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Ind. Eng. Chem. Res. 1997, 36, 76-82
MATERIALS AND INTERFACES Catalytic Cracking of Linear Paraffins: Effects of Chain Length John Abbot* and Paul R. Dunstan Scientific Services, Ampol Refineries (Queensland) Ltd., Lytton, Queensland, Australia 4178
The influence of chain length on cracking reactions of linear paraffins on acid catalysts has been examined. Whether studied as individual reactants, as components of simple mixtures of paraffins, or as minor constituents of complex industrial FCCU feedstocks, increasing chain length of n-paraffins is associated with increasing cracking reactivity. This increase in reactivity cannot be accounted for solely by considering the increase in the number of carbon atoms or the number of crackable bonds in the molecular structure. However, by simultaneously taking into account adsorption phenomena and crackable bonds, good correlations between observed rates and predicted behavior are obtained, both for cracking of individual paraffins and for components of simple paraffin mixtures. Reasonable correlations are also observed for cracking rates of linear paraffins as components of industrial FCCU feedstocks. The presence of other components in the feed can influence the crackability of linear paraffins but does not have a major influence on the relative cracking rates of linear paraffins due to chain-length effects. Introduction For each major representative class of hydrocarbons in crude oil, including monoalkylbenzenes, naphthenes, and n-paraffins (Venuto and Habib, 1979), the observed conversion under cracking conditions increases with the number of carbon atoms in the molecular structure. This trend in crackability with increasing chain length has also been established for cracking of olefins (Buchanan et al., 1996; Abbot and Wociechowski, 1988a-c), which, although not a component of crude oil, are important products from cracking of other hydrocarbon types (Venuto and Habib, 1979). Paraffins, both linear and branched, comprise significant proportions of many FCCU feeds (Fischer, 1990; Venuto and Habib, 1979). It is well established that the crackability of linear paraffins generally increases with chain length (Buchanan et al. 1996; Groten and Wojciechowski, 1993; Abbot and Wojciechowski, 1989), although some studies have concluded that this trend levels off at about C16 (Nace, 1969; Kissin, 1990). Some studies have tried to relate this effect linearly to either the number of carbon atoms, C-C bonds, or crackable bonds in the paraffin chain. Other studies have emphasized the need to take into account adsorption phenomena, as well as intrinsic reactivity of the paraffin molecule, as cracking is a heterogeneously catalyzed reaction process (Weller, 1995). In this paper, we have reexamined reported data on rates of cracking of individual linear paraffins and propose that most results can be accounted for by assuming a dependence of both factors. There have been comparatively few reported studies of cracking of simple hydrocarbon mixtures, where effects of chain length have been investigated (Guerzoni and Abbot, 1993). The presence of other molecular species in a mixture of hydrocarbons can influence the rate of cracking of paraffinic species, with both accelerating and inhibiting phenomena having been observed (Abbot and Head, 1990; Zhao et al., 1993a,b; Guerzoni and Abbot, 1993). Accelerating effects are generally only observed for cracking of paraffins which S0888-5885(96)00255-2 CCC: $14.00
contain hydrogen atoms attached to tertiary carbon atoms (i.e., branched paraffins), so that cracking reactions involving chain reactions are important. In this paper we have reexamined previously reported results for cracking of highly paraffinic mixtures (Guerzoni and Abbot, 1993), to compare chain-length effects with results for individual compounds. We have also reported new experimental data for cracking of paraffinic industrial FCCU feedstocks on equilibrium catalysts to examine the influence of this more complex environment on reactivity of linear paraffins under cracking conditions. Experimental Section Cracking of FCC Feedstocks. The microreactor methodology at Ampol is based on ASTM D3907. The initial temperature in the catalyst bed was 537 °C. A preheating zone for the feed was provided with 10 g glass beads located above the catalyst zone. Experiments were carried out using 5.0 g of catalyst, a feed flow rate of 0.05 g‚s-1, and time-on-stream of 20-30 s. The reactor was purged with dry nitrogen gas, prior to reaction, and for 10 min after feed injection. Overall mass balances of between 95 and 105% were achieved, with most between 98 and 102%. The catalyst used was a sample of regenerated Grace Orion/XPD equilibrium catalyst withdrawn from the FCCU inventory at the Ampol refinery. Analysis Procedures. Gas samples were analyzed using a Carle Series 400 AGC gas chromatograph, with thermal a conductivity detector to establish the total mass of gaseous hydrocarbon products formed and also molecular hydrogen. Gas samples were also analyzed using a Perkin-Elmer gas chromatograph, equipped with a capillary column and a flame ionization detector to provide distribution of hydrocarbon products collected in the gas phase. Liquid samples were also analyzed using the PerkinElmer Autosystem GC with a Restek MXT-1 capillary column, allowing a complete distribution of products to © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 1, 1997 77
