434
Ind. Eng. Chem. Res. 1995,34, 434-439
KINETICS, CATALYSIS, AND REACTION ENGINEERING Matrix Acidity Determination: A Bench Scale Method for Predicting Resid Cracking of FCC Catalysts Saeed Alerasool,* Patricia K. Doolin, and James F. Hoffman Research and Development Department, Ashland Petroleum Company, P.O. Box 391, Ashland, Kentucky 41114
Novel methods for determination of matrix acidity of cracking catalysts are discussed. Virgin matrix acidity is determined after complete destruction of the zeolite phase by acid treating the catalyst at pH 2. Properties of acid-treated catalyst closely approximate those of pure matrix. However, the virgin matrix acidity does not correlate with resid cracking performance of the catalyst under commercial or pilot plant conditions. A second method has been developed that facilitates the measurement of matrix acidity on a laboratory deactivated sample with properties similar to that in the commercial cracking unit. The approach is based on total destruction of the zeolite by steaming a t 870 "C for 5 h. Acidity of the steamed catalyst closely resembles that of steamed pure matrix and has a good correlation with the resid cracking activity in the pilot plant testing. Preservation of matrix acid sites during hydrothermal deactivation was shown to be an important requirement for achieving high resid cracking activity.
Introduction Modern cracking catalysts contain REY or USY zeolite embedded in an inorganic oxide matrix. The zeolite provides most of the cracking activity while the matrix possesses physical as well as some catalytic functions (Scherzer, 1989). The most important physical role of matrix is its ability to bind the zeolite crystallites together in a microsphericalcatalyst particle hard enough to withstand intraparticle and reactor wall collisions in a commercial catalytic cracking unit (Plank, 1983). In addition, matrix with its large number of mesopores (> 20 A) provides a medium for the diffusion of feedstock molecules and cracked products. It also provides a means for heat transfer during reaction and regeneration thereby protecting the zeolite structure from structural damage by increasing its hydrothermal stability (Scherzer, 1989; Plank, 1983; Rajagopalan and Habib, 1992). Migration of sodium ions from zeolite into the matrix at high temperatures has also been suggested as a cause of improved stability (Plank, 1983). In addition to the physical functions of the matrix, it can also play a catalytic role in the cracking reaction. Especially in cracking applications where resid is mixed in with the feed, the large higher boiling point hydrocarbons in the resid (bp >540 " C ) cannot enter the internal structure of zeolite. An active matrix with large mesopores can precrack these molecules into smaller fragments before they enter the zeolite and are further cracked into more valuable products. Figure 1 schematically illustrates the function of an active matrix in a cracking catalyst. Precracking of large hydrocarbons (i.e., resids) is expected to be catalyzed by the surface acid sites of the matrix. Therefore, the activity of a matrix as a catalytic substrate should be related to the density, type, and strength of its acid sites. In order to evaluate and
* To whom correspondence should be addressed. E-mail:
[email protected]. 0888-588519512634-0434$09.00/0
0
Active Matrix
(Q
t L #
El
Zeolite Particle
1
Figure 1. Simplified schematic of a cracking catalyst with an acidic matrix.
predict the resid cracking performance of matrix, it is important to characterize its acidic properties. Preferably, this characterization should provide information on the acidity of the matrix after it has been equilibrated at conditions close to those of FCC unit. Although the most convenient way of characterizing matrix acidity is t o measure the acidity of matrix before it is incorporated into the catalyst, this does not necessarily provide a realistic measure of matrix acidity because interactions between matrix and zeolite during their hydrothermal deactivation can lead to modification and even generation of new acid sites (Suzuki et al., 1986). Furthermore, refiners are usually not provided with the matrix when catalyst vendors submit their catalyst for evaluation. Therefore, development of a reliable method
0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34,No. 2, 1995 436 for an "in situ" measurement of matrix acidity can be a valuable tool for both catalyst manufacturers and refiners. The task of measuring matrix acidity in the presence of zeolite is a challenging one. The zeolite with its high acid density and strength easily masks the acidity of matrix. While the topic of matrix acidity and indirect ways of characterizing it have been discussed in the literature (Corma, et al., 1990; Benesi, 19751, very few studies have specifically addressed the question of measuring matrix acidity and distinguishing it from the acidity of the zeolite present in the cracking catalyst. In a recent paper by Pereira and Gorte (Pereira and Gorte, 1992), the use of large molecules of cyclooctylamine and 2-ethylhexylamine with mixtures of zeolites and silica-alumina was reported to facilitate direct measurement of acid sites in silica-alumina without detecting the zeolite acidity. The approach was based on catalytic decomposition of these amines into an alkene and ammonia at the Bronsted acid centers of silica-alumina. Determination of the total number of decomposed molecules allowed the authors to calculate the number of Bronsted acid sites present on the nonzeolitic silica-alumina phase. Molecules of cyclooctylamine and 2-ethylhexylamine were presumably too large to penetrate the zeolite structure. If the values obtained are used as a measure of matrix acidity, it should be assumed that only Bronsted sites of the matrix participate in the cracking of large resid molecules. However, some forms of alumina, known to contain only Lewis acidity, have been found t o be active for catalytic precracking of large molecules (Humphries, 1989). The object of this study was t o develop methods by which the acidity of matrix can be measured on a virgin catalyst and on a laboratory-modified sample with a matrix that closely simulates the matrix of a catalyst in the commercial FCC reactor. The former is referred to as "virgin matrix acidity" while the latter is called "pseudoequilibrium acidity".
