Energy & Fuels 1997, 11, 463-469
463
Temperature-Programmed Oxidation of Coke Deposited on Cracking Catalysts: Combustion Mechanism Dependence Cam Le Minh, Rodney A. Jones, Ian E. Craven, and Trevor C. Brown* Department of Chemistry, University of New England, Armidale, NSW 2351, Australia Received July 24, 1996X
Coke deposited on catalytic cracking catalysts has been investigated by continuously monitoring evolved CO and CO2 during temperature-programmed oxidation (TPO) in a 1% O2/N2 mixture. Analyses were carried out on spent samples recovered from an industrial catalytic cracker and on coke prepared in the laboratory by exposure of fresh catalyst to 1-hexene and cyclohexene at 500 °C. A maximum of three peaks in the rate of carbon dioxide evolution and one carbon monoxide peak are apparent in the TPO spectra. The small lowest temperature CO2 peak, which is only observed for the TPO of the spent catalyst, is assigned to either highly reactive coke or coke in the vicinity of trace metals. Two larger overlapping CO2 peaks occur at higher temperatures and are attributed to competing coke oxidation mechanisms. The first of these larger peaks coincides with the CO evolution, while for the final peak only CO2 evolution prevails. This reduction in CO and increase in CO2 are also observed for the TPO of powdered charcoal. The temperature dependence of carbon oxide evolution is attributed to changes in the combustion rate-determining steps due to different pre-exponential factors and activation energies. This indicates that the combustion reaction mechanism can affect the shape of the TPO spectra and so must be included in the interpretation of catalytic coke oxidation data.
Introduction Formation of coke (carbonaceous compounds) on the surface of catalytic cracking catalysts is an inevitable chemical reaction that occurs during the conversion of hydrocarbon feed to products. As the catalyst is deactivated by the coke, a regeneration phase, involving coke combustion, is generally employed. Combustion being an exothermic reaction produces the heat necessary for the industrial catalytic cracking process.1 Coke, therefore, has two controlling influences: it regulates both the activity of the catalyst and the available heat. Amount, type, and location of coke on the catalyst surface are fundamental properties that depend on the process conditions, catalyst type, and feed composition and which can ultimately affect the apparent combustion kinetics. Many studies have been reported on amorphous carbon and coal-char oxidation kinetics, while information on catalytic coke combustion is limited but has been steadily increasing since the early work of Haldeman and Botty2 and Weisz and Goodwin.3,4 Massoth5,6 investigated the isothermal oxidation of coked silicaalumina catalysts and observed a rapid temperature rise with concomitant hydrogen oxidation, during the
initial stages of oxygen exposure. This they attributed to active hydrogen-rich species on the coke surface. Carbon combustion studies have indicated that the high oxidation rate is due to either surface oxide formation or transient reactions with labile carbon atoms.7 Problems with temperature fluctuations during the initial oxygen exposure can be obviated by using temperatureprogrammed oxidation (TPO). By converting the CO2, which forms during the TPO of coke on a re-forming catalyst, to methane then analyzing by flame ionization detection, Fung and Querini8-10 observed up to four peaks in the resultant spectra. Simulations indicate that these peaks can be the result of (i) different rate coefficient parameters due to different oxidation properties of the coke deposits, (ii) the range of coke particle sizes, or (iii) varying reaction order with either coke conversion or coke particle geometry.10 Augustine et al.11 used an on-line mass spectrometer to study evolved CO2 and H2O from coke deposited on a Pt/Al2O3 catalyst that had been doped with Re, S, and Cl. This provided information on coke distribution and location. Similar use of mass spectrometry was made by both Wrammerfors and Andersson12 and Bartholdy et al.13,14 to monitor CO2 and other evolved species during TPO of coke from various metal silica and alumina catalysts. Others have
* Author to whom correspondence should be addressed (telephone +61 67 73 2872; fax +61 67 73 3268; e-mail
[email protected]). X Abstract published in Advance ACS Abstracts, January 15, 1997. (1) Biswas, J.; Maxwell, I. E. Appl. Catal. 1990, 63, 197-258. (2) Halderman, R. G.; Botty, M. C. J. Phys. Chem. 1959, 63, 489496. (3) Weisz, P. B.; Goodwin, R. B. J. Catal. 1963, 2, 397-404. (4) Weisz, P. B.; Goodwin, R. B. J. Catal. 1966, 6, 227-236. (5) Massoth, F. E. Ind. Eng. Chem. Process Des. Dev. 1967, 6, 200207. (6) Massoth, F. E.; Menon, P. G. Ind. Eng. Chem. Process Des. Dev. 1969, 8, 383-385.
