably of hydrogen, into the inertinite is highly restricted at low temperature ( < 400C)20 and consequently hydrogenation takes place only at particle surfaces. However, as the temperature increases the partiallyhydrogenated zones gradually advance towards the cores of the inertinite particles. Partially-hydrogenated inertinite shows optical anisotropism. Again compact and high-reflectance inertinite particles are more difficult to hydrogenate. Short residence time hydrogenation with ZnCl,
The hydrogenation model can also be used to explain the genesis of various types of residue particles from the ZnCl,-catalysed hydrogenation of coal in a short residence time continuous reactorig. At the University of Utah, ZnCl,-coated powdered coals were hydrogenated without solvent in a tubular reactor with a variable length of 12-36 m (6 mm i.d.)21*22. Coal particles were carbonized at various stages of the hydrogenation’g*20 on coming into contact with the hot reactor wall.When vitrinite particles came into contact with the reactor wall soon after entering the reactor tube, they were carbonized rather than hydrogenated, thus forming primary semicoke (2-12 in Figure 2 and 1 in Figure 2) and leading to very low yields of liquid and gaseous products. When vitrinite particles were hydrogenated first and then carbonized (2-3-5 in Figure 1 and 2 in Figure 2) secondary semi-coke was formed, and slightly larger yields of liquid and gaseous products were obtained. However, when vitrinite particles remained in the reactor without being subjected to drastic heating (2-34 in Figure 2 and 3 in Figure 2) they were hydrogenated and largely converted to liquid and gas. When ZnCl, was not added to the raw coal, vitrinite particles did not become plastic quickly enough to allow some adhesion to the reactor wall and consequently they passed through the reactor rapidly (4 in Figure 2).
In this process the hydrogenation of inertinite was slower than that of vitrinite” and its conversion to liquid and gas was lower. Partially hydrogenated inertinite became secondary semi-coke which was anisotropic as in the case of inertinite hydrogenated with tetralin. APPLICATIONS MODEL
OF THE HYDROGENATION
Assessment of coal hydrogenation performance Hydrogenation catalysed by metal halides
The catalysed coal hydrogenation process without hydrogen donor solvent can also be described using a simplified version of the model. In this case both hydrogen and the metal halide catalyst penetrate vitrinite18-20. In these experiments the vitrinite grains are first coated with
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The microscopic analysis of residue samples leads to a greater understanding of the overall coal hydrogenation process. For example, the presence of both primary and secondary vitroplast3*14 in the solid residues from the hydrogenation of vitrinite using tetralin indicates incomplete hydrogenation and that improvement in conversion yields is possible. However, the formation of anisotropic
Model for the coal hydrogenation process: M. Shibaoka and S. Heng
coal hydrogenation system. The carbonaceous deposits are usually secondary semi-coke or unreacted fine solid particles (e.g. mineral and inertinite particles) bound by secondary semi-coke. The formation of the secondary semi-coke (3-5 in Figure 2) can be caused by localized overheating of the slurry combined with prolonged heating due to stagnation of the slurry flow. Stagnation can occur for many reasons, but one of the more common causes is an increase in slurry viscosity due to vitroplast formation (2-3 in Figure 1).
