Failure Knowledge Database - 100 Selected Cases

Explosion of Acetylene Hydrogenation Section in Ethylene Plant【July 7th, 1973 T okuyama, Yamaguchi, Japan】
Hiroshi Koseki (National Research Institute of Fire and Disaster)
Mitsuo Kobayashi (Graduate School of New Frontier Sciences, University of Tokyo)
Masamitsu Tamura (Graduate School of New Frontier Sciences, University of Tokyo)

1973 may have been a special year forthe chemical industry of Japan. M

accidents happened successively from the west to the east

ajor

from the spring till theautumn of that year . The accident shown in this article was the first accident oftheseries.
At around 22:13 on July 7th, 1973, a large explosion and fi re accident occurred at thesecond ethy lene manuf acturing plant of th e Idemitsu Petrochemicals Co., Ltd. in
Tokuyama, Yamaguchi. The p lant started an em ergency shutd own procedure a s aresult of a n instrum ent air (IA) failure. When th e su pply of IA was restarted,

preparatory work for th e re-startup was carri ed out at the p lant after a few minutes ofan emergency shutdown. At that time, a runaway re action occurre d at the ace tylenehydrogenation r eactor, gas leaked from th e o utlet p iping, and an exp losion and fireoccurred. Acetylene is a trace impurity in the product of ethylene. The fire was initiallyextinguished at around 12:00 the next day , but the complete extinguis hing of the fire
was not achieved until 10 p m on July 1 1th, 3.5 d ays after the oc currence of th e fire,
because the propylene used in the process began to spout out as a result of the fire, andit took many hours to stop the spouting.
The hu man d amage of this acc ident consis ted of thedeath of on e p erson. T hephysical d amage was t he bur ning of about on e tenth of th e enormous ethylene p lant,
and the direct mon etary damage was calc ulated to be 25 hundred mi llion yen in thecurrency va lue at the time. H

owever, the damage was much larger when therestoration expense and profit loss from th e long-term no-operation period were added.
In addition, the ethylene plant supplie d each company in t he industrial complex withraw material through the piping, and so the plants of eight companies were forced to
shutdown or at least d ecrease their production rate as an effect of th e shutdown of theethylene plant.
The d irect cause of th e accid ent i s as fo llows: 1) at th e re-start of t he ac etylenehydrogenation reactor after the emergency shutdown, excess hydrogen was supplied, 2)
the ethyl ene that had accumulated in thereactor was hydrogenated, while originally1

Failure Knowledge Database - 100 Selected Cases

only a s mall amount of acetylene was hyd rogenated, 3) exc ess heat was gen erated by
the ethylene hydrogenation. However, it is more important why the IA failure occurred,
and why the acetylene hydrogenation reaction was restarted just after the re-supply of
IA started. In this series of events, it is shown that facility management and education
are fundamentally important for operating a chemical plant.
Hydrogenation : One of additional reactions; hydrogen atoms are added to atoms at
the both en ds of unsaturated bo nd such as C= C double bond, C ≡C triple bond,
C=O double bond (from the “essential chemistry dictionary”). The hydrogenation
process used in the ethylene plant is described in the Event section.
1. Event
At ethylene manufact uring plants in Japan , naphtha is a main feedstock, and it iscracked thermally at a high temp erature of around 800 ℃ with steam in a tubular
cracking reactor. Products such as ethylene and propylene are obtained through more
complicated refining passes. The ethylene plant block flow sheet is shown in Fig.1.

Many isom ers c oexist i n the p roduct material because of the th ermal cracking of
naphtha. E thane and acetylen e are contained in ethylen e, and p ropane, p ropadiene,
and p ropyne ar e co ntained in p ropylene. Although the paraffin ingred ient suc h as
ethane and propane can be separated from ethylene and propylene by distillation using
a slight boiling point difference, acetylenesand diene are removed by conversion tomono-olefin through hydrogenation. The section that carries out this hydrogenation forC2 fraction, mainly co mposed of e thylene, is called "acetylene hydrogenation". Thishydrogenation is a larg e exothermic r eaction, and coexisti ng mate rials of C 2 frac tion

