EVAPORATION

RECOMMENDED TEXTBOOKS

• Unit Operations in Food Engineering. Albert Ibarz and Gastavo Barbosa-Canovas

• Food Process Engineering and Technology. Zeki Berk

• A raw material may contain more water than is required in the finalproduct. When the raw material or foodstuff is liquid, the easiest method ofremoving the water is to apply heat to evaporate it.

• It is the separation of a volatile liquid from a non-volatile solid based on theprinciple of vapourization.

• It is an important unit operation commonly employed to remove waterfrom dilute liquid food to obtain concentrated liquid products.

• Evaporation or concentration, partial removal of water from liquid byboiling off water vapour, thus, increases the solid content and hence,preserves it

Major problem associated with evaporation is loss of volatile aromacomponents

The main objectives of evaporation in the food industry are:

● Mass and volume reduction, resulting in reduced cost of packaging,transportation and storage

● Preservation, by virtue of the reduced water activity.

● Preparation to subsequent treatment such as crystallization (sugar,citric acid), precipitation (pectin, other gums), coagulation (cheese,yogurt), forming (candy), dehydration (milk, whey, coffee soluble)

● Building a desired consistency (jams and jellies, tomato concentrates,ketchup).

• Disadvantage Evaporation is more expensive in energy consumption than other methods

of concentration (membrane and freeze concentration)

• Separation is achieved by exploiting differences in the vapour pressure (volatility) of the components and using heat to remove one or more from the bulk of the food.

• The industrial equipment used for this process is called evaporator

• Evaporators are designed to give maximum heat transfer to a low densityfeed material in order to remove liquid and produce more dense or thickproduct.

Theory

• During evaporation, sensible heat is transferred from steam to the food, toraise the temperature to its boiling point.

• Latent heat of vaporisation is then supplied by the steam to form bubblesof vapour, which leave the surface of the boiling liquid.

• The rate of evaporation is determined by both the rate of heat transferinto the food and the rate of mass transfer of vapour from the food

• These processes are represented schematically in Fig. 1

• mf (kg/s), mass transfer rate of feed liquor; mp (kg/s), mass transfer rate of product;

• Xf, solids fraction of feed liquor; Xp, solids fraction of feed product;

• mv (kg/s), mass transfer rate of vapour produced; ms (kg/s), mass transfer rate of steam used;

• f (ºC), initial feed temperature; b (ºC), boiling temperature of food;

• s (ºC), temperature of steam.

Fig. 1: Steady state operation of an evaporator

Heat and mass balances

• Heat and mass balances are used to calculate the degree of concentration, energy use and processing times in an evaporator. The mass balance states that ‘the mass of feed entering the evaporator equals the mass of product and vapour removed from the evaporator’.

• For the water component, this is given by:• Equation 1

• For solutes, the mass of solids entering the evaporator equals the mass of solids leaving the evaporator:

Equation 2

• The total mass balance is Equation 3

• Assuming that there are negligible heat losses from the evaporator, the heat balance states that ‘the amount of heat given up by the condensing steam equals the amount of heat used to raise the feed temperature to boiling point and then to boil off the vapour’:

Equation 4

That is:

Heat supplied by steam = Sensible heat + Latent heat of vapourisation

Consider the continuous evaporation process described above

• Overall material balance gives:F = V + C Equation 5

Where F = mass flow of feed, kg/sV = mass flow of vapor, kg/sC= mass flow of concentrate (product), kg/s

Material balance for the solid is written as:F.xF = C.xC Equation 6

where: xF and xC =mass fraction of solids in feed and concentrate, respectively

Combining Eqs. (5) and (6) and defining concentration ratio as R = xC/xF

𝑉 = 𝐹(1 −1

𝑅) Equation 7

Equation 8

Equation 9

Equation 10

Design of an evaporator

Evaporator consist of a heat exchanger enclosed in a large chamber. The heat exchanger

provides means to transfer heat from low pressure steam to the product. The product in

the evaporation chamber is kept under vacuum. The vacuum causes the temperature

difference between the steam and the product to increase. The product, thus, boils at

relatively low temperature minimizing heat damage.

Usually the liquid being concentrated flows through a tube while heat is applied to the

outside of the tube. The solvent boils and is separated from the concentrated liquid. Most

foods are damaged by heat, so they are normally evaporated under vacuum conditions

with a low boiling point. The latent heat of vaporization of water is high but by reusing

energy in multiple stages or with vapor recycle, good energy efficiency can be obtained.

Figure 2: Scheme of an evaporator

The main components of evaporators are:

• A feed preheater to bring the feed close to the boiling point

• A feed distribution system to distribute the feed equally between the tubes

• An energy supply, usually steam or electricity

• A method of heat transfer to the boiling liquid

• Vapor/liquid separators to separate the vapor with minimal liquid carryover

• A vacuum system to keep the boiling temperature low

• A condenser to remove energy from the vapor and/or to help maintain the vacuum

Evaporator effect

• An evaporator may be designed for use as a single effect system or several evaporator bodies connected together to form a multiple effect system

• Multiple effect evaporators are widely used in large operation

Design of a single-effect evaporator

• In a single-effect evaporator, dilute liquid feed is pumped into the heatingchamber, where it is heated indirectly with steam. Steam is introducedinto the heat exchanger, where it condenses to give up its heat ofvaporization to the feed, and exits the system as condensate. Thetemperature of evaporation (T1) is controlled by maintaining vacuuminside the heating chamber. The vapors leaving the product are conveyedthrough a condenser to a vacuum system, usually a steam ejector or avacuum pump. The concentrated product is then pumped out of theevaporator system.

