Effect of Co-Feeding Inorganic and Organic Molecules in the Fe and Co Catalyzed Fischer–Tropsch Synthesis: A Review

Abstract

This short review makes it clear that after 90 years, the Fischer–Tropsch synthesis (FTS) process is still not well understood. While it is agreed that it is primarily a polymerization process, giving rise to a distribution of mainly olefins and paraffins; the mechanism by which this occurs on catalysts is still a subject of much debate. Many of the FT features, such as deactivation, product distributions, kinetics and mechanism, and equilibrium aspects of the FT processes are still subjects of controversy, regardless of the progress that has been made so far. The effect of molecules co-feeding in FTS on these features is the main focus of this study. This review looks at some of these areas and tries to throw some light on aspects of FTS since the inception of the idea to date with emphasis and recommendation made based on nitrogen, water, ammonia, and olefins co-feeding case studies.

1. Introduction

The addition of molecules other than syngas in the reactor during Fischer–Tropsch synthesis (FTS) is considered co-feeding of that molecule to the FTS. These molecules, usually with lower molecular weights, could be water, organic or inorganic additives. For instance, there have been controversies about the co-feeding of water to the FT reactor. Several researchers have investigated the addition of water to elucidate the water effect on both activity and selectivity [1,2,3,4] on the catalyst deactivation [5,6,7], the kinetics and mechanism [8,9,10], and product distribution [3]. Due to the different views on the effect of water co-feeding in syngas during FTS, some authors have focused their studies on shedding light on the subject of positive effects of water co-feeding on the FT rate, which was observed for some, but not all, Co-based catalysts [9]. Reports on the negative effects of water on FT processes are also available [11]. It is worth noting that the FTS mechanism is not entirely clear, and the distribution of products does not typically follow a typical Anderson–Schulz–Flory (ASF) distribution. As such, various molecules have been used in the FT reactions in an attempt to better the understanding of the reaction mechanism and explain the deviations observed from the ASF distribution [12,13,14]. In particular, the additives that have been co-fed to syngas to investigate their effects on the mechanism and influence on product distribution in FTS include light olefins [14,15,16,17], alcohols [18,19], water [1,20,21,22,23,24,25,26], CO₂ [27,28,29,30,31], and many more, as illustrated in Figure 1.

Several researchers [32,33,34] have reported the effect of ammonia in syngas on the FT reaction. The authors studied the effect to explore possible phase changes of the Fe catalyst with change in time on the stream. What they also assessed was the deactivation mechanism of the Fe catalyst by poisoning during the FT reaction [32]. The effect of co-feeding ammonia on the conversion of syngas using traditional cobalt catalyst was investigated to probe the catalyst deactivation and ability to affect the selectivity of methane and C₅₊ hydrocarbon formation [33]. Sango et al. [34] also reported the co-feeding of ammonia in different concentrations to a slurry phase, iron-based FT synthesis to study catalyst deactivation, methane selectivity, the chain growth probability, the olefin selectivity, and the FT reactions mechanisms. In a separate experimental study, controlled and verified quantities of hydrogen sulfide and ammonia were added into the feed gas. Hydrogen sulfide was employed to probe the impact of irreversible catalyst poisoning relative to the intrinsic deactivation profile, whereas ammonia co-feeding was done to quantify the impact of regenerable catalyst poisoning. The combined effects provided insight regarding catalyst deactivation and possibly prediction of time dependant catalyst performance during FTS [35].

