85 results found
Wehbe M, Haslam AJ, García-Muñoz S, et al., 2023, Thermodynamic modelling of the nature of speciation and phase behaviour of binary and ternary mixtures of formaldehyde, water and methanol, Molecular Physics, ISSN: 0026-8976
Formaldehyde is a highly reactive chemical that is usually sold and processed in the form of aqueous solutions, with methanol added for stability. In these solutions, formaldehyde reacts with the solvents to form a variety of reaction products, including oligomers. These chemical reactions can occur in the liquid and vapour phases and have a significant influence on the properties of formaldehyde-containing solutions. Of particular interest to industrial applications is the prediction of the vapour–liquid equilibria (VLE) in formaldehyde solutions, considering the chemical reactions. We use the SAFT-γ Mie group-contribution (GC) equation of state to obtain the fluid-phase behaviour of binary and ternary mixtures of formaldehyde with water and methanol. The oligomerisation reactions taking place in aqueous and methanolic solutions of formaldehyde are modelled implicitly using a physical approach, which is possible within the SAFT-γ Mie framework by adding association (reactive) sites that mediate the formation of the reaction products. Using this approach, the nature of the chemical speciation in formaldehyde + water, formaldehyde + methanol and formaldehyde + water + methanol mixtures is studied. A new group, CH (Formula presented.) O, characterising formaldehyde within the SAFT-γ Mie GC approach, is developed. Experimental data for the VLE in binary mixtures of formaldehyde + water and formaldehyde + methanol are used to obtain the optimal unlike interaction parameters between the corresponding SAFT-γ Mie groups. The newly developed parameters are used to predict the VLE of ternary formaldehyde + water + methanol mixtures for a wide range of temperatures and pressures, with excellent agreement to experimental data. Additionally, the SAFT-γ Mie approach is shown to provide accurate predictions of the distribution of reaction species (oligomers) in binary and ternary mixtures containing formaldehyde.
Kournopoulos S, Santos MS, Ravipati S, et al., 2022, The Contribution of the Ion-Ion and Ion-Solvent Interactions in a Molecular Thermodynamic Treatment of Electrolyte Solutions, JOURNAL OF PHYSICAL CHEMISTRY B, Vol: 126, Pages: 9821-9839, ISSN: 1520-6106
Walker PJ, Zhao T, Haslam AJ, et al., 2022, Ab initio development of generalized Lennard-Jones (Mie) force fields for predictions of thermodynamic properties in advanced molecular-based SAFT equations of state, JOURNAL OF CHEMICAL PHYSICS, Vol: 156, ISSN: 0021-9606
Kournopoulos S, Haslam AJ, Jackson G, et al., 2022, Molecular theory of the static dielectric constant of dipolar fluids, The Journal of Chemical Physics, ISSN: 0021-9606
Ravipati S, Simoes Santos M, Economou I, et al., 2021, Monte Carlo molecular simulation study of carbon dioxide sequestration into dry and wet calcite pores containing methane, Energy and Fuels, Vol: 35, Pages: 11393-11402, ISSN: 0887-0624
We perform grand canonical Monte Carlo (GCMC) simulations to study the adsorption of carbon dioxide in a calcite slit pore. The injection of carbon dioxide is simulated by increasing the chemical potential of carbon dioxide, which allows for an investigation of adsorption under varying carbon dioxide loadings. The study is carried out for three different environments: an empty pore; a pore containing methane; and a pore containing methane with trace amounts of water. We systematically investigate the impact of the presence of these other fluids on carbon dioxide adsorption. We study the influence of carbon dioxide loading on fluid density in the pore and examine individual fluid-density profiles (in the direction normal to the fluid–solid interface). The order of fluid adsorption affinity to the surface is found to be water > carbon dioxide > methane. The interpretation of our results is informed by the examination of free-energy-averaged fluid–substrate potentials, which are computed independently from the simulations. Our observations suggest that ignoring the presence of water could lead to overestimation not only of methane availability but also of carbon dioxide storage capacity in pores, with important consequences in, for example, modeling carbon dioxide sequestration in calcite-rich reservoirs. Ultimately, it is hoped that the molecular-level insights from this study will aid the multiscale modeling of reservoir fluids in the context of enhanced oil recovery and carbon dioxide sequestration.
