EVAPORATION:EQUIPMENT DESIGN

-MEESHA SINGH M.TECH FPT

Parts of an evaporator

• Heat exchanger (k/a calandria): transfers heat from steam to the food

• Means of separating the vapors produced• Vacuum pump: Mechanical or steam

ejector

Selection criteria• Operating capacity• Degree of concentration required• Heat sensitivity of product• Requirement for facilities to recover volatiles• Ease of cleaning• Reliability and simplicity of operation• Size of evaporator• Capital and operating costs

Types of evaporators

Open/Closed pan evaporator

Short tube evaporator

Long tube evaporator

Climbing-film evaporator• Thin film of liquor is forced up the evaporator tubes• For low-viscosity foods (for example milk)

Falling-film evaporator• Feed is introduced at the top of the tube bundle• For more viscous foods• For heat sensitive foods: yeast extracts, fruit juices

Forced circulation evaporators

Plate evaporators

Expanding-flow evaporator• Uses similar principles to the plate evaporator but has a

stack of inverted cones instead of a series of plates. • The vapor-concentrate mixture leaves the cone assembly

tangentially.

ADVANTGES• Compactness• short residence times• high degree of flexibility achieved by changing the number

of cones.

Scraped/wiped-surface evaporators

Effect on foodsAroma compounds that are more volatile than

water are lost during evaporation.• This reduces the sensory characteristics of most

concentrates• In fruit juices this results in a loss of flavor• In some foods the loss of unpleasant volatiles

improves the product quality (for example in cocoa)

Evaporation darkens the color of foodsBecause of:• increase in concentration of solids• reduction in water activity

Other changes• Vitamins A and D and niacin are unaffected.• Additional vitamin losses occur during storage

Distillation• When a food that contains components having

different degrees of volatility is heated, those that have a higher vapor pressure (more volatile components) are separated first.

• Distillate: components that are separated first and are more volatile

• Bottoms or residues: components that have a lower volatility

• Feed liquor flows continuously through the column.• As it is heated, volatiles are produced and separated at

the top of the column as distillate.• The residue is separated at the base.• In order to enhance the separation of these components,

a proportion of the distillate is added back to the top of the column (reflux) and a portion of the bottoms is vaporized in a reboiler and added to the bottom of the column.

• Columns are filled with either a packing material (typically ceramic, plastic or metal rings) or fitted with perforated trays, both of which increase the contact between liquid and vapor phases.

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