Swissi Safety 3

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Thermal Safety Case Study- Lesson 3This case study shows through an example of a batchreaction how to systematically assess the thermal risk

related to runaway reactions.It was prepared by R.Perrayon and P.Lerena based onthe knowledge of the Swiss Safety Institute (Basel,Switzerland)

Lesson 3 summarizes the results obtained in the previous

lessons through the construction of a COOLING FAILURESCENARIO and the assessment of its CRITICALITY.


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Temperature

Time

TMRad = h

DecompositionReaction

Tmax = C

DesiredReaction

MTSR = C

Temperature ofprocess = C

Time of coolingfailure: h

Risk analysis form 3 : Cooling failure scenarioTemperature

Time

TMRad = h


Tmax = C

DesiredReaction

MTSR = C

Temperature ofprocess = C

Time of coolingfailure: h

Risk analysis form 3 : Cooling failure scenario


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Lesson 3

Remember in lesson 2...

...the results of the reactioncalorimetry experiment

available from the thermalsafety laboratory.

Lesson 3

The highest potential of theprocess is attained when thereaction mass temperature isabout to reach 195 C.


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0

10

20

30

40

50

60

70

2 3 4 5 6 7

(W/kg)

120

140

160

180

200

220

240

260

T (C)

Results of the reaction calorimetry experiment

Tprocess

Tprocess + Tad, acc (T)

MTSR

Time (h)

Heat release rate0

10

20

30

40

50

60

70

2 3 4 5 6 7

(W/kg)

120

140

160

180

200

220

240

260

T (C)


Tprocess

Tprocess + Tad, acc (T)

MTSR

Time (h)

Heat release rate


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Temperature

Time

TMRad = 10 mn


Tmax = 399 C

DesiredReaction

MTSR = 256 C

Temperature ofprocess = 195 C

Time of coolingfailure: 3 h


Write data inrisk analysisform 3

Temperature

Time

TMRad = 10 mn


Tmax = 399 C

DesiredReaction

MTSR = 256 C




Write data inrisk analysisform 3


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Lesson 3

RemarkRemark

A time of 10 mn as TMRad istoo short for a productionprocess because it needstime to recognize the failure,

to organize the necessarymeasures and additional timewill pass until the measureswill show an effect.

Lesson 3

To complete the cooling failure

scenario you should draw a line torepresent the position of theMaximum Temperature for Technicalreasons (MTT).

For the closed system under study, itis the temperature at which thepressure reaches the maximum

permissible value , i.e the valveaperture temperatureof 200C.

In this case, the MTT can serve as asafety barrier, because thistemperature is lower than the MTSR.


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Temperature

Time

TMRad = 10 mn


Tmax = 399 C

DesiredReaction

MTSR = 256 C




MTT

Cooling FailureScenario is nowcomplete

Temperature

Time

TMRad = 10 mn


Tmax = 399 C

DesiredReaction

MTSR = 256 C




MTT

Cooling FailureScenario is nowcomplete


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Lesson 3

Beside of triggering a possibledecomposition reaction, thepressure build up caused by

uncontrolled heat release ofthe desired reaction is animportant thermal riskespecially in industrialsynthesis reactions performedunder pressure.

The consequences of the highpressure build up in the caseof a cooling failure are nowdiscussed.

Lesson 3

Reaction calorimetry experiment

A reaction calorimetry experimentfollowing the procedure for theprocess is available from thethermal safety laboratory.

The temperature dependence ofthe vapor pressure of the solvent(aqueous ammonia), evaluatedfrom the results of thisexperiment, will be shown on thenext screen.


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Temperature (C)

Presssure (bar)

0

20

40

60

80

100

120

40 80 120 160 200 240 MTSR


Temperature (C)

Presssure (bar)

0

20

40

60

80

100

120

40 80 120 160 200 240 MTSR



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Case study :Batch reaction

Lesson 3

Lesson 3

The vapor pressure at reaction

temperature is 42 bar but in thecase of a cooling failure thetemperature will rise up to 256 Caccompanied by an increase ofthe system pressure to 100 bar.

Consequences...

A failure of the cooling system orthe stirrer can lead to adangerous situation, but we willdevelop it later...


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Assessment

of criticality


Lesson 3

Lesson 3

For the assessment of criticality weconsider the relative position of fourtemperature levels:

Process temperatureMaximum temperature ofsynthesis reactionTemperature at which TMRad is24 h

Maximum Temperature forTechnical reasons

This allows classification of thescenarios into 5 different classesranging from the least critical (1) to

the most critical (5).


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Temperature

Temperatureat which TMRadis 24 h

ProcessTemperature

Lesson 3Assessment of criticality

Criticality index:(Class)

1

MTSR

MTT

Lesson 3

Description of criticality classes

Class 1

After loss of control of the synthesis

reaction, the maximum temperature fortechnical reasons (MTT) cannot be reachedand the decomposition reaction cannot betriggered. Only if the reaction mass ismaintained for a long time under heataccumulation conditions, can the MTT be

reached and even then the evaporativecooling would serve as a safety barrier. Theprocess is thermally safe.


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Temperature


ProcessTemperature



MTSR

MTT

2

Lesson 3


Class 2


reaction, the maximum temperature fortechnical reasons (MTT) cannot be reachedand the decomposition reaction cannot betriggered. The situation is similar to theclass 1 but if the reaction mass ismaintained for a long time under heat

accumulation conditions, the decompositionreaction could be triggered and the MTT bereached. In this case, reaching themaximum temperature for technical reasonsmay be a hazard if the boiling rate is toohigh. For normal process times, the process

can be considered thermally safe.