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Failure Knowledge Database - 100 Selected Casesconsists of mainly ethylene and a slightamount of acetyl ene. If the hydrogenation

conditions are not ideal, ethylene is hydrogenated and may generate a large amount ofheat. In th e w orst cas e, it might cause an

exothermic decomposition reaction of

ethylene. Therefore, this hydrogenation process is the most problematic section of theethylene ma nufacturing p rocess, as well as the high-temp erature crac king furnace inthe ethylene plant. The important points in the control of hydrogenation are to supply
only a nec essary amou nt of hyd rogen for ac etylene hyd rogenation an d to raise theselectivity o f acetylen e hydrogenation. Th e t emperature ra nge p ermitted to maint ain
the best s electivity is v ery narrow , so ca reful attention is required f or temperature
control, and so on.
On the night of July 7th, suddenly IA supply stopped. IA is used as a driving force foradjusting controllers such as a flow controller and adjusting an automatic control valve(CV). There fore, the IA failure cau sed an im mediate em ergency s hutdown, and al l of
the CVs took the failure position.

At that moment, the flare stack for burning theexcess gas began to kindle a large flame.
Flare stack: the equipment for burning the unnecessary gas that is discharged from aplant. In Japan, a chimney-shaped flare stack is most often used. It is a necessary
piece of safety equipment for an ethylene plant because a la rge amount of gas isdischarged during an emergency shutdown, etc.

After a f ew minutes, the supply of IA was re started. Preparations for restarting theoperation started while the causeof the IA failure was investigated. A few minutes

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Failure Knowledge Database - 100 Selected Cases

after the in struments returned to normal, th e ethylen e flow rate and hyd rogen fl ow
rate wer e o bserved to have d ecreased, so a c ontrol op erator chang ed the flow rat e
control from automatic to manual, and closed the ethylene feed CV. Then he also closed
the hyd rogen C V. As th e t emperature at th e u pper p art of the middle se ction of t hereactor was rising, the contr ol op erator intr oduced ethyl ene into th e reactor fro m adifferent et hylene p lant for low ering the te mperature. Then the te mperature rose
rapidly, so the block val ve of the hy drogen C V was p erfectly closed . Th e temp erature
indicator ex ceeded the scale limit – it was later found to be 970℃ when it was

confirmed by the computer – and afterw ards the temp erature dropped back to 750 ℃.
Meanwhile, the outlet piping of the acetylene hydrogenation reactor B division became
red-hot, and the gas that leaked from the valve flange caught fire. Moreover, the piping
ruptured as a result of the high te mperature, and a large exp losion and fire occu rred.
This fire destroyed a large amount of equi pment, including some distillation columns
and heat exchangers. A large fireball with a 60m diameter arose at this time.
The fire wa s got under control at 6:00 am

the next morning, but spouting andcombustion of the p ropylene refrigerant c ontinued, and the fire was extinguished atlast on July 11th.
2. Course
At around 18:50, instruments ofthe co ntrol room all

failed simultaneously . The

control op erators who came in fo r the e mergency shutdown work could not find thecause. Few minutes before the incident, a field operator opened the six inch valve of thework air piping (Y A) and clo sed the fo ur inch IA valve near th e six inch valv e by
mistake, instead of cl osing the two inch YA valve fo r d e-coking the cra cking furna ce.
Therefore, t he c ontrol i nstruments too k the f ailure p osition, th e p lant ente red an
emergency shutdown, a nd black s moke rose f rom the flare stack. T he operator was
surprised at the black smoke, and returned the IA valve to the open position.
De-coking: coke is formed inside the crac king tubes of the cracking furnace, and it
adheres to the tube wall. The coke lowers the cracking efficiency and brings about
a rise in differential pressu re. The work for removing the adherent coke is called
de-coking. The decoking work is n ot only done f or cracking furnaces but also forheating furnaces in general. G enerally decoking work is execut ed in st eam-air or
nitrogen-air condition.
Failure position: the failure position is the position of the CV – full open, full close, or
stay – which is set beforehand so that when the IA supply is cut off, the plant will
be placed in a safe condition.

