Jer Ru Maa

Removal of copper ions and dissolved phenol from water using micellar-enhanced ultrafiltration with mixed surfactants ☆

A laboratory study was conducted to evaluate the effects of composition and concentration of mixed anionic/nonionic surfactants on the efficiency of a micellar-enhanced ultrafiltration (MEUF) operation in removing metal ions/organic solutes from aqueous solutions. Based on the analysis of surface tensions and micelle sizes, it was found that for mixed sodium dodecylsulfate (SDS)/Triton X-100 surfactants, the critical micelle concentration (cmc) was significantly lower than that of SDS and mixed micelles formed. The mixed surfactant system was then applied in a cross-flow mode of MEUF, in which the concentration polarization can be neglected, to remove Cu2+ from aqueous solutions. With a surfactant concentration of 10 mM, the Cu2+ rejection was negligible by using pure Triton X-100 and increased with increasing SDS mole fraction with a value as high as 85%, which suggests that the rejection of Cu2+ was due to the electrostatic attraction between Cu2+ and SDS. Furthermore, pronounced Cu2+ rejection was obtained for a 5 mM SDS solution, which was attributable to a decrease in the cmc of SDS by the existence of Cu2+. When the MEUF technique was applied to remove Cu2+ and phenol simultaneously from aqueous solutions, the Cu2+ rejection was slightly enhanced in the presence of phenol. However, the rejection of phenol was comparatively low, approximately 27%, which may be caused by its relatively hydrophilic characteristic.

Bubble coalescence in dilute surfactant solutions

Abstract In order to understand the behaviors of the stretching gas-liquid interface and the effects of surfactants, the coalescence of nitrogen bubble pairs issued from neighboring nozzles submerged in water has been studied by means of high-speed photography. It is shown that small amount of surface active additive is able to increase the coalescence time greatly. The transition from 0 to 100 percent bubble coalescence during their stay at the nozzle mouths happens over a very narrow concentration range. The central value of this concentration range increases with the increase of bubbling frequency, which is proportional to the heat flux in the case of nucleate boiling.

Drop size distribution and heat flux of dropwise condensation

Abstract A population balance equation is derived for dropwise condensation which considers both the drop growth due to direct condensation and the coalescence between drops. This equation is solved numerically to obtain drop size distributions for various conditions. The agreement between the calculated results and the available experimental data is satisfactory.

Surface diffusion and heterogeneous nucleation on solid substrates

Abstract The adsorption of a vapor on a chilled solid surface and the growth of the condensate crystals are studied by solving the two-dimensional diffusion equation and the population balance equation. This enables one to compute the growth rate and size distribution of the crystals if various vapor-crystal-substrate interfacial properties are available. These properties are: the sticking coefficient of the vapor molecules on the crystal surface, the crystal-substrate contact angle, the two-dimensional diffusion coefficient, the root mean square diffusion distance of the adsorbed monomer, and the equilibrium concentration of the adsorbed monomer. Some of these properties may be estimated based on the experimentally measured growth rate and size distribution of the crystals. It is shown that, for the case of the condensation of iodine vapor on glass, the diffusion of the adsorbjed iodine molecules is more important than the direct addition of the vapor molecules to the growth of the crystals when they are small. The contribution of surface diffusion is still significant when the crystal are 1 mm in size. The sticking coefficient of iodine molecules on its own crystal surface is also estimated.

Lower limit of the possible nucleate pool-boiling enhancement by surfactant addition to water

The heat transfer of saturated nucleate pool boiling in aqueous solutions of Triton SP-190 and SP-175 on a horizontal tubular heater was experimentally investigated. Equilibrium and dynamic surface...

Adsorption and Nucleation on Smooth Surfaces

The nucleation and growth of condensate nuclei on smooth surfaces, e.g., an immiscible liquid or a smooth solid, can occur both by the direct addition of molecules from the vapor and from those adsorbed on the substrate. We show how to generalize nucleation theory to allow for the simultaneous occurrence of both mechanisms. The vapor-condensate-substrate interfacial forces, the contact angle, the critical supersaturation, and the coefficient in the adsorption isotherm are different ways of expressing the affinity between vapor molecules and the substrate surface. The critical supersaturations for nucleation on the surface of an immiscible liquid and nucleation on the surface of a perfectly smooth solid are predicted in terms of these parameters and the relationships among them. For most values of these parameters we find that adsorbed molecules are usually far more important to the nucleation process than those in the vapor phase.

