1

Contents

1. Definition Of Evaporation ............................................................................................................ 2

2. The Standard Evaporator ............................................................................................................ 2

3. History Of Multiple Effect Evaporator ....................................................................................... 3

4. Working Of Multiple Effect Evaporators (quadruple) ............................................................. 3

5. Multiple Effect Evaporator Operation ....................................................................................... 4

6. Condenser ...................................................................................................................................... 4

7. Vacuum Pump ............................................................................................................................... 4

8. Entrainment Prevention ............................................................................................................... 5

9. Heat Transmission ........................................................................................................................ 5

10. Gases During Evaporation ....................................................................................................... 5

11. Fouling of Evaporators ............................................................................................................. 5

12. Removal of Scale ....................................................................................................................... 5

13. Prevention of Scale Formation ................................................................................................. 6

14. Insolation of Evaporators and Vacuum Pans ......................................................................... 6

15. Determination of Evaporation Rate In Multiple Effect Evaporator .................................... 6

16. Referances ................................................................................................................................ 13

2

The juice from the clarification system contains the natural water expressed from the cane

together with part of imbibitions water, say an average composition of 85% water and 15%

solids. The major portion of water must now be removed to yield syrup of 60% solid or more.

For the removal of this major portion of water (making syrup) evaporation is done by

the help of evaporators.

1. Definition Of Evaporation

Evaporation is the process of removing water from a solution by vaporization through

the application of heat. In other words we can say that Evaporation is a type

of vaporization of a liquid that occurs from the surface of a liquid into a gaseous phase that is

not saturated with the evaporating substance.

Evaporation is similar to drying in that both driven off volatiles, but is different in that

the product is a liquid. Evaporation differs from distillation because both components in a

distillation system are volatile. Evaporation normally produces a single vapor fraction,

distillation several.

2. The Standard Evaporator

An evaporator consists of a heat exchanger for boiling the solution and a means to

separate the vapor from the boiling liquid. Different types are categorized by the length and

alignment (horizontal or vertical) of the evaporator tubes. The evaporation tubes may be

located inside or outside of the main vessel where the vapor is driven off. Because many

materials cannot tolerate high temperatures, evaporators often operate at reduced pressure so

that the boiling point will also be reduced. In most cases, evaporators operate under a

vacuum. This means that a vacuum pump or jet ejector vacuum system is required on the last

effect.

Evaporators are commonly used in the inorganic and organic chemical, pulp and

paper, and food industries (especially sugar).

Evaporator is made up of two closed spaces, separated from one another by thin metal

walls in the form of tubes, coils or plates, called the heating surface. In sugar industries, there

is also possibility of constructing spear heater so that each unit can be cleaned without

retarding the operation of mill. The evaporator consists of a closed, cylindrical, vertical

vessel, the lower part of which is a dished bottom. It has openings for feed and drain. There is

a large steam or vapor inlet at the side (steam enters on of this spaces at a fixed temperature

and pressure where it condenses, thus giving up its latent heat),and drain pipes at the bottom

and vent pipes as well as vapor outlet at the top, leading to the outside and eventually

connecting to condenser. Heating surface (copper tubes are used because of better heat

3

transmission) are present down constituting 4 to 6 ft whereas the top height is perhaps 10 ft,

including sight glasses, thermometer and other accessories. With the dome or top cover,

catchall is attached which is designed to catch the drops of juice which might be entrained by

the rapid vapor current and lost. This catchall has a drain and a vapor outlet at the side.

In cane sugar industry, multiple effect evaporator is used which consist of three, four,

or five evaporators connected in series. The individual evaporator is called effect bodies, cells

unit, pans according to local custom.

3. History Of Multiple Effect Evaporator

The invention of the multiple effect evaporator is generally credited to Norbert

Rillieux. Rillieux developed a multiple pan evaporation system for use in sugar refining.

Rillieux was born in Louisiana and trained in France. Most of his working career was spent in

the U.S., although he later returned to Europe where he is buried in the famous Pere Lachaise

cemetery in Paris. Rillieux's achievements were little acknowledged during his lifetime,

because according to the laws of the time he was "a free person of color." He was also the

first cousin, once removed, of the painter Edgar Degas.

