Article pubs.acs.org/jced
An Experimental Investigation of the Influence of Vanadium Pentoxide on the Solubility of CO2 in Aqueous Solutions of Potassium Carbonate Dirk Schaf̈ er, Á lvaro Pérez-Salado Kamps, Bernd Rumpf, and Gerd Maurer* Department of Mechanical and Process Engineering, University of Kaiserslautern, P.O. Box 30 49 D-67653 Kaiserslautern, Germany ABSTRACT: The “hot-potash” process is one of the most commonly used processes to remove carbon dioxide from gases in the chemical industry. The solvent of that absorption process is an aqueous solution of potassium carbonate. Dissolving carbon dioxide in an aqueous solution of K2CO3 results in the conversion of carbonate and CO2 to bicarbonate; that is, carbon dioxide is predominantly dissolved chemically. In industrial applications a single substance or a mixture of several substances are added to the solvent to improve the performance of the process. The specific role of such additives is often unknown, but it is well-established that some additives improve the performance of the “hot-potash” process. One of those additives is vanadium pentoxide. The influence of a small amount of vanadium pentoxide on the equilibrium solubility of carbon dioxide in aqueous solutions of potassium hydroxide was investigated at conditions that are typical for a “hot-ash” process. The experimental results reveal that vanadium pentoxide reduces the solubility of carbon dioxide; that is, they reveal that adding vanadium pentoxide alone has a negative influence on the equilibrium solubility of carbon dioxide in the “hotpotash” process.
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acid is an additive in the so-called Lurgi process,3 and arsenic trioxide or glycine are the published additives in the Giammarco-Vetrocoko process.1 Publications on the often used Benfield process cite diethanolamine3 or vanadium pentoxide4 as the essential additives. One of the most commonly applied activated hot-potash processes for the removal of CO2 is the Catacarb process.1 The additives used in the Catacarb process are proprietary, and their identities have not been disclosed. The favorable influence of such additives on the absorption/desorption of CO2 in aqueous solutions of K2CO3 in industrial processes is well-established, although there are no openly accessible publications on the influence of these additives neither on the equilibrium solubility of CO2 nor on the reaction paths and reaction kinetics. In a previous publication we investigated the influence of boric acid (H3BO3) on the equilibrium solubility of CO2 in aqueous solutions of K2CO3.5 In the investigations presented here boric acid was replaced by vanadium pentoxide (V2O5). Experimental results are reported for the influence of V2O5 on the equilibrium solubility of CO2 in an aqueous solution of KOH (or K2CO3) at 343 K and at 383 K, that is, at temperatures that are typical for CO2 removal and solvent regeneration, respectively, in the “hotpotash” process. The contents of potassium and V2O5 in the solvent are expressed by the stoichiometric molalities of potassium hydroxide m̅ KOH and vanadium pentoxide m̅ V2O5 in
INTRODUCTION Aqueous solutions of potassium carbonate are often used to remove carbon dioxide from gas streams. One important example for such an application is the “hot-potash” process.1 In the “hot-potash” process carbon dioxide is typically removed by absorption into an aqueous solution of K2CO3 at low temperatures (around 310 K), and the loaded solvent solution is regenerated in a desorption step at higher temperature (around 400 K). As long as the amount of dissolved CO2 is considerably below the amount of potassium ions, CO2 is predominantly dissolved chemically as bicarbonate HCO3− (as the carbonate ions CO32− are converted to HCO3−). When nearly all carbonate is converted to bicarbonate more carbon dioxide can only be dissolved physically; that is, more carbon dioxide has to be dissolved as neutral species in an aqueous solution of potassium bicarbonate. Increasing the temperature shifts the chemical reaction equilibrium in favor of CO2 (i.e., decreases the “chemical” solubility of CO2) which is essential for the regeneration of the CO2-loaded solvent. In previous work, we performed experimental work on the equilibrium solubility of CO2 in aqueous solutions of K2CO3 and developed a thermodynamic model to describe the combined “chemical” and “physical” solubility of CO2 in aqueous solutions of potassium carbonate.2 However, industrially used solvents for the “hot-potash” process contain some further “additives” which are described as “activators” (to speed up the conversion of CO2 to HCO3−) and/or “corrosion inhibitors” (to reduce corrosion, i.e., to allow cheaper construction materials). Examples for a variety of such additives have been reported. However, only a few ones are really used. For example, boric © 2012 American Chemical Society
