Comprehensive kinetic model for acetylene pretreated mesoporous silica supported bimetallic Co-Ni catalyst during Fischer-Trospch synthesis
Graphical abstract
A new comprehensive kinetic model incorporating porosity field during acetylene pretreatment (PT) is proposed for quantitatively describing the product distributions during Fischer-Trospch (FT) synthesis using mesoporous supported Co-Ni bimetallic catalyst. By assuming the acetylene pretreated catalyst forming active carbidic intermediates engaging with subsequent CO dissociation and 1-olefin re-adsorption associated secondary reactions during FT synthesis, this new comprehensive kinetic model is capable of predicting the increased olefin to paraffin ratios (OPR) caused by acetylene pretreatment.
Introduction
Although Fischer-Trospch (FT) synthesis has been reported over 90 years, it still remains one of the most fascinating and intriguing topics for both academia and industry (Boller et al., 2019, Jiao et al., 2016). Over the vicissitudes of research and development, FT synthesis has witnessed its metamorphism from a lab prototype into a commercially viable process for liquid fuel productions (Baliban et al., 2013, Elia et al., 2011, Li et al., 2018, Niziolek et al., 2014, Sun et al., 2017c, Wang et al., 2018). In the meanwhile, the development of FT synthesis technology is frequently stumbled by the technical hurdles such as easiness of catalyst deactivation, poor product selectivity, and complexities of downstream processing (Jahangiri et al., 2014, Sage and Burke, 2011). One practical approach to tackle the aforementioned problems is to manipulate the product distributions, such as tailoring the heavier products into lighter liquid products, which results in simplification of downstream process (Sage et al., 2017). To simultaneously realize the goal of suppressing catalyst deactivation and shifting product selectivity, the strategies i.e. development of metallic alloy catalyst, high performance supports with less tendency to sinter, and alteration of operation approaches, have been proposed (Chen et al., 2012, Zhang et al., 2010). Apart from the above mentioned approaches, the addition of C2 unsaturated hydrocarbons have been found to effectively skew the product distribution towards lighter hydrocarbon during FT synthesis (Kibby et al., 2009). The merit of this approach lies in the fact that instead of moving towards the tedious processes of selecting new catalyst, the existing catalyst can be in-situ pretreated prior to the FT synthesis, a route that can be easily retrofitted to the existing process (Sage and Burke, 2011, Sage et al., 2017). Investigating the deposition of short chain unsaturated hydrocarbons such as acetylene on the metallic catalyst does present practical and fundamental interest, especially in the area of reducing the amount of waxy hydrocarbons (C22+) and increasing the amount of light liquid hydrocarbons (Shafer et al., 2019). A good modeling and understanding of this process will ultimately benefit the downstream process for product fractionation. However, the current status quo of investigation of acetylene deposition mainly focuses on the pyrocarbon deposition, which is widely practiced in the chemical vapor deposition (CVD) and chemical vapor infiltration (CVI) processes for the reinforced carbon fiber composites manufacturing industries (Ducrot-Boisgontier et al., 2009, Tan et al., 2019). These processes often involve with typical higher temperature (over 1000 °C) at ambient pressure, which makes gaseous reaction and surface chemistry in heterogeneous pyrocarbon deposition being essentially different to acetylene catalyst pretreatment. For commercial FT synthesis, cobalt (Co) and iron (Fe) are the most commonly used catalysts. But the problems of using Co catalyst lies in its inherent high cost and complexity of downstream process required for the isomerization and cracking to generate finished fuel (Nikparsaa et al., 2014). Although iron based catalyst can be operated at wider range of conditions, its high levels of gasoline (C2-C5) olefinic selectivity and rapid deactivation limits its commercial applications (Ghorbani et al., 2021). The practical ways to tackle these problems include: designing a new material that mimics the electronic structure of an existing or ideal catalyst, and finding the inexpensive metallic alloys to replace current expensive catalysts (van Helden et al., 2020). In this study, we focus on the latter technical route. Albeit the strong tendency of generating CH4 through methanation by using Ni metal, recent studies have suggested the Ni metal a good replacement of rhenium (Re) as a reduction and activity promoter to their Co catalyst during FT synthesis. With addition of Ni, the deactivation and run activities have been appreciably improved (Badoga et al., 2020). In addition, studies also indicated that Ni particles facilitate the cobalt dispersion, by serving as seeding sites for the formation of cobalt oxide crystals on the support surface, lowering reduction temperature, and tend to form a hydrogen spill over mechanism (Voss et al., 2015). Apart from above features that Co-Ni alloy catalyst possess, alkene selectivity (small olefinic fraction) were also appreciably increased, resulting in C3 olefin to paraffin ratio reaching around 6 to 8 (Karthikeyan K. Ramasamy, 2015). Although there have been some reports of using metal alloys of Co-Ni for FT synthesis, a further step forward to pretreat this type of metallic alloy followed by subsequent FT synthesis has seldom been trialed before. In addition, to obtain insightful understanding of this coupled process and for the sake of reactor sizing, the quantitative linkages between the critical operational parameters (i.e. pretreatment temperature, duration, partial pressures of reagents of CO, H2 during FT synthesis etc) and kinetics (rates of reagents, products formation rate with different carbons numbers) during FT synthesis is pivotal. In fact, there are rare attempts to construct a modeling tool that describes the product distributions during FT synthesis when mesoporous silica supported bimetallic Co-Ni catalyst was pretreated by the acetylene. Thus, it initiates our attempt to couple these two processes and establish a quantitative correlation between the acetylene PT and subsequent kinetics during FT synthesis. To the best knowledge of the authors, this modeling approach have been seldom reported before.
Section snippets
Experimental works
In this work, the mesoporous silica supported bimetallic Co-Ni catalyst was pretreated by the acetylene. The catalyst was prepared by the reported methods (Sun et al., 2017b). The obtained catalyst was sieved to achieve particle range of 37–75 μm. The porosity parameters were determined by nitrogen gas adsorption–desorption technique (ASAP 2020 Automated Gas Sorption System, USA). The specific surface area (SSA) was calculated using Brunauer-Emmett-Teller (BET) approach by assuming the
Theoretical background
Carbon deposition on catalytic surface is a complicated process. According to the consecutive reaction pathway model, the pyrolytic carbons from acetylene are formed by assuming all surface reaction kinetics zero residence time during the CVD process (Huttinger, 1998). For chemical vapor infiltration (CVI), the multi-step reaction coupled with hydrogen inhabitation model is found to best describe the process (Becker et al., 2000). The acetylene pretreatment of catalyst (such as nickel, cobalt),
Model validation and simulation during PT
Different experimental works were used to validate the model during PT. At BL condition, the effects of meshing upon predicting breakthrough curves were carried out and results are shown in Fig. 3a. The raw meshing (4526 cells) and fine mesh (18106 cells) show similar predicting profile for breakthrough curves when TOS is over 1 h. Appreciable differences, especially at the beginning of pretreatment (TOS < 0.5 h) were observed between these two meshing. At the beginning of the pretreatment, the
Conclusion
The comprehensive kinetic model by incorporating porosity field function during acetylene pretreatment with product distribution was proposed in this paper. The statistical homogeneous porous media being altered by the reactive acetylene deposition at the investigated conditions (relative lower temperature 180–210 °C with10 bar) is found to yield a good agreement with the obtained experimental results, i.e., breakthrough curves, permeability, and pressure drops. The acetylene pretreated
CRediT authorship contribution statement
Yong Sun: Investigation, Methodology, Formal analysis, Writing - original draft, Project administration, Funding acquisition. Yixiao Wang: Data curation, Investigation. Jun He: Writing - review & editing. Abubakar Yusuf: . Yunshan Wang: Investigation, Formal analysis. Gang Yang: Project administration, Funding acquisition. Xin Xiao: Conceptualization, Validation, Supervision, Writing - review & editing.
Declaration of Competing Interest
The authors declare no competing financial interest.
Acknowledgements
Authors would like to sincerely appreciate the critical and insightful comments that raised from those anonymous reviewers in significantly improving the quality of this work. Financial support for this project: Qianjiang Talent Scheme (QJD1803014), Faculty Inspiration Grant of University of Nottingham (FIG2019), Ningbo Municipal Commonweal Key Program (2019C10033 & 2019C10104), UNNC FoSE New Researchers Grant (NRG I01210100011). The kind help for language polishing from Adrian Styles and
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