Hybrid liquid membrane modules

Hydrophilic (or ion-exchange) membranes were used for designing rotating disk, creeping film, hybrid liquid membrane, and multimembrane hybrid membrane systems, hoUow-fiber LM modules. Hydrophobic membranes were used for designing hybrid liquid membrane, multimembrane hybrid system, flowing LM, hoUow-fiber contained LM, capiUary liquid membrane modules (or contactors). Below, some of these systems are referenced and described shortly. 

The works, in which the authors prove the overaU equilibrium state, are not referenced here. The works, in which the equilibrium state is not proved, or cannot be reached (e.g., the process where the feed-membrane-strip flows are arranged in one continuously operating module) are referenced in the next section as hybrid liquid membrane or hoUow-fiber liquid membrane processes. 

Promising results are shown by recently developed integrated SLM-ELM [84, 85] systems. These techniques are known as supported liquid membrane with strip dispersion (SLMSD), pseudo-emulsion-based hollow fiber strip dispersion (PEHFSD), emulsion pertraction technology (EPP), and strip dispersion hybrid Hquid membrane (SDHLM). AH techniques are the same the organic phase (carrier, dissolved in diluent) and back extraction aqueous phase are emulsified before injection into the module and can be separated at the module outlet. The difference is only in the type of the SLM contactors hoUow fiber or flat sheet and in the Hquid membrane (carrier) composition. These techniques have been successfuUy demonstrated for the removal and recovery of metals from wastewaters. Nevertheless, the techniques stiU need to be tested in specific apphcations to evaluate the suitabUity of the technology for commercial use. 

Raghuraman and Wiencek [11] developed a hybrid technique where an emulsion is fed into a hollow fiber contactor on the tube side. Since the solid membrane support is hydrophobic, the continuous phase of the water-in-oil emulsion easily wets the pores of the tube wall and permeates to the shell side. On the shell side of the hollow fiber, the aqueous feed phase is exposed and held at an elevated pressure that prevents the permeating liquid membrane phase from exiting the pores. Thus, extraction occurs on the shell side, and stripping on the tube side of the hollow fiber membrane module. This methodology is closely related to SLMs, but the key difference is the presence of the emulsion on the tube side, which allows for long-term stability because the membrane liquid is continuously replenished to make up for any loss by solubility. 

Membranes can also be used to purify a mixmre and attain composition beyond the azeotropic composition. The pervaporation process features a liquid feed, a liquid retentate, and a vapor permeate. While gas-phase membrane processes are essentially isothermal, the phase change in the pervaporation process produces a temperature decrease as the retentate flows through the unit. Since flux rates decrease with decreasing temperature, the conventional pervaporation unit consists of several membrane modules in series with interstage heating. The vapor permeate must be condensed for recovery and recycle, and refrigeration is usually required. Hybrid systems of distillation columns and pervaporation units are frequently used in situations where distillation alone is impossible or very expensive. An important application is the removal of water from the ethanol-water azeotrope. Chapter 14 will discuss the details of design and control of such processes. 

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