The Reactivity of Butadiene with Acetylenic Hydrocarbons Via Adabatic Calorimetry
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: [email protected]
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. Rate [psi/min] 19,8002 29,6002 33,2002 22.8 5.1 12.8 Temp at Max Self-Heat Rate [°C] 893 533 898 200 264 280 Temp at Max Press. Rate [°C] 367 486 680 200 264 280 Thermal Inertia, ~b 1.19 1.16 1.17 1.16 1.17 1.17 Comments BD includes 0.40 g inhibitor soln; Bomb rupture, RD burst ll~eached equipment maximum reading 2Self-heat and pressurization rates sufficiently high to cause possible temperature, pressure reading lags ~cq 0 I go i Z [-.- ~i < ::hi: , i i | i | | | ! | | i i i
c,I
t.D o o ~< N N i i , i i i i , ! | , , , ! i
• . • | i i , , , , , , ~ ! , , , ,
¸
! • • , , , | | , | | i , ! i | i |
• e,~ uq
, ~
0 • • , i i | | i i i | , ~ | | ! | ('Dr-- t~ 0 c,1
..~_.~ ' ~
n ea o ,---' ,'-' ',.~ I • ,-" "-' ""
= = , I
==h I < ~ ~ i
= = i | | , , , , , | ~ d * I | | ' | ~.)
, el ~-1
~ 0 cq ~
, , , | | | , | , , | , , | 0 = ' ' | | | | i = , , | | , , , | , = .... • ~.=..I
o g • ~, _. ~g~~ ~< ~:~o~o o. ",-', 0 ,--, c..l 133iE~ ~-~ 0 rq o ~" ..~ : : , | | ! | | , , i | I I I ! I • I t I I I I i I I i g ~
0o ',D ~ ,-.-.>"
<< o~ ° i ~'5 , • ,r,D , ! ! i i | | | .! ! | i i ! , ~_.,. ~ ,...-
0 o~'~ ~o ~ o "CL . E~::::I "" ~'--" ~ ~ '-- '--" ~ I::: . ,..., 0
m ~ E.~
~i ~ _ !!! -- ; /.i - ~~; ~ ~, -=~
¢)C
m r.n r.n m ~q ~ ~ ~ ~ RESULTS AND DISCUSSION
Butadiene-Propyne Tests
For all 1,3-butadiene-propyne mixtures investigated, exothermic activity is evident by the accelerating rise in temperature with time (once the stair-step feature of the heat-wait-search mode has been completed) exhibited in Figure 1. The greatest degree of temperature rise occurs when little propyne is present with the butadiene. Temperatures approaching 900°C are observed in one case of neat butadiene and also for a 75%w butadiene- 25%w propyne mix. However, this may be an artifact of the experiment shutdown parameters. Rather low shutdown criteria (in particular, maximum exotherm temperatures and pressures) were selected to avoid the possibility of vapor-phase decomposition of propyne accompanied by possible propagation or even detonation [ 11 ]. As a result, temperatures, pressures, self-heat rates, and pressurization rates for many of the tests were deliberately limited. However, relative reactivities of the mixtures as a function of propyne content are still revealed.
Onset temperatures are seen to increase steadily with increasing propyne weight fraction. Reaction for neat butadiene takes place in the vicinity of 100°C (sometimes lower due to instrument "drift") while for a mix comprised of 65%w propyne, the onset temperature is as high as 160°C. Note that in previous work with neat alkynes [ 10], 2-butyne, 1-pentyne, and 2-pentyne exhibited reaction only after temperatures above 210°C were attained.
The accompanying pressure profiles for the butadiene-propyne tests are displayed in Figure 2. In general, higher initial pressures are found in the tests containing greater percentages of propyne due to the higher vapor pressure associated with propyne. It is also apparent from the pressure profiles that the rate of pressure rise with time is steepest for mixtures containing little or no propyne. Again, however, the ultimate pressure and rate of pressure rise developed in some tests cannot be discerned from the data on account of the premature shutdowns programmed into the tests.
