Thermodynamic Study of a Complex System for Carbon Capture

Article pubs.acs.org/jced

Thermodynamic Study of a Complex System for Carbon Capture: Butyltriacetonediamine + Water + Carbon Dioxide Dan Vasiliu,† Amir Yazdani,† Nichola McCann,† Muhammad Irfan,‡ Rolf Schneider,‡ Jörn Rolker,§ Gerd Maurer,† Erik von Harbou,*,† and Hans Hasse† †

Laboratory of Engineering Thermodynamics, University of Kaiserslautern, Erwin-Schrödinger-Strasse 44, 67663 Kaiserslautern, Germany ‡ Evonik Technology & Infrastructure GmbH, Rodenbacher Chaussee 4, 63457 Hanau-Wolfgang, Germany § Evonik Performance Materials GmbH, Rodenbacher Chaussee 4, 63457 Hanau-Wolfgang, Germany ABSTRACT: Different thermodynamic properties of aqueous solutions of butyltriacetonediamine (BuTAD), unloaded and loaded with carbon dioxide, are studied experimentally. For unloaded mixtures of BuTAD and water, protonation equilibrium constants between 283 and 333 K and liquid−liquid equilibria between 313 and 353 K, including the lower critical point, are determined at atmospheric pressure. Furthermore, the solubility of carbon dioxide in aqueous solutions of BuTAD between 313 and 393 K is determined at low loadings with an analytic method based on headspace gas chromatography and at high loadings with a synthetic method using a high pressure view cell. In the loaded system, solid precipitation, liquid−liquid phase split, and the presence of metastable states are observed in certain ranges. The data is interesting for assessing the aqueous solution of BuTAD as solvent for carbon capture. A short-cut method is used for comparing the new solvent with an aqueous solution of monoethanolamine (MEA) with respect to the energy requirement of an absorption/ desorption process for CO2 scrubbing. Furthermore, a new and simple gas chromatographic method for determining the loading with carbon dioxide of aqueous solutions of amines is described.

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INTRODUCTION

Two strategies have been pursued to achieve a synergistic effect by combining the advantages of different types of amino groups while avoiding the disadvantages. First, a great deal of blends of especially tertiary with primary or secondary amines was formulated and investigated.3 Here, the carbamate-forming primary and secondary amines, which are in lower concentrations, act as catalysts. Second, the so-called sterically hindered amines were developed and investigated. Sterically hindered amines have a bulky chemical group adjacent to the amino group. The bulky group, usually a tertiary carbon, hinders the formation of carbamates.4 The unstable carbamates will act only as catalysts, being hydrolyzed very fast to free amines and carbonates.4 Sterically hindered amines can combine thus the advantages of fast reacting primary amines with the high absorption capacity of tertiary amines.4,5 An interesting class of sterically hindered amines is derived from triacetoneamine (TAA, 2,2,6,6-tetramethyl-4-piperidinone, CAS: 826-36-8, cf. Figure 1a), in which the oxygen atom of the carbonyl group of TAA is replaced by a nitrogen atom with two different substituents, which can be tailored for attaining desired properties.6,7

Aqueous solutions of amines, especially alkanolamines, have been used extensively for the last 80 years for removal of acidic components from gaseous streams.1 The process is used in large-scale applications like purification of natural gas or synthesis gas and flue gas treatment. Accordingly, many different amines have been studied regarding their suitability as solvents in gas treatment. The thermodynamics and the kinetics of the chemical system amine/water/CO2 define the suitability of an amine from the point of view of the energy consumption of a process employing that respective amine. Simple amines show an increase of their CO2 absorption capacity from primary through secondary to tertiary amines, whereas the CO2 absorption rate varies the other way around. The main structural features of the amines and their effect on the chemistry of the system amine/ water/CO2 in the liquid phase can explain these variations. First, it is the increase of basicity with the number of electron releasing substituents that favors the formation of carbonates. Second, it is the decrease of the number of available hydrogen atoms necessary for the formation of carbamates, which in the case of the tertiary amines is impossible. The formation of carbonates, whereby one molecule of CO2 requires one amino group, is typically slower than the formation of carbamates, whereby one molecule of CO2 requires two amino groups.1,2 © XXXX American Chemical Society

Received: June 2, 2016 Accepted: October 5, 2016

A

DOI: 10.1021/acs.jced.6b00451 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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chosen BuTAD concentration is optimal for carbon capture, but it is in the range that is common in aqueous amine solvents used for that purpose. A series of chemical reactions takes place in aqueous solution of BuTAD. BuTAD can be protonated at both amino groups (cf. Reactions I and II). Because of the presence of more electron releasing alkyl groups in the α position, amino group 1 is less acidic than amino group 2 and, hence, it is protonated first. Additionally, water undergoes autoprotolysis (cf. Reaction III).

Figure 1. Structural formulas. (a) Triacetoneamine (TAA). (b) Butyl triacetonediamine (BuTAD). Numbers are assigned to the two amino groups of BuTAD.

BuTAD + H3O+ ⇌ BuTADH+ + H 2O

(I)

BuTADH+ + H3O+ ⇌ BuTADH 2 2 + + H 2O

The present work reports on results from a study of physicochemical properties of one member of the family of the TAA derivatives, butyltriacetonediamine (BuTAD, N-butyl2,2,6,6-tetramethylpiperidin-4-amine, CAS 36177-92-1, cf. Figure 1b). BuTAD is not the most promising member of that family with respect to CO2 scrubbing, but its unloaded and carbon-dioxide-loaded aqueous solutions show a variety of interesting effects including solid precipitation, metastable states, and liquid−liquid phase split, so that the study can be considered as a pilot study for investigations of other members of the TAA derivatives family and other amines used for carbon capture. There is some discussion about whether solid precipitation and liquid−liquid phase split are positive or negative regarding the use of solvents for carbon capture. These effects could either compromise the operation of absorption/desorption cycles or, on the other hand, they could enable new process options.8,9 We do not enter into that discussion in the present work and simply present the results of the investigations of the relevant physicochemical properties of the system BuTAD/ water/CO2: vapor pressure of amine, protonation constants, liquid−liquid equilibria, and gas solubilities. Furthermore, a new short-cut method, named NoVa,10 is used to assess an aqueous solution of BuTAD as solvent for carbon capture and storage (CCS) in absorption/desorption processes. It is a modification of the method of Notz et al.11 and serves for ranking solvents based on gas solubility data. Thus, it can be applied in early stages of a process of screening of solvents for CCS. In the present work, NoVa was used for comparing an aqueous solution of BuTAD with wBuTAD = 0.3 g· g−1, which is named here BuTAD30 to a well-investigated aqueous solution of monoethanolamine (MEA) with wMEA = 0.3 g·g−1, which is named here MEA30.

