Temperature control in a multi-tubular fixed bed Fischer-Tropsch reactor using encapsulated phase change materials

The Fischer-Tropsch synthesis is a highly exothermic, indirect, catalytic, gas (syngas) liquefaction chemical process. Temperature control is particularly critical to the process in order to ensure longevity of the catalyst, optimise the product distribution, and to ensure thermo-mechanical reliability of the entire process. This thesis proposes and models the use of encapsulated, phase change material, in conjunction with a supervisory temperature control mechanism, as diluents for the catalytic, multi-tubular fixed bed reactor in order to help mitigate the heat rejection challenges experienced in the process. The modelling was done using the Finite Element Analysis (FEA) software, COMSOL Multiphysics. In the main, three studies were considered in this thesis. In the first study, a two dimensional quasi-homogeneous, reactor model, without and with the dissipation of the enthalpy of reaction into a near isothermal phase change material (silica encapsulated tin metal) heat sink, in a wall-cooled, single-tube fixed bed reactor was implemented and the results were presented. The encapsulated phase change material was homogeneously mixed with the active catalyst pellets. The thermal buffering provided by the phase change material were found to induce up to 7% increase in selectivity towards the C5+ and a 2.5% reduction in selectivity towards CH4. Although there was a reduction in the conversion per pass of the limiting reactant and hydrocarbon productivity due to a reduction in reactor temperature, it was observed that for a unit molar reduction in the productivity of C5+, there was a corresponding 1.5 moles reduction in methane production. In the second study, a modified, one dimensional, α-model was derived which accounted for the heat sink effect of the phase change material diluent. The resulting, less computationally cumbersome, yet sufficiently accurate model was benchmarked against the more rigorous two-dimensional quasi-homogeneous model in order to check its fidelity in predicting the reactor performance. As in the first case study, a homogeneous distribution of the phase change material and active catalyst pellets was assumed. The α-model was able to approximate the reactor temperature profile of the 2D-quasi-homogeneous reactor model to within 4% error, and consistently, slightly over-predicted the limiting reactant conversion by about 3%. Based on these comparisons, the α-model was deemed sufficiently accurate to predict the reactor performance in place of the 2D model for the optimisation simulation in the third study. The third case study entailed simultaneously maximising the production of long chain hydrocarbon molecules and ensuring proper heat rejection from the reacting system, two desirable yet often conflicting operational requirements. The homogeneous distribution of the active catalyst pellets and the phase change material diluents was abandoned for a multi-zonal axial distribution in which, individual zones of the catalyst bed were diluted to varying extents. The best dilution and distribution “recipe” was determined using optimisation techniques and the previously derived modified α-model. The multi-zonal axial dilution of the catalyst bed brought about a marked increase (up to 19%) in the productivity of the long chain hydrocarbons, while ensuring a more judicious use of the catalyst bed in contrast to the homogeneous catalyst/phase change material arrangement in the previous two studies. The latent enthalpy of the metallic phase change material combined with its good thermal conductivity helped push the limits of the catalyst bed by increasing the conversion per pass beyond the typical 20-30% reported in literature, with less likelihood of either early catalyst deactivation or thermal unreliability of the reacting system. In the main, it was observed that the overall productivity of the desired C5+ could be enhanced by reducing the quantity of the catalyst pellets by a pre-defined reactor volume. In addition, the reactor productivity benefits from a highly active zone situated at the reactor entrance, immediately followed by a less reactive zone. This arrangement has the effect of ramping the reaction rate (and in effect the reactor temperature) early on, and this is kept in check by the less reactive zone immediately adjacent to the reactive one at the reactor entrance

Progress in Reactors for High-Temperature Fischer-Tropsch Process: Determination Place of Intensifier Reactor Perspective

High-temperature Fischer-Tropsch (HTFT) process aims to produce lighter cuts such as gasoline and diesel. For many years there have been studies and improvements on HTFT process to make the existing reactors more efficient. Recent studies proposed new configurations such as dual-type membrane reactor and coupling configurations reactor, which improved the performances of this process. This achievement persuades us to update the existing knowledge about the available reactors for HTFT process. In this article, features and performances overview of two classes of reactors are reviewed. The first class consists of the reactors which are based on older studies, and the second one includes recent studies which are called product intensifier reactors. Finally, it is shown that the product intensifier reactors have higher CO conversions and lower selectivity of undesired by-products which results in higher production yield of gasoline. Furthermore, the place of product intensifier reactor among common reactors with regard to the influence of the process parameters on the product distribution has been estimate

