Water vapor partial pressure effect

As we can see from relations such as Equation 8.2 (J = gjAcj = ACjlrj), the conductances or the resistances of the various parts of the pathway determine the drop in concentration across each component when the flux density is constant. Here we will apply this condition to a consideration of water vapor concentration and mole fraction in a leaf, and we will also consider water vapor partial pressures. In addition we will discuss the important effect of temperature on the water vapor content of air (also considered in Chapter 2, Section 2.4C). 

These experiments show that addition of water vapor to the reactant mixture has a strong inhibitory effect on the production of hydrocarbons. The C02 yield is increased owing to the reaction H20 + CO — C02 + H2. When the water vapor partial pressure is approximately equal to the CO partial pressure in the reactant gas mixture, no hydrocarbons are produced at all. The pronounced inhibitory effect of H20 vapor on hydrocarbon formation in this reaction may explain some of the scatter of the data shown in Figures 1, 2, and 3. If small (and variable) amounts of H20 vapor were adsorbed on the walls of the reactor tubes, this would decrease the hydrocarbon yield in (some of) the runs by a varying amount which would show up as scatter in the data. However, there are other, as yet unknown, sources of variability in the experimental conditions which also undoubtedly contribute to the scatter in the data. 

Figure 1. Effect of argon carrier flow role upon the deposit oxidation by water vapor at 1056°C. Key to water vapor partial pressure O, 58 mm Hg and , 362 mm Hg.

Figure 3. Effect of the water vapor partial pressure (Ph2o) on deposit oxidation at 1056°C.

Figure 18. The effect of water vapor partial pressure ( h2o) on the weight of PBI/PPy(50)coPSF SO/SO-HaPO/ with initial doping level DL = 180%wt at (— —) 150°C, (— —) 160°C and (—A—) 170°C and Copolymer ll6o (Fig. 9) with initial doping level DL = 230%wt at (-u-)150°C, (- -) 160°C, (-A-) 170°C and (- -) 180°C. The dash and dotted lines corresponds to the weight of the doped blend membranes at 95°C (see Fig. 17) imbibed with 100% H3PO4.

The positive effect of water on the fuel cell performance was further proven by introducing several H2O/H2 mixtures into the anode compartment. I-V curves were recorded for each water vapor partial pressure as shown in Fig. 22. I-V curves were recorded fast, within a time interval less than 30 s, so that the water produced at the cathode would not equilibrate with the membrane. It is evident in Fig. 22 that by increasing the water partial pressure a threefold increase of the cell performance is observ ed, thus proving the vital importance of steam for the efficient operation of the phosphoric acid imbibed high temperature MEA. 

Fig. 5.14 The effect of water vapor partial pressure (PH20)...

Giberson RC, Walker JP. Reaction of nuclear graphite with water vapor. Part 1. Effect of hydrogen and water vapor partial pressure. Carbon l966 3(4) 521-525. 

The density of bubbles increases with water vapor partial pressure. An example of bubbles formed in a silica scale from oxidation of SiC in 90% H2O/10% O2 at 1200 °C for 64 h is shown in Fig. 7-17. These bubbles are not observed at 1100 °C nor are they observed for crystalline scales formed at 1400 °C. Again, these bubbles are an effect of gaseous oxidation products formed by the... 

Because water will be present in this system, and is assumed immiscible with the other components, it will exert its own vapor pressure. This situation is similar to many systems where the liquid to be flashed enters below its dew point, and hence requires the use of steam to heat (sensible + latent) as well as steam for the partial pressure effect... 

The effect of plasticizers and temperature on the permeabiUty of small molecules in a typical vinyUdene chloride copolymer has been studied thoroughly. The oxygen permeabiUty doubles with the addition of about 1.7 parts per hundred resin (phr) of common plasticizers, or a temperature increase of 8°C (91). The effects of temperature and plasticizer on the permeabiUty are shown in Figure 4. The moisture (water) vapor transmission rate (MVTR or WVTR) doubles with the addition of about 3.5 phr of common plasticizers (92). The dependence of the WVTR on temperature is a Htde more comphcated. WVTR is commonly reported at a constant difference in relative humidity and not at a constant partial pressure difference. WVTR is a mixed term that increases with increasing temperature because both the fundamental permeabiUty and the fundamental partial pressure at constant relative humidity increase. Carbon dioxide permeabiUty doubles with the addition of about 1.8 phr of common plasticizers, or a temperature increase of 7°C (93). 