Figure 1" Temperature History of Butadiene + Propyne Mixes in the APTAC 9OO
80O
7OO
,--, 600 0 .o. 500
~.. 400
1-- 300
200
100
o 0 1O0 200 300 400 500 600 700 800 900 1000 11 O0 1200 1300 1400 1500
Time [mini Figure 2: Pressure History of Butadiene + Propyne Mixes in the APTAC
2200 + Inhibited Butadiene (A00287) _! _ _+_ _ 2000 X Inhibited Butadiene + 411 ppm TBC added (A00375) A75%w Butadiene - 25%w Propyne (A00424) _ _+ ...... 13 58%w Butadiene - 42%w Propyne (A00420) -~
1800 <:.~42%w ButadieneBUtadiene- 65%w58%wPropyne (A00428a) _ ~.' "' 1600 "' O 35%w - Propyne (A00429) ' -i:-" i- >~' _/o
,--, 1400 ~1 ' : : ...... , ...... , ...... ~ ...... ~ ...... , ...... ~ ...... ~__~ _ n ....~:¢i::;, )il •~- 1200 ' ' i il2~ :___ ~0 ...... ~:~i:::i:::i:i:;ii::i::iii~ ...... ,:::., ,_ ...... ~ ,~ -
,- ...... ! - - - - -I:,:. :..:,~iiiiili~li~ii~:~ __ !. -~ ! :~iiii~#"~::~:~:~ .... t ~" 800 , . ..'.....~ ~iiU~" ...... /1 ...... ~" ......
600 ,~ ~ I I I ...... ~:~::!:~:#ii~ii~:i:;:J:~:~ ...... ~lL~ ..~
0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 Time [min]
The qualitative kinetic behavior of the sample is more directly illustrated in the self-heat rate plot in Figure 3. This graph displays temperature rise rate vs. negative reciprocal temperature (with the corresponding temperatures in degrees Celsius shown). The direct link to an Arrhenius plot can be obtained by multiplying the self-heat rate by the sample heat capacity and dividing by the heat of reaction to yield the observed reaction rate. Figure 4 displays the same information as Figure 3, except that the initial heat-wait-search steps have been removed for clarity.
Self-heat rates in excess of 10,000°C/min are found for neat butadiene and for 25% propyne in butadiene. Presumably, this would occur for higher propyne charges, if allowed. A clear trend of decreasing temperature rise rate with increasing propyne content at a given temperature is seen in Figure 4. This is also reflected in the onset temperatures at a threshold of 0.06°C/rain; however, instrument "drift" at this self-heat rate level appears to flatten the slope of these profiles. As a result, regression of the linear portion of the data at higher self-heat rate (without the apparent drift) was carried out to determine the most appropriate onset temperatures. The onset temperatures derived in this manner are shown in Figure 5 and, again, show that increasing the propyne content causes the overall reactivity to diminish. The same trend carries over to the pressurization rate data, as exhibited in Figure 6. At a chosen temperature, the pressurization rate is higher for mixtures containing less propyne.
The pressure vs. temperature plot of Figure 7 shows more complex behavior. Three different trends are found in this plot. For neat butadiene in test A00287 and for 25%w propyne in butadiene, the residual pressure upon cool-down is substantially higher than the initial pressure. At 50°C, the final test pressure was about 1000 psia versus 40-60 psia initially. This residual pressure can be attributed to the formation of non-condensable species from the decomposition that occurs as temperatures proceed toward 900°C in these tests. In contrast, the other propyne- butadiene tests (A00420, A00428a, A00429) show cool-down trends that coincide with the pressure-temperature behavior during heatup. This is not surprising since in these tests, the temperatures never exceed 300°C where no decomposition is encountered. In test A00375, the Figure 3" Self-Heat Rate-Temperature Profiles of Butadiene + Propyne Mixes in the APTAC 10000 + Inhibited Butadiene (A00287) X Inhibited Butadiene + 411 ppm TBC added (A00375) /x 75%w Butadiene - 25%w Propyne (A00424) 13 58%w Butadiene - 42%w Propyne (A00420) 1000 ;:~ 42%w Butadiene - 58%w Propyne (A00428a) O 35%w Butadiene - 65%w Propyne (A00429) ._
lOO
i.....i .... t ...... -~-l- Heat-Wait-Search Step E ¢o °,9_,
I-
0.1
Exotherm detected- o.ol ,~~ -~-,.' ~- oC -3.5 -3.0 -2.5 -2.0 -1.5 -1.0
-1000IT [K 4]
Figure 4" Self-Heat Rate-Temperature Profiles ofButadiene + Propyne Mixes in the APTAC Heat-Wait-Search Steps Removed
10000
lOOO
lOO
C
o .o. lO
"oi.-
0.1
0.01 20 40 60 80 100 120 140 160180 2OO 300 400 500 oC -1000/T [K 41 Figure 5" Dependence of Butadiene - Propyne Reactivity on Concentration
180 356
170 ...... •~.'...... Regressed line 160 4i~ ,/ 320 • ~./..o.:,~,'.#'<*J: " ! ; '- 150 O i J I,I. O 140 284 .o. i,..
