Thermal Hazard Testing of Processes

Thermal Hazard Testing of Processes

Prof. Attilio Citterio

Dipartimento CMIC “Giulio Natta”

http://iscamap.chem.polimi.it/citterio/dottorato//

PhD

IN INDUSTRIAL CHEMISTRY AND CHEMICAL ENGINEERING (CII)

Causes of Accidents

In Chemical Sector approximately:

• 25 % lack of chemical knowledge

• 25 % lack of heat transfer capacity

• 25 % other design faults

• 25 % manual operating errors

44

22

12 11

5 5

1

Accidents (%)

Process Design

Nitration 33 Polymerisation phenolic resins 12 non-phenolic resins 16 other than resins 13

Sulfonation 12

Hydrogenation 7

Chlorination 6

Hydrolysis 6

Condensation 4

Oxidation 4

Alcoholysis 3

Amination 3

Cyclization 3

Bromination 3

Diazotization 2

Esterification 2

Isomerization 2

Methylation 2

Electrolysis 1

P F Nolan and J A Barton, J. Hazardous Materials, 1987, 14, 233

Analysis by Type of Process

BASF, Oppau/Ludwigshafen, September 21, 1921

Crater: 80 m diam., 16 m deep 450 dead

AZF, Toulouse,

September 21, 2001 Crater: 50 m diam., 10 m deep

29 dead

Survey of Incidents in the Chemical Industry

1. Basic lack of knowledge of chemistry/thermochemistry 34 2. Mischarging - omission of reactant 2

- undercharging of reactant 7

- overcharging 14

- charging too quickly 7

- charging at wrong temperature 3 - not following precise instructions 5

TOTAL 38

3. Accumulation of reactants 7

4. Impurities - raw materia1 changes 9

- water 10

- other 14

TOTAL 33

5. Delays in processing 9 6. Inadequate cooling - due to incorrect scale-up 9

- due to faulty instrumentation,

supply 12

TOTAL 21

7. Agitation failure/stoppage - inadequate agitation 4

- breakdown 6

- failure to start agitator 10 - intentional stoppage 4

TOTAL 24

Survey of Incidents in the Chemical Industry

8. Inadequate temperature control

- due to method of operation 14 - due to instrumentation 8

TOTAL 22

9. Problems with valves, leaks and blockages 31 10. Opening wrong manhole or reactor 2 11. Failure of auxiliary equipment 10 12. Case histories with insufficient detail 32

GRAND TOTAL 24

Survey of Incidents in the Chemical Industry

Process Changes Which May Be Hazardous

 Quality of raw materials, solvents, intermediates

 Material of construction of vessel (heat transfer, metal contamination)

 Change of scale (agitation, mass transfer, cooling capacity)

 Vessel shape, size, configuration (agitation, heat transfer)

Temperature (°C)

Heat flow (mW)

0 100 200 300 400 500

0 10 20 30 40 50 60

Crude DMU

pure DMU

Cl

Cl NH

N O CH₃

OCH₃ 3-(3,4-dichlorophenyl)-1-methoxy- 1-methylurea

(weedkiller)

DSC Data for DMU

Temperature (°C)

Heat flow (mW)

0 100 200 300 400 500

0 10 20 30 40 50 60

Profile B (purity 94 % w/w) J. Thermal Anal., 37, 1991, 10

Profile A (purity 98 % w/w)

70 80

DSC Data for β-Lactam Precursor

Principles of Hazard Testing

 Identify hazards

 What if?

 Can hazards be controlled?

 Consequences of equipment failure

 Evaluate for thermal safety

- Raw materials and solvents - Intermediates and products - By-products

- Distillation residues

Effect of Changing Circumstances

 Order of addition of reagents

 Omitting one component

 Under- or overcharging reagents

 Incorrect pH

 Overheating the batch

 Extended time

Process Definition

 Define all parameters

- Temp, conc., rates of addition, times

 Define limits within which NO corrective action will be taken

- Re normal operating variables - e.g. temp 50°C +/- 5°C

 Define possible failure situations

 Safety test to each limit

Only then can safety assessment give assurance that the process is safe

HEAT

REMOVAL

dQ/dt

Rate of heat production r_QR = V ∆H_R(-r_A)

Rate of heat loss r_QW = UA (T-T_A)

A

C k

B

1

2

T_a T_az T_a1 T_C

Likelihood of loss of control increases with scale

Temperature gradients increase too

∝

∝

TOTAL HEAT GENERATED

TOTAL VOLUME

(AND CONCENTRATION) (i.e. CUBE DIAMETER)

SURFACE AREA

AVAILABLE FOR COOLING (i.e. SQUARE OF DIAMETER)

Thermal Hazard

Changes in Raw Material Spec

Rate of reaction may be affected by

 Amount of sodium in Lithium

- (D. Service, Chem. Brit., 1987, 27)

 Particle size / grade of aluminium chloride

 Grade of catalyst (esp. Raney Nickel)

 Water content of solvents

- e.g. Grignard reactions - water delays initiation Too much reagent then added

When exotherm starts, goes out of control

Assessment Strategy Flow Diagram

Possible explosive compound

or mixture

Yes Specialist testing Explosive?

