Limiting radial temperature

Radial temperature gradients are more critical than axial gradients. In order to limit the effects of heat transport limitations on the observed rates to 5% it is sufficient that for a single reaction with the following kinetics ... 

A radial temperature gradient with a maximum at the wall is observed at the reactor entrance. Further away from the reactor entrance, the radial profile is flat. The mole fraction profiles also contain marked radial gradients within the first 1.5 meters of the reactor. The radial gradients observed in the species concentration profiles are caused by the limited heat flux added to the reactor through the wall. The reactions are endothermic and the heat transferred through the wall and/or from the wall into the bed is not sufficient to smooth out the temperature profile, thus the chemical conversion becomes non-uniform. 

Limit the height of the bed to keep temperature increase <50°C to minimize effects of radial temperature gradients. Bed can be shallow and wide. Quench can include injection of cold reactants, internal or external heat exchangers. 

For adiabatic operation with exothermic reactions, limit the height of the bed to keep temperature increase < 50 °C. Tube diameter < 50 mm to minimize extremes in radial temperature gradient. For fast reactions, catalyst pore diffusion mass transfer may control if the catalyst diameter >1.5 mm. 

In case of failure of the main heat removal system decay heat removal is achievable by conduction and radiation via radial reflector and pressure vessel to surface coolers installed around the pressure vessel. These surface coolers limits the temperatures of the pressure vessel and the concrete structure. 

There is no upper limit on the scale of fluidized-bed reactors that can be constructed and operated. Hence reactors can be many meters in diameter, generally much larger than for competing types of reactor, where factors like radial temperature variations tend to limit the reactor diameter. 

Lenz et al. [73] described the development of a 3 kW monolithic steam-supported partial oxidation reactor for jet fuel, which was developed to supply a solid oxide fuel cell (SOFC). The prototype reactor was composed of a ceramic honeycomb monolith (400 cpsi) operated between 950 C at the reactor inlet and 700°C at the reactor outlet [74]. The radial temperature gradient amoimted to 50 K which was attributed to inhomogeneous mixing at the reactor inlet. The feed composition corresponded to S/C ratio of 1.75 and O/C ratio of 1.0 at 50 000 h GHSV. Under these conditions, about 12 vol.% of each carbon monoxide and carbon dioxide were detected in the reformate, while methane was below the detection limit. Later, Lenz et al. [74] described a combination of three monolithic reactors coated with platinum/rhodium catalyst switched in series for jet fuel autothermal reforming. An optimum S/C ratio of 1.5 and an optimum O/C ratio of 0.83 were determined. Under these conditions 78.5% efficiency at 50 000 h GHSV was achieved. The conversion did not exceed 92.5%. In the product of these... 

Tubular reactors with both axial and radial temperature gradients In many exothermic processes the reactor temperature has to be controlled within much narrower limits. This is particularly true for many catalytic gas phase reactions. The reasons are usu y that undesired side reactions have to be avoided, or that the catalyst has to be protected against sintering. There are two reactor types for solid/gas-reactions that make good temperature control possible ... 

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