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Production of lower alkenes and light fuels by gas phase oxidative cracking of heavy hydrocarbons
Fuel Processing Technology 87 (2006) 649 – 657 www.elsevier.com/locate/fuproc
Production of lower alkenes and light fuels by gas phase oxidative cracking of heavy hydrocarbons Haiou Zhu, Xuebin Liu, Qingjie Ge, Wenzhao Li ⁎, Hengyong Xu Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, PR China Received 26 September 2005; received in revised form 11 January 2006; accepted 24 January 2006
Abstract The gas phase oxidative cracking (GOC) and non-oxidative pyrolysis of heavy hydrocarbons were investigated, with decalin (decahydronaphthalene) and tetralin (tetrahydronaphthalene) as the model compounds for naphthenic hydrocarbon and aromatic hydrocarbon, respectively. Unlike pyrolysis, the ring rupture of decalin or tetralin molecule and the decoking ability of system were significantly enhanced due to the introduction of O2 in GOC. For GOC of decalin, both the lower alkenes and the light fuels were obtained. At lower temperatures the light fuels mainly contained alkyl benzene, alkyl cyclohexane and isoparaffins, while it was rich in BTX (benzene, toluene and xylenes) at higher temperatures. A 38.9% yield of lower alkenes and 48.0% yield of light fuels (BTX mass content: 59.9%) at 100% decalin conversion were obtained under the conditions of 800 °C and decalin / O2 = 0.5. For GOC of tetralin, both the dehydrogenation and the cracking reactions dominated the reaction routes, resulting in a high mass content of alkyl naphthalene and alkyl benzene in the light fuels. The estimation of O2 distribution in the products demonstrated that O2 participated primarily in the oxydehydrogenation reactions at low temperatures, while mainly in the partial oxidation reactions at high temperatures to produce COx (x = 1, 2). © 2006 Elsevier B.V. All rights reserved. Keywords: Decalin; Tetralin; Gas phase oxidative cracking; Pyrolysis; Lower alkenes; Light fuels
1. Introduction Lower alkenes are commercially produced by the pyrolysis of hydrocarbons in tubular reactor coils installed in externally fired heaters. Feedstock plays a vital role in the production cost of the lower alkenes. Although lower alkanes such as ethane and propane are much preferable for the production of lower alkenes, those heavy feedstocks, such as naphtha, diesel oil, crude oil and residual oil, are also to be widely used in some regions where there are abundant resources of heavy stuff [1]. The heavy feedstock generally consists of paraffins, isoparaffins, naphthenes and aromatics, the mass concentrations of which greatly depend on the producing areas. For example, naphthene content in virgin naphtha of Taching oilfield (China) is as high as 46 wt.% [2]. Those naphthenic and aromatic compositions, however, are not suitable for the pyrolysis process to produce lower alkenes, due to their high molecule stability and the tendency to agglomeration. Therefore, how to take ⁎ Corresponding author. Tel.: +86 411 84379152; fax: +86 411 84691570. E-mail address: [email protected] (W. Li). 0378-3820/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2006.01.009
Conversion & Yield /%
100
80
60
40
20
0 600
650
700
750
800
850
Temperature /oC Fig. 1. Conversions and selectivities versus temperature for non-oxidative pyrolysis of decalin and tetralin with a nitrogen dilution ratio of 90% and feed flow rate of 100 ml/min (STP). Decalin: ■, conversion; ●, yield of lower alkanes; ▲, yield of lower alkenes; ▼, yield of light fuels. Tetralin: □, conversion; ○, yield of lower alkanes; △, yield of lower alkenes; ▽, yield of light fuels.
650
H. Zhu et al. / Fuel Processing Technology 87 (2006) 649–657
Table 1 Light fuel yields and compositions for non-oxidative pyrolysis of decalin (90% nitrogen dilution ratio and 100 ml/min (STP) feed flow rate) T (°C)
700 750 800 860 a
Fuel yield (%)
Fuel composition (wt.%)
8.2 34.2 47.6 50.9
Paraffins
Isoparaffins
Naphthenes
Alkylbenzenes (BTX a)
Olefins
Others
0.0 2.0 0.0 0.0
3.5 4.2 3.1 0.4
34.2 28.2 16.7 8.4
59.1 (9.0) 63.9 (33.7) 79.6 (41.6) 90.7 (66.3)
2.8 0.9 0.6 0.5
0.4 0.8 0.0 0.0
Benzene, toluene, xylenes.
Table 2 Light fuel yields and compositions for non-oxidative pyrolysis of tetralin (90% nitrogen dilution ratio and 100 ml/min (STP) feed flow rate)
700 750 800 860
Fuel yield (%)
12.2 47.0 79.0 85.1
Fuel composition (wt.%) Paraffins
Isoparaffins
Naphthenes
Alkylbenzenes
Alkylnaphthalenes
Olefins
Others
0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0
48.1 38.4 37.3 38.9
51.9 61.6 62.5 60.8
0.0 0.0 0.0 0.0
0.0 0.0 0.2 0.3
full advantage of those feedstocks (rich in naphthenes and aromatics) becomes a challenging topic for the production of lower alkenes from heavy stuff. The gas phase oxidative dehydrogenation (ODH) of C2–C4 alkanes [3–5] and gas phase oxidative cracking (GOC) of higher hydrocarbons [6,7] to lower alkenes have attracted much attention since the early 1990s. Burch et al. [3] found that some homogeneous conversion of ethane to ethylene occurred in the oxidative coupling of methane, and their successive work on ODH of ethane [8] and propane [9] showed that the homogeneous ODH reactions were competitive with the heterogeneous catalytic reactions over the best catalysts available at that time. Lemonidou and Stambouli [5] compared the pyrolysis and the catalytic and non-catalytic ODH of n-butane, and found that nearly 90% selectivity to lower alkenes with 22% conversion of butane can be achieved in non-catalytic ODH process, which was much higher than that either in the pyrolysis or in the catalytic route under comparable conditions. Most of the work in ODH of lower alkanes focused on the break of C–H bonds; however, the situation should be somewhat different when higher hydrocarbons are employed, because the cracking of C–C bonds in their molecules becomes more remarkable. The work of Liu et al. [10,11] on GOC of hexane and cyclohexane showed that much high cracking performances can be found at low temperatures. For example, at 750 °C the GOC conversion of hexane and lower alkene yield were 85% and 51%, respectively, which were significantly higher than those in the pyrolysis process: 29% and 21%. The similar reaction performance was also found in GOC of cyclohexane (87% conversion and 44% lower alkene yield at 750 °C), while its pyrolysis conversion and alkene yield were only 8% and 6%, respectively. It is evident that, when compared with pyrolysis process, GOC process exhibits much distinct advantage in the case of inferior feedstock such as cycloalkane. In this paper, decalin (decahydronaphthalene) and tetralin (tetrahydronaphthalene) were employed as the model compounds for heavier naphthene and arene, and their GOC processes were investigated for lower alkene production. The ring rupture or ring
opening of these multi-ring compounds are much more relevant to the upgrading of fuel oils [12], for example, the ring rupture of decalin to n-decane will increase the cetane number of kerosene fractions, while its ring opening to alkyl benzene will improve the octane number of gasoline fractions. Therefore, GOC of decalin or tetralin to light fuels (gasoline and kerosene) were also studied in this paper. 2. Experimental 2.1. Experimental apparatus The reactions were conducted in a straight tubular quartz reactor (6 mm inner diameter × 300 mm) under atmospheric pressure. A chromel–alumel thermocouple inside a closed quartz thermowell (2 mm outer diameter × 240 mm), which was located in the center of the reactor, was used to record the reaction temperatures freely along the reactor. The reactor was mounted in a programmable tubular furnace (120 mm long), 100
Conversion & Yield /%
T (°C)
80 60 40 20 0 400
500
600
700
800
900
Temperature /oC Fig. 2. Conversions and yields versus temperature for GOC of decalin with a decalin / O2 molar ratio of 0.5 and feed flow rate of 100 ml/min (STP). □, Conversion of decalin; ○, conversion of O2; ▲, yield of CO; ▼, yield of CO2; ▶, yield of lower alkenes; ◀, yield of light fuels.
651
100
1.0
2.3. Analytical procedure
80
0.8
60
0.6
40
0.4
20
0.2
Experimental data obtained from two GC were linked through CH4 in the products. The introduction of O2 and derived COx complicated the quantitative analysis. Thus the following adjustment was made. In order to estimate the weight distribution of carbon atoms in the products, the weight of CO or CO2 were converted to the weight of CH2. Based on above consideration, come the following definition of conversion, selectivity and yield,
0
n(feed)
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0.0 400
500
600
700
800
860
XO2 ¼ ðmTO2 −mRO2 Þ=mTO2 100%
Temperature /oC Fig. 3. Conversions and yields versus temperature for GOC of decalin, after excluding COx from GOC process, with a decalin / O2 molar ratio of 0.5 and feed flow rate of 100 ml/min (STP). Parameter n(feed) refers to the ratio of the part of decalin feed, participating in the partial oxidation reaction, to the other part of decalin feed in GOC of decalin. □, Conversion of decalin; ○, conversion of O2; ▤, yield of light fuels; ▧, yield of lower alkenes; ■, n(feed).
and the isothermal zone for reactor was 20 mm long. The furnace, reactor and gas lines were closed into a self-made constant temperature cabinet to prevent the condensation of liquid reactants and products. The flow rates of O2 and N2 in the feed were controlled with mass-flow controllers, and decalin or tetralin was introduced with a syringe pump, vaporized, and mixed with the previously mentioned gases in series. Two Varian model 3800 gas chromatographs (GC) were used to analyze the products online: one used a 100 m PONA capillary column with a flame ionization detector (FID) as the detector, to analyze the total organic compounds in the products; and the other used a 1.8 m HAYESEP C packed column that was connected serially with a 1.2 m MOLESIEVE 13X packed column, with a thermal conductivity detector (TCD) as the detector, for gas products (H2, O2, N2, CO, CO2, and CH4). 2.2. Experimental procedure The gas flow was interrupted between each analysis. The reactant gases were switched on for 30 min (long enough for stabilizing the reaction and obtaining reproducible experimental data) before taking a sample for analysis, and switched off immediately after sampling; then an argon stream (40 ml/min) flowed through the reactor to purge the reaction system until the next run began.
