The global population is continuously increasing, and it is foreseen that it will exceed 10 billion by 2050 [1]. In this scenario, natural gas (NG) exploration and production attract interest in meeting the growing world energy demand [2]. Moreover, NG plays a key role in the transition to renewable energy sources [3] as NG is the cleanest fossil fuel due to its high hydrogen/carbon ratio [4].

Proven NG reserves around the world are approximately 196.9 trillion cubic meters [4], but onshore NG reserves—with simpler production that is closer to consumers—are decreasing, while offshore NG reserves far from the coast and in deep waters are growing [5]. In this regard, oil production with associated NG in deep-waters and ultra-deep-waters usually presents high carbon dioxide (CO₂) content and gas-to-oil ratio, which entail difficulties for gas processing in offshore rigs where there are weight/footprint limitations [6]. Due to complex logistics for NG production/transportation in the context of deep-water offshore rigs, high costs lead to significant offshore CO₂-rich NG reserves being left unexplored. Albeit the existence of efficient technologies for onshore NG production, innovative low-cost technologies are necessary for deep-waters processing of CO₂-rich NG [5].

The concept of carbon capture utilization and storage (CCUS) is necessary to allow a smooth transition from fossil energy to renewable energy to keep a profitable industrial operation. In CO₂-rich NG processing, CO₂ removed from NG is transported to a storage site [7]. Promising CO₂ utilizations are enhanced oil recovery (EOR) in oil-and-gas reservoirs [8] or enhanced gas recovery (EGR) in gas reservoirs [9]. In CCUS-EOR or CCUS-EGR, CO₂ is injected into reservoirs to increase oil and gas production. If the increase is high enough, the extra revenues can offset CO₂ capture/transportation costs entailing profits [7].

Raw NG primarily contains methane (CH₄) and secondarily ethane, propane, butanes, and heavier hydrocarbons. Other common constituents are CO₂, hydrogen sulfide (H₂S), water (H₂O), nitrogen (N₂), and trace components. Raw NG purification is necessary to meet gas pipeline specifications and market/environment regulations [10].

NG processing usually comprises: (i) H₂S removal [11]; (ii) water dew-point adjustment (WDPA) via dehydration [12]; (iii) hydrocarbon dew-point adjustment (HCDPA) [10]; and (iv) CO₂ removal. H₂S causes pipeline/equipment corrosion, it is undesirable in combustion, and threatens human health and the environment [11]. WDPA is necessary mainly because water condensed from NG may form solid gas-hydrates under high-pressure and low-temperature typically found in subsea pipelines [12]. HCDPA is necessary to avoid hydrocarbon condensation in pipelines (safety and efficiency issues) and to recover valuable NG liquids [13]. Besides, HCDPA removes hydrocarbons to meet the heating value specification (Wobbe Indexes), ensuring adequate gas–turbine and combustion equipment operations minimizing emissions [5]. Finally, CO₂ content is reduced to 2–3 mol%, increasing NG heating value, preventing solid CO₂-hydrates, avoiding corrosion, and avoiding occupying pipeline capacity with inert [14].

Several technologies exist for carbon capture from CO₂-rich NG. Pressure-swing adsorption (PSA) prescribes selective CO₂ adsorption onto solid adsorbents at high-pressure conjugated with low-pressure CO₂ desorption [15]. PSA issues comprehend the methane purity-recovery tradeoff [16] and the low-pressure CO₂ release entailing compression costs [17].

Cryogenic distillation is considered for CO₂ ≥ 10 mol% due to lesser energy consumption and footprint compared to chemical absorption [18]. However, since cryogenic distillation operates at temperatures below the CO₂ triple-point [19], there is a risk of CO₂ freeze-out (dry-ice) inside the equipment, causing blockage [18]. Moreover, high-pressure cryogenic columns are protected against pressure surges by blowdown valves, which under cryogenic conditions can lead to dry-ice formation via the Joule–Thomson effect [20].