Experimental Section Catalysts. Twenty catalysts were obtained from five different manufacturers. A number of these catalysts were commercially produced while others were in the exploratory or developmental stage. All 20 catalysts had been well characterized and tested on the pilot scale prior to this study. Selected physical and chemical properties of each catalyst are listed in Table 1. The catalysts contain between 10 and 25% REY or REUSY zeolite. Relative zeolite intensity (ZI), as measured by X-ray diffracation (ASTM D3906-911, is directly related to the zeolite content. Total specific surface area varied between 157 and 252 m2/g. Rare earth oxide contents ranged beween 0.4 and 3.8 wt %. The unit cell sizes of the catalysts in this series varied between 2.453 and 2.472 nm. Specific pore volume determined by mercury porosimetry is also listed in Table 1. This value closely approximates the porosity of the matrix since mercury does not penetrate the small pores of the zeolite. Samples of corresponding active matrices for five of the cracking catalysts, provided by the vendors, were also included in this study to aid in validating the underlying assumptions of the newly developed methods. Sample Preparation for Virgin Matrix Acidity Determination. About 25 g of virgin cracking catalyst was added to approximately 500 mL of distilled water
Table 1. Selected Properties of Virgin Catalysts catalyst UCSa ZIb SAc MSAd %RE@ H g PVf 0.4 0.42 131 A 2.467 11.2 207 1.5 0.44 112 B 2.465 12.5 252 C 2.462 13.3 194 109 1.5 0.40 D 2.464 16.1 213 80 2.3 0.43 E 2.462 11.0 210 117 1.7 0.40 F 2.460 12.0 203 69 1.2 0.23 1.0 0.28 64 G 2.468 14.3 230 40 3.3 0.29 H 2.470 12.5 157 128 1.4 0.30 7.4 174 I 2.453 J 2.463 10.9 180 33 1.4 0.31 1.4 0.31 90 K 2.467 16.6 159 L 2.467 17.1 255 118 3.8 0.27 M 2.477 13.6 218 105 3.5 0.36 124 0.1 0.28 N 2.456 17.1 230 0 2.473 13.7 217 124 3.4 0.31 3.4 0.40 94 P 2.472 8.5 186 2.457 8.0 195 111 2.0 0.60 Q 54 2.7 0.27 2.456 16.3 211 R S 2.461 15.7 203 67 2.6 0.29 T 2.472 8.3 210 114 2.3 0.29 a UCS: unit cell size (nm). b ZI: relative zeolite intensity determined by XRD. SA total surface area (m2/g). MSA mesopore surface area (m2/g).e %REO: weight percent rare earth oxide, determined by XRF. f H g P V mercury pore volume (cm3/ g), determined by mercury porosimetry.
