Comparison of normal phase operation and phase reversal studies in a pulsed sieve plate extraction column
Highlights
► Comparative study of normal phase and phase reversal in pulsed sieve plate extraction column (PSPEC). ► Describes hydrodynamic phenomena occurring in a PSPEC for different operating conditions. ► Normal phase operation is recommended for extraction. ► Data can be used for extraction and to develop mathematical model.
Introduction
Liquid–liquid extraction usage in industry for separation processes has been hampered by the poor extraction efficiencies associated with gravity-operated towers. The relatively small driving force for phase dispersion in this column is because of the small density difference between the two liquids. To aid phase dispersion an external means of agitation were provided, which evolved new class of liquid–liquid extractors i.e. pulse column introduced by Van Dijck in 1935. Various methods have been developed for mechanically agitating extractor column for increasing degree of turbulence which in turn increases rate of extraction (Goldberger and Benenati, 1959). The pulsed sieve plate extraction columns are especially attractive for fuel reprocessing operations as it can be operated remotely. Moreover, they have fewer moving parts and short residence times (Van Dijck, 1935, Jaradat et al., 2011). Its mechanical simplicity contributes to easier maintenance and their short residence time minimizes the solvent degradation due to intense radioactivity of the irradiated fuel.
The study of hydrodynamic characteristic plays an important role in the design of PSPEC. The literature on the PSPEC has been primarily concerned with the evaluation of the column operating variables (flow rates, nature of continuous phase, pulse amplitude, frequency); geometrical parameters (the number, spacing, and material of construction of the perforated plates; plate perforation diameter and per cent free area; column height and diameter); and fluid properties (viscosity, density, interfacial tension) for determination of the hydrodynamic characteristics like holdup, drop size and flooding.
The two phase flow is found in a wide range of industrial applications in column contactors. In a system of two immiscible liquids, usually an aqueous and an organic liquid, there are two general types of dispersions which can occur in the system. Water-in-oil (w/o) dispersion is a dispersion formed when the aqueous phase is dispersed in the organic phase and oil-in-water (o/w) dispersion is a dispersion formed when the organic phase is dispersed in the aqueous phase (Torab Mostaedi et al., 2008).
However, there is often an ambiguity in the usage of the terms; phase reversal and phase inversion associated with two phase flow. The phase reversal refers to a phenomenon where, the dispersed phase reversed to become the continuous phase and vice versa under conditions determined by system properties, phase ratio and energy input. A few authors [Li and Newton (1957), Sobotik and Himmelblau (1960) and Ikeda and Suzuki (1995)] have studied the phase reversal phenomenon for evaluating the effect of plate wetting characteristics. A controlled phase reversal is a desirable and essential step in certain industrial processes like extraction and stripping. It is intentionally created in the column by simply interchanging the continuous and dispersed phases to determine its effects.
Phase inversion is the phenomenon wherein, under particular system conditions, the dispersed phase coalesces to form the continuous phase and simultaneously the continuous phase breaks into droplets to form dispersed flow (Torab Mostaedi et al., 2008, Tidhar et al., 1986, Ioannou et al., 2005). There is a significant change in pressure drop and viscosity of the two-phase mixture in the ambivalent zone marking the transition between the two dispersions (De et al., 2010, Yeo et al., 2002). An uncontrolled phase inversion has to be prevented in all processes. The main difference between phase reversal and phase inversion is that the former is intentionally created whereas the latter is accidental phenomenon.
The objective of this research is to contribute more data of hydrodynamic parameters needed for developing and/or improving design strategies and scale up procedures of pulsed columns. In present study emphasis is given on the finding experimentally hydrodynamic parameters using an original system. Geier (1954) studied the general aspects of pulsed column used for the reprocessing of nuclear spent fuel to recover uranium and plutonium in the Purex process. In the Purex process, the aqueous phase is the dispersed phase to the extraction column; the organic phase becomes the dispersed phase to the stripping column. This paper aims towards understanding basic behaviour of phase reversal operation in a pulsed sieve plate extraction column (PSPEC) so as to study possible applicability in the Purex process.
Section snippets
Materials
Depending on the operational conditions, either of the two fluids involved can form the dispersed phase.
System-1: The continuous heavy phase is 0.3 M nitric acid (aqueous phase), and the dispersed phase is 30% tributyl phosphate (TBP) in NPH (organic phase) for normal phase operation and vice versa for phase reversal studies.
System-2: Normal phase operation uses kerosene (organic) as dispersed and water as continuous phase conversely phase reversal studies were carried out using water (aqueous
Experimental setup
The schematic diagram of the PSPEC used for the present work is shown in Fig. 1. The glass column consisted of 0.076 m in internal diameter and cylindrical shell of 1 m in length. The plate cartridge of SS-316 was fitted in the column with twenty plates; having each plate thickness 0.0016 m and hole diameter of 0.0015 m and 0.003 m at 0.005 m triangular pitch. Two types of fractional free area of the perforations, 10% and 20% were used for the experimentation. The plates were arranged at 0.05 m
Results and discussion
Different experiments were performed in order to study the effect of operating parameters on drop size, dispersed phase holdup and flooding for both the cases i.e. normal phase operation and phase reversal. The operating parameters are pulse velocity, Af (0.55–3.3 cm/s), superficial velocity of continuous phase, Vc (0.12–0.76 cm/s), and superficial velocity of dispersed phase, Vd (0.11–0.8 cm/s). The geometrical parameters include plate spacing (5 cm), perforation size (0.15 and 0.3 cm) and
Conclusions
The results of the experimental studies have shown that the hydrodynamic parameters are related to the properties of the system, pulsation conditions, and the plate geometry. For normal as well as phase reversal studies, drop size decreases with increasing pulse velocity while it is independent of continuous phase superficial velocity. Drop size is invariant with dispersed phase superficial velocity and increases only slightly. The drop size reduction occurs along the flow of dispersed phase,
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