venting a runaway rxn

RELIEF OF CHEMICAL REACTORSRELIEF OF CHEMICAL REACTORSRELIEF OF CHEMICAL REACTORSRELIEF OF CHEMICAL REACTORSUNDER RUNAWAY CONDITIONS UNDER RUNAWAY CONDITIONS -- IntroductionIntroduction

DR JASBIR [email protected] helgroup comwww.helgroup.com

Harry Yang, HEL Beijing

([email protected])TEL : 010 8210 1033

HEL IncNew Jersey, USA

HEL ChinaChna

HEL IndiaIndia

HEL AGGermany

HEL LtdLondon, UK

REACTOR PROTECTIONREACTOR PROTECTIONTECHNIQUESTECHNIQUESTECHNIQUESTECHNIQUES

1. Reduce the potential for an incident

Choice of chemistry, operating conditions Choice of solvent Inventory etc etc Inventory etc. etc.

2. Provide automated controls/alarms

High temperature and pressure shut offs Intelligent controls

3. Design to cope with the residual hazard

Final back-stop is Venting (relief) of reactor

REACTOR VENTINGREACTOR VENTINGREACTOR VENTINGREACTOR VENTING

Pressure Relief ValvePressure Relief Valve

or

Bursting disk

oror

If pressure too high, valve or vent opens ..

3

p essu e too g , va ve o ve t ope s ..relieves the pressure and protects reactor

VENTING SIZING CRITERIAVENTING SIZING CRITERIA

Vapour/gas removal rate vapour/gas generation rateVapour/gas removal rate vapour/gas generation rate If this is not satisfied .. Pressure increases in reactor ..

Explosion results!

4

Phenolic Resin runaway .. Vent not large enough

better chemistry - faster

TWO DECISIONS THAT DETERMINE VENT SIZETWO DECISIONS THAT DETERMINE VENT SIZETWO DECISIONS THAT DETERMINE VENT SIZETWO DECISIONS THAT DETERMINE VENT SIZE

Maximum Rate of Gas/Vapor generation

What is the fluid entering the vent only gas/vapour or liquid too? What is the fluid entering the vent .. only gas/vapour or liquid too?

6

Venting Common liquids ... subjected to fireVenting Common liquids ... subjected to fire

Before During After

P P P

Time

PRESSURE RELIEF OF PROCESSPRESSURE RELIEF OF PROCESSEQUIPMENTEQUIPMENTEQUIPMENTEQUIPMENT

Maximum allowable pressure, Pm

V t t li f t P Overpressure

U

R

E

Vent opens at relief set pressure, Ps Overpressure

P

R

E

S

S

U

Vent opensNormalOperation

FaultconditionOperation develops

8

TIME

BASIC CALCULATIONBASIC CALCULATIONOF RELIEF SIZINGOF RELIEF SIZINGOF RELIEF SIZING OF RELIEF SIZING

Problem specification Identify the source term of vapour/gas (e.g. external heat input from ay p g ( g pfire) .. Worst Credible Maloperation (WCM)

CalculationCalculation Size the vent to pass the maximum required vapour/gas rate (ie at WCM)

9

WHAT IS THE WORST CREDIBLEWHAT IS THE WORST CREDIBLEMALOPERATION (WCM)MALOPERATION (WCM)MALOPERATION (WCM)MALOPERATION (WCM)

loss of cooling? external fire? recipe errors? raw material impurities? raw material impurities? blockages? loss of, or resumed, agitation?, , g temperature too high? temperature too low? leaks?