Adsorption and nucleation of water vapor on smooth solid substrates

Abstract The critical supersaturation, ( p p e ) c , values for the condensation of water vapor on various smooth organic substrates and the values of contact angle, θ, between water and these substrates were determined by simple experimental techniques. The experimental relationship between ( p p e ) c and θ agrees satisfactorily with the classical nucleation theory of spherical cap model. In the theoretical calculation, both the direct addition of the vapor molecules and the indirect addition of the adsorbed molecules by two-dimensional migration to the condensate nuclei were considered; and it was assumed that the adsorbed vapor molecules on the substrate surface behaved like two-dimensional gas. The coefficients ka in the adsorption isotherm and the contact angle are essentially two different ways of expressing the affinity between the vapor molecules and the substrate surface. The theoretical relationship between ka and θ based on the classical nucleation theory is also presented.

The role of interfaces in heat transfer processes

Abstract An understanding of the role of interfaces between phases is essential to true understanding of many heat transfer processes. In the cases of evaporation ion and condensation in single component systems, the evaporation or condensation coefficient represents the transmitance of the vapor-liquid interface itself or the maximal possible rate of interfacial heat as well as mass transfer. Jet tensimetry is a simple but powerful technique for the study of the resistance of the interface in the evaporation and condensation processes, with or without chemical reaction, involving the interface between phases. This technique also eliminates most of the complications of the bulk phases on both sides of the interface. Get tensimetry and other recent experimental data verify that the evaporation coeffecients of water and other common liquids are unity or nearly unity; the interface does not constitute a significant resistance to the heat transfer processes. Boiling and condensation are two processes with very high heat transfer coefficients, especially in the cases of nucleate boiling and dropwise condensation. An examination of the vapor-liquid interface under equilibrium conditions shows that there are many similarities between these two processes from the thermodynamic point of view. However, since the creation of a new phase is a problem of stability, more attention should be paid to the dynamic rate of the nucleation of the vapor bubbles and condensate droplets. The solid heating and cooling surfaces introduce more complications because they are usually not smooth and homogeneous. Vapor bubbles and condensate droplets are often generated at the position of active sites. Because most of the solid heat transfer media are hydrophilic, the nucleation theory shows that from the point of view of interfacial phenomena: 1) nucleate boiling is usual but film boiling is exceptional: and 2) film condensation is usual but dropwise condensation is exceptional. Dropwise condensation can be realized by making the solid cooling surface hydrophobic with the help of promoters. Its mechnism is complicated but relactively well understood after the experimental studies and theoretical simulation carried out by many authors. Their conclusions are discussed with emphasis on the solid-liquid-vapor interfacial properties. The condensation on the surface of an immiscible liquid is also discussed because of its similarity to dropwise condensation. The treatment of dropwise condensation problems by nucleation theory and statistical methods is also introduced.

Effects of polymer additives on pool boiling phenomena

Abstract The addition of small amount of soluble polymer makes the boiling behavior significantly different from that of pure water. The polymer additives reduce the tendency of coalescence between vapor bubbles. Consequently, they become smaller in size, larger in number, and tend to stay on the heating surface in a relatively orderly manner. No significant improvement in heat transfer rate caused by the polymer additives was observed in this work; the critical burn out heat flux was reduced slightly.

Drop formation from flat tip nozzles in liquid-liquid system

Correct estimation of interfacial area for heat or mass transfer is of primary importance in liquid-liquid contacting systems. But the existing correlations for the estimation of the sizes of drops formed by nozzles at low flow rate are based on experimental data using nozzles with tips filed down to sharp or bevel or a very thin thickness, and the inside diameters are used as the characteristic diameters. These correlations would underestimate the size of aqueous drops formed from flat tip metallic nozzles considerably, because as the drops are detached from the nozzle tips, the liquid-liquid-solid contact lines are not just located at the inside edge of the opening. In this work, the formation of drops of aqueous solutions of NaCl and AlCl3 from flat tip stainless steel nozzles of various dimensions into a pool of n-dodecane was studied experimentally. It was found that the deviation of the estimated drop sizes by the correlation of Kagan differs only slightly from that of Scheele and Meister. The deviations of the estimated values are not affected by the dissolved salts within the range of concentrations studies. And most importantly, the mean deviation of the correlation of Scheele and Meister can be reduced to a few percents if different characteristic diameters, dn, are used in the computation: dn = O.D. for the cases of relatively large nozzles of about 1cm O.D. with wall thickness between 0.098 and 0.170cm; dn = 1.15×O.D. for small thin wall nozzles of O.D. < 0.35 cm with wall thickness < 0.036cm.