Cryogenic Technologies for Biomethane Production: A Critical Review of Processes, Performance and Prospects

1. Introduction

The global energy landscape is at a critical crossroads, driven by the urgent need to reconcile the ever-growing demand for energy with pressing environmental imperatives. Rising population and industrial expansion continue to fuel global energy consumption, with fossil fuels like crude oil and natural gas still dominating the mix. After the temporary downturn caused by the COVID-19 pandemic, global oil demand has continued its strong recovery. In 2024, it reached approximately 103.9 million barrels per day, and is projected to rise to 105 million b/d in 2025 [1]. Similarly, global natural gas demand grew by 2.7% in 2024, corresponding to an increase of around 115 billion cubic metres, and is expected to maintain a similar growth rate in 2025 [2]. Yet this reliance on fossil energy is increasingly unsustainable, given its major contribution to greenhouse gas emissions and climate change. This situation has catalyzed a fundamental transformation of the energy paradigm. Global attention has shifted toward clean, low-carbon energy sources, and within this context, biomethane has emerged as a renewable vector of particular promise. Derived from biogas—produced via anaerobic digestion of organic residues including sewage sludge, agricultural waste, and energy crops [3]—biomethane offers both environmental and energy security advantages. The anaerobic process not only diverts waste from landfills but simultaneously transforms it into usable energy, aligning with circular economy principles. The substrates for anaerobic digestion are diverse and locally available, including livestock manure, agro-industrial residues, source-separated organic municipal solid waste and wastewater sludge. This flexibility supports decentralized energy models and allows tailoring the process to local biomass availability.

Biomethane’s versatility further reinforces its role in decarbonization. It can serve diverse end-uses: industrial cogeneration, heating, domestic energy supply, and transportation fuel, effectively replacing natural gas [4]. Additionally, biomethane utilization directly contributes to the reduction of GHG emissions. The typical production pathway includes feedstock pretreatment, anaerobic digestion, and subsequent upgrading to produce grid-quality or liquefied biomethane, depending on the intended application. However, realizing this potential hinges on addressing the critical technical challenge of biogas purification. Biogas typically contains a mixture of methane (CH₄), carbon dioxide (CO₂), nitrogen, hydrogen sulfide, oxygen, and water vapor, which must be efficiently removed to obtain grid-quality biomethane [5]. Cryogenic upgrading technologies, based on extremely low-temperature processing, stand out for their ability to produce high-purity biomethane, often exceeding 99% CH₄. [6]. These processes enable effective methane-CO₂ separation and often facilitate CO₂ recovery as a valuable byproduct [7].

Recent studies highlight the competitive advantages of cryogenic upgrading when integrated with liquefaction of natural gas (bio-LNG) routes. Compared to conventional methods such as membrane separation or chemical scrubbing, cryogenic systems can simultaneously deliver high methane purity and energy efficiency, while producing a liquefied product with increased energy density. For long-distance transport, bio-LNG (with ~21 MJ/L) is significantly more efficient than compressed biomethane [8]. Moreover, the potential for recovering liquid CO₂ as a co-product enhances the environmental and economic performance of these systems [9,10].

The growing interest in biogas upgrading is also reflected in the diversification of technologies adopted across Europe. While membrane separation currently dominates the market, cryogenic methods are gaining momentum due to their integration potential and purity levels [11]. In parallel, favorable policies—such as the EU’s target to achieve 15% renewable fuels in transport by 2030—continue to push for biomethane adoption at scale [12]. In 2023, combined biogas and biomethane production in Europe reached 22 billion cubic metres (bcm), equivalent to approximately 234 TWh, representing about 7% of the EU's natural gas consumption. Biomethane production alone grew to 4.9 bcm, marking a 21% year-on-year increase, and installed capacity rose to 6.4 bcm/year by Q1 2024, the highest annual increase recorded to date [13].

