Mixed xylene (CAS 1330-20-7; EINECS 215-535-7; UN 1307) is the C8 aromatic cut recovered from catalytic reforming, steam-cracker pyrolysis gasoline hydrotreating, and toluene disproportionation. A direct-plant stream typically contains 18–24 wt% ethylbenzene, 18–24 wt% p-xylene, 38–45 wt% m-xylene, and 18–24 wt% o-xylene, with the balance distributed among toluene, benzene, C9+ aromatics, and non-aromatic hydrocarbons. Because the ethylbenzene-to-p-xylene ratio depends on reactor severity and distillation cut points, the certificate of analysis is not a static document; it is the lot-specific analytical boundary that determines suitability for p-xylene extraction, solvent use, or gasoline blending. Buyers requiring Mixed Xylene COA and Export Packaging Direct from the Plant should expect loading-line sampling, batch-lock traceability, and packaging records generated at the filling point rather than terminal averages. This requirement shifts the quality-control burden from post-arrival inspection to direct plant verification at the gantry.

What Does a Direct-Plant Mixed Xylene COA Actually Certify?

A direct-plant COA under ASTM D5211-19 or ISO 5280:1999 is structured around fit-for-purpose parameters. For industrial mixed xylene, appearance is reported as clear and free of suspended matter and free water; quantitative limits are set for color, density, distillation range, sulfur, benzene, non-aromatics, and C9+ aromatics. Color is measured by ASTM D1209 and is typically controlled at ≤20 Pt-Co for export solvent grade. Density at 15.6°C is determined by ASTM D4052 and usually falls between 0.862 g/cm³ and 0.872 g/cm³. Distillation range by ASTM D850 is tightly controlled because a low initial boiling point indicates loss of C6–C7 aromatics and a high dry point signals C9+ carryover; a representative export limit is initial boiling point ≥137°C and dry point ≤143°C.

Gas chromatographic composition by ASTM D7504 is used for benzene, toluene, ethylbenzene, xylene isomers, non-aromatic hydrocarbons, and C9+ aromatics. Benzene is typically limited to ≤0.05 wt% when the material is destined for downstream solvent applications that require reduced aromatic toxicity. Total non-aromatic hydrocarbons are controlled at ≤1.0 wt% because excess paraffinic and naphthenic co-boilers reduce extraction selectivity in downstream p-xylene recovery. Total sulfur by ASTM D5453 is commonly limited to ≤1 mg/kg for catalytic-unit protection; higher sulfur shortens catalyst cycle length in isomerization and disproportionation reactors. Acidity is reported as no free acid by ASTM D847. The COA should also carry batch number, production date, shore tank or day tank identifier, loading date, and the signature of the plant quality-control chemist.

The ASTM D7504 gas chromatographic method uses effective carbon number response factors and a non-polar capillary column, typically 100 m × 0.25 mm × 0.5 µm with a 100% dimethylpolysiloxane stationary phase. This column provides separation of ethylbenzene from p-xylene and m-xylene; unresolved isomer pairs are quantified using effective carbon number logic. In direct-plant QC laboratories, the GC is calibrated with a gravimetric blend of C6–C9 aromatics and non-aromatics at concentrations bracketing the COA limits. The limit of quantitation for benzene in mixed xylene by this method is typically 0.01 wt%; for total non-aromatics, 0.05 wt%. This resolution is necessary because a benzene shift of 0.02 wt% can alter downstream regulatory classification in some jurisdictions.

Representative direct-plant mixed xylene COA matrix; contract limits may be tighter or wider depending on intended downstream unit.

Loading-line sampling rather than shore-tank average is the core difference between direct-plant COA and a trader-issued generic document. At the gantry, a composite sample is withdrawn after the transfer line has been flushed with the export lot and before the first container seal is applied. Sampling follows ISO 3171 for petroleum liquids, using either a permanently installed automatic sampler or a manual continuous drip into a 1 L amber borosilicate glass bottle with a PTFE-lined cap. A pneumatic piston sampler located downstream of the final filter delivers the sample into a cooled receiver at 4°C to minimize volatile loss; if the receiver is uncooled during warm loading, benzene and toluene may vent and generate biased COA data. The sample is split into three portions: plant retained sample, buyer sample, and third-party inspection reserve. Each portion is labeled with the COA number, container seal number, and UTC time of collection. The retained sample is held for at least 90 days after loading, or as defined by the purchase contract. Batch-lock traceability links the COA to individual drum numbers, ISO tank numbers, or the marine vessel bill of lading; this prevents commingling with off-spec terminal volumes and provides a defensible chain of custody if a shore-side quality dispute arises. On direct-plant drum lines, a recurring bottleneck is the lag between the loading-line composite and the finished drum batch; if the line is not flushed, residual previous lot can shift color and sulfur by more than the COA method uncertainty.

