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A comparative study on deposited and mixed iron oxide catalysts for hydrogenation of coal
Fuel Processing Technology, 8 (1984) 283--291
283
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
A COMPARATIVE STUDY ON DEPOSITED AND MIXED IRON OXIDE CATALYSTS FOR HYDROGENATION OF COAL
J.R. MITRA, P.B. CHOWDHURY and D.K. MUKHERJEE
Central Fuel Research Institute, P.O. FRI, Dhanbad 828108, Bihar (India) (Received April 5th, 1982; accepted September 8th, 1983)
ABSTRACT The activity and selectivity of two modes of using iron oxide catalysts for hydrogenation of coal have been compared. In one m o d e of application, the hydrated oxide of iron was deposited on coal by keeping it suspended in the aqueous solution of an iron salt and adding a m m o n i u m hydroxide to it. In the other case, the hydrated oxide was precipitated under exactly similar conditions in the absence of coal and was mixed with the powdered coal. For the same concentration of iron and under identical conditions of reaction, the overall efficiency of the deposited form of catalyst was found to be superior to the mixed type.
INTRODUCTION
Historically, iron catalysts have played a significantrole in the coal hydrogenation process. Iron oxides from different sources were used in commercial plants in Germany for liquid phase hydrogenation of coal to replace expensive molybdenum and tin compounds. Though comparatively less active, iron sulphide in various forms is even n o w considered as a potential disposable catalyst. The modern-day research interest is therefore guided either towards improvement of the catalytic activity of iron sulphides [ 1--4] or process innovations to compensate for the lower activity [ 5, 6]. Investigations in this laboratory showed that the catalytic property of iron, used initially as hydrated iron oxide and transformed subsequently into nonstoichiometric iron sulphide, can be enhanced when the hydrated oxide is deposited on coal by precipitation instead of mechanically mixing the powdered oxide with coal [7]. It was also observed that the deposited iron oxide was more effective than impregnated iron sulphate under identical atomic ratio of sulphur to iron or partial pressure ratio of hydrogen sulphide to hydrogen for the same concentration of metal [8]. In practice the present method of deposition of hydrated iron oxide on coal may be reckoned as an improved substitute for red mud, iron ore and similar other forms of mechanically mixed iron catalysts, as also impregnated iron salts for production of coal-liquids, the desired distillate fractions of
0378-3820/84/$03.00
© 1984 Elsevier Science Publishers B.V.
284
which may be further upgraded to the commercial petroleum substitutes. However, in order to study the relative effects of deposition and mixing, hydrated iron oxide of the same origin as prepared in the laboratory was used. The results of more detailed work with a less amenable coal under different operating conditions are presented. EXPERIMENTAL
A stock sample of washed coal (Kottadih, Raniganj) was used after pulverization through 200 mesh. The analysis of the coal sample is given in Table 1. The powdered coal was kept suspended in warm aqueous ferric nitrate solution by agitation in the presence of small amounts of nitric acid and ammonium nitrate. The hydrated oxide was precipitated by gradual addition of 1:1 NH4OH till a pH of 8--9 was attained. The material was filtered after settling, washed and dried. The coal mass contained 2.52% Fe on dry basis consisting of 1.96% from ferric nitrate and 0.56% from coal mineral matter. Hydrated iron oxide was also prepared in a similar way without using coal in suspension, powdered to below 200 mesh and mixed with the coal. TABLE 1
Analysis of washed coal (Kottadih colliery,Samla seam, Raniganj field) As received basis (wt. %)
Moisture Ash
4.0 10.9
DMMF basis (wt. %)
C H S N O (by diff.)
