Eli Ruckenstein

Carbon dioxide reforming of methane over nickel alkaline earth metal oxide catalysts

The CO2 reforming of CH4 over reduced NiO/alkaline earth metal oxide catalysts was investigated. A CO yield of 95% was obtained from a stoichiometric feed mixture of CH4 and CO2, at high GHSV (60 000 cm3 g−1 h−1), over a reduced NiOMgO with a weight ratio of 0.2. In addition, the catalyst had excellent stability, since the CO yield remained unchanged during 120h. Compared to the reduced NiOMgO catalyst, the reduced NiOCaO,NiO SrO NiO BaO catalysts had low CO yields and very low stabilities. The TPD of CO over the reduced NiOMgO catalyst indicated a lower decomposition of CO to CO2 than over the other catalysts investigated, hence that MgO inhibits the disproportionation reaction 2CO → C + CO2 over Ni. The behavior of the reduced NiOMgO catalyst is probably due to the formation of a NiOMgO solution, as a result of the similar crystalline structures of NiO and MgO.

Catalytic Conversion of Methane to Synthesis Gas by Partial Oxidation and CO2 Reforming

Abstract The preparation of synthesis gas from natural gas, which is the most important step in the gas-to-liquid transformation, has attracted increasing attention in the last decade. Steam reforming, partial oxidation, and CO 2 reforming are the three major processes that can be employed to prepare synthesis gas. Because steam reforming was reviewed recently in this series [Adv. Catal. 47 (2002) 65], this chapter deals only with the latter two processes. The history of the development of methane conversion to synthesis gas is summarized as an introduction to the partial oxidation of methane, which is reviewed with emphasis on hot spots in reactors, major developments in the reduction of O 2 separation costs, and reaction mechanisms. The various catalysts employed in CO 2 reforming are examined, with emphasis on inhibition of carbon deposition.

BINARY MgO-BASED SOLID SOLUTION CATALYSTS FOR METHANE CONVERSION TO SYNGAS

The excellent catalytic performance and high stability of MgO–NiO solid solution catalysts in CH4 conversion to syngas generated the recent outburst of interest for the binary MgO-based solid solutions. This review will focus on the relationship between the catalytic performance of the binary MgO-based solid solution and its properties in the CO2 reforming, the partial oxidation and the steam reforming of methane. First, the development of methane conversion to syngas will be summarized. Second, the role of the basicity and of the solid solution in the design of a catalyst that can inhibit carbon deposition and active metal sintering will be examined. Third, the main results regarding the catalytic performance of the MgO-based solid solutions will be presented. Fourth, detailed information regarding the effects of the NiO/MgO composition, surface area, pore distribution, crystal lattice parameter, precursors, and preparation condition on its catalytic behavior will be provided.

Spontaneous rupture of thin liquid films

The rupture of a liquid film on a solid surface and of a free liquid film have been studied using hydrodynamic stability theory. The films are not thicker than several hundred Angstrom. A small perturbation applied to the free interface generates motions in the film, and the assumption is made that the Navier–Stokes equations can be used to describe them. The difference in forces acting upon an element of liquid in a thin film and in a bulk fluid is accounted for by introducing a body force in the Navier–Stokes equations. This force is calculated from the potential energy per unit volume in the liquid caused by the London–van der Waals interactions with the surrounding molecules of the liquid and with those of the solid. If the perturbation grows, it leads to the rupture of the film. The range of wavelengths of the perturbation for which instability occurs is established and the time of rupture is evaluated. The effect of insoluble and soluble surface active agents is analyzed. Available experimental data concerning condensation on a solid surface and coalescence of bubbles are explained on the basis of the obtained results.

Membrane Chromatography: Preparation and Applications to Protein Separation

As a result of the convective flow of solutes through porous membranes, membrane chromatography has a higher capture efficiency and a higher productivity than column chromatography and shows most promising industrial applications for the recovery, isolation, and purification of proteins and enzymes. This paper presents a comprehensive review of the methods for preparation of adsorptive membranes (such as surface modification, in situ copolymerization, direct formation from hydrophilic materials, and functionalized particulate‐entrapped membranes) and deals particularly with novel macroporous chitin and chitosan membranes for protein separations developed by the authors.

Thermal and dynamic mechanical analysis of PVA/MC blend hydrogels

New blend hydrogels based on poly(vinyl alcohol) (PVA) and methylcellulose (MC) were prepared by crosslinking in an aqueous solution with glutaraldehyde (GA) in the presence of HCl. The state of the miscibility of the blend hydrogel films was examined over the entire composition range by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Depressions of the melting and crystallization temperatures of PVA were observed with increasing MC content and crosslinking density via the DSC. The determination of the glass transition temperature of blend hydrogels by DMA indicated that they exhibit a higher miscibility than the non-crosslinked blends.

Spreading kinetics of liquid drops on solids

Abstract During spreading of a liquid drop on a solid surface, there is an initial stage where several factors can be important, then a stage where gravity and fluid viscosity are the chief factors promoting and resisting spreading, and finally a stage where intermolecular forces at the drops periphery replace gravity as the main factor causing spreading. Approximate analyses for the last two stages are developed. The results are in excellent agreement with available experimental data on spreading rates. The use of intermolecular forces acting throughout a finite region near the drops edge is superior to the use of a line force acting at the edge itself in predicting experimental spreading rates for the last stage.

