ConcentrationApplication :

effects on food and equipments

By- ketaki b.patil

Introduction • Frequently in the food industry a raw material or a

potential foodstuff contains more water than is required in the final product.

• When the food stuff is liquid, the easiest method of removing the water, in general, is to apply heat to evaporate it.

• Evaporation is thus a process which is often used by the food technologist.

Evaporation

• Evaporation is a physical separation process, which

removes a volatile component from a liquid solution

or mixture by vaporization, obtaining a concentrated

product of the nonvolatile components.

• For liquid foods, evaporation removes most of water

resulting in concentrated product which may be used

as such or processed further, e.g., by drying.

Cont …• Evaporation is used extensively in concentrating fruit

and vegetables juices, milk, coffee extracts and in refining sugar and salt.

• Reduction of the water content reduces weight and volume of the product, cutting storage and transportation costs, and improving the storage stability of the product.

• Evaporation is established as the major process of concentrating liquid foods, although some new methods offer special advantages, such as freeze concentration and reverse osmosis.

Difference between Evaporation and Vaporization

Evaporation Vaporization

Process by which liquid changes to its vapors at a temperature below its

Boiling Point

Process by which liquid changes to its vapors at its Boiling Point

Takes place at all temperatures Takes place at only fixed temperature i.e. its boiling point temperature

Temperature may changes during evaporation

Temperature during vaporization does not change

A slow and silent process A fast and violent process

Evaporation takes place only at surface of liquids

Takes place over the entire mass of the liquid

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• While the design criteria for evaporators of the industry, two questions always exist:

1)Is this equipment best suited for the duty?

2) Is the equipment arranged for the most efficient and economical use?

Energy Saving Evaporation Systems.

• Single effect evaporators

• Multiple effect evaporators

• Vapor Recompression Evaporators

The single effect evaporator

• The typical evaporator is made up of three functional sections:

• The heat exchanger• The evaporating section – where liquid boils and

evaporates• Separator – in which vapor leaves the liquid and

passes off to the condenser or the other equipment. • In many evaporators all these three sections are

contained in a single vertical cylinder.

Construction

• In the centre of cylinder there is steam heating section, with pipes passing through it in which the evaporating liquors rise.

• At the top of the cylinder the baffles which allow the vapours to escape but check liquid droplets which may accompany the vapors from the liquid surface this type of evaporator which may be called conventional evaporator.

Mechanism • In the heat exchanger section, called a

calandria in this type of evaporator, steam condenses in the jacket and the liquid being evaporated boils on the inside of the tubes and in the space above the upper tube plate.

• The circulation of liquids greatly affects evaporation rates but circulation rates and pattern are very difficult to predict in any detail.

• As evaporations proceeds the remaining liquors become more concentrated and because of this the boiling temperature rise.

Cont…

• The rise in temperature of boiling reduces the available temperature drop, assuming no change in the heat source. And so the total rate of heat transfer will drop accordingly.

• Also the viscosity of liquid will increase, often quite substantially, this affects circulations and heat transfer coefficients leading again to lower rates of boiling

Multiple effect evaporator

• Multi-Effect Evaporation is an important procedure adopted

to economize the consumption of energy.

Cont….

• Multi effect evaporation system is based on the

repeated use of the water vapors from one

evaporation unit (effect) to heat the next effect, which

operates at a lower pressure.

• Thus, 1kg of steam can evaporate more water,

depending on the number of effects and the operating

pressures.

• The feed for the multi-effect type evaporator is

generally transferred from one effect to another. This

leads to the ultimate product concentration to reach

only in the one effect of evaporator.

Steam Economies (SE) of evaporator systems

Evaporator system SE, kg water / kg steam

Single effect 0.90- 0.98

Double effect 1.70- 2

Triple effect 2.40- 2.80

Six effect 4.6- 4.9

Thermo compressor, 3 effect 4-8

Mechanical vapor recompression 10-30

feeding operations• There are two feed operations - backward feed and

forward feed operations.

Backward operation

• In the backward operation, the raw feed enters the last (coldest) effect and the discharge from this effect becomes a feed for the next to last effect.

• This technique of evaporations is advantageous, in case the feed is cold, as much less liquid must be heated to the higher temperature existing in the early effects. The procedure is also used if the product is viscous and high temperatures are required to keep the viscosity low enough to produce good heat transfer coefficients.

forward feed operation

• In the case of a forward feed operation, the raw feed

is introduced in the first effect and is passed from

effect to effect parallel to steam flow. The product is

withdrawn from the last effect. This procedure is

highly advantageous if the feed is hot. The method is

also used if the concentrated product may be

damaged or may deposit scale at high temperature.

