Browsing by Author "Novais, Augusto Q."
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- Analysis of processing systems involving reaction and distillation : the synthesis of ethyl acetatePublication . Filipe, Rui M.; Castro, Pedro; Matos, Henrique A.; Novais, Augusto Q.The integration of reaction and separation into a single process unit, i. e., reactive destillation, may offer several advantages over conventional systems that use a reactor followed by a distillation column. In this paper we explore the operational characteristics of reactive distillation and highlight some of this potential benefits, using the production of ethyl acetate as an illustrative example. With this aim, the two types of system are compared employing different reactor types and a number of performance indicators, such as yield, conversion, purity, specific energy consumption and residence time. A sensitivity analysis is carried out on some variables and parameters, in order to explore and define the distillation columns operating conditions. As expected, results point to a clear advantage of reactive distillation allowing for the azeotrope to be surpassed and for the overcoming of chemical equilibrium, favouring an increase in conversion and product purity, along with reduced operating costs.
- Assessing cell polarity reversal degradation phenomena in PEM fuel cells by electrochemical impedance sectroscopyPublication . Travassos, Maria Antónia; Lopes, Vitor V.; Silva, R. A.; Novais, Augusto Q.; Rangel, C. M.Electrochemical impedance spectroscopy (EIS) is identified as one of the most promising in-situ diagnostics tools available for assessing fuel cell ageing and degradation. In this work, the degradation phenomena caused by cell polarity reversal due to fuel starvation of an open cathode 16 membrane electrode assembly (MEA) – low power (PEM) fuel cell (15 W nominal power) – is reported using EIS as a base technique. Measuring the potential of individual cells, while the fuel cell is on load, was found instrumental in assessing the “state of health” of cells at fixed current. Location of affected cells, those farthest away from hydrogen entry in the stack, was revealed by very low or even negative potential values. EIS spectra were taken at selected break-in periods during fuel cell functioning. The analysis of impedance data was made using an a priori equivalent circuit describing the transfer function of the system in question –equivalent circuit elements were evaluated by a complex non-linear least square (CNLS) fitting algorithm, and by calculating and analyzing the corresponding distribution of relaxation times (DRT). Results and interpretation of cell polarity reversal due to hydrogen starvation were complemented with ex-situ MEA cross section analysis, using scanning electron microscopy. Electrode thickness reduction and delamination of catalyst layers were observed as a result of reactions taking place during hydrogen starvation. Carbon corrosion and membrane degradation by fluoride depletion are discussed.
- Assessing cell polarity reversal degradation phenomena in PEM Fuel Cells by electrochemical impedance spectroscopyPublication . Travassos, Maria Antónia; Lopes, Vitor V.; Novais, Augusto Q.; Rangel, C. M.The mechanisms of fuel cell degradation are multiple and not well understood. Irreversible changes in the kinetic and/or transport properties of the cell are fostered by thermal, chemical and mechanical issues which constrain stability, power and fuel cell lifetime. Within the in-situ diagnostics methods and tools available, in-situ electrochemical impedance spectroscopy (EIS) is within the most promising to better understand and categorize changes during fuel cell ageing. In this work, the degradation phenomena caused by cell polarity reversal due to fuel starvation of an open cathode 16 MEA (membrane-electrode assembly) –low power PEM fuel cell (15 W nominal power) is reported using EIS as a base technique. A frequency response analyzer from Solartron Model 1250 was used connected to an electrochemical interface also from Solartron, Model 1286. The range of covered frequencies spans from 37000 Hz to 0.01Hz. Hydrogen is supplied from a metallic hydride small reactor with a capacity of 50 NL H2 at a pressure of 0.2 bar. Measuring the potential of individual cells, while the fuel cell is on load, was found instrumental in assessing the “state of health” of cells at fixed current. Location of affected cells, those farthest away from hydrogen entry in the stack, was revealed by the very low or even negative potential values. EIS spectra were taken at selected break-in periods during fuel cell functioning. The analysis of impedance data is made using two different approaches: using an a priori equivalent circuit describing the transfer function of the system in question -equivalent circuit elements were evaluated by a complex non-linear least square (CNLS) fitting algorithm, and by calculating and analyzing the corresponding distribution of relaxation times (DRT) -avoiding the ambiguity of the a priori equivalent circuit and the need for provision of the initial fitting parameters. A resistance and two RQ elements connected in series are identified as describing the impedance response of the cell during normal functioning. A constant phase element (CPE) was chosen to describe the impedance observed behavior. The quality of the fit was evaluated by analysis of the residuals between the fit result and the measured data at every single point. Consistency and quality of the impedance data were established by Kramers-Kronning validation. With continuous operation, using a reduced hydrogen flow, an inversion of polarity was observed in the 16th cell of the stack, evident in the potential measurement of individual cells as a result of insufficient hydrogen to reach the last cells. EIS data analyses suggest that water electrolysis happens at the anode judging by the appearance of an intermediate semicircle associated to a marked change in resistance and capacitance values. The presence of an inductive loop at low frequencies is now evident, which cannot be explained by the relaxation of reaction intermediates involved in the oxygen reduction reaction [1]. It is to be noticed that when the incursion into the negative potential values is not too marked the phenomenon is partially reversible, so it is suggested that the relaxation is due to intermediates in the water electrolysis process. The anode potential rose to levels compatible with the oxidation of water. Once the phenomenon is made irreversible and when water is no longer available, oxidation of the carbon support is favored accelerating catalyst sintering. Ex-situ MEA cross section analysis, under a scanning electron microscope, confirmed it. Electrode thickness reduction and delamination of catalyst layers were observed as a result of reactions taking place during hydrogen starvation. Carbon corrosion and membrane degradation are analyzed, according to evidence by SEM.
- Design and planning of green supply chains: a fuzzy approachPublication . Pinto, Tânia; Barbosa-Póvoa, Ana Paula; Novais, Augusto Q.Green SC can be seen as logistic structures that guarantee production and global distribution of products in an environmental manner. To achieve this goal companies must invest on the optimal design and planning of their logistic structures, while accounting for the trade-off between profits and environmental impacts. This is addressed using a generic and uniform mathematical framework, the RTN. For this bi-level optimization a SFLP approach is applied, where those objectives are treated as constraints and replaced by a new one, the aspiration level, which embodies a compromise between them.
- Deterministic optimization of the thermal Unit Commitment problem: A Branch and Cut searchPublication . Marcovecchio, Marian G.; Novais, Augusto Q.; Grossmann, Ignacio E.This paper proposes a novel deterministic optimization approach for the Unit Commitment (UC) problem, involving thermal generating units. A mathematical programming model is first presented, whichincludes all the basic constraints and a set of binary variables for the on/off status of each generator ateach time period, leading to a convex mixed-integer quadratic programming (MIQP) formulation. Then,an effective solution methodology based on valid integer cutting planes is proposed, and implementedthrough a Branch and Cut search for finding the global optimal solution. The application of the pro-posed approach is illustrated with several examples of different dimensions. Comparisons with other mathematical formulations are also presented.
- Electrochemical impedance spectroscopy modeling using the dis-tribution of relaxation times and error analysis for fuel cellsPublication . Lopes, Vitor V.; Rangel, C. M.; Novais, Augusto Q.This paper proposes a new approach to determine the distribution of relaxation times (DRT) directly from the electro-chemical impedance spectroscopy (EIS) data, i.e. without the use of an equivalent electrical circuit model. The method uses a generalized fractional-order Laguerre basis to represent EIS where both the parameters of the basis and their co-efficients are estimated by solving a nonconvex minimization problem. Furthermore, the DRT confidence region is de-termined to assess the accuracy and precision of the DRT estimate. The approach is applied to analyze the dominant dynamic properties of an open-cathode hydrogen fuel-cell under different current and air-flow conditions. Results showed that the estimated DRT closely reconstructs EIS data even when there is a higher variance at smaller relaxation times.
