Boiler Water Treatment Chemical Guidelines Part 1

Transcription

1 Boiler Water Treatment Chemical Guidelines Part 1 AWT Boiler Water Subcommittee The choice of boiler water treatment depends upon two main considerations: (1) boiler operating pressure and (2) boiler feedwater hardness. As the pressure increases, greater care in control of treatment, cycles, etc., is required. Also, since boiler operation is much more critical at higher pressures, less hardness and total dissolved solids can be tolerated. Generally, less use of combined (or multifunctional) treatments occurs at higher pressures. Boiler water treatments are used to prevent scale, corrosion, and carryover/foaming. Scale is generally a result of poor control of calcium, magnesium, and silica in the boiler water. There will almost always be iron present in the scale deposit as well. This can be from iron in the makeup water or a result of corrosion in the system. Corrosion is usually due to oxygen in the system, while foaming or carryover is usually a result of high total dissolved solids in the boiler water. Scale control is accomplished by either internal treatment or a combination of internal and external treatment. The higher the pressure, the more external treatment is utilized. Oxygen corrosion is prevented by chemically reducing or scavenging the oxygen in the system, and by mechanically deaerating the feedwater before it enters the boiler. The higher the pressure, the more likely there is a deaerator in the system. Foaming or carryover can be controlled by blowdown or with chemicals. It is almost always more economical to use a chemical to prevent foaming or carry-over. Preboiler Corrosion Protection Metals that are corrosion resistant and/or can be protected by adequate chemical programs are used in the preboiler section of the system. Iron, stainless steels, and copper alloys are most commonly used. 29 the Analyst Volume 19 Number 4

2 Feedwater ph control is very important. To minimize the corrosion of steel, a minimum ph of 8.5 is required. The optimum range is For copper alloys, the preferred ph range is Usually there are combination of both metals in the system, and a compromise range of is recommended. This ph adjustment can be accomplished with caustic, ammonia, or amines, depending upon the overall water treatment objectives. where it is heated and scrubbed to release gases. As fresh steam enters the unit, water undergoes a second agitation to liberate most remaining impurities. Tray deaerators direct feedwater into a series of cascading trays. Water falls from tray to tray by overflowing or passing through small holes. Steam, which engulfs the trays, heats and deaerates the feedwater as it falls. Control of dissolved oxygen is the most frequently encountered problem. While it is very corrosive by itself, it magnifies the corrosion caused by carbon dioxide and ammonia. For example, CO 2 corrosion is times more detrimental in the presence of oxygen. This being the case, dissolved oxygen is removed by either chemically reducing the oxygen, mechanical deaeration, or in most cases, a combination of the two approaches. One of the greatest single causes for failure in a boiler system is the improper operation of the deaerator heater. As the name implies, a deaerator is an exchanger, which both heats and removes dissolved gases from the feedwater. Deaerators also provide a certain degree of feedwater storage. The deaerator takes advantage of the natural gas laws to regulate the temperature and pressure of the feedwater system in such a way that the noncondensable gases are removed from the feedwater. The most common type of deaerators are either the spray or tray type. A spray deaerator sprays feedwater into a steam filled space 31 the Analyst Volume 19 Number 4