2H 2O ⇌ H3O+ + OH−

(II) (III)

Also, the chemical reactions in the system water/CO2 have to be considered (cf. Reactions IV and V). CO2 + 2H 2O ⇌ HCO3− + H3O+

(IV)

HCO3− + H 2O ⇌ CO32 − + H3O+

(V)

Carbamate formation (cf. Reaction VI) is possible in principle at both amino groups of BuTAD. We studied the composition of BuTAD30 loaded with CO2 (αCO2 ≈ 1 molCO2· mol−1 BuTAD) [the loading αCO2 is defined in the section Results and Discussion by eq 1] at room temperature using 13C NMR spectroscopy and found that no CO2 bounds as carbamate at the sterically hindered position 1 and only a small amount −1 (nCO2,carbamate·n−1 CO2,total < 0.07 mol·mol ) at position 2 (cf. Figure 1b)

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BuTAD + CO2 + OH− ⇌ BuTADCOO− + H 2O

(VI)

CHEMICALS BuTAD was delivered by Evonik Industries AG (wBuTAD = 0.993 g·g−1). Carbon dioxide (xCO2 > 0.99995 mol·mol−1) and the mixture CO2/N2 (xCO2 = 0.1996 mol·mol−1 ± 0.4%) were purchased from Air Liquide. A list of the pure components used in this work is given in Table 1. Hydrochloric acid 1 M ± 0.2% and sodium hydroxide 1 M ± 0.2% were purchased from Carl Roth GmbH + Co. KG. All reagents necessary for Karl Fischer titration (Hydranal−Methanol dry, Hydranal−Composite 5, Hydranal−Water standard 10.0) were purchased from Fluka Analytical. 1-Propanol AnalaR NORMAPUR (w1−propanol > 0.999 g·g−1) was purchased from VWR Chemicals. Water was purified in our laboratory using a purification system from Siemens AG (TWF/EI-Ion UV Plus TM). Evonik Antifoam A, a 0.7 g·g−1 active antifoam concentrate based on modified siloxanes, was delivered by Evonik Industries AG. All reagents except the water were used as delivered without further purification.

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CHEMICAL SYSTEM BuTAD (cf. Figure 1b) is a sterically hindered amine with two secondary amino groups. Amino group 1 is located inside a piperidinic ring and amino group 2 connects the piperidinic ring with an n-butyl group. In case nothing else stated, in the present work the studied system is the aqueous solution BuTAD30 (wBuTAD = 0.3 g·g−1). We do not claim that the Table 1. Provenance and Purity of the Pure Components

a

chemical name

source

initial fraction purity

purification method

final fraction purity

analysis method

BuTAD CO2 H2O

Evonik Industries Air Liquide SWKc

0.993a 0.99995b n.a.

none none ion-exchange and filtration

0.993a 0.99995b >0.99999a

stated by supplier stated by supplier conductometry

Mass fraction. bMole fraction. cStadtwerke Kaiserslautern. B



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Titrino Plus and 870 KF Titrino Plus) from Metrohm AG (Herisau, Switzerland). The mass fractions of both amines and water were determined with an uncertainty of ±2%. The temperature of the water bath was determined with an uncertainty of ±0.1 K. Liquid Phase Speciation. The chemical speciation of liquid mixtures of BuTAD/water/CO2 was investigated at 293 K by 13C NMR spectroscopy with an Avance III HD 400 MHz spectrometer (BBO KryoProbe Prodigy, 1H broadband decoupled inverse gated pulse sequence, 30° flip angle, 200 ppm sweep width, 64 scans, relaxation delay 60 s) from Bruker (Rheinstetten, Germany). Experiments with Flow Saturator. A flow saturator apparatus was designed and built for screening experiments. Figure 2 depicts a scheme of the apparatus.

All of the unloaded aqueous solutions of BuTAD were prepared gravimetrically by mixing degassed water and BuTAD in an evacuated glass cylinder. After mixing, the cylinder was shaken for 4 to 5 h and stored subsequently at room temperature for at least 12 h. The CO2 loaded aqueous solutions of BuTAD (between 10 and 15 mL) were prepared gravimetrically, typically by adding pressurized CO2 from a gas cylinder to the unloaded BuTAD in a nonmagnetic stainless steel cell (volume about 30 mL) equipped with an on−off gas valve. The cell, which contained a magnetic stir bar, was kept on a hotplate magnetic stirrer at around 40 °C during loading with CO2 to increase the absorption rate of CO2. The uncertainty of the gravimetric measurements was about ±2 mg. The uncertainties (U) are reported in this work as estimates including both systematic and statistical errors. We refrain from specifying confidence intervals for uncertainties, which include estimations for systematical errors.

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EXPERIMENTAL SECTION Amine Vapor Pressure. The vapor pressure of BuTAD was determined by ebulliometry. The ebulliometer consists of a flask (volume about 100 cm3), a reflux condenser, a heating jacket, a vacuum system, and sensors for temperature and pressure. The boiling temperature was measured for a given pressure in the range between 1 and 100 kPa. About 70 cm3 of the liquid sample is placed in the flask. Then the pressure is reduced and heating is started so that the liquid boils and gases are removed. After adjusting the pressure to a set point, the heating is adjusted so that a reflux rate between one and two drops per second is achieved. Temperature and pressure are recorded after steady conditions are obtained. The pressure is increased stepwise and more readings are taken. The temperature was measured with a Pt-100 thermometer with an uncertainty of ±0.1 K. The pressure was measured with an absolute pressure gauge with an uncertainty of ±0.01 kPa, which led to estimated relative uncertainties for the final values of vapor pressure between ±1% and ±5%. Amine Protonation Equilibria. First, 20 mL samples of 0.02 M aqueous solution of BuTAD were treated with an excess of HCl to obtain the dihydrochloride (BuTADH22+ Cl22−). The dihydrochloride was titrated against 1 M aqueous solution of NaOH until both amino groups were deprotonated. The titrations were conducted at constant temperature, which was controlled by circulating a thermal agent through the double jacket of the titration beaker. In order to avoid the contact with the atmospheric CO2, the titrations were performed in a glovebox under a constant flow of dry nitrogen. The titration was performed automatically using an 848 Titrino Plus from Metrohm AG (Herisau, Switzerland). Before the measurements, a calibration of the electrode was performed with a strong acid−base titration. The temperature was determined with an uncertainty of ±0.1 K. Liquid−Liquid Equilibria. First, 30 mL closed vials containing solutions of aqueous BuTAD were thermostated at different temperatures for more than 3 h. The vials were manually shaken from time to time and placed back into the thermostatic bath. In preliminary experiments it was checked that equilibrium is reached in that procedure. Samples were taken with syringes, ensuring that only the desired phase was withdrawn. Each phase was analyzed by titration against 1 M hydrochloric acid to determine the concentration of amine and by Karl Fischer titration to determine the concentration of water. The titrations were performed using titrators (848

Figure 2. Scheme of the flow saturator used for screening solvents for CO2 capture. (T1) source of CO2; (T2) presaturator containing brine; (T3) saturator containing the tested solvent; (T4) thermostated water bath; (T5) pressure fluctuation damper; (P1) gas pump; (H1) heating element; (V) valves; (PI) pressure indicator; (FI) flow rate indicator; (TIC) temperature indicator and controller; (PIC) pressure indicator and controller.

