Ruth E. Baltus

Examination of the potential of ionic liquids for gas separations

Abstract: Ionic liquids have received increasing interest in recent years for “green” synthesis and separations because they have essentially no vapor pressure. We have begun an investigation of the potential of ionic liquids for gas separations, including the removal of carbon dioxide from stack gas generated in coal-fired power plants. In this paper, we report results from measurements of the permeance of nitrogen and carbon dioxide in supported ionic liquid membranes. Preliminary results for a porous alumina membrane saturated with l-butyl-3-methyl imidazolium bis[trinuoromethylsulfonyl] amide yielded a CO2 : N2 selectivity of 127. Using previously reported measurements of CO2 solubility in ionic liquids (1) and the measured membrane transport characteristics, a preliminary economic analysis of a separation process based on supported ionic liquid membranes has been performed. A comparison of cost estimates for this membrane-based separation to cost estimates reported for carbon dioxide removal using a conventional amine scrubbing operation shows that, with continued technology development, an ionic liquid membrane process may potentially be economically competitive with amine scrubbing. A preliminary cost estimate for an ionic liquid scrubber indicates that an ionic liquid absorption process shows less favorable economics than a supported ionic liquid membrane or an amine scrubber. However, results indicate that a more comprehensive technical and economic assessment is warranted.

Fabrication and characterization of single layer and multi-layer anodic alumina membranes

Abstract Porous alumina films containing parallel capillary pores of uniform size were fabricated by anodically oxidizing high purity aluminum films in phosphoric acid and sulfuric acid solutions. These films were formed into membranes by post-oxidation processing that removes unoxidized aluminum as well as a barrier layer of alumina from the base of the pores. Symmetric membranes were made by oxidizing at constant current density conditions. Two layer composite membranes were made by changing current density during the oxidation process. The thickness, pore density and porosity of each membrane were predicted from the relationships between structural characteristics and processing conditions that were developed in previously reported kinetic studies of anodic oxidation of aluminum. Each membrane was then characterized using permeability measurements. The hydraulic permeability of membranes formed in phosphoric acid and the diffusive permeability of membranes formed in sulfuric acid were measured. A comparison of the measured permeability values to those predicted using the structural characteristics calculated using relationships developed in the kinetic studies shows excellent agreement. These results illustrate that porous alumina membranes can be fabricated with transport characteristics that can be predicted from the processing conditions used during membrane formation.

Transport studies with porous alumina membranes

Abstract In recent years, there has been a growing interest in utilizing inorganic membranes, particularly microporous membranes, for numerous biotechnology applications, inorganic catalysis and separation processes. In order to conduct a fundamental study of any membrane process, a well defined membrane system is required. In this paper, results from characterization experiments performed on porous Anotec alumina membranes containing uniform, parallel pores are reported. Scanning electron microscopy was used to determine pore density and pore length. Solvent (water) flow measurements and benzoic acid diffusion experiments were used to determine the pore radius. The pore radius values determined from these measurements were in good agreement, indicating the validity of the methods as well as pore radius uniformity. The application of this work for preparing a well defined immobilization membrane system is discussed.

Growth Kinetics and Morphology of Porous Anodic Alumina Films Formed Using Phosphoric Acid

The growth kinetics of porous alumina films formed by anodic oxidation of aluminum in phosphoric acid under galvanostatic conditions was studied. Scanning electron microscope measurements, Faradays law, and oxide film mass measurements was used to analyze the growth kinetics and obtain film growth rates, pore density, and porosity. Current efficiency was also determined from these measurements. The effect of current density and solution temperature on the oxide film growth rate and morphology was examined. The rate of growth of the alumina film was found to increase with an increase in current density. The rate of growth was observed to increase with temperature at low current densities (7.5 mA/cm 2 ), but was found to exhibit a maximum as temperature was varied at high current densities (17.5 mA/cm 2 ), The pore density was found to decrease with an increase in current density and with a decrease in temperature. The porosity and the average cross-sectional pore area of the films were found to increase with anodization time and decrease with an increase of current density and temperature. These results can be explained by considering the interplay between film growth and oxide dissolution that controls the resulting film morphology. The understanding of the effect of process variables on film morphology that is gained by this study provides the information that is needed to prepare alumina films with well-defined and well-characterized morphology.

