Article pubs.acs.org/IECR
Continuous Age Distribution Method for Catalytic Cracking. 1. Proof of Principle David M. Stockwell* BASF Corporation, 25 Middlesex-Essex Turnpike, Iselin, New Jersey 08830, United States S Supporting Information *
ABSTRACT: A novel steam-deactivation method is described and demonstrated that is able to produce in the laboratory a mixture of cracking catalyst having the same distribution of zeolite surface area as found in the refinery catalytic cracking unit. Both physical and catalytic properties can be closely matched to the refinery catalyst. Based on fundamental kinetic and reactor models, the method has the potential to be quantitatively predictive of refinery results. Deviations from ideal first order tetrahedral Si decay however prevent a priori prediction of refinery performance at this time. We observed slight deviations within the age distribution, but more significant deviations in overall zeolite crystallinity retention were found as a function of refinery fresh catalyst makeup rate, rendering the method semiquantitative at present. The results constitute proof of principle, however, for the method, and further progress on the found nonidealities is anticipated.
1. INTRODUCTION In the fluidized catalytic cracking (FCC) unit, gas oil or residual feeds are combined in the riser with faujasite-based microspheroidal cracking catalyst, whereupon lower molecular weight hydrocarbon products are produced together with the deposition of coke, Ni, V, Na, and S on the catalyst. After cracking, the coke is removed by fluidized bed regeneration in air at 950−1000 K, leading to the formation of steam that causes irreversible dealumination1,2 and crystallinity loss in the zeolite,3,4 the chemistry of which is complicated by the presence of Na, V, and SOx.5−8 Typically the circulating catalyst spends 3 s in riser cracking, 1 min in hydrocarbon stripping, and 8 min in regeneration, so the irreversible catalyst deactivation taking place in the regenerator is most important. The kinetics of these processes are only partially understood, however, and laboratory procedures to reproduce or predict this deactivation are commonly empirical. It is therefore an objective of this paper to provide a sound theoretical basis for FCC catalyst deactivation procedures in the laboratory. With the loss of zeolite crystallinity and accumulation of Ni and V comes the degradation of catalyst activity and selectivity. To overcome this, a few percent of the circulating catalyst inventory is replaced daily, leading to a continuous distribution of catalyst ages in the equilibrated working catalyst (E-cat).9−13 The variation in physical and chemical properties within equilibrium catalysts has been revealed by density separation,7,14−18 showing that zeolite Y tetrahedral aluminum loss (AlT, dealumination) is much faster than tetrahedral zeolite framework Si (SiT) collapse under regenerator conditions. Since many commercial catalysts also contain hydrothermally stable transition alumina matrix,19−21 it is readily apparent that both the framework Si/Al ratio, as measured22 by unit cell size (UCS), and the ratio of cracking activity of zeolite to alumina matrix will vary with the age of a given microsphere. Both of these have a strong influence on cracking selectivity,21,23 so it is reasonable to assume that the selectivity of the working refinery catalyst may not be accurately portrayed by laboratory © 2015 American Chemical Society
deactivation methods unless those methods correctly reproduce the properties within the age distribution.3,13,24 A single hydrothermal catalyst deactivation may be employed for FCC25,26 that will match the averaged working catalyst properties but not the age distribution.3,21,24 One may add 5% fresh catalyst to steamed for cracking selectivity evaluation, but this is already acknowledged to overestimate the role of the newest catalyst in the inventory.12 Alternatively, fresh catalyst may be steamed at increasing severity,3,13,24 but this increases complexity of experimentation and no systematic guidance has been provided on the laboratory conditions to use for a given FCC unit. Analytic9−11 and numerical3,13 models have been reported, but in most cases these are activity-based instead of property-based and so do not provide any fundamental insight into catalyst deactivation or selectivity changes. Current industrial practice thus lacks a quantitative understanding of E-cat zeolite properties on average and within the age distribution that is based on first principles, as well as quantitative methodologies to predict and reproduce these properties in the laboratory. Once developed, such methods could be used, for example, to fully understand and reproduce in the laboratory seemingly mundane but economically significant changes in catalyst replacement rate or the rare earth stabilization of zeolite. It is the goal of this paper to describe and demonstrate proof of principle for a novel continuous age distribution method (CADM) of FCC catalyst deactivation which reproduces in a single steaming the zeolite micropore surface area (ZSA) on average and within the age distribution of a given E-cat. In support of that end, we will first derive mathematical models to predict and explain the refinery ZSA and confirm them with density separation data from the literature. The development Received: Revised: Accepted: Published: 5921
