Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 63 (2014) 781 – 786
GHGT-12
Intensification of CO2 Stripping from Amine Solutions by Ultrasonic Jiru Ying*, Dag A. Eimer, Anette Mathisen, Henriette Sørensen, Hans Aksel Haugen Tel-Tek, Porsgrunn 3918, Norway
Abstract Ultrasound was introduced to enhance CO2 stripping from loaded amine solutions in this work. The effects of ultrasound on desorption of CO2 from loaded amine solutions, i.e. MEA (Monoethanolamine) and MDEA (Methyldiethanolamine), at various temperature, amine concentration, CO2 loading and energy input of ultrasound were investigated at ambient pressure. Energy consumption of CO2 stripping assisted by ultrasound was estimated. The data are useful as a platform for further ultrasonic related research in CO2 capture plant. © 2014 Published by Elsevier This isLtd. an open access article under the CC BY-NC-ND license 2013Authors. The Authors. Published by Ltd. Elsevier © The (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility the Organizing Committee of GHGT-12 Selection and peer-reviewofunder responsibility of GHGT. Keywords: Ultrasound; CO2 stripping enhancement; Amine solutions
1. Introduction One of the main challenges for post combustion CO2 capture is the large energy consumption in the regeneration unit. This contributes to high capture costs and is considered to be one of the main economical challenges for large scale post combustion CO2 capture. Ultrasound can be used for medical imaging, detection, measurement and cleaning, and also for enhancement of mass transfer rates in biological and other industrial field [1-4]. In the present work, ultrasound was introduced to enhance CO2 stripping from loaded amine solutions.
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1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Organizing Committee of GHGT-12 doi:10.1016/j.egypro.2014.11.087
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Ultrasound leads to cavitation and nucleation in the liquid and thus the formation of bubbles. Once formed, it could be relatively easy for bubbles to grow as more gas diffuses to the bubbles and becomes part of the bubble. In this way ultrasound makes it easy for gas to escape in the form of bubbles. Using ultrasound is expected to contribute to a significant reduction of the CO2 loading in lean amine absorbent without increasing the specific energy input in reboiler (kg steam/kg CO2 desorbed). Therefore a reduction of equipment sizes is expected, and the enhanced desorption at lower temperatures which will lead to a reduction of the CO2 capture cost. Furthermore the problems related to degradation of amine solvents may be reduced. In this work, the effects of ultrasound on desorption of CO2 from loaded amine solutions (MEA (Monoethanolamine) and MDEA (Methyldiethanolamine))at various values of parameters (temperature, amine concentration, CO2 loading, energy input) were investigated at ambient pressure. Nomenclature α CO2 loading in amine solution, (mol CO2 / mol amine) MEA Monoethanolamine MDEA Methyldiethanolamine
2. Experimental Section 2.1. Experimental set-up A semi-batch rig, seen in figure 1, was employed to investigate the effect of ultrasound on desorption of CO2 from amine solutions. In the rig, the loading of amine solution is a batch operation and CO 2 outlet is continuous. 50mL CO2 loaded solution in the flask is heated by an oil bath to a desired temperature, and then treated by an ultrasonic processor (Hielscher, UP200Ht) with a frequency 26 kHz. The desorbed CO2 from the solution passes through a condenser and a buffer/scrubbing bottle, where vapours of water and amine are removed, and the gas temperature drops to room temperature. Then the degassed CO2 flow rate is measured by a digital mass flow meter (Top-Trak, Sierra 820). All these data including degassing flow rate, liquid temperature, room temperature, and the gas pressure in the flask are recorded by a data logger and collected by a computer. Lastly, the degassed CO 2 is collected into a gas bag which can be used to verify measurement of the rate of CO2 desorption. Liquid analysis can be conducted by the method from Weiland and Trass [5] for determining the CO2 loading before and after the experiment. The rig is easy to operate, it has good accuracy and repeatability, low solution requirement implying quick preparation of runs thus enabling many runs. Its operation is low cost, and results can be obtained quickly.
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Figure 1. Semi-batch apparatus for ultrasound assisted stripping of CO2.