(7) Tucker, B. G.; Mulcahy, M. F. R. Trans. Faraday Soc. 1969, 65, 274-286. (8) Fung, S. C.; Querini, C. A. J. Catal. 1992, 138, 240-254. (9) Querini, C. A.; Fung, S. C. J. Catal. 1993, 141, 389-406. (10) Querini, C. A.; Fung, S. C. Appl. Catal. A 1994, 117, 53-74. (11) Augustine, S. M.; Alameddin, G. N.; Sachtler, W. M. H. J. Catal. 1989, 115, 217-232. (12) Wrammerfors, Å.; Andersson, B. J. Catal. 1994, 147, 82-87. (13) Bartholdy, J.; Zeuthen, P.; Massoth, F. E. Appl. Catal. A 1995, 129, 33-42. (14) Zeuthen, P.; Bartholdy, J.; Massoth, F. E. Appl. Catal. A 1995, 129, 43-55.
S0887-0624(96)00114-4 CCC: $14.00
© 1997 American Chemical Society
464
Energy & Fuels, Vol. 11, No. 2, 1997
Le Minh et al.
measured coke weight loss during TPO to determine oxidation kinetic parameters, information on coke location, and carbon type on various catalysts.15-19 Moljord et al.20,21 have measured CO2, CO, and H2O formation by trapping and weighing the oxidized gases over a range of temperatures. This technique indicated that the nature or location of coke molecules within HY zeolites had little influence on the oxidation kinetics, while the density of framework aluminum species had a significant effect. Radical cation intermediates are proposed to explain the primary steps of coke oxidation. This paper reports on an investigation into CO and CO2 evolution during TPO of coke deposited on cracking catalysts. These catalysts typically consist of an ultrastable Y zeolite in an active alumina, silica-alumina, clay, and rare earth oxide matrix.1,22,23 The matrix serves as a support for the zeolite, increases the hydrothermal stability of the catalyst, and acts as a catalytic surface for breaking down the larger hydrocarbon components of the reduced crude feed which are unable to diffuse into the zeolite micropores. The majority of the active surface area of the cracking catalyst, however, lies within the zeolite micropores, and it is within these pores that most of the octane enhancement occurs. Experimental Section The coking and TPO experimental system is drawn schematically in Figure 1. It consists of a flow reactor surrounded by a thermostatically controlled furnace. At the center of the furnace is a boat in which is placed 500 mg (nominal) of the fresh catalyst for coking or 100 mg (nominal) of the coked catalyst for oxidation studies. The small quantities are to ensure the endothermicity of the cracking reaction and the exothermicity of the combustion process do not cause large temperature fluctuations in the catalyst bed. Temperature is measured by a thermocouple placed immediately below the boat, and pressure by a pressure gauge at the outlet to the reactor. Fresh catalyst is initially calcined for 2 h at 800 °C, which is the typical pretreatment temperature prior to employment in industrial catalytic crackers.1 For in situ preparation of coked substrates a known volume of single-component hydrocarbon feed [1-hexene (Aldrich, 97%) or cyclohexene (Aldrich, 99%)] was injected, using a KD Scientific syringe pump, over fresh catalyst at a controlled injection rate (1 mL/ min) at 500 °C. As the liquid feed passes through the 1 mm i.d. stainless steel tube leading to the catalyst, it vaporizes and reaches thermal equilibrium before flowing over the substrate. Immediately following feed