Prototype of phosphate sludge rotary dryer coupled to a parabolic trough collector solar loop: Integration and experimental analysis

Abstract

This study presents an experimental analysis of a phosphate sludge drying prototype that uses a Parabolic Trough Collector (PTC) solar technology as an energy source. The interconnection of the system's three components, namely the PTC loop, oil-air heat exchanger (HX), and rotary dryer, is described. Experiments were carried out for each sub-system separately considering three air and Heat Transfer Fluid (HTF) flowrates, a drying temperature up to 135 °C, and a phosphate sludge flowrate of 5 kg/h. Results show that to dry sludge at the target humidity (≤7%), air at 105 °C is sufficient when operating at the average flowrate, this temperature increases to 123 °C at lower flowrate. The overall system was tested under the Moroccan semi-arid conditions for three typical days. All temperature profiles follow similar trends as Direct Normal Irradiation (DNI) variation; this latter influence the final sludge moisture value. During the sunny day, 32 kg of sludge was dried at the target moisture content throughout the day, with a relatively stable temperature (130 ± 4 °C). This amount decreased to 7 kg on the cloudy day while the target moisture content was achieved only 25% of the drying time. A back-up energy source or a thermal storage system is recommended to improve the drying performance during cloudy periods.