Membrane contactors mechanism

This book provides a general introduction to membrane science and technology. Chapters 2 to 4 cover membrane science, that is, topics that are basic to all membrane processes, such as transport mechanisms, membrane preparation, and boundary layer effects. The next six chapters cover the industrial membrane separation processes, which represent the heart of current membrane technology. Carrier facilitated transport is covered next, followed by a chapter reviewing the medical applications of membranes. The book closes with a chapter that describes various minor or yet-to-be-developed membrane processes, including membrane reactors, membrane contactors and piezodialysis. 

Cross-section structure. An anisotropic membrane (also called asymmetric ) has a thin porous or nonporous selective barrier, supported mechanically by a much thicker porous substructure. This type of morphology reduces the effective thickness of the selective barrier, and the permeate flux can be enhanced without changes in selectivity. Isotropic ( symmetric ) membrane cross-sections can be found for self-supported nonporous membranes (mainly ion-exchange) and macroporous microfiltration (MF) membranes (also often used in membrane contactors [1]). The only example for an established isotropic porous membrane for molecular separations is the case of track-etched polymer films with pore diameters down to about 10 run. All the above-mentioned membranes can in principle be made from one material. In contrast to such an integrally anisotropic membrane (homogeneous with respect to composition), a thin-film composite (TFC) membrane consists of different materials for the thin selective barrier layer and the support structure. In composite membranes in general, a combination of two (or more) materials with different characteristics is used with the aim to achieve synergetic properties. Other examples besides thin-film are pore-filled or pore surface-coated composite membranes or mixed-matrix membranes [3]. 

In Figure 22.2 the picture of a membrane contactor manufactured by GVS and used for the demonstration tests reported in this chapter is shown. The mechanism of the capture of a specific gas molecule by the extractant is also schematically illustrated. The molecule is captured by the extractant without any contact between the stream to be treated and the extractant stream. While the gas or liquid polluted stream passes through the contactor flowing tangentially to one side of the membrane, the gas molecule to be captured passes through the pores of the membranes and is captured at the other side of membrane interface by the extractant solution. Periodically, the extractant is regenerated to release the absorbed molecules and to be reused in the system. 

A historical perspective on aqueous-organic extraction using membrane contactor technology is available in Refs. [1,6,83]. The mechanism of phase interface immobilization was first explored in Ref. [84], while application of membrane solvent extraction for a commercial process was first explored in Ref. [85]. Two aspects of liquid-liquid contact in membrane contactors that are different from typical gas-liquid contact are (1) the membrane used could be hydrophobic, hydrophdic, or a composite of both and (2) the membrane mass transfer resistance is not always negligible. Ensuring that the right fluid occupies the membrane pores vis-a-vis the affinity of the solute in the two phases can minimize membrane resistance. These aspects have been discussed in detail in Refs. [6,86,87]. 

This chapter presents an overview of different membrane processes and a description of all of the chapters presented in this edition. Chapter 2 focuses on updated information of utility to UF and NF membrane research and development, particularly in the preparation of new types of UF/NF membranes with improved performances. Chapter 3 presents a comprehensive review on RO membrane, the latest developments in the field, important installations demonstrating this technology, and future scope of RO processes. Chapter 4 presents the potential of membrane contactors, especially hollow fiber contactors in the field of chemical and nuclear industry along with their applications, performance, and current challenges faced by indnstry. This chapter also gives an introduction to membrane contactors, their principles of operation and associated mechanisms (where chemical reactions are involved), and fntnre scope of these contactors. 

In the second part of this chapter, three key membrane contactor operations are described, examining the basics, mechanisms and applications of each. 

Gableman, A., and Hwang, S.-T. (1999). Hollow fiber membrane contactors. J. Membr. Sci. 159, 61. Gableman, A., Hwang, S.-T., and Krantz, W. B. (2005). Dense gas extraction using a hoUow fiber membrane contactor Experimental results versus model predictions. J. Membr. Sci. 257, 11. Hestekin, J. A., Bachas, L. G., and Bhattacharyya, D. (2001). Poly(amino acid) functionalized cellulosic membranes Metal sorption mechanisms and results. Ind. Eng. Chem. Res. 40, 2668. Imai, M., Furusaki, S., and Miyauchi, T. (1982). Separation of volatile materials by gas membranes. Ind. Eng. Chem. Process Des. Dev. 21, 421. 

FIGURE 9.1 Schematic representation of (a) CO2 absorption process and (b) CO2 stripping mechanism through a gas-liquid membrane contactor. 

Numerous researchers have studied the potential of commercial membrane such as PTFE and PP membrane for long-term stability performance. Some of these commercial membranes are reliable in other applications however, when contacted with conventional liquid absorbent such as aqueous amine solution in a membrane contactor system, the membranes became susceptible to wetting and lost their initial microstructure. This was unexpected as the commercial membranes have high hydrophobicity properties and high chemical tolerance. Fundamental study on the wetting mechanism is required as the detail of the process is still ambiguous. 

The authors of this paper have proposed the application of novel high performance membranes with the dense thin top-layer made of the glassy polymer PTMSP with the highest gas permeance among known polymers. It was already shown that PTMSP as a membrane material possesses long-term chemical and mechanical stability at typical MGD conditions - amine-based solvent, trans-membrane pressure up to 40 bar and temperature 100 °C [7]. Furthermore, PTMSP is a barrier material towards chemical solvents such as aqueous solutions of alkanolamines [7] and some physical solvents like water [8] and ionic liquid [9]. Details about the development of these membranes are described elsewhere [10], In this paper, the focus is on experimental work on using these membranes in contactors and the implications for application in natural gas processing. 

For ELM applications, conventional solvent extraction contactors, e.g., mixer-settlers and mechanically agitated columns, have been generally employed. Recently, Raghuraman and Wiencek (63) investigated ELM extractions in a microporous hollow-fiber contactor. The hollow-fiber contactor may improve the efficiency of extraction by decreasing membrane swelling and leakage, particularly for poorly formulated ELMs with stability problems. 

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