Microencapsulated phase-change materials as heat transfer media in gas-fluidized beds

The objective of this research is to investigate microencapsulated phase-change materials as heat-transfer media in gas-fluidized beds. The effective specific heat of an encapsulated material undergoing phase-transition is several-fold larger than the specific heat of the same material undergoing sensible heat changes. Because heat transfer coefficients in a fluidized bed increase with specific heat of the particulate phase, encapsulated phase-change materials are expected to enhance heat transfer rates in a fluidized bed. Several types of microencapsulated products were evaluated for their usefulness as heat transfer media in fluidized beds, including shells of polymethylene-urea, cross-linked nylon, and gelatin and cores of octadecane and paraffin. Microcapsules with shells of gelatin and cores of octadecane proved most useful in this study. Heat transfer enhancements of 30% were observed for these microcapsules although enhancements of at least 85% were expected based on the effective specific heats of the microcapsules. The thermal resistance of the gelatin shell or octadecane core may be responsible for these results. Another possibility is that particle residence times in the fluidized bed were shorter than estimated, which would reduce the effect of increasing thermal time constant of capsules. (C) 1998 Elsevier Science S.A. All rights reserved.