. Without Zn CL2 ////I/,/////////// *5
(b)
ACKNOWLEDGEMENTS Primary semicoke Secondary semicoke Partially converted coal Plas ticoal
The authors are grateful to R. J. Tyler, J. F. Stephens, Dr N. R. Foster and Dr M. A. Wilson for their helpful discussions and constructive criticisms. This work was supported in part under the National Energy Research Development and Demonstration Program administered by the Commonwealth Department of National Development and Energy. REFERENCES 1 Shibaoka, M. and Russell, N. J. ‘Proc. Int. Conf. Coal Sci., Diisseldorf, 1981, p. 453
Figure 2 A simplified model for the catalysed coal hydrogenation process without hydrogen donor solvent in an entrained flow reactor. (a) Particle flow regime in reactor with and without added ZnCI, catalyst; (b) simplified model of hydrogenation process in entrained flow reactor
mesophase and secondary semi-coke indicates that the hydrogenation of the vitrinite had been carried out under hydrogen-deficient conditions or that the reaction temperature was too high. However, it is important to note that the presence of anisotropic particles in the residues does not necessarily indicate unfavourable hydrogenation conditions since, as shown in Figure I, anisotropic secondary semi-coke is formed from inertinite even when hydrogenation conditions are favourable. Normally it is not easy to differentiate between vitrinite-derived semicoke and inertinitederived semi-coke. It requires careful investigation of the constitution of inertinite macerals in the original coal, and the properties and proportion of anisotropic material in the residues. Factors underlying reactor blockages
Blockages are often caused by the deposition of solid carbonaceous materialZ3 when coal-solvent slurry is heated in a tubular reactor or preheater of a continuous
2 Shibaoka, M., Russell, N. J. and Bodily, D. M. Fuel 1982,62, 201 3 Shibaoka, M. Fuel 1981.60.240 4 Mitchell, G. D., Davis, A. and Spa&man, W. in ‘Liquid Fuels from Coal’ (Ed. E. T. Ellington), Academic Press, New York, 1977, p. 255 5 Shibaoka, M. and Ueda, S. Fuel 1978,57,667 6 Shibaoka, M., Heng, S. and Okada, K. ‘Proc. Int. Conf. Coal Sci., Pittsburgh’, 1983, p. 699 7 Heng, S. and Shibaoka, M. Fuel 1983,62,610 8 Shibaoka, M., Stephens, J. F. and Russell, N. J. Fuel 1979,58,515 9 Brenner, D. ‘Proc. Int. Conf. Coal Sci., Dusseldorf, 1981, p. 163 10 Francis, W. ‘Coal - Its Formation and Composition’, Edward Arnold, London, 1961, p. 814 11 Drvden. I. G. C. Fuel 1951.30. 145 12 Whitehurst, D. D. and Mitchell, T. 0. Am. Gem. Sot., Div. Fuel Gem., Preprints 1976, 21, 121 13 Medeisos, D. and Petersen, E. E. Fuel 1979, Ss, 531 14 Shibaoka, M. Fuel 1981,60,945 15 Plett, E. G., Alkidas, A. C., Rogers, F. E. and Summerfield, M. Fuel 1977,56,241 16 Shibaoka, M. Fuel 1982,61,303 17 Brooks, J. D. and Taylor, G. H. ‘Chemistry and Physics of Carbon’, Vol. 4 (Ed. P. L. Walker, Jr). Edward Arnold. New York. 1968. D.243 18 Shibadka, M., Ueda, S.. and Russell, N. J. Fuel 1980,&, 11” 19 Bodily, D. M., Shibaoka, M. and Yoshida, R. ‘Proc. Int. Conf. Coal Sci., Dtisseldorf, 1981, p. 350 20 Bodily, D. M. and Shibaoka, M. Fuel (in preparation) 21 Wood, R. E. and Wiser, W. H. Ind. Eng. Chem., Proc. Des. Deu. 1976, 15, 144 22 Lytle, J. M., Wood, R. E. and Wiser, W. H. Fuel 1980,59,471 23 Shibaoka, M., Foster, N. R., Okada, K., Russell, N. J. and Clark, K. N. Fuel Proc. Technol. (submitted)
FUEL,
1984,
Vol
63,
February
177
Shibaoka
and Sammy
process
Heng
CSIRO Division of Fossil Fuels, PO Box 136, North Ryde, NS W 2 113, Australia (Received 4 March 1983; revised 14 June 1983)
