process risk analysis summary risk characterization, assessment … · 2017-05-12 · thermal...

Process Risk Analysis

K. Hungerbühler: Risk Analysis of Chemical Processes and Products, FS 2017 1

IV: qualitative; example CCSummary

Content of last lecture

• Technical reliability R(t)

– Failure rate z(t) with “bathtub” curve, Probability of Failure F(t), Failure

Density Function f(t), Mean Time Between Failure (MTBF), Probability of

Failure on Demand (PFD), Probability and MTBF of coincidence

• Human reliability analysis (HRA)

• Quantitative impact characterization

– Fault and event tree of incidental substance release

– Substance spreading

• Heavy gas model, gas diffusion model, timescale for global mass transport

• Risk assessment and management

– p/i-chart, Protection Layer Concept, Ignition sources and their avoidance,

Cost efficiency of safety measures

• Assessment of residual risk

• PRA Exercise: Hydrogenation of nitrobenzene to aniline

Ris

kch

ara

cte

rizati

on

Risk characterization, assessment & management

Thermal Process Safety


Introduction: example of accidentsLecture overview

Date Sem. Week Topics

21.02.2017 1 1: Introduction

28.02.2017 2 2: Life-Cycle Assessment

07.03.2017 3 2: Life-Cycle Assessment

14.03.2017 4 No Lecture

21.03.2017 5 2: Life-Cycle Assessment 3A: Product Risk Analysis

28.03.2017 6 3A: Product Risk Analysis



18.04.2017 9 Easter Holidays

25.04.2017 10 3B: Product Risk Analysis

02.05.2017 114: Process Risk Analysis (Chem./Chem. Ing.)

4a: Human Exposure: Theory and Case Studies (UWIS)

09.05.2017 124: Process Risk Analysis (Chem./Chem. Ing.)


16.05.2017 135: Thermal Process Safety (Chem./Chem. Ing.)


23.05.2017 14 Risk and Ethics

30.05.2017 15 Excursion and Presentation

3

Risk Analysis of Chemical Processes and Products

Part 5: Thermal Process Safety

K. Hungerbühler: Risk Analysis of Chemical Processes and Products, FS 2017

Self-

acceleration

Increase in reac-

tion temperature

Increase in

reaction rate

Increase in

reaction heat

Thermal runaway


4

!

IDLH

- Process

RA**in terms of thermal

process safety

Safety, health and environmental risk (exemplified by human toxicity)

Aim of thermal process safety: to avoid thermal runaway

accidents at a systemic level by securing

(i) a stable heat balance (in case of “as usual” scenario)

(ii) a manageable thermal risk (in case of “cooling

failure” scenario)


- Product RA

“accidental” scenario

short-term direct exposure

(worker/resident)

R = f(probability, impact)

“as usual” scenario

long-term direct exposure

(worker/consumer)

R = f(exposure, dose-effect)

“as usual” scenario

long-term indirect exposure

(specific population)

R = f(contribution to effect class)

Part 5: Thermal Process Safety

safety

environment

duration of exposure

health

MIC



Accident:

Ciba Basel, 23.12.1969,

Azo-dye production K-90

Damage from a thermal explosion

of a 2.8 m3 Emaille-diazotization-reactor

Introduction: example of accidents

Qr = 60 kJ/kg

Reaction:

diazotization of 6-chloro-2,4-dinitro-aniline

(40% NSA)

Introduction Example: diazotization



Introduction: example of accident

Diazotization of 6-chloro-2,4-dinitro-aniline in sulfuric acid/

nitrosyl sulfuric acid (40% NSA)

Introduction Example: diazotization

0

500

1500

1000

1000 200 300 °C

heat flow

W/kg

microthermal analysis 4°C/min

nitro-

decomposition

-380 kJ/kgdiazo-

decomposition

-360 kJ/kg

diazotization

-60 kJ/kg

reactant mixture

temperature

DTad

(diazotization) = 35°C cp = 1.75

kJ

kg K



Definition of safe thermal process conditions

I normal case: therm. reactor stability

(focus: desired reaction)

- mass and heat balance; stability criteria

II accidental case: thermal runaway – scenario

after cooling failure (focus: decomposition reaction)

- characterization of risk by impact and probability

safety measures (primarily inherently safe process

conditions)

Thermal process safety as part of process risk analysis

Thermal safety data collection

(phys.-chem., reaction)

Definition of thermal safe process

conditions

Thermal hazard identification

Thermal risk characterization by

impact and probability

Thermal risk assessment and

management

Assessment of residual risk

!

