Ind. Eng. Chem. Res. 2008, 47, 7419–7423
7419
Screening Test of Solid Amine Sorbents for CO2 Capture Seungmoon Lee,† Thomas P. Filburn,*,† Mac Gray,‡ Jin-Won Park,§ and Ho-Jun Song§ Clean Energy Institute, Department of Mechanical Engineering, UniVersity of Hartford, 200 Bloomfield AVenue, West Hartford, Connecticut 06117, National Energy Technology Laboratory, U.S. Department of Energy, P.O. Box 10940, Pittsburgh, PennsylVania 15236, and Department of Chemical Engineering, Yonsei UniVersity, 134 Shinchon-dong, Seodaemun-ku, Seoul, 120-749, Korea
In this study, the absorption properties of CO2 in seven different solid amine absorbents were measured. The specific characteristics, such as absorption capacity, absorption/desorption rate, cyclic capacity, cycle decay effect, temperature of reaction in the absorber, and optimal conditions for the CO2 absorption process were measured. This absorption process occurred at two temperaturesseither 296.15 or 313.15 Kswhile the desorption process temperature was raised to 348.15 K. The solid amines (TEPAN and E-100AN) demonstrated higher cyclic capacity than others under these conditions. Our testing showed high reproducibility for the CO2 capture of TEPAN and E-100AN in measuring their cyclic capacity. Diminished cyclic capacities were noted for the MEA and 194B materials, with measured drops at the third cycle of about 16.17% and 15.79% lower than those in the first cycle, respectively. 1. Introduction The atmospheric levels of many greenhouse gases are increasing, especially that of carbon dioxide, which has increased by 30% over the past 200 years.1 A wide variety of processes have been developed for the removal of acid gases such as carbon dioxide and hydrogen sulfide from gas streams including physical/chemical absorption, adsorption, membrane processing, and oxygen recovery from O2/CO2 recycle combustion. The most common option for separating CO2 from flue gases or other gas streams is scrubbing the gas stream using liquid amine sorbents. The aqueous amine sorbents that are based on chemical absorption have certain advantages over traditional alkanolamines such as higher surface tension, very low volatility, resistance to degradation in the oxygenated elevated temperature flue gas, thermal stability, low solubility of hydrocarbons, and high alkalinity.2-4 These same aqueous amine solutions have some inherent disadvantages such as corrosion, high regeneration energy, and fouling of the process equipment.5-7 To avoid these problems, many researcher have investigated operating with various solid amine sorbents relying on supports such as silica gels,8-10 fly ash carbon,11 molecular sieves,12 activated carbon,12,13 and polymers to provide contact between the gas phase and the active amine material.14,15 These solid amines can be found with the amine functionality chemically bonded to the support or represent liquid amines immobilized within a porous support. The solid amine sorbents based on high surface area supports have the advantage of recovery of gases at low pressure, low capital cost, and low regeneration energy compared to the large amount of energy required to heat the water solution for the aqueous amine sorbent process.12,16 The objective of this research is to study the total absorption/ desorption capacity, cyclic capacity, absorption/desorption rate, and effect of blending various amines within a single support type on the CO2 capture capacity. These sorbents include immobilized monoethanolamine (MEA), tetraethylenepentamine (TEPA), reaction-modified tetraethylenepentamine with acry* To whom correspondence should be addressed. Tel.: (860) 7684843. Fax: (860) 768-5073. E-mail:
[email protected]. † University of Hartford. ‡ U.S. Department of Energy. § Yonsei University.
