Ind. Eng. Chem. Res. 2008, 47, 7917–7926
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Online NMR Spectroscopic Study of Species Distribution in MDEA-H2O-CO2 and MDEA-PIP-H2O-CO2 Wolfram Bo¨ttinger,† Michael Maiwald,‡ and Hans Hasse*,§ Institute of Thermodynamics and Thermal Process Engineering, UniVersita¨t Stuttgart, Germany
Quantitative online nuclear magnetic resonance (NMR) spectroscopy was used to study the species distribution in solutions of carbon dioxide (CO2) in aqueous N-methyldiethanolamine (MDEA), and MDEA + piperazine (PIP). The mass fraction of MDEA in the unloaded ternary solution was 0.2, 0.3, and 0.4 g/g. In quaternary solutions, the mass fraction of MDEA was 0.3 g/g, and that of PIP was 0.1 g/g. The temperature ranged from 293 K to 333 K, and the overall CO2 loading was up to 1.4 molCO2/molamine. For the measurements, a special apparatus was used that allowed the mixtures to be prepared gravimetrically and applied pressures up to 25 bar to keep the CO2 in solution. It was coupled to a 400 MHz NMR spectrometer by heated capillaries. Using both 1H and 13C NMR spectroscopy, quantitative information on the concentrations of the following species was obtained: amines, carbamates, bicarbonate, and CO2. Because of the fast proton transfer between molecular and protonated amines, only the sum of their concentrations can be determined. Furthermore, a byproduct was observed and quantified. The experimental data were used to develop a thermodynamic model of the studied electrolyte solutions, based on the extended Pitzer GE-model. In the model development, vapor-liquid equilibrium (VLE) data from the literature also were included. The model describes both the species distribution and the VLE of the studied mixtures. The properties of the quaternary system are predicted from information on the subsystems. Introduction Aqueous solutions of alkanolamines are widely used as absorbents to remove acid gases (such as carbon dioxide (CO2) and hydrogen sulfide (H2S)) from natural gas, synthesis gas, flue gas, and various refinery streams. Currently, among the greenhouse gases, CO2 receives the most attention, and the use of alkanolamine processes to remove CO2 from the flue gas of fossil-fueled power plants is presently being studied intensively.1-7 The absorption of CO2 into aqueous alkanolamine solutions is a chemisorption that can only be modeled properly by explicitly taking the chemical reactions in theses solutions into account. CO2 reacts with the alkanolamines either directly or via an acid-base buffer mechanism, forming ionic species. Industrially, besides monoethanolamine (MEA), often N-methyldiethanolamine (MDEA) is used for the removal of CO2. MDEA has a high capacity, but, as a tertiary amine, it cannot form carbamates; therefore, the overall kinetics of the CO2 absorption is unfavorable when MDEA is used alone. For that reason, blends of MDEA with primary amines are used. In BASF’s widely used MDEA process, piperazine (PIP), which is a cyclic primary diamine, is used as a so-called “activator”, together with MDEA. In a recent paper,8 we showed how quantitative online 1H and 13C nuclear magnetic resonance (NMR) spectroscopy can be used to obtain reliable information on the species distribution in aqueous solutions of a primary amine (MEA) and a secondary amine (diethanolamine (DEA)). The present study extends this work to the tertiary amine MDEA and blends of MDEA with PIP. The experimental studies were conducted using a novel * To whom correspondence should be addressed. Tel.: 49-631-2053464. Fax: 49-631-205-3835. E-mail address:
[email protected]. Homepage: http://thermo.mv.uni-kl.de. † Currently with Wacker Chemie AG, Burghausen, Germany. ‡ Currently with BAM Federal Institute for Materials Research and Testing, Berlin, Germany. § Currently with Laboratory of Engineering Thermodynamics, University of Kaiserslautern, Germany.
