Environmental Benefits of the Fischer-Tropsch Reactor
1. Reduction in Greenhouse Gas Emissions
The Fischer-Tropsch (FT) reactor enables the production of synthetic fuels using syngas
derived from renewable sources, such as biomass or captured CO2, combined with
green hydrogen. This approach significantly reduces the reliance on fossil fuels and
minimizes CO2 emissions, contributing to global decarbonization efforts.
2. Utilization of Waste Carbon Sources
The process can repurpose industrial CO2 emissions or biogenic carbon sources,
preventing these greenhouse gases from entering the atmosphere and fostering a
circular carbon economy.
3. Cleaner Fuel Production
FT fuels, such as synthetic diesel and kerosene, are sulfur-free and have lower
particulate matter and NOx emissions compared to conventional fuels. Their cleaner
combustion reduces air pollution, improving air quality and public health.
4. Compatibility with Existing Infrastructure
The liquid fuels produced by FT synthesis can be directly integrated into existing
transportation and energy systems without requiring significant modifications. This
reduces environmental costs associated with infrastructure overhaul.
5. Energy Storage Potential
FT fuels serve as efficient long-term energy storage solutions, stabilizing energy grids
by storing excess renewable energy as chemical energy. This mitigates the variability of
renewable energy sources like wind and solar.
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Safety Methods for Conducting Fischer-Tropsch Operations
1. Pressure and Temperature Control
FT reactors operate at high pressures (20–30 bar) and temperatures (~210°C).
Continuous monitoring and automated control systems are essential to prevent runaway
reactions or over-pressurization.
Pressure relief valves and safety interlocks must be installed to mitigate risks.
2. Handling Flammable Gases
Syngas (H₂ and CO) is highly flammable and potentially explosive. Gas detectors and
alarms for H₂ and CO leaks should be strategically placed throughout the facility.
All equipment and piping must adhere to fireproofing standards and use materials rated
for flammable gases.
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3. Catalyst Safety
Catalysts, such as cobalt-based materials, may pose toxicity risks during handling and
disposal. Proper protective equipment (PPE) and protocols must be followed for catalyst
loading, unloading, and waste management.
Catalyst deactivation and regeneration should be conducted in controlled environments
to prevent accidental exposure.
4. Efficient Heat Removal
The highly exothermic FT reaction necessitates robust cooling systems to avoid
localized overheating, which could lead to thermal degradation or reactor damage.
Regular maintenance of heat exchangers and cooling systems is critical.
5. CO Exposure Mitigation
Carbon monoxide is toxic at low concentrations. Workspaces should be equipped with
CO detectors, and personnel must be trained to recognize and respond to exposure
symptoms.
Adequate ventilation systems must be in place to prevent gas accumulation.
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6. Emergency Shutdown Systems
Automated emergency shutdown systems should be designed to safely halt operations
in case of abnormal conditions, such as temperature spikes, pressure surges, or
equipment failures.
7. Explosion-Proof Equipment
Electrical and mechanical systems must be explosion-proof and meet the requirements
for hazardous area classifications to avoid ignition of flammable gases.
8. Training and Safety Drills
All operators and personnel should undergo regular training on FT reactor operations,
emergency response, and handling of hazardous materials.
Periodic safety drills ensure readiness for potential incidents.
By integrating these environmental and safety considerations, the Fischer-Tropsch
process can contribute to sustainable energy production while ensuring a safe working
environment
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Introduction (extensive)
Catalyst Selection and Validation
Cobalt-based catalysts are widely recognized as the standard for low-temperature
Fischer-Tropsch synthesis (LTFT), particularly in gas-to-liquid processes targeting liquid
fuels such as diesel and kerosene. Compared to iron-based catalysts, cobalt offers
superior performance in hydrocarbon selectivity, lower water-gas shift activity, and
higher resistance to deactivation by water, a by-product of the reaction. These
characteristics are critical for ensuring high productivity and long catalyst life, minimizing
operational downtime and maintenance costs.
The alumina (Al₂ O₃ ) support provides a robust structure for cobalt dispersion, ensuring
effective catalyst activity through enhanced metal-support interactions. Alumina’s high
thermal and mechanical stability makes it suitable for high-pressure packed-bed
reactors, where temperature gradients and mechanical stresses are significant
concerns. Additionally, alumina supports enhance the thermal conductivity of the
catalyst bed, aiding in heat dissipation during the highly exothermic FT reaction.
The validation of this catalytic system is supported by its widespread adoption in
industrial FT plants and experimental studies. Models such as those developed by Ma
et al. (2014) and validated in this project have shown high correlation coefficients
between experimental and simulated data, confirming the reliability of cobalt on Al₂ O₃
for FT synthesis.
Physical Model and Reactor Type
The multi-channel packed-bed reactor was selected for its ability to maintain precise
control over reaction conditions. Its modular structure allows for uniform distribution of
reactants and facilitates efficient heat removal via internal or external cooling
mechanisms. This configuration minimizes temperature hotspots, which can lead to
catalyst sintering or uneven reaction rates.
From a modeling perspective, a one-dimensional plug-flow reactor model is employed,
incorporating kinetic data for cobalt-based catalysts. The Anderson-Schulz-Flory (ASF)
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model is used to predict hydrocarbon chain distribution, while pressure drop and energy
balances are rigorously calculated to reflect the reactor's macro-scale behavior.
Inlet Conditions
The inlet conditions for the reactor design are adapted from validated kinetic and
thermodynamic studies. A syngas feed of 13 tons per hour, with a molar H₂ :CO ratio of
2:1, at 210°C and 20–30 bar, serves as the basis for the design. These conditions
optimize the selectivity for middle distillates, such as kerosene, while ensuring stable
reactor operation.
