4 phosphate chelants

8/2/2019 4 Phosphate Chelants

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PHOSPHATE

PURPOSE: To remove boiler water calcium as sludge instead of hard scale.

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Some calcium is present in the feed water in almost all steam generating plants. In the processof steam generation, calcium is concentrated in company with other components of the boilerwater, tending to precipitate eventually as calcium scale (calcium carbonate, calcium sulfate,and/or calcium silicate). In the absence of protective internal treatment, the rate of scale

accumulation depends upon the calcium content of the feed water and the rate of steamgeneration. Calcium scale is extremely objectionable; it may obstruct heat transfer and causemetal to fail by over heating, or it may slough off and then collect in places where it canseriously interfere with heat transfer and/or fluid flow.

Phosphate is used specifically to remove calcium from solution so it cannot produce calciumscale. Under proper conditions, the reaction between calcium and phosphate produces afinely divided, nonadherent sludge (calcium phosphate) which remains suspended in the boilerwater until removed through blow down. Prevention of calcium scale requires that the boilerwater contains a small reserve of soluble phosphate at all times. A carefully designedphosphate test is used to determine this important constituent in the boiler water; and it is most

essential that this test be performed on perfectly clear samples which contain no particles of"spent" phosphate in the form of suspended calcium phosphate sludge.

The amount of reserve phosphate required in boiler water depends upon the calcium contentof the feed water, boiler pressure and the type of boilers involved, blow down procedure, andthe chemical feeding method employed. Maximum and minimum phosphate control limits arenormally 10-60 ppm. Loss of the reserve of the phosphate must almost inevitably result indeposition of calcium scale. Therefore, you should never hesitate to increase the feed ofphosphate whenever boiler water phosphate concentrations run near or below the specifiedminimum.

Maintaining unnecessarily high phosphate reserves is wasteful of phosphate, a relatively



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expensive treating chemical. Furthermore, if the feed water contains magnesium--as mostfeed waters do-- high phosphate may result in precipitation of magnesium phosphate, a veryobjectionable type of adherent sludge. In general, therefore, it is undesirable to maintainphosphate reserves in excess of the recommended maximum limit. Reduce the rate ofphosphate addition whenever boiler water phosphate concentrations approach or exceed theupper limit.

The purpose of phosphate treatment is to precipitate all boiler water calcium in the form ofcalcium phosphate sludge. Although calcium phosphate is normally nonadherent, it may comedown in adherent form if boiler water alkalinity is too low; therefore, proper conditioning withphosphate also requires that boiler water alkalinity be maintained within specified limits.Prevention of objectionable magnesium phosphate necessitates that a certain relationshipexist between boiler water phosphate and silica concentrations; this frequently calls for the useof a silicate chemical in conjunction with phosphate, the purpose being to favor precipitation ofdesirable magnesium silicate in preference to undesirable magnesium phosphate. A suitabledispersive material is sometimes used to help keep the precipitated sludge from settling outor otherwise adhering to the boiler surfaces.

Phosphate may be fed in solution with almost any combination of other water conditioningchemicals. Either continuous or intermittent feeding is appropriate to introduce phosphatesolution directly to boilers. Unless feed water hardness is quite low (1 to 2 ppm at most)phosphate should not be fed continuously to feed water systems; the danger of feed line scaledeposition would be to great. However, solutions of sodium hexametaphosphate can be fedintermittently in shots to almost any feed water system,provided each shot is added quickly(within 3 minutes) with no " dribbling" at the beginning or end of the feeding period. Whenphosphate is fed intermittently, the duration of any one shot should not exceed 5% of the timeinterval between shots. The usual method for feed of phosphate, is to tie the chemical feedpump into the feed water pump. In this manner, the phosphate is fed when the boiler makes up.

CHELANTS

PURPOSE: To remove cationic deposit forming salts, as calcium and magnesium,



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through the formation of a soluble by-product.

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Some cationic salts of calcium, magnesium and in many cases iron, are present in the boilerfeed water. These salts will concentrate up in the presence of other boiler water componentsduring the process of steam generation. In that these materials are inversely soluble in wateras the water temperature increases (as the water temperature increases, calcium magnesiumand iron lose the ability to remain in solution and drop out as a hard solid precipitate). Thisinverse solubility causes these salts to drop out of solution in the boiler water and form aninsulative deposit on the boiler metal surfaces. This insulative deposit will then obstruct heattransfer across the tube which leads to; metal failure due to overheating, fuel wastage, andinhibition of fluid flow.

A chelant prevents the formation of these deposits by reacting with the cations (positivelycharged particles of calcium, magnesium and iron) in the feed water, to form a soluble salt.Hence, if a test tube of hot feed water were to have a scoop of calcium carbonate added to it,

it would appear to have a heavy adherent white sludge on the bottom of the test tube. Add tothis test tube a sufficient amount of chelant to react with the calcium carbonate and the testtube will appear to have nothing but clear feed water in it. The chelant reaction has created asoluble salt with the calcium and this reaction has made the calcium soluble enough to bebrought back into solution.