130 i L L • 11~ 120 i 248 E~" 1- I--• ~ 110 1 I ~ $ 10o 212 e- 0 OO E E 9o L L e" ~- 80 176 o X 0 UJ ~ 70
60 -: 140
50
40 " 104 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Wt. Percent Propyne
Figure 6" Pressurization Rate-Temperature Profiles of Butadiene + Propyne Mixes in the APTAC
100000 + Inhibited Butadiene (A00287) X Inhibited Butadiene + 411 ppm TBC added (A00375) •75%w Butadiene - 25%w Propyne (A00424) El 58%w Butadiene - 42%w Propyne (A00420) 10000 ~', 42%w Butadiene - 58%w Propyne (A00428a) O 35%w Butadiene - 65%w Propyne (A00429)
1000
C E 100 o-- Q.
"O 1 ...... I 10
0.1
0.01 20 40 60 80 100 120 140 160 180200 300 400 500 600 700 °C
-1000IT [K 4] cool-down pressure falls rapidly to atmospheric pressure indicating that the test bomb ruptured during the height of the exotherm, also causing the containment vessel rupture disk to burst. The time-to-maximum rate profiles in Figure 8 illustrate from another perspective the difference in reactivities for the butadiene-propyne mixtures. It is quite apparent that, for mixtures with propyne comprising greater percentages, more time is required to reach peak reactivity- at least when considering reactivity in the 100 to 250°C region. The limited temperatures in tests A00420, A00428a, and A00429 combined with the associated absence of decomposition behavior causes an artificial flattening for the time-to-maximum rate profiles for these mixtures.
In summary, the addition of propyne to butadiene serves to reduce the reactivity of the system, suggesting that propyne is acting as a thermal diluent in the mixture. Most likely, butadiene preferentially undergoes Diels-Alder reaction with other butadiene molecules and occasionally reacts in this manner with propyne.
It is important to stress, however, that the reactivity reflected in these tests does not include, nor is intended to include, the contribution from vapor-phase propyne decomposition. The mechanism for such decomposition and the potential impact of its propagation should be addressed separately from the condensed-phase behavior explored in this work.
Figure 7" Pressure-Temperature Profiles of Butadiene + Propyne Mixes in the APTAC
10000 ...... ,3-Butadiene Vapor
1000 Cooling
~.ill 100
__ Cooling .....
10 +Inhibited Butadiene (A00287) t:-:--tLL:-t L---I- X Inhibited Butadiene + 411 ppm TBC added (A00375) t:----t:---4-:--7- A75%wButadiene-25%wPropyne(A00424) It ..... l .... ] .... [- CI58%wButadiene-42%wPropyne(A00420) I1..... / .... / .... /- <>42%wButadiene-58%wPropyne(A00428a) II ..... 1 .... t .... t- O 35%w Butadiene - 65%w Propyne (A00429)
20 40 60 80 100 120 140 160180 200 300 400 500 600 700 °C
-1000/T [K "1] Figure 8" Time to Maximum Rate Profiles of Butadiene-Propyne Mixes in the APTAC
500 + Inhil~ited Butadiene (A00287) X Inhibited Butadiene + 411 ppm TBC added (A00375) A 75%w Butadiene - 25%w Propyne (A00424) C158%w Butadiene - 42%w Propyne (A00420) ,.::~ 42%w Butadiene - 58%w Propyne (A00428a) 400 O 35%w Butadiene - 65%w Propyne (A00429) X--- T---:-- i- ~,- F ~,-: -', ...... F---:-- ,'- ;-',- ; i ~, ...... ',--- ~--',--:- ~,-F F ; ..... ~,---F--:--:- :-;-:-: ...... F---:--T-~-7~, -~, ...... :.... ',--F-F-:-F ~, ', --~-!---i--!z~-~~', ...... i.... i--!-!-i-!!! ...... !---!- i i! i!! ..... ! --!--!-i !!i! ...... ! i !!!!!i ..... i ---~ !!! !! ..... " - -X',- - " - -~- ~- -,'-, ...... ',.... ',--'-'-',- ~ ~ ~ ...... :- - - " - -:- -:- " -',- ,'- " ..... "---1---:--:-'-:-1-: ...... :----:-- +- -' -:- +-:-: ..... t---,- -:--:--: : ~
0 .o. 300
&
200 !.-
100
0 0.01 0.1 1 10 100 1000 10000 Time to Maximum Rate [min]
Butadiene-Methyl Vinyl Acetylene Tests
In this phase of the study, it was decided to relax the test shutdown parameters. Moreover, no constraints were placed on the mixture compositions since neat methyl vinyl acetylene had already been tested. Methyl vinyl acetylene molar percentages in the vicinity of 25%, 50%, and 75% were chosen to complement already-acquired data for neat methyl vinyl acetylene and neat 1,3 butadiene.
