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Solubility of Hydrogen Sulfide in Aqueous Solutions of N‑Methyldiethanolamine and Piperazine Dirk Speyer, Arne Böttger, and Gerd Maurer* Department of Mechanical and Process Engineering, University of Kaiserslautern, P.O. Box 30 49, D-67653 Kaiserslautern, Germany ABSTRACT: The solubility of hydrogen sulfide in an aqueous mixture of 2,2′-methyliminodiethanol (N-methyldiethanolamine, [CAS 105-59-9], MDEA) and 1,4-diazacyclohexane (piperazine, [CAS 110-85-0], PIPH2) was measured over a wide range of gas-loadings (stoichiometric molar ratios of hydrogen sulfide to (MDEA + PIPH2) between about 0.014 and 1.67) at three temperatures (about 313 K, 353 K, and 393 K) by headspace gas chromatography as well as by a synthetic gas solubility method. The molality of MDEA in the aqueous mixture was about 4.5 mol·(kg water)−1 and that of PIPH2 about 1.6 mol·(kg water)−1. The headspace gas chromatography technique was applied in the low pressure range (partial pressure of H2S between 0.64 and 125.7 kPa, stoichiometric molar ratio of H2S to (MDEA + PIPH2) between about 0.014 and 0.73). The synthetic method was applied in the high pressure range (total pressure between 0.2 and 5.5 MPa, stoichiometric molar ratio of H2S to (MDEA + PIPH2) between about about 0.51 and 1.67). The new experimental results are compared to prediction results from a thermodynamic model that was parametrized using only experimental results for the solubility of H2S data in the aqueous solution of the single amines in the high pressure range.

1. INTRODUCTION In many processes, acid gases, such as CO2 and/or H2S, have to be removed from gas streams. One of the most important examples is the removal of such sour gases from natural gases where the global production amounted to 3 × 1012 m3 in 20081). Carbon dioxide must be removed, as it reduces the calorific value of gas and/or is responsible for corrosion in pipes and fittings. Hydrogen sulfide is very corrosive, toxic, and malodorous also at low concentrations. The most common technique for the removal of acid gases from gaseous streams is a “chemical” absorption by an aqueous solution of (alkanol)amines. Aqueous solutions of (alkanol)amines absorb acid gases at around ambient temperatures almost completely converting the sour gases into ionic species via an acid−base buffer mechanism. The sour gases are released (and the solvent is regenerated) at higher temperatures (about 120 °C) where the chemical reaction equilibrium is shifted from ionic to neutral species (of sour gases and amines). Aqueous solutions of the tertiary amine MDEA are often used in a kinetically controlled process for the selective removal of H2S in the presence of CO2. Aqueous solutions of N-methyldiethanolamine (MDEA) and aqueous solutions of mixtures of MDEA and piperazine, socalled “activated MDEA” solutions, are commonly applied absorption solvents in particular when carbon dioxide and hydrogen sulfide are to be removed simultaneously.2 Hydrogen sulfide reacts quickly with MDEA to form ionic components (hydrosulfide/sulfide), whereas the conversion of neutrally dissolved carbon dioxide to ionic species (hydrogen carbonate/ carbonate) is much slower. Therefore, H2S can be separated from CO2. Higher reaction rates are required for the removal of CO2. Such higher reaction rates are achieved by “activating” an aqueous solution of MDEA with modifiers. Primary or secondary amines are typical modifiers. They react quickly with CO2 to form carbamates. Piperazine (PIPH2) is a secondary amine and a common activator. © 2012 American Chemical Society

The competitive chemical absorption of hydrogen sulfide and carbon dioxide is kinetically controlled. However, deviation from equilibrium provides the driving force in that kinetically controlled process. Therefore, for a successful basic engineering of such absorption and desorption processes, the thermodynamic equilibrium properties have to be known.3 In particular, the vapor−liquid equilibrium (including the chemical reaction equilibrium in the liquid phase) and the enthalpy changes on vaporization and condensation of such mixtures have to be known reliably. The information on the energy requirements can be obtained from a reliable thermodynamic model for the vapor−liquid equilibrium. However, reliable methods for correlating/predicting the simultaneous solubility of H2S and CO2 in aqueous solutions of (MDEA + PIPH2) can only be developed when sufficient and accurate experimental data on the solubility of the single gases in aqueous solutions of the single amines and of the amine mixtures are available in a wide range of temperature and pressure. The present work continues a series of experimental and theoretical investigations on the solubility of a single sour gas in aqueous solutions of the single amines (MDEA and PIPH2) and their mixtures: {(H2S + MDEA + H2O),4−6 (H2S + PIPH2 + H2O),8,9 (H2S + MDEA + PIPH2 + H2O),8 (H2S + MDEA + H2SO4/NaSO4 + H2O),11 (CO2 + MDEA + H2O),4−7 (CO2 + PIPH2 + H2O),9,12,13 (CO2 + MDEA + PIPH2 + H2O),12−15 (CO2 + MDEA + H2SO4/NaSO4 + H2O)16}. At first, the thermodynamic model for the vapor−liquid equilibrium of the system (H2S + MDEA + PIPH2 + H2O) was based only on experimental information for the solubility of H2S in aqueous solutions of the single amines in the high gas-loading Received: Revised: Accepted: Published: 12549

June 22, 2012 August 22, 2012 August 29, 2012 August 29, 2012 dx.doi.org/10.1021/ie301657y | Ind. Eng. Chem. Res. 2012, 51, 12549−12556