Modelling reaction and diffusion in a wax-filled hollow cylindrical pellet of Fischer Tropsch catalyst

Previous modelling of fixed-bed, Fischer-Tropsch (FT) reactors has demonstrated the advantages relative to spherical pellets of using cylindrical and shaped pellets to provide improved transport attributes under conditions relevant to industrial operation. However, mass transport models have focussed on the investigation of transport within pellets with spherical symmetry, whilst detailed investigations of more complex shapes have not been undertaken. Here, a pseudo-isothermal, steady-state, two-dimensional model was investigated for catalyst pellets of cylindrical form, both solid and hollow. A cobalt-based catalyst was considered at conditions where the rate of condensable hydrocarbon generation is large enough to result in the accumulation of liquid hydrocarbons in the pores of a catalyst. It was found that effectiveness factors were bounded by those of sphere and slab above and below Thiele moduli of ~0.75 and ~1.15, respectively, for the conditions examined, with the effectiveness factors exceeding those of both sphere and slab models between these moduli. Here, comparisons were made on the basis of the characteristic diffusion length, the catalyst particle’s volume divided by its external surface area. However, values of the FT chain growth parameter, α, between these values of Thiele modulus were lower than both those of sphere and slab geometry, and thus under these conditions hollow cylinders gave the greatest methane selectivity

A Computational Model of Networked Small-Scale Fuel Synthesis Demonstrating Greater Production Flexibility and Specificity

The rapid pace of industrial change over the past hundred years has led to any number of paradigm shifts in the way business is conducted and technologies are applied, but economies of large scale have persisted in the energy sector. In an age of automation and mass-production of small units, however, complex networking of many small energy systems can permit novel application of established technologies. This dissertation explores how established fuel synthesis technologies might behave in an automated network in which familiar units are arranged in unfamiliar ways. The flexibility afforded by automation and small scale operation allows for potentially complementary means of exploiting the fungible nature of hydrocarbon resources. Beyond any benefits of small-scale incurred from mass production and learning, fuel synthesis is a process with sensitivities to input streams that a network could exploit in a nuanced way. The completed work demonstrates that a network of small-scale fuel synthesis reactors and thermal crackers, based on current industrial practices at large monolithic scale, can be networked to dramatically sharpen the chemical spectrum they produce. In order to study the behavior of such a network in ways that are unavailable in current software, a hierarchical numerical modeling code was developed to offer greater flexibility to nest and optimize network configurations within network configurations, reflecting the modularity of the networks it is meant to simulate. This new code is capable of simulating aggressively numerically constrained networks, dynamically substituting various configurations while optimizing them across user-specified variables. Various weighting schemes were developed to facilitate more rapid convergence to a numerical solution so that highly constrained recycling schemes could be reconciled to a steady state that would produce the specified output spectrum. Modular units were coded to simulate the essential properties of real processes and technologies, with close attention paid to the sensitivity of these processes to input conditions, so that these units could be assembled in various configurations and subjected to user-specified constraints. Coded modules were designed under the principle that these individual units need not be custom-made or technologically ahead of their time; the benefits explored by network simulations are incurred not by dramatically upgrading the processes being simulated, but rather by directing and redirecting the chemical streams which are subject to those processes to tailor the outcome to the desired product. This principle was applied to chemical separation in an analytical framework in order to derive how unremarkable separators might be networked to produce remarkable precision of separation. Such precision is important because the direction and redirection of chemical streams is predicated on the ability to select the destination of a particular chemical. The effect of networking fuel synthesis reactors and thermal crackers was studied for unidirectional flows in order to understand how repeated applications of these units at smaller scale sharpen the spectrum relative to single large scale application. These fuel synthesis reactors and thermal crackers were also configured in aggressively recycled networks, imposing more severe constraints on the output spectrum. This work demonstrated that fuel synthesis at industrial output scales need not operate in monolithic units and can benefit dramatically from judicious networking, to the point that a network of units that would otherwise have produced a broad spectrum of chemical flavors can be configured to produce only a single user-specified output chemical

