The per-pass conversion of toluene in hydrodealkylation is governed by an operational envelope in which the principal reaction competes with aromatic ring saturation rather than by the equilibrium of the main dealkylation reaction. The primary conversion path follows C₆H₅CH₃ + H₂ → C₆H₆ + CH₄, while the competing saturation path consumes three moles of hydrogen to form methylcyclohexane, C₆H₅CH₃ + 3H₂ → C₆H₁₁CH₃. The distinction matters because the main dealkylation is strongly exothermic and entropically favored at temperatures above approximately 600°C, whereas ring saturation is exothermic but entropically disfavored at those same temperatures. Commercial reactors therefore do not run to full equilibrium conversion; they are deliberately constrained to a single-pass window of 65% to 80% toluene conversion so that methylcyclohexane, dimethylcyclopentane, and other saturation products remain below a tolerable concentration in the crude reactor effluent. Once methylcyclohexane exceeds 1.0 wt%, the downstream benzene fractionation train must operate with higher reflux ratios, and the benzene product may fail to meet non-aromatic specifications under ASTM D2359-19. Thus, the per-pass limit is best understood as a selectivity-driven limit, not a thermodynamic limit for benzene formation.

In a thermal fired-coil hydrodealkylation unit, the reaction is carried out in a radiant-coil furnace with a coil outlet temperature of 680°C to 720°C and a total pressure of 35 bar to 45 bar. The coil is typically fabricated from 9Cr-1Mo or Type 321H stainless steel, with tube outside diameters of 100 mm to 150 mm and gas-phase residence times of 10 seconds to 30 seconds. Under these conditions, the equilibrium conversion of toluene to benzene and methane is essentially complete, but the practical outlet conversion is deliberately held at 70% to 75%. Higher conversion requires higher coil outlet temperatures, which accelerate the formation of biphenyls, fluorene, and heavy polynuclear aromatics through secondary coupling reactions. The benzene selectivity can fall from 96 mol% to below 90 mol% when the coil outlet temperature is raised from 700°C to 730°C. In addition, the creep life of the radiant tubes is governed by API 530 calculations, and the maximum continuous tube wall temperature is typically limited to 750°C. This metallurgical ceiling prevents the per-pass conversion from being increased through temperature alone. The overall result is that non-catalytic thermal hydrodealkylation operates at a conversion where the cost of toluene recycle is lower than the cost of selectivity loss and accelerated furnace tube retirement.

Is Per-Pass Conversion Solely Equilibrium Limited in Non-Catalytic HDA?

The short answer is no. At 700°C and 40 bar, the standard Gibbs energy change for the main dealkylation reaction is sufficiently negative that complete conversion is thermodynamically possible, but the kinetic window for selective conversion is narrow. The competing ring saturation reaction has a large negative entropy change because it consumes three moles of hydrogen per mole of toluene, and its equilibrium constant declines sharply as temperature rises. At 600°C and 40 bar hydrogen partial pressure, methylcyclohexane formation is thermodynamically constrained but not entirely negligible; at 700°C, the equilibrium concentration drops to a very small value. However, aromatic ring saturation is observed in commercial units even when the bulk reaction temperature is well above 650°C. This occurs because saturated products are kinetically generated in cold zones such as the quench exchanger inlet, the transfer line, or inter-bed regions where the gas temperature passes through 400°C to 500°C. The per-pass conversion limit is therefore set by the need to keep the entire reaction effluent above the saturation-active temperature window until it is rapidly quenched. The main dealkylation itself is not the limiting reaction; the side reactions that become competitive at high conversion are the true constraint.

The standard enthalpy of reaction for toluene dealkylation to benzene and methane at 298 K is approximately -42 kJ mol⁻¹, while the ring saturation enthalpy is approximately -205 kJ mol⁻¹. The large difference in heat release means that local temperature excursions in a catalyst bed or furnace coil can drive ring saturation even when the average temperature is high. In a non-catalytic thermal coil, the temperature profile is not uniform across the tube cross-section. The gas near the tube wall is hotter than the bulk gas, while the gas in the centre of the tube may be 10°C to 20°C cooler. This radial temperature gradient is normally beneficial for avoiding wall coking, but it also creates a region where methylcyclohexane can form if the hydrogen partial pressure is sufficient. The practical response is to operate the coil outlet at a temperature high enough to suppress ring saturation, but not so high that heavy aromatic condensation and coke deposition accelerate. The resulting conversion window is narrow; published process design literature indicates that the upper limit of the per-pass conversion is typically 75% to 80% for thermal HDA, with benzene selectivity of 95 mol% to 98 mol%.