Profiles of temperature vs. time for the butadiene- methyl vinyl acetylene mixtures are shown in Figure 9. With the exception of two tests programmed to end prematurely for liquid sampling (A00409b and A00425c), all mixtures containing methyl vinyl acetylene exceed 700°C when allowed. No clear trend in the effect of methyl vinyl acetylene content on maximum temperature shows up, presumably due to the extremely high temperatures, pressures, and pressurization rates encountered, triggering instrument shutdown. Trends in onset temperatures with composition in this plot are difficult to distinguish since the onset temperatures are all in a relatively narrow range between 80°C and 110°C. No decrease in activity appears upon adding methyl vinyl acetylene to butadiene.
The corresponding pressure histories are depicted in Figure 10. As can be noted from Table 2, in all cases where the tests are not deliberately ended early, pressures greater than 1500 psig result. Again, there appear to be no trends in maximum pressure with methyl vinyl acetylene content, reflecting the high pressures reached and their dependence on engagement of the instrument shutdown parameters.
Self-heat rates in excess of 1000°C/min and in most cases, above 15,000°C/min are seen for these mixtures. Inspection of the temperature rise rate data of Figures 11 and 12 reveals generally greater reactivity with increasing methyl vinyl acetylene content. In addition, the slope of the self-heat rate vs. reciprocal curves increases with higher methyl vinyl acetylene percentage Figure 9: Temperature History of Butadiene-Methyl Vinyl Acetylene Mixes in the APTAC
900 + Inhibited' Butadiene (A(~0287) ' ' × Inhibited Butadiene + 411 ppm TBC added (A00375) II! 72%w Butadiene - 28%w Methyl Vinyl Acetylene (A00408) -~ .... []+ i 800 • 47%w Butadiene - 53%w Methyl Vinyl Acetylene (A00425) 46%w Butadiene - 54%w Methyl Vinyl Acetylene (A00407) A 46%w Butadiene - 54%w Methyl Vinyl Acetylene (A00419b) 22%w Butadiene - 78%w Methyl Vinyl Acetylene (A00410) 700 El Methyl Vinyl Acetylene (A00320) ..... liB, o Methyl Vinyl Acetylene (A00322)
600 o ~- ~---',~ -= -I .o...... [3 '.'.> -- -N ...... I,,,. 500 <.:~, .. ~. ,..1 ,~ _~;>___~.~_ ,~ _ .4.,, LIo J O. 400 E .... .A_ 1- Heat-Wait-Search Step _ Programmed ___ Exotherm __ 300 T_::_mi nat!o_n :::: :!: :~
200
100
0 100 200 300 400 500 600 700 800 900 1000 Time [min]
Figure l 0" Pressure History of Butadiene-Methyl Vinyl Acetylene Mixes in the APTAC
2200 ' +Inhibited Butadiene (AOC)287) X Inhibited Butadiene + 411 ppm TBC added (A00375) ~- - 1 72%w Butadiene - 28%w Methyl Vinyl Acetylene (A00408) 2000 • 47%w Butadiene - 53%w Methyl Vinyl Acetylene (A00425) 46%w Butadiene - 54%w Methyl Vinyl Acetylene (A00407) -~- 1800 ~3426et%yl Biu!iiicntil!!%(v~AoM3~i W=nnyll ACeett~llenn ee IA0004~ 9 ~) i A ~ -i!