A stream of a gaseous mixture of CO2 and N2 (xCO2 = 0.2 mol·mol−1) is taken from the gas cylinder T1. For the experiments at partial pressures of CO2 above 20 kPa, a stream of pure CO2 is used instead. The gas is first saturated with water upon passing an aqueous solution of sodium chloride (wNaCl = 0.056 g·g−1) in the presaturator T2. The concentration of sodium chloride is chosen so that the activities of water in the NaCl solution and in the studied solvent BuTAD30 are approximately equal to avoid water losses from the studied amine solutions during the experiments. The water-saturated gas then passes the studied solvent in the saturator T3 where the CO2 is absorbed. In order to reduce the chance of occurrence of metastable states, glass pearls were added as crystallization germs to the studied solvent at the beginning of the experiment. The saturators T2 and T3 are glass bottles with volumes of about 120 mL. Both are filled with 100 mL of liquid and the gas is fed to their bottoms via a disk-shaped fritted glass. T2 and T3 are immersed in the water bath T4, in which the temperature is controlled by the thermostat TIC1. The gas flow rate and the total pressure in the saturator T3 are regulated by means of the manual valve V3 and controller PIC5. After passing the buffer vessel T5, the gas passes the vacuum pump P1, which enables operation at subatmospheric pressures. The experiments were carried out at constant gas flow rate (typically 100 Ncm3·min−1), constant pressure, and constant temperature in the saturator T3. The temperature of the water bath was determined with an uncertainty of ±1 K. Samples were taken periodically from the liquid solvent in the saturator C



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group. 12,16,17 The working principle of headspace gas chromatography is based on measuring the partial pressure of the gas above a preloaded sample of solvent (10 to 16 mL) for a known amount of dissolved gas in the solvent. A thermostated vial (volume about 30 mL) holder, which enables the parallel handling of eight samples, is connected through an automated thermostated multiposition sample valve system to a gas chromatograph (GC) 6890 from Agilent (Santa Clara, U.S.A.). The GC is equipped with a GS-Q capillary column from J&W Scientific (Folsom, U.S.A.) and a thermal conductivity detector, which analyses the gas phase from the headspace. The peak area in the chromatogram corresponding to CO2 is related to the partial pressure of CO2 by calibration. Before each series of measurements at a new temperature, a calibration was performed by filling the sample cells with pure CO2. The relative deviation between the partial pressure of CO2, calculated from the calibration curve and the measured partial pressure of CO2 was in each case better than ±2%. A calibrated pressure sensor of type 690A13TRA from MKS (Andover, U.S.A.) enabled the measurement of the total pressure with a relative uncertainty of ±0.05%. The temperature of the oil bath was measured with a calibrated platinum resistance thermometer with an uncertainty better than ±0.1 K. Analysis of Precipitated Solids. Precipitated solids from the flow saturator experiments were filtered using a Büchner flask and funnel system coupled to a water jet vacuum pump. The overall water content of the crystals was checked from time to time by Karl Fischer titration. The filtration was stopped when the measured water content did not vary anymore in consecutive determinations. Subsequently, the dried crystals were analyzed. Elemental analysis of crystals was performed to determine the molar ratio of the elements C:H:N, using a vario MICRO cube from Elementar Analysensysteme (Hanau, Germany). Crystals were analyzed using titrators (848 Titrino Plus and 870 KF Titrino Plus) from Metrohm AG (Herisau, Switzerland). The amine content was determined by titration against HCl 1 M. The water content was determined by Karl Fischer titration.

T3 with a syringe. The sampling was performed every 30 min for the first 2 h of the experiment and every 60 min afterward. The CO2 loading of the solvent is determined by a simple and robust GC analysis method that was developed in this work and it is described in Appendix A. During the experiments, either foaming or solid precipitation may occur in the saturator T3. In case of foaming, a few drops (one to five) of antifoaming agent (Evonik Antifoam A) were added to the solvent and the experiment was resumed. Foaming could always be suppressed by that procedure. If solid precipitation occurs, the experiment is stopped. Otherwise, the experiment is conducted until the equilibrium is reached, that is, the CO2 loading is constant. To test whether the equilibrium is stable, extra crystallization germs (porous stones and seed crystals) were added to the equilibrated solvent and the experiment was resumed for 30 min. Additionally, the wetted wall of the saturator T3 was rubbed with a glass rod. The equilibrium was considered as stable if no solid precipitation occurred under these conditions. The analytical results give the loading as a function of time and enable assessing the overall absorption kinetics as well as the equilibrium loading. The overall absorption kinetics considers both the chemical reactions and the mass transfer. All of the information on the absorption kinetics provided in this work refers to the overall absorption kinetics. The absorption rate is calculated by differentiation of the timeloading profile with respect to time. A new three-parameter empirical correlation for CO2 absorption kinetics, which is presented in Appendix B, was used for describing the timeloading profile. Gas Solubility by Synthetic Method. The experimental setup and procedures used in the present work for the gas solubility measurement with the synthetic method were the same as described in previous works of our group.12,13 The central part of the apparatus is a jacketed high-pressure view cell (volume about 30 cm3), which was filled at the beginning with a known amount of pure gas (between 0.6 and 3.3 g). To absorb the gas, an appropriate amount of unloaded solvent (about 28 g) was injected into the cell by means of a highpressure spindle press. When the gas was completely absorbed into the liquid solvent, incremental amounts of liquid were removed from the cell by relaxing the spindle press. When the first stable gas bubble emerged from the liquid, the total pressure was recorded. It is the solubility pressure. The total amount of absorbed CO2 was determined from the known volume of the cell and the measured temperature and pressure with the Wagner equation of state.14 Numerical solutions of the Wagner equation of state were retrieved using the software RefProp.15 The temperature was measured with calibrated platinum resistance thermometers with an uncertainty better than ±0.1 K. The pressure was measured by calibrated pressure gauges from WIKA GmbH (Klingenberg, Germany) coupled with a mercury barometer from Lambrecht (Göttingen, Germany). The uncertainty was better than ±0.3 MPa for the gauge sensors and negligible (less than 10 Pa) for the mercury barometer. The relative uncertainty of the mass of solvent in the view cell was ±0.2%. The relative uncertainty of the molality of CO2 in the loaded solvent was about ±0.65%. Gas Solubility by Headspace Gas Chromatography. The experimental setup and the experimental procedures for the gas solubility measurement by gas chromatography are the same as described in detail in previous publications of our

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RESULTS AND DISCUSSION Amine Vapor Pressure. The vapor pressure of BuTAD was measured at temperatures between 390 and 530 K. The melting point of BuTAD is 210 K.18 Experimental results are reported in Table 2. A modified Antoine function to describe the temperature dependence of the vapor pressure was parametrized using the present experimental data. The results are given in Table 3. The mean relative deviation between the Table 2. Experimental Data of the Vapor Pressure ps of Pure BuTAD at Temperature Ta T/K

ps/kPa

U(ps)/kPa

398.7 418.2 429.7 445.6 462.1 479.5 525.3

1.9 4.5 6.7 11.4 19.4 31.9 99.8

0.1 0.2 0.3 0.5 0.8 1.2 1.0

a

The estimated uncertainty for temperature is U(T) = 0.1 K. The estimated uncertainties for vapor pressure U(ps) are given in the table. D



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experimental data and the correlation is 2.5%. A graphical comparison is shown in Figure 3.

