Mass-Transfer Performance for CO2 Absorption by 2-(2

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Mass Transfer Performance for CO2 Absorption by 2-((2aminoethyl)amino) ethanol Solution in Rotating Packed Bed Shu-ying Wu, Liang-Liang Zhang, Baochang Sun, Haikui Zou, Xiaofei Zeng, Yong Luo, Qiang Li, and Jian-Feng Chen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03002 • Publication Date (Web): 19 Nov 2017 Downloaded from http://pubs.acs.org on November 25, 2017

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Mass Transfer Performance for CO2 Absorption by 2-((2-aminoethyl)amino) ethanol Solution in Rotating Packed Bed

Shuying Wu1, Liangliang Zhang1,*, Baochang Sun 1, Haikui Zou1, Xiaofei Zeng 1,2,*,

Yong Luo1, Qiang Li 3, Jianfeng Chen 1,2

1

Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing, 100029, PR China 2

State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing, 100029, PR China 3

Xinjiang DunHua Co., LTD., Kelamayi, 834000, PR China

Abstract: The emission of CO2 leads to serious global climate change, which has attracted increasing attention. In this work, rotating packed bed (RPB) was employed as a high effective reactor to intensify the CO2 absorption in alkanolamine solution, which mainly contained 2-((2-aminoethyl)amino) ethanol (AEEA). The effects of important operation conditions, such as high gravity level, amine solvent concentration, gas-liquid flow ratio, CO2 inlet concentration, absorption temperature and CO2 loading in amine solvent, on gas-phase volumetric mass transfer coefficient (KGa) and CO2 capture efficiency were investigated. Results indicated that the high gravity level and CO2 inlet concentration had significant effects on KGa, and the experimental value of KGa was about 1.42-2.86 kmol∙m-3∙h-1∙kPa-1 in RPB, which was an order of magnitude higher than that in a conventional packed column. Furthermore, an artificial neural network (ANN) model was applied to predict the mass transfer *

Corresponding author. Tel: +86-10-64443134; fax: +86-10-64434784. E-mail address: [email protected] and [email protected]

1

ACS Paragon Plus Environment

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performance. The predicted values from the ANN model were in good agreement with experimental data (±10%).

Keywords: Mass transfer, CO2 capture, RPB, ANN

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1 INTRODUCTION With increasing global energy consumption, carbon dioxide (CO2) emissions from industry and fossil fuel-fired power plant bring about worrying environmental impacts, especially global warming1, which greatly challenges the sustainable development of the world. Accordingly, intensive researches have focused on reducing carbon emissions into the atmosphere, hoping to solve this thorny problem. In view of the large emission reductions needed from the coal and gas power generation plants that will remain a feature of the electricity mix for the foreseeable future, it is widely believed that CO2 capture and storage (CCS) is one of the promising methods in limiting future temperature increases to 2°C, which is the consensus and goal set in Paris Agreement2,3. Nowadays, several technologies have been widely investigated for CO2 capture on the laboratory and industrial scales, such as absorption4, adsorption5,6, membrane based separation7, biological separation8 and cryogenic separation9. Among them, the absorption process seems to be the established leading candidate in industrial CO2 capture technologies due to its high absorption reactivity, high selectivity, scale-up feasibility and relatively low cost10-12. Alkanolamines, which were discovered in the late 1920s by Bottoms13, are the most commonly used absorption solvents. Among the alkanolamines, the monoethanolamine (MEA) scrubbing to capture CO2 is firstly commercialized and is regarded as a standard to evaluate the overall CO2 capture performance of various absorbents. However, the regeneration cost to capture CO2 from the flue gas of power plants is very high when using MEA scrubbing for its high regeneration temperature14-16. Thus, more and more studies are focused on the research and development of new or mixed amine absorbents with high reaction rate, high CO2 capacity and low regeneration cost. For example, the mixing amines, such