In offshore CO₂-rich NG processing, membrane permeation (MP) is adopted due to its low footprint, modularity, and flexibility to feed composition changes [21] and is recommended for high CO₂ content NG [22]. However, thanks to its capacity-selectivity tradeoff [23], permeate methane losses are remarkable, and the low-pressure CO₂-rich permeate imposes high compression costs [24].

Physical absorption (PhA) prescribes a physical solvent that dissolves CO₂ at high-pressure [25], requiring high CO₂ fugacity for high CO₂ loadings [26]. PhA is much less energy-intense for solvent regeneration compared to chemical absorption at the expense of a low-pressure CO₂ stripping [6]. PhA drawbacks comprise (i) low CO₂/CH₄ selectivity entailing CH₄ losses in the stripped gas; (ii) low-pressure stripped gas entailing compression costs for EOR destination.

The most mature technology for CO₂ and H₂S removal from CO₂-rich NG is chemical absorption (ChA), which is based on CO₂ reactions with aqueous-alkanolamine solvents [27]. Aqueous-monoethanolamine (aqueous-MEA) and aqueous-methyl-diethanolamine (aqueous-MDEA) are the most used chemical solvents [28]. The main advantage is the high CO₂/CH₄ selectivity in a wide range of operation pressures [27] and this selectivity is particularly high at low CO₂ fugacity [22]. ChA generally operates with packing columns that have beds of variously-shaped packings—e.g., Pall rings or Berl saddles—providing a high specific surface area for CO₂ transfer. Packing columns are fed with lean solvent at the top and with raw NG at the bottom in countercurrent flows [29]. ChA drawbacks comprise: (i) high heat-ratio (kJ/kg^CO2) solvent regeneration [28]; (ii) solvent degradation (oxidation and thermal degradation) causing corrosion and efficiency loss; (iii) low-pressure CO₂ stripping entailing compression costs for EOR; (iv) hydraulic issues (e.g., foaming, channeling, flooding and entrainment) [29].

The gas-liquid membrane contactor (GLMC) combines MP and ChA advantages—without the respective drawbacks—becoming a promising solution for CO₂-rich NG decarbonation [30]. In GLMC, the hollow-fiber membrane (HFM) provides a non-dispersive gas-liquid contact avoiding packing-columns hydraulic problems like foaming, channeling, flooding, and entrainment [31]. Comparatively, with MP, the selective chemical solvent and thousands of HFM inside the GLMC shell ensure high CO₂/CH₄ selectivity with low CH₄ loss, high mass transfer area per shell, compactness, modularity, and easy scale-up, all desired attributes for CO₂ removal from CO₂-rich NG with aqueous-amine solvents in offshore rigs [22]. Comparatively, with conventional CO₂ absorption packing columns, GLMC reaches reductions up to 70% and 66% in size and weight, respectively [32]. Angular indifference is another advantage of GLMC vis-à-vis the sway of floating offshore rigs [33].

