Andrew Bayly

Tapping characterisation of high shear mixer agglomerates made with ultra-high viscosity binders

Abstract The time-dependent consistency regimes produced during high shear mixing of zeolite powder and an ultra-high viscosity binder (linear alkylbenzene sulphonate (LAS) paste) were quantitatively described by the bulk aerated and tapped densities, which were determined using both hand and automated tapping techniques. The Hausner ratio was calculated, providing information on the inter-granular friction and cohesivity of the granular bulk as a function of mixing time. An agglomeration mechanism is proposed based upon the trend in Hausner ratio, which was confirmed by optical micrographs and granule size distribution data. The suggested mechanism comprises layering of zeolite particles onto an LAS paste core, breakage of powder-coated paste granules, micro-mixing of the granules, granule growth via coalescence, and finally granule consolidation. The bulk tapping data were analysed using the Kawakita equation and a logarithmic compaction approach. Three distinct compaction regions were identified with the latter analysis, the first of which was related to weak agglomerate break up, and the second and third to granule movement involving elastic and plastic granule deformations respectively during rearrangement. A variety of bulk compaction parameters were obtained, and their variation with mixing time is discussed. At least 10 times as many automated taps were required to reach the final tapped density in comparison to the hand-tapping procedure, and the final density was always lower than that obtained via hand-tapping. When the automated tapping data were scaled in terms of total number of taps and analysed, the parameters describing the bulk compressibility showed similar trends to those obtained from the hand-tapping procedure.

Observing diffusive exchange between surfactant and aqueous domains in detergents.

Nuclear magnetic resonance (NMR) two-dimensional relaxation correlation experiments have been used to study an industrially relevant formulation of surfactant multilamellar vesicles (MLVs) in an aqueous solution. By correlating transverse T2 relaxation measurements before and after a storage interval, diffusive exchange of water molecules between domains can be observed. Two average exchange times of 0.04 and 0.83 s were determined by solving the one-dimensional Fredholm integral form of a model for region-to-region exchange. Diffusion coefficients for the restricted and free water fractions within the mixture determined via chemically selective pulsed field gradient (PFG) NMR measurements allowed exchange distances of 6.2 and 81 microm to be determined from the exchange times. These exchange distances are associated with the average MLV diameter and speculatively with the size of MLV clusters, respectively.

CFD modelling of a pilot-scale counter-current spray drying tower for the manufacture of detergent powder

ABSTRACT A steady-state, three-dimensional, multiphase computational fluid dynamics (CFD) modeling of a pilot-plant countercurrent spray drying tower is carried out to study the drying behavior of detergent slurry droplets. The software package ANSYS Fluent is employed to solve the heat, mass, and momentum transfer between the hot gas and the polydispersed droplets/particles using the Eulerian–Lagrangian approach. The continuous-phase turbulence is modeled using the differential Reynolds stress model. The drying kinetics is modeled using a single-droplet drying model, which is incorporated into the CFD code using user-defined functions (UDFs). Heat loss from the insulated tower wall to the surrounding is modeled by considering thermal resistances due to deposits on the inside surface, wall, insulation, and outside convective film. For the particle–wall interaction, the restitution coefficient is specified as a constant value as well as a function of particle moisture content. It is found that the variation in the value of restitution coefficient with moisture causes significant changes in the velocity, temperature, and moisture profiles of the gas as well as the particles. Overall, a reasonably good agreement is obtained between the measured and predicted powder temperature, moisture content, and gas temperature at the bottom and top outlets of the tower; considering the complexity of the spray drying process, simplifying assumptions made in both the CFD and droplet drying models and the errors associated with the measurements.

The design and scale-up of spray dried particle delivery systems

ABSTRACT Introduction: The rising demand for pharmaceutical particles with tailored physicochemical properties has opened new markets for spray drying especially for solubility enhancement, improving inhalation medicines and stabilization of biopharmaceuticals. Despite this, the spray drying literature is scattered and often does not address the principles underpinning robust development of pharmaceuticals. It is therefore necessary to present clearer picture of the field and highlight the factors influencing particle design and scale-up. Areas covered: The review presents a systematic analysis of the trends in development of particle delivery systems using spray drying. This is followed by exploring the mechanisms governing particle formation in the process stages. Particle design factors including those of equipment configurations and feed/process attributes were highlighted. Finally, the review summarises the current industrial approaches for upscaling pharmaceutical spray drying. Expert opinion: Spray drying provides the ability to design particles of the desired functionality. This greatly benefits the pharmaceutical sector especially as product specifications are becoming more encompassing and exacting. One of the biggest barriers to product translation remains one of scale-up/scale-down. A shift from trial and error approaches to model-based particle design helps to enhance control over product properties. To this end, process innovations and advanced manufacturing technologies are particularly welcomed.