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as MEA/PZ, MEA/DETA17 and DETA/PZ18 are found to have an improvement of either the average absorption rate or saturated CO2 loading. It also has been found that 2-((2-aminoethyl)amino)-ethanol (AEEA) as a diamine offers high absorption rate combined with high net cyclic capacity, which is significantly higher than that of MEA19. Besides, the AEEA rich solution can be desorbed at a lower temperature than MEA solution, which favors the regeneration process and will greatly reduce the regeneration cost due to less water evaporated from the solution. An industrial plant with 100,000 tons CO2 capacity using this AEEA-based absorbent in conventional packed tower has been established in Xinjiang, China, to capture the CO2 from purge gas of a chemical plant. Over the last few decades, gas-liquid contact approaches such as packed tower, spray column, bubble column and tray tower have been successfully developed in CO2 capture using alkanolamines in both bench-scale investigations and pilot-scale applications20-22. However, process intensification and performance improvement of these conventional gas-liquid mass transfer processes still face great challenges, as mass transfer limitations will hinder CO2 capture performance and do not allow for device size reduction23-25. Gao et al.26 studied the removal of CO2 by MEA absorption in microporous tube-in-tube microchannel reactor, which demonstrates a new process intensification technique for CO2 capture with a relatively large throughput compared to the conventional microchannel reactors. Zanfir et al.27 proved that falling film microreactors provided intensification of CO2 absorption by increasing interfacial area and minimizing waste of liquid reactant. However, the maximum throughputs of these reported technologies are much smaller than those of conventional gas-liquid reactors, which is hard to meet the demand of industrial applications. Additionally, these reactors are hard to scale up due to their specific structure. Consequently, it is urgent

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to develop efficient and scalable alternative technologies to overcome the above drawbacks. As one of the cutting-edge process intensification technologies, Higee (High gravity) has received considerable attention. The key point of the Higee technology relies on the simulated high-gravity environment created by the centrifugal force of the rotating packed bed (RPB) 28,29. In the RPB, liquid sprays radially via the liquid distributor and then flows through the packing due to the centrifugal force under high gravity field. The liquid will be spread or split into very fine liquid elements by the packing, including droplets, threads and films, which lead to the intensification of mass transfer performance. It has been evidenced that the volumetric mass transfer coefficient in a PRB is 1 to 3 orders of magnitude higher than that in a conventional packed tower30. Consequently, the total amount of packing required in RPB can be greatly reduced, and the investment and size of the equipment will also be minimized31-33. Thus, it has been extensively explored in many industrial operations, including absorption of H2S34,35 and SO236. Nevertheless, CO2 capture by RPB using AEEA solution has not been reported, especially its process intensification, operation characteristics and model of the mass transfer performance in the RPB. The motivation of this study focuses on the mass transfer performance of the CO2 absorption process using AEEA solvent in RPB. The performance of the process, which was shown in terms of the volumetric overall mass transfer coefficient (KGa) and CO2 capture efficiency, was investigated experimentally under various conditions to evaluate the effects of operating variables, including high gravity level, gas-liquid ratio, inlet CO2 concentration, absorbent concentration, lean CO2 loading and operating temperature. In addition, an artificial neural network (ANN) model was developed to predict the CO2 absorption performance. The accuracy of the ANN

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model was examined by comparing the model results with the experimental data. Finally, the process intensification characters of RPB was evaluated by comparing the mass transfer coefficient in RPB and packed tower.

2 EXPERIMENTAL SECTION 2.1 Chemicals The 2-(2-aminoethylamino) ethanol (AEEA) was supplied by Dalian University of Technology, and diluted by de-ionized water prepared from our own laboratory. Both of CO2 and N2 gas were purchased from Beijing Ruyuanruquan Technology Co., LTD., China, with a purity of 99.9%. 2.2 Experimental procedure The experimental set-up for CO2 absorption was shown in Figure 1.

1-CO2 cylinder, 2-N2 cylinder, 3,4-gas valve, 5,6,8-gas flowmeter, 7-gas mixer, 9,14-CO2 analyzer, 10-liquid waste tank, 11-liquid valve, 12-absorbent tank, 13-pump, 15-gas outlet, 16-RPB

Figure 1. The experimental set-up for CO2 absorption The CO2 removal experiment was performed as follows. The CO2 gas from the gas cylinder was firstly mixed with the N2 gas to simulate stack gas. The flow rate of CO2 and N2 gases were controlled by gas flowmeters to adjust the CO2 content in the 6


