Dry solids in rotary drums

EXPERIMENTAL STUDIES, 196 Flow of Dry Solids in Rotary Drums, 196 Flow of Highly Settling Slurries in Rotary Drums, 196 Flow of Slightly Settling Slurries in Rotary Drums, 198... 

FLOW OF DRY SOLIDS IN ROTARY DRUMS, 199 Prediction of Solids Hold-up, 199... 

This chapter is divided into four parts. In the first part, a description of experimental studies on flow of solids and slurries in rotary drums is given. In the second part, the flow of dry solids in rotary drums will be examined. In the third part, flow of highly settling slurries in rotary drums will be reviewed. In the last part, flow of slightly settling slurries will be discussed. 

EXPERIMENTAL STUDIES Flow of Dry Solids in Rotary Drums... 

There are two types of solids motion in a horizontal rotary drum longitudinal (axial) and transverse (radial). For a horizontal drum without lifters, solids transport in the axial direction occurs as a result of the difference in the bed height at the inlet and outlet ends of the drum. Afacan and Masliyah discussed various models to predict dry solids transport in rotary drums as a result of the bed height axial gradient [16]. 

Flow of dry solids in a lightly loaded horizontal open-end rotary drum is given by Kramers and Croockewit [44] as ... 

Flow of dry solids in horizontal rotary drums is governed by many factors. Solids hold-up in rotary drums is a function of drum speed, drum design, and solids properties. Solids hold-up for lightly loaded drums, at n < 0.4, can be accurately predicted using the Kramers and Croockewit equation [45]. More experimental and theoretical work are needed to predict solids hold-up for highly loaded drums. 

A slurry containing 40 per cent by mass solid is to be filtered on a rotary drum filter 2 m diameter and 2 m long which normally operates with 40 per cent of its surface immersed in the slurry and under a pressure of 17 kN/m2. A laboratory test on a sample of the slurry using a leaf filter of area 200 cm2 and covered with a similar cloth to that on the drum, produced 300 cm3 of filtrate in the first 60 s and 140 cm3 in the next 60 s, when the leaf was under pressure of 84 kN/m2. The bulk density of the dry cake was 1500 kg/m3 and the density of the filtrate was 1000 kg/m3. The minimum thickness of cake which could be readily removed from the cloth was 5 mm. 

At steady state, the product stream is continuously removed from the fermentor at a rate equal to feedrate. The product stream is concentrated in a desludging separator to a paste of 20 to 30% dry solids. After washing with warm water to remove adsorbed or absorbed hydrocarbons, a second desludging separator produces a cleaned paste that is ftirther dehydrated in a rotary vacuum filter. A final drying step in an atmospheric drum dryer reduces the moisture content to less than 10%, and the powdered product is packaged for the customer. 

Conduction or indirect dryers are more appropriate for thin products or for very wet solids. Heat for evaporation is supplied through heated surfaces (stationary or moving) placed within the dryer to support, convey, or confine the solids. The evaporated moisture is carried away by vacuum operation or by a stream of gas that is mainly a carrier of moisture. Vacuum operation is recommended for heat-sensitive solids. Because the enthalpy lost with the drying air in convective dryers is large, their thermal efficiency tends to be low. For conduction dryers the thermal efficiency is higher. Paddle dryers for drying of pastes, rotary dryers with internal steam tubes, and drum dryers for drying thin slurries are examples of indirect dryers. 

A. Richard, M. Tomczak, M. Aubran, and E. Ronat, An approach to automation of rotary drum dryers, in Comparative Study of Industrial Applications in Drying of Solids, A.S. Mujumdar (Ed.), Sarita Prakashan, New Delhi, India, pp. 253-260 (1990). 

Washing and adsorption The belt filters are used for washing and dewatering of fine solids in the manufacture of catalysts, zeolites, alumina and other crystalline substances, and so on. The belt filter requires the associated equipment — several vacuum pumps and separation drums. Through-circulation tray driers and through-circulation rotary driers are used to dry solids. All these are continuous crosscurrent processes. These could be replaced with a moving-bed, Hildebrand (screw) conveyer or Kennedy extractor [18], each of which is a countercurrent process. Their potential for PI has not been explored. 

Slurry flow in a horizontal rotary drum can be compared to that of dry solids, slurry flow in closed conduits, or open channels. A brief discussion will be given in this section for each of these transport modes. 

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