In recent studies, the synthesis gas mainly obtained from the reforming of coal, biomass, and natural gas contains significant amounts of carbon dioxide (CO₂) [36]. CO₂ removal from the synthesis gas is quite complex and expensive, hence, consequently, takes part in the FT catalytic activity [36]. A series of FTS experiments, which entail the co-feeding of CO₂ were conducted in a fixed bed reactor over a cobalt-based catalyst to investigate the catalyst deactivation, catalytic activity, and product selectivity and formation rates [28]. The influence of CO₂ content in the feed stream was shown to improve the product distribution toward valuable hydrocarbons and conversion rates [36]. CO₂ was added to the FT feed to investigate the CO conversion, chain growth probability, the products distribution, and FT reaction mechanisms [37,38]. Further investigations of the CO₂ influence in the feed gas relate to the economy and sustainability developments of FT technology [29,39,40]. Still, on the cost-effectiveness of the FT system, it has been reported that the use of nitrogen-rich syngas could be a better approach than the classical processes where syngas is free of nitrogen [39,40]. Inert gases, such as nitrogen, have also been co-fed to the FT reactor. The effects of co-feeding nitrogen (N₂) in the FTS in a fixed bed reactor over a Co/TiO₂ catalyst was investigated by Muleja et al. (2016) to elucidate the effect on the catalytic activity, selectivity, and the implication on the FTS kinetics [41]. Nitrogen-rich syngas was used to gain an understanding of its behavior in thermochemical catalytic conversion to gasoline range hydrocarbon [42]. As such, researchers [43] have recently proposed that when operating a cobalt-based FTS process, the olefin and paraffin formation rates should be considered separately. The reason being that the rates before, during, and after deactivation during FTS are comparatively inconclusive, for instance, in the studies by [41,43] the olefin formation rate was fairly constant over time, and in some cases even increased, while the paraffin formation rate dropped, indicating that the deactivation mainly affects the paraffin formation rate. This review is undertaken to summarize and consolidate the co-feeding work done so far and to try explaining findings in terms of controlling the catalytic activity, the selectivity of desired products, products distributions, understanding the mechanism, kinetics, and/or thermodynamics of the Fischer–Tropsch (FT) processes.

2. Co-Feeding in Fischer–Tropsch Synthesis

Syngas is a mixture of carbon monoxide (CO) and hydrogen(H₂) often used as the feed to the FT process. As the feed in FTS is central to the process, a plethora of studies has been conducted on the feed composition and co-feeding of different molecules with the syngas for the FT reactions [39,44,45]. In terms of co-feeding, extensive investigations have been conducted, and some of the molecules that have been co-fed include water [1,11,45,46,47,48] additives, such as CO₂ [28,30,31,36,49,50], and hydrocarbons, such as olefins [51,52,53,54,55,56]. Although researchers [32,33,34,57] have published findings on nitrogen-containing compounds including ammonia, inorganic compounds, such as nitrogen gas, have not been reported on intensively. But to show the importance of co-feeding in FT reaction, already in the 1950s, scientists had investigated FT processes with both inorganic and organic compounds, to be exact CO₂, CH₄, and N₂, and they have shown that the addition of these diluents poses a negative effect on FT reaction rate [58]. Some major co-feeding effect studies are summarized in Table 1.

The co-feeding of molecules to the syngas during FTS is important. However, each of these molecules has advantages and inconveniences. As such, the choice of one molecule over another additive is dictated by the aim of the study. For example, water is an inherently oxygen-containing by-product in the FTS. Oxygen atoms furnished by the CO in the feed are mainly removed as water in FT reactions. The water may negatively affect syngas conversion as it contributes to catalyst deactivation by oxidation. In addition, the products selectivity product distribution, and secondary reactions and catalyst longevity is also affected. [46]. It is, therefore, necessary to identify the aspect of the FT process that requires improvement to devise an appropriate experiment for the co-feeding of water into the syngas for FT reactions.

In FTS, high methane selectivities are generally not preferred as it is comparatively a low-value product than higher olefins and paraffins. Recycling the formed methane back to syngas would be complex and uneconomical, therefore, suppressing its formation would be key [46]. Most studies should, therefore, focus on reducing the production of methane while increasing heavy hydrocarbons formation.

The literature findings indicate that the effect of the co-feeds, whether organic or inorganic, are almost the same regardless of the operating condition or the catalyst used (See Table 1).

Inorganic co-feeds are often incorporated in FTS to enhance selectivity to higher hydrocarbons. In the case of N₂, however, peer-reviewed publications indicate negative effects of the addition, as in the case of H₂O and CO₂. The inorganic feed H₂O source affected catalyst durability; however, overall co-feed type differences in effects were significantly different with the likelihood of affecting the FTS performance upon feeding.

