Gas Phase Saturation Research Articles

Fluid phase equilibria in supercritical fluid chromatography with CO2‐based mixed mobile phases: A review

AbstractIn supercritical fluid chromatography (SFC), cosolvents (modifiers) have been added to CO2 in order to deactivate the analytical column and to improve the solvating power of CO2 for extending the utility of SFC to polar and high molecular weight solutes. However, the use of modifiers has proven to be difficult, and irreproducibility in retention and selectivity is often observed.This comprehensive review focuses on the phase behavior of analytically useful CO2‐based mixed mobile phases in SFC. All of the CO2‐based mixed mobile phases which have been used in packed or open tubular column SFC are reviewed. All of the available fluid phase equilibria data for such binary fluids are discussed, and the P‐T‐x region which is applicable to SFC is compiled in both graphs and tables. Single phase regions (pressure and temperature) are listed by mol% modifier. However, only mixtures of CO2 + benzene, + ethanol, + hexane, + methanol, or + toluene have enough P‐T‐x data points to enable extrapolation to any other conditions used in conventional SFC (0–20 mol% modifier, 40–400 atm, and 0–200°C). Problems associated with estimating the critical parameters of the pure components or the mixtures are reviewed. Phase transitions in the syringe pump, transfer lines, injector, column, and restrictor are delineated. Equilibration problems associated with the preparation of mixed mobile phases (premixed cylinders, liquid or gas phase saturation, and dual pumps with on‐column mixing) and phase transitions during the operation of SFC instrumentation are discussed. Finally, future research needs are outlined.

The hydrodynamics of trickling flow in packed beds. Part II: Experimental observations

AbstractThe liquid holdup and the pressure drop for two‐phase trickling flow in a packed bed were measured experimentally. Different values of those parameters were obtained as the liquid flow rate was increased and then decreased, indicating a multiplicity in hydrodynamic states. This behavior was observed even when the gas flow rate was zero in the bed. It was determined that the hysteresis exhibited by the process is due to imperfect wetting of the packing and to the difference between advancing and receding contact angles at the gas‐liquid‐solid contact lines. The reduced wetting conditions are also the cause of the increase in liquid holdup observed when the surface tension of the liquid is decreased. In this case, the amount of liquid retained in the bed is controlled by the extension of the wetted regions due to the more favorable contact angles as the surface tension decreases. The transition from the trickling to the pulsing flow regime was also dependent on the history of the process.A new correlation for predicting liquid holdups and pressure drops for trickling flow in packed beds is proposed for the decreasing liquid flow rate operating mode. This correlation is based on the experimental determination of the liquid phase relative permeability as a function of the the liquid phase reduced saturation and the determination of the gas phase relative permeability as a function of the gas phase saturation and the gas phase Reynolds number. The new data are analyzed in the light of the theory developed in the first part of this paper.

Undersaturation dependence of the evaporation rate of potassium chloride

Theories of ledge dynamics for crystals evaporating by the stepwise mechanism predict a change from crystal edge control to dislocation control of the evaporation kinetics as the gas phase saturation is reduced. This effect was studied by measuring the rate of evaporation from the (100) face of high purity KCl crystals vs gas phase saturation with a specially designed spherical effusion cell of variable orifice area. The experiments minimized or eliminated variation in sample temperature, area, dislocation density, and purity and gave results of sufficient precision to confirm the theoretical predictions. Ledges arising from dislocations do not influence the evaporation rate of KCl if the incident to equilibrium flux ratio (Vi/Veq) exceeds 0.93±0.02, at 460 ° C. This conclusion is based on an observed change in slope of evaporation rate versus incident flux at a value of Vi/Veq consistent with the theory of stepwise evaporation and independent estimates or measurements of surface energies, surface reaction activation energies, and ledge spacings on evaporated crystals.

The viscosity of fluid mercury to 1520 K and 1000 bar

AbstractThe viscosity of liquid and gaseous mercury has been measured at saturation conditions and in the expanded gas phase to 1520 K and nearly 1000 bar. Measurements were performed with an oscillating cup viscometer. Small cylindrical, closed autoclaves (“cups”) of tungstenrhenium or pure molybdenum were suspended inside a heated vacuum chamber on tungsten rhenium wires.The viscosity at 1520 K is 0.58mPa · s in the saturated liquid and 0.16 mPa · s in the saturated gas phase as compared with 2.0 mPa · s for liquid Hg at the melting point. Results are discussed in terms of a modified Enskog hard‐sphere transport theory for dense fluids. An internally consistent description of data is obtained with temperature‐dependent hard‐sphere diameters of Hg atoms and ions derived from PVT and neutron scattering data. A viscosity curve for mercury along the whole coexistence line to the critical point (Tc = 1760 K) is given. The analysis is applied to correlate data for fluid Hg, Pb and Na.

