Xylene dependence

The pathway for the degradation of the xylenes depends critically on the orientation of the methyl groups, and o-xylene is considered to be the most recalcitrant since xylene monooxygenase cannot hydroxylate one of its methyl groups ... 

High-purity benzene and xylenes are products of aromatics complexes having several interconnected processes and unit operations (22). In 1998, the market demand for benzene, on a world-wide basis, was 27.4 million metric tons per year, mostly for styrene. By comparison, the demand for para-xylene was 16.1 million metric tons. Ortho-xylene demand was lower, at 3 million metric tons. The market for meta-xylene was even lower, at about 300,000 metric tons. Because of these relative market requirements, most aromatics complexes are designed for benzene and para-xylene. Depending on local situations, they may also produce orthoxylene, which can be separated by fractionation, and/or meta-xylene. Process units that can be integrated into UOP aromatics complexes are described in Figure 4.17. 

The degradation of dialkylbenzenes such as the dimethylbenzenes (xylenes) depends critically on the position of the methyl groups (Baggi et al. 1987). Two distinct pathways have been found for the 1,4-isomer (Davey and Gibson 1974 Gibson et al. 1974) and these are illustrated in Figure 6.28. 

It is seen that for all compositions the selectivity of p-xylene is higher than the equilibrium values [23], Selectivity of p-xylene was found to be maximum when x = 2.5 except for the ammonium salt. Yashima and coworkers [24] have reported that in the case of Y zeolite,the formation of p-xylene depends on the strength and density of Bronsted acid centres. The results obtained in this study suggest that high surface Bronsted acidity is responsible for high para selectivity. Fig. 3 shows the correlation of p-xylene selectivity with strong acid centres with H <-5.6 on CsxHs-xPWnO o-... 

Treatment with inorganic oxides or weak acids such as H3PO4 are widely reported, particularly for ZSM-5 where considerable improvements in shape selectivity are demonstrated [67]. The increased shape selectivity, for example in the methylation of toluene to produce xylene, depends upon limiting the extent of reaction on outer-surface sites which are flee fiom pore constraints. A reaction/diffiision model... 

Epoxy esters. Similar solvents are used as for alkyds, e.g. white spirit or xylene, depending on their type and amount of fatty acid, eventually with a small amount of alcohol. 

The mechanical encapsulations of guest EtOH molecules in the closed channels were clarified by measuring the amount of EtOH molecules released from the solid sample of 6b/6a in m-xylene depending on the temperature (Figure 8). Although the release of EtOH molecules from the closed channels of 6b was not detected below -20 °C, they were suddenly released when the temperature was increased to —20 °C because of the conversion to 6a bearing open channels. These results clearly demonstrate that guest EtOH molecules are captured in the closed channel mechanically. 

More precisely, the rate of ozone formation depends closely on the chemical nature of the hydrocarbons present in the atmosphere. A reactivity scale has been proposed by Lowi and Carter (1990) and is largely utilized today in ozone prediction models. Thus the values indicated in Table 5.26 express the potential ozone formation as O3 formed per gram of organic material initially present. The most reactive compounds are light olefins, cycloparaffins, substituted aromatic hydrocarbons notably the xylenes, formaldehyde and acetaldehyde. Inversely, normal or substituted paraffins. 

Mobil s Low Pressure Isomerization Process (MLPI) was developed in the late 1970s (123,124). Two unique features of this process are that it is Operated at low pressures and no hydrogen is used. In this process, EB is converted to benzene and diethylbenzene via disproportionation. The patent beheved to be the basis for the MLPI process (123) discusses the use of H-ZSM-5 zeoHte with an alumina binder. The reaction conditions described are start-of-mn temperatures of 290—380°C, a pressure of 273 kPa and WHSV of 5—8.5/h. The EB conversion is about 25—40% depending on reaction conditions, with xylene losses of 2.5—4%. The PX approach to equiHbrium is about 99 ndash 101%. The first commercial unit was Hcensed in 1978. A total of four commercial plants have been built. 

A second Mobil process is the Mobil s Vapor Phase Isomerization Process (MVPI) (125,126). This process was introduced in 1973. Based on information in the patent Hterature (125), the catalyst used in this process is beHeved to be composed of NiHZSM-5 with an alumina binder. The primary mechanism of EB conversion is the disproportionation of two molecules of EB to one molecule of benzene and one molecule of diethylbenzene. EB conversion is about 25—40%, with xylene losses of 2.5—4%. PX is produced at concentration levels of 102—104% of equiHbrium. Temperatures are in the range of 315—370°C, pressure is generally 1480 kPa, the H2/hydrocatbon molar ratio is about 6 1, and WHSV is dependent on temperature, but is in the range of 2—50, although normally it is 5—10. 

