Advances in Water Treatment and Enviromental Management

GAS LIQUID MIXING AND MASS TRANSFER 1192. The scale of operation. The size of operation of waste treatment equipmentmay be orders of magnitude greater than a chemical production plant,although the largest vessels may be a few 10’s of metres in diameter.Direct scaleup of a chemical/pharmaceutical plant geometry to thatrequired for water treatment may not be feasible from a physical/mechanical engineering standpoint. Additionally, at large scale, waterdepth considerations become very important, because of the increasedpressure required from any air supply to cope with the head of water.3. The rate of mass transfer and intensity of mixing employed. For watertreatment, only slow mass transfer is required to supply the biologicaldemands. Typical values of K a are of the order of 10 s -1 . For chemical/Lpharmaceutical operations, values up to 0.1 s -1 are common.These differences will be considered further in the Discussion Section.4. Chemical and Pharmaceutical Techniques4.1 Agitated VesselsThe “workhorse” of the chemical process industry for gas liquid mass transfer isthe agitated mixing vessel. Although a wide range of geometries are used, two of themost common are illustrated in Figures 1 and 2. Tall vessels, eg Figure 2, have agreater surface area/unit volume than the standard geometry vessels, Figure 1,and are therefore used when heat transfer is critical, as is the case in somepharmaceutical processes.Figure 1: Standard geometry vessel with a single impeller