Mixed xylene yield after transalkylation of toluene-rich benzene cuts is a process-defined metric that integrates catalytic conversion, thermodynamic equilibrium, and downstream separation losses. The feed is typically a heartcut from reformate or pyrolysis gasoline processing that contains benzene, toluene, ethylbenzene, and variable C9 aromatics; the boiling range is commonly between 80 °C and 140 °C at atmospheric pressure. Mixed xylene yield is reported on a stabilized liquid product basis as the mass of C8 aromatic isomers—para-xylene, meta-xylene, ortho-xylene, and ethylbenzene when present—per 100 kg of fresh aromatic feed, corrected for unconverted toluene and C9 aromatics and for C8 aromatics already present in the feed. In transalkylation service, two simultaneous reactions dominate: the disproportionation of toluene to benzene and xylene, represented as 2 C7H8 ⇌ C6H6 + C8H10, and the transalkylation of toluene with trimethylbenzene, represented as C7H8 + C9H12 ⇌ 2 C8H10. The second reaction is the preferred route for mixed xylene production because it does not generate benzene as a co-product; however, its contribution is limited when the feed already contains benzene and the methyl-to-ring ratio falls below the stoichiometric requirement.

Commercial fixed-bed transalkylation units processing toluene-rich benzene cuts usually operate with a hydrogen-rich recycle gas to suppress coke formation and with a catalyst bed temperature between 360 °C and 450 °C. The liquid hourly space velocity is commonly set between 1.0 h⁻¹ and 3.0 h⁻¹, while the hydrogen-to-hydrocarbon molar ratio is maintained between 3:1 and 6:1. Under these boundaries, the per-pass mixed xylene yield is strongly dependent on the concentration of C9 aromatics in the combined feed. Published data from commercial licensors indicate that a feed containing 20–30 wt% C9 aromatics and 5–10 wt% benzene can produce a per-pass mixed xylene yield in the range of 28–34 wt%, with C9 conversion of 45–60% and toluene conversion of 20–35%. If the C9 aromatic content is below 5 wt%, the disproportionately slow rate of toluene disproportionation becomes the limiting path, and the per-pass xylene yield may fall below 15 wt% even when the reactor temperature is increased to 440 °C. These figures are indicative rather than universal, because catalyst composition, binder content, and upstream splitter efficiency introduce significant batch-to-batch variance.

Mixed Xylene Selectivity Is Governed by the Feed Methyl-to-Ring Ratio, Not Conversion Alone

Feed methyl-to-ring ratio is calculated as the total number of methyl substituents divided by the total number of aromatic rings, and it provides a more reliable predictor of maximum xylene yield than conversion alone. Benzene contributes no methyl groups, toluene contributes 1 methyl group, xylene contributes 2, and trimethylbenzene contributes 3. For a feed containing 15 wt% benzene, 55 wt% toluene, 10 wt% C8 aromatics, and 20 wt% C9 aromatics, the molar methyl-to-ring ratio is close to 1.0 due to the balancing effects of benzene and trimethylbenzene. At this ratio, the thermodynamic equilibrium for transalkylation permits significant C8 aromatic formation, and the observed xylene selectivity is limited primarily by the approach to equilibrium and by zeolite pore constraints rather than by a fundamental methyl deficiency. As the benzene content increases without a corresponding increase in C9 aromatics, the ratio drops below 0.9, and benzene production from toluene disproportionation becomes increasingly unfavourable because benzene is now present as a product inhibitor. This shifts the reaction network toward undesirable dealkylation to light gases and polycyclic aromatic condensation.

The equilibrium distribution of C8 aromatic isomers at 400 °C is approximately 24% para-xylene, 51% meta-xylene, and 25% ortho-xylene on a thermodynamic basis, but the actual isomer distribution exiting a shape-selective transalkylation catalyst can be richer in para-xylene when medium-pore zeolites are used. However, shape-selective benefits are often partially negated by subsequent isomerization in the downstream xylene loop. For mixed xylene yield accounting, the para-xylene content of the C8 aromatic product is critical because downstream para-xylene recovery by adsorption or crystallization imposes a minimum feed purity. If ethylbenzene is present in the C8 fraction above 5 wt%, the para-xylene separation route becomes more energy-intensive, and some operators hydroisomerize ethylbenzene to xylenes or remove it by fractionation before transalkylation. The mixed xylene yield reported in licensor technical bulletins therefore often excludes ethylbenzene when a separate ethylbenzene removal column is installed.

In fixed-bed pilot evaluations using commercial mordenite extrudates with a binder content of 20 wt% and a crush strength of approximately 1.2 kg/mm, the transalkylation reactor is configured as a single downflow bed with a length-to-diameter ratio between 6:1 and 12:1. The feed is preheated to 350 °C before entering the reaction zone, and the catalyst bed is divided into three independently controlled heating zones to reduce adiabatic temperature rise. The observed temperature exotherm under normal operation is less than 12 °C because the hydrogen-rich gas acts as a heat sink and the transalkylation reactions are only mildly exothermic. The pressure drop across the catalyst bed at start-of-run is typically below 1.5 bar; at end-of-run, when high-boiling polycyclic aromatics accumulate at the inlet, the pressure drop may reach 3.5 bar and the unit must be scheduled for regeneration. Mixed xylene yield is monitored on the stabilized liquid product by gas chromatography using ASTM D5134 for C6–C9 aromatic distribution and ASTM D2306 for C8 aromatic isomer split. These analyses are performed on both the feed and the product so that the net xylene yield is corrected for C8 aromatics already present in the feed.

