Influence of concentration waves on the fischer—tropsch reaction

Investigation of the hydrocarbon rate formation behaviour during the Fischer—Tropsch synthesis under periodic concentration forcing reveals the existence of resonance peaks for certain choices of cycle parameters. These peaks appeared under symmetrical cycling with a mean feed composition of 17–5% CO (balance H₂) at both low (less than 0-001 cps) and high (greater than 0-01 cps) frequencies for the alkanes. Although methane was more strongly stimulated under periodic operation than other hydrocarbons, the rate enhancement observed for C₂ to C₆ products signalled the potential for ‘tuning’ selectivity in this mode of operation.     

The effects of catalyst surface heterogeneity on the fischer-tropsch synthesis

We find that the surface energetic heterogeneity of Fischer-Tropsch catalysts is either not important kinetically or may be masked by the effect of increasing molecular size, so that the classical Anderson-Schulz-Flory distribution is routinely observed in product distributions. The anomalous yields of C₁ and C₂ observed in many cases may also be due to surface heterogeneity but can in practice be readily, and more conveniently, accounted for by assigning individual rate constants to their formation. The “break” sometimes observed in ASF distributions cannot be explained by surface heterogeneity and is best explained by the presence of two distinct modes of product formation.     

Composition modulation of the fischer-tropsch synthesis over a supported cobalt catalyst

Different cycling strategies are explored to see if carbon chain growth can be enhanced and methane formation suppressed. Of the strategies considered, bang-bang cycling between H₂ and CO feeds substantially increased the consumption of CO and H₂, but the formation of higher hydrocarbons and olefins was reduced. The best strategy for longer chain hydrocarbons suitable for jet or diesel fuels was found to be cycling between syngas mixtures. None of the cycling strategies was able to produce C₈₊ or low-molar mass olefin yields that matched yields found in steady-state operation.     

Rate and selectivity modification in fischer—tropsch synthesis over charcoal supported molybdenum by forced concentration cycling

Forced concentration cycling of the feed between pure CO and pure H₂ was used to successfully change both the selectivities and reactivities of promoted and unpromoted charcoal supported molybdenum catalysts in Fischer-Tropsch synthesis. It was found that with the unpromoted catalyst the rate enhancement increases with temperature and selectivity shifts towards methane. At the lower temperatures concentration cycling increases selectivity to ethane and higher hydrocarbons to levels only achievable with promoted catalysts. Periodic operation with the potassium promoted catalyst results in small rate enhancements but the olefin to paraffin ratio is dramatically changed without changing the carbon number distribution.     

Rate equations for the fischer‐tropsch reaction on a promoted iron catalyst

Intrinsic rates for the Fischer‐Tropsch synthesis reaction over a promoted iron catalyst fabricated at the Research Institute of the Petroleum Industry (RIPI) have been obtained in the temperature range of 290°C to 310°C, pressure range of 1500 to 2300 kPa, molar hydrogen to carbon monoxide ratio of 0.76 to 1.82, and a space velocity of 3300 h^?1 under conditions of constant catalyst activity. To this end, the initial reaction rates have been measured at constant temperature (±1°C) in the absence of diffusion limitations, and power‐law equations have been fitted in terms of the hydrogen and carbon monoxide partial pressures for the reaction rates.     

Dynamics of the fischer-tropsch synthesis over a supported cobalt catalyst

Step-change experiments between H₂, CO, and syngas mixtures with time resolution of ca. 0.3 s were undertaken to critically test mechanisms proposed in the literature for the Fischer-Tropsch synthesis. A silica-supported cobalt catalyst was used. Results suggest C₂₊ olefins and branched paraffins form from a carbon deposit on the catalyst surface. Two pathways appear to exist for methane formation. The first of these is from the carbon deposit through direct hydrogenation and through hydrogenolysis of the long-chain materials formed. The second pathway is hydrogenation of strongly adsorbed CO.     

