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In this study we investigate performance characteristics (catalyst effectiveness, CH4 selectivity, and hydrocarbon product distribution) of with a highly active Co/Re/Al(2)O(3 )catalyst particle for Fischer-Tropsch synthesis. In numerical simulations we utilize kinetic parameters for CO consumption rate, CH4 formation rate and hydrocarbon formation rates (C-2+ hydrocarbons) determined from experiments with this catalyst to study effects of catalyst activity, catalyst particle shape (sphere, slab, solid and hollow cylinder), size (i.e. diffusion length), catalyst distribution (uniform vs. eggshell type distribution for a spherical particle) and process conditions (temperature, pressure, syngas composition and conversion level) on the catalyst performance. With increase in Thiele modulus (i.e. particle size at a fixed set of process conditions) we observe increasing H-2/CO ratio profile towards the center of the particle resulting in increase of local and average CH4 selectivity. The goal is to find conditions which allow one to use sufficiently large particles to reduce pressure drop, while avoiding negative influence of diffusional limitations on selectivity and activity. For each catalyst particle shape we determined values of Thiele modulus, i.e. characteristic length of diffusion, corresponding to the upper limit of the kinetic region, and investigated how it changes with operating conditions. We found that simultaneous increase of pressure and the use of syngas with H-2/CO feed ratio of 1.4-1.7 is the best strategy for mitigating the negative impact of intraparticle diffusional limitations on CH4 selectivity. For a spherical particle of 1 mm in diameter, one can achieve CH4 selectivity of 5.6% with catalyst effectiveness factor of 1.07 at the reactor inlet by operating at 50 bar, 473 K and H-2/CO = 1.4.

Quantitative data comparing iron and cobalt based catalysts for the Fischer-Tropsch synthesis (FTS) are scarce due to the fact that these two kinds of catalysts typically utilize different process conditions. This paper focuses on studying the catalytic behavior of two highly active iron and cobalt based research catalysts at clean conditions and during poisoning with common syngas contaminants. The catalyst activity and selectivity at identical conditions, the CO conversion effect, and the effect of poisons on iron and cobalt catalysts were systematically explored in a quantitative manner. At a set of identical FTS conditions, the cobalt catalyst was 2.5 times as active as the iron catalyst with higher CH4 and C-5+ selectivities but much less olefins and lower CO2 selectivity. Cobalt based catalysts are more susceptible to deactivation by oxidation at high CO conversions (e.g. gt 80%) due to the high partial pressure of water (P-H2O) in the reactor, while the iron catalyst can be stabilized at a high conversion level. Under clean FTS conditions, the cobalt catalysts displayed a more pronounced CO conversion effect on stability and selectivity; on the other hand, a combination of effects (i.e. from CO conversion and the nature of the catalyst) were observed for the iron catalysts. The sensitivities of the Fe and Co catalysts to the typical contaminants (i.e., H2S and NH3) present in the syngas derived from coal, natural gas or biomass were compared. Iron and cobalt catalysts exhibited similar resistance to the H2S poison (i.e. threshold levels 25-50 ppb), but the iron catalyst was found to be much more resistant to ammonia than the cobalt catalyst (i.e., threshold levels of 80 ppm and 45 ppb, respectively). Co-feeding 150-200 ppm ammonia lowered CH4 selectivity and 2-olefin content (suppressing secondary reactions of 1-olefin) on both types of catalysts. In contrast, co-feeding up to 1 ppm H2S significantly increased CH4 formation only on cobalt catalysts but had a

