Ind. Eng. Chem. Res. 1999, 38, 3255-3260
3255
Kinetics of Gaseous Product Formation in the Coke Combustion of a Fluidized Catalytic Cracking Catalyst Jose´ M. Arandes,* In ˜ aki Abajo, Inmaculada Ferna´ ndez, Danilo Lo´ pez, and Javier Bilbao Departamento de Ingenierı´a Quı´mica, Universidad del Paı´s Vasco, Apartado 644, 48080 Bilbao, Spain
Combustion of the coke deposited on a commercial catalytic cracking catalyst is studied by differential scanning calorimetry. The conditions of the industrial operation in a fluidized catalytic cracking unit causing deactivation (feed, contact time, and temperature, C/O ratio) and those used for regeneration (catalyst stripping and temperature) have been reproduced. Combustion heat of coke is strongly dependent on combustion temperature and on deactivation severity (reaction conditions). Kinetic equations for the formation of gaseous combustion products, CO, CO2, and H2O, have been determined, which are useful in the design of industrial regenerators. Introduction Because of the importance of the fluidized catalytic cracking (FCC) unit in the global economy of refineries, numerous models have been proposed in the literature for the design of both the reactor and the regeneration section, which operates in a bubbling fluidized bed.1-5 With models established for each one of these sections being assembled, integrated models for the whole reactor-regenerator systems have been proposed.6-11 The design of the regeneration section and of the reactor-regenerator system requires the knowledge of the heat released and of the kinetics of the gaseous products CO, CO2, and H2O, formed in the combustion of coke. These data are needed in the heat balance of the regenerator unit, which is part of the global energy balance of the FCC unit. Despite their interest, there is little information in the literature on these subjects and it is somewhat adventurous to use information obtained for amorphous silica-alumina catalysts, which are not presently used, and under reaction conditions different from those corresponding to the present FCC commercial units.12-16 Le Minh et al. made a review of a great number of papers on catalyst regeneration, which correspond to a wide range of catalysts, feeds, and operating conditions.17 In this review it is concluded that coke combustion is sensitive to several factors, of which the following are worth mentioning: coke nature, catalyst porous structure, coke relation to catalyst acidic sites, and activity of CO combustion promoters. Consequently, in the kinetic studies of catalyst regeneration carried out on a laboratory scale, it is important to reproduce the industrial conditions of reaction and regeneration in order for the results to be valid for the design of the commercial unit. The aim of this paper is the application of methodologies which have been successfully used for the kinetic study of regeneration of Y and ZSM-5 zeolites deactivated on the laboratory scale to the regeneration of a commercial catalyst used in a FCC unit.17,18 The study has been carried out by reproducing the reaction and * To whom correspondence should be addressed. Telephone: 34-4-6012511. Fax: 34-4-4648500. E-mail: iqparesj@ lg.ehu.es.
