Energy & Fuels 1991,5,139-744
739
Transfer Hydrocracking of Heavy Oil and Its Model Compound Kohjiroh Aimoto, Ikusei Nakamura,t and Kaoru Fujimoto* Department of Synthetic Chemistry, Faculty of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan Received February 11, 1991. Revised Manuscript Received May 13, 1991 Transfer hydrocracking of Kuwait atmospheric residue (RC-KW) and its model compounds with nickel supported on active carbon catalyst was examined under several reaction conditions. In the case of RC-KW cracking, the yield of asphaltene in product oil is very low (almost zero) with small amount of hydrogen consumption (100m3(STP)/kL of oil, kL = m3),whereas coke yield is relatively high (about 10 w t ?%1. As the hydrogen pressure increased, the level of hydrogen consumption and sulfur removal increased, while the coke yield and the conversion of residual oil decreased. These results suggest that transfer hydrocracking of residual oil is composed of thermal cracking of residual oil and hydrogen transfer from precoke compounds such 89 asphaltene which was adsorbed on active carbon surface to form coke and hydrogen atoms. In order to clarify the reaction mechanism, model reactions of diphenylpropane (DPP) as hydrogen acceptor with tetrahydronaphthalene (THN) or decahydronaphthalene (DHN) as hydrogen donor were conducted with Ni/AC (AC = active carbon) catalysts. Metal-free active carbon showed an activity for hydrogen-transfer reaction to some extent and supported Ni enhanced it, remarkably. With increasing hydrogen pressure, the level of dehyon of hydrogen donor was decreased and the consumption of hydrogen was increased. These ggest that hydrogen in the gas phase, on the active carbon surface, and in the hydrogen donor was able to move between these phase reversibly, by spillover and reverse spillover of hydrogen.
Introduction The development of advanced technologies for upgrading heavy oils to high-quality distillates has been collecting much attention as the key technology for promoting the utilization of petroleum residue, oil sand bitumen, shale oil, and coal liquid. Among these technologies, the solidcatalyzed hydrocracking process is considered to be a promising technology because of ita high product quality. However, this technology has several problems to be solved, such as low conversion level, low LHSV,low quality of residue, high operational pressure, and large hydrogen consumption. The present authors have already reported a new method for upgrading heavy oils, which is composed of the thermal cracking of heavy hydrocarbons and hydrogen transfer from asphaltene to product Also, we have clarified that metal-supported active carbon catalysts show excellent activities for this reaction and suggested that spillover and reverse spillover of hydrogenM plays important roles in the catalysis. In this paper, we conducted transfer hydrocracking of Kuwait atmospheric residue with nickel on an active carbon (Ni/AC, AC = active carbon) catalyst and the same reaction of diphenylpropane with tetralin or decalin as the hydrogen donor on the same catalyst to clarify the role of catalysts of active carbon, spillover, reverse spillover, and transfer of hydrogen on this reaction. Experimental Section Reaction Apparatus and Procedure. (a) Reaction of Kuwait Atmorpheric Residue. Reactions were conducted in a semibatch apparatus which was composed of a magnetically stirred autoclave with an inner volume of 150 mL and a gas flow controller shown in Figure 1. Kuwait atmospheric residue (RC-KW) was employed as the feed oil in this experiment. Ret Present address: Central Research Laboratories, Idemi6 K&an Co., Ltd., 1280, Kamiizumi, Sodegawa, Kimitu, Chiba, Japan 299-02.
