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Vaporization theory

Tomas Kovacs 021027

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Vaporizors

Boiler

Reboiler

Steam generator

Ref. Evaporator

Evaporator

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Pool boiling at heated wire

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Pool Boiling

ABNatural Convection

BCSubcool Boiling

CDSaturated Nucleate Boiling

DEPartial Film Boiling

EFStable Film Boiling

FGFilm Boiling with Radiation

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Pool boiling reboilers

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Film boiling !

Could occur in S&T at high T

Could occur in SHE at high T

Has never been seen in PHE or CP

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Convective

boiling

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Vaporizing profile

1 2 3 4 5

K 471 437 437 437 437

1 14186 14350 14282 13597 12971

2 494 458 458 459 459

wall/ 28371 31570 28564 28427 31130

A 1.00 0.63 0.63 0.64 0.64

6 7 8 9 10 11

5126 5378 4557 4132 2950 2622

12586 12140 11655 11129 10634 10064

12248 14376 10031 8443 4742 4028

29435 29415 29455 29677 29400 29594

0.05 0.05 0.06 0.07 0.08 0.07

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Two-phase boiling heat transfer

correlations

tot = sp+ nb+ con

totoverall heat transfer coefficient

spsingle phase heat transfer coefficient

nbnucleate boiling heat transfer coefficient

contwo-phase convective heat transfer coefficient

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Convective versus nucleate

Plate 70-80% convective 15-20% nucleate

RF S&T 50-60% convective 30-40% nucleateFF S&T 100% convective

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Two-phase pressure drop calculations

Homogeneous model

Equal vapor and liquid velocities

Ordinary model

Different velocities for the vapor and liquid phases

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Pressure Drop

Frictional

Gravitational (Static head)

Accelerational

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Boiling point elevation (BPE)

orBoiling point rise (BPR)

BPR = BP for solution - BP for solvent

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What is a Reboiler?

A vaporiser at the bottom of a distillationcolumn, stripping column or reactor. Its main

purpose to supply heat (vapour) to the column.

Normally heated by steam.

A reboiler is normally controlled by a control

valve on the steam inlet. However AL prefersliquid level control on Compabloc reboilers

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S&T Reboilers

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Compabloc reboiler

Vapour/liquid mix

Liquid

Steam

Condensate

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Compabloc used as

thermosiphonereboiler

Condenser

Reboiler

Distillation

column

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AlfaVap reboiler

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What is Evaporation?

Removal of a solvent by heating a diluted solutionin order to increase concentration of dissolved

matter

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Applications

Sugar, starch, glucose, fructose, dextrose, sorbitol

NaOH in Chlorine-Alkali, Alumina, textile, pharma

CaCl2 in Soda Ash, HCl neutralization

Soap

Polyols e.g. Ethylene glycol, Propanediol, Glycerine

Waste streams in all industries

Steam generators in all industries

Other inorganic e.g. KOH, MgCl2, FeCl3, NH4SO4, NH4Cl, pigments,

electrolyte baths, spent acids Other organic e.g. Protein, caprolactam...

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The AlfaVap

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Heat transfer comparison

0 10 20 30 40 50 60 70 80 90 100

5000

4000

3000

2000

1000

0

Roberts

AlfaVap

Falling film

K[W/(m2K)]

Weight % sugar

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MVR

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TVR

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Multi Effect Evaporation (MEE)

1

Feed

TS

PS

V0

V0

2

VN

VN-1

V1

V1

3

V2

N

V2

V3T1

P1

T2

P2

T3

P3

TN

PN

T1+

1T

2+

2T

3+

3T

N+

N

Fresh

steam

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Optimum effects?

Energy cost

Investment cost

Total cost

Cost

Number of effects

Optimum

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Counter-current product flow

1

JN, in

V1, in

2

J1, o

VN, o

3 N

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Mixed product flow

1

J3, in

V1, in

2

J2, o

VN, o

3 N

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Controlling an evaporation system

Steam pressure

Condenser pressure

Sucking in air before vacuum pump

Vacuum pump frequency control

Cooling water flowrate control

Feed flowrate

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If no inerts specified - add 0.5% air to simulate vent control

Cyclone

Separator

Cooling water

at full flowCondenser

PI

Vacuum

pump

LC

Air from

atmosphere

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Considerations for Evap.

Solution Physical Properties

BPEs

Temperature sensitivity

Fouling & Scaling

Corrosion on plate and gasket

As much info as possible on existing S&T