Advanced treatment systems for coking unit wastewater from delayed coking and fluid coking operations, addressing high phenol, COD, and ammonia concentrations.
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Coking operations represent one of the most challenging refinery wastewater streams, requiring specialised treatment approaches to handle extreme contaminant loads.
Coking is a severe thermal cracking process used in petroleum refineries to convert heavy residual oils into lighter, more valuable products such as naphtha, diesel, and gas oils. The process leaves behind petroleum coke, a solid carbonaceous material. There are two primary coking technologies employed in modern refineries: delayed coking and fluid coking. Both processes operate at temperatures between 480°C and 520°C and generate significant volumes of process water that become heavily contaminated with complex organic and inorganic compounds.
Delayed coking, the most widely used variant, involves heating residuum in furnace tubes and then discharging it into large coke drums where cracking and polymerization reactions continue. Fluid coking uses a fluidized bed of hot coke particles where feedstock is sprayed and cracked in a continuous process. Flexicoking adds a gasification step to convert coke into a low-BTU fuel gas. Each variant produces distinct wastewater characteristics requiring tailored treatment strategies.
Coker wastewater originates from multiple process points. The fractionator overhead receiver produces sour water containing hydrogen sulphide, ammonia, and light hydrocarbons. Coke drum quench water, used to cool and fracture the coke mass, becomes laden with oil, suspended solids, and dissolved organics. Blowdown from overhead accumulators, stripper reflux drums, and product rundown systems contributes additional contaminated streams. Steam condensate from turbine drivers and pump steam jets adds thermal load and dissolved solids.
The combined coker wastewater stream typically exhibits extreme variability in flow and composition, with flow rates fluctuating based on drum switching cycles, quench sequences, and unit turndown operations. This variability necessitates robust equalization and buffering capacity in the treatment system design.
Coker effluent is characterised by some of the highest pollutant concentrations found in refinery wastewater. Chemical Oxygen Demand (COD) frequently exceeds 10,000 mg/L and can reach 50,000 mg/L during upset conditions. Phenol concentrations range from 200 to 2,000 mg/L, representing one of the most toxic and difficult-to-treat components. Ammonia nitrogen levels typically fall between 500 and 3,000 mg/L, while hydrogen sulphide and cyanide present additional toxicity concerns. Free oil, emulsified oil, and tar-like substances contribute elevated hydrocarbon content, often exceeding 500 mg/L total petroleum hydrocarbons (TPH).
Other constituents include thiocyanates, mercaptans, pyridines, cresols, and polynuclear aromatic hydrocarbons (PAHs). The high temperature of some streams introduces thermal shock potential for advanced biological treatment systems, requiring careful heat recovery or cooling integration.
Four critical contaminant categories drive treatment system design for coking operations.
Phenol and substituted phenols (cresols, xylenols) constitute the primary toxicological concern in coker wastewater. Concentrations from 200 to 2,000 mg/L inhibit conventional advanced biological treatment at levels above 50 mg/L. Phenols bioaccumulate and exert acute toxicity to aquatic life at concentrations below 1 mg/L. Specialised extraction or advanced biological acclimation is required to achieve discharge standards. Phenol removal also reduces effluent colour and odour problems associated with coking operations.
Coker wastewater COD values ranging from 5,000 to 50,000 mg/L represent among the highest organic loads in refinery operations. The COD consists of complex mixtures of aliphatic and aromatic hydrocarbons, organic acids, heterocyclic nitrogen compounds, and polymerization byproducts. High COD imposes massive oxygen demand on biological systems and can overwhelm conventional activated sludge processes without significant dilution or staged treatment. Fractionation of COD into biodegradable and refractory components guides technology selection.