81.1 5.4 0.3 2.4 10.8
Visible MM F basis (vol. %)
Vitrinite Exinite Inertinite
83.4 6.5 10.1
The hydrogenation tests were carried o u t in the presence of two types of pasting media or solvent. Though largely non-catalytic, the transfer of hydrogen from the solvent is influenced b y the catalyst. In the present comparative study, therefore, a variation was introduced by using tetralin and tar oil (unneutralized, 230--330°C) as pasting media having different hydrogen donating properties. The charge for each experiment consisted of 72 g coal (dry basis), 123 g pasting medium, 1.8 g total Fe (as oxide and from mineral matter) and 4.67 g sulphur, made up of 4.50 g elemental sulphur and 0.17 g organic sulphur from coal. The atomic ratio of sulphur to iron was thus 4.34. The experiments were conducted in a one-litre rocking autoclave under a constant h o t pressure of 250-+2 kg/cm 2 (24.5 MPa) at 450°C maintained by
285 intermittent addition of hydrogen. The rate of heating of the autoclave to reach the reaction temperature was maintained as uniform as possible. Separate experiments were conducted for different durations starting from zero experimental time (time to reach 450°C) to a maximum of 90 minutes. The gas was released after cooling the autoclave to room temperature, metered and analysed. The contents of the autoclave were quantitatively recovered. Conversion of coal to gas and liquid was calculated on the basis of benzene solubility. The asphaltene content in the liquid was estimated using petroleum ether (60--80°C). The carbon and hydrogen estimations were carried out by Liebig's method. RESULTS AND DISCUSSION
Conversion and asphaltene yield The comparative data on total conversion and yields of asphaltene are graphically represented by the respective regression lines in Figs. 1 and 2. As it is not possible to evaluate separately the contributions of coal and pasting oil to the yield of oil inclusive of the distillate, gas and liquor, the theoretical composite yield of the three is evaluated by deducting the yield of asphaltene from total conversion. Under the selected operating conditions and in the presence of tetralin, the total conversion with both deposited and mixed catalysts attained almost the limiting value even at 30 minutes (Fig. 1). The lower values at zero times were also comparable for the two modes of catalyst usage. A perceptable difference is, however, noticed for the asphaltene yields. In both cases, the asphaltene contents showed gradual decrease and it seems that the peak, as expected for an intermediate product like asphaltene, occurred even before zero experimental time. The deposited catalyst nevertheless showed a lower asphaltene yield or a higher conversion of asphaltene, and the difference varied from about 7% at zero time to 9% at 90 minutes as compared with the mixed catalyst. The results of the experiments in the presence of tar oil as pasting medium are shown in Fig. 2. In this case, also, the conversions with the two forms of catalysts were comparable. The pattern of change in asphaltene yield was somewhat different, however, both the catalysts apparently exhibited the expected maxima. The asphaltene yield was observed to be more in the case of the deposited catalyst as compared with the mixed form up to a reaction time of about 60 minutes, but the rate of decay of asphaltene was also higher. Beyond 60 minutes, the deposited catalyst yielded less asphaltene and the asphaltene yield with this catalyst was 10--13% lower than that with the mixed iron oxide at 90 minutes. Considering the repeatability of the asphaltene determination method, this difference in yields is significant. Logically, the calculated composite yields of oil, gas and liquor show minima opposite to the asphaltene peaks. They, however, appear to have significance,
286
100
100
80
- 9 ( ) c~
J 0 ~J
8o
60 c~
Z Q
40
k~J
20
I 0
I
I
I
20
I
I
40 TIME
IN
I 60
I
I, 80
I
I 10Q
MINUTES
Fig. 1. Variation of conversion and yields in tetralin medium, o, =, • and o, o, n: total conversion, yield of asphaltene and yield of other products, with mixed and deposited catalysts, respectively.
as similar trends were observed in a separate work [9] where it was shown that asphaltenes are probably formed in two ways: directly from coal through benzene-insoluble intermediates or preasphaltenes, and from primary oils. As regards the comparative activities of the two forms of catalysts in the tar oil medium it thus appears that the mixed type of catalyst has a relatively greater influence up to about 60 minutes, although the corresponding asphaltene yield of 35--40% is much above the tolerance range. As the liquor yield does not vary considerably, the superiority of the mixed catalyst can only be justified if its selectivity towards oil formation is more than that of the deposited type. This, in other words, means that the gas yield with the mixed catalyst should be equal to or lower than that from the deposited catalyst. This aspect is discussed below.
