Effect of CO2 Concentration and Liquid to Gas Ratio on CO2 Absorption from Simulated Biogas Using Monoethanolamine Solution

In industrial scale, removal of CO2 by chemical absorption from raw biogas represents an imperative treatment and the cutting-edge technology towards improvement of its quality and heat value. In this work, CO2 absorption studies were conducted in an absorption column packed with Sulzer metal gauze packing with simulated biogas absorbed using 30 wt. % of monoethanolamine (MEA) solution. Experimental works were conducted to determine the influence of different CO2 concentrations in feed gas (30 % and 40 %) and L/G ratio (0.6 and 0.7) and subsequently assessed in terms of CO2 absorption efficiency along the column. The results showed that 30 % CO2 in feed gas has higher removal efficiency as compared to 40 % CO2 with the ability to remove 94 % CO2 during the process. In addition, the CO2 absorption studied on the L/G ratio proved that CO2 removal was improved at higher L/G ratio of 0.7.


Introduction
Biogas represents a renewable energy source that has good calorific value, produced under anaerobic condition from decomposition of organic matter. The principal components of biogas consist of methane (CH4) (40 % -75 %) and carbon dioxide (CO2) (15 % -60 %) with other trace gasses such as hydrogen sulphide (H2S) [1]. CO2 contained in the biogas is in terms of combustion, an inert gas. Thus, removal of CO2 through biogas treatment is an essential process to improve its heat value. For enhancement of biogas quality, several CO2 capture technologies such as adsorption [2], absorption [3] and membrane separation [4] may be implemented. Among these processes, chemical absorption stands out as the most well-established approach for CO2 removal in industries due to its high removal efficiency, fast absorption rate and high product purity [5], [6]. Alkanolamines group of solvent has shown an effective absorption performance in CO2 removal application. A primary amine, monoethanolamine (MEA) is a commercial solvent that has been widely used for the CO2 absorption process which offers high reaction rate while being cost-effective [7].
The absorption performance in packed column is strongly influenced by the operating conditions. It has been reported for simulated flue gas treatment, removal efficiency is primarily affected by the CO2 concentration in feed gas, followed by liquid flow rate (L) and gas flow rate (G). Under low pressure  [3 -5]. A similar conclusion was reached by Hairul et al. [11] using natural gas (NG) containing 30 -50 % CO2 at high pressure conditions up to 4.04 MPa. In addition, for optimization of operating conditions and costs, the L/G ratios in the column have also been deemed essential for consideration [7,8]. However, the influence of L/G ratios in the previous researches were mostly focused on the flue gas treatment of feed gas containing low CO2 concentrations (1 -15 %).
Hence, this study sought to understand the performance of CO2 absorption from simulated biogas into MEA solution, conducted at relatively higher CO2 concentrations (30 % and 40 %). In addition, the effect of L/G ratio towards the efficiency of CO2 absorption was also studied via control of both flow rates entering the absorption column. The absorption performance was then quantified by degree of CO2 removal (%) along the height of column.

Chemical and gasses
Monoethanolamine (MEA) (99 % purity) was purchased from Merck, Germany. The CO2 along with the natural gas (NG) which consists of CH4 (97 %), CO2 (2 %) and heavier hydrocarbon (1 %) were purchased from Air Product Malaysia and Petronas Dagangan Bhd, respectively. All chemicals and gasses for experimental works were used without additional purification.

Equipment and procedures
A fabricated packed absorption column with total height of 2.04 m and internal diameter of 0.046 m was used for all experiments. Sulzer metal gauze packing (Sulzer Chemtech Pte Ltd., Winterthur, Switzerland) with approximate surface area of 500 m 2 /m 3 was packed in the column with 6 sampling points placed vertically at 0.34 m intervals.
Experimental setup of the process is as depicted in Figure 1. The simulated biogas was prepared as a mixture of NG and CO2. Composition of the feed gas was determined by setting the mass flow controller of NG and CO2 gases independently.  The mixed gas was pressurized up to 30 bar and kept in a gas vessel. From the bottom of the column, the simulated biogas was then introduced to flow upwards, controlled by a gas flow controller at a desired flow rate. Liquid phase was instead introduced from top of the column to travel downwards, thus establishing counter-current contact with the gas. Steady state condition of the absorption process was deemed reached upon operation for 30 minutes. Sampling was then conducted using CO2-CH4 IR Gas Analyzer (Fuji Electric Instrument, Japan) at each level of the absorption column under steady state condition. The CO2-rich solvent was discharged from the bottom part of the absorption column and collected in a CO2-rich solvent tank. 30 wt. % MEA solution with the simulated biogas containing either 30 % or 40 % CO2 in NG gas acted as the liquid phase and gas phase, respectively. The CO2 concentrations of 30 % and 40 % in NG were chosen to represent the intermediate concentration of raw biogas in real application. For process screening purpose, L/G ratios used were set at 0.6 and 0.7 by altering the gas flow rate while keeping constant the liquid flow rate. The experiments were conducted at operating pressure of 1.5 -2 bar in the column. The absorption performance was finally quantified in terms of CO2 removal percentage along the column and calculated using Equation (CO2 removal (%) = − 0  100 % (1)), where and 0 represent the CO2 mole fraction at the inlet and outlet of the column, respectively.

The influence of CO2 concentration in feed gas on the CO2 removal efficiency
The CO2 removal at each level of the column at a fixed gas and liquid flow rate for both CO2 concentrations is as depicted in Figure 2. From the graph, the feed gas containing 30 % CO2 and 40 % CO2 can be seen to have achieved 94 % and 82 % of CO2 removal, respectively. Lower removal efficiency at higher CO2 concentrations under the same operating conditions may be due to the increased CO2 mole fraction to react with limited free active amines in the liquid phase [14]. Moreover, similar increasing trend of CO2 removal was demonstrated by both CO2 concentrations. The reactive area with highest removal can be seen in the middle section of the column in between 0.68 m and 1.36 m. In this section, the concentration of CO2 in the gas phase decreases as it moves upward along the column due to absorption of several CO2 molecules into the liquid phase. In the meantime, CO2 loading in the solvent gradually increases as the liquid moving downwards and limits the reaction with CO2 molecules in the gas phase. Hence, most reactions between amine and CO2 molecules occurred in the middle section of the column with significant increment in the removal efficiency [15].   Figure 3 shows the effect of L/G ratio at 0.6 and 0.7 on the CO2 absorption performance using 30 wt.% MEA solution. The CO2 removal at 0.7 ratio was about 73 %, which was 15 % higher than 0.6 ratio. This observation was possibly due to higher availability of amine molecules at the higher ratio to react with CO2 molecules, consequently leading to higher absorption efficiency. Subsequently, at lower L/G ratios (higher gas flow rate), shorter residence time of gas in the absorption column further limits the reaction between CO2 molecules and amine solvent. The same observation was also demonstrated by Zeng et al. [8] and Kasikamphaiboon et al. [16].

Conclusion
The CO2 absorption process was carried out in a packed column using 30 wt.% MEA solution with the influence of CO2 concentration and L/G ratio on the removal performance investigated. This study proved that the efficiency of MEA as an absorbent was enhanced at low CO2 concentration (30 % CO2) in the feed gas and the parametric study on the effect of L/G ratio at 0.7 was higher than that at 0.6. Hence, this study concludes that both CO2 concentration in feed gas and L/G ratio significantly affect the absorption performance and therefore play an important role in improving the removal process.