I
DONALD E. SWARTS and MILTON ORCHIN Applied Science Department, University of Cincinnati, Cincinnati, Ohio
Spontaneous Ignition Temperature of Hydrocarbons Correlation with Behavior on Vapor-Phase Oxidation in a flowing System A three-dimensional diagram illustrates the dependence of spontaneous ignition temperatures on surface-volume ratio of the apparatus and the effect of pressure
EARLIER
Mark in this laboratory (6) has demonstrated that the spontanrous ignition temperatures (SIT) of a series of hydrocarbons determined in thc sainc apparatus are dependent o n the structure of the hydrocarbon. Thus hydrocarbons with long methylene chains have relatively low spontaneous ignition temperatures; branching raises the tcmperature. A great deal of \\ark has been reported (2, J ! 7! 27) on the relative ease of oxidation of hydrocarbons. These studies have shoivn that the rate of oxidation is also dependent on the structure of the hydrocarbon; the number and kind of hydrogen atoms (hydroqens attached to primary. secondar!.. and tertiary carbon atoms) determine the ease of oxidation. An attempt \vas made to relate the spontaneous ignition remperature of a srries of hydrocarbons with the behavior of these same hydrocarbons toward vapor-phase oxidation in a flowing system under conditions such thar only a sniall portion of the hydrocarbon is attacked. Apparatus and Procedure T h e apparatus used to determine spontaneous ignition teinperaturrs ( 6 ) consists essentially of a 43-cc., cylindrical, electrically heated, stainless-steel cup. T h e temperature is measured potentiometrically with a thermocouple and the fuel-air ratio is varied by a regulated flow of preheated air. T h e hydrocarbon to be tested is added dropwise a t intervals by a hypodermic syringe or a medicine dropper as the cup and furnace cool. The temperature below which no further ignition is observed is defined as the
432
spontancous ignition teinperature (if t h c ~ hydrocarbon a t the rate of air floiv used. Reproducibility t o within 1 O C:. can bc secured. Vapor-Phase Oxidation in Reactor Tubes. 'The apparatus emplo!.ed in rhe reactor-tube exprriments for controlled oxidation (7) \vas modified to permit a variation of stoichiornetry of the charged gaseous niisture as drsircd. T h e three main units of the apparatus a r r thc hydrocarbon-oxygen-nitrugen mixing tube; the oxidation chamber, consisting of a glass tube 1 , ' 1 6 inch in inside diametrr and 13 inchrs long in an elcctrically hratrd furnace; and cold traps ( -'0" C , ) for rapid quenching ofthe reaction products. .4n alcoholic solution of 2,4-dinitro€)henylliydrazine is used in a final scrubber to trap any volatile carbonyl compounds. Oxyqrn and nitrogen are dricd by passing them through separate columns of Drirrite and the gases are conducted through separate calibrated Ho\vineters to a mixing tube containing a 6-inch length of glass helices. This gaseous mixture is then bubbled through the hydrocarbon, ivhich is held at a constant temperature by a vapor bath in order to secure a knoivn fuel-oxygen-nitroSeri mixture. T h e ratio of oxygen to nitrogen is varied to obtain 1>0.2. 0.1> 0.0.5of the stoichiometric amount of oxygen in each run as desired. T h e fuel-oxygen-nitrogen mixture is then passed through the reaction chamber a t a selected contact time and temperature and the reaction products are quenched in tlvo cold traps, the second of which contains loosely packed glass wool. Total peroxide, hydrogen peroxide, and acids are determined in the oxidation products (21). .4 solution of
INDUSTRIAL AND ENGINEERING CHEMISTRY
Discussion and Results Effect of Surface-Volume Ratio. I1iscrcpancit.s in reporrcd hl'l' va1uc.s of a qiven compound frcqurritl> arc due i o tlifTerences in surface-volritnc. (Si'\.) t,atios of tht- \ ~ s s e l sernploycd hy \rarious investigators. I o r example. the spontanrous ignition trmipt'ratiirc of benzenr has been variously repor,ted as 04.5O (6).i 9 2 O ( / I ) . 5x2' (?,I). and 362' C'. 1-3 I ) . I n all thcse de~trrnin;itions air ivas used and t h c ratt' of air l l o ~ v\vas zero. escept for the (14.5O \.aluc \vhic.li \cas ircd a t ;in air tlo\\ o l 12.5 cc, 1)rr 1111r. ~ \ l i c . nrhc, riitc rjf i i i r I l u \ v was iiced to 2.5 r e . p(.r ininurr. this valrit: \vas decrrasccl only 0': ttic h i g h spontaneotis ignition trinperaturc'? ncrordinql!.. is not d i i c . Ijt,incilially LO the air flutv, T'hr. S '1.l a t i c i s iiwd i n thc. differrnt expt'rirnrnts ivtai' from thc: drscription of 11ic literature. 