One-dimensional pseudo-homogenous dynamic reactor model, incorporating detailed Fischer-Tropsch kinetics, was applied in a theoretical analysis of forced periodic operations. A milli-scale fixed-bed reactor was analyzed, using design and operation parameters, obtained previously in a steady-state optimization. Dynamic optimization and NLP methods were utilized to obtain optimal values of amplitude(s), frequency and phase shift(s) of sine-wave variation of inputs, around the corresponding optimal steady-state values, which maximize the productivity of C5+ hydrocarbons. Inlet variables that were modulated are: coolant temperature, reactants molar ratio, mass flow rate and pressure. In addition to the single input forcing, simultaneous modulations of multiple inputs were also considered, with combinations of the listed inlet variables. Among the single input cases, periodic variation of the coolant temperature resulted in the highest relative improvement of C5+, productivity by 30%. Multiple inputs forcing showed additional potential for improvement, resulting in relative c(5+) productivity increase of 52% for synchronized modulation of the coolant temperature, reactants molar ratio and mass flow rate. However, the increase in C5+ productivity is accompanied with relative increase in methane selectivity of 22-33% (relative to the steady-state value). The results suggest that, in the case of multiple input variations with high amplitudes, modulation of the inlet reactants molar ratio mainly contributes to the increase of CO conversion (e.g. reaction rate), the coolant temperature forcing slightly increases selectivity towards the desirable higher hydrocarbons (C5+), while the variation of the inlet mass flow rate enables better reaction temperature control and prevents a thermal runway.

Predictions of vapor liquid equilibrium for Fischer-Tropsch mixtures were com-pared using the classical Soave-Redlich-Kwong (SRK) and cubic-plus-asso-ciation (CPA) equations of state. The performance of the two equations of state was evaluated based on comparison with results from eight sets of exp-erimental runs in which different process conditions (pressure, reactants feed ratio, space velocity) were used. Flash calculations were used to determine the phase split at defined process conditions, whereas the phase equilibrium was defined utilizing the concept of equal fugacities in the vapor and the liquid phase for all components. A total of 75 components were considered in the reaction mixture: CO, H-2, H2O, CO2, C-1-C-57 paraffins and C-2-C-15 olefins. All calculations were performed in MATLAB. The results showed that both equat-ions of state had similar performance regarding the hydrocarbons, whereas CPA gave better results with inorganic components and SRK with prediction of the composition of the liquid phase. Computational time for CPA was sub-stantially (100 times with the CPU used) higher than that for SRK. Overall, the use of CPA did not improve VLE prediction for FTS systems significantly enough to be recommended for use in FTS reactor models.

A two-dimensional pseudo-homogeneous model of wall-cooled fixed bed Fischer-Tropsch synthesis (FTS) reactor with Co/Re/gamma-Al2O3 catalyst was developed to study the effect of process and design parameters on heat generation and removal characteristics. The influence of liquid-phase formation on heat transport was accounted for by using two-phase correlations. The effect of intraparticle diffusion on heat generation was considered. Detailed numerical simulations were performed to analyze the effect of process and design parameters on the reactor performance in terms of heat management. Results show that thermal behavior of FTS fixed bed reactors is very sensitive and any large disturbances can lead to temperature runaway. Large tube diameters are shown to be particularly unfavorable, with d (t) gt 5 cm resulting in axial and radial gradients greater than 20 K and 13 K, respectively. The importance of detailed reactor modeling when designing and optimizing FTS fixed bed reactors is highlighted.

Our previously developed mathematical model is used for parametric sensitivity and optimization study of conventional and milliscale fixed-bed reactors (FBRs) for Fischer-Tropsch synthesis (FTS). Five indicators are chosen to analyze the influence of eight parameters on the FBRs performance. The results show the scale of the effects caused by changing single parameter values and highlight the most important ones. Subsequently, the model is used to perform a rigorous multivariable optimization of the FBRs performance in the steady state. Three optimization functions are used, depicting different reactor costs. Four design parameters (tube length and diameter, particle diameter, and catalyst layer thickness) and five operating parameters (inlet and wall temperature, inlet pressure, H-2/CO ratio, velocity) are optimized simultaneously. The results indicate that optimal results, in terms of reactor design and operating parameters and FBR performance, highly depend on the selected objective function and values of constrained parameters (especially methane selectivity and the partial pressure of water).

