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Fischer-Tropsch Synthesis under Periodic Operations Proposal

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Updated: Apr 17th, 2022

Executive Summary

The primary aim of this project was to provide and in-depth analysis of the Fischer-Tropsch Synthesis with a specific focus on the impact of periodic operations on the FTS. The review of literature showed that there is an increasing preference on the use of Fischer-Tropsh process in the present days. Such change of preference has resulted from the increasing demand of fuels that have zero percentage of sulfur in their composition. In addition, trend about the demand of fuels has showed signs of shifting from oil to natural gas. For this reason, there is need for a process that can be used for large-scale production of liquid fuels in the future. The selective Fisher-Tropsch has been cited as the suitable methods that can be adopted in the production of the required fuels, since it is economical and avoids the production of unnecessary gases such as C1-C4 gases.

Additionally, the review of literature showed that even though the use of bi-functional catalysts has been in existence for long, it is not economically successful and hence a suitable alternative is needed. The periodic Fischer-Tropsch reactor operation can be used in the place of bi-functional catalysts since, the periodic operation take into consideration the time-average rate as opposed to steady-state conditions, promotes the life of any catalyst used, as well as enhances selectivity during the process. Besides, the periodic operation is a suitable substitute of bi-functional catalysts in that it enhances the removal of any hydrocarbon chains that may form on the surface of the catalyst used in FTS. Generally, the review of literature showed that periodic operation in Fischer-Tropsch process helps to maximize the yield of the fuel.

A number of outcomes are expected from the overall project. First, it is expected that the periodic operation with hydrogen, as the reagent will lead to an increase in the rate of Co conversion. In addition, the selectivity of the catalyst will be increased following increased exothermic reaction. Such conditions will affect the activity of the catalyst with the focus being on the possibility of achieving the steady state value. This implies that the selectivity of methane can be restored after a while in the case of periodic operation. Secondly, it is expected that there will be a correlation between the formation of the hydrocarbons (in this case methane) and the concentration of the pulsing reagent. Increasing the temperatures of the synthesis will have an enormous effect in the activity of the catalysts, as well as in the formation of the concerned hydrocarbon since high temperatures favor the production of CO2 and water. In addition, such a condition is expected to increase the ratio of olefin and paraffin in cases involving light hydrocarbons. However, it is expected that the increase of temperatures alongside a periodic operation reagent will decrease the olefin/paraffin ratio.

As such, the project aims to present findings that show the effect of the concentration of syngas on the of olefin/paraffin ratio, as well as on the chain growth probability. In this case, it is expected that decreasing the concentration of the syngas will strongly affect the ratio of hydrocarbons that have a low molecular weight. On the other hand, an increase in the concentration of syngas is likely to cause an increase in the chain growth probability, which will provide the basis for analyzing the effect of periodic operation on Fischer-Tropsch.

Project Description

Introduction

The Fischer- Tropsch synthesis often abbreviated as FT has been used for long in the syngas transformation to form valuable petrochemicals or even liquid fuels of high quality (Baron, Larsen and Wajc 2000). The Fischer-Tropsch process is exothermic which necessitates careful temperature control especially in the reactor for purposes of ensuring that the process is safe and successful. For this reason, the synthesis requires the use of catalysts, which are often based on elements in group 8 of the periodic table. Such elements include Ru, Ni, Co, and Fe. However, iron and cobalt are commonly used in the FT synthesis. Nowadays, Fisher- Tropsch has become significant following the increase in demand for middle distillate fuel (ranging between C10 and C20), as well as the view that in future the preference of natural gas would exceed that of oil all over the world (Elvira, Solvas, Wootton and deMello 2013). The use of selective Fisher-Tropsch in the production of certain fuels such as gasoline C5-C11 or even diesel C10-C20 has been considered economically attractive, and is achieved through changing the Schultz-Flory-Anderson distribution. On the other hand, the production of high-octane gasoline range hydrocarbons using bi-functional catalysts is nowadays considered uneconomical since it leads to the production of C1-C4 gases which are not desirable.

In the present analysis, the interest has shifted to the maximization of the amount of high-cetane that the Fischer-Tropsch synthesis can produce (Inderwildi, Jenkins and King 2008). In addition, there has been an increase in the demand for fuel that has low quantity of sulfur. This demand has put a lot of weight in the significance of developing low-sulfur products through the Fischer-Tropsch process. The periodic Fischer-Tropsch reactor operation, also referred to as pulsing, is considered to be a suitable substitute of the bi-functional catalysts. The periodic operations in FT involve the promotion of catalyst life, selectivity during the process, as well as time-average rate (Rouge, Spoetzl, Gebauer, Schenk and Renken 2001).

