Batch drying studies of solids in a multiple draft tube spouted bed

Abstract

Solids drying is a unit operation carried out with solids either in immobile state (as packed bed) or in an agitated state. Since this operation involves simultaneous transfer of heat and mass, it will be beneficial when the bed of solids is kept in continuous circulation in order to improve the transfer rates. Freshly harvested agricultural grains are typically coarse, granular and fibrous in nature. Fluidised beds and spouted beds can be considered for drying solid materials. However, when coarse granular solids like agricultural grains, whose particle sizes are more than 1mm, fluidised beds are not efficient, whereas spouted beds seem to be more suitable. A conventional Spouted Bed (CSB) have some inherent limitations for scale up. A Multiple Spouted Bed (MSB) can handle larger capacities and provision of draft tubes (DTs) in the bed can help in overcoming the limitation of maximum spoutable bed depth(MSBD). In addition, DTs act as vertical baffles to avoid hydrodynamic instability of MSB when it is to be operated at higher gas velocities as well as help in achieving better control of gas residence times and solids cycle times. Presence of holes on DTs leads to movement of gas into annulus, which will be useful in an operation like solids drying. A multiple porous draft tubes spouted bed (MPDTSB) was considered as a gas-solid contactor for carrying out experimental studies on grain drying. The focus of this research work was to find the applicability of proposed design of the contactor as a grain dryer and consider it for scale up to large-scale application. In addition, two different single porous draft tube spouted bed (SPDTSB) dryers were used for comparing the performance of multiple and single spouted bed dryers. Drying experiments were carried out using ragi (Eleusine coracana), barley and wheat grains at temperatures of 40,45, 50, 55 and 60 0C, inlet air flow rates of 30, 33 and 36 m3/h. The initial moisture contents of grains used were 0.15, 0.20, 0.25, 0.30 and 0.35 kg moisture/ kg dry solids with bed mases varying between 1 and 8 kg. The fluid inlet sizes used were 8, 15 and 21 mm. Three different draft tubes having inside diameters of 21, 26 and 39 mm having hole sizes of 2 and 4 mm were used in the study. Also, non-porous draft tubes were used in the study. The batch drying times, moisture diffusivities in grains, thermal efficiencies and gas - solid heat transfer coefficient were evaluated. They were influenced by the operatingii variables, design variables and properties of grains. In addition, the performance of multiple porous draft tube spouted bed dryer was compared with the performance of two different single porous draft tube spouted bed dryers. Using the experimental data an empirical equation was proposed to predict batch-drying times in MPDTSB. The following important conclusions could be drawn based on the results obtained in this study. • It was possible to overcome MSBD limitation and larger bed masses could be handled in multiple porous draft tube dryer. • The operating parameters influenced the batch drying time. A higher air inlet temperature and air rate resulted in lower batch drying times. • The batch drying time would decrease with increase in draft tube diameter and hole size/ area. • Porous draft tubes helped in reducing the batch drying time. • Batch drying time decreased as the fluid inlet size decreased for a given bed mass. • The moisture diffusivities were found to be dependent on both operating and design variables of the dryer. • The thermal efficiencies and gas-solid convective heat transfer coefficients for MPDTSB were found to be higher when compared to those for SPDTSBs. • The overall performance of the proposed design of Multiple Porous Draft Tube Spouted Bed seemed to be good for solids drying. Scale up of this design for largescale operations should be possible.