Analysis of the Maximum Efficiency and the Maximum Net Power as Objective Functions for Organic Rankine Cycles Optimization.

Maximum efficiency and maximum net power output are some of the most important goals to reach the optimal conditions of organic Rankine cycles. This work compares two objective functions, the maximum efficiency function, ß, and the maximum net power output function, ω. The van der Waals and PC-SAFT equations of state are used to calculate the qualitative and quantitative behavior, respectively. The analysis is performed for a set of eight working fluids, considering hydrocarbons and fourth-generation refrigerants. The results show that the two objective functions and the maximum entropy point are excellent references for describing the optimal organic Rankine cycle conditions. These references enable attaining a zone where the optimal operating conditions of an organic Rankine cycle can be found for any working fluid. This zone corresponds to a temperature range determined by the boiler outlet temperature obtained by the maximum efficiency function, maximum net power output function, and maximum entropy point. This zone is named the optimal temperature range of the boiler in this work.

Molecular modelling techniques for predicting liquid-liquid interfacial properties of methanol plus alkane (n-hexane, n-heptane, n-octane) mixtures.

In this work, the liquid-liquid interfacial properties of methanol plus n-alkane (n-hexane, n-heptane, n-octane) mixtures are investigated at atmospheric pressure by two complementary molecular modelling techniques; namely, molecular dynamic simulations (MD) and density gradient theory (DGT) coupled with the PC-SAFT (perturbed-chain statistical associating fluid theory) equation of state. Furthermore, two molecular models of methanol are used, which are based on a non-polarisable three site approach. On the one hand, is the original (flexible) TraPPE-UA model force field. On the other hand, is the rigid approximation denoted as OPLS/2016. In both cases, n-alkanes are modelled using the TraPPE-UA model. Simulations are performed using the direct coexistence technique in the ensemble. Special attention is paid to the comparison between the estimations obtained from different methanol models, the available experimental data and theoretical calculations. In all cases, the rigid model is capable of predicting the experimental phase equilibrium and interfacial properties accurately. Unsurprisingly, the methanol-rich density and interfacial tension are overestimated using the TraPPE model combined with Lorentz-Berthelot mixing rules for predicting the mixture behaviour. Accurate comparison between MD and DGT plus PC-SAFT requires consideration of the cross-interactions between individual influence parameters and fitting the ßij values. This latter aspect is particularly important because it allows the exploitation of the link between the EOS model and the direct molecular simulation of the corresponding fluid. At the same time, it was demonstrated that the key property defining the interfacial tension value is the absolute concentration of methanol in the methanol-rich phase. This behaviour indicates that there are more hydrogens bonded with each other, and they interact favourably with an increasing number of carbon atoms in the alkane.

On the phase and interface behavior along the three-phase line of ternary Lennard-Jones mixtures: a collaborative approach based on square gradient theory and molecular dynamics simulations.

This work focuses on the application of a two-way approach, where Molecular Dynamics (MD) simulations and the Square Gradient Theory (SGT) have been used for describing the phase and interface behavior of binary and ternary Lennard-Jones (LJ) mixtures, along a condition of three-phase equilibrium. The unequivocal correspondence between MD and SGT has been achieved by using the global phase diagram of binary mixtures composed by equally sized Lennard-Jones molecules, from which representative molecular parameters for Type-I, Type-II, and Type-III systems have been determined. The so selected binaries have been used then to scale the behavior of a ternary mixture characterized by complex phase equilibrium patterns. For the case of the theoretical SGT approach applied to the Lennard-Jones equation of state was used for predicting phase equilibrium and interfacial properties. In addition the corresponding MD simulations of these macroscopic properties have been conducted for the LJ potential by using equivalent molecular parameters and conditions than in the theoretical approach. Excellent agreement has been observed between the predictions obtained from theory and simulations. Particularly, our results concerning the characterization of the three phase line of a binary Type-III mixture indicate that the bulk liquid (α) and the bulk gas (G) regions are sharply separated by a bulk liquid region (ß) for all the explored temperature, pressure, and concentration conditions. The structural analysis of these bulk phases reveals that a secondary liquid phase (ß) perfectly wets the liquid-gas interface (α-G), as previously found for Type-II mixture [A. Mejía and L. F. Vega, J. Chem. Phys. 124, 244505 (2006)]. The exploration along the three-phase line for the ternary mixture shows good agreement between SGT and MD. Particularly, we observed the specific influence of a third component in the phase and interface behavior. From all the previous results, we conclude that the SGT applied to an EoS with appropriate mixing rules produces reliable predictions of the properties of ternary mixtures.

Molecular Insights into the Wettability and Adsorption of Acid Gas-Water Mixture.

Sequestration of acid gas in geological formations is a disposal method with potential economic and environmental benefits. The process is governed by variables such as gas-water interfacial tension, wetting transition, and gas adsorption into water, among other things. However, the influence of the pressure and temperature on these parameters is poorly understood. This study investigates these parameters using coarse-grained molecular dynamics (CG-MD) simulations and density gradient theory (DGT). Simulations were carried out at 313.15 K and a pressure range of 0-15 MPa. A comparison was made against H2S-water systems to clarify the effects of adsorption on interfacial tension due to vapor-liquid-liquid equilibrium. The predicted H2S-water interfacial tension and phase densities by CG-MD and DGT matched the experimental values well. The adsorption can be quantified via the Gibbs Adsorption function Γ12, which correlated well with the three-phase transition. On the one hand, pressure increments below the three-phase transition revealed a significant adsorption of H2S. On the other hand, above the three-phase transition, the Gibbs Adsorption capacity remained constant, which indicated a saturation of H2S at the water surface due to liquid-liquid equilibrium. Finally, H2S behaves markedly differently in wetting transition, rather than the involved for CO2 to different molecular layers beneath the surface of aqueous solutions. In this respect, H2S is represented by a first-order wetting transition while CO2 presents a critical wetting. Finally, it has also been found that the preferential adsorption of H2S over the H2O interface is greater if compared to that of CO2, due to its strong interaction with water. In fact, we have also demonstrated that CO2 under triphasic conditions strongly influences the wetting of the ternary system.

Accurate and Model-Free Control Function for a Single Stage Transcritical Refrigerator Cycle.

Transcritical cycles are a successful and probed system in engineering practices, particularly in refrigeration. Therefore, their optimization is a critical factor in the design, control, and operation in order to maximize the coefficient of performance (COP) and to find the optimal pressure operating conditions. Often, this labor is faced using empirically based correlations, which are limited by their origin and the configuration of the cycle. In this regard, this work is devoted to the development of a rigorous and general framework in order to characterize the behavior and performance of a simple transcritical refrigeration cycle. An accurate mathematical expression for the COP depending on the compressor efficiency and the properties of the working fluid is presented. The expression proposed has no approximations and is relevant to any model depending on the Helmholtz variable group, being easy to combine with any equation of state (EOS), regardless of its complexity. From this expression, it is possible to derive a simple control function for transcritical refrigeration cycles. As an example, the expression is combined with the Span-Wagner EOS, presenting a comprehensive application for a transcritical cycle using CO2 and N2O as working fluids.