Comparison of moving bed dryers of solids operating in parallel and counterflow modes

Jaakko Saastamoinen

Research output: Contribution to journalArticleScientificpeer-review

9 Citations (Scopus)

Abstract

The drying rates in moving bed dryers are compared. The gas and the solids to be dried are in parallel flow or counterflow. A new simplified method to simulate the drying in parallel and counterflow moving beds is developed. This model is based on the solution of arbitrary experimental or theoretical drying rate Equations of single solid particles (or thin-layer drying rate equation) coupled with heat and mass conservation Equations of the dryer. The solution is presented in an integral form of the drying equation showing the relation between time or location in the dryer and degree of drying. The method allows rapid calculation of the moisture, vapor mass fraction, and temperature distributions along the dryer in drying with moist air or steam. The model is demonstrated by using a result based on the receding front evaporation model as the specific thin-layer drying equation in the moving bed model. Wood chips are chosen as an example of the substance to be dried, but the method applies also for other medium, if the dependence of the drying rate on moisture and ambient temperature and humidity (thin layer drying rate) is known. The size of the dryer needed to reach the same degree of drying operating in the parallel mode is much greater than that of counterflow type, when the drying medium is air or flue gases. The reason for the poorer drying in parallel flow is mainly the unfavorable distribution of the evaporation temperature. In steam drying, the difference in the size is not so great, since the evaporation takes place approximately at constant temperature.
Original languageEnglish
Pages (from-to)1003 - 1025
Number of pages23
JournalDrying Technology
Volume23
Issue number5
DOIs
Publication statusPublished - 2005
MoE publication typeA1 Journal article-refereed

Fingerprint

drying apparatus
Driers (materials)
counterflow
drying
beds
Drying
parallel flow
Evaporation
Parallel flow
Steam
evaporation
moisture
steam
Moisture
flue gases
conservation equations
air
Air
Flue gases
gases

Keywords

  • particles
  • wood chips
  • forest chips
  • mathematical model
  • air drying
  • steam drying

Cite this

Saastamoinen, Jaakko. / Comparison of moving bed dryers of solids operating in parallel and counterflow modes. In: Drying Technology. 2005 ; Vol. 23, No. 5. pp. 1003 - 1025.
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Comparison of moving bed dryers of solids operating in parallel and counterflow modes. / Saastamoinen, Jaakko.

In: Drying Technology, Vol. 23, No. 5, 2005, p. 1003 - 1025.

Research output: Contribution to journalArticleScientificpeer-review

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AB - The drying rates in moving bed dryers are compared. The gas and the solids to be dried are in parallel flow or counterflow. A new simplified method to simulate the drying in parallel and counterflow moving beds is developed. This model is based on the solution of arbitrary experimental or theoretical drying rate Equations of single solid particles (or thin-layer drying rate equation) coupled with heat and mass conservation Equations of the dryer. The solution is presented in an integral form of the drying equation showing the relation between time or location in the dryer and degree of drying. The method allows rapid calculation of the moisture, vapor mass fraction, and temperature distributions along the dryer in drying with moist air or steam. The model is demonstrated by using a result based on the receding front evaporation model as the specific thin-layer drying equation in the moving bed model. Wood chips are chosen as an example of the substance to be dried, but the method applies also for other medium, if the dependence of the drying rate on moisture and ambient temperature and humidity (thin layer drying rate) is known. The size of the dryer needed to reach the same degree of drying operating in the parallel mode is much greater than that of counterflow type, when the drying medium is air or flue gases. The reason for the poorer drying in parallel flow is mainly the unfavorable distribution of the evaporation temperature. In steam drying, the difference in the size is not so great, since the evaporation takes place approximately at constant temperature.

KW - particles

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KW - forest chips

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