Industrial & Engineering Chemistry Research

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(i.e., high-pressure) region. These measurements were performed by a synthetic method where the total pressure was determined that is required to dissolve a known amount of sour gas in a known amount of solvent at a given temperature and solvent composition. Such models (that are based only on experimental data for the solubility of a sour gas in an aqueous solution of a single amine) can be used to predict the gas solubility in aqueous solutions of amine mixtures. However, as in such predictions all parameters for interactions between species that originate from one amine with species that originate from another amine must be estimated or neglected, it is very difficult to estimate the quality of such predictions. The assessment/ development of improved models requires reliable experimental data for the solubility of H2S in aqueous solutions of amine mixtures both in the high gas-loading (high-pressure) as well as in the low gas-loading (low partial pressure) regions. Experimental results for the solubility of H2S in aqueous solutions of MDEA have been reported by various research groups. However, the experimental information on the solubility of H2S in the mixed amine system (MDEA + PIPH2 + water) is scarce. Only Xia et al.8 reported some data for the solubility of H2S at 353 K in about (2 + 2) molal aqueous solutions of MDEA and PIPH2 in the high-pressure region. The present work is to extend these results to other aqueous solutions (different molalities of MDEA and PIPH2) as well as to lower partial pressures of H2S.

Figure 1. Scheme of the headspace chromatographic (HS-GC) arrangement: (CH) liquid-thermostatted cell holder (temperature T1), (VH) liquid-thermostatted sampling valve holder (temperature T2 > T1), (A) buffer tank (higher pressure), (B) buffer tank (lower pressure), (GC) gas chromatograph, (He) helium (carrier gas), (SC) sample cell, (MV) multiposition valve, (S1−S8) sample positions, (P1−P8) purge positions, (SV) sample valve, (SL) sample loop.

CP-Pora Plot U, length 27.5 m, inner diameter 0.53 mm, thickness of capillary coating 20 μm) and a thermal conductivity detector}. In an experiment, eight sample cells (vial volume: 30 cm3, made of stainless steel and coated with a protection layer: Sulfinert by Restek Corporation) were partially filled (to about 75% of the total volume) at room temperature with the loaded amine solution (H2S + MDEA + piperazine + H2O) and mounted in the cell holder. Only one of those sample cells is shown in Figure 1. The composition of the liquid mixture was known from its gravimetric preparation (see below). The temperature was measured with a calibrated platinum resistance thermometer in the liquid bath that is used to control the temperature of the cell holder. The overall uncertainty of the temperature measurement was less than 0.1 K. During equilibration, very small amounts of the volatile components evaporate into the vapor phase (headspace). After equilibration, the cell was pressurized (from buffer tank A) with nitrogen to a constant pressure {(0.2, 0.26, and 0.45) MPa for the measurements at (313, 353, and 393) K, respectively} for 2 min. Then the sample loop was filled from the vapor phase. That task was achieved by connecting the vapor phase of the cell to buffer tank B. That tank is also pressurized to a constant but smaller pressure than in tank A {(0.17, 0.2, and 0.37) MPa for the measurements at (313, 353, and 393) K, respectively}. Each of the eight sample cells were connected (by stainless steel capillaries that were also coated with the above-mentioned protection layer) via a multiposition valve (Valco Instruments Co. Inc., type 2CSD16MWE-HC) to the sample loop (sample volume 100 μL). The other eight positions of the multiposition valve were used for purging. The multiposition valve and the sample valves (Valco Instruments Co. Inc., type ZVIC6WE-HC) were operated pneumatically via an electronic controller. The sample was transferred to the gas chromatograph, and afterward the sampling system was purged with pure nitrogen. To avoid condensation in the sampling system, the temperature of the valve holder was kept at (15 to 20) K above the temperature of the cell holder. The line to the gas chromatograph was also kept at a higher temperature. The primary data collected in a HS-GC experiment is the peak area of the mixture components. The peak area of hydrogen sulfide is proportional to the mass of the gas in the sample loop. That mass is proportional to the partial pressure of the gas in the

2. EXPERIMENTAL SECTION 2.1. Apparatus and Methods. As no single experimental technique is available for a precise determination of “chemical” gas solubility from pressures far below to well above atmospheric pressure, two different techniques were applied in the experimental work. A headspace gas chromatographic technique (HS-GC, i.e., a direct analytical method for analyzing the partial pressure of H2S) was used to determine the solubility of hydrogen sulfide in aqueous solution of (MDEA + PIPH2) in the low gas-loading (low partial pressure) region whereas a synthetic gas solubility method (SGSM) that determines the total pressure that is required to dissolve a known amount of the gas in a known amount of the solvent was applied for the gas solubility experiments in the high gas-loading area. That synthetic technique is not well suited for investigations at low pressures (below about 0.2 MPa), because the experimental results for the total pressure in that low gas-loading region are subject to large experimental uncertainties caused, for example, by the presence of small amounts of other dissolved gases. The experimental equipment and the experimental procedures used in the present work were the same as in previous investigations such as, for example, on the solubility of CO2 in aqueous solutions of (MDEA + PIPH2) in the high gas-loading area14 as well as in the low gas-loading area13 and on the solubility of H2S in aqueous solutions of (MDEA + PIPH2) in the high gas-loading area.8 Therefore, we restrict the description to the main features and amend the specific differences that result from the particular systems investigated. 2.1.1. Apparatus and Method Used in the Low GasLoading Area. Figure 1 shows a scheme of the experimental arrangement of the headspace gas chromatographic technique. The main components are a thermostatted cell holder, a thermostatted sampling-valve holder (containing a multiposition valve and the sampling system), two large buffer tanks (volume ≈50 dm3 each) filled with high-purity nitrogen, and a gas chromatograph {Agilent (type 6890), which was equipped with a capillary column (Varian Chromapack Capillary Column, 12550

