Dynamic modeling and feedback control for conveyors-belt dryers of mate leaves

Abstract A semi-empirical model is proposed to reproduce the kinetics of drying in a continuous shallow packed bed dryer of mate leaves at transient conditions. The mathematical expression representing the dynamic model, which was obtained from a mass balance for water in the solid phase of the drying chamber, was validated at steady-state condition in an industrial continuous dryer. The transient model was solved with the numerical method of lines by involving a backward differentiation formula (BDF) to approximate the first order spatial and time derivative. Based upon this reliable model, a control strategy was suggested to maintain the discharge moisture content in the acceptable range of 2.4–3.4% (dry basis) by adjusting the velocity of conveyor-belt to compensate disturbances in the operating conditions. The performance of a proportional-integral-derivative (PID) and a proportional-integral (PI) controller was verified by a comparison between open- and closed-loop responses of discharge moisture content to random changes in the feed moisture content, drying temperature and air velocity. The simplex method was applied during the tuning procedure of the controller parameters by minimizing the integral squared error (ISE) of the process output when a step change in the set-point was imposed.

[1]  G. M. White,et al.  Predicting Equilibrium Moisture Properties of Soybeans , 1989 .

[2]  Stephanopoulos.  Chemical Process Control , 2008 .

[3]  A.J.B. van Boxtel,et al.  Control of fluid bed tea dryers : controller design and tuning , 2000 .

[4]  E. Zanoelo.  A theoretical and experimental study of simultaneous heat and mass transport resistances in a shallow fluidized bed dryer of mate leaves , 2007 .

[5]  E. Zanoelo.  Equilibrium Moisture Isotherms for Mate Leaves , 2005 .

[6]  Y. Mizukami,et al.  Moisture Content Measurement of Tea Leaves by Electrical Impedance and Capacitance , 2006 .

[7]  S. J. Temple.  Control of fluidized bed tea drying , 2000 .

[8]  D. Goldenberg.  Maté: a risk factor for oral and oropharyngeal cancer. , 2002, Oral oncology.

[9]  Paul Lewin,et al.  Experimental comparison of the performance of different chain conveyor controllers , 2000 .

[10]  A.J.B. van Boxtel,et al.  Modelling of Fluidized-bed Drying of Black Tea , 1999 .

[11]  Victor George Jenson,et al.  Mathematical Methods in Chemical Engineering , 1978 .

[12]  Zhongwei Tang,et al.  Thin-layer drying of spent grains in superheated steam , 2005 .

[13]  L. Cardozo-Filho,et al.  SUPERHEATED STEAM‐DRYING OF MATE LEAVES AND EFFECT OF DRYING CONDITIONS ON THE PHENOL CONTENT , 2006 .

[14]  P. Panchariya,et al.  Thin-layer modelling of black tea drying process , 2002 .

[15]  S. Prachayawarakorn,et al.  Comparative study of heating processes for full-fat soybeans , 2004 .

[16]  A.J.B. van Boxtel,et al.  Control of fluid bed tea dryers: controller performance under varying operating conditions , 2000 .

[17]  Jean-Claude Cheftel,et al.  Introducción a la bioquímica y tecnología de los alimentos , 1980 .

[18]  Alastair Robertson,et al.  Near-infrared reflectance prediction of quality, theaflavin content and moisture content of black tea , 1988 .

[19]  A.J.B. van Boxtel,et al.  Thin Layer Drying of Black Tea , 1999 .

[20]  A. Kobayashi,et al.  Volatile constituents of green mate and roasted mate , 1991 .

[21]  Everton Fernando Zanoelo,et al.  Drying kinetics of mate leaves in a packed bed dryer , 2007 .