Several processes are available that use the basic action of various amines. These amines can be categorized as primary, secondary, or tertiary according to the number of organic groups bonded to the central nitrogen atom.
Primary amines are stronger bases than secondary amines, which are stronger than tertiary amines. Amines with stronger base properties will be more reactive toward CO2 and H2S gases and will form stronger chemical bonds.
A typical amine system is shown in Figure 7-4. The sour gas enters the system through an inlet separator to remove any entrained water or hydrocarbon liquids. Then the gas enters the bottom of the amine absorber and flows counter-current to the amine solution. The absorber can be either a trayed or packed tower. Conventional packing is usually used for 20-in. or smaller diameter towers, and trays or structured packing for larger towers. An optional outlet separator may be included to recover entrained amines from the sweet gas.
The amine solution leaves the bottom of the absorber carrying with it the acid gases. This solution containing the CC>2 and IH^S is referred to as the rich amine. From the absorber the rich amine is flashed to a flash tank to remove almost all the dissolved hydrocarbon gases and entrained hydrocarbon condensates. A small percentage of the acid gases will also flash to the vapor phase in this vessel. From the flash tank the rich amine proceeds to the rich/lean amine exchanger. This exchanger recovers some
of the sensible heat from the lean amine stream to decrease the heat duty on the amine reboiler. The heated rich amine then enters the amine stripping tower where heat from the reboiler breaks the bonds between the amines and acid gases. The acid gases are removed overhead and lean amine is removed from the bottom of the stripper.
The hot lean amine proceeds to the rich/lean amine exchanger and then to additional coolers to lower its temperature to no less than 10°F above the inlet gas temperature. This prevents hydrocarbons from condensing in the amine solution when the amine contacts the sour gas. The cooled lean amine is then pumped up to the absorber pressure and enters the top of the absorber. As the amine solution flows down the absorber it absorbs the acid gases. The rich amine is then removed at the bottom of
the tower and the cycle is repeated.
Of the following amine systems that are discussed, diethanol amine (DBA) is the most common. Even though a DBA system may not be as efficient as some of the other chemical solvents, it may be less expensive to install because standard packaged systems are readily available. In addition, it may be less expensive to operate and maintain because field personnel are likely to be more familiar with it.