The periodic operation in FT has been known to provide a platform through which the chain of hydrocarbon that grows in the surface of catalyst is removed. Usually, the role of periodic operation in Fischer-Tropsch process is to help in maximizing the production of diesel. The focus of this project is on the investigation of the role of periodic operation on the Fisher-Tropsch synthesis, especially on product distribution and activity of the catalysts used in the synthesis.

Motivation of the project

Following the increased demand for fuel products that are low in sulfur, there is need for a process that will ensure that such demands are met. Even though FT has been considered one of the most reliable processes in the transformation of liquid fuels of high value, it is important to consider the use of alternative approaches in FTS to ensure that the final products aligns with the market demand (Silveston, Hudgins and Renken 2000). For this reason, periodic operation is considered a suitable approach in this case. However, few studies have addressed the impact of periodic operation on Fisher-Tropsch Synthesis. Thus, this project is motivated by the need to provide in-depth analysis of Fischer-Tropsch Synthesis under periodic operations and cobalt support on silica nanosprings in micro reactor. As such, the project will analyze the Fischer-Tropsch synthesis with a key focus on the effect of periodic operation on Fischer-Tropsch Synthesis.

Literature Review

Introduction

The Fischer-Tropsch Synthesis refers to the process in which a mixture of gases is converted to several liquid fuels (Song, Ramkrishna, Trinh and Wright 2004). Usually, such fuels are obtained from different sources such as biomass, natural gas, or even coal. When the process of synthesis and separation is complete the FTS is upgraded and the resultant liquid used in transportation as fuel. On the other hand, products that are not fuels are often cracked in the presence of hydrogen to achieve the respective fuels, or even in some cases recycled back into the FTS (Kengne and others 2015).

The reaction during the Fischer-Tropsch Synthesis results in the production of water and hydrocarbons in the presence of a catalyst. Often, the hydrocarbons produced during the reaction are α-olefins and n-paraffins, ranging from methane to heavy waxes. In spite of this, the process can also produce some other molecules like aldehydes, alcohols, and branched paraffins alongside the α-olefins and n-paraffins.

Over the recent decades, theories and experiments have been used by researchers and scholars to explain the concept of periodic operation in Fischer-Tropsch synthesis (Ma, Ding, Yang, Liu, Lin 2005). In most of these studies, it has been showed that careful consideration of composition is likely to cause an improvement of the selectivity and rates for any given catalytic reaction with comparison to steady-state operations. The FT synthesis is very useful in the production of fuels from natural gas. In the modern days, the Fisher-Tropsch process has been improved following the introduction of an active catalyst that is based on high wax selectivity. For this reason, supported cobalt has become commonly used in the production of paraffin. This can be attributed to their comparative low price, low water-gas shift activity, high selectivity, and high activity.

Nanosprings and supported Co catalyst

It is important to determine the conditions required, as well as the products desired for purposes of choosing the right catalyst. For example, the use of Co is most preferred in cases involving low temperatures because there is production of methane at high temperatures (Spivey and Egbebi 2007). Often, Fischer-Tropsch synthesis at low temperatures leads to production of hydrocarbons of high molecular weight as opposed to when high temperatures are involved to produce gasoline alongside olefins that have low molecular weight. As such, whenever in need of maximum production of gasoline, Fe catalyst is most preferred (Murata, Okabe, Takahara, Inaba and Saito 2007). On the other hand, the Co catalyst is chosen when the process aims at maximum production of diesel products.

Cobalt is relative expensive than iron. However, to reduce the cost of the catalyst, the cobalt metal is often dispersed onto a support, which should be inert in nature, thermally and mechanically stable, possess balanced metal-support interaction, as well as have a surface area that is solvent-accessible. The implication is that the dispersed metal should not form a metal alloy with the support. Often, Co catalysts are supported by metal compounds such as TiO2, AL2O3, and SiO2. In spite of these, silica nanosprings (NS) has been cited to go past the criteria employed in the case of TiO2, AL2O3, or even SiO2. According to recent research to compare the catalytic performance of Fischer-Tropsch, it was evident that there was high Fischer-Tropsch Synthesis activity whenever CO/NS was used, as opposed to the case of silica gel catalysts (Pintar 2000). Often, the conversion rates of CO are influenced by a number of factors such as the failure of Co specie to be completely reduced, sintering and coking of the nanoparticles of CO.

