(712b) CFD Simulation of Flow and Heat Transfer in Fixed Beds with Axial Thermowells

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.¹

Particle-resolved simulations of fixed beds using computational
fluid dynamics (CFD) is an approach that can improve our understanding of
fluid flow and heat transfer by solving for flow and heat transfer around the
particles, coupled to conduction, diffusion and reaction inside them. Using
particle-resolved CFD, the local effects of a thermowell on the positioning of
the particles and on the flow and temperature fields can be found.  In this
study simulation results using our validated CFD methods² are
presented for flow with heating from the tube wall for random packings of spheres
inside tubes with tube-to-particle diameter ratios (N) of 3.96, 5.96 and 7.99,
respectively. Each tube-to-particle ratio (N) includes cases with inner radius
(thermowell radius) to outer radius (tube radius) ratio of 0 (full bed), 0.05,
0.1, 0.2 and 0.3.

 Figure 1: Temperature contours in N = 3.96 bed, comparing full bed to bed with
0.05 thermowell

Simulation results show that
the presence of thermowells did change the structure, velocity field and heat
transfer inside the fixed beds, mainly at the center of the tube. Pressure drop
is only slightly affected. There is no clear trend with size of thermowell, the
main influence on the transport is the local structure of the bed due to the
arrangement of the spherical particles, which can change drastically with only
a small change in thermowell size. Systematic differences are found between the
centerline temperature of the tube with no thermowell, and the thermowell
temperature (see Figure 1) in all cases. The changes in axial temperature occur
because the configuration of particles in the bed is rearranged, even by a thin
thermowell, causing changes in voidage and flow pattern and also due to axial
thermowell conduction.

1. Landon V
G. Temperature correction for packed bed multitubular reactors with concentric
axial thermowells. Comput. Chem. Eng, 1996, 20, 475-481.

2. Dixon A G, Walls G,
Stanness H, Nijemeisland M, Stitt E H. Experimental
validation of high Reynolds number CFD simulations of heat transfer in a
pilot-scale fixed bed tube. Chem. Eng. J. 2012, 200, 344-356.