Comparison of the effect of catalysts in coal liquefaction with tetralin and coal tar distillates

ELSEVIER

Fuel Vol. 76, No. 13, pp. 1309-1313, 1997 © 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0016-2361/97 $17.00+0.00

Plh S0016-2361(97)00017-3

Comparison of the effect of catalysts in coal liquefaction with tetralin and coal tar distillates J. Andres Legarreta, Blanca M. Caballero, Isabel de Marco, M. Jesus Chom6n and Pedro M. Uria Departamento de Ingenieria Ouimica y de/Medio Ambiente, Escuela de Ingenieros de Bilbao, Universidad del Pals Vasco, Alda, Urquijo s/n, 48013 Bilbao, Spain (Received 22 October 1996; revised 23 December 1996) Special CoMo/AI203 catalysts were prepared for testing in coal liquefaction: a conventional CoMo/AI203 catalyst, one containing Zn as a second promoter and one having the alumina acidified with fluorine. Their activities were compared with that of red mud. The experiments were conducted in a stirred autoclave with a subbituminous coal and solvent (tetralin, anthracene oil or creosote oil) at 425°C and 17 MPa. The liquefaction products were fractioned into oils, asphaltenes and preasphaltenes with pentane, toluene and THF. The Co(Zn)Mo/Al203 catalysts have far higher activities than red mud. Zn and fluorine have beneficial effects on the catalyst activity. Coal tar distillates give higher conversions and oil + gas yields than tetralin when the prepared catalysts are used. © 1997 Elsevier Science Ltd. (Keywords: coal liquefaction; C o M o c a t a l y s t s ; subbituminous coal)

The role and importance of catalysts in coal liquefaction is well known. The addition of active catalysts enhances total conversion and selectivity (increase in yield of oils, which are the most desirable products). This results in more efficient hydrogen consumption. In addition, effective catalysts permit a reduction of reaction severity: lower temperature, pressure and reaction time. The choice of catalyst for coal liquefaction depends on several factors. Disposable catalysts such as red mud or other iron oxide-containing products have the advantage of being inexpensive or even free of charge, and do not need to be recovered. In contrast, the more specialized catalysts, such as those supported on alumina, are more expensive and should be recovered, but may well give better results. The combined cost of catalyst and chemicals has been estimated to be - 8 % of the net operating costs for several liquefaction processesk Therefore catalyst cost itself may not be an effective argument against exploring the use of more expensive materials. The catalyst cost may well be justified if substantial reductions in capital investment and operating costs are made possible. The objective of this study was to determine the activity in coal liquefaction of a series of catalysts prepared specifically for this work: a conventional CoMo/A1203, and two more novel preparations, one which has half the amount of Co replaced by Zn, and another which has the alumina acidified with fluorine and also contains Zn. The interest in testing these catalysts arose from the following facts. It has been reported 2'3 that dopping C o - M o catalysts with Zn enhances their hydrodesulfurization (HDS) activity. This has been attributed to the fact that Zn has a greater tendency than Co to occupy tetrahedral positions inside the

alumina 2. The remaining Co tends to form less of the inactive COA1204, consequently increasing the proportion of octahedrally coordinated Co, which is the active phase 4. On the other hand, experiments with model compounds such as cyclohexene and thiophene have shown that fluorination of alumina increases its surface acidity and consequently increases the cracking, hydrocracking, HDS and hydrogenation activities of the catalyst 5-7. The same effects of Zn and of F on the activities of catalysts in coal liquefaction might be expected. The prepared CoMo/AI203 catalysts were tested in coal liquefaction with a typical H-donor solvent (tetralin) and two coal tar distillates (anthracene oil and creosote oil). The activities of these catalysts were compared with that of red mud, an inexpensive iron oxide unsupported catalyst traditionally used in coal liquefaction.

