(329d) Do Axial Thermowells Measure the True Temperature for Highly Exothermic Reactions in Fixed Beds? a CFD Analysis for Phthalic Anhydride Synthesis.

Axially-centered thermowells are commonly used for monitoring temperatures in lab scale and pilot plant fixed bed tubes used in catalytic reactors for highly exothermic partial oxidation reactions. Previous analyses of the effects of including thermowells inside the tube have used effective medium models which do not represent changes in particle location and bed structure that must result from use of a thermowell.

Particle-resolved simulations of fixed beds using computational fluid dynamics (PRCFD) can improve our understanding of transport and reaction on multiple scales by solving for flow around the particles, coupled to conduction, diffusion, and reaction inside them. Using validated PRCFD, the local effects of a thermowell on the positioning of the particles and on the flow, species and temperature fields can be found.

The present study analyses the thermowell system for monitoring temperatures in a fixed bed catalytic reactor for the highly exothermic partial oxidation of o-xylene to phthalic anhydride. PRCFD simulations are presented for randomly-packed spheres inside cylindrical tubes with tube-to-particle diameter ratios (N) of 3.96, 5.96 and 7.99. Results are obtained for each N value for inner thermowell radius to outer tube radius ratios of 0.0 (no thermowell), 0.05, 0.1 and 0.2. Comparisons of radial void fraction and axial velocity profiles, axial pressure drop and axial and radial temperature profiles under o-xylene oxidation are presented.

Changes in measured axial temperature occurred because the local configuration of particles in the bed was rearranged, by even the thinnest thermowell, causing large-scale changes in voidage, flow pattern and distribution of temperature and reaction inside the bed. This work implies that measurements made in low-N beds of temperatures resulting from a highly-exothermic reaction should be treated cautiously, as the actual hot-spot and its location in an un-instrumented bed may differ from those measured in a tube with a thermowell.