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EvaporatorERT 216 HEAT & MASS TRANSFER

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1. Introduction & 2. General Types of Evaporators3. Methods of Operation of

Evaporators4. Overall Heat-Transfer

Coefficient In Evaporators5. Calculation Methods for Single

Effect Evaporators

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5.1 Effects of Processing Variables on Evaporator Operation

5.2 Boiling-Point Rise of Solutions5.3 Enthalpy-Concentration Charts

of Solutions6 Evaporation of biological

materials

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Evaporation:The vapor from a boiling liquid solution

is removed and a more concentrated solution remains.A separation process of removing water

from an aqueous solution.Examples: Concentrated aqueous solutions

of….

Sugar

Sodium chloride Glycerol Milk

Orange juice

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Evaporation processes: to evaporate seawater to provide

drinking water have been developed and used.

to concentrate a solution so that upon cooling, salt crystals will form and separated (crystallization).

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Physical & chemical properties of the solution being concentrated and of the vapor being removed are influenced by the type of evaporator used and the pressure and temperature of the process.Some of the properties which affecting

the processing methods:i. Concentration in the liquid. Liquid feed to an evaporator is

relatively dilute (its viscosity is low), relatively high heat transfer coefficients are obtained.

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As evaporation proceeds, the solution may become very concentrated & quite viscous, causing heat transfer coefficient to drop markedly.

Adequate circulation &/or turbulencemust be present to keep the coefficient from becoming too low.

ii. Solubility As solutions are heated and the

concentration of the solute or salt increases, the solubility limit of material in solution may be exceeded and crystals may form.

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This may limit the maximum concentration in solution which can be obtained by evaporation.

Fig 1: Solubility

curves for some typical salts in

water.

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iv. Temperature sensitivity of materialsFood & biological products may be

temperature sensitive & degrade at higher temperatures or after prolonged heating.The amount of degradation is a

function of the temperature and the length of time.

v. Foaming & frothingCaustic solutions, food solutions (e.g.

skim milk), & fatty acid solutions form a foam or froth during boiling.

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This foam accompanies the vaporcoming out of the evaporator and entrainment losses occur.

vi. Pressure & temperature The boiling point of the solution is

related to the pressure of the system. The higher the operating pressure of

the evaporator, the higher the temperature at boiling.

As the concentration of the dissolved material in solution increases, the boiling temperature may rise.

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vii. Scale deposition & materials of construction

Some solutions deposit solid materials called scale on the heating surfaces.

Scale could be formed by decomposition products or by decreases in solubility.

The overall heat-transfer coefficient decreases, the evaporator must eventually be cleaned.

Material of construction of the evaporator should be chosen to minimize corrosion.

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Evaporator:The heat is generally provided by the condensation of a vapor (e.g. steam) on one side of a metal surface, with the evaporating liquid on the other side.The type of equipment used depends

primarily on the configuration of the heat-transfer surface & on the meansused to provide agitation or circulationof the liquid.

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1. Open kettle or pan:The simplest evaporator (inexpensive).It consists of an open pan

(or kettle) in which the liquid is boiled.The heat is supplied by

condensation of steam in a jacket or in coils immersed in liquid or direct-fired.

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2. Horizontal-tube natural circulation evaporator

The horizontal bundle of heating tubes is similar to the bundle of tubes in a heat exchanger.The steam enters

the tubes, where it condenses.The steam condensate

leaves at the other end of the tubes.

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It is relatively cheap & is used for nonviscous liquids with high heat transfer coefficients and liquid that do not deposit scale.It is not suitable for viscous liquids due to

poor circulation.Usually, the feed enters at a constant rate

& the concentrate leaves at a constant rate (most types of evaporator).

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3. Vertical-type natural circulation evaporatorVertical rather than horizontal tubes are

used; the liquid is inside the tubes and the steam condenses outside the tubes.The liquid rises in

the tubes by natural circulation due to boiling and decreases in density.The liquid flows

downward through a large, central open space or downcomer.

Short tube

evaporator

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This natural circulation increases the heat-transfer coefficient.It is not used with viscous liquids.A variation is the basket type, the vertical tubes

are used but the heating element is held suspended in the body so there is an annular open space as the downcomer.

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It differs in such a way where it has a central instead of annular open space as the downcomer.It is used in the sugar, salt & cautic soda

industries.

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4. Long-tube vertical type evaporatorSince the heat-transfer

coefficient on the steamside is very highcompared to that on the evaporating liquid side, high liquid velocities are required.In this type, the liquid

is inside the tubes (3 – 10 m long).

