Sebastian C. Reyes

Transport-enhanced α-olefin readsorption pathways in Ru-catalyzed hydrocarbon synthesis

Residence time and. cofeed studies show that olefins and paraffins are primary products in Rucatalyzed hydrocarbon synthesis. Olefins readsorb and initiate surface chains that are indistinguishable from those formed directly from CO/H2 and that continue to grow and ultimately desorb as higher molecular weight hydrocarbons. Transport-enhanced α-olefin readsorption leads to an increase in chain growth probability (a) and in paraffin content with increasing pore and bed residence time. Deviations from conventional (Flory) polymerization kinetics and the increasing paraffinic content of higher hydrocarbons are quantitatively described by transport effects on the residence time of intermediate olefins, without requiring the presence of several types of chain growth sites. Our transport-reaction model combines a description of diffusive and convective transport with a mechanistic kinetic model of olefin readsorption and of CO hydrogenation and chain growth. It quantitatively describes carbon number, site density, pellet size, and space velocity effects on hydrocarbon synthesis rate and product distribution. The model is consistent with the experimentally observed maximum C5+ selectivities at intermediate values of site density and pellet size. These intermediate values permit extensive readsorption of α-olefins without significant CO arrival transport limitations.

Selectivity Control and Catalyst Design in the Fischer-Tropsch Synthesis: Sites, Pellets, and Reactors

Publisher Summary This chapter focuses on selectivity control and catalyst design in the Fischer-Tropsch (FT) synthesis. Chain growth during the FT synthesis is controlled by surface polymerization kinetics that place severe restrictions on our ability to alter the resulting carbon number distribution. Intrinsic chain growth kinetics are not influenced strongly by the identity of the support or by the size of the metal crystallites in supported Co and Ru catalysts. Transport-limited reactant arival and product removal, however, depend on support and metal site density and affect the relative rates of primary and secondary reactions and the FT synthesis selectivity. Diffusion-limited removal of products from catalyst pellets leads to enhanced readsorption and chain initiation by reactive α-olefins. Diffusive and convective transport processes introduce flexibility in the design of catalyst pellets and in the control of FT synthesis selectivity. The model is proposed in the chapter that describes the catalytic behavior of more complex Fe based materials, where several chain termination steps and highly non-uniform and dynamic surfaces introduce additional details into the models required to describe FT synthesis selectivity models.

Estimation of effective transport coefficients in porous solids based on percolation concepts

Abstract Present available means to predict mass transport of gases in porous solids are inaccurate as a consequence of the inherent difficulties encountered in properly relating the local transport coefficients to the highly complex pore space. Within the concepts of percolation theory we make use of a network model of pore topology, a Bethe lattice, to simulate pore space related properties in porous solids. This network allows an exact evaluation of effective transport coefficients for binary mixtures, without resorting to tortuosity factors. In addition, it fundamentally accounts for the influences of narrow necks, tortuous paths and dead ends. A general form of the “dusty gas” model is used to describe transport at a pore level. The method is simple to use and model predictions are in good agreement with published experimental data.

Percolation concepts in modelling of gas-solid reactions—II. Application to char gasification in the diffusion regime

Abstract The application of percolation theory to modelling of char gasification in the diffusional regime is presented. The Bethe network description of the pore space developed in Part I is used to evaluate effective diffusivities and permeabilities during char gasification. This approach fundamentally accounts for the influence of narrow necks, tortuous paths and dead ends. The gasification model also incorporates the effects of pore enlargement, pore coalescence and closed pores on the evolution of accessible porosity and surface area. In addition, the role of pore topology on perimeter fragmentation of the gasifying particle is included. These phenomena could have implications in the operation of gasifiers. Model predictions are in good agreement with experimental data.

Effective diffusivities in catalyst pellets: new model porous structures and transport simulation techniques

Compressed and sintered porous solids are simulated by random-loose aggregates of spheres that are distributed in size and partially overlapped to achieve the required porosity. The resulting porous networks closely capture the morphological details of diffusing channels within granular materials commonly used as catalyst supports. Effective diffusivities in these model solids are calculated by Monte Carlo techniques that allow the probing of representative regions of the void space throughout the Knudsen, transition, and molecular diffusion regimes. Simulated diffusivities and tortuosity factors are in excellent agreement with experimental observations. These simulations also allow the calculation of accurate pore-size distributions and of transition-region diffusivities, previously estimated by simple geometric arguments and by the Bosanquet approximation, respectively. Mean pore radii calculated from surface area (S) and porosity ({Phi}{sup A}) data ({bar r}{sub p} = 2 {Phi}{sup A}/S) closely resemble the exact values obtained in our simulations for compressed solids but less so for sintered materials. The simulations show that tortuosity factors, when properly defined and calculated, are intrinsic properties of porous solids, and identical in the Knudsen and molecular diffusion regimes.