Beyond technical merit, cryogenic upgrading technologies are increasingly evaluated from a sustainability and techno-economic perspective. Recent assessments have compared the energy and exergy efficiencies of different cryogenic routes, such as distillation, anti-sublimation, and controlled freeze zone (CFZ) systems, in both standalone and integrated bio-LNG configurations. Reported efficiencies for optimized configurations exceed 95%, with payback periods under 2.5 years in favorable economic scenarios [14,15,16].

Against this backdrop, the present article aims to provide a critical and comprehensive review of cryogenic upgrading as a cornerstone technology in the biomethane value chain. The underlying principles, key configurations, and recent technological advances in cryogenic separation are analyzed in detail. Special attention is paid to comparing cryogenic options with conventional upgrading methods in terms of energy consumption, methane purity, carbon capture potential, and integration feasibility with downstream processes such as liquefaction. Furthermore, the review identifies knowledge gaps related to the comparative performance of emerging cryogenic techniques (e.g., CFZ vs. anti-sublimation) and their scalability. It also examines the challenges associated with high energy demands in refrigeration cycles, the behavior of contaminants under cryogenic conditions, and prospects for optimizing process efficiency through heat integration or hybridization. In summary, this review positions cryogenic upgrading as a key enabler in the large-scale deployment of biomethane, contributing not only to energy diversification but also to climate mitigation. It aims to support informed decision-making by policymakers, industry actors, and researchers seeking high-performance solutions in the transition toward a low-carbon energy system.

2. Principles and Configurations of Cryogenic Biogas Upgrading

Cryogenic upgrading of biogas is founded on the principle of differential condensation behavior of its constituents under extremely low temperatures. Unlike conventional separation techniques that rely on chemical affinity or physical differences such as molecular size or permeability, cryogenic methods exploit the distinct phase-change thresholds of each compound present in biogas when progressively cooled. The aim is to isolate methane—biogas’s energy-rich component—by cooling the gas mixture to specific temperatures where impurities condense or solidify while methane remains in the vapor phase or is selectively liquefied [17]. The most common contaminants in biogas—such as carbon dioxide, water vapor, hydrogen sulfide, and siloxanes—exhibit varying condensation and solidification points. This variation enables a staged cooling strategy for their selective removal. For instance, water vapor tends to condense at relatively higher temperatures, allowing for early separation. Impurities like hydrogen sulfide and siloxanes can be extracted in subsequent steps as their respective liquefaction or freezing points are reached. Carbon dioxide, which is present in significant quantities (up to 50%), typically requires lower temperatures to undergo phase transition and is often the most technically challenging component to separate.

The efficiency of the process depends critically on maintaining precise temperature gradients across multiple stages of compression and refrigeration [18]. Advanced temperature regulation ensures that each target compound transitions phase at its specific thermal threshold, thereby maximizing methane purity while reducing energy demand [19]. In standard configurations, the process begins at a working pressure of about 10 bar. Sequential cooling steps are then applied: at approximately −25°C, most of the water vapor, hydrogen sulfide, and volatile siloxanes are removed; at around −55°C, carbon dioxide begins to condense, and final CO₂ purification is typically achieved at −85°C when solidification is complete [20]. The solid or liquid CO₂ recovered through this process may be valorized as a commercial by-product, enhancing the economic feasibility of the system [21]. Alternatively, some systems employ a configuration involving initial gas drying and multistage compression up to 80 bar, which allows operation at moderately higher temperatures (−45°C to −55°C). However, this approach requires intermediate cooling stages during compression, which increases process complexity and energy consumption [17].

