Browsing by Author "Cox, Kenneth R."
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Item High pressure measurements and molecular modeling of the water content of acid gas containing mixtures(Wiley, 2015) Fouad, Wael A.; Yarrison, Matt; Song, Kyoo Y.; Cox, Kenneth R.; Chapman, Walter G.Water content of three carbon dioxide containing natural gas mixtures in equilibrium with an aqueous phase was measured using a dynamic saturation method. Measurements were performed up to high temperatures (477.6 K = 400°F) and pressures (103.4 MPa = 15,000 psia). The perturbed chain form of the statistical associating fluid theory was applied to predict water content of pure carbon dioxide (CO2), hydrogen sulfide (H2S), nitrous oxide (N2O), nitrogen (N2), and argon (Ar) systems. The theory application was also extended to model water content of acid gas mixtures containing methane (CH4). To model accurately the liquid-liquid equilibrium at subcritical conditions, cross association between CO2, H2S, and water was included. The agreement between the model predictions and experimental data measured in this work was found to be good up to high temperatures and pressures.Item Isolating the non-polar contributions to the intermolecular potential for water-alkane interactions(AIP Publishing LLC., 2014) Ballal, Deepti; Venkataraman, Pradeep; Fouad, Wael A.; Cox, Kenneth R.; Chapman, Walter G.Intermolecular potential models for water and alkanes describe pure component properties fairly well, but fail to reproduce properties of water-alkane mixtures. Understanding interactions between water and non-polar molecules like alkanes is important not only for the hydrocarbon industry but has implications to biological processes as well. Although non-polar solutes in water have been widely studied, much less work has focused on water in non-polar solvents. In this study we calculate the solubility of water in different alkanes (methane to dodecane) at ambient conditions where the water content in alkanes is very low so that the non-polar water-alkane interactions determine solubility. Only the alkane-rich phase is simulated since the fugacity of water in the water rich phase is calculated from an accurate equation of state. Using the SPC/E model for water and TraPPE model for alkanes along with Lorentz-Berthelot mixing rules for the cross parameters produces a water solubility that is an order of magnitude lower than the experimental value. It is found that an effective water Lennard-Jones energy εW/k = 220 K is required to match the experimental water solubility in TraPPE alkanes. This number is much higher than used in most simulation water models (SPC/E—εW/k = 78.2 K). It is surprising that the interaction energy obtained here is also higher than the water-alkane interaction energy predicted by studies on solubility of alkanes in water. The reason for this high water-alkane interaction energy is not completely understood. Some factors that might contribute to the large interaction energy, such as polarizability of alkanes, octupole moment of methane, and clustering of water at low concentrations in alkanes, are examined. It is found that, though important, these factors do not completely explain the anomalously strong attraction between alkanes and water observed experimentally.Item Multi-Scale Molecular Modeling of Phase Behavior and Microstructure in Complex Polymeric Mixtures with Nanoparticles(2013-06-05) Feng, Zhengzheng; Chapman, Walter G.; Biswal, Sibani Lisa; Barrera, Enrique V.; Cox, Kenneth R.The phase behaviors and microstructures of various realistic and model mixtures of macro and micro molecules, such as polyolefin solutions and nanoparticle block copolymer composites, have been accurately predicted by the application of Statistical Associating Fluid Theory (SAFT) based approaches through various extensions that improve both the physical description of molecular interactions and efficiency of computations. The extensions are presented in a generic sense that is applicable to other studies. These rigorously derived theories have been demonstrated to capture material structure-property relationships and can be applied broadly to other fields including biology, medicine and energy industry. On the phenomenogical scale, the novel SAFT-Dimer equation of state has been extended to study the liquid-liquid phase boundary (cloud point) in polyolefin solutions. A simplified model of the polyolefin molecules has been followed and the effect of various parameters, such as temperature, molecular weight, solvent quality and comonomer content, on the phase behavior has been successfully captured by the theoretical model through comparison with experimental measurements. The presented approach requires less parameters than previous methods and is of critical value to the industrial productions of polymers, especially polyolefins with long branches. On the molecular scale, the interfacial SAFT (iSAFT) Density Functional Theory (DFT) has been extended to include a dispersion free energy functional that explicitly accounts for molecular correlations. The Order-Disorder Transition (ODT) between lamellar and disordered phase has then been investigated for pure block copolymer and copolymer nanocomposite systems. The extension has been shown to dramatically improve the ODT predictions of iSAFT as well as the self assembled microstructures in nanocomposites over previous DFT calculations, in comparison to coarse grained molecular simulations. The behavior of the equilibrium spacing of ordered structures is also examined against the variation of copolymer size and interactions. An efficient numerical scheme, Fast Fourier Transform (FFT), has been implemented and shown to drastically increase the computation efficiency. The theory has then been extended to study block copolymer morphologies with density variations in multiple dimensions. Comprehensive phase diagrams including lamellar, cylindrical and disordered phases have been obtained for copolymer nanocomposites for the first time using a single framework molecular theory. In addition, the nanoparticle induced morphological transition between cylindrical and lamellar phase has been studied using a pseudo arc-length continuation method. Transition evolution