Thermal Degradation Pathways in Chromia-Alumina Systems

Catalytic hydrodealkylation replaces the empty coil with a fixed bed of chromia-alumina catalyst, usually containing 10 wt% to 20 wt% Cr₂O₃ on a γ-Al₂O₃ support with a surface area of 40 m²/g to 60 m²/g and a pore volume of 0.35 cm³/g to 0.45 cm³/g. Chromia is chosen because it is less active for aromatic ring hydrogenation than nickel, cobalt, or platinum group metals, but it still produces measurable amounts of methylcyclohexane at the lower operating temperatures required by the catalyst. The catalytic route operates at 600°C to 650°C, which is lower than the thermal route, and this lower temperature makes the equilibrium for ring saturation less unfavourable. At the same time, chromia catalysts slowly deactivate by carbon deposition on acid sites and by the progressive loss of surface area during regeneration. Regeneration with dilute air or steam is performed at 450°C to 550°C, but chromium oxidation to Cr(VI) must be carefully managed because of occupational exposure and environmental restrictions under REACH Annex XVII. The deactivation rate and ring saturation tendency both limit the per-pass conversion in catalytic HDA to 70% to 80%, with methylcyclohexane yields generally held below 1.0 wt% by rapid post-bed quench.

On a world-scale unit processing 200,000 t/year of toluene feed, the selection of thermal versus catalytic hydrodealkylation is determined by hydrogen purity, benzene product specifications, and utility integration. A thermal unit can tolerate lower hydrogen purity because no catalyst is present to poison, whereas a catalytic unit requires hydrogen with less than 5 mol% methane and no more than 10 ppmv hydrogen sulfide to protect the chromia phase. In both configurations, the aromatic ring saturation issue is managed by maintaining the bulk reaction zone above 650°C and reducing the effluent temperature below 300°C within 2 seconds to 5 seconds. This is accomplished with a quench exchanger or cold hydrogen injection point located within 3 m of the reactor outlet. If the quench is delayed and the gas remains in the 400°C to 500°C range, methylcyclohexane and dimethylcyclopentane isomers form in the transfer line. The exact magnitude of this effect depends on the local hydrogen partial pressure; published data for this specific quench-delay configuration is limited, but operating experience consistently identifies the quench residence time as a critical control variable. The quench exchanger is often a shell-and-tube unit with Type 321 stainless steel tubes and an inlet plenum designed to minimise stagnant gas volume.

What operational boundaries prevent methylcyclohexane co-production above 1 wt%?

The primary boundary is hydrogen partial pressure. Ring saturation consumes three moles of hydrogen per mole of toluene, so the equilibrium concentration of methylcyclohexane is highly sensitive to hydrogen partial pressure. In a thermal HDA reactor, lowering total pressure from 50 bar to 35 bar reduces the thermodynamic driving force for ring saturation, but hydrogen partial pressure cannot be reduced below about 25 bar to 30 bar without increasing coking and reducing the main dealkylation rate. The practical compromise is to operate at a total pressure of 35 bar to 45 bar with a hydrogen-to-toluene molar ratio of 3:1 to 5:1. At these conditions, the methylcyclohexane yield in a thermal unit is typically below 0.5 wt%. In a catalytic HDA unit, the lower reactor temperature of 600°C to 650°C makes ring saturation more favourable, so the hydrogen-to-toluene ratio is often increased to 5:1 to 6:1 to suppress coke, but this also increases the partial pressure of hydrogen and therefore requires a more aggressive quench. The methylcyclohexane yield is generally below 1.0 wt% when the catalyst bed outlet remains above 620°C and the effluent is quenched within 5 seconds. Any excursion of the catalyst bed outlet below 600°C or a hydrogen partial pressure above 40 bar can increase the methylcyclohexane yield above the 1.0 wt% threshold and force the benzene column to operate at an elevated reflux ratio.

The downstream separation of unconverted toluene from benzene and methylcyclohexane is a further constraint on the per-pass conversion limit. Benzene boils at 80.1°C, toluene at 110.6°C, and methylcyclohexane at 101.0°C, so methylcyclohexane is not easily separated from toluene by simple distillation. When the methylcyclohexane content in the reactor effluent exceeds 1.0 wt%, the benzene column cannot achieve the required purity without a higher reflux ratio or an additional light-ends fractionation step. The aromatics fractionation train therefore imposes a practical ceiling on the amount of ring saturation that can be tolerated. Unreacted toluene is recovered and recycled to the reactor feed, and the recycle rate is fixed by the per-pass conversion. At 70% conversion, the recycle flow is equivalent to 30% of the fresh toluene feed; at 80% conversion, it falls to 20%. However, the small saving in recycle pump and preheater duty is usually outweighed by the selectivity loss and the additional fractionation energy when ring saturation products rise. This is why the per-pass conversion limit is maintained even though the main reaction equilibrium would allow near-total conversion.

