Vapor Recovery Units (VRU)
Engineered Packaged Solutions for Vapor Control
What Are Vapor Recovery Units (VRU’s)?
A Vapor Recovery Unit (VRU) is an engineered compression package, which aims to lower emissions levels coming from the vapors of gasoline or other fuels while recovering valuable hydrocarbons to be sold or reused as fuel onsite. A package for vapor recovery is designed to capture about 95% of Btu-rich vapors, generating many benefits, guaranteeing less air pollution, and recovering gasoline vapors to be used as fuel.
Benefits of a VRU System
- Profitability & Reduced Operating Costs: Recovered vapors can be re-ejected in oil wells, sent to users, or used as fuel, resulting in increased revenue or cost reduction for the facility.
- Social and Environmental Responsibility: Venting is eliminated and emissions of dangerous pollutants are reduced up to 95%. This demonstrates a commitment to social responsibility and environmental protection.
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Vapor Recovery System Applications
How Vapor Recovery Units Work
How Vapor Recovery Units for Tank Farms or Upstream Process Separators Work
When crude oil is stored in storage tanks, the vapor space above the liquid will become saturated with VOCs to reach equilibrium. Changing liquid levels due to loading and unloading of the tank will cause changes in the vapor space.
As the tank is unloaded (drained) vapor must be added into the tank above the liquid to maintain a slightly positive pressure inside the tank to keep it from collapsing as the liquid is evacuated. As the tank is loaded (filled) the vapor above the liquid must be removed to keep the tank from over pressuring and potentially rupturing.
Ambient temperature changes can also change the vapor pressure inside the tank above the liquid and can lead to the venting of the VOC saturated vapor to the atmosphere or allow to air ingress into the tank, if the tank is rapidly cooled, forming a potentially explosive atmosphere inside the tank. The VOC containing vapor that is vented through boil-off (hot ambient days) or through loading operations can be recovered and compressed.
Tank Vapor Recovery Unit - Process Flow
The gas (1) coming from the tank farms enters
the Liquid Ring Compressor (2) along with the process water (3).
After the
compression phase, the gas, water and hydrocarbon mixture enters the separator
(4) where the three elements are parted: the gas passes through a demister (5)
to remove water droplets and leaves the vessel from the top, while
condensed hydrocarbons and water are separated from the vapor stream by gravity
due to reduced gas speed.
The water is sent back to the compressor, after
being cooled by a cooler (6). Moreover, continuous process water make up line
(7) in the compressor, suction is provided to ensure a continuous water ring in
the compressor. The condensed hydrocarbons are discarded or put back into the
tanks (8). The gas exiting the separator finally heads to the Gas Gathering
System (9).
Which Types of Compressors for Vapor Recovery
Which Types of Compressors for Vapor Recovery
The most commonly used compressors (rotary and mechanical) used for VRUs include:
- Liquid Ring Compressors
- Flooded rotary screw
- Rotary sliding vane
Garo Vapor Recovery Units are built around the liquid ring compressor, which compresses the gases using a seal liquid. The process liquid is fed into the machine and, by centrifugal acceleration, forms a moving cylindrical ring, which takes the shape of the double-lobe eccentric casing.
This liquid ring creates a series of seals in the space between the impeller vanes forming the compression chamber. The gas is fed into the machine by a distribution cone and it is compressed by the rotation of the impeller inside the seal liquid.
Liquid ring gas compressors have been used successfully in vapor recovery systems for more than a decade. There are distinct advantages of using Garo liquid ring compression for vapor recovery. The compression occurs inside a liquid bath (typically water), making the compression cycle intrinsically safe.
The liquid bath also absorbs the majority of the heat generated by compression, actively cooling the gas vapors during the compression cycle. The seal liquid is also cooled to remove heat and maintain near isothermal compression within the system.The liquid bath is also effective at removing fine particulate matter entrained in the vapor stream. Garo liquid ring compressors can also withstand some small amount of free liquids in the incoming gas stream, potentially removing the need for a suction knock-out.
Are there different types of Vapor Recovery Units(VRUs)?
Yes, there are two completely different pieces of engineered equipment for different applications that are both referred to as Vapor Recovery Units.
What is the difference between a mechanical and a carbon bed or adsorption type vapor recovery unit(VRU)?
A mechanical VRU is a compression package that is often used to recover tank vapors- gas formed when liquid in a storage tank is heated by outdoor temperatures and collects in a space at the top of the tank. Tank vapor is routed to a mechanical VRU, where it is then compressed and sent to a pipeline that delivers the gas to another part of the facility or to a sales pipeline.
A carbon bed or adsorption type VRU is essentially a large filtration system. These are used to handle vapors that are offset during liquid loading of trucks, railcars, marine vessels, or tanks. Vapor is routed to the VRU, where it passes through a bed of activated carbon that adsorbs hydrocarbons and allows clean air to exit the system. When the carbon bed reaches its maximum capacity, a vacuum pump can then extract the hydrocarbon vapor, send it to an absorber tower, and return the vapor back to a liquid state, so it can be put back into the tanks.
