Convective drying characteristics of sludge from treatment plants in tomato processing industries

The present work is mainly focused on the study of the thin layer drying behaviour of sludge from water treatment plants in tomato processing industries, using a convective dryer. The drying experiments were conducted at inlet temperatures of drying air of 30 °C, 40 °C and 50 °C and at an airflow rate of 0.9 m/s and 1.3 m/s. The drying rate was found to increase with temperature and velocity, hence reducing the total drying time. In particular, as drying temperature was raised from 30 °C up to 50 °C, the time period needed to reduce the moisture content of the sample from 173 wt% down to 7 wt% (dry basis) was observed to decrease from more than 760 min to 470 min (0.9 m/s) and from 715 min to 295 min (1.3 m/s).Using a non-linear regression (Marquart's method) together with a multiple regression analysis, a mathematical model for the thin-layer convective drying process of sludge from treatment plants in tomato processing industries was proposed. The values of the diffusivity coefficients at each temperature were obtained using Fick's second law of diffusion, and varied from 6.11 × 10−10 m2/s to 2.54 × 10−9 m2/s over the temperature and velocity range. The temperature dependence of the effective diffusivity coefficient was described following an Arrhenius-type relationship. The activation energy for the moisture diffusion was determined as 30.15 kJ/mol and 36.70 kJ/mol, for airflow rates of 0.9 m/s and 1.3 m/s respectively. Air temperature 40 °C and drying airflow rate 1.3 m/s were found adequate to reduce drying energy consumption as well as to optimise the dryer loading/unloading periods.

► This work shows the most interesting of drying of sludge from treatment plants in tomato processing industries.
► We propose Page's model for the drying kinetic in our article, which combines simplicity and precision with r2 values over 0.99, within a drying temperature range from 30 °C to 50 °C and airflow rates of 0.9 m/s and 1.3 m/s.
► The estimated values of effective diffusivity range between 6.11 × 10 −10 m2/s and 2.54 × 10−9 m2/s and the activation energy from 30.15 kJ/mol to 36.70 kJ/mol.
► The optimal drying conditions from and energy consumption point of view were obtained for a temperature of 40 °C and a drying airflow rate of 1.3 m/s.