Screening test of amino acid salts for CO2 absorption at flue gas

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Screening test of amino acid salts for CO2 absorption at flue gas temperature in a membrane contactor Feijie He, Tao Wang, Mengxiang Fang, Zhen Wang, Hai Yu, and Qinhui Ma Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02578 • Publication Date (Web): 21 Dec 2016 Downloaded from http://pubs.acs.org on December 26, 2016

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Screening test of amino acid salts for CO2 absorption at flue gas temperature in a membrane contactor Feijie Hea, Tao Wanga,*, Mengxiang Fanga, Zhen Wanga,b, Hai Yub, Qinhui Maa a

State Key Laboratory of Clean Energy Utilization, Zhejiang University, 310027, P.R. China b

CSIRO Energy Centre, Mayfield West, 2304, Australia *Corresponding authors: [email protected]

Abstract: CO2 absorption at the temperature of flue gas inlet could reduce the costs related to flue gas cooling system and improve the economic feasibility of the Post-combustion Carbon Capture (PCC) processes. Amino acid salts are considered as promising absorbents to absorb CO2 in a membrane contactor at elevated temperatures due to their advantages of lower volatility, less degradation and higher surface tension. In this study, 24 common amino acids have been screened for their potential to absorb CO2 at the temperature of flue gas inlet in a membrane contactor. These screening processes involved examination of the water solubility of amino acids, measurement of surface tension and viscosity of their potassium salts, CO2 capacity and CO2 membrane absorption test. Taurine, sarcosine and glycine were identified as performing well in all the screening tests and were further investigated for CO2 membrane absorption at high temperatures up to 80 °C and various CO2 loadings in a polypropylene (PP) hollow fiber membrane module. The results show that those 1

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amino acid salts are feasible to absorb CO2 at high temperatures in a membrane contactor. Potassium sarcosinate (PS) is identified as the most promising absorbent for high-temperature CO2 absorption with a better absorption performance than MEA and other amino acid salts.

Keywords: Carbon dioxide; Membrane; Absorption; High temperature; Amino acid salt

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1

Introduction

Among all CO2 capture technologies, amine-based post-combustion capture (PCC) is a well-established technology for CO2 capture. Due to its tail-end technology, it can be retrofitted to existing power plants and has successful commercial applications in many industries.1 For most of the commercially available amine-based PCC processes, CO2 absorption at low temperature, ranging from 283 K to 313 K, is usually recommended to obtain a good balance between solubility and reaction kinetics.2,3 However, the temperatures of flue gases coming from coal-fired power stations are generally higher than the absorption temperatures in the capture plant.4,5 For flue gas without FGD (e.g. Australian power station system6), the flue gas temperature could be more than 393 K which generates a significant gap between CO2 absorption temperature and flue gas temperature. Moreover, for flue gas with semi-dry FGD and bag filter technologies, the temperature at the exit is as high as 353 K.7,8 Although additional flue gas polishing for lower SO2 concentration or lower temperature may benefit the CO2 absorption performance, the polishing scrubbing could result in a significant increase of capital cost. On the other hand, the heat of reaction released during CO2 absorption can also cause a significant temperature increase of 15-20 K for solvent.9,10 Therefore, additional cooling systems are employed to decrease the temperature of flue gas and solvent in amine-based PCC processes, such as absorber intercooling.11 This will lead to an increase in capital cost and energy penalty. Fisher et al.12 have made a techno-economic assessment of integration of a monoethanolamine (MEA) based PCC process to a coal-fired power 3


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plant integrating. It has been found that capital costs of cooling system and associated pumps account for 4% of total equipment cost, and energy penalty for this cooling system is about 10% of total energy consumption.12 As such, CO2 absorption at flue gas temperature could reduce the costs related to flue gas cooling system and improve the economic feasibility of the PCC processes. However, due to their high volatilities and thermal degradations, amine based solvents are not suitable for absorption of CO2 at high temperatures.13-15 Amino acid salts (AASs) are reported as promising and environmentally friendly absorbents for CO2 removal from flue gas with the advantage of fewer volatility and degradation issues.16-19 The reaction of CO2 with AASs can be described by a two-step mechanism proposed by Caplow originally.20 Reaction (1) and (2) give the reaction scheme for CO2 absorption in aqueous AAS solution. A zwitterion is formed in the first step (reaction 1), followed by deprotonation of the zwitterion by a base B to form carbamate in the second step (reaction 2). k2  → − O 2 CR 1R 2 NH 2 + COO − CO 2 + − O 2 CR 1R 2 NH 2 ← k −1