Thermal Process safety Basic structure

Thermal process data (ΔHR, ΔHdec, cp, therm. reactor power etc.)



“As usual” and “runaway” scenario as starting points for thermal

risk analysis

p. 181

(I) Thermal reactor stability“as usual” scenario for desired reaction

in normal operation (batch-/semibatch

process etc.)

Normalfall (isotherm)

1) erwünschte

Reaktion

2) Zersetzungs-

reaktion

II)

I)process

cooling failure

temperature

1) desired

reaction

II)

2) decomposition

reaction

thermal explosion

Introduction OverviewThermal Process safety Basic structure

I)

!

(II) Thermal runaway“cooling failure” scenario resulting in

adiabatic decomposition reaction

risk = f(impact, probability)

where

Tad : adiabatic temperature increase

(proxy measure for impact)

TMRad : time to maximum rate under

adiabatic conditions

(proxy measure for probability)



630-Liter reactor. Stainless steel with

- double-walled heating (cooling)

- stirring system

- distillation system

(Pharma Pilotplant Novartis Basel)

I)

II)

Batch/semibatch process

II)

temperature

1) desired

reaction

2) decomposition

reaction

cooling failure thermal explosion

I Thermal Reactor Stability Batch/semibatch

I)



Definition of batch reaction process:

All reactants are fed into the reactor. The

start of the reaction is carried out by:

• increasing the temperature or

• adding catalyst or

• adding reaction components

simultaneously

I: batch process

Assumptions:

• reactor: no heat storage capacity

• reactor content : neither axial nor radial

concentration or temperature gradient

• cooling jacket: no temperature gradient

between input and output

• process parameter: ρ, cp, ΔHR and U

temperature independent

a) Batch reactor

A BV [m3]

T [K]

cp [J/kg/K]

ρ [kg/m3]

Tc [K]

A [m2]

U [W/m2/K]

- rA [mol A/m3/s] (= k ∙ cA ∙ cB)

- ΔHR [J/mol A]

-rA

-ΔHR

e.g .

!

Batch processI Thermal Reactor StabilityI Thermal Reactor Stability Batch

A cooling area

CA concentration of species A

CB concentration of species B

cp specific heat capacity of reaction

mixture

ΔHevap solvent evaporation heat

ΔHR reaction enthalpy

solvent reflux

mevap [kg/s]

𝑄evap [J/kg]

k kinetic constant

𝑛𝑒𝑣𝑎𝑝 evaporation rate

rA reaction rate for A

T reaction temperature

Tc cooling temperature

U heat transfer coefficient

V reaction volume

ρ density of reaction mixture

where

E-1



I: batch processBatch processI Thermal Reactor StabilityI Thermal Reactor Stability Batch

𝑘 𝑇 = 𝑘0𝑒− 𝐸𝐴

𝑅𝑇

−𝑟𝐴 = 𝑘 𝑇 · 𝑓 𝑐𝑜𝑛𝑐.mol

m3·s

(Arrhenius equation)

where

n=0n=1

n=2

e.g.: −𝑟A = 𝑘 · 𝐶A𝑛

; n: reaction order

1

𝜌−

𝑑𝐶𝐴

𝑑𝑡=

1

𝜌−𝑟A

mol

kg·s

!1. Mass balance (for A)



I: batch process

p. 177/179

!


p. 177/179

!Semenov diagram: (reaction versus cooling power)

external cooling

Assumption:

ρ, cp, ∆HR, U ≠ f(T)

Tc

stable

instable

Tc,critical

qC

•

2. Heat balance

Batch cooled by external heat transfer

𝑐P𝑑𝑇

𝑑𝑡=

1

𝜌−𝑟A −∆𝐻R −

𝑈

𝜌

𝐴

𝑉𝑇 − 𝑇C

𝑊

𝑘𝑔

𝑞R 𝑞C 𝑞accaccumulation reaction

where

A/V specific heat transfer area [m2/m3]

cP specific heat capacity of reaction mixture [J/kg/K]

∆HR reaction enthalpy [J/mol]

𝑞 thermal power [W/kg]

rA reaction rate for A [mol/m3/s]

t time [s]