February 18, 2015 May 5, 2015 May 14, 2015 May 14, 2015 DOI: 10.1021/acs.iecr.5b00666 Ind. Eng. Chem. Res. 2015, 54, 5921−5934
Article
Industrial & Engineering Chemistry Research
separation data shown in Figure 1 was made possible by making the assumptions (i) mesoporous matrix surface area (MSA) of
will be restricted to low vanadium ( B > C) but only when using age distribution. The results of Figure 2 indicate that the age distribution method (ADM) has in this particular instance enhanced activity and selectivity performance differentiation in a plausible way. 3.2. CADM Calibration. As noted earlier, the Arrhenius method27 is the most straightforward way to obtain the kinetic parameters needed for the CADM temperature ramp of eq 9. Toward this end, steamings were conducted on three unrelated commercial REUSY FCC catalysts, with cat. G being steamed in 40% steam and the measured rate constant being divided by 0.4 before plotting in Figure 3. None of the Arrhenius data were linear, indicating substantial deviations from first order decay (eq 3). The Ea estimated from high temperature range was also at least 35% higher than reported by Pine5 for zeolite Y powder. The massive scale (∼7 × 108 kg/year worldwide, >450 FCC units) of consumption of these and related catalysts and the results of Figure 2 however require that more practical and informative deactivation methods should be developed. For the purposes of the present paper, reconcilliation of the deviations in Figure 3 was deferred and the method of section 2.3 and eq 11 were employed instead. Twenty-one-hour CADM experiments were run on cat. E using five different Ea hypotheses, these ranging between 126 and 628 kJ/m. The ZSA measured after a conventional steaming was sufficient to determine a reasonable corresponding value of ko to use in each of the five cases. The CADM tests were then run using 100% steam, and catalyst aliquots were taken from the steamer at intermediate times. The experimental ZSA results are shown in Figure 4 (solid symbols), where they are plotted against the fraction of the total catalyst added to the steamer, ωL. Also shown are the ZSA estimates from eq 11 (open symbols), where the predicted SiTSA was increased to account for AlTSA using the experimentally observed UCS and the Jorik
Figure 3. Arrhenius plot for SiT decay for cat. E (△), cat. F (●), and cat. G (□).
Figure 4. Calibration by Ea hypothesis testing using eq 11. L = 21 h and Ea = 126 (●), 251 (▲), 377 (■), or 502 (◆) kJ/m; target = 108 m2/g (- - -). Solid symbols indicate measured results, and open symbols indicate results from model. The 7-day test is also shown (∗).
correlation. After least-squares optimization of ln(ko,T) and Ea,T, a reasonable fit was obtained. This suggests that eq 11 provides an adequate representation of ZSA during CADM and that the CADM calibration can in fact be adjusted via eq 11 to give ZSA that is constant with time. The kinetic parameter values after optimization were Ea,T = 490 kJ/m and ko,T = 2.45 × 1022 h−1. This calibration was then evaluated in a series of runs with addition times as long as 7 days, and these results, also shown in Figure 4, confirmed that the ZSA of the blend in the steamer remained nearly constant for 7 days and essentially equal to the E-cat target value of 108 m2/g. When the kinetic parameters were optimized yet again in consideration of the 7-day tests, the results were revised slightly to 473 kJ/m and 3.68 × 1021 h−1. A second set of stability tests 5925
DOI: 10.1021/acs.iecr.5b00666 Ind. Eng. Chem. Res. 2015, 54, 5921−5934
Article
Industrial & Engineering Chemistry Research Table 1. Comparison of CADMa to Conventional Steaming and E-catb
a
method
conditions
ending T [K]
BET [m2/g]
ZSA [m2/g]
ZSA/MSA
UCS [Å]
CADM CADM CADM CADM CADM steamed E-cat
L=5h L = 12 h L = 21 h L = 72 h L = 168 h 1089 K, 6 h refinery
1103 1084 1073 1048 1032 1089 ∼995
147 153 155 149 150 152 164
94 100 100 103 102 98 108
1.8 1.9 1.8 2.1 2.1 1.8 1.9
24.30 24.30 24.28 24.29 24.28 24.25 24.27
CADM runs at 473 kJ/m. bE-cat contains 111 ppm of Ni and 467 ppm of V.
