Heat transfer characterization of gas–liquid flows in a trickle-bed

Abstract

Instantaneous local fluid–solid heat transfer coefficient ( h t ) in a laboratory scale trickle-bed was measured using a constant-voltage anemometry technique. It was observed that convective heat transfer rate in the liquid-rich pulses was approximately 4 times that in gas-continuous bases for the air–water system. Time-averaged heat transfer rate was found to be positively influenced by both gas and liquid flow rates, with a stronger dependence on the latter. Heat removal efficiency, taking pressure drop penalty into account, suggested an optimum at intermediate liquid flow rate. Based on the measurements, a four-parameter heat transfer model featuring heat transfer coefficients in liquid-rich pulses ( h t p ) and gas-continuous bases ( h t b ), pulsing frequency and pulse fraction was developed to characterize transient h t under various flow regimes. This model can be used in any trickle-bed reactor simulation that accounts for the dynamic interactions of catalytic reactions and heat transfer. It was found that while h t p and h t b correspond to liquid–solid and gas–solid heat transfer, respectively, and are determined mainly by the fluid properties, pulsing frequency and pulse fraction are the factors characterizing different flow regimes. Pulsing frequency, which can significantly impact reaction, may be tuned by selecting appropriate packing size, since smaller sizes generate higher frequency pulses. For example, a two-fold higher frequency was detected in 6 mm packing as compared to that with 8 mm packing. Flow regime evolution along the column axial location was identified visually, while the dispersed bubbling flow retreating to pulsing flow owing to gas bubble coalescence was evidenced by the heat transfer measurements.