Reference temperature, condensation

Dephlegmation, or partial condensation, refers to the process in which a vapor stream is cooled to a desired temperature such that a portion of the less volatile components of the stream is removed from the vapor by condensation. 

What is unique about metal particles burning in oxygen is that the flame temperature developed is a specific known value—the vaporization-dissociation or volatilization temperature of the metal oxide product. This temperature could be referred to as a boiling point. This interesting observation is attributable to the physical fact that the heat of vaporization-dissociation or decomposition of the metal oxide formed is greater than the heat available to raise the condensed state of the oxide above its boiling point. That is, if <2r is the heat of reaction of the metal at the reference temperature 298 K and (H° - H gi) is the... 

Heat-transfer coefficients in this book have the units of Btu/[(h)(ft2)(°F)], where the ft2 term refers to the surface area of the surface condenser. The °F term refers to the condensing steam temperature, minus the average tube-side cooling-water temperature. 

First, the use of the condensing solvent vapors as a reference temperature can lead to considerable uncertainty, particularly with non polymeric solutes, because changes in the reference temperature, which... 

Condensation refers to the formation of a liquid or solid directly from a gas. (Sometimes, forming a solid directly from a gas is called deposition.) The opposite processes are evaporation or sublimation, respectively. During all of these processes, pressure and temperature cannot be varied separately. If one is fixed, so is the other. It is common experience that water boiling at 1.0 atm pressure remains at 100°C as heat is added and its volume increases, until there is no more liquid remaining in the system. During the process of vaporization, liquid and gaseous water simultaneously exist in the system. The 1.0-atm isobar of water is shown in Fig. 7. 

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-... 

Example 4.25 Column Exergy efficiency Propylene-propane mixture is a close boiling mixture. A reflux ratio of 15.9 (close to minimum) and 200 equilibrium stages are necessary. Table 4.13 shows the enthalpy and entropies of the saturated feed and saturated products from the simulation results with the Redlich-Soave equation of state. The reboiler and condenser duties are 8274.72 and 8280.82 kW, respectively. The reference temperature is 294 K. The lost work ZTFis obtained from Eq. (4.198) as... 

Controlling temperature and humidity of process air or ambient air is another unique application of membrane contactors. Membranes are used to humidify or dehumidify air by bringing air in contact with water or a hygroscopic liquid. Mass transfer in such processes is very fast since mass transfer resistance in the liquid phase is negligible. Heat transfer and mass transfer are directly related to these processes, since latent heat of evaporation (or condensation) creates temperature profiles inside the contactor. Some of the references in Literature are shown in Refs. [78-79]. Application of such processes has been proposed for conditioning air in aircraft cabins [80], in buildings or vehicles [81], or in containers to store perishable goods [82]. 

Rohsenow [13] showed that if the condensate temperature profile was allowed to be nonlinear to account for convection effects in the condensate film, an improved correction term, i tg = itg + 0.68c,f( Ts - TJ) results. Another correction pertains to the variation of viscosity with temperature. For the assumed linear temperature profile in the condensate, Drew [14] showed that if l/pf is linear in temperature, then the condensate viscosity should be calculated at a reference temperature equal to T, - Yt(T, - Tw). 

Shang and Adamek [15] recently studied laminar film condensation of saturated steam on a vertical flat plate using variable thermophysical properties and found that the Nusselt theory with the Drew [14] reference temperature cited above produces a heat transfer coefficient that is as much as 5.1 percent lower than their more correct model predicts (i.e., the Nusselt theory is conservative). 

In many practical appliances, which range from steam engines to internal combustion engines, the carbon dioxide and water products are released to the atmosphere at comparatively high temperatures and the heat that these gases hold above the reference temperature of 298.15 K is lost. This heat consists of both the sensible heat i.e., the heat carried by the substance - its heat capacity ) and the latent heat of condensation of the steam. For practical engineering purposes, it has become customary, therefore, to define a lower heating value (LHV), which corresponds, arbitrarily, to the maximum heat recoverable when the reaction products are emitted at 423.15 K. 

