Модель внешнего массообмена в системе квазиплоских частиц

Abstract

Рассмотрен процесс внешнего массообмена между поверхностью частиц и жидкостью. Совокупность частиц образует слабоконсолидированное или неконсолидированное тело. Частицы имеют квазиплоскую форму. Величина отклонения формы частиц от идеально плоской порядка величины, равной толщине частицы. Частицы имеют на поверхности микромасштабную неоднородность формы. Поровое пространство, формируемое поверхностями частиц, состоит из каналов двух масштабов: больших и малых. Толщина больших каналов порядка толщины частицы. Толщина малых каналов порядка масштаба микрошероховатости. В такой двухфазной системе изучен процесс внешнего массообмена и определены его характеристики. Результаты работы могут использоваться в предпроектных расчетах режимов процессов экстрагирования в промышленных экстракторах пищевой и парфюмерно-косметической промышленности.

The paper considers the process of external mass transfer between the surface of the particles and the liquid. A collection of particles is a weakly consolidated or non-consolidated body. Particles have a quasi-flat form. The magnitude of the deviation of the particle shape from an ideally flat order of magnitude equal to the thickness of the particle. Particles have microscale irregularities on the surface. The pore space formed by the surfaces of the particles consists of channels of two scales: large and small. The thickness of large channels is of the order of the particle thickness. The thickness of the small channels is of the order of the scale of microroughness. In such a two-phase system, the process of external mass transfer is studied and its characteristics are determined. These characteristics are obtained by solving various versions of the convective diffusion equation. The list of these options includes convective diffusion equation in the approximation of the diffusion boundary layer, the convective diffusion equation under conditions of restriction of the diffusion flow, the molecular diffusion equation for the bilayer flat region in two cases: in the first, both layers are solid, and in the second, one layer is solid and the other is liquid, the equation of convective diffusion in a bilayer system for a collector region in which the collector channels are liquid, and in which the liquid is stationary, and with the collector is also liquid, and in which the fluid moves. The above options reflect the differences in mass transfer conditions on the surface of a quasi-flat particle and the two scale character of the structure of the pore space. An important feature of this work is an attempt to take into account the inertial character of the motion of the liquid phase through large channels. In the paper the listed options are considered taking into account two possible types of flow in the channels: stabilized and unstabilized. The equations of convective diffusion for a stabilized flow lead to mass transfer coefficients, depending on the average value of the flow rate of th e fluid in the large channel. The convective diffusion equations for an unstabilized flow in a large channel lead to mass transfer coefficients, which depend not only on the mean velocity, but also on the Reynolds number. This dependence reflects the restructuring of the velocity profile of the fluid along the channel. It has been established that despite the differences in the scale of thicknesses of large and small channels, the contributions of the interphase boundaries of these groups of channels can be comparable. It was also established that the internal overflows of the substance extracted from the particles have a significant effect on the kinetics of external mass transfer. These flows occur between areas which surfaces border large canals and areas that border small canals. The emergence of such flows is due to a faster depletion of the extracted substance areas, bordering the large channels in comparison with areas bordering the small channels. For all the listed cases and options, the kinetics of all the flows of the extracted substance was calculated, groups of the corresponding dimensionless complex were selected and characteristic times of the rates of external mass transfer processes were determined. The results of the work can be used in pre-design calculations of the modes and design parameters of industrial extractors of any construcion that use interfacial mass transfer with a dispersed solid phase in the form of quasi-planar particles with a two-scale pore space.