Effect of high solids concentration on hydrodynamics and mass transfer of a three-phase airlift reactor for marine sediment bioremediation

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The more restrictive limits for environmental pollution control imposed nowadays require the development of new biological technologies. Multiphase reactors including slurry reactors have become reactors of choice for chemical and biochemical applications. In the present work, three-phase airlift reactors are proposed as an alternative to mechanically stirred systems and slurry bubble columns for destruction of contaminants sorbed to solids. That application of airlift reactors is based on the advantages provided by these reactors, such as high mass transfer rates, minimum gas velocity needed for complete solids suspension (Heck and Onken, 1987; Siegel and Merchuk, 1987), and lower shear rate which is attractive for systems with biofilm particles (Heijnen et al., 1997). For the optimum design and operation of these reactors it is important to know hydrodynamic and mass transfer characteristics.

In airlift reactors, the sole energy input is that needed to inject the aeration gas. The sparged air supplies the oxygen required for both bacterial respiration and particles suspension. The riser is aerated and due to the resulting density difference between the riser and downcomer sections, a liquid circulation is developed. The liquid circulating velocity is a crucial parameter in the design of the airlift reactor because it determines liquid mixing, gas hold-up and solids suspension. The liquid velocity will be determined by reactor geometry, gas supply rate and solids loading and properties.

In biological operations, oxygen transfer is one of the most important parameter influencing metabolic activity, process efficiency as well as energy costs (Lazarova et al., 1997). The reactor should have a maximum mass transfer rate, with efficient mixing, at minimum energy input. Although stirred tank reactors are successfully used for bioremediation of soils and sediment (Mueller et al., 1991), power input requirements have made these reactors difficult to operate at high loading. Stratification and solids settling occurs at higher (>30%) solids loading in stirred tanks (Apitz et al., 1994). Several studies have demonstrated the airlift reactor's ability to provide high mass transfer and efficient mixing.

Many works could be found in the literature describing hydrodynamic and mass transfer characteristics for twophase airlift reactors, but only few studies have used three-phase airlift reactors and, nearly none of them corresponded to actual processes (Heijnen et al., 1997). The solid phase was usually glass beads, aluminium, etc.  (Livingston and Zang, 1993). In those works using soil or sediment, the effect of high solids concentrations on hydrodynamic and mass transfer parameters was rarely examined.

The purpose of this paper is to describe the hydrodynamic and mass transfer characteristics including gas hold-up, liquid velocity and volumetric mass transfer coefficient of a split-vessel airlift reactor as a function of the operation conditions (power input per volume of degassed liquid and solids loading). Several high solids concentrations are examined over a range of superficial gas velocities.

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