Article pubs.acs.org/IECR
Application of Inverse Gas Chromatography for Diffusion Measurements and Evaluation of Fluid Catalytic Cracking Catalysts Dieter Wallenstein,* Christoph Fougret, Stefan Brandt, and Ulrike Hartmann Grace GmbH & Co. KG, Postfach 1445, 67545 Worms, Germany ABSTRACT: The diffusion rates of feedstocks and intermediate products in fluid catalytic cracking (FCC) catalysts have relevance for the product selectivities obtained from the conversion of feedstocks in the FCC process. A simple and robust method was developed to determine the diffusion of probe molecules in such catalysts by means of inverse gas chromatography. This procedure is described in detail, and its relevance in FCC is illustrated by several examples. It is shown that catalytic properties of FCC catalysts can be rationalized with the diffusion coefficient parameter in cases where traditional analytical tools (nitrogen sorption and X-ray diffraction) reach their limits.
1. BACKGROUND Fluid catalytic cracking (FCC) catalyst particles have on average a diameter of 70 μm containing around 20−40% Y zeolite crystals of about 1 μm diameter dispersed in a matrix.1 The Y zeolite is the main catalytically active component in the FCC catalyst. Regarding its pore structure, it has openings of about 7.4−12 Å supercages, allowing only penetration of molecules smaller than 10.2 Å.2 During their life in FCC units, the catalysts undergo significant changes. The catalysts are exposed to a hydrothermal atmosphere and contaminant metals, which degrade both the zeolite and matrix structures of the catalysts.3,4 In order to counteract the deleterious effects of contaminant metals and hydrothermal conditions on the FCC catalyst performance, the catalyst is continuously withdrawn from FCC units and replaced by a fresh catalyst, resulting in a catalyst inventory with an age distribution.4 Hence, catalysts taken from an FCC unit inventory are in dynamic equilibrium and are commonly known as equilibrium catalysts. As a consequence of these catalyst withdrawal and addition rates, continuums of pore openings and pores of different sizes evolve in the catalyst inventory of FCC units, whereby the pores ascribed to the mesopores of the zeolite (20−500 Å) are enlarged with the particle age. Such a continuum of the poresize distribution, determined with nitrogen sorption on an equilibrium catalyst separated by density separation in age fractions,5 is illustrated in Figure 1. The pore structure displayed originates from two different catalyst components, the zeolite and the matrix. The zeolite mesopores that evolve during the degradation process are represented by the peaks between 100 and 1000 Å. The matrix contributes to the whole >50 Å region, and in the given example, its contribution is pronounced in the 50−100 Å range. The peaks in the 20−50 Å region are ascribed to the tensile strength effect.6 In the example shown, the pores of the zeolite evolving from © 2016 American Chemical Society
Figure 1. Pore-size distributions (nitrogen sorption) of age fractions of an equilibrium catalyst, demonstrating the impact of the FCC catalyst residence time in the FCC units on the pore-size distribution.
