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Application 1029

Channelling effects common in columns removed with rotating bed reactors

Fixed bed reactors (FBRs), also known as packed bed reactors (PBRs), are frequently used for purification, ion-exchange processes, or heterogeneous catalysis. Though relatively easy to construct, they are tedious to charge with solids in an effective way. A poorly packed FBR is subject to channelling, where passages are formed in the bed. These passages offer less resistance to the liquid, which will preferentially travel through those regions. In this work, the robustness with respect to channelling of a packed bed reactor was compared to a rotating bed reactor (RBR) using computational fluid dynamics (CFD).

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By design, the rotating bed reactor is unaffected by channelling and homogeneously utilizes the entire bed, even when very irregularly loaded. The RBR was found to be extremely robust with respect to the level of packing of the solid phase within, while the FBR was strongly negatively affected by channelling. As a result, the FBR will exhibit breakthrough before the solid-phase media has been fully exploited. Avoiding this requires painstakingly careful packing of the FBR.

Diagram showing the effect of channelling on the flow rate through a packed bed inside a rotating bed reactor and fixed bed reactor, respectively

Diagram showing the effect of channelling on the flow rate through a packed bed inside an RBR and FBR, respectively.

The robustness with respect to packing yields the capability to use an RBR with any rate of loading for processes such as:

Being able to load the reactor with different amounts of solid phase means that the same hardware can be used for multiple applications. Multi-purpose reactors will in turn reduce the capital expenditures for the process owner, who can invest in fewer pieces of equipment. In conclusion, the RBR offers:

  • Dramatically simplified packing operations
  • Complete and homogeneous utilization of solid-phase media
  • Flexibility with respect to rate of loading

 

Further reading

Topics Support, Reactor Engineering

Products SpinChem® RBR S2

Details

ANSYS Fluent was used to make simulations of a spinning RBR and a stationary FBR with the same geometry. The simulated bed was in both cases split into two halves so that a loosely packed region could be modelled alongside an optimally packed region. The flow rate determined for the RBR at optimal packing was pumped through the FBR for all degrees of packing. Homogeneous packing was simulated by assigning the region a specific permeability coefficient corresponding to that of an ion exchange resin of uniform particle size. This value served as a reference value for the simulations, and corresponds to a pressure drop of 0.054 bar/m for water at 20°C with a linear velocity of 1 mm/s. Moderate and severe channelling was assigned specific permeability values corresponding to 133% and 200% of that of the reference, respectively.