Heat and Mass Transfer Enhancement in a Packed Bed, Low Temperature Fischer-Tropsch Reaction Via Ring-Disc Passive Flow Disturbers

Heat and Mass Transfer Enhancement in a Packed Bed, Low Temperature Fischer-Tropsch Reaction via Ring-Disc Passive Flow Disturbers.

Steven Wiryadinata¹, and Paul Erickson¹

¹ Energy Research Laboratory, Department of Mechanical and Aerospace Engineering, University of California, Davis

This work investigates the effects of ring-disc flow disturbers on conversion and C₅₊ selectivity of the low temperature Fischer-Tropsch (FT) reaction in a packed-bed tubular reactor with an aspect ratio (length to inner diameter, L/D) of 52. The flow disturbers directly influence the flow characteristics, temperature distribution and species concentrations within the bed, which, in turn, affects the distribution of products (ranging from CH₄ to heavy hydrocarbon waxes and water) from the conversion of reactant syngas (H₂ and CO). The packed bed for FT synthesis is known to suffer from severe radial and axial heat limitation manifesting as large temperature gradients which lead to lower C₁₀₊ selectivity and increased risk of catalyst degradation. Even though the packed bed affords some flexibility in the reduction of catalyst size to minimize the mass transfer resistance and improve conversion, it is limited by pressure drop considerations, and by the catalyst attrition specifically in terms of reduced structural strength as particle size decreases.

Industrial scale packed bed FT reactor has employed high aspect ratio reactors (L/D in excess of 150) in order to achieve acceptable conversion levels, and significant product liquid recycle in order to remove the heat of reaction [1]. Other smaller scale experimental studies has demonstrated the benefits to conversion and selectivity of inter-stage hydrocarbon products and H₂O removal to maintain high reactant concentration at each reaction state[2], and of the use of helical mixers to promote mixing and enhance the radial heat transfer[3]. The slurry bubble column [4] and the microchannel reactors [5] are alternate reactor designs which address the heat transfer limitation of the conventional packed bed, but introduces other issues such as product-catalyst separation especially with finer particles to reduce mass transfer resistances, catalyst inventory in reaction channel with small hydraulic diameter, and manufacturing and operational cost. With the highest catalyst inventory of all reactor types, the conventional packed bed has the highest productivity but only if heat and mass transfer resistances are minimized.

The ring-disc flow disturbers change the flow characteristics within the bed by inducing mixing and redirecting flow radially to minimize axial channeling. Radial heat transfer is no longer dependent solely on diffusion, and heavy hydrocarbons formed in the vicinity of the relatively cool wall region are directed to the warmer center axis to assist in the heat removal process. For a given inlet mass flow, the rings and discs induce a higher local flow velocity which increases both the convection heat transfer and the external mass transfer.

The experimental setup consists of feed gas conditioning systems, tubular reactor with external cooling jacket, two-stage liquid trap with condenser, and product analysis subsystems.  A reactant stream is formulated from bottled gases in 2:1 H₂/CO ratio and heated to 210^oC. A 2.1 cm diameter stainless steel tube houses 15 wt% Co/Al₂O₃ catalysts in pellet form (average diameter of 1 cm) with varying numbers of disturber packages (up to 8 alternating ring-disc packages) in equidistant packing. Saturated propylene glycol maintains the reactor wall at 200^oC. A pressure transmitter and back pressure regulators downstream of the hot trap maintain the reactor pressure at 5 bar. The hot trap is operated at a minimum temperature of 150 ^oC. The effluent line is heat traced to maintain the specified temperature. Hot trap effluents are expanded to atmospheric pressure and cooled in a condenser at 2^oC. The hot trap vapor is directed to a 2^nd trap after cooling to separate non-condensable species for online gas analysis of H₂, CO, CO₂ and CH₄. An SRI gas chromatography unit with a 13X molecular sieve and MXT-1 capillary column was used to speciate condensed products.

The flow disturber technique presented here is expected to improve productivity of the packed bed by reducing heat and mass transfer resistances. Optimization of the ring-disc geometry, reactor aspect ratio and catalyst size has the potential of further performance improvements.

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