regeneration conditions of the FCC units. With the aim of obtaining reproducible results, recent conclusions on the complexity of coke combustion in microporous acidic catalysts have been taken into account.19 Likewise, given that in the FCC units the deactivated catalyst is subjected to stripping to eliminate coke light components, this stripping step has been studied. Experimental Section The reaction conditions of the industrial unit have been reproduced in riser simulator equipment, which allows for obtaining samples of the catalyst deactivated under different operating conditions when different feeds are used. The reactor is discontinuous for the catalyst and is based on internal circulation, as is the Berty reactor,20 but in contrast to this, the bed is fluidized (to avoid coke profiles and gas shortcuts as happens in fixed beds due to the small particle diameter).21-23 The feed is injected into the reactor with a glass syringe, which in turn is fed from an externally heated 40-cm3 vessel (to ensure the liquid state) by means of a three-way valve. The sample injected is instantaneously vaporized in the reactor (temperature above 500 °C), and a turbine drives it through the bed. At the same time, as the sample is introduced into the reactor, a timer is activated, which once the preset reaction time has elapsed (between 1 and 10 s) opens a four-way valve to let the cracking products pass into the analysis system. This system consists of a vacuum chamber (528 cm3) and a gas chromatograph (PerkinElmer 8500 provided with a FID and Tracer highresolution capillary column of methyl silicone. The carrier gas used is helium with a velocity of 20 cm/s, which is set by maintaining a pressure of 40 psig at the column head. The identification of the products was carried out using three methods: (1) infrared spectrophotometry (FTIR Nicolet 740 SX, provided with a GC-IR thermostated interface); (2) injection of pure standards (Alphagaz PIANO calibration standards from Air Liquide); (3) mass spectrometry (5989B from Hewlett-Packard). The catalyst studied (an OCTYDINE 1169 BR from Engelhardt) is a commercial one used in a Spanish FCC unit. Consequently, the results obtained for this catalyst will be representative of the regeneration in the FCC
10.1021/ie980764b CCC: $18.00 © 1999 American Chemical Society Published on Web 08/07/1999
3256 Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999 Table 1. Catalyst Properties
Table 2. Composition of the Feeds (wt %)
physical properties particle size, µm average particle diameter, µm bulk density, cm3/g BET surface area, m2/g micropores volume, cm3/g BET surface area, m2/g mesopores volume, cm3/g BET surface area, m2/g average diameter, Å unit cell size, Å
0-20 0-40 0-80
component 0 wt % 6 wt % 60 wt % 75 0.94 205 0.02 40 0.25 176 42.3 24.28
composition Cu Ni V Fe Na C Al2O3 ReO
10 ppm 270 ppm 1200 ppm 0.35 wt % 0.17 wt % 0.10 wt % 48.0 wt % 1.26 wt % catalytic properties
MAT activity CF (Carbon Factor) GF (Gas Factor)
71 wt % 0.64 0.93
unit. Catalyst properties are shown in Table 1. The porous structure was studied from isotherms of adsorption-desorption of N2 (mesopores) and of Ar (micropores) in a Micromeritics ASAP 2000. Three feeds have been used: a light cycle oil (LCO), a refinery vacuum gas oil (usual feed, VGO), and a paraffinic vacuum gas oil (PVGO), which has been prepared by mixing the previous VGO with its saturated fraction. The characterization of the feeds was carried out by simulating distillation (Hewlett-Packard 6890 chromatograph with FID) and subsequently following the ASTM D 2549-85 standard to separate and determine the aromatic and nonaromatic fractions. Subsequently, the two fractions were analyzed following ASTM D 2786-86 and ASTM D 3239-86 standards, respectively. Both standards are based on the analysis by mass spectrometry with high ionization voltage. The results of the analysis are shown in Table 2 The runs for obtaining a deactivated catalyst have been carried out at 500 and 550 °C, with contact times of 1 and 10 s and with C/O ) 4. The rapid evacuation of the reaction gases from the reactor avoids their subsequent cracking once the reaction is finished, and so the catalyst coke content on