action conditions were as follows: RC-KW, 40 g; catalyst, 8 g; hydrogen pressure, Q-7.5 MPa; reaction temperature, 435 "C; process time, 60 min; stirring rate, 600 rpm; and outlet gas flow rate, 120 mL/min. (b) Reaction of Model Compounds. Reactions were conducted in a batch apparatus using a conventional shaking autoclave with an inner volume of 75 mL. A glass insert was employed to minimize the effect of the autoclave wall. Three grams (1.53 "01) of diphenylpropane(DPP) was used as a reactant and 4.04 g (3.06 mmol) of tetralin (THN) or 2.11 g (1.53 mmol) of decalin (DHN) was used as a hydrogen donor. The Ni/AC (Ni 5 w t %) catalyst (0.60 g) and mixture of model compounds were charged to the autoclave, air was replaced by hydrogen, and then the autoclave was charged to initial pressure. Commerically available pure-grade THN and DHN were used without any purification. Reaction conditions were as follows: process time, 1 h; reaction temperature, 380-420 "C; initial hydrogen pressure (at room temperature), 0-2.2 MPa. Catalyst Preparation. The catalysts were prepared by impregnating a commercially available active carbon (Takeda ShirasagiC, made from wood, 1500 m2/g),which had been crushed and sieved between no. 40 and 60 mesh, with an aqueous solution of nickel nitrate. The catalyst precursor was dried in air at 120 "C, calcined in nitrogen at 450 "C for 3 h, and activated by reduction in flowing hydrogen at 450 "C for 1 h, and then sulfided with an H p H a mixed gas (411 mole ratio) at 400 OC for 1 h. The nickel content was 1 w t % (the catalyst for the residual oil reaction) or 5 w t 70 (the catalyst for the model compound reaction). Analysis. Gaseous products were analyzed by using gae chromatography. Liquid products from the reaction of RC-KW were distilled to naphtha (initial boiling point 171 "C), kerosene (172-232 "C), gas oil (233-343 "C), and residue (343 "C+) fraction (1) Fujimoto, K.; Nakamura, I.; Tominaga, H. J . Jpn. Pet. Inet. 1988, 31,410-415. (2) Nakamura, I.; Aimoto, K.; Fujimoto, K. AIChE Symp. Ser. 1989, 85,273, 15-20. (3) Sermon, P. A.; Bond, G. C. Cotol. Reu. 1973,8,211-239. (4) Pajonk, G. M.; Teichner, S. J.; Germain, J. E. Spillover of Adsorbed Species; Eleevier Science: Amsterdam, 1983. (6)Conner, W . C. Jr.; Pajonk, G. M.; Teichner, S. J. Aduonces in Catalysis; Academic Press: New York, 1986; 94, pp !-79. (6) Conner. W. C. Jr. Hydrogen in Cotolysur; Marcel Dekker: - Effects .. New York, 1988; p 311-346.
Q887-0624/91/ 25Q5-Q739$02.5Q/Q 0 1991 American Chemical Society
740 Energy & Fuels,
Aimoto et aZ.
VoZ. 5, No.5,1991
Rcssure regulator
n
E c .-E
100 80
60
Q)
8 u
40 20
-3
0 E 100
Flowmeter
Autoclave
%S Trap
Gapbag
50 40
2
50 25
30
G
8
2m
Cake .-
>
2
20
0
80
-
37 :: 2.g
I TA
E
v)
%i2
wm
n
90
3
Sa
60
S removal
6
-25
Olefin
hamtics
5 0
75
Figure 1. Apparatus for cracking of RC-KW.
Gas o i l
Coke yield
5 .-
I=
U
c
PP kr
Eo%
U&
70
L
0 0
Paraffin
2.0
4.0
6.0
8.0
H2pressure (MPa)
Figure 3. Effects of hydrogen pressure. Reaction conditions: RC-KW 40 g, catalyst 8 g, reaction temperature 435 "C,process time 1 h.
Ni1A.C. 104 Nm3/kl a )
Gas Nap Ker Gas o i l 4
I
I Sa
YA DA
TA PP As
Coke Noncat.
0 Nm3/kl a )
N i / S i O -A1 0 114 nm / k l 'a?
5
Figure 2. Praduct distribution of cracking of RC-KW by boiling point and hydrocarbon type. Naphtha (initial boiling point 171 "C),kerosene (172-232 "C),gas oil (233-343 "C),residue (343 OC+). Sa, saturates; MA, monoaromatics;DA, diaromatics; TA, triaromatics; PP,polyaromatics and polar compounds; As, asphaltene. (a) H2consumption; reaction conditions: RC-KW 40 g, catalyst 8 g, reaction temperature 435 "C,hydrogen pressure 7.5 MPa, process time 1 h. following the standard distillation method after removing the catalyst and solid product by centrifugation. The contents of paraffin, olefin, and aromatics were determined by FIA method (JIS-2536)for each fraction. The amount of coke, asphaltene, and maltene were determined as toluene insoluble (TI), toluene soluble-pentane insoluble (TS-PI)and pentane soluble (PS), respectively. Maltene was analyzed by silica-alumina column chromatograph for saturate (SA), monoaromatics (MA), diaromatics (DA), triaromatics (TA), and polyaromatics and polar compounds (PP).The desulfurization rate was calculated from the amount of Ha in the outlet gas which was trapped by aqueous lead acetate as PbS. Products from the reaction of the model compound were analyzed by gas chromatography using active alumina column for gaseous products and OV-17 for liquid products.