A coal hydrogenation model has been formulated which incorporates both chemical and microscopic experimental data. In this generalized model, carbonization and hydrogenation areviewed asconcurrent processes in the liquefaction of coal. Insufficient hydrogen availability, rapid heating rates and long reaction times at elevated temperatures can promote carbonization reactions. The model describes in detail the reaction pathways involved in the hydrogenation of both inertinite and vitrinite. When vitrinite is hydrogenated in the presence of a hydrogen donor solvent, a plastic material called vitroplast is formed. The vitroplast is either converted to liquid and gaseous products when hydrogen availability is high or becomes mesophase and then semicoke when hydrogen availability is low. Even under favourable hydrogenation conditions, the major reaction pathway in the hydrogenation of inertinite is one of initial mild carbonization followed by hydrogenation. It is evident that the difference in hydrogenation behaviour between vitrinite and inertinite is due, in part, to the ability of the hydrogen donor solvent to penetrate vitrinite but not inertinite particles. The hydrogenation model is useful for explaining various phenomena that occur during hydrogenation, such as the formation of mesophase and semicoke, and the blockage of reactors and preheaters. (Keywords: coal; hydrogenation; models)
Essentially coal hydrogenation is a chemical process. However, it is greatly influenced by the physical attributes of the reactants. At the early stages of the coal hydrogenation process the physical state of coal particles is one of the most important factors affecting its chemistry. Furthermore, changes in the physical state of the coal particles, which can be observed microscopically, provide important information on the coal hydrogenation process. As coal is usually highly heterogeneous at the microscopic level, individual coal particles, or different parts of single coal particles, are expected to show significantly different hydrogenation behaviour. Furthermore, the reactions which occur in the core of a solid coal particle may be quite different from those which occur at its margins in immediate contact with solvent, gas and catalyst. Most chemical techniques do not detect this microscopic differentiation in the hydrogenation process, and only the overall chemical transformations are determined. Therefore it is essential to combine the results from both chemical and microscopic investigations to derive a correct understanding of the hydrogenation process. The hydrogenation model developed in this Paper is based on the results of recent investigations into the hydrogenation behaviour of the vitrinite and inertinite macerals of Australian bituminous coals. These coals are mainly of Permian and Triassic ages and therefore differ from the common Carboniferous coals of the Northern Hemisphere in several respects, especially their generally higher content of low-reflectance inertinite mace&. Investigations in this laboratory have shown that the low-reflectance inertinite macerals, particularly semifusinite, can make significant contributions to the yields of liquid and gaseous products though with somewhat 0016-2361/84/020174-04$3.00 @ 1984 Butterworth & Co. (Publishers)
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greater difficulty than the accompanying vitrinite. High reflectance inertinite in Australian coals is as unconvertible as that of Northern Hemisphere Carboniferous coals. The model presented in this Paper sets out the different behaviour patterns of the vitrinite and inertinite macerals and explains them in terms of the interaction between two concurrent processes, carbonization and hydrogenation192. In this Paper carbonized material derived directly from unhydrogenated coal is defined as primary semi-coke whereas that derived from mesophase and partiallyhydrogenated anisotropic inertinite is defined as secondary semi-coke l*’ . Primary semi-coke derived from vitrinite is either isotropic or exhibits a line-grained anisotropic texture, depending on the rank of the original coal. Primary semi-coke derived from inertinite is usually isotropic while secondary semi-coke usually exhibits a medium- to coarse-grained anisotropic mosaic texture. Many semi-cokes, which are of intermediate form, in terms of genesis and optical properties, also occur. HYDROGENATION
MODEL
The model describing the overall coal hydrogenation process using hydrogen donor solvents is illustrated in Figure 1.