Table 5. Correlation of the Acidity Constants of BuTAD, ln(Ka,j) = Aj − Bj·(RT)−1, Describing the Acidity Constant Ka,j as a Function of the Temperature Ta

Table 3. Correlation of the Vapor Pressure of Pure BuTAD, ln(ps/kPa) = A + B·(T/K + C)−1, Describing the Vapor Pressure ps as a Function of the Temperature Ta

a

amino group j

Aj

Bj/(kJ·mol−1)

U(Bj)/(kJ·mol−1)

1 2

6.96 2.08

−45 −40

2 1

a

A

B

C

12.98

−3308.33

−131.15

The parameters are estimated using the data reported in Table 4. R is the universal gas constant and T is the temperature. The estimated uncertainties U(Bj) for the parameter Bj, which corresponds to the enthalpy of protonation, are given in the table. The amino groups are assigned as shown in Figure 1b.

Parameters are estimated using the data reported in Table 2.

Figure 3. Vapor pressure of BuTAD. (○) Experimental results; () correlation cf. Table 3.

Figure 4. Temperature dependence of the protonation constants of BuTAD. Experimental results: (□) amino group 1; (○) amino group 2 (cf. Figure 1b); () correlation cf. Table 5.

Amine Protonation. Dihydrochloride of BuTAD was titrated against aqueous NaOH at different temperatures between 283 and 333 K. Both acidity constants Ka,1 and Ka,2 (the indices correspond to the numbering given in Figure 1b) were determined by fitting an activity-based model to the experimental data. The model considers the equilibria Reactions I to III, the species balance, and the charge balance. The activity coefficients of the ionic species were calculated using a simple Debye−Hückel approximation.19 Because the calculated activity coefficients are close to unity, a molaritybased model would result in almost the same equilibrium constants. Table 4 lists the results for pKa,1 and pKa,2 at the studied temperatures. The van’t Hoff equation was fitted to the results. The fitted parameters are given in Table 5. The maximum absolute deviation between the correlated pKa values and the corresponding experimental data was ±0.05. A graphical comparison between the experimental and the correlated data is shown in Figure 4. The enthalpy of the

first protonation stage of BuTAD (Δ1hBuTAD = −45 kJ·mol−1) is similar to that of the first protonation stage of piperazine (PZ) (Δ 1hPZ = −44 kJ·mol−1)20 and between the numbers commonly observed for primary amines (e.g., MEA, Δ1hMEA = −51 kJ·mol−1)20 and that of tertiary amines (e.g., Nmethyldiethanolamine (MDEA), Δ1hMDEA = −34 kJ·mol−1).20 The enthalpy of the second protonation stage of BuTAD (Δ2hBuTAD = −40 kJ·mol−1) is about two times the enthalpy of the second protonation stage of piperazine (Δ2hPZ = −22 kJ· mol−1).20 Liquid−Liquid Equilibrium. Binary liquid−liquid equilibria (LLE) for the unloaded system BuTAD/water were measured at atmospheric pressure and temperatures between 313 and 353 K. The experimental values are given along with their uncertainties in Table 6. The LLE was modeled using the isoactivity condition. Parameters of the UNIQUAC model were determined from a fit to the experimental data of the present work. Temperature dependent parameters were used as this improved the quality of the fit significantly. The results are given in Table 7. The lower critical solution temperature (LCST) and the composition at the critical point were determined from the fit to be 316.9 K and wBuTAD = 0.313 g· g−1. Experimental data are depicted in Figure 5. LLE occurs in the studied system at temperatures above about 317 K, that is, in the operating range of typical PCC plants.21 Furthermore, the presence of the miscibility gap, suggests that BuTAD forms a heteroazeotrope with water. The fact that such a low-boiling azeotrope exists has consequences for the solvent losses via the lean gas in the absorption column. However, preliminary estimates show that at a typical absorber temperature of 313 K the losses are low due to the low vapor

Table 4. pKa Values of BuTAD in Aqueous Solution at Temperature Ta T/K

pKa,1

U(pKa,1)

pKa,2

U(pKa,2)

283.15 293.15 303.15 313.15 323.15 333.15

11.36 11.00 10.72 10.50 10.30 10.09

0.02 0.02 0.03 0.03 0.02 0.06

8.38 8.07 7.83 7.61 7.40 7.27

0.03 0.03 0.04 0.04 0.02 0.08

a

The estimated uncertainty for temperature is U(T) = 0.1 K. The estimated uncertainties for the pKa values U(pKa) are given in the table. The indices correspond to the amino groups indicated in Figure 1b. E



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not accurate. Anyway, considering the magnitude of the calculated ratios, they show clearly that the amine losses are lower in the case of BuTAD30 compared to the benchmark MEA30. It is only mentioned here that recent work24 indicates that amine losses do not mainly depend on the amine partial pressure above the solution in equilibrium but rather result from aerosol formation in the supersaturated vapor. CO2 Solubility. The solubility of CO2 in mixtures of BuTAD and water was investigated with different experimental methods at temperatures between 313 and 393 K and partial pressure of CO2 between 1 kPa and 10 MPa. The experimental results are presented in Table 8 for the flow saturator, in Table

Table 6. Liquid−Liquid Equilibrium in the System BuTAD/ Water at Atmospheric Pressure, ptot = 99 ± 1 kPa, and Temperature Ta aqueous phase −1

T/K

wBuTAD/g·g

318.0 323.2 328.3 333.4 343.6 353.8

organic phase

U(wBuTAD)/g·g

0.136 0.071 0.052 0.022 0.015 0.011

−1

0.013 0.023 0.017 0.007 0.005 0.003

wBuTAD/g·g−1

U(wBuTAD)/g·g−1

0.456 0.667 0.755 0.812 0.865 0.906

0.012 0.007 0.005 0.004 0.003 0.002

a

The estimated uncertainty for temperature is U(T) = 0.1 K. The estimated uncertainties U(wBuTAD) for the mass fractions wBuTAD are given in the table.