Cooling System Design
Isothermal operation is essential for maintaining reaction efficiency and ensuring the
longevity of the catalyst. The reactor incorporates a controlled cooling system
employing a counter-current coolant flow. Among potential coolants, molten salts and
Dowtherm are preferred due to their high heat capacity, thermal conductivity, and
stability at elevated temperatures. These properties enable effective heat removal,
maintaining the reactor at the desired temperature profile and preventing thermal
runaway.
The cooling system design also integrates variable flow control, ensuring adaptability to
changes in heat production during operation. By maintaining a precise temperature
range, the cooling system enhances overall reactor performance and product quality.
Extent of Design Detail
This project covers all major aspects of reactor design, including the determination of
material and energy balances, catalyst loading, pressure drop calculations, and thermal
stability analysis. The design also incorporates safety and scalability considerations,
ensuring that the reactor can operate reliably under industrial conditions. Advanced
simulation tools, such as Aspen Custom Modeler, are employed for rigorous validation
of the design, ensuring accuracy and reliability for scaling to full industrial operations.
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This comprehensive design aims to bridge the gap between theoretical modeling and
practical implementation, providing a scalable and efficient solution for Fischer-Tropsch
synthesis in industrial settings.
Let me know if you need additional elaboration on specific sections or further details!
Introduction (brief)
Here’s the introduction section for your Fischer-Tropsch reactor design report based on
the provided details and the information in the uploaded thesis:
Introduction
The design of a Fischer-Tropsch (FT) reactor requires careful consideration of catalyst
selection, reactor configuration, and operational conditions to ensure optimal
performance. For this project, a cobalt-based catalyst supported on alumina (Al₂ O₃ )
was selected due to its high activity, stability, and selectivity for long-chain
hydrocarbons, making it ideal for the production of synthetic fuels. The alumina support
provides excellent mechanical strength and thermal stability, which are crucial in highpressure operations. Additionally, the cobalt catalyst’s resistance to deactivation by
water ensures a prolonged lifespan, further justifying its industrial relevance.
The validation of this catalyst choice is well-documented in the literature, particularly in
its ability to achieve high conversion rates under conditions suitable for FT synthesis.
The Anderson-Schulz-Flory (ASF) model supports its application for hydrocarbon
distribution, ensuring reliable prediction of product selectivity. These properties align
with the objectives of this macro-scale design project.
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A multi-channel packed bed reactor was chosen for its ability to handle high throughput
while maintaining uniform temperature distribution, a critical factor given the exothermic
nature of the FT reactions. The inlet conditions, adapted from existing validated studies,
include a feed rate of 13 tons per hour of syngas, composed of CO and H₂ at a molar
ratio of 2:1, operating at a pressure range of 20–30 bar and a temperature of 210°C.
This design incorporates a controlled cooling system to maintain isothermal operation,
which is vital for catalyst performance and product selectivity. A counter-current cooling
approach is employed using a coolant with high thermal conductivity and heat capacity,
such as molten salt or Dowtherm, ensuring efficient heat removal and stable
temperature profiles. The reactor design details extend to material and energy
balances, pressure drop analysis, and kinetic modeling to achieve a comprehensive
engineering solution.
The extent of this design emphasizes macro-scale considerations, including pressure
drop management and thermal stability, which are essential for scaling up from labscale data to industrial applications. This systematic approach ensures that the
proposed reactor is robust, efficient, and capable of sustained operation under the given
conditions.
If you would like more specific sections or adjustments, let me know!
Background
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The chart below (reference) show’s South Africa’s CO2 emissions per capita.
This graph is an indication of the country’s need for environmentally friendly and
sustainable fuel production, specifically from biomass. Environmentally friendly and
sustainable jet fuel production from biomass, can reduce the net amount of CO2
emissions by the industry by reducing it’s need for fossil fuels. This reports point of
focus is the design of a Fischer – Tropsch reactor, production of jet fuel from syngas
that is produced from biomass.
Introduction (CPJ Design)
Fischer-Tropsch synthesis (FTS) has regained interest during the last decade as an
option for the production of clean fuels and chemicals from either coal or natural gas
because of changes in fossil energy reserves and environmental demand [1]. Because
the FT reactions are highly exothermic, it is important to remove the heat of the reaction
rapidly from the catalyst bed in order to avoid overheating of the catalyst, which would
otherwise cause the deactivation of the catalyst because of sintering and fouling, and
also an increase in the undesirable methane formation. The extensive research,
therefore, has been focused on the improvement of heat removal of catalyst bed in the
FT process [2].
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Due to the era of high oil prices recently, the importance of synthetic oil manufacturing
technology using natural gas is emerging gradually and the necessity of an
environment-friendly fuel leads to the development of BTL (Biomass-to-liquid), CTL
(Coal-to-liquid), and GTL (gas-to-liquid) processes. Among many processes for
producing synthetic fuel, GTL process for producing synthetic fuel from natural gas has
received much attention for eco-friendly fuel which contains less sulfur and aromatic
components [1]-[3]. Since the most of gas fields in the world are small and mediumsized, GTLFPSO (Floating Production Storage and Offloading) process can be applied
for these fields. Fischer-Tropsch process was developed in 1920s by Franz Fischer and
Hans Tropsch and it plays an important role in conversion of synthesis gases in
chemical industries [4]-[6].
The Fischer - Tropsch reaction process was developed by F. Fischer and H. Tropsch, in
the 1920’s[ ]. The main part of this process is a collection of multiple chemical reactions
converting H2 and CO syngas into a mixture of paraffins, olefins and oxygenates[ ].