degradation increase, whereas those of the matrix decrease after an initial increase with the catalyst particle age. Recent efforts have been published to characterize the changes to the pore system by highly sophisticated methods like synchrotron-based IR spectroscopy7 and ptychographic−Xray tomography.8 While these investigations provide insight into the fundamental changes of the pore system during the lifetime of selected FCC catalyst particles, those methods lack quantitative information about the pore system structure of the bulk equilibrium catalyst representing all age fractions. The diffusion in equilibrium catalysts is classified as three distinct resistances to mass transfer depending on the pore Received: Revised: Accepted: Published: 5526
February April 21, April 22, April 22,
2, 2016 2016 2016 2016 DOI: 10.1021/acs.iecr.6b00470 Ind. Eng. Chem. Res. 2016, 55, 5526−5535
Article
Industrial & Engineering Chemistry Research size:9 (1) micropore resistance of the zeolite; (2) mesopore and macropore diffusional resistance ascribed to the matrix; (3) resistance related to the outer layer of the zeolite (surfacebarrier resistance). The pore-size distribution illustrated in Figure 1 shows the pore structure of equilibrium catalysts in the mesopore and macropore ranges. The pore sizes of the zeolite and matrix are overlapping, rendering discrimination of their individual contributions to zeolite and matrix diffusion in equilibrium catalysts difficult. In the FCC process, diffusion is supported by flow and concentration gradients; this kind of diffusion is the so-called transport diffusion.10 The diffusion rates obtained from inverse gas chromatography (iGC) represent such a diffusion type. iGC refers to interaction of the stationary phase with a pulse of probe molecules moving through it in a carrier gas flow. This technique is named inverse gas chromatography (iGC) because, unlike in the classical gas chromatography (GC), the stationary phase of the system is the object of investigation. iGC is described as an appropriate tool for measuring transport diffusion9,10 because the diffusion is supported by concentration gradients and the flow of the carrier gas; i.e., iGC measurements mimic the FCC conditions to a certain degree. The most common methods for the determination of diffusion in FCC catalysts do not provide such a conceptually good applicability for FCC catalysts. Techniques like pulsedfield-gradient nuclear magnetic resonance and quasi-elastic neutron scattering measure self-diffusion.10 The uptake rate technique (sorption rates in batch systems) measures transport diffusion, as is done by GC methods. However, it is a static measurement and suffers from resistances at the external surface area of the zeolite crystals to heat and mass transfer,2,10 which are less pronounced in fast-diffusing systems like the FCC process. In contrast to such uptake measurements, the high gas flow in GC methods (like iGC) helps to minimize the impact of transport resistances at the external surface area of the zeolite crystals. A GC method that has attracted considerable attention is the so-called zero-length-column gas chromatography (ZLC) method.11 The main concern is the very small sample quantity used in this technique because a certain amount of equilibrium catalyst is required to represent its individual fractions (additives and age fractions) correctly. Determination of the Thiele modulus describing the relationship between diffusion and the reaction rate and applied for determination of the catalyst effectiveness factor12 best reflects the FCC process; the disadvantage of this method is its complexity regarding testing and evaluation. According to these considerations, iGC appears to be the most appropriate method for diffusion investigations on equilibrium FCC catalysts because it is a transport-diffusion-type measurement simulating diffusion in the respective commercial process and it measures the properties of the bulk equilibrium catalyst across all age fractions. The measured diffusion in FCC catalysts describes diffusion through the pore space of porous media. It is macroscopic in nature because it does not consider the individual pores but only transport-available pores. This equals the total pores minus the pores that, because of their size, are not accessible to the diffusing particles and minus dead-end and blind pores (i.e., pores without being connected to the rest of the pore system). In this body of work, the diffusion coefficients were measured with an iGC method optimized in terms of the probe molecule, column geometry, catalyst pretreatment, catalyst quantity,
carrier gas, and temperature. The diffusion coefficients obtained with this method correlated well with the catalytic findings. Moreover, the catalytic data could even be rationalized in cases where no reasonable interpretations could be derived based on the levels of surface areas, pore-size distributions, and unit cell sizes of the compared catalysts.