3. Water Co-Feeding in FTS

Water is of particular interest because the FTS produces a significant volume of water [61]. Conventional and green processes rely on accurate accounts of products and by-products [62]. An account of all large quantities of produced material should be thoroughly conducted to ensure the integration of various scenarios happening at different levels [63]. This is crucial not only for the FT reaction but also for downstream processes [64,65,66]. Okoye-Chine et al. [67] recently proposed that phase of H₂O impacts FT reaction in their report about the effect of water co-feeding on a cobalt-based FT catalyst. They have also proposed that the relationship between water affinity and/or resistance behavior of the supported catalyst affect the activity and selectivity, which could assist the explanation of the kinetic influence of H2O in FT reactions. Fratalocchi et al. [3] found that added water reduced the selectivity to CH₄ and alcohol and increased C₂₅₊ hydrocarbon, olefin, and CO₂ selectivities, which they ascribed to the suppression of hydrogenation reactions by water. The decrease of the selectivity to methane was also observed with in situ experiments designed for laboratory X-ray diffractometers in the laboratory [68]. The ability to influence FT selectivity could be useful in the design of intensified processes, such as FT reactive distillation (RD) systems [69], which would enable streams to be directed to particular trays in an RD to achieve the desired product specification on that tray.

Iglesia et al. [70] have reported that water influences chain growth, selectivity, and depending on the catalyst, the co-feeding of water also affects the rate of the initial CO hydrogenation reaction. When the pressures of the reactants or the reactants conversions are low, the addition of water improves the FTS reaction productivity (rates and selectivity) of olefins and C5+ hydrocarbons. Furthermore, at low CO conversion, the effects of water co-feeding in FTS are influenced by the type of supports and lead to support effects. The pressure is one of the most critical operating conditions of the FT process. The feed partial pressure is equally important. The effect of water co-feeding stands to affect the performance of catalysts. Scientists [1] have studied the effect of water while changing the conversion and thereby, raising the partial pressure of water. They have also investigated the influence with the co-feeding of water to the feed gas. Upon increasing the water partial pressures, a shift from the ASF distribution was observed with the FTS total product distribution, the olefin reinsertion caused typical deviations, to a much narrower distribution. The sole C₁-wise chain growth process cannot be explained by such mechanisms. As such, another product formation route which takes into consideration the combination of adjacent alkyl chains leading to paraffin (“reverse hydrogenolysis”), has, therefore, been suggested [2]. Certain features (physico–chemical properties) of the catalysts affect the catalytic activity behavior in an FT reactor. The reduction in the reaction rates was found when the water was co-fed with an inlet partial pressure ratio (P_H2O/P_H2 = 0.4) for the narrow-pore catalysts. Whereas, the reaction rates increased for the larger pores catalysts with the same water pressure. Overall, when the quantity of water added was equal to P_H2O/P_H2 = 0.7 at the reactor inlet, the reaction rates were suppressed, resulting in permanent deactivation. Furthermore, the addition of water improved the selectivity to C₅₊ and decreased the selectivity to CH₄ selectivity for all type of catalysts. Hence, the pore characteristics appear to define the impact of water on the rates [23]. Bertole et al. [47] added water at partial pressures amounting to 8 bar to an FT functioning unsupported cobalt catalyst and managed to increase the reactivity of CO which was adsorbed on the surface without affecting the reactivity of the active surface of carbon intermediate. An increase in surface concentration was obtained for the monomeric carbon precursors leading to hydrocarbon formation [47]. The interaction of water with the catalysts differs in many ways. The effect of water with Co particle sizes, adsorbed species on the catalyst surface, in terms of secondary reactions, and diffusion in liquid-filled is reported in the literature [1]. The complexity of the effect of water on the product distribution has also been reported [3]. It has been revealed that water has both irreversible and reversible effects. The former becomes evident upon raising the concentration of co-fed water, and the latter is clearly visible, starting from small concentrations of co-fed water. These two effects result in the decline of the selectivity to the undesired CH4 and alcohol while the selectivity to C₂₅₊ hydrocarbon, olefin, and those for CO₂ increase. The authors concluded that these findings can be described by an assumption that water suppresses hydrogenation reactions [3]. Another negative effect of co-feeding water to syngas is the deactivation of the catalysts. At high partial pressures, the presence of water deactivates unsupported cobalt catalysts. The deactivation of the catalyst is noticeable through the reduction of site activity of the catalyst and/or lower CO surface inventory. The treatment of the catalyst with hydrogen generally recovers site activity, and it does not affect surface inventory. The latter observation indicates that cobalt surface loss is due to sintering. This sintering process is prominently facilitated by high water partial pressure, such as >4 bar [47]. Claeys and van Steen [8] reported a noticeable increase in the formation rates of the product and significant changes in the selectivity to the FT product, in particular, lower methane selectivity and enhanced chain growth ensuing from the effect of water. The flow rate of the main reactants (CO and H₂) is another FT operating process parameter that affects the performance of the FT processes. Li et al. [71,72] investigated the effects of adding water to the feed gas by changing the values of the space velocity. They found that the addition of water did not affect the CO conversion significantly at higher syngas space velocity values. At lower space velocity, the water co-feeding decreased the CO conversion; yet, the negative effect was reversible with the catalyst quickly recovering the activity obtained before the feeding of water. When the space velocity is low, the CO conversion is high, and researchers have found that the addition of water permanently deactivates the catalyst [71,72]. The quantity of water added to the syngas feed also plays a role in the catalytic activity. For example, it was found that co-feeding of small amounts of water slightly affects the CO conversion, but interestingly the effect was reversible. However, a large amount of added water which could equal the partial pressures P_H2O/P_CO ∼ 1 in the feed permanently deactivated the catalyst. When the selectivity to CO₂ is higher, it indicated that cobalt oxide or another catalytic form of cobalt, such as cobalt aluminate, was formed in the presence of higher water partial pressure [11]. Dalai and Davis [46] reviewed the influence of water on the performances of numerous cobalt catalysts for FTS and confirmed that the effect of co-feeding water into an FT reactor is quite complicated. They found that the influence of water addition depends on various aspects, including the catalyst support and its nature, the Co metal loading, the promoters used with noble metals, and also the preparation procedure.