Among the multiple cryogenic technologies under development or commercial use, two broad categories dominate. Systems based on cryogenic distillation facilitate separation through differential boiling points, allowing methane and carbon dioxide to be isolated as liquid phases under well-controlled thermal conditions [22]. Meanwhile, anti-sublimation technologies rely on the direct solidification of CO₂ onto cold surfaces, bypassing the liquid phase and offering potential benefits in compactness and reduced equipment footprint [23]. According to thermodynamic analyses, anti-sublimation configurations can operate efficiently under sub-atmospheric temperatures, offering CO₂ capture with high selectivity and minimal methane loss [10]. These systems are especially attractive for distributed biomethane applications where on-site CO₂ valorization is feasible, and transport of compressed gas is limited. Cryogenic packed beds have also been proposed as an alternative configuration. In these systems, the gas passes through a bed of cryogenically cooled material, where specific components are trapped via deposition [24]. Computational models have been developed to optimize key design parameters such as bed geometry, flow rate, and heat transfer conditions, aiming to maximize methane recovery while ensuring stable operation. Recent studies have shown that careful control of temperature gradients and packing structure is essential to avoid blockages caused by frost accumulation, and to improve the efficiency of contaminant capture [25]. Simulation tools also support the evaluation of system behavior during transient conditions, contributing to the development of robust and energy-efficient configurations [26].

3. Cryogenic Biogas Upgrading Technology

Cryogenic biogas upgrading capitalizes on the distinct phase-change characteristics of each gas constituent at cryogenic temperatures. Instead of relying on chemical reactions or membrane-based selectivity, this approach exploits the physical transitions—condensation or solidification—that occur when the biogas stream is cooled in a controlled sequence [27]. By progressively lowering the temperature under defined pressure conditions, impurities such as CO₂, H₂O, H₂S, and siloxanes can be effectively removed, resulting in a methane-enriched stream that meets biomethane quality standards.

A widely implemented operational regime maintains the system at approximately 10 bar, under which cooling stages are configured to selectively condense and separate unwanted compounds at their characteristic temperature thresholds [28]. Water vapor, hydrogen sulfide, and siloxanes are typically eliminated at around −25 °C, while carbon dioxide requires lower temperatures—liquefying near −55 °C and ultimately solidifying at −85 °C to achieve complete separation [29]. As an alternative, some configurations apply initial drying followed by multi-stage compression up to 80 bar. This allows operation at moderately higher cryogenic temperatures, between −45 °C and −55 °C, but requires intermediate cooling during compression steps, increasing system complexity and energy demands [30]. To improve process flexibility and economic performance, numerous modifications and design alternatives have been proposed in recent years, ranging from novel cycle architectures to hybrid and intensified systems [17,31,32]. These technological developments are examined in detail throughout the following subsections.

3.1. Cryogenic Distillation Process

Cryogenic distillation is a thermally-driven technology used to separate carbon dioxide from biogas by exploiting the specific phase-change behavior of CO₂ at low temperatures and elevated pressures. The process enables CO₂ to be recovered in liquid form while producing biomethane of high purity. Traditionally, this method is associated with high energy demands, which can negatively impact its economic competitiveness compared to other upgrading techniques [33]. To address these limitations, recent developments have focused on integrating process intensification techniques and hybrid systems that significantly improve energy performance. Reported results indicate that, when properly designed, hybrid cryogenic distillation schemes can achieve up to 70% energy savings compared to baseline configurations [34].

Although most cryogenic distillation systems have been validated under natural gas purification scenarios, several authors have outlined their potential applicability to biogas upgrading [35,36]. Figure 1 shows a representative setup in which the gas stream is pre-treated to remove trace contaminants and then cooled in two stages before entering a distillation column. Inside the column, separation is based on boiling point differences: methane, as the lightest compound, is collected at the top via a partial condenser, while CO₂ is recovered as a bottom product in liquid form. A fraction of the CO₂ stream is recycled to assist in the reboiler duty of the pre-vaporization unit, whereas the rest is extracted for commercial use, thereby enhancing the system’s economic viability.