Table 8. Solubility of CO2 in Aqueous Solutions of BuTAD, mBuTAD = 2.02 ± 0.15 mol·kgH2O−1, Determined with the Flow Saturator at 313 K ± 1 Ka

Table 7. UNIQUAC42 Parameters for Describing the LLE in the System BuTAD (1)/Water (2)a geometric parameters

interaction parameters

q1

q2

r1

r2

a12

a21

b12

b21

7.96

1.4

9.8286

0.92

−9.6832

1.9842

3111.4

−664.38

a

Geometric parameters are from UNIFAC.42 Temperature-dependent interaction parameters are given by ln (τi,j) = ai,j + bi,j·(T/K)−1.

mCO2/mol·kgH−12O

U(mCO2)/mol·kg−1 H2O

pCO2/kPa

U(pCO2)/kPa

2.000 2.050 2.079 2.119 2.234 2.313

0.085 0.087 0.089 0.091 0.096 0.099

5.52 6.52 7.52 8.52 11.52 13.52

0.50b 0.50b 0.50c 0.50c 0.50c 0.50c

a

The estimated uncertainties U(mCO2) for the molalities mCO2 and U(pCO2) for the partial pressure pCO2 are given in the table. The corresponding type of state of each measured point is given in the last column. bStable VLE. cMetastable VLE.

9 for the headspace GC, and in Table 10 for the high-pressure view cell. Depending on the conditions, different types of phase equilibria were observed: vapor−liquid equilibria (VLE), vapor−liquid−liquid equilibria (VLLE), and vapor−liquid− solid equilibria (VLSE). Additionaly, metastable VLE were observed in some regions. The molalities m (i.e., the amount of CO2 or BuTAD, respectively, per kilogram of water) reported here normally refer to the liquid phase. In case of liquid−liquid phase split or solid precipitation they are overall molalities in which both condensed phases (liq + liq or liq + solid) are lumped together. From the molalities the molar ratio of CO2 to BuTAD, αCO2, is readily found using eq 1. m̅ CO2 αCO2 = m̅ BuTAD (1) Figure 6 represents the experimental CO2 solubilities in BuTAD30 at partial pressures of CO2 below 70 kPa. The lines shown in Figure 6 are calculated from a new empirical correlation called SolSOFT, which is presented in Table 11 and discussed by Vasiliu.10 At 333 K, a single liquid phase is observed and a VLE is established. At higher temperature (393 K), a liquid−liquid phase split occurs and a VLLE is established. At lower temperatures (313 and 323 K), the solubility isotherms exhibit single branches at lower loadings corresponding to stable VLE. At higher loadings the isotherms exhibit two branches. The lower branch corresponds to stable VLSE states and the upper branch to metastable VLE states. The existence of competing metastable states at 313 K was confirmed in experiments carried out with both headspace GC and flow saturator. As described above, both techniques were adapted to increase the probability of reaching stable VLSE. Seed crystals were used in the experiments with flow saturator. In the experiments with headspace GC, an aqueous solution

Figure 5. Liquid−liquid equilibrium in the system BuTAD/water at atmospheric pressure. (○) Experimental data; () UNIQUAC fit; (■) critical point.

pressure of BuTAD. On the other hand, at typical low temperatures of the condenser installed at the top of the desorber, namely toward 283 K, the losses are low due to the high affinity of BuTAD to water. A comparison to the benchmark solvent MEA30 was performed. The vapor pressure of BuTAD extrapolated with the correlation given in Table 3 is about 6 Pa at 313 K and 0.2 Pa at 283 K. The molar fraction based activity coefficient of BuTAD in BuTAD30 calculated with the UNIQUAC model with the parameters given in Table 7 is about 1.02 at 313 K and 0.0006 at 283 K. The vapor pressure of MEA calculated with an extended Antoine equation22 is 164 Pa at 313 K and 13 Pa at 283 K. The molar fraction based activity coefficient of MEA in MEA30 calculated with the UNIQUAC model23 is about 0.39 at 313 K and 0.31 at 283 K. The mass ratio of the amine losses above the washing sections of absorbers that operate in the same conditions with MEA30 and BuTAD30, respectively, was estimated to be almost 10 gMEA·g−1 BuTAD at 313 K and around −1 30 000g MEA ·g BuTAD at 283 K. We are aware that the extrapolation induces errors and the calculated numbers are F



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Table 9. Solubility of CO2 in Aqueous Solutions of BuTAD, mBuTAD = 2.02 ± 0.01 mol·kg−1 H2O, Determined with Headspace GC in the Low Pressure Range at 313 K, 323 K, 333 K, and 393 Ka T/K 313.15

323.15

mCO2/mol·kg−1 H2 O 1.684 1.989 2.186 2.314 2.412 2.838 3.096 2.054 2.174 2.235 2.462 2.547 2.654 2.892 2.908 3.053 3.265 1.160 1.610 1.914 2.005 2.270 2.349 2.670 2.716 2.455 2.480 2.694 2.872

U(mCO2)/mol·kg−1 H2 O 0.007 0.008 0.008 0.008 0.008 0.008 0.008 0.007 0.007 0.008 0.007 0.007 0.008 0.008 0.008 0.009 0.009 0.006 0.006 0.011 0.007 0.008 0.008 0.006 0.007 0.007 0.007 0.008 0.007

pCO2/kPa 3.64 7.09 11.27 14.11 17.56 34.66 62.10 7.33 8.65 9.70 10.55 12.20 14.94 19.01 16.87 21.19 23.59 3.46 6.87 12.30 14.05 23.77 26.68 50.23 54.69 35.55 35.95 38.31 42.18

U(pCO2)/kPa

T/K

b

0.08 0.15c 0.23c 0.29c 0.36c 0.70c 1.25c 0.15d 0.18d 0.20d 0.22d 0.25d 0.30d 0.39d 0.34d 0.43d 0.48d 0.07b 0.14b 0.25b 0.29b 0.48b 0.54b 1.01c 1.10c 0.72c 0.72c 0.77d 0.85d

333.15

393.15

mCO2/mol·kg−1 H2O

U(mCO2)/mol·kg−1 H2O

pCO2/kPa

U(pCO2)/kPa

2.875 2.975 3.097 3.112 3.277 3.292 0.802 1.100 1.312 1.592 1.760 2.060 2.077 2.106 2.276 2.351 2.476 2.537 0.021 0.058 0.091 0.106 0.142

0.007 0.006 0.007 0.007 0.009 0.007 0.006 0.006 0.006 0.007 0.007 0.007 0.007 0.006 0.007 0.007 0.007 0.006 0.001 0.001 0.001 0.001 0.001

42.94 51.91 49.18 47.04 56.67 56.89 6.64 7.69 10.29 14.99 20.07 32.04 33.24 32.96 43.12 48.82 66.24 68.17 5.88 13.98 23.58 31.54 56.78

0.86d 1.04d 0.99d 0.95d 1.14d 1.14d 0.14b 0.16b 0.21b 0.30b 0.41b 0.65b 0.67b 0.66b 0.87b 0.98b 1.33b 1.37b 0.12e 0.28e 0.48e 0.64e 1.14e

a

The estimated uncertainty for temperature T is U(T) = 0.1 K. The estimated uncertainties U(mCO2) for the molalities mCO2 and U(pCO2) for the partial pressure pCO2 are given in the table. The corresponding type of state of each measured point is given in the last column. b Stable VLE. cMetastable VLE. dStable VLSE. eStable VLLE.

