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Performance of CO2 absorption in a diameter-varying spray tower
Performance of CO 2 absorption in a diameter-varying spray tower Xiaomei Wu, Yunsong Yu, Zhen Qin, Zaoxiao Zhang PII: DOI: Reference:
S1004-9541(16)30936-3 doi:10.1016/j.cjche.2017.03.013 CJCHE 776
To appear in: Received date: Revised date: Accepted date:
25 September 2016 27 February 2017 21 March 2017
Please cite this article as: Xiaomei Wu, Yunsong Yu, Zhen Qin, Zaoxiao Zhang, (2017), Performance of CO2 absorption in a diameter-varying spray tower, doi:10.1016/j.cjche.2017.03.013
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ACCEPTED MANUSCRIPT Performance of CO2 Absorption in a Diameter-varying Spray Tower* Xiaomei Wu1,2, Yunsong Yu1, Zhen Qin1,2, Zaoxiao Zhang1,2 ** 1
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School of Chemical Engineering and Technology，Xi’an Jiaotong University，Xi’an 710049，
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Shannxi，China 2
State Key Laboratory of Multiphase Flow in Power Engineering，Xi’an Jiaotong University，Xi’an
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710049，Shannxi，China
Abstract The application of spray towers for CO2 capture is a development trend in recent years.
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However, most of the previous jobs were conducted in a cylindrical tower by using a single spray nozzle, whose configuration and performance is not good enough for industrial application. To solve
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this problem, the present work proposed a diameter-varying spray tower and a new spray mode of dual-nozzle opposed impinging spray to enhance the heat and mass transfer of CO2 absorption process. Experiments were performed to investigate the mass transfer performance (in terms of the
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CO2 removal rate (η) and the overall mass transfer coefficient (KGae)) of the improved spray tower
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under various operating conditions. Experimental results showed that the liquid to gas ratio and mole ratio of MEA to CO2 are major factors which affect the absorption performance and the maximums of
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η and KGae are 94.0% and 0.574 kmol·m-3·h-1·kPa-1 respectively, under the experimental conditions. Furthermore, new correlations to predict the mass transfer coefficient of the proposed spray tower are developed in various CO2 concentrations with a Pearson Correlation Coefficient over 90%.
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Keywords carbon dioxide, absorption, diameter-varying spray tower, mass transfer
1 INTRODUCTION Global warming and the greenhouse effect have become a huge challenge for the sustainable development of the world [1-3]. It is well known that CO2 is the major greenhouse gas that contributes to global warming more than 60%, resulting in the necessity to reduce the CO2 emission [4,5]. Currently, post-combustion CO2 capture is a promising choice in the near-to middle-term, since it can be retrofitted to the existing power plants compared to the other approaches [6,7]. Among all the technologies,
Received 2016-00-00, accepted 2016-00-00. * Supported by the National Natural Science Foundation of China (51276141). ** Corresponding author. Email: [email protected] 1
ACCEPTED MANUSCRIPT chemical absorption is generally recognized as the most mature technology for industrial application, and monoethanolamine (MEA) is the most widely used absorbent [8,9].
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Conventional MEA absorption process suffers from high energy consumption due
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to its immense steam consumption in the regeneration process, leading to the extremely
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high operating cost [3]. In order to reduce the cost, apart from choosing good absorbent, it is very important to select effective reactors and proper operating conditions. The application of a spray tower instead of a packed reactor for CO2 capture is a relatively
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recent development. Javed et al. [10] studied the low-concentration CO2 spray absorption with NaOH aqueous solution and the experimental results drew a
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conclusion that the existing of nozzle has greatly improved the mass transfer performance. Kuntz et al. [11,12] compared the mass transfer efficiency of spray
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towers with packed columns for CO2 absorption into MEA solvent and declared that
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the spray tower was capable of removing CO2 from gas mixture at a higher rate than that of the packed column. Niu et al. [13,14] conducted an experiment for CO2
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absorption into MEA solution in a spray tower. The experimental results showed that the mole ratio of MEA to CO2 was the main factor for absorption performance, and the spray tower can achieve more than 95% CO2 removal rate, which verify the feasibility
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of the spray tower used in CO2 capture. Zeng et al. [15] studied the absorption of CO2 into aqueous ammonia, and found that the performance of spray towers varies with the operating parameters. The overall mass transfer coefficient was measured to provide reference data for the future industrial design. Lim et al. [16] studied the relationships between the capture efficiency and the operating parameters and also reported the optimum tower diameter for a given spray nozzle. However, most of the previous experiments were conducted in a cylindrical tower by using a single spray nozzle, whose configuration differs from that used in actual industry, making the results of these studies far from application. This paper focuses on the enhancement of CO2 absorption process by using an improved diameter-varying spray tower. As mentioned in literature, absorption in spray 2
ACCEPTED MANUSCRIPT tower mainly occurred in the nozzle exit, hence increasing the space of nozzle exit is a feasible way to improve the absorption performance. The reaction sections of the proposed spray tower are composed of two parts: the cylindrical section and the conical
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section. The existence of the conical section would increase the effective contacting
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area and gas-liquid contacting time, which will benefit the absorption performance. A
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new spray mode of dual-nozzle opposed impinging spray was also proposed to replace the single nozzle spray method, aiming to enhance heat and mass transfer performance. When droplets from two opposite spray nozzles impinge and exchange momentum in
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the center of the tower, the droplets breakup into smaller size which would cause a rapid increase of interfacial area leading to better heat and mass transfer performance.
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Experiments were carried out in the spray tower under a wide range of operating conditions to investigate the effects of various operating parameters, including CO2
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inlet concentration, total gas flow rate, liquid flow rate, MEA concentration, liquid to
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gas ratio and mole ratio of MEA to CO2 on absorption performance. The performance of the proposed spray tower was evaluated in terms of the CO2 removal rate and the
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overall mass transfer coefficient. Additionally, empirical correlations for the mass transfer coefficient of the proposed diameter-varying spray tower absorption system
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were developed to predict the experimental results.
2 EXPERIMENTAL METHOD 2.1 System description The CO2 absorption experimental setup is shown in Fig. 1(a). It mainly comprises the spray tower, the absorbent distribution system, the flue gas distribution system and the gas analyzing system. The diameter-varying spray tower is uniquely fabricated with two spray nozzles locating on the opposite side and at the upper part. Fig. 1(a) only shows one side of the gas and liquid inlets for clear process description. The detailed structure of the diameter-varying spray tower are shown in Fig 1(b). During the experiment, aqueous MEA solution is pumped through the spray nozzles (0.5 mm
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ACCEPTED MANUSCRIPT orifice diameter, 60 deg spray angle) to form droplets, then droplets from two opposite spray nozzles impinge and exchange momentum in the center of the tower. The droplets would breakup into smaller size, then contact with the gas mixture entered
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from the bottom of the tower. MEA solution is piped to the spray nozzles using a
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plunger metering pump (0.8-1.0 MPa), and the flow rate is measured with a calibrated
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rotameter. CO2 and N2 are mixed to certain concentrations (8 vol%, 12 vol%, 16 vol% or 18 vol%) before entering the absorber to act as the flue gas. The flow rate of gas mixture is controlled by a mass flow controller and the gas mixture is fed through a gas
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mixing tank to ensure a uniform distribution of species in the gas. Then the gas is introduced into the tower from two bottom inlets and reacts with MEA solution. After
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absorption, the vent gas from the top of the absorber is dried throw a drying tower with anhydrous silica gel. Then the CO2 concentration in the gas mixture is continuously
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measured at both gas inlets and gas outlets, using an infrared gas analyzer (model
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IRME-S, Xi 'an Weichuang Instrument Inc.). The reading range of the analyzer is 0-20.0% of CO2 by volume with the accuracy of 0.1% of the full-scale reading.
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Experiments are repeated to validate the reproducibility of the results.
