R. Semiat

The roles of gas bubbling, wall crystallization and particulate deposition in CaSO4 scale formation

Abstract The objective of this work was to seek conditions leading to rapid deposition of a tenacious calcium sulfate scale layer from a falling film flowing on a heated stainless steel surface. The main results were as follows: (A) The rate of calcium sulfate scale deposition was considerably enhanced (by a factor of 5–10) when boiling occurred on the film flowing on the heat transfer surface; (B) Filtration of suspended calcium sulfate particles from the feed solution led to a very large reduction (by a factor of 20–30) in the rate of scale deposition. This result is entirely opposite to that encountered in the formation of calcium carbonate scales where particle filtration greatly enhances the rate of deposition of a calcium carbonate scale layer and increases considerably scale tenacity. These observations suggest that the dominant mechanisms in calcium sulfate formaiton is particulate deposition while the dominant mechanism in calcium carbonate scale formation is wall crystallization

Parameters affecting the properties of dynamic membranes formed by Zr hydroxide colloids

Membranes exhibiting separation properties similar to those of conventional UF membranes can be dynamically prepared on a porous substrate by in-situ deposition of particles from a circulating colloidal suspension of certain hydrous oxides, notably Zr hydroxide. The objective of this work is to elucidate parameters affecting Zr hydroxide colloidal properties in order to determine optimal conditions for dynamic membrane (DM) formation on substrates of different porosity. The main parameters studied were the effects of pH, Zr concentration and ionic strength on colloidal properties. The main results were as follows. Over the pH range from 3 to 4, a stable mono-disperse colloidal dispersion was obtained, composed of sub-micron particles. The particle size increased systematically with the pH level of the suspension. At a pH value of near 4, a bi-modal distribution was obtained, composed of both sub-micron particles and very large agglomerates. At pH values higher than about 4.5, large agglomerates formed instantly. Particle agglomeration was also affected by the ionic strength of the solution and the procedure of the colloid preparation. The above results find full support in the flow and separation properties displayed by DM prepared with Zr suspensions of different chemistries (notably the pH level) on two types of substrate: a porous fabric having a pore size of tens of microns and a microfiltration membrane having a pore size of a few microns.

Polymerization of isobutylene and copolymerization of isobutylene with isoprene promoted by methylalumoxane

Abstract Methylalumoxane (MAO) is an active pre-catalyst for the polymerization of isobutylene and the copolymerization of isobutylene with isoprene at ambient temperature. The absence of any proton-containing substances suggests that the solvent dichloromethane reacts with the MAO forming the active cationic species. In the presence of toluene, as the solvent, the reaction rate is extremely low. The increase of the MAO concentration in the reaction mixture causes a large increase in the isobutylene conversion, although the molecular weight of the polymers sharply decreases. The use of CD2Cl2 as a solvent induces a large kinetic isotope effect (KIE=7). In the copolymerization of isobutylene with large amounts of isoprene a cross-linkage copolymer is formed. The polymerization and copolymerization reactions were also studied in the presence of a mixture of MAO with the complex (acac)2TiCl2 (acac=acetylacetonate).

Theoretical analysis of horizontal condenser-evaporator conduits of various cross-sections

Abstract An analysis of simultaneous condensation inside and evaporation outside a horizontal conduit has been performed for various shapes of conduits. The study is aimed at improving the performance of horizontal evaporator-condenser water desalination units by establishing the most promising shape. Comparison of various shapes indicates that vertically oriented elongated elliptical conduits yield the highest overall heat transfer coefficients. The effects of intermittent removal of the condensate film by internal horizontal films decreases as the vertical to horizontal axis ratio increases. Effect of non-condensables are included in the analysis.

Transfer characteristics of convex and concave rivulet flow on inclined surfaces with straight-edged grooves

Abstract The flow and temperature fields are solved for convex rivulet flow on the crest of a straight-edged groove and for concave rivulet flow in the bottom of the groove. The effect of the plane inclination in the range of 0

Enhanced heat transfer in horizontal evaporator- condensers with straight-edged grooved tubes

Abstract The flow characteristics on a grooved surface depend on the external feeding rate and the overflow from the crest to the valley between the crests. Energetic considerations affecting transition from convex to concave rivulets are presented. The limiting bounds for minimum and maximum flow with full wetting of the groove determine the desired range of practical operation with different groove dimensions. The corresponding average groove-side heat transfer coefficients are presented. Overall heat transfer coefficients for the simultaneous evaporation-condensation process were calculated for various operating conditions and different grooved surfaces. Comparison with experimental data is satisfactory, but nevertheless indicates lack of availability of reliable data for the dynamic solid-liquid contact angle.

Simultaneous film evaporation and condensation on a two-directionally inclined surface☆

Abstract A theoretical analysis of the effects of inclination on the heat transfer rate during the simultaneous film evaporation on one side and film condensation on the other side of a solid flat plate is presented. The decrease of the evaporating film thickness due to the two-dimensional nature of the flow is relatively small. Hence, the corresponding increase in the overall transfer coefficient is practically insignificant. For a system in which the evaporating film is sustained by a vertically down-flowing liquid, the main effect of the inclination of the plane from the horizontal is due to the formation of an unwetted area. Some practical conclusions related to a bundle of conduits are suggested. In view of the conclusions, the extension of the flat plane analysis by sectional integration to inclined conduits is not warranted.