ABSORPTION AND DESORPTION TO AND FROM LIQUID FILMS ON INCLINED PLANES

Two published theoretical models are examined and applied to experimental results for absorption and desorption. The system used was CO₂/H₂O and studies were made for liquid film flow down inclined planes. Experimental results give “Reduced” values of mass ransfer rates.

Interferometric studies give interfacial concentration, penetration and film depths, and take-up of carbon dioxide. In the case of desorption the interferograms are distorted by “deflections.”

All the experimental values for absorption and desorption differ from those calculated from theoretical models.

Desorption is not a mirror image of absorption, and it is approximately 75% of the transfer rate of absorption over a wide operating range.

A comparison is made of the behaviour of static pools and flowing liquid films.     

HEAT EFFECTS FOR PHYSICAL AND CHEMICAL ABSORPTION IN TURBULENT LIQUID FILMS

An eddy diffusivity model was used to describe simultaneous heat and mass transfer for chemical absorption in turbulent liquid films. For absorption accompanied by a first-order reaction an approximate expression for the mass transfer coefficient is derived and shown to be in excellent agreement with exact numerical calculations. An equation is developed for the temperature rise at the free surface due to both the heat of reaction and the heat of solution. Relationships are also developed for the concentration of the liquid phase product at the free surface and the depletion of the liquid phase reactant at the free surface.

The temperature rise at the free surface for gas absorption accompanied by an instantaneous reaction due to both the heat of solution and the heat of reaction was determined. An equation is also derived for the concentration of liquid phase product at the free surface for the case of an instantaneous reaction.     

HEAT EFFECTS FOR PHYSICAL AND CHEMICAL ABSORPTION IN TURBULENT LIQUID FILMS

An eddy diffusivity model was used to describe simultaneous heat and mass transfer for chemical absorption in turbulent liquid films. For absorption accompanied by a first-order reaction an approximate expression for the mass transfer coefficient is derived and shown to be in excellent agreement with exact numerical calculations. An equation is developed for the temperature rise at the free surface due to both the heat of reaction and the heat of solution. Relationships are also developed for the concentration of the liquid phase product at the free surface and the depletion of the liquid phase reactant at the free surface.

The temperature rise at the free surface for gas absorption accompanied by an instantaneous reaction due to both the heat of solution and the heat of reaction was determined. An equation is also derived for the concentration of liquid phase product at the free surface for the case of an instantaneous reaction.     

ON THE MECHANISM OF CO2 ABSORPTION AND DESORPTION

Recent work by Thomas et al. (1982) showed a significant difference in mass transfer rates between absorption and desorption for a CO₂-water system in a film flow, and attributed the phenomena to density-driven convection. Our experiment with a flat-surface stirred cell shows no enhancement of oxygen absorption in simultaneous absorption with CO₂ under a turbulent flow condition. Also, a comparison of liquid velocities measured extremely close to the gas-liquid interface with a laser doppler anemometer reveals negligible difference between absorption of air and that of CO₂. These results indicate that there is practically no detectable density-driven convection in CO₂ absorption.     

DLVO AND NON-DLVO FORCES IN THIN LIQUID FILMS FROM RHAMNOLIPIDS

Microscopic foam films (r?=?100?μm) stabilized with a single rhamnolipid with a well-known structure (α-L-rhamnopyranosyl-β-hydroxydecanoyl-β-hydroxydecanoate (R1)) are investigated, and the obtained results are compared with results obtained from studies of foam films formed from solutions of rhamnolipid mixtures. The studies are carried out employing the Scheludko-Exerowa microinterferometric method. The dependence of foam film thickness on the electrolyte concentration (C _el ) of the solution is monitored, and formation of common films (CF), common black films (CBF) and Newton black films (NBF) is found. The continuous CBF-to-NBF transition is considered as evidence of the action of repulsive forces that are not described by the classic Derjaguin–Landau–Verwey–Overbeek (DLVO) theory of colloid stability. These non-DLVO repulsive forces lead to an additional positive component of the disjoining pressure. To understand better the surface forces operating in the rhamnolipid foam films, direct measurements of the disjoining pressure/film thickness (Π(h)) isotherms are carried out employing the thin liquid film–pressure balance technique. The comparison of the obtained experimental Π(h) isotherm for CF (C _el ?=?10^?3 mol dm^?3 NaCl) to the DLVO theoretical predictions yields a diffuse electric layer potential of?～?5?mV and surface charge density of?～?50?mC?m^?2 at the film solution–air interfaces. The deviation of the experimental curve from the theoretical one found for films thinner than about 40?nm evidences the action of non-DLVO surface forces. The experimental steplike Π(h) isotherms obtained for the CBF (C _el ?=?0.15?mol?dm^?3 NaCl) are considered to result from an aggregation process, leading to the formation of lamellar structures in the foam film. The obtained results show that the surface forces operative in rhamnolipid foam films are determined not only by the structure and organization of the adsorbed layers but also by the molecular state of the bulk solution.     

