Browsing by Author "van Steen, Eric"

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    Open Access
    A study of configurational alternatives of a gas-to-liquids process based on Fischer-Tropsch technology
    (2018) Khazali, Ashcaan Tendo; Moller, Klaus; van Steen, Eric
    Environmental concerns, associated legislation, limited oil reserves and fluctuating crude oil prices are some of the factors that highlight the need for alternative and environmentally friendly routes to fuels. One alternative is to use Fischer-Tropsch Synthesis (FTS) as the major technology in conversion of carbon containing feedstock to transportation fuels. The FTS product, called syncrude, can be refined to high quality transportation fuels in Coal, Gas or Biomass to liquid plants (denoted as CTL, GTL, and BTL, respectively, and collectively referred to as XTL). The economic viability of XTL processes is generally subject to the present price of crude oil and past studies show that traditional refining is generally more economically viable. However, XTL processes have been shown to be more economical and in some cases more environmentally friendly than conventional options when legislative measures aiming to curb traditional fossil fuel usage are considered. This study explores XTL process configurations that can improve plant carbon efficiency to diesel and liquids. The configuration encompasses technologies used, operating conditions, and layout of unit operations. A basic GTL process configuration consists of an Air Separation Unit (ASU), Auto-Thermal Reforming (ATR), syngas cleaning, full conversion Low Temperature Fischer-Tropsch (LTFT) and wax hydrocracking (WHC). These operations are modeled individually and combined to produce a plant model for study with the aim of determining the effects of configurational alternatives on the process efficiency to liquids and diesel. Furthermore, given that the ASU is a major contributor to costs the effect of using oxygen-enriched or pure air is investigated. Since production of heavy wax is prioritized, FTS represents the use of cobalt catalyst in LTFT operation. Where air is used, FTS is run to high conversion in once through mode to avoid the unfavorable economics of recycling nitrogen. After separation of the syncrude, the light fraction is reformed back to syngas in order to maximize carbon efficiency. The heavy wax is hydrocracked to maximize distillate range material. The light products from the WHC are combined with the lights from FTS and the heavy wax is recycled. Carbon efficiency, liquid selectivity and diesel yield are the means of assessing performance. The Scilab programming language is used along with physical properties estimated using the COCO/ChemSep pure component database as a starting point. Estimation of properties for alkanes and olefins of carbon chain length up to C200 has been carried out. The presence of 25% nitrogen in the ATR was found to beneficial to the H2 : CO ratio in the resulting syngas. Furthermore, in FTS the presence of 10-20% nitrogen produced the lowest reduction in carbon monoxide conversion and _FTS. In general, the introduction of nitrogen resulted in decreased conversion of methane in the ATR and both decreased _FTS and conversion in FTS. WHC performance was found to benefit from alpha being as high as possible; however, when the heavy wax recycle was inactive the optimal value was 0.92. The OOT80 configuration was found to have the highest liquid selectivity, while the efficiency to diesel was maximized for the OIRC40 configuration.
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    Open Access
    Aromatic hydroxylations over titanium-substituted crystalline silicates
    (2001) Wilkenhöner, Uwe; van Steen, Eric; Gammon, David
    This work focuses on aromatic hydroxylations over titanium-substituted zeolites of different pore sizes (TS-1, Al-free Ti-beta and Ti-HMS) using aqueous H2O2 as the oxidant. The aim was to gain a deeper understanding of the key factors controlling activity and selectivity, which implied an investigation of the reaction kinetics and the adsorption and diffusion properties of reactants and products in the various catalysts using different solvents. The role of shape selectivity inside the micropores of the solids and the reaction at their external surface was also examined.