Before drying can begin, a wet material must be heated to such a temperature that the vapor pressure of the contained Hquid exceeds the partial pressure of vapor already present in the surrounding atmosphere. The effect of a dryer s atmospheric vapor content and temperature on performance can be studied by constmction of a psychrometric chart for the particular gas and vapor. Figure 2 is a standard chart for water vapor in air (6). 

Both of the above chemical studies point towards the increased importance of the burning process at 285°C in determining the initial rate of heat production. The role of water as yet remains undefined other than at the higher temperature of 285°C it appears to have the opposite effect on the bitumen sample compared to the process at 225°C i.e., it appears that water vapor encourages pathways by which the various components of bitumen react with oxygen. Preliminary calculations of the total heats evolved during the wet oxidation of bitumen sands indicate that they are independent of the partial pressure of oxygen in the system at... 

The nucleation of these decomposition processes was studied by means of thermomicroscopy on single crystal cleavage plates of calcite, magnesite, dolomite and smithsonite (Fig. 59). The shape of the nuclei was found to be different for these carbonates, which might be also of importance for the decomposition mechanism. The partial pressure of water vapor has a pronounced effect on the decomposition of transition metal carbonates such as ZnC03 and CdC03. The evolution of C02 is probably catalyzed in the presence of water vapor and shifted to considerably... 

The ammonia partial pressures given in Tables 1 and 2 are based on the concentration of ammonia found in the vapor stream times the total pressure. The actual pressures applied at each run condition are summarized in Table 3 where the pressures varied from 15 psia at 80°C to 90 psia at 120°C. Because nitrogen was used as a pressurizing fluid, the partial pressure of water and the total pressure excluding nitrogen have been computed in Tables 1 and 2 based on Raoult s law for water as noted at the bottom of Table 1. Raoult s law applies for the partial pressure of water because the activity coefficient of water is virtually unity at the low levels of ammonia used in the liquid phase. Minor effects due to vapor non- ideality have not been applied. 

It was found that a nickel-activated carbon catalyst was effective for vapor phase carbonylation of dimethyl ether and methyl acetate under pressurized conditions in the presence of an iodide promoter. Methyl acetate was formed from dimethyl ether with a yield of 34% and a selectivity of 80% at 250 C and 40 atm, while acetic anhydride was synthesized from methyl acetate with a yield of 12% and a selectivity of 64% at 250 C and 51 atm. In both reactions, high pressure and high CO partial pressure favored the formation of the desired product. In spite of the reaction occurring under water-free conditions, a fairly large amount of acetic acid was formed in the carbonylation of methyl acetate. The route of acetic acid formation is discussed. A molybdenum-activated carbon catalyst was found to catalyze the carbonylation of dimethyl ether and methyl acetate. 

Azeotropic and Partially Miscible Systems. Azeotropic mixtures are those whose vapor and liquid equilibrium compositions are identical. Their x-y lines cross or touch the diagonal. Partially miscible substances form a vapor phase of constant composition over the entire range of two-phase liquid compositions usually the horizontal portion of the x-y plot intersects the diagonal, but those of a few mixtures do not, notably those of mixtures of methylethylketone and phenol with water. Separation of azeotropic mixtures sometimes can be effected in several towers at different pressures, as illustrated by Example 13.6 for ethanol-water mixtures. Partially miscible constant boiling mixtures usually can be separated with two towers and a condensate phase separator, as done in Example 13.7 for n-butanol and water. 

Figure A2.1.2 Effect of temperature on vapor pressure measurement. The upper curve is the vapor pressure of pure water, pw°. The lower curve is a system whose partial water vapor pressure, pw, is always a constant fraction of the vapor pressure of pure water. See text for details.

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