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gas mixture within a range of 8-20% of volumetric fraction. The fed alkanolamine aqueous solution was prepared by adding a predetermined amount of alkanolamine into de-ionized water, and then pumped into the liquid inlet. The gas mixture stream as the continuous phase flowed inward from the outer edge of the RPB while the absorbent as the dispersed phase was sprayed through 4 holes (the diameter is 1 mm) in the liquid distributor at the center of the RPB and moved outward via a centrifugal force. Gas and liquid streams had a countercurrent contact in the RPB. Consequently, CO2 in the gas stream dissolved in the liquid stream, and reacted with the absorbent before the gas and liquid streams left the RPB from gas outlet and liquid outlet, respectively. The RPB packed with stainless wire mesh possesses an inner and outer diameter of 5 and 15 cm, respectively, and a height of 5.3 cm. Meanwhile, the surface area per unit volume of the packing is 650 m2/m3, and the porosity of packing is 95%. 2.3 Sample analysis The CO2 concentrations in the gas phase for both the inlet and outlet were measured by an infrared CO2 analyzer (model GXH-3010F, Beijing Huayun Analytical Instrument Research Institute). The inlet and outlet concentrations of CO2 were monitored by analyzers ranging of 0-20vol% with a resolution of 0.1% and 0-10 vol% with a resolution of 0.01% separately. The CO2 loading in absorbents and the concentration of the alkanolamine were measured by neutralization titration37.

3 RESULTS AND DISCUSSION The CO2-AEEA-H2O is a reactive system. As AEEA is a diamine containing one primary and one secondary amine group, the reactions among the system are very complicated. Based on the study of Ma’mum et al.38,39, the mechanism of absorption process is described as Eqs. (1)~(6) Diffusion of CO2 from the gas phase into liquid phase:

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CO2 (g) ⇌ CO2 (l)

(1)

Protonated alkanolamine formation: AEEA + H3 O+ ⇌ AEEAH + + H2 O

(2)

Diprotonated alkanolamine formation: AEEAH + + H3 O+ ⇌ +HAEEAH + + H2 O

(3)

Carbamate formation: − + 2AEEA + 2CO2 (l) + 2H2 O ⇌ AEEACOO− p + AEEACOOs + H3 O

(4)

Protonated carbamate formation: − − + + + − AEEACOO− p + AEEACOOs + H3 O ⇌ HAEEACOOP + HAEEACOOs + H2 O

(5) Dicarbamate formation: − − − + AEEACOO− p + AEEACOOs + 2CO2 (l) + 2H2 O ⇌ 2( OOCAEEACOO ) + 2H3 O

(6) Here, subscripts p and s denote bonding to the primary and secondary amine group, respectively. The protonation in the solution is instantaneously formed, while the formation of primary(p) and secondary(s) carbamate is also simultaneously37. Compared with MEA19, the reactions between the AEEA and CO2 are much faster, causing the physical mass transfer inside the liquid film to be the limitation process of the entire CO2 capture. According to the two-film theory, the mass flux per unit volume (𝑁𝐶𝑂2 𝑎) in terms of the overall gas-side mass transfer coefficient (𝐾𝐺 𝑎) can be written as Eq.(7) ∗ 𝑁𝐶𝑂2 𝑎 = 𝐾𝐺 𝑎 ∙ 𝑃(𝑦𝐶𝑂2 − 𝑦𝐶𝑂 ) 2

(7)

∗ where P represents the total pressure, 𝑦𝐶𝑂2 and 𝑦𝐶𝑂 are the mole fraction and 2

equilibrium mole fraction of CO2 in the gas phase, respectively. Considering a differential volume with cross-sectional area 2𝜋𝑟𝐻 and radial 8


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thickness of dr in the RPB, the mass balance can be given as Eq.(8) 𝑦𝐶𝑂2

𝑁𝐶𝑂2 𝑎 ∙ 2𝜋𝑟𝐻 ∙ 𝑑𝑟 = 𝐺𝐼 𝑑(1−𝑦

𝐶𝑂2

)

(8)

where 𝐺𝐼 is the inert gas molar flow rate, H is an axial height of the packing. Substituting Eq.(7) into Eq.(8) and giving the following equation Eq.(9) 𝑦𝐶𝑂2

∗ 𝐾𝐺 𝑎 ∙ 𝑃(𝑦𝐶𝑂2 − 𝑦𝐶𝑂 ) ∙ 2𝜋𝑟𝐻 ∙ 𝑑𝑟 = 𝐺𝐼 𝑑(1−𝑦 2

𝐶𝑂2

)

(9)

Thus the 𝐾𝐺 𝑎 can be derived from Eq.(10) by integrating the equation from rin to rout. 𝐺

𝐾𝐺 𝑎 = 𝜋𝑃𝐻(𝑟 2 𝐼

∫𝑦

2 𝑜𝑢𝑡 −𝑟𝑖𝑛 )