at room temperature. Concentrated (9.0 M) sulfuric acid was added to the catalyst mixture while monitoring the pH with an electronic pH meter. The addition was stopped when a stable pH of 2.0 was reached. Normally, 5-10 mL of 9.0 M acid was needed for this step. The catalyst was washed with at least 4 L of deionized water and was subsequently dried at 120 "C in a vacuum oven. Crystallinity of the sample was then examined by X-ray diffraction (Phillips XRG3100), using ASTM D3906-91 method, t o assure that the sample was completely amorphous. Each of the acid-treated samples was also analyzed for elemental sulfur using an infrared sulfur detector (LECO, Model SC-432DR). Sample Preparationfor PseudoequilibriumMatrix Acidity Determination. Approximately 100 g of virgin catalyst was calcined in air at 600 "C for at least 4 h. The catalyst was then treated with 100% steam at 870 "C and 1 atm for 5 h using a procedure similar to ASTM D4463-91. An X-ray diffraction pattern of the steamed sample was recorded t o assure that steaming had completely destroyed the zeolite crystal structure. Acidity Measurements. A 50 mg aliquot of steamed catalyst or matrix was loaded in the aluminum sample pan of a thermogravimetric analyzer (TGA 951, TA Instruments). The sample was heated to 600 "C at a constant heating rate of 10 "C/min under a dry nitrogen purge and was held isothermal for 2 h. It was then cooled to 100 "C, and the nitrogen flow was redirected to pass through a saturator containing pyridine (Fisher Scientific, 99.9% purity) before entering the TGA. This configuration allowed the sample to be exposed to a constant flow of nitrogedpyridine mixture (Ppyridine = 20 mmHg) at an adsorption temperature of 100 "C. The adsorption of pyridine continued for approximately 1.5 h or until no additional weight gain was noted. At the conclusion of this step, the flow of pyridindnitrogen was discontinued and was replaced with a flow of pure nitrogen at 100 "C. This step was required for removing reversibly and physically bound pyridine from the surface. The desorption step continued until no additional weight loss was noted. Values of total acidity were determined from the following equation:
436 Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995
total acidity = {[(W, - Wi)/WiYMW} x
lo6
1.50
(1)
130 ..
4..
I10 - -
where Wf and Wi correspond to the values of catalyst weight after pyridine desorption and prior to pyridine adsorption, respectively, and M w is the molecular weight of pyridine. The steam-treated samples were also characterized by nitrogen BET to determine their total surface area and mesopore surface area. The mesopore area was determined by using a t-plot analysis (Anderson, 1985). Pilot Plant Evaluation. To evaluate the methods discussed in this study, the matrix acidity data were correlated with the existing pilot plant performance results. The pilot scale data have been gathered over an 8-year period. The operating conditions of the pilot plant unit were selected carefully to mimic those of the commercial reduced crude conversion (RCC) unit. The RCC is a patented fluidized catalytic cracking unit specifically designed to process heavy feedstocks (Myers, 1981; Zandona et al., 1982). The key elements of the RCC process include two-stage regeneration, external catalyst coolers, lift gas catalyst contacting, and a vented riser to limit oilkatalyst contact time (Shaffer, 1990). In addition, the catalyst used in the RCC is designed to endure the high metal and sulfur content of heavier crude fractions as well as the more severe thermal conditions required for processing these fractions than those in a conventional FCC unit. The pseudoequilibrium catalyst samples for pilot plant testing were prepared by metalating to a final total metal (nickel plus vanadium) content of 8000 ppm using an accelerated aging technique. A reduced crude with a Ramsbottom carbon of 6.6 wt %, API gravity of 17.0, nickel and vanadium concentrations of 10 and 35 ppm, respectively, was used as the feedstock for the pilot plant tests. At the conclusion of pilot scale evaluation of each catalyst, yield (selectivity) curves were constructed for several key product components as a function of conversion. Points on these performance curves were obtained by conducting experiments a t different catalyst-to-oil ratios and temperatures. Among the performance curves constructed from the pilot plant test, the plot of weight percent light cycle oil yield to weight percent slurry oil yield ratio (LCO/SO) as a function of volume percent conversion was chosen as the best indicator of resid cracking capability. A typical LCO/SO plot obtained from the pilot plant tests on two of the catalysts used in this study is shown in Figure 2. Slurry oil (SO) is defined as the fraction of hydrocarbons with boiling point above 332 "C while light cycle oil (LCO) is the fraction of product which boils between 221 and 332 "C. Because many of the hydrocarbon molecules in the resid are too large to penetrate the zeolite internal structure, they are expected to remain unconverted in the slurry oil if the catalyst contains an inactive matrix. With an active matrix, the large resid molecules are cracked into hydrocarbons that are in the LCO boiling range. This would in turn increase the yield of light cycle oil (LCO) at the expense of slurry oil (SO). As a result, when a feedstock is cracked by a catalyst with high matrix acidity, a high LCO/SO ratio is expected. Although this ratio is calculated for a range of conversions during the pilot plant evaluation, a single value of LCO/SO at 75% conversion is used in this study because the conversion achieved in the catalytic cracking unit is in the 70-80% range. A detailed description of the pilot plant setup
-
0
s3 ow-4--070
--
0 5 0 --
J
0.30 50
55
60
-
65
70
75
80
85
Volume Percent Conversion Catalyst A -Catalyst
H
Figure 2. Typical LCO/SO plot obtained from pilot plant testing on catalysts A and H. Table 2. Effect of Acid Treatment on Physical and Acidic Properties of Catalysts and Their Corresponding Matricesu virgin acid-treated acid-treated catalyst matrix matrix catalyst TA SA MSA TA SA MSA TA SA MSA ZI A 91 104 104 80 99 99 100 94 94 0 32 0 32 20 40 H 32 38 34 26 34 N 209 164 164 186 152 151 171 125 112 0 L 155 125 125 160 115 115 157 98 95 0 0 184 128 128 163 113 113 140 114 114 0 TA: total acidity, ,umoVg. S A surface area, m2/g. MSA mesopore surface area, m2/g. ZI: relative zeolite intensity.