It is, therefore, generally accepted by researchers [1,9,11,45,46,47,48] that the effects of water on FTS are quite complex. Dalai and Davis [46] summarised the effects of water in FTS with three scenarios:

• (i). An oxidation process for the cobalt supported catalyst with the extent of oxidation being a function of features of the cobalt namely the crystallite size but also the ratio values of the reactor partial pressures of hydrogen and water (P_H2O/P_H2).

• (ii). The average support pore diameter influences the water-co-feeding.

• (iii). The effects could be kinetic in particular the CO dissociation by direct interaction with co-adsorbed CO can be lowered with water co-feeding while the secondary hydrogenation of olefin products can be inhibited as a result of competitive adsorption of water [46].

On the other hand, Yan et al. used Fe-Pd/ZSM-5 catalyst (a bi-functional catalyst) which yielded relatively high activity and selectivity in producing liquid hydrocarbons when the FT reaction was carried out with nitrogen-rich syngas [73]. Conversely, Visconti and Mascellaro [74] co-fed nitrogen to the reactor, increasing the nitrogen content (23.5% to 45.1%) and kept the total pressure unchanged and found that the CO conversion dropped. It is most likely that the CO conversion decreased because the reactant partial pressures values declined when adjustment was made to maintain the total pressure while increasing the nitrogen content. Nevertheless, researchers who investigated the effect FTS with nitrogen-rich syngas have reported that such FT process is feasible because it could potentially be cost-effective [39,75,76,77,78]. The addition to the FTS of molecules, namely olefins, alcohols, carbon dioxide, water, and isotope markers, has contributed to a better understanding of the reaction mechanism. These same molecules were also used by researchers who were studying the deviations observed from the ASF distribution [12].

4. Organic Co-Feeds

Small hydrocarbons are an important organic additive in FTS. The most commonly used co-feeds include acetylene (C₂H₂), ethylene (C₂H₄), ethane (C₂H₆), propyne (C₃H₄), propene (C₃H₆), and propane (C₃H₈). These hydrocarbons are needed for elucidating reaction mechanisms. These molecules exist in the gas-phase during the reaction and through a series of temperatures used in FTS.