We investigate effects of catalyst activity, catalyst particle shape (sphere, slab, and hollow cylinder), size (i.e., diffusion length), catalyst distribution (uniform vs eggshell type distribution for a spherical particle), and process conditions (temperature, pressure, syngas composition, and conversion level) on catalyst effectiveness factor and methane selectivity inside the catalyst pellet. In numerical simulations we utilize kinetic parameters for CO consumption rate and CH4 formation rate determined from experiments with a highly active Co/Re/gamma-Al2O3 catalyst. It is found that the use of small spherical particles (0.2-0.5 mm) or eggshell distribution for larger spherical particles with catalyst layer thickness less than approximately 0.13 mm is needed to avoid negative impact of diffusional limitations on CH4 selectivity under typical Fischer Tropsch synthesis operating conditions. For monolith reactors with wash-coated catalyst, diffusional limitations can be avoided by using a catalyst layer thickness less than 0.11 nun at base case conditions (473 K, 25 bar, and H-2/CO molar ratio of 2).

A multiphase fixed-bed reactor (FBR) model for Fischer Tropsch Synthesis has been developed. A high level of details is considered for description of the phenomena on the reactor and particle scale. Detailed kinetics is used, with parameters estimated from experiments with a cobalt-based catalyst. Model robustness has been validated using literature data. Performance analysis was made for a conventional scale FBR with egg-shell distribution of catalyst and a millimeter-scale FBR with small particles and uniform distribution. In both cases, diffusion limitations are almost eliminated due to use of small diffusion lengths. For similar qualitative results, a milli-scaled design would result in a significantly lower reactor volume, but the capital costs could be high due to large wall area and a vast number of tubes. Heat removal is efficient in both cases, and pressure drop in the milli-scale reactor is low due to the use of a shorter bed and lower velocity.

Fischer-Tropsch synthesis (FTS) is a heterogeneously catalyzed chemical reaction in which a mixture of hydrogen and carbon-monoxide is converted into an array of hydrocarbon products. These products can be used as synthetic liquid fuels (gasoline, diesel, kerosene) or feedstock for the chemical industry. As such, FTS is a key step in the conversion of coal, natural gas and biomass into liquids in large plants all over the world, including plants in Qatar, Malaysia, South Africa etc. Even though FTS has been commercially used for most of the 20th century, many fundamental aspects of this reaction are unclear. The main objectives of the work included in this thesis were: to study the effect of process conditions on FTS product selectivity and distribution with regards to hydrocarbon chain length, develop a detailed model of FTS kinetics (capable of predicting both reactant disappearance and product formation rates) and apply the detailed kinetic model in modeling and optimization of a multi-tubular fixed-bed reactor for FTS. Most often used catalysts for FTS are iron and cobalt. The experimental data used in this study were obtained in collaboration with Texas A&M University and Center for Applied Energy Research, University of Kentucky, for iron- and cobalt-based catalyst, respectively. The analysis of experimental data for both catalysts showed dependence of product selectivity from process conditions inside the reactor (temperature, pressure, reactant feed ratio and conversion level). These dependences were shown to be related to the chain-growth probability factor (α), which also varies with carbon number. Over the cobalt catalyst, the significant variation of C1 intermediate growth probability (α1) at different process conditions and a lack of the same variations for higher C2+ chain intermediates, was interpreted as evidence for the existence of separate methanation pathways. The same data presented evidence that the secondary 1-olefin readsorption does not play, Fischer-Tropsch sinteza (FTS) je heterogeno katalizovana hemijska reakcija u kojoj se smeša vodonika i ugljen-monoksida pretvara u niz ugljovodoničnih proizvoda. Ti proizvodi se mogu koristiti kao sintetička tečna goriva (benzin, dizel i kerozin) ili sirovine za hemijsku industriju. Kao takva, FTS je ključni korak u konverziji uglja, prirodnog gasa i biomase u tečnosti u velikim postrojenjima širom sveta, uključujući postrojenja u Kataru, Maleziji, Južnoj Africi itd. Iako je FTS veći deo dvadesetog veka bila u komercijalnoj upotrebi, mnogi fundamentalni aspekti ove reakcije su nejasni. Glavni ciljevi istraživačkog rada u sklopu ove disertacije su bili: ispitivanje efekata procesnih uslova na selektivnost i raspodelu FTS proizvoda u odnosu na dužinu ugljovodoničnog lanca, razvoj detaljnog kinetičkog modela FTS (sposobnog da ujedno predskaže brzine nestajanja reaktanata i nastajanja proizvoda) i primena detaljnog kinetičkog modela u modelovanju i optimizaciji višecevnih FTS reaktora sa pakovanim slojem katalizatora. Najčešće korišćeni katalizatori za FTS su gvožđe i kobalt. Eksperimentalni podaci za gvozdeni i kobaltni katalizator korišćeni u ovoj studiji su dobijeni u saradnji sa Texas A&M Univerzitetom (Texas A&M University) i Centrom za primenjeno istrazivanje energije Univerziteta u Kentakiju (Center for Applied Energy Research, University of Kentucky), respektivno. Analiza eksperimentalnih podataka za oba katalizatora je pokazala zavisnost selektivnosti proizvoda reakcije of procesnih uslova unutar reaktora (temperature, pritiska, odnosa reaktanata u ulaznoj smeši i konverzije reaktanata). Pokazano je da su ove zavisnosti povezane sa verovatnoćom rasta ugljovodoničnog lanca (α), koja se takođe menja sa brojem atoma ugljenika. Značajne varijacije u vrednosti verovatnoće rasta C1 lančanih intermedijera pri različitim procesnim uslovima, zabeležene sa kobaltnim katalizatorom, kao i odsustvo istih varijacija za duže C2+ lančane intermedijere, su interpretirane kao do