XC10 ¼ ðmT −mR Þ=mT 100% SCOx ¼ mCH2 =ðmT −mR Þ 100% SP ¼ mR =ðmT −mR Þ 100% YP ¼ XC10 SP 100% where XO2 and XC10 are the conversions of O2 and decalin (or tetralin), respectively; mTO2 and mT are the total reactant O2 and total weight of decalin (or tetralin), respectively; mRO2 and mR are the remained O2 and remained decalin (or tetralin) weight in the products, respectively; mCH2 is the adjusted weight of CO (or CO2); SCOx and SP are the selectivity to CO and CO2 and the selectivity to other organic product, respectively; YP is the product yield. Besides lower hydrocarbons (such as C1∼C4 alkanes, C2∼C4 alkenes), a large amount of higher hydrocarbons (carbon atoms ≥ 5), which were generally called light fuels in this paper, were also produced in GOC of decalin or tetralin. For the sake of clarity, we grouped these light fuel products according to their carbon chain skeleton in molecules: (a) paraffins (straight-chain alkanes, such as n-pentane, n-hexane, and etc.); (b) isoparaffins (branched-chain alkanes, such as methyl pentane, ethyl pentane, and etc.); (c) naphthenes (such as methylcyclopentane, cyclohexane, methylcyclohexane, and etc.); (d) alkylbenzenes (such as benzene, toluene, ethylbenzene, butylbenzene, and etc.); (e) alkylnaphthalenes (such as naphthalene, methylnaphthalene, dimethylnaphthalene, and etc.); (f) olefins (straightchain alkenes, such as pentene, hexene, and etc.) and (g) others (some oxygenates, such as phenol, benzaldehyde, and etc.).
Table 3 Light fuel yields and compositions for GOC of decalin (molar ratio of decalin / O2 = 0.5 and feed flow rate = 100 ml/min) T (°C)
Fuel yield (%)
Fuel composition (wt.%) Paraffins
Isoparaffins
Naphthenes
Alkylbenzenes (BTX)
Olefins
Others
400 500 600 700 800 860
46.3 49.1 50.3 51.3 50.0 48.4
0.0 0.2 1.0 0.2 1.0 0.0
3.9 8.7 10.0 8.9 1.1 0.5
29.4 23.5 24.0 19.1 7.2 4.9
65.8 (0.6) 65.2 (2.6) 60.9 (15.5) 67.6 (17.9) 87.2 (50.5) 92.1 (57.8)
0.4 0.7 1.5 1.4 1.7 0.0
0.5 1.7 2.6 2.8 1.8 2.5
100
1.0
80
0.8
60
0.6
40
0.4
20
0.2
n(feed)
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Conversion & Yield /%
652
0.0
0 0.3
0.4
0.5
0.7
1
Decalin/O2 molar ratio Fig. 4. Conversions and yields versus molar ratio of decalin to O2 for GOC of decalin, after excluding COx from GOC process, with a reaction temperature of 600 °C and a feed flow rate of 100 ml/min (STP). □, Conversion of decalin; ○, conversion of O2; ▤, yield of light fuels; ▧, yield of lower alkenes; ■, n(feed).
In order to estimate the fraction of some decalin (or tetralin), which converts to a certain above grouped product, in the total feeding decalin (or tetralin), we defined the relative conversion (Xr), Xr ¼ mc =mT 100% where mc is the converted decalin (or tetralin), to form a certain grouped product. In order to estimate the fraction of converted O2 in the COx, the following definition (D) was made, D ¼ O2ðCOxÞ =ðXO2 mTO2 Þ 100% where O2(COx) is the amount of O2 distributed in the COx and equals to 1 / 2 × mole number of CO plus mole number of CO2.
3. Results and discussion 3.1. Non-oxidative pyrolysis of decalin and tetralin The conversions and product yields in non-oxidative pyrolysis of decalin and tetralin at various temperatures are shown in Fig. 1. The conversions of decalin and tetralin were unconspicuous at temperatures below 650 °C, above which both conversions greatly increased, and decalin exhibited a slightly
higher activity than tetralin. Lower alkenes (C2∼C4 alkenes) and light fuels were the main products, and their yields significantly increased as the reaction temperature rose. A lower alkene yield of 35.3% and light fuel yield of 47.7% can be achieved at 800 °C in pyrolysis of decalin, while light fuels greatly dominated the pyrolysis products in the case of tetralin and a 79.0% light fuel yield and only a 3.4% lower alkene yield were obtained at 800 °C. For all the temperatures, the yields of lower alkanes (C1∼C4 alkenes) were less than 11%, though they slightly improved with the rising temperature. The variations in light fuel compositions with increasing temperature in non-oxidative pyrolysis of decalin and tetralin are displayed in Tables 1 and 2, respectively. According to Table 1, the light fuels in pyrolysis of decalin mainly comprised naphthenes and alkylbenzenes, total mass contents of which reached above 92%. The increase of temperature greatly improved the alkylbenzene content but decreased the naphthene content in the light fuels. At 860 °C the contents of alkylbenzenes and naphthenes were 90.7% and 8.4%, respectively. Just as shown in Table 2, it is the alkylbenzenes and alkylnaphthalenes, but not the naphthenes that constituted the light fuel in pyrolysis of tetralin. The mass contents of alkylbenzenes and alkylnaphthalenes nearly unchanged at above 750 °C. It can be deduced that the hydrogen abstraction reaction and alkyl transfer reaction play an important role in pyrolysis of tetralin. 3.2. Gas-phase oxidative cracking of decalin 3.2.1. Effect of temperature The results for GOC of decalin at a temperature range of 400–860 °C are shown in Fig. 2, with a decalin / O2 molar ratio of 0.5, feed flow rate of 100 ml/min (STP). The O2 conversion was N 98% for all temperatures. The decalin conversion increased with the increasing temperature and exceeded 99% at above 800 °C. Compared with the results for pyrolysis of decalin in Fig. 1, the ring cleavage of decalin was greatly accelerated in the presence of O2 and high cracking severity was obtained at a low temperature. For example, at 600 °C, the conversion, lower alkene yield and light fuel yield were 94.0%, 28.5% and 41.0%, respectively, which are much higher than those in pyrolysis at 750 °C: 61.2%, 22.1% and 34.1%, respectively. It is obvious that the introduction of O2 changed the thermodynamic equilibrium and greatly enhanced the conversion of decalin.
Table 4 Light fuel yields and compositions for GOC of decalin (reaction temperature = 600 °C and feed flow rate = 100 ml/min) Decalin / O2
1.0 0.7 0.5 0.4 0.3
Fuel yield (%)
43.5 44.7 41.0 41.1 30.6
Fuel composition (wt.%) Paraffins
Isoparaffins
Naphthenes
Aromatics (BTX)
Olefins
Others
0.0 0.0 1.0 0.0 0.9
7.8 8.6 10.0 10.3 9.5
28.6 27.7 24.0 22.0 14.7
61.8 (8.1) 60.7 (10.9) 60.9 (19.4) 62.2 (25.9) 68.3 (29.1)
0.6 1.3 1.5 2.1 1.9
1.2 1.7 2.6 3.4 4.7
H. Zhu et al. / Fuel Processing Technology 87 (2006) 649–657
Fig. 5. Conversions and yields versus molar ratio of decalin to O2 for GOC of decalin, after excluding COx from GOC process, with a reaction temperature of 800 °C and feed flow rate of 100 ml/min (STP). □, Conversion of decalin; ○, conversion of O2; ▤, yield of light fuels; ▧, yield of lower alkenes; ■, n(feed).