Green emitting Zn-diffused LPE GaP diodes

The growth of liquid phase epitaxial (LPE) layers of n-type GaP on GaP substrates was investigated using a multi-wafer growth system constructed of fused quartz which had provisions for gas phase saturation and doping of the gallium melt. The effect of growth temperature, substrate orientation, doping level, and repeated use of the same melt on the properties of zinc diffused electroluminescent diodes fabricated from these epitaxial layers were investigated. After an initial increase, the carrier concentration remained relatively constant (1.7 × 1017/cc) throughout a series of eighteen runs from the same melt. Using conventional commercial techniques, zinc diffused diodes with efficiencies of 0.05% at 30A/cm2 and a brightness of 1200 fL at 10A/cm2 were produced. These diodes had a limited area n and p-type contact and had an epoxy dome. Layers grown on the 〈 111 〉P orientation had the best surface quality whereas those grown on the 〈 100 〉 plane incorporated less background impurities. The use of relatively low growth starting temperatures (∼ 920°C) was found to minimize the background impurity of the layers and the substrate surface deterioration due to the reaction with ammonia.

Simultaneous heat and mass transfer from multicomponent condensing vapor‐gas systems

AbstractMulticomponent vapor condensation in the presence of noncondensing gas has been analyzed using numerical solutions to a set of linearized rate equations that describe the simultaneous heat and mass transfer to the condensate film. For specific examples of methanol and water and ethanol and toluene condensing from turbulent air streams in vertical tube and on horizontal tube condensers, it is shown that the alcohol concentrations in both the liquid and gas phases increase as condensation from initially superheated, alcohol‐rich gas phases progresses; the liquid phases also become richer in alcohol. The mass fluxes of the vapors vary with their respective mass transfer coefficients and local concentration driving forces. In the vertical tube condensers, the flux of the alcohols go through maximums. Gas phase saturation is approached as the mixtures cool and the final traces of vapor become more difficult to condense. Interface properties are computed from limited VL equilibrium data, heat of solution data, and a predictive equation. The techniques of solution and analysis have general application to the design of cooler‐condensers in either vertical or horizontal configurations.

An apparatus for thermometric determination of the activity of water in solutions and in biological fluids.

An apparatus for thermometric determination of the activity of water in solutions and in biological fluids is described, based on measurement of the temperature differences between the solutions or fluids and water suspended on thermistors in a saturated gas phase with forced convection. Thc precision and accuracy of the equipment is evaluated by determining the activity of water in sodium chloride solutions. The ratio between the measured temperature differences and the differences of the activity of water in these solutions was in this study found to be constant, extending the rangc of concentration in which Raoult's law is valid.

Effect of temperature on "saturation currents" in gas phase radiolysis cells: the conductance of Pyrex and other cell materials

When gas-filled Pyrex cells were heated above about 50°, the saturation currents obtained in a gamma radiation field apparently decreased. At temperatures above 100° good saturation currents were not obtained. At temperatures below 50° the conductance of Pyrex was negligible, and "true" saturation currents were measured. At temperatures greater than 50° the Pyrex conducted appreciably, so the total current flowing in the circuit was appreciably greater than the gas phase saturation current (absorbed dose rate 1010–1013 eV/ml s). If the outer surface of the cell was grounded, for example by ionized air, part of the current by-passed the measuring ammeter, thereby decreasing the "apparent" saturation current. The magnitude of the temperature effect was greatly decreased by wrapping the cell with teflon tape, thereby insulating it from the ionized air.The effect of temperature on apparent saturation currents measured in quartz cells was much smaller than that in Pyrex cells, because the conductance of quartz was much less than that of Pyrex at temperatures above 50°.When a metal cell that contains a Pyrex insulator between the electrodes is used, the temperature of the Pyrex should be kept below about 50°.

Some features of the gas-phase saturation of a two-phase boundary layer

In [1] the author reported that, as follows from the hydrodynamic theory of burnout, the processes of boiling and bubbling are physically related with respect to the general development of the critical transition from a nucleate to a film boundary layer structure. This paper presents certain results of a study of the variation of the gas content of a two-phase boundary layer in both boiling and bubbling through a porous plate over a wide range of loads.