Slip Factor and Average Voidage

An important concern in FCC riser submodels is how to calculate the slip factor, ip, and the average voidage, s, of the riser. The slip factor is simply defined as the ratio between gas velocity and catalyst particle velocity. The slip factor plays an important part in determining the residence time of reactions, and thus, affects the overall conversion in the riser. Harriot describes a slip factor range of 1.2 to 4.0 for most FCC risers but also indicates that there is no reliable correlation available for prediction [44]. Previous authors have used a variety of approaches including constant shp factor [45], multiple slip factors [46] and correlations [47]. An alternative approach is to include additional momentum balance equations for the gas phase and catalyst phase [48]. This approach allows users to calculate velocity profiles for each phase and the overall pressure drop in the riser directly. 

Aspen HYSYS uses a custom correlation based on fully-developed flow (away from the catalyst particle acceleration zone) that accounts for various angles of riser inclination. We present a similar correlation from Bolkan-Kenny et al. [47] in Eq. (4.1) using dimensionless Froude numbers, Eqs. (4.2)-(4.3). This correlation is essentially a function of riser diameter, D gravitational constant, g superficial gas velocity, u and Up terminal settling velocity of the catalyst particle. 

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