RUNAWAY REACTIONS Experimental Characterization and Vent Sizing

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1 RUNAWAY REACTIONS Experimental Characterization and Vent Sizing Ron Darby Professor of Chemical Engineering Texas A&M University College Station, TX (979)

2 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

16 Notation A V ρ Cv T λ Ps R T s P b M wv C D = vent area (m 2 ) = reactant volume (m 3 ) = reactant density (kg/m 3 ) = liquid specific heat (J/(kg K)) = self-heat rate (K/s) = latent heat (J/kg) = relief set pressure (Pa) = gas constant (8314 Pa m 3 /(K kmol)) = relief set temperature (K) = backpressure (Pa) = molecular weight of vapor (kg/kmol) = discharge coefficient (-)

17 Gassy Critical Flow A V = C D v ρ P M m P RT t wg 1 2 where: P m t P M wg v = 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 10-4 m 3 )

19 Sample Exothermic Reactions Methanol/Acetic Anhydride (Tempered Vapor System) (CH 3 CO) 2 O + CH 3 OH CH 3 CO 2 CH 3 + C 2 H 4 O 2 Peroxide Decomposition (Hybrid) H 2 O 2 H 2 O + 1/2 O 2

20 Example -Tempered System Scenario: Loss of cooling for a 1500 kg batch of methanol/acetic anhydride in a 2.3 m 3 (600 gal) vessel. MAWP is 300 psig, set pressure is 15 psig. Fill fraction of 81%.

21 Test #1 (300 psig) Chilled 10g sample (3.86g methanol, 6.14g acetic anhydride) Containment vessel pressurized to 300 psig Scan temperature at 2 C/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)

22 Methanol/Acetic Anhydride psig

23 Methanol/Acetic Anhydride psig

24 Self-heat rate at P=15 psig (T=95 C) is about 20 C/min 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 95 C Compare self-heat rate for both tests

25 Methanol/Acetic Anhydride - 15 psig

26 Methanol/Acetic Anhydride

27 General Screening Equation Vapor, Gassy or Hybrid Critical Flow A V = C C D P ( ) T + P where: T P C A/V = 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)

29 Methanol/Acetic Anhydride is Tempered (Vapor System): where: A = D C T V C P S 6 ρ c RT S C = 8.0x10 v λ Mwv 1 2 (Factor of 2 included for foamy behavior)

30 Example Results Use properties of methanol at 95 C V M wv ρ C v λ T P s T s = (0.81)(2.3) = 1.86m 3 = kg/kmol = (1500)/(1.86) = 800 kg/m 3 = 3200 J/ kg K = 1.0 x 10 6 J/kg = 20 C/min = 15psig = 29.7psia = 95 C = 368K Results: C = 6.3 x 10-3, A / V = 4.2 x 10-3 m -1 d = 3.9 in

31 Use Properties of Mixture at 50% Conversion M wv C v λ = 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

32 Using Screening Equation and Properties of Water (Do not use P 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 nonfoamy behavior. Thus the value of C in each equation should be multiplied by 0.5 and the diameter divided by 2 1/2.

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

34 REFERENCES 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, , FAX , Fauske@Fauske.com)

35 Questions??

4 th Edition Chapter 9

Insert Page 547A 4 th Edition Chapter 9 In summary, if any one of the following three things had not occurred the explosion would not have happened. 1. Tripled production 2. Heat exchanger failure for