Efficient filtration system for paraffin-catalyst slurry separation

The filtration efficiency for separating liquid paraffin (or water) from a slurry consisting of 25 weight% spherical alumina in a Slurry Bubble Column Reactor (SBCR) comprised of a cylindrical tube of 10 cm diameter and 150 cm length was studied. Various differential pressures (ΔP) were applied to two separate tubular sintered metal stainless steel filter elements with nominal pore size of 4 and 16μm. The experimental results disclosed that the rate of filtrations increased on applying higher differential pressure to the filter element. Albeit this phenomenon is limited to moderate ΔPs and for ΔP more than 1 bar is neither harmful nor helpful. The highest filtration rates at ΔPs higher than 1 bar were 170 and 248 ml/minute for 4 and 16μm respectively. Using water as the liquid in slurry the rate of filtration enhanced to 4 folds, and this issue reveals impact of viscosity on filtration efficiency clearly. In all situations, the total amount of particles present in the filtrate part never exceeded a few parts per million (ppm). The statistical analysis of the SEM image of the filtrate indicated that by applying higher pressure difference to the filter element the frequency percent of larger particle size increases. The operation of filter cake removing was performed with back flashing of 300 ml of clean liquid with pressures of 3-5 bar of N2 gas

An Energy Integrated Approach to Design Supercritical Fischer-Tropsch Synthesis Products Separation and Solvent Recovery System

Extensive research has been done in the advancement of gas-to-liquid (GTL) technology for producing a cleaner source of energy through the conversion of natural gas into ultra-clean fuels and value-added chemicals. The Fischer-Tropsch (FT) synthesis, which is a catalytic process that converts synthesis gas (or syngas, which is a mixture of CO and H2) into longer chain hydrocarbons is considered to be the heart of the GTL process. Conventional FT processes are currently utilizing two most common types of reactors: the multi-tubular fixed bed reactor (in which the reaction takes place in a gas phase medium) and the slurry bubble column reactor (where the reaction takes place in a liquid phase medium). However, they possess heat transfer and mass transfer limitations, respectively. In order to avoid the challenges, the application of a supercritical fluid (SCF) solvent in the Fischer-Tropsch synthesis was introduced. The SCF-FT process, in essence, combines the benefits of the two major reactor technologies used in conventional GTL processes due to the SCF’s gas-like diffusivity, liquid-like solubility and heat transfer. The SCF-FT synthesis involves co-feeding the SCF solvent along with the syngas into the reactor at a specific solvent to syngas ratio (set as 3:1 in this work). Introducing the supercritical solvent (which was selected to be n-hexane in this work) requires adjustments in the SCF-FT products’ separation sequence due to the significantly large amount of solvent available in the process. The major additional costs associated with the SCF-FT synthesis is in the product separation and solvent recovery. For SCF-FT to be adopted on a large-scale, the economics from operation under high pressure supercritical conditions must exceed the additional cost required for the separation of the solvent. The aim of this work is to construct an optimum separation design to target the separation of synthetic crude oil (or syncrude) obtained from SCF-FT synthesis while recovering the supercritical solvent. Aspen Plus® was used as the process simulator to determine the energy consumption and quantify the sensitivity of the various parameters on the solvent recoverability, purity, product yield, and operation feasibility while comparing it to the typical FT process. Three separation sequences were developed using existing GTL plants as references. The three scenarios were compared with regards to their energy requirements. The simulation results showed that despite the addition of a large amount of solvent, the separation of the products, water, and the recovery of the solvent was achieved

Low severity Fischer-Tropsch synthesis for the production of synthetic hydrocarbon fuels

