Performance Assessment of a Membrane Gas Absorption (MGA) Process for Biogas Upgrading with Potassium-Based Solvents

Abstract

Global energy instability has increased interest in biomethane production through biogas upgrading as a renewable alternative to natural gas. This study experimentally investigated membrane gas absorption (MGA) for CO₂ capture using aqueous potassium hydroxide (KOH) and potassium carbonate (K₂CO₃) in a hydrophobic polypropylene hollow fiber membrane contactor under once-through and recycle operating modes. KOH demonstrated faster reaction kinetics and superior mass transfer performance, whereas K₂CO₃ exhibited slower but steadier CO₂ absorption behavior. Under recycle mode operation, both absorbents reached a comparable saturation capacity of approximately 1 mol of CO₂ per mol of absorbent despite their different absorption profiles. The membrane showed no measurable performance decline or morphological changes, confirming good membrane stability and compatibility with both solvents. The results demonstrate the potential of MGA as an effective and flexible approach for small- to medium-scale decentralized biogas upgrading using alkaline solvents with improved chemical stability, lower toxicity, and potential economic advantages over conventional amine-based absorbents.

1. Introduction

The growing instability of global energy markets, coupled with ongoing geopolitical disruptions, has intensified the need for secure and domestically sourced energy systems, thereby enhancing the strategic role of biogas as a renewable energy carrier. In this context, biomethane, derived from purified biogas, has attracted significant attention as a renewable alternative to natural gas. − The relevance of such alternatives is further underscored by recent trends, as global energy-related CO₂ emissions increased by 0.8% in 2024, reaching an all-time high of 37.8 Gt of CO₂, with emissions from natural gas rising by approximately 2.5% in 2024 (about 180 Mt of CO₂), representing the largest contribution to global carbon emissions growth. Upgrading biogas to biomethane contributes to greenhouse gas (GHG) emissions mitigation by substituting fossil natural gas and supporting low-carbon energy systems. Biogas is produced through anaerobic digestion of organic waste and typically contains 50%–75% methane (CH₄), 25%–50% CO₂, and trace gases such as hydrogen sulfide (H₂S). , The high CO₂ content reduces the calorific value, narrows flammability limits, decreases flame speed, and promotes corrosion of pipelines and equipment. , Therefore, efficient CO₂ removal is essential to improve biogas quality and enable its utilization as a substitute for natural gas.

Building on the need for effective CO₂ removal, several separation technologies have been developed for biogas upgrading, among which membrane-based processes have emerged as a promising and increasingly commercialized option. Their modularity, compact design, and operational flexibility render them attractive for efficient and scalable biomethane production. , Membrane-based CO₂ separation technologies can be broadly classified into two main categories: (i) gas separation membranes, which rely on selective permeation, and (ii) membrane gas absorption (MGA) systems, where membrane contactors are combined with chemical absorption using a liquid solvent. The latter approach integrates the advantages of conventional absorption with those of membrane technology, offering enhanced mass transfer performance and process intensification. In MGA systems, a porous hydrophobic membrane acts as a nondispersive interface between the gas and liquid phases, enabling a high specific interfacial area and independent control of gas and liquid flow rates within a compact configuration. Consequently, common operational limitations of conventional absorption columns (e.g., flooding, foaming, and gas–liquid entrainment) are inherently avoided. − Moreover, MGA systems exhibit a fast dynamic response to fluctuations in operating conditions, ensuring high on-stream reliability and reducing downtime, ,, which is particularly advantageous in applications where energy efficiency and cost-effectiveness are critical. Additionally, MGA can be readily operated in stripping mode (reverse operation) after the absorption process, where a vacuum can be applied on the other side of the membrane contactor plus mild heating of the solvent in order to aid CO₂ desorption and solvent regeneration; note that the regeneration process can be facilitated more effectively in the physical absorption solvent case, with minimal energy requirements. − A summary of the advantages and limitations of each technology is presented in Table . Industrial applications of membrane contactors include water deionization, pharmaceutical and biotechnological processes, as well as applications in the beverage, oil, and gas industries, among others, as discussed by Sengupta and Peterson.

