Dynamics of CO2 Absorption and Desorption Processes in

Article pubs.acs.org/IECR

Dynamics of CO2 Absorption and Desorption Processes in Alkanolamine with Cosolvent Polyethylene Glycol Jun Li,† Chenjia You,† Lifang Chen,† Yinmei Ye,† Zhiwen Qi,*,† and Kai Sundmacher‡,§ †

State Key Laboratory of Chemical Engineering, School of Chemical Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China ‡ Max Planck Institute for Dynamics of Complex Technical Systems, Sandtorstrasse 1, D-39106 Magdeburg, Germany § Process Systems Engineering, Otto-von-Guericke University Magdeburg, Universitätsplatz 1, D-39106 Magdeburg, Germany ABSTRACT: To reduce the high energy consumption and equipment corrosion in conventional processes of CO2 capture with aqueous amine solutions, the mixed nonaqueous solvents of monoethanolamine (MEA), diethanolamine (DEA), and diglycolamine (DGA) with polyethylene glycol (PEG) as cosolvent were explored for CO2 capture. The dynamic experiments of CO2 absorption and desorption were carried out to evaluate the performance of the studied nonaqueous solutions. It demonstrated that the mixed solutions of amines and PEG exhibited higher CO2 cyclic capacity and regeneration efficiency compared with the only aqueous amine solutions. Especially, the solution of 3 mol/L DGA-PEG200 exhibits a high cyclic capacity of 0.438 mol CO2/mol DGA and a high regeneration efficiency of 94.6%, which indicates its great potential in industrial application. Moreover, the very low vapor pressure of PEG helps the mixed solution for CO2 capture with reduced corrosion, energy consumption, and environmental pollution. to generate a large amount of water evaporation.12−14 The reaction heat of primary amines aqueous solutions is about 80− 90 kJ/mol CO2, while those of the secondary and tertiary amines are about 70−75 kJ/mol CO2 and about 55 kJ/mol CO2, respectively.13,15,16 Besides the specific heat capacity of water, the energy required to produce enough water evaporation for removing 1 mol CO2 from the MEA solution could reach up to 92.4 kJ. Hence, the latent heat of water vaporization leads to the excessive energy consumption, and the higher content of water in solvent requires more energy for regeneration.17 On the other hand, in order to recover the lean solvent, the regeneration temperature is generally operated at as high as 110−125 °C, which further accelerates the corrosion and degradation of aqueous alkanolamine. In brief, the disadvantages of the conventional CO2 capture processes with aqueous amine solutions are commonly ascribed to water of the solutions. If water is replaced by other types of organic solvents, the alkanolamine process would be more energy efficient and less corrosive. In practice, many nonaqueous processes have been extensively investigated for acid gas treatment. Some common organic solvents such as methanol, ethanol, and ethylene glycol have been suggested as potential cosolvents for CO2 capture with MEA, DEA, and MDEA.18−20 However, these nonaqueous mixture systems usually have relatively high vapor pressure and thus are not very suitable for CO2 capture from flue gases under the condition of high temperature and low pressure, i.e., 40− 100 °C and ambient pressure.21 In recent years, ionic liquids (ILs) as novel solvents have been intensively developed for