4. Working Of Multiple Effect Evaporators (quadruple)

At the present time the quadruple (four cell evaporator) is most commonly in use,

although sextuple effects are not rare. The ordinary practice is as follow:

The juice enters cell No.1 and covers the heating tubes, to which is admitted sufficient steam

generally exhausted from the engine to cause the liquid to boil. The steam or vapor liberated

from the first boiler is conducted through the vapor pipe directly into the heating tubes of cell

No. 2, while juice from cell No.1 is passed into the second, or cell No.2, and surrounds the

surface which contains the hot vapor given off from the same juice in cell No.1. As there is

little or no pressure above the liquid in the first cell, the juice boils at 105˚ C. by maintaining

a vacuum of 4-5 inches in the second cell, the temperature at which the liquid will boil will

reduce to 96˚C and the vapor at cell No.1 is hot enough to boil the juice at cell No. 2 without

any addition of heat. The vapor at cell No. 2 in the same way enters the heating tubes of cell

No. 3, while the juice entering this cell is exposed to a vacuum of 15 inches, which reduces

the boiling temperature to 82 C, so that the difference of -3.88 ˚C between the condition of

cell No. 2 and cell No. 3causes a third boiling and evaporation without any additional steam

being added. A vacuum of 26 inches in the last cell (cell No. 4) brings the final boiling to

52˚C. the vapor from this cell enters a condenser, where it is exposed to a spray of cold

water, is condensed and passes down a pipe not less than thirty feet long, terminating in a

4

water seal, and called the Torricellian tube. Juice is feed to the first effect and passed onto the

second, the third, the forth etc, by appropriate feed pipes with control valves, and the

concentrated syrup is removed from the last effect by a pump. The juice passing through

these evaporating cells is boiled to a syrup containing about 35% of water and 65% of solid

matter (65˚Brix). It is pumped out to the fourth cell into the receiving tank for the vacuum

pan.

5. Multiple Effect Evaporator Operation

There are two feed operations - backward feed and forward feed operations. A brief

explanation of these operations:

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.

In the case of a forward feet 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.

6. Condenser

In order to obtain vacuum the vapor evaporated by the apparatus at the corresponding

temperature must be condensed. A condenser is a closed cylindrical vessel into the upper part

of the last body in which cold water is admitted through close pipes. The water comes in

contact with hot water, condenses it, and thus increases its own temperature. This hot water is

discharged.

7. Vacuum Pump

A powerful vacuum pump draws the air and other in condensable gases from the

condenser and maintains the vacuum, which is applied to the necessary extent in each cell. In

commercial operation a vacuum greater than 28 inches is seldom required, as this is sufficient

for all practical purposes. The degree of vacuum for any container can be varied easily by

mechanical manipulation, so that a vacuum anywhere from 1 to 28 inches may be maintained.

5

8. Entrainment Prevention

In all evaporator, the possibility exists of loss of sucrose by entrainment, the carrying

over of small drops of juice or syrup by rapidly moving current. The separator are catchall is

used to separate the entrainment. The operating principle of all separator is a change of

direction permitting the droplets of entrained liquid to veer away from the path of vapors,

whereupon impingements on a wetted surface, they can be salvaged and return to the

evaporator.

9. Heat Transmission

In an evaporator, the resistance to the flow of heat is made up of three distinct parts:

the steam film, the metal resistance and the liquid film resistance. Naturally, the lower these

influences, the higher the heat transmission will be. As a solution grows more viscous, heat

transmission becomes more and more difficult.

10. Gases During Evaporation

Ammonia is liberated from the juice by braking up of organic compounds, incident to

alkalization, heating and concentration. Gases from whatever sources accumulate, must be

removed continuously by means of vent pipes properly located and ultimately discharging

into the condensers. Ammonium gas is lighter than steam and will tend to rise, whereas air

carbon dioxide are heavier and will tend to accumulate in the lower part.

11. Fouling of Evaporators

The syrup from the evaporators in modern factories has a concentration of at least 60

brix and generally higher. A considerable amount of impurities especially mineral salts

becomes less soluble as the concentration of the juice progresses and part of these impurities

deposits on the heating surface of the evaporators forming a hard scale. This scale is a poor

conductor of heat and must be removed periodically to maintain the efficiency of effects.

The scale in the first cell may be of calcium phosphate, calcium sulphate, carbonates

and silicates. The scale in the last body is the thickest and most obstinate scale and may

consist of largely of silica, with calcium oxalate, silicate, carbonate, sulphate and phosphate

also present.

12. Removal of Scale

Many procedures have been advocated for cleaning evaporator tubes and heating

surfaces, the commonest practice being to boil for several hours with caustic soda solution

(30˚), or a mixture of caustic soda and soda ash. Then wash with water and boil with dilute

6

HCl acid. The caustic soda solution is stored and used repeatedly, the acid discarded after

use.

13. Prevention of Scale Formation

Efforts to prevent the formation of scale by adding tetra phosphoglucosate of lime to

the clarified juice have met with varied success.

14. Insolation of Evaporators and Vacuum Pans

Heat losses of considerable magnitude will occur unless the bodies of evaporators and

vacuum pans are insulated. To cover expose surfaces, wood or asbestos magnesia blocks can

be used painting with aluminum paint has also a high insolating value.