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    Comparing the performance of different catalyst packings for Fischer-Tropsch synthesis
    (2021) Mhinga, Masana; van Steen, Eric
    The increase in global population accompanied by depleting fossil fuel reserves and rising fuel demand is prompting the need to increase and diversify fuel production rates. In addition, increasing global pressure to reduce the contribution of greenhouse gas emissions to global warming and as well as rising oil prices are driving efforts to improve efficiency and environmental sustainability of fuel production. Low temperature Fischer-Tropsch synthesis is an attractive alternative to crude refining. During Fischer-Tropsch synthesis (FTS), synthesis gas, a mixture of H2 and CO is converted to a wide range of value-added long chain hydrocarbon products in a polymerisation reaction: (2n+1) H2 + n CO → CnH2n+2 + n H2O -∆H= 165 − 214 kJ ∙ mol!" 0-1 The benefit of Fischer-Tropsch synthesis is that any carbonaceous feedstock can be used to generate synthesis gas, including natural gas, coal and biomass. However, exploitation of biomass requires decentralised, small-scale units, to reduce the complexity of a variable feed supply. To make this feasible, small-scale FT reactors must be simple to operate and achieve high single-pass conversion. Multi-tubular fixed-bed reactors are ideal for realising this as they are relatively easy to scale-up and operate when compared to other reactor technologies. However, conventional catalyst packings for fixed-bed reactors result in high pressure drop. Structured packings offer an attractive solution as their high porosity (ca. 70%-90%) when compared to that of conventional pellet- or particle packed beds (ca. 40%) results in significant reductions in pressure drop, which lowers gas compression costs. Large pellets are typically used in fixed-bed reactors to reduce pressure drop, resulting in increased internal diffusion limitations which may result in increased CH4 selectivity and reduced C5+ selectivity, the desired product. In contrast, a thin layer of the catalytically active material applied on the surface of the structured substrate results in lower diffusion limitations, and thus maintaining a favourable product distribution. Furthermore, open-cell foams reactors have the potential to enhance external mass in fixed-bed reactors because of their tortuous flow channels and high geometric surface area. The aim of this study was to compare the performance of catalyst coated on a ceramic open cell foam with that of catalyst powder, pellets and catalyst coated on a ceramic honeycomb monolith in terms of CO conversion, CH4 and C5+ productivity per gram catalyst using an alumina-supported catalyst containing 22 wt.% Co and 0.05 wt.% Pt. Furthermore, the effect of catalyst layer thickness on catalyst performance was also evaluated. It was shown that the pellets performed poorly when compared to the catalyst powder due to its much higher CO2 and CH4 selectivity, which were attributed to poor temperature control, induced by large diffusion length (i.e., pellet diameter). The foams and monoliths also performed poorly when compared to the catalyst powder. For monoliths and foams wash coated with ca. 0.5 g of catalyst, it was seen that CO conversion achieved was on average ca. 40% lower than that achieved over the catalyst powder at similar space velocities. The product selectivity was also poor. Furthermore, it was seen that despite the tortuous flow channels of the foams, which were initially thought to produce better mass transfer than the flat-wall channels of the monoliths, the performance of the foams was not better than that of the monoliths. Thus, it is concluded that the use of monoliths and foams does not improve performance in Fischer-Tropsch synthesis. The poor performance may be attributed to poor heat and mass transfer properties in the reactor set-up, and possibly aggravated by catalyst deactivation incurred during reactor startup.
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    Open Access
    Design, construction and commissioning of a packed bed reactor system for methane to methanol conversion
    (2018) Guo, Junfeng; van Steen, Eric
    The direct conversion of methane to methanol can have great economic implications and have been under extensive research for the past century. It is speculated that platinum-based catalysts may achieve this due to its ability to adsorb molecular oxygen as reactive surface oxygen species that may react with methane to form methanol. The selective conversion to methanol over such a catalyst might be possible through site blocking action with in presence of steam and/or high oxygen partial pressures as well as the presence of a promoter. Thus, a packed bed reactor system capable of safely operating under high pressures (< 50 bar) is designed and constructed to investigate the performance of platinum-based catalysts in the direct oxidative conversion of methane to methanol whilst co-feeding steam. The design procedure is carried out from flowsheet development to the detailed design of individual units of the reactor system. The constructed reactor system is built around a quartz lined microreactor 200 mm long and 2.4 mm in diameter to minimize risks associated with the flammability of methane and oxygen mixtures. A method for complete product analysis of all carbon containing reaction products using gas chromatography with flame ionization detector in conjunction with an oxidizer-methanizer microreactor is developed which is capable of quantifying minor carbon containing reaction products formed at low conversions with yields greater than 10 μmol/mol. Flow, pressure and temperature controls are also developed for the reactor system to ensure steady state operation.