the catalyst will not be higher than that corresponding to the reaction conditions. The effect of stripping in coke combustion has been studied in a differential scanning calorimeter (DSC 111 from Setaram) using a T-t ramp. Kinetics of coke combustion has also been studied by carrying isothermal experiments in the same equipment at 620, 660, and 720 °C. In these experiments, the combustion gases have been measured by FTIR analysis (Nicolet 740 SX). The quantitative analysis of the absorption bands is carried out by calculating by integration the areas under the curves for the wavelength range corresponding to each product (CO2, 2400-2200 cm-1; CO, 2200-2006 cm-1; H2O, 2006-1250 cm-1).18,19 The coke combustion heat has been determined in each of these isothermal
Saturated C(n)H(2n+2) paraffins C(n)H(2n) monocycloparaffins C(n)H(2n-2) dicycloparaffins C(n)H(2n-4) tricycloparaffins C(n)H(2n-6) tetracycloparaffins C(n)H(2n-8) pentacycloparaffins C(n)H(2n-10) hexacycloparaffins C(n)H(2n-12) heptacycloparaffins Total Aromatics monoaromatics C(n)H(2n-6) alkylbenzenes C(n)H(2n-8) benzocycloparaffins C(n)H(2n-10) benzodicycloparaffins diaromatics C(n)H(2n-12) naphthenes C(n)H(2n-14) C(n)H(2n-16) triaromatics C(n)H(2n-18) C(n)H(2n-22) tetraaromatics C(n)H(2n-24) C(n)H(2n-28) Total Sulfur Compounds C(n)H(2n-4)S thiophenes C(n)H(2n-10)S benzothiophenes C(n)H(2n-16)S dibenzothiophenes C(n)H(2n-22)S naphthobenzothiophenes Total
LCO
VGO
PVGO
7.9 5.6 2.7 4.5 0.0 0.0 0.0 0.0 20.7
12.5 13.8 7.9 4.9 0.0 0.0 0.0 0.0 39.1
22.1 24.4 14.0 8.6 0.0 0.0 0.0 0.0 69.1
17.8 12.2 3.0
9.3 4.8 3.7
4.6 2.4 1.9
21.5 5.3 6.3
5.4 5.2 9.0
2.7 2.6 4.5
0.8 0.0
5.8 3.7
2.9 1.9
0.0 0.0 66.9
0.8 0.0 47.7
0.4 0.0 23.9
0.4 0.3 12.0 8.3 0.0 4.6 0.0 0.0 12.4 13.2 100 100
0.6 4.1 2.3 0.0 7.0 100
Figure 1. Calorimetry of coke combustion. Reaction conditions: VGO charge; C/O ratio ) 4; 500 °C; contact time ) 1 s.
experiments. The total coke content was determined by thermogravimetry (Setaram TAG 24). Results Coke Aging. The aim of the study is to determine the role played by stripping prior to coke combustion carried out in the commercial FCC units and to determine the degree of aging needed to obtain reproducible coke combustion results, which are useful in the design of the regenerator. As an example of the stripping effect on coke combustion, Figure 1 compares the temperature-programmed oxidation (TPO) results of combustion with air corresponding to a sample of an unaged deactivated catalyst with those of the same sample once it has been aged by a stripping treatment with a He stream at 500 °C for 20 min. The results correspond to the heat released by the mass unit of catalyst with coke. The sample was used under the conditions shown in the figure captions,
Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999 3257 Table 3. Combustion Heats and Properties of Coke Samples for Different Reaction and Aging Conditions reaction conditions
CO2/CO ratio charge
regen. conditions
T, °C
t, s
stripping
500 500 500 500 500 500 550 550 550 550 550 550
1 1 1 1 1 1 10 10 10 10 10 10
no no no yes yes yes no no no yes yes yes
n charge
combustion heat, J/g of coke charge
T, °C
LCO
VGO
PVGO
LCO
VGO
PVGO
LCO
VGO
PVGO
620 660 700 620 660 700 620 660 700 620 660 700
0.47 0.34 0.27 0.47 0.34 0.27 0.43 0.34 0.27
0.45 0.32 0.28 0.45 0.32 0.28 0.47 0.38 0.25 0.42 0.38 0.25
0.46 0.32 0.28 0.46 0.32 0.28 0.46 0.32 0.28 0.46 0.32 0.28
1.0 1.0 1.0 0.8 0.8 0.8 0.9 0.9 0.9
0.6 0.6 0.6 0.5 0.5 0.5 0.6 0.6 0.6 0.4 0.4 0.4
0.8 0.8 0.8 0.4 0.4 0.4 0.75 0.75 0.75 0.4 0.4 0.4
26 170 23 630 20 917 24 737 21 956 19 845 25 396 23 104 20 143
20 832 20 634 20 384 20 543 19 062 18 856 21 784 21 422 19 437 19 038 18 473 16 471
23 422 22 474 22 132 20 142 19 473 18 559 21 621 21 276 20 449 19 579 19 288 18 936
