Investigation Report
Chemical plant explosion
caused by runaway reaction
after stirring was suspended

National Institute of Occupational
Safety and Health, Japan1. Overview
1.1. Disaster
An emergency shutdown of all processes in a high-purity terephthalicacid manufacturing plant was initiated because the steam supply stopped
quickly. The cause of the trouble originated in the power plant.
Approximately 2 hours and 45 minutes later, an explosion and fire occurredin the resorcin (adhesive material) plant, and the fire spread to the cymene
plant, the power plant, and piping racks. After about 6 hours, a tank in theresorcin plant exploded. As a result, one worker died, seven were injured(two seriously) in the workplace. Two workers outside the workplace and 14
people in nearby neighborhoods were also injured.
1.2. Outline of resorcin manufacturing process

Purifying

Decomposition

Material
m-DIPB

Neutralization
Concentration

Oxidation

An outline of the process for resorcin production is shown in Figure 1.

Resorcin

Figure 1. Outline of whole resorcin production process
1.3. Outline of the oxidation process
Inresorcin

production,

twometa-position

propylgroupsof

meta-di-isopropyl-benzene are peroxidized with oxygen, and the peroxide
forms a hydroxyl group through acid decomposition (Figure 2). This reaction
is called the (applied) cumene process and is widely used industrially. The
decomposition reaction of peroxide is highly selective, depending on theacidity (Figure 3).1,2)O2oxidization

m-DIPB

O2oxidization

Mono-hydro-peroxide

Aciddecomposition

Di-hydro-peroxide

Resorcin

AcetoneFigure 2. Oxidation of m-DIPB

O2oxidization

CumeneAciddecomposition

Cumene-hydro-peroxide

PhenolAcetoneFigure 3. Reaction mechanism during cumene process(phenol and acetone formation)
The oxidation process was carried out in a column reactor, as shown in
Figure 4.

Water + NaOH(aq) 35.6 t/Bt
(add in the reaction)
Level in the reaction(include GHU)
Stationary level

Outlet 2 ofcirculation water

Inlet 2 ofcirculation water
Outlet 1 ofcirculation water
m-DIPB
121 t/Bt
Air for reaction
4800 Nm3/h

Inlet 1 ofcirculation water

Air for stirring
1600 Nm3/h
Outlet ofproducts

Figure 4. Outline of oxidation reactor
In operation, 121 t of m-DIPB (as the material), 10.2 t of sodiumhydroxide aqueous solution, and 25.4 t of process water are introduced intoan oxidation reactor. Then, 4,800 Nm3/h of air at a pressure of 0.52 MPa isoxidized in the mixture after raising the temperature to 96ºC. Mechanicalstirring by a motor, etc. cannot be done in the reactor because the reaction isunder high pressure. Therefore, air (1,600 Nm3/h) was supplied from thebottom of the oxidation reactor, which stirred the materials (airlift method).
The organic acid generated as the reaction progressed, but air oxidationneeds a weak alkalinity atmosphere. So adding a sodium hydroxide aqueoussolution must neutralize the acid to make the oxidation reaction more
efficient (Table 1).
Table 1. Materials sequentially added to the reactor
Sodium hydroxide aqueous solution

Process water

T-HPO (%)*

Input (t/h)

T-HPO (%)*

Input (t/h)

45 ~ 650.78

122 ~ end of process

0.44

65 ~ 1000.92

100 ~ 1150.65

115 ~ 1220.36

122 ~ end of process

0.19*T-HPO

isaperoxide

concentration

convertedtohydro-peroxide

concentration.
1.4. Timelines
A timeline leading up to the disaster is shown in Table 2. A timeline ofthe plant data is shown in Figure 5.
Table 2. Timeline up to the disaster
Time
23:20

Operation
70% of the plants sharing the same steam system experienced anemergency shutdown due to a problem in the power plant

23:32 ~ESD (emergency shut down) (primary operation) began at the resorcin
plant
Automatic remote control operation by ESD switch
Interlock of the oxidation reactor
Reaction suspension
Refrigerant entered the internal coil (circulating water -> emergency
cooling water)
Air compressor stopped supplying air to oxidation reactor
Nitrogen supply started (stirring bynitrogen)

23:32

Secondary ESD operation (onsite)
Operations began for safe suspension of all equipment (heat source
block, closing of receiving valve, etc.)*There was no secondary operation for the oxidation reactor.

