Transport of gaseous species

Consider the transport of gaseous species A from a bulk gas to a bulk liquid, in which it has a measurable solubility, because of a difference of chemical potential of A in the two phases (higher in the gas phase). The difference may be manifested by a difference in concentration of A in the two phases. At any point in the system in which gas and liquid phases are in contact, there is an interface between the phases. The two-film model (Whitman, 1923 Lewis and Whitman, 1924) postulates the existence of a stagnant gas film on one side of the interface and a stagnant liquid film on the other, as depicted in Figure 9.4. The concentration of A in the gas phase is represented by the partial pressure, pA, and that in the liquid phase by cA. Subscript i denotes conditions at the interface and 8g and are the thicknesses of the gas and liquid films, respectively. The interface is real, but the two films are imaginary, and are represented by the dashed lines in Figure 9.4 hence, Sg and 8( are unknown. 

In full-scale Are modeling, a diffusion flame structure is usually assumed. However, in many fire situations, such as underventilated fires, premixed or partially premixed flame theory may be more appropriate. The Burke-Schumann description of the diffusion flames can be used to conveniently represent the transport of gaseous species by a single scalar quantity called mixture fraction. For a simple one-step reaction ... 

The overall model of a P-CVI process is as shown in Figure 5.43. During a P-CVI process the mass transport of gaseous species can be divided into two stages. In the first stage mass transport takes place by forced convection within a very short period of a few hundredths or tenths of a second. In the second stage of the duty cycle the mass transport is dominated by the diffusion from the free space of the reaction chamber into the pores of the fibre preform. The temperature is kept... 

The numerical model for reaction-transport processes in the fractured welded tuffs must account for the different rates of transport in fractures, compared to a much less permeable rock matrix. Transport rates greater than the rate of equilibration via diffusion leads to disequilibrium between waters in fractures and matrix. Because the system is unsaturated, and undergoes boiling, the transport of gaseous species, especially CO, is an important consideration. The model must also capture the differences in initial mineralogy in fractures and matrix and their evolution. 

A kinetic model, based on the FACSIMILE code, has been developed to study the effects of composition change during reactor scenarios. Vaporisation flux and vapour pressure data have been used in a preliminary study of transport of gaseous species from the external surface of the fuel and... 

Concerning the diffusion overpotential, the main relation with the fuel utilization is present at the anode side. The model equations and hypothesis are assumed by literature [21,25,26]. Transport of gaseous species usually occurs by binary diffusion, where the effective binary diffusivity is a function of the fundamental binary diffusivity and the... 

Using as a basis the single-phase model presented in section 3, a multi-phase model has been developed that accounts for both the gas and liquid phase in the same computational domain and thus allows for the implementation of phase change inside the gas diffusion layers. The model includes the transport of liquid water within the porous electrodes as well as the transport of gaseous species, protons, energy, and water dissolved in the ion conducting polymer. 

In liquid electrolyte fuel cells, the reactant gases diffuse through a thin electrolyte film that wets portions of the porous electrode and react electrochemically on their respective electrode surface. If the porous electrode contains an excessive amount of electrolyte, the electrode may "flood" and restrict the transport of gaseous species in the electrolyte phase to the reaction sites. The consequence is a reduction in electrochemical performance of the porous electrode. Thus, a delicate balance must be maintained among the electrode, electrolyte, and gaseous phases in the porous electrode structure. 

The gas diffusion layer (GDL) consists of two regions (1) a void region for gas transport and (2) a solid region for the transport of electrons. Due to the porous nature of the gas diffusion layer, the transport of gaseous species is also governed by Pick s second law of diffusion as follows ... 

The transport of the gas species is not as straightforward and is highly dependent on the computational domain of interest. The flow channels are grooved into the bipolar plate and their design can vary depending on the performance desired. When solving for the mass transport of gaseous species in the flow channels, the momentum and mass should be conserved as follows ... 

The transport in the gas diffusion layer is similar to that described earlier for the CO poisoning case. The transport of gaseous species and electrons should be considered. In the void region, the gases are transported following Fick s second law of diffusion ... 

One of the main goals of the GDL is the transport of gaseous species. From an experimental point of view the determination of the gas permeability (pressure-driven flow) is easier than the determination of the diffusion (driving force concentration difference) of gases. This might be the reason that, in the literature, values for the permeability for GDL material can be found much more often than diffusion coefficients or structural parameters of the materials necessary for the determination of the effective diffusion coefficient. Especially when looking in the specification for GDLs provided by manufacturers, one will find values for gas permeation but no data relevant for gas diffusion. 

Chemical contaminants in the atmosphere can be deposited to surfaces in association with aerosol particles or falling rain and snow. These are advective transport processes, since the chemical moves in association with aerosol particles, raindrops, or snowflakes. This chapter describes methods for estimating chemical fluxes associated with deposition of aerosol particles and precipitation, and provides recommended values for mass transfer coefficients for a range of environmental conditions. In this chapter we do not consider transport of gaseous species in the atmosphere and adjacent surfaces. These convective transport processes, termed dry deposition of gases, are covered in Chapter 2, Section 2.5.6 and Chapter 7, Section 7.3. exchange between air and plants in Chapter 7, air and water in Chapter 9, and air and snow in Chapter 18. 

