Xylenes by distillation

In one process (Fig. 2), a light fraction (boiling range 65 to 175°C) from a straight run petroleum fraction or from an isocracker is fed to a catalytic reformer and is followed by fractionation and extraction. The mixed xylenes stream is then processed further to produce high-purity p-xylene and/or o-xylene. Because of the close boiling points of p-xylene and m-xylene, production of high-purity p-xylene by distillation is impractical and methods such as crystallization and adsorption are used. 

Small differences in the boiling points of the xylene isomers make the preparation of pure m- xylene by distillation impossible, Nevertheless, boiling ranges of fractions suitable for nitration can be established. 

It is obviously impossible to separate m-x ene and p-xylene by distillation, ance their boiling points differ by only (L8 C However, this separation method is feasible to isolate o-xylene and ethylbenzene, although supeifiactionations are required. Further more, the melting point of p-xylene is much higher than that of the other Cg compounds. This is why crystallization was initially employed to recover p-xylene from the mixture, which may be previously rid of ethylbenzene and o-xylene. 

It is possible to effect only partial replacement of the bromine of 2,4,6-tri-bromopyrimidine by alkoxyl when a solution of two equivalents of sodium in anhydrous ethanol is used in benzene as solvent and room temperature is not exceeded, then the product is 4-bromo-2,6-diethoxypyrimidine (89% yield).738 If the excess of ethanol is removed from a solution of sodium in ethanol and xylene by distillation and the residue is heated with 4-chloro-6-methoxy-2-pyrimidinamine for 2 hours under reflux, 4-ethoxy-6-methoxy-2-pyrimidin-amine is formed 739 but if more ethanol is present the methoxyl group is also replaced by ethoxyl and the product is 4,6-diethoxy-2-pyrimidinamine. 

In a conventional scheme comprising the separation of ethylbenzene and o-xylene by distillation, and.of a large fraction of p-xylene by crystallization or nearly completely by adsorption, a mother liquor with a high m-xylene content remains after these operations. It can be upgraded as a solvent or employed in high octane gasolines. Depending on market requirements, however, this Ca cut can be used to boost the production of o-xylene and p-xylene by catalytic isomerization. 

The boiling point of o-xylene is around 5 °C higher than that of m-xylene, its nearest co-boiling compound. o-Xylene is therefore recovered by fractional distillation in columns with 100 to 150 trays, which requires a reflux ratio of 8-lO.i.The high-boiling aromatics are separated from the o-xylene by distillation in the o-xylene column with 40 to 60 trays and a reflux ratio of around 1 1. 

Figure 4.15 shows a typical flow diagram for the separation of o-xylene by distillation, yielding a product of over 95% purity this concentration is sufficient for the main use of o-xylene, the production of phthalic anhydride. 

In the Parex process, the p-xylene is separated at 120 to 175 °C by selective adsorption. Separation of the Cg-aromatics is effected by an adsorbing solid material (adsorbant) and a suitable liquid. The process is based on the fact that the various components, adsorbed to a different degree, are recovered from the active surface of the adsorbant. A synthetic zeolite is used as the adsorption agent, the active centres of which are formed by cations from the first and second group (K, Ca) of the periodic table. A hydrocarbon with a low adsorption capacity, such as toluene or p-diethylbenzene is used for desoiption, and can easily be separated from p-xylene by distillation. In this process, the liquid phases are fed through a... 

The stock solution of quinoline-sulphur poison is prepared by refluxing I g. of sulphur with 6 g. of quinoline for 5 hours and diluting the resulting brown liquid to 70 nJ. with xylene which has been purified by distilling over anhydrous aluminium chloride. The addition of the quinoline - sulphur poison ensures that the reduction does not proceed beyond the aldehyde stage it merely slows up the reaction and has no harmful effects. 

Hydrazine hydrate of 95-100 per cent, concentration is a commercial product. The 40-60 per cent, solution may be concentrated to 80-85 per cent, strength by distillation with xylene in an all-glass apparatus. 