1 ∗ 𝐶𝑂2 −𝑦𝐶𝑂2

𝑦𝐶𝑂2

𝑑(1−𝑦

𝐶𝑂2

)

(10)

Because of the fast reaction rate between CO2 and aqueous amine solution, the absorbed CO2 in the solution is consumed immediately in the process. Therefore, ∗ 𝑦𝐶𝑂 is very close to zero and usually negligible. Then the 𝐾𝐺 𝑎 can be obtained by 2

integrating the Eq.(10) from 𝑦 = 𝑦𝑖𝑛 to 𝑦 =𝑦𝑜𝑢𝑡 and written as Eq.(11) 𝐺

𝐾𝐺 𝑎 = 𝜋𝑃𝐻(𝑟 2 𝐼

2 𝑜𝑢𝑡 −𝑟𝑖𝑛 )

𝑦𝐶𝑂2 ,𝑖𝑛 (1−𝑦𝐶𝑂2 ,𝑜𝑢𝑡 )

× [𝑙𝑛 𝑦

𝐶𝑂2 ,𝑜𝑢𝑡 (1−𝑦𝐶𝑂2 ,𝑖𝑛

𝑦𝐶𝑂2 ,𝑖𝑛

+ (1−𝑦 )

𝐶𝑂2 ,𝑖𝑛

𝑦𝐶𝑂2 ,𝑜𝑢𝑡

− 1−𝑦

𝐶𝑂2 ,𝑜𝑢𝑡

)]

(11)

where 𝑦𝑖𝑛 and 𝑦𝑜𝑢𝑡 denote mole concentration of gas phase CO2 entering and leaving the RPB, respectively. rin and rout are the inner and outer radii, respectively. Additionally, the CO2 capture efficiency (η) can be expressed by Eq. (12): (1−𝑦𝑖𝑛 )

𝑦

𝜂 = [1 − 𝑦𝑜𝑢𝑡(1−𝑦 𝑖𝑛

] × 100%

𝑜𝑢𝑡 )

(12)

3.1 Effect of high gravity level on KGa and CO2 capture efficiency The high gravity level means the multiple of centrifugal acceleration in RPB, which is equal to acceleration of gravity. The numerical amounts of gravity level can be calculated by the following equation: gravity level = ω2 r/g

(13)

The ω is the angular speed of the RPB, r is the geometric average radius of the packing and g is the acceleration of gravity (9.8 m·s-2).

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Figure 2 shows the dependence of KGa and CO2 capture efficiency on high gravity level at different inlet CO2 concentrations for a gas-liquid ratio of 150 L/L and 25 wt% alkanolamine solution at ambient temperature (about 17°C). When the high gravity level varied from 22 to 87, the KGa increases obviously, and then the increasing trend has slowed down and tends to keep constant as the high gravity level is over 87. This is because that the effectiveness of process intensification with RPB highly depends on liquid flow pattern especially liquid forms. It can be observed by visual experiments that a number of liquid forms exist in the packing: pore flow (at gravity levels of 15-60), droplet flow (at gravity levels beyond 100), and film flow that exists on the packing surface and coexists with both of pore flow and droplet flow 40,41

. As the high gravity level increases, the absorbent in RPB obtains higher

centrifugal acceleration which gives rise to enhancement of the cutting and breaking effect. This effect contributes to thinner liquid films and smaller liquid droplets, eventually generating large gas-liquid contact areas and fast interface refresh rate, which favor the mass transfer. Thus the KGa firstly increases with the increase of high gravity level. The film thickness decreases as the high gravity level increases till a point (87 in the present work) and then approaches a constant state at higher gravity level, thus the high gravity level has little effect on the KGa afterwards. It is reported that the liquid film thickness in RPBs achieving high gravity fields is usually found to average 100μm42. Furthermore, high gravity level means more energy consumption, resulting in more operating cost. Based on the KGa and energy saving, the optimal gravity level of 87 is therefore used in following study.

