Spray dryer modelling

Both spraying and drying are critical to spray dryer performance. Models are developed which explain the very different performance of a spray dryer when large droplets of film forming materials are created using a Rayleigh resonance atomiser. The droplet diameter distribution from this "Acoustic Atomiser" is inadequately described by previously reported spray size distribution functions, but well described by the Stable distribution. The alpha parameter of this distribution was found to tend towards the Gaussian limit for low viscosity fluids and the Lorentz limit with increasing viscosity, consistent with behaviour as a simple and damped forced harmonic oscillator respectively, and hence with the physics of the atomisation process. Droplet drying kinetics dominate model predictions. A device using an ultramicrobalance to measure droplet drying kinetics with unprecedented accuracy and range has been designed. A scaling and residence time analysis model was able to account for experimental spray dryer observations. Sprayability even of complex fluids is predicted adequately by the Ohnesorge diagram, provided that extensional rather than shear viscosity is plotted. A new determination of the transient apparent extensional viscosity from arbitrary CaBER time-diameter curves has successfully been used for fluids too complex to analyse using previously published rheological models

Spray dryer modelling

Both spraying and drying are critical to spray dryer performance. Models are developed which explain the very different performance of a spray dryer when large droplets of film forming materials are created using a Rayleigh resonance atomiser. The droplet diameter distribution from this "Acoustic Atomiser" is inadequately described by previously reported spray size distribution functions, but well described by the Stable distribution. The alpha parameter of this distribution was found to tend towards the Gaussian limit for low viscosity fluids and the Lorentz limit with increasing viscosity, consistent with behaviour as a simple and damped forced harmonic oscillator respectively, and hence with the physics of the atomisation process. Droplet drying kinetics dominate model predictions. A device using an ultramicrobalance to measure droplet drying kinetics with unprecedented accuracy and range has been designed. A scaling and residence time analysis model was able to account for experimental spray dryer observations. Sprayability even of complex fluids is predicted adequately by the Ohnesorge diagram, provided that extensional rather than shear viscosity is plotted. A new determination of the transient apparent extensional viscosity from arbitrary CaBER time-diameter curves has successfully been used for fluids too complex to analyse using previously published rheological models.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Studying the effects of elongational properties on atomization of weakly viscoelastic solutions using Rayleigh Ohnesorge Jetting Extensional Rheometry (ROJER)

The extensional rheological properties of dilute polymer solutions play a dominant role in many commercial processes such as air-assisted atomization. This is a high deformation rate process important in application of diverse materials such as paints, fertilizer sprays and delivery of airborne drugs. Dilute polymeric solutions which have identical values of high shear-rate viscosity (HSV) often exhibit different values of Sauter Mean Diameter (SMD) in their spray size distributions as a result of differing extensional rheological properties. We explore the atomization of a series of model Poly(ethylene oxide) (PEO) solutions dissolved in water/glycerol mixtures. Each solution is sprayed with an air-assisted spray gun under similar conditions and imaged with a commercial spray measurement system. The values of HSV for PEO solutions are close to the solvent viscosity and matched to those of typical ink or paint samples. The surface tensions of the fluids are also tuned to be very similar, however both the SMD and the droplet size distribution change considerably. For the highest molecular weight PEO systems, interconnected beads-on-string structures are observed at different positions of the spray fan. Capillary Break-up Extensional Rheometry (CaBER) can be used to measure the extensional properties of the more viscous solutions, but the well-known limitations of this approach include inertially-induced asymmetries, gravitational sagging and the very short filament lifetimes of low viscosity samples all of which constrain the range of relaxation times that can be probed. Consequently we also explore the use of Rayleigh Ohnesorge Jet Elongational Rheometry (ROJER) to probe the extensional response of these viscoelastic solutions at realistic timescales and deformation rates. A cylindrical liquid jet is excited by a piezo-actuator at a known frequency as it exits a micromachined nozzle, and stroboscopic imaging provides high temporal and spatial resolution in the break-up process. Analyzing the evolution in the jet diameter before break-up enables meaningful measurement of relaxation times down to values as small as 60 μs, and these values can be directly correlated with the differences in the final spray size distributions and the mean diameters. We outline a simple model for the fluid dynamics of the thinning filaments close to breakup that accurately describes the variation of the average droplet diameter as a function of the elongational relaxation time measured for each fluid