Minireview: Intensified Low-Temperature Fischer–Tropsch Reactors for Sustainable Fuel Production

Abstract

Low-temperature Fischer–Tropsch (LTFT) synthesis converts syngas to diesel/wax at 200–250 °C. The LTFT reaction has recently received renewed interest, as it can be used for converting syngas from renewable sources (biomass and waste) to high-value fuels and chemicals. Conventional LTFT reactors, such as fixed-bed and slurry reactors, are not entirely suitable for bio-syngas conversion due to their smaller scale compared to fossil fuel-based syngas processes. This review explores advancements in intensifying LTFT reactors suitable for bio-syngas conversion, enabling smaller scale and dynamic operation. Various strategies for enhancing heat and mass transfer are discussed, including the use of microchannel reactors, structured reactors, and other designs where either one or both the heat and mass transfer are intensified. These technologies offer improved performance and economics for small LTFT units by allowing flexible operation, with increased syngas conversion and reduced risk of overheating. Additionally, this review presents our outlook and perspectives on strategies for future intensification.

1. Introduction

Fischer–Tropsch synthesis (FTS), a pioneering process initially developed for fossil fuel production, has gained renewed interest due to its potential to convert synthesis gas (syngas), a mixture of carbon monoxide and hydrogen, into liquid hydrocarbons from renewable feedstocks. Bio-syngas, produced by gasifying biomass and plastic waste, offers a promising alternative to fossil-based syngas. Life cycle assessment (LCA) studies have demonstrated the efficiency of gasification for extracting chemicals and energy from plastic waste [1,2,3] and biomass [2,3,4], particularly when minimizing feedstock transportation (note that waste and biomass have a low energy density in comparison with common fuels). This results in decentralized syngas production from biomass in The Netherlands, for example [5]. To effectively produce renewable fuels and chemicals from bio-syngas (the last step of the biomass to liquid BTL process), intensified and decentralized FTS reactor designs are crucial, as current industrial FTS reactors are not efficient for this purpose. Similarly, transporting natural gas from rural areas to the market can be expensive and challenging, as natural gas transportation is significantly more expensive and complicated than liquid fuel transportation, which ends up with undesired gas flaring [6,7,8]. In this context, the modular design of small-scale gas-to-liquid (GTL) plants utilizing the Fischer–Tropsch reaction holds promise as a viable solution to address this issue. This minireview focuses particularly on small-scale FTS reactors, which can be used in both GTL and BTL units and are designed to operate under mild conditions of low temperature and pressure while exhibiting enhanced versatility. BTL via FTS has gained growing attention from both academic and industrial sectors due to its capability to generate carbon-neutral and eco-friendly chemicals and fuels. These products are essential for addressing rising global energy demand and complying with stringent environmental regulations [9], which further highlights the importance of developing new reactors for FTS. In small-scale BTL units, the efficiency of biomass-to-syngas conversion (e.g., gasification and subsequent processing) plays a crucial role in economic feasibility. However, these aspects fall beyond the scope of this manuscript, which is specifically focused on novel intensification strategies for low-temperature FTS reactors.

2. Fischer–Tropsch Synthesis Reaction

As mentioned, FTS is a highly exothermic reaction (enthalpy change of ca. 165–180 kJ/mol of CO converted depending on the product [10,11,12]). Heat dissipation is, therefore, a major challenge in FTS reactors. If the heat is not dissipated effectively, it can lead to several problems, including hotspot formation, catalyst deactivation, thermal runaway, temperature heterogeneity, and undesired product formation. Commercial catalysts for FTS are Co, Fe, Ni, and Ru (Ru mostly as a promoter), and the most common supports are alumina, titania, and silica in both pure and modified forms fused with K. The product distribution for conventional FTS catalysts obeys the Anderson–Schulz–Flory (ASF) law of polymer chemistry [13,14]. This model describes that the molar fraction (Mn) of a product (with carbon length of n) is only determined by the chain growth probability (α), which depends on the chain growth rates and chain termination, as expressed by Equation (1):

M_n = (1 − α)α^{n − 1}

(1)

This equation can also be expressed in another form [15] to better understand how much carbon is going to each Cn:

F_n = n(1 − α)²α^{(n − 1)}

(2)

In the above equation, F_n is the weight fraction of the carbon number within the hydrocarbon chain containing n carbon atoms with respect to total product. According to this law, lighter hydrocarbons (C₁–C₄) are more likely to form at lower α values, while heavier hydrocarbons (C₂₁₊) are produced at higher α values. However, the middle distillate, which is the generally preferred product, cannot be obtained with high selectivity under this law. The α value itself is a function of temperature, pressure, and catalyst type. Maintaining isothermicity is crucial for achieving stable and uniform selectivity of products in FTS, as the exothermic nature of the reaction can cause local hotspots and undesired product formation. As mentioned, the carbon number distribution of FTS products can be predicted by the ASF distribution [6]; however, it poorly predicts the C₁–C₂ content in the low-temperature Fischer–Tropsch (LTFT) process [16]. Shape selectivity and spatial confinement of newly developed catalysts can also change the product distribution to a more desired carbon number range. Work from the Tsubaki [14,17,18,19,20,21] group has focused on the production of jet fuel directly from syngas through FTS without an additional refining/hydrotreatment step. They clearly showed that their product distribution deviated strongly from the ASF distribution. The group achieved this either by use of modified mesoporous zeolite with a confined space and tandem reaction [22], or by the addition of small amounts of 1-olefin, or by the use of a core–shell catalyst [20] as a miniaturized capsule catalyst. Similar efforts for altering the selectivity of FTS are nicely reviewed in Refs. [23,24].