dx.doi.org/10.1021/ie301657y | Ind. Eng. Chem. Res. 2012, 51, 12549−12556


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stoichiometric molalities (defined as number of moles of a solute per kilogram of water) is less than 0.002 m for each of the amines. The amount of mass of the solvent filled into the cell was calculated from the volume displacement in a calibrated spindle press (from which the solvent is charged into the cell) and the solvent density. The density of the aqueous amine solution was determined in separate measurements with a vibrating tube densimeter (model DMA 60/DMA 602 HAT, Anton Paar GmbH, Graz, Austria) with an uncertainty of less than 0.001 g/cm3. The relative uncertainty of the mass of solvent in the view cell is about 0.14%. The relative uncertainty of the stoichiometric molality of hydrogen sulfide in the solvent is about 1%. Two calibrated platinum resistance thermometers in the thermostatted jacket of the view cell were used to determine the temperature with an uncertainty below 0.1 K. The solubility pressure was measured with two precise pressure transducers (WIKA GmbH, Klingenberg, Germany; full scale (2.5 and 10) MPa, respectively) in connection with a mercury barometer (Lambrecht, Göttingen, Germany). All pressure transducers were calibrated against a high-precision pressure balance (Desgranges & Huot, Aubervilliers, France) before and after each measurement series. The maximum uncertainty in the solubility pressure measurement results from the intrinsic uncertainty of the pressure transducers (i.e., 0.1% of the transducer’s full scale), an additional contribution of about 0.01 MPa from a small temperature drift inside the isolated (high-pressure) tubes filled with the solvent, which connect the view cell with the pressure transducers, and the difference of the pressure transducer’s readings before and after the last expansion step. 2.2. Materials, Sample Preparation, and Experimental Uncertainties. Hydrogen sulfide (mass fraction ≥0.99) was purchased from Air Liquide Deutschland GmbH, Krefeld, Germany. N-Methyldiethanolamine (MDEA, mass fraction ≥0.985) was purchased from Riedel de Haën (Seelze, Germany); anhydrous piperazine (PIPH2, mass fraction ≥0.99) was purchased from Sigma-Aldrich Chemie GmbH (Munich, Germany). Both amines (i.e., liquid N-methyldiethanolamine and solid, scalelike piperazine anhydride) were degassed under vacuum before dissolving in water. That water was deionized, distilled, and degassed prior to use. For both types of investigations (by HS-GC and SGSM), typically about 1.4 dm3 of an aqueous solution of (MDEA + PIPH2) was gravimetrically prepared in an evacuated storage tank of glass by dissolving known amounts of the amines in water. For all HS-GC experiments, about 0.25 dm3 of such an aqueous solution was transferred to a smaller (also previously evacuated) storage tank (stainless steel, volume ≈0.3 dm3) and charged with known amounts of hydrogen sulfide at room temperature. The exact amount of mass of dissolved H2S was determined by weighing that storage tank before and after the addition of the sour gas. The charged storage tank was shaken for about 5 h and finally stored for at least another 24 h to reach thermodynamic equilibrium. The vials of the headspace apparatus were then filled at room temperature with the liquid mixture and mounted in the cell holder, where they were thermostatted to the experimental temperature for another 12 h. Corrections of the stoichiometric molalities of the solutes were applied to account for the transfer of H2S and water to the vapor phase (in both the second storage tank and the vials). In principle, both amines might also be present in the vapor phase in a vial. As the saturation pressures of MDEA17 and PIPH218 are small, the transfer of amines into the small vapor phase was neglected. The vapor phase volumes in all containers were estimated using

cell. The relation between peak area and partial pressure in the cell was determined by calibration measurements. Calibration was performed by charging the cells with pure hydrogen sulfide and measuring its pressure with a high-precision (absolute) pressure transducer (Schäfer Datametrics, Langen, Germany, type 590A1000T-2Q1-V1X-4D). In the calibration, the pressure of H2S ranged from (7 to 70) kPa. In that range the peak area of hydrogen sulfide was proportional to its pressure. The maximum systematic uncertainty in the pressure measurement ranges from about ±0.05 kPa at p ≈ 10 kPa to about ±0.15 kPa at p ≈ 70 kPa. The maximum relative deviation between the measured pressures and the calibration line amounts to 2%. 2.1.2. Apparatus and Method Used in the High GasLoading Area. The pressure is measured that is required to dissolve a precisely known amount of a gas in a precisely known amount of solvent that fills a high-pressure view cell. Figure 2

Figure 2. Apparatus for measuring the solubility of a single gas in a solvent by the synthetic method (SGSM) at elevated pressures: (A) cylindrical high-pressure equilibrium view cell with two sapphire windows and magnetic stirrer, (B) thermostat, (C) container for the gas, (D) pressure transducers, (E) tank for rinsing water, (F) tank for solvent mixture, (G) high-pressure spindle press, (H) AC-bridge with three platinum resistance thermometers, (I) solution outlet, (J) cooling trap, (K) vacuum pump.