2. EXPERIMENTAL SECTION 2.1. Diffusion Measurements. 2.1.1. Instrumentation. The chromatographic experiments were performed with an HP 6890 gas chromatograph equipped with a flame ionization detector (FID), electronic pressure control for maintaining the correct flow rates, and automated sample injection. Helium was used as the carrier gas. Quartz glass columns with the following dimensions were used: length, 120 mm; external diameter, 8 mm; internal diameter, 3 mm. 2.1.2. FCC Catalyst Samples. The FCC catalyst samples used were equilibrium catalysts from commercial applications in different refinery’s FCC units. The respective fresh catalysts were formulated by combining rare-earth-exchanged Y zeolites, clay, special alumina matrixes, and an alumina solid binder followed by spray drying and calcination. More details are given in ref 13. The properties of these equilibrium catalyst samples are provided in Tables 1−6. The differences in their properties are attributed to differences in the deactivation conditions inside the commercial process units and to their catalyst design. 2.1.3. FCC Catalyst Pretreatment. Prior to diffusion measurements, the equilibrium catalyst samples were heated in air for 3 h at 540 °C in a fixed bed. With this procedure, catalyst inhibitors like coke and nitrogen compounds were removed from the sample. 2.1.4. Column Packing and Sample Conditioning. The columns were packed with 270 mg of FCC catalyst, fitted in the oven of the gas chromatograph, and subsequently conditioned at 300 °C for 30 min under a carrier gas flow. Thereafter, the samples were preequilibrated by 15 injections of the probe molecule. 2.1.5. Diffusion Measurements. 1,2,4-Trimethylcyclohexane and phenanthrene dissolved in carbon disulfide (5 and 8 wt %, respectively) were used as individual probe molecules. The injection volume was 0.5 μL. For each molecule, the retention times and full widths at halfmaximum values of the peaks were determined at five different flow rates between 30 and 65 mL/min. The measurements with 1,2,4-trimethylcyclohexane were performed at 200 °C oven temperature and those with phenanthrene at 350 °C. These temperatures are above the respective boiling point of the probe molecules, and thus it is ensured that the molecules are in the gas phase. The diffusion coefficient was measured at temperatures much lower than those typically used in FCC operation (>500 °C) because under FCC temperatures the probe molecules would be converted to other products, rendering diffusion measurements impossible. For the same reason, a paraffinic and an aromatic compound were chosen because of their lower reactivity compared to olefinic compounds. For determination of the average linear velocity of the carrier gas (see paragraph 3, eq 4), the retention times of methane were measured for each flow rate and temperature at which the probe molecule experiments were conducted; methane is 5527
DOI: 10.1021/acs.iecr.6b00470 Ind. Eng. Chem. Res. 2016, 55, 5526−5535
Article
Industrial & Engineering Chemistry Research
3. CALCULATIONS The concept of peak broadening was used to determine the diffusion coefficients. The most widely employed theory of peak broadening in GC for packed columns is that proposed by van Deemter, Zuiderweg, and Klinkenberg.15 The well-known equation16 is given in eq 1 and shown in Figure 2.
considered to be a nonadsorbing and non-diffusion-hindered molecule. The diffusion measurement conditions and probe molecules given in sections 2.1.3−2.1.5 resulted from experiments performed for determination of the most appropriate test parameters in terms of the peak symmetry, peak size, and catalyst differentiation. 2.2. Catalytic Experiments. Four different case studies were performed within the scope of this work. The catalytic properties of the equilibrium catalysts were determined with an Advanced Cracking Evaluation (ACE) unit.14 Each catalyst was tested at six different catalyst-to-oil ratios ranging from 3.5 to 7.5 g/g to determine conversion and yield response curves. The catalyst-to-oil ratios were varied by changing the mass of the catalyst, while the total amount of the feedstock and time-on-stream were kept constant at 1.5 g and 75 s, respectively. The catalyst bed temperature was adjusted to 540 °C. GC methods were used for analysis of both the gaseous and liquid products. Hydrogen and C1−C6 products were analyzed with an Agilent HP7890 gas chromatograph equipped with a thermal conductivity detector for hydrogen analysis and an FID for hydrocarbon analysis. The liquid products were analyzed with an Agilent HP7890 gas chromatograph for the fractions of gasoline, light cycle oil, and heavy cycle oil using the Simulated Distillation ASTM D2887 method and cut points at 221 and 338 °C. The yields were calculated as the weight percent of the feedstock. Conversion is defined as
HETP = A + Bμ−1 + C μ
(1)
Figure 2. van Deemter plot reproduced from the literature19 demonstrating the three individual terms of the van Deemter equation.
where HETP = height equivalent to a theoretical plate [cm], μ = average linear velocity of the carrier gas [cm/s], A = eddy diffusion, B = longitudinal diffusion, and C = mass-transfer resistance in the stationary phase. Because B/μ converges to zero at higher average carrier gas velocities, the equation simplifies to the linear equation (2) in the range of high μ, from which the parameter of interest, the diffusion coefficient of FCC catalysts, is derived.
