Dehydration and drying poly(vinyl)chloride (PVC) porous grains: 1. Centrifugation and drying in controlled humid atmospheres

Abstract

Abstract Concentration of aqueous suspensions of solid particles (called slurries in industrial processes) is achieved by centrifugation at high accelerations and it is an important step in dry powder productions; dehydration precedes drying. In aqueous suspensions of porous particles, water is both the interstitial fluid, which disperses the particles and the imbibition fluid, which fills the pores inside particles. This is the case in the pastes of poly(vinyl)chloride (PVC) polymerized in suspensions after centrifugation. PVC grains are non-colloidal particles with diameters close to 150 μm and variable inner porosity, which are synthetized in aqueous solutions using dispersants such as various poly(vinyl)alcohols (PVA). In this paper we determine the dehydration by centrifugation of different grades PVC suspensions with laboratory scale experiments. It is shown that the humid pastes reach a pendular state at high accelerations and that the compaction of the grains and their surface properties determine the final retention of the water by capillary forces. In such conditions, internal water, inside the pores of the grains can be eliminated only by evaporation. Drying was investigated in controlled relative humidity atmospheres ( RH ) in desiccators by measuring the equilibrium moisture content of the grains and the evaporation rates. The evaporation rate of the superficial water is similar to pure water and can be interpreted using Stefan's equation, whereas substantial differences exist between the total drying times of grades.