Dear Rahma Alhosni,

Deaeration Process:

A Deaerator is a device that is widely used for the removal of oxygen and other dissolved gases from the feed water to steam-generating boilers.

In particular, dissolved oxygen in boiler feed waters will cause serious corrosion damage in steam systems by attaching to the walls of metal piping and other metallic equipment and forming oxides (rust).

Dissolved carbon dioxide combines with water to form carbonic acid that causes further corrosion. Most deaerators are designed to remove oxygen down to levels of 7 ppb by weight (0.005 cm³/L) or less as well as essentially eliminating carbon dioxide.

There are two basic types of deaerators, the tray-type, and the spray-type:

• Tray-Type Deaerator
• Spray-Type Deaerator
• Vacuum Deaerator
• Ultrasonic Deaerator
• Forced Draft Degasifier
• Oxygen Scavenger

The deaerator system works according to the principle of two laws. They are

• Henry’s Law: In conformity to Henry, the pressure available inside the equipment is directly proportional to the solubility of gases. Whenever the pressure present inside is decreased such that the solubility of gases also gets reduced. Hence, the pressure is reduced by the introduction of steam inside the deaerator.
• Inverse solubility Law: In conformity to Inverse solubility Law, the temperature inside the equipment is inversely proportional to the solubility of gases. Whenever the temperature is increased such that the number of dissolved gases inside will be reduced. Thus, the temperature is increased by the introduction of steam inside the deaerator. Therefore, this is the basic principle of how does a deaerator system work by removing the dissolved gases from the feedwater before supplying it to the boiler. They are removed either by decreasing pressure or by increasing temperature but both will be practiced by the introduction of steam.

The Spray-Type Deaerator:

Preheating section: In the preheating section, the water is heated by steam until the temperature of the water reaches its saturation point. At this high temperature, the feed water containing dissolved gases will be eliminated. The high-temperature water converted to steam is sent to the deaeration section.

Deaeration section: In the deaeration section, steam and water droplets will be separated. The separated dissolved gases are escaped out through the Air vent. And the water droplets free from gases are collected at the bottom of the vessel. The deaerator boiler feed water is fed to the boiler by using a pump. This water is used as the feed water to the boiler which helps in avoiding the corrosion.

Low-pressure steam:

Low-pressure steam enters below the perforated tray & flow upwards. This arrangement provides better contact & miking of steam with boiler water.

Steam stripped out the dissolved gases from boiler feed water& these gas are exit out via vents which are given at top of the horizontal vessel. The horizontal vessel also has a sparger system that keeps boiler feed water warm.

Types of Deaerator:

1. Tray-Type Deaerator:

The Tray-Type Deaerator also called the cascade-type deaerator is a vertical domed deaeration section mounted on top of a horizontal cylindrical vessel that serves as the deaerated boiler feedwater storage tank.

The typical tray-type deaerator has a vertical domed deaeration section mounted above a horizontal boiler feedwater storage vessel. Boiler feedwater enters the vertical deaeration section above the perforated trays and flows downward through the perforations.

Low-pressure deaeration steam enters below the perforated trays and flows upward through the perforations. Some designs use various types of packed beds, rather than perforated trays, to provide good contact and mixing between the steam and the boiler feedwater.

The steam strips the dissolved gas from the boiler feedwater and exits via the vent valve at the top of the domed section.

The deaerated water flows down into the horizontal storage vessel from where it is pumped to the steam-generating boiler system. Low-pressure heating steam, which enters the horizontal vessel through a sparger pipe in the bottom of the vessel, is provided to keep the stored boiler feedwater warm.

2. Spray-Type Deaerator:

The Spray-Type Deaerator consists of a horizontal or vertical cylindrical vessel which serves as both the deaeration section and the boiler feedwater storage tank.

The typical spray-type deaerator is a horizontal vessel which has a preheating section and a deaeration section. The two sections are separated by a baffle. Low-pressure steam enters the vessel through a sparger in the bottom of the vessel.

The boiler feedwater is sprayed into a section where it is preheated by the rising steam from the sparger.

The purpose of the feedwater spray nozzle and the preheat section is to heat the boiler feedwater to its saturation temperature to facilitate stripping out the dissolved gases in the following deaeration section.

The preheated feedwater then flows into the deaeration section, where it is deaerated by the steam rising from the sparger system.

The gases stripped out of the water exit via the vent at the top of the vessel. The deaerated boiler feedwater is pumped from the bottom of the vessel to the steam-generating boiler system.

3. Vacuum Deaerator:

Vacuum Deaerator or degasifier is designed to effectively remove select non-condensable gases from the liquid stream. A vacuum is pulled on a stream of water, and the vacuum draws the dissolved gas out of solution, removing it from water.

There are two basic systems available:

• Vacuum Deaerator Tower
• Membrane Deaerator

Vacuum Deaerator

A vacuum deaerator is typically used as part of a demineralizer system to remove entrained gasses like CO2 and oxygen from boiler-feed water for high-pressure boilers.

The liquid is sprayed into a vessel onto selected packing material through fine nozzles to increase the water/gas transfer surface. The vacuum causes the gas to be released, and it is extracted from the system by the suction.

To enable the most efficient removal, the size of the transfer surface and the contact time are optimized. Vacuum deaerator is utilized to remove O2, CO2 and N2 and is capable of producing water with part-per-billion levels of these dissolved gasses.

In a vacuum deaerator, water flows by gravity down through a tower filled with packing as a vacuum is drawn on the tower. The packing in the tower has a very high surface area, disperses the water very effectively, thereby enhancing the removal of O2, CO2, and N2.

Performance is further enhanced by using atomizing nozzles as the water enters the tower. Since the vessel has to bear full vacuum, a steel ASME code vessel with a rubber lining is used.

Polypropylene tower packing is utilized in the same fashion as forced draft degasifiers. A vacuum pump with or without an ejector typically is used on the vessel.

Membrane Deaerator

A Membrane Deaerator is equipment where liquid flows across one side of a membrane surface while a vacuum is drawn on the other side. Jet educators or liquid ring pumps are used to generate the vacuum required.

Gas-Liquid Separation membranes allow gas but not liquid to pass through. Flowing a solution inside a gas-liquid separation membrane makes the dissolved gas go out through the membrane with the used of the vacuum.

This method has the advantage of being able to prevent redissolution of the gas, so it is used to produce very pure solvents.

Source: https://www.stscanadainc.com/vacuum-deaerator.html

I hope the information will help you.

With Best Wishes,

Samir G. Pandya