Carbon formation pressure effects

In a natural gas fueled PAFC, water is condensed out of the fuel stream going to the fuel cell to increase the partial pressure of hydrogen. In a coal gasification MCFC, water often is added to the fuel stream prior to the fuel cell to prevent soot formation. The addition of excess steam not only prevents the soot formation, but also causes a voltage drop of approximately 2 mV per each percentage point increase in steam content (45). The use of zinc ferrite hot gas cleanup can aggravate the soot formation problem because of the catalytic effect of the sorbent on carbon formation, and requires even higher moisture levels (46). 

It was found that a nickel-activated carbon catalyst was effective for vapor phase carbonylation of dimethyl ether and methyl acetate under pressurized conditions in the presence of an iodide promoter. Methyl acetate was formed from dimethyl ether with a yield of 34% and a selectivity of 80% at 250 C and 40 atm, while acetic anhydride was synthesized from methyl acetate with a yield of 12% and a selectivity of 64% at 250 C and 51 atm. In both reactions, high pressure and high CO partial pressure favored the formation of the desired product. In spite of the reaction occurring under water-free conditions, a fairly large amount of acetic acid was formed in the carbonylation of methyl acetate. The route of acetic acid formation is discussed. A molybdenum-activated carbon catalyst was found to catalyze the carbonylation of dimethyl ether and methyl acetate. 

Recently Lee et al (Ref 3) re-examined the behavior of PETN under 10 to 50 kbars of external pressure. They also find a reduction in decomposition rate with increasing applied pressure. HMX behaves similarly to PETN. TNT whose explosion products contain a high proportion of solid carbon, as expected from LeChatelier s Principle, shows little pressure effect on its thermal decomposition. Nitro-methane, however, appears to decompose more rapidly under an external pressure of 50 kbars than 10 kbars. This effect is not completely understood but Lee et al suggest that high pressure may favor the formation of the thermally less stable aci form of Nitromethane ... 

Effect of Temperature. Figure 23 shows the effect of temperature on n-Ci6 POX over a temperature range of 500 to 1,000°C, at an O/C of 1.2 and a pressure of 1 atm. At all temperatures, n-Ci6 conversion was complete. H2 and CO selectivities increase from 500 to 750°C, after which they remain essentially constant. Carbon formation is avoided by operating above 750°C. From 500 to 750°C, selectivities toward H2O and CO2 diminish. Selectivity toward CH4 also diminishes from 500 to 825°C, where it essentially reaches zero. 

The TEOM results presented in Figure 9.4 showed that CO in the product stream of C02 reforming might represent another major component contributing to carbon formation. Consequently, in an effort to further elucidate the contribution of CH4 and CO to carbon formation, a kinetics study on the effect of the partial pressure of CH4 or CO (PCH4 or PCo) on the carbon formation rate was conducted under conditions of 0.1 MPa and 923 K. 

When the pressure of C02 in a carbonate-oxide system is equal to the equilibrium pressure pe, no net reaction occurs. When p < pe, the thermodynamic driving force favors oxide formation conversely, when p > pe, carbonate formation is favored. In the actual system the favored reaction may not occur, however, because kinetic factors prevent it. Particularly when p is not far from pe, the reaction may not proceed because some rate-limiting process, such as nucleus formation, is proceeding too slowly. The resulting spurious equilibria15 give rise to hysteresis effects, i.e., decomposition stops for some p < pe, recombination stops for some p > pe. It is for this reason that this work relies largely on thermodynamic methods for the calculation of equilibrium pressures. 

Alkane dehydroeyelization with Pt-Sn-alumina catalysts—Continued pressure effect, 120 PtSn alloy formation, 117-118 role of Sn, 117 Sn vs. carbon deposition, 120 Sn vs. coking, 118-119 Sn vs. n-octane conversion, 120-122 Sn vs. selectivity, 118 temperature effect, 119 Alkene hydroformylation, asymmetric catalysis, 24... 