shows the scheme of the apparatus. Its central component is a cylindrical, high-pressure view cell (cell volume about 30 cm3) with a sapphire window on each end. This view cell is thermostatted by a liquid that flows through the cell’s annular jacket. That jacket is much longer than the view cell itself and well insulated so that temperature differences within the thermostatting liquid in the annular jacket are smaller than the uncertainty of the temperature measurement (cf., below). The evacuated cell was charged with the water−amine mixture and the sour gas H2S so that a homogeneous liquid phase existed at a pressure that was somewhat above the solubility pressure. Then very small amounts (volume expansion of about 0.012 cm3) of the liquid water−amine mixture were withdrawn step by step until the first (very small) stable gas bubbles appeared. The pressure at which the degassing starts is the solubility pressure of the mixture. That pressure was calculated as the arithmetic average of the pressures before and after that last step. The mass of hydrogen sulfide in the view cell was determined gravimetrically by weighing a gas condenser (from which the cell was charged) before and after the charging process on a high precision balance. The gravimetric uncertainty amounts to 0.01 g. The solvent amine mixtures were prepared gravimetrically (see below). The experimental uncertainty of the 12551



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Table 1. Experimental Results for the Solubility of Hydrogen Sulfide in Aqueous Solutions of MDEA and PIPH2 in the Low Gas-Loading Areaa T, K 313.3 313.3 313.3 313.3 313.3 313.3 313.3 313.1 313.3 353.4 353.4 353.4 353.4 353.4 353.4 353.4 353.4 353.4 353.4 393.2 393.2 393.2 393.2 393.2 393.2 393.2 393.2 393.2 393.2 393.2 393.2 393.2

m̅ MDEA , mol·(kg water)−1 4.506 4.506 4.506 4.506 4.523 4.523 4.523 4.523 4.523 4.463 4.463 4.538 4.538 4.463 4.463 4.538 4.538 4.524 4.463 4.149 4.494 4.149 4.471 4.149 4.149 4.471 4.659 4.659 4.494 4.659 4.149 4.471

m̅ PIPH2 , mol·(kg water)−1 1.579 1.579 1.579 1.579 1.599 1.599 1.599 1.599 1.599 1.545 1.545 1.590 1.590 1.545 1.545 1.590 1.590 1.599 1.545 1.461 1.541 1.461 1.548 1.461 1.461 1.548 1.612 1.612 1.541 1.612 1.461 1.548

m̅ H2S , mol·(kg water)−1 1.045 ± 0.002 1.360 ± 0.002 2.029 ± 0.003 2.035 ± 0.003 2.418 ± 0.004 3.117 ± 0.007 3.390 ± 0.007 4.137 ± 0.014 4.446 ± 0.019 0.320 ± 0.001 0.601 ± 0.002 1.048 ± 0.003 1.382 ± 0.003 1.761 ± 0.004 1.779 ± 0.004 2.041 ± 0.005 2.034 ± 0.005 2.417 ± 0.008 2.618 ± 0.010 0.081 ± 0.001 0.155 ± 0.001 0.265 ± 0.002 0.318 ± 0.002 0.356 ± 0.002 0.541 ± 0.003 0.595 ± 0.003 0.733 ± 0.004 0.896 ± 0.005 1.003 ± 0.005 1.290 ± 0.007 1.425 ± 0.009 1.707 ± 0.015

pH S , kPa

ΔpH S,repr , kPa

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.04 0.04 0.04 0.11 0.07 0.3 0.8 2.1 1.8 0.03 0.07 0.13 0.2 0.3 0.2 0.6 1.2 0.7 1.3 0.01 0.03 0.10 0.15 0.23 0.2 0.5 1.1 1.5 2.2 1.5 4.1 1.8

2

1.26 2.36 5.89 5.98 9.08 17.6 21.9 43.6 56.7 0.90 3.00 8.63 14.6 25.6 25.9 34.2 34.9 52.8 66.1 0.64 1.98 4.99 6.21 8.11 15.5 17.1 25.8 36.7 45.0 71.1 91.4 125.7

0.03 0.06 0.15 0.15 0.23 0.50 0.64 1.5 2.3 0.02 0.08 0.22 0.4 0.7 0.7 0.9 0.9 1.7 2.0 0.02 0.06 0.14 0.18 0.23 0.4 0.5 1.0 1.4 2.3 2.1 3.0 5.7

2

Partial pressure of hydrogen sulfide pH2S (and its standard deviation ΔpH2S,repr) above aqueous solutions. The composition is given by stoichiometric molalities of MDEA (m̅ MDEA), PIPH2 (m̅ PIPH2), and H2S (m̅ H2S) (ΔT = ± 0.1 K, Δm̅ MDEA/m̅ MDEA = ± 0.1%, Δm̅ PIPH2/m̅ PIPH2 = ±0.1%).

a

the amount of totally (i.e., physically and chemically) dissolved moles of H2S per kilogram of water. The experimental results are reported together with their uncertainties: The absolute uncertainty for the partial pressure of H2S was estimated from ΔpH2S = (ΔPH*2S + acalpH2S). The first contribution ΔPH*2S accounts for uncertainties in temperature as well as in the composition of the liquid mixture (i.e., in gas and amine molalities). It is determined from a Gauss error propagation calculation (by applying the thermodynamic model). The second contribution results from the maximal uncertainty of the calibration experiments (acal = 0.02). Each experiment was repeated four to eight times. The absolute standard deviation (ΔpH2S,repr) from the reported (average) numerical value for the partial pressure of hydrogen sulfide is also given. As expected, for most experimental data points ΔpH2S,repr is less than ΔpH2S. That finding supports the estimation for the experimental uncertainties. The stoichiometric molar ratio of H2S to (MDEA + PIPH2), i.e., the loading αH2S ranges from 0.17 to 0.73 at 313 K, from 0.05 to 0.44 at 353 K, and from 0.014 to 0.27 at 393 K. The experimental results for the partial pressure of H2S ranges from about 1.26 kPa to about 98.7 kPa. In Figure 3 the new experimental results for the partial pressure of hydrogen sulfide from the investigations in the low gas-loading

