Isomerisation Grade Xylene for PX Plants: Feed Quality Requirements and Penalties is not a single specification but a set of process-defined limits applied to the C8 aromatic cut charged to the fixed-bed isomerisation reactor. The stream typically originates from reformate splitters, selective toluene disproportionation, or transalkylation distillation; it contains para-xylene, meta-xylene, ortho-xylene, ethylbenzene, and controlled residuals of toluene, C9+ aromatics, non-aromatics, sulfur, nitrogen, water, and bromine-reactive unsaturates. Feed quality is determined by ASTM D5134 high-resolution capillary GC and by trace impurity methods because each contaminant acts through a different mechanism: noble-metal site blockage, acid-site neutralisation, chloride displacement, coke precursor formation, or heat exchanger fouling. Penalties are not uniform; they scale with recycle loop composition, catalyst type, space velocity, and reactor inlet temperature.

The limits in the table are typical for Pt/mordenite or Pt/Al₂O₃-Cl isomerisation catalysts and are adjusted to licensor-specific design bases. Published data for site-specific catalyst performance at high contaminant levels is limited, so deviations above the maxima are generally evaluated through pilot-plant or adiabatic bench-scale tests rather than linear extrapolation.

What Feed Contaminants Produce Irreversible Catalyst Deactivation in PX Isomerisation?

Total sulfur is the primary irreversible poison. The feed specification is typically 0.5 mg/kg maximum by ASTM D7039 or ASTM D7504, but sulfur speciation is equally important; thiophenic sulfur can survive upstream hydrotreating at low severity. Sulfur occupies Pt sites, and the resulting activity loss cannot be recovered by hydrogen stripping alone. Nitrogen compounds neutralise acid sites; the typical total nitrogen limit is 0.5 mg/kg by ASTM D4629, because pyridine and nitriles decompose to ammonia and displace chloride from chlorided alumina catalysts. Water is controlled to 1.0 mg/kg by ASTM D6304; on Pt/Al₂O₃-Cl systems, water hydrolyzes surface chloride and shifts the chloride balance to HCl, causing lower acid-site density and a drop in para-xylene approach to equilibrium. Oxygenates are controlled to 1.0 mg/kg where measured; methanol and MTBE decompose to water and carbon oxides, amplify coke, and interfere with chloride management. Continuous on-line analysis is typically limited to water and sulfur analysers; full trace oxygenated and nitrogen profiles are determined by laboratory GC in UOP 744 or ASTM D7504 extended methods.

When Ethylbenzene Concentration Moves Above the Unit Design Basis

In dealkylation-dominated units, ethylbenzene is the most consequential bulk compositional variable because it participates in both acid-catalysed isomerisation and metal-catalysed hydrogenolysis. The reaction consumes 1.0 mol H₂ per 1.0 mol ethylbenzene; a 1.0 wt% excess above the design basis increases hydrogen demand by approximately 2.1 Nm³/t feed and yields 0.74 wt% benzene and 0.28 wt% ethane. On a 100 t/h charge rate, a 1.0 wt% excess ethylbenzene corresponds to 1.0 t/h additional EB and approximately 211 Nm³/h hydrogen consumption. If the unit is not hydrogen-limited, the excess ethylbenzene still occupies catalytic sites and may suppress the para-xylene approach to equilibrium. Feed ethylbenzene is commonly specified at 5–15 wt%; reformate-derived C8 cuts are near the upper end, while selective toluene disproportionation cuts are often below 5 wt%. For each 1.0 wt% ethylbenzene above design, the xylene potential loss is approximately 1.0 wt% on a feed basis unless the catalyst successfully isomerises ethylbenzene to xylenes through naphthene intermediates. That pathway is kinetically slower and becomes less favourable at low hydrogen partial pressure; recycle gas partial pressure is typically raised or feed rate reduced to hold EB conversion above the threshold required to prevent ethylbenzene accumulation in the loop. Published data for specific catalyst configurations under high-EB operation is limited; the economic penalty is therefore calculated from a combination of hydrogen consumption, benzene credit, and lost para-xylene margin.