- A goal programming approach for the retrofit of supply chain networksPublication . Pinto, Tânia; Barbosa-Póvoa, Ana Paula; Novais, Augusto Q.In order to achieve sustainability, the design and planning of a supply chain has to fulfil economic, social and environmental objectives. Traditionally the design of supply chains has been based on economic objectives. As societal environment concerns grow, environmental aspects are also emerging, not only at the industry level, but also within the context of supply chain management. The investment towards logistics structures that consider both economic and environmental performance is nowadays an important research topic. However, much is still to be done. This paper, addresses the retrofit of supply chain networks where planning aspects are also considered. The supply chain network design and planning is modeled through a Resource-Task-Network (RTN) methodology. A mixed integer linear programming (MILP) multi-objective approach is developed, which attempts to simultaneously maximize the annual profit of the supply chain, taking into account the network retrofit, while environmental impacts are minimized. The environmental impacts are accounted for through the Eco-indicator 99 methodology. Profit and environmental impacts are balanced through the use of goal programming. The model applicability is illustrated through the solution of an example.
- Heuristic algorithm for the piecewise linear segmentation of multiple time-series for solar thermal systems inverse modellingPublication . Lopes, Vitor V.; Ferro, Filipa; Carvalho, Maria João; Novais, Augusto Q.This paper presents a novel algorithm for the piecewise linear segmentation of multivariate time-series and proposes its application to the analysis of hot water thermal solar systems (TSS). The ISO 9459-5:2007 norm describes a non-intrusive dynamic test for the performance assessment of TSS. This allows to characterize the system heat losses and the thermal stratification properties, as well as to predict its long-term performance. The application of this norm requires an inverse modeling approach where the parameters of a simplified plug flow storage model, based on simulation runs, are determined through an optimization procedure aiming at the adjustment of the predicted results to those obtained by a predefined experimental test sequence (3-5 days). This paper proposes a new method to decrease the computation time required for the model simulation, which is based on the segmentation of the multivariate time-series into a piecewise linear approximation, where the number of segments is critically selected. An illustrative example is presented consisting in the simulation of a real 3-day experimental dataset with 26873 points and a 15 s sampling rate.
- Hydrogen PEMFC stack performance analysis : a data-driven approachPublication . Lopes, Vitor V.; Novais, Augusto Q.; Rangel, C. M.For low power fuel cells, it is paramount that management of reactants, water and heat, be realized in a passive fashion in order to minimize parasitic losses. Effective fuel, oxygen supply and water management for reliable performance are also greatly affected by cell geometry and materials. Fuel cells are complex systems to optimize on a mere experimental basis. As an aid to this goal, data-driven analysis techniques, requiring no mathematical model to be fixed a priori, are gaining a reputation in other fields of work, where a phenomenological modeling approach might be intractable. This work presents a characterization study of a 12W PEMFC series stack by means of a new data-driven technique, M-NMF. The stack was developed for low temperature operation, uses own designed flow field plates, integrated in a series configuration, and is operated for 12 combinations of hydrogen/air flowrate ratios, generating as many polarization curves. M-NMF is applied, in combination with an alternating least squares algorithm, to the analysis of the overvoltage data matrix derived from the original experimental polarization data. From this analysis, it is possible to group and differentiate data according to similar overvoltage patterns and gain insight into their relative contribution to fuel cell performance immunization.
- Interdisciplinary approach to automated negotiation: a preliminary reportPublication . Lopes, Fernando; Novais, Augusto Q.; Coelho, HelderAutonomous agents with negotiation competence are becoming increasingly important and pervasive. This paper follows an interdisciplinary approach to build autonomous negotiating agents by considering both game-theoretic techniques and bargaining procedures from the social sciences. The paper presents a generic model that handles bilateral multi-issue negotiation, describes equilibrium strategies for the bargaining game of alternating offers, and formalizes important strategies used by human negotiators. Autonomous agents equipped with the model are able to negotiate under both complete and incomplete information, thereby making them very compelling for automated negotiation.
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