Product Distribution And Mechanistic Kinetic Modeling Of The Fischer-Tropsch Synthesis

The Fischer-Tropsch Synthesis?FTS?is a catalytic reaction which can convert coal,natural gas and biomass into urgently needed liquid fuels and valuable chemicals under the catalyst,and it has a wide spread application prospect.Considering the difficulties of the FT product collection and data analysis,and the complex of the active sites of the catalysts,the FT reaction mechanism and reaction kinetics based on the mechanism are still in controversy.It is hoped to promote the development and optimization of catalysts and providing accurate information to reactor design,simulation,and optimization through FT reaction mechanism and reaction kinetics study.An accurate FT product distribution is the basis of mechanism study and the key of the product prediction using FTS kinetic models,The main purpose work of this thesis is aimed at the FT product distribution and the related kinetic modeling.This work replaced the traditional multiple-step collection of Fischer-Tropsch products by an one-step cooling method.The products(C₁-C₃₀ hydrocarbons and alcohols)in the gas phase were analyzed by on-line GCs with high-temperature heating,while the liquids(C₁₀-C₇₀ hydrocarbons)were analyzed by off-line GCs.The gases and the liquids were accurately measured by using internal standards,respectively.The work checked the accuracy and the stability of the high-temperature on-line analysis of the gas components.Some more reliable experimental results,e.g.the paraffin,olefin and alcohol formations catalyzed by an industrial iron-based catalyst,were given by this high-efficiency equipment.A correlation between the carbon number and the detailed products was developed.Furthermore,the work estimated the flash loss in product collecting and also the water condensation in the separator with the varied operating conditions by Aspen Plus modeling.It showed that the flash loss is negligible,and the water condensed in the separator,which will be mixed with the liquid wax,could enlarge the yields of the components in C₁₀-C₂₀ region.The separator running at higher temperature and lower pressure would be necessary to avoid the water condensation in the liquid wax.The chain growth mechanism of the F-T reaction have been investigated by ethylene adding under different reaction conditions in a high-pressure fixed-bed reactor over an industrial iron-based catalyst.The details are as following:?1?.C₁ monomer could be appeared by C-C bond cleavage of ethylene under the induction of hydrogen environment over an iron catalyst.the C₁ monomer could be hydrogenated to CH₄ the high-carbon-number hydrocarbons are produced by C-C coupling,it reinforces the surface carbide mechanism.?2?.Ethylene can easily be hydrogenated into ethane in high ratio of H₂/C₂H₄,this is the reason why alkene/alkane ratio decreases sharply in F-T reaction.?3?.The molar selectivity of hydrocarbon increased with the increase of reaction temperature and the ratio of H₂/C₂H₄,and the selectivity of hydrocarbon with higher carbon number increased.Based on the accuracy of experimental data with the product distribution and the carbide mechanism of the model compounds addition,the paper based on the hydrogen Assisted CO Insertion Mechanism,we assumed that alkyl generated paraffin via hydrogenation,alkyl generated olefin via?-elimination,and the intermediate species which contain oxygen generated alcohol via hydrogenation.The rate parameter of rate equation simplify the number of independent parameter,through the Single-Event method,and we have the detailed and accurate FT synthesis product kinetics prediction with 20parameters.