'I'1ic.w \.slurs ;I Tablt. I and plotted against sjiontaiicous irc i n o n r o f the curves ut'c 1 iilsu contains a 1' of four u t h r r h!,drocarbons. :\lthough thr ciii'v('s for the five h),drocarbons indicatt. a linrar. relationship, the proportionality cannot hold ovrr the entire S \- rangr. ' I h e physical limitations of thr surfacr disappear a t very low *S,\' ratios and a t high ratios t h r necessary hiqh temperatures promote activation of oxygrn
540-
.,
/ ,: SOOL
500 i,
$ e
c
460-
Q,
420-
~
Q
W
0
c
c
2 c 380S .-cn
n 2 -00
0
0,
,001
t-
A 200ml.conical flask~ef.24 V I50rnl. conical flask,ref.23 W I25 rn I, conica I flask,ref. I4
4 125ml.conical f losk,ref.l9 fn
.43rnl.cylindrical
300
I
H- HeDtane I
2 340-
Q,
C
0
c
C
g 300-
v,
cup ,ref.6
.
A
v
v)
Y
220
Hexodecane
0
20
40
, 60
1
80
100
Per cent ( b y volume) of 2,2,4- trimethylpentane Surface-volume ratio Figure 1. Effect of surface-volume ratio of spontaneous ignition temperature and hydrocarbons, and these effects overshadow the chain-breaking effect of the surface.
Effect of Hydrocarbon-Oxygen Ratio. I n general, SIT values are lower in stagnant air (zero air flow) than in flowing air. Notable exceptions are the diesters (Q), the organic phosphorus esters (77), and blends of commercial oils with hydrogenated polyisobutylene (70, 77). Maximum flammability for various hydrocarbons in air has been reported (5) when about 120% of the stoichiometric amount of oxygen is present. T h e rate of vaporization and hence the degree of mixing of fuel and air have a striking effect on the spontaneous ignition temperature. Thus spray injection (8) rather than dropwise addition often lowers the spontaneous ignition temperaThe curves i n , tures as much as 100'. Figure 2 illustrate this effect with heptane-2,2,4-trimethylpentane blends. The characteristic S-shaped curves are lower with the spray method than with the drop method. The smaller fueloxygen and surface-volume ratios of the larger cup (200-cc.) (24)are more favorable for lower spontaneous ignition temperature compared to the ratios of the smaller (43-cc.) cup. Effect of Other Variables. The chief variables affecting the ignition behavior of a compound are temperature, S/V ratio, HC/02 ratio, pressure (75), ignition-vessel material, induction time (73), and sample purity. Although
(iso-octane) in mixtures with heptane. Figure 2. Effect of surface-volume ratio (vessel size) and hydrocarbon-oxygen ratio (method hydrocarbon addition) on various 2,2,4-trimethylpentane (iso-octane)-heptane mixtures 0 43 ml. cylindrical cup (dropwise addition) ( I 4) A3 ml. cylindrical cup (spray injection) ( I 4) A 200 ml. conical Aask (dropwire addition) ( I 3)
these variables have been studied separately or in groups, oxidation phenomena in which all these essential variables are considered and related have not been investigated. If one compound were studied extensively using a complete set of conditions, it would be possible to construct three-dimensional solid models with SIT-S/V ratio and pressure as the coordinates. A model for a hydrocarbon such as 2,2,4-trimethylpentane is shown in Figure 3. T h e highest spontaneous ignition temperature would occur a t high S/V ratios and low pressures and the lowest a t low S/V ratios and high pressures. Intermediate values would occur a t the other extremes a t low S/V ratio and low pressure and high S/V ratio
and high pressure. The fold in the surface represents the nonignition zone in going from high to low spontaneous ignition temperature. A number of surfaces of this type could be constructed for HC/OI ratios on either side of and including stoichiometric mixtures. I n view of all the variables, it is probable that no minimum spontaneous ignition temperature has been reported for any particular compound. Results. Eight hydrocarbons, representing a variety of structural types, (Table 11) were chosen for the present study, in which the spontaneous ignition temperature and behavior toward vaporphase oxidation in a flowing system were investigated.
