chapter 2 evaporation.pdf

DKK 2453 UNIT OPERATION

CHAPTER 2: EVAPORATION

Prepared by: SITI NORAISHAH ISMAIL

Lecturer,

Gas Engineering Department, FKKSA, UMP

25/02/2014 1 Siti Noraishah Ismail

At a glance

25/02/2014 2

3.Type of Evaporator 1. Concept/basic

operation

4.Method of Operation

2. Processing Factor

1. Concentration in liquid

2. Solubility

3. Temperature sensitivity of materials

4. Foaming or frothing

5. Pressure and temperature

6. Scale deposition and materials

7. of construction

1. Single effect evaporators

2. Forward feed multiple effect evaporators

3. Backward feed multiple effects evaporators

4. Parallel feed multiple effect evaporators

1. Open kettle or pan

2. Horizontal-tube natural circulation evaporator

3. Vertical-type natural circulation evaporator

4. Long-tube vertical-type evaporator

5. Falling-film type evaporator 6. Forced-circulation-type evaporator

7. Agitated-film evaporator

8. Open-pan solar evaporator

5. Calculation of single

& multiple effect

3

Introduction Evaporation is achieved by adding heat to the solution to vaporize

the solvent.

Vapor (usually water) from a boiling liquid solution is removed and a more concentrated solution remains.

Heat is provided by the condensation of a vapor (such as steam) on one side of a metal surface with the evaporating liquid on the other side

The normal heating medium is low pressure exhaust steam from turbines, special heat transfer fluids or flue gases.

Example: concentration of aqueous solutions of sugar, sodium chloride, glue, milk and orange juice.

In some case, the purpose of evaporation is to concentrate the solution so that upon cooling, salt crystal will be formed and separate

25/02/2014 Siti Noraishah Ismail 4

Basic Operation of Evaporator The typical evaporator is made up of three functional

sections: the heat exchanger, the evaporating section, where the liquid boils and evaporates, and the separator in which the vapour leaves the liquid and passes off to the condenser or to other equipment.

In many evaporators, all three sections are contained in a single vertical cylinder.

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

At the top of the cylinder, there are baffles, which allow the vapours to escape but check liquid droplets that may accompany the vapours from the liquid surface.

In the heat exchanger section, called a calandria in this type of evaporator, steam condenses in the outer jacket and the liquid being evaporated boils on the inside of the tubes and in the space above the upper tube plate.

The resistance to heat flow is imposed by the steam and liquid film coefficients and by the material of the tube walls.

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Basic Operation of Evaporator

The circulation of the liquid greatly affects evaporation rates, but circulation rates and patterns are very difficult to predict in any detail.

With dissolved solids in increasing quantities as evaporation proceeds leading to increased viscosity and poorer circulation, heat transfer coefficients in practice may be much lower than this.

As evaporation proceeds, the remaining liquors become more concentrated and because of this the boiling temperatures rise. The rise in the 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, with increasing solute concentration, the viscosity of the liquid will increase, often quite substantially, and this affects circulation and the heat transfer coefficients leading again to lower rates of boiling.

Yet another complication is that measured, overall, heat transfer coefficients have been found to vary with the actual temperature drop, so that the design of an evaporator on theoretical grounds is inevitably subject to wide margins of uncertainty.

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Processing Factor in Evaporation

7


2. Solubility


4. Foaming or frothing


6. Scale deposition and materials of

construction

8


Usually liquid feed to evaporation is relatively dilute and has a lower viscosity and higher heat transfer coefficient, h

As evaporation proceeds, the solution become more concentrate and high viscosity, then will drop the heat transfer coefficient value.

Therefore, adequate circulation and turbulence must be present to keep the h value becoming too low.

2. Solubility

As solutions are heated, the concentration of solute increase and solubility is decrease and can be exceed the solubility limit of the solution, then the crystal formed.

Solubility is increase as temperature increase. This means when hot concentrated solution from evaporation is cooled to room temperature, crystallization may occur.


Many food products or biological materials may be temperature sensitive and degrade at higher temperatures or after prolonged heating.

Must be considered in the operation of evaporation.