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300

100 3

G/L=150L/L, G=2m /h 25wt%AEEA T=290K

98

260

96

240

94

220

92

16% inlet CO2 concentration

200

90

CO2 capture efficiency(%)

280

KGa(kmolm-3h-1atm-1)

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12% inlet CO2 concentration 180

88 20

40

60

80

100

120

140

160

High gravity level

Figure 2. Effect of high gravity level on KGa and CO2 capture efficiency 3.2 Effect of gas-liquid ratio on KGa and CO2 capture efficiency Figure 3 illustrates the effect of gas-liquid ratio on KGa and CO2 capture efficiency with high gravity level of 87, absorbent concentration of 25 wt% and ambient temperature. The gas-liquid ratio is controlled by changing the liquid flow rate under a fixed gas flow rate of 2 m3/h. The curves show that the KGa decreases with increasing gas-liquid ratio. Because the increase of gas-liquid ratio will result in lower distribution of liquid per unit volume of packing, which weakens the driving force in liquid phase and leads to the decrease of mass transfer coefficient. However, a smaller gas-liquid ratio means more absorbent feeding, which may lead to increase of cycling amount of the absorbent and subsequent energy cost of the absorbent regeneration, as the gas amount and gravity level are particular values.

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High gravity level=87 25wt%AEEA, T=290K

260

98

240 96 220

94

200

92

16% inlet CO2 concentration

CO2 capture efficiency (%)

100

KGa (kmolm-3h-1atm-1)

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12% inlet CO2 concentration 180

90 100

120

140

160

180

200

G/L (LL)

Figure 3. Effect of gas-liquid ratio on KGa and CO2 capture efficiency 3.3 Effect of inlet CO2 concentration on KGa and CO2 capture efficiency Considering that the industrial stack gas contains 10-20 vol% of CO2, the inlet CO2 concentrations of 8, 12, 16, 20 vol% were studied with the high gravity level of 87, gas-liquid ratio of 150 L/L, absorbent concentration of 25 wt% and ambient temperature. As shown in Figure 4, an increased inlet CO2 concentration leads to the decrease of both the KGa and CO2 capture efficiency, but high CO2 capture efficiency can still be achieved more than 92% among the industrial stack gas concentrations. The increase of CO2 partial pressure theoretically enhances the reaction rate of CO2 with AEEA and allows more CO2 molecular to travel from gas bulk to the gas-liquid interface according to the two-film theory. This is beneficial for reducing the mass transfer resistance of the gas phase. However, the KGa represents the mass transfer rate per unit driving force, and the increase of the driving force with the CO2 partial pressure leads to the decrease of the KGa. As a whole, the increase of CO2 partial pressure has a negative effect on KGa and the CO2 capture efficiency, which is similar to many experimental results reported in the literature43,44.

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92 280 90

High gravity level=60 High gravity level=87

260

88


High gravity level=87 G/L=150 L/L, 25 wt%AEEA 94 T=290K

300


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86 8

12

16

20

Inlet CO2 concentraion(%)

Figure 4. Effect of inlet CO2 concentration on KGa and CO2 capture efficiency 3.4 Effect of absorbent concentration on KGa and CO2 capture efficiency The absorbent concentration is closely related to the KGa, which is presented in Figure 5. Based on the mechanism of mass transfer between AEEA and CO2, AEEA-CO2 is an activate system and the reaction takes place on the interface of gas and liquid phase. Thus absorbents present in high concentration enhance mass transfer and increase the liquid-side volumetric mass transfer coefficient, leading to the increase of overall mass transfer. However, the carbonation reaction will reach the reaction limit under specific conditions, the KGa tends to keep constant under relatively high absorbent concentration. For AEEA, high concentration will increase the viscosity of the absorbent, which is likely to offset the benefit of increased free-amine molecules and leads to the reduced KGa and CO2 capture efficiency. What’s more, the increased concentration of alkanolamine solution will aggravate the corrosion of equipment in the long-term operations. Thus a critical absorbent concentration value occurs in the curves, 25 wt% alkanolamine solution is obtained as the optimized condition.

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260

G/L=150 L/L yin=16%, T=290K

240

95

220 90 200


100


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High gravity level=60 High gravity level=87 180 15

20

25

30

Absorbent concentration (wt%)

