Adding Water Vapor Research Articles

Selectivity to cyclohexene in the gas-phase hydrogenation of benzene over ruthenium

A study has been made of the heterogeneous gas-phase hydrogenation of benzene to cyclohexene and cyclohexane in a continuous-flow apparatus over non-supported Ru, at 0.11 MPa total pressure, in the range of temperatures from 273 to 320 K. When working with highly purified reactants, no formation of cyclohexene is detected. On adding water vapour to the feed a selectivity to cyclohexene is gradually built up over a period of more than 10 h. If Ru is pre-covered with adsorbed water, the selectivity to cyclohexene is effected immediately, and on drying the feed to zero partial water-vapour pressure, the selectivity towards cyclohexene falls away rapidly.

The effect of steam on flame temperature, burning velocity and carbon formation in hydrocarbon flames

The effect on flame temmperature, burning velocity and carbon limit of adding water vapor to a premixed flame has been investigated using a Bunsen-type burner operated at atmospheric pressure and employing propane and ethylene as fuels. The results indicate that water vapor does not act as an inert diluent but instead inhibits carbon formation and gives rise to a greater heat release than in its absence. The additional heat release, in turn, partly counteracts the cooling effect of the added steam, so that the flame temperature and burning velocity do not decrease with steam addition to the extent they would if water vapor were an inert heat sink, provided the equilibrium state is attained. The shift of the carbon limit towards higher C O ratios with steam addition gives evidence of the major role of OH radicals in inhibiting carbon formation.

Atomic beam velocity distributions with a cooled discharge source

Time-of-flight measurements have been made for hydrogen, nitrogen, and metastable helium, argon, and nitrogen beams emerging from a cooled discharge source, in order to determine the velocity distributions in these beams. We observe typically 85% H and 15% H2 relative beam number densities from a hydrogen discharge, and 10% N and 90% N2 relative beam number densities from a nitrogen discharge. The results show the effects of variation of the source gas pressure, microwave power, cooling rate, and in the case of a hydrogen discharge, the effect of adding water vapor to enhance dissociation. Argon seeding of the hydrogen discharge has been investigated as a means of determining the beam temperature without resorting to direct velocity or time-of-flight measurements. Some results are also given for room-temperature and low-temperature beams, obtained without a discharge.

A technique for introducing low concentrations of additives to high pressure gas streams

A technique is described for continuously adding low concentrations of additives to high pressure, low flow rate gas streams. The method utilizes the diffusion of gases and vapours through plastics and avoids the problems of mixing two gas streams at very low flow rates. It is shown to be successful in adding water vapour (0-1000 vpm) to dry carbon dioxide and carbon dioxide (0-100 vpm) to pure helium.

Stratospheric Ozone with Added Water Vapor: Influence of High-Altitude Aircraft

Simple, steady-state models for ozone photochemistry, radiative heat balance, and eddy-diffusive mass transport can be combined to estimate water-induced changes in the stratospheric ozone concentrations and temperatures, the integrated ozone column, the solar power transmitted to the earth's surface, and the surface temperature. These changes have been computed parametrically for mixing fractions of water vapor between 3 x 10(-6) and 6.5 x 10(-6). With added water from the exhausts of projected fleets of stratospheric aircraft, the ozone column may diminish by 3.8 percent, the transmitted solar power increase by 0.07 percent, and the surface temperature rise by 0.04 degrees K in the Northern Hemisphere. Due to a cancellation of terms, temperatures in the lower stratosphere remain essentially unchanged. These results are sensitive to the form of the water profile and emphasize the potential role of convective transients near 30 kilometers.

High-Temperature Evaporation and Thermodynamic Properties of Zirconium Diboride

Vapor pressures of zirconium over zirconium diboride have been measured by the Knudsen technique over the temperature range 2150° to 2475°K. A new type of apparatus was constructed and used successfully in the study. Zirconium diboride was determined to evaporate congruently at a composition of ZrB1.906(+0.025 or —0.010) by heating solid pressed plugs of both zirconium-rich and boron-rich material to constant composition at 2400°C to 2500°C. The over-all reaction is ZrB1.906(s)=Zr(g)+1.906 B(g)Three series of measurements were made using tungsten crucibles and different orifice sizes. Second-law and third-law treatments of the data did not agree. Thermodynamic calculations were made which indicated that water vapor at low background pressures would produce volatile oxides of both zirconium and boron. This reaction was investigated by adding water vapor to the system and the increased transport of zirconium was clearly demonstrated. Accordingly, each pressure measurement was corrected by a factor β, constant for each series, related to the background pressure. From the corrected pressures, values of ΔH0o for the reaction were computed by the third-law treatment. From the three series a vaporization coefficient of 0.025±0.010 was computed, leading to an equilibrium ΔH0o of 458.3±6.5 kcal/mole for reaction 1, or a partial pressure of Zr over ZrB1.906 of 2.38×10—10 atm±20% at 2000°K. The variance between this and an expected value of 477.4 kcal/mole is presumed to be related, at least in part, to discrepancies in the heat of vaporization of zirconium. The results verify the prediction that the Zr–B system exhibits a congruently evaporating phase and suggest that the congruently evaporating composition in many high-temperature systems will occur at nonstoichiometric compositions.

Atomic hydrogen II—surface effects in the discharge tube

In the production of atomic hydrogen by means of the glow discharge, Wood (1921) found that a long discharge tube, e. g. 2m., was desirable, and that a trace of water vapour (or oxygen) in the hydrogen was necessary in order to obtain appreciable yields. Langmuir (Wood 1922) suggested that the effect of water vapour was to form on the walls of the tube a film very much less active than the glass surface as a catalyst for the recombination reaction H + H→H 2 . Wood also suggested that the beneficial effect of using a long tube was due to the removal of the catalytically active electrodes to a con­siderable distance from the point of emergence of the gas (usually near the middle of the tube). Bonhoeffer (1925) recommended a long preliminary discharge to “burn out” active areas of the glass surface. Wrede (1929) found that preliminary cleaning of the tube with chromic acid and hydrofluoric acid was desirable, v. Wartenberg and Schultze (1930) showed that a coating of meta-phosphoric acid inside the discharge tube, if used in conjunction with moist hydrogen, resulted in a great improvement in yield, presumably owing to the hygroscopic character of the acid. The latter workers also studied quantitatively the effect of increasing the amount of added water vapour, and showed that the atom output for a constant current rose rapidly from a very low value until about 2% of water vapour had been added, after which the yield was practically constant. It has been customary, therefore, to introduce about 2·5% of water vapour, and this is very easily done by bubbling the hydrogen through water at atmospheric temperature and pressure.