1600
,_, 1400 .m .~. 12oo ...... iiii iiiiiii!iiiiiiiiiiiiiiiiiiiiiiiiiiiiiii i!iiiiiiiiiiiiiiiiii 1000
O. 800
600 ~ >:
400
200
0 100 200 300 400 500 600 700 800 900 1000 Time [min] Figure 11" Self-Heat Rate-Temperature Profiles of Butadiene-Methyl Vinyl Acetylene Mixes in the APTAC
10000 + Inhibited Butadiene (A00287) X Inhibited Butadiene + 411 ppm TBC added (A00375) i 72%w Butadiene - 28%w Methyl Vinyl Acetylene (A00408) 47%w Butadiene - 53%w Methyl Vinyl Acetylene (A00425) 46%w Butadiene - 54%w Methyl Vinyl Acetylene (A00407) • . 46%w Butadiene - 54%w Methyl Vinyl Acetylene (A00419b) 1000 <> 22%w Butadiene - 78%w Methyl Vinyl Acetylene (A00410) [] Methyl Vinyl Acetylene (A00320) O Methyl Vinyl Acetylene (A00322)
100
......
e" ;::-::-:--__-_-::- Heat-Wait-Search Step ::-:: .m to .o. 10 _:__ ! __-__-_-__------...... _:--_-
=- _- _: - __-- ___--~,~
/__~1 ......
'o.....%_ 0.1 !!ff!!!!!!!!!-:~ -:!!!! l !-~!l
!)~ !! ! :_:_ i ! ! ! !,¢! ?!,_-- :_ ! ---I
...... r }.... D - - - ..... Exotherm detected -I '' 0.01 ! ~ r"l , / • • ~ r ~ ~ o C 2o 4(~ ~ zo 1 40 160 180 200 30 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0
-1000IT [K 4]
Figure 12: Self-Heat Rate-Temperature Profiles of Butadiene-Methyl Vinyl Acetylene Mixes in the APTAC; Heat-Wait-Search Steps Removed
...... :!:: 10000 1 '+Inhibited Butadiene (A00287) .... ------! t Xlnhibited Butadiene + 411 ppm TBC added (A00375) ------;------; ] [] 72%w Butadiene - 28%w Methyl Vinyl Acetylene (A00408) I- -] ...... / 4,47%w Butadiene - 53%w Methyl Vinyl Acetylene (A00425) ...... , .... ,~ ] ~ 46%w Butadiene - 54%w Methyl Vinyl Acetylene (A00407) I- -I ...... | A 46%w Butadiene - 54%w Methyl Vinyl Acetylene (A00419b) 1000 "t ,:,>22%w Butadiene - 78%w Methyl Vinyl Acetylene (A00410) : : z _- z - z - - _- : ] 13 Methyl Vinyl Acetylene (A00320) [- -/: -- : - : -- : -- : ...... - . ~ ...... ooii O Methyliiii Vinyl Acetylene iii (A00322)iiii iiiiii ii iii iiiiii___J- --/- ~ ~ --i I------I~ ...... ~ ~22
_ .o. 10 -~:
.~!
...... Exotherm detected ...... 1 ", ' ---- .... -_Z_-
.... .__
0.1 ....
--_----_-- ...... ::-:-: ::::-::-- :--:: :--:-:--:--:-- Exotherm threshold - ......
...... L___I___I__~__[__I__L_ I_l_ .... 1 ...... oC 0.01 20 40 ~0 84~ 100 120 1~,0 160 11 0 2) -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -1000/T [K t] Figure 13" Dependence of Butadiene - Methyl Vinyl Acetylene Reactivity on Concentration
180
170
e~ E 160 O $ 150 O
oL 140
N--o ~' 130 o
L 120 :3 ,- ~ 110 Q. E 100 1-- 90 "= -- Regressed line --
C O 80 E 70 c- o x 60
50
40 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Wt Percent Methyl Vinyl Acetylene reflecting an increase in activation energy associated with the methyl vinyl acetylene. Self-heat rates an order-of-magnitude above those for butadiene can eventually develop. Regressed onset temperatures are presented in Figure 13 as a function of methyl vinyl acetylene composition. The decrease in onset temperature (reflecting increasing reactivity) with methyl vinyl acetylene percentage can be described by a linear function. Thus, the reactivity of 1,3-butadiene/methyl vinyl acetylene mixtures can be expressed in a simple manner in terms of the concentration. It is also important to note that the onset temperatures shown in Figure 13 for the entire concentration range (including neat 1,3-butadiene) are about 50°C lower than those given previously in the literature for butadiene with vinyl acetylene [9]. The reason for this is likely the improvement in sensitivity for detecting reaction onset in laboratory equipment in the years since the original study was published in 1971.