lonitrile (TEPAN ) 1:1 molar ratio TEPA:AN), ethyleneamine (E-100), reaction-modified ethyleneamine with acrylonitile (E100AN ) 1:1 molar ratio E-100: AN), and 194A (DBN). All these amines were used with either poly(methyl methacrylate) (PMMA) or 194B (DBU + DBN + THP + APD) material and used a S-7 support. The hope is to create an improved CO2 capture material which will allow for improved acid-gas treatment equipment when operated in coal-fired power plants. 2. Experimental Section All chemicals used in this study were of reagent grade such as MEA (Sigma-Aldrich, Inc., purity >99+%), AN (SigmaAldrich, Inc., purity >99+%), TEPA (MGA I Co., purity >99+%), and E-100 (Huntsman Co., purity >99+%). The polymeric support PMMA (HP-2MG) was purchased from Supelco Co. with the following manufacturer specifications:17 effective particle size 0.5 mm; pore volume 1.2 mL/g; specific surface area 470 m2/g; average pore radius 14 nm; density 0.29 g/mL. The process gases, N2 and CO2, were of high purity (>99%) for all experiments. The composition of flue gases in coal-fired power plants is expected to be approximately 79% N2, 15% CO2, 4% O2, and trace quantities of other gas species (among them HCl, SOx, and H2O). Therefore, this study has begun an initial evaluation of these solid CO2 capture sorbents with a gas composition of approximately 82.4% N2, 15% CO2, and 2.6% H2O (vapor). The syntheses of modified solid amine sorbents are fabricated in the manner described by Filburn17 and Schladt.18 The apparatus used for the screening tests of solid amine sorbents used in this study is shown in Figure 1. The equilibrium breakthrough curve was measured using an equilibrium cell at atmospheric pressure, a saturated reactor (500 mL), and a precision digital thermometer (to within (0.05 K), respectively. The equilibrium testing used a cylindrical reactor (inner diameter 1.7 cm and length 8 cm) located inside a water bath, and was made of stainless steel. Reticulated aluminum foam and porous quartz frit were installed inside the top and bottom of the equilibrium cell to hold the beads in place. Before beginning the experiment, N2 was fully supplied into the equilibrium cell to remove any gaseous contaminants. The gases (N2, CO2) were kept at the same ambient temperature as the experimental setup
10.1021/ie8006984 CCC: $40.75 2008 American Chemical Society Published on Web 08/27/2008
7420 Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008 Table 2. Cycle Effect Parameters Obtained for CO2 Capture at Low Temperature (296.15 K) max. rich min lean cyclic loading loading capacity av absorption av desorption (R), (R), (∆R), rate, (mol/ rate, (mol/ mol/kg mol/kg mol/kg g · s) × 10-5 g · s) × 10-5 First Cycle MEA TEPAN E-100AN 194B
15.74 14.22 12.70 11.60
7.28 4.01 1.44 4.83
MEA TEPAN E-100AN 194B
10.30 11.90 12.47 9.24
3.11 2.54 1.82 3.55
8.46 10.21 11.26 6.77
6.49 5.69 4.70 4.64
3.36 4.08 4.15 2.71
4.12 4.75 4.82 3.70
2.87 3.72 4.07 2.27
3.36 4.56 4.26 2.97
2.83 3.98 4.17 2.13
Second Cycle
Figure 1. Schematic diagram of screening test system.
Third Cycle
Table 1. Properties of Solid Amines at Different Absorption Temperatures for CO2 Capture cyclic max rich min lean capacity av absorption av desorption loading loading (∆R), rate, (mol/ rate, (mol/ (R), mol/kg (R), mol/kg mol/kg g · s) × 10-5 g · s) × 10-5 296.15 K MEA TEPA TEPAN E-100 E-100AN 194A 194B
15.74 21.45 14.22 18.58 12.70 11.05 11.60
7.28 13.88 4.01 11.00 1.44 3.66 4.83
MEA TEPA TEPAN E-100 E-100AN 194A 194B
13.85 21.07 13.21 18.2 11.92 10.82 11.46
6.20 14.03 3.02 11.03 1.86 3.53 5.10
MEA TEPA TEPAN E-100 E-100AN 194A 194B
13.7 20.85 12.91 17.74 10.80 10.39 10.86
6.14 12.70 2.70 10.58 1.33 3.58 4.60
8.46 7.57 10.21 7.58 11.26 7.39 6.77
6.49 8.58 5.69 7.43 4.77 4.41 4.64
3.36 3.02 4.08 3.02 4.15 2.83 2.70
5.54 8.42 5.20 7.28 4.70 4.33 4.59
3.05 2.80 3.82 2.77 4.00 2.97 2.54
5.48 8.34 5.16 7.10 4.32 4.16 4.35
3.00 3.24 4.07 2.86 3.76 2.71 2.49
303.15 K 7.65 7.04 10.19 7.17 10.06 7.29 6.36 313.15 K 7.56 8.15 10.21 7.16 9.47 6.81 6.26
7.19 9.36 10.65 5.69
and were precisely injected into the equilibrium cell via individual mass flow controllers (OMEGA, FMA-5400). The chemical absorbent/support ratio (mass amine/mass support) for the sorbents MEA, TEPA, TEPAN, E-100, and E-100AN was 0.7:1.0. Those for 194A and 194B were 0.667:1.0 and 0.833: 1.0, respectively. The gas residence time was approximately 1.47 s. The CO2-rich amine absorption occurred at two temperaturesseither 296.15 or 313.15 K. The desorption temperature of the CO2-lean amine was kept constant at an elevated temperature, 343.15 K, in the stripper. The gas flow rate was controlled during experiment using a mass flow controller. An exit CO2 gas analyzer (Vaisala Co. Ltd., GMT 221 CO2 analyzer) was used to measure the concentration of CO2 with the balance being assumed to be chiefly N2. 3. Results and Discussion The immobilized solid amine sorbents were absorbed and desorbed in CO2 for 25 min, respectively. The rich amine CO2 loading and lean amine CO2 loading were determined by the area of the curve formed by the CO2 outlet concentration [%]