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    Design, construction and commissioning of an automated optical fibre catalyst coating process for use in photocatalytic reactor systems
    (2020) Harrisankar, Naomi; van Steen, Eric; Levecque, Pieter
    Climate change is one of the greatest challenges facing humanity. Fossil fuels are the primary source of energy on Earth. Since the global economic growth is closely linked to the global energy demand, fossil fuel usage remains the largest contributor to the steadily increasing atmospheric carbon dioxide concentration (CO2). CO2 mitigation through carbon capture and conversion are of great interest. Capturing CO2 from point source emitters is possible by absorption in a basic, sodium hydroxide (NaOH) containing solution, which is then converted into sodium bicarbonate (NaHCO3). Conversion of CO2 is thermodynamically demanding as it will require a large amount of energy, which renders currently used technologies infeasible. A promising alternative is the conversion of captured NaHCO3 into useful hydrocarbons at moderate operating conditions using solar energy, by a process called photocatalysis. Photocatalysis is the acceleration of a photo-induced reaction in the presence of a catalyst. Photocatalytic reactors have not yet been commercialised due to suboptimal catalyst and reactor designs. The typically low catalyst activity has to be countered by efficiently loading a large amount of catalyst in the reactor. This results in a problem regarding the photon transfer limitations to the catalytically active site, which limits illumination of the catalyst in the reactor. This can be overcome by using optical fibre to guide photons, which are coated with the photocatalyst. However, it is estimated that a reactor containing ca. 1 g of catalyst will require ca. 1.8 km of identically coated optical fibre. The aim of the project is to design, construct and commission an automated controllable process to increase the production volume of catalyst coated optical fibre using either a solgel suspension or a slurry containing P25 (TiO2). A multi-step optical fibre coating process was developed to achieve the desired coated optical fibre as a product. It consists of 6 major units that process raw (polymer-coated) optical fibre into catalyst coated optical fibre. The steps include the 4 essential steps required for optical fibre preparation by-hand, these steps are stripping, washing, coating and heat treatment. This automated optical fibre catalyst coating process (AOFCCP) can make the coating of optical fibres time-efficient and controllable. The latter can be achieved by controlling the effect various process parameters affecting the coating thickness and homogeneity of the coating, such as pH, heat treatment, catalyst slurry concentration as well as pulling speed. The AOFCCP produced coating thicknesses ranging from 0.47 µm - 0.59 µm and 0.37 µm - 0.46 µm for the P25 slurry and sol-gel coating methods respectively. The pH of the P25 slurry was found to have a negligible effect on both the coating thickness and surface morphology, therefore is no longer regarded as a process variable in the AOFCCP. The thickness of the coating increased with an increase in P25 slurry concentration with a maximum achievable coating thickness of 0.87 µm using a slurry concentration of 20 wt.-%. The temperature of heat treatment which was tested showed different relationships between the coating methods. For the sol-gel coating method, the increase in temperature resulted in a decrease in coating thickness possibly due to the decrease in porosity whereas for the P25 slurry method the increase in temperature showed an increase in coating thickness possibly due to the higher evaporation rates. An increase in the pulling speed in the AOFCCP resulted in an increase in coating thickness on the optical fibre independent of the coating method; coating thicknesses ranging from 0.41 µm - 0.71 µm and 0.23 µm - 2.14 µm were obtained using the P25 slurry and sol-gel coating methods, respectively, by varying the pulling speed. The critical cracking thickness is defined as the thickness of the film, produced by the sol-gel method, at which coating deformations become observable which was found to be 0.37 µm at 600 °C, and 0.77 µm at a pulling speed of 2.30 mm.s -1 . The results obtained from the commissioning experiments showed that the AOFCCP can produce coated optical fibre with controllable thickness. The controllability was discovered to be in the adjustment of the process variables investigated which showed a significant effect on the coating thickness, except for pH. Based on the statistical analysis that was performed, it was confirmed that the results obtained from the system were repeatable and that the coating was uniform for all process variables that were investigated except for sol-gel coating at high speeds of 2.88 mm.s -1 – 3.46 mm.s -1 . The system was able to produce fibre with coating thickness's between 0.4 – 1.1 µm. It is recommended that a combination of the process variables be used in order to achieve better controllability in the process and to achieve thicker coating layers. Furthermore, the operating ranges of the process variables should be increased in order to determine the extent of the relationship between the process variable and the coating thickness and surface morphology.
