Effect of Temperature and Gas Flow Rate on CO2 Capture

Accepted: 15 Dec. 2021 In this study, the removal of carbon dioxide (CO2) that has a huge contribution to global warming from gas emissions was performed using absorption method. Effect of operational parameters such as temperature and gas flow rate on the absorption capacity (g CO2/kg solvent and mol CO2/mol MEA) was investigated in a bubble column reactor with a semi-batch operation. The monoethanolamine (MEA) was used as a solvent and absorption capacity was determined at different gas flow rates (2.5 and 5 L/min) and temperatures (25-35-45 oC). Because of the study, absorption capacities of 74.71 g CO2 / kg MEA and 0.51 mol CO2/mol MEA were obtained at 5 L / min gas flow rate, 25oC and 20% solvent concentration. The absorption capacity increases as the temperature decreases and as the flow rate increases. Because of the study, it can be concluded that the capture of CO2 into the MEA solution at bubble column can be successfully achieved with high absorption capacity.


INTRODUCTION
Because of the increasing industrialization and the need for energy in the world, our dependence on fossil fuels has increased, which causes an increase in carbon dioxide concentration in the atmosphere. According to NASA, over the past 171 years, the global average amount of carbon dioxide is raised with human activities by 48% above pre-industrial levels found in 1850 and hit a record high in 2021 as 416 parts per million (NASA, 2021). Carbon dioxide (CO2) is an important greenhouse gas that causes global warming effects (NASA, 2021). To reduce global warming, reduction of CO2 emissions is critical and considering this energy needs supplied from fossil fuels, CCS (carbon capture storage) is the best way to maintain CO2 level under control.
Various methods have been studied to reduce postcombustion CO2 emissions, such as chemical absorption (Heydarifard et al., 2020;Pang et. al., 2021), physical absorption (Elhambakhsh et al., 2020), membrane separation (Widakdo et al., 2021;Yu et al., 2021), adsorption (Wu et al., 2021), cryogenic separation (Bi and Ju, 2021), algal system (Judd et al., 2015). In these processes, chemical absorption is extensively used for the removal of CO2 from flue gas by chemical reaction with a solvent (Chen et al., 2008). The drawbacks of CO2 adsorption method include adsorption mechanism only occurring on the material surface, and that regeneration would require a stoppage to the adsorption process. Cryogenic CO2 separation also requires significant amounts of energy for the cooling of CO2 down to its liquefaction temperature at 78.5 °C. CO2 capture with a membrane is an interesting field of study but it still faces challenges in the form of the unavoidable tradeoff between permeability and selectivity, as well as poor resistance towards acid gases (Wibowo et al., 2021).
Absorption-based technology is the most widely used worldwide, accounting for 64% of total CO2 capture capacity in these facilities and is used in 72% of the facilities. Aminebased solvent alone is used in 69% of the facilities, accounting for 55% of total CO2 capture capacity. The popularity of aminebased absorbent, is most due to its high absorption efficiency, low energy consumption, high economic value, and simple operation (Peng et al., 2012;Wibowo et al., 2021). In the field of CO2 absorption, innovations can be made on the searching solvents and scrubber types. Several scrubbers are used to capture CO2, such as packed bed column, sieve tray column and bubble column. Bubble columns have several advantages, such as high mass and heat transfer rates, simple operation without any moving parts and the flexibility to adjust various residence time requirements.
Various solvents are used in the bubble column for CO2 absorption based on published data. Heydarifard et al. focus on CO2 reactive absorption using Piperazine aqueous solutions in a stirrer bubble column (Heydarifard et al., 2018). Bai and Yeh (1997) studied the CO2 capture by ammonia in bubble column reactor. In a stirred bubble column, Pashaei et al. (2017) investigated the solubility and chemical absorption rate of carbon dioxide into aqueous solutions of diethanolamine. Monoethanolamine (MEA) is frequently used to evaluate experimental setups and as a reference. Aronu et al. (2011) and Maneeintr et al. (2009)  In this study, the absorption capacity of MEA solution was investigated in a bubble column reactor. The main objective of this study is to determine the effect of temperature and gas flow rate on absorption capacity given as g CO2/kg solvent and mol CO2/mol MEA.

REACTION MECHANISM
MEA is an important solvent in the CO2 removal process because it reacts quickly with carbon dioxide due to its primary amine characteristics (Maceiras et al., 2008).
Different reaction mechanisms are proposed based on the number of amine functionality. Two different mechanism named zwitterion and termolecular mechanism is proposed to describe reaction mechanisms between CO2 and primary amine (RNH2) solutions. Because of the reaction between CO2 and MEA solution, the carbamate (RNHCOO − ) is formed in two steps according to the zwitterion mechanism, in one step according to the termolecular mechanism.

