FCC Slurry Oil Coking Experiments: Laboratory Evaluation of Coking Tendency and Fouling Behavior

FCC slurry oil coking experiments are laboratory studies that evaluate the thermal stability, coking tendency, and fouling behavior of fluid catalytic cracking (FCC) slurry oil under simulated refinery process conditions. These experiments help refineries diagnose slurry circuit fouling, screen antifoulant additives, optimize operating parameters for slurry steam generators and heat exchangers, and qualify slurry oil as feedstock for downstream processes. This article provides a comprehensive overview of sample types, test items, experimental methods, applicable standards, and industrial applications of FCC slurry oil coking research.

Technical Overview

Fluid catalytic cracking (FCC) slurry oil, also known as clarified oil or decanted oil, is the heaviest liquid product withdrawn from the bottom of the FCC main fractionator. It is characterized by a high concentration of condensed polycyclic aromatics, resins, trace asphaltenes, and suspended catalyst fines. When slurry oil is circulated through the fractionator bottom, slurry pumps, heat exchangers, and slurry steam generators, the heavy aromatic and colloidal components undergo thermal condensation, polymerization, dehydrogenation, and oxidation reactions. These reactions generate insoluble mesophase precursors and coke deposits that adhere to hot metal surfaces, causing reduced heat transfer efficiency, elevated pressure drop, accelerated equipment corrosion, and in severe cases unplanned unit shutdowns.

An FCC slurry oil coking experiment reproduces these degradation mechanisms under controlled laboratory conditions, typically at temperatures between 280 °C and 450 °C. By quantifying coke yield, coking induction period, deposition rate, and the physicochemical changes of the oil before and after thermal stressing, the experiment provides actionable data for fouling mitigation, additive selection, and process optimization. Both static (batch autoclave) and dynamic (circulating loop) test geometries are used, supported by standardized carbon residue and sediment measurements in accordance with ASTM, GB, ISO, and related standard methods.

Testing Samples

Fresh FCC Slurry Oil

Representative samples taken directly from the fractionator bottoms or slurry circulation line of an operating FCC unit. Sampling is performed in sealed containers with minimal headspace to prevent oxidation and loss of light components. These samples establish the baseline coking tendency of the slurry under actual operating conditions.

Additive-Treated Slurry Oil

Slurry oil dosed with antifoulant or coking inhibitor additives at graduated concentrations, typically 50 ppm to 500 ppm. Paired testing of treated and untreated aliquots allows quantitative evaluation of additive effectiveness in suppressing coke formation and surface deposition.

Fractionated and Blended Samples

Narrow-boiling-range cuts and SARA fractions (saturates, aromatics, resins, asphaltenes) separated from the slurry oil, used to identify which fractions dominate coke formation. Blends of slurry oil with vacuum residue, decanted heavy aromatics, or delayed coking feed are also tested to assess coking behavior when slurry is routed to downstream conversion units.

Fouled Deposits and Field Samples

Coke deposits and foulant layers recovered from slurry steam generator tubes, heat exchanger bundles, and piping during turnarounds. These field samples are analyzed to determine deposit composition, catalyst fines content, and the mechanism responsible for fouling, providing a link between laboratory results and plant observations.

Testing Items

The core measurable parameters in FCC slurry oil coking experiments include:

• Coke yield, expressed as the mass percentage of coke formed relative to the initial oil charge
• Coking induction period, the time interval before rapid coke generation begins under isothermal conditions
• Coke deposition rate and coupon weight gain on heated metal surfaces
• Carbon residue values, including micro carbon residue, Conradson carbon residue, and Ramsbottom carbon residue
• Organic and total sediment content, including n-heptane insolubles and toluene insolubles
• SARA compositional changes before and after thermal aging
• Kinematic viscosity and density changes of the thermally stressed oil
• Catalyst fines content, ash content, and particle size distribution
• Elemental composition (carbon, hydrogen, nitrogen, sulfur) of both the oil and the generated coke
• Coke morphology and microstructure by optical microscopy and scanning electron microscopy (SEM)
• Thermal stability indicators, such as the onset temperature of rapid mass loss determined by thermogravimetric analysis (TGA)

Testing Methods

Static Autoclave Coking Test

A weighed charge of slurry oil is thermally stressed in a sealed batch autoclave under nitrogen at 300 °C to 450 °C for a defined duration. After cooling, the reactor contents are filtered and washed with solvents such as toluene and n-heptane to isolate the toluene-insoluble coke fraction. The coke is dried to constant mass and weighed to calculate coke yield and coking rate. Time-series experiments at multiple residence times establish the coking induction period and kinetic constants.

