Vapor composition, sources

Detonation arresters are typically used in conjunction with other measures to decrease the risk of flame propagation. For example, in vapor control systems, the vapor is often enriched, diluted, or inerted, with appropriate instrumentation and control (see Effluent Disposal Systems, 1993). In cases where ignition sources are present or pre-dic table (such as most vapor destruct systems), the detonation arrester is used as a last-resort method anticipating possible failure of vapor composition control. Where vent collec tion systems have several vapor/oxidant sources, stream compositions can be highly variable and... 

The vapor sample under investigation may not eontain only one kind of speeies. It is desirable to learn as mueh as possible about the vapor composition from independent sources, but here the different experimental conditions need to be taken into account. For this reason, the vapor composition is yet another unknown to be determined in the electron diffraction analysis. Impurities may hinder the analysis in varying degrees depending on their own ability to scatter electrons and on the distribution of their own intemuclear distances. In case of a conformational equilibrium of, say, two conformers of the same molecule may make the analysis more difficult but the results more rewarding at the same time. The analysis of ethane-1,2-dithiol data collected at the temperature of 343 kelvin revealed the presence of 62% of the anti form and 38% of the gauche form as far as the S-C-C-S framework was concerned. The radial distributions calculated for a set of models and the experimental distribution in Figure 6 serve as illustration. 

A comparison of columns 4 and 8 reveals no clear pattern, which is perhaps of greater significance. The use of raw data yields smaller values of the vapor composition sample deviations in four out of six cases, but the effects are small and could be masked by errors in the vapor compositions themselves. It seems likely that the greatest source of error lies in determination of vapor composition. Thus there is very little difference in using raw or smoothed data. A typical example of the fit is shown in Figure 2. The optimum smoothing parameters used in run 1 were found to be the same as required for run 2, and are listed in columns 11 and 12 of Table II. 

A second explanation involves the relative accuracy of the first three data points. Examination of the Ay values (Figure 3) reveals that the contribution of these three values distorts the final result. By ignoring them we can reduce the sample deviation from 0.1381 to 0.0387. The All values (Figure 4) show no particular bias hence we might conclude that a likely source of error is in the experimental vapor compositions. In practice, the attainment of true equilibrium... 

Flammability limits The range of gas or vapor compositions in air that will bum or explode if a flame or other ignition source is present. Important The range represents an unsafe gas or vapor mixture with air that may ignite or explode. Generally, the wider the range the greater the fire potential. 

Select the criterion to be used for thermodynamic consistency. Deviations from thermodynamic consistency arise as a result of experimental errors. Impurities in the samples used for vapor-liquid equilibrium measurements are often the source of error. A complete set of vapor-liquid equilibrium data includes temperature T. pressure P. liquid composition x, and vapor composition y,. Usual practice is to convert these data into activity coefficients by the following equation, which is a rearranged form of the equation that rigorously defines K values (i.e., defines the ratio y, /x, under Related Calculations in Example 3.1) ... 

In the calculation of total pressure and vapor composition from boiling point data using the indirect method, the greatest source of error lies in the liquid-phase composition. We have attempted to characterize the frequency distribution of the error in the calculated vapor composition by the standard statistical methods and this has given a satisfactory result for the methanol- vater system saturated with sodium chloride when the following estimates of the standard deviation were used x, 0.003 y, 0.006 T, 0.1° C and tt, 2 mm Hg. This work indicates that in the design of future experiments more data points are required and, for each variable, a reliable estimate of the standard deviation is highly desirable. 

There are three sources of error in the calculated vapor composition when these are calculated from boiling point data random error in each experimental observation systematic error in one or more of the observations and the model is imperfect (this is particularly true for isobaric data because use is made of the Gibbs-Duhem equation which was derived for constant temperature and pressure). In the present work we shall assume that the only error in the data is caused by randomness. 

Table II gives the standard deviations of pressure, vapor composition and temperature, and the corresponding bias and D-value as each variable is changed randomly and then as all four are changed simultaneously. We see that the random error of x contributes ca. 75% of the induced error in the value of the standard deviation of both the pressure and temperature while the random error of T and tt only contribute about 12% each. On the other hand the random errors of x and y contribute equally to the induced-vapor composition standard deviation with the pressure making a negligible contribution. The bias values are negligibly small except for the pressure standard deviations where they are still not large. The final column has D-values at least equal to two and this gives one confidence in the model and suggests it is adequate for good quality data as in this particular case the only source of error is caused by random behavior.

Several conclusions can be drawn from this work. First, in the calculation of total pressure and vapor composition from boiling point data the greatest source of error lies in the liquid-phase composition, particularly at low concentration. Second, the estimates of the standard deviation for vapor composition and temperature of 0.006 and 0.1°C,... 