1. Comparison between Packed Column and MGA Units for Gas Absorption–Regeneration Applications.

Technology	Advantages	Limitations
Packed column absorption units	• Well-established technology	• Solvent foaming
	• Flexible use of different solvents depending on the application	• Column flooding
		• Large equipment size
		• Solvent losses (particularly during solvent regeneration)
Membrane gas absorption units	• High gas–liquid interfacial area	• Membrane pore wetting
	• Compact equipment size	• Long-term membrane stability and durability
	• No foaming or flooding	• Solvent compatibility restrictions
	• Simple and flexible operation
	• Modular and scalable design
	• Energy efficient regeneration

The selection of the chemical solvent is a key factor governing CO₂ absorption performance in MGA systems. Conventional amine-based absorbents, such as monoethanolamine (MEA) and diethanolamine (DEA), are widely considered as benchmark solvents due to their fast reaction kinetics, high CO₂ removal efficiency, and successful industrial implementation in packed column absorption processes over several decades. − Amine solutions in MGA systems have been used in a number of cases; − see, for example, some remarks in the study by Pantoleontos et al. especially regarding the use of MEA solutions in MGA systems and their limitations.

However, despite their excellent absorption characteristics, amine-based solutions suffer from several limitations, including equipment corrosion, solvent degradation and oxidation in the presence of O₂, NO _x , and SO _x , toxicity concerns, susceptibility to membrane wetting, and high regeneration energy demand due to the elevated temperatures required for solvent recovery. − To address these challenges, increasing attention has been directed toward alternative solvents for CO₂ capture, such as potassium-based alkaline absorbents like potassium hydroxide (KOH) and potassium carbonate (K₂CO₃). − Compared to amine-based solutions, these solvents exhibit improved chemical stability and compatibility with the membranes, lower regeneration energy demands, lower cost, and reduced environmental impact. Among them, potassium carbonate is especially attractive due to its high CO₂ loading capacity, low regeneration energy requirements, reduced toxicity and corrosivity, and enhanced thermal stability, although it is generally characterized by slower reaction kinetics compared to potassium hydroxide. ,− Furthermore, it should be noted that absorption of CO₂ into K₂CO₃ leads to the formation of less soluble KHCO₃. While aqueous solutions containing more than 40% w/w K₂CO₃ may promote potassium bicarbonate crystallization, the stable operation of membrane contactors can be achieved by maintaining operating conditions below the precipitation values, thereby preventing membrane fouling, pore blockage, and increased mass transfer resistance. , Moreover, these solvents are increasingly being considered in advanced CCUS schemes involving either electrochemical regeneration of the solvent or the direct electrochemical conversion of captured CO₂ to potassium formate, , thereby offering opportunities for flexible, electricity-driven operation under conditions of surplus renewable power.

In this study, the performance of an MGA process for CO₂ capture and biogas upgrading is evaluated using hydrophobic polypropylene (PP) hollow fiber membranes as the gas–liquid contactor and aqueous KOH and K₂CO₃ solvents at a concentration of 1.0 M. Experiments are conducted under both once-through and solvent recycle modes to systematically assess absorption performance and operational behavior under conditions relevant to biogas upgrading. Although alkaline solvents have been extensively investigated for CO₂ capture in conventional absorption processes, their application in MGA configurations, particularly under biogas-relevant conditions, remains relatively underexplored. Therefore, this study aims to define the performance envelope of KOH- and K₂CO₃-based MGA systems and elucidate their potential for efficient and scalable CO₂ removal in biogas upgrading applications.

2. Experimental Section

2.1. Experimental Setup

The piping and instrumentation diagram of the MGA unit is illustrated in Figure . The experimental setup is designed to enable controlled gas–liquid contact within a hollow fiber membrane module operating in a counter-current flow configuration. Figure illustrates a typical configuration of the contactor. The gas phase is typically introduced through the lumen side (inner channel) of the fibers, while the liquid absorbent flows through the shell side (outer channel).

1.

