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Heat transfer and pressure drop in fixed beds of wood chips Chow, Bosco


Heat transfer from a flowing gas to a fixed bed of dried Dougas-fir wood chips has been studied by a transient method. Hot air at about 130°C flowed upward through 0.2 m dia x 1 m deep beds of commercially prepared wood chips which had been screened for thickness. Four different wood chip sizes were used, which varied in mean thickness from 2.44 to 7.26 mm. The thickest chips were 18.4 mm wide x 36.3 mm long. Gas temperatures were measured at a number of axial positions as the bed temperature rose from its initial temperature of about 20°C. Heat transfer coefficients were calculated by fitting the air temperature profiles to a transient mathemical model for plug flow of gas through a bed of slab-shaped particles with finite internal thermal resistance. The heat transfer model was solved analytically using an approach pioneered by Amundson (10) for fixed beds of spherical particles and based on Rosen's (6,7) function. This solution has not appeared elsewhere in the literature, and is shown to converge to that of Anzelius (1) if the Biot number for the particle approaches zero. Experiments were done at a series of air velocities with four wood-chip thicknesses and with spherical catalyst particles to provide a check on the technique. The effect on heating rate of 30% by volume steam in the incoming air was investigated. For selected experiments, solid temperatures within the wood chips were measured. A correlation of the heat transfer coefficients is presented. Pressure drop was measured as a function of air velocity for different sizes of wood chips at room temperature and the results are compared with predictions of the Ergun equation.

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