Inguva PK, Walker PJ, Yew HW, et al., 2021, Continuum-scale modelling of polymer blends using the Cahn-Hilliard equation: transport and thermodynamics, Soft Matter, Vol: 17, Pages: 5645-5665, ISSN: 1744-683X
The Cahn–Hilliard equation is commonly used to study multi-component soft systems such as polymer blends at continuum scales. We first systematically explore various features of the equation system, which give rise to a deep connection between transport and thermodynamics-specifically that the Gibbs free energy of mixing function is central to formulating a well-posed model. Accordingly, we explore how thermodynamic models from three broad classes of approach (lattice-based, activity-based and perturbation methods) can be incorporated within the Cahn–Hilliard equation and examine how they impact the numerical solution for two model polymer blends, noting that although the analysis presented here is focused on binary mixtures, it is readily extensible to multi-component mixtures. It is observed that, although the predicted liquid–liquid interfacial tension is quite strongly affected, the choice of thermodynamic model has little influence on the development of the morphology.
Haslam AJ, Gonzalez-Perez A, Di Lecce S, et al., 2020, Expanding the Applications of the SAFT-gamma Mie Group-Contribution Equation of State: Prediction of Thermodynamic Properties and Phase Behavior of Mixtures, JOURNAL OF CHEMICAL AND ENGINEERING DATA, Vol: 65, Pages: 5862-5890, ISSN: 0021-9568
Walker PJ, Haslam AJ, 2020, A New Predictive Group-Contribution Ideal-Heat-Capacity Model and Its Influence on Second-Derivative Properties Calculated Using a Free-Energy Equation of State, JOURNAL OF CHEMICAL AND ENGINEERING DATA, Vol: 65, Pages: 5809-5829, ISSN: 0021-9568
Wehbe M, Haslam A, Adjiman CS, et al., 2020, Predicting optimal salt forms for active pharmaceutical ingredients using the SAFT-y mie equation of state
Ravipati S, Galindo A, Jackson G, et al., 2019, An investigation of free-energy-averaged (coarse-grained) potentials for fluid adsorption on heterogeneous solid surfaces, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 21, Pages: 25558-25568, ISSN: 1463-9076
Campos-Villalobos G, Ravipati S, Haslam AJ, et al., 2019, Modelling adsorption using an augmented two-dimensional statistical associating fluid theory: 2D-SAFT-VR Mie, Molecular Physics: An International Journal at the Interface Between Chemistry and Physics, Vol: 117, Pages: 3770-3782, ISSN: 0026-8976
We present an extension of the SAFT-VR Mie approach to model adsorption of molecular fluids based on a two-dimensional (2D) approximation to describe the adsorbed fluid. Analytical results are provided for the first- and second-order perturbation terms of the free energy for the 2D system. The adsorption model is based on the assumption that the particle pair interactions in the adsorbed and bulk phases are described with the same Mie potential exponents λa and λr, in contrast with the square-well version of the 2D-SAFT-VR approach in which it is considered necessary to modify the attractive ranges of the SW interactions. This important difference between the two approaches leads to a reduction in the number of molecular parameters to be determined. In order to demonstrate the performance of the 2D-SAFT-VR Mie approach, we present results for the the modelling of carbon dioxide (CO2) and methane (CH4) adsorbed onto dry coal.
