Fluidized drying curves

Example 20 Scaling of Data An experimental batch drying curve has been measured at 100°C, and drying time was 2 h. Estimate the drying time at la) 100°C and (b) 150°C for (i) a fluidized-bed dryer and (ii) a conical vacuum dryer at 100-mbar absolute pressure, for a batch size 10 times greater than that of the test. Assume for the fluidized bed that temperature driving forces are proportional to and batch drying time is proportional to bed depth, and for... 

Here X(t) is the drying curve, corrected as before to the new scale and new operating conditions, and Eft) is the residence time function, which must be known. This approach has been used successfully for well-mixed fluidized beds. For pure plug flow, Eft) is a spike (Green s function) andX =X(t). 

Provided that a batch drying curve X t) is available, the performance of a continuously well-mixed fluidized bed dryer can be predicted, at least in principle, by a so-called integral model of drying first proposed by Vanecek et al. (1964). The model is based on the following equation for calculating the average outlet moisture content, Xq, from a continuous dryer ... 

The functional progression dependency of the Sh-number on gas mass flow, expressed by the Re-number, is illustrated for experiment V-9 in Fig. 16.25. The curve has a maximum which is caused by the higher expansion (relative porosity) of the fluidized bed [66], In contrast to fixed beds, the energy input in fluidized beds is higher because of higher gas throughputs, and consequently the drying is faster. 

Liteanu et al. [75] studied the isothermal dehydration of (NH4)2HP04 in a fluidized bed. The ar-time curves were fitted by the contracting volume equation, and the rate of dehydration in air increased with decreasing particle size. For a fixed particle size, the reaction rates and apparent values of increased with variation in carrier gas in the sequence air < methane < hydrogen. The apparent values of E were 80 4,93 3 and 104 3 kJ mol", respectively. The reaction rate also increased with water vapour pressure in the air, but E decreased from 84.5 0.5 in dry air to 64.0 0.5 kJ mof when the water vapour pressure was 17.7 Torr. Water vapour on the grain surfaces promoted dissociation ... 

Figure 5 represents a typical textbook drying rate curve obtained from a batch of wet particles fluidized with hot air of fixed temperature, humidity, and flow rate. After an initial transient period (generally a heatup period) if the particles are very wet, i.e., covered with a liquid film, the drying rate depends entirely on the external heat and mass transfer rates, e.g., a rise in air temperature or air flow rate or a reduction in air... 

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