CHE_0407_Facts
2
Department Editor: Rebekkah Marshall Preventing Runaway Reactions general considerations [ 1 ] A process is considered to be thermally safe only if the reactions can easily be controlled, and if the raw material, the products, the intermediates and the re- action masses are thermally stable under the considered process conditions. Check into the process equipment, its design, its sequence of operation and the control strategies. In addition to the engineer- ing aspects, get detailed information on thermodynamic and kinetic properties of the substances involved, such as the reaction rates or heat-release rates as a function of process conditions. Deter- mine the physical and chemical proper- ties, as well. Understanding of thermal-hazard po- tential requires knowledge of various skills and disciplines [3]. These include: Operating mode: The mode of opera- tion is an important factor. For instance, a batch reaction, where all the reactants are charged initially, is more difficult to control than a semi-batch operation in which one of the reactants is charged progressively as the reaction proceeds (for more, see Design Options). Engineering: Design and layout of the plant and equipment and its built-in con- trols impact the entire process. The ca- pacity of the heating or cooling system is important in this context. Process en- gineering is used to understand the con- trol of the chemical processes on a plant scale. It determines which equipment should be used and how the chemical processes should be performed. In ad- dition, take into account technical failure of equipment, human errors (deviations from operating instructions), unclear operating instructions, interruption of energy supply, and external influences, such as frost or rain (for more, see De- sign Options). Chemistry: The nature of the process and the behavior of products must be known, not only under reaction conditions, but also in case of unexpected deviations (for example, side reactions, instability of intermediates). Chemistry is used to gain information regarding the reaction pathways that the materials in question follow. Physical chemistry and reaction kinetics: The thermophysical properties of the reac- tion masses and the kinetics of the chemi- cal reaction are of primary importance. Physical chemistry is used to describe the reaction pathways quantitatively. data collection The following data are especially rel- evant in avoiding runaway reactions: • Physical and chemical properties, ig- nition and burning behavior, electro- static properties, explosion behavior and properties, and drying, milling, and toxicological properties • Interactions among the chemicals • Interactions between the chemicals and the materials of construction • Thermal data for reactions and de- composition reactions • Cooling-failure scenarios design options [2 ] If a reaction is has the potential for runaway, the following design changes should be considered: • Batch to continuous. Batch reactors require a larger inventory of reac- tants than continuous reactors do, so the potential for runaway in continu- ous systems is less by comparison • Batch to semi-batch. In a semi-batch reaction, one or more of the reactants is added over a period of time. There- fore, in the event of a temperature or pressure excursion, the feed can be switched off, thereby minimizing the chemical energy stored up for a sub- sequent exothermic release • Continuous, well-mixed reactors to plug flow designs. Plug-flow reactors require comparatively smaller volumes and therefore smaller (less dangerous) inventories for the same conversion • Reduction of reaction inventory via increased temperature or pressure, changing catalyst or better mix- ing. A very small reactor operating at a high temperature and pressure may be inherently safer than one operating as less extreme conditions because it contains a much lower in- ventor y [ 3 ]. Note that while extreme conditions often result in improved reaction rates, they also present their own safety challenges. Meanwhile, a compromise solution employing mod- erate pressure and temperature and medium inventory may combine the worst feature s of the ext remes [ 3 ]. • Less-hazardous solvent • Externally heated or cooled to inter- nally heated or cooled thermal stability criteria [1 , 4 ] As a guideline, three levels are sufficient to characterize the severity and prob- ability of a runaway reaction, as shown in the Table. Defining hi gh, meDium anD low risk [1] Severity Probability High ΔT ad > 200K TMR ad < 8 h Medium 50K < ΔT ad < 200K 8 h < TMR ad < 24 h Low ΔT ad < 50K and the boiling point cannot be sur- passed TMR ad > 24 h a u The adiabatic temperature rise is calculated by dividing the energy of reaction by the specific heat capacity as shown in Equation (1). ΔT ad = 1,000Q r /C p (1) where: ΔT ad = adiabatic temperature rise, K Q r = energy of reaction, kJ/kg C p = heat capacity, J/(kg)(K) t xu (tmr) TMR ad (the time to maximum rate, adiabatic) is a semiquantitative indicator of the probability of a runaway reaction. Equation (2), defining TMR ad in hours, is derived for zero-order reaction kinetics: TMR ad = C p RT o 2 /3,600q o E a (2) where: R = gas constant, 8.3 14 J/molK T o = absolute initial temperature, K q o = specific heat output at To, W/kg E a = activation energy, J/mol Th e TMR value provides operating personnel with a measure of response time. Knowledge of the TMR allows decisions to be based on an understanding of the time-frame available for corrective measures in case heat transfer is lost during processing. References 1. V enugopal, Bob, Avoiding Runaway Reac- tions, Chem. Eng., June 2002, pp. 54–58. 2. Smith, Robin, ”Chemical Process Design,” McGraw-Hill, New York, 1995. 3. Kletz, T. A., “Cheaper, Safer Plants,” IChemE Hazard Workshop, 2d., IChemE, Rugby, U.K., 1984. 4. Gygax, R., Reaction Engineering Safety, Chem. Eng. Sci., 43, 8, pp. 1759–71, Au- gust 1998.
