Separation 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. 

Both para-xylene selectivity and r2/D (tQ 3) increase smoothly with MgO level for a series of large crystal, Mg modified HZSM-5 catalysts, and again para-xylene selectivity increases with tQ 3 (Figure 13, Table IV). However, these catalysts appear to be significantly different from the catalysts just discussed, defining a separate functional dependence on r2/D (tQ 3). These differences will be shown to be attributable to differences in acid activity of this series of catalysts. 

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. 

Ortho-xylene can be separated by distillation ethylbenzene is only 3.9°F from para-xylene, but by using very tall, multitrayed distillation columns (200 feet high with 300 trays), it too can be separated fairly... 

F, in a holding tank. At that temperature, para-xylene crystals form and grow in a liquid-solid mixture like slush. The key to good solid-liquid separation is large crystal growth. The larger the crystals, the better the separation because of the next step. 

When the crystals have grown sufficiently, the slush is put in a centrifuge. The spinning action permits the para-xylene to separate from the mother liquor, so-called because the crystals come out of the liquid. At this stage, the para-xylene crystals, C2[ tA filter cake 2.x. this point, have a purity of 80-90%, due to the mother liquor that coats the crystal surface. (Thafs the reason for big crystals—less surface area for the mother liquor to coat.)... 

Licensors offer a variety of catalysts to promote the isomerization— silica alumina by itself or enhanced with a noble metal like platinum or a non-noble metal like chromium. Another uses hydrofluoric acid with boron trifluoride In the case of the noble metal catalytic process, the feed enters a vessel with a fixed catalyst bed at 850°F and 14.5 psi. As is often the case, a small amount of hydrogen is present to reduce the amount of coke laying down on the catalyst. The effluent is processed in a standard fashion to separate the hydrogen, the para- and ortho-xylene, and any unreacted or miscellaneous compounds. Yields of para-xylene are in the 70% range. 

A small amount of EB is present in crude oil and also is formed in cat reforming. You might recall from Chapter 3 that there is only a 4°F difference between the boiling points of EB and para-xylene Consequently, a superdistillation column is needed for the separation-. In process engineers terms, it would have about 300 theoretical trays, be about 200 feet tall, and even then have a high reflux ratio to accomplish the separation. All this is necessary because the EB stream must be quite pure to be used for styrene manufacture. 

Zinnen, H.A. (1990) Zeolitic para-xylene separation with diethyltoluene heavy desorbent. U.S. Patent 4,864,069. 

Kulprathipanja, S. (1998) Adsorptive separation of para-xylene using isopropylbenzene desorbent. U.S. 

Cain, J.J. (2002) Process for separation of para-xylene using an ether as desorbent. E.P. Patent 1,165,471. 

Distillations are perfect candidates for Raman process monitoring since many potential fluorescent or interfering species are reduced or removed. The Institut Francais du Petrole holds two relevant patents discussing the use of Raman spectroscopy to control the separation of aromatics by adsorption on simulated moving beds and the separation of isomers, particularly aromatic hydrocarbons with 8-10 carbon atoms such as para-xylene and orffio-xylene.67 68... 

Reactive crystallization/precipitation plays a role in a number of industrially relevant processes, such as liquid-phase oxidation of para-xylene to produce technical-grade terephthalic acid, the acidic hydrolysis of sodium salicylate to salicylic acid, and the absorption of ammonia in aqueous sulfuric acid to form ammonium sulfate (135). Reactive crystalhzation/precipitation is also widely applied in the pharmaceutical industry, to facilitate the resolution of the enantiomers (diastereomeric crystallization). Here, the racemate is reacted with a specific optically active material (resolving agent) to produce two diastereomeric derivatives (usually salts) that are easily separated by crystallization ... 

Analytical Properties Separation of meta- and para-xylene in nematic region Reference 5... 

The mixture of aromatics is typically referred to as BTX and is an abbreviation for benzene, toluene, and xylene. The first two components, benzene and toluene, usually are separated by distillation, and the isomers of the third component, xylene, are separated by partial crystallization.17 Benzene is the starting chemical for materials such as styrene, phenol, and many fibers and plastics. Toluene is used to make a number of chemicals, but most is blended into gasoline. Xylene usage is dependent on its isomer. Para-xylene (p-xylene) is a precursor compound for polyester. Ortho-xylene (o-xylene) is the building block for phthalic anhydride. Both compounds are widely used to manufacture consumer products. 

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 Parex and MX-Sorbex processes are both members of UOP s family of Sorbex processes. The MX-Sorbex process for the separation of high-purity meta-xylene was introduced in 1998. Five MX-Sorbex units were commercialized between 1998 and 2001. The market for meta-xylene is expected to grow to 800,000 metric tons per year by 2009 (23). The Parex process was introduced in 1971 for the separation of para-xylene, and 71 units were commercialized by 2001. 

Diphenyl carbonate from dimethyl carbonate and phenol Dibutyl phthalate from butanol and phthalic acid Ethyl acetate from ethanol and butyl acetate Recovery of acetic acid and methanol from methyl acetate by-product of vinyl acetate production Nylon 6,6 prepolymer from adipic acid and hexamethylenediamine MTBE from isobutene and methanol TAME from pentenes and methanol Separation of close boiling 3- and 4-picoline by complexation with organic acids Separation of close-boiling meta and para xylenes by formation of tert-butyl meta-xyxlene Cumene from propylene and benzene General process for the alkylation of aromatics with olefins Production of specific higher and lower alkenes from butenes... 

Two alternative commercial operations have been developed to perform the separation. In one, a mixture of the isomers is contacted with a molecular sieve that has pores large enough to accommodate para-xylene but not the meta or ortho isomers. This operation is referred to as adsorption. In another process, the difference in freezing points of the three isomers (para-xylene freezes at 13.3°C, ortho at —25.2°C, and meta at -47.9°C) forms the basis of a crystallization operation. The mixture is cooled to a temperature at which para crystallizes and can then be separated physically from the remaining ortho and meta liquid. 

Wytcherly RW and McCandless FP. The separation of meta- and para-xylene by pervaporation in the presence of CBr4, a selective feed-complexing agent. J Membr Sci 1992 67 67-75. 

Parex process for separation of para-xylene from other Cs-aromatics. 

Fractional Crystallization. The separation of p-xylene from a Hydroformate by fractional crystallization has been described in the literature (24)- This process is operated commercially to produce para xylene as a raw material for Dacron fiber manufacture. 

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. 

In the present study, silicon and transition metal substituted aluminophosphate molecular sieves have also been evaluated for activity and selectivity for para-xylene production via Cg aromatic isomerization. In commercial practice, Cg aromatic cuts are obtained from reformate gasoline and from pyrolysis naphtha streams. Both feeds contain a significant fraction of ethylbenzene which is difficult to separate from xylenes by physical techniques,... 

The efficiency of gas-liquid chromatography with capillary columns is exemplified by its accomplishing the separation of meta- and- para- xylenes, which had proved to be an extremely difficult separation in earlier attempts made with conventional columns. Various mixtures of acids and esters were resolved chromatographically for the first time making use of capillary columns, a noteworthy example being the resolution of a mixture of methyl elaidate and methyl oleate which are cis- and /ram-isomers. 

When considering chemical reactions involving isomers— for example, ortho-, meta-, and para-xylene—one proceeds as described here, treating each isomer as a separate chemical species. 

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