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Temperature


ProcessTemperature



3

MTSR

MTT

Lesson 3


Class 3


reaction, the maximum temperature fortechnical reasons (MTT) will be reached, butthe decomposition reaction cannot betriggered. The safety of the situation willdepend on the heat release rate of thesynthesis reaction at the boiling point

(evaporation rate vs. condensation and/orrelief capacity).


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Temperature


ProcessTemperature



4

MTT

MTSR

Lesson 3


Class 4


reaction, the maximum temperature fortechnical reasons (MTT) will be reached andthe decomposition reaction couldtheoretically be triggered. The safety of thesituation depends on the heat release rate ofboth the synthesis reaction and the

decomposition reaction at the boiling point.The evaporative cooling may serve as asafety barrier, depending on the capacity ofthe heat exchanger available.


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Temperature


ProcessTemperature



5

MTSR

MTT

Lesson 3


Class 5

After loss of control of the synthesisreaction, the decomposition reaction will berapidly triggered and the maximumtemperature for technical reasons (MTT)will be reached during the runaway of thedecomposition reaction. It is very unlikelythat the evaporative cooling can serve as a

safety barrier in this case. The heat releaserate of the decomposition at the boilingpoint determines the thermal safety of theprocess. This is the most critical of allscenarios.


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MTSR

MTT

Temperature

Tprocess

Criticality Index:

T(TMRad = 24 hr)

MTSR MTSR

MTSR

MTT

MTT

MTT

Criticality

1

Tick apropriate box

2 3 4 5

Remember: In the previous lessons the following temperatureswere determined:

Process temperature: 195C MTSR: 256CTemperature at which MTT: 200CTMRad is 24 h: 175C

Lesson 3: Assessment of criticalityAnswer this question:Which criticality class do

you assign to the coolingfailure scenario (1 to 5)?

Right answer:The criticality of the

cooling failure scenariocorresponds to class

number4

MTSR

MTT

MTSR

MTT

Temperature

Tprocess

Criticality Index:

T(TMRad = 24 hr)

MTSR MTSR

MTSR

MTT

MTT

MTT

Criticality

1

Tick apropriate box

2 3 4 5

Remember: In the previous lessons the following temperatures

were determined:Process temperature: 195C MTSR: 256CTemperature at which MTT: 200CTMRad is 24 h: 175C

Lesson 3: Assessment of criticalityAnswer this question:Which criticality class doyou assign to the coolingfailure scenario (1 to 5)?

Right answer:The criticality of thecooling failure scenario

corresponds to class

number4

MTSR

MTT


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MTSR

MTT

Temperature

Tprocess

Criticality Index:

T(TMRad = 24 hr)

MTSR MTSR

MTSR

MTT

MTT

MTT

Criticality

1

Tick apropriate box

2 3 4 5

The criticality of the cooling failure scenario corresponds toclass 4.

Lesson 3: Assessment of criticality

MTT

MTSR

MTSR

MTT

Temperature

Tprocess

Criticality Index:

T(TMRad = 24 hr)

MTSR MTSR

MTSRMTT

MTT

MTT

Criticality

1

Tick apropriate box

2 3 4 5

The criticality of the cooling failure scenario corresponds toclass 4.

Lesson 3: Assessment of criticality

MTTMTSR


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Are you sure thatthe criticality of thecooling failure scenariocorresponds toclass number 4 ?

Lesson 3 Lesson 3

As a matter of fact, the

criticality class of thecooling failure scenario is

out ofout ofrankingranking.

The combined potential ofthe desired and thedecomposition reactionsadded to the high pressure

provide a very high criticalityclass.


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Lesson 3

The consequences ofloss of control of thedesired reactionshould be described

in risk analysis form1.

Lesson 3

The consequences of loss of

control of the desired reactionare that the temperature will riseto 256C accompanied by anincrease of the system pressureto 100 bar.

At this temperature, thedecomposition is fast enough toresult in a thermal explosion, ina short time period.


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CONSEQUENCES(Tick all appropriate Boxes, taking into account the control by addition if available)

No critical consequence Boiling Gas release

Decomposition Other Pressure build up

DESCRIPTION:

In case of a cooling failure at 195C, there will be a runaway time of the desiredreaction not exceeding one hour. The temperature will within this time rise up to

260C, at which temperature decomposition is fast enough to result in a thermalexplosion. Morever, the vapor pressure will be about 100 bars at this temperature.

Lesson 3 : Consequences of loss of control of the desired reaction

Form 1

CONSEQUENCES(Tick all appropriate Boxes, taking into account the control by addition if available)

No critical consequence Boiling Gas release

Decomposition Other Pressure build up

DESCRIPTION:

In case of a cooling failure at 195C, there will be a runaway time of the desiredreaction not exceeding one hour. The temperature will within this time rise up to260C, at which temperature decomposition is fast enough to result in a thermal

explosion. Morever, the vapor pressure will be about 100 bars at this temperature.

Lesson 3 : Consequences of loss of control of the desired reaction

Form 1


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Consequencesof a coolingfailure


Lesson 3

Lesson 3

The consequences of

triggeringdecompositionshould be described

in risk analysis form2.


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Consequences:(e.g. Thermal explosion, gas generation, pressure build up, vapour pressures,etc...)

Pressure build up, fast thermal explosion. There is not enough time to take

counter - measures. In case of rupture of the bursting disc due to the highpressure, we have in addition to expect a total discharge of the reactor.

Lesson 3 : Consequences of triggering decomposition

Form 2Consequences:(e.g. Thermal explosion, gas generation, pressure build up, vapour pressures,

etc...)

Pressure build up, fast thermal explosion. There is not enough time to take

counter - measures. In case of rupture of the bursting disc due to the highpressure, we have in addition to expect a total discharge of the reactor.

Lesson 3 : Consequences of triggering decomposition

Form 2

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