and test conditions is found elsewhere (Mitchell et al., 1988, 1993).
Results and Discussion Virgin Matrix Acidity. This method makes use of the fact that the crystal structure of the zeolitic portion of a cracking catalyst can be completely destroyed by treatment with a concentrated acid without significantly modifying the acidic and physical properties of the matrix. The measured acidity on an acid-treated cracking catalyst should therefore correspond to the acidity of its pure matrix phase. To be able to rely on the proposed method for the determination of matrix acidity, it should be demonstrated that the values obtained closely approximate the acidity of the matrix component of the virgin catalyst. To demonstrate this point, the following assumptions have to be validated. First, the zeolite crystallinity should totally vanish upon acid treatment. Second, the properties of acid-treated catalysts should be very similar t o those of pure matrices. Third, acid treatment of an amorphous matrix should not significantly modify its surface characteristics. The validity of these assumptions was established using the set of five catalysts whose pure matrices were available. Selected properties of the acid-treated catalysts and their corresponding matrices are listed in Table 2. As shown in this table, XRD did not detect any zeolite peaks on the acid-treated catalysts. This suggests that the first assumption of the method is valid; namely, the zeolite is completely destroyed upon acid treatment. Furthermore, according to Table 2, values of total surface area and mesopore surface area of acid-treated catalysts are very close to one another. Since most of the micropores are believed to be associated with the zeolite phase, this is a further indication that acid treatment does in fact destroy the zeolite structure. To demonstrate that the acidity values determined by this method actually correspond to the matrix acidity,
Ind. Eng. Chem. Res., Vol. 34, NO. 2, 1995 437 Table 3. Selected fioperties of Acid-Treated and Steamed Catalysts virgin steamed at 870 "C catalyst ZP SAb MSAc acidity 21 SA MSA acidity LCO/S@ A 0 125 125 139 0 59 57 48 1.17 1.24 B 0 126 126 48 151 0 59 59 50 1.25 C 0 97 92 100 0 66 66 41 1.15 D 0 104 102 118 0 54 52 39 1.19 E 0 90 87 89 0 64 50 0
50
100
150
200
250
Virgin Matrix Acidity (pmoldg)
F G H I
Figure 3. Effect of acid treatment on the acidity of catalyst and matrix.