Not easy to decide which will need largest VentNot easy to decide which will need largest Vent

10

PROCESS CONDITIONSPROCESS CONDITIONSDURING RELIEFDURING RELIEFDURING RELIEF DURING RELIEF

Relief Pressure

Ps = Maximum Allowable Working Pressure (MAWP)

Pm = 110% of Ps (in barg) (BS 5500), up to 121% of Ps for fire relief ( ASME codes)

Information/data needed

From any value of pressure obtain the corresponding temperature (byi t ti ti th d )experiment or estimation methods)

Find physical property data at any temperature

11

TYPICAL APPROACHTYPICAL APPROACHTYPICAL APPROACHTYPICAL APPROACHTO RELIEF SIZING : Fire CaseTO RELIEF SIZING : Fire Case

For a Fire :

Vapour generation rate = Heat from firelatent heat of vapourisation

Vapour/gas removal rate vapour/gas generation rateUse compressible gas flow equations to calculate that this

vapour rate can be removed

At the relief conditions

12

TYPICAL APPROACHTYPICAL APPROACHTYPICAL APPROACHTYPICAL APPROACHTO RELIEF SIZING: Reaction HazardTO RELIEF SIZING: Reaction Hazard

For a chemical reaction:

Vapour generation rate = (reaction rate reaction enthalpy)latent heat of vapourisation

Vapour/gas removal rate vapour/gas generation rate

Use compressible gas flow equations to calculate that thisvapour rate can be removed

BUT .. Is this correct?How is data obtained?

13

DATA .. Vent Sizing for runaway reactionsDATA .. Vent Sizing for runaway reactions

Is the rate of reaction known? Incorrect temperature? I i ? Incorrect recipe?Wrong order of addition? Contamination, etc. ?

Is the reaction enthalpy known? New reactions may be promoted in the runaway

Is the pressure-temperature relationship known? Composition dependence during the reaction Non-condensable gas generation in addition to vapour?

Is vapor still vented? What happens if liquid is entrained?

VENTING REACTIONS: Data NeededVENTING REACTIONS: Data Needed

Is the rate of reaction known?R lRarely

Is the reaction enthalpy known?U lik l f ll iUnlikely for all scenarios

Is the pressure-temperature relationship known?U lik l f l i iUnlikely for real mixtures in vent

Is vapor still vented? Unlikely

Hence ... conventional vent sizing approach NOT CORRECTWhat next?

15

DIERS RESEARCHDIERS RESEARCHDesign Design instinst for emergency Relief Systemsfor emergency Relief Systems

The DIERS research programme in the United States. 29 companies formed DIERS in 1976 in collaboration with AIChE $ 1 6 millionformed DIERS in 1976 in collaboration with AIChE. $ 1.6 million (1979 money) programme: still continuing.

M h d lMethodology

Equipment (VSP, Phi-tec II)

Software (Safire, Superchems for DIERS)

Workbooks for chemical reactor relief system sizing. More digestible re-statement (with some updating) of the material presented in the DIERS manual. Willday and Etchells, ISBN 0 7176 1389 5, HSE Booksmanual. Willday and Etchells, ISBN 0 7176 1389 5, HSE Books

KEY STEPS TO VENT SIZINGKEY STEPS TO VENT SIZINGBased on DIERS methodology (I)Based on DIERS methodology (I)Based on DIERS methodology (I)Based on DIERS methodology (I)

Establish main source of pressure rise during runaway- is pressure rise due to vapour pressure increase, or- is it primarily due to gas generation (typical decompositions) p y g g ( yp p )

How? using low-phi-adiabatic calorimeter

Establish vapor/liquid ratio entering vent- Is it all gas (or vapour)- homogeneous two-phase mixture- somewhere in between.

How? using low-phi adiabatic calorimeter g p

KEY STEPS TO VENT SIZINGKEY STEPS TO VENT SIZINGBased on DIERS methodology (II)Based on DIERS methodology (II)Based on DIERS methodology (II)Based on DIERS methodology (II)

To size Vent needed to protect reactorp

- Determine the maximum gas or vapour generation rate.Perform Tests to determine worst case (WCM )( )

- Select the equation for gas or appropriate two-phase flow and size the vent to cope with maximum rate of gas/vapourp g p

- Obtain physical property data (mostly direct from tests)

How?Well established DIERS methodology.

Requires extensive training.

KEY FINDING OF DIERS methodology (III)KEY FINDING OF DIERS methodology (III)

- Traditionally, assumed that vapor/gas will separate from liquid y, p g p qand enter the vent.