Ammonia nitrogen in coker sour water frequently exceeds 1,000 mg/L as N, arising from organic nitrogen conversion during thermal cracking and steam stripping operations. Cyanide concentrations range from 10 to 100 mg/L, particularly in fluid coking effluents. Both compounds are highly toxic to aquatic ecosystems and human health. Ammonia stripping or nitrification-denitrification are required for nitrogen removal, while alkaline chlorination, ozonation, or biological degradation address cyanide destruction. Thiocyanate, a related compound, adds further nitrogen load and toxicity.
Coker wastewater contains free oil, emulsified oil, and heavy tar-like residues from coke drum quenching, fractionator overheads, and product separation. These hydrocarbons foul downstream treatment equipment, inhibit oxygen transfer in biological reactors, and can pass through conventional separation devices when emulsified. Heavy polyaromatic components and asphaltenic material contribute to sludge generation and disposal requirements. Effective oil separation at the front end of treatment protects downstream phenol and ammonia removal processes.
A five-stage integrated approach engineered to handle the extreme variability and contaminant concentrations characteristic of coking operations.
Flow and load equalization using covered basins with mechanical mixing, pH adjustment, and temperature moderation. Buffer capacity of 24 to 72 hours dampens drum cycle shocks and enables consistent downstream operation. Volatile organic compound (VOC) containment systems prevent emissions.
API separators, corrugated plate interceptors (CPI), and dissolved air flotation (DAF) units remove free and emulsified oil down to <15 mg/L. Chemical demulsification with polyelectrolytes improves separation efficiency for stable coker oil emulsions. Recovered oil is routed back to the refinery slop system.
Phenol-rich streams undergo solvent extraction using methyl isobutyl ketone (MIBK) or liquid-liquid extraction with activated carbon adsorption. Alternatively, highly acclimated activated sludge systems with extended aeration (SRT >20 days) achieve phenol biodegradation. Sequencing batch reactors (SBR) provide flexibility for shock load handling.
Air or steam stripping in packed columns removes ammonia from high-strength sour water at pH >10.5. Stripped ammonia is recovered as ammonium sulphate fertilizer or destroyed via catalytic oxidation. For lower concentrations, biological nitrification-denitrification in moving bed biofilm reactors (MBBR) achieves nitrogen removal without chemical addition.
Sand or multimedia filtration removes biological solids and residual suspended matter. Granular activated carbon (GAC) polishing adsorbs residual phenolics, COD, and colour. Optional self-cleaning filters provide continuous operation with minimal maintenance. Effluent meets refinery discharge or reuse specifications.
Design basis and performance specifications for coking effluent treatment systems.
| Parameter | Raw Coker Effluent | After Oil Separation | After Biological Treatment | Final Effluent |
|---|---|---|---|---|
| Flow Rate (m³/h) | 10 – 150 | 10 – 150 | 10 – 150 | 10 – 150 |
| COD (mg/L) | 5,000 – 50,000 | 4,500 – 45,000 | 400 – 800 | < 300 |
| BOD5 (mg/L) | 2,000 – 20,000 | 1,800 – 18,000 | < 50 | < 30 |
| Phenol (mg/L) | 200 – 2,000 | 180 – 1,800 | < 1.0 | < 0.5 |
| Ammonia-N (mg/L) | 500 – 3,000 | 500 – 3,000 | < 50 | < 10 |
| Cyanide (mg/L) | 10 – 100 | 10 – 100 | < 1.0 | < 0.2 |
| Oil & Grease (mg/L) | 100 – 1,000 | < 15 | < 5 | < 5 |
| TSS (mg/L) | 200 – 2,000 | < 100 | < 30 | < 20 |
| Temperature (°C) | 60 – 95 | 35 – 45 | 25 – 35 | 20 – 30 |
| pH | 7.0 – 9.5 | 7.0 – 8.5 | 6.5 – 8.0 | 6.5 – 8.5 |
| Sulphide (mg/L) | 50 – 500 | 50 – 500 | < 1.0 | < 0.5 |
| Thiocyanate (mg/L) | 20 – 200 | 20 – 200 | < 5 | < 2 |
Tailored solutions for every coking technology and wastewater source within the refinery coking complex.