Chelants can therefore provide improved boiler e fficiency over most other internal programs,by removing the deposition potential from the boiler water. In addition, a chelant will seekcalcium and magnesium in deposits that have previously formed in the boiler. This procedureis called a "CHELANT CLEAN UP" and it essentially is the process whereby old deposits are

slowly removed by solubilizing the deposits binders (calcium and magnesium), thus allowing

the deposit to slowly slough off of the tubes. Chelant clean up programs are enhanced with theuse of dispersants. Dispersants improve the ability of the chelant to remove old deposits byassisting in the suspension of the sludge formed from the chelant clean up of deposits. It isimportant that the dispersant being used in this case is specific to the type of deposit that isbeing removed, for optimum performance.

The two chelants usually used in boiler water treatment are: EDTA (ethylenediaminetetraaceticacid) and NTA (nitrilotriacetic acid). The molecules that these chelants form with boiler watercations are very stable, so they stay in solution. This removes the cations from availability toreact and form; calcium carbonate, calcium sulfate, magnesium hydroxide, magnesiumsilicate, and iron compounds.

The small molecular size of the chelant molecule is what allows the combination of feed water



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cations and chelants to be so stable and as such so soluble in the boiler water. Therefore, thisreaction creates a soluble salt rather than a solid precipitant in the boiler. This solid saltformation is one that reduces the level of solids in the boiler. Reduction of solids will then allowthe system to reduce the quantity of blow down from the boiler. All of this relates into; less fuelwasted in blow down, a cleaner boiler, and with a cleaner boiler, a more efficient fuel to steamoperation.

A chelant should be fed through a stainless steel quill. The quill is to be placed in the feedwater line, down stream of the feed water pump and at least five feet up stream from any bendin the feed water line. The proper installation will prevent copper loss from the feed water pumpimpeller as well as any erosion of the feed water piping at elbow bends, while allowingmaximum contact between the chelant used and the feed water that is to be treated.

Feeding a chelant to the feed water line has the following advantages:

* The chelant can react to form a soluble molecule prior to entering the

boiler, thus, drastically reducing the potential for boiler deposit

formation.

* EDTA breaks down at 300 psi and NTA will break down at 900 psi.

Their chelant/cation salt complexes are both stable up to 1300 psi.

Thus, the maximum benefit from using a chelant can be obtained

from the use of feed water line application.

To realize the greatest benefit both technically and economically from the use of a chelant, thefeed water must be of high quality. The allowable maximum limits for contaminants in the feedwater for the effective use of a chelant are:

Total Hardness 0-2 ppm

Iron 0-2 ppm

Must Be Oxygen Free

Should the contaminant levels in the feed water be higher than the above limits, then a chelantprogram will most likely not be economical for use in the boiler water treatment program.

Chelants can also be used in combination with other internal treatment programs, based upon

the system conditions. In a plant with high quality feed water and good control, achelant/polymer program would be the most effective. Where feed water quality and control aregood, but not of the highest quality, then a phosphate/chelant/polymer program should be usedby the boiler plant.

Chelant/polymer programs have the potential for being the most effective scale preventativeprograms for boilers that are supplied with high quality feed water. The polymer (dispersant)



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used in this type of program should be selected based upon the type of contaminants that arefound in the particular feed water that is being treated, or the type of deposit that has beenfound in the boiler. This selection will allow the polymer to compliment the chelant properly.

Phosphate/chelant/polymer programs are usually used where the chelant is of a sufficient levelto react with the majority of the scale forming agents in the feed water, and the phosphate isthen used to tie up the remaining calcium salts, while acting as a product level indicator.

This type of a chelant program approach is referred to as "sub stoichiometric" chelant use.The basic concept is that the total chelant requirement for the boiler to be treated is calculated,and then 80% of this chelant requirement is fed. The approach yields practically every benefitachieved with a chelant program, without the possibility of a potential chelant overfeed. Themethod is also used because of its ease of testing. The phosphate in the product allows for asimple method for the testing of product level, while acting as a buffer against chelantcorrosion.

Chelants are very different to test for accurately. Due to this problem, many times, a plant willshow adequate chelant levels while in reality the chelant demand is much higher than the tests

indicate. Even more serious, many of the chelant tests used today will show insufficient levelsof chelant when there is actually an overfeed situation occurring in the boiler. In this case, theboiler metal will be eaten away while the operator is increasing chelant feed rates to overcome what they think is a scaling condition.