T temperature of reaction mixture [K]

Tc temperature of cooling material [K]

U heat transfer coefficient [W/m2/K]

ρ density of reaction mixture [kg/m3]



Orders of magnitude for the heat transfer (liquid/liquid)

I: batch process

p. 178

d wall thickness of the reactor [m]

U heat transfer coefficient [W/m2/K]

𝛼ext heat transfer coefficient external [W/m2/K]

𝛼int heat transfer coefficient internal [W/m2/K]

𝜆w heat conductivity of the wall [W/m/K]

* e.g. stirring failure

Heat exchange during cooling process

I Thermal Reactor Stability Batch

𝛼int

[W/(m2·K)]

𝜆/d

[W/(m2·K)]

𝛼ext

[W/(m2·K)]

U

[W/(m2·K)]

normal case 1000 5000 2000 600

accidental case* 100 800 100 50

1

𝑈=

1

𝛼int+

𝑑

𝜆w+

1

𝛼ext

where

d

Tc

T

λw

αint αext

reactor

site

cooling

site

!



I: batch processBatch processI Thermal Reactor StabilityI Thermal Reactor Stability Batch

Batch cooled by solvent reflux

(at boiling temperature Tb of reaction solvent)

Total heat of reaction (Qreact,tot) is equal to total heat of evaporation:

𝑄react,tot = 𝑚 0

∞

𝑞𝑅𝑑𝑡 = 𝑉 · 𝑐AO· −∆𝐻R = 𝑚evap,tot · 𝑄evap 𝐽

𝑚evap,tot = 𝑉 · 𝑐AO· −∆𝐻R / 𝑄evap kg

m reaction mass

𝑚evap solvent reflux stream [kg/s]

mevap,tot total mass of evaporated solvent [kg]

Qevap specific evaporation heat of solvent [J/kgsolvent]

where

!



0 0 0

0

0

0

1 10 0 0 0 0 0

0

0

CC

A

TT t, ,

T T t

R A,*

P E RT* n n

A, A,

*

P

A

H ·CB

·c ·T

D t ·k T ·C t ·k ·e ·C

UAtS

·V·cE

RT

Mass and heat balance in dimensionless form

I: batch process

Dimensionless parameters:

( for nth order reaction with CA = CA,0(1-XA) )

ChemIng

only

Ratio between:

- heat of reaction and thermal capacity of the fluid

- two temperature or time scales

- time of process and time of reaction

- heat transferred and thermal capacity of the fluid

- activation energy and potential energy along the reaction path


1

1

1

1

n*AA

n* * *A C

dX· X

d

dB · · X S ·

d

mass: D e

heat: D e

!



Definition of semibatch

reaction process:

At least one reactant is fed into or one

product removed from the reactor. For

reaction start and control at least one

other reactant is subsequently added

over a certain time period (usually

evenly dosed).

I: semibatch process

b) Semibatch reactor

Assumptions:

• reactor: no heat storage capacity

• reactor content : neither axial nor

radial concentration or temperature

gradient

• cooling jacket: no temperature

gradient between input and output

• process parameter: ρ, cp, ΔHR and U

temperature independent

• dosing rate m ≠ f(t) for t tDos

(t) V(t)

(t) m(t) ρ . V(t)

Tc

e.g .

Semibatch processI Thermal Reactor StabilityI Thermal Reactor Stability Semibatch

R

feed

feed

R

R

feed

dos

!


17

Assumption: cp,RM = cp,feed = cp (ρ = ρRM ) (RM : reaction mass)

1. Mass balance (for B)

2. Heat balance


( m ≠ f(t) for t tDos,

otherwise m = 0 )



𝒒acc 𝒒R 𝒒C

𝒒feed

𝑑

𝑑𝑡𝑉 · 𝑐B =

𝑚

𝜌feed𝑐B, 0− 𝑉 · −𝑟B

mol

s

1

𝜌−𝑑𝑐B𝑑𝑡

=1

𝜌−𝑟B −

𝑚

𝜌feed

1

𝜌 · 𝑉𝑐B, 0− 𝑐B

mol

kg · s

𝑑

𝑑𝑡𝑚 · 𝑐P · 𝑇 = 𝑉 · −𝑟B −∆𝐻R − 𝑈 · 𝐴 · 𝑇 − 𝑇C + 𝑚 · cP · 𝑇feed [𝑊]

𝑐P𝑑𝑇

𝑑𝑡=1

𝜌−𝑟B −∆𝐻R −

𝑈 · 𝐴

𝜌 · 𝑉𝑇 − 𝑇C −

𝑚 · 𝑐P𝜌 · 𝑉

𝑇 − 𝑇feed𝑊

𝑘𝑔

!



