A novel selectivity test for the evaluation of FCC catalysts

Applied Catalysis A: General 231 (2002) 227–242

A novel selectivity test for the evaluation of FCC catalysts Dieter Wallenstein a,∗ , Mark Seese b , Xinjin Zhao b a

b

GRACE GmbH & Co. KG, In der Hollerhecke 1, 67545 Worms, Germany W.R. GRACE and Co.-Conn., 7500 Grace Drive, Columbia, MD 21044-4098, USA

Received 3 December 2001; received in revised form 11 January 2002; accepted 18 January 2002

Abstract Microactivity testing is the most commonly used procedure for determining the selectivities of FCC catalysts and researchers have followed several strategies in order to optimise this approach. Grace Davison developed the short contact-time microactivity test (SCT-MAT) and catalyst evaluations performed with this method have been shown to agree well with those obtained from circulating riser pilot units. Nevertheless, the experimental setup of the SCT-MAT still has further possibilities of improvement, which was the motivation for this work. In the presented work unconventional concepts were followed to upgrade the SCT-MAT method. A novel product collection system was developed and modifications were made to the reactor design as well as the method of feed injection. The operational parameters which ensured the superior accuracy of the original SCT-MAT, such as contact-time, reaction temperature and catalyst bed geometry, were not altered. Experimental results obtained by the novel selectivity test demonstrated a considerably improved precision and a better discrimination of the catalytic properties of FCC catalysts over the original SCT-MAT without any trade-offs in accuracy. The overriding characteristic of the novel selectivity test is its simplicity which imparts a high precision to this equipment. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Fluid catalytic cracking; Microactivity test; FCC catalyst evaluation; Selectivity

1. Introduction The fluid catalytic cracking of gas–oil is one of the most intensively used processes in refining industry; this process produce not only gasoline and diesel fuel but also raw materials, such as light olefins for a number of petrochemical processes. The latter application is gaining increasing importance [1]. For the catalyst manufacturer, efficient catalyst development is only possible if new ideas and hypotheses can be rapidly evaluated. Therefore, an accurate screening of novel FCC catalyst materials is of utmost importance. ∗ Corresponding author. Tel.: +49-6241-403-312; fax: +49-6241-403-90368. E-mail address: [email protected] (D. Wallenstein).

For the estimation and prediction of the commercial performance of FCC catalysts it is also crucial to have the appropriate tools available. One convenient approach for evaluating the catalytic performance of FCC catalysts is microactivity testing. The steadily increasing requirements on this test have propelled the expectations of this technique far beyond its original application. The initial ASTM-microactivity test has been shown to produce data of limited precision and accuracy [2–5]. Therefore, researchers have sought to improve this test in several directions. A wide variety of reactor-types, including riser reactors [6], fixed fluidised bed and fixed bed reactors have been found to be applicable for microactivity testing [7]. Grace Davison developed a Riser Pilot Plant known as Davison’s Circulating Riser (DCR)

0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 2 ) 0 0 0 5 2 - 2

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pilot unit, for catalyst evaluation [8]. However, such units are expensive to operate and therefore, microactivity testing has remained an important screening tool. A new development in MAT-testing is the advanced catalyst evaluation (ACE) technique, which reflects the renewed interest in precise and accurate MAT-scale tools. ACE-testing is performed in a fixed fluidised bed reactor, the strength and weaknesses of this method are discussed in [7]. During the 1990s, Grace Davison developed the SCT-MAT technique to better simulate riser reactors. In this extensive development programme, the experimental settings of Grace Davison’s ASTM-type MAT, such as (i) reaction temperature, (ii) feed pre-heat, (iii) catalyst-time-on-stream, (iv) receiver, (v) catalyst bed geometry and (vi) reaction severity adjustment were modified and significant improvements were thus achieved [9,10]. No fundamental alterations were made with regard to the ASTM procedure of the separated collection of liquid and gaseous products, of the reactor-insert construction and of the transition state of the feed injection to the stripping phase and this work was focused on improving these items. In order to achieve this goal, an unconventional strategy was followed: a single receiver for the collection of both liquids and gases was developed, a reactor with no dead volume was designed and the pressure in the catalyst bed was increased. Experiments were performed with the original and novel SCT-MAT apparatus to contrast the results from the two approaches. The data clearly exhibited substantial improvements in precision and a more distinct differentiation of the catalytic properties of FCC catalysts by the improved SCT-MAT equipment.