EXPERIMENTAL The coal used in most of the liquefaction experiments was Encasur, a Spanish subbituminous A coal from Ciudad Real. Homogeneous coal samples for liquefaction were prepared as follows. The coal was first dried in air, and then ground and sieved under nitrogen to a particle size of 100-200 ~tm. The proximate, ultimate and maceral analyses of the prepared Encasur samples are presented in Table 1. Additionally, to establish whether the activity of the catalysts depended on the coal used, they were tested with a standard coal, Vouters-140 FR 36, a French highvolatile bituminous coal, provided by the European Centre for Coal Specimens (SBN), the Netherlands. It was tested

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The effect of catalysts in coal liquefaction: J. A. Legarreta et al. Table 1 Analyses of the coals used for the experiments Petrographic analysis (vol.% mmf)

Proximate and ultimate analyses (wt%)" Moisture

Ash

VM

GCV b

C

H

N

S

Oc

Vitrinite

Exinite

Inertinite

4.5 d 2.3

22.3 5.8

25.4 35.0

23.20 30.86

61.5 78.1

4.1 5.1

1.2 0.9

0.4 0.9

6.0 6.9

73.5 90.4

8.9 5.8

17.6 3.8

Encasur Vouters

~Air-dried basis for Encasur; as-receivedbasis for Vouters bGrosscalorific value (MJ kg-I) "By difference;includes chlorine etc. and errors dTotal moisture(as-received) 11.1 wt%

Table 2

Characteristics of anthracene and creosote oils used for the experiments Boiling Viscosity, range (°C) 20°C (mPa s)

Anthracene oil b Creosote oil

Ultimate analysis (wt%) C

H

N

S

O~

H/C atomic ratio

Compounds with <- 2 rings (wt%)

Compounds with -> 3 rings (wt%)

280-400

31.0

90.8

7.3

0.6

0.2

1.1

0.96

15.8

84.2

220-290

6.5

89.6

7.9

0.6

0.3

1.6

1.1

60.0

40.0

"By difference hHydrogenated

as-received. Its proximate, ultimate and maceral analyses are also shown in Table 1. The CoMo/AI203 supported catalysts were prepared specifically for this study in the Catalysis Institute of Madrid (Spain): (1) CMA, which is a conventional CoMo/ A1203 catalyst containing 4 wt% of CoO and 12 wt% of MOO3; (2) CZMA, a novel preparation with half the amount of Co replaced by a second promoter, Zn; (3) CZMFA.7, which has the same proportions of metal oxides as CZMA but has the alumina acidified with 0.7 wt% of fluorine. These three catalysts were prepared by multistep impregnation and had a particle size of < 50/~m. The method of preparation and detailed characteristics of these catalysts have been presented elsewhere s. It is worth mentioning that other fluorinated catalysts with different fluorine contents, from 0.4 to 2 wt%, were also prepared and were tested in hydrotreatment processes as reported previously9"~°; it was found that the fluorine content affects the catalyst activity in hydrodesulfurization (HDS) and hydrodenitrogenation (HDN). However, they were tested by the authors in coal liquefaction and no differences among them were found; hence in this paper only the results obtained with one of these catalysts, chosen at random, are presented. Red mud is an inexpensive by-product of the aluminium industry that contains - 3 6 wt% of Fe203 mixed with alumina and other oxides. It has a particle size of 5-100/zm. Due to its activity and low price, it has been traditionally used in coal liquefaction. The solvents used for the experiments were tetralin (1,2,3,4-tetrahydronaphthalene), which is a typical H-donor solvent widely used in coal liquefaction research, and two coal tar distillates, anthracene oil and creosote oil. Both oils are complex mixtures of hydrocarbons derived from the distillation of coal tar and are therefore better analogues of the recycle oils used as solvents in coal liquefaction plants. The anthracene oil, provided by a Spanish tar distillation company (Industrial Qufmica del Nal6n), was rather viscous and contained some solid particles. Such oil when used as-received was difficult to handle and did not give reproducible results in coal liquefaction, probably due to its heterogeneous nature. Therefore it was decided to

