Membrane pores, mass transfer processes

Dispersion free extraction in hollow fiber (HF) membrane utilizes immobilized liquid-liquid interface at the pore mouth of a microporous membrane to effect phase to phase contact and the mass transfer process. HF module can be con-... 

There are a number of different membrane techniques which have been suggested as alternatives to the SPE and LLE techniques. It is necessary to distinguish between porous and nonporous membranes, as they have different characteristics and fields of application. In porous membrane techniques, the liquids on each side of the membrane are physically connected through the pores. These membranes are used in Donnan dialysis to separate low-molecular-mass analytes from high-molecular-mass matrix components, leading to an efficient cleanup, but no discrimination between different small molecules. No enrichment of the small molecules is possible instead, the mass transfer process is a simple concentration difference over the membrane. Nonporous membranes are used for extraction techniques. 

In a wet or dry-wet process of phase inversion, the thermodynamic properties of the polymer solution and gelation medium give us some information on the overall porosity of a final membrane but not on the pore size and its distribution. The pore size and its distribution are mainly controlled by kinetic effects. This means that upon the immersion of polymer solution into a coagulation bath, mass transfer mainly determines the asymmetric structure of the membrane. The mass transfer is normally expressed by the exchange rate of solvent/nonsolvent at the interface between the polymer solution and the gelation medium. This exchange rate depends upon the nonsolvent tolerance of the polymer solution, the solvent viscosity and so on [14]. 

The resistance offered by the membrane with liquid-fiUed pores wiU be different (generally higher) from the gas-tilled pores, due to the different effective diffusion coefficients. There are two absorption processes operating it is either by physical or chemical absorption. To predict the overall mass transfer coefficient the membrane, liqnid mass transfer coefficient, and Henry s law constant must be known. The mass transfer coefficient is a function of the systan geometry, fluid properties, and flow velocity [1]. It can be expressed by a resistance in series model [9] ... 

Mass transfer through dense polymeric membranes is nowadays accepted to be described by the sorption-diffusion mechanism. According to this, the species being transported dissolve (sorb) in the polymer membrane surface on the higher chemical potential side, diffuse through the polymer free volume in a sorbed phase, and pass into the fluid phase downstream of the membrane (lower chemical potential side). In the case of dense polymeric membranes the polymer is an active participant in both the solution and diffusion processes. However, since in many porous membranes the mass transfer takes place mainly in the pores, the membrane material is not an active participant and only its pore structure is important. ... 

In a MD process, a microporous hydrophobic membrane is in contact with an aqueous heated solution on the feed or retentate side. The hydrophobic nature of the membrane prevents the mass transfer in liquid phase and creates a vapor/liquid interface at the entrance of each pore. Here, volatile compounds (typically water) evaporate, diffuse and/or convert across the membrane, and are condensed and/or removed on the permeate or distillate side. 

Vapor-Induced Phase Separation During the VIPS process, phase separation is induced by penetration of nonsolvent vapor, into the homogeneous polymer solution consisting of polymer and solvent(s). Mass transfer is usually much slower than that in the wet casting process thus, the VIPS process has been used to obtain membranes with symmetric, cellular, and interconnected pores [86,87],... 

Application of polymer membranes to separation of aqueous and organic phases in liquid-liquid extraction processes is called microporous membrane liquid-liquid extraction (MMLLE). An organic acceptor solvent, filling the pores of the hydro-phobic membrane, stays in direct contact with the aqueous phase near the membrane surface, where mass transfer takes place. This kind of extraction is similar to SEME, but takes place in a two-phase system and is slower and less selective because of the absence of carrier agent. Because the polymer membranes are insoluble, an arbitrary combination of aqueous and organic phase is possible and the extraction efficiency mainly depends on the partition coefficient. 

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. 

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]. 

The first awaited advantage of the process is to be able to work with a high permeate flux because of the low viscosity of SC CO2 (10 times lower than for water). Indeed the membrane used in the process is a hybrid nanofiltration element, constituted from an inorganic substrate (Ti02 with a mean pore diameter of about 10 run) on which a nation layer had been deposited, the prevailing mass transfer mechanism of which is convection. This fact could be checked through experiments conducted successively with water and SC CO2, flows obtained being in the opposite ratio of fluid viscosities within less than 10%. 

The SGMD is a temperature driven process, and it involves (a) evaporation of water at the hot feed side, (b) transport of water vapor through the pores of hydrophobic membrane, (c) collection of the permeating water vapor into an inert cold sweeping gas, and (d) condensation outside the membrane module. A decrease in driving force has been observed due to polarization effects of both temperature and concentration [80,82]. To calculate both heat and mass transfer through microporous hydrophobic membrane as well as the temperature and concentration polarization layer, the theoretical model suggested by Khayet et al. [58] can be written as... 

Wankat and Koo (110) have shown that the efficient mass transfer achievable with small ( 10 micron diameter) monodisperse packing can provide excellent resolution on very short columns, even when adsorption isotherms are nonlinear. For high-throughput processes, the most efficient columns resemble squat disks or pancakes (109). The ultimate "column" geometry may well be a membrane or consolidated packing with mobile phase flow through monodisperse pores. 

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