Determination of Coking Characteristics of Crude Oil During Thermal Conversion

Determination of coking characteristics during crude oil thermal conversion is a specialized laboratory evaluation that quantifies the coke formation tendency of crude oils and heavy residues when heated under controlled cracking conditions. By measuring parameters such as coke yield, induction period, and coking onset temperature through autoclave conversion tests, carbon residue analysis, and thermal analysis techniques, this testing provides essential data for refinery furnace design, heat exchanger fouling control, and optimization of thermal upgrading processes such as visbreaking and delayed coking.

Technical Overview

Thermal conversion refers to the chemical transformation of crude oil and its heavy fractions when they are subjected to elevated temperatures, typically in the range of 350 °C to 500 °C. Under these conditions, the large hydrocarbon molecules in resins and asphaltenes undergo a complex sequence of reactions, including thermal cracking, dehydrogenation, condensation, polymerization, and aromatization. As these reactions progress, the heaviest molecules grow and aggregate until they become insoluble in the surrounding hydrocarbon medium and precipitate as a solid carbonaceous phase known as coke.

The determination of coking characteristics is a critical evaluation for any refinery processing heavy or unconventional feedstocks. Excessive coke formation leads to fouling of preheat exchangers, coking of furnace tubes, increased pressure drop, reduced unit run length, and unplanned shutdowns. Conversely, in deliberate carbon rejection processes such as delayed coking, accurate knowledge of coke formation behavior is essential for yield prediction and unit design. A comprehensive coking characteristics determination program therefore measures not only the final coke yield, but also the kinetics of coke formation, the induction period before coke initiation, and the threshold temperature at which significant coke generation begins.

Testing is typically performed by accredited third-party laboratories or in-house refinery research facilities using bench-scale autoclave reactors, micro carbon residue apparatus, thermogravimetric analyzers, and supporting compositional characterization methods. The resulting data are applied to establish maximum allowable furnace outlet temperatures, evaluate crude oil compatibility, assess fouling potential, and optimize the severity of thermal conversion units.

Testing Samples

Crude Oils and Heavy Feedstocks

Straight-run crude oils of all grades, including conventional light crudes, medium and heavy crudes, extra-heavy crudes, and high Total Acid Number (TAN) crudes, can be evaluated. Heavy feedstocks such as oil sand bitumen and vacuum gas oil blends are also commonly tested, as their high asphaltene and resin contents make them particularly prone to thermal coke formation.

Residual Fractions

Atmospheric residue (long residue), vacuum residue (short residue), and visbreaker residues are among the most frequently submitted sample types. These bottom-of-the-barrel fractions contain the highest concentration of coke precursors and are the primary feeds for delayed cokers, visbreakers, and residue hydroprocessing units, making accurate coking characterization essential for process design and operation.

Processed, Blended, and Specialty Streams

Thermally or catalytically processed streams, including slurry oil, clarified oil, deasphalted oil, visbreaker and coker distillates, and solvent deasphalting asphalts, may be evaluated to assess their stability and coking tendency. Blend samples of two or more crude oils or residues are routinely tested to identify synergistic or antagonistic interactions that affect sediment and coke formation during heating and storage.

Testing Items

Typical measurement parameters included in a coking characteristics determination program are listed below:

• Coke yield at specified conversion temperatures and residence times (mass percent of feed)
• Coking induction period, defined as the time elapsed before detectable coke formation begins
• Threshold (onset) temperature for coke formation under the selected test conditions
• Rate of coke formation and apparent kinetic parameters, including activation energy and reaction order
• n-Heptane insolubles (asphaltenes) before and after thermal treatment
• Toluene insolubles and quinoline insolubles as indicators of converted coke and mesophase content
• Micro Carbon Residue (MCR) and Conradson Carbon Residue (CCR) of feed and converted products
• Total sediment existent and total sediment potential after thermal aging
• SARA composition (saturates, aromatics, resins, asphaltenes) before and after conversion
• Yields of gas, naphtha, and distillate fractions generated during thermal conversion
• Colloidal stability parameters and peptizability indices of the feed and product oils

Testing Methods

High-Temperature Autoclave Thermal Conversion

The core of the determination is typically a batch autoclave or stirred high-pressure microreactor test. A weighed quantity of sample is charged into the reactor, purged with nitrogen to exclude oxygen, and heated to a target temperature between approximately 360 °C and 480 °C for a defined residence time. After rapid cooling, the reactor contents are recovered and separated by filtration and solvent extraction to quantify toluene-insoluble coke, maltenes, and volatile products. By conducting a matrix of tests at different temperatures and reaction times, the induction period, threshold temperature, and coke formation kinetics can be established.