100 wt % − (light cycle oil, wt % + heavy cycle oil, wt %)
The data reported in this paper were obtained from interpolations at constant conversion. The feedstocks used for the different studies are compiled in the respective tables. 2.3. Physical Characterization Methods. 2.3.1. FCC Catalysts. The surface areas and pore-size distributions of the catalysts were determined by nitrogen sorption using a Micromeritics Tristar 3000 unit. The zeolite and matrix surface areas were calculated by the t-plot method (Harkins/Jura) using the pressure range p/p0 = 0.06−0.35. The nitrogen pore volume was calculated from the volume of nitrogen adsorbed at the pressure point of p/p0 = 0.97. The pore-size distribution was calculated from the desorption branch according to Barrett, Joyner, and Halenda. The particle size distribution and average particle size were determined by laser diffraction with a LS13320 apparatus from Beckmann Coulter applying Fraunhofer theory. The unit cell size of zeolite Y was determined by X-ray diffraction using a Bruker AXS D8 Advance analyzer applying Rietveld refinement and whole powder pattern decomposition techniques. The water pore volume was determined by adding water to a dry sample of a given weight. The water between the particles was removed by centrifugation. The water pore volume represents the volume of water that displaces the air in the pores. With this method, the sum of the micropore, mesopore, and macropore volumes is measured. 2.4. Density Separation. Separation of equilibrium catalysts into age fractions was performed by density separation in a sodium metatungstate solution, as described in ref 5. 2.5. Feedstocks. The percentage of carbon atoms in aromatic structures (Caromatic) was calculated according to ASTM D 3238-95.
HETP = A + C μ
(2)
HETP characterizes the separation efficiency of GC columns and is related to the column length and the ratio of peak broadening to retention time. This parameter is calculated according to eq 3.16 HETP = Catblb0.52(8 ln 2)−1(tr prmol)−2
(3)
where Catbl = catalyst bed length [cm], b0.5 = full width at halfmaximum values of the peak [s], and tr,prmol = retention time probe molecule [s]. The van Deemter model is a continuative consideration of the plate theory. It involves the dynamic response of HETP as a function of the average linear velocity of the carrier gas (see Figure 2), thereby allowing one to distinguish the three diffusion types: eddy diffusion, longitudinal diffusion, and masstransfer resistance. The average linear velocity μ is calculated according to eq 4.
μ = Catbl(tr,methane)−1
(4)
where Catbl = catalyst bed length [cm] and tr,methane = retention time methane [s]. The diffusion process of interest in FCC catalysts is characterized by the slope of the linear part of the van Deemter model as described above. HETP is therefore measured at several flow velocities μ in the linear part of the van Deemter model, allowing estimation of the slope C by 5528
DOI: 10.1021/acs.iecr.6b00470 Ind. Eng. Chem. Res. 2016, 55, 5526−5535
Article
Industrial & Engineering Chemistry Research linear regression. A corresponding example is given in case study I. The diffusion coefficients are derived according to eq 5. D = 16APS2 k[π C(1 + k)]−1
Table 1. Reproducibility of iGC Measurements Catalyst A Diffusion Coefficients iGC probe molecule 1,2,4-trimethylcyclohexane no. of measurements 24 diffusion coefficient, mean (cm2/s) 1.1 × 10−2 relative standard deviation (%) 3 iGC probe molecule phenanthrene no. of measurements 24 diffusion coefficient, mean (cm2/s) 3.0 × 10−3 relative standard deviation (%) 6 Physical and Chemical Properties surface area (m2/g) 151 unit cell size (Å) 24.27 water pore volume (mL/g) 0.39 average particle size (μm) 88 Al2O3 (wt %) 45.1 RE2O3 (wt %) 1.4 Na (wt %) 0.24 V (mg/kg) 1007 Ni (mg/kg) 514 Fe (wt %) 0.45
(5)
where D = diffusion coefficient [cm2/s], APS = average particle size [cm], and k = (tr,prmol [s] − tr,methane [s])/tr,methane [s] with tr,prmol and tr,methane from a flow rate of 0.667 cm3/s. More details on the van Deemter model and its application for the determination of diffusion can be found in refs 2 and 17−19.