Mass and heat balance in dimensionless form

Dimensionless parameters:

( for nth order reaction with CB = CB,0(1-XB) )

ChemIng

only

Ratio between:

- heat of reaction and thermal capacity of the fluid

- two temperatures or time scales

- time of process and time of reaction

- heat transferred and thermal capacity of the fluid

- activation energy and potential energy along the reaction path

- thermal capacity of dosing and thermal capacity of

reaction mass

I: semibatch processSemibatch processI Thermal Reactor StabilityI Thermal Reactor Stability Semibatch

n* *BBB

n* * * *B C feed

dX· X Q ·X

d

dB · · X S · Q ·

d

mass: D e

heat: D e

1

1

1

1

0 0 0 0

0

0

0

1 10 0 0 0 0 0

0

0

0

C feedC feed

A

T TT t, , ,

T T T t

R B,*

P E RT* n n

B, B,

*

P

*

A

H ·CB

·c ·T

D t ·k T ·C t ·k ·e ·C

UAtS

·V·cm·t

Q·V

E

RT

!



Semibatch cooled by solvent reflux

I: semibatch processSemibatch processI Thermal Reactor StabilityI Thermal Reactor Stability Semibatch

(at boiling temperature Tb of reaction solvent)

𝑞𝑅 =1

𝜌−𝑟B −∆𝐻𝑅 =

𝑚 · 𝑐p

𝜌 · 𝑉𝑇b − 𝑇feed +

𝑚evap

𝜌 · 𝑉𝑄evap

W

kg

For Tfeed = Tb total heat of reaction (Qreact,tot) is equal to total heat of evaporation:

𝑄react,tot = 𝑚(𝑡𝑑𝑜𝑠) 0

∞

𝑞𝑅𝑑𝑡 = 𝑚 · 𝑡dos𝜌feed

𝑐BO −∆𝐻R = 𝑚evap,tot · 𝑄evap 𝐽

𝑚evap,tot = 𝑚 · 𝑡dos𝜌feed

𝑐BO −∆𝐻R / 𝑄evap kg

∆Hevap (solvent) evaporation heat [J/mol]

𝑚evap solvent reflux stream [kg/s]

mevap,tot total mass of evaporated solvent [kg]

Qevap specific evaporation heat of solvent [J/kgsolvent]

where

!




ni : number of moles of reactant i in the reactor at time t (i = A, B)

(balance equations, see

"semibatch – feed cooling”)

(balance equations, see “batch“) (balance equations, see

“semibatch“)

e.g. nitration:

AB

A: already fed into the reactor

B: addition (20 mol% excess with respect to A)

react.

time(t) time(t) time(t)

dos. )

tdos tdos tdos

Semibatch processI Thermal Reactor Stability

Concentration profile (A + B P)

I Thermal Reactor Stability Semibatche.g. nitration:

AB

Complete dosing control

𝒕 𝟏 𝟐𝒓𝒆𝒂𝒄𝒕 ≪ 𝒕 𝟏 𝟐

𝒅𝒐𝒔

Complete reaction control

𝒕 𝟏 𝟐𝒓𝒆𝒂𝒄𝒕 ≫ 𝒕 𝟏 𝟐

𝒅𝒐𝒔

Partial dosing control

𝒕 𝟏 𝟐𝒓𝒆𝒂𝒄𝒕 ≅ 𝒕 𝟏 𝟐

𝒅𝒐𝒔

!




