Nonreactive gas mixture

For simplicity, we shall again employ assumptions 1-5 of Section 1.3, the hypotheses of steady, low-speed flow, and assumptions 1-4 of Section 3.1.2. The other assumptions in Section 1.3 are not necessary here because we are studying a binary, nonreacting gas mixture and are not considering... 

Consider the four-component nonreactive gas mixture of methane, isobutane, carbon disulfide, and chloromethane. The diffusivity of methane and its total mass transfer resistance due to convection and ordinary molecular diffusion are calculated at 25°C and 1 atm. The data listed in Table 21-1 are available. 

TABLE 21-1 Nonreactive Gas Mixture Composition and Pure-Component Lennard-Jones Parameters for the 6-12 Intermolecular Potential... 

TABLE 21-2 Individual Mass Transfer Resistances, including Convection and Diffusion, for Methane in a Four-Component Nonreactive Gas Mixture"... 

Determination of Change in Entropy for Nonreacting Gas Mixture For a species in a mixture we have... 

The volume fractions and mole fractions become identical in ideal gas mixtures at fixed conditions of pressure and temperature. In an isolated, nonreactive system, the molar composition does not vary with temperature. 

The definition of, and the necessary conditions for, the existence of an arheotrope are based on the material balances of a batch distillation process in the presence of a stagnant or flowing sweep gas (Fig. 4.13). Let us first consider a nonreactive liquid mixture. In this case, the component and total mass balances are given by ... 

The equilibrium constant of a gas-phase reaction increases as temperature is increased. When the nonreacting gas neon is admitted to a mixture of reacting gases (holding the temperature and the total pressure fixed and increasing the volume of the reaction vessel), the product yield is observed to decrease. 

The theory describing diffusion in binary gas mixtures at low to moderate pressures has been well developed. Modem versions of the kinetic theory of gases have attempted to account for the forces of attraction and repulsion between molecules. Hirschfelder et al. (1949), using the Lennard-Jones potential to evaluate the influence of intermolecular forces, presented an equation for the diffusion coefficient for gas pairs of nonpolar, nonreacting molecules ... 

Comparison between Rigorous and Approximate Calculations Based on Ordinary Molecular Diffusion in Nonreactive Multicomponent Gas Mixtures... 

The gas mixtures described in this chapter are assumed to be mixtures of nonreacting gaseous substances. 

The study of mixtures of ideal gases is important for two reasons first, these systems serve as a reference system for the study of real mixtures and solutions and second, these systems are special cases of systems in which a chemical reaction occurs. As we shall see in section 2.4, a system of reacting species in an ideal-gas mixture reduces to a nonreacting ideal-gas mixture whenever we block the flow of material from one species to another— e.g., by adding an inhibitor or by removing a catalyst. 

Nonreacting Ideal Gas Mixture Calculation In many cases relevant to fuel cells, we must deal with mixtures rather than pure gases. Thermodynamic properties of mixtures can be easily calculated based on the mole or mass fractions of the constituents. A good example of this is air, which is a nonreacting mixture of mostly nitrogen and oxygen. For these mixtures, we can assume each species in the mixture is occupying the total volume but at a partial pressure in the mixture, where the partial pressure is defined as... 

Example 3.5 Determination of Nonreacting Ideal Gas Mixture Properties Given a mixture of air (21% oxygen and 79% nitrogen by volume) at 2 atm pressure and 350 K, find... 

For a nonreacting mixture, determination of the change in enthalpy (or other thermodynamic properties) is the same as for a single ideal gas species, but follows ideal gas mixture property relations discussed in this section and exemplified in the following example. 

Example 3.6 Change in Properties for a Nonreacting Ideal Gas Mixture Find the... 

Methods for calculating for nonreacting ideal gas mixtures, reacting ideal gas mixtures, liquids, and solids are based on determination of entropy and enthalpy changes in previous sections. Examples of the calculation are given in Section 3.6, where the Gibbs function is used to determine the expected open-circuit voltage (OCV) of a fuel cell. 

Psychrometrics is the study of nonreacting moist air mixtures and is critical to understand the water balance in low-temperature PEFCs. Nonreacting moist mixtures can be evaluated exactly like other nonieacting gas mixtures, but since engineering with moist air mixtures is so common, additional parameters and special charts have been developed to aid in calculation and analysis, hi most fuel cells, water is produced as a product and must be removed from the fuel cell as part of the effluent mixture. In low-temperature PEFCs, the water balance is critical to maintain proper electrolyte conductivity while avoiding electrode flooding. 

Stripping or elution methods, also widely used, are based on the saturation of the melt with the gas. The melt is stripped by an inert gas carrier and the evolved gas mixture is measured and analyzed.In this method it is assumed that the inert gas removes completely the dissolved reactive gas by displacing the equilibrium representing the solubility reaction. However, this method, although applicable to nonreactive systems, does not appear to be reliable with reactive systems. 

The choice of a liquid absorbent depends on the concentrations in the feed gas mixture and on the percent removal desired. If the impurity concentration in the feed gas is high, perhaps ten to fifty percent, we can often dissolve most of the impurity in a nonvolatile, nonreactive liquid. Such a nonreactive liquid is called a physical solvent. If the impurity concentration is lower, around one to ten percent, we will tend to use a liquid capable of fast, reversible chemical reaction with the gas to be removed. Such a reversibly reactive liquid is referred to as a chemical solvent. If the concentration of the gas to be removed is lower still, we may be forced to use an adsorbent that reacts irreversibly, an expensive alternative that may produce solid waste. 

Adiabatic Reaction Temperature (T ). The concept of adiabatic or theoretical reaction temperature (T j) plays an important role in the design of chemical reactors, gas furnaces, and other process equipment to handle highly exothermic reactions such as combustion. T is defined as the final temperature attained by the reaction mixture at the completion of a chemical reaction carried out under adiabatic conditions in a closed system at constant pressure. Theoretically, this is the maximum temperature achieved by the products when stoichiometric quantities of reactants are completely converted into products in an adiabatic reactor. In general, T is a function of the initial temperature (T) of the reactants and their relative amounts as well as the presence of any nonreactive (inert) materials. T is also dependent on the extent of completion of the reaction. In actual experiments, it is very unlikely that the theoretical maximum values of T can be realized, but the calculated results do provide an idealized basis for comparison of the thermal effects resulting from exothermic reactions. Lower feed temperatures (T), presence of inerts and excess reactants, and incomplete conversion tend to reduce the value of T. The term theoretical or adiabatic flame temperature (T,, ) is preferred over T in dealing exclusively with the combustion of fuels. 

The gas phase polymerization of diisobutylene with boron fluoride also did not occur unless a third component was present. The addition of water or acetone caused the mixture to react rapidly, the boron fluoride combining instantaneously with the vapor of the third component in approximately equimolecular quantities. Certain substances, oxygen, hydrogen sulfide, and hydrogen chloride, did not produce a rapid polymerization of diisobutylene these substances did not combine with the boron fluoride. In each of these cases, the final addition of water to the nonreacting mixture resulted in rapid polymerization. Ammonia formed an addition compound with the boron fluoride in approximately equimolecular quantities, but did not bring about the polymerization of the diisobutylene until water vapor was added, after which rapid reaction occurred (Evans and Weinberger, 85). 

We consider a nonreacting mixture such as iso-propanol (1) and water (2), evaporating into ambient air at constant temperature. Assuming the physical equilibrium condition (VLE) at the vapor-liquid interface and applying the Stefan-Maxwell-flux equations to the liquid phase and linear flux equations to the air-diluted gas phase, we derive the following expression for the relative flux XT... 

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