Continuous absorption of CO2 in packed column using MDEA solution for biomethane preparation

Nowadays, the energy consumption in Indonesia is increasing. Raising of energy consumption force Indonesia to find other energy resources. Biogas is one of the renewable energy, which was developed in anticipation to the fossil energy reduction. Reducing the content of impurities in biogas may reduce the corrosion impact and increase the combustion efficiency. The biomethane can be utilised as fuel for generator in small and medium scale industries (IKM). Continuous CO2 absorption in packed column using MDEA solution as absorbent is studied for biomethane preparation. CO2 absorption experiments was performed continuously in the packed absorption column with a diameter of 6 cm and 75 cm length. Gas is sparged from the bottom of the column while the liquid is pumped through the top of the column. The concentration of CO2 at exit gas is analysed by GC and recorded as a function of time. The flowrate of the inlet gas was varied at 1 LPM; 1.5 LPM; and 1.8 LPM. Variation of MDEA solution concentration used was 20% and 35.31%. Mathematical model for unsteady state CO2 absorption in packed column was developed. The reaction rate constant (k) and mass transfer coefficient KGa were determined by fitting the outlet CO2 concentration data as a function of time to the model solution with smallest Sum of Square of Errors (SSE). The experimental data shows that absorption of 1 LPM gas flow rate with 0,15 LPM MDEA solution flow rate may reduce 40 % CO2 to be 17 % CO2 in outlet gas. The steady state process reaches at 10 minutes. Increasing gas flow rates shows the higher overall mass transfer coefficient. The reaction rate constant is not affected by gas flow rate variation.


Introduction
Energy scarcity is a latent risk from our dependence in nonrenewable energy source. In order to anticipate energy scarcity, renewable energy source development is a must. Biogas promises a sustainable energy source, is produced from biomass anaerobic bacterial digestion. Biogas consists of methane, carbon dioxide, hydrogen sulfide and some trace element. Biogas quality is determined by methane content. Generally CO 2 is the biggest impurity of biogas, hence biogas purification is best achieved by CO 2 removal. The chosen absorbent must be economic, nontoxic, having high CO 2 absorption ability, and easy to regenerate. Abharchaei, 2010 studied the absorption of CO 2 by 2(Methyl)-Aminoethanol (MEA) solution, but the solution is corrosive [1]. Normal methyldiethanolamine (MDEA) is a widely used in industry due to high absorption capacity, especially for acid gases including CO 2 . In addition to the large absorption capacity, MDEA also has degradation proof property due to exposure to heat and direct contact with chemicals [4]. Another reason that encourages the use of MDEA extensively on industrial scale is a low corrosivity and low vapor pressure of MDEA [2]. The advantage of chemical absorption is the bond formed between CO 2 with MDEA is stronger compared to physical absorption. But the chemical absorption has shortcomings such as high energy requirements for regeneration of solvent [5]. CO 2 absorption with MDEA is an exothermic reaction with heat of reaction -72 kJ / mol. During absorption process CO 2 rich gas is contacted directly with absorbent. During this process, CO 2 is transferred from gas bulk into liquid body [6]. This condition continues until liquid reaches a saturated point where absorption can no longer take place since absorbent has already lost its absorption capability. In this paper effect of gas flow rate dan MDEA concentration on absorption process of CO 2 in MDEA solution is studied.
The chemical absorption of CO 2 in MDEA solution follows this reaction (Gao et al, 2013).
Mathematical model of unsteady state continuous absorption in packed column is developed by applying mass balance.
Mass balance of CO 2 in gas phase: Mass balance of CO 2 in liquid phase : IC : t = 0, z = z, C AL = 0, C AG = 0 BC : t = t, z = 0, C AL = C AL (0,t), C AG = C AG (0,t) z = L, C AL = C AL (L,t), C AG = C AG (L,t) Differential Equations (2), (3) and (4) with boundary conditions was solved using MATLAB. The solution are C AG and C AL as function of time, for a certain value of k and K G a. The values of k, K G a and H will be applied to absorber design.

Method
Continuous absorption experiments was conducted in the absorption column as shown in Figure 2.   The unsteady state absorption of CO 2 mathematical model was developed. Together with Initial and boundary condition, the differential equations were solved. The solution were outlet CO2 concentration as a function of time. Equilibrium parameter (H) was taken from previous experiment which are 37.64 atm.L/mol. Reaction rate constant (k) and mass transfer coefficient KGa were determined by fitting the outlet CO2 concentration data as a function of time to the model solution with smallest Sum of Square of Errors (SSE). Figure 3 and Figure 4 show the experimental data for CO2 absorption for initial gas content 40% at various gas flow rate which are 1 LPM, 1.5 LPM and 1.8 LPM with 0.15 LPM MDEA solution flow rate at different MDEA solution concentrations which are 20 % and 35.3 %. The outlet CO2 concentrations for different MDEA concentration have only 1 % difference, it is not significant. The absorption experimental data for various gas flow rate is shown in Figure 5 for 40 % inlet CO2 concentration and Figure 6 for 74 % inlet CO2 concentration. At 0.15 LPM MDEA flowrate with inlet gas flow rate 1 LPM, the outlet CO2 concentration drop from 40 % to 17 %. It meets the requirement to be the fuel of electric generator which is less than 20 % CO2. By increasing the flow rate of MDEA solution and conduct the absorption at several columns, it is possible to get the composition of biomethane more than 90%, and fulfill the requirement as generator fuel.  The experiments were conducted at isothermal condition, the values of reaction rate constant is almost constant. The overall mass transfer coefficient are affected by gas flow rate. The higher the flow rate, the higher the mass transfer coefficient.

Conclusion
Continuous absorption of CO 2 using MDEA solution at ambient temperature in packed column is possible to reduce the 40 % CO 2 concentration to 17 % CO 2 . By applying some stages of this absorption method, it is possible to get biomethane from biogas, and to be utilised as fuel for electrical generator in the small and medium scale industries. 20 % MDEA solution is not significantly different from 35.3 % MDEA solution in absorbing CO 2 , therefore based on economical point of view, 20 % MDEA is more feasible.