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hydrogenate it prior to liquefaction, using the following operating conditions: 250 mL autoclave, 120 g of solvent, 1.2 g of a CoMo/AI203 catalyst, 12 MPa initial hydrogen pressure, 400 rev min -j stirring speed, 425°C and 4 h reaction time. Immediately afterwards, the anthracene oil was filtered to eliminate any residual solid particules, and after checking that it had an H/C atomic ratio of - 0 . 9 6 H/C, it was saved for liquefaction. The creosote oil was provided by another Spanish company (Sociedad Bilbaina de Maderas y Alquitranes SA). It was tested in coal liquefaction as-received, since it was lighter and much less viscous, did not contain solids and did not lead to reproducibility problems in the liquefaction results as did the as-received anthracene oil. The characteristics and composition of both oils as used for the experiments are presented in Table 2. The liquefaction experiments were conducted in a 250 mL magnetically stirred autoclave. On the basis of previous studies by the authors 11-13 of the influence of operating conditions on coal liquefaction and its relation to catalyst effects, the following procedure was used for testing the catalysts. The autoclave was charged with H2, 10 g of coal, 30 g of solvent, the catalyst in a proportion of 1.2 wt% (based on daf coal) as active metal oxides (Fe203 for red mud and CoO + ZnO + MoO3 for the supported catalysts) and elemental sulfur, added to activate the catalyst, in a proportion of 157 wt% with respect to active metal oxides (equivalent to 50 wt% with respect to red mud). The operatin~ conditions were 425°C, 1 h reaction time, 400 rev min- stirring speed and 17 MPa operating pressure. The liquefaction products were separated into gases, liquid products and solid residue (unconverted coal ÷ mineral matter). The liquid products were fractionated into preasphaltenes (THF-soluble, toluene-insoluble), asphaltenes (toluene-soluble, pentane-insoluble) and oils (pentane-soluble), as follows. The reactor contents were extracted with toluene in a Soxhlet apparatus for 24 h when the solvent was tetralin or 48 h when the solvent was either of the oils, and then the Soxhlet thimble was dried and weighed. Toluene was evaporated from the extract and pentane added to precipitate the asphaltenes, which were

The effect of catalysts in coal liquefaction: J. A. Legarreta et al. filtered, dried and weighed. The Soxhlet thimble with the toluene-insolubles was again extracted with THF for 24 h to dissolve the preasphaltenes, and was again dried and weighed. Total conversion and yields of the different types of products obtained were calculated using the following formulae:

Total conversion (wt% THF solubles)=

Preasphaltenes (wt%) =

Asphaltenes ( w t % ) =

than red mud not only in terms of total conversion but also in selectivity and quality of the products. All the supported catalysts give similar results. Therefore it seems that there is no beneficial effect of Zn on the catalyst activity. However, it must be taken into account that Zn, which is a cheaper metal than Co, is supposed to be

wt coal(daf) - wt THF insolubles(daf) 100 wt coal(daf)

wt toluene insolubles(daf) - wt THF insolubles(daf) 100 wt coal(daf)

wt asphaltenes 100 wt coal(daf)

Pentane solubles (oils + gases) (wt%)=

wt coal(daf) - wt toluene insolubles(daf) - wt asphaltenes 100 wt coal(daf)

The liquefaction yields (wt%) obtained in equivalent experiments seldom differed by more than two points. Each result presented in this paper is the mean value of the data obtained in at least two equivalent experiments and therefore the estimated error of such results is _ 1 wt%. RESULTS AND DISCUSSION The liquefaction yields obtained with the four catalysts when Encasur coal and tetralin were used are presented in Table 3. It can be seen that the three Co(Zn)Mo/AI203 catalysts give higher conversions and oils + gases yields than red mud. This is as expected, since Mo metal is more active than Fe, and supported catalysts have a higher surface/volume ratio than unsupported ones. Total conversion increases by - 8 - 1 0 points with the Co(Zn)Mo/A1203 catalysts and 6 points with red mud. As regards product distribution, the oil + gas yield increases only slightly with red mud, whereas the supported catalysts increase the yield by - 8 - 1 0 points. Asphaltenes and preasphaltenes are lower with the Co(Zn)Mo/A1203 catalysts than with red mud. Therefore the Co(Zn)Mo/AI203 catalysts give better results

inactive. Therefore CZMA, being cheaper, is as active as CMA even though it contains half the amount of Co. The explanation may be the one reported in the literature for HDS reactions by other authors,2 as mentioned in the introduction to this paper. In summary it can be stated that the addition of Zn leads to better use of Co, since the part of Co that would form an inactive phase is replaced by Zn. On the other hand, the acidification of the alumina with fluorine, which was suspected to enhance the cracking and hydrocracking capacity of the catalyst and consequently its activity in coal liquefaction, does not show any beneficial effect, since similar results are obtained with the fluorinated catalyst (CZMFA.7) to those with the unfluorinated ones (CMA and CZMA). However, it must be taken into account that the solvent used, tetralin, is a strong H-donor solvent, which without catalyst already gives very high coal conversions; consequently the effects of the catalysts are rather low and it seems probable that the differences among them are masked. The liquefaction yields obtained with Encasur coal when anthracene oil is used are presented in Table 4. It can be seen that now the coal conversion and oil + gas yield obtained without catalyst are rather low, much more so than those

