Heat transfer with single- and dual-gas distribution in a pressurised bubble column

The effects of the heat transfer coefficient variance on microbubble formation in a pressurised bubble column and its variance in the dual gas distribution were investigated. The tests were performed with a cylindrical column with an inner diameter of 0.097 m and a height of 1.8 m with air–kerosene media. The heat transfer coefficient was measured using various distributors with different numbers of orifices, and its size was fixed. For an opening fraction of 0.223 %, the heat-transfer coefficient increased with increasing superficial gas velocity (U_g) under all pressure conditions in the tested range. The amount of microbubble generation increased with an increase in pressure and U_g when the opening fraction decreased. To analyse the effect of the kinetic energy rate at the orifice on the number of microbubbles, the gas holdup was measured according to the increase in U_g and system pressure. When the total kinetic energy rate was equal to or higher than 1 J/s, the gas holdup rapidly increased owing to microbubble generation. To increase the heat transfer coefficient in the presence of microbubbles, a dual gas distribution was applied by inserting a single nozzle with different orifice hole diameters (1.7, 2.0, 2.5, and 3.46 mm). In this system, the heat transfer coefficient increased by approximately 25 % compared with the dispersion in a single distributor.