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• Methane selectivity increases sharply beyond a critical value of Thiele modulus. • Kinetic region extends with increase in pressure or decrease in H2/CO feed ratio. • Methane selectivity decreases with increase in conversion and/or decrease in H2/CO. • Multiple steady-state solutions are possible for some values of Thiele modulus. • Hybrid kinetic model was used to predict hydrocarbon product distribution. In this study we investigate performance characteristics (catalyst effectiveness, CH 4 selectivity, and hydrocarbon product distribution) of with a highly active Co/Re/Al 2 O 3 catalyst particle for Fischer-Tropsch synthesis. In numerical simulations we utilize kinetic parameters for CO consumption rate, CH 4 formation rate and hydrocarbon formation rates (C 2 + hydrocarbons) determined from experiments with this catalyst to study effects of catalyst activity, catalyst particle shape (sphere, slab, solid and hollow cylinder), size (i.e. diffusion length), catalyst distribution (uniform vs. eggshell type distribution for a spherical particle) and process conditions (temperature, pressure, syngas composition and conversion level) on the catalyst performance. With increase in Thiele modulus (i.e. particle size at a fixed set of process conditions) we observe increasing H 2 /CO ratio profile towards the center of the particle resulting in increase of local and average CH 4 selectivity. The goal is to find conditions which allow one to use sufficiently large particles to reduce pressure drop, while avoiding negative influence of diffusional limitations on selectivity and activity. For each catalyst particle shape we determined values of Thiele modulus, i.e. characteristic length of diffusion, corresponding to the upper limit of the kinetic region, and investigated how it changes with operating conditions. We found that simultaneous increase of pressure and the use of syngas with H 2 /CO feed ratio of 1.4–1.7 is the best strategy for mitigating the negative impact of intraparticle diffusional limitations on CH 4 selectivity. For a spherical particle of 1 mm in diameter, one can achieve CH 4 selectivity of 5.6% with catalyst effectiveness factor of 1.07 at the reactor inlet by operating at 50 bar, 473 K and H 2 /CO = 1.4. [ABSTRACT FROM AUTHOR]

Two NiO based oxygen carrier materials (OCMs) were synthesized and tested for use as potential materials in chemical looping reforming applications. Redox properties of these materials were evaluated in successive methane reduction – air oxidation (redox) cycles in a thermogravimetric analyzer unit (TGA) and an in situ magnetometer. Zirconia supported (Ni–Zr) OCM exhibited excellent redox activity (high degree of reduction and oxidation) and stability during ten CH 4 reduction-air oxidation cycles. The degree of reduction of the alumina supported (Ni–Al) OCM increased gradually during cycling experiments, due to the formation of easily reducible NiO from nickel aluminate species with successive reduction/re-oxidation. The Ni–Al OCM exhibited excellent stability with respect to oxidation resulting in nearly complete oxidation of reduced Ni in all cycles. Results from measurements in the magnetometer were in good agreement with those in the TGA for the Ni–Zr OCM (both with regards to the degree of reduction and oxidation) and the degree of oxidation of the Ni–Al OCM. A moderate crystallite growth with cycling was observed for Ni–Al, whereas a decrease in nickel crystallite size was observed for Ni–Zr. • Degree of reduction of Ni–Al oxygen carrier (OC) increased with cycles. • Ni–Zr carrier had a high degree of lattice oxygen utilization in multiple redox cycles. • Crystallite size of Ni on Ni–Zr OC decreased continuously during 10 cycles. • Carbon deposition did not have a negative effect on redox stability of OCs. • Magnetometer is a valuable tool to study redox and sintering characteristics of OCs. [ABSTRACT FROM AUTHOR]