be determined. Liquid products were also analyzed using a Hewlett Packard 5850 gas chromatograph with FID using an OV-1 packed column to obtain a simulated distillation (ASTM D2887) profile, enabling conversions to be calculated as 100 - % LCO - % CLO. Coke yields were determined using a Leco WR12 carbon determinator. The total coke was calculated by addition of both coke on the catalyst and thermal coke deposited on the glass beads in the preheater zone. Generally, about 10% of the total coke produced was deposited on the glass beads, and both types of coke were included in the calculation of the total mass balances. Adsorption Processes As recently pointed out by Pruski et al. (1995), cracking reactions are often interpreted in the technical literature as pseudohomogeneous reaction processes for purposes of kinetic modeling. However, catalytic cracking reactions involve the combined effects of both reaction and adsorption phenomena, and adequate kinetic modeling requires a consideration of both processes (Pruski et al., 1995). In a series of detailed kinetic analysis of cracking of n-paraffins at 400 °C (Abbot and Wojciechowski, 1987a), it was concluded that adequate fitting of results required consideration of the Langmuir adsorption isotherm. Incorporating adsorption parameters into the kinetic model (Abbot and Wojciechowski, 1987a) provided a superior fitting to experimental data compared to utilization of a simple first-order kinetic expression (Corma et al., 1985). Kissin (1990) also concluded that the apparent rate constants obtained should be regarded as a function of two parameters, the equilibrium constant for hydrocarbon adsorption on the active sites and the rate constant for ionization of the adsorbed hydrocarbon molecule. Early studies by Barrer and Sutherland (1956, 1972) showed that heats of adsorption of paraffins on faujasite zeolites increase approximately linearly with chain length. More recently, Stockenhuber (1994) showed that heats of adsorption of n-alkanes on HSM-5 increase with chain length, and adsorption can be described by a set of Langmuir isotherms. Nareshuber et al. (1995) concluded that at high temperatures, where low alkane coverages prevail, the adsorption can be described by a Langmuir isotherm in its reduced form, so that the apparent rate constants of the hydrocarbon reactions may be expressed as the product of the adsorption constant and the rate constant of the corresponding cracking process. Adsorption constants have been incorporated into a four-lump model for gas oil cracking, demonstrating that there are consistent differences between adsorption constants for gas oil, gasoline, and light gases (Farag et al., 1994). The gas oil lump yielded the largest adsorption constants, gasoline intermediate values, and light gases the smallest values. On the basis of these considerations, we have made the approximation that
K∝N
(1)
where K is the Langmuir absorption constant, and N is the number of carbon atoms in the linear paraffin. Kinetics of Paraffin Cracking Processes There have been numerous kinetic studies of cracking of individual n-paraffins on solid acid catalysts (Abbot
and Wojciechowski, 1987a; Corma et al., 1984). Some studies (Abbot and Wociechowski, 1987a) have attempted to apply the Langmuir adsorption isotherm as presented in eq 2:
∑
-d[C] K ki[C] ) dt 1 + K[C]
(2)
where paraffin C is present at concentration [C] at time t, ki are the individual rate constants for i parallel modes of cracking, and K is the Langmuir adsorption constant for paraffin C. Detailed kinetic models for mixtures of paraffins have also been developed by extending eq 2 (Abbot and Wociechowski, 1986), but the usefulness of these models is limited by the required number of parameters, even for a simple binary mixture (Abbot and Wociechowski, 1986). Others (Corma et al., 1984; Corma, 1985) have fitted experimental data for cracking of individual n-paraffins using a reduced form of the Langmuir expression, by assuming 1 . K, so that eq 2 can be rewritten as:
∑ki[C] ) k[C]
-d[C] )K dt
(3)
where the measured apparent first-order rate constant, k would be assumed to be directly proportional to both K and ∑ki. In the case of mixtures of hydrocarbons, Kissin (1990) assumed that the ratio of first-order rate constants relative to a marker compound can be expressed as:
ki/kT ) ln(1 - Xi)/ln(1 - XT)
(4)
