Undesirable compounds

Chemical composition does not generally come into play, except for the case where it is necessary to establish maximum specifications for undesirable compounds such as sulfur, nitrogen, and metals, or even more unusually, certain compounds or families of compounds such as benzene in premium gasolines. By tradition, the refiner supposedly possesses numerous degrees of freedom to generate products for which the properties but not the composition are specified. 

Liquid solvents are used to extract either desirable or undesirable compounds from a liquid mixture. Solvent extraction processes use a liquid solvent that has a high solvolytic power for certain compounds in the feed mixture. For example, ethylene glycol has a greater affinity for aromatic hydrocarbons and extracts them preferentially from a reformate mixture (a liquid paraffinic and aromatic product from catalytic reforming). The raffinate, which is mainly paraffins, is freed from traces of ethylene glycol by distillation. Other solvents that could be used for this purpose are liquid sulfur dioxide and sulfolane (tetramethylene sulfone). 

The synthesis of a-substituted phosphonates 89, via the electrophilic addition of phosphorylated C-radicals 88 (generated by reaction of BujSnH to the readily accessible a-phosphoryl sulfides (or selenides)) and electrophilic addition to electron rich alkenes, has been described [57] (Scheme 26). A large excess of alkene is necessary to minimize the competitive formation of the undesired compound 90 resulting from direct reduction of the initial radical 88. The ratio 89/90 has been measured for each example. The synthesis of the a-mono- or a,a-di-substituted (R or phosphonates 89 shows that the free radical approach... 

Treatment of the iminoacetate 152 with 4-chlorobenzoyl chloride in the presence of a base gave the desired ethyl 5-(4-chlorophenyl)-l-(2,4-dichlorophenyl)-17/-l,2,4-triazolecarboxylate 153 and the uncyclised ethyl 2-(4-chlorobenz-amido)-2-(2-(2,4-dichlorophenyl)hydrazinyl)acetate 154 in 36% and 6% yield, respectively. The undesired compound 154 was subsequently converted to 1,2,4-triazole 153 in 93% yield by further treatment with pyridine (Equation 47) <2006EJM114>. 

The Brellochs reaction represents a significant breakthrough in monocarbaborane chemistry, the exploration of which was previously limited by multistep routes. For example, the important carborane anion CBnFI12 can now be made in two steps with an overall yield of 66%.88 Its synthesis previously required three steps involving undesirable compounds such as hydrogen cyanide, dimethylsulfate, and trialkylamine. (COMC (1982) 5.4.2.6.2) The arachno-cztbotznc... 

Tab. 18.4 Functional groups used to filter out undesirable compounds [24, 78, 86, 87, 155]...

A great attraction of hydrogen is pollution-free combustion while a host of undesirable compounds are emitted from gasoline and diesel fuel vehicles, or formed from their emissions. The main combustion product of hydrogen is water ... 

The characterisation of health hazards of food contaminants, the assessment of the occurrence of undesirable compounds in food and the estimation of the dietary intake are key issues in the risk assessment. In 2000, the European Commission published a White Paper on Food Safety, which underlined the importance of ensuring the highest possible standards of food safety and proposed a new approach to achieve them. Recently, PFCs have gained increased scientific and socioeconomic interest as emerging environmental contaminants due to the unique combination of persistence, toxicity and environmental prevalence. Risk assessment of the dietary exposure to PFCs, however, is hampered by the lack of sufficient data about the occurrence of these contaminants in food. 

Beavers, C. canadensis store branches for weeks and months in the water as winter food caches. They also soak logs and sticks of less-palatable trees in their pond, often for 2-3 days, before they eat them (Fig. 11.24). Field and laboratory experiments suggest that in this way undesirable compounds leach out from the bark (Muller-Schwarze etal, 2001). 

The simultaneous oxidations of various desired or undesired compounds are usually necessary in our research, and in order to understand the phenomena better, it is essential that they be dealt with as complex co-oxidations. The success obtained by using simple co-oxidation for analyzing elementary processes proves that we have a method, which when applied to actual complex transformations, makes possible a more quantitative understanding of industrial operations. 

Removal of the objectionable odors due to the presence of H2S and mercaptans is the objective of the fuel sweetening process. Several methods can be utilized to remove these undesirable compounds including caustic washing, copper chloride sweetening, sulfuric acid treating, Merox processing, and hydrotreating. These methods will be discussed below. 

During extraction processes, many undesirable compounds are also released from the sample matrix these must be removed to obtain quantitative results from certain... 

In the context of integrated chemical processes however, reactive filtration should be specified as the combination of the solids handling unit operation filtration device and the unit operation chemical reactor . This comprises the separation of solids or aerosols from a fluid stream and the chemical conversion of undesired compounds carried by the stream in one instead of two unit operations. The chemical reaction can proceed either continuously during the separation or stepwise after a certain amount of solids or aerosols has been deposited. Two general cases for the application of a reactive filtration unit may be distinguished ... 

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