In the work by Yousef et al. [37], cryogenic distillation was optimized to avoid CO₂ freezing within the column, a common issue that may disrupt process performance. They investigated the influence of feed pressure, CO₂ concentration, temperature, reflux ratio, and feed location within the column. Simulation results demonstrated that a CO₂ purity of 99.5 mol% could be attained at a specific energy consumption of 0.31 kWh/Nm³ of raw biogas, while methane purity reached up to 97 mol% without CO₂ solidification. Notably, the tendency for CO₂ freezing was found to remain constant across a wide range of inlet CO₂ concentrations, suggesting robustness in process operation. In a complementary study [38], a novel design combining a four-stage compression train, a distillation column, and an integrated flash separator was proposed to optimize energy recovery and separation efficiency. This configuration yielded a record CH₄ purity of 97.2 mol%, outperforming previous single-column systems. At the same time, a valuable CO₂ co-product of more than 99% purity was generated. The associated energy penalty was just 0.38 kWh/Nm³ of clean gas, positioning the system among the most efficient reported to date in terms of methane recovery, CO₂ valorization, and overall process economics.

Moreover, recent numerical studies have explored the feasibility of process intensification in cryogenic distillation by adopting column internals that improve mass and heat transfer performance. For instance, the use of structured packing and enhanced heat integration has been shown to reduce condenser and reboiler loads, especially when CO₂-rich feeds are treated at optimized pressure levels [39]. Other experiments have demonstrated that cascading refrigeration stages and adjusting the CO₂ reflux ratio can further suppress freezing risk while maintaining operational efficiency [40]. These innovations underscore the ongoing evolution of cryogenic distillation from an energy-intensive method to a viable and scalable solution for biogas upgrading.

3.2. Dual-Pressure Cryogenic Distillation Process

The process of removing carbon dioxide from gas through a dual-pressure low-temperature (or cryogenic) distillation process represents a significant advancement in gas purification, especially in fields containing acid gas, for example, natural gas that is typically laden with acidic compounds like CO₂ and H₂S. These fields, accounting for about 40% of the globe's natural gas reserves, span continents from Europe and America to Asia. Yet, to harness these resources effectively, eliminating acidic components is paramount [36]. The process proposed by Pellegrini et al. [41] tackles these hurdles with an innovative approach, employing two distillation stages at varying pressures alongside an intermediary heating phase (Figure 2). Initially, at high pressure, the feed mixture undergoes distillation, facilitating CO₂ separation in liquid form. Subsequently, at lower pressure, further methane purification is achieved.

More specifically, purification occurs within two distillation units: the first operates at high pressure (50 bar), while the second operates below methane's critical pressure (40 bar). The high-pressure section functions akin to the stripping section of a conventional distillation column, where liquid reflux is facilitated by recycling the flow from the low-pressure section's bottom. Similarly, the low-pressure section acts as the enrichment section: it features a partial condenser at the top, and the feed gas flow originates from the high-pressure section's output. The gas stream exiting the top of the low-pressure section meets the required methane purity specifications. Liquid biomethane (LBM) is subsequently generated via an appropriate liquefaction process. The high-pressure distillation section's bottom product comprises highly pure carbon dioxide. The incoming biogas feed is precooled in a primary heat exchanger utilizing the available cooling capacity from an intermediate process stream, necessitating preheating before entering the low-pressure section. After precooling, the biogas is compressed to 50 bar and further cooled to its dew point at that pressure before entering the high-pressure distillation section. Compression follows precooling to reduce the compression power by lowering the feed stream's temperature.

The high-pressure section effectively eliminates input CO₂: the bottom stream is high-pressure liquid CO₂, while the top product stream is methane-rich (approximately 6.5 mol% CO₂). [41]. Since the high-pressure section operates above methane's critical pressure, obtaining pure methane through distillation in a single 50-bar unit is infeasible. Hence, final purification occurs in the low-pressure section at 40 bar. Outputs from the low-pressure section include an upper methane gas flow and a lower methane-rich liquid flow pumped back to the high-pressure section. The feed flow enters the high-pressure section on the fourth tray from the top, while liquid reflux from the low-pressure section's bottom is pumped and introduced into the first tray from the top. The upper gas flow from the high-pressure section undergoes splitting into two streams. Before entering the low-pressure section's bottom, a portion of the high-pressure section's upper product flow is heated and expanded to the low-pressure section's operating pressure, ensuring a temperature higher than its dew point. This prevents solid phase formation during expansion, with heat sourced from the input raw biogas stream precooled prior to compression. The remaining upper product flow from the high-pressure section is cooled to 50 bar (below CO₂'s solubility limit) and expanded to the low-pressure section's operating pressure, yielding a liquid flow at its bubble point, fed into the low-pressure section one theoretical plate above the gas feed flow [42].

