Emulsion liquid membranes hydrocarbon separations

Liquid membrane (LM) separation provides a potentially powerful technique for effecting diverse separation operations. Compared to conventional processes, emulsion liquid membrane (ELM) and liquid surfactant membrane (LSM) processes have some attractive features, for example, simple operation, high efficiency, extraction and stripping in one stage, larger interfacial area, scope of continuous operation. The ELM technique has great potential for recovery and removal of different metal ions and hydrocarbons from wastewater where conventional methods provide lower separation efficiency. 

Emulsion liquid membranes (ELM) are double emulsions formed by mixing two immiscible phases and then dispersing the resulting emulsion in another continuous phase under agitation. Proposed applications for emulsion liquid membranes have included selective recovery of metal ions (1-12), separation of hydrocarbons (13 16), removal of trace organic contaminants (17-27), and encapsulation of reactive enzymes or whole cells (28-36). 

Emulsion liquid membranes have been formulated for the removal of species without chemical carriers. These systems rely on solubility differences between permeant species. The first work performed by Li (73) dealt with the separation of a bineiry mixture of aromatic eind paraffinic hydrocarbons. The hydrocarbons were encapsulated in an aqueous liquid membrane forming an emulsion which was distributed in a hydrocarbon solvent and mixed. The aromatic hydrocarbons preferentially permeated through the aqueous liquid membrane phase due to solubility differences. Cahn and Li (7 ) describe a liquid membrane formulation for phenol removal in which sodium hydroxide is encapsulated by an organic liquid membrane. The resulting membrane phase is mixed with a continuous aqueous phase containing phenol. 

This paper presents an emulsion liquid membrane (ELM) system capable of extracting both Se(IV) and Se(VI) from an aqueous waste stream containing large amounts of innocuous anions like sulfate. In an emulsion liquid membrane, small drops of an aqueous stripping phase are contained (emulsified) inside an organic hydrocarbon (oil) drop, llie surfactant-stabilized oil acts as a membrane separating the wastewater (the continuous, bulk phase solution) from encapsulated droplets of aqueous solution (the internal phase) as shown in Figure 3. 

Ultrasound-assisted emulsification in aqueous samples is the basis for the so-called liquid membrane process (LMP). This has been used mostly for the concentration and separation of metallic elements or other species such as weak acids and bases, hydrocarbons, gas mixtures and biologically important compounds such as amino acids [61-64]. LMP has aroused much interest as an alternative to conventional LLE. An LMP involves the previous preparation of the emulsion and its addition to the aqueous liquid sample. In this way, the continuous phase acts as a membrane between both the aqueous phases viz. those constituting the droplets and the sample). The separation principle is the diffusion of the target analytes from the sample to the droplets of the dispersed phase through the continuous phase. In comparison to conventional LLE, the emulsion-based method always affords easier, faster extraction and separation of the extract — which is sometimes mandatory in order to remove interferences from the organic solvents prior to detection. The formation and destruction of o/w or w/o emulsions by sonication have proved an effective method for extracting target species. 

Cassamatta G, Chavarie C, and Angelino H. Hydrocarbon separation through a liquid water membrane Modeling of permeation in an emulsion drop. AIChE J 1978 24 945-949. 

Liquid membranes are double emulsions formed when a water-in-oil emulsion (w/o) is gently dispersed in a second aqueous phase, the external aqueous phase. The internal (emulsified) and external aqueous phases are kept separate by a layer of hydrocarbon, forming the liquid membrane. Since the two aqueous phases are not in contact, LM systems can be useful for separation processes as well as for enzyme immobilization separation is accomplished by selective transport of solutes across the hydrocarbon "membrane," and enzyme immobilization is accomplished by encapsulating enzyme(s) via emulsification of an aqueous enzyme solution. It is in fact possible to combine enzymatic reaction(s) with separations in a single LM system. Figure 1 depicts an LM-enzyme system. 

Liquid membranes can be of three types—bulk liquid membrane, immobilized on a solid supported hquid membrane, and liquid membrane as double emulsions. Of these three types, ELMs can achieve much higher mass transfer area than the other two membranes. ELMs were first used by Li [1] for separation of hydrocarbons. Since then, considerable work has been done to demonstrate qualitatively the feasibihty of performing separations with specific formulations. 

---