Residence time distribution dryers

Variations may be narrowed by various devices, but never eliminated completely. Figure 12 depicts relative particle residence time distributions among four dryers. 

Brod, F.P.R., Park, K.J. and de Almeida, R.G., Image analysis to obtain the vibration amplitude and the residence time distribution of a vibro-fluidized dryer. Food Bioprod. Proc., 82 (2004) 157-163. 

Figure 9.5. Residence time distribution in particle dryers, (a) Four types of dryers (McCormick, 1979). (b) Residence time distribution cf air in a detergent spray tower example shows that 27% (difference between the ordinates) has a residence time between 24 and 32 sec [Place et al., Trans. Inst. Chem. Eng. 37, 268 (1959)]. (c) Fluidized bed drying of two materials (Vanacek et al., Fluidized Bed Drying, 1966).

Well-mixed continuous fluidized bed dryers are most commonly used if the final product moisture content distribution is permitted to vary somewhat due to the inherent residence time distribution in the dryer. Equilibration occurs during storage, resulting in uniform product moisture. 

The pressing need of product quality in the plastics industry also forces one to consider residence time distribution of the product when comparing dryers. 

Benefits achieved from such dryers are (1) uniform residence time distribution regardless of particle size (2) ability to handle polydisperse solids (3) ability to operate at low aeration rates and hence lower pressure drops (4) gentle handling of product (5) higher heat transfer and drying rates, and others. Since the equipment is mounted on resonance springs, the power consumption for vibration is minimal for well-designed VFB [6]. 

To understand the behavior of the fluidized bed, one can determine the average residence time or the residence time distribution (RTD) from the tracer technique. For instance, RTD in fluidized bed dryer is usually carried out by means of the stimulus-response technique, in which an impulse of solids marked with some appropriate tracer is fed to the dryer and its time of elution and concentration measured at the exit of the dryer. The material of the tracer has to be such that it can be detected and does not react with the substrate material, and its form of application and response are well known (Levenspiel, 1972). 

It should be noted that a simulation results of the proposed solids transport model was used as a guide to determine when the dryer outlet stream should be sampled, in order to catch the peak of the residence time distribution (RTD) curve. In this way an information-rich signal was gained, which was invaluable for model validation and parameter estimation purposes, while at the same time reducing experimentation costs. 

In this type of dryer (Fig. 2), the bed temperature is uniform and is equal to the product and exhaust gas temperatures. However, due to inherent product residence time distribution, product moisture content will span the range from inlet moisture content to a lower value. One advantage of the perfect mixing dryer is that the feed falls into a bed of relatively dry material and so is easy to fluidize. 

In a continuously well-mixed dryer, where the residence time distribution is that of a perfectly mixed vessel, Eq. (3) becomes... 

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