exposure, residual reactants and products in the reactor are flushed out with nitrogen at 200 sccm. These liquids and gases were collected and stored for future analysis. For the current investigations 1 mL of the feed was used for coking as this quantity produced a similar coke coverage on the fresh catalyst as was found for the spent (0.15 ( 0.01 m2 of coke/m2 of catalyst), that is, significantly less than a monolayer coverage. In the determi(15) Pieck, C. L.; Verderone, R. J.; Jablonski, E. L.; Parera, J. M. Appl. Catal. 1989, 55, 1-10. (16) Galuszka, J.; Sano, T.; Sawicki, J. A. J. Catal. 1992, 136, 96109. (17) Gayubo, A. G.; Arandes, J. M.; Aguayo, A. T.; Olazar, M.; Bilbao, J. Chem. Eng. J. 1994, 54, 35-40. (18) Fernandes, V. J.; Araujo, A. S. Thermochim. Acta 1995, 255, 273-280. (19) Bond, G. C.; Dias, C. R.; Portela, M. F. J. Catal. 1995, 156, 295-297. (20) Moljord, K.; Magnoux, P.; Guisnet, M. Catal. Lett. 1994, 25, 141-147. (21) Moljord, K.; Magnoux, P.; Guisnet, M. Appl. Catal. A 1995, 121, 245-259. (22) Avidan, A. A. Oil Gas J. 1992, 59-67. (23) Chen, N. Y.; Degnan, T. F. Chem. Eng. Proc. 1988, 32-41.
Figure 1. Schematic diagram of coking and controlled combustion experimental system. nation of coke coverage the cross-sectional area of a coke unit is estimated to be one-sixth the area of a benzene molecule (3.75 Å2), calculated using the Lennard-Jones diameter of benzene.24 The 500 mg of coked catalyst is removed from the furnace and divided into 100 mg batches for oxidation studies and surface area measurements. Simultaneously with starting a linear temperature program from 200 to 1000 °C, at 5 or 10 °C/min, a controlled flow of 0.939 ( 0.019% oxygen in nitrogen is passed over the deactivated catalyst to burn off the layer of coke. A low O2 partial pressure was chosen to minimize both heat release during oxidation and secondary reactions such as CO oxidation. The maximum oxygen conversion in any of the reported TPO experiments is 10%. Evolved carbon monoxide and carbon dioxide are converted to methane over a ruthenium catalyst (maintained at 300 °C) and continuously monitored with two flame ionization detectors (FID). One FID measures the CO plus CO2 emanating from the reactor, while the other measures only the CO by absorbing CO2 onto ascarite (Aldrich) before conversion to CH4. Standardized cylinders of 119 ( 2 ppm CO in N2 and 519 ( 10 ppm CO2 in N2 were used to calibrate the evolved combustion gases before and after each analysis. Also, evolved CO and CO2 from different weights of calcium oxalate observed during linear temperature programs were measured to confirm the calibration, to check the linearity of the detection system, and to ensure the response times of the two FID signals are equivalent. The units for the rate of CO and CO2 evolution [µmol (g of char)-1 °C-1] are calculated from the calibrated FID signals, pFID, the standardized mass flow rate m ˘ in sccm, and the weight of catalyst wcat.
CO and CO2 rate [µmol (g of catalyst)-1 °C-1] ) 2 × pFID × m ˘ × 60 R × 273 K × wcat × β Here R is the ideal gas constant, β is the heating rate in °C/ min, and the rate is multiplied by two as the flow is split between the two FIDs. Data acquisition consists of a Straw(24) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids, 4th ed.; McGraw-Hill: New York, 1987.