−

kb O 2 CR1R 2 NH 2 + COO − + B  → − O 2CR 1R 2 NHCOO − + BH +

(1) (2)

Wei et al. investigated the CO2 absorption with AASs at high temperatures.21 In their study, potassium taurinate was chosen as the high temperature absorbent for absorption of CO2. It has been found that CO2 solubility of taurinate solutions is comparable to that of alkanolamines at high temperatures, and the overall mass transfer coefficients of CO2 in taurinate solutions are high at high temperature up to 353K. 4


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Another advantage of AASs over amines is their high surface tension, which makes them suitable for use in CO2 membrane absorption.22,23 Membrane contactors are a strong candidate for CO2 absorption compared with conventional packed columns.24-27 Due to controlling gas and liquid fluids independently, membrane contactors offer more operational flexibility and can solve some operating problems, such as foaming, flooding, and entrainment, encountered in conventional contacting devices. Moreover, a membrane contactor could provide much larger gas-liquid interfacial area than a conventional packed column.26,27 So far, most studies on CO2 membrane absorption using AASs have focused on low temperature absorption (i.e. 313K), with limited research on high temperature.22,28,29 In this work, 24 common AASs have been screened for their potential to absorb CO2 at flue gas temperatures in a membrane contactor. The screening processes involved ranking of the water solubility of amino acids, measurements of surface tension and viscosity of their potassium salts, CO2 capacity and CO2 membrane absorption test. Three of these amino acids, taurine, sarcosine and glycine, were identified as performing well in the screening tests and were further investigated for CO2 membrane absorption at temperatures up to 80 °C. The objective of this work was to prove the concept of CO2 membrane absorption at high temperatures using aqueous AASs, and to identify some promising absorbents used for high temperature absorption.

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2

Experimental

2.1 Chemicals In this study, AASs were prepared by neutralizing amino acid solutions with an equimolar potassium hydroxide. All the amino acids used, including sarcosine (purity: >98%), β-Alanine (purity: 99%), L-Arginine (purity: ≥98%), L-Proline (purity: >99%), L-Threonine (purity: >99%), glycine (purity: ≥99%) and taurine (purity: ≥99%), were purchased form Sigma-Aldrich. In surface tension measurement, potassium hydroxide of 99.99% purity was used; in the absorption and regeneration experiments, the potassium hydroxide with the purity of 90% was used. Both were also purchased from Sigma-Aldrich. 2.2 Surface tension measurement The surface tensions of different AASs solutions at different temperatures were analyzed by Sigma700/701 Force Tensiometer. Du Nouy ring method was employed to measure the surface tension, and at least 10 runs were carried out for each measurement. The temperatures of solutions were controlled by a water bath connected to the liquid chamber of Sigma Tensionmeter. 2.3 Viscosity measurement Viscosities of AASs and MEA solvents at various concentrations, temperatures and loadings were measured by viscometer (DV-II+ pro, USA Brookfield Co., Ltd.). At least 3 runs were carried out for each measurement. The solution temperature was controlled by an oil bath connected to the viscometer. 6


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2.4 Screening experiments of CO2 absorption at room temperature The screening test of various AASs was conducted in a solvent continuous cycle membrane absorption apparatus in which the simulated flue gas is fed in continuously while the solvent is circulated in a closed loop. In this test, the simulated flue gas (pressure: 110 kPa) at a total gas flow of 2 L/min were prepared by mixing CO2 and N2. The CO2 concentration was controlled at 12.5% by mass flow controllers. The mixed stream was saturated with moisture by bubbling through a pre-wet vessel and then flowed into the shell side of the hollow fiber membrane contactor (HFMC). Solvents of known volume (400 ml) stored in stirring vessel were pumped to the tube side of HFMC at a flow rate of 20 ml/min and the temperature of 298K. The CO2 absorbed solvent was then sent back to the stirring vessel and well mixed with the remaining solvent in the vessel prior to continuous cyclic absorption. The outlet CO2 concentration in gas was monitored by a FTIR gas analyzer (GASMET FTIR Dx4000). As the outlet concentration of CO2 reached 95% of the inlet CO2 concentration, we ended the test considering that the solvent has almost reached its capacity for CO2 absorption. The specific parameters and schematic diagram of this apparatus are available in our previous work.25 CO2 absorption rate was calculated by the following equation,

