To refine vacuum gas oil (VGO) efficiently, engineers must know its physical and chemical characteristics. Vacuum Gas Oil properties determine the best processing route (hydrocracking vs. FCC), the required pre-treatment, and the final product yields. The Vacuum Gas Oil Market trades VGO from different crude sources, each with distinct properties. For refinery process engineers, crude oil traders, and quality control managers, understanding these properties is essential for optimizing operations and maximizing profitability. This guide provides a detailed overview of key VGO properties and their impact on refining.

How VGO is Produced: [Vacuum Gas Oil distillation]
Vacuum Gas Oil distillation is the primary production method. After atmospheric distillation, the reduced crude bottoms are fed to a vacuum distillation tower operating at 10-50 mmHg absolute pressure. The low pressure allows heavy hydrocarbons to vaporize at lower temperatures (350-400°C), preventing thermal cracking. The vapors are condensed, yielding VGO (boiling range 340-550°C / 650-1020°F). The residue (vacuum residue) is used for asphalt, fuel oil, or coking. The cut point (initial and final boiling point) determines whether the VGO is Light VGO (LVGO) or Heavy VGO (HVGO). LVGO is lighter (lower boiling point) and easier to crack; HVGO is heavier and more aromatic.

Key Vacuum Gas Oil Properties

1. Density (API Gravity)

• Definition: A measure of how heavy or light the VGO is relative to water. Higher API gravity (e.g., 25-30) indicates lighter, more paraffinic VGO. Lower API (e.g., 10-20) indicates heavier, more aromatic VGO.

• Typical range: 10-35° API.

• Impact on processing:

  • Lighter VGO (higher API): Easier to crack; produces higher yields of gasoline and diesel. Lower coke formation. Preferred for FCC.

  • Heavier VGO (lower API): Requires more severe cracking; produces more coke and hydrogen (if hydrocracking). Can lead to catalyst deactivation in FCC (due to metals).

• Relationship to other properties: Higher API correlates with lower sulfur, lower metals, and lower Conradson carbon residue (CCR).

2. Sulfur Content (wt%)

• Definition: The percentage of sulfur in the VGO. Ranges from 0.1% (low sulfur) to 3-4% (high sulfur). Crude source determines this: Light sweet crude yields low-sulfur VGO; heavy sour crude (e.g., Arab Heavy, Maya) yields high-sulfur VGO.

• Impact:

  • FCC: High sulfur VGO produces high-sulfur FCC gasoline and light cycle oil (LCO), requiring expensive post-treatment (hydrodesulfurization, HDS) to meet gasoline and diesel sulfur specs (10 ppm).

  • Hydrocracking: The process itself removes >99% of sulfur, producing ultra-low sulfur diesel (ULSD) directly. However, high sulfur feeds increase hydrogen consumption and may poison noble metal catalysts. Pre-treatment (hydrotreating) is often used.

• Sulfur compliance: Refiners must comply with IMO 2020 (marine fuel sulfur 0.5%) and Tier 3 gasoline (10 ppm). High sulfur VGO is less desirable for FCC due to post-treatment costs.

3. Viscosity

• Definition: Resistance to flow. Measured in centistokes (cSt) at 50°C or 100°C.

• Typical range: 10-200 cSt at 50°C.

• Impact:

  • Higher viscosity VGO requires higher pumping energy and may cause poor atomization in the FCC riser. It can lead to incomplete vaporization and increased coke formation.

  • For storage and transport, viscosity determines heating requirements. High viscosity VGO may need to be heated (>50°C) for pipeline transport.

4. Conradson Carbon Residue (CCR), wt%

• Definition: A measure of the carbon-forming tendency of the VGO under thermal cracking conditions. Indicates the amount of coke that will form in the FCC or hydrocracker.

• Typical range: 0.1-5 wt%.

• Impact:

  • High CCR (>2%): Leads to high coke yield in FCC, increasing regenerator temperature (which can damage catalyst) and reducing liquid yield. It may also deposit metals on the catalyst.

  • Hydrocracker: High CCR requires more hydrogen to saturate aromatic rings and can plug the catalyst bed if not properly pre-treated.

• CCR is critical for FCC operators. High CCR VGO is often sent to a hydrocracker or coker instead of FCC.

5. Metals Content (Nickel, Vanadium, Iron, ppm)

• Definition: Trace amounts of metals from the crude oil. Nickel (Ni) and Vanadium (V) are the most problematic.

• Typical range: Ni+V: 1-50 ppm.

