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

A new mathematical model for nucleation of spherical agglomerates by the immersion mechanism

Initial wetting of crystals by binder droplets is a key rate process in spherical agglomeration, however there are no models to predict the kinetics and formation of agglomerate nuclei. Two new mathematical models are introduced for agglomerate nucleation by an immersion mechanism; immersion rate limited model and collision rate limited model. The agglomerate nucleation number developed in this work predicts different regimes; immersion rate limited, collision rate limited and intermediate. In an immersion rate limited regime, agglomerate size increases with square root of time. In a collision rate limited regime, size increases linearly with time if the bulk crystal volume fraction, φPb, is constant, or with an exponential decay rate for batch crystallisation with decreasing φPb. The timescale for nucleation is less than ten minutes for a broad range of conditions, significantly less than most crystallisation timescales. These models have great promise for population balance modelling and spherical agglomeration optimisation

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

3D printed elastic mould granulation

YesIn the pharmaceutical industry, enhanced process understanding resulting in superior control of product attributes, has the potential to save up to 20% of process engineering and product development costs during drug development. With the aim of achieving enhanced process understating, a novel approach for granulation of fine powders is presented. First, a mould with the desired particle shape and size is created using 3D printing followed by casting using elastomeric material. The formulation is prepared through wet massing and tested as a thin film on flat elastomeric membranes. The thin film itself can be a product but it also gives a good indication of coating performance before coating the patterned elastic membrane with the formulation i.e., 3D printed elastic mould granulation. Results show that following granulation and drying, granules of controlled size and shape (e.g. cubic and 500 μm), strength, friability and flowability can be formed. The method presented may allow for more robust process development in particle engineering.Research Development Fund Publication Prize Award winner, December 2018

Attrition of alumina catalyst carrier beads.

Attrition of alumina catalyst carrier beads in reforming units causes operational problems and the loss of the catalyst particles due to the formation of fines and small fragments. This thesis addresses the characterisation and optimisation of the mechanical strength of these beads in collaboration with Institut Francais du Petrole (IFP) and Rhodia. A methodology was devised to test and improve the particle strength at various strain rates using both single particle and multiple particle tests by considering the mechanical stresses prevailing in industrial units. This methodology was tested with a commercial sample and then used to assess the strength of new samples for which the bead structure was modified by changing the filler concentration and type, the macroporosity, the drying regime and the surfactant concentration. A significant increase in the particle strength was achieved in comparison with the commercial samples. The mean crushing strength increased by a factor of about three and the extent of impact attrition was significantly decreased, e.g. by a factor of 30 for normal impacts at 20 m s-1. For single particle testing, a relationship between quasi-static and impact results was obtained when the impact breakage was compared with the percentage of weak particles obtained from the side crushing strength (SCS) test. This suggests that for this type of material the particle strength is not sensitive to the strain rate. Multiple particle tests confirmed the results obtained by single particle tests for two samples for which sufficient quantity of test material was available. In order to relate the extent of attrition in a particle assembly under compressive loading to the single particle properties, the BCS test was simulated by distinct element analysis using the TRUBAL code. Trends similar to the experimental work were obtained for the simulation of the attrition. However, the simulations tend to underestimate slightly the extent of attrition, which is highly dependent on the particle strength distribution and on the contact force distribution within the particle assembly. As a result of this work, the manufacture of the alumina catalyst carrier beads used in reforming units has been significantly improved

Attrition of alumina catalyst carrier beads.

Attrition of alumina catalyst carrier beads in reforming units causes operational problems and the loss of the catalyst particles due to the formation of fines and small fragments. This thesis addresses the characterisation and optimisation of the mechanical strength of these beads in collaboration with Institut Francais du Petrole (IFP) and Rhodia. A methodology was devised to test and improve the particle strength at various strain rates using both single particle and multiple particle tests by considering the mechanical stresses prevailing in industrial units. This methodology was tested with a commercial sample and then used to assess the strength of new samples for which the bead structure was modified by changing the filler concentration and type, the macroporosity, the drying regime and the surfactant concentration. A significant increase in the particle strength was achieved in comparison with the commercial samples. The mean crushing strength increased by a factor of about three and the extent of impact attrition was significantly decreased, e.g. by a factor of 30 for normal impacts at 20 m s-1. For single particle testing, a relationship between quasi-static and impact results was obtained when the impact breakage was compared with the percentage of weak particles obtained from the side crushing strength (SCS) test. This suggests that for this type of material the particle strength is not sensitive to the strain rate. Multiple particle tests confirmed the results obtained by single particle tests for two samples for which sufficient quantity of test material was available. In order to relate the extent of attrition in a particle assembly under compressive loading to the single particle properties, the BCS test was simulated by distinct element analysis using the TRUBAL code. Trends similar to the experimental work were obtained for the simulation of the attrition. However, the simulations tend to underestimate slightly the extent of attrition, which is highly dependent on the particle strength distribution and on the contact force distribution within the particle assembly. As a result of this work, the manufacture of the alumina catalyst carrier beads used in reforming units has been significantly improved

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)