Dr. L. Antonio Estévez

- Green Chemistry and Supercritical Fluids

Supercritical fluids (SCFs) have found many applications in pursuing greener processes. This project has to do with the use of a SCF as a reaction medium in biorefineries. The idea is to obtain products with high market value that can overcome the higher cost of biofuels production costs. One of the most common unsaturated fatty acids is oleic acid, and it can be oxidized with an ozone/oxygen mixture to produce azelaic acid and pelargonic acid. Since the ozone/oxygen mixture is a gas and oleic acid is a liquid under reaction conditions, mass transfer limitations exist. However, a reduction of the mass-transfer limitations can be achieved if the reactants coexist in a single phase. When supercritical carbon dioxide (sc-CO2) is used as the reaction medium, it is possible for both oleic acid and the ozone/oxygen mixture to both exist in the same phase at the same time. Use of supercritical carbon dioxide also provides the possibility of product fractionation, depending upon the solubility of the products in sc-CO2. Previous studies have shown pelargonic acid to have a significantly higher solubility than azelaic acid in sc-CO2, which indicates the compounds could be separated following their formation. Although the original scope of this research was to use ozone as oxidant, it has been found out that oxidation of oleic acid with potassium permanganate in sc-CO2 resulted in higher oleic acid conversion and increased yields of azelaic acid and pelargonic acid compared to the oxidation without sc-CO2.

- Surface Modification of Nanoporous Films Using Organosilanes Dissolved in Supercritical Carbon Dioxide

In the microprocessor industry, it is well known that as the number of transistors increases, the capacity of chips to manage information also increases. To continue with the increment in the number of transistors per integrated circuit (IC), manufacturers have followed a continuous reduction in size of these units. This reduction in size comes linked to negative effects, such as resistance-capacitance-delay (RC-delay), cross talk, and power consumption increase. Integrated circuits manufacturers have changed to new materials that diminish these negative effects, but incompatibilities with the damascene process used for the fabrication of metallic interconnections, have slowed down the increasing trends of transistor density. A recognized alternative for the recovery of electrical properties of damaged silicon-based films is the use of organosilanes. Organosilanes dissolved in supercritical carbon dioxide have been considered an enhanced alternative for the recovery of these damaged films. In this work, a thorough study of the different factors that can affect the repaired properties of damaged methylsilsesquioxane films was performed. Fluid phase equilibria of chlorosilanes-carbon dioxide mixtures, electric and hydrophobic properties recovery by the use of different organolsilanes, control and analysis on thicknesses of deposited films by different silylation reaction mechanisms, determination of kinetic parameters for the rate-law expression of different organosilane silylating reactions, and a comprehensive infrared analyses of the different bonds formed in gas and solid phases by the silylating reaction were studied in this work. Experimental apparatuses were designed for these studies, which include a new real-time, infrared system capable of studying the solid-gas interaction at high pressure conditions. This study led to a thorough understanding of the reaction mechanisms that takes place in the gas and solid phases. Consequently, the effect of controllable parameters such as pressure, temperature, and concentration of organosilanes is now better understood and specific results in the repairing process can now be obtained.