experimental data for the volumes of the containers and for the density of the solvent mixtures. As all vapor phase volumes are small and the partial pressures of hydrogen sulfide and water (which were either calculated from the previously published thermodynamic model or known from the experiments) are also small, the corrections to the stoichiometric molality of all solutes are small (between 0.04% and 1.1% for H2S). The uncertainty in the stoichiometric molalities of MDEA and PIPH2 from the gravimetric preparation does not surmount 0.04%. The total uncertainty of the molality of MDEA and PIPH2 in the liquid phase in a vial is estimated to be smaller than 0.1%. The relative uncertainty of the stoichiometric molality of hydrogen sulfide in such a liquid phase ranges from about 0.16% up to about 0.85%. It was estimated from the filling procedure described before including all corrections by means of a Gauss error propagation calculation. 2.3. Experimental Results. The results of the HS-GC experiments for the partial pressure of hydrogen sulfide above an aqueous blend of MDEA and PIPH2 (stoichiometric molalities m̅ MDEA ≈ 4.5 mol·(kg water)−1; m̅ PIPH2 ≈ 1.6 mol·(kg water)−1) are given in Table 1 for three temperatures (313, 353, and 393) K. In Table 1 (as well as in Table 2), the content of H2S in the aqueous solution is also given as its stoichiometric molality, i.e., 12552



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Table 2. Experimental Results for the Pressure above Aqueous Solutions of H2S, MDEA, and PIPH2 in the High Gas-Loading Areaa T, K

m̅ MDEA , mol· (kg water)−1

m̅ PIPH2 , mol· (kg water)−1

313.2

4.446

1.538

353.1

4.446

1.538

393.2 393.2 393.1 393.1 393.2 393.2 393.2 393.2 393.2 393.2 393.2 393.2 393.2 393.2 393.2

4.446 4.446 4.446 4.446 4.446 4.446 4.446 4.446 4.446 4.499 4.446 4.499 4.499 4.499 4.499

1.538 1.538 1.538 1.538 1.538 1.538 1.538 1.538 1.538 1.543 1.538 1.543 1.543 1.543 1.543

m̅ H2S , mol· (kg water)−1 5.460 6.422 6.422 7.016 7.214 7.214 7.872 8.168 8.508 8.968 9.660 3.494 3.493 3.490 3.691 3.692 4.246 4.259 5.094 5.624 5.746 6.857 6.861 8.162 8.162 9.641 9.642 10.01 10.01 3.076 3.076 3.899 3.900 4.476 4.476 4.816 5.206 5.206 5.494 6.115 6.376 6.908 6.994 6.993

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.055 0.064 0.064 0.069 0.072 0.072 0.077 0.080 0.086 0.088 0.095 0.034 0.034 0.034 0.036 0.036 0.043 0.043 0.049 0.055 0.057 0.068 0.068 0.081 0.081 0.095 0.096 0.099 0.100 0.031 0.031 0.037 0.038 0.045 0.045 0.048 0.052 0.052 0.055 0.062 0.063 0.069 0.069 0.069

p, MPa 0.225 ± 0.011 0.609 ± 0.011 0.603 ± 0.011 0.914 ± 0.014 1.104 ± 0.014 1.122 ± 0.014 1.532 ± 0.014 1.810 ± 0.014 2.134 ± 0.014 2.471 ± 0.014 2.852 ± 0.014 0.204 ± 0.011 0.207 ± 0.011 0.206 ± 0.011 0.250 ± 0.011 0.251 ± 0.011 0.303 ± 0.011 0.327 ± 0.011 0.523 ± 0.011 0.760 ± 0.014 0.844 ± 0.014 1.770 ± 0.014 1.762 ± 0.014 3.184 ± 0.014 3.186 ± 0.014 5.045 ± 0.02 5.045 ± 0.02 5.473 ± 0.02 5.477 ± 0.02 0.568 ± 0.011 0.568 ± 0.011 0.866 ± 0.014 0.865 ± 0.014 1.210 ± 0.014 1.212 ± 0.014 1.374 ± 0.014 1.655 ± 0.014 1.655 ± 0.014 1.844 ± 0.014 2.542 ± 0.014 2.779 ± 0.014 3.353 ± 0.014 3.572 ± 0.014 3.572 ± 0.014

Figure 3. Solubility of H2S in aqueous solutions of (MDEA + PIPH2) in the low gas-loading area. New experimental results for the partial pressure of H2S above aqueous solutions of MDEA and PIPH2 (m̅ MDEA ≈ 4.5 mol·(kg water)−1 + m̅ PIPH2 ≈ 1.6 mol·(kg water)−1) at 313.3 K (■), 353.4 K (□), and 393.2 K (●) compared to prediction results (). αH2S = m̅ H2S/(m̅ MDEA + m̅ PIPH2).

