Processing, June 2018

Powder Handling F i x i n g fluidization Using dynamic powder testing to understand and improve the performance of fluidized bed processes By Jamie Clayton Freeman Technology Ray Cocco Ph D Particulate Solid Research Inc anaging solid loss rates from a fluidized bed is a critical aspect of design and operation particularly in catalytic processes where annualized losses can add up to M millions of dollars in a suboptimal system 1 Effective sizing of the cyclones and diplegs associated with the return of entrained solids is crucial but relies on having a reliable estimate of particle size and loading in the disengagement zone freeboard above the bed Conventionally particles in a fluidized bed are assumed to behave independently once suspended but particle clustering has long been recognized as a phenomenon with significant potential to affect entrainment behavior 2 A better understanding of the impact of particle clustering and of the factors that affect it is becoming increasingly important to support the growing use of computational fluid dynamics CFD to tackle fluidized bed challenges Fluid catalytic cracking FCC the conversion of high boiling high molecular weight hydrocarbon fractions to gasoline olefin gases and other valuable products is one of the most commercially important applications of fluidized bed technology Currently around 350 FCC units are operating worldwide with a total processing capacity of more than 147 million barrels per day typical catalyst circulation rates are in excess of 50000 metric tons of catalyst per day Controlling fluidization behavior and managing solids losses in both the FCC and the associated catalyst regenerator is crucial for beneficial economic performance In this study the dynamic flow shear and bulk properties of FCC catalysts with different fines content were characterized using an FT4 Powder Rheometer from Freeman Technology with the aim of developing insight into particle clustering behavior A key goal was to use dynamic powder testing to investigate how fines influence the characteristics of a powder bed and fluidization performance to elucidate the potential impact of particle clustering Understanding fluidized bed processes In a fluidized bed particles are suspended in an upward flowing stream of fluid a gas vapor or liquid to form a highly mobile mixture Particles vary significantly in terms of their response to fluidization behavior notably classified by the Geldart groupings which are used extensively in fluidized bed design This classification of fluidization behavior divides powders into four groups A through D which differ primarily in terms of particle size and density Most FCC catalysts fall into group A which includes particles with a size range of 20 to 100 μm and a density less than 14 g cc A typical particle size distribution for FCC catalysts ranges from 10 to 150 μm while density tends to be less than 1g cc 080 to 096 g cc Group A powder beds typically exhibit significant expansion when fluidized reducing substantially in bulk density before transitioning to a steady bubbling bed state 2 Solids losses are a result of the entrainment of powder from the fluidized bed which is directly influenced by gas velocity and by particle properties such as size shape and surface morphology Particle clustering has the potential to reduce the rate of entrainment at a given gas velocity by effectively increasing particle size and is a behavior particularly associated with Group A powders a finding attributed to the fact that powders in this group contain relatively high levels of fines 10 Solids losses can also be influenced by particle attrition the breakdown of primary catalyst particles by collisions and or the shear force associated with fluidization Higher levels of fines in the fresh catalyst or as a result of attrition might be expected to increase entrainment but in fact the opposite has been observed an effect similarly attributed to the propensity of finer particles to form clusters Clusters tend to consist of several smaller particles adhering to just one or two larger particles but are also prone to break down above a certain critical size 3 11 13 These multiple competing mechanisms that influence particle characteristics within the bed may help to explain the variability seen in predictions of entrainment rate 1 1 variability that directly impacts the optimization of solids management equipment Particle clusters have been observed in the freeboard of fluidized beds containing FCC catalysts but the electrostatic and physical mechanisms that drive their formation are not yet fully resolved There is evidence that cluster formation is influenced by the level of fines in the bed 1 1 but it is still unclear Where clusters form Solids concentration in the freeboard is low suggesting that clusters are more likely to form in the bed itself If clusters form in the bed then whether their formation is limited to the region around and below bubbles in which case clustering would just affect entrainment rates or whether the process occurs throughout the bed thereby also impacting its hydrodynamics How fines affect cluster formation and associated fluidization behavior Historically fluidized bed processes have largely 32 Processing JUNE 2018 Figure 1 Bulk property data for the three catalyst samples shows that the Medium Fines sample is more permeable and less compressible than would be expected from the remaining data suggesting it results in a very efficiently packed powder bed All graphics courtesy of Freeman Technology