Evaporation - PowerPoint Presentation

Slide1

EvaporationSlide2

Introduction

The objective of evaporation is to concentrate a solution consisting of

a nonvolatile

solute and a volatile solvent. In the overwhelming majority of

evaporations the

solvent is water.

When the liquid

phase is agitated, mass-transfer in the liquid phase

is sufficiently

rapid that the rate of evaporation of solvent

can be

determined by the rate of heat transfer from the

heating medium

, usually condensing steam, to the

solution.

Evaporation differs from

drying in that the residue is a liquid-sometimes a highly viscous

one-rather than

a

solid.

it

differs from distillation in that the vapor usually is a

single component

, and even when the vapor is a mixture, no attempt is made in

the evaporation

step to separate the vapor into

fractions.

Mineral-bearing

water often is evaporated to give a solid-free product

for boiler

feed, for special process requirements, or for human consumption.

This technique

is often

called

water distillation, but technically it is evaporation.Slide3

Liquid characteristics

Concentration

The density

and viscosity

increase with solid content until either the solution becomes

saturated or

the liquor becomes too viscous for adequate heat transfer

.

continued boiling of

a saturated solution causes crystals to form; these must be removed or the

tubes clog.

The

boiling point of the solution may also rise considerably as the

solid content

increases, so that the boiling temperature of a concentrated solution

may be

much higher than that of water at the same pressure

.

2. Foaming

A

stable foam accompanies the vapor out of the evaporator, causing

heavy entrainment

. In extreme cases the entire mass of liquid may boil over into

the vapor

outlet and be lost

.

3.

Temperature

sensitivity

4. Scale

رسوب

5. Materials

of

construction

6. Toxicity,

explosion hazards, radioactivity, and

necessity for

sterile

operationSlide4

Once-through circulation evaporators

These evaporators are

well adapted to multiple-effect

operation

Agitated-film

evaporators are

always operated

once

through

Falling-film

and climbing-film evaporators can also

be operated

in this

way

Useful

for heat-sensitive

materials, By

operating under high vacuum, the temperature of the liquid can be kept

low

With a single rapid passage through the tubes the thick liquor is at the

evaporation

Temperature

but a short time and can be quickly cooled as soon as it leaves

the evaporatorSlide5

Circulation evaporators

Although the average residence time of the liquid in the heating

zone may

be short, part of the liquid is retained in the evaporator for a

considerable time

. Prolonged heating of even a small part of a heat-sensitive material like

a food

can ruin the entire

product.

Climbing-film

evaporators are

usually

circulation

units.Slide6

Continuous flow evaporators

1. Horizontal-tube

evaporator.

inside of which

steam condenses and outside of which the

solution to

be concentrated boils

.

Agitation is provided

only by the movement of the

bubbles formed

. Therefore, this type of unit is only suitable for low-viscosity solutions that do not deposit scale on the heat-transfer surfaces.Slide7

Continuous flow evaporators

2. Short-vertical-tube

evaporator.

solution

inside the tubes and

steam condensing

outside.

Boiling inside

the tube causes the solution to circulate,

thus providing

additional

agitation

not

suitable for very viscous solutions.Slide8

Continuous flow evaporators

3. Long-vertical-tube evaporator

Higher

tube-entering liquid velocity

Higher

heat-transfer

coefficient

For liquids

that tend to foam.Slide9

Continuous flow evaporators

4. Forced-circulation evaporator

Very

viscous solutions

A

pump is used to force the

solution upward

Through relatively short tubes

Because

of the high velocities in a

forced-circulation evaporator

, the residence time of the liquid in the tubes is short-about 1

to 3 s-so that moderately heat-sensitive liquids can be concentrated in them.

Salting

liquors or those that tend to foam.Slide10

Continuous flow evaporators

5. Falling-

f

ilm evaporator

heat-sensitive

solutions such as

fruit juices

The flows

as a

film inside

walls of the

tubesThe concentrate and

the vapor produced are separated at the bottomSlide11

Continuous flow evaporators

6

. AGITATED-FILM EVAPORATOR

The principal resistance to overall heat

transfer from

the steam to the boiling liquid in an evaporator is on the liquid side.

One way

of reducing this resistance, especially with viscous liquids, is by

mechanical agitation

of the liquid

film.

This is a modified falling-film evaporator with a single jacketed tube containing an internal agitator.

High rates of heat transfer with viscous liquids

Viscous

heat-sensitive products as gelatin, rubber latex, antibiotics, and fruit juices.Slide12

Performance of tubular evaporators

Capacity is defined as the number of

kilograms

of

water vaporized per hour.

Economy

is the number of kilograms vaporized

per

kilogram

of steam fed to the

unit.

In a single-effect evaporator the economy is nearly always less than 1, but in multiple-effect equipment it may be considerably greater.

The steam consumption, in kilograms per hour, is also important. It equals the capacity divided by the economy.

