Abstract Rotating Disc Contactor
Rotating Disc Contactor (RDC) is a continuous contacting device used for liquid-liquid extraction operation. A typical RDC column consists of a series of rotating, flat discs connected to a central shaft. Stator rings are mounted on the column, centered between the discs. In this paper, we have consolidated the available data on hydrodynamics of rotating disc contactor. The empirical correlations found in literature are compared for their efficacy in estimation of hydrodynamic parameter of interest such as liquid holdup, pressure drop, and bubble size, across different liquid-liquid systems. Attempts were also made to improve the predictive capability of available empirical correlations by tuning them to available data from literature. Based on the comprehensive information studied, gaps in existing work on hydrodynamics of RDC are identified.
Keywords: Rotating disc contactor, Liquid-liquid extraction, Hydrodynamics, Liquid hold-up
Liquid-liquid extraction is a unit operation that describes the separation of components from homogeneous liquid mixtures. In this process a mass separating agent (solvent) is added which is partially miscible or immiscible with the feed. The component to be extracted (solute) gets distributed unequally between the two immiscible phases. This separation process is used when distillation of the mixture is not feasible or heat sensitive non-volatile materials have to be separated. Liquid-liquid extraction is used in numerous chemical industries to produce chemical compounds ranging from pharmaceuticals to biochemicals and to separate and purify various reaction products. Demand on the purity of chemical products has added significance to the process and the relative advantage of this separation process is governed by the choice of the right extractor.
The rotating disc contactor (RDC) for extraction process has evolved from the basic column type extractors. However, the performance of this equipment is found to be better than the conventional extractors in that it exhibits higher throughput, lower cost for solvent inventory and maintenance. In RDCs, horizontal discs that are mounted on a centrally supported shaft acts as agitating elements. Stator rings are mounted on the column centered between the rings. The device uses shearing action of the rotating discs to disperse the solute between the two phases. The rotating discs create both turbulence and large interfacial area required for high mass transfer rates. Rotating disc contactor is an appropriate choice for a wide range of liquid-liquid systems (with low to medium interfacial tension) that are encountered commonly in industries. It is one of the widely used agitated extractors.
Investigation hydrodynamic parameters such as liquid holdup, pressure drop and bubble size, across different liquid-liquid systems is fundamental for RDC design, for which several empirical correlations are published by researchers.
The available correlations for dispersed phase holdup may not applicable for all the systems. In this work, the empirical correlations developed for the hydrodynamic behavior of a traditional Rotating disc contactor (RDC) is studied and consolidated.
Literature study of correlations for dispersed phase holdup
Several correlations for dispersed phase holdup for various systems are available in literature of which some of them are listed in table below.
The correlation developed by Kasatkin et al. (1962) was the first one to be reported in terms of the system dimensions (column diameter, compartment height and disc dimensions), operating conditions (agitator speed, flow rates of continuous and dispersed phase), and physical properties of the system (density, viscosity and interfacial tension). Murakami et al. (1978) developed a correlation for holdup which is applicable for the data on measurements in column diameters up to 300 mm.
Correlations reported by Kumar and Hartland for dispersed phase holdup both with and without mass transfer were based on experimental data from various literature sources. For Kerosene-Water system, Moris et al. (1997) published a correlation for holdup based on experimental data of a pilot-plant sized RDC. Dispersed phase holdup correlations, for both no solute and solute transfer conditions under different regions of operation were established by Kalaichelvi and Murugesan (1998). Correlations developed by Al-Aswad (1982) predicted high values of holdup due to very high pre-constant value.