Transfer of Gases

Mass Transfer and Kinetics in Rotary Kilns. The rates of mass transfer of gases and vapors to and from the sohds iu any thermal treatment process are critical to determining how long the waste must be treated. Oxygen must be transferred to the sohds. However, mass transfer occurs iu the context of a number of other processes as well. The complexity of the processes and the parallel nature of steps 2, 3, 4, and 5 of Figure 2, require that the parameters necessary for modeling the system be determined empirically. In this discussion the focus is on rotary kilns. 

Land disposal sites result in soil contamination through leachate migration. The composition of the substances produced depends principally on the type of wastes present and the decomposition in the landfill (aerobic or anaerobic). The adjacent soil can be contaminated by direct horizontal leaching of surface runoff vertical leaching and transfer of gases from decomposition by diffusion and convection. The disposal of... 

The upper and lower respiratory tracts respond differently to the presence of toxicants. The upper respiratory tract is affected mostly by toxicants that are water soluble. These materials either react or dissolve in the mucus to form acids and bases. Toxicants in the lower respiratory tract affect the alveoli by physically blocking the transfer of gases (as with insoluble dusts) or reacting with the wall of the alveoli to produce corrosive or toxic substances. Phosgene gas, for example, reacts with the water on the alveoli wall to produce HC1 and carbon monoxide. 

Low-temperature adsorption is also useful for the quantitative transfer of gases into special apparatus. For example, a small quantity of activated molecular sieve in a freeze-out tip on an infrared cell may be used for the identification of methane or, when placed in a freeze-out tip on a manometer and calibrated volume, the molecular sieves permit the measurement of the quantity of noncon-densable gas. 

As cataluminescence (CTL) is emitted from the excited species produced in the course of the catalytic oxidation of gas, the CTL intensity is limited by the total reaction rate. As we described in the previous section, the total reaction rate is limited by the rate of the slower process between the surface reaction and the transfer of gases from the gas phase to the catalyst surface. The rate of diffusion is characteristics of the gas itself, so that the maximum gas sensitivity is limited by the flow-dynamical condition and not by the activity of... 

Heterogeneous reactions begin with the gas molecule striking the surface of the droplet and entering into tne liquid phase. In this connection the basic parameter determining the transfer of gases into the droplet is the mass accommodation or "sticking" coefficient, which is simply the probability that the molecule which strikes the surface enters into the bulk liquid. 

Knudsen coefficient — The term relates to a particular type of mass transfer of gases through the pores of a specific porous medium. The gas transport characteristics depends on the ratio of the mean free path for the gas molecule, A, to the pore diameter, dpore, which is called the Knudsen number, Kn (Kn = -7 —). 

Because an air packet and the molecules within it move as a unit, the eddy diffusion coefficients for different gaseous species are equal. In fact, Kj is often assumed to be the same for the transfer of gases, heat, and momentum (expressed in the same units), a relation that is referred to as the similarity principle. Therefore Kj is generally measured for the most... 

The placenta develops from a portion of the zygote and thus has the same genetic endowment as the developing fetus (78). The embryonic/fetal component consists of trophoblastic-derived chorionic villi/ which invade the maternal endometrium and are exposed directly to maternal blood in lake-like structures called lacunae. These villi create the large surface area necessary for maternal-fetal transfer in what becomes the intervillous space of the placenta. Here the maternal blood pressure supplies pulsatile blood flow in jetlike streams from the spiral arteries of the endometriunX/ to bathe the chorionic villi and allow for transfer of gaseS/ nutrientS/ and metabolic products. Biologically/ the human placenta is classified as a hemochorial placenta because maternal blood is in direct contact with the fetal chorionic membrane. It is this membrane that determines what is transferred to the fetus. 

The transfer of gases at the base of the euphotic zone is by a combination of advection and diffusion processes and in reality is probably dependent on mechanisms that are intermittent rather than constant in time. Here we write the flux as a simple one-dimensional diffusion process dependent on the concentration gradient at the base of the euphotic zone and a parameter that is assumed to be analogous to molecular diffusion, an eddy diffusion coefficient, K ... 

One of the most important results of including the bubble mechanism in the description of gas exchange is that it explains why gases are supersaturated when there is no net gas flux at the air-water interface. This situation is illustrated by considering a body of water in contact with the atmosphere at constant temperature where there is a sustained, constant brealdng of waves that creates a transfer of gases by bubbles (Fig. 10.11). At steady state there will be no net flux of inert gases across the air-water interface. The flux to the water by bubbles will be exactly balanced by the flux out via diffusive transfer across the surface (Ft = 0, Fawi=EB) and from Eqs. (10.1) and (10.33)... 

The limited data for mass transfer of gases to pure organic liquids show trends similar to those for oxygen in water. Sridhar and Potter [33] followed the transient absorption of oxygen in cyclohexane in a 2-L stirred vessel and reported that kj a varied with Pg/Vf As shown in... 

Fig. 1. Processes that influence transfer of gases across the natural air-water interface. Some of these include wind shear, waves, Langmuir circulation, turbulent mixing, and bubbles injected by breaking waves. In addition, films of organic surface active materials reduce direct gas transfer through the air water interface and through bubble surfaces...

The effect of surface active materials on bubble mass transfer must also be important. There is little doubt that small bubbles become coated with surface active matter and rise and exchange gases at rates below those for bubbles that are clean [81-84]. Johnson and Slauenwhite (unpublished data) found that mass transfer of gases from bubbles in a diatom culture medium occurred at rates substantially below those for bubbles in water that had been treated to remove most organic matter. In some cases, measured rates of dissolution of bubbles rising in the culture medium were less than rates predicted for pure diffusion alone. 

However, the situation changes, if we want to place a porous diaphragm or a permeable membrane instead of the piston in Fig. 6.2. In order to prevent the transfer of gases, no driving force should be there, which implies that the chemical potentials of the cases in both chambers must be equal. Therefore, in this case, in addition... 

MASS TRANSFER OF GASES TO GROWING WATER DROPLETS... 

Mass Transfer of Gases to Growing Water Droplets. 61... 

Permeability (P) is another important property, and measures the rate of transfer of gases and vapors through a layer of polymer (mainly a film). It is expressed as P = SD, where S = solubility and D = diffusivity. The character of the gas, its chemical affinity to the polymer, the structure of the polymer and its degree of crystallinity — all strongly affect the permeability (which drops with an increase of crystallinity). This property plays a major role in the packaging of food. 

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