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Steam cracking of coal-derived oils and model compounds
Steam cracking of coal-derived model compounds 2. Cracking Wolfgang
of hydrogenated
Hillebrand,
Werner
oils and
coal-derived
Hodek* and Georg
Uhde GmbH, Dortmund and *Bergbau-Forschung (Received 26 June 1983)
GmbH,
oils
KGlling* Essen. West Germany
Coal-derived oilsof differentdegreesof hydrogenation and of different boiling rangesaswellasa petrolbased gas oil (for comparison) have been thermally cracked at 830°C with a residence time of 0.14s. Hydrogenated coal-derived middle oil yields less light olefins than gas oil (32 versus 40wt%) but substantially more BTX aromatics (21 versus 7wt%). The amount of olefins (42 wt%) from hydrogenated coal-derived light oil is higher and that of BTX aromatics (16wt%) is slightly less than obtained from middle oil. Perhydrogenated coal-derived middle oil (>99%saturated compounds) has been cracked within a wide temperature- and residence time range to verify the applicability of two crack severity functions designed to represent the product distribution. The latter are compared with that oftdecalin and naphtha. (Keywords:
coal; coal-derived oils; model compounds)
Coal liquefaction was practised on a large scale before the Second World War but it is only recently that it has been reactivated at high expenditure. The concept of using coal-derived oil as a feed for steam cracking is, however, new. In 1976 the first paper on this subject’ was published and the interest in pyrolysis of coal-derived oil has increased ever since’-‘. Early experiments used coal-derived oil from the ‘synthoil’ process’. This coal-derived oil (boiling range between 250 and 380°C) was further hydrogenated and then characterized as given in Table 1. The hydrogenated coal-derived oil was thermally cracked at 875°C with a residence time of 0.3 s and a steam/hydrocarbon ratio of 1. The main product yields were compared with those having been obtained from the cracking of gas oil and decalin under almost identical conditions. The authors came to the following conclusions. The yield in light Table 1 Pyrolysis of coal-derived
oils
Feed Density
(g cm-?
Average b.p. (‘Cl H (wt%) c (wt%) Saturates (wt%) Aromatics (wt%) Conditions Temperature (“Cl Residence time (s) Partial pressure (kPa) Yields IL@%) Ethene Propene BTX 0016-2361/84/060762~5$3.00 @ 1984 Butterworth & Co. (Publishers)
FUEL,
Ltd.
1984, Vol 63, June
(2)
(1) Hydrogenated heavy oil
(3) Hydrogenated extract
(4) Hydrogenated light oil
(5) Hydrogenated heavy oil
0.895 (15W 285 12.8 87.1 84.1 12.8
0.851 (2OW 140 11.3 87.1 59.3 38.3
0.818 (15W 150 12.5 84.7 71 26
0.906 (15W 300 11.6 85.5 52 48
?
875 0.3 10 (steam)
800 1.2 10 (steam)
783 0.09
750 0.06
860 0.4 ?
(NE)
(N:,
18.8 7.8 6.2 22.3
c4
762
olefins from coal-derived oil is slightly less than that from gas oil, whereas almost double the amount of BTX aromatics can be obtained. The product distribution from coal-derived oil was nearly identical to that from decalin, except for the fuel oil which was twice as high with coalderived oil. This phenomenon is attributed to the influence of hydroaromatics which are dehydrogenated during cracking. The principal conclusion of their work was that coal-derived oil has to be completely hydrogenated to give a good pyrolysis feed. In a study3 of the influence of temperature and residence time on the product distribution in the pyrolysis of a hydrogenated coal extract, the extract (boiling range 8& 230°C) was cracked in a quartz tube at temperatures between 700 and 800°C and with residence times of 1.2 and 2.5 s. The maximum ethylene yield amounted to 2 18 wt%.
18 5 4 14
26 8.4 2.1
11.6 5.7 5.3
Hydrogenated extract
140-250 13.2 86.4 95.5 2.2
23.1 8.0 25.4
Steam cracking of coal-derived oils: W. Hillebrand et al.
by secondary hydrogenation on a nickel contact. Coalderived oils of different degrees of hydrogenation were characterized using elemental analysis, ‘H n.m.r. spectroscopy and h.p.1.c. as well as by their density and boiling behaviour. Details of the hydrogenation and characterization of these coal oils are presented elsewhere6. The properties of the oils and those of a petrol-based gas oil are summarized in Table 2. Thermal cracking of these oils and analyses of the cracking products were carried out as described previously’ for the cracking of t-decalin.
