Failure Knowledge Database - 100 Selected Cases

Explosion of Methanol Distillation Column of Detergent Manufacturing Plant【June 26th, 1991

Ichihara, Chiba, Japan】

Mitsuru Arai (Graduate School of New Frontier Sciences, University of Tokyo)
Mitsuo Kobayashi (Graduate School of New Frontier Sciences, University of Tokyo)
Masamitsu Tamura (Graduate School of New Frontier Sciences, University of Tokyo)

On June 26th 1991, an explosion and fire occurred during the shutdown work at themethanol rectifying column of the surfactant manufacturing plant in the Chiba factoryof Lion Co., Ltd. in Ichihara, Chiba. The surfactant produced at the plant was "fatty
acid methyl ester ethyl sodium sulphonate (hereinafter referred to as α-SF)", which
this company had newly developed. The human damage by this explosion and fireconsisted of two employees killed and thirteen employees wounded. As for the physicaldamage, the methanol rectifying column in which the accident occurred was destroyed,
319 spots in the factory were damaged by the blast and the fragments scattered by the
explosion, and seventeen offices of nearby companies were damaged. Althoughaccording to the Fire and Disaster Management Agency investigation, the monetarydamage to the factory reached 884 million yen and the monetary damage to the nearby
factories totaled 129 million yen, this estimate may not have included other monetarydamages such as restoration expenses and non-operation losses of the plant, so thetotal monetary damage is likely to be much larger.
The plant, which was changed from its original use as facilities for manufacturing
alkylbenzene sulphonate, started manufacturing of α-SF from natural fat on February
11th, 1991, the year of the accident. After several operations, stops and repairs wererepeated, the plant finished an operation that had started on June 19th, and the
explosion and fire occurred at 10:15 on June 26th during the shutdown work for that
operation. It was supposed that the scale of the explosion was equivalent to 10 to 50 kg
of TNT, and the pressure of the explosion was over 16 MPa at the center of the
explosion and over 11 MPa at the lower part of the explosion.
One of the main causes of this explosion and fire was the fact that organic peroxide
methyl hydroperoxide (MHP) is slightly formed and then it decreases in the following
neutralization process was overlooked during the research and development phase.
During several hours before stopping, as pH control in the neutralization process wasinsufficient because of a failure of the pH sensor, the situation in which MHP cannot
decompose or decrease easily was generated. Therefore, the recovered methanol that
included slight larger amounts of MHP than normal was fed to the column where the1Failure Knowledge Database - 100 Selected Cases

accident occurred. After stopping the methanol feed to the column, the hold operationand total reflux operation of the column were continued. The hold operation is a specialname used at the factory, which means drawing-off of refined methanol from the top of
the column after stopping the feed. It is assumed that during those operations, MHP
accumulated at the specific trays, reached rather concentration and exploded as aresult of a self-exothermic decomposition reaction. The boiling point of MHP is between
the distillate methanol and the bottom products of the column (water in this case).
One of the primary causes of the accident was that the generation of the organic
peroxide MHP was not found. Therefore, it was not known that MHP was decomposedin the neutralization process. As pH indications became incorrect due to a failure of the
pH sensor, the flow rate of a sodium hydroxide aqueous solution for neutralization
lowered extremely and the neutralization operation was carried out under acidic
condition. Therefore, decomposition of MHP became difficult, and rather a large
quantity of MHP remained. The MHP accumulated more than in the usual shutdown
operation in the column and MHP was concentrated at a certain trays during total
reflux operations and so on. pH control is a very important process for quality control
in manufacturing of surfactant. If the pH control is not performed properly, MHP is notdecomposed sufficiently. From matters mentioned above, it is one of the primary causesthat the pH control was not performed sufficiently.
As a new analysis method for MHP was established two months after the accident,
the existence of MHP was discovered. Considering the fact that the existence of MHP
could not be found at the time of this accident, it seems to show the difficulties in
prediction of hazards belong to the newly development technology. However, from theviewpoint of the operation, there was a problem that the pH sensor did not indicate thecorrect value because of an electrode failure, so that automatic pH control could not be
performed sufficiently. As a result, the flow rate of an alkali solution that was used forneutralization decreased, and neutralization operation was done in an acidic condition.
The problem was thought that the operation continued for several hours in the acidic
condition without noticing the failure of the pH sensor. Although the stability of pH
control in this process is not known, considerable fluctuation may have occurred as aresult of using a strong alkali sodium hydroxide. Besides the weakness of the pH
electrode against fouling was well known. If the importance of the pH sensor was
realized, a doubled redundancy of the pH sensor should have been prepared.
MHP: The abbreviation of Methyl-hydroperoxide. It is organic peroxide with thestructure of CH3OOH in which one oxygen is added to an OH base of methanol. It
was confirmed after the accident that MHP is formed by the reactions of methyl