The process is highly exothermic, this requiring the removal of large amounts of heat of
reaction[ ]. A controlled cooling system is required to effectively remove the heat from
the system [ ].
Depending on specific design requirements, the carbon chain lengths may differ for the
quality of kerosene required [ ]. In this design, the range of paraffins of interest is
C12H26 to C20H44. Due to this design decision, operating the reaction at a low
temperature would be ideal, since the production of longer chain lengths is of interest [ ].
Conducting the Fischer – Tropsch reaction at a low temperature, such as 210 0 C,
favours the production of longer chain compounds, as compared to short chains, on a
mass basis [ ].
The choice of operating the Fischer – Tropsch reaction at lower pressures, doesn’t just
reduce the chances of excessive amounts of products in the liquid phase, which
reduces reactor model complexities, it has several other advantages such as lower
methane selectivity that can be induced by the low temperature [ ], a longer catalyst
lifespan, since high temperatures lead to rapid catalyst deactivation and hot spots within
the reactor [ ]. The Fisher – Tropsch reaction selectivity towards each product is not
heavily dependent on pressure but on temperature, which allows the optimization of the
process by adjusting other process variables [ ]. Working at lower pressures is safer, as
it reduces the chances of catastrophic failure within the plant [ ] . The lower pressures
are easier to integrate into the downstream processes, require less complex
engineering equipment and overall being a financially suitable option [ ]. An inlet
reaction pressure of 20 bar is therefore chosen in this design.
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The type of catalyst chosen is a cobalt – based catalyst ( Co) supported on alumina
(Al2O3). Cobalt catalysts are generally preferred at low temperatures, they produe a
higher selectivity of long – chain paraffins, minimal oxygenates and are almost inactive
to the water – gas shift reaction and are more stable in the presence of water, compared
to other catalysts [master, using].
The alumina support Al2O3 was chosen over the other common catalyst support for
such a process, such as SIO2 due to its various advantages, though SiO2 may be
cheaper. The Al2O3 catalyst support helps stabilize the cobalt particles, thus
maintaining their dispersion over each tube in the bed and preventing aggregation, thus
keeping the active catalyst surface area constant [ ]. The Al2O3 support also provides
good mechanical stability, which is essential in ensuring that the bed structure does not
fail under the operating conditions [ ]. The support also provides good heat transfer
properties which facilitate efficient heat dissipation over the reactor bed, thus reducing
the chances of hotspots which could lead to runaway reactions [ ]. The support also
provides small pores, due t its particle size, thus exposing more of the Co particles
surface area for the surface reaction [ ]. Al2O3 is inert to water, which is a significant
product in this process [ ].
The kinetic model was the most key part of this design, with multiple sources consulted
during the design decision making, in order to ensure that the design model produces
the correct ratio of products, to make the refinery process easier and to meet the design
objectives. The kinetics were validated and compared using multiple source and
experimental data. The formation of oxygenates is negligible under the design
conditions and is consequently not a huge factor in this design [ ].
A multi - tubular packed bed reactor (MT – PBR) with a cooling system was chosen to
carry out the reaction under isothermal conditions [ ]. The catalyst hydrodynamics were
optimized for a minimal pressure drop while meeting the design requirements. Using an
MT – PBR allows for enhanced and precise temperature control to avoid the formation
of hotspots and to prevent catalyst deactivation [ ]. The tubes are designed to have a
thin diameter, surrounded by a cooling medium, to ensure the effective heat removal
and a uniform temperature distribution along the packed bed reactor length [ ]. This also
maximizes the selectivity of the required carbon chain lengths [ ]. The multi – tubular
design, with many thin tubes, allows for efficient heat transfer using either counter –
current or co- current flow, compared to a single packed bed reactor [ ]. The multi –
tubular design is scalable for larger throughput, with this design having a 13 ton hour of
gas mixture and the large surface area to volume ratio allows for the reaction to occur at
this large scale without compromising the reaction and heat transfer [ ]. This design
promotes catalyst longevity and efficient usage of catalyst surface area by minimizing
channeling and bypassing effects [ ]. This design can accommodate various operating
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conditions, which makes it easier to control to ensure that the kinetics are adhered to,
thus making it a robust design [ ].
1. Enhanced Temperature Control
The FT process is highly exothermic, requiring precise temperature regulation to avoid
hotspot formation that could deactivate the cobalt catalyst.
MT-PBRs use small-diameter tubes surrounded by a cooling medium, ensuring effective
heat removal and uniform temperature distribution across the reactor. This prevents
thermal runaway and maximizes selectivity for desired hydrocarbons.
2. Improved Heat Transfer Efficiency
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The reactor design’s modular nature, with numerous small tubes, allows for better heat
transfer compared to a single large packed bed reactor.
Counter-current or co-current cooling systems in the shell side can efficiently manage
the significant heat released during the FT reactions.
3. Scalability for Large Throughput
MT-PBRs are ideal for large-scale operations like processing 13 tons/hour of syngas, as
they provide high surface area-to-volume ratios, allowing better management of reaction
and heat transfer at scale.
The modular arrangement makes it easier to scale up production by adding more tubes
without compromising reactor performance.
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4. Catalyst Utilization and Longevity
The packed bed design ensures uniform flow distribution of syngas through the catalyst,
minimizing channeling or bypassing effects.
This uniformity enhances catalyst utilization efficiency and prolongs its operational
lifespan by reducing thermal stress.
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5. Flexibility in Operating Conditions
MT-PBRs accommodate variations in operating conditions, such as temperature,
pressure, and syngas composition. This flexibility is crucial for maintaining optimal
reactor performance under real-world conditions.
