Abstract:

The product synthetic gas compositions in methane combined reforming are governed by thermodynamic equilibrium and the relevant coke deposition reactions are also dependent on operation conditions. Hence, quantitative thermodynamic analysis of the influences of operation conditions (feedstock composition, temperature, pressure) on product compositions and coke formation reactions are desirable for process design. In this contribution, the RGibbs reactor in Aspen Plus software was used to calculate Gibbs free energies at varied temperatures, the influence of feeding compositions, temperature and pressure on the compositions of equilibrated reaction mixtures by taking into consideration all relevant reactions. Both methane and carbon dioxide conversions increased as a consequence of temperature increase or pressure decrease, as reflected by the endothermic nature of the volume expansion reaction. When stoichiometric feed [CH₄∶(CO₂+H₂O)=1∶1] was adopted, the influence of different conditions on the composition of reaction products was explored by changing the reaction temperature, pressure and feed composition and the equilibrium conversion of CH₄ and CO₂ increased with increasing temperature and tended to the limit with increasing temperature for all stoichiometric ratios. Meanwhile, methane conversion increased and the carbon deposition increased with increasing carbon dioxide to methane ratios, while methane conversion elevation and suppression of carbon deposition could be achieved by increasing the proportion of steam, even under pressurized operations. The H₂/CO ratios in the product syngas could be manipulated to meet the required low concentrations for unconverted methane. With respect to specified downstream use of syngas, to generated methanol, ethanal, acetic acid and Fischer-Tropsch synthesis, the optimized operation conditions was identified. These calculations provided a thermodynamic basis for selection of bi-reforming conditions, process and catalyst design.

Key words: thermodynamics, Aspen Plus, methane combined reforming, Gibbs free energy, carbon dioxide, natural gas

摘要：

甲烷联合重整产物合成气的组成受热力学平衡控制，反应中积炭反应也受操作条件的影响，因此，计算反应条件（原料组成、温度、压力）对产物组成和积炭形成的影响，进行定量热力学分析是工艺设计所需要的。本文使用Aspen Plus软件中的RGibbs模块，建立甲烷联合重整热力学反应模型，采用吉布斯自由能最小化法，考 虑所有相关反应的进料组成、温度和压力对平衡反应混合物组成和积炭的影响。首先，设定化学计量比 CH₄∶（CO₂+H₂O）=1∶1进料，调控CO₂、H₂O相对进料比例，结果发现：CO₂/CH₄比增加时，CH₄和CO₂转化率升高，但积炭量也相应增加，H₂/CO比减小；CH₄和CO₂的平衡转化率随温度升高而升高，且随着温度升高趋于极限，可通过调节温度从而调控H₂/CO比达到所需生产要求。然后，研究了压力对反应平衡的影响，结果表明CH₄和CO₂的热力学平衡转化率随压力升高而降低，积炭量增加。最后，结合进料比、温度和压力的影响，针对合成气制下游化学产品的不同需求以及特定氢碳比精准调控，模拟计算出合适的反应进料比例，为将来实际生产调控提供了理论计算依据。

关键词: 热力学, Aspen Plus, 甲烷联合重整, 吉布斯自由能, 二氧化碳, 天然气