Zhejiang Petroleum & Chemical Co., Ltd. (ZPC) operates an integrated refining and petrochemical complex in the Zhoushan archipelago, Zhejiang Province, People's Republic of China. Publicly disclosed design capacity of the site totals 40,000,000 t/a of crude processing, divided into two phases each with 20,000,000 t/a of nameplate throughput, and includes downstream units for aromatics, olefins, and polymer production. The complex is operated under Chinese petroleum refining and petrochemical pollutant emission standards GB 31570-2015 and GB 31571-2015, and product specification testing is aligned with ASTM and ISO methods where applicable. The co-processing of refinery naphtha, liquefied petroleum gas, and aromatics intermediates creates hydrogen balance constraints, sulfur distribution obligations, and aromatic saturation requirements that propagate through every downstream unit. The site processes medium-sour imported crudes and is designed to shift naphtha between catalytic reforming and steam cracking depending on paraxylene and ethylene demand. These operating modes require continuous monitoring of desalter performance, overhead corrosion, reformer catalyst condition, steam cracker tube metallurgy, and polymer extrusion melt stability.

Crude Preheat Trains Are Subject to Naphthenic Acid Corrosion and Asphaltene Deposition

Atmospheric and vacuum distillation are the first thermal separation stages. Heat transfer in the crude preheat train is controlled to avoid high tube-wall temperatures that accelerate asphaltene flocculation and iron sulfide/coke formation in crude oil fouling. For medium-sour crudes with total acid number above 0.5 mg KOH/g, naphthenic acid corrosion is suppressed by maintaining transfer-line velocities below 2 m/s in hot sections and by specifying heater tube metallurgy of 9Cr-1Mo or 316L in side-cut and atmospheric residue service. Desalter outlet total salt content is maintained below <2 mg/L to limit hydrolytic hydrogen chloride generation in the atmospheric tower overhead. Overhead system pH in the reflux drum separated water is controlled between 6.0 and 7.5 using neutralising amine and filming inhibitor injection. Atmospheric tower overflash is maintained between 1 vol% and 3 vol% to prevent heavy fraction entrainment into gas oil products while avoiding excessive heater fuel consumption. Vacuum distillation is operated with flash zone pressure below 4 kPa to separate heavy gas oil and vacuum residue without excessive thermal cracking. Plant-specific fouling rate data for the ZPC crude unit are not publicly disclosed; however, comparable units processing medium-sour crudes typically adjust desalter wash-water flow to maintain water content in desalted crude below 0.3 vol% to reduce chloride carryover into the overhead system.

Continuous catalytic reforming of full-range naphtha at the complex reduces sulfur in naphtha hydrotreating to below 0.5 mg/kg as measured by ASTM D5453 before the platinum-promoted chlorided alumina catalyst is contacted. The CCR reactors are operated at high severity to maximize reformate aromatics yield while maintaining refinery hydrogen balance; the reformate is fractionated into light and heavy cuts, and the benzene-toluene-xylene fraction is routed to aromatic extraction. Liquid-liquid extraction with a selective polar solvent separates aromatic hydrocarbons from non-aromatics, with extract purity maintained by controlling solvent water content, extract reflux ratio, and solvent regeneration. Transalkylation and disproportionation units convert C7 and C9 aromatics to xylenes, and xylene isomerization recycles meta-xylene and ortho-xylene to para-xylene. The integrated aromatics loop is constrained by aromatic ring preservation, naphthene dehydrogenation, and transalkylation catalyst deactivation from trace sulfur and chloride breakthrough. Recycle gas water content in the CCR reformer is controlled below 5 µL/L to maintain chloride retention on the catalyst without excessive acid site loss. Aromatics composition in the reformate is measured by ASTM D5580 for benzene, toluene, and xylene distribution.

What Limits Ethylene Furnace Run Length in Naphtha and LPG Co-Cracking?

Ethylene furnace run length in naphtha and LPG co-cracking service is governed by radiant coil coke accumulation, tube metal temperature, and transfer line exchanger fouling. The cracking reaction is carried out with radiant coil outlet temperature between 820 °C and 870 °C and steam-to-hydrocarbon ratio between 0.3 kg/kg and 0.5 kg/kg; these variables are adjusted per feedstock slate to maintain product yield while reducing the rate of catalytic and radical coke formation. Tube metallurgy for modern liquid feedstock crackers is typically 35/45 Ni-Cr microalloyed with niobium and rare earth elements to form a protective chromium oxide scale; maximum tube skin temperature is normally held below 1,070 °C to avoid oxide scale spalling and aggressive carburisation. Run length is terminated when measured pressure drop across the coil increases by 30–50% relative to clean-coil baseline or when tube metal temperature reaches the operational limit. Decoking is performed with steam-air mixtures at controlled oxygen concentration, with air flow staged to avoid thermal runaway and localized tube overheating. The transfer line exchanger outlet temperature is monitored for fouling because polymerized heavy products in cracked gas can reduce heat transfer and increase pressure drop. Publicly available data for the ZPC ethylene furnace run length are limited; however, operation of liquid feedstock cracking furnaces without gas-to-liquid ratio adjustment is known to reduce run length by increasing catalytic coke accumulation from low-sulfur feedstocks.

