Calculating Boiler Chemical Requirements

CHEMICAL EQUIVALENTS

A. Equivalents and equivalent weights

The concept of equivalent weights, equivalents, and equivalents per million (epm) is most useful in calculating basic chemical dosages. The equivalent weight of any ion, radical or compound is that weight which will combine with or replace a unit weight of hydrogen. All ions (and compounds) react together in the ratio of their respective equivalent weights. For example, one equivalent weight of sodium (23) reacts with one equivalent of chloride (35.5) to form one equivalent of sodium chloride (58.5).

The equivalent per million (epm) is determined by dividing the concentration in ppm of a substance by its equivalent weight. Once this is determined we can calculate the amount of any chemical required to react with that ion. This is done by multiplying the epm of the ion by the equivalent weight of the treatment chemical. The results provides the chemical dosages in ppm.

B. Purity or active ingredients of a treatment chemical

In making chemical calculations we must adjust equivalent weights of substances to reflect their active content or purity. For example, the equivalent weight of hydrated lime (calcium hydroxide) is 37.1, assuming the lime is 100% active. However, hydrated lime is only 90% active. The equivalent weight of hydrated lime must by adjusted by dividing the equivalent weight of pure lime (37.1) by the activity (0.90) which yields an equivalent weight for the product of 41.1.

C. Analyses in terms of calcium carbonate

In making field analyses, we report hardness and alkalinity in terms of calcium carbonate equivalents, instead of as calcium or magnesium ions. To determine the epm of hardness or alkalinity, we divide the concentration in ppm as calcium carbonate by 50, which is the equivalent weight of calcium carbonate.

Examples:

1. Calcium as Ca ion = 100 ppm

Equivalent weight of calcium carbonate (CaCO3) = 50

Equivalent weight of calcium (Ca) = 20

Calcium as CaCO3 = 100 ppm X 50 = 250 ppm

20 ppm

The multiplying factor to convert from calcium as the ion to the calcium carbonate equivalent is 2.5 which is the ratio of the equivalent weights.

2. To go from ppm as CaCO3 to Ca, reverse the process.

250 ppm calcium as CaCO3 X 20 ppm = 100 ppm as Ca

50 ppm

SCAVENGING DISSOLVED OXYGEN

A. Estimating the dissolved oxygen content of boiler feedwater

The dissolved oxygen content of feedwater can be estimated from the temperature of the water and the working pressure of the deaerator. The following table presents the dissolved oxygen concentration at a given feedwater temperature and pressure.

Maximum Expected Dissolved Oxygen Level

at Listed Deaerator Temperatures and Pressures

T

e

m

p

e

r

t

u

r

e

Deaerator Working Pressure, psig

Dissolved Oxygen

0

2

5

7

10

Cc/liter

ppm

212

218

227

232

239

0.0

0.0

205

212

221

227

235

0.5

0.7

197

204

215

221

230

1.0

1.4

188

195

208

215

224

1.5

2.2

178

186

200

208

218

2.0

2.9

164

175

191

200

211

2.5

3.6

148

162

180

190

204

3.0

4.3

133

149

169

181

196

3.5

5.0

118

134

155

170

186

4.0

5.7

B. Sodium sulfite dosage

Once the oxygen content of the feedwater has been estimated, the sulfite dosages can be calculated. The sulfite dosage is the sum of the ppm needed to neutralize the dissolved oxygen, plus additional amounts needed to produce an acceptable boiler water residual. For low to moderate pressures, sulfite residuals range from 20 to 40 ppm . The excess required depends on the residual desired in the boiler, and the number of feedwater concentrations maintained in the boiler as controlled by blowdown.

The theoretical dosage of sodium sulfite (100% purity) is 8 ppm sulfite for every 1 ppm (0.7 cc per liter) of dissolved oxygen. However, correction must be made for the activity or purity of the commercial sulfite, which is about 90%, and for the efficiency of the scavenging reaction. From a practical viewpoint, the sulfite dosage is 10 ppm per ppm of dissolved oxygen in the feedwater. Additional sulfite must then be added to produce the required sulfite residual

C. Sample Calculation

Assume boiler is operating at 10 cycles of concentration with feedwater at 205 oF. This equates to a dissolved oxygen content of 0.7 ppm or 0.5 cc per liter. Assume a feedwater demand of 500,000 lbs per day. The desired sulfite residual is 30 ppm.