0:40Interlock was released and refrigerant in the cooling coil was switched(emergency cooling water -> circulating water)
By releasing the interlock, nitrogen supply
was stopped automatically (stop stirring bynitrogen)(Temperature rose in the oxidation reactor)

1:39

Cooling water was introduced from the top of the oxidation reactor

2:11

Air compressor was started, but flow rate was not directed

2:15

Explosion occurred

About 0:35
Stop nitrogen

About 2:10
Explosion

Temperature (ºC)

Temperature

Flow rate

Temperature in the oxidation reactor

Flow rate of nitrogen to stir

3
Flow rate (Nm /h)

About 23:30
ESD Nitrogen supply

Figure 5. Timeline (plant data)

2. Investigation of Disaster Factors
It was determined that, in this disaster, stirring had stopped, the heat ofthe decomposition reaction rose in the system, then the overall temperaturerose, the decomposition reaction accelerated, and there was a runaway
reaction. Analysis results with the sources for drawing conclusions areshown as follows.
2.1. Identification of chemical reactions in the oxidation reactor
In Japan’s Industrial Safety and Health Act, organic peroxides are
categorized as “explosive.” In the oxidation reactor, m-DIPB was oxidized
with air to produce m-DIPB peroxide. Therefore, m-DIPB peroxide wasdetermined to be the cause of the explosion. The risk associated withm-DIPB peroxide was defined using:1) thermal analysis2) measuring the thermal behavior under adiabatic conditions.
2.1.1. Thermal analysis of m-DIPB peroxide by differential
scanning calorimeter
The appearance of the sample is shown in Figure 6.

Figure 6. Sample of m-DIPB peroxide
A thermal analysis test of m-DIPB peroxide was carried out using adifferential scanning calorimeter (DSC-1) manufactured by Mettler Toledo
Inc. The sample was kept in a SUS 303 sealed container. About 1 mg of thesample was sealed in an air atmosphere. The temperature increase rate was
10 K/min, and the range of the measurement temperature was 25 ~ 500ºC.
The results of the exothermic behavior obtained are shown in Figure 7.
Heat generation started around 110ºC. Near 150ºC, the exothermic rate
sharply increased. The exotherm was about 1.46 kJ/g, measured up to about
200ºC.
The guidelines for estimating the explosion risk, calculated from heating
value (QDSC), are shown as follows.3)i) Power: Detonation

Criteria: 2.1 kJ/g ≤ QDSCii) Power: Deflagration or detonation

Criteria: 1.3 kJ/g ≤ QDSC < 2.1 kJ/giii) Power: Continuous decomposition or combustion
Criteria: 0.4 kJ/g ≤ QDSC < 1.3 kJ/giv) Power: Non-continuous decomposition Criteria: QDSC < 0.4 kJ/g
Based on these guidelines, m-DIPB peroxide has a risk of deflagration or

Heating rate (W/g)

detonation.

Temperature (ºC)
Figure 7. Thermal analysis curve of m-DIPB peroxide

2.1.2. Runaway reaction test using adiabatic calorimeter
It was estimated that decomposition heat rose when stirring wassuspended, the temperature increased, decomposition accelerated, and arunaway reaction occurred. To analyze these processes, it is necessary tomeasure exothermic behavior under adiabatic conditions. By simulating thissituation, an adiabatic calorimeter can measure the behavior as the reactionaccelerates and a runaway reaction occurs.
Therefore, the thermal behavior of m-DIPB peroxide under adiabatic
conditions was measured by an adiabatic calorimeter (d-ARC) manufactured
by OmniCal Inc. The amount of the sample was about 5 g.
Figure 8 shows the entire exothermic behavior and Figure 9 shows thepart of the behavior at temperatures greater than the onset of heatgeneration (95ºC or higher). It was determined that m-DIPB peroxide started
to generate heat at 95ºC, the temperature rose at an accelerated pace with arise in pressure, and the runaway reaction occurred. Also, the maximum
pressure went above the safe limit of the reactor: about 12 MPa.
Figure 10 shows a comparison between the temperature data of the DCS(distributed control system) and the results of the adiabatic test. The sample
was an m-DIPB peroxide of hydro-peroxide equivalent with a T-HPO
concentration of 114%. There is about a 60-minute difference in the time to
the runaway reaction. At the disaster, the time to reach runaway of theperoxide was estimated to be shorter than the time to runaway of the sample
peroxide (experiment) because the higher T-HPO (134%) of the peroxide in
the oxidation reactor has a faster rate of decomposition reaction than theT-HPO (114%) in the sample.
Based on these results, it was determined that the m-DIPB peroxide in
the oxidation reactor caused the disaster.