The chain growth and chain termination of the FTS are a function of the reaction mechanism on a molecular scale. However, there are limited in situ studies on the FTS catalyst during the reaction by different micro-spectroscopical approaches, which are necessary to understand reaction mechanisms at the molecular scale. This is due to the dark and covered surface of the catalyst during the reaction, either by coke and other hydrocarbons, which may dynamically convert to each other [25]. Consequently, the exact mechanism of FTS is still unclear, and different groups proposed different contradictory reaction pathways to explain the observed results. The main reaction pathways considered are the carbide mechanisms, CO insertion, and the hydroxycarbene mechanism [6,26]. The CO insertion mechanism suggests that the C−C bond is the result of the constant CO insertion in the previously formed metal−alkyl bond [6]. In addition to the CO insertion mechanism, CO* can also be hydrogenated to form HCO*. The hydrogenation results in a hydroxycarbene (HCOH*). In the hydroxycarbene mechanism, condensation and polymerization of the formed hydroxycarbene result in the growing of the RCOH* chain. This mechanism is rarely justified by experiment [26]. The carbide mechanism is based on metal carbide formation as an intermediate, which later hydrogenizes and polymerizes to form a hydrocarbon [6]. In a recent Science paper, Zhang et al. [27] used a nearly gradient-free microreactor and studied the temperature changes during the FTS on a modified Co catalyst (Ce modified), and they observed a non-isothermal rate and selectivity oscillations that are self-sustained over prolonged time on the catalyst surface. They attribute these oscillations to the adsorption (at low T) or desorption of the reactant (at high T, heat produced by the FTS) depending on the temperature, which increases or decreases the reaction speed and product yields [27]. Using a nearly temperature-gradient-free microreactor, they conclude that the reaction mostly occurred by CO insertion post hydrogenation, and it has a formate-like intermediate, with the carbide mechanism being less probable (as mentioned, the carbide mechanism proposes that CO quickly decomposes on the surface to create local carbon storage, which then further reacts with H₂ to form hydrocarbons) [6]. Another mechanism recently proposed in the literature is a vinylene mechanism that can explain initiation, propagation, and chain-growth during FTS on Co catalysts [28,29]. This model consists of four main stages. First, carbon monoxide is activated by hydrogen to form formyl, a reactive intermediate. Next, the hydroxycarbene intermediate, a key molecule in the process, receives hydrogen from either water or hydroxyl, generating CH* species on the surface of the cobalt catalyst. These CH* species are the building blocks for longer hydrocarbon chains. In the third stage, methylidyne (CH*) is added to an alkylidene (CHCH2R) chain to form a vinylene (CHCHR) end unit, extending the chain. Finally, hydrogenation either completes the chain-building step or terminates the chain, depending on whether hydrogen is added to the β- or α-carbon atom, respectively. Iglesia et al. [30] proposed a mechanism that explains the increased reactivity of low-index facets of Co and the high turnover rates observed in large Co and Ru nanoparticles. This mechanism involves hydrogen- and water-aided CO activation pathways. Compact adlayers reduce CO* binding through intermolecular repulsion, making CO* more reactive and simpler to desorb. This creates space for the transition state required for H-assisted CO* activation steps. The discontinuation of these adlayers (due to the initial formation of a chain) helps transition state structures to form adjacent to a chain without the need for CO* desorption to build extra space.

Although FTS is similar to other industrial exothermic reactions, including methanation and ammonia synthesis, in that it releases heat and requires effective heat dissipation, it differs significantly as it produces a range of products and is not thermodynamically and kinetically limited (see following for further explanation) [31].

The generally accepted kinetic expression for a Co-based FTS reaction by Yates and Satterfield [32] is expressed by Equation (3):

−R_CO = aP_COP_H₂/(1 + bP_CO)²

(3)

a = kinetic parameter (temperature-dependent), b = adsorption coefficient (temperature-dependent), R_CO = rate of CO consumption.

This kinetic expression nicely shows the absence of water and the fact that the H₂/CO ratio is more important than the absolute pressure (See also

Table 1. Effect of different processes.

Parameter	Effect on Product Distribution	Effect on CO Conversion/Activity
Temperature	Higher temperatures and small α and less C5+ products [13,28].	Higher temperatures favor higher conversion [13,28,33].
Pressure	Higher pressures and high α and more C5+ products [34]. Gas composition has a superior impact than pressure, especially in the case of the Co catalyst.	Higher pressures favor higher conversion [34,35].
Flow rate of reactants	Basically an effect of residence time. A long residence time (low flow rate) increases α and C5+ products [36].	High flow rate and short residence time allow conversion [36].
Gas composition	Higher H2/CO favors paraffins and low chain products (small α) [34,37].	Higher H2/CO increases the conversion but favors methane and undesired product formation [34,37].
Catalyst composition	Co-based catalysts favor paraffins and middle distillate (i.e., diesel, kerosene) yield (high α); Fe-based catalysts favor olefins, oxygenates; Ni-based catalysts are less selective (more CH4 formation) but more active. Fe and Ni are also active for water gas shift (WGS) and reverse WGS in case of low H2/CO ratio or CO2 presence in the feed gas [28,38]. Ru is more active than these metals; however, due to its high price, it only used as a promoter [28].	These catalysts have different temperature windows (conversion is not comparable) [28,38].
Support material	Affects the dispersion of the metal particles, reducibility, and heat conduction, which can impact the activity of the catalyst [39,40].	Acid–base character of the support and its porosity affect both conversion and selectivity. TiO2 supports and coating result in outstanding yield and C13+ productivity [40].
Promoter addition	Can improve the activity, reducibility, and selectivity of the catalyst [28,41]. Ru is one of the most common promoters, especially for a Co catalyst, as it enhances the reducibility of Co [42].	Can affect stability, interaction, and dispersion of NPs and selectivity, as well as conversion and lifetime of the catalyst [41].

) in the case of Co-based catalysts for FTS reactions. It can also be inferred from this equation that for the LTFT synthesis by a Co-based catalyst, heat dissipation is the most important factor for reactor intensification since there is no limitation by formed water, while in the case of an Fe-based catalyst, the expression also includes partial H₂O pressure, as partial water pressure limits the FTS reaction in the presence of iron [31]. Moreover, FTS is a complex process that is more sensitive to reactor temperature and pressure, and diverse products can be obtained in this process; in addition, the products are heavier and can be liquefied (Table 1), which means mass transport can be the limiting parameter in FTS reactors. There are several technologies that can be used to dissipate heat effectively in FTS reactors, including internal cooling, external cooling, and highly (thermal) conductive materials. The reaction parameters of the FTS significantly impact the product distribution, conversion, and selectivity. Table 1 summarizes how different parameter of FTS reactors affect the reaction.

