Partial oxidation Soot removal

The partial combustion (partial oxidation) of natural gas (Fig. 1) is probably the most widely used method of producing acetylene. The overall reaction of the methane (combustion and splitting) is 90 to 95 percent whereas the oxygen is 100 percent converted. The residence time is 0.001 to 0.01 seconds. The acetylene and gases are cooled rapidly by quench oil or water sprays to 38°C and have the following typical composition (percent by volume acetylene, 8 to 10 hydrogen, 50 to 60 methane, 5 carbon monoxide, 20 to 25 and carbon dioxide, <5. The soot is removed in a carbon filter and the clean gases are compressed to 165 psi (1.14 MPa). 

Partial oxidation (POX) of natural gas with oxygen is carried out in a high-pressure, refractory-lined reactor. The ratio of oxygen to carbon is carefully controlled to maximize the yield of CO and H2 while maintaining an acceptable level of C02 and residual methane and minimizing the formation of soot. Downstream equipment is provided to remove the large amount of heat generated by the oxidation reaction,... 

The raw gas from the partial oxidation contains soot, about 0.8 wt% of the hydrocarbon feed. Soot particles together with ash are removed mainly in the venturi scrubber downstream of the quench. The soot slurry from quench and venturi is sent to the metals ash recovery system (MARS) Figure 58. First the soot slurry is flashed to atmospheric pressure and then filtered, leaving a filter cake with about 80 % residual moisture. The filter cake is subjected to a controlled combustion in a multiple hearth furnace. Under the conditions applied, a metal oxide concentrate containing 75 wt% of vanadium, together with some nickel and iron, is obtained which can be sold to metal reclaimers. The MARS ist practically autothermal as the heat of combustion is sufficient to evaporate the moisture of the filter cake. 

Partial oxidation units may operate with feedstock, ranging from natural gas to heavy oil fractions such as asphalt. Heavy feedstock may contain large amounts of both sulfur and heavy metals. In many cases these compounds are removed from the raw syngas downstream the POX reactors. Some residual carbon or soot may be formed in the combustion chamber of the reactor.To avoid the carbon lay-down in the heat exchanger downstream the POX reactor, special coils and high gas velocity are used. The carbon may be removed downstream the heat exchanger in a suitable water wash or scrubbing system. ... 

If the partial oxidation unit is integrated in a coal gasification complex - as suggested in Chap. 7 - and the liquid products of coal gasification are used as a feedstock, the ash content of this feedstock will be extremely low and gasification can be performed in such a way that even less than 0.5 wt % of soot occur in the reformed gas. In this case, the expenditure for a carbon recycling system can be saved as the small carbon quantities can be more easily and with less expenditure be removed in the waste water treatment system of the complex. 

Foams were proved to be highly suitable as catalytic carrier when low pressure drop is mandatory. In comparison to monoliths, they allow radial mixing of the fluid combined with enhanced heat transfer properties because of the solid continuous phase of the foam structure. Catalytic foams are successfully used for partial oxidation of hydrocarbons, catalytic combustion, and removal of soot from diesel engines [14]. The integration of foam catalysts in combination with microstructured devices was reported by Yu et al. [15]. The authors used metal foams as catalyst support for a microstructured methanol reformer and studied the influence of the foam material on the catalytic selectivity and activity. Moritz et al. [16] constructed a ceramic MSR with an inserted SiC-foam. The electric conductive material can be used as internal heating elements and allows a very rapid heating up to temperatures of 800-1000°C. In addition, heat conductivity of metal or SiC foams avoids axial and radial temperature profiles facilitating isothermal reactor operation. 

The non-catalytic partial oxidation [486] (POX, Texaco, Shell) needs high temperature to ensure complete conversion of methane and to reduce soot formation. Some soot is normally formed and is removed in a separate soot scrubber system downstream of the partial oxidation reactor. The thermal processes typically result in a product gas with H2/C0=1.7—1.8. Gasification of heavy oil fractions, petcoke, coal and biomass may play an increasing role as these fiaetions are becoming more available and natural gas (NG) less available. 

Thermal partial oxidation (TPOX) requires high operating temperatures, between 1250-1500 °C and pressures of between 3-12 MPa, which means that a catalyst is not required. Due to the exothermic nature of the process additional heat is not required, as the heat produced is sufficient to maintain the operating conditions within the reactor. However, unlike SMR the process produces soot, which means that an additional cleaning process is necessary to remove solid particulates from the gas (Gupta, 2008 Holladay et al., 2009). 

Teraoka et al. [15] also reported that Co, Mn, and Fe perovskite-type and Cu-based I<2NiF4-type oxides (all with La partially substituted by alkaline metal or alkaline earth metal cations) catalyze the simultaneous removal of NO and diesel soot particulates and that these perovskite-related oxides are superior to transition metal simple oxides and Pt/Al203 with regard to their selectivity toward NO reduction. 

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