On the optimal design of gas-expanded liquids based on process performance

AbstractGas-expanded liquids (GXLs) are mixed solvents composed of an organic solvent and a compressible gas, usually carbon dioxide (CO2) due to its environmental and economic advantages. The best choice of GXL, as defined by the specific organic solvent and the CO2 composition, depends strongly on the process in which the solvent is to be used. Given the large range of possible choices, there is a need to predict the impact of GXL design on process performance from economic and environmental perspectives. In this work, we present a design methodology in which limited experimental data are used to build a predictive model which allows a wider design space to be assessed. The proposed methodology for the integrated design of CO2-expanded solvent and process is applied to the Diels–Alder reaction of anthracene and 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD). Three organic co-solvents are studied: acetonitrile, methanol and acetone. Given that the process cost is sensitive to the operating pressure and reactor volume, a trade-off between reaction rate constant and solubility is required in order to design an optimal process from a cost perspective. From a total cost perspective and in terms of energy consumption, it is found that designs with small amounts of CO2 or, in the case of acetone, without any CO2, offer the best performance. However, CO2 use is found to lead to a significant reduction in organic solvent inventory, up to 70 % in some cases. In this work the importance of taking multiple performance criteria, including process metrics, into account when designing GXLs is demonstrated

Extending Wertheim’s perturbation theory to the solid phase of Lennard-Jones chains: Determination of the global phase diagram

Wertheim’s first order thermodynamic perturbation theory (TPT1) [M. S. Wertheim, J. Chem. Phys. 87, 7323 (1987)] is extended to model the solid phase of chains whose monomers interact via a Lennard-Jones potential. Such an extension requires the free energy and contact values of the radial distribution function for the Lennard-Jones reference system in the solid phase. Computer simulations have been performed to determine the structural properties of the monomer Lennard-Jones system in the solid phase for a broad range of temperatures and densities. Computer simulations of dimer Lennard-Jones molecules in the solid phase have also been carried out. The theoretical results for the equation of state, the internal energy, and the sublimation curve of the dimer model in the solid phase are in excellent agreement with the simulation data. The extended theory is used to determine the global (solid–liquid–vapor) phase diagram of the LJ dimer model; the theoretical estimate of the triple point temperature for the LJ dimer is T*=0.653. Similarly, Wertheim’s TPT1 is used to determine the global phase diagram of chains formed by up to 8 monomer units. It is found that the calculated triple point temperature is hardly affected by the chain length, and that for large chain lengths the fluid–solid equilibrium coexistence densities are virtually independent of the number of monomers in the chain when the densities are expressed in monomer units. This is in agreement with experimental indications observed in polyethylene, where both the critical and the triple point temperatures tend to finite values for large molecular weights.Financial support is due to Project No. BFM-2001-1420- C02-01 and BFM-2001-1420-C02-02 of the Spanish DGICYT (Dirección General de Investigación Científica y Técnica). F.J.B. would like to acknowledge the Universidad de Huelva and Junta de Andalucía for additional financial sup- port. A.G. would like to thank the Engineering and Physical Sciences Research Council for the award of an Advanced Research Fellowship

Global fluid phase behavior in binary mixtures of rodlike and platelike molecules

Published versio

Classical density functional theory for the prediction of the surface tension and interfacial properties of fluids mixtures of chain molecules based on the statistical associating fluid theory for potentials of variable range

The statistical associating fluid theory for attractive potentials of variable range (SAFT-VR) density functional theory (DFT) developed by [G. J. Gloor et al., J. Chem. Phys. 121, 12740 (2004)] is revisited and generalized to treat mixtures. The Helmholtz free-energy functional, which is based on the SAFT-VR approach for homogeneous fluids, is constructed by partitioning the free-energy density into a reference term (which incorporates all of the short-range interactions and is treated locally) and an attractive perturbation (which incorporates the long-range dispersion interactions). In this work, two different functionals are compared. In the first, one uses a mean-field version of the theory to treat the long-range dispersive interaction, incorporating an approximate treatment of the effect of the correlations on the attractive energy between the segments by introducing a short-range attractive contribution in the reference term. In the second, one approximates the correlation function of the molecular segments in the inhomogeneous system with that of a homogeneous system for an average density of the two positions, following the ideas proposed by Toxvaerd [S. Toxvaerd, J. Chem. Phys. 64, 2863 (1976)]. The SAFT-VR DFT formalism is then used to study interfacial properties and adsorption phenomena at the interface. A detailed analysis of the influence of the molecular parameters on the surface tension and density/composition profiles of the mixtures is undertaken for binary mixtures of molecules of different chain length, segment diameter, dispersive energy, and attractive range. The effect of the asymmetry of the molecular species on the adsorption phenomena is examined in some depth. The adequacy of the approach is demonstrated by comparing the theoretical predictions with the interfacial properties of some real mixtures. The relative merits of the two approximate free-energy functionals are assessed by examining the vapor-liquid interfacial tension of selected mixtures of n-alkanes. The theory generally provides an excellent description of the interfacial properties of the mixtures without the need for further adjustment of intermolecular parameters obtained from an examination of the bulk fluid-phase behavior alone.F.J.B. acknowledges the financial support from the Spanish Dirección General de Investigación (Project No. FIS2007-66079-C02-02) and the Proyecto de Excelencia from Junta de Andalucía (Grant No. P07-FQM02884). Support from Universidad de Huelva and Junta de Andalucía is also acknowledged. F.L. thanks Shell for funding a research fellowship. Additional funding from the Engineering and Physical Sciences Research Council of the U.K. (EPSRC Grant No. EP/E016340) is also gratefully acknowledged by the Molecular Systems Engineering Group, as is the award of a refurbishment grant for the Royal Society-Wolfson Foundation