Processing Factor in Evaporation

9

Processing Factor in Evaporation 4. Foaming and frothing

Caustic solutions, some food solutions such as milk, some fatty acid solutions form foam/froth during boiling.

This foam will losses from the solution by the vapor comes out from the evaporation.


Higher operating pressure, higher boiling temperature of the solution

As concentration of the solution increased by evaporation, the temperature of boiling may rise- called boiling point rise (BPR)

To keep the temperatures low in heat sensitive materials, it is often necessary to operate under 1 atm (i.e under vacuum)

6. Scale deposition and materials of construction

Some solid material can be deposit on the heating surface of the evaporation, this will reduce the overall heat transfer coefficient and cleaning is necessary.

Material for construction of evaporation must be minimize corrosion phenomena.

Rate of Evaporation The basic factors that affect the rate of evaporation are the:

rate at which heat can be transferred to the liquid

quantity of heat required to evaporate each kg of water

maximum allowable temperature of the liquid

pressure at which the evaporation takes place

changes that may occur in the foodstuff during the course of the evaporation process.

Important practical considerations in evaporators are the:

maximum allowable temperature, which may be substantially below 100C.

promotion of circulation of the liquid across the heat transfer surfaces, to attain reasonably high heat transfer coefficients and to prevent any local

overheating,

viscosity of the fluid which will often increase substantially as the concentration of the dissolved materials increases,

tendency to foam which makes separation of liquid and vapour difficult.

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Type of Evaporator

Open kettle or pan

Horizontal-tube natural circulation evaporator

Vertical-type natural circulation evaporator

Long-tube vertical-type evaporator

Falling-film type evaporator

Forced-circulation-type evaporator

Agitated-film evaporator

Open-pan solar evaporator

11

Open Kettle/Pan Evaporator

heat is supplied by

condensation od steam in a

jacket or in coils immersed in

the liquid

in some cases, kettle is direct

fired

inexpensive and simple to use

heat economy is poor

in some cases, paddles or

scrapers are used for agitation

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9/ 12/ 12 12:07 PM

Page 1 of 1f ile:/ / / Users/ smsaufi/ Documents/ 00%20Sugay%20Sync/ Akademik/ 2012- 2013- I/ Unit%20Operation/ Evaporator2.swf

http://rpaulsingh.com/animated%20figures/fig8_4.htm

Horizontal Tube Natural Circulation Evaporator

The horizontal bundle of heating tubes

similar to heat exchanger is used

The steam enters the tubes, where it

condenses, leaves at the other end of the

tubes.

The boiling liquid solution covers the

tubes.

The vapor leaves the liquid surface, often

goes through some de-entraining device

such as baffle to prevent carryover of

liquid droplets, and leaves out the top.

Relatively cheap, used for non-viscous

liquids with high heat-transfer coefficient

and liquid that do not deposit scale.

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Vertical Type Natural Circulation Evaporator

The liquid is inside the tubes and

the steam condenses outside the

tubes

Because of boiling and decreases

in density, the liquid rises in the

tubes by natural circulation, and

flows downward through a large,

central open space or

downcomer.

Often called as short-tube

evaporator

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Long Tube Vertical Type Evaporator

The tubes are 3 to 10 m long and the formation of vapor bubbles inside the tubes causes a pumping action, which gives quite high liquid velocities

Liquid passes through the tubes only once and is not recirculates. Contact time can be quite low in this type of evaporator.

In some cases, as when the ratio of feed to evaporation rate is low, recirculation is made by adding large pipe connection between the outlet concentrate line and the feed line

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Falling Film Type Evaporator Liquid is fed to the top of the tubes and flows down the walls as thin film

V-L separation take place at the bottom

widely used for concentrating heat sensitive materials such as fruit juices

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http://rpaulsingh.com/animated%20figures/fig8_7.htm http://www.niroinc.com/evaporators_crystallizers/falling_film_ev

aporators.asp

Forced Circulation Type Evaporator

Used pump to circulate the liquid

Increase liquid-film heat transfer

Use for viscous liquids

17

http://www.niroinc.com/evaporators_crystallizers/forced_circulation_e

vaporator.asp

Agitated Film Evaporator Mechanical agitation of liquid

film to increase turbulence in this film, and hence the heat transfer coefficient

Modification of falling film evaporator with only a single , large, jacketed tube containing an internal agitator.

