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A General Correlation for AccuratePrediction of the Dew Points of AcidicCombustion Gases in Petroleum IndustryB. ZareNezhada

a Faculty of Chemical, Petroleum and Gas Engineering, SemnanUniversity, Semnan, IranPublished online: 04 Jun 2014.

To cite this article: B. ZareNezhad (2014) A General Correlation for Accurate Prediction of the DewPoints of Acidic Combustion Gases in Petroleum Industry, Petroleum Science and Technology, 32:16,1988-1995, DOI: 10.1080/10916466.2012.730596

To link to this article: http://dx.doi.org/10.1080/10916466.2012.730596

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Petroleum Science and Technology, 32:1988–1995, 2014Copyright C© Taylor & Francis Group, LLCISSN: 1091-6466 print / 1532-2459 onlineDOI: 10.1080/10916466.2012.730596

A General Correlation for Accurate Prediction of the DewPoints of Acidic Combustion Gases in Petroleum Industry

B. ZareNezhad1

1Faculty of Chemical, Petroleum and Gas Engineering, Semnan University, Semnan, Iran

A general correlation has been proposed for the first time for accurate prediction of the acidic combustiongases dew points to mitigate the corrosion potential in pollution control and energy recovery equipments.Acidic combustion gases can cause rapid corrosion when they condense on the surfaces of the heatrecovery and flue gas treatment equipments. The most important acidic gases, namely, SO3, SO2, NO2,HCl, and HBr, are considered in this investigation. The presented correlation can be used for accurateprediction of the flue gas acid dew point temperatures over wide ranges of acid and water vaporconcentrations in oil and gas industries.

Keywords: acid dew point, combustion gas, correlation, corrosion, energy recovery

1. INTRODUCTION

Acid dew point corrosion results from condensation of flue gas acid species on low temperature gas-path surfaces. This kind of corrosion differs from general atmospheric corrosion and causes heavycorrosion not only of ordinary steels but even stainless steels (Rockel and Bender, 2008). Corrosionfailures often occur because of condensing flue gasses containing SO3, SO2, NO2, HCl, HBr, andH2O. Dew point temperature is a function of the water vapor concentration and the concentrationof acid species in the flue gas (Blanco and Pena, 2008). The condensed acids are corrosive to steeland almost all plastics, as well as hydraulic cement composites. Further, gas cooling below this dewpoint by radiation or convection forms a mist of corrosive acid droplets that is highly detrimental tothe stack and heat recovery equipment (Cherif et al., 2002). Many of the processes for improving thethermal efficiency of combined cycle plants can also result in lower flue gas temperatures leaving theheat recovery steam generators or increased flue gas moisture content. These conditions can increasethe potential for corrosion of the low temperature gas-path components (Lins and Guimaraes, 2007)

Sulfuric acid and hydrochloric acid dew point corrosion occur in flue gas treatment systems ofwaste incineration facilities. For a typical flue gas (SO3 = 3 ppmv; HCl = 300 ppmv; H2O = 30vol%), the H2SO4 and HCl dew points are about 136 and 72◦C, respectively. At temperatures between72 and 136◦C, the H2SO4 condensation is the main cause of steel corrosion failure. Improvementof the thermal efficiency of flue gas treatment equipment may need further cooling to a temperatureas low as 72◦C. At flue gas temperatures below 72◦C, the HCl condensation is also an importantinfluencing factor regarding corrosion failure.

Address correspondence to B. ZareNezhad, Faculty of Chemical, Petroleum and Gas Engineering, Semnan University,PO Box 35195–363, Semnan, Iran. E-mail: Prof.b.zarenezhad@gmail.com

1988

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ACCURATE PREDICTION OF ACID DEW POINTS 1989

The following correlation (Verhoff and Banchero, 1974) was provided for predicting flue gassulfuric acid dew point:

(1)

where TDew is the sulfuric acid dew point in K and the partial pressures (pSO3 and pH2O) are in mmHg.

The previous correlation (the Verhoff and Banchero [V.B] correlation) does not apply to halo-genated or nitrated acid gases. To overcome this shortcoming. Kiang (1981) proposed differentcorrelations for estimating the NO2, HCl and HBr dew points. However there are usually significantdeviations between predicted and experimental dew points over the concentrations prevailing incombustion gases.