in which ki and kT are the rate constants for cracking of paraffin i or reference compound T. Under conditions where eq 3 is valid for cracking of a pure hydrocarbon or where eqs 3 and 4 are valid for a hydrocarbon mixture, we could postulate that measured first-order rate constants would depend directly both on the magnitude of adsorption of the feed molecule and on a term which depends on the number of individual cracking modes. On the basis of this assumption, we have attempted to explain observed trends in cracking rate as the chain length of linear paraffins is increased. Models for Cracking Rate Based on Molecular Structure Recently, Sie has addressed the problem of interpreting the relative rates of cracking of linear paraffins as the chain length increases (Sie, 1992), by considering previously published results from the literature. Models presented assumed that the cracking rate should be either proportional to the number of secondary carbon atoms (N - 2) or the number of cleavable bonds (N 1), where N is the number of carbon atoms in the linear paraffin. However, neither function provided a good correlation with experimental data. Sie (1993a,b) also suggested a new mechanism for cracking of linear paraffins based on the assumption of initial formation of a carbenium ion via hydride ion abstraction, followed by rearrangement to a nonclassical protonated cyclopropyl structure. Based on this assumption, it was proposed that an explanation for the sharp increase in reaction rate with increasing carbon number of normal paraffins above C7, as well as the
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virtual absence of C1 and C2 cracking products. Sie proposed that cracking reactivity should be proportional to N - 6, and this concept has also recently been used by Corma et al. (1994) in his interpretation of cracking rates of linear paraffins on USY and ZSM-5 zeolites. As noted by Sie (1992, 1993a,b), the yields of C1 and C2 products during cracking of linear paraffins are indeed much lower than for larger fragments (Abbot and Wojciechowski, 1987b). This has also been shown (Abbot and Wojciechowski, 1985) for cracking of a Fischer-Tropsch synthesis product containing over 92% linear paraffins, on both HY and ZSM-5. It has also been shown (Abbot and Wojciechowski, 1988a,b) that, although methane is observed as a product from reaction of n-hexane on HY at 500 °C, it is formed catalytically as a product of secondary reactions, with thermal cracking accounting for initially observed methane. Recent theoretical studies (Collins and O’Malley, 1995) have also concluded that cracking of central C-C bonds in a linear paraffin is energetically favored over cracking of outer C-C bonds. We can proceed with the assumption that
∑ki ∝ number of modes of cracking ∝ N - 5
Figure 1. Normalized rate constants for cracking of linear paraffins on acid catalysts as a function of chain length, with the function N - 6 (all data normalized to C12).
(5)
The concept of crackable bonds in a linear paraffin (Abbot and Wojciechowski, 1989; Groten and Wojciechowski, 1993) can now be combined with the requirement of providing a dependence on adsorption characteristics to give an overall function describing cracking rate, as chain length is increased, so that combining eqs 1, 3, and 5, we have
∑ki ∝ N(N - 5)
k∝K
(6)
We are therefore proposing that the measured firstorder rate constant is proportional to both the number of carbon atoms in the chain (N) and the number of crackable bonds in the chain (N - 5), i.e., proportional to N(N - 5).
Figure 2. Normalized rate constants for cracking of linear paraffins on HZSM-5 (all data normalized to C7).
Cracking of Individual Paraffins Liguras and Allen (1989) have calculated rate constants for cracking reactions of n-paraffins at 500 °C, based on the published results of Greensfelder and coworkers (Greensfelder and Voge, 1945a-d; Greensfelder et al., 1949) for cracking of pure compounds up to C25 on a silica-alumina-zirconia catalyst. The reported rate constants are shown plotted as a function of carbon number in Figure 1, with values shown normalized at C12. Clearly, linear paraffins at C6 and below crack at very slow rates, while increasingly higher rates are observed as chain length increases. Corma et al. (1994) have recently investigated the effect of chain length for cracking of linear paraffins between C7 and C14 on USY and ZSM-5 at 500 °C, while Abbot and Wojciechowski (1989) have reported rate constants for cracking of n-octane, n-dodecane, and n-hexadecane on HY zeolite at 400 °C. In each of these studies, a decay function was assumed to account for loss of catalyst activity (Wojciechowski and Corma, 1986) as catalyst decay rates and coke formation can vary significantly as chain length varies (Abbot and Wojciechowski, 1989; Corma et al., 1994). The reported rate constants should therefore reflect intrinsic cracking reactivity of individual n-paraffins. It is evident from Figure 1 that the influence on cracking rate, as chain length increases,
Figure 3. Normalized rate constants for cracking of linear paraffins: (i) a mixture of eight n-paraffins and (ii) a Gippsland reduced crude. Also shown is the function N(N - 5) (all data normalized at C25).