Catalyst makers also succeeded in minimizing the activity reducing effect of the potassium in the alkalized catalysts [430], Pre-reduced primary reforming catalysts are now also marketed (ICI Katalco, Topsoe) [430], and splitloading of reformer tubes with more than one type of catalyst has now become very common. The benefitial effects concern pressure drop at increased plant load, carbon formation potential, catalyst activity, catalyst cost, and desired catalyst life. For example, a reformer tube may be loaded with 15 % alkali-free catalyst in pre-reduced form (top-section), 25 % unreduced alkali-promoted (middle section) and 60% alkali-free unreduced catalyst (bottom section). 

Today contractors and licensors use sophisticated computerized mathematical models which take into account the many variables involved in the physical, chemical, geometrical and mechanical properties of the system. ICI, for example, was one of the first to develop a very versatile and effective model of the primary reformer. The program REFORM [361], [430], [439] can simulate all major types of reformers (see below) top-fired, side-fired, terraced-wall, concentric round configurations, the exchanger reformers (GHR, for example), and so on. The program is based on reaction kinetics, correlations with experimental heat transfer data, pressure drop functions, advanced furnace calculation methods, and a kinetic model of carbon formation [419],... 

The rate at which S( Z)) atoms react with ethylene, ethane, and COS are all of the same order of magnitude. Thus some approximate preliminary relative rate constant values are ethyl mercaptan formation 1.0 vinyl mercaptan formation 0.80 abstraction from COS, 2.0 deactivation by CO2, 0.4 and deactivation by COS, 0.06. In addition, preliminary data seem to indicate that the reactivity of sulfur atoms, formed in the photolysis of COS, increases with increasing alkyl substitution on the doubly bonded carbon atoms. However, these rate studies have proven to be more complex than anticipated in that there is an apparent pressure effect on the rate constant values. 

For glucose, complete conversion and constant gas phase composition is already obtained at 30 s (FigureS). At feed concentrations higher than 0.6 M and the low potassium concentrations used, a considerable decrease of the gasification efficiency as well as small formation of soot and tar is observed at a residence time of 120 sec. A marked pressure effect is observed for the CH4 yield. The CH4 content in the product gas increases from about 3 vol% at 250 bar to about 8 vol% at 450 bar along with a decrease of H2 of about 60 vol% at 450 bar to about 50 vol% at 250 bar. For these experiments, the closure of the carbon balance is better than 96% and no or only traces... 

Effects of HjO were studied at 843 K, 883 K and 923 K at 3.5 bar CH. The experiments were carried out at differential conditions in the catalyst bed. Some results are given in Fig. 5. The observed rate of carbon formation is strongly dependent upon temperature, partial pressure of HjO and catalyst composition. The carbon limit was found to be close to 0.8 bar HjO for the unpromoted catalyst. Ca lowers the limit to 0.4 - 0.5 bar, whereas Mg increases the limit to above 2 bars. 

Hj at partial pressures of up to 0.9 bar (at 1.28 bar CH4) effectively increases the observed rate of carbon formation from CH4 at higher Ph2 carbon formation rate decreases, although carbon is still formed at 2.1 bar Hj. Small amounts of HjO also increases the observed carbon formation rate, although a rate maximum is observed at about 0.15 bar HjO for 3.5 bar CH4. 

The formation of carbon to carbon bonds in the lattice of the catalyst must occur to obtain elemental carbon from metal carbide. If the diffusion of carbidic carbon to nuclei where such carbon to carbon bonding has started is retarded by the penetration of the lattice by atomic hydrogen, the rate of elemental carbon formation would decrease with increasing partial pressure of hydrogen. This explanation, based on rate of diffusion of carbidic carbon in the lattice, may serve also to account for the effect of alkali and other promoters or impurities. Thus the presence of alkali tends to preserve a structure similar to that of the spinels, and it is possible that carbidic carbon diffuses more readily through such a lattice than through that obtained when little or no alkali is present. 

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