PERFORMANCE EVALUATION OF CASSAVA FLASH DRYING IN A VERTICAL UPWARD PNEUMATIC CONVEYING DRYER

Author: Otuu Obinna Otuu
Department: Mechanical Engineering
Affiliation: Nnamdi Azikiwe University Awka

One-dimensional steady-state non-equilibrium two-phase model has been developed to simulate the convective drying of cassava cultivar, TMe 419 in a vertical upward pneumatic conveying dryer. The model takes into account the momentum, heat and mass transfer between the continuous phase and the dispersed phase. The work determined the physical thermal and aerodynamic properties of cassava cultivar TMe 419. The results indicated that particles of grated cassava are irregular and their size distribution is grater-specific. The density of dewatered mash was determined to be 866.82 kg/m3 while the particle hardness was 0.769 kg/mm2, way below the abrasive threshold of 800 kg/mm2. The terminal velocity of TMe 419 particle is correlated to the particle diameter by the expression 􀝒􀯧 = 0.0813􀝀􀯣 􀬷 − 0.6624􀝀􀯣 􀬶 + 2.9718􀝀􀯣 + 0.3967 . The specific heat capacity, thermal conductivity and thermal diffusivity of TMe 419 were determined to be 3.1422 􀝇􀜬/(􀝇􀝃°􀜥), 0.4634 􀜹 􀝉 °􀜥 and 0.1164 (􀯠􀰮􀯦 ) respectively. The drying curve for TMe 419 also showed that the expected moisture content is lower than the critical moisture content. This implies that flash drying is carried out within the constant-rate drying period. These data were inputted into the model which was solved numerically using fourth order Runge-Kutta implemented on ComsolScript platform for the dispersed phase. The data generated from the solution of the gas phase was used to determine the state of the solid phase by simulation on ComsolMultiphysics platform based on finite element method of analysis. The implementation of the ComsolScript allowed the investigation of the effects of different variables on the operating conditions during pneumatic drying and also on each other. Dryer variables like air inlet velocity, temperature and pressure drop is required in the selection of an appropriate blower and heat exchanger rating. Coupling the data from the gas phase to a finite element model of the particle, on ComsolMultiphysics platform predicted the moisture concentration as drying progresses and enables the prediction of optimal flash tube height. Overall the work has provided a tool for gaining insight into the workings of pneumatic conveying drying of TMe 419 but could easily be adapted for other material or operating conditions by simply changing the relevant input data. Here a tool for the design and performance assessment of existing pneumatic conveying dryers has been developed.

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