1. INTRODUCTION Greenhouse gases (GHGs) emission is believed to give rise to a serious climate change. The combustion of fossil fuels accounts for around 86% of anthropogenic GHGs emission, while CO2 represents over 80% of GHGs.1 State-of-the-art amine-based solvents have been widely applied for treating acid gases of oil refineries and nature gas, and it is currently the only available technology for postcombustion capture of CO2 that can be retrofitted for the existing power plants.2,3 The major aqueous amine absorbents applied in industrial processes are monoethanolamine (MEA), diethanolamine (DEA), diglycolamine (DGA), and N-methyldiethanolamine (MDEA) with excellent advantages in the high absorption rate and CO2 capacity.4−6 However, the aqueous amine processes usually suffer from high equipment corrosion and amine degradation at high temperature.7−9 Moreover, the high energy consumption for solvent regeneration, which accounts for 25−40% of the power output loss, impedes the wide application of aqueous amines in CO2 capture.10,11 The wet CO2 corrosion could occur at the bottom of the absorber and the upper part of the regenerator in the conventional process of postcombustion CO2 capture, which is ascribed to the electrochemical corrosion of carbon steel and CO2 in an aqueous environment. In addition, the high operation temperature and the high conductivity of the aqueous solution could cause corrosion of amine solution at the bottom of the regenerator. The heat of regeneration for rich amine solutions, consisting of the sensible heat, the reaction heat, the latent heat of partial water, and solvent vaporization, contributes 50−80% of the total energy consumption in conventional aqueous amine processes. The heat is first utilized to increase the solvent temperature, which is determined by the specific heat capacity of the solvent. Meanwhile, a large amount of energy is consumed to promote the desorption reaction and © 2012 American Chemical Society

Received: Revised: Accepted: Published: 12081

May 4, August August August

2012 17, 2012 28, 2012 28, 2012

dx.doi.org/10.1021/ie301164v | Ind. Eng. Chem. Res. 2012, 51, 12081−12088

Industrial & Engineering Chemistry Research

Article

CO2 capture from flue gases, and they exhibit good solubility and high selectivity to CO2.22−24 The mixed solvent of ILs and alkanolamine with efficient and reversible performance for CO2 capture shows more commercial potential. However, the comparatively high viscosity, price, and their unknown toxicity impede ILs for large-scale industrial application. Polyethylene glycols (PEGs) have the molecular structural formula of H(OCH2CH2)nOH, with n being the number of ethylene oxide.25 PEGs have competitively excellent properties to ILs, such as having very low vapor pressure, being nontoxic and inexpensive, and having chemical and thermal stability, which indicates PEGs are qualified to be a physical solvent for gas separation even at high temperature.26−28 In addition, the properties of PEGs could be tuned, attributed to their great choice, by varying the molecular weights. Therefore, PEGs have been widely used in pharmaceutical, cosmetics, and food industries.29 As an excellent cosolvent, PEGs show the good prospect for improving the efficiency of CO2 capture and reducing the consumption energy of regeneration.30,31 In this work, the dynamics of CO2 absorption and desorption processes in mixed solvents of alkanolamine and PEG were investigated. Since there was no reaction occurring for ternary amine in nonaqueous solvents, primary alkanolamine (MEA, DGA) and secondary alkanolamine (DEA) were chosen as candidate amines. The cyclic CO2 capacity and regeneration efficiency of the mixed solvents were obtained and compared to the aqueous amine solutions.

online gas chromatograph (Fuli, 9790II, GC). It can be operated at pressure up to 6 bar and temperature up to 200 °C. Considering the practical situation of flue gases, the absorption of CO2 from SFG and the regeneration with N2 as stripping gas are performed at 40 and 80 °C with ambient pressure, respectively. Prior to the experiment, the GC was calibrated using the SFG at a controlled flow rate of 200 mL/min. After purging the vessel by N2, the fresh mixed solvents of alkanolamine and PEGs with volume of 150 mL and a specific concentration were fed into the glass vessel. Then, the solution was heated to 40 °C and bubbled by N2 at a flow rate of 50 mL/min to purge a possible trace of CO2 for about 0.5 h. When the temperature was stabilized, the SFG with a flow rate of 200 mL/min was accessed into the vessel and the outlet gas concentration was online measured by GC at intervals of 6 min. The mechanical agitator was set at 500 r/min to ensure intensive mixing of the gas and liquid. When the concentration of CO2 of the outlet was approximately equal to the inlet CO2 concentration, the system was considered to achieve equilibrium. Subsequently, the regeneration experiment was conducted. Once the solution was heated to 80 °C, the solution was stripped with N2 at a flow rate of 200 mL/min, and the outlet gas concentration could be measured by GC. The regeneration is terminated when the molar ratio of CO2 detected in the outlet gas was lower than 0.5%. The second cyclic experiment was processed after the solution was cooled down to 40 °C. This approach can provide firsthand knowledge of the dynamic behavior of the absorption and desorption processes with different solvent mixtures. On the basis of the experimental data, the cyclic CO2 capacity is easily obtained. Moreover, for a better comparison of the cyclic performance, reference experiments were performed with the most commonly used MEA aqueous solvents at the concentration of 2 and 2.5 mol/L in this work.32−34 It should be noted that the comparison is semiquantitative for the uncertainty of gas− liquid interfacial area, viscosity, and interfacial tension. 2.3. Calculation Approach. Assuming the inert gas N2 is not absorbed by solvents, the absorption and desorption rates of CO2 at a given time can be calculated as follows:

2. EXPERIMENTAL SECTION 2.1. Materials. Polyethylene glycol 200 (PEG200), polyethylene glycol 300 (PEG300), polyethylene glycol 400 (PEG400), monoethanolamine (MEA, 99%), and diethnaolamine (DEA, 99%) were obtained from Sinopharm Chemical Reagent Co., Ltd. in Shanghai. Diglycolamine (DGA, 98%) was purchased from Aladdin Chemistry Co., Ltd. in Shanghai. Nitrogen (N2) with purity of 99.999% and simulated flue gas (SFG) with 84.8% N2 and 15.2% CO2 (molar fraction) were supplied by Shanghai Wetry Standard Reference Gas Co., Ltd. All materials were used as received without further treatment. 2.2. Apparatus and Procedures. The experiment was carried out in an apparatus, as shown in Figure 1. The apparatus consists of a glass vessel (Büchiglassuster, type I, 1.0 L) with a mechanical agitator (cyclone 075, 0−3000 r/min) and a condenser, a circulation thermostat (Huber, ministat 230, ±0.1 °C), a mass flow controller (Sevenstar, CS200A, MFC), and an

out xCO = 2

out xCO = 2

in − rabs VSFGxCO 2

VSFG − rabs rdes VN2 + rdes

(1)

(2)

where rabs and rdes are the absorption rate and desorption rate, respectively; V is the flow rate and it was set to 200 mL/min in this work; xinCO2 and xout CO2 are the CO2 molar fraction of the inlet and outlet gas, respectively. By simple transformation, one can get rabs =

rdes =

in out − xCO VSFG(xCO ) 2 2 out 1 − xCO 2

(3)

out VN2xCO 2 out 1 − xCO 2

(4)

By integrating rabs and rdes with time, the accumulated amount of CO2 (nCO2) can be calculated. Since the absorption and desorption experiments were cycled at least two times, the

Figure 1. Schematic diagram of the experimental setup. 12082



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Figure 2. CO2 absorption rate and capacity of amine-PEG200 solutions at 313 K: (a) 0.5 M; (b) 1 M; (c) 2 M; (d) 3 M.

solution cyclic capacity was obtained directly from the capacity of the second absorption process. The CO2 capacity (αCO2) in solvent is calculated by eq 5. αCO2 =

nCO2 namine

=

As the concentration of the investigated amine solvents is increased from 0.5 to 3 M, their absorption rates are enhanced, as illustrated in Figure 2a−d. Take MEA as an example; the absorption rates increase from 26 to 31 mL/min. It maintains for about 45 min of the 3 M solution at its maximal absorption rate, indicating that the studied amine solution has great potential to effectively capture CO2 from flue gases that have high CO2 content and flow rate. For the CO2 capacity, 2 M solutions show maximal ability, which is limited by 0.5 mol CO2/mol amine due to the stoichiometry of the chemical absorption reaction. As seen, the CO2 capacity increases from 0.45 to 0.5 mol CO2/ mol MEA as the concentration increases from 0.5 to 2 M, while it reduces slightly to 0.48 mol CO2/mol MEA as further increased to 3 M. This might be attributed to the intensive interaction between amine molecules and the higher mass transfer resistance caused by the change of the solution viscosity at higher concentration. It is worth mentioning that the CO2 capacity is valid under the CO2 partial pressure of 15.2 kPa and temperature of 40 °C in this study. From the absorption experiments, it turns out that mixed solvents of MEA and PEG200 exhibit better performance of absorption rate and rich CO2 capacity (the maximal CO2 capacity in solvent) than DEA and DGA. However, for industrial application, the performance of solvent should be evaluated under a dynamic environment taking both absorption and desorption processes into account. Hence, the cyclic

