Aromatics: benzene, toluene and xylene production

BTX from reformate and pyrolysis gasoline: sulfolane extraction, para-xylene recovery, xylene isomerisation, toluene disproportionation and hydrodealkylation, with stoichiometric balances.

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Why it matters

Benzene, toluene and the xylenes (BTX) are the aromatic building blocks for polystyrene, phenol resins, nylon, polyester (PTA from para-xylene) and plasticisers (phthalic anhydride from ortho-xylene). India's large PTA and polyester industry runs on para-xylene, so an "aromatics complex" downstream of a refinery reformer is a common workplace. The technology is a showcase of separations that cannot be done by plain distillation.

Key ideas

Sources of BTX.

  • Catalytic reformate: about 60–70 % aromatics, mainly toluene and xylenes; the main source.
  • Pyrolysis gasoline from naphtha steam cracking: rich in benzene, but contains dienes and styrene, so it needs two-stage hydrotreating (first stage Pd or Ni at mild conditions to saturate dienes; second stage Co–Mo to saturate olefins and remove sulphur) before extraction.
  • Coal tar light oil from coke ovens (minor today).

Aromatics extraction. Aromatics form azeotropes and have boiling points overlapping with paraffins and naphthenes of similar carbon number, so they cannot be separated from non-aromatics by ordinary distillation. A selective polar solvent is used:

  • Liquid–liquid extraction with sulfolane (or glycols in the older Udex process): aromatics dissolve in the solvent; the raffinate (paraffins, naphthenes) is washed with water to recover solvent. The extract is stripped to recover aromatics.
  • Extractive distillation (sulfolane, N-formylmorpholine) for benzene-rich narrow cuts: the solvent lowers the volatility of aromatics so non-aromatics distil overhead. The aromatic extract is then fractionated in benzene and toluene columns; the bottoms give mixed C₈ aromatics (o-, m-, p-xylene and ethylbenzene) and C₉+.

Boiling points (°C): benzene 80.1, toluene 110.6, ethylbenzene 136.2, p-xylene 138.4, m-xylene 139.1, o-xylene 144.4. o-Xylene can be taken by a large "xylene splitter", but p- and m-xylene are too close to distil apart.

Para-xylene recovery.

  • Adsorption in a simulated moving bed (UOP Parex, Axens Eluxyl) on a zeolite (Ba/K-exchanged X or Y) that selectively adsorbs p-xylene; desorbent such as p-diethylbenzene. Over 99.7 % purity and about 97 % recovery per pass.
  • Crystallisation: p-xylene freezes at 13.3 °C while m- and o-xylene freeze far lower (−47.9 and −25.2 °C), so p-xylene is crystallised at sub-zero temperatures. Recovery per pass is limited by the eutectic.

Conversion processes that rebalance supply to demand (demand is mainly for benzene and p-xylene; toluene is in surplus):

  • Xylene isomerisation: the p-xylene-depleted raffinate is isomerised back towards equilibrium (about 24 % para) over an acidic zeolite with H₂, and ethylbenzene is converted or dealkylated; the stream is recycled to the p-xylene unit.
  • Toluene disproportionation (TDP): 2C₇H₈ ⇌ C₆H₆ + C₈H₁₀ over a zeolite (e.g. ZSM-5 or mordenite) at about 400–470 °C with H₂ to limit coking; per-pass conversion about 40–50 %. Selective TDP over modified ZSM-5 gives xylenes richer in the para isomer (shape selectivity).
  • Transalkylation: toluene + C₉ aromatics → xylenes.
  • Hydrodealkylation (HDA): C₇H₈ + H₂ → C₆H₆ + CH₄, thermal (about 600–700 °C, 30–50 bar) or catalytic (Cr or Mo oxide). Converts surplus toluene to benzene when benzene is in demand.

Uses. Benzene → ethylbenzene/styrene, cumene/phenol/acetone, cyclohexane (nylon), linear alkylbenzene; toluene → benzene/xylenes, TDI, solvent; p-xylene → PTA; o-xylene → phthalic anhydride; m-xylene → isophthalic acid. Benzene is a carcinogen, so fuels limit benzene and plants control VOC emissions.