To quote from Schreckenback, Wolff, and Ziegler[67] chemical shifts are known to be sensitive to everything . Their list of factors include the following (1) relativity, (2) quantum mechanical approximation, (3) gauge problem, (4) basis set, (5) geometries, (6) reference compound, (7) condensed phase, temperature, and pressure. Relativistic effects, which are particularly important for heavy nuclei or light nucleus attached to a heavy one, will be discussed later. 

The 673 K temperature was selected as the reference point for calculating the 1° KDIE at standard temperature because it is the maximum condensed phase temperature experimentally measured during combustion of a nitramine propellant formulation (see ref 24). 

Temperature difference to the reference temperature (0 °C), K hmiv- Higher heating value of hydrogen at reference temperature, J kg hfg Enthalpy of condensation for water at reference temperature, J kg A,- Stoichiometric factor for component i, —... 

Here the enthalpy of the substance is referred to that of a reference standard for that material chosen at any convenient reference temperature T and pressure Pi. This formulation is particularly useful for condensed phases. In the approximation of Dulong and Petit, the reference value for atomic solids at sufficiently high temperatures is approximated by 3nR. The partial derivative for volume in the integrand may be replaced by aV, where a is the isobaric coefficient of thermal expansion, which is fairly insensitive to pressure changes. For such materials, we cast the above in the form... 

First of all, it should be noted that the commonly used term heat of combustion is equivalent to the gross calorific value or HHV. In contrast to the HHV, which incorporates the condensation enthalpy of all kinds of water from the coal, the LHV or net calorific value accounts for all water being in the gas phase at the same reference temperature (25 °C). Consequently, the LHV is lower than the HHV by the latent heat of condensation of the coal moisture (2442kJ/kg. ... 

The condensate temperature is chosen as the reference temperature for energy balances. 

In condensation-corrosion by gases, there are three possible modes of interaction of a gas with the external and internal surfaces of a refractory. First, if the gas is condensable (referred to as a vapor), in the course of penetration of the porosity and moving down the temperature gradient in a wall, the gas may literally condense or liquefy progressively. Second, the gas may condense by virtue of dissolving in the refractory and third, it may condense... 

However, if the liquid solution contains a noncondensable component, the normalization shown in Equation (13) cannot be applied to that component since a pure, supercritical liquid is a physical impossibility. Sometimes it is convenient to introduce the concept of a pure, hypothetical supercritical liquid and to evaluate its properties by extrapolation provided that the component in question is not excessively above its critical temperature, this concept is useful, as discussed later. We refer to those hypothetical liquids as condensable components whenever they follow the convention of Equation (13). However, for a highly supercritical component (e.g., H2 or N2 at room temperature) the concept of a hypothetical liquid is of little use since the extrapolation of pure-liquid properties in this case is so excessive as to lose physical significance. 

Appendix C-6 gives parameters for all the condensable binary systems we have here investigated literature references are also given for experimental data. Parameters given are for each set of data analyzed they often reflect in temperature (or pressure) range, number of data points, and experimental accuracy. Best calculated results are usually obtained when the parameters are obtained from experimental data at conditions of temperature, pressure, and composition close to those where the calculations are performed. However, sometimes, if the experimental data at these conditions are of low quality, better calculated results may be obtained with parameters obtained from good experimental data measured at other conditions. 

The way in which these factors operate to produce Type III isotherms is best appreciated by reference to actual examples. Perhaps the most straightforward case is given by organic high polymers (e.g. polytetra-fluoroethylene, polyethylene, polymethylmethacrylate or polyacrylonitrile) which give rise to well defined Type III isotherms with water or with alkanes, in consequence of the weak dispersion interactions (Fig. S.2). In some cases the isotherms have been measured at several temperatures so that (f could be calculated in Fig. 5.2(c) the value is initially somewhat below the molar enthalpy of condensation and rises to qi as adsorption proceeds. In Fig. 5.2(d) the higher initial values of q" are ascribed to surface heterogeneity. 

The fractionation of these refractory elements is beheved to be the result of relative efficiencies of incorporation of condensed sohds rich in early high temperature phases into the meteorite parent bodies at different times and locations in the solar nebula. The data are taken from Reference 3. 

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