P-Xylene Manufacturer and Supplier: Bulk Para-Xylene for Sale

p-Xylene, CAS 106-42-3, is the para-disubstituted C8 aromatic isomer recovered from catalytic reformate, pyrolysis gasoline, and toluene disproportionation streams. The trade material for oxidation-grade terephthalic acid production is controlled primarily by ASTM D5136, with polymer-grade p-xylene typically supplied at 99.7 wt% minimum purity. The compound has a normal boiling point of 138.35°C, a freezing point of 13.25°C, a closed-cup flash point of 27°C, and a density at 20°C of 0.861 g/cm³ by ASTM D4052. Its vapor pressure at 20°C is approximately 0.87 kPa, which places storage and loading operations in the flammable-liquid envelope. Because the boiling point gap between p-xylene and m-xylene is only 0.75°C, distillation alone cannot produce polymer-grade material; industrial separation relies on selective adsorption or fractional crystallization combined with xylene isomerization. The largest use of p-xylene is continuous oxidation to purified terephthalic acid and dimethyl terephthalate, which are then polymerized into polyethylene terephthalate for bottle resin, fiber, and film. This downstream chemistry is highly sensitive to trace isomers and oxygenated impurities. The supply chain therefore treats p-xylene as an intermediate with assay-controlled limits rather than a generic solvent, and bulk terminal operations are designed around fire prevention, static control, and moisture exclusion. Certificates of analysis for export-grade p-xylene are issued against the methods shown in the specification table in the next section; the table is not a theoretical construct but a consolidation of commonly used contract limits for oxidation-grade product. Bulk shipments are moved in parcel tankers, barges, railcars, and dedicated pipeline systems; each transfer mode introduces different contamination and moisture risks that are managed by sampling under ASTM D4057 and by nitrogen-padded transfer equipment. The trade term “polymer-grade” is not a universal standard; it is defined by the intersection of ASTM D5136 limits and the purchaser’s specific PTA oxidation process. In regions where downstream PET is sold into food-contact applications, additional attention is given to trace benzene, total sulfur, and halide content because these can affect both oxidation catalyst life and final resin compliance. p-Xylene’s freezing point of 13.25°C is high enough to create cold-climate handling difficulties; railcars and storage tanks in northern locations require heating coils or recirculation systems to maintain a minimum loading temperature of 15–20°C without overheating the material. The product is sparingly soluble in water, with solubility below 200 mg/L at ambient temperature, but moisture ingress into storage is a batch-quality issue because free water in the cargo can be difficult to remove and can shift the acetic acid water balance in downstream oxidation. Thus the technical focus in bulk p-xylene supply is not simply the molecule itself but the preservation of its assay through the entire logistics chain.

What Are the Commercial Purity Requirements for Terephthalic Acid Feedstock?

The specification for oxidation-grade p-xylene is one of the tightest among bulk aromatic hydrocarbons because the downstream PTA process is operated at high throughput and low intermediate inventory. A typical export specification is provided in the following table. Individual purchase agreements may tighten the m-xylene limit to 0.15 wt% or lower when the PTA plant uses a fixed-bed hydrogenation purification step with limited capacity for isophthalic acid co-oxidation products. The method codes are contract reference methods; alternate methods may be used only after correlation under ASTM D6708 or an equivalent validation protocol. This prevents disputes when two laboratories report different values for the same cargo.

