Hydrazine oxygen scavenger

The hydrazine-oxygen scavenging reaction is pH-dependent (like sulfite), and an increase in pH from 8 to 9 produces a threefold increase in reaction rate and a further three fold increase from pH 9 to 10. 

Oxygen scavengers other than hydrazine are also used, especially catalyzed sodium sulfite, which reacts rapidly with oxygen even at room temperatures to form sodium sulfate. Catalyzed hydrazine formulations are now commercially available that react with oxygen at ambient temperatures at rates comparable to catalyzed sulfite (189). At elevated temperatures, the reaction rates are all similar. Table 14 Hsts the standard hydrazine solution products offered by Olin Corp. for sale to the water-treatment market. Other concentrations are available and other companies offer similar products. 

Concern for personnel exposure to hydrazine has led to several innovations in packaging to minimize direct contact with hydrazine, eg, Olin s E-Z dmm systems. Carbohydrazide was introduced into this market for the same reason it is a soHd derivative of hydrazine, considered safer to handle because of its low vapor pressure. It hydrolyzes to release free hydrazine at elevated temperatures in the boiler. It is, however, fairly expensive and contributes to dissolved soHds (carbonates) in the water (193). In field tests, catalyzed hydrazine outperformed both hydrazine and carbohydrazide when the feedwater oxygen and iron levels were critical (194). A pubUshed comparison is available (195) of these and other proposed oxygen scavengers, eg, diethyUiydroxylarnine, ydroquinone, methyethylketoxime, and isoascorbic acid. 

There are several oxygen scavengers commercially available, such as hydrazine and sulfites, with the most cost-effective being sulfites. Sulfites react with oxygen in the following manner ... 

Catalysis plays an important part in the hydrazine/oxygen reaction. Copper salts were formerly added for this purpose, but in recent years certain organic substances, e.g. quinhydrone, have been employed and a number of proprietary activated hydrazines have been available. These are useful at low temperatures above 150°C scavenging rates with normal hydrazine are such that no great benefit is achieved by their use. 

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. 

Demineralized water is often employed for HTHW systems, and here it is preferable to employ an organic oxygen scavenger. Hydrazine, although a health hazard, is often used, although diethylhydroxylamine (DEHA) is preferred from an overall safety position or where there are high levels of nonferrous metals present. The pH typically is maintained at 8 to 10, depending on specific circumstances. 

Despite the potential carcinogenic nature of hydrazine, it is still widely employed as an oxygen scavenger and is effective at higher temper-... 

Oxygen scavengers include inorganics such as sodium sulfite and organics such as hydrazine. 

The hydrolysis products of the hydrolyzable tannins are not dissimilar to some of the novel oxygen scavengers developed to replace hydrazine. Oxygen reaction rates and application rates are also similar. The oxygen reaction time for tannin blends varies with pH levels and temperature but at 45 °C (113 °F) is on the order of 75% complete within 5 seconds and 90% complete within 10 seconds. 

All-volatile treatment essentially consists of adding a volatile oxygen scavenger, such as hydrazine, diethylhydroxylamine (DEHA), car-... 

In fact, following oxygen scavenging, the balance of hydrazine then proceeds to reduce any cupric oxide to reform a protective, passivated cuprous oxide layer. This premise is, of course, dependent on effective post-boiler scavenging. 

Where copper corrosion occurs, the problem usually can be traced back to an excess feed of hydrazine, DEHA, or similar product, coupled with inadequate post-boiler oxygen scavenging. 

Overall, DEHA is a low toxicity product, performs extremely well as a rapid oxygen scavenger and metal passivator, and is an excellent replacement for hydrazine. In addition, it is competitively priced and consequently a very popular product. 

Hydroquinone is a good oxygen scavenger, even in cold water, so much so that it is commonly employed as a catalyst for hydrazine, DEHA, and carbohydrazide. 

Carbohydrazide was the first novel oxygen scavenger of any significance to provide a safe handling replacement for hydrazine it was patented as an oxygen scavenger as Nalco ELIMIN-OX in 1981. 

