Fischer-Tropsch synthesis steady-state

Fuels and petrochemicals from CO2 via Fischer-Tropsch synthesis - steady state catalyst activity and selectivity... 

Luo, M., O Brien, R.J., Bao, S., and Davis, B.H. 2003. Fischer-Tropsch synthesis Induction and steady-state activity of high-alpha potassium promoted iron catalysts. Appl. Catal. A Gen. 239 111-20. 

Dautzenberg et al. (3) have determined the kinetics of the Fischer-Tropsch synthesis with ruthenium catalysts. The authors showed, that because the synthesis can be described by a consecutive mechanism, the non steady state behaviour of the catalyst can give information about the kinetics of the process. On ruthenium they found that not only the overall rate of hydrocarbon production per active site is small, but also that the rate constant of propagation is low. Hence, Dautzenberg et al. find that the low activity of Fischer-Tropsch catalysts is due to the low intrinsic activity of their sites. On the other hand, Rautavuoma (4) states that the low activity of cobalt catalysts is due to a small amount of active sites, the amount being much smaller than the number of adsorption sites measured. 

Although metals or even promoted metals have very low sulfur tolerances in synthesis reactions, other materials, such as metal oxides, nitrides, borides, and sulfides, may have greater tolerance to sulfur poisoning because of their potential ability to resist sulfidation (18). The extremely low steady-state activities of Co, Ni, and Ru metals in a sulfur-contaminated stream actually correspond to the activities of the sulfided metal surfaces. However, if more active sulfides could be found, their activity/selectivity properties would be presumably quite stable in a reducing, H2S-containing environment. This is, in fact, the basis for the recent development of sulfur active synthesis catalysts (211-215), which are reported to maintain stable activity/ selectivity properties in methanation and Fischer-Tropsch synthesis at H2S levels of 1% or greater. Happel and Hnatow (214), for example, reported in a recent patent that rare-earth and actinide-metal-promoted molybdenum oxide catalysts are reasonably active for methanation in the presence of 1-3% H2S. None of these patents, however, have reported intrinsic activities... 

Selectivity control continues to be a critical issue in Fischer-Tropsch chemistry, a catalytic process that dates back more than seventy years [1]. Operating conditions can be adjusted to control selectivities but overall effects are limited [2-4]. During Fischer-Tropsch synthesis with conventional bulk iron catalysts, various phases, including metal, metal carbides and metal oxides are present at steady-state catalytic conditions [5-7]. 

The effect of water on the Fischer-Tropsch synthesis over alumina-supported cobalt catalysts has been studied using isotopic transient kinetic methods (SSITKA) in combination with steady-state measurements. Water has been introduced to the catalytic system as a pretreatment procedure as well as under reaction conditions. The SSITKA results showed a decrease in the number of active surface sites, but no change in the specific site activity. 

The compositional modulation technique has been applied to the Fischer-Tropsch synthesis (FTS) reaction [2-5], It was found that the cyclic feeding of CO/H2 had an influence on the selectivity of the FTS products. Among the conclusions was that for an iron catalyst the selectivity for methane increased under periodic operation compared to the steady state operation [5], In the study [5] it was found that the propane/propene ratio increased under periodic operation and the largest changes were with periods between one and ten minutes. Due to the limitations of the anal5dical technique utilized, they could not separate ethane and ethene so that the selectivity basis was for the C3 hydrocarbons. In this study the analytical procedure permitted analysis of products only to the Cg-compoimds. 

The feasibility of increasing the selectivity of the Fischer-Tropsch synthesis by periodic operation is investigated. The process is modeled in a dynamic form using a CSTR reactor. The dynamic behavior of the model corresponds with in literature reported experimental results. The simulation results show a 20% increase in selectivity to Diesel range products compared to the best steady-state solution using a blockprofile. The profile has a cycle time of 1.1 O seconds and consists of a base composition of almost pure carbon monoxide and a pulse of 95% hydrogen and 5% carbon monoxide during 10.8% of the cycle time. 

This study explores the potential of periodic operation for the Fischer-Tropsch synthesis aiming at Diesel range products. The approach followed is modeling the process in a dynamic form using a simple CSTR reactor configuration. The kinetic scheme is based on steady-state data reported in literature. The steady-state behavior is in agreement with experimental observations reported earlier by various research groups. 

Liu QS, Zhang ZX, Zhou JL. Steady-state and dynamic behavior of fixed-bed catalytic reactor for Fischer-Tropsch synthesis I Mathematical model and numerical method. Journal of Natural Gas Chemistry 1999 8 137-150. 

Lang X, Akgerman A, Bukur DB. Steady state Fischer-Tropsch synthesis in supercritical propane. Ind. Eng. Chem. Res. 1995 34 72-77. 

This XPS investigation of small iron Fischer-Tropsch catalysts before and after the pretreatment and exposure to synthesis gas has yielded the following information. Relatively mild reduction conditions (350 C, 2 atm, Hg) are sufficient to totally reduce surface oxide on iron to metallic iron. Upon exposure to synthesis gas, the metallic iron surface is converted to iron carbide. During this transformation, the catalytic response of the material increases and finally reaches steady state after the surface is fully carbided. The addition of a potassium promoter appears to accelerate the carbidation of the material and steady state reactivity is achieved somewhat earlier. In addition, the potassium promoter causes a build up on carbonaceous material on the surface of the catalysts which is best characterized as polymethylene. 

The FTS was conducted at varying temperatures (from 483 to 513 K) over approximately 50 h of reaction time in order to investigate the reaction kinetics achieved with the respective catalysts. A typical conversion curve using the Co/ HB catalyst as an example is shown in Figure 2.3. After a short settling phase (caused by the pore filling of liquid Fischer-Tropsch products) of only about 4 h, steady-state conditions were reached. In the observed synthesis period of 50 h no deactivation of the catalysts was detected. However, industrially relevant experiments over several weeks are still outstanding. 

Eliason, S.A., and Bartholomew, C.H. 1997. Temperature-programmed reaction study of carbon transformations on iron Fischer-Tropsch catalysts during steady-state synthesis. Stud. Surf. Sci. Catal. 111 517-26. 

Temperature-Programmed Reaction Study of Carbon Transformations on Iron Fischer-Tropsch Catalysts During Steady-State Synthesis... 

Another example of the power of this method was its application to the Fischer Tropsch(90) synthesis. This reaction was first run with CO and H2 until a steady state was attained. The reactive gases were flushed off with helium and deuterium was then introduced. The hydrocarbons produced were all completely deuterated suggesting that the active intermediate on the surface was carbon. The deuterium tracer work was carried out extensively in England by C. Kemball and G. C. Bond and in the United States by R. L. Burwell, Jr. 

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