Xylene, meta

Properties There are three xylene isomers, commonly known as orr/zo-xylene, meta-xylene, and para-xylene. They are all colorless liquids. The orr/zo-isomer boils at 144°C, the meta- at 139.1°C, and the para-at 138.5°C. 

Another possibility is to measure the interaction between liquid crystal26 and two xylene isomers, para- and meta-xylene, which is demonstrated in Figure 13. 

In a study of the iodination of meta xylene by ICI at 20 C the reaction was followed by the analysis of unreacted ICI (cited by Pannetier, 1967). Two different solvent mixtures were used, (a) 60 vol% m-xylene and 40 vol%... 

FIGURE 6.2 Representation of multivariate data by icons, faces, and music for human cluster analysis and classification in a demo example with mass spectra. Mass spectra have first been transformed by modulo-14 summation (see Section 7.4.4) and from the resulting 14 variables, 8 variables with maximum variance have been selected and scaled to integer values between 1 and 5. A, typical pattern for aromatic hydrocarbons B, typical pattern for alkanes C, typical pattern for alkenes 1 and 2, unknowns (2-methyl-heptane and meta-xylene). The 5x8 data matrix has been used to draw faces (by function faces in the R-library Tea-chingDemos ), segment icons (by R-function stars ), and to create small melodies (Varmuza 1986). Both unknowns can be easily assigned to the correct class by all three representations. 

The isomers, called ortho-xylene, meta-xylene, and para-xylene, each have unique properties. Two such properties are the freeze points, at which xylenes turn from liquid to crystals, and the boiling points, at which xylenes turn from liquid to vapor. These two properties figure importantly in the apparatus used to separate xylene isomers from each other. Mixed xylenes, a commonly traded commodity, is a combination of the three isomers. 

You might think youVe finished your introductions to the aromatics family, having dealt with benzene, xylene, and the xylene triplets, ortho-, para-, and meta-xylene. There s another isomer in the closet, ethylbenzene. 

Even though the boiling temperatures of meta- and para-xylene are close together, their freezing points, i.e., the temperatures at which the liquid starts freezing, i.e., turning to crystals, are not. Meta-xylene crystallizes at -54.2 F and para-xylene at +55.9°F, a spread of more than 100 degrees. 

To further purify the para-xylene, the crystals are again melted, cooled— this time to about -40 F—and crystallized once again. Centrifuging this time results in about 99% pure para-xylene. The meta-xylene from both centrifuges is about 85.%, the rest para-xylene. 

A commercial process using a material called molecular sieve can also separate para-xylene from meta-xylene. Molecular sieves are marble-sized, pellets that have millions of pores, all of a size that para-xylene molecule can fit in but the meta-xylene molecule cannot. The pore sizes are so small they are measured in Angstroms, which are 1x10" centimeters (0.00000001 cm). Molecular sieves of varying pore sizes are used in many other applications as well. 

Mixed xylenes are used as an octane improver in gasoline and for commercial solvents, particularly in industrial cleaning operations. By far, most of the commercial activity is with the individual isomers. Para-xylene, the most important, is principally used in the manufacture of terephthalic acid and dimethyl terephthalate en route to polyester plastics and fibers (Dacron, films such as Mylar, and fabricated products such as PET plastic bottles). Ortho-xylene is used to make phthalic anhydride, which in turn is used to make polyester, alkyd resins, and PVC plasticizers. Meta-xylene is used to a limited extent to make isophthahc acid, a monomer used in making thermally stable polyimide, polyester, and alkyd resins. 

Can a mixture of benzene, toluene, and meta-xylene be separated by cryogenic crystallization Whats the usual (more economic) way ... 

The stepsister of the other two phthalic acids is isophthalic acid made from meta-xylene. The applications are similar, but the commercial demand is smaller. If it weren t for the fact that m-xylene is a coproduct of the other xylenes, no one would invent it. The other xylenes and phthalic acids would probably suffice. 

Benzene, toluene, orth-xylene, meta-xylene, para-xylene, and ethylbenzene... 

Sure, because the freezing points of benzene, toluene, and meta-xylene are 167.2, 231.4, and 282.4°F, and far enough apart that distilling by freezing will work. The more usual method is fractional distillation, which is cheaper in both capital cost and operating cost. 

Benzene and para-xylene are the most sought after components from reformate and pygas, followed by ortho-xylene and meta-xylene. While there is petrochemical demand for toluene and ethylbenzene, the consumption of these carmot be discussed in the same way as the other four. Toluene is used in such a large quantity in gasoline blending that its demand as a petrochemical pales in comparison. Fthylbenzene from reformate and pygas is typically dealkylated to make benzene or isomerized to make xylenes. On-purpose production of petrochemical ethylbenzene (via ethylene alkylation of benzene) is primarily for use as an intermediate in the production of another petrochemical, styrene monomer. Ethylbenzene plants are typically built close coupled with styrene plants. 

Significant progress is being made in using m-xylene as the primary monomer in other resins with low oxygen permeability such as meta-xylene diamine [40-42]. 

Neuzil, R. (1982) Process for the separation of meta-xylene. US Patent 4,325,092. 

Meta-xylene isomerization to ortho- and para-xylene over 10- and 12-MR zeolites is another illustration of product shape selectivity effects [13]. The two products are essentially equally favorable from the standpoint of thermodynamics. With decreasing pore size, however, kinetics come into play and the selectivity to para-xylene increases, as illustrated in Figure 13.37 for results obtained at 317-318°C, 0.5 kPa meta-xylene pressure (in the presence of He carrier gas) and 10% conversion [64]. While the para ortho ratio is typically 1.0-1.5 with multi-dimensional... 

As an example for aromatic transformation the mechanism for meta-xylene disproportionation to toluene -i- trimethylbenzene is illustrated in Figure 13.46. In the first step the zeolite extracts a hydride from meta-xylene to form a carbenium ion at one of the methyl groups, presumably the rate-controlling step. This mechanism is likely to involve a Lewis acid site. The carbenium ion then adds to a second... 

Over the past several decades, there has been a continuing growth in the worldwide demand for plastics, films and fibers, particularly polyesters. The raw materials that make up these polymers are based primarily on the C8 family of aromatics (C8A)-ethylbenzene (EB), para-xylene (PX), meta-xylene (MX), and ortho-xylene (OX). Polyester (PET plastic), derived from PX, in particular has experienced rapid growth and is projected to see continue rapid growth as many developing countries desire to have the lifestyle flexibility that such readily available, versatile plastics support. While the markets for MX- and OX-derived plastics are smaller (plasticizers and specialty polyesters, respectively), all C8A markets continue to increase with population growth [59],... 

---