Water vapor pressurized hydrogen

Phosphoric Acid Fuel Cell. Concentrated phosphoric acid is used for the electrolyte ia PAFC, which operates at 150 to 220°C. At lower temperatures, phosphoric acid is a poor ionic conductor (see Phosphoric acid and the phosphates), and CO poisoning of the Pt electrocatalyst ia the anode becomes more severe when steam-reformed hydrocarbons (qv) are used as the hydrogen-rich fuel. The relative stabiUty of concentrated phosphoric acid is high compared to other common inorganic acids consequentiy, the PAFC is capable of operating at elevated temperatures. In addition, the use of concentrated (- 100%) acid minimizes the water-vapor pressure so water management ia the cell is not difficult. The porous matrix used to retain the acid is usually sihcon carbide SiC, and the electrocatalyst ia both the anode and cathode is mainly Pt. 

In static systems hydrogen and mercuric oxide were found as decomposition products (59,73). Hydrogen peroxide formation was not investigated. The quantum yield for a water vapor pressure of 8.5 mm. varied from 0.02 at 45°C. to 0.04 at 580°C. (59). In flow systems about 27% oxygen was found in the gaseous products, but no hydrogen peroxide was found (9). The flow system results have been substantiated in a more recent study (7) in which the quantum yields were found to be comparable to those obtained in static systems (59). 

The three-phase (Lw-V-Lhc) pressure-temperature line is approximated by the vapor pressure (Vhc-Lhc) locus for the pure component due to two effects, both of which are caused by the hydrogen-bond phenomenon described in Chapter 2. First, hydrogen bonds cause almost complete immiscibility between the hydrocarbon liquid and the aqueous liquid, so that the total pressure may be closely approximated by the sum of the vapor pressures of the hydrocarbon phase and that of water. Second, hydrogen bonds cause such a self-attraction of the water molecules that the water vapor pressure is very low, composing only a small fraction of the total vapor pressure at any temperature. Because each immiscible liquid phase essentially exerts its own vapor pressure, and because the water vapor pressure is very small, the hydrocarbon vapor pressure is a very good approximation of the three-phase (Lw-V-Lhc) locus. 

Utilizing the free energy concept, and disregarding departure from ideal behavior, answer the following questions (i) Will a mixture consisting of equimolecular proportions of water vapor and hydrogen at 1 atm. total pressure be capable of re-... 

A condenser operates with a feed vapor consisting of ammonia(l)-water vapor(2)-hydrogen(3) at a pressure of 340 kPa. At one point in the condenser the mole fractions in the bulk vapor are y n, = 0.30, y2o = 0.40, and y3o = 0.30. The liquid on the condensing surface at this point is at 93.3°C and contains 10 mol% ammonia and 90 mol% water, with negligible hydrogen. The composition of the vapor-gas mixture at the liquid surface, assumed to be in equilibrium with the liquid surface of the stated composition, is y g = 0.455, y2g = 0.195, and y33 = 0.35. Employ the exact matrix solution of the Maxwell-Stefan equations to estimate the rate of condensation of water relative to that of ammonia. 

As discussed previously (2), the contribution to 0 from hydrogen chemisorption must be negligible. By means of Equations (2) and (6), values for the equilibrium constant, Ki, have been determined for palladium black and palladium-silver alloys of different compositions at various temperatures and are collected in Fig. 2. For every sample, data were obtained at two different water-vapor pressures. The lines drawn through the experimental points were taken as straight lines, whose slopes were used to com-... 

A constant-potential coulometric determination of copper is being done using a mercury-pool cathode and a water coulometer. A volume of 32.14 ml of hydrogen-oxygen mixture is obtained. The temperature of the gas is 24.0°C and the barometric pressure in the room is 752.0 mm of mercury. The water vapor pressure above the 0.1 M sodium sulfate solution in the coulometer is as follows ... 

Figure 5.9. Water partial pressures and / as a function of position in the gas channel. The feed is countercurrent with dry hydrogen and air with the conditions T = 80°C, i = 0.6 A/cm, psi—pc= 1.5 bar, hydrogen and air stoichiometries of 4 and 2, respectively. The partial pressures are given for the anode and cathode gas channels (aGC and cGC) and GDL / membrane interfaces (aM and cM). The water vapor pressure at this temperature is about 0.2 bar. (The figure is reproduced from Ref. [71] with permission of The Electrochemical Society, Inc.)...

If water vapor and hydrogen are the only gases present, the sum of their partial pressures is equal to the total pressure P = / h2 + H20. For total pressure of 1 bar, the equilibrium constant of reaction (2.20) is expressed as ... 

Water vapor in hydrogen inlet Higher pressures lower water vapor content of gases so when pressure increases water vapor content of hydrogen decreases as shown in the Figure VL34. 

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