FCC Catalyst Accessibility—A Review

Round 1

Reviewer 1 Report

This is a good overview work for FCC catalyst.  It contains a lot of basic and well-known info about FCC which are maybe non necessary.

Since the review topic is on FCC catalyst accessibility I would expected more emphasis on the accessibility of FCC catalysts towards especially to the effect of accessibility on catalyst performance based on experimental results.

Very little are also mentioned on the way to experimentally measure the accessibility of FCC catalyst. Thus, I would expected a little more about the AAI method and other efforts and how they are correlated with catalyst performance.

I would expect also some more industrial data of the effect of catalyst accessibility on the performance of FCC catalysts.   

Author Response

This is a good overview work for FCC catalyst.  It contains a lot of basic and well-known info about FCC which are maybe non necessary.

Point 1: Since the review topic is on FCC catalyst accessibility I would expected more emphasis on the accessibility of FCC catalysts towards especially to the effect of accessibility on catalyst performance based on experimental results.

Response 1:

Weckhuysen et al. [1], [2], [3] have characterized commercial e-cats and artificially deactivated and metallated catalysts using the techniques of X-ray micro- and nano-tomography, as well as micro-XRF and m-XRD. They have concluded that both contaminant metals Fe and Ni gradually incorporate almost exclusively near the external surface regions of the FCC catalyst particles in a shell not thicker than two μm (Figure 1, adapted from [1]), thus severely limiting the macropore accessibility as metal concentrations increase. Pore-blocking prevents feedstock molecules from reaching the catalytically active sites. Consequently, metal deposition reduces the catalytic conversion with increasing time on stream because although the internal pore system remains unobstructed, it becomes mostly inaccessible.

They carried out a catalyst performance test (ACE) on the e-cats with different metal levels. The catalysts were tested for activity using VGO at a cracking temperature of 538°C and at four different catalyst-to-oil (CTO) ratios: 3, 4, 5, and 6. The performance testing results show a clear correlation between catalyst deactivation/age (and therefore the metal levels of Ni and Fe in the sample) and catalytic activity: the older the catalyst, the lower the bottoms conversion [1].

Point 2: I would expect also some more industrial data of the effect of catalyst accessibility on the performance of FCC catalysts.   

Response 2:

 FCC unit yields are directly related to accessibility. Figure 2 and Figure 3 (adapted from [4]) show the increase in conversion and gasoline yield as the catalyst ages in a commercial FCCU [4].

Nickell [5] has documented commercial cases illustrating a tangible correlation between AAI and unit performance. Figure 4 (adapted from [5]) shows the decrease in activity (bottoms conversion) as the catalyst ages in the FCCU, with the consequent decrease in accessibility. It is noticeable that the catalyst fell below the refiner's "critical AAI"; a point where the bottoms increased remarkably as the conversion correspondingly decreased.

Empirical observations consistently demonstrate that many refiners experience a "critical accessibility level". This critical level is extremely unit-specific and is a function of feed quality, feed–catalyst contact efficiency, riser residence time, equilibrium catalyst metal levels, and regenerator conditions. Operating with accessibility levels below this point results in significant losses in FCC unit conversion and transportation fuel production. This observation does not show in traditional equilibrium catalyst laboratory testing [4].

Point 3: Very little are also mentioned on the way to experimentally measure the accessibility of FCC catalyst. Thus, I would expected a little more about the AAI method and other efforts and how they are correlated with catalyst performance.

Response 3: AAI measurement is based on the liquid-phase diffusion of large organic molecules into the catalyst. A probe molecule and solvent circulate through a stirred vessel and an inline spectrophotometer. The large probe molecule cannot enter the zeolite channels, and its effective diffusivity strongly depends on the catalyst's pore system. The test determines the catalysts' initial mass transfer characteristics with no reaction involved. The AAI is thus a relative measure of the penetration rate [6], [5].

Bibliography

[1]       F. Meirer et al., “Life and death of a single catalytic cracking particle,” Sci. Adv., vol. 1, no. 3, 2015, doi: 10.1126/sciadv.1400199.