The gas chromatographic method must be calibrated with certified reference materials that contain benzene, toluene, ethylbenzene, para-xylene, meta-xylene, ortho-xylene, 1,2,4-trimethylbenzene, and 1,3,5-trimethylbenzene. A three-point calibration covering the expected concentration range is required, and the calibration curve is revalidated after every 200 injections or whenever the flame ionization detector response factor for any target aromatic deviates by more than 5%. Failure to account for C9 aromatic isomers in the feed leads to an overestimation of mixed xylene yield because partial conversion of trimethylbenzenes can produce xylene through multiple routes. If published data for a specific configuration are limited, laboratory-scale microreactor testing with the same catalyst extrudate and feed blend is recommended before scaling to a commercial unit.

What Limits Liquid Hourly Space Velocity When Benzene Cuts Are Processed Without Pre-Fractionation?

Liquid hourly space velocity is constrained by the rate of aromatic ring saturation and by the equilibrium approach of the transalkylation reaction. In a toluene-rich benzene cut that has not been fully dehexanized, the presence of residual non-aromatic hydrocarbons in the C6–C7 boiling range accelerates catalyst deactivation because these compounds crack exothermically and generate olefins that alkylate the zeolite surface. A typical lower limit for liquid hourly space velocity is 0.8 h⁻¹; below this value, the residence time is long enough for undesired hydrogenation of the aromatic ring to cyclohexane and methylcyclohexane, which reduces ring retention and mixed xylene yield. At the upper end, liquid hourly space velocities above 3.0 h⁻¹ reduce the conversion of trimethylbenzene because the residence time is shorter than the characteristic diffusion time for C9 aromatics in the zeolite micropore network. In pilot evaluations, the optimal liquid hourly space velocity for a mordenite-based transalkylation catalyst with 20 wt% binder is between 1.5 h⁻¹ and 2.5 h⁻¹ when the feed contains 10 wt% benzene and 25 wt% C9 aromatics. Within this window, the C9 aromatic conversion remains above 50% and the net mixed xylene yield is stable over a run length of 1000 h.

The hydrogen-to-hydrocarbon molar ratio also sets an operational boundary. At values below 2:1, the catalyst surface becomes hydrogen-lean and the rate of polycyclic aromatic formation increases, as evidenced by an increase in the reactor pressure drop and a decline in C9 conversion. At values above 8:1, excess hydrogen cools the catalyst bed and increases recycle compressor energy demand without improving xylene yield. The preferred operating window is therefore 3:1 to 6:1, with the lower value applied when the feed benzene content is low and the upper value applied when the feed contains measurable olefins or sulfur. Furthermore, water in the feed must be controlled below 25 mg/kg because water competes with aromatics for acid sites and, at concentrations above 50 mg/kg, causes a reversible loss of catalytic activity that can be mistaken for permanent deactivation.

Separation of the transalkylation effluent begins with a stabilizer column that removes light gases and any dissolved hydrogen; the stabilizer overhead is sent to fuel gas after amine treating, while the bottoms stream enters an aromatics fractionation train. The C8 aromatic heartcut is withdrawn from a rerun column operating at a reflux ratio between 2.0 and 4.0, and the mixed xylene yield determined at this point is sensitive to the column top pressure because benzene and toluene carryover into the C8 fraction inflates the apparent xylene mass but contaminates downstream para-xylene recovery. In commercial para-xylene units, the benzene content of the C8 feed to adsorption is kept below 200 mg/kg and the toluene content below 500 mg/kg to maintain reliable separation performance. If the transalkylation reactor is operated with a benzene-rich feed, the stabilizer and rerun column duties increase because the benzene/toluene/xylene separation becomes more difficult; this non-catalytic loss of mixed xylene yield through fractionation is frequently overlooked in bench-scale yield comparisons but can reduce the net xylene recovery by 1–3% in commercial operation.

Catalyst Deactivation Profiles and Aromatic Ring Loss in Heavy Reformate Service

Catalyst deactivation in transalkylation of toluene-rich benzene cuts occurs through three parallel mechanisms: coking, dealumination by trace water, and metal sintering when a hydrogenation promoter is present. The coking rate is highest in the inlet section of the catalyst bed where the concentration of trimethylbenzene is greatest; heavy trimethylbenzene isomers such as 1,2,4-trimethylbenzene and 1,2,3-trimethylbenzene are more reactive toward cyclization than 1,3,5-trimethylbenzene. At a reactor temperature of 420 °C and a hydrogen-to-hydrocarbon ratio of 4:1, the carbon content of the spent catalyst at end-of-run can reach 8–12 wt%, concentrated in the first 20% of the bed. This axial coke profile requires moving-bed or swing-bed operation in larger units, because a fixed-bed unit with a single reactor cannot maintain the required temperature and conversion throughout the run. The ring retention of the catalyst is defined as the percentage of aromatic rings in the fresh feed that remain as aromatic rings in the liquid product; published data for modern transalkylation catalysts indicate ring retentions above 97%, with losses primarily to light gases and polycyclic aromatics.

