Industrial Feasiblity of Direct Methane Conversion to Hydrocarbons over Fe-Based Fischer Tropsch Catalyst

Recently, as a direct consequence of the dwindling world oil reserves and the growing awareness of the environmental problems associated with the use of coal as energy source, there is growing interest in cheaper, abundant and cleaner burning methane. The Gas-to-Liquid technology offers perhaps the most attractive routes for the exploitation of the world huge and growing natural gas resources. Using this process the erstwhile stranded gas is converted to premium grade liquid fuels and chemicals that are easily transported. However, a widespread application of the GTL process is being hampered by economical and technical challenges. The high cost of synthesis gas, for instance, weighs heavily on the economics and competitiveness of the process limiting its wider application. This work presented a modified Gas-to-Liquid process that eliminates the costly synthesis gas production step. The proposed process utilized an alternative pathway for methane activation via the production of chloromethane derivatives which are then converted to hydrocarbons. It established that hydrocarbons mainly olefins can be economically produced from diand tri-chloromethanes over a typical iron-based Fischer Tropsch catalysts in a moving bed reactor at industrially relevant conditions. Some of the attractions of the proposed process include a) the elimination of the costly air separation plant requirement b) high process selectivity and c) significant reduction of carbon dioxide emissions thereby saving on feedstock loss and the costly CO2 removal and isolation processes.


Introduction
The discovery of more stranded gas resources and dwindling oil reserves, have spurred the growing interest in the Gas-to-Liquid (GTL) technology [1,2].The world proven reserve of natural gas at the end of 2012 stands at 6800 trillion cubic foot (tcf) [3,4] that is, about 3196 billion barrels of oil equivalent.This estimates more than double the crude oil reserves of around 1669 billion barrels [3].However, a total of 2612 tcf of this reserve [5], more than 38 per cent are stranded [6], that is, in remote locations far from existing markets, transportation and processing infrastructures.In addition, a large amount of gas is typically co-produced with crude oil as associated gas.In many cases, the gas is routinely flared resulting in a huge loss of revenue and serious environmental consequences due to the greenhouse gases produced.It has been estimated that the amount of gas being flared in Africa could produce 200 Terawatt hours (TWh) of power, about 50 per cent of the whole continent's power consumption [9,10].Natural gas flaring worsens the global warming problem by significantly increasing the emis-sion of greenhouse gases and volatile organic compounds.A World Bank report estimated that natural gas flaring is responsible for about 10 per cent of the global CO 2 emission [9].Converting the natural gas resources on-site into liquid products becomes attractive.Compare to natural gas, the liquid products are relatively easier to handle, they can be transported using existing infrastructures [7,8,11] and have a higher energy density.One of the most attractive routes for conversion of natural gas is the Gasto-Liquids technology [12,13].The growing world gas resource has resulted in construction of more GTL plants [11,14].
The GTL technology includes a group of processes that convert natural gas into liquid products [15].Most of these processes are scalable and hence offers an attractive route to monetize even a relatively small deposit of natural gas.The technology involves direct [16,17] and indirect [7,18,19] conversion of methane to synthetic liquid products.The direct route involves methane direct conversion into valuable chemicals and chemical intermediates, for instance catalytic oxidation of methane to produce methanol.The indirect conversion via synthesis gas has gained more attention for the production of high premium transportation fuels.Rapid technology development has led to significant technological advances in the GTL technology.While these advancements have added new insights resulting in overall process improvement [20], the heavy dependency on the cost of raw materials still remains an economic challenge [21,22].The production of syngas together with the required air separation unit and the eventual CO 2 removal, is the most capital and energy intensive of the overall GTL process [22][23][24], accounting for between 55% -70% of the total capital investment and running costs of the plant [24,25].Another drawback of the syngas conversion route is the production and emission of relatively large amounts of CO 2 .In the reforming plant, about 20% of the carbon is converted to CO 2 [26].
This study is motivated by the economic challenges posed by the costly synthesis gas production step, the loss of feedstock as carbon dioxide, and the environmental challenges of the CO 2 emission, associated with typical FT plants.A modified Gas-to-Liquid technology that eliminates the use of synthesis gas is proposed.This process provides insight into a potential route for commercial application of a modified GTL process where the costly synthesis gas is replaced by chloromethanes and directly converted to hydrocarbons, either in the presence or absence of hydrogen.The proposed process exhibits important advantages over the typical process.It eliminates the air separation unit which in a GTL plant is the unit with the highest capital investment.Apart from the significant environmental benefit due to zero CO 2 emissions, the process obviates the need for the costly CO 2 removal and isolation plants.The result is a process that is greener, more energy efficient and economical for natural gas conversion to liquid fuels and chemicals.

