Equation 2-8: Partition coefficient
ci: concentration of component i in phase α or β
An uneven distribution of a solute leads to a - 3 value lower or greater than zero.
In this thesis, the partition coefficient was measured towards the evaluation of different two-phase systems as suitable for liquid-liquid extraction. Hence, the major points for the design of extraction processes are presented in chapters 2.3 and 2.4.
2.3 LIQUID-LIQUID EXTRACTION
The principle of a liquid-liquid extraction is the mass transfer of a target substance (solute) from one liquid phase (feed) to another (solvent). Therefore, the solute must have a higher solubility in the solvent than in the feed phase and both liquid phases must be either entirely or at least partially immiscible. The resulting solute-rich solvent is referred to as an extract and the second phase, lean in solute, is referred to as a raffinate. Liquid-liquid extraction finds typical applications in the treatment of wastewater since the volatility of the impurities does not differ much to the one of water. The Udex process is also an example of an industrial separation of aromatics from aliphatics by liquid-liquid extraction. Further, the separation of heat-sensitive substances may be more cost-effective in two-phase extraction systems than in adsorption units [10,14].
A crucial issue for the design of an extraction process is the right solvent choice.
The requirement for a suitable solvent are listed below [14]:
pronounced selectivity for the target substance,
high solubility of the substance of interest;
high capacity for solute accumulation;
low or no solubility in the raffinate;
cost-efficient recovery from the extract;
high density difference compared to the feed;
low interfacial tension;
low toxicity.
The simple approach of applying the suitable solvent for the separation of the target substance is the single theoretical stage extraction. This is a typical laboratory technique of stripping the extract from raffinate after reaching the
thermodynamic equilibrium [14]. Hence, the accumulation of the solute is limited by the partition coefficient (Equation 2-7 or Equation 2-8).
Multistage extraction processes are introduced to reach better recovery of the target substance. These consist of multiple extraction stages, with concurrent, crosscurrent, or countercurrent flow direction as depicted in Figure 2.1.
Figure 2.1: Schematic illustration of (a)con-, (b) cross- and (c) countercurrent extraction [15].
One extraction stage consists of feed (F), solvent (S), extract (E), and a raffinate (R). The crosscurrent technique is advantageous for laboratory applications since the sampling of extract and raffinate is possible after each stage. However, crosscurrent extraction demands high solvent amount. Hence, this scheme is less economically attractive. On the other hand, depending on the solvent and feed properties, the countercurrent process can be more beneficial, since it is known to reach up to 12 equivalents of the equilibrium stage in industrial scale. The apparatus, utilized for the countercurrent liquid-liquid extraction are classified to stage-wise (mixer-settler) and differential (continuous) contactors [14].
Widely used differential contactors with introduced stirring are the rotating disc contactor (RDC), the Kühni, the Scheibel and the Oldshue-Rushton (ORC) column types. These apparatus have in common that they consist of several compartments. In each compartment mixing energy is introduced by a rotated, centrally located agitator [10]. Two Lightnin Mixer (Oldshue-Rushton) extraction columns of different scale were subject to investigation in this work and therefore are presented more detailed.
Figure 2.2: Scheme of an Oldshue-Rushton column (ODC) with a heating jacket.
An extraction column of the type applied in this thesis is presented in figure Figure 2.2. The continuous contactor is equipped with a central agitator (in green) and stators (in red). Agitators and stators are alternating along the height of the mixing zone. On both ends, the mixing zone is connected to the top and bottom parts, where the settling of the phases takes place. The light and the dense phase enter the column at the bottom and the top, respectively. Then, the raffinate and extract exit the column vice-versa. In this way, the column is operated in a countercurrent mode. Depending on its density, the solvent can be introduced at the top or at the bottom of the contactor. Further, the solvent is the disperse phase in most columns. Hence, the agitation in the mixing zone leads to the dispersion of the solvent in the feed [13]. It is important to note, that in case of poor solute concentration in the disperse phase the extract flow can be recycled as solvent until the desired concentration is reached [10]. In this way, the stage efficiency is expected to increase [14].
The countercurrent extraction in contactors with stirring is well studied in organic but also in aqueous media. For instance, a multistage countercurrent contactor is applicable for the purification of enzymes in the aqueous two-phase system of polyethylene glycol [16]. Furthermore, the biotransformation of penicillin G is possible in a modified Kühni extractor, which contains the carrier Amberlite LA 2 loaded with enzyme [5]. A further contribution to the application of aqueous
biphasic mixtures is presented in this work, focusing on aqueous surfactant-based systems for the continuous extraction.
Stirred columns allow density differences between feed and solvent greater 0.05 g·cm-3 and viscosity of the continuous phase similar to water [17]. Hence, the column type was suitable for the investigations in this thesis.
The optimal operation in a stirred column can be reached by varying the mechanical energy input, as well as the feed and solvent flows. The agitation speed directly influences the extraction efficiency in an ODC. The stirrer provides the needed mass transfer area between the two phases by dispersing the solvent in droplets among the feed [18]. Hence, the droplet size distribution, as well as the breakage or coalescence, are influencing the enrichment of solute. Moreover, through the axial forces of the stirrer blade, small droplets are held up in the mixing zone. This phenomenon is referred to as backmixing [10]. A simplified representation of the influence of the agitation speed on the extraction efficiency is presented in Figure 2.3.
Figure 2.3: Influence of the agitation speed on the droplet size, backmixing of the continuous phase and the extraction efficiency (own simplified representation according to [18,19]).
The droplet size does not decrease with intensive stirring until a critical value of the agitation speed is reached. Then, the droplets are getting smaller, and thus the mass transfer is elevated. However, the adverse axial mixing effect is also more pronounced at higher stirring speeds. Therefore, it is essential to define a
process window providing highest mass transfer surface at less significant amount backmixing [18].
The feed and solvent flow are further parameters, which provide the amount of free phase accepting the target substance from the feedstock. The feed-to-solvent ratio and the capacity are the two primary values used to characterize extraction processes.
The feed-to-solvent ratio in stirred columns usually ranges from 1 to 10 [17]. The ratio is referred to as 4 and is defined as:
4 =56
6