费托合成工艺热力学性质的计算化学研究

本文采用高精度的G4方法,全面计算了不同反应条件下费托合成工艺中可能的1287个产物的热力学数据,然后用这些数据得到的热力学量用于分析实际化工生产的热力学和分析费托合成的产物分布.结果表明:降温、加压和增大氢碳比(H_2/CO)时,热力学上可能生成的产物数目增多.在低温、高压和高碳氢比下,很多产物都在热力学上可以生成,其中产物的选择性主要由动力学因素控制.另一方面,升温或者降压可以提高小分子产物的选择性.值得注意的是在降温、加压和增大碳氢比到一定条件时,产物的平衡产率会达到最大值并且不随条件改变而变化,这说明优化条件改变产率是有一定限度的.热力学分析同样对设计和评价费托合成的反应机理有重大意义,其中甲醛的平衡产率很低,可以排除含有甲醛的反应路径.近期有一些采用金属氧化物-分子筛双功能催化剂高选择性获得C_(2-4)烯烃和芳烃的报道,其中有很多可能进入分子筛孔道的中间体,分析结果显示乙烯酮、甲醇和二甲醚是可能的中间体.

Computational Study on Thermodynamic Properties of Fischer-Tropsch Synthesis Process

Using the highly accurate G4 method, we computed the thermodynamic data of 1287 possible reaction products under a wide range of reaction conditions in the Fischer-Tropcsh synthesis (FTS) process. These accurate thermodynamic data provide basic thermodynamic quantities for the actual chemical engineering process and are useful in analyzing product distribution because FTS demonstrates many features of an equilibrium-controlled system. Our results show that the number of thermodynamically allowed products to increase when lowering temperature, raising pressure, and raising H₂/CO ratio. At low temperature, high pressure and high H₂/CO ratio, many products are thermodynamically allowed and the selectivity of product has to be controlled by kinetic factors. On the other hand, high selectivity of lighter products can be realized in thermodynamics by raising temperature and lowering pressure. We found that the equilibrium product yield will reach a maximum and remain unchanged when lowering temperature, raising pressure, and raising H₂/CO ratio to some limits, implying that optimizing reaction conditions has no effect on equilibrium product yields beyond these limits. The thermodynamic analysis is also useful in designing and evaluating FTS reaction mechanisms. We found that reaction pathways through formaldehyde should be discarded because of its extremely low equilibrium yield. Recently, in the FTS process using metal-oxide-zeolite catalysts for the highly selective production of C₂-C₄ olefins and aromatic hydrocarbons, there are several guesses on the possible reaction intermediates entering the zeolite channel. Our results show that ketene, methanol, and dimethyl ether are three possible reaction intermediates.