apparatus that allows an accurate gravimetric preparation of the CO2-containing samples and pressurization. That apparatus was coupled online with a NMR spectrometer that was equipped with a pressure-resistant flow cell. The applied method is unfortunately not fast enough to be suited for kinetic experiments in the studied solutions (cf ref 9). Therefore, only equilibrium species distributions were measured. The results for these distributions are directly reported here, together with their correlation. Because of their technical importance, aqueous solutions of MDEA and PIP that contain CO2 have been studied previously, using NMR spectroscopy. Bishnoi and Rochelle10,11 and Ermatchkov et al.12 determined equilibrium constants for the carbamate formation of PIP, but from a database that is distinctly narrower than that supplied here. Poplsteinova6 recently presented results from a more comprehensive study and also reported experimental results for the species distribution, similar to the way they are presented here. All studies from the literature were conducted using the conventional NMR sample tube technique. Furthermore, deuterium oxide (D2O) was used instead of water, as was done in the present study. The experimental results are interpreted in the present work using the well-known reaction scheme that describes the main reactions in the studied systems. In addition to these reactions, side reactions also occur, one of which also has been observed and quantified in the present study. Chakma and Meisen13,14 performed extensive studies on the degradation of MDEA. Using harsh experimental conditions with temperatures up to 200 °C and very high CO2 partial pressures, they determined 17 degradation products, using gas chromatography and mass spectroscopy. Based on those data, they proposed an extended reaction network and a kinetic model. In addition to that, Dawodu and Meisen3 investigated degradation in blends of amines, which also included MDEA. Degradation reactions of PIP do not seem to have been studied systematically in the literature.
10.1021/ie800914m CCC: $40.75 2008 American Chemical Society Published on Web 09/24/2008
7918 Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008
Chemical equilibria in the studied amine solutions should be modeled in a thermodynamically consistent way.15 This requires also a model for the nonideality of the liquid phase. Presently, for that purpose mainly, the electrolyte NRTL model of Chen and Evans16 and the extended Pitzer model of Edwards et al.17 are used in the literature. The parameters for describing the nonideality are generally determined from vapor-liquid equilibrium (VLE) measurements. For the systems studied in the present work, different parameters sets are available: Austgen et al.18 and Bishnoi and Rochelle11 reported sets for the electrolyte NRTL model, and Pe´rez-Salado et al.19,20 reported sets for the extended Pitzer model. These papers also included information on the chemical equilibrium, so that the models can also be used to calculate the species distribution in the studied solutions and the results can be compared to the experimental data from the present study. Because of the fact that the comparison revealed major differences, parameters were readjusted in the present work. Chemistry In aqueous solutions, carbon dioxide reacts through an acid-base buffer mechanism with alkanolamines. The equilibrium reactions can be written as follows: Dissociation of water: H2O h H+ + OH-
(I)
Formation of bicarbonate: CO2 + H2O h H+ + HCO3 (II) 2+ Formation of carbonate: HCO3 h H + CO3
(III)
Dissociation of protonated amine: R′′R′RNH+ h H+ + R′′R′RN (IV) Piperazine has two N atoms and, therefore, it can form five different species. It can be protonated once or twice: PIPH+ h PIP + H+ +
+
