Chapter 8 - Liquid mixing

Abstract

This chapter focuses on the process of liquid mixing. The simplest process of liquid mixing where two or more miscible liquids are mixed together to develop a product of a desired specification and which does not involve either heat or mass transfer, or even a chemical reaction, faces problems when the liquids have vastly different viscosities, or if density differences are sufficient to lead to stratification. When mixing two or more immiscible liquids, they are stirred together and then one liquid becomes dispersed as droplets in the second liquid to form a continuous phase. The two liquids are then brought into contact with a solvent that selectively dissolves one of the components present in the mixture and then agitation causes one phase to disperse in the other and when the droplet size is small, a high interfacial area is created for interphase mass transfer. The production of stable emulsions encountered in food, brewing, and pharmaceutical applications is an important example of dispersion of two immiscible liquids. Gas–liquid dispersion and mixing are used in various processing operations involving chemical reactions, such as aerobic fermentation, wastewater treatment, oxidation, hydrogenation, or chlorination of hydrocarbons to produce a large interfacial area by dispersing bubbles of the gas into the liquid. Various gas–liquid reactions of industrial significance are carried out in the agitated tank reactors, where energy efficiency is the most important consideration, and large tanks giving long hold-up times are used. For gas dispersion into highly viscous media, two inherently conflicting requirements are met in practice by using a combination of two impellers mounted on a single shaft.