(359c) Development of a Comprehensive and Flexible Model for Fixed-Bed Fisher-Tropsch Reactors

Liquid fuels synthesized from renewable CO and H₂ could displace petroleum-based fuels to provide clean, renewable energy for the future.   Small scale (<100 bbl/day) reactors for synthetic liquid fuels production (e.g. methanol, Fischer-Tropsch diesel) are an emerging development area that may enable the use of diverse, renewable, carbon-neutral feedstocks, such as biomass.  For small scale production of synthetic liquid fuels from syngas, fixed-bed reactors are well-suited and have high potential productivity per reactor volume.  To fully realize the potential of these reactors, four key issues must be addressed: 1) minimize temperature gradients due to the highly exothermic reactions, 2) maximize diffusion of reactants and products, 3) prevent heavy hydrocarbons (waxes) from accumulating in the catalyst pellets, reducing conversion and 4) management of lifecycle degradation. 

Over the last 90 years, the determination of optimal Fischer-Tropsch synthesis (FTS) reactor conditions has evolved starting with primarily experimentally determined inputs, to increased use of empirical modeling, to incorporation of more theory so that optimal conditions can be predicted and laboratory and pilot plant tests are no longer needed.  However, although reactor size and capacity has increased over the years, the same general reactor design today is not much different than that used in the 1940’s, especially in the case of fixed-bed reactors.  This has led, rightly so, to models that explicitly or implicitly take advantage of this wealth of experience and knowledge with some degree of empiricism.   Although they give very accurate results for modeling traditional reactors, the existing models are unable to examine significant design changes and innovations that may be needed to solve the four challenges listed above.

The goal of the model developed in this work is to improve upon the fidelity and flexibility of current models, by reducing the level of empiricism, to enable examination of small-scale reactors or unique designs.  Accomplishing this requires validated, coupled, heat and mass transport and chemical kinetics models derived from a component-based, dynamic (time-varying) two-dimensional heterogeneous model that accounts for interfacial and intra-particle gradients.  There is no evidence in the open literature of a model of this type being successfully developed. 

This paper presents our model development progress in the following areas:

1. Two-phase interparticle (bulk) flow
2. Interparticle (bulk) heat transfer
3. Intraparticle mass transfer (within pore)
4. Intraparticle heat transfer (within solid and pore)
5. FTS chemical kinetics
6. Time-dependent catalyst deactivation by coke deposition, hydrothermal sintering, liquid/wax accumulation, and poisoning.
7. Combination of the various phenomena into a single comprehensive model.

The successful, validated model of this type should enable accurate simulation of innovative and non-traditional reactors.