Gas Phase Concentrations

Gauges. Because there is no way to measure and/or distinguish molecular vacuum environment except in terms of its use, readings related to gas-phase concentration ate provided by diaphragm, McCleod, thermocouple, Pitani gauges, and hot and cold cathode ionization gauges (manometers). 

Height of an overall transfer unit, gas phase concentrations Hq] Height of an overall transfer unit,... 

The most commonly used remediation technique for the recovery of organic contaminants from ground water has been pump- and-treat, which recovers contaminants dissolved in the aqueous phase. In this regard, the application of carbon adsorption has found extensive, but not exclusive use. Vacuum extraction (also called soil venting) has also become popular for removal of volatile organic contaminants from the unsaturated zone in the gaseous phase. Both of these techniques can, in the initial remediation phase, rapidly recover contaminants at concentrations approximately equal to the solubility limit (pump-and-treat), or the maximum gas phase concentration of the contaminant (vacuum extraction). The... 

The mass spectrometer sampling capillary or the dispersive infra-red analyzers used for continuous analysis and monitoring of the gas phase composition are situated between the reactor and the sampling valve, as close to the reactor as possible, in order to avoid any delay in the recording of changes in the composition of reactants or products. This delay should be taken into account when plotting simultaneously the time dependence of catalyst potential or current and gas phase concentration of the reactants or products. 

Strictly gas-phase CSTRs are rare. Two-phase, gas-liquid CSTRs are common and are treated in Chapter 11. Two-phase, gas-solid CSTRs are fairly common. When the solid is a catalyst, the use of pseudohomogeneous kinetics allows these two-phase systems to be treated as though only the fluid phase were present. All concentration measurements are made in the gas phase, and the rate expression is fitted to the gas-phase concentrations. This section outlines the method for fitting pseudo-homogeneous kinetics using measurements made in a CSTR. A more general treatment is given in Chapter 10. 

All these steps can influence the overall reaction rate. The reactor models of Chapter 9 are used to predict the bulk, gas-phase concentrations of reactants and products at point (r, z) in the reactor. They directly model only Steps 1 and 9, and the effects of Steps 2 through 8 are lumped into the pseudohomoge-neous rate expression, a, b,. ..), where a,b,. .. are the bulk, gas-phase concentrations. The overall reaction mechanism is complex, and the rate expression is necessarily empirical. Heterogeneous catalysis remains an experimental science. The techniques of this chapter are useful to interpret experimental results. Their predictive value is limited. 

This kinetic relationship provides the necessary link between the gas-phase concentration ai and the concentration of A in its adsorbed form, which is denoted as [AS]. The units for surface concentration are moles per unit area of catalyst surface. S denotes a catalytically active site on the surface, also with units of moles per area of catalyst surface. 

Step 6. The products are desorbed to give the gas-phase concentrations pi and qi. The desorption mechanism is written as... 

When the mass transfer resistances are eliminated, the various gas-phase concentrations become equal a/(/, r, z) = j(r, z) = a(r, z). The very small particle size means that heat transfer resistances are minimized so that the catalyst particles are isothermal. The recycle reactor of Figure 4.2 is an excellent means for measuring the intrinsic kinetics of a finely ground catalyst. At high recycle rates, the system behaves as a CSTR. It is sometimes called a gradientless reactor since there are no composition and temperature gradients in the catalyst bed or in a catalyst particle. 

Suppose a gradientless reactor is used to obtain intrinsic rate data for a catalytic reaction. Gas-phase concentrations are measured, and the data are fit to a rate expression using the methods of Chapter 7. The rate expression can be arbitrary ... 

Example 10.1 Consider the heterogeneously catalyzed reaction A —> P. Derive a plausible form for the intrinsic kinetics. The goal is to determine a form for the reaction rate that depends only on gas-phase concentrations. 

Equation (10.12) is the simplest—and most generally useful—model that reflects heterogeneous catalysis. The active sites S are fixed in number, and the gas-phase molecules of component A compete for them. When the gas-phase concentration of component A is low, the k a term in Equation (10.12) is small, and the reaction is first order in a. When a is large, all the active sites are occupied, and the reaction rate reaches a saturation value of kjkd-The constant in the denominator, is formed from ratios of rate constants. This makes it less sensitive to temperature than k, which is a normal rate constant. 

Note that [ A]heterogeneous has uuits of mol/(m s) but remains a function of gas-phase concentrations. The composite term of Chapter 9 and Equation... 

Solution Since = —kuout for a CSTR, the rates in the previous example are just divided by the appropriate exit concentrations to obtain k. The ordinary, gas-phase concentration is used for the pseudohomogeneous rate ... 

The conversion is carried out using the equilibrium relationship between the gas- and liquid-phase concentrations. Usual practice is to assume Henry s law. Thus, the gas-phase concentration that is equivalent to u is Kh ai, where Kh is... 

Equation (11.1) replaces the liquid-phase concentration with an equivalent gas-phase concentration. It is obviously possible to do it the other way, replacing the gas-phase concentration with an equivalent liquid-phase concentration. Then... 

Henry s law constant is dimensionless when Ug and ay are in mol/m, but conventional units for Kyy are atmospheres or torr per mole fraction. Thus, the gas-phase concentration is expressed in terms of its partial pressure and the liquid-phase concentration is expressed as a mole fraction. 

The initial conditions are a = 0.219 and analytical solution is possible but messy, values of kiAj and Vi/V (as in Example 11.3) and on the value of VgjQg. In essence. Example 11.3 assumed VgjQg Q so that the gas-phase concentration quickly responded to the change in inlet concentration. 

In Eq. (1.5) the surface coverage is given by 9c, and 9c is related to parameter X of Eq. (1.7). Equation (1.5) can be rewritten to show explicitly its dependence on gas-phase concentration. Equation (1.17a) gives the result. This expression can be related to practical kinetic expressions by writing it as a power law as is done in Eq. (1.18b). Power-law-type rate expressions present the rate of a reaction as a function of the reaction order. In Eq. (1.17b) the reaction order is m in H2 and —n in CO. 

Note that in this case, the gas phase concentration, Cq, relates to the total mixed gas phase volume Vq, whereas the mass transfer capacity coefficient term is more conveniently related to the liquid volume, Vl-... 

For gas absorption, this problem can often be circumvented by the assumption of a quasi-steady-state condition for the gas phase. In this, the dynamics of the gas phase are effectively neglected and the steady state, rather than the dynamic form of component balance is used to describe the variation in gas phase concentration. 

The diffusion system. Figure 8.31(B), is a useful and simple apparatus for preparing mixtures of volatile and moderately volatile vapors in a gas stream [388]. The method is based on the constant diffusion of a vapor from a tube of accurately known dimensions, producing a gas phase concentration described by equation (8.12). 

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