A detailed thermodynamic analysis of the sorption enhanced chemical looping reforming of methane (SE-CL-SMR), using CaO and NiO as CO 2 sorbent and oxygen transfer material (OTM) respectively, was conducted. Conventional reforming (SMR) and sorption enhanced reforming (SE-SMR) were also investigated for comparison reasons. The results of the thermodynamic analysis show that there are significant advantages of both sorption enhanced processes compared to conventional reforming. The presence of CaO leads to higher methane conversion and hydrogen purity at low temperatures. Addition of the OTM, in the SE-CL-SMR process concept, minimizes the thermal requirements and results in superior performance compared to SE-SMR and SMR in a two-reactor concept with use of pure oxygen as oxidant/sweep gas. [ABSTRACT FROM AUTHOR]

• NiO reduction mechanism is significantly affected by the nature of the support. • A reaction rate-based modeling determines the prevalent NiO reduction reactions. • NiO on ZrO 2 is almost exclusively reduced by total and partial methane oxidation. • NiO reduction proceeds partially by formed CO/H 2 on SiO 2 and Al 2 O 3 supports. • Al 2 O 3 support facilitates Ni-catalyzed CH 4 decomposition towards H 2 production. The focus of this study is to assess the reaction rate-based kinetics of three NiO-based oxygen transfer materials (OTMs) supported on ZrO 2 , Al 2 O 3 and SiO 2 under CH 4 reduction. Both NiO weight change data and gaseous phase composition from TGA-MS tests are utilized for the elucidation of the reaction scheme. We show that mainly CH 4 , as well as CO and H 2 (primary NiO reduction products by CH 4), are involved in the reduction process under a varying extent that depends on the support used. The developed kinetic model is first order dependent on the concentration of the main reductant at each reaction and a function of the NiO/Ni concentration. On ZrO 2 , NiO is exclusively reduced by CH 4 via both total and partial oxidation. On the other hand, in addition to CH 4 , CO and H 2 also contribute to NiO reduction on Al 2 O 3 and SiO 2. Still, total and partial CH 4 oxidation are the dominant routes. In the case of SiO 2 in particular, the reduction rate of NiO by CO and H 2 is the highest among all OTMs. The CH 4 decomposition reaction, catalyzed by the metallic nickel sites, also takes place via the Langmuir-Hinshelwood mechanism after the completion of the NiO reduction. The rate of this reaction correlates with the surface area of the materials due to the higher population of active metallic nickel sites on the surface, leading to carbon deposition in the order NiO/Al 2 O 3 > NiO/SiO 2 > NiO/ZrO 2. [ABSTRACT FROM AUTHOR]

• Forced periodic operations in milli-fixed-bed FTS reactor can be beneficial. • High amplitude modulations are optimal for maximization of C 5+ productivity. • Optimal forcing of coolant temperature resulted in 30% gain of productivity. • Three inputs forcing resulted in highest C 5+ productivity improvement of 52%. • Methane selectivity increased relatively less than C 5+ productivity in optimal FPO. One-dimensional pseudo-homogenous dynamic reactor model, incorporating detailed Fischer-Tropsch kinetics, was applied in a theoretical analysis of forced periodic operations. A milli-scale fixed-bed reactor was analyzed, using design and operation parameters, obtained previously in a steady-state optimization. Dynamic optimization and NLP methods were utilized to obtain optimal values of amplitude(s), frequency and phase shift(s) of sine-wave variation of inputs, around the corresponding optimal steady-state values, which maximize the productivity of C 5+ hydrocarbons. Inlet variables that were modulated are: coolant temperature, reactants molar ratio, mass flow rate and pressure. In addition to the single input forcing, simultaneous modulations of multiple inputs were also considered, with combinations of the listed inlet variables. Among the single input cases, periodic variation of the coolant temperature resulted in the highest relative improvement of C 5+ productivity by 30%. Multiple inputs forcing showed additional potential for improvement, resulting in relative C 5+ productivity increase of 52% for synchronized modulation of the coolant temperature, reactants molar ratio and mass flow rate. However, the increase in C 5+ productivity is accompanied with relative increase in methane selectivity of 22–33% (relative to the steady-state value). The results suggest that, in the case of multiple input variations with high amplitudes, modulation of the inlet reactants molar ratio mainly contributes to the increase of CO conversion (e.g. reaction rate), the coolant temperature forcing slightly increases selectivity towards the desirable higher hydrocarbons (C 5+), while the variation of the inlet mass flow rate enables better reaction temperature control and prevents a thermal runway. [ABSTRACT FROM AUTHOR]