The rate of pressure generation for the butadiene- methyl vinyl acetylene mixtures appears in Figure 14. Overall, the presence of methyl vinyl acetylene causes a significant increase in the pressurization rate as well as the slope of pressurization rate with reciprocal temperature. As was seen for self-heat rate, pressurization rates for methyl vinyl acetylene-containing mixes can be an factor of 10 higher than those of butadiene. It also appears that the pressurization rate profiles tend to "clump" together for the various concentrations ranging from 28% methyl vinyl acetylene up to neat methyl vinyl acetylene. This seems to suggest a weak dependence of the pressurization rate on the actual composition. One final observation: the rates of pressure rise of several of the tests (tests A00407, A00408, and A00410, A00419b in Figure 14) experience sharp, upward increases in the range of 210 to 260°C. This behavior may indicate the occurrence of vapor- phase decomposition undergoing propagation.
Pressure-temperature profiles are given in Figure 15. Two primary trends can be seen: either substantial residual pressure is generated and retained during the course of the exotherm (tests A00287 and A00419b) or the pressure rapidly falls to atmospheric pressure. The latter case Figure 14: Pressurization Rate-Temperature Profiles of Butadiene-Methyl Vinyl Aeetvlene Mixes in the APTAC 100000
10000
1000
C 100
10
0.1
0.01 20 40 60 80 100 120 140 160 180200 300 400 500 600 700 °C
-1000IT [K 4]
Figure 15: Pressure-Temperature Profiles of Butadiene-Methyl Vinyl Acetylene Mixes in the APTAC 10000 1,3-Butadiene Vapor Pressure + Pad !!!!!!!!t!!!!!!! ¸ - Cooling .... i i¢~i:i::..-J .+++-~+++, .k.:~I -- U, 1000 !!!!I!!!!!!!!!! -- ,~
.... i ...... Heating C/) 100 t ,....., a...... "-I ..... Cooling ...... -I ...... [] ..... J ...... I ......
+ Inhibited Butadiene (A00287) X Inhibited Butadiene + 411 ppm TBC added (A00375) [] 72%w Butadiene - 28%w Methyl Vinyl Acetylene (A00408) @ 47%w Butadiene ° 53%w Methyl Vinyl Acetylene (A00425) 46%w Butadiene - 54%w Methyl Vinyl Acetylene (A00407) ,t 46%w Butadiene - 54%w Methyl Vinyl Acetylene (A00419b) •~ 22%w Butadiene - 78%w Methyl Vinyl Acetylene (A00410) [] Methyl Vinyl Acetylene (A00320) O Methyl Vinyl Acetylene (A00322)
20 40 60 80 100 120 140 160180 200 300 400 500 600 700 °C
-IO00/T [K 1] echoes the behavior seen before at times for butadiene - propyne mixtures in that pressure was generated at a rate sufficient to overcome the instrument's ability to balance the pressure, thus causing failure of the sample bomb and the containment vessel rupture disk. Except for the runs that were terminated early for liquid sampling, none of the tests retrace the heatup portion of the pressure-temperature curves due to decomposition arising at high temperatures. Just as occurred for the neat methyl vinyl acetylene runs in the previous study, in all of the tests generating high residual pressure leading to failure of the sample bomb, a fine, light, black powder was generated. This powder managed to infiltrate much of the APTAC's ancillary tubing as well as be expelled from the system when the rupture disk burst.
The final graph that is presented characterizing the behavior of butadiene- methyl vinyl acetylene mixtures (Figure 16) shows the time-to-maximum rate profiles for the various compositions. As expected based on the already-discussed rate data, addition of methyl vinyl acetylene to butadiene causes a progressive reduction in the time required to reach the peak reaction rate and a reduction in the time-temperature slope. (For the tests that were aborted early for liquid sampling, a downward deviation is observed relative to the trends for those runs going to completion reflecting the lack of attainment of peak rate.)