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2.2. Reagents and sample preparation Table 1 presents the sample description of the chemicals used. The water from a Milli-Q integral water purification system (18.2 MΩ cm) was degassed using the rotary evaporator. The binary mixtures (amine + H2O) were prepared by mixing degassed water and amine using the analytical balance with an accuracy of ±1∙10-6 kg. Table 1. Chemical Sample Descriptions Name
CAS-no.
Manufacturer
Initial mole purity
MEA
141-43-5
Sigma Aldrich
≥ 0.95
MDEA
105-59-9
Sigma Aldrich
≥ 0.99
CO2
124-38-9
AGA
0.9999
The selected unloaded amine samples were introduced by bubbling CO2 using a glass pipe with a sinter to prepare the high loaded amine samples. The loaded amine sample was analysed by the method from Weiland and Trass [5] which measures CO2 loading through the reaction of BaCl2 and CO2 and thus forming BaCO3 precipitate. 3. Results and discussions The experiments were performed on 30 wt% and 70 wt% MEA solutions with a 0.51 loading (mole CO2/ mole MEA) of CO2 and on 50 wt% MDEA solutions with a 0.45 loading. The CO2 stripping processed by only heat or by heat + ultrasound (26 kHz). 3.1. Comparison of heat only and ultrasound assisted for desorption Figure 2 shows the CO2 desorption behaviour in a 50 mL 50 wt% MDEA solution when it is heated from 40 to 60 °C over a period of 12 min, and the CO2 stripping flow rate was approximate 0.015 sL/min when the temperature is 60 °C. However, in Figure 3, it can be found that the CO2 stripping flow rate reached 3.9 sL/min for the same solution assisted by ultrasonic at 60 °C. It can be seen that CO2 desorption amount by heating is only 0.057 sL over 12 min, but the CO2 desorption amount treated by ultrasonic reached 0.171 sL in 5 s. This typical experiments show that the effects of ultrasound on the stripping of CO2 is very significant. 4.0
To integrate the flow rate-time curve, the CO2 desorption amount:0.0571sL
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Figure 2. CO2 desorption behaviour from 50 mL 50 wt% MDEA solution under heating from 40 to 60 °C
1015 0
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20
Time (s)
Figure 3. CO2 desorption behaviour from 50 mL 50 wt% MDEA solution treated by ultrasound at 60 °C, 50 W
Figure 4 shows the CO2 desorption behaviour from the same solution as above during the second heating period. The temperature is increased from 61.5 to 64 °C after the first ultrasonic treatment shown in Figure 3. Figure 5 shows the CO2 desorption from the solution treated by ultrasound at 64 °C. The results mirror the first observation,
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the CO2 desorption rate when the solution is treated with ultrasound is considerably higher than when the same solution is only subjected to heat. The situation is the same for MEA solutions, desorption of CO2 due to ultrasound is considerably stronger than from heat only. These results manifest that ultrasound could enhance the CO2 desorption from the aqueous amines solutions significantly. 0.0025
5
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68
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o
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Figure 4. CO2 desorption behaviour from 50 mL 50 wt% MDEA solution under heating from 61.5 to 64 °C
Figure 5. CO2 desorption behaviour from 50 mL 50 wt% MDEA solution treated by ultrasonic at 64 °C, 50 W
3.2. The effect of ultrasonic intensity on the desorption Figure 6 shows the effects of ultrasonic intensity on desorption of CO2 from a loaded 70 wt% MEA solution at 90 °C. The intensities shown are 50 and 100 W and when comparing the results, it is clear that the CO2 desorption rate increases when increasing the ultrasonic intensity as expected. The flow rate of CO2 desorbed at 100 W reached 3.10 sL/min, and 1.38 sL/min at 50 W.