Research and analysis has showed that the involvement of chain growth mechanism in the FT synthesis can be considered to be a challenge in that it involves limited selectivity with respect to particular outcomes (Ying and others 2015). This is the case when considering large-scale use of the chain growth mechanism. Often, during the Fischer-Tropsch mechanism the formation of the final product follows a method whereby the chain of hydrocarbons grows with each C1 addition.

To improve the selectivity of products the periodic operation method can be used, especially in commercial use. In other cases, the use of a catalyst that has a higher chain probability is preferred.

Periodic Operation

Usually, most of the processes in FT synthesis involve conditions that are in steady states. In spite of this, there are processes that are carried out in unsteady-sates, and are considered to have a lot of benefits in terms of selectivity and productivity.

Periodic operation is a tool that is used in controlling the selectivity and conversion during the Fischer-Tropsch Synthesis. Periodic operation covers several techniques involving the variation of the inputs involved in the system. Several inputs are available for variations, such as temperature, pressure, flow direction, and concentrations. Usually, most of the cases show a trend whereby there is uniformity in the variations of flow rate such that the reactor’s space velocity is always constant. However, such condition is not highly significant. Different studies have noted that there is a great difference between steady-states values and time-averages in cases whereby the feed conditions are identical (Zhang, Kang and Wang 2010). During the comparison of steady-states and periodic operations enchantment is a performance ratio represented as follows:

µ= r/rss

Where r refers to the average time in a periodic operation, and rss is the steady state rate. Periodic operation is significant nowadays because it has the potential of offering mechanistic insight into the conditions occurring prior to responses becoming cycle-invariant. In addition the exploration of periodic operation is based on the need for increased catalyst activity.

The use of hydrogen as a reagent in a periodic operation of FTS has significant effect on the yield of C10-C20. In addition, hydrogen pulse decreases significantly the activity of CO conversion, as well as causes a considerable increase in methane and CO2 selectivity, which is often not desired. On the other hand, periodic operation that involves CO positively influences the production of C10-C20 hydrocarbons. However, CO pulse does not show any effect on methane and CO2 selectivity, but a moderate decrease in the conversion of CO is noticed.

H2 pulsing has a significant effect on the performance of FTS under the activity of cobalt based catalyst. Increasing the frequency of H2 significantly increases the quantity of CO conversion that can be achieved (Zhao and Lu 2009). In addition, the concentration of the syngas determines the growth probability of the chain of hydrocarbons. The use of periodic operations in Fischer-Tropsh Synthesis ensures that there is no loss of shift, as well as activity in the selectivity of olefin and paraffin. However, in order to achieve the desired yield, the pulse duration and pulse frequency should be at optimum levels.

Proposed Research Plan

This project will seek to provide an overview of the Fischer-Tropsch Synthesis with respect to periodic operations and cobalt support on silica nanospring in micro reactor. The choice of the topic was informed by the fact that there exists a knowledge gap on the effects of periodic operations on the Fischer-Tropsh Synthesis. Technically, the project aims at establishing whether or not there are any improvements that can be done on the performance of FTS (Ying and others 2015). This concept is feasible in that improving the performance of FTS has the implication of increased yield of the FTS products.

The research project is based on the hypothesis that periodic operations improve the catalytic performance of Fischer-Tropsch Synthesis. This forms the basis of investigation of the research problem.

The project considers the use of periodic substitution in Fischer-Tropsch Synthesis and the investigation of the flow of reactants whenever similar flow of pulse gas was involved. The resulting changes will be monitored based on product distribution as well as productivity changes in the presence of certain Fischer-Tropsch Synthesis catalyst. In order to evaluate the effect of various catalysts, the focus of the project will be on maximum production of the product yield, in this case, C10-C20. In addition, the research will also investigate the characteristics of pulse sequence such as gas concentration, gas type, duration, as well as the frequency. Several catalysts will be used such as cobalt support on silica nanosprings in micro reactor. The fixed-bed reactor will have a pulsing capacity.

The research problem under investigation is the availability of long chains of hydrocarbons that continuously grow during the Fischer-Tropsh synthesis. Often, when these hydrocarbons grow on the surface of the catalyst, they tend to limit the activity, selectivity and the life of the catalyst. The implication is that the quantity of yield from the FTS is reduced. There has been growing concern on the effects of long-chain hydrocarbon on the efficacy of different catalysts, well as on the overall performance of Fischer-Tropsch synthesis as a suitable method in the conversion of natural gas and biomass to valuable liquid fuels. For this reason, this project will be very significant in that, it will highlight the challenges involved in Fischer-Tropsch Synthesis as well as ways in which the reliability of the FTS can be improved.

