Drying recycled fiber rejects in a bench-scale cyclone: Influence of device geometry and operational parameters on drying mechanisms

Significant amounts of waste sludge and rejects are generated by pulp and paper mills, and stricter environmental regulations have made waste handling a global challenge. Thermochemical conversion of mechanically dewatered by-products is expensive and inefficient due to their high moisture content; therefore drying is a vital unit operation in waste management. This paper reports results from drying of light coarse fiber reject in a bench-scale cyclone that allows changes in geometry. For the sake of comparison, convective fixed-bed drying tests were also performed. The results showed that the drying rate in the cyclone was hundreds of times higher than in the fixed-bed. For cyclone drying, the inlet air velocity was the most important factor in both determining the drying rate and residence time of the material. This led to the hypothesis that grinding of the reject particles due to particle-wall and particle-particle collisions play a crucial role in enhancing the efficiency of heat and mass transfer. In addition to inlet air velocity, cyclone geometry was the main factor that determined particle residence time, as drying air temperature mainly determined drying rate.