287
100 -
qlO0
-90! -
-
g
? ~"
§
20
I
0
I
I
2O TIME
I
I
40 IN
I
I
60
I
I
B0
I
I
100
MINUTES IIL
Fig. 2. V a r i a t i o n o f c o n v e r s i o n and y i e l d s in tar o i l m e d i u m . , , , , • and o, o, z~: t o t a l con-
version, yield of asphaltene and yield of other products, with mixed and deposited catalysts, respectively.
Gas yield As it is not possible to differentiate between the gas yields from coal and pasting oil separately, the total yields of hydrocarbons up to C4 are given in Table 2 for both tetralin and tar oil runs. The gas yield with the deposited catalyst did not show a continuous rise from zero to 90 minutes as in case of the mixed catalyst, although there were slopes from 30 to 90 minutes. Quite characteristically, the yields at zero experimental time with this catalyst were higher than those at 30 minutes and also than the initial yields with the mixed catalyst. Though it is difficult to explain this p h e n o m e n o n from the available information it appears that the nature and contact of the deposited catalyst was such that non-catalytic thermal degradation predominated and progressed to a considerable extent even during the cooling period at zero time. In the case of 30-minute experiments, the catalyst probably played a role in changing the course of the reaction during the corresponding pc-
288
TABLE 2 Yield of hydrocarbon gases (C,---C4) Experimental time (min)
0 30 60 90
Yield (g) Tetralin medium
Tar oil medium
Mixed catalyst
Deposited Mixed catalyst catalyst
Deposited catalyst
1.78 2.33 5.35 5.34
4.80 2.83 4.28 4.88
8.85 6.63 10.72 a 16.95
7.82 12.20 16.36 a 18.61
a50 minutes.
riod at the reaction temperature. Further, a lag in the formation of the active phase of the deposited catalyst or higher initial activity of the mixed catalyst in hydrogenating the pasting oil, due to better initial contact between the two, may be the reason for the difference in the gas yields for the respective catalysts at zero time. With the progress of the reaction, however, the gas yields with the mixed catalyst increased and surpassed those from the deposited catalyst. Due to this trend the apparent advantage of lower asphaltene yields up to 60 minutes with the mixed catalyst in presence of tar oil loses its significance as higher amounts of the converted coal appear as gas.
Product quality The quality of the benzene soluble product with tar oil medium was assessed from the respective ratios of atomic hydrogen to carbon. The comparative data for the two catalyst forms (Table 3) show that the atomic H/C ratio was for all experimental times higher in the case of the deposited catalyst. Starting from zero experimental time, a maximum improvement by only two H atoms per 100 C atoms was observed at 30 minutes for the mixed catalyst, and the hydrogen content gradually depleted with longer duration of reaction. The deposited catalyst showed a continuous improvement with time up to a maximum of five H atoms per 100 C atoms at 50 minutes, which also decreased thereafter. It is to be noted that the difference between the maximum values of H/C ratio of the products obtained with the two catalysts correspond to eight H atoms per 100 C atoms. As the decrease in H/C ratio occurred in spite of the decrease in asphaltene yield and increase in hydrogen consumption, it appears that there was a greater shift towards pyrolytic rupture of saturated rings and dealkylation, and thus there was a proportionally higher loss of hydrogen as hydrocarbon gases. The comparative data, however, show that the product quality is more favourable in the case of the deposited catalyst.
