Catalyst placement

Many of the earlier studies on inorganic membrane reactors use a bed of catalyst particles contained inside the membrane element Transport of the reaction components into and out of the catalyst particles mostly rely on diffusion. In addition, transport of the reaction product(s) out of the catalyst bed may not be as efficient as the other modes of catalyst placement relative to the membrane. 

The choice of the above three modes of catalyst placement relative to the membrane can significantly affect the reactor performance. From the analysis of catalytically active and passive (inert) membrane reactors [Sun and Khang, 1988], it appears that the critical parameter determining the choice is the reaction residence time. At low residence times, the difference between a catalytically active and a catalytically passive membrane is not significant. However, as the reaction residence time becomes high, the catalytically active membrane shows a higher reaction conversion. 

DOCs also convert sulfur dioxide to sulfur trioxide, which forms sulfuric acid droplets or solid sulfate particles. These add to the amount of particulates emitted and can put an engine out of compliance. One approach to this problem is to lower fuel sulfur from the 1994 level to 0.01 wt% or less. A second approach is to develop a catalyst that oxidizes HC and CO but not sulfur dioxide. Newly developed catalysts that make almost no sulfate at temperatures as high as 400 C have brought this approach a step closer to reality. A third approach concerns catalyst placement. Sulfur dioxide oxidizes above 350-400°C over a EXX, while hydrocarbons do so below this temperature. A DOC could be located to have an inlet temperature favoring HC conversion. 

A catalyst placement inside the MS module working at 600°C could be considered. At this temperature, an equilibrium conversion for the H2S decomposition reaction is about 3%. If equilibrium is reached inside the MS containing the catalyst, then an additional shift towards the H2S decomposition products will be expected. 

Catalyst placement in extractor, distributor and two contactor-type membrane reactors (A, B reactants P product) (Miachon and Dalmon, 2004). 

The placement of the NOx bed ahead of the oxidation bed causes a delay of the warm-up of the oxidation bed from a cold start. Since many of the materials considered for the reduction of NO are also excellent oxidation catalysts, the NOx bed is often used as the oxidation bed by the injection of secondary air during the first two minutes from a cold start. After the oxidation bed is warmed up, the secondary air is diverted from upstream of the first bed to upstream of the second bed. This procedure helps the emission reduction when the catalysts are fresh, but hastens the aging of the NOx catalyst as it is being exposed repeatedly to oxidation and reduction conditions. 

The placement of catalysts/carriers in micro channels can be done by various means. In a conventionally oriented variant, catalyst powders or small grains are inserted as mini fixed beds [7]. However, more specific catalyst arrangements are also known, originally designed for novel ways of processing at the macro scale, such as catalyst filaments [8], wires [9] and membranes (Figure 3.2) [10, 11]. 

In atactic polymers, side groups are irregularly positioned on either side of the chain, as illustrated schematically in Fig. 1.8 c). A truly atactic polymer would comprise a random distribution of steric centers. In practice, atactic polymers typically show some preference for either meso or racemic placement The tendency towards stereoregularity is due to the fact that polymerization catalysts often contain steric centers, which tend to direct the incoming monomers and the growing chain into preferred configurations. 

Butadiene-Styrene Copolymers from Ba-Mg-Al Catalyst Systems. Figure 13 shows the relationship between copolymer composition and extent of conversion for copolymers of butadiene and styrene (25 wt.7. styrene) prepared in cyclohexane with Ba-Mg-Al and with n-BuLi alone. Copolymerization of butadiene and styrene with barium salts and Mg alkyl-Al alkyl exhibited a larger initial incorporation of styrene than the n-BuLi catalyzed copolymerization. A major portion of styrene placements in these experimental SBR s are more random however, a certain fraction of the styrene sequences are present in small block runs. 

The P-alkoxy elimination pathway is important during the incorporation of oxygen-containing monomers. Therefore, it is often necessary to provide distance between the olefin and the polar group, or to prevent chain walking close to the group that can be eliminated by the placement of a quaternary carbon spacer [87], The incorporation of acrolein dimethyl acetal is accompanied by reduced activity and full catalyst... 

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