Fig. 1. The schematic of experimental setup of CO2 absorption by aqueous MEA in the diameter-varying dual-nozzle opposed impinging spray tower (a), the geometry of proposed spray tower (b). (1. N2 cylinder; 2. CO2 cylinder; 3. Gas mixing tank; 4,8,12. Pressure gauge; 5,13. Gas flow meter; 6. dual-nozzle opposed impinging spray tower; 7. Liquid receiver; 9. Liquid flow meter; 10. Pump; 11. Feed receiver; 14. Drying tower; 15. CO2 analyzer; 16. Computer.)
2.2 Experimental conditions 4
ACCEPTED MANUSCRIPT The geometry of the diameter-varying dual-nozzle opposed impinging spray tower and the operating parameters in experiments are listed in Table 1.
Geometry 200, 120, 160
h1, h2, h3, h4 /mm
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d1, d2, d3 /mm
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Table 1 Geometry and operating parameters of the proposed spray tower. Parameter Value
50, 200, 300, 235 S1
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Gas outlet Liquid inlets
S2, S3
Gas inlets
S4, S5
Liquid outlet
S6
Orifice diameter /mm
0.5 60
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Spray angle /deg Operating conditions MEA flow rate, L /L·h-1 MEA concentration, CL /wt %
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Gas flow rate, G /m3·h-1 CO2 concentration, CG /vol %
10-40 1-5 8-18 20 0.1
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Temperature, T /℃ Pressure, P /MPa
40-120
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2.3 Mass transfer model
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The CO2 removal rate and the overall mass transfer coefficient are chosen to evaluate the absorption performance of the improved spray tower. 2.3.1 The CO2 removal rate
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The removal rate ( ) defines the percentage of CO2 in the gas stream that is removed during absorption process, and it is simply determined by the difference between the amounts of CO2 entering and leaving the spray tower, which can be expressed by the following equation:
=
Y1 Y2
100%
(1)
Y1
where the mole ratio of CO2 to N2, can be calculated by Y
y 1 y
(2)
Finally, the CO2 removal rate can be obtained by combining the Equations (1) and (2), which can be expressed by the following equation:
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y1 y2 y1 (1 y2 )
100%
(3)
2.3.2 The overall mass transfer coefficient
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The overall mass transfer coefficient ( K G ae ) is a lumped parameter that represents the absorption performance per unit volume of reactor. It is a combination of
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thermodynamics, kinetics, and hydrodynamics of CO2 absorption system. Thus, it is really necessary to introduce the overall mass transfer coefficient to qualify the mass transfer performance of the improved spray tower. The material balance of the spray
yCO )dZ GI dYCO *
2
,G
2
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K G ae P( yCO
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tower can be expressed as
2
,G
(4)
where G is the inert gas flow rate, P( yCO ,G yCO ) is the mass transfer driving force I
*
2
2
of gas phase, Z is the height of the tower, YCO ,G is the mole ratio of CO2 in gas
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2
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phase.
as
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According to equation (4), the overall mass transfer coefficient can be expressed
dYCO ,G P( yCO ,G yCO ) dZ GI
K G ae
2
*
2
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2
(5)
As it is a diameter-varying spray tower, the equivalent cross-sectional area can be
calculated by integrating cylindrical section and conical section, which is expressed as
S 1S1 2 S2
1 d12 4
2 d22 4
(6)
where λ 1, λ 2 are proportionality coefficient, d1 is the diameter of cylindrical absorption section, d2 is the equivalent diameter of the conical absorption section.
3 RESULTS AND DISCUSSIONS 3.1 Effect of liquid flow rate The effects of liquid flow rate on the CO2 removal rate and the overall mass
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ACCEPTED MANUSCRIPT transfer coefficient were investigated. As can be seen from Fig. 2, the CO2 removal rate and the overall mass transfer coefficient increase from 62.1% to 93.1% and 0.167 to 0.452 kmol·m-3·h-1·kPa-1 respectively, as the liquid flow rate increases from 40 L·h-1 to
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100 L·h-1. This give rise to the number of droplets produced by the spray nozzles
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increases and the size of droplets becomes smaller, as the liquid flow rate increases. In
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this sense, the interfacial area between the gas and liquid phases increases, leading to a better mass transfer performance between MEA and CO2 molecules. Besides, with the increase of liquid flow rate, the droplets flow rate increased and the boundary layer of
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liquid phase decreased. So the resistance for gas diffusion to the liquid phase decreased and the mass transfer performance is enhanced. As has mentioned above, both the CO2
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removal rate and the overall mass transfer coefficient increased with the liquid flow rate. However, the increasing tendency dropped rapidly at the higher range of liquid
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flow rate, this is because the reduction in droplet size by the increasing of liquid flow
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rate becomes insignificant and the increase of effective interfacial area is limited.
0.45
85
0.40
80
0.35
75
0.30
70
0.25
65
0.20
-1
90
-1
0.50
-3
95
KGae/kmol·m ·h ·kPa
CO2 removal rate/%
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flow rate.
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Hence, the mass transfer performance can not be enhanced furthermore at higher liquid
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0.15 2
3
4
5 3
6 -2
7
-1
Liquid flow rate/m · m · h
Fig. 2. Effect of liquid flow rate on the CO2 removal rate and the overall mass transfer coefficient. (CL =30 wt%, G =3 m3·h-1, T =20 ℃, CG =8 vol%)
3.2 Effect of MEA concentration Fig. 3 shows the profile of the CO2 removal rate and the overall mass transfer coefficient under different MEA concentrations. As shown in Fig. 3, the CO2 removal
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ACCEPTED MANUSCRIPT rate and the overall mass transfer coefficient increase from 84.2% to 94.0% and 0.312 to 0.472 kmol·m-3·h-1·kPa-1 respectively, as the MEA concentration increases from 10 wt% to 40 wt%. This is attributed to the fact that the increase of the MEA
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concentration yields more active MEA molecules available to diffuse toward the
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gas-liquid surface and then react with CO2 molecules, which will enlarge the reaction
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enhancement factor and lead to a better mass transfer performance. Nevertheless, from the point of industrial application, the viscosity of solution increases significantly at higher MEA concentration. As for the packed tower, the increase of liquid viscosity
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seriously affects the distribution of absorbents on the packing, which would block the absorption process. For the spray tower this side effect becomes insignificant because
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the absence of packing. However, the increase of liquid viscosity would do harm to the droplets distribution of spray nozzles and severe corrosion would occur in the
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equipment (like pipes, pumps, nozzles, and tower). These side effects would block the
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improvement of absorption performance and increase the capital cost for the maintenance. Hence, the absorption rate and cost should be balanced when increasing
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the concentration of MEA.
95.0
0.475 0.450 -1 -3
0.400
-1
0.425
90.0
KGae/kmol·m ·h ·kPa
CO2 removal rate/%
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92.5
0.375
87.5
0.350 85.0 0.325 82.5
0.300 10
15
20
25
30
35
40
MEA concentration/wt%
Fig. 3. Effect of MEA concentration on the CO2 removal rate and the overall mass transfer coefficient. (L=80 L·h-1, G =3 m3·h-1, T =20 ℃, CG =8 vol%)
3.3 Effect of gas flow rate Fig. 4 shows the effect of gas flow rate on the CO2 removal rate and the overall mass transfer coefficient. The experimental results show that when the gas flow rate
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ACCEPTED MANUSCRIPT increases from 1.0 m3·h-1 to 5.0 m3·h-1, the overall mass transfer coefficient increases from 0.150 to 0.574 kmol·m-3·h-1·kPa-1, however, the CO2 removal rate decreases from 93.5% to 86.8%. According to the gas-liquid mass transfer theory, the mass transfer
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coefficient increases with the increase of gas flow rate. This is because as the total gas
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flow rate increases, the amount of CO2 molecules available to contact and react with
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MEA molecules increased, which will lead to an increase of the overall mass transfer coefficient. However, the mole ratio of MEA to CO2 decreases with the increasing total gas flow rate, which means that more CO2 molecules will contact and react with
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limited MEA molecules bringing about a decrease of the CO2 removal rate. Therefore, in order to keep the CO2 removal rate at a higher value, it is important to maintain the
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mole ratio of MEA to CO2 at a suitable point.
