Xylene cleaning process

The second process investigated was an alternative for a xylene cleaning process. The dual-cycle (two step) process, as described previously in this chapter, is used due to the nature of the contaminant being removed. Figure 7 shows a bar chart of the annual costs (normalized) for the two processes. Once again the key differences are in the equipment and consumables. 

Figure 7. Normalized annual costs for a 5-year accounting life of the equipment. Comparison betweena xylene solvent cleaning pFooessand atwo-stq> supercritical fluid cleaning process.

Tables 14.13.1 and 14.13.2 give information on the reported solvent releases and transfers from the iron and steel industries. Not all the solvents listed in the tables are used in processing. Some are by-products of coke manufacture from coal. Benzene and polycyclic aromatics compounds are by-products. Strong solvents such as methyl ethyl ketone, toluene, xylene, and trichloroethylene are typical of those used in cleaning processes. There is no program formulated by the industry to reduce amounts of solvents used.

Sodium xylene sulfonate (SXS), short-chain alcohols, or glycol ethers help make the formulation homogeneous as well as solubilize water during the cleaning process. Ethylene glycol monobutyl ether can serve the function of hydrotrope, organotrope, and cosolvent. 

There are yet more similarities between protein and dairy products. For example, highly processed protein products (e.g., ground, mixed and canned) also contain disinfection byproducts. Thus, hot dogs, ground beef, bologna, salami, and canned tuna are contaminated with chloroform and sometimes with bromodichloromethane. Benzene, toluene, ethyl benzene, and xylenes, as well as the other detected chlorinated petroleum solvents (e.g., PCE, TCA, and TCE), also occur in protein products because these chemicals were in products used to lubricate or clean machinery that contacted the dairy products, or because these chemicals were in the ambient air of the meatpacking plant. Protein products also contain styrene from contact with plastics. Finally. PCB (a banned industrial chemical) was found to occur in every Market Basket sample of salmon. 

Component 1 in Singapore buildings was correlated with compounds associated with humans and their activities. Human effluents have been reported to contain isoprene (Ellin et al, 1974) while tetrachloroethylene is a VOC found in dry-cleaned clothes worn by building occupants (Wallace, Pellizzari and Wendel, 1991) or from the use of consumer products (Sack et al., 1992). Tetradecane, benzaldehyde, o-xylene, naphthalene are emissions from dry process photocopiers (Leovic et al., 1996). Component 2 with high loadings ofn-decane, n- undecane, toluene, styrene, n-nonane, 1,2,4-trimethyl benzene probably reflects the emissions of carpets and vinyl floorings (Yu and Crump, 1998). Component 3 was primarily correlated with heptane and methylcyclopentane, which could be due to the emissions of water-based paints. Finally, component 4 was associated with 2-methylpentane, hexane, cyclohexane, methylcyclohexane and limonene, which is reflective of the emissions of air fresheners and cleaning products (Sack et al., 1992). 

One of the major uses of activated carbon is in the recovery of solvents from industrial process effluents. Dry cleaning, paints, adhesives, polymer manufacturing, and printing are some examples. Since, as a result of the highly volatile character of many solvents, they cannot be emitted directly into the atmosphere. Typical solvents recovered by active carbon are acetone, benzene, ethanol, ethyl ether, pentane, methylene chloride, tetrahydrofuran, toluene, xylene, chlorinated hydrocarbons, and other aromatic compounds [78], Besides, automotive emissions make a large contribution to urban and global air pollution. Some VOCs and other air contaminants are emitted by automobiles through the exhaust system and also by the fuel system, and activated carbons are used to control these emissions [77,78],... 

The best approach to improving separations is to work toward reactions that achieve 100% yields at 100% conversions. Frequently, this will require more selective catalysts. The previous chapter contained an example moving in this direction. Toluene was disproportionated to benzene and xylenes using a silica-modified zeolite catalyst.23 After removal of benzene and unchanged toluene by distillation, the xylene remaining was a 99% para-isomer. It was clean enough to put directly into the process of oxidation to terephthalic acid. This avoided the usual separation of xylenes by crystallization or by a molecular sieve. There are times when an equilibrium can be shifted by removal of a product or by-product continuously to give 100% conversion. The familiar esterification with azeotropic removal of water or removal of water with a molecular sieve is an example. 

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