Ibrahim D, Oyewunmi O, Haslam A, et al., 2019, Computer-aided working fluid design and optimisation of organic Rankine cycle (ORC) systems under varying heat-source conditions, 32ND INTERNATIONAL CONFERENCE ON EFFICIENCY, COST, OPTIMIZATION, SIMULATION AND ENVIRONMENTAL IMPACT OF ENERGY SYSTEMS
Ibrahim D, Oyewunmi O, Haslam A, et al., 2019, COMPUTER-AIDED WORKING FLUID DESIGN AND POWER SYSTEM OPTIMIZATION USING THE SAFT-γ MIE EQUATION OF STATE, 4th Thermal and Fluids Engineering Conference (TFEC)
Brumby P, Wensink R, Kournopoulos S, et al., 2019, Simulation of the surface tension of a nematic liquid crystal in contact with a planar solid
White M, Oyewunmi OA, Chatzopoulou M, et al., 2018, Computer-aided working-fluid design, thermodynamic optimisation and technoeconomic assessment of ORC systems for waste-heat recovery, Energy, Vol: 161, Pages: 1181-1198, ISSN: 0360-5442
The wider adoption of organic Rankine cycle (ORC) technology for power generation or cogeneration from renewable or recovered waste-heat in many applications can be facilitated by improved thermodynamic performance, but also reduced investment costs. In this context, it is suggested that the further development of ORC power systems should be guided by combined thermoeconomic assessments that can capture directly the trade-offs between performace and cost with the aim of proposing solutions with high resource-use efficiency and, importantly, improved economic viability. This paper couples, for the first time, the computer-aided molecular design (CAMD) of the ORC working-fluid based on the statistical associating fluid theory (SAFT)-γ Mie equation of state with thermodynamic modelling and optimisation, in addition to heat-exchanger sizing models, component cost correlations and thermoeconomic assessments. The resulting CAMD-ORC framework presents a novel and powerful approach with extended capabilities that allows the thermodynamic optimisation of the ORC system and working fluid to be performed in a single step, thus removing subjective and pre-emptive screening criteria that exist in conventional approaches, while also extending to include cost considerations relating to the resulting optimal systems. Following validation, the proposed framework is used to identify optimal cycles and working fluids over a wide range of conditions characterised by three different heat-source cases with temperatures of 150 °C, 250 °C and 350 °C, corresponding to small- to medium-scale applications. In each case, the optimal combination of ORC system design and working fluid is identified, and the corresponding capital costs are evaluated. It is found that fluids with low specific-investment costs (SIC) are different to those that maximise the power output. The fluids with the lowest SIC are isoheptane, 2-pentene and 2-heptene, with SICs of £5620, £2760 an
van Kleef LMT, Oyewunmi OA, Harraz AA, et al., 2018, Case studies in computer-aided molecular design (CAMD) of low- and medium-grade waste-heat recovery ORC systems, ECOS 2018 - 31st International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems, Publisher: ECOS
Organic Rankine cycle (ORC) engines are suitable for theconversion oflow-grade heat into useful power. While numerous substances are available asORC working-fluid candidates, computer-aided molecular design (CAMD) techniques allow the rigorous selection of an optimal working fluid during system optimisation. The aim of this present study is to extend an existing CAMD-ORC framework [1,2] by incorporating, in addition to thermodynamic performance objectives, economic objectives when determining the optimal systemdesign, while maintaining the facility of selecting optimal working fluids. The SAFT-γ Mie equation of state is used to predictthethermodynamic properties of theworking fluids(here, hydrocarbons)that are relevant to the systems’economic appraisalsand critical/transport properties are estimated using empirical group-contribution methods. System investment costs are estimated with equipment cost correlations for the key system components, andthe stochastic NSGA-II solver is used for system optimisation. From a set of NLP optimisations, it is concluded that the optimal molecular size of the working fluid is linked to the heat-source temperature. The optimal specific investment cost (SIC) values were £10,120/kW and£4,040/kW when using heat-source inlet temperatures of 150°Cand250°C (representative of low-and medium-gradeheat) respectively, andthe corresponding optimal working fluids were propane, 2-butane and 2-heptene.
Jackson G, dufal S, Lafitte T, et al., 2018, Corrigendum: The A in SAFT: developing the contribution of association to the Helmholtz free energy within a Wertheim TPT1 treatment of generic Mie fluids, Molecular Physics, Vol: 116, Pages: 283-285, ISSN: 0026-8976
brumby PE, wensink HH, haslam AJ, et al., 2017, Structure and interfacial tension of a hard-rod fluid in planar confinement, Langmuir, Vol: 33, Pages: 11754-11770, ISSN: 0743-7463
The structural properties and interfacial tension of a fluid of hard-spherocylinder rod-like particles in contact with hard structureless flat walls are studied by means of Monte Carlo simulation. The calculated surface tension between the rod fluid and the substrate is characterized by a non-monotonic trend as a function of bulk concentration (density) over the range of isotropic bulk concentrations. As suggested by earlier theoretical studies, a surface-ordering scenario can be confirmed from our simulations: the local orientational order close to the wall changes from uniaxial to biaxial nematic when the bulk concentration reaches about 85% of the value at the onset of the isotropic-nematic phase transition. The surface ordering coincides with a wetting transition whereby the hard wall is wetted by a nematic film. Accurate values of the fluid-solid surface tension, the adsorption, and the average particle-wall contact distance are reported (over a broad range of densities into the dense nematic region for the first time), which may serve as a useful benchmark for future theoretical and experimental studies on confined rod fluids. The simulation data are supplemented with predictions from a second-virial density functional theory, which are in good qualitative agreement with the simulation results.
