Design of gas-inducing reactors

A gas-inducing impeller enables efficient recycling of gas from the headspa ce into the Liquid. Historically, these impellers were used for the first t ime in froth flotation machines. The various designs of gas-inducing impell ers (including those used in froth flotation) could be classified into thre e categories, depending on the flow pattern coming into and leaving the imp eller zone. These are denoted as type If, type 12, and type 22 systems. The critical impeller speed for the onset of gas induction (N-CG) is governed by a balance between the velocity head generated by the impeller and the hy drostatic head above the impeller. A number of correlations (for types 11 a nd 22) are based on this balance (Bernoulli's equation). The rate of gas in duction (Q(G)) for the type If system can be accurately determined by equat ing the pressure difference (between the impeller zone and the headspace) g enerated by the impeller and the pressure drop required for the flow of gas . For type 22 systems, the correlations for Q(G) are mainly empirical in na ture. Correlations for the power consumption, fractional gas holdup, mass-t ransfer coefficient, and so forth are also available in the literature, alt hough these studies on are not comprehensive. A process design algorithm ha s been presented for the design of gas-inducing impellers. The algorithm co nsists of the determination of the rate-controlling step, selection of geom etry and the operating conditions, and an economic analysis to choose the o ptimum design. Guidelines have been given about the desired geometry of gas -inducing impellers for achieving different design objectives such as heat transfer, mass transfer, mixing, solid suspension, froth flotation, and so forth. It has been shown that the use of a gas-inducing impeller in a conve ntional stirred vessel can lead to a substantial increase in the productivi ty. It has been shown that the optimum geometry may not correspond to the m aintenance of equal power consumption per unit volume, or equal tip speed o n scale-up. Suggestions have been made for future work in this area.