Water vapor concentration

The characteristics of WC, especially grain size, are determined by purity, particle shape and grain size of the starting material, and the conditions employed for reduction and carburization. The course of the reaction WO3 — W — WC is dependent on temperature, gas flow rates, water-vapor concentration in the gas, and the depth of the powder bed. All these factors affect the coarsening of the grain. 

Thermal Decomposition of GIO2. Chloiine dioxide decomposition in the gas phase is chaiacteiized by a slow induction period followed by a rapid autocatalytic phase that may be explosive if the initial concentration is above a partial pressure of 10.1 kPa (76 mm Hg) (27). Mechanistic investigations indicate that the intermediates formed include the unstable chlorine oxide, CI2O2. The presence of water vapor tends to extend the duration of the induction period, presumably by reaction with this intermediate. When water vapor concentration and temperature are both high, the decomposition of chlorine dioxide can proceed smoothly rather than explosively. Apparently under these conditions, all decomposition takes place in the induction period, and water vapor inhibits the autocatalytic phase altogether. The products of chlorine dioxide decomposition in the gas phase include chlorine, oxygen, HCl, HCIO, and HCIO. The ratios of products formed during decomposition depend on the concentration of water vapor and temperature (27). 

The real atmosphere is more than a dry mixture of permanent gases. It has other constituents—vapor of both water and organic liquids, and particulate matter held in suspension. Above their temperature of condensation, vapor molecules act just like permanent gas molecules in the air. The predominant vapor in the air is water vapor. Below its condensation temperature, if the air is saturated, water changes from vapor to liquid. We are all familiar with this phenomenon because it appears as fog or mist in the air and as condensed liquid water on windows and other cold surfaces exposed to air. The quantity of water vapor in the air varies greatly from almost complete dryness to supersaturation, i.e., between 0% and 4% by weight. If Table 2-1 is compiled on a wet air basis at a time when the water vapor concentration is 31,200 parts by volume per million parts by volume of wet air (Table 2-2), the concentration of condensable organic vapors is seen to be so low compared to that of water vapor that for all practical purposes the difference between wet air and dry air is its water vapor content. 

A typical dehydration. specification in the U.S. Gulf Coast is 7 lb of water vapor per MMsef of gas (7 Ib/MMscf). This gives a dew [lomi ot around 32 f for 1,000 psi gas. In the northern areas of the U.S. and Canada the gas contracts require lower dew points or lower water vapor conceniia tions ill (he gas. Water vapor concentrations of 2—4 Ib/MMscf are comnioii II I he g. as is to be proces.sed at very low temperatures, as in a cryogenic gas pl.mi. w ater vapor removal down to 1 ppm may be required. 

R02./R02 Recombinations. Another area of uncertainty is the peroxyl radical recombination reactions described above, which become especially significant when the NO concentration is low. This can occur late in the photooxidation of polluted air undergoing transport, as in some rural environments (60,85) and in clean air. Although reactions of H02 with itself (R33) are reasonably well understood (their rate depends upon total pressure and upon water vapor concentration), reactions of H02 with R02 species and the R02 self reaction are much less well quantified. Since these serve as important radical sink processes under low NO. conditions, their accurate portrayal is important for accurate prediction of HO, concentrations. 

Kinetic analysis based on the Langmuir-Hinshelwood model was performed on the assumption that ethylene and water vapor molecules were adsorbed on the same active site competitively [2]. We assumed then that overall photocatalytic decomposition rate was controlled by the surface reaction of adsorbed ethylene. Under the water vapor concentration from 10,200 to 28,300ppm, and the ethylene concentration from 30 to 100 ppm, the reaction rate equation can be represented by Eq.(l), based on the fitting procedure of 1/r vs. 1/ Ccm ... 

The permanent gases must be pumped off at the lowest position in the condenser. They are more dense than water vapor, concentrate at the bottom of the condenser and fill up the condenser housing in time. This permanent gas reduces the vapor transport to the cold surfaces and form a snow ice as can be seen in Fig. 2.17. A condenser (Fig. 2.18.1) meets these requirements in general, but other designs are possible (see Fig. 2.52 B). 

Such special atmosphere is often necessary in decomposition and rehydration studies of hydroxides, hydrates etc. Furthermore also the catalytic effect of water vapor on certain reactions is of interest. For such studies the gas is saturated with water, or other vapors e.g. D20, alcohol, CS2, etc. When higher water vapor concentrations are required special furnaces are available (see Sect. 2.4). 

FIG. 24-56 Calculated sulfuric acid dew points, as a function of SO3 content, for various flue gas water vapor concentrations. Courtesy W. M. M. Huijbregte,... 