A pivotal advantage of this method lies in its adeptness at handling sour gas, even with substantial CO₂ and other impurities like higher hydrocarbons and hydrogen sulfide. This adaptability stems from meticulously tailored pressure and temperature conditions at each distillation phase, preventing solid CO₂ formation and ensuring efficient acid component separation. Moreover, the process exhibits versatility, proving equally effective for both conventional natural gas and biogas derived from organic waste. This adaptability renders it a multipurpose solution for gas purification across diverse industrial and environmental settings [41]. Recent literature supports the industrial relevance and scalability of this approach. According to Pellegrini et al. [19], dual-pressure cryogenic distillation demonstrates strong operational efficiency: it utilizes only 14 % of the produced biomethane as internal energy—significantly lower than anti-sublimation (~22 %) and MEA (monoethanolamine) scrubbing (~29 %) schemes. Furthermore, most of this energy (61 %) is used for cooling, with no requirement for high-temperature reboiler heat. In terms of process stages, about 38 % of energy is consumed in compression, ~32 % in upgrading, and ~30 % in liquefaction, while CO₂ pressurization is effectively zero since CO₂ is handled as a pressurized liquid.

3.3. Anti-Sublimation Process

The production of liquefied biomethane via the anti-sublimation process involves the utilization of heat exchange surfaces to refine biogas, operating within the solid-vapor equilibrium region at atmospheric pressure [43]. In this process, CO₂ freezes directly within the gas stream, resulting in methane enrichment. This innovative approach, patented by Clodic et al. [44], is based on a specially designed cold chamber where CO₂ solidifies upon contact with cryogenic refrigerants. Its modular configuration allows for the decoupling of vaporization and solidification stages in separate chambers, thus enhancing operational flexibility. The process configuration is illustrated in Figure 3.

In the anti-sublimation process, purification is achieved through dry ice formation within a sealed cryogenic system. Two dynamic heat exchangers (HE in Figure 3) alternate between purification and regeneration phases, enabling continuous operation. Figure 3 depicts solid lines for the operational phase and dashed lines for the regeneration cycle. The incoming raw biogas, typically at ambient temperature and pressure, is first routed through a pre-cooling exchanger that recovers cold from the liquid CO₂ stream in the recovery loop. This precooled gas then passes through the recovery exchanger, where residual dry ice from the previous cycle contributes additional cooling. By melting this layer of CO₂, temperatures as low as –51 °C are attained, and the resulting CO₂ is collected in liquid form. The liquid CO₂ is then pressurized to 50 bar and reheated in the recovery exchanger. To reach the anti-sublimation threshold, further cooling is required to bring the gas temperature down to approximately –160 °C, enabling the direct solidification of CO₂ from the gas phase. This is achieved with an external refrigeration system where nitrogen acts as the final cooling agent [16]. Inside the anti-sublimation exchanger, a stable layer of solid CO₂ accumulates, reducing CO₂ concentration in the methane-rich stream to below 50 ppm at atmospheric pressure. The purified gas is subsequently liquefied in a dedicated cryogenic condenser for downstream storage and use.

The anti-sublimation process requires precise control of pressure and temperature, guided by the CH₄–CO₂ phase diagram, to ensure efficient CO₂ solidification. Simulation studies conducted in Aspen Plus indicate that by reducing feed gas temperature to the CO₂ sublimation point, extraction purities up to 90% can be achieved [45]. Energy consumption ranges from 0.286 to 1.3 kWh/kgCO₂ removed, depending on operational conditions and the degree of thermal integration. Under optimal configurations, the lowest consumption case also corresponds to a specific cost of approximately 0.1 €/kg of CO₂ [16,45].

