Depolymerization of Butadiene Dimer THOMAS F. DOUMANI, ROLAND F. DEERING,
AND
ART C. McKINNIS
Union Oil Company of California, Wilmington, Calij.
Depolymerization of dimeric butadiene has been carried out at temperatures from 510' C. (950' F.) to 704' C. (1300' F.) and at atmospheric pressure both in the presence and absence of steam. A gaseous depolymerization product containing 90-93% butadiene with 95-98qo butadiene in the C, cut can be obtained at temperatures from about 558" C. (lOOOo F.) to 649' C. (1200" F.) with contact times from about 0.2 to 1.0 second. With these reaction conditions the amount of dimeric butadiene
pyrolyzed per pass should be less than about 25% to maintain a high concentration of dimer in the recycle product. The first-order reaction rate in the absence of steam is 8.37, where Tis expressed given by log k = -14,18O/T~ in O Rankine. When T i s given in ' Kelvin, this equation becomes log k = - 7 8 8 0 / T ~ 8.37, from which the activation energy E was calculated to be 36,000 calories per mole. Trimeric and tetrameric butadiene are more difficult to depolymerize than the dimeric form.
B
based on the feed pyrolyzed. Kistiakowsky and Ransom (2) calculated the rate of dissociation of vinylcyclohexene (dimeric butadiene) from its entropy of dissociation and the entropy of a proposed intermediate free radical for the following reaction:
+ +
UTADIENE forms polymers when employed in various chemical reactions and even during its handling and storage a t ordinary temperatures. These polymers are formed more rapidly a t elevated temperatures and are of two principal types, liquid products and plastic polymers. The former products arise by thermal polymerization of butadiene, whereas the latter are catalyzed by oxygen or peroxides. The Sormation of the rubberlike polymers can be largely controlled by the addition of suitable inhibiters; however, the prevention of formation of the liquid polymers has not been accomplished, These liquid polymers are principally dimers. Thus, the formation and accumulation of dimeric butadienes by various processes and means makes desirable a study of their depolymerization to monomeric butadienq, Rice and Murphy (a) recently pyrolyzed 3-vinylcyclohexene a t 1292' F. (700' C.) to the extent of 61% a t a contact time of 0.03 second, forming 80% of the theoretical 1,3-b;tadiene
P H 2 \
HC
CH-CH=CHz
liC
CHz
I
+ 2CH-CH-CH=CHz
\CHI' 4-Vinylcyclohexene
'
THE
1,a-Butadiene
This paper deals with the depolymerization of dimeric britadiene a t temperatures from 510' C. (950' F.) to 704' C. (1300' F.) and a t atmospheric pressure, both in the presence and absence of steam. The reaction conditions studied were in the range where only a portion of the dimer was cracked and the uncracked dimer was not changed unduly. This work was confined largely to these conditions so that the dimer could be used in a cyclic process; this might not be possible if the unconverted dimer were transformed to a recycle product which does not form butadiene upon. pyrolysis. APPARATUS AND MATERIALS
Figure 1presents a sketch of the apparatus used. The stainless steel reaction tube was of 29-inch (73.7-cm.) length and 0.626inch (1.58cm.) inside diameter, with a 0.250-inch (0.635-cm.) outside diameter thermocouple well extending throughout its length. The net internal volume of the reaction tube was 120 ml. The hydrocarbon and water were pumped from separate reservoirs into the preheater a t the desired rate by means of a Manzel chemical feeder. The preheater and the ?eaction tube were electrically heated, the temperature being regulated and recorded by means of Leeds & Northrup Micromax controllers. The preheater was maintained a t approximately 454' C. (849' F.) for all of the runs. After leaving the reaction tube, the products were cooled in a water-cooled condenser and then collected in a flask which was held at 90-95' C. by means of a steam jacket. This steam-jacketed flask removed most of the water and uncracked hydrocarbons without retaining an appreciable amount of the lighter gases. The uncondensed gases were passed through a glass wool fog remover which was maintained a t 90-95" C. by means of a steam jacket, cooled to 20' C. in a water-cooled con-
TEST
Figure 1. Depolymerization Unit for Dimeric Butadiene
89
INDUSTRIAL AND ENGINEERING CHEMISTRY
90
\
/'
as shown in Figure 2, and the average reaction rate was determined from this plot. The average rate selected is such that the area of the shaded portion above the average rate line is equal to the area of the shaded portion below the line. The temperature corresponding to this rate is the true average temperature and in all cases was nearly 11' F. lower than the arithmetical average temperature. In Figure 3 the k values were plotted using the true average temperature determined in the manner described; the first-order reaction rate equationis expressed as follows:
\ \
\
1100
10 .