[2]       M. Gambino et al., “Mimicking industrial aging in fluid catalytic cracking: A correlative microscopy approach to unravel inter-particle heterogeneities,” J. Catal., vol. 404, pp. 634–646, 2021, doi: https://doi.org/10.1016/j.jcat.2021.10.012.

[3]       E. T. C. Vogt and B. M. Weckhuysen, “Fluid catalytic cracking: recent developments on the grand old lady of zeolite catalysis,” Chem. Soc. Rev., vol. 44, no. 20, pp. 7342–7370, 2015, doi: 10.1039/c5cs00376h.

[4]       Albemarle, “Quantifying FCC catalyst accessibility.” https://www.albemarle.com/storage/wysiwyg/mib-aai_final.pdf (accessed Mar. 30, 2023).

[5]       R. Nickell, “The advantages of catalyst accessibility (TIA),” PETROLEUM TECHNOLOGY QUARTERLY, vol. 21, no. 3, p. 132, 2016.

[6]       A. C. Psarras, E. F. Iliopoulou, L. Nalbandian, A. A. Lappas, and C. Pouwels, “Study of the accessibility effect on the irreversible deactivation of FCC catalysts from contaminant feed metals,” Catal. Today, vol. 127, no. 1–4, pp. 44–53, 2007, doi: 10.1016/j.cattod.2007.05.021.

[7]       D. Wisser and M. Hartmann, “129Xe NMR on Porous Materials: Basic Principles and Recent Applications,” Adv. Mater. Interfaces, vol. 8, no. 4, pp. 1–19, 2021, doi: 10.1002/admi.202001266.

[8]       L. Vásárhelyi, Z. Kónya, Á. Kukovecz, and R. Vajtai, “Microcomputed tomography–based characterization of advanced materials: a review,” Mater. Today Adv., vol. 8, p. 100084, 2020, doi: https://doi.org/10.1016/j.mtadv.2020.100084.

[9]       M. Sun et al., “A review of small angle scattering technique,” J. Nat. Gas Sci. Eng., vol. 78, no. March, p. 103294, 2020, doi: 10.1016/j.jngse.2020.103294.

[10]     Z. Yuxia, D. Quansheng, L. Wei, T. Liwen, and L. Jun, “Chapter 13 Studies of iron effects on FCC catalysts,” Stud. Surf. Sci. Catal., vol. 166, pp. 201–212, 2007, doi: 10.1016/S0167-2991(07)80196-0.

Author Response File: Author Response.pdf

Reviewer 2 Report

Authors review FCC catalyst accessibility in technical paper and patent literatures over the past three decades. The accessibility characterization, its effect on the FCC catalyst performance and the methodology of accessibility improvement are discussed. Thank authors for systematically selecting the references and very hard working on reviewing.

I would like to suggest that the authors should consider the following aspects to improve the quality of manuscript for publishing.

1.      The grammar errors and mistyping words were found. Please check carefully the manuscript again. Moreover, I suggest that author should use the professional English editing service (for example: editage, etc.) or the manuscript should be checked by English speaker before publishing.

2.      In the section 1.2.3 to 1.2.6, please add the characterization methods to measure catalyst properties.

3.      In the section 4.5, “O'Connor et al. [48] revealed a proprietary method to characterize catalyst accessibility based on the non-steady state diffusion of hydrocarbons into FCC particles. This technique can be used to quantify catalyst accessibility and optimize catalyst effectiveness in an FCCU operation. They have observed a significant loss of accessibility quantified by an Albemarle Accessibility Index (AAI) as a function of catalyst age and metals content.” was written.

Please add more detailed information of AAI, including graphs (if it is possible).

4.      In the section 4.5.2, please explain the effectiveness of each characterization methods.

5.      In FCC process, the acidity of catalyst is very important factor. Is there any relationship between acidity and accessibility? This should be additionally discussed.

6.      The effect of accessibility on the catalyst life should be discussed in more detailed.  

Author Response

Authors review FCC catalyst accessibility in technical paper and patent literatures over the past three decades. The accessibility characterization, its effect on the FCC catalyst performance and the methodology of accessibility improvement are discussed. Thank authors for systematically selecting the references and very hard working on reviewing.