Kinetics modelling of Fischer-Tropsch synthesis over an industrial Fe-Cu-K catalyst

The kinetic experiments of Fischer-Tropsch synthesis (FTS) over an industrial Fe-Cu-K catalyst are carried out in a micro-fixed-bed reactor under the conditions as follows: temperature of 493-542 K, pressure of 10.9-30.9 bar, H₂/CO feed ratio of 0.98-2.99, and space velocity of 4000-10?000 h⁻¹. The effects of secondary reactions of olefins are investigated by co-feeding C₂H₄ and C₃H₆. A detailed kinetics model taking into account the increasingly proven evidence of the olefin re-adsorption mechanism is then proposed. In this model, different sites are assumed for FTS reactions and water gas shift (WGS) reaction, respectively. Rate expressions for FTS reactions are based on the carbide polymerisation mechanism, in which olefin re-adsorption is considered to be a reverse step of olefin desorption reaction. Rate expression for WGS reaction is based on the formate mechanism. An integral reactor model considering both FTS and WGS kinetics is used to describe the reaction system, and the simultaneous estimation of kinetic parameters is conducted with non-linear regression procedure. The optimal model shows that the rate determining steps in FTS reactions proceed via the desorption of hydrocarbon products and the adsorption of CO and the slowest step in WGS reaction is the desorption of gaseous carbon dioxide via formate intermediate species. The activation energies of FTS reactions and WGS reaction are in good agreement with literature values.     

钌基氨合成催化剂研究进展

钌基催化剂被称为继铁基催化剂之后第二代氨合成催化剂。文中介绍了钌基氨合成催化剂的载体、钌的母体化合物和促进剂的研究进展以及在钌基催化剂上氨合成反应动力学。     

负载型纳米钌催化剂催化加氢合成间苯二胺工艺研究

以高分子/二氧化硅双重负载的纳米钌为催化剂,由间二硝基苯液相催化加氢制备间苯二胺,考察了反应温度、压力、催化剂用量、溶剂等对催化反应的影响,同时对催化剂的稳定性进行了研究。结果表明,该催化剂具有很高的催化活性,以甲醇为溶剂,间二硝基苯初始浓度0.5 mol/L,温度80℃,压力1.5MPa,反应1.5 h后间二硝基苯转化率和间苯二胺选择性分别达到95.6%和99%以上;同时,催化剂具有良好的稳定性,经过25次套用实验后,催化剂的反应性能未发生明显变化。     

Ammonia synthesis over the Ba-promoted ruthenium catalysts supported on boron nitride

Barium promoted ruthenium catalysts deposited on the boron nitride supports were characterised (XRD, O₂ and CO chemisorption) and tested in NH₃ synthesis. Prior to use, the raw BN materials marked as BNS (Starck, 96 m²/g) and HCV (Advanced Ceramics Corporation Cleveland USA, 40 m²/g) were heated in an ammonia stream at 700–800 °C for 120 h. As a result, the oxygen content was reduced from 7.0 at% (BNS) to 3.5 at% (BNS_NH3) and from 3.8 to 2.7 at% (HCV_NH3), as evidenced by XPS. The kinetic studies of NH₃ synthesis (63 or 90 bar; H₂:N₂ = 3:1) revealed that the catalysts based on the modified supports were more active, respectively, than those derived from starting nitrides, the difference being especially pronounced in the case of BNS and BNS_NH3. Studies of the catalysts activation have shown, in turn, that the stabilisation in a H₂:N₂=3:1 mixture at 1bar is very slow, i.e. the reaction rate increases slowly versus time on stream even at a high temperature of 550 – 600°C. Stabilisation is faster and the NH₃ synthesis rates are higher when the activation is performed with an ammonia rich mixture (10% NH₃ in H₂:N₂=3:1) flowing under high pressure of 90 bar. It is suggested that boron oxide (an impurity) acts as a deactivating agent for Ba–Ru/BN and that the reaction between NH₃ and B₂O₃ (B₂O₃+2NH₃=2BN +3H₂O) is responsible for the activity increase. A poisoning mechanism of B₂O₃ is discussed.     

Structure sensitive reactions over supported ruthenium catalysts during Fischer-Tropsch synthesis

