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Page 1: 03 Cryogenic Separation

1. General points2. Theoretical principles3. Cryogenic separation

1. The distillation flask2. Liquid / vapor equilibrium of a mixture3. Principle of fractional distillation4. Single column model5. Double column model6. Operating conditions

4. Heat Exchange5. Air Purification6. Cold production7. Air and products compression8. Air separation unit process control9. Argon, Xr&Xe, He&Ne extraction

Air Distillation

Hamilton Sept. 2009

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

COMPRESSION

AIR

OXYGEN

NITROGEN

PURIFICATION

COLDPRODUCTION

(option 1)

DISTILLATIONCRYOGENIC

NON-CRYOGENIC

HEAT EXCHANGE

WASTE

COLDPRODUCTION

(option 2)

Distillation in the air separation process

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

The distillation flask

Mixture

water + alcohol

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

The distillation flask

Cooler

water + alcohol

vapor phase

Liquid phase

Distillate

Heater

vapor richerin alcohol

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Partial pressure definition

The partial pressure of a component in a gas mixture is the pressure this component would have if it alone occupied the same volume at the same temperature as the mixture.

P = P «   » + P «   » P = P «   »

In case of red

component alone

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Equilibrium - Partial pressure

0.79bar1bar10079PN2

0.21bar1bar10021PO2

Mixture

Partial pressures:

Total P = 1 bara

N2 = 79 %

O2 = 21 %

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Liquid-Vapor Equilibrium

Composition of the liquid

79% N2 and 21% O2 Temperature: - 183 °C

What are the pressure and the composition of the vapor phase?

Liquid

Vapor

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Determining of the vapor composition At T = -183 °C:

P0O2 = 1 bar abs => PO2 = 0.21 x 1 = 0.21 bar

P0N2 = 3.6 bar abs => PN2 = 0.79 x 3.6 = 2.84 bar

P = PO2 + PN2 = 0.21 + 2.84 = 3.05 bar

Dalton’s law : [O2]vapor = 0.21 / 3.05 = 7 % [N2]vapor = 2.84 / 3.05 = 93 %

Liquid-Vapor Equilibrium: Raoult law

Liquid

Vapor

7% O2

93% N2

21% O2

79% N2

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Equilibrium ratio of a component

K (most volatile component) > 1and

K (less volatile component) < 1

component content in the vapor phaseK = component content in the liquid phase

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Phase diagram of a mixtureO2-N2 diagram under constant pressure

YXYZLiq%

YXZXVap%

% O2

100 %XZ0 % Y

PPV

Q2V

Q2LQ2

Q1

Q3

Q41

Q0T

PL[L] [V]

L

V2

Appearance of the the first gas bubble in the liquid“boiling point curve”

Appearance of the first liquid drop in the vapor“dew point curve”

Q3V

Q3L

Q1V Q1L

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Phase diagram of a mixture O2-N2 diagram under constant pressure

Effect of pressure:

P2 > P1

% O2100 %

T

0 %

P2

P1

Page 12: 03 Cryogenic Separation

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Phase diagram of a mixtureO2-N2 diagram at constant temperature

YXYZLiq%

YXZXVap%

% O2100 %

P

PV

Q2V

Q2L

Q2

Q3

Q1

Q0

1

Q4

P

0 %

PL2 [L]

[V]

X Z Y

L

V

Oxygen content ofthe first liquid drop

Oxygen content ofthe last gas bubble

Q1V

Q1L

Q3V

Q3L

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Phase diagram of a mixtureO2-N2 diagram at constant temperature

Effect of temperature:

T2 > T1

100 %0 % % O2

P

T2

T1

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

1 bar abs

P bars abs

%O2

T °C

5 bar abs

Liquid / vapor equilibrium curve of pure O2

Liquid / vapor

equilibrium curve of pure N2

Phase diagram of a mixture

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Exercise 1

We consider an exchanger made of a pipe fed by a liquid nitrogen flow under 1 bar abs.

This pipe is going through a jacket fed by de-carbonated and dry air maintained at a constant pressure of 1 bar abs.

• What happens to the air ?