propylene to x 5 wt%, C, olelins to 6 wt%, BTX to 14 wt%. Furthermore, the hydrogenated coal extract was separated into boiling range fractions which were added separately to the pyrolysis process and it was established that the lower-boiling fractions gave higher yields of light olefins. Pyrolysis of the non-hydrogenated extract resulted in poor olefm yields: ethylene 7 wt”/O,propylene 2 wt%, C4 olefins 1 wt%. This emphasized the significance of secondary hydrogenation. Pyrolysis of a light oil fraction of a hydrogenated lowtemperature carbonization tar was the subject of a further study4, in which experiments were carried out in a temperature range between 500 and 780°C with residence times from 0.01-0.12 s. The maximum yields of ethylene, propylene and C4 olelins were 25, 11 and 5 wto/,, respectively. Pyrolysis of a heavy oil fraction of the same coal-derived oil (boiling range between 200 and 400°C) gave a yield of 12 wt% ethylene, 6 wt% propylene and 5 wt% C4 hydrocarbons’. Finally, there is a study in which coal extracts of different degrees of hydrogenation were thermally cracked at temperatures between 800 and 960°C and with 0.4 s residence time*. While the fractions of a lesser degree of hydrogenation yielded relatively small amounts of BTX and ethylene, the most highly hydrogenated fraction gave 27 wt% BTX and 25 wt% ethylene. 16 wt% BTX and 30wt% ethylene were recovered from a North Sea gas condensate cracked in similar conditions. It is difficult to compare the results of the different studies with one another because in most cases the reaction conditions, i.e. temperature and residence time, are not clearly defined. Moreover, the crack severity functions applied for correlation purposes in the cracking of naphtha or gas oils have not been tested for their applicability to results with coal-derived oil. Finally, no attempt has yet been made to predict product yields using characterization factors as is common practice in the mineral oil sector.
RESULTS AND DISCUSSION 17hermalcracking of a perhydrogenated coal-derived middle oil
A total of 22 tests was carried out with perhydrogenated coal-derived middle oil (VH 6). The temperature was varied between 700 and 890°C and the residence time was between 0.08 and 0.35s. The liquid products were analysed by means of gas chromatography, the amount of BTX aromatics being determined by means of a standard and the overall product being classified into two fractions, T,< 180°C and T,> 180°C which correspond to the common classification in gasoline and fuel oil cracking. The properties of the feed oil are given in Table 2. Product distribution in relation to the crack severity function
To represent the measured results, a check was made of the applicability of two ‘crack severity functions’ which are used in steam-cracking naphtha and gas oil. For gas oils the following relation is generally used’*‘: S=T.z”
where: T is the reference temperature, 7 the residence time, and m an empirical parameter. In the literature a value of 0.06 for m is often given but for the results obtained in this study a value of 0.02 gave the best representation. As shown in Figure 1, ethylene yields from hydrogenated coal-derived middle oil increased with increasing crack severity. The maximum ethylene yield was ~25 wt%. Propylene and the C4 olelins passed
EXPERIMENTAL Coal-derived oil from sump phase hydrogenation was subjected to hydrotreating on a Ni/W catalyst, followed Table 2 Characterization Coalderived middle oil
Oil Densitv (2OW’ Average b.p. (W BMCI UOP-K R Elemental analysis (wt%) C H N s fa
0 Saturates content (wt%)
0.970
of various pyrolysis
VR6 0.933
VHl
0.937
feedstocks
VH2
0.932
226
224
226
226
100 9.9 165
83 10.4 167
85 10.3 161
82 10.4 157
88.9 9.2 0.6 0.2
0.57 0.30
7
90.5 10.0 0.1 0.01 0.49 0.37 12
89.1 10.5 0.1 0.01 0.42 0.44
17
89.1 11 .o 0.1 0.01 0.40 0.42 19
VH3
0.895 220 66 10.7
177
86.9 12.0
VH4
0.897 225 66
10.7 174
86.5 12.7
0.1 0.01
0.1 0.01
0.17 0.54 60
0.17 0.61 62
VH5
0.887 211 64 10.8 168
VH6
0.874 208 59 10.9 187
86.8
86.8
12.9
13.5 0.1 0.01 0.01 99
0.2 0.01 0.14 0.64 80
Hydrogenated coal-derived light oil
0.785
Hydrogenated coal-derived heavy oil
Gas oil
0.826
0.905
119
279
256
39
60
26
11 .o 218
11.9 215
11.3
115
85.3 14.7 0.1 0.01 0.01
99
86.7 12.8 0.1
0.07 0.01 99
FUEL, 1984, Vol63,
86.4 13.7 0.1 0.2 0.08 0.62 92
June
763
Steam cracking of coal-derived oils: W. Hillebrand et al.
through a yield maximum of 7 wt% and 5 wt%, respectively. The benzene yield increased with increasing crack severity, whereas toluene and the C, aromatics passed through a maximum at S = 840. The maximum BTX yield amounted to ~30 wt% (Figure 2). Gas and gasoline increased at the expense of the fuel oil up to S=850 (Figure .?), after which the proportions remained approximately constant. Gas formation attained a maximum of 50 wt% and gasoline from the pyrolysis attained 40 wt%. At low crack severities the fuel oil fraction consisted mostly of the unreacted parts of the feed middle oil. When comparing these results with those obtained by others there emerges the following pattern (see Table I).