J K L
total acidities of treated catalysts have been plotted against fresh matrix acidities in Figure 3. A good linear relationship was found between the two sets of values (r2 = 0.90). This confirms that the acidity measured on the treated catalyst is a direct contribution of the matrix. The deviation from a perfectly linear relationship is caused by a number of factors: First, since different catalysts contain varying amounts of zeolite and the zeolite relic has a dilution effect on the acidtreated catalyst, the net effect of this dilution is expected to vary from catalyst to catalyst. Second, some synergy between matrix and zeolite can create additional acidity as they are incorporated into the cracking catalyst (Rajagopalan and Habib, 1992). This synergy can, in turn, modify the acidic properties of the matrix. To show that the acid treatment of matrix does not significantly vary its acidic properties, the five matrices were also treated with sulfuric acid according to the above outlined procedure. Total acidities of the acidtreated matrices were measured and compared t o those of their respective virgin matrices. These values are listed in Table 2. As shown in this table, acid-treated matrices have values of acidity equal or slightly lower than those of fresh matrices. A good linear relationship is observed (r2 = 0.93) when these values are plotted against one another, as shown in Figure 3. The small loss in surface acidity as a result of washing with sulfuric acid can be attributed to partial dissolution of the matrix. However, the extent of dissolution is not large enough to substantially affect the total acidity of the matrix. The acid-treated samples were also analyzed for sulfur. The sulfur levels were found to vary between 0.1 and 0.3 w t %. These sulfur levels were not significant since the fresh catalyst typically contains 0.1-0.3 wt % sulfur. Since the above findings provided the evidence needed for validating the approach, the method was then applied to the remaining 15 catalysts for which matrices were not available, their virgin matrix acidities were determined, and the results were included in Table 3. Once the virgin matrix acidities of all catalysts are determined, the relationship between these values and resid cracking can be examined. However, when the values of virgin matrix acidity are plotted against the resid cracking performance of the catalysts, represented by the LCO/SO ratio, virtually no correlation is observed between matrix acidity and resid cracking under pilot plant testing conditions as shown in Figure 4. These findings imply that virgin matrix acidity cannot predict the resid cracking of the catalyst. We then postulated that due to differences in the hydrothermal stability of different matrices, acidity measurement should be performed on a catalyst sample that has experienced some of the severe conditions
N
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
M
0
P Q
R S
T
76 97 32 140 88 82 116 93 125 94 85 146 59 87 115
106 110 20 171 120 64 170 113 171 140 134 96 77 127 122
62 77 29 126 64 68 111 93 112 94 85 124 54 77 107
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
45 53 5 33 25 20 34 33 21 45 37 26 16 26 35
49 53 5 41 25 20 34 33 37 45 38 35 16 36 35
25 42 5 44 25 22 23 50 21 38 31 21 15 20 21
1.02 1.17 0.80 1.19 1.08 0.90 1.02 1.12 0.98 1.15 1.05 1.00 0.84 0.90 1.01
ZI: relative zeolite intensity determined by XRD. Total surface area (m2/g). Mesopore surface area (m2/g). Total acidity, umolle. e LCO/SO: Ratio of lieht cvcle oil to slum, oil (both in weig& percent) obtained in tGe piiot plant reactor at 75 vol conversion. 1.30 I
.
I
. I
1.
I
I
m I
I
I
I
s
I
I
lr'=023/
I
0.70
4 0
20
40
60
80
100
120
140
160
180
Virgin Matrix Acidity (rmoldg)
Figure 4. blationship between resid cracking and virgin matrix acidity.
existing in the commercial unit. Since under actual RCC conditions, the catalyst is exposed to steam, nickel, vanadium, iron, sodium, and heat, it is logical to assume that its physical and acidic properties will change as a result of being exposed to such adverse conditions. Catalyst manufacturers use a variety of preparation methods and compositions. As a result, in all likelihood, different catalysts will have various degrees of hydrothermal stability. These arguments led to the advent of the "pseudoequilibriummatrix acidity" method which is discussed in the following section. PseudoequilibriumMatrix Acidity. The approach used in the development of this method is based on the premise that severe steaming of a cracking catalyst not only leads to total destruction of the zeolitic portion of the catalyst and its associated acid sites, but it also leaves behind a matrix with physical and acidic properties very similar to that equilibrated under commercial FCC conditions. Therefore, if resid cracking is catalyzed by the matrix acid sites, the acidity measured on such a matrix should have a good correlation with the resid cracking performance of the catalyst. Once again, to prove this hypothesis, the underlying assumptions should be validated. First, it should be demonstrated that a 5-h steaming at 870 "C does lead t o total destruction of the zeolite and its acid sites. The XRD data presented in Table 3 clearly show that the crystal structure of the zeolite is destroyed upon steaming at
438 Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995
0
10
20
30
50
40
60
Acidity of Catalyst Steamed at 870°C (pmoldg)
Figure 5. Comparison of steamed catalyst acidity and steamed matrix acidity.