MEA TEPAN E-100AN 194B
9.02 11.71 12.04 7.43
1.93 1.68 1.82 1.73
7.09 10.03 10.88 5.70
and reaction time [min] for these absorbents. The CO2 capacity data in these solid amine sorbents at low temperature (absorption) and high temperature (deosrption) were measured and are tabulated in Table 1 and shown graphically in Figure 2. It was found that the CO2 loading capacity increased linearly with an increase in absorption time and decreased with a reduction in desorption time. The CO2 loading capacity in our absorption reaction linearly increased in a rapid reaction of CO2 and the immobilized solid amine sorbent during the initial 7.5 min but then showed very little capacity after this 7.5 min point. For the lower temperature absorption case (296.15 K), the maximum of CO2-rich loading of MEA, TEPA, TEPAN, E-100, E-100AN, 194A, and 194B for CO2 capture was 15.74, 21.45, 14.22, 18.58, 12.70, 11.05, and 11.60 mol/kg, respectively. The minimum of CO2-lean loading of MEA, TEPA, TEPAN, E-100, E-100AN, 194A, and 194B at 343.15 K was 7.28, 13.88, 4.01, 11, 1.44, 3.66, and 4.83 mol/kg, respectively. Similar trends were reported in the literature for MEA.19,20 The cyclic capacity for CO2 capture can be calculated from the maximum CO2-rich loading and minimum CO2-lean loading. It is an important parameter for estimating the working sorbent capacity and will allow designers to accurately predict the size and energy requirements for pilot- or full-scale CO2 capture systems. The cyclic capacities for these sorbents at the lower absorption temperature (296.15 K) follow the order E-100AN > TEPAN > MEA > TEPA ) E-100 > 194A > 194B. The solid amines (TEPAN and E-100AN) demonstrated higher cyclic capacities than the others under these temperature and gas flow conditions. As the absorption temperature was increased, the CO2 loading capacity decreased for the same CO2 absorption partial pressure. As CO2 absorption is exothermic in nature, an increase in temperature should decrease the extent of chemical absorption (if reaction kinetics are not strongly affected by the change in temperature) in accordance with Le Chatelier’s principle.3,29 The variation in the measured CO2 equilibrium breakthrough curves as a function of time is shown in Figure 3. The CO2 loading capacity increased with an increase in the area below the inlet CO2 concentration of 15%. However, it is the difference in the area of the equilibrium breakthrough curves between the absorption and desorption curves that determines the overall equilibrium capacity. We would expect that the area of the equilibrium breakthrough curves would be opposed to the absorption temperature trend. That is, as the absorption tem-
Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008 7421
Figure 2. Value of CO2 loading of solid amine at low ambient temperature (296.15-313.15 K) and high temperature (348.15 K).
perature increased, we would predict a drop in the equilibrium capacity. That trend is seen in Figure 3: the area of the equilibrium breakthrough curves during absorption decreased with an increase in absorption temperature. Similar trends were reported in the literature for DBU.21 The support impregnated with the amine TEPA displayed the highest capacity for CO2 capture during absorption, and E-100AN displayed the highest capacity for evolving CO2 during desorption. It was found that these results are consistent with the results of our CO2 cyclic capacity testing. Figure 4 shows the absorption rate and desorption rate as a function of time for all the solid amine sorbents tested under this program. The absorption rate and desorption rate for CO2 capture were defined by the cyclic capacity [mol/kg] per unit of reaction time [min] for these absorbents. TEPA shows the highest absorption rate at the 3.5 min point in the absorption cycle. E-100AN shows the highest desorption rate at the 26.75 min point in the cycle (1.75 min into the desorption 1/2 cycle). The average absorption reaction rate for the solid amines decreased with an increase in the absorption temperature tested in Table 1. The average absorption rate for these sorbents at 296.15 K follows the order TEPA > E-100 > MEA > TEPAN > E-100AN > 194B > 194A, and the average desorption rate for these same sorbents at 343.15 K follows the order E-100AN > TEPAN > MEA > TEPA ) E-100 > 194A > 194B. It was found that the cyclic capacity increased with an increase in the average absorption rate and a drop in the absorption temperature.
