Liquid membranes terminology

The concept of emulsion liquid membranes (ELM) was first proposed by Li in 1968 [1]. Since their inception in the late 1960s they have been referred to as surfactant liquid membranes, double emulsion membranes or ELM. Regardless of the terminology used, the workings of such systems are as follows they consist of an emulsion formed by an organic solvent and water, which can be stabilized by the addition of surfactant. This emulsion is then contacted with a continuous phase containing the desired solute, stirred to yield globules, and transported across the extremely thin membrane layer that separates internal phase droplets... 

As the membrane is in the fluid state of matter (i.e. smectic A in liquid crystal terminology), it cannot withstand shear in its plane. Moreover, the solubility of the double chain phospholipids is extremely low. Therefore, there is practically no exchange of material between membrane and solution. This fact, together with the small compressibility of the membrane, implies that for almost all phenomena the membrane can be considered as locally incompressible. 

The methods and literature are briefly reviewed for solid-suspension separations, solution-phase separations, liquid-phase separations, and gas-phase separations. In the terminology used, the objective is to separate a feed stream (or streams) into a permeate phase and a reject phase, either of which may contain the compon-ent(s) of more interest. For a single membrane, say, the permeate phase remains on the feed side or high-pressure side of the membrane, and is subsequently discharged, whereas the reject or raffinate phase builds up on the opposite or low-pressure side of the membrane, and is then discharged. 

In another type of membrane extraction devices, porous polypropylene hollow fibers are used, often in a disposable way, which minimizes carryover problems and reduces costs [26-33]. On the other hand, manual manipulations are needed, limiting the possibility for automation. With these devices, the extraction can be carried out in a static mode, either in large sample volumes, where the extraction is not intended to be complete, or in small volumes aiming for complete extraction. Usually, stirring is applied to increase the speed of mass transfer. Some typical practical arrangements are shown in Figure 12.2. This type of SLM extraction is often called hollow fiber liquid phase microextraction, or three-phase liquid phase microextraction or two-phase liquid phase microextraction but the terminology in this active field of research has not been settled. Also hollow fibers can be connected in flow systems [34,35]. 

The OMD process can be included in the group of processes under membrane distillation [38], because it meets the terminology for membrane distillation that was decided by the expert committee at the workshop on membrane distillation in Rome in 1986 [39]. Membranes used in MD need to satisfy certain conditions For example, the membrane should be porous and should not be wetted by the process liquids no capillary condensation should take place inside the pores of the membrane the membrane must not alter the vapor-liquid equilibrium of the different components in the process liquids at least one side of the membrane should be in direct contact with the process liquid for each component the driving force of this membrane operation is partial pressure gradient in vapor phase. 

Membrane filtration has many similarities to conventional filtration, and the mathematical description of the process uses many ccmcepts already introduced in Chapter 2. However, there are rignificant differences in the terminology enqiloyed the filtrate is referred to as the permeate , the residual slurry or suspension from the filtration is called the retentate and the permeate filtration rate is the flux rate , which in microfiltration is conventionally reported in the emits of litres per square metre of membrane area per hour (1 m h ). This rate is equivalent to the superficial liquid velocity through the menibrane. In nearly aU the instances of constant-pressure... 

Other procedures and calculation techniques have been developed for both stagewise and differential permeation, such as those presented S-T Hwang and K. Kammermeyer, but they are not pursued here, inasmuch as the analogy is to be made specific to vapor-liquid mass transfer unit operations. In this way, the conventions and techniques already developed for mass transfer operations can be more readily utilized. Also note that the symbols and terminology used for membrane permeation have evolved through the years and vary from one author to another. 

The selection of a membrane for a particular type of separation is usually determined by its average pore size e.g., 10-100 yam is useful for conventional filtration, 0.1-10 yam for microfiltration, 50-1000 A for ultrafiltration, and less than 50 A for reverse osmosis, gas separation, and pervaporation, as depicted in Fig. 33.2. The latter are also described as nonporous membranes and depend on molecular interactions between the permeant and the membrane itself to affect separation. The basic terminology and theory of membrane-based separation systems are similar for both gases and liquids and are therefore treated together in this section. 

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