RESUMO Neste trabalho investigou-se a eficiência computacional na solução do problema de equilíbrio termodinâmico sólido-líquido (ESL) em óleos parafínicos. Foram avaliados diversos modelos termodinâmicos para a fase líquida: a equação de estado de Peng-Robinson (PR-EOS) e os modelos de coeficiente de atividade Wilson preditivo, UNIversal QUAsiChemical (UNIQUAC) preditivo e UNIQUAC Functionalgroup Activity Coefficient (UNIFAC), além do modelo de solução ideal. A fase sólida é caracterizada pelo modelo multisólido e a previsão de formação de fase sólida é inicialmente avaliada por um teste de estabilidade termodinâmica. Posteriormente, o sistema de equações não-lineares (SENL) formado pelas equações de equilíbrio termodinâmico e de balanço material é resolvido pelo método de Newton multivariável. Até onde pudemos verificar, não há estudos sistemáticos sobre o efeito do tipo de modelo (e sua complexidade) no tempo de computação necessário para a solução do problema. Por outro lado, tendo em conta o elevado número de componentes nas frações de petróleo estudadas, aspectos como a robustez (frente a diferentes estimativas iniciais) e tempo de computação são aspectos importantes neste contexto e que foram analisados neste trabalho.

ABSTRACT In this paper, the computational efficiency in the solid-liquid equilibrium (SLE) problem for paraffinic oils was examined. Several different thermodynamic models were evaluated for liquid phase: the Peng-Robinson equation of state (PR-EOS), and the activity coefficient models for ideal solution, the predictive Wilson model, the predictive Universal Quasi-chemical (UNIQUAC) model, and UNIQUAC Functional-group Activity Coefficient (UNIFAC) model. The solid phase is characterized by the multisolid model and the prediction of solid phase formation is initially estimated by a thermodynamic stability test. Afterwards, the systems of non-linear equations (SNLE) formed by the thermodynamic equilibrium and the material balance equations was solved using Newton-Raphson method (multivariate). As far as we can see, there are no systematic studies addressing the effect of the model (and its complexity) on the computational effort of the algorithm. On the other hand, considering the high number of different components in the oil fractions studied, the robustness (due to different initial estimates), and the computational time are important aspects and were analyzed in this work.

RESUMO. A análise de consistência e as condições de estabilidade da equação de difusão com fluxo bimodal são tratadas neste trabalho. Foram, também, apresentados diversos detalhes sobre a maneira como a equação diferencial foi discretizada no âmbito do método de Volumes Finitos incluindo a forma que as condições de contorno foram definidas nas equações discretizadas.

ABSTRACT Consistency analysis and stability conditions of the bi-flux diffusion equation are discussed in this paper. Several details are presented on how the differential equation has been discretized based on the Finite Volume Method.

In the present work, the mechanical behavior of stress corrosion cracking phenomenon is described. Such phenomenon presents strong complexity due to metallurgic and electrochemical aspects. A methodology for modeling both SSR (Slow Strain Rate) and CL (Constant Load) tests based upon thermodynamics of continuum solids and elastoplastic damage is proposed. In this macroscopic approach, besides the classical variables (stress, total strain, plastic strain), an additional scalar variable related to the damage induced by stress corrosion is introduced. An evolution law depending on the corrosive environment parameters is proposed for this damage variable. The model accounts for the stress corrosion effect through a reduction of the mechanical resistance of the material induced by the damage variable. The model prediction is compared with the curves obtained experimentally in different acid solutions at room temperature showing a good agreement. The alloy/environment system studied here is an AISI 304 austenitic stainless steel in acid aqueous solution containing sodium chloride.