Table I. Dependence of Spontaneous Ignition Temperature of Benzene on Surface-Volume Ratio of Apparatus SIT, Vessel Shape Material S/V,Cm.-' O C. Cylinder Stainless steel 1.55 645 Erlenmeyer" (14) Borosilicate 1.14 592 flask glass ($8) 150 Erlenmeyer" Borosilicate 1.08 582 flask glass ($44) 200 Erlenmeyer" Quartz 0.98 562 flask 9 and V calculations made by assuming flask a right circular cone; V calculation checks well with experiment. Reference (6)
Vessel Sizes, Cc. 43 125
~~~~~~~~
VOL. 49,
NO. 3
MARCH 1957
433
Table II.
Spontaneous Ignition Temperatures _ ~ ~ _ _ _ _ _Ppotitaneoii. foilition Teinp., C . ~
.lir
~~
Compo u II d
Toluene 2,2-Dimethylbutane 2,2,4-Trimethylpentane (iso-octane) Z,3-Dimethylbutane 2,2,5-Trimethylhexane Methylcyclohexane 3-Heptene Heptane a
V a l u e spprosiiiintecl froin 1,'iguw 2
12.5 cu.!min. 43-rc. cup
635 524
2.5 w. " i i i i i i . ZI-cr. VUlJ"
580 565
515
497 493 323 294
540
Oxygen, 25 r c . 'inin. 21-cc. cup4
310 318
328
2 50 (l(j).
T h e values for three of the hydrocarbons have been determined together ( 7 6 ) , but in smaller apparatus. -4s might be expected from the smaller cup size ( 2 1 cc,). these previously reported values, although in the same relative order, are all at higher temperatures than those reported in Table 11. It is of intermt that lvith pure oxygen rather than air, the order of the three SIT values is reversed. an effect which might he due to the loiver fuel-oxygen ratio in the system with pure oxygen. Cnder conditions of great selectivity, 1;2-dimethylbutane is 1 2 times more reactive tmvard oxidation than the 2,j-isomer ( 3 ) . Holvever;
434
~~
a t high trmpei~tturesand lo\ver ox concentrations, p!-rolytic reaction overshadoiv osidative ones and the more rasily pyrolyrd ditertiarb- compound disappears faster.
Oxidation of ~
~
Toluene. Thr data from the vaporphase oxidation of toluene are sho\vn in Table 111. .I stoichiornetric tolueneoxygen mixture \\'as made to react a t 550' C:. and at coniac( rimes of 0.16 and 0.64 second. Little iir no appreciable oxidation occurs !\.it11 toluene under these condition:.
INDUSTRIAL AND ENGINEERING CHEMISTRY
d
~
~
~
~
Table IV. 1
Oxidation of Methylcyclohexane for Various Stoichiometries Reaction Mixture 2 3 4 5 6 7
Conditions" Fraction of stoichiometry 1.0 1.0 0.2 Temperature, a C. 350 400 500 Analysisb Hydrogen peroxide None 120 104 Organic peroxides None None None Acids None 54 29 Olefins Qualitative test (Brz in CClr) Relative amount (Ana2)c 0 0.0063 0.0053 a Contact time: 0.16 second. Millimole product/mole methylcyclohexane charged. Increase in refractive index of product.
+
a nonignition region under certain conditions. The lesser amounts of oxidation products observed a t 450' and 500" C. may correspond to this region-that is, a zone where the predominant influence of the low-temperature oxidation mechanism has begun to fade and that of the high-temperature mechanism has not yet become effective. With a reaction tube of Il/l-inch inside diameter and contact time of 10 to 13 seconds (77), this maximum occurs near 325' C., or about 75' lower than that observed with the tube '/IO inch in inside diameter. Conceivably, this difference is due to the effect of surface-volume ratio. There is some doubt, however, as to the proper interpretation of the observed decrease in rate with increase in temperature. Recent work (72) indicates that a t somewhat longer contact times, the decrease in rate may be owing to the complete consumption of all the available oxygen. Whether or not all the oxygen is used u p in the short contact times has not been established.
Spontaneous Ignition Temperatures and Vapor-Phase Oxidotion The spontaneous ignition temperatures of the eight hydrocarbons examined in the present study are shown in Table
+
0.1 350
0.1 400
2.3
56 None 26
0.8
-
27 None 7.9
44 None 5
+
0.0012
9
0.1 550
24 None 2.4
+
4-
0.0038
0
0.1 500
0.1
450
8
0.0009
+
0.0025
0.05
550 24 None 14
+
0.0013
Effect of Stoichiometry and Temperature on Oxidation of Methylcyclohexane" Millimoles* of Product per Mole of Methylcyclohexane Charged Product Stoich. 350' C. 400' C. 450' C. 500' C. 550' C. Hydrogen peroxide 1.0 None 120 .. 0.2 104 0.1 2.3 56 27 24 44 ... 24 0.05 1.o None 54 ... .. Acids 0.2 29 0.1 0.8 26 7.9 2.4 5 0.05 14 Table V.