3 Finally, combinations of spray and tray type heaters are used. Feedwater is first sprayed into a steam filled space, and then rains down on a series of trays through which it passes for further agitation and scrubbing. Live steam entering the deaerator meets the hottest water first, stripping it of dissolved gases. Carrying the gases along, the steam moves across and up through cascading water, gathering additional impurities as it flows. Steam is gradually condensed as it moves upward in the deaerator. Noncondensable gases are drawn off at a high point in the unit, where little steam remains. Sulfite The oxygen scavenging chemical reaction of sodium sulfite is as follows: SO 3 + 1/2O 2 (1) The disadvantage of sodium sulfite is its thermal instability at high pressures and its addition to the dissolved solids in the boiler water. This thermal decomposition results in the formation of sulfur dioxide and hydrogen sulfide according to the following equations: One additional type of deaerator should be noted here: the vacuum deaerator. In the vacuum deaerator, water cascades over packing such as raschig rings, etc. The pressure of the environment above the water is reduced using vacuum pumps, steam ejectors, etc. The degree of oxygen removal obtained in vacuum degasification will depend on a variety of variables. Therefore, finished oxygen limits have not been established. Typical degasification achieved with vacuum type deaerators will be in the area of 50 µg/l (ppb) to 500 µg/l of dissolved oxygen by volume. Because of these limits, boiler systems which utilize vacuum deaerators, will generally have another type of deaerator following the vacuum unit. SO 3 O + Heat SO 2 + 2NaOH (2) 4 SO 3 O + Heat 3 + 2NaOH S (3) Most low-pressure systems (less than 300 psig) use sodium sulfite because of cost and ease of handling. As pressures and temperatures increase, the breakdown potential increases and sulfite is not usually recommended. Use is usually limited to less than 600 psig. Above this pressure breakdown can occur. The breakdown products are corrosive gases (SO 2 and H 2 S) and can cause corrosion in the condensate system. Table 1 lists the attributes of sodium sulfite. Feedwater leaving the deaerator is usually specified as having mg/l (7 µg/l) of dissolved oxygen or less, assuming the deaerator is functioning per design specifications. Further reduction of oxygen levels requires the use of chemical reducing agents. Free carbon dioxide levels should be reduced to zero. Chemical Removal of Oxygen Sodium sulfite has historically been used to remove the last traces of oxygen remaining after good mechanical deaeration. Catalyzed forms are used to speed up the reaction with oxygen from 10 to 100 times the normal reaction time. Cobalt is the most common catalyst for sodium sulfite, although there is some use of copper catalysts. Sulfite should be fed as far back in the system as possible, preferably into the storage section of the deaerator, so that the entire feedwater system will be protected from oxygen attack. Temperature has an important influence on the reaction times, but a holding time of minutes is the minimum suggested for sulfite. Table 1: Sodium Sulfite/Catalyzed Sodium Sulfite Advantages Reacts at room temperature quickly Simple handling requirements Disadvantages Adds dissolved solids to the boiler water Cobalt catalyst or other transition metal catalysts can be precipitated in boiler water Not toxic Excess sulfite can decompose at 254 ºC (490 ºF) to produce acidic gases Sludge Conditioning Sludge conditioning either follows one of two directions: It is either conditioned and precipitated, or solubilized. The precipitating type of program either removes the hardness by forming calcium carbonate or by forming calcium phosphate and then conditioning it so that is a nonadherent sludge. Carbonates are formed from the soluble bicarbonates (found in the makeup water) reacting with sodium hydroxide, which is added into the boiler as a part of the treatment program. 33 the Analyst Volume 19 Number 4

4 If the phosphate sludge technique is used, then soluble sodium phosphates are added that react within the boiler to form calcium phosphate, which is conditioned to sludge. The conditioning chemicals used are soluble organic chemicals that change the precipitates formed (either carbonate or phosphate) to a non-adherent sludge. The solubilizer type program ties up all of the hardness through the use of a chelating agent. This will keep all of the hardness soluble despite the severe conditions that exist in a boiler. The most commonly used chelants are EDTA (Ethylene Diamine TetraAcetic Acid) and NTA (Nitrilotriacetic Acid). Phosphonates, which are weak chelating type chemicals are beginning used. The EDTA and NTA chelants are used at stoichiometric dosages. Polymer type dispersants are used as a backup should some of the hardness come out of solution. The phosphonates are always used with a polymer dispersant and are generally maintained at a substoichiometric dosage. General Internal boiler water treatment methods for scale and deposit control consist of precipitating water hardness in the boiler and adding a sludge conditioner to prevent scale buildup. In general, magnesium hardness is precipitated as magnesium silicate or magnesium hydroxide. Calcium is precipitated as either calcium carbonate using a carbonate cycle program or as tricalcium phosphate using a phosphate cycle program. Phosphate programs, especially when used with a polymer dispersant, give excellent scale and deposit control. However, stricter chemical controls or balances are necessary in the boiler water to obtain the desired results. Without tight control of the phosphate treatment program, undesirable deposits such as the very sticky magnesium phosphate sludge, along with calcium carbonate, will form. The carbonate cycle is important since it offers an opportunity of treating a boiler without any phosphate being present in the blowdown discharge. This could allow direct discharge to a water source, which restricts discharge. In addition, testing is simplified, no phosphate test is required since there is no phosphate excess being 35 the Analyst Volume 19 Number 4