De-carbonatedand dry air

Liquid nitrogen

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

First stacking model

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

First stacking model ISOBARIC SYSTEM :

P1= P2= P3 = P4

TEMPERATURE GRADIENT : T1 > T2 > T3 > T4

P1,T1

P2,T2

P3,T3

P4,T4

TEMPERATURE INCREASES

TEMPERATURE DECREASESEnrichment by morevolatile constituant

Enrichment by lessvolatile constituant

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Second stacking model

P1,T1

P2,T2

P3,T3

P4,T4

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Third stacking model

Condenser

Boiler

Page 20: 03 Cryogenic Separation

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Distillation column

Most volatile element

Less volatile element

CONDENSER

Trays

BOILER

Enric

hmen

t in

Lig

ht

Enric

hmen

t in

Hea

vy

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Distillation tray

vapor V1

vapor V0

vapor V2

liquid L2

liquid L1

liquid L0

Liquid – vaporcontact

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Distillation packing

NITROGEN

OXYGEN

PACKING

Continuous liquid – vapor contact

VAPO

R R

ICH

ER A

ND

RIC

HER

IN N

ITR

OG

EN

LIQ

UID

RIC

HER

AN

D R

ICH

ER

IN O

XYG

EN

Page 23: 03 Cryogenic Separation

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Total reflux column

V

L

Total condenser

Total boiler

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Boiler / condenser column introduction

The more volatile product (N2)

The less volatile product(O2)

Mixture to be treated (O2+N2)

Condenser

Boiler

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Single boiler column

liquide

VAPOR

LIQUID

LIQUID

Mixture to be treated (O2+N2)

Residual product (O2+N2)

Boiler

The less volatile product (O2)

Page 26: 03 Cryogenic Separation

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Single condenser column

VAPOR

LIQUID

LIQUID

Mixture to be treated (O2+N2)

Residual product (O2+N2)

Condenser

The more volatile product (N2)

Page 27: 03 Cryogenic Separation

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Double column model

GOX

LOX

GAN

LIN

Heat exchangeLOX

Waste

Waste

AIR

LIN

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Double column model - Vaporizer

EXCHANGER

GAN

LIN

Upper column

Lower column

LOX

GOX

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Double column – Vaporizer-CondenserP (b abs)

T (°C)

1.0

N2

O2

-196 -183 -179 -177

T= 2°C

1.4

3.5

5.6

Page 30: 03 Cryogenic Separation

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Non-condensable gases

GAN

LIN

T= 2°C

Non condensable gases

GOX

LP column

HP column

-179°C1.4b

-177°C5.6b

Page 31: 03 Cryogenic Separation

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Double column model

LOX

GAN LIN

LP column

HP column

vaporizer-condenser

GOX

Liquid rich in O2

Waste (N2+O2)

Blow-off of theNon-condensable

Gaseous AIR

O2 content ~ 15 %

O2 content ~ 40 %

Page 32: 03 Cryogenic Separation

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Second configuration

LOX

GAN LIN

LP column

HP column

vaporizer-condenser

GOX

Rich Liquid

Lean Liquid

Waste: Impure nitrogen

Gaseous AIR

O2 content < 0.5 %

O2 content ~ 1 %

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Liquid / Vapor ratio

Liquid / Vapor ratio =

Liquid flow (Nm3/h)

Vapor flow V (Nm3/h)

Liquid flow L

Vapor flow V

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Effect of the variation of reflux ratio

Initialstate

Firstmodified

case

Secondmodified

case

L flowNm3/h

1000 1000 1000

V flowNm3/h

1000 1100 900

L / V 1 0.91 1.11

BottomV purity

%O221 21 21

TopV purity

%O210.5 > 10.5 < 10.5

V L

V L

10.5% O2

21% O2

(purities written are only given as an example)

Initial purities

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

L / V

Top product

N2 purity (V)

Bottom product

O2 purity (L)

Effect of the variation of reflux ratio

V L

V L

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Double column model - with top hat

LIN

LOX

HP GAN

LP column

HP column

GOX

Rich liquid

Lower

Impure nitrogen

LP GAN

Top hat

Upperlean liquid

lean liquid

Gaseous AIR

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Material balance

Oxygen balance:

Σ(outlets)Σ(inlets)