Temperature

Temperature (ºC)

Gauge pressure (MPa)

Pressure

Time (min)Figure 8. Temperature and pressure measured by adiabatic
calorimeter (all results)

Temperature

Temperature (ºC)

Gauge pressure (MPa)

PressureTime (min)Figure 9. Temperature and pressure measured by adiabatic
calorimeter (more than about 95ºC)

Temperature (ºC)

Adiabatic
calorimeter
DCSTime (min)Figure 10. Comparison of measurement results of adiabatic
calorimeter and DCS data
3. Prevention of a Reoccurrence of the Oxidation Reactor Disaster3.1. Substances
m-DIPB peroxide can deflagrate and detonate from the energy (1.5 kJ/g)
generated by thermal decomposition. Its self-accelerating decomposition
temperature is 95ºC to runaway reaction. So, under the conditions in thereactor for oxidization (reaction temperature: 96ºC) the decomposition
reaction occurred at the same time as the oxidation reaction. Therefore, thisprocess is highly risky, because suspending cooling by stirring causes
decomposition leading to a runaway reaction.
•It is important to reduce the temperature for safe handling of m-DIPB
peroxide

because theself-decomposition

reactionofperoxideissuppressed at room temperature.
•

By setting a lower temperature for oxidation than the current condition,
self-decomposition of m-DIPB peroxide by suspension of stirring becomesslower and enough time for taking measurements can be secured.
3.2. Equipment
From a past case (previous) accident report prepared by the company,
after that emergency shutdown the temperature did not fall rapidly. Thiswould not secure safety as soon as possible. Further, the concentration ofT-HPO at this disaster was higher (134%) than the concentration during theprevious emergency shutdown. The circumstances in this study resulted inhigh heat and difficulty with cooling.
In addition, the cooling coil at the bottom of the oxidation reactor cannot
produce natural convection. This means that the bottom of the reactor was ata low temperature and the upper part was at a high temperature and thesetwo parts were separated without forced flow by stirring. A cooling coil must
be designed for safe operation taking into consideration this convection andthe specific emergency shutdown as this disaster.
To prevent decomposition of m-DIPB peroxide and to secure safety, it isnecessary to consider the following measures:•The flow rate (700 Nm3/h-N2) for stirring gas during the emergency
shutdown was lower than 1,600 Nm3/h-Air, the rate during the oxidation
reaction. The effectiveness of the stirring during the emergency shutdown
was also lower. A larger amount of nitrogen gas is necessary for stirringand cooling because the viscosity of m-DIPB mixture increases byperoxidation.

•Nitrogen gas must not be stopped automatically when canceling anemergency shutdown based on a wrong decision or operation error. It isnecessary to design a manual stop.

•In normal stop operations, an oxidation reactor is effectively cooled by air.
However, for cooling with nitrogen gas during an emergency shutdown, amethod to cool by air must also be designed (multiplexing).

•To prevent an explosion in the oxidation reactor caused by failure of thesemeasures, additional equipment, such as an emergency buffer tank witha cooling system, must be installed.

3.3. Human factors
In this case, releasing the emergency stop by a worker onsite caused thisdisaster. Therefore, the following measures are necessary.
•

Determine the criteria, such as a decreasing ratio of temperature, forfailure an emergency shutdown, and workers must confirm it.

•Do not determine procedures onsite for emergencies. Follow the planned
release procedure for an emergency shutdown.

•Determine a procedure for a manual stop when an emergency shutdowndoes not work.

References
1) Yoshio Kamiya, Organic Oxidation Reaction: Theory and Application ofAutoxidation, Gihodo Shuppan, Tokyo, pp. 224–231, (1973)
2) Organic Peroxide Research Group, Organic Peroxide: Its Chemistry andIndustrial Application, Kagaku Kogyo Sha, Tokyo, pp. 193–202, (1972)
3) Tadao Yoshida and Dayu Ding, Safety Technology for Chemical

Substances, Tokyo Progress System, Tokyo, pp. 14–15, 94–96 (1996)