289 TABLE 3 Atomic H/C ratio of benzene-soluble products with tar oil medium Experimental time (rain)
Mixed catalyst
Deposited catalyst
0 30 50 90
1.23 1.25 1.24 1.22
1.28 1.31 1.33 1.29
Hydrogen consumption The data given in Table 4 show that on the basis of the total charge, the hydrogen consumption was higher with the deposited catalyst in the presence of tetralin. There was, of course, higher yield of hydrocarbon gases up to 30 minutes, but the general trend of higher hydrogen consumption with this catalyst may be mainly attributed to the desired lower yields of asphaltene, especially when the lower yields of gases at 60 and 90 minutes are considered. In tar oil medium the consumption of hydrogen was higher in the case of the mixed catalyst up to 50 minutes. This may be the reason for lower asphaltene yields up to about 60 minutes, but the undesirable higher yields of hydrocarbon gas are also to be taken into consideration. The higher initial hydrogen consumption with this catalyst may be due to higher hydrogenation of the tar oil because of better initial contact between the two. The increased hydrogen consumption in the case of the deposited catalyst at 90 minutes having gas yields lower than that from the mixed catalyst is obviously due to higher conversion of asphaltene. The deposited catalyst was thus found to be more efficient in the overall reaction for the desired yields of products. TABLE4
Consumption of hydrogen Experimental
Yield (g)
time (rain)
Tetralin medium
Tar oil medium
Mixed catalyst
Deposited catalyst
Mixed catalyst
Deposited catalyst
0.98 1.87 2.21 2.94
2.16 2.97 3.50 4.61
2.66 3.53 3.68 a 3.96
2.08 2.93 2.93 a 4.65
0 30 60 90
a50 minutes.
290 CONCLUSION The study shows that for the same concentration of metal and under identical conditions of all other reaction parameters, the overall activity and selec. tivity of the hydrated oxide of iron is better if it is deposited on coal than when the two are mechanically mixed. The superiority of the deposited catalyst becomes particularly prominent under those conditions where a reasonably low yield of asphaltene is desired. Under such conditions, the deposited catalyst produced n o t only less asphaltene but also less gas and a better-quality product in terms of atomic H/C ratio, viz., at 90 minutes in tar oil medium. In the presence of tetralin, it was observed that the asphaltene yield was lower and the consumption of hydrogen was higher with the deposited catalyst as compared with the mixed catalyst. Although the atomic H/C ratio of the products from the experiments with tetralin medium were not determined, the deposited catalyst produced less hydrocarbon gases at 60 and 90 minutes, i.e., in the region of the desired coal and asphaltene conversions. The variations in the composition of the iron sulphide phases with time for the two catalyst modes are presently under investigation using X-ray diffraction and MSssbauer spectroscopic techniques. ACKNOWLEDGEMENT The authors wish to t h a n k Mr. T.K. Goswami and Dr. K.N. Bhattacharyya for gas analysis and the Analytical Section of the Institute for carbon--hydrogen estimations. Thanks are also due to Mr. U.N. Sharma and Mr. S.R. Rudra for technical assistance. REFERENCES
1 Andres, M. and Charcosset, H., 1979. Catalyse de la liquefaction des charbons par hydrogenation sous pression. J. Chim. Phys., 76 (10): 887--901. 2 Lambert, J.M., Simkovich, G. and Walker, Jr.,P.L., 1980. Production of pyrrhotites by pyrite reduction. Fuel, 59: 687--690. 3 Moroni, E.C. and Fisher, R.H., 1980. Disposable catalysts for coal liquefaction. Amer. Chem. Soc., Div. Fuel Chem., Prepr., 25 (1): 16. 4 Stahl, F.V. and Granoff, B., 1981. The relationship between the properties of iron sulfides and their catalytic activity. Paper presented at the AIChE Spring National Meeting & 11th Petro Expo, Houston, Texas, April 5--9. 5 Wiirfel,H.E., 1979. A further development of proven I.G. Farben technology. Fuel
Processing Technology, 2: 227--233. 6 Morita, M., Hashimoto, T., Sato, S., Toyoshima, H. and Ito, T., 1980. Reduction of reaction time on direct liquefaction of coal by using disposable catalysts. J. Chem. Soc. Jpn., 6: 931--938. 7 Mukherjee, D.K., Chowdhury, P.B., Sama, J.K., Basu, A.N., Basak, N.G. and Lahiri, A. Indian Patent 126,709 (May 18, 1970); U.S. Patent 3,775,286 (November 27, 1973). 8 Mukherjee, D.K., Sama, J.K., Chowdhury, P.B. and Lahiri, A., 1972. Hydrogenation of coal with iron catalysts. Proceedings Symposium Chemicals and Oil from Coal, Central Fuel Research Institute, India. Paper 8, pp. 116--127.