2. GLMC: Principles, Materials, and Configurations

GLMC is modular equipment configured in batteries. Each GLMC module contains thousands of hydrophobic HFM tubes fixed into a shell-and-tube arrangement ^[34][51] in Figure 1. Equipment inclination translates into GLMC angular indifference [33]. GLMC is configurable as an absorber (for CO₂ capture) or as a stripper (CO₂ stripping for solvent regeneration). In both cases, GLMC has two feeds (where L is the molar flowrate of the GLMC liquid stream and V is the gas counterpart): (i) GLMC-absorber: NG feed (Vinabsorber); lean aqueous-amine solvent (Linabsorber) ^[35][52]; (ii) GLMC-stripper: Sweep-gas (nitrogen ^[36][53] or steam ^[37][54]) (Vinstripper); rich aqueous-amine solvent (Linstripper). GLMC modules can operate with parallel V and L flows (co-current contact) ^[38][55] or opposed V and L flows (counter-current contact) ^[39][56]. Whichever the case, V flows inside HFM tubes or on the shell-side, while L respectively flows on the shell-side or inside HFM tubes. Consequently, there are four different GLMC configurations shown in Figure 1: (a,b) apply counter-current contact; (c,d) apply parallel contact; (a,c) have L in shell-side and V inside HFM tubes; (b,d) have L inside HFM tubes and V in shell-side ^[40][57]. Since HFM is a physical barrier against phase dispersion, additional phase separation equipment is not necessary ^[41][37]. HFM also increases the interfacial area reducing equipment size ^[42][58]. GLMC is based on reactive-vapor-liquid-equilibrium (RVLE) since V and L streams are kept in direct contact through the HFM pores and relatively rapid chemical reactions occur in the liquid. In GLMC-absorber with chemical solvent, high CO₂/CH₄ selectivity is imposed by the aqueous solvent thanks to CH₄ hydrophobicity [33] and selective CO₂ conversion via liquid phase chemical reactions. When crossing the membranes, CH₄ rapidly reaches its low solubility in the aqueous solvent, such that CH₄ bubbles as a small vapor phase may form on the liquid side. As absorption chemical reactions between the solvent and CO₂ (or H₂S) are exothermic, the liquid temperature continuously increases, reducing CH₄ solubility and raising CH₄ fugacity over the liquid. Therefore, an L stream can be a single-phase liquid or a two-phase stream. Moreover, the CH₄ bubbles on the solvent side contribute to stripping some CO₂/H₂S from the solvent, at the same time raising trans-membrane transport via reduction of CO₂/H₂S fugacities in the liquid [33]. On the other hand, in GLMC-absorber with physical solvent, CO₂ does not react with the solvent, so species mass transfers and CO₂/CH₄ selectivity depend only on gas diffusivities into the solvent and solute solubilities in the solvent. In this case, CO₂ Henry’s Law is frequently used for VLE predictions ^[43][59]. In GLMC, convective heat transfers occur due to temperature differences between (i) V and L streams across HFM; (ii) shell-side phase and shell outside. As molar isobaric heat capacity (C¯¯¯P) values imply VC¯¯¯PV<<LC¯¯¯PL, the L stream controls the temperature distribution; consequently, L and V temperature difference is near zero |TV−TL|≈  0 at a given GLMC axial position due to convective interfacial heat transfer. Water can transfer L→V or V→L, depending on the interfacial difference of H₂O fugacities fˆVH2O−  fˆLH2O, influenced by the water content of feeds and absorption heat effects. For counter-current GLMC, the lowest TV,  TL values occur at the L inlet, which is also the V outlet position. As V approaches its outlet at water saturation, its temperature lowers and condensation can occur, turning V into a two-phase flow [33]. Additionally, the large absorption heat release can vaporize solvent on the L side ^[44][60]. Considering all these thermal effects, both L and V can be single-phase or two-phase streams at some GLMC point [33]. GLMC-stripper regenerates physical solvents (L) with nitrogen or steam as sweep-gas (V). A vacuum-driven pump can also be employed to reduce V pressure. Since CO₂/H₂S fugacities in the sweep-gas are lower than in the solvent, CO₂/H₂S move L→V enabling separation without heat supply ^[37][54]. As Figure 2 shows, CO₂/H₂S move V→L in GLMC-absorber and L→V in GLMC-stripper, while H₂O moves V→L or L→V whether fˆVH2O−  fˆLH2O>0 or fˆVH2O−  fˆLH2O<0. N₂ transfer in GLMC-stripper is negligible since N₂ is highly hydrophobic ^[36][53].

