evaporator.pdf

7/27/2019 evaporator.pdf

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ECH711: Food Process Engineering L-XXI

Food Preservation with Removal of Water

(a) Evaporation

(b) Dehydration

(c) Freeze Drying &

Freeze Concentration


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Evaporation

A food preservation technique in which

dilute liquid foods are concentrated by theevaporation of water, with the aim ofincreasing microbial stability and shelf-life ofthe food.

Reduction in the bulk volume of the foodalso reduces the transport and storagecosts.

Evaporation is also used to concentrate theliquid foods prior to their dehydration,

particularly by spray drying.


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Comparative cost of water removal

Separation cost per unit

volume of waterremoved (arbitrary units)

Spray drying 17-50

Drum drying 10-25

Centrifugation 0.1-10

UF/RO 0.2-7

Evaporation 0.2-5


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Evaporation is extensively used for theconcentration of milk, fruit and vegetable juices and

sugar solutions. Lengthy exposure to heat should be avoided to

prevent thermal degradation of the food.

Residence time of the food in the evaporator is

reduced by maximizing the rate of heat transfer byusing thin liquid fi lms rather than add heat to liquidsin bulk.

Evaporators are usually operated under vacuum toreduce the boiling point of the solution. Thisincreases the driving force between the steam andthe boiling liquid and reduces thermal degradation.

Increased concentration increases the viscositysubstantially resulting in problems associated withpumping and poor heat transfer.

Deposition of fouling layers on heat transfer

surfaces reduces the heat transfer coefficient.


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Equipment for Evaporation

Natural Circulation Evaporators

- Horizontal Tube

- Vertical Tube

Forced Circulation Evaporators

Thin Film Evaporators


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Horizontal tube natural circulation

evaporator


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Vertical tube natural circulation

evaporator


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Climbing film evaporator


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Sizing of a Single Effect Evaporator

- Flow rates of the feed, vapour andconcentrated liquor

- Steam flow rate

- Area of heat transfer surface

44Simultaneous solution of a materialbalance, an enthalpy balance and a heattransfer rate equation.


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Single effect evaporator: material and

enthalpy balance


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Evaporator Efficiency

refers to mass of vapours

generated per unit mass of

steam admitted to the

calendria.

Economy = V / S(9)

Boiling point Elevation

difference between the

boiling point of the

solution and that of pure

water.


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Illustration-I

A single effect evaporator is to be used toconcentrate a fruit juice containing 15% (w/w)

dissolved solids to 50% solids. The feed stream

enters the evaporator at 291K with a feed rate of 1.0kg-1. Steam is available at a pressure of 2.4 bar and

at absolute pressure of 0.07 bar is maintained in the

evaporator. Assuming that the properties of thesolution are the same as those of water, and taking

the overall heat transfer coefficient to be 2,300 W.m-

2

.K-1

, calculate the rate of steam consumption andthe necessary heat transfer area and the steam

economy in the evaporator.


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Improving Evaporator Efficiency

In a single stage evaporator, the enthalpy of the

vapour is partially wasted because the vapour is

either vented to atmosphere or condensed, resulting

in poor steam economy. Reusing the vapour, either

by cycling it to the calendria or by passing it to the

calendria of a second evaporator can improve the

steam economy greater than unity.

1. Vapour recompression

(a) Mechanical recompression (b) Steam jet ejector

2. Multiple effect evaporation

(a) forward feed (b) backward feed (c) Mixed feed

An example: Concentration of tomato juice.


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Mechanical vapour recompression


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Steam injector vapour recompression


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Triple effect evaporator: forward feed


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Triple effect evaporator :backward

feed


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Illustration-2

A 4% (w/w) aqueous food solution is fed to a forwardfeed double effect evaporator with equal surface

area at a rate of 2.0kg.s-1 and a temperature of 700C.

The solution is concentrated to 20% (w/w). Thesecond effect is maintained at a pressure of 20kP

with a boiling point elevation of 8 K. Steam at240 kP

is available. The heat transfer coefficients in the first

and second effects are 2.20 and 1.50 kW.m-2.K-1,respectively. Heat capacity of each liquid stream

may be assumed to be 4.18 kJ.kg -1.K-1. Calculate the

heat transfer surface area of each effect and thesteam economy

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