Carbon limit temperature

Fig. 7. Carbon Limit Temperatures and Nickei Crystal Size (ref. 17)... 

The decomposition of carbon monoxide (Reactions R7 and R8, Table 5.2) may take place without catalyst on the surfaces of the equipment (e.g. heat exchangers). This may also lead to metal dusting corrosion [121] [211] [535]. Reaction R7 in Table 5.2 appears to be involved [211] [293]. It means that for a given gas composition (and pressure) there will be potential for metal dusting below the carbon limit temperature (see Example 5.1). At very low temperature, the rate will be too small. 

When cooling this gas, the carbon limit temperature is calculated from the equilibrium quotient for Reaction R8 in Table 5.2 ... 

Figure 5.17 Principle of equilibrated gas. Carbon limit temperatures and Ni-crystal size [382]. Conditions H20/CFLt=1.8, C02/CH =2.2, P=18 bar abs.

Carbon limits depend on deviation from graphite thermod5mamics, meaning that a eatalyst with small nickel crystals can operate at more critical conditions. This is illustrated in Figure 5.16 for conditions in a prereformer [415] [451]. The upper carbon limit temperature depends on the steam-to-carbon ratio and the nickel crystal size of the catalyst. 

Figure 5.18 Radial gradients and carbon limits [389]. Steam reforming of methane (H20/CH4=3.5, F=33 bar) [389] 3 meters from tube inlet. Tu is the carbon limit temperature for methane decomposition. There is potential for carbon formation when the catalyst temperature Tcat > m. Reproduced with the permission of Springer.

The overall benefits of this high efficiency combustor over a conventional bubbling- or turbulent-bed regenerator are enhanced and controlled carbon-bum kinetics (carbon on regenerated catalyst at less than 0.05 wt %) ease of start-up and routiae operabiUty uniform radial carbon and temperature profiles limited afterbum ia the upper regenerator section and uniform cyclone temperatures and reduced catalyst iaventory and air-blower horsepower. By 1990, this design was well estabUshed. More than 30 units are ia commercial operation. 

Figures 6.30 and 6.31 present the same information for saturated hydrocarbons. In Figure 6.30, the saturated liquid state is on the lower part of the curve and in Figure 6.31 it is on the upper part of the curve. Below T y, the line width changes, indicating that the liquid probably does not flash below that level. Note that a line has been drawn only to show the relationship between the points a curve reflecting an actual event would be smooth. Note that a liquid has much more energy per unit of volume than a vapor, especially carbon dioxide. Note It is likely that carbon dioxide can flash explosively at a temperature below the superheat limit temperature. This may result from the fact that carbon dioxide crystallizes at ambient pressure and thus provides the required number of nucleation sites to permit explosive vaporization.

Various other classes of catalysts have been investigated for NH3-SCR, in particular, metal-containing clays and layered materials [43 15] supported on active carbon [46] and micro- and meso-porous materials [31b,47,48], the latter also especially investigated for HC-SCR [25,3lb,48-53], However, while for NH3-SCR, either for stationary or mobile applications, the performances under practical conditions of alternative catalysts to V-W-oxides supported on titania do not justify their commercial use if not for special cases, the identification of a suitable catalyst, or combination of catalysts, for HC-SCR is still a matter of question. In general terms, supported noble metals are preferable for their low-temperature activity, centred typically 200°C. As commented before, low-temperature activity is a critical issue. However, supported noble metals have a quite limited temperature window of operation. 

Relaxation data for the methyl carbon could be measured only down to ca. -125°C below this temperature line broadening was too severe to obtain results. The near equal values observed for the Ti and Tip over much of the limited temperature interval is in accord with the methyl motion being on the high temperature side of the Ti-minimum. 

The inherent problem of limited thermal and chemical stability of metal anchoring sites on carbon limits not only the activation temperature but also the application conditions [185]. In oxidizing environments carbon supports are much less stable than the macroscopic bum-off temperature (sec Figs 28 and 17). In order to... 

Prior to the main study the experimental conditions for performing kinetic measurements were established. CH4 conversions of below 10% (reactants diluted with H O) could be obtained at temperatures below 923 K. The carbon limits, which correspond to the maximum partial pressure and minimum temperature under which formation of carbon could be observed as a weight increase by the microbalance, were also determined. In addition, test runs at the most severe conditions, i.e. 1073 K and 4.3 bar CH4, revealed that the support as well as the quartz and the alumina surfaces are sufficiently inert, and that gas phase activation of CH4 is not important. 

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. 

Example 1-1 Ethylene oxide is produced by direct oxidation with air using a bed of catalyst particles (silver on a suitable carrier). Suppose that the stream enters the flow reactor at 200°C and contains 5 mole % ethylene and 95% air. If the exit temperature does not exceed 260°C, it is possible to convert 50% of the ethylene to the oxide, although 40% is also completely burned to carbon dioxide. How much heat must be removed from the reaction, per mole of ethylene fed, in order not to exceed the limiting temperature The average molal heat capacity of. ethylene may be taken as 18 Btu/(lb mole) (°R) between 25 and 200°C and as 19 Btu/(lb mole)(°R) between 25 and 260°C. Similar values for ethylene oxide are 20 and 21 Btu/(lb mole)(°R). The pressure is essentially atmospheric. 

Flexibility - Can operate at varying temperatures, pressures and steam to carbon ratios because catalyst is not carbon limited. 

T Tc for each species however, (<5 — 82) is often nearly temperature independent, at least over limited temperature ranges. Therefore, the solubility parameters listed in Table 9.6-1 may be used at temperatures other than the one at which they were obtained (see Eq. 9.3-22, however). Also, referring to Eq. 9.6-10, it is evident that liquids with very different solubility parameters, such as neopentane and carbon disulfide, can be expected to exhibit highly nonideal solution behavior (i.e., y > 1), whereas adjacent liquids in Table 9.6-1 will-form nearly ideal solutions. This is useful information. 

In one studyOPPs were extracted with SPME (85 /rm of polyacrylate coating) by the immersion technique at 75°C for 60 min. Desorption was done in a desorption device by supercritical fluid carbon dioxide (temperature 50°C pressure 306 atm) prior to online introduction into LC. The detection limits were 300 /rg/l for diazinon, 40 /rg/l for EPN, and 60 /rg/1 for chlorpyrifos, with recoveries ranging from 62 to 64%. 

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