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Transcript of CHE_0407_Facts
7/31/2019 CHE_0407_Facts
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Department Editor: Rebekkah Marshall
Preventing
Runaway
Reactions
general considerations [1 ]
A process is considered to be thermallysafe only if the reactions can easily be
controlled, and if the raw material, theproducts, the intermediates and the re-action masses are thermally stable underthe considered process conditions. Checkinto the process equipment, its design, itssequence of operation and the controlstrategies. In addition to the engineer-ing aspects, get detailed information onthermodynamic and kinetic propertiesof the substances involved, such as thereaction rates or heat-release rates asa function of process conditions. Deter-mine the physical and chemical proper-ties, as well.
Understanding of thermal-hazard po-tential requires knowledge of variousskills and disciplines [3]. These include:
Operating mode: The mode of opera-tion is an important factor. For instance,a batch reaction, where all the reactantsare charged initially, is more difficult tocontrol than a semi-batch operation in
which one of the reactants is chargedprogressively as the reaction proceeds(for more, see Design Options).Engineering: Design and layout of theplant and equipment and its built-in con-
trols impact the entire process. The ca-pacity of the heating or cooling systemis important in this context. Process en-gineering is used to understand the con-trol of the chemical processes on a plant scale. It determines which equipment should be used and how the chemicalprocesses should be performed. In ad-dition, take into account technical failureof equipment, human errors (deviationsfrom operating instructions), unclearoperating instructions, interruption of energy supply, and external influences,such as frost or rain (for more, see De-
sign Options).Chemistry: The nature of the process andthe behavior of products must be known,not only under reaction conditions, but also in case of unexpected deviations(for example, side reactions, instabilityof intermediates). Chemistry is used togain information regarding the reactionpathways that the materials in questionfollow.
Physical chemistry and reaction kinetics: The thermophysical properties of the reac-tion masses and the kinetics of the chemi-
cal reaction are of primary importance.Physical chemistry is used to describe thereaction pathways quantitatively.
data collection
The following data are especially rel-evant in avoiding runaway reactions:
• Physical and chemical properties, ig-nition and burning behavior, electro-static properties, explosion behaviorand properties, and drying, milling,and toxicological properties
• Interactions among the chemicals
• Interactions between the chemicalsand the materials of construction
• Thermal data for reactions and de-composition reactions
• Cooling-failure scenarios
design options [2 ]
If a reaction is has the potential forrunaway, the following design changesshould be considered:
• Batch to continuous. Batch reactorsrequire a larger inventory of reac-tants than continuous reactors do, sothe potential for runaway in continu-ous systems is less by comparison
• Batch to semi-batch. In a semi-batchreaction, one or more of the reactantsis added over a period of time. There-
fore, in the event of a temperature orpressure excursion, the feed can beswitched off, thereby minimizing thechemical energy stored up for a sub-sequent exothermic release
• Continuous, well-mixed reactors to plug flow designs. Plug-flow reactorsrequire comparatively smaller volumesand therefore smaller (less dangerous)inventories for the same conversion
• Reduction of reaction inventory via increased temperature or pressure,changing catalyst or better mix-
ing. A very small reactor operatingat a high temperature and pressuremay be inherently safer than oneoperating as less extreme conditionsbecause it contains a much lower in-
ventory [3 ]. Note that while extremeconditions often result in improvedreaction rates, they also present theirown safety challenges. Meanwhile, acompromise solution employing mod-erate pressure and temperature andmedium inventory may combine the
worst features of the extremes [3 ].
•Less-hazardous solvent
• Externally heated or cooled to inter- nally heated or cooled
thermal stabilitycriteria [1 , 4 ]
As a guideline, three levels are sufficient
to characterize the severity and prob-ability of a runaway reaction, as shownin the Table.
Defining high, meDiumanD low risk [1]
Severity Probability
High ΔT ad > 200K TMR ad < 8 h
Medium 50K < ΔT ad < 200K 8 h < TMR ad <24 h
Low ΔT ad < 50K andthe boiling pointcannot be sur-passed
TMR ad > 24 h
a u The adiabatic temperature rise is calculatedby dividing the energy of reaction bythe specific heat capacity as shown inEquation (1).
ΔT ad = 1,000Q r /C p (1)
where:
ΔT ad = adiabatic temperature rise, K
Q r = energy of reaction, kJ/kg
C p = heat capacity, J/(kg)(K)
t xu (tmr)TMR ad (the time to maximum rate, adiabatic) isa semiquantitative indicator of the probabilityof a runaway reaction. Equation (2), definingTMR ad in hours, is derived for zero-orderreaction kinetics:
TMR ad = C pRT o 2/3,600qo E a (2)
where:
R = gas constant, 8.314 J/molK
T o = absolute initial temperature, K
qo = specific heat output at To, W/kg
E a = activation energy, J/mol
TheTMR value provides operating personnelwith a measure of response time. Knowledgeof the TMR allows decisions to be based onan understanding of the time-frame availablefor corrective measures in case heat transferis lost during processing.
References
1. Venugopal, Bob, Avoiding Runaway Reac-tions, Chem. Eng., June 2002, pp. 54–58.
2. Smith, Robin, ”Chemical Process Design,”McGraw-Hill, New York, 1995.
3. Kletz, T. A., “Cheaper, Safer Plants,”IChemE Hazard Workshop, 2d., IChemE,Rugby, U.K., 1984.
4. Gygax, R., Reaction Engineering Safety, Chem. Eng. Sci., 43, 8, pp. 1759–71, Au-gust 1998.