Pantaleo AM, markides, Oyewunmi, et al., 2017, Integrated computer-aided working-fluid design and thermoeconomic ORC system optimisation, ORC-2017, Publisher: Elsevier, Pages: 152-159, ISSN: 1876-6102
The successful commercialisation of organic Rankine cycle (ORC) systems across a range of power outputs and heat-source temperatures demands step-changes in both improved thermodynamic performance and reduced investment costs. The former can be achieved through high-performance components and optimised system architectures operating with novel working-fluids, whilst the latter requires careful component-technology selection, economies of scale, learning curves and a proper selection of materials and cycle configurations. In this context, thermoeconomic optimisation of the whole power-system should be completed aimed at maximising profitability. This paper couples the computer-aided molecular design (CAMD) of the working-fluid with ORC thermodynamic models, including recuperated and other alternative (e.g., partial evaporation or trilateral) cycles, and a thermoeconomic system assessment. The developed CAMD-ORC framework integrates an advanced molecular-based group-contribution equation of state, SAFT-γ Mie, with a thermodynamic description of the system, and is capable of simultaneously optimising the working-fluid structure, and the thermodynamic system. The advantage of the proposed CAMD-ORC methodology is that it removes subjective and pre-emptive screening criteria that would otherwise exist in conventional working-fluid selection studies. The framework is used to optimise hydrocarbon working-fluids for three different heat sources (150, 250 and 350 °C, each with mcp = 4.2 kW/K). In each case, the optimal combination of working-fluid and ORC system architecture is identified, and system investment costs are evaluated through component sizing models. It is observed that optimal working fluids that minimise the specific investment cost (SIC) are not the same as those that maximise power output. For the three heat sources the optimal working-fluids that minimise the SIC are isobutane, 2-pentene and 2-heptene, with SICs of 4.03, 2.22 and 1.84 £/W res
Schoen M, Haslam AJ, Jackson G, 2017, Perturbation Theory versus Thermodynamic Integration. Beyond a Mean-Field Treatment of Pair Correlations in a Nematic Model Liquid Crystal., Langmuir, Vol: 33, Pages: 11345-11365, ISSN: 0743-7463
The phase behavior and structure of a simple square-well bulk fluid with anisotropic interactions is described in detail. The orientation dependence of the intermolecular interactions allows for the formation of a nematic liquid-crystalline phase in addition to the more conventional isotropic gas and liquid phases. A version of classical density functional theory (DFT) is employed to determine the properties of the model, and comparisons are made with the corresponding data from Monte Carlo (MC) computer simulations in both the grand canonical and canonical ensembles, providing a benchmark to assess the adequacy of the DFT results. A novel element of the DFT approach is the assumption that the structure of the fluid is dominated by intermolecular interactions in the isotropic fluid. A so-called augmented modified mean-field (AMMF) approximation is employed accounting for the influence of anisotropic interactions. The AMMF approximation becomes exact in the limit of vanishing density. We discuss advantages and disadvantages of the AMMF approximation with respect to an accurate description of isotropic and nematic branches of the phase diagram, the degree of orientational order, and orientation-dependent pair correlations. The performance of the AMMF approximations is found to be good in comparison with the MC data; the AMMF approximation has clear advantages with respect to an accurate and more detailed description of the fluid structure. Possible strategies to improve the DFT are discussed.
Jimenez-serratos M, Herdes C, Haslam A, et al., 2017, Group contribution coarse-grained molecular simulations of polystyrene melts and polystyrene solutions in alkanes using the SAFT-γ force field, Macromolecules, Vol: 50, Pages: 4840-4853, ISSN: 0024-9297
A coarse-grained (CG) model for atactic polystyrene is presented and studied with classical molecular-dynamics simulations. The interactions between the CG segments are described by Mie potentials, with parameters obtained from a top-down approach using the SAFT-γ methodology. The model is developed by taking a CG model for linear-chain-like backbones with parameters corresponding to those of an alkane and decorating it with side branches with parameters from a force field of toluene, which incorporate an “aromatic-like” nature. The model is validated by comparison with the properties of monodisperse melts, including the effect of temperature and pressure on density, as well as structural properties (the radius of gyration and end-to-end distance as functions of chain length). The model is employed within large-scale simulations that describe the temperature–composition fluid-phase behavior of binary mixtures of polystyrene in n-hexane and n-heptane. A single temperature-independent unlike interaction energy parameter is employed for each solvent to reproduce experimental solubility behavior; this is sufficient for the quantitative prediction of both upper and lower critical solution points and the transition to the characteristic “hourglass” phase behavior for these systems.