3. Commercial FTS Reactors

As mentioned, FTS is an exothermic process; generated heat must be effectively dissipated to prevent hotspots and thermal runaway of the reactor. Such thermal runaway could lead to rapid local temperature increases, catalyst deactivation, sintering, undesirable product formation, and reactor failure. Mass transfer in an FTS reactor is another important factor, which can limit the reaction efficiency, especially since the products of FTS are mid/long chain hydrocarbons. Commercial reactor designs for FTS involve a variety of configurations to deal with the exothermicity of reactions and mass transfer limitations in large scale production. These includes multi-tubular fixed-(trickle)-bed reactors (or in general, multi tubular fixed-bed reactors (MTFBRs)), fixed and circulating fluidized-bed reactors (F/C-FLBRs), and slurry bubble column reactors (SBCRs) [28]. Each type of these reactors offers distinct advantages and disadvantages, and the optimal choice depends on the specific application, scale, and desired product distribution. MTFBRs are typically packed with a cobalt-based catalyst (and sometimes iron), such as Co/Al₂O₃ and Co/K, and are operated at low temperatures (T = 200–250 °C, P = 2–40 bar, α = 0.85–0.95 [6]) to produce heavy hydrocarbons and waxes. These reactors, while simple in design, are tolerant to coking due to the length of the reactor tube and the presence of non-coked catalyst at the end of catalyst bed [43]. This design relies primarily on narrow reactor tubes and high gas flow rates for heat dissipation [24]. Research indicates that catalyst contact with the reactor wall is crucial for efficient heat transfer [44,45]. This is because larger diameter tubes, like those used in industrial fixed-bed reactors (around 50 mm [44]) struggle to maintain uniform temperature throughout the reactor (isothermal operation). The shell middle distillate process (SMDS) and a Sasol plant in Sasolburg are well known to use industrial fixed-bed reactors in their unit [6,28,46]. F/C-FLBRs can be packed with a variety of catalysts, such as iron-based catalysts (Fe/Al₂O₃) and nickel-based catalysts (Ni/Al₂O₃), and to a lesser extent this includes cobalt-based catalysts (the use of Co-based catalysts in HTFT (high-temperature Fischer–Tropsch) reactors is limited due to its activity for methanation at high temperatures [13]), though iron-based ones are more common in these reactors. These reactors are operated at higher temperatures (300–350 °C; HTFT, α = 0.7–0.75 [6]) to produce a wider range of hydrocarbons, including short/medium-chain paraffins and olefins. FLBRs offer improved heat transfer and mass transfer characteristics (excellent gas–solid contact) leading to higher conversion and increased product yields without hot spot formation and ease of solid handling. However, FLBRs come with higher capital costs, increased operational complexity, and erosion problems due to high linear velocities. Additionally, particle separation is required following the reactor. Sasol’s advanced Synthol technology utilizes fluidized-bed reactor systems [6]. SBCRs are typically packed with a cobalt-based catalyst supported on a metal oxide, such as Co/ZrO₂. These reactors can operate at a wide temperature range (200–350 °C, low, medium, and high temperature ranges for FTS), and they offer improved mass and heat transfer, an absence of moving parts, simplicity of operation, and low operating and maintenance costs [47]. However, they suffer from back mixing, which can also reduce the conversion [47], and their design is very complex (due to complex hydrodynamics in the churn-turbulent flow regime needed for satisfactory conversion levels), especially during the scale up [46,47]. Sasol SPD [48] and ExxonMobil AGC-21 [49] are two well-known technologies using slurry reactor technology. Slurry reactor technology is more common than other commercial counterparts due to better heat management and high stability and other beforementioned advantages [24]. Figure 1 schematically shows how different commercial FTS reactors and parameters of the reaction affect the products [28,50].

The LTFT processes with fixed-bed and in some cases slurry reactors have gained more attention for converting biomass-derived syngas and GTL processes due to their lower capital investment requirements, simplicity, and suitability for smaller-scale operations (

Table 2. Comparison of the commercialization scale of syngas conversion in fossil fuel-based units, biomass production units (BTLs), and small scale GTL units.

Type of Syngas	Production Scale of Fuel
Fossil fuels	15,000–140,000 barrel per day (BPD) [7,56] (≈0.9–8.2 GW, assuming gasoline as fuel, 0.0583 MW/BPD)
Biomass BTL unit	≈21.4–≈342 BPD (1.25 [51]–20 MW [57]) * 500–2000 BPD [56] (≈29–116 MW)
Small scale GTL	1000–2000 BPD [56] (≈58.3–116.6 MW)

* The 20 MW value is the capacity of the largest biomass gasifier in the world for CNG production. The process can be adopted for FTS.

) [38], though these reactors and the catalyst are often optimized for feed gas composition. For instance, the catalyst instead of a Co-based one is a bimetallic catalyst having at least one other metal, which is also active for water gas shift reactions to deal with the generally low hydrogen concentration of bio-syngas [38]. Although, this is not always the case, and in more commercial intensified reactors, Co-Al₂O₃ is still the preferred catalyst since Co catalysts, especially with particle sizes above 4.5 nm, are more stable in concentrated steam conditions, which are common inside intensified reactors (a high amount of water is produced in single-pass conversion mode, which is common for intensified reactors) [51]. It is believed by some researcher that water can re-oxidize small Co particles and cause deactivation of the catalyst [51,52,53], while more recent studies show that hydrothermal sintering [53,54] and cobalt aluminate formation [52] can also occur when a high partial pressure of CO and water exist in the gas stream, resulting in catalyst deactivation. It is also reported that small Co particles (e.g., 1.6 nm) favor the reverse water gas shift reaction [55].

4. Aim of This Minireview

The composition of biobased syngas and its processing scale differ from fossil fuel-derived syngas, necessitating FTS process optimization, as conventional FTS reactor efficiencies will not be suitable for efficient bio-syngas conversion [58]. Since the energy density of biomass and plastic waste are lower than fossil fuels (19 MJ/kg and 54 MJ/kg of natural gas [59]), their transportation has a huge impact on the process cost and should be minimal. Similarly, transportation of natural gas from rural areas is costly, and natural gas flaring is well known problem [8]. Therefore, decentralized [51,60] and modular [6] BTL and GTL units are crucial for economical fuel and chemical production. Further research and development are essential for advancing novel small-scale FTS reactors with advanced heat dissipation and mass transfer technologies to ensure uniform product composition under economically viable conditions. The aim of this minireview, therefore, is to review the intensified FTS reactors, which have the potential for more dynamic and small-scale bio/fossil-based syngas conversion. While several groups have reviewed reactors and catalyst design for intensified FTS [10,28,61,62], advancements in the field since the publication of those papers need to be summarized specifically for bio-syngas conversion. The aim of this minireview, therefore, is to provide an overview of recent technology developments for improved heat dissipation, mass transfer, and loading in these new FTS reactors. The efficiency, flexibility, real time parameter control of the reactors, scale-up potential, and commercial viability will be compared.

5. Catalyst for Decentralized and Intensified LTFT Reactors

It should be noted that different thermochemical and biochemical processes exist for converting biomass and waste to bio-syngas, including gasification, pyrolysis, and digestion. Usually, these units need gas conditioning, reforming, and purification to obtain syngas with a desired composition. The process can also be adapted for hydrogen-rich gas production, using sorption-enhanced gasification processes and post purification [63]. However, producing hydrocarbons (chemicals and fuels) or energy in a gasifier is now more economical than hydrogen, considering the market size and current demand. Co-based catalysts are the most common ones for small intensified units, as they are stable, work in LTFT units, and have no water inhibition [31]. It should be noted that Cobalt-based catalysts are normally used with a H₂/CO ratio of 2.15:1 [64,65,66] in commercial fixed-bed reactors. In the case of bio-syngas, if not upgraded, the hydrogen amount is low (biomass on average contains 6.3% of H₂ [67], and the resulting syngas contains between 20 and 60% of H₂ depending on the process [67]), and CO₂ exists in the produced gas (up to 40% for an oxygen-blown one and generally around 15–24% [68]). WGS and reverse-water gas shift (rev-WGS) reactivity play an important role in CO₂ hydrogenation and balancing of the bio-syngas composition for FTS [9]. Promoting Co/alumina catalysts by Mn/Ti is reported in the literature, especially for intensified reactors [69]. The Co/Mn/Ti/alumina catalyst activity is enhanced by 25% in comparison with conventional Co/alumina, while the selectivity of an undesired product like methane decreased with a similar percentage.