Dissociation of protonated amine DEA: DEAH+ h H+ + DEA (XI) Carbamate formation from bicarbonate and DEA: DEA + HCO3 h DEACOO + H2O (XII) Note that some more components were observed in very small amounts. For example, the carbon spectra showed an additional peak, which also has been observed by Poplsteinova.6 She described it as an impurity. Because of the fact that it could neither be identified nor quantified, it was neglected in the chemistry model. Experimental Section A special apparatus was designed for conducting the online NMR spectroscopic investigations of the species distribution in reactive gas solutions. (See Figure 1 and refs 8 and 9.) Using that apparatus, CO2-containing solutions can be prepared gravimetrically. Furthermore, it allows pressurization so that the liquid phases can be studied at temperatures above their normal boiling points. The apparatus mainly consists of a thermostated view cell that contains ∼350 mL of the studied solution. The pressure in this cell is controlled by a plunger that is driven by pressurized nitrogen. The cell is suited for temperatures up to 120 °C and pressures up to 100 bar. For the connection to the NMR spectrometer, liquid-thermostated pressure-resistant capillaries (up to more than 100 bar) are used. The NMR spectrometer is a 400 MHz Unity Inova (Varian, USA), which was equipped with a Varian 90 µL inverse gated triple resonance flow probe head that was suited for temperatures up to 120 °C and pressures up to 30 bar. For loading, the cell is first evacuated. Gravimetrically prepared mixtures of amine and water then are supplied from a reservoir that is weighed
(V) +
PIP(H )2 h PIPH + H
(VI)
It also can form a carbamate, as well as a bicarbamate: PIP + HCO3 h PIPCOO + H2O
(VII)
PIPCOO- + HCO3 h PIP(COO )2 + H2O
(VIII)
Protonation and carbamate formation may result in the formation of zwitterions, which is described here using the backward reaction (IX) PIPH+COO- h PIPCOO- + H+ As described by several authors, MDEA degrades in an extended reaction scheme. Only reactions starting from amine, water, and carbon dioxide must be considered here. As Chakma and Meisen16 have suggested, one MDEA molecule and one protonated MDEA molecule form a dimethyl-diethanolamine ion (DMDEA+) and one molecule of diethanolamine (DEA). The latter can be protonated itself and forms a carbamate.16 Both DMDEA+ and DEA were observed in the present study, although Chakma and Meisen16 stated that DMDEA+ is an unstable intermediate. Components that might be produced from DMDEA+ as an instable intermediate were not detected. Therefore, only the following reversible reactions are included in the model: Formation of DMDEA+ and DEA: MDEA + MDEAH+ h DMDEA+ + DEA (X)
Figure 1. Apparatus for measuring the species distribution in aqueous amine solutions loaded with carbon dioxide (CO2) by NMR spectroscopy.
Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 7919
before and after filling. The same holds for the CO2 that comes from a small steel bottle. The cell contains a magnetically coupled stirrer (Cyclone 075, Bu¨chi, Switzerland) that allows rapidly dissolving CO2. The homogeneous mixture is transported to the NMR spectrometer in the capillary by a high-performance liquid chromatography (HPLC) pump (Multitherm 2251, Bischoff, Germany). MDEA (>98.0%) and PIP (>98.0%) were purchased from Merck, Germany. Water was doubly distilled in our laboratory. The unloaded solutions, which contained water and amine, were degassed under vacuum before use. CO2 (>99.995%) was purchased from Messer Griesheim, Germany. The amounts of CO2 and amines in the cell were determined by weighing and applying corrections for the rest of the materials remaining in the feed lines. The accuracy of the overall mole fractions of amine, water, and CO2 in the studied samples is better than 1% (relative deviation). The purity of the unloaded amine solution was checked by a 1H NMR spectrum. The pressure level was controlled by nitrogen that had been separated from the fluid in the cell by a plunger. The accuracy of the pressure measurement is not critical, because the pressure was mainly applied to keep the CO2 in solution; it was ∼1%. The pressure was controlled by nitrogen that had been separated from the fluid in the cell by a plunger. Its initial position was chosen to be sufficiently high to allow the cell volume to be reduced until all the CO2 was dissolved. Through a sapphire window, it was verified that no gas phase was present in the cell after the filling. After initial thermal equilibration in the cell, the solution was pumped to the NMR probe head via the heated-loop capillaries. Steady-state conditions were attained in the loop after another 1-2 h. Then, for each CO2 loading, a 1 H NMR spectrum and a 13C NMR spectrum were acquired; optionally, two-dimensional (2D) correlation spectra also were acquired. After that spectra acquisition, the stationarity was checked by pumping for another 30 min and acquiring an additional 1H NMR spectrum. The speciation in the MDEA-H2O-CO2 system was investigated for 0.2, 0.3, and 0.4 g/g MDEA in the unloaded solution at 293, 313, and 333 K. The CO2 loading ranged up to 1.37 molCO2/molMDEA, and the pressure range was 5-25 bar. In the quaternary MDEA-PIP-H2O-CO2 system, the speciation was studied for 0.3 g/g MDEA and 0.1 g/g PIP in the unloaded solution at 293, 313, and 333 K. The CO2 loading was up to 0.776 molCO2/molamine. BASF’s aMDEA contains less PIP; a loading of 0.1 g/g was chosen to achieve a better quantification of the NMR signals, relative to PIP. The procedure for the acquisition of the NMR spectra is the same as that described in refs 8 and 9. The experiments were conducted with water (H2O), not deuterium oxide (D2O). To understand the NMR measurements, one must consider that undiluted undeuterated solutions were studied. In common NMR experiments, the sample is highly diluted in D2O, which enables easy shimming and locking of the magnetic field. Because of the fact that the solutions studied in the present work contained no deuterium, locking was not possible. Because of the very high field stability of the superconducting magnet, this does not adversely affect the quality of the spectra (or for 13C NMR spectra, acquired with many scans (see below)). Before spectra were taken, the magnetic field around the probe head was carefully homogenized. Doing this manually is very timeconsuming; therefore, proton field mapping was used, which enables an automation of this step. (For further details, see refs 8, 9, and 21-23.)
In NMR spectra, one can distinguish signals from different functional groups within molecules. Depending on the type of NMR spectrum, either protons or C atoms were observed. Combining the information from both spectra, most of the relevant components in the studied solutions could be identified and quantified. 1H NMR spectra are easy to acquire; in most cases, one scan was sufficient and they are sensitive enough to allow the resolution of even byproducts in very low concentrations. 13C NMR spectra suffer from the fact that only the 13C isotope can be observed. For a reasonable signal-to-noise ratio, up to 512 scans were applied. Because of the long spin-lattice relaxation times of some peaks, the holding time between the scans were up to 1 min. Therefore, the acquisition of such a spectrum can take up to 8 h. This effort was undertaken because it is the only possible way to observe bicarbonate and molecular carbon dioxide directly under the experimental conditions. Assigning peaks to functional groups of the molecules was done using information from the literature, from the concentration-dependent studies of the present work, and from 2D correlation spectra. (Also see ref 9.) It is possible to identify and quantify the following components: (i) MDEA/protonated MDEA, (ii) PIP/protonated PIP, (iii) PIP monocarbamate/ protonated PIP monocarbamate, (iv) PIP bicarbamate, (v) DMDEA/protonated DMDEA, (vi) DEA/protonated DEA, (vii) bicarbonate/carbonate, and (viii) carbon dioxide. Furthermore, also small amounts of a product formed from DEA (most likely DEA carbamate) and an unidentified byproduct were observed but not quantified. Because of the fast proton transfer between molecular and protonated amines, only the sum of their concentrations can be observed. Examples of 1H and 13 C NMR spectra for MDEA-PIP-H2O-CO2 are given in Figures 2 and 3; the typical chemical shifts are given in Table 1. Note that these shifts vary slightly with temperature and composition. The peak areas were evaluated as described in refs 8 and 9; they are directly proportional to the amount of nuclei observed. With the peak areas, the chemical structure of the observed molecules, and the overall composition being known, the mole fraction of all observable species can be calculated without any calibration or further assumptions. Combining both 1H and 13C NMR spectra, information on all relevant components is available. The results are presented in Tables 2 and 3. The accuracy of the experimental data can be estimated from several tests. Ratios of areas of different peaks within the same molecule typically are constant within 1% for 1H spectra and 5% for 13C NMR spectra. Furthermore, the overall amine and CO2 mole fractions from sample preparation can be compared to the spectroscopic results. Relative deviations are typically
