Handling and processing of particulate and powdered materials is of critical importance to the process and chemical industries world-wide. An understanding of the properties and processing characteristics of these materials is an essential requirement in process design, process performance evaluation and troubleshooting.
The Department of Process & Chemical Engineering has been conducting research in particle and powder technology for a number of years. The aim of the research effort is
- to measure particle and powder physical properties
- to evaluate how these properties and processing conditions affect handling and processing operations involving particles and powders.
- to identify how this information can be applied in process development, improvement and design.
- to educate students in particle and powder technology
The measurement of physical properties is important because these properties help define the particle / powder system and because they intrinsically affect its behaviour during storage, handling and processing.
Particle properties measured include:
- particle size distribution
- fractal dimension
- particle density
- particle strength
Bulk powder properties measured include:
- flow properties
- moisture content
- cake strength
- water sorption isotherms
- bulk density
- DSC thermograms
- compression properties
There are a number of on-going projects in the area of probabilistic analysis of particle technology with an emphasis on fluidised bed operations:
- Simulation of the granulation process in a re-circulatory fluidised bed using Markov Chains.
- Analysis of the randomness in particle motion in fluidised beds.
- Development of a general approach to model particle agglomeration based on Population Balances and Markov Chains.
- Stochastic modelling of particle motion along a sliding conveyor.
More recently, the group has also become involved with some research in the area of spray drying (examining crust formation on a particle as a Stefan type problem), encapsulation (of functionally active food micro-ingredients) and co-melt fluidised granulation.
Science and Technology for Manufacturing Aggregate Food Products
Particle agglomeration is the means by which food products such as granola are manufactured. Work in this area is focussed on developing and optimising processes to manufacture granola using in turn both high shear and fluidised bed granulation. Particle breakage properties of the products are also being analysed through purpose built processing rigs and through tensile testing to determine particle friability. Shelf life studies on the products are also being carried out. Important particle parameters that are being considered include aggregate size, PSD, strength and porosity.
Work is being carried out in the department in collaboration on co-melt fluidised bed granulation. The effects of various key operating parameters are being evaluated on ultimate product quality parameters for this novel granulation technique.
Particle Breakage during Conveying of Infant Formula Powders
Infant formula is typically produced by wet-blending ingredients including milk proteins, lactose, vegetable oils, minerals and vitamins before a drying process to produce agglomerates which are packaged. The powder is reconstituted by the consumer directly before use by adding water. Properties of individual agglomerates determine bulk powder properties such as powder flowability, bulk density and solubility. These are important product quality characteristics since for example, product is measured out on a volume basis (in scoops) whereas nutritional requirements are provided on a mass basis. Thus any changes in particle properties post drying, during conveying and packaging, will have implications. Work at the department is being carried out to understand, quantify and model these effects through a purpose-built pneumatic conveying rig and by evaluating key quality parameters such as particle size distribution, particle density, bulk density and wettability. This will enable prediction of particle properties under given key operating conditions and rig design configurations. Particle breakage at a particulate level is also being investigated.
Particle technology as applied to precipitation & crystallisation
The particle size distribution (PSD) of precipitates and crystals has a major influence on their ease of separation from liquids and is also a very important property of the dried powder.
Particle breakage: Particle breakage will have a major influence on PSD. It can occur during particle formation, transport and separation. Work in this area focuses on the breakage of whey protein precipitates during transport and separation where it was investigated experimentally how flow through different geometries affected the breakage of these particles. Following this, computational fluid dynamics (CFD) was used to quantify the fluid energy dissipation rate in the flow geometry and to use this for predicting particle breakage in other flow geometries.
Dextrose Crystallisation: The PSD of dextrose crystals is an important product quality parameter and greater control of the crystallisation process is required to achieve consistent desirable dextrose PSD while achieving high crystal yields. This work is investigating the potential for on-line monitoring of dextrose PSD, and how seeding, feed impurities and the temperature-time profile affect dextrose crystal PSD and yield, and how this can be applied in crystalliser control.
Powder Caking and Flowability
The caking of food powders is often influenced by the glass transition properties of certain components, such as lactose in dairy powders, which can have a major influence on the stickiness and caking of food powders. Above the glass transition temperature (Tg), powder particles often become stickier and may even sinter together forming very strong cakes. Increasing water content reduces the Tg and if Tg is reduced below the temperature of the powder then caking may occur. Maltodextrins are commonly used in food ingredient mixes, and work is currently being carried out to measure water sorption behaviour, Tg vs water content and the influence of water sorption and glass transition on the caking behaviour of maltodextrin DE21. A novel force displacement technique has also been developed to quantitatively measure powder cake strength and investigate the caking behaviour of milk powders, maltodextrin, common salt and their mixes.
The flowability of powders and their flow behaviour under pressure, temperature and humidity are important in handling and processing operations, such as storage in hoppers and silos, transportation, formulation, compression and packaging. Experimental work has focused on investigating the effect of powder properties and storage conditions on the flow properties of a wide variety of food powders. Powder flow properties measured were flowfunction and effective angle of internal friction (using an annular shear cell), and wall friction (using a Jenike-type half shear cell).