Piping and instrumentationdiagram of the membrane gas absorption (MGA) unit for carbon capture.

2.

Schematic representation of the membrane gas absorption (MGA) process. The figure shows the gas–liquid interface within the pores of hydrophobic hollow fibers, enabling selective chemical absorption.

The gas is delivered from gas cylinders through pressure regulators to the mass flow controllers (EL-Flow Select, Bronkhorst), which accurately adjust and maintain the desired inlet gas flow rates. The volumetric gas flow rates at the inlet and outlet of the membrane module may be continuously measured using a drum-type flowmeter (Ritter TG1/5), while gas composition is monitored using an online infrared biogas analyzer (Hubei Cubic-Ruiyi, Gasboard-3200), with measurement ranges of CH₄, 0–100%; CO₂, 0–50%; H₂S, 0–9999 ppm; and O₂, 0–25%. The liquid solvent is supplied from an external tank (maximum capacity: 4 L) and delivered to the system using a peristaltic pump (Ismatec BVP-Z). A set of dedicated tubes and valves allows flexible operation, enabling the gas and liquid streams to be directed either through the membrane contactor or to bypass it, depending on the required experimental configuration. Temperature, pressure, and flow rates are continuously monitored throughout all experiments. This configuration enables the systematic investigation of both steady-state and dynamic operating conditions relevant to biogas upgrading. Specifically, under the once-through operation, the solvent exiting the module is collected in a downstream container, while in the recycle mode, the solvent is continuously returned to the feed reservoir, allowing the evaluation of the absorption performance under progressively increasing CO₂ loading.

2.2. CO₂ Capture Experiments

A commercial 3M Liqui-Cel hollow fiber membrane contactor, model MM-1.7X5.5, was employed as the gas–liquid contacting unit. The module comprises approximately 7400 X50 porous hydrophobic polypropylene hollow fibers, each with an outer diameter (OD) of 300 μm and an inner diameter (ID) of 220 μm, providing an effective membrane surface area (OD-based) of approximately 0.79 m².

Each experimental run commenced with the preparation of fresh solvent solutions by dissolving potassium hydroxide (KOH) pellets (≥85% purity; Chem-Lab) and potassium carbonate (K₂CO₃) powder (≥98% purity; Chem-Lab) in deionized water to obtain 1 M aqueous solutions. The solvents were introduced into the system with the dedicated pump at pressures 0.2–0.3 barg, slightly higher than the gas-phase pressure, ensuring complete filling of the shell side while avoiding undesired phase dispersion and minimizing long-term membrane wetting effects.

Prior to each experiment, the gas analyzers were warmed up and calibrated using standard gas mixtures representative of the operating conditions. The gas phase consisted of premixed simulated biogas of 45% CO₂ in CH₄, supplied from a gas cylinder (Air Products and Chemicals, Inc.). The feed gas composition and flow rate were set and verified before the gas entered the membrane module, with initial stabilization performed via a bypass line. Following stabilization, the gas stream was directed into the lumen side of the membrane module under counter-current flow conditions. For each set of operating parameters, the outlet gas composition and flow rate were continuously monitored until either (i) steady-state conditions were achieved in once-through operation mode (typically within ∼10 min) or (ii) the gas outlet concentration of CO₂ approached that of the gas inlet in recycle operation mode.