NCO 2 =

Qg (Cin − Cout ) 273 P × × Vm T 101.3

(3)

where NCO2 is instantaneous absorption rate, mol/min; Cin and Cout are the inlet and outlet CO2 concentrations in vol% in the gas phase, respectively; Qg is the total gas

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flow rate, L/min; Vm is standard molar volume of an ideal gas, 22.4 L/mol; T is the absorption temperature, K; P is pressure in gas stream, kPa. 2.5 Absorption-desorption tests Bubbling absorption and thermal regeneration were used to determine the effective CO2 capacity of AASs and MEA at high temperature. Figure 1 shows the experimental apparatus and procedure to carry out the CO2 absorption tests of the absorbents. The designed absorption temperature was maintained at 80 °C to mimic the real flue gas circumstance without cooling. The desired CO2 volumetric concentration of 12.5% in the simulated flue gas was attained by using the mass flow controllers (MFC, D07-7B, Beijing Sevenstar Electronics Co., Ltd.) to respectively control the CO2 and N2 flow rates. The total flow rate was fixed at 2 L/min. The simulated gas was bubbled into the solvent in the glass reactor vessel at ambient pressure. The solvent concentration and volume were fixed at 2 kmol/m3 and 150 ml, respectively. CO2 concentrations of inlet and outlet gas were measured with an infrared (IR) analyzer. It is expected that the absorption equilibrium reaches when this CO2 concentration difference is less than 0.5 vol%. Figure 2 presents the experimental setup of regeneration tests. The CO2 after regeneration was firstly condensed by a condenser and then flowed through the mass flow controller. The flow rate of pure CO2 stream was recorded every 10 s. Regeneration experiments were terminated when the CO2 flow rate was less than 10 ml/min. 30% MEA (i.e. 5M MEA), as a widely accepted industrial benchmark for CO2 absorption, was also used to carry out the tests for comparison. 8


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Figure 1. Experimental setup for CO2 absorption from simulated flue gas

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Figure 2. Experimental setup of regeneration tests

CO2 absorption rate, CO2 loading at any absorption time and saturated CO2 absorption loading can be determined by using the following Eqs. (4)-(6) respectively17

nCO2 ,in − RCO2 =

abs φCO , out × nN ,in abs 1 − φCO , out

αR

2

2

V t

α abs

2

∫R (t ) = 0

CO2

× 1000

(4)

dt (5)

1000C

∫ =

t0

0

RCO2 dt

(6)

1000C

where RCO2 is CO2 absorption rate, mmol/(L·s); nCO2 ,in and nN 2 ,in are CO2 and N2 abs molar flow rates of the inlet gas in mol/s, respectively; φCO represents the CO2 2 ,out

volumetric concentration of outflow gas; V is the solvent volume, L; αabs (t ) is CO2 10


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loading of solvent at any absorption time t, mol CO2 per mol absorbent (mol/mol);

α R is the saturated CO2 absorption loading, mol/mol; C represents the molar concentration of solvent, mol/L; t and t0 represent the any given absorption time and time absorption ceased in s, respectively. CO2 regeneration rate, CO2 loading of lean solvent and CO2 regeneration efficiency can be calculated by using the following Eqs. (7)-(9)17, SCO2 =

nCO2 , reg 60V

× 1000 t

α reg (t ) = α R η (%) = (1−

∫S − 0

CO2

(7)

dt (8)

1000C

αL ) × 100 αR

(9)

Where SCO2 is regeneration rate, mmol/(L·s); nCO2 ,reg is regeneration molar flow rate, mol/s; V is solvent volume; α reg (t ) is loading of any regeneration time t, mol/mol; C represents molar concentration of solvent, mol/L; α R is loading of rich solvent, mol/mol; η is regeneration efficiency, %; α L is equilibrium CO2 loading value of lean solvent in mol/mol, which can be achieved when the term t in Eq. 8 is replaced by the time regeneration terminated (t1 in s). The effective CO2 capacity ( ∆α ) in mol/mol is an important parameter to assess the comprehensive CO2 absorption–regeneration performance from flue gas. ∆α can be determined through the CO2 loading difference between after absorption and after regeneration.