Delayed coking is the dominant coking technology worldwide, producing wastewater from fractionator overhead systems, coke drum quenching, and product stripping. Our treatment systems handle the extreme variability associated with batch drum cycles, providing equalization capacity and robust biological acclimation for phenol and COD removal. Integrated vapour recovery prevents VOC emissions during quench events.
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Fluid coking operates continuously in a fluidized bed reactor, generating steady-state wastewater with elevated cyanide and thiocyanate concentrations compared to delayed coking. Our designs incorporate dedicated cyanide destruction stages using alkaline chlorination or advanced oxidation. Continuous operation allows optimised advanced biological treatment without the equalization demands of batch coking systems.
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Flexicoking extends fluid coking with a coke gasification section, producing a low-BTU fuel gas and reducing solid coke yield. Wastewater from flexicokers contains additional gasification-derived contaminants including higher ammonia loads and gasifier blowdown constituents. Treatment systems integrate ammonia stripping with sour water stripper (SWS) compatibility for combined processing with refinery sour water.
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Fractionator overhead receivers generate high-ammonia sour water streams requiring dedicated stripping before advanced biological treatment. Our designs route fractionator sour water through single or dual-stage sour water strippers to remove H2S and NH3, producing stripped water suitable for biological processing or reuse as desalter wash water. Stainless steel construction handles sour water corrosion.
Sour Water Treatment
Process blowdown from coker overhead accumulators, compressor knockouts, and seal pots contains concentrated sour water with intermittent slugs of liquid hydrocarbons. Three-phase separators with level control and interface detection separate oil, water, and solids before routing to downstream treatment. Automated diversion systems protect biological reactors from toxic upsets.
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Coke drum quench water becomes heavily contaminated with suspended coke fines, oil, and dissolved organics during cooling cycles. Closed-loop quench water treatment systems using sand traps, hydrocyclones, and lamella separators remove solids and enable water recycle. Blowdown from the quench loop is treated alongside other coker wastewater streams for compliance discharge.
Explore Oilgas RefineryEngineered solutions that deliver reliability, compliance, and operational efficiency for the most challenging refinery wastewater stream.
Systems designed with 3:1 turndown ratios and extended equalization handle the severe flow and concentration variability inherent to batch coking operations without process failure.
Acclimated biomass and optional solvent extraction consistently achieve >99.5% phenol removal, meeting the most stringent discharge limits and protecting receiving waters.
Treated coker effluent meets specifications for cooling tower makeup, boiler feed, or desalter wash water, reducing overall refinery freshwater consumption by up to 40%.
Containerised and skid-mounted process units enable phased installation, capacity expansion, and retrofitting into existing wastewater treatment plants without extended shutdowns.
Covered equalization basins, vapour recovery units, and enclosed biological reactors prevent benzene, toluene, and xylene emissions, ensuring compliance with MACT standards.
SCADA-integrated systems with online phenol, ammonia, and COD analysers provide real-time optimisation, early warning of upsets, and automated chemical dosing adjustment.
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High temperature (80–95°C), particulate-laden, oil-bearing. Solids settling pond + DAF + flash cooler for biological feed. Coke fines abrade pumps and piping.
High-pressure water jets release coker tube deposits. Stream: 500–2,500 mg/L TSS, oil, phenols, cyanide. Sedimentation lagoon → oil-water separator → biological.
Hard-cutting jets create coke fines suspension. Large settling basin; collected coke fines sold or recycled. Recycle clarified water back to cut.
Coke abrasion + high temperature + sulphide demand 316L or duplex in wetted parts. Pump impellers in hard-coated SS or chrome. Refractory-lined sumps near drums.
UV/H₂O₂ or ozone destroy phenol residuals upstream of biological to prevent bio inhibition. Target <50 mg/L before biological feed.
Modern cokers run closed-loop quench-cut-cool with continuous filtration and cooling. Reduces fresh water demand 80–90% vs once-through.
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