Computational fluid mixing

Computational fluid dynamics (CFD) is an extremely powerful tool for solving problems associated with flow, mixing, heat and mass transfer and chemical reaction. Although the equations of motion for fluid flow were established in the first half of the nineteenth century (e.g. Navier, 1822; Stokes, 1845), it was not until the arrival of digital computers in the 1960s and 1970s that it became feasible to perform numerical simulations of complex engineering flows. In these early days, CFD was a very much a research tool and most of the early work was aimed at developing numerical methods, solution algorithms and Reynolds-averaged turbulence models. However, in the 1980s, the first commercial codes emerged — e.g. PHOENICS, FLUENT, FIDAP, Star-CD, FLOW3D (which later became CFX) — providing general purpose software packages for both academic and industry users. The aerospace and automotive industries were amongst the first to embrace the use of CFD in engineering design, but from the 1990s onwards commercial codes have found widespread applications, for example in: biomedical engineering, environmental and atmospheric modelling, meteorology, chemical reaction engineering and more recently in the food and beverage industries. This chapter will focus on mixing vessel applications for the last two of these industry sectors, where CFD is increasingly used to provide process understanding and semi-quantitative analysis. In their review, Norton and Sun (2006) presented a graph showing the very significant increase in the number of peer-reviewed papers related to CFD applications to food process engineering. Figure 0.1 is an updated version of this graph, containing more recent data and showing that the number of papers that specifically analyse food mixing operations using CFD is still relatively small. In contrast, there are a vast numbers of papers on CFD simulation of (i) other food process operations, (e.g. drying, sterilisation, thermal treatment and extrusion, many of which are described by Sun (2007)) and (ii) more conventional mixing operations in the chemicals and specialty product industries (see for example, Marshall and Bakker (2004)). This chapter will outline the background knowledge required for CFD studies, present some examples of CFD modelling of mixing vessel flows and finally will discuss the current difficulties in applying this approach to food mixing processes

Developments in fluidised bed freeze drying

Developments in fluidised bed freeze dryin

An experimental study of microneedle-assisted microparticle delivery

A set of well-defined experiments has been carried out to explore whether microneedles (MNs) can enhance the penetration depths of microparticles moving at high velocity such as those expected in gene guns for delivery of gene-loaded microparticles into target tissues. These experiments are based on applying solid MNs that are used to reduce the effect of mechanical barrier function of the target so as to allow delivery of microparticles at less imposed pressure as compared with most typical gene guns. Further, a low-cost material, namely, biomedical-grade stainless steel microparticle with size ranging between 1 and 20 μm, has been used in this study. The microparticles are compressed and bound in the form of a cylindrical pellet and mounted on a ground slide, which are then accelerated together by compressed air through a barrel. When the ground slide reaches the end of the barrel, the pellet is separated from the ground slide and is broken down into particle form by a mesh that is placed at the end of the barrel. Subsequently, these particles penetrate into the target. This paper investigates the implications of velocity of the pellet along with various other important factors that affect the particle delivery into the target. Our results suggest that the particle passage increases with an increase in pressure, mesh pore size, and decreases with increase in polyvinylpyrrolidone concentration. Most importantly, it is shown that MNs increase the penetration depths of the particles

Break up of silica nanoparticle clusters using ultrasonication

This study is concerned with the deagglomeration of hydrophilic silica nanoparticle clusters (Aerosil® 200V) in water using an ultrasonicator operated in batch mode. An impeller was also present in the tank to ensure homogeneity. The effect of power input was studied in the range of 18 to 77 W (9 to 39 kW m-3) on the kinetics and mechanisms of deagglomeration and the dispersion fineness. The effect of particle concentration was also studied in the range of 1 to 15% wt. The process was monitored through the evolution of particle size distribution (PSD), which indicated erosion as the dominant mechanism of breakup. The smallest attainable particle size was found to be independent of power input and solid concentration. Faster break up kinetics were noted as the power input was increased whereas increasing the solids concentration to 15% wt. slowed the process. It could also be shown that processing concentrated dispersions can be beneficial as the break up rate assessed on the basis of energy per unit mass of solids was faster for increased particle concentration

Microneedle assisted micro-particle delivery by gene guns: mathematical model formulation and experimental verification

Gene gun is a micro-particles delivery system which accelerates DNA loaded micro-particles to a high speed so as to enable penetration of the micro-particles into deeper tissues to achieve gene transfection. Previously, microneedle (MN) assisted micro-particles delivery has been shown to achieve the purpose of enhanced penetration depth of micro-particles based on a set of laboratory experiments. In order to further understand the penetration process of micro-particles, a mathematical model for MN assisted micro-particles delivery is developed. The model mimics the acceleration, separation and deceleration stages of the operation of a gene gun (or experimental rig) aimed at delivering the micro-particles into tissues. The developed model is used to simulate the particle velocity and the trajectories of micro-particles while they penetrate into the target. The model mimics the deceleration stage to predict the linear trajectories of the micro-particles which randomly select the initial positions in the deceleration stage and enter into the target. The penetration depths of the micro-particles are analyzed in relation to a number of parameters, e.g., operating pressure, particle size, and MNs length. Results are validated with experimental results obtained from the previous work. The results also show that the particle penetration depth is increased from an increase of operating pressure, particle size and MN length. The presence of the pierced holes causes a surge in penetration distance. © 2014 Elsevier Ltd. All rights reserved

State feedback linearization and adaptive model predictive control applied to a simulated MSMPR crystalliser [Abstract]

State feedback linearization and adaptive model predictive control applied to a simulated MSMPR crystalliser [Abstract