Figure 3. Equilibrium breakthrough curves between CO2 and solid amine with temperature.
MEA, TEPA, and E-100 show the highest absorption rates for these materials, and it is surmised that this is due to the alkyl group attached to the amine itself. The absorption rates of the other materials were markedly slower. One theory for this result relates to the size of the substituents attached to the amine functional group. It is surmised that an increase in internuclear distance diminished internuclear attraction and decreased the reaction between CO2 and the amine group. 4,23-26 The absorption rate profile shifts lower toward the left (in the time domain), thereby increasing slightly the peak rate, when the absorption temperature is increased. It was theorized that the reaction rate constant (k) between CO2 and the amine increases linearly with an increase in temperature.4,22 We also studied the decay of CO2 capacity for the solid amine sorbents MEA, TEPAN, E-100AN, and 194B based on cyclic operation. Figure 5 and Table 2 show the effect of cyclic capacity and average reaction rate with cycle number for these solid amines at the low temperature (296.15 K) absorption and high temperature (343.15 K) desorption conditions. After the first cycle, TEPAN and E-100AN maintain their cyclic capacities, which were shown in the reaction rates and CO2 capture breakthrough curves. However, MEA and 194B both demonstrated a large decrease in absorption rate and capacity (area of breakthrough curve) after the first cycle. The trend for the CO2 loading in MEA and 194B in the absorption phase showed a reduction in capacity for all three cycles as well as a reduction in the instantaneous absorption rate. The
7422 Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008
Figure 4. Absorption/desorption rates of solid amines as a function of absorption temperature.
average absorption rate and desorption rate decreased with each additional cycle, which is quantified in Table 2. The cycle decay effect for these sorbents follows the sequence E-100AN > TEPAN > MEA > 194B. Our testing showed high repeatability for the cyclic CO2 capture capacity of TEPAN and E-100AN. The ability to maintain working capacity was the highest for TEPAN and E-100AN. The drop in cyclic capacities of MEA, TEPAN, E-100AN, and 194B at the cycle effect of the third cycle was 16.17%, 1.73%, 3.0%, and 15.79%, respectively. However, the cyclic capacities of MEA, TEPAN, E-100AN, and 194B only saw a reduction of about 2% between the second cycle and third cycle. It is not advisable to use MEA for absorbing carbon dioxide over a long period of time because the residual products of MEA/CO2 include a cyclic carbamate or urea that is created by heat. The cyclic carbamate cannot absorb adequate quantities of carbon dioxide because as the cyclic carbamate content increases, CO2 solubility decreases.27,28 In the case of TEPAN and E-100AN, it is not unreasonable to postulate that the large amount of acrylonitrile added to the amine results in the conversion of primary amines to secondary amines.14
Figure 5. Cycle decay effect of solid amines at low temperature (296.15 K).
materials. These two solid amine sorbents, TEPAN and E-100AN, have shown a large increase in CO2 cyclic capacity compared to the commercial amine MEA, also impregnated into the same support material. The conclusion which could be drawn from this study of a screening test for CO2 capture are these: First, the solid amines (TEPAN and E-100AN) demonstrated higher cyclic capacities than others under the same absorption/desorption conditions. Second, the initial rate of absorption and desorption was shifted earlier in time and the CO2 equilibrium breakthrough capacity was slightly decreased when the temperature was increased. Third, the cyclic reproducibilities of TEPAN and E-100AN seemed superior compared to those of the other materials tested. This study lays the foundation for future work on thermodynamics properties, solubility with pressure, and foundation process design of solid amine sorbents using the PSA process. Acknowledgment This work was supported by the Korea Research Foundation Grant funded by the Korean Government (KRF-2007-357D00049) and the U.S Department of Energy. Literature Cited
4. Conclusion We have investigated the CO2 capture capacity of several novel solid amine sorbents with at least two different support
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ReceiVed for reView April 29, 2008 ReVised manuscript receiVed July 3, 2008 Accepted July 11, 2008 IE8006984