Direct-plant export gantries are often shared with toluene, benzene, or reformate. Positive isolation of the mixed xylene line is maintained by double block-and-bleed valves and a dedicated product filter. The export lot is cleared only after a line-flush sample confirms that the previous product has dropped below the COA limit for benzene or toluene. If the flush volume exceeds 2 m³ without meeting the target, the lot is diverted back to the day tank for re-certification. This step prevents a direct-plant COA from certifying a packaged lot that is contaminated by heel material in the transfer line.

The direct-plant COA is transmitted as a secure PDF with timestamped digital signature, and the COA number is stenciled on the drum label or ISO tank placard. A paper copy is placed in a weather-resistant envelope on the load. The electronic record includes the analyzer serial number, calibration date, and injection time. This level of metadata supports downstream audit under ISO 9001 and avoids the commercial ambiguity of an unsigned terminal summary.

Export Packaging Configurations Direct from the Producing Train

Mixed xylene is loaded direct from the plant into marine bulk, ISO tank container, drum, or IBC modes. The selection depends on flash point, vapor pressure, loading temperature, and product-contact compatibility. Flash point by ASTM D56 is approximately 25–27°C closed cup, which places mixed xylene in UN 1307, Class 3, Packing Group III. Vapor pressure at 20°C is approximately 0.8 kPa, requiring vapour recovery during open loading and inert padding for extended storage. Marine bulk parcels are loaded via articulated arms with vapour balancing; vessel tanks are either stainless steel or coated mild steel with epoxy-phenolic or zinc silicate linings. ISO tank containers are equipped with 316L stainless steel or lined carbon steel tanks. Direct-from-plant drum filling is performed on calibrated automatic scales rather than volumetric fill lines.

Mixed xylene is filled into tight-head steel drums rated UN 1A1 or open-head steel drums rated UN 1A2, with internal epoxy-phenolic lining. Filling by weight is preferred over volumetric filling because density changes with ambient temperature; a 200 L drum is typically filled to a maximum of 98% of capacity at 55°C under IMDG 4.1.1.4. For 1,000 L composite IBCs, the packaging code UN 31HA1 is acceptable for Packing Group III flammable liquids. Gaskets and seals in direct-plant loading equipment are specified as PTFE, fluorocarbon, or epoxy-phenolic; EPDM, nitrile, and natural rubber are unsuitable for prolonged contact because mixed xylene swells and extracts plasticizer fractions, causing seal failure and product contamination. Trace chloride is not routinely reported on a standard mixed xylene COA but becomes relevant for export packaging into bare stainless steel. If chloride is present above 2 mg/kg, pitting corrosion can occur in 304L stainless steel under stagnant water layers; therefore export tanks are dried and purged to a dew point below -20°C before loading. Lined carbon steel or 316L stainless steel is preferred when chloride data is unavailable.

Marine Bulk, ISO Tank, and Drum Export Compliance Matrix

The export documentation package changes with packaging mode, but the direct-plant COA remains the analytical anchor. For marine bulk, the COA is supported by the vessel tank cleaning certificate, last-cargo compatibility statement, loading-arm sample log, and bill of lading. For ISO tank and drummed shipments, the COA is linked to the container number or drum seal list and the packing certificate. Table 2 summarizes the common compliance references.

When the COA Must Distinguish Solvent Grade from p-Xylene Feedstock Grade

A single direct-plant stream may simultaneously meet solvent-grade and p-xylene feedstock-grade requirements, but the contractual COA must state which grade applies. Solvent-grade mixed xylene is governed primarily by color, distillation range, sulfur, and benzene; non-aromatic content is less critical. p-Xylene feedstock under ASTM D5211-19 requires tight p-xylene content and low C9+ aromatics because heavy aromatics depress separation efficiency in adsorption or crystallization trains. If a buyer attempts to use a solvent-grade COA for extraction-feedstock qualification, non-aromatic and C9+ data are often absent or reported with insufficient resolution. Direct-plant COA formats therefore separate the two grades: feedstock grade reports p-xylene content by ASTM D7504 with expanded isomer resolution, while solvent grade reports distillation and color with tighter aromatic-purity limits. Published data for a specific plant’s exact COA template is limited because each licensor and refinery configures the format to downstream unit constraints.

Direct-plant export packaging imposes operational boundaries that are not present in terminal liftings. The product should not be loaded during thunderstorm static-risk periods unless grounding and bonding resistance is verified below 10 Ω. The loading line should be flushed with the export lot until density and color match the COA within method repeatability. Mixed xylene should be stored away from strong oxidizers and open flames, with tanks grounded and nitrogen-blanketed if retention exceeds 30 days. Avoid contact with EPDM and nitrile elastomers. If the material is held in unlined low-density polyethylene totes, transfer periods should not exceed 24 h because xylene permeation causes swelling and loss of aromatic integrity. The direct-plant loading record should include the maximum filling temperature, density at loading, COA number, seal list, and the name of the gantry operator. This record, not a marketing summary, is the contractually enforceable quality boundary.