As an excellent free radical initiator [13], O2 could greatly reduce the activation energy of the initial step of chain reactions, facilitate the formation of hydrocarbon radicals, and accelerate the subsequent C–C bond scission reactions in GOC of decalin, which contribute to the much higher activity in GOC than that in pyrolysis process. Meanwhile, O2 tends to attack C–C bonds, resulting in the inevitable formation of COx. According to Fig. 2, a range of 12∼19% CO yield and less than 3% CO2 yield were obtained in GOC of decalin. In principle, the formation of COx in GOC process would play two important roles: one is internally providing a large amount of heat for GOC running in an autothermic way; the other is eliminating the carbon deposition in GOC system. It was found that the reactor wall in GOC always remained rather clean after repeated runs, while apparent blacking of the reactor could be found in the pyrolysis. One should note that pyrolysis reaction is a high-energy consumption process, and a great deal of fuels has to be combusted externally to obtain a high cracking temperature inside, leading to enormous discharge of COx. Therefore, it’s reasonable to compare with pyrolysis of decalin (or tetralin) that the GOC conversion and product yield were calculated by excluding formed COx from the products. Based on the above assumption, in GOC the hydrocarbon feed will be divided to two parts, one for the COx (mainly CO) formation by partial oxidation reactions; the other for producing lower alkenes or light fuels by cracking reactions. The ratio of the combusted feed to the cracked feed was defined as “n(feed)”,
653
to evaluate the proportion of feed used for internal heating. After the exclusion of COx (the conversion and selectivity were recalculated only by dealing with cracked feed), the effect of reaction temperature on GOC of decalin is shown in Fig. 3. Just as shown in Fig. 3, the decalin conversion was so close to that in Fig. 2 especially at those temperatures above 600 °C. The yields of lower alkenes and light fuels increased with the rising temperature until 600 °C, after which lower alkene yield slightly dropped but light fuel yield nearly unchanged. Contrast to the pyrolysis of decalin (see Fig. 1), the GOC of decalin could obtain higher yields of lower alkenes and light fuels at a low temperature. For example, the conversion, lower alkene yield and light fuel yield in GOC at 600 °C were 92.7%, 34.9% and 50.3%, respectively, which are comparable to the results for pyrolysis at 800 °C: 92.1%, 35.3% and 47.7%, respectively. It could also be known in Fig. 3 that the value of parameter n (feed) varied from 0.18 to 0.25 with the temperature increasing from 400 to 860 °C. The increment of n(feed) indicates that more feed is involved in partial oxidation reactions to produce COx. Considering coking tendency becomes more obvious at high temperatures in pyrolysis, the increasing COx formation in GOC with the temperature might be attributed to the decoking performance of O2. As narrated in literature [10], the fuel consumption for firing a steam-cracker is usually equivalent to 15–18% of the feedstock (i.e., n(feed) = 0.15∼0.18). For naphthene feeds, especially multi-ring naphthenes, more heat would be needed for the pyrolysis reactions because their molecules are more difficult to crack than linear or branched chain alkanes; thus the parameter n(feed) for pyrolysis of decalin would be close to that for GOC process (0.18∼0.25), and their feedstock utilizations in both processes would be somewhat comparable. The effect of temperature on light fuel yield and compositions in GOC of decalin is displayed in Table 3. Most of light fuels were naphthenes and alkylbenzenes. The naphthene mass content gradually reduced with the increasing temperature, accompanied by the sharp increase of alkylbenzene content. When compared with the results in Table 1, the variation of light fuel compositions with temperature in GOC is so similar to that in pyrolysis. Therefore, it is deduced that the cracking routes of decalin in both GOC and pyrolysis would be similar. Firstly, decalin proceeds in the ring opening reactions to form alkyl cyclohexane and low molecules such as lower alkenes, since little dehydrogenation products (tetralin and naphthalene) were found in light fuel product. Secondly, the followed alkyl cyclohexane would go on with the cracking reactions to low
Table 5 Light fuel yields and compositions for GOC of decalin (reaction temperature = 800 °C and feed flow rate = 100 ml/min) Decalin / O2
1.0 0.7 0.5 0.4 0.3
Fuel yield (%)
49.2 49.0 48.0 45.5 40.5
Fuel composition (wt.%) Paraffins
Isoparaffins
Naphthenes
Aromatics (BTX)
Olefins
Others
1.1 0.9 0.8 0.7 0.3
1.0 0.8 0.9 6.8 4.5
10.5 9.0 7.7 1.1 0.0
86.4 (65.3) 87.6 (64.2) 87.0 (59.9) 87.4 (59.6) 90.7 (65.6)
0.2 0.0 0.0 0.0 0.0
0.8 1.7 3.6 4.0 4.5
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H. Zhu et al. / Fuel Processing Technology 87 (2006) 649–657
Fig. 6. Conversions and yields versus temperature for GOC of tetralin, after excluding COx from GOC process, with a tetralin / O2 molar ratio of 0.5 and feed flow rate of 100 ml/min (STP). □, Conversion of tetralin; ○, conversion of O2; ▤, yield of light fuels; ▧, yield of lower alkenes; ■, n(feed).
molecules; at the same time, dehydrogenation of alkyl cyclohexane to alkylbenzenes would occur. The above two reactions become intense at higher temperatures, thus resulting in a decreasing content of naphthenes and an increasing content of alkylbenzenes in light fuels with the rising temperature. Because aromatic ring in benzene is so difficult to crack to linear hydrocarbons in either GOC or pyrolysis [14], the dealkylation reactions of alkylbenzene will become more preferential at a high temperature. Therefore, the higher reaction temperature, the more BTX in light fuels. 3.2.2. Effect of decalin / O2 molar ratio The ratio of hydrocarbon to O2 is a key parameter for oxidation processes. For GOC of decalin, the right select of decalin / O2 molar ratio is crucial to the realization of its autothermic state. In this paper, the decalin / O2 molar ratio was varied between 0.3 and 1.0, which is far from its stoichiometric ratio of 0.07 for total oxidation of decalin. Fig. 4 illustrates the effect of decalin /O2 molar ratio on GOC of decalin at 600 °C, after excluding COx from GOC process. The ratio played a remarkable role in the conversion of decalin, which decreased from 94.9% at a decalin / O2 ratio of 0.3 to 67.0% at a ratio of 1.0. The O2 conversion was N 96% for all the investigated decalin /O2 ratios. The yield of lower alkenes dropped as the feed became more fuel rich, while at 0.4∼0.7 decalin /O2 molar ratios the high light fuel yields of N 50% was obtained. The thermodynamic calculation, expounded in literature [10], shows that the
autothermic state for GOC of decalin can be realized at a decalin/ O2 molar ratio of 0.5. At this decalin/ O2, a lower alkene yield of 34.9% and light fuel yield of 50.3% at a decalin conversion of 92.7% can be achieved. The compositions for the light fuels are given in Table 4. It can be known that the decrease of decalin /O2 molar ratio slightly increased the arene content in light fuels but greatly reduced the naphthene content, with the isoparaffin content nearly unchanged. Obviously, the introduction of more O2 in GOC of decalin enhances the overall cracking severity of decalin. Considering those light fuels, containing more than 60% aromatics and 7∼11% isoparaffins and less than 3% olefins, would hold a high octane number, the low temperature GOC process of heavy hydrocarbons, rich in multi-ring naphthene feeds, would be promising in producing lower alkenes and high octane rating gasoline. Fig. 5 exhibits the effect of decalin / O2 molar ratio on GOC of decalin at 800 °C, after excluding COx from GOC process. The conversions of decalin and O2 were N 98% for all molar ratios of decalin / O2. As the feed became more fuel lean, the lower alkene yield increased gradually and exceeded the fuel yield at decalin / O2 = 0.3. Just as shown in Table 5, over 85% compositions in light fuels are aromatics, and the mass content of BTX in light fuels exceeded 59% for any a decalin / O2 ratio. Under the conditions of 800 °C and a decalin / O2 ratio of 0.5, GOC reaction of decalin can obtain a 38.9% lower alkene yield and a 48.0% light fuel yield, in which the mass content of BTX was 59.9%. Considering BTX is also a fundamental chemical like lower alkenes, the production of lower alkenes can also be accompanied by BTX production in the high temperature GOC process of heavier hydrocarbons rich in multi-ring naphthene feeds. 3.3. Gas-phase oxidative cracking of tetralin GOC of tetralin were performed under the same experimental conditions as those in the case of decalin. The effect of reaction temperature on GOC of tetralin was investigated at a tetralin / O2 molar ratio of 0.5 and the results are shown in Fig. 6. Compared with the results for pyrolysis in Fig. 1, the conversion of tetralin was greatly increased in the presence of O2 especially at low temperatures. For example, at 600 °C, the conversions of tetralin and O2 were 84.5% and 74.0% in GOC process, respectively, while tetralin remained no conversion in pyrolysis. In the GOC products, the light fuels dominated and its yield was significantly higher than that of lower alkenes by an order of magnitude. Below 700 °C, the yields of both light fuels and
Table 6 Light fuel yields and compositions for GOC of tetralin (molar ratio of tetralin / O2 = 0.5 and feed flow rate = 100 ml/min) T (°C)
Fuel yield (%)
Fuel composition (wt.%) Paraffins
Isoparaffins
Naphthenes
Alkylbenzenes
Alkylnaphthalenes
Olefins
Others
500 550 600 700 800 850
3.4 15.3 77.3 83.5 87.4 87.5
0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.1 0.0 0.0 0.0 0.0
0.0 0.0 0.3 0.3 0.7 1.0
18.4 33.7 32.0 35.7 34.6 35.0
81.6 62.7 60.2 57.9 57.8 58.8
0.0 0.0 0.4 0.0 0.0 0.0
0.0 3.5 7.1 6.1 6.9 5.2
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yield increased with the depressing tetralin / O2 ratio from 1.0 to 0.4. Evidently, the ring opening reactions to alkylbenzenes and the dehydrogenation reactions to alkylnaphthalenes were somewhat comparably facilitated in GOC of tetralin, which is so different from GOC process of decalin and will be discussed in the following Section 3.4. 3.4. Comparisons of decalin and tetralin in gas-phase oxidative cracking processes
Fig. 7. Conversions and yields versus molar ratio of tetralin to O2 for GOC of tetralin, after excluding COx from GOC process, with a reaction temperature of 600 °C and feed flow rate of 100 ml/min (STP). □, Conversion of tetralin; ○, conversion of O2; ▤, yield of light fuels; ▧, yield of lower alkenes; ■, n(feed).