loo-
80-
20-
o! 650
700
900 S=T. too2
Figure 3 Yields of A, gas; B, gasoline (Cs-180°C); and C, fuel oil (> 78OC) as a function of crack severity. Yield is given by the difference between the curves
650
700
800
Sal
SzT.too2
Figure 1 Gas yields from hydrogenated coal-derived middle oil as a function of crack severity. 0, Ethene; A, propene; 0, C,-olefins 30
The product distribution is most similar to that quoted by Bernhardt et aL2 for cracking of a hydrogenated extract. The total yield of light olefins is slightly higher in the present study (34 wt% uersus 32 wt%); the same applies to the BTX yield (30 wt% versus 25 wt%). The coal-derived oils used are very similar in their boiling range, hydrogen content and proportion of saturated compounds. They were, however, obtained by different processes. Similar results were obtained also by Korosi et al.’ who used a hydrogenated heavy oil. In their case the ethylene yield is lower (19 wt%), the propylene and C4 yields are higher (14 wt% uersus 9 wt%), whereas the BTX yield is again lower (22 wt% uersus 30 wt%). The feed oil was of a higher boiling range and contained slightly more aromatic compounds and, accordingly, slightly less hydrogen. Despite these differences there is one feature common to all previous studies in that the coal-derived oils, notwithstanding their different origins and different pyrolysis conditions, yield a similar product distribution providing they are highly saturated and they have a similar boiling range.
20
Comparison of product distributions from perhydrogenated coal-derived oil, t-decalin and naphtha
IO
To compare the product distribution from coal-derived middle oil to that obtained from t-decalin, a different crack severity function, the so-called ‘kinetic severity function’ (KSF), was applied. It is defined by the conversion of a key component, i.e. pentane in the case of naphtha pyrolysis”: kdz
S=
T.too2
Figure 2 BTX Yield as a function of crack severity. x, BTX; 0, benzene; A, toluene; n , Cs-aromatics
764
FU’EL, 1984, Vol 63, June
The advantage of this method is that to derive the KSF value it is only necessary to determine the original and the final concentration of pentane. As t-decalin is the main component of hydrogenated middle oil, the t-decalin conversion of the middle oil was taken as the basis for deriving the KSF value. This also
Steam cracking of coal-derived oils: W. Hillebrand et al.
40
30
20
fraction during hydrogenation of the coal-derived middle oil, was subjected to thermal cracking at 3 different crack severities. The main analytical data are summarized in Table 2. Figure 6 shows the product distribution for the coal-derived light oil and middle oil as a function of crack severity. The hydrogenated coal-derived light oil yields significantly more ethylene than the hydrogenated middle oil (32 wt% versus 25 wt%). The propylene yields are slightly higher (maximum 9 wt% uersus maximum 8 wt%) and the amounts of BTX obtained from the middle oil are also higher (30 wt% versus 26 wt%). The formation of undesirable ‘fuel oil’ boiling at > 180°C is twice as high from the middle oil as from coalderived light oil.
10
0
0 0
1
2 KSF
3
Figure 4 Product yield depending on the kinetic severity function. ---, t-Decalin; -, hydrogenated coal-derived middle oil (VH 6); 0, ethene; A and 0, BTX; W and v methane; + and 0, propene
allows direct comparison with the cracking of pure tdecalin. When comparing the product distribution in the cracking of hydrogenated coal-derived middle oil and t-decalin as a function of crack severity the following is evident (Figure 4). The ethylene yield is identical (within the accuracy of measurement) for both feed materials. The BTX and C4 yields are slightly higher for t-decalin, while slightly more methane and propylene is recovered from coal-derived middle oil. The latter is understandable as hydrogenated coal-derived middle oil contains a number of methyl decalins which during thermal cracking may either eliminate methyl radicals contributing to methane formation or generate propylene, e.g. according to following reaction : I 2 KSF
I
1
In contrast, t-decalin would in the analogous reaction be converted to ethylene and cyclohexane. The yields of the three main products, ethylene, propylene, benzene, are plotted in Figure 5 as a function of the crack severity for a typical naphtha” as well as for hydrogenated coal-derived middle oil. Here, identical KSF values mean identical conversion of the key components pentane or t-decalin. However, this should not be taken for identical reaction conditions (Q) as the kinetic constants for the decomposition of the two substances are different. Coal-derived oil yields slightly less ethylene (25 wt% uersus x 30 wt%), less than half the amount of propylene, twice as much benzene and approximately identical amounts of C4 oletins. Comparison coal-derived
of product distribution from perhydrogenated light and middle oils
A perhydrogenated coal-derived light oil (boiling range 5O-180°C) which had been formed as a low-boiling
I
3
4
Figure 5 Product yield from naphtha” (-), and hydrogenated coal-derived middle oil (---) as a function of crack severity
7-----l1
$F-
3001
Ethene
8 lJl20g P, * lo-
O-%-k+-+0
1
2
3
4
50
KSF
KSF
Figure 8 Comparison of yields from a hydrogenated coalderived middle oil (0) and from a hydrogenated coal-derived light oil (A)
FUEL, 1984, Vol63,
June
765
Steam cracking
of coal-derived
oils: W. Hillebrand
et al.
