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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 𝑣𝑡 = 0.0813𝑑𝑝3− 0.6624𝑑𝑝2+ 2.9718𝑑𝑝+ 0.3967 .

The specific heat capacity, thermal conductivity and thermal diffusivity of TMe 419 were determined to be 3.1422 𝑘𝐽/(𝑘𝑔°𝐶), 0.4634 ^𝑊

𝑚 °𝐶 and 0.1164 (^𝑚2

𝑠 ) 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 and agrees with the assertion in literature that flash drying remove surface moisture. These data were inputted into the model which was solved numerically using fourth order Runge-Kutta implemented on

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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. One of the significant results of the investigations is the „Design Curve‟ for pneumatic conveyance of TMe 419. It involves the plot of the pressure drop at different air inlet velocities and material feed rate. It generated a family of curves and each curve drawn for each material flow rate. The graphs are discontinuous in areas where the subsisting air inlet velocity and pressure could not support the material flow rate under dispersed flow situation. It was also observed that gas temperature continuously decreases along the dryer, while the solid phase temperature increases continuously until ( if the residence time allows) it attains the wet bulb temperature. The increase in the velocity of the gas phase is as a result of the continuous influx of material into the flash tube which far outweighs that of pressure drop due to non-slip boundary condition. The velocity of the particle increases from zero or near zero at the point of introduction accelerates for a while, and attains and maintains the terminal velocity for the rest of its journey through the flash tube. This explains why some of the models reviewed assumed that the particle travels at a uniform velocity across the flash tube and do not account for the initial particle acceleration. This assumption may be

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valid for pneumatic conveying because it happens over a considerable distance and the residence time is much more when compared to pneumatic conveying drying or flash drying which happens within a very short interval of time. The very short resident time makes that initial interaction significant because it is at that point that the value of slip velocity and of course heat transfer between the air stream and the particle are at a maximum. Coupling the data from the gas phase to a finite element model of the particle, on Comsol Multiphysics 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 audit of existing pneumatic conveying dryers has been developed.

5.2 Contribution to Knowledge

This research work made the following contributions to knowledge:

 Generated data for the physical, thermal and aerodynamic properties of TMe 419.

 Adapted the formulation for two-fluid model in developing a model for convective drying of TMe 419.

 Generated a “Design Curve” for Flash drier design handling TMe 419.

 Provided a tool for designing new flash drying plants

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 Provided a tool for performance assessment and upgrade of existing flash drier.

5.3 Recommendations

The validation of the model which ignored the mass and heat transfer needs to be revisited. Though good agreement was obtained when the model was modified for pneumatic conveyance only but the task was for pneumatic conveying drying and so the validation of the model must consider mass and heat transfer to obtain the needed assurances before the model is deployed for actual design or performance assessment of an existing plant. This can be done by designating an existing dryer for experimental purpose or constructing a small scale experimental flash dryer for the purpose with all the required instrumentation.

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