When Pressure Reduction Improves Aromatic Selectivity But Sacrifices Hydrogen Partial Pressure

Lowering the total reactor pressure shifts the ring saturation equilibrium away from methylcyclohexane because the reaction consumes three moles of hydrogen and reduces the total number of gas-phase moles. At the same time, lower pressure reduces the hydrogen partial pressure available for the main dealkylation reaction and for coke suppression. In a thermal coil reactor, reducing pressure from 45 bar to 30 bar can reduce the equilibrium methylcyclohexane concentration substantially, but the coking rate on the radiant coil surface may increase if the hydrogen-to-toluene ratio is not raised accordingly. In a catalytic fixed-bed reactor, lower pressure also reduces gas density and can lead to maldistribution across the catalyst bed, especially in a radial-flow or multi-bed design. The allowable operating pressure is therefore bounded on the low side by hydraulic constraints and coke formation, and on the high side by ring saturation equilibrium and reactor wall thickness. The optimum total pressure for thermal HDA is usually cited as 35 bar to 45 bar, while catalytic HDA units operate at 35 bar to 50 bar. The corresponding hydrogen partial pressure is 25 bar to 35 bar, which keeps the equilibrium methylcyclohexane concentration below 0.5 wt% at 680°C. Published data for this specific configuration is limited because most operators optimise pressure through proprietary furnace model simulations and catalyst vendor contracts.

The recycle hydrogen compressor and furnace design further set practical per-pass conversion limits. The recycle gas stream contains 60 mol% to 80 mol% hydrogen and 20 mol% to 30 mol% methane, with smaller amounts of ethane and ethylene from hydrocracking. A centrifugal compressor with dry gas seals is standard, and its capacity is specified for the maximum recycle rate corresponding to a per-pass conversion of 60% at turn-down. If the per-pass conversion is increased beyond 80%, the required recycle rate falls, but the methane production increases, raising the molecular weight of the recycle gas and moving the compressor operating point toward surge. The anti-surge control system must therefore be designed for a wide range of gas molecular weights, typically 4 g mol⁻¹ to 8 g mol⁻¹. The furnace radiant coil is fabricated from 9Cr-1Mo or Type 321H stainless steel, and the tube wall temperature is controlled to a maximum of 750°C by API 530 calculation. Above this wall temperature, the creep life of the radiant tubes declines rapidly, so the furnace outlet temperature cannot be increased indefinitely to drive higher conversion. In practice, the maximum continuous coil outlet temperature is limited to 720°C to 730°C, which corresponds to a toluene conversion of 70% to 75% in a single pass.

Catalytic Bed Quench Geometries and Hydrogen Recycle Constraints

In a catalytic HDA unit, the fixed-bed reactor often consists of two or three adiabatic beds with intermediate hydrogen quench. Each bed contains chromia-alumina extrudates with a diameter of 3 mm to 5 mm and a bed density of 800 kg/m³ to 900 kg/m³. The pressure drop across a 15 m bed is typically 0.5 bar to 1.5 bar at a gas hourly space velocity of 700 h⁻¹. Inter-bed quench hydrogen at 250°C to 300°C is used to control the outlet temperature of each bed, but this injection creates a local cold zone where ring saturation can occur. To minimise this effect, the quench gas is preheated to 400°C or a static mixer is used to disperse the cold gas over a length of less than 1 m. The last bed outlet is directly connected to a waste heat boiler or quench exchanger with a residence time of less than 3 seconds in the 400°C to 500°C range. If the quench exchanger has a large inlet plenum, the gas residence time in the saturation-active temperature window increases, and methylcyclohexane yield can rise from less than 0.5 wt% to more than 1.0 wt%. Operators monitor the gas temperature at the exchanger inlet and outlet using Type K thermocouples with an accuracy of ±1.5°C, and methylcyclohexane concentration is measured by online gas chromatography using ASTM D7504-21 methodology.

The hydrogen recycle stream is treated in an amine absorber to remove hydrogen sulfide and in adsorption beds for chloride and water removal before returning to the reactor. The methane byproduct from hydrodealkylation must be purged from the recycle gas loop to prevent accumulation. The purge rate is directly proportional to the per-pass conversion; at 75% toluene conversion, the methane production is roughly one mole per mole of benzene produced, and the purge gas contains 30 mol% to 40 mol% methane, 50 mol% to 60 mol% hydrogen, and 5 mol% to 10 mol% C₂+ hydrocarbons. The purge gas is sent to the plant fuel system or to a hydrogen recovery unit using pressure swing adsorption. If the per-pass conversion is increased above 80%, the purge gas becomes richer in methane and lower in hydrogen, reducing its fuel value and increasing the load on the pressure swing adsorption unit. This is another reason the per-pass conversion limit is not set solely by the main reaction equilibrium but by the integrated energy and separation balance. The aromatic ring saturation constraint is therefore embedded in the overall process design, not as a single thermodynamic threshold but as a set of interacting limits on temperature, hydrogen partial pressure, quench residence time, and downstream distillation capacity.