Assessment of a phase change regenerator for batch industrial dryers

Waste heat recovery in the industrial sector plays a major role for reducing the use of primary energy. This work assesses a phase change regenerator for heat recovery from industrial dryers operating in a batch mode. The chosen architecture is a fixed bed type consisting of two vertical stoves with horizontal rod bundles, each rode being a hollow steel cylinder filled with either of two paraffins selected from the market. A model was developed and implemented in MATLAB to describe the mass balances, the energy balances, the heat transfer coefficients, and the pressure drops of the flows and the cylinders within the stoves. The heat transfer coefficients can be computed for both non-condensing and condensing exhaust gas from the dryer. The phase change materials are modelled in a lumped parameter approach, while ambient air and wet exhaust gas via a simplified indirect method. For the case study of a real natural gas-fired batch dryer for cotton flat fabrics with an exhaust gas at 1 kg/s and at a temperature ranging from 40 to 140 °C, a parametric analysis is executed to identify trends and optimal configurations for the two paraffins. Then, a detail analysis is conducted on the temperature distribution for the flow and the cylinder within the stove for the hot and the cold blows. The paraffins with the higher fusion temperature at 118 °C is best. The regenerator 500 m wide and deep and 4 m high has an optimal configuration characterized by a cylinder diameter of 9 mm and a number of cylinder per row of 24, yielding an annual savings of 2260 euros. A regenerator with the same paraffin for all rows is not optimal as only the first and the last row change actually phase. Hence, the performance can be improved optimizing the selection of different paraffins along the rows.