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

The concentrated solution leaves at the bottom and vapor leaves through a separator and out the top.

18

http://www.technoforce.net/agitated-thin-

film-evaporators.html

http://distilleryplants.tradeindia.com/agitated-thin-

film-evaporator-355261.html

Method of Operation of Evaporators

Single effect evaporators

Forward feed multiple effect

evaporators

Backward feed multiple effects

evaporators

Parallel feed multiple effect evaporators

19

1. Single Effect Evaporators

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Single Effect Evaporators The solution in the evaporator is assumed to be completely mixed, the

concentrated product and the solution in evaporator have the same

composition and temperature T1, which is the boiling point of solution at P1.

The temperature of the vapor is also at T1, since it is equilibrium with the

boiling solution.

The pressure is P1, which is the vapor pressure of the solution at T1.

Often used when the required capacity of operation is relatively small and

the cost of steam is relatively cheap compared to the evaporator cost

However, energy utilization is poor since the latent heat of the vapor leaving

is not used but is discarded.

23

Feed, F TF , xF , hF.

Steam, S TS , HS

Concentrated liquid, L

T1 , xL , hL

Condensate, S TS , hS

Vapor, V T1 , yV , HV

P1

T1

heat-exchanger tubes

to condenser

The rate of heat transfer (q : W, btu/h)

U : overall heat transfer coefficient, W/m2.K; btu/h.ft2.F

A : heat transfer area, m2; ft2

Ts, T1 : in K; F

Ts is temperature of condensing steam

q =UADT =UA(Ts -T1)

2. Forward Feed Multiple Effect Evaporators

The fresh feed is added to the first effect and flows to the next in the same

direction as the vapor flow.

Used when the feed hot or when the final concentrated product might be

damaged at high temperature.

At steady-state operation, the flow rates and the rate of evaporation in each

effect are constant.

The boiling temperature decrease from effect to effect, cause pressure also

decrease (e.g. if first evap is at 1 atm the last evap. will be under vacuum).

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steam, TS

feed, TF

concentrate from first

effect.

vapor T1

(1)

T1

(2)

T2

(3)

T3

concentrate from second

effect.

concentrated product

condensate

vapor T2 vapor T3

to vacuum condenser

1 kg of steam will evaporate 1 kg of

water in each evaporation

The 1st evap. operates at a T high

enough that the evaporated water

serves as the heating medium to the

2nd evap.

Very rough estimation, 3kg water will

be evaporated for 1 kg steam

Steam economy (kg vapor

evaporated/kh steam used) is

increased

3. Backward Feed Multiple Effect Evaporators

Fresh feed enters the last and coldest effect and continues on until the concentrated product leaves the first effect.

Advantageous when the fresh feed is cold or when concentrated product is highly viscous.

Liquid pump are used in each effects, since the flow is from low to high pressure.

The high temperature in the first effect reduce the viscosity and give reasonable heat-transfer coefficient.

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steam, TS

feed, TF

vapor T1

(1)

T1

(2)

T2

(3)

T3

concentrated product

condensate

vapor T2 vapor T3

to vacuum condenser

4. Parallel Feed Multiple Effect Evaporators

Involves the adding of fresh feed to each

effect and the withdraw of concentrated

product from each effect.

However, the vapor from each effect is still

used to heat the next effect

Mainly used when the feed is almost

saturated and solid crystal are the product, as

in the evaporation of brine to make salt

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Overall Heat Transfer Coefficients in Evaporator

Components contribute to the overall heat transfer coefficient , U in

evaporator

steam-side condensing coefficient can be predicted using Eqs 4.8-20 to 4.8-26.

metal wall resistance usually negligible due to high thermal conductivity of metal; increase velocity to decrease the rate of scale formation

resistance of the scale on the liquid side cannot be predicted

liquid film coefficient, h - usually inside the tube - can be predicted using various eq depend on type of tubes configuration/evaporator type

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Calculation Method for Single Effect Evaporator (additional notes)

31

MATERIAL BALANCE

Total mass balance

F = L +V

Balance on solute/solids

FxF = LxL

hF and hL often not available, enthalpy-

concentration data are available for

only few substance, some

approximation are made:

Using latent heat of evaporation of 1 kg water from from steam table

at solution boiling temperature, T1

Calculate using heat capacity, cpF and cpL if available

32

)(

evaporatorfor eqution design general Also

S=)h-S(H = q

evaporator thefer toHeat trans

)h-H=( steam ofheat latent is ;

steam condensedin Heat +in vapor Heat + liquid edconcentratin Heat = steamin Heat + feedin Heat

1

ss

ss

TTUATUAq

VHLhSFh

ShVHLhSHFh

S

vLF

svLsF

BALANCE ENERGY

Calculation Method for Single Effect Evaporator (additional notes)

Example 8.4-1 Heat-Transfer Area in Single-Effect Evaporator.

A continuous single-effect evaporator concentrates 9072 kg/h of a 1.0 wt % salt

solution entering at 311.0 K (37.8 C) to a final concentration of 1.5 wt %. The

vapor space of the evaporator is at 101.325 kPa (1.0 atm abs) and the steam

supplied is saturated at 143.3 kPa. The overall coefficient U = 1704 W/m2 .K.

calculate the amounts of vapor and liquid product and the heat-transfer area

required. Assumed that, since it its dilute, the solution has the same boiling point

as water.

33

Effect of Processing Variables on Evaporator

Operation.

Feed temperature, TF TF < Tbp, some of latent heat of steam will be used to

heat up the cold feed, only the rest of the latent heat of steam will be used to vaporize the feed.

feed is under pressure & TF > Tbp, additional vaporization obtained by flashing of feed.

Evaporator pressure, P1

desirable T [q = UA(TS T1)], A & cost . T1 depends on P1 - will P1 T1 then T (e.g under

vacuum) .

Steam pressure, PS

PS will TS but high-pressure steam is costly. Optimum TS by overall economic balances are need.

38

Boiling Point Rise & Heat of Solution

Majority cases, solutions in evaporator are not dilute, thus thermal properties of the solution being evaporated may differ considerably with water.

Dhrings rule a straight line of solution boiling point against water boiling point at the same pressure for a given concentration at different pressures

Heat of solution must be considered in heat balance for the substance that give a considerable temperature rise during dissolve in water.

39

Duhrings Plot (example)

40

Enthalpy-Concentration Chart (example)

41

Example 8.4-3 An evaporator is used to concentrate 4536 kg/h of a 20 % solution of NaOH in water entering at 60 C to a product of 50 % solid. The pressure of the saturated steam used is 172.4 kPa and the pressure in the vapor space of the evaporator is 11.7 kPa. The overall heat-transfer coefficient is 1560 W/m2.K.

Calculate:

1. steam used

2. steam economy in kg vaporized/kg steam used

3. heating surface area in m2

42

U = 1560 W/m2 T1 A = ?

P1 = 11.7 kPa

F = 4536 kg/h TF = 60 C xF = 0.2

hF.

S = ? TS , HS

PS = 172.4 kPa

L, T1 , hL xL = 0.5

S, TS , hS

V, T1, HV

Solution Example 8.4-3

Refer to Fig. 8.4-4, for flow diagram for this solution.

For the total balance, F = 4536 = L + V

For the balance on the solute alone, F xF = L xL

4536 (0.2) = L (0.5)

L = 1814 kg/h of liquid

Substituting into total balance and solving,

V = 2722 kg/h of vapor

43


44

Duhrings Plot

45


46


47


48

Calculation Method for Multiple Effect Evaporator

The calculation are done using material balance, heat balance and heat capacity equation (q=UAT) for each effect. Normally using trial and error method.

Objective to calculate

Area (A) in each effect

Amount of steam (S) need

Amount of vapor (V) leaving each effect

Usually given or known value

Steam pressure in first effect

Final pressure in the vapor space of last effect (P3)

First condition and flow to first effect (F, XF)

Final concentration of the liquid leaving on the last effect (X3)

Physical properties such as enthalpies or heat capacity of the liquid and vapor

Overall heat transfer coefficient on each effect, normally the value is same in each effect, U

49

(3)

U3

(2)

U2

S

PS1

T3 T1 T2 F

xF

TF

T1 , L1 , x1

V1 = F L1

(1)

U1

V2 = L1 L2 V3 = L2 L3

TS1 TS3 TS2

T2 , L2 , x2

T3

L3

x3

P3


Assumption made in operation;

no boiling point rise.

no heat of solution.

neglecting the sensible heat necessary to heat the feed to the boiling point.