1. For fixation of dissolved oxygen: 0.7 ppm x 10 ppm = 7 ppm

7 ppm sulfite/120 = 0.0583 lbs per 1000 gallons

2. For sulfite residual at 10 cycles: 0.0250 lbs per 1000 gallons

3. Total sulfite requirement = 0.0833 lbs / 1000 gallons

4. Sulfite required per day is:

500,000 lbs/day X 0.0833 lbs/1000 gal = 5.0 lbs/day

8.34 lbs/gal

D. Hydrazine Calculations

The theoretical dosage of hydrazine (100% active) is 1 ppm per ppm of dissolved oxygen. Because of the explosive nature of pure hydrazine, it is available for industrial use in 35% active solutions. It, therefore, takes 3 ppm of 35% active hydrazine to neutralize 1 ppm of dissolved oxygen. In addition, a low residual of from 1 to 3 ppm is required in the boiler to accelerate the reduction of ferrous oxides.

ADJUSTING BOILER WATER ALKALINITY

A. Soda Ash Requirement

Soda ash (Na2CO3) may be used to increase the boiler water alkalinity. It partially decomposes at boiler temperatures and pressures to produce caustic soda (NaOH) and carbon dioxide (CO2) gas. Since the carbon dioxide adds to the neutralizing amine demand in the condensate, the use of caustic soda as the alkalinity builder is preferred.

The soda ash requirement for treatment of a given feedwater is determined from the non-carbonate hardness (H-M) plus the required excess over and above the non-carbonate hardness. “H” and “M” values are determined as calcium carbonate from feed water analysis.

Example: Field tests on a given feedwater show H = 84 ppm and M = 59 ppm. Non-carbonate hardness (H-M) = 25 ppm as CaCO3.

Assume boiler operates at 10 cycles of concentration, and the residual boiler water alkalinity, M, is 250 ppm. To develop this alkalinity, we need 25 ppm soda ash (as CaCO3) in the feedwater (250 ppm boiler alkalinity divided by 10 concentrations), over and above the amount required to neutralize non-carbonate hardness in the feedwater.

From the preceding data, soda ash required is non-carbonate hardness plus excess = 25 ppm + 25 ppm = 50 ppm as CaCO3. Dividing 50 ppm by 50 (equivalent weight of CaCO3) we find 1 epm of soda ash is required.

The equivalent weight of soda ash (Na2CO3) is 53.0. So, 1 epm required X 53 = 53 ppm soda ash needed. If we divide 53 ppm by 120 we determine that 0.44 lbs. per 1000 gallons of soda ash is required.

B. Caustic Soda

Caustic soda is commonly used for direct upward adjustment of the total alkalinity in the boiler. The equivalent weight of caustic soda (NaOH) is 40. It takes 0.8 ppm NaOH (as 100% active) to produce a 1 ppm increase in total alkalinity as CaCO3. Caustic soda is commonly purchased as 50% liquid solution. In this case, 1.6 ppm of 50% liquid caustic is required to produce a 1 ppm increase in total alkalinity as CaCO3.

Example: The boiler feedwater is zeolite softened (0 total hardness). Total alkalinity of the feedwater is determined from field tests to be 15 ppm. The desired alkalinity in the boiler is 400 ppm “M” alkalinity. The boiler operates at 20 cycles of concentration. What is the required dosage of caustic soda when purchased as a 50% liquid solution.

The total alkalinity required in the feedwater is 400 ppm divided by 20 cycles of concentration = 20 ppm. Natural alkalinity is 15 ppm. The supplemental alkalinity requirement is, therefore, 20 ppm minus 15 ppm = 5 ppm required as calcium carbonate.

Dividing 5 ppm by 50 (equivalent weight of calcium carbonate) gives 0.10 epm caustic soda (as 100% active), or 0.20 epm as 50% active. The equivalent weight of NaOH is 40, so 0.20 epm X 40 = 8 ppm caustic soda (50%) required. Dividing 8 ppm by 120 gives 0.067 lbs of 50% caustic soda per 1000 gallons of feedwater is required to produce 400 ppm total alkalinity as CaCO3 in the boiler.

PHOSPHATE REQUIREMENTS

A. Phosphate products

Many products are available, both in powder and liquid form, to provide the required phosphate residual in the boiler. In most cases, the available phosphate in a water treatment product will be given as percent P2O5, but the specific mixture of phosphates used will not be disclosed. The P2O5 content and equivalent weights for various phosphates are tabulated in the following table.

Equivalent Weights of Phosphate Products

Phosphate Compound Approx. %P2O5 Equiv. Wt.

