IMPROVEMENT OF THE HEAT TRANSFER IN CATALYTIC FIXED BED REACTORS BY MEANS OF STRUCTURED PACKINGS
T. Schildhauer , E. Newson, A.Wokaun
Laboratory for Energy and Material Cycles, Paul Scherrer Institute CH-5232 Villigen-PSI, Switzerland
Overview
A novel type of structured catalyst support for heterogeneous catalysis in industrial tube-bundle reactors has been developed. Compared to conventional fixed beds, it shows a lower pressure drop but an increased heat transfer through the reactor wall. This can improve the performance of reactors used for strongly endo- or exothermic reactions by decreasing the development of cold or hot spots and lowering the risk of thermal run-aways in the latter case. These improvements have been shown by flow visualization and in heat transfer experiments and validated in experiments under dehydrogenation reaction conditions combined with a reactor simulation.
Acknowledgments
The project was supported by the Swiss Federal Office of Energy (BFE). Commercial catalysts were supplied by UOP Ltd. (UK) and Sulzer Chemtech (CH) under confidentiality agreements. P. Binkert (PSI) was responsible for construction work. Visualization experiments were carried out at the water channel of Institut für Fluiddynamik, ETH Zürich.
Comparison of experimental points and reactor simulation
Introduction
Conventional open cross flow structures show three main fluid paths:
The first follows the orientation of the "valleys" in the corrugated sheets, which alternately transport the fluid from one reactor wall through the structure to the opposite reactor wall in one layer, where the heat transfer occurs, and from left to the right in the neighbouring layers (case a) in the right figure).
The second main fluid path stays always inside the packing, winding itself around the crosspoints of the sheet’s corrugations and ensuring the good mixing behaviour and the heat and mass transfer between fluid flow and structure surface (case c) in the right figure).
The third fluid path, not shown in the figure, is the by-passing flow in the gap between structure and reactor wall.
To increase the poor heat transfer to the reactor wall, the second main fluid path is closed in the improved structured packings developed in this work, by flat sheets between the corrugated layers as seen in the left figure. This forces the whole fluid flow to contact the reactor wall where the heat transfer is improved.
flow direction
flow direction commercial packing
optimised packing structure
gap wall
flow direction structure
wide gap wall
low Redhydr(170), small gap
high Redhydr(500), wide gap commercial packing optimised packing
view direction of the photographs tracer injection
transparent walls
model of the structure
112 mm
10 mm
5 or 10 mm
(figure from Disseration Gerd Gaiser, Univ. Stuttgart, 1990)
Flow visualization
Heat transfer experiments (radial gradients in the structures neglected, Nu-Pr-Re-analogies describing heat transfer behaviour obtained)
• Endothermic Dehydrogenation of Methylcyclohexan as example reaction (∆Hr298K = 205 kJ/mol):
• structure (optimized packing) coated with Pt/Al2O3-catalyst
• compared to commercially available spherical Pt/Al2O3-catalyst
• pseudohomogeneous first order model including found heat transfer characteristics, heat transfer by radiation and mass transfer limitation due to film diffusion
Conclusions and Outlook
Optimised structured catalyst supports improve the heat transfer in tube-bundle fixed bed reactors by 25 - 30% without higher pressure drop.
The validity of the Nu-Re-Pr-analogy under reaction conditions could be shown by comparison of polytropic experiments with reactor simulation.
The use of optimised packing may decrease operating costs and permits higher space-time-yields both for endo- and exothermic reactions.
19
PI TI
2.5
350 thermo-well
sand electrical
heating catalyst
Inside Wall Heat Transfer Coefficients α
αCP = 60.2 G0.55 αOP = 78.6 G0.53
αSC = 50.2 G0.61 10
100 1000
0.1 1 10
Specific Mass Flow G [kg/m2s]
HT-Coefficient α [W/m2K] Optimised Packing
Commercial Packing Spherical Catalyst (d = 1.75 mm)
+ 3 H
2280 320 360 400
0 0.1 0.2 0.3 0.4
reactor length [m]
temperature [°C]
optimised packing, measured optimised packing, calculated spherical catalyst, measured spherical catalyst, calculated
