Atmospheric Distillation Unit vs. Vacuum Distillation Unit for Crude Oil

Crude oil refining is an intricate process involving the separation of hydrocarbons into various fractions based on their boiling points. Two key units that play an integral part in this process include Atmospheric Distillation Units (ADU) and Vacuum Distillation Units (VDU), although both operate similarly under different circumstances for distillation purposes and serve distinct needs.

This article describes the differences between ADU and VDU, how they operate together, and their applications within the oil and gas industry.

What is an Atmospheric Distillation Unit (ADU)?

Atmospheric Distillation Units (ADU) serve as the initial separation stage in crude oil refining, where raw crude is heated and broken down into various hydrocarbon fractions. Operating at normal atmospheric pressure, ADU uses heat and fractional distillation technology to separate crude into usable products by their boiling points.

Inside an ADU, crude oil is first heated to between 350- 400 °C using a furnace before entering a tall distillation column for further distillation. As its vapors rise upwards, they cool and condense at different heights to form distinct layers which ultimately separate into distinct strata.

• Light ends (gases like propane and butane)
• Naphtha (used for gasoline and petrochemicals)
• Kerosene (jet fuel and diesel)
• Gas oil (diesel and heating oil)
• Atmospheric residue (heavy bottoms sent for further processing)

The ADU is crucial because it extracts the most valuable light and mid-distillates before heavier components move to the Vacuum Distillation Unit (VDU). Without the ADU, refineries couldn’t efficiently produce fuels like gasoline, diesel, or aviation kerosene.

What is a Vacuum Distillation Unit (VDU)?

 A Vacuum Distillation Unit (VDU) is a secondary refining unit that processes the heavy atmospheric residue left over from the Atmospheric Distillation Unit (ADU). Since some high-boiling-point hydrocarbons break down at extreme temperatures, the VDU operates under reduced pressure (vacuum), typically between 10-40 mmHg. This lower pressure allows the unit to separate heavier components at moderate temperatures (400-500°C) without causing thermal cracking.

The VDU further refines the residue into valuable products such as:

• Vacuum Gas Oil (VGO) – Used as feedstock in catalytic crackers to produce gasoline and diesel products.
• Lubricating Oil Base Stocks – These are used as raw materials to produce engine oils and greases.
• Heavy Distillates – Pending further processing in hydrocrackers or cokers for further conversion, heavy distillates may be sent directly to them for extra conversion.
• Vacuum Residue – Can be further processed into bitumen or used for delayed coking applications.

By operating under vacuum, VDU maximizes hydrocarbon recovery from heavy crude fractions for maximum refinery efficiency and reduced waste production. As such, this equipment plays a vital role in making sure even heavy parts of crude oil can be utilized effectively.

Key Differences Between ADU and VDU

Atmospheric Distillation Units (ADUs) and Vacuum Distillation Units (VDUs) are both essential in crude oil refining, but they serve distinct purposes and operate under different conditions. Understanding their differences is crucial for optimizing refinery processes and maximizing product yields.

1. Operating Pressure

ADU and VDU differ primarily by their operating pressures. An ADU operates at atmospheric pressure (1 bar), enabling initial separation of crude oil into lighter fractions such as naphtha, kerosene, and diesel. In comparison, VDUs operate under deep vacuum (10- 40 mmHg), which prevents thermal cracking of heavy hydrocarbons while simultaneously permitting their distillation at lower temperatures.

2. Temperature Range

ADUs typically heat crude oil between 350-400degC, enough to vaporize lighter components while leaving heavier residues untouched by decomposition at higher temperatures. To mitigate decomposition risks associated with heavier fractions, VDUs operate between 400 and 500 °C under vacuum and reduce their boiling points for safer separation and safe separation of heavy fractions.

3. Feedstock and Products

ADU processes raw crude oil into gases, naphtha, diesel, and atmospheric residue for processing by VDU; remaining atmospheric residue from ADU processing can then be further broken down to produce vacuum gas oil (VGO), lubricant base stocks (LBSTs), and vacuum residue products.