Additional insights from Naquash et al. [10] further support the practical implementation of this process. In comparative analyses, anti-sublimation exhibited the highest refrigeration demand among cryogenic systems, with over 99% of total energy consumption allocated to cooling duties, largely due to the necessity of reaching extreme cryogenic temperatures. Despite this, it enables CH₄ recoveries of over 98% and achieves CO₂ concentrations well below 100 ppm, making it suitable for high-purity biomethane production. However, the lack of process flexibility under variable flow rates and the need for robust frost management strategies were identified as key limitations. The design's integrated heat exchange between incoming gas and recovered CO₂ stream plays a pivotal role in energy efficiency, particularly in reducing the refrigeration load of the final purification stage. By leveraging the cold potential of sublimated or liquefied CO₂, the system reduces external cooling demands during startup and regeneration cycles. This integrated thermodynamic behavior illustrates the technological ingenuity of anti-sublimation and underscores its relevance for both bio-LNG production and high-value CO₂ recovery.

3.4. Controlled Freeze Zone Process

Controlled Freeze Zone, or CFZ technology, is a cryogenic one-step distillation process developed by Exxon Mobil over four decades ago to efficiently separate CO₂ and other acid gases from natural gas streams [46]. Originally demonstrated in a pilot unit with a processing capacity of 0.02 MNm³/day and CO₂ concentrations ranging from 15% to 65%, this technology achieved outlet gas CO₂ levels as low as 100 ppm. Following initial success, it was scaled up to accommodate natural gas flows up to 30 MNm³/day while maintaining output specifications. The key innovation of CFZ lies in its integration of vapor–liquid–solid equilibrium within a single distillation column. The column is structured into three main zones: an upper rectifying section (packed bed), a central CFZ chamber, and a lower stripping section. The central freezing zone contains a spray subzone for distributing liquid CO₂, a freezing zone where solid CO₂ forms, and a melting zone where the solids are recovered as liquid. Pre-treated and dehydrated natural gas enters the upper section. As methane rises and vaporizes, a CO₂-enriched liquid is sprayed into the CFZ chamber, where it solidifies under cryogenic conditions. The solid CO₂ accumulates, melts, and is withdrawn as a high-purity liquid at the bottom, while purified CH₄ exits at the top. The process is illustrated in Figure 4, which depicts the structural layout of the integrated CFZ column.

According to Exxon Mobil, this configuration reduces capital costs by 10–30% compared to conventional absorption systems [47], with added operational benefits such as simplified process flow and reduced refrigerant demand. However, its performance diminishes in gas mixtures with high ethane or heavier hydrocarbons, which tend to co-solidify and hinder selectivity, thereby increasing separation complexity and cost. A comparative techno-economic and performance analysis between CFZ and dual-pressure cryogenic distillation (Pellegrini process) was conducted on the Natuna and Bojonegoro gas fields in Indonesia [48]. At Natuna, CFZ achieved a CH₄ purity of 95.62% and CO₂ recovery of 99.04%, outperforming Pellegrini’s scheme in CO₂ separation but slightly lower in methane purity (91.98% CH₄, 97.86% CO₂ recovery). At Bojonegoro, however, Pellegrini’s approach surpassed CFZ in both parameters, reaching 99.91% CH₄ purity and 99.99% CO₂ recovery, compared to CFZ’s 97.99% and 98.45%, respectively. Despite these mixed results in purity, CFZ demonstrated a lower CO₂ capture cost, at 7.9 USD/tonne, compared to 10.26 USD/tonne for Pellegrini distillation. This cost advantage is attributed to the integration of freezing and melting within a single column, reduced reboiler duty, and minimized refrigerant inventory. According to additional insights from Spitoni et al. [31], CFZ technology exhibits superior thermodynamic integration and a compact footprint, which enhances modularity and scalability—particularly in offshore or decentralized applications. Moreover, the direct collection of liquid CO₂ facilitates its downstream valorization for industrial use.