Trisodium phosphate – 12 H2O 18.7% 126.7

Trisodium phosphate (anhydrous) 43.2% 54.7

Disodium phosphate – 12 H2O 19.8% 119.4

Disodium phosphate (anhydrous) 50.0% 47.3

Monosodium phosphate – 1 H2O 51.4% 46.0

Monosodium phosphate (anhydrous) 59.1% 40.0

Sodium hexametaphosphate (anhydrous) 67.5 to 69.0% 34.0

Sodium tripolyphosphate (anhydrous) 57.0 to 58.0% 40.9

P2O5 100% 23.7

From these figures we can determine the equivalent weight of any phosphate or phosphate mixture from which the P205 content is known. Divide the equivalent weight of P2O5 (23.7) by the percentage of P2O5 in the product.

B. Phosphate Calculation

Phosphate reacts with feedwater hardness and OH alkalinity to produce hydroxyapatite, an insoluble sludge. If we write the chemical equation for hydroxyapatite as 3Ca3(PO4)2 * Ca(OH)2 and calculate the molecular weight of the compound, we see that only a little over 90% of the calcium reacts with the phosphate. The remainder combines with OH alkalinity to form calcium hydroxide. Since only 90% of the calcium hardness reacts with phosphate, the calculated amount of phosphate can be reduced by 10%.

Example: Determine the amount of disodium phosphate required to precipitate 30 ppm calcium (as Ca) to form hydroxyapatite.

30 ppm calcium as calcium ion equals 75.o ppm calcium as calcium carbonate, or 1.5 epm. Anhydrous disodium phosphate has an equivalent weight of 47.3. Therefore, to produce hydroxyapatite, multiply the epm calcium times the equivalent weight of disodium phosphate time 0.90 to determine the parts per million disodium phosphate required. The 0.90 factor reflects that only 90% of the calcium reacts with the phosphate. Completing this equation we have:

1.5 X 47.3 X 0.9 = 64 ppm disodium phosphate (anhyd)

This is the amount of phosphate required to react with 30 ppm calcium ion. 64 ppm disodium phosphate divided by 120 equals 0.533 lbs disodium phosphate per 1000 gallons treated water.

If we compare the calcium content expressed as calcium carbonate (75 ppm) with the amount of disodium phosphate required (64 ppm) we see that 0.85 ppm disodium phosphate is required to react with each ppm calcium expressed as calcium carbonate. Or, if we express calcium as the ion, about 2.13 ppm phosphate is required to react with each ppm calcium.

SLUDGE CONDITIONERS

A. Polymers

Natural and synthetic polymers are routinely used to condition the sludge produced by the hardness precipitation reactions with phosphate and alkalinity in the boiler. Natural tannins, lignins, and synthetic acrylates and polyacrylates are examples. Typical polymer dosages are between 5 and 25 ppm as 100% active polymer. Boiler polymers are marketed as dilute solutions of these active ingredients, however, so overall product dosages are between 100 and 500 ppm.

Since boiler polymers are non-volatile, they concentrate in the boiler. Chemical procedures are available for estimating the polymer residual, but the tests are difficult to perform, and the accuracy is not very good. For these reasons, the dosage of polymeric sludge conditioners are frequently determined by direct calculation from steam production and cycles of concentration data. The dosage of the polymer product is adjusted by regulating the output of the chemical pump, or boiler blow down.

B. Chelants

Chelants such as Na4EDTA and Na3NTA are used as internal treatments for scale control in boilers. Chelants must be used with oxygen scavengers, alkalinity builders, antifoam, and polymer additives for a complete boiler water treatment program. The major advantage of chelant treatment is that no insoluble deposits form. Calcium and magnesium are held in solution.

Four (4) parts of Na4EDTA (as the dry salt) are required to complex 1 part of calcium hardness. Liquid solutions of chelant require a proportionately higher dosage. Because of the expense of chelants compared to phosphate-based programs. Chelant treatment is only appropriate for high quality feedwaters averaging less than 2 ppm total hardness; preferably less than 1.0 ppm. EDTA must be added after feedwater has been deaerated and oxygen traces scavenged with catalyzed sulfite. Otherwise, EDTA reacts with dissolved oxygen. (1 ppm O2 destroys 100 ppm EDTA)

EDTA is used in boilers up to 1200 psig. The practical limit for NTA programs is about 850 psig. For practical purposes, NTA is no longer commonly used in boiler applications.

This material originally prepared by:

J. Fred Wilkes

Consulting Chemical Engineer

P.O. Box 2320

Titusville, FL 32781-2320

Revised and edited by:

William F. Harfst

Chemical Consultant

Harfst and Associates, Inc.

P.O. Box 276

Crystal Lake, Illinois 60039

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s