Received: July 26, 2012 Accepted: September 7, 2012 Published: September 14, 2012 2902
dx.doi.org/10.1021/je3008364 | J. Chem. Eng. Data 2012, 57, 2902−2906
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Article
repeated. The main part of the apparatus is a thermostatted high-pressure cell with an internal volume of 2.0 dm3. A magnetically coupled stirrer, a liquid level indicator, and all lines to and from the cell are mounted at the top cover of the cell. The stirrer was designed to pump the vapor through the liquid for a rapid equilibration of the coexisting phases. The liquid level indicator consists of an electrically isolated rod through the top cover of the cell, a device to move that rod vertically, and a positioning meter. A small electrical voltage is applied between the rod and the cell. An amperemeter is used to indicate an electric current between the rod and the cell housing. There is no electric current as long as the tip of the rod is not immersed in the liquid. Therefore, the device is used to determine the position of the liquid level in the cell andvia a calibration curvethe volume of the vapor phase. The bottom of the cell consists of a thin metal foil that transfers the pressure to silicon oil which again transfers the pressure to calibrated pressure transducers. The previously evacuated cell was charged with the solvent from a buret. The amount of solvent (typically about 1.5 kg) was determined by weighing the buret before and after the filling with an uncertainty of less than 0.2 g. Then, CO2 was added stepwise from a small tank. The amount of CO2 was also determined by weighing that tank. In the first (each further) step about 80 g (between 10 and 25 g) of CO2 were charged to the cell. The mass of CO2 charged in a single step was determined with an experimental uncertainty of less than 0.05 g. After each addition, the phases were equilibrated before temperature, pressure, and vapor-phase volume were determined, and small vapor phase samples were taken and analyzed by online gas chromatography. The temperature was measured with an uncertainty of 0.1 K with a calibrated platinumresistance thermometer placed in the thermostatting liquid. The estimated uncertainty for the solubility pressure is 0.5 % of the actual pressure reading. In a typical experiment the vapor phase volume was about 500 cm3. It was determined with an estimated uncertainty of less than 5 cm3. The volume of the liquid phase (typically about 1.5 dm3) was determined as the difference between the total volume of the cell and the volume of the vapor phase. Its estimated uncertainty is also about 5 cm3. The vapor phase samples were analyzed online in a gas chromatograph (HP Agilent, type 6890) equipped with a capillary column (Alltech, type Heliflex AT-Q 30 m, 0.32 mm I.D.) and a thermal conductivity detector. A correlation between the primary data collected in the chromatographic measurements (i.e., the peak areas of water or carbon dioxide) and the vapor phase mole fraction of water yH2O (or of CO2 yCO2) was determined in calibration measurements with binary gaseous mixtures of (carbon dioxide + water). In the first steps of an experimental run at constant temperature (i.e., where the amount of added CO2 is well below the amount of potassium ions) the estimated relative uncertainty of the experimental results for the mole fraction of carbon dioxide in the vapor phase can be as large as 20 %. That uncertainty decreases with increasing solubility pressure to about 2 % in the last steps of an experimental run, where the molar ratio of CO2 to potassium in the liquid phase approaches 1. Vice versa, the experimental uncertainty of the mole fraction of water in the vapor phase is very small in the first steps of an experimental run (< 1 %) but reaches about 10 % in the last steps of such an isothermal run. The experimental results for the composition of the vapor phase, temperature, pressure, and volume of the vapor phase
water. To discuss the influence of vanadium pentoxide on the equilibrium solubility of CO2 in aqueous solutions of K2CO3 the new results are compared with calculation results from a previously published model2 for the solubility of CO2 in additive-free aqueous solutions of K2CO3. Substances and Sample Pretreatment. Details (CAS No., purity, molar mass, and supplier) of all materials are given in Table 1. Carbon dioxide was used without further Table 1. Sample Description relative molar mass
chemical
CAS No.
