Vapor Standards

Another approach to vapor standards is to use the diffusion of vapor through a capillary to add small amounts of vapor to a flowing gas stream (31-33). The theory and practice are reasonably well defined. The concentration is determined by knowing the rate of diffusion and using the following equation ... 

It is with gaseous materials that preparation of extremely dilute standards presents real difficulties. For example how can one be certain that a syringe pulled back to 1 cm3 volume in fact contains 1 cm3 of the test gas and not 0.5 cm3 of that gas, plus 0.5 cm3 of air or 1 cm3 of the gas at reduced pressure Furthermore, since the concern is for quantities in the parts-per-million range, a 1-cm3 volume of a standard gas is not a reasonable volume to be considered since it would imply the use of a 1000-liter container. Therefore, a variety of alternative methods has been devised, though the serial dilution of a known gas volume is also used. This is defined as a static gas standard, while methods for the preparation of dynamic flowing concentrations of trace levels of the sample gas in a carrier have also been well defined. The variety of techniques for preparation of gas or vapor standards is fully covered in Chapter 4. 

Define both formally (in terms of internal energies and enthalpies) and in words a high school senior could understand the variables Cy(T) (heat capacity at constant volume), Cp(T) (heat capacity at constant pressure), AH (heat of fusion or heat of melting), A//v (heat of vaporization), standard heats of fusion and vaporization, and AHj (heat of solution or heat of mixing). 

Fig. 9-15 shows one of the standard connections for cryogenic oxygen fluid transfer from CGA V-6, and Fig. 9-16 shows one of the liquid and vapor standard connections for carbon dioxide from CGA V-6.1. 

Low range water vapor standards may be obtained by the use of water permeation tubes. Permeation rates must be... 

Compressed gas water vapor standards may be used, provided they are checked by an independent method once a month. 

In Equation (24), a is the estimated standard deviation for each of the measured variables, i.e. pressure, temperature, and liquid-phase and vapor-phase compositions. The values assigned to a determine the relative weighting between the tieline data and the vapor-liquid equilibrium data this weighting determines how well the ternary system is represented. This weighting depends first, on the estimated accuracy of the ternary data, relative to that of the binary vapor-liquid data and second, on how remote the temperature of the binary data is from that of the ternary data and finally, on how important in a design the liquid-liquid equilibria are relative to the vapor-liquid equilibria. Typical values which we use in data reduction are Op = 1 mm Hg, = 0.05°C, = 0.001, and = 0.003... 

Enthalpies are referred to the ideal vapor. The enthalpy of the real vapor is found from zero-pressure heat capacities and from the virial equation of state for non-associated species or, for vapors containing highly dimerized vapors (e.g. organic acids), from the chemical theory of vapor imperfections, as discussed in Chapter 3. For pure components, liquid-phase enthalpies (relative to the ideal vapor) are found from differentiation of the zero-pressure standard-state fugacities these, in turn, are determined from vapor-pressure data, from vapor-phase corrections and liquid-phase densities. If good experimental data are used to determine the standard-state fugacity, the derivative gives enthalpies of liquids to nearly the same precision as that obtained with calorimetric data, and provides reliable heats of vaporization. 

Convergence is usually accomplished in 2 to 4 iterations. For example, an average of 2.6 iterations was required for 9 bubble-point-temperature calculations over the complete composition range for the azeotropic system ehtanol-ethyl acetate. Standard initial estimates were used. Figure 1 shows results for the incipient vapor-phase compositions together with the experimental data of Murti and van Winkle (1958). For this case, calculated bubble-point temperatures were never more than 0.4 K from observed values. 

Correlations for standard-state fugacities at 2ero pressure, for the temperature range 200° to 600°K, were generated for pure fluids using the best available vapor-pressure data. 

Subroutine VLDTA2. VLDTA2 loads the binary vapor-liquid equilibrium data to be correlated. If the data are in units other than those used internally, the correct conversions are made here. This subroutine also reads the estimated standard deviations for the measured variables and the initial parameter estimates. All input data are printed for verification. 

SDZ(I) cols 31-40 standard deviation of vapor composition measurement... 

The computer subroutines for calculation of vapor-phase and liquid-phase fugacity (activity) coefficients, reference fugac-ities, and molar enthalpies, as well as vapor-liquid and liquid-liquid equilibrium ratios, are described and listed in this Appendix. These are source routines written in American National Standard FORTRAN (FORTRAN IV), ANSI X3.9-1978, and, as such, should be compatible with most computer systems with FORTRAN IV compilers. Approximate storage requirements and CDC 6400 execution times for these subroutines are given in Appendix J. 

The Reid vapor pressure characterizes the light petroleum products it is measured by a standard test (refer to Chapter 7) which can be easily simulated. 

Tbe ASTM D 323 standard describes a method for determining the vapor pressure employing two chambers, A and B the volume of chamber A is four times that of chamber B. 

The criterion retained up to now in the specifications is not the true vapor pressure, but an associated value called the Reid vapor pressure, RVP. The procedure is to measure the relative pressure developed by the vapors from a sample of motor fuel put in a metallic cylinder at a temperature of 37.8°C. The variations characteristic of the standard method are around 15 millibar in repeatability and 25 millibar in reproducibility. 

In the standard method, the metal enclosure (called the air chamber) used to hold the hydrocarbon vapors is immersed in water before the test, then drained but not dried. This mode of operation, often designated as the wet bomb" is stipulated for all materials that are exclusively petroleum. But if the fuels contain alcohols or other organic products soluble in water, the apparatus must be dried in order that the vapors are not absorbed by the water on the walls. This technique is called the dry bomb" it results in RVP values higher by about 100 mbar for some oxygenated motor fuels. When examining the numerical results, it is thus important to know the technique employed. In any case, the dry bomb method is preferred. 

The V/L ratio is a volatility criterion seldom used in France but is used in Japan and in the United States where it has been standardized as ASTM D 2533. At a given temperature and pressure, the V/L ratio represents the volume of vapor formed per unit volume of liquid taken initially at 0°C. 

Safety standards govern the manipulation and storage of crude oil and petroleum products with regard to their flash points which are directly linked to vapor pressure. 

The standard entropy of adsorption AS2 of benzene on a certain surface was found to be -25.2 EU at 323.1 K the standard states being the vapor at 1 atm and the film at an area of 22.5 x T per molecule. Discuss, with appropriate calculations, what the state of the adsorbed film might be, particularly as to whether it is mobile or localized. Take the molecular area of benzene to be 22 A. ... 

The column (or line entry) headed a gives the volume of gas (in milliliters) measured at standard conditions (0°C and 760 mm or 101.325 kN dissolved in 1 mL of water at the temperature stated (in degrees Celsius) and when the pressure of the gas without that of the water vapor is 760 mm. The line entry A indicates the same quantity except that the gas itself is at the uniform pressure of 760 mm when in equilibrium with water. 

Some solid materials are very intractable to analysis by standard methods and cannot be easily vaporized or dissolved in common solvents. Glass, bone, dried paint, and archaeological samples are common examples. These materials would now be examined by laser ablation, a technique that produces an aerosol of particulate matter. The laser can be used in its defocused mode for surface profiling or in its focused mode for depth profiling. Interestingly, lasers can be used to vaporize even thermally labile materials through use of the matrix-assisted laser desorption ionization (MALDI) method variant. 

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