This device has not reached commercialization, no doubt in part because bulk electrochemical transport of major gaseous components will rarely be economical compared with more standard separation processes. It is in the transport of minority species from low partial pressure to high (e.g. 02 from seawater, C02 from air) where the benefits of the electrochemical driving force, as detailed at the outset of this chapter can best be exploited. Two final examples of contaminant control of great commercial interest demonstrate this principle. 

Mass accommodation coefficients (a) represent the probability of reversible uptake of gaseous species colliding with the condensed surface of interest. For liquid surfaces this process is associated with interfacial (gas-to-liquid) transport and is generally followed by bulk liquid solvation. The term sticking coefficient is often used for mass accommodation on solid surfaces where physisorption or chemisorption takes place. 

Road transport is an important contributor to primary emissions of PM (soot, wear particles and road dust) and also a source of secondary particles formed by condensation of gaseous species (mainly S- and N-compounds and organics) emitted by the tailpipe and partly also by the wear of brakes and tyres. Thus, PM emissions from road traffic are responsible for an important proportion of the exceedances of the PM10 and PM25 Air Quality Limit Values established by the European legislation for the protection of the human health (2008/50/EC [17]). The daily (50 pg m-3) and annual (40 pg m 3 ) limit values for PM10 (atmospheric particles with mean aerodynamic diameter <10 pm) and the annual limit value for PM2.5 (25 pg m-3) (in force from 2015) concentrations in ambient air are indeed exceeded mostly in the urban areas (Fig. 1 [17]). 

The above equations for the diffusion coefficients do not take into account the volume fraction of porosity and the tortuous nature of the path through porous bodies. When the transport occurs through a porous body, as in fuel cell electrodes, effective diffusion coefficients accounting for the interaction of gaseous species with the porous matrix must be employed. Different theoretical approaches for the determination of the effective diffusion have been proposed in the literature. The Bruggemann correction allows the evaluation of these coefficients, through the following expression [47] ... 

An important implication of the data obtained with both a tubular reactor and a bell jar reactor is that the polymer deposition onto a stationary substrate cannot be uniform due to the diffusional transport of polymer-forming species and the path-dependent growth mechanism. The variation of polymer deposition rates at various locations becomes smaller as the system pressure decreases because the diffusional displacement distance of gaseous species increases at lower pressure. It is important to recognize that a certain degree of thickness variation always exists when the plasma polymer is deposited onto a stationary substrate regardless of the type of reactor and the location of the substrate in the reactor. 

Increasing the pressure of the gaseous reactant not only increases the amount present in the gas phase but also increases gas/liquid transport and the solubility of the gas in the liquid phase. This, in turn, facilitates liquid/solid transport of this species. All of these factors increase the availability of the gaseous reagent to the catalyst. Fig. 5.11 shows a typical plot for the relationship between hydrogen pressure and the reaction rate at a fixed catalyst quantity and agitation rate.28 At lower values an increase in pressure promotes an increase in rate but above a given value further increases in pressure have little or no effect on the rate. In the... 

Convection The transport of a species in a liquid or gaseous medium by stindng, mechanical agitation, or temperature gradients, Coordination compounds Species formed between metal ions and electron-pair donating groups the product may be anionic, neutral, or cationic. 

A recent study (12) has shown that Nafion is also suitable for use in water electrolyzers with alkaline solution as the supporting electrolyte. The major charge carrier is the alkali metal ion because of the negligible IT " ion concentration in alkaline solution and the Off ion rejection capability of the cation exchange membrane. The current efficiency of the cell is related to the inhibition of the transport of gaseous products across the separator. Thus, the ionic groups of the membrane are not important, in this case, because alkaline solution is the major electrolyte, and the migration of any ionic species across the membrane would not affect the current efficiency of the cell (33). 

Deposition includes dry and wet processes by which gaseous pollutants and aerosols are removed from the atmosphere [39]. Dry deposition is the transport of gaseous and particulate species from the atmosphere onto underlying surfaces. The intensity of dry deposition is expressed by the vertical flux, F, which represents the amount of pollutants deposited on a unit surface area per unit time. The proportionality constant between flux and concentration in the air of the surface layer, C, has units of length per unit time and is named the deposition velocity, v ... 

After establishing the rates of reaction, the governing equations of the gas, electron, and proton transport can be formulated. Due to the porous structure of the catalyst layer. Pick s second law is usually used to describe the transfer of gaseous species as follows ... 

It is important to consider the mass transport in the flow channels as well as the bipolar plate. The flow channels are used to transport the gaseous species to the gas diffusion layer, while the electrons travel through the bipolar plate toward the gas diffusion layer. Ohm s law governs the transport of electrons in the bipolar plate as follows ... 

The diffusion coefScient may or may not vary in the direction of diffusion. More on transport of a species through a solid material is provided in Section 3.4.2. The solid material considered there is in the form of a thin membrane with or without holes, pores or defects. Difiiision in liquid and gaseous phases through a porous solid material/membrane is considered in Sections 3.1.3.2.3 and 3.I.3.2.4. Evaporative flux of molecules from a free liquid surface under high vacuum is described in Section 3.I.3.2.5. 

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