The design Is robust, handling the high pressures (~20 bar) and temperatures (~210°C)
typical for FT synthesis.
6. Ease of Maintenance and Modularity
Tubes in the MT-PBR can be individually replaced or cleaned without dismantling the
entire reactor, reducing downtime.
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Modular design also facilitates easier maintenance and replacement of damaged
components.
7. High Selectivity for Desired Products
Precise temperature control and uniform syngas flow enable better adherence to kinetic
models, such as the Ma et al. model discussed in the thesis. This maximizes the yield of
long-chain hydrocarbons like kerosene while minimizing unwanted products like
methane.
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8. Reduced Pressure Drops
With optimized tube dimensions and catalyst particle sizes, MT-PBRs exhibit lower
pressure drops compared to single large packed beds. This feature is particularly
beneficial for large-scale operations, ensuring energy-efficient gas flow through the
reactor.
9. Industrial Precedence and Proven Design
MT-PBRs are well-established in the chemical industry for similar exothermic and largescale reactions (e.g., ammonia synthesis and methanol reforming). Their proven
reliability and adaptability make them a preferred choice for FT processes.
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These benefits highlight why an MT-PBR is well-suited for your Fischer-Tropsch
process, particularly at a scale of 13 tons/hour syngas throughput. If you need additional
details or specific calculations, let me know!
Here are the references for the benefits of using a multi-tubular packed bed reactor in
the Fischer-Tropsch process:
1. Rouxhet, Antoine. Modelling and kinetic study of a Fischer-Tropsch reactor for
the synthesis of e-kerosene. Master’s Thesis, University of Liège, 2021-2022.
Available at: http://hdl.handle.net/2268.2/14391.
2. Zhu, Jimin, et al. Modeling and Design of a Multi-Tubular Packed-Bed Reactor for
Methanol Steam Reforming over a Cu/ZnO/Al2O3 Catalyst. Energies, 2020,
13(3), 610. Available at: https://www.mdpi.com/journal/energies.
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3. Oyegoke, T., et al. Design and Fabrication of a Multi-Tubular Fixed Bed Reactor
for Acetone Production as a Pilot Plant Model for Chemical Engineering Training
in Developing Nations. Journal of Pakistan Institute of Chemical Engineers, 2022.
DOI: 10.54693/piche.05021.
If you need formatted references in a specific citation style, let me know!
The alumina (Al₂ O₃ ) support enhances the performance of cobalt-based catalysts in
the Fischer-Tropsch process through several mechanisms:
1. Stabilization of Cobalt Particles: Alumina helps stabilize cobalt particles,
preventing aggregation and maintaining their dispersion, which is essential for
maintaining active surface area.
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2. Mechanical Strength: It provides excellent mechanical properties and high
attrition resistance, ensuring the catalyst remains effective under the process’s
operational conditions.
3. Thermal Management: Alumina facilitates effective heat dissipation in exothermic
reactions like the Fischer-Tropsch synthesis, minimizing temperature hotspots
that could deactivate the catalyst.
4. Control Over Pore Size: The support’s pore size influences cobalt particle size,
with wider pores leading to larger particles and narrower pores producing
smaller, more active particles.
5. Durability in Harsh Environments: Alumina is chemically stable and resistant to
water, which is crucial since water is a significant byproduct of the FischerTropsch process.
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These properties make alumina a preferred support material in industrial applications of
cobalt-based Fischer-Tropsch catalysts.
Here are references to sources discussing alumina as a support for cobalt catalysts in
Fischer-Tropsch synthesis:
1. Sumbal, R., Arshad, M., Khan, Z., et al. (2017). “Preparation and characterization
of cobalt catalysts supported on γ-alumina for Fischer-Tropsch synthesis.”
Biofuels Engineering.
Link to article
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2. Boukha, Z., Brahmi, T., Montero, C., et al. (2018). “Effect of surface modification
on the performance of cobalt catalysts supported on alumina for Fischer-Tropsch
synthesis.” Journal of Catalysis and Reaction Engineering.
Link to article
3. Myhrvold, B., Elmi, K., and Holmen, A. (2020). “The role of α-alumina as a
support material for cobalt catalysts in Fischer-Tropsch synthesis.” Catalysis
Today.
Link to paper
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Aim and Design Objectives
Aim: The main aim of this report is to design a functional Fischer – Tropsch reactor that
maintains a constant selectivity and weight fraction of kerosene ( paraffin molecules)
ranging from 12 to 20 carbon atoms in the man chain for the ease of integration with
downstream refinery processes.
Objectives
One of the main objectives of this design is to produce a constant selectivity of jet fuel (
kerosene).
Maintaining the reactor at isothermal conditions to produce a constant reaction rate and
prevent runway reactions.
Another objective is to design cooling a system that allows for effective and efficient
cooling that prevents hotspots within the catalyst and inhibit catalyst deactivation.
Designing to operate at design conditions that inhibit catalyst deactivation, thus allowing
for longevity in the catalyst utilization. This, in hand, significantly reduces operating
costs due to the exorbitant costs of catalyst particles with a well engineered support.
Designing for ease or maintenance, safe and environmentally friendly operating, with an
orientation and reactor type that is financially suitable to construct and operate.
Lastly, to design the reactor in such a way that it is robust, allowing for optimization by
varying minimal variables.
Literature
The Fischer – Tropsch process can significantly reduce the aviation industry’s
dependence on fossil fuels. This reduces their overall carbon dioxide emissions, as this
promotes recycling CO2 that is already in the environment, rather than releasing
additional CO2 that has been locked away for long periods of time within fossil fuels [ ].
