Experimental studies on mass transfer during convective drying of spent coffee grounds generated in the soluble coffee industry

Abstract

Drying is a very important stage in the treatment of spent coffee grounds destined to biofuels production. The mass transfer during the convective drying of spent coffee grounds generated in the soluble coffee industry (SCG-SCI) is analyzed. An experimental design from sixteen isothermal drying experiments for different sample thicknesses (5mm, 10mm, 15mm and 20mm) and drying air temperatures (100ºC, 150ºC, 200ºC and 250ºC) using a drying air velocity of 1m/s was proposed. Drying times, drying rates and effective diffusivity coefficients were obtained. Drying curves were fitted with the main mathematical model proposed in the literature and drying rates were studied from the moisture ratio and the drying air temperature. Constant and time-dependence effective diffusivity were evaluated using polynomial surface models. Drying times range between 18 minutes (test at 5 mm and 250ºC) and 3 hours (test at 20 mm and 100ºC). Drying rate and effective diffusivity values were found between 0.0000226 and 0.001722 s-1 and 1.79·10-9 and 29.1·10-9 m2/s, respectively. The main differences between these experiments and those carried out by the same authors about the drying of spent coffee grounds obtained in the services sector (SCG-SS) were studied and analyzed.