Figure 3 Isobars representing locus of values for dissolved H2O and CO2 in basaltic melt in equilibrium with H2O-CO2 vapor at 1,200 °C and selected pressures. Similarly, isopleths represent locus of basaltic melt compositions in equilibrium with given vapor compositions (20 mol.%, 50 mol.%, and 80 mol.% H2O) at 1,200 °C (source Newman and Lowenstern, 2002). See also http //wrgis.wr.usgs.gov/docs/geologic/jlwnstm/other/software jbl.html.

The spectra were similar with the LC-SAX column in line. The minor changes in ion relative intensity when FIA spectra were compared to LC/MS spectra are not uncommon in TSP mass spectrometry. TSP spectra are concentration dependent and with the column in line detector concentration is reduced due to band broadening. Thermospray mass spectrometry exhibits limited day-to-day reproducibility of ionization efficiency and fragmentation patterns, and a dependence of ion intensity on flow rate (21). TSP spectra also are affected by pressure, temperature, and vapor composition (22.231 and apparently also the design of the TSP source (24). 

FIG. 2-7 Enthalpy-concentration diagram for aqueous ammonia. From Thermodynamic and Physical Properties NH3-H20, Int Inst. Refrigeration, Paris, France, 1994 (88 pp.). Reproduced by permission. In order to determine equilibrium compositions, draw a vertical from any liquid composition on any boiling line (the lowest plots) to intersect the appropriate auxiliary curve (the intermediate curves). A horizontal then drawn from this point to the appropriate dew line (the upper curves) will establish the vapor composition. The Int. Inst. Refrigeration publication also gives extensive P-v-xtah es from —50 to 316°C. Other sources include Park, Y. M. and Sonntag, R. E., ASHRAE Trans., 96,1 (1990) 150-159 x, h, s, tables, 360 to 640 K) Ibrahim, O. M. and S. A. Klein, ASH E Trans., 99, 1 (1993) 1495-1502 (Eqs., 0.2 to 110 bar, 293 to 413 K) Smolen, T. M., D. B. Manley, et al.,/. Chem. Eng. Data, 36 (1991) 202-208 p-x correlation, 0.9 to 450 psia, 293-413 K)i Ruiter, J. P, 7nf. J. R rig., 13 (1990) 223-236 gives ten subroutines for computer calculations. 

Evaporation of metals is generally straightforward because they evaporate either as atoms or as clusters of atoms. However, most compounds dissociate when heated and therefore the vapor composition will be different from that of the source. Consequently, the stoichiometry of the deposited film will also be different from that of the source. An example of an oxide that dissociates on heating is ZrOi... 

Most ceramics have very high Tb. Consequently their vapor pressures are negligible at room temperature and become appreciable only at high temperature. Also very few ceramics vaporize without a molecular change. The result is that the vapor composition is usually not the same as that of the original liquid or solid. A practical consequence is that when we try to grow ceramic thin films by evaporation as described in Chapter 28 the film may have a stoichiometry different from that of the source. Some of the phenomena that can occur when compounds evaporate are shown in Table 34.5. 

A second method of studying vapor composition is to trap the atoms in a cold, inert material such as frozen argon or xenon. This can be done by employing liquid helium as a coolant (4 K) or special refrigeration units that can go down to below 20 K. A cold window can be employed inside a vacuum chamber, and the vapors coming off the hot metal source can be condensed on the cold window simultaneously with excess argon gas (atoms). In this way the metal particles can be surrounded and trapped in a frozen inert ice. Then the trapped atoms can be analyzed spectroscopically. This method is called matrix isolation spectroscopy and will be discussed in more detail in Section IL.B.2. 

Metal alloys, such as Al u, Co r or Ni-Cr, can generally be evaporated directly from a single heated source. If two constituents of the alloy evaporate at different rates causing the composition to change in the melt, two different sources held at different temperatures may be employed to ensure uniform deposition. Unlike metals and alloys, inorganic compounds evaporate in such a way that the vapor composition is usually different from that of the source. The resulting molecular structure causes the film stoichiometry to be different from that of the source. High purity films of virtually all materials can be deposited in vacuum by means of electron beam evaporation. 

Another source of error in the system is possible because the condensate returned to the still is of a different composition from the liquid in the still and in general is of lower boiling point. If this vaporizes before it is completely mixed with all of the liquid in the still, this vapor composition will not be an equilibrium vapor. 

Alloys. Alloys consist of two or mote elements of different vapor pressures and hence different evaporation rates. As a result, the vapor phase and therefore the deposit constantiy vary in compositions. This problem can be solved by multiple sources or a single rod- or wire-fed electron beam source fed with the alloy. These solutions apply equally to evaporation or ion-plating processes. 

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