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    Formation of Pt-Based Alloy Nanoparticles Assisted by Molybdenum Hexacarbonyl
    (2021-07-14) Leteba, Gerard M; Mitchell, David R G; Levecque, Pieter B J; van Steen, Eric; Lang, Candace I
    We report on an optimized, scalable solution-phase synthetic procedure for the fabrication of fine-tuned monodisperse nanostructures (Pt(NiCo), PtNi and PtCo). The influence of different solute metal precursors and surfactants on the morphological evolution of homogeneous alloy nanoparticles (NPs) has been investigated. Molybdenum hexacarbonyl (Mo(CO)6) was used as the reductant. We demonstrate that this solution-based strategy results in uniform-sized NPs, the morphology of which can be manipulated by appropriate selection of surfactants and solute metal precursors. Co-surfactants (oleylamine, OAm, and hexadecylamine, HDA) enabled the development of a variety of high-index faceted NP morphologies with varying degrees of curvatures while pure OAm selectively produced octahedral NP morphologies. This Mo(CO)6-based synthetic protocol offers new avenues for the fabrication of multi-structured alloy NPs as high-performance electrocatalysts.
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    Selective oxidation of methane in a trickle bed reactor over a platinum-based catalyst
    (2022) Hytoolakhan, Lal Mahomed Nasseela; van Steen, Eric
    The direct on-site conversion of methane to methanol could provide a more cost-effective and less energy-intensive utilization of natural gas compared to the industrial two-step syngas based route. Despite the numerous approaches investigated for direct methane to methanol (DMTM) conversion in the last century such as homogeneous gas-phase, homogeneous liquid partial oxidation and heterogeneous catalytic partial oxidation, none of them were deemed successful for commercialization due to either the use of expensive reagents such as H2O2 and H2SO4, low yields and conversions, or inefficient system requirements. This has been mainly attributed to the thermodynamic challenge of breaking the strong C-H bond (bond dissociation energy of 435 kJ/mol) in the highly stable methane molecule and due to the higher reactivity of the product methanol leading to the formation of COx products. Based on density functional theory (DFT) calculations, it was postulated that the selective oxidation of methane to methanol was possible with weakly adsorbed oxygen species over transition metal surfaces such as Ag and Pt at high oxygen and water partial pressures. Recent experimental studies also indicated that the presence of water through site blocking action had beneficial effects on catalytic activity and methanol selectivity. Since the role of liquid water in a single continuous-flow reactor has not been well established, evaluating its effect on catalytic performance and product selectivity in the direct conversion of methane over platinum-based catalysts was the prime focus of this research. Therefore, a specifically designed trickle bed reactor (TBR) system capable of operating continuously and safely at high pressures was constructed to investigate the selective oxidation of methane over platinum-based catalysts in the presence of liquid water. Methane, oxygen and helium were pressurized and flow-metered prior to entering the reactor while the flow of deionized liquid water was controlled through a pump. The reactor assembly consisted of a quartz tube liner sealed at the top with a fluorocarbon O-ring in a stainless-steel shell (545 mm in length and 15.8 mm in inside diameter). A central thermowell fixed along the reactor tube which shielded an internal thermocouple was also quartz-sheathed to prevent undesired side reactions on the metal surface. Pelletized platinum catalyst supported on titania (ca. 1.5 g, dp = 150-250 µm) was packed in the isothermal region (ca. 100 mm) of the reactor with silicon carbide granules (dp ~ 1000 µm) filling the top and bottom void spaces. The reactor body was heated through a five-zone electric furnace. Pressure-controlled argon was allowed to flow between the annular region of the quartz liner and reactor shell at the same pressure as inside the quartz tube liner. The flowrate of argon was set approximately 2-3 times higher than the total gas flowrate through the catalyst bed to completely vaporize the effluent stream. The