2.3. Membrane Compatibility Assessment Tests

Membrane compatibility with the alkaline solvents was evaluated through dedicated exposure experiments using two 3M Liqui-Cel MM-0.5X1 series membrane contactors with an effective membrane area of 0.01 m² operating in a continuous solvent recycle mode for a total of 4 days. One module was tested with 1 M KOH, and the other was tested with 1 M K₂CO₃. During the first 2 days, each system was operated under an inert nitrogen atmosphere (100% N₂), while during the subsequent 2 days the gas feed was switched to simulated biogas composition containing 45% CO₂, allowing the membranes to be exposed to both fresh and CO₂ loaded absorbents. Throughout these experimental tests, gas and liquid flow rates were maintained at 0.16 L/min and 0.1 L/min, respectively. After completion of the exposure tests, membrane samples were extracted and analyzed using scanning electron microscopy to assess potential changes in the membrane surface morphology and structural integrity. A pristine, unexposed membrane contactor of the same series was used as a reference comparison. Scanning electron microscopy (SEM) sample characterization was performed using a TESCAN Gaia 3 FIB/SEM instrument. The microscope hosts a 30 kV Triglav electron column and a Cobra Focused Gallium Ion Beam column. SEM images of the sample surfaces were acquired using the two in-beam secondary electron (SE) and backscattered electron (BSE) detectors located inside the electron column. Samples were sputter-coated with a conductive metal and mounted on stubs. SEM images of the tubular structures were acquired at 5 kV using the in-beam SE detector, highlighting the surface morphology in detail.

2.4. Analysis of Experimental Measurements

CO₂ removal using aqueous solutions of KOH and K₂CO₃ proceeds via chemical absorption, where CO₂ reacts with the alkaline species to form bicarbonate ions. While the current study is focused on the experimental procedures and performance assessment, the reactions of carbonate solutions merit discussion, as they provide insights into the CO₂ removal efficiency of KOH and K₂CO₃ systems. The overall mechanism is governed by a set of coupled reactions. The primary absorption reaction involves the formation of bicarbonate from dissolved CO₂ and hydroxide ions:

$${\mathrm{CO}}_2+{\mathrm{OH}}^{-}\leftrightarrow {{\mathrm{HCO}}_3}^{-}$$ AR1

In addition, the carbonate–bicarbonate equilibrium contributes to the generation of hydroxide ions:

$${{\mathrm{CO}}_3}^{2-}+{\mathrm{H}}_2\mathrm{O}\leftrightarrow {{\mathrm{HCO}}_3}^{-}+{\mathrm{OH}}^{-}$$ AR2

Reaction is typically assumed to be at equilibrium. The availability of reactive species depends on the dissociation of the respective potassium-based solvents:

$$\mathrm{KOH}\to {\mathrm{K}}^{+}+{\mathrm{OH}}^{-}$$ AR3

$${\mathrm{K}}_2{\mathrm{CO}}_3\to 2{\mathrm{K}}^{+}+{{\mathrm{CO}}_3}^{2-}$$ AR4

Complete dissociation is generally assumed for both of the salts. In KOH solutions, hydroxide ions are directly available for the reaction with CO₂ via reaction . In contrast, in K₂CO₃ systems, hydroxide ions are generated indirectly through the carbonate equilibrium (reaction ), making the carbonate concentration a key parameter governing hydroxide availability. In both cases, bicarbonate (HCO₃ ^–) is the dominant reaction product, formed through the combined contribution of the kinetic absorption reaction (reaction ) and the equilibrium reaction (reaction ).

Experimental data, combined with mass balance relationships, were used to determine the key performance indicators of the process, including the CO₂ removal efficiency, solvent loading, CO₂ absorption flux, and overall apparent mass transfer coefficient.

The CO₂ removal efficiency (eq ) is calculated as

$${\mathrm{CO}}_2\mathrm{removal}(\%)=\frac{Q_{\mathrm{CO}2,\mathrm{in}}-Q_{\mathrm{CO}2,\mathrm{out}}}{Q_{\mathrm{CO}2,\mathrm{in}}}\times 100$$ 1

where Q _CO2,in and Q _CO2,out are the molar flow rates (mol/s) of CO₂ at the inlet and outlet of the membrane module, respectively. The volumetric gas flow rate was converted to molar flow rate using the ideal gas law, accounting for the operating temperature and pressure.

The solvent loading, a (mol of CO₂ per mol of absorbent), is defined as

$$a=\frac{{\mathrm{mol}}_{\mathrm{CO}2,\mathrm{L}}}{{\mathrm{mol}}_{\mathrm{absorbent}}}=\frac{Q_{\mathrm{CO}2,\mathrm{in}}-Q_{\mathrm{CO}2,\mathrm{out}}}{C_{\mathrm{absorbent}}\times F_{\mathrm{solvent}}}$$ 2

where mol_CO2,L is the moles of CO₂ absorbed in the solvent (mol), mol_absorbent is the moles of the absorbent in the solvent (mol), C _absorbent (mol/L) is the concentration of the absorbent in the solvent, and F _solvent (L/s) is the volumetric flow rate of the solvent, which is an experimental parameter.

