Ind. Eng. Chem. Res. 1996, 35, 2121-2126
2121
Catalytic Steam Cracking of n-Heptane with Special Reference to the Effect of Calcined Dolomite Georgios Taralas* Bioresource Technology Unit (BTU), Department of Chemical Engineering, Division IV, National Technical University of Athens, Zografou Campus, GR-15700 Athens, Greece
The catalyzed steam cracking of n-heptane was carried out using a fixed-bed reactor and different, commercially available limestone [CaCO3], dolomites [CaMg(CO3)2], and NiMo/γ-Al2O3 as the catalysts. The steam cracking of n-heptane was investigated by variation of water and n-heptane partial pressure and by the additions of hydrogen and carbon dioxide to the water vapor at a total pressure of 101.3 kPa. In the presence of calcined dolomite (1073 K), the partial pressure of water vapor (pH2O ) 9.1-34.2 kPa) enchances cracking efficiency, whereas hydrogen (pH2 ≈ 33 kPa) and carbon dioxide (CO2 ) 39 vol %), when added to the cracking mixture (973 K), do suppress conversion of n-heptane. Introduction
Table 1. Characteristics of Katjenfine 153S-1.5E (NiMo/ γ-Al2O3) Catalyst
A number of investigations have been published emphasizing the ability of low-cost metal oxides prepared from naturally occurring minerals such as dolomite [CaMg(CO3)2], for purification of tarry fuel gas during thermal gasification and pyrolysis of biomass, oil shale, lignite, and coal at low-pressure conditions (Yeboah et al. 1980; Floess et al., 1985; Alde´n et al., 1988; Sjo¨stro¨m et al., 1988; Taralas, 1990; Delgado et al., 1991; Corella et al., 1991; Vassilatos et al., 1992). However, because of the complex composition of tar, the reaction phenomena in catalytic gasification and pyrolysis processes with dolomite required use of previous cracking studies with well-defined model compounds (Taralas et al., 1991, 1993; Alde´n et al., 1994). Nevertheless, in the present study the steam cracking of n-heptane over calcined dolomites [CaMg(O)2], limestone [CaO], and a commercial NiMo catalyst was investigated further. The effect of water vapor and n-heptane (n-C7H16, a saturated hydrocarbon without π-electrons) partial pressure as well as the influence of other reaction products (carbon dioxide, hydrogen) added to the reacting mixture has been studied. Experimental Section Experimental Setup. The experimental apparatus, downflow continuous operation, and analysis arrangement used were similar to that described in a previous study (Taralas et al., 1991). n-Heptane (Merck, 99% purity) was used as the feed in all tests. After the course of a run, the catalyst was weighed and, in some cases, was analyzed for “coke” content. Catalysts. The calcined mineral stones (particle size diameter between 1.4 and 1.5 mm), Limhamn limestone, and Sala, Larsbo, and Glanshammar dolomites are commercial Scandinavian quarried products (Taralas et al., 1991, 1993). The NiMo/γ-Al2O3 catalyst, Katjenfine 153S-1.5E, was obtained from Degussa AG. The commercially available NiMo on γ-alumina catalyst was selected since this type of catalyst exhibited tar conver* On leave from the Royal Institute of Technology (KTH), Department of Chemical Engineering and Technology, Chemical Technology, Stockholm, Sweden.
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MoO3 (wt %)a NiO (wt %)a Na2O (wt %)a Fe (wt %)a SO4 (wt %)a loss on ignition activated alumina (%)b nominal size (mm) shape, textural features SBET - area (m2/g) av pore diam (Å) a
15.0% 3.1 0.08 0.03 0.7 2.0% 1.50 extrudate, pellets 217.56 74.95
Dry base. b Wet base; 1 h, 823 K.
sion as a cracking catalyst (Baker and Mudge (1987) Pedersen (1994) and references therein). The NiMo catalyst was used in its extrudate form and was pretreated in a hydrogen stream (773 K). Table 1 tabulates the values of the physical and chemical makeup of the NiMo catalyst. Calcination of Dolomite. Calcination of dolomite is a complex process which at higher temperatures causes a gradual rearrangement of the dolomitic material and includes two different decomposition stages (Hehl et al., 1983). Dolomite decomposes at a relatively low temperature (
