Condensation coefficient definition

However, a different definition was used for [426]. This agrees well with the fact that the chemisorption of oxygen on metals usually takes place with only small activation energy [423,427]. The temperature dependence also supports this fact. A decrease of the reaction was observed with increasing temperature of the substrate surface. Ritter [293,298] explains this observation with a reduction of the condensation coefficient a of oxygen. 

In condensation problems dealing with very small droplets (with radius far below the mean free path) Gyarmathy s equations converge with the much simpler Hertz-Knudsen model involving the condensation coefficient a. This model is widely used although the condensation coefficient is not known. Mostly it is taken to be unity, however values as low as 0.01 are perpetuated [2]. Some of the uncertainty may be due to considerable confusion in the literature regarding its definition. Here we deal with it because our results render valuable information on its magnitude. 

For the Hertz-Knudsen model and the definition of the condensation coefficient we follow Hill [4]. The bombardment of small droplets by vapor molecules is described by the impingement rate... 

In this equation, refers to the condensation coefficient (c.f.. Chap. 3) and Ml is Avogadro s number. By definition, the surface coverage with oxygen is equal to do = Wo/Wi, where Wq represents the number of sites occupied by the oxygen atoms, and Wt is the total number of adsorption sites. 

It is strictly for convenience that certain conventions have been adopted in the choice of a standard-state fugacity. These conventions, in turn, result from two important considerations (a) the necessity for an unambiguous thermodynamic treatment of noncondensable components in liquid solutions, and (b) the relation between activity coefficients given by the Gibbs-Duhem equation. The first of these considerations leads to a normalization for activity coefficients for nonoondensable components which is different from that used for condensable components, and the second leads to the definition and use of adjusted or pressure-independent activity coefficients. These considerations and their consequences are discussed in the following paragraphs. 

Since the AT across the film is unknown, it is best eliminated from Equation 15.80. By definition of the condensing film coefficient ... 

The aim of this Chapter is the development of an uniform model for predicting diffusion coefficients in gases and condensed phases, including plastic materials. The starting point is a macroscopic system of identical particles (molecules or atoms) in the critical state. At and above the critical temperature, Tc, the system has a single phase which is, by definition, a gas or supercritical fluid. The critical temperature is a measure of the intensity of interactions between the particles of the system and consequently is a function of the mass and structure of a particle. The derivation of equations for self-diffusion coefficients begins with the gaseous state at pressures p below the critical pressure pc. A reference state of a hypothetical gas will be defined, for which the unit value D = 1 m2/s is obtained at p = 1 Pa and a reference temperature, Tr. Only two specific parameters, Tc, and the critical molar volume, VL, of the mono-... 

In this paper, we now report measurements of heat transfer coefficients for three systems at a variety of compositions near their lower consolute points. The first two, n-pentane--CO2 and n-decane--C02 are supercritical. The third is a liquid--liquid mixture, triethylamine (TEA)--H20, at atmospheric pressure. It seems to be quite analogous and exhibits similar behavior. All measurements were made using an electrically heated, horizontal copper cylinder in free convection. An attempt to interpret the results is given based on a scale analysis. This leads us to the conclusion that no attempt at modeling the observed condensation behavior will be possible without taking into account the possibility of interfacial tension-driven flows. However, other factors, which have so far eluded definition, appear to be involved. 

LT is the standard cell potential difference, which is determined only by the reactants in definited standard states. This quantity results as the difference of standard electrode potentials. The power term Ila contains the corrected composition quantities a, (fugacities and activities) with the stoichiometric coefficients v, of the gases and condensed substances taking part in the cell reaction [10,12]. If a sensor at equilibrium delivers signals in agreement with Equation (25-7) then we have a reaction celt. In this case at solid electrolytes with oxide ion vacancies Vo> two reactions can be found besides... 

For condensation of methylene chloride in water, in cocurrent downflow 4-in. and 6-in. diam columns packed with i-in. Intalox saddles, the volumetric transfer coefficients reported (HlOa) were less than half those obtained with the sieve-plate column. The difference may be due partially to the different definition of the temperature driving-force applied for these two columns. (The log-mean AT was used for the packed bed, and a 2-in. transfer height was assumed.) The volumetric heat transfer coefficients increased with the 0.4-0.6 power of the liquid rate from 65,000 to 150,000 Btu/hr/ft /°F with the liquid rate increasing from 1 to 4 x 10" Ib/hr/ft. Contrary to the sieve-plate and spray-column studies, no effects of the vapor flow rate (from 1100 to 2500 Ib/hr/ft ) on the heat-transfer coefficient were noted in the packed bed study. 

However, if mass exchange takes place, this definition is only valid for molecules which, after falling on the surface, are not condensed and are reflected or re-emitted from it. Obviously their accommodation coefficient cannot be determined directly by bulk measurements. 

The discharge capacity of the trap depends on the flow area of the valve orifice, the pressure drop across it, and the inlet temperature of the condensate. There is a considerable problem in measuring the pressure drop because hot condensate flashes as it passes through the valve orifice. Trap capacity is not truly defined by orifice size and pressure differential. Pressures upstream and downstream of the trap are also subject to variation, depending on calandria performance, flowrates, temperatures, and system back pressure. The orifice may never be fully open to flow because of the valve design. Nor can flow coefficients be measured with the same precision as for control valves, since the valve stem in the trap is often not definitely located with reference to the orifice. 

In Section II. 1 the heat capacity is considered from the viewpoint of pheno-menolo cal thermod3mamics. In Section II.l.l the usual definitions are ven for the heat capacity Cy or Cp at constant volume V respectively constant pressure and constant quantity of matter. They are valid for thermodynamically simple systems. As far as liquids and solids are concerned (and polymers are alwaj in a condensed state) Cp is the quantity more available from the experiment and Cy that more available from the theory. In consequence, one is always constrained to convert both quantities. Such a conversion is rendered postible by Eq. (25a) if the thermal expansion coefficient a, the isothermal compressit ty x and the volume V (or the mass densitiy) of the system are known. In default of these data the formula (25b) of Nemst and lindemann is often used which, as approximation, follows from (25a). 

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