RSST ARSST EXPERIMENTS, APPLICATIONS AND VENT SIZING

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RSST (ARSST) EXPERIMENTS, APPLICATIONS AND VENT
SIZING

• Amy E. MillerChemical EngineerFauske
  Associates, Inc.Burr Ridge, IL www.Fauske.com

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ARSST CALORIMETER

• Advanced Reactive System Screening Tool
• Screen Chemicals for Reactivity
• Determine Onset Temperature for Exothermic
  Reactions
• Estimate 1st Order Kinetic Parameters
• Determine Self Heat and Pressure Rise Rates
• Obtain Data Needed to Size Relief Devices
• Distinguish Between Foamy and Non-Foamy
• Data is Directly Scaleable to Plant Process

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RSST Experimental Data

• RSST is adiabatic calorimeter
• Conservative estimate for heat of reaction
• RSST has a low phi-factor 1.04
• Allows data to be directly scaled up

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ARSST Containment Vessel and Control Box(shown
with Super Magnetic Stirrer)
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ARSST Containment Vessel
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Schematic of ARSST containment vessel
Test cell assembly
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Runaway Reaction Classes
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Runaway Reaction Classes

• Vapor system only non-condensable gas is
  generated
• Gassy system pressure rise is generate by
  non-condensable gas
• Hybrid system pressure increase is due to both
  vapor pressure of system and non-condensable gas
  that is generated

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Vent Sizing Terms

• Back pressure upstream pressure that will be
  imposed if a rupture disk breaks
• MAWP design pressure of the vessel
• Overpressure relative to the set pressure
• Tempering point when the reaction rate
  approaches zero (typically during boiling)
• Foamy behavior

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Vent Size Determination
Tempered, Critical Flow
Tempered, Subcritical (Low Pressure) Flow
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Notation

• vent area (m2) reactant volume
  (m3) reactant density (kg/m3) liquid
  specific heat (J/(kg K)) self-heat rate
  (K/s) latent heat (J/kg) relief set
  pressure (Pa) gas constant
• (8314 Pa m3/(K kmol)) relief set
  temperature (K) backpressure (Pa)
  molecular weight of vapor (kg/kmol) disch
  arge coefficient (-)

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Gassy Critical Flow

• where
• maximum rate of pressure rise (Pa/s)
• test sample mass (kg)
• maximum allowable accumulated pressure,
  MAAP (Pa)
• molecular weight of gas (kg/kmol)
• ARSST containment volume
• (3.5 x 10-4m3)

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Hybrid, Critical Flow

• Hybrid, Subcritical (Low Pressure) Flow

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(CH3CO)2O CH3OH
Sample Exothermic Reactions
Methanol/Acetic Anhydride (Tempered Vapor
System)
CH3CO2CH3 C2H4O2

• Peroxide Decomposition (Hybrid)

H2O2
H2O 1/2 O2
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Example -Tempered System

• Scenario
• Loss of cooling for a 1500 kg batch of
  methanol/acetic anhydride in a 2.3 m3(600 gal)
  vessel.
• MAWP is 300 psig, set pressure is 15 psig.
• Fill fraction of 81.

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Test 1 (300 psig)

• Chilled 10g sample (3.86g methanol, 6.14g acetic
  anhydride)
• Containment vessel pressurized to 300 psig
• Scan temperature at 2C/min for 30 min
• Computer records T and P vs time
• Plot self-heat rate and pressure rate vs inverse
  temperature
• Exotherm at about 25 min, final P 300 psig (no
  gas)

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Methanol/Acetic Anhydride - 300 psig
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Methanol/Acetic Anhydride - 300 psig
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Test 2 (15 psig)

• Determine boiling point and vapor pressure at 15
  psig
• Same as Test 1, but with back pressure at 15
  psig (A relief valve can be used to control the
  back pressure during the runaway)
• Computer records T and P vs time
• Mixture tempers at about 95C
• Compare self-heat rate for both tests
• The self-heat rate at P15 psig (T95C) is about
  20C/min

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Methanol/Acetic Anhydride - 15 psig
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Methanol/Acetic Anhydride
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General Screening Equation Vapor, Gassy or Hybrid
Critical Flow

• where
• self-heat rate (C/min)
• pressure rise rate (psi/min)
• 3.5 x 10-3 (7 x 10-3 if foamy)
• 1/m (vent area / reactor volume)

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General Screening EquationVapor, Gassy or Hybrid
for Subcritical Flow
where self-heat rate (C/min)
pressure rise rate (psi/min) 4.0 x 10-4
(8.0 x 10-4 if foamy) 1/m (vent area
/ reactor volume)
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Methanol/Acetic Anhydride is Tempered (Vapor
System)
where
(Factor of 2 included for foamy behavior)
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Example Results Use properties of methanol at
95C

• (0.81)(2.3) 1.86m3
• 32.04 kg/kmol
• (1500)/(1.86) 800 kg/m3
• 3200 J/ kg K
• 1.0 x 106 J/kg
• 20C/min
• 15psig 29.7psia
• 95C 368K
• Results
• C 6.3 x 10-3 , A / V 4.2 x 10-3 m-1
• d 3.9 in

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• Use Properties of Mixture at 50 Conversion
• 53.4 kg/kmol
• 2500 J/ kg K
• 583,000 J /kg
• Results
• C 6.6 x 10-3 , A / V 4.4 x 10-3 m-1
• d 4.1 in
• Use Properties of Water at Ambient Conditions
• Results
• C 6.4 x 10-3 , A / V 4.3 x 10-3 m-1
• d 4.0 in

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Using Screening Equation and Properties of Water

• (Do not use term, since system is
  tempered) C 7.0 x 10-3 , A / V 4.7 x
  10-3 m-1
• d 4.2 in
• Foam Test
• The Flow Regime Detector indicates non-foamy
  behavior.
• Thus the value of C in each equation should be
  multiplied by 0.5 and the diameter divided by
  21/2.

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Summary

• The ARSST is a very useful device for
• Screening Chemicals and Chemical Mixtures for
  Reactivity
• Obtaining Data Needed to Size Relief Vents for
  Runaway Reactions
• Illustrating the Relationship between Runaway
  Reaction Kinetics and Safety
• Providing Students With a Valuable Experience in
  these Methods Via a Realistic Laboratory
  Experiment

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REFERENCES

• Burelbach, J.P., Vent Sizing Applications for
  Reactive Systems, Process Plant Safety
  Symposium, AIChE National Meeting, Houston, TX,
  April 2001
• Fauske, H.K., Properly Size Vents for
  Nonreactive and Reactive Chemicals, CEP, 17-29,
  February, 2000
• Fauske, H.K., The Reactive System Screening Tool
  (RSST), U.S. Patent 5,229,074, July, 1993
• Darby, R., A Unit Operations laboratory
  Experiment for Runaway Reactions, AIChE Annual
  Meeting, Paper T1303H, Los Angeles, CA, November
  2000
• (Note Additional details, pricing, etc. can be
  obtained from Fauske Associates, Inc., Burr
  Ridge, IL, 630-323-8750, FAX [phone],
  Fauske_at_Fauske.com)

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Questions??