is very similar in each case, which includes reaction on an amorphous catalyst as well as large- and mediumpore zeolites. Figure 1 shows a theoretical line generated by assuming Sie’s (N - 6) relationship, normalized at C12. It is apparent that, while this line can be regarded as an adequate representation of cracking
Ind. Eng. Chem. Res., Vol. 36, No. 1, 1997 79 Table 1. Composition of a Linear Paraffin Mixture component
formula
mixture composition (mol %)
dodecane tetradecane pentadecane eicosane docosane tricosane tetracosane pentacosane hexacosane
n-C12H26 n-C14H30 n-C15H32 n-C20H42 n-C22H46 n-C23H48 n-C24H50 n-C25H52 n-C26H54
30.27 26.46 26.25 11.22 0.74 1.59 1.48 0.68 1.31
Figure 4. Effect of catalyst-to-oil ratio on conversion of FCCU-1 and FCCU-2.
rates for paraffins in the range C6-C12, it significantly underestimates the cracking rates for the higher homologues. Figure 2 shows published results for rates of cracking of linear paraffins on HZSM-5 normalized at C7 (Corma et al., 1994; Buchanan et al., 1996), and the general behavior, as chain length is increased, is very similar to that observed in Figure 1. This figure also shows the behavior of the function N(N - 5), indicating that there is a reasonable degree of correlation between this function and experimental data. Figure 3 shows a similar comparison between the N(N - 5) function and data from cracking of individual n-paraffins on largepore or amorphous catalysts as in Figure 1. Again, there is a good correlation between the two sets of points. Cracking of Highly Paraffinic Mixtures Two feedstocks, predominantly containing mixtures of linear paraffins, were studied under cracking conditions at 500 °C on HY zeolite. The first mixture contained nine n-paraffins from n-dodecane (C12H26) through to n-hexacosane (C26H54), with the composition of the mixture shown in Table 1. The second paraffinic mixture was a highly paraffinic Gippsland reduced crude, containing linear paraffins, ranging from C14 to >C32, with a maximum concentration at C25. A full description of the properties of the reduced crude and experimental procedures used has been reported previously (Guerzoni and Abbot, 1994). Using the approach of Kissin (1990), we have previously shown that the calculated ratios for pairs of n-paraffins are independent of conversion in the range 30-75% (Guerzoni and Abbot, 1993). Figure 3 shows results plotted for cracking rates for linear paraffins in the two paraffinic mixtures, with C25 taken as the reference compound. The general correspondence be-
Figure 5. Distributions by weight percent according to carbon number for feedstocks FCCU-1 and FCCU-2 for (a) total hydrocarbons, (b) linear paraffins, and (c) nonlinear components.
tween the data points for cracking of paraffins in the mixtures and those for individual paraffins indicates that relative cracking rates are not strongly influenced by the presence of other paraffins in the mixture. Cracking of Industrial FCC Feeds Two FCCU feedstocks used in test runs at the Ampol refinery, designated as FCCU-1 and FCCU-2, were studied. FCCU-1 was derived from a blend of crudes originating predominantly from Kutubu, Lalang, and Bekapai, while FCCU-2 originated mainly from Gippsland, Kutubu, and Belida. Both feedstocks were paraffinic in character, as indicated by respective UOP-K factors of 12.05 and 12.26. Cracking of FCCU feedstocks was carried out using a microreactor with
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Figure 6. Profiles of feed and product according to carbon number for cracking of (a) FCCU-1 and (b) FCCU-2.
Figure 7. Distributions of linear paraffins above C13 for cracking products from FCCU-1 and FCCU-2.
