Gustavo Mendes Platt

Theoretical and Experimental Aspects of the Corrosivity of Simulated Soil Solutions

Corrosion of buried steel pipe is a permanent engineering problem and, albeit the counter measures against degradation, when the corrosion process takes place, the damage has costly impact. In order to study the corrosion behavior of pipelines, it is possible to use actual soil extracts or simulated soil solutions. The extract is much related to specific sites and consequently too strict to permit a general understanding. The simulated soil presents, as advantage, its inorganic characteristic and easy preparation. In this paper, we present some theoretical results concerning the chemical equilibria of NS1, NS2, NS3, and NS4 simulated soil solutions. Besides, we have studied the effect of the above four media in corrosion behavior and polarization curves were performed for an API 5L X65 steel. The theoretical findings show that each ionic concentration varies for a 6–12 pH range. The experimental data suggested that the corrosion currents decrease as high is the pH and increase as high is the chloride content. Notwithstanding these facts, for multielectrolyte solutions, a simple correlation with a given ion is not straightforward but the complementary approaches used here give useful insights.

The Luus-Jaakola algorithm applied to a nuclear reactor core design optimisation

The Luus-Jaakola (LJ) algorithm is a random search optimisation method that has been successfully employed mainly in chemical engineering problems. In this paper, we apply this algorithm to an optimisation problem solved previously with genetic algorithms, particle swarm optimisation and Metropolis algorithms. This problem consists of adjusting several reactor cell parameters, such as dimensions, enrichment and materials, in order to minimise the average peak factor in a three-enrichment-zone reactor, considering restrictions on the average thermal flux, criticality and submoderation. LJ is compared with the aforementioned optimisation algorithms and is shown to perform well, thus demonstrating its potential for other applications.

Modeling of Closed-Vessel Experiments and Ballistic Parameter Estimation

ABSTRACT Closed vessels have been used for the regression of lumped ballistic parameters for decades. If material and energy balances are coupled with burning rate empirical correlations, uncorrelated parameters can be adjusted, describing accurately the thermochemical behavior of the generated gases, even if the chemical composition of the propellant is unknown (aging of the propellant). This work presents a system of differential Equations integrated to produce pressure profiles within the vessel that are highly dependent on the choice of empirical parameters. Such parameters are manipulated via maximum likelihood statistical procedures, leading to the best set of parameters to describe the propellant.

A study of equivalent electrical circuit fitting to electrochemical impedance using a stochastic method

Display Omitted A mathematical development regarding the minimization of residual least squares is presented.A statistically valid objective function of the complex nonlinear regression problem is obtained.A global search stochastic optimization algorithm is used on an electrochemical impedance system modelling problem.The population based stochastic method was used directly to obtain confidence regions to the estimated parameters.A Sensitivity Analysis was performedto assess the influence of each parameter onreal and imaginary parts of the model. Modeling electrochemical impedance spectroscopy is usually done using equivalent electrical circuits. These circuits have parameters that need to be estimated properly in order to make possible the simulation of impedance data. Despite the fitting procedure is an optimization problem solved recurrently in the literature, rarely statistical significance of the estimated parameters is evaluated. In this work, the optimization process for the equivalent electrical circuit fitting to the impedance data is detailed. First, a mathematical development regarding the minimization of residual least squares is presented in order to obtain a statistically valid objective function of the complex nonlinear regression problem. Then, the optimization method used in this work is presented, the Differential Evolution, a global search stochastic method. Furthermore, it is shown how a population-based stochastic method like this can be used directly to obtain confidence regions to the estimated parameters. A sensitivity analysis was also conducted. Finally, the equivalent circuit fitting is done to model synthetic experimental data, in order to demonstrate the adopted procedure.

Numerical experiments with new metaheuristic algorithms in phase equilibrium problems

In this work, we presented some numerical experiments with two recently proposed metaheuristics (symbiotic organisms search and water wave optimisation) in the parameter estimation of an excess Gibbs free energy model (the Wilson model, using vapour-liquid equilibrium data) and in the calculation of azeotropes (non-reactive and reactive azeotropes). Both problems are characterised by nonlinear algebraic systems with multiple solutions, converted into optimisation problems with non-negative merit functions. The numerical results indicated that these new stochastic optimisation algorithms are capable to find the expected solutions in these real-world problems with robustness and accuracy. Moreover, fluid phase equilibrium problems can be viewed as challenging and useful tests for new metaheuristics.