2WasteFlow2LOXFlow2GOXFlow2AirFlow OOOOWASTELOXGOXAIR

Total balance:

WNFlowGANFlowLINFlowLOXFlowGOXFlowAirFlow

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Material balance

GOXQ

LOXQ

AIRQ

With: = Gaseous oxygen production flow

= Liquid oxygen production flow

= Air feed flow

0.2096 = Mole fraction of oxygen in air

O2η = Oxygen extraction ratio

AIR

LOXGOXO2 0.2096Q

QQ100η

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Material balance

WNFlow

NFlowAIRFlow0.782

WNFlowOFlowAIRFlow0.21

2

NFlowOFlowAirFlowWNFlow

2

waste

2

waste

22

N

O

Page 40: 03 Cryogenic Separation

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Cold balance

Liquid production(Cold output)

Cold production(Cold input)

Cold losses(Delta T exchangers,radiation losses…)

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Losses of cold: Losses by radiation

Losses by the gap at the warm end of the main exchanger

Losses by liquid production

Cold balance

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Cold balance

Cold production or cold inputs: Expansion through a turbine

Liquid assist

Free gas expansion

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Pressure chart

vapo ΔT

Air HP

waste

Atm P

top LP P

vapo P

cond P

purification ΔP

exchanger ΔP

LP column ΔP

O2 bubble T

N2 dew T

HP column

ΔP

bottom HP P

compressor P

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

P out / P in

Q air

Pressure drops

Compressor curves

Cold Box operating curves

Pressure chart

ANTI S

URGE

SURGE

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Pressure chart

1.013 abs bar

1.463 abs bar

1.513 abs bar

6.6 abs bar 150 mbars

300 mbars

50 mbars

-178 °C

-175 °C3°C

100 mbars

6.7 abs bar

7 abs bar 300 mbars

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Adjusting the L/V ratio 600

LOX

GAN LIN

GOX

RL

LL

Impure nitrogen

1000

L/V = 0.450

L/V = 0.800

L/V = 1.333

L/V = 0.583

450

200

0

800

200

600 350

600 800

1000 800

1000 450

450

350

0

350

Gaseous AIR

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Adjusting the L/V ratio600

LOX

GAN LIN

GOX

RL

LL

Impure nitrogen

1000

L/V = 0.500

L/V = 0.800

L/V = 1.333

L/V = 0.500

500

200

0

800

200

600 300

600 800

1000 800

1000 500

500

300

0

300

Gaseous AIR

Increase in the oxygen contained

Drop of reflux ratio

Rise of reflux ratio

Decrease in the oxygen contained

Decrease in lean liquid

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Adjusting the L/V ratio

LOX

GAN LIN

GOX

RL

LL

Impure nitrogen

800

L/V = 0.450

L/V = 0.800

L/V = 1.333

L/V = 0.583

360

160

0

640

160

480 280

480 640

800 640

800 360

480

360

280

0

280

Gaseous AIR

Conservation of the rates of reflux

Decrease in air flow

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Liquid draw off

LOX

GaN LiN

GOX

RL

LL

Impure nitrogen

1000

L/V = 0.450

L/V = 0.800

L/V = 1.333

L/V = 0.583

450

200

200

800

0

600 350

600 800

1000 800

1000 450

600

450

350

0

350

Gaseous AIR

Liquid falling into the vaporizer = 800 Nm3/h

Liquid vaporized + drawn off = 1000

Nm3/h

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Liquid draw off

LOX

GN

LIN

GOX

RL

LL

Impure nitrogen

1000

L/V = 0.450

L/V = 0.800

L/V = 1.333

L/V = 0.583

450

400

200

600

0

600 350

600 800

1000 800

1000 450

600

450

350

0

350

Gaseous AIR

Liquefier

200

LIN

200

450

LOX

GN LIN

GOX

RL

LL

800L/V = 0.312

L/V = 0.750

200

200

600

0

800 600

800 250

0

350

Gaseous AIR200

Liquid AIR

Impure nitrogen

L/V = 1.333

L/V = 0.583 600 350

600 800

450

350

800 250

600

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Content profile

LIN

GOX

RL

LL

Impure nitrogen

Gaseous AIR

Content of the LL

%O2

Decrease of the GOX draw off

Increase of the GOX draw off

Content of the GOX

20 40 60 80 1000

Number of

tray

GAN

LP

RL

LOX

Content n of the GAN or tthe LIN

Content of the RL AIR

Content of the impure nitrogen

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Exercise

a HPN is fed by an air flow of 1000 Nm3/h at 6 bar a.