For instance, the influence of low molecular weight olefins on FTS has been examined with the aim of either understanding the mechanism of chain growth or influencing the distribution of products formed [14]. Experiments conducted with co-feeding of ethene, 1-alkenes, and diazomethane as a source of surface methylene have been undertaken to strongly support the hypothesis of two independent mechanisms during FTS with the methylene insertion mechanism as one of them [13]. Furthermore, FT experiments with 1-alkene and ethane co-feeding in a reactor loaded with cobalt catalysts and iron catalysts were carried out to study the product distributions, chain growth probabilities, and different chain growth mechanisms [16,79,80]. In brief, olefins, alcohols or CO₂ additives have all been co-fed to investigate the chain growth and mechanism during FTS [12,14,18,81], the ASF distribution [13], and the role of secondary reactions of olefins [15,17]. Specifically, olefins have been used as co-feeds in FT which resulted in enhancing the selectivity to hydrocarbons (C₈–C₁₆), while at the same time, the formation of light hydrocarbons, such as CH₄ and CO₂ was suppressed [60].

On the other hand, when additives, such as small 1-alkenes, alcohols, or CO₂, were co-fed into the FT reactor, they acted as chain initiators for the FTS. The probe-initiated and conventional FTS progress concurrently, with their separate products overlapping. It has been shown, overwhelmingly, that co-feeding additives do not contribute to chain growth, although when olefins and alcohols were added, their incorporation into FTS products was evident usually through chain initiation [12]. However, up to now, the effects and influences of the additives reported [12] on the overall FT product distributions and on the FT catalysts activity are yet to convince the majority of researchers universally. However, Sage and Burke [12] reported in their review that olefins adsorb on the catalyst surface influenced the overall FTS product distribution. As such, it could be assumed that co-feeding olefins might be useful for FTS to modify the catalyst surface. For example, the catalyst surface modification could be achieved by selectively binding to and/or inhibiting certain active sites favorable to chain growth of higher hydrocarbons, or reactions, such as secondary hydrogenation, to improve the overall FT efficiency and selectivity of the process [12].

Alkane, such as n-hexane, has also been co-fed into the FT reaction. The addition of this molecule to the FTS feed under realistic conditions will not result in supercritical FT conditions. However, such an addition will change the composition of the liquid phase in the FT reactor leading in a higher liquid flow rate and a greater diffusivity of reactants (H₂ and CO). Therefore, the effectiveness of the process was increased if the FTS is conducted under internal mass transport limiting conditions [82]. Patzlaff et al. [79] conducted experiments with 1-alkene and ethane co-feeding using cobalt and iron catalysts and revealed superimposed distributions with different chain growth probabilities resulted from different chain growth mechanisms. When alcohols were co-fed, the results showed reliance on the ratio of the two distributions on the reactant pressures while the promoter effect on iron catalysts also supported the hypothesis of the two mechanisms. They also suggested the CO insertion mechanism as the second approach that is exhibited by the higher growth probability of the resulting ASF distribution [13].

It is known that the FT synthesis is highly exothermic, which makes the heat removal and control of the reaction temperature critical steps since the damage of the catalyst at high temperatures decreases the conversion rate. Product selectivity also shifts with increasing temperature towards the production of more unwanted short-chain hydrocarbons, including methane [75]. FT processes are also categorized based on the temperature at which the operation is being carried out, and the products obtained differ. For example, the syncrude produced during high temperature (300–350 °C) Fischer–Tropsch (HTFT) synthesis has more light hydrocarbons, hence, is gaseous at reaction conditions; more than one product phase is formed on cooling. Low temperature (200–260 °C) Fischer–Tropsch (LTFT) syncrude is acknowledged to be a two-phase mixture at reaction conditions, but at ambient conditions, the syncrude from LTFT synthesis consists of four different product phases: gaseous, organic liquid, organic solid (wax), and aqueous [83]. Medium temperature Fischer–Tropsch (MTFT) ranges from 270 to 300 °C, the product phases are similar to those of LTFT. Although FTS products are useful, it is usually more profitable to obtain fewer light hydrocarbons, such as methane and oxygenate products, which are often unwanted. It has been reported that the commercialization of the FT technology suffers from two of major limitations. These are limited selectivity for the main products since it produces a broad spectrum of products and catalyst deactivation, which leads to frequent replacement of the catalyst [39]. Cobalt-based catalysts are only used in LTFT processes as at the higher temperatures excess CH₄ is produced [84].