First, the outcomes of the research project will on to the existing knowledge on Fischer-Tropsch Synthesis with respect to periodic operations and cobalt support on silica nanospring in micro reactor. This is informed by the fact that there is a research gap on the effects of periodic operations on the efficiency and performance of Fischer-Tropsch. Secondly, the outcomes will provide an in-depth analysis and solution to the problem of long-chain hydrocarbons on the surface of catalysts. In this case, the results will provide a solution on how such a problem can be minimized or even eliminated for purposes of efficient FTS, and maximum diesel yield.

In addition, the research project and its outcomes are very significant in that it will outline some of the problems associated with FTS, along with possible solutions. For example, bi-functional catalysts have been commonly used in FTS, however, in this project; the outcomes will show that bi-functional catalysts are not significant anymore in FTS since they lead to the formation of C1-C4 hydrocarbons that are not desired. The research project therefore, will provide significant analysis and advantages of using periodic operation in FTS, and its significance in enhancing FTS catalytic performance.

References

Baron G, Larsen K, Wajc S. 2000. Selectivity enhancement of consecutive catalytic reactions by periodic operation. Bull. Soc. Chim. Belges, 87(2): 105-13.

Elvira K, Solvas X, Wootton R, deMello A. 2013. The past, present and potential for microfluidic reactor technology in chemical synthesis. Nature Chemistry, 5(11): 905-15.

Inderwildi O, Jenkins S, King D. 2008. Fischer-Tropsch Mechanism Revisited: Alternative Pathways for the Production of Higher Hydrocarbons from Synthesis Gas. J. Phys. Chem. C, 112(5): 1305-07.

Kengne B, Alayat A, Luo G, McDonald A, Brown J, Smotherman H, McIlroy D. 2015. Preparation, surface characterization and performance of a Fischer-Tropsch catalyst of cobalt supported on silica nanosprings. Applied Surface Science, 359 (1):508-14.

Ma W, Ding Y, Yang J, Liu X, Lin L. 2005. Study of activated carbon supported iron catalysts for the Fischer-Tropsch synthesis. Reaction Kinetics and Catalysis Letters, 84(1): 11-9.

Murata K, Okabe K, Takahara I, Inaba M, Saito M. 2007. Fischer-Tropsch synthesis over Ru/Al2O3 catalysts. Reaction Kinetics and Catalysis Letters, 90(2): 275-83.

Pintar A. 2000. Catalytic hydrogenation of aqueous nitrate solutions in fixed-bed reactors. Catalysis Today, 53(1): 35-50.

Rouge A, Spoetzl B, Gebauer K, Schenk R, Renken A. 2001. Microchannel reactors for fast periodic operation: the catalytic dehydration of isopropanol. Chemical Engineering Science, 56(4): 1419-27.

Silveston P, Hudgins R, Renken A. 2000. Periodic operation of catalytic reactors: Introduction and overview. Catalysis Today, 25(2): 91-112.

Song H, Ramkrishna D, Trinh S, Wright H. 2004. Operating strategies for Fischer-Tropsch reactors: A model-directed study. Korean Journal of Chemical Engineering, 21(2): 308-17.

Spivey J, Egbebi A. 2007. Heterogeneous Catalytic Synthesis of Ethanol from Biomass-Derived Syngas. ChemInform, 38(50): 1-2.

Ying X, Zhang L, Xu H, Ren Y, Luo Q, Zhu H, Qu H, Xuan J. 2015. Efficient Fischer’s Tropsch microreactor with innovative aluminizing pretreatment on stainless steel substrate for Co/Al2O3 catalyst coating. Fuel Processing Technology, 143 (1): 51-9.

Zhang Q, Kang J, Wang Y. 2010. ChemInform Abstract: Development of Novel Catalysts for Fischer-Tropsch Synthesis: Tuning the Product Selectivity. ChemInform, 42(1):1-2

Zhang Q, Kang J, Wang, Y.2010. Development of Novel Catalysts for Fischer-Tropsch Synthesis: Tuning the Product Selectivity. ChemCatChem, 2(9):1030-58.

Zhao H, Lu H. 2009. Effect of preparation methods on Co/ZrO2 catalysts in Fischer’s Tropsch synthesis. Reaction Kinetics and Catalysis Letters, 97(2): 289-93.

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