Explosion Safety in EthoxylationReactors

M Braithwaite(1) & A Pekalski(3)

(1) Imperial College, London, UK

(2) Shell Global Solutions, Chester, UK

Ethoxylation Reactor Study (1997-2000)• many reactor types/ different bases of safety/

reactor conditions/ products

•relievers•containers

• Review of EO HazardsJ L Gustin, IChemE Hazards XV (April 2000)

this study solely concerned with reactor issues - ieexcludes EO/air interactions, toxicity, downstream EO handling,liquid phase EO chemistry, corrosion etcdecomposition flame in the vessel and not detonation or events external to the reactor is the prime concern

Combustion Characteristics of Ethylene Oxide

Molecular Weight 44.05Density (relative to air) 1.5Boiling Point 10.4 Deg C at 1 ataStoichiometric Concentration in air 7.72 % v/vHeat of Formation at 25 Deg C 52.7 kJ/ moleHeat of Combustion at 25 Deg C 1217 kJ/moleAutoignition Temperature 429 Deg C at 1 ataFlash Point Deg C < - 18 Deg CLower Explosion Limit 3.6 % v/vUpper Explosion Limit 100 % v/v

Basis of Safety in Ethoxylation Reactors

• containmentmaximum pressure governed by EO concentration. T, P

• inertinguse on inert (eg N2) to render system non-reactive

• relief + venting• emergency (via bursting disk)• controlled (via relief valve) - smaller capacity

• suppressionrapid release of powder/ inert gas

• avoidance of ignitionbest practice but not a B o S

Recent Plant Accidents involving Ethylene Oxide

Year Country Company Cause1987 Switzerland Sandoz External fire

1987 Belgium BP External hot spot

1987 Netherlands DOW Failure of heat exchanger

1989 Belgium BASF Hairline crack, EO distillation column

1989 Japan Asahi Denka EO leakage

1991 China Lu Shun Reactor malfunction

1991 USA Union Carbide Iron oxide impurities

Batch ethoxylators --- generic chemical engineering

v v v v v v v v v v

EO / PO FEED

SUBSTRATE+CATALYST FEED VENT TO HEADGEAR

DIFFUSIONAL MASS TRANSFER(DISSOLUTION OF GASES)

REACTION KINETICS

HEAT TRANSFER

PRODUCT

BULK MASS TRANSFER

MIXING

DIFFUSIONAL MASS TRANSFER(SOLUTION OF GASES)

VVV

Ethylene oxide reactionsTYPICAL RELATIVEHEAT OF REACTION MOLAR

CHANGEKCal/Mole

HYDROLYSIS 21.4 CONTRACTION

22.4 CONTRACTIONPOLYMERISATION

NO CHANGE28.1ISOMERISATION

31.0 EXP. 1.5 --->1.75DISPROPORTIONATION

32.7 EXP. 2.0 --->2.5DECOMPOSITION (FLAME)

EXP. 1.1COMBUSTION 291.2

This Study - one componentof complete engineering overview of

ethoxylation reactors (1996-2000)

• revisit thermodynamic (conservative) estimates of maximumoverpressures in EO/ EO+N2 mixtures

• assess literature information of gross decomposition rates• develop simple venting model based on above to evaluate :

gaps in current understandingneeds for more experimental datafeasibility of some envisaged engineering solutions

eg N2 dilutionuse of relief valves only in existing plantalternative diluents/ operating conditions

Published EO studies

Author/ Ref GAS

Initial Pressure bara

Initial Temperature Deg C

Scale of experiment (litres)

Flow conditions & Vessel type

R Friedman & E Burke EO 0.4 - 2.0 60 – 140 - Laminar flame J H Burgoyne & K E Bett et al

EO 0.3 - 9.3 20 – 100 2.44 Static/ cylinder

R K June and R F Dye EO/N2

1.7 – 6.5 60 – 190 2.00 Static/ cylinder

R Siwek & E Rosenberg EO 0.5 – 4.0 40 – 200 20 – 10(3) Various/ Various

T Ogawa, A Miyake & H Matsuo

EO/ N2/PO

1 – 5 20 – 180 0.765 Static/ Cylinder

M.Braithwaite, A.Pekalski & J Zevenbergen

EO/N2 4 100 20 Static/ turbulentsphere

15

20

25

30

35

40

45

50

55

20 40 60 80 100 120 140 160 180 200Initial temperature [C]

Pm

ax [

bara

]

100%EO

80%EO

60%EO

50%EO

Effect of initial temperature and mixture composition on the maximum equilibrium pressureSolid line model with soot, dashed line model without soot, Pini= 4 bara, Tini= 100 oCSTANJAN Chemical Equiibrium Code (Stanford University (Reynolds)

a• 20 liter explosion sphere• Pressure-time curve:

Pmax, (dP/dt)max

• Cube-root law:

K = (dP/dt)max * V1/3

= constant ?

0

5

10

15

20

25

30

35

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5Time [s]

P [b

ara]

Ignition 100% EO

80% EO

60% EO

Pressure-time history for EO-nitrogen mixtures of different initial composition; Pini= 4 bara, Tini = 100 ºC, IE= 180 J.

Ethylene Oxide: Maximum rate of pressure rise &

maximum pressures - from 373 K and 4 bara

CompositionEO/N2[EO%]

IgnitionEnergy

[J]

(dP/dt)max[bar/s]

Pmax[bara]

60 180 25.01 17.4680 180 36.80 25.65

100 0.72 41.98 31.83100 180 77.80 33.65100 540 169.6 36.38

Explosion severity data of pure EO, Pini= 4 bara, Tin = 100 oC, IE= 250J, V= 0.02 m3 [Bartknecht]

Mixture status

(dP/dt)max [bar / s] Kd [bar m / s]

Quiescent 96 26

Turbulent 1500 407

EO Decomposition Model -Decomposition Flame

Laminar Flame velocity (T,P) - experimental

Simple Flamelet ModelSeff = Su x (Re/Rec)ΠSu - laminar burning velocity at T,PRec, Π - adjustable constants

Large scale studies - KD - experimentalKD - reduced maximum rate of pressure rise

EO Decomposition Model - Assumptions

• ideal gas (EO, N2: EO gaseous decomposition products)• infinitessimal reaction film - 2 zone model• adiabatic process• mechanical equilibrium throughout• products in chemical equilibrium• flame velocity simulated by simple expression• spherical flame• vented gas either reactant or product - not mixed• no additional turbulence generated by venting process

Typical Pressure-Time History for EO decomposition in a 5 m3 vessel: initial

conditions 373 K and 4 bara: confined and vented explosions

Time (secs)1 2 3

Pres

sure

(Bar

a)

0

10

20

30

40

50

Maximum Pressure in 5 m3 vented EO vessel: initial conditions - 373 K 4 bara

Ignition (1-base, 2-middle, 3-top)1 2 3

Pres

sure

(bar

a)

30

32

34

36

38

40

Vessel Aspect Ratio0.2 0.4 0.6 0.8 1.0

Pres

sure

(Bar

a)

31

32

33

34

No of Relief Valves0 1 2 3

Pres

sure

(Bar

a)101520253035404550

EO Analysis (pre-SAFEKINEX) Conclusions

• limited rate data• simple turbulence model• 20 litre explosion data best available

Conservative design for explosion relief

(Containment preferred anyway for EO)