GAS ABSORPTION INTO WAVY AND TURBULENT FALLING LIQUID FILMS IN A WETTED-WALL COLUMN

Experiments are conducted for gas absorption in a long wetted-wall column. Liquid-side mass transfer coefficients are measured for absorption of CO₂ and O₂ into falling water films on the outside of a stainless steel pipe 2.72 cm OD and 183 cm absorption length. The liquid film Reynolds number ranges from 129 to 10500 which encompasses the wavy-laminar, wavy-transition and turbulent flow regimes. The experimental data are correlated by a dimensionless equation of the form kt = (k_tD) (v²/g) ^1/3 = a-Re^p-Sc^1/2. The correlation is well supported by a viscosity-damped turbulence model at the gas-liquid interface which tends to confirm that viscosity is probably the major mechanism causing eddy damping and not surface tension as proposed by Levich and Davies. The form of the above correlation also represents previous experimental work at different temperatures and for different gases quite well.     

GAS ABSORPTION INTO WAVY AND TURBULENT FALLING LIQUID FILMS IN A WETTED-WALL COLUMN

Experiments are conducted for gas absorption in a long wetted-wall column. Liquid-side mass transfer coefficients are measured for absorption of CO₂ and O₂ into falling water films on the outside of a stainless steel pipe 2.72 cm OD and 183 cm absorption length. The liquid film Reynolds number ranges from 129 to 10500 which encompasses the wavy-laminar, wavy-transition and turbulent flow regimes. The experimental data are correlated by a dimensionless equation of the form kt = (k_tD) (v²/g) ^1/3 = a-Re^p-Sc^1/2. The correlation is well supported by a viscosity-damped turbulence model at the gas-liquid interface which tends to confirm that viscosity is probably the major mechanism causing eddy damping and not surface tension as proposed by Levich and Davies. The form of the above correlation also represents previous experimental work at different temperatures and for different gases quite well.     

GAS ABSORPTION INTO WAVY AND TURBULENT NON-NEWTONIAN FALLING LIQUID FILMS IN A WETTED-WALL COLUMN

Average Him thickness and liquid-side mass transfer coefficient for the absorption of oxygen in wavy and turbulent power-law model falling liquid films are measured. Considerable reduction in (he film thickness with resulting increase in the mass transfer coefficient are obtained as compared to that predicted by the theoretical laminar flow equation assuming no slip at the wall.     

STABILITY OF AXISYMMETRIC WAVES ON LIQUID FILMS FLOWING DOWN A VERTICAL COLUMN TO AZIMUTHAL AND STREAMWISE DISTURBANCES

Axisymmetric film waves on the outside surface of a vertical column have been observed to be more susceptible to azimuthal disturbances as the column radius increases. We scrutinize this phenomenon here and show that, for liquids with high surface tension, infinitesimal azimuthal disturbances first become unstable when the flow rate per unit azimuthal length exceeds (5συ/6pr ² g) where r is the column radius and σ is the interfacial tension. However, even beyond the onset of these azimuthal disturbances, only long, finite-amplitude, streamwise axisymmetric waves are unstable to such disturbances. We also show that, in contrast to earlier results, relatively short waves near the neutral curve are unstable to the classical streamwise Eckhaus sideband instability. There is a window of streamwise wavenumbers, including the wave number k_m corresponding to the maximum growth rate, that is stable to both instabilities. This window is reminiscent of the “Busse balloon” in Rayleigh-Benard instability and we study their size and shape as a function of column radius. In all cases, finite-amplitude axisymmetric waves with the maximum growing streamwise wavenumber k_m are stable to both sideband instabilities. This then suggests that there are two necessary conditions for the appearance of finite-amplitude azimuthally varying waves—the destabilization of the infinitesimally small azimuthal modes and the nonlinear excitation of axisymmetric waves longer than the maximum-growing ones. We suggest that subharmonic instability of the maximum-growing waves can provide the second mechanism.     