2. Experimental 2.1. SCT-MAT equipment: modifications The individual hardware elements of the original SCT-MAT and the modifications performed are described and discussed below. The SCT-MAT equipment was thoroughly examined on parts whose modifications could lead to improvements: three elements, viz., the product collection with its three different compartments, reactor-insert design and feed injection were identified

and the separated product collection system for gases and liquids was deemed to be the most important part to be worth a redesign. This system has a variety of variables which can represent sources of error and therefore, impair precision. The product gas volume is determined by water displacement and cooling and condensation are performed by three different media; air cooled to 273 K, ice water and dry ice. Only the gaseous products trapped in the gas collection vessel are analyzed, whereas the amount of gaseous compounds in the receiver are calculated by use of a correlation describing the ‘rate of hydrocarbon transfer from the receiver to the gas collection bottle as a function of carbon number’. The syncrude contains a significant amount of liquified petroleum gas (LPG) which in turn complicates the simulated distillation analysis. The ASTM-reactor-insert construction has a void between insert wall and reactor wall. This void represents a dead volume which is not purged by nitrogen during the cracking experiment, entailing product losses by condensation of larger molecules at insert wall and reactor wall, in particular light cycle-oil (LCO) and heavy cycle-oil (HCO) range compounds. The third modification was the transition state from the feed injection to the stripping phase, where an interruption of feed injection into the catalyst bed occurs. In order to improve the above-described items, a new MAT apparatus was developed and Fig. 1 contrasts the improved and original SCT-MAT equipment. The figure clearly shows that the hardware of the original SCT-MAT was substantially simplified; this fact is the key to a more robust MAT operation with excellent precision. Briefly, the optimized MAT has a receiver for the collection of both, liquid and gaseous products. Only cooling by air (283 K) is used to condition the receiver to a constant temperature during the cracking experiment. The ASTM reactor insert construction was replaced by a reactor having a thread positioned directly above the catalyst bed. Both reactors are compared in Fig. 2 and this comparison shows that the entire volume of the new reactor is purged by nitrogen during the stripping step; no losses in LCO and HCO by condensation in dead volume zones can occur. A continuous feed injection without any interruptions was achieved by a nitrogen blanket over the feed which ensures (i) that the feed is injected completely into the catalyst bed prior to the switch to the stripping phase and (ii) that no reflux of feed into the nitrogen piping

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Fig. 1. MAT units: (a) novel SCT-MAT; (b) original SCT-MAT.

caused by backpressure from the evaporating and reacting feed in the catalyst bed can occur. In order to improve the discrimination of catalyst properties, the pressure and thus the concentration of hydrocarbons in the catalyst bed was enhanced by narrowing the reactor outlet. This modification enhances the kinetic chain

length (KCL); this expression is defined as the total rate of reactant conversion (initiation + propagation) divided by the rate of initiation reactions [11]. Propagation reactions are bimolecular reactions and therefore, the rates of such reactions rise at higher reactant concentrations. Differences in catalytic properties of

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restricted flow of products from the reactor into the receiver, the negative pressure changes very rapidly to a positive pressure following feed injection; the negative pressure in the catalyst bed prevails only about 1 s in the initial part of the cracking experiment. The benefits of the evacuated catalyst bed are enhanced feed vaporization and a better feed dispersion. The feed penetrates more rapidly into the catalyst bed and cracking reactions start in each part of the catalyst bed, roughly simultaneously. As a result more homogeneous temperature profile will be achieved. The restriction of the reactor exit entails a better flow control of products from the reactor into the receiver. Thus, the formation of channels in the catalyst bed is reduced, which in turn improves precision. 2.2. FCC catalyst types and catalyst deactivation Fig. 2. MAT reactors: (a) ASTM-MAT reactor insert construction; (b) new reactor.

FCC catalysts are primarily governed by secondary and bimolecular reactions and thus an increase in the KCL is expected to improve the discrimination of the catalytic properties between different catalyst types. In order to enable the collection of products without using the original concept of water displacement, a receiver with a volume of about 700 ml, evacuated prior to the cracking experiment was used and the following effects were expected from this modification: The negative pressure in the receiver facilitates the stripping of LPG from the syncrude fraction, which improves the precision of the simulated distillation analysis. The gas sample taken for analysis represents the C-number distribution in the whole product collection system and this accurate sampling improves both precision and accuracy. The reactor, including the catalyst bed, was evacuated prior to the cracking experiment and our hypotheses for commencing the cracking experiment under negative pressure are: Under the conditions of Grace Davison’s SCT-MAT, about 150 ml crack-gas is formed from the feed. However, only a space of about 3 ml is available for gas expansion in the catalyst bed and moreover the restricted reactor outlet (described earlier) prevents a quick escape of the crack-gas from the catalyst bed. Due to the small space available for gas expansion together with the

The FCC catalysts used in this work contained REUSY type zeolites dispersed in an alumina sol matrix. They varied in rare-earth levels and matrix type. These catalyst grades were chosen in order to investigate the potential of the novel SCT-MAT concerning an accurate catalyst ranking and discrimination of catalytic properties. Prior to microactivity testing the catalysts were deactivated in the presence of contaminant metals to unit-cell sizes, surface areas and activities typical for commercially equilibrated catalysts. The deactivations were performed by cyclic propylene steaming (CPS). The experimental conditions were as follows: the samples were calcined for 3 h at 813 K prior to loading with vanadium- and nickel-naphthenates. After impregnation, the organic components were burned off with air at 523 K for 3 h followed by 973 K for 3 h in a shallow bed. Thereafter, the samples were deactivated by CPS as described below • Heat-up phase: ◦ room temperature to 989 K at 8.6 K/min, N2 purge. • Two reduction–oxidation cycles as described below during the heat-up phase from 989 to 1077 K at 1.1 K/min: ◦ 30 min; 50%, 5% propylene in nitrogen + 50% steam; ◦ 2 min; 50% nitrogen + 50% steam;