Types of evaporator • Batch pan

• Forced circulation

• Natural circulation

• Wiped film

• Rising film tubular

• Plate equivalents of tubular evaporators

• Falling film tubular

• Rising/falling film tubular

Food Evaporators

1)Long Residence –Time evaporator

a)Jacketed vessel evaporator

b)Coil evaporator

c)Short tube evaporator

2) Short Residence-Time Evaporator

a) Long tube vertical evaporators

b) Plate evaporator

c) Agitated film evaporators

d) Centrifugal film evaporator

Falling Film Evaporators Rising Film Evaporator

Thin Film Evaporators Forced Circulation Evaporators

Multi Effect Evaporators

Falling film evaporation.• The liquid feed is pumped through a preheater to the

top of the evaporator tube, and it is distributed as a liquid film on the tube walls.

• The main body of the evaporator consists of a vertical tube, surrounded by a steam jacket.

• Evaporation takes place as the liquid film lows down the tube, and the generated vapors flow concurrently with the liquid into a vapor-liquid separator, located at the bottom of the tube.

• pressure drop (up to 100 psig), measured with a pressure gauge at the entrance of the preheater.

Batch pan evaporator

T U B U L A RE V A P O R A T O R S

NATURAL CIRCULATION

RISING FILM TUBULAR

FALLING FILM TUBULAR

FORCED CIRCULATION

Vacuum evaporation• Vacuum evaporation is the process of causing the

pressure in a liquid-filled container to be reduced below the vapor pressure of the liquid, causing the liquid to evaporate at a lower temperature than normal.

• Although the process can be applied to any type of liquid at any vapor pressure.

• It is generally used to describe the boiling of water by lowering the container's internal pressure below standard atmospheric pressure and causing the water to boil at room temperature .

Freeze evaporation

• Evaporation is considered to be the most economical and most widely used method of concentration. However, it is not suited for food products with very delicate flavors. Commercial processes for the concentration of such products by membrane separation techniques are not yet available.

Cont…• As compared to the conventional evaporation

processes, concentration by freezing is potentially a superior and economic process for aroma-rich liquid foods.

• In the past, the process, however, was seldom used because of the investment cost and the considerable loss of concentrate in the withdrawn ice, and hence, the quality.

• Recent technological developments have minimized these two drawbacks associated with the earlier freeze concentration processes.

• In the coming decade, freeze concentration is seen as a potentially attractive method for the concentration of aroma-rich liquid foods, including fruit juices, coffee, tea, and selected alcoholic beverages.

• In this article, several aspects of the theoretical considerations behind freeze concentration of fruit juices, the development of new and cheaper designs, and commercially available freeze concentration processes are reviewed.

Reverse osmosis

• Evaporation results in a loss of fresh juice flavors, color degradation and “cooked” taste due to the thermal effects.

• Methods using less heat for thermal damage reduction must be investigated to establish parameters for future commercial processes.

• The promising alternative is Reverse Osmosis (RO),but it cannot achieve concentrations greater than 30ºBrix.

• RO has advantages over traditional evaporation techniques in removing water. Because less heat is used, thermal damage to products is generally eliminated.

Vapour Recompression

• Vapour recompression is an alternative method to

multiple effect to save energy and that can be

achieved with two different technologies :

1) Thermal Vapour Recompression (TVR) and

2) Mechanical Vapour Recompression (MVR).

Thermal Vapour Recompression (TVR)

• The main features of TVR are as follow :

• Usable when high pressure steam is available

• Single or multiple effect

• Energy and cooling water savings

• No moving parts in steam jet ejector

• Operating close to design conditions

• Suitable for low boiling elevation and non-incrusting

products

Mechanical Vapour Recompression (MVR).

• The main features of MVR are as follow : • Most energy efficient evaporation technology • Flexible and simple to use

• Generally single effect• Almost no steam and cooling water consumption

(boiler and cooling tower cost reduction) • Suitable for moderate boiling elevation products • Capital cost vs. operating cost to be studied

Application of evaporators • Chemical Industry –

Dyes, Pickle Liquor, Plating bath, Salt, Glycerin, Sodium Nitrate, Sodium Sulphate, Paints, Pigments.

• Food Industry -Milk, Fruit Juice, Pulps, Tomato Puree, Herbal Extracts, Coffee, Tea, Protein, Gelatin, Coconut Water.