lower alkenes increased as the temperature rose, while above 700 °C they kept nearly unchanged and were ca. 88% and 8%, respectively. The value of n(feed) increased gradually with the rising temperature, and reached ∼0.30 at temperatures above 700 oC, which was a bit higher than that in GOC of decalin. The variation of light fuel compositions with temperature is shown in Table 6. Most of fuels are alkylbenzenes and alkylnaphthalenes, the mass contents of which nearly unchanged at temperatures above 550 °C. Those product distributions in Table 6 are so similar to those in pyrolysis process shown in Table 2. It can be known that the presence of O2 also facilitates the hydrogen abstraction reaction and alkyl transfer reaction in GOC of decalin, besides the ring opening reactions to alkylbenzenes. The effect of tetralin / O2 molar ratio on GOC of tetralin was investigated at 600 °C and the results are illustrated in Fig. 7. It can be seen that the conversion of tetralin was greatly enhanced from 42.0% to 92.9%, when tetralin / O2 ratio was decreased from 1.0 to 0.3. The conversion of O2 increased significantly with the decreasing tetralin / O2 ratio from 1.0 to 0.7, and then leveled off at around 79%. In GOC products, the yield of lower alkenes dropped as the feed became fuel rich, and the highest fuel yield of 81.0% occurred at the tetralin / O2 molar ratio of 0.4. Those light fuel compositions are shown in Table 7. It can be known that the variation of tetralin / O2 ratio has no obvious influence on the light fuel compositions, though light fuel
In principle, there were mainly three parallel grouped reactions in GOC of decalin and tetralin: (1) dehydrogenation reactions (decalin to tetralin or naphthalene, tetralin to naphthalene); (2) cracking reactions (decalin to alkyl cyclohexane/ benzene and small molecules such as lower alkenes, tetralin to alkylbenzenes and small molecules); (3) oxidation reactions to COx (mainly CO). The sketches for those grouped reactions are shown in Fig. 8. In order to compare the relative reactivity of above three grouped reactions in GOC of decalin or tetralin, we defined the relative conversion (the fraction of some decalin (or tetralin), which participates in a certain grouped reaction, in the total feeding decalin (or tetralin)); the results are shown in Fig. 9, in which Xr1 (Xr1′), Xr2 (Xr2′) and Xr3 (Xr3′) refer to the above mentioned grouped dehydrogenation, cracking, and oxidation reactions in GOC of decalin (tetralin), respectively, and the sum of Xr1 (Xr1′), Xr2 (Xr2′) and Xr3 (Xr3′) should equal to the conversion of decalin (tetralin). Just as shown in Fig. 9, decalin gave much higher relative conversions than tetralin at the temperatures less than 600 °C because of its high cracking severity. Above 600 °C, those relative conversions kept constant except for the continuously increasing Xr3′ of tetralin. Even though decalin and tetralin exhibited a nearby oxidation conversion at temperatures ≥ 600 °C, Xr1 of decalin was significantly lower than Xr1′ of tetralin, but Xr2 was twice as high as Xr2′. It can be known decalin exhibits much higher cracking performance than tetralin in GOC. On the other hand, for decalin molecules, the cracking reaction of double naphthenic rings was much preferable to the dehydrogenation reaction, and its Xr2 was significantly higher than Xr1 by an order of magnitude. For tetralin molecule, the dehydrogenation and cracking reactions were comparable, and the Xr1′ was close to Xr2′. Fig. 10 shows the variation in the relative conversions of decalin and tetralin with the molar ratio of hydrocarbon to O2 at 600 °C. In their GOC processes, Xr1′ of tetralin was 3∼10 fold
Table 7 Light fuel yields and compositions for GOC of tetralin (reaction temperature = 600 °C and feed flow rate = 100 ml/min) Tetralin / O2
1.0 0.7 0.5 0.4 0.3
Fuel yield (%)
39.0 57.6 77.3 81.0 79.0
Fuel composition (wt.%) Paraffins
Isoparaffins
Naphthenes
Alkylbenzenes
Alkylnaphthalenes
Olefins
Others
0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0
0.2 0.2 0.3 0.5 1.0
37.7 33.7 32.0 36.1 35.1
57.2 60.4 60.2 58.7 55.5
0.0 0.4 0.4 0.4 0.4
4.9 5.3 7.1 4.3 8.0
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Fig. 8. The sketches for three grouped reactions in GOC of decalin and tetralin.
higher than Xr1 of decalin, while Xr2 of decalin was about 2 times of Xr2′ of tetralin; and their oxidation conversions (Xr3 and Xr3′) were somewhat comparable for all the investigated hydrocarbon / O2 molar ratios. For decalin molecule, its cracking performance was so conspicuous and the Xr2 greatly exceeded Xr1 by an order of magnitude. For tetralin molecule, the Xr1′ was approximately equal to Xr2′. In a decalin or tetralin molecule, there exist 11 C−C bonds. Considering the little effect of O2 on the cleavage of C−C or C−H bondsofaromaticringinGOCprocessofbenzene[14],therewould be 11 C−C bonds and 18 C−H bonds involved in GOC of decalin, but 5 C−C bonds and 8 C−H bonds involved in GOC of tetralin. As aneffectiveinitiator offreeradicals, O2 can facilitatethe cleavageof C−C bonds of hydrocarbon in GOC process. For GOC of decalin and tetralin, the involved C−C bonds in a decalin molecule is just 2.2 times of those in a tetralin molecule, this may result in the cracking conversion of decalin (Xr2) being 2 times of that of tetralin (Xr2′). In other word, the naphthene rings in decalin and tetralin molecules exhibit a similar ring-rupture performance in GOC. In principle, the dehydrogenation conversion of decalin (Xr1) should be 2∼3 fold higher than that of tetralin (Xr1′) according to their C−H bond numbers in their molecules. However, as is shown in Figs. 9 and 10, the dehydrogenation conversion of decalin was significantly lower than that of tetralin. This may
attribute to the existence of aromatic ring in tetralin molecule, whose conjugated π bond would facilitate the cleavage of C−H bonds to form the steadier molecule naphthalene. Thus GOC of tetralin gave a comparable dehydrogenation conversion (Xr1′) to the cracking conversion (Xr2′), but in the case of decalin the dehydrogenation conversion (Xr1) significantly dropped behind the cracking conversion (Xr2). In GOC process of hydrocarbon, O2 would attack the C−H bonds in its molecule to produce H2O; at the same time O2 would attack the C−C bonds to form some COx (mainly CO). In order to investigate the effect of O2 on the cleavage of C−H bonds and C −C bonds, we compared the parameter D (ratio of the amount of O2 distributed in COx to the amount of converted reactant O2) in GOC of decalin and tetralin, and the results are illustrated in Fig. 11. Fig. 11 shows the variation in parameter D with the GOC conversion of decalin or tetralin, where the conversions were obtained at various temperatures. The D value greatly increased with the conversion, which shows that O2 mainly participates in the dehydrogenation reactions at lower conversions (or at low temperatures) but took part in the partial oxidation reactions at higher conversions (or at high temperatures). The higher D was obtained in GOC of tetralin than that in GOC of decalin for all conversions. This indicates that more O2 is needed to attack the
Fig. 9. The relative conversions in GOC of decalin and tetralin (Xr1, Xr1′ for grouped dehydrogenation reactions; Xr2, Xr2′ for cracking reactions; Xr3, Xr3′ for oxidation reactions) versus reaction temperature at a 0.5 molar ratio of hydrocarbon to O2. ○, Decalin: Xr1, Xr2, Xr3; ■, Tetralin: Xr1′, Xr2′, Xr3′.
Fig. 10. The relative conversions in GOC of decalin or tetralin (Xr1, Xr1′ for grouped dehydrogenation reactions; Xr2, Xr2′ for cracking reactions; Xr3, Xr3′ for oxidation reactions) versus molar ratio of hydrocarbon to O2 at 600 °C. ○, Decalin: Xr1, Xr2, Xr3; ■, Tetralin: Xr1′, Xr2′, Xr3′.
H. Zhu et al. / Fuel Processing Technology 87 (2006) 649–657
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naphthalene and alkyl benzene in the light fuels. The estimation of O2 distribution in the products demonstrated that O2 participated primarily in the oxydehydrogenation reactions at low temperatures, while mainly in the partial oxidation reactions at high temperatures to produce COx; more O2 should be involved in COx formation for tetralin molecule to obtain the same cracking severity as decalin molecule. Acknowledgements The authors are grateful to SINOPEC for its financial support (under Grant No. X504012). References Fig. 11. Parameter D (the ratio of O2 amount distributed in products COx to the converted reactant O2 amount) versus hydrocarbon conversion at a molar ratio of hydrocarbon to O2 of 0.5. ■, Tetralin; ○, Decalin.
C−C bonds in tetralin molecule to produce COx in order to obtain the same cracking severity as decalin molecule. Our previous work [14] on GOC of hexane, cyclohexane and benzene revealed that the COx formation has a correlation with the coking tendency. The hydrocarbon molecule, which has a low pyrolysis performance and is prone to the formation of coke in its pyrolysis reaction, would bring about much COx formation in its GOC reaction. For GOC of decalin and tetralin, the formation of COx is necessary for guaranteeing no carbon deposition in their GOC system, and the more COx formation means the higher decoking ability. Therefore, compared with decalin, tetralin has a higher coking potentiality and O2 exhibits a higher decoking capacity in its GOC process, and this is just the reason why the n(feed) of tetralin was a bit higher than that of decalin in GOC (see Figs. 3 and 6). 4. Conclusions The presence of O2 in gas phase oxidative cracking (GOC) process facilitated the ring rupture of decalin or tetralin molecule and enhanced the decoking ability. For GOC of decalin, both the lower alkenes and the light fuels were obtained. At lower temperatures the light fuels mainly contained alkyl benzene, alkyl cyclohexane and isoparaffins, while it was rich in BTX (benzene, toluene and xylenes) at higher temperatures. This indicates that the low temperature GOC process of heavy hydrocarbons, rich in multi-ring naphthene feeds, would be promising in producing lower alkenes and high octane rating gasoline, while the high temperature GOC process would produce lower alkenes, accompanied by BTX. For GOC of tetralin, both the dehydrogenation and the cracking reactions dominated the reaction routes, resulting in a high mass content of alkyl
[1] Y.C. Hu, Unconventional olefin processes, Hydrocarbon Process. 11 (1982) 109–116. [2] B. Chen (Ed.), Ethylene Engineering (Chinese), Chemical Industry Press, Peking, 1997, p. 87. [3] R. Burch, E.M. Crabb, G.D. Squire, S.C. Tsang, The importance of heterogeneous and homogeneous reactions in oxidative coupling of methane over chloride promoted oxide catalysts, Catal. Letters 2 (1989) 249–256. [4] Q. Chen, E.J.A. Schweitzer, P.F.V.D. Oosterkamp, R.J. Berger, C.R.H.D. Smet, G.B. Marin, Oxidative pyrolysis of ethane, Ind. Eng. Chem. Res. 36 (1997) 3248–3251. [5] A.A. Lemonidou, A.E. Stambouli, Catalytic and non-catalytic oxidative dehydrogenation of n-butane, Appl. Catal., A Gen. 171 (1998) 325–332. [6] J.J. Krummenacher, K.N. West, L.D. Schmidt, Catalytic partial oxidation of higher hydrocarbons at millisecond contact times: decane, hexadecane, and diesel fuel, J. Catal. 215 (2003) 332–343. [7] N. Kijima, K. Matano, M. Saito, T. Oikawa, T. Konishi, H. Yasuda, T. Sato, Y. Yoshimura, Oxidative catalytic cracking of n-butane to lower alkenes over layered BiOCl catalyst, Appl. Catal., A Gen. 206 (2001) 237–244. [8] R. Burch, E.M. Crabb, Homogeneous and heterogeneous contributions to the catalytic oxidative dehydrogenation of ethane, Appl. Catal., A Gen. 97 (1993) 49–65. [9] R. Burch, E.M. Crabb, Homogeneous and heterogeneous contributions to the oxidative dehydrogenation of propane on oxide catalysts, Appl. Catal., A Gen. 100 (1993) 111–130. [10] X.B. Liu, W.Z. Li, H.Y. Xu, Y.X. Chen, A comparative study of nonoxidative pyrolysis and oxidative cracking of cyclohexane to light alkenes, Fuel Process. Technol. 86 (2004) 151–167. [11] X.B. Liu, W.Z. Li, H.Y. Xu, Y.X. Chen, Production of light alkenes with low CO2 emission from gas phase oxidative cracking (GOC) of hexane, React. Kinet. Catal. Lett. 81 (2) (2004) 203–209. [12] M. Santikunaporn, J.E. Herrera, S. Jongpatiwut, D.E. Resasco, W.E. Alvarez, E.L. Sughrue, Ring opening of decalin and tetralin on HY and Pt/ HY zeolite catalysts, J. Catal. 228 (2004) 100–113. [13] M.M. Bhasin, J.H. McCain, B.V. Vora, T. Imai, P.R. Pujado, Dehydrogenation and oxydehydrogenation of paraffins to olefins, Appl. Catal., A Gen. 221 (2001) 397–419. [14] H.O. Zhu, X.B. Liu, W.Z. Li, Q.J. Ge, H.Y. Xu, Production of lower alkenes by gas phase oxidative cracking of C6 hydrocarbons, Petrochemical Technology (Chinese), vol. 34(9), 2005, pp. 813–817.