Thermodynamic simulation of phosphate precipitation based on ion-selective microelectrode measurements

Simulated body fluids (SBFs) are largely used as biocompatibility screening and also as an alternative process to the plasma spray coating. Biomimetic processes occur heterogeneously on metallic surface; therefore, the use of a microelectrode technique is highly advisable to detect surface changes. In this work, the local pH was monitored during the first 44 h of deposition of calcium phosphates on titanium samples with a simplified SBF solution. Three-dimensional data (local pH, x-y coordinates) were used to simulate equilibrium concentrations of all species. Based on these quantities, relative supersaturations and Gibbs energy variations of hydroxyapatite and octacalcium phosphate were evaluated. The conjoint experimental and theoretical approach suggested that neither local alkalinity nor free calcium concentration alone produces an adequate scenario of precipitation conditions, but the use of both procedures results in interesting findings not achieved using only a single approach.

Double azeotropy: calculations with Newton-like methods and continuous GRASP (C-GRASP)

Azeotropy is a thermodynamic phenomenon where a boiling liquid produces a vapour with identical composition. This situation is a challenge for separation engineering, since distillation processes explore the differences among relative volatilities and, thus, an azeotrope can be a barrier for the separation. In binary mixtures, the azeotropy calculation problem is characterised by a 2 × 2 non-linear system with pressure or temperature and a molar fraction as unknowns. An interesting and rare case is the so-called double azeotropy, when this non-linear system has two solutions, corresponding to two distinct azeotropes. In this paper, we present some numerical aspects of double azeotropy using classical approaches (Newton’s methods with globalisation techniques) and a new non-linear optimisation algorithm (the C-GRASP method).

Estudo teórico e experimental da Solução Avaliadora de Bioatividade (SAB)

The contact of materials such as metals used as biomaterials in simulated body solutions (SBF) that mimetize the inorganic and acellular part of blood plasma allows the evaluation of its capacity of calcium phosphate precipitation. These phosphates can precipitate by biomimetic route in the interface materialtissue, being important when the material is in vivo implanted. In addition, there are several solutions that favor this biomimetic deposition such as named as KOKUBO, NEUMAN and RINGER, and they are used in the promotion of precipitation. These solutions employed several reagents; some of them are not necessary in the stoichiometry of the main calcium phosphates. From a theoretical study that showed the possibility of using a simplified solution, we decide to investigate this solution in vitro precipitation conditions. In this way, samples of Ti were grinding, treated with 1M NaOH and subsequently exposed to simplified SBF. After the exposure of surfaces, they were analyzed by SEM/EDS and XRD to characterize the deposits. The experimental results show that the precipitated phase is octacalcium phosphate, which corroborate theoretical findings.

Designed Solution to Coat Titanium Sheets with Octacalcium Phosphate

In this work, a solution able to precipitate calcium phosphate in titanium samples was studied. At first, a thermodynamic analysis of the proposed solution was conducted using a computational simulator that considers most of chemical reactions and evaluates parameters such as activity of species. After this procedure, experimental tests were performed in order to confirm this precipitation. With the use of TRIS at concentration of 50mM, the deposits were basically composed of octacalcium phosphate, as confirmed in some characterization techniques. The deposit presents a thickness of approximately 15μm after a 7-day exposure in the designed solution.

An Overview of the Use of Metaheuristics in Two Phase Equilibrium Calculation Problems

In this chapter, we presented an overview of metaheuristic algorithms applied in several problems of phase equilibrium calculation, particularly in: (1) the prediction of reactive azeotropy (mainly when two reactive azeotropes occur); and (2) the calculation of dew point pressures in systems with double retrograde vaporization. These problems are frequently addressed in the literature, and several metaheuristics (for instance, Differential Evolution, Flower Pollination Algorithm , Symbiotic Organisms Search , among others) were applied in order to obtain all the solutions for each problem. We conducted a review of previously published results, as well as we applied a statistical analysis to compare the performance of different algorithms.