Designing a Fischer – Tropsch reactor that operates at isothermal conditions supports
constant molar fractions within the product mixture, suppressing complications with
integrating the system with the downstream refinery processes [ ].
Most dehydrogenation reactions require….
Cobalt – based catalysts promote a higher selectivity of long – chain hydrocarbons
when operated at low temperatures and a lower methane, CH4 (a greenhouse gas)
selectivity when operated at lower pressures between 20 and 30 bar. Selecting the
operating inlet conditions of 210 0C and 20 bar, would produce a significant molar
fraction of the desired C – 12 to C – 20 paraffin molecule [ ]. These conditions also limit
the reaction rate of the water gas shift reaction which produces undesired water and
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CO2, especially, which is another greenhouse gas, thus making the system
environmentally friendly.
Using the heuristics of catalyzed dehydrogenation reactions, the assumption is made
that at steady – state operation, the overall reaction is surface reaction limited [ ].
Equation 1, below, shows the steady – state reaction rate of the Fischer – Tropsch
process [ ].
Diffusion effects, both external and internal diffusion, of the syngas particles particles to
the surface of the porous cobalt – based catalyst, supported on alumina (Al2O3). Due
the electron configuration of cobalt particles, a significant amount of empty d – orbitals,
that are available as active sites for the rapid chemisorption of syngas molecues H2 and
CO into active sites in the cobalt surface, for the surface reaction to occur, specifically
m, 10^14 to 10^15 active sites per m2 [ ]. The average chemisorption rate of the syngas
particles onto solid cobalt particles ranges from 0.001 to 0.1 molecules per second.
Equation 2 shows the average chemisorption rate of syngas particles onto the the
surface of cobalt particles, based on previous experimental results [ ].
It is also notable to cnsider the expected diffusion rates of the syngas to the surface
cobalt particles within the reactor pores. The average internal diffusion rate takes priority
in such a case, since the point of concern is the diffusion from the bed pores to the
catalyst surface, while dispersing the syngas molecules almost evenly on the catalyst
surface, which is where the active sites are located []. Equation 3 to Equation X shows
the determination of the expected internal diffusion rate on cobalt based catalyst
particles supported by alumina [ ].
The overall reaction kinetics of such a design, could reach a reaction rate of …The
kinetics therefore have to be selected, analyzed and validated with grave
circumspection [ ]. The overall CO consumption rate in such a process, as defined by
numerous studies, ranges from 10 to 100 mmol/gcat.h of CO, for temperatures and
pressures between [ ]. The assumption of the surface reaction rate being the limiting
step in the overall reaction rate is therefore justified [ ].
A multi – tubular packed bed reactor has the advantages the support the design
requirements of this reactor in particular such as…
It also has other advantages such as ….
H2 to CO ratio…[ ].
The Reaction Kinetics and their Validation
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The consumption rates of CO and H2 can be defined using a ….type of relationship for
the reaction rate (r’) and rate constant (ki). The general empirical equation that the rate
functions of the syngas and products in this study, Equation Emp. Is shown below.
This information was obtained from an experimental study for sustainable kerosene
production.
The reactions considered in this study are the paraffin formation, the water gas shift
reaction and olefins. Oxygenates were excluded from this study de to their low
selectivity, especially of moderate to long chains [ ].
Reaction X
Reaction Y
Reaction Z
A maximum carbon chain length of 45 was chosen for the kinetic modelling, to account
for a large number of wax products that form. The reaction mixture flowing up each tube
of the packed bed reactor consists of 99.6 percentage vapour and 0.4 % liquid. The
olefins themselves are produced in smaller quantities,thus only estimated from the
weight fractions distribution.
The Fischer – Tropsch reaction rate is thus modelled with the reaction rate expression
below.
Reaction X
The reaction rate constant can be calculated with Equation X below.
Methane, due to it’s relatively high molar selectivity compared to other paraffin products
is modelled with the rate expression below. Accounting for the overall formation of CH4.
This rate expression has its own rate constant equation, Equation C.
Table X shows the meaning, values and units for al the parameters mentioned above.
The Anderson – Schultz Flory Distribution Model
The chain – growth probability determination technique, the ASF model significantly
estimates the weight and molar fractions of each carbon chain length of either paraffins,
olefins and oxygenates. The weight fraction of each carbon – chain length can be well
modelled with an alpha value of 0.9 to give a larger fraction of the desired moderate to
long chain paraffins, C12 to C20. This can lead to a high kerosene production, with the
possibility of hydrocracking to make more of the moderately lond chain products.
Equation X below shows how the weight fractions are distributes in the product mixture.
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Graph A below shows the ASF distribution of paraffins, due to the catalysts available in
and commonly used in industry, iron – based catalysts (Fe), cobalt – based catalysts,
ruthenium – based catalysts and modified.
It is apparent from these plots that ruthenium would yield most desirable weight
fractions of the desired products but is too rare and expensive to use. Cobalt gives a
weight distribution sufficient for this design. The cobalt – based catalyst is therefore
chosen to model the kinetics of the reactor.
Graph D below shows the ASF distribution for the cobalt based catalyst used in this
design, on a molar basis.
Graph H shows the expected mol fractions of the paraffin molecules, in the paraffin
product mixture.
The methane formation, from past experimental tests, tends to deviate from the
theoretical ASF model, as shown by Graph C below. It is therefore modelled differently
from the other paraffin products.
The Rate Expressions for other Paraffin Products
The overall chemical reaction for the formation of paraffin molecules can be written in
the form below, to expose the stoichiometric coefficients of each paraffin product.
Reaction T
The reaction rate expression for this reaction is therefore modelled as follows (Equation
c) Equation c
The methane formation can be defined independently, to account for deviation from the
theoretical ASF model. The reaction rate for this reaction can be defined by the model
below (Equation g).