∆α = α R − α L

(10)

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2.6 CO2 membrane absorption experiments at high temperatures A schematic diagram of the experimental apparatus for CO2 membrane absorption at high temperatures is shown in Figure 3. Commercially available polypropylene (PP) microporous membranes provided by Hangzhou Jiemi Enviromental Technology Co., Ltd. were used in this work. The detailed specifications of membrane are listed in Table 1. In order to control the absorption temperature, the membrane module and solvent reservoirs were immersed into a water bath. The liquid was pumped into the tube side of membrane contactor at the flow rate of 8 ml/min by a peristaltic pump. A back-pressure valve placed on the liquid outlet was employed to prevent gas bubble formation in the liquid phase. A simulated flue gas (pressure: 110 kPa) which flowed into the shell side of HFMC was prepared by diluting CO2 with N2. The flow rates of CO2 and N2 gas streams were adjusted by mass flow controllers to achieve a CO2 inlet concentration of 11% by volume. This CO2 concentration was chosen as it closely resembles that of a coal fired power station flue gas. The CO2 inlet and outlet concentrations in the gas phase were measured using the FTIR gas analyzer.

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Figure 3. Schematic diagram of the experimental setup for high temperature CO2 membrane absorption

Table 1. Specifications of hollow fiber and membrane module Item

Parameters

Value

Fiber I.D. (µm)

170

Polypropylene

Fiber O.D. (µm)

250

membrane fiber

Average membrane pore diameter (µm)

0.0317

Porosity (%)

30

Effective Length (mm)

210

Module I.D. (mm)

12

No. of fibers

50

Membrane module

3

Results and discussion

3.1 Solubility of amino acids in water We selected suitable amino acids for CO2 membrane absorption based on their 13


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solubilities in water. High water solubility allows high solute concentrations and avoids the precipitation in liquid. Figure 4 is a plot of the solubilities of 24 common amino acids in water versus molecular weight at 25 oC. The solubility values were all taken from literatures.30 These amino acids can be divided into four classes: linear amino acids; cyclic amino acids; poly amino acids; and sterically hindered amino acids. For all the amino acids investigated, the solubility in water appears to show a decreasing correlation with molecular weight in general. Of them, seven amino acids have solubilities higher than 100 g/kg water and were chosen in the surface tension measurement and screening absorption test They include one poly amino acid (L-Arginine), one sterically hindered amino acid (L-Threoine), one cyclic amino acid (L-Proline) and four linear amino acids (β-Alanine, glycine, sarcosine and taurine). Their structures are given in Figure 5.

Figure 4. Solubilities of 24 common amino acids in water at 25 oC

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Figure 5. Chemical structures of seven amino acids with good solubilities in water

3.2 Surface tension screening test Figure 6 shows the surface tensions of aqueous potassium salts of 7 selected amino acids at the temperatures of 25 to 70 oC. The surface tensions of 1.0 kmol/m3 aqueous monoethanolamine (MEA) solutions and pure water were also measured for comparison purpose. The reliability of the measurement was validated by the good agreement between the measured surface tensions of pure water and reference values at various temperatures. The surface tensions of some AASs were reported at room temperature,18 but to the best of our knowledge, the surface tensions of AASs at high temperature are not available in open literatures. In order to avoid precipitation issue for some AASs with low solubility and give a fair comparison of surface tension, all the AASs solutions was prepared as 1.0 kmol/m3. It should be noted that the potassium L-Argininate is 0.5 kmol/m3 because 15