With this type of difficulty, many programs are designed for base feed. This uses the quantityof make up or feed water used in the boiler in a day, and the total ppm of cations in the makeup or feed water to establish a feed rate for the chelant. In most cases, this procedure isaccomplished using a simple chart. In order to simplify matters, many chelant programs goback to the phosphate/chelant/dispersant approach for accuracy of product level. They alsouse this approach, because of the additional benefit of anion competition between the chelant

and the phosphate. this competition creates a condition where the phosphate will act as abuffering agent against chelant attack on the boiler metal, in the event of an accidental chelantoverfeed.

Chelant use is extremely beneficial, as long as it is used properly and under the properconditions.

DISPERSANTS/SLUDGE CONDITIONERS

PURPOSE: To minimize tendency of sludge to adhere to boiler surfaces. To fluidize

all solids, thus, enhancing solids removal from the boiler through blow down.

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Hardness and other solids entering the boiler will tend to be precipitated as insoluble solid



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particles. The calcium is precipitated as calcium phosphate while the magnesium comesdown as magnesium silicate or occasionally, magnesium hydroxide. Representing the desiredend products of good internal treatment, the calcium phosphate and magnesium silicatenormally occur as finely divided sludge, most of which remains suspended in the boiler wateruntil removed through blow down. If a chelant is utilized, the volume of precipitant will be greatlyreduced as EDTA will combine with hardness to form a soluble salt. In any case, properdispersant program will greatly enhance the effectiveness of any treatment program.

In specific cases, sludge may tend to remain in the boiler, either as a shoal or sediment in aheader or as an adherent coating on boiler surfaces, especially near the point where mixing offeed water with boiler water occurs. Use of a dispersive represents one means employed tokeep sludge from accumulating in boilers. Disperants behave something like this: when

particles of sludge first start to form and are still very tiny, they become encased in a

sheath of organic material which keeps them from growing larger or combining with

one another to form adherent or rapidly- settling masses.

These groups attach themselves to an active site in the growing crystalline scale. Thepresence of the large polymer molecule on the growing crystal alters its growth. The result is

crystal distortion and particle repulsion of the subsequently formed colloids in the boiler water.The effectiveness of a polymer is determined by its type, molecular weight and concentration.

Generally speaking, the higher the molecular weight of a polymer, the greater its ability to actas a coagulant. On the other hand, the lower the molecular weight, the more a polymer will actas a dispersant. It is the anticoalescent or dispersancy characteristics of the polymer whichenable it to function as a boiler water sludge conditioner. The proper application of a syntheticpolymer in a boiler water treatment program will change a poorly hydrated, granular andadhesive boiler sludge into a light and fluffy, highly hydrated sludge which is non-adherent towaterside surfaces.

Dispersants in general are synthetic polymers. A synthetic polymer is a long chain molecule ofrepeating units, with each unit being an active group.

The most commonly used synthetic dispersants are:

POLYACRYLATE Good for calcium carbonate conditioning. Forms a light

non-adherent sludge. Can disperse phosphate salts in low

alkalinity conditions.

POLYMALEIC Crystalline modifier for calcium carbonate and phosphate

salts. Excellent clean up material for boilers with calcium

or phosphate based scale. Very stable polymer that can



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bring scale off of metal surfaces by pulling the scale

binder out of the scale into the boiler water.

POLYMETHACRYLATE A second generation acrylate that reacts similar to

Polyacrylate. Should not be over fed as it could break down.

More active than Polyacrylate on calcium salts.

SULFONATED Highly charged anionic dispersant that can specificallyCOPOLYMER attach to metallic ions as soluble iron in the boiler.

CARBOXYLATED Extremely active dispersant for the suspension of calcium salts.

COPOLYMER Also, very effective on iron deposits.

TERPOLYMER A combination polymer containing, carboxylated, sulfonated

and nonionic plane repulsion members. This makes up a

dispersant with three dispersant groups. A strong acid

(sulfonate) for surface charge repulsion on the particle, a weak

acid (carboxylate) for surface charge attachment on the

particle, and a nonionic dispersant for broad range dispersancy

of particle charge surfaces. It will disperse both hydrated and

nonhydrated iron oxide sludge as well as phosphate and

calcium salts in both the tricalcium phosphate and the

hydroxyapatite forms.

Dispersants should be fed on a continuous basis to the boiler feed water in order to be able tocontinuously supply fresh product to the region of the system where precipitating reactions aretaking place. Since there are no satisfactory field tests for the control of a dispersant, they areusually fed in a proportional manner, using the amount of make up water, feed water or in

relation to another product which there is a easy field test for.

Thus, although the minimum and the maximum limits are specified for the dispersiveconcentration in the boiler water, treatment is actually based on the feed of (X) number ofpounds of dispersant per million pounds of Make Up water, with consideration for the amountof blow down from the boiler.

The addition of dispersant is based on:

Required dosage in ppm X Millions of pounds of Make Up per Day = #/Day DispersantCycles

Make Up Cycles are found by:



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Boiler Neutralized Conductivity = Make Up Cycles

Raw Water Conductivity

4 phosphate chelants

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