Formulas

C₇H₈ + H₂ → C₆H₆ + CH₄ (HDA) 2C₇H₈ ⇌ C₆H₆ + C₈H₁₀ (TDP: 2 mol toluene give 1 mol benzene + 1 mol xylene) C₇H₈ + C₉H₁₂ ⇌ 2C₈H₁₀ (transalkylation)

Selectivity S = mol desired product / mol reactant converted (with stoichiometric factor: for TDP, S_benzene,max = 0.5)

Product = (m_feed / M_feed) × X × S × M_product

Recovery = aromatics in extract / aromatics in feed

Molar masses: benzene 78.11, toluene 92.14, xylene 106.17, CH₄ 16.04, H₂ 2.016 g/mol.

Worked examples

Example 1 (standard): hydrodealkylation. 1000 kg of toluene is fed to an HDA reactor; conversion is 90 % and selectivity to benzene 98 % (mol benzene per mol toluene converted). Find the benzene produced and the H₂ consumed by the main reaction.

  1. Toluene = 1000 / 92.14 = 10.853 kmol; converted = 0.90 × 10.853 = 9.768 kmol.
  2. Benzene = 0.98 × 9.768 = 9.572 kmol × 78.11 = 747.7 kg.
  3. H₂ (main reaction, 1 : 1 with benzene) = 9.572 kmol × 2.016 = 19.3 kg.

Answer: about 748 kg benzene; about 19.3 kg H₂ for the main reaction.

Example 2 (GATE level): toluene disproportionation. Toluene is fed at 100 kmol/h; per-pass conversion is 45 % with no side reactions. The xylene product has an equilibrium-like para content of 24 mol %. Find the benzene, total xylenes and p-xylene produced, in kg/h, and check the mass balance.

  1. Toluene converted = 45 kmol/h; benzene = xylene = 45 / 2 = 22.5 kmol/h.
  2. Benzene = 22.5 × 78.11 = 1757 kg/h.
  3. Xylenes = 22.5 × 106.17 = 2389 kg/h; p-xylene = 0.24 × 2389 = 573 kg/h.
  4. Unconverted toluene = 55 × 92.14 = 5068 kg/h.
  5. Check: 1757 + 2389 + 5068 = 9214 kg/h = 100 × 92.14. Balanced.

Answer: 1757 kg/h benzene, 2389 kg/h xylenes, of which 573 kg/h p-xylene.

Common mistakes

  • Using 1 mol toluene → 1 mol benzene for disproportionation; it is 2 → 1 + 1.
  • Assuming reformate BTX can be separated from non-aromatics by distillation; extraction is needed first.
  • Trying to separate p- and m-xylene by distillation (boiling points differ by 0.7 °C).
  • Confusing HDA (makes benzene, consumes H₂) with TDP (makes benzene + xylenes, no net H₂ consumption).
  • Forgetting ethylbenzene in the C₈ aromatic cut.

For GATE CH

Expect questions on BTX sources, why solvent extraction is needed and which solvents are used, how p-xylene is separated and why, the reactions of TDP, transalkylation, isomerisation and HDA, and end uses of each aromatic. Numericals: stoichiometric yields with conversion and selectivity, extraction recovery and simple aromatic complex balances.

Quick check

  1. Name a solvent used for aromatics extraction.
  2. Why is adsorption or crystallisation needed for p-xylene?
  3. What are the products of toluene disproportionation?
  4. Which xylene isomer is the feed for PTA?

Answers: 1. sulfolane (also glycols, NMP, N-formylmorpholine); 2. p- and m-xylene boil within 0.7 °C of each other; 3. benzene and mixed xylenes; 4. para-xylene.

Interview questions

All Chemical Technology interview questions

Try answering each one aloud before you open it.