THE SIGNIFICANCE OF END-EFFECTS ON THE MEASUREMENT OF MASS TRANSFER TO A FLOWING LIQUID ON AN INCLINED PLANE

The nature of the hydrodynamic flow of water down an inclined plane in the presence of static gas is discussed. The effects of liquid flow rates and angles of inclination of the plane on the film thickness and ripple inception are recorded. The ripples differ in number and character from those usually associated with liquid flow on a vertical wall

Mass transfer of carbon dioxide into a water film on an inclined plane is determined more accurately than hitherto by introduction of modifications to the apparatus and the use of novel methods for determining end-effects. This makes possible more meaningful comparisions between experimental results and model theory. Penetration concentration profiles in the falling liquid film are presented     

TEMPERATURE-PROGRAMMED DESORPTION OF MOLECULES FROM SOLID SURFACES: EFFECTS OF SURFACE SITE DISTRIBUTION AND LATERAL INTERACTIONS ON DESORPTION SPECTRA

The effects of surface site distribution are examined and compared using thermal desorption techniques (in particular, Monte Carlo simulation schemes to simulate temperature-programmed desorption). Specifically, the effects of lateral interactions among adatoms, surface heterogeneity, initial surface coverage, and desorption energy have been studied. Surface heterogeneity effects have been examined for desorption of a single species only. The effects of lateral interactions on the desorption spectra for different types of site distributions (in this case, a 10-site patch distribution, an alternating site distribution, random site distributions, and for a cluster of molecules on the center of the surface surrounded by molecules of a different type) are studied. Lateral interactions among adatoms provided a broad range of results depending on sign and magnitude of the interaction. It is also found that as the site energy increased, a molecule was held to the surface much more strongly, and observed desorption did not occur until temperature increased. Results are shown for a single layer distribution.     

TEMPERATURE-PROGRAMMED DESORPTION OF MOLECULES FROM SOLID SURFACES: EFFECTS OF SURFACE SITE DISTRIBUTION AND LATERAL INTERACTIONS ON DESORPTION SPECTRA

The effects of surface site distribution are examined and compared using thermal desorption techniques (in particular, Monte Carlo simulation schemes to simulate temperature-programmed desorption). Specifically, the effects of lateral interactions among adatoms, surface heterogeneity, initial surface coverage, and desorption energy have been studied. Surface heterogeneity effects have been examined for desorption of a single species only. The effects of lateral interactions on the desorption spectra for different types of site distributions (in this case, a 10-site patch distribution, an alternating site distribution, random site distributions, and for a cluster of molecules on the center of the surface surrounded by molecules of a different type) are studied. Lateral interactions among adatoms provided a broad range of results depending on sign and magnitude of the interaction. It is also found that as the site energy increased, a molecule was held to the surface much more strongly, and observed desorption did not occur until temperature increased. Results are shown for a single layer distribution.     

WAVE EFFECT ON THE TRANSFER PROCESSES IN LIQUID FILMS

The superimposed oscillations method for generating waves of many different types was used in the laboratory to study the wave effect on the desorption of carbon dioxide from falling liquid films. The mechanisms for heat mass transfer augmentation by waves were analyzed, taking into consideration the processes of absorption for a dilute gas, non-isothermal absorption, evaporation, and condensation.     

WAVE EFFECT ON THE TRANSFER PROCESSES IN LIQUID FILMS

The superimposed oscillations method for generating waves of many different types was used in the laboratory to study the wave effect on the desorption of carbon dioxide from falling liquid films. The mechanisms for heat mass transfer augmentation by waves were analyzed, taking into consideration the processes of absorption for a dilute gas, non-isothermal absorption, evaporation, and condensation.     