Climbing film evaporator

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Vapor bubbles are formed iside the tubes causes a pumping action, which gives quite high liquid velocities.Generally the liquid passes through the

tubes only once and it is not re-circulated.Contact times can be quiet low.In some cases (e.g. condensed milk), when

the ratio of feed to evaporation rate is low, natural recirculation of the product through the evaporator is affected by adding a large pipe connection between the outlet concentrate line & feed line.

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5. Falling-file type evaporatorA variation of long-tube type evaporator.The liquid is fed to the top of the tubes &

flows down the walls as a thin film.Vapor liquid separation usually takes place

at the bottom.It is widely used for concentrating heat

sensitive materials (e.g. fruitjuice) because the holdup time is very small & heat transfer coefficients are high.

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Falling film Evaporator Agitated-film Evaporator

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6. Agitated-film evaporatorHeat transfer in an evaporator is limited

on the liquid side.By actual mechanical agitation of the

liquid film, turbulence in film and hence the heat transfer coefficient is increased. This is done in a modified falling-film

evaporator with only a single, large, jacketed tube containing an internal agitator.

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Liquid enters at the top of the tube & as it flows downward, it is spread out into a turbulent film by vertical agitator blades.The concentrate leaves at the bottom &

vapor leaves through a separator and out the top.It is very useful with highly viscous

materials and heat-sensitive viscous materials (e.g. rubber latex, gelatin, antibiotics, fruit juices).It has a high cost & small capacity.

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7. Forced circulation type evaporator

The liquid film heat transfer coefficient can be increased by pumping to cause food circulation of the liquid inside the tubes.

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The long tube vertical type is modified by adding a pipe connecting a pump between the outlet concentrate line and the feed line.The vertical tubes of this type are usually

shorter than the long-tube type.This type is very useful for viscous liquids.

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1. Single effect evaporators:The feed enters at TF (K) and saturated

steam at TS enters the heat exchanger section.Condensed steam leaves as condensate or

drips.

Simplified diagram of

single effect

evaporator

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Solution in the evaporator is completely mixed: Concentrated product & the solution in

the evaporator = T1 (~ boiling point). Vapor = T1 = boiling solutionPressure (P1) is the vapor pressure of the

solution at T1.An overall heat transfer coefficient is

used, the rate of heat transfer in an evaporator:

)TT(UATUAq S 1Δ

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Single effect evaporators are ofen used when the required capacity of operation is relatively small and/or the cost of stream is relatively cheap compared to the evaporator cost.However, for large-capacity operation,

using more than one effect will markedly reduce steam costs.

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2. Forward-feed multiple effect evaporatorsA single effect evaporator is wasteful of

energy because the latent heat of the vapor leaving is not used but is discarded. This latent heat can be recovered &

reused by employing multi effect evaporator.

Simplified diagram of forward-

feed triple effect

evaporator

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The first effect operates at a temperature that is high enough that the evaporated water serves as the heating medium to the second effect. Almost 3 kg of water will be evaporated

for 1 kg of steam in a three-effect evaporator.

The steam economy is increased.

usedsteamkg

evaporatedvaporkgeconomySteam

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Forward-feed operation:The fresh feed is added to the first

effect and flows to the next in the same direction as the vapor flow.

It is used when the feed is hot or the final concentrated product might be damaged at high temperature.

The boiling temperatures decreasesfrom one effect to another effect.

If the first effect (P1) is 1 atm, then the last effect (P3) will be under vacuum.

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3. Backward-feed multiple-effect evaporatorsThe fresh feed enters the last & coldest

effect and continues on until the concentrated product leaves the first effect - ”Reverse feed”.

Simplified diagram of backward-feed triple-effect evaporator

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It is advantageous when the fresh feed is cold. Liquid pumps must be used in each

effect, since the flow is from low to high pressure.

It is used when the concentrated product is highly viscous.

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In making preliminary designs or cost estimation, it is helpful to have available overall heat transfer coefficients usually encountered in commercial practice. Refer to Table 8.3-1However, detailed calculation is needed

for actual evaporator design and/or for evaluating the effects of changes in operating conditions on the coefficients.

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Table 8.3-1: Typical Heat Transfer Coefficients for Various Evaporators

* Generally, nonviscous liquids have the higher coefficient than viscous liquids in the ranges given.

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5.1 Heat & Material Balances for EvaporatorsBasic equation for solving for the capacity

of a single effect evaporator:

q could be determined by making a heat & material balance on the evaporator.Material balance: F = L + VFor a balance on the solute (solids) alone,

TUAq Δ The difference in temperature between the condensing steam and the boiling liquid in the evaporator.