291 9 Gun, S.R., Chowdhury, P.B., Sama, J.K., Mukherjee, S.K. and Mukherjee, D.K., 1979. Kinetics of growth and decay of asphaltenes during hydrogenation of coal. Paper No. F-6, presented at the International Symposium on Coal Science and Technology for the Eighties, Central Fuel Research Institute, India, February 1--3.
283
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
A COMPARATIVE STUDY ON DEPOSITED AND MIXED IRON OXIDE CATALYSTS FOR HYDROGENATION OF COAL
J.R. MITRA, P.B. CHOWDHURY and D.K. MUKHERJEE
Central Fuel Research Institute, P.O. FRI, Dhanbad 828108, Bihar (India) (Received April 5th, 1982; accepted September 8th, 1983)
ABSTRACT The activity and selectivity of two modes of using iron oxide catalysts for hydrogenation of coal have been compared. In one m o d e of application, the hydrated oxide of iron was deposited on coal by keeping it suspended in the aqueous solution of an iron salt and adding a m m o n i u m hydroxide to it. In the other case, the hydrated oxide was precipitated under exactly similar conditions in the absence of coal and was mixed with the powdered coal. For the same concentration of iron and under identical conditions of reaction, the overall efficiency of the deposited form of catalyst was found to be superior to the mixed type.
INTRODUCTION
Historically, iron catalysts have played a significantrole in the coal hydrogenation process. Iron oxides from different sources were used in commercial plants in Germany for liquid phase hydrogenation of coal to replace expensive molybdenum and tin compounds. Though comparatively less active, iron sulphide in various forms is even n o w considered as a potential disposable catalyst. The modern-day research interest is therefore guided either towards improvement of the catalytic activity of iron sulphides [ 1--4] or process innovations to compensate for the lower activity [ 5, 6]. Investigations in this laboratory showed that the catalytic property of iron, used initially as hydrated iron oxide and transformed subsequently into nonstoichiometric iron sulphide, can be enhanced when the hydrated oxide is deposited on coal by precipitation instead of mechanically mixing the powdered oxide with coal [7]. It was also observed that the deposited iron oxide was more effective than impregnated iron sulphate under identical atomic ratio of sulphur to iron or partial pressure ratio of hydrogen sulphide to hydrogen for the same concentration of metal [8]. In practice the present method of deposition of hydrated iron oxide on coal may be reckoned as an improved substitute for red mud, iron ore and similar other forms of mechanically mixed iron catalysts, as also impregnated iron salts for production of coal-liquids, the desired distillate fractions of
0378-3820/84/$03.00
© 1984 Elsevier Science Publishers B.V.