In the low humidity cases, the cell does not produce liquid water immediately but after the water vapor concentration in the gas reaches the saturation value therefore, the condensation front is pushed downstream, and its location is directly related to cathode inlet relative humidity. In the 20% cathode relative humidity case, it is found that the first two-thirds of... 

Gieseler et al. utilized tunable diode laser absorption spectroscopy to detect water vapor concentrations, gas velocities and mass flow during freeze-drying of pure water at different pressure and shelf temperature settings and of a 5%w/w mannitol solution. The analyzer was interfaced to the spool that connected the dryer chamber to the condenser. The reported method was advantageous in that primary and secondary drying end-point control based upon mass flow rate was independent of freeze-dryer size and configuration. ... 

For these conditions, we have the following water vapor concentrations ... 

But, in addition, it was shown that the water vapor concentration was without the effect on hydrogen peroxide formation which should be expected from this mechanism. The suggestion was made that the reaction might perhaps be better represented by the mechanism suggested by Taylor and Marshall ... 

The ammonium nitrate formed in reaction (54) can exist either as a solid particle or in solution, and since this reaction is an equilibrium, it can redissociate to form the reactants. The deliquescence point for NH4N03 at 25°C is 62% RH i.e., at a water vapor concentration corresponding to 62% RH, the solid particle dissolves to form a concentrated liquid solution. 

At water vapor concentrations above the deliquescence point, the equilibrium is that between the reactant gases and aqueous ammonium nitrate. As treated in detail by Mozurkewich, the equilibrium constant, K 4-54, then depends on the solution concentrations or activities ... 

Water vapor concentrations have also been used to show that stratospheric air in the midlatitudes cannot all have originated via the tropical pump, i.e., path I in Fig. 12.3. For example, Dessler et al. (1995b) have shown that water vapor concentrations in the lowermost stratosphere at 37.4°N, 122.1°W are higher than expected for an air mass that has passed through the cold tropical tropopause. Their data are consistent with path II, although as they point out, these measurements do not exclude path III, which represents convective transport from the troposphere to the stratosphere at mid and high latitudes. Lelieveld et al. (1997) report aircraft measurements of CO, 03, and HNO-, over western Europe that suggest that tropospheric air can be mixed into the lower stratosphere. 

Explain the effect of the water vapor concentration on the NO formation. Hint Look at the O atom profile as function of time. 

A particular, and unusual, atmospheric application of such data involves the formation of noctilucent clouds (NLC s) in the vicinity of the mesopause (at 82 km, in the summer hemisphere, where temperatures can fall as low as 130 K, and ice can exist even at the miniscule ambient water vapor concentrations found there). The presence of laige water-aggregated hydronium ions led to the suggestion [e.g., 63-65] that these provide condensation sites for ice particles. Detailed simulation studies bore out the likely relationship between positive ion nucleation and the behavior of some NLC s [66], notwithstanding a strong possibility that meteoritic dust and smoke also had a dominant role [67], ITie contribution to NLC formation of hydronium-ion/electron... 

Although the troposphere has the characteristic of containing a high relative concentration of water vapor (10 5-10-2), the stratosphere is dry and the water vapor concentration is only a few parts in a million. However, the oxidation of methane by hydroxyl radical must be intro-... 

For the system described in problem 3.3 develop a general expression for computing the water vapor concentration profile through the stagnant air film. 

During cell/stack operation, water content in the membrane is affected by the local intensive variables, such as local temperature, water vapor concentration in the gas phase, gas temperature and velocity in the channel, and the properties of the electrode and gas diffusion media. The power fluctuation can result in temperature variation inside the cell/stack, which will subsequently change the local membrane water content. As the water content in the membrane tends to be non-uniform and unsteady, this results in operation stresses. When the membrane uptakes water from a dry state, it tends to expand as there is no space for it to extend in plane and it can wrinkle up as schematically shown in Fig. 4 when the membrane dries out, the wrinkled part may not flatten out, and this ratcheting effect can cause the pile up of wrinkles at regions where membrane can find space to fold. The operation stress is typically cyclic in nature due to startup-shutdown cycles, freeze-thaw cycles, and power output cycles. 

Recently Meng17 developed a transient, multiphase, multidimensional PEFC model to elucidate the fundamental physics of cold start. The results showed the importance of water vapor concentration in the gas channels, which implies that large gas flow rates benefit cold-start performance. They also found that ice growth in the cathode catalyst layer during cold start was faster under the land than under the gas channels, and accumulated more at the interface between the cathode catalyst layer and GDL. 

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