Mamine × nCO2 mamine

(5)

3. RESULTS AND DISCUSSION 3.1. Absorption Rate and CO2 Capacity. The absorption rates and CO2 capacities with respect to time at 0.5, 1, 2, and 3 M alkanolamines (i.e., MEA, DEA, DGA) using PEG200 as cosolvent are accordingly shown in Figure 2a−d. As seen in Figure 2a, the absorption rate of MEA solution (26 mL/min) is just slightly higher than that of DGA (25.2 mL/min), while the rate of DEA is only about 17.6 mL/min. According to the difference of the absorption rate, the reactivity of the studied nonaqueous alkanolamine solutions is ordered as MEA ≈ DGA > DEA. The competitive reactivity of MEA and DGA solutions is attributed to their similar molecule structure. However, the DEA solution shows lower reactivity, which is ascribed to the reduction in steric hindrance around the amine group. In addition, the low absorption rate and short absorption time of DEA result in lower CO2 capacity (0.23 mol CO2 /mol amine), which is about 60% of those of MEA and DGA. The determined CO2 capacities of MEA and DEA are in good agreement with the data reported by Camper et al.22 12083



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Figure 3. Absorption and desorption rates and CO2 capacity in mixed solvents of 0.5 M amine and PEG200 in the dynamic process. (a) 0.5 M MEAPEG200; (b) 0.5 M DEA-PEG200; (c) 0.5 M DGA-PEG200.

conducted at the absorption temperature of 40 °C and the desorption temperature of 80 °C using N2 as stripping gas. As can be seen in Figure 3a, in the first circulation, the absorption rate decreases from 30 to 0 mL/min, indicating the solution is saturated and the first absorption process is finished. As the solvent temperature increases from 40 to 80 °C, the desorption process starts. The desorption rate declines from 85 to 0 mL/ min. When the desorption rate is approaching to be constant and close to 0 mL/min, the regeneration process is considered

dynamic operation of absorption and desorption processes was carried out. 3.2. Cyclic Absorption and Desorption Operation. Since the 2 M solutions possess the maximal CO2 capacity, in this section, two solutions of 0.5 and 2 M alkanolamine were selected to investigate the influence of solvent concentration on the absorption and desorption rates and cyclic CO2 capacities in dynamic processes. As presented in Figure 3, three circulations with 0.5 M amine-PEG200 solvents were 12084



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Figure 4. Absorption and desorption rates and CO2 capacity in mixed solvents of 2 M amine and PEG200 in the dynamic process. (a) 2 M MEAPEG200; (b) 2 M DEA-PEG200; (c) 2 M DGA-PEG200.

to be complete. After the solution is cooled down to 40 °C, it is ready for processing the second circulation, and the further circulation can be continuous following the same procedure. As seen, for the second and third circulations, the performance is competitive to the first one except very slight derivation caused by experimental operation and the corresponding CO2 capacity increases from 0 to 0.45 mol CO2/mol MEA in the absorption process and then decreases to 0 in the desorption process. The desorption rate is associated with the CO2 capacity in solution and the regeneration time, from which one can

evaluate the difficulty of regenerating the solvent. As seen, the initial desorption rate of 0.5 M MEA, DEA, and DGA solutions are accordingly about 85 mL/min (Figure 3a), 50 mL/min (Figure 3b), and 70 mL/min (Figure 3c), respectively. When N2 is introduced to strip the solution, the initial desorption rates are unusually high because of the accumulation of desorbed CO2 during the solvent heat-up period and the high CO2 capacity at the start of desorption. Due to the low CO2 capacity of DEA, it takes only about 25 min to completely regenerate the solution (Figure 3b), while it is more than 45 12085