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Failure Knowledge Database - 100 Selected Cases

At around 18:58, the instruments returnedt o n ormal, and the c ontrol operators

began adjusting the instruments for re-start ing the p lant. The sup ply of ethylen e andhydrogen t o the acetyl ene hyd rogenation react or had been started , but the ethyl ene
supply stopped due to t he pressure balance of the ethylene system. At 19:02, a con trol
operator switched the controller of the ethylene fe ed to t he r eactor t o manual a ndclosed the CV. Then he noticed that the hydrogen supply was still continuing. At 20:08,
he closed the hydrogen CV.
At 21:00, although the normal operating temperature was 60℃ at the middle sectionof the hydrogenation reactor B, it h ad risen t o 90℃, but th e c ontrol operator se emed
not to be aware of it. At 21:30, the thermometer at the hydrogenat ion reactor B middle
section sho wed a te mperature of 120 ℃. Th e c ontrol operator, wh o d iscovered theabnormally high te mperature, intr oduced ethylene gas fro m anoth er plant into t hehydrogenation reactor for cooling. Simultaneously, the required amount of hydrogen forthe hydrogenation process was supp lied by the automatic control. At 21:38, the control
operator noticed the rapid temperature rise, and he stopped the supply of the hydrogen
manually.
When the supply of eth ylene started, the temperature indicated by the thermomet er
started to increase rapidly. The temperature recorder was sc aled out ove r a maximu mtemperature of 200 ℃. At the op erator’s console, a temp erature of 970℃ was observedat the middle section thermometer at 21:45. Th e middle section thermometer lowered
to 750℃ at 22:00, but the ther mometer of the l ower section showed a temp erature of
896℃.
After 22:00, the temperatur

e at t he l ower s ection of h ydrogenation r eactor B

exceeded 1000℃, the outlet piping became red-hot, and the gas leaked from the valve
flange and ignited. At

around 22:15, the hydrogenation

rector B outle t pipe wasdestroyed by the high t emperature, the gas that leaked from the p ipe exploded, and alarge-scale fire occurred.
After that, the fire spread and was extinguished at last – it is omitted here.
3. Cause
The cause of the exp losion was d etermined to be the mist aken closin g of the LA
valve. During the restar t-up operation, a contr ol operator mistakenly closed the valve
by mistake , ethyl ene w as hyd rogenated in th e acetylene h ydrogenation r eactor, a ndthen the temperature of the re actor became high. As a result of the high temperature,
an exothermic decomposition of ethylene occurred. The outlet piping was broken by the5Failure Knowledge Database - 100 Selected Cases

high temp erature, and a large a mount of h ot gas leak ed from the p iping. The gas
ignited spontaneously by the high temperature of the piping.
Although, only a slight amount of acetylene was to be hydrogenated selectively inthe reactor, excess hydrogen was supplied when the crude ethylene supply was stopped.
So nearly the entire ethylene in th e reactor was hydrogenated. Furthermore, althoughsome metal based catalysts are u

sed for th is hydrogenation, it ishydrogenation catalyst of p alladium, which was us

known that aed, causes an exothermic

decomposition reaction of ethylene at temperatures over 400℃.
The true cause is considered below. The accident was regarded to have been causedby the combination of the hu man error of th e field op erator wh o mistook the valve
operation, the judgment error of th e control operator who allowed only hydrogen to besupplied at the re-startup of the hyd rogenation reactor, and the jud gment error of thecontrol op erator who in troduced ethylene fr om another p lant. Howe ver, actually thetrue cause of the accident seems t o have been the following: insufficient considerat ion
in the mechanical design ph ase and defects in the oper ation management, together
with a shortage of education and training that led to a lack of und erstanding of theessence of the plant and the importance of reconfirming the plant condition prior to there-startup after the emergency shutdown.

The acciden t was triggered when t he IA was stopped by closing the I

A valve by

mistake instead of the YA valve. Therefore, it is also possible to consider the accident tobe caused by typical human error. However, what must be considered here is why thehuman er ror was caused , and whether or n ot the huma n err or c ould have bee n

