Mass transport, gas-phase

Similarly to mass diffusion in electrolytes, gas-phase mass diffusion is driven by concentration gradients. If a spray of perfume is released at the front of a classroom, the students 

The other species present. The other molecules will collide with the diffusing species and affect the net rate of motion. 

The number of molecules, which corresponds to pressure. The greater the number of molecules, the greater the number of collisions, which reduces the average diffusion rate. 

The size and mass of the molecules (e.g., molecular collision diameter and molecular weight). 

We shall see that the diffusion coefficient for bulk diffusion is indeed a function of pressure, temperature, molecular size, and weight. Diffusion is a spontaneous process that is a result of the second law of thermodynamics. The second law of thermodynamics requires that thermodynamic processes proceed in a way that maximizes entropy. This ultimately requires uniform mixing of everything in the universe. When there is nonuniform mixing, diffusion occurs to eliminate concentration gradients and can be written as 

---

To make the model less empirical, gas-phase mass-transport limitations can be incorporated into the modeling equation explicitly ... 

Relative importance of interfacial mass transport limitation to gas-phase mass transport limitation 4 Dg/aau [Pg.164]

A comprehensive analysis of solid oxide fuel cells phenomena requires an effective multidisciplinary approach. Chemical reactions, electrical conduction, ionic conduction, gas phase mass transport, and heat transfer take place simultaneously and are tightly coupled. 

To be able to predict the release of SVOCs from a material to the indoor environment it is important to understand the fundamental mechanisms in order to mathematically model the emissions. The emission behavior of DEHP from PVC in the FLEC and CLIMPAQ experiments (Clausen et al., 2004) have now been successfully modeled (Xu and Little, 2006). Fluid building materials such as paints (Clausen, 1993 Xu and Little, 2006) and wood oil (Clausen, 1997) may also emit SVOCs and are usually used on large indoor surfaces such as walls, ceilings and floors. Such wet materials may be applied on substrates like wood or plaster board. The emission of for example, Texanol from water-based paint was found likely to be limited by gas phase mass transport (Clausen, 1993) similar to the DEHP emission from PVC (Clausen et al., 2004). 

This criterion for gas-phase diffusion limitation is illustrated in Figure 12.7 for a series of droplet diameters and an arbitrarily chosen sg = 0.1. In Figure 12.7 the inequality (12.85) corresponds to the area below and to the left of the lines. For a given situation if the point (k, H K) is to the left of the corresponding line in Figure 12.7, then gas-phase mass transport limitation does not exceed 10%. Similar plots, introduced by Schwartz (1984), provide an easy way to ascertain whether there is a mass transport limitation for a given condition of interest. 

Despite the fact that for typical cloud droplets gas-phase mass transport is in the continuum regime, mass transport across the air-water interface is, ultimately, a process involving individual molecules. Therefore the kinetic theory of gases sets an upper limit to the flux of a gas to the air-water interface. This rate is given by (12.25) and depends on the value of the accommodation coefficient. 

Gas-Phase limitation The problem of coupled gas-phase mass transport and aqueous-phase chemistry was solved in Section 12.3.1 resulting in (12.82). Solving for the aqueous-phase reaction term / aq and noting that in this case Henry s law will be satisfied at the interface (pA(Rp) = Caq/// ) ... 

This equation indicates that when the aqueous-phase reaction rate is limited by gas-phase mass transport, at steady-state the aqueous-phase reaction rate is only as fast as the mass transport rate. 

We see that there is a critical value of the Henry s law constant, HAfint = kG/kL, for which the gas- and liquid-phase resistances are equal. For HA <, HA crit (i.e., a slightly soluble gas), liquid-phase mass transport is controlling. If HA HA ml (i.e, a very soluble gas), gas-phase mass transport controls the deposition process, and the transport is independent of the value of HA. The two limiting cases are... 

