Degassing of Water and Boiler Feedwater
The degassing of water serves to remove dissolved gases such as oxygen (O2) and carbon dioxide (CO2). These gases can cause corrosion in boiler systems, steam generators, and industrial processes, and significantly shorten the service life of the equipment.
RWT GmbH supplies high-performance degassing systems for feedwater treatment and process water treatment. Depending on the requirements, thermal degassing, vacuum degassing, membrane degassing, or chemical post-degassing are employed.
Applications and Methods for Degassing
Common degassing processes are:
- Thermal degassing (Pressure degassing, vacuum degassing)
- Membrane degassing
- Chemical degassing
Degassing systems are used for feedwater treatment in power plants, heating plants, and steam boiler systems, as well as for process water treatment in the food industry, chemical industry, and many other industrial sectors.
Thermal degassing
Thermal degassing (pressure degassing) of boiler feedwater requires that the water be boiling. The mixture of condensate and fresh water is fed into the degassing dome at the top. Heating up to the boiling temperature occurs with heating steam, which flows into the degassing unit at the bottom and flows upward against the current. The gases driven out as a result are lead away with the exhaust vapours via the exhaust vapour outlet in the degassing dome. The degassed water is collected in the feedwater tank.
Thermal degassing occurs at temperatures just above 100°C and operating pressures around 0.2 to 0.3 bar. Thus an oxygen content under 0.02 mg O2/l and a carbon dioxide content of less than 1 mg CO2/l are possible in the feedwater.
Vacuum Degassing
With vacuum degassing, water can also be degassed below 100°C by boiling. Creating a corresponding negative pressure (partial vacuum) in the degassing unit enables the boiling point to be reached as early as in the 30-80 °C range. Here degassing takes place in a percolator with a spraying unit, packing and supply section. The gases are removed via a vacuum pump with the resulting water vapour. It is preferable to use an exhaust vapour condenser to enable the vacuum pump's intake volume flow to be kept low. The residual oxygen content of 0.02 mg/l can also be achieved here.
Vacuum degassing is used for treating supplementary water for heating systems.
Membrane degassing
Membrane degassing enables dissolved carbon dioxide (CO2) to be removed from the water in pure stripping air mode down to values of < 2 mg/l (ppm) or, in combination with a vacuum pump, down to values of < 1 mg/l (ppm). Using industrial nitrogen as the stripping medium and in combination with a vacuum pump enables oxygen to be reduced to residual values of < 1 µg/l (ppb).
Creating a corresponding partial pressure gradient as the driving force (stripping with air/gas and/or creating a vacuum) causes the gases to diffuse out of the liquid and into the gas-bearing membrane fibres through microporous, hydrophobic (water-impermeable) hollow-fibre membrane modules, and to be transported away with the stripping gas. The individual hollow-fibre membranes are combined in modules (known as membrane contactors or as membrane degassing units = MDU).
In the case of demineralization with ion exchangers, membrane degassing allows the load to be relieved on the anion exchanger. This makes membrane degassing a good alternative to the CO2 percolator, but with the advantages of requiring much less space and energy (e.g. no pressure increase required). In addition, membrane degassing is useful between EDI (electrode ionisation) and reverse osmosis.
Chemical degassing
Chemical degassing or subsequent degassing occurs through the addition of chemicals. These chemicals are dissolved in dilution water and are suitable for removing or converting the oxygen contained in the water. The reaction time differs when different chemicals are used. In general, the greater the water temperature, the shorter the reaction time.
Due to the heavy use of chemicals, chemical degassing is of interest only at low boiler pressures and a low system performance. In medium-sized and large systems, chemical degassing is used only when it is necessary to achieve residual oxygen contents of < 10 µg/l.
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