Recommend route/process

changes Yes

No Thermal Stability

Does thermal runaway occur?

Recommend max. safe operating

temp

Is max safe operating temp above reflux temp or max heat supply temp?

Recommend modifications

or control No

Yes No

Yes

Reaction Exotherm Can reflux or max safe operating temp be reached?

No

Recommend control

Can accumulation occur such that reflux or max safe operating temp be reached?

Recommend modifications No

Yes Yes

Recommend minimum operating Gas Evolution temp

Can max rate be

vented/scrubbed safely?

No

Recommend emergency venting

or modifications Yes

Gas Evolution Can max rate be

vented/scrubbed safely?

Assessment complete

Check subsequent changes to

assessed process No

Desk Screening

 Presence of unstable groups

- Acetylene, nitro, azide, perchlorate, diazonium, peroxide and similar groups

 Comparison with known explosives or potentially explosive materials

 Calculation of oxygen balance

 Thermodynamic data calculation (CHETAH)

- From molecular formula calculate heats of formation and reaction - Accurate within an order of magnitude

- Positive or small negative heats of formation indicate instability - Large negative heats of reaction indicate high output

oxygen Balance = 1600 [ 2x + (y/2) - z ] / M

M = molecular weight x = N° of carbon atoms y = N° hydrogen atoms z = N° oxygen atoms

(other heteroatoms are ignored)

Hazard potential oxygen balance

High da - 80 a + 120

Medium da + 240 a + 120

da - 160 a - 80

Low > + 240

< - 160

Oxygen Balance

 OXYGEN BALANCE 128 (Medium/high Hazard Potential)

 DSC-TGA Exotherm at 190-200°C

 SIKAREX/ARC Exotherm at 110°C Destruction at 130°C

Heat of reaction - 800 kJ·mol^-1 Pressure rise to 2000 psi

THEREFORE SAFETY LIMIT OF 60°C PLACED ON DRYING

Oxmetidine Intermediate

O

N N⁺ O O^- O

O O

O

N S N

N

O O

O

Ranetidine Intermediate

CS₂ +

CH₃NO₂

2 (CH3)2SO₄ C C

H NO₂ KS

KS

C C H NO₂ H₃CS

H₃CS H

H₃C S O

NO₂ H₃CS

KOH

H₂O₂

DSC Experiments

 Sealed pan

- Sharp melting endotherm at 147.9°C - Exotherm with onset at 160°C

- Sample carbonized

- Pan seal ruptured and pan displace from sensor

 Unsealed pan

- Sharp melting endotherm at 147.3°C - Exotherm with onset at 160°C

- Generates 954 J·g^-1 - Sample carbonized

 Conclusion

- In sealed pan material slowly self-heats before melting

Temperature (°C)

50 90 130 170 210 250

18

Heat flow (mW)

22 26

30 34

38 42

160 J·g^-1 146.3°C

175.6°C

147.3°C

202.8°C 954 J·g^-1

DSC for SF&F Intermediate

H H₃C S

O

NO₂

H₃CS

ARC Experiments

 Sample heated in 10° increments from 50 °C

 Searching for self-heat rate of 0.02 °C·min^-1 during 10 min

 After 239 minutes, melting endotherm sensed at 139.5 °C

 Step-heating continued to 150 °C

 After 3 minutes, sample detonated and bomb destroyed

 Last recorded temperature: 321°C

 Self-heat rate – 54 °C·min^-1

Examples: (a) AIBN; (b) styrene monomer; (c) gunpowder Showing: onset of exotherm, simple or complex reactions,

rate at any temperature (kinetics), total heat

output (thermodynamics), maximum self heat rate

40 140 240

0.01 0.1 1 10 100

(a)

(b)

(c)

Temperature (°C)

Heat rate (deg/min)

Self Heat Rate

Pressure transducer Heaters Top zone Bomb thermocouple

Jacket Bomb Radiant Heater Bottom zone Heater

Accelerating Rate Calorimeter

Time (min)

Temperature (°C)

search

search wait

wait

Adiabatic conditions

begin

Initial heating

ARC and the Heat-Wait-Search Operating Mode

Adiabatic Heating exotherm potential:

equipment assembly

TO POWER SUPPLY CONTROLLER LOW WATTAGE HEATER

BUNG

1 LITRE OR 500 ml DEWAR FLASK STIRRER GUIDE

STIRRER RECORDER

THERMOCOUPLE

Dewar Calorimeter

 Advantages

- cheap and simple - Adiabatic

- Sensitive

 Disadvantages

- Sample size

- Time consuming to use

Reaction Calorimeter (RC1)