Due to the global growth in production of synthetic fuels via the Gas-to-Liquid (GTL), Coal-To-Liquid (CTL) and Biomass-To-Liquid (BTL) processes, academic and industrial interest in Fischer-Tropsch synthesis (FTS) research has increased during the past decade. The undesired product of FTS is methane and it is formed in amounts higher than expected according to the current understanding of the FTS mechanism. Therefore, it is important to gain better understanding of methane formation in order to optimize the FTS process. In this review we discuss the reasons responsible for higher than expected methane selectivity under FTS conditions over cobalt-based FTS catalysts and describe novel microreactors for use in FTS. These novel reactors could help improve reaction selectivity and yield, as well as offer significant economic benefits. Recommendations are given for intensification of FTS in terms of product selectivity by improved selection of catalysts, process conditions and reactor configurations.

Two kinetic models of Fischer-Tropsch product selectivity have been developed based on reaction networks from the literature. The models were fitted to experimental data obtained using commercial iron-based catalyst in a stirred tank slurry reactor and under a wide range of process conditions. Results showed that both of the rival models were able to provide a satisfactory prediction of the experimental product distribution for n-paraffin, 1- and 2-olefin. The simpler of the two models, a reaction network with a single type of active sites and solubility enhanced 1-olefin readsorption term, was chosen as more adequate for practical use.

The kinetic models of Fischer-Tropsch synthesis (FTS) product distribution can be classified into two major groups: hydrocarbon selectivity models and detailed Langmuir-Hinshelwood-Hougen-Watson (LHHW) kinetic models. In this study the two approaches to FTS product distribution modeling are presented and compared using the experimental data obtained in a stirred tank slurry reactor with promoted iron catalyst over a wide range of process conditions. Positive deviations from the classical Anderson-Schulz-Flory distribution and an exponential decrease in olefin-to-paraffin ratio with carbon number are predicted by the inclusion of solubility-enhanced 1-olefin readsorption and/or chain length dependent 1-olefin desorption concepts. In general the agreement between the model predictions and experimental data was very good, and modeling approaches are discussed in terms of fit quality, physical meaningfulness and practical utility.