Figure 5. Effect of absorbent concentration on KGa and CO2 capture efficiency 3.5 Effect of lean CO2 loading on KGa and CO2 capture efficiency In practical industry, the absorbents are regenerated from the regeneration units and then cycled to the absorption units, thus the absorbents are usually lean solutions absorbed with partial CO2. It’s meaningful to investigate the effects of lean CO2 loading on the KGa and CO2 capture efficiency to be a reference for the regeneration sections. Figure 6 shows the KGa and CO2 capture efficiency with different CO2 loadings of the absorbent. For the fresh absorbent (as absorbed small amount of CO2 in the environment, with a loading of 0.026 mol CO2/mol amine), the CO2 capture efficiency is all above 92%. For the solution with CO2 loading of 0.274 mol CO2/mol amine (nearly 26% of the saturated loading), the effectiveness of CO2 capture is a little weakened, but it still can achieve more than 80% CO2 capture efficiency. However, when the CO2 loading reaches 0.541 mol CO2/mol amine (nearly half of the saturated loading), the CO2 capture efficiency reduces to 60-73%. The lower CO2 loading means more free-amine in the solution having greater absorption capacity. According to vapor-liquid equilibrium (VLE) theory, the mole fraction in equilibrium ∗ with the bulk concentration (𝑦𝐶𝑂 ) decreases as the CO2 loading reduces, enlarging 2

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∗ the driving force (𝑦𝐶𝑂2 − 𝑦𝐶𝑂 ) for CO2 to transfer from gas phase into liquid phase 2

and then resulting in a higher KGa and CO2 capture efficiency.

100

80

200 60

160

120

80 0.0

G/L=160 L/L G/L=140 L/L

40


High gravity level=87 3 G=2m /h, 25 wt%AEEA yin=16%

240


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20

0.2

0.4

0.6

Lean CO2 loading(mol CO2/mol amine)

Figure 6. Effect of lean CO2 loading on KGa and CO2 capture efficiency 3.6 Effect of temperature on KGa and CO2 capture efficiency The absorbent temperature is also an important parameter influencing the CO2 capture efficiency, which can be observed from Figure 7. Absorbent dissolved with 0.333 mol CO2/mol amine was investigated at a high gravity level of 87, inlet CO2 concentration of 12 vol%. It is illustrated that the KGa increases over the temperature range of ambient to 323K, and the increasing trend turns to be much slower when the temperature is over 303K. This can be explained from two aspects: the reaction of CO2 with the absorbent and the dissolution of CO2 in the absorbent. On the one hand, the AEEA-CO2 system is a reversible and exothermic reaction. Increasing temperature activates reactant molecules and causes more effective collision, which speed up the reaction rate. On the other hand, according to the reaction thermodynamics, high temperature also gives rise to the reversed reaction. When the gas pressure and concentration of CO2 are decided, the dissolution of CO2 in the absorbent thermodynamically decreases with increasing temperature, holding back the mass 15


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transfer. Initially, the acceleration of absorption reaction is dominant over the drawback of dissolution of CO2 and the reverse reaction. Thus, the KGa increases obviously as the temperature increases. Then the benefits of increasing temperature to improve absorption rate offsets the adverse factors as the effects of dissolution of CO2 in the absorbent and reverse reaction rate tend to strengthen, indicating a slow increase of KGa. Therefore, the effect of temperature on KGa is a combined outcome.

200

High gravity level=87 3 G=2m /h, yin=12%

95

180 90 160

G/L=160 L/L G/L=140 L/L

140

85


100


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80 290

300

310

320

T(K)

Figure 7. Effect of temperature on KGa and CO2 capture efficiency 3.7 Comparison of KGa between the RPB and the packed column As CO2 capture using AEEA solution has been studied using packed column in previous work, Table 1 presents the comparison of KGa between the RPB in this work and the packed column. Shen et al.45 studied the mass transfer performance of AEEA-CO2 system in a packed column using θ ring packing with a diameter of 8cm and height of 120cm, which was much higher than the packing in the RPB. In both situations, the gas-liquid ratio is kept the same, ranging from 100 to 200 L/L, and the absorbents concentration is similar as well. As can be observed that the KGa values calculated from the RPB are within the range of 1.42-2.86 kmol∙m-3∙h-1∙kPa-1, which are an order of magnitude higher than that in the packed column of 0.15-0.26 16


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kmol∙m-3∙h-1∙kPa-1. As the mass transfer is greatly intensified in the RPB, it is feasible to apply RPB to CO2 capture using AEEA solution in order to scale up and reduce the equipment size. Table 1. Comparison of KGa between the RPB and the packed column Type of

Type of

Reactor

Packing

Packed column RPB

θ ring wire mesh

Amine

20% AEEA +2% MDEA

G

L

KGa

(m3∙h-1)

(L∙h-1)

(kmol∙m-3∙h-1∙kPa-1)