4. RESULTS AND DISCUSSION The following investigations were performed to validate the iGC method: (i) applicability of the van Deemter model for diffusion measurements on FCC catalysts; (ii) reproducibility of the iGC method; (iii) impact of the molecule size and temperature on diffusion. Following these fundamental investigations, four case studies were performed reflecting different events in FCC units such as catalyst changes and the severity of hydrothermal deactivation. For this purpose, corresponding sets of equilibrium catalysts were characterized on diffusion properties, catalytic performance, and conventional physical properties such as the zeolite and matrix surface areas, pore-size distribution, particle size distribution, water pore volume, and unit cell size of the zeolite Y component. Regarding the discussion of the catalytic data in relation to the catalyst diffusion properties, the size of the probe molecule 1,2,4-trimethylcyclohexane is representative for the diffusion of product molecules of similar or smaller sizes such as C1−C4 and gasoline. Therefore, only these products and hydrogen transfer in the C4 range at constant conversion are discussed. The catalyst activity was not evaluated in this respect because the corresponding conversions are calculated from product fractions with much higher boiling points (>338−600 °C) than that of the probe molecule (142 °C). 4.1. Applicability of the van Deemter Model for Diffusion Measurements on FCC Catalysts. HETPs were determined over the full flow range applicable for the GC equipment used for this work (10−100 mL/min) on an equilibrium catalyst (catalyst A, Table 1). The chromatograms of the probe molecule and of methane obtained from iGC analysis are shown in Figure 3. For the sake of clarity, only each second peak in the intermediate flow range (30, 40, 50, 60, 70, 80, and 90 mL/min) is given. The shapes of the peaks demonstrate a high symmetry, indicating that the concentrations of the molecules in the gas phase are in the infinite concentration range, i.e., in Henry’s law region. Here interactions of the molecules are assumed to be negligible, and the GC signals (peaks) obtained are only affected by molecule− FCC catalyst interactions. Such conditions are required for an accurate determination of the diffusion coefficients.19 The van Deemter plot generated from these data is illustrated in Figure 4. These experimental data show that the three diffusion types reported for the van Deemter model (eddy diffusion, longitudinal diffusion, and mass-transfer resistance) were obtained. The diffusion type of interest is the mass-transfer resistance representing the interaction between molecules (mobile phase) and the stationary phase (FCC catalyst). Figure 4 demonstrates that the experimental data cover a wide range for this diffusion type. Thus, these
Figure 3. GC chromatograms of the (a) probe molecule 1,2,4trimethylcyclohexane and of the (b) molecule for linear velocity determination (methane) used for calculation of the van Deemter plot shown in Figure 4. The different colors of the individual peaks represent the peaks obtained at the different flow rates.
findings demonstrate that this model can be applied for diffusion measurements of FCC catalysts. 4.2. Reproducibility. The relative standard deviations obtained from 24 measurements on an equilibrium catalyst (catalyst A) are given in Table 1 and indicate good reproducibility for the iGC measurements. Because each 5529
DOI: 10.1021/acs.iecr.6b00470 Ind. Eng. Chem. Res. 2016, 55, 5526−5535
Article
Industrial & Engineering Chemistry Research
lowered by an increase of the molecule size, which is expected in the Knudsen regime.20,21 4.4. Case Study I. Refinery W switched from catalyst type C to type D. The catalytic and physical properties as well as the diffusion coefficients of the corresponding equilibrium catalysts taken from the FCC unit inventory are given in Table 3. The properties of the feedstock used for the catalytic experiments are also compiled in Table 3. Table 3. Case Study I: Effects of the Catalyst Change in Refinery W on the Catalyst Diffusivities and Catalytic Performance catalyst
Figure 4. van Deemter plot of catalyst A (probe molecule: 1,2,4trimethylcyclohexane) demonstrating that the three diffusion types shown in Figure 2 are obtained for FCC catalysts.