The

rate of heat transfer

q through the heating surface of an

evaporator

is

the

product of

three factors: the area of the heat-transfer surface

A, the overall

heat-transfer

coefficient

U, and the overall temperature drop

Δ

T.

q = UA

Δ

TSlide13

Evaporator

Economy

Influencing factor

1. Number of effects

By

proper design the enthalpy of vaporization of the steam to the

first effect

can be used one or more times, depending on the number of effects.

2. Temperature

of the

feedSlide14

Effect of the feed state on the capacity

If the feed to the evaporator is at the boiling temperature corresponding

to the absolute

pressure in the vapor space, all the heat transferred through

the heating

surface is available for evaporation and the capacity is proportional to

q

.

If the feed is cold, the heat required to heat it to its boiling point may be

quite large

and the capacity for a given value of

q is reduced accordingly, as heat used to heat the feed is not available for

evaporation.if the feed is at

a temperature

above the boiling point in the vapor space, a portion of the

feed

evaporates spontaneously by adiabatic equilibration with the vapor-space

pressure and

the capacity is greater than that corresponding to

q. This process is called

flash evaporation.Slide15

Boiling-point elevation

(BPE

)

For a given pressure in the vapor space

of

an evaporator, the boiling temperature of an aqueous solution will be equal to that of pure water if the solute is not dissolved in

the

water

but rather consists of small, insoluble, colloidal material

.

If the solute is soluble, the boiling temperature will

be greater than that of pure water by an amount known as

the boiling-point elevation of the solution.

In

actual evaporators, however, the

boiling point

of a solution is affected by two factors, boiling-point elevation and

liquid head

.

If, as is usually

the case

, the solute has little or no vapor pressure, the

evaporator

pressure is equal to the partial pressure of the water in

the solution

. Then, by a modified

Raoult's

law:Slide16

Diihring

chart for aqueous solutions of

sodium hydroxide

.Slide17

Nomograph

for

boiling-point

elevation of

aqueous

solutionsSlide18
Slide19

Effect of liquid head and friction on temperature drop

If

the depth

of liquid in an evaporator is appreciable, the boiling point

corresponding to

the pressure in the vapor space is the boiling point of the surface layer of

liquid only

.

The average boiling point of the liquid in the tubes is higher than the boiling point corresponding to the pressure in the vapor because:

pressure

of the vapor

space head of Z meters or feet

of liquid frictional loss in the tubes increases the average pressure of the

liquid (large liquid velocity).

This increase in boiling point lowers the average temperature drop

between the

steam and the liquid and reduces the

capacity.

The

amount of reduction

cannot be

estimated quantitatively with precision, but the qualitative effect of liquid

head, especially

with high liquor levels and high liquid velocities, should not be

ignored.

The

temperature drop is fixed by the

properties of

the steam and the boiling liquid and

except for

the effect of hydrostatic

head is

not a function of the evaporator construction.Slide20

Heat transfer coefficients

The overall coefficient, on

the other

hand, is strongly influenced by the design and method of operation of

the evaporator.

the overall resistance to heat

transfer between

the steam and the boiling liquid is the sum of five individual resistances:

The steam-film resistance (not important, the presence of non-condensable gas seriously reduces the steam-film coefficient)

Inside

and outside the

tubesThe tube-wall resistance (not important)

The resistance from the boiling liquid.Slide21

The liquid-side coefficient

The liquid-side coefficient depends to a large extent

on the

velocity of the liquid over the heated surface. In most evaporators,

and especially

those handling viscous materials, the resistance of the liquid side

controls the

overall rate of heat transfer to the boiling liquid

.

Forced circulation gives high liquid-side coefficients even though

boiling inside

the tubes is suppressed by the high static head.

The formation of scale on the tubes of an evaporator adds a thermal resistance equivalent to a fouling factor.Slide22

single-effect evaporation

When a single evaporator is used, the vapor from the boiling liquid

is condensed

and discarded

.

This method is called

single-effect evaporation,

and

although

it is simple, it utilizes steam ineffectively

.

To evaporate 1 kg of water from a solution calls for from 1 to 1.3 kg of steam.Slide23

weight-fraction solute:

wf

mass

flow

rate:

mf

1. The thin-liquor feed has only one volatile

component, e.g

., water.

2. Only the latent heat of the heating steam at T, is

available for

heating and vaporizing the solution in the evaporator.3. The boiling action on the heat-exchanger surfaces

agitates the solution, in the evaporator, sufficiently to achieve perfect mixing Te= Tp

and

Tv

= Tp.

4. Driving force for heat

transfer =

Δ

T

= Ts -

Tp

5. The

Δ

T

is high enough to

achieve nucleate

boiling and not so

high as

to cause film

boiling

6. No

heat loss from the

evaporator

Continuous-flow, steady-state model evaporatorSlide24

Continuous-flow, steady-state model evaporatorSlide25

Enthalpy-concentration diagram for sodium hydroxide-water system

.Slide26

Multiple-Effect Evaporator

Systems

When condensing steam is used to evaporate water

from an

aqueous solution, the heat of condensation of the

higher temperature condensing

steam is less than the heat of

vaporization of

the lower-temperature boiling

water. consequently, less

than 1 kilogram of vapor is produced

per kilogram condensation of heating steam. This ratio is called the economy.