The Modified Gas-to-Liquid Process
The modified process for the conversion of natural gas to transportation fuels and chemicals consists of three principal steps-production of chloromethane compounds; conversion of the chloromethanes to hydrocarbons, and lastly, chlorine recovery.The first step involves gas phase thermal or catalytic selective chlorination of methane to predominantly di-chloromethane and tri-chloromethane.The monochloromethane is separated and recycled.Secondly, the chloromethane is fed into a moving-bed reactor packed with iron-based FT catalyst and wherein it is converted to predominantly olefinic hydrocarbons FT products and HCl gas, at industrial relevant conditions.The HCl by-product is separated from the FT products to obtain premium fuels with permissichlorine content.The process features a close chlorine loop.The Deacon reac-tion is used to recover chlorine from the hydrogen chloride byproduct, so that effectively there is no net consumption of chlorine in the overall process.Finally, the plant employs a hydrolyser to regenerate the chloride catalyst.The process flow diagram is presented in Figure 1.The overall reaction can be represented as: Consisting of the following independent steps: The process features an excellent process integration to minimize utilities requirement.The process consists of established technologies hence only salient points as relate to choice of reacting system and operating conditions will be mentioned as appropriate.The production of hydrocarbon over the iron catalyst will be mentioned in details however.

Production of Chloromethanes
Methane (typical of alkanes) undergoes very few reactions.One of these reactions is halogenation, or the substitution of hydrogen with halogen to form a halomethane.This is a very important reaction providing alternative pathway for methane activation for the production of synthetic crude, fuels and chemicals.Industrial use of this process will not only eliminate the expensive air separation plants, but as well produce far less greenhouse gases.Gas phase thermal oxidation [27,28] and catalytic oxidative methanation [29] process are suitable for industrial application.The proposed process is based on elimination of need for air separation for oxygen production, hence the gas phase thermal chlorination is selected.
Methane chlorination is a radical reaction characterised by poor selectivity [27], forming a products stream consisting of equilibrium concentration of all the chloromethanes as shown in Equations 2: One way to influence the product ratios is to control the moles of chlorine used and the process conditions.It has been reported that, except for reaction 2.4, the products will contain all the chloromethanes [6,27,30,31].The process conditions chosen to maximize the proportions of di-and tri-chloromethanes were as reported by Rozanov and Treger [27].Methyl chloride was separated from the products and recycled with unreacted methane.Over Fe- based catalysts, polymerisation of methylene chloride (CH 2 Cl 2 ) and CHCl 3 to hydrocarbons (mainly olefins) was achieved in preliminary work as detail in the next section.Hence, the focus is to maximize the yield and recovery of these compounds for the feasibility of this process.

Production of Hydrocarbons
This study established that hydrocarbons mainly olefins are produced from the polymerisation of CH 2 Cl 2 and CHCl 3 over a typical Fe-based Fischer Tropsch catalysts at industrially relevant conditions.The catalysts were reduced in-situ in the presence of hydrogen.
Catalyst Preparation-A Fe/Al 2 O 3 /Cu catalysts prepared by the method of co-precipitation was employed in this study.Details of the catalyst formulation and preparation steps are reported elsewhere [32].The precipitate is dried cake and crushed into particles of pre-determined size.The resulting oxyhydroxides were calcined in a fluidized bed reactor using Ar (flow rate 60 ml/min at NTP) at 350˚C for 16 hrs using a heating rate of 1˚C/min.The calcination step removes interstitial water and other volatiles from the solid precursor.
Catalyst Characterization-Nitrogen chemisorption, SEM-EDX and AAS analyses were employed to obtain the surface area, pore size and pore size distribution, pore volume, particle size, elemental distribution and compo-sition of the crystallites respectively.The bulk phases present was studied with XRD while the reducibility of the metal oxides under the reaction conditions was studied using the TPR method (see Rabiu et al. [32] for details).
Reactions-The reactions were conducted in a fixedbed tubular reactor made up of a ¾-inch OD stainless steel tube and 30 cm long.The required amount of the calcined catalyst is diluted with enough silicon carbide and packed into the isothermal zone of the tube and secured in place with a glass wool.The catalysts are reduced in-situ with hydrogen flowing at 60 ml/min (STP) at a temperature ramped at 10˚C/min to 350˚C for 16 hrs.Upon completing the reduction, for the reaction with hydrogen, a 3-way valve is used to direct the hydrogen gas to the reactor via a saturator filled with CH 2 Cl 2 (and later replaced CHCl 3 ).The temperature of the saturator is pre-determined such that the desired vapour of the chloromethanes is carried with the hydrogen into the reactor maintained at 1 atm and 240˚C.
Products Analysis-The setup is fitted with an online GC-TCD to study the activity and an ampoule sampling point (for offline GC-FID analysis) for full products sampling.The GC-TCD is used to obtain the H 2 conversions and the methane yield.A N 2 -cyclohexane mixture was used as internal standard for the GC-FID analysis.
Results-The XRD pattern confirmed that the dominant phase in the calcined samples was haematite.The elemental composition was confirmed with AAS and EDX studies (Figure 2).The TPR spectra revealed that a high degree of reduction (~85%) was obtained for the samples reduced at 350˚C.From the GC-FID chromatogram only olefins were observed with ethylene been the most abundant.
Thermodynamic calculation predicts the transformation of the catalyst in the presence of chlorine gas to various iron chlorides: FeOCl, FeCl 2 and FeCl 3 .The catalyst expectedly suffers rapid deactivation.In Figure 3, the conversion of the hydrogen co-fed to monitor activity declined rapidly after about 22 hrs on stream.XRD analysis of the spent catalysts shown that the catalyst confirmed the presence of FeCl 2 and FeCl 3 (Figure 4).The catalyst was found to regain its activity after steam was sent to the reactor (as discussed in detail in section 2.4).

