RUNAWAY REACTIONS Experimental … REACTIONS Experimental Characterization and Vent ... the reactivity and for sizing relief vents in ... A foamy system will result in two-phase
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RUNAWAY REACTIONS Experimental Characterization and Vent Sizing Ron Darby Professor of Chemical Engineering Texas A&M University College Station, TX 77843-3122 (979) 845-3301 [email protected]
This presentation describes the equipment and procedures for a laboratory experiment that allows the student to measure the time-dependent (pressure and temperature) characteristics of a runaway reaction, and illustrates how this information can be used to estimate the required area of a relief vent that would be adequate to control the reactor pressure during the runaway. The scope and time duration of the experiment is ideal for a typical unit operations laboratory, and the topic provides the student with valuable experience that will be very useful in an industrial setting. An example is given with data taken in the apparatus for a tempered runaway reaction. The equations for estimating the size of the relief vent required for a large scale reactor are applied to these data to illustrate the method. This or other reactions can be run in the laboratory experiment, and/or the sample data given here can be given to students for an exercise in how to treat the data and size relief vents.
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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It is imperative that the potential reactivity of every chemical involved in a plant or process be understood, and the conditions under which the reactivity could get out of control identified. The Advanced Reactive System Screening Tool (ARSST) provides a method for screening potentially reactive chemicals, as well as for the acquisition of quantitative data needed to characterize the reactivity and for sizing relief vents in the case of runaway reactions.
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This is a photograph of the ARSST containment cell, stirrer and computerized control module. The stainless steel containment cell has an internal volume of 350 ml.
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This is a close-up of 350 ml containment cell showing the sample ports and transducer leads.
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This cut-away view of the containment cell shows the arrangement of the connections to the cell and the 10 ml test cell, including the heaters, thermocouples, and inlet and outlet ports.
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This expanded view of the 10 ml test cell shows the arrangement of the thermocouples, heaters, insulation and feed port.
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Whether or not the liquid exhibits foamy behavior is important in determining the phase properties of the vent discharge fluid. A foamy system will result in two-phase flow, whereas a non-foamy system may be either single- or two-phase flow. This slide illustrates the difference between all vapor venting with complete disengagement of liquid from the vapor, which will result in all vapor flow, and a foamy system which results in a two-phase fluid mixture leaving the vent.
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A special foam detector is an optional accessory to the system, and consists of a separate heater and thermocouple mounted above the sample in the neck of the test cell. The heater keeps this thermocouple at a temperature which is approximately 100o F above that of the sample. If foam is generated in the cell, the foam contacting the thermocouple detector will cool or “quench” the thermocouple. Thus a drop in this detector temperature indicates the presence of a foamy sample.
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This photograph illustrates the difference in appearance between non-foamy and foamy test fluids.
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There are three possible conditions that can be encountered with a runaway reaction which can result in an overpressure situation: A vapor (or tempered) condition, in which the rise in pressure is due only to the increase in vapor pressure of the liquid in the reactor as the temperature increases from the heat evolved from the exothermal reaction. A gassy condition, in which the pressure rise is due to the evolution of a non-condensable gas by the reaction (e.g. a decomposition reaction). A hybrid condition, in which the pressure increase results from both the increase in vapor pressure as well as evolution of a non-condensable gas. The nature of the temperature vs time relation gives an indication of which type of condition exists in the reactor.
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Observation of the pressure before and after the run, as well as the pressure and temperature rise rates, tells whether the reaction is tempered (vapor), hybrid, or gassy: If the pressure at the end of the run (after cool-down) is the same as at the start, then no non-condensable gas has been evolved and the system is tempered (vapor) If the final pressure is greater than the initial pressure, but the pressure and temperature both level off at the same time (same temperature), then the system is gassy. If the final pressure is greater than the initial pressure, but the pressure and temperature level off at different times (temperatures), then it is a hybrid system. (Note: PMAAP represents the Maximum Allowable Accumulated Pressure for the reactor)
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For a vapor system, if the total mass lost from the liquid is less than 1% of the initial mass, and there are no pressure spikes during the run, the reaction is not likely to result in a serious overpressure condition (this can be confirmed if the vapor pressure of the liquid as a function of temperature is known). If the mass loss is significant, and/or a pressure spike occurs, a relief vent must be sized properly and installed on the reactor. This requires a knowledge of the vapor pressure versus temperature properties of the liquid, which can be obtained from either an independent knowledge of the liquid vapor pressure vs temperature, or it can be measured in a separate test run with pressures up to the relief set pressure.