reactor effluent was throttled to atmospheric pressure and heated via heating cords to ensure no condensation of products prior to analysis via an existing online GC-Polyarc™-FID system. Long-term catalytic experiments were performed on platinum impregnated TiO2(P25) and TiO2(rutile) in the newly constructed trickle bed reactor at 220 °C and 30 bar. The inlet partial pressures of methane and oxygen were kept constant at 0.5 bar and 1.5 bar, respectively, while the feed rate of liquid water was varied from 0 - 0.3 ml/min with intermittent return to baseline condition (i.e., at PH₂O = 6.9 bar) to monitor catalyst activity. The results of this investigation indicated that co-feeding liquid water has beneficial effects on the activity of these catalysts as well as the selectivity towards C1 oxygenated products. Both catalysts were able to selectively convert methane to C1 oxidation products in the trickle bed reactor with the highest activity and selectivity obtained while operating in the flooding region whereby water exists in the liquid phase (PH₂O > 23.1 bar). Pt/TiO2(P25)selectively formed methanol and methoxy methanol with a maximum selectivity of ca. 10% at a methane conversion of 0.6%, corresponding to an average turnover frequency of 0.25 h-1. Carbon dioxide was the major product formed over Pt/TiO2(P25). Conversely, over Pt/TiO2(rutile), formaldehyde was the exclusive selective oxidation product formed with a remarkable selectivity of 90% at a methane conversion of 1%, corresponding to an average turnover frequency of 0.43 h-1. Carbon dioxide and carbon monoxide were also detected in the product spectrum. Thus, under similar operating conditions, Pt/TiO2(rutile) exhibited a significantly higher activity and improved selectivity, greatly favouring the formation of C1 oxygenates compared to Pt/TiO2(P25). The novel trickle bed reactor system has demonstrated great ability in operating continuously at steady state for long-term runs for the selective oxidation of methane in the presence of liquid water at high pressure (< 50 bar) and moderate temperature (< 400 °C). Although the findings in this research indicate that liquid water facilitates the direct conversion of methane to selective oxidation products such as methanol and formaldehyde, the role of liquid water is not still obvious and has not been explicitly studied. Therefore, it is recommended that more catalytic testing with optimised reactor conditions and/or addition of promoters (such as Ag, Ni, Pd, Mo, Co and Cu) should be done in order to improve methane conversion, as well as yield for C1 oxidation products. Furthermore, to enhance the performance of the reactor and better understand the interaction of liquid water with the solid catalysts in the tri-phasic system, hydrodynamics studies should be performed.
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    Strong-metal–support interaction by molecular design: Fe–silicate interactions in Fischer–Tropsch catalysts
    (Elsevier, 2012) Mogorosi, Ramoshibidu P; Fischer, Nico; Claeys, Michael; van Steen, Eric
    Metal–support interactions in the form of iron–silicate were investigated by an inverse approach, that is, modification of nano-sized iron oxide with surface silicate groups. The presence of surface silicate groups in the calcined catalyst precursor was confirmed using diffuse reflectance infra-red Fourier transform analysis. The genesis of the various iron phases in the presence of surface silicate groups after H2-activation and the Fischer–Tropsch synthesis was followed. The surface silicate groups are preserved after a hydrogen treatment at 350 C for 16 h, and these surface ligands are associated with the residual iron oxide phase, wüstite. During the Fischer–Tropsch synthesis, a-Fe is mostly converted into v-Fe5C2, whereas FeO is the main source for e-Fe2C. The activity per unit surface area of hexagonal carbide, eFe2C, is ca. 25% higher than that of v-Fe5C2. The presence of surface silicate ligands on e-Fe2C results in a further enhancement of the rate per unit surface area of e-Fe2C by a factor of ca. 3. This is being ascribed to the enhanced availability of hydrogen on the surface due to the presence of the surface silicate groups, which also results in an increase in the methane selectivity, a decrease in the olefin content and a decrease in formation of branched product compounds.