4. Role in Refining

The ADU serves as the initial refining step, producing direct fuel products and prepping residue for subsequent processing. Meanwhile, VDU improves refinery efficiency by extracting additional value from heavy residues that might otherwise go to waste.

5. Equipment Design

Due to the high temperatures and corrosive nature of heavy residues, VDUs require specialized metallurgy and larger column diameters to handle the vacuum conditions and prevent fouling. ADUs, in comparison, have a more straightforward design optimized for atmospheric distillation.

Comparison Summary

Here is a table summarizing the key points:

Feature	Atmospheric Distillation Unit (ADU)	Vacuum Distillation Unit (VDU)
Operating Pressure	Atmospheric (~1 bar)	Vacuum (10-40 mmHg)
Temperature Range	350-400°C	400-500°C
Feedstock	Raw crude oil	Atmospheric residue from ADU
Primary Products	Naphtha, kerosene, diesel, gas oil	VGO, lubricants, heavy distillates
Key Function	Initial crude separation	Further refining of heavy residues

While the ADU and VDU both rely on distillation principles, their operational differences allow refineries to efficiently process crude oil across its entire boiling range. The ADU handles lighter fractions at atmospheric pressure, while the VDU recovers heavier components under vacuum, ensuring minimal waste and optimal resource utilization. Together, they form the backbone of modern crude oil refining.

How ADU and VDU Work Together in Crude Oil Refining

Refining crude oil requires careful coordination among multiple units that work in unison to maximize efficiency and product yield, including the Atmospheric Distillation Unit (ADU) and the Vacuum Distillation Unit (VDU). Although each performs distinct functions, their integration ensures that every part of crude oil can be utilized efficiently.

Step 1: Initial Separation in the ADU

The ADU serves as the primary distillation stage, where crude oil is heated to high temperatures (350–400°C) at atmospheric pressure. This process separates lighter hydrocarbons, such as liquefied petroleum gas (LPG), naphtha, kerosene, and diesel, from heavier fractions. These lighter products are sent for further treatment or blending, while the remaining atmospheric residue (a thick, high-boiling-point mixture) is transferred to the VDU for deeper processing.

Step 2: Secondary Processing in the VDU

Since the atmospheric residue contains valuable hydrocarbons that would degrade if distilled at higher temperatures, the VDU operates under a vacuum (50–100 mmHg). This reduced pressure lowers the boiling points of heavy molecules, allowing them to vaporize without cracking. The VDU separates the residue into vacuum gas oil (VGO), lubricating oil stocks, and vacuum residue (bitumen or asphalt).

Step 3: Downstream Optimization

The outputs from both units feed into subsequent refining processes:

ADU products (naphtha, diesel) are hydrotreated or reformed to improve quality.

VDU outputs (VGO) are processed in catalytic crackers or hydrocrackers to produce additional gasoline and diesel.

Vacuum residue can be further converted into heavy fuel oil or asphalt, or processed in cokers to extract even more light hydrocarbons.

Synergy for Maximum Efficiency

By working together, the ADU and VDU ensure that no fraction of crude oil is wasted. The ADU handles the initial separation, while the VDU recovers additional value from heavier components that would otherwise be discarded. This integrated approach enhances refinery profitability, reduces waste, and meets the growing demand for high-quality fuels and petrochemical feedstocks.

Ultimately, the collaboration between ADU and VDU exemplifies the sophistication of modern refining, where every drop of crude oil is optimized for maximum utility.

To Sum Up

Atmospheric Distillation Units (ADUs) and Vacuum Distillation Units (VDUs) both play integral parts in oil refining processes, with ADUs performing initial separation under atmospheric pressure before heavier residues are processed under vacuum to extract valuable products that add further value.

Together, they ensure efficient refining by minimizing waste and increasing production of fuels, lubricants, and petrochemical feedstocks. A thorough understanding of their differences and synergies enables refinery operations to function more efficiently for improved profitability and greater success.