 Measure

- Overall heat of reaction - Heat of reaction with time

- Start of reaction (delay, accumulation) - Heat transfer during reaction

Maximum cooling capacity required

 Disadvantages

- Size of sample

- Cannot allow runaway to proceed

 Advantages

- Close to batch conditions - Shows when heat released - Cost

T_c(in) M_c

C_pc T_R

T_j(in)

T_j(out) M•_j

C_pj

M_R C_pR

Condenser

Reactor Jacket

Vacuum insulator

T_c(out)

Heating/Cooling System

Circulating pump

•

q_R = q_ex + q_accu + q_reflux + q_dos + q_loss Heat balance

T_R = temp in the reaction mass T_{J in}= temp of the cooling inlet T_{J out}= temp of the cooling outlet T_{c in}= temp of the cooling inlet T_{c out}= temp of the cooling outlet

Principle of RC1 Calorimeter

Addition (g)

0 -50 100 200 300

530 570 610 650 690 730 770 810

Time (min.)

530 570 610 650 690 730 770 810 0

40 20 60 80

Time (min.) Heat release rate (W/Kg)

20 100 140 60 180

Integrated Heat kJ/Kg)

Standard Results - Curves from Reaction Calorimeter

Disadvantages of Adiabatic Calorimeters

 Expense € 100,000 +

 Not truly adiabatic

 Agitation

 Not entirely suitable for reaction mixtures

Conclusions

 No single piece of equipment or technique will give all the answers

 Dewar calorimeters give adequate results and are cheap

 Combined with DSC, should give information for many applications

 ARC and similar instruments give more detailed data and can detect sensitive exotherms

 Reaction calorimeters measure heat evolved at each point and more accurately mimic plant conditions

 Importance of understanding reaction

 Thermal hazard, exotherms, runaway scenario

 Rates of reaction, kinetics

 Equipment choice

Dyestuff Intermediate - Ciba Geigy

6 H₂, catalyst, 55-60°C NO₂

NO₂ RO

RO

NH₂

NH₂ RO

RO

Mechanism and Thermochemistry

RNO₂ + H₂ RNO + H₂O ∆H = -134 kJ·mol^-1 RNO₂ + H₂ RNHOH ∆H = -155 kJ·mol^-1 RNHOH + H₂ RNH₂ + H₂O ∆H = -259 kJ·mol^-1

Overall - 548 kJ·mol^-1

Different rates for each step

Different catalyst requirements for each stage Must compromise

RNO₂ RNO RNHOH RNH₂

RN=NR O

RN=NR

RNH-NHR RNH₂

H₂ H₂ H₂

H₂

H₂

RNO

Reaction Pathways

Reduction of Nitro Compounds

Intermediates

Nitroso Extremely reactive

Short-lived under normal conditions Hydroxylamines More stable

Reduce more slowly Accumulate in reaction

Particularly if catalyst poisoned by sulphides, nitrates, metals

Problems if : Temperature is too low Poor agitation

Catalyst is of wrong type

Catalyst contaminated by aqueous nitrate

Du Pont Explosion 1976

• For reduction to occur, species must be absorbed on the catalyst ArNO > ArNO₂ > ArNHOH > ArNH₂

• Thus ArNHOH is displaced by the other intermediates and reaction may stop at ArNHOH

Cl Cl

NO₂ NH₂

Cl Cl

Cl Cl

NHOH Raney Nickel

At Du Pont, when reaction slower

- operator increased temperature by 10°C

- thermal reaction set in - no control - Explosion after 3 - 4 min

At 260°C, the nitro compound itself

decomposes, ∆H = 1000 - 2000 kJ·mol^-1

2 RNHOH 2 RNO + RNH₂ + H₂O ∆H = -55 kJ·mol^-1 H₂ Overall ∆H = -259 kJ·mol^-1

100 200 300

1 2 3 4 5

minutes Temp. (°C)

Disproportionation

Dinitro Reduction - Differences

•

•

•

•

•

M

Preparation of reaction mixture charging of reactor autoclave with raw materials feed pump

Preparation of reaction mixture the top reactor desired level by

recirculation through

Catalyst addition to reaction through a suspension of raw materials or solvent

Recicl. pump Recicl. pump

1 2 3

Process Sequences

(hydrogenation reaction 1-7, and catalyst separation 10-12)

M

Preparation of reaction mixture charging of reactor autoclave with raw materials feed pump

Preparation of reaction mixture the top reactor desired level by recirculation through reaction heat exchange

Process Sequences

Key Reference:

Solvent Recovery Handbook I. Smallwood, Arnold, London

Solvent Recovery

•

•

•

•

Both caused by lack of attention to detail in solvent recovery process

Two Typical Incidents

•

•

•

•

•

Angus

C C H NO₂ KS

KS

C C H NO₂ H₃CS

H₃CS CS₂ KOH

+

CH₃NO₂ 2 (CH3)2SO₄

Shell Fluoroaromatics Explosion

•

Fluoroaromatics plant was ruptured by the pressure generated by a runaway reaction

•

•

•

•

•

HALEX REACTION

CENTRIFUGATION

BATCH

DISTILLATION

HYDROGENATION &

CENTRIFUGATION

FINAL WORK UP OF 2,4-DFA

Pt/C catalyst Hydrogen Methanol

Recycle DMAc

Potassium fluoride 2,4-DCNB

TMAC catalyst DMAc solvent

Reacted KCl

Intercuts for distillation

Residue for incineration

Spent catalyst for recovery

Difluoroaniline Block Diagram

Cl Cl

NO₂

NH₂ Cl

Cl Cl

NHOH H₂

H₂

Plant - Halex Reaction

•

•

•

- Pressure held at 0.2 bar maximum

- Pressure build-up had never been observed during the 14 years operation of the Halex plant

•

The Plant - DFNB Distillation

•

•

•

•

•

CUT Pressure Temp (mbar) (deg °C) 1. A toluene/water azeotrope to remove water 200 76

2. “Dry” toluene for re-use - -

3. A toluene/DMAc intercut for redistillation - - 4. DMAc for re-use in the Halex reaction 100 110 5. A DMAc/DFNB intercut for redistillation - -

6. DFNB for hydrogenation 50 125

7. Monofluorinated chloronitrobenzene

(CFNB) for recycle - -

8. Heavy residues for third party incineration - -

Distillation Fractions

Events Leading to the Explosion

 Campaign 4 months old - 39 batches run

 Weeks prior to incident, water entered distillation pot - water carne from washing centrifuge to remove salts

 Contents of distillation vessel were separated, some water removed

 Batch 40 proceeded OK and DMAc recovered and appeared to be in spec

 Batches 41 and 42 used DMAc recycled before the water incursion and were normal

 Batch 43 used some DMAc recycled after water incursion and was normal

 Batch 44 used DMAc, all of which was recycled after the water incursion

Batch 44 - Abnormal Reaction

 Heated to 165°C but temperature continued to rise

 Pressure was rising, but pressure not shown on VDU used by operators

 Another operator alerted colleagues to the abnormally high pressure but, before they could take action, the relief valve lifted and the

reactor exploded

 Reactor torn in 3 pieces

 One piece travelled 200 meters

 Plant buckled beyond recognition

 Reactor contents formed a fireball which caused secondary fires

 6 Operators injured - one died later in hospital

Original Process Development (to 1982)

 Recycling of DMAc

- No impurities after 10 recycles - No acetic acid present !!!!

- Use tests with recycled DMAc normal

 Stability of reaction mixture - SIKAREX studies

- Storage or reaction mixture at 150°C gave no change, with or without iron present

- At 180°C for 20 hours there was some decomposition but no

exotherm (DFNB and DMAc decompose under these conditions - Distillation residues decomposed slowly at 200°C with a small

exotherm which would not runaway

Development Work 1989

 No DFA was made between 1982 and 1989

 Reaction was optimised at 165°C compared to the original 145°C to increase throughput

 Because thermal stability testing had previously been carried out at 180°C no further work was considered necessary

Chemical Investigation

 Presence of acetic acid in Halex reaction caused temperature to rise from 160 to 240°C

 At 240°C a second exotherm starts (decomposition of DFNB, DCNB)

 Temperature profile of runaway batch mimicked in laboratory by addition of acetic acid

 Acetic acid formed from DMAc by hydrolysis, but only in the presence of DFNB, which scavenges dimethylamine

 DMAc-acetic acid-water azeotropes at the same

temperature as DMAc, thus water cannot be separated out once acetic acid has formed

 DMAc charge vessel contained acetic acid

Lessons

 Need to rigorously check for build-up of impurities in recycle streams

 Analytical methods must be adequate for contro1 of the specification !

 Changes in reaction conditions should be re-evaluated in the light of newer techniques

 Old processes should be re-evaluated in the light of changing standards

0 50 100 150 200 1

2 3

0 20 40 60 80 100 120 140 160 180 200

time (min)

temp (°C)

2,4-DFNB Batch 44

Stanlow 20 March 1990 Coil temperature

reaction temperature

pressure

pressure (bar)

Process Conditions - Runaway Batch

CH₂=C=O chetene

(b.p. -56 °C)

Role of Acetic Acid

+ CH₂=C=O

X

X NO₂

X

OCOCH₃ NO₂

∆

X

OH NO₂

X

O

X NO₂ NO₂ HOAc

KF

Further reaction

etc.