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    Open Access
    The mobility of oxygen containing species (OCS*) over Pt-based catalyst surfaces: Impact on the oxygen reduction reaction (ORR) activity
    (2020) Gambu, Thobani G; van Steen, Eric; Petersen, Melissa
    The growing need to curb greenhouse gas emissions has made low-temperature proton exchange membrane fuel cells (PEMFCs) more attractive for automotive application. One of the major problems facing PEMFCs is the sluggish kinetics of the oxygen reduction reaction (ORR). To further enable wide-scale commercialisation of PEMFCs for automotive applications, major improvements in the ORR catalyst are therefore needed. An in depth understanding of the ORR mechanism over Pt surfaces can enable rational approaches in the search for more active ORR catalysts. The ORR occurs over multi-faceted Pt nanoparticles which predominantly expose Pt{111} and Pt{100} facets. Most studies have modelled the overall ORR activity over multi-faceted surface assuming that the Pt{111} and Pt{100} facets are kinetically isolated. Density functional theory (DFT) studies have shown that Pt(111) surfaces can efficiently facilitate OH* hydrogenation to H2O* but not the hydrogenation of O* to OH*, whereas Pt(100) surfaces can facilitate O* hydrogenation to OH* better than OH* hydrogenation to H2O*. If O* intermediates can readily diffuse from Pt{111} to Pt{100} facets and OH* from Pt{100} to Pt{111} facets, the ORR activity on Pt{111} and Pt{100} facets of multi-faceted surfaces may no longer be limited by O* and OH* hydrogenation steps, respectively. This study uses DFT and microkinetic models to investigate the nature of inter-facet cooperation and how it influences the ORR activity under dry conditions, i.e. catalyst surface exposed to a gas mixture of 33% O2 and 67% H2 at 1 bar. Under these conditions, it is assumed that the Langmuir-Hinshelwood kinetics are dominant. Using DFT, the adsorption, diffusion and reaction energetics of various reaction intermediates and reaction steps were calculated. The Pt{111} and Pt{100} facets were modelled as Pt(111)-p(3x3) and Pt(100)-p(3x3) slabs, respectively. The edge was modelled using a rhombic nanowire model with alternating Pt{111} and Pt{100} facets. Edge sites were found to adsorb oxygen containing species strongly. Consequently, the diffusion barriers of O* and OH* from edge sites towards terrace sites were much higher than the diffusion on the terraces and even higher than the activation barrier for reaction in the ORR. Replacing the edge Pt atoms with Au and Ag atoms weakens the adsorption of both O* and OH* on edge sites. Microkinetic analyses of ORR requires the inclusion of lateral interactions, since otherwise a full coverage of the surface with O* is predicted. Higher ORR rates are obtained on Pt(100) surfaces and --(vi)-- Pt{100} facets than on Pt(111) surfaces and Pt{111} facets. The ORR activity on Pt(111) and Pt(100) is limited by O* hydrogenation at T < 480 K and O2* dissociation at high temperatures. The ORR pathway varies greatly over these surfaces. On Pt(111), the ORR follows a peroxyl pathway at T < 500 K and a dissociative pathway at T > 700 K. On Pt(100) surface H2O* is formed via O* hydrogenation to OH* followed by 2OH* coupling to H2O* and O*. The ORR activity on multifaceted Pt surfaces was shown to be dependent on the ratio of edge sites to Pt{111} sites Modelling the inter-facet exchange of ORR intermediates based on data generated using Au and Ag modified nanowires could improve inter-facet cooperation. The most interesting case was Ag modified systems where inter-facet exchange of OH* occurs at temperatures as low as 360 K. On these systems, the ORR pathway on Pt{111} involves OH* diffusion from edge followed by OH* hydrogenation to H2O*. No O2 adsorbs on the Pt{111} facet. Edge modification has the ability to selectively enable inter-facet exchange of some reaction intermediates whilst inhibiting others. Therefore, it should be explored in rational catalyst design.
