- Title
- Catalytic hydrogenation of CO and CO₂ in the presence of light hydrocarbons
- Creator
- Shadravan, Vahid
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2018
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Carbon oxides emission, as by-products of many industrial synthetic hydrocarbon processes, causes serious environmental issues and negatively affects commercialisation of some new processes (e.g. OCM). Thus, producing CO and CO₂ (COx) free (or with minimal amount of COx) synthetic hydrocarbon streams is necessary to facilitate commercialization of these new processes as large scale industrial plants. Moreover, due to the significant environmental effect of COx, it is critical to develop processes to convert COx and reduce their emission into the atmosphere. In this thesis, catalytic hydrogenation of COx in the presence of light hydrocarbons (methane, C₂-C₃ alkane and alkene) was studied. The feasibility of converting COx (where the residual concentration of both CO and CO₂ in the product gas stream are less than 1 ppm) without reducing the inlet concentration of feed hydrocarbons was initially investigated over a bench-mark hydrogenation catalyst (Ni/Al₂O₃). It is found that the inlet species were consumed and converted in different temperature ranges. For feed compositions containing COx and C₁-C₃, the consumption of carbon monoxide, carbon dioxide and C₂/C₃ paraffins was observed and their maximum conversion was attained over different temperature ranges, in the following order: CO (150 – 250 °C) < CO₂ (250 – 350 °C) < C₂/C₃ paraffins (275 – 400 °C). Moreover, Olefins were converted under all reaction conditions at lower temperatures (below 150 °C) due to the hydrogenation reaction which resulted in the formation of saturated hydrocarbons. Furthermore, the studies on COx hydrogenation in the presence of light hydrocarbons were extended to the development of catalysts to enhance the total outlet concentration of light hydrocarbons in a COx-free product stream. The effect of different transition metals (i.e. Fe, Co, Cu, Cr, Mn, Zn, Ru, Rh, Ag and Cd) on the catalytic performance of a Ni/Al₂O₃ catalyst was studied. Different and distinct promoting or inhibiting influence was observed (e.g. maximum C₂-C₄ yield of production increased from 6% for Ni/Al₂O₃ to 12% for Ni-Mn/Al₂O₃). The characteristics of partially charged active sites of the catalysts were studied by employing different techniques (i.e. in situ NO-FTIR, CO-/H₂-TPD and chemisorption). It is found that the addition of transition metals to Ni/Al₂O₃ markedly changed the structure of the active sites on the primary catalyst. For example, addition of copper resulted in increasing the ratio of Carbon-accepting to Oxygen-accepting sites (i.e. NO linear/bent adsorption increased from 7.70 for Ni/Al₂O₃ to 24.89 for Ni-Cu/Al₂O₃), which is probably increased the chance of linear CO adsorption that needs higher temperature for C – O cleavage. In contrast, by adding manganese to Ni/Al₂O₃ catalyst the ratio of electron accepting to donating sites balanced on the catalyst surface. Thus, most probably the number active carbon and hydrogen species increased on the surface. Promoting effect of manganese on Ni/Al₂O₃ was further investigated. Catalyst activity measurements as well as various characterisation techniques (such as XRD, CO and H₂ chemisorption, in situ NO-FTIR and TPR) were performed for a series of Ni-Mn/Al₂O₃ catalysts with different nickel and manganese contents. It is considered that there is an optimum amount of Mn added (i.e. bi-metallic Ni-Mn/Al2O3 catalyst with 8 wt% of nickel and 4 wt% of manganese) to the primary catalyst which enhanced the catalyst activity and selectivity. Moreover, the more hydrogen amount in the feed stream improved the catalyst activity for COx hydrogenation and selectivity toward C₂-C4 production (i.e. maximum C₂-C₄ yield of production increased from 1.5% for ~9.5 kPa H2 to 6.5% for ~37.8 kPa H2 in the feed stream over Ni/Al₂O₃). According to investigation of the catalysts’ electronic properties with different Ni and Mn contents, changes in catalytic activity (for COx hydrogenation) and selectivity (for light hydrocarbons formation) can be interpreted as being due to the effect of different electronic structure of the catalysts with variety of Ni/Mn ratios. The electrostatic properties of crystalline nickel and nickel-manganese particles was studied by computational methods (i.e. KS-DFT). Finally, this study continued on investigating the catalytic hydrogenation of COx in an industrial gas mixture containing light hydrocarbons. Complete removal of COx present in an ethane offgas (ExxonMobil refinery, Altona, VIC) via catalytic hydrogenation (over Ni-Mn/Al₂O₃) was studied. The effects of adding extra hydrogen and pre-treatment of the feed stream on the process was analysed. It is found that the addition of hydrogen gas into the feed reduced the concentration of CO and CO₂ to below the detection limits. By adding 25% and 40% of extra H₂ to the feed stream no COx were detected in the outlet. Moreover, pre-treatment of the offgas using molecular sieves to remove water vapour from the feed gas stream did not affect COx hydrogenation at low temperatures (below 300 °C). However, pre-treatment resulted in a significant reduction in CO and CO₂ concentrations at temperatures above 300 °C. The results also confirmed the saturate gas plant ethane offgas can considerably deactivate the Ni-Mn/Al₂O₃ catalyst. The effect of ethane and ethylene in the feed gas stream on catalytic hydrogenation of low concentration CO and CO₂ has also been investigated. Ethane addition did not influence the hydrogenation of COx at 180 °C while it inhibited the hydrogenation reaction at 320 °C. On the other hand, ethylene addition inhibited CO and CO₂ hydrogenation at both 180 °C and 320 °C.
- Subject
- hydrogenation; nickel catalysts; bimetallic catalysts; in-situ FTIR; FCC offgas; CO; CO2
- Identifier
- http://hdl.handle.net/1959.13/1393112
- Identifier
- uon:33500
- Rights
- Copyright 2018 Vahid Shadravan
- Language
- eng
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