• Heat and mass balances conducted on the evaporator system allowdetermination of various design and operating variables. Such variablesmay include mass flow rates, final concentration of product, and heat-exchanger area.

• Figure 2: Single effect evaporator

• Mass balance: wA = wC + V• wAXA = wCXC

• Where XA and XC are the mass fractions of solute in food and concentrated solution stream respectively

Example 1• A single effect continuous evaporator is used to concentrate a fruit juice

from 15 Bx to 40 Bx. The juice is fed at 25°C, at the rate of 5400 kg/h (1.5kg/s). Calculate:

a. the concentration ratio R

b. the required evaporation capacity V (kg/s)

Read on Brix

• Apple juice is being concentrated in a natural-circulation single effect evaporator. At steady state conditions, dilute juice is the feed introduced at a rate of 0.67 kg/s. The concentration of the dilute juice is 11% total solids. The juice is concentrated to 75% total solids. Calculate the mass flow rate of concentrated product and vapour.

• Solution• Mass flow rate of feed mf =0.67 kg/s

• Concentration of food xf = 0.11

• Concentration of product xp = 0.75

Multiple effect evaporators

• Several evaporators (or ‘effects’) are connected together.

• Multiple effect evaporation is evaporation in multiple stages, whereby thevapours generated in one stage serve as heating steam to the next stage. Thefirst stage acts as a steam generator for the second which acts as a condenserto the first and so on. The first effect is heated with boiler steam and thevapours released from the last effect are sent to the condenser.

Design of a multiple (triple) effect evaporator• In a triple-effect evaporator, dilute liquid feed is pumped into the

evaporator chamber of the first effect. Steam enters the heat exchangerand condenses, thus discharging its heat to the product. The condensateis discarded. The vapors produced from the first effect are used as theheating medium in the second effect, where the feed is the partiallyconcentrated product from the first effect. The vapors produced from thesecond effect are used in the third effect as heating medium, and thefinal product with the desired final concentration is pumped out of theevaporator chamber of the third effect. The vapors produced in the thirdeffect are conveyed to a condenser and a vacuum system. In the forwardfeed system shown, partially concentrated product from the first effect isfed to the second effect. After additional concentration, product leavingthe second effect is introduced into the third effect. Finally, product withthe desired concentration leaves the third effect.

The vapor released each time is at a lower temperature and pressure

t1,t2,t3 are the boiling temperatures of the solutions leaving the evaporation chambers of the first, second and third effects, respectively tb1, tb2, tb3 are the boiling temperature of pure water at pressures P1, P2, P3, respectively

Triple effect evaporator

Arrangements of multiple effect evaporators

• Parallel system

The food is distributed in different streams fed into each of the effects whilethe concentrated solution stream of each effect is gathered in only onestream, which will be the final concentrate.

• Forward Feed

The diluted stream is fed into the first effect, while the concentrate streamleaving each effect is fed into the following effect. It can be observed that thevapor and concentrate streams of each effect are parallel flows. This passsystem is frequently used for solutions that can be thermally affected, sincethe most concentrated solution is in contact with the vapor at the lowesttemperature.

• Backward Feed

The flows of the solutions to concentrate and of vapor are countercurrent. Thediluted solution is fed into the last effect, where the vapor has less energy, andthe concentrated solution leaving this effect is used to feed the previous effectand so on. This type of arrangement should be carefully used in the case offood solutions, since the solution with the highest concentration gains heatfrom the vapor at the highest temperature, which may affect the food.

• Mixed Feed

In this type of arrangement, the diluted solution can be fed into any of theeffects, while the concentrated solutions can be used to feed a previous orfollowing effect. In the below illustration, the diluted solution is fed into thethird effect, while the solution leaving this effect is used to feed the first one.The food stream feeding the second effect is the concentrated solution thatleaves the first effect, obtaining the final concentrated solution at the secondeffect.

Effect of Evaporation on Food Quality• Thermal effects

During evaporation, foods are susceptible to thermal damage, depending on thetime–temperature profile of the process. The rate and extent of browningdiscoloration is concentration dependent. As the concentration of the foodincreases during evaporation, so does its sensitivity to high temperature.Thermal damage can be drastically reduced by lowering the evaporationtemperature, i.e. by operating the evaporator under vacuum. On the otherhand, too low evaporation temperature may result in longer residence time. Inthe case of lycopene loss in tomato juice, it has been found that hightemperature– short time evaporation results in better retention of the lycopenepigment.

Some examples of thermal damage types associated with evaporation:• Non-enzymatic (Maillard) browning• Induction of ‘ cooked taste ’ in fruit juices• Loss of carotenoid pigments (e.g. lycopene in tomato juice)• Protein denaturation (milk).

• Loss of volatile flavour components

A proportion of the desirable volatile components, known as the ‘aroma’,‘fragrance’ or ‘essence’ are lost when fruit juices or coffee extract areconcentrated by evaporation. The extent of aroma loss depends on thevolatility of the flavour substances in relation to that of water. Differentcomponents an aroma do not have the same volatility, thus, duringevaporation, some components are lost to a greater extent than others.

Fruit aromas are classified into four groups, with respect to their relativevolatility:

• High volatility aromas, such as apple aroma, which are practically lost completelywhen only 15% of the juice has been evaporated

• Medium volatility aromas (e.g. plum, grape), lost almost completely when 50% of thejuice is evaporated

• Low volatility aromas (peach, apricot), lost to the extent of about 80% when 50% ofthe juice is evaporated

• Very low volatility aromas (strawberry, raspberry), of which 60–70% or less are lostwhen 50% of the juice is evaporated.