Introduction and Literature Review
1. Introduction
1.1 Liquid membrane
1.1.4 Types of LM
LMs are of three types based on their configurations: bulk liquid membrane (BLM), emulsion liquid membrane (ELM) and supported liquid membrane (SLM). According to the geometry SLM is again of three types, flat sheet, hollow fiber and spiral wound membranes. The other types are electrostatic pseudo LM and contained liquid membrane. Fig. 1.3 shows about the various configurations of LM. This research work is more concerned with BLM and two types of SLMs viz. flat sheet supported liquid membrane (FS-SLM) and hollow fiber supported liquid membrane (HF-SLM). BLM is studied for initial estimation of the various parameters related to mass transfer and transport feasibility of solute. As SLM is the most promising for commercial applications, it is studied in detail. In the following sub-sections, BLM and SLMs have been introduced in brief.
Chapter-I
Figure 1.3: Family of liquid membranes
1.1.4.1 Bulk liquid membrane (BLM)
BLM is the simplest of all configurations of LMs. In a BLM setup, two aqueous phases are separated with a solid barrier (e.g. a glass wall) in a vessel and the membrane phase, in bulk volume, shares an interface with both these aqueous phases as shown in the schematic representations in Fig. 1.4(a-b). The Fig. 1.4 (a) shows the case when membrane phase is lighter than the feed and strip phases and Fig. 1.4 (b) shows the other case when membrane phase is heavier than feed and strip phases. Separation study with BLM is very important and relevant because, it is the simplest of all configurations of LM and it provides the understanding of the separation feasibility of a LM for any system of concern. However, up-gradation of the laboratory scale BLM to a pilot/commercial scale happens to be practically ineffective mainly
Introduction and Literature Review due to very low mass transfer area per unit volume as well as for the longer diffusion path of the solute or solute-carrier complex.
(a)
(b)
Figure 1.4: Schematic of BLM: (a) for lighter ML (b) for heavier ML
1.1.4.2 Supported liquid membrane (SLM)
A solid micro-porous polymeric membrane holds the ML in its pores by capillary force. The typical thickness of the supports are in the range of 25-170 Β΅m (for polymeric membranes) and the average diameters of the pores are in the range of 0.075-0.45Β΅m [20]. Thus, a SLM consists
Chapter-I of three main parts (i) support material (ii) diluent (solvent) and (iii) carrier. The membrane phase is prepared by dissolving the carrier agent into the solvent and immobilized into the pores using the wetting characteristic of the ML and by capillary forces. The porous support material serves as a framework or supporting layer for the membrane phase. The porous support can be inorganic or organic (polymer) with compatible chemical properties and mechanical stability.
The novelty of the SLM techniques lies in the fact that the diffusion path (L) of the solute can be designed to be very small provided that the stability of membrane is not compromised.
According to the Fickβs law, rate of diffusion of a solute through a medium (liquid) is inversely proportion to the diffusion path. Hence, to increase the rate of solute diffusion, thickness of the LM should be kept as minimum as possible. All these factors result in the increase in the solute flux. However, the success of SLM techniques depends on the judicious selection of support, ML and the stripping agent for solute of interest. Important properties of ML in view of membrane stability are viscosity of ML, interfacial tension between membrane phase and the aqueous phases, solubility of ML in aqueous phases, etc. In addition, the support material needs to be compatible with the membrane phase. The compatibility is of two types viz. physical and chemical compatibility. Physical compatibility includes thickness, pore size, shape of the pores, tortuosity etc. On the other hand, relative hydrophobicity of ML and the support material falls in chemical compatibility.
Ideally, the pores should be of identical in size and cylindrical in shape. But, very often, they are away from this uniqueness and regular shape. Moreover, the supports need to be of higher porosity so that the effective surface area for the solute diffusion is high. Therefore, the porosity, thickness, shapes and sizes of the pores of the supports as a whole contribute to the performance
Introduction and Literature Review of solute transportation. The effective characteristic is measured as the tortuosity (π) of the support which is defined as follows [6]:
π =1 + ππ 1 β ππ
(1.2)
Where ππ is the volume fraction of the polymeric framework and again defined as:
ππ = 1 β π, where π is porosity of the support membrane.
Based on the geometry of the supports, SLM is in general of three types viz. FS-SLM, HF-SLM and spiral wound. FS-SLM, as the name implies, uses support material in sheet form. It is simple in structure and of low cost but has very low mass transfer area per unit volume as well. A schematic of FS-SLM is shown in Fig. 1.5.
Figure 1.5: Schematic of FS-SLM
HF-SLM is just like a shell and tube heat exchanger, thus compact with increased mass transfer area per unit volume [21]. The typical mass transfer area to volume ratio of commercially available HFM module is 2930 m2.m-3 [22]. HF-SLM has an outer shell and there are large
Chapter-I number of thin porous fibers in the shape of tubes (ID: 0.5-1.0 mm) inside the shell [23]. The pores of the fibers are immobilized with the ML. The feed phase and strip phase are separately passed through either fiber and shell side or through shell side and fiber respectively along the length. A schematic representation of HF-SLM is shown in Fig. 1.6.
Figure 1.6: Schematic of HF-SLM