LF xLxF

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Fig 1: Heat & Mass balance for single effect evaporator

Enthalpy

Enthalpy

Enthalpy

Enthalpy

Enthalpy

Mass fraction

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The condensed steam leaving of S kg/h is assumed usually to be at TS (saturation temperature) and enthalpy hS.The steam gives off only its latent heat:

For heat balance, total heat entering = total heat leaving

SS hHλ

Heat in feed + Heat in steam

Heat in concentrated liquid + Heat in vapor +

Heat in condensed steam

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Assumes no heat by radiation or convection:

Hence,

The q transferred in the evaporator,

SVLSF ShVHLhSHFh

VLF VHLhλSFh

λShHSq SS

Note: The latent heat (λ) of steam at the saturation temperature TS can be obtained from steam tables.

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5.1 Effects of processing variables on evaporator operation:

1. Effect of feed temperature:The inlet temperature of the feed (TF)

has a large effect on the operation on the evaporator.Example 8.4-1:The feed temperature was cold (311.0 K)

as compared to the boiling temperature (373.2K).About ¼ of the steam was used to heat

the cold feed to the boiling point.

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Only about ¾ of the steam was left for vaporization of the feed.Preheating the feed can reduce the size

of evaporator heat-transfer area needed.

2. Effect of pressure:In many cases a large ∆T (>10K) is

desirable, since, as ∆T increases, the heating surface area A and cost of the evaporator decrease.To reduce the pressure (< 101.32kPa – to

be in vacuum), a condenser and vacuum pump can be used.

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when P ↓, boiling point ↓, ∆ T ↑

3. Effect of steam pressure:Using higher pressure, saturated steam

increased ∆T, which decreases the size and cost of the evaporator.

Although high-pressure steam is more costly, more valuable power source.

Overall economic balances are really needed to determine the optimum steam pressures.

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5.2 Boiling point rise of solutions: In most cases, the thermal properties of

solution being evaporated may differconsiderably from those of water.The processed solutions are not dilute

solutions.The concentration of the solutions are

high enough that the heat capacity & boiling point are quite different from those for water.

The boiling point rise for strong solutions of dissolved solutes cannot be predicted.

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A useful empirical law (Duhring’s rule) can be used.

Fig 2: Duhringlines for aqueous

solutions of sodium

hydroxide (at constant

pressure)

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5.3 Enthalpy-concentration Charts of solutions:

Heat of solution - It is found a considerable temperature rise occur, heat is evolved when pellets of NaOH are dissolved in a given amount of water.

Enthalpy-concentration chart for NaOH:It is not made for solutions having

negligible heats of solution, since the heat capacities can be easily used to calculate enthalpies.

Such charts are available for only a few solution.

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Fig 3: Enthalpy-

concentration chart for the

system NaOH-water.

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Evaporation of biological materials frequently differs from the evaporation of inorganic materials and organic materials. Inorganic: NaCl, NaOH Organic: Ethanol, acetic acid Biological: pharmaceuticals, milk, citrus

juices, vegetable extracts.

The degradation of biological materials on evaporation is a function of temperature, and time length.

Heat sensitive, & often contain fine particles of suspended matter in solution.

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To keep the temperature low, the evaporation must be done under vacuum, which reduces the boiling point of the solution.To keep the time of constant low, the

equipment must provide for a low holdup time (contact time) for the material being evaporated.Examples: Long tube vertical evaporator:

condensed milk. Falling film evaporator: fruit juices

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Agitated-film (wiped-film) evaporator: rubber latex, gelatin, antibiotics, fruit juices.

Heat-pump cycle evaporator: Fruit juices, milk, pharmaceuticals.

Fig 4: A rising film cassette evaporator for orange juice production.

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6.1 Fruit JuicesFruit juices are heat-sensitive and the

viscosity increases greatly as concentration increases. Solid suspended matter in fruit juices

has a tendency to cling to the heating surfaces, thus causing over-heatingwhich leads to burning and spoilage of the matter.

To avoid this tendency of stick and to reduce residence time, high rates of circulation over the heat-transfer surface are necessary.

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A fruit juice concentration plant usually employs a single and not a multiple evaporation unit.Vacuum is used to reduce the

temperature of evaporation.A typical fruit juice evaporation system

using the heat-pump cycle employs low -temperature ammonia as the heating fluid.

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6.2 Sugar solutionsSugar (Sucrose) is obtained by primarily

from sugarcane and sugar beets.Sugar tends to caramelize if kept at high

temperatures for long periods.The tendency is to use short-tube

evaporators of the natural circulation type.

The feed is first preheated by exhaust system and then typically enters a six-effect forward-feed evaporator system.

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6.3 Paper-pulp waste liquorsIn the manufacture of paper pulp in the

sulfate process, wood chips are digested or cooked and spent black liquor is obtained after washing the pulp.This solution contains primarily sodium

carbonate and organic sulfide compounds.

It is concentrated by evaporation in six effect system.