extending out to 7 days was then run (Table 1), and these gave ZSA that initially increased slightly with time, arriving incrementally low to target. Table 1 also shows that the steamed ZSA/MSA and UCS are in good agreement with the E-cat target. These two extended time tests demonstrate that the CADM calibration can be tuned to give accurate and remarkably consistent ZSA, ZSA/MSA, and UCS. Conventional steaming can of course also be empirically adjusted to give comparable results, but the latter lacks age distribution and any formal linkage to the refinery. 3.3. Proof of Principle for CADM. So far we have established that the ZSA of the contents of the CADM steamer can be made to stay constant over time and continually match the target E-cat ZSA. Proof of principle for the CADM technique will have been achieved if we can show that the distribution of CADM SiTSA follows eq 6 (Figure 1) and that the finished ZSA responds to changes in τR in the same way that the refinery is presumed to respond (eq 5). In pursuit of the former, CADM runs were made on the catalyst of Figure 4 using the 490 kJ/m calibration, where the catalyst feeder was run to only produce 10 wt % segments of the age distribution. The SiTSA results plotted in Figure 5 give satisfactory agreement with eq 6, but some systematic deviation appears to be present, as might be expected. Also shown in Figure 5 are the UCS results, and these generally agree with the density separation data of Figure 1. However, it does appear that the higher values of UCS extend more deeply into the age distribution in 21 h CADM than is found in E-cat (Figure 1) and that the UCS values are less flat in the middle of the CADM distribution than found in E-cat (Figure 1). Since inclusion of excessive amounts of high UCS material is known to exaggerate the role of fresh catalyst UCS,12 we conducted a second series of tests to examine the first 10 wt % of the age distribution more closely, this time with L = 50 days so that the last of the catalyst would be added at regenerator temperature (978 K). Since no catalyst would have been added during the first 45 days of the temperature ramp, the computer was caused to jump ahead to tL = 45 days, so that preparation of the 8−10 wt % segment required only 5 days instead of the full 50 days. The results in the lower portion of Figure 5 show that, at least when 1 atm of steam is applied, fewer than 2% of the age distribution has UCS significantly different than the E-cat and suggest that the UCS of the middle portion of the age distribution will perhaps be flatter and more like E-cat for longer, lower temperature deactivations where the ending temperature TL(L) is closer to actual regenerator temperatures. We next used CADM to explore the effect of varying the refinery catalyst inventory turnover time τR on catalyst properties. A single lot of NaY FCC catalyst was exchanged to contain 0, 1, 3, or 5 wt % of mixed rare earth oxides (REO,
Figure 5. SiTSA (◇) and UCS (○) within the age distribution determined experimentally (points) or by eq 6 (solid curve). Ea = 490 kJ/m, τR = 36 days, τSi = 49 days, Tmax = 1200 K, and L = 21 h (open symbols) or 50 days (bottom, solid symbols). Horizontal error bars indicate the percentile range of the age distribution segment.
primarily La) and then CADM-steamed using the parameters in Figure 5, except for changing to L = 46 h and varying τR 5926
DOI: 10.1021/acs.iecr.5b00666 Ind. Eng. Chem. Res. 2015, 54, 5921−5934
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Figure 6. Response of % AlT, UCS, AlTSA, and SiTSA to changes in τR at 0% (◆), 1% (■), 3% (▲), and 5 wt % (●) REO.