CFZ remains a promising option for high-volume natural gas upgrading, especially where cryogenic utilities are already present or CO₂ recovery for reuse is a priority. Nevertheless, its implementation in biogas treatment remains limited, primarily due to the lower scale and compositional variability of typical biogas streams. However, its technical underpinnings continue to inform the development of hybrid cryogenic–adsorption systems being considered in recent research for biomethane production [49].

3.5. Ryan-Holmes Process

The Ryan-Holmes process [50] represents a cryogenic biogas upgrading route based on extractive distillation, which facilitates carbon dioxide removal by modifying the phase behavior of the CH₄–CO₂ mixture. This is achieved by incorporating an entrainer that elevates the critical locus and displaces the CO₂ freezing boundary toward lower pressures and temperatures, thus minimizing the risk of CO₂ solidification [19]. The process typically employs heavier hydrocarbons, such as n-butane, as entraining agents. These components interact with CO₂ to depress its freezing point and promote efficient separation from methane. The overall system comprises five principal units: (i) biogas compression, (ii) extractive distillation, (iii) entrainer recovery and regeneration, (iv) biomethane liquefaction, and (v) CO₂ condensation and pumping.

Initially, the raw biogas feed is compressed to approximately 40 bar to prepare it for extractive distillation. Within the distillation column, n-butane is injected as a solvent. Its presence shifts the critical region of the gas mixture, enabling lower-temperature operation without encountering the risk of CO₂ freeze-out. The biogas feed enters the column at an upper tray, optimizing the contact with the descending entrainer and maximizing separation efficiency. The overhead product from this column is primarily composed of methane and undergoes further cooling and depressurization via a liquid expander and condensation stage, ultimately yielding liquefied biomethane at near-atmospheric pressure. Conversely, the bottom stream from the extractive distillation unit—rich in CO₂ and n-butane—is first depressurized to 30 bar and then introduced into a regeneration column. In the regeneration section, thermal input enables the selective desorption and recovery of CO₂ in liquid form. The n-butane, now separated, is condensed, cooled, and recycled back to the extractive distillation stage, thereby closing the entrainer loop and minimizing solvent losses. The extractive distillation column itself operates under a temperature gradient that decreases from bottom to top—heated at the reboiler and cooled at the condenser—allowing stable phase separation throughout the column height. In contrast, the regeneration column maintains relatively higher temperature levels to ensure effective desorption and recovery of the entrainer. The Ryan-Holmes process is distinguished by its relatively high energy consumption, largely attributable to the reboiler duty required for entrainer regeneration and the condenser duty associated with methane product recovery. Nevertheless, its ability to circumvent CO₂ solidification without operating at extreme pressures or employing anti-sublimation steps makes it a promising alternative for biogas upgrading.

Originally developed for sour gas treatment in enhanced oil recovery operations, the Ryan-Holmes process was specifically designed to address two critical technical challenges: the formation of a CO₂–C₂H₆ azeotrope and the high freezing point of CO₂, both of which compromise the performance of conventional cryogenic distillation. By incorporating an extractive recycle loop, this process suppresses CO₂ solid formation and breaks the azeotropic barrier, enabling efficient separation of methane, CO₂, and light hydrocarbons [51].

Numerous large-scale natural gas processing facilities have implemented this technology successfully, including the Seminole Unit operated by Amerada Hess, the Willard Unit by ARCO, the GMK South Field by Mobil Oil Corporation, and the Wasson Denver Unit by Shell Oil. These applications confirm the maturity and industrial viability of the process under high CO₂ concentration conditions. In comparison with amine scrubbing systems, the Ryan-Holmes process presents multiple advantages: lower dehydration requirements, elimination of corrosive solvents, the use of natural gas liquids as solvents, and the production of high-pressure CO₂ streams that are readily suitable for geological reinjection. Additionally, the process achieves high recovery rates of heavier hydrocarbons—such as ethane and propane—providing an important revenue stream, particularly in scenarios where methane content in the feed gas is reduced [51,52].