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◦ 6 min; 50%, 4000 ppm SO2 in air + 50% steam; ◦ 2 min; 50% nitrogen + 50% steam. • 29 reduction–oxidation cycles as described above at 1077 K. • 30 min; 50%, 5% propylene in nitrogen + 50% steam. • Cool down under nitrogen flow. The coke-on-catalyst formed during CPS deactivation was burned off with air at 773 K, for 3 h in a shallow bed [12,13]. 2.3. Cracking experiments Cracking experiments were performed with the original SCT-MAT and the novel selectivity test apparatus, in order to contrast the results from the two approaches. 2.3.1. Original SCT-MAT method Complete details and a thorough discussion of this method are given in reference [9]. Briefly, the SCT-MAT has an annular fixed bed. To achieve different conversions necessary for the evaluation of product selectivities, the reaction severity was varied by changing the catalyst-to-oil (c/o) ratio. This ratio was varied by changing the mass of the catalyst while the amount of feed and catalyst time-on-stream were kept constant at 1.5 g and 12 s, respectively. The volume of the catalyst bed was 10 ml at each c/o ratio and this was achieved by diluting of the catalyst with glass beads of 0.2–0.3 mm in diameter. For each cracking experiment a new portion of catalyst and glass beads were used. The feed was pre-heated prior to injection into the catalyst bed to about 600 K by just passing the oil injection capillary heated by the oven; the oven temperature was 833 K. The feed which is still in the oil injection capillary after completion of feed injection by the syringe drive was injected into the catalyst bed by nitrogen purge starting with an over-pressure of 0.6 bar following feed injection. Thus a reflux of the feed into the nitrogen piping was prevented. After passing through the catalyst bed the liquid products were collected in a receiver with a volume of 180 ml. During feed injection the interior of the receiver was cooled by air of 273 K flowing through the internal tube of the receiver. The liquid recovery system was cooled with ice (receiver) and

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dry ice (trap); see Fig. 1. The gaseous products passed through the liquid products and receiver and were collected in a vessel over water. The catalyst bed and the liquid products were then stripped with a stream of 30 cm3 /min nitrogen to a total gas volume (product gas + nitrogen) of 600 ml which was determined by the amount of displaced water. During the stripping phase no cooling was performed to allow the products to warm up to room temperature and thus to enhance stripping efficiency. The gaseous compounds in the gas collection vessel were analyzed by taking a sample from this vessel whereas those in the receiver were determined by calculation using a correlation of ‘hydrocarbon transfer from the receiver to the gas collection vessel as a function of molecular weight’. This correlation was determined by analyzing the product gas in the gas collection vessel and receiver at different conversion levels and for different catalysts. The ratio of the compounds analyzed for the gas collection vessel content to the compounds analyzed for the receiver content generally decreased with increasing molecular weight; the ratios obtained for H2 , C1 , C2 , C3 , C4 , C5 and C6 were 4.1, 4.1, 4.6, 4.4, 2.8, 1.2, 0.7, respectively. These ratios were virtually the same at all conversion levels and for the different catalysts, i.e. these numbers represent constants. These ratios were used to calculate the concentration of the individual gases in the receiver and the calculated data were added to the compounds analyzed for the gas collection vessel. The properties of the feed are given in Table 1. 2.3.2. Modifications For the experiments performed with the novel selectivity test the following experimental conditions were the same as those described above: Feed, reaction temperature, catalyst time-on-stream, variation of c/o ratio and catalyst bed geometry. Different from the original protocol were the following items: the feed syringe contained 3 ml nitrogen and 2 ml feed and was oriented vertically so that the nitrogen was above the feed. Consequently, the syringe drive injected the feed, immediately followed by nitrogen injection and thus a continuous injection of the total feed quantity without any delay was achieved. Products were collected in a receiver with a volume of 700 ml. The receiver was conditioned to 283 K during feed injection and stripping phase. Receiver and

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Table 1 Feedstock properties API gravity at 289 K Aniline point (K) Sulfur (wt.%) Total nitrogen (wt.%) Conradson carbon (wt.%) Average molecular weight Caromatic (%) Cparaffinic (%) Cnaphthenic (%) D-1160 simulated distillation IBP (K) 5 vol.% (K) 20 vol.% (K) 40 vol.% (K) 60 vol.% (K) 80 vol.% (K) 95 vol.% (K) FBP K-factor