The overall molar flux of CO_2, J _CO2 (mol s^–1 m^–2), through the membrane into the shell side is given by

$$J_{\mathrm{CO}2}=\frac{Q_{\mathrm{CO}2,\mathrm{in}}-Q_{\mathrm{CO}2,\mathrm{out}}}{A}$$ 3

where A (m²) is the effective membrane surface area.

The overall apparent mass transfer coefficient, K _OG (mol m^–2 s^–1 Pa^–1), is calculated as

$$K_{\mathrm{OG}}=\frac{J_{\mathrm{CO}2}}{{\Delta P}_{\mathrm{CO}2,\mathrm{lm}}}$$ 4

where ΔP _CO2,lm (Pa) is the logarithmic mean CO₂ partial pressure difference between the gas and liquid phase:

$${\Delta P}_{\mathrm{CO}2,\mathrm{lm}}=\frac{(P_{\mathrm{CO}2,\mathrm{in}}-{P_{\mathrm{CO}2,\mathrm{L},\mathrm{out}}}^{*})-(P_{\mathrm{CO}2,\mathrm{out}}-{P_{\mathrm{CO}2,\mathrm{L},\mathrm{in}}}^{*})}{\ln \left(\frac{P_{\mathrm{CO}2,\mathrm{in}}-{P_{\mathrm{CO}2,\mathrm{L},\mathrm{out}}}^{*}}{P_{\mathrm{CO}2,\mathrm{out}}-{P_{\mathrm{CO}2,\mathrm{L},\mathrm{in}}}^{*}}\right)}$$ 5

whereP _CO2,in and P _CO2,out (Pa) are the CO₂ partial pressures in the gas phase entering and exiting the module, respectively, which are experimentally recorded, while P _CO2,L,in* and P _CO2,L,out* (Pa) are the partial equilibrium pressures of CO₂ at the liquid inlet and outlet streams, respectively.

Due to the fast reaction between CO₂ and alkaline solvents, the concentration of physically dissolved CO₂ in the liquid phase is typically negligible. Under this assumption, the mass transfer driving force can be expressed using the logarithmic mean partial pressure difference based on the corresponding CO₂ mole fractions (LMPD). Consequently, eq may be accordingly simplified to eq :

$${\Delta P}_{\mathrm{CO}2,\mathrm{lm}}=\mathrm{LMPD}=P_{\mathrm{inert}}\frac{Y_{\mathrm{in}}-Y_{\mathrm{out}}}{\ln \left(\frac{Y_{\mathrm{in}}}{Y_{\mathrm{out}}}\right)}$$ 6

where Y _in and Y _out $\left(\frac{{\mathrm{mol}}_{\mathrm{CO}2}}{{\mathrm{mol}}_{\mathrm{inert}}}\right)$ are the mole ratios of CO₂ to inert gas in the gas phase, while P _inert (Pa) is the partial pressure of the inert gas.

3. Results and Discussion

3.1. Solvent Once-Through Absorption Tests

The CO₂ absorption performance of aqueous KOH and K₂CO₃ solutions was experimentally evaluated through both solvent once-through and recycle mode tests. Initially, key performance indicators, including the CO₂ removal efficiency, CO₂ absorption flux, and overall apparent mass transfer coefficient (K _OG), were determined as a function of the gas and liquid flow rates. Subsequently, each solvent’s saturation behavior was investigated under the solvent recycle operation mode.