Rssults and Discussion Reaction of RC-KW.The results of RC-KW cracking with various supported Ni catalysts and without a catalyst are given in Figure 2. Carrier materials such as active carbon (AC), @"-A1203,which is a typical basic support, and Si02-A1203,which is a typical acidic support, were employed. The yield of gas products was in the older Si02-A1203 > noncatalyzed > A.C.>/3"-A120,. In the Ni/AC-catalyzed reaction, the yield of gaseous hydrocarbons was lower than that of noncatalyzed or Ni/SiO2-Al2Os-catalyzedreaction, whereas the conversions of RC-KW was similar for these reactions. The olefin content in the distilled oil was pl'A1203 > noncatalyzed > Si02-A12031 AC I0, although
the yield of naphtha, kerosene, and gas oil in distilled oil were similar in these systems: naphtha 35-39%, kerosene 21-25%, gas oil 39-42%, respectively. ?'he properties of residue (343 OC+) were characteristic for each reaction system. The asphaltene yield was noncatalyzed I@"-A1203 >> AC 1 Si02-A12032 0, although the coke yield was AC honcatalyzed 2 Si02-A1203>> p"-A12032 0. The amount of asphaltene in the residue with noncatalyzed and r A1203system was similar to that of RC-KW fed. The characteristics of the Ni/AC-catalyzed reaction are summarized as follows: (1)The yield of gaseous hydrocarbons was lower than that of noncatalyzed or Ni/ Si02-A1203-catalyzedreaction, whereas the conversions of RC-KW were similar for these reactions. (2) The content of olefins in the distilled oil was very low. (3) The coke yield was highest in these systems but the yield of asphaltene was almost zero. These results suggest that the asphaltene in the residual oil was adsorbed on or decomposed on the catalyst surface and was then converted to coke in the case of the carbon-catalyzed system. At the same time olefins were hydrogenated to paraffins. The effects of hydrogen pressure on the Ni/AC-catalyzed reaction of RC-KW are shown in Figure 3. With increasing hydrogen pressure, hydrogen consumption increased linearly and conversion of residual oil decreased slightly. Also, the degrees of desulfurization increased and coke yield decreased with the increase in hydrogen pressure up to 3.0 MPa, but they were almost constant over 3.0 MPa. Since the degrees of desulfurization and the coke yield are considered a measure of the hydrogenation of product oil, it is reasonable that the hydrogenation of produced oil reached the same level in the cases of 3.8 and 7.5 MPa of hydrogen pressure. The fact that hydrogenation of product oil was at the same level for reactions a t hydrogen pressures of 3.8 and 7.5 MPa, while the hydrogen consumption at 3.8 MPa was much lower than at 7.5 MPa, suggests that gaseous hydrogen was mainly consumed in the hydrogenation of the products at 7.5 MPa. On the other hand, when the hydrogen pressure was low, the dehydrogenation of asphaltene to coke was enhanced and the hydrogen atom on the catalyst, which was formed from asphaltene coking, reacted with sulfur compounds or oil without desorbing to the gas phase. In other words, under high hydrogen
Energy & Fuels, Vol. 5, No. 5, 1991 741
Transfer Hydrocracking of Heavy Oil catalyst atmosphere initial pressure, MPa DPP conversion, % PhC2H6/PhCH3mole ratio H, to gas phase, mmol product yield: mol % benzene toluene ethylbenzene naphthalend
Table I. none' HZ 3.6 93.0 0.37
1.1 93.6 34.7
Transfer Hydrocracking of DPP with THN" Ni/ACba nonec none Raney Ni H2 H2 HZ NZ 3.6 34.6 0.88 -6.9
1.5
1.0 74.2 0.21 2.3
3.6 75.4 0.27 5.0
AC NZ 3.6 66.7 0.54 1.9
1.0
0.6 91.2 19.2 6.7
tr 78.0 21.4 8.7
0.4 79.0 42.6 16.9
1.0
0.1 25.5 22.4
Ni/AC N2 3.6 59.0 0.89 13.0
Ni/ACd H2 3.6 21.2 1.0 9.8
1.0 64.6 57.4 35.1
tr 20.6 20.6 25.2
OReaction conditions: temperature 400 "C, time 1 h, DPP 15.3 mmol, THN 30.6 mmol. *Reaction temperature 360 "C. Without DPP. dReaction temperature 380 OC. eBased on DPP. /Based on THN. #Without THN.