Olefins
1.0 0.2 0.1 0.05
... ...
... ...
...
...
... ... 0 ...
... 63 ... 38 ...
0
... ... ... ... ... ... ... 12 ... e . .
...
..
..
... ...
.. ..
53 9
25 13
...
" Contact time, 0.16 second. Olefins measured by change of refractive index of product, An'; X 10' given in table.
VI, with information on their behavior toward vapor-phase oxidation. Judged by the amounts of oxidation products, the resistance to vapor-phase oxidation shows the same general order for the compounds tested as obtained for the spontaneous ignition temperature, with some doubt as to the exact order of 2,2-dimethylbutane, 2,2,4trimethylpentane, and 2,3-dimethylbutane. At 350" C. and 0.04second contact time, 3-heptene yields only about one third as much product as heptane (compare runs 10 and 9), while
methylcyclohexane a t 350' C. and 0.16 second shows no attack (run 8). However, a t 400" C. and 0.16 second, methylcyclohexane is oxidized to a much greater extent than 2,2,5-trimethylhexane at 450' C. and 0.16 second (compare runs 7 and 6). Similarly, 2,2,5-trimethylhexane a t 550" C. and 0.08 second is more readily attacked than 2,3-dimethylbutane, 2,2,4-trimethylpentane, or 2,2dimethylbutane (compare run 5 with runs 4, 3, and 2). Toluene at 550" C. and 0.16 second (run 1) is oxidized least
Comparison of Spontaneous Ignition Temperatures with Vapor-Phase Oxidation Behavior Conditions' SIT, O C., Contact Product Analysis, Mmole/Mole Hydrocarbon Run a t 125 Temp., time, Hydrogen Organic Total Compound No. Cc./Min. OC. sec. peroxide peroxide Olefins carbonyl Bldehydesb Trace Toluene 1 635 550 0.16 0.6 None Trace 2 524 500 0.08 1.2 ' 0.8 3.6 0.73 0.23 2,2-Dimethylb~tane~ 0.08 1.6 0.16 3.5 1.1 0.6 Iso-octaned 3 515 550 Trace 7,3-Dimethylb~tane~ 4 497 500 0.08 0.57 0.76 2.9 Trace 6.8 2,2,5-Trimethylhexaned 5 493 550 0.08 7.5 4.3 6.1 70 6 493 450 0.16 17 13 51 15 M ethylcyclohexane 7 323 400 0.16 120 None 8 323 350 0.16 None None 3-Heptenec 9 294 350 0.04 27 4.5 220 110 %-Heptaned 10 250 350 0.04 79 2.1 78 360 Stoichiometry hydrocarbon-oxygen compositions, 1:l. For simplicity, derivative mixture of aldehydes considered to have molecular weight of HCHO derivative (292). Table VI.
...
... ...
...
... ...
...
Acids None 0.40
...
None 2.8 7.3 54 None 28 60
(81).
