Vapors versus gases

The equilibrium adsorption characteristics of gas or vapor on a solid resemble in many ways the equilibrium solubility of a gas in a liquid. Adsorption equilibrium data are usually portrayed by isotherms lines of constant temperature on a plot of adsorbate equilibrium partial pressure versus adsorbent loading in mass of adsorbate per mass of adsorbent. Isotherms take many shapes, including concave upward and downward, and S-curves. Equilibrium data for a given adsorbate-adsorbent system cannot generally be extrapolated to other systems with any degree of accuracy. 

The method of generating N, (moles of adsorbed gas or vapor per unit gram of polymer) versus p (partial pressure of gas or vapor) by IGC is based on the method described by Mohlin and Gray (16). This technique is the same as Elution by Characteristic Point (ECP) Method described by Conder and Young (18). 

The adsorption-desorption method is a popular and commonly used method for characterization of surface and structural properties of porous materials, allowing the determination of their surface area, pore-size distribution, pore volume and adsorption energy distribution. Nitrogen is often used for the adsorbent gas but other adsorbent gases such as argon can also be used. According to this method, adsorption-isotherm (amount of adsorbed gas versus relative pressure [pressure/saturation vapor pressure of the adsorbent]) is drawn and the data are analyzed by assuming capillary condensation. 

Figure 9-51. Characteristics of Koch/Sulzer packing, Gas loading factor, F, versus HETP, pressure drop, and packing hold-up. Note Vs = superficial gas velocity, ft/sec and pv = vapor density, Ib/fl. Used by permission of Koch Engineering Co., inc.. Bull. KS-1 and KS-2.

In most of today s FCC operations, the desired reactions take place in the riser. In recent years, a number of refiners have modified the FCC unit to eliminate, or severely reduce, post-riser cracking. Quick separation of catalyst from the hydrocarbon vapors at the end of the riser is extremely important in increasing the yield of the desired product. The post-riser reactions produce more gas and coke versus less gasoline and distillate. Presently, there are a number of commercially proven riser disengaging systems offered by the FCC licenser designed to minimize the post-riser cracking of the hydrocarbon vapors. 

The vapor pressure of water, which is 24 mm Hg at 25°C, becomes 92 mm Hg at 50°C and 1 atm (760 mm Hg) at 100°C. The data for water are plotted at the top of Figure 9.2. As you can see, the graph of vapor pressure versus temperature is not a straight line, as it would be if pressure were plotted versus temperature for an ideal gas. Instead, the slope increases steadily as temperature rises, reflecting the fact that more molecules vaporize at higher temperatures. At 100°C, the concentration of H20 molecules in the vapor in equilibrium with liquid is 25 times as great as at 25°C. 

Temperature Tgo in the range between 3.0 and 24.5561 K is defined in terms of 3He or 4He constant volume gas thermometers (CVGT), calibrated at the triple points of Ne and H2, and at a temperature between 3.0 and 5.0 K that has been obtained from vapor pressure versus temperature relations for He. 

Solid-Fluid Equilibria The solubility of the solid is very sensitive to pressure and temperature in compressible regions, where the solvent s density and solubility parameter are highly variable. In contrast, plots of the log of the solubility versus density at constant temperature often exhibit fairly simple linear behavior (Fig. 20-19). To understand the role of solute-solvent interactions on sofubilities and selectivities, it is instructive to define an enhancement factor E as the actual solubihty divided by the solubility in an ideal gas, so that E = ysP/Pf, where P is the vapor pressure. The solubilities in CO2 are governed primarily by vapor pressures, a property of the solid... 

The Clausius-Clapeyron equation implies that if we plot the natural log of the pressure of the gas phase versus inverse temperature, the slope of the resulting line is the heat of vaporization divided by the gas constant (R). A plot of In P (vapor pressure of water) versus inverse temperature is given in Figure 3. The calculated heat of vaporization (determined by multiplying the slope by R) is 10,400 cal/mol. The important aspect of Eq. (10) with regard to moisture sorption is the fact that increasing the temperature also increases the vapor pressure. 

In addition to the thermal effects of the reaction, pressure data are acquired from an ARC experiment. As in the closed Dewar flask tests, the pressure is the result of (1) the heating of the free-board gas, (2) the vapor pressure, and (3) the reaction-produced gases. With the pressure-temperature versus time curve, the gas generation of the substance can be calculated (mol gas/mol substance). This is possible if enough knowledge of the gas solubility in the liquid and the vapor pressure of the sample are available. Such a calculation is useful for gas venting estimates for the process. 

