Mixed xylene is not a single aromatic isomer but a boiling-range mixture of ethylbenzene, para-xylene, meta-xylene and ortho-xylene. The commercial product is defined less by total C8 aromatic content than by impurity limits in ASTM D5211-19 for xylenes used as p-xylene feedstock and ASTM D843 for nitration-grade xylene. Ethylbenzene, nonaromatic hydrocarbons, sulfur, water and acid wash color are the controlling specifications because downstream isomer recovery by crystallization or adsorptive separation is highly sensitive to these contaminants. The production of mixed xylene follows three principal industrial routes: catalytic reforming of naphtha, toluene disproportionation and coal-tar aromatic recovery. The route determines not only the xylene isomer distribution but also the trace heteroatom burden and the unit operations required to reach specification material.

Under what reforming severity does the C8 aromatic yield peak?

In semi-regenerative and continuous catalytic reforming units, mixed xylene is generated as part of the liquid reformate aromatic pool. The naphtha feed is normally hydrotreated to sulfur below 0.5 mg/kg because platinum-based reforming catalysts are sulfur-sensitive. Nitrogen and oxygenate limits are also reduced to low single-digit mg/kg levels. A mid-cut naphtha with a boiling range of 90–160 °C typically produces reformate containing 58–72 vol% total aromatics, with mixed xylenes in the 20–28 vol% range depending on naphthene content, reactor pressure and feed cut point. The principal reactions are naphthene dehydrogenation, paraffin dehydrocyclization and isomerization. Dehydrogenation is strongly endothermic, producing inter-reactor temperature drops of 20–70 °C. Inlet temperatures are held at 480–540 °C in semi-regenerative units and 500–530 °C in continuous catalyst regeneration units. Pressure is a decisive variable: semi-regenerative units commonly operate at 1.4–2.8 MPa, while CCR units operate at 0.35–0.7 MPa to favor aromatics formation and suppress hydrocracking. Hydrogen-to-hydrocarbon molar ratio is maintained at 4–6 in semi-regenerative service and 2–3 in CCR service. High severity increases dehydrocyclization but also increases benzene yield, light gas production and coke deposition. Mixed xylene yield therefore peaks when C8 naphthenic intermediates are mostly converted while C8 paraffin cracking remains limited.

Commercial CCR reactors use stacked radial-flow moving-bed designs with continuous catalyst circulation. The catalyst is typically chlorided platinum-tin on high-purity alumina with platinum loading in the 0.2–0.4 wt% range and surface area of 150–220 m²/g. The regenerator removes coke in a burn zone with oxygen held below 1.0 vol% at the entry to limit temperature excursion and metal agglomeration. Regenerator peak bed temperatures are maintained below 550–600 °C. After reforming, the C8 aromatic cut is separated from nonaromatic raffinate by extractive distillation or liquid-liquid extraction using sulfolane or tetraethylene glycol. Solvent-to-feed ratios in the extractor commonly range from 3:1 to 7:1 by weight, and extractive distillation columns operate under vacuum to avoid solvent degradation above 180–200 °C. Residual nonaromatics in nitration-grade mixed xylene are controlled below 0.5 wt%, and sulfur is reduced to below 10 mg/kg.

Zeolitic disproportionation and xylene isomer distribution control

Toluene disproportionation proceeds over acidic zeolites by the stoichiometric conversion of two moles of toluene to one mole of benzene and one mole of mixed xylene. Because the reaction is acid-catalyzed and equilibrium-limited, per-pass toluene conversion is usually controlled at 30–50 wt% to limit heavy aromatic formation and catalyst coking. Operating temperatures range from 400–500 °C, with total pressure of 2–4 MPa and hydrogen-to-hydrocarbon molar ratios of 4–8. Hydrogen is not consumed stoichiometrically but is required to suppress polynuclear aromatic deposits and maintain acid-site activity. Weight hourly space velocity is commonly held at 2–4 h−1. Industrial fixed-bed units such as UOP Tatoray and ExxonMobil MTDP use modified mordenite or ZSM-5 catalysts, often promoted with nickel, molybdenum or platinum to improve hydrogenation performance and cycle life. The xylene isomer distribution from unselective disproportionation approaches thermodynamic equilibrium, with meta-xylene dominant. Near 450 °C, the C8 aromatic product typically contains 50–55 wt% meta-xylene, 22–25 wt% para-xylene, 20–24 wt% ortho-xylene and an ethylbenzene content below 5 wt% when ethylene and C8 naphthene impurities are excluded.