Decay of standing foams: drainage, coalescence and collapse

Abstract A summary of recent theoretical work on the decay of foams is presented. In a series of papers, we have proposed models for the drainage, coalescence and collapse of foams with time. Each of our papers dealt with a different aspect of foam decay and involved several assumptions. The fundamental equations, the assumptions involved and the results obtained are discussed in detail and presented within a unified framework. Film drainage is modeled using the Reynolds equation for flow between parallel circular disks and film rupture is assumed to occur when the film thickness falls below a certain critical thickness which corresponds to the maximum disjoining pressure. Fluid flow in the Plateau border channels is modeled using a Hagen-Poiseuille type flow in ducts with triangular cross-section. The foam is assumed to be composed of pentagonal dodecahedral bubbles and global conservation equations for the liquid, the gas and the surfactant are solved to obtain information about the state of the decaying foam as a function of time. Homogeneous foams produced by mixing and foams produced by bubbling (pneumatic foams) are considered. It is shown that a draining foam eventually arrives at a mechanical equilibrium when the opposing forces due to gravity and the Plateau-border suction gradient balance each other. The properties of the foam in this equilibrium state can be predicted from the surfactant and salt concentration in the foaming solution, the density of the liquid and the bubble radius. For homogeneous foams, it is possible to have conditions under which there is no drainage of liquid from the foam. There are three possible scenarios at equilibrium: separation of a single phase (separation of the continuous phase liquid by drainage or separation of the dispersed phase gas via collapse), separation of both phases (drainage and collapse occurs) or no phase separation (neither drainage nor collapse occurs). It is shown that the phase behavior depends on a single dimensionless group which is a measure of the relative magnitudes of the gravitational and capillary forces. A generalized phase diagram is presented which can be used to determine the phase behavior. For pneumatic foams, the effects of various system parameters such as the superficial gas velocity, the bubble size and the surfactant and salt concentrations on the rate of foam collapse and the evolution of liquid fraction profile are discussed. The steady state height attained by pneumatic foams when collapse occurs during generation is also evaluated. Bubble coalescence is assumed to occur due to the non-uniformity in the sizes of the films which constitute the faces of the polyhedral bubbles. This leads to a non-uniformity of film-drainage rates and hence of film thicknesses within any volume element in the foam. Smaller films drain faster and rupture earlier, causing the bubbles containing them to coalesce. This leads to a bubble size distribution in the foam, with the bubbles being larger in regions where greater coalescence has occurred. The formation of very stable Newton black films at high salt and surfactant concentrations is also explained.

Aggregation of Amphiphiles as Micelles or Vesicles in Aqueous Media

The physical factors responsible for the aggregation of amphiphiles in aqueous media are examined and expressions for their contribution to the attractive or repulsive components of the free energy change of aggregation are established. Whereas in previous treatments, an arbitrary repulsive force was necessary to explain the behavior of nonionic systems, no such ad hoc assumption is made here. Rather the free energy changes due to interfacial tension at the hydrocarbon core (of the aggregates)-water interface and to the loss of a part of translational and rotational degrees of freedom of the amphiphiles when they aggregate are the two main repulsive contributions. On the other hand, the factors favoring aggregation are: (i) the van der Waals interactions between the hydrocarbon tails of the amphiphiles, and (ii) the structural changes in water and the changes in the interactions between amphiphiles and water resulting from aggregation. For ionic and zwitterionic amphiphilar systems additional free energy contributions are included to account for the repulsive electrostatic interactions between the head groups. For vesicles, the repulsion caused by the overlapping electrical double layers inside the vesicles is also considered. The expressions established for the various free energy changes associated with aggregation are used to examine the formation of micelles and vesicles, from single and double chain amphiphiles with nonionic, ionic, or zwitterionic head groups. In general, single chain amphiphiles aggregate as micelles, rather than as vesicles, for all types of polar head groups. Depending upon the nature of the head groups small and/or large micelles can form. Nonionic amphiphiles which have head groups of small cross-sectional areas form large micelles, whereas those with large cross-section aggregate as small micelles. This happens because the repulsion caused by the loss of translational degrees of freedom is larger in the latter of the two cases. Ionic or zwitterionic amphiphiles form small micelles even though they have small head groups because of the electrostatic repulsion between the head groups. At large ionic strengths, large micelles can form because the repulsive interactions between the head groups are small. Nonionic double chain amphiphiles aggregate predominantly as vesicles. Ionic or zwitterionic double chain amphiphiles aggregate as micelles when the electrostatic repulsion between the head groups is large and as vesicles when this repulsion is small. However for intermediate values of these interactions both micelles and vesicles form depending upon the length of the hydrocarbon tail. Most biologically significant double chain amphiphiles have long, complex polar head groups and they aggregate as micelles when the hydrocarbon tail length is short, even if the electrostatic repulsion between the head groups is weak; but they aggregate as vesicles when the hydrocarbon tail length is long. The size distributions calculated for different types of amphiphiles can be unimodal, representing a single population of aggregates, bimodal, or trimodal representing the coexistence of two or three distinct populations of aggregates. The three possible populations are small micelles, large micelles, and vesicles.

Specific ion effects via ion hydration: I.Surface tension

A simple modality is suggested to include, in the framework of a modified Poisson-Boltzmann approach, specific ion effects via the change in the ion hydration between the bulk and the vicinity of the surface. This approach can account for both the depletion of the interfacial region of structure-making ions as well as for the accumulation of structure-breaking ions near the interface. Expressions for the change in interfacial tension as a function of electrolyte concentrations are derived. On the basis of this theory, one explains the dependence of the surface potential on pH and electrolyte concentration, the existence of a minimum in the surface tension at low electrolyte concentrations and the linear dependence, with a positive or sometimes negative slope, of the surface tension on the electrolyte concentration at sufficiently high ionic strengths.