2Failure Knowledge Database - 100 Selected Cases

sulfuric acid and hydrogen peroxide, which are by-products of the bleachingprocess. MHP explodes instantly when it reaches concentrations of over 25%, so it
is not a desirable by-product.
1. Event
On June 26th 1991, a large explosion and fire occurred at a surfactant plant of the
major toiletry company Lion Co., Ltd. at the Chiba factory in the seaside complex in
Ichihara, Chiba Prefecture. The main product of the factory was surfactant
manufactured by a sulfonation reaction of alkylbenzene. There were several surfactant
plants in the factory. One of the plants was changed to manufacture α-SF by asulfonation reaction of fatty acid methyl ester using a process that was developed in
the company. The explosion occurred at the methanol rectifying column used for theprocess during the shutdown procedure.

The block flow sheet of the plant where the accident occurred is shown in Fig.1.
The manufacturing process of α-SF has been divided into the following five processes:"the reaction process", "the bleaching process", "the neutralization process" ,"theconcentration process", and "the methanol rectification process". The accident occurredat the methanol rectifying column in the methanol rectification process. In the first“reaction process”, fatty acid methyl ester sulfonic acid is synthesized by sulfonation of
raw material fatty acid methyl ester with sulfuric anhydride. In the next “bleachingprocess”, oxidation bleaching is done for color improvement of the fatty acid methyl
ester sulfonic acid. Next, the bleached methyl ester sulfonic acid is sent to the“neutralization process”, where it is saponified by sodium hydroxide, becoming α-SF. At
this point, however, the product is still in a solution of water-methanol. This solution is3

Failure Knowledge Database - 100 Selected Cases

sent continuously to the “concentration process”, where it is separated intomethanol-water vapor and product α-SF slurry, and the α-SF slurry is rundown to the
product tank. The separated methanol-water vapor is sent to the “methanol
rectification process” to recover methanol for reusing at the “bleaching process”. Hereby distillation, the feed liquid is separated into refined methanol and water and thewater is sent outside of the process.
The operation was carried out entirely by the continuation method and automatic
control.
The use of methanol and an aqueous solution of hydrogen peroxide together
simultaneously for bleaching was a distinctive feature of this process, and it wasrecognized as an original technology developed by the company.
2. Course
2.1. Operation course to accident generation
On October 27th, 1990, a "water dummy run" was performed on the methanol
rectifying column. A water dummy run is a confirmation operation using water insteadof the actual liquid to be processed, which is performed when a plant is almost
completed.
The complete survey was conducted by the fire fighting authorities, and the plantstood it on January 9th, 1991. From January 14th, a "trial liquid dummy run" wasperformed at the methanol rectifying column. During the trial liquid dummy run, thecolumn was filled with a liquid that has nearly the same composition as the actual
liquid to be processed, so the run conditions were almost the same as the actual
operation conditions. This dummy run is conducted for checking the functioning of arotating machine, instruments, and so on and also for training the operators.
From January 22nd to January 31st, trial runs were carried out intermittently. The
first "production runs" started on February 11th, and the first shutdown was carriedout on February 13th. In total, six production runs were carried out by June. A large
number of operation runs were believed to be necessary for training the operators andadjusting the quantity of materials to be stored. During each operation period, theplant was shut down many times. Almost all of the causes of shutdowns seemed to betrouble at the concentration process. Although a temporary shutdown of the plantmight be inevitable at the initial start-up especially at in-house developed plants, too
many temporary shutdowns seems a little strange.
2.2 Operation course of just before the accident
From 21:35 on June 19th, the eighth production run started. The methanol