24.0 365 0.301 0.10 0.34 388 18.1 61.4 20.5 513 584 640 684 724 770 820 895 11.78

reactor were evacuated to 70 mbar prior to feed injection. During the cracking reactions the pressure in the reactor was enhanced by a restriction of the reactor outlet. Hydrocarbon stripping of catalyst bed and syncrude was performed with a stream of 30 cm3 /min nitrogen until atmospheric pressure in the receiver was achieved, i.e. the volume of the receiver represents the gas volume used for the calculation of the yields of the individual compounds in the gas-phase. The liquids were taken from the receiver by a syringe and thereafter, the receiver was rotated for 10 min prior to gas sampling in order to ensure a homogeneous distribution of the individual gaseous compounds in the receiver. 2.4. Analysis of products After the cracking experiment the FCC catalyst was separated from the glass beads by sieving and the coke on catalyst was determined by a carbon analyzer type CS-344 [14]. The gas-chromatographic methods employed for analysis of the products obtained from the cracking experiments, such as hydrogen, C1 –C6 gases, gasoline, LCO and HCO fractions, gasoline composition and gasoline octane numbers have been described previously [14]. The yields were calculated as weight

percent of reactant. The fractions of gasoline, LCO and HCO were determined by the cut points at 489 and 611 K. Conversion is defined as 100 wt.% ff(LCO, wt.% ff + HCO, wt.% ff). 2.5. Data analysis To simplify reporting, yields were compared at an interpolated constant conversion value rather than showing all the measured data by means of selectivity plots. The approach used for the evaluation of the cracking experiments and the computational method of calculating yields at constant conversion has been described previously [15]. Briefly, conversions and yields were modeled with functions derived from reaction kinetics using the c/o ratio as the independent variable. Interpolated yields at constant conversion were calculated with the parameters obtained by regression analysis of the experimental data with these functions. The interpolated data represent yields at conversions in the steep part of the exponential function describing the dependence of conversion on c/o ratio. Interpolations at higher conversions in the flat part of this curve could misrepresent the corresponding c/o values and influence the ranking of selectivities due to overcracking effects. 3. Results and discussion 3.1. Enhancement of the KCL Based on commercial experience, the product olefinicities obtained by the original SCT-MAT method are higher than in commercial FCC units and therefore, this development work was also focused on lowering the olefin selectivities. Decreasing product olefinicities indicate an increase in the KCL of feed molecules which in turn is expected to improve the differentiation of the individual product selectivities of FCC catalysts. In the presented work the product olefinicities were used to identify the changes in KCL. A decrease in olefin selectivity can be achieved by the following strategies: (i) lower reaction temperature, which favors exothermic H-transfer reactions and the exothermic formation of carbenium ions by protonation of olefins;

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(ii) longer oil injection times at constant feed mass, which increases the contact-time of the feed molecules with the catalyst, this however, also increases catalyst time-on-stream; (iii) increase of the contact-time of the feed molecules with the catalyst at constant catalyst time-on-stream. Items (i) and (ii) are steps back to the approaches used in selectivity testing with ASTM-type microactivity tests and fixed fluidised bed MAT units whose shortcomings were amply demonstrated [2–5]. Therefore, the strategy described by item (iii) was followed. Two routes are accessible for the realization of item (iii), either the increase of the catalyst bed diameter at constant catalyst bed height or the use of a lower feed mass at constant oil injection time. Experiments were performed following both approaches. The first one was realized by replacing the rod in the catalyst bed by glass beads. The weight hourly space velocities (WHSVs) were calculated by the sums of catalyst mass + glass bead mass. For the experiments following the second approach the feed quantity was reduced from 2 to 1 ml without changing the oil injection time of 12 s. The findings obtained by the two methods were compiled in Table 2. The data clearly showed that both modifications resulted in higher product olefinicities indicating that both modifications lowered the rates of hydrogen transfer reaction further. In both approaches the WHSV was reduced which in turn increased the contact-time of the feed molecules with the catalyst, and the longer contact-time was expected to increase hydrogen transfer. On the other hand, the concentration of hydrocarbons in the catalyst bed was lowered by either modification entailing lower gas-phase concentration of reactants and lower population of carbenium ions on the catalyst surface. Hydrogen transfer reactions are bimolecular; carbenium ions react with gas-phase molecules and therefore, hydrogen transfer rapidly slows down with a decrease in reactant concentration. Obviously, the concentration of hydrocarbons in the catalyst bed is a more crucial determinant for hydrogen transfer reactions than the contact-time of the feed molecules with the catalyst under the experimental conditions applied. Therefore, no increase in KCL was observed. In order to achieve higher H-transfer reaction rates, the partial pressure of hydrocarbons in the catalyst bed

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Table 2 Influence of ‘feed: FCC catalyst contact-time’ at constant catalyst time-on-stream on product olefinicity Variation of space velocity by variation of catalyst bed diameter (sample: equilibrium catalyst A) Physical properties Unit-cell size (Å) 24.27 Zeolite surface area (m2 /g) 126 33 Matrix surface area (m2 /g) MAT conditions Catalyst bed Feed quantity (ml) Feed injection time (s) WHSV (h−1 )