  1. 1.What are aromatics, and why are benzene, toluene, and xylene important in chemical engineering?Concept

    Aromatics are a class of hydrocarbons characterized by the presence of one or more benzene rings. Benzene, toluene, and xylene are important because they serve as key raw materials in the production of various chemicals and polymers. Benzene is used to produce styrene, phenol, and aniline. Toluene is used as a solvent and in the production of benzene and xylene. Xylene is used in the production of terephthalic acid and dimethyl terephthalate, which are precursors for polyester.

  2. 2.Explain the process of catalytic reforming in the production of aromatics.Concept

    Catalytic reforming is a chemical process used to convert low-octane naphtha into high-octane reformate, which contains a high concentration of aromatics like benzene, toluene, and xylene. The process involves the use of a catalyst, typically platinum-based, under high temperature and pressure. The catalyst facilitates the rearrangement of hydrocarbon molecules, increasing the octane number and producing hydrogen as a byproduct. This process is crucial for producing high-quality gasoline and aromatic compounds.

  3. 3.How is benzene separated from a mixture of benzene, toluene, and xylene?Concept

    First the aromatics must be separated from the paraffins and naphthenes in reformate or pyrolysis gasoline, which boil in the same range and form azeotropes; this is done by solvent extraction or extractive distillation with sulfolane or a similar polar solvent. The aromatic extract is then fractionated: benzene (80.1 °C) overhead in the benzene column, toluene (110.6 °C) in the next column, and mixed C8 aromatics as bottoms. The xylenes themselves need adsorption or crystallisation to isolate para-xylene, since p- and m-xylene boil only 0.7 °C apart.

  4. 4.Why is toluene used as a solvent in chemical processes?Application

    Toluene is used as a solvent because it has a good ability to dissolve a wide range of organic compounds, is relatively inexpensive, and has a moderate evaporation rate. It is less toxic than benzene, making it a safer alternative in many applications. Toluene's chemical stability and ability to mix well with other solvents also make it a versatile choice in paints, coatings, and adhesives.

  5. 5.Describe the environmental concerns associated with the production of benzene.Application

    The production of benzene poses several environmental concerns, including the release of volatile organic compounds (VOCs) and hazardous air pollutants (HAPs). Benzene is a known carcinogen, and its release into the atmosphere can contribute to air pollution and pose health risks to humans. Proper containment, monitoring, and treatment of emissions are necessary to minimize environmental impact. Additionally, benzene production processes must comply with environmental regulations to ensure safe handling and disposal.

  6. 6.Calculate the amount of benzene produced from 1000 kg of toluene by hydrodealkylation if the conversion is 85% and selectivity to benzene is 100%.Numerical

    HDA: C7H8 + H2 -> C6H6 + CH4, 1 mol benzene per mol toluene converted. Toluene = 1000 / 92.14 = 10.853 kmol; converted = 0.85 x 10.853 = 9.225 kmol. Benzene = 9.225 x 78.11 = 720.6 kg. (By toluene disproportionation the same toluene would give only half as many moles of benzene, plus xylenes.)

  7. 7.What are the safety precautions necessary when handling xylene in an industrial setting?Application

    When handling xylene in an industrial setting, it is important to use personal protective equipment (PPE) such as gloves, goggles, and protective clothing to prevent skin and eye contact. Adequate ventilation is necessary to avoid inhalation of vapors, which can cause respiratory irritation. Xylene should be stored in a cool, well-ventilated area away from sources of ignition, as it is flammable. Emergency procedures should be in place to address spills or leaks, and workers should be trained in the safe handling and disposal of xylene.

  8. 8.Explain the role of hydrogen in the catalytic reforming process.Concept

    In the catalytic reforming process, hydrogen plays a crucial role in maintaining the activity and longevity of the catalyst. It helps to prevent the formation of coke on the catalyst surface, which can deactivate the catalyst. Hydrogen is also a byproduct of the reforming reactions and can be recycled within the process or used in other refinery operations. The presence of hydrogen ensures that the reforming reactions proceed efficiently and that the desired aromatic compounds are produced.

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