Kinetic-transport models of bimodal reaction sequences. I: Homogeneous and heterogeneous pathways in oxidative coupling of methane

Abstract A reaction-transport model that combines gas-phase reactions occurring within interstitial and intrapellet voids with surface reactions occurring on catalytic sites was used to describe the oxidative coupling of methane in packed-bed reactors, a typical example of bimodal (homogeneous-heterogeneous) reaction systems. A kinetic model for gas-phase reactions was assembled from available literature data; it describes well experimental results in empty reactors. Simulations suggest that C 2 yields greater than 8–9% are unattainable with CH 4 /O 2 mixtures in homogeneous reactors. Staging the introduction of the oxygen reactant along the reactor length minimizes secondary oxidation reactions by lowering the local O 2 pressures, and leads to a slight increase in maximum yield (12% for 200 injection points) but also to much larger required reactor volumes. The introduction of an ideal catalytic function (methyl and ethyl radical formation without full oxidation) also increases maximum C 2 yields by increasing the concentration of methyl radicals involved in bimolecular coupling steps. However, C 2 yields greater than 30% require selective catalysts with very high turnover rates (100 s −1 ); higher rates become ultimately limited by intrapellet diffusion rates. Again, staging oxygen by multiple injection schemes increases attainable yields but requires larger reactor volume and catalytic sites with low reaction order (⪡ 1) in oxygen. For optimum conditions, staged oxygen injection techniques lead to C 2 yields as high as 50%.

Monte carlo simulations of structural properties of packed beds

Abstract Monte Carlo simulations are used to obtain void fraction (Φ) and particle—particle contacts radial profiles for nondeformable spheres contained within cylinders with impernetrable walls. Simulated packings of monosize and multisize spheres are created by their sequential random placement within a prescribed enclosure using packing rules consistent with loading procedures in packed bed reactors. The resulting random-loose packings (Φ = 0.42) differ significantly in local and global properties and in the extent of ordering from random-close packings (Φ = 0.36). Void fraction and particle—particle contact radial profiles in monosize packings show a heavily damped decay behavior near the wall, extending only about two sphere diameters away from it. The damped oscillatory behavior characteristic of random-close packings is not observed. Similar trends are observed for multisize sphere distributions. Void fraction profiles for multisize spheres prescribed by continuous distributions are accurately described by a simple linear combination of the profiles of the individual size components. In bidisperse size distributions, similar superimposition rules also describe the qualitative details of the void fraction profiles, but do not account for compaction effects caused by penetration of small spheres into the interstices between larger spheres. Void fraction and contacts in segregated packings, in which monosize spheres of different size are dropped into annular and core regions in the cylindrical enclosure, are also described. These simulations provide the basis for our current studies of heat transport properties in packed beds, and for the generation of realistic pore structure models of overlapping randomly-arranged spheres.

Percolation concepts in modelling of gas-solid reactions-III. Application to sulphation of calcined limestone

A Bethe network description of porous solids is used to model pore structure changes, including pore plugging, in the sulphation of calcined limestone. This model accounts for diffusion of SO2 in the shrinking pore space as well as in the product layer. The model predictions clearly demonstrate the increasing diffusion resistance and isolation of partially reacted pores causing incomplete conversion of the solid. The importance of an accurate description of pore space topology both in the interpretation of Hg porosimetry data and in transport calculations is illustrated. The model simulations show excellent agreement with published experimental observations.

Kinetic-transport models and the design of catalysts and reactors for the oxidative coupling of methane

The design of catalytic pellets and reactors using detailed kinetic-transport models is illustrated for the oxidative coupling of methane to form ethane and ethylene. Oxygen sieving within diffusion-limited pellets and staged oxygen injection reactors increase C2 selectivity by inhibiting full oxidation homogeneous pathways that lead to CO and CO2 products. Our simulations suggest that high densities of surface sites with kinetics that depend weakly on oxygen concentration are required to benefit from oxygen-sieving catalyst and reactor schemes. These sites favor beneficial surface activation processes even at the low oxygen concentrations present within staged injection reactors and diffusion-limited pellets. Controlled introduction of stoichiometric oxygen reactants leads to C2 yields as high as 50%; the reactions, however, occur at much slower rates and require much greater reactor volumes than in conventional cofeed reactors.

Phase transition and steady-state multiplicity in a trickle-bed reactor

Abstract A pseudo-homogeneous model is developed to describe the wet-to-dry phase transition in a laboratory trickle-bed reactor, observed by Hanika and coworkers in 1976 for cyclohexene hydrogenation. Model predictions of the location of the dryout point, the maximum temperature rise, and a reaction rate hysteresis are compared with observations. The comparison implies that the reaction rate is sensitive to internal wetting of the catalyst and that the hysteresis loop can be attributed to thermal effects. It is hoped that the model will help interpret and scale up laboratory data and serve as a basis for the development of more rigorous models.