•For a feed with H2/CO>1.7 the single pass syngas conversion is less than 50–60%.•Partial pressure of water increases with conversion which causes deactivation.•With CO rich feed high single pass ...syngas conversion (>80%) can be achieved.•Water-gas-shift activity increases with temperature and decrease in pressure.•The H2/CO usage ratio depends strongly on syngas feed ratio and syngas conversion.
Water-gas-shift (WGS) reaction plays a significant role in industrial application of Fischer–Tropsch synthesis (FTS) for coal-to-liquid (CTL) processes with iron-based catalysts. This reaction provides necessary hydrogen for synthesis gas with low H2/CO molar ratio, and has influence on concentrations of reactants, water and carbon dioxide, which in turn has effect on product distribution, rate of FTS and catalyst deactivation. We provide information on the effect of process conditions (H2/CO feed ratio, reaction temperature and pressure), syngas conversion, and catalyst composition and activation procedure on the WGS activity. H2/CO consumption (or usage) ratio and the exit H2/CO ratio vary with conversion and the extent of WGS reaction. The extent of variation is much greater for H2/CO feed ratios greater than 1.7, than it is for the CO rich syngas (H2/CO=0.5–1). This in turn places limits on maximum practical single pass conversion which can be achieved with different feed compositions and results in different types of operation (low single pass conversion with tail gas recycle, and high once through single pass conversion).
Component carbon number distribution.
•Methane selectivity decreases with decrease in temperature and H2/CO feed ratio.•Space velocity and pressure have a small effect on hydrocarbon product ...distribution.•Oxygenates participate in secondary reactions.•Olefin content decreases with increase in residence time and H2/CO feed ratio.
The effect of process conditions on product selectivity of Fischer–Tropsch synthesis (FTS) over industrial iron-based catalyst (100 Fe/5 Cu/4.2K/25 SiO2) was studied in a 1-L stirred tank slurry reactor. Experiments were performed over a range of different reaction conditions, including three temperatures (T=493, 513 and 533K), four pressures (P=0.8, 1.5, 2.25 and 2.5MPa), two synthesis gas feed molar ratios (H2/CO=0.67 and 2) and gas space velocity from 0.52 to 23.5Ndm3/g-Fe/h. The effect of process conditions on reaction pathways of FTS and secondary 1-olefin reactions was analyzed by comparing product selectivities, chain growth probabilities and ratios of main products (n-paraffin, 1- and 2-olefin). Reduction of methane production and increase of C5+ products was achieved by decreasing temperature, inlet H2/CO ratio and/or increasing pressure. Overall selectivity toward methane and C5+ did not show significant changes with variations in residence time. All of the product selectivity variations were shown to be related to changes in chain length dependent growth probabilities.
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 CH4 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 NiAl2O4 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.
•Ni–Zr oxygen carrier exhibited stable redox properties during 20 cycles in a TGA.•Reducibility of Ni–Al oxygen carrier increased with cycles.•Methane decomposition was the major pathway to form C and H2 on Ni–Al and Ni–Zr.•Ni–Zr had superior stability relative to Ni–Al in chemical looping reforming.
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•Metallic Ni on ZrO2, Al2O3 and NiAl2O4 is active and stable in reforming.•Use of SiO2 and TiO2 support results in fast catalyst deactivation.•Ni on TiO2 easily reoxidizes to NiO ...during reforming conditions.•NiO/ZrO2 shows excellent activity and stability in chemical looping steam reforming.
This study evaluates the performance of NiO-based oxygen carriers supported on ZrO2, TiO2, SiO2, Al2O3 and NiAl2O4 as conventional reforming catalysts at low temperature (650°C) as well as oxygen transfer materials (OTMs) for chemical looping steam methane reforming. Conventional reforming experiments with pre-reduced OTMs demonstrated satisfactory performance of NiO/Al2O3, NiO/ZrO2 and NiO/NiAl2O4 at 650°C, with less than 8% relative decrease in conversion after 10h time-on-stream. On the other hand, NiO/SiO2 and NiO/TiO2 were found to have low activity (<50% initial CH4 conversion) and deactivated rapidly, eventually dropping to less than 10% CH4 conversion after 2h on stream. The three most promising materials were tested under chemical looping steam methane reforming conditions for twenty consecutive redox cycles in a fixed bed flow unit. NiO/ZrO2 exhibited good activity with initial CH4 conversion higher than 80% and had very good stability. Deactivation was minimal after 20 consecutive redox cycles, corresponding to 20h exposure to CH4/steam. NiO/Al2O3 and NiO/NiAl2O4 demonstrated similar high initial activity, but also high deactivation, leading to methane conversion of 59 and 63% conversion respectively at the end of the test.
