Search

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.

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).