1. It produces 200 Nm3/h of pure nitrogen. What is the oxygen content of the rich liquid?

2. What is the maximum nitrogen production of such a plant? (assume the oxygen content of the liquid in equilibrium with the air at 6 bar is 40% O2)

GANAIR

RL

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Annular trays AT

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Weired trays WT

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Packing

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Packing manufacturing

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Liquid distributor

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Assembly of a column in workshop

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Air Separation Unit in Europe

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

ASU in South Africa

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Packing columns

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

O2/N2 concomitant phases diagram

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

McCabe - Thiele Diagram

1

1

0

Tray number : n

LV

Vn-1

VnLn+1

Ln

xn

yn

xn

ynY (fraction in vapor phase)

X (fraction in liquid phase)

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

McCabe - Thiele Diagram

Tray 1

Tray 2

Tray 3

Tray 4

Tray 5

V1

V2

V3

V4

V5

L2

L3

L4

L5

L6

x1 x2 x3

y3

1

1

0

y1

y2

Total condenser

Total boiler

y N2 fraction in vapor phase

x N2 fraction in liquid phase

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

P = constant

% O2

T (°C)

V

L + V

L

1000

O2-N2 Mixture phase diagram

Dew point curve

Boiling point curve

P = 1 atm-183

-196

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

O2-N2 Mixture phase diagram

% O2

T (°C)

V

zL5

100x

vL

L

y0

V1

P = constant

2

4

3

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

P = constant = P1

P = constant = P2

P2 > P1

% O2

T (°C)

1000

O2-N2 Mixture phase diagram

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68This document is the property of the AIR LIQUIDE Group and may not be communicated to any party without its formal authorization – This is an uncontrolled copy, the only controlled copy is kept in Alexandria data base

Hamilton Sept. 2009 Air Distillation – Cryogenic separation

O2-N2 Mixture phase diagram

% O2

P(bar)

L

L+V

V

1000

Dew point curve

Boiling point curve

T = constant

T = - 183 °C3.5

1.013

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

O2-N2 Mixture phase diagram

P (bar)

% O21000

L5

V1

V

z x

VL

L

y

T = constant

2

34

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

% O2

P (bar) T = constant = T1

T = constant = T2

T2 > T1

1000

O2-N2 Mixture phase diagram

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71This document is the property of the AIR LIQUIDE Group and may not be communicated to any party without its formal authorization – This is an uncontrolled copy, the only controlled copy is kept in Alexandria data base

Hamilton Sept. 2009 Air Distillation – Cryogenic separation

O2-N2 Mixture phase diagram

% O2

T°C

1000

Li+1

Li

Vi

Vi-1

yi-1 z xi+1 xiyi

V yi-1 + L xi+1

= V yi + L xi

= (V+L) z

P = constant

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72This document is the property of the AIR LIQUIDE Group and may not be communicated to any party without its formal authorization – This is an uncontrolled copy, the only controlled copy is kept in Alexandria data base

Hamilton Sept. 2009 Air Distillation – Cryogenic separation

O2-N2 Mixture phase diagram

yO2

xO2

1

1

Li+1

Li

Vi

Vi-1

V yi-1 + L xi+1

= V yi + L xi

= (V+L) z00

xi+1

yi-1

z

zyi

xi

operating straight line of - V / L slope

P = constant

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

O2-N2 Mixture phase diagram

Ln+1

Q

Vn

LiVi-1

Vaporizer

V = Q + L

V yi-1 = Q yn + L xi

yO2

xO2

1

1

00

xi

yi-1

z

yi

xi-1

operating straight line of V / L slope passing through

(yn,yn)

P = constant

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

Safety at Work – how not to do it

4th place

Page 75: 03 Cryogenic Separation

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Hamilton Sept. 2009 Air Distillation – Cryogenic separation

This concludes the Section on Distillation


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