Influence of temperature during pyrolysis of Fe-alginate: Unraveling the pathway towards highly active Fe/C catalysts

Transition metals supported on carbons play an important role in catalysis and energy storage. By pyrolysis of metal alginate, highly active catalysts for the Fischer-Tropsch synthesis (FTS) can be produced. However, the evolution of the carbon (alginate) and transition metal (Fe3+) during pyrolysis remains largely unknown and was herein corroborated with several advanced in situ techniques. Initially, Fe3+ was reduced to Fe2+, while bound to alginate. FeO nucleated above 300 °C, destabilizing the alginate functional groups. Increasing temperatures improved carbonization of the carbon support, which facilitated reduction of FeO to α-Fe at 630 °C. Catalysts were produced by pyrolysis between 400 and 700 °C, where the highest FTS activity (612 µmolCO gFe−1 s−1) was achieved for the sample pyrolyzed at low temperature. Lower metal loading, due to less decomposition of alginate, moderated sintering and yielded larger catalytic surface areas. The results provide valuable knowledge for rational design of metal-alginate-based materials.publishedVersio

Modeling Fischer–Tropsch kinetics and product distribution over a cobalt catalyst

A detailed kinetic model describing the consumption of key components and productdistribution in the Fischer–Tropsch synthesis (FTS) over a 20%Co/0.5Re γ-Al2O3commercial catalyst is developed. The developed model incorporates the H2O-assisted CO dissociation mechanism developed by Rytter and Holmen and a novelapproach to product distribution modeling. The model parameters are optimizedagainst an experimental dataset comprising a range of process conditions: total pres-sure 2.0–2.2 MPa, temperature 210–230C, CO conversion range of 10%–75% andfeed with and without added water. The quality of the model fit measured in termsof mean absolute relative residuals (MARR) value is 23.1%, which is comparable to lit-erature reported values. The developed model can accurately describe both positiveand negative effects of water on the rate kinetics, the positive effect of water on thegrowth factor, temperature and syngas composition on the kinetics and product dis-tribution over a wide range of process conditions, which is critical for the design andoptimization of the Fisher–Tropsch reactors.publishedVersio

Deactivation and Regeneration of Commercial Type Fischer-Tropsch Co-Catalysts—A Mini-Review

Deactivation of commercially relevant cobalt catalysts for Low Temperature Fischer-Tropsch (LTFT) synthesis is discussed with a focus on the two main long-term deactivation mechanisms proposed: Carbon deposits covering the catalytic surface and re-oxidation of the cobalt metal. There is a great variety in commercial, demonstration or pilot LTFT operations in terms of reactor systems employed, catalyst formulations and process conditions. Lack of sufficient data makes it difficult to correlate the deactivation mechanism with the actual process and catalyst design. It is well known that long term catalyst deactivation is sensitive to the conditions the actual catalyst experiences in the reactor. Therefore, great care should be taken during start-up, shutdown and upsets to monitor and control process variables such as reactant concentrations, pressure and temperature which greatly affect deactivation mechanism and rate. Nevertheless, evidence so far shows that carbon deposition is the main long-term deactivation mechanism for most LTFT operations. It is intriguing that some reports indicate a low deactivation rate for multi-channel micro-reactors. In situ rejuvenation and regeneration of Co catalysts are economically necessary for extending their life to several years. The review covers information from open sources, but with a particular focus on patent literature

Perspectives on the Effect of Water in Cobalt Fischer-Tropsch Synthesis

Water is an inherent component in Fischer–Tropsch synthesis on Co catalysts, and the effect of water is discussed with an emphasis on alumina and aluminates as support materials. Water may affect the selectivity, activity, deactivation, and state of the catalyst, and without exception, water is known to enhance the C5+ selectivity by increasing the chain propagation α value. The effect of water depends on the catalyst. Small-pore γ-Al2O3 is less efficient at high water content than large-pore γ-Al2O3. The effect of water on selectivity is independent of its origin: i.e., adding water to the feed has the same effect as water produced by the reaction. Arguments are provided for the effect of water being partially mechanistic in nature and occasionally due to pore condensation. In particular, we introduce water-assisted CO activation to methylidyne as an option for generation of polymerization monomers. An additional factor that needs to be considered is that high water partial pressure is concurrent with reduction in hydrogen partial pressure. The influence of water strongly depends on the type of reactor employed

Water as key to activity and selectivity in Co Fischer-Tropsch synthesis: γ-alumina based structure-performance relationships

Thirteen γ-alumina supports with different pore sizes and pore size distributions have been impregnated with cobalt and rhenium. The catalysts were tested for Fischer-Tropsch synthesis (FTS) under dry and enhanced water vapor pressure conditions. It is shown that there is a positive correlation between pore size and cobalt crystallite size of the reduced catalyst after incipient wetness impregnation as long as the pore size distribution is sufficiently narrow. There is a concurrent increase in C5+ with pore diameter due to higher chain propagation probabilities α3+. Higher α values are ascribed to larger cobalt crystallites that promote CO activation resulting in higher surface coverage of CHx monomers. Adding water vapor to the syngas feed has a strong positive effect on selectivity to higher hydrocarbons and activity. Water imposes a significant enhancement of α1 that is attributed to higher relative cobalt surface coverage of H2O(OH−) and CHx relative to hydrogen. Small pores are susceptible to condensation of water during FTS.acceptedVersio

Boosting carbon efficiency of the biomass to liquid process with hydrogen from power: The effect of H2/CO ratio to the Fischer-Tropsch reactors on the production and power consumption

Carbon efficiency of a biomass to liquid process can be increased from ca. 30 to more than 90% by adding hydrogen generated from renewable power. The main reason is that in order to increase the H2/CO ratio after gasification to the value required for Fischer-Tropsch (FT) synthesis, the water gas shift reaction step can be avoided; instead a reversed water gas shift reactor is introduced to convert produced CO2 to CO. Process simulations are done for a 46 t/h FT biofuel production unit. Previous results are confirmed, and it is shown how the process can be further improved. The effect of changing the H2/CO ratio to the Fischer-Tropsch synthesis reactors is studied with the use of three different kinetic models. Keeping the CO conversion in the reactors constant at 55%, the volume of the reactors decreases with increasing H2/CO ratio because the reaction rates increase with the partial pressure of hydrogen. Concurrently, the production of C5+ products and the consumption of hydrogen increases. However, the power required per extra produced liter fuel also increases pointing at optimum conditions at a H2/CO feed ratio significantly lower than 2. The trends are the same for all three kinetic models, although one of the models is less sensitive to the hydrogen partial pressure. Finally, excess renewable energy can be transformed to FT syncrude with an efficiency of 0.8–0.88 on energy basis