Backmixing in channel reactors with high-strength bottom gas injection

Abstract

Many metallurgical processes involve continuous countercurrent contact between two immiscible liquids. In the ideal operation of a continuous refining system, plug flow should be attained in the horizontal direction and local equilibrium in the vertical direction. This would allow mass transfer to take place between the two liquid phases and still maintain concentration gradients along the reactor, which are required for its efficient operation. Top blowing over a channel reactor, as in the WORCRA process, [~-4] showed that this goal can be approached. More recently, bottom gas injection into a channel reactor has been applied to the QSL leadmaking process [Sj and the Hismelt steelmaking process.[ 6] Preliminary indications are that this goal is better achieved by bottom gas injection. Mixing of the two liquids to promote phase equilibration is possible by either mechanical agitation or by using gas jets. High-strength submerged gas injection has an advantage over mechanical agitation in that it can produce very fine emulsions, if so desired, while providing process gases. This article deals with the backmixing observed in a horizontal countercurrent channel reactor with high-strength bottom gas injection into two immiscible liquids. The degree of backmixing can be quantified using the dispersed plug flow model or the dispersion model, derived for a vessel with no stagnant pockets and no gross short-circuiting of the fluid. [7] These conditions exist in the physical model for the channel reactor used in this work and schematically illustrated in Figure 1. The cold model studies were carried out in a 0.635-cm-thick circular cylindrical plexiglas channel 0.3 m ~b x 2.4 m long, with a slope of 1 deg to the horizontal. The vessel has holes along the bottom to accommodate gas injectors at variable intervals. Compressed air was used as the injection gas. An air manifold was fabricated out of mild steel to serve as a reservoir for compressed air at a constant pressure. The air manifold had several slots for tapping the air supply for the bottom injection. The air flow rates were controlled using flow meters and kept constant during each experiment. The gas injectors were fabricated of 2.5-cm-thick acetyl resin formed by the polymerization of formaldehyde (commercially known as Delrin) and had a converging section to accelerate the gas. The local gas Mach number could not exceed unity with this injector because the injector only had a converging section. Thus, the gas is compressed, increasing its density to maintain the mass flow rate at high rates