Gas hold-up and liquid circulation velocity in a Modified Airlift Loop Reactor with double draft tubes

The overall gas holdup, the riser and downcomers gas holdups, the liquid velocities in the riser and two downcomers, and the circulation regimes in a modified airlift reactor with double draft tubes were studied in this work. Air and water were used as gas and liquid media. In order to determine the superficial liquid velocities, the solid tracer method was used. The gas holdups were measured using manometric and the volume expansion technique. Based on gas holdup in all sections of the reactor, two regimes (homogeneous bubble and heterogeneous churn-turbulent) of the two-phase flow were observed. The superficial liquid velocities and gas holdups increased with increasing superficial gas velocity (Ug), and the increasing rate of each parameter was related to the changes in circulation and flow regimes, which have been described here. The εg and εgr increased with Ug by an exponent of 1.303 and 1.220, respectively. Also (εgdi + εgdo) increased with εgr by a slope of 0.164. The value of the driving force (εgr – εgd) was found to be important only for low values of gas flow rate, because at high values, the circulation velocity seemed to be governed mostly by friction in the reactor vessel. The outer downcomer had higher gas holdup and consequently lower liquid velocity, than the inner one. The downcomers liquid velocities showed a linear relationship with the riser gas holdup.

Keywords:

Modified Airlift Loop Reactor , Double draft tubes , Gas holdup , Liquid circulation velocity , Hydrodynamics