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    The oxidative reforming of methane to synthesis gas on a commercial steam reforming catalyst
    (1997) Theron, Johannes Nicolaas; Fletcher, Jack; O'Connor, Cyril; Dry, Mark; van Steen, Eric
    The oxidative reforming of methane to predominantly carbon monoxide and hydrogen was studied over a commercial steam reforming catalyst. The said reaction was performed in an integral fixed-bed reactor at temperatures between 575°C and 650°C at a total pressure of 200 kPa(a). The primary objective of the study was the design and construction of equipment to facilitate firstly, the measurement of axial bed temperature profiles, and secondly, the investigation of transport effects during the oxidative reforming of methane. The absence of internal or external temperature- and concentration gradients were tested by subjecting the experimental results to theoretical criteria that had previously been derived to check for transport limitations. In all cases the correlations confirmed that the experimental system was free of transport resistances. Having obtained differential reaction rates from the integral data, two LangmuirHinshelwood models were developed and fitted to the data. The model which was derived from the assumption that methane adsorption on a single active site was the rate-determining step, gave the best fit to the experimental data. Adsorption constants correlated in broad terms with the reaction orders of the species which were previously determined by the method of initial rates. The differential kinetic data that were obtained from the fitting of exponential curves to the integral data gave the best correlation between predicted and measured reaction rates. Subsequent to the differential treatment of the data, an attempt was made to correlate the integral data by means of an integral reaction model. A combination of the total oxidation-, oxidative reforming- and steam reforming of methane, as well as the water-gas shift reaction, resulted in the best fit between measured- and predicted data. The predicted methane, carbon monoxide- and hydrogen partial pressures correlated well with experimental data, but that of oxygen, carbon dioxide and water were less accurately predicted by the model. The lack of any comparable study in the literature made it impossible to compare adsorption- and rate constants to other work. X-ray diffraction results showed that the active catalyst bed consisted of a top layer of nickel oxide on alumina and zero-valent nickel on alumina deeper into the bed. Evidence from thermogravimetric experiments revealed that carbon formation was inhibited by H2 cofeeding and high 0 2 inlet partial pressures, but that it was enhanced by CO co-feeding.
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    Theoretical feasibility of CO-activation and Fischer–Tropsch chain growth on mono- and diatomic Ru complexes
    (Elsevier, 2008) Welker, Cathrin; Phala, Noko S; Moss, John R; Claeys, Michael; van Steen, Eric
    Thermodynamic analyses of different proposed reaction pathways to determine the thermodynamic feasibility of FT reactions on a mono- and diatomic Ru-complex as model catalysts were performed. Ru(CO)5 and Ru2(CO)9 were taken as starting complexes. The calculations illustrate that a minimum of two adjacent metal atoms is required for C O bond cleavage and chain growth in the Fischer–Tropsch synthesis. The CO-insertion mechanism seems to be thermodynamically most feasible reaction pathway on diatomic Ru-clusters.
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    Waste to fuel: designing a cobalt based catalyst and process for once-through Fischer-Tropsch synthesis operated at high conversion
    (2020) Tucker, Chelsea Lyn; van Steen, Eric
    The production of fuels from waste on a small-scale decentralized level may enhance the liquid fuel security of Sub-Saharan Africa. The Fischer-Tropsch process can be used to convert waste into drop-in fuels. However, operating at small scale in remote locations requires a plant design with lower capital requirements, a greater level of simplicity and utility self-sufficiency. A plant design using an air-fed biogas reformer (without an air separation unit) and a single pass Fischer-Tropsch configuration is proposed. A fundamental requirement of this particular design is that it needs to operate at a higher FischerTropsch conversion than typically seen in industry (55 – 65%). High conversion conditions result in a high partial pressure of H2O and low partial pressures of CO and H2 within the Fischer-Tropsch reactor. These conditions have been reported to negatively affect the activity, selectivity and stability of cobalt-based catalysts. To date, no study has investigated the cause of this phenomenon, nor has a catalyst been developed specifically to operate under high conversion conditions. The objective of this study is to investigate the mechanisms behind these phenomena and provide catalyst design improvements that facilitate operation at high conversion conditions. Furthermore, a detailed design of the proposed once-through Fischer-Tropsch biogas-to-fuel plant will be evaluated using data from