Highly dispersed ruthenium catalysts can be prepared on alumina by aqueous impregnation of ruthenium. EXAFS at the K-edge showed that this type of catalyst, after calcination and reduction, consisted of ruthenium particles, which were about 0.8 nm in size. When highly dispersed on alumina, ruthenium appears to catalyze the water-gas shift reaction, which occurs subsequent to Fischer-Tropsch synthesis. The hydrocarbons produced had low olefinicity, possibly because ofin situ production of hydrogen via the water-gas shift reaction. Highly dispersed ruthenium was not stable on alumina during Fischer-Tropsch synthesis. The ruthenium agglomeration on the alumina surface, as well as overall ruthenium loss from the catalyst, was attributed to the formation of a volatile ruthenium carbonyl species.Catalysts with about 85% of the ruthenium in the form of 3–7 nm particles were prepared on alumina by reverse micelle impregnation of ruthenium. These larger particles were stable against ruthenium carbonyl formation and, therefore, did not exhibit ruthenium agglomeration or loss of ruthenium. Catalysts with 3–7 nm ruthenium particles displayed a higher turnover number for hydrocarbon synthesis, higher olefinicity, and chain-growth probability and did not exhibit water-gas shift activity in contrast to ruthenium particles which were about 0.8 nm in size.The CO disproportionation measurements showed much less CO dissociation over highly dispersed ruthenium relative to 3–7 nm ruthenium particles. This phenomenon is consistent with the low activity, the low chain-growth probability and may also relate to the tendency to form ruthenium carbonyl that is observed with small ruthenium particles. The apparent water-gas shift activity of highly dispersed ruthenium can be explained by the low CO dissociation efficiency as well as by the proposed ability to dissociate the water molecule.     

氨合成铁、钌催化剂联用工艺

在实验室和工业侧线装置上考察了FA-Ru型氨合成钌催化剂与铁系A202型催化剂的性能差异,以及铁催化剂和钌催化剂联用工艺与单铁催化剂工艺对氨合成效果的影响。结果表明,FA-Ru催化剂在低温（375～425℃）、低压（10～15 MPa）、低氢氮比（R=1.5～2.3）和合成气高氨浓度（10%～16 %,体积分数）条件下,活性比A202催化剂相对提高44%～75%。铁催化剂与钌催化剂混装工艺的氨合成率随着钌催化剂装量的增加而增加,比单铁催化剂的氨合成率提高24.5%～44.8%。铁催化剂串钌催化剂工艺的氨合成率同样随着钌催化剂装量的增加而增加,比单铁催化剂的氨合成率提高27.7%～58.8%。对于铁、钌催化剂联用的氨合成工艺,在实验条件下,当钌催化剂用量达铁催化剂用量1/2以上时,催化剂的最高活性点温度降至400℃。工业侧线实验表明,FA-Ru催化剂在13.0 MPa、10000～15000 h-1条件下的氨合成率可达到铁催化剂在相同空速、26 MPa 压力下的水平。根据不同工况,铁催化剂串钌催化剂生产工艺比单铁催化剂生产工艺氨合成率可相对提高43%～56%。     

超声波法研究F-T合成铁基催化剂机械强度

采用超声波法研究了微颗粒催化剂磨损机械强度及其磨损机制,考察了超声介质、超声功率、固液比和超声时间等对磨耗率的影响,并在相同测试条件下,比较了熔铁催化剂和沉淀铁催化剂磨耗强度。结果表明,熔铁催化剂还原后耐磨强度略降,与此相反,沉淀铁催化剂还原后耐磨强度略有提高。由球磨法得到的熔铁催化剂存在较多微细粉末的粘附与团聚,影响测定结果,但熔铁催化剂的耐磨强度高于沉淀铁催化剂。基于形态和粒度分布的研究表明,超声波作用下熔铁催化剂主要磨损机制为剥层磨损,沉淀铁催化剂为体断裂或破碎机制。     

The role of C2 intermediates in Fischer-Tropsch synthesis over ruthenium

The C₂ products formed over Ru during Fischer-Tropsch synthesis often lie well below the Anderson-Schulz-Flory line describing the C₄₊ products. This has led to speculation that either the surface precursor to C₂ hydrocarbons is exceptionally long lived, or that the ethylene formed by CO hydrogenation readsorbs and thereby reenters the chain growth process. In this study, the role of ethylene readsorption on the dynamics of chain initiation and growth is investigated using¹³CO/H₂ and¹²C₂H₄ to differentiate between the carbon sources. Ethylene addition is found to suppress the rate of methanation and increase the rates of formation of C₃₊ hydrocarbons. Ethylene serves as an effective chain initiator, as well as a source of C₁ monomer species which participate in chain propagation. No evidence is seen, though, for the participation of C₂ species in chain propagation.     