purity mass fraction
CO2
124-38-9
≥0.99995
44.01
KOH
1310-58-3
56.11
V2O5
1314-62-1
>0.85 (residue: H2O) >0.99
H2O
7732-18-5
181.88 18.015
supplier Messer-Griesheim, Ludwigshafen, Germany Riedel-de Haen, Seelze, Germany Merck, Darmstadt, Germany University of Kaiserslautern, Germany
purification. The amount of water in (degassed) KOH was determined by potentiometric titration and taken into account in the calculations for the composition of the aqueous solvent feed. Vanadium pentoxide was degassed under vacuum. Water was double-distilled and degassed before use. The CO2-free solvent mixture was prepared by dissolving known amounts of KOH (typically about 250 g) and V2O5 (typically about 35 g) in degassed water (typically about 1.5 kg). The uncertainty in the amount of KOH is estimated to 2 % . That uncertainty nearly completely results from the experimental uncertainty in the potentiometric determination of the water content of the sample. The amount of V2O5 was determined with an uncertainty of less than 0.01 g; that is, the uncertainty of the amount of V2O5 is dominated by its purity (mass fraction impurities < 0.01). The amount of water was determined with an uncertainty of less than 0.1 g. Apparatus and Measuring Technique. A schematic of the apparatus is shown in Figure 1. Details of that apparatus were reported before.6−16 Therefore, only a short description is
Figure 1. Schematic of the experimental equipment: (A) thermostat; (B) equilibrium cell; (C) solvent tank; (D) container for CO2; (E) magnetically coupled stirrer; (F) liquid level indicator; (G) pressure transducing device; (H) pressure gauge; (I) sampling valve; (J) gas chromatograph; (K) platinum resistance thermometer; (L) automatic AC-bridge; (M) positioning meter; (N) stirrer motor. 2903
dx.doi.org/10.1021/je3008364 | J. Chem. Eng. Data 2012, 57, 2902−2906
Journal of Chemical & Engineering Data
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Table 2. Experimental Results for the Solubility of Carbon Dioxide in Aqueous Solutions of KOH (m̅ KOH = (3.04 ± 0.06) mol·(kg H2O)−1) and V2O5 (m̅ V2O5 = (0.119 ± 0.001) mol·(kg H2O)−1) at 343.2 Ka m̅ CO2 mol·(kg H2O)−1
p
pCO2
pH2O
MPa
MPa
MPa
± ± ± ± ± ± ± ±
0.0289 ± 0.0001 0.0357 ± 0.0002 0.0487 ± 0.0003 0.0919 ± 0.0005 0.202 ± 0.001 0.395 ± 0.002 0.541 ± 0.003 0.687 ± 0.004
0.0004 ± 0.0001 0.0079 ± 0.0007 0.0232 ± 0.0012 0.0689 ± 0.0018 0.180 ± 0.002 0.371 ± 0.002 0.519 ± 0.002 0.664 ± 0.002
0.0294 ± 0.0001 0.0278 ± 0.0006 0.0255 ± 0.0012 0.0229 ± 0.0018 0.022 ± 0.002 0.023 ± 0.002 0.022 ± 0.002 0.023 ± 0.002
1.631 2.179 2.456 2.679 2.794 2.852 2.875 2.894
0.004 0.007 0.014 0.018 0.020 0.021 0.023 0.027
m̅ KOH, m̅ V2O5, and m̅ CO2 = stoichiometric molalities of KOH, V2O5, and CO2, respectively; p = pressure; pCO2 (= yCO2·p), pH2O (= yW·p) are the partial pressures of CO2 and water, respectively. yi is the mole fraction of component i in the vapor phase.
a
Table 3. Experimental Results for the Solubility of Carbon Dioxide in Aqueous Solutions of KOH (m̅ KOH = (3.05 ± 0.06) mol·(kg H2O)−1) and V2O5 (m̅ V2O5 = (0.120 ± 0.001) mol·(kg H2O)−1) at 383.2 Ka m̅ CO2 mol·(kg H2O)−1
MPa
MPa
MPa
± ± ± ± ± ± ± ±
0.1340 ± 0.0008 0.1362 ± 0.0008 0.1470 ± 0.0009 0.1795 ± 0.0011 0.2540 ± 0.0015 0.357 ± 0.002 0.584 ± 0.004 0.686 ± 0.004