284
which may be further upgraded to the commercial petroleum substitutes. However, in order to study the relative effects of deposition and mixing, hydrated iron oxide of the same origin as prepared in the laboratory was used. The results of more detailed work with a less amenable coal under different operating conditions are presented. EXPERIMENTAL
A stock sample of washed coal (Kottadih, Raniganj) was used after pulverization through 200 mesh. The analysis of the coal sample is given in Table 1. The powdered coal was kept suspended in warm aqueous ferric nitrate solution by agitation in the presence of small amounts of nitric acid and ammonium nitrate. The hydrated oxide was precipitated by gradual addition of 1:1 NH4OH till a pH of 8--9 was attained. The material was filtered after settling, washed and dried. The coal mass contained 2.52% Fe on dry basis consisting of 1.96% from ferric nitrate and 0.56% from coal mineral matter. Hydrated iron oxide was also prepared in a similar way without using coal in suspension, powdered to below 200 mesh and mixed with the coal. TABLE 1
Analysis of washed coal (Kottadih colliery,Samla seam, Raniganj field) As received basis (wt. %)
Moisture Ash
4.0 10.9
DMMF basis (wt. %)
C H S N O (by diff.)
81.1 5.4 0.3 2.4 10.8
Visible MM F basis (vol. %)
Vitrinite Exinite Inertinite
83.4 6.5 10.1
The hydrogenation tests were carried o u t in the presence of two types of pasting media or solvent. Though largely non-catalytic, the transfer of hydrogen from the solvent is influenced b y the catalyst. In the present comparative study, therefore, a variation was introduced by using tetralin and tar oil (unneutralized, 230--330°C) as pasting media having different hydrogen donating properties. The charge for each experiment consisted of 72 g coal (dry basis), 123 g pasting medium, 1.8 g total Fe (as oxide and from mineral matter) and 4.67 g sulphur, made up of 4.50 g elemental sulphur and 0.17 g organic sulphur from coal. The atomic ratio of sulphur to iron was thus 4.34. The experiments were conducted in a one-litre rocking autoclave under a constant h o t pressure of 250-+2 kg/cm 2 (24.5 MPa) at 450°C maintained by
285 intermittent addition of hydrogen. The rate of heating of the autoclave to reach the reaction temperature was maintained as uniform as possible. Separate experiments were conducted for different durations starting from zero experimental time (time to reach 450°C) to a maximum of 90 minutes. The gas was released after cooling the autoclave to room temperature, metered and analysed. The contents of the autoclave were quantitatively recovered. Conversion of coal to gas and liquid was calculated on the basis of benzene solubility. The asphaltene content in the liquid was estimated using petroleum ether (60--80°C). The carbon and hydrogen estimations were carried out by Liebig's method. RESULTS AND DISCUSSION
Conversion and asphaltene yield The comparative data on total conversion and yields of asphaltene are graphically represented by the respective regression lines in Figs. 1 and 2. As it is not possible to evaluate separately the contributions of coal and pasting oil to the yield of oil inclusive of the distillate, gas and liquor, the theoretical composite yield of the three is evaluated by deducting the yield of asphaltene from total conversion. Under the selected operating conditions and in the presence of tetralin, the total conversion with both deposited and mixed catalysts attained almost the limiting value even at 30 minutes (Fig. 1). The lower values at zero times were also comparable for the two modes of catalyst usage. A perceptable difference is, however, noticed for the asphaltene yields. In both cases, the asphaltene contents showed gradual decrease and it seems that the peak, as expected for an intermediate product like asphaltene, occurred even before zero experimental time. The deposited catalyst nevertheless showed a lower asphaltene yield or a higher conversion of asphaltene, and the difference varied from about 7% at zero time to 9% at 90 minutes as compared with the mixed catalyst. The results of the experiments in the presence of tar oil as pasting medium are shown in Fig. 2. In this case, also, the conversions with the two forms of catalysts were comparable. The pattern of change in asphaltene yield was somewhat different, however, both the catalysts apparently exhibited the expected maxima. The asphaltene yield was observed to be more in the case of the deposited catalyst as compared with the mixed form up to a reaction time of about 60 minutes, but the rate of decay of asphaltene was also higher. Beyond 60 minutes, the deposited catalyst yielded less asphaltene and the asphaltene yield with this catalyst was 10--13% lower than that with the mixed iron oxide at 90 minutes. Considering the repeatability of the asphaltene determination method, this difference in yields is significant. Logically, the calculated composite yields of oil, gas and liquor show minima opposite to the asphaltene peaks. They, however, appear to have significance,
286
100
100
80
- 9 ( ) c~
J 0 ~J
8o
60 c~
Z Q
40
k~J
20
I 0
I
I
I
20
I
I
40 TIME
IN
I 60
I
I, 80
I
I 10Q
MINUTES
Fig. 1. Variation of conversion and yields in tetralin medium, o, =, • and o, o, n: total conversion, yield of asphaltene and yield of other products, with mixed and deposited catalysts, respectively.