1.6-2.8

15-30

0.15-0.2642

2

10-20

1.42-2.86

15-30% AEEA

4 ARTIFICIAL NEURAL NETWORK (ANN) MODEL OF KGa 4.1 General description of ANN model It is of great importance to predict the mass transfer performance of CO2 capture using alkanolamine solution in RPB under practical industry situations. The gas-phase volumetric mass transfer coefficient is chosen to quantify the mass transfer between the gas and liquid in the RPB. Due to the number of influencing factors and complexity of gas-liquid mixing, mass transfer and reaction processes in high gravity-based RPB, the relationship between the performance parameters and the influencing factors are highly nonlinear and implicit. Therefore, an artificial neural network model is carried out to predict the mass transfer performance. ANN model evolves from artificial intelligence applications and is able to recognize the underlying linear and nonlinear relationships among input and output data46. Figure 8 shows brief structure of ANN model for KGa prediction. A common ANN model contains input layer, hidden layer and output layer, the layers are

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connected by transfer function, while the neurons among different layers are adjusted by weight and bias. Then the data collected are split into two parts- the training and testing data. Finally it is necessary to modify the parameters among the network to obtain an optimized ANN model using training data, and verify the model with testing data.

Figure 8. Brief structure of ANN model 4.2 ANN model of KGa for AEEA-CO2 system In the present work, back-propagation (BP) algorithm based on error-correction learning rule, which is one of the most classic training algorithms, was applied in the AEEA-CO2 system. After the validity of the developed ANN model was confirmed, the prediction of KGa for the AEEA-CO2 system was carried out using the experimental data in this work via 79 (75%) datasets for training and 27 (25%) for testing. The main influencing factors, such as high gravity level, gas-liquid ratio, inlet CO2 concentration and so on, were selected as input parameters, while the KGa was chosen as an output variable. The finally selected network was found for a network topology of 6-11-1. The parity plot in Figure 9 shows that the predicted results are in good agreement with the experimental values with a deviation less than 10%, which is in an acceptable range for mass transfer studies. 18


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Predicted KGa(kmol·m-3·h-1·atm-1)

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280 240

+10% -10%

200 160 120 120

160 200 240 280 Experimental KGa(kmol·m-3·h-1·atm-1)

Figure 9. Comparison of predicted and experimental KGa values

5 CONCLUSION In this work, the mass transfer between the CO2 and AEEA solution in a RPB was studied. The influencing factors of high gravity level, gas-liquid ratio, inlet CO2 concentration, absorbent concentration, lean CO2 loading and temperature were investigated, all of which have significant effects on the gas-phase volumetric mass transfer coefficient (KGa) and CO2 capture efficiency. Within the range of this experimental investigation, the high gravity level and temperature had an obviously positive influence while the gas-liquid ratio and lean CO2 loading had a negative effect on the KGa and capture efficiency, the optimal operations for high gravity level of 87 and absorbent concentration of 25 wt% were obtained as well. The CO2 capture efficiency in RPB are more than 90% in most situations, while an order of magnitude higher KGa can be achieved in a RPB compared with the packed column, showing good CO2 capture performance. Additionally, the predicted values of KGa from the ANN model are in good agreement with experimental data, with a deviation of ±10%.

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ACKNOWLEDGEMENTS This work was financially supported by National Key R&D Program of China (No. 2017YFB0603300) and the fundamental research funds for the central universities (JD1706). We are thankful to Prof. Yongchun Zhang in Dalian University of Technology for providing the AEEA-based absorbent, and to Prof. Shaoyun Chen in Dalian University of Technology for useful discussion on the topic of absorbent improvement and the industrial application of AEEA-based absorbent.

NOMENCLATURE Yin

inlet mole fraction of CO2

Yout

outlet mole fraction of CO2

g

acceleration of gravity, m·s-2

G

gas flow rate, m3·h-1

GI

inert gas flow rate, m3·h-1

L

liquid flow rate, L·h-1

N

rotating speed, r·min-1

r

geometric average radius of the packing, m

KGa

gas-phase volumetric mass transfer coefficient

H

axial length of the packing

rin

inner radii of the packing

rout

outer radii of the packing

Greek symbols ω

angular speed, rad·min-1

η

CO2 capture efficiency, %

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100, 195-202.

Graphic Abstract

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Mass-Transfer Performance for CO2 Absorption by 2-(2

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