Volatile Organic Compounds (VOC) are light components generated from oil-based products such as gasoline, naphtha, diesel or crude oil.The typical sources of crude oil vapors are the filling of Floating Storage and Offloading vessels (FSO) near the oil platforms. Also vessels or tankers at onshore terminals or tank farms can generate vapors during the loading and unloading operations.
For what concerned refined petroleum products, typical sources of condensed vapors are:
A vapor treating system is fundamental equipment to handle these vapors, recognized as being dangerous for the enviroment and human health. Currently, the Best Available Technique to recover these vapors is the Vapor Recovery Unit (VRU) with Activated Carbon Beds. They rely on Active Carbon Technology (also called Carbon Vacuum Adsorption), which is the most applicable solution for high volatile products.
Gasoline, diesel, naphtha, condensates, methanol, ethanol, benzene, toluene, xylene, and also crude oil are compatible with Activated Carbons. From a design point of view, an active carbon based vapor recovery unit is equipped with the following main elements:
These three main components reflect the three main process steps featuring the working principles of a Carbon Bed VRU:
The vapors from loading or storage operations arrive at the Vapor Recovery Unit (VRU). They enter the carbon bed vessel set in adsorption/receiving mode. The hydrocarbons in the vapor are adsorbed onto the Activated Carbons (AC) and the clean air is released in the atmosphere.
When the ACs are saturated, after 10 to 15 min on average, the vapors are directed to the second vessel. The first, saturated, vessel is isolated and put under vacuum by one or more vacuum pumps. By doing this, the adsorbed hydrocarbons are removed from the AC and, through the vacuum pumps, they enter the absorber column. In the absorber column, an absorbent fluid (e.g. gasoline) is showered down, while the extracted and highly concentrated vapors go in reverse direction.
Most of the hydrocarbons in the vapor are absorbed in the absorbent liquid. The not absorbed vapors are redirected to the AC vessel in adsorption/receiving mode through a recycle line. The absorber with the recovered product is returned to the storage tank or pipeline. When the complete cycle is finished, the AC is clean and ready to receive a new stream of vapor. For this reason, the AC vessels switch function and the vapors are directed to the clean AC vessel in receiving mode. This sequence repeats itself every 10-15 minutes as long as hydrocarbon vapors are entering the Vapor Recovery System.
One of the main drivers for installing a VRU is the necessity of complying with environmental legislation. For what concerns crude oil, the standard is related to the recovery efficiency, which is usually required to be between 90-95%.
For oil-based products every country has its own standard, many countries require <10 mg/Nm3 according to the signed Gothenburg Protocol. Some countries go even lower with emission limits from 5 grams, 2 grams up to 150 or 50 mg/Nm3.Aromatics components standards range between 10mg/Nm3 up to 1mg/Nm3.
A Carbon Bed Vapor Recovery System reduces emissions levels, while recovering valuable hydrocarbons. They are able to capture about 99% of Volatile Organic Compounds generated.At a glance, a Carbon Bed VRU benefits are:
Since 1947, Garo has collaborated with companies around the world to custom design compressors that enable safe handling of toxic and corrosive gases. More than 70 years of engineering expertise allows us to support clients from concept/FEED studies through project execution to site services and more.