Design of gas-inducing reactors

Abstract

A gas-inducing impeller enables efficient recycling of gas from the headspace into the liquid. Historically, these impellers were used for the first time in froth flotation machines. The various designs of gas-inducing impellers (including those used in froth flotation) could be classified into three categories, depending on the flow pattern coming into and leaving the impeller zone. These are denoted as type 11, 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 hydrostatic head above the impeller. A number of correlations (for types 11 and 22) are based on this balance (Bernoulli's equation). The rate of gas induction (Q_G) for the type 11 system can be accurately determined by equating the pressure difference (between the impeller zone and the headspace) generated 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 nature. Correlations for the power consumption, fractional gas holdup, mass-transfer coefficient, and so forth are also available in the literature, although these studies on are not comprehensive. A process design algorithm has been presented for the design of gas-inducing impellers. The algorithm consists of the determination of the rate-controlling step, selection of geometry and the operating conditions, and an economic analysis to choose the optimum 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 conventional stirred vessel can lead to a substantial increase in the productivity. It has been shown that the optimum geometry may not correspond to the maintenance of equal power consumption per unit volume, or equal tip speed on scale-up. Suggestions have been made for future work in this area.