Vol. 39, No. 1
\
where k = reaction constant in reciprocal seconds t = contact time in seconds a = initial concentration of dimeric butadiene under the reaction conditions in moles per liter x = amount'oI dimer depolymerized at any time 1 .
The equation in Figure 3 is
..
0
4 E 12 16 20 24 DISTANCE FROM REACTOR BOTTOM (INCHES)
log k =
Figure 2. Reaction Tube Temperatures and Corresponding Relative Reaction Rate Constants
- 14,18O/T~+ 8.37
where T R is expressed in 'Rankine. When Tis given in 'Kelvin this equation becomes
denser, and finally passed through a wet test meter saturated with the gas to determine the volume of gases produced. Experiments on the dimerization of butadiene indicated that the constitution of the compounds formed depend on the dimerization conditions. A dimer of practically constant boiling point and refractive index was obtained when butadiene was heated in closed vessels in the liquid phase a t temperatures from 110" C. (230' F.) to 150" C. (302"F.), Data on the kinetics of this dimerization have been published (4). When higher dimerization temperatures-such as 399' c. (750' F.) to 482' c. (900' F.)-were employed, the product consisted apparently of a mixture of isomers, as indicated by the variation in refractive index with boiling point. The low temperature dimer was used to obtain the kinetic data. The high temperature dimer, the crude dimer-polymer mixture obtained by various polymerization conditions, as well as the trimer and tetramer of butadiene were studied to determine whether they could be depolymerized to ' butadiene. The trimer and tetramer were accumulated from crude dimerization products. The gases formed by cracking the dimer were analyzed by a mass spectrometer. Analyses for butadiene pere by the method of Cuneo and Switzer ( 1 ) . The dimeric but,adiene was analyzed by fractionation and determination of the refractive index and by density of the fractionated product. ~
log k = -7880/T f 8.37 from which the activation energy E was calculated to be 36,000 calories per mole. EFFECT OF DEPOLYMERIZATION CONDITIONS
When steam was employed with the dimer, a reaction rate was obtained which was greater than that obtained in the absence of steam (Figure 4). These data were not sufficiently good to determine the order of the reaction but were plotted as first order for comparison with the results in the absence of steam (the straight line of Figure 4). The use of steam tended to suppress the deposition of carbonaceous materials on the tube walls. Increasing the contact time for the depolymerization of dimeric butadikne a t temperatures of 532' C. (990' F.) and 560' C. (1040' F.) in the absence of steam (Figure 5) caused the production of a less pure butadiene, less dimer in the liquid products, and a larger conversion to gas. The per cent dimer in the liquid product was an indication of the quality of the hydrocarbon for ' recycle. The average molecular weight of the gas found was a criterion of the per cent butadiene in the gas.
147
2.0 1
REACTION TEMPERATURES AND RATE CONSTANTS
To determine the true or effective temperature in the reaction tube, the reaction rates were plotted throughout the reaction tube ai
,am
-
w 0
2
v! 0.02 0.01
1
,
6.6
01 .
NUMBER WITH DATA POINT SIGNIFIES PER CENT DEPOLYMERIZED.
I '
6.7
-
DIMER, MM.Hg. CIRCLED NUMBERS SIGNIFY PER CENT DEPOLYMERIZED.
I
I
I
I
I
6s
6.9
7.0
5.8
6.0
( I I T ) (I04),'RANKINE
Figure 3. Depolymerization of Dimeric Butadiene in Absence of Steam
I
I
6.2 6.4 ( I / T ) (IO'),*RANKlNE
I
6.6
6.8
Figure 4. Depolymerization of Dimeric Butadiene in Presence of Steam
91
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1947
55
*-----
100
-*Q'
PER CENT DIMER IN
T, THE uaum PRODUCT
AVERAGE MOL.WT. OF GAS
60
.
