Compressed gases: risks of compressing CO2 & H2O mixtures
In the realm of engineering, the compression of gas mixtures is a common yet complex process that requires a deep understanding of the properties and behaviours of the involved gases. Today, we delve into the specifics of compressing carbon dioxide (CO2) when mixed with water (H2O), a scenario that presents unique challenges and risks.
The nature of CO2
CO2 is an odorless and invisible gas that is heavier than ambient air.
At room temperature (20°C) and pressure (1 bara), it exists as a gas, but its behaviour changes when combined with water. In case the H2O concentration is greater than 2,33% vol, the water will start to condense and form liquid droplets.
Condensation of gaseous H2O also happens for example when a hot, saturated gas mixture is cooled down with an inter -or aftercooler after compression.
It is when liquid H2O is present, the mixture forms carbonic acid (H2CO3), which is a balance between CO2, liquid H2O, and HCO3- ions. This equilibrium is influenced by the partial pressure of CO2, which determines the amount of CO2 that remains as a gas or converts into HCO3-ions in the condensate.
The more HCO3-ions are dissolved, the more acidic the condensate becomes.
The acidic risks involved
The primary risk in compressing a CO2 and H2O mixture lies in the formation of acids. When CO2 comes into contact with water, it forms carbonic acid, which can have corrosive effects on the materials used in the compression chamber or any location where condensation can occur.
This is why the use of stainless steel in the construction of compressors and related components is crucial. Stainless steel offers resistance to the corrosive nature of the acids formed during compression and cooling, ensuring the longevity and reliability of the machinery.
Stainless steel
Stainless steel is renowned for its corrosion resistance, which is primarily due to the presence of chromium. By definition, stainless steels must have a minimum of 10.5% chromium content by weight. The corrosion resistance of stainless steel can be further enhanced with the addition of other alloying elements such as nickel, molybdenum, nitrogen, and titanium.
For instance, a common type, AISI 304L stainless steel, contains 18.111% chromium and 8.074% nickel, offering good corrosion resistance and mechanical properties, with a yield strength of 351 N/mm² and tensile strength of 619 N/mm². It is also noted for its low carbon content, which helps prevent intergranular corrosion after welding.
These properties make stainless steel an ideal material for many applications, including those in the medical, food processing, and construction industries, where durability and hygiene are paramount.
The liquid risks involved
A second, and evenly important, risk to consider when dealing with gas mixtures which include H2O as part of the mix, is related to the formation of liquid droplets before compression. These liquid droplets are far less compressible compared to a gas. When they would enter a compression chamber of a volumetric compressor, the forces that would be required to compress them, could be far beyond for what the gas compressor is designed for.
This could result in crankshaft failures, piston rod damage or other mechanical breakdowns.
To mitigate the risks associated with compressing wet CO2 – and especially saturated CO2 ,it is mandatory to use
an inlet separator.
- This device prevents liquid water from entering the compression chamber, protecting cylinders, valves, and pistons from damage.
- It also guarantees the reliable operation of the gas compressor under demanding applications.
The thermal risks involved
Another aspect to consider is the specific heat of gas mixture. The specific heat indicates how much energy is required to change the temperature of the gas. Compressing the same amount of ambient air or pure CO2 will result in a different temperature of the gas at the same outlet pressure.
A thorough understanding of this property is essential for fine-tuning the compression process and related cooling requirements to avoid any risks associated with temperature variations.
A properly sized inter-and aftercooler, will make a compressor run in the most efficient way possible and reduce operating costs to the minimum.
Conclusion
Compressing CO2 and H2O mixtures is a task that demands respect for the properties of the gases involved.
By using the right materials, such as stainless steel, and incorporating safety measures like inlet separators, engineers can effectively manage the risks and ensure safe and efficient operations.