Maximum drying time

To solve this problem, we will use the equations in Table 8.4 to determine the maximum drying time, r, for the various rate determining steps. In these equations the surface temperature of the water is not known. This problem shows a way in which the surface temperature can be determined. Using the terminal settling velocity, V, ... 

Determine the maximum drying time for an aqueous 1.5% volume solid AI2O3 gel droplet 100 ttm in diameter at 20°C in air at 100°C if the pores are like that shown in Figure 8.17(a). Assume that the size of the droplet does not change during drying. 

Maximum drying time This is the period of time which only just enables a bonded joint. If the maximum drying time is exceeded, the polymer layers will already have solidified to such an extent that the cohesion strength of the adhesive layer may be affected. An exception are the contact adhesives (Section 5.3)... 

Consider open assembly time, minimum drying time, maximum drying time (Figure 5.2). 

To reduce drying time, maximum airflows are used during the first portion of the kilning cycle until the exit air is no longer saturated with moisture. Airflow then is reduced or recirculated to conserve energy. Average fuel consumption for a United States kiln is ca 5.9 x 10 kJ/t (1.4 x 10 kcal/t) ) malt, with a range of 2.9-10 X 10 kJ/t of malt. 

Heat transfer from the shelves to the sublimation front depends on the pressure and the distance between shelf and product (Fig. 1.58). Mass transfer (g/s) increases with the pressure, but also depends on the flow resistance of the already dry product and of the packing of the bones. If the maximum tolerable Tke is defined, the drying time depends only on the two processes mentioned above. It cannot be shortened under a given geometric situation and the chosen Tke. This method of Tkt control does not require thermocouples, and does not contaminate the product. 

The possible drying times for the main drying are estimated in Section 1.2.1 and complemented by examples. The decisive qualities are the heat transfer coefficient from the shelf to the sublimation front of the ice (Ktot). The heat conductivity in the product does normally not play an important part (see Fig. 1.67), except that a granulated product is dried from the surface to the center (see Fig. 1.68). The shortest possible main drying time can be estimated with 5 or 10 % error, if the dimensions of the product and the maximum tolerable TKe (e. g. -10 °C) are given (Eq. (12), (12 a-c) in Section 1.2.1). 

The maximum temperature at which the drying material may be held is controlled by the thermal sensitivity of the product and this varies inversely with the time of retention. Where lengthy drying times are employed, as for example in a batch shelf dryer, it is necessary to operate under vacuum in order to maintain evaporative temperatures at acceptable levels. In most continuous dryers, the retention time is very low, however, and operation at atmospheric pressure is usually satisfactory. As noted previously, dryer selection is considered in some detail in Volume 6. 

Facilitate broad coverage while providing maximum surface drying time (before tablets on the surface of the bed fold under and get mixed into the tablet mass)... 

Equipment capability/capacity The load that can be efficiently dried within the unit needs to be known. A larger load will require more moisture to be removed on drying and will affect the drying time. In the case of fluid bed drying, a maximum dryer load is that load above which the dryer will not fluidize the material. 

Fig. 1.99.2. Top RM measured by KF as a function of drying time. Ten vials were each closed at the time indicated by mean RM data. The cake from 5-7 vials out of each 10 vials closed was analyzed with 3-5 samples from each vial. The average data shown in the top graph are the mean values of 15 or more measurements. The bars are not standard deviations they mark the minimum and the maximum of all measured data. ( ) Run 1, mean Tice = -38.56 °C, SA = 0.38 °C. (+) Run 2,...

Tsh,MD = 0 °C- Pc = 008 mbar. Tsh,SD = 40 °C Below dW data calculated from DR measurements during secondary drying as a function of drying time. ( ) Mean dW of all three runs bars, minimum and maximum of all calculated dW... 

M = maximum amount of product (L) tMD = main drying time (h)... 

Table maximum water vapor flow-rates given by the hydrodynamic data of the plant. The maximum sublimation rates depend also on the heat transfer from the brine to the sublimation front of the ice [see Eq. (12)]. The main drying time will generally be governed above 0.06 mbar by the heat transfer and below 0.06 mbar by the water vapor transport, as shown in the example below ... 

---