TO GAS
.
1050
1200
1100 1150 TEMPERATURE,'E
4 0 0 0
a5
ID
1.5
Figure 6. Effect of Temperature on Depolymerization of Dimeric Butadiene
25
2.0
CONTACT T I M E , SECONDS
Figure 5. Effect of Contact Time on Depolymerization o$ Dimeric Butadiene
Contact time, 0.M second; dimer partial pressure, 130 mm.; diluent, steam
The effect of temperature on the depolymerization of dimeric butadiene is given for contact times of 0.40 second (Figure 6) and 0.70 second (Figure 7) a t dimer partial pressures of 130 mm. and 110 mm., respectively. The contact times used in this work were limited by the capacities of the feed pumps and the heating capacity of the cracking furnace. With these contact times, temperatures above 649" C. (1200" F.) did not appear feasible, since a good recycle product could not be obtained with conversions of more than about 25%. The possibility of obtaining improved results by employing higher temperatures and shorter contact times appears to be of interest. A gas containing a high percentage of butadiene (90 to 95%) was obtained for all conditions where the uncracked hydrocarbon was substantially the same as the feed. Table I lists a number of gas analyses for various feeds. Column 2 shows the results of a run made with mixed butadiene trimer and tetramer feed. The conditions used for this run do not afford a good comparison with the other runs; however, thg results indicate that trimer and tetramer were definitely inferior feeds compared with butadiene dimer. It can be seen that a
mixed feed of butadiene dimer and higher polymers was cracked to yield a C4 fraction (Kydrocarbons consisting of 4 carbon atoms) containing 90-98% butadiene, but the best feed was the pure butadiene dimer which yielded a gas containing 90-93q6 butadiene with 95-98% butadiene in the C4 cut. Dimeric butadiene formed a t high temperatures (750-900' F.) was equivalent to the loy temperature (about 300' F.) dimer for depolymerization to monomeric butadiene, even though it apparently consisted of dimer isomers. Table I1 shows the results of three runs made using the condensed hydrocarbon from one run as the feed for the next. The results show that conversions of less than about 25% would have to be used to obtain a satisfactoryrecycle product. DISCUSSION OF RESULTS
The results obtained in this work indicate the possibility of obtaining practically pure butadiene by depolymerising dimeric butadiene. It was found that the temperature
TABLEI. ANALYSES OF GAS FROM DEPOLYMERIZATION OF BUTADIENE POLYMERS Peed
80% dimer 20%
+higher
polymers
Temp., F. Temp., C. Contact time. sec. Conversion to gas, w4. % Partial pressure of hydrocarbon, mm. Boilin range of liquid procfuct, 0 C .
1306 708 0.37
Trimkr -Itetramer, impure
Dimer (b.p.,
1309 710 0.62
1178 637 0.70
95% dimer Dimer (b.p., Dimer (b.p., 70% dimer Dimer (b.p., Dimer ( b . p 128-132' C . ) (b.p., 125129-131' C.) 129-131° C.) 30% 125-131' C.) 125-135 C.) 135' C . ) higher
+
polymers
48
51
45
51
150
109
1099 593 0.72 23.4 122
1043 562 1.70 13.7 760
.....
. .. . .
-
-
-
-
-
100.0
100 0
100,o
100.0
100.0
I35 125-135 (10%) (24.9%); >135 (30.1%); loss (11.1%)
997 536 3.2 14.6 760
1049 565 0.94 15.7 115
1044 563 0.96 21.2 129
I32 >133 (8.9%) (6.7%) (22.7%) (47.8%)
1065 575 0.77 17.37 110
...
Anal., wt. % of gas H2
CHI
CzHi C1H6 C3H6 C3He C4Hs C4Hs C4HlO
100.0
-
-
-
100.0
100.0
100.0
92
INDUSTRIAL AND ENGINEERING CHEMISTRY
so
100
--*----, --\
PER CENT DIMER IN \, LlOUlD PRODUCT+
v)
THE
u)
0 U
I-
e
0 W
'5 0
a
w W 4
,
W
-
w
3 20
40
-
0 1 ' 1000
I
1050
I
I
I100 I150 TEMPERATURE,*F.