The difference in these product yields is due to the different chemical compositions of the two oils. The main constituents of coal-derived light oil, l-ring naphthenes and paraffins, are mostly cracked into ethylene and other gaseous products, whereas only part of the 2-ring naphthenes of the middle oil will be cracked according to the mechanism described, whilst a part of them will, through cleaving of one ring and dehydrogenation of the second, be converted into BTX aromatics.
m
Benzene
Comparison of product yields from the various oils
The oils characterized in Table 2 were subjected to thermal cracking under identical conditions to examine the influence exerted by the properties of different feed oils on the product yields during thermal cracking. The experimental conditions (830°C 0.14 s) correspond to medium crack severity as carried out in conventional tube furnaces and maximize the yields in propylene and C, olefins. The maximum ethylene and BTX yields, however, are attained at higher crack severities. The yields of the main products are plotted in Figures 7 and 8. The yield of light olefins (Figure 7) is highest from hydrogenated coal-derived light oil (42 wt%), followed by that from gas oil (40 wt%). Gas oil produces less ethylene and C4 olefins but more propylene. The coal-derived middle oils of most advanced hydrogenation give 32 wt% olelins which is significantly less than that recovered from coal-derived light oil and gas oil. However, the ratio among light olefins is similar for coal-derived light oil and middle oil with a clearly higher selectivity for C, olelins in the case of coal-derived light oil. The hydrogenated coalderived heavy oil yields ~27 wt% of light olefins which is slightly less than the coal-derived middle oil although the diIference is not as great as that between light and middle oils. From a comparison of the coal-derived middle oils of different degrees of hydrogenation it is evident that there is a marked tendency for hydrogenation to favour the
0 1
2
3
4
5
6
7
Feedstcck
Figure 8 7)
Yields in BTX aromatics from different oils (see Figure
production of light olefins, i.e. there is a doubling of the yield (from 15 wt% to 32 wt%) from the least to the highest hydrogenated oil. The yield of BTX aromatics is highest (21 wt%) for the most hydrogenated coal-derived middle oil (VH 6) (Figure 8). Here again, hydrogenation leads to more than doubling of the BTX yield. Hydrogenated coal-derived heavy oil gives slightly lower BTX yields (18 wt%). The BTX yield from coal-derived light oil is z 16 wt% and that from gas oil is 8 wt%. In conclusion, the following order of product yields occurs: Olefin yield: Coal-derived light oil > gas oil > hydrogenated coal-derived middle oil > hydrogenated coal-derived heavy oil BTX yield: Hydrogenated coal-derived middle oil > hydrogenated coal-derived heavy oil > hydrogenated coal-derived light oil > gas oil ACKNOWLEDGEMENT The authors thank the Commission Communities for financial support.
of the European
REFERENCES 1 2 3 4 5 6 1
2
3
4 Feedstock
5
6
7
Figure 7 Light olefin yields from different feed oils. 1, VH 1; 2, VH 3; 3, VH 5; 4, VH 6 (VH 1, 3, 5 and 6, hydrogenated coalderived middle oil); 5, hydrogenated coal-derived heavy oil; 6. hydrogenated coal-derived light oil; 7, gas oil
766
FUEL, 1984, Vol63,
June
7 8 9 10 11
Korosi, A., Woebcke, H. N. and Virk, P. S. Am. Chem. Sot. Dia. Fuel Chem. Preprints 1976, X(6), 190 Bemhardt, R. S., Ladner, W. R., Newman, J. 0. H. and Sage, P. W. Am. Chem. Sot. Div. Fuel Gem. Preprints 1980,25(l), 188 Taniewski, M. and Krajewski, J. Przemysl Chemiczny 1978,57, 121 Krishnamurthy, S., Shah, Y. T. and Stiegel, G. J. Ind. Eng. Chem. Proc. Des. Deu. 1979, 18, 254 Krishnamurthy, S., Shah, Y. T. and Stiegel, G. J. Ind. Eng. Chem. Proc. Des. Dev. 1979, 18,466 Hillebrand, W. A., Liphard, K.-G., Strobe], B., Hodek, W. and Kolling, G. ErdGl und Kohle 1981, 34,443 Hillebrand, W., Hodek, W. and Kolling, G. Fuel 1984,63,756 Linden, H. R. and Peck, R. E. Ind. Eng. Chem. 1955,47,2470 White, L. R., Davies, H. G., Keller, G. E. and Rife, R. S. 63rd AIChE Meeting, Chicago, Nov. 1970 Zdonick, S. B., Green, E. J. and Hallee, L. P. Oil GasJ. 1968,19,91 Chambers, L. E. and Potter, W. S. Hydrocarbon Processing 1974, 53, 121
