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On Thermodynamic Equilibrium of Carbon Deposition from Gaseous C-H-O Mixtures: Updating for Nanotubes

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On Thermodynamic Equilibrium of Carbon Deposition from Gaseous C-H-O Mixtures: Updating for Nanotubes Rev Chem Eng 2017; 33(3): 217–235 Open Access Zdzisław Jaworski*, Barbara Zakrzewska and Paulina Pianko-Oprych On thermodynamic equilibrium of carbon deposition from gaseous C-H-O mixtures: updating for nanotubes DOI 10.1515/revce-2016-0022 Received April 29, 2016; accepted July 29, 2016; previously published 1 Introduction online August 26, 2016 The world energy infrastructure currently uses mainly Abstract: Extensive literature information on experimental fossil fuels such as hydrocarbons and various types of thermodynamic data and theoretical analysis for deposit- coal with steadily growing contribution from renewable ing carbon in various crystallographic forms is examined, energy sources as the energy economy based on hydrogen and a new three-phase diagram for carbon is proposed. as energy carrier is still in its infancy. One of the key prob- The published methods of quantitative description of lems associated with carbonaceous fuels is unfavorable gas-solid carbon equilibrium conditions are critically deposition of solid carbon from gaseous carbon-hydro- evaluated for filamentous carbon. The standard chemi- gen-oxygen (C-H-O) mixtures met in a range of industrial cal potential values are accepted only for purified single- processes (Rostrup-Nielsen 1997). The main sources of walled and multi-walled carbon nanotubes (CNT). Series depositing carbon were described in the literature as (i) of C-H-O ternary diagrams are constructed with plots of molecules of elemental carbon created in thermal crack- boundary lines for carbon deposition either as graphite or ing reactions in gas phase (Jankhah et al. 2008) and (ii) nanotubes. The lines are computed for nine temperature C-H-O compounds of various proportions of the elements, levels from 200°C to 1000°C and for the total pressure of decomposed on catalyst surfaces (Chen et al. 2011, Korup 1 bar and 10 bar. The diagram for graphite and 1 bar fully et al. 2012). Carbon deposition in (i) consists in direct- conforms to that in (Sasaki K, Teraoka Y. Equilibria in fuel phase transformation of the element molecules from cell gases II. The C-H-O ternary diagrams. J Electrochem gaseous state into the solid form. Soc 2003b, 150: A885–A888). Allowing for CNTs in carbon Decomposition of several hydrocarbons was often deposition leads to significant lowering of the critical car- used to produce the so-called pyrolytic carbon (pyrocar- bon content in the reformates in temperatures from 500°C bon), which found significant applications initially in the upward with maximum shifting up the deposition bound- space and aircraft industries. Nowadays, carbon nano- ary O/C values by about 17% and 28%, respectively, at 1 materials are being manufactured mainly in the form of and 10 bar. single- or multi-walled nanotubes (Dasgupta et al. 2011, Prasek et al. 2011). In contrast, carbon deposition process Keywords: carbon deposition; C-H-O reformates; CNT (coking, fouling) is highly undesirable in petrochemistry effects; thermodynamic equilibrium. (Benzinger et al. 1996) at cracking and dehydrogenation processes and also in the solid oxide fuel cell technology. Likewise, combustion of carbon containing fuels is fre- quently accompanied by soot formation due to incom- plete oxidation (Wijayanta et al. 2012). However, this study is focused on conditions of carbon *Corresponding author: Zdzisław Jaworski, Department of Chemical deposition from reformed fuels that is detrimental mainly Technology and Engineering, West Pomeranian University of to catalyst surfaces, depending on the morphology of the Technology, Szczecin, 71065 Szczecin, Poland, surface carbon (Pakhare and Spivey 2014). Therefore, the e-mail: [email protected] phenomenon of solid carbon formation both in syngas pro- Barbara Zakrzewska and Paulina Pianko-Oprych: Department of Chemical Technology and Engineering, West Pomeranian University duction and in preprocessing of fuels for solid oxide fuel of Technology, Szczecin, 71065 Szczecin, Poland cells (SOFCs) is of profound importance. The presence of ©2016, Zdzisław Jaworski et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License. 