An investigation of spray-freezing and spray-freeze-dryings
Drying is an important process for a variety of industries such as pharmaceuticals, food, and chemicals, and produce products with low bulk density, good shelf stability, economical storage and transport, and in some cases unique structural qualities. Of the various drying methods available, freeze-drying is the most beneficial for heat sensitive products that susceptible to thermal degradation. Freezedrying also confers a porous structure on the material result from the voids left after subliming the ice crystals. Freeze-drying however is used mainly for high value products due to the high capital and operational costs. The cycle time of such a drier may take several hours. The spray-freeze-drying process has evolved in an attempt to shorten the freezedrying process time. The process atomises a liquid feed to increase the heat and mass transfer surface area. The spray is then frozen in a very cold gas or a cryogenic liquid to form solid particles, which are then freeze-dried. Some processes incorporate a - fluidised bed freeze-drier to reduce the drying time by forced convection heat and mass transfer. This project aim to develop the Spray Freeze- Drying process and examine the effect of the process on the drying entities and resulting product quality, followed by modelling of the spray freezing operation in an attempt to optimise the operation. This thesis is divided into two main areas of investigations; Spray freezing investigation as an influential on the resulting product size and structure. The experiments performed using Phase Doppler Anemometry technique to measure the particle size distributions and velocities in a specially constructed spray freezing chamber with incorporated windows. A spray freeze-drying chamber was constructed composed of three parts co-current spray freezing, gas - particle separation, and fluidisation freeze-drying unites. Successfully spray characterisation measurement was carried in both ambient and sub-ambient temperatures, producing data of the drop size distribution and velocities from refractive scattered laser PDA measurement. The technique produced valuable information about the spray size distribution and velocity, the application of this technique appear was a novel approach in the spray freezing process measurement. The result were used in the established spray freezing model derived from Pham (1984) freezing model incorporating with a recalescence stage from the Hindmarsh (2003) spray freezing model. The current model predicted the droplet freezing time, which is influenced by the droplet diameter, velocity and freezing gas temperature. The results also shows an agreement between the freezing time predicted and the PDA recognition of bursts rejected due to their possible phase change. The spray-freezing-drying chamber was operated in vacuum conditions to produce freeze-dried whey protein powder. The powder characteristic results shows an physical properties such as density, solubility and particle sizes in comparison to spray drying and cryo-spray freeze-drying. The morphology of the spray freeze-dried whey powder is substantially different to that produced by spray drying and displays a porous microstructure. The operation of the freeze-drying unites requires an further investigation for temperature, pressure, and flow control to maintain the freezedrying.