Catalyst combustion 434 - performance

Fig. 15.11 (a),(b) Fraction of CNT catalysts combusted after 24 h time on stream in a 02/He gas mixture (a) CNTs, (b) 5 wt% P205/CNTs. (a),(b) Reprinted with permission from [25]. Copyright (2011) American Chemical Society, (c) Catalytic performance of B203-modified CNT catalysts in the ODH of propane. Propene selectivity at 5 % propane conversion ( ) and reaction rate (o) as a function of B203 loading, (d) Reaction scheme of CNT-cataiyzed ODH. (c),(d) Reprinted with permission from [61]. Copyright (2009) Wiley VCH. 

In Section 17.8.3 we discussed the catalytic combustion of methane within a single one of the tubes in a honeycomb catalyst, illustrated in Fig. 17.18. The high velocity, and thus the dominance of convective over diffusive transport, makes the boundary layer approximations valid for this system. We will model the catalytic combustion performance in one of the honeycomb channels in this problem. 

Transient experiments were performed on a 12 1 heavy duty diesel engine, with a 24 1 monolithie catalytic converter connected to the exhaust pipe. The eatalytic converter contained two different catalysts, both supplied by Johnson Matthey. These were an 18 1 high temperature active (HT) catalyst placed upstream and a 6 1 low temperature active (LT) catalyst placed downstream. The HT catalyst provides the main capacity for NOx reduction. The LT catalyst combusts unreacted hydrocarbon from the HT catalyst and contributes with some NOx reduction at lower temperatures. As this study only concerns the performance of the HT catalyst, the LT catalyst will not be discussed further, and the HT catalyst will be referred to simply as the catalyst. 

The coke content of the catalysts was measured by combustion-volumetry after the run. The differential thermal analysis (DTA) of the used catalysts was performed with an Aminco Thermoanalyzer using oxygen as dynamic gas. The soluble fraction of coke from used catalysts was extracted with solvents and the solutions were analyzed by several techniques in order to determine their nature. 

In general, the following steps are used to ensure optimum regeneration and catalyst performance recovery alow temperature first pass to remove low boiling point hydrocarbons and other volatile matter, an initial combustion step to remove a portion of the sulfur and carbon, and a final combustion step to remove the remaining carbon to the target level. 

In the second phase, performed at a maximum temperature of about 370°C, the sulfur and a portion of the coke are removed by combustion. The rate and exothermicity are controlled by limiting the flow of combustion gas through the catalyst. Spent base metal catalysts may have sulfur levels of from 6 to 12 wt % in the form of metal sulfides. A high degree of sulfur removal must be achieved in these first two regeneration steps to avoid the formation of sulfate on the support during the final combustion step. Such a formation causes a loss of catalyst activity. 

Catalyst Function. Automobile exhaust catalysts are perfect examples of materials that accelerate a chemical reaction but are not consumed. Reactions are completed on the catalyst surface and the products leave. Thus the catalyst performs its function over and over again. The catalyst also permits reactions to occur at considerably lower temperatures. For instance, CO reacts with oxygen above 700°C at a substantial rate. An automobile exhaust catalyst enables the reaction to occur at a temperature of about 250°C and at a much faster rate and in a smaller reactor volume. This is also the case for the combustion of hydrocarbons. 

Sulfur oxides resulting from fuel sulfur combustion often inhibit catalyst performance in Regions II, III, and a portion of Region IV (see Fig. 7) depending on the precious metals employed in the catalyst and on the air/fuel ratio. Monolithic catalysts generally recover performance when lower sulfur gasoline is used so the inhibition is temporary. Pd is more susceptible than Rh or Pt. The last is the most resistant. Pd-containing catalysts located in hotter exhaust stream locations, ie, close to the exhaust manifold, function with Httie sulfur inhibition (72—74). 

Metals in the platinum family are recognized for their ability to promote combustion at lowtemperatures. Other catalysts include various oxides of copper, chromium, vanadium, nickel, and cobalt. These catalysts are subject to poisoning, particularly from halogens, halogen and sulfur compounds, zinc, arsenic, lead, mercury, and particulates. It is therefore important that catalyst surfaces be clean and active to ensure optimum performance. 

The metallic catalysts for exliaust pollution control are designed to perform three functions. The air/fuel ratio employed in combustion engines creates exhaust products which are a mixture of hydrocarbons, carbon oxides, and niU ogen oxides. These must be rendered environmentally innocuous by reactions on the catalyst such as... 

Finally, there is active interest in developing catalyst systems, both ballistic and polymerization, that would promote combustion stability at high pressures (especially in metal-free systems for smokeless applications) and allow processing lattitude for relatively large motors. The ferric-based systems currently being used fall short of these performance measures. Compounds that form complex structures with the metal chelate to reduce its activity to acceptable levels seem to be most promising. Interestingly, the use of an antibiotic has been cited in this context [19],... 

As discussed in Chapter 1, a portion of the feed is converted to coke in the reactor. This coke is carried into the regenerator with the spent catalyst. The combustion of the coke produces H2O, CO, CO, SO2, and traces of NOx. To determine coke yield, the amount of dry air to the regenerator and the analysis of flue gas are needed. It is essential to have an accurate analysis of the flue gas. The hydrogen content of coke relates to the amount of hydrocarbon vapors carried over with the spent catalyst into the regenerator, and is an indication of the rcactor-stripper performance. Example 5-1 shows a step-by-step cal culation of the coke yield. 

A discussion with 14 refs on expls and proplnts considering the thermodynamic characteristics of expl substances, the kinetics of combustion of powders and the effects of catalysts, corrosion, and instability on the kinetics, the occurrence of deflagration on detonation, and forms of solid mixts in view of the augmentation of their performance and the extension of conditions used in their mixts. The importance of modern methods of calcn is stressed... 

Principles and Characteristics Combustion analysis is used primarily to determine C, H, N, O, S, P, and halogens in a variety of organic and inorganic materials (gas, liquid or solid) at trace to per cent level, e.g. for the determination of organic-bound halogens in epoxy moulding resins, halogenated hydrocarbons, brominated resins, phosphorous in flame-retardant materials, etc. Sample quantities are dependent upon the concentration level of the analyte. A precise assay can usually be obtained with a few mg of material. Combustions are performed under controlled conditions, usually in the presence of catalysts. Oxidative combustions are most common. The element of interest is converted into a reaction product, which is then determined by techniques such as GC, IC, ion-selective electrode, titrime-try, or colorimetric measurement. Various combustion techniques are commonly used. 

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