Computational Flow Modeling of Multiphase Mechanically Agitated Reactors

Mixing and dispersion of solids and gases in liquids in mechanically agitated reactors is involved in about 80% of the operations in the chemical industries, including processes ranging from leaching and complete dissolution of reagents to suspension of catalysts and reaction products, such as precipitates and crystals (Smith, 1990). This is one of the most widely used unit operations because of its ability to provide excellent mixing and contact between the phases. An important aspect in the design of solids suspension in such reactors is the determination of the state of full particle suspension, at which point no particle remains in contact with the vessel bottom for more than 1 sec. Such a determination is critical because until such a condition is reached the total surface area of the particles is not efficiently utilized, and above this speed the rate of processes such as dissolution and ion exchange increases only slowly (Nienow, 1968). Despite their widespread use, the design and operation of these agitated reactors remain a challenging problem because of the complexity encountered due to the three-dimensional (3D) circulating and turbulent multiphase flow in the reactor. Mechanically agitated reactors involving solid–liquid flows exhibit three suspension states: complete suspension, homogeneous suspension and incomplete suspension, as depicted in Figure 1 (Kraume, 1992). A suspension is considered to be complete if no particle remains at rest at the bottom of the vessel for more than 1 or 2 sec. A homogeneous suspension is the state of solid suspension, where the local solid concentration is constant throughout the entire region of column. An incomplete suspension is the state, where the solids are deposited at the bottom of reactor. Hence, it is essential to determine the minimum impeller speed required for the state of complete off-bottom suspension of the solids, called the critical impeller speed. It is denoted by Njs for solid suspension in the absence of gas and by Njsg for solid suspension in the presence of gas. A considerable amount of research work has been carried out to determine the critical impeller speed starting with the pioneering work of Zwietering (1958) who