• Co catalysts are 2.5 times as active as Fe catalysts based on the TOF of clean conditions. • Fe and Co catalysts have similar CO conversion effect below 50% conversion. • Co and Fe catalysts have similar H 2 S threshold limit, 25–50 ppb. • NH 3 threshold limits were determined to be 45 ppb for Co and 80 ppm for Fe. • NH 3 and H 2 S deactivated catalysts through an adsorption mechanism. Quantitative data comparing iron and cobalt based catalysts for the Fischer-Tropsch synthesis (FTS) are scarce due to the fact that these two kinds of catalysts typically utilize different process conditions. This paper focuses on studying the catalytic behavior of two highly active iron and cobalt based research catalysts at clean conditions and during poisoning with common syngas contaminants. The catalyst activity and selectivity at identical conditions, the CO conversion effect, and the effect of poisons on iron and cobalt catalysts were systematically explored in a quantitative manner. At a set of identical FTS conditions, the cobalt catalyst was 2.5 times as active as the iron catalyst with higher CH 4 and C 5 + selectivities but much less olefins and lower CO 2 selectivity. Cobalt based catalysts are more susceptible to deactivation by oxidation at high CO conversions (e.g. > 80%) due to the high partial pressure of water (P H2O) in the reactor, while the iron catalyst can be stabilized at a high conversion level. Under clean FTS conditions, the cobalt catalysts displayed a more pronounced CO conversion effect on stability and selectivity; on the other hand, a combination of effects (i.e. from CO conversion and the nature of the catalyst) were observed for the iron catalysts. The sensitivities of the Fe and Co catalysts to the typical contaminants (i.e., H 2 S and NH 3) present in the syngas derived from coal, natural gas or biomass were compared. Iron and cobalt catalysts exhibited similar resistance to the H 2 S poison (i.e. threshold levels 25–50 ppb), but the iron catalyst was found to be much more resistant to ammonia than the cobalt catalyst (i.e., threshold levels of 80 ppm and 45 ppb, respectively). Co-feeding 150–200 ppm ammonia lowered CH 4 selectivity and 2-olefin content (suppressing secondary reactions of 1-olefin) on both types of catalysts. In contrast, co-feeding up to 1 ppm H 2 S significantly increased CH 4 formation only on cobalt catalysts but had a minor effect on CH 4 selectivity with iron catalysts. It increased 2-olefin content (enhanced secondary reactions of 1-olefin) regardless of catalyst type. H 2 S and NH 3 have different impacts on H 2 , CO adsorption, and the formation of sulfur and nitride compounds have been proposed to explain these dissimilar effects. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Milli fixed bed Fischer-Tropsch reactors exhibit fast and highly nonlinear dynamics. • Thermal runaway may occur with disturbance of coolant temperature and feed flowrate. • Forced periodic operation may enhance Fischer-Tropsch reactor performance. • Temperature control is crucial – advanced control systems are recommended. Abstract Dynamic performance of a milli-scale fixed bed reactor for Fischer-Tropsch synthesis (FTS) was studied using a dynamic pseudo-homogeneous 1D reactor model. The model uses detailed kinetics to describe the rates of FTS product formation. Dynamic responses of the process variables and main performance indicators, including productivity of C 5+ hydrocarbons and CH 4 selectivity, to input step changes, were analyzed. Total of 7 inlet variables were used in step-change-response analysis, with different magnitudes of change and for two initial steady-state conditions. Reactor simulations show highly nonlinear behavior due to phenomena coupling and fast dynamics due to system small scale and intensified rates within. In addition, reactor model shows instability related to thermal runaway with certain magnitudes of step change of coolant temperature and feed flow rate. The analysis outlines the effects of potential process disturbances on operation of milli-scale fixed bed reactor for FTS and provides general guidelines for control systems. [ABSTRACT FROM AUTHOR]