Figures and present the experimental results as a function of the liquid flow rate, for two different gas flow rates, using 1 M KOH and 1 M K₂CO₃, respectively. In all cases, a simulated biogas stream (45% CO₂ in CH₄) was used as feed gas. The operating conditions were selected to capture the observed trends over a practically relevant range of CO₂ removal efficiencies. Furthermore, for the KOH system operating at a gas flow rate of 2 L/min, an additional point was included to extend the analysis toward lower CO₂ removal efficiencies. Nevertheless, in all experiments, a minimum CO₂ removal efficiency of 30% was ensured. Notably, substantially lower gas flow rates were required for the 1 M K₂CO₃ absorbent compared to 1 M KOH in order to achieve the same removal efficiencies, owing to the slower reaction kinetics of the former.

3.

Effect of gas and liquid flow rate on CO₂ removal (a), CO₂ absorption flux (b), and overall mass transfer coefficient (c) for solvent 1 M KOH at different inlet gas flow rates (1.5 and 2 L/min).

4.

Effect of gas and liquid flow rate on CO₂ removal (a), CO₂ absorption flux (b), and overall mass transfer coefficient (c) for solvent 1 M K₂CO₃ at different inlet gas flow rates (0.5 and 1 L/min).

For both solvents, increasing the liquid flow rate resulted in higher CO₂ removal efficiencies, increased CO₂ absorption fluxes, and enhanced overall mass transfer coefficients. This behavior is attributed to improved local liquid-side mass transfer, driven by higher liquid velocities, and reduced solvent saturation, as the reactive species in the solvent are ubiquitous, thus promoting CO₂ absorption. , For KOH, the CO₂ flux exhibited a steep increase as the liquid flow rate increased from 0.05 to 0.16 L/min, beyond which a plateau was observed, indicating that beyond this point the available reactive species are in excess and the liquid-phase mass transfer resistance is negligible compared to the combined membrane and gas-phase resistance. A similar behavior has been observed for fast-reacting solvents, such as monoethanolamine (MEA). In contrast to KOH, no clear plateau was observed for K₂CO₃ under the investigated conditions, suggesting that in the whole range of the liquid flow rate tested, the liquid-phase mass transfer resistance remains comparable to the combined membrane and gas-phase resistance, due to the slower absorption kinetics.

The effect of the gas flow rate on the CO₂ absorption efficiency is also evident in Figures and . Increasing the gas flow rate led to a decrease in the CO₂ removal efficiency, primarily due to the reduced gas residence time and the shorter gas–liquid contact. This behavior is in good agreement with previously reported studies. , For the KOH system, higher gas flow rates resulted in increased CO₂ flux (Figure b) due to enhanced gas–liquid mass transfer driven by increased gas velocities (i.e., increased local mass transfer coefficients) and driving forces (due to lower removal efficiencies). This trend was not observed for K₂CO₃ (Figure b), further confirming that the rate-limiting step in carbonate-based systems is governed by the slower reaction kinetics at the gas–liquid interface rather than by gas-phase mass transfer limitations. This interpretation is further supported by the substantially lower mass transfer coefficients calculated for K₂CO₃ (Figure b), confirming that slower liquid-side reaction kinetics constitute the controlling step for the carbonate-based solvent.

5.

Comparison of the overall CO₂ removal efficiency (%) (a) and mass transfer coefficients (b) using 1 M KOH and 1 M K₂CO₃ solutions in a polypropylene (PP) hollow fiber membrane contactor for different gas-to-liquid flow rate ratios (G/L) at different inlet gas flow rates (0.5–2 L/min).

