Abstract:
Orthodox broken type teas, currently producing in tea factories in Sri Lanka, have on average smaller size particles than that of tea produced a decade ago. However most of the tea factories still use the conventional fluid bed dryers and hence they are experiencing difficulties in achieving fluidization with required co-existence of continuous phase and bubble phase. In the present study, fluidization behavior of Orthodox broken type tea was examined in a pilot-scale fluid bed dryer. Six different bedplate configurations were evaluated against the conventional bedplate having perforations of 36 mm 0.5 mm with 3.4 % opening area. The bedplate of 36 mm 0.6 mm with 5 % opening area gave the best performance. A stable fluidized tea-bed with co-existence of continuous phase and bubble phase was achieved without stagnation and entrainment at higher loadings of 44.5 – 50.5 kg/m2 than the conventional loading of 38.5 kg/m2. Further, the fluctuations were found to be minimized for a wide range of fluidizing velocities of 1.3 - 1.9 m/s.
A new mathematical model was developed to predict the minimum fluidization velocity by correlating dimensionless Archimedes number, Reynolds number and moisture ratio. The variations in particle size and particle density due to shrinkage during the drying process were incorporated in the new model. The predicted fluidization velocity was found to be in good agreement with the experimental data and the difference was below 10 % for majority of the cases. An empirical model relating the dimensionless moisture ratio to an easily measurable parameter, tea-bed temperature, was also proposed and validated.
Drying characteristics of Orthodox broken type tea and the quality variations with the fluidization parameters were also examined using a laboratory-scale fluid bed dryer. Page model was found to give better predictions than the other thin-layer drying models. Free moisture was found to be present above moisture contents of 60 % (w/w, dry basis) and the effective diffusivity was found to be 2.52 x 10-11 m2/s. During the final stage of drying, effective diffusivity was found to vary between 2.660 x 10-11 m2/s and 2.782 x 10-11 m2/s. Quality variations were examined by the method of chemical analysis and organoleptic analysis. The results indicated that better quality tea could be achieved with higher loadings than conventional loading of 38.5 kg/m2 and lower hot air temperatures than the conventional temperature of 124 °C. However, the drying time was found to increase by 2233 % for higher loadings and by 12 – 77 % for lower hot air temperatures.
