D₂-tracer Study Of The Fischer-Tropsch Synthesis For Product Distribution

In the Fischer-Tropsch synthesis (FTS) process, synthesis gas, a mixture of predominantly CO and H₂, is converted to a multi-component mixture of hydrocarbons on catalysts. Currently, a promising topic in the catalysis and energy industry is the conversion of coal or natural gas to environmentally clean fuels and specialty chemicals with the FTS process. In this process, the development of the catalysts and the industrial reactor are the key problems. It is well known that the product distribution of FTS is determined by Anderson-Schulz-Flory (ASF) rule and then the high selective formation of hydrocarbons is difficult. Base on this,the study for the product distribution and the selectivity to hydrocarbon is very important for the study in FTS reaction. If only we are familiar with the product distribution, we can get the product selectivity by controlling the component of the catalyst, the praparation of the catalyst and the reaction conditions.The earlier study suggested that the product obtained during FTS followed an Anderson-Schulz-Flory (ASF) distribution and theαvalue,the probability of chain growth, was a constant for hydrocarbon. But the deviations from this ASF distribution were observed. To explain these deviations, some theories and models were proposed. However, none of these theories can adequately explain all of these chain-length related problems.In order to obtain the correct product distribution data and paraffin to olefin ratio for a FTS reaction test, we conducted a Co catalyzed Fischer-Tropsch synthesis in a fixed bed reactor (FBR) as well as in a continuously stirred tank reactor (CSTR) using D₂/H₂ switching method. In addtion, the isotopic effect of using D2 and H2 during Fischer-Tropsch synthesis was investigated over Fe and Co catalyst. The main results are as following:1. The Fischer-Tropsch reaction was carried out in a large continuously stirred tank reactor but also in a small fixed bed reactor by using D₂/H₂ switching method and we found that product accumulation does occur and we obtained the correct product distribution data and paraffin to olefin ratio for a FTS reaction test1) Product accumulation in Fischer-Tropsch synthesis (FTS) occurs not only in large continuously stirred tank reactors, but also in small fixed bed reactors. The product accumulation problems in FBR runs are less problematic than in CSTR runs. The fn value is the percentage of products produced that can be collected during each sample collection period and the fn value is a function of Henry's law constant and the reaction conditions such as gas flow and the reactor type.2) After accumulation corrections, the product distribution obtained by the D2/H2 switching method for a Co catalyzed FTS reaction conducted in the FBR reactor follows the Anderson-Schulz-Flory (ASF) distribution and there is only oneαvalue.3) After accumulation and secondary hydrogenation corrections, the paraffin to olefin ratio for the hydrocarbons produced by the FTS reaction is much smaller than the values obtained by the conventional method.2. The isotopic effect of the Fischer-Tropsch synthesis was studied over Fe and Co catalyst.1) The Fischer-Tropsch reaction was carried out in a large continuously stirred tank reactor but also in a small fixed bed reactor by using D2/H2 switching method. When the feeds were switched from CO/H2/N2 to CO/D2/N2, CO conversation increased and an inverse isotopic effect was observed.2) The isotopic effects of using D2 and H2 during Fischer-Tropsch synthesis were investigated using Fe and Co catalysts. The magnititude of the isotopic effect observed over both Fe and Co catalyst is no more than 1 and noticeable inverse isotopic effects have been observed.3) The inverse isotopic effects maybe arise from a complex combination of the kinetic and thermodynamic isotope effects.