Table 3 Liquefaction yields of Encasur coal with tetralin (wt% daf coal) Catalyst

None

Red mud

CMA

CZMA

CZMFA.7

THF-solubles Oils + gases Asphaltenes Preasphaltenes

73.7 39.0 29.6 5. l

79.5 39.5 33.5 6.5

82.8 47.9 32.3 2.6

83.3 47.7 32.8 2.8

82.5 47.5 32.6 2.4

Table 4 Liquefaction yields of Encasur coal with anthracene oil (wt% daf coal) Catalyst

None

Red mud

CMA

CZMA

CZMA.7

THF-solubles Oils + gases Asphaltenes Preasphaltenes

56.7 22.5 27.1 7.1

74.6 37.5 33.1 4.0

83.6 53.4 28.2 2.0

84.3 54.7 27.3 2.3

87.0 58.3 27.8 0.9

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The effect of catalysts in coal liquefaction: J. A. Legarreta et al. obtained with tetralin. This is logical, since anthracene oil is a coal tar distillate that, even though it had been thoroughly hydrogenated prior to liquefaction, contains less donatable hydrogen than tetralin. Consequently, active catalysts are required to continuously replenish anthracene oil with Hdonors during the reaction and make it an effective solvent for coal liquefaction. Again the Co(Zn)Mo/AI203 catalysts give better results than red mud, but additionally, the catalytic effects are much more pronounced than with tetralin, so that now there are differences among the Co(Zn)Mo/AI203 catalysts. CZMA gives equal results to CMA. Therefore it is confirmed that there is a beneficial effect of Zn, which makes the catalyst cheaper and maintains its activity. CZMFA.7 clearly gives higher conversion and oil + gas yield than CMA and CZMA. Therefore the acidification of the alumina with fluorine enhances the activity and the selectivity of the catalyst in coal liquefaction. The effect of fluorine may be attributed, as already mentioned, to the fact that surface acidity increases cracking and hydrogenation activities. On the other hand, comparing the results obtained with anthracene oil with those obtained with tetralin, the following aspects must be mentioned. First, when no catalyst is used, tetralin gives far better conversion than anthracene oil. Second, when red mud is used, the yields in both solvents are much closer. Third, with any of the Co(Zn)Mo/AIzO3 catalysts, conversion with anthracene oil is greater than that obtained with tetralin. This increase may not be very significant with CMA and CZMA, but it is quite clear with CZMFA.7. Therefore anthracene oil can be a better hydroliquefaction solvent than tetralin if very active catalysts, which efficiently promote the hydrogenation and dehydrogenation of the solvent during the liquefaction reaction, are used, provided that the operating conditions are appropriate. The results obtained with the novel Co(Zn)Mo/A1203 catalysts are very promising. However, the fact that a nondonor solvent gave better results than a typical H-donor solvent seems somewhat unusual. This is why the catalysts were tested with another coal tar distillate, creosote oil. The results obtained with this solvent are presented in Table 5. It can be seen that the relative behaviour of the catalysts is equivalent to that previously observed with anthracene oil. Again the Co(Zn)Mo/AI203 catalysts have higher activities than red mud, and once more CZMA gives the same results as CMA, and the fluorinated catalyst gives better results than the two others. It is worth mentioning that with the coal

