Catalytic cooling between beds

This confirms the efficacy of multi-bed catalytic oxidation with gas cooling between beds. 

Fig. 13.1. Schematic of 1st and 2nd catalyst beds with gas cooling between. The cooling system cools lsl catalyst bed exit gas in preparation for more catalytic S02 oxidation in a 2nd catalyst bed. Industrial catalyst bed arrangements are discussed in Chapters 7 and 8. Gas cooling is discussed in Chapter 21.

Rgure 11.2 presents the formation of NO during normal combustion of sulfur. There are some special methods of sulfur combuslion that minimize the formation of nitric oxides. The catalytic conversion of SO2 to SO3 usually is carried out in three or more steps (catalyst beds) the gas is cooled between steps to keep the temperature within the desired range of 420 -450. The conversion steps are carried out by contact of the gas with successive beds of vanadium oxide catalyst, which are often arranged in sections of a single tower. 

The single absorption contact process for sulfuric acid is characterized by four main process steps gas drying, catalytic conversion of S02 to S03, absorption of S03, and acid cooling. The maximum S02 conversion for a single absorption plant is about 97.5-98 percent. By adding a second S03 absorber with one or two catalyst beds between absorbers, the S02 conversion can be increased to 99.5-99.8 percent or even as high as 99.9 percent with a cesium-promoted catalyst, resulting in lower S02 emis-... 

The Lurgi LPM process involves the same basic steps as the ICI processes. The two processes differ mainly in their reactor designs and the way in which the produced heat is removed as shown in Figure 12.18. The ICI design consists of a number of adiabatic catalytic beds, and cold gas is used to cool the reactant gases between the beds. The highest temperature is reached in the first catalyst bed. The Lurgi... 

The multi-tube reactor is more common than the other two fixed bed designs because many of the important heterogeneous catalytic processes require effective heat transfer between the mobile fluid, catalyst bed and heat-ing/cooling media. 

Catalytic activity measurements were carried out in a fixed bed quartz reactor of inner diameter 6 mm. The catalyst (usually 200 mg, 125-212 pm fraction) was placed between two glass wool plugs with a bed length of 10-20 mm. For the oxidation of CO and H2, a standard gas containing 1 vol% CO or H2 in air was dried in a silica gel column cooled down to 0°C or -77° C and passed through the catalyst bed at a space velocity of 20,000 h-iml/g-cat.. For the partial... 

Radial dispersion of mass and heat in fixed bed gas-solid catalytic reactors is usually expressed by radial Peclet number for mass and heat transport. In many cases radial dispersion is negligible if the reactor is adiabatic because there is then no driving force for long range gradients to exist in the radial direction. For non-adiabatic reactors, the heat transfer coeflScient at the wall between the reaction mixture and the cooling medium needs also to be specified. 

The gas is cooled in a boiler, w ch generates steam, to about 420°C, the desired inlet temperature to the catalytic cmvcrter Some of the excess air may be bypassed around the burner and boiler to control the combustion chamber temperature and to decrease the required size of the boiler. Alternatively, excess air may be added between catalyst beds to control the temperature during conversion. 

Heat transfer between packed beds and the external column wall has been widely studied because of its relevance to the design and operation of wall-cooled catalytic reactors. In the one-dimensional model, which is the basis of Eq. (7.17), the overall heat transfer resistance may be represented as the sum of the internal, external, and wall resistances ... 

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