RESUMO

The literature reveals the application of the laws of thermodynamics for predicting life span and the effects of the physical activity level on longevity. But the vastly simplified literature models seem to suggest a reduction in duration of life with increased activity level, which is the opposite of medical recommendations, that means that exercises increase the longevity. The main objectives of this paper are to re-present the previous model, check and confirm the previous results and improve the model by formulating a simplified phenomenological relation between life span, specific entropy generation of the body (SEG-life, in MJ/kg.K) and physical activity level. The model was validated considering different individuals. In this study, it is suggested that the principle of cumulative entropy generation limit should be relaxed in function of lifestyle and type of exercise performed during life, differently from what it is defined by the literature. So, it is proposed a relax to the limit on SEG-life as a result of various physical activity levels.

Assuntos

RESUMO

A model including simultaneous droplet heating and water evaporation is proposed to simulate temperature, shrinking and mass profiles of a spherical droplet subjected to convective drying, being valid for the first drying stage. Experimental data on drying skim milk and colloidal silica obtained in the literature were used for validation, but there is no restriction in the model that prevents it from being suitable for other materials. There were not significant differences observed concerning to the droplet components (dissolved or insoluble materials). The initial heating time of the particle upon reaching the constant temperature is relatively short ($\Delta t\approx7s$) for both simulated materials and water evaporation during the first drying stage occurs mostly at the wet bulb temperature of the air. Discrepancy between simulated and experimental values did not exceed $9\%$ for skim milk and $7\%$ for colloidal silica in this first stage, indicating good applicability of the model. Considering the applicability of the model in a more generic way, Whitaker correlation evaluated at the film temperature showed better results. Finally, the small discrepancy found is discussed and some improvements are proposed.

Assuntos

RESUMO

Cement is one of the most used building materials in the world. The process of cement production involves numerous and complex reactions that occur under different temperatures. Thus, there is great interest in the optimization of cement manufacturing. Clinker production is one of the main steps of cement production and it occurs inside the kiln. In this paper, the dry process of clinker production is analysed in a rotary kiln that operates in counter flow. The main phenomena involved in clinker production is as follows: free residual water evaporation of raw material, decomposition of magnesium carbonate, decarbonation, formation of C3A and C4AF, formation of dicalcium silicate, and formation of tricalcium silicate. The main objective of this study was to propose a mathematical model that realistically describes the temperature profile and the concentration of clinker components in a real rotary kiln. In addition, the influence of different speeds of inlet gas and solids in the system was analysed. The mathematical model is composed of partial differential equations. The model was implemented in Mathcad (available at CCA/UFES) and solved using industrial input data. The proposal model is satisfactory to describe the temperature and concentration profiles of a real rotary kiln.