Open pore structure

The family of agarose-based gels, Sepharose, Sepharose CL, and Sepharose Fast Flow, are bead-formed gels prepared from 2, 4, or 6% agarose solutions. The matrix porosity decreases and rigidity of the bead structure increases with increasing agarose concentrations. The open pore structure and broad... 

FIGURE 4-10 The open-pore structure of reticulated vitreous carbon. 

Cancrinites are one of the rarest members of the feldspathoid group, classified as such due to its low silicon content. However, cancrinite is also classified as a zeolite, due to its open pore structure, which confers molecular sieve properties [1], Likewise, variable sodium carbonate and NaOH concentrations in the hydrothermal synthesis of cancrinite could direct the synthesis of the intermediate phase or the disordered cancrinite formation [2], The intermediate phase is described as a phase between cancrinite and sodalite [3], The disordered cancrinite is an intermediate phase which is much closer to the cancrinite structure than sodalite structure [2],... 

A completely different approach was taken by Koresh and Soffer (1980, 1986, 1987). Their preparation procedure involves a polymeric system like polyacrylonitrile (PAN) in a certain configuration (e.g. hollow fiber). The system is then pyrolyzed in an inert atmosphere and a dense membrane is obtained. An oxidation treatment is then necessary to create an open pore structure. Depending on the oxidation treatment typical molecules can be adsorbed and transported through the system. 

From the mercury porosimetry data, porosity can be calculated. A higher porosity means a more open pore structure, thus generally providing a higher permeability of the membrane. Porous inorganic membranes typically show a porosity of 20 to 60% in the separative layer. The porous support layers may have higher porosities. 

The same approach was applied to enhance and extend resid catalyst technology. We gave considerable attention to specific grades of Kaolin to assure a very good pore structure and to keep costs down For a combination of low cost, good open pore structure, low iron content, and low attrition, we preferred the smaller, spherical equivalent Kaolin crystals with a high diameter-to-thickness ratio. 

Due to their defined monomodal macropore distribution (see Section 1.2.1), monolithic stationary phases, based on polymerization of organic precursors, are predestined for efficient and swift separation of macromolecules, like proteins, peptides, or nucleic acids, as their open-pore structure of account for enhanced mass transfer due to convection rather than diffusion. In fact, most of the applications of organic monolith introduced and investigated in literature are directed to analysis of biomolecule chromatography [29]. 

Therefore, we could say that the reticulated scaffold chosen was appropriate based on its open pore structure and physical strength, but as we have seen in other applications, its chemistry is less than ideal. With this conclusion and our earlier work with Trudell that indicated that our chemistry was correct, we developed a process that combined the two aspects into a composite. We will discuss this in detail later in this chapter. 

Table IV. The total pore volume is only a little changed, and the surface area is diminished a good deal. Nevertheless, activity greatly improves because large diameter pores are created in place of the small ones. This undoubtedly makes the gel more fragmentable during polymerization, and the more open pore structure permits an easier escape of the polymer chains from the fragments. Note (Table IV) that most of the pore volume of the active ID hydrogel is in pores with diameters in the range 100-1000 A.

Stiff lightweight structures such as aircraft wings are made from sandwiches of continuous sheets filled with foams or honeycombs. Open porous structures can form frameworks for infiltration by other materials leading to application of biocompatible implants. Open pore structures are used as supports for catalysts. 

Even if this does not reflect the swollen state porosity, it would lead to increased diffusional limitations and a larger specific surface area. The photopolymers probably have a more open pore structure in the swollen state giving the template more rapid access to the sites, which are in this case confined to a smaller surface area. The difference in the conversion of pendant double bonds, and thereby the difference in cross-linking densities between the two types of materials, is probably also a factor that comes into play. An increase in chain flexibility at the sites is likely to cause an increase in the template adsorption-desorption rate coefficients. In this context it is interesting to note that increased rate enhancements were observed upon controlled hydrolysis of the polymer backbone of an imprinted esterase model [73]. 

Type Y zeolite has a much more open pore structure than most other zeolites, but counterdilfusion through its pore structure of small aromatic hydrocarbons can still be quite slow and depends strongly upon the nature of cations present. Counterdilfusion in the type Y zeolite appears to be modeled best as diffusion over periodic high energy barriers which are the pore apertures joining the supercages. The activation energy is... 

At lower temperatures hydrated lime, Ca(OH)2, can be injected into the flue gas stream near the economiser zone (300-650 C). In this temperature range, CaCOj can be formed, which is undesirable because it not only consumes sorbent but pore closure also blocks the access of SO2 to the active sorbent surface. Carbonation significantly increases with reaction temperature, and therefore, the flue gas duct process where the temperature is about 150 C, may be more effective. This process yields S02-removal efficiencies of approx, 80% in actual commercial installations if small particles with an open pore structure are applied. 

The second scale which determines the relation between the selectivity and conversion is the diffusion of the reactants through the catalyst poes. Model calculations conducted by McCarty indicated that at 10 atm die coupling of methyl radicals occurs preferentially inside the pores in a particle of 25 mm in diameter. The effect of this time scale is shown in Figure 10(a) in terms of the intraphase and interphase profiles for methane and ethane inside a catalyst pore. Clearly, higha C2 selectivities are obtained on catalysts with an open pore structure and low surface area. A majority of the literature results have been obtained using powdered catalysts in which diffusional effects are not in rtant however, such effects could be relevant at high pressure in fixed-bed reactors requiring the use of catalysts in a pelletized form. 

A macroporous silica can be synthesized through a slow sedimentation of colloidal particles onto a template. In a fast, single-step reaction (see Figure 8.37), the monomeric alkoxide precursors permeate the array of bulk polystyrene spheres and condense in air at room temperature. Close-packed, open-pore structures with 320-360 nm voids are obtained after calcination of the organic component at high temperatures. 

Fig. 1. Illustration of porous matrix formed via templating. The initial configuration of particles shown in a is equilibrated at a high temperature without a template and then quenched to yield the structure shown in b. The initial configuration in a has the same density of matrix particles (small circles) as in a, but template particles (large circles) are also present in this system. The template particles are removed from the quenched equilibrated matrix + template system (a") to yield the structure shown in b. It is clear to the eye that the structure in b possesses a more open pore structure with more available void volume than the structure in b [55]. (Reproduced with permission from S. Ramalingam. D. Maroudas. and E. S. Aydil. Atomistic simulation study of the interactions of Sill radicals with silicon surfaces. Journal of Applied Physics, 1999 86 2872-28SS. Copyright 1999, American Institute of Physics.)...

Prerequisite for the infiltration is a open porosity. Concentration areas below 10 vol.-% of the lower melting component needs in this case a below 10 % porosity an open pore structure. But this is not realizable because the content of closed pores into the direction of full density increase seriously from this value of porosity on. The area and the function of the concentration gradient are fixed by the processing of the porosity. 

Capillary Model. In substances with a large open-pore structure and in beds of particulate material, the liquid flows from regions of low concentration to those of high concentration by capillary action. Based on this mechanism, the instantaneous drying rate is given by ... 

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