Análise termodinâmica das reformas do metano (oxidativa e autotérmica), do etanol e da nafta

Abstract

Catalytic reforming of hydrocarbons or other organic species is a technology usually employed in either improving traditional energy sources or producing alternative energy sources. Amongst the technologies employed in improving traditional energy sources, one can include catalytic reforming of petrochemical naphtha. Although being also employed to produce hydrogen, the main goal of this reforming is to transform naphthenic compounds and paraffins in branched-chain isoparaffins and aromatic compounds, used to produce polymers and to increase gasoline octane rating. When the reforming processes are applied to produce alternative energy sources, attention has mostly been given to hydrogen production. Hydrogen is mainly produced from non-renewable fossil fuels, especially by means of steam catalytic reforming of methane, main component of natural gas, and in refineries, through oxidative reforming of higher hydrocarbons. More recently, hydrogen production from catalytic reforming of ethanol has also been studied. It has the advantage of reduced carbon dioxide emission, easiness of storage and distribution of ethanol and a higher yield of hydrogen. The composition of the reformate is very dependent on the variables involved in the process, such as pressure, temperature and reactant feed ratios. The effect of these variables can be studied by means of definition and analysis of some performance parameters such as conversion, yield, selectivity, coke deposition and others. Usually, the first step for this type of investigation is the accomplishment of a thermodynamic analysis for each process through methods of Gibbs free energy minimization. These methods result in non-linear algebraic equation systems, solved numerically with the aid of appropriated software. In this work, a Lagrange Multipliers method based thermodynamic analysis is conducted for the oxidative and autothermal reforming of methane; steam, dry, oxidative and autothermal reforming of ethanol; and naphtha reforming. The main goals of this work are to determine the linearly independent reactions which represent the chemical equilibrium of each reforming system and to foresee the best conditions in which each reaction system should be operated to reach specific goals. To validate the simulations, the results are compared with experimental and simulation data specific of chemical equilibrium. The set of linearly independent reactions of each reforming system was determined and validated through mole balances for each species involved in the process. Ethanol reforming systems showed higher hydrogen yields compared to those of methane reforming. Among all methane reforming systems, oxidative reforming showed the higher yield, with a value of 200%, for a temperature of 1273 K, atmospheric pressure, without feeding oxygen. For ethanol reforming systems, the higher hydrogen yield, with a value of 479%, was obtained for steam reforming in a temperature of 1110 K, 1 atm of pressure and H2O/C2H5OH feed ratio equal to 6. Among the methane reforming systems, autothermal reforming deposited the lesser amount of coke (0,03 moles). Concerning the ethanol reforming systems, autothermal reforming also deposited the lesser amount of coke, with a value of 0,02 moles. For naphtha reforming, it was verified that hydrogen and methane are the most abundant species constituting the reformate. Carbon formation increased a lot when increasing the operational temperature, but it was possible to decrease it in the same magnitude increasing the operational pressure and the H2/Naphtha feed ratio. To increase the aromatics yield, it was necessary to raise both the temperature and the pressure. The resolution of the non-linear algebraic equation systems was carried out with the open-source software Scilab.