Acid Dew Point Temperature T dp

First one should have an idea of the acid dew point temperature T dp to find out if corrosion is a possibility. A practical way to determine T dp is using a dew point meter. Another method is to use correlations available in the literature. Presented in the following text are a few correlations for sulfuric acid available in the literature. Table 1.20 shows the dew point correlations for a few acid vapors.

TABLE 1.20

Dew Points of Acid Gases a Hydrobromic acid

1000/T dp = 3.5639 – 0.1350 ln P H 2 O – 0.0398 ln P HBr + 0.00235 ln P H 2 O ln P HBr Hydrochloric acid

1000/T dp = 3.7368 – 0.1591 ln P H 2 O – 0.0326 ln P HCl + 0.00269 ln P H 2 O ln P HCL Nitric acid

1000/T dp = 3.6614 – 0.1446 ln P H 2 O – 0.0827 ln P HNO 3 + 0.00756 ln P H 2 O ln P HNO 3 Sulfurous acid

1000/T dp = 3.9526 – 0.1863 ln P H 3 O + 0.000867 ln P SO 2 – 0.000913 ln P H 2 O ln P SO 2 Sulfuric acid

1000/T dp = 2.276 – 0.0294 ln P H 2 O – 0.0858 ln P H 3 SO 4 + 0.0062 ln P H 2 O ln P H 2 SO4

Source: Okkes, A.G., Hydrocarbon Process., July, 1987; Hsiung, K.Y., Chem.

Engineer ., 1981, 127; Ganapathy, V., Oil Gas J., 1978, 105. a T dp is dew point temperature (K), and P is the partial pressure (mmHg).

Compared with published data, the predicted dew points are within about 6 K of actual values except for H 2 SO 4 , which is within about 9 K.

Combustion Calculations

Example 1.19

Flue gas analysis in a boiler is as follows: % H 2 O = 12, SO 2 = 0.02, HCl = 0.0015, and rest

oxygen and nitrogen. Gas pressure = 250 mm wc. Compute the dew points of sulfuric and hydrochloric acids given that 2% of SO 2 converts to SO 3 . To use the correlations in Table 1.20, one must convert the gas pressure to mmHg (mm mercury).

250 mm wc = 250/13.6 = 18.38 mm Hg. Absolute pressure = 760 + 18.38 = 778.38 mm Hg or 778.38/760 = 1.02457 atm abs. p w = 0.12 × 1.02457 × 760 = 93.44 mm Hg. ln p w = 4.537 p HCL = 0.0015 × 778.38 = 0.1168 mmHg. ln p HCl = –2.1473 p SO3 = 0.02 × 0.0002 × 778.38 = 0.0031 mmHg. ln p SO3 = –5.7716

For HCl, T dp = 3.7368 – 0.1591 × 4.537 + 0.0326 × 2.1473 – 0.00269 × 4.537 × 2.1473 = 3.0588 or T dp = 327 K = 54°C = 129°F. Note that HCl dew point is low and close to water dew point, while sulfuric acid dew point is much higher.

For sulfuric acid (Verhoff correlation), 1000/T dp = 2.276 – 0.0294 × 4.537 + 0.0858 × 5.7716 – 0.0062 × 4.537 × 5.7716 = 2.4755 or

T dp = 404 K = 131°C = 268°F. The following correlation for sulfuric acid was proposed by Okkes [5]:

T dp = 203.25 + 27.6 log p w + 10.83 log p SO3 + 1.6 (log p SO3 + 8) 2.19

p SO3 = 0.0031 mmHg = 4.1 × 10 –6 atm. Log p SO3 = –5.3872 p w = 93.44 mmHg = 0.1229 atm. Log p w = –0.9104

T dp = 203.25 – 27.6 × 0.9104 – 10.83 × 5.3872 + 1.6(2.6128) 2.19 = 128.4°C = 263°F

Haase and Borgman proposed the following correlation based on experimental data [3]:

T dp = 255 + 27.6 log (PH 2 O)+18.7 log (PSO 3 ) where T dp is in °C, gas partial pressures in atm. Using this, we get T dp = 255 – 27.6 × 0.910 – 18.7 × 5.387 = 120°C = 264°F.

Once an idea of the acid dew point temperature is obtained, one may proceed to compute the water dew point also as discussed later and evaluate the energy recoverable. Suitable materials such as alloy 22 steels or 304 stainless steels may be selected for the exchanger depending on the acid concentration. Calculation of tube wall temperatures at the cold end of an economizer is shown in Chapter 3 using which method one may establish how many rows of tubes are prone to condensation of acid or water vapor. Similarly, calculation of cold-end temperatures at normal and part loads is shown for a tubular air heater in Chapter 4. Plant engineers may perform similar calculations and ensure that the lowest tube wall temperatures are above the dew point of acid or water vapors to ensure that no corrosion occurs. The other option is to select materials to minimize corrosion for such equipment. This is discussed in Chapter 6.

40 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

References

1. V. Ganapathy, Applied Heat Transfer, Pennwell Books, Tulsa, OK, 1982, p13. 2. Babcock and Wilcox, Steam: Its Generation and Use, 38th edn., New York, 1978. 3. V. Ganapathy, Industrial Boilers and HRSGs, CRC Press, Boca Raton, FL, 2003, p238. 4. ASME, Power Test Code. Performance test code for steam generating units, PTC 4.1, New York,

ASME 1974. 5. AG Okkes, Get acid dew point of fluegas, Hydrocarbon Processing, July 1987. 6. K.Y. Hsiung, Predicting dew point of acid gases, Chemical Engineering, Feb. 9,1981, p127. 7. V. Ganapathy, Estimate combustion gas dew point, Oil and Gas Journal, April 1978, p105. 8. V. Ganapathy, Cold end corrosion—Causes and cures, Hydrocarbon Processing, Jan. 1989, p57.

Acid Dew Point Temperature T dp