(448y) Comparing the Flow Behaviour of FCC Catalysts Using the FT4 Powder Rheometer®

The Fluid Catalytic Cracking (FCC) unit remains the primary hydrocarbon conversion unit in the modern petroleum refinery. It is widely used to convert the high-boiling, high-molecular weight hydrocarbon fractions of petroleum crude oils, for example straight-run atmospheric gas oil, vacuum gas oils and heavy stocks recovered from other operations to more valuable gasoline, olefin gases, and other products [1]. There are currently around 350 FCC units operating worldwide with a total processing capacity of over 14.7 million barrels per day [2]. The profitability of an FCC unit depends largely on the type of feed being processed and the FCC catalyst employed, with a typical unit processing up to 75,000 barrels per day and circulating over 55,000 tonnes of catalyst per day [3].

Modern FCC catalysts are fine powders with typical bulk densities of 0.80 to 0.96 g/cm³ and a particle size distribution ranging from 10 to 150 µm with an average of 60 to 100μm. The main active component is usually a stabilized form of zeolite Y but other components may include clay, alumina and silica sources as well as compounds for trapping metals [2, 4]. The components are typically mixed in aqueous slurry, and then spray-dried to form near uniform spherical particles.

During the cracking processes, the hydrocarbon vapours "fluidize" the powdered catalyst and the mixture of hydrocarbon vapours and catalyst flows upward to enter the reactor at a temperature of about 700^oC and a pressure of about 1.72bar. The regenerated catalyst is then circulated back to the riser reactor. This is all done continuously at circulating rates ranging from 5 to 25 tons per min. As such it is essential that the selected FCC catalyst exhibits the desired fluidisation behaviour. This is especially true in the catalyst regenerator were poor fluidization can result in higher temperatures, lower productivity and higher NOx emissions. Furthermore, as the catalyst will undergo a large number of collisions during this process the catalyst particles must also be resistant to attrition[4].

In this study, a range of FCC catalysts with different fines content were characterised using an FT4 Powder Rheometer[5] with the aim of understanding the observed variation in process behaviour. Measurements included evaluating dynamic flow, bulk and shear properties of the powders including an assessment of fluidisation behaviour and permeability.

The FT4 data demonstrated clear differences in both the dynamic flow and bulk properties of the catalysts, explaining why they performed differently in the process. The results also highlighted that an increase in fines content can actually lead to improved flow properties in some applications as the fine particle fraction can act as a lubricant but it can also result in high levels of particle clustering in other cases.

References

[1] Coker, A.K., Ludwig's applied process design for chemical and petrochemical plants. (Elsevier Science), 2014.

[2] Sadeghbeigi, R., Fluid catalytic cracking handbook: An expert guide to the practical operation, design, and optimization of fcc units. (Butterworth-Heinemann), 2012.

[3] Gary, J.H. Handwerk, G.E., Petroleum refining. (Taylor & Francis), 2001.

[4] Vogt, E.T.C. Weckhuysen, B.M., Fluid catalytic cracking: Recent developments on the grand old lady of zeolite catalysis. 2015:44:7342-7370.

[5] Freeman, R., Measuring the flow properties of consolidated, conditioned and aerated powders â?? a comparative study using a powder rheometer and a rotational shear cell. Powder Technol.2007:174:25-33.