Advancements in the theory of solvent extraction and the characterization of annular centrifugal contactors
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Date
2025-09-02
Authors
Hamamah, Zaid Alkhier
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Publication Type
Dissertation
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Abstract
Solvent extraction (SX) process, also known as liquid-liquid (L-L) extraction, is one of the most important separation processes. Despite being widely implemented in industry, SX is still less researched and understood compared to other unit operations like distillation, leaving ample room for investigation. The presented work addresses both the theoretical and applied aspects of SX process and provides answers for several research questions. In the theory, the graphical concepts of the minimum (S/F)min and maximum (S/F)max solvent-to-feed ratios for countercurrent processes of ternary systems are elaborated on. These limits, with their corresponding number of stages, define the operation boundaries of the separation task; thus, they are crucial in the design of SX processes. For (S/F)min, typically associated with an infinite number of stages and a pinch point, it is shown that the state-of-the-art graphical method to determine it is applicable only within a limited region of the ternary diagram. In that, the method depends on finding the tie line that passes through the feed point when extended; since tie lines exist only below the tangent of binodal curve at the plait point, the method fails for the feeds located above this tangent. Although these feeds are quite significant in SX context, the literature lacks a graphical method to find their respective (S/F)min. The presented work bridges this knowledge gap by introducing a novel graphical technique and a three-region division of the ternary diagram, extending the current method and making it possible to find (S/F)min for any potential feasible feed. To verify the developed theory, rigorous simulations are performed to examine process behavior close to the (S/F)min determined by the proposed extension. The obtained results of simulations prove the introduced graphical technique and show the typical asymptotic behavior of infinite stages when (S/F)min is approached. For (S/F)max, it is consensually defined in literature as the amount of solvent needed to fully dissolve the feed, and is always associated with a single required theoretical stage. Yet, it is not clear how this one-stage requirement is universal and whether the term "SX stage" can even be used with (S/F)max since it yields one homogeneous phase as defined. By discussing (S/F)max from different perspectives, its definition is refined, allowing for the determination of the corresponding minimum stages that can differ from one. The discussions on (S/F)max are also supported by a case study simulation. In the applied aspect of the work, the focus shifts to SX technologies to characterize a major type of centrifugal extractor, the so-called annular centrifugal contactors (ACCs). These devices consist of an annulus chamber where two liquid phases are vigorously mixed, followed by a rotor chamber where the L-L separation occurs under centrifugation. Due to their superior features compared to standard SX equipment, ACCs have been increasingly implemented across various fields, ranging from traditional applications in nuclear industry and pharmaceuticals to sustainable ones like biotechnology and wastewater treatment. Nevertheless, the current industrial employment of ACCs still falls short of their potential, largely due to their complex fluid dynamics and the interconnected effects of operational variables on performance. These aspects hinder ACCs’ acceptance and prevent realizing their prospects. The presented work aims to resolve the aforesaid issues and enable the practical and informed operation of ACCs to leverage their capabilities. This is achieved by characterizing them through two research work packages, using their prominent variant CINC V 02. The first package aims to establish a fundamental nderstanding of ACCs hydrodynamics using one-phase flow, whereas the second addresses the L-L operation using two-phase flow. For the latter, the ternary mixture water + n-propanol + toluene is utilized as a model system for experiments. Initially, to obtain the system equilibria under the conditions of interest, 20 C and atmospheric pressure, the well-known databases DDB and NIST were consulted; unfortunately, no relevant datasets were found. Therefore, an additional dedicated study was carried out to generate the liquid-liquid equilibria (LLE) at these conditions. In this LLE study, the binodal curve data of the system were produced along with twelve tie lines covering the miscibility gap. The compositions of the conjugate phases were determined using high-precision refractive index measurements. By applying the techniques of conjugate lines and mass balances, the data quality was verified, with further discussions presented to emphasize the incorrect usage of empirical correlations in the literature for that purpose. Three different methods to extrapolate and locate the plait point were then employed and contrasted. Finally, a thermodynamic correlation using two activity coefficient models, UNIQUAC and NRTL, was performed on the produced dataset along with another dataset from the literature generated at a different temperature. While UNIQUAC did not function, NRTL effectively modeled the system LLE and adapted to temperature changes. The work then continues with ACC research and begins with the first work package that investigates ACCs hydrodynamics. First, several sources of holdup variation
are identified and quantified, with the help of a model developed in MATLAB® to simulate the liquid surface inside the rotor chamber and calculate its holdup. The findings provide explanations for unclear deviations reported earlier in the literature. By implementing the model together with pressure measurements, the factors controlling the liquid content in both chambers are defined and the underlying functionalities are understood. Eventually, because the standard procedure of literature for measuring holdups failed to reflect the impact of flow rate on ACCs’ liquid content, a new procedure is introduced. A comparison between the measurements from both procedures reveals significant differences at high throughputs, indicating that the flow-dependent portion of holdup can be captured only using the introduced approach. Building on the above findings, the second work package on ACCs’ L-L operation was initiated. A primary outcome here is the development of a procedure to define the operation window of the ACC for a certain SX task. The procedure integrates a hydraulic substudy, focusing on L-L separation quality, with a statistical one, modeling
the mass transfer efficiency. Therefore, the resulting window outlines the feasible ranges of operational variables to accomplish the task in terms of efficiency and separation quality, and it also shows the maximum achievable throughput. The developed procedure is designed to apply to any ACC or SX system, representing a versatile tool for practical handling of ACCs despite their complexities. Using the 3D plots of the mass transfer model, the interlinked impacts of the variables are visualized, allowing for the prediction of process behavior in response to the variables’ fluctuations. To facilitate the transfer of knowledge to end users, the whole concept is converted into an interactive user-interface software explained within the manuscript and appended to it, serving as an inclusive standalone unit. Additionally, throughout the substudies, in-depth insights into ACCs’ hydraulics and mass transfer dynamics are presented, including the time required to reach steady state and the identification of internal dead zones. The work finally concludes with a detailed analysis of the variables’ interplay and introduces practical guidelines to readily realize successful operating
points.
Description
Faculties
Fakultät für Ingenieurwissenschaften, Informatik und Psychologie
Institutions
Institut für Chemieingenieurwesen
Citation
DFG Project uulm
EU Project THU
Other projects THU
License
CC BY 4.0 International
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DOI external
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Degree Program
DFG Project THU
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Keywords
Solvent-to-feed Ratio, Annular Centrifugal Contactors, CINC V 02, Thermodynamic Correlation, Flüssig-Flüssig-Extraktion, Solvent extraction, Liquid-liquid equilibrium, DDC 620 / Engineering & allied operations, DDC 540 / Chemistry & allied sciences