4Failure Knowledge Database - 100 Selected Cases

rectifying column was on stand-by during the total reflux operation. The “total reflux
operation” means the operation where there is no supply to the column, the vapor
evaporated by heating at the bottom reboiler is entirely condensed and it is all
returned to the column without being drawn off and outside the process. At first, thesulfonation process started, and the processes down stream also started in order. At
around 02:30 on June 20th, the feed to the methanol rectifying column in the final
process started.
At around 01:20 on June 26th, the pH sensor of the neutralization process wasfound to be failed. Before that finding, pH control had been conducted automatically
based on incorrect indication values. During this period, it is supposed that theneutralization operation was performed in acidic conditions. After finding out the
failure of the pH sensor, the operation was changed to manual operation, and bleaching
agents and sodium hydroxide were fed in proportion to measured of pH.
From 08:29, the shut down started from the reaction section in order to adjust thestorage volume. At around 8:45, the neutralization process was shut down temporarily
for repairing the pH sensor. The supply of the recovered methanol to the methanol
rectifying column was stopped at around 09:15. From that point, the column was in thestate of total reflux and in the so called “hold operation” in which the methanol in the
reflux drum was rundown to the tank.
At around 10:15, an explosion occurred in the upper part of the methanol rectifying
column. The upper part of the column and the ancillary piping fractured, and thefractured parts were scattered by the explosion. Simultaneously, a fire occurred near
the column by methanol.
3. Cause
The cause of the accident was finally estimated as follows: MHP generated in thebleaching process could not be decreased during the neutralization process because ofincorrect pH control caused due to a failure of the pH sensor, and as a result, methanol
with a slightly higher content of MHP than normal operation conditions was fed to thecolumn. The MHP was concentrated at specific trays during the total reflux operationand the hold operation in the shutdown procedure. This concentrated MHP caused self
exothermic decomposition, which led to a thermal explosion as a result of the heataccumulation.
It seems many causes that were related to each other caused the accident. As forthe operation, improper pH control, which was a basic item to maintain the products
quality, was a problem. The neutralization operation continued in acidic conditions for5

Failure Knowledge Database - 100 Selected Cases

several hours due to a failure of the pH sensor. Methanol including high concentrated
MHP which was not decreased sufficiently was fed to the column just before the
shutdown operation. If the existence of MHP had not been known, and if the original
operation conditions had been kept, the explosion would not have occurred. Since pH
control is very important for quality control, redundancy of the pH sensor should havebeen prepared or the functions of the pH sensor should have been checked.
Next main problem was that the generation and decomposition of MHP was notfound at the research and development stage. However, when it is considered that theexistence of MHP was found two months after the investigation of the cause of theaccident started by the establishment of a new analysis technology of peroxides,
estimation of the formation of MHP might have been difficult. The existence ofperoxide was known from before, but it was difficult to distinguish organic peroxidefrom the hydrogen peroxide that was used in the process. Therefore, it can be said that
unknown phenomena caused the accident.
The third cause was the operation method for a shutdown of the column. The
causes of a lot of chemical accidents are the related to the concentration of impurities.
Distillation is one of the concentration methods, and the concentration of impurities
might become high in the case of total reflux operation and hold operation. Total reflux
and hold operations were not regarded as suitable shutdown methods for the first plantof the company’s in-house development, since the existence of impurities is notconfirmed. Though it seemed to be a general operation method at that time, it would bebetter to consider another shutdown operation method.
Based on the considerations above, it can be said that the accident originated fromthe cause of "irresistible force" or "unknown phenomena". However, an attitude of thecompany toward the in-house developed process was also a problem. Thoroughly
verification should have been conducted. For example, there were many emergency
shutdowns due to troubles at the concentration process. If many thermometers andmany sampling points had been mounted in the column, might some other information
about the operation have been gathered? It can be estimated that there was a
maximum of 5 kg of MHP remained in the column at a normal shutdown and 40 to 50
kg of MHP remained at the accident. The estimated temperature profile during thedistillation at the column is shown in Fig.4. It shows a distinct difference intemperature profiles between just before the accident and in normal operations. If thisdata had been available before the accident, the existence of something different fromslight ester or surfactant could have been estimated.

