Microporous solids, mass transport

Many reaction processes of industrial importance occur in microporous solids (catalysis, electrolysis, shale retorting, etc.). Access to the interior of the solid is by diffusional transport and the transport of mass is usually described by Fick s law. The parameter describing the ease of mass transport is the effective diffusivity, De, where... 

A number of methods are used for studying the sorption of basic probe molecules on zeolites to learn more about zeolite acidity. A common disadvantage of all the examinations is that adsorbed basic probe increases the electron density on the solid and, thereby, change the acidic properties of the sites examined. From this aspect it seems advantageous to probe the acid sites with a weak base, e. g., with a hydrocarbon. It was shown that adsorption of alkanes is localized to the strong Brdnsted acid sites of H-zeolites [1, 2]. However, recent results suggest that usually the diffusion in the micropores controls the rate of hydrocarbon transport [3-5]. Obviously, the probe suitable for the batch FR examination of the sites has to be non-reactive and the sorption dynamics must control the rate of mass transport. The present work shows that alkanes can not be used because, due to their weak interaction with the H-zeolites, the diffusion is the slowest step of their transport. In contrast, acetylene was found suitable to probe the zeolitic acid sites. The results are discussed in comparison with those obtained using ammonia as probe. Moreover, it is demonstrated that fundamental information can be obtained about the alkane diffusivity in H-zeolites... 

For membrane processes involving liquids the mass transport mechanisms can be more involved. This is because the nature of liquid mixtures currently separated by membranes is also significantly more complex they include emulsions, suspensions of solid particles, proteins, and microorganisms, and multi-component solutions of polymers, salts, acids or bases. The interactions between the species present in such liquid mixtures and the membrane materials could include not only adsorption phenomena but also electric, electrostatic, polarization, and Donnan effects. When an aqueous solution/suspension phase is treated by a MF or UF process it is generally accepted, for example, that convection and particle sieving phenomena are coupled with one or more of the phenomena noted previously. In nanofiltration processes, which typically utilize microporous membranes, the interactions with the membrane surfaces are more prevalent, and the importance of electrostatic and other effects is more significant. The conventional models utilized until now to describe liquid phase filtration are based on irreversible thermodynamics good reviews about such models have been reported in the technical literature [1.1, 1.3, 1.4]. 

The neutral, microporous films represent a very simple form of a membrane which closely resembles the conventional fiber filter as far as the mode of separation and the mass transport are concerned. These membranes consist of a solid matrix with defined holes or pores which have diameters ranging from less than 2 nm to more than 20 //m. Separation of the various chemical components is achieved strictly by a sieving mechanism with the pore diameters and the particle sizes being the determining parameters. Microporous membranes can be made from various materials, such as ceramics, graphite, metal or metal oxides, and various polymers. Their structure may be symmetric, i.e., the pore diameters do not vary over the membrane cross section, or they can be asymmetrically structured, i.e., the pore diameters increase from one side of the membrane to the other by a factor of 10 to 1,000. The properties and areas of application of various microporous filters are summarized in Table 1.1. 

The diffusion coefficient (or diffiisivity) and viscosity represent transport properties which affect rates of mass transfer. In general, these properties are at least an order of magnitude higher and lower, respectively, compared with liquid solvents. This means that the diffusion of a species through an SCF medium will occur at a faster rate than that obtained in a liquid solvent, which implies that a solid will dissolve more rapidly in an SCF. In addition, an SCF will be more efficient at penetrating a microporous solid structure. However, this does not necessarily mean that mass transfer limitations will always be absent in an SCF process. For example, in the extraction of a solute from a liquid to an SCF phase, the resistance to diffusion in the liquid phase will probably control the overall rate of mass transfer. Stirring will therefore continue to be an important factor in such systems. 

The main focus of this volume is on imderstanding the transport of molecules in microporous solids such as zeolites and carbon molecular sieves, and the kinetics of adsorption/desorption. This subject is of both practical and theoretical interest, since the performance of zeohte-based catalysts and adsorbents is strongly influenced by resistances to mass transfer and intracrystalline diffusion. However, at an even more basic level, the performance of microporous catalysts and adsorbents depends on favorable adsorption equilibria for the relevant species, so a general imderstanding of the fundamentals of adsorption equilibrium is a necessary prerequisite for understanding kinetic behavior. This chapter is intended to provide a concise summary of the general principles of adsorption equiHbriiun and of the main features of sorption kinetics in microporous solids, which generally depend on a combination of both equilibriiun and kinetic properties. 

Studies on mass transport processes of sorbates in bidispersed porous solid materials, exhibiting macro- or mesopores between crystals and micropores inside the crystals, are of considerable importance in obtaining a better understanding of the separation and catalytic processes involved in the systems. Such systems have not, however, been studied in depth. As discussed in Sect. 3.2.5, the FR technique provides a realistic way to investigate the dynamics processes taking place in such biporous systems. 

Diffusivity values are higher in the supercritical phase than in the Hquid phase, so that species wiU diffuse faster through a supercritical fluid (SCF) than through a Hquid, implying faster solubility of solids in SCFs than in more normal liquids, and that SCFs wiU be more efl dent at penetrating through microporous materials thereby increasing the rate of mass transport... 

Here M-g is the mass of the molecules of the component B. The factor can be expected between 0.3 and 1. Things are quite different when the molecules have to stay in the immediate neighborhood of solid pore walls because the micropores or slits are so narrow. In this case the vibration of the solid molecules with the frequency V is the decisive transport mechanism. The following equation is assumed to be a reasonable approach ... 

---