This study investigates the redox kinetics of four NiO-based oxygen transfer materials (OTMs) supported on Al 2 O 3 , TiO 2 , SiO 2 and ZrO 2 . The OTMs were tested and evaluated under twenty consecutive methane reduction/air oxidation cycles. Several solid-state kinetic models were consecutively and optimally screened for each OTM at different redox cycles. The use of different supports resulted in different forms of the redox kinetics. NiO/Al 2 O 3 and NiO/TiO 2 reduction kinetics were rate-determined by chemical reaction and fitted via the Unreacted Shrinking Core Model. On the other hand, NiO/ZrO 2 and NiO/SiO 2 reduction was found to proceed via nucleation and subsequent nuclei growth and was suitably described with Avrami-Erofeev models. The use of different types of kinetic models is attributed to differences in strength of metal-support interactions. Specifically, the strong interaction between NiO and Al 2 O 3 and TiO 2 creates NiO-support interfaces where Ni nuclei are formed very fast, rendering the chemical reaction the governing step of the reduction process. When NiO is supported on SiO 2 and ZrO 2 , the interaction is weak and NiO basically behaves like free NiO, reducing via the slow formation of homogeneous nuclei on the surface and the subsequent faster growth of Ni domains. Regarding Ni oxidation kinetics, all OTMs were rate-determined by nucleation and nuclei growth. NiO/Al 2 O 3 and NiO/TiO 2 OTMs followed an Avrami-Erofeev model approach for all considered cycles, while NiO/ZrO 2 oxidation kinetics were described via a Prout-Tompkins model that considered a short but still, significant nucleation period. [ABSTRACT FROM AUTHOR]

A multi-scale pseudo-homogeneous one-dimensional model for the Fischer Tropsch fixed-bed reactor has been developed in this study using a detailed mechanistic kinetic scheme proposed in an earlier study (Todic et al., 2013). The developed model is capable of predicting the concentration and temperature profiles at the micro-(catalyst pores) level as well as the macro-(reactor bed) level for a cobalt-based catalyst (15 wt% Co/Al 2 O 3 ). The uniqueness of this model is that it tries to combine different levels of complexity (product distribution modeling, particle diffusion and reactor bed modeling) into one single model. The predictability of this model has been validated experimentally using an advanced high-pressure FTS reactor unit over a wide range of testing conditions. It quite accurately predicts experimentally measured CH 4 selectivity at different gas hourly space velocities but less accurately predicted CO conversion. On the other hand, a hydrocarbon product distribution has been predicted using a MATLAB ® code that was written to estimate the FTS kinetic model’s parameters (Todic et al., 2013) based on the experimental data collected using bench scale FTS fixed-bed reactor. The optimization of this model was done using a Genetic Algorithm (GA). The findings showed excellent predictability of the experimentally measured hydrocarbon product distribution profile of the catalyst, specifically paraffin formation rates which are the main products of the cobalt-based catalyst. This comprehensive model also involved modified thermodynamic equation of state and currently is upgraded to enable a direct comparison of the gas-phase and supercritical solvent-assisted FTS reactions under a variety of conditions using the experimental data reported in a recent study by our team (Kasht et al., 2015). [ABSTRACT FROM AUTHOR]

Two nickel based oxygen transfer materials (OTMs) supported on alumina and zirconia were tested for their potential use in chemical looping reforming. Zirconia supported (Ni–Zr) OTM exhibited good redox activity and stability during multiple CH 4 reduction-air oxidation cycles in a thermogravimetric unit. Alumina supported OTM (Ni–Al) showed increasing degree of reduction during the first 12 cycles, due to reduction of NiAl 2 O 4 species, followed by stable performance during the last 8 redox cycles. Hydrogen was produced by methane decomposition catalyzed by Ni on both OTMs. Decrease in carbon deposition and hydrogen production with increasing number of cycles is attributed to sintering of nickel during redox cycles. Steam methane reforming experiments in a fixed bed reactor showed that Ni–Zr OTM is superior to the Ni–Al OTM in terms of stability during 20 cycles of reduction/reforming and reoxidation at 650 °C and 850 °C respectively. [ABSTRACT FROM AUTHOR]

