Macropore with surface resistance

Fig. 6. Concentration profiles through an idealized biporous adsorbent particle showing some of the possible regimes. (1) + (a) rapid mass transfer, equihbrium throughout particle (1) + (b) micropore diffusion control with no significant macropore or external resistance (1) + (c) controlling resistance at the surface of the microparticles (2) + (a) macropore diffusion control with some external resistance and no resistance within the microparticle (2) + (b) all three resistances (micropore, macropore, and film) significant (2) + (c) diffusional resistance within the macroparticle and resistance at the surface of the...

A porous adsorbent in contact with a fluid phase offers at least two and often three distinct resistances to mass transfer external film resistance and intraparticle diffusional resistance. When the pore size distribution has a well-defined bimodal form, the latter may be divided into macropore and micropore diffusional resistances, Depending on the particular system and the conditions, any one of these resistances may be dominant, or the overall rate of mass transfer may be determined by the combined effects of more than one resistance. The magnitude of the intraparlicle diffusional resistances, or any surface resistance to mass transfer, can be conveniently determined by measuring the adsorption or desorption rate, under controlled conditions, in a batch system. 

Using the computer programs discussed above, it is possible to extract from these breakthrough curves the effective local mass transfer coefficients as a function of CO2 concentration within the stable portion of the wave. These mass transfer coefficients are shown in Figure 15, along with the predicted values with and without the inclusion of the surface diffusion model. It is seen that without the surface diffusion model, very little change in the local mass transfer coefficient is predicted, whereas with surface diffusion effects included, a more than six-fold increase in diffusion rates is predicted over the concentrations measured and the predictions correspond very closely to those actually encountered in the breakthrough runs. Further, the experimentally derived results indicate that, for these runs, the assumption that micropore (intracrystalline) resistances are small relative to overall mass transfer resistance is justified, since the effective mass transfer coefficients for the two (1/8" and 1/4" pellets) runs scale approximately to the inverse of the square of the particle diameter, as would be expected when diffusive resistances in the particle macropores predominate. 

The porosity of the beads used is the result of a lot of optimization, and is formed both by macropores with pore diameters exceeding 0.1 pm and by micropores with a pore diameter less than 20 nm. The micropores give the high BET surface area, whereas the macropores assure a high intraparticle mass transfer rate as well as a resistance against deactivation by poisoning. 

The FR measurements have been carried out for several systems using bidispersed structured sorbents [64,76,77]. All the spectra, however, indicate that either micropore diffusion or macropore diffusion, with or without a surface resistance, was the rate-controlUng step for these systems. 

Ammonia, which possesses a large dipole moment, has been used extensively as a probe molecule for the characterisation of both Lewis and Bronsted acidic sites. Figure 22 shows the significant difference in the FR data between ammonia in zeohte crystals and in pellets. The FR spectra of ammonia in zeolite crystals demonstrated that the rate of the ammonia adsorption on different acidic sites in the crystals controls the overall dynamics of the processes occurring in the systems, hi the case of pellets, the rate-controlhng step was found to be macropore diffusion with (Fig. 22a,2,b,2) or without (Fig. 22c,2) surface resistances [77]. 

Important physical properties of catalysts include the particle size and shape, surface area, pore volume, pore size distribution, and strength to resist cmshing and abrasion. Measurements of catalyst physical properties (43) are routine and often automated. Pores with diameters <2.0 nm are called micropores those with diameters between 2.0 and 5.0 nm are called mesopores and those with diameters >5.0 nm are called macropores. Pore volumes and pore size distributions are measured by mercury penetration and by N2 adsorption. Mercury is forced into the pores under pressure entry into a pore is opposed by surface tension. For example, a pressure of about 71 MPa (700 atm) is required to fill a pore with a diameter of 10 nm. The amount of uptake as a function of pressure determines the pore size distribution of the larger pores (44). In complementary experiments, the sizes of the smallest pores (those 1 to 20 nm in diameter) are deterrnined by measurements characterizing desorption of N2 from the catalyst. The basis for the measurement is the capillary condensation that occurs in small pores at pressures less than the vapor pressure of the adsorbed nitrogen. The smaller the diameter of the pore, the greater the lowering of the vapor pressure of the Hquid in it. 

Macroporous resin beads, due to their mode of preparation, consist of a macroporous internal structure and highly cross-linked areas (>5%). The latter impart the resin with rigidity, whereas the porous areas provide a large internal surface for functionalization, even in the dry state. These macroporous polystyrene-based resins are subsequently modified in various manners, which render them compatible with numerous organic solvents. Furthermore, they show high resistance toward osmotic shock, but can be brittle when not manipulated carefully. 

The chiral recognition ability of the insoluble (+)-l was estimated by HPLC using a column packed with small particles of l.25 However, this column showed a poor efficiency because of a low theoretical plate number. This defect was overcome by coating soluble poly(TrMA) with a DP of 50 on macroporous silica gel.26 The 1-coated silica gel had higher resistance against compression and longer lifetime than the CSP of insoluble 1. Moreover, the two 1-based CSPs show quite different chiral recognition for several race-mates, which may be attributed to the different orientation of 1 in bulk and on the surface of the silica gel.27... 

