Use of injected air to reduce Taylor dispersion in a flowing liquid

A study is presented of liquid mixing during upward gas-liquid flow in a 0.019 m i.d. vertical column. The superficial velocities were in the range 0-0.14 m/s, for the gas, and 3.5 × 10-3 -9.0 × 10-3 m/s, for the liquid. These conditions were chosen to achieve simultaneously laminar flow of the liquid away from the slugs and regular slugging, with virtually no coalescence, along the column. Dispersion of the liquid along the 3.44 m long column was studied by means of a 'stimulus-response' technique; the variation in concentration of a tracer was continuously monitored at the column outlet, following a step change at its inlet. A detailed physical model was developed to predict the variation in tracer concentration at the outlet and its predictions are shown to agree remarkably well with the experimental results. The theoretical model predicts tracer progressions along the column to result from Taylor dispersion, due to laminar flow between successive slugs, combined with intense mixing in the wake to each slug. An alternative interpretation of the experimental results is presented, based on a simplified model of plug flow of the liquid with superimposed axial dispersion; the dispersion coefficient is then calculated from the variance of residence times of tracer in the column. The dependence of the dispersion coefficient on gas flowrate, for constant flowrate of the liquid, is seen to show a curious behaviour; for low flowrates of the gas, the dispersion coefficient decreases at first, with increasing gas flowrate, goes through a minimum and then start to increase. The value of the dispersion coefficient is well predicted by the equation τD/H2 = 0.173(τφ)-1 + 0.50(τφ)(lw/H)2 which was suggested from the asymptotic behaviour of the physical model presented.