When vitrinite is heated in tetralin, highly-gelilied vitrinite macerals (e.g. collinite) gradually become plastic (2+3 in Figure I), while poorly-gelitied vitrinite macerals (e.g. telinite) remain as fibrous materia13. The plastic material is called vitroplast4 while the fibrous material is called vitrofibre. The lower molecular weight compounds formed during vitroplast hydrogenation form liquid and gaseous products (34). As the vitrolibre is usually line,
Model for the coal hydrogenation process: M. Shibaoka and S. Heng 12 (Primory
. ,#5 ._-
( Secondary semicokel
Temperoture (or time) * Model for the coal hydrogenation process with solid; -. -. -, mesophasa; hydrogen donor solvent. -, liquid and gas. Numbers are referred to in ., plastic; -, text
most of it is dissolved and then hydrogenated to liquid and gas relatively quickly. Vitrofibre hydrogenation is not shown in Figure I to avoid complicating the diagram. During the formation of vitroplast (2-3) and vitrofibre and their conversion to liquid and gaseous products (3+4) the rate of carbonization can exceed that of hydrogenation, particularly if the level of hydrogen availability falls’ or if the heating rate is excessively rapid. As a result, mesophase is formed from the vitroplast, and further heating under hydrogen-deficient conditions will bring about the formation of secondary semi-coke (3-5). If hydrogen deficiency occurs at point 2, repolymerization and solid-state molecular reorganization commence and accelerate suddenly when the temperature reaches certain levels (6) to yield primary semi-coke (2&12). Under conditions of good hydrogen availability the hydrogenation and dissolution of very line particles of the low-reflectance inertinite and peripheral parts of larger inertinite particles (6-+7) can occur but at a higher temperature (6) than that for vitrinite. The hydrogenation of high-reflectance inertinite and core parts of the large low-reflectance inertinite particles (849) takes place at higher temperature and is preceded by a slight carbonization reaction (6-+8)6. Under favourable hydrogenation conditions a significantly large part of the low-reflectance inertinite is converted to liquid and gaseous products’ (9+10) while the rest becomes secondary semi-coke (9+11). However, under hydrogen-deficient conditions where the rate of carbonization exceeds the rate of hydrogenation, inertinite becomes primary semi-coke (6 and 8-12). At every bifurcation beyond point 2 in the hydrogenation model (i.e. 3,6,8 and 9), the course of the process depends on both hydrogen availability and the rate of thermal bond-breaking in the coal. EXPERIMENTAL DEVELOPMENT MODEL
BASIS UNDERLYING OF HYDROGENATION
of uitroplast and vitrofibre (2-3 in Figure 1) When small blocks of coal are treated with organic solvents (including tetralin) at their boiling points, exten-
Formation
sive swelling of the vitrinite occurs’. Brenner (1981)’ reported that substantial swelling of vitrinite occurs on exposure to n-propylamine vapour at room temperature. The swelling of vitrinite can be reversed by desorption of the solvents, although in some cases residual deformation of the vitrinite layers may be observed8v9. The extent of swelling of the vitrinite depends on coal rank and the properties of the solvent u~ed~-‘~. The swelling of vitrinite is closely related to changes in its pore structure. The porosity is increased by solvents at elevated temperatures giving the residual coal sample a much larger total surface area than that of the parent coal”. When the vitrinite from a high volatile bituminous coal was treated with tetralin at 335-34O”C, collinite became slightly plastic and large vesicles were formed. Telinite and telinite/collinite were partially dissolved, leaving a network of coaly material which consisted mainly of cell wall-derived material (vitrofibre). It is reasonable to assume that a considerable amount of tetralin is present within the vitrinite particles, filling both the network and submicroscopic pores. While thermal fragmentation of the coal matrix at these temperatures may not be extensive, should this occur and active sites be formed, then they would be rapidly stabilized by the hydrogen from the tetralin within the vitrinite, and consequently the development of plasticity in the vitrinite would be enhanced. The tetralin within the vitrinite particles is most useful at higher temperatures (>4OO”C) when extensive thermal fragmentation of the vitrinite molecular structure begins to take place and stabilization of active sites becomes important 3*14.Thus, the influence of particle size in the hydrogenation of vitrinite must be reduced14v15 when the hydrogen donor solvent can penetrate, react with and disperse the coal substance. Vitroplast is essentially a mixture of asphaltenes and preasphaltenes with inclusions of tetralin and/or naphthalene. Further hydrogenation of vitroplast will depend largely on the diffusion of fresh tetralin into the vitroplast or on the extent of the dissolution of the vitroplast in the tetralin. If reaction temperature is lowered before most of the vitroplast material dissolved in solvent is hydrogenated to oil and gas, this material is precipitated as secondary vitroplast3s14.