Annular 2 12 60

Cylindrical 2 12 33

Data interpolated at 65% conversion C3 olefinicity (%) C4 olefinicity (%) Gasoline olefinicity (%)

90.2 73.8 31.7

91.1 74.6 33.2

Variation of space velocity by variation of feed quantity (sample: catalyst A) Physical properties following CPS deactivation Unit-cell size (Å) 24.24 Zeolite surface area (m2 /g) 138 Matrix surface area (m2 /g) 39 MAT conditions Catalyst bed Feed quantity (ml) Feed injection time (s) WHSV (h−1 )

Annular 2 12 51

Annular 1 12 30

Data interpolated at 65% conversion C3 olefinicity (%) C4 olefinicity (%) Gasoline olefinicity (%)

89.0 74.4 30.1

90.6 76.2 34.8

was increased by reducing the diameter of the reactor outlet. Consequently, the stream of hydrocarbons from the catalyst bed into the product collection system was restricted and thus the pressure in the reactor, i.e. the concentration of hydrocarbons in the catalyst bed increased. Experiments were performed with several reactor outlet diameters and the impact of pressure on H-transfer reactions is demonstrated by the C4 olefinicities illustrated in Fig. 3 and the product slates at 65% conversion compiled in Table 3. Clearly, the product olefinicities decreased and aromaticity of the gasoline increased with decreasing diameter of the reactor-outlet; the increase in concentration of the hydrocarbons in the catalyst bed increased the rates of H-transfer reactions significantly. The

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Fig. 3. Effect of different reactor outlet diameters on C4 -olefinicity.

targeted olefinicities are also given in Table 3. The reactor-outlet diameter of 0.6 mm matched best these targets and this diameter was used for the experiments reported and discussed in the following sections. 3.2. Temperature profile in the catalyst bed A temperature profile measured during a cracking experiment in the catalyst bed of the novel selectivity test is illustrated in Fig. 4. The temperature of the feed after passing the oil injection capillary and before entering the catalyst bed was about 600 K which is in the range of commercial FCC unit and DCR operation. The feed/catalyst mixing temperature at the top of the catalyst bed (thermocouple was placed 5 mm below the top of the catalyst bed) was 803 K, 6 s after start of feed injection, which means that the temperature dropped by 30 K. The corresponding temperature drops in the middle and at the bottom of the catalyst

Fig. 4. Temperature profiles in catalyst bed.

bed were 12 and 9 K, respectively. The temperature profile suggests that this fixed bed provides an isothermicity equivalent to riser and fixed fluidised bed reactors where due to the endothermic cracking reactions, temperature drops also exist. 3.3. Precision The relative standard deviations and concentration of LPG in syncrude obtained for the original SCT-MAT and novel SCT-MAT are compiled in Table 4. The number of measurements used in either method was 50. The syncrude obtained by the novel SCT-MAT is virtually free of LPG and on average the scatter could be reduced by 38%.

Table 3 Influence of pressure in the catalyst bed, adjusted by the diameter of the reactor outlet, on the reaction rates of hydrogen transfer in the original and the novel MAT methodsa

C3 olefinicity (%) C4 olefinicity (%) Gasoline olefinicity (%) Gasoline aromaticity (%) Coke (wt.% ff) Hydrogen (wt.% ff) a

Original method

Novel method (increasing pressure →)

2 mm

2 mm

87.5 68.1 30.4 29.0 1.8 0.13

Data interpolated at 65% conversion 90.2 87.3 73.8 66.7 31.7 26.3 30.8 32.7 2.0 2.0 0.10 0.11

Sample: equilibrium catalyst A (for physical properties see Table 2).

0.6 mm

Target 0.4 mm 81.4 59.6 21.6 35.2 2.2 0.12

∼83 ∼64 ∼28 ∼30 ∼2.2 ∼0.11

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Table 4 R.S.D. of data interpolated at 65% conversion and LPG concentration in syncrudea

Conversion (%) Hydrogen (wt.% ff) Coke (wt.% ff) C1 + C2 (wt.% ff) C3 olefinicity (%) C4 olefinicity (%) LPG (wt.% ff) LPG olefinicity (%) Gasoline (wt.% ff) LCO (wt.% ff) HCO (wt.% ff) LPG in syncrude (%) a b

Novel SCT-MAT (nb = 50)

Current SCT-MAT (nb = 50)

Improvement (%)

1.6 4.4 2.5 2.8 1.9 1.8 1.9 1.3 0.6 1.2 1.3 <0.1

2.5 6.5 6.5 4.5 3.0 2.5 2.5 1.5 1.0 2.1 1.9 2

36 32 61 37 37 28 24 14 40 43 32

Sample: equilibrium catalyst A (for physical properties: see Table 2). n: Number of measurements.

