Deaerators oxygen removal

The mechanical deaeration oxygen removal process is not 100% efficient, however, and all boiler FW must be further deoxygenated by some form of chemical oxygen scavenger. 

Oxygen Control. To meet industrial standards for both oxygen content and the allowable metal oxide levels in feed water, nearly complete oxygen removal is required. This can be accompHshed only by efficient mechanical deaeration supplemented by an effective and properly controlled chemical oxygen scavenger. 

In almost all industrial circumstances, a deaerator is a cost-effective oxygen-removing proposition that complements primary pretreatment equipment such as water softeners. 

All large boiler plants and many smaller units employ mechanical deaerators to remove oxygen. In addition, various oxygen-scavenging chemicals are employed as consumable treatments such as sodium sulfite-bisulfite, hydrazine, various novel chemistry organics (hydrazine replacements), and certain tannins. 

NOTE The injection point of chelant should be into the boiler FW as far downsteam as possible from the deaerator and chemical oxygen scavenging point to permit sufficient time for effective oxygen removal to take place. 

Conversely, larger and higher pressure boilers tend to employ mechanical deaeration systems and supplement this by using a smaller quantity of oxygen-removing chemical as a true scavenger. This is added to the base of the deaerator (the deaerator storage section). 

Given the importance of deaerators in removing oxygen from FW, it is important to understand the steam-heat energy relationship. The following example uses this relationship and refers to Table 3.2 (Steam Tables Suitable for Pressure Deaerators). 

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. 

The use of deaeration to remove oxygen from solution is probably the most effective way of combating corrosion and will probably make possible the use of ordinary steels. 

Eliminates the need for feed deaerator for removal of oxygen... 

The softened water is now in its most corrosive state, being still saturated with DO and having no hardness to form a protective scale. For reasons of economy, the removal of DO is usually first effected in part by thermomechanical deaeration. The boiler feedwater is preheated, then flashed in a deaerator to remove any free carbon dioxide and most of the dissolved oxygen. Then the last traces of DO are chemically scavenged with sodium sulfite (Na SO ) that reacts with oxygen to form sodium sulfate (Na SO ) as described in Eq. (8.22) ... 

The second approach, changing the environment, is a widely used, practical method of preventing corrosion. In aqueous systems, there are three ways to effect a change in environment to inhibit corrosion (/) form a protective film of calcium carbonate on the metal surface using the natural calcium and alkalinity in the water, (2) remove the corrosive oxygen from the water, either by mechanical or chemical deaeration, and (3) add corrosion inhibitors. 

Deaeration (mechanical or chemical) removes the corrosive substance—oxygen. 

When selecting a suitable feed symp, the main criteria are optimization of enzyme productivity and minimization of the formation of by-products. Typical feed symp specifications are shown in Table 5. Higher symp concentration and higher viscosity results in a reduced isomerization rate due to diffusion resistance in the pores of the immobilized enzyme. A deaeration step is desirable to remove dissolved oxygen that would otherwise iacrease the formation of by-products. The pH is adjusted to the optimum level for the productivity of the enzyme. 

Deaerators not only effectively remove dissolved oxygen and other noncondensable gases but also provide the benefit of FW heating. Also, deaerators do not add dissolved solids to the FW, as happens with sulfite-based oxygen scavengers. 

All large industrial WT and power boiler plants utilize deaerators and supplement the DO removal process by means of a suitable chemical oxygen scavenger. Many midsize factories operating FT boilers or FT-WT boiler combinations also employ mechanical deaeration. This is especially common in the United States, Germany, and several other industrialized countries where boiler design custom and practice virtually dictate that a deaerator be included in almost every midsize and larger boiler plant facility. 

For these medium to large operations, apart from taking measures to ensure the continuous efficiency of deaeration and oxygen scavenging, little more can be said because all that can be done practically to remove DO is being done. The problems of inadequate FW deaeration, the resultant corrosion risks entailed, and the consequent need for constant vigilance primarily affects those owners and operators of small to midsize steam-raising boiler plants where mechanical deaeration is not available. 

Smaller or lower pressure boilers usually do not have mechanical deaeration and thus employ only a chemical reactant for DO removal, which can be expensive if the FW temperature is low and less than totally effective if the feedlines are very short. Treatment is by the direct injection of a suitable oxygen-scavenging chemical into the FW system. The feed rate requirement is based on combining a quantity proportional to the level of oxygen present, plus a further amount that contributes to a BW reserve. 

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