• Corn Industry -Whey, Gelatin, Malt Extract, Glucose, Fructose, Dextrose, Corn Steep Liquor, Sorbitol, Maltodextrine.

• Waste Water Treatment Blow Down, RO reject, Floor washings, Process effluents, Black Liquor, Fish Sick Water.

• Pharmaceutical Industry -Bulk drugs, Glycerin, Sweet water, Yeast Extract, Protein Hydrolysate

Typical evaporator applications

• Product concentration

• Dryer feed pre-concentration

• Volume reduction

• Water / solvent recovery

• Crystallization

Products which are treated to evaporation

• Apple juice • Coffee • Pear juice

• Amino acids • Fruit purees • Pectin• Beef broths • Gelatin

• Pharmaceutical products

• Chicken broth• Mango juice

• Beet juice • Grape juice • Pineapple juice• Betacyclodextrin • Lime juice

• Skim milk• Caragenan • Liquid egg • Sugars• Cheese whey

• Low alcohol beer

Factors

• Physical properties :-– The physical properties of the liquid, which are of

direct importance to evaporation, are the – Viscosity (or rheological constants)– Thermal conductivity – Density– Specific heat – Surface tension – Boiling point elevation

• Surface tension of water decreases significantly when organic components are present in the aqueous system .

• The surface tension of liquid food material is lower due to the surface active component present.

• Boiling Point Elevation (BPE)is caused by solute/water interaction and it is undesirable in evaporation, since it requires a higher temperature of heating medium to affect the same driving force (temperature differences)

• It is particularly high in concentrated aqueous solutions of salts and alkalis (e.g., sodium hydroxide)

Fauling in Evaporators

• Fouling is the formation of deposits on the heat transfer surface, which reduces heat transfer and evaporation rates and may damage the quality of the concentrated products.

• Fauling is costly to industrial operations because of the higher capital cost (oversized plants), the energy losses, the higher maintenance, and the loss of production during shutdown for cleaning.

• Fauling includes scaling, which is the precipitation of inorganic salts on the heating surface.

Food quality consideration

• Evaporation should minimize the undesirable changes in quality and nutritive value of heat sensitive food materials. Such changes are losses of organoleptic quality( flavor, aroma, and color), losses of vitamins, and production of undesirable compounds( browning, Caramalization)

• Losses of aroma components during evaporation of fruit juices and aqueous extracts can be restored by aroma recovery in the distillate or add-back of fresh juice.

Evaporator control

• Efficient operation of evaporation system requires process control, which is achieved mainly by simple or advanced control system.

• The control system should respond to :-• Changes in feed composition • Conditions, • Change the heat supply

Safety

• Testing of evaporators :-– The testing and evaluation performance of

evaporators can be performed using a procedure developed by the American Institute Of Chemical Engineering's.

– Standardized measurements are made, after the evaporator has reached steady-state operation.

– It includes feed, product, steam rates, temperature, evaporator body, and condenser pressure, cooling water temperature, boiling point elevation etc

• Preventing excessive vacuum• Excessive vacuum can produce too much vapor flow

for processing by the emissions control system. Control of the vacuum can significantly decrease emissions.

• Air drawback into the evaporator after the vacuum pump is shut down can result in unsafe conditions when flammable solvents are distilled. A flammable mixture can be created in the equipment that is a potential source for an explosion.

Hygienic consideration

• Hygienic (sanitary) guidelines and codes for food processing equipment are of particular importance to the design, operation, and cleaning of food evaporators.

• The most important hygienic problems is the fouling of evaporation surfaces, caused by deposition of food colloids and other food components, which reduces heat transfer and evaporation rate(capacity).

• Fouling is an empirical function of the operation time, and periodic cleaning is necessary.

Cont…

• The operation cycle may be long (a week) for sugar evaporators, and shorter for dairy units (a day), due to the precipitation of milk heat sensitive proteins.

• CIP cleaning systems can be used for all parts of food contact equipment.

References

• Evaporator - Wikipedia, the free encyclopedia• Evaporator_Handbook_10003_01_08_2008_US• Calandria Evaporator Swenson Technology, Inc_• Principles of Food Processing - Dennis R_ Heldman, Richard

W Hartel - Google Books• Applications of Evaporators - Comp Engineering and Exports,

Pune_

• Food, Dairy and Beverage Evaporator Applications• Handbook of food processing equipment by George

D.Saravacos• Unit operations in food processing by R.L.EARLE

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