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of the unstable surface tension measurement at above 0.5 kmol/m3. We believe that it is because surface tension measurement is very sensitive to the impurities in the L-Argininate solution. L-Arginine has a much larger molecular weight than other amino acids, and thus if 1.0 kmol/m3 of potassium L-Argininate was used, the weight concentration of impurities in aqueous L-Argininate will be high enough to affect the surface tension measurement even we bought the highest purity of L-Argininate in the market. Figure 6 shows that all the AASs solutions have much higher surface tensions than aqueous MEA. The high surface tensions will reduce the wettabilities of membrane pores in CO2 membrane absorption process. If the membrane pores are wetted by the liquid, CO2 will be partially concentrated near the pores and the mass transfer resistance of membrane phase will increase significantly.31,32 The tested 7 AASs, except potassium L-prolinate, presented higher surface tensions than pure water at 25 o

C. The aqueous potassium glycinate had the greatest surface tension. However,

surface tension is a strong decreasing function of temperature. As the temperature increased from 25 oC to 70 oC, for example, the surface tension of potassium taurinate decreased from 77.20 mN/m to 66.76 mN/m. It indicates that membrane wetting issue will be more serious at high temperatures. Therefore, choosing absorbents with high surface tensions is important for CO2 membrane absorption at high temperatures. At 70 oC, potassium glycinate and taurinate displayed the highest surface tension among all the AASs investigated.

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Figure 6. Surface tension of AASs and MEA solutions at different temperatures

3.3 CO2 membrane absorption screening experiments at room temperature CO2 membrane absorption experiments at room temperature were also conducted to screen the absorption kinetics and capacity of these seven selected AASs. In the screening test, the concentration of the aqueous AAS solutions was 1.0 kmol/m3, and the absorption temperature was 25 oC. Figure 7 plots the CO2 absorption rate per unit mole of absorbent (NCO2/nAb) versus the CO2 loading (molar ratio of CO2 to AAS) in absorbent. Potassium L-Argininate showed the fastest absorption rate and CO2 capacity among all the AAS solutions studied. The reason is that potassium L-Argininate has two active amine functional groups in its chemical structure, which can provide more available reaction site for 17


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CO2 absorption. For the other AASs with mono-amine group, their absorption rate curves were similar and potassium L-Prolinate and sarcosinate presented slightly faster absorption rate than others.

Figure 7. Plot of the CO2 absorption rate per unit mole of absorbent (NCO2/nAb) versus the CO2 loading (moles of CO2 in unit mole of absorbent) at 25 oC

In industry, it is more common to evaluate the CO2 absorption rate or capacity based on the mass of the absorbent. Figure 8 shows mass based CO2 absorption rate (NCO2/mAb) versus mass based CO2 loading. Potassium glycinate had the highest absorption rate per unit weight of absorbent, and potassium sarcosinate was second highest. Potassium L-Argininate, which had outstanding absorption rates per unit mole of absorbent, had inferior performance in per unit weight of absorbent, as shown in Figure 8.

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Figure 8. Plot of the CO2 absorption rate per unit kg of absorbent (NCO2/mAb) versus the CO2 loading (moles of CO2 in unit weight of absorbent) at 25 oC

3.4 Basic properties of selected solvents Based on the solubility of the amino acids and surface tension measurement as well as the mass based absorption test results, three AASs, potassium taurinate (PT), potassium glycinate (PG) and potassium sarcosinate (PS), were selected for further CO2 membrane absorption tests at high temperatures. PG and PS have excellent performance for both surface tension and CO2 absorption rate. For PT, although the performance of membrane absorption is not among top three, its surface tension is the second largest which is important for preventing membrane wetting. Figure 9 presents the viscosities of these three AAS solutions compared to MEA. It can be observed that the viscosity is a strong function of temperature and 19


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concentration. An increase of temperature and a decrease of concentration will result in a lower viscosity. AASs have slightly higher viscosities than MEA at the same molar concentration and temperature. With the increase of temperature, viscosity reduced significantly. For example, the viscosity of 3M PS reduced by 58% as temperature was raised from 30 to 80 oC. Smaller viscosity number indicates less CO2 mass transfer resistance in liquid phase during CO2 absorption. Therefore, increase of absorption temperature will facilitate CO2 absorption mass transfer from this point of view. Figure 10 shows the viscosities of 2M AASs and 2M MEA of varied loadings at 80 oC. It can be concluded that viscosity of solvents increased with an increase of CO2 loading. For instance, the viscosity of 2M PS increased by 15% when CO2 loading rose from 0 to 0.3 mol/mol. The effect of CO2 loading on viscosity is insignificant compared to the effect of temperature.