Beyond its traditional configuration, the Ryan-Holmes process has also been explored in hybrid schemes combined with membrane technologies to improve cost-effectiveness and operational flexibility. Membrane units may be installed upstream for bulk CO₂ removal prior to cryogenic distillation, reducing the load on the distillation system and optimizing overall energy use. Alternatively, membranes can be integrated downstream of the CO₂ stripper, particularly in the overhead stream rich in CO₂ and ethane, to further purify the CO₂ product and break azeotropic mixtures. This hybrid approach has been demonstrated to lower utility costs by up to 20% and capital expenditure by approximately 25% compared to stand-alone cryogenic systems [51,53].

3.6. Hybrid Cryogenic Processes

Building upon the foundational concepts discussed in the Ryan-Holmes process and its membrane-integrated variants, hybrid cryogenic systems have emerged as a promising direction to overcome some of the limitations associated with conventional cryogenic separation. These integrated configurations aim to improve energy efficiency, process flexibility, and cost-effectiveness by combining cryogenic methods with other separation technologies, such as membranes, adsorption, or absorption, to create synergistic effects. Cryogenic technologies alone may face several barriers to effective application, including high energy consumption, production costs, and the availability of cooling energy sources [54]. In this context, hybrid cryogenic processes, such as those integrating cryogenic adsorption with membrane separation, have been proposed to mitigate these drawbacks and enable more sustainable biogas upgrading or CO₂ capture routes [55]. For instance, an experimental study assessed the energy-saving opportunities of a cryogenic–membrane hybrid process designed for biogas refining applications [56]. In this system, membrane modules were evaluated under a range of operating temperatures, revealing excellent energy performance—0.8 MJ per kilogram of CH₄—and high product purity, reaching 98% for methane and 99.8% for CO₂, when operated at temperatures between −30 and 40 °C. In another work based on simulation, a hybrid system combining vacuum cryogenic adsorption with subsequent CO₂ liquefaction was examined [57]. The results showed that a total energy input of 1.4 GJ per tonne of CO₂ was required to achieve a product stream with 98.2% CO₂ purity, indicating the potential of such configurations under moderate thermal demand. From an economic perspective, a techno-economic assessment evaluated the cost implications of a hybrid membrane–cryogenic process for CO₂ capture, reporting that the avoided CO₂ cost associated with an 85% capture ratio was approximately 9% higher than that of a traditional amine absorption process [58]. Although this figure reflects a relative disadvantage in terms of direct cost, the hybrid system demonstrated favorable performance when factors such as solvent regeneration and environmental compatibility were considered. A comparative study further analyzed cryogenic hybrid processes for biogas upgrading, showing that while conventional membrane-integrated systems achieved lower overall costs, the cryogenic hybrid alternatives offered notable advantages in terms of plant-level benefits and system robustness under variable feed conditions [59]. These findings underline the potential of hybridization as a design principle in advanced CO₂ separation systems.

Beyond membrane integration, cryogenic hydrate-based processes also present a viable option for high-purity CO₂ recovery. These systems rely on hydrate formation at low temperatures and high pressures, following a preliminary cryogenic separation stage that reduces the CO₂ content of the feed stream [60]. The sequential operation allows efficient impurity removal and CO₂ capture through solid hydrate formation. Similarly, cryogenic absorption processes using ammonia (NH₃) have gained attention as an alternative for biogas upgrading. NH₃-based absorption offers several advantages, such as low regeneration energy requirements, compatibility with low-temperature operation, and the commercial availability of NH₃ as a working fluid [61]. However, the practical implementation of these emerging hybrid processes still requires further optimization to enhance selectivity, scalability, and cost competitiveness under real operating conditions.