equilibrium catalyst, initially at 537 °C. The variation in the total conversion with catalyst-to-oil ratio is shown in Figure 4, reflecting the higher UOP-K value of FCCU2. At a given catalyst-to-oil ratio, the conversion of FCCU-2 was consistently 4-5% higher than for cracking of FCCU-1. Ampol refinery test runs also showed that feedstock FCCU-2 was more easily cracked than FCCU-1, with a conversion 4-5% higher under typical heat-balanced FCCU operating conditions. At similar catalyst-to-oil ratios, respective refinery conversions of FCCU-1 and FCCU-2 were 72% and 77%. The total amount of linear paraffins in each feed, as determined by GC, was very similar, 26.9% and 25.7%
for FCCU-1 and FCCU-2, respectively. Figure 5a shows that the distributions of linear paraffins by carbon number in the two samples were also very similar. Parts b and c of Figure 5 show distributions by carbon number for nonlinear components and total hydrocarbons in each feed. In the present study, “nonlinear” components were defined as all hydrocarbons present other than linear paraffins. Parts a and b of Figure 6 show distributions of total hydrocarbons before and after cracking FCCU-1 and FCCU-2, respectively. The origin of differences in the gasoline range products, particularly noticable at C5 and C6, must lie in the identity of the nonlinear components present in the two feedstocks. Figure 7 shows the distribution of linear paraffins above C13 in the cracked products from FCCU-1 and FCCU-2. Parts a and b of Figure 8 show the conversion of linear paraffins and nonlinear components as a function of carbon number. The slower relative rate of cracking of the linear paraffins in FCCU-1 can be attributed to other components present in the feedstocks, particularly the presence of unsaturated species such as aromatics. The influence of 1-methylnaphthalene on cracking of n-hexadecane has been investigated on HY zeolite at 400 °C (Guerzoni and Abbot, 1993). The rate of cracking of n-hexadecane was significantly reduced in the binary mixture, and this could be attributed to inhibition due to preferential adsorption of the unsaturated species at Bronsted sites, or associated with increased catalyst decay, by significantly higher coke yields. Coke yield from microreactor cracking of FCCU-1 was found to be consistently higher than that from FCCU-2 (5 vs 4%), and this trend in coke formation from the two feedstocks was also apparent in the FCCU refinery test runs. Parts a and b of Figure 8 show that the rates of linear paraffin cracking relative to other feed components can differ significantly. However, as shown in Figure 9, the relative rates of cracking of the linear paraffins in FCC1-1 and FCCU-2 (normalized at C25) do not appear to be significantly influenced by the presence of other components. Indeed, the dependence of cracking rate of chain length of the n-paraffins is similar to that in the linear paraffin mixture, which is also illustrated in Figure 9. These observations are of some significance in developing a philosophy to adjust catalyst formulations to maintain conversion levels, when refineries are faced with heavier, more aromatic feedstocks. Enhancement of the activity of the amorphous matrix component of the catalyst, with control of the distribution of mesoand macropores, is one approach (Alerasool et al., 1995; Scherzer, 1993), targeting the large residual molecules. However, targeting the residual linear paraffins by addition of the medium-pore zeolite (Dwyer and Degnan, 1993) may be a viable alternative strategy. Effects of Branching in Paraffins Although branched isomers are also often a significant component of paraffinic feedstocks, there have been comparatively few detailed studies on their cracking behavior (Abbot and Wojciechowski, 1988b). Kissin found that the reactivity of dimethyl-substituted alkanes in the range C7-C13 was generally higher than that of the monosubstituted isomer, which was, in turn, more reactive than the corresponding linear paraffin using an industrial catalyst containing a rare-earth exchanged Y zeolite. Abbot and Wojciechowski (1985) have also reported that, for reaction of a Fischer-
Ind. Eng. Chem. Res., Vol. 36, No. 1, 1997 81
Figure 8. Conversion of linear paraffins and nonlinear components for cracking of (a) FCCU-1 and (b) FCCU-2.
cracking of individual linear paraffins, simple mixtures of linear paraffins, and complex mixtures such as FCC feeds containing diverse types of molecular species. The rate of cracking of a linear paraffin may be retarded by the presence of other feedstock components, through catalyst aging or inhibition. However, the relative cracking rates for a series of n-paraffins is not strongly influenced by the presence of other components in the feed. Literature Cited
Figure 9. Normalized rate constants for cracking of linear paraffins in feeds: FCCU-1, FCCU-2, and linear paraffin mixture (all data normalized to C25). Table 2. Concentrations of n-C19 and Pristane in FCCU-2 FCCU-2 product
n-C19 paraffin (wt %)
pristane X (wt %)
1.148 0.852
0.502 0.232
Tropsch product, isoparaffins in the range C14-C20 react more readily than the linear isomers on HY zeolite at 400 °C, although the opposite effect is observed on ZSM5. In general, it is very difficult to observe the removal of individual branched paraffins in an FCCU feedstock, as their concentrations are very small. However, it was found possible to identify the GC peak corresponding to the isoprenoid compound pristane (2,6,10,14-tetramethylpentadecane), alongside the peak for the C17 linear paraffin (Elliott and Melchior, 1982). Table 2 shows the concentrations of pristane and the n-C19 paraffin isomer in the FCCU-2 feed and after cracking at 71% conversion. Using eq 4, the branched isomer is 1.7 times more reactive than the linear isomer. Conclusions The cracking rate for linear paraffins increases continuously with hydrocarbon chain length, and this cannot be accounted for by simple correlations with the number of crackable bonds in the linear paraffin. However, assuming a proportionality between cracking rate and both the number of crackable bonds and adsorption constant which depends on molecular size yields a function N(N - 5). Good correlations between this function and observed cracking rates are found for
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Received for review May 7, 1996 Revised manuscript received October 2, 1996 Accepted October 4, 1996X IE960255E
X Abstract published in Advance ACS Abstracts, November 15, 1996.