I would like to suggest that the authors should consider the following aspects to improve the quality of manuscript for publishing.

Point 1: The grammar errors and mistyping words were found. Please check carefully the manuscript again. Moreover, I suggest that author should use the professional English editing service (for example: editage, etc.) or the manuscript should be checked by English speaker before publishing.

Response 1: We have submitted the manuscript to extensive grammar, usage, and style check by a native English-speaking person and we have complied to all recommendations we were given.

Point 2: In the section 1.2.3 to 1.2.6, please add the characterization methods to measure catalyst properties.

Response 2:

1.2.3.    Particle size distribution (PSD)

ASTM D4464-15(2020) Standard Test Method for Particle Size Distribution of Catalytic Materials by Laser Light Scattering describes the standard procedures to determine FCC PSD, applied with minor adaptations from site to site. PSD can also be determined by a set of wire mesh sieves with openings from 20 to 149 µm.

1.2.4.    Textural properties

Area and porosity are determined by nitrogen adsorption methods described in standard procedures:

• ASTM D4222-20 Standard Test Method for Determination of Nitrogen Adsorption and Desorption Isotherms of Catalysts and Catalyst Carriers by Static Volumetric Measurements: Specific Area, SA - total area per unit mass of the catalyst (m²/g)
• ASTM D4365-19 Standard Test Method for Determining Micropore Volume and Zeolite Area of a Catalyst: Micropore Volume (MiPV), which is the total volume of micropores per unit mass (cm³/g), associated with the available zeolite content in the catalyst, and meso Specific Area (mSA), external and mesopores area per unit mass (m²/g) of the material.

1.2.5.    Accessibility

AAI measurement is based on the liquid-phase diffusion of large organic molecules into the catalyst. A probe molecule and solvent circulate through a stirred vessel and an inline spectrophotometer. The large probe molecule cannot enter the zeolite channels, and its effective diffusivity strongly depends on the catalyst's pore system. The test determines the catalysts' initial mass transfer characteristics with no reaction involved. The AAI is thus a relative measure of the penetration rate [6], [5].

1.2.6.    Attrition Resistance

The attrition index determination procedure is described in ASTM D5757-11(2017) Standard Test Method for Determination of Attrition of FCC Catalysts by Air Jet. In the test method, the sample is subjected to vigorous fluidization by a constant high-flow air stream, which causes shear and breakage of the particles that collide at high speed with each other and with the walls of the test tube. The fines formed are removed from the attrition zone by elutriation, collected in an extraction cartridge and weighed after 5 and 20 h of testing, and the attrition is calculated by dividing these values by the initial mass of the catalyst.

Point 3: In the section 4.5, "O'Connor et al. [48] revealed a proprietary method to characterize catalyst accessibility based on the non-steady state diffusion of hydrocarbons into FCC particles. This technique can be used to quantify catalyst accessibility and optimize catalyst effectiveness in an FCCU operation. They have observed a significant loss of accessibility quantified by an Albemarle Accessibility Index (AAI) as a function of catalyst age and metals content." was written.

Please add more detailed information of AAI, including graphs (if it is possible).

Response 3:

A special apparatus circulates a probe molecule and solvent through a stirred vessel and a spectrophotometer. The probe molecule is chosen to simulate high molecular weight hydrocarbons with a boiling point well above 480°C. Such molecules are typically too large to enter the zeolite supercage and have an effective diffusivity highly dependent on the catalyst's pore architecture [5].

Please find a few graphs illustrating AAI features under item Point 6 below.

Point 4: In the section 4.5.2, please explain the effectiveness of each characterization methods.

Response 4: ¹²⁹Xe NMR allows probing pore sizes and pore connectivity or pore blocking. These parameters make the technique particularly interesting for investigating hierarchical functional materials on different length scales. Obtaining sufficient signal intensity in a reasonable experimental time remains a limitation of the method, which can be overcome by hyperpolarization techniques [7].

3D imaging in a transmission electron microscope is called electron tomography. Transmission images of the sample acquired from at least a hundred different angles can be reconstructed into a 3D model of the sample. This technique has a very high resolution (tens of nanometers) but is only suitable for tiny samples [8].