Figure 9. Viscosities of AASs and MEA solvents of different concentration at varied 20


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temperature

Figure 10. Viscosities of 2M AASs and 2M MEA of varied CO2 loadings at 80 oC

The absorption-desorption performance of these three AASs were analyzed before high-temperature CO2 membrane absorption test. The absorption rates at 80 °C versus loading plots for 2.0 kmol/m3 AASs, 2.0 kmol/m3 MEA and 30% MEA are presented in Figure 11(a). Under high absorption temperature condition, PS had a distinctively higher absorption rate than the other solvents and also attained the highest saturated loading of 0.42 mol/mol. PG showed a similar absorption curve with MEA but had a slightly higher initial absorption rate than MEA. PT presented a slowest absorption rate and achieved a slightly lower saturated loading than MEA at 80 °C. 30% MEA had a lower initial absorption rate than 2M PS, but higher than the other three absorbents. Its saturated loading was 0.348 mol/mol, similar with 2M PT. 21


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AASs and MEA rich solvents after absorption were regenerated at 102 °C. Their regeneration curves are compared in Figure 11(b). It is obviously seen that regeneration rate decreased with the decrease of CO2 loading. In contrast with absorption curves, PS showed the slowest desorption rate and highest lean loading after regeneration. The lowest lean loading of about 0.10 mol/mol was attained by PT, which also showed the highest regeneration rate. PS which had the highest absorption rate and highest saturated loading only could reach a lean loading of 0.25 mol/mol. PG with a similar absorption performance to MEA had a lower regeneration rate and higher lean loading. 30% MEA had higher regeneration rate than 2M MEA and achieved a lower lean loading. Figure 12 shows effective CO2 capacities and regeneration efficiencies of the four absorbents for high temperature absorption. The highest effective CO2 capacity was achieved by PT, thanks to its highest regeneration efficiency of 71%, despite its lower rich loading. Although PS had lower regeneration efficiency compared to PG, PS presented a higher effective capacity than PG because of its higher rich loading. 30% MEA showed the second highest capacity and regeneration efficiency, which were slightly higher than those of 2M MEA.

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Figure 11. Absorption and desorption curves: (a) absorption rates as a function of CO2 loading at 80 °C (b) regeneration rates as a function of CO2 loading at 102 °C

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Figure 12. Effective capacities and regeneration efficiencies of five absorbents

3.5 CO2 membrane absorption at high temperature High-temperature membrane absorption tests were carried out. Figure 13 shows the CO2 absorption rates of these three AAS solutions at the amino acid salt concentration of 2.0 kmol/m3, CO2 loadings of 0, 0.2 and 0.4 mol CO2/mol absorbent and temperatures varying from 25 to 80 °C. The variation of CO2 absorption rate with temperature depends on nature of AASs and CO2 loading, which can be explained by Eq. (11), * RCO2 =KG A(PCO2 - PCO ) 2

(11)

24


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where RCO2 is CO2 absorption rate, mol/s; KG is the overall mass transfer coefficient, * mol/(s·m2·kPa); PCO2 is the partial pressure of CO2 in flue gas, kPa; PCO is the CO2 2

equilibrium partial pressure in the solvent, kPa; A is the effective interfacial surface area, m2. Although a greater KG could be achieved at a higher absorption temperature due to the lower viscosity and faster reaction kinetics,21 the CO2 solubility of the absorbent will decrease with an increase in temperature. As the negative influence of * CO2 absorption driving force PCO2 -PCO overwhelms the positive influence of KG, a 2

decreasing function of absorption rate with the temperature will be presented. On the contrary, as the positive influence of KG is dominated, CO2 absorption rate will increase with increasing temperature.

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Figure 13. CO2 absorption rate as a function of temperature at the amino acid salt concentration of 2M and various CO2 loading (a): Potassium Taurinate; (b) Potassium Glycinate; (c) Potassium Sarcisinate.

