Master of Philosophy (M.Phil.)

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Browsing Master of Philosophy (M.Phil.) by Author "Amarasinghe, ADUS"

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    item: Thesis-Full-text
    Drying kinetics of coir pith and the performance in flash drying
    Fernando, JAKM; Amarasinghe, ADUS; Jayasundara, JMMA
    Drying and retting can be identified as the most important factors affecting the quality variations in dried coir pith which directly affects the final quality of compressed coir pith products. A pilot scale flash dryer was designed and fabricated to examine the effect of hot air temperature and velocity on the drying behavior of coir pith. Hot air drying was carried out to examine the drying kinetics by allowing the coir pith particles to fluidize and circulate inside the drying chamber. The physico-chemical properties of volume expansion ratio (VE), water retention capacity (WRC), bulk density, pH and electrical conductivity (EC) of compressed coir pith discs were measured. Scanning electron microscopy was used to analyze the microstructures of dried coir pith. The results were compared with the two other drying techniques namely sun drying and oven drying. The effect of time duration for retting and the method of retting the coconut husk were also examined. The optimum temperature for coir pith drying was found to be 140 C. The most suitable range of particle size and the range of moisture content in dried coir pith were identified as 0.5 – 6.3 mm and 12 - 23% (w/w, dry basis) respectively. The VE, WRC, pH and EC of coir pith dried in the flash dryer at the optimum temperature of 140 C was found to be 5.01 ± 0.21, 4.02 ± 0.10 (w/w), 5.95 ± 0.08 and 330 ± 16 μs/cm respectively. These values were comparable with those of the sundried coir pith. Oven drying caused rupturing the cells and case hardening of coir pith. Similar effect was observed with temperatures > 140 C for hot air drying and flash drying. VE and WRC of coco discs were found to increase significantly, pH to change marginally and EC to drop significantly with the increase of retting time. The effective moisture diffusivity was found to increase from 1.18 × 10-8 to 1.37 × 10-8 m2/s with the increase of hot air velocity from 1.4 to 2.5 m/s respectively. Correlation analysis and residual plots were used to determine the adequacy of existing mathematical models for describing the drying behavior of coir pith using hot air. A new mathematical model was proposed and it gave the best correlation between observed and predicted moisture ratio with high value of coefficient of determination (R2) and lower values of root mean square error (RMSE), reduced chi-square (χ2) and mean relative deviation (E %). Wang and Singh model and Linear model were also found to be adequate for accurate prediction of drying behavior of coir pith. Since the experimental setup of this study closely simulated the particle motion and heat and mass transfer in flash drying due to induced fluidization and circulation, the new model has a great potential in designing and modeling of the flash drying of coir pith.
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    item: Thesis-Abstract
    Ultrasonic devulcanization of sulfur vulcanized natural rubber
    Maduwage, S; Amarasinghe, ADUS; Munidradasa, DAI
    The high-energy ultrasound could be used to devulcanize rubber as it can focus energy into localized sites for selective bond rupture. The research work reported to date suggests that the ultrasonic technology is more suited to convert rubber waste to a usable material efficiently, effectively and environmental friendly. The ultrasonic devulcanization reactor consisted of three main sections, namely a power source, ultrasonic transducer with sample holding unit, and a monitoring system to measure the amplitude, frequency and power. N-cyclohexyl-2-benzthiazyl sulfenamide (CBS) accelerated unfilled natural rubber vulcanized with conventional sulfur vulcanizing system and with efficient sulfur vulcanizing system were used as the model rubber compounds in these experiments. 2 mm thick vulcanized rubber sheets were directly kept on the vibrating diaphragm of the ultrasonic transducer. The frequency of ultrasonic wave was varied in a range of 20 to 50 kHz and the power level was varied up to 800 watt. The treatment time was limited to 10 minutes when treated at high power levels. The vibrating amplitudes were measured at different power levels with the variation of ultrasonic frequency. Curing behaviour, gel content and cross-link density were studied for rubber samples devulcanized at different process conditions. The increase in cross-link density and gel content of the samples treated at lower amplitudes indicated the formation of additional cross-links. However, the higher vibrational energies associated with high amplitudes resulted in lower cross-link densities and gel contents indicating a breakdown of bonds. Cure curves of virgin and devulcanized NR samples suggested that the fast initial curing of devulcanized NR was due to the presence of active sufidized rubber molecules formed due to break down of some cross-links during devulcanization. The lower maximum torque values observed in the devulcanized samples were due to the partial breakdown of C-C bonds in the main chain. The tensile properties of the revulcanized samples gave comparable results with that of virgin rubber. A theoretical process model was developed to express the extent of devulcanization in terms of cross-link density. It was based on the vibrational energy transfer mechanism. The model treated the vulcanized rubber as a pure elastic solid containing void regions. Experimental and theoretical values lied within ± 10% error limits. The model showed that the media effect on the nature of void excitation was significant and the viscoelasticity was also considerable. However, the effect due to surface tension was negligible.