Incidentally, these features cannot be accounted for by assuming different values of macropore radius or tortosity factor in the predictive equations. Even with the assumptions of negligible Knudsen resistance (rpore- ) and no tortuosity (rw = D, the predicted macropore resistances (excluding surface effects) would be lowered by only forty percent, which is still insufficient to account for the low LUB values, at least in the 5A and 13X systems. There appears, therefore, to be a fairly strong case for the presence of a surface diffusion effect in these systems, with the possibility of such an effect in the CO2/ air/4A system as well. 

Although the systems investigated here exhibited predominantly macropore control (at least those with pellet diameters exceeding 1/8" or 0.32 cm), there is no reason to believe that surface diffusion effects would not be exhibited in systems in which micropore (intracrystalline) resistances are important as well. In fact, this apparent surface diffusion effect may be responsible for the differences in zeolitic diffusion coefficients obtained by different methods of analysis (13). However, due to the complex interaction of various factors in the anlaysis of mass transport in zeolitic media, including instabilities due to heat effects, the presence of multimodal pore size distribution in pelleted media, and the uncertainties involved in the measurement of diffusion coefficients in multi-component systems, further research is necessary to effect a resolution of these discrepancies. 

It is believed that, when steaming the gel at high temperatures, the V0+i attacks and breaks the Si-O-Al bonds promoting mullite formation and the collapse of the gel macroporous structure (3). The XRD pattern in Fig. 2B shows that mullite formation in the gel can be observed with only 1.5% V and when this occurs, there is a 81% decrease in surface area, Table 1. Mullite level increased with V-loadings, see Fig. 2. Data in the literature (20) indicates that when the steaming temperature is decreased to 730 C from 760 C (as in the present work) gel stability to V improved and only a 23% reduction in surface area was observed in a similar gel loaded with 1.5% V. Aluminosilicate gels are clearly less resistant than aluminas to V attack at hydrothermal conditions, Table 1. 

Ma et al. [104] attributed a decrease in diffusivity with an increase in initial concentration to pore diffusion effects. Because zeolites are bi-dispersed sorbents, both surface and pore diffusions may dominate different regions. In micropores, surface diffusion may be dominant, while pore diffusion may be dominant in macropores. This, therefore, supports the use of a lumped parameter (De). To explore further the relative importance of external mass transfer vis-a-vis internal diffusion, Biot number (NBl — kf r0/De) was considered. Table 9 summarizes the NBi values for the four initial concentrations. The NBi values are significantly larger than 100 indicating that film diffusion resistance was negligible. 

The zeolite-carbon adsorbents from mineral-carbon adsorbents group are novel and exhibit not quite well recognized properties with their unique, modified porous structure. The characteristic structures for zeolite, active carbon and intermediate structure exist in these materials. Such a structure results fi-om the modification of a surface of a mineral matrix by depositing carbon material. The efifectivity of enrichment of the structure of zeolite-carbon adsorbents (in relation to crystalline zeolite structure) in hydrophobic micropores (0.4 - 2 nm) and macropores (above 50 nm) is proportional to the fi action of carbon phase. Such combination of hydrophilic properties of mineral phase and hydrophobic properties of organic phase results in various sorptive properties of the material and the range of their application can be consequently extended. Additionally, the chemical resistance of these adsorbents for their exploitation in aggressive conditions takes place. 

Monocrystalline, macro- and mesoporous silicon were used for the electrochemical deposition of Pt. A 10 pm thick macroporous silicon layer was formed by anodizing of p-type Si wafers of 12 Ohm-cm resistivity in an aqueous solution of HF acid and DMSO (10 46 by volume parts) at the current density of 8 mA-cm [1]. Pore channels distributed with the surface density of 6T0 cm look like long straight holes with inlet diameters of 1.5 pm. An uniform 1 pm thick mesoporous silicon layer was fabricated by anodizing of n" -type Si wafers of 0.01 Ohm-cm resistivity in a solution of HF acid, water and isopropanol (1 3 1 by volume parts) at the current density of 60 mA-cm . The mesoporous silicon sample formed looks like Si layer perpendicularly pierced through by pore channels with diameter of about 20 nm. The number of pores per square centimetre is up to 2-10 [2]. 

Two-Layer PS. Two-layer PS with a micro PS on top of a macro PS layer is formed on lowly doped p-Si or illuminated n-Si. For lowly doped p-Si, two-layer PS can form when the conditions are such that the space charge layer and the resistive layer differ in dimension by several orders of magnitude and both are significantly involved in the rate-limiting process due to the effect of surface curvature on the current flow near the surface and in the substrate. For n-Si, two-layer PS can form on a front-illuminated substrate as long as the conditions exist for the formation of macropores. It may also form in the dark under conditions similar to those for the formation of two-layer PS on lowly doped p-Si. 

The impurities stick to the periphery of particle pores making the gas flow into the catalyst difficult or impossible. This in turn leads to a considerable increase in the diffusion resistances during the catalytic process. One way of fighting this phenomenon is to use double-porosity alumina. Micropores of about 20 nm are always useful to develop the specific surface area necessary for a good dispersion and stability of the catalytic phase. Macropores over 100 nm in diameter help to diffuse the reagents within the particles. However, the proportion of macropores must not be too great, as that would diminish the mechanical properties of the support correspondingly. For this reason, Rhone Poulenc has, since 1974, developed and marketed various exhaust, gas catalyst supports with specific surface areas of 2... 

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