Wall Heat Transfer Properties of Metal Microfiber Catalyst Support Structures

Abstract

A renewed interest in highly exothermic and endothermic reactions for scalable energy technology have demanded a need for a catalyst support structure that is capable of managing the thermal requirements of the reaction while promoting a high level of reaction efficiency. A novel catalyst support structure Microfibrous Entrapped Catalysts (MFEC) consists of a sintered network of highly thermally conductive micron-diameter metal fibers that entrap catalyst particles. The MFEC’s thermal properties and tube-wall interactions were studied to quantify and predict effective thermal conductivities and wall heat transfer coefficients.

Various fiber diameters were tested at the same volume percent (8.8 vol%) and gave thermal conductivities in a range of 3.15 to 6.80 [Wm-1K-1] and wall contact conductances in a range of 712 to 2939 [Wm-2K-1]. Stainless steel tubes were lathed with an internal screw pattern to give additional surface area (up to 41.6%) for the MFEC to contact the tube wall. In all tests, the MFEC was determined to have fibers that were unable to elastically conform to the grooves cut into the tube wall, leading to a reduction of the wall heat transfer coefficient by approximately 61%. Flowing gas tests were also utilized showing wall heat transfer coefficients up to 350 [Wm-2K-1] at a GHSV of 13,700 [h-1]. Metal volume percent was kept constant for all fiber diameters. Using a thermal interface material tester was used to test the axial properties of the MFEC. Axial thermal conductivities ranged from 0.320 to 0.493 [Wm-1K-1], and confirmed using a thermal effusivity tester. Thermal resistance network diagrams were constructed to show that wall heat transfer coefficient is limited by the thermal conductivity of the interstitial gap. Once the interstitial fluid’s thermal resistance is greater than 2.07E-4 [K/W], the stainless steel wall becomes the limiting step.