0.60 0.55
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93
91
0.45
-1
0.50
-1
0.35
89
0.30
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0.40
90
-3
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CO2 removal rate/%
92
KGae/kmol·m ·h ·kPa
94
0.25
88
0.20
87
0.15
86
100
150
200
250 3
-2
300
0.10 350
-1
Gas flow rate/m · m · h
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50
Fig. 4. Effect of gas flow rate on the CO2 removal rate and the overall mass transfer coefficient. (L=80 L·h-1, CL =30 wt%, T =20 ℃, CG =8 vol%)
3.4 Effect of CO2 concentration The effect of CO2 concentration on the CO2 removal rate and the overall mass transfer coefficient was shown in Fig. 5. Experimental results show that the CO2 removal rate and the overall mass transfer coefficient decrease from 92.2% to 84.0% and 0.427 to 0.292 kmol·m-3·h-1·kPa-1 respectively, as the CO2 concentration increases from 8 vol% to 18 vol% in a fixed 80 L·h-1 liquid flow rate. In general, according to the two-film theory, the gas phase driving force and gas phase mass transfer coefficient increase with the increase of CO2 concentration, which will enhance the absorption process. Whereas, the mole ratio of MEA to CO2 decreased with the increasing CO2 9
ACCEPTED MANUSCRIPT inlet concentration, which means more CO2 molecules react with limited active MEA molecules and this will cause the reduction of CO2 removal rate. Thus, the CO2 removal rate decreased slightly with the increasing of CO2 inlet concentration in the
2
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2
T
* spray tower. Moreover, the gas phase driving force P( yCO ,G yCO ) increased with the
increase of CO2 concentration, which will lead to the decrease of the overall mass
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transfer coefficient.
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Fig. 5. Effect of CO2 concentration on the CO2 removal rate and the overall mass transfer coefficient. (CL =30 wt%, G =3 m3·h-1, T =20 ℃, L =80 L·h-1, CG =8 vol%)
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3.5 Effect of liquid to gas ratio As have been mentioned above, the liquid to gas ratio affect the absorption
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performance to some extent. The effect of liquid flow rate and inlet gas flow rate discussed above can be summarized as the effect of liquid to gas ratio. As is depicted in Fig. 6, the CO2 removal rate and the overall mass transfer coefficient increase from 62.1% to 93.1% and 0.167 to 0.452 kmol·m-3·h-1·kPa-1 respectively, as the liquid to gas ratio increases from 0.0136 to 0.0335. Due to the increase of liquid to gas ratio, the thinner boundary layer of liquid phase and larger contacting area would decrease the mass transfer resistance and accelerate the reaction process. However, under the larger values of liquid to gas ratio, the grow tendency becomes slow.
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IP
T
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Fig. 6. Effect of liquid to gas ratio on the CO2 removal rate and the overall mass transfer coefficient. (CL=30 wt%, T=20 ℃, CG=8 vol%)
3.6 Effect of mole ratio of MEA to CO2
As have been mentioned above, the mole ratio of MEA to CO2 also affect the
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absorption performance obviously. The effect of MEA concentration and CO2
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concentration discussed above can be summarized as the effect of mole ratio of MEA
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to CO2. Fig. 7 shows that in a fixed liquid to gas ratio of 0.0267, the CO2 removal rate and the overall mass transfer coefficient increase from 81.7% to 89.9% and 0.278 to 0.371 kmol·m-3·h-1·kPa-1 respectively, as the mole ratio of MEA to CO2 increases from
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6.39 mole/mole to 25.5 mole/mole. At the same liquid to gas ratio, the increase of MEA to CO2 mole ratio allows more active MEA molecules to contact and react with CO2 molecules, causing the increase of CO2 removal rate and the overall mass transfer coefficient. It can be concluded that both the liquid to gas ratio and mole ratio of MEA to CO2 are key factors which affect the performance of CO2 absorption process.
11
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IP
T
ACCEPTED MANUSCRIPT
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Fig. 7. Effect of mole ratio of MEA to CO2 on the CO2 removal rate and the overall mass transfer coefficient. (CL=30 wt%, T=20 ℃, CG=8 vol%, L/G=0.0267)
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4 MASS TRANSFER CORRELATIONS
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Mass transfer coefficient correlation is considered to be a very important parameter for the absorption column design, as well as for effectively operating and the
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prediction of experimental results. The equation of K G ae used in this paper has been widely accepted and applied in both packed columns and spray towers [17-21].
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However, K G ae varies with the types of absorber, types of packing, and operating conditions. Therefore, it is necessary to develop an effective predictive correlation of K G ae for the improved spray tower.
4.1 Development of correlations for the improved spray tower Many researchers have developed correlations to predict the mass transfer performance for different packings and systems. The total resistance of absorption process consists of gas phase resistance and liquid phase resistance, which can be presented as 1 KG
1 kG
H
kL
0
(7)
The right-hand side term of the equation represents the gas and liquid film
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ACCEPTED MANUSCRIPT resistance, respectively. When the system is controlled by the resistance in the liquid phase, the equation can be simplified as K G ae kL ae 0
T
(8)
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Furthermore, according to Astarita [22], the enhancement factor of chemical
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reaction can be expressed as ( eq )C PCO
(9)
2
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where eq represents CO2 loading of solution in equilibrium with PCO2, represents the CO2 loading in solution.
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When the gas film controls the system, the equation can be simplified as K G ae kG ae
(10)
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The correlation for overall mass transfer coefficient K G ae is expressed as K G ae L G
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b
c
(11)
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where L represents liquid flow rate, G represents gas flow rate, b and c are the coefficients. When the liquid film controls the system, the value of b is 0.3-0.7 and the value of c is only 0.06-0.08. However, when the gas film controls the system, the
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value of c increases to 0.67-0.80 [23]. Based on the correlations discussed above, the K G ae can be expressed as K G ae (
( eq )C PCO
+B）Lb G c
(12)
2
b
The relationship can be expressed by plotting the term of K G ae / L G
c
against the
term of ( eq )C / PCO . By trial and error, the optimum values of b and c were 2
found to be b = 0.68 and c = 0.075, and the plot is shown in Fig. 8. Based on linear regression analysis, the predictive correlation for K G ae for CO2 absorption into aqueous MEA in the diameter-varying dual-nozzle opposed impinging spray tower is expressed as
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ACCEPTED MANUSCRIPT K G ae L G 0.68
0.075
[A
( eq )C PCO
B]
(13)
2
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The predicted K G ae are in good agreement with the experimental results under
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varies CO2 concentrations and the corresponding values of A and B are shown in Table
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2. However, this model is provided only for the purpose of predicting unknown K G ae values based on the experimental conditions as shown in Table 1 and only for this type
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of spray tower.
b
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Fig. 8. Relationship between K G ae / L G
c
and ( eq )C / PCO
for the diameter-varying 2
dual-nozzle opposed impinging spray tower.
