(32c) A Comprehensive Heat Transfer Model for the Fischer-Tropsch Tubular Fixed Bed Reactor

INTRODUCTION: The chemistry of the Fischer-Tropsch synthesis
can be described as the polymerization of single carbon units on a catalytic site. The
FTS reaction is highly exothermic and has an adiabatic temperature rise of up
to 1750K. Efficient heat removal is of
crucial importance because too high temperatures may lead to a loss in
selectivity, possible thermal runaways and rapid deactivation of the catalyst. The
investigation of the heat transfer and accurate heat transfer model are crucial
when a tubular fixed bed reactor needs to be designed for this synthesis. The current heat transfer models are
for classical packed bed reactors. When a fixed bed reactor is used for FTS,
there may be liquid involved. A study from a colleague (Xiaojun
Lu et al 2010) showed
that water might be both in liquid and vapor phases under FT reaction
conditions. The interaction between reaction, heat transfer, VLE, condensation, and
boiling is very complex and is not covered in the current models. Thus, it
would not be appropriate to apply these models in this situation. What we are
attempting to do in this paper is to set up a heat transfer model, considering
the heat transfer by the conduction mechanism and the condensation and evaporation
of the material, to offer a better simulation for FT reactors.

METHODS: In this
study, a tubular reactor with an inner diameter of 23 mm is used to investigate
the radial heat transfer in a fixed bed FTS reactor with TiO₂ supported cobalt
catalyst (10% Co/90% TiO₂, BET area 28.6 m²/g,
average pore diameter 35.8 nm). Thermo-wells were built into the reactor at
different radial positions as indicated in Figure 1. Thermocouples could be
moved up and down to measure the temperature at different axial positions. Multiple heating jackets offered
a flat axial temperature profile (<0.5C)
under no flow conditions. FTS reaction was performed at typical low
temperature FTS conditions. Different control
temperatures and flow rates were chosen

The model being
developed currently regards the heat transferred in the bed as a combination of
the conduction mechanism and the heat flux due to the radial flow of the
materials in the reactor as a consequence of condensation and evaporation that
is governed by vapour liquid equilibrium.

Fig.1: A temperature profile of the catalytic bed when the FT
reaction took place (Twall=200oC, P=20bar, SV=2.25 NL/(h-gcat))

Fig. 2 The k_effof the catalyst bed at different FT reaction rates

RESULTS: A two dimensional temperature profile of the
catalyst bed is presented in Figure 1. When wall control temperature was at 200^oC,
the maximum temperature rise in the bed was as high as 27^oC with a
SV of 2.25 NL/(h-gcat). In Figure 2, calculated effective thermal conductivity
coefficients (k_eff) of the catalyst bed are plotted in respect of
the reaction rates that were derived at different reaction conditions. A higher
reaction rate results in a lower k_eff.

DISCUSSION & CONCLUSIONS: The measured
temperature profiles of the bed show that the temperature rise in the catalyst
bed is very sensitive to the wall control temperature, and a small increment
(from 200^oC to 205^oC) of T_wall could make the
maximum temperature rise increase from 27^oC to 45^oC. An
obvious conclusion from the k_eff values derived for the different
temperatures suggests that an assumption of constant heat transfer characteristics
for the whole bed is not sufficient and even incorrect The complexity of the vapour
liquid equilibrium and the condensation and evaporation of the products in
different parts of the bed limits the application of the classic heat transfer
model from the literature. The preliminary simulation shows promising results
when using the model taking both the conduction and the heat flux carried by
the side flow of the stream with condensation and evaporation into
consideration.

References:

Xiaojun
Lu, Diane Hildebrandt, and David Glasser

Study of
Fischer-Tropsch Synthesis (FTS) in a Batch Reactor with TiO2 supported Co
catalyst Abstract AIChE Spring Meeting 2010