- High-Pressure Phase Equilibria for Chlorosilane + Carbon Dioxide Mixtures

Chlorosilanes have been considered in the microprocessor industry to repair surfaces after harsh processes such as CMP. The most effective way to bring chlorosilanes into contact with the solid surface to be repaired is by dissolving them in supercritical fluids. To design these repair processes, fluid-phase-equilibrium behavior has to be known. In this project, fluid-phase equilibria, including dew points, bubble points, and critical points were measured for four binary systems composed of a chlorosilane and carbon dioxide. The measurements were carried out in a constant-composition, variable-volume cell equipped with a sapphire window, which allowed visual observation of the phases in the cell. A syringe pump was used to inject the CO2 into the cell and to control its pressure. Methyltrichlorosilane, dimethyldichlorosilane, trimethylchlorosilane, and diethyldichlorosilane up to about 0.14 mole fraction were studied in this apparatus and a total of 243 phase-boundary points were obtained. Displacements in the critical point with respect to pure CO2 of up to 11.81 MPa and 348.05 K were observed. Modeling of the fluid-phase equilibria for three of the four binary systems was done using the Peng-Robinson equation of state, standard van der Waals mixing rules with two binary interaction parameters, and a φ-φ formulation of the equilibrium. The binary interaction parameters were obtained by fitting the model to the experimental data. The model produced excellent agreement between computed and experimental data. Graphical representations of the modeling results are presented and compared to experimental results. The results indicate that the largest chlorosilane (diethyldichlorosilane) produced the largest shift in critical pressure and critical temperature with respect to pure CO2.

- Solubility of Pharmacological Drugs in Supercritical Fluids

Supercritical fluids (SCFs) have attracted variable attention during the last 30 years. Recent developments in SCF technology have shown that it is capable of solving challenging specialty separations. An interesting application of particular interest in Puerto Rico is related to the pharmaceutical industry. A possible application could be visualized as the substitution of a series of conventional processes by a simpler stage involving supercritical fluids. To this end, an extensive database of basic properties has to be developed. These properties include phase equilibria or solubility data as well as transport properties (i.e.,viscosities, and diffusivities). The supercritical-fluids group at University of Puerto Rico has engaged in a number of challenging projects in this area. These include the measurement of solubility of a number of drugs in supercritical carbon dioxide, the development of various models based on equations of states to describe these solubilities, and the measurement of diffusion coefficients of liquid species in supercritical carbon dioxide.

- Natural-Convection Heat Transfer in Supercritical Fluids

The main objective of this study is to estimate the heat-transfer coefficients for natural convection from a heated, vertical flat plate into a supercritical fluid. In the first part of this study, an equation for the coefficient of thermal expansion or thermal expansivity for a van der Waals gas was derived. The van der Waals expansivity and the expansivity from accurate, tabulated PVT data were calculated for carbon dioxide, butane, and water. The trends of both expansivities were similar and both diverged at the critical point. These features confirm the validity of the equation obtained in this work.

In the second part of this study, the van der Waals expansivity was used in the momentum equation, which, along with the energy and continuity equations, forms a set of coupled equations. This set of equations was solved numerically by finite differences. A FORTRAN code was written to obtain the velocity and temperature profiles along the plate. The local Nusselt number was then calculated and plotted as a function of the local Rayleigh number for five pre-selected supercritical conditions and one low-pressure condition far from the critical point. A curve corresponding to the Churchill and Chu's empirical correlation was added as a reference to all plots. It was observed in these plots that the curve obtained at conditions far from the critical point approaches the empirical-correlation curve. But most importantly, the curves at supercritical conditions are significantly above the empirical-correlation curve, indicating that the natural convection heat-transfer phenomenon is considerably enhanced in the critical region.

- Molecular Simulation of the Sorption and Swelling Phenomena for Polymer-Supercritical Fluid Systems

Contact between polymers and supercritical fluids occurs in several new technologies such as polymer impregnation. There is ample experimental evidence that polymers swell upon absorbing a supercritical fluid (SCF). Attempts to use fundamental or basic principles to describe these phenomena have not been fully satisfactory. Molecular simulation is becoming an increasingly popular tool in various fields such as in phase-equilibrium calculations. In this project, Monte Carlo simulations is being used to describe the sorption and swelling processes that occur when a polymer is brought into contact with a supercritical fluid, e.g., supercritical carbon dioxide. In a first stage, a bead-spring model has been used in the simulations to study the thermodynamics of simple polymer systems. Properties such as mean radius of gyration, mean end-to-end distance, and phase transitions have been studied using a parallel tempering Monte Carlo algorithm. In a second stage, the Monte Carlo Gibbs ensemble method will be used to model the sorption of supercritical fluids in polymers.