as similar trends were observed in a separate work [9] where it was shown that asphaltenes are probably formed in two ways: directly from coal through benzene-insoluble intermediates or preasphaltenes, and from primary oils. As regards the comparative activities of the two forms of catalysts in the tar oil medium it thus appears that the mixed type of catalyst has a relatively greater influence up to about 60 minutes, although the corresponding asphaltene yield of 35--40% is much above the tolerance range. As the liquor yield does not vary considerably, the superiority of the mixed catalyst can only be justified if its selectivity towards oil formation is more than that of the deposited type. This, in other words, means that the gas yield with the mixed catalyst should be equal to or lower than that from the deposited catalyst. This aspect is discussed below.
287
100 -
qlO0
-90! -
-
g
? ~"
§
20
I
0
I
I
2O TIME
I
I
40 IN
I
I
60
I
I
B0
I
I
100
MINUTES IIL
Fig. 2. V a r i a t i o n o f c o n v e r s i o n and y i e l d s in tar o i l m e d i u m . , , , , • and o, o, z~: t o t a l con-
version, yield of asphaltene and yield of other products, with mixed and deposited catalysts, respectively.
Gas yield As it is not possible to differentiate between the gas yields from coal and pasting oil separately, the total yields of hydrocarbons up to C4 are given in Table 2 for both tetralin and tar oil runs. The gas yield with the deposited catalyst did not show a continuous rise from zero to 90 minutes as in case of the mixed catalyst, although there were slopes from 30 to 90 minutes. Quite characteristically, the yields at zero experimental time with this catalyst were higher than those at 30 minutes and also than the initial yields with the mixed catalyst. Though it is difficult to explain this p h e n o m e n o n from the available information it appears that the nature and contact of the deposited catalyst was such that non-catalytic thermal degradation predominated and progressed to a considerable extent even during the cooling period at zero time. In the case of 30-minute experiments, the catalyst probably played a role in changing the course of the reaction during the corresponding pc-
288
TABLE 2 Yield of hydrocarbon gases (C,---C4) Experimental time (min)
0 30 60 90
Yield (g) Tetralin medium
Tar oil medium
Mixed catalyst
Deposited Mixed catalyst catalyst
Deposited catalyst
1.78 2.33 5.35 5.34
4.80 2.83 4.28 4.88
8.85 6.63 10.72 a 16.95
7.82 12.20 16.36 a 18.61
a50 minutes.
riod at the reaction temperature. Further, a lag in the formation of the active phase of the deposited catalyst or higher initial activity of the mixed catalyst in hydrogenating the pasting oil, due to better initial contact between the two, may be the reason for the difference in the gas yields for the respective catalysts at zero time. With the progress of the reaction, however, the gas yields with the mixed catalyst increased and surpassed those from the deposited catalyst. Due to this trend the apparent advantage of lower asphaltene yields up to 60 minutes with the mixed catalyst in presence of tar oil loses its significance as higher amounts of the converted coal appear as gas.