6Failure Knowledge Database - 100 Selected Cases

prevented. The piping and valves concerning the accident are shown in Fig.4. Although
the reason for mistaking the valve, which was mounted at interval of 1 00 m cannot beknown f or certain, some problem s can bepointed out regarding th e design of the

facilities and the condition of the management.
To begin with, all of th e valves w ere m ounted on racks, u nder th e flo or or in th eceiling. As the de-coking of t he cracking furnace is executed several times per year, theYA valve is an often used. Therefore the valve should have been placed where operationand confirmation could be performed easily.
Next, "why the IA valve was cl osed" is a problem in the operation cont rol aspect.
Closing the IA valve is absolutely forbid den when th e plant is on stream. For a va lvethat is absolutely fo rbidden to be turned on and off d uring nor mal operation, t heperson responsible for the operation must take measures to prohibit the turning of thevalve. At least, col or coding or ha nging cards on the val ve must b e conducted for
capture the attention of the workers. Ideally, the valve should be sealed in the presenceof a manager or supervisor after th e valve was set in the correct position. Valves thatare absolute ly forbid den to b e turn ed are the valves that may cause damage to t hesafety or c ontrol of the p lant if turned from t he sp ecified p osition, in cluding the IA
valve and the block valve of the pressure safety valve. When the author joined a certain
petrochemical company in 1966, the mark “CSO(C)” was put near some of the valves in
the Piping and Instrument Diagram (P&ID), which is one of the basic design drawings
for a chemical plant. The author

was taught that this mark means(Closed)”, ind icating that the marked

valves had to be sealed“Car Seal Openbec ause theyareforbidden to be turned.
It can be g uessed that ther e we re som e problems in the work instruction. The
person in c harge of th e valve operation had to instruct th e corr ect val ve cl early, a nd
make the fi eld op erator of the valv e repeat it. It is sup posed that if the p erson, who
indicated the valve had warned of the position of the two inch valve, the accident would
not have occurred.
It is more i mportant to examine w hy the plant operators hurried to re-start theplant. When the IA failure occurs, the plant shutdown occurs aut
refineries and chemical plants, not only at

omatically at all

ethylene plants. At that ti me, all of t hecontrol valves are switched to their failure positions that had been set beforehand, suchas full op en, full clos ed o r stay as it was. At an ethyl ene p lant, in t he event of an
emergency shutdown, g as is discharged from many press ure control valves (PCV) to
the flare st ack, becaus e the safety of the plant is the fi rst priority . Therefore, thecondition of the entire plant iscompletely d ifferent from that during the no rmal

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Failure Knowledge Database - 100 Selected Cases

operation. The pressure balance cannot be maintained, and all temperat ures gradually
approach to ambient te mperature. In such a p lant, even if the IA failure time is only a
few m inutes, the plant m ust not be restarted without a detailed confirmation a ndchecking of the situation. More over, the hyd rogenation sect ion must no t be restart ed
without checking carefully , becaus e a runaw ay reaction can be ca

used if excesshydrogenation happens.
It is supposed that managers and operators wanted to restart the plant as soon as
possible because it takes a few days to restar t an ethylene plant and the non- operation
loss is several tens of

millions of yen pe r day . However , the careless restart wit h

insufficient knowledge about the plant caused the accident.
4. Immediate action
All manufacturing plants e xcept the boiler were stopped immediately. At the plant
where the accident oc curred, water spraying from a stationary fireplug, separating of
the burning secti on fr om the ot her secti ons, d ischarge of t he c ombustible gas to t heflare stack, and various other measures includ ing introd uction of nitro gen gas to t heplant and extraction of the liquefied gas were conducted.
The op eration of th e other p lants was also stop

ped for p reventing d angerous

materials fr om catchin g fire. Furthermore, shipping through thepipe line to o ther

facilities of the companies of the complex was stopped.
During the fire fighting, theevacuation of the n eighborhood inhabitantswasconsidered because the p lant was rather near the p rivate houses, but in the end, theevacuation was judged not to be necessary.
5. Countermeasure
The report lately made by the company was describing the countermeasures ranging
over six pages. At thenvironment ande headquarters, th e sa fety cou ntermeasure center and thep rotection ro om was es

tablished, andstreng thening an d

improvement of the environment and peace section of the factory were conducted.
In the subject of the facilities, many kinds of alarms at t he reactors were improved
and the vessels that contained large amounts of liquefied gas were re modeled so thatde-pressure or discharge could be ca rried out rapidly. Labeling and other measures todistinguish the piping and valve s were do ne more tho roughly in o rder to pre ventmistakes in operation. Finally, the w ater sprinkling system and other s ystems for fire
prevention were improved.
The p ersonnel allocatio n was mad e p roper, and the pers onnel were gi ven complete

