CHAPTER 8
EVAPORATION (cont'd)

EVAPORATION OF HEAT SENSITIVE MATERIALS

In evaporators which have large volumes into which incoming feed is mixed, the retention time of a given food particle may be considerable. The average retention time can be obtained simply, by dividing the volume of the evaporator by the feed rate, but a substantial proportion of the liquor remains for much longer than this. Thus with heat sensitive materials a proportion may deteriorate and lead to general lowering of product quality.

This difficulty is overcome in modern high flow-rate evaporators; in which there is a low hold up volume and in which little or no mixing occurs. Examples are long-tube evaporators with climbing or falling films, plate evaporators, centrifugal evaporators, and the various scraped-plate thin-film evaporators.

EXAMPLE 8.6. Concentration of tomato juice in a climbing film evaporator
Tomato juice is to be concentrated from 12% solids to 28% solids in a climbing film evaporator, 3 m high and 4 cm diameter. The maximum allowable temperature for tomato juice is 57°C. The juice is fed to the evaporator at 57°C and at this temperature the latent heat of vaporization is 2366 kJ kg^-1.

		Solids	Liquids	Total
	Feed	12	88	100
	Product	12	31	43
	Evaporation			57

Heat balance
Area of evaporator tube pDHL
                                    = p x 0.04 x 3
                                    = 0.38 m²

Condensing steam temperature at 170 kPa (abs) = 115°C from Steam Tables.
Making a heat balance across the evaporator

                          q = UA DT
                             = 6000 x 0.38 x (115 - 57)
                             = 1.32 x 10⁵ J s^-1
Heat required per kg of feed for evaporation
                             = 0.57 x 2366 x 10³
                             = 1.34 x 10⁶ J

Rate of evaporation = (1.32 x 10⁵)/ 1.34 x 10⁶)
                             = 0.1 kg s^-1
Rate of evaporation = 360 kg h^-1
Quantity of tomato juice feed per hour = 360 kg

Evaporation > EVAPORATION EQUIPMENT

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