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Abstract

A radio frequency (RF) heating system has been designed to heat electrolytic liquids and sludges in a pressurized test vessel. The RF heating system is designed to heat a 400 ml load volume at a frequency of 13.56 MHz using a 1 kW RF generator. The primary application for the heating system is to conduct experimental work on the pretreatment of waste activated sludge, a byproduct of wastewater treatment plants. The dielectric properties of waste activated sludge were analyzed and it was determined that ionic conduction heating at a low frequency would be more efficient than microwave heating at a frequency of 2.45 GHz. The RF heating system was tested with sludge and salt water loads over a temperature range of 20°C to 120°C. Power transfer efficiency from the generator to the load was excellent and exceeded 86% over the entire temperature range for both loads. An important part of this work was the design of the RF applicator. The applicator consists of two circular electrodes mounted inside a Teflon cylinder to uniformly heat the load. The Teflon cylinder has seals and can heat loads under pressure. The load cylinder is enclosed in a coaxial housing and the geometry of the load cylinder and enclosure are designed to provide a nominal impedance of 50 Ω to match the RF generator impedance. The applicator has a thermocouple embedded in an electrode and a software control system is implemented in LabView to adjust the generator power for a target ramp rate or to maintain a steady state temperature. Extensive electromagnetic simulations were done to verify and optimize the applicator design. Analytic, simulation and experimental results are compared and very good agreement is obtained.