CFD Modeling of gas-solid reactors

Ranade, V.V. (1999), Modelling of gas-solid flows in FCC riser reactor fully developed flow, 2nd International Conference on CFD, Melbourne. 

The EMMS model was proposed for the time-mean behavior of fluidized beds on the reactor scale. A more extensive application of the EMMS model to gas-solid flow is through its coupling with the two-fluid CFD approaches, which brings about an EMMS-based multi-scale CFD framework for gas—solid flow. For this purpose, Yang et al. (2003) introduced an acceleration, a, into the EMMS model to account for the... 

Prediction of gas solid flow fields, in processes such as pneumatic transport lines, risers, fluidised-bed reactors, hoppers and precipitators are crucial to the operation of most process plants. Up to now, the inability to accurately model these interactions has limited the role that simulation could play in improving operations. In recent years, CFD software developers have focused on this area to develop new modelling methods that can simulate gas-liquid-solid flows to a much higher level of reliability. As a result, process industry... 

Our discussion of multiphase CFD models has thus far focused on describing the mass and momentum balances for each phase. In applications to chemical reactors, we will frequently need to include chemical species and enthalpy balances. As mentioned previously, the multifluid models do not resolve the interfaces between phases and models based on correlations will be needed to close the interphase mass- and heat-transfer terms. To keep the notation simple, we will consider only a two-phase gas-solid system with ag + as = 1. If we denote the mass fractions of Nsp chemical species in each phase by Yga and Ysa, respectively, we can write the species balance equations as... 

The list is merely suggestive. Complexity of reactive flows may greatly expand the list of issues on which further research is required. Another area which deserves mention here is modeling of inherently unsteady flows. Most flows in engineering equipment are unsteady (gas-liquid flow in a bubble column reactor, gas-solid flow in a riser reactor and so on). However, for most engineering purposes, all the details of these unsteady flows are not required to be known. Further work is necessary to evolve adequate representation of such flows within the CFD framework without resorting to full, unsteady simulations. This development is especially necessary to simulate inherently unsteady flows in large industrial reactors where full, unsteady simulations may require unaffordable resources (and therefore, may not be cost effective). Different reactor types and different classes of multiphase flows will have different research requirements based on current and future applications under consideration. 

While the lower order models described in Section 6.3 are useful for the quick prediction of the overall performance of a reactor, these models often rely on simplified flow approximations and often fail to account for change in the local fluid dynamics or transport processes during the presence of internal hardware or changes in flow regimes. Moreover, these models are also based on empirical knowledge (as discussed in Section 6.4) of several parameters such as interfacial area, dispersion coefficients, and mass transfer coefficients. Some of these limitations may be avoided by using CFD models for simulations of gas-liquid-solid flows in three-phase slurry and fluidized bed. 

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