Recovery of Chlorine
During the gas-phase methane chlorination reaction, a significant amount of the (natural gas) feedstock is converted to hydrogen chloride gas (as a by-product).Catalytic oxidation, electrolysis and cyclic oxidization processes provide routes for the recovery of chlorine from the   HCl [33].Of particular importance is the heterogeneous catalytic gas-phase oxidation of HCl with air or oxygen to produce chlorine gas [34,35] as shown in Equation (3): The so-called Deacon process is a well-established and matured industrial process for large scale manufacturing of high purity chlorine from hydrogen chloride.Compared to the competing electrolytic processes, the Deacon process requires lower energy input [36] and thermal management requirements [37].A highly active catalyst is required to make the operation of the oxidation process feasible at relatively low temperatures.The original catalyst developed for the Deacon process based on CuO/ CuCl 2 suffers from low stability and low activity as a result of formation of volatile copper chloride species due to volatilization of the active phase [38] and the highly corrosive mixture formed by the unreacted HCl in the presence of water [34].These have resulted into a limited application of the Deacon process, and the dominance of the electrolytic process for large-scale recovery of chlorine from HCl [33,38].
The modified-FT process proposed aims to eliminate the use of pure oxygen and the associated cost for air separation facilities.A low temperature process developed by Sumitomo Chemicals employed a highly stable and active TiO 2 rutile-supported RuO 2 catalyst in a fixedbed reactor configuration [37,38].The uniqueness of the process is that the RuO 2 /TiO 2 catalyst exhibited a high thermal conductivity and high activity for HCl oxidation at a relatively low temperature.The catalyst gave very high HCl conversion up to 90% at reaction temperatures between 200˚C and 350˚C [34,39].The tendency to promote selective and self-regulating surface chlorination while at the same time suppressing in-depth chlorination [40,41] make the catalyst highly stable [39].The Sumitomo's process requires lower energy input compared with the electrolysis process.A drawback however is the high price of ruthenium, hence this needs to be investigated relative to its benefit.An alternative process that employs active but cheaper metal oxide was suggested by Moser [42].The study investigated various catalyst supports for CeO 2 and found that Zirconia gave the best stability and activity.The catalyst was reported to possess excellent thermal conductivity, which reduces hot spots within the catalyst layer.It is therefore proposed for this process and employed at a temperature of 350˚C.

Catalysts Regeneration
As was mentioned earlier, XRD analyses of the spent catalysts shown that the reduced Fe crystallites are readily oxidized to FeCl 2 and FeCl 3 and probably FeOCl, due to the prevalence of HCl and gaseous chlorine in the reactor.When steam was passed over the catalyst at elevated temperature, it readily reduced to magnetite according to equation 4. The catalyst deactivation-regeneration follows: where x = 2 and 3 Finally, the FT product is further treated to remove trace of HCl to permissible level.The HCl is recovered and recycled.

Conclusion
It could be seen that the process configuration proposed in this study offers a potential industrial greener process for the direct conversion of natural gas to highly priced olefins which can be further treated for production of high premium gasoline or as chemicals intermediates.The process generated far less carbon dioxide and the products stream is well defined.

Figure 1 .
Figure 1.The schematic diagram for the modifies gas-to-liquid technology.

Figure 4 .
Figure 4. XRD pattern for the spent catalyst.