pressure, hydrogen molecule in the gas phase was adsorbed and dissociated on the metal particle, and diffused to active carbon surface. On the other hand, under low hydrogen pressure, hydrogen on active carbon surface migrated to the metal particle and then combined to form hydrogen molecules.1o The former phenomenon has been attributed to hydrogen spillover, and the latter phenomenon has been attributed to reverse hydrogen spillover. These results suggest that the reaction model of transfer hydrocracking of heavy oil is summarized as follows:'2 (1)Carbon-carbon bondings are thermally cracked to form olefin rich oil. (2) Precoke compounds like asphaltene are adsorbed on the active carbon surface to be converted to coke and hydrogen. (3) Hydrogen atom generated from asphaltene moves across for active carbon surfaces to metal particles and then hydrogenates cracked fragments, olefins, and sulfur compounds. When the pressure of hydrogen is high, hydrogen in the gas phase is consumed for hydrogenation, whereas, when the pressure of hydrogen is low, hydrogen on the active carbon surface should be desorbed to the gas phase by reverse spillover. Model Reaction of DPP-THN System. In order to clarify the reaction model previously shown, model reactions were conducted using a similar catalyst. (a) Pyrolysis of Diphenylpropane. It is considered that pyrolysis of DPP proceeds through a radical chain mechanism as shown in Figure 4a." Based on this mechanism, the product made by pyrolysis of DPP should be an equimolar mixture of toluene and styrene (Figure 4b). However, when DPP was pyrolyzed alone, toluene and only a small amount of styrene were obtained as shown in Table I. This is explained by the fact that styrene is 80 reactive under the reaction conditions that it quickly polymerizes to higher hydrocarbons or coke. (b) Catalytic Transfer Hydrogenolysis. If the hydrogen-transfer reaction occurs quickly after DPP pyrolysis, styrene or 0-phenylethyl radical should be hydrogenated to form ethylbenzene which is stable under reaction conditions. So in this case styrene produced by DPP pyrolysis is detectable as ethylbenzene. So if hydrogen transfer occurs sufficiently, a mixture of equal molar toluene and ethylbenzene should be obtained as shown in Figure 4c. Thus,one can define the ethylbenzene to toluene ratio (E/T ratio) as a measure of hydrogenation level. (7) Asaoka, S.;Maruunizu, K.;Fujimoto, K.; Kunugi, T. J.Chem. SOC. Jpn. 1976,3,388-393. (8) Fujlmoto, K.J. Jpn. Pet. Inet. 1984,27,463-471. (9) Fujimoto, K.;Toyoshi, S. h o c . 7th Int. Congr. Cotal. 1980,
235-246. (10) Fujimoto, K.; Ohno, A.; Kunugi, T. J. Jpn. Pet. Inst. 1983,26, 339-343. (11)Rice, F. 0.J . Am. Chem. SOC.1931,53,1969-1972.
I )W
o n "8of DPP pyrolysis
Initiation
@%*+*%-O Tamirvrion
b) CnCWW Of DPP
Figure 4. Reaction scheme of DPP cracking.
