Quantitative plane-resolved crystal growth and dissolution kinetics by coupling in situ optical microscopy and diffusion models : the case of salicylic acid in aqueous solution

The growth and dissolution kinetics of salicylic acid crystals are investigated in situ by focusing on individual microscale crystals. From a combination of optical microscopy and finite element method (FEM) modeling, it was possible to obtain a detailed quantitative picture of dissolution and growth dynamics for individual crystal faces. The approach uses real-time in situ growth and dissolution data (crystal size and shape as a function of time) to parametrize a FEM model incorporating surface kinetics and bulk to surface diffusion, from which concentration distributions and fluxes are obtained directly. It was found that the (001) face showed strong mass transport (diffusion) controlled behavior with an average surface concentration close to the solubility value during growth and dissolution over a wide range of bulk saturation levels. The (1̅10) and (110) faces exhibited mixed mass transport/surface controlled behavior, but with a strong diffusive component. As crystals became relatively large, they tended to exhibit peculiar hollow structures in the end (001) face, observed by interferometry and optical microscopy. Such features have been reported in a number of crystals, but there has not been a satisfactory explanation for their origin. The mass transport simulations indicate that there is a large difference in flux across the crystal surface, with high values at the edge of the (001) face compared to the center, and this flux has to be redistributed across the (001) surface. As the crystal grows, the redistribution process evidently can not be maintained so that the edges grow at the expense of the center, ultimately creating high index internal structures. At later times, we postulate that these high energy faces, starved of material from solution, dissolve and the extra flux of salicylic acid causes the voids to close

Flows of granular material in two-dimensional channels

Secondary cone-type crushing machines are an important part of the aggregate production process. These devices process roughly crushed material into aggregate of greater consistency and homogeneity. We apply a continuum model for granular materials (`A Constitutive Law For Dense Granular Flows', Nature 441, p727-730, 2006) to flows of granular material in representative two-dimensional channels, applying a cyclic applied crushing stress in lieu of a moving boundary. Using finite element methods we solve a sequence of quasi-steady fluid problems within the framework of a pressure dependent particle size problem in time. Upon approximating output quantity and particle size we adjust the frequency and strength of the crushing stroke to assess their impact on the output

Methodology for Investigating the Mechanical Strength of Reforming Catalyst Beads

Reforming catalyst beads must exhibit strong resistance to the mechanical and thermal stresses they experience during their lifetime in a continuous regenerative catalytic unit. An inventory of the mechanical stresses, e. g. compressive, impact and shearing, is presented. It shows that a multitest approach must be designed in order to measure the particle strength and then optimise the production process to enhance their strength. This approach combines measurements reproducing the different types of stress generated in the catalytic process with an extensive characterisation of the physical and mechanical properties of the porous solid such as Young's modulus, hardness and fracture toughness. The methodology outlined here on alumina beads goes beyond the common practice of evaluating catalyst strength based on a comparative study using a single-crushing test and a bulk-crushing test. Prediction of bulk attrition and breakage behaviour, based on the above properties, is achieved using distinct element analysis (DEA)

Compressive Properties of Micro-spherical SiO2 Particles

Micron-sized, spherical SiO2 particles are important various industrial applications, such as in heterogeneous catalyst preparation. In particular, many of industrially relevant olefin polymerization catalysts are currently prepared using micro-spherical silica as catalyst support. In large-scale catalytic polyolefin production, the quality of the final product, as well as the process efficiency is crucially dependent on overall consistency, quality, physico-chemical properties of the catalyst. As the catalyst particle experiences various stresses during the polymer particle growth, mechanical properties of catalyst play a key role in its performance in the polymerization process. However, there is currently a lack of experimental mechanical property measurements of micron-sized, spherical SiO2 particles relevant for the polyolefin catalyst production. In this work, compressive properties of commercial porous micro-spherical silicas were studied using a quasi-static micro-compression method. The method includes compressing single, micron-sized particles in controlled loading conditions. From the measurements, the compressive elastic-plastic properties of these particles can be determined.Peer reviewe