Reaction Y
Equation G
The Fischer – Tropsch reaction rate expression is therefore written as a sum of these
tow rate expression.
Equation D
The CO consumption is therefore modelled using to the same expression.
Equation E
Determination of the Stoichiometric Coefficients
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Equation F to Equation H below shows how the stoichiometric coefficients we derived
for this model. They are modelled using a C – atom mass balance.
The overall stoichiometric coefficients of the paraffins and olefins, individual products
based on their carbon – chain length, can be calculated using Equation T below.
For any chain length of products, the ratio of the stoichiometric coefficients of the
olefins to paraffins, of the same chain length can be defined with Equation H below.
The stoichiometric coefficients can then be fully defined with Equation H and Equation
G.
Graph V shows the stoichiometric coefficients of each carbon chain length in the
paraffin and olefins mixture.
Determination of the Hydrogen Usage Ratio
The hydrogen usage ratio is modelled using a hydrogen atom balance to yikes the
expression, Equation O, below.
The hydrogen usage ratio determined, is UH2 = 2.057.
Kinetic Model Validation
These kinetics are verified using experimental data obtained from the work by Ma et al [
]. Their managed to obtain experimental data for isothermal and isobaric conditions,
using the H2/CO ratio of 2.1.
The experimental results are compared to what the reactor kinetic model in this design,
in Graph J.
A parity plot for the Fisher – Tropsch reaction rate and for the CH4 formation is the main
tool used to compare the rates against experimental data.
Graph J shows the parity plot for the Fischer – Tropsch reaction rate.
Graph G shows the parity plot for the CH4 reaction rate.
The squared coreelation coefficient r2, is also calculated to show comparison between
the experimental rates and the rates used in this model. Equation H shows how this
correlation was obtained.
Equation K
With r2 values that are close to 1, it can be concluded that the reaction rate expressions
in this model, fairly describe the process.
Table G shows the r2 values for each reaction rate
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The Reactor Model
The reactor model is defined using 3 fundamental equations, the mol balance, the
energy balance and the pressure drop down each tube, to mathematically model the
operation of the MB – PBR reactor.
Feed Conditions
The symbol, meaning and units of the feed properties is shown in Table U below.
Table U
Preliminary Reactor Sizing
The system was first treated as a packed bed reactor with no tubes to estimate the
catalyst requirements on a mass basis. The conditions were kept isothermal and
isobaric, to favour a design with a minimal pressure drop. The system designed is
expected to have a CO conversation of 50 %.
The following flow profiles, temperature profile and pressure profiles were obtained. The
expected total mass of catalyst was then determined.
The Mol Balance
The mol balance was done for all reactants and paraffin components. The olefins
production can be estimated using the ASF model. The mass balance is shown with the
series of equations, below.
Equations
The resulting flow profiles are shown below. Graph A shows the depletion of products
and H2O and CH4 formation. Graph 7 shows the flow profiles of products C12H26 to
C20H42. Graph Y shows the flow profile of the waxes while Graph T shows the
formation of the shot chain fuels.
The Energy Balance
The reactor is operated under isothermal conditions, with the energy balance using the
equations below.
Equations
The resulting temperature profile is shown in Graph A. It is worth notifying that an
effective and efficient , rapid cooling system is a part of this design.
Graph A
The Physical Model of The Reactor
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As in the experimental runs, the catalyst used in this design is a cobalt – based catalyst
supported on Al2O3. The design catalyst properties are shown in Table 1.
Table 1
The Reactor Configuration and Specifications of Dimensions
The schematic Figure A, shows a simplified version of the M – PBR.
The tube diameters typically range from 30 to 50 mm for effective heat transfers radially,
across each tube, to prevent heat transfer vertically. Due to the heat transfer from the
tubes being a significant part of this design, the internal tube diameter of 40 mm is
chosen for uniform distribution of catalyst particles in the tube and for effective heat
transfer. The wall thickness of ….mm allows for effective heat transfer.
The Modified Mol Balance per Tube
The mol balance was then repeated for each tube, based on the preliminary sizing.
Graph G shows the conversion of CO, with catalyst mass, within each tube. The system
is still kept isothermal. The graphs below show some of to the flow profiles of concern.
Pressure Drop Along Each Tube
The large MT – PBR surface area and low height, is a design that favours minimal
pressure drop along the reactor tubes. The pressure profile is shown below. Using the
heauristics of pressure drop, the overall pressure drop of ….. is within the allowable
pressure drop in a PBR.
Final Reactor Sizing
The conversation CO conversation of 50% is now obtained at a catalyst mass of ….
Which is higher than expected, due to the pressure losses, reactants probably require a
higher residence time within each tube. Table B below shows the final specifications.
Optimization
Equipment Selection and Material Selection
The shell side of the reaction is designed for cooling purposes. The design of the
cooling is excluded from this study.
Design Costs
Design Recommendations
Conclusion
References
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A good range for the percentage of the total internal bed surface area In multi-tubular
packed bed Fischer-Tropsch (FT) reactors, the typical percentage of cross-sectional
area occupied by the tubes usually ranges between 25% and 40%. This percentage
depends on reactor design considerations, including the desired heat transfer, flow
distribution, and pressure drop.
Key Design Considerations:
1. Heat Management: FT reactions are highly exothermic, so the reactor must
efficiently transfer heat away. A higher tube-to-total cross-sectional area ratio
ensures better heat removal through the walls of the tubes.
2. Pressure Drop: A higher tube density increases pressure drop due to the
narrower flow passages for the coolant or gas, which must be balanced with
operational constraints.