Fischer-Tropsch synthesis on ruthenium supported ETS-10 titanium silicate catalysts

Titanium silicate ETS-10 was found to be a suitable metal catalyst support, having high surface area, high ion exchange capacity and no acidic functions. In this work, ETS-10 was used as a support in preparing ruthenium supported catalyst for Fischer-Tropsch synthesis. Ru/METS-10 catalytic systems (M standing for Na or K) showed some important characteristics, as good metal dispersion and shape selectivity. Moreover, no side reactions due to acidic functions were evidenced; indeed readsorption of olefins on active metal centers was found to control the activity of the catalysts.In part presented at 10th IZC, Garmisch-Partenkirchen, July 1994.     

Studies of the fischer-tropsch synthesis on a cobalt catalyst II. Kinetics of carbon monoxide conversion to methane and to higher hydrocarbons

Six Langmuir-Hinshelwood-Hougen-Watson models have been derived for the kinetics of conversion of carbon monoxide to hydrocarbons in the Fischer-Tropsch synthesis. The models were fitted to experimental data obtained in an internal recycle reactor over a wide range of operating conditions. Two models, one based on the hydrogenation of surface carbon and the other on a hydrogen-assisted dissociation of carbon monoxide as rate limiting steps were both able to provide a satisfactory fit to the experimental rate data. A general model was also developed for the rate of methanation in the presence of higher hydrocarbons. The same two rate limiting assumptions as those used in formulating the rate of total CO conversion are used in these models. The two models were fitted to experimental data for methane formation. It was the model assuming CH formation as rate limiting that showed the best fit for both CO conversion for CH₄ formation.     

Studies of the fischer-tropsch synthesis on a cobalt catalyst i. evaluation of product distribution parameters from experimental data

Two models describing the distribution of linear and monomethyl alkanes in Fischer-Tropsch products from a cobalt catalyst are formulated. Distribution parameters for monomethyl isomers are derived by assuming either a post-branching effect on linear chain growth or a decreasing reactivity toward branching with increasing chain length. Distribution parameters for each of the models have been extracted from an extensive experimental data set. We find that the experimental results are well represented by both the models. A “break” in the distribution of linear paraffins has also been observed and is modelled as a sum of the yields of two Anderson-Schulz-Flory distributions. We postulate that the two chain growth processes which produce the distributions are governed by two kinds of termination.     

Transient and resonant behavior for no reduction by CO over a Pt/Al2O3 catalyst during forced composition cycling

The reduction of NO by CO over a Pt/Al₂O₃ catalyst has been investigated using the technique of forced concentration cycling in an isothermal recycle reactor at 485 K. Time-average conversions exhibit resonant behavior with increasing frequency. Maximum time-average NO conversion of 78%, compared with the steady-state conversion of 3.8%, was attained during out-of-phase feed concentration cycling. The effect of the phase angle between the NO and CO feed cycles has been examined. Higher conversions are obtained by decreasing the NO phase lead below 180°. The convergence to cycle-invariance was slow for high frequency cycling.     

Effect of manganese on a potassium-promoted iron-based Fischer-Tropsch synthesis catalyst

The effects of manganese promoter on the reduction–carburization behavior, surface basicity, bulk phase structure and their correlation with Fischer-Tropsch synthesis (FTS) performances have been emphatically studied over a series of spray-dried Fe–Mn–K catalysts with a wide range of Mn incorporation amount. The catalysts were characterized by means of H₂ and CO temperature-programmed reduction (TPR), CO₂ temperature-programmed desorption (TPD), Mössbauer spectroscopy etc.. The results indicated that small amount of Mn promoter can promote the reduction of the catalyst in H₂. However, FeO phase formed during reduction is stabilized by MnO phase with the further increase of Mn content, making FeO phase difficult to be reduced in H₂. The addition of Mn promoter can stabilize the Fe²⁺ and Fe³⁺ ions, and suppresses the reduction and carburization of the catalyst in syngas and CO. Mn promoter can also enhance the amount of the basic sites and weaken the strength of the basic sites, which possibly come from the reason that the Mn–K interaction is strengthened with the addition of Mn promoter. The change of surface basicity can modify the selectivity of hydrocarbons and olefins, and the change of bulk structure phase derived from the addition of Mn promoter will affect the catalyst activity and run stability. The synergetic effects of the two main factors result in an optimized amount of Mn promoter for the highest catalyst activity and heavy hydrocarbon selectivity in slurry FTS reaction of Fe–Mn–K catalysts.