Table I shows the results of the DPP-THN reaction with and without a catalyst such as Raney Ni, active carbon, and Ni/active carbon. Only naphthalene was obtained as the dehydrogenation product of THN. Toluene, ethylbenzene, and a trace of benzene were obtained as pyrolysis products of DPP. No styrene was detected by gas chromatography because styrene is so reactive under the reaction conditions that it was quickly hydrogenated or polymerized to higher hydrocarbons or coke. In the case of the noncatalyzed reaction, although the simple dehydrogenation of THN hardly proceeded without DPP, the dehydrogenation of THN and the decomposition of DPP proceeded, to some extent. This result suggests that the direct hydrogen transfer between free radicals (benzyl radical or 0-phenylethyl radical) and THN proceeded. But the E / T ratio was low (0.21), which means that the transfer hydrogenation of styrene scarcely proceeded. The decomposition of DPP proceeded with a considerable degree either in the presence or in the absence of THN. The results on the Raney Ni catalyst in a N2 atmosphere were quite similar to those of the noncatalyzed DPP-THN reaction, which means that the Raney Ni catalyst is almost inactive for the total reaction. When the nickel-free active carbon was used as a catalyst, naphthalene yield was 16.9% and the E / T ratio was 0.54. It is clear that active carbon itself shows catalytic activity for the hydrogen-transfer reaction.l0 Furthermore, dehydrogenation of THN apparently proceeds more than in the case of noncatalyzed or Raney Ni-catalyzed case. When the Ni/AC catalyst was used, naphthalene yield
742 Energy & Fuels, Vol. 5, No. 5, 1991
Aimoto et al. 1
1
1
1 00
Noncat.
' 8 & -2
0.8 60
o - o - o ~ o ~ o -
0)
\
80
0
0
*
. I
Conversion
0.6
0.4
1 .o
40
1
&o--o--'o--L,~ I
I
I
1
.i 3
20
0
0.2
I
0 0
I
I
60
Naphthalene yield
Naphthalene yield (%)
Figure 5. Transfer hydrocracking of DPP with THN. Reaction conditions: reaction temperature 400 "C, process time 1 h.
I
50
50
40
40
30
30
20
20
h
5 z! .d
Table 11. Effect of Hydrogen Acceptor on Dehydrogenation of THN" AC Ni/AC none DPP none DPP naphthalene yield, % (THN base) 7.7 16.9 36.4 35.2 H2 to gas phase, mmol 3.0 1.9 20.4 13.0 OReaction conditions: N2 3.6 MPa, temperature 400 "C, time 1 h.
increased to 35.7% and the E / T ratio increased to 0.89. The amount of hydrogen released to the gas phase was about 7 times larger than that of the active carbon catalyzed reaction. When the reaction temperature was 380 "C, the initial hydrogen pressure was 1.1MPa, the E / T ratio reached 1.0, and the ethylbenzene yield was the same for the conversion of DPP. Figure 5 shows the E / T ratio and the DPP conversion for a variety of reaction systems as a function of naphthalene yield. It is clear that, with increasing naphthalene yield, the E / T ratio increased and the DPP conversion decreased. This means that the transfer of hydrogen from THN to styrene proceeded by utilizing the proper catalyst even in the absence of gaseous hydrogen. The fact that the DPP conversion decreased with increased hydrogen transfer will be discussed later. Table I1 shows the effects of DPP addition on THN dehydrogenation. When the Ni/AC catalyst was used, the yield of naphthalene was independent of the presence of DPP. However, in the case of the metal free AC catalyst, added DPP promoted the THN dehydrogenation. These results are explained as follows. On the Ni/AC catalyst system, the hydrogen generated by dehydrogenation of THN migrates on the active carbon surface to reach the Ni particle loaded on it. Some part of the surface hydrogen reacts with the hydrogen acceptor such as radicals or olefins, while the other part of the hydrogen combines to form hydrogen molecules and is then desorbed into the gas phase. Thus, the hydrogen concentration on the active carbon surface is kept at a low level.1° However, on the metal-free active carbon, hydrogen on the carbon surface is hard to desorb as a hydrogen molecule, whereas it transfers from the active carbon surface to a hydrogen acceptor (free radical or olefin) to keep the hydrogen concentration on the surface at a relatively low level. The low concentration of hydrogen results in the accelerated dehydrogenation of the hydrogen donor (THN) as demonstrated in Table 11. It can be concluded that the dehydrogenation of THN on active carbon is accelerated by reverse spillover or the transfer of hydrogen (as demonstrated in Figure 9).
