Effect of loading density on quality of dried teas in fluidized bed drying

Loading density is a critical parameter that affects the drying behaviour of teas in a fluidized bed dryer and hence the final product quality. During fermentation process, polyphenols available in teas are converted into theaflavin (TF) and thearubigin (TR). However, the final percentages of TF and TR in teas are determined by the post fermentation reactions which take place within the first few minutes of drying. Effect of high loading densities on quality of dried teas was examined for 27, 29 and 31 kg/m2 against the control of 25kg/m2 using orthodox broken type teas. Quality parameters mainly TF%, TR% and tea tasters score were analysed. An increase in both TF% and TROT ratio was observed in teas dried at higher loading densities but the difference from the control sample was not statistically significant at p=0.05. However, professional tea tasters gave higher scores for overall quality of teas dried at higher loading densities based on infused leaf appearance, liquor colour, liquor strength and liquor quality. Results suggest that loading densities may be increased even up to 31 kg/m2 with a marginal improvement to the overall quality of orthodox broken type teas

Drying characteristics of orthodox broken type tea

Drying characteristic of Orthodox broken type tea was examined using a laboratory-scale fluid bed tea dryer. Drying experiments were carried out at loading of 29 kg/m2 and moisture content of tea was reduced from 106% to 7% on dry basis. Hot air temperature was varied in the range of 108 - 127 °C as applicable for industrial type tea dryers. Semi-theoretical thin-layer drying models of Lewis, Henderson and Pabis, Logarithmic, Page, Modified Page and Two-term exponential model were tested. Page model gave better predictions than other models, and satisfactorily described drying characteristics of Orthodox broken type tea. Results suggested that internal mass transfer resistance has fully controlled the drying process after 4 minutes of drying. The effective diffusivity of water in tea during latter stage of drying was found to be 3.796 x 10-11 (m/s) and 5.062 x 10-11 (m/s) at hot air temperatures in the range of 108–121 °C and 124–127°C respectively. An empirical model was proposed to describe the variation of moisture content with tea-bed temperature and the predicted values were in close agreement with the measured data. The bulk density of the tea bed was found to increase with time and that might have reduced the channeling effect during latter stages of drying

A Study on fluidization behaviour of orthodox-rotorvane teas

Orthodox-Rotor-vane teas dried in liquidized bed driers in tea factories at present have, on average, smaller size particles than that of teas produced a decade ago. Further particles are distributed over a wide size range. As a result, achieving desired fluidization was found to be difficult in the drier and that affected quality of the product. Therefore, a detailed study was conducted to understand the fluidizing behaviour of tea particles. Fluidization behaviour of drier-mouth teas from a commonly used drier called TRICCC FBD was studied using six different bedplates having varying opening area percentages and number of perforations. The study was conducted using a pilot scale fluidized bed unit. Desired bubbling phase in tea-bed was observed with three different bedplates: bedplate with perforations of 0.5 mm wide, opening area percentage of 4 and 2,217 perforations per square meter, bedplate with perforations of 0.5 mm wide, opening area percentage of 5 and 2,779 perforations per square meter and bedplate with perforations of 0.6 mm wide, opening area percentage of 5 and 2,269 perforations per square meter. These bedplates could be used in the drier to dry and improve the quality of the teas produced at present. A statistical analysis conducted on the data of bed pressure drop with varying loading rates of teas indicates that desired bubbling phase in tea-bed is achievable using these bedplates. This was confirmed bystudying change in bed pressure drop and fluidization velocity when airflow for fluidization of teas varied. The bedplate with perforations of 0.5 mm wide, opening area percentage of 4 and 2,217 perforations per square meter was found to be most suitable for the drier as desired bubbling phase in tea-bed could be achieved at lowest fluidization velocity for a wide range of loading rates, 21 - 29 kg/ m2. Low fluidization velocity may lead to a reduction in energy consumption for drying the teas in the drier. 28

Study on bedplate configuration for fluidizing orthodox broken type tea

Orthodox broken type tea produced in Sri Lanka is popular for its distinct characteristics. Fluid bed dryers which are widely used in manufacturing of tea has been changed over the years and consequently, drying the tea with required fluidizing condition with co-existence of continuous phase and bubble phase in the conventional fluid bed dryers became difficult. In this 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. Bedplate of 36 mm × 0.6 mm with 5 % opening area facilitated a stable fluidized tea-bed with coexistence of continuous phase and bubble phase without stagnation and entrainment. Higher loadings of 44.5 – 50.5 kg/m2 than the conventional loading of 38.5 kg/m2 was achieved. Mean particle size was found to reduce from 1801 to 462 μm and particle density increased from 1336 to 1600 kg/m3 during drying. Minimum fluidization velocity was varied between 0.74 and 1.0 m/s and an empirical model relating the moisture ratio to the tea-bed temperature was proposed and validated for fluidized bed drying of dhool using air at 124 ± 1 °C and for loadings in the range of 44.5 – 50.5 kg/m2