of hydrogenated
Hillebrand,
Werner
oils and
coal-derived
Hodek* and Georg
Uhde GmbH, Dortmund and *Bergbau-Forschung (Received 26 June 1983)
GmbH,
oils
KGlling* Essen. West Germany
Coal-derived oilsof differentdegreesof hydrogenation and of different boiling rangesaswellasa petrolbased gas oil (for comparison) have been thermally cracked at 830°C with a residence time of 0.14s. Hydrogenated coal-derived middle oil yields less light olefins than gas oil (32 versus 40wt%) but substantially more BTX aromatics (21 versus 7wt%). The amount of olefins (42 wt%) from hydrogenated coal-derived light oil is higher and that of BTX aromatics (16wt%) is slightly less than obtained from middle oil. Perhydrogenated coal-derived middle oil (>99%saturated compounds) has been cracked within a wide temperature- and residence time range to verify the applicability of two crack severity functions designed to represent the product distribution. The latter are compared with that oftdecalin and naphtha. (Keywords:
coal; coal-derived oils; model compounds)
Coal liquefaction was practised on a large scale before the Second World War but it is only recently that it has been reactivated at high expenditure. The concept of using coal-derived oil as a feed for steam cracking is, however, new. In 1976 the first paper on this subject’ was published and the interest in pyrolysis of coal-derived oil has increased ever since’-‘. Early experiments used coal-derived oil from the ‘synthoil’ process’. This coal-derived oil (boiling range between 250 and 380°C) was further hydrogenated and then characterized as given in Table 1. The hydrogenated coal-derived oil was thermally cracked at 875°C with a residence time of 0.3 s and a steam/hydrocarbon ratio of 1. The main product yields were compared with those having been obtained from the cracking of gas oil and decalin under almost identical conditions. The authors came to the following conclusions. The yield in light Table 1 Pyrolysis of coal-derived
oils
Feed Density
(g cm-?
Average b.p. (‘Cl H (wt%) c (wt%) Saturates (wt%) Aromatics (wt%) Conditions Temperature (“Cl Residence time (s) Partial pressure (kPa) Yields IL@%) Ethene Propene BTX 0016-2361/84/060762~5$3.00 @ 1984 Butterworth & Co. (Publishers)
FUEL,
Ltd.
1984, Vol 63, June
(2)
(1) Hydrogenated heavy oil
(3) Hydrogenated extract
(4) Hydrogenated light oil
(5) Hydrogenated heavy oil
0.895 (15W 285 12.8 87.1 84.1 12.8
0.851 (2OW 140 11.3 87.1 59.3 38.3
0.818 (15W 150 12.5 84.7 71 26
0.906 (15W 300 11.6 85.5 52 48
?
875 0.3 10 (steam)
800 1.2 10 (steam)
783 0.09
750 0.06
860 0.4 ?
(NE)
(N:,
18.8 7.8 6.2 22.3
c4
762
olefins from coal-derived oil is slightly less than that from gas oil, whereas almost double the amount of BTX aromatics can be obtained. The product distribution from coal-derived oil was nearly identical to that from decalin, except for the fuel oil which was twice as high with coalderived oil. This phenomenon is attributed to the influence of hydroaromatics which are dehydrogenated during cracking. The principal conclusion of their work was that coal-derived oil has to be completely hydrogenated to give a good pyrolysis feed. In a study3 of the influence of temperature and residence time on the product distribution in the pyrolysis of a hydrogenated coal extract, the extract (boiling range 8& 230°C) was cracked in a quartz tube at temperatures between 700 and 800°C and with residence times of 1.2 and 2.5 s. The maximum ethylene yield amounted to 2 18 wt%.
18 5 4 14
26 8.4 2.1
11.6 5.7 5.3
Hydrogenated extract
140-250 13.2 86.4 95.5 2.2
23.1 8.0 25.4
Steam cracking of coal-derived oils: W. Hillebrand et al.
by secondary hydrogenation on a nickel contact. Coalderived oils of different degrees of hydrogenation were characterized using elemental analysis, ‘H n.m.r. spectroscopy and h.p.1.c. as well as by their density and boiling behaviour. Details of the hydrogenation and characterization of these coal oils are presented elsewhere6. The properties of the oils and those of a petrol-based gas oil are summarized in Table 2. Thermal cracking of these oils and analyses of the cracking products were carried out as described previously’ for the cracking of t-decalin.