6. Emerging Intensified LTFT Reactors

Different heat dissipation and mass transport enhancement technologies are used to intensify LTFT reactors, including single [70] and multiline microchannel reactors (also known as micro reactors in the literature) [51,71,72], monolithic reactors [73,74], open-cell foams (sometimes referred as structured packing reactors) [12,75,76,77], cross flow structures [78], phase change materials [79], membrane reactors [80,81], and coupled reactors (coupling endothermic and exothermic reactions) [82,83]. As mentioned, some of these designs are reviewed in Refs. [10,28,62]. In this minireview, we tried to cover additional emerging technologies like heat dissipation by phase change materials (PCMs) and 3D-printed metallic and nonmetallic cells. These intensified reactors can be categorized also based on the heat or mass transfer enhancement approaches used in their design, as shown in Figure 2. Note that some of these reactors take advantage of multiple heat and mass transfer approaches. For example, the microchannel reactors not only include a high active catalyst surface with improved mass transfer (diffusion enhancement), but they can also have cooling channels for heat dissipation by convection, and the main structure is made from steel (or similar metals), and therefore they also benefit from high-heat conduction materials (heat dissipation). As discussed earlier, sorption-enhanced intensification, especially sorption of water, which is a common intensification technology for methanol and DME production, cannot improve the LTFT (Co-based) reactor, considering the kinetic model (Equation (3)). A small improvement is reported in the literature [84] (e.g., 1–3% improvement in C₅₊ selectivity); however, this cannot address future needs.

6.1. Heat Conducting Support

One of the simple approaches for improving the heat dissipation of catalysts is to use heat conduction support instead of a common, less conducting ceramic material as a catalyst support. This can be done either by full replacement of the catalyst support or just by diluting, preparing the composite catalyst and mixing the catalyst with heat-conducting materials. Mixing, diluting, and packing the catalyst with heat-conducting materials, for instance SiC, C, and Al [60,85], directly increase the heat dissipation in the packed bed reactor. For instance, it has been reported that the heat conduction of Co/Al₂O₃ catalysts on an Al support is superior than that of pure Co/Al₂O₃ [85]. The SiO₂-modified Al₂O₃@Al-supported cobalt for FTS [86] is also reported. Laser flash measurements revealed that the heat transfer coefficients of Co/SiO₂-Al₂O₃@Al were over 30 times higher than those of commercial Co/Al₂O₃. The thermal conductivity study indicated that Co/SiO₂-Al₂O₃@Al maintained a uniform radial temperature gradient in the catalyst bed, even without a diluter, demonstrating excellent thermal conductivity within the reactor. In another study, embedding SiC nanoparticles in a Co/Al₂O₃-Al-based core–shell type catalyst is reported, which improved heat conduction of the catalyst support and resulted in improved FTS performance [87]. Carbon materials such as nanofibers and flakes are suggested as supports to create heat-conducting catalyst beds, as they also improve the dispersion and reducibility of Co nanoparticles [39]. Osa et al. [88] compared cobalt supported on different materials, and they concluded that above 235 °C, the activity of the catalysts follows the trend TiO₂ < Al₂O₃ < bentonite < SiC. They also observed that different supports have different working temperatures and selectivity. For instance, catalysts with bentonite supports have good catalytic activity at low temperatures; however, oxygenate formation is boosted on this catalyst.

Fratalocchi et al. [89] showed that diluting the catalyst bed with separate conductive pieces does not result in the same isothermicity and temperature control as when the catalyst is filled inside the connected cellular/porous structure with the same composition. Therefore, more promising results are obtained by using different monolithic heat-conducting materials inserted inside the reactor to improve the heat conduction performance, which will be reviewed in following sections.

6.2. Microchannel Reactors

Microchannel reactors, or simply microreactors, are compact FTS reactors that have micrometer range channels. The small microchannels dissipate heat faster than conventional reactors with larger channels, with sizes in the range of 20–30 mm (i.e., an inch) so that higher catalysts can be loaded. As a result, microreactors often achieve conversions around 70% per pass. This limit is maintained to prevent catalyst deactivation at high water vapor partial pressures. Microreactors are designed for cost-effective small-scale production. While a single micro-channel reactor may be limited to producing up to 50 BPD of liquid fuel, they can be scaled up to produce over 500 BPD [71]. The surface-to-volume ratio increases by decreasing the channel diameter, leading to inhibition of gas-phase-free-radical reactions and intensification of heat transfer and, consequently, improved conversion and selectivity of hydrocarbons. Nevertheless, it is crucial to emphasize that this approach introduces additional struggles during the modeling. Therefore, it becomes essential to empirically re-estimate the parameters needed for pressure drop calculations [6]. Catalytic microstructures boast an exceptional external surface area, ranging from approximately 20,000 to 50,000 catalyst surfaces per reactor volume, in units of m²/m³. Commercial tubular monoliths with a hydraulic diameter of 0.9 mm packed into 50 mm inner diameter tubes have this value in the range of 2600 m²/m³ [90]. A traditional commercial reactor provides a surface area of around 100 m²/m³, and in optimal conditions, it reaches a maximum of 1000 m²/m³. This enhanced surface area per volume of microreactor, coupled with an integrated cooling system, dramatically improves heat transfer within microreactors [6]. Microreactors also exhibit a narrow residence time distribution (uniform flow rate), which allows for sequential processes to reach high selectivity for targeted products. The intricate shapes of microchannels improve catalytic activity by facilitating the use of different catalyst coating techniques at this scale, which can also reduce the mass transfer resistance [6].

A single microchannel reactor with cooling is demonstrated by Cao et al. [70] The heat distribution isotherm is maintained by oil circulation around the reactor and the small size of the microchannel (gap width of 0.508 mm). Outstanding FTS activity is observed in this mini reactor, which results in a reported productivity of 2.14 g C₂₊/(per gcat and time of 1 h), while still maintaining low methane production and high chain growth, with a gas hourly space velocity (GHSV) up to 60,000 h⁻¹. They also perform simulations and studied temperature gradients inside single catalyst particles and intraparticle temperature gradients. They showed that temperature gradients in the intraparticle region are negligible for particles below 150 μm, while 6 degrees of gradient could be observed in particles with a size around 1 mm (parameters of simulation: T (particle surface) = 232 °C, GHSV = 21,500 h⁻¹, H₂/CO = 2, total conversion = 60%, particle property: d_p = 150 μm, ρ_p = 2.5 g/cm³, k_eff = 0.2 W/m K). In the conventional fixed-bed reactors, particles are at the mm scale to inhibit pressure build-up, and smaller particles cannot be used, which indicates that even inside one particle there is heterogeneity of the FTS product. In the same year, Myrstad et al. [60] also reported on a multichannel reactor made by etched foil arrangements. The foils had a pillar structure and hexagonal arrangement (400 μm depth and 800 μm distance between the pillars). An 800 μm channel height was obtained by arrangement of the pillars. The catalyst channels were sandwiched between the cross flow oil channel for heat exchange. The isothermal behavior of the microchannel reactor and acceptable pressure drop below 1.5 bar/m, even in a high GHSV of 20,000 h⁻¹, confirm the promising function of such multichannel reactors for FTS.