100W 70MEA 50W 70MEA
60c 50W 50MDEA 70c 50W 50MDEA
6
Flow rate (sL/min)
Flow rate (sL/min)
3
2
1
4
2
0
0 0
10
20
30
40
Time (s)
Figure 6. The effect of ultrasonic intensity on the CO2 desorption from 30 mL loaded 70 wt% MEA solution at 90 °C
0
5
10
15
20
25
Time (s)
Figure 7. The effect of temperature on the CO2 desorption from 50 mL loaded 50 wt% MDEA solution, 50 W
3.3. The effect of solution temperature on the desorption The effect of solution temperature on the desorption rate of CO2 from 50 mL loaded 50 wt% MDEA solution
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treated with ultrasound (50 W) is shown in Figure 7. The result shows that the flow rate of CO2 desorbed at 70 °C is over 6 sL/min (the actual value was beyond the range of the flow meter, cannot be logged), and the value at 60 °C is 3.83 sL/min. This because the reverse reaction for producing CO2 is high at high temperature and the free CO2 amount in liquid at higher temperature is higher than that of at low temperature. 3.4. The effect of CO2 loading on the desorption Figure 8 shows the influence of CO2 loading on the CO2 desorption rate. The solution, a 50 mL 50 wt% MDEA solution with variable CO2 loading, is treated with ultrasound at 50 W and 64 °C. The result shows that the flow rate of CO2 desorption increasing with the increases CO2 loading of the solution.
50W 50W 50W 50W
4
50wt MDEA,0.42 loading 50wt MDEA,0.39 loading 50 wt MDEA,0.35 loading 50wt MDEA,0.25 loading
70 MEA 0.51 loading 50mL,85C,50W 30 MEA 0.51 loading 50mL,85C,50W
3
Flow rate (sL/min)
Flow rate (sL/min)
3
2
1
2 amount CO2: 0.238 sL
1
amount CO2: 0.072 sL
0
0
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Figure 8. The effect of CO2 loading of the solution on the CO2 desorption from 50 mL 50 wt% MDEA solution, 50 W, 64 °C
0
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Figure 9. Comparison aqueous MEA solutions with varying MEA concentrations, α = 0.5, 50 W, 85 °C
3.5. The effects of MEA concentration on the desorption Figure 9 shows the CO2 desorption rate dependence on MEA concentration. Two aqueous MEA solutions with 30 and 70 wt% MEA both with 0.51 CO2 loading was treated by ultrasound at 50 W and 85 ̊C. It is interesting to note that the CO2 desorption rate from the 30 wt% MEA solution is higher than that of the 70 wt% MEA solution during the ultrasound treatment. This may be because that the viscosity of 70 wt% MEA solution at 85 °C is too high and causes the diffusion of CO2 in the bubbles is slow and the CO2 stripping becomes more difficult than low viscosity solution. However, the CO2 desorption amount is considerable for the 70 wt% MEA than 30 wt% MEA solution. This is because the 70 wt% MEA solution has higher CO2 capacity and consequently more CO2 can be desorbed during ultrasonic treatment, but due to the viscosity of the solution the release of bubbles will take longer. 3.6. Energy consumption Actually, ultrasound as employed in this work does not change the thermodynamics of the system. Free gas in the solution ready to escape must be available for the ultrasonic stripping. The reverse reaction from carbamate to CO2 (aq) could be accelerated due to a larger amount of CO2 (aq.) transferring to the gas phase. A lot of experiments have been done to estimate the energy consumption of CO2 stripping assisted by ultrasound. Figure 10 shows that the trend of energy consumption per kg CO2 stripping vs. ultrasonic energy input in the solutions. It indicates that too much ultrasonic energy input will lead to a situation where most of the ultrasonic energy acts as heating which is an expensive way of providing desorption energy.
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Cons. energy (MJ/kg CO2)
12 10 8 6 4 2 0 0
100
200
300
400
500
600
Net energy input in the liquid by US (Ws)
Figure 10. Energy consumption of CO2 stripping vs. ultrasonic energy input for aq. 30 wt% MEA, 0.25 initial loading, 100 °C
4. Conclusions Ultrasound can enhance CO2 desorption from amine solutions significantly. The stripping rate and the amount of CO2 desorbed increase with increasing temperature, energy input of ultrasound and CO 2 loading, however, the stripping rate decreases with an increase of concentration of amine solution due to a higher viscosity causing more difficulty in formation of bubbles and CO2 diffusion. The introduction of ultrasound in the solution should be properly, too much ultrasonic energy input will lead to a situation where most of the ultrasonic energy acts as heating and causes a low efficiency of ultrasonic enhancement. Acknowledgements This work was funded by Norwegian Research Council, Statoil ASA, Norway (project No. 2212050). References 1.
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