To reduce the amount of steam required and, thereby, increase the economy, a series of evaporators, called effects, can be

used.

The

increased economy is achieved by operating the

effects

at different pressures, and thus at different boiling

temperatures, so

that vapor produced in one effect can be

condensed to

supply the heat in another effect.Slide27

Multiple-Effect Evaporator

Systems

1. Forward-feed,

triple-effectSlide28

Multiple-Effect Evaporator

Systems

1. Forward-feed,

triple-effect

This pattern of liquid flow is the simplest.

One-third

of

the total

evaporation occurs in each effect

.

To achieve a temperature-driving force for

heat transfer in the second effect, the pressure of the second effect, P2, is lower than that of the first effect. This

procedure is repeated in the third effect.For three effects, the flow

rate of

steam entering the first is only about one-third of

the amount

of steam that would be required if only one

effect were used.

the

temperature-driving force in

each of

the three effects is only about one-third of that in a

single effect.

Therefore

, the heat transfer area of each of the

three evaporators

in a triple-effect system is approximately

the same

as for the one evaporator in a single-effect

unit.

It

requires a pump for

feeding

dilute solution to the first

effect, since

this effect is often at about atmospheric pressure, and a pump to

remove thick

liquor from the last effect. The transfer from effect to effect, however, can

be done

without pumps, since the flow is in the direction of decreasing pressure,

and control

valves in the transfer line are all that is required

.Slide29

Multiple-Effect Evaporator Systems

2. Backward-feed,

triple-effectSlide30

Backward-feed, triple-effect

When the temperature of the fresh feed is

significantly below

its saturation temperature corresponding to the

pressure in

the first effect,

backward-feed operation is

desirable.

Th

e

cold fresh feed is sent to

the third effect, which operates at the lowest pressure and, therefore, the lowest temperature.

Unlike the forward-feed system, pumps

are required to move the concentrate from

one effect

to the next because

PI >

P

2

> P3.

Backward

feed often gives a higher capacity than forward feed when the

thick liquor

is viscous, but it may give a lower economy than forward feed when

the feed

liquor is cold.Slide31

Multiple-Effect Evaporator Systems

3.

mixed

feedSlide32

Multiple-Effect Evaporator Systems

4.

parallel feedSlide33

CAPACITY AND ECONOMY OF MULTIPLE-EFFECT

EVAPORATORS

The total capacity of a multiple-effect evaporator is usually no

greater than

that of a single-effect evaporator having a heating surface equal to one

of the

effects and operating under the same terminal conditions, and, when there

is an

appreciable boiling-point elevation, is often considerably smaller

.

When

the boiling-point elevation is negligible, the effective overall ΔT

equals the sum of the ΔT's in each effect, and the amount of water evaporated per unit area of surface

in

an

N-effect multiple-effect evaporator is approximately

1/Nth

that in the

single effect

. Slide34

Effect of boiling-point elevation on capacity of evaporatorsSlide35

Effect of boiling-point elevation on capacity of evaporators

Consider an evaporator that is concentrating a solution with a

large boiling-point

elevation. The vapor coming from this boiling solution is at

the solution

temperature and is therefore superheated by the amount of the

boiling point elevation.

Superheated steam is

essentially equivalent to saturated steam at the same pressure when used as

a heating

medium.

The temperature drop in any effect, therefore, is calculated from the temperature of saturated steam at the pressure of the steam chest, and

not from the temperature of the boiling liquid in the previous effect. This means that the boiling-point elevation in any effect is lost from the total available

temperature drop

. This loss occurs in every effect of a multiple-effect evaporator, and

the resulting

loss of capacity is often important

.

The boiling-point elevation tends

to make

the capacity of a multiple-effect evaporator less than that of the

corresponding single

effect

.

In a single-effect unit producing 50 percent

NaOH

,

for example

, the overall coefficient

U for this viscous liquid would be small. In

a

triple-effect

unit, the coefficient in the final effect would be the same as that in

the single

effect, but in the other effects, where the

NaOH

concentration is much

lower than

50 percent, the coefficients would be greater. Thus the average coefficient

for the

triple-effect evaporator would be greater than that for the single effect

.Slide36

The

economy of a

multiple effect

evaporator

depends on heat-balance considerations and not on the rate

of heat

transfer.

The

capacity

is reduced by the

boiling-point

elevation.The capacity, on the other hand, is reduced by the

boiling-point elevation. The capacity of a double-effect evaporator concentrating a solution with a boiling-point elevation is generally less than half the capacity of two single

effects, each

operating with the same overall temperature drop. The capacity of a

triple effect

is generally less than one-third that of three single effects with the

same terminal

temperatures

.

The optimum number of effects must be

found from

an economic balance between the savings in steam obtained by

multiple-effect operation

and the added investment required.Slide37