- However, most chemicals especially mixtures under pressure will not vent as gas/vapor

- Instead, mixture of liquid with gas/vapour will enter the vent

This has profound implications ...most vents are UNDERSIZED ..

by factor of 10 !

VENTING REACTIVE LIQUIDS .. Under runaway reaction VENTING REACTIVE LIQUIDS .. Under runaway reaction

P P PP

20Time

IMPORTANCE OF LIQUID ENTRAINMENTIMPORTANCE OF LIQUID ENTRAINMENT

Pressure rises when vapour is generated by liquid heating.g

Venting of Vapour ... reduces pressure.

Venting of same mass of liquid .. makes hardly any difference to pressuredifference to pressure

So liquid essentially blocks the vent .. stopping vapour q y pp g pescape

As a result, a ten-fold increase in vent area may be needed

21

FLOW REGIME ON PRESSURE RELIEFFLOW REGIME ON PRESSURE RELIEFFLOW REGIME ON PRESSURE RELIEFFLOW REGIME ON PRESSURE RELIEF

Many reacting systems generate a two-phase flow on pressure relief. This is accentuated by high reactor fill levels, high relief set pressures, high superficial velocities, naturally foamy systems, etc.

Possible flow regimes are:

All gas/vapour flowAll gas/vapour flow Churn turbulent flow (bubbles coalesce, increase in size and

rise fast) Bubbly flow (small discrete bubbles rising relatively slowly) Bubbly flow (small discrete bubbles rising relatively slowly) Homogeneous two-phase flow (naturally foamy)

22

FLOW REGIME ON PRESSURE RELIEFFLOW REGIME ON PRESSURE RELIEF

For high velocity venting, characteristic final void fractions are:PPP P

Homogeneousor foamy Bubbly

Churnturbulent

All gas orvapour

f ~ = 0.65f ~ = 0.8f ~ = 1 f ~ = start

I i i f f h d i i d23

Increasing size of vent for the same depressurisation duty

THE COMPLETE DISPOSAL SYSTEMTHE COMPLETE DISPOSAL SYSTEM

Atmosphere,

THE COMPLETE DISPOSAL SYSTEMTHE COMPLETE DISPOSAL SYSTEM

p ,scrubber, flareincinerator,etc

Cooling

ReactorCatchpot,

dump tank,quench tank Vapour condensation

24

Vapour condensation

VENTING RUNWAY REACTIONS .. DATA...VENTING RUNWAY REACTIONS .. DATA...

Modelling runway reactions very time consuming and i h i t d trequires much input data

In most cases, input data is not available

Anyway, experimental verification is needed

Hence, widely accepted solution is direct testing usinglow-phi, adiabatic calorimeter.p ,

25

PHI- Factor

(Mass X specific heat) test cell

(Mass X specific heat) reactantsPhi-factor = 1 +

( p )

Testing in Standard ARC 2 + <

Data for relief sizing: < 1.1


Data from ARC devices ...cannot be used for vent sizing

HEL Phi-Tec I (ARC)

DIERS developed concept of low-phi using pressure compensation

low phi-factor with PHI-TEC II

Pressure Compensation allows low phi-factor test cells


PHI-TEC Pressure Vessel


PHI TEC II ADIABATIC CALORIMETERPHI TEC II ADIABATIC CALORIMETERPHI TEC II ADIABATIC CALORIMETERPHI TEC II ADIABATIC CALORIMETER

Effect of PHI-Factor Decomposition of TBP(20%) in Toluene(20%) in Toluene

Higher phi-factor leads to artificially slower rise in temperature and a reduced maximum temperature, low phi-factor resembles response of real plant


Pressure-Time data for NMTS (20%) in Dioxane

Typical data in different test cells


Rate data NMTS (20%) in Dioxane

6

)

d

T

d

t

(

=

1

.

5

6

d

d


CONCLUSIONSCONCLUSIONS

Most existing vents in industry are too small!

V i i f h i l i i l ! Vent sizing for runaway chemical reactions is complex!

Training courses lasting 2-3 days are normally needed Training courses lasting 2-3 days are normally needed

Requires a combination of chemical engineering with q g gcalorimetry data

venting a runaway rxn

Documents