Gas diffusion process

In Equation 24, t is the tortuosity, a term well established for gas diffusion into porous materials (10). It is unfortunate, but necessary to introduce t into our model. The value of t cannot be obtained a-priori cuid must be obtained experimentally since it is an almost impossible task to describe the complicated pore geometry in a gel. Given idealised perfect pore geometry it has been possible to estimate t for gas diffusion processes. In our work, of necessity x becomes an adjustable pareuneter to help achieve better agreement between the model predictions eind experimental results. Since x is unmeasurable our only concern has been to use reasonable values in our simulations. 

Quantitative study of gas diffusion process from a single bubble under the action of capillary pressure is a more complex task. Bubble sizes at which diffusion rate would be sufficiently high and when it is possible to observe the decrease in their size, are very small (R < 0.1 cm). The only reliable method for determination of the thickness of films of such bubbles is the optical one. However, it should be kept in mind that the film thickness of... 

The year 1980 marked the entry of a new type of commercial ceramic membrane into the separation market. SPEC in France introduced a zirconia membrane on a porous carbon support called Carbosep. This was followed in 1984 by the introduction of alumina membranes on alumina supports, Membralox by Ceraver in France and Ceraflo by Norton in the U.S. With the advent of commercialization of these ceramic membranes in the eighties, the general interest level in inorganic membranes has been aroused to a historical high. Several companies involved in the gas diffusion processes were responsible for this upsurge of interest and applications. 

For gas transport in small pores (say, less than the 10 nm range) the sizes of which are no longer much larger than those of the gas molecules, the contribution of viscous flow can be neglected and other considerations need to be factored in the model. First, the gas molecules are considered to be hard sphere with a finite size and the gas diffusion process is assumed to proceed in the membrane pores by random walk. The membrane pores are assumed to consist of smooth-wall circular capillaries. In addition to gas molecules colliding with the membrane pore walls, adsorption on the pore wall and the associated surface flow or diffusion are considered. Adsorption also effectively reduces the pore size for diffusion. 

The membranes used for analytical pervaporation are hydrophobic membranes of the types usually employed in ultrafiltration and gas-diffusion processes. In practice, PTFE is the most frequently used membrane material, followed by hydrophobic polyvinylidene-fluoride (PVDF). Ultrafiltration membranes are very thin, which, in combination with the large surface area of both the donor and acceptor chamber, leads to their easy bending. This results in changes in the ffux of the permeating component through an altered membrane area and hence in changes in the efficiency of the process. As a result, membranes must be replaced fairly often. Because of their thickness, gas-diffusion membranes are not so easily bent, so the same membrane can be used over long periods. The pore size of the... 

The major advantage of the gas diffusion process is that it removes the analyte of interest from the sample matrix. The analyte is transferred to a new matrix, the acceptor stream, which contains no chemical or physical interferents. An additional advantage is that the acceptor stream can be configured so that the optimized conditions for the detection process are realized. For example, optimized reaction conditions for the formation of the analyte-reagent complex can be maintained in the acceptor stream. 

In water. The two principle controlling factors for gas diffusion processes in liquids are the gas mass and its activation energy for diffusion. The diffusion activation energy in turn is controlled by the extent of interaction of the gas molecule or atom with the liquid phase. For noble gases, because they are monatomic and have a stable electron shell, there is little interaction with water and the rate of diffusion is almost entirely controlled by their respective masses. This is in contrast with species such as CO2 and CH4 where interaction occurs with water molecules through induced dipole-dipole moments. Because this is in addition to mass, these species diffuse significantly more slowly in water than noble gases of similar mass (Table 5, Fig. 11). 

Natural uranium is mostly nonfissionable it contains only about 0.7% of fissionable For uranium to be useful as a nuclear fuel, the relative amount of must be increased to about 3%. This is accomplished through a gas diffusion process. In the diffusion process, natural uranium reacts with fluorine to form a mixture of UFg(g) and UFg(g). The fluoride mixture is then enriched through a multistage diffusion process to produce a 3% nuclear fuel. The diffusion process utilizes Graham s law of effusion (see Chapter 5, Section 5.7). Explain how Graham s law of effusion allows natural uranium to be enriched by the gaseous diffusion process. 

