Falling-rate period

Critical moisture content is that obtained when the constant rate period ends and the falling rate periods begin. Second critical moisture content specifies that remaining in a porous material when capillary flow dominance is replaced by vapor diffusion. 

Falling-rate period is a drying period during which the instantaneous diying rate continually decreases. 

Estimates of both the constant-rate and the falling-rate periods are needed to estimate the total drying time for a given diying operation. If estimates for these periods are available, the total diying time is estimated by summing as... 

The total drying time is the sum of the durations of the eonstant rate and falling rate periods and is often simplified into two linear portions (Figure 4.27). The period taken for X to reaeh Xc is known as the eonstant rate time, Oqr, whilst that to go from Xc to X is the falling rate time, 0pR. The sum of these two quantities gives the total drying time 6i. 

Fig. 7.10 Effect of different orifice diameter on the consumption of Ca(OH)2 slurry at different time (showing constant rate period and falling rate period), (4% Ca(OH)2 and 5 L/min C02 flow rate) [46]...

Falling needle viscometer, 27 738 Falling rate drying, 9 109-110, 113-115 Falling rate period, 9 97 Falling rod viscometer, 27 738 False Irish moss, common and scientific names, 3 188t... 

Fire retardant treatment, for wood, 26 348 Fire science, 11 450 Fire test methods, 11 449—450 Fire test terminology, 19 588 Fire-tube furnaces, 12 319—320, 327 Firing, of ferrites, 11 73 Firming agents, 12 32 as food additives, 12 57 First aid and rescue, 21 858 First aid, for nitric acid exposure, 17 192 First failure, time to, 26 987 First falling rate period, 23 67 First-generation ionic liquids, 26 837-838, 841, 865... 

Secondary wastewater treatment, 25 888 Second falling rate period, 23 66 Second-generation ionic liquids, 26 838, 847, 865... 

At the conclusion of the first falling rate period it may be assumed that the surface is dry and that the plane of separation has moved into the solid. In this case, evaporation takes place from within the solid and the vapour reaches the surface by molecular diffusion through the material. The forces controlling the vapour diffusion determine the final rate of drying, and these are largely independent of the conditions outside the material. 

The rate of drying Rc over the constant rate period is equal to the initial rate of drying in the falling rate period, so that Rc = mfc. 

In the falling-rate period, the surface is no longer completely wetted and the rate of drying steadily falls. In the previous analysis, it has been assumed that the rate of drying per unit effective wetted area is a linear function of the water content, so that the rate of drying is given by ... 

For the falling rate period the batch drying time is given by... 

Diying Time over Constant and Falling Rate Periods with Constant Gas Conditions... 

When drying is entirely in the falling rate period with rate equation... 

Drying Rates. Drying a frozen material proceeds initially at a constant rate with rapid evolution of water vapor. As Ihe sublimation interface recedes within the product, water-vapor evolution decreases. This is the start of the falling-rate period. When only bound water remains within the cellular structure of the product, the desorption period begins. During the constant-rate period, the sublimation rate can be expressed in terms of the heat of sublimation of ice and the heat-rale equation ... 

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