The advantages of catalyst accessibility (TIA)
Fluid catalytic cracking is a diffusion limited process; nevertheless, the industry has evolved towards short contact time to minimise thermal cracking while simultaneously using a high catalyst to oil ratio (CTO) to maximise influence of the catalyst’s cracking and selectivity characteristics.
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With this elevated impact of catalyst architecture on yields, the opportunity cost of poor catalyst design is significant for the modern refiner. A key factor in catalyst architecture is accessibility. Accessibility is conceptually defined as the ability of large molecular structures to interact with the active cracking sites present on catalyst within a given time frame. While active sites do exist on the catalyst surface, the bulk remain buried inside a tortuous pore system with often limited accessibility. These sites go unused in poor catalyst designs. Albemarle offers refiners an efficient and unique test to quantify catalyst accessibility.
Albemarle reports catalyst accessibility on a scale of 0-36 with a numerical value called the Albemarle Accessibility Index (AAI). Accessibility can be measured on FCC catalyst from any supplier under unbiased, normalised conditions. A special apparatus is used to circulate a probe molecule and solvent through both 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. Upon catalyst addition, a signal appears on the spectrophotometer since the probe molecule begins immediate diffusion into the catalyst pores. The response at short contact time is that which is relevant to FCC conditions. Based on analysis of the fundamental diffusion equations, the AAI is reported as the initial slope of the probe’s changing concentration vs the square root of time. An objective measurement is ensured with a standardised catalyst quantity and particle size. The test may be run on fresh, laboratory deactivated, and equilibrium catalyst. Further technical details of this analytical method are presented elsewhere.1
The accessibility of all FCC catalysts drops from exposure to metal contaminants present in the feed. Sodium, calcium, and nickel all impact accessibility, but Figure 1 shows the particularly important effect of iron. Healthy equilibrium catalyst has a visibly porous surface and shows, in this example, an AAI greater than 5. The adjacent catalyst with nodules is heavily poisoned by iron, lacks porosity, and presents a glazed surface appearance. AAI of this poisoned catalyst is below 2. The morphology of iron contamination is furthermore illustrated in Figure 2, a cross-section micrograph with false colour mapping. The iron clearly presents a defined ring at the surface. As an example, bulk iron of a catalyst might measure less than 1 wt% by traditional XRF though the actual surface ring is well over 6 wt%.
The phenomenon of iron poisoning shows why traditional, static methods to assess catalyst architecture are incomplete. Nitrogen adsorption is most common and provides valuable information on catalyst surface area, mesoporosity, microporosity, and pore size distribution. This information can be misleading, though, especially with equilibrium catalyst. Having a very small kinetic diameter, nitrogen can penetrate even small surface openings remaining in glazed, contaminated catalyst. Furthermore, adsorption methods utilise an equilibrated gas-solid system which inherently fails to account for diffusion. The net result is that a refiner needs to know both the total porosity remaining in his catalyst (from adsorption) and the accessible porosity (from Albemarle’s AAI test).
Albemarle has documented repeated commercial cases illustrating a tangible correlation between AAI and unit performance. Figure 3 is a pointed example because the catalyst fell below the refiner’s ‘critical AAI’; a point where the bottoms increased remarkably as the conversion correspondingly decreased. This critical AAI value will vary from unit to unit depending on feed, operating conditions, and catalyst design. Figure 4 is a commercial example comparing a premium, high AAI catalyst from Albemarle and a competitor. This FCC unit is a high turnover unit and completed three comparisons in a 70-day period. Throughout this period, the unit ran a high iron feed and recorded a relatively stable iron on Ecat of ~0.75%. In all three comparisons, the refiner witnessed an increase in conversion and drop in slurry with the high accessibility catalyst from Albemarle.
Accessibility should be a key consideration during all parts of the catalyst selection and evaluation process. In the selection process, refiners should be aware that AAI generally increases in laboratory deactivation protocols while it decreases in the field. Corrections are not straightforward, and Albemarle recommends field trials to evaluate catalyst performance. Refiners opting for this recommended approach can work with technical service to select an optimal accessibility catalyst (Upgrader, Action, AFX, or Coral). During the evaluation and monitoring process, refiners can learn what metals most influence their AAI and establish action levels. For example, a refiner knowing that calcium impacts his AAI can proactively adjust catalyst addition rates. Furthermore, refiners can re-formulate to higher AAI catalyst when planning acquisition of heavy, high metals feeds. Refiners choosing high accessibility catalyst will be reap optimal profits in today’s dynamic marketplace of challenging, unconventional feedstocks.
Reference: 1 US Patent: 6828153
This short case study originally appeared in PTQ's Technology In Action feature - Q2 2016 issue.
For more information: Ryan.Nickell@Albemarle.com
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