Heat balances for multiple/triple-effect evaporator.

Heat is same in all effect: q = U1 A1 T1 = U2 A2 T2 = U3 A3 T3

Areas in all effects are equal,: q/A = U1 T1 = U2 T2 = U3 T3

The temperature drops in evaporator (no BPR),

T = T1 + T2 + T3 = TS T3

The temperature drops in evaporator (with BPR),

T = T1 + T2 + T3 = TS Tsat@P3 (BPR1+BPR2+BPR3)

hence we know that T are approximately inversely proportional to the values of U,

similar equations can be written for T2 and T3

if we assumed that the value of U is the same in each effect, the capacity equation,

q = U A (T1 + T2 + T3 ) = UA T

50

321

11

111

1

UUU

UTT


For the given x3 and P3 and find

BPR3 if exist

From an overall MB , determine VT = V1 + V2 + V3

(1st trial assumption V1=V2=V3)

Calculate the amount of concentrated solutions

(L1,L2,L3) & their concentrations (X1,X2,X3) in each effect using

MB

Find BPR & T in each effect & T.

If the feed is very cold, the portions may be modified

appropriately, calculate the boiling point in each effect.

Calculate V and L in each effect through MEB

If the amounts differ significantly from the assumed values in step 2;

step 2,3 and 4 must be repeated with the amounts just calculated.

Using heat transfer equations for each effect, calculate A required for each effect. Then calculate Am = (A1+A2+A3)/3. Repeat second trial if the area is not reasonably

close to each other

For second trial, using new value of L1,L2,L3, V1,V2,V3

and calculated solid concentration in each effect

Obtain new values T1= T1A1/Am, , T2, T3, then

determine new T for find new are as step 4.

51

52

Find T3, BPR3 and TS3

Assume V1=V2=V3

Calc. L1,L2,L3,X1,X2,X3 from MB

Compare A1,A2,A3 with Am

Calc. q1, q2, q3 and solve A1,A2,A3

Find Am

Compare V1,V2,V3from MB with V1,V2,V3 from EB

Find H1,H2,H3, s1,s2,s3

Find T1,T2,T3,Ts1,Ts2,Ts3

Calc. T, T1, T2, T3

Adjust for cold feed

Calc. BPR1, BPR2, BPR3

From EB, calc. new V1,V2,V3, L1,L2,L3,

STOP

>10%

>10%

Example 8.5-1 A triple-effect forward-feed evaporator is being used to evaporate a sugar solution

containing 10 wt% solids to a concentrated solution of 50 %. The boiling-point rise of the solutions (independent of pressure) can be estimated from (BPR C = 1.78x + 6.22 x2 ), where x is wt fraction of sugar in solution. Saturated steam at 205.5 kPa and 121.1C saturation temperature is being used. The pressure in the vapor space of the third effect is 13.4 kPa. The feed rate is 22 680 kg/h at 26.7 C. the heat capacity of the liquid solutions is cP = 4.19 2.35x kJ/kg.K. The heat of solution is considered to be negligible. The coefficients of heat transfer have been estimated as U1 = 3123, U2 = 1987, and U3 = 1136 W/m

2.K. If each effect has the same surface area, calculate the area, the steam rate used, and the steam economy.

53

(3)

U3=1136

(2)

U2=1987

S = ?

TS1 = 121.1 C

PS1 = 205.5 kPa

T3

T1 T2 F = 22680

xF = 0.1

TF = 26.7 C

T1 , L1 , x1

V1 = 22,680 L1

(1)

U1=3123

V2 = L1 L2 V3 = L2 - 4536

TS1 TS3 TS2

T2 , L2 , x2

T3

L3 = 4536

x3 = 0.5

P3 = 13.4 kPa

QUESTION & ANSWER SESSION

25/02/2014 54 Siti Noraishah Ismail

THANK YOU!!

chapter 2 evaporation.pdf

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