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If the system is gassy, a relief vent can be sized based on the PMAAP and the maximum pressure rise rate during the run. If the resulting required vent area is excessively large, the possibility of a hybrid system can be determined by running a second test up to the set pressure. If the maximum temperature reached during this second test is less than that reached during the initial test, vapor (I.e. a tempered system) is present. If this occurs, both the pressure rise rate and the temperature rise rate at the set pressure are required to size the vent (see equations to follow later). If the maximum temperature from the second test is not less than that from the first test, the system is gassy and the vent can be sized using only the maximum pressure rise rate at the PMAAP (see equations to follow.)
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If the initial screening indicates the presence of a hybrid system, a second test should be run for pressures up to the set pressure. As before, if the maximum temperature reached during this second run is less than that reached in the first run, vapor (tempering) is present and the vent must be sized for a hybrid system using both the pressure rise rate and the temperature rise rate at the set pressure (and the corresponding temperature). If the maximum temperature reached in the second run is not less than that reached in the first run, no vapor (tempering) is present and the vent can be sized for a gassy system using only the maximum pressure rise rate at the PMAAP.
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The equations that determine the required vent area (A) for a given volume (V) of reaction mass are derived from an energy balance on the reactor. The energy released (generated) by the reaction must be removed by mass flow from the reactor at a rate at least equal to the rate at which it is generated to prevent over-pressuring the reactor. The energy balance on the reacting mass determines the required rate of mass removal from the reactor. This is coupled with the mass flux capacity of the vent, which is determined from an energy balance on the vent (assumed it to be an isentropic nozzle), to determine the area of the vent which permits the required flow rate for the given driving force. The driving force for flow in the vent depends upon whether or not the velocity reaches the speed of sound (I.e. critical (choked) or sub-critical flow). For a tempered (vapor) system, the driving force is the saturation vapor pressure in the reactor (Ps) at the relief pressure/temperature. If the flow is critical (choked), the vent flow rate is independent of the exit or back pressure (Pb), but for sub-critical flow the back pressure does affect the flow rate.
= 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) = discharge coefficient (-)
Notation AVρ
vCTλ
sPR
sTbPwvM
DC
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The Notation defines the terms in the equations (SI or any consistent system of units can be used, with appropriate conversion factors as needed)
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)
21
wg
tD RTM
PmPρv
C0.611
VA
=
�Ptm
P
wgMv
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For a gassy system, the reaction rate and required mass flow rate are directly proportional to the rate of gas release, as measured by the pressure rise rate. The relief area is determined by this mass flow rate and the mass flux capacity of an isentropic (ideal gas) nozzle. For critical (choked) flow, the driving force is the relieving pressure only, independent of the exit back pressure. It is noted that sub-critical gassy flow is very rarely, if ever, encountered. (Note that this equation includes v, the ARSST containment volume, because the pressure is measured in this volume and the maximum pressure rise rate is used to calculate the corresponding rate of mass release by the reaction. If a containment volume other than 350 cc is used, this number must be modified accordingly)
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For a hybrid system the energy released by the reaction is removed by both the vapor generated by the evaporating or flashing liquid (which is proportional to the temperature rise rate), as well as the rate of non-condensable gas evolution (which is proportional to the pressure rise rate). For critical (choked) flow, the vent driving force is independent of the exit back pressure, but for sub-critical flow the back pressure is important.
A good example of a tempered (vapor) reaction that can be used in a laboratory experiment within the usual time span allowed, and which gives consistent results, is the reaction of methanol and acetic anhydride to produce methyl acetate and acetic acid. Another convenient non-toxic and non-hazardous reaction that illustrates a hybrid system is the decomposition of hydrogen peroxide to water and oxygen. This reaction is catalyzed by iron, so the addition of a small amount of iron filings can significantly alter the onset temperature and reaction rate. This can be used to provide variety for the lab experimental conditions.
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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This shows a hypothetical scenario which requires the acquisition of data for the sizing of a vent for a possible runway reaction involving methanol and acetic anhydride. (note: MAWP stands for Maximum Allowable Working Pressure)
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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This is the procedure for running the ARSST for a test up to the MAWP (300 psig) of the reactor vessel. The reactant charge contains excess methanol, which insures a vapor (tempered) system while all of the acetic anhydride is consumed.
Methanol/Acetic Anhydride - 300 psig
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This shows the measured temperature and pressure versus time for the reaction. Note the rapid temperature rise at about 50 minutes, indicating a runaway reaction. After the reaction is complete and the reactor cools down, the final pressure reached was essentially the same as the initial containment pressure (300 psig), indicating a tempered (vapor) reaction.