Dynamic Circulating Fouling Test

Slurry oil is circulated through a heated test section or pilot loop that simulates slurry steam generator or heat exchanger service. The decline of the overall heat transfer coefficient and the increase in fouling resistance are monitored continuously, while coupon and tube-section weight gains quantify deposit accumulation. This method reproduces the combined effects of fluid shear, surface temperature, and bulk oil aging on deposition behavior.

Hot Surface and Thin Film Coking Tests

A thin film of slurry oil is held on a heated metal plate, test tube, or rotating cylinder at controlled surface temperatures. Deposited carbon is quantified gravimetrically or by reflectance measurement. Thermal oxidation stability of lighter slurry fractions can additionally be screened using procedures adapted from the jet fuel thermal oxidation test principle described in ASTM D3241.

Standardized Characterization Methods

Coke precursor potential is measured as micro carbon residue per ASTM D4530, Conradson carbon residue per ASTM D189 or GB/T 268, and Ramsbottom carbon residue per ASTM D524. Asphaltene content is determined per ASTM D4124, and SARA separation is performed per ASTM D2007 or SH/T 0509. Ash content is measured per ASTM D482 or GB/T 508. Sulfur content is determined per ASTM D4294. Thermogravimetric characterization of oils and coke follows ASTM E1131 and ASTM E2550. Kinematic viscosity is measured per ASTM D445, density per ASTM D4052, and catalyst fines particle size distribution per ISO 13320 laser diffraction. Carbon, hydrogen, and nitrogen are determined per ASTM D5291.

Deposit and Morphology Analysis

Recovered coke and field deposits are examined by optical microscopy and SEM to distinguish between shot coke, sponge coke, and anisotropic mesophase structures. Energy-dispersive spectroscopy attached to the SEM identifies entrained catalyst fines (aluminum and silicon signals) and metal contamination, which clarifies whether fouling is driven by thermal coke formation, fines settling, or a combination of both.

Applications

FCC slurry oil coking experiments support a wide range of refinery engineering and research objectives:

• Screening and ranking of antifoulant additives, including determination of optimal dosage and injection location
• Troubleshooting of chronic fouling in slurry steam generators, slurry exchangers, and fractionator bottoms circuits
• Optimization of operating variables such as slurry circulation rate, furnace outlet temperature, residence time, and oxygen exclusion
• Qualification of slurry oil as feedstock for delayed coking, needle coke production, carbon black manufacture, and slurry-phase hydrocracking
• Estimation of maximum allowable tube metal temperatures and fouling margins for equipment design and revamps
• Development of coking kinetic models used in refinery simulation and fouling prediction software
• Root cause analysis of deposit samples collected during unit turnarounds

Accredited third-party laboratories and refinery research centers routinely perform these experiments to support operational decisions, additive procurement, and long-term asset reliability programs.

FAQ

What temperature range is typically used in FCC slurry oil coking experiments?

Static autoclave tests are commonly conducted between 300 °C and 450 °C, which brackets the range from fractionator bottoms conditions to steam generator tube metal temperatures. Dynamic fouling tests usually operate at bulk temperatures of 250 °C to 360 °C with surface temperatures elevated by 30 °C to 80 °C above the bulk oil temperature.

How is the coking induction period determined?

Parallel isothermal tests are run at several residence times, and the toluene-insoluble coke yield is plotted against time. The induction period is identified as the time before which coke formation remains negligible, followed by a rapid increase in coke generation. Antifoulant effectiveness is often quantified by the extension of this induction period.

What is the difference between static and dynamic coking tests?

Static autoclave tests emphasize bulk liquid phase aging and total coke yield, making them suitable for kinetic studies and additive screening. Dynamic loop tests add fluid velocity, shear, and a controlled heat transfer surface, so they better reproduce deposit adhesion and fouling resistance behavior in actual slurry heat exchanger service.

How much sample is required for a typical test program?

A single static coking run generally requires 30 g to 200 g of slurry oil, while a dynamic loop campaign may require 5 L to 20 L depending on loop volume. A complete screening program with replicate conditions typically consumes 1 kg to 5 kg of representative sample.

Which standards govern the supporting analyses?

Coke precursor and sediment analyses follow ASTM D4530, ASTM D189, ASTM D524, ASTM D482, ASTM D4124, ASTM D2007, ASTM D445, and the corresponding GB/T and SH/T methods. Thermogravimetric studies follow ASTM E1131 and ASTM E2550, and particle size determination follows ISO 13320.

Can the experiments distinguish between thermal coking and catalyst fines fouling?

Yes. Combining coke yield data with ash analysis, SEM imaging, and elemental mapping of aluminum and silicon allows the laboratory to separate organic thermal coke from inorganic catalyst fines deposition, which is essential for selecting the correct mitigation strategy.