Figure 13(a) shows the CO2 membrane absorption of 2M PT at different 26


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temperatures. It should be noted that due to the precipitation in the CO2 loaded taurinate at low temperatures, the absorption rate was thus measured from 40 °C to 80 °C for PT solution at CO2 loading of 0.4 mol/mol. As expected, higher loadings leaded to slower absorption rates. For the fresh PT solution, CO2 absorption rate varied a little as the absorption temperature increased. However, for the loaded solutions, a decreasing trend of absorption rate with increasing temperature was observed. Especially, as the absorption temperature reached 80 °C, CO2 desorption occurred in the 0.4 mol/mol taurinate solvent. CO2 membrane absorption of 2M PG was shown in Figure 13(b). For the solvents with CO2 loadings of 0 mol/mol and 0.2 mol/mol, the absorption rates increased initially before reaching a maximum value, and then declined after this point as the absorption temperature was raised. The highest CO2 absorption rates can be found at 60 °C and 40 °C, respectively. For aqueous glycinate with CO2 loading of 0.4 mol/mol, the increasing temperature resulted in a decline in absorption rate. But compared with PT in Fig. 13(a), this decline is much more slowly. Figure 13(c) shows the change of CO2 membrane absorption rate with temperature for 2M PS solvent. It has been found that the variation of absorption rate with temperature followed different patterns under three different CO2 loading conditions. For the fresh sarcosinate solvent, the CO2 absorption rate became faster as the absorption temperature rose. In the case of CO2 loading of 0.2 mol/mol, the absorption rate maintained almost constant with rising temperature. As the CO2 loading reached 0.4 mol/mol, the CO2 absorption rate of sarcosinate solvent displayed 27


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a slight decrease trend with increasing of absorption temperature. This indicated that for PS, the negative effect of high temperature was much weaker than those of PT and PG solvents. Even in some cases, elevating temperature led to improvement on CO2 absorption. Figure 14 compared the absorption rates of these three AASs at the absorption temperature of 80 °C. Obviously, PS always presented the fastest absorption rates at various CO2 loadings. Therefore, PS is the most suitable absorbent for high temperature CO2 absorption in a membrane contactor among these screened AASs.

Figure 14. CO2 absorption rate as a function of CO2 loading at the amino acid salt concentration of 2M and the temperature of 80 °C

4

Conclusions

In the present work, we have screened 24 common amino acids for their potential 28


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to absorb CO2 at flue gas temperatures in a membrane contactor. By considering their performance in terms of their solubilities in water, surface tension and CO2 absorption rate, three candidates, which were taurinate, sarcosinate and glycinate, were selected for the experiments of CO2 membrane absorption at elevated temperatures. Viscosity measurement showed that amino acid salts had larger viscosity than MEA at same concentration and temperature. Viscosities decreased with increase of temperature, indicating less CO2 absorption mass transfer resistance at high temperature. In membrane absorption experiments at elevated temperature, it has been observed that potassium sarcosinate was the most promising absorbent for high temperature absorption. Increase in temperature led to an obvious improvement on CO2 absorption rate in the fresh potassium sarcosinate solution and had a much weaker negative effect on CO2 absorption rate in the CO2 loaded potassium sarcosinate than in the other CO2 loaded amino acid salts. For future work, a more delicate model with quantitative comparing would be helpful for efficient solvent screening. Moreover, it should be more rigorous to screen solvents for high temperature absorption based on systematic long term test. Further interesting study also includes the AASs based technology which combines the simultaneous removal of CO2/SO233 and high temperature absorption together for a wide application in power plants.

Acknowledgement This work was financially supported by the National Key Projects for Fundamental Research and Development of China (2016YFB0600904) and the National Natural 29


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Science Foundation of China (No. 51276161). Zhen Wang is grateful for financial support from the Australian Government under the Australia–China Joint Coordination Group on Clean Coal Technology Partnership Fund which allows him to carry out part of work in CSIRO.

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Table 1. Specifications of hollow fiber and membrane module Item

Parameters

Value

Fiber I.D. (µm)

170

Polypropylene

Fiber O.D. (µm)

250

membrane fiber

Average membrane pore diameter (µm)

0.0317

Porosity (%)

30

Effective Length (mm)

210

Module I.D. (mm)

12

No. of fibers

50

Membrane module

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Screening test of amino acid salts for CO2 absorption at flue gas

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