the catalytic experiments. An investigation into the effect of high conversion on the activity and selectivity of 0.05Pt-22Co/Al2O3 was conducted in a slurry bed reactor at T = 220°C, P = 20 bar, with a feed simulating synthesis gas generated from air-blown reforming (H2:CO:N2= 4:2:6). Space velocity was decreased to increase conversion to between XCO = 40% and XCO = 97%. The rate of CO consumption decreased with increasing conversion. Increasing the CO conversion was found to have negligible effect on CO2 selectivity (an unwanted by-product) up to a CO conversion of 75%, after which a strong increase was observed. This was attributed to the enhanced of water-gas shift activity of Co0 under hydrothermal conditions. The production of CO2 raised the H2/CO ratio within the reactor resulting in a large increase in the CH4 selectivity (an unwanted by-product), a decrease in the chain growth probability and thus a decrease in the C5+ selectivity (fuel product). In order to improve unfavourable selectivity obtained at high conversion in the Fischer-Tropsch synthesis, the effect of adding manganese (Mn) to 0.05Pt-22Co/Al2O3 was explored. The catalyst (0.05Pt22Co/Al2O3) was impregnated with increasing amounts of manganese, resulting in six Mn-Pt-Co/Al2O3 catalysts with Mn:Co mass ratios of 0, 0.04, 0.09, 0.14, 0.23 and 0.47:1. The optimal level of manganese promotion was found at a Mn:Co mass ratio of 0.14:1. At this level of manganese promotion, CO2 and CH4 selectivity was decreased by up to 6 C-% and 12 C-% (XCO = 90%) respectively whilst turn-over frequency was improved by 100%. The maximum in the C5+ yield as a function of CO-conversion was shifted from XCO = 78% to XCO = 91%, thus making operation at high conversion feasible from an activity and selectivity perspective. Operating Pt-Co/Al2O3 at conversion levels higher than XCO = 70% was shown to lead to rapid irreversible deactivation, with a total activity loss of 50% between XCO = 70% and 97%. Using a combination of spent catalyst characterisation via transmission electron microscopy, temperature programmed reduction/hydrogenation as well as an in-situ magnetometer, this irreversible deactivation was attributed to both sintering and cobalt aluminate formation. At very high conversion (XCO > 97%) enhanced reversible deactivation was exhibited due to the oxidation and re-reduction Co0 to Co(II)O. This oxidation/reduction cycle is the result of a thermodynamic conversion limit: at a mean Co0 crystallite size of 6 nm (as obtained with Pt-Co/Al2O3), the maximum achievable conversion (assuming a lognormal distribution of crystallites, σ = 0.5) is XCO = 88%. A log-normal distribution of cobalt crystallites with an average size of 8 nm (and the same variance) is required to obtain a maximum conversion of up to XCO = 98%. In order to limit deactivation due to cobalt aluminate formation at conversions higher than XCO = 70%, zinc aluminate was investigated as a novel support material for a platinum promoted cobalt catalyst. Zinc aluminate thermodynamically limits the formation of cobalt aluminate and facilitates the formation of larger sized cobalt crystallites. The catalyst, 0.04Pt-23Co/ZnAl2O4 exhibited minimal signs of irreversible deactivation at high conversion with a total rate loss of 0.08 mmol /min/g (0.62 to 0.54 mmol /min/g), whilst the rate loss over 0.05Pt-22Co/Al2O3 amounted to 0.47 mmol /min/g (from 0.74 to 0.27 mmol /min/g). The zinc aluminate supported catalyst exhibited equal selectivity towards CO2, CH4 and C5+ as PtCo/Al2O3 and an improved turnover frequency, thus making it a viable replacement support for cobalt under high conversion conditions. A once-through waste-to-fuel process (using biogas from the anaerobic digestion of waste as a raw material) was designed using the experimentally determined selectivity and activity data from the FischerTropsch synthesis. The syngas generation step of this design incorporates a tri-reformer and water-gas shift reactor. Syngas is then fed into the Fischer-Tropsch reactor, which produces largely waxy products at lower conversions (XCO = 60%) and largely naphtha/distillate products at higher conversions (XCO > 80%). The Fischer-Tropsch products were partially refined to distillate (low density diesel) by means of flash tanks and an atmospheric distillation column. At lower conversion levels, a hydrocracker must be used to improve distillate yields. All light hydrocarbons and syngas are fed to a combined cycle power plant, which produced electricity for the plant, thus satisfying the plant's utility self-sufficiency objective. The plant design was evaluated to find an optimal conversion at which to operate, and to gauge the effectiveness of the catalyst design improvements. An optimal conversion of XCO = 80% was found for Mn-Co/Al2O3 (Mn:Co = 0.14) at a production level of 329 bbl/day distillate from a feed of 16 tonnes of municipal solid waste per hour. This represents a 12% increase in production of distillate when compared to Pt-Co/Al2O3 at the same conversion. A shift from an alumina support to a zinc aluminate support will be necessary as the optimal conversion for this process lies above the XCO = 70% deactivation threshold.