5 maintained. As a result, carbonate cycle programs have a unique place in the industry use in low to medium pressure boilers (0-250 psig), operating on medium to high feedwater hardness (30 mg/l). Carbonate Technology Most natural waters have dissolved salts in the range of 50 to 500 mg/l. The cations that most frequently occur are sodium, calcium, and magnesium. The anions are chlorides, sulfates, silicates, and bicarbonates. In many cases, the bicarbonates form 50% or more of the total anions. When water containing these materials is heated, there are natural chemical reactions that occur. These include the following: 2NaH (released) (4) Ca(H Ca (released) (5) Mg(H Mg (released) (6) O 2NaOH + CO 2 (released) (7) Mg + 2NaOH Mg(OH (precipitated) + (8) It will be seen that the reaction of heat with bicarbonates causes a change to the carbonate. In the case of sodium and magnesium bicarbonate, the still completely soluble sodium and somewhat soluble magnesium carbonate are formed. In the case of the calcium bicarbonate, an insoluble product, calcium carbonate, occurs. Equation (4) shows that, upon further heating the sodium carbonate breaks down to form sodium hydroxide. This, in turn, can react with the magnesium (see equation 5) to form the insoluble magnesium hydroxide. These equations show that water, which is high in bicarbonate content, can provide its own carbonate and hydroxide alkalinity to precipitate its calcium and magnesium. Also note that all of these reactions release carbon dioxide gas, which normally passes over with the steam to form a corrosive acidic solution of carbonic acid. H 2 H 2 (9) The rate and degree to which equations 4-7 proceed is primarily a function of the temperature involved. In a deaerator, it is possible that most of the bicarbonates are converted to the carbonate form before they reach the boiler. However, the further reaction converting carbonate to hydroxide does not occur until higher boiler water temperatures are obtained. Many boiler feedwaters have enough natural alkalinity to theoretically precipitate their own calcium and magnesium. However, in actual practice, it is usually necessary to add either or both carbonate and hydroxide to the boiler water. This will always be necessary if the water does not contain enough natural alkalinity. Also, it often must be done to provide the excess alkalinity which is desired in boiler water. This excess is maintained for several reasons. First is to rapidly cause the reactions noted in equation 5 and 8. Another reason is that alkaline conditions in a boiler help to prevent corrosion problems from occurring. When maintaining alkalinity in a boiler, there are several points that should be considered. Note that equations 7 and 8 show that sodium carbonate can be used to precipitate magnesium hydroxide in a two-step reaction. However, carbon dioxide is formed in this reaction, which will carry over into the steam distribution system and form carbonic acid, and therefore it is often preferred that sodium hydroxide be added for the precipitation of magnesium salts. The range of alkalinity that can be maintained in a boiler water system when using the carbonate cycle is quite broad. However, very high alkalinities can be a cause of boiler foaming and carryover. Very high hydroxide alkalinity can be the cause of caustic embrittlement. However, these are alkalinities not normally encountered under usual operating conditions. As a broad control range, it is recommended that the Phenolphthalein or Total alkalinity be maintained in a range of mg/l. Some systems will require the hydroxide or OH alkalinity to be calculated as (2P-M) and maintained in a range of mg/l. Phosphate Technology Phosphate residual treatment programs are usually preferred over carbonate cycle programs for feedwaters that are consistently below 60 mg/l in hardness, have very low magnesium, and in which the silica content is more than ⅓ of the magnesium. Phosphate treatment is always preferred over coagulation treatments when the boiler pressure is more than 350 psig. 39 the Analyst Volume 19 Number 4

