Lean compounding

Spook W. Lean compounding the key to survival. Plastics Addit Compound [trade journal—Elsevier Ltd.] July/August 2008. 

The autoignition temperature is the minimum temperature required for self-sustained combustion in the absence of an external ignition source. The value depends on specified test conditions. Tht flammable (explosive) limits specify the range of concentration of the vapor in air (in percent by volume) for which a flame can propagate. Below the lower flammable limit, the gas mixture is too lean to burn above the flammable limit, the mixture is too rich. Additional compounds can be found in National Fire Protection Association, National Fire Protection Handbook, 14th ed., 1991. 

Some of these compounds could be considered as dietary additives, but various other terms, including pesticides, can also be used. They can have beneficial effects on the environment and this aspect will be discussed later. The ionophore monensin, which is an alicyclic polyether (Figure 1), is a secondary metabolite of Streptomyces and aids the prevention of coccidiosis in poultry. Monensin is used as a growth promoter in cattle and also to decrease methane production, but it is toxic to equine animals. " Its ability to act as an ionophore is dependent on its cyclic chelating effect on metal ions. ° The hormones bovine somatotropin (BST) and porcine somatotropin (PST), both of which are polypeptides, occur naturally in lactating cattle and pigs, respectively, but can also be produced synthetically using recombinant DNA methods and administered to such animals in order to increase milk yields and lean meat production. "... 

Chapters Three, Five and Six have covered the synthesis of physical mass-exchange networks. In these systems, the targeted species were transferred from the rich phase to the lean phase in an intact molecular form. In some cases, it may be advantageous to convert the transferred species into other compounds using reactive MSAs. Typically, reactive MSAs have a greater capacity and selectivity to remove an undesirable component than physical MSAs. Furthermore, since they react with the undesirable species, it may be possible to convert pollutants into other species that may either be reused within the plant itself or sold. 

The NOx storage-reduction (NSR) catalyst, developed by Toyota and other companies, offers a solution based on a two step process, in which the engine switches periodically between a long lean-burn stage and a very short fuel-rich stage. The NSR catalyst combines the oxidation activity of platinum with a NOx storage compound based on barium oxide. Figure 10.10 illustrates the principle of operation. 

It is finally easy to understand why this simultaneity of functions 2 and 3 can lead to a complete misunderstanding of the DeNOx reaction. As function 2 is producing RNOx compounds, simultaneously with function 3 which is producing N2, it is rather difficult to discriminate between the elementary steps leading to either RNOx or N2. The present chapter will demonstrate that, in lean conditions, RNOx leads to [CxH),Oz + NO] and N2 is formed in another catalytic cycle. 

One of the most promising processes is the active DeNO based on NO -trap materials. It has been developed for lean-burn gasoline engines. Cerium compounds are thought to intervene in different steps of the whole process (1) NO oxidation, (2) NO storage, (3) Nitrate desorption and NO reduction. Most probably, the main role of OSC materials is to accelerate HC partial oxidation during rich-spikes (giving CO and H2 as NO reducers). However, this beneficial effect of OSC compounds competes with a detrimental reaction,... 

Figure 11 shows the results of these experiments for the same pelleted Pt/Pd/Rh/Ce/A O catalyst used for the data in Figure 9. The oxygen content of the catalyst bed increased linearly with pulse duration and reached the steady-state lean level within 0.5 s. The oxygen capacity of the catalyst was associated primarily with oxidation of the 190 umol of Ce contained in each gram of catalyst the catalyst contained only 8 umol of precious metal per gram. A comparison of the oxygen capacity to the amount of Ce in the catalyst suggests that about 76% of the Ce could change between the +3 and +4 oxidation states (the most common oxidation states of Ce). The identities of the Ce compounds which undergo oxidation and reduction in exhaust are not known, however, dispersed hydroxides and oxy-hydroxides are likely candidates (18).

When elemental sulfur or a sulfur-bearing compound is present in any combustion system, the predominant product is sulfur dioxide. The concentration of sulfur trioxide found in combustion systems is most interesting. Even under very lean conditions, the amount of sulfur trioxide formed is only a few percent of that of sulfur dioxide. Generally, however, the sulfur trioxide concentration is higher than its equilibrium value, as would be expected from the relation... 

Table 16.2 Plasma concentrations of some compounds during starvation in normal lean adult males...

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