Production of lower alkenes and light fuels by gas phase oxidative cracking of heavy hydrocarbons Haiou Zhu, Xuebin Liu, Qingjie Ge, Wenzhao Li ⁎, Hengyong Xu Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, PR China Received 26 September 2005; received in revised form 11 January 2006; accepted 24 January 2006
Abstract The gas phase oxidative cracking (GOC) and non-oxidative pyrolysis of heavy hydrocarbons were investigated, with decalin (decahydronaphthalene) and tetralin (tetrahydronaphthalene) as the model compounds for naphthenic hydrocarbon and aromatic hydrocarbon, respectively. Unlike pyrolysis, the ring rupture of decalin or tetralin molecule and the decoking ability of system were significantly enhanced due to the introduction of O2 in GOC. For GOC of decalin, both the lower alkenes and the light fuels were obtained. At lower temperatures the light fuels mainly contained alkyl benzene, alkyl cyclohexane and isoparaffins, while it was rich in BTX (benzene, toluene and xylenes) at higher temperatures. A 38.9% yield of lower alkenes and 48.0% yield of light fuels (BTX mass content: 59.9%) at 100% decalin conversion were obtained under the conditions of 800 °C and decalin / O2 = 0.5. For GOC of tetralin, both the dehydrogenation and the cracking reactions dominated the reaction routes, resulting in a high mass content of alkyl naphthalene and alkyl benzene in the light fuels. The estimation of O2 distribution in the products demonstrated that O2 participated primarily in the oxydehydrogenation reactions at low temperatures, while mainly in the partial oxidation reactions at high temperatures to produce COx (x = 1, 2). © 2006 Elsevier B.V. All rights reserved. Keywords: Decalin; Tetralin; Gas phase oxidative cracking; Pyrolysis; Lower alkenes; Light fuels
1. Introduction Lower alkenes are commercially produced by the pyrolysis of hydrocarbons in tubular reactor coils installed in externally fired heaters. Feedstock plays a vital role in the production cost of the lower alkenes. Although lower alkanes such as ethane and propane are much preferable for the production of lower alkenes, those heavy feedstocks, such as naphtha, diesel oil, crude oil and residual oil, are also to be widely used in some regions where there are abundant resources of heavy stuff [1]. The heavy feedstock generally consists of paraffins, isoparaffins, naphthenes and aromatics, the mass concentrations of which greatly depend on the producing areas. For example, naphthene content in virgin naphtha of Taching oilfield (China) is as high as 46 wt.% [2]. Those naphthenic and aromatic compositions, however, are not suitable for the pyrolysis process to produce lower alkenes, due to their high molecule stability and the tendency to agglomeration. Therefore, how to take ⁎ Corresponding author. Tel.: +86 411 84379152; fax: +86 411 84691570. E-mail address: [email protected] (W. Li). 0378-3820/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2006.01.009
Conversion & Yield /%
100
80
60
40
20
0 600
650
700
750
800
850
Temperature /oC Fig. 1. Conversions and selectivities versus temperature for non-oxidative pyrolysis of decalin and tetralin with a nitrogen dilution ratio of 90% and feed flow rate of 100 ml/min (STP). Decalin: ■, conversion; ●, yield of lower alkanes; ▲, yield of lower alkenes; ▼, yield of light fuels. Tetralin: □, conversion; ○, yield of lower alkanes; △, yield of lower alkenes; ▽, yield of light fuels.
650
H. Zhu et al. / Fuel Processing Technology 87 (2006) 649–657
Table 1 Light fuel yields and compositions for non-oxidative pyrolysis of decalin (90% nitrogen dilution ratio and 100 ml/min (STP) feed flow rate) T (°C)
700 750 800 860 a
Fuel yield (%)
Fuel composition (wt.%)
8.2 34.2 47.6 50.9
Paraffins
Isoparaffins
Naphthenes
Alkylbenzenes (BTX a)
Olefins
Others
0.0 2.0 0.0 0.0
3.5 4.2 3.1 0.4
34.2 28.2 16.7 8.4
59.1 (9.0) 63.9 (33.7) 79.6 (41.6) 90.7 (66.3)
2.8 0.9 0.6 0.5
0.4 0.8 0.0 0.0
Benzene, toluene, xylenes.
Table 2 Light fuel yields and compositions for non-oxidative pyrolysis of tetralin (90% nitrogen dilution ratio and 100 ml/min (STP) feed flow rate)
700 750 800 860
Fuel yield (%)
12.2 47.0 79.0 85.1
Fuel composition (wt.%) Paraffins
Isoparaffins
Naphthenes
Alkylbenzenes
Alkylnaphthalenes
Olefins
Others
0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0
48.1 38.4 37.3 38.9
51.9 61.6 62.5 60.8
0.0 0.0 0.0 0.0
0.0 0.0 0.2 0.3
full advantage of those feedstocks (rich in naphthenes and aromatics) becomes a challenging topic for the production of lower alkenes from heavy stuff. The gas phase oxidative dehydrogenation (ODH) of C2–C4 alkanes [3–5] and gas phase oxidative cracking (GOC) of higher hydrocarbons [6,7] to lower alkenes have attracted much attention since the early 1990s. Burch et al. [3] found that some homogeneous conversion of ethane to ethylene occurred in the oxidative coupling of methane, and their successive work on ODH of ethane [8] and propane [9] showed that the homogeneous ODH reactions were competitive with the heterogeneous catalytic reactions over the best catalysts available at that time. Lemonidou and Stambouli [5] compared the pyrolysis and the catalytic and non-catalytic ODH of n-butane, and found that nearly 90% selectivity to lower alkenes with 22% conversion of butane can be achieved in non-catalytic ODH process, which was much higher than that either in the pyrolysis or in the catalytic route under comparable conditions. Most of the work in ODH of lower alkanes focused on the break of C–H bonds; however, the situation should be somewhat different when higher hydrocarbons are employed, because the cracking of C–C bonds in their molecules becomes more remarkable. The work of Liu et al. [10,11] on GOC of hexane and cyclohexane showed that much high cracking performances can be found at low temperatures. For example, at 750 °C the GOC conversion of hexane and lower alkene yield were 85% and 51%, respectively, which were significantly higher than those in the pyrolysis process: 29% and 21%. The similar reaction performance was also found in GOC of cyclohexane (87% conversion and 44% lower alkene yield at 750 °C), while its pyrolysis conversion and alkene yield were only 8% and 6%, respectively. It is evident that, when compared with pyrolysis process, GOC process exhibits much distinct advantage in the case of inferior feedstock such as cycloalkane. In this paper, decalin (decahydronaphthalene) and tetralin (tetrahydronaphthalene) were employed as the model compounds for heavier naphthene and arene, and their GOC processes were investigated for lower alkene production. The ring rupture or ring
opening of these multi-ring compounds are much more relevant to the upgrading of fuel oils [12], for example, the ring rupture of decalin to n-decane will increase the cetane number of kerosene fractions, while its ring opening to alkyl benzene will improve the octane number of gasoline fractions. Therefore, GOC of decalin or tetralin to light fuels (gasoline and kerosene) were also studied in this paper. 2. Experimental 2.1. Experimental apparatus The reactions were conducted in a straight tubular quartz reactor (6 mm inner diameter × 300 mm) under atmospheric pressure. A chromel–alumel thermocouple inside a closed quartz thermowell (2 mm outer diameter × 240 mm), which was located in the center of the reactor, was used to record the reaction temperatures freely along the reactor. The reactor was mounted in a programmable tubular furnace (120 mm long), 100
Conversion & Yield /%
T (°C)
80 60 40 20 0 400
500
600
700
800
900
Temperature /oC Fig. 2. Conversions and yields versus temperature for GOC of decalin with a decalin / O2 molar ratio of 0.5 and feed flow rate of 100 ml/min (STP). □, Conversion of decalin; ○, conversion of O2; ▲, yield of CO; ▼, yield of CO2; ▶, yield of lower alkenes; ◀, yield of light fuels.