Table 10. Solubility of CO2 in Aqueous Solutions of BuTAD, −1 mBuTAD = 2.018 ± 0.005 mol·kgH , Determined with the 2O Synthetic Method in the High Pressure Range at 333 and 393 Ka T/K

mCO2/mol·kg−1 H2O

U(mCO2)/mol·kg−1 H2 O

ptot/kPa

U(ptot)/kPa

333.15

3.334 3.369 3.537 3.589 3.617 4.239 0.519 0.707 0.822 1.126 1.482 2.490 2.792 2.975 2.984

0.022 0.023 0.023 0.024 0.024 0.028 0.004 0.005 0.006 0.008 0.010 0.017 0.019 0.020 0.020

0.441 0.467 0.942 0.671 0.728 2.437 0.557 0.755 0.886 1.097 1.615 3.301 4.409 5.577 5.656

0.011b 0.011b 0.011c 0.011d 0.011d 0.011d 0.011e 0.011e 0.011e 0.011e 0.011e 0.011b 0.012b 0.012b 0.012b

393.15

Figure 6. Partial pressure of carbon dioxide above liquid mixtures of BuTAD/water/CO2 (mBuTAD = 2.018 mol·kgH−12O) at different temperatures, measured with flow saturator (pentagrams) and headspace gas chromatography (rest of symbols). Temperature: circles and stars 313 K; diamonds - 323 K; squares - 333 K; triangles - 393 K. Type of equilibrium: empty symbols - stable VLE; horizontal strike through symbols - stable VLLE; vertical strike through symbols and pentagram with solid lines - metastable VLE; full symbols - stable VLSE. SolSOFT correlations: () VLE; (− −) VLSE; (·−) VLLE.

a

The estimated uncertainty for temperature T is U(T) = 0.1 K. The estimated uncertainties U(mCO2) for the molalities mCO2 and U(ptot) for the total pressure ptot are given in the table. The corresponding type of state of each measured point is given in the last column. b Stable VLE. cMetastable VLE. dStable VLSE. eStable VLLE.

with high BuTAD concentration (0.4 g·g−1 < wBuTAD < 0.6 g· g−1) was first loaded with CO2 until solid precipitated. Afterward the heterogeneous mixture was diluted with water G



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Table 11. SolSOFT Correlation10 Describing the Solubility of CO2 as a Function of the Partial Pressure of CO2: αCO2 = pmCO2·Kp−m + n·pnCO2·(pnCO2+Knc )−1a system

T/K

m

n

ln(Kp/bar)

ln(Kc/bar)

BuTAD30

313 323 333 393 313* 323*

0.078 0.641 0.329 0.516 0.395 0.493

0.941 1.023 1.148 1.222 2.812 1.048

2.070 0.909 3.456 4.899 −2.646 −0.642

−1.714 −3.385 −1.905 3.184 4.022 −0.835

metastability. At lower loadings, no solid precipitation can occur.

a

The parameters (m, n, Kp, and Kc) of the SolSOFT equation for VLE in the system CO2/BuTAD30 are given for different temperatures. The rows marked with * give the SolSOFT parameters fitted to the VLSE experimental data.

to reach the desired BuTAD/water ratio. Upon the dilution, the crystals did not dissolve. In Figure 7, the two isotherms at 333 and 393 K are shown both in the low and in the high pressure region. For the data

Figure 8. Estimated solid solubility limit in the system BuTAD/water/ CO2 for mBuTAD = 2.018 mol·kgH−12O at different temperatures. (○) values read from Figures 6 and 7; () guide to the eye.

Analysis of Solid Crystals. Dried precipitated crystals obtained in flow saturator experiments were analyzed for apparent species (BuTAD and H2O) and for elements (C, H, and N). Experimental results, averaged from three solid samples obtained in independent experiments performed at 313 K and a partial pressure of CO2 of 100 kPa, are reported in Table 12. The reproducibility of the results with regard to the molar fractions of analytes is about 4.5%. The results are interpreted in a first step by determining an integer value ratio of mole numbers of the apparent species BuTAD:H2O:CO2. The simplest ratio giving a good fit of the data is 3:10:5. A simple way to explain these findings would be that the solid is a mixture of two species Sol1 ([BuTADH+][HCO3−]·H2O) and Sol2 ([BuTADH22+][HCO3−]2·2H2O) in a molar ratio Sol1:Sol2 of 0.5. The molar fractions of the analytes determined from that ratio are reported in Table 12. They deviate from the experimental values by less than 4.5%. This hypothesis is in line with different independent findings obtained in the present study. The results obtained for the amine protonation show that BuTAD is present in the solution in the studied pH range in its two protonated forms (BuTADH+ and BuTADH22+). In the 13C NMR spectroscopic measurements, only a negligible amount of carbamates (BuTADCOO−) was detected (cf. Figure 9). Furthermore, in the studied pH range CO2 is present mainly as HCO3−, the amount of CO32− being negligible as well. Ranking of BuTAD30 against MEA30 by a Short-Cut Method. One common way to rank solvents for carbon capture is based on comparing the heat duty and solvent circulation rates of absorption/desorption processes that employ those particular solvents and are conducted under the same conditions.25,26 Plots of heat duty for desorption (qReb) against the mass ratio of liquid to gas (L/G) in the absorber at fixed composition and flow-rate of the gas and fixed CO2 removal rate are often used for the comparison of results found for different solvents.25−27 Notz et al.11 have proposed a shortcut method yielding such curves for different solvents based on gas solubility data and simple assumptions regarding caloric properties. The method has been shown to be useful and reliable for ranking solvents.28

Figure 7. Partial pressure of carbon dioxide above liquid mixtures of BuTAD/water/CO2 (mBuTAD = 2.018 mol·kgH−12O) at different temperatures, measured with headspace gas chromatography at total pressures below 0.1 MPa and with synthetic method at total pressure above 0.4 MPa. Temperature: squares - 333 K; triangles - 393 K. Type of equilibrium: empty symbols - stable VLE; horizontal strike through symbols - stable VLLE; vertical strike through symbols - metastable VLE; full symbols - stable VLSE. SolSOFT correlations: () 333 K, VLE; (·−) 393 K, VLE and VLLE.

measured with the high pressure view-cell, the partial pressure of CO2 was calculated by subtracting the estimated partial pressures of water and of BuTAD from the measured total pressure, assuming ideal behavior of BuTAD and water in the liquid, which is sufficient for the present purpose. At 333 K, the same type of branching as observed for the lower temperatures (cf. Figure 6) occurs, however, at higher loadings. This result confirms the above inferences concerning metastability. At 393 K and low partial pressure of CO2, a VLLE is established, whereas at high partial pressure of CO2 a VLE is established. These observations show that the presence of CO2 reduces the miscibility gap in aqueous solutions of BuTAD. The three branching points at 313, 323, and 333 K were visually determined from Figures 6 and 7. The three points represent the solid solubility limit in the system BuTAD30/ CO2 and are shown together in Figure 8. If the loading exceeds the limit, solid precipitation occurs unless hindered by H



