Testing & Measurements of Particle Attrition II

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Particle Attrition: Mechanisms and Methods to Determine Attrition Indices

Reddy Karri, PSRI

Particle attrition can be a major issue in using catalyst particles in fluidized beds and circulating fluidized beds.  Particles tend to break down via two mechanisms – abrasion and fragmentation.  Abrasion can be defined as constituent particles attriting off the surface of the parent particles.  The resulting size distribution of parent particles shows almost no change in cut sizes under this abrasion attrition mechanism.  With the fragmentation attrition mechanism, the parent particles are breaking down to intermediate cut sizes and also generating constituent particles.  This results in a shift in the size distribution of parent particles in addition to producing finer constituent particles.

Several methods[1][2] available in characterizing attrition characteristics of catalyst particles.  Jet cup[3] attrition testing is a common method for ranking relative particle attrition of several catalyst materials. An attrition index, calculated from jet cup data, is used to compare with one or more reference materials. However, this method is far from perfect despite its popularity. Results obtained at Particulate Solid Research, Inc. (PSRI) in different-sized jet cups and a 29-cm (11.5-in.)-diameter fluidized bed test unit did not provide the same ranking of catalyst with respect to particle attrition. To obtain a better understanding of attrition in a jet cup, both computational fluid dynamics (CFD) and cold flow studies were performed with a 2.5-cm (1-in.) diameter Davison-type jet cup and PSRI's cylindrical 7.6-cm (3-in.) diameter jet cup. Results showed that a significant amount of material in the Davison and PSRI jet cup remained stagnant. Based on these results and additional CFD modeling, PSRI designed a new jet cup, where most of the material was hydrodynamically active. The new jet cup showed a 25% increase in attrition compared to PSRI's cylindrical jet cup under similar conditions and run times. Results were also compared to cyclone attrition data for several materials at PSRI. The new jet cup provided data that correlated with attrition results from the 29-cm (11.5-in.)-diameter fluidized bed unit.

Benjamin Amblard, IFP Energies Nouvelles

Chemical Looping Combustion (CLC) is an oxy-combustion like technology where oxygen-carrying particles are used to supply oxygen for combustion. The process uses dense group B metal oxide as an oxygen carrier to transfer oxygen from an air reactor to a fuel reactor in a circulating fluidized bed. Apart from reactivity and oxygen transfer capacity, it is also important to study the oxygen carrier resistance to attrition. Indeed during their life-time, particles go through high mechanical, chemical and thermal stresses due to their circulation between the different reactors. Particles with low mechanical resistance will produce fines which can cause many problems such as inventory losses and solids circulation disturbances. Furthermore, attrition may also impact significantly the operating costs of an industrial unit. Therefore, it is important to estimate in the early stages of the process development (where little amount of solids is available) the issues related to attrition for a given oxygen carrier.

In order to evaluate the mechanical resistance to attrition of the different oxygen carriers available, most of the time in small quantities, we proposed to use Fluid Catalytic Cracking (FCC) catalyst as a reference with which many industrial data are available. Indeed, the CLC process is similar in its configuration to the FCC process with a controlled solid flow circulating in a loop between different reactors. Therefore, in both processes, particles go through a similar mechanical stress which is proportional to the solid circulation between reactors.

FCC catalyst and CLC oxygen carriers belong respectively to the Group A and Group B of the Geldart classification. In order to compare the mechanical resistance to attrition of solids from different fluidization groups, we developed a method using a lab scale Jet Cup rig. First, we defined a new attrition index  that does not depend upon a given particle size. It was then found that attrition in the jet cup is proportional to the contact frequency between the air jet and the particles. Therefore it is necessary to estimate the particles circulation within the air jet for each solid tested in order to make sure that particles go through the same mechanical stress. A CFD study showed that in order to have the same circulation of particles in the air jet, therefore the same mechanical stress, tests should be carried out with the same initial volume of particles. Then, the particles mechanical resistance was characterized by the TPGI increase rate with respect to the jet cup test duration. This parameter was then used to compare the solids tested An application of this method will be provided by presenting the results obtained through the EU FP7 Project SUCCESS with the comparison of CLC oxygen carriers produced by the project.

It is important to notice that this method allow to compare solids relatively, meaning that it is possible to know that one solid will behave better with respect to the other in terms of attrition. However it is difficult to get quantitative attrition data at industrial scale from this test. A step forward will be to actually connect the jet cup attrition tests, to the attrition occurring in the principal sources of attrition met in the CLC process such as cyclones and gas injectors. The test results could then be transposed to the attrition at industrial scale using a population balance model.
           

Overcoming the Limitations of Today’s Particle Characterization Technology

Paul Freud, Microtrac

Static Light Scattering (Laser Diffraction), Dynamic Light Scattering, and Dynamic Image Analysis have been applied to the characterization of particles for over 40 years. Each method has limitations for particle size range, particle concentration, and sample preparation. This presentation will explain the theoretical basis for these limitations and will also describe the criteria for choosing the appropriate method for characterizing various applications.