This significant improvement in precision is attributed to the following: • The simplification of the product recovery system, i.e. the reduction in sources of error. • The restriction of the reactor outlet which ensures a more controlled flow of hydrocarbons from the reactor into the receiver and thus the formation of channels in the catalyst bed is reduced. • The dead-volume-free reactor which prevents losses of LCO and HCO by condensation, as it is on the walls of the ASTM-reactor insert construction. • The collection of all products in one receiver, i.e. the gas sample taken for analyses represents the accurate distribution of the individual products in the whole MAT unit. Such an accurate sampling cannot be obtained by ASTM-type- and by the original SCT-MAT or by small-scale FFB units. • The syncrude is virtually free of LPG, which improves the precision of the simulated distillation analysis. 3.4. Applications In order to verify the novel SCT-MAT method and to analyze its potential in terms of the degree of differentiation of catalytic properties, catalyst types whose catalytic performance is well understood were tested in either SCT-MAT method. Changes in product selectivities as a function of the concentration of rare-earth on zeolite have been

described in several publications [16,17] and the interpretation of such data is accepted. There is also ample commercial experience with different matrix types in FCC catalysts [18,19]. Therefore, the variation of rare-earth and matrix are appropriate means to validate MAT test methods. Four catalysts only differing in the concentration of rare-earth on zeolite, and six catalysts only differing in matrix type were tested in either method. Prior to microactivity testing, the samples were deactivated by CPS in the presence of metals to unit-cell sizes typical for commercially equilibrated catalysts. 3.4.1. Case study I: variation of rare-earth concentration The key physical properties of the catalysts compared in this section and their product selectivities interpolated at constant conversion are given in Table 5. The following trends were observed: The unit-cell sizes following deactivation increased with increasing rare-earth content. In each test protocol the cracking activity was seen to increase with increasing unit-cell size and the degree of differentiation was similar. The LPG and gasoline olefinicities decreased with increasing unit-cell size in either method and the data demonstrate that the novel selectivity test provides a better discrimination than the original SCT-MAT method. The changes in gasoline olefinicity were accompanied by the expected changes in gasoline i-paraffins. The increase in the rates of H-transfer reactions at high unit-cell size was also reflected by

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increasing gasoline aromaticity. Gasoline selectivity increased and LPG selectivity decreased with increasing unit-cell size and the data displayed a moderate improvement in differentiation of catalyst properties for these selectivities by the novel selectivity test, too. Graphical representations of product olefinicity, gasoline and LPG yields are shown in Figs. 5–7. Fig. 5 suggests that the novel selectivity test discriminated better product olefinicity than the original SCT-MAT over the whole conversion range. Figs. 6 and 7 also demonstrate the better differentiation of LPG and gasoline selectivity achievable by the novel MAT.

Fig. 6. Influence of rare-earth concentration on LPG selectivity. Selectivity plots obtained by the: (a) novel SCT-MAT; (b) original SCT-MAT.

Fig. 5. Influence of rare-earth concentration on C4 -olefinicity. Selectivity plots obtained by the: (a) novel SCT-MAT; (b) original SCT-MAT.

The changes in reaction pathways as a function of rare-earth content are relevant for this work. Therefore, some of the key findings obtained by previous investigations [16,17] are briefly reviewed: During hydrothermal deactivation low rare-earth catalysts are more strongly dealuminated than high rare-earth catalysts, hence, they have fewer active sites resulting in a lower catalyst activity. The higher degree of dealumination attenuates the interaction between framework aluminium atoms and therefore amplifies the acid strength of the active centers [20]. Protolytically induced reactions are enhanced by increasing acid strength, consequently, more methane and ethane are

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Fig. 7. Influence of rare-earth concentration on gasoline selectivity. Selectivity plots obtained by the: (a) novel SCT-MAT; (b) original SCT-MAT

formed. Higher unit-cell sizes increase acid site density and thereby the population of carbenium ions at the catalyst surface. At high unit-cell size the acid strength of the active centers is lower, which increases the residence times of adsorbed carbenium ions on the active centers favoring bimolecular H-transfer over cracking reactions. Since the ␤-scission loop is terminated at higher molecular weight, gasoline yield is increased; as a consequence the formation of C3 and C4 compounds decreases. The ranking of catalysts obtained by either MAT method followed the trends described in the literature, whereby this differentiation was more accentuated by the novel selectivity test.

