The Drying of Vegetable Materials in a Stream of Air

Smith, James Caldwell (1959) The Drying of Vegetable Materials in a Stream of Air. PhD thesis, University of Glasgow.

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A rotary drier is a rotating cylinder through which passes a steady stream of material which is agitated and showered through hot air by lifting flights to secure efficient heat and mass transfer. Although this drier is now well established in chemical industry, virtually all the design methods and complete operational studies available are restricted to the treatment of materials which have only surface moisture and are therefore easily dried. Vegetable materials are representative of the substances whose drying rates decrease as they lose moisture, i.e. they dry in the so-called falling rate period. They have therefore been chosen to study the counter-current operation of a small rotary drier built for this purpose. The operation of the drier is controlled by air mass flow and temperature, the speed and slope of the tube, by the lifting flights,and the feed rate of the material being dried. The effects of these on the drying rates of three vegetable materials which were available in bulk, viz. brewers' spent grain, barley grain and granulated cork, were studied, and to allow for the effects of the time of contact in the drier, the conveying properties of the unit were checked with both wet and dry materials. As considerable deviation from an average contact time is possible, which may lead to scorching and degradation of part of the feed, the effects of some of the operating factors on dispersion were studied in a smaller unit. Because of the interdependence of many of the factors involved, interpretation of the results was occassionally difficult, but where possible, particularly in tests on inlet air temperature and velocity, drying rates are compared with fundamental or basic figures obtained from through circulation drying of thin layers. After allowance had been made for the considerable variation in material velocity through the rotary drier, in both types of drying the rate of removal of moisture is shown to be proportional to the moisture content of the material, i.e. dW/dtheta -m.W , or Log eW = -m.theta + k The values of m, the falling rate constant, are employed to compare the effects of each of the operating variables on drying rates. Those calculated from the rotary drier are about one tenth of the values obtained from single layers under comparable conditions of temperature and airflow. Effects of variables controlling rotary drying: Tests on contact, or retention time and loading show that although relations already proposed for dry materials can be generally accepted, with wet feed considerable differences are observed. The effects of the operating factors are detailed and discussed. A general correlation for deviation from mean retention time has been developed explaining the effects of the factors, viz. flight action and bouncing, which apparently cause this. Inlet air temperature: In studies in the range 120

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Agricultural engineering, Mechanical engineering, Food science
Date of Award: 1959
Depositing User: Enlighten Team
Unique ID: glathesis:1959-79341
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 05 Mar 2020 10:40
Last Modified: 05 Mar 2020 10:40

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