Computed tomography (CT) is a non-destructive 3D imaging technique based on the different X-ray attenuation of materials. Its non-destructive nature allows temporal investigation (4D imaging, where the fourth dimension is time), and the examined samples remain unchanged, can be further investigated (even in situ), or can be put to use. Virtually no sample preparation is required. The main limitations of CT are related to the high Z (atomic number) contrast necessary for good imaging quality. Low-Z materials and samples with low X-ray attenuation contrast are challenging to measure, whereas very high-Z materials (e.g. metals) can introduce severe artifacts and worsen image quality. Micro-CT stands for high-resolution CT. With the decrease in focal spot size and increased resolution, developers could achieve submicron resolution and started referring to devices capable of this high resolution as nano-CT. After some time, with the use of synchrotron radiation and special X-ray optics, even better resolution (below 100 nm) became obtainable

Small-angle X-Rays scattering (SAXS) technique uses X-ray beams to penetrate the materials and obtain information on the pore structure by measuring the intensity of scattered radiation within a specific range of scattering angles (0.1 to 5°). It is a non-destructive measurement of the total porosity, including both open and closed pores, providing information on pores from angstrom to micron scales (from ca. 5 Å to 20 μm) [9]. In mercury porosimetry, the lower limit of the detectable pore throat is approximately 3 nm, whereas helium pycnometry method is capable of obtaining gas-accessible porosity but not total porosity; the SAXS method, on its part, includes both accessible (open) and inaccessible (closed) porosity – although this is not a relevant information for catalytical purposes.

Point 5: In FCC process, the acidity of catalyst is very important factor. Is there any relationship between acidity and accessibility? This should be additionally discussed.

Response 5: The relationship between acidity and accessibility should be discussed with different approaches, namely, at the catalyst preparation level (ingredients and compounding) and the catalyst application.

• At the ingredient level

Zeolite micropores pose the main restriction to diffusion in a catalyst, and the ways to increase it belong to the nanoscale manipulation. Soft or hard templates can be added to the zeolite synthesis to introduce mesopores, thus increasing accessibility to the zeolite active (acid) sites located in the framework (bottom-up methods). Virtually the same framework composition as the strictly microporous reference zeolite can be obtained; hence, the mesoporous zeolite will bear the same acidity in terms of number and strength as the reference material.

Top-down methods, on the other hand, create mesopores at the cost of micropores destruction. They involve dealumination or desilication. In both cases, acidity will change, reflecting the framework composition change (higher or lower SAR).

• At the catalyst assembling level

A more accessible zeolite does not mean a more accessible catalyst. It is useless to embed a hierarchical zeolite if the large molecules of the feed cannot reach the crystal. The key to accessibility increase lies in the compounding technology, the way the ingredients are packed, arranged, and bound in the catalyst particle. Here we deal with mesoscale phenomena, in which the extent of macroscale mass transfer within the catalyst pore network will change the product yield, selectivity, or yield no product.

• During catalyst application/operation

Calcium and heavy metals contaminants (iron, nickel, vanadium) present in crude oils as porphyrins, naphthenates, or inorganic compounds can deposit on the FCC catalyst surface during cracking and destroy the crystalline structure, block pore channels and cover the catalyst's active sites. The catalyst's selectivity and activity decrease, and coke and dry gas yield are higher with a simultaneous decrease in liquid fuel yield [10].

Yuxia et al. [10] have impregnated FCC catalysts with two kinds of iron species (iron chloride and iron naphthenate) to simulate contamination sources from FCC feedstocks. Fresh FCC catalyst and iron-contaminated samples were steam-deactivated, and their performance was carried out on an ACE unit.

The contamination of iron chloride showed little influence on the catalyst performance, while the activity of the catalyst contaminated with iron naphthenate decreased with increasing iron content.

Scanning Electron Microscopy with Energy Dispersive X-Ray Analysis (SEM-EDAX) revealed that the distribution of iron on the surface of the catalyst contaminated with iron chloride is uniform, and the iron contents in the exterior and interior of the catalyst particles are close, suggesting the absence of local enrichment of iron deposits. The catalyst contaminated using iron naphthenate presented a lack of uniformity of iron distribution, with the  iron content in the exterior of the FCC particle markedly higher than that in its interior.