Run No. Feed type Temp., F. Temp., C. Contact time, sec. Partial pressure of hydrocarbon, mm. Conversion to gas, wt. % Av. mol. wt. of gas Dimer in condensed hydrocarbon, wt. % Cd hydrocarbons in gas, wt..% C4Hs in C4 cut, wt. %
a
40
Vol. 39, No. 1
DIMER
GAS
I
a
'
TABLE11. EFFECTOF RECYCLEON DEPOLYMERIZATION OF
AVERAGE MOL.WT.
OF
I
I
l2W
Figure 7. Effect of Temperature on Depolymerization of Dimeric Butadiene Contact time, 0.70 second; dimer partial pressure, 110 mm.; diluent, steam
c-33 c-34 c-35 Dimer (b.p., Condensed Condensed 129-131' C.) hydrocarbons hydrocarbon from C-33 from C-34 1078 1076 1078 582 581 682 ' 0.48 0.48 0.49 109 19.8 52.6
110 17.3 60.6
113 16.0 48.7
95
90
68
95 99
93 98
89
...
ture should be as close to the initial depolymerization (decomposition) temperatures as possible and still provide for a practical conversion per pass. I n practice this makes necessary the employment of temperatures somewhat higher than the initial decomposition temperature. LITERATURE CITED
gradient in the reaction tube was quite sensitive to changes in feed rate. The partial pressure of the hydrocarbon gases varies a$ the cracking takes place. The partial pressures r$ported are the arithmetic averages of the partial pressures a t the entrance and at the exit end of the reaction tube. I n general, low temperatures and relatively small conversions produced butadiene in the highest over-all yields, with a high percentage of dimer in the recycle product. The required tempera-
(1) Cuneo, J. F.,and Switzer, R. L., IND.ENQ.CHEM.,ANAL.ED.,15, 508 (1943). (2) Kistiakowsky, G. B., and Ransom, W. W., J . 'Chem. Phys., 7, 734 (1939). (3) Rice, F. O., and Murphy, M. T., J . Am. Chem. SOC.,66, 766 (1944). (4) Robey, R. F.. Wiese, H. K., and MorreI1, C. E., IND.ENQ. CHEM.,36,3 (1944). PREBENTED before the Division of Petroleum Chemistry at the 109th Meeting of the AMHIRICAN CHEMICAL SOCIETY, Atlantic City, N. J.
Formation of Static Electric Charges on
Agitating Petroleum ,Products with Air by C. M. KLAEKNER agitators, as demonstrated by the secured electric sparks require not Magnolia Petroleum Company, B ~ T~~~~ ~ following ~typical results ~ ~ with a Detroleum distillate of only the existence of high voltages 96" F. flash point: but also a certain current density and the presence of an explosive mixture. Formation of an Solvent Temperature, Relative Explosibility of Vapors explosive mixture in air agitation depends on the vapor pressure F. in Commercial Agitatorsn of the liquid. If the vapor pressure is low, the concentration of 88 0.62 92 0 85 the vapors in the air may be less than that corresponding to the 96 1.00 low explosibility limit, whereas if the vapor pressure is very high, Explosive limit = 1.00. the concentration of vapors may be above the high explosibility limit and, in either case, no explosion occurs. Explosive mixIt is believed, therefore, that the explosive temperature is withtures of hydrocarbons of known composition can be calculated in less than 1' F. of the flash point of the petroleum distillates. with a fair degree of accuracy (d), but these calculations are not The rate of air blowing should not have an appreciable effect applicable to commercial petroleum products which contain small on the explodbility limits, provided sufficient time is allowed to quantities of volatile components. For these reasons the flash reach the equilibrium conditions. In plant practice the depth of point remains the most reliable guide for such estimates. the liquid in agitators is sufficient to allow the attainment of Since the flash point determination is affected by the type of equilibrium conditions during the passage of air-bubbles. This procedure selected, experiments were made for determining the may be shown by the following typical data obtained by varying relation between the Pensky-Martