Thermal Degradation of Aqueous Piperazine for CO2 Capture. 1

Apr 28, 2012 - In this manuscript, a method is presented that allows the analysis and direct comparison of thermal degradation data obtained under hig...
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Thermal Degradation of Aqueous Piperazine for CO2 Capture. 1. Effect of Process Conditions and Comparison of Thermal Stability of CO2 Capture Amines Stephanie Anne Freeman† and Gary Thomas Rochelle*,† †

Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712, United States ABSTRACT: The effect of process conditions on the rate of thermal degradation of concentrated, aqueous piperazine (PZ) was investigated. At 150 °C, 8 m (m) PZ degrades with a first order rate constant, k1, of 6.1 × 10−9 s−1. Thermal degradation of 8 m PZ with 0.3 mol CO2/mol alkalinity demonstrated an Arrhenius dependence on temperature with an activation energy of 184 kJ/ mol. Degradation at 175 °C was negligible with no dissolved CO2, while the k1 increased from 65 to 71 × 10−9 s−1 at 0.1 to 0.4 mol CO2/mol alkalinity and decreased to 24 × 10−9 s−1 at 0.47 mol CO2/mol alkalinity. In an industrial system with a simple stripper, losses due to thermal degradation are expected to be 0.043 mmol PZ/mol CO2 captured. In the case of a 2-stage flash, losses are expected to be only 0.0086 mmol PZ/mol CO2 captured. A Maximum Estimated Stripper Temperature (MEST) was calculated for a variety of amines to provide the same thermal degradation rate of MEA at 120 °C based on first order rate constants for amine loss during thermal degradation and the expected Arrhenius dependence on temperature for all amines. Substituted and unsubstituted 6-member amine rings were found to be the most thermally stable. ance.9,11,12 Studies on the behavior of diethanolamine (DEA), methyldiethanolamine (MDEA), and 2-amino-2-methyl-2propanol (AMP) have also elucidated the impact of process conditions on amine performance at high temperature.9,13−18 The ability of increased temperature to accelerate amine degradation has been well established,9,13,15,17,19−21 even for concentrated PZ systems.10 Arrhenius temperature relationships have been observed in a variety of amines including MEA, 3-amino-1-propanol, 4-amino-1-butanol, AMP, 2-amino-1propanol, 1-amino-2-propanol, and blends of MEA with PZ, morpholine, diglycolamine (DGA), or AMP, and EDA.9,17,21 However, most previous studies of thermal degradation of amines for CO2 capture have not significantly varied temperature to observe this effect.6,16,22−24 The ability of CO2 to enhance thermal degradation has been previously observed and quantified for a variety of CO2 capture amines.6,9,18,21,24 Kennard observed an enhancement in DEA thermal degradation up to 0.48 mol CO2/mol alkalinity in 30 wt % DEA.18 Davis determined that degradation of MEA was approximately first order in CO2 concentration with MEA loss doubled with a doubling of CO2 loading in 7 m MEA degraded at 135 °C.9 Multiple authors have found that all amines tested for thermal degradation, including MEA, AMP, DEA, MDEA, and ethylenediamine (EDA), demonstrated a noted increase in the level of degradation in the presence of CO2 compared to without CO2.6,24 The catalytic effect of CO2 on each amine was slightly different, but all showed marked increases in degradation. The effect of CO2 on PZ thermal degradation has not been studied previously in literature, but it was

1. INTRODUCTION Amine-based absorption-stripping is the current state-of-the-art technology for postcombustion carbon dioxide (CO2) capture from coal-fired power plants. A crucial step in utilizing this technology, however, is solvent selection. The most desirable solvent for this application should have fast CO2 absorption rate, high CO2 capacity, high heat of absorption for CO2, limited or controllable oxidation and thermal degradation, low amine volatility, and favorable physical properties. The traditional baseline solvent investigated for this application is monoethanolamine (MEA). In comparison with MEA, concentrated, aqueous piperazine (PZ) is an advanced solvent that has double the CO2 absorption rate and CO2 capacity, limited rate of oxidation and thermal degradation, and lower amine volatility.1−4 The rate of thermal degradation of a CO2 capture amine is crucial in solvent selection. Solvents can spend over one-third of the residence time of an industrial system well above 100 °C. Thermal degradation in the stripping section of a CO2 capture system will reduce CO2 capacity, increase the steam requirement for stripping, and increase the cost of amine replacement, reclaiming, and disposal. Thermal degradation can also lead to environmental issues relating to the unknown health effects and reactivity in the environment of many of the volatile degradation products.3,5−8 Low concentration PZ solutions were first identified as thermally resistant during screening efforts for CO2 capture amines.9 The resistance to thermal degradation of concentrated, aqueous PZ systems has been reported previously under a limited set of conditions.10 The effect of varying process conditions on the high temperature stability of any CO2 capture amine is crucial to understanding the solvent behavior in the reboiler and reclaiming sections of an absorber-stripping system. Fundamental studies on MEA loss at high temperature have provided an understanding of its expected perform© 2012 American Chemical Society

Received: Revised: Accepted: Published: 7719

August 30, 2011 April 27, 2012 April 28, 2012 April 28, 2012 dx.doi.org/10.1021/ie201916x | Ind. Eng. Chem. Res. 2012, 51, 7719−7725

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3. RESULTS AND DISCUSSION 3.1. First Order Rate Constant Analysis. The thermal degradation rate of aqueous PZ solutions was studied over a range of conditions. It has been shown previously and will be shown in this manuscript that the loss of PZ during thermal degradation is approximately first order in amine concentration at 175 °C (section 3.2).10 This observation is supported by the fact that most thermal degradation concentration profiles for PZ can be well characterized by an exponential regression. In order to compare thermal degradation data effectively, a first order rate constant, k1, was calculated for all experiments in the same manner as described previously.1,10 This approach assumes a first order loss in PZ as shown by the following rate equation where CPZ is the concentration of PZ in any appropriate units (eq 1). Integrating eq 1 provides an equation in terms of the initial PZ concentration, CPZ,0 and experimental time, t (eq 2) dC − PZ = k1CPZ (1) dt

hypothesized to follow the trends found with other amines and alkanolamines. The effect of amine concentration on thermal degradation was investigated for MEA where the loss rate of MEA was determined to be only slightly more than first order in MEA loss between 3.5 and 11 m MEA.9 Additional studies conducted to date have not varied amine concentration significantly, so the effect of other amines is not clear.10,16,22−24 Independent comparisons of the thermal stability of numerous CO2 capture amines have been investigated through the use of screening studies.6,9,22−25 The experimental conditions, however, can differ greatly between authors which have not allowed a unified examination of thermal degradation tendency. In this manuscript, a method is presented that allows the analysis and direct comparison of thermal degradation data obtained under highly variable conditions through the application of a first order rate constant and estimation of the maximum expected stripper temperature (MEST). More details on this work are reported in the dissertation by Freeman (2011).1

CPZ = CPZ,0 ·e−k1·t

2. METHODS AND MATERIALS

(2)

Each set of thermal degradation data was regressed with an exponential trendline that matched this form of the rate equation, and a value of k1 was extracted. As an example of this analysis, the PZ loss data for the degradation of 8 m PZ with 0.3 mol CO2/mol alkalinity at 175 °C are shown in Figure 1.

2.1. Aqueous Solution Preparation and Total Inorganic Carbon (TIC) Assay. Concentrated, aqueous piperazine solutions were prepared as described in detail previously.1,2,10,26 Aqueous PZ solutions were heated to dissolve solids and gravimetrically sparged with CO2 to attain the desired CO2 loading. Anhydrous piperazine (IUPAC: 1,4diazacyclohexane, CAS 110-85-0, purity 99%, Acros Organics N.V., Geel, Belgium) and CO2 (CAS 124-38-9, purity 99.5%, Matheson Tri Gas, Basking Ridge, NJ) were obtained from commercial sources and used with distilled, deionized water for experimental solutions. The CO2 loading of aqueous solutions was quantified using a total inorganic carbon (TIC) assay that has been described previously.1,2,10,27 Phosphoric acid was used to liberate CO2 from solution, while an infrared detector (Horiba Instruments Inc., Spring, TX) is used to quantify CO2 concentration. A calibration curve generated from an inorganic carbon standard (Ricca Chemical Company, Pequannock, NJ) was used to calculate CO2 concentrations, which are reported in units of mol CO2/mol alkalinity (mol CO2/mol equivalent amine). 2.2. Thermal Degradation Cylinders. Thermal degradation was performed in 5 in. long 316SS cylinders with Swagelok end-caps as described previously.1,2,9,10 Multiple cylinders were filled with the amine solution of interest, sealed according to Swagelok specifications, and placed in forced convection ovens for up to 72 weeks. Cylinders were removed from the oven periodically to sample the experiment. The amine solution was removed from sealed cylinders and analyzed for PZ, other amines, if blended, and CO2 concentration. 2.3. Cation Ion Chromatography. Cation IC was used to quantify PZ and other amine concentrations using a Dionex ICS-2100 Integrated Reagent-Free IC system with AS autosampler, 4-mm Cationic Self-Regenerating Suppressor (CSRS), and conductivity detector as described in detail previously (Dionex Corporation, Sunnyvale, CA).1,2,10 Separation was achieved in an IonPac CG17 guard column (4 × 50 mm) and IonPac CS17 analytical column (4 × 250 mm) using a gradient of methanesulfonic acid in analytical grade water.

Figure 1. An example of the determination of k1 value from thermal degradation of 8 m PZ with 0.3 mol CO2/mol alkalinity at 175 °C.

These data are collated from multiple experiments performed with the same conditions, and an exponential regression has been fit for the entire set. The regression fits the data well as evidenced by the high coefficient of determination (r2). Based on the form of eq 2, the k1 for this set of condition is 8.0 × 10−2 wk−1 or 1.3 × 10−7 s−1. A comparison of k1 values will be used throughout this manuscript to analyze the effect of process conditions on PZ thermal degradation (section 3.2) and to provide a basis for comparison of CO2 capture amines (section 3.4). 3.2. Effect of Process Conditions on PZ Thermal Degradation. The effect of temperature, CO2 concentration, and PZ concentration on the rate of thermal degradation rate of aqueous, concentrated PZ was studied using thermal cylinders. Experiments were conducted from 135 to 175 °C, 0 to 0.47 mol CO2/mol alkalinity, and 4 to 20 m PZ. The basis for 7720

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alkalinity. A maximum in k1 was observed at or around 0.4 mol CO2 per mole alkalinity and is likely related to the speciation present at this CO2 concentration. Based on modeling work on the speciation of loaded 8 m PZ at 175 °C, the concentration of free PZ and PZ carbamate (PZCOO−) decreases above 0.4 mol CO2/mol alkalinity, while the concentration of bicarbonate (HCO3−) and protonated PZ carbamate (H+PZCOO−) increase. As demonstrated in previous work and in Part 2 of the current study,28 the active species for degradation are thought to be H+PZ, PZ, and PZCOO−1. As loading increases, less active species accumulate (H+PZCOO−), while more reactive species are present in lower concentrations. Freeman (2012) measured little degradation in solutions prepared from KHCO3 and PZ. However, PZ partially neutralized with sulfuric acid degraded at a rate approximately half that of CO2-loaded PZ.1 The k1 values for the thermal degradation of 4, 8, 12, and 20 m PZ with 0.3 mol CO2/mol alkalinity at 175 °C are shown in Table 1. As observed with MEA, the thermal degradation of aqueous PZ is slightly more than first order between the concentrations of 4 to 12 m PZ. There is a slight increase in the k1 value for increased PZ concentration in this range, demonstrating a small effect of PZ concentration. In comparison with changes in CO2 loading and temperature, which produce changes in k1 over 2 orders of magnitude, the effect can be reported as only minor with regards to PZ concentration. In addition, changes in PZ concentration during operation of an industrial system will not likely vary significantly and are well represented by the data from 4 to 12 m PZ. The change in k1 for 20 m PZ is significant in comparison with the 4 to 12 m PZ data and reflects the drastic changes in solution composition and speciation expected with this concentrated (53 wt % PZ) and highly viscous solution. 3.3. Degradation Rates in PZ-Based Industrial Systems. The experimental measurements of thermal degradation allow the prediction of the anticipated losses of PZ due to thermal degradation in a full-scale industrial CO2 capture absorption stripping system using concentrated PZ. The thermal stability of PZ allows for the design of PZ systems with a 150 °C reboiler in the stripping section. An estimate of PZ loss for an 8 m PZ system due to thermal degradation at 150 °C was done assuming the use of a simple stripper, a residence time of 10 min in the stripping section, and using the previously published CO2 capacity of 8 m PZ of 0.79 mol CO2/ kg (PZ + water).29 The losses due to thermal degradation will be 0.043 mmol PZ/mol CO2 captured. This estimate was based on a single pass or cycle of solvent and should be applicable to pilot, demonstration, and full scale installations. A novel stripping design is the use of two flash stages at high temperature to separate and help compress CO2 instead of a simple stripper.30 In the designs previously discussed for 8 m PZ, the two flash tanks would be operated at 150 °C with a total residence time around 2 min expected at this high temperature. In this case, the estimated losses due to thermal degradation would be 0.0086 mmol PZ/mol CO2 captured. 3.4. Maximum Estimated Stripper Temperature (MEST) for Comparison of CO2 Capture Amines. The thermal stability of PZ is a clear advantage when compared to other CO2 capture amines. Direct comparison of k1 values for 8 m PZ and 7 m MEA, as described previously, showed a two magnitude increase in thermal stability for the PZ system from 135 to 150 °C.10 The comparison of k1 values is limited to the existence of data for multiple amines obtained at the same

comparison between conditions is a comparison of the k1 value extracted from the PZ concentration data over the experimental time. The k1 values are tabulated in Table 1. Table 1. Tabulated k1 Values for 4 to 20 m PZ from 135 to 175 °C k1 × 109 (s−1) PZ conc (m)

CO2 loading (mol/ mol alk)

135 (°C)

150 (°C)

165 (°C)

175 (°C)

4 8 8 8 8 8 8 12 20

0.3 0 0.1 0.2 0.3 0.4 0.47 0.3 0.3

1.0 -

0.1 6.1 7.9 24

36 0.8 18 31 41 50 -

114 7.0 66 79 132 171 24 156 269

The effect of temperature and CO2 concentration on the rate of thermal degradation are demonstrated in Figure 2. In this

Figure 2. Effect of CO2 loading (α = mol CO2/mol alkalinity) and temperature on thermal degradation of 8 m PZ. Data: 0 (●), 0.1 (○), 0.2 (■), 0.3 (□), 0.4 (▲), and 0.47 (Δ) mol CO2/mol alkalinity. Lines: exponential regressions to demonstrate Arrhenius temperature behavior. Thick tick marks are included to indicate important temperatures in °C.

figure, the k1 value is plotted against inverse temperature on a semilog scale, while temperature units in °C are included for clarity. The k1 of PZ thermal degradation has an Arrhenius dependence on temperature, as expected, demonstrated by the straight exponential trendlines in Figure 2. Based on an Arrhenius relationship, the natural log of k1 is related to a constant, A, the activation energy or degradation, EA, the universal gas constant, R, and the temperature in Kelvin, T (eq 3). The Arrhenius fit allows the determination of the EA for a given condition when experiments were performed at multiple temperatures. For 8 m PZ with 0.3 mol CO2/mol alkalinity, the EA was found to be 184 kJ/mol Ln(k1) = Ln(A) −

EA 1 · R T

(3)

The effect of CO2 concentration was more complex as increased CO2 enhanced degradation but then was seen to plateau and decreased above a CO2 loading of 0.4 mol/mol 7721

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degrade thermally without having to account for experiment length or complex mechanistic differences. This analysis is intended to be a tool to quickly assess the thermal stability of an amine in relation to other systems tested under similar conditions. Also, the MEST is presented as a maximum temperature because numerous factors would disallow operation at a rate to match that of MEA. Cost of the amine, for example, was not considered in this analysis and for expensive amines, such as PZ or other advanced amines, a lower k1 value may be the target thermal degradation rate for an optimized design. The MEST is meant to provide a reasonable starting point for stripping design and maximum suggested temperature based on the propensity of that amine to thermally degrade. A MEST value was calculated for all thermal degradation data available in the literature.1,6,9,13,16,20−24,33 For all amines, k1 values were calculated based on fitting eq 2 to the reported experimental data. When experiments were performed at three or more temperatures, an Arrhenius fit for the temperature dependence of that amine was performed to determine the EA for k1 based on eq 3 and the MEST was calculated from the regression. When only one or two temperatures were examined in a particular study, the EA of either PZ or MEA was used to extrapolate to the target k1 value. Values of EA have not been found to vary drastically for the most part, as demonstrated previously, and will provide a reasonable approximation.1,10 The EA values of 8 m PZ with 0.3 mol CO2/mol alkalinity and 7 m MEA with 0.4 mol CO2/mol alkalinity were determined to be 184 and 157 kJ/mol, respectively. The EA of the molecule that was the most similar in chemical structure, either PZ or MEA, was used when an independent EA was not able to be calculated. The k1 values, and therefore the MEST, calculated for blended solvents were based on the total loss of amine rather than the individual loss of constituent amines in order to simplify the analysis. As an estimate of error, the MEST for 8 m PZ with 0.3 mol CO2/mol alkalinity was predicted using the lowest and highest EA values calculated for any amine in this study. For EA values ranging from 105 to 184 kJ/mol, the estimated MEST for PZ ranges from 173 to 163 °C. This provides an estimate of error for all of the amines without enough data to determine their own EA value. An error in 10 °C in reboiler temperature represents a reasonable error considering the analysis performed to obtain the MEST. The MEST for a selection of important CO2 capture amines is presented in Table 2. The MEST for blended CO2 capture amines is presented in Table 3. A full table of MEST values with chemical structures can be found in the dissertation by Freeman (2011).1 In each table, the amines are arranged in order of decreasing MEST or decreasing thermal stability. The amine and CO2 concentrations for each system are presented in units of molal (m) and mol CO2/mol alkalinity, respectively, unless otherwise noted. Molal as is defined as the moles of amine per kilogram of water. Alkalinity in this use refers to the number of amino functions on the amine of interest. For example, a standard solution of 8 m PZ has 8 mols of PZ per kg of water or 40 wt % PZ. CO2 loading reported for PZ solutions takes into account the two amino functions that PZ contains compared to only one found on such amines as MEA and MDEA. The range of temperature investigated in this study or found in the literature is indicated as well as the source of experimental data. Morpholine (Mor) was found to be the most stable CO2 capture amine tested with a MEST of 170 °C. Piperidine (PD),

temperature. Thermal degradation data have been obtained from 100 to 200 °C in the literature and is problematic to compare directly. In order to alleviate the difficulty in assessing thermal stability, an analysis based on the calculation of maximum estimated stripper temperature (MEST) is proposed. The intention of this analysis is to provide a concrete number to allow direct comparisons of a variety of CO2 capture amines based on degradation data obtained at any temperature. The calculation of the MEST is based on the thermal stability of MEA, the baseline CO2 capture solvent. Davis optimized the pressure and temperature of a simple stripper with 7 m MEA based on MEA thermal degradation rates from his work.9 The optimization balanced steam stripping energy costs based on stripper temperature with cost estimates for amine replacement due to degradation, reclaiming, and disposal.9 The optimum based on minimum cost was to operate a simple stripper of a 7 m MEA system at 122 °C. This result is supported by the historical operation of MEA systems successfully at temperatures between 115 and 120 °C.31,32 The basis of the MEST analysis is the calculated k1 value for 7 m MEA degraded at 120 °C with a lean loading of 0.4 mol CO2/mol alkalinity.9 The lean loading is used since the reboiler, where the leanest solution is present, is expected to be the hottest part of the stripping section. The k1 value for this condition calculated from Davis is 2.9 × 10−8 s−1 (1.1 × 10−4 hr−1, 2.5 × 10−3 d−1, or 1.8 × 10−2 wk−1).9 This k1 value was used as the target to extrapolate the data of other systems in order to determine at which temperature the rate of degradation matched that of MEA at the baseline conditions. The temperature at which a system will have a k1 value of 2.9 × 10−8 s−1 is the MEST for that amine. An example of how the MEST was calculated for 8 m PZ with 0.3 mol CO2/mol alkalinity is shown in Figure 3. Also demonstrated in Figure 3 is the thermal stability of PZ compared with 7 m MEA which degrades with a k1 value up to 2 orders of magnitude higher.

Figure 3. Example of the determination of the MEST of 8 m PZ with 0.3 mol CO2/mol alkalinity and 7 m MEA with 0.4 mol CO2/mol alkalinity with the goal k1 value of 2.9 × 10−8 s−1 (dashed line). Thick tick marks are included to indicate important temperatures in °C.

The MEST analysis involves assumptions that limit its application. All amines are appropriately modeled with a first order rate constant. For most amines discussed, the first order approximation fits well either because the data suggest the amine behavior is first order in amine loss, or because data are too limited to suggest another reaction order. This overall approach allows an assessment of the tendency of an amine to 7722

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Table 2. MEST for CO2 Capture Amines amine

abbreviation

morpholine piperidine piperazine hexamethylenediamine 2-methylpiperazine 1-methylpiperazine 5-amino-1-pentanol pyrrolidine homopiperazine 2-amino-2-methyl-1-propanol 1,4-dimethylpiperazine 2-amino-2-methyl-1-propanol 4-amino-1-butanol diglycolamine hexamethyleneimine N-(2-hydroxyethyl)piperazine N-methyldiethanolamine N-methyldiethanolamine 3-amino-1-propanol 2-piperidineethanol N,N,N′,N′-tetramethylethylenediamine 3-aminopropanol ethylenediamine 2-amino-2-methyl-1-propanol DL-1-amino-2-propanol N,N-dimethylethanolamine monoethanolamine N-(2-aminoethyl)piperazine monoethanolamine ethylenediamine diethylenetriamine N,N-dimethylethanolamine ethylenediamine 3-(methylamino)propylamine 6-amino-1-hexanol monoethanolamine monoethanolamine N-(2-hydroxyethyl)ethylenediamine 3-(methylamino)propylamine DL-2-amino-1-propanol (MIPA) 2-piperidinemethanol diethylenetriamine N-methylethanolamine diethanolamine 2-methylaminoethanol N-(2-hydroxyethyl) ethylenediamine

Mor PD PZ HMDA 2-MPZ 1-MPZ 5a1p Pyr HomoPZ AMP 1,4-DMPZ AMP 4a1b DGA HMI HEP MDEA MDEA 3a1p 2-PE TMEDA AP EDA AMP DLAP DMMEA MEA AEP MEA EDA DETA DMAE EDA MAPA 6a1h MEA MEA HEEDA MAPA MIPA 2 p.m. DETA MAE DEA MMEA HEEDA

conca (m) loading (mol/mol alk) 8 8 8 8 8 8 7 8 8 4.81 4* 7 7 7 8 7 4* 7 7 8 4* 5.71 3.5 4* 7 4.81 3.5 2.33 7 7.14 4.15 4* 8 4.81 7 4* 7.01 4.12 9 7 7 7 4* 4.08 5.71 3.5

0.3 0.3 0.3 0.3 0.3 0.3 0.4 0.3 0.3 0.5 2 MPa 0.4 0.4 0.4 0.3 0.4 2 MPa 0.1 0.4 0.4 2 MPa 0.5 0.4 2 MPa 0.4 0.5 0.4 0.4 0.4 0.25 0.17 2 MPa 0.4 0.25 0.4 2 MPa 0.5 0.25 0.4 0.4 0.4 0.4 2 MPa 0.5 0.5 0.4

T range of data (°C)

act. energyb (kJ/mol)

MEST (°C)

refc

175 175 135−175 175 150 150 135−150 175 175 135 140 120−150 120−150 135 175 135 140 150 100−150 135−150 140 135 135−150 140 100−150 135 100−135 135 100−150 135 135 140 100−135 135 100−150 140 135 135 135 100−150 135 135−150 140 135 135 135

PZ PZ 184 PZ PZ PZ MEA PZ PZ MEA PZ 112 161 MEA PZ PZ MEA MEA 117 MEA MEA MEA PZ MEA 106 MEA 129 PZ 157 MEA MEA MEA 154 MEA 148 MEA MEA MEA MEA 105 MEA MEA MEA MEA MEA MEA

170 166 163 160 151 148 145 142 140 140 138 137 133 132 131 130 129 128 127 127 127 126 125 123 123 122 122 121 121 121 118 118 117 117 117 116 115 114 114 114 109 108 105 105 102 101

F1 F1 F2 F2 F1 F1 D F1 F1 EH L1 D D D F1 D L1 C D Z L3 EH D L1 D EH D D D EH EH L1 Z EH D L1 EH EH V D D D L1 EH EH D

a

Concentrations indicated with asterisks (*) are in units of mole amine/kg solution, not molal (m). bEA used to for MEST calculation; PZ = 184 kJ/ mol; MEA = 156 kJ/mol. cReference: CM;13 C;20 D;9 EH;6 F1;25 F2;1 L1;24 L3;22 RT;16 V;33 Z.21

reported that PZ is preferentially degraded when blended with both MEA and MDEA.9,20 Analysis of k1 values indicates that PZ is degraded 12 to 15 times faster than MDEA and 4 to 8 times faster than the total amine concentration when in a 7 m MDEA/2 m PZ blend (data not shown). In a 7 m MEA/2 m PZ blend, k1 values indicate PZ degrades only 1.2 to 1.7 times faster than MEA and 1.1 to 1.9 times faster than the total amine. Based on overall amine loss, as shown in Table 3, the blend of MEA and PZ is seen as significantly less stable with a MEST 34 °C lower than the MDEA/PZ blend because MEA and PZ are both highly unstable in this blend. In the MDEA/

PZ, 1-methylpiperazine (1-MPZ), and 2-methylpiperazine (2MPZ) closely followed Mor and indicate that the most resistant amines are unsubstituted and substituted 6-member rings with one or two amino functions. More unstable amines with lower MEST values were found to be alkyl chain amines with hydroxyl functions, methyl groups, and multiple amine functions such as HEEDA, DEA, and diethylenetriamine (DETA). The MEST results for the blended CO2 capture solvents shed interesting light on the interaction of PZ and other amines in a blend at high temperature (Table 3). It has been previously 7723

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Table 3. MEST for Blended CO2 Capture Solvents Based on Total Amine Losses aminea

conc (m)

loading (mol/mol alk)

T range of data (°C)

act. energyb (kJ/mol)

MEST (°C)

refc

PZ/1-MPZ/1,4-DMPZ PZ/1-MPZ/1,4-DMPZ PZ/1-MPZ PZ/2-MPZ MDEA/DEA MDEA/PZ PZ/AMP MEA/AMP MEA/DGA MEA/Mor MEA/PZ

3.9/3.9/0.2 3.75/3.75/0.5 4/4 4/4 5.3/1.7 7/2 6/4 7/2 7/2 7/2 7/2

0.3 0.3 0.3 0.3 0.27 0.11 0.4 0.4 0.4 0.4 0.4

150 150 150 150 200 135−150 135−150 100−150 100−150 100−150 100−150

PZ PZ PZ PZ MEA PZ PZ 146 94 97 84

160 159 156 155 151 138 134 123 112 108 104

F3 F3 F2 F2 RT C Z D D D D

a

See Table 2 for chemical abbreviation definitions. bEA used to for MEST calculation; PZ = 184 kJ/mol; MEA = 156 kJ/mol. cReference: C;20 D;9 F2;1 F3;34 RT;16 Z.21

■

PZ blend, however, the MDEA degrades at similar rates as it does alone while PZ loss is accelerated. As with the trends seen with amines alone, blends of (un)substituted 6-membered rings with two amino functions were significantly more stable than blends with predominantly long-chain alkyl and alkanolamines. The exceptions are blends of MEA/Mor and MEA/PZ in which the presence of a strong nucleophile (Mor or PZ) accelerates thermal degradation of both species.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS The Luminant Carbon Management Program provided support of this work.

4. CONCLUSIONS

REFERENCES

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The rate of thermal degradation of PZ was studied as a function of process conditions to predict the expected degradation in a full-scale industrial system. Thermal degradation data were well fit by a first order rate constant, k1, for the loss of PZ. At 150 °C, PZ degrades with a k1 of 6.1 × 10−9 s−1. Thermal degradation of PZ demonstrated an Arrhenius dependence on temperature with an activation energy of 184 kJ/mol for 8 m PZ with 0.3 mol CO2/mol alkalinity. Degradation was found to be a strong function of CO2 concentration which also depends on speciation of the solution. Degradation increased from 0 to 0.4 mol CO2/mol alkalinity but decreased rapidly above 0.4 mol CO2/mol alkalinity, likely due to the presence of HCO3− and low concentrations of reactive species. In the case of a simple stripper at 150 °C with a residence time of 10 min, losses due to degradation are expected to be 0.043 mmol PZ/mol CO2 captured. In the case of a 2-stage flash with a residence time of 2 min, losses are expected to be only 0.0086 mmol PZ/mol CO2 captured. A Maximum Estimated Stripper Temperature (MEST) was calculated for a variety of amines in order to compare and evaluate the thermal stability of each. The MEST analysis was based on the thermal degradation rate of MEA at 120 °C, first order rate constants for amine loss during thermal degradation, and the expected Arrhenius dependence on temperature for all amines. Morpholine, piperidine, and piperazine were found to be the most thermally stable amines tested along with other substituted six-member ring amines with MESTs above 160 °C. Unstable amines were found to be alkyl chain or alkanolamines with varying combinations of methyl substitution, hydroxyl substitution, and amino functions. The MEST analysis provides a straightforward way to both compare amines in terms of thermal stability and provide a starting point for the temperature used in stripper designs. 7724

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