In the past 5 years, membrane ultrafiltration has gained increasing prominence as a simple and convenient process for concentrating, purifying, and fractionating solutions of moderate-to-high molecular weight solutes and colloids, and for purifying water and other solvents containing such solutes. The emergence of this new molecular separation technique for both laboratory and industrial applications is almost entirely attributable to the development of a family of uniquely structured polymeric membranes which display extraordinarily high hydraulic permeabilities coupled with the capacity to retain even quite small solute molecules.

Solute polarization and cake formation in membrane ultrafiltration: causes, consequences, and control techniques

Knowledge of particle−liquid mass transfer coefficient (kSL) is important for a rational design of a solid−liquid/solid−liquid−gas stirred tank reactor (STR). Considerable efforts have been made by different investigators to study and correlate the behavior of kSL in STRs for a variety of geometric and operating conditions. This review attempts to put together and analyze the information available in the literature on kSL in STRs systematically. Different approaches used by researchers to correlate kSL are critically discussed. The effect of system configurations (type of the impeller, D/T, and C/T), operating parameters (impeller speed/power input, particle loading, and gas flow rate), and physical properties (dP, Dm, μ, and Δρ) on kSL have been discussed elaborately. Some new data for six-bladed impellers have been generated to determine the optimum geometric configuration and operating conditions for particle−liquid mass transfer in three-phase STRs. It is concluded that kSL in two-/three-phase STRs ca...

Particle-liquid mass transfer coefficient in two-/three-phase stirred tank reactors

Publisher Summary This chapter focuses on the effects of substituents present in a pyridine nucleus upon the position and ease of attack at carbon by a suitable heterolytic or homolytic reagent. Reactions involving pyridine N-oxides and pyridinium salts are discussed. The various theoretical approaches agree with the simple resonance theory representation of the valence structure of pyridine in predicting that nucleophilic substitution should take place readily at the 2-, 4-, or 6-positions but not at the 3- or 5-position. The pyridinium cation should be even more susceptible to nucleophilic attack. Again, differences of opinion exist as to whether the 2- or the 4-position should be more reactive. Some calculations indicate that the attack at C-4 can occur more readily than at C-2, while others predict exactly the opposite, depending not only upon the use of ground state π-electron densities or atom localization energies for predictive purposes, but also upon the values of the parameters used in the calculations. Finally, the chapter illustrates intramolecular cyclizations onto a pyridine ring.

Substitution in the Pyridine Series: Effect of Substituents

https://pure.tue.nl/ws/files/2161195/621185.pdf

Unsteady-state flux behaviour in relation to the presence of a gel layer

Pore size analysis from retention of neutral solutes through nanofiltration membranes. The contribution of concentration–polarization

Studies of mass transfer between solids and liquids for agitated vessels have been reviewed. Data from the literature correlated empirically by the Gilliland-Sherwood correlation, regardless of the actual geometry of the apparatus, in two main categories.



The velocity term in the Re number was defined as the velocity of the rotating device, i.e., impeller, cylinder or rotating annulus. The characteristiclength term was taken as the impeller diameter in the cases of mass transfer from suspended pellets or flat surfaces, the gap-width in the case of rotating cylinders, and the hydraulic radius of the annulus in the case of rotating vessels.



The empirical correlations were found to be in general agreement with the turbulent boundary layer mechanism of mass transfer.



Les etudes relatives au transfert de masse entre solides et liquides dans des recipients sous agitation ont ete examinees. Les valeurs obtenues dans la bibliographie, comparees a l'aide de l'equation de Gilliland-Sherwood, sont exprimees en deux categories principales, quelle que soit la geometrie du recipient.



Le terme “vitesse” dans le nombre de Reynolds est defini par la vitesse de l'impulseur, cylindre ou anneau rotatif. Le terme “diametre” a ete pris comme etant le diametre de l'impulseur, dans les cas de transfert de masse depuis les granules en suspension ou les surfaces planes, la largeur de l'espacement dans le cas de cylindres en rotation et le rayon hydraulique de l'anneau dans le cas de recipients en rotation.



Les correlations empiriques sont en accord general avec le mecanisme de transfert de masse en couche limite turbulente.

A correlation of mass transfer data of solid‐liquid systems in agitated vessels

This paper reports the results of a theoretical study on mass transfer from solid surfaces to fluids in turbulent flow in circular or rectangular conduits. A combined molecular and eddy diffusion mechanism was assumed; three different cases of eddy-distribution functions were considered.

Analytical results were obtained for concentration profiles, mass transfer coefficients and reaction factors in cases of mass transfer without reaction, with rapid chemical reaction, and with first-order chemical reaction. Mass transfer coefficients were shown to vary linearly with . The exponent n increased from 0.5 to unity as eddy motion assumed near the wall decreased to zero. The reaction factor was shown to increase with decreasing turbulence near the interface, with the reactant concentration ratio, CBL/CA1 and with the Schmidt number ratio, ScA/ScB9 to the power n.

J. Marangozis

Papers

A correlation of mass transfer data of solid‐liquid systems in agitated vessels

Mass transfer in turbulent flow with and without chemical reaction