Table 2 Calculated coefficient for predictive mass transfer correlation. A
B
Pearson Correlation Coefficient/ %
8
0.30
0.026
90
12
0.39
0.024
92
16
0.49
0.023
90
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CO2 concentration/ vol %
5 CONCLUSIONS A diameter-varying spray tower and a new spray mode of dual-nozzle opposed impinging spray have been developed to enhance the performance of CO2 absorption process. Experiments were performed to validate its mass transfer performance, in terms of the CO2 removal rate (η) and the overall mass transfer coefficient (KGae). The experimental results indicate that the liquid to gas ratio and mole ratio of MEA to 14
ACCEPTED MANUSCRIPT CO2 are major factors affecting the absorption performance. Both the CO2 removal rate and the overall mass transfer coefficient increase with the liquid flow rate, MEA concentration, liquid to gas ratio and mole ratio of MEA to CO2 and decrease with the
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inlet CO2 concentration. However, with the increase of total gas flow rate, the overall
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mass transfer coefficient increases, but the CO2 removal rate decreases. Under the
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experimental conditions, the maximums of η and KGae are 94.0% and 0.574 kmol·m-3·h-1·kPa-1 respectively. Furthermore, new correlations were developed to predict the overall mass transfer coefficient under different CO2 concentrations for the
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diameter-varying dual-nozzle opposed impinging spray tower absorption system in this study. The predicted results are in good agreement with the experimental results, which
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can be used in the tower design, effectively operating and the prediction of
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experimental results.
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ACKNOWLEDGEMENTS Financial support of the National Natural Science Foundation of China (No.
CE P
51276141) is gratefully acknowledged. This work is also supported by the Natural Science Basic Research Plan in Shaanxi Province of China (No. 2015JQ5192) and
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“Fundamental Research Funds for the Central Universities”.
NOMENCLATURE ae
effective contacting area, m2·m-3
A
coefficient
b
coefficient
B
coefficient
C
amine concentration, kmol·m-3
c
coefficient
d1
the diameter of cylindrical absorption section, m
d2
the equivalent diameter of the conical absorption section, m
G
gas flow rate, m3·m-2·h-1
GI
inert gas flow rate, kmol·m-2·h-1
H
Henry’s coefficient, kPa·m3·kmol-1
KG
overall mass transfer coefficient of gas phase, kmol·m-2·h-1
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ACCEPTED MANUSCRIPT volumetric overall mass transfer coefficient, kmol·m-3·h-1·kPa-1
kG
gas phase mass transfer coefficient, kmol·m-2·h-1
k L0
liquid phase mass transfer coefficient, kmol·m-2·h-1
L
liquid flow rate, m3·m-2·h-1
PCO2
CO2 partial pressure, kPa
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T
K G ae
system pressure, kPa
the cross-sectional area of cylindrical absorption section, m2
S2
the equivalent cross-sectional area of the conical absorption section, m2
Y1, Y2
inlet and outlet mole ratio in gas phase
YCO2 ,G
CO2 mole ratio in gas phase
y1, y2
inlet and outlet mole fraction in gas phase
y *CO
equilibrium mole fraction of CO2
2
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CO2 mole fraction in gas phase
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yCO2 ,G
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P S1
height of the spray tower, m
CO2 removal rate
eq
CO2 loading of solution in equilibrium with PCO2(mole CO2/mole amine)
solution CO2 loading(mole CO2/mole amine)
enhancement factor of chemical reaction
1
proportionality coefficient
2
proportionality coefficient
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TE
D
Z
REFERENCES
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[1] W.M. Budzianowski, Explorative analysis of advanced solvent processes for energy efficient carbon dioxide capture by gas–liquid absorption, Int. J. Greenh. Gas Con. 49 (2016) 108-120.
[2] G. Manzolin, E. Macchi, M. Binotti, Integration of SEWGS for carbon capture in natural gas combined cycle. Part A: Thermodynamic performances, Int. J. Greenh. Gas Con. 5 (2) (2011) 200-213. [3] G.T. Rochelle, Amine Scrubbing for CO2 Capture, Science 25 (2009) 1652-1654. [4] X.M. Wu, Y.S. Yu, C.Y. Zhang, G.X. Wang, B. Feng, Identifying the CO2 Capture Performance of CaCl2-Supported Amine Adsorbent by the Improved Field Synergy Theory, Ind. Eng. Chem. Res. 53 (24) (2014) 10225-10237. [5] Y.S. Yu, Y. Li, H.F. Lu, Z.X. Zhang, Synergy Pinch Analysis of CO2 Desorption Process, Ind. Eng. Chem. Res. 50 (24) (2011) 13997-14007. [6] B.Y. Li, Y.H. Duan, D. Luebke, Advances in CO2 capture technology: A patent review, Appl. Energ. 102 (2013) 1439-1447. [7] M. Wang, A.S. Joel, C. Ramshaw, Process intensification for post-combustion CO2 capture with chemical absorption: a critical review, Appl. Energ. 158 (2015) 275-291. [8] P. Brown, B.E. Gurkan, T.A. Hatton, Enhanced gravimetric CO2 capacity and viscosity for ionic liquids with cyanopyrrolide anion, AIChE. J. 61(7) (2015) 2280-2285. 16
ACCEPTED MANUSCRIPT [9] Y.S. Yu, Y. Li, H.F. Lu, Z.X. Zhang, Multi-field synergy study of CO2 capture process by chemical absorption, Chem. Eng. Sci. 65 (10) (2010) 3279-3292. [10] K.H. Javed, T. Mahmud, E. Purba, The CO2 capture performance of a high-intensity vortex spray scrubber, Chem. Eng. J. 162 (2) (2010) 448-456. [11] J. Kuntz, A. Aroonwilas, Performance of spray column for CO2 capture application, Ind. Eng.
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Chem. Res. 47 (1) (2008) 145-153.
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[12] J. Kuntz, A. Aroonwilas, Mass-transfer efficiency of a spray column for CO2 capture by MEA, Energy Procedia 1 (1) (2009) 205-209.
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[13] Z.Q. Niu, Y.C. Guo, W.Y. Lin, Experimental study on absorption of carbon dioxide in flue gas by monoethanolamine fine spray, Proc. Chin. Soc. Electrical Eng. 32 (2010) 41-45. [14] Z.Q. Niu, Y.C. Guo, W.Y. Lin, Carbon dioxide removal efficiencies by fine sprays of MEA, NaOH and aqueous ammonia solution, J. Tsing hua Univ. 07 (2010) 1130-1134.
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[15] Z.Q. Niu, Y.C. Guo, W.Y. Lin, Comparison of capture efficiencies of carbon dioxide by fine spray of aqueous ammonia and MEA solution, Chem. J. Chinese U. 03 (2010) 514-517. [16] Y. Lim, M. Choi, K. Han K, Performance Characteristics of CO2 Capture Using Aqueous
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Ammonia in a Single-Nozzle Spray Tower, Ind. Eng. Chem. Res. 52 (43) (2013) 15131-15137. [17] K. Maneeintr, R.O. Idem, P. Tontiwachwuthikul, Comparative mass transfer performance studies of CO2 absorption into aqueous solutions of DEAB and MEA, Ind. Eng. Chem. Res. 49(6) (2010) 2857-2863.
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[18] A. Aroonwilas, P. Tontiwachwuthikul, Mass transfer coefficients and correlation for CO2 absorption into 2-amino-2-methyl-1-propanol (AMP) using structured packing, Ind. Eng. Chem.
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Res. 37(2) (1998) 569-575.
[19] A. Aroonwilas, A. Veawab, Characterization and comparison of the CO2 absorption
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performance into single and blended alkanolamines in a packed column, Ind. Eng. Chem. Res. 43(9) (2004) 2228-2237.
[20] A. Naami, M. Edali, T. Sema, R. Idem, P. Tontiwachwuthikul, Mass transfer performance of CO2 absorption into aqueous solutions of 4-diethylamino-2-butanol, monoethanolamine, and N-methyldiethanolamine, Ind. Eng. Chem. Res. 51(18) (2012) 6470-6479.
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[21] K. Fu, T. Sema, Z. Liang, Investigation of mass-transfer performance for CO2 absorption into diethylenetriamine (DETA) in a randomly packed column, Ind. Eng. Chem. Res. 51(37) (2012) 12058-12064.