tar distillates, either anthracene oil or creosote oil, coal conversion with the fluorinated catalysts is - 8 7 % , which is more than the proportion of reactive macerals (vitrinite + exinite), which can be estimated as 84-85 wt%, considering that the densities of vitrinite, exinite and inertinite are - 1.3, 1.2 and 1.5 g cm -3 respectivelyl4; so not only have vitrinite and exinite been converted, but also some of the inertinite. On the other hand, if the results obtained with creosote oil are compared with those obtained with tetralin, it can be seen that without catalyst or with red mud, tetralin gives better conversion and oil + gas yield than creosote oil, but with any of the Co(Zn)Mo/A1203 catalysts, especially with CZMFA.7, the conversion and oil + gas yield obtained with creosote oil are greater than those obtained with tetralin. Hence again a coal tar distillate non-donor solvent gives better liquefaction yields than a typical H-donor. There are several possible reasons for this: (1) Coal tar distillates have greater solvent power for coal and coal products than tetralin, according to the simple dictum that 'like dissolves like'; however, this fact is independent of whether catalyst has been used or not, so this is not a sufficient reason why with active catalysts the tar distillates give better liquefaction yields than tetralin, if without catalysts they do not. (2) It may be considered that since there is only 30 g of solvent in a 250 mL autoclave, at 425°C much of the tetralin, which boils at 207.5°C, will be in the vapour phase, giving very low liquid solvent/coal ratio compared with anthracene oil and creosote oil, which are higher boiling (280-400 and 220-290°C respectivelylfi However, previous studies carried out by the authors using the same experimental procedure as in this study, with 10 g of coal and different amounts of tetralin, showed that at 425°C above a 3:1 solvent/coal ratio there was no influence of the amount of tetralin on the liquefaction yields. Therefore although when tetralin is used more solvent is in the vapour phase than when the tar distillates are used, the amount of tetralin in the liquid phase is sufficient for this solvent to be effective, since greater amounts of tetralin in the autoclave do not enhance coal conversion. Therefore the liquid/vapour ratio under the experimental conditions does not provide an effective argument to explain the poor performance of tetralin compared with creosote oil and anthracene oil in catalytic coal liquefaction. (3) It has been reported by several authors I1A5-17 that three- and four-ring condensed aromatics ,:an accept

Table 5 Liquefaction yields of Encasur coal with creosote oil (wt% daf coal) Catalyst

None

Red mud

CMA

CZMA

CZMFA.7

THF-solubles Oils + gases Asphaltenes Preasphaltenes

41.5 10.0 23.8 7.7

55.5 21.3 28.1 6.1

83.6 54.3 27.3 2.0

84.8 56.2 26.8 1.8

86.3 57.9 27.5 0.9

Table 6 Liquefaction yields of Vouters (standard coal) with tetralin (wt% daf coal) Catalyst

None

Red mud

CMA

CZMA

CZMA.7

THF-solubles Oils + gases Asphaltenes Preasphaltenes

87.1 46.5 38.6 2.0

87.4 46.7 39.7 1.0

90.1 52.3 37.0 0.8

90.6 52.5 37.2 0.9

91.1 52.9 37.5 0.7

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The effect of catalysts in coal liquefaction: J. A. Legarreta et al. Table 7 Liquefaction yields of Vouters (standard coal) with anthracene oil (wt% daf coal) Catalyst

