LDPE Technology

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Content

What is LDPE Application History Process theory Autoclave process Tubular process Process steps Process control Process safety Key equipment Investment cost Cost of production

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What is LDPE?

Low Density PolyEthylene

Typical properties

Density 0,915-0,935

Melt index 0,3->20 g/10 min (190 C/2,16 kg)

Melting point 120 C

Polydispersity 5,5 – 6 (TVK tubular grades)

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Application

LDPE end-use

64,12,8

5,4

10,8

0,1

9,5

7,2

film

blow moulding

injection moulding

extrusion coating

rotomoulding

other extrusion

others

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Application by Properties

0,1

1

10

100

0,915 0,92 0,925 0,93 0,935

EXTRUSION COATING

FILM EXTRUSION

INJECTION MOULDINGEXTRUSION COATING

BLOW MOULDING

MI, g

/10 m

in/1

90 C

Density, kg/dm3

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History

LDPE was discovered by ICI in 1933 → autoclave process in 1938

BASF developed the first tubular process during WW II High variation of autoclave and tubular processes by

different licensors Nowadays licences available up to 400 kt/y plant

capacity Consumption in 2012

Global: 19 million t Domestic: 60 thousand t

TVK LDPE plants 1970 - ICI autoclave process 24 kt/y – debottlenecked to 50 kt/y 1991 - BASF tubular process 60 kt/y

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Process Theory 1

Free radical reaction Initiation

I → R· + R·

Initiators: typically organic peroxides

Propagation R· + CH2CH2 → RCH2CH2·

R(CH2CH2)n-1CH2CH2· + CH2CH2 → R(CH2CH2)nCH2CH2·

Termination Combination

Rx· + Ry·→ Px+y

Disproportioning

Rx· + Ry·→ Px + Py

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Process Theory 2

Other reactions

Chain transfer – important to control molecular weight

Rx· + CH2CH2→Px + R1· by monomer

Rx· + M→Px + M· by modifier

Cracking – results in shorter chains

Rx·→Py + Rx-y·

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Process Theory 3

Branching – LDPE characterised by high degree of short and long chain branching

Short chain branching – responsible for density

Intramolecular chain transfer and copolymerisation result in short chain branches

Long chain branching

Intermolecular chain transfers give long chain branches

C

C

H

C C

C

C C C C

C C C

H

C C C

C C C

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LDPE - Autoclave Process

Initiators

(Peroxides)

Autoclave

reactor

Secondary

compressor

Primary

compressor

280 bar

250 C

HP separator

Wax

separator

0,5 bar

250 C

LP separator

Extruder

Ethylene

Oil

separator

250 bar

1400 bar

170 C

240 C

280 C

Bagging

Bulk loading

Degassing

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Autoclave Reactor

Autoclave reactor MK 10

L = 4750 mm d = 18”

Thermocouple 1

(1st T control)

Thermocouple 2

Thermocouple 4

(2nd T control)

Thermocouple 5

Thermocouple 6

Thermocouple 7

(control 2nd zone T)

Thermocouple 8

Thermocouple 9

(control 3rd zone T)

Thermocouple 10

(control 4th zone T)

Thermocouple 3

Top Zone

2 nd Zone

3 rd Zone

4 th Zone

Total 725 l

ethylene

peroxides

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LDPE - Tubular Process

Modifier

Fresh Ethylene

Purge

Initiator

(Oxygen)

Primary

Compressor

Secondary

Compressor

Preheater

Precooler

Reaction

Zone I

Reaction

Zone II

HP Separator

LP Separator

Extruder

HP RecycleLP Recycle

280 bar

250 C

0,5 bar

250 C

2400-2900 bar

180-310 C

250 bar

Bagging

Bulk loading

Degassing

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Autoclave vs. Tubular Process

Autoclave Conversion up to 21% -

adiabatic, reaction heat removed by reactant only

1300-2000 bar operating pressure

Higher capacity of hyper compressor

Organic peroxide initiators only Specialty polymer capability –

EVA copolymers over 40% vinyl acetate

Lower reactor capacity – 150 kt/y

Tubular Conversion up to 36% -

reaction heat partly removed by coolant

2500-3200 bar operating pressure

Lower capacity of hyper but higher load

Cheaper oxygen initiator possible

Film grades with higher clarity, EVA up to 10% vinyl acetate

Reactor capacity up to 400 kt/y

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Process Steps 1

Compression Increase make up ethylene

pressure to reaction pressure

Recycle unreacted ethylene Inject modifier and

comonomer

Reaction Injection of initiators Control temperatures for

required product properties One phase reaction – two

phase only at lower pressure and temperature

Short residence time – some minutes

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Process Steps 2

HP separator Separate melt LDPE and

unreacted ethylene

LP separator Remove unreacted ethylene

Feed tank for extruder

HP recycle Remove low molecular weight

polymer (wax)

Cool down recycle ethylene

LP recycle Remove low molecular weight

polymer (wax, oils)

Cool down recycle ethylene

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Process Steps 3

Extrusion

Homogenization

Additive dosing

Pelletizing

Degassing

Remove residual ethylene (generally less than 1000 ppm) from LDPE – to avoid explosive gas mixtures

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Melt index (Molecular weight) P↑ MI↓ (Mw↑)

T↑ MI↑ (Mw↓)

[Modifier]↑ MI↑ (Mw↓)

Density P↑ D↑

T↑ D↓

Pressure range: 1300-3000 bar

Temperature range: 160-310 C

Process Control

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Risk of decomposition Decomposition: ethylene or polyethylene decomps to C and H2

High temperature, high pressure, contaminations favour decomposition

Reactor and HP separator in confined area – generally behind concrete wall

Interlock system to avoid decomposition and

protect equipment

Special metal gaskets at high pressures

Gas detectors

Fire fighting system

Process Safety

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Key Equipment

Compressors

Primary compressor (Booster+Primary)

5 stages

0,1 bar suction; 250 bar discharge

Secondary compressor (Hyper)

Two stages

250 bar suction; 3000 bar discharge

Extruder Hot melt extruder for homogenization

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Hyper Compressor Arrangement

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Hyper Compressor Cylinder

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Extruder Arrangement

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Single-screw Extruder

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Investment cost Basis: WE 2010 Q1

Process Tubular Autoclave

Capacity, thousand ton/year 300 100

Capital cost, € million

Battery Limits (ISBL): 98 55

Offsites (OSBL): 49 28

Total Fixed Investment 146 83

Specific Investmant Cost, €/ton 488 830

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Cost of Production Basis: WE 2010 Q1

Process Autoclave Tubular

Capacity 100 kt 300 kt

Ethylene 939,8 925,1

Comonomer

Catalysts&chemicals 23 12,5

Total raw materials 962,8 937,6

Power 63,8 56,6

Other utilities 22,4 7,4

Steam credit -9,9

Total utilities 86,2 54,1

Direct fix costs 48,8 18,8

Allocated fix costs 33,7 15,1

Total fixed costs 82,5 33,9

Total cash costs 1131,5 1025,6

EUR/t

30%

40%

50%

60%

70%

80%

90%

100%

Autoclave Tubular

Total f ixed costs

Total utilities

Total raw materials

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Appendix: Metal Gasket

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Appendix: Hyper compressor cylinder

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Appendix: Blown Film Extrusion

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Appendix: Extrusion Coating