Refrigeration - Condensation Vapor Recovery Equipment
Process Description
Refrigeration Condensation, along with Lean Oil Absorption, was one of the first vapor recovery technologies to be utilized in the terminals industry. The theory of operation is relatively straight forward, the air hydrocarbon stream (the vapor) is chilled to a temperature where the hydrocarbons condense out of the vapor stream.
In practice however, it is a bit more complex since the vapor stream usually consists of not only air and hydrocarbon but also contains water vapor. If the vapor stream is chilled to a temperature below the freezing point of water the coils may become plugged with hydrates (water and hydrocarbon "ice" mixture) thus reducing the usefulness of the system.
To circumvent the hydrate formation, the designer will usually chill the vapor stream in stages. The first stage is used to condense the bulk of the water vapor from the vapor mixture. A small amount of hydrocarbon is also condensed along with the water. The liquid water/hydrocarbon is collected, the hydrocarbon is decanted and the contaminated water is sent to wastewater treatment. The temperature attained in the first stage is usually just above the freezing point of water.
In the second stage, the temperature of the vapor stream is dropped to a level such that the hydrocarbon remaining in the vapor stream is within the least stringent of the mandated emission limits. If more stringent emission limits are required, a third stage can be used and the temperature reduced further. The temperatures attained are dependent upon what hydrocarbons are present in the vapor and what emission limits are required.
Other methods of attaining the more stringent emission limits have been attempted, including the use of a circulating cryogenic liquid (usually liquid nitrogen) to attain the extremely low temperatures required to condense minute concentrations of hydrocarbon.