Temperature-Programmed Oxidation of Coke
Energy & Fuels, Vol. 11, No. 2, 1997 465
Table 1. N2 BET Surface Areas, Weight Percent Carbon, and Coverage for Each of the Analyzed Catalystsa substrate fresh spent 1-hexene coked cyclohexene coked
heating rate (°C/min)
surface area (m2/g of catalyst)
CO carbon (g%/g of catalyst)
CO2 carbon (g%/g of catalyst)
total carbon (g%/g of catalyst)
coke coverage
5 10 5 10 5 10
211 ( 5 105 ( 5 105 ( 5 213 ( 5 213 ( 5 125 ( 5 125 ( 5
0.43 ( 0.02 0.36 ( 0.02 0.72 ( 0.04 0.58 ( 0.04 1.67 ( 0.15 1.24 ( 0.13
0.39 ( 0.02 0.46 ( 0.02 0.79 ( 0.04 0.95 ( 0.04 1.69 ( 0.15 1.57 ( 0.13
0.82 ( 0.04 0.82 ( 0.04 1.51 ( 0.08 1.53 ( 0.08 3.4 ( 0.3 2.8 ( 0.3
0.15 ( 0.01 0.15 ( 0.01 0.13 ( 0.01 0.13 ( 0.01 0.30 ( 0.04 0.25 ( 0.03
a The fresh and spent catalysts were supplied by Ampol Refinery, Lytton, Queensland, Australia, while the 1-hexene and cyclohexene coked fresh catalysts were prepared in the laboratory. Coke coverage was estimated assuming a cross-sectional area for a coke unit of 3.75 Å2.
berry Tree’s analog input and digital input/output board, associated software, and personal computer which continuously records data from the two FIDs and the furnace thermocouple. BET surface areas were measured using equipment constructed and calibrated in our laboratory. Fresh and spent cracking catalysts were provided by the Ampol Research and Development Laboratories, Lytton, Queensland, Australia, with the spent catalyst taken from the output of their catalytic cracker. The Si/Al ratio of the fresh catalyst is 2.8, and the spent catalyst is 2.3 as determined by X-ray fluorescence (XRF). Other elements detected in both substrates by XRF are Ti (1.0 wt %), La (1.0 wt %), and Nd (0.3%). In addition, significantly higher concentrations of Fe (5600: 2000 ppm), Ni (1544: 14 ppm), Zn (265: 13 ppm), and V (543: 113 ppm) were detected in the spent catalyst when compared with the fresh catalyst. The average particle size for both fresh and spent catalysts is 75 µm, which is a diameter at which gas-phase diffusion processes should not be controlling the observed kinetics.3 Nitrogen is passed over an Oxy-Trap cartridge (Alltech Associates, Inc., Deerfield, IL) to reduce the oxygen content to ca. 1 ppm, while water vapor was removed from all gases by passing through Drierite and 5Å molecular seive (Alltech). All gases were supplied and analyzed by BOC Gases and gas flows stabilized by Bronkhorst mass-flow controllers. Charcoal (BDH) was used without further treatment.
Results The percentage weight of carbon per gram of catalyst, N2 BET surface areas, and coke coverage for each of the cracking catalyst substrates are listed in Table 1. Weight percent carbon is calculated by integrating under the CO and CO2 curves of the TPO spectrum, with errors determined from the maximum deviations found for repeat coking and oxidations under identical conditions. These values agree to within 10% of those measured on samples from the same coked batch using a Carlo Erba CHNS-O elemental analyser. The difference in surface area between the fresh and spent catalysts is large, i.e., 106 m2/g. Cyclohexene coking also resulted in a significant decrease in surface area, i.e., -86 m2/g. In contrast, the surface areas of the 1-hexene coked and fresh substrates are equal within experimental error. The surface areas of the spent catalyst before and after TPO (to 800 °C) are identical, while under the same treatment the surface area of the cyclohexene coked catalyst returned to 211 ( 5 m2/g. Ozawa and Bischoff25 have shown that