5. Effect of Nitrogen as a Co-Feed

In addition to the contribution of cost reduction during syngas preparation, the presence of nitrogen in the syngas is significant for safety purpose, mass balance determination, and controlling the heat in the FT system. Jess and co-workers [77] suggested a process with nitrogen co-feeding to the syngas. They reported that the proposed process with syngas co-fed does not utilize a recycle loop, hence, skipping any build-up of nitrogen in the system. This configuration is probably cost-effective because a recycle compressor is not required. The presence of the syngas with nitrogen plays a significant role which removes substantial amounts of heat generated during the FT reaction [85]. Furthermore, researchers have reported that α-olefins formation was enhanced by the two processes between FTS and N₂ purging. The improved result was achieved, when the liquid filled in pores of the catalyst purged with nitrogen, prompting the release of olefins from liquid to gas phase and minimizing its secondary reaction [86]. On the other hand, Jess et al. [75,77] and Xu et al. [78] separately conducted studies on the effect of nitrogen co-feeding in FT reactor and found that nitrogen only dilutes syngas. The presence of nitrogen in the reactor has little influence on the kinetics if the reactants (carbon monoxide and hydrogen) partial pressures are kept constant. However, Lu et al. [42] used feed composed of nitrogen-rich syngas in FTS and obtained more C₁₀ hydrocarbons and smaller amounts of C₈ hydrocarbons which were more when compared to FTS carried out with pure syngas. These results are different from the findings of Jess et al. [75] and Xu et al. [78]. Lu et al. [42] findings are partially in agreement with conclusions from Muleja et al. [41] studies. Muleja et al. [41] investigated the effect of nitrogen co-feeding to an FT reactor while maintaining the partial pressures of reactants and concluded that the selectivity to all light hydrocarbons decreased while the selectivity to C₅₊ in mainly C₅–C₁₉ fraction is enhanced. They [41], therefore, suggested such findings could not be explained by kinetics alone but compared the vapour–liquid equilibrium (VLE) and boiling effect and drew attention to the boiling phenomenon during FTS which is depicted in Figure 2.

However, it is relatively agreed that FTS is a polymerization process leading to a distribution of mainly olefins and paraffins, although the mechanism by which this occurs on catalysts is still a subject of much debate. Todic et al. [87] reported well detailed schematic reactional representation of the FTS process as depicted in Figure 3.

On the other hand, van Steen and Schulz [88] have simplified the scheme representing (Figure 4) the formation of chain initiators and insertion into the developing chains. This is what is used to show the growth of rate equations using the polymerization principle.

Furthermore, Muleja et al. [89] attempted to explain the effect of nitrogen co-feeding into the FT reactor with a reactive distillation mapping leading to the suggestion that further to kinetics, thermodynamic equilibrium and VLE should potentially be considered for FT processes modeling.

6. Concluding Remarks and Recommendations

Maximizing the yield of high-value product is a critical factor for the commercialization and successful implementation of the FT process. Coal or biomass gasification and natural gas reforming generally produce syngas that contains some short-chain hydrocarbons and impurities [90]. Some feed streams have contaminants that affect the FT reactor in various ways. The ability to integrate some contaminants, such as CO₂, into the process design might be crucial to the success of a new design. Taking advantage of N₂ that is inherent in air could save capital installation costs in eliminating the need for cryogenic air separation units. The by-product water could be used to absorb the heat of reaction upon vaporization in an intensified unit, such as an FT reactive distillation system [91]. Since olefin readsorption has been demonstrated in an FT reactor [17,92,93,94], the light gas stream could be recycled to various trays in a reactive distillation column to influence product selectivity [95]. Insights gained from FT co-feeding studies are not only applicable to FTS but could also be extended to methanol-to-hydrocarbons synthesis and other hydrocarbons processing systems [96]. There are some ongoing ethylene co-feeding studies to try to understand the impact on olefin selectivity over Co-based catalysts [97]. Thus, insight from co-feeding studies leads to process synthesis and integration opportunities that were not envisaged by the researchers probing various phenomena.

Author Contributions

This manuscript went through various iterations. All the authors collected articles on the topic, reviewed them, and edited each other’s contributions on the manuscript.

Funding

This research was partially funded by the National Research Foundation of South Africa, Grant Numbers: 113652 and 120270.