The Velocys microchannel reactor system, combined with a unique catalyst from Oxford catalysis, is designed for a highly efficient and stable FTS system. The microchannel structure effectively removes heat, enabling consistent 80% conversion in a single pass. The catalyst demonstrates stability under these demanding conditions, even with high water content (H₂O/H₂ ratios of 7–8). Moreover, the catalyst’s performance can be completely restored using an oxidative regeneration approach, which can be repeatedly applied without detrimental effects [72].

INERATEC GmbH (Karlsruhe, Germany) has been working on developing compact and efficient Fischer–Tropsch reactors for producing carbon-neutral liquid fuels from renewable hydrogen and biomass. The company has demonstrated the feasibility of modularization and has scaled up its reactors by a factor of more than 5000 from the lab-scale to reactors that can produce up to 2 BPD of hydrocarbon product. Power-to-X (PtX) plants using these reactors have been manufactured in sizes of up to 1 MW of electrolysis. The company is currently developing 1.25 MW reactors for further plant scale-up. The produced fuels can be used for long-term energy storage, as sustainable alternatives to fossil fuels, and for chemical applications [51]. The experimental results by Yu et al. [91] demonstrated that the performance of the FTS reactor in omega-shaped microchannels surpasses that of conventional straight or zigzag-shaped microchannels. The omega-shaped design increases contact time and provides a larger surface area, resulting in higher conversion rates and enhanced hydrocarbon chain growth.

Todic et al. [92] categorized the monolith and heat conducting foam reactors as microreactors. However, for this minireview, we will discuss them separately in next section, as they have a key difference from microchannel reactors. For example, the heat-conducting porous structure can be inserted inside the common fixed-bed reactor, and in some cases they can be packed by commercial catalysts, while this is not the case for microchannel reactors. Moreover, the microchannel reactor can have cooling channels near the process channels, while this is less practical in the case of heat-conducting inserts.

6.3. Structured Reactors (Monoliths, Heat-Conducting Inserts, and …)

Traditionally, monolithic structures (especially ceramic ones) are produced by extrusion or corrugation and post spindling, and they are vastly used in environmental catalysis [93] as supports of the catalyst in the exhaust system in cars and other emission sources for toxic gas emission abatement. Their channels are usually in the shape of a honeycomb or are tubular; however, with 3D printing, more elaborate shapes of the channels and their connectivity inside the monolithic reactors can be obtained. Concepts of monolithic loop reactors for FTS are suggested by Deugd et al. [73], and they explain that such reactors demonstrate a high conversion and reasonable pressure drop, while maintaining high selectivity through short diffusion lengths and minimal temperature increases in the reactor. Earlier, in 1999, Mesheryakov et al. [94] used gas-lift reactors with monolith-type catalyst packing, and their calculation shows that this type of reactor can outperform the traditional slurry and fixed-bed reactors. In the follow-up, Deugd et al. [74] showed that catalyst activity dropped only 7% after 200 h of operation under stoichiometric and hydrogen-rich feeds, and at different temperatures up to 230 °C; however, extensive deactivation is observed in CO-rich conditions (76% of activity drop). The use of structured and foamed ceramics for different catalytic applications is reviewed by Twigg et al. [95], and they conclude improved mass transfer and heat conduction in FTS (due to less pressure drop and higher heat flux) and methane reforming processes by these structured ceramics in comparison with packed-bed catalysts. The technique for successful loading of active sites in such ceramics is also summarized in this review paper. According to this review, the CH₄ formation can be decreased from 52–54% for packed powder and pellets to 24% in the case of ceramic foam loaded with FTS active metals, while productivity is doubled [95]. Despite these results, and although simulations and experiments [96] confirm that by using a monolithic design the pressure drop and mass transfer (as well as heat conduction) can be improved in comparison with packed catalysts, the heat conduction of ceramic supports (i.e., porous alumina) and formed catalysts are generally low (0.2 W m⁻¹K⁻¹ in the case of a Co-Al₂O₃ catalyst) [70], since they are designed to have a high surface area (porous structure), and they have a heterogenous composition. Consequently, it can be concluded that the use of different shaping techniques like 3D printing or extrudate to prepare monoliths of conventional catalysts would not significantly improve the heat transfer of the reactor. Replacing alumina with SiC as a monolithic support in LTFT reactors is also reported by Lacroix et al. [97], which resulted in increased CO conversion up to 70%. They observed a significant difference in terms of the C₅₊ selectivity in alumina-supported and SiC-based monoliths (i.e., 80% for Co/SiC and 54% for Co/Al₂O₃), which specify that under harsh FTS conditions the SiC seems to be a more suitable foam support. Tronconi et al. (Eni S.P.A. and Politecnico Di Milano) [98] patented the application of a honeycomb monolithic structure with a high thermal conductivity (it can be Al or other metal alloys based on their patent) filled with catalyst in granule form for different endo/exothermic reaction intensification. In the follow-up patent, their group [99] mentioned the difficulty of preparing tubular monoliths of different sizes, and they suggested the assembly of two or more thermoconductive monolith bodies to extend them in a longitudinal direction. This way, they have further flexibility to obtain the desired size of the multi-structured tubular element (monolith).

In additional to traditional honeycomb and tubular monoliths, other conductive inserts, including conductive microfibrous structures [100,101,102], knitted wires, open-cell foams, open and closed cross flow structures [103,104], and cooling inserts [105], are also reported in the literature for heat dissipation from FTS reactors. Barrera et al. [105] from OxEon Energy LLC. (North Salt Lake, Utah, USA)performed simulations to determine the optimal shape of cooling inserts for efficient heat removal (minimal ∆T), and they showed that for a higher working temperature (e.g., T = 217 °C instead of 212 °C), more intricate hierarchical fins with tapers toward the center of the reactor are necessary to obtain high heat transfer of the cooling inserts.

Harmel et al. [106] prepared cobalt nanowires on the copper foams in monolithic form, and the prepared Co/Cu_foam catalyst was studied for FTS in an FBR, showing stability and higher activity and C₅₊ selectivity in comparison with a Co/SiO₂-Al₂O₃ reference catalyst. Wang et al. [107] studied the effect of axial gradient of Co-loading on FeCrAl monolith catalysts, and they showed that a decreasing gradient is beneficial in a high GHSV of 28,000 h⁻¹ for better thermal management inside high-flux LTFT reactors.

The main challenge of monolithic catalytic design is to have an even dispersion of active species on the monolith’s surface. Oxidation or self-activity of metal parts in carbon monoxide reactions are also possible [108]. Gyraznov et al. [108] report embedding Al flakes and exfoliated graphite in extrudate catalyst [108], where Al embedding improved the conductivity of original extrudate catalyst (containing 20% wt. boehmite and 80 wt%. zeolite) from 0.3 Wm⁻¹K⁻¹ to 4.02 Wm⁻¹K⁻¹, while in the case of exfoliated graphite, it increased up to 8.99 Wm⁻¹K⁻¹. The exfoliated graphite-based catalyst was tested at a high syngas GHSV (1000–4000 h⁻¹) in a scaled-up reactor (12 × 1 × 6000 mm), and superior C₅₊ selectivity and activity were observed for this catalyst.