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Failure Knowledge Database - 100 Selected Cases

4. Process of cause elucidation
The elucidation of the cause was difficult. At first, the facts related to the column
where the accident occurred were arranged in the usual way. At this point, it wasnoticed that the scattered fragments had come from only the upper part of the column,
from around the fourth tray to the 26th tray from the top. In addition, the time courseof the entire operation was arranged.
Here, the damage to the methanol rectifying column is shown in Fig.2 and thephotograph of the column after the accident is shown in Fig.3.

In addition, the explosion pressure was estimated to be 16 MPa at the center of the
explosion and 11 MPa at the lower part of the explosion, and the TNT equivalent of the
explosion was judged to be 10 to 50 kg.
From the facts above, the possibilities of a vapor phase explosion, a liquid phase
explosion and a solid phase explosion were examined. However, the data on theoperating conditions and the experimental results denied all three kinds of explosions.
It was suggested that if explosive organic peroxide had concentrated and deposited in
the column, it might explode. In order to check this idea, investigators searched inside
the column thoroughly, but they could not find any trace of the deposit.

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Failure Knowledge Database - 100 Selected Cases

Two months after the accident, there a discovery of a new method for analyzingorganic peroxide components in recovered methanol. This discovery became a trigger
for reexamining the cause of the accident. The total quantity of the peroxide wasmeasured by three kinds of analysis methods, and the results were almost equal.
However, the component ratio of organic peroxide and hydrogen peroxide could not be
determined. It was proven that the hydroperoxide existed in the recovered methanol as
the investigation was continued with the use of various peroxide measuring methods.
Simultaneously, there were peaks of both gas chromatography and NMR, the origin of
which had been unclear, and it was proven that the peaks were originated fromperoxide, and that the hydroperoxide was derived from peroxide having a molecularweight of 48 and that belongs to a CH3OO- functional group. This peroxide was later
identified as MHP. The object of examinations was limited to MHP after this, and theestimation of the concentration of MHP in the rectifying column, the mechanism of thegeneration of MHP and its explosion risk were examined.
The samples in the circumference of the methanol rectifying column were analyzed,
and it was confirmed that 1700ppm MHP existed in the feed to the column, 5200 ppm
MHP existed in the reflux of the column at the time of the accident, and 0 ppm MHP
existed in the refined methanol during the normal operation.
When the distillation was simulated in the MHP-methanol-water system, thesimulation result showed that MHP is concentrated in the column and reaches
concentrations of over 10% in the total reflux operation or the high reflux ratio
operation. When the rectifying column was checked, discoloration of column material
was observed. The discoloration seemed to be caused by MHP, and the position of the
discoloration accorded almost exactly with the place where the MHP concentration was
highest in the simulation. The concept is shown in Fig.4.
The location where MHP was generated was estimated next. The MHP
concentration in each place of the process was measured by analyzing the samples,
which were taken at intervals during the operation and stored. The results of theanalysis showed that the highest concentration was at the bleaching process outlet,
and the concentration decreased to about 1/4 to 1/3 in the neutralization process, and
MHP was not detected in the product α-SF at the outlet of the concentration process.
It was proven that almost all of the MHP entered into the recovered methanol that was
supplied to the methanol rectifying column, because the methanol was the only otherproduct of the concentration process. Regarding the generation of MHP, reproduction
tests were carried out under some conditions, and it was confirmed that the possibilityof MHP generation during the bleaching process was high.

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It was a question why MHP did not cause an accident during the previous
shutdowns and caused the accident on June 26th. As the operation conditions were
reexamined, in the operation conducted from midnight to dawn on June 26th pH value
was incorrect due to a failure of the pH sensor and the flow rate of an aqueous solutionof sodium hydroxide was extremely low. According to a literature, MHP cannot be
decomposed easily in acidic conditions. So it was considered that all of the MHP, which
was generated in the bleaching process, did not decrease in the neutralization process
and was fed to the column, then the MHP was concentrated at specific trays in thecolumn during the total reflux operation and hold operation.
In the column, 30 to 40 kg of MHP might be accumulated during the shutdown
when the accident occurred. Only a maximum of 5kg of MHP might be accumulated
during previous shutdowns, so MHP might not reach the required concentration for an
explosion at that time.
The danger of MHP was confirmed by various thermal analyses and simulations.
The results of these studies showed that detonation of MHP may occur atconcentrations of over 40%, and a decomposition reaction with a great heat occurs
instantly at concentrations of over 25%. As MHP of about 23.5% concentration
decomposes, rapid evaporation of methanol on the tray instantaneously could occur by
an exothermic reaction of MHP, generating a pressure of about 15.5 MPa which is large
enough to cause an explosion.
The explosion is considered to have progressed as follows. MHP was locally9Failure Knowledge Database - 100 Selected Cases

