CO2 Capture with Potassium Carbonate ​ ​ and Alcohol

† † † Olajumobi Akeeb ,​ Connor Junes ,​ Zachary Luedtke † ​ ​ ​ ​ ​ Swenson​ College of Science & Engineering, Department of Chemical Engineering, University of Minnesota Duluth, 1303 Ordean Ct, Duluth, Minnesota 55812, United States. ​ ​

Abstract

Traditional CO capture technologies using amine-based sorbents are very energy ​2 intensive and result in secondary environmental pollution originating from amine degradation. In this work, a low temperature, energy saving and environmentally friendly CO capture method ​2 has been investigated using potassium carbonate-alcohols-water mixtures as sorbents. The addition of certain amounts of alcohols, especially methanol to the potassium carbonate solution can significantly increase the CO absorption and desorption amounts, as well as the capture ​2 capacity. The absorbent containing 50 wt% (~90 g) methanol, ~20 g potassium carbonate, and

~90 g water has proven to be the best mixture for CO2 capture using this inorganic capture ​ ​ method. This research demonstrates a catalytic CO capture route that is promising to be ​2 economical as well as environmentally safe, and energy saving.

1. Introduction

The emission of greenhouse gases (GHG) has seen a significant increase since the start of the industrial revolution. A significant amount of attention has been drawn to the rising levels of

CO2 in the atmosphere, causing an increase in the global temperature and noticeable climate ​ ​ changes [1]. CO2 capture and storage or utilization is one of the methods that can be used to ​ ​ reduce the amount of GHG in the atmosphere, minimize their warming effects and fight climate change. Carbon capture and storage (CCS) technologies are used to reduce the emissions of

GHG by capturing the CO2 gas. Post-combustion capture removes CO2 from flue gas (usually ​ ​ ​ consisting primarily of CO2, water vapor, O2, N2, trace gases and fly ash) after the fossil fuel has ​ ​ ​ ​ ​ ​ been burned. At present, conventional coal-fired power plants combust primary coals to directly generate power [2]. Because of this, post-combustion capture is the most viable choice for existing coal-fired power plants. Currently, amine-based CO capture using aqueous monoethanolamine (MEA) mixtures ​2 is the leading technology in the industry [3]. Among the various aqueous alkanolamines including diethanolamine (DEA) and methyldiethanolamine (MDEA), MEA is considered to be the main capture solvent because of its high absorption capacity, low solvent cost and low regeneration heat requirement [4]. However, this state-of-the-art technology (MEA mixtures) has a few shortcomings, it has a high-energy requirement [4] and results in secondary environmental pollution originating from the degradation of amines [5]. The latter could potentially result in health and ecosystem damages, and this issue cannot be neglected when CO capture is widely ​2 applied all over the world [6,7]. In order to overcome the shortcomings of the traditional capture method, new technology has to be investigated. Alkali metal carbonates have been recognized as potential absorbents for CO capture [8]. ​2 Potassium carbonate (K2CO3) and sodium carbonate (Na2CO3) have high CO2 capture capacity ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ and low cost, but K2CO3 sorbents have been found to exhibit better capture performance than ​ ​ ​ ​ Na2CO3 [9]. While K2CO3 has several advantages as a CO2 absorbent, its slow reaction kinetics ​ ​ ​ ​ ​ ​ ​ ​ prevents it from being a practical reagent [10]. Although the addition of catalysts like piperazine [11,12], boric acid [13], and amine[14], have been shown to improve the reaction kinetics and efficiency[5]. This work provides a potential alternative to the traditional capture method. It involves low temperature CO2 absorption using K2CO3 as the primary capture material aided by ​ ​ ​ ​ ​ ​ organic alcohols (Methanol, Ethanol, Isopropanol and Butanol) acting as catalysts. It opens up a new avenue for energy saving and reduction in secondary pollution yielding from the capture process.

Table 1. Summary of the effects of alcohol on CO2 capture with K2CO3-based absorbent. ​ ​ ​ ​ ​ ​

Maximum CO2 Maximum CO2 CO2 capture ​ ​ ​ ​ b b b K2CO3 Solvents Absorbed Desorbed capacity ​ ​ ​ ​ ​ ​ ​

K2CO3-H2O 50.99 0.00099 50.989 ​ ​ ​ ​ ​ ​

a K2CO3-H2O-MeOH 141.62 14.22 127.40 ​ ​ ​ ​ ​ ​ ​

a K2CO3-H2O-EtOH 101.7 0.0017 101.698 ​ ​ ​ ​ ​ ​ ​

a K2CO3-H2O-Isopropanol 50.28 0.00083 50.279 ​ ​ ​ ​ ​ ​ ​

a K2CO3-H2O-Butanol 26.6 0.00164 26.598 ​ ​ ​ ​ ​ ​ ​ a ​ 50 wt% alcohols b (mmol​ CO2/mmol K2CO3) ​ ​ ​ ​ ​ ​