3.4.2. Case study II: variation of matrix type The key physical properties and selectivities of the catalysts compared in this section are compiled in Table 6. The following trends were observed: The unit-cell sizes of the catalysts used in Comparison III (samples H, I) and Comparison IV (samples K, L) following deactivation were the same due to the same rare-earth content. The catalysts formulated with the additional matrix compounds (samples I, L) had higher matrix surface areas than the counterparts with no added matrix; the zeolite surface areas of the corresponding samples with no added matrix were similar. The data obtained by microactivity testing showed the following shifts caused by matrix addition: The gasoline olefinicity increased in the novel selectivity test in Comparisons III and IV, whereas no changes in gasoline olefinicity was obtained by the original method. The data in Table 6 and Figs. 8 and 9 explicitly reveal that bottoms cracking differentiation is more accentuated by the novel SCT-MAT technique. These findings are consistent with the literature which says: the incorporation of special matrices into FCC catalysts improves bottoms cracking [18]. In catalysts with no added matrix the cracking of larger molecules (bottoms cracking) is more influenced by cracking them in the mesopores of the zeolite rather than by matrix cracking. In case of matrix incorporation into FCC catalysts the surface area available for the cracking of large molecules is increased, which enhances the rate of pre-cracking of the feed by the matrix. Thus, the diffusion of molecules to the zeolite is improved, which entails a more effective bottoms cracking. An enhanced matrix contribution to the overall reaction rate increases olefin production [19] without consuming the olefins by hydrogen-transfer reactions; matrix components do not have H-transfer activity. Collectively, matrix addition enhances bottoms cracking in either protocol. The experimental modifications in the novel selectivity test, such as better feed vaporization and feed dispersion, and the higher reactant concentration in the catalyst bed are presumed to be the reasons for the better differentiation because mass transport is improved. Moreover, the improved precision certainly contributes to the better differentiation as well.

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Table 6 Differentiation of FCC catalysts by the novel and original SCT-MAT method; Influence of matrix modification on activity and product distribution Comparison III Catalyst H

Comparison IV Catalyst I

 Relative (%)

Catalyst K

Catalyst L

 Relative (%)

LCO yields at 65% conversion (wt.% ff) Novel MAT 19.9 Original MAT 18.2

20.7 18.8

4.0 3.3

19.6 17.8

20.4 18.4

4.1 3.4

HCO yields at 65% conversion (wt.% ff) Novel MAT 15.1 Original MAT 16.8

14.3 16.2

−5.3 −3.6

15.4 17.2

14.6 16.6

−5.2 −3.5

Percentage olefins in gasoline at 65% conversion Novel MAT 24.8 26.5 Original MAT 35.7 34.5

6.9 −3.4

20.5 34.1

23.4 31.9

14.1 −6.5

24.31 127 26 2.7 41.5

24.31 133 51 2.8 47.7

Catalyst properties following metallation to 3000 ppm V + 2000 ppm Ni, CPS UCSa (Å) 24.26 24.26 Z-SAb (m2 /g) 157 139 M-SAc (m2 /g) 30 61 1.1 1.1 RE2 O3 (wt.%) Al2 O3 (wt.%) 42.5 48.3 a

Unit-cell size. Zeolite surface area. c Matrix surface area. b

3.5. Reactant concentration and reactant temperature For a better understanding of the presented findings the discussion was extended to the influence of temperature on FCC reactions although this parameter was not altered in this work. The increase in reaction temperature of approximately 60 K was one of the essential modifications when switching from the ASTM-type MAT to the original SCT-MAT method reported previously [9,10]. The higher temperature provided advantages in terms of a better feed vaporization which reduced coke formation by condensation reactions of poorly vaporized feed molecules. Both the higher temperature and the resulting lower coke formation enhanced the diffusion of feed molecules. A disadvantage might be a decline of exothermic reactions which could lower the discrimination of the catalytic properties of FCC catalysts in SCT-MAT operation in terms of the yield differences. In the novel SCT-MAT method the temperature was not changed to keep the advantages of better feed vaporization, lower coke formation and thus, higher diffusion rates. In order to improve the differentiation of catalysts, the pressure in the catalyst bed was enhanced (this

operational parameter is a proxy for the concentration of feed molecules in the catalyst bed). Consideration of these two operational parameters in context of their influence on overall activity and selectivities led to the following interpretation: Initiation of reactions by protolysis of feed molecules and the rate of ␣- and ␤-scission of the protolysed molecules are boosted at higher temperatures resulting in the formation of olefins from ␤-scission. The protonation of these olefins, in turn, i.e. the formation of carbenium ions is decelerated with increasing temperature because this reaction is exothermic. Propagation reactions, such as disproportionation reactions of adsorbed protolysed molecules with gas phase molecules, also decrease at higher temperatures. For these reasons the variation of temperature affects feed conversion in the following manner: increased temperature enhances the overall conversion of gas–oils by enhancing the rates of protolysis of paraffins and decomposition reactions as ␣- and ␤-scission. On the other hand, propagation reactions of the products decrease, and thus the KCL of feed molecules is shortened. The protonation of olefins formed by ␤-scission decreases, and therefore the KCL of these ‘chain transfer compounds’

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Fig. 8. Influence of SAM-100 addition on bottoms cracking. Selectivity plots of LCO obtained by the: (a) novel SCT-MAT; (b) original SCT-MAT.