It should be noted that there are some disagreements between the experimental data and the Verhoffand Banchero (1974) correlation especially at low acid concentrations and high H2O contents. Dewpoints predicted in the range of 120–140◦C have a positive deviation of 4◦C and more. Also inthe range of 100–121◦C, the predicted dew points are usually 4◦C low. Okkes (1987) proposed acorrelation to overcome some of these shortcomings, which can be written as follows:

(2)

Where the partial pressures are expressed in atmosphere and the sulfuric acid dew point is in◦C. Although this correlation is more accurate at H2O concentrations higher than 25%, but itsignificantly underpredicts the sulfuric acid dew points at low H2O concentrations prevailing inoil and gas industries. Since the above correlations are not accurate enough for proper design ofcombustion equipments and heat recovery systems, a general correlation has been presented foraccurate prediction of the most important acid gases dew points.

PROPOSED CORRELATION

In this work, a general correlation based on all verified experimental data is proposed for accurateprediction of flue gas acid dew points. A set of 940 dew point temperatures is determined for themost important acidic gases namely, SO3, SO2, NO2, HCl, and HBr by using the most accuratevapor-liquid equilibrium data (Perry et al, 1999; Ullmann’s, 2001; Gmehling et al., 2004; NipponOil Company, 2008). Studies have found that the following general correlation can be used foraccurate prediction of the dew points of acidic combustion gases:

(3)

where TDew is the acid dew point in ◦C and the partial pressures (pAcid and pH2O) are expressed in mmHg. It should be noted that the parameters Ai, Bi, Ci, and Di are not dependent on acid species andare always fixed. The parameter, λ and molecular weight, Mw are given for the relevant condensedacid. The optimum values of all parameters determined by the genetic algorithm (Goldberg, 2007)are displayed in Table 1.

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1990 B. ZARENEZHAD

TABLE 1The Optimum Values of Parameters in Eq. (3)

CondensedAcids H2SO4 H2SO3 HNO3 HCl HBr

λ 0.011283 0.005164 −0.00614 −0.04165 0.00466Mw 98.0 82.0 63.0 36.5 80.9Indices i = 0 i = 1 i = 2 i = 3 i = 4A 160.4 −45.57 −1320 372.4 770.2B −1.365 4.031 138 −34.88 −78.01C 18.93 −7.466 −115.1 31.96 65.95D −0.4365 0.1829 3.622 −0.9925 −2.165

Equation (3) with the parameters displayed in Table 1 yields the smallest sum of the squared errorsfor the whole range of acid dew point temperatures. The performance of the proposed correlation iselaborated in the following section.

RESULTS AND DISCUSSION

Predicted flue gas sulfuric acid dew points by using the proposed correlation (Eq. [3]) are comparedwith experimental data at different acid and moisture concentrations in Figure 1. Whenever fossilfuels containing sulfur are fired in heaters or boilers, sulfur dioxide, and to a small extent sulfurtrioxide are formed in addition to carbon dioxide and water vapor. The SO3 combines with watervapor in the flue gas to form sulfuric acid and condenses on heat transfer surfaces that could lead tocorrosion and destruction of the surfaces. This condensation occurs on surfaces that are at or belowthe dew point of the acid gas.

As shown in Figure 1, the acid dew point is very sensitive to the flue gas SO3 concentrationsuch that a small increase in SO3 concentration leads to a significant increase in sulfuric acid dewpoint at a given H2O concentration. The moisture content is also an important influencing parameter.However, as the moisture concentration is increased, effect of vapor H2O on sulfuric acid dew point

FIGURE 1 Comparison of the proposed correlation with experimental sulfuric acid dew points at different moistureand acid concentrations.

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ACCURATE PREDICTION OF ACID DEW POINTS 1991

FIGURE 2 Comparison of the proposed correlation with experimental sulfurous acid dew points at differentmoisture and acid concentrations.

is gradually reduced. Since the sulfuric acid, which is formed through the reaction of SO3 and H2Oin the flue gas, is usually condensed at relatively high temperatures, the corrosion risk in the wasteheat recovery system is too high. As shown, the vapor phase sulfur trioxide concentration has avery strong influence on flue gas dew point especially at SO3 concentrations below 100 ppmv. Thusthe accurate prediction of sulfuric acid dew points at low SO3 concentrations prevailing in processindustries is very important to control the corrosion problems in thermal waste treatment plants andenergy recovery equipments. In most installations the formation of sulfuric acid is prevented bykeeping the temperature above the sulfuric acid dew point as well as possible.