Product Analysis In the forerunner to this study [10], GC/IR/MS, GC/FID, and NMR analysis of liquid product from a terminated methyl vinyl acetylene test (A00322) revealed the presence of both aromatic and linear C~0H~2 dimer. The aromatic product appeared to be the predominant species, suggesting that a Diels-Alder reaction pathway for methyl vinyl acetylene dimerization was present. Further support for the possibility of Diels-Alder reaction for acetylenic compounds is found in the open literature [12]. This reaction pathway is the same followed by 1,3-butadiene. When both 1,3-butadiene and methyl vinyl acetylene are present, it is possible to make the co- dimers of these species in addition to the dimers of the neat species. Liquid from test A00425c (taken from an approximately equimolar mix of butadiene and methyl vinyl acetylene) was analyzed by GC/MS and GC/FID and found to contain about 3-4%w total dimer and nearly 1%
Figure 16" Time-to-Maximum Rate Profiles of Butadiene-Methyl Vinyl Acetylene Mixes in the APTAC
500 i i i ! i i!! ! i i !!!i ! ! !!!! I{ +inhiloite~ene (A0()287) ...... ~---'--~-~-~-:-:-: ...... :.... :--,~-,~-:-;~,ql ...... :---~--:--:--:-:-:-~li XlnhibitedButadiene+411 p_pm TBCadded(A00375) ..... ~ - --:- - -'- -'- "- ...... '--'- ..... -'~ ...... " ...... "|1 1 72%w Butadiene - 28%w Methyl Vinyl Acetylene (A00408) : ', ', : : : ', :. : : " : " : : :/ : " : : " : " "II ~1,47%w Butadiene - 53%w Methyl Vinyl Acetylene (A00425c) ...... : -- -',- -",- ",- ,~ ~,-- 1 ...... ,~ - - -:- - ,~ - ,~ -:- ,~ ", ~, 1 ...... :--- ;--:--:- ~,-,~ ,~ ; II ~ 46%w Butadiene - 54%w Methyl Vinyl Acetylene (A00407) ...... I ...... | ...... [/ .L 46%w Butadiene - 54%w Methyl Vinyl Acetylene (A00419b) ~',-",,-!-!~ II ~ 22%w Butadiene - 78%w Methyl Vinyl Acetylene (A00410) 400 ...... [] Methyl Vinyl Acetylene (A00320) I ,' ,' : ,': ', ', ', II oMeth-I Vinl Acet-lene A00322 .... -x-i---i--i-i-i i, ...... ~--- :---:- -:-" -:-:-: ...... :---~--:--:--:- :- "-:
r-.I t.1 .O. 300 !-- = ..... ~,~, .... !-!-!!-i-i ...... i---!--! -!-i-! ! !
Q. E 2oo
100
0 0.01 0.1 10 100 1000 10000 Time to Maximum Rate [mini total trimer. One would expect a total conversion of nearly 5%w, based on the observed temperature rise (26°C) and the heat of dimerization of butadiene as a reference. This is consistent with the sum of the dimer and trimer (counted as dimer and trimer) amounts.
The dimer from A00425c is found to be roughly comprised of an even distribution of C8, C9, and C 10 species. Since the probability of forming co-dimer based on the equal availability of diolefin and alkyne is twice as high as for the neat dimers, the even distribution actually observed suggests a lower propensity for co-dimerization than for dimerization of the neat species. In contrast to this inference, considerably more C 14 trimer is found than other trimer. This may be an artifact, however, of the analysis since little C 12 or C 13 material was detected.
Implications for Butadiene - Vinyl Acetylene Reactivity As mentioned earlier, the lack of readily-available vinyl acetylene prompted the use of methyl vinyl acetylene as a substitute in this study. It is expected, based on the similarity between the reactivity of isoprene to 1,3-butadiene, that vinyl acetylene alone and in mixtures with 1,3- butadiene will exhibit reactivity behavior comparable to that described above for methyl vinyl acetylene alone and in mixtures with 1,3-butadiene. In other words, vinyl acetylene may show a higher reactivity toward dimerization than butadiene and when in mixtures with butadiene, a reactivity between that of the two species.
Thermal Inertia Effects
In this study's experiments, the thermal inertia or phi factor, qb, ranges between 1.14 to 1.42 (see Tables 1 and 2). This means that the sample container has a thermal capacitance of 14-42% of that of the sample, or expressed differently, about 12-30% of the total of the bomb plus sample. The actual temperature rise experienced in a large-scale adiabatic environment, in which the relative wall thermal capacitance might be very small, would be higher by the thermal inertia factor or an additional 14-42%. Thus, the extent of the each exotherm would be greater. Furthermore, a greater pressure build-up can be expected to accompany the increased temperature rise.
In addition to the impact of thermal inertia on exotherm temperature rise, the self-heat rates associated with the exotherms would be greater at the commercial scale than those observed in the experiments by a factor larger than the thermal inertia factor. This means that the timeframe for a temperature/pressure excursion beginning at some initial temperature would be correspondingly shorter.
To adjust the current study's results properly for equipment with a lower thermal inertia, a dynamic simulation that takes into account the observed reaction kinetics coupled with material and equipment properties would be required.
Sensitivity Considerations
Reactions following Arrhenius-type kinetics will exhibit ever-lower self-heat rates as the temperature is lowered. Instead of predicting a reaction to "shut off" at some low temperature, it may be more reasonable to picture the generation of heat from the reaction being "masked" by equipment heat losses or other phenomena (e.g., agitation energy input).