6 The use of phosphate in boiler chemistry is predicated on the extremely low solubility product for tricalcium phosphate. Hence, the addition of phosphate to boiler water, when tightly controlled, removes calcium so completely and efficiently that calcium sulfate, calcium carbonate, and calcium silicate scale can be entirely prevented. In the presence of sufficient alkalinity, the actual phosphate precipitation formed is hydroxyapatite, which is a less sticky, more readily conditioned reaction product than tricalcium phosphate. Although phosphates can also precipitate magnesium as magnesium phosphate, proper boiler water chemistry will preferentially precipitate magnesium as less adherent and more easily conditioned magnesium hydroxide and/or magnesium silicate (preferred). Calcium can be precipitated with the carbonate ion as calcium carbonate through the addition of soda ash to the boiler. Phosphates, where economically and environmentally justifiable, are preferred because of better thermal stability (no objectionable CO 2 release into the steam). Phosphates are always preferred over the carbonate cycle where the feedwater hardness is low. There are numerous available chemicals that furnish the phosphate radical necessary for internal softening treatment. Compounds of orthophosphate ion are the most widely used, namely mono, di, trisodium phosphates, and phosphoric acid. Other phosphate ions are the poly and pyro. Some of these salts are called polyphosphates because they form inorganic polymers. Among these are the glassy sodium polyphosphates (hexametaphosphate - SHMP) and crystalline sodium polyphosphates such as the sodium tripolyphosphates and tetrasodium pyrophosphate. The poly and pyro phosphates can be described as molecularly dehydrated phosphates. Upon addition to water and at a rate that is temperature dependent, these phosphates rehydrate to the orthophosphate form from which they were derived. The precipitation of tricalcium phosphate (or hydroxyapatite) in the boiler can only occur when the phosphate in use has been converted to trisodium orthophosphate by heat and reaction with boiler water alkalinity. If a product is to be fed directly into the boiler, for example into the steam drum, an orthophosphate based 41 the Analyst Volume 19 Number 4

7 product should be used because the conversion to trisodium orthophosphate is extremely rapid. Conversely, feeding a poly or pyro form polyphosphate directly to the boiler may delay chemical reactions and allow undesirable reactions to take place. When feeding the product into the feedwater system, then a polyphosphate product would be the product of choice in order to avert the possibility of softening reactions occurring in the feedwater lines, economizers, etc. An orthophosphate product would not be suitable for feedwater application at all since quick reactions might allow precipitation in the feedwater lines. Each of the phosphate types commonly used will have an effect upon the boiler water alkalinity as well. When hardness in the feedwater is precipitated by direct reaction with alkalinity, a decrease in available alkalinity generally results: Ca(H Ca (precipitated) + CO 2 (released) O (10) Ca + Ca + (11) Once the phosphate has been converted to trisodium orthophosphate the reactions with calcium hardness proceed as follows: 2Na 3 + 3Ca ( (17) 2Na 3 + 3Ca ( (18) In practice, an additional amount of phosphate is carried as a residual, which means that an additional amount of alkalinity will be consumed (equations 14-16) for each mg/l of phosphate residual that is maintained. The net gain or loss in available alkalinity becomes a function of: (1) type of phosphate used, (2) amount and form of calcium hardness, (3) level of magnesium hardness and (4) the level of residual phosphate carried in the boiler. These principles can be demonstrated perhaps somewhat more clearly by considering typical calcium phosphate reactions as one, rather than as two separate steps: 2Na 3 + 3Ca ( (19) Mg(H + 4NaOH Mg(OH + 2 O (12) MgCl 2 + 2NaOH Mg(OH + 2NaCl (13) However, when phosphates are used to precipitate the calcium hardness, some of the alkalinity associated with calcium hardness is retained in the form of sodium alkalinity in the boiler. The amount of alkalinity retention depends upon the type of phosphate being used. Since phosphates other than trisodium orthophosphate must be converted to trisodium orthophosphate in the boiler before precipitation of calcium occurs, some alkalinity consumption results: Na 5 P 3 O NaOH 3Na 3 O (14) (Tripoly) H + NaOH Na 3 O (15) (Disodium) NaH 2 + 2NaOH Na 3 O (16) (Monosodium) 2 H + 3Ca ( + CO O (20) (Disodium) 2NaH 2 + 3Ca ( + +2CO 2 O (21) (Monosodium) 2Na 5 P 3 O Ca + 2NaOH 3Ca 3 ( CO 2 O (22) (Tripoly) 2 H + 3Ca + 2NaOH ( O (23) (Disodium) The above equations (19-23) demonstrate the effect on retained sodium alkalinity. To these equations must then be added those controlling magnesium precipitations (equations 12-13) and those controlling phosphate conversions for phosphate residuals (equations 14-16). While this is complex, the above reactions are extremely important. And, when coordinated phosphate ph technology is used, they assume even greater importance. 43 the Analyst Volume 19 Number 4