3.4. Emulation of Refinery Rare Earth and Catalyst Replacement Rate Changes. Its limitations notwithstanding, we next sought to demonstrate the utility of CADM by evaluating the activity and selectivity effects of rare earth (RE) exchange and fresh catalyst makeup rate variations as they would occur in the refinery for an FCC unit limited by its coke burning capacity, a very common scenario. For FCC units like these, net product value is best represented by the yield slate obtained when coke has reached the limiting value. Catalysts having low coke yield at a given conversion (low coke selectivity) thus generate higher product value by driving the process to higher conversion to reach the required rate of coke generation. The cracking results obtained from the catalysts of Figure 6 are therefore reported as those obtained by regression to a constant coke yield of 3.89 wt %, the coke yield where the 1% REO catalyst had reached 75% conversion and was near the point of maximum gasoline yield. The FFB results shown in Figure 7 have RE increasing activity and hydrogen transfer, resulting in higher gasoline as well as lower LPG (C3 + C4 hydrocarbons) and LPG olefins, consistent with expectations.23,36 While the catalyst at 5% REO may have the highest gasoline selectivity, it does not give the
between 12 and 60 days. Figure 6 shows the UCS and the fraction of AlT in the framework22 as well as the AlTSA and SiTSA we then calculated from the measured ZSA. As should be expected from Figure 1 and Figure 5, the measured UCS of the full CADM mixture is insensitive to the small fraction of higher UCS at the front end of the age distribution, so UCS hardly responds to τR. The values we obtained are in good agreement with commercial experience however. Although the total ZSA gave the directionally correct response of decreasing with decreasing catalyst replacement rate, the SiTSA, response was weaker than expected for ideal first order decay (eq 5, dashed curves for SiT in Figure 6). So while the results of Table 1 together with Figures 5 and 6 constitute proof of principle for CADM, the deviations from ideal first order decay in Figures 3, 5, and 6 render the technique only semiquantitative at this time. Consequently, laboratory kinetic data obtained by the Arrhenius method do not so far successfully predict the rate constants obtained from the refinery via eq 10. In effect the first-principles approach so far fails in practice, so the τSi needed for the CADM temperature ramp (eq 9) must be determined from previous refinery experience and eq 10 instead of calculated a priori. 5927
DOI: 10.1021/acs.iecr.5b00666 Ind. Eng. Chem. Res. 2015, 54, 5921−5934
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Figure 7. Effect of τR and RE on conversion at 5 cat/oil (top left) and then conversion, LPG, gasoline, propylene, and catalyst/oil, each being evaluated at 3.89 wt % coke: 0% (◆), 1% (■), 3% (▲), and 5% (●) REO.
The final plot of Figure 7 shows the catalyst/oil weight ratio (C/O) evaluated at constant 3.89 wt % coke, which indicates that the combined effects of lower coke selectivity and lower specific activity would, through the refinery enthalpy balance,39 result in the highest FCC unit catalyst circulation rate when using about 1 wt % REO. The FFB results are of course not quantitatively predictive of the FCC unit. Taking the data at face value, however, this is of great practical significance, since not all FCC units would have the capacity to circulate such large amounts of catalyst. If catalyst circulation became limiting,
highest gasoline yield at constant 3.89 wt % coke, owing to its higher coke selectivity (lower conversion at 3.89 wt % coke). Thus, the loss of conversion at constant coke overwhelms the gasoline selectivity effect. The highest gasoline yield at constant coke is instead obtained at 3% REO, which results from a compromise between higher gasoline selectivity at high REO and lower coke selectivity at lower REO. Conversion at constant coke is maximized (coke selectivity optimized) near 1% REO and 24.28 Å (Figure 6), also consistent with the literature.36−38 5928
DOI: 10.1021/acs.iecr.5b00666 Ind. Eng. Chem. Res. 2015, 54, 5921−5934
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Industrial & Engineering Chemistry Research this final plot also suggests how much the catalyst replacement rate 1/τR would have to be increased in order to bring C/O back within circulation limits. For example, a typical circulationlimited FCC unit running a 3 wt % REO catalyst at a 60-day turnover time would require at least triple the fresh catalyst makeup rate when using a 1 wt % REO catalyst. These curves already account for any changes in coke selectivity associated with the replacement rate change. Ironically then, the results are projecting that the net rate of RE consumption would not be lowered using a catalyst containing the lower RE content. If the replacement rate is instead not changed, then the conversion would begin to decline, unless of course the gas oil throughput rate was deliberately reduced. At a typical 50 000 barrels per day, such considerations can result in enormous economic impact. CADM provides a potentially rigorous method to make such determinations under the well-controlled conditions of the laboratory, enabling refiners to make more reliable process projections for potential operating changes, ultimately leading to improved decision-making and profitability. 