-8 f
2 10
tp
10
/-
,/
0
0.5
I
I
I
l
1.0
1.5
2.0
2.5
o
Initial hydrogen pressure (MPa)
Figure 6. Effects of hydrogen pressure on transfer hydrocracking of DPP with THN on Ni/AC catalyst. Reaction conditions: reaction temperature 400 "C, process time 1 h.
1 .o 1.6 2.2 Hydrogen pressure (MPa)
0.5
H2 transferred to products. H2 desorbed to gas phase.
Figure 7. Hydrogen destination in transfer hydrocracking of DPP with THN on Ni/AC catalyst. Reaction conditions: reaction temperature 400 "C, process time 1 h.
Figure 6 shows the effect of the initial hydrogen pressure on the DPP-THN reaction over the Ni/AC catalyst. With increasing initial hydrogen pressure from 0.5 to 2.2 MPa, the conversion of DPP decreased slightly, while the yield of naphthalene remarkably decreased. At 0 MPa of initial hydrogen pressure the yield of naphthalene was smaller than that at 0.5 MPa of initial hydrogen pressure. This should be attributed to deactivation of Ni/AC by coke formation on it. Although the amount of hydrogen charged in the reactor is proportional to the initial hydrogen pressure, the amount of hydrogen recovered after the reaction was almost independent of the initial hydrogen pressure except for the hydrogen-free case. Figure 7 shows the relationship between hydrogen supplied from THN, which is calculated by naphthalene yield, released into the gas phase and consumed to hydrogenate products. The s u m of hydrogen released into the gas phase and hydrogen consumed for hydrogenation was in fair agreement with the amount of hydrogen supplied from THN. The amount
Transfer Hydrocracking of Heavy Oil
Energy & Fuels, Vol. 5, No. 5, 1991 743
Table 111. Transfer Hydrocracking - of DPP with DHN" catalyst nand Ni/ACbJ noneC none AC Ni/AC atmomhere Ho Ho Ho N, No H, initial-pressure, 3.6 3.6 1.6 3.6 3.6 3.6 MPa 81.1 85.0 11.5 DPP conversion, 70 93.0 34.6 PhC*Hb/PhCHS 0.31 0.88 0.23 0.54 0.85 mole ratio -6.9 1.0 0 0 1.8 H2 to gas phase, mmol product yield: mol 9% benzene 1.1 0.1 93.6 25.5 toluene 34.1 22.4 ethylbenzene naphthalene'
1.0
0.2 90.2 20.8 1.0
0.2 19.0 42.6 2.6
0.2 18.2 66.8 8.4
"Reaction conditions: temperature 400 OC, time 1 h, DPP 1 mmol, DHN 15.3 mmol. bReaction temperature 360 "C. e Without DPP. dBased on DPP. 'Based on DHN. 'Without DHN.