Fluid–solid equilibria of flexible and linear rigid tangent chains from Wertheim’s thermodynamic perturbation theory

An extension of Wertheim’s first-order thermodynamic perturbation theory is proposed to describe the global phase behavior of linear rigid tangent hard sphere chains. The extension is based on a scaling proposed recently by Vega and McBride [Phys. Rev. E 65, 052501 (2002)] for the equation of state of linear chains in the solid phase. We have used the Einstein-crystal methodology, the Rahman–Parrinello technique, and the thermodynamic integration method for calculating the free energy and equation of state of linear rigid hard sphere chains with different chain lengths, including the solid–fluid phase equilibria. Agreement between the simulation data and theoretical predictions is excellent in all cases. Once it is confirmed that the proposed theory can be used to describe correctly the equation of state, free energy, and solid–fluid phase transitions of linear rigid molecules, a simple mean-field approximation at the level of van der Waals is included to account for segment–segment attractive interactions. The approach is used to determine the global phase behavior of fully flexible and linear rigid chains of varying chain lengths. The main effect of increasing the chain length in the case of linear rigid chains is to decrease the fluid densities at freezing, so that the triple-point temperatures increase. As a consequence, the range of temperatures where vapor–liquid equilibria exist decreases considerably with chain length. This behavior is a direct result of the stabilization of the solid phase with respect to the liquid phase as the chain length is increased. The vapor–liquid equilibria are seen to disappear for linear rigid chains formed by more than 11 hard sphere segments that interact through an attractive van der Waals mean-field contribution; in other words, long linear rigid chains exhibit solid–vapor phase behavior only. In the case of flexible chains, the fluid–solid equilibrium is hardly affected by the chain length, so that the triple-point temperature reaches quickly an asymptotic value. In contrast to linear rigid chains, flexible chains present quite a broad range of temperatures where vapor–liquid equilibria exist. Although the vapor–liquid equilibria of flexible and linear rigid chain molecules are similar, the differences in the type of stable solid they form and, more importantly, the differences in the scaling of thermodynamic properties with chain length bring dramatic differences to the appearance of their phase diagrams.Financial support is due to project Nos. BFM-2001- 1420-C02-01 and BFM-2001-1420-C02-02 of the Spanish DGICYT (Dirección General de Investigación Científica y Técnica). F.J.B. would like to acknowledge Universidad de Huelva and Junta de Andalucía for additional financial support. E.S. would like to thank the Ministerio de Educación y Cultura for the award of a PhD studentship (FPU). A.G. also thanks the Engineering and Physical Sciences Research Council for the award of an Advanced Research Fellowship

Measurement & Prediction of Phase Behaviour of Carbon Dioxide Mixtures

Acquiring a comprehensive understanding of the behaviour of carbon dioxide under reservoir conditions is essential for optimizing its usage in enhanced oil recovery (EOR) and for developing sequestration schemes. In order to obtain this understanding, it is necessary to study the physical properties and phase behaviour of mixtures of carbon dioxide with hydrocarbons and brines under conditions of high pressure. In this work we are addressing both the experimental and the theoretical aspects of this problem. A new apparatus, based on the static-analytical method, has been developed to measure phase equilibrium. The equipment comprises a high-pressure cell with sapphire windows for visual observation and phase sampling, with on-line gas chromatography analysis, for measuring the phase compositions. The experimental work is complemented with a theoretical modelling for these mixtures, using the statistical association fluid theory for potentials of variable range (SAFT-VR). As an example of the predictive capabilities of the equation, the fluid phase behaviour of the mixture (carbon dioxide + n-decane) is presented

