Ultraclean Transportation Fuels - American Chemical Society

Chapter 7

Multifunctional Catalyst for Fischer-Tropsch Synthesis Mitrajit Mukherjee and Sankaran Sundaresan

Downloaded by COLUMBIA UNIV on March 3, 2015 | http://pubs.acs.org Publication Date: April 12, 2007 | doi: 10.1021/bk-2007-0959.ch007

Exelus Inc., 99 Dorsa Avenue, Livingston, NJ 07039

A first-of-a-kind multifunctional catalytic system is being developed to convert synthesis gas into synthetic crude via the Fischer-Tropsch reaction. This is achieved through intensification of chemical reaction and heat and mass transport processes within the catalyst system. The synergistic integration of intensified unit operations with chemical reaction leads to enhanced catalyst performance and significant economic advantages.

Gas-To-Liquids (GTL) technologies offer opportunities to convert natural gas into premium transportation grade fuels. The GTL process first converts natural gas to synthesis gas (a mixture of CO & H ) by partial oxidation of methane, steam reforming of methane or a combination of both called autothermal reforming. Syngas (synthesis gas) is then upgraded into a range of products termed 'synthetic crude' via the Fischer-Tropsch (FT) reaction. The major advantage of the GTL process is the exceptional quality of its products, which can easily meet current and future anticipated clean fuels specifications. The initial gas-to-liquids complexes implemented in the 1980s and early 1990s were not commercially successful for a number of reasons - the main one being that they were far too expensive. Although improvements in technology have been achieved during the past decade, GTL complexes are still very expensive propositions. The other problem with GTL projects is scalability. The 2

© 2007 American Chemical Society

In Ultraclean Transportation Fuels; Ogunsola, O., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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76 lowest capacity of the three commercial GTL processes available requires throughput of at least lOOxlO ft /day of natural gas to be within its viable range and have minimum estimated installed costs of about $1 billion for a 34,000 barrels/day facility (/). This paper summarizes the development of a breakthrough FT technology that has the ability to significantly improve the economics of the process using a multi-functional catalytic system. The cost reduction potential using this next-generation FT technology provides a very strong springboard for overall GTL process viability. 6

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Fischer-Tropsch Process Technical Challenges The Fischer-Tropsch process presents many technical challenges due to its unique reaction chemistry and high heats of reaction. The selectivity of the process is usually described by the Anderson-Schulz-Flory (ASF) distribution and the characteristic chain growth probability, a. High chain growth probabilities (a >0.90) are regarded desirable as they lead to high selectivities of heavy components (2), which subsequently can be converted to liquid fuels rather easily. Product distribution or product selectivity (a) depends heavily on the type and structure of the catalysts and on the reactor type and operating conditions.

FT Catalyst Design Issues The most common FT catalysts are Group VIII metals (Co, Fe, Ru, etc.) (5). Most early FT catalysts were Fe-based systems mainly due to its low cost. Cobalt catalysts give the highest yields, longest lifetimes and yield predominantly linear alkanes. A large number of papers have focused on the optimization of metal. However limitations in pore diffusion within the support lead to lower C product selectivity. Even though the reactants are in the gas phase, the pores of the catalyst are filled with liquid products. The diffusion rates in the liquid phase are typically three orders of magnitude slower than in the gas phase, and even slow reactions may be diffusion limited in the liquid phase. As the ratio between hydrogen and carbon monoxide concentration is important, the difference in diffiisivity between hydrogen and carbon monoxide result in selectivity changes. Increasing the transport limitations leads to CO depletion and enhanced hydrogénation reactions, resulting in lower selectivities to C . Hence, key to an effective FT catalyst design lies in optimizing both the catalyst composition and its diffusional characteristics. 5 +

5+


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FT Reactor Design Issues The FT synthesis reaction is strongly exothermic, requiring highly effective heat transfer. Multi-tubular fixed-bed and slurry bubble columns (4) have been the reactors of choice for low-temperature Fischer-Tropsch synthesis. The large support particles infixed-bedreactors result in poor intra-particle mass transfer characteristics and the space-time yield is limited by heat transfer in the catalyst bed. The slurry system gives rise to significantly improved mass transfer characteristics within the catalyst particles, but the separation of the catalyst from the product can be troublesome. Back-mixing renders the slurry reactor less efficient in terms of reactor volume than a plug flow reactor. A principal advantage of slurry bubble column reactors (BCR) over fixed-bed reactors is that the presence of a circulating/agitated slurry phase greatly increases the heat transfer rate to cooling surfaces built into the reactor. As a result, conventional reactors are unable to satisfy all requirements simultaneously.

New Development Approach In commercial-scale FT reactors, non-idealities in flow distribution and inefficient inter-phase transport rates reduce the effectiveness of the catalyst significantly. As a result, large improvements in catalyst performance alone do not translate to large improvements in reactor performance. Researchers at Exelus are developing afirst-of-a-kindcatalytic system for multi-phase reactions that blurs the line of distinction between the reactor and catalyst by incorporating many features one associates with a reactor within the catalyst body. The new multifunctional catalyst system, called the HyperCat-FT, has a controlled structure at scales ranging from the nano- (pore level) to the macro-scale (reactor level). By creating 'structure' on various length scales, the HyperCat-FT system is able to achieve superior control of the microenvironment at the catalytic sites through higher rates of gas-liquid and liquidsolid heat and mass transfer while minimizing pore-diffusion limitations. The high level of control allows ideal reaction conditions to prevail at the active sites, enhancing catalyst performance leading to significantly higher activity and product selectivity. As a result, reactor productivity is enhanced and the active catalyst volume can now be reduced by 50% or more over a conventional slurry bubble column reactor. The higher levels of inter-phase heat transfer also allow steam coils (a low-cost item) to be used for removing reaction heat instead of expensive shell-and-tube reactor designs. These characteristics result in a significant reduction in the cost of the Fischer-Tropsch reactor, leading to a corresponding decrease in the cost of a GTL complex.


78 Conventional Catalytic Process


HyperCat

Residence Time Distribution

Figure 1. Concept for a new multifunctional FT-catalyst.

Results and Discussion Gas Hold-Up Gas hold-up is a critical parameter in characterizing the hydrodynamic behavior and hence the performance of a bubble column reactor. It determines: a) the reaction rate by controlling the gas-phase residence time and b) the masstransfer rate by governing the gas-liquid interfacial area. It is mainly a function of the gas velocity and the liquid physical properties. The gas hold-up of the HyperCat system was studied using an air-water system. The gas superficial velocities were varied from 0.1-1.0 m/s. The results are shown in Figure 2. The HyperCat gas hold-up is slightly lower than that in a conventional bubble column reactor (~ 30% reduction) and comparable to that in a slurry bubble column with 30-35% solids concentration (4).

Liquid-Phase Axial Dispersion The liquid phase axial dispersion in bubble column reactors is very high making them effectively back-mixed reactors for industrial-scale systems.


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Ultraclean Transportation Fuels - American Chemical Society

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