[22] G. Astaria, D.W. Savage, A. Bisio, Gas treating with chemical solvents, (1983). [23] A. Benamor, M.K. Aroua, Modeling of CO2 solubility and carbamate concentration in DEA, MDEA and their mixtures using the Deshmukh-Mather model, Fluid Phase Equilib. 231(2) (2005) 150-162.
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ACCEPTED MANUSCRIPT Performance of CO2 Absorption in a Diameter-varying Spray
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Tower
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Xiaomei Wu1,2, Yunsong Yu1, Zhen Qin1,2, Zaoxiao Zhang1,2 **
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Graphical abstract:
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S1004-9541(16)30936-3 doi:10.1016/j.cjche.2017.03.013 CJCHE 776
To appear in: Received date: Revised date: Accepted date:
25 September 2016 27 February 2017 21 March 2017
Please cite this article as: Xiaomei Wu, Yunsong Yu, Zhen Qin, Zaoxiao Zhang, (2017), Performance of CO2 absorption in a diameter-varying spray tower, doi:10.1016/j.cjche.2017.03.013
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ACCEPTED MANUSCRIPT Performance of CO2 Absorption in a Diameter-varying Spray Tower* Xiaomei Wu1,2, Yunsong Yu1, Zhen Qin1,2, Zaoxiao Zhang1,2 ** 1
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School of Chemical Engineering and Technology，Xi’an Jiaotong University，Xi’an 710049，
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Shannxi，China 2
State Key Laboratory of Multiphase Flow in Power Engineering，Xi’an Jiaotong University，Xi’an
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710049，Shannxi，China
Abstract The application of spray towers for CO2 capture is a development trend in recent years.
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However, most of the previous jobs were conducted in a cylindrical tower by using a single spray nozzle, whose configuration and performance is not good enough for industrial application. To solve
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this problem, the present work proposed a diameter-varying spray tower and a new spray mode of dual-nozzle opposed impinging spray to enhance the heat and mass transfer of CO2 absorption process. Experiments were performed to investigate the mass transfer performance (in terms of the
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CO2 removal rate (η) and the overall mass transfer coefficient (KGae)) of the improved spray tower
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under various operating conditions. Experimental results showed that the liquid to gas ratio and mole ratio of MEA to CO2 are major factors which affect the absorption performance and the maximums of
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η and KGae are 94.0% and 0.574 kmol·m-3·h-1·kPa-1 respectively, under the experimental conditions. Furthermore, new correlations to predict the mass transfer coefficient of the proposed spray tower are developed in various CO2 concentrations with a Pearson Correlation Coefficient over 90%.
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Keywords carbon dioxide, absorption, diameter-varying spray tower, mass transfer
1 INTRODUCTION Global warming and the greenhouse effect have become a huge challenge for the sustainable development of the world [1-3]. It is well known that CO2 is the major greenhouse gas that contributes to global warming more than 60%, resulting in the necessity to reduce the CO2 emission [4,5]. Currently, post-combustion CO2 capture is a promising choice in the near-to middle-term, since it can be retrofitted to the existing power plants compared to the other approaches [6,7]. Among all the technologies,
Received 2016-00-00, accepted 2016-00-00. * Supported by the National Natural Science Foundation of China (51276141). ** Corresponding author. Email: [email protected] 1
ACCEPTED MANUSCRIPT chemical absorption is generally recognized as the most mature technology for industrial application, and monoethanolamine (MEA) is the most widely used absorbent [8,9].
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Conventional MEA absorption process suffers from high energy consumption due
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to its immense steam consumption in the regeneration process, leading to the extremely
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high operating cost [3]. In order to reduce the cost, apart from choosing good absorbent, it is very important to select effective reactors and proper operating conditions. The application of a spray tower instead of a packed reactor for CO2 capture is a relatively
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recent development. Javed et al. [10] studied the low-concentration CO2 spray absorption with NaOH aqueous solution and the experimental results drew a
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conclusion that the existing of nozzle has greatly improved the mass transfer performance. Kuntz et al. [11,12] compared the mass transfer efficiency of spray
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towers with packed columns for CO2 absorption into MEA solvent and declared that
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the spray tower was capable of removing CO2 from gas mixture at a higher rate than that of the packed column. Niu et al. [13,14] conducted an experiment for CO2
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absorption into MEA solution in a spray tower. The experimental results showed that the mole ratio of MEA to CO2 was the main factor for absorption performance, and the spray tower can achieve more than 95% CO2 removal rate, which verify the feasibility
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of the spray tower used in CO2 capture. Zeng et al. [15] studied the absorption of CO2 into aqueous ammonia, and found that the performance of spray towers varies with the operating parameters. The overall mass transfer coefficient was measured to provide reference data for the future industrial design. Lim et al. [16] studied the relationships between the capture efficiency and the operating parameters and also reported the optimum tower diameter for a given spray nozzle. However, most of the previous experiments were conducted in a cylindrical tower by using a single spray nozzle, whose configuration differs from that used in actual industry, making the results of these studies far from application. This paper focuses on the enhancement of CO2 absorption process by using an improved diameter-varying spray tower. As mentioned in literature, absorption in spray 2
ACCEPTED MANUSCRIPT tower mainly occurred in the nozzle exit, hence increasing the space of nozzle exit is a feasible way to improve the absorption performance. The reaction sections of the proposed spray tower are composed of two parts: the cylindrical section and the conical
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section. The existence of the conical section would increase the effective contacting
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area and gas-liquid contacting time, which will benefit the absorption performance. A
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new spray mode of dual-nozzle opposed impinging spray was also proposed to replace the single nozzle spray method, aiming to enhance heat and mass transfer performance. When droplets from two opposite spray nozzles impinge and exchange momentum in
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the center of the tower, the droplets breakup into smaller size which would cause a rapid increase of interfacial area leading to better heat and mass transfer performance.
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Experiments were carried out in the spray tower under a wide range of operating conditions to investigate the effects of various operating parameters, including CO2
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inlet concentration, total gas flow rate, liquid flow rate, MEA concentration, liquid to
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gas ratio and mole ratio of MEA to CO2 on absorption performance. The performance of the proposed spray tower was evaluated in terms of the CO2 removal rate and the
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overall mass transfer coefficient. Additionally, empirical correlations for the mass transfer coefficient of the proposed diameter-varying spray tower absorption system
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were developed to predict the experimental results.
2 EXPERIMENTAL METHOD 2.1 System description The CO2 absorption experimental setup is shown in Fig. 1(a). It mainly comprises the spray tower, the absorbent distribution system, the flue gas distribution system and the gas analyzing system. The diameter-varying spray tower is uniquely fabricated with two spray nozzles locating on the opposite side and at the upper part. Fig. 1(a) only shows one side of the gas and liquid inlets for clear process description. The detailed structure of the diameter-varying spray tower are shown in Fig 1(b). During the experiment, aqueous MEA solution is pumped through the spray nozzles (0.5 mm
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ACCEPTED MANUSCRIPT orifice diameter, 60 deg spray angle) to form droplets, then droplets from two opposite spray nozzles impinge and exchange momentum in the center of the tower. The droplets would breakup into smaller size, then contact with the gas mixture entered
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from the bottom of the tower. MEA solution is piped to the spray nozzles using a
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plunger metering pump (0.8-1.0 MPa), and the flow rate is measured with a calibrated
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rotameter. CO2 and N2 are mixed to certain concentrations (8 vol%, 12 vol%, 16 vol% or 18 vol%) before entering the absorber to act as the flue gas. The flow rate of gas mixture is controlled by a mass flow controller and the gas mixture is fed through a gas
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mixing tank to ensure a uniform distribution of species in the gas. Then the gas is introduced into the tower from two bottom inlets and reacts with MEA solution. After
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absorption, the vent gas from the top of the absorber is dried throw a drying tower with anhydrous silica gel. Then the CO2 concentration in the gas mixture is continuously
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measured at both gas inlets and gas outlets, using an infrared gas analyzer (model
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IRME-S, Xi 'an Weichuang Instrument Inc.). The reading range of the analyzer is 0-20.0% of CO2 by volume with the accuracy of 0.1% of the full-scale reading.