The most consequential impurity for PET color is m-xylene. In the p-xylene oxidation reactor, m-xylene is oxidized to isophthalic acid, which can co-crystallize with terephthalic acid and cannot be eliminated by standard crude TPA hydrogenation. o-Xylene oxidation produces phthalic acid species and oxygenated intermediates that alter crystal shape, reduce bulk density, and increase the load on the purification step. Ethylbenzene is more resistant to total oxidation under typical p-xylene oxidation conditions; it can persist as partially oxidized aromatic compounds in the acetic acid recycle loop and increase solvent purification requirements. Non-aromatic hydrocarbons contribute to lower oxidation selectivity and may appear in off-gas or in recycled acetic acid as light ends. Bromine index is used as a surrogate for trace olefins that can form gum deposits in heat exchangers and air feed compressors. The 1.0 mg/kg total sulfur limit is applied because sulfur compounds can accumulate in acetic acid recycle and may affect catalyst performance or contribute to corrosion in titanium and high-alloy piping. For plants producing bottle-grade PET, the m-xylene limit is most often cited as the primary supply risk; a sustained excursion above 0.20 wt% can shift the melt-phase resin color and increase the required solid-state polycondensation residence time. Published data for this specific configuration is limited for individual continuous PET lines, but licensors generally recommend holding m-xylene in the p-xylene feed below 0.15 wt% for bottle-grade operations. The certificate of analysis should also report density and distillation data because those values confirm that the cargo is not contaminated with lighter non-aromatics or heavier C9+ aromatics. The density window is narrow because p-xylene density is sensitive to isomeric contamination; a density outside 0.861–0.862 g/cm³ at 20°C can indicate m-xylene or o-xylene excursions even when GC purity is within limits due to co-elution or sampling bias.

In dedicated p-xylene oxidation units producing purified terephthalic acid, the reactor is typically a vertical bubble column made of titanium or titanium-clad carbon steel because the bromide-promoted catalyst system is highly corrosive to unprotected stainless steel. The liquid phase contains p-xylene, acetic acid, cobalt acetate, manganese acetate, and bromide; air or oxygen-enriched air is sparged through the liquid at 1.0–2.0 MPa gauge and 150–200°C. The reaction proceeds through p-toluic acid and 4-carboxybenzaldehyde intermediates. The 4-carboxybenzaldehyde content of crude TPA is the critical analytical parameter because residual aldehyde groups act as chain terminators in PET polymerization and increase color formation. The purification section employs catalytic hydrogenation in hot water to reduce 4-carboxybenzaldehyde to p-toluic acid and to convert colored impurities into water-soluble compounds; therefore, the oxidation unit is designed around an integrated mass balance rather than a single-pass conversion target. The oxygen concentration in the reactor off-gas is maintained below the flammability limit, and carbon dioxide and carbon monoxide selectivity are monitored as indicators of solvent burning. Acetic acid and water are recovered by distillation; the water content in the recycle acid is maintained within tight bounds because excess water above 5 wt% slows oxidation kinetics, while too little water increases solvent dehydration duty and raises corrosion rates on titanium-clad internals. Feed p-xylene purity interacts with this balance: m-xylene and o-xylene oxidize to isomeric benzene dicarboxylic acids that consume oxidant, consume catalyst, and alter crystallization behavior in the product crystallizers. Ethylbenzene and non-aromatic impurities tend to partition into the acetic acid recycle or the off-gas, increasing purge requirements. This is why continuous PTA plants require not only a certificate of analysis at the receiving tank but also routine in-line density and water monitoring of the feed line before the oxidation reactor. Batch-to-batch variance in m-xylene content of more than ±0.02 wt% relative to the previous cargo is operationally significant for a bubble column reactor running at high liquid level, because the concentration gradient in the column is already steep and the purification unit is not designed for rapid changes in by-product load. The equipment-failure modes observed on industrial manufacturing lines include titanium-clad weld corrosion at the acetic acid vapor/liquid interface, slurry pump wear due to crystal solids, and air distributor plugging from corrosion products and polymer-like fouling. Published data for the optimum bromide-to-metal ratio is licensor-specific, but peer-reviewed literature describes the general kinetics as first-order in p-xylene and limited by oxygen mass transfer at high conversion.