Wei et al. (Tsubaki group) [109] used selective laser sintering (SLS) to fabricate a Fe, Co, and Ni self-catalytic reactor (SCR) for harsh catalytic reactions such as FTS, CO₂ hydrogenation, and methane dry reforming. They showed high performance and reusability [109,110] of CO-SCR for FTS, especially when a flower-like cross-section is 3D printed, which increases the C₅₊ selectivity. This result can be compared with the results obtained by Yu et al. [91] for microchannel reactors, where they showed higher activity for omega-shaped channels. An indirect path of reactant and product inside the reactor increases the contact time, and therefore the FTS yield and selectivity, to higher hydrocarbons.

Random and Ordered Porous Metallic Structures

Open cell metallic foams are industrially available in the market (especially aluminum), and since they are built from highly heat-conducting metals (Al, Cu, …, see

Table 3. Heat conduction of typical conductors used as foam support for FTS reactor.

Material 1	Heat Conduction Wm−1 K−1
Al	237 [113]
Cu	402 (at 27 °C) [113]
AlSi7Mg0.6/EN AC-42200	150–170 [114]
SiC	98.6 [115]
Ni	93 (at 7 °C) [113]
Stainless Steel (AISI 304 L)	16–17 [116]
Silica and coerdierite	1–3 [117]
Typical FTS catalyst	0.2 [70]

¹ For materials where the measurement temperature is not specified, thermal conductivity values are often assumed to be at room temperature (approximately 20–25 °C) unless otherwise noted.

), they are a good candidate as a heat exchanger inside the FTS reactors. Egaña et al. [75] used Al and FeCrAl open cell structures inside FTS reactor for intensification of the process. They used a wash coat as an approach for loading the catalyst. They observed improved heat dissipation and FTS efficiency on Al foam compared to steel and parallel channel aluminum monoliths. The use of Al foam, FeCrAl alloy, and structured Al monoliths for LTFT reactors is also reported by Almeida et al. [77,111], and they compare them with a microchannel block. They conclude the following order for efficiency of different intensified reactors: microchannels block > micromonoliths > monoliths > foams. On the other hand, they found that increasing the CO conversion resulted in a lower C₅₊ selectivity, which is attributed to the higher diffusivity of H₂ in liquid products inside the pores than CO and possible WGS reactions, which affect the CO/H₂ ratio. Other heat-conducting foams, including SiC [97], Ni [76], and Ni-Cr Alloy [112] foams, are also studied for heat dissipation from FTS reactors. In general, aluminum and copper are the best choices for intensification, as their heat conduction is very high, and they are both commercially available. Though copper is more conductive than aluminum, and the use of copper can result in even more isothermal conditions, Al has the advantage that it forms an oxide layer on the surface, and the formed layer has an affinity with the catalyst support for FTS. Therefore, in the case that different coating technologies are applied for catalyst loading, the use of Al would be preferred.

Packed-bed FTS reactors containing an open cell aluminum foam are tested by Fratalocchi et al. [12], and they showed that by having conductive foam (Figure 3a), they can enrich outstanding performances of 1300 kW/m³ heat dissipation with CO conversions >65%. In the follow-up, Fratalocchi et al. [11] showed that a highly conductive periodic open cellular structure (POCS, 3D-printed alloys of AlSiMg) packed with catalyst pellets is a promising strategy to increase heat exchange in FTS reactors (Figure 3b) [11]. POCSs are designed with uniform pores in comparison to foams with a wide distribution of pore sizes, which results in an improved radial conductivity up to 30% in comparison with traditional foams, according to the simulation by Bracconi et al. [118]. Moreover, having a uniform pore design means that the catalyst and inert loading inside the heat-conducting structure are more predictable. In another follow-up, Tronconi showed that adding an outer metallic skin to the conductive cellular internals (Figure 3c) is necessary to dissipate the heat to the walls of the reactor and better maintain heat isothermicity, which further extends the operational temperature window of the reactor (temperature difference below 10 °C for volumetric heat duty of 2000 kW/m³) [119].

In a pilot-scale study, Tronconi’s group [120] further validate their previous findings that a POCS with skin can significantly enhance the performance of catalytic reactors, particularly for compact-scale applications (Figure 4). They achieve overall heat transfer coefficients of up to 1300 W/m²/K (reactor tube diameter of 28.80 mm), which is significantly higher than traditional reactors. In a Fischer–Tropsch experimental campaign, their packed-POCS reactor achieved CO conversions above 70% at a GSHV exceeding 4000 cm³ (STP)/h/gcat, with C₅₊ selectivity above 0.35 g/h/gcat and CH4 production below 15%. These results were among the best results for a compact FTS reactor according to them.

As part of the GLAMOUR project [121], we also explore intensified reactor concepts for converting syngas from biogenic residues into liquid fuels via FTS. To achieve high productivity per reactor volume, two strategies were investigated, the use of 3D-printed catalysts and thermally conductive aluminum and copper contactors filled with catalyst particles. The best performance was achieved with aluminum foam and 3D-printed copper contactors, yielding heat duties of 880 kW m⁻³ and 1238 kW m⁻³, respectively, far surpassing the 185 kW m⁻³ from 3D-printed catalysts and 218 kW m⁻³ from conventional packed beds. The ordered 3D-printed copper contactors achieved a productivity of at least 0.85 gC_5⁺ gcat⁻¹ h⁻¹ without skin in the design, attributed to the superior thermal conductivity of Cu [122].

6.4. Cross Flow Structures

Nekhamkina et al. [78] simulated the application of cross flow reactors (CFRs) for FTS, highlighting their advantages over packed-bed reactors (PBRs). These reactors have a uniform gas flow composition throughout the entire catalyst bed by applying cross flow configurations. Nekhamkina et al. [78] demonstrated that CFRs could achieve superior activity and selectivity due to their ability to maintain uniform conditions throughout the reactor, and with minimal pressure drop and compositional variation. The analysis is based on published kinetics and a 1D dynamic model, which reveals the potential for traveling fronts or pulses in the reactor. The authors conclude that CFRs offer several benefits but require a more complex reactor design [78]. They also recommend the separation of endothermic WGS reactions from exothermic FTS if they are happening on separate catalysts, for better performance.

6.5. Heat Adsorbing Materials

The concept of using a phase change material (PCM) for heat stabilization in FTS reactors is reported by Odunsi et al. [123]. They present a 2D axisymmetric, pseudo-homogeneous, steady-state model of an Fe-catalyzed LTFT reactor, incorporating temperature variation through the integration of a PCM (Figure 5) and conventional water wall cooling, though modeled PCM materials are not yet applied in LTFT reactors.

6.6. Coupled Reactors

Rahimpour et al. [124] simulated a decalin dehydrogenation reaction coupled with FTS reaction to intensify both endothermic and exothermic reactions. Their design was based on thermally coupled multi-tubular reactors. In other research, they [83] also reported a thermally coupled intensified reactor with FTS as an exothermic reaction and dehydrogenation of cyclohexane as an endothermic reaction. A hydrogen membrane was used in their design to separate the produced hydrogen from the dehydrogenation process. Comparison between different modes in terms of temperature, conversion, methanol production rate, as well as hydrogen permeation rate shows that the reactor in co-current configuration results in less productivity and hydrogen permeation rates than a countercurrent configuration; however, catalysts last longer in this condition [125].