of hydrogen consumed for hydrogenation of products was almost independent of the initial hydrogen pressure; however, the amount of hydrogen released into the gas phase decreased markedly with increasing initial pressure. It is estimated from Figure 6 that apparently no hydrogen is desorbed into the gas phase at about 2.5 MPa, which means that neither hydrogen formation nor hydrogen consumption apparently occurs. Therefore, when the hydrogen pressure in the gas phase reaches an appropriate value, the dehydrogenation of THN does not proceed further; that is, some dynamic equilibrium is reached. Therefore, it is clear that hydrogen can reversibly transfer between the gas phase and THN depending on the hydrogen pressure in the gas phase in the presence of a Ni/active carbon catalyst. It should be reminded that the same phenomenon was observed in the reaction of RC-KW (Figure 3). Reaction of the DPP-DHN System. (a) Catalytic Activity for the Hydrogen-Transfer Reaction. The results of the DPP-decahydronaphthalene (DHN) reaction are shown in Table 111. Little gaseous products were formed. Although, the noncatalyzed hydrogen-transfer reaction hardly proceeded, metal-free active carbon showed some activity for the reaction, and the supported Ni remarkably enhanced the activity, similar to the case of THN. These results shows that the active carbon catalyst has an excellent activity to abstract hydrogen atoms even from paraffinic hydrocarbon^.'^^ Its active site for dehydrogenation is not the sulfided Ni particle but the active carbon surface The DPP conversion in the DPP-DHN system was higher and the DHN conversion was lower than in the DPP-THN system. The lower conversion of DHN suggested that DHN had a lower reactivity than THN as a hydrogen donor. (b) The Effect of Initial Hydrogen Pressure. Figure 8 shows the effect of initial hydrogen pressure on the reaction of the DPP-DHN system with Ni/AC catalyst. With increasing initial hydrogen pressure from 0.5 to 1.0 MPa, naphthalene yield decreased monotonously. The small yield of naphthalene a t 0 MPa of initial hydrogen pressure should be attributed to coke formation on the catalyst. The amount of hydrogen recovered after the reaction increased with the increase in the initial hydrogen pressure but its level was much lower than for the case of the DPP-THN system. It is estimated that the amount of hydrogen recovered reaches the same level as that of the charge at 0.9 MPa of initial hydrogen pressure. This critical equilibrium point for DHN is much lower than that for THN at 2.5 MPa.
&,I
0
,
,
0.5
1 .o
Initial hydrogen pressure (MPa)
Figure 8. Effects of hydrogen pressure on transfer hydrocracking of DPP with DHN on Ni/AC catalyst. Reaction conditons: reaction temperature 400 "C,process time 1 h.
Active Carbon Figure 9. Reaction model of transfer hydrocracking of DPP.
Reaction Model The reaction model of the catalyzed transfer hydrocracking of DPP is shown in Figure 9. First, DPP was decomposed thermally to form radicals and/or olefins as shown in the figure. A hydrogen donor such as THN or DHN is dehydrogenated on the active carbon surface (not on the Ni site) to form naphthalene and surface hydrogen atoms. Hydrogen formed on the surface partly reach with radicals or olefins but mostly moves to reach metal particles. Hydrogen on the active carbon and hydrogen in the gas phase transfer reversibly by spillover and the reverse spillover. At the metal particles, some hydrogen reacts with these radicals or olefins to form alkyl aromatics. If free radicals, which are the chain carrier of the DPP cracking (Figure 4a), are hydrogenated to form toluene or ethylbenzene, the chain reaction is quenched, to lower the DPP conversion. The results shown in Figure 5, which demonstrate that, with increasing naphthalene yield, DPP conversion decreased, are very consistent with the previously discussion. Therefore, some of the hydrogen combines to form hydrogen molecules and is desorbed into the gas phase (reverse spillover). Under high hydrogen pressure conditions, the desorption of hydrogen molecules to the gas phase is suppressed by the spillover effect and thus
744
Energy & Fuels 1991,5, 744-748
the dehydrogenation of the hydrogen donor is depressed because of the high hydrogen concentration on the active carbon surface. Therefore, the consumption of hydrogen in the gas phase is superior to that from the donor. On the contrary, at low hydrogen pressure, the dehydrogenation of a donor is superior to the reaction of the surface hydrogen acceptor (fragment of DPP decomposition) and thus some of hydrogen atoms supplied from the donor come out into the gas phase (reverse spillover), which results in the amount of recovered excess of that charged.
Conclusions The results from pyrolysis of RC-KW and its model compounds with a Ni/AC catalysts can be summarized as follows:
1. Hydrogen-transfer thermal cracking of residual oil is composed of two reactions; one is pyrolysis of hydrocarbons and the other is the hydrogen transfer on the catalyst from a hydrogen donor (such as asphaltene) to the cracked oil. 2. In the presence of a catalyst, hydrogen is able to move between the catalyst surface and gas phase reversibly by the spillover and reverse spillover effects. The dehydrogenation of a hydrogen donor can be controlled by hydrogen pressure in the gas phase. As a result, hydrogenation of product oil can be achieved with almost no consumption of hydrogen in the gas phase. Registry No. Ni, 7440-02-0;C, 7440-44-0;SO2, 7631-86-9; A1203,1344-28-1;8”-A1203,12005-16-2;DPP,1081-75-0;THN, 119-64-2;DHN, 91-17-8;benzene, 71-43-2;toluene, 108-88-3; ethylbenzene, 100-41-4;naphthalene, 91-20-3.