Conversion of vitroplast to liquid and gas or mesophase and secondary semi-coke (3-4 or 5 in Figure 1)
The fate of the vitroplast is governed by the outcome of the competition between the polymerizationcarbonization and hydrogenation reactions14. If the heating rate is too rapid and/or the final temperature is too high (e.g. >47o”C), the rate of thermal repolymerization will exceed the rate of hydrogenation even when good hydrogen-donor solvent and catalysts are employed. Consequently, mesophase is often formed in the nucleus of massive vitroplast. Mesophase will form even if the vitroplast is maintained at relatively low temperatures (e.g. 410°C) for long periods of time without sufficient agitation. Essentially mesophase formation can occur at any stage of vitroplast hydrogenation (2+3)16. The conversion of vitroplast to liquid products can be promoted in several ways, such as by employing efficient hydrogen-donor solvents and catalysts, and by improving mass transfer with vigorous agitation.
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Model for the coal hydrogenation process: M. Shibaoka and S. Heng Hydrogenation of inertinite (6-7 and 6-8-9
Figure
1)
The macro- and microporosity of inertinite macerals have not yet been investigated in detail. However, results indicate that inertinite, even low-reflectance inertinite, does not undergo any obvious deformation in the presence of organic solvents at temperatures < 375°C 6. The degree of swelling of the inertinite in response to solvent treatment appears to be negligible when compared with that of vitrinite at the same temperature8. There are large cavities such as cell lumens in inertinite, but the population of micropores and the total surface area generated by these cavities is small when compared to the population of micropores in vitrinite and their total surface area. Therefore, even if tetralin can penetrate the inertinite particles, the quantity of tetralin likely to be retained within the inertinite will be relatively small compared to that in vitrinite. Due to the limited penetration of tetralin into inertinite, the hydrogenation reaction and, thus, dissolution of inertinite, is limited to the external surfaces of the inertinite particles6. In such situations, the rate of hydrogenation at an early stage of the reaction is dependent on the external surface area so that smaller and finely structured inertinite particles are usually converted earlier (67). Because of the limited penetration of tetralin into the inertinite particles, stabilization of active sites formed during fragmentation of the coal structure cannot occur to any significant extent, particularly in the cores of the inertinite particles. Thus, slight repolymerizationcarbonization is likely to occur. Unlike the hydrogenation of vitrinite, the inertinite particles do not become plastic at temperatures < 400°C. Instead, the inertinite undergoes solid-state molecular reorganization during the hydrogenation process, i.e., the limited mobility of the aromatic ring units at these temperatures only permits in-situ reorientation of the aromatic ring units which results in the development of weak local anisotropism. Temperatures >42o”C result in gradual hydrogenation of the inertinite particles, beginning mainly at the margins of the particles. Partly hydrogenated inertinite becomes slightly plastic and develops further anisotropism. The process involved in the development of anisotropism in inertinite during hydrogenation is different from that involved in the development of anisotropism due to mesophase formation in vitroplast. In inertinite, the aromatic ring systems do not have great mobility which permits them to stack together to form spherically shaped mesophase. The mesophase spheres in vitroplast are able to coalesce and migrate within the vitroplast while in inertinite such phenomena do not occur. In the process of mesophase development, the isotropic vitroplast and anisotropic mesophase exist side by side separated by a sharp boundary l’. In inertinite the boundary between anisotropic and isotropic regions is usually broad and poorly deIined6.