propylene to x 5 wt%, C, olelins to 6 wt%, BTX to 14 wt%. Furthermore, the hydrogenated coal extract was separated into boiling range fractions which were added separately to the pyrolysis process and it was established that the lower-boiling fractions gave higher yields of light olefins. Pyrolysis of the non-hydrogenated extract resulted in poor olefm yields: ethylene 7 wt”/O,propylene 2 wt%, C4 olefins 1 wt%. This emphasized the significance of secondary hydrogenation. Pyrolysis of a light oil fraction of a hydrogenated lowtemperature carbonization tar was the subject of a further study4, in which experiments were carried out in a temperature range between 500 and 780°C with residence times from 0.01-0.12 s. The maximum yields of ethylene, propylene and C4 olelins were 25, 11 and 5 wto/,, respectively. Pyrolysis of a heavy oil fraction of the same coal-derived oil (boiling range between 200 and 400°C) gave a yield of 12 wt% ethylene, 6 wt% propylene and 5 wt% C4 hydrocarbons’. Finally, there is a study in which coal extracts of different degrees of hydrogenation were thermally cracked at temperatures between 800 and 960°C and with 0.4 s residence time*. While the fractions of a lesser degree of hydrogenation yielded relatively small amounts of BTX and ethylene, the most highly hydrogenated fraction gave 27 wt% BTX and 25 wt% ethylene. 16 wt% BTX and 30wt% ethylene were recovered from a North Sea gas condensate cracked in similar conditions. It is difficult to compare the results of the different studies with one another because in most cases the reaction conditions, i.e. temperature and residence time, are not clearly defined. Moreover, the crack severity functions applied for correlation purposes in the cracking of naphtha or gas oils have not been tested for their applicability to results with coal-derived oil. Finally, no attempt has yet been made to predict product yields using characterization factors as is common practice in the mineral oil sector.
RESULTS AND DISCUSSION 17hermalcracking of a perhydrogenated coal-derived middle oil
A total of 22 tests was carried out with perhydrogenated coal-derived middle oil (VH 6). The temperature was varied between 700 and 890°C and the residence time was between 0.08 and 0.35s. The liquid products were analysed by means of gas chromatography, the amount of BTX aromatics being determined by means of a standard and the overall product being classified into two fractions, T,< 180°C and T,> 180°C which correspond to the common classification in gasoline and fuel oil cracking. The properties of the feed oil are given in Table 2. Product distribution in relation to the crack severity function
To represent the measured results, a check was made of the applicability of two ‘crack severity functions’ which are used in steam-cracking naphtha and gas oil. For gas oils the following relation is generally used’*‘: S=T.z”
where: T is the reference temperature, 7 the residence time, and m an empirical parameter. In the literature a value of 0.06 for m is often given but for the results obtained in this study a value of 0.02 gave the best representation. As shown in Figure 1, ethylene yields from hydrogenated coal-derived middle oil increased with increasing crack severity. The maximum ethylene yield was ~25 wt%. Propylene and the C4 olelins passed
EXPERIMENTAL Coal-derived oil from sump phase hydrogenation was subjected to hydrotreating on a Ni/W catalyst, followed Table 2 Characterization Coalderived middle oil
Oil Densitv (2OW’ Average b.p. (W BMCI UOP-K R Elemental analysis (wt%) C H N s fa
0 Saturates content (wt%)
0.970
of various pyrolysis
VR6 0.933
VHl
0.937
feedstocks
VH2
0.932
226
224
226
226
100 9.9 165
83 10.4 167
85 10.3 161
82 10.4 157
88.9 9.2 0.6 0.2
0.57 0.30
7
90.5 10.0 0.1 0.01 0.49 0.37 12
89.1 10.5 0.1 0.01 0.42 0.44
17
89.1 11 .o 0.1 0.01 0.40 0.42 19
VH3
0.895 220 66 10.7
177
86.9 12.0
VH4
0.897 225 66
10.7 174
86.5 12.7
0.1 0.01
0.1 0.01
0.17 0.54 60
0.17 0.61 62
VH5
0.887 211 64 10.8 168
VH6
0.874 208 59 10.9 187
86.8
86.8
12.9
13.5 0.1 0.01 0.01 99
0.2 0.01 0.14 0.64 80
Hydrogenated coal-derived light oil
0.785
Hydrogenated coal-derived heavy oil
Gas oil
0.826
0.905
119
279
256
39
60
26
11 .o 218
11.9 215
11.3
115
85.3 14.7 0.1 0.01 0.01
99
86.7 12.8 0.1
0.07 0.01 99
FUEL, 1984, Vol63,
86.4 13.7 0.1 0.2 0.08 0.62 92
June
763
Steam cracking of coal-derived oils: W. Hillebrand et al.
through a yield maximum of 7 wt% and 5 wt%, respectively. The benzene yield increased with increasing crack severity, whereas toluene and the C, aromatics passed through a maximum at S = 840. The maximum BTX yield amounted to ~30 wt% (Figure 2). Gas and gasoline increased at the expense of the fuel oil up to S=850 (Figure .?), after which the proportions remained approximately constant. Gas formation attained a maximum of 50 wt% and gasoline from the pyrolysis attained 40 wt%. At low crack severities the fuel oil fraction consisted mostly of the unreacted parts of the feed middle oil. When comparing these results with those obtained by others there emerges the following pattern (see Table I).
