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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 5136-5139 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this short note we describe a novel method, based upon the Kirkwood equation, for doing free energy simulations in asymmetric molecular systems typical of those encountered with supercritical fluid mixtures involving large organic species solvated in a lighter supercritical gas. The rationale behind this modified Kirkwood approach has been to develop simulation procedures that are both accurate and stable, by minimizing the perturbation to the system that is implicit to all perturbation-based simulation methods. A good example of an algorithm in this latter class of methods is the free energy perturbation method (FEP) which has been used in this work for comparative purposes with our proposed technique which we refer to as the modified Kirkwood coupling method (MKC). The MKC approach described here effectively makes the perturbation process in the simulation process vanishingly small and appears to result in a stable, accurate algorithm for free energy calculations in chemical mixtures. Numerical results supporting these conclusions are provided in two Lennard-Jones model systems at supercritical conditions.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 5438-5449 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this series of papers we provide new results related to critical phenomena in binary mixtures particularly with regard to the phenomenon of critical azeotropy. In Part I we carefully develop a set of necessary and sufficient conditions guaranteeing the existence of critical azeotropy in these mixtures. The sufficiency conditions are particularly useful for determining whether or not a critical azeotrope exists in a system, without having to actually find this point itself, typically a difficult experimental problem. Classical critical exponents of various thermodynamic properties along various paths are determined for the critical azeotrope. These are compared to the analogous situation near a pure fluid critical point in a dilute binary mixture. Both situations display analogies as well as differences that are discussed at some length. In Part II we concentrate upon describing interesting synergistic solvation effects in these systems using many of the results presented in Part I.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 5450-5461 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this paper we investigate the properties of near-critical mixtures as solvent media, with particular regard to the critical azeotrope. The results from Part I are used in conjunction with integral equation theory calculations to demonstrate that the critical azeotrope has a number of unique properties as a solvent medium. Volumetric and thermal effects related to the solvation of dilute solutes are greatly enhanced and these can be related to higher order divergences of these properties near a critical azeotrope. Finally, we consider synergistic solvent effects near a mixture critical point, showing that solvent mixtures exhibit cooperative behavior that affect solvation thermodynamics in interesting ways that have been little discussed to date.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 30 (1984), S. 669-672 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 32 (1986), S. 1665-1676 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A procedure is presented for designing supercritical gas extraction processes using a pure solvent fluid to effect the separation. The necessary conditions for the procedure to be applicable are the existence of crossover behavior and separable crossover pressures in the fluid phase mixture. Data are presented to support the existence of such phenomena in ternary systems, and an actual separation process has been designed and operated based upon the data. For systems exhibiting this behavior, process synthesis principles are developed to illustrate the evolution of a process flowsheet for the separation of a mixture into its pure components.
    Additional Material: 15 Ill.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 33 (1987), S. 976-986 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: An analysis of parallelism for modular process flowsheet calculations is given. The central idea is to examine various algorithms that allow one to decouple and decentralize flowsheet calculations for computation in a multiprocessor environment. Specific algorithms are evaluated which make use of quasi-Newton updates to converge the flowsheet equations. From the analysis presented and example problems solved, it is clear that parallel processing can be used to enhance the computational speed of modular flowsheet calculations. In particular, various formulations of the simultaneous modular approach are analyzed to show how parallelism may be taken advantage of in flowsheet solutions.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 31 (1985), S. 2039-2051 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A model reduction technique based upon modal analysis is presented for modeling the dynamics of general, large equilibrium staged cascades encountered in chemical processes. The analysis is general enough to encompass quite complex cascades involving multiple fee and/or product streams with nonideal thermodynamic behavior. The low-order models proposed have a number of desirable features: the state variables have physical significance and model parameters are clearly related to the fundamental process parameters which are easily obtainable from steady-state plant data. Quantitatively it is shown that the low-order models proposed are in excellent agreement with the complete (high-order) dynamic models on a range of problems some of which involve complex, nonideal cascades.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 41 (1995), S. 389-401 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: An asymptotic analysis based on Taylor series expansions is used for first-order correction terms to the Henry's law approximation to describe solvation phenomena in multiple solute-multiple solvent systems. The magnitude of these correction terms in solvent systems very near their critical points is of particular concern, as shown in model fluid calculations with the aid of integral equation theory. The results clearly demonstrate that close proximity to the critical point in pure and mixed solvent systems causes the Henry's law approximation to show large errors in predicting solubilities, especially near the critical azeotrope of a mixed solvent system. Theoretical results also show that cross solubility enhancements in a two solutesupercritical solvent system cause cooperative synergism (both solute solubilities are increased relative to the corresponding single solute situations) or reverse synergism (both depressed relative to the single solute situation). It appears to be consistent with the available data. In computer simulations, the solute's infinitely dilute reference state is often used as a basis for describing solute thermodynamic behavior. These simulations are best achieved in the canonical ensemble because of the weak composition dependence of free energies in terms of characteristic variables of this ensemble.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 35 (1989), S. 981-987 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: This paper presents the experimental technique and data acquired in the evaluation of a separation process in the crossover region of a model supercritical fluid system consisting of benzoic acid, 1-10 decanediol, and carbon dioxide. The results show that by exploiting the crossover effect it is possible to obtain essentially pure benzoic acid (〉99.5% purity) with a single temperature cycle. A number of other issues are also discussed including the tradeoff between yield and purity and the manner in which the process variables are specified, which involves the interplay of experiment and model calculations.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 39 (1993), S. 1985-1994 
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Computer simulation results are presented for the residual chemical potential μ1r, local solvent density enhancement Δρloc, and average solute potential energy u1 of a solute molecule immersed in afluid very close to its critical point. The simulation algorithm involves the Monte Carlo method in conjunction with free energy perturbation ideas. A number of ideas related to the feasibility of computer simulations in near-critical solvents are investigated. Due to long-range correlations near the critical point, it is not clear that simulations in this regime can be expected to yield accurate results using ensemble sizes of the magnitude typically used in simulations. Examination of system size and number of sampling steps on the simulations shows that the accuracy of the results depend to a large extent on the nature of the thermodynamic property being investigated. The solute chemical potential is suited particularly to simulation since it depends primarily on short-range structure in the system. A comparison of the simulation results with integral equation theory calculations shows both approaches agree well with each other.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
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