Methanol/Acetic Anhydride - 300 psig
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This shows the same data, plotted as the derivatives of the raw data versus time (I.e. the self heat rate, or temperature rise rate, and the pressure rise rate versus time). Note that the maximum self heat rate exceeds 100oC/min, and the maximum pressure rise rate is approximately 10 psi/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
Self-heat rate at P=15 psig (T=95°C) is about 20°C/min
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Since this is a vapor (tempered) system, it is necessary to determine the temperature at which the vapor pressure reaches the vent relief pressure. This could be calculated if independent information about the liquid vapor pressure versus temperature is known, or it can be determined from a second run at a pressure equal to the relief pressure. This slide outlines the procedure for doing the second run to get this information at a relief pressure of about 15 psig. The relief pressure is controlled to approximately 15 psig by a relief vent on the containment vessel that opens near this pressure.
Methanol/Acetic Anhydride - 15 psig
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This is the raw data from the second run. The maximum pressure was limited to about 17 psig by using a pre-set relief valve on the containment cell.
Methanol/Acetic Anhydride
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This shows the self heat rate for the second (low pressure) run superimposed on the first (high pressure) run Note that the self heat rate falls to zero at about 100oC, at which temperature the maximum self heat rate is approximately 20oC/min. This is used as the design point for sizing the required relief vent for the reactor.
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)
( )PTPC
CVA
D
+=
�T�P
CA/V
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A convenient form of the hybrid system equation for critical (choked) flow is shown here. It has been noted that the numerical value of the coefficients of the pressure rise rate and the temperature rise rate terms is very nearly the same regardless of the specific fluids used for most materials, when the units shown above are used. These coefficients, which are groupings of the fluid properties, are collected and factored out as the coefficient C, which has very nearly the value indicated above for many different fluids, when the indicated units are used. Note that the value of C also includes the volume of the containment cell from the coefficient of the pressure rise rate term. This would result in a different value of C if the 350 cc cell is not used.
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This the form of the general screening equation for sub-critical flow, for which the exit back pressure is important.
Methanol/Acetic Anhydride is Tempered (Vapor System):
=
D S
A C TV C P
− =
12
6 Sv
wv
RTρ cC 8.0x10λ M
where:
(Factor of 2 included for foamy behavior)
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Since our methanol/acetic anhydride system is tempered (vapor), the pressure rise rate term is not needed in the screening equation. The fluid properties needed to calculate the constant C are shown here. The numerical coefficient includes the volume of the 350 cc containment cell, as well as a factor of 2 to account for the larger vent area that would be required for two-phase flow if the system is foamy. (Note: To determine the foamy nature of the system, an independent test is run with half the reactant mass in the test cell. This test shows that the methanol/acetic anhydride system is not foamy, so the resulting calculated value of A must be adjusted accordingly – this is done later).
Example Results Use properties of methanol at 95°C = (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 = 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
VwvM
ρ
vCλTsPsT
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The values of the system parameters and fluid properties required in the vent sizing equation are given here. Using these values predicts a required vent diameter of 3.9 in.
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
wvMvCλ
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Since the exact properties of the reaction mixture at the runaway point are not often known, an estimate can be made using the properties of the reaction mixture at 50% conversion, as shown here. The resulting predicted vent diameter is 4.1 in. on this basis, as compared with 3.9 in. previously determined. However, as was noted earlier, the particular collection of fluid properties that are included in the constant C has a value that is almost independent of the specific fluid. To illustrate this, the corresponding properties of water at ambient conditions can be used to calculate the value of C, which gives a vent diameter of 4.0 in. (essentially the same as that determined from the actual reaction mixture properties).
(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.
Using Screening Equation and Properties of Water
P
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If the general screening equation is used, with the “universal” value of C, the calculated vent diameter is 4.2 in., which is within 5% of the previous values. Since this value of C includes a factor of 2 to account for foamy behavior, the results for our non-foamy system must be adjusted accordingly, by dividing the calculated diameter by 21/2.
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
Characterizing Runaway Reactions
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In summary, the ARSST is a convenient, relatively inexpensive and relatively simple device that can be used to screen chemicals for their potential reactivity and to obtain quantitative data on the characteristics of a runaway reaction that can be used to determine the required relief vent size for a reactor containing the reaction. Using various well-characterized reactions, the equipment is ideally suited for use as a laboratory experiment in a Unit Operations Laboratory. In the time frame of a typical lab experiment, the students can learn how to test chemicals for their potential reactivity, obtain the pertinent data need to characterize a runaway reaction, and how to use these data to estimate the relief vent size required for a commercial scale reactor containing the reaction.
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
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