Figure 4.1. Heterogeneous biological processes utilizing a biological film (a biodisk reactor). A group of disks is fastened to a horizontal shaft. Each disk supports the growth of the desired culture. The disks are turned through the liquid and gas phases. Mass transport is subject to the same limitations as given in Fig. 2.3 for pseudo-homogeneous processes. With the biodisk, however, step 4 (limitation in the solid phase) becomes significant. (From Moser, 1981.)...

The adsorption of gaseous iodine species on surfaces has received much attention in the reactor safety literature. Varieties of metal, concrete and painted surfaces are in a reactor containment building. The gaseous forms of iodine will interact with these surfaces. The interactions are usually thought to involve a rapid physical adsorption followed by a slower chemical reaction. Dry (or moist, but not wet) surfaces of both stainless steel and organic (vinyl-, epoxy-, polyurethane-) paint have large capacities for absorbing molecular iodine. The adsorption rate of molecular iodine on the dry surfaces is close to the limit dictated by gas-phase mass transport. Intermediate-scale studies performed in the Radioiodine Test Facility have shown that about 75 to 95% of airborne molecular... 

There have been some efforts to develop paint with a high adsorption capacity for iodine. However, such paint may not help much in terms of the overall iodine sorption during an accident. The overall adsorption rate on dry surfaces under accident conditions is limited by the gas phase mass transport. Furthermore, because water films may cover surfaces, there appears to be limited scope for using surface reactions to immobilise iodine and reduce the gas phase concentrations. 

The kinetics of chemical interactions between fission product vapor and aerosols is mainly controlled by gas phase mass transport, by the kinetics of the chemical reaction, and by mass transport in the condensed phase. Another factor potentially influencing the kinetics of vapor deposition is that the heat liberated by condensation or by chemical reaction of vapor with aerosol must be disposed of. Because of their small masses, aerosol particles have only limited capacity for conducting away this heat, compared with the structures within the reactor coolant system. This problem may arise particularly in the deposition of water vapor onto aerosol particles which have been previously covered by hygroscopic or water-soluble compounds such as CsOH. 

The oxygen transport is based on several mechanisms. The gaseous reactants enter the fuel cell typically through the gas channels, and then diffuse through the GDL and into the catalyst layer. The gases diffuse into the catalyst layer either to the catalyst surface where they react or are dissolved into the ionomer in the catalyst layer. The gas phase mass transport in the GDL is due to diffusion governed by Pick s law, in which the rate of diffusion is a function of the concentration gradient, the thickness of the GDL, and the diffusion coefficient for the species [21]. 

Recall that the mass balance equations of Eqs. (1.1a) and (1.1b) incorporate not only terms for internal chemical reactions but also terms for physical mass transport across the boundaries of the control volume. Often, useful control volume boundaries coincide with boundaries between phases, such as between air and water or between water and solid bottom sediment, as discussed for the lake control volume in Section 1.3.1. Note, however, that the terms "environmental media" and "phases" are not interchangeable. For example, chemicals in the gas phase can refer to chemicals present in gaseous form in the atmosphere or in air bubbles in surface waters or in air-filled spaces in the subsurface environment. Chemicals in the aqueous phase are chemicals dissolved in water. Chemicals in the solid phase include chemicals sorbed to solid particles suspended in air or water, chemicals sorbed to soil grains, and solid chemicals themselves. In addition, an immiscible liquid (i.e., a liquid such as oil or gasoline that does not mix freely with water) can occur as its own nonaqueous phase liquid (NAPL, pronounced "napple"). Some examples of mass transport between phases are the dissolution of oxygen from the air into a river (gas phase to aqueous phase), evaporation of solvent from an open can of paint (nonaqueous liquid phase to gas phase), and the release of gases from new synthetic carpet (solid phase to gas phase). Mass transport between phases is affected both by physics and by the properties of the chemical involved. Thus, it is important to imderstand both the types of chemical reactions that are common in the environment, and the relative affinities that various chemicals have for gas, liquid, and solid phases. 

Gas-Phase Mass Transport 213 Table 5.6 Some Representative Solid-Phase Diffusion Coefficients... 

---