Comparison of the Effectiveness of a Fluidized Sand Bath and a Steam Chamber for Reactor Heating

Both fluidized sand baths and steam chambers have been used to heat laboratory reactors, in particular for studies of biomass pretreatment. In this study, several aspects of the heating performance of these devices were compared: time to heat reactors to reaction temperature, the stability of reactor temperature, and the convection coefficient. The convection coefficient was determined using correlations and multiple analyses of empirical data. On the basis of the results presented in this study, the steam chamber can heat reactors to temperature in a tenth of the time sand baths can, can maintain a more stable temperature during pretreatment, and has a convection coefficient one to two magnitudes greater than that of the sand bath. Therefore if heat transfer is critical, a steam chamber is advantageous. ■ INTRODUCTION Heating laboratory reactors can be accomplished by a wide variety of devices. The choice of heating system is often dictated by the available equipment. A current field of research is the development of a pretreatment to alter the composition and/or structure of lignocellulosic biomass such as poplar for rapid and effective enzymatic hydrolysis as part of the efficient production of ethanol from cellulose and hemicellulose. Pretreatments using water at elevated temperatures have been shown to be effective and have been conducted in batch reactors heated in a fluidized sand bath or more recently in a 96 well plate heated in a steam chamber with improved penetration of the heat transfer fluid into the interstices between wells. This new heating device prompted an investigation into the differences in heat transfer in the traditional fluidized sand bath and the novel steam chamber in order to determine the optimal choice. Consequently, the objectives of this study were to compare (1) the time for reactors to reach reaction temperature in each device, (2) the temperature stability of the reactors during pretreatment, and (3) the convection coefficients that would describe heat transfer in each device. ■ CALCULATION OF CONVECTION COEFFICIENTS The convection coefficient, h, which depends on fluid properties, surface geometry, and flow conditions can be calculated with an appropriate correlation or from experimental data. Calculation of Convection Coefficient from Literature CorrelationsSand Bath. The convection coefficient from a gas fluidized bed to a solid rod can be calculated as the sum of a gas convective component, hg, a particle convective component, hp, and a radiative component, hr. 14−16 = + + h h h h SB r g p (1) Since hr is only significant at T > 600 °C, it was considered negligible in this study. One correlation for hg developed by Martin is