6.7. Fixed-Bed Membrane Reactor

Rahimpour et al. [81,82,126,127] conceptualized a few types of Fischer–Tropsch reactors based on water and hydrogen permeative membranes, with Pd–(23%)Ag alloys used as an H₂ permeative membrane. Hydroxy sodalite is a suitable membrane for H₂O removal from LTFT reactors based on Rohde et al. [80] and Rahimpour et al. [126]. A two-membrane reactor is suggested by Rahimpour et al. [82] for simultaneous hydrogen injection and in situ H₂O removal from an FTS reactor (Figure 6). This reactor was designed to couple the FTS reaction as an exothermic reaction, with decomposition of ammonia as an endothermic reaction. Their simulation shows a 27% enhancement in the gasoline yield and a decrease in CO₂ yield of 35.2% in comparison with conventional fixed-bed reactors. The membrane-based reactor in a way is similar to a crossed flow reactor, since in both cases the idea is to have a uniform feed of gas, though in the membrane reactor, the produced gases can also exit uniformly from the reactor column.

6.8. Catalyst Loading

The catalyst can be loaded in different ways inside the microreactors and reactors with heat-conducting inserts. One simple approach is similar to traditional catalyst packing used in fixed-bed reactors; the only difference is that here the catalyst particles should be very small (150 μm vs. 1 mm, as mentioned in Ref. [70]). Although, Cao et al. [70] used catalyst packing to load microtubular reactors; however, in their case they had only one tube, and the more commercialized microchannel reactor usually used a coating approach for catalyst loading [51]. Washcoating is a well-known procedure for coating of the catalyst both in microchannel and monolithic reactors. Adhesion of catalyst coating is an important parameter to obtain a stable coating, and in the case of metal supports, pretreatment on the metal support (tube or monolith) is necessary to increase the affinity and adhesion of metals with the catalyst. Anodizing of alumina, used by Aguirre et al. [128], increases the aluminum oxide layer thickness on the surface of the aluminum support and, therefore, increases the adhesion of the catalyst and support during the washcoat process. Pack cementation is another approach reported to coat alumina on stainless steel with high adhesion. This alumina layer serves as a support for metals like cobalt, thereby enhancing catalytic activity. The pack cementation technique involves mixing powders of Al, Fe, Al₂O₃, and NH₄Cl, followed by heating the mixture to high temperatures to promote the formation of a strong bond between the alumina and the stainless steel surface. Other common techniques for catalyst coating include metal encapsulation, which involves depositing a thin layer of metal nanoparticles (mainly Fe, Co, and Rh) onto a sol-gel support (such as titanium or silica) and then fixing the layer through calcination (heating to high temperatures). Another approach involves using centrifugal force to achieve a more uniform and dense coating of the catalyst material.

The advantage of washcoating small catalyst particles is the proximity of the catalyst to the cooled reactor wall. Moreover, Aguirre et al. [128] showed both by simulation and also experiment that above a critical thickness (60 μm in their case), a further increase of thickness will deteriorate the selectivity of heavy products (C₅₊ hydrocarbon selectivity) in favor of methane formation. The same results were also reported by Egaña et al. [75], with the only difference that in their case the critical thickness is reported to be 70 μm. Tronconi’s group [129] compared the washcoated foam with packed foam for methane steam reforming, and they conclude that at high temperature (T > 600 C), washcoating is not any more superior to packed foam, as the majority of heat conduction is trough radiation [129], although this is not the case for LTFT reactors. Regardless, another major limitation of coating approaches is that the amount of loaded catalyst is limited, which results in lower productivity per reactor volume. In packed beds, however, larger catalyst particles/pellets must be applied in a trade-off between selectivity and pressure drop. The possibility of packing with catalyst micro-pellets inside highly conductive structured inserts was a game changer and highlights the advantage of such a system over microtubular reactors with limited catalyst loading [11,119]. The packing is specially facilitated in 3D-printed POCS, since the pores are well designed for easy catalyst packing [119,130].

It should be noted that the amount of catalyst loading plays an important role when comparing different reactors. For instance, Guettel and Turek [131] performed a simulation to compare some of the intensified reactors mentioned here with commercial FTS reactors, including a micro-structured reactor, fixed bed reactor, monolithic reactor, and slurry bubble column reactor. Their results indicate that microreactors have the highest productivity per gram of catalyst used, followed by SBCR and monolith reactors. The fixed-bed reactor showed low productivity in their simulation, considering severe mass transfer limitations. However, the microreactor showed low productivity per reactor unit due to an exceptionally small catalyst loading and small reactor volume, similar to that of the fixed-bed reactor. Conversely, the specific productivities of SBCR and monolith reactors are higher (up to one order of magnitude).

6.9. Industrialization of Intensified LTFT Technologies

Tronconi illustrates the pilot-scale POCS with skin [120] in a reactor with a diameter of 28.8 mm and heat dissipation capability of 1300 W/m²/K with a GSHV of 4000 h⁻¹ and CO conversion above 70%. The full product amount, however, is not mentioned/calculatable from the data.

The integrated microchannel FTS pilot reactor developed by Velocys features 40 process microchannels and 425 coolant microchannels, achieving a CO conversion of 70% and methane production of 9%. These results demonstrate superior conversion and comparable selectivity compared to commercial fixed-bed FTS reactors of similar processing capacity (28–44 BPD). The FTS catalyst used by Velocys is developed by the Oxford Catalysts Group, and it allows 95% CO conversion and has a long lifetime despite the regeneration cycles [132,133]. Velocys, Ltd. (Oxford, UK) also tested another reactor coated by the catalyst from Oxford Catalysts, Ltd. (Oxford, UK) [134] containing 276 parallel process channels (∼17 cm in length) and a catalyst bed with a length of 4 to 62 cm and 132 coolant channels. Microchannel FTS enhances reactor productivity and realizes scale economies at low capacity [132]. The following is a list of installations by Velocys/Oxford Catalysts as reported in the report of Myrstad et al. [56]:

• 2010: 1 BPD SGC Energia, Güssing, Austria; 2011: Petrobras, Fortaleza, Brazil, 6 BPD GTL; and 2012: SGC Energia, Brazil, 50 BPD BTL
• Greyrock (London, UK) patent claims their microreactor unit can produce 1000–10,000 BPD of diesel per day [135]. CompactGTL Ltd. (London, UK) [136] also presented an industrial microreactor unit for a GTL unit. In 2010, CompactGTL installed a demo unit of 20 BPD at Petrobras, Aracaju-Brazilm [56].

