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Evaluation of flow nonuniformity and its effects on solid-liquid extraction in a continuous countercurrent system
Abstract
A laboratory-scale, horizontal, continuous, countercurrent solid-liquid extractor was constructed and used in experiments to determine: (1) axial dispersions in solid phase, (2) convective mass-transfer coefficients, (3) axial dispersions in liquid phase, and (4) extraction yields and solute concentration profiles in liquid and solid phases at various stripping factors, and Fick's numbers. Exhaustively extracted and sucrose-infused roasted and ground coffee with a narrow particle size range was used as solid carrier in experiments to determine axial dispersions in the liquid phase, extraction yields, and concentration profiles. All experiments were carried out at room temperature, i.e. between 17$\sp\circ$C and 23$\sp\circ$C. The solid-phase superficial axial dispersion coefficients ranged between 8.37 $\times$ 10$\sp{-6}$ m$\sp2$/sec and 3.85 $\times$ 10$\sp{-6}$ m$\sp2$/sec, and the corresponding 1/Pe for the solid phase were between 0.0050 and 0.0065. At such low 1/Pe, the effect of solid-phase axial dispersion on extraction efficiency is insignificant. The measured convective mass-transfer coefficients ranged between 2.6 $\times$ 10$\sp{-5}$ m/sec and 5.2 $\times$ 10$\sp{-5}$ m/sec, and corresponding mass transfer Biot numbers were between 500 and 1100. High axial dispersion in liquid phase was obtained by using liquid recirculation to examine the effects of liquid-phase axial dispersion on extraction efficiency. Liquid-phase axial dispersion coefficients increased 4.2 to 11.6 times when recirculation was used, and other flow conditions were maintained constant, and extraction yields decreased by 1.75% to 4.83%. Diffusion partial differential equation solutions which account for liquid-axial dispersion can be used to improve predictions of extraction yields and concentration profiles. Experimental yields ranged between 87.0% and 101.7% as high as the predicted yields. Major factors which probably contributed to experimental yields being lower than predicted yields were: (1) partial bypassing of liquid due to cross-sectional nonuniformity in solid bed packing, and (2) decreases of mass-transfer driving force due to cling. Solute concentration profiles predicted agreed well with experimental data when the Fick's number (Fo) was small, and average discrepancies of 7.4% occurred at high Fo.
Subject Area
Chemical engineering|Agricultural engineering
Recommended Citation
Lee, Jeong Cheol, "Evaluation of flow nonuniformity and its effects on solid-liquid extraction in a continuous countercurrent system" (1990). Doctoral Dissertations Available from Proquest. AAI9110173.
https://scholarworks.umass.edu/dissertations/AAI9110173