Researchers use X-ray nanotomography to identify key mechanisms of FCC catalyst aging; could lead to more efficient gasoline production
Scientists at Utrecht University and the US Department of Energy’s SLAC National Accelerator Laboratory have used X-ray nanotomography to identify key mechanisms of the aging process of catalyst particles that are used to refine crude oil into gasoline. This advance could lead to more efficient production of gasoline. (Tomography reconstructs a sliceable, virtual 3D copy of an object under study from 2D images.)
Their recent experiments studied fluid catalytic cracking (FCC) particles that are used to break heavy long-chain hydrocarbon fractions in crude oil into lighter, more valuable hydrocarbons such as gasoline and propylene. During FCC, the heavy hydrocarbons are vaporized and cracked into short-chain fractions by billions of tiny, fairly spherical catalyst particles with diameters ranging from 50–150 µm. FCC particles account for 40-45% of worldwide gasoline production.
In FCC, long-chain feedstock molecules are cracked into smaller, more valuable ones by hierarchically structured multicomponent catalyst particles consisting of zeolite, matrix, filler, and binder. Microporous crystalline zeolites provide most of the catalytic activity and product selectivity. They are embedded in the particle matrix, which itself plays an active role in precracking large hydrocarbons to appropriate size so that they can enter zeolite micropores for further selective cracking.
During commercial FCC, the particles accumulate metals from crude oil and/or upstream processes and equipment. Ni and V contaminants are predominantly present in conventional heavy crude oils, whereas Fe contaminants are known to be present at relative high concentrations in tight and/or shale oils. Metal accumulation occurs via deposition and incorporation into the catalyst body and can cause unwanted shifts in product distribution, destabilize zeolite domains, and reduce bulk accessibility. We summarize these effects as “catalyst aging.”—Meirer et al. (2015b)
A major problem is that these catalysts quickly age and lose their activity, so tons of fresh catalysts have to be added to a reactor system every day. We are trying to understand how this aging happens, and we’re working with companies that produce these FCC catalysts to make the process more efficient.—Florian Meirer, assistant professor of inorganic chemistry and catalysis at Utrecht University, lead researcher
In experiments using X-ray beams at SLAC’s Stanford Synchrotron Radiation Lightsource (SSRL), a DOE Office of Science User Facility, the researchers studied FCC catalysts of various ages to better understand the effects of aging. They were able to image whole catalyst particles with high resolution so they could also see the catalysts’ internal structure—like taking a panoramic landscape photograph where you can zoom in to see the ants.
We have been able to localize the metal poisons that are a leading cause of catalyst aging and also determine how they influence the materials. It’s been studied in the past, but not at this resolution and not on the single particle level. That is the beauty of what we have done.—Bert Weckhuysen, professor of inorganic chemistry and catalysis at Utrecht University
|Reconstructed 3-D image of a single catalyst particle, showing where iron (orange) and nickel (blue) deposited on the surface and inside the catalyst. The movie shows a rotating particle to view the entire catalyst surface, and then cross sectional slices to see inside the catalyst. (Florian Meirer/Utrecht University)|
The problem of catalyst aging is widespread and costly. Worldwide, about 400 reactor systems refine crude oil into gasoline, and each system requires 10 to 40 tons of fresh FCC catalysts daily.
Crude oil is contaminated with metals, mostly iron, nickel and vanadium. These metals accumulate in catalysts during refining and eventually deactivate them. This is particularly an issue for low-quality crude oil, the largest available oil supply.
In the SSRL study, the research team took a series of two-dimensional images of catalyst particles at various angles and used software they developed to combine them into three-dimensional images of whole particles. These images show the 3-D distribution of iron and nickel in catalysts of various ages.
The researchers determined that the metals quickly accumulate on the outer surface of a catalyst, blocking the crude oil molecules from traveling through catalyst pores to reach deeper into its still-active core. They also showed that some catalyst particles stick together in clusters, which disturbs the fluidity of the catalysts and lowers gasoline production yields.
… the findings reported in this work provide an unprecedented, detailed view on the changing internal macropore structure of aging FCC particles. We were able to correlate the 3D distribution of Ni and Fe to changes in porosity and pore connectivity in fresh and aged catalysts, in turn linking reduced catalytic activity to highly localized pore clogging. The results elucidate an important, as yet unknown aspect of metal poisoning: Fe and Ni accumulation is strongest early in cycle life and is concentrated at the particle surface and in a near-surface layer, whereas the inner core remains relatively clear of these metals, resembling the pore structure of a fresh particle.
This inner, relatively unimpeded and interconnected pore network remains functional for reaction and is likely still accessible, although less efficiently, by meso- and micropores. Its presence explains the remaining functional activity of the catalyst, as confirmed by a small but quantifiable accessibility index and a measurable catalytic conversion rate, even for particles with the highest metal loading. Our results complement previous work that has shown that V (not studied in this work) can penetrate more deeply into the FCC particle, mainly affecting zeolite structures, rather than macropore radius, connectivity, and accessibility.—Meirer et al. (2015b)
The Utrecht researchers are already working with companies to redesign these FCC catalysts.
The research team also included Sam Kalirai at Utrecht and Darius Morris, Yijin Liu and Joy C. Andrews at SSRL. This research was supported by the NWO Gravitation program, Netherlands Center for Multiscale Catalytic Energy Conversion, Netherlands Research School Combination-Catalysis and a European Research Council Advanced Grant.
F. Meirer et al. (2015a) “Agglutination of single catalyst particles during fluid catalytic cracking as observed by X-ray nanotomography” Chemical Communications doi: 10.1039/c5cc0040b
F. Meirer et al. (2015b) “Life and death of a single catalytic cracking particle” Science Advances doi: 10.1126/sciadv.1400199
F. Meirer et al. (2015c) “Mapping Metals Incorporation of a Whole Single Catalyst Particle Using Element Specific X-ray Nanotomography” Journal of the American Chemical Society doi: 10.1021/ja511503d)