Predicted and measured flue gas sulfurous acid dew points are compared in Figure 2. As shownin this figure, the sulfurous acid condensation temperature is mainly influenced by the water vaporconcentration. Since the sulfurous acid is always formed through the dissolution of SO2 in condensedwater below the water dew point, the corrosion risk in the stack is lower as compared to the caseof sulfuric acid. However the sulfurous acid formation at the temperatures around 60◦C speeds upthe metallic corrosion in heat recovery systems. The trend of dew point variation with respect to the

FIGURE 3 Comparison of the proposed correlation with experimental nitric acid dew points at different moistureand acid concentrations.

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1992 B. ZARENEZHAD

FIGURE 4 Comparison of the proposed correlation with experimental hydrochloric acid dew points at differentmoisture and acid concentrations.

moisture content is similar to the case of sulfuric acid such that the H2O content is less influentialat higher moisture concentration. According to Figure 2, the sulfurous acid dew point temperatureis always below 70◦C as long as the moisture concentration is not higher than 30 vol%.

Predicted flue gas nitric acid dew points by using the proposed correlation are compared withexperimental data at different acid and moisture concentrations in Figure 3. The NO2 condenses asnitric acid below the nitric acid dew point or it dissolves in the condensed water below the water dewpoint to form nitric acid solution causing very severe stress corrosion cracking. In a gas, containing50 ppmv NO2 and 5 vol% water (a typical high NO2 containing GT gas) the NO2 dew point (28◦C)is lower than the water dew point (33◦C). During operation of the Heat-Recovery Steam Generators(HRSGs), the inlet temperature is at least 70◦C, which excludes water and nitric acid condensing.In this case, only during shutdowns, nitric acid can be formed by solving of the gas in the waterdroplets. At high flue gas NO2 concentrations, the NO2 dew point is usually higher than water dewpoint such that the nitric acid condensation takes place directly from the gas phase below the nitric

FIGURE 5 Comparison of the proposed correlation with experimental hydrobromic acid dew points at differentmoisture and acid concentrations.

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ACCURATE PREDICTION OF ACID DEW POINTS 1993

FIGURE 6 Comparison of the proposed correlation with Verhoff and Banchero (V.B) and Okkes correlations andexperimental data.

acid dew point. Predicted nitric acid dew points are in good agreement with the experimental dataas shown in Figure 3.

Predicted flue gas hydrochloric and hydrobromic acid dew points by using the proposed correla-tion are also compared with measured data at different acid and moisture concentrations in Figures 4and 5, respectively. In municipal solid waste fired plants, in addition to sulfuric acid, one has todeal with hydrochloric and hydrobromic acid formation. Also the halogenated compounds used invarious types of plastics are released in waste incinerators such that the dew point corrosion due toHCl or HBr condensation may take place. Similar to the previous cases, effect of H2O concentrationon acid dew point is more important at low moisture concentrations as shown in Figures 4 and 5.According to these figures, the flue gas hydrochloric and hydrobromic acid dew points are usuallybelow 80◦C in most practical cases. Predicted results are in good agreement with experimental dataas shown in Figures 4 and 5.

The proposed correlation (Eq. [3]) is compared with the V.B (Eq. [1]) and Okkes (Eq. [2])correlations regarding the prediction of sulfuric acid dew points at a moisture concentration of

FIGURE 7 Comparison between the proposed correlation predictions with the experimental acid gases dew points.

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1994 B. ZARENEZHAD

20 vol% in Figure 6. As shown in this figure, the Verhoff and Banchero correlation leads to aconsiderable acid dew point overprediction. In such a case the operator may incorrectly increase theair preheating level to combat the cold-end corrosion problem. On the contrary, using the Okkescorrelation leads to a significant dew point underprediction such that the air preheating level may notbe adequate to overcome the corrosion risk. The dew points predicted by the proposed correlation(Eq. [3]) are much more accurate than those predicted by the V.B and Okkes correlations and are inexcellent agreement with measured acid dew points as shown in Figure 6. According to Figures 1–5,the limiting dew point is that due to sulfuric acid and thus any heat transfer surface should be keptabove H2SO4 dew point if acid condensation is to be avoided. The accurate prediction of flue gas aciddew point is quite important for optimization of energy consumption in combustors and incinerators.

The average absolute deviation (AAD%) in sulfuric acid dew point predictions is about 0.5541%,which is small enough for design calculations. Since the sulfuric acid dew point is usually higher thanthat of the other acids, it is important to predict the H2SO4 condensation temperature accurately. Infact the sulfuric acid dew point temperature is the main bottleneck for increasing the performance ofheat recovery systems. The overall AAD%, RMSD and R2 values regarding the proposed correlationare about 1.055%, 0.9815, and 0.9994 respectively, suggesting that Eq. (3) is accurate enough forpredicting the dew points of the different acid gases.

