The Reactivity of Butadiene with Acetylenic Hydrocarbons Via Adabatic Calorimetry
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Second Annual Symposium, Mary Kay O'Connor Process Safety Center "Beyond Regulatory Compliance: Making Safety Second Nature" Reed Arena, Texas A&M University, College Station, Texas October 30-31, 2001 The Reactivity of Butadiene with Acetylenic Hydrocarbons via Adabatic Calorimetry M. I_evin and A.D. Hill Equilon, Inc , P O Box 1380 Houston TX 772511380 Phone: (281) 544-7575 Email: melevin@equilon.com ABSTRACT Building upon recent studies of the behavior of conjugated diolefins and alkynes individually, a study involving adiabatic calorimetry has been undertaken to characterize the reactivity of combinations of 1,3- butadiene with selected alkynes. Mixtures of 1,3-butadiene with methyl vinyl acetylene (a conjugated alkene-yne) along with mixtures of 1,3-butadiene with methyl acetylene have been tested in the Automatic Pressure Tracking Adiabatic Calorimeter. Results from these tests are compared with the reactivity of the individual species. In the case of butadiene plus methyl vinyl acetylene, mixtures exhibit exotherm onset temperatures intermediate between those of the pure components. Compositional analysis of reaction product has been conducted to elucidate the role of the Diels-Alder condensation pathway in these tests. THE REACTIVITY OF BUTADIENE WITH ACETYLENIC HYDROCARBONS VIA ADIABATIC CALORIMETRY by M.E. LEVIN and A.D. HILL Equilon Enterprises, LLC ABSTRACT Building upon recent studies of the behavior of individual conjugated diolefins and alkynes, adiabatic calorimetry has been undertaken to characterize the reactivity of combinations of 1,3- butadiene with selected alkynes. Mixtures of 1,3-butadiene with methyl vinyl acetylene (a conjugated alkene-yne) along with mixtures of 1,3-butadiene with methyl acetylene have been tested in the Automatic Pressure Tracking Adiabatic Calorimeter. Results from these tests are compared with the reactivity of the individual species. In the case of butadiene plus methyl vinyl acetylene, mixtures exhibit exotherm onset temperatures intermediate between those of the pure components. Compositional analysis of reaction product has been conducted to elucidate the role of the Diels-Alder condensation pathway in these tests. INTRODUCTION Background Chemical plants handling 1,3-butadiene have a variety of chemistries that should be addressed during facility safeguarding. Dimerization/trimerization/oligomerization, peroxidation upon exposure to oxygen, "popcorn" polymer growth, free radical-catalyzed polymerization, and polymer decomposition are among the reactions that can take place, posing risk to a butadiene unit [ 1-6]. Moreover, in the extraction of butadiene from light hydrocarbon streams, reaction of other species such as triple-bonded compounds (alkynes) should not be neglected. This is amply illustrated by the 1969 event at the Union Carbide Texas City event which destroyed a relatively- new butadiene facility [7,8]. That event has been attributed to accumulation and reaction of excess vinyl acetylene in the presence of 1,3-butadiene. The Texas City incident spawned an investigation into the kinetics of vinyl acetylene with butadiene [9]. In all, few studies examining the reactivity of vinyl acetylene with butadiene are available, especially given the advances in adiabatic calorimetric instrumentation developed in the last decade or two. The same is true for combinations of propyne (also known as methyl acetylene) - a common species in butadiene extraction units - and butadiene. A recent study [10] of various olefins, diolefins, and alkynes tested individually by adiabatic calorimetry demonstrated that straight-chain alkynes such as 2-butyne, 1-pentyne, and 2-pentyne exhibit reactivity when heated to above 200°C. However, the conjugated double-triple bond compound, methyl vinyl acetylene (also known as 2-methyl-l-buten-3-yne) shows dimerization activity that is even faster than 1,3-butadiene and that the reaction pathway is consistent with a Diels-Alder mechanism. In principle, conjugated unsaturates, such as 1,3-butadiene, vinyl acetylene, and methyl vinyl acetylene, should be able to co-dimerize with other unsaturated species. The present study investigates, through use of adiabatic calorimetry, the reactivity of methyl vinyl acetylene with 1,3-butadiene and of propyne with 1,3-butadiene. Methyl vinyl acetylene has been chosen for study as an analog to vinyl acetylene as a result of the observation that isoprene exhibits behavior similar to 1,3-butadiene and since methyl vinyl acetylene can be readily purchased. The approach of measuring kinetics through heat release and pressure generation provides the opportunity to observe and characterize in a laboratory setting the accelerating reaction environment that might be experienced in a commercial-scale incident. EXPERIMENTAL Equipment Testing for this study was carried out in the Automatic Pressure