- Measuring and Modeling the Axial Dispersion Coefficient in a Bubble Column with a Non-Newtonian Liquid Phase

The experimental characterization and modeling of the mixing in bubble columns when the liquid phase has a non-Newtonian rheological behavior undoubtedly needs more attention given the broad range of industrial applications of bubble columns, e.g., as bioreactors and for hydrotreatment of petroleum fractions. The non-Newtonian nature of the liquid phase is usually caused by the presence of live microorganisms and/or micro- or nanoparticles. Despite the relevance of the subject, studies emphasizing the non-Newtonian behavior of the liquid phase have been investigated only sparsely through the years. Therefore, the objective of the this research is to perform a holistic study of the mixing phenomenon in the liquid phase of bubble columns with a non-Newtonian liquid phase. In this approach, all aspects are to be taken into account: rheology, careful experimentation, and rigorous mathematical analysis.

The experimental part will include: (a) the measurement of the rheological properties of the solutions or suspensions to be used (e.g., carboxymethylcellulose solutions or micro or nanosuspensions); and (b) experimentation to characterize the mixing of the liquid phase and to measure the gas-phase holdup. This approach will allow relating the hydrodynamic behavior of the liquid phase to its rheological properties.

- Hydrodynamic Studies of Slurry Bubble Columns for Water Treatment Applications

There is no question that water quality is of utmost importance to society and that good quality water is becoming a precious commodity. These facts have prompted creativeness in developing ways to recycle and reuse water, and therefore existing water treatment techniques must be in continuous improvement and new, more effective techniques have to be developed. Many of the emerging and classical techniques involve the contact of water with a gas phase (e.g., aeration) or with a gas and solid phases. On the other hand, bubble columns are devices extensively used in chemical industry and their potential has not been fully exploited in the water-treatment field. Many of the techniques involve three-phase contact, and slurry bubble columns are particularly suitable for that type of application. Therefore, the use of slurry bubble columns for water treatment is being explored. This will be done in two stages: first, the hydrodynamics of slurry bubble columns will be studied, and then actual treatment techniques using slurry bubble columns will be proposed. Three-phase processes are probably more relevant to high-BOD waters, such as industrial wastewaters coming from industries such as pharmaceutical, food, or the like. Therefore, places or regions such as Puerto Rico, with a high concentration of this type of industries, will be particularly benefited.

- Stripping VOCs from Water Bubble Columns

The presence of volatile organic components (VOC's) in water is a problem that has been considered since more than three decades. There are many technologies that offer different approaches to solve this problem. Some of these technologies are packed-tower aeration, granular activated carbon, and powered activated carbon. An experimental setup has been used to carry out experiments on the removal of THM from water. The apparatus consists of a 2.5-m-high, 20-cm-diameter, Plexiglas bubble column where steady-state concentration profiles were measured. A mathematical model with no adjustable parameters was developed. The experimental results and the model prediction coincided very nicely.

- Model of Efficiences of Distillation Sieve Trays

Despite the development of new separations processes, distillation remains as the most important one based on the use and impact on the economy of the chemical process industry. Design and simulation of distillation columns require accurate algorithms to compute the number of theoretical stages and accurate methods to predict the Murphree tray efficiency in terms of the operating conditions. While plate-to-plate computational methods are well developed, estimation models for tray efficiencies available today are not necessarily accurate enough. Fair (1987) states that this has long been a nebulous area for the design of distillation columns, and it would appear that improved models for contacting efficiency can represent major advances in the state of distillation design technology. The main difficulty in the prediction of distillation tray efficiencies results from the complexity of the gas-liquid contact in the tray, and from the high number of variables and effects affecting the efficiency. One of the most difficult effects to account for is the liquid phase velocity pattern. This has been subjected to numerous studies since Kirschbaum (1934) first pointed out that liquid flow maldistribution could cause a reduction on tray efficiency.