2. Experiment

2.1. Materials

Ethyl Alcohol (200 Proof) Isopropanol(99.9%) Methanol 1-Butanol

H2O ​ ​ K2CO3 ​ ​ ​ Air gas(10% CO2, 10% O2, 80% N2 molar concentration) ​ ​ ​ ​ ​ ​ N2 gas ​ ​

Materials listed above were used without further purification in this work. Each trial run had a total of 200g in each. Blank trials were prepared with 20g K2CO3 and 180g H2O. ​ ​ ​ Remaining trials were prepared with 20g K2CO3 and the remaining 50wt% H2O and 50wt% ​ ​ ​ specific alcohol. Air and Nitrogen gas were purchased from Praxair. Air was used as simulated atmospheric air. Nitrogen was used as a purge gas in setup shown in Figure 1. A red-y mass flow controller by vogtlin was used to control gas flow from gas cylinders to reactor. The CAI ZPA gas analyzer was used to analyze gas as the process is running so we are able to monitor gas levels being absorbed. And Monarch DataChart 2000 was used to read the data.

2.2. Procedure

CO capture tests were performed using a setup as shown in Figure 1. 200g trials were ​2 run using a 500mL batch reactor with a magnetic stirrer. Tests with and without the additions of different alcohols were performed to evaluate the effect of alcohols on absorption and desorption. All CO absorption tests were performed at 25 °C. The simulated atmospheric air ​2 was Air gas(10% CO2, 10% O2, 80% N2 molar concentration) with a flow of 500 μm/min was ​ ​ ​ ​ ​ employed for all absorption tests. The gas entering the reactor is percolated into the solution through a muffler. The CO concentration in the outlet gas was measured using a CAI ZPA gas ​2 analyzer and data recorded per second with a DataChart data recorder.

The CO desorption experiments were performed by heating the spent sorbents to 75 °C ​2 without the continued introduction of the carrier gas into the reactor. A 1000 μm/min of N2 was used to mix with desorbed CO2 gas exiting the reactor and then the mixture gas was analyzed by the gas analyzer. The measured CO2 concentration of the mixed gas was recorded by the data recorder. The cooling liquid from a cooling unit with its temperature being set to 15 °C was used to condense vapors in the condenser. The time for the desorption step was 40 min.

Figure 1: Complete Experimental Setup

3. Results and discussion

3.1. Effects of different alcohols on CO2 capture ​ ​

The CO capture experiments were performed in two stages. The first stage that will be ​2 discussed in this section was done with ~20 g of K2CO3 mixed with a 180 g solution of 50 wt% ​ ​ ​ ​ H2O (90 g) and 50 wt% alcohol (90 g). The CO2 absorption and desorption with and without the ​ ​ ​ use of methanol (MeOH), ethanol (EtOH), isopropanol (IPA), and butanol were conducted to evaluate the effect of alcohols on the CO2 capture performance using ~20 g K2CO3. Table 1 ​ ​ ​ ​ ​ above summarizes the effects of alcohol on the capture capacity, under the conditions of 2400 s o o of absorption at room temperature (~25 C)​ and 2400 s of desorption at ~75 C.​ As shown in Table ​ ​ 1, K2CO3-H2O-alcohol sorbents with 50 wt% of MeOH, EtOH, IPA and Butanol all show ​ ​ ​ ​ ​ ​ increased amounts of CO2 absorption and desorption within the 2400 s time. ​ ​ The CO2 absorption and desorption experiments were conducted in five distinct methods ​ ​ as previously mentioned above. One of them involved running a blank trial with a solution of

~20 g K2CO3 and 180 g of H2O, this allows for a baseline to be established. The four other ​ ​ ​ ​ ​ ​ methods involved using ~20 g K2CO3 mixed with 50 wt% H2O (90 g) and 50 wt% alcohol (90 g). ​ ​ ​ ​ ​ ​ From that we could establish a trend of the effects of the alcohols. At least three trials of each method were performed so we could use the average values of absorption and desorption during data analysis. From Figure 2 and Figure 3 below, it can be concluded that MeOH has the best absorption and desorption capacity. The next best alcohol for absorption as shown on Figure 2 is EtOH. The least effective alcohol in absorption is butanol. As shown on Figure 4, all alcohols excluding MeOH performed very poorly during the desorption process. The absorption as previously mentioned was conducted at room temperature for 40 minutes (2400 s). The desorption process however, involved heating the spent K2CO3 solution to ​ ​ ​ ​ o ~75 C​ for 40 minutes. The maximum amount of CO2 absorbed by the K2CO3/MeOH solution ​ ​ ​ ​ ​ ​ ​ after 2400 s was ~141.62 mmol compared to the maximum amount absorbed by the K2CO3/H2O ​ ​ ​ ​ ​ ​ solution, ~50.99 mmol. The maximum amount of CO2 desorbed by the K2CO3/MeOH solution ​ ​ ​ ​ ​ ​ after 2400 s was ~14.22 mmol compared to the maximum amount desorbed by the K2CO3/H2O ​ ​ ​ ​ ​ ​ solution, ~0.00099 mmol. These results clearly demonstrate that the addition of alcohols, especially MeOH can significantly improve the CO2 absorption and desorption of K2CO3 based ​ ​ ​ ​ ​ ​ sorbents.