Figure 4. Solubility of H2S in aqueous solutions of (MDEA + PIPH2): New experimental results for the solubility pressure (total pressure) (m̅ MDEA ≈ 4.5 mol·(kg water)−1 + m̅ PIPH2 ≈ 1.5 mol·(kg water)−1) 313.2 K (■), 353.1 K (□), and 393.2 K (●) compared to prediction results ().

slightly as sulfide), resulting in very small partial pressures of the sour gas at low gas-loadings. But when the amine has been spent by chemical reactions, newly added sour gas can no longer be absorbed “chemically” but has to be dissolved “physically”. At higher temperatures the chemical reaction equilibrium is shifted in favor of neutrally dissolved sour gas, resulting in a higher partial pressure of hydrogen sulfide at constant gas-loading. The new experimental results for the total pressure above an aqueous (MDEA + PIPH2) solution from the SGSM-experiments (i.e., in “high gas-loading region”) are given in Table 2 together with their uncertainties. The gas-loading factor αH2S ranges from 0.91 to 1.61 for the experiments at 313 K, from 0.58 to 1.67 at 353 K, and from 0.51 to 1.17 at 393 K. The corresponding solubility pressure ranges from 0.23 to 5.5 MPa. Figure 4 shows the new experimental results for the solubility pressure (total pressure) from the SGSM-experiments plotted against the stoichiometric molality of H2S for the investigated

Total pressure p (and its uncertainties) {(ΔT = ±0.1 K, Δm̅ MDEA/ m̅ MDEA = ±0.002, Δm̅ PIPH2/m̅ PIPH2 = ±0.002)}.

a

region (and prediction results from a thermodynamic model (see below)) are plotted versus the stoichiometric molar ratio αH2S. Figure 3 (and also Figure 4) reveals the typical behavior when a sour gas is dissolved in an aqueous amine solution: the partial pressure of the sour gas (and also the total pressure) at first only very slightly increases with increasing molar ratio of gas to amines (i.e., increasing stoichiometrić molality of the gas in the liquid). This is due to the basic character of the amine. The sour gas is at first predominantly dissolved chemically (i.e., in nonvolatile ionic form, here predominantly as bisulfide (hydrosulfide) and only 12553



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“activated” MDEA solution at (313, 353, and 393) K together with prediction results from the thermodynamic model (see below).

H2O, MDEA, PIPH2, and H2S, and ξR is the extent of reaction R. Solving this set of equations for a given temperature and stoichiometric amounts of substances n̅i of components H2O, MDEA, PIPH2, and H2S results in the speciation, i.e., the “true” composition of the liquid phase (the amount of substance ni of all species present). That speciation is required to determine the composition of the vapor phase. The model applies the extended Henry’s law on the molality scale to describe the partial pressure of H2S (i.e., the product of total pressure p and vapor phase mole fraction yH2S) above the aqueous solution.

3. COMPARISON WITH PREDICTIONS FROM A THERMODYNAMIC MODEL FOR THE VAPOR−LIQUID EQUILIBRIUM The thermodynamic model for the solubility of hydrogen sulfide in aqueous solutions of one of the single amines (MDEA or PIPH2) was described and parametrized in previous publications.4−9 A detailed description of the model can be found there. In addition, all further thermodynamic properties were reported by Kuranov et al.4 and by Perez-Salado et al.6 The parametrization of the model was based only on experimental data for the total pressure above aqueous solutions of the single amines (MDEA or PIPH2) in the high gas-loading range at temperatures between 313 K and 393 K. The model can be extended to describe the solubility of H2S in an aqueous blend of (MDEA and PIPH2). Figure 5 shows a scheme of the extended model.

⎡ vH∞ S(p − p s ) ⎤ W ⎥a H2S = yH S pφH S kH,H2S exp⎢ 2 2 2 ⎥⎦ ⎢⎣ RT

(3)

and the extended Raoult's law to describe the partial pressure water: s pW φWs

⎡ v W (p − p s ) ⎤ W ⎥aW = yW pφW exp⎢ RT ⎦ ⎣

(4)

kH,H2S is Henry’s constant of H2S in water at temperature T, R is the universal gas constant, and pSW is the vapor pressure of water. vw and vH∞2S are the molar volume of liquid water and the partial molar volume of H2S at infinite dilution in water, respectively. ϕH2S and ϕW are the fugacity coefficient of component H2S and water, respectively, in the gaseous mixture that coexists with the liquid. ϕSW is the fugacity coefficient of water in its saturation state, and ai is the activity of species i in the liquid state. The activity of species i is normalized according to Henry’s law on the molality scale: a i = miγi(m)

(5)

γ(m) i are

where mi and the molality and the molality-based activity coefficient of solute species i. The activity of the solvent (water) is normalized according to Raoult’s law. It is calculated via the Gibbs−Duhem equation from the equations for the activities of all solute species. The activity coefficients of all solute species are calculated with a modification of Pitzer’s equation for the excess Gibbs energy of aqueous electrolyte solutions. That equation requires binary and ternary parameters for interactions between all solute species. These interaction parameters were adopted from Ermatchkov et al.7,9 They are given in Table A1 in the appendix. No experimental results for the solubility of H2S in aqueous solutions of the mixed amines (MDEA + PIPH2) were available for the previous parametrization of the model. Therefore, all parameters for interactions between species that originate from MDEA on one side and species that originate from PIPH2 on the other side (binary parameters as well as all ternary parameters where at least two of the three species originate either from MDEA or PIPH2) had to be neglected. Although that set of model parameters can be used to predict the solubility of H2S in aqueous solutions of the mixed amines (MDEA + PIPH2), the quality of such predictions is unknown. However, previous investigations showed that good predictions were possible for the solubility of CO2 in such aqueous solutions.12,14,15 These predictions were made by applying the corresponding model for the solubility of CO2 with a parameter set that was only adjusted to experimental results for the solubility of CO2 in aqueous solutions of the single amines. Caused by the formation of carbamates, the chemical reaction equilibrium in (CO2 + H2O + MDEA + PIPH2) is more complicated than in the system (H2S + H2O + MDEA + PIPH2). Therefore, the expectation was that model predictions for the

Figure 5. (H2S +MDEA + PIPH2 + H2O): Model for the vapor− liquid-equilibrium with all considered chemical reactions.