Characterization of the pore area distribution in porous membranes using transport measurements

An approach originally proposed by Mason and coworkers has been applied to model porous membranes to show that transport measurements with small and large solutes can be used to distinguish between porous membranes with the same average pore size but different pore size distributions. In addtion, it is shown that such measurements can be used to account for membrane heteroporosity when predicting the sieving characteristics of a membrane. This is done by applying moment theory to results from flux measurements for a small solute at Pe ≈ 1 or to results from measurements of the reflection coefficient for a large solute at infinite Pe. No a priori assumptions about the nature of the distribution of pore areas are necessary. n nIn this paper, the results from calculations performed with three different model membranes with log-normal pore size distribution are reported. These results show that one can begin to distinguish between membranes by measuring the hydraulic and diffusive permeability and performing at least one additional flux measurement — with either a small, non-hindered solute at Pe ≈ 1 or a large solute at infinite Pe. Results also show that a fairly narrow window can be placed on the sieving curve for a heteroporous membrane without performing any sieving measurements. This is an interesting and encouraging result because it means that many of the problems that arise from measuring and interpreting pore size distributions using more traditional techniques can be avoided by using small solute flux measurements to predict the separation characteristics of many porous membranes.

Measurement of diffusion coefficients of oil residual fractions using porous membranes

Abstract Diffusion coefficients of narrow size range fractions of Cold Lake vacuum bottoms (+ 525 °C) have been determined experimentally using porous polyester membranes. These measurements were performed in tetrahydrofuran solvent at 10 °, 20 ° and 30 °C and in 1-methylnaphthalene solvent at 50 °C. The results indicate that the effective size of these fractions is significantly smaller than those of similar materials whose diffusion properties were determined by other investigators. The results are also in poor agreement with the diffusion properties predicted for model structures that have been proposed to describe the intrinsic viscosity of these fractions under the same conditions 1 . We believe that these observations are due to association between the constituents of these materials during the diffusion experiments.

Characterization of oil residual fractions using intrinsic viscosity measurements

Experimental measurements of molecular weight and intrinsic viscosity of bitumen fractions of Cold Lake vacuum bottoms (+525 °C) are used to characterize the macroscopic structure of these materials in tetrahydrofuran solvent at 10 °C and 20 °C and in 1-methylnaphthalene solvent at 50 °C. A multisubunit model is proposed to describe the characteristics of these materials. These models are rigid structures assembled from identical spherical subunits. The theory of Garcia de la Torre and Bloomfield1 is used to determine the model structure that best describes the observed intrinsic viscosity for each fraction. The results show that the hydrodynamic properties of these fractions are in agreement with properties predicted for model structures of planar geometry.

The use of moment theory to interpret diffusion and sieving measurements in terms of the pore size distribution in heteroporous membranes

Moment theory has been applied to model porous membranes to show that one can place reasonable bounds on the cumulative pore size distribution, the hindered diffusivity or the reflection coefficient of large solutes in a heteroporous membrane by measuring the diffusive permeability to a small solute, the hydraulic permeability and one or two additional transport characteristics. These additional measurements involve either the flux of a small solute at Pe∼1, the hindered diffusivity of a large solute or the reflection coefficient of a large solute at Pea1. Membrane heteroporosity is incroporated in the predicted bounds without requiring one to make any a priori assumptions about the nature of the pore size distribution. In this paper, the results from calculations performed with different model membranes containing log-normal pore size distributions are reported. A comparison of the results obtained with three different membranes shows that one can distinguish between membranes with the same average pore size but different pore size distributions by measuring either the hindered diffusion coefficient or the reflection coefficient of two different sized solutes. A comparison of the bounds on D and the bounds on σ predicted from different types of transport measurements shows that, under certain conditions, one can place tighter bounds on one transport characteristic by measuring a different one.

Characterization of Asphaltenes and Heavy Oils Using Hydrodynamic Property Measurements

Heavy oils and residua are characteristically more difficult to process catalytically than lighter petroleum fractions because of their large molecular size, polarity and heteroatom and metals content. The component of these materials which is generally accepted to be the most refractory is the asphaltenes, which are defined as a solubility class, typically the n-pentane or n-heptane insoluble fraction. There have been extensive efforts focused on developing an understanding of the chemical and the physical characteristics of entire resids as well as the asphaltene fraction.

Effect of electrostatic interactions on rejection of capsular and spherical particles from porous membranes: theory and experiment.

HYPOTHESESnParticle rejection from porous membranes will increase when particle and membrane carry like charges. The influence of charge on particle rejection can be modeled by first solving the Poisson-Boltzmann equation for the electrostatic particle-pore wall interaction energy, enabling one to predict the cross sectional particle concentration in a pore. Rejection coefficients can then be predicted by combining the Boltzmann factor with a hydrodynamic lag coefficient.nnnEXPERIMENTSnRejection experiments were conducted with three different spherical colloidal silica particles, a spherical virus (PRD1) and gold nanorods of two different aspect ratios (ratio of length to diameter). Track-etched polycarbonate microfiltration and ultrafiltration membranes having nearly parallel pores of cylindrical cross-section were used. Experiments were conducted under conditions where both particle and membrane carried a negative charge as well as under conditions where surface charges had minimal impact. Experiments were designed to cover a broad range of dimensionless particle sizes under conditions when convection dominated particle transport.nnnFINDINGSnModel predictions and experimental measurements demonstrate that particle rejection can be enhanced significantly when particle and pore carry like charges.