Catalyst Deactivation and Hydrogen Cofeed Constraints Are More Severe Than Feed Purity Tables Suggest

The recycle loop of a paraxylene complex contains a xylene isomerization reactor that re-equilibrates m-xylene and o-xylene in the raffinate or mother liquor. The thermodynamic ceiling for p-xylene in the isomerate is approximately 23 wt% at 380°C and declines to approximately 21 wt% at 450°C. This creates a process conflict: higher reactor temperature improves ethylbenzene conversion and reduces catalyst deactivation from coke, but it also reduces the equilibrium concentration of p-xylene. Bifunctional catalysts containing a hydrogenation metal such as platinum on a zeolitic acid support operate at 350–450°C, 0.8–2.0 MPa gauge, a hydrogen-to-hydrocarbon molar ratio of 2–6, and a liquid hourly space velocity of 2–10 h⁻¹. The hydrogen cofeed is not an inert utility; it hydrogenates coke precursors on the metal sites and maintains the metal function in a reduced state. When hydrogen partial pressure is insufficient, olefinic intermediates oligomerize, heavy aromatics condense on the acid sites, and reactor pressure drop rises. In industrial units, the earliest detectable symptom is often an increase in first-bed pressure drop of 0.1–0.3 MPa from the clean-bed baseline, followed by a decline in p-xylene approach to equilibrium and a rise in benzene yield from ethylbenzene dealkylation. Sulfur in the isomerization feed is controlled below 0.5 mg/kg for platinum-containing catalysts because even low sulfur levels selectively deactivate the metal function, leaving an acid-only catalyst that produces more cracking and lower xylene retention. Water and oxygenates are also controlled because high-temperature steam can dealuminate the zeolite and permanently reduce acid-site density. The practical catalyst bed operating window is narrower than the broad ranges in licensor manuals; for some mordenite-based catalysts, a reactor inlet temperature deviation of more than ±5°C during start-up can accelerate dealumination and shorten cycle length. This is especially true in units that process pyrolysis gasoline-derived C8 aromatics with higher olefinic or oxygenated carryover. The decision to increase throughput by raising liquid hourly space velocity must be balanced against approach to equilibrium; increasing LHSV from 6 h⁻¹ to 9 h⁻¹ may raise p-xylene production capacity at the reactor, but the p-xylene concentration in the isomerate may fall by 1–2 wt% absolute, increasing recycle volume and separation energy. Published data for this specific configuration is limited to process licensor reports because the kinetic parameters of commercial catalysts are not fully public. The general mechanism is documented in peer-reviewed zeolite catalysis literature: strong acid sites catalyze xylene isomerization and ethylbenzene transformation, while the metal function supplies hydrogenation-dehydrogenation activity and coke precursor removal.

When Ethylbenzene Content Exceeds 0.8 wt% in Mixed Xylene Feed, Splitter Performance Degrades

In the C8 aromatic fraction, ethylbenzene boils at 136.2°C, p-xylene at 138.35°C, m-xylene at 139.1°C, and o-xylene at 144.4°C. The relative volatility between ethylbenzene and p-xylene is close enough that producing an ethylbenzene-free p-xylene stream by distillation requires high reflux ratios and a large number of theoretical stages. When the mixed xylene feed to the p-xylene recovery section contains more than 0.8 wt% ethylbenzene, the xylene splitter is forced to reject ethylbenzene in the raffinate or extract depending on the sequencing, and this can degrade both product recovery and reboiler efficiency. The reason is that ethylbenzene is only slightly lighter than p-xylene; achieving the typical p-xylene product ethylbenzene specification of 0.05 wt% from an 0.8 wt% feed shifts the column operating point toward high reflux and increases the sensitivity of overhead and bottoms composition to reboiler steam fluctuations. In simulated moving bed adsorption units, ethylbenzene competes for adsorbent capacity and increases desorbent circulation. Licensor designs often allow feed ethylbenzene up to 5–15 wt%, but the actual cost penalty depends on whether the ethylbenzene is dealkylated in the isomerization reactor or purged. In fractional crystallization units, ethylbenzene is rejected to the mother liquor and can be more easily tolerated, but the mother liquor volume increases and the recycle to isomerization becomes richer in ethylbenzene. A particular operational boundary is reached when the ethylbenzene content in the mixed xylene loop exceeds 20 wt%; below this value, isomerization with ethylbenzene dealkylation is typically used, while above this value, selective ethylbenzene removal or a separate hydrodealkylation step may be required to prevent the loop from becoming ethylbenzene-limited. The failure mode in a xylene splitter is not usually catastrophic but economic: a rising ethylbenzene fraction in feed increases reboiler steam demand and can limit net p-xylene production because column capacity is consumed by an unwanted component. Operating experience on large aromatics complexes shows that the xylene splitter is often the first bottleneck when refinery reformate or pygas quality shifts toward higher ethylbenzene. The splitter internals are usually high-efficiency trays or structured packing designed for low pressure drop; fouling from olefins or gums in the C8 feed reduces tray efficiency and can produce a product that is off-spec for ethylbenzene even when reflux is increased. For this reason, feed bromine index and oxygenate content are as important as the GC assay in preserving splitter performance. Published data for the exact hydraulic limit of a given splitter is manufacturer-specific, but the separation challenge is well documented in standard distillation texts and licensor articles on C8 aromatic processing.