Carbohydrazide is a derivative of hydrazine that hydrolyzes in the boiler to produce hydrazine, although CHZ is an oxygen scavenger in its own right. Other derivations of this oxygen-scavenging chemistry now exist, including polyacrylic hydrazide. 

There is nothing particularly special about the product except its feed rate, which is fairly low in comparison to some of the other novel oxygen scavengers. It exhibits passivation characteristics (forming magnetite in a way similar to hydrazine), coupled with good corrosion and iron transport control. 

Novel oxygen scavengers and polymer-based deposit control programs hardly figure in the operation of these large boilers because the treatment regimen is often simply hydrazine and ammonia. 

In many of the largest boiler plants around the world AYT programs are often employed that commonly provide for hydrazine (as an oxygen scavenger) and a volatile amine such as morpholine (to boost the pH level). Where funds are limited, however, some facilities instead use ammonia as a pH booster because it is a low-cost item with a low equivalent weight and a high DR. Unfortunately, this approach may lead to downstream problems as ammonia becomes less chemically bound with increases in FW pH and a weaker base as temperatures rise. Consequently, a point is reached when ammonia ceases to further influence the pH level upward. 

Amines can be blended into multifunctional product formulations containing alkali, polymer, and phosphate and where sulfite- or hydrazine-based oxygen scavengers are part of the formula, but not where tannins are employed. Amines must be fed separately from tannins. 

Dissolved oxygen (DO) in the FW should be reduced to the lowest practical level before adding an oxygen scavenger. Hydrazine is recommended as an alternative to sodium sulfite only when it is dosed to the FW at a substantial distance from the boiler and where the FW is sufficiently hot to enable the removal of oxygen to be 90% complete before entry to the boiler. 

Oxygen corrosion is often underestimated. Studies have shown that the corrosion can be limited when proper oxygen scavengers are used. Hydrazine leads the group of chemicals that are available for oxygen removal. Because of... 

D. Sikora. Hydrazine—a universal oxygen scavenger (hydrazyna— uniwersalny inhibitor korozji tlenowej w pluczkach wiertniczych). Nafta Gaz (Pol). 50(4) 161-168, April 1994. 

In Methods 1 through 3, either sulfite (up lo 1800 psi 122 aim) or hydrazine can be used as the oxygen scavenger. Above 1800 psi (122 atm), or with the volatile treatment (Method 4), hydrazine is used. 

Volatile Treatment. This method of treatment may be used for units operating above 2000 psi (136 atm) drum pressure. In this method, no Solid chemicals are added lo either the boiler 01 pie-boiler cycle. By eliminating solid treatment, the volatile carry-over of solids is eliminated and consequently turbine deposits are avoided. Cycle pH is controlled at 9.0 to 9.5 with a volatile amine such as ammonia. Hydrazine is added as an oxygen scavenger in quantity sufficient to provide a concentration of 20 to 30 ppb at the economizer inlet. 

Dissolved oxygen should be monitored at the condensate pump discharge and the deaerator outlet. Sulfite or hydrazine can be used for oxygen scavenging. Over 1800 psi (122 atm) drum pressure, sulfite should not he used only hydrazine is recommended. Sulfite or hydrazine can be added to the condensate on a manual or automated basis. 

Recommended limits should be low because all solids in the feedwater will either deposit in the boiler or be earned over with the steam to the turbine. Consequently, water-treatment chemicals must be volatile. All cycles should have condensate-polishing systems to meet the limits show ll in Table 3. A schematic diagram is shown in Fig. 11. Laboratory tests as well as field studies show that high-flow-rate condensate-polishing systems 25 to 50 gal per min per sq ft (1015-2030 liters/minute/square meter) of cross-sectional bed area] perform as filters of suspended material and ionized particles. Ammonia is added to control the pH in die system. Fig. 12 indicates the amount of ammonia required, in terms of ppm or solution conductivity, to give a certain pH in the system. Hydrazine is added to the cycle for oxygen scavenging. 

Attempts are made to minimize corrosion by controlling the chemistry of the coolant water. Adding 7LiOH raises the pH to 8. One can use oxygen scavengers such as hydrazine to reduce the oxygen concentration. 

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