INERATEC GmbH (Karlsruhe, Germany) [137] also reported building a couple of microreactors at a pilot or near-commercial scale. One of their reactors is reported to have 2.1 gC₅₊gcat⁻¹h⁻¹ of catalyst activity (C₅₊ per catalyst mass) and 1785 kg m⁻³h⁻¹ of space-time yield (C₅₊ per reactor volume). More recently, they built a 1 MW reactor and planned to build a 1.25 MW reactor (Figure 7). They experimentally validated 2 barrel/day production (total hydrocarbon, which can be further scaled up by a factor of 5000) [51]. This is comparable to Velocys reactor, where a 1600 space-time yield [kg m⁻³h⁻¹] is reported, and also Sasol’s Oryx GTL unit with capacity of 20.6 [kg m⁻³h⁻¹] [137]. INERATEC GmbH also build a microreactor for 2 barrel-per-day (total hydrocarbon product) production, and they are further scaling it up to 1.25 MW (≈18 BPD) [51].

As listed in Table 2, a small commercialization scale is needed for GTL (1000–2000 BPD) and BTL units (500–2000 BPD), and it can also indicate that the gap of the pilot-scale microchannel reactor is getting closer to the commercialization scale needed for BTL and GTL units. From an economical point of view, the microchannel FTS reactor achieves cost efficiencies at lower production scales (i.e., 500 BPD) than traditional technologies (>10,000 BPD). This advantage causes microchannel FTS to be economical for BTL, distributed, and offshore GTL applications.

Additionally, microchannel reactors offer significant advantages in terms of compactness and material efficiency compared to conventional reactors. For instance, a microchannel reactor with a production capacity of 1000 BPD requires only 15 tons of catalyst and 490 tons of metal, whereas a conventional reactor with a capacity of 5000 BPD typically uses 90 tons of catalyst and 340 tons of steel. This translates to a 2–3 times higher productivity for the microchannel reactor while using six times less steel. [56] Microchannel reactors offer catalyst productivity exceeding ten times that of slurry and fixed-bed reactors [72,132]. It should, however, be considered that the amount of catalyst loading is limited in these reactors, since a thin layer of catalyst can be coated in the microchannels, and replacing the coated catalyst is not trivial [60,132]. INERATEC GmbH does not use a catalyst layer but instead loads catalyst powder into the microchannels [51,138]. However, loading powder into the microchannels remains challenging due to the small size of the channels.

The main limitation of microchannel reactors is the catalyst loading, which in most of mentioned cases is through coating, which limits the amount of loaded catalyst, but also limit the catalyst replacement possibility. Loading micro-powders results in too high a pressure drop and makes it difficult to handle. Therefore, heat-conducting inserts are more flexible because they can be used in conventional fixed-bed reactors and the newly developed POCSs can be easily packed with different catalysts. While intensified reactors are quickly developing for FTS, several groups like the Tsubaki group still use conventional slurry-based reactors for BTL applications, and they installed several BTL units, including a 10 BPD (barrel per day) pilot plant in Hokkaido. They also installed a 500 BPD (80,000 L per day) diesel demonstration plant in Niigata city as well as a biodiesel plant with a capacity of 4.5 BPD in Akita and a biomass gasification and sustainable aviation fuel plant at the Mitsubishi Heavy Industry Company, with a capacity of 240 kg/D. This group used slurry FTS reactors and not the new intensification strategy; instead, they focused on catalyst design [21,139]. We estimated the technology readiness levels (TRLs) of different intensified FTS reactors based on the reviewed literature in this study (

Table 4. TRL (technology readiness level) of different intensified FTS reactors.

Type of the Reactor	TRL Level	Example of Heat Duties kW/m3
Fixed (packed)-bed reactor	10	79–218 [89,122] 3
Heat-conducting support	3	251 [89] 3
Microchannel reactor	7 (8) 1	Similar or higher than POCS 3
Structured reactors—POCS	5	800–2000 [119,120,122] 3
Cross flow structures	2	N/A
Heat-adsorbing materials (PCMs)	2	N/A
Coupled reactors	2	N/A
Fixed-bed membrane reactors	2 (4) 2	N/A

¹ The TRL in the bracket is based on the plan of INTRATEC to produce 2500 tons of e-fuels per year using catalyst from Sasol in Frankfurt, Germany [139]. ² The reactor is used for other reactions (i.e., methanation) and not for FTS. ³ Heat duties in Fischer–Tropsch synthesis (FTS) reactors are influenced by various factors, including CO conversion, GHSV, and others. Here, we provide examples of heat duties from the literature prior to thermal runaway, illustrating the typical heat dissipation efficiency of reactors in this category. Although we could not find specific data for microchannel reactors, we expect them to exhibit similar heat duties to structured reactors, as they share a similar design with catalysts on heat-conducting metals.

). Microchannel reactors and reactors with heat-conducting inserts are the most developed technology so far, and they also offer excellent heat dissipation (heat duties above 800 kW/m³ are reported for these reactors in the literature). Each of them has their own advantages and disadvantages, as mentioned in previous sections, though reactors with heating inserts have a unique advantage, which is that they can be operated in conventional fixed-bed reactors, and their loading process is comparatively simple.

Recently, the push toward decentralized fuel production has accelerated, with notable advancements in the commercialization of microchannel reactors. While full-scale industrial BTL plants utilizing microchannel technology are not yet widespread, recent developments by INERATEC, Velocys, and CompactGTL indicate a growing trend toward modular, scalable designs. INERATEC has expanded its operations with new Power-to-Liquid (PtL) plants, targeting synthetic fuel production at a commercial scale [139]. Similarly, Velocys’ Bayou Fuels project in the U.S. aims to scale up its microchannel-based FT process for sustainable aviation fuel (SAF) production [56,140]. The industrialization of intensified FTS reactors is expected to continue evolving, driven by increasing regulatory support for sustainable fuels and advancements in process integration.

7. Conclusions

This minireview highlights the critical role of modularization and decentralization for achieving small-scale, commercially viable BTL and GTL units. Intensified Fischer–Tropsch (FTS) reactors are essential for this purpose, driven by advancements in both heat dissipation and mass transfer. Progress has been made in reactor designs with advanced heat dissipation and reactors with POCS and skin (contactor), as well as microchannel reactors reaching pilot and near-commercial scales. However, reactor designs that are only focused on mass transfer enhancement, such as cross flow and membrane reactors, require further experimental exploration and development. The concept of coupled reactors, while promising for efficiency gains and successfully tested for several chemical reactions, still lacks experimental development involving FTS. Among near-commercial options, insertion of conductive heat dissipation structures offers retrofitting flexibility for existing fixed-bed reactors. Microchannel reactors offer optimal heat management, and cooling channels can be used in addition to reaction channels; however, catalyst loading is restricted in them. Microchannel reactors also boast superior catalyst efficiency and reduced metal usage compared to conventional reactors. Based on this review, we can suggest that future efforts should be focused more on experimental validation of mass-transfer-based intensification concepts (e.g., membrane, cross flow, and coupled reactors) and trying to combine them with existing heat dissipation technologies. Additionally, optimizing catalyst loading strategies for microchannel reactors seems to be crucial for their wider adoption. By addressing these challenges, intensified FTS reactors can pave the way for decentralized, sustainable BTL and GTL production.