None

Red mud

CMA

CZMA

CZMFA.7

THF-solubles Oils + gases Asphaltenes Preasphaltenes

68.7 31.4 34.7 2.6

83.2 45.3 35.1 2.8

93.7 62.5 29.5 1.7

94.0 63.3 29.8 0.9

94.7 64.9 28.9 0.9

and redistribute hydrogen more readily than two-ring aromatics. This may be the explanation for the better performance of anthracene and creosote oils in comparison with tetralin. When no catalyst is used, since the oils contain less donatable hydrogen than tetralin, they give lower liquefaction yields, but when very active catalysts are used, there are compounds in the oils (three- and four-ring condensed aromatics) which hydrogenate and transfer the hydrogen to coal free radicals more efficiently than tetralin, consequently giving higher coal conversion. Finally, the catalyst activities in coal liquefaction with both tetralin and anthracene oil were compared using the standard Vouters 140 FR 36 coal. The results are presented in Tables 6 and 7 respectively. It can be seen that the conversion and oil + gas yield for Vouters are always higher than those obtained with Encasur. Vouters is therefore a more reactive coal than Encasur, and this is as expected, since it has higher H/C ratio, VM (daf) and reactive macerals (vitrinite + exinite) content than Encasur. If the catalyst activities are considered, the same trends and behaviour as observed with Encasur can be seen. Again the catalyst effects are much more pronounced with anthracene oil than with tetralin, with both solvents the Co(Zn)Mo/A1203 catalysts have far better activities than red mud, and when the Co(Zn)Mo/A1203 catalysts are used, coal conversion and oil + gas yield with anthracene oil are higher than with tetralin. However, now the differences among the catalytic effects are smaller, even when anthracene oil is used, and consequently the beneficial effect of F is hardly noticeable. However, it must be taken into account that the conversion is extremely high, - 9 4 % with anthracene oil, so the convertibility limit of the coal is presumably being approached, since vitrinite + exinite account for 96.2 vol.%; therefore higher catalyst activities cannot be observed and consequently the differences among catalysts are reduced or masked. CONCLUSIONS The CoMo/AI203 catalysts prepared for this study, either the conventional CMA or the novel CZMA and CZMFA.7, which contain the second promoter Zn and the alumina acidified with fluorine, have proved to be very active in coal liquefaction, so active that with them, coal tar distillates such as anthracene or creosote oils, which are non-H-donor solvents, give higher liquefaction yields than a typical Hdonor such as tetralin. This conclusion has been proved to be independent of the coal used.

ACKNOWLEDGEMENTS The authors would like to thank the Commission of the European Communities, as well as the UPV (Universidad del Pais Vasco) for financial assistance for this work.

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9 10 l1 12

13 14 15 16 17

El Sawy, A., Gray, D., Talib, A. and Tomlinson, G., A Techno-Economic Assessment of Recent Advances in Direct Coal Liquefaction. Sandia National Laboratories, 1986. Fierro, J. L. G., L6pez Agudo, A., Grange, P. and Delmon, B., in Proceedings of the 8th International Congress on Catalysis. Verlag Chemie, Berlin, 1984, pp. 363-373. Kibby, C. L. and Swift, H. E., Journal of Catalysis, 1976, 45, 231. Boorman, P. M., Kidd, R. A., Sarbak, Z. and Somogybary, A., Journal of Catalysis, 1985, 96, 115. Boorman, P. M., Kidd, R. A., Sarbak, Z. and Somogybari, A., Journal of Catalysis, 1987, 106, 544. Jiratova, K. and Kraus, M., Applied Catalysis, 1986, 27, 21. Ramirez, J., Cuevas, R. L., Lrpez Agudo, A., Mendioroz, S. and Fierro, J. L. G., Applied Catalysis, 1990, 57, 223. Cambra, J. F., Desarrollo de nuevos catalizadores de hidrodesulfuracion. Estudio de la influencia del promotor Zn, y de la acidificacidn del soporte alumfnico. Thesis, Universidad del Pals Vasco, 1989. Cambra, J. F., Gi.iemez, M. B., Arias, P. L., Legarreta, J. A. and Fierro, J. L. G., Industrial & Enginering Chemistry Research, 1991, 30, 2365. Gaemez, M. B., Cambra, J. F., Arias, P. L., Legarreta, J. A. and Fierro, J. L. G., Fuel, 1995, 74, 285. Marco, I., Caballero, B., Chom6n, M. J., Legarreta, J. A. and Urfa, P. M., Fuel Processing Technology, 1993, 36, 169. Legarreta, J. A., Arias, P. L., Marco, I., Chom6n, M. J., Caballero, B. M., Cambra, J. F., Gtiemez, M. B. and Fierro, J. L. G., International Journal of Energy Research, 1994, 18, 145. Marco, I., Caballero, B. M., Chom6n, M. J. and Legarreta, J. A., in 1995 International Conference on Coal Science. Elsevier Science, Amsterdam, 1995, pp. 1283-1286. Van Krevelen, D. W., Coal. Elsevier, Amsterdam, 1961, pp. 313-324. Bockrath, B. C., in Coal Science, Vol. 3, ed. M. L. Gorbaty, J. W. Larsen and I. Wender. Academic Press, London, 1983, pp. 65-120. Davidson, R. M., Mineral Effects in Coal Conversion. ICTIS/TR22, IEA Coal Research, London, 1983. Derbyshire, F. J., Catalysis in Coal Liquefaction. IEACR/ 08, lEA Coal Research, London, 1988.

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