2.1. HFM for GLMC

Contrarily to MP, which uses dense selective membranes, GLMC adopts non-selective microporous HFM to allow V-L contact and selectivity is imposed by the solvent ^[45][61]. Figure 3 depicts how HFM works as a mass transfer barrier preventing phase dispersion ^[46][62]. Figure 3a represents direct V-L contact in tray columns or packing columns with phase dispersion. Figure 3b shows microporous HFM with gas-filled pores ensuring V-L contact with low mass transfer resistance ^[47][63]. However, during GLMC operation, the solvent may fill the pores (Figure 3c), increasing mass transfer resistance drastically and reducing transfer fluxes. This undesirable phenomenon compromises GLMC performance and is called membrane pore wetting (MPW) ^[46][62]. MPW is influenced by membrane characteristics (pores diameter, tortuosity, porosity, surface roughness), solvent surface tension, and operating conditions, while some solvents may favor MPW by affecting HFM hydrophobicity, morphology, and roughness. To avoid MPW, HFM material must be hydrophobic such as polypropylene (PP), polyvinylidene fluoride (PVDF), polytetrafluorethylene (PTFE), polyether ether ketone (PEEK), and solvents should have high surface tension, negligible vapor pressure and should not chemically attack HFM ^[48][41]. Figure 3d shows another solution against MPW wherein dense HFM CO₂ permeable and solvent impermeable are used. However, dense HFM presents high transfer resistance and does not fit in the original GLMC concept of non-dispersive V-L contact ^[46][62]. A compromise solution uses composite HFM (Figure 3e) consisting of an ultrathin dense skin layer onto microporous support. The ultrathin layer is generally a fluorine-based hydrophobic material, while the microporous support—made of a cheaper material like PP—should be mechanically resistant, chemically stable, and highly porous to minimize transfer resistance ^[46][62].

2.2. Solvents for GLMC CO

₂

 Absorption

Hereafter, physical and chemical GLMC solvents are discussed.

2.2.1. Physical Solvents

PhA relies on CO₂ solubility, which follows Henry’s law, such that the absorption loading is favored by high CO₂ fugacity and low temperature. Thus, PhA can perform CO₂ removal from high-pressure raw NG. Comparatively to ChA, the main PhA advantage is the lower heat ratio for solvent regeneration ^[49][64]. Physical solvents are listed in Table 14 for GLMC CO₂ removal from NG. Some promising GLMC physical solvents are ionic liquids, which are salts of organic cations with organic anions. Ionic liquids exhibit melting points below 100 °C, negligible vapor pressure, high CO₂ solubility, and thermal stability but present high viscosity as a disadvantage ^[55][70].

2.2.2. Chemical Solvents

ChA is characterized by high CO₂/CH₄ selectivity and limited absorption loadings (kg^CO2/kg^Solvent), both due to chemical reactions of the solvent with CO₂. Aqueous-amine solvents are the most used in ChA for CO₂ capture from NG due to high reactivity, low cost, easy regeneration and high CO₂/CH₄ selectivity ^[56][71]. Aqueous-MEA, aqueous-DEA (aqueous-diethanolamine) and aqueous-MDEA are examples of aqueous-amine solvents extensively employed for NG decarbonation since 1930 ^[57][72]. However, aqueous-amines exhibit drawbacks such as high heat-ratio (kJ/kg^CO2) solvent regeneration, thermal/oxidative degradation, corrosiveness and limited CO₂ loading ^[58][73]. Aqueous-MEA is the most used for ChA CO₂ capture, exhibiting high reactivity at low-pressure or high-pressure ^[59][74], while aqueous-MDEA is adequate at high pressure despite lower absorption rate ^[60][75], which recommends using reactivity promoters. Table 25 lists literature studies on chemical solvents for GLMC NG decarbonation.

2.2.3. Solvent Blends

The literature presents CO₂ capture studies (for GLMC or not) with several blended solvents ^[72][87], namely: (i) aqueous-MDEA-PZ (PZ promoter) ^[73][88]; (ii) aqueous-MEA-MDEA, aqueous-MEA-DIPA and several combinations of aqueous-amines ^[74][89]; (iii) aqueous-PZ-K₂CO₃ (PZ promoter) ^[75][90]. Physical and chemical solvents are also blended, such as NMP with aqueous-ethanolamine [26] and ionic-liquid with aqueous-ethanolamine ^[76][91].