loo-
80-
20-
o! 650
700
900 S=T. too2
Figure 3 Yields of A, gas; B, gasoline (Cs-180°C); and C, fuel oil (> 78OC) as a function of crack severity. Yield is given by the difference between the curves
650
700
800
Sal
SzT.too2
Figure 1 Gas yields from hydrogenated coal-derived middle oil as a function of crack severity. 0, Ethene; A, propene; 0, C,-olefins 30
The product distribution is most similar to that quoted by Bernhardt et aL2 for cracking of a hydrogenated extract. The total yield of light olefins is slightly higher in the present study (34 wt% uersus 32 wt%); the same applies to the BTX yield (30 wt% versus 25 wt%). The coal-derived oils used are very similar in their boiling range, hydrogen content and proportion of saturated compounds. They were, however, obtained by different processes. Similar results were obtained also by Korosi et al.’ who used a hydrogenated heavy oil. In their case the ethylene yield is lower (19 wt%), the propylene and C4 yields are higher (14 wt% uersus 9 wt%), whereas the BTX yield is again lower (22 wt% uersus 30 wt%). The feed oil was of a higher boiling range and contained slightly more aromatic compounds and, accordingly, slightly less hydrogen. Despite these differences there is one feature common to all previous studies in that the coal-derived oils, notwithstanding their different origins and different pyrolysis conditions, yield a similar product distribution providing they are highly saturated and they have a similar boiling range.
20
Comparison of product distributions from perhydrogenated coal-derived oil, t-decalin and naphtha
IO
To compare the product distribution from coal-derived middle oil to that obtained from t-decalin, a different crack severity function, the so-called ‘kinetic severity function’ (KSF), was applied. It is defined by the conversion of a key component, i.e. pentane in the case of naphtha pyrolysis”: kdz
S=
T.too2
Figure 2 BTX Yield as a function of crack severity. x, BTX; 0, benzene; A, toluene; n , Cs-aromatics
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FU’EL, 1984, Vol 63, June
The advantage of this method is that to derive the KSF value it is only necessary to determine the original and the final concentration of pentane. As t-decalin is the main component of hydrogenated middle oil, the t-decalin conversion of the middle oil was taken as the basis for deriving the KSF value. This also
Steam cracking of coal-derived oils: W. Hillebrand et al.
40
30
20
fraction during hydrogenation of the coal-derived middle oil, was subjected to thermal cracking at 3 different crack severities. The main analytical data are summarized in Table 2. Figure 6 shows the product distribution for the coal-derived light oil and middle oil as a function of crack severity. The hydrogenated coal-derived light oil yields significantly more ethylene than the hydrogenated middle oil (32 wt% versus 25 wt%). The propylene yields are slightly higher (maximum 9 wt% uersus maximum 8 wt%) and the amounts of BTX obtained from the middle oil are also higher (30 wt% versus 26 wt%). The formation of undesirable ‘fuel oil’ boiling at > 180°C is twice as high from the middle oil as from coalderived light oil.
10
0
0 0
1
2 KSF
3
Figure 4 Product yield depending on the kinetic severity function. ---, t-Decalin; -, hydrogenated coal-derived middle oil (VH 6); 0, ethene; A and 0, BTX; W and v methane; + and 0, propene
allows direct comparison with the cracking of pure tdecalin. When comparing the product distribution in the cracking of hydrogenated coal-derived middle oil and t-decalin as a function of crack severity the following is evident (Figure 4). The ethylene yield is identical (within the accuracy of measurement) for both feed materials. The BTX and C4 yields are slightly higher for t-decalin, while slightly more methane and propylene is recovered from coal-derived middle oil. The latter is understandable as hydrogenated coal-derived middle oil contains a number of methyl decalins which during thermal cracking may either eliminate methyl radicals contributing to methane formation or generate propylene, e.g. according to following reaction : I 2 KSF
I
1
In contrast, t-decalin would in the analogous reaction be converted to ethylene and cyclohexane. The yields of the three main products, ethylene, propylene, benzene, are plotted in Figure 5 as a function of the crack severity for a typical naphtha” as well as for hydrogenated coal-derived middle oil. Here, identical KSF values mean identical conversion of the key components pentane or t-decalin. However, this should not be taken for identical reaction conditions (Q) as the kinetic constants for the decomposition of the two substances are different. Coal-derived oil yields slightly less ethylene (25 wt% uersus x 30 wt%), less than half the amount of propylene, twice as much benzene and approximately identical amounts of C4 oletins. Comparison coal-derived
of product distribution from perhydrogenated light and middle oils
A perhydrogenated coal-derived light oil (boiling range 5O-180°C) which had been formed as a low-boiling
I
3
4
Figure 5 Product yield from naphtha” (-), and hydrogenated coal-derived middle oil (---) as a function of crack severity
7-----l1
$F-
3001
Ethene
8 lJl20g P, * lo-
O-%-k+-+0
1
2
3
4
50
KSF
KSF
Figure 8 Comparison of yields from a hydrogenated coalderived middle oil (0) and from a hydrogenated coal-derived light oil (A)
FUEL, 1984, Vol63,
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765
Steam cracking
of coal-derived
oils: W. Hillebrand
et al.