Advanced thermodynamic and processing modelling integration for amine scrubbing in post-combustion CO~2~ capture

The reduction in CO~2~ emissions from anthropogenic sources has become a topic of widespread interest over the past number of years. As the power generation sector is by far the largest stationary-point-source of CO~2~, being responsible for approximately 35% of total global CO~2~ emissions^1^ this question has special relevance for this industry. As the inclusion of carbon capture facilities incurs a significant energy penalty on the efficiency coal-fired power-stations, there is a strong requirement for the improvement of these systems in terms of the minimisation of operation and maintenance costs, capital costs and the maximisation of efficiency and flexibility. This last issue has relevance for start-up times and ramp-rates. Post-combustion capture methods based on the chemisorption of CO~2~ in aqueous amine solutions are among the most mature and accepted technologies for CO2 capture from power plants^2^. However, amines are complex, associating solvents requiring a sophisticated thermodynamic model, capable of modelling the hydrogen bonding interactions that occur in these systems. One such model is provided by the statistical associating fluid theory (SAFT^3^). This is a molecular approach, specifically suited to hydrogen-bonding, chain-like fluids. In this contribution we use the SAFT approach for potentials of variable range (SAFT-VR^4^) to model the thermodynamics and phase equilibria of a number of amines including ammonia and monoethanolamine. The molecules are modelled as homonuclear chains of tangentially bonded square-well segments of variable range, and a number of short-ranged off-centre attractive square-well sites are used to mediate the anisotropic effects due to association in the fluids. We also determine values of the binary parameters for mixtures and then use these parameters to predict the phase equilibria of amine+water, amine+carbon dioxide as well as water+carbon dioxide mixtures. We then consider the phase equilibria of the ternary mixtures of amine+water+carbon dioxide and finally that of quaternary mixtures of amine+water+carbon dioxide+nitrogen. A good quantitative understanding of the phase behaviour of these quaternary mixtures is essential for accurate modelling of absorption processes for carbon dioxide capture. 

1. Steeneveldt, R., Berger, B. & Torp, T.A., ChERD, 84(A9): 739-763, 2006
2. Rao, A.B.; Rubin, E.S., 2002. A Technical, Economic, and Environmental Assessment of Amine-Based CO2 Capture Technology for Power Plant Greenhouse Gas Control. Environ. Sci. Technol. 36, 4467-4475
3. Chapman, W.G., Gubbins, K.E., Jackson, G. & Radosz, M., Ind. Eng. Chem. Res., 1990. 29, 1709-1721
3. Gil-Villegas, A., Galindo, A., Whitehead, P. J., Mills, S. J. & Jackson, G., J. Chem. Phys. 106 (10), 8 March 199

Application of the generalised SAFT-VR approach for long-ranged square-well potentials to model the phase behaviour of real fluids

In a recent generalisation of the SAFT-VR equation of state the method was extended so as to deal with wide square-well ranges, namely, 1.2 ≤ λ ≤ 3.0 [B. H. Patel, H. Docherty, S. Varga, A. Galindo, and G. C. Maitland. Mol. Phys., 103(1), 129–139, 2005.]. In this work, this equation is used to revisit the adjustment of intermolecular model parameters, with special emphasis on substances where the upper boundary of the potential range (λ = 1.8) has been previously reported or may be expected on grounds of the polar nature of the molecules. For this purpose, we follow the work of Clark et al. [G. N. I. Clark, A. J. Haslam, A. Galindo, and G. Jackson. Mol. Phys., 104(22-24), 3561–3581, 2006] and study a relative least squares objective function and the percentage absolute average deviation (%AAD) to determine the intermolecular model parameters (m, λ, σ, ε/kB, εhb/kB and rc) by comparison to experimental vapour-pressure and saturated liquid density data. In order to ensure in each case that the global minimum is identified, the dimensionality of the problem is reduced by discretising the parameter-space. Applying this method to the study of argon, nitrogen, benzene, carbon dioxide, carbon monoxide, n-alkanes, the refrigerant R1270, water, hydrogen chloride and hydrogen bromide, we find that the optimal models always present square-well ranges λ < 1.8, meaning that an upper bound value of λ = 1.8 (as set in the original approach) for the square-well range is sufficient to model real fluids. Accurate intermolecular potential models with ranges higher than 1.8 are also identified, but we find that these do not usually correspond to the global minimum of the objective function considered.M.C.dR. acknowledges the Programme Alßan from European Union Programme of High Level Scholarships for Latin America (identification number E03D21773VE) for a Fellowship. Authors also acknowledge financial support from project number FIS2007- 66079-C02-02 of the Spanish Dirección General de Investigación. Additional support from Universidad de Huelva and Junta de Andalucía is also acknowledged