Chris D. Rielly

Effects of Process Variables on the Denaturation of Whey Proteins during Spray Drying

The effects of varying feed concentration (20–40% w/v) and outlet temperature (60 to 120°C) on whey protein denaturation (determined by DSC) and solubility (at pH 4.6) have been investigated in a pilot-scale co-current spray dryer. The study confirms that low outlet gas temperatures (60 and 80°C) produce the lowest amount of denaturation and solubility loss, with almost complete denaturation observed at 120°C along with a less dramatic reduction in solubility. Slightly more denaturation and loss of solubility was found with a 40% feed concentration. It is hypothesized that crust formation, resulting in high particle temperatures while still maintaining a wet core, is likely to lead to high levels of denaturation.

Proving of bread dough: Modelling the growth of individual bubbles.

Proving of bread dough was modelled using classical one-component diffusion theory, to describe the rate of growth of bubbles surrounded by liquid dough containing dissolved carbon dioxide. The resulting differential equation was integrated numerically to predict the effect of initial bubble size and system parameters (carbon dioxide concentration, surface tension at the bubble interface, temperature) on bubble growth. Two situations exist, potentially; the dough could be either supersaturated or subsaturated with carbon dioxide. When the dough is supersaturated, the model predicts a critical bubble size above which bubbles grow indefinitely,while belowthe critical bubble size bubbles reach a limiting size and stop growing. The critical bubble size decreases with increasing carbon dioxide concentration and increases with increasing surface tension.Below saturation, all bubbles reach an upper size limit proportional to their initial size. The final bubble size increases with carbon dioxide concentration and decreases with increasing surface tension. Higher temperatures increase the rate of bubble growth and reduce the critical bubble size for supersaturated doughs, by increasing the value of Henry’ s Law constant. Higher temperatures also increase the final bubble size for subsaturated systems. The model could be extended to include yeast kinetics and entire bubble size distributions, to develop a full simulation of the proving operation.

Gas-inducing impeller design and performance characteristics

A theoretical and experimental study on the design and performance characteristics of gas-inducing impellers is presented. In particular, the model developed by Evans et al. (1991, A.I.Ch.E. Spring National Meeting, Houston, TX, Paper 33e) is critically reviewed and, as a result, improvements to the kinetic energy pressure loss analysis and to the initial conditions are proposed. In addition, the model is successfully extended to account for multiple gas outlet orifice on each blade. Experimental measurements of the power consumption, rate of gas induction, mass transfer coefficient and detached bubble size for a partially optimised, 0.154 m diameter, six-bladed concave gas-inducing impeller are presented. A significant increase in the induced gas rate is observed by adding more outlet orifices to each blade. The principal advantage of using multiple orifices is that similar size bubbles are produced, compared to a single orifice, but larger interfacial areas are generated; the aerated power input is only slightly reduced from its ungassed value. Mass transfer coefficients, kLa, of the order of 0.02 s−1 are attainable for a single outlet orifice on each blade; kLa is significantly increased by using multiple orifices. The dimensionless bubble size distributions, ddgm independent of the impeller speed over the range 4–8 rps, and can be successfully represented by a log-normal distribution.

Free jet expansion and gas entrainment characteristics of a plunging liquid jet

Abstract The change in effective jet diameter is measured as a function of free jet length for vertical liquid jets passing through air. The data are incorporated into a model to predict the rate of gas entrainment for a liquid jet plunging into a confined column of liquid. In the model it was assumed that the total gas entrainment rate included gas contained within (1) the effective diameter of the free jet at the plunge point and (2) an annular film adjacent to the surface of the jet, where the outer boundary of the film was defined to be the separating streamline between the entrained and unentrained components of the moving gas boundary layer. It was further assumed that the radial location of the separating streamline was independent of both liquid and gas flow rates and system geometry. Excellent agreement between model predictions and gas entrainment measurements were obtained once a number of experimental parameters were determined.

Automated direct nucleation control for in situ dynamic fines removal in batch cooling crystallization

Secondary nucleation (e.g. due to attrition) and accidental seeding (e.g. from crust on the wall of vessels) are undesired events that often take place during industrial crystallization processes. The crystal size distribution is greatly affected by these events. The typically used open loop control strategies fail to respond to these dynamic changes resulting in undesired end product properties. Variations in seed quality during seeded operations (e.g. initial breeding) can also result in undesired product quality. The automated direct nucleation control (ADNC) approach presented in this paper automatically detects any changes in the system and removes fines in situ. The approach is tested for external seed additions and accidental seeding scenarios. The results show that the ADNC approach is able to detect any changes in the metastable zone width and drive the system accordingly to dissolve unwanted fines providing an automatic in situ fines removal mechanism.