651
100
1.0
2.3. Analytical procedure
80
0.8
60
0.6
40
0.4
20
0.2
Experimental data obtained from two GC were linked through CH4 in the products. The introduction of O2 and derived COx complicated the quantitative analysis. Thus the following adjustment was made. In order to estimate the weight distribution of carbon atoms in the products, the weight of CO or CO2 were converted to the weight of CH2. Based on above consideration, come the following definition of conversion, selectivity and yield,
0
n(feed)
Conversion & Yield /%
H. Zhu et al. / Fuel Processing Technology 87 (2006) 649–657
0.0 400
500
600
700
800
860
XO2 ¼ ðmTO2 −mRO2 Þ=mTO2 100%
Temperature /oC Fig. 3. Conversions and yields versus temperature for GOC of decalin, after excluding COx from GOC process, with a decalin / O2 molar ratio of 0.5 and feed flow rate of 100 ml/min (STP). Parameter n(feed) refers to the ratio of the part of decalin feed, participating in the partial oxidation reaction, to the other part of decalin feed in GOC of decalin. □, Conversion of decalin; ○, conversion of O2; ▤, yield of light fuels; ▧, yield of lower alkenes; ■, n(feed).
and the isothermal zone for reactor was 20 mm long. The furnace, reactor and gas lines were closed into a self-made constant temperature cabinet to prevent the condensation of liquid reactants and products. The flow rates of O2 and N2 in the feed were controlled with mass-flow controllers, and decalin or tetralin was introduced with a syringe pump, vaporized, and mixed with the previously mentioned gases in series. Two Varian model 3800 gas chromatographs (GC) were used to analyze the products online: one used a 100 m PONA capillary column with a flame ionization detector (FID) as the detector, to analyze the total organic compounds in the products; and the other used a 1.8 m HAYESEP C packed column that was connected serially with a 1.2 m MOLESIEVE 13X packed column, with a thermal conductivity detector (TCD) as the detector, for gas products (H2, O2, N2, CO, CO2, and CH4). 2.2. Experimental procedure The gas flow was interrupted between each analysis. The reactant gases were switched on for 30 min (long enough for stabilizing the reaction and obtaining reproducible experimental data) before taking a sample for analysis, and switched off immediately after sampling; then an argon stream (40 ml/min) flowed through the reactor to purge the reaction system until the next run began.
XC10 ¼ ðmT −mR Þ=mT 100% SCOx ¼ mCH2 =ðmT −mR Þ 100% SP ¼ mR =ðmT −mR Þ 100% YP ¼ XC10 SP 100% where XO2 and XC10 are the conversions of O2 and decalin (or tetralin), respectively; mTO2 and mT are the total reactant O2 and total weight of decalin (or tetralin), respectively; mRO2 and mR are the remained O2 and remained decalin (or tetralin) weight in the products, respectively; mCH2 is the adjusted weight of CO (or CO2); SCOx and SP are the selectivity to CO and CO2 and the selectivity to other organic product, respectively; YP is the product yield. Besides lower hydrocarbons (such as C1∼C4 alkanes, C2∼C4 alkenes), a large amount of higher hydrocarbons (carbon atoms ≥ 5), which were generally called light fuels in this paper, were also produced in GOC of decalin or tetralin. For the sake of clarity, we grouped these light fuel products according to their carbon chain skeleton in molecules: (a) paraffins (straight-chain alkanes, such as n-pentane, n-hexane, and etc.); (b) isoparaffins (branched-chain alkanes, such as methyl pentane, ethyl pentane, and etc.); (c) naphthenes (such as methylcyclopentane, cyclohexane, methylcyclohexane, and etc.); (d) alkylbenzenes (such as benzene, toluene, ethylbenzene, butylbenzene, and etc.); (e) alkylnaphthalenes (such as naphthalene, methylnaphthalene, dimethylnaphthalene, and etc.); (f) olefins (straightchain alkenes, such as pentene, hexene, and etc.) and (g) others (some oxygenates, such as phenol, benzaldehyde, and etc.).
Table 3 Light fuel yields and compositions for GOC of decalin (molar ratio of decalin / O2 = 0.5 and feed flow rate = 100 ml/min) T (°C)
Fuel yield (%)
Fuel composition (wt.%) Paraffins
Isoparaffins
Naphthenes
Alkylbenzenes (BTX)
Olefins
Others
400 500 600 700 800 860
46.3 49.1 50.3 51.3 50.0 48.4
0.0 0.2 1.0 0.2 1.0 0.0
3.9 8.7 10.0 8.9 1.1 0.5
29.4 23.5 24.0 19.1 7.2 4.9
65.8 (0.6) 65.2 (2.6) 60.9 (15.5) 67.6 (17.9) 87.2 (50.5) 92.1 (57.8)
0.4 0.7 1.5 1.4 1.7 0.0
0.5 1.7 2.6 2.8 1.8 2.5
100
1.0
80
0.8
60
0.6
40
0.4
20
0.2
n(feed)
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Conversion & Yield /%
652
0.0
0 0.3
0.4
0.5
0.7
1
Decalin/O2 molar ratio Fig. 4. Conversions and yields versus molar ratio of decalin to O2 for GOC of decalin, after excluding COx from GOC process, with a reaction temperature of 600 °C and a feed flow rate of 100 ml/min (STP). □, Conversion of decalin; ○, conversion of O2; ▤, yield of light fuels; ▧, yield of lower alkenes; ■, n(feed).
In order to estimate the fraction of some decalin (or tetralin), which converts to a certain above grouped product, in the total feeding decalin (or tetralin), we defined the relative conversion (Xr), Xr ¼ mc =mT 100% where mc is the converted decalin (or tetralin), to form a certain grouped product. In order to estimate the fraction of converted O2 in the COx, the following definition (D) was made, D ¼ O2ðCOxÞ =ðXO2 mTO2 Þ 100% where O2(COx) is the amount of O2 distributed in the COx and equals to 1 / 2 × mole number of CO plus mole number of CO2.
3. Results and discussion 3.1. Non-oxidative pyrolysis of decalin and tetralin The conversions and product yields in non-oxidative pyrolysis of decalin and tetralin at various temperatures are shown in Fig. 1. The conversions of decalin and tetralin were unconspicuous at temperatures below 650 °C, above which both conversions greatly increased, and decalin exhibited a slightly
higher activity than tetralin. Lower alkenes (C2∼C4 alkenes) and light fuels were the main products, and their yields significantly increased as the reaction temperature rose. A lower alkene yield of 35.3% and light fuel yield of 47.7% can be achieved at 800 °C in pyrolysis of decalin, while light fuels greatly dominated the pyrolysis products in the case of tetralin and a 79.0% light fuel yield and only a 3.4% lower alkene yield were obtained at 800 °C. For all the temperatures, the yields of lower alkanes (C1∼C4 alkenes) were less than 11%, though they slightly improved with the rising temperature. The variations in light fuel compositions with increasing temperature in non-oxidative pyrolysis of decalin and tetralin are displayed in Tables 1 and 2, respectively. According to Table 1, the light fuels in pyrolysis of decalin mainly comprised naphthenes and alkylbenzenes, total mass contents of which reached above 92%. The increase of temperature greatly improved the alkylbenzene content but decreased the naphthene content in the light fuels. At 860 °C the contents of alkylbenzenes and naphthenes were 90.7% and 8.4%, respectively. Just as shown in Table 2, it is the alkylbenzenes and alkylnaphthalenes, but not the naphthenes that constituted the light fuel in pyrolysis of tetralin. The mass contents of alkylbenzenes and alkylnaphthalenes nearly unchanged at above 750 °C. It can be deduced that the hydrogen abstraction reaction and alkyl transfer reaction play an important role in pyrolysis of tetralin. 3.2. Gas-phase oxidative cracking of decalin 3.2.1. Effect of temperature The results for GOC of decalin at a temperature range of 400–860 °C are shown in Fig. 2, with a decalin / O2 molar ratio of 0.5, feed flow rate of 100 ml/min (STP). The O2 conversion was N 98% for all temperatures. The decalin conversion increased with the increasing temperature and exceeded 99% at above 800 °C. Compared with the results for pyrolysis of decalin in Fig. 1, the ring cleavage of decalin was greatly accelerated in the presence of O2 and high cracking severity was obtained at a low temperature. For example, at 600 °C, the conversion, lower alkene yield and light fuel yield were 94.0%, 28.5% and 41.0%, respectively, which are much higher than those in pyrolysis at 750 °C: 61.2%, 22.1% and 34.1%, respectively. It is obvious that the introduction of O2 changed the thermodynamic equilibrium and greatly enhanced the conversion of decalin.
Table 4 Light fuel yields and compositions for GOC of decalin (reaction temperature = 600 °C and feed flow rate = 100 ml/min) Decalin / O2
1.0 0.7 0.5 0.4 0.3
Fuel yield (%)
43.5 44.7 41.0 41.1 30.6
Fuel composition (wt.%) Paraffins
Isoparaffins
Naphthenes
Aromatics (BTX)
Olefins
Others
0.0 0.0 1.0 0.0 0.9
7.8 8.6 10.0 10.3 9.5
28.6 27.7 24.0 22.0 14.7
61.8 (8.1) 60.7 (10.9) 60.9 (19.4) 62.2 (25.9) 68.3 (29.1)
0.6 1.3 1.5 2.1 1.9
1.2 1.7 2.6 3.4 4.7
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Fig. 5. Conversions and yields versus molar ratio of decalin to O2 for GOC of decalin, after excluding COx from GOC process, with a reaction temperature of 800 °C and feed flow rate of 100 ml/min (STP). □, Conversion of decalin; ○, conversion of O2; ▤, yield of light fuels; ▧, yield of lower alkenes; ■, n(feed).