C iGC Testing with 1,2,4-Trimethylcyclohexane diffusion coefficient (cm2/s) 4.8 × 10−3 ACE Testing: Yields at 70% Conversion C1−C4 (wt %) 18.3 C4 olefinicity (%) 52 gasoline (wt %) 44.9 light cycle oil (wt %) 11.8 heavy cycle oil (wt %) 18.2 FCC Catalyst Physical and Chemical Properties zeolite surface area (m2/g) 90 matrix surface area (m2/g) 29 unit cell size (Å) 24.32 water pore volume (mL/g) 0.31 average particle size (μm) 82 Al2O3 (wt %) 47.7 RE2O3 (wt %) 2.8 Na (wt %) 0.38 V (mg/kg) 3920 Ni (mg/kg) 3829 Sb (mg/kg) 1334 Fe (wt %) 0.66 Feedstock from Refinery W API Gravity at 15 °C 24.6 density at 15 °C (kg/m3) 905.8 average molecular weight (g/mol) 400 Caromatic (%) 21.3 final boiling point (°C) 624 UOP K factor 12.03
measurement was performed with a new portion of 270 mg of this equilibrium catalyst, the good reproducibility indicates that the 270 mg batches well represent the bulk properties of the equilibrium catalyst. 4.3. Impact of the Molecule Size and Temperature on Diffusion. The diffusion coefficients of 1,2,4-trimethylcyclohexane and phenanthrene were determined for an equilibrium catalyst (catalyst B), the former at two temperatures. The results obtained and the catalyst characterization data are shown in Table 2. Table 2. Impact of the Probe Molecule Size and Temperature on Diffusion Catalyst B Diffusion Coefficients 1,2,4-trimethylcyclohexane at 200 °C (cm2/s) 1,2,4-trimethylcyclohexane at 350 °C (cm2/s) phenanthrene at 350 °C (cm2/s) Physical and Chemical Properties zeolite surface area (m2/g) matrix surface area (m2/g) unit cell size (Å) water pore volume (mL/g) average particle size (μm) Al2O3 (wt %) RE2O3 (wt %) Na (wt %) V (mg/kg) Ni (mg/kg) Fe (wt %)
1.7 × 10−3 7.0 × 10−3 1.0 × 10−3 107 26 24.29 0.35 78 49.9 3.3 0.34 5258 3316 0.68
D 1.0 × 10−2 16.1 59 48.2 14.5 15.5 78 37 24.34 0.31 59 47.0 2.9 0.27 3320 3551 27 0.47
The van Deemter curves obtained for the two catalysts are illustrated in Figure 5. These data show a high linearity being necessary for reliable estimations of the slope by linear regression and diffusion coefficients. The catalytic data show catalyst D to produce more gasoline and higher product olefinicity than catalyst C. Normally such differences are explained by changes in the unit cell size and zeolite surface area whereby the former is the main driving force for product selectivities. Changes in the unit cell size of the FCC catalyst reflect changes in the number of framework alumina atoms in the Y zeolite, which can be calculated with the equation proposed by Breck and Flanigen23 given in eq 6.