is tracked and metastable morphologies are examined and compared with existing experimental reports and theoretical calculations. With these extensions, iSAFT offers a powerful prediction tool that closely relates molecular structure to thermophysical properties and provides an efficient alternative to screen parameter space for specified material properties.Item Re-Engineering the alkanolamine absorption process to economize carbon capture(2013-09-16) Warudkar, Sumedh; Hirasaki, George J.; Wong, Michael S.; Chapman, Walter G.; Cox, Kenneth R.; Billups, W. EdwardClimate change caused by carbon dioxide (CO2) released from the combustion of fossil fuels threatens to have a devastating impact on human life. Power plants that burn coal and natural gas to produce electricity generate more than half of global CO2 emissions. Separating the CO2 emitted at these large sources of emission, followed by long term storage has been proposed as short to medium term solution to mitigate climate change. Implementation of this strategy called 'Carbon Capture and Storage' would allow the continued use of fossil fuels while simultaneously reduce our CO2 emissions. Technologies such as the alkanolamine absorption process, used to separate CO2 from gas mixtures already exist. However, it is presently infeasible to use them for Carbon Capture and Storage due to their relatively large energy consumption. It is estimated that even with the use of state-of-the-art technology, the cost of electricity will increase by around 90%. The research presented in this dissertation is focused on developing novel strategies to limit the increase in the cost of electricity due to implementation of Carbon Capture and Storage. In order to achieve this objective, a process simulation software; ProMax® has been used to optimize the alkanolamine absorption process to suit Carbon Capture application. A wide range of process operating conditions has been analyzed for their effects on energy consumption. Included in this study are process conditions under which waste heat can be utilized for providing energy instead. Based on this analysis, some of the most energy efficient process configurations have been identified for an economic evaluation of their capital costs. This research has also led to the invention of novel absorbent blends which involve the replacement of water used in CO2 absorbents with alcohols. It has been shown that the use of these absorbents can significantly reduce energy consumption and thereby limit the increase in cost of electricity.Item Structure and thermodynamics of a mixture of patchy and spherical colloids: A multi-body association theory with complete reference fluid information(AIP Publishing LLC., 2016) Bansal, Artee; Asthagiri, D.; Cox, Kenneth R.; Chapman, Walter G.A mixture of solvent particles with short-range, directional interactions and solute particles with short-range, isotropic interactions that can bond multiple times is of fundamental interest in understanding liquids and colloidal mixtures. Because of multi-body correlations, predicting the structure and thermodynamics of such systems remains a challenge. Earlier Marshall and Chapman [J. Chem. Phys. 139, 104904 (2013)] developed a theory wherein association effects due to interactions multiply the partition function for clustering of particles in a reference hard-sphere system. The multi-body effects are incorporated in the clustering process, which in their work was obtained in the absence of the bulk medium. The bulk solvent effects were then modeled approximately within a second order perturbation approach. However, their approach is inadequate at high densities and for large association strengths. Based on the idea that the clustering of solvent in a defined coordination volume around the solute is related to occupancy statistics in that defined coordination volume, we develop an approach to incorporate the complete information about hard-sphere clustering in a bulk solvent at the density of interest. The occupancy probabilities are obtained from enhanced sampling simulations but we also develop a concise parametric form to model these probabilities using the quasichemical theory of solutions. We show that incorporating the complete reference information results in an approach that can predict the bonding state and thermodynamics of the colloidal solute for a wide range of system conditions.Item Sustainable Production of Biofuels: Plant Optimization and Environmental Impact(2012-09-05) Rigou, Venetia; Zygourakis, Kyriacos; Gonzalez, Ramon; Cox, Kenneth R.; Cohan, Daniel S.Many recent studies on the relative costs and benefits of biofuels have raised the need for a detailed and rigorous analysis of the operations of a biorefinery that is focused on optimization. The current thesis concentrates on the design and optimization of plants for producing biodiesel and ethanol from cellulosic biomass. We have performed numerical simulations combined with systematic parametric analyses to investigate the effect of various parameters on the overall material and energy balances of each biorefinery. The efficiency of the simulated processes was investigated by introducing and/or estimating various metrics in order to select the more beneficial directions for process improvements. Particular emphasis has been paid on heat integration and the design of highly efficient combined heat and power (CHP) units that generate the steam and electricity needed for the purification of biofuels and their co-products. The first part of the thesis is focused on biodiesel production via transesterification of soybean oil with methanol, under alkali-catalyzed conditions. We have analyzed the performance of several reactor configurations in order to improve the conversion of the reversible transesterification reactions. The effect of the oil to alcohol ratio has also been extensively explored. Furthermore, the energy requirements of the simulated process have been rigorously calculated. Since biodiesel facilities can be used either for small-scale, distributed applications or for large-scale production, we have explored whether it is more energy efficient to burn the glycerol-rich stream in a combined heat and power (CHP) plant, or purify the glycerol