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Experiments are repeated to validate the reproducibility of the results.
Fig. 1. The schematic of experimental setup of CO2 absorption by aqueous MEA in the diameter-varying dual-nozzle opposed impinging spray tower (a), the geometry of proposed spray tower (b). (1. N2 cylinder; 2. CO2 cylinder; 3. Gas mixing tank; 4,8,12. Pressure gauge; 5,13. Gas flow meter; 6. dual-nozzle opposed impinging spray tower; 7. Liquid receiver; 9. Liquid flow meter; 10. Pump; 11. Feed receiver; 14. Drying tower; 15. CO2 analyzer; 16. Computer.)
2.2 Experimental conditions 4
ACCEPTED MANUSCRIPT The geometry of the diameter-varying dual-nozzle opposed impinging spray tower and the operating parameters in experiments are listed in Table 1.
Geometry 200, 120, 160
h1, h2, h3, h4 /mm
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d1, d2, d3 /mm
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Table 1 Geometry and operating parameters of the proposed spray tower. Parameter Value
50, 200, 300, 235 S1
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Gas outlet Liquid inlets
S2, S3
Gas inlets
S4, S5
Liquid outlet
S6
Orifice diameter /mm
0.5 60
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Spray angle /deg Operating conditions MEA flow rate, L /L·h-1 MEA concentration, CL /wt %
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Gas flow rate, G /m3·h-1 CO2 concentration, CG /vol %
10-40 1-5 8-18 20 0.1
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Temperature, T /℃ Pressure, P /MPa
40-120
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2.3 Mass transfer model
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The CO2 removal rate and the overall mass transfer coefficient are chosen to evaluate the absorption performance of the improved spray tower. 2.3.1 The CO2 removal rate
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The removal rate ( ) defines the percentage of CO2 in the gas stream that is removed during absorption process, and it is simply determined by the difference between the amounts of CO2 entering and leaving the spray tower, which can be expressed by the following equation:
=
Y1 Y2
100%
(1)
Y1
where the mole ratio of CO2 to N2, can be calculated by Y
y 1 y
(2)
Finally, the CO2 removal rate can be obtained by combining the Equations (1) and (2), which can be expressed by the following equation:
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y1 y2 y1 (1 y2 )
100%
(3)
2.3.2 The overall mass transfer coefficient
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The overall mass transfer coefficient ( K G ae ) is a lumped parameter that represents the absorption performance per unit volume of reactor. It is a combination of
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thermodynamics, kinetics, and hydrodynamics of CO2 absorption system. Thus, it is really necessary to introduce the overall mass transfer coefficient to qualify the mass transfer performance of the improved spray tower. The material balance of the spray
yCO )dZ GI dYCO *
2
,G
2
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K G ae P( yCO
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tower can be expressed as
2
,G
(4)
where G is the inert gas flow rate, P( yCO ,G yCO ) is the mass transfer driving force I
*
2
2
of gas phase, Z is the height of the tower, YCO ,G is the mole ratio of CO2 in gas
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2
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phase.
as
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According to equation (4), the overall mass transfer coefficient can be expressed
dYCO ,G P( yCO ,G yCO ) dZ GI
K G ae
2
*
2
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2
(5)
As it is a diameter-varying spray tower, the equivalent cross-sectional area can be
calculated by integrating cylindrical section and conical section, which is expressed as
S 1S1 2 S2
1 d12 4
2 d22 4
(6)
where λ 1, λ 2 are proportionality coefficient, d1 is the diameter of cylindrical absorption section, d2 is the equivalent diameter of the conical absorption section.
3 RESULTS AND DISCUSSIONS 3.1 Effect of liquid flow rate The effects of liquid flow rate on the CO2 removal rate and the overall mass
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100 L·h-1. This give rise to the number of droplets produced by the spray nozzles
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increases and the size of droplets becomes smaller, as the liquid flow rate increases. In
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this sense, the interfacial area between the gas and liquid phases increases, leading to a better mass transfer performance between MEA and CO2 molecules. Besides, with the increase of liquid flow rate, the droplets flow rate increased and the boundary layer of
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liquid phase decreased. So the resistance for gas diffusion to the liquid phase decreased and the mass transfer performance is enhanced. As has mentioned above, both the CO2
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removal rate and the overall mass transfer coefficient increased with the liquid flow rate. However, the increasing tendency dropped rapidly at the higher range of liquid
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flow rate, this is because the reduction in droplet size by the increasing of liquid flow
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rate becomes insignificant and the increase of effective interfacial area is limited.
0.45
85
0.40
80
0.35
75
0.30
70
0.25
65
0.20
-1
90
-1
0.50
-3
95
KGae/kmol·m ·h ·kPa
CO2 removal rate/%
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flow rate.
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Hence, the mass transfer performance can not be enhanced furthermore at higher liquid
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0.15 2
3
4
5 3
6 -2
7
-1
Liquid flow rate/m · m · h
Fig. 2. Effect of liquid flow rate on the CO2 removal rate and the overall mass transfer coefficient. (CL =30 wt%, G =3 m3·h-1, T =20 ℃, CG =8 vol%)
3.2 Effect of MEA concentration Fig. 3 shows the profile of the CO2 removal rate and the overall mass transfer coefficient under different MEA concentrations. As shown in Fig. 3, the CO2 removal
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ACCEPTED MANUSCRIPT rate and the overall mass transfer coefficient increase from 84.2% to 94.0% and 0.312 to 0.472 kmol·m-3·h-1·kPa-1 respectively, as the MEA concentration increases from 10 wt% to 40 wt%. This is attributed to the fact that the increase of the MEA
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concentration yields more active MEA molecules available to diffuse toward the
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gas-liquid surface and then react with CO2 molecules, which will enlarge the reaction
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enhancement factor and lead to a better mass transfer performance. Nevertheless, from the point of industrial application, the viscosity of solution increases significantly at higher MEA concentration. As for the packed tower, the increase of liquid viscosity
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seriously affects the distribution of absorbents on the packing, which would block the absorption process. For the spray tower this side effect becomes insignificant because
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the absence of packing. However, the increase of liquid viscosity would do harm to the droplets distribution of spray nozzles and severe corrosion would occur in the
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equipment (like pipes, pumps, nozzles, and tower). These side effects would block the
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improvement of absorption performance and increase the capital cost for the maintenance. Hence, the absorption rate and cost should be balanced when increasing
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the concentration of MEA.
95.0
0.475 0.450 -1 -3
0.400
-1
0.425
90.0
KGae/kmol·m ·h ·kPa
CO2 removal rate/%
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92.5
0.375
87.5
0.350 85.0 0.325 82.5
0.300 10
15
20
25
30
35
40
MEA concentration/wt%
Fig. 3. Effect of MEA concentration on the CO2 removal rate and the overall mass transfer coefficient. (L=80 L·h-1, G =3 m3·h-1, T =20 ℃, CG =8 vol%)
3.3 Effect of gas flow rate Fig. 4 shows the effect of gas flow rate on the CO2 removal rate and the overall mass transfer coefficient. The experimental results show that when the gas flow rate
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ACCEPTED MANUSCRIPT increases from 1.0 m3·h-1 to 5.0 m3·h-1, the overall mass transfer coefficient increases from 0.150 to 0.574 kmol·m-3·h-1·kPa-1, however, the CO2 removal rate decreases from 93.5% to 86.8%. According to the gas-liquid mass transfer theory, the mass transfer
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coefficient increases with the increase of gas flow rate. This is because as the total gas
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flow rate increases, the amount of CO2 molecules available to contact and react with
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MEA molecules increased, which will lead to an increase of the overall mass transfer coefficient. However, the mole ratio of MEA to CO2 decreases with the increasing total gas flow rate, which means that more CO2 molecules will contact and react with
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limited MEA molecules bringing about a decrease of the CO2 removal rate. Therefore, in order to keep the CO2 removal rate at a higher value, it is important to maintain the
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mole ratio of MEA to CO2 at a suitable point.