870 "C. Furthermore, the surface area data in Table 3 show that virtually all of the microporosity associated with the zeolite has been destroyed by the steaming. On the other hand, when catalyst H, which is known to contain a virtually non-acidic matrix phase, was steamed at 870 "C it only left behind 5 pmollg of acidity. This is one of the catalysts for which the pure matrix was available for this study. When the acidity of the pure steamed matrix for catalyst H was measured, it adsorbed 8 pmollg of pyridine. It can be inferred that the acid sites detected on the steamed catalyst H correspond to the matrix, and even if a small number of acid sites remains on the zeolite, it is below the detection limit of the thermogravimetric method. Once it is established that the zeolite and its acid sites can be effectively destroyed by severe steam treatment, it should be shown that the acid centers detected on the steamtreated catalyst do belong to the matrix. To prove this point, acidities of the five available matrices and their corresponding catalysts were measured and compared after steaming at 870 "C. As shown in Figure 5 , the acidity of each steamed catalyst was quite similar to its matrix. Although this is a small set of samples, the strong correlation (r2= 0.98) observed is a convincing indication that the acidity measured on the severely steamed catalyst corresponds to its matrix acidity. Another underlying assumption of this method is that the deactivation experienced by the matrix as a result of such severe steam treatment results in a set of physical and acidic properties similar to those found on the matrix after it equilibrates in the RCC unit. This is a difficult assumption to prove. Both the pilot plant and commercial catalysts contain high levels of metal contaminants which interfere with acidity measurements due to the adsorption of pyridine on metal sites. Therefore a direct comparison of matrix acidity is not possible. Only indirect evidence can be provided at this point. Figure 6 shows a comparison between matrix
Catalyst C
surface areas of two catalysts that, based on commercial RCC unit data, have high resid cracking activity. According to Figure 6, the matrix surface area of the equilibrium commercial catalyst is measurably lower than both the catalyst steamed a t 780 "C and the pilot plant prepared catalyst. In contrast, close agreement was found between the matrix surface area of commercially equilibrated sample and the sample steamed at 870 "C. Although a comparison of the matrix acidities of these samples is not possible due to the presence of metals on the commercial and pilot plant catalysts, the surface area data strongly suggest that, in the commercial RCC, the combined destabilizing effects of metal contaminants and steam on the matrix are similar to the impact of laboratory steaming at 870 "C. We believe the above arguments provide the necessary justifications for the use of 870 "C steaming as a way of mimicking the equilibrium matrix in the laboratory and the measured acidity should closely approximate that of the active matrix after it has been equilibrated in the commercial unit. Figure 7 shows the linear relationship (r2 = 0.85) between resid cracking observed in the pilot plant testing of catalysts, represented by the LCO/SO ratio a t 75% conversion, and the pseudoequilibrium matrix acidity. This is in marked contrast with the lack of dependence observed between virgin matrix acidity and resid cracking (Figure 4). These data confirm that an active matrix not only needs to be acidic, it should also be able to preserve a significant fraction of its acidity even after being exposed to such severe conditions as those encountered in the commercial scale process. Since the pseudoequilibrium matrix acidity accounts for both stability and acidity, it provides a reasonably accurate prediction of resid cracking performance. The pseudoequilibrium matrix acidity method can be a valuable tool for quick and inexpensive evaluation and screening of catalysts to predict their resid cracking activity before performing more time-consuming and expensive pilot plant tests. It should be noted, however, that the pilot plant data used in this study were obtained under conditions chosen to mimic the Ashland RCC unit. The RCC unit was designed for cracking feeds containing heavy fractions. Thus the matrix acidity method proposed here cannot necessarily be applied to catalysts used in the conventional FCC units that handle lighter fractions or operate under widely different conditions. The method presented in this paper is based on a thermogravimetric measurement of total number of acid sites. As is the case in most solid acid catalyzed reactions, the strength and type of acid sites also affect
Catalyst D
0Virgin After Pilot Plant Test H Steamed @ 870 "C I3 Commercial Equilibrium Figure 6. Surface area of virgin, pilot plant prepared, and commercial equilibrium catalyst.
1.30 1
I
m---
Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995 439 I
I
The authors thank Ashland Petroleum Company for granting them permission to publish this work. The assistance of David E. Day with the numerous thermogravimetric measurements involved in this study is also greatly appreciated.
r'=0.85
am
Acknowledgment
Literature Cited 0.10
I
4 0
10
20
M
40
50
00
Pseud+Equilibrlum Malrlx AcldHy @mole.&)
Figure 7. Relationship between pseudoequilibrium matrix acidity and resid cracking.
the activity and selectivity of the catalyst. The study of type and strength of matrix acid sites is part of our ongoing efforts in this area. The incorporation of these variables into our existing body of knowledge is certainly expected to deepen our insight in the area of resid cracking. This in turn will provide better tools for designing superior catalysts that can meet the challenges of reformulated gasoline era.