Currently, the main source for the production of liquid transportation fuels is petroleum, the continued use of which faces many challenges including depleting oil reserves, significant oil price rises, and environmental concerns over global warming which is widely believed to be due to fossil fuel derived CO2 emissions and other greenhouse gases. In this respect, lignocellulosic or plant biomass is a particularly interesting resource as it is the only renewable source of organic carbon that can be converted into liquid transportation fuels. The gasification of biomass produces syngas which can then be converted into synthetic liquid hydrocarbon fuels by means of the Fischer-Tropsch (FT) synthesis. This process has been widely considered as an attractive option for producing clean liquid hydrocarbon fuels from biomass that have been identified as promising alternatives to conventional fossil fuels like diesel and kerosene. The resulting product composition in FT synthesis is influenced by the type of catalyst and the reaction conditions that are used in the process. One of the issues facing this conversion process is the development of a technology that can be scaled down to match the scattered nature of biomass resources, including lower operating pressures, without compromising liquid composition. The primary aims of this work were to experimentally explore FT synthesis at low pressures for the purpose of process down-scaling and cost reduction, and to investigate the potential for obtaining an intermediate FT synthetic crude liquid product that can be integrated into existing refineries under the range of process conditions employed. Two different fixed-bed micro-reactors were used for FT synthesis; a 2cm3 reactor at the University of Rio de Janeiro (UFRJ) and a 20cm3 reactor at Aston University. The experimental work firstly involved the selection of a suitable catalyst from three that were available. Secondly, a parameter study was carried out on the 20cm3 reactor using the selected catalyst to investigate the influence of reactor temperature, reactor pressure, space velocity, the H2/CO molar ratio in the feed syngas and catalyst loading on the reaction performance measured as CO conversion, catalyst stability, product distribution, product yields and liquid hydrocarbon product composition. From this parameter study a set of preferred operating conditions was identified for low pressure FT synthesis. The three catalysts were characterized using BET, XRD, TPR and SEM. The catalyst selected was an unpromoted Co/Al2O3 catalyst. FT synthesis runs on the 20cm3 reactor at Aston were conducted for 48 hours. Permanent gases and light hydrocarbons (C1-C5) were analysed in an online GC-TCD/FID at hourly intervals. The liquid hydrocarbons collected were analyzed offline using GC-MS for determination of fuel composition. The parameter study showed that CO conversion and liquid hydrocarbon yields increase with increasing reactor pressure up to around 8 bar, above which the effect of pressure is small. The parameters that had the most significant influence on CO conversion, product selectivity and liquid hydrocarbon yields were reactor temperature and catalyst loading. The preferred reaction conditions identified for this research were: T = 230ºC, P = 10 bar, H2/CO = 2.0, WHSV = 2.2 h-1, and catalyst loading = 2.0g. Operation in the low range of pressures studied resulted in low CO conversions and liquid hydrocarbon yields, indicating that low pressure BTL-FT operation may not be industrially viable as the trade off in lower CO conversions and once-through liquid hydrocarbon product yields has to be carefully weighed against the potential cost savings resulting from process operation at lower pressures

Multiphase Equilibrium of Fluids Confined in Fisher-Tropsch Catalytic Systems

Energy supply and security imposes a significant challenge in our modern world stemming from our dependence on depleting resources such as petroleum and oil. Fischer-Tropsch synthesis (FTS) is considered as a great energy alternative which can significantly reduce our dependence on oil, improve rural economics, reduce greenhouse emissions, and promise energy security. It is a key technology for converting syngas, produced from coal, biomass or natural gas, into a variety of hydrocarbon products. Although this technology was discovered in 1923, commercialization and scale up are limited to the use of few reactor configurations (e.g. multi-tubular fixed-bed reactor, Slurry-bubble column reactor, and fluidized bed reactors). In order to improve the limitations in both reactor configurations, on lab scale near critical media was utilized, since it offers a great combination of the advantages of both the gas-phase reaction (multi-tubular fixed-bed reactor) and the liquid-phase reaction (slurry-bubble column reactor), while simultaneously overcoming their limitations. This work focuses on modeling the phase behavior of the FTS mixture in fixed bed reactor in the bulk phase inside the reactor bed or inter-particle and then zoom into the catalyst (confined phases within the catalyst pores or intra-particle). This is done by using an extended Peng-Robinson (PR) equation of state (EOS) that is capable of accounting for the fluid behavior inside confined pores as well as in the bulk phases. The PR Equation of state model extended to confined fluid (PR-C) has been utilized in multiphase equilibrium algorithm using FORTRAN. The simulation results provide the composition and the condition of each bulk phase and pore phase for a given initial mixture. Two different scenarios were studied for fixed bed reactor: the first one is the conventional gas phase FTS and the second one is for the supercritical phase FTS (SCF-FTS). In each case, the phase behavior of the mixture of the reactants and products was investigated at different conversions along the bed length. The simultaneous assessment of both gas phase FTS and SCF-FTS phase behavior and reaction performance open the door for optimizing the design FTS reactor and enhance the efficiency of the process. Preferential adsorption of hydrogen has been observed and this could be due to the small size of the hydrogen molecules compared to those of the other components. Our studies suggested that the supercritical phase provides superior heat dissipation due to the existence of denser phase in the bulk and the confined regions than the conventional gas phase. On the other hand in the gas phase and for limited carbon number (up to C8) the pore phase is found to be in a vapor state which should provide higher diffusivity of the reactant than that in the supercritical phase. Our study will continue by integrating the developed phase behavior studies in the reactor design model

In-situ H2O removal via hydorphilic membranes during Fischer-Tropsch and other fuel-related synthesis reactions

The general objective of this thesis was to explore the potential of in-situ H2O removal during fuel-related synthesis reactions with focus on in-situ H2O removal by hydrophilic membranes and by chemical reaction. It is demonstrated that in-situ H2O removal through vapour permeation during CO2 hydrogenation to Fischer-Tropsch hydrocarbons and during DME/DEE synthesis leads to increased conversion and yield levels, which are directly linked to the degree of H2O recovery