Monolith void fraction

The shape of the catalyst pellets. The shape (cylinders, rings, spheres, monoliths) influence the void fraction, the flow and diffusion phenomena and the mechanical strength. 

A monolith catalyst has a much higher void fraction (between 65 and 91 percent) than does a packed bed (which is between 36 and 45 percent). In the case of small channels, monoliths have a high geometric surface area per unit volume and may be preferred for mass-transfer-limited reactions. The higher void fraction provides the monolith catalyst with a pressure drop advantage compared to fixed beds. 

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. 

However, carbon formation and destruction of the cordicntc support were both found to have taken place over the course of vanous test conditions These findings indicated that while the heat transfer into the monolithic catalyst bed improved, (1) the gas-phase hexane cracking reaction that produced carbon precursor species (due to the high void fraction and mass transfer limitation) still existed, and (2) the combination of sustained high temperature and high steam density on cordiente warranted use of only metal monoliths for this application However, the relatively low loading of nickel in the monolith catalyst and the mass transfer limitation still resulted in equivalent conversion under conditions similar to those found in industrial practice... 

The relative methane conversions for the entire series of tests are shown in Fig. 12. From these data the heat transfer coefficients that were calculated (Table 3) show a significant improvement of the monolith bed over the packed bed. It is apparent that a combination of (1) the endothermic reaction rate and (2) heat transfer via conduction and convection are balanced over these flow rates in this monolith bed. However, based on the unfavorable shift in the extent of methane conversion from the monolith to the packed bed as flows increase, shown in Fig. 12, it appears that the mass transfer of reactants to the catalyst surface in this monolith bed may be rate-limiting at flow rates at and above 0.50 L/sec, probably a function of the high void fraction of this bed design. 

Monolithic catalytic converters continue to receive attention in the literature because of their applications in air pollution control and clean energy production. They differ from packed-bed reactors in their configuration as there are many parallel channels coated with a layer of catalyst. The flow in the channels is typically laminar. Because of its large void fraction, it is expected that the temperature transients will exhibit a significant impact on the performance of the monolith, particularly with respect to thermal stability. 

Hydraulic diameters of monolithic channels range between ca. 3x 10 m and 6x 10 m [8]. Combination of such small diameter channels leads to surfece areas per reactor volume in the order of 10 m /m (which is 10 m /m for PBRs) and void fractions up to 75% (which is -40% for PBRs). As shown in Figure 1.10 [9], these design properties allow monolith reactors to operate with pressure drops that are up to three orders of magnitude less than those observed in PBRs. 

Reactor diameter Reactor length Monolith void fraction Pitch... 

Indications have been obtained for an appropriate selection of the channel size and of the void fraction in monolithic supports in order to optimize their overall heat transfer properties. 

It is easy to use this definition for large scale fixed beds, because the porosity of the catalyst and of the catalyst bed are easy to measure and in most instances the reactor housing contributes to the overall volume only to a minor extent. However, the definition becomes doubtful when the catalyst is coated onto a monolith or foam. Here the question arises as to which volume is then being referred to the volume of the catalyst coating itself, of the monohth void fraction (channels) or of the entire monolith All these questions need to be clarified before a fair comparison of catalytic activity is feasible when gas hourly space velocity is applied for the calculations. 

Figure 6.3 Normalised axial and radial heat conductivity Ice.ax and ke r of a cylindrical monolith versus monolith void fraction [457. ...

Metal gauzes, foam monoliths, sponges and traditional honeycomb monoliths have been successfully applied as catalysts or catalyst supports. The main advantages offered by the structured catalysts with high void fractions are represented by the reduced pressure drop and, in the case of foams, by the even distribution of the reactant flow across the fixed bed. 

---