Vacuum deaerators, also known as vacuum degasifiers, use a vertical pressure vessel, multiple vacuum stages and vacuum pumps to continuously remove objectionable gases from liquids. Packed towers, supported by powerful vacuum pumps, reduce the surrounding vapor partial pressure which promotes the mass transfer from the liquid to the gas phase.
A vacuum deaerator system consists of the tower with integral catch basin, vacuum pump and forwarding pump. Demisters and condensers on the vacuum lines are employed to protect the vacuum pumps from corrosion and damage by moisture entrained in the gas stream. Water seals are an integral design of the forwarding pumps to prevent the re-introduction of air into the water. Finally, level transmitters are utilized to control the feedwater flow rate, prevent flooding and protect the forwarding pumps from cavitation.
Veolia can provide stand-alone degassers with capacities ranging from 100 to 10,000 gpm for a single unit, or complete systems that can include vacuum towers, liquid ring vacuum pumps, ejectors, mass transfer packing, chemical oxygen scavenger dosing skids and transfer pumps. Veolia selects materials of construction to provide effective corrosion resistance under the most aggressive service conditions.
Veolia’s proprietary system design provide the best value in the market by optimizing the tower diameter, number of stages, packed bed geometry and vacuum capacity on the basis of the salinity, temperature, and other conditions of the influent water.
In vacuum degasification, uniform feed-water distribution and downflow hydraulic balancing from the top of the tower and over its cross section is a key design consideration as the water flows through a bed of mass transfer packing that maximizes the liquid/vapor-phase contact area and, thus, reduces residence times and lowers vacuum requirements. As the vacuum system withdraws all the gasses from the vapor phase, it reduces the system pressure below atmospheric pressure which promotes the dissolved gas molecules in the water to diffuse into the vapor phase. Thus, the concentration of gas in the water is reduced. Under this condition, the dissolved gasses in the liquid are extracted and evacuated from the top of the tower. The intensity of the vacuum and the volume of vapor removed from the system determine the degree of removal.
For some applications where extremely low levels of dissolved gases are required, two-stage towers are designed to optimize the extraction process by phasing the removal process with increasingly higher degrees of vacuum. These systems employ a taller tower that has two independent beds of random packing with dedicated vacuum pumps for each bed. The two stage vacuum deaerators have specially designed internals to ensure high efficiency gas removal without flooding or short-circuiting. An effective pressure sealing method between these two stages guarantees the pressure differential between them. This feature in the design achieves the removal of gases more efficiently than in a single-stage step which would require much larger vacuum capacity.
The last traces of dissolved gases are the most difficult to remove and the concentration can never be reduced to zero. Generally, 50 parts per billion (ppb) of oxygen is the lowest practical level that can be achieved with vacuum degasification. After the second-stage packing, the liquid is allowed to settle in a sump section to allow for the sulfite-based oxygen scavenger chemical to react and reduce the residual oxygen concentration even further, if required. The sump or storage section, located at the bottom of the tower, collects the de-gasified water and provides the net positive suction head requirements (NPSHR) of the forwarding pump.
Vacuum deaerators are used primarily on water streams to remove dissolved gases including oxygen, nitrogen, carbon dioxide and volatile organics that can produce corrosion, scaling and plugging of piping and injection systems. When used in boiler feedwater treatment systems, they provide a cost-effective means of removing carbon dioxide between cation and anion exchange columns.
Veolia’s vacuum degasification systems are also commonly used to remove oxygen as part of the Merrill Crowe process in gold and silver mining. Other applications include oxygen removal for corrosion protection in enhanced oil recovery using seawater, as well as Process Water Oxygen Removal in the Power industry.