Dissolved oxygen removal methods

Corrosion may be described as the undesirable reaction of a metal or alloy with its environment and it follows that control of the rate of process may be eflFected by modifying either of the reactants. In corrosion inhibition , additions of certain chemicals are made to the environment, although it should be noted that an aqueous environment can, in some cases, be made less aggressive by other methods, e.g. removal of dissolved oxygen or adjustment of pH. 

Having removed the suspended solids and dissolved salts, the water then needs to have the dissolved gases removed, principally, oxygen and carbon dioxide, which would otherwise cause corrosion in the steam boiler. The usual method to achieve this is deaeration, which removes dissolved gases by raising the water temperature1,2. 

Figure 19.3—Polarographic cell and diffusion current. Dissolved oxygen, which leads to an interfering double wave, has to be removed from the sample solution by degassing. On the right features of the diffusion current are shown. These increase with time for every drop of mercury in a static (unstirred) solution. Direct polarography is a slow method of analysis. More than 100 droplets are needed to record the voltammogram.

Dissolved oxygen and other gases can be removed from the solvent by the freeze-pump-thaw (FPT) method, or by the expansion of gases above the solvent into an evacuated container on the vacuum line. It is customary to freeze-pump-thaw the solvents before vapor transfer from the storage container to the vacuum electrochemical cell. Such FPT cycles should be done as soon as the solvent is brought into contact with the drying agent. The FPT method involves three basic steps (i.e., one cycle), which are often repeated ... 

The removal of oxygen by the expansion of gases above the solvent on a vacuum line is usually done directly in the vacuum electrochemical cell [37]. This method is much faster than the FPT method and allows one to monitor the amount of dissolved oxygen by electrochemical means. 

Other methods have been developed for the removal of oxygen (particularly from flowing streams).These include the use of electrochemical or chemical (zinc) scrubbers, nitrogen-activated nebulizers, and chemical reduction (by addition of sodium sulfite or ascorbic acid). Alternately, it may be useful to employ voltammetric methods that are less prone to oxygen interference. The background-correction capability of modern (computerized) instruments is also effective for work in the presence of dissolved oxygen. 

A recirculating methodology is an attempt to maintain the water quality of the test solution without altering the toxicant concentration. A filter may be used to remove waste products or some form of aeration may be used to maintain dissolved oxygen concentration at a specified level. The advantage of this system is the maintenance of the water quality of the test solution. Disadvantages include an increase in complexity, an uncertainty that the methods of water treatment do not alter the toxicant concentration, and the increased likelihood of mechanical failure. 

The VODC process provides specific steel qualities, which cannot be obtained by other methods. The total oxygen content is also lower than what is typical for electric arc melted and AOD-treated steel, because a high level of oxide inclusions are removed form the melt during the VODC process, and most of the dissolved oxygen is further removed during the degassing phase. 

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