White MT, Oyewunmi OA, Haslam A, et al., 2017, Exploring optimal working fluids and cycle architectures for organic Rankine cycle systems using advanced computer-aided molecular design methodologies, 13th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics (HEFAT2017), Publisher: ICHMT
The combination of computer-aided molecular design(CAMD) with an organic Rankine cycle (ORC) power-systemmodel presents a powerful methodology that facilitates an in-tegrated approach to simultaneous working-fluid design andpower-system thermodynamic or thermoeconomic optimisation.Existing CAMD-ORC models have been focussed on simplesubcritical, non-recuperated ORC systems. The current workintroduces partially evaporated or trilateral cycles, recuperatedcycles and working-fluid mixtures into the ORC power-systemmodel, which to the best knowledge of the authors has not beenpreviously attempted. A necessary feature of a CAMD-ORCmodel is the use of a mixed-integer non-linear programming(MINLP) optimiser to simultaneously optimise integer working-fluid variables and continuous thermodynamic cycle and eco-nomic variables. In this paper, this feature is exploited by in-troducing binary optimisation variables to describe the cycle lay-out, thus enabling the cycle architecture to be optimised along-side the working fluid and system conditions. After describingthe models for the alternative cycles, the optimisation problemis completed for a defined heat source, considering hydrocar-bon working fluids. Two specific case studies are considered,in which the power output from the ORC system is maximised.These differ in the treatment of the minimum heat-source outlettemperature, which is unconstrained in the first case study, butconstrained in the second. This is done to replicate scenariossuch as a combined heat and power (CHP) plant, or applicationswhere condensation of the waste-heat stream must be avoided.In both cases it is found that a working-fluid mixture can per-form better than a pure working fluid. Furthermore, it is foundthat partially-evaporated and recuperated cycles are optimal forthe unconstrained and constrained case studies respectively.
White MT, Oyewunmi OO, Haslam AJ, et al., 2017, Industrial waste-heat recovery through integrated computer-aided working-fluid and ORC system optimisation using SAFT-γ Mie, Energy Conversion and Management, Vol: 150, Pages: 851-869, ISSN: 0196-8904
A mixed-integer non-linear programming optimisation framework is formulated and developed that combines a molecular-based, group-contribution equation of state, SAFT-γγ Mie, with a thermodynamic description of an organic Rankine cycle (ORC) power system. In this framework, a set of working fluids is described by its constituent functional groups (e.g., since we are focussing here on hydrocarbons: single bondCH3, single bondCH2single bond, etc. ), and integer optimisation variables are introduced in the description the working-fluid structure. Molecular feasibility constraints are then defined to ensure all feasible working-fluid candidates can be found. This optimisation framework facilitates combining the computer-aided molecular design of the working fluid with the power-system optimisation into a single framework, thus removing subjective and pre-emptive screening criteria, and simultaneously moving towards the next generation of tailored working fluids and optimised systems for waste-heat recovery applications. SAFT-γγ Mie has not been previously employed in such a framework. The optimisation framework, which is based here on hydrocarbon functional groups, is first validated against an alternative formulation that uses (pseudo-experimental) thermodynamic property predictions from REFPROP, and against an optimisation study taken from the literature. The framework is then applied to three industrial waste-heat recovery applications. It is found that simple molecules, such as propane and propene, are the optimal ORC working fluids for a low-grade (150 °C) heat source, whilst molecules with increasing molecular complexity are favoured at higher temperatures. Specifically, 2-alkenes emerge as the optimal working fluids for medium- and higher-grade heat-sources in the 250–350 °C temperature range. Ultimately, the results demonstrate the potential of this framework to drive the search for the next generation of ORC systems, and to
