Deaeration
Technical Profile
The objective of this process is the removal of dissolved oxygen in seawater. Oxygen promotes bacterial growth and equipment corrosion in injection systems. Some bacteria produce sulfide that can sour oil and gas with H2S.
In general, dissolved oxygen can be removed in one of two ways. It may be stripped out with another gas, or it may be stripped out with water vapor. Forced draft deaeration is done by contacting gas and water counter-currently in a tower filled with packing material. The flow of the stripping gas provides the energy for mass transfer of dissolved gas into the stripping gas flow. An alternative to forced draft deaeration is vacuum deaeration.
Vacuum deaeration is a common and economic process to remove oxygen from seawater. Seawater enters the top of the deaeration tower where it is distributed evenly over internal packing. As water flows over the packing it forms a thin film, which maximizes its contact with the vapor phase within the deaerator. A vacuum system extracts the vapor phase in order to lower the partial pressure of oxygen. Oxygen, dissolved in the water phase, diffuses into the vapor phase where it is swept out with the vapor. High dissolved oxygen contents may require multistage deaeration to meet requirements. Vacuum as low as 20 to 50 mm mercury absolute is used for deaeration.
There is debate on the placement of deaeration in the circuit for process and injection water treatment. Since aerated water is more corrosive to downstream equipment than deaerated water, often deaerators are located upstream of membrane systems such as sulfate removal packages and seawater RO (SWRO). Locating the deaerator downstream of sulfate removal packages, however, allows for a smaller unit since the reject will not be processed and lower energy requirements for high pressure pumping. Unit size, weight and energy requirements are always issues of concern for offshore use.
