KINETICS OF DRYING OF DISPERSED MATERIAL IN THE APPARATUS OF PERIODIC ACTION WITH FLUIDIZED BED

Abstract

A theoretical method for calculating a periodically operating fluidized bed dryer is presented based on the zonal method for describing kinetics and an example of calculation is given in relation to the drying of pea grain. In the calculation, it is assumed that the solid phase in the apparatus is completely mixed, and the gas is ideally displaced, the particles have a spherical shape and are the same in size. Accounting for changes in the parameters of the drying agent in the layer is carried out on the basis of the equations of material and thermal balance for the layer with the division of the entire range of changes in the moisture content of the material into a number of concentration zones. The change in the temperature of the dried particles during the process is calculated based on the analytical solution of the linear heat conduction problem at a constant ambient temperature and the condition that moisture evaporation occurs at the surface of the particles (there is no internal evaporation). The calculation is carried out using the method of successive approximations. First, the duration of drying of the material in the concentration zone under consideration is set, then the average air parameters are determined by the height of the layer and by the drying time, and, using these data, by solving the problem of mass conductivity (moisture diffusion), the drying duration in the zone under consideration is determined and compared with the preset one. If there is a significant discrepancy, a second calculation iteration is carried out, taking the drying time found in the first calculation iteration. Calculations are carried out until an acceptable match between the previously accepted and calculated drying time. Then they move on to the second concentration zone, for which they do the same. The total drying time is equal to the sum of the times in the concentration zones. This technique was tested using the example of calculating the drying of pea grains. In the calculations, we used data previously found by the authors on the coefficient of mass conductivity (moisture diffusion), as well as literary reference data on the coefficients of thermal conductivity, thermal diffusivity and heat capacity of this material. Heat and mass transfer coefficients were calculated using criterion equations given in the literature. To select the operating air speed, the speed of the beginning of fluidization was calculated; the operating air speed was taken at a fluidization number of 1.05. The calculation of the Biothermal and mass transfer criteria showed that the thermal problem is mixed (the Biothermal number is 5.81), i.e. the heat transfer process is influenced by both internal and external heat transfer. The problem of mass transfer is purely internal (the modified mass transfer Bio number is 122.5), which was taken into account when calculating the drying time. The calculated drying curve was compared with the experimental one. A diagram of the experimental setup is given and a description of the experiment is given. Satisfactory agreement between the calculated and experimental drying curves indicates the correctness of the calculation method under consideration, which can be recommended for engineering use.

For citation:

Rudobashta S.P., Zueva G.A., Babicheva E.L. Kinetics of drying of dispersed material in the apparatus of periodic action with fluidized bed. ChemChemTech [Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol.]. 2024. V. 67. N 6. P. 109-118. DOI: 10.6060/ivkkt.20246706.6968.