Note: Symbol list, see prior dimensionless representation of the semibatch balance equations

e.g. nitration:

AB

1) ChemIng only

1. Mass balance (for B; within the period 0 ≤ t ≤ tSE )

2. Heat balance (isotherm, cooling only by feed stream)

( reaction rate is decoupled from temperature)

( Tfeed for autothermal process)

tSE

1

𝜌−𝑟B =

𝑚

𝜌feed·

1

𝜌 · 𝑉· 𝑐B, 0

time(t)tdos


A: already fed into the reactor

B: addition (20 mol% excess with respect to A)

SE: stochiometric equivalence

Feed cooling in case of complete dosing control

(A + B P)

(for t < tSE : dcB/dt =0 and cB << cB,0)

(ρ = ρRM )

e.g. nitration:

AB

1)

1)

*

feedfeed

J ; B

kg

1

1n* *

B

mol ; D e X Q

kg s

1

𝜌feed· 𝑐B, 0 −∆𝐻R = 𝑐P(𝑇 − 𝑇feed)

1)

!



tDos: dosing time (up to equimolar addition of reactants)


heat production

𝒒𝒓 [W/kg]

Definition of thermal conversion (Xtherm):

t

R0

therm

R0

q dt

X t =

q dt

time

t [s]

Heat production and thermal conversion


with 𝑞𝑅=1

𝜌−𝑟B (−∆𝐻R)

Controlling step xtherm (t = tDos)

Complete dosing control ~ 1

Complete reaction control ~ 0

Partial dosing control 0 < xtherm< 1

!



-HR = 100 – 200 kJ/mol

I: semibatch process – example 1

Reaction procedure:

first R-OH/OOH/NH2 is fed into the reactor, and then dosing of ethylene oxide is started

Alkoxylation

reactions of fatty alcohols, fatty acids and

fatty amines with ethylene oxide (propylene

oxide and butylene oxide) non-ionic

surfactants

reactor with circulation cooling system

(V=10 m3, A= 200 m2)

Example 2I Thermal Reactor StabilityI Thermal Reactor Stability Semibatch – example 1



therm. scale-up

effect (∞ qR/qc)qR ~rB·

A

B

(i) Batch/semibatch reaction mode

!

I: comparison of reactor types

••

c) Comparison of different reactor types

high(at t=0)

low

low medium

to large

large

variable

c

batch

Comparison of reactor typesI Thermal Reactor StabilityI Thermal Reactor Stability Comparison of reactor types

type of reactor



qR ~rB·

A

B

ISTR: ideally stirred tank reactor

I: comparison of reactor types

therm. scale-up

effect (∞ qR/qc)••

(ii) Continuous reaction mode

mediummediumlow

high

(at the

entrance)high small

smallhighvariable

cross flow

c

plug-flow

ISTR

Comparison of reactor typesI Thermal Reactor StabilityI Thermal Reactor Stability Comparison of reactor types

!

type of reactor



II Runaway

Risk R = f(p,i)

II Thermal runaway: basic scenario for risk analysistemperature

Probability (p)

Imp

act

(i)

1) desired

reaction

2) decomposition

reaction

I)

OverviewII Thermal RunawayII Thermal Runaway Basic scenario

!



Recipe:

1. 250 ml H2O2 35% fed into Dewar (T0 22 ºC)

2. 0.074 g KI added (@ t=0) as decomposition catalyst

3. stop of adiabatic decomposition (@ T 80 ºC) through addition of ice

II: example H2O2

Adiabatic decomposition of hydrogen peroxide

Example 4II Thermal RunawayII Thermal Runaway Demo example

2 2 2 22 2 H O aq O g H O l

2 2 2

2 2 2 2

H O aq + I aq IO aq H O l

H O aq + IO aq I aq H O l O g



∆Tad as indicator for the impact of a thermal runaway

p. 183/184

!

II: impact

a) Impact of thermal runaway

ImpactII Thermal RunawayII Thermal Runaway Impact

∆𝑇ad=𝑄dec𝑐P

𝐾 =−∆𝐻dec · 𝐶0

𝜌 · 𝑐P

C0 educt concentration @ t=0 [mol/m3]

cP specific heat capacity of reaction mass [J/kg/K]

H2O cP = 4.2 [kJ/kg/K]

organic solvent cP = c.a. 1.7 [kJ/kg/K]

H2SO4 (100%) cP = 1.3 [kJ/kg/K]

∆Hdec reaction enthalpy [J/mol]

Qdec specific reaction heat [J/kg] (analog for decomposition Qdec)

∆Tad adiabatic temperature rise [K]

ρ density of reaction mass [kg/m3]

where

Classification of Tad in terms of impact

* additional condition: boiling point not exceeded

Tad [ºC] Impact

> 200 high

50 < Tad < 200 medium

< 50* low



Calculation of standard reaction enthalpy from standard

enthalpies of formation of reactants and products

II: impact

online database for standard formation enthalpy e.g. http://webbook.nist.gov/chemistry/