As instrument design and detection limits improve, onset temperatures for reactions can be expected to fall, as already noted earlier for the butadiene-methyl vinyl acetylene system. The lower limit of detection for an exothermic reaction in the APTAC is quoted at 0.04°C/min. This is generally considered to be near the state-of-the-art (e.g., 0.02°C/min for the Accelerating Rate Calorimeter TM) for laboratory-scale adiabatic calorimeters. Nevertheless, when interpreting raw or thermal inertia-adjusted onset rates, it is important to recognize that rates at even the best laboratory-scale detection limit might be unacceptable in commercial-scale applications. For example, a 0.04°C/min rate in the APTAC corresponds to a 58°C/day self-heat rate. A piece of equipment in an operating facility experiencing this magnitude heating rate would likely necessitate some form of intervention. In short, care must be exercised when referencing reaction onset temperatures. Such onset temperatures usually do not necessarily represent a lower threshold below which reaction does not occur.
CONCLUSIONS
An investigation of the reactivity of mixtures of 1,3-butadiene with two selected alkynes has revealed seemingly contrasting trends as a function of concentration. Butadiene in combination with propyne (a.k.a. methyl acetylene) shows a decrease in reactivity as the propyne concentration increases. When compared to the reactivity of other alkynes such as 2-butyne, 1- pentyne, and 2-pentyne, this behavior - which should not be confused with the propensity for vapor-phase decomposition of propyne with possible propagation or detonation- suggests no substantial interaction between the butadiene and propyne.
Combinations of 1,3-butadiene and methyl vinyl acetylene (a.k.a. methyl buten-yne) display an increase in activity with higher methyl vinyl acetylene concentration. This result is not surprising since neat methyl vinyl acetylene has already been found to have greater reactivity than 1,3- butadiene. In fact, the onset temperature for reaction of butadiene-methyl vinyl acetylene mixtures is adequately described by a linear function of the composition. Compositional analysis of product from a deliberately-terminated butadiene-methyl vinyl acetylene test indicates that the degree of co-dimerization is limited. Thus, as in the case of butadiene with propyne, butadiene with methyl vinyl acetylene mixtures show little reactivity between these species.
Finally, the results of these tests show that the onset temperature of reaction involving butadiene and/or methyl vinyl acetylene can be considerably lower than values previously reported.
REFERENCES
1. R.F. Robey, H.K. Wiese, C.E. Morrell, Ind. Eng. Chem., 36(1), pp.30-7 (1944).
, D.G. Hendry, F.R. Mayo, and D. Schuetzle, Ind. Eng. Chem. Res. and Dev., 7(2), pp. 136-145 (1968).
. H. E. Fried, D.K. Schisla, J.F. Zoeller, and M.E. Levin, "A Study of Free-Radical and Thermally-Initiated Butadiene Polymerization," Process/Plant Safety Symposium, AIChE, Houston, TX (April 1-2, 1996).
. H.G. Fisher, G.A. Melhem, M.E. Levin, and J.C. Leung, "International Symposium on Runaway Reactions, Pressure Relief Design, and Effluent Handling," AIChE (March 11-13, 1998), pp. 445-460. 5. M.E. Levin and A.D. Hill, "Further Calorimetric Evaluation of Polymer/Oligomer Decomposition: APTAC Testing," Proceedings of the 1999 Mary Kay O'Connor Process Safety Symposium (October 26-27, 1999).
6. R.T. Morrison and R.N. Boyd, "Organic Chemistry," Allyn & Bacon, 3 rd Edition, 1973. 7. H.C. Jarvis, Chem. Eng. Progr., 67 (6), pp. 41-44, 1971. 8. R.H. Freeman and M.P. McCready, Chem. Eng. Progr., 67 (6), p. 45-60, 1971.
, R.G. Keister, B.I. Pesetsky, and S.W. Clark, Loss Prevention Symposium, Vol. 5, pp. 67-75, 1971. 10. M.E. Levin and A.D. Hill, "Reactivity of Unsaturated Hydrocarbons via Adiabatic Calorimetry," Proceedings of the 2000 Mary Kay O'Connor Process Safety Center Annual Symposium. 11. P.G. Urben (Ed.), "Bretherick's Handbook of Reactive Chemical Hazards," Fifth Edition, Butterworth-Heinemann, 1995. 12. H.G. Viehe I(Ed.), "Chemistry of Acetylenes," Marcel Dekker, New York, 1969.