Carbon Residue Determination

Carbon residue testing complements conversion experiments by providing a standardized measure of coke-forming propensity. Widely used methods include the Conradson Carbon Residue test (ASTM D189, GB/T 268), the Micro Carbon Residue test (ASTM D4530, GB/T 17144), and the Ramsbottom Carbon Residue test (ASTM D524). Comparing the carbon residue of the feed with that of thermally aged or converted samples reveals how heating history alters the coking tendency of the oil.

Thermogravimetric and Calorimetric Analysis

Thermogravimetric analysis (TGA) monitors sample mass loss under controlled temperature programming in nitrogen, allowing identification of volatile loss, cracking, and coke oxidation zones, along with the estimation of kinetic parameters. Pressure differential scanning calorimetry (PDSC) detects exothermic condensation and polymerization events associated with coke precursor formation, providing a rapid screening method for comparing feedstocks and additive effectiveness.

Sediment and Colloidal Stability Testing

Sediment formation is an early indicator of instability during thermal conversion. Hot filtration testing (ASTM D4870) quantifies total sediment, while the spot test (ASTM D4740) rates blend stability and aging behavior. Optical stability instruments (ASTM D7061) and the automated stability parameter method (ASTM D7157) measure the peptizing power of the oil medium and the flocculation resistance of asphaltenes, both of which correlate with coking tendency.

Compositional Characterization

SARA fractionation (ASTM D4124) separates the sample into saturates, aromatics, resins, and asphaltenes, while n-heptane insolubles determination (ASTM D6560, SH/T 0265) measures the asphaltene fraction directly. Tracking compositional changes before and after thermal treatment identifies the degree of resin conversion and asphaltene aggregation that drives coke precipitation. Elemental analysis of the produced coke, including carbon, hydrogen, sulfur, nitrogen, and metals content, further characterizes the coke quality and its behavior in downstream units.

Applications

The results of coking characteristics determination are applied across the entire value chain of heavy oil processing:

• Establishing maximum allowable crude preheat and furnace outlet temperatures that keep operation within the coke induction period, protecting fired heater tubes from coking
• Ranking crude oils and residues by fouling potential to guide feedstock selection and purchase decisions
• Evaluating crude blend compatibility to prevent asphaltene precipitation, sediment formation, and exchanger fouling in the preheat train
• Optimizing visbreaker severity to maximize conversion while avoiding coke formation in the soaker and downstream equipment
• Predicting coke yield and volatile product distribution for delayed coker design, throughput planning, and economic evaluation
• Screening antifoulant and dispersant additives by comparing coke and sediment formation with and without chemical treatment
• Supporting residue hydroprocessing operations by defining feed coking propensity and guarding against catalyst bed fouling
• Providing kinetic data for process simulation models used in the design of new thermal conversion units

FAQ

What is the coking induction period, and why is it important?

The coking induction period is the time interval between the moment a feedstock reaches conversion temperature and the first detectable formation of toluene-insoluble coke. During this period, cracking and condensation reactions occur in the liquid phase without solid precipitation. Operation within the induction period, which is strongly temperature dependent, allows refiners to heat heavy feeds aggressively while avoiding fouling and coke deposition in furnace tubes and transfer lines.

How does the autoclave coking test differ from a standard carbon residue test?

Carbon residue tests such as ASTM D189 and ASTM D4530 apply a fixed, severe heating program and report only the final carbonaceous residue, making them rapid indicators of coking propensity. Autoclave thermal conversion tests, by contrast, simulate process-relevant temperatures, residence times, and pressure conditions, and they measure the full time and temperature dependence of coke formation. The two approaches are complementary: carbon residue values correlate with ultimate coke potential, while autoclave data reveal the kinetics and induction behavior needed for equipment design and operating limits.

What temperature and pressure conditions are typically used?

Thermal conversion tests are commonly conducted between 360 °C and 480 °C under the sample autogenous pressure, or under an inert nitrogen blanket at elevated pressure, with residence times ranging from several minutes to several hours. The exact conditions are selected to match the severity of the process being simulated, such as crude preheating, visbreaking, or delayed coking.

How much sample is required for a complete evaluation?