∆𝐻𝑅𝜃=

𝐽

𝜈𝐽 · ∆b𝐻𝐽𝜃

J index for specific reactant or product

∆𝐻𝑅θ standard reaction enthalpy

∆𝑏𝐻𝐽θ standard formation enthalpy

θ standard conditions (298 K, 105 Pa)

ν stoichiometric coefficient

where

ReactionReaction heat

(-∆HR) [kJ/mol]

neutralisation (HCl) 55

diazotization 65

amination 120

nitration 130

diazo decomposition 140

sulfonation (SO3) 150

vinylpolymerisation 50-200

nitro decomposition 400

hydrogenation (nitroaromate) 560


Typical functional groups of unstable compounds


Polymerizable compounds

!

II: impact

reductants(in contact with oxidizing agents)

metals (e.g. Na, Zn)

metalorg. compounds (e.g. BuLi)

hydride (e.g. LiAlH4, NaBH4)

silane

hydrogen

oxidants(in contact with reducing agents)

HClO4 (conc.) / perchlorate

HNO3 (conc.) / nitrate

CrO3 / chromate

KMnO4

chlorate

nitric/sulfuric acid (HNO3/H2SO4)

alkylnitrite

H2O2

organic and inorganic peroxide

SO3 / oleum

oxygen / ozone

chlorine

chemical class unstable group

1 perchlorate –ClO4

2 nitro –NO2

3 polynitro

4 nitroso –N=O

5 N-oxide

6 hydroxylamine,

oxime

>N–OH

7 tetrazole

8 triazene, triazole –N=N–N<

9 azo –N=N–

10 hydrazine –NH–NH2,

>N–NH2

chemical class unstable group

11 substituted

hydrazine

–NH–NH–, >N–

N<

12 –N–N– in a ring

13 2 –N–N– in a ring

14 imidazole N–C–N (ring)

15 oxazole N–C–O (ring)

16 thiazole N=C–S (ring)

17 acetylide –C≡C–

18 halogen/nitrogen

compounds

>N–X

19 nitric acid esters –ONOx

20 peroxides –O–O–

-+

olefine (X = e.g. –F, –Cl, –CN)

epoxidediketene




Impact according to Tad

-Hdec= 98 kJ/mol

impact = high

II: impact – example H2O2

Adiabatic decomposition of hydrogen peroxide

(35 wt %)

Example 4II Thermal RunawayII Thermal Runaway Demo example

1010

2803 6

decad

P

kJ Q kgT K

kJc . kg·K

2 H2O2 aq → O2 g + 2 H2O(l)



decomposition power qdec:dependent on decomposition rate and

decomposition enthalpy

where T0 determined by residual

conversion of the desirable reaction:

(for batch reaction)

TMRad as indicator for the probability of a thermal runaway

In case of cooling failure (at tcooling failure)

TMRad is defined by the temperature dependent decomposition power and the starting temperature of

decomposition.

II: probability

b) Probability of thermal runaway

.

ProbabilityII Thermal RunawayII Thermal Runaway Probability

p. 185/188

𝒒𝐝𝐞𝐜 =𝟏

𝝆−𝒓𝐝𝐞𝐜 −∆𝑯𝐝𝐞𝐜

𝑇0 ≅ 𝑇process + 𝑡cooling failure

∞ 𝑞𝑅 𝑡 𝑑𝑡

𝑐𝑝

starting temperature T0 of

decomposition:determined by Tprocess and residual

conversion of the desired reaction

(for batch mode)

𝑞dec(𝑇)

𝑞dec(𝑇0)= exp

𝐸A𝑅·

1

𝑇0−1

𝑇

!



1st approximation for TMRad: 10-degree rule for doubling of reaction rate

II: probability

temperature

7/8 …


[-]

0 102ad T KTMR t

0 10T Kt

!

p. 186



2nd approximation for TMRad:

II: probability

Adiabatic heat balance for decomposition reaction

(assumption: 0th order reaction; ρ, cp, ΔHdec ≠ f(T))


!

