Unit Operations in Food Processing

So far the drying considered has been by hot air.
Other methods of drying which are quite commonly encountered are drying by contact with a hot surface; a continuous version of this is the drum or roller dryer where the food is coated as a thin paste over the surface of a slowly revolving heated horizontal cylinder. In such a case, the food dries for as much of one revolution of the cylinder as is mechanically feasible, after which it is scraped off and replaced by fresh wet material.
The amount of drying is substantially controlled by the rate of heat transfer and estimates of the heat transfer rate can be used for calculations of the extent of drying.

EXAMPLE 7.19. Moisture content of breakfast food after drum drying
A drum dryer is being used to dry a starch-based breakfast food. The initial moisture content of the food is 75% on a wet basis, the drum surface temperature is 138°C and the food layer outer surface 100°C. The estimated heat transfer coefficient from the drum surface to the drying food is 800 J m^-2 s^-1 °C^-1.

Assume that the thickness of the food on the drum is 0.3 mm and the thermal conductivity of the food is 0.55 J m^-1 s^-1°C^-1. If the drum, 1 m diameter and 1 m in length, is rotating at 2 rev/min and the food occupies three-quarters of the circumference, estimate the moisture content of the film being scraped off. Assume the critical moisture content for the food material is 14% on a dry basis, and that conduction heat transfer is through the whole film thickness to give a conservative estimate.

Initial moisture content = 75 % wet basis
= 0.75/(1 -0.75)
= 3 kg kg^-1 dry basis.

Total quantity of material on drum

                                  = (p x D x 3/4) x 1 x 0.0003 m³
                                  = p x 1 x 3/4 x 1 x 0.0003
                                  = 7.1 x 10^-4 m³.

Assuming a density of the food paste of 1000 kg m³,
Weight on drum = 7.1 x 10^-4 x 10³
= 0.71 kg.

Overall resistance to heat transfer, 1/U

                                  = 1/800 + 0.0003/0.55
                                  = 1.25 x 10^-3 + 0.55 x 10^-3
                                  = 1.8 x 10^-3
                Therefore U = 556 Jm^-2 s^-1 °C^-1
                               q = UA DT
                                   = 556 x p x D x 1 x 0.75 x (138 - 100)
                                  = 49.8 kJ s^-1.
Latent heat of evaporation of water
                                = 2257 kJ kg^-1
Rate of evaporation       = q /l
                                  = 0.022 kg s^-1.

Residence time of food on drum: at 2 rev min

1 revolution takes 30s, but the material is on for ¾ rev.

Residence time = (3/4) x 30
= 22.5 sec.

Water removed            = 22.5 x 0.022
                                 = 0.495 kg.
Initial quantity of water = 0.71 x 0.75
                                 = 0.53 kg
and dry solids              = 0.71 x 0.25
                                  = 0.18kg.

Residual water = (0.53 - 0.495)
= 0.035 kg.

Water content (wet basis) remaining
= 0.035/(0.18 + 0.035)
= 16%

Drying > DRYING EQUIPMENT

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