SnCl, or ZnClz by dipping the sample into a SnCl,- or ZnCl,-ethanol paste. When thermal decomposition of vitrinite begins, active sites thus formed are stabilized by hydrogen, and plasticoa12 (equivalent to vitroplast in a solvent system) is formed. The aromatic ring systems in the plasticoal are randomly orientated and their mobility is enhanced by smaller aliphatic fragments and gas molecules. As with vitroplast, the plastic&’ is converted to either mesophase or liquid and gas, depending on hydrogen availability. In contrast, the penetration of metal halides, and probably of hydrogen, into the inertinite is highly restricted at low temperature ( < 400C)20 and consequently hydrogenation takes place only at particle surfaces. However, as the temperature increases the partiallyhydrogenated zones gradually advance towards the cores of the inertinite particles. Partially-hydrogenated inertinite shows optical anisotropism. Again compact and high-reflectance inertinite particles are more difficult to hydrogenate. Short residence time hydrogenation with ZnCl,
The hydrogenation model can also be used to explain the genesis of various types of residue particles from the ZnCl,-catalysed hydrogenation of coal in a short residence time continuous reactorig. At the University of Utah, ZnCl,-coated powdered coals were hydrogenated without solvent in a tubular reactor with a variable length of 12-36 m (6 mm i.d.)21*22. Coal particles were carbonized at various stages of the hydrogenation’g*20 on coming into contact with the hot reactor wall.When vitrinite particles came into contact with the reactor wall soon after entering the reactor tube, they were carbonized rather than hydrogenated, thus forming primary semicoke (2-12 in Figure 2 and 1 in Figure 2) and leading to very low yields of liquid and gaseous products. When vitrinite particles were hydrogenated first and then carbonized (2-3-5 in Figure 1 and 2 in Figure 2) secondary semi-coke was formed, and slightly larger yields of liquid and gaseous products were obtained. However, when vitrinite particles remained in the reactor without being subjected to drastic heating (2-34 in Figure 2 and 3 in Figure 2) they were hydrogenated and largely converted to liquid and gas. When ZnCl, was not added to the raw coal, vitrinite particles did not become plastic quickly enough to allow some adhesion to the reactor wall and consequently they passed through the reactor rapidly (4 in Figure 2).
In this process the hydrogenation of inertinite was slower than that of vitrinite” and its conversion to liquid and gas was lower. Partially hydrogenated inertinite became secondary semi-coke which was anisotropic as in the case of inertinite hydrogenated with tetralin. APPLICATIONS MODEL
OF THE HYDROGENATION
Assessment of coal hydrogenation performance Hydrogenation catalysed by metal halides
The catalysed coal hydrogenation process without hydrogen donor solvent can also be described using a simplified version of the model. In this case both hydrogen and the metal halide catalyst penetrate vitrinite18-20. In these experiments the vitrinite grains are first coated with
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FUEL, 1984, Vol 63, February
The microscopic analysis of residue samples leads to a greater understanding of the overall coal hydrogenation process. For example, the presence of both primary and secondary vitroplast3*14 in the solid residues from the hydrogenation of vitrinite using tetralin indicates incomplete hydrogenation and that improvement in conversion yields is possible. However, the formation of anisotropic
Model for the coal hydrogenation process: M. Shibaoka and S. Heng
coal hydrogenation system. The carbonaceous deposits are usually secondary semi-coke or unreacted fine solid particles (e.g. mineral and inertinite particles) bound by secondary semi-coke. The formation of the secondary semi-coke (3-5 in Figure 2) can be caused by localized overheating of the slurry combined with prolonged heating due to stagnation of the slurry flow. Stagnation can occur for many reasons, but one of the more common causes is an increase in slurry viscosity due to vitroplast formation (2-3 in Figure 1).