As an excellent free radical initiator [13], O2 could greatly reduce the activation energy of the initial step of chain reactions, facilitate the formation of hydrocarbon radicals, and accelerate the subsequent C–C bond scission reactions in GOC of decalin, which contribute to the much higher activity in GOC than that in pyrolysis process. Meanwhile, O2 tends to attack C–C bonds, resulting in the inevitable formation of COx. According to Fig. 2, a range of 12∼19% CO yield and less than 3% CO2 yield were obtained in GOC of decalin. In principle, the formation of COx in GOC process would play two important roles: one is internally providing a large amount of heat for GOC running in an autothermic way; the other is eliminating the carbon deposition in GOC system. It was found that the reactor wall in GOC always remained rather clean after repeated runs, while apparent blacking of the reactor could be found in the pyrolysis. One should note that pyrolysis reaction is a high-energy consumption process, and a great deal of fuels has to be combusted externally to obtain a high cracking temperature inside, leading to enormous discharge of COx. Therefore, it’s reasonable to compare with pyrolysis of decalin (or tetralin) that the GOC conversion and product yield were calculated by excluding formed COx from the products. Based on the above assumption, in GOC the hydrocarbon feed will be divided to two parts, one for the COx (mainly CO) formation by partial oxidation reactions; the other for producing lower alkenes or light fuels by cracking reactions. The ratio of the combusted feed to the cracked feed was defined as “n(feed)”,
653
to evaluate the proportion of feed used for internal heating. After the exclusion of COx (the conversion and selectivity were recalculated only by dealing with cracked feed), the effect of reaction temperature on GOC of decalin is shown in Fig. 3. Just as shown in Fig. 3, the decalin conversion was so close to that in Fig. 2 especially at those temperatures above 600 °C. The yields of lower alkenes and light fuels increased with the rising temperature until 600 °C, after which lower alkene yield slightly dropped but light fuel yield nearly unchanged. Contrast to the pyrolysis of decalin (see Fig. 1), the GOC of decalin could obtain higher yields of lower alkenes and light fuels at a low temperature. For example, the conversion, lower alkene yield and light fuel yield in GOC at 600 °C were 92.7%, 34.9% and 50.3%, respectively, which are comparable to the results for pyrolysis at 800 °C: 92.1%, 35.3% and 47.7%, respectively. It could also be known in Fig. 3 that the value of parameter n (feed) varied from 0.18 to 0.25 with the temperature increasing from 400 to 860 °C. The increment of n(feed) indicates that more feed is involved in partial oxidation reactions to produce COx. Considering coking tendency becomes more obvious at high temperatures in pyrolysis, the increasing COx formation in GOC with the temperature might be attributed to the decoking performance of O2. As narrated in literature [10], the fuel consumption for firing a steam-cracker is usually equivalent to 15–18% of the feedstock (i.e., n(feed) = 0.15∼0.18). For naphthene feeds, especially multi-ring naphthenes, more heat would be needed for the pyrolysis reactions because their molecules are more difficult to crack than linear or branched chain alkanes; thus the parameter n(feed) for pyrolysis of decalin would be close to that for GOC process (0.18∼0.25), and their feedstock utilizations in both processes would be somewhat comparable. The effect of temperature on light fuel yield and compositions in GOC of decalin is displayed in Table 3. Most of light fuels were naphthenes and alkylbenzenes. The naphthene mass content gradually reduced with the increasing temperature, accompanied by the sharp increase of alkylbenzene content. When compared with the results in Table 1, the variation of light fuel compositions with temperature in GOC is so similar to that in pyrolysis. Therefore, it is deduced that the cracking routes of decalin in both GOC and pyrolysis would be similar. Firstly, decalin proceeds in the ring opening reactions to form alkyl cyclohexane and low molecules such as lower alkenes, since little dehydrogenation products (tetralin and naphthalene) were found in light fuel product. Secondly, the followed alkyl cyclohexane would go on with the cracking reactions to low
Table 5 Light fuel yields and compositions for GOC of decalin (reaction temperature = 800 °C and feed flow rate = 100 ml/min) Decalin / O2
1.0 0.7 0.5 0.4 0.3
Fuel yield (%)
49.2 49.0 48.0 45.5 40.5
Fuel composition (wt.%) Paraffins
Isoparaffins
Naphthenes
Aromatics (BTX)
Olefins
Others
1.1 0.9 0.8 0.7 0.3
1.0 0.8 0.9 6.8 4.5
10.5 9.0 7.7 1.1 0.0
86.4 (65.3) 87.6 (64.2) 87.0 (59.9) 87.4 (59.6) 90.7 (65.6)
0.2 0.0 0.0 0.0 0.0
0.8 1.7 3.6 4.0 4.5
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Fig. 6. Conversions and yields versus temperature for GOC of tetralin, after excluding COx from GOC process, with a tetralin / O2 molar ratio of 0.5 and feed flow rate of 100 ml/min (STP). □, Conversion of tetralin; ○, conversion of O2; ▤, yield of light fuels; ▧, yield of lower alkenes; ■, n(feed).
molecules; at the same time, dehydrogenation of alkyl cyclohexane to alkylbenzenes would occur. The above two reactions become intense at higher temperatures, thus resulting in a decreasing content of naphthenes and an increasing content of alkylbenzenes in light fuels with the rising temperature. Because aromatic ring in benzene is so difficult to crack to linear hydrocarbons in either GOC or pyrolysis [14], the dealkylation reactions of alkylbenzene will become more preferential at a high temperature. Therefore, the higher reaction temperature, the more BTX in light fuels. 3.2.2. Effect of decalin / O2 molar ratio The ratio of hydrocarbon to O2 is a key parameter for oxidation processes. For GOC of decalin, the right select of decalin / O2 molar ratio is crucial to the realization of its autothermic state. In this paper, the decalin / O2 molar ratio was varied between 0.3 and 1.0, which is far from its stoichiometric ratio of 0.07 for total oxidation of decalin. Fig. 4 illustrates the effect of decalin /O2 molar ratio on GOC of decalin at 600 °C, after excluding COx from GOC process. The ratio played a remarkable role in the conversion of decalin, which decreased from 94.9% at a decalin / O2 ratio of 0.3 to 67.0% at a ratio of 1.0. The O2 conversion was N 96% for all the investigated decalin /O2 ratios. The yield of lower alkenes dropped as the feed became more fuel rich, while at 0.4∼0.7 decalin /O2 molar ratios the high light fuel yields of N 50% was obtained. The thermodynamic calculation, expounded in literature [10], shows that the
autothermic state for GOC of decalin can be realized at a decalin/ O2 molar ratio of 0.5. At this decalin/ O2, a lower alkene yield of 34.9% and light fuel yield of 50.3% at a decalin conversion of 92.7% can be achieved. The compositions for the light fuels are given in Table 4. It can be known that the decrease of decalin /O2 molar ratio slightly increased the arene content in light fuels but greatly reduced the naphthene content, with the isoparaffin content nearly unchanged. Obviously, the introduction of more O2 in GOC of decalin enhances the overall cracking severity of decalin. Considering those light fuels, containing more than 60% aromatics and 7∼11% isoparaffins and less than 3% olefins, would hold a high octane number, the low temperature GOC process of heavy hydrocarbons, rich in multi-ring naphthene feeds, would be promising in producing lower alkenes and high octane rating gasoline. Fig. 5 exhibits the effect of decalin / O2 molar ratio on GOC of decalin at 800 °C, after excluding COx from GOC process. The conversions of decalin and O2 were N 98% for all molar ratios of decalin / O2. As the feed became more fuel lean, the lower alkene yield increased gradually and exceeded the fuel yield at decalin / O2 = 0.3. Just as shown in Table 5, over 85% compositions in light fuels are aromatics, and the mass content of BTX in light fuels exceeded 59% for any a decalin / O2 ratio. Under the conditions of 800 °C and a decalin / O2 ratio of 0.5, GOC reaction of decalin can obtain a 38.9% lower alkene yield and a 48.0% light fuel yield, in which the mass content of BTX was 59.9%. Considering BTX is also a fundamental chemical like lower alkenes, the production of lower alkenes can also be accompanied by BTX production in the high temperature GOC process of heavier hydrocarbons rich in multi-ring naphthene feeds. 3.3. Gas-phase oxidative cracking of tetralin GOC of tetralin were performed under the same experimental conditions as those in the case of decalin. The effect of reaction temperature on GOC of tetralin was investigated at a tetralin / O2 molar ratio of 0.5 and the results are shown in Fig. 6. Compared with the results for pyrolysis in Fig. 1, the conversion of tetralin was greatly increased in the presence of O2 especially at low temperatures. For example, at 600 °C, the conversions of tetralin and O2 were 84.5% and 74.0% in GOC process, respectively, while tetralin remained no conversion in pyrolysis. In the GOC products, the light fuels dominated and its yield was significantly higher than that of lower alkenes by an order of magnitude. Below 700 °C, the yields of both light fuels and
Table 6 Light fuel yields and compositions for GOC of tetralin (molar ratio of tetralin / O2 = 0.5 and feed flow rate = 100 ml/min) T (°C)
Fuel yield (%)
Fuel composition (wt.%) Paraffins
Isoparaffins
Naphthenes
Alkylbenzenes
Alkylnaphthalenes
Olefins
Others
500 550 600 700 800 850
3.4 15.3 77.3 83.5 87.4 87.5
0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.1 0.0 0.0 0.0 0.0
0.0 0.0 0.3 0.3 0.7 1.0
18.4 33.7 32.0 35.7 34.6 35.0
81.6 62.7 60.2 57.9 57.8 58.8
0.0 0.0 0.4 0.0 0.0 0.0
0.0 3.5 7.1 6.1 6.9 5.2
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yield increased with the depressing tetralin / O2 ratio from 1.0 to 0.4. Evidently, the ring opening reactions to alkylbenzenes and the dehydrogenation reactions to alkylnaphthalenes were somewhat comparably facilitated in GOC of tetralin, which is so different from GOC process of decalin and will be discussed in the following Section 3.4. 3.4. Comparisons of decalin and tetralin in gas-phase oxidative cracking processes
Fig. 7. Conversions and yields versus molar ratio of tetralin to O2 for GOC of tetralin, after excluding COx from GOC process, with a reaction temperature of 600 °C and feed flow rate of 100 ml/min (STP). □, Conversion of tetralin; ○, conversion of O2; ▤, yield of light fuels; ▧, yield of lower alkenes; ■, n(feed).