Dealumination of the zeolite framework is controlled by feed drying below 25 mg/kg water and by operating with a small continuous water addition in some commercial units to minimize acid site loss; however, this practice must be balanced against the reduction in acid site density. Metal-promoted catalysts containing rhenium, molybdenum, or platinum are used to hydrogenate coke precursors and extend run length, but they are sensitive to sulfur in the feed. A sulfur concentration above 0.5 mg/kg in the combined feed can produce hydrogen sulfide, which suppresses the hydrogenation function and increases the deactivation rate. Consequently, heavy reformate-derived feeds are typically hydrotreated to reduce sulfur below 0.1 mg/kg before entering the transalkylation unit. The combination of high benzene content and high trimethylbenzene content can create a process conflict: high benzene raises the equilibrium benzene concentration and reduces xylene yield, while high trimethylbenzene accelerates coke formation at the bed inlet. This conflict limits the allowable concentration of trimethylbenzene in the feed to approximately 30 wt% unless a guard bed is installed.

Quantitative determination of mixed xylene yield in routine plant service requires the use of internal standardization with a known weight of n-heptane or n-octane added to both feed and product samples. The internal standard is injected into the gas chromatograph using a split ratio of 100:1 and a flame ionization detector, with the inlet temperature set at 250 °C and the detector temperature at 300 °C. The oven program used with ASTM D5134 is typically a 50 m × 0.20 mm × 0.5 μm methylsiloxane capillary column, but equivalent columns may be used if resolution between meta-xylene and para-xylene is adequate. If the analytical method does not resolve meta-xylene and para-xylene, the xylene isomer distribution cannot be used for para-xylene yield calculations, and ASTM D2306 with a high-polarity column is required. The reported mixed xylene yield is then calculated from the normalized mass percentages of C8 aromatics in the product minus the mass of C8 aromatics in the feed, divided by the mass of fresh feed, multiplied by 100%. This procedure removes the contribution of ethylbenzene and C8 naphthenes only when the analytical method explicitly quantifies them. The repeatability of ASTM D5134 for C8 aromatics is typically in the range of 0.1–0.5 mass% absolute, depending on the compound and the column condition; therefore, yield differences of less than 1 mass% between two pilot runs may not be statistically significant. Published data for specific yield improvements at low benzene feed concentrations are limited when the feed contains more than 10 wt% benzene, because most published studies use pure toluene or toluene/C9 blends without benzene. This absence of benzene-containing feed data makes pilot verification under the exact feed composition essential.

When Trimethylbenzene Concentrations Exceed 10 wt%, Transalkylation Reactor Effluent Requires Partial Condensation Ahead of the Stabilizer

At trimethylbenzene concentrations above 10 wt% in the reactor feed, the transalkylation effluent contains elevated concentrations of unconverted C9 aromatics and partially hydrogenated C9 naphthenes. If the effluent is sent directly to a stabilizer column without partial condensation, the column top pressure must be increased above 0.5 MPa to prevent benzene and toluene losses to the fuel gas, which reduces the mixed xylene yield determined from the liquid product. A partial condenser operating at 40 °C to 60 °C separates the reactor effluent into a liquid aromatic stream and a hydrogen-rich off-gas stream before stabilization; the liquid stream contains the majority of C9 aromatics and C8 aromatics, while the off-gas stream is recycled to the reactor after amine scrubbing. In this configuration, the stabilizer column can be operated at a top pressure of 0.2 MPa to 0.4 MPa, and the C8 aromatic recovery from the stabilizer bottoms is typically above 98%. If the trimethylbenzene concentration is below 10 wt%, the partial condenser can be bypassed without a significant loss of aromatic yield.

The recycle of unconverted C9 aromatics from the partial condenser to the reactor inlet increases the effective trimethylbenzene concentration and improves the methyl-to-ring ratio. The recycle loop must be purged continuously, however, because polycyclic aromatic compounds formed by condensation of trimethylbenzenes accumulate in the recycle stream. The purge rate is typically maintained at 2–5% of the recycle flow, and the purge stream is returned to the heavy reformate fractionator or removed as fuel oil. Without this purge, the recycle stream darkens and the concentration of polycyclic aromatics exceeds 1 wt% within 500 h, leading to fouling of the preheater and reactor inlet distributor. Operational boundaries for the partial condenser include a maximum allowable pressure drop of 0.1 MPa across the condenser and a minimum liquid level in the separator to avoid hydrogen carryover to the stabilizer. The mixed xylene yield for a configuration with partial condensation is reported on a net basis after subtracting the C9 aromatic recycle purge, but the magnitude of the correction is typically below 0.5 wt% of the fresh feed.