Conclusions The crystal structure of the zeolite in a cracking catalyst can be completely destroyed by acid treatment at pH 2 without significantly modifying the acidic properties of the matrix. Thermogravimetric measurement of acidity by pyridine adsorption on an acidtreated cracking catalyst provides an accurate estimate for virgin matrix acidity. Although this was shown to be a reliable method for determining virgin matrix acidity, no correlation was found between the measured values and the resid cracking activity of the catalyst. Similar pyridine adsorption measurements on catalysts steamed a t 870 "C provided good estimates for the equilibrium matrix acidity of the catalysts. These values were found to have a linear relationship with resid cracking activity of the catalyst. Therefore, the pseudoequilibrium matrix acidity method can be used t o estimate the resid cracking of a catalyst. Since the method is quick and inexpensive, it can be a valuable screening tool for both refiners and catalyst manufacturers. Type and strength of matrix acid sites are important variables that are also believed to affect the resid cracking performance of a catalyst. A clear understanding of the importance of matrix acidity in resid cracking can pave the way for marked advances in cracking catalyst technology.
Anderson, J. R.; Pratt, K. C. Introduction to Characterization and Testing of Catalysts;Academic Press: Sydney, Australia, 1985. Benesi, H. A. Determination of Zeolite Contents of Zeolitic Catalysts from Vaporization Measurements. J. Cutal. 1976,38, 307. Corma, A.; Grande, M.; Fornes, V., Cartledge S.; Shatlock, P. Interaction of Zeolite Alumina with Matrix Silica in Catalytic Cracking Catalysts. Appl. Cutal. l9W,66,45. Humphries, A.; Wilcox, J. R. Zeolite Components and Matrix Composition Determine FCC Catalyst Performance. Oil Gus J. 1989,87 (61, 45. Mitchell, M. M., Jr.; Moore, H. F. Protocol Development for Evaluation of Commercial Catalytic Cracking Catalysts. Prepr.-Am. Chem. SOC.,Div. Pet. Chem. 1988,33(4), 547. Mitchell, M. M., Jr.; Hoffman, J. F.; Moore, H. F. Residual Feed Cracking Catalysts. Stud. Surf. Sci. Cutul. 1993,76, 293. Myers, G. D.; Busch, L. E. Carbo-Metallic Oil Conversion with Controlled C0:COz Ratio in Regeneration. U.S. Patent 4,299,687,1981. Myers, G. D.; Walters, P. W.; Cottage, R. L. Apparatus for Separating Solid Dispersoids from Gaseous Streams. U.S. Patent 4,070,159, 1977. Pereira, C.; Gorte, R. Method for Distinguishing Bronsted-Acid Sites in Mixtures of H-ZSM-5, H-Y and Silica-Alumina. J . Appl. Catul. A: Gen. 1992,90, 145. Plank, C. J. The Invention of Zeolite Cracking Catalysts-A Personal Viewpoint. ACS Symp. Series. 1983,222,253. Rajagopalan, K.;Habib, E. T. Understand FCC Matrix Technology. Hydrocarbon Process. 1992,71 (91, 43. Schemer, J. Octane-Enhancing Zeolite FCC Catalysts: Scientific and Technical Aspects. Catal. Rev. Sei. Eng. 1989,31,215. Shaffer, A. G., Jr.; Hemler, C. L. Seven Years of Operation Prove RCC Capability Oil Gus J. 1990,88(22), 62. Suzuki, I.; Oki, S.; Namba, S. Determination of External Surface Area of Zeolites. J . Cutul. 1986,100,219. Zandona, 0. J.; Busch, L. E.; Hettinger, W. P., Jr. Reduced Crude Conversion: An Inexpensive Route to High Octane Gasoline. NPRA Annu. Meet., 1982,Paper No.AM-82-61. Received for review May 24, 1994 Revised manuscript received October 5, 1994 Accepted October 27, 1994@ IE940332A
* Abstract published in Advance ACS Abstracts, January 15, 1995.
