Heat and mass transfer analysis in a spouted bed dryer coupled with a cyclone for pepper drying

Abstract

This thesis outlines the analysis of heat and mass transfer process of drying of black_pepper in a spouted bed coupled with a cyclone separator. Black pepper is a widely used spice and is well-known for its pungency and aroma. Experiments conducted with wetted black pepper reveals that conventional spouted bed has the higher drying rate, higher moisture diffusivity, lower drying time and lower activation energy compared to those of spouted_beds equipped with porous_draft tube and non-porous draft tube. Therefore, the conventional spouted bed was used to conduct the experiments to achieve the objectives of the study since it was identified as the optimum configuration of the spouted bed. Spouting behaviour of raw black pepper was studied in the conventional spouted bed. The minimum spouting velocities of particular bed heights at ambient temperature were determined. Minimum spouting velocity of raw black pepper in the conventional spouted_bed versus static bed height was correlated to a power form of function and the exponent of static bed height was 0.67. The effect of operational conditions; drying air temperature, air velocity and static bed height on drying kinetics of raw black pepper was analysed. The higher values of moisture removal rates were achieved when the dryer was operated with moderately high air temperatures, high air flow rates and shallow beds. Drying kinetic data of raw black pepper in the conventional spouted bed shows only falling rate periods. Effective moisture diffusivity values increased with increasing drying air temperature. The highest effective moisture diffusivity of 2.03×10-10 m2/s was obtained at air temperature of 75 ᵒC. Activation energy for drying of raw black pepper in conventional spouted bed was 38.59 kJ/kmol. Specific energy consumption was calculated and the specific energy consumption values increased with increasing air velocity and decreased gradually with increasing the static bed height. Drying_kinetic data obtained from experiments were fitted to five thin_layer drying models. Results show that the Logarithmic model gives the best fit. In addition, four models were developed for black pepper drying in the conventional spouted bed by correlating drying constants and coefficients of Logarithmic model to stagnant bed height at specified temperatures; 45 ᵒC, 55 ᵒC, 65 ᵒC, and 75 ᵒC. Developed models can be used to estimate the drying time of black pepper in the conventional spouted bed dryer for given moisture reduction in the ranges of 0.14-0.22 m stagnant bed heights and in 2.37 m/s air velocity at specified temperatures. Essential_oil was extracted using hydro distillation from black pepper dried at five different drying conditions. The analysis of components by Gas Chromatography Mass Spectrometry technique shows that the black pepper essential oil comprised mainly monoterpenes and sesquiterpenes. Analysis of variance was conducted and the results show that variation of sesquiterpenes concentration in black_pepper essential oil was significant while variations of monoterpenes, oxygenated terpenes and caryophyllene concentration were non-significant in black pepper essential oil with drying air temperature. In addition, the variation of essential oil yield is statistically significant with drying air temperatures. 65 ᵒC drying air temperature provides consistent quality essential oil with high percentage of caryophyllene and higher oil yield. Heat transfer coefficients for black_pepper drying in the conventional spouted dryer were estimated for different drying conditions of unsteady state drying of black pepper dried from initial moisture content to final moisture content of 15% dry basis. Heat transfer coefficient varied from 35-68 W/m2K for the conditions under the investigation. Dimensional analysis iii was carried out and important dimensionless numbers were identified. A correlation was developed for heat_transfer process as a function of dimensionless groups namely Reynolds number, Nusselt_number, Gukhman_number and static_bed height to particle_diameter with 0.791 of coefficient of determination. Heat transfer coefficients predicted from the developed correlation show a good agreement with the experimentally determined heat transfer coefficients.