Bubble formation from cylindrical, flat and concave sections exposed to a strong liquid cross-flow

The results of an experimental study on gas bubble formation from a submerged orifice on a cylindrical, flat or concave blade section, which is exposed to a strong liquid cross-flow, are presented. The effects of the gas velocity in the orifice (3–45 m s-1), the liquid cross-flow velocity (0.5–4 m s-1) and the blade configuration on the mode of bubble formation and the detached bubble size are investigated using high-speed flash photography. The results show that when the orifice is positioned in, or close to, an unseparated flow region the predominant bubbling mode is jetting. In contrast, when the orifice is positioned within the wake region behind the blade, the bubbles are generated individually at the orifice. For a fixed gas velocity, the detached bubble size decreases significantly with increasing liquid velocity, by approximately 50%, between 1 and 3 m s-1; at liquid velocities greater than 3 m s-1, the bubble size decreases more slowly. For a fixed liquid velocity, the bubble size increases approximately linearly with increasing gas velocity through the orifice.

A particle's eye view of crystallizer fluid mechanics

Abstract It has been recognized for some time that flow and mixing in industrial crystallizers has an effect on the kinetics of growth, nucleation and agglomeration and consequently on the crystal size distribution. Yet, a common assumption in population balance modelling is that the fluid mechanical environment experienced by growing crystals is uniform. In practice, however, industrial crystallizers provide extremely varied flow conditions, with local velocities, shear rates and energy dissipation rates varying by orders of magnitude throughout the vessel. A computational fluid dynamics simulation of the flow in a stirred tank was used to illustrate that, in a Lagrangian sense, the particles experienced regions with very different local micromixing characteristics, mean velocities, slip velocities, shear rates and turbulence levels; the sampling of these regions depended only slightly on the particle size and, for the flow considered here, the Eulerian and particle Lagrangian statistics were similar. However, the distribution of slip velocities experienced by the crystals was strongly dependent on the particle microscale and macroscale Stokes numbers. The consequent effects for the estimation of the average growth, nucleation and agglomeration kinetics used in population balances were also considered.

Investigation of the riddle of sulfathiazole polymorphism

Since the discovery of sulfathiazole as an antimicrobial agent in 1939, numerous works in the screening for its different polymorphic forms, which is an essential part of drug development, have been conducted and published. These works consequently result in the availability of various methods for generating a particular polymorph. By following these methods, however, one cannot be guaranteed to obtain the intended pure polymorph because most of the methods do not clearly and adequately describe the crystallisation conditions, such as cooling rates and initial solute concentrations. In this paper, the available methods for generating all the known polymorphs of sulfathiazole are reviewed and selected methods for generating certain polymorphs, performed with their processes monitored using process analytical technology tools, i.e. focussed beam reflectance measurement and attenuated total reflectance ultraviolet spectroscopy, are presented. The properties of the obtained crystals, examined using various characterisation methods, are also presented and whenever possible, are compared with those of other workers.

A Study of Particle Histories during Spray Drying Using Computational Fluid Dynamic Simulations

Computational fluid dynamics (CFD) models for short-form and tall-form spray dryers have been developed, assuming constant rate drying and including particle tracking using the source-in-cell method. The predictions from these models have been validated against published experimental data and other simulations. This study differs from previous work in that particle time histories for velocity, temperature, and residence time and their impact positions on walls during spray drying have been extracted from the simulations. Due to wet-bulb protection effects, particle temperatures are often substantially different from gas temperatures, which is important, because the particle temperature–time history has the most direct impact on product quality. The CFD simulation of an existing tall-form spray dryer indicated that more than 60% of the particles impacted on the cylindrical wall and this may adversely affect product quality, because solids may adhere to the wall for appreciable times, dry out, and lose their wet-bulb protection. The model also predicts differences between the particle primary residence time distributions (RTD) and the gas phase RTD. This study indicates that a short-form dryer with a bottom outlet is more suitable for drying of heat-sensitive products, such as proteins, due to the low amounts of recirculated gas and hence shorter residence time of the particles.

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.