0.60 0.55
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93
91
0.45
-1
0.50
-1
0.35
89
0.30
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0.40
90
-3
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CO2 removal rate/%
92
KGae/kmol·m ·h ·kPa
94
0.25
88
0.20
87
0.15
86
100
150
200
250 3
-2
300
0.10 350
-1
Gas flow rate/m · m · h
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50
Fig. 4. Effect of gas flow rate on the CO2 removal rate and the overall mass transfer coefficient. (L=80 L·h-1, CL =30 wt%, T =20 ℃, CG =8 vol%)
3.4 Effect of CO2 concentration The effect of CO2 concentration on the CO2 removal rate and the overall mass transfer coefficient was shown in Fig. 5. Experimental results show that the CO2 removal rate and the overall mass transfer coefficient decrease from 92.2% to 84.0% and 0.427 to 0.292 kmol·m-3·h-1·kPa-1 respectively, as the CO2 concentration increases from 8 vol% to 18 vol% in a fixed 80 L·h-1 liquid flow rate. In general, according to the two-film theory, the gas phase driving force and gas phase mass transfer coefficient increase with the increase of CO2 concentration, which will enhance the absorption process. Whereas, the mole ratio of MEA to CO2 decreased with the increasing CO2 9
ACCEPTED MANUSCRIPT inlet concentration, which means more CO2 molecules react with limited active MEA molecules and this will cause the reduction of CO2 removal rate. Thus, the CO2 removal rate decreased slightly with the increasing of CO2 inlet concentration in the
2
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2
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* spray tower. Moreover, the gas phase driving force P( yCO ,G yCO ) increased with the
increase of CO2 concentration, which will lead to the decrease of the overall mass
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transfer coefficient.
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Fig. 5. Effect of CO2 concentration on the CO2 removal rate and the overall mass transfer coefficient. (CL =30 wt%, G =3 m3·h-1, T =20 ℃, L =80 L·h-1, CG =8 vol%)
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3.5 Effect of liquid to gas ratio As have been mentioned above, the liquid to gas ratio affect the absorption
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performance to some extent. The effect of liquid flow rate and inlet gas flow rate discussed above can be summarized as the effect of liquid to gas ratio. As is depicted in Fig. 6, the CO2 removal rate and the overall mass transfer coefficient increase from 62.1% to 93.1% and 0.167 to 0.452 kmol·m-3·h-1·kPa-1 respectively, as the liquid to gas ratio increases from 0.0136 to 0.0335. Due to the increase of liquid to gas ratio, the thinner boundary layer of liquid phase and larger contacting area would decrease the mass transfer resistance and accelerate the reaction process. However, under the larger values of liquid to gas ratio, the grow tendency becomes slow.
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Fig. 6. Effect of liquid to gas ratio on the CO2 removal rate and the overall mass transfer coefficient. (CL=30 wt%, T=20 ℃, CG=8 vol%)
3.6 Effect of mole ratio of MEA to CO2
As have been mentioned above, the mole ratio of MEA to CO2 also affect the
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absorption performance obviously. The effect of MEA concentration and CO2
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concentration discussed above can be summarized as the effect of mole ratio of MEA
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to CO2. Fig. 7 shows that in a fixed liquid to gas ratio of 0.0267, the CO2 removal rate and the overall mass transfer coefficient increase from 81.7% to 89.9% and 0.278 to 0.371 kmol·m-3·h-1·kPa-1 respectively, as the mole ratio of MEA to CO2 increases from
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6.39 mole/mole to 25.5 mole/mole. At the same liquid to gas ratio, the increase of MEA to CO2 mole ratio allows more active MEA molecules to contact and react with CO2 molecules, causing the increase of CO2 removal rate and the overall mass transfer coefficient. It can be concluded that both the liquid to gas ratio and mole ratio of MEA to CO2 are key factors which affect the performance of CO2 absorption process.
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Fig. 7. Effect of mole ratio of MEA to CO2 on the CO2 removal rate and the overall mass transfer coefficient. (CL=30 wt%, T=20 ℃, CG=8 vol%, L/G=0.0267)
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4 MASS TRANSFER CORRELATIONS
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Mass transfer coefficient correlation is considered to be a very important parameter for the absorption column design, as well as for effectively operating and the
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prediction of experimental results. The equation of K G ae used in this paper has been widely accepted and applied in both packed columns and spray towers [17-21].
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However, K G ae varies with the types of absorber, types of packing, and operating conditions. Therefore, it is necessary to develop an effective predictive correlation of K G ae for the improved spray tower.
4.1 Development of correlations for the improved spray tower Many researchers have developed correlations to predict the mass transfer performance for different packings and systems. The total resistance of absorption process consists of gas phase resistance and liquid phase resistance, which can be presented as 1 KG
1 kG
H
kL
0
(7)
The right-hand side term of the equation represents the gas and liquid film
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(8)
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Furthermore, according to Astarita [22], the enhancement factor of chemical
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reaction can be expressed as ( eq )C PCO
(9)
2
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where eq represents CO2 loading of solution in equilibrium with PCO2, represents the CO2 loading in solution.
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When the gas film controls the system, the equation can be simplified as K G ae kG ae
(10)
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The correlation for overall mass transfer coefficient K G ae is expressed as K G ae L G
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b
c
(11)
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where L represents liquid flow rate, G represents gas flow rate, b and c are the coefficients. When the liquid film controls the system, the value of b is 0.3-0.7 and the value of c is only 0.06-0.08. However, when the gas film controls the system, the
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value of c increases to 0.67-0.80 [23]. Based on the correlations discussed above, the K G ae can be expressed as K G ae (
( eq )C PCO
+B）Lb G c
(12)
2
b
The relationship can be expressed by plotting the term of K G ae / L G
c
against the
term of ( eq )C / PCO . By trial and error, the optimum values of b and c were 2
found to be b = 0.68 and c = 0.075, and the plot is shown in Fig. 8. Based on linear regression analysis, the predictive correlation for K G ae for CO2 absorption into aqueous MEA in the diameter-varying dual-nozzle opposed impinging spray tower is expressed as
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0.075
[A
( eq )C PCO
B]
(13)
2
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The predicted K G ae are in good agreement with the experimental results under
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varies CO2 concentrations and the corresponding values of A and B are shown in Table
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2. However, this model is provided only for the purpose of predicting unknown K G ae values based on the experimental conditions as shown in Table 1 and only for this type
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of spray tower.
b
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Fig. 8. Relationship between K G ae / L G
c
and ( eq )C / PCO
for the diameter-varying 2
dual-nozzle opposed impinging spray tower.
Table 2 Calculated coefficient for predictive mass transfer correlation. A
B
Pearson Correlation Coefficient/ %
8
0.30
0.026
90
12
0.39
0.024
92
16
0.49
0.023
90
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CO2 concentration/ vol %
5 CONCLUSIONS A diameter-varying spray tower and a new spray mode of dual-nozzle opposed impinging spray have been developed to enhance the performance of CO2 absorption process. Experiments were performed to validate its mass transfer performance, in terms of the CO2 removal rate (η) and the overall mass transfer coefficient (KGae). The experimental results indicate that the liquid to gas ratio and mole ratio of MEA to 14
ACCEPTED MANUSCRIPT CO2 are major factors affecting the absorption performance. Both the CO2 removal rate and the overall mass transfer coefficient increase with the liquid flow rate, MEA concentration, liquid to gas ratio and mole ratio of MEA to CO2 and decrease with the
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inlet CO2 concentration. However, with the increase of total gas flow rate, the overall
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mass transfer coefficient increases, but the CO2 removal rate decreases. Under the
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experimental conditions, the maximums of η and KGae are 94.0% and 0.574 kmol·m-3·h-1·kPa-1 respectively. Furthermore, new correlations were developed to predict the overall mass transfer coefficient under different CO2 concentrations for the
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diameter-varying dual-nozzle opposed impinging spray tower absorption system in this study. The predicted results are in good agreement with the experimental results, which
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can be used in the tower design, effectively operating and the prediction of
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experimental results.