The difference in these product yields is due to the different chemical compositions of the two oils. The main constituents of coal-derived light oil, l-ring naphthenes and paraffins, are mostly cracked into ethylene and other gaseous products, whereas only part of the 2-ring naphthenes of the middle oil will be cracked according to the mechanism described, whilst a part of them will, through cleaving of one ring and dehydrogenation of the second, be converted into BTX aromatics.
m
Benzene
Comparison of product yields from the various oils
The oils characterized in Table 2 were subjected to thermal cracking under identical conditions to examine the influence exerted by the properties of different feed oils on the product yields during thermal cracking. The experimental conditions (830°C 0.14 s) correspond to medium crack severity as carried out in conventional tube furnaces and maximize the yields in propylene and C, olefins. The maximum ethylene and BTX yields, however, are attained at higher crack severities. The yields of the main products are plotted in Figures 7 and 8. The yield of light olefins (Figure 7) is highest from hydrogenated coal-derived light oil (42 wt%), followed by that from gas oil (40 wt%). Gas oil produces less ethylene and C4 olefins but more propylene. The coal-derived middle oils of most advanced hydrogenation give 32 wt% olelins which is significantly less than that recovered from coal-derived light oil and gas oil. However, the ratio among light olefins is similar for coal-derived light oil and middle oil with a clearly higher selectivity for C, olelins in the case of coal-derived light oil. The hydrogenated coalderived heavy oil yields ~27 wt% of light olefins which is slightly less than the coal-derived middle oil although the diIference is not as great as that between light and middle oils. From a comparison of the coal-derived middle oils of different degrees of hydrogenation it is evident that there is a marked tendency for hydrogenation to favour the
0 1
2
3
4
5
6
7
Feedstcck
Figure 8 7)
Yields in BTX aromatics from different oils (see Figure
production of light olefins, i.e. there is a doubling of the yield (from 15 wt% to 32 wt%) from the least to the highest hydrogenated oil. The yield of BTX aromatics is highest (21 wt%) for the most hydrogenated coal-derived middle oil (VH 6) (Figure 8). Here again, hydrogenation leads to more than doubling of the BTX yield. Hydrogenated coal-derived heavy oil gives slightly lower BTX yields (18 wt%). The BTX yield from coal-derived light oil is z 16 wt% and that from gas oil is 8 wt%. In conclusion, the following order of product yields occurs: Olefin yield: Coal-derived light oil > gas oil > hydrogenated coal-derived middle oil > hydrogenated coal-derived heavy oil BTX yield: Hydrogenated coal-derived middle oil > hydrogenated coal-derived heavy oil > hydrogenated coal-derived light oil > gas oil ACKNOWLEDGEMENT The authors thank the Commission Communities for financial support.
of the European
REFERENCES 1 2 3 4 5 6 1
2
3
4 Feedstock
5
6
7
Figure 7 Light olefin yields from different feed oils. 1, VH 1; 2, VH 3; 3, VH 5; 4, VH 6 (VH 1, 3, 5 and 6, hydrogenated coalderived middle oil); 5, hydrogenated coal-derived heavy oil; 6. hydrogenated coal-derived light oil; 7, gas oil
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Korosi, A., Woebcke, H. N. and Virk, P. S. Am. Chem. Sot. Dia. Fuel Chem. Preprints 1976, X(6), 190 Bemhardt, R. S., Ladner, W. R., Newman, J. 0. H. and Sage, P. W. Am. Chem. Sot. Div. Fuel Gem. Preprints 1980,25(l), 188 Taniewski, M. and Krajewski, J. Przemysl Chemiczny 1978,57, 121 Krishnamurthy, S., Shah, Y. T. and Stiegel, G. J. Ind. Eng. Chem. Proc. Des. Deu. 1979, 18, 254 Krishnamurthy, S., Shah, Y. T. and Stiegel, G. J. Ind. Eng. Chem. Proc. Des. Dev. 1979, 18,466 Hillebrand, W. A., Liphard, K.-G., Strobe], B., Hodek, W. and Kolling, G. ErdGl und Kohle 1981, 34,443 Hillebrand, W., Hodek, W. and Kolling, G. Fuel 1984,63,756 Linden, H. R. and Peck, R. E. Ind. Eng. Chem. 1955,47,2470 White, L. R., Davies, H. G., Keller, G. E. and Rife, R. S. 63rd AIChE Meeting, Chicago, Nov. 1970 Zdonick, S. B., Green, E. J. and Hallee, L. P. Oil GasJ. 1968,19,91 Chambers, L. E. and Potter, W. S. Hydrocarbon Processing 1974, 53, 121