(7). VOL. 49, NO. 3
MARCH 1957
435
of all. T h e difference in susceptibility to oxidation of 2,3-dimethylbutanel 2,2,4trimethylpentane, and 2,2-dimethy1butane is slight and, as no comparable run of iso-octane a t 500" C. and 0.08 second is available the exact order of resistance to oxidation is difficult to evaluate. T h e greater ease of oxidation of 3,2dimethylbutane compared \vith the 7:3isomer is the reverse of the spontaneous ignition temperatures. Ho\vevt.r. the present spontaneous ignition temperature determinations !\.ere made in air, while reactor tube experiinentsrvere made in pure oxygen. T h e spontanrous ignition temperatures with oxygen are near 300" C . : the region of the lotv-tempcrature oxidation mechanism. Those in air are near 500" C. and ignite via the high-temperature mechanism. Seemingly, the structural differences \vhich determine selectivity at l o ~ v temperatures are of lesser importance at high temperatures. Comparison of the vapor-phase rcactor-tube experiments trith the spontaneous ignition temperature in pure oxygen (Table 11. column 3 ) shoivs tliat the order in Table 1.1 is the samt: for both types of experiments. h*o revcrsal is noted uith the o t h r r compounds listed whose spontaneous ignition temperatures were detcririined in air. T h e \vide spread in spontaneous ignitioii rempcratiires reduces this possibility. This is not to say that an inversiori in order couid not occur Lvith these compounds. C:hangrs in other variables such as in surfacevolume ratio of h e ignition chamber 0 1 ' pressure: as \vel1 as in hydrocarbonoxygen ratio. may effect such ret Generally. the ease OF ignition increases lvith the length of uninterrupted methylene chain. .I notable escrption is methylcyclohexane. Although i t is isomeric Lvith heptane and both havr a chain of five uninterrupted methylrne groups, methylcyclohrsanr has a spontaneous ignition trmperature of 323" C., 73' higher than that of hrptane. Reaction-tube experiments under similar conditions (compare runs 1 0 and 8: Table \-I) also sho\v this large dili'tirncc. O n e reason for this greater resist aim. of merhylcyclohexane is the rrlativelv greater strength of the srrainlrss, cyclic carbon skeleton compared Lvith that of the straight-chain configuration. Sclialla and McDonald (78) report the carboncarbon bond strrngth for and hexane to be 80 and 67 kcal. per mole, respectively. By analogy. riieth!,lcyclohexane has a greater carbon-carbon bond strength than heptane (69 kcal. per mole). Hence, a main reaction i n the oxidation process, rhe scission of a carbon-carbon bond in the decomposition of an alkoxy free radical (7): RO.
--L
R'. - R2"CO
Lvould occur \vith more difficulty in the
436
cyclic than in the straight-chain structure. Similarly, the carbon-hydrogen bonds in the cyclic methylene groups are and stronger than those of heptane (XI) more resistant to initiation reactions. A l though the tertiary radical produced by hydrogen abstraction from methylcyclohexane would be more stable than the secondary radicals from paraffins, the relative inertness of methylcyclohesane is more plausiblv related to its cyclic structure and to the absence of selectivity of tertiary attack a t reaction temperature. I n 3-lieptenc there are only three uninterrupted methylene groups and ii central double bond; these structural features increase its spontaneous ignition temperature 44' C:. over that of heptane (250" C . ) Toluene has a spontaneous ignition temperature of 63.3" C. and is the most stable o f the series toivard oxidation. T h e fact that the spontancoiis ignition temperature of toluene is 1 o0 C:. lower than that of benzene m a y be partly attributed to the rncthyl hydrogens. .\r these ternjieratttres the 1 ' hydrogeiis arc' contributr to the drgradation of tolucric,. Pyrolysis can niore easily occt~rberiveen the meth!,l-ring carbons than t)et\veen trvo ring carbons as i n benzcnc-. In the initial steps of oxidation of hydrocarbons a t I o ~ v tempc*ratiirrs ( < j O O o C:.)- for rxaniple. 3-lirptmr arid heptane--- cqganic ijemxidcs ai-? forined in large amounts. Becausc: of t h t s rciative stability of the alkylperoxy radicals at these teinperaturas. they reniain i n tact long cnouqh to abstract a Iiydroqcw atom and form the alk!-l hydroprrosidr, ROO.
+ RH
-+
KOOH
+ K.
'At higher temperatures (>4OOo C , ) tlir first products formed are hydmgen peroxide and oldins. Here, the concentrations of the acti\.e oxygen dirndical .OO. and the active .OOH radical make thr follo\vinq initiation steps predoniinant : RH RH K.
+ .OO.
+ .OOH + .OOH
-+ -+
-+
li. f .OON
K , 4- H!O? IZ'C:H=C:H?
--L
H20:
I'OL the hYdt(Jcarhol1i tcStcd.
lli? Lillal1tity of hydrogen peroxide i n the product r s c e r d s t l i a i of KC>OH as the oxidation proct-eds. T h e ROOF3 dccomposcs rathrr rapidl!.:
KOOH
-+
li0. -C
.OH
while the morc srahlcz hydroyen 1x1oxide (22) decomposes mort' sio~vly. .\s lhr extent of oxidation increases. carbonyl compounds are found in large quantities. The tables indicate that the (1rdt.r of decreasing resistance ( ( 1 both spontaneous ignition and preflarne controlled vaporphase oxidation rjf the h!.drocarbon structures testrd is aromatic > branched > cyclic > straight-chain oicfin > straightAs the spontanrous chain paraffin. ignition rempcraturc for these hydro-
INDUSTRIAL AND ENGINEERING CHEMISTRY
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