Drying rate

drying rate

1. DRYING RATE Mamta Sahurkar

2. Drying curve D rying curve is the plot of the drying rate Φ or N versus the remaining water content X. Water content X is expressed as kg of water per kg of dry matter. Drying process of a material can be described as a series of steps in which drying rate plays a key role. Following figure shows typical drying rate curve for a constant drying conditions.

3. Point B represents equilibrium temperature conditions of the product surface. Section B to C of the curve, known as the constant rate periods, represents removal of unbound water from the product. The water acts as if the solid is not present . The surface of the product is very wet and water. And the water activity is equal to one . The constant rate period continues as long as the amount of water evaporates is equal to the amount of water supplied to the surface of the material. Constant rate period

4. In the constant-rate period, the water is being evaporated from what is effectively a free water surface. The rate of removal of water can then be related to the rate of heat transfer, if there is no change in the temperature of the material and therefore all heat energy transferred to it must result in evaporation of water. The rate of removal of the water is also the rate of mass transfer, from the solid to the ambient air. These two - mass and heat transfer - must predict the same rate of drying for a given set of circumstances.

5. The falling rate period is reached when the drying rate starts to decrease, and the surface water activity falls to less than one. The rate of drying is governed by the internal flow of liquid or vapor. This point is represented by C in the figure. At this point there is not enough water on the surface to maintain a water activity value of one. Falling rate period

6. The falling rate period can be divided into two steps. First falling drying rate Second falling drying rate

7. A first falling drying rate occurs when wetted spots in the surface continually diminish until the surface is dried (Point D). First falling drying rate

8. Second falling rate period begins at point D when the surface is completely dry . The plane of evaporation recedes from the surface . Heat required for moisture removal is transferred through the solid to the vaporization of moisture in the solid and the vapor moves through the solid into air stream. Second falling drying rate

9. The amount water removed in this period can be relatively small compared to the constant rate and first falling rate period. However this period may take much longer than constant rate period because the drying is slow.

10. Drying rate experiments can be simply done by measuring the weight change during drying. The material to be dried is placed on the tray. The tray is suspended from a balance and exposed to air flow in drying tunnel. Drying rate experiments

11. Water removed/time can be easily determined

12. Drying time

13. Drying rates, once determined experimentally or predicted from theory, can then be used to calculate drying times so that drying equipment and operations can be designed. In the most general cases, the drying rates vary throughout the dryer with time as drying proceeds, and with the changing moisture content of the material. So the situation is complicated. Calculation of Drying Times

14. However, in many cases a simplified approach can provide useful results. One simplification is to assume that the temperature and RH of the drying air are constant. In this case, for the constant-rate period, the time needed to remove the quantity of water which will reduce the food material to the critical moisture content  X c  (that corresponding to the end of the constant-rate period and below which the drying rate falls) can be calculated by dividing this quantity of moisture by the rate.

15. In this case, for the constant-rate period, the time needed to remove the quantity of water which will reduce the food material to the critical moisture content Xc (that corresponding to the end of the constant-rate period and below which the drying rate falls) can be calculated by dividing this quantity of moisture by the rate.

16. and Xo is the initial moisture content and Xc the final moisture content (the critical moisture content in this case ) both on a dry basis, w is the amount of dry material in the food and (dw/dt) const is the constant-drying rate .

17. Where the drying rate is reduced by a factor  f  then this can be incorporated to give:                      D t  =  w  ( D X )/  f (d w  /d t ) const .     and this has to be integrated piecemeal down to X = X f  where subscript f denotes the final water content, and  f  expresses the ratio of the actual drying rate to the maximum drying rate corresponding to the free surface-moisture situation.

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