Figure 2. Average Cumulative Absorption

Figure 3. Average Cumulative Desorption

Figure 4. Average Cumulative Desorption Rate

Although the quantity of CO2 desorbed is important, the desorption rate reflecting the ​ ​ CO2 kinetics is more important to the application of the sorption based CO2 capture technology ​ ​ ​ ​ [15]. Figure 4 shows the rates of the desorption based on the type of alcohol used in the solution. From the figure, it can be concluded that the addition of MeOH led to a great improvement in

CO2 desorption. ​ ​

3.2. Effects of different Methanol concentrations on CO2 capture ​ ​

K2CO3 based sorbents (~20 g K2CO3) with different MeOH concentrations from 10 to 60 ​ ​ ​ ​ ​ ​ ​ ​ wt% were used to investigate the effect of MeOH concentration on CO capture. There was a ​2 significant difference in the amount absorbed and desorbed by the 10 and 30 wt% MeOH solution as shown in table 2 below. The 60 wt% MeOH solution absorbed roughly the same amount of CO2 as the 50 wt% MeOH solution, but desorbed a significantly smaller amount of ​ ​ CO2. These results lead to the conclusion that a higher alcohol concentration increases the ​ ​ absorption and desorption amount, but if the alcohol concentration is too high ( >50 wt%), precipitation of K2CO3 in the solution during the absorption step could reduce the amount of CO2 ​ ​ ​ ​ ​ desorbed.

Table 2. Summary of the effects of different wt% MeOH on CO2 capture with K2CO3-based ​ ​ ​ ​ ​ absorbent.

Maximum CO2 Maximum CO2 CO2 capture ​ ​ ​ ​ a a a K2CO3 Solvents Absorbed Desorbed capacity ​ ​ ​ ​ ​ ​ ​

K2CO3-H2O 50.99 0.00099 50.989 ​ ​ ​ ​ ​ ​

K2CO3-H2O-10 wt% MeOH 89.44 0.0046 89.435 ​ ​ ​ ​ ​ ​

K2CO3-H2O-30 wt% MeOH 87.75 0.0027 87.747 ​ ​ ​ ​ ​ ​

K2CO3-H2O-50 wt% MeOH 141.62 14.22 127.40 ​ ​ ​ ​ ​ ​

K2CO3-H2O-60 wt% MeOH 140.06 1.35 138.71 ​ ​ ​ ​ ​ ​ a (mmol​ CO2/mmol K2CO3) ​ ​ ​ ​ ​ ​

Figure 5. Effect of different MeOH wt% on CO2 Desorption ​ ​

Figure 6. Average Cumulative Absorption with Varying Amounts of MeOH

Figure 7. Average Cumulative Desorption with Varying Amounts of MeOH

Figure 8. Average Cumulative Desorption Rate with Varying Amounts of MeOH

The rates of CO2 desorption from the spent K2CO3 solution with the varying amounts of ​ ​ ​ ​ ​ ​ alcohol excluding 50 wt% MeOH is shown in Figure 8 above. The rate of desorption observed in the figure is significantly less that the rate for 50 wt% (shown in figure 3 above). Only one trial of the 10, 30, and 60 wt% MeOH was conducted, so the conclusions made in this report are based on only one set of data for each parameter. This is not ideal but conclusions have to be made and based on what has been observed, 50 wt% MeOH in the K2CO3 solution will be the ​ ​ ​ ​ best option for CO2 capture. 50 wt% MeOH increases the amount of CO2 absorbed at room ​ ​ ​ ​ temperature by the K2CO3 solution from 50.99 mmol (blank solution) to 141.62 mmol, and ​ ​ ​ ​ increases the amount desorbed from 0.00099 mmol (blank solution) to 14.22 mmol. It absorbs and desorbes the highest amount of CO2 making it the favored catalyst to improve CO2 capture. ​ ​ ​ ​

Conclusion

As shown above 50 wt% MeOH in the K2CO3 solution has proven to be the best mixture ​ ​ ​ ​ for CO2 capture using this inorganic capture method. The 50 wt% MeOH solution increases the ​ ​ amount of CO2 absorbed at room temperature by the K2CO3 solution from 50.99 mmol (blank ​ ​ ​ ​ ​ ​ solution) to 141.62 mmol, and increases the amount desorbed from 0.00099 mmol (blank solution) to 14.22 mmol. These results demonstrate that the addition of MeOH can significantly ​ improve the CO2 absorption and desorption of K2CO3 based sorbents. This research demonstrates ​ ​ ​ ​ ​ ​ a catalytic CO capture route that is promising to be economical as well as environmentally safe, ​2 and energy saving. References

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Low-energy-consumption and environmentally friendly CO2 capture via blending ​ ​ alcohols into amine solution. Applied Energy 254 (2019) 113696, https://doi.org/10.1016/j.apenergy.2019.113696