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Table 12. Composition of the Precipitated Solids from a Flow Saturator Experiment Performed with BuTAD30 at 313 K and a Partial Pressure of CO2 of 100 kPaa analytical technique

analyte

xexp/mol·mol−1

U(xexp)/mol·mol−1

xcalc/mol·mol−1

elemental analysis

C H N BuTAD H2O

0.251 0.591 0.036 0.167 0.562

0.003 0.011 0.001 0.008 0.023

0.254 0.595 0.035 0.167 0.556

titration

Comparison between experimental data (xexp) and data obtained, assuming that there are two solids: Sol1 = [BuTADH+][HCO3−]·H2O and Sol2 = [BuTADH22+][HCO3−]2·2H2O in a molar ratio of Sol1:Sol2 = 0.5 (xcalc). The estimated expanded uncertainties U(xexp) for the molar fractions xexp are given in the table. a

Figure 9. 13C NMR spectrum of an aqueous solution of loaded BuTAD (mBuTAD = 2.018 mol·kgH−12O, αCO2 = 1 mol·mol−1 BuTAD) at 298 K and corresponding peak assignment.

That method, however, has some shortcomings: the equilibrium curves are only represented pointwise and linear interpolations are used, which introduces ambiguity and may cause numerical trouble. Furthermore, arbitrary assumptions for the stage numbers in the absorber and desorber are used. In order to overcome these problems, we used a modification of the method of Notz et al. The modified short-cut method is called NoVa (from Notz and Vasiliu) and it is presented in detail by Vasiliu.10 The only physicochemical properties required for this short-cut method as input are (a) correlations describing the CO2 solubility at the absorption and desorption conditions (usually at constant temperature); (b) the overall enthalpy of absorption of CO2 at desorber conditions; (c) the heat capacity of the solvent. Differently from the original method,11 in the calculations of the NoVa method, the number of stages both in the absorber and in the desorber is assumed to be infinite. In this work, the NoVa method was applied to BuTAD30 and to the reference solvent MEA30 (aqueous solution of MEA, wMEA = 0.3 g·g−1) for the process conditions listed in Table 13. The flue gas conditions correspond to a reference brown coal power plant11 and the operating parameters are typical for common PCC plants.21 The solubilities of CO2 in BuTAD30 are those determined in this work. Only VLE data was considered at 313 K, metastability was ignored. VLLE data at 393 K was treated as VLE data without any modification of the method. VLE data for MEA30 was taken from Wagner et al.12

Table 13. Boundary Conditions Used in the Calculations with the NoVa Short-Cut Methoda flue gas

operating parameters

parameter/unit

value

molar mass/kg·kmol−1 molar fraction of CO2 CO2 removal rate/% total pressure in the absorber/bar total pressure in the desorber/bar temperature in the absorber/°C temperature of the rich solvent/°C temperature of the lean solvent/°C temperature in the condenser/°C L/G/kg·kg−1

28.88 0.136 90 1.039 2 40 110 120 20 2−6

The flue gas conditions correspond to a reference brown coal power plant.11 a

The SolSOFT equation was used for determining the enthalpy of absorption of CO2 in BuTAD30 from the influence of the temperature upon the solubility of CO2. For that purpose, a correlation with temperature-dependent parameters was developed (extending the information presented in Table 11). An analogous procedure was applied for MEA30. For details see Vasiliu.10 The resulting enthalpies of absorption used for NoVa are −1 ΔabshBuTAD30 = −83 kJ·mol−1 CO2 and ΔabshMEA30 = −88 kJ·molCO2. −1 −1 The heat capacity cp,S = 4.05 kJ·kg K was considered the same for both BuTAD30 and MEA30, as it corresponds to an aqueous solution and it does not differ significantly. I



Article

The resulting qReb vs L/G plots are shown in Figure 10. The minimum specific energy consumption of BuTAD30, deter-

energy requirement in an adsorption/desorption process. The short-cut method was used to compare the performance of a BuTAD-based solvent with the performance of an aqueous solution of MEA. The results indicate that this aqueous solution of BuTAD is potentially attractive for the use as solvent in a reactive absorption process of CO2. BuTAD, however, is just a first example for the large class of TAA-derivatives and other more interesting candidates exist, out of which several are currently investigated. The methods described here can also be used to study them. By a solvent design, potential problems due to liquid−liquid phase split or solid precipitation can be avoided. At the same time, should processes be established in which such phase-changes phenomena can be exploited, the TAA-derivatives can be tailored to deliver them.

■

APPENDIX A. ANALYSIS OF TOTAL DISSOLVED CO2 IN LIQUID PHASE Fast and reliable analysis methods for the determination of the total amount of dissolved CO2 in the liquid phase of loaded amine-based aqueous solvents are important, for example, for screening of solvents for CCS processes. Three categories of methods prevail in the literature: (a) precipitation methods,29−32 based on precipitation of CO2 as carbonates (e.g. BaCO3, SrCO3) and the subsequent handling of solids; (b) acidification methods,4,33−35 based on releasing the CO2 in gaseous form by treating the samples with an acidic solution (e.g., H3PO4, HCl) and the subsequent handling of the released gas; (c) wet chemistry titration methods, 36 based on identification of the equivalent points from titration curves. The known methods have shortcomings that include long times and much manual labour needed for the analysis, high consumption of analytical chemicals, and operator-dependent outcomes. Because of the high degree of automation, gas chromatography is attractive, especially where large numbers of samples have to be analyzed. Also, gas chromatography has been used for analyzing CO2 in aqueous amine solutions. Several calibration methods to correlate the peak area of CO2 obtained in the chromatogram with the concentration of CO2 in the sample are reported: (a) volumetric techniques based on injection of gaseous CO2 (time consuming and possibly operator-dependent37,38); (b) calibration against literature values39 (not always available); (c) calibration based on the estimation of the relative response factor of CO2 with respect to water40 (assuming that the amount of water is constant, which may not be true). Therefore, we developed a robust method for determining the CO2 concentration in aqueous amine solutions based on gas chromatography that can be used to determine the total amount of dissolved CO2 over an wide range of CO2 loading. An internal standard approach is used for the calibration. 1Propanol is selected as internal standard (ISTD) due to its stability and different volatility compared to the other constituents of the loaded solutions under the measurement conditions. Solutions for calibration with different CO2 loadings were prepared gravimetrically as described in the Chemicals section. CO2 concentrations were chosen between zero and the saturation solubility at loading conditions. Preloaded samples, about 1000 mg, are mixed with about 100 mg ISTD. The samples containing ISTD are analysed by GC. The specifications of the GC method are listed in Table 14. In this work, the weighing of the sample and of the ISTD as well as the mixing were carried out manually. To reduce the manual

Figure 10. Specific energy consumption of MEA30 and BuTAD30 as functions of L/G calculated with NoVa short-cut method for the boundary conditions given in Table 13.

mined by NoVa is 2.6 GJ·t−1 CO2, about 20% smaller than the corresponding number for MEA30, which is 3.3 GJ·t−1 CO2. Also L/G in the minimum is lower for BuTAD30 (3.1 kg·kg−1) compared to MEA30 (4.5 kg·kg−1) by 20%. Since the short-cut method is intended only for ranking of solvents, the conclusion is plain and qualitative: BuTAD30 is potentially more attractive than MEA30. Nevertheless, caution should be exercised, since solid precipitation and the occurrence of a liquid−liquid phasesplit were ignored in the calculations.

■

CONCLUSIONS Aqueous solutions of hindered amines derived from triacetoneamine (TAA) could be interesting for carbon capture. BuTAD was studied in detail as an example. The system BuTAD/water/ CO2 exhibits complex thermodynamic behavior, which involves multiphase equilibria (e.g., LLE, VLE, VLLE, VLSE) and chemical reactions. The vapor pressure of pure BuTAD was determined experimentally. For the unloaded system BuTAD/ water, the acidity constants and the composition of each of the two coexisting phases were determined. For the loaded system BuTAD/water/CO2, qualitative aspects such as the tendency to solid precipitation and foaming were investigated with a flow saturator, which also provides information on the CO2 solubility. In this context, a new and simple gas chromatographic method for the analysis of dissolved CO2 was developed. Highly accurate CO2 solubilities were measured in the low pressure range (pCO2 < 80 kPa) with a method based on headspace gas chromatography and in the high pressure range (pCO2 > 0.4 MPa) with a synthetic method in a high pressure view cell. A new short-cut method (NoVa) coupled with an empirical correlation for describing gas solubilities (SolSOFT) was applied for assessing solvents with respect to their specific J



Article

Table 14. Summary of the Gas Chromatography Method for the Analysis of CO2 from Liquid Samples gas chromatograph separation column injector detector temperature programming carrier gas column flow programming injection volume

Agilent 7890 A with autosampler capillary column Restek R RTX 5-Amine, length 30 m, diameter 250 μm, film thickness 1 μm split-injector at 280 °C, split ratio 75:1, septum purge 3 mL·min−1 thermal conductivity detector at 300 °C, makeup flow 9 mL·min−1, reference gas flow 15 mL·min−1 7 min at 40 °C, increase with 100 °C·min−1 to 120 °C, 2.5 min at 120 °C, increase with 120 °C·min−1 to 280 °C, 4 min at 280 °C helium 4.6 min at 0.5 mL·min−1, increase with 1 (mL·min−1)· min−1 to 1.2 mL·min−1, 1.7 min at 1.2 mL·min−1, decrease with 5 (mL·min−1)·min−1 to 0.5 mL·min−1 0.6 μL

Figure 12. Calibration of the gas chromatograph for CO2 analysis from liquid samples of loaded MEA30 using 1-propanol as internal standard (ISTD). (○) Experimental data; () correlation. The relative deviations are below 4% for wCO2 > 0.015 g·g−1.

labour further, these preparation steps can be carried out very accurately by a laboratory robot as described by Werner et al.41 The possibility to conduct the whole analysis process including the sample preparation and the GC analysis automatically makes this method very interesting when large numbers of samples have to be investigated such as in screening experiments. A typical chromatogram of the analysis of CO2 in MEA30 is shown in Figure 11. It was observed that for systems which

■

APPENDIX B. EMPIRICAL CORRELATION OF CO2 LOADING IN FLOW SATURATOR An empirical three parameter correlation for the CO2 loading as a function of time (time-loading profile) in absorption kinetics experiments is proposed (cf. eq 3)

Figure 13. Time profiles of CO2 loading and CO2 absorption rate during an experiment carried out with the flow saturator with BuTAD30 at 313 K, 35 kPa, and 100 Ncm3·min−1 CO2/N2 mixture (xCO2 = 0.2 mol·mol−1). (□) Experimentally determined CO2 loading; (○) numerically determined rate of CO2 absorption; () CO2 loading calculated with eq 3; (−−) rate of CO2 calculated with eq 5.

Figure 11. Characteristic chromatogram for the GC analysis of CO2 from a liquid sample of loaded MEA30 (wCO2 = 0.0985 g·g−1) obtained with the method characterized in Table 14. The signal of the amine (at τMEA ≈ 9.6 min) is not shown.

∞ αCO2(τ ) = αCO · 2

form no carbamates, the calibration curves are typical straight lines passing through the origin. On the other hand, for systems which form carbamates, the calibration curves are nonlinear at low concentrations of CO2, like in the case of MEA30 (cf. Figure 12). For the correlation, a three-parameter empirical equation (cf. eq 2) was used, which is reported here for the first time, where a, k, and b are the parameters. For BuTAD an ordinary linear calibration curve is obtained mCO2 mISTD

= a ·(1 − e−k·ACO2 / AISTD) + b·

(3)

αCO2 is the CO2 loading which is given here as molar ratio of CO2 to amine, τ is the time, α∞ CO2 a parameter with loading units corresponding to equilibrium loading (τ → ∞), k a parameter expressed in reciprocal time units corresponding to a time constant and q ≠ 0 a dimensionless parameter. For q → 0 the correlation approaches a first order kinetics law (cf. eq 4).

A CO2 AISTD

ln(eq + e−kτ − e(q − kτ)) q

(2)

∞ lim αCO2(q , τ ) = αCO (1 − e−kτ ) 2

q→0

The method was tested on more than ten different aminebased aqueous solvents. In all cases, for concentrations of CO2 above 0.03 g·g−1, the maximum relative deviation between the resulted calibration curve and the data was below 5%.

(4)

The CO2 absorption rate rCO2 is calculated with eq 5, where the initial rate of CO2 absorption r0CO2 is given by eq 6 for q ≠ 0. K


Journal of Chemical & Engineering Data rCO2(τ ) = 0 rCO = 2

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0 rCO 2

1 + eq(ekτ − 1)

eq − 1 ∞ kαCO2 q

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(5)

(6)

One experiment with BuTAD30 carried out at 313 K, 35 kPa, and 100 Ncm3·min−1 CO2/N2 gaseous mixture (xCO2 = 0.2 mol· mol−1) is here considered as a show-case. The fitted parameters −1 −1 are α∞ and q = CO2 = 0.99 molCO2·molBuTAD, k = 0.0316 min 0 −5.32. The initial rate of CO2 absorption is rCO2 = 5.85 −1 mmolCO2·mol−1 BuTAD·min . The time-loading profile and the calculated CO2 absorption rate are shown in Figure 13.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

We gratefully acknowledge the financial support of this work by Federal Ministry for Economic Affairs and Energy of the Federal Republic of Germany under the project 03ET1098A-C. Notes

The authors declare no competing financial interest.

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REFERENCES

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Article

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Thermodynamic Study of a Complex System for Carbon Capture

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