3. Tube Packing Efficiency: Optimal packing geometries, such as triangular or
square arrangements, influence the achievable percentage.
Typical Range by Design:
Lower end (20%-25%): Used in designs prioritizing low pressure drop and uniform flow
distribution.
Higher end (35%-40%): Used in high-heat flux designs where effective heat removal is
critical.
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If you are working on a specific reactor design, these values can be adjusted based on
the tube diameter, number of tubes, and the reactor shell diameter. Would you like to
perform a detailed calculation for a particular case?
Integration of the o
Here are some key references that discuss active sites on cobalt for chemisorption in
the Fischer-Tropsch reaction:
1. Bartholomew, C. H. (2001). "Mechanisms of catalyst deactivation." Applied Catalysis
A: General, 212(1-2), 17-60.
This paper discusses catalyst deactivation, active site properties, and chemisorption
mechanisms.
2. Anderson, R. B. (1984). "The Fischer-Tropsch Synthesis." Academic Press.
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A classic book covering the Fischer-Tropsch process, including active sites and surface
reactions on cobalt catalysts.
3. Van Steen, E., & Claeys, M. (2008). "Fischer-Tropsch catalysts for the bio-to-liquids
(BTL) process." Chemical Engineering and Technology, 31(5), 655-666.
Discusses cobalt-based Fischer-Tropsch catalysts, their active sites, and interactions
with CO and H₂ .
4. Dry, M. E. (2002). "The Fischer–Tropsch process: 1950–2000." Catalysis Today,
71(3-4), 227-241.
A detailed review of Fischer-Tropsch catalysis, focusing on cobalt and iron catalysts.
5. Rostrup-Nielsen, J. R., & Sehested, J. (2002). "Hydrogen and Synthesis Gas by
Steam and CO2 Reforming." Advances in Catalysis, 47, 65-139.
Contains information on syngas interactions with catalysts, including cobalt surfaces.
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6. Shroff, M. D., et al. (1995). "Surface site interactions during Fischer-Tropsch
synthesis on cobalt catalysts." Journal of Catalysis, 156(2), 185-207.
Focuses on the nature and quantity of active sites for syngas chemisorption.
If you need more specific details from these references or additional ones, let me know!
The chemisorption rates of H₂ and CO onto cobalt particles in the Fischer-Tropsch
reaction are typically measured in terms of turnover frequency (TOF), which represents
the number of molecules adsorbed or reacted per active site per second. The specific
rates depend on various factors, including temperature, pressure, catalyst preparation,
and the cobalt particle size. Below are general values from the literature:
1. Hydrogen (H₂ ) Chemisorption Rate:
The TOF for H₂ on cobalt catalysts is typically in the range of 0.01 to 1 s⁻¹ under typical
Fischer-Tropsch conditions (220–250°C and 10–40 bar).
Hydrogen chemisorption is faster due to its dissociative adsorption mechanism and high
diffusivity.
2. Carbon Monoxide (CO) Chemisorption Rate:
The TOF for CO adsorption is generally lower, around 0.001 to 0.1 s⁻¹, as CO
adsorption involves stronger bonding and requires specific surface orientations of cobalt
atoms.
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CO often competes with H₂ for active sites, and its adsorption is rate-limiting for
Fischer-Tropsch synthesis.
Reference Data:
1. Van der Laan, G. P., & Beenackers, A. A. C. M. (1999). "Kinetics and selectivity of the
Fischer-Tropsch synthesis: A literature review." Catalysis Reviews, 41(3-4), 255-318.
This review provides data on chemisorption rates and kinetics for cobalt catalysts.
2. Schulz, H. (1999). "Short history and present trends of Fischer–Tropsch synthesis."
Applied Catalysis A: General, 186(1-2), 3-12.
Discusses the role of adsorption rates in Fischer-Tropsch reaction mechanisms.
3. Van Steen, E., & Claeys, M. (2008). "Fischer-Tropsch catalysts for the bio-to-liquids
(BTL) process." Chemical Engineering and Technology, 31(5), 655-666.
Focuses on the interaction of H₂ and CO with cobalt surfaces.
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If you need detailed equations or further assistance calculating chemisorption rates, let
me know!
These references provide detailed insights into how alumina influences the performance
and characteristics of cobalt catalysts in Fischer-Tropsch reactions.
In-Text Citations
The cobalt-based catalyst used in the Fischer-Tropsch process is supported on alumina
(Al₂ O₃ ), chosen for its excellent mechanical properties, high attrition resistance, and
ability to stabilize cobalt particles. Alumina’s pore size influences cobalt particle size,
enhancing performance. Silica (SiO₂ ) is another common support, offering weaker
metal-support interactions, improving cobalt reducibility and increasing active site
availability.
References List
1. Rouxhet, A. (2021-2022). Modelling and kinetic study of a Fischer-Tropsch
reactor for the synthesis of e-kerosene. Master’s Thesis, University of Liège.
Retrieved from http://hdl.handle.net/2268.2/14391.
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Unlike the temperature, the effect of the reactor operating pressure on the methane and
C12-C20 mass fractions is less obvious, as shown by the orange and grey curves in
Figure IV.16. The operating pressure should not influence the Fischer-Tropsch product
distribution, as the chain growth probability only depends on temperature and syngas
ratio. However, there is still a slight observable variation in the plot. It can presumably
be explained by the fact that the temperature and syngas ratio variations along the
reactor are not similar for each operating pressure. For each studied case, the initial
temperature and syngas ratio are the same and correspond to the base case
conditions, but the cooling system parameters are the same in each case as well. As
the CO conversion varies, the heat released by the reaction also varies, thus explaining
a different temperature variation in the reactor for the diverse operating pressures and
then different selectivities obtained at the outlet. On top of this explanation, another
effect might explain the slight observed variation. It turns out that the methane specific
reaction rate associated with the additional methane production (r′ 2) is larger than the
one concerning the Ideal ASF production at 210°C, i.e. the base case temperature, and
even more at low pressure. It might explain a larger methane mass fraction at low
pressure. Lastly, it turns out that the molar production of the C12-C20 fraction is
maximised at high pressure, given that its mass fraction in the product spectrum is
relatively stable and that the CO conversion continually rises.
