Membrane contactors mass transfer process

Kreulen H, Smolders CA, Versteeg GF, and van Swaaij WPM, Microporous boUow fiber membrane modules as gas-bquid contactors. Part I, Physical mass transfer processes. Journal of Membrane Science 1993, 78, 197-216. 

Creeping film process (CFP) is a liquid membrane technique for simultaneous removal and concentration of dissolved species from their diluted aqueous solutions. CFP contactor is presented schematically in Figure 13.9. Feed and strip solutions flow down the vertical hydrophilic porous membrane sheets. A mobile organic LM is interposed between two creeping aqueous films. CFP is a continuous mass transfer process in which eddy diffusion controls the mass fluxes in all three liquid films. 

In principal, the ability of a membrane contactor to remove dissolved gases from an aqueous solution, by absorption or stripping mode, involves a mass transfer process between two phases. According to Henry s law, the amount of gas that will dissolve... 

H. Kreulen, C.A. Smolders, G.R Versteeg, W.RM. van Swaaij, Microporous hollow fibre membrane modules as gas- liquid contactors. Part 1. Physical mass transfer processes A specific application Mass transfer in highly viscous hquids, J. Memb. Sci. 78 (1993) 197-216. 

The introduction of membrane contactors in industrial cycles might represent an interesting way to realize the rationalization of chemical productions in the logic of the process intensification. Membrane contactors are, in fact, highly efficient systems for carrying out the mass transfer between phases and achieving high removals. They also present lower size than conventional apparatus. Commercial applications are already present (e.g., the electronics industry or bubble-free carbonation lines), however, some critical points must be still overcome and several are the research efforts needed for their further implementation at industrial level, as summarized below ... 

The membrane in a contactor acts as a passive barrier and as a means of bringing two immiscible fluid phases (such as gas and hquid, or an aqueous hquid and an organic hquid, etc.) in contact with each other without dispersion. The phase interface is immobilized at the membrane pore surface, with the pore volume occupied by one of the two fluid phases that are in contact. Since it enables the phases to come in direct contact, the membrane contactor functions as a continuous-contact mass transfer device, such as a packed tower. However, there is no need to physically disperse one phase into the other, or to separate the phases after separation is completed. Several conventional chemical engineering separation processes that are based on mass exchange between phases (e.g., gas absorption, gas stripping, hquid-hquid extraction, etc.) can therefore be carried out in membrane contactors. 

Controlling temperature and humidity of process air or ambient air is another unique application of membrane contactors. Membranes are used to humidify or dehumidify air by bringing air in contact with water or a hygroscopic liquid. Mass transfer in such processes is very fast since mass transfer resistance in the liquid phase is negligible. Heat transfer and mass transfer are directly related to these processes, since latent heat of evaporation (or condensation) creates temperature profiles inside the contactor. Some of the references in Literature are shown in Refs. [78-79]. Application of such processes has been proposed for conditioning air in aircraft cabins [80], in buildings or vehicles [81], or in containers to store perishable goods [82]. 

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]. 

Membrane-assisted solvent extraction processes have known an increasing number of applications in the last decades [1 ]. This technique not only overcomes the limitations of conventional liquid extraction, such as flooding, intimate mixing, limitations on phase flow rate variations, and requirement of density difference but also provides a large surface area of mass transfer per volume of contactor [5]. Excellent reviews of the technology and its applications were presented by Ho and Sirkar in 1992 [6], and by Gabelman and Hwang [7]. 

Membrane contactor (MC) is a phase-contacting device for use in gas absorption and stripping (degassing) processes as well as in biomedical gas transfer processes [44, 46]. The function of the membrane is to facilitate diflfusive mass transfer between contactir phases such as liquid-liquid, gas-liquid and gas-gas. The membrane phase contactor uses polyolefins, e.g., polypropylene (PP) microporous hollow fibres membranes, which are packed densely in a high surface area module. Since membranes are hydrophobic and have small pores (0.05—0.1 3m), water does not pass through the membrane pores easily. The pressure required to force water to enter the pore is called the breakthrough pressure, which for a PP membrane with a pore size of 0.05 pm is greater than 10 bar g. 

A MC module contains thousands of microporous hollow fibres, which are knitted into a fabric that is wound around a distribution tube with a central baffle as shown in Figure 1.15. The baffle ensures the water is distributed across the fibres, and also results in reduced pressure drop across the contactor. The hollow fibres are packed densely in a membrane module with a surfrce area of up to 4000 n / m. The liquid flows outside (shell side) the membrane, while vacuum is appHed on the inside of the fibre (tube side) forming a film across the pores of the membrane. Mass transfer takes place through this film and the pores due to the difference in the gas partial pressure between the shell side and tube side. Since the membranes are hydrophobic, they are not wetted by water, thereby, efiectively blocking the flow of water through the membrane pores. The membrane provides no selectivity. Rather its purpose is to keep the gas phase and the Hquid phase separated. In effect, the membrane acts as an inert support that allows intimate contact between gas and liquid phases without dispersion. Vacuum on the tube side of the membrane increases the mass transfer rate as in a vacuum tower. The efficiency of the process is enhanced with the aid of nitrogen sweep gas flowing on the permeate side of the membrane. 

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