218 Z. Jaworski et al.: Carbon deposition: updating for nanotubes C1 to C4 hydrocarbons and CO in reformate gases was found decomposition at catalyst surfaces, and associated mech- to be a major precursor of carbon formation in SOFCs (Bae anisms of filamentous deposit growth were proposed (de et al. 2010). On the other hand, the nucleation of graphite Bokx et al. 1985, Snoeck et al. 1997a, Wagg et al. 2005). over a catalyst is extremely sensitive to its nanostructure A range of carbonaceous deposit types, both amor- (Bengaard et al. 2002). The deposition usually occurs on phous and crystalline, was found in broad studies of carbon solid surfaces, but nuclei of solid carbon may also form in deposition. The deposits widely differ in morphology and the fluid phase (Guo et al. 2013, Lemaire et al. 2013). reactivity (Bartholomew 2001, Wagg et al. 2005, Diaz Alva- This review starts from a general description of litera- rado and Gracia 2012). Carbon deposition can result in che- ture data on deposition of various forms of solid carbon mical reactions over a catalyst surface, where several forms from gaseous reformate mixtures. Based on the literature including atomic carbon can appear (Kee et al. 2005), or in experimental data, an original three-phase carbon diagram gas phase due to free-radical reactions ultimately forming is proposed for a wide P-T range followed by validated math- polyaromatic compounds (Sheng and Dean 2004). McCarty ematical expressions for equilibrium carbon vapor pressure and Wise (1997) observed four main types of carbon on a over carbon solid forms. In the following chapter, quantita- nickel catalyst. Other early classifications (Rostrup-Nielsen tive description of thermodynamic equilibrium is included 1979, 1984) proposed three types of solid carbon formed at for a range of allotropic forms of solid carbon, including two reforming of fuels: (i) encapsulating film, (ii) whisker-like filamentous forms. Then the published models of deposi- (filamentous), and (iii) pyrolytic carbon. Those carbon tion equilibrium are briefly presented. Based on minimiza- forms were also observed by Sehested (2006). The encapsu- tion of the Gibbs energy of the considered C-H-O systems, lating tar-like carbon film (i), according to Rostrup-Nielsen it was possible to compute equilibrium phase concentra- (1979, 1984), is formed by slow polymerization of higher tions and detect the depositing carbon form. Series of such hydrocarbon radicals at temperatures below 773 K. Accord- simulations for varied temperature and pressure allowed to ing to Sehested (2006), the encapsulating carbon (gum) construct ternary C-H-O plots showing boundaries of both deposits consist of a thin film of hydrocarbon or a few layers graphitic and filamentous carbon deposition zones. of graphite. Formation of the filamentous, graphitic type in the form of tubular filaments (whiskers, fibers ii) prevails in the temperature range from 720 K up to 870 K (Rostrup- Nielsen, 1979, 1984), whereas at the latter temperature, the 2 Carbon deposits formation of pyrolytic carbon (soot, iii) begins to predomi- nate as result of thermal cracking of hydrocarbons. The Three major types of industrial reforming of hydrocar- mechanism of pyrolytic carbon deposition in the range of bon/alcohol fuels can be distinguished. Those are steam 1073–1273 K was studied by Zheng et al. (2013). Alstrup et al. reforming (SR), catalytic partial oxidation (CPOX), and (1998) found that carbon filaments are formed on nickel dry reforming (DR). While SR and DR are endothermic catalysts when hydrogen is present in the gas phase while and CPOX is exothermic, a combination of SR and CPOX is encapsulating carbon dominated in pure CO reactions. named oxidative SR and its specific energy-neutral version Filamentous carbon deposits were detected more is called autothermal reforming (Diaz Alvarado and Gracia than 120 years ago by Hughes and Chambers (1889) during 2012). Depending on the process conditions, all those their search for carbon filaments for electric lighting. The reforming processes can result in formation of carbonic finding was confirmed by Davis et al. (1953) by means of solid. Thermal decomposition of hydrocarbons and alco- electron micrography. Sehested (2006) underlines the hols proceeds along complex paths, and the mechanism detrimental role of whisker carbon for catalysts and links details are still searched for (Alberton et al. 2007, Dufour development of pyrolytic carbon with the appearance of et al. 2009, Wang et al. 2009). Most of the studies of carbon higher hydrocarbons at elevated temperature. Generally deposition indicate formation of elemental carbon in the similar results were reported by Alberton et al. (2007) and reforming reactions as the main source of carbon deposi- He and Hill (2007) indicating the importance of catalyst tion, however, without supplying a validated mechanism. type and the highly destructive role of carbon dissolved Two models were considered for pyrolytic carbon (Zheng within the catalyst structure above 873 K. During cata- et al. 2013): a deposition droplet model proposed by Shi lytic and thermal cracking of ethanol in the temperature et al. (1997) and the combined mechanism of nucleation range from 523 K to 1123 K, Jankhah et al. (2008) found on graphene and of carbon growth at the edge of gra- that carbon
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