Oxygen unconverted

Carbon Conversion. Carbon conversion on a once-through basis is a function of the coal composition and is strongly influenced by the oxygen/coal ratio. For some coals, the conversion pattern is also affected by the level of steam in the blast. Another factor is fly slag recycle, which raises the carbon conversion by recycling the unconverted carbon, most of which resides on the fly slag. This results in an overall carbon conversion greater than 99%. 

Concerning the reaction pathway, two routes have been proposed the sequence of total oxidation of methane, followed by reforming of the unconverted methane with CO2 and H2O (designated as indirect scheme), and the direct partial oxidation of methane to synthesis gas without the experience of CO2 and H2O as reaction intermediates. The results obtained by Schmidt and his co-workers [4, 5] indicate that the direct reaction scheme may be followed in a monolith reactor when an extremely short contact time is employed at temperatures in the neighborhood of 1000°C. However, the majority of previous studies over numerous types of catalysts show that the partial oxidation of methane follows the indirect reaction scheme, which is supported by the observation that a sharp temperature spike occurs near the entrance of the catalyst bed, and that essentially zero CO and H2 selectivity is obtained at low methane conversions (<25%) where oxygen is not fully consumed [2, 3]. A major problem encountered... 

When air is used as the oxidant, the gas cannot be recycled since inert nitrogen would accumulate in the system. The one-stage process requires recycling of the gas, because it contains unconverted ethene and thus oxygen is used. 

Possible solutions to overcome this problem are (1) decrease the residence time the decrease of conversion is more than compensated by an increase of selectivity (due to the lower extent of methacrylic acid combustion), and in overall the productivity increases (2) increase the total pressure, while simultaneously increasing both the oxygen and the isobutane partial pressure, as well as the total gas flow (so as to keep a constant contact time in the reactor). A higher pressure also implies smaller reactor volume, and hence lower investment costs. Under these circumstances, productivity as high as 6.4 mmol/h/gcat was reached, which is acceptable for industrial production. The additional heat required for the recirculation of unconverted isobutane and for increased pressure would be equalized by the higher heat generated by the reaction. 

The first phase described occurs very close to the burner tip, and it has not been possible either to sample the gas accurately or measure the temperature at this point. However, indirect measurements, with well designed burners, have indicated a flame burst temperature in the range of 3200° to 3600° F. with roughly 40% of the methane unconverted when oxygen consumption is complete. With poorly designed burners, the overburning in the first phase can, of course, become intolerable the worst conceivable case is the complete combustion of approximately one fourth of the methane. 

Figure 9.5 illustrates a simplified scheme of the Mitsubishi process. The process includes the recycle of unconverted propane, and the BOC-PSA technology for rejection of N2 the latter is present in the feed, which contains oxygen-enriched air, and is also generated in the reactor by ammonia combustion. The unconverted hydrocarbon is recovered and recycled to the reactor [23fig], One Mitsubishi patent claims the differentiation of ammonia along the catalytic bed [23d], This might... 

Oxygenated compounds and unconverted ethylbenzene, collected at the bottom of the second depropanizer, are first distilled under vacuum to recover the propylene oxide and lighter components at the top. This effluent is nd in succession of the acetaldehyde and propionaldehyde it contains by simple distillation, and then of methyl formate by extractive dbtiUation with ethylbenzene. The latter b then purified and recycled. The operation b terminated by a final column whidi produces propylene oxide to commercial specifications. 

The chemical character of the unconverted asphaltenes is also a function of processing. Both the H/C and O/C atomic ratios declined in a regular manner as conversion progressed. In some cases, the oxygen content was reduced to that typically found for pentane-soluble oils (about 2%). At the same time, the atomic H/C ratio of the residual asphaltenes was reduced to values considerably lower (<0.7) than that of the coal. The relative oxygen content may be used as a crude indicator of the severity of hydroprocessing experienced by a particular asphaltene. 

The ammoxidation of benzene is described in some papers and patents [124, 128, 131, 132]. Whilst benzene has been reported to remain unconverted in the presence of ammonia and oxygen with a V2O5 catalyst [124], it was efficiently transformed to nitriles with catalysts based on mixed molybdates [128]. 

---