Oyewunmi OA, white MT, Chatzopoulou M, et al., 2017, Integrated Computer-Aided Working-Fluid Design and Power System Optimisation: Beyond Thermodynamic Modelling, 30th International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems (ECOS 2017), Publisher: ECOS-2017
Improvements in the thermal and economic performance of organic Rankine cycle (ORC) systems are requiredbefore the technology can be successfully implemented across a range of applications. The integration ofcomputer-aided molecular design (CAMD) with a process model of the ORC facilitates the combinedoptimisation of the working-fluid and the power system in a single modelling framework, which should enablesignificant improvements in the thermodynamic performance of the system. However, to investigate theeconomic performance of ORC systems it is necessary to develop component sizing models. Currently, thegroup-contribution equations of state used within CAMD, which determine the thermodynamic properties of aworking-fluid based on the functional groups from which it is composed, only derive the thermodynamicproperties of the working-fluid. Therefore, these do not allow critical components such as the evaporator andcondenser to be sized. This paper extends existing CAMD-ORC thermodynamic models by implementinggroup-contribution methods for the transport properties of hydrocarbon working-fluids into the CAMD-ORCmethodology. Not only does this facilitate the sizing of the heat exchangers, but also allows estimates of systemcosts by using suitable cost correlations. After introducing the CAMD-ORC model, based on the SAFT-γ Mieequation of state, the group-contribution methods for determining transport properties are presented alongsidesuitable heat exchanger sizing models. Finally, the full CAMD-ORC model incorporating the componentmodels is applied to a relevant case study. Initially a thermodynamic optimisation is completed to optimise theworking-fluid and thermodynamic cycle, and then the component models provide meaningful insights into theeffect of the working-fluid on the system components.
White M, Oyewunmi OA, Haslam A, et al., 2017, Incorporating Computer-Aided Working-Fluid Design in the System Optimisation of an Organic Rankine Cycle Using the SAFT-γ Mie Equation of State, SAFT 2017 Conference
White MT, Oyewunmi OA, Haslam AJ, et al., 2017, High-efficiency industrial waste-heat recovery through computer-aided integrated working-fluid and ORC system optimisation, The 4th Sustainable Thermal Energy Management International Conference (SusTEM 2017)
In this paper, we develop a mixed-integer non-linear programming optimisation framework that combines working-fluid thermodynamic property predictions from a group-contribution equation of state, SAFT- Mie, with a thermodynamic description of an organic Rankine cycle. In this model, a number of working-fluids are described by their constituent functional groups (i.e., -CH3, -CH2, etc.), and integer optimisation variables are introduced in the description of the structure of the working-fluid. This facilitates combining the computer-aided molecular design of novel working-fluids with the power system optimisation into a single framework, thus removing subjective and pre-emptive screening criteria, and simultaneously moving towards the next generation of tailored working-fluids and optimised organic Rankine cycle systems for industrial waste-heat recovery applications. The thermodynamic model is first validated against an alternative formulation that uses (pseudo-experimental) thermodynamic property predictions from REFPROP, and against an optimisation study taken from the literature. Furthermore, molecular feasibility constraints are defined and validated in order to ensure all feasible working-fluid candidates can be found. Finally, the optimisation problem is formulated using the functional groups from the hydrocarbon family, and applied to three industrial waste-heat recovery case studies. The results demonstrate the potential of this framework to drive the search for the next generation of organic Rankine cycles, and to provide meaningful insights into which working-fluids are the optimal choices for a targeted application.