The model neglects the volatilities of both amines. Caused by chemical reactions in the liquid phase, hydrogen sulfide is dissolved not only in neutral, volatile form but also in nonvolatile, ionic form (predominantly as hydrosulfide HS−), and both amines are protonated. The following reversible chemical reactions (chemical equilibrium) are taken into account: the autoprotolyses of water (R1), the formations of hydrosulfide (bisulfide) and sulfide (R2, R3), the protonation of MDEA (R4), and the first and second protonation of PIPH2 (R5, R6). The temperature-dependent chemical reaction equilibrium constants on molality scale (K1−K6) were adopted from previous publications.4,6 The condition for chemical equilibrium for a chemical reaction R (= 1, ..., 6) is: KR (T ) =

∏ aiν

i ,R

i

(1)

where KR(T) is the chemical reaction equilibrium constant for reaction R, ai is the thermodynamic activity of species i in the liquid phase, and νi,R is the stoichiometric coefficient of species i in reaction R. The influence of pressure on KR is neglected. The balance equation for the amount of substance (mole number ni) of species i in the liquid phase is:

ni = ni̅ +

∑ νi ,RξR R

(2)

where ni̅ is the amount of substance i in the liquid feed, i.e., ni̅ = 0, for all species besides the four components of the mixture 12554



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limited experimental information for that property. It is shown that predictions for the partial pressure of H2S (and the total pressure) by a previously parametrized thermodynamic model are in reasonable agreement with the new experimental data. However, there seems to still be some space for improvements.

solubility of H2S in aqueous solutions of that amine mixture, with a set of parameter that is solely based on solubility data for H2S in aqueous solutions of the single amines, are reliable. That expectation proved to be correct as can be seen from Figures 3 and 4, where the full lines represent prediction results from that model. The average relative (absolute) deviation between the new experimental data in the low gas-loading region (cf., Figure 3) and the prediction of the model for the partial pressure of H2S amounts to 3.5% (0.4 kPa) at 313.3 K and 5.5% (1.3 kPa) at 353.4 K. At 393.2 K, the differences between the experimental data and the model predictions are somewhat larger. The average relative (absolute) deviation amounts to 33% (8.6 kPa). The average relative (absolute) deviation between new experimental data and prediction results for the total pressure in the high gas-loading region (cf., Figure 4) is about 13% (0.17 MPa) at 313.2 K, about 9% (0.08 MPa) at 353.1 K, and about 7.5% (0.12 MPa) at 393.2 K. Figure 6 shows calculation results for the speciation in

■

APPENDIX

The platinum resistance thermometers used in the work by Xia et al. for investigations on the solubility of hydrogen sulfide in aqueous solutions of piperazine as well as in aqueous solutions of (MDEA + PIPH2) were recalibrated straight after those experimental results had been published, showing that a correction had to be applied to the temperature values. The corresponding corrections to the experimental results for the solubility of hydrogen sulfide in aqueous solutions of (MDEA + PIPH2) are given in Table A2. Table A1. Interaction Parameters in Pitzer’s GE Equation for the System (H2S + MDEA + PIPH2 + H2O) from Ermatchkov et al.7,9 q2 f (T ) = q1 + (T / K ) q1

q2

T, K

βH(0)S,H S a 2 2

−0.26156

69.751

283−453

βH(0)S,HS−

0.0096582

−18.988

313−413

(0) βMDEA,HS −

0.21482

−55.014

313−395

(0) β MDEA + ,HS−

0.039284

−

(1) β MDEA + ,HS−

4.4771

−1466.8

βH(0)S,PIPH+

0.16812

−48.836

(0) βPIPH + ,HS−

0.027680

−

(1) βPIPH + ,HS−

1.4921

−469.89

(0) βPIPH 2+ ,HS−

0.33892

−97.698

(1) βPIPH 2+ ,HS−

0.93222

−

μH S,PIPH+,HS−

−0.0089545

2.1507

μH S,H S,PIPH+

0.0062141

−

parameter

2

Figure 6. Predicted species distribution in the system [H2S + MDEA+ H2O] at 313.15 K. (m̅ MDEA = 4.5 mol·(kg water)−1); m̅ PIPH2 = 1.5 mol· (kg water)−1).

aqueous solutions of (H2S + MDEA (4.5 m) + PIPH2 (1.5 m)) at 313 K. Two different regions are distinguished. Region I corresponds to the low gas-loading area (i.e., αH2S < 1) whereas region II is the high gas-loading area (αH2S ≥ 1). Chemical absorption dominates the solubility in region I whereas in region II the dominant phenomena is “physical” gas solubility in an aqueous electrolyte solution. Figure 6 shows that when small amounts of H2S are added to the aqueous amine solution, the decrease of the molality of PIPH2 is steeper than that of MDEA, i.e., H2S reacts preferentially with PIPH2. In the low gas-loading range, the deviations between the experimentally determined partial pressures of H2S and the prediction results might be assigned to missing parameters for interactions between (MDEA and MDEAH+) on one side and (PHIPH2 and its protonated species) on the other side. The deviations between the experimental results and the prediction results in the high gas-loading range cannot be caused by those missing parameters, as the amount of neutrally dissolved amines is very low. These deviations might be caused by uncertainties in the chemical reaction equilibrium constants for the protonation reactions, which might have been compensated by fitting interaction parameters in the development of the models for the solubility of H2S in the aqueous solutions of the single amines.

