Improved Efficiency with 5 Catalyst Beds

Improved efficiency with five catalyst beds... 

This term embraces a wide class of potentially efficient techniques combining chemical reaction and separation in a catalytic reactor. If the reaction products are able to be adsorbed on the catalyst to different extents and for different lengths of time, these products can be separated from each other. This feature of catalytic processes can be used for enhancement of the reaction rate or selectivity, or for improvement of the quality of a desirable product. The process can be arranged in various ways, e.g. as a system with a fixed catalyst bed operated with periodic changes of the inlet composition or as various types of reactors with moving beds. To improve the separation, a mixture of catalyst and adsorbent can be loaded in the fixed bed reactor, or adsorbent can be fed into the reactor. 

Double-Bed Catalysts. Because the temperature of the colder section in the nonisothermal catalyst bed could not be readily controlled, an apparatus was constructed that contained two separate furnaces, each containing 20 g of Surinam red mud. The temperature of the first bed was varied to determine the optimum operating conditions with an inlet gas of 0.57% sulfur dioxide, 0.89% carbon monoxide, and 3% water vapor in helium. The exhaust gas analyses from the first furnace are shown in Figure 6. These results indicate that the hydrogen sulfide and sulfur dioxide removal efficiency increases with temperature up to about 400 °C. Beyond this temperature there is little improvement. 

When a metal-catalyzed reaction is so fast that external mass transfer controls, several layers of fine wire screen can be used as the catalyst bed. The catalytic oxidation of ammonia to nitric oxide, which is the first step in nitric acid production, is carried out with screens (called gauzes) of Pt/Rh alloy, and very high ammonia conversions are obtained. Similar gauzes are used in the Andrussov process for manufacture of HCN from CH4, NH3, and O2. Wire screens are also used for catalytic incineration of pollutants and in improving combustion efficiency in gas burners. 

The efficiency of diffusion between the main gas stream and the surface of the catalyst will be favored by a high linear velocity, because high velocity results in greater turbulence and a thinner laminar film around the particles than would be present at low linear velocity. For a given space velocity, the linear velocity varies directly with the length of the catalyst bed. For the two samples shown here, the 1/4 inch tube with 3 cc of catalyst results in a linear velocity about twice that of the 3/8 inch tube with 5 cc of catalyst. It seems possible that the improved performance of the small converter could be due to a difference in diffusion rates, as well as, or in addition to the difference in heat transfer efficiency. 

Speed-up of mixing is known not only for mixing of miscible liquids, but also for multi-phase systems the mass-transfer efficiency can be improved. As an example, for a gas/liquid micro reactor, a mini packed-bed, values of the mass-transfer coefficient K a were determined to be 5-15 s [2]. This is two orders of magnitude larger than for typical conventional reactors having K a of 0.01-0.08 s . Using the same reactor filled with 50 pm catalyst particles for gas/Hquid/solid reactions, a 100-fold increase in the surface-to-volume ratio compared with the dimensions of laboratory trickle-bed catalyst particles (4-8 mm) is foimd. 

A fixed-bed reactor often suffers from a substantially small effectiveness factor (e.g., 10 to 10 for a fixed-bed steam reformer according to Soliman et al. [1988]) due to severe diffusional limitations unless very small particles are used. The associated high pressure drop with the use of small particles can be prohibitive. A feasible alternative is to employ a fluidized bed of catalyst powders. The effectiveness factor in the fluidized bed configuration approaches unity. The fluidization system also provides a thermally stable operation without localized hot spots. The large solid (catalyst) surface area for gas contact promotes effective catalytic reactions. For certain reactions such as ethylbenzene dehydrogenation, however, a fluidized bed operation may not be superior to a fixed bed operation. To further improve the efficiency and compactness of a fluidized-bed reactor, a permselective membrane has been introduced by Adris et al. [1991] for steam reforming of methane and Abdalla and Elnashaie [1995] for catalytic dehydrogenation of ethylbenzene to styrene. 

Conversion efficiency is definitely affected by the large void fraction, which is apparent in the results from changes in the total throughput, or space velocity (0.56 versus 1.11 sec ), shown in Fig. 7. In this comparison, the concentration of unconverted hexane increased tenfold when the flow rate was doubled. The impact of improvements in conductive heat transfer, combined with the mass transfer limitations associated with the cell size and honeycomb design, and a catalyst loading that was nearly one-half Chat of commercial pellet catalysts (average, 11.5% versus 19.2%) suggested that both carbon formation and steam/hydrocarbon reactions were better controlled with monolithic supports under the conditions employed. This comparison was made where the extent of the endothermic reaction is equal between the pellet bed and the hybrid cordierite/metal monolith bed. 

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