Figure presents an attempt to summarize the CO₂ removal efficiency and the apparent overall mass transfer coefficient per solvent, as a function of the gas-to-liquid (G/L) flow rate ratio. This approach was not successful for the CO₂ removal efficiency, as this metric reflects overall process performance and is independently affected by multiple operating parameters, including the gas flow rate, liquid flow rate, and available membrane area. Consequently, the results could not be consistently grouped by solvent using the G/L ratio as the sole variable. As shown in Figure a, the CO₂ removal efficiency varied substantially at the same G/L ratio, even for the same solvent, when different gas flow rates were applied. The figure also shows that, with 1 M K₂CO₃, CO₂ removal efficiencies above 70% were achieved at considerably lower gas flow rates than those used with 1 M KOH. In contrast, the grouping approach proved more suitable for the apparent overall mass transfer coefficient (K _OG), which is a more fundamental parameter reflecting the combined effects of mass transfer and reaction kinetics and therefore allows a more direct comparison between solvents. As illustrated in Figure b, K _OG values obtained at different gas flow rates could be grouped reasonably well by solvent using the G/L ratio alone. Moreover, regardless of the CO₂ removal efficiency achieved, KOH consistently exhibited substantially higher K _OG values (16.7 × 10^–9–20.9 × 10^–9 mol m^–2 s^–1 Pa^–1) than K₂CO₃ (2.6 × 10^–9–4.7 × 10^–9 mol m^–2 s^–1 Pa^–1) across all G/L ratios, confirming the faster reaction kinetics of KOH with CO₂. For both solvents, K _OG decreased with increasing G/L ratio (with a steeper decline observed for KOH), indicating a higher overall mass transfer resistance (inverse K _OG) at increased G/L flow conditions.

3.2. Solvent Recycle Mode Absorption Tests

Two recycle mode experiments, one with 1 M KOH and one with 1 M K₂CO₃, were conducted to assess the solvent loading behavior and long-term CO₂ absorption performance. In each recycle experiment, the simulated biogas stream was continuously fed to the lumen side, while the solvent was continuously recirculated on the shell side. Specifically, the solvent exiting the membrane contactor was returned to the feed tank and recirculated back to the membrane, in contrast to the once-through experiments, where the solvent was discharged after a single pass and collected as waste. The outlet CO₂ concentration was monitored over time to assess the absorption performance (Figure ), with gas flow rates set to 1 L/min for KOH and 0.33 L/min for K₂CO₃, to ensure comparable initial conditions, i.e., an initial outlet CO₂ concentration below 10%, which facilitated data interpretation and minimized measurement uncertainty.

6.

Solvent recycle mode performance during CO₂ absorption using 1 M KOH (a,c,e) and 1 M K₂CO₃ (b,d,f). Inlet and outlet CO₂ concentrations (a,b), solvent loading (c,d) , and CO _² absorption flux­(e,f).

Figure a,b shows the measured outlet CO₂ concentration as a function of time, while Figure c,d presents the corresponding solvent loading profiles. The 1 M KOH solution exhibited a rapid initial CO₂ uptake due to the fast reaction with CO₂ to form bicarbonates. As KOH was consumed, the reaction rate declined, and the system gradually entered a slower regime. Consequently, the loading curve asymptotically approached a plateau near 1 mol of CO₂ per mol of KOH. In contrast, 1 M K₂CO₃ exhibited a slower but steady increase in the CO₂ loading, consistent with its slower intrinsic reaction kinetics associated with the carbonate-to-bicarbonate pathway. Nevertheless, the loading curve also approached asymptotically similar saturation levels, around 1 mol of CO₂ per mol of K₂CO₃.

The corresponding CO₂ flux profiles (Figure e,f for 1 M KOH and 1 M K₂CO₃, respectively) further illustrate these differences. In all cases, the CO₂ absorption flux decreased as the CO₂ loading of the solvent increased, in agreement with trends reported in the literature. ,, For the KOH system, the CO₂ flux was initially high, indicating strong absorption driven by the rapid reaction with KOH, but it decreased rapidly as the KOH was progressively consumed. After this point, the flux stabilized at a lower level until a new slow decline was observed as the solvent approached saturation. For the K₂CO₃ system, the initial CO₂ flux was moderately high due to the favorable concentration gradient and the availability of fresh absorbent. As absorption progressed and the solvent became increasingly loaded, the flux gradually decreased, reflecting the diminishing driving force and the reduced availability of reactive carbonate ions. Eventually, the flux stabilized at a low level and finally declined as the solvent approached full saturation, indicating the exhaustion of its CO₂ absorption capacity.

