Straight-pore membrane

The microstnictuie of a porous membrane can vary according to the schematic in Figure 1.2. The shape of the pores is strongly dictated by the method of preparation which will be reviewed in Chapter 3. Those membranes that show essentially straight pores across the membrane thickness are referred to as straight pore or nearly straight pore membranes. The majority of porous membranes, however, have interconnected pores with tortuous paths and are called tortuous pore membranes. 

Figure 2.1. Schematic representation of main types of pore structures and membranes. A and B homogeneous unsupported straight pores C supported asymmetric, interconnected pores D a photograph of a membrane of the type (c). (SCT-support+y-AljOj top layer UT Twente)...

Pores with a very regular, linear shape can be produced by the track-etch method (Quinn et al. 1972). Here a thin layer of a material is bombarded with highly energetic particles from a radioactive source. The track left behind in the material is much more sensitive to an etchant in the direction of the track axis than perpendicular to it. So etching the material results in straight pores of uniform shape and size with pore diameters ranging between 6 nm and 1200 nm. To avoid overlap of pores only 2-5% of the surface can be occupied by the pores. This process has been applied on polymers (e.g. Nuclepore membranes) and on some inorganic systems like mica. Membranes so obtained are attractive as model systems for fundamental studies. 

The subscript 0 indicates that this flux is for solvent A without the presence of rejected solute B. This model is appropriate for membranes that have straight pores however, such membranes are not typical of most industrial membranes. Not only are the rate limiting pores in the actual working skin layer tortuous, but a complex porous support layer is generally present that supports the working skin layer. In well-made sieving membranes, the porous support is ideally invisible to the transport process, as is illustrated in Fig. 2. The support, therefore, simply serves as a scaffold for the ideal selective layer. Formation of such structures requires some care, but technology exists to achieve this requirement as is described elsewhere (Koros and Pinnau, 1994). 

Two well known processes have been used to prepare inorganic membranes with nearly straight pores anodic oxidation and track etching. The former method has been practiced commercially while the latter remains as a laboratory investigation. 

Other metals such as zirconium, in principle, can also be synthesized in the same way to form oxide membranes with essentially straight pores. 

Other methods have also been adopted to produce inorganic membranes with essentially straight pores. Witte [1988] subjected a metal foil such as nickel to a two-step photolithographic procedure. The pore density of the final membrane exceeds 150,(X)0/cm and the pore sizes form fall under the microfiltration and ultrafiltration ranges. Although both flat and cylindrical shapes can be handled, the flat shape is preferred. 

Opposiny-reactants mode. When immobilized with a catalyst or enzyme, the interconnected tortuous pores or the nearly straight pores of a symmetric inorganic membrane provides a relatively well controlled catalytic zone or path for the reactants in comparison with the pellets or beads in a fixed or fluidized bed of catalyst particles. This unique characteristic of a symmetric membrane, in principle, allows a novel reactor to be realized provided the reaction is sufficiently fast. The concept applies to both equilibrium and irreversible reactions and does not utilize the membrane as a separator. Consider a reaction involving two reactants, A and B ... 

Using nearly straight-pore alumina membrane plates as both a separator and a catalyst, Fumeaux et al. [1987] demonstrated the dehydration of isopropanol to form propene and hydrogenolysis of ethane to make methane. No quantitative information, however, was provided for the conversion or yield of the associated reactions. 

It is interesting to note that, due to its uniform and nearly straight pore structure, planar anodic alumina membranes have been used as the probes for monitoring catalyst deactivation at the single pore level [Nourbaksh et al, 1989a]. The nearly idealized structure is suitable for various surface analysis techniques such as scanning and transmission election microscopy and associated EDX and XPS to be applied to fresh as well as spent catalysts. In the case of hydrotreating of crude-derived heavy oils, catalyst... 

All current MF membranes may be classified as either "tortuous-pore" or "capillary-pore" membranes (see Figure 2.1). The "capillary-pore" structure is distinguished by its straight-through cylindrical capillaries, whereas the "tortu-ous-pore" structure resembles a sponge with a network of interconnecting tortuous pores. 

In a similar approach, nanoporous membranes have also been prepared without the use of a template [75]. These membranes were also built from a mixture of two LCs, an H-bonded dimer and a covalent linker. The membranes were created by photopolymerization to lock the smectic structure into a network followed by an alkaline treatment to create the pores (Fig. 2.17a) [76]. The H-bonds were formed between two benzoic acid monomers and after deprotonating of the acid moieties the H-bonds break and a carboxylate pore interior was created. The smectic nature of the material resulted in straight pores with a 2D geometry and transmission electron microscopy (TEM) (Fig. 2.17e) revealed a periodicity of approximately 3 nm and pores around 1 nm. 

The second hydrodynamic model is the Hagen-PoiseuiUe model. It is used to macroporous membranes with cylindrical straight pores of same diameter ... 

For straight pores, the tortuosity weighting parameter is unity, and equivalent area fraction of pores (<) in the membranes with a let equal to ilw mentbrane thickness. FOr tortuous pores, it is not generally possible to sepatme decreases in porosity ( ) from increases in tortuosity (q), so that die composite parameter given in this table is reported. 

Ito Y, Kotera S, Ibana M, Kono K and Imanishi Y, Control of pore size of polycarbonate membrane with straight pores by poly(acrylic acid) . Polymer, 1990, 31, 2157. 

DP (A) has two different pore mouths one a large (pore a) and the other a small mouth (pore b). ZP (B) has zigzag shaped pores whose sizes (diameters) are all the same at the pore entry. SP (C) has straight pores which can be called slit-shaped pores. The minimum pore width (W ) is set equal to 0.5 nm for all three types of pores. The membrane thickness, given in Fig. 8.12 asXZ was set equal to 6.3,4.6 and 5.2 nm for DP, ZP and SP membranes. These values are based on the arrangement of micro-graphite crystallites (shown as a nodule in the figiue) whose unit size was fixed at 2.7 x 3.4 x 1.3 nm for the DP and ZP membranes. 

Efforts to overcome the limitations of the fragile membranes (as delicate as soap bubbles) have evolved with the use of membrane supports, such as polycarbonate filters (straight-through pores) [543] or other more porous microfilters (sponge-like pore structure) [545-548]. 

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