lower alkenes increased as the temperature rose, while above 700 °C they kept nearly unchanged and were ca. 88% and 8%, respectively. The value of n(feed) increased gradually with the rising temperature, and reached ∼0.30 at temperatures above 700 oC, which was a bit higher than that in GOC of decalin. The variation of light fuel compositions with temperature is shown in Table 6. Most of fuels are alkylbenzenes and alkylnaphthalenes, the mass contents of which nearly unchanged at temperatures above 550 °C. Those product distributions in Table 6 are so similar to those in pyrolysis process shown in Table 2. It can be known that the presence of O2 also facilitates the hydrogen abstraction reaction and alkyl transfer reaction in GOC of decalin, besides the ring opening reactions to alkylbenzenes. The effect of tetralin / O2 molar ratio on GOC of tetralin was investigated at 600 °C and the results are illustrated in Fig. 7. It can be seen that the conversion of tetralin was greatly enhanced from 42.0% to 92.9%, when tetralin / O2 ratio was decreased from 1.0 to 0.3. The conversion of O2 increased significantly with the decreasing tetralin / O2 ratio from 1.0 to 0.7, and then leveled off at around 79%. In GOC products, the yield of lower alkenes dropped as the feed became fuel rich, and the highest fuel yield of 81.0% occurred at the tetralin / O2 molar ratio of 0.4. Those light fuel compositions are shown in Table 7. It can be known that the variation of tetralin / O2 ratio has no obvious influence on the light fuel compositions, though light fuel
In principle, there were mainly three parallel grouped reactions in GOC of decalin and tetralin: (1) dehydrogenation reactions (decalin to tetralin or naphthalene, tetralin to naphthalene); (2) cracking reactions (decalin to alkyl cyclohexane/ benzene and small molecules such as lower alkenes, tetralin to alkylbenzenes and small molecules); (3) oxidation reactions to COx (mainly CO). The sketches for those grouped reactions are shown in Fig. 8. In order to compare the relative reactivity of above three grouped reactions in GOC of decalin or tetralin, we defined the relative conversion (the fraction of some decalin (or tetralin), which participates in a certain grouped reaction, in the total feeding decalin (or tetralin)); the results are shown in Fig. 9, in which Xr1 (Xr1′), Xr2 (Xr2′) and Xr3 (Xr3′) refer to the above mentioned grouped dehydrogenation, cracking, and oxidation reactions in GOC of decalin (tetralin), respectively, and the sum of Xr1 (Xr1′), Xr2 (Xr2′) and Xr3 (Xr3′) should equal to the conversion of decalin (tetralin). Just as shown in Fig. 9, decalin gave much higher relative conversions than tetralin at the temperatures less than 600 °C because of its high cracking severity. Above 600 °C, those relative conversions kept constant except for the continuously increasing Xr3′ of tetralin. Even though decalin and tetralin exhibited a nearby oxidation conversion at temperatures ≥ 600 °C, Xr1 of decalin was significantly lower than Xr1′ of tetralin, but Xr2 was twice as high as Xr2′. It can be known decalin exhibits much higher cracking performance than tetralin in GOC. On the other hand, for decalin molecules, the cracking reaction of double naphthenic rings was much preferable to the dehydrogenation reaction, and its Xr2 was significantly higher than Xr1 by an order of magnitude. For tetralin molecule, the dehydrogenation and cracking reactions were comparable, and the Xr1′ was close to Xr2′. Fig. 10 shows the variation in the relative conversions of decalin and tetralin with the molar ratio of hydrocarbon to O2 at 600 °C. In their GOC processes, Xr1′ of tetralin was 3∼10 fold
Table 7 Light fuel yields and compositions for GOC of tetralin (reaction temperature = 600 °C and feed flow rate = 100 ml/min) Tetralin / O2
1.0 0.7 0.5 0.4 0.3
Fuel yield (%)
39.0 57.6 77.3 81.0 79.0
Fuel composition (wt.%) Paraffins
Isoparaffins
Naphthenes
Alkylbenzenes
Alkylnaphthalenes
Olefins
Others
0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0
0.2 0.2 0.3 0.5 1.0
37.7 33.7 32.0 36.1 35.1
57.2 60.4 60.2 58.7 55.5
0.0 0.4 0.4 0.4 0.4
4.9 5.3 7.1 4.3 8.0
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Fig. 8. The sketches for three grouped reactions in GOC of decalin and tetralin.
higher than Xr1 of decalin, while Xr2 of decalin was about 2 times of Xr2′ of tetralin; and their oxidation conversions (Xr3 and Xr3′) were somewhat comparable for all the investigated hydrocarbon / O2 molar ratios. For decalin molecule, its cracking performance was so conspicuous and the Xr2 greatly exceeded Xr1 by an order of magnitude. For tetralin molecule, the Xr1′ was approximately equal to Xr2′. In a decalin or tetralin molecule, there exist 11 C−C bonds. Considering the little effect of O2 on the cleavage of C−C or C−H bondsofaromaticringinGOCprocessofbenzene[14],therewould be 11 C−C bonds and 18 C−H bonds involved in GOC of decalin, but 5 C−C bonds and 8 C−H bonds involved in GOC of tetralin. As aneffectiveinitiator offreeradicals, O2 can facilitatethe cleavageof C−C bonds of hydrocarbon in GOC process. For GOC of decalin and tetralin, the involved C−C bonds in a decalin molecule is just 2.2 times of those in a tetralin molecule, this may result in the cracking conversion of decalin (Xr2) being 2 times of that of tetralin (Xr2′). In other word, the naphthene rings in decalin and tetralin molecules exhibit a similar ring-rupture performance in GOC. In principle, the dehydrogenation conversion of decalin (Xr1) should be 2∼3 fold higher than that of tetralin (Xr1′) according to their C−H bond numbers in their molecules. However, as is shown in Figs. 9 and 10, the dehydrogenation conversion of decalin was significantly lower than that of tetralin. This may
attribute to the existence of aromatic ring in tetralin molecule, whose conjugated π bond would facilitate the cleavage of C−H bonds to form the steadier molecule naphthalene. Thus GOC of tetralin gave a comparable dehydrogenation conversion (Xr1′) to the cracking conversion (Xr2′), but in the case of decalin the dehydrogenation conversion (Xr1) significantly dropped behind the cracking conversion (Xr2). In GOC process of hydrocarbon, O2 would attack the C−H bonds in its molecule to produce H2O; at the same time O2 would attack the C−C bonds to form some COx (mainly CO). In order to investigate the effect of O2 on the cleavage of C−H bonds and C −C bonds, we compared the parameter D (ratio of the amount of O2 distributed in COx to the amount of converted reactant O2) in GOC of decalin and tetralin, and the results are illustrated in Fig. 11. Fig. 11 shows the variation in parameter D with the GOC conversion of decalin or tetralin, where the conversions were obtained at various temperatures. The D value greatly increased with the conversion, which shows that O2 mainly participates in the dehydrogenation reactions at lower conversions (or at low temperatures) but took part in the partial oxidation reactions at higher conversions (or at high temperatures). The higher D was obtained in GOC of tetralin than that in GOC of decalin for all conversions. This indicates that more O2 is needed to attack the
Fig. 9. The relative conversions in GOC of decalin and tetralin (Xr1, Xr1′ for grouped dehydrogenation reactions; Xr2, Xr2′ for cracking reactions; Xr3, Xr3′ for oxidation reactions) versus reaction temperature at a 0.5 molar ratio of hydrocarbon to O2. ○, Decalin: Xr1, Xr2, Xr3; ■, Tetralin: Xr1′, Xr2′, Xr3′.
Fig. 10. The relative conversions in GOC of decalin or tetralin (Xr1, Xr1′ for grouped dehydrogenation reactions; Xr2, Xr2′ for cracking reactions; Xr3, Xr3′ for oxidation reactions) versus molar ratio of hydrocarbon to O2 at 600 °C. ○, Decalin: Xr1, Xr2, Xr3; ■, Tetralin: Xr1′, Xr2′, Xr3′.
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naphthalene and alkyl benzene in the light fuels. The estimation of O2 distribution in the products demonstrated that O2 participated primarily in the oxydehydrogenation reactions at low temperatures, while mainly in the partial oxidation reactions at high temperatures to produce COx; more O2 should be involved in COx formation for tetralin molecule to obtain the same cracking severity as decalin molecule. Acknowledgements The authors are grateful to SINOPEC for its financial support (under Grant No. X504012). References Fig. 11. Parameter D (the ratio of O2 amount distributed in products COx to the converted reactant O2 amount) versus hydrocarbon conversion at a molar ratio of hydrocarbon to O2 of 0.5. ■, Tetralin; ○, Decalin.
C−C bonds in tetralin molecule to produce COx in order to obtain the same cracking severity as decalin molecule. Our previous work [14] on GOC of hexane, cyclohexane and benzene revealed that the COx formation has a correlation with the coking tendency. The hydrocarbon molecule, which has a low pyrolysis performance and is prone to the formation of coke in its pyrolysis reaction, would bring about much COx formation in its GOC reaction. For GOC of decalin and tetralin, the formation of COx is necessary for guaranteeing no carbon deposition in their GOC system, and the more COx formation means the higher decoking ability. Therefore, compared with decalin, tetralin has a higher coking potentiality and O2 exhibits a higher decoking capacity in its GOC process, and this is just the reason why the n(feed) of tetralin was a bit higher than that of decalin in GOC (see Figs. 3 and 6). 4. Conclusions The presence of O2 in gas phase oxidative cracking (GOC) process facilitated the ring rupture of decalin or tetralin molecule and enhanced the decoking ability. For GOC of decalin, both the lower alkenes and the light fuels were obtained. At lower temperatures the light fuels mainly contained alkyl benzene, alkyl cyclohexane and isoparaffins, while it was rich in BTX (benzene, toluene and xylenes) at higher temperatures. This indicates that the low temperature GOC process of heavy hydrocarbons, rich in multi-ring naphthene feeds, would be promising in producing lower alkenes and high octane rating gasoline, while the high temperature GOC process would produce lower alkenes, accompanied by BTX. For GOC of tetralin, both the dehydrogenation and the cracking reactions dominated the reaction routes, resulting in a high mass content of alkyl
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