0

0

0

0

1

Aad ad

P dec dec

ET T TMRdecRT

PT

dT Wc = k T - H q

dt kg

k - He dT = dt

·c

TMRad ≅𝑐P

𝑞dec 𝑇 = 𝑇0

𝑅𝑇02

𝐸A[𝑠]

p. 187



Isothermal DSC measurements for parameter determination of TMRad

qdec·

[W/kg](-EA/RT)

II: probability

time

temperature [˚C]

plot:

dec

dec


TMRad ≅𝑐P

𝑞dec 𝑇 = 𝑇0

𝑅𝑇02

𝐸A

!



With the approximations

II: probability

ChemIng

only

ProbabilityII Thermal Runaway

Adiabatic heat balance for the decomposition in dimensionless form(For nth order decomposition with cA=cA,0 (1-xA))

1

1

1 00

* *

TMR TMR* *ad adA

dB · ·

d

e d B ·D d (for X )

D e

* *adTMRB ·D

·

1

1

1

TMRad

* *

e d

B ·D

0 0 0

0

0

T t A, ,T t

dec B,*

P

E

RT

H ·CB

·c ·T

1

11

1 1

TMRade

0

0

0

ad

ad

adTMR

adTMR

TMR

tT T

T

0 10 0 0

AE

RT*A,D t ·k ·e ·Cwhere

!



assumption: cP=2 kJ/kg/K, decomposition reaction 0th order with EA= 100 kJ/mol

Example: temperature dependence of TMRad

p. 188

II: probability

0

dec 0

pT=T

q (T=T )dT =

dt c

*(according to adiabatic heat balance)


T0 [˚C] 𝑞dec(T=T0)

[W/kg] 𝑑𝑇𝑑𝑡 |𝑇=𝑇0[K/h]* TMRad [h]

100 1 1.8 6.4

75 0.1 0.18 56

53 0.01 0.018 490



Classification for TMRad in terms of risk

p. 187

II: probability

TMRad as critical time

constant for effectiveness

of safety measures

ProbabilityII Thermal Runaway

temperature

time

TMRad [h] Probability

< 8 high

8 <TMRad < 24 medium

> 24 low

!



Probability according to TMRad

1st approximation: 10ºC rule

2nd approximation: adiabatic heat balance

II: probability – example H2O2

Adiabatic decomposition of hydrogen peroxide:

(35 wt %)

Demo exampleII Thermal Runaway

TMRad = 2 ·

TMRad =𝑐P

𝑞dec(𝑇 = 𝑇0)

𝑅𝑇02

𝐸A=

⋯

t T0+10K = …



II: risk assessment

not acceptable

acceptable

medium

high

low

> 8

< 8

< 24

> 24

< 50 > 50 <200 > 200

highmediumlow ∆Tad [K]

TMRad [h]

probability

impact

c) Thermal runaway risk: classification by ∆Tad and TMRad

Risk assessmentII Thermal Runaway Risk assessmentII Thermal Runaway

acceptability range



T0: max. attainable temperature after runaway of the desired reaction

Tb: boiling point of the reaction mixture

Thermal runaway risk: classification by criticality classes

p. 190

Stoessel, F, 1993. Chemical Engineering

Progress 89, 68-75.

II: risk assessment

criticality

class

tem

pera

ture

Risk assessmentII Thermal Runaway

b

b

b

b

b

!



II: case study 1

(from ortho-nitro-toluene (ONT); DNDB is key intermediate for pharmaceutical production)

Production of dinitrodibenzyl (DNDB)

Reaction

alcoholate

solvent

Case study 1II Thermal Runaway Example1: DNDBII Thermal Runaway

Goal

a) assessment of thermal risk potential

b) safety measures planning

Lab process

(at T=12 ˚C; isotherm; semibatch process)

1. 500 g solvent is already fed in the reactor. (bp. ~90 ˚C)

2. add 1 mol alcoholate.

3. add 0.5 mol ONT within 70 min.

4. addition of ONT stops for 15 min.

5. add 1 mol ONT within 140 min.