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while it is over 94.6% for DGA and 100% for DEA. In addition, 3 M DGA solution has the highest cyclic capacity (0.438 mol CO2/mol amine) and cyclic loading (1.208 mol CO2/kg solution) compared with other studied solvents. From Table 2

min for MEA and DGA (Figure 3a−c). In addition, the heat of reaction of DEA is lower than those of MEA and DGA,16 which is also ascribed to the rapid desorption of DEA. Moreover, it is worth noticing that all of the 0.5 M alkanolamine solutions exhibit high desorption ability even after three circulations. For 2 M alkanolamine solutions, the cyclic rates of absorption and desorption and CO2 capacity are illustrated in Figure 4. Compared to the low concentration amine-PEG200 solutions, i.e., 0.5 M in Figure 3, it shows much higher desorption rate attributed to their high CO2 capacity in solution. The desorption rates of MEA, DEA, and DGA are accordingly about 147 mL/min (Figure 4a), 195 mL/min (Figure 4b), and 225 mL/min (Figure 4c), respectively. For the MEA solution, the cyclic CO2 capacity is only 0.397 mol CO2/ mol MEA, although 2 M MEA possesses the highest rich capacity. In contrast, DEA and DGA show excellent cyclic desorption rate and CO2 capacity even under the high concentration. As seen, for 2 M DEA and DGA solutions, their cyclic capacities are 0.3 mol CO2/mol DEA and 0.47 mol CO2/mol DGA, and the times to reach complete desorption are about 60 and 120 min, respectively. Furthermore, the cyclic CO2 capacity of DGA is higher than that of MEA. From the dynamic experiments, the nonaqueous solvents of DEA and DGA show the superiority compared with MEA in the dynamic process, which is different from the discussion in Section 3.1. Therefore, further evaluation of the mixed solvents is required. 3.3. Evaluation of Mixed Solvents Performance. The performance of each mixed nonaqueous solution is evaluated using selected criteria, i.e., rich CO2 capacity (αrich, defined as mol CO2/mol amine), cyclic capacity (αcyc, defined as mol CO2/mol amine), cyclic loading (mcyc, defined as mol CO2/kg solvent), and the regeneration efficiency. Generally, the cyclic capacity, cyclic loading, and regeneration efficiency are of particular interest to the industrial application as they are strongly related to the energy consumption. Table 1 summarizes the results calculated according to the dynamic absorption−desorption experiments. As already discussed in Section 3.2, both DEA and DGA exhibit excellent desorption ability. In Table 1, as the solvent concentration increases from 0.5 to 3 mol/L, the regeneration efficiency of MEA solution decreases from 99.8% to 76.9%,

Table 2. Boiling Temperature (Tb), Vapor Pressure (p), Heat of Vaporation (Hvapor), and Viscosity (η) of Amines and PEG20012,14,26,35 Tb (°C, at 1 atm) p (Pa, at 20 °C) Hvapor (kJ/kg, at 1 atm) η (mPa·s, at 20 °C)

MEA +PEG200

DEA +PEG200

DGA +PEG200

Camine αrich (mol/L) (mol/mol)

acyc (mol/mol)

mcyc (mol/kg)

regeneration efficiency, %

0.5

0.449

0.448

0.200

99.8

1 2 3 0.5

0.468 0.496 0.484 0.239

0.427 0.397 0.372 0.239

0.387 0.721 1.016 0.109

91.2 80.0 76.9 100

1 2 3 0.5

0.281 0.300 0.288 0.422

0.281 0.300 0.288 0.421

0.253 0.549 0.794 0.190

100 100 100 99.8

1 2 3

0.467 0.474 0.463

0.463 0.467 0.438

0.425 0.864 1.208

99.1 98.5 94.6

DEA

DGA

268.8

Dynamics of CO2 Absorption and Desorption Processes in

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