The magnitude of the diffusion coefficients categorizes them to be in the Knudsen regime. This regime is characterized by the fact that the pore diameter is smaller than the mean free path of the probe molecule used, and collisions between the probe molecules and pore walls occur more frequently than among the molecules themselves.20,21 The Knudsen regime can be expected in mesopores and macropores of FCC catalysts under typical FCC conditions.22 Hence, the intraparticle diffusion in FCC catalysts does not depend on the diffusion in micropores; such an intracrystalline surface diffusion and solid diffusion in the zeolitic part of the FCC catalyst would result in lower diffusion coefficients.20,21 Regarding the impact of the temperature and probe molecule size, diffusion was accelerated by a temperature increase and
NFAUC = 115.2(unit cell size − 24.191)
(6)
where NFAUC = number of framework alumina atoms per unit cell. Framework alumina represents the acidic centers in the zeolite, i.e., the catalytically active centers in the FCC catalyst. It 5530
DOI: 10.1021/acs.iecr.6b00470 Ind. Eng. Chem. Res. 2016, 55, 5526−5535
Article
Industrial & Engineering Chemistry Research
Table 4. Case Study II: Effects of the Catalyst Change in Refinery X on the Catalyst Diffusivities and Catalytic Performance catalyst E iGC Testing with 1,2,4-Trimethylcyclohexane diffusion coefficient (cm2/s) 3.8 × 10−3 ACE Testing: Yields at 70% Conversion C1−C4 (wt %) 19.1 C4 olefinicity (%) 51 gasoline (wt %) 45.0 light cycle oil (wt %) 16.4 heavy cycle oil (wt %) 13.6 FCC Catalyst Physical and Chemical Properties zeolite surface area (m2/g) 99 matrix surface area (m2/g) 24 unit cell size (Å) 24.33 water pore volume (mL/g) 0.34 average particle size (μm) 82 Al2O3 (wt %) 46.4 RE2O3 (wt %) 2.9 Na (wt %) 0.25 V (mg/kg) 5180 Ni (mg/kg) 2840 Sb (mg/kg) 666 Fe (wt %) 0.44 Feedstock from Refinery X API gravity at 15 °C 21.2 density at 15 °C (kg/m3) 925.7 average molecular weight (g/mol) 405 Caromatic (%) 26.7 final boiling point (°C) 643 UOP K factor 11.80
Figure 5. van Deemter plots generated from data being in the linear regime of the van Deemter model for determination of the slope for calculation of the diffusion coefficients. The FCC catalysts were from case study I. Probe molecule: 1,2,4-trimethylcyclohexane.
can be deduced from eq 6 that the number of framework alumina atoms, i.e., the number of acidic centers in the zeolite, increases with increasing unit cell size. Two recent papers on the acidity measurements of Y zeolite confirmed this classical unit cell size−framework alumina relationship;24,25 it was shown that the number of acidic centers increases with increasing unit cell size. The effects of unit cell size variation, i.e., changes in the number of acidic centers in the zeolite on the catalytic performance of the FCC catalyst, are as follows: Increasing the unit cell size enhances the hydrogen-transfer rates. As a consequence, gasoline olefins are reduced, leading to a lower crackability of this fraction. Thus, higher gasoline yields are expected with increasing unit cell size.26,27 In this case study, the C4 olefinicities, which reflect the olefinicity in the gasoline range, are enhanced with increasing unit cell size; i.e., the ranking is reversed to that expected from changes in the unit cell size. Another phenomenon is that the sample that produced more olefins, i.e., gasoline of higher crackability, retained the higher gasoline yield. The conflicting unit cell size versus selectivity findings can be rationalized by the diffusion coefficients. The diffusion coefficients given in Table 3 suggest faster diffusion of C4 and gasoline molecules through catalyst D and thus less hydrogen transfer and secondary cracking. Hence, higher product olefinicity, more gasoline, and less C1−C4 were obtained in the case of the high unit cell size catalyst. 4.5. Case Study II. Refinery X switched from catalyst type E to type F. The catalytic and physical properties as well as the diffusion coefficients of the corresponding equilibrium catalysts taken from the FCC unit inventory are given in Table 4. The properties of the feedstock used for the catalytic experiments are also compiled in Table 4. In this example of catalyst changeover, the physical and textural properties (unit cell size, zeolite, and matrix surface area) of both catalyst types are virtually identical. The catalytic data, however, show differences in C1−C4, gasoline, and hydrogen transfer, which again, as in case study I, cannot be explained with changes in the unit cell size and zeolite surface area. Catalyst F has a faster diffusion coefficient than catalyst E, and the arguments given in case study I for the selectivity differences hold in this comparison too.
