Fluidised beds

In a fluidised bed a solid material is maintained in a fluid like state by passing a gas or liquid up through the bed at sufficient velocity to maintain fluid behaviour. They are often used in reactions where the solid is a catalyst or a heat transfer medium. When optimised, the advantages are excellent mixing and a well mixed temperature profile.

In operation the challenges are to maintain the bed fluidised, while ensuring the solid particles are not “carried over” due to too high a gas velocity and allowing any waste reaction products such as carbon to be removed. The largest use is the fluidised cat cracker (FCC riser) in a refinery.

Fluidised bed with ECT sensor placed part way down

Again the measurement requirement is to measure the homogeneity of the fluidised bed and the presence of any waste products that may need removing. This is very difficult from inside the vessel as any measurement system potentially disturbs the fluidity.

Considerable use has been made of process tomography in the investigation of different types of fluidised beds.  This work has spanned the spectrum of highly theoretical studies to pragmatic large scale optimisation projects.

Powder flow

Electrical Capacitance Tomography (ECT) is a measurement technique for obtaining information about the contents of process vessels and pipelines. Multiple electrodes are arranged around the boundary of the vessel at fixed locations in such a way that they do not affect the flow or movement of materials. For electrically insulating pipes the electrodes can be mounted externally and for electrically conducting pipes the electrodes must be mounted internally.

A typical application is real time monitoring of multicomponent flows within pipelines. Specific applications where ECT has been successfully exploited include solid/gas and liquid (organic)/gas systems such as fluidised beds, pneumatic conveying and multi-phase flow. In principle, ECT can be used to investigate and monitor any process where the main continuous phase is non- conducting and the other phases and components have differing values of permittivity.

Key benefits include:

• shorten cycle times by measuring reaction conditions through bed

• increase yield by measuring phase concentration and boundaries

• measure flow velocities to identify areas of good and poor contact

• development of new distributor arrangements

• ability to monitor gas and liquid flows

"Tomography is one of the few tools that gives information about what is actually happening inside"

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Publications:

Qui C, Joachim R, Karri R, (2011) Understanding Standpipe Hydrodynamics using Electrical Capacitance Tomography, 10th International Conference on Circulating Fluidized Beds and Fluidization Technology - CFB-10 

Wang SJ, Geldart D, Beck MS and Dyakowski, T (2000) A behaviour of a catalyst powder flowing down in a dipleg, Chemical Engineering Journal, Vol. 77, 51-56

For more information about this paper, please contact ITS.

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