• Impact:

  • FCC: Nickel and vanadium deposit on the catalyst, causing deactivation (poisoning) and increasing hydrogen and coke production. Vanadium also destroys the zeolite structure. High metals VGO requires catalyst metals traps (e.g., antimony, bismuth) or is sent to a hydrocracker (which can tolerate metals to some extent).

  • Hydrocracker: Metals deposit on the catalyst, causing pressure drop and requiring catalyst replacement. A guard bed (demetallization catalyst) can be used upstream.

6. Nitrogen Content (wt%)

• Definition: Percentage of nitrogen in the VGO. Can be basic nitrogen (amines) or non-basic.

• Typical range: 0.1-1.0 wt%.

• Impact:

  • FCC: Basic nitrogen poisons the acid sites of the zeolite catalyst, reducing conversion (less gasoline, more coke).

  • Hydrocracker: Nitrogen is converted to ammonia (NH₃) which can react with H₂S to form ammonium bisulfide (NH₄HS), causing corrosion and fouling in downstream equipment. Must be removed via wash water.

7. Aromatic Content (wt%)

• Definition: Percentage of aromatic hydrocarbons (single rings, polycyclic aromatics).

• Typical range: 20-70%.

• Impact:

  • High aromatics: Leads to lower hydrogen content, higher density, higher CCR. Produces less gasoline/diesel in FCC; in hydrocracker, consumes more hydrogen and produces lower-quality products (unless severe hydrotreating is applied).

  • Low aromatics (paraffinic): Higher hydrogen content, easier to crack, yields more valuable products.

8. Aniline Point (°C)

• Definition: The temperature at which equal volumes of aniline and the VGO sample become miscible. It indicates the solubility of the VGO in aniline, which correlates with aromaticity.

• Typical range: 60-100°C.

• High aniline point (less aromatic): Better cracking feedstock, produces higher yields.

• Low aniline point (more aromatic): Poorer cracking feedstock.

9. Hydrogen Content (wt%)

• Definition: The percentage of hydrogen in the VGO (by weight). Typically 10-13 wt%.

• Impact: Higher hydrogen content means less coke formation and higher product yields. Hydrocracking adds hydrogen to the feed, so low hydrogen feeds require more hydrogen input (higher operating cost).

Impact of Properties on [Vacuum Gas Oil price]
Vacuum Gas Oil price is highly dependent on its properties. Low sulfur, low metals, high API, low CCR VGO (sweet, light) commands a premium because it yields more high-value products with less processing. High sulfur, high metals, high CCR VGO (sour, heavy) is discounted because it requires expensive hydrotreating or yields poorer products. The price spread between high-quality and low-quality VGO can be $50-150 per metric ton.

Property Interrelationships and Trade-offs

• Low API ↔ High density ↔ High CCR ↔ High metals ↔ High sulfur ↔ High viscosity. These properties often occur together (heavy, sour crude).

• High API ↔ Low density ↔ Low CCR ↔ Low metals ↔ Low sulfur ↔ Low viscosity. Light, sweet crude.

• Refiners must analyze VGO properties holistically. A VGO that is high in sulfur but low in metals and CCR may be suitable for hydrocracking. A VGO that is low in sulfur but high in CCR may be sent to a coker rather than FCC.

VGO Property Specifications for Different Processes

Testing and Analysis of VGO Properties

• ASTM D287: API gravity.

• ASTM D4294: Sulfur (X-ray fluorescence).

• ASTM D445: Viscosity.

• ASTM D189: Conradson carbon residue.

• ASTM D5185: Metals (ICP).

• ASTM D4629: Nitrogen (chemiluminescence).

• ASTM D6379: Aromatic content (HPLC).

• ASTM D611: Aniline point.

Conclusion
Vacuum Gas Oil properties determine its value as a refinery feedstock. Key properties include API gravity (density), sulfur content, viscosity, Conradson carbon residue (CCR), metals (Ni, V), nitrogen content, and aromaticity. These properties impact the choice between Vacuum Gas Oil hydrocracking vs fluid catalytic cracking. Lighter, sweeter VGO (low sulfur, low metals, low CCR) is preferred for FCC and commands a premium Vacuum Gas Oil price. Heavier, sour VGO is often sent to hydrocracking or coking. Vacuum Gas Oil distillation cut points determine the boiling range and influence properties. A thorough understanding of VGO properties allows refiners to optimize their crude slate, select the appropriate processing technology, and maximize profitability. The Vacuum Gas Oil Market is driven by the demand for these intermediate feedstocks.

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