Tracking Adiabatic Calorimeter (APTAC TM) available from Arthur D. Little, Inc. The instrument is designed to allow a laboratory-scale sample undergoing an exothermic reaction to self-heat at a rate and extent comparable to that in a commercial-scale, adiabatic environment. This is accomplished by heating the gas space surrounding the bomb to match the sample temperature, thereby minimizing the heat loss from the sample and sample bomb. Balancing of the pressure outside of the sample bomb to match the internal pressure of the sample bomb enable the apparatus to probe sample pressures as high as 2000 psig without resorting to a thick-walled sample container. This feature, along with the 130 cc spherical sample bomb capacity, provide for a low thermal inertia factor, qb, (expressing the amount of heat absorbed by the sample plus container relative to the sample) ~)- 1 + mbCpb msCp~ where m denotes the mass, Cp the heat capacity, subscript b the bomb+stir bar, and s the sample. Values of qb in the APTAC are typically 1.15 - 1.30. The APTAC can match temperature and pressure rise rates of up to 400°C/rain and 10,000 psi/min, respectively. Operation is automated and typically starts in the heat-wait-search mode with an exotherm threshold of 0.05 - 0.06°C/min. Stirring is accomplished via a teflon-coated magnetic stir bar inserted in the sample bomb. More details about the operating principles, construction, and operation of the APTAC have been described previously [ 10]. Samples Materials for this study, 1,3-butadiene, propyne (methyl acetylene), and methyl vinyl acetylene (2-methyl-l-buten-3-yne), were obtained from Aldrich, as indicated in Tables 1 and 2. In one neat butadiene test, extra tertiary butyl catechol inhibitor was added to preclude free-radical polymerization. In another neat butadiene test, commercial-grade butadiene was also employed. Procedures To preclude reaction of the unsaturated hydrocarbon species with oxygen, steps were taken in all experiments to purge air from the system. A summary of test characteristics may be found in Tables 1 and 2. No gas samples for compositional analysis were taken at the end of any of the tests; however some butadiene-methyl vinyl acetylene tests were aborted prematurely to allow sampling and analysis of the liquid. The alkynes and 1,3-butadiene samples were prepared by first connecting the appropriate compressed gas sample can to a dry ice/acetone-cooled condenser. Once sufficient material collected in the condenser's graduated trap, the material was transferred by cannula under nitrogen pressure into a dry ice/acetone-cooled titanium APTAC sample bomb (fitted with a septum and purged with nitrogen). This procedure was repeated for a second species for tests involving mixtures. The sample bomb and its dry ice/acetone bath were then placed in a glove bag. After attaching the glove bag to the APTAC containment vessel head and purging with five vacuum/nitrogen cycles, the septum was then removed from the sample bomb and sample bomb attached to the vessel head. Total sample weights ranged from 19.5 to 52.0 while the total weight of titanium bomb and stir bar ranged between 33.4 and 35.9 g. Table 1" Summary of APTAC Test Conditions and Results for 1,3-Butadiene Combined with Propyne Run ID A00287 A00375 A00424 A00420 A00428a A00429 Butadiene Mass [g] 34.86 38.94 28.94 22.91 14.54 12.50 Commercial Aldrich Aldrich Aldrich Aldrich Aldrich Grade #29,503-5 #29,503-5 #29,503-5 #29,503-5 #29,503-5 Propyne Mass [g] 0.0 0.0 9.86 16.81 20.08 23.60 Aldrich Aldrich Aldrich Aldrich #48,098-3 #48,098-3 #48,098-3 #48,098-3 Propyne Percentage [%wt] 0.0 0.0 25.4 42.3 58.0 65.4 Propyne Percentage [%mol] 0.0 0.0 31.5 49.8 65.1 71.8 Total Sample Mass [g] 34.86 38.8 39.72 34.62 36.10 Sample Pad Gas Nitrogen Nitrogen Nitrogen Nitrogen Nitrogen Nitrogen Sample Bomb Mass [g] 31.01 31.10 32.01 32.19 30.72 32.44 Sample Bomb Material Titanium Titanium Titanium Titanium Titanium Titanium Stir Bar Mass [g] 3.47 3.41 3.53 3.53 3.47 3.48 Stirring Rate (magnetic) [rpm] 300 550 500 500 500 500 Expt (Search) Start Temp [°C] 40 60 80 60 215 120 Expt Final or Max Temp [°C] 460 395 180 180 260 200 Heat-Wait-Search Increment [°C] 10 Expt Duration (before S/D) [min] 734 899 771 1090 1329 1109 ExptExr~t Shutdown Cause S/D P Rate Exo T Limit Highg P S/D Exo T Limit i Unclear Exo T Limit Expt Exotherm Limit (N2) [°C] 430 290 260 200 300 280 Expt Temperature Shutdown [°C] 460 300 280 280 310 300 Expt Pressure Shutdown [psia] 1800 1800 1500 1400 1800 1800 Expt Heat Rate Shutdown [°C/mini 2000 800 1000 1000 800 800 Expt Press Rate Shutdown [psi/min] 10,000 5000 4000 4000 2000 2000 Exotherm Threshold [°C/min] 0.05 0.05 0.05 0.05 0.05 0.05 | i Number of Exotherms 1+ 1 1 1+ 1 1 Regressed Onset Temp (0.06°C/min) [°C] 103 108 118 122 156 160 Max Observed Yemp [°C] 8941 533 8981 200 264 280 Max Observed Pressure [psi] 2089 2159 2439 1286 1481 1712 Max Obs'd Self-Heat Rate [°C/min] 61002 10,3002 13,6002 2.6 1.1 2.4 Max Obs'd Press.