Selective toluene disproportionation, often abbreviated STDP, uses medium-pore ZSM-5 modified with phosphorus, boron, silicon or controlled coke to narrow effective pore openings and suppress meta- and ortho-xylene formation. Para-xylene selectivity in the xylene stream can exceed 90 wt%, but this is achieved at lower per-pass conversion, often below 20 wt%, and at higher catalyst aging rates. Sulfur above 1 mg/kg in the toluene feed accelerates deactivation and shortens cycles. Toluene disproportionation is frequently integrated with transalkylation of C9 and C10 aromatics. In a mixed C7/C9/C10 feed, toluene reacts with trimethylbenzenes and methylethylbenzenes to produce xylenes while reducing the heavy-aromatic purge. C9+ content in the feed is typically limited to 20–40 wt% to prevent excessive coking and to maintain reactor effluent benzene-to-xylene molar ratios close to 1:1. The resulting mixed xylene tends to be lower in ethylbenzene than reformate-derived material, which reduces loading on downstream para-xylene separation units.

When coke-oven light oil enters the aromatics extraction train

Coal-tar sources generate mixed xylene as part of high-temperature carbonization byproduct recovery. Crude light oil from coke ovens distills below 170 °C and contains benzene, toluene, xylene, ethylbenzene, cumene, styrene, indene, pyridine bases, phenol, carbon disulfide and thiophene homologs. The C8 fraction from coal-tar operations differs from petroleum reformate in two important respects: ethylbenzene content is often higher because of thermal cracking of alkyl aromatics, and sulfur, nitrogen and olefinic impurities are present at levels requiring aggressive hydrorefining. Light-oil yield is low relative to petroleum routes; published values for coke-oven light oil are generally below 1.5 wt% of dry coal charge. The combined BTX fraction after primary distillation may account for 60–75 wt% of light oil, but this depends on coking temperature, coal rank and byproduct recovery settings.

Acid washing with sulfuric acid was historically used to remove pyridine bases, thiophenes and unsaturated compounds. Modern plants instead use hydrotreating over sulfided Co-Mo/Al₂O₃ or Ni-Mo/Al₂O₃ catalysts at 300–400 °C, 4–8 MPa and liquid hourly space velocity of 1–3 h−1. Hydrotreating reduces thiophenic sulfur from several thousand mg/kg to below 10 mg/kg and saturates styrene and indene to prevent gum formation in downstream extraction and distillation. Coal-tar light oil hydrotreaters are often smaller than refinery naphtha hydrotreaters and exhibit higher exotherm because olefins can represent 10–20 wt% of the light oil. First-bed inlet temperature is usually limited to 250–300 °C, with interbed quench to prevent runaway polymerization of styrene and dicyclopentadiene. After hydrotreating and stabilization, the C8 cut is recovered by extractive distillation or liquid-liquid extraction. If thiophenic sulfur is not reduced, it concentrates with the xylene fraction because of boiling-point proximity and degrades acid wash color and downstream isomerization catalyst performance. Published data for trace oxygenates and nitrogen compounds in modern coal-tar mixed xylene are limited; operational boundaries for a specific light-oil fraction should be confirmed by pilot testing before full-scale recovery.

Fractional isolation of para-xylene and ortho-xylene from mixed xylene is constrained by boiling-point differences below 1 °C between para-xylene and meta-xylene. Conventional distillation is therefore not used for isomer separation. Para-xylene recovery relies on crystallization at temperatures near −40 to −60 °C or adsorptive separation over zeolitic sorbents such as KX or BaX. In crystallization, para-xylene is recovered as a solid from a feed containing about 18–23 wt% para-xylene, with mother liquor sent to isomerization. The eutectic limit prevents recovery beyond about 60–65% of the para-xylene in a single-stage crystallizer. Commercial crystallizers use scraped-surface or suspension-based designs to control crystal size and wash efficiency. Ortho-xylene is recovered by super-fractionation because its boiling point of 144.4 °C is approximately 5 °C higher than that of meta-xylene. Ortho-xylene columns may require more than 100 theoretical stages and reflux ratios above 10:1. Meta-xylene is generally not isolated unless isomerization to para- and ortho-xylene is economically justified. Throughout these operations, mixed xylene feed is pre-dried over molecular sieves to maintain water below 50 mg/kg when adsorptive separation is used, because moisture reduces sorbent capacity and promotes corrosion. Routine analytical control uses ASTM D7504-18 for trace hydrocarbon impurities by gas chromatography and ASTM D848 for acid wash color.