The predicted flue gas acid dew points according to Eq. (3) are also compared with all availableexperimental data regarding all aforementioned acid gases in Figure 7. As shown, the proposedcorrelation can be used for accurate prediction of the flue gas acid dew point temperatures over wideranges of moisture and acid gas concentrations.

4. CONCLUSIONS

A general correlation has been presented for accurate prediction of the flue gas acidic dew points overwide ranges of moisture and acid gas concentrations. Acidic combustion gases (SO3, SO2, NO2, HCland HBr) can cause rapid corrosion when they condense (as H2SO4, H2SO3, HNO3, HCl and HBrrespectively) on the surfaces of the heat recovery and flue gas treatment equipments. The accurateprediction of flue gas acid dew point is quite important for optimum control of corrosion rate andenergy consumption in combustors and incinerators. The proposed correlation (Eq. 3) outperformsthe other alternatives regarding the prediction of acid dew points in chemical process industries.The predicted flue gas acidic dew points are in excellent agreement with experimental data with theoverall average absolute deviation of 1.055%.

ACKNOWLEDGMENTS

The authors would like to thank the cooperation of National Iraninan Gas Company (NIGC) duringthe preparation of the manuscript.

REFERENCES

Blanco, J. M., and Pena, F. (2008). Increase in the boiler’s performance in terms of the acid dew point temperature:Environmental advantages of replacing fuels. Appl. Therm. Eng. 28:777–784.

Cherif, M., Mgaidi, A., Ammar, M. N., Abderrabba, M., and Furst, W. (2002). Representation of VLE and liquid phasecomposition with an electrolyte model: application to H3PO4-H2O and H2SO4-H2O. Fluid Phase Equilib. 194:729–738.

Gmehling, J., Menke, J., Krafczyk, J., and Fischer, K. (2004). Azeotropic Data (2nd ed.). Berlin: Wiley-VCH.Goldberg, D. E. (2007). Genetic Algorithms in Search, Optimization, and Machine Learning. Boston: Addison Wesley.

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ACCURATE PREDICTION OF ACID DEW POINTS 1995

Kiang, Y. H. (1981). Predicting dew points of acid gasses. Chem. Eng. 9:127–128.Lins, V. F. C., and Guimaraes, E. M. (2007). Failure of a heat exchanger generated by an excess of SO2 and H2S in the Sulfur

Recovery Unit of a petroleum refinery. J. Loss Prevention Proc. Indust. 20:91–97.Nippon Oil Company. (2008). Sulfuric acid dew point measurement. Technical document. Tokyo, Japan.Okkes, A. G. (1987). Get acid dew point of flue gas. Hydrocarb. Process. 7:53–55.Perry, R. H., Green, D. W., and Maloney, J. O. (1999). Chemical Engineers Handbook (7th ed.). New York: McGraw-Hill.Rockel, M. B., and Bender, R. (2008). Corrosion Handbook. Frankfurt, Germany: Society for Chemical Engineering and

Biotechnology.Ullmann’s. (2001). Ullmann’s Encyclopedia of Industrial Chemistry (6th ed.). New York: Wiley.Verhoff, F. H., and Banchero, J. T. (1974). Predicting dew points of flue gases. Chem. Eng. Prog. 70:71–72.

NOMENCLATURE

TDew sulfuric acid dew pointtemperature,◦C

pH2O partial pressure of H2Oin the flue gas, mm Hg

pSO3 partial pressure of SO3 inthe flue gas, mm Hg

pAcid gas partial pressure of acidgas (SO3, SO2, NO2,HCl, HBr), mm Hg

yH2O concentration of H2O inthe flue gas, vol%

ySO3 concentration of SO3 inthe flue gas, ppmv

ySO2 concentration of SO2 inthe flue gas, ppmv

yNO2 concentration of NO2 inthe flue gas, ppmv

yHCl concentration of HCl inthe flue gas, ppmv

yHBr concentration of HBr inthe flue gas, ppmv

RMSD root mean square devia-tion

AAD% percent of average abso-lute deviation

R2-value correlation coefficienti index (from 0 to 4) in Eq.

(3)Ai, Bi, Ci, Di and λ parameters defined in

Eq. (3)Mw molecular weight

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