Acknowledgments

We gratefully acknowledge the partial financial support from the University of South Africa and the National Research Foundation of South Africa Grant Numbers: 113652 and 120270. The opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors. The funding bodies accept no liability whatsoever in this regard.

Conflicts of Interest

The authors declare no conflict of interest.

Figures and Table

Figure 1. Illustration of co-feeding components considered in this review.

Figure 2. A schematic diagram for the comparison between the pressure in vapour–liquid equilibrium and boiling [41]. PTotal is the total pressure of the system, PInerts are the total partial pressures of all the virtually insoluble gases, and PSolubles is the total partial pressure of the soluble gases.

Figure 3. Schematic representation of Fischer–Tropsch synthesis (FTS) (chain propagation, hydrogenation to n-paraffins, and desorption to 1-olefins) and 1-olefin secondary reaction (hydrogenation, isomerization, and readsorption) [87].

Figure 4. Simplified kinetic scheme of the successive hydrogenation of surface carbon yielding chain starters and incorporation into growing chains [88].

Table 1

Summary of some effects of the co-feeds used in Fischer–Tropsch synthesis (FTS). The Co-feeds are grouped as organic or inorganic.

Nature of Co-Feed	Co-Feed	Catalyst Used	Reaction Conditions	Effect in FTS	Amount Used	Ref
Inorganic Co-feeds	Water	Pt (0.5%)-15%Co/A12O3	2.93 MPa, and H2/CO was 2.0.	Decreased CO conversion, permanent deactivation of the catalyst.	3–25 vol%	[6]
	Water	0.27%Ru-25%Co/Al2O3 25%Co/γ-Al2O3	205−230 °C, 1.4−2.5 MPa, H2/CO = 1.0−2.5, and 3−16	Reduction in the CH4 rate by 12% catalyst deactivation observed during the addition of water.	10 vol%	[10]
	CO2	Co/γ-Al2O3	(H2/CO = 2), fixed-bed reactor; 220 °C, 20 bar, and SV (L/kg cat/h) = 2000.	Decrease in CO conversion and C5+ selectivity partial oxidation of surface cobalt metal	20 vol%	[30]
	Nitrogen	Co/TiO2	220 °C; 60 (NTP)mL/min to 75 (NTP) mL/min; 20.85 bar abs 25.85 bar abs	Reduced selectivity to the undesired light hydrocarbons (mainly CH4)	28% N2	[41]
	Ammonia	100Fe/5.1Si/2.0Cu/3.0K	220–270 °C and 1.3 MPa using a 1-L slurry phase reactor.	Rapidly deactivation of catalyst simultaneously changed the product selectivities.	0.1–400 ppm	[32]
	Ammonia	iron-catalyst	250 °C, 75 mL NTP/min, H2:CO = 2 and 5 bar, respectively.	The selectivities toward nitrogen-containing compounds enhanced with increasing NH3 content. Rates of formation of alcohols, aldehydes, and organic acids decreased	0–10 vol%	[59]
Organic co-feeds	Ethene	62 wt% cobalt oxide and was supported on kieselguhr	473 K and 110 kPa pressure.	The selectivity of the higher hydrocarbons was improved.	1% to 2%	[16]
	Ethanol addition	10% Co/TiO2 catalyst	T = 220 °C, P = 8 bar, H2/CO = 2)	The selectivity to light products increased, as well as the olefin to paraffin ratio. A significant decrease in the catalyst activity.	2 vol% and 6 vol%)	[19]
	Small oxygenates	Iron catalysts. gas with a mol ratio H2:C0 = 0.5	2.0 MPa; 543 K flow (VHSV = 1000)	Aldehydes suppress and entirely change normal synthesis behavior.	10 mol% dimethyl ether (DME). Diethyl ether (DEE) is 3.3 mol%. Acetaldehyde is 3 mol%	[44]
	1-olefins as additives	Co/ZrO2–SiO2 bimodal catalyst	513 K, 1.0 MPa, W/F Syngas = 10 g − cat h/mol.	Resulted in an anti-Anderson–Schulz–Flory (anti-ASF) product distribution. 1-decene and 1-tetradecene mixed with the volume ratio of 1:1, showed the highest selectivity to jet-fuel-like hydrocarbons. Formation rates of CH4 and CO2, as well as light hydrocarbons (C2–C4), suppressed	20 mol%	[60]