Abstract
The process of evaporation is employed in the food industry primarily as a means of bulk and weight reduction for fluids. The process is used extensively in the dairy industry to concentrate milk; in the fruit juice industry to produce fruit juice concentrates; in the manufacture of jams, jellies, and preserves to raise the solids content necessary for gelling; and in the sugar industry to concentrate sugar solutions for crystallization. Evaporation can also be used to raise the solids content of dilute solutions prior to spray drying, vacuum belt drying, or freeze-drying.
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Suggested Reading
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Problems
Problems
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11.1.
A single-effect falling film-type evaporator is used to concentrate orange juice from 14% to 45% solids. The evaporator utilizes a mechanical refrigeration cycle using ammonia as refrigerant, for heating and for condensing the vapors. The refrigeration cycle is operated at a high pressure of 200 psia (1.379 MPa) and a low side pressure of 50 psia (344.7 kPa). The evaporator is operated at a vapor temperature of 90 °F (32.2 °C). Feed enters at 70 °F (21.1 °C). The ratio of insoluble to soluble solids in the juice is 0.09, and the soluble solids may be considered as glucose and sucrose in 70:30 ratio. Consider the ΔT as the log mean ΔT between the liquid refrigerant temperature and the feed temperature at one point and the hot refrigerant gas temperature and the concentrated liquid boiling temperature at the other point. The evaporator has a heat transfer surface area of 100 ft2 (9.29 m2), and an overall heat transfer coefficient of 300 BTU/(h $ ft 2 $ °F) or 1703 W/(m2 $K) may be expected. Calculate:
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(a)
The evaporator capacity in weight of feed per hour
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(b)
Tons of refrigeration capacity required for the refrigeration unit based on the heating requirement for the evaporator
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(c)
Additional cooling required for condensation of vapors if the refrigeration unit is designed to provide all of the heating requirements for evaporation
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(a)
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11.2.
Condensate from the heating unit of one effect in a multiple-effect evaporator is flashed to the pressure of the heating unit in one of the succeeding effects. If the condensate is saturated liquid at 7.511Â psia (51.79Â kPa) and the heating unit contains condensing steam at 2.889Â psia (19.92Â kPa), calculate the total available latent heat that will be in the steam produced from a unit weight of the condensate.
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11.3.
A single-effect evaporator was operating at a feed rate of 10,000 kg/h concentrating tomato juice at 160 °F (71.1 °C) from 15% to 28% solids. The ratio of insoluble to soluble solids is 0.168, and the soluble solids may be assumed to be hexose sugars. Condensing steam at 29.840 psia (205.7 kPa) was used for heating, and the evaporator was at an absolute pressure of 5.993 psia (41.32 kPa). It is desired to change the operating conditions to enable the efficient use of a vapor recompression system . The steam pressure is to be lowered to 17.186 psia (118.37 kPa). Assume there is no change in the heat transfer coefficient because of the lowering of the heating medium temperature.
Calculate:
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(a)
The steam economy for the original operating conditions
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(b)
The capacity in weight of feed per hour under the new operating conditions
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(c)
The steam economy of the vapor recompression system
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(a)
Express the steam economy as the ratio of the energy required for concentration of the juice to the energy required to compress the vapor assuming a mechanical efficiency of 50% for the compressor. Assume condensate from the heating element is added to the superheated steam to reduce temperature to saturation.
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Toledo, R.T., Singh, R.K., Kong, F. (2018). Evaporation. In: Fundamentals of Food Process Engineering. Food Science Text Series. Springer, Cham. https://doi.org/10.1007/978-3-319-90098-8_11
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DOI: https://doi.org/10.1007/978-3-319-90098-8_11
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