Fuel Instability Model Studies: The Liquid-Phase Cooxidation of Thiols and Indene by Oxygen Robert E.Morris* and George W . Mushrush Naval Technology Center for Safety and Survivability, Code 6180, Fuels Section, Naval Research Laboratory, Washington, D.C . 20375 Received December 21, 1990. Revised Manuscript Received May 17, 1991 Instability problems in middle distillate fuels have been correlated with the presence of both active olefin species and heteroatomic compounds such as thiols. It has been demonstrated that the type of sulfur compound rather than the total sulfur concentration is the key to fuel instability reactions. Research has shown that low concentrations of thiols will act as radical traps to inhibit autoxidation. When added to a fuel, thiols accelerated the rate of oxygen reaction without a commensurate increase in peroxidation. Evidence for the oxidative addition of thiols to olefins has been found to occur by studying the addition of thiophenol to indene in a model fuel during stressing in both a model system at temperatures in the 100-120 “C range and in the JFTOT apparatus at temperatures up to 320 “C. Similarities and differences were found in the two systems, with the product distribution being temperature dependent. This could account, in part, for the differences in thiol influences on autoxidation observed in model systems and in fuels.
Introduction Predictive model systems to explain fuel instability reactions represent an attractive concept for research. Model systems have both advantages and disadvantages in a complex reaction medium such as fuels and other natural products. The major advantage of model systems is that they permit the determination of trace reaction products that a complex fuel would mask. A major disadvantage of a model system is that interactive effects between various fuel components cannot be readily determined. The detailed understanding of the influence of various heteroatoms on fuel instability is of continuing interest and can only be deciphered by a combination of both model and real system studies. The use of jet fuel as a heat-transfer medium can result in many deleterious side effects. Jet fuels subjected to this additional thermal stress undergo considerable chemical changes. This thermal stress leads to the formation of detrimental deposits on filters, in nozzles, and on heat exchanger surfaces.’F2 This observed deposit formation ~
(1) Hazlett, R. N.; Hall, J. M. Jet Aircraft Fuel System Deposits in Chemistry of Engrne Combustion Deposits; Ebert, L. B., Ed.; Plenum Press: New York, 1985; pp 245-261.
is the consequence of free-radical autoxidation reactions. Catalysts, free-radical initiators, or free-radical inhibitors that are naturally present in fuels can significantly alter both the rate of oxidation and the product distribution.36 Present knowledge has suggested that for, some fuels, sulfur heterocycles may play a role in the formation of filterable sedimenb and adherent gums under oxidative conditions. Previous research has indicated that it was the type of organosulfur compound present rather than the total quantity of sulfur that was the controlling factor in fuel degradation.6 Sulfur is the most abundant heteroatom present in military jet fuel with up to 0.4% total sulfur by weight allowed (MJL-T-5624M). For commercial jet fuel, the ASTM Standard Specification for Aviation Turbine Fuels permits up to 0.3% total sulfur by weight.’ (2) Scott, G. Atmospheric Oxidation and Antioxidants; Eleevier: Amsterdam, 1965; Chapter 3. (3) Benbon, S. W.; Shaw, R. Organic Peroxides; Wiley-Interscience: New York, 1970; Chapter 2. (4) Howard, J. A,; Ingold, K. U. Can. J . Chem. 1969,47,3793-3796. (5) Richardson, W. H.J . Am. Chem. SOC.1965,87, 1096. (6) Mushrush, G. W.; Watkins, J. M.; Hazlett, R. N.; Hardy, D. R.; Eaton, H. C . Fuel Sci. Technol. Int. 1989,6, 165-183. (7) ASTM ‘Standard Specification for Aviation Turbine Fuels”. In Annual BOOKof ASTM Standards; ASTM: Philadelphia, 1987; Part 23, ASTM D1655-82.
This article not subject to U S . Copyright. Published 1991 by the American Chemical Society
