Engineering design of direct contact counter current moving bed heat exchangers

•Model prediction of both solid and gas temperatures along height of moving bed•Heat loss to wall considered in the model•Model application to scale-up to semi-industrial and industrial operations•Identified operating conditions to minimise solid or maximise gas outlet temperature•Established appropriate operating window for cooling slag granules to recover heat

A heat transfer model was applied to simulate heat exchange in a direct contact counter current moving bed (CCMB) heat exchanger associated with a dry slag granulation process developed by CSIRO. Cold air is used to recover heat from hot solid slag granules in the heat exchanger. Experimental data was used to calibrate the heat transfer model. The model was then used to predict the solid and gas temperature distributions along the height of the bed, the heat loss, and the operating and design conditions for scale-up of the CCMB to semi-industrial and industrial operations.Two scenarios were examined using the model to determine conditions for i) minimising the solid outlet temperature and ii) maximising the gas outlet temperature. The model can also be used to determine the nominal bed height to ensure that the designed gas outlet temperature is achieved. The modelling results suggest that a relatively small bed height (below 0.5 m) will be sufficient for heat exchange. The model also suggests that a solid to gas mass flow rate ratio of 0.8 should be used to minimise the solid outlet temperature, while a ratio of 1.0 should be used to maximise the gas outlet temperature. Therefore an appropriate operating window for a CCMB heat exchanger for recovering heat from hot slag was found with a solid to gas mass flow rate ratio between 0.8 and 1.