and the results have been obtained by carrying out the combustion in the differential scanning calorimeter following a temperature ramp of 5 °C min-1. As is observed in Figure 1, the sample that was not subjected to stripping has an initial combustion corresponding to the combustion of a more hydrogenated fraction of coke. Nevertheless, this peak is not observed when the sample is subjected to stripping. Concerning the characteristics of the TPO, it is noteworthy that the intensity of the peak for the hydrogenated coke is smaller than that corresponding to supported metallic catalysts, where the hydrogenated coke is deposited on metallic sites and where combustion is accelerated by the catalytic effect of the metal.24 The TPO data in Figure 1 seem to indicate a heterogeneous nature of the coke, which has a considerable hydrogenated fraction, rather than two different kinds of coke. Moreover, the area of the first stretch of the TPO curve is smaller than that observed for coke deposited on HZSM-5 zeolites in the MTG (methanol to gasoline) process,19 because of the fact that the FCC coke is less heterogeneous and less hydrogenated, which may be attributed to the higher reaction temperature in the FCC, to a heavier feed with aromatics, and to the greater pore size of the HY zeolite, which are aspects that favor coke evolution over a short time as that of the FCC reactor (1 s for the results in Figure 1). The influence of aging on the characteristics of the coke deposited was also observed in the isomerization of cis-butene on silica-alumina25 and in the dehydrogenation of butane to butadiene over a Cr2O3/alumina catalyst.26 It must be emphasized that the aforementioned effect of the stripping is less important for the catalyst sample deactivated under the more severe conditions, which correspond to a more developed coke, with a lower H/C ratio.27 Comparing the results of combustion of unaged coke with those corresponding to the same coke once it has been subjected to aging, Figure 2, the effect of aging attenuates for the coke deposited at a higher reaction temperature 550 °C and at a longer contact time (10 s). The explanation lies in the fact that this is a more developed coke (higher molecular weight and lower H/C than that of Figure 1) and, consequently, it is less sensitive to the aging treatment prior to combustion. When different conditions have been tried, it has been determined that the stripping treatment with He at 500 °C for 20 min is the minimum one, yet severe enough for the coke to have reproducible combustion kinetics. When calorimetric measurements of isothermal combustion of the coke subjected to this stripping treatment are carried out, combustion heat data for coke samples obtained under different reaction conditions have been
Figure 2. Calorimetry of coke combustion. Reaction conditions: VGO charge; C/O ratio ) 4; 550 °C; contact time ) 10 s.
obtained. The results for the three charges are set out in Table 3 for combustion temperatures of 620, 660, and 720 °C. It can be appreciated that the combustion heat varies between 16 471 and 26 170 J (g of coke)-1, depending on the reaction conditions. The effect of the reaction conditions on coke combustion is explained by the nature of the coke (n ) H/C ratio). The H/C ratio of the coke and the CO2/CO ratio were measured from the results of the formation of CO, CO2, and H2O in the stream of combustion gases. As is observed in Table 3, the combustion heat of the coke depends on its H/C ratio (which varies between 0.6 and 1.0, depending on the severity of the reaction for which the coke was deposited) and on the CO2/CO ratio in the combustion gases. The more hydrogenated coke is the one corresponding to the sample obtained by feeding LCO charge to the reactor and so, logically, the combustion heat of the corresponding coke is higher than that of coke deposited for other charges. These results can be attributed either to a lower content in triaromatics and negligible content in tetraromatics (both responsible for coke formation) in the LCO or to a lower boiling range and a lower molecular weight of the LCO charge. Likewise, as the reaction temperature is increased from 500 to 550 °C and as the contact time is increased from 1 to 10 s, the coke deposited is less hydrogenated and its combustion heat is lower. As mentioned above, the previous aging of the coke decreases its H/C ratio, on one hand, which gives way to a considerable decrease in combustion heat. On the other hand, as the combustion temperature is increased, the CO2/CO ratio decreases and, consequently, combustion heat also decreases. This result is opposed to that observed in the industrial unit, which is explained because the experimentation carried out here corre-