II: case study 1

DTA measurement 1

DTA measurement 2

q•

q•

Temperature

Temperature

Temperature ramp:

4 ̊C/min

*

*

* Differential Thermo-Analyser

educts cold mixed

educts cold mixed, residual heat after 21 hours at 0 ºC

DTA Measurements

-320KJ/KG

-410KJ/KG -170KJ/KG

dec

dec

Case study 1II Thermal Runaway Example1: DNDBII Thermal Runaway



II: case study 1

*Isothermal heat flow reaction calorimetry at T = 12 ˚C

accumulated heat of reaction

measured heat of reaction

time

rea

ctio

n e

ne

rgy [

J/k

g]

(th

ou

sa

nds)

total produced heat

of reaction with

ideal dosing control

Heat accumulation of DNDB lab process (semibatch mode)

Case study 1II Thermal RunawayII Thermal Runaway Example1: DNDB



II: case study 1 – video clip 9Case study 1II Thermal RunawayII Thermal Runaway

Adiabatic conversion of ONT to DNDB (video clip 9)

Example1: DNDB



II: case study 2

Risk assessment after explosion of a pesticide-intermediate

A pesticide-intermediate is crystallized from a melt on a cooling belt and bottled as a

granulate formulation in 20 kg containers. As a result of a technical failure on the

cooling belt 2600 kg of this melt (melting point 80 ˚C) must be stored at 90 ˚C over the

weekend in a heated unstirred tank.

product loss = 1% per day

storage conditions: tank= 4 m3

accident:

thermal explosion after 50 hours

How to explain the cause of this accident?

Case study 2II Thermal Runaway Example2: pesticideII Thermal Runaway



Assumption: cp= 2 kJ/(kg·K)

with 1% decomposition per day

Doubling the rate of decomposition when the temperature rises by 10 ˚C :

90 ˚C 0.2 ˚C per hour



:

etc.

1) Impact

2) Probability

DTA measurement: temperature ramp = 4 K/min

qdec

•

temperature (˚C)

II Thermal RunawayII Thermal Runaway Example2: pesticide

d𝑇

d𝑡=4°C

day≅

0.2°C

h(@90°C)

∆𝑇ad =800

kJkg

2kJ

kg · K

= 400K



time [h]

140

130

120

110

100

90 T0

50 h = TMRad1)

25 h = TMRad/2 12.5 h 6.25

150

0.2 ˚C/h

0.4 ˚C/h

temperature

1) according to upper slope (=0.4 ˚C/h)

TMRad: 10-degree rule for doubling of reaction rate

Case study 2II Thermal RunawayII Thermal Runaway

T [˚C]

Example2: pesticide



Process alternative: after technical fault of the cooling belt storage of the melt in a

stirred double-walled tank (4 m3) at 90˚C with warm water cooling (TC = 85˚C)

decomposition power:

stirring power:

with: Tmax = 5 K

U = 200 W/m2/K

m = 2600 kg

A = 8 m2

cooling power:

cooling power > decomposition power + stirring power

stable heat balance

II: case study 2Case study 2II Thermal RunawayII Thermal Runaway

𝑞cooling = 𝑈 · 𝐴 ·∆𝑇

𝑚≅ 3W/kg

Example2: pesticide

𝑞dec ≅ 0.1 W/kg

1 kW/𝑚3 → 𝑞stir ≅ 1W/kg



II: case study 2Case study 2II Thermal RunawaySummary Thermal process safety

Content of this lecture

• “As usual” and “runaway” scenario

• Thermal reactor stability: Batch and semibatch process

– Mass and heat balance; Semenov diagram

– Concentration profiles, heat production and thermal conversion (semibatch

only)

• Comparison of reactor types batch, semibatch, ISTR, plug-flow and cross

flow

• Thermal runaway: Basic scenario for risk analysis

– ∆Tad as impact indicator

– Typical functional groups of unstable compounds

– TMRad as probability indicator. Approximation with 10-degree rule or

adiabatic heat balance

– Risk classification by criticality classes



II: case study 2Case study 2II Thermal RunawayReview questions Thermal process safety

You should be able to answer the following questions

• Draw the “as usual” and “runaway” scenario in a time-temperature profile.

• How are batch and semibatch reaction processes defined? Sketch the two reactor

types. What assumptions are made?

• Compare mass balance, heat balance and cooling process for a batch and

semibatch reactor.

• How are mass and heat balances defined in dimensionless form (ChemIng only)

• Draw possible concentration profiles for a semibatch reactor. Which assumptions

are made?

• Compare batch, semibatch and continuous reaction modes regarding qR, qC and

thermal scale-up effect.

• What parameter is an indicator for the impact of a thermal runaway? Which

classes are here differentiated?

• Give some examples of typical functional groups of unstable compounds.

• What parameter is an indicator for the probability of a thermal runaway? How can

this value be approximated?

. .

process risk analysis summary risk characterization, assessment … · 2017-05-12 · thermal...

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