Simulated Moving Bed Adsorption and Crystallization Integration

The two major industrial routes for p-xylene recovery differ in separation principle, equipment failure mode, and utility consumption. Simulated moving bed adsorption uses a zeolitic adsorbent with a cyclic port-switching sequence to simulate counter-current contact between the solid adsorbent and the liquid C8 feed. The para-isomer is selectively retained within the zeolitic micropores, while the meta- and ortho-isomers pass to the raffinate. The extract stream is then fractionated to separate p-xylene from the desorbent, which is recycled. Fractional crystallization uses the difference between the freezing points of p-xylene and its isomers; p-xylene crystals are formed in scraped-surface or suspension crystallizers, separated by centrifugation, and reslurried or washed to remove adhering mother liquor. The comparative data in the following table are typical licensor-reported ranges, not guarantees for any single unit.

The recovery technologies are not standalone; each is embedded in a loop with xylene isomerization. Raffinate from SMB adsorption contains mostly m-xylene and o-xylene and is sent to isomerization. Mother liquor from crystallization is similarly recycled. The choice between the two technologies depends on feed ethylbenzene content, available low-temperature refrigerant, plot area, and the downstream PTA plant’s tolerance for trace desorbent or crystal-wash impurities. An SMB unit can achieve single-pass recovery above 97% at 99.9 wt% p-xylene purity when the adsorbent water content, desorbent composition, and port pressure drop are maintained within licensor limits. The characteristic failure modes are rotary valve leakage, adsorbent fines generation from pressure cycling, and column internals damage from hydrate formation if water enters the C8 feed. A crystallization unit is simpler in rotating equipment but more constrained by the eutectic composition; cooling below the eutectic temperature produces mixed crystals and can require reslurry washing to reach polymer-grade purity. If the reslurry ratio is too low, trapped mother liquor raises m-xylene and o-xylene carryover into the product. If the reslurry ratio is too high, p-xylene yield drops and refrigeration demand increases. In both systems, trace desorbent carryover must be measured. For SMB, extract or raffinate desorbent above 50 mg/kg indicates a fractionation problem or adsorbent bed disturbance. For crystallization, residual crystal-wash solvent in the product can interfere with PTA oxidation. Published data for specific configuration optima is limited outside licensor documentation, but the operating boundaries described above are consistently reported in public technical literature on industrial xylene separation.