Atmospheric dispersion of any rupture disc discharges would result in a vapor cloud with gas concentrations above the lower explosive limit. Thus, such releases must be avoided, and other mitigation procedures should be used. However, as an additional check on the situation, mapping of the potential gas cloud versus the plant layout was conducted with the conclusion that no ignition sources were likely to be present in the region where the vapor cloud would be flammable. 

Hydrocarbon releases in the petroleum industry are either gaseous, mists or liquids and are either atmospheric releases or pressurized. Gas and mist releases are considered more significant since they are readily ignitable since they are in the gas state and due to the generation of vapor clouds which if ignited are instantly destructive in a widespread nature versus liquid fires that may be less prone to ignition, generally localized and relatively controllable. 

A common consideration is the presence of water vapor, H20(g). Water generates a vapor pressure, which varies with the temperature. Daltons law is used in these cases to adjust the pressure of a gas sample for the presence of water vapor. The total pressure (normally atmospheric pressure) is the pressure of the gas or gases being collected and the water vapor. When the pressure of an individual gas is needed, the vapor pressure of water is subtracted from the total pressure. Finding the vapor pressure of water requires measuring the temperature and using a table showing vapor pressure of water versus temperature. 

This chapter focuses on gas-liquid chromatography, in which compounds in a sample are separated based on vapor pressures and differences in affinity for the stationary phase (a high boiling point liquid) versus the gaseous mobile phase. The time between sample injection and detection of the individual compound eluting from the column is called the retention time. Compounds that have limited solubility in the stationary phase will exit the column quickly as a large proportion will remain in the mobile phase. Compounds with polarity similar to that of the stationary phase will have longer retention times and potentially broader peaks, due to increased interaction with the stationary phase. 

The purchaser will advise the HPRT manufacturer whether any portion of the process stream entering the HPRT will flash to vapor and whether absorbed gas in the stream will evolve at any pressure less than the inlet pressure. In either case, the purchaser must specify the volume percentage of vapor or gas or both at the turbine outlet and the pressure and temperature at which the vapor will flash off. When known, the fluid composition, and the liquid and vapor (or gas) density versus pressure should also be specified. 

Take, for example, the plot of G versus temperature for elemental sulfur, represented by the bottom diagram in Figure 2.1. We know from experiments and observation that there are four phases we have to consider for sulfur two solid forms (a low-temperature orthorhombic form, R, and high-temperature monoclinic form, M), liquid (L), and vapor (or gas, V). The lines of G versus T for each phase, which are partially solid and continue on as dashed lines, are constructed at constant pressure using Eq. (2.10),... 

The charcoal tubes were broken open and the charcoal transferred into a stoppered glass test tube. One milliliter of carbon disulfide was pipetted into each tube, and 1.5 ml of carbon disulfide was pipetted into each 3M vapor monitor badge. After 30 minutes, aliquots of carbon disulfide were injected into the gas chromatograph and compared versus hydrocarbon standards prepared in carbon disulfide. The total areas of the sample and standard peaks were measured by the data system. 

Moreover, this relation between chemical structure and vapor pressure also holds because enthalpies and entropies of vaporization are directly related, in general. Recall that the entropy of vaporization reflects the difference of a molecule s freedom in the gas phase versus the liquid phase (A pS = Si% - SiL). At ambient pressures, we may assume that differences in Avap5) between different compounds are primarily due to differences in molecular freedom in the liquid phase. (The freedom of the molecules in the gas phase is not that different between compounds). Hence, not surprisingly, molecules that exhibit stronger intermolecular attractions... 

Figure 28 shows the result of the kinetic in-cell investigation. For three anodic overpotentials there are plotted the obtained current densities versus gas composition, which simulates different degrees of hydrogen conversion. The gas composition changes from pure hydrogen (left) to H20/C02 = 1/1 mixtures. As predicted by Eq. (30), not only for complete conversion but also for vanishing conversion the current approaches zero as neither vapor nor carbon dioxide is present. 

The water content of the vapor phase in (H-V) equilibrium is very small (typically less than 0.001 mole fraction) and therefore difficult to measure accurately. As a consequence, in the history of gas processing, semilogarithmic straight lines (gas water content versus reciprocal absolute temperature) from the Lw-V region were extrapolated into the H-V region with limited justification. 

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