Oxidation. Acetaldehyde is readily oxidised with oxygen or air to acetic acid, acetic anhydride, and peracetic acid (see Acetic acid and derivatives). The principal product depends on the reaction conditions. Acetic acid [64-19-7] may be produced commercially by the Hquid-phase oxidation of acetaldehyde at 65°C using cobalt or manganese acetate dissolved in acetic acid as a catalyst (34). Liquid-phase oxidation in the presence of mixed acetates of copper and cobalt yields acetic anhydride [108-24-7] (35). Peroxyacetic acid or a perester is beheved to be the precursor in both syntheses. There are two commercial processes for the production of peracetic acid [79-21 -0]. Low temperature oxidation of acetaldehyde in the presence of metal salts, ultraviolet irradiation, or osone yields acetaldehyde monoperacetate, which can be decomposed to peracetic acid and acetaldehyde (36). Peracetic acid can also be formed directiy by Hquid-phase oxidation at 5—50°C with a cobalt salt catalyst (37) (see Peroxides and peroxy compounds). Nitric acid oxidation of acetaldehyde yields glyoxal [107-22-2] (38,39). Oxidations of /)-xylene to terephthaHc acid [100-21-0] and of ethanol to acetic acid are activated by acetaldehyde (40,41). 

Alkylated aromatics have excellent low temperature fluidity and low pour points. The viscosity indexes are lower than most mineral oils. These materials are less volatile than comparably viscous mineral oils, and more stable to high temperatures, hydrolysis, and nuclear radiation. Oxidation stabihty depends strongly on the stmcture of the alkyl groups (10). However it is difficult to incorporate inhibitors and the lubrication properties of specific stmctures maybe poor. The alkylated aromatics also are compatible with mineral oils and systems designed for mineral oils (see Benzene Toulene Xylenes and ethylbenzene). ... 

The solubiHty of phosphoms in water is about 3 ppm. However, process water used in phosphoms manufacture or handling often catties larger amounts of phosphoms as particulates or small droplets, depending on the water temperature. Phosphoms-contaminated water is commonly called phossy water. Phosphoms has low solubiHty in most common solvents, but is quite soluble in carbon disulfide and some other special solvents. The solubiHty in CS2 and benzene was formerly used in phosphoms analyses, but toxicity and increasing waste disposal costs have led to mote use of toluene and xylene, and mote tecentiy to the use of nonchemical turbidity measurements. 

Economic Aspects. Isophthahc acid in North America sold for 1.19— 1.32/kg in 1994, depending on the shipment method. The price of xylene plays a role, although not to the same extent as -xylene in terephthaUc acid. The far lower production volumes and smaller plant sizes for isophthahc acid do not give the same economies of scale. 

The speed of the reaction depends both on the metal and on the alcohol, increasing as electropositivity iacreases and decreasiag with length and branching of the chain. Thus sodium reacts strongly with ethanol, but slowly with tertiary butyl alcohol. The reaction with alkaU metals is sometimes carried out ia ether, ben2ene, or xylene. Some processes use the metal amalgam or hydride iastead of the free metal. Alkaline earth metals and aluminum are often covered with an oxide film which hinders the reaction. 

Toluene, Benzene, and BTX Reeoveiy. The composition of aromatics centers on the C - and Cg-fraction, depending somewhat on the boihng range of the feedstock used. Most catalytic reformate is used directiy in gasoline. That part which is converted to benzene, toluene, and xylenes for commercial sale is separated from the unreacted paraffins and cycloparaffins or naphthenes by hquid—hquid extraction or by extractive distillation. It is impossible to separate commercial purity aromatic products from reformates by distillation only because of the presence of azeotropes, although comphcated further by the closeness in boihng points of the aromatics, t/o-paraffin, and unreacted C -, C -, and Cg-paraffins. 

Transall lation. Two molecules of toluene are converted iato one molecule of benzene and one molecule of mixed xylene isomers ia a sequence called transalkylation or disproportionation. Economic feasibiUty of the process strongly depends on the relative prices of benzene, toluene, and xylene. Operation of a transalkylation unit is practical only when there is an excess of toluene and a strong demand for benzene. In recent years, xylene and benzene prices have generally been higher than toluene prices so transalkylation is presendy an attractive alternative to hydrodealkylation (see also Btx... 

The open tack time of the CR adhesives partially depends on the evaporation rate of the solvent blend. If a solvent evaporates slowly, the CR adhesive will retain tack longer, whereas if the solvent evaporates quickly, the cohesive strength will develop more rapidly. According to Table 13, addition of small amounts of xylene (generally lower than 5%) will increase the open time of CR adhesives. 

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