Fig. 9. Influence of matrix modification on bottoms cracking. Selectivity plots of light cycle-oil obtained by the: (a) novel SCT-MAT; (b) original SCT-MAT.

also becomes shorter. These considerations suggest a better differentiation in terms of catalyst activity at higher temperature, but due to the shorter KCLs a less perceptible catalyst differentiation with regard to the LPG and gasoline fraction as well as to product olefinicity might be obtained. The better differentiation of activity was clearly observed at the 60 K higher reaction temperature in the original SCT-MAT vis a vis that of ASTM-type microactivity testing. The differentiation of the individual selectivities was influenced by the changes in temperature and catalyst-to-oil adjustment procedure (severity effects) and therefore, the influence of temperature

alone on product distribution could not be discussed in detail. The increase of the concentration of hydrocarbons in the catalyst bed was quantified in Case Studies I and II and the findings can be understood as follows: The propagation term is of second order and at a similar coverage of active sites by activated hydrocarbon species, the gas phase concentration of hydrocarbons, i.e. the pressure in the reactor should influence the reaction rates by a first order law. In fact, the product olefinicities in the presented catalyst comparisons indicated higher rates of propagation reactions when increasing the concentration of hydrocarbons in the

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reaction zone. This implies an increase in the KCL of reactants in the novel selectivity test which in turn is responsible for the more pronounced discrimination of product olefinicity and product selectivities (LPG, gasoline) for the different catalyst types. Bottoms cracking is more governed by initiation reactions which are dependent on the first power of gas-phase feed concentration. Therefore, the more distinct differentiation in bottoms cracking by the novel MAT can be partially attributed to the higher pressure, too. Thus, in the novel SCT-MAT the beneficial effects of high temperature and high reactant concentration could be combined and the differentiation potential of the original SCT-MAT could be further improved.

4. Conclusions A novel selectivity test has been developed which provides a better discrimination of the catalytic properties of FCC catalysts as well as an improved precision vis a vis the original SCT-MAT method. This novel selectivity test is operated with (i) one compartment for the collection of both liquids and gases, (ii) a reactor in which no product losses by condensation in ‘dead volume zones’ can occur and (iii) continuous feed injection. A better feed dispersion in the catalyst bed was achieved by increasing the pressure differential between the feed injection capillary exit and reactor. These modifications account for the improvements in precision. Moreover, the experimental setup of the novel selectivity test was streamlined to the fundamental requirements for catalyst testing. Each part which was deemed to present a source of error was removed or changed and this reorganization also promises a better precision. In order to accentuate the differentiation of catalysts further, the pressure in the reactor was increased which translates to a higher concentration of hydrocarbons in the catalyst bed. Thus, the KCL of reactants was increased, and by the same token a better differentiation for product olefinicities, LPG- and gasoline selectivity of different FCC catalyst types was achieved. The fundamental advantage of the novel selectivity test over other microactivity testing techniques is that the better differentiation was simply achieved by increasing the pressure, i.e. by increase of the reactant concentration in the catalyst bed. This modification in operational conditions further moves

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the novel selectivity test closer to DCR and commercial operation where the reactions take place at higher hydro-carbons partial pressure as well. This concept has never been applied before, and therefore, represents a breakthrough in microactivity testing. In contrast to other micro-scale tests, the increase in KCL was achieved on relatively fresh FCC-catalysts where the impact of catalyst decay on product selectivities plays a subordinate role. Other microactivity test techniques, such as ASTM-type MAT and fixed fluidised bed (ACE [21], FACT [22,23]) testing, involve longer feed injection times. In these approaches the reactions are more diffusion-controlled rather than kinetically controlled which means that these approaches involve selectivity changes due to increasing steric constraints by coke formation. The fact that fixed fluidised bed testing is performed under atmospheric pressure might additionally increase the disparity between this technique and riser testing. The novel selectivity test also provides economic benefits. The experiments are automated, and 12 MAT runs per 8 h shift can be performed on a routine basis. Reactor and receiver, that means all MAT parts in which cracking reactions and product separation proceed, are cleaned after each experiment, which ensures high reproducibility of experiments. Moreover, the periodically necessary maintenance work which has to be performed for ASTM-type and FFB-MAT units is therefore obviated. The novel selectivity test also provides the option of further reducing the catalyst time-on-stream by simply decreasing the feed mass at constant feed injection rate, i.e. this MAT equipment provides a high versatility. References [1] J. Biswas, J.E. Maxwell, Recent process and catalyst-related development in FCC, Appl. Catal. 63 (1990) 197–258. [2] A.V. Sapre, T.M. Leib, Translation of laboratory fluid cracking catalyst characterization tests to riser reactors, ACS Symp. Series FCC2 (1991) 144–161. [3] J.L. Mauleon, J.C. Courcelle, FCC heat balance critical for heavy fuels, OGJ 21 (1985) 64–70. [4] G.W. Young, Realistic assessment of FCC catalyst performance in the laboratory, Stud. Surf. Sci. Catal. 76 (1993) 257–292. [5] A. Corma, A. Martinez, L.J. Martinez-Triguero, Limitations of the microactivity test for comparing new potential cracking

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