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April 11, 2002 High pressure gas cylinders for the carbon dioxide beverage marketAuthor: Jim Barker (Technical Manager, Luxfer Asia Pacific)
SUMMARY Carbon dioxide for the beverage market is contained in both steel and aluminium high pressure gas cylinders. Whilst the carbon dioxide supply remains dry, and no liquids contaminate the cylinders, both types of cylinders will give satisfactory service. However, when the carbon dioxide becomes moist through wet gas supply or mixing with entrapped liquid in the cylinders, the carbonic acid that is formed will corrode a steel cylinder. Rust develops that will contaminate the carbon dioxide gas used to dispense the beverage, and can lead to cylinder failure if undetected. Aluminium cylinders, on the other hand, are not affected by carbonic acid, and no corrosion will occur. In the very rare event of highly caustic (alkaline) or acidic washing solutions being accidentally introduced into the cylinder, both aluminium and steel cylinders will corrode. The corrosion pits formed in an aluminium cylinder will be self-limiting, and often look worse than they really are. This paper provides detailed information on carbon dioxide, and the compatibility with steel, aluminium and brass.
CARBON DIOXIDE GAS Carbon dioxide is a colorless, odorless, slightly acid gas, which is approximately 50% heavier than air. The chemical formula is CO2. Carbon dioxide can exist as a solid, liquid, gas, or supercritical fluid depending upon conditions of temperature and pressure. It is nonflammable and will not support combustion. Some uses for carbon dioxide are for fire extinguishing, for beverage dispensing, as a shielding gas for welding, for food freezing and as dry ice (in solid form). Dry (anhydrous) carbon dioxide is non-corrosive, hence common materials like aluminium, steel, brass and plastic are compatible. Dry carbon dioxide is at least 99.9% CO2, and has less than 50 ppm moisture. Wet carbon dioxide is slightly corrosive (as carbonic acid), hence carbonic acid resistant materials such as aluminium, stainless steel, brass and certain plastics are preferred.
CARBON DIOXIDE MANUFACTURE Unrefined carbon dioxide can be sourced from underground reserves or from by-product waste gas streams. Typical waste stream sources are chemical manufacturing facilities such as ethylene oxide, ammonia/urea plants and synthetic natural gas production. Fermentation processes like those in breweries can also provide suitable feed gas streams of carbon dioxide.
COMPATIBILTIY OF CARBON DIOXIDE WITH STEEL, ALUMINIUM AND BRASS
Carbon dioxide gas reacts with water to form carbonic acid. According to the ASM (American Society for Metals) Metals Handbook (1,2,3), aluminium and brass are not attacked by carbonic acid, but steel will corrode. The American Compressed Gas Association, in Pamphlets G-6 and G-6.3 (4,5), warns that wet carbon dioxide will cause severe corrosion in steel cylinders. Only dry carbon dioxide should ever be filled into steel cylinders. Additionally, precautions should be taken to drain cylinders suspected of containing liquids before filling, and special care should be taken when retesting steel cylinders that have been used in carbon dioxide service. Luxfer’s representative in Germany has reported that the German Authorities were so concerned with the safety of steel carbon dioxide cylinders after several accidents that they insisted on every steel carbon dioxide cylinder being opened and checked before refilling. The German Gas Industry is resisting this instruction, but they have decided not to use light, thin wall alloy steel cylinders for carbon dioxide. The decision of the Gas Industry not to use thin wall steel cylinders was based on the experience that corrosion of these cylinders may have catastrophic consequences.
“Carbon dioxide is only about one tenth as corrosive as oxygen, and in natural waters, the carbon dioxide concentration is important only as it affects the solubility and precipitation of calcium carbonate. The chief detrimental effects of carbon dioxide arise from its great solubility in water and high chemical reactivity with water. When process water comes in contact with a continuing or concentrated source of carbon dioxide, such as occurs in certain steam-powered systems or in some chemical or manufacturing plants, the solubility and reactivity of carbon dioxide lead to formation of dilute carbonic acid solution. At 20 degC (68degF), the solubility of carbon dioxide in water is 900 mL/L – by contrast, the solubility of oxygen is about 6 mL/L. Because of its high solubility, sufficient carbon dioxide can be dissolved in pure water to drive the pH below 6.0, where acid corrosion and hydrogen evolution can occur when the water is in contact with steel.” The following statement is quoted from Clause 6.3.5.1 of the Carbon Dioxide Pamphlet G-6 – 1997, issued by the American Compressed Gas Association (4). “The hydrostatic test shall be performed at 5/3 of the stamped service pressure. The internal visual inspection shall be carefully performed to detect harmful corrosion because wet carbon dioxide can rapidly corrode a steel cylinder.” The following statement is quoted from Clause 5.6 of the Carbon Dioxide Cylinder Filling Pamphlet G-6.3 – 1995, issued by the American Compressed Gas Association (5). “ Carbon dioxide cylinders are used extensively in the manufacturing and distribution of carbonated beverages. It is possible to have water flow back into the cylinders in some circumstances. When water collects in the bottom of the cylinder it combines with carbon dioxide to form carbonic acid. This causes internal corrosion and potential failure”.
“Corrosion resistance of aluminium alloys in high-purity water is not significantly decreased by dissolved carbon dioxide or oxygen in the water or, in most cases, by the various chemicals added to high-purity water in the steam power industry to provide the required compatibility with steel.” The following is quoted from Page 930 of Volume 1 of the ASM Metals Handbook Eighth Edition (3). “Carbonic acid has negligible action on aluminium. Aluminium equipment is in use handling carbonated beverages. Aluminium steam-condensate lines have replaced steel in installations where carbon dioxide is associated with the steam.”
All copper alloys are deemed to be “suitable under most conditions of use with dry carbon dioxide, moist carbon dioxide and carbonated beverages”.
CONTAMINATION OF CARBON DIOXIDE BEVERAGE CYLINDERS Carbon dioxide cylinders can be contaminated through suck-back with the drinks that are being dispensed, and thus beer, soft drink syrup concentrates and water can be found in the cylinders. Cylinders can also be contaminated with cleaning solutions if valves are not closed during equipment flushing. The most common contamination is the suck-back of drinks. In addition to affecting the purity of the carbon dioxide gas, such contaminants react with the carbon dioxide to form carbonic acid, and the rust corrosion of steel cylinders will result. Corrosion with this type of contamination is negligible in aluminium cylinders containing a positive pressure of carbon dioxide. Corrosive attack will generally be limited when aluminium alloys are exposed to these types of environments. A much less common type of contamination is from cleaning solutions when cylinders with open valves are still attached to lines when they are being flushed. All care should be taken to prevent this, but it sometimes occurs. If water is being used to flush the lines, then it will create a serious problem for steel, but none for aluminium. If caustic solutions are being used, the alkaline liquid will cause corrosion in aluminium cylinders, as well as steel, if they are left standing for a long period.
SUMMARY TABLE – EFFECTS OF CONTAMINATION
1. Metals Handbook Ninth Edition, Volume 1, Properties and Selection: Irons and Steels. Published by the American Society for Metals. 2. Metals Handbook Ninth Edition, Volume 2, Properties and Selection: Nonferrous Alloys and Pure Metals. Published by the American Society for Metals. 3. Metals Handbook Eighth Edition, Volume 1, Properties and Selection of Metals. Published by the American Society for Metals. 4. CGA G-6-1997, Carbon Dioxide. Published by the American Compressed Gas Association, Inc. 5. CGA G-6.3-1995, Carbon Dioxide Cylinder Filling and Handling Procedures. Published by the American Compressed Gas Association, Inc.
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