Paraxylene recovery in the integrated complex is performed by simulated moving bed adsorptive separation using zeolitic adsorbent; the separation is operated at moderate temperature and pressure with a desorbent that is selected to produce a para-xylene-rich extract and a para-xylene-lean raffinate. Feed to the adsorptive separation unit must be free of oxygenates and water to avoid degradation of the adsorbent and desorbent; typical feed water concentration for comparable units is maintained below 10 mg/kg. Para-xylene product purity is typically required to exceed 99.8 mass% for subsequent purified terephthalic acid production, measured by gas chromatographic methods such as ASTM D5307. The main operational risks are adsorbent channeling, bed packing heterogeneity, and desorbent contamination by heavy aromatics. The isomerization loop operates at near-equilibrium conversion with selective catalysts to return meta-xylene and ortho-xylene to the adsorptive separation feed; loss of xylene occurs through ring saturation and transalkylation side reactions. Detailed configuration data for the ZPC simulated moving bed unit are not publicly disclosed, but comparable paraxylene units must maintain paraxylene recovery above 95% and paraxylene purity above 99.8 mass% to avoid excessive xylene recycle and PTA feedstock impurity accumulation. The heavy raffinate stream from the separation unit is routed to isomerization or to transalkylation after stabilizer removal, which reduces ethylbenzene and cumene concentration in the recycle loop.

Polyolefin Melt Stability and Pelletizing Torque Control

Polypropylene and polyethylene powder from downstream polymerization reactors are converted into pelletised product through twin-screw extrusion, melt filtration, and underwater pelletizing. Extrusion lines for polyolefin compounding are configured with L/D ratios between 40:1 and 52:1 and screw speeds from 200 rpm to 600 rpm, depending on polymer grade and melt index. Melt pressure at the gear pump inlet is maintained between 8 MPa and 18 MPa to ensure uniform flow to the die plate. Primary antioxidant and secondary phosphite stabilizer loadings are controlled by gravimetric feeders to prevent melt oxidation during high-shear processing; acid scavengers such as zinc stearate are added at 0.05–0.15 wt% to neutralise residual catalyst chloride. Melt flow rate is measured on a melt flow indexer according to ISO 1133-1:2022 for polypropylene and ASTM D1238-23 for polyethylene, with die temperature 230 °C and load 2.16 kg for polypropylene, and 190 °C with load 2.16 kg for polyethylene. Additive packages must avoid amine-based stabilizers in polypropylene because amine residues can cause discoloration and die plate build-up. Pre-drying of hygroscopic additives is required when ambient relative humidity exceeds 60%. Pelletizing torque is monitored continuously; excursions above the baseline by more than 10–15% indicate die plate hole blockage, melt temperature variation, or filler agglomeration in filled grades. Underwater pelletizer water temperature is maintained within 20–40 °C to control pellet surface crystallization and prevent die hole freezing.

Emissions and aqueous discharge from the integrated complex are controlled under Chinese refining and petrochemical pollutant standards. Sulfur recovered from hydrotreating off-gases and sour water stripper acid gas is routed to Claus sulfur recovery units; tail gas treating is employed to achieve sulfur recovery efficiency above 99.9%. Sour water from crackers and hydrotreaters is stripped to remove ammonia and hydrogen sulfide before biological wastewater treatment. The wastewater treatment train includes oil-water separation, dissolved air flotation, and biological nutrient removal; discharge parameters for chemical oxygen demand, ammonia nitrogen, and total phosphorus are monitored under GB 31571-2015. Volatile organic compound control includes flare gas recovery, floating-roof storage tanks, and vapor recovery on loading racks. Spent catalysts, heavy residues, and off-specification waxes are segregated and managed according to hazardous waste inventory requirements. Site-level discharge concentration data for ZPC are subject to provincial verification but are not published in detail.

Product quality and environmental monitoring methods applicable to ZPC process streamsProcess streamParameterStandard methodAcceptance rangeNaphtha hydrotreater feedTotal sulfurASTM D5453<0.5 mg/kgReformate aromaticsBenzene, toluene, xylene compositionASTM D5580product-slate-specificParaxylene productPara-xylene purityASTM D5307≥99.8 mass%Polypropylene meltMelt flow rateISO 1133-1:2022grade-specificPolyethylene meltMelt flow rateASTM D1238-23grade-specificAqueous effluentChemical oxygen demandGB 31571-2015discharge standard