15. Determination of Evaporation Rate In Multiple Effect Evaporator

In a triple-effect evaporator, dilute liquid feed is pumped into the evaporator chamber

of the first effect. Steam enters the heat exchanger and condenses, thus discharging its heat to

the product. The condensate is discarded. The vapors produced from the first effect are used

as the heating medium in the second effect, where the feed is the partially concentrated

product from the first effect. The vapors produced from the second effect are used in the third

effect as heating medium, and the final product with the desired final concentration is

pumped out of the evaporator chamber of the third effect. The vapors produced in the third

effect are conveyed to a condenser and a vacuum system. In the forward feed system shown,

partially concentrated product from the first effect is fed to the second effect. After additional

concentration, product leaving the second effect is introduced into the third effect. Finally,

product with the desired concentration leaves the third effect. Design expressions for

multiple-effect evaporators can be obtained in the same manner as for a single-effect

evaporator.

7

Conducting mass balance analysis on the flow streams,

Where mf is the mass flow rate of dilute liquid feed to the first effect (kg/s); mv1, mv2, and mv3

are the mass flow rates of vapor from the first, second, and third effect, respectively (kg/s);

and mp is the mass flow rate of concentrated product from the third effect (kg/s).

Using mass balance on the solids fraction in the flow streams,

Where xf is the solid fraction in the feed stream to be consistent with the first effect

(dimensionless) and xp is the solid fraction in the product stream from the third effect

(dimensionless).

We write enthalpy balances around each effect separately.

Where the subscripts 1, 2, and 3 refer to the first, second, and third effect, respectively. The

other symbols are the same as defined previously for a single-effect evaporator. The heat

transfer across heat exchangers of various effects can be expressed by the following three

expressions:

8

The steam economy for a triple-effect evaporator as shown in

Example: Illustrates the use of these expressions in evaluating the performance of multiple-

effect evaporators.

Calculate the steam requirements of a double-effect forward-feed evaporator to concentrate a

liquid food from 11% total solids to 50% total solids concentrate. The feed rate is 10,000 kg/h

at 20°C. The boiling of liquid inside the second effect takes place under vacuum at 70°C.

The steam is being supplied to the first effect at 198.5 kPa. The condensate from the first

effect is discarded at 95°C and from the second effect at 70°C. The overall heat-transfer

coefficient in the first effect is 1000 W/ (m2°C); in the second effect it is 800 W/ (m2°C). The

specific heats of the liquid food are 3.8, 3.0, and 2.5 kJ/ (kg °C) at initial, intermediate, and

final concentrations. Assume the areas and temperature gradients are equal in each effect.

9

Approach Since this is a double-effect evaporator, we will use modified forms of Equations

(8.8), (8.9), (8.10), (8.11), (8.13), and (8.14). Enthalpy values of steam and vapors will be

obtained from steam tables.

Solution

1. From Equation (8.9),

2. From Equation (8.8),

Thus, the total amount of water evaporating is

3. Steam is being supplied at 198.5 kPa or 120°C, the temperature in the second effect is

70°C, and thus the total temperature gradient is 50°C.

Assuming equal temperature gradient in each evaporator effect,

10

4. The area of heat transfer in the first and second effects are the same. Thus, from

Equations (8.13) and (8.14),

5. To use Equations (8.10) and (8.11), we need values for enthalpy of product.

In addition, from steam tables

6. Thus, substituting enthalpy values from step (5) in the equation given in step (4),

11

7. Using Equations (8.10) and (8.11),

8. Let us assemble all equations representing mass flow rates of product, feed, vapor, and

steam.

9. In step (8),we have five equations with five unknowns,namely, mp,mv1,m v2,ms, and mf1.

We will solve these equations using a spreadsheet procedure to solve simultaneous equations.

The method described in the following was executed on Excel.

10. The simultaneous equations are rewritten so that all unknown variables are collected on

the right-hand side. The equations are rewritten so that the coefficients can easily be arranged

in a matrix. The spreadsheet method will use a matrix inversion process to solve the

simultaneous equations.

12

11. Enter the coefficients of the left-hand side of the preceding equations in arrayB2:F6; enter

the right-hand side coefficients in a column vector H2:H6.

12. Select another arrayB9:F13 (by dragging the cursor starting from cell B9).

Type ₊MINVERSE (B2:F6) in cell B9 and press the CTRL, SHIFT, and ENTER keys

simultaneously. This procedure will invert the matrix B2:F6 and give the coefficients of the

inverted matrix in arrayB9:F13.

13. Highlight cells H9:H13 by dragging the cursor starting from cell H9. Type ₊ MMULT

(B9:F13, H2: H6) into cell H9; press the CTRL, SHIFT, and ENTER keys simultaneously.

The answers are displayed in the column vector H9:H13.

Thus,

14. The steam requirements are computed to be 1.43 kg/s.

15. The steam economy can be computed as

13

16. Referances

http://www.compevaporators.com/multi-effect-evaporators.html

http://www.geape.fr/nfruk/cmsdoc.nswf/WebDoc/webb8mujc

http://facstaff.cbu.edu/rprice/lectures/evap1.html

ethesis.nitrkl.ac.in/3890/1/GHOSHNA_JYOTI-_THESIS.pdf