Moisture drying time calculation

An example is given in Fig. 2.38 plot 1 is extrapolated to DR < 0.1 %/h. At this time, one more hour drying time would contribute to the desired moisture content of e. g. I % by only 0.1 %. If the computer starts the integration from this calculated time the result is the residual moisture content as a function of time. 

A 100 kg batch of granular solids containing 30 per cent of moisture is to be dried in a tray dryer to 15.5 per cent moisture by passing a current of air at 350 K tangentially across its surface at the velocity of 1.8 m/s. If the constant rate of drying under these conditions is 0.7 g/sm2 and the critical moisture content is 15 per cent, calculate the approximate drying time. It may be assumed that the drying surface is 0.03 m2/kg dry mass. 

The above equations can be used to calculate capsule drying time. Suppose a soft shell capsule reaches the required moisture level after 24 h of drying. If the capsule shell wall thickness is increased 20%, how long will it take to dry the new capsules to the same moisture content c Solution Using Equations (It) and (12), it is seen that if h is increased by 20% to 1.2 h, then a increases by a factor of (1.2) = 1.44. For c — c )/ Co — Cgc) to have the same value as for the thinner capsule shell, r must also be increased by a factor of 1.44 (i.e., increased so that t/a remaias the same). The drying lime for the thicker shell, therefore is 1.44(24 hr) = 35 hr. 

Falling-Rate Kinetics To correct from a calculated constant-rate (unhindered) drying time ten to first-order falling-rate kinetics, the following equation is used, where Xi is the initial, X2 the final, and X the equilibrium moisture content (all must be dry-basis) ... 

If the drying rate curve (TV versus X) is known, the total drying time required to reduce the solid moisture content from X to X2 can be simply calculated by... 

This equation can be used for calculation of drying time in batch drying. Substituting this equation into Equation 3.112 and integration from the initial Xq to final moisture content Xf, the drying time is obtained... 

Equation T5.10.1 calculates the vapor pressure at drying temperature, whereas Equation T5.10.2 is the psychro-metric equation. Equations T5.10.1 and T5.10.2 are used to calculate the water activity at drying conditions (i.e., temperature T and air humidity Y). Equation T5.10.3 calculates the equilibrium material moisture content at drying conditions, whereas Equation T5.10.4 estimates the drying time constant at drying conditions. Both Equations T5.10.3 and T5.10.4 are used in Equation T5.10.5, which calculates the required drying time. 

Once adequate airflow rate, temperature, and moisture are defined to intensify the external exchange processes, drying operation usually becomes completely dependent on internal mass transfer rate. To calculate the effective diffusivity, Aff. one has to combine experimental results with Fick-type model. However, diffusion phenomenon has to be studied out the starting drying time (1 0). Hence, a starting accessibility is defined as the difference between the initial moistme content Wj and the value Wq calculated by extrapolating the diffusion model till f = 0. 

---