The total acidity of the iron-contaminated samples was measured with NH3-TPD methods, and results were compared with the fresh catalyst's. The authors noticed significant acidity loss in iron naphthenate-contaminated catalysts and that acidity loss is negligible when iron chloride is used.

Point 6: The effect of accessibility on the catalyst life should be discussed in more detailed.  

Response 6: FCC unit yields are directly related to accessibility. Figure 2 and Figure 3 show the increase in conversion and gasoline yield as the catalyst ages in commercial FCCU [4].

Nickell [5] has documented commercial cases illustrating a tangible correlation between AAI and unit performance. Figure 4 shows the decrease in activity (bottoms conversion) as the catalyst ages in the FCCU, with the consequent decrease in accessibility. It is noticeable that the catalyst fell below the refiner's "critical AAI", a point where the bottoms increased remarkably as the conversion correspondingly decreased.

Empirical observations consistently demonstrate that many refiners experience a "critical accessibility level". This critical level is extremely unit-specific and is a function of feed quality, feed–catalyst contact efficiency, riser residence time, equilibrium catalyst metal levels, and regenerator conditions. Operating with accessibility levels below this point results in significant losses in FCC unit conversion and transportation fuel production. This observation does not show in traditional equilibrium catalyst laboratory testing [4].

Bibliography

[1]       F. Meirer et al., “Life and death of a single catalytic cracking particle,” Sci. Adv., vol. 1, no. 3, 2015, doi: 10.1126/sciadv.1400199.

[2]       M. Gambino et al., “Mimicking industrial aging in fluid catalytic cracking: A correlative microscopy approach to unravel inter-particle heterogeneities,” J. Catal., vol. 404, pp. 634–646, 2021, doi: https://doi.org/10.1016/j.jcat.2021.10.012.

[3]       E. T. C. Vogt and B. M. Weckhuysen, “Fluid catalytic cracking: recent developments on the grand old lady of zeolite catalysis,” Chem. Soc. Rev., vol. 44, no. 20, pp. 7342–7370, 2015, doi: 10.1039/c5cs00376h.

[4]       Albemarle, “Quantifying FCC catalyst accessibility.” https://www.albemarle.com/storage/wysiwyg/mib-aai_final.pdf (accessed Mar. 30, 2023).

[5]       R. Nickell, “The advantages of catalyst accessibility (TIA),” PETROLEUM TECHNOLOGY QUARTERLY, vol. 21, no. 3, p. 132, 2016.

[6]       A. C. Psarras, E. F. Iliopoulou, L. Nalbandian, A. A. Lappas, and C. Pouwels, “Study of the accessibility effect on the irreversible deactivation of FCC catalysts from contaminant feed metals,” Catal. Today, vol. 127, no. 1–4, pp. 44–53, 2007, doi: 10.1016/j.cattod.2007.05.021.

[7]       D. Wisser and M. Hartmann, “129Xe NMR on Porous Materials: Basic Principles and Recent Applications,” Adv. Mater. Interfaces, vol. 8, no. 4, pp. 1–19, 2021, doi: 10.1002/admi.202001266.

[8]       L. Vásárhelyi, Z. Kónya, Á. Kukovecz, and R. Vajtai, “Microcomputed tomography–based characterization of advanced materials: a review,” Mater. Today Adv., vol. 8, p. 100084, 2020, doi: https://doi.org/10.1016/j.mtadv.2020.100084.

[9]       M. Sun et al., “A review of small angle scattering technique,” J. Nat. Gas Sci. Eng., vol. 78, no. March, p. 103294, 2020, doi: 10.1016/j.jngse.2020.103294.

[10]     Z. Yuxia, D. Quansheng, L. Wei, T. Liwen, and L. Jun, “Chapter 13 Studies of iron effects on FCC catalysts,” Stud. Surf. Sci. Catal., vol. 166, pp. 201–212, 2007, doi: 10.1016/S0167-2991(07)80196-0.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Thanks authors for revising very carefully the manuscript and the response to the reviewers' comments. The revised manuscript is well-organized and carefully written.