Membrane pores, mass transfer

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. ... 

The thud step gives a polymer-rich phase forming the membrane, and a polymer-depleted phase forming the pores. The ultimate membrane structure results as a combination of phase separation and mass transfer, variation of the production conditions giving membranes with different separation characteristics. Most MF membranes have a systematic pore structure, and they can have porosity as high as 80%.11,12Figure 16.6 shows an atomic force microscope... 

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-... 

Figure 8.1 shows, in graphical terms, the concentration gradients of a diffusing solute in the close vicinity and inside of the dialyzer membrane. As discussed in Chapter 6, the sharp concentration gradients in liquids close to the surfaces of the membrane are caused by the hquid film resistances. The solute concentration within the membrane depends on the solubility of the solute in the membrane, or in the liquid in the minute pores of the membrane. The overall mass transfer flux of the solute J(kmol h m" ) is given as... 

The results in sections 2 and 3 describe the adsorption isotherms and diffusivities of Xe in A1P04-31 based on atomistic descriptions of the adsorbates and pores. The final step in our modeling effort is to combine these results with the macroscopic formulation of the steady state flux through an A1P04-31 crystal, Eq. (1). We make the standard assumption that the pore concentrations at the crystal s boundaries are in equilibrium with the bulk gas phase [2-4]. This assumption cannot be exactly correct when there is a net flux through the membrane [18], but no accurate models exist for the barriers to mass transfer at the crystal boundaries. We are currently developing techniques to account for these so-called surface barriers using atomistic simulations. 

Mechanical forces can disturb the elaborate structure of the enzyme molecules to such a degree that de-activation can occur. The forces associated with flowing fluids, liquid films and interfaces can all cause de-activation. The rate of denaturation is a function both of intensity and of exposure time to the flow regime. Some enzymes show an ability to recover from such treatment. It should be noted that other enzymes are sensitive to shear stress and not to shear rate. This characteristic mechanical fragility of enzymes may impose limits on the fluid forces which can be tolerated in enzyme reactors. This applies when stirring is used to increase mass transfer rates of substrate, or in membrane filtration systems where increasing flux through a membrane can be accompanied by increased fluid shear at the surface of the membrane and within membrane pores. Another mechanical force, surface... 

As organic and aqueous phases are macroscopically separated by the membrane, HFM offer several hydrodynamic advantages over other contactors, such as the absence of flooding and entrainment, or the reduction of feed consumption (160, 161). The flowsheets tested in HFM were similar to those developed for centrifugal contactor tests. Computer codes based on equilibrium (162) and kinetics data, diffusion coefficients (in both phases and in the membrane pores), and a hydrodynamic description of the module, were established to calculate transient and steady-state effluent concentrations. It was demonstrated that, by selecting appropriate flow rates (as mass transfer is mainly controlled by diffusion), very high DFs (DI A 11 = 20,000 and DFrm = 830) could be achieved. Am(III) and Cm(III) back-extraction efficiency was up to 99.87%. 

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. 

The most commonly used hard templates are anodic aluminum oxide (AAO) and track-etched polycarbonate membranes, both of which are porous structured and commercially available. The pore size and thickness of the membranes can be well controlled, which then determine the dimension of the products templated by them. The pores in the AAO films prepared electrochemically from aluminum metals form a regular hexagonal array, with diameters of 200 nm commercially available. Smaller pore diameters down to 5 nm have also been reported (Martin 1995). Without external influences, capillary force is the main driving force for the Ti-precursor species to enter the pores of the templates. When the pore size is very small, electrochemical techniques have been employed to enhance the mass transfer into the nanopores (Limmer et al. 2002). 

A high electroosmotic flow through the stationary-phase particles may be created when the appropriate conditions are provided. This pore flow has important consequences for the chromatographic efficiency that may be obtained in CEC. From plate height theories on (pressure-driven) techniques such as perfusion and membrane chromatography, it is known that perfusive transport may strongly enhance the stationary-phase mass transfer kinetics [30-34], It is emphasised... 

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. 

Figure 7 presents the parameters of the reactor model and the operating conditions simulated. A packed bed of catalyst with a loading of catalyst equivalent to 0.11 gr of Shell 105 catalyst/void cc of reactor is assumed. A membrane 5 microns thick with 20 A pore radii is placed on inner side of a 3/8" I.D. porous support. The pore size of this support is considered to be large enough that it does not exhibit any resistance to mass transfer. A tortuosity... 

---