Product quality The quality of the benzene soluble product with tar oil medium was assessed from the respective ratios of atomic hydrogen to carbon. The comparative data for the two catalyst forms (Table 3) show that the atomic H/C ratio was for all experimental times higher in the case of the deposited catalyst. Starting from zero experimental time, a maximum improvement by only two H atoms per 100 C atoms was observed at 30 minutes for the mixed catalyst, and the hydrogen content gradually depleted with longer duration of reaction. The deposited catalyst showed a continuous improvement with time up to a maximum of five H atoms per 100 C atoms at 50 minutes, which also decreased thereafter. It is to be noted that the difference between the maximum values of H/C ratio of the products obtained with the two catalysts correspond to eight H atoms per 100 C atoms. As the decrease in H/C ratio occurred in spite of the decrease in asphaltene yield and increase in hydrogen consumption, it appears that there was a greater shift towards pyrolytic rupture of saturated rings and dealkylation, and thus there was a proportionally higher loss of hydrogen as hydrocarbon gases. The comparative data, however, show that the product quality is more favourable in the case of the deposited catalyst.
289 TABLE 3 Atomic H/C ratio of benzene-soluble products with tar oil medium Experimental time (rain)
Mixed catalyst
Deposited catalyst
0 30 50 90
1.23 1.25 1.24 1.22
1.28 1.31 1.33 1.29
Hydrogen consumption The data given in Table 4 show that on the basis of the total charge, the hydrogen consumption was higher with the deposited catalyst in the presence of tetralin. There was, of course, higher yield of hydrocarbon gases up to 30 minutes, but the general trend of higher hydrogen consumption with this catalyst may be mainly attributed to the desired lower yields of asphaltene, especially when the lower yields of gases at 60 and 90 minutes are considered. In tar oil medium the consumption of hydrogen was higher in the case of the mixed catalyst up to 50 minutes. This may be the reason for lower asphaltene yields up to about 60 minutes, but the undesirable higher yields of hydrocarbon gas are also to be taken into consideration. The higher initial hydrogen consumption with this catalyst may be due to higher hydrogenation of the tar oil because of better initial contact between the two. The increased hydrogen consumption in the case of the deposited catalyst at 90 minutes having gas yields lower than that from the mixed catalyst is obviously due to higher conversion of asphaltene. The deposited catalyst was thus found to be more efficient in the overall reaction for the desired yields of products. TABLE4
Consumption of hydrogen Experimental
Yield (g)
time (rain)
Tetralin medium
Tar oil medium
Mixed catalyst
Deposited catalyst
Mixed catalyst
Deposited catalyst
0.98 1.87 2.21 2.94
2.16 2.97 3.50 4.61
2.66 3.53 3.68 a 3.96
2.08 2.93 2.93 a 4.65
0 30 60 90
a50 minutes.
290 CONCLUSION The study shows that for the same concentration of metal and under identical conditions of all other reaction parameters, the overall activity and selec. tivity of the hydrated oxide of iron is better if it is deposited on coal than when the two are mechanically mixed. The superiority of the deposited catalyst becomes particularly prominent under those conditions where a reasonably low yield of asphaltene is desired. Under such conditions, the deposited catalyst produced n o t only less asphaltene but also less gas and a better-quality product in terms of atomic H/C ratio, viz., at 90 minutes in tar oil medium. In the presence of tetralin, it was observed that the asphaltene yield was lower and the consumption of hydrogen was higher with the deposited catalyst as compared with the mixed catalyst. Although the atomic H/C ratio of the products from the experiments with tetralin medium were not determined, the deposited catalyst produced less hydrocarbon gases at 60 and 90 minutes, i.e., in the region of the desired coal and asphaltene conversions. The variations in the composition of the iron sulphide phases with time for the two catalyst modes are presently under investigation using X-ray diffraction and MSssbauer spectroscopic techniques. ACKNOWLEDGEMENT The authors wish to t h a n k Mr. T.K. Goswami and Dr. K.N. Bhattacharyya for gas analysis and the Analytical Section of the Institute for carbon--hydrogen estimations. Thanks are also due to Mr. U.N. Sharma and Mr. S.R. Rudra for technical assistance. REFERENCES
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