8Failure Knowledge Database - 100 Selected Cases

education and training. In addition, the oper ating standards were dras tically revised.
In short, the safety conditions were totally reviewed, and it was improved in all fields.
6. Knowledge
1) It was reconfirmed that the safety of a chemical plant fundamentally depends on
education and training.
2) It is important todesign plants so that t

he possibili ty of huma n error would

decrease, and it is also important that the management which can suppo rt the design
is carried out. In this ex ample, the accident would not have happened if the valve th atmust not be turned on or off was controlled so that the valve could not be turned, and if
the plant had been designed so that mistakes in operation would not occur.
3) If a plant stops once, the situation is diffe rent from that of the plant before it stops.
It is necessary t o conduct the re-startup of a plant only after careful investigation a nd
confirmation of th e cau se of the sh utdown as well as confirmation of the situation of
the entire plant.
7.

Influence of failure

1) As for the human damage, one person was injured.
2) As for th e p hysical d amage, a to tal of 94 p ieces of equi pment, incl uding twelv edistillation and reaction columns

and seve n heat exchangers, w ere damaged. The

direct monetary damage was estimated at 25 hundred million yen (in t he value of the
yen at that time). However, considering the restoration expenses, non-operation losses,
and so on, the monetary damage wasenormous. In additio n, there we re som e

non-operation losses resulting from the stopping of other equipment in the factory.
3) Among t he neighboring facilities, twelve plants in eight companies were forced tostop or t o r educe operation because the sup ply of ethylen e and other r aw materials
was stopped.
8.

SequelMultiple accidents related to petrochemical plants and a large fuel oil leakage to the

Seto Inland Sea that occurred in the next year , 1974, triggered a major revision of thefire protection law and the high pressure gas law in the second half of the 1970’s, which
led to the establishment of present safety regulations.
9.

By the side
Among the accid ents of che mical p lants, there are9many examp les where aFailure Knowledge Database - 100 Selected Cases"runaway reaction" caused the accident, includ ing this accident. A runaway reaction iscaused by insufficient c ontrol of th e h eat g eneration by an ex othermic r eaction, andthere are some typical cases. Some of those are introduced here. To begin with, there isa case where the cooling function is lost. The loss of cooling function can originate fromblockage in the heat exchanger for cooling, a temperature rise of cooling water, stops ofthe cooling water, agitation, and so on. Sometimes a runaway reaction is caused by therestart of th e agitation, although there was no problem in the stop of agitation. This
runaway reaction is th e result of the separation of the contents into two phases whi leagitation stops. It led t o a rapid exothermic reaction when mixing is restarted quickly.
In a selective reaction where a small p art of the feed material is reacted using a slight
condition d ifference, someti mes t he entire contents of the feedmaterial r eact

completely when the conditions of composition, temperature, and flow rate are changed,
resulting in a runaway reaction.
The accident at the ethylene plant introduced here is an example where a selective
reaction changed to a runaway reaction. Another example of a selective reaction is asfollows; a selective hydrogenation

reaction of a pyrolysis gasoline fraction usingasulfurized metal catalyst changed to a runa way reaction by the removal of sulfur . In
the composition of the feedstock, some runaway reacti ons occurred when an oxidizing
agent was i njected over the regulated rate.

In short, there are many possibilities inwhich a runaway reaction can be caused, if temperature control is insufficient.
References
・The High Pressure Gas Safety Inst. of Jap an, “Accident investigation report at N o.2
ethylene plant of Tokuyama factory, Idemitsu Petrochemicals Co.,Ltd. On July 28th,
1973” (1973)
・ Fire-Defense Headquarter of T okuyama Ci ty, “Outline of fire at No.2 ethyl

enemanufacturing plant in Tokuyama factory, Idemitsu Petrochemical Co., Ltd.” (1973)
・Tetsuzo Kitagawa, “Ethylene leakage due to the instrument air failure”, “Analysi s ofthe explosion Accidents”, p166-167 Nikkan Kogyo(1980)
・The High Pressure Gas Safety Ins t. of Japan, “Tokuyama-Shin-nanyo and IwakuniOtake complex safety investigation report”, p56-57(1982)

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