Carbon monoliths reaction rates

FIGURE 32 Initial rates of reaction in catalysis by free lipase and immobilized lipase (Novozyme, and catalyst supported on 200 cpsi carbon monolith) in the acylation of butanol with vinyl acetate in an organic medium at 300 K. 

Using the reaction rate observed (in moFs m ataiyst) in experiments performed at 150 rpm, the Wheeler-Weisz modulus was calculated for aU monoUths by estimating the layer thickness (L) from the carbon yield. inside the carbon layers was estimated to be 1 to 3 x 10 ° m /s inside the varions carbons. The valnes for are presented in Table 11.5. For 1-Al, consisting mainly of macroporons carbon, it is likely that internal diffusion limitations are present inside the carbon walls (where the lipase is located). This was also indicated by the decreased activity per gram of enzyme for these monoliths (Table 11.5). For ceramic monoliths, all adsorbed enzyme is used effectively (a constant tnmover freqnency for all carbon types), as confirmed by the low valnes for... 

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. 

Summary data for different conversion/temperature conditions are provided in Tables 5-7. Rate constants were calculated from these data, and it was determined that although the operations at 140 kPa were influenced by mass transfer, this was not the rate-limiting step however, the reaction was mass transfer limited at 1000 kPa. The higher carbon monoxide conversion values and methane production observed for the monolith-supported nickel compared to pellets were explained to be due to the provision by the monoliths of smaller pore diffusion resistance and higher mass transfer rates at higher temperatures, primarily a result of shorter diffusion paths in thin alumina coatings on the monolith walls. 

Carbon coating can be achieved using pyrolysis of hydrocarbons at elevated temperatures [69]. Figure 2 shows a device used for carbon coating via hydrocarbon pyrolysis. In the example described here, an alumina-washcoated monolith is covered with carbon by pyrolysis of cyclohexene. A gas mixture of cyclohexene in nitrogen is passing the reactor at a certain flow rate. The monolith block to be coated is placed in the middle of the heated tubular reactor. The reaction takes place at 873-973 K, and the amount of carbon deposited can be controlled by the temperature and the time on stream. Up to 3-10 wt% carbon can be homogeneously coated onto the monolith in this way. It appears that the surface area of the carbon-coated alumina-washcoated cordierite monolith is of... 

The catalytic oxidation of hydrocarbons on monolithic catalysts for a short time proceeds efficiently in small-sized simple-design reactors, without formation of solid carbon. Characteristic features of these processes are large time ( 10 K/s) and space ( 10 K/cm) gradients of temperature. These autothennal processes occur in nearly adiabatic conditions, since exothermic reactions rapidly heat the catalyst and gas to about 1000 °C, whereas the heat release rate is too high to allow the gas mixture to be effectively cooled through the walls. 

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