These low-temperature reactors are characterised by the potential presence of three
phases: the gaseous reagents and part of the products, another part of the products
that are liquids (waxes) and the solid catalyst.
They are operated in the same pressure range as the LTFT process with exclusively Febased catalysts, as the utilisation of Co-based catalysts at this temperature would
induce a too large methane selectivity. The products yielded at high temperature are
typically shorter than at low temperature, which is induced by the ASF distribution they
follow (see Section IV.2.1).
The utilisation of such reactors presents the advantage of being easy to manufacture
and facilitating the design transition from one tube to several tubes. They do not require
a separation of the liquid products and the catalyst performance is predictable from the
results of tests performed on lab-scale reactors. However, the catalytic bed is composed
of large catalyst particles to avoid too nefast pressure drop, which has a negative
impact on intra-particles transport properties. On an industrial scale, where the tube
diameters are large, the apparition of hot spots is possible, which is detrimental to the
catalyst. Their manufacturing cost is also generally consequential. The way of cooling
brings an inherent problem of the apparition of axial and radial temperature gradients
affecting the conversion. This issue is mitigatable by recirculating unreacted syngas,
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which enhances the overall conversion and increases the gas velocity, which is
beneficial for heat transfer (Steynberg et al., 2004; Guettel et al., 2008; Knutsen, 2013).
Shell applies this technology in their MDS process in Bintulu, Malaysia, which was the
first-ever GTL commercial plant (Shell, 2022).
The hydrocarbon chain lengths range that constitutes kerosene varies depending on the
consulted source. Doliente et al. (2020) consider that hydrocarbons having between 10
and 18 carbon atoms are found in jet fuel, Kallio et al. (2014) take a broader range with
the distribution varying between 6 and 16 carbon atoms, Liu et al. (2013) claim that a
typical carbon range is C9-C15 and Shepherd et al. (2000) say that species between 5
and 20 carbon atoms can be found in jet fuel. The presence of hydrocarbon chains as
long as 20 carbon atoms has also been measured elsewhere (Smith and Bruno, 2007).
For this work, it is considered that the fraction of interest contains between 12 and 20
carbon atoms to match with the hypothesis previously taken by Morales and Léonard
(2022). According to Doliente et al. (2020), the desired composition of jet fuel should be
between 75 and 85 vol% of paraffins, including straight-chained, branched, and cyclic
paraffins and the remaining being 15 to 25 vol% of olefins and aromatics
Based on the provided documents, here is a concise guide for designing a multi-tubular
packed bed reactor (MT-PBR) for the Fischer-Tropsch (FT) process:
1. Objective and Scope
The MT-PBR design focuses on maximizing the selectivity for desired hydrocarbons
(e.g., e-kerosene) while ensuring efficient heat and mass transfer and maintaining
structural integrity under operating conditions. Consider the low-temperature FT process
using a cobalt-based catalyst.
2. Key Design Considerations
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a. Catalyst and Reaction Mechanism
Catalyst: Use cobalt-based catalysts, which are efficient for high chain growth and low
water-gas shift activity.
Kinetics: Implement the Ma et al. (2014) kinetic model, which accounts for water’s
positive kinetic effect and the Anderson-Schulz-Flory distribution for product yields.
b. Reactor Configuration
Opt for a multi-tubular packed-bed configuration to ensure uniform temperature
distribution.
Include heat exchange systems (shell-side cooling or internal heat exchangers) to
manage the exothermic nature of FT reactions and maintain optimal reaction
temperature (~210°C).
c. Operating Conditions
Temperature: Maintain ~210°C for optimal kerosene selectivity.
Pressure: Operate at 20-30 bar.
Syngas Ratio: Maintain an H₂ :CO ratio slightly above 2 to maximize performance.
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3. Geometric and Structural Parameters
a. Tubular Design
Use small-diameter tubes to minimize radial temperature gradients and enhance heat
transfer.
Ensure tube bundle configuration allows uniform cooling fluid distribution.
b. Catalyst Bed
Optimize particle size to balance pressure drop and reaction efficiency.
Use high porosity for uniform reactant distribution.
4. Heat and Mass Transfer
Evaluate heat transfer using L/D ratios (length-to-diameter) and optimize coolant flow
rate for efficient temperature control.
Consider the Ergun equation to calculate pressure drops in the bed and ensure
adequate flow distribution.
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5. Modeling and Simulation
Use tools like Aspen Custom Modeler for reactor modeling and sensitivity analysis.
Simulate axial and radial temperature profiles, syngas conversion, and product
distributions.
6. Fabrication and Scale-Up
Design the reactor considering scalability for industrial applications.
Include modular components to facilitate maintenance and catalyst replacement.
This guide combines insights from theoretical modeling, practical design considerations,
and experimental data. Would you like detailed calculations or simulations for any
specific aspect?
Why a multi tubular packed bed?
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Using the uploaded documents, here are the benefits of employing a multi-tubular
packed bed reactor (MT-PBR) for the Fischer-Tropsch (FT) process, designed to
process 13 tons per hour of syngas under similar conditions as described in the thesis:
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