Vapor Recovery System Applications

There are different applications of the Vapor Recovery systems:

• Oil tank vapors
• Marine loading terminals
• Storage terminals
• Truck loading
• Railcar loading
• Barge degassing
• Vapor particulate filter

Garo is specialized in designing and engineering custom made Tank Vapor Recovery Systems for the Oil & Gas operations, using liquid ring compressors as the means of compression.

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

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.