concentrated in the column, and an exothermic decomposition reaction started. The
reaction rate was increased by self-heating decomposition, and the generated large
heat caused an explosion with a temperature rise (thermal explosion). Methanol on the
tray instantaneously vaporized, causing a large pressure rise and detonation.
5. Immediate action
The private fire brigade turned out, and the fire was extinguished after 10 minutes.
6. Countermeasure
As countermeasures for the prevention of recurrence of the accident, the following
items were indicated.
1) Improvement in the process to avoid MHP formation
The operation conditions are to be changed to avoid formation of by-productsthat may produce MHP and a process to decompose MHP when it is produced
should be added.
2)

Improvement in the operation methods
Monitoring for confirming that there is no MHP in the column feed, is to be

performed. Samples are to be taken periodically for checking the existence of
MHP. The pH meter is to be sufficiently maintained, and the plant should be
stopped in the event a failure of the pH meter if necessary.
3) The improvement on the safety management system.
When a new product is produced by the newly constructed plant, a priori
checking system that studies the safety of the plant during the development andresearch stage and conducts test runs beforehand is to be introduced andincluded in the safety management system. Furthermore, it was attempted tomake the operation manual more thorough.
Items above mentioned were a matter of cause from the view point that the
explosion occurred at the plant. As a general rule in the development of a new plant,
particularly if it is the first plant to be constructed using new technologies createdin-house, safety countermeasures, including for intermediates, should be considered, atechnical follow-up should be considered for a once completed plant and/or process, andfurther development of the plant should be executed. As discussed before, there may beproblems that cannot be foreseen before an accident occurs in a newly developed plantor process. After the accident, the money and time that are required for remodeling andimprovement are very large.

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Failure Knowledge Database - 100 Selected Cases

7. Knowledge
1)

In the development of a new process, the safety measures might not be sufficienteven if sufficient examination and analysis is performed. Based on the issues
mentioned above, it is necessary to thoroughly investigate and examine each of
the reactions and physical properties involved in the process. At that the time, it
is also necessary to consult with the specialists from outside of the company.
Some phenomena only become known after performing continuous operation ina large-scale plant. In order to prepare for unexpected phenomena, it isdesirable to mount many sampling points and measuring instruments, which
may appear to be unnecessary at first.

2)

In the distillation of a multi-component system that includes a middle fraction,
the middle fraction can be concentrated to unexpectedly high concentration levelas shown Fig.4 during the total reflux operation when the distillate is not
rundown outside. It is necessary to sufficiently understand this fact.

3)

It is necessary to keep the operation as decided, and it is important to prepare
the facilities and operation management to maintain the decided items.

8. Influence of failure
As for the human damage, two persons died instantly when fragments scattered by
the explosion hit them on the head, and thirteen persons were injured.
The monetary damage was reported to be about 880 million yen; however, thisamount may only be the direct monetary damage such as damaged buildings anddamage to the plant. For the entire enterprise, the monetary damage becomes large

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when other types of damage such as non-operation losses caused by the stopping of thewhole factory are considered. Additionally, seventeen nearby companies in theindustrial area suffered damage. The direct loss to the adjacent factories was 130
million yen. Fortunately, there was no damage to private houses because the plant was
located in the seaside industrial area.
The product of this plant was a main raw material of new goods that the enterpriseintended to sell in memory of 100 years anniversary of the enterprise. The accident
might bring considerable large damage for the enterprise.
References
・Fire and Disaster Management Agency, “Explosion and fire of methanol rectifying
column on stream”, “Accident case of the dangerous substances”, p 64-67(1991)
・Tadao Yoshida et all, “Explosion in the methanol rectifying column caused by organichydroperoxide ”, J, Japan Soc. for Safety Engineering, 35, 370-378 (1996)
・Safety Research Association on dangerous materials in Kawasaki City, “Explosion andfire of the methanol rectifying column of synthetic detergent manufacturing plant”,
Accident examples of dangerous facilities with FTA immediately, p8-10(1997)

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