Calculation of the Limiting Pore Diameter Qualifying the Pore as a Capillary Under Conditions of Centrifugal Drying of Sediments

Abstract

Mechanical drying in the rotating rotor of a filtering centrifuge is accompanied by the formation of a capillary rim, an annular layer of pores of the sediment upstream of the filtering surface. The rim of adjacent chains of pores, which are filled with the filtrate and tend to a radial arrangement, limits the decrease in the moisture content of the sediment. Consideration is made of a statistically ordered pore space of the sediment in which chains of communicating pores of each size can form. The rim includes a filtrate-unsaturated inner annular layer with a branched structure in the form of a fringe on the side of the rotor and may include a filtrate-saturated outer bottom layer adjoining the filtering surface. The filtrate filling each chain of pores ends at both ends with menisci. The larger the diameter of pores, the shorter their chain filled with the filtrate. The condition under which only menisci remain in the largest pores is considered. The diameter of such pores is called the limiting diameter. If the sediment contains pores whose size exceeds the limiting diameter, then even menisci cannot form in them, and chains of such pores will remain unfilled. In the rim, through channels appear, through which the air flow circulates, enhancing the drying of the sediment in the rotating rotor. The purpose of the study was to obtain an analytical formula for calculating the limiting diameters of pores in sediments in filtering centrifuges. A formula for calculating the limiting diameter was obtained. This formula also allows determining the operating parameters of the centrifuge to control the limiting diameter up to the disappearance of the bottom layer and the occurrence of a ventilation effect. An example of calculation was performed. It showed the possibility of the appearance or disappearance of the bottom layer by selecting the angular velocity and the radius of the filter surface. This creates practical opportunities for the calculation-assisted control of the moisture created by the capillary rim either by organizing a ventilation effect through the thickness of the sediment with its thermodynamic drying, or by creating a bottom layer preventing ventilation to maintain a certain moisture content.