| abstract |
A system for providing improved bulk liquid mixing and effective gas-liquid contacting for mass transfer of the gas to the liquid, especially a non-Newtonian liquid, the viscosity of which decreases when under shearing conditions (shear thinning), in an upright tank. A process, such as fermentation which produces commercial quantities of polysaccharides such as xanthan gum. may be carried out in the tank. An upright draft tube is mounted within the tank and has a lower end spaced from the tank bottom and an upper end spaced below the surface of the liquid in the tank. A plurality of mixing impellers in the draft tube are sufficiently close to each other to establish a field or pattern of agitation to cause shear thinning and upflow throughout the draft tube and which may produce turbulence at the liquid surface. A plurality of radially inwardly projecting. circumferentially spaced baffles extend from the draft tube and are proximate the mixing impellers to prevent swirling of the liquid within the draft tube. Gas may be sparged into the vessel in or adjacent to the lower end of the draft tube. A circulating co-current flow of gas and liquid, with intimate gas-liquid contact, is induced up through the draft tube and out the upper end of the draft tube so as to provide a high rate of mass transfer from the gas to the liquid. The flow including entrained gas turns down through the annular region between the tank wall and the draft tube for recirculation of the liquid and gas above the surface of the liquid, resulting in high gas holdup and gas-liquid contact in the downflow annular region and in the upflow draft tube region, without gas flooding of the impellers. The concentration of dissolved gas and the effectiveness of the system for gas-phase mass transfer is determined by reaeration of a test sample of the liquid. Additional surface aeration can be provided with a surface impeller having a plurality of blades with vertical portions and outwardly inclined portions, the vertical portions being inclined with respect to radial lines from the axis of rotation of the impeller. |