Overall, the combined once-through and recycle mode results demonstrate the superior performance of KOH in MGA, which is attributed to its rapid reaction kinetics with CO₂. Under identical operating conditions (gas and liquid flow rates), the CO₂ removal achieved with 1 M KOH was found to be comparable to, and in some cases slightly higher than, that previously reported for a 0.25 M DEA solution in the same MGA configuration by our group, indicating that KOH can provide competitive separation performance while offering additional advantages, such as lower solvent cost, higher chemical stability, reduced solvent degradation, and lower regeneration energy requirements. In contrast, K₂CO₃ exhibited a slower absorption performance, resulting in lower CO₂ removal under identical conditions. This kinetic limitation could be effectively addressed through the use of promoters , or blended KOH/K₂CO₃ mixtures, which have been studied and shown to substantially enhance CO₂ absorption kinetics while retaining the favorable characteristics of K₂CO₃. Despite their different absorption rates, both alkaline solvents exhibited similar saturation capacities, approaching approximately 1 mol of CO₂ per mol of KOH or K₂CO₃, highlighting the long-term potential of potassium-based absorbents for membrane gas absorption applications.

3.3. Membrane Compatibility Investigation

Throughout the experimental test campaign, no significant performance loss was observed over time for the single membrane module (3M Liqui-Cel MM-1.7X5.5), indicating good membrane compatibility with the selected solvents. This observation was further supported by dedicated membrane exposure tests aiming to qualitatively assess the effects of chemical exposure and gas–liquid interactions on membrane structural integrity and surface morphology. Following the 4 day exposure period described in Section , three hollow fiber samples, one from the pristine membrane contactor and two from the membrane contactors exposed to 1 M KOH and 1 M K₂CO₃, respectively, were analyzed via scanning electron microscopy (SEM). The obtained SEM images are presented in Figure . No noticeable changes in the membrane’s surface morphology were observed after exposure to either solvent. In addition, no potassium bicarbonate crystallization particles were detected on the membrane surface, implying that pore blockage or significant membrane degradation did not occur during the 4 day exposure period.

7.

SEM images of the membrane contactor’s hollow fibers before (a) and after 4 days of exposure to aqueous solvents of 1 M KOH (b) and 1 M K₂CO₃ (c) at different magnifications.

4. Conclusions

The present study experimentally evaluated membrane gas absorption (MGA) for CO₂ capture from simulated biogas streams by using aqueous KOH and K₂CO₃ solutions. The performance of both absorbents was investigated through once-through and recycle mode operation tests using a hydrophobic polypropylene hollow fiber membrane contactor.

In the once-through operation, both solvents showed improved CO₂ removal efficiency, CO₂ flux, and apparent overall mass transfer coefficient (K _OG) with increasing liquid flow rate, confirming the importance of enhanced liquid-side hydrodynamics and reduced solvent saturation (excess reactive species). KOH generally exhibited a higher mass transfer performance than K₂CO₃, with substantially higher K _OG values across all investigated gas-to-liquid ratios, reflecting its faster reaction kinetics with CO₂. For KOH, the CO₂ flux reached a plateau at higher liquid flow rates, indicating that liquid-phase resistance became negligible relative to the membrane and gas-phase resistances. By contrast, no such plateau was observed for K₂CO₃, suggesting that the liquid-phase resistance remained significant throughout the investigated range due to the slower reaction kinetics.

In the recycle mode operation, the two solvents exhibited distinct loading and flux evolution profiles, but both approached a similar saturation capacity of approximately 1 mol of CO₂ per mol of solvent. KOH showed rapid initial CO₂ uptake and high initial flux, followed by a marked decline as the active hydroxide was consumed, whereas K₂CO₃ displayed a slower but more gradual and stable absorption behavior consistent with the carbonate-to-bicarbonate reaction pathway. These findings indicate that KOH is better suited to applications requiring high absorption rates and superior mass transfer performance, while K₂CO₃ may remain attractive where slower but steadier absorption, together with a more favorable health, toxicity, and corrosivity profile, is desirable.

Finally, the absence of measurable membrane performance loss and morphological changes after solvent exposure indicate good membrane stability and compatibility with both absorbents.

The open access publishing of this article is financially supported by HEAL-Link.

The authors declare no competing financial interest.