Heat and Mass Transfer During Fry-Drying of Sewage Sludge

Abstract

Deep-frying, which consists of immersing a wet material in a large volume of hot oil, presents a process easily adaptable to dry rather than cook materials. A suitable material for drying is sewage sludge, which may be dried using recycled cooking oils (RCO) as frying oil. One advantage is that this prepares both materials for convenient disposal by incineration.

This study examines fry-drying of municipal sewage sludge using recycled cooking oil. The transport processes occurring during fry-drying were monitored through sample weight, temperature, and image analysis. Due to the thicker and wetter samples than the common fried foods, high residual moisture is observed in the sludge when the boiling front has reached the geometric center of the sample, suggesting that the operation is heat transfer controlled only during the first half of the process followed by the addition of other mechanisms that allow complete drying of the sample. A series of mechanisms comprising four stages (i.e., initial heating accompanied by a surface boiling onset, film vapor regime, transitional nucleate boiling, and bound water removal) is proposed. In order to study the effect of the operating conditions on the fry-drying kinetics, different oil temperatures (from 120 to 180°C), diameter (D = 15 to 25 mm), and initial moisture content of the sample (4.8 and 5.6 kg water·kg⁻¹ total dry solids) were investigated.

Keywords:

• Boiling
• Drying
• Frying
• Incineration
• Impregnation
• Recycled cooking oil

Notes

¹Currently, anaerobic digestion is not the most widely practiced treatment for sludge in France. However, since 1998, new policies regarding the practices of land spreading and landfill of sewage sludge, combined with recent technological progress in biogas production and valorization (in particular in northern Europe and the United States) give new impetus to these processes in the French context. ^[28]

²Proximate analysis is a thermal gravimetric analysis that describes the total solids content (TS), total volatile solids content (TVS), and total fixed solids content (TFS) in a sample. TS constitutes the remaining residue after drying of the wet sample at 105°C, TVS those solids that can be volatilized and burned off when TS are ignited at 550°C, and TFS is the residue that remains after ignition.

³The ultimate analysis gives the elemental (C, H, O, N, S) compositions of the total solids matter.

*Calculated according to Tsen et al. ^[33]