Eriksen DK, Lazarou G, Galindo A, et al., 2016, Development of intermolecular potential models for electrolyte solutions using an electrolyte SAFT-VR Mie equation of state, Molecular Physics, Vol: 114, Pages: 2724-2749, ISSN: 1362-3028
We present a theoretical framework and parameterisation of intermolecular potentials for aqueous electrolyte solutions using the statistical associating fluid theory based on the Mie interaction potential (SAFT-VR Mie), coupled with the primitive, non-restricted mean-spherical approximation (MSA) for electrolytes. In common with other SAFT approaches, water is modelled as a spherical molecule with four off-centre association sites to represent the hydrogen-bonding interactions; the repulsive and dispersive interactions between the molecular cores are represented with a potential of the Mie (generalised Lennard-Jones) form. The ionic species are modelled as fully dissociated, and each ion is treated as spherical: Coulombic ion–ion interactions are included at the centre of a Mie core; the ion–water interactions are also modelled with a Mie potential without an explicit treatment of ion–dipole interaction. A Born contribution to the Helmholtz free energy of the system is included to account for the process of charging the ions in the aqueous dielectric medium. The parameterisation of the ion potential models is simplified by representing the ion–ion dispersive interaction energies with a modified version of the London theory for the unlike attractions. By combining the Shannon estimates of the size of the ionic species with the Born cavity size reported by Rashin and Honig, the parameterisation of the model is reduced to the determination of a single ion–solvent attractive interaction parameter. The resulting SAFT-VRE Mie parameter sets allow one to accurately reproduce the densities, vapour pressures, and osmotic coefficients for a broad variety of aqueous electrolyte solutions; the activity coefficients of the ions, which are not used in the parameterisation of the models, are also found to be in good agreement with the experimental data. The models are shown to be reliable beyond the molality range considered during parameter estimatio
White MT, Oyewunmi OA, Haslam AJ, et al., 2016, Integrated working-fluid design and ORC system optimisation for waste-heat recovery using CAMD and the SAFT-γ Mie equation of state, 3rd Annual Engine ORC Consortium Workshop
Kirmse CJW, Oyewunmi, Taleb A, et al., 2016, A two-phase single-reciprocating-piston heat conversion engine: Non-linear dynamic modelling, Applied Energy, Vol: 186, Pages: 359-375, ISSN: 0306-2619
A non-linear dynamic framework is presented for the modelling of a novel two-phase heat engine termed ‘Up-THERM’, which features a single solid moving-part (piston). When applied across the device, a constant temperature difference between an external (low- to medium-grade) heat source and an external heat sink is converted into sustained and persistent oscillations of pressure and volumetric fluid displacement. These oscillations are transformed in a load arrangement into a unidirectional flow from which power is extracted by a hydraulic motor. The Up-THERM engine is modelled using a system of first-order differential equations that describe the dominant thermal/fluid processes in each component of the device. For certain components where the deviations from a linear approximation are non-negligible (gas spring in the displacer cylinder, check valves and piston valve, and heat exchangers), a non-linear description is employed. A comparison between the linear and non-linear descriptions of the gas spring at the top of the displacer cylinder reveals that the non-linear description results in more realistic predictions of the oscillation frequency compared to experimental data from a similar device. Furthermore, the shape of the temperature profile over the heat-exchanger surfaces is modelled as following a hyperbolic tangent function, based on findings from an experimental investigation. Following the validation of these important device components, a parametric study is performed on the Up-THERM engine model with the aforementioned non-linear component descriptions, aimed at investigating the effects of important geometric parameters and of the heat-source temperature on key performance indicators, namely the oscillation frequency, power output and exergy efficiency of the engine. The results indicate that the geometric design of the displacer cylinder, including the height of the gas spring at the top of the cylinder, and the heat-source temperature hav
Kirmse CJW, Oyewunmi OA, Haslam AJ, et al., 2016, Comparison of a novel organic-fluid thermofluidic heat converter and an organic Rankine cycle heat engine, Energies, Vol: 9, ISSN: 1996-1073
The Up-THERM heat converter is an unsteady, two-phase thermofluidic oscillator that employs an organic working fluid, which is currently being considered as a prime-mover in small- to medium-scale combined heat and power (CHP) applications. In this paper, the Up-THERM heat converter is compared to a basic (sub-critical, non-regenerative) organic Rankine cycle (ORC) heat engine with respect to their power outputs, thermal efficiencies and exergy efficiencies, as well as their capital and specific costs. The study focuses on a pre-specified Up-THERM design in a selected application, a heat-source temperature range from 210 °C to 500 °C and five different working fluids (three n-alkanes and two refrigerants). A modeling methodology is developed that allows the above thermo-economic performance indicators to be estimated for the two power-generation systems. For the chosen applications, the power output of the ORC engine is generally higher than that of the Up-THERM heat converter. However, the capital costs of the Up-THERM heat converter are lower than those of the ORC engine. Although the specific costs (£/kW) of the ORC engine are lower than those of the Up-THERM converter at low heat-source temperatures, the two systems become progressively comparable at higher temperatures, with the Up-THERM heat converter attaining a considerably lower specific cost at the highest heat-source temperatures considered.
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