3258 Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999
sponds to coke combustion in the dense phase of the industrial regenerator. Consequently, the experimental results correspond to this phase without considering CO conversion in the freeboard of the industrial regenerator. This aspect must be taken into account when these data are to be used in the simulation of the industrial unit. The results of Table 3 are in agreement with previous results in the literature on the effect of the reaction conditions on the structure of the coke deposited on cracking catalysts,27 and on the effect of the coke characteristics on its combustion, which was observed in both the regeneration of HZSM-5 zeolites used in the MTG process18 and the regeneration of Y zeolites used in the isobutane alkylation.28 Several authors pointed out that in regeneration by coke combustion the active sites of the catalyst play an important role.17,19,29 The slightly evolved coke is partially or totally adsorbed on the active sites of the catalyst, which explains the low combustion rate of the coke deposited on HZSM-5 zeolites in the MTG process at temperatures lower than 400 °C.19,29 The coke deposited on the HY zeolite in this study is more evolved and the C-C bond predominates over the C-active site bond, which explains the high combustion rate.17 From the results of Table 3 it is concluded that there is a strong dependency of reaction and regeneration conditions on coke combustion kinetics. Also, the value of coke combustion heat is calculated for it to be used in the design of the industrial regenerator. Under the conditions of the industrial operation a value between 16 400 and 19 000 J (g of coke)-1 is the suitable one, depending on the characteristics of the feed. These values are those corresponding to the more severe reaction and regeneration conditions among those shown in Table 3. Kinetic scheme. It includes the following reactions,
[
]
β+2 n CHn + O f + 2(β+1) 4 2 β n 1 CO + CO + β+1 β+1 2 2H2O
Table 4. Kinetic Constants for Regeneration constants
values
β kC k a (order for O2) b (order for CO) c (order for H2O)
2.66 × exp(-14700 ( 200/RT) 4.71 × 102 exp(-26200 ( 300/RT) 3.81 × 1016 exp(-51500 ( 500/RT) 1 1 0 104
tion in the calorimeter (fixed bed) for O2, CO, CO2, and H2O. Assuming plug flow for the gas flowing through the calorimeter, pseudo steady state and constant pressure, mass conservation equations are
for O2: ∂NO2 ∂t
)
{
1
[(
∂NO2
-v
∂z n β+2 RT(1-)FkcCcNO2 - vA + L 4 2β+2 PM Q
)
]}
c a NO 2k(RT)a+b+cNbCONH 2O 2
Qa+b+c for CO:
{
[(
∂NCO ∂NCO 1 -v ∂z β RT(1-)FkcCc NO2 ) + vA ∂t L β+1 PM Q
)
]}
2kNbCONcH2ONaO2 Qa+b+c for CO2: ∂NCO2 ∂t
)
{
1
[(
∂NCO2
-v
∂z L
+ vA
)
]}
a c NO 2kNbCONH 2O 2
where CHn is the coke, C, and β is the CO/CO2 molar ratio in the combustion product stream, which follows the following relationship with temperature:
Qa+b+c
( )
(1)
for H2O: ∂NH2O ∂t
For the previous scheme the following kinetic equations are established:
Coke combustion: -rC ) kCCCPO2
(2)
a Pb Pc CO disappearance: -rCO ) 2kPO 2 CO H2O
(3)
Equation 2, first-order with respect to each one of the reactants, coke and oxygen, is widely accepted in the literature when coke combustion is not under diffusional restrictions.12-16,18,30-35 Methodology. It consisted of fitting the experimental results of the coke content remaining in the catalyst and the flow rates of O2, CO, CO2, and H2O in the outlet stream, to eq 2 and to the equations of mass conserva-
)
[
1
∂NH2O
-v
∂z L
]
n RT(1-)FkcCc NO2 2 PM Q
()
+ vA
(5)
1 RT(1-)FkcCc NO2 + β+1 PM Q
CO + 1/2O2 f CO2
-Eβ β ) kβ0 exp RT
(4)
(6)
(7)
The values of the kinetic constants and of the reaction orders have been calculated by means of a routine written in FORTRAN, which uses the COMPLEX method with reparametrization of the constants and minimizes the following objective function:
OF ) [(NCOcalc - NCOexp)2 + (NCO2calc - NCO2exp)2 + (NH2Ocalc - NH2Oexp)2]/Nexp (8) Kinetic Constants. The values of the kinetic constants calculated are set out in Table 4. These results correspond to the fitting of kinetic data obtained for all the experimental systems.
Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999 3259
sense, to attain a better fitting, a more complex kinetic model would be required, which has as a counterpart a greater complexity in the design of the FCC unit. Conclusions The study of coke combustion kinetics is affected by the coke nature, which is characterized in this paper by the H/C ratio. Consequently, for the kinetic results to be reproducible, a previous aging treatment of the coke is needed. It has been proven that a sweeping treatment with He at 500 °C for 20 min is enough for result reproducibility. The effect of coke stabilization obtained in this treatment is equivalent (although it is carried out under less severe conditions) to the treatment of stripping, to which is subjected the catalyst deactivated in commercial catalytic cracking units, FCC. The H/C ratio of the coke greatly affects heat combustion, which also depends on the severity of the reaction conditions (feed composition, temperature, contact time, and C/O ratio). Nevertheless, the variable of greater incidence in coke combustion heat is the combustion temperature. Combustion heat decreases as the temperature is increased, which is a consequence of the decrease in the CO2/CO ratio in the combustion gases. The results of combustion heat and the kinetic equations for the formation of the different combustion gases, that is, CO, CO2, and H2O, are very useful for the design of the industrial regeneration unit. It is noteworthy that the kinetic constants calculated suitably fit the data of combustion of the coke deposited under different reaction conditions and for several feeds, which augers well for its application to the calculated kinetic model. Acknowledgment This work was carried out with the financial backing of the Ministry of Education and Culture of the Spanish Government (Project DGICYT PB94-1355). Notation Figure 3. Evolution with time of coke combustion at 660 °C. Points, experimental results. Lines, calculated with the kinetic model. Reaction conditions: C/O ratio ) 4; 500 °C; contact time ) 1 s. Samples with previous stripping. (a) LCO charge; (b) VGO charge; (c) PVGO charge.
In Figure 3, each plot corresponds to a different feed. The experimental results (points) of evolution with combustion time of the concentration of the combustion gases (expressed as molar fraction) are plotted together with the concentrations calculated with the kinetic model for combustion and the calculated kinetic constants (lines). The results shown are an example corresponding to certain operating conditions. As is observed, the fitting between the experimental results and the values calculated is suitable, which also shows the validity of the procedure followed in the experimentation, of the data analysis and of the kinetic constants calculated. The fact that the same values for the kinetic parameter are obtained for all the operating conditions and the different feeds is interesting for the application of these parameters in the design of the industrial regenerator and shows the importance of carrying out the kinetic study of coke combustion subsequently to stripping for homogenization of coke properties. In this
A ) cross-sectional area of the reactor, m2 a, b, c ) orders for O2, CO, and H2O in their respective reactions Cc ) coke content, kg of coke (kg of catalyst)-1 k ) constants for homogeneous combustion of CO, kmol m-3 kPa-2 s-1 kC ) coke combustion constant, kPa-1 s-1 L ) reactor length, m Nexp ) number of experimental data NO2, NCO, NH2O, NCO2 ) molar flow, kmol s-1 n ) hydrogen/carbon atomic ratio in the coke PO2, PCO, PH2O, PCO2 ) partial pressure, kPa PM ) molecular weight of coke, kg kmol-1 Q ) volume flow of gases, m3 s-1 R ) constant of gases, kPa m3 kmol-1 K-1 -rC ) coke combustion rate, kg of coke (kg of catalyst)-1 s-1 -rCO ) CO combustion rate, kmol m-3 s-1 T ) temperature, K v ) gas linear velocity, m s-1 z ) longitudinal coordinate, dimensionless
3260 Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999 Greek Letters β ) CO/CO2 ratio ) bed voidage F ) bulk density of the bed, kg m-3
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Received for review December 3, 1998 Revised manuscript received June 8, 1999 Accepted June 16, 1999 IE980764B