Unique adsorption selectivities are employed in the separation of Cg aromatic isomers, a classical problem that caimot be easily solved by distillation, crystallisation, or solvent extraction (10). Although -xylene [106-42-3] can be separated by crystallisation, its recovery is limited because of the formation of eutectic with / -xylene [108-58-3]. However, either -xylene, / -xylene, (9-xylene [95-47-6] or ethylbensene [100-41-4] can be extracted selectively by suitable modification of seoUtic adsorbents. 

Hydrochloric acid [7647-01-0], which is formed as by-product from unreacted chloroacetic acid, is fed into an absorption column. After the addition of acid and alcohol is complete, the mixture is heated at reflux for 6—8 h, whereby the intermediate malonic acid ester monoamide is hydroly2ed to a dialkyl malonate. The pure ester is obtained from the mixture of cmde esters by extraction with ben2ene [71-43-2], toluene [108-88-3], or xylene [1330-20-7]. The organic phase is washed with dilute sodium hydroxide [1310-73-2] to remove small amounts of the monoester. The diester is then separated from solvent by distillation at atmospheric pressure, and the malonic ester obtained by redistillation under vacuum as a colorless Hquid with a minimum assay of 99%. The aqueous phase contains considerable amounts of mineral acid and salts and must be treated before being fed to the waste treatment plant. The process is suitable for both the dimethyl and diethyl esters. The yield based on sodium chloroacetate is 75—85%. Various low molecular mass hydrocarbons, some of them partially chlorinated, are formed as by-products. Although a relatively simple plant is sufficient for the reaction itself, a si2eable investment is required for treatment of the wastewater and exhaust gas. 

The mixed monocyclic aromatics are called BTX as an abbreviation for ben2ene, toluene, and xylene (see Btxprocessing). The benzene and toluene are isolated by distillation, and the isomers of the xylene are separated by superfractionation, fractional crystallisation, or adsorption (see Xylenes and ethylbenzene). Bensene is the starting material for styrene (qv), phenol (qv), and a number of fibers and plastics. Toluene (qv) is used to make a number of... 

Initial production of the dimethyl terephthalate started with the oxidation of -xylene to terephthaUc acid using nitric acid both companies reportedly used similar technology (43—45). Versions of the nitric acid oxidation process, which has been abandoned commercially, involved the use of air in the initial oxidation step to reduce the consumption of nitric acid (44,46,47). The terephthaUc acid was then esterified with methanol to produce dimethyl terephthalate, which could be purified by distillation to the necessary degree (48). 

Herm/es/Djnamit JS obe/Process. On a worldwide basis, the Hercules Inc./Dynamit Nobel AG process is the dorninant technology for the production of dimethyl terephthalate the chemistry was patented in the 1950s (67—69). Modifications in commercial practice have occurred over the years, with several variations being practiced commercially (70—72). The reaction to dimethyl terephthalate involves four steps, which alternate between liquid-phase oxidation and liquid-phase esterification. Two reactors are used. Eirst, -xylene is oxidized with air to -toluic acid in the oxidation reactor, and the contents are then sent to the second reactor for esterification with methanol to methyl -toluate. The toluate is isolated by distillation and returned to the first reactor where it is further oxidized to monomethyl terephthalate, which is then esterified in the second reactor to dimethyl terephthalate. 

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. 

UOP s Parex Process can be used to purify -xylene by adsorption (38). Toray has a similar process. These processes take advantage of the fact that %xylene is adsorbed more easily than the other Cg aromatics by a suitable molecular sieve. The -xylene is desorbed by either a lighter or heavier hydrocarbon which is subsequently removed by distillation. -Xylene is recovered in about 97% yield (see Adsorption). 

Antimony trichloride [10025-91-9] M 228.1, m 73", b 283", pK 1.4, pK j ll.O (11.8), PKI 12.95 (for Sb " " aquo). Dried over P2O5 or by mixing with toluene or xylene and distilling (water is carried off with the organic solvent), then distd twice under dry nitrogen at 50mm, degassed and sublimed twice in a vacuum into ampoules. Can be crystd from CS2. Deliquescent. Fumes in moist air. 