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ACKNOWLEDGEMENTS Financial support of the National Natural Science Foundation of China (No.
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51276141) is gratefully acknowledged. This work is also supported by the Natural Science Basic Research Plan in Shaanxi Province of China (No. 2015JQ5192) and
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“Fundamental Research Funds for the Central Universities”.
NOMENCLATURE ae
effective contacting area, m2·m-3
A
coefficient
b
coefficient
B
coefficient
C
amine concentration, kmol·m-3
c
coefficient
d1
the diameter of cylindrical absorption section, m
d2
the equivalent diameter of the conical absorption section, m
G
gas flow rate, m3·m-2·h-1
GI
inert gas flow rate, kmol·m-2·h-1
H
Henry’s coefficient, kPa·m3·kmol-1
KG
overall mass transfer coefficient of gas phase, kmol·m-2·h-1
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ACCEPTED MANUSCRIPT volumetric overall mass transfer coefficient, kmol·m-3·h-1·kPa-1
kG
gas phase mass transfer coefficient, kmol·m-2·h-1
k L0
liquid phase mass transfer coefficient, kmol·m-2·h-1
L
liquid flow rate, m3·m-2·h-1
PCO2
CO2 partial pressure, kPa
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K G ae
system pressure, kPa
the cross-sectional area of cylindrical absorption section, m2
S2
the equivalent cross-sectional area of the conical absorption section, m2
Y1, Y2
inlet and outlet mole ratio in gas phase
YCO2 ,G
CO2 mole ratio in gas phase
y1, y2
inlet and outlet mole fraction in gas phase
y *CO
equilibrium mole fraction of CO2
2
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CO2 mole fraction in gas phase
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yCO2 ,G
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P S1
height of the spray tower, m
CO2 removal rate
eq
CO2 loading of solution in equilibrium with PCO2(mole CO2/mole amine)
solution CO2 loading(mole CO2/mole amine)
enhancement factor of chemical reaction
1
proportionality coefficient
2
proportionality coefficient
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REFERENCES
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[1] W.M. Budzianowski, Explorative analysis of advanced solvent processes for energy efficient carbon dioxide capture by gas–liquid absorption, Int. J. Greenh. Gas Con. 49 (2016) 108-120.
[2] G. Manzolin, E. Macchi, M. Binotti, Integration of SEWGS for carbon capture in natural gas combined cycle. Part A: Thermodynamic performances, Int. J. Greenh. Gas Con. 5 (2) (2011) 200-213. [3] G.T. Rochelle, Amine Scrubbing for CO2 Capture, Science 25 (2009) 1652-1654. [4] X.M. Wu, Y.S. Yu, C.Y. Zhang, G.X. Wang, B. Feng, Identifying the CO2 Capture Performance of CaCl2-Supported Amine Adsorbent by the Improved Field Synergy Theory, Ind. Eng. Chem. Res. 53 (24) (2014) 10225-10237. [5] Y.S. Yu, Y. Li, H.F. Lu, Z.X. Zhang, Synergy Pinch Analysis of CO2 Desorption Process, Ind. Eng. Chem. Res. 50 (24) (2011) 13997-14007. [6] B.Y. Li, Y.H. Duan, D. Luebke, Advances in CO2 capture technology: A patent review, Appl. Energ. 102 (2013) 1439-1447. [7] M. Wang, A.S. Joel, C. Ramshaw, Process intensification for post-combustion CO2 capture with chemical absorption: a critical review, Appl. Energ. 158 (2015) 275-291. [8] P. Brown, B.E. Gurkan, T.A. Hatton, Enhanced gravimetric CO2 capacity and viscosity for ionic liquids with cyanopyrrolide anion, AIChE. J. 61(7) (2015) 2280-2285. 16
ACCEPTED MANUSCRIPT [9] Y.S. Yu, Y. Li, H.F. Lu, Z.X. Zhang, Multi-field synergy study of CO2 capture process by chemical absorption, Chem. Eng. Sci. 65 (10) (2010) 3279-3292. [10] K.H. Javed, T. Mahmud, E. Purba, The CO2 capture performance of a high-intensity vortex spray scrubber, Chem. Eng. J. 162 (2) (2010) 448-456. [11] J. Kuntz, A. Aroonwilas, Performance of spray column for CO2 capture application, Ind. Eng.
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Chem. Res. 47 (1) (2008) 145-153.
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[12] J. Kuntz, A. Aroonwilas, Mass-transfer efficiency of a spray column for CO2 capture by MEA, Energy Procedia 1 (1) (2009) 205-209.
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[13] Z.Q. Niu, Y.C. Guo, W.Y. Lin, Experimental study on absorption of carbon dioxide in flue gas by monoethanolamine fine spray, Proc. Chin. Soc. Electrical Eng. 32 (2010) 41-45. [14] Z.Q. Niu, Y.C. Guo, W.Y. Lin, Carbon dioxide removal efficiencies by fine sprays of MEA, NaOH and aqueous ammonia solution, J. Tsing hua Univ. 07 (2010) 1130-1134.
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[15] Z.Q. Niu, Y.C. Guo, W.Y. Lin, Comparison of capture efficiencies of carbon dioxide by fine spray of aqueous ammonia and MEA solution, Chem. J. Chinese U. 03 (2010) 514-517. [16] Y. Lim, M. Choi, K. Han K, Performance Characteristics of CO2 Capture Using Aqueous
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Ammonia in a Single-Nozzle Spray Tower, Ind. Eng. Chem. Res. 52 (43) (2013) 15131-15137. [17] K. Maneeintr, R.O. Idem, P. Tontiwachwuthikul, Comparative mass transfer performance studies of CO2 absorption into aqueous solutions of DEAB and MEA, Ind. Eng. Chem. Res. 49(6) (2010) 2857-2863.
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[18] A. Aroonwilas, P. Tontiwachwuthikul, Mass transfer coefficients and correlation for CO2 absorption into 2-amino-2-methyl-1-propanol (AMP) using structured packing, Ind. Eng. Chem.
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Res. 37(2) (1998) 569-575.
[19] A. Aroonwilas, A. Veawab, Characterization and comparison of the CO2 absorption
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performance into single and blended alkanolamines in a packed column, Ind. Eng. Chem. Res. 43(9) (2004) 2228-2237.
[20] A. Naami, M. Edali, T. Sema, R. Idem, P. Tontiwachwuthikul, Mass transfer performance of CO2 absorption into aqueous solutions of 4-diethylamino-2-butanol, monoethanolamine, and N-methyldiethanolamine, Ind. Eng. Chem. Res. 51(18) (2012) 6470-6479.
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[21] K. Fu, T. Sema, Z. Liang, Investigation of mass-transfer performance for CO2 absorption into diethylenetriamine (DETA) in a randomly packed column, Ind. Eng. Chem. Res. 51(37) (2012) 12058-12064.
[22] G. Astaria, D.W. Savage, A. Bisio, Gas treating with chemical solvents, (1983). [23] A. Benamor, M.K. Aroua, Modeling of CO2 solubility and carbamate concentration in DEA, MDEA and their mixtures using the Deshmukh-Mather model, Fluid Phase Equilib. 231(2) (2005) 150-162.
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ACCEPTED MANUSCRIPT Performance of CO2 Absorption in a Diameter-varying Spray
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Tower
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Xiaomei Wu1,2, Yunsong Yu1, Zhen Qin1,2, Zaoxiao Zhang1,2 **
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CE P
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Graphical abstract:
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