1.1 Introduction
1.1.4 Types of LM
LMs are broadly classified into three types according to their configurationviz. bulk liquid membrane (BLM), supported liquid membrane (SLM) and emulsion liquid membrane (ELM) [30]. As per the geometry of SLM, it is classified into three types
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(Figure 1.5), viz. flat sheet supported liquid membrane (FS-SLM), hollow fibre sup- ported liquid membrane (HF-SLM) and spiral wound supported liquid membrane (SW-SLM). Two other types of membrane are also there, electrostatic pseudo LM and contained liquid membrane. This research work is focussed on BLM and FS-SLM.
BLM process is well suited for initial study about various parameters related to mass transfer and transport feasibility of solute, where as SLM process is most promising towards commercial application, it is studied in detail. The following sub-sections, BLM and SLM have been discussed in brief.
Figure 1.5: Family of liquid membranes
1.1.4.1 Bulk liquid membrane (BLM)
In BLM, the feed and strip phases are separated with a solid barrier (e.g. glass wall) and bulk amount of organic phase is placed in such a way that the liquid is in contact with both feed and strip solutions. A schematic diagram is in Figure 1.6. The difference in density between membrane and aqueous phases is the determining factor i.e. membrane phase density is lower than feed and strip phase (refer Figure 1.6a) and membrane phase density is heavier than feed and strip phase (refer Figure 1.6b).
The BLM is simplest among other LMs. BLM process is helpful to determining rate constants and distribution coefficients of novel carrier and it is suitable for selection of stripping solution. But BLM has comparably small surface area to volume ratio and longer diffusion path of the solute or solute-carrier complex which limits its industrial usefulness.
(a) Lighter LM
(b) Heavier LM
Figure 1.6: Schematic of BLM
1.1.4.2 Supported liquid membrane (SLM)
The SLM uses a thin micro-porous solid support where organic/membrane phase is immobilized within the pores by capillary forces. The solid support containing organic/membrane phase act as barrier between feed and strip phases. The porous support can be inorganic or organic (polymer) depending on the mechanical stability, chemical properties and application. In polymeric support, typical thickness varies in the range of 25-170 µm and the average diameters of the pores are in the range
1.1. Introduction 13
of 0.075-0.45 µm [45]. The novelty of SLM lies in the diffusional path (L) of solute transport. The diffusional path can be designed very small provided the stability of membrane is not compromised. Fick’s law of diffusion says the rate of diffusion of solute through medium (here liquid) is inversely proportional to the length of diffusional path. Hence, to increase the solute diffusion the length of diffusion path should be kept as small as possible. These factors are important to increase the solute flux. The success of SLM technique depends also on the judicious selection of support, proper selection of LM and the stripping agent for solute of interest.
The stability of organic phase in SLM is also an important part for transporting any solute. The stability of LM or organic phase mainly depends on the viscosity of LM, interfacial tension between aqueous phases and organic phase, solubility of LM in aqueous phases etc. The compatibility between LM and solid support is also a key factor for stability of LM in SLM process. Mainly two types of compatibility are involved i.e. physical and chemical compatibility. Physical compatibility lies mainly in the support characteristics such as thickness, pore size, shape of the pores, tortuosity etc. Whereas, relative hydrophobicity of LM and the support material involves in chemical compatibility.
Ideally, the pores should be of identical in size and cylindrical in shape. But in real case support pores are neither identical nor regular in shape. The support should have higher porosity which would provide higher effective surface area for solute diffusion.
The tortuosity (τ) of the solid support is an effective membrane characteristics which can be defined as the following equation [46]:
τ = 1 +Vp
1−Vp (1.2)
Where, Vp = 1−ε, Vp is the volume fraction of the polymeric framework and ε is porosity of the support membrane.
Based on the geometry of the support and applications, SLM can be designed in a
variety of different configurations, namely, flat sheet-SLM (FS-SLM), hollow fiber- SLM (HF-SLM) and spiral wound-SLM. Simplest is the flat sheet configuration, where the membrane phase is held in a porous sheet separating the compartments of the feed and receiving phases (see Figure 1.7). This configuration is simple to construct but has a fairly low mass transfer area per unit volume.
The other advantages of using SLM are:
X It incurs less capital, operating, maintenance and energy costs X Amount of LM consumption is very less
X Easy to operate and scale-up
X Can be used for high selective transport
X Operating procedure is simpler than other LM processes
However, the main problem of the SLM technology is the instability of the membrane phase viz. the chemical stability of the LM, the mechanical stability of the support pores and limited membrane life.
The reason behind the instability of SLM are [35]:
X Loss of membrane solution from the pores of the solid support due to wetting of the pores by aqueous phases
X Effect of trans-membrane and osmotic pressure X Emulsion formation in the LM phase
X Blockage of membrane pores by contamination
1.1. Introduction 15
Figure 1.7: Schematic of flat sheet supported liquid membrane