Chloro-1 -methyl-5-phenyl-s-trizolo[4,3-a]quinoline A stirred mixture of 6triethyl-orthoacetate (0.925 g,0.0057 mol) and xylene (100 ml) was refluxed, under nitrogen, for 2 hours 40 minutes. During this period the ethanol formed in the reaction was removed by distillation through a short,glass helix-packed column. The mixture was concentrated to dryness In vacuo and the residue was crystallized from methanol-ethyl acetate to give 1.28 g of 7-chloro-1-methyl-5-phenyl-s-triazolo[4,3-a]-quinoline (83.9% yield). The analytical sample was crystallized from methylene chloride methanol and had a melting point 252.5°-253.5°C. 

A mixture of 100 g of 5,5-dimethylacridan, 20 g of pulyerized sodium amide and 6.5 g of 1 -chloro-3-dimethylaminopropane in 50 ml of xylene is heated at reflux with stirring for one hour. To the cooled reaction mixture is added one yolume of water. The organic layer is separated and extracted several times with diluted lactic acid. The acidic extracts are combined, washed with ether and neutralized by alkali. The crude 10-(3 -dimethylaminopropvl)-5,5-dimethylacridan is isolated by ether extraction and purified by distillation in a high vacuum. The yield is 6,4 g BP 170°-1 B0°C/0.005 mm. n = 1.5990. 

A suspension of NaOEt, prepared by adding EtOH (2 rnL) to a suspension of xylene-washed Na (0.77 g, 33.5 mmol) in anhyd Et20 (90 mL), was added to a solution of diethyl 4-(chloromethyl)-2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxylate (1 10 g, 33 mmol) in anhyd El20 (600 mL) whereupon afine precipitate appeared immediately. The mixture was stirred and heated under reflux for 18 h, then cooled, and the precipitate of NaCl was removed by filtration. The volume of the filtrate was reduced to 100 mL, and the filtrate was washed with cold H20, then dried (MgS04). The Et2C) was evaporated under reduced pressure to yield the crude product as a golden oil [yield 7.3 g (83%)] which was purified by distillation bp 142-145 C/0.1 Torr. 

The 42 per cent hydrazine hydrate solution supplied by the Eastman Kodak Company is too dilute for use as such, but may be concentrated by distillation with xylene.1 A mixture of r44 cc. (150 g.) of the 42 per cent solution and 230 cc. of xylene is distilled from a 500-cc. flask through a 17-cm. Hempel column fitted into a cork covered with tin foil. After distillation of the xylene, with about 85 cc. of water, the residue yields on distillation 45-50 g. of 80-85 per cent hydrazine hydrate. This material, assayed best by titration with standard acid using methyl orange as indicator, may be used as such or concentrated further (see Note 6). 

Soya oil, 88.6 g, 20.0 g of pentaerythritol, and 0.06 g of lithium hydroxide monohydrate are weighed into a 250-mL three-necked round-bottom flask fitted with a magnetic stirrer, a Dean-Stark separator, and nitrogen inlet and outlet. The reaction mixture is heated to 250°C for 30 min under nitrogen (note a below), then cooled to 200°C. Phthalic anhydride, 34.6 g, 0.70 g of maleic anhydride, and 8.0 g of xylene are added. The reaction mixture is heated to 230°C and the toluene-water azeotrope is removed by distillation. The reaction is stopped when the acid number of reaction medium is lower than 10 (note b below). After cooling to room temperature, 52 g of white spirit is added. 

Manufacture The xylenes are obtained with benzene (and toluene) from the catalytic reforming of naphtha and separated from the aromatic mixture by distillation. From the mixed isomers, the ortho- can be obtained by distillation because its boiling point is sufficiently different. The meta- and para- are separated by either selective adsorption or by crystallization. 

Because orr/zo-xylene is more readily isolated and purified (by distillation), it costs less than para-xylene. Like all petrochemicals, prices depend on the price of crude oil but in early 2001, mixed-xylene was about 17 cents/lb while para-xylene was only about 15 cents due to high manufacturing capacity and low demand for use for making terephthalic acid. In the extremely high volumes in which such chemicals are sold, fractions of a penny difference in price can be very important. 

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