A critical investigation into the spray-drying of hydroxyapatite powder for thermal spray applications
Murtaza, Qasim (2006) A critical investigation into the spray-drying of hydroxyapatite powder for thermal spray applications. PhD thesis, Dublin City University.
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This work examines the investigation of the spray drying process of Hydroxyapatite powder (HA) used as a thermal spray deposit in the application of orthopaedic femoral implants. In this research, the Niro- Minor™ mixed spray dryer was used for both modelling and experimental studies. The process parameters investigated included HA slurry viscosity, temperature, and air flowrate. Computational Fluid Dynamic (CFD) modelling and validation of the spray drying of HA powder was performed. An analysis of the spray drying of the HA slurry, was performed using the UMETRI AB, MODDE 7 software.
For the CFD analysis, the Spray dryer was divided into the three parts; two-fluid nozzle, the drying chamber, and atomisation. The Standard K-e, Reliable K-e and Reynolds Stress Model models were used to predict velocity profiles of the air, feed pipe of the two external nozzle and temperature profile for the drying chamber. Different model results were compared, studied and compared with experimental results. The standard K-e method is found to have good agreement with the experiment data in predicting the air and feed nozzle velocities, and the Reliable K-e simulated the temperature profile of the drying chamber. These results were also used to predict atomisation modelling. The models hence have proved to be an innovative method of understanding the dynamics of the spray drying technique.
In the statistical analysis of the spray drying process, factors such as temperature and flowrate of the inlet hot air in the spray dryer, viscosity of feed/ HA and responses (chamber powder size, cyclone powder size, deposition of powder on the wall of spray dryer and overall thermal efficiency) were determined using a Multiple Linear Regression (MLR) method and the statistical analysis of main and interaction effects were quantified using the ANOVA test. For the chamber particle size, the statistical analysis showed that the viscosity of the HA slurry is most significant and for the cyclone particle size, the main affects are temperature, viscosity and flow rate, and also the interaction effect of temperature and viscosity were significant. Wall deposition is influenced by temperature and the interaction of both temperature and viscosity. The spray dried HA powders were also studied in terms of morphology. The two main shapes observed are a doughnut and solid sphere shape as a result of the different input parameters. A solid sphere of HA spray dried powder with pores was observed when a viscosity of 75 mPa.s was applied to all three levels of drying temperature. Doughnut shaped particles were observed when a slurry viscosity of 50 mPa.s was utilised. This doughnut phenomenon was more pronounced with an increase in the spray drying air temperature (461K) in the chamber powders. While a viscosity of 50 mPa.s and temperature of 461K yielded the ideal particle size and range, in terms of HA morphology, where a mix of solid and doughnut shape powder was produced. This is beneficial for HA thermal spray coatings as they require porous coatings to help the growth of the cells inside the coating to provide a strong bioactive bond between the implant and bone. This research provides a deeper understanding into the spray drying of hydroxyapatite powders providing data to improve its application in the use of HA deposits to anatomically join femoral implants to human tissue.
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