Solids Flow Pattern in Gas – Flowing Solids – Fixed Bed Contactors
Advancing the chemical engineering fundamentals
Multifase Flows - III (T2-5c)
Keywords: multiphase systems, gas – flowing solids – fixed bed contactors, residence time distribution, solids flow model, axial dispersion
Gas – flowing solids – fixed bed contactors are multiphase systems with countercurrent flow of gas and fine particles through a packed bed of larger particles. These contactors can be considered as two phase or three phase systems. In the first case, gas and flowing solids are the active phases, while the packing serves only to enable better contact between the flowing phases. In such a case there are no limitations in geometry and design of packing elements. In the second case, the packing elements present the third active phase, as in heterogeneous catalytic reactors with in situ separation. In these cases the geometry of packing elements is limited by the process requirements, i.e. by the shape of catalyst particles. However, in both cases solids flow pattern is an important characteristic of the system. In this paper it will be interpreted by a differential, macroscopic mathematical model.
Residence time distribution (RTD) measurements confirmed that the flow of particles in gas – flowing solids – fixed beds is close to plug flow. Deviation from desirable (plug) flow pattern origins from two sources: back-mixing and the presence of stagnant solids which are not completely inactive. A differential model based on material balances was developed to describe these phenomena. Model assumes two flowing solids zones: dynamic for fast moving (freely suspended) particles and stagnant for particles temporarily settled on the packing. Furthermore model presumes plug flow with axial dispersion for dynamic zone and exchange of particles between dynamic and stagnant zones.
Complex hydrodynamic behavior of the stagnant particles was represented by an approximate exchange model. Its optimized parameters (exchange rates) were obtained on the base of the tracer experiments in static holdup. In the balance equation for the dynamic zone, the optimized parameter was Peclet number which corresponded with an axial mixing. Well known Dankwert’s boundary conditions were used for this model. The other physical properties of the particle flow pattern: dynamic and static holdup and “dead” zone were determined from additional experiments, so they were not considered as model parameters. The simulation results are in very good agreement with the experimentally obtained RTD curves, as well as with the static holdup response curves.
Different operational conditions were used in calculations according to experimental data (four different solids fluxes and four gas velocities). The model simulations confirmed a minor influence of gas velocity and considerable influence of solids flux on solids flow patter. These previously withdrawn conclusions were here quantified through the values of model parameters. Sensitivity analysis was performed in order to allocate the significance of model parameters.
Presented Tuesday 18, 09:45 to 10:05, in session Multifase Flows - III (T2-5c).