EFFECTS OF SURFACTANT ADSORPTION—DESORPTION KINETICS AND INTERFACIALRHEOLOGICAL PROPERTIES ON THE RATE OFDRAINAGE OF FOAM AND EMULSION FILMS

An analysis has been developed to describe the rate of drainage of an axi-symmetric plane-parallel foam and emulsion films associated with bubble-bubble and drop-drop coalescence phenomena. The present analysis extends the earlier work of Zapryanov et al(1983) and of others which was limited to the diffusion controlled case of the surfactant transport from the bulk phase onto the interface. Here the surfactant transport from the bulk phase to the interface is accounted for by a two step mechanism: (i) diffusion of the surfactant from the bulk to the sublayer, followed by (ii) the adsorption of the surfactant from the sublayer onto the interface. The results of a parametric study indicate that the rate of drainage of surfactant stabilized films is strongly dependent upon the surfactant adsorption-desorption kinetics, selective surfactant solubility, and interfacial Theological properties such as elasticity and interfacial viscosity. Predictions of the film drainage time are compared with our experimental data to establish the range of the applicability of the present model with respect to film radius.     

PHYSICAL ABSORPTION AND SURFACE RESISTANCE IN A FLOWING LIQUID

Absorption of carbon dioxide by water films flowing in a channel inclined at angles up to 5^° to the horizontal has been carried out. Gas take-up and concentrations in the liquid were obtained by chemical analysis and the use of an interferometer based on the Michelson principle. The concentrations were related to the gas-liquid contact times. Application of theoretical equations to the results is reported. The conclusion is that under all conditions a surface resistance exists which is time dependent and of an appreciable magnitude.     

PHYSICAL ABSORPTION AND SURFACE RESISTANCE IN A FLOWING LIQUID

Absorption of carbon dioxide by water films flowing in a channel inclined at angles up to 5^° to the horizontal has been carried out. Gas take-up and concentrations in the liquid were obtained by chemical analysis and the use of an interferometer based on the Michelson principle. The concentrations were related to the gas-liquid contact times. Application of theoretical equations to the results is reported. The conclusion is that under all conditions a surface resistance exists which is time dependent and of an appreciable magnitude.     

水平管外壁液膜流动状态及其对传热的影响

采用智能化薄膜厚度测试仪对水平管外液膜流动状态进行了研究 ,得到液膜厚度的几率分布和平均厚度 .液膜的平均厚度不仅随液体负荷的增加而加大 ,而且与管外壁不同角度有关 ;液膜的波动状态受制于液体负荷与管径的共同影响 .定义了量纲 1波高 ,并通过实验数据回归得到量纲 1波高的经验关系式 .还进行了初步的传热实验 ,得到水平管外蒸发侧传热膜系数随量纲 1膜厚和量纲 1波高的变化 ,为进一步揭示蒸发侧传热的决定因素提供实验依据     

射频磁控溅射法在玻璃上制备紫外吸收和透明导电双功能薄膜

制备了SnO2:Sb(6%,摩尔分数)靶材和一系列不同摩尔比的CeO2-TiO2靶材;以所制靶材用射频磁控溅射法在普通玻璃基片上沉积了CeO2-TiO2单层和CeO2-TiO2/SnO2:Sb双层薄膜.用紫外-可见光谱、Raman光谱、X射线光电子能谱和X射线衍射对CeO2-TiO2薄膜进行了表征.结果表明:在CeO2/TiO2摩尔比为0.5:0.5和0.6:0.4时沉积得到的CeO2-TiO2薄膜为非晶态结构,并具有高的紫外吸收(平均值＞99%)和高的可见光透过率(平均值＞80%).在CeO2-TiO2薄膜表面存在Ce4 ,Ce3 和Ti4 .对CeO2-TiO2/SnO2:Sb双层薄膜,除具有高的紫外吸收(平均值＞99%)和可见光透过率(平均值＞75%)性能外,还具有导电性,方块电阻在80～90 Ω/口之间,电阻率在(3.2～3.6)×10-3Ω·cm之间.该双层镀膜玻璃具有截止紫外线和透明导电双功能.     

液相原料制备PTC粉体及其性能研究

本文介绍了一种通过液相原料形式引入以草酸盐共沉淀来制备ＰＴＣ粉体的新方法。即ＢａＴｉＯ３、Ａｌ２Ｏ３、ＳｉＯ２、ＭｎＯ２、Ｙ２Ｏ３等均采用液相原料引入,然后用草酸进行共沉淀,通过灼烧制得ＰＴＣ粉体。用这种粉体制得的ＰＴＣ陶瓷,其晶粒均匀性和电性能都优于传统固相法制得的ＰＴＣ陶瓷。