Pores adsorption

Fluids in narrow pores adsorption, capillary condensation and critical points,... 

For a surface to be detected in adsorption measurements, the adsorbed gas must be able to penetrate the pores. Adsorption methods thus will not detect closed pores. Also, nitrogen at low temperatures may not penetrate pores as small as those which carbon dioxide can enter at room temperature. 

Evans, R., Marconi, M.B., and Tarazona, P. (1986). Fluids in narrow pores adsorption, capiUary condensation, and critical points. J. Chem. Phys., 84(4),... 

Again, all graphs show linear relationships, indicating the possible presence of all four mechanisms to some extent. This could be true as the aggregation is most likely incomplete, leaving some small colloids for contribute to pore adsorption and pore blocking. 

The cut-off, which was between 6 and 8 nm under unfouled conditions, has shifted to >20 nm under fouling conditions. The increase in rejection can be attributed to the larger solute size due to aggregation and pore adsorption which was demonstrated for UF in Chapter 6. This shows a clear increase in membrane rejection as fouling proceeds. 

Blocking law analysis in UF indicated internal pore adsorption to be the principal mechanism by which natural organic aggregates were retained and resulted in a pore size reduction. When ferric chloride is added cake filtration becomes important. 

A number of key issues are highlighted through direct comparison of the three processes. First, a cutoff for natural organics rejection at a pore diameter of clean membranes of about 6 to 8 nm is observed. Below this pore diameter rejection is <20%, while above it is mostly >80%. All twelve membranes lie on this cut-off curve. When the membranes are fouled this cut-off is shifted to >20 nm and the cut-off itself is not as distinct. This can be attributed to a pore size reduction due to internal pore adsorption, the formation of self-rejecting cake layers, and solute-solute interactions in the boundary lay er. If ferric chloride is added this cut-off ceases to exist. With chemical addition, rejection... 

Activated carbon, wi de-pore Adsorption and recovery of high-boiling hydrocarbons 300 - 400 0.3 -0.5 0.5-1.1 1000-1500 850... 

Pore adsorption (c oiute < pore) Colloids or solutes adsorb on the niembrane walls, effective pore size is restricted and flux declines... 

This reaction consists of several steps starting with the diffusion of CH3 I from the gas flow to the surface of the charcoal grains and from there into the pores, adsorption of CH3 I and isotopic exchange with the KI impregnant, followed by desorption of CH3 I and diffusion of this compound out of the pores and away from the grain surface. Due to the complexity of this process, which generally is an irreversible reaction of (pseudo)-first order, its progress may be influenced by several parameters. 

The r-plot is involved in most of the methods for the reason that on a relatively flat surface in the absence of pores, adsorption occurs and the adsorbed film becomes several molecular layers thick before the vapor pressure reaches p/p l.O for the bulk liquid. Obviously, in the multilayer film the properties of the nitrogen are not the same as in the bulk. As already pointed out, determination of pore size requires not only the Kelvin equation to calculate the size of pores that fill with bulk liquid- nitrogen but also the thickness of the adsorbed film on the inner surface of pores that are not filled. 

Kobayashi, S, et al., Adsorption behavior of chlorofluorocarbons in zeolitic pores Adsorption isotherm, Ind. 

Theoretical treatments discuss adsorption in terms of 1) formation of a monolayer of adsorbate on the surface, 2) formation of multilayers on plane surfaces, and 3 capillary condensation in small pores. Adsorption of a gas is considered the result of van der Waal s forces of the same type involved in its condensation. The magnitude of the interactions resulting in adsorption is indicated by the fact that heats of adsorption are generally comparable to the heat of condensation for a given gas. Experimental isotherms n ay conform over wide ranges to theoretically derived equations (as the hyperbolic isotherm) for suitable values of the constants in the equation. However, values of the constants thus obtained are often not easily related to the physical models used in the derivations. For discussion of adsorption in general and... 

Below a critical size, pores do not undergo capillary condensation, but fill continuously as the pressure is increased without a discontinuity in the single pore adsorption isotherm [8]. 

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