Calculation of Absorption Capacity
The absorption capacity is the maximum amount of pollutant absorbed per amount of solvent. The absorption capacity of continuous system can be calculated using the effluent CO2 concentration vs time graph. The area in Figure  1 between the curves representing the inlet and outlet concentrations of CO2 is used to determine the amount of total absorbed CO2. The upper line is actually the CO2 concentration in the inlet, which was kept constant during a single run. At the outlet, concentration changes alongside the saturation of the MEA absorbent solution. The difference between these two values shows the absorbed amount. The input flow rate of CO2 can be calculated using the total flow rate and inlet CO2 concentration. The outlet flow rate of CO2 can be calculated using a fixed flow rate of N2, which is an inert compound and effluent CO2 concentration. Eq. (7) was used to calculate the CO2 outlet flow rate: In this equation, 2 is the outlet flow rate (l/min); is the total flow rate (l/min); 2 is N2 mole fraction in the gas inlet; 2 is CO2 mole fraction in the gas outlet; 2 is N2 mole fraction in the gas outlet.
The volumetric flow rates are converted to molar mass flow rate using conversion factors and assuming an ideal gas equation of state where each mole of gas occupies 22.4 liters at standard temperature (273 K) and pressure (1 atm). Then, the effluent concentration (ppm)-time graph can be replotted for mass flow rate-time. The rate of absorbed CO2 at each reading interval is then calculated using Eq. (8): where, Rco2 is the rate of absorbed CO2 ( l/min), ̇2 is the mass of absorbed CO2 in the gas inlet, and ̇2 is the mass of absorbed CO2 in the gas outlet.
The amount of absorbed CO2 for each time interval was calculated using Eq. (9): where, MCO2 is the mass of absorbed CO2 (mol CO2), Rco2 is the rate of absorbed CO2 (l/min), and t is the time (0-250 min).
The absorption capacity of the absorbent was calculated using Eq. (10): . =

Experimental Set-up
In this study, the bubble column absorption column shown in Figure 2 was used. The column has a height of 1.00 m and diameter of 5.0 cm. The process was performed in semi batch mode where liquid did not flow (batch) and the gas phase was feeding continuously into the solution. Column surrounded with heat jacket that keeps the solution temperature constant in the column. In each experiment, the temperature of the heat jacket was adjusted to the desired temperature 25 min before the run. However, the MEA solution was heated to the desired temperature on the hot plate and pour into the reactor immediately. A 1L MEA solution is used in each experiment. The gas mixture that consists of CO2 and N2 was prepared using two separate mass flow controllers (ALICAT Scientific Mass Flow Controller, Range:0-10L/min, accuracy; %0,2 of fullscale). The gas combination (nitrogen and carbon dioxide) is so dry, a continuous flow of gas through the column would cause the solution to evaporate significantly. This is a negative consequence since the solution's concentration and absorption capacity will change. As a result, before introducing gas to the absorption column, a humidifier must be used to saturate the incoming gas stream with humidity. The initial CO2 concentration was adjusted to 50,000 ppm using mass flow controllers. At the beginning of the experiment gas mixture was fed to the CO2 analyzer to ensure its concentration and then fed to the column filled with MEA solution. The gas mixture was bubbled using diffuser in the column and the carbon dioxide concentration of the effluent gas was monitored using an inline CO2 gas analyzer (Vernier, USA). The absorption process continued until there was no further absorption. This was confirmed by the concentration/time profile as shown in Figure 1.

The Effect of Temperature on Absorption Capacity
CO2 absorption capacity is an essential feature for the successful CO2 removal in a bubble column. The absorption capacity can be expressed in two units as mol of CO2 absorbed per mol of amine solution and as g CO2 absorbed per kg of amine solution.
The effect of temperature on the absorption capacity is shown in Figure 3 and Figure 4. The absorption capacity of 66.27 g CO2/kg solvent and 0.456 mol CO2/mol MEA decreases to 45.67 g CO2/kg solvent and 0.32 mol CO2/mol MEA with increasing temperature from 25 to 45 o C when using %20 MEA solution. This was due to the thermodynamics of the exothermic CO2 absorption system that could cause reversible reactions when the temperature was too high. The increase in temperature could also increase the CO2 vapor pressure over the solution that leads to decrease in the physical solubility of CO2 in the solvent (Tan et al., 2012).

The Effect of Gas Flow Rate on Absorption Capacity
The gas flow rate determines the detention time of the gas component and effects the mixing regime of the solution. The effect of gas flow rate on the absorption capacity is shown in Figure 5 and With the increasing gas flow rate, detention time of the gas into the solution was decreased. However, a higher flow rate creates turbulent conditions in the column, which is favorable for mass transfer. With increasing gas flow rates, the driving force between CO2 and water is strengthened and more CO2 molecules transfer from the bulk gas phase to the gas-liquid boundary layer, causing an increase in mass-transfer coefficient.

CONCLUSIONS
In this study, the effect of temperature and gas flow rate on the absorption capacity (g CO2/kg solvent and mol CO2/mol MEA) determined using bubble column reactor. Experiments were performed at semi-batch mode and 20% MEA solution was used as a solvent. Because of the study, the following findings were obtained: It can be concluded that capture of CO2 into the MEA solution at bubble column can be successfully achieved with the high absorption capacity. Development of a solvent for a high absorption capacity is one of the most crucial issues for post-combustion capture. Mixed amine solutions can be used to achieve high absorption capacity. Conway, W., Bruggink, S., Beyad, Y., Luo, W., et al. (2015). CO2 absorption into aqueous amine blended solutions containing monoethanolamine (MEA), N, Ndimethylethanolamine (DMEA), N, Ndiethylethanolamine (DEEA) and 2-amino-2-methyl-1propanol (AMP) for post-combustion capture processes.