F 1.2 × 10−2 18.4 54 46.1 16.7 13.3 99 24 24.33 0.34 85 49.6 3.7 0.24 5184 2973 519 0.36
The catalyst with the faster diffusion coefficient had, because of shorter residence times of the molecules in the FCC particle, less secondary cracking reactions and hydrogen transfer. Thus, this sample produced higher product olefinicity as well as more gasoline and light cycle oil and less C1−C4. 4.6. Case Study III. Refinery Y switched from catalyst type G to type H. The catalytic and physical properties as well as the diffusion coefficients of the corresponding equilibrium catalysts taken from the FCC unit inventory are given in Table 5. The properties of the feedstock used for the catalytic experiments are also compiled in Table 5. The most striking issue in this example is the large unit cell size difference between catalysts G and H. From the differences in the unit cell sizes, we would expect catalyst H to produce much lower product olefinicity and higher gasoline yield.26,27 However, these selectivities show, with the exception of gasoline, tendencies in the opposite direction. Again, a reasonable explanation can be derived from the diffusion coefficients. The probe molecule diffuses faster through the catalyst with the high unit cell size (catalyst H). Hence, less secondary cracking reactions and hydrogen transfer and thus higher product olefinicity, more gasoline, and less C1−C4 were obtained. 4.7. Case Study IV. Catalysts I and J are equilibrated catalysts of the same catalyst type from refinery Z. Their catalytic and physical properties and diffusion coefficients and the properties of the feedstock used for the catalytic experiments are also compiled in Table 6. 5531
DOI: 10.1021/acs.iecr.6b00470 Ind. Eng. Chem. Res. 2016, 55, 5526−5535
Article
Industrial & Engineering Chemistry Research Table 5. Case Study III: Effects of the Catalyst Change in Refinery Y on the Catalyst Diffusivities and Catalytic Performance
Table 6. Case Study IV: Effects of the Catalyst Change in Refinery Z on the Catalyst Diffusivities and Catalytic Performance
catalyst
catalyst
G iGC Testing with 1,2,4-Trimethylcyclohexane diffusion coefficient (cm2/s) 5.3 × 10−3 ACE Testing: Yields at 70% Conversion C1−C4 (wt %) 19.1 C4 olefinicity (%) 55 gasoline (wt %) 46.9 light cycle oil (wt %) 15.6 heavy cycle oil (wt %) 14.4 FCC Catalyst Physical and Chemical Properties zeolite surface area (m2/g) 100 matrix surface area (m2/g) 28 unit cell size (Å) 24.32 water pore volume (mL/g) 0.36 average particle size (μm) 89 Al2O3 (wt %) 46.8 RE2O3 (wt %) 2.8 Na (wt %) 0.50 V (mg/kg) 951 Ni (mg/kg) 1857 Sb (mg/kg) 14 Fe (wt %) 0.56 Feedstock from Refinery Y API gravity at 15 °C 25.0 density at 15 °C (kg/m3) 903.4 average molecular weight (g/mol) 378 Caromatic (%) 22.0 final boiling point (°C) 612 UOP K factor 11.96
H
I iGC Testing with 1,2,4-Trimethylcyclohexane diffusion coefficient (cm2/s) 2.0 × 10−3 ACE Testing: Yields at 70% Conversion C1−C4 (wt %) 15.5 C4 olefinicity (%) 64 gasoline (wt %) 48.3 light cycle oil (wt %) 17.9 heavy cycle oil (wt %) 12.1 FCC Catalyst Physical and Chemical Properties zeolite surface area (m2/g) 74 matrix surface area (m2/g) 75 unit cell size (Å) 24.28 water pore volume (mL/g) 0.23 average particle size (μm) 74 Al2O3 (wt %) 54.7 RE2O3 (wt %) 2.1 Na (wt %) 0.25 V (mg/kg) 1325 Ni (mg/kg) 1717 Sb (mg/kg)