and use it a feedstock for producing higher-value chemicals with further biotechnological processes. The second part of the thesis focuses on the production of cellulosic ethanol. Having developed the process model, a detailed parametric analysis was carried out to determine how the energy balances and overall efficiency of the biorefinery were influenced by changes in (a) the composition of the biomass feedstock, and (b) the conversion levels of the hydrolysis and fermentation stages. Furthermore, the requirements of the utility section of the ethanol plant were calculated. The utility section included a combined heat and power unit where by-product streams of the production process were utilized for energy generation. The parametric analysis indicated that these streams were in most cases an insufficient fuel source for meeting the energy requirements of the plant and thus, additional fuel was required (biomass, coal, or natural gas). The calculations of this section indicated a significant trade-off between ethanol production and external energy inputs, thus casting some doubt on the ultimate effectiveness of efforts to develop genetically modified energy crops (with high carbohydrate content) in order to maximize fuel production.Item Thermodynamic Perturbation Theory for Associating Fluids: Beyond First Order(2014-04-23) Marshall, Bennett Davis; Cox, Kenneth R.; Chapman, Walter G.; Kolomeisky, Anatoly B.; Verduzco, RafaelAssociation interactions such as the hydrogen bond are a key component in many physical and biological systems. For this reason accurate theories are needed to describe both the thermodynamics and self – assembly of associating species. Applications of these theories range from those of industrial importance, such as equations of state for process simulations, to the realm of materials science where these theories can be used to predict how molecular structure determines the self – assembly of associating species into advanced supramolecular materials. Semi – empirical equations of state based on chemical or lattice theories do not contain the molecular level detail to make predictions on how molecular structure affects self – assembly of associating species. For this, one needs a theory whose starting point is the interaction potential between two associating species which includes this molecular level detail. Development of accurate molecular theories for associating fluids is hampered by the strength, directionality and limited valence of the association interaction. This has proven particularly true in the extension of Mayer’s cluster theory to these associating fluids. This problem was largely solved by Wertheim in the 1980’s who developed an exact cluster expansion using a multi – density formalism. Wertheim’s cluster theory incorporates the geometry of the association interaction at an early point in the derivation. This allowed Wertheim to develop the theory in such a way that accurate and simple approximation methods could be applied such as thermodynamic perturbation theory (TPT). When treated at first order in perturbation (TPT1), Wertheim’s theory gives a simple and general equation of state which forms the basis of the statistical associating fluid theory (SAFT) free energy model. SAFT has been become a standard in both academia and industry as an equation of state for associating (hydrogen bonding) fluids. While simple in form and widely applied, the development of TPT1 rest on a number of, sometimes severe, simplifying assumptions: no interaction between associated clusters beyond that of the reference fluid, association sites are singly bondable, no double bonding of molecules, no cycles of association bonds, no steric hindrance between association sites, association is independent of bond angle and there is no bond cooperativity. The purpose of this dissertation is to relax these assumptions. Chapters 2 – 4 extend TPT to allow for multiple bonds per association site. Chapter 2 focuses on the case of associating spheres with a doubly bondable association site as a model for patchy colloids with a multiply bondable patch. Chapters 3 – 4 extend TPT to associating mixtures of spheres where the first component has directional association sites and the second component has spherically symmetric association sites. This theory is applicable as a model for mixtures of patchy and spherically symmetric colloids and ion – water association. Chapters 5 – 6 extend TPT to account for the effect of relative association site location. In chapter 5 the case of associating hard spheres with two association sites is considered. For this case the angle between the centers of the association sites (bond angle) is treated as a independent variable. This is the first application of TPT which has included the effect of bond angle. It is shown that as bond angle is decreased the effects of steric hindrance, ring (cycle) formation and eventually double bonding of molecules must be accounted. The developed theory accounts for each of these higher order interactions as a function of bond angle. The resulting theory is shown to be accurate over the full bond angle range for both the distribution of cluster types (chains, rings, double bonded) as well as the equation of state. In chapter 6 this theory is extended to the case there are more than two association sites. In chapter 7 TPT is extended to account for hydrogen bond cooperativity for the case of molecules with two association sites. The derived theory is shown to be highly accurate for molecules which exhibit positive or negative hydrogen bond cooperativity. Finally, in chapter 8 the case of associating fluids in spatially uniform orienting external fields is considered. An example system for this case would be associating dipolar molecules in a uniform electric field. By employing classical density functional theory in the canonical ensemble exact results are obtained for the orientational distribution function. These exact results contain the monomer fraction which must be approximated in TPT1. The resulting theory is in good agreement with simulation data for the prediction of the effect of a linear orienting field on the chain length and orientation of associated chains of spheres with two association sites.