Drying kinetics of granulated cork: Effect of air drying stream conditions and granule size

Abstract Granulated cork is a by-product of natural cork stoppers manufacturing which is incorporated into highly valued products such as agglomerated cork stoppers and expanded cork insulation. Prior to this, the cork granules must be dried. This work presents an experimental study for the characterization of wet granulated cork drying kinetics with different morphologies: a powder and granules with different sizes (2–2.8, 2.8–4 and 4–5.6 mm). A drying process with hot air stream flowing tangentially to the solid was used for a systematic analysis of the influence of air stream conditions (temperature and flowrate) on drying kinetics. Under these conditions, the influence of the stream temperature was much more relevant than its velocity with the drying time almost halving when the temperature rose from 35.0 °C to 56.5 °C. The drained humid solid with higher granulometry was dried entirely during a falling drying rate period. All others products presented a constant drying rate period before the falling drying rate period. The results revealed that the falling drying rate period is conditioned by water migration through the solid and presents a similar shape for all products. Thus, the behavior can be described by a single characteristic drying curve, allowing predicting drying rate as a function of the solid normalized humidity, despite the cork granules dimensions. This normalized curve is a useful tool for dryer equipment dimensioning in the cork manufacturing industry. To predict the time of drying for defined air drying conditions, a simple procedure was proposed using the characteristic drying rate obtained.