Fluidized-bed driers

The heat required to evaporate moisture may be obtained by eonveetion from the gas stream, by eonduetion e.g. on heated trays, in pneumatie risers, fluidized beds, spray towers, or by radiation. A typieal fluidized bed drier is depieted in Figure 4.28. The hydrodynamie prineiples of fluidization were summarized in Chapter 2. Within a fluid bed drier, heated air, or other hot... 

Fluidized-bed driers are also widely used due to their large heat- and mass-transfer coefficients. However, materials of even moderate adherence and cohesiveness cannot be dried in a fluid bed. The same applies to materials that are sensitive to oxygen, especially at elevated temperatures. Vacuum drying is often necessary for oxygen sensitive materials and this is not easy to realize in fluid-bed driers, although there are systems to deal with this problem. Fluid-bed driers are not as easy to clean as shelf driers or rotary driers. 

Hanni et al. (1976) described a continuous fluidized bed drier using the centrifugal principle. It consisted of a perforated horizontal cylinder rotating inside a plenum with hot air blown across the outside of the cylinder and perpendicular to the axis of rotation (Figure 1.20). Vanes placed in the air inlet allowed the incident angle of the air flow to be varied from 0° to 45° to the perpendicular. Particles were fluidized inside the Teflon-coated stainless steel cylinder which could be tilted by up to 6° in order to control particle residence time. The cylinder, of... 

An overall material balance across a fluidized bed drier may be written... 

Figure 4.4 Fluidized bed drier material and enthalpy balance.

There are fwo fundamenfally different designs of fluidized bed the well-mixed bed and the plug flow bed. In addition, various combinations of fluidized bed drier may be used fo meef particular applications Kunii and Levenspiel (1991) illustrate a series of such designs (see Variations in fluidized bed drier design, below). 

Figure 4.5 Schematic diagram of plug flow fluidized bed drier.

Roberts, R.L., Carlson, R.A. and Farkas, D.F., Preparation of a quick-cooking brown rice product using a centrifugal fluidized bed drier, /. Food Sci., 45 (1980) 1080-1081. 

Instantisation, in which the fluidized bed is responsible for a significant change to the structure and size of the granules, should not be confused with the use of an integrated fluidized bed drier immediately following spray drying. There are many descriptions in the literature of such processes a recent example of a novel application of this technique (Jha et al, 2002) is in the production of a shelf-stable powdered kheer mix, an Indian dessert which consists of a sweetened mixture of cooked partially concentrated milk and rice flour. This is spray dried, the particles are then dried in a fluidized bed and finally dry blended with sugar. 

Liedy W, Hilligardt K. Contribution to the scale-up of fluidized bed driers and conversion from batchwise to continuous operation. Chem Eng Process 1991 30(l) 51-58. 

In the experiments use was made of a three-section fluidized-bed drier whose characteristics are given below. 

The Soviet-made granulation lines comprise the following main assemblies (Fig. 13) screw feeder (3), granulator (4), fluidized-bed drier (6) and sizer (13). 

The fluidized-bed drier is a welded square chamber accommodating meshes and separated by two vertical partitions into three sections having individual inputs suited to receive a heat-transfer agent. T wo of the sections are used for drying granules and the third one is a cooling section. The granules are loaded and unloaded by sluice feeders. 

Ziegler catalysts are also used for the manufacture of HDPE at low (40-150 psi) or medium (290-580 psi) pressures. The polymerization is carried out in a stirred reactor at about 80-90°C and 145 psi, or in a loop reactor at about 450 psi and temperatures between 65 and 110°C. Since the catalyst is very efficient, its concentration is kept very low, obviating the need for catalyst deactivation and removal steps. Conversions are very high. The solvent is removed by centrifugation, while the polymer is dried in a fluidized bed drier. 

There are many fields of drying which require much more work before proper design is possible. Very little work is available in most driers such as fluidized bed driers, steam tube driers, or almost any production unit. 

A more traditional means of drying is the tray drier. Hot air flows over a series of shelves on which the wet material is spread. Compared to the fluidized bed drier, fhe solid-air interface is smaller, and water molecules... 

Agitated batch driers consist of a jacketed cylindrical vessel with agitator blades designed to scrape the bottom and walls. They may operate at atmospheric pressure or under vacuum. Pasty materials that could not be handled in tumbling or fluidized-bed driers, may be successfully dried at rates higher than can be achieved in an oven. 

FIGURE 6.10 Fluidized bed drier. Indirectly heated air rather than the combustion gases from burning natural gas may also be used for bed fluidization and for cleaner drying action. 

Pressurized batch hot water (PBHW) pretreatment was conducted in a 2-1 pressure vessel (Model 4600 Parr Instrument Co., Moline, IL), surrounded by retractable ceramic heaters [12]. Tifton 85 bermudagrass (15 g) was placed in a 500-p.m (35 mesh) stainless steel basket, which was immersed in 1,450 ml of deionized water in the vessel. The sample was treated at 230°C for 2 min essentially as previously described [12]. Hydrolyzed solids were dried at 40°C for 90 min using a fluidized bed drier (Endecott, FBD2000, London, UK), and stored at 4°C until further use. 

Drying phase The suspension is dried in a spray-dryer or in a fluidized bed drier. Spray-drying is a suitable method for heat-sensitive products. The atomized particles assume a spherical shape. 

M. Alden, P. Torkington, and A.C.R. Strutt, Control and instrumentation of a fluidized-bed drier using the temperature difference technique I. Development of a working model, Powder Tech., 54 15-25,1988. 

Eventually, the crystal mass having still residual moisture of some 2% is unified with the filter cake of granular salt from the low-speed centrifuge. The mixed crystal mass is then dried in a fluidized bed drier to below 0.1% H2O. 

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