(30f) 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.

 ADDIN EN.REFLIST 1.         Dry, M.E., The Fischer-Tropsch process: 1950-2000. Catalysis
Today, 2002. 71: p. 15.

2.         Dai, X.P.,
et al., Fischer-Tropsch synthesis in a bench-scale
two-stage multitubular fixed-bed reactor: Simulation
and enhancement in conversion and diesel selectivity. Chemical
Engineering Science, 2014. 105: p. 11.

3.         Narataruksa, P., et al., Conversion enhancement of tubular
fixed-bed reactor for Fischer-Tropsch synthesis using
static mixer. Journal of Natural Gas Chemistry, 2012.
21: p. 10.

4.         Krishna, R.
and S.T. Sie, Design and scale-up of the Fischer-Tropsch bubble column slurry reactor. Fuel
Processing Technology, 2000. 64: p. 33.

5.         LeViness, S., et al., Velocys
Fischer-Tropsch Synthesis Technology - New Advances
on State-of-the-Art. Top Catalysis, 2013. 57: p. 8.