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► Effect of Pd, Pt, Re, and Ru on the performance of 25% Co/Al2O3 was studied. ► Catalysts performance was examined in a 1-L continuously stirred tank reactor (CSTR). ► Noble metal ...promoters improved initial CO conversion of the Co catalysts. ► Noble metal promoters changed hydrocarbon distribution to some extent.
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.
A kinetic model for Fischer–Tropsch synthesis is derived using a Langmuir–Hinshelwood–Hougen–Watson approach. Experiments were conducted over 25% Co/0.48% Re/Al2O3 catalyst in a 1 L slurry reactor ...over a range of operating conditions (T = 478, 493, 503 K; P = 1.5, 2.5 MPa; H2/CO = 1.4, 2.1; WHSV = 1.0–22.5 NL/(gcat·h)). Rate equations were based on the elementary reactions corresponding to a form of well-known carbide mechanism. The 1-olefin desorption rate constant was assumed to be a function of carbon number due to the effect of weak interaction of the hydrocarbon chain with the catalyst surface. Values of estimated activation energies are in good agreement with those reported previously in the literature. The kinetic model was able to correctly predict all of the major product distribution characteristics, including the increase in chain growth probability and decrease in olefin-to-paraffin ratio with carbon number, as well as formation rates of methane and ethylene.
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 CH4 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.
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•Nature of support influences NiO reduction mechanism.•Shrinking core model best describes NiO reduction on strongly interacting support.•On supports with weak interaction, NiO is ...reduced by nucleation and nuclei growth.•NiO structural modifications with increasing cycles reduce the support effect.
This study investigates the redox kinetics of four NiO-based oxygen transfer materials (OTMs) supported on Al2O3, TiO2, SiO2 and ZrO2. 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/Al2O3 and NiO/TiO2 reduction kinetics were rate-determined by chemical reaction and fitted via the Unreacted Shrinking Core Model. On the other hand, NiO/ZrO2 and NiO/SiO2 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 Al2O3 and TiO2 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 SiO2 and ZrO2, 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/Al2O3 and NiO/TiO2 OTMs followed an Avrami-Erofeev model approach for all considered cycles, while NiO/ZrO2 oxidation kinetics were described via a Prout-Tompkins model that considered a short but still, significant nucleation period.
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► H
2-activated Co/ZnO catalyst was more active than the CO-activated catalyst. ► H
2-activated catalyst had higher olefin content and lower methane selectivity. ► Both cobalt carbide ...(Co
x
C;
x
=
2 or 3) and metallic cobalt were found after CO pretreatments.
The effect of catalyst pretreatment, using hydrogen or carbon monoxide, on the activity and selectivity of cobalt on ZnO catalyst (10
wt% Co/ZnO) during Fischer–Tropsch synthesis was studied in a fixed bed reactor. Catalyst reduced with hydrogen had higher activity and higher olefin content, and produced less methane than the CO activated catalyst. Catalysts were characterized by means of Brunauer–Emmett–Teller (BET), X-ray diffraction (XRD), temperature programmed reduction (TPR), and hydrogen chemisorption (H
2-TPD) with pulse re-oxidation. Reduction of Co
3O
4 occurs in two stages during H
2-TPR and CO-TPR. In the latter case CoO, metallic cobalt, cobalt carbides (Co
x
C;
x
=
2 or 3) and carbon (from CO disproportionation) are formed during the reduction process. The existence of Co
0 along with CoO and Co
x
C, indicates a possibility of two reaction paths for formation of Co
x
C from CoO: the direct path (CoO
→
Co
x
C) and a series reaction via Co
0 (i.e. CoO
→
Co
0
→
Co
x
C).
We investigate effects of catalyst activity, catalyst particle shape (sphere, slab, 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 catalyst effectiveness factor and methane selectivity inside the catalyst pellet. In numerical simulations we utilize kinetic parameters for CO consumption rate and CH4 formation rate determined from experiments with a highly active Co/Re/γ-Al2O3 catalyst. It is found that the use of small spherical particles (0.2–0.5 mm) or eggshell distribution for larger spherical particles with catalyst layer thickness less than approximately 0.13 mm is needed to avoid negative impact of diffusional limitations on CH4 selectivity under typical Fischer–Tropsch synthesis operating conditions. For monolith reactors with wash-coated catalyst, diffusional limitations can be avoided by using a catalyst layer thickness less than 0.11 mm at base case conditions (473 K, 25 bar, and H2/CO molar ratio of 2).
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