3.5. Effect of Accelerating Deactivation on Selectivity. As far as the CADM theory is concerned, the five CADM runs of Table 1 should give identical samples. Table 1 shows only slight variations in physical properties. The temperature trajectory over time in each case was the same; however, the longer CADM runs were terminated at progressively lower ending temperatures. During the search for a suitable segmented age distribution protocol for the experiment of Figure 2, much shorter conventional steamings at a higher temperature (1144 K) were tested for their ability to represent the oldest 50% the age distribution. However, these and other more accelerated deactivations were found (Supporting Information) to give unusually high specific activity and coke selectivity in FFB testing, as well as lower olefinicity, so the longer 30 h steaming at 1089 K was adopted for the experiment of Figure 2. To see if similar catalytic effects would be operative during CADM, the samples of Table 1 were also evaluated by FFB testing. The results shown in Figure 8 indicate higher catalyst activity, higher coke with less gasoline, and lower C3 olefinicity upon accelerating CADM from L = 7 days to L = 5 h. These findings thus agree with the earlier segmented age distribution method results, and because these changes occur whether (Figure 8) or not (Figure S2) age distribution is present, we can safely conclude that the effects are associated with accelerating deactivation, not with the presence or absence of age distribution. In catalyst evaluations made to support the operation of specific FCC units, considerable effort is often expended trying to match the properties and performance of a lab-deactivated catalyst to the corresponding E-cat. In the present case, Figure 8 also shows the results for the E-cat control from Table 1, which activity and selectivity lie between the CADM results for L = 21 h and 3 days. Although a 2-day CADM protocol like Figures 6 and 7 may have matched this E-cat rather closely, it is important to recall that there is no a priori reason for the performance of the CADM-deactivated catalysts to be changing in the first place. Any optimization of the catalyst addition time L to match E-cat selectivity is therefore an empirical exercise, albeit a potentially useful one. In the present case, the increase in coke caused by accelerating from a typical 80 days in the refinery to just 2 days of CADM deactivation may simply be offsetting a small amount of contaminant coke from Ni and V on the E-cat. The H2 yields from the FFB were about 0.10 wt %
Figure 8. Effect of accelerating CADM deactivation on FFB activity and selectivity, L = 5 h (■), 21 h (◆), 3 days (●), 7 days (△), or conventional at 1089 K for 6 h (×) or E-cat (○). WHSV = 60/(C/O).
after CADM, as compared to 0.25 wt % from the E-cat, so the presence of a small amount of contaminant coke is at least plausible. In the earlier segmented ADM testing (Figure S1), the lower LPG olefinicity observed was easily attributed to the higher UCS we had obtained from a shorter, hotter steaming. However, the higher UCS due to CADM acceleration (Table 1) should have increased gasoline incrementally in Figure 8 instead of decreasing it23,36 (Figure 7), and the UCS variation 5929
DOI: 10.1021/acs.iecr.5b00666 Ind. Eng. Chem. Res. 2015, 54, 5921−5934
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Industrial & Engineering Chemistry Research
Figure 9. Acceleration effect on N2 PSD: fresh (−); 1089 K for 6 h (×); CADM at L = 23 (◇), 89 (△), 161 h (○); E-cat (●).
agrees on average with the E-cat but very likely agrees within the age distribution itself. The vacancy migration hypothesis was investigated further by TEM, now using conventionally steamed products so only one catalyst age would be present. The micrographs in Figure 10 compare to pristine NaY, three samples steamed at 1033, 1089, or 1144 K for times that resulted in equivalent ZSA and UCS. Although the N2 PSDs are certainly more reliable indicators of zeolite texture, the TEM investigation was easily able to provide evidence for variations in vacancy coalescence, as the crystallites steamed for longer times at lower temperatures were more rounded and contained clearly identifiable mesopores with diameters as large as 500 Å, near the limit of analysis for N2 PSD. In summary then, it appears that a difference in activation energies between vacancy migration and SiT collapse leads to narrower zeolite mesopores during more accelerated deactivations, which in turn leads to increases in coke selectivity that can help match CADM-deactivated coke selectivity to refinery E-cats with low contaminant metals levels. Were it not for the hypothetical contribution of the contaminant coke, we would anticipate better agreement between the optimal CADM addition times for selectivity (∼2 days) and zeolite texture (7 days).
seems small in Table 1. While more high UCS material is present in the front end for more accelerated CADM steamings according to Figure 5, the front end UCS at L = 21 h is still consistent with REY E-cat and high gasoline, so good selectivity is still expected and another explanation is needed. These front end UCS effects do explain the trend in CADM C3 olefinicity, however, which in all cases were lower than the conventionally steamed control (Figure 8), as desired. In order to gain some further understanding of why gasoline and coke selectivity may vary with the degree of acceleration of the steam-deactivation process, deactivated samples were characterized by N2 pore size distribution (PSD) and transmission electron microscopy (TEM). Figure 9 provides PSD for catalyst aliquots taken during the stability testing of Table 1, and these show that higher temperature, more accelerated aging results in narrower mesopores than E-cat, perhaps indicating a cause for the activity and selectivity effects since mesopores improve mass transfer into the zeolite.40 The pore volume below 5 nm diameter was reduced while volume above 20 nm increased on E-cat and all steaming methods, as compared to fresh, owing to the well-known40−42 widening of USY mesopores on steam-deactivation. The remaining mesoporosity in the
