|Content Type||Conference Paper|
|Title||The Controversy of CO2 Solubility in Water|
|Authors||Bob Fu, Nalco/Exxon Energy Chemicals; Andrew James McMahon and K. Blakley, BP International Ltd.|
|Source||CORROSION 98, March 22 - 27, 1998 , San Diego Ca|
|Copyright||1998. NACE International|
|Keywords||carbon dioxide, volubility, Henry?s constant, sweet corrosion|
A study was conducted to measure the volubility of C02 in water. The objective was to examine the effect of various experimental parameters on the C02 volubility. The results confirmed that water vapor pressure must be considered when determining the dissolved C02 concentration at elevated temperatures (>30°C). The gas-sparging rate has no effect on the steady state C02 volubility as long as the sparging time exceeds 1 hour. Indeed, the steady state concentration was established within the first 30 minutes of sparging at a rate of 100 ml/min. The evolution rate of dissolved CO2 was found to be considerably lower than the dissolution rate of CO2 gas.
Although the volubility of C02 measured at 80C agreed reasonably well with the literature, the value obtained at room temperature was significantly lower. The steady state concentration of C02 ranges from 265 to 320 ppm at 80°C and 1 bar total pressure, as compared to the calculated literature value of 307 ppm. Conversely, the concentration measured at 20°C (1125 ? 1400 ppm) was significantly lower than the literature value (1720 ppm). Several measurement methods were attempted in order to resolve the difference; however, the results were consistently lower than those previously reported in the literature. The explanation to the discrepancy is yet to be identified.
Many corrosion tests in the laboratory, such as flow loop tests and bubble tests, involve the dissolution of C02 in brine for simulating sweet conditions in oilfields. This is typically achieved by constantly sparging the test fluid with C02. The amount of dissolved C02 is critical to the test because it directly affects the corrosivity of the test fluid.?-? An inconsistent amount of dissolved C02 often leads to a large data variation and thus the reproducibility of the test data is significantly reduced. The variation also further complicates the data interpretation. In an effort to control the amount of C02 dissolved in the test fluid, a series of experiments were carried out in this work to study the effect of experimental variables on the volubility of COZ. One important aspect in addressing the CO, volubility is the water vapor pressure. The presence of water vapor reduces the partial pressure of C02 coexisting in the headspace and consequently the concentration of dissolved C02 is reduced.
This phenomenon is more pronounced at elevated temperatures because of the higher water vapor pressure. For example, in an open (to the air) sparging process, water vapor pressure is 0.47 bar at 80°C and the remaining 0.53 bar is balanced by the C02 partial pressure. The lower C02 partial pressure results in a lower equilibrium concentration of dissolved C02 according to Henry?s law (e.g., 307 ppm with water vapor vs. 577 ppm without water vapor at 1 bar and 80°C). The difference is negligible at low temperatures (<300C), but becomes quite significant at higher temperatures, as illustrated in 20 40 60 80 100 Figure 1. Temperature(C)
Figure 1. Comparison of literature data and estimated volubility
ofCO2 in water at 1 bar with and without water pressure
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