2

313−393

3

3 3

4

4

2

2

3

2

3

a

Based on experimental results for the solubility of H2S in water (cf., Kuranov et al.)5

Table A2. Solubility of H2S in Aqueous Solutions of MDEA and PIPH2a T, K 354.4 ± 0.1

4. CONCLUSIONS The new experimental results for the solubility of H2S in aqueous solutions of (MDEA and PIPH2) extend the rather

m̅ MDEA , mol·(kg water)−1 1.975

m̅ PIPH2 , mol·kg−1

m̅ H2S , mol·kg−1

p, MPa

1.966

2.600 3.440 4.479 5.145 6.572 7.594 7.680

0.1364 0.3454 1.243 2.154 4.392 6.142 6.207

a

Experimental results from Xia et al.8 after applying a small temperature correction.

12555



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Article

(17) Xu, S.; Qing, S.; Zhen, Z.; Zhang, C.; Carroll, J. J. Vapor pressuremeasurements of aqueous N-methyldiethanolamine solutions. Fluid Phase Equilib. 1991, 67, 197−201. (18) Walton, J. Vapour pressures and critical points of liquids. XI: Heterocyclic nitrogen compounds; Engineering Sciences Data Unit: London, U.K., 1977; Vol. 77019.

AUTHOR INFORMATION

Corresponding Author

*Tel. +49 631 205 2410. Fax +49 631 205 3835. E-mail: gerd. [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS Financial support by BASF SE, Ludwigshafen a.Rh., Germany, for some parts of this investigation is gratefully acknowledged. REFERENCES

(1) BP. Statistical Review of World Energy, June 2009, http://bp.com/ statisticalreview. (2) Kohl, A. L.; Nielsen, R. B. Gas purification, 5th ed.; Elsevier: New York, 1997. (3) O’Connell, J. P.; Gani, R.; Mathias, P. M.; Maurer, G.; Olson, J. D.; Crafts, P. A. Thermodynamic property modeling for chemical process and product engineering: some perspectives. Ind. Eng. Chem. Res. 2009, 48, 4619−4637. (4) Kuranov, G.; Rumpf, B.; Maurer, G.; Smirnova, N. A. VLE modeling for aqueous systems containing methyldiethanolamine, carbon dioxide and hydrogen sulfide. Fluid Phase Equilib. 1997, 136, 147−162. (5) Kuranov, G.; Rumpf, B.; Smirnova, N. A.; Maurer, G. Solubility of single gases carbon dioxide and hydrogen sulfide in aqueous solutions of N-methyldiethanolamine in the temperature range 313−413 K at pressures up to 5 MPa. Ind. Eng. Chem. Res. 1996, 35, 1959−1966. (6) Pérez-Salado Kamps, Á .; Balaban, A.; Jödecke, M.; Kuranov, G.; Smirnova, N. A.; Maurer, G. Solubility of single gases carbon dioxide and hydrogen sulfide in aqueous solutions of N-methyldiethanolamine at temperatures from 313 to 393 K and pressures up to 7.6 MPa: New experimental data and model extension. Ind. Eng. Chem. Res. 2001, 40, 696−706. (7) Ermatchkov, V.; Pérez-Salado Kamps, Á .; Maurer, G. Solubility of carbon dioxide in aqueous solutions of N-methyldiethanolamine in the low gas loading region. Ind. Eng. Chem. Res. 2006, 45, 6081−6091. (8) Xia, J.; Pérez-Salado Kamps, Á .; Maurer, G. Solubility of H2S in (H2O + piperazine) and in (H2O + MDEA + piperazine). Fluid Phase Equilib. 2003, 207, 23−34. (9) Ermatchkov, V.; Pérez-Salado Kamps, Á .; Speyer, D.; Maurer, G. Solubility of carbon dioxide in aqueous solutions of piperazine in the low gas loading region. J. Chem. Eng. Data 2006, 51, 1788−1796. (10) Speyer, D.; Maurer, G. Solubility of hydrogen sulfide in aqueous solutions of piperazine in the low gas-loading region. J. Chem. Eng. Data 2011, 56, 763−767. (11) Anoufrikov, Y.; Pérez-Salado Kamps, Á .; Rumpf, B.; Smirnova, N. A.; Maurer, G. Solubility of H2S in H2O + N-methyldiethanolamine + (H2SO4 or Na2SO4). Ind. Eng. Chem. Res. 2002, 41, 2571−2578. (12) Pérez-Salado Kamps, Á .; Xia, J.; Maurer, G. Solubility of CO2 in (H2O + piperazine) and in (H2O + MDEA + piperazine). AIChE J. 2003, 49, 2662−2670. (13) Ermatchkov, V.; Pérez-Salado Kamps, Á .; Speyer, D.; Maurer, G. Solubility of carbon dioxide in aqueous solutions of piperazine in the low gas loading region. J. Chem. Eng. Data 2006, 51, 1788−1796. (14) Speyer, D.; Ermatchkov, V.; Maurer, G. Solubility of carbon dioxide in aqueous solutions of N-methyldiethanolamine and piperazine in the low gas loading region. J. Chem. Eng. Data 2010, 55, 283−290. (15) Böttger, A.; Ermatchkov, V.; Maurer, G. Solubility of carbon dioxide in aqueous solutions of N-methyldiethanolamine and piperazine in the high gas loading region. J. Chem. Eng. Data 2009, 54, 1905−1909. (16) Pérez-Salado Kamps, Á .; Rumpf, B.; Maurer, G.; Anoufrikov, Y.; Kuranov, G.; Smirnova, N. A. Solubility of CO2 in H2O + Nmethyldiethanolamine + (H2SO4 or Na2SO4). AIChE J. 2002, 48, 168−177. 12556


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