Abstract: The effect of noble metal promoters (atomic ratio of promoter to Co=1/170) on the activity and selectivity of a 25%Co/Al2O3 catalyst was studied at a similar CO conversion level of 50% at 493K, 2.2MPa and H2/CO=2.1 using a 1-L continuously stirred tank reactor (CSTR). The results show that all promoted catalysts exhibited markedly higher initial CO conversion rates on a per gram catalyst basis than the unpromoted one, which was ascribed to increased Co site density when the promoters were present. This is because the Re, Ru, Pt and unpromoted Co catalysts give essentially the same initial Co TOF values of 0.092–0.105s−1 (based on hydrogen-chemisorption). However, the initial Co TOF value for the Pd-Co catalyst was about 40% lower, which might be caused by Pd atoms segregating on the Co surface and partially blocking Co sites. At 50% CO conversion, Re and Ru promoters decreased CH4 selectivity and increased C5+ selectivity by nearly the same extent, whereas the opposite effect was observed for Pd and Pt promoters. The Re and Ru promoters had less of an impact on C2–C4 olefin selectivity (7.5–60%), but suppressed the secondary reaction of 1-C4 olefin (from 14.3 to 9%) compared to the unpromoted one; however, the addition of Pd and Pt promoters resulted in lower olefin selectivity (4.4–55%) but higher 2-C4 olefin selectivity (14.3 to 27–31%). Pt promotion had a negligible effect on C4 olefin isomerization. The selectivity results were reproducible. Both Pt and Pd promoters slightly increased WGS activity, whereas Re and Ru promoters had a negligible effect. The Pd and Pt promoters were observed to slightly enhance oxygenate formation, while Re and Ru slightly decreased it. [Copyright &y& Elsevier]

Abstract: CO hydrogenation over alumina-supported Mo carbide catalyst synthesized by temperature-programmed carburization of the impregnated MoO3 utilizing H2/C3H8 feed has been investigated. Samples were prepared following a 23-factorial design with temperature, hydrocarbon composition and reaction time as the carburization factors. Catalysts were characterized using N2 physisorption, NH3 temperature-programmed desorption and solid total organic carbon (TOC) analysis. Solid-state carburization kinetics was studied in a thermogravimetric analyzer at different heating rates. Statistical analysis revealed that carburization temperature was a strong determinant of key physicochemical properties (BET, pore volume, acid site strength and concentration). Increasing the H2:C3H8 ratio in the carburizing gas reduced the acid site strength and concentration but improved overall hydrocarbon synthesis rate, olefin-to-paraffin ratio and specific activity. All catalysts possessed Lewis acid centers. The carburizing temperature was also a statistically significant factor in the determination of the FT reaction metrics. A two-stage process involving the transformation of Mo oxide to the MoC1− x carbide via an intermediate oxycarbide phase was observed. The solid-state kinetics was captured by a contracting volume model and Arrhenius treatment implicated the occurrence of a ‘compensation effect’ for the carbothermal synthesis. Although the rate-composition envelope exhibited qualitatively similar features for the entire Mo carbide class, the optimum feed composition for maximum hydrocarbon formation rate varied between 0.67 and 0.75 depending on the catalyst preparation conditions and the carbon number. The olefin-to-paraffin ratio has a reciprocal relationship to the H2 partial pressure but experienced an exponential decay with carbon number. Catalyst with 15wt.% Mo loading produced the highest hydrocarbon synthesis rate and chain growth probability. [Copyright &y& Elsevier]