Microneedle assisted micro-particle delivery from gene guns: experiments using skin-mimicking agarose gel

A set of laboratory experiments has been carried out to determine if micro-needles (MNs) can enhance penetration depths of high-speed micro-particles delivered by a type of gene gun. The micro-particles were fired into a model target material, agarose gel, which was prepared to mimic the viscoelastic properties of porcine skin. The agarose gel was chosen as a model target as it can be prepared as a homogeneous and transparent medium with controllable and reproducible properties allowing accurate determination of penetration depths. Insertions of various MNs into gels have been analysed to show that the length of the holes increases with an increase in the agarose concentration. The penetration depths of micro-particle were analysed in relation to a number of variables, namely the operating pressure, the particle size, the size of a mesh used for particle separation and the MN dimensions. The results suggest that the penetration depths increase with an increase of the mesh pore size, because of the passage of large agglomerates. As these particles seem to damage the target surface, then smaller mesh sizes are recommended; here, a mesh with a pore size of 178 μm was used for the majority of the experiments. The operating pressure provides a positive effect on the penetration depth, that is it increases as pressure is increased. Further, as expected, an application of MNs maximises the micro-particle penetration depth. The maximum penetration depth is found to increase as the lengths of the MNs increase, for example it is found to be 1272 ± 42, 1009 ± 49 and 656 ± 85 μm at 4.5 bar pressure for spherical micro-particles of 18 ± 7 μm diameter when we used MNs of 1500, 1200 and 750 μm length, respectively

Potential of microneedle-assisted micro-particle delivery by gene guns: a review

Abstact Context: Gene guns have been used to deliver deoxyribonucleic acid (DNA) loaded micro-particle and breach the muscle tissue to target cells of interest to achieve gene transfection. Objective: This article aims to discuss the potential of microneedle (MN) assisted micro-particle delivery from gene guns, with a view to reducing tissue damage. Methods: Using a range of sources, the main gene guns for micro-particle delivery are reviewed along with the primary features of their technology, e.g. their design configurations, the material selection of the micro-particle, the driving gas type and pressure. Depending on the gene gun system, the achieved penetration depths in the skin are discussed as a function of the gas pressure, the type of the gene gun system and particle size, velocity and density. The concept of MN-assisted micro-particles delivery which consists of three stages (namely, acceleration, separation and decoration stage) is discussed. In this method, solid MNs are inserted into the skin to penetrate the epidermis/dermis layer and create holes for particle injection. Several designs of MN array are discussed and the insertion mechanism is explored, as it determines the feasibility of the MN-based system for particle transfer. Results: This review suggests that one of the problems of gene guns is that they need high operating pressures, which may result in direct or indirect tissue/cells damage. MNs seem to be a promising method which if combined with the gene guns may reduce the operating pressures for these devices and reduce tissue/cell damages. Conclusions: There is sufficient potential for MN-assisted particle develivery systems

Pharmaceutical crystallisation processes from batch to continuous operation using MSMPR stages: modelling, design, and control

In pharmaceuticals manufacturing, the conversion of conventional batch crystallisations to continuous mode has the potential for intensified, compact operation and more consistent production via quality-by-design. A pragmatic conversion approach is to utilise existing stirred tank batch crystallisers as continuous mixed-suspension mixed-product removal (MSMPR) stages. In this study, a rigorous and general mathematical model is developed for a pharmaceutical crystallisation process under continuous MSMPR operation. In the proposed changeover from batch to continuous operation, concentration control (C-control), which has been well accepted in batch crystallisation operation, is further extended to facilitate the convenient design of the steady-state operating point of a continuous MSMPR crystalliser; an objective is to ensure that the start-up procedures and on-line control conditions fall within the design-space of the original batch operation. Both single-stage and cascaded two-stage MSMPR crystallisers were investigated and compared to the conventional batch operation. It was observed that despite the production of a smaller number-based mean crystal size, the proposed continuous MSMPR operation achieved higher production capacity with shorter mean residence time and comparable product yield to the batch operation. Lastly, the robustness of C-control strategy against uncertainties in crystallisation kinetics was also demonstrated for the proposed continuous MSMPR operation

A freeze-drying microscopy study of the kinetics of sublimation in a model lactose system

Freeze drying microscopy has been used to probe the lyophilisation kinetics of lactose solutions of various concentrations, at temperatures ranging from -50. °C to -30. °C and under a constant pressure of 1. Pa. Sublimation front velocities were determined by recording a sequence of video images of the sublimation and analysing the frontal progression using MATLAB. Initial experiments showed poor reproducibility. To combat this, silver iodide (AgI) was added as an ice nucleator, which raised nucleation temperatures and improved reproducibility when compared to non-AgI experiments. The lower supercooling on nucleation when AgI was used also produced larger ice crystals, which enabled the crystal microstructure of the more dilute samples to be more clearly observed. This showed long thin crystals, and the orientation of these crystals with respect to the direction of the frontal movement strongly affected frontal progression rates, which explained the earlier reproducibility problems. A twin resistance mass transfer model, comprising a fixed edge resistance and a resistance which increased with frontal depth, was able to describe the sublimation kinetics. The edge resistance first increased and then decreased with solids content. The resistance per unit depth increased exponentially with solids content, so much so that there is an optimal solids content in relation to the rate of production of dried material. Resistances were also much higher when crystals were oriented with their major axis perpendicular to the direction of frontal movement. Freeze drying rates were approximately proportional to the saturation vapour pressure of water, however the long-held belief that water vapour pressure is the main driving force for mass transfer in freeze-drying systems may be an oversimplification as this only reflects driving forces in the vapour phase (pores) rather than within the solid