The most frequently used detector in FI systems with gas-diffusion separation is the spectrophotometer. Quite often the gas-diffusion process offers sufficient selectivity to allow relatively non-specific chemical reactions in the acceptor stream to detect the analyte. Thus, carbon dioxide, sulfur dioxide, hydrogen sulfide, ammonia may all be determined using suitable acid-base indicators in appropriate buffer solutions used as the acceptor streams. The concentration of the buffer solutions may be adjusted to suit a certain concentration range for the analyte. In order to further enhance the selectivity and/or sensitivity more specific reagents may be introduced in the acceptor streams. In the previously mentioned example on the determination of cyanide [20] a modified pyrazolone-isonicotinic acid reaction was used for such purposes. Interferences due to Schlieren effects seem not to have been reported in gas diffusion spectrophotometric systems. This is understandable, since the matrix composition of acceptor streams is usually quite uniform, and the refractive index is little affected after absorbing the gaseous analytes. 

The combination of FI gas-diffusion separation with chemiluminescence has produced selective methods for the determination of chlorine and chlorinated species. Hollowell ct al.[33] determined chlorine dioxide by a chemiluminescent reaction with luminol, following a gas-diffusion separation. A T-spiral flow cell was mounted directly in front of the photomultiplier to maximize the detection of the light emission. Potential interferences from transition metals were removed by the gas-diffusion process, since they do not pass... 

The area specific resistances for the composite LSM-YSZ symmetrical cell, which was obtained from impedance spectroscopic measurements, are shown in Figure 3-24. The and were attributed to electrode overpotentials (the charge transfer and the gas diffusion process), while the... 

The understanding and exploiting all of these features requires further development of this technology. For example, the microstructure of the composite electrodes can be optimized or the composition of composite electrodes can be carefully selected in order to decrease the electrode overpotential resulted from the gas diffusion process or from the activation of electrochemical reactions, respectively. 

There are three procedures that are used for the enrichment of U (Choppin and Rydberg 1980 Pickert and Zech 1981) (1) gas diffusion process, (2) gas centrifugation process, and (3) nozzle process. All three processes use the fact that UFg with the isotope U is lighter by three mass units than the compound with U. 

In the gas diffusion process, UFs in gaseous form is pumped along some microporous diaphragms, and the fact that UFs diffuses more rapidly through the diaphragm than UFg is used. 

For instance, quantification of ultratrace levels of sulfide is feasible with inexpensive flow analyzers relying on the catalytic action of the target analyte on the hydrogen peroxide-luminol chemiluminescent (CL) reaction. Furthermore, in combination with online gas diffusion processes, it is possible to suppress matrix effects, e.g. metals, ions, oxidants, and organic compounds, and improve selectivity as a result. The MSFIA-CL method depicted in Figure 7.9 features a linear working range... 

A hybrid SIA-FIA spectrophotometric method for the determination of total sulfite in white and red wines has been reported (Tzanavaras et al., 2009). The assay was based on the reaction of sulfite with o-phthalaldehyde (OPA) and ammonium chloride. Upon online alkalization with NaOH, a blue product was formed having an absorption maximum at 630 nm. Sulfite was separated from the wine matrix through an online gas-diffusion process incorporated in the SI manifold, followed by reaction with OPA in the presence of ammonia. The reaction mixture merged online with a continuously flowing of NaOH prior to detection. The SIA-FIA manifold is illustrated in Figure 2.8. 

By contrast porous ceramic membranes had found application since the 1960s in the field of large-scale gas diffusion processes for uranium isotope separation. It was only in the 1980s that porous ceramic membranes found other non-nuclear industrial applications, mainly oriented towards microfiltration and ultrafiltration water treatment processes. 

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