venting a runaway rxn
DESCRIPTION
.TRANSCRIPT
-
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
better chemistry - faster
-
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
better chemistry - faster
-
PHI-TEC Pressure Vessel
better chemistry - faster
-
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
better chemistry - faster
-
Pressure-Time data for NMTS (20%) in Dioxane
Typical data in different test cells
better chemistry - faster
-
Rate data NMTS (20%) in Dioxane
6
)
d
T
d
t
(
=
1
.
5
6
d
d
better chemistry - faster
-
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