In fluidised bed processes, the solids are in vigorous motion and thus inevitably subjected to mechanical stresses due to inter-particle and particle-wall impacts. These stresses lead to wear, erosion and in extreme case breakage of the particles which is quite often an unwanted phenomenon that is therefore termed attrition.
Processes like Fluid Catalytic Cracking (FCC) and Chemical Looping Combustion (CLC) are based on the circulating fluidised bed design where solids are fluidised, entrained, collected in cyclones and sent to a second fluidised bed, usually a regenerator, after which the solids are sent back to the initial bed. Therefore, there is plenty of chances for attrition, either from mechanical, thermal or chemical sources. In particular, the main cause of attrition is related to the mechanical stresses.
Significant contributions of attrition come from the air jets of the fluidised bed distributor, the bubbling bed and the cyclones.
In the first case, particles are entrained in the air jets, where they get accelerated and impacted onto the fluidised bed suspension. The jet induced attrition only affects part of the bed which is of course limited by the jet length, where the mode of attrition is believed to be mainly collisional.
The cyclones are another source of attrition because particles experience significant shear and high velocity impact at the entrance.
Inter-particle collisions in the bubbling bed occur at smaller velocities due to the rise of bubbles in the bed; so it is usually less disruptive than jet regions and cyclones but still considerable, given the scale of the these processes.
Analysis of the complex process of attrition in these systems is carried out at several levels: (i) Single particle, (ii) bulk attrition testers, (iii) modelling of attrition in large scale operation, (iv) experimental trials at pilot scale at IFPEn, Solaize, France. CLC single particle properties and propensity to breakage are evaluated experimentally by the Single Particle Impact test and the Scirocco disperser Impact test, in order to obtain a kernel of breakage, e.g. extent of breakage as function of size and impact velocities. Gravimetric extent of breakage of CLC particles for the full range of velocities. Such kernels will be coupled with a scale down fluid-dynamic analysis of the aforementioned regions by CFD-DEM simulations in order to describe the attrition locally and eventually develop a population balance model able to predict the attrition extent and the evolution of the particle sizes in the system. Validation of such models will be carried out using small scale fluidised beds and cyclones. The population balance model should be eventually able to predict attrition for the actual scale process via a scale-up validation.