Stacking of monoliths

Fig. 8.11. Stacking of monoliths for extension of flooding limits, residence time distribution (RTD) and mass transfer by remixing the laminar layers. Short pieces with a higher hydraulic diameter, turned through 45°, are stacked between longer monoliths with a small hydraulic diameter. (From Ref. [23].)...

However, the mass transfer coefficients found are clearly lower than those reported for distillation packings [26, 27]. This can be explained by the flow patterns in distillation packings, where the films constantly are disturbed and remixed, and therefore a completely developed laminar profile is never present. The mass transport is dominated by convection, not diffusion. It would be expected that remixing of the film layers, as accomplished by the stacking of monoliths (see Section 8.23) improves not only the RTD but also the mass transfer performance of monoliths. 

Cocurrent downflow with slug or Taylor flow has been most widely used. Other possible designs, e.g., cocurrent upflow and froth flow, have to our knowledge been tested only in laboratory and pilot plant reactors. Consequently, we will focus on downward slug flow, and the main areas of interest are scale-up, liquid distribution, space velocity, stacking of monoliths, gas-liquid separation, recirculation, and temperature control. 

Several uncertainties in this periodic process have not been resolved. Pressure drop is too high at SV = 10,000 h 1 when packed beds of carbon are used. Study of carbon-coated structured packing or of monoliths with activated carbon washcoats is needed to see if lower pressure drops at 95% SO2 removal can be achieved. Stack gas from coal or heavy oil combustion contains parts-per-million or -per-billion quantities of toxic elements and compounds. Their removal in the periodically operated trickle bed must be examined, as well as the effect of these elements on acid quality. So far, laboratory experiments have been done to just 80°C use of acid for flushing the carbon bed should permit operation at temperatures up to 150°C. Performance of periodic flow interruption at such temperatures needs to be determined. The heat exchange requirements for the RTI-Waterloo process shown in Fig. 26 depend on the temperature of S02 scrubbing. If operation at 150°C is possible, gas leaving the trickle bed can be passed directly to the deNO, step without reheating. 

The concept of monolithic module design is associated with Argonne National Laboratories [111, 112]. Power density of about 8 kW/kg or 4kW/1 and fuel efficiency over 50% are expected to be achieved with monolithic SOFCs. The monolithic structure started with a co-flow version where the cell consists of a honeycomb-like array of adjacent fuel and oxidant channels that look like corrugated paperboard, as shown in Fig. 42. Multilayer laminates of the active cell components (anode-elec-ttolyte-cathode) are appropriately corrugated and stacked alternatively between flat multilayer laminates in the following sequence anode- interconnection material-cathode. Tape casting [111] and hot-roll calendering [113, 114] are used to fabricate the monolithic structure. A cross-flow version where oxidant and fuel channels are... 

As a prototype for such a reactor, the monolithic heat exchanger in Figure 4.112, was manufactured from a stack of micro structured foils and laser welded at the front and side faces [169], The flanges were welded manually. 

Figure 4.112 Monolithic counter-current heat exchanger manufactured from a stack of micro structured plates and sealed by laser welding (source IMM).

In order to improve the flooding behavior at the point where two pieces of monolith are stacked on each other, it was suggested [17] that short pieces of monoliths... 

The above requirements are to some extent contradictory, which has led to the proposition of a large number of different catalyst shapes and arrangements. However, only a few of these have proved really effective in practical operation. Suitable catalyst forms and arrangements include random packings of spheres, solid cylinders, and hollow cylinders, as well as uniformly structured catalyst packings in the form of monoliths with parallel channels, parallel stacked plates, and crossed, corrugated-plate packets (Fig. 3). 

The most crucial step in the design of monolith reactors is the proper distribution of fluids over the reactor cross section. However, the available information on the gas-liquid distribution over monoliths is limited, and additional research and development are needed in this field. It should be noted that distributing the fluid al the top of the reactor is not the end of the problem, and redistributions may be needed. This is due to the fact that the monoliths can only be manufactured in short pieces, and the desired length of the catalyst in the reactor is usually obtained by stacking monolith pieces on top of each... 

In this equation, it is assumed that both the gas and the liquid move in slugs at the same velocity through the orifice. The contribution of the orifice effects to the total pressure drop is small and is of significance only in monolith blocks with high cell density and at high liquid flow rates. The frictional and static pressure drops prevail. The orifice effects may arise if blocks of monoliths stacked on top of each other are used. The contribution of these effects will depend on the extent of obstruction and the length of individual blocks. 

This phenomenon is the same for ail types of geometric shapes of catalysts, monoliths, pellets or nets, In a monolith the mass transfer, between gas bulk and the outer surface of catalyst, is not particular good since the flow, at least in the boundary layers, tends to become laminar, but the pressure drop is low. In a packed bed with pellets the mass transfer is very good in general, but the pressure drop is high. A stack of nets, however, combine good mass transfer and low pressure drop, it is in between a monolith and a packed bed. 

High efficiency solar cells are produced from crystalline binary or multinary compounds of elements from Groups III and V, almost entirely for space applications. While a wide range of materials and methods have been employed, the monolithic tandem stack of a Ga cIni (P cell, where x 0.516, above GaAs and Ge cells is a commercially produced, indicative example.Monolithic tandem structures present several challenges. The materials used in the various junctions should have similar thermal expansion coefficients and... 

Fixed-Bed Reactors - Monoliths 1189 Figure 9.10 Stack of SCR honeycombs. 

Sedriks et al. [20] and Bakulin et al. [19] found that Ecggn of B/Al MMCs was active to that of their monolithic matrix aUojrs in aerated NaCl solutions. That behavior does not appear to comply with the mixed-potential theory. Bakulin et al. [79], however, found that hot-pressed stacks of aluminum foil processed in the same way as the MMC (but without the BFs) have Ecorr values that are active to the MMCs. The only difference between the monolithic aluminum and the hot-pressed stacks of aluminum foil was crevices in the diffusion bonds between adjacent foils. The crevices, which are sources of additional anodic sites, can polarize the stacks to active potentials. Thus, the B/Al MMCs are actually noble to the matrix material processed in the same way, emd the Ecorr values actually coincide with the mixed-potential theory. Sedriks et al. [20] found that increasing the volume fi ac-tion of BF caused anodic current densities (w.r.t. matrix area) to increase. This implies that BF-matrix interfaces, which increase with BF content, were also sources of anodic sites. Evans and Braddick [89] also reported that BF-matrix interface regions were severely attacked in an oxygenated NaCl solution. These reports indicate that the BF-matrix and foil-foil interfaces are major causes of corrosion. 

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