. Without Zn CL2 ////I/,/////////// *5
(b)
ACKNOWLEDGEMENTS Primary semicoke Secondary semicoke Partially converted coal Plas ticoal
The authors are grateful to R. J. Tyler, J. F. Stephens, Dr N. R. Foster and Dr M. A. Wilson for their helpful discussions and constructive criticisms. This work was supported in part under the National Energy Research Development and Demonstration Program administered by the Commonwealth Department of National Development and Energy. REFERENCES 1 Shibaoka, M. and Russell, N. J. ‘Proc. Int. Conf. Coal Sci., Diisseldorf, 1981, p. 453
Figure 2 A simplified model for the catalysed coal hydrogenation process without hydrogen donor solvent in an entrained flow reactor. (a) Particle flow regime in reactor with and without added ZnCI, catalyst; (b) simplified model of hydrogenation process in entrained flow reactor
mesophase and secondary semi-coke indicates that the hydrogenation of the vitrinite had been carried out under hydrogen-deficient conditions or that the reaction temperature was too high. However, it is important to note that the presence of anisotropic particles in the residues does not necessarily indicate unfavourable hydrogenation conditions since, as shown in Figure I, anisotropic secondary semi-coke is formed from inertinite even when hydrogenation conditions are favourable. Normally it is not easy to differentiate between vitrinite-derived semicoke and inertinitederived semi-coke. It requires careful investigation of the constitution of inertinite macerals in the original coal, and the properties and proportion of anisotropic material in the residues. Factors underlying reactor blockages
Blockages are often caused by the deposition of solid carbonaceous materialZ3 when coal-solvent slurry is heated in a tubular reactor or preheater of a continuous
2 Shibaoka, M., Russell, N. J. and Bodily, D. M. Fuel 1982,62, 201 3 Shibaoka, M. Fuel 1981.60.240 4 Mitchell, G. D., Davis, A. and Spa&man, W. in ‘Liquid Fuels from Coal’ (Ed. E. T. Ellington), Academic Press, New York, 1977, p. 255 5 Shibaoka, M. and Ueda, S. Fuel 1978,57,667 6 Shibaoka, M., Heng, S. and Okada, K. ‘Proc. Int. Conf. Coal Sci., Pittsburgh’, 1983, p. 699 7 Heng, S. and Shibaoka, M. Fuel 1983,62,610 8 Shibaoka, M., Stephens, J. F. and Russell, N. J. Fuel 1979,58,515 9 Brenner, D. ‘Proc. Int. Conf. Coal Sci., Dusseldorf, 1981, p. 163 10 Francis, W. ‘Coal - Its Formation and Composition’, Edward Arnold, London, 1961, p. 814 11 Drvden. I. G. C. Fuel 1951.30. 145 12 Whitehurst, D. D. and Mitchell, T. 0. Am. Gem. Sot., Div. Fuel Gem., Preprints 1976, 21, 121 13 Medeisos, D. and Petersen, E. E. Fuel 1979, Ss, 531 14 Shibaoka, M. Fuel 1981,60,945 15 Plett, E. G., Alkidas, A. C., Rogers, F. E. and Summerfield, M. Fuel 1977,56,241 16 Shibaoka, M. Fuel 1982,61,303 17 Brooks, J. D. and Taylor, G. H. ‘Chemistry and Physics of Carbon’, Vol. 4 (Ed. P. L. Walker, Jr). Edward Arnold. New York. 1968. D.243 18 Shibadka, M., Ueda, S.. and Russell, N. J. Fuel 1980,&, 11” 19 Bodily, D. M., Shibaoka, M. and Yoshida, R. ‘Proc. Int. Conf. Coal Sci., Dtisseldorf, 1981, p. 350 20 Bodily, D. M. and Shibaoka, M. Fuel (in preparation) 21 Wood, R. E. and Wiser, W. H. Ind. Eng. Chem., Proc. Des. Deu. 1976, 15, 144 22 Lytle, J. M., Wood, R. E. and Wiser, W. H. Fuel 1980,59,471 23 Shibaoka, M., Foster, N. R., Okada, K., Russell, N. J. and Clark, K. N. Fuel Proc. Technol. (submitted)
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