Because a C8 aromatics certificate based only on total aromatics purity can pass while the distillation endpoint fails, the boiling-range specification carries independent penalty weight. A typical isomerisation-grade C8 cut is controlled by ASTM D850, with an IBP not below 136.0 °C and a dry point not above 145.0 °C. A dry point above 145.0 °C signals C9+ aromatics carryover; each 0.1 wt% increment of C9+ aromatics increases transalkylation products and coke precursors. In production-scale equipment, C9+ carryover is often first observed as a rise in stabiliser bottoms flow, an increase in feed/effluent exchanger pressure drop, and a change in recycle gas molecular weight.

Non-aromatics are limited to 0.5 wt%. Paraffins and cycloparaffins crack on acid sites, consume hydrogen, and reduce the concentration of C8 aromatics reaching the isomerisation zone. Toluene is limited to 0.5 wt% because toluene disproportionates to benzene and xylenes and transalkylates with C9+ aromatics, disturbing the xylene pool composition. The penalty is not linear: a feed containing 0.5 wt% toluene and 0.5 wt% C9+ aromatics may produce more benzene than the sum of the two individually because the two components react with each other across acid sites. This interaction is one reason feed quality clauses in licensor guarantees are expressed as a combined maximum for toluene plus C9+ aromatics, typically 1.0 wt%.

Bromine Index, Colour Bodies, and Feed/Effluent Exchanger Fouling

Within the preheat train, bromine index is controlled at 20 mg Br/100 g by ASTM D5776 or ASTM D1492 because bromine-reactive diolefins and styrenics form gums after repeated exposure to elevated temperatures. Dissolved oxygen in tankage is an indirect specification; a maximum of 1.0 mg/kg oxygen is often set to limit colour body formation. The fouling mechanism is not instantaneous catalyst deactivation but progressive heat-transfer loss: a fouled feed/effluent exchanger raises the fired heater duty, increases the reactor inlet temperature deviation, and can force an early shutdown for cleaning. Free water is removed by tank dewatering, but soluble water at 1.0 mg/kg is damaging on chlorided alumina catalysts because it promotes chloride loss. Some operators install a 0.3 nm molecular sieve dryer or a brine dryer upstream of the reactor to hold water below 1.0 mg/kg during humid ambient conditions. Colour bodies and dissolved oxygen are monitored by ASTM D1209 platinum-cobalt colour and by oxygen-specific electrochemical or GC methods, respectively; published data for the exact fouling rate as a function of oxygen concentration in this service is limited.

In isomerisation unit troubleshooting, the feed sample is drawn from the flowing charge pump discharge rather than a static storage tank bottom because water, chloride, and particulates settle in stagnant zones. On-line feed analysers are usually placed after the feed dryer and before the feed/effluent exchanger; sample conditioning must avoid dead legs and degassing losses. Water analysers based on Karl Fischer are preferred over capacitance probes because the C8 aromatic matrix has low electrical conductivity. Sulfur analysis by ASTM D7039 has a lower detection limit suitable for 0.5 mg/kg control, but laboratory confirmation by ASTM D7504 extended GC is recommended when sulfur peaks are observed. The sampling interval for nitrogen and oxygenates is generally shortened during upstream unit upsets; a single grab sample per day is insufficient to catch intermittent carryover from reformate splitter flooding or transalkylation stabiliser upsets.

The penalty for feed quality deviations is calculated from incremental hydrogen consumption, benzene byproduct value, xylene yield loss, and catalyst cycle shortening. Operational boundaries vary with catalyst type: chlorided alumina catalysts require continuous chloride injection and strict water/oxygen limits, while zeolitic Pt/mordenite catalysts tolerate slightly higher water but remain sensitive to sulfur and C9+ coke. A feed with 0.5 mg/kg sulfur may pass a one-time certificate but cause cumulative irreversible damage if the excursion is repeated. For this reason, feed quality monitoring includes both online total sulfur and water analysers and daily laboratory confirmation by ASTM D7039 and ASTM D6304. Incompatibilities include the combination of high oxygenates with chlorided alumina systems, where decomposition water strips chloride and forms hydrochloric acid in downstream overhead condensing sections. No additive or guard bed fully compensates for poor upstream distillation; the reactor feed remains a distillate-defined cut, not an easily reconditioned stream.