Bulk p-xylene is transported as a flammable liquid under UN 1307, Class 3, Packing Group III, and is assigned to MARPOL Annex II as a Category Y noxious liquid substance for bulk chemical shipments. The closed-cup flash point of 27°C means that normal ambient cargo temperatures are below the flash point in many climates, but marine and rail transfer operations still require vapor collection and flame control because the vapor is heavier than air and can accumulate in sumps and pump pits. Storage terminals use fixed-roof tanks with internal floating roofs or nitrogen blanketing at 1.0–2.5 kPa gauge to minimize oxygen ingress and reduce VOC emissions. Transfer lines are constructed from carbon steel or 316L stainless steel; carbon steel is acceptable for dry, oxygen-free service, while 316L is preferred when chloride or water contamination is possible. Soft goods are specified as PTFE or flexible graphite; EPDM, nitrile, and natural rubber elastomers are unsuitable because xylene causes swelling, seal leakage, and premature gasket failure. Loading pumps use magnetic or tandem mechanical seals to reduce fugitive emissions. During the initial stage of tank filling, the maximum linear velocity is restricted to 1 m/s until the fill pipe is submerged, and the receiving tank is grounded and bonded to prevent electrostatic discharge. The material’s electrical conductivity is low enough to be classified as static-accumulating, so relaxation time is designed into the transfer system. Moisture pickup is managed by closed-loop nitrogen padding and by dry connections; if water content rises above 100 mg/kg, downstream PTA operators may need to dry the feed or adjust the acetic acid dehydration column balance. The shelf life of p-xylene in a sealed, nitrogen-blanketed storage tank is typically set at 12 months before reinspection because slow peroxide formation can occur if oxygen has entered. For pipeline export, previous cargo compatibility is critical: oxygenates such as MTBE, chlorinated solvents, amines, or heavy pygas must be cleared by line flushing and interface detection. Failure to do so can introduce trace components that are not visible in a standard p-xylene GC purity check but are potent catalyst poisons in isomerization or oxidation. Railcars and tank trucks are inspected for heel material, water bottoms, and valve integrity before loading. The high freezing point of 13.25°C requires heated loading arms or recirculation loops in cold regions; product temperature at the loading flange is typically maintained at 15–20°C to prevent crystallization in uninsulated lines. Published data for absolute shelf life is limited because it depends on tank history and ambient conditions, so the 12-month reinspection interval is an industry practice rather than a chemical stability limit.

Managing Trace Sulfur, Olefinic Carryover, and Exposure Limits in Export Documentation

The export dossier for bulk p-xylene includes a certificate of analysis, safety data sheet, REACH registration confirmation where applicable, and transport classification. The analytic methods should be referenced with full designations so that cargo disputes can be resolved. For purity and isomer distribution, ASTM D5136 is the primary contract method. Total sulfur is determined by ASTM D5453, bromine index by ASTM D1492, color by ASTM D1209, density by ASTM D4052, distillation by ASTM D850, and water by ASTM E1064. Sampling is performed under ASTM D4057 because the volatility and flammability of p-xylene make open sampling dangerous and analytically biased. The product is classified under the Globally Harmonized System as Flammable Liquid Category 3, Aspiration Hazard Category 1, Acute Toxicity Category 4 for inhalation, Skin Irritation Category 2, Eye Irritation Category 2, and Specific Target Organ Toxicity Single Exposure Category 3 for respiratory irritation. Occupational exposure limits in the United States are an OSHA PEL of 100 ppm as an 8-hour time-weighted average and an ACGIH TLV-TWA of 100 ppm, with an ACGIH STEL of 150 ppm. These limits are not product specifications but are used in the SDS and in terminal hygiene programs. Trace benzene is not a routine p-xylene specification, but it may be reported in some export dossiers because it affects waste-water stripping and personnel exposure at jetty operations. The most common analytical disputes in bulk p-xylene transactions are m-xylene co-elution, total sulfur repeatability at the 1 mg/kg level, water content differences between shore tanks and ship tanks, bromine index method bias, and sampling line contamination. Each dispute is resolved by referencing the named method and by comparing sample chain of custody under ASTM D4057, not by renegotiating the specification. For REACH compliance, the legal entity placing the substance on the European market must verify that the registration dossier covers the imported grade and impurity profile; the exact registration number is entity-specific and cannot be transferred in a generic document. For food-contact PET applications, downstream converters often request a statement that the p-xylene was produced and transported without contact with prohibited substances; this is a supply-chain documentation requirement rather than a chemical property of p-xylene. The operational boundary for trace sulfur is not only catalyst life in downstream isomerization; sulfate and sulfonic acid species can partition into acetic acid recycle and interact with titanium metallurgy, so the certificate of analysis should be retained for the full life of the cargo batch. Olefinic carryover measured by bromine index is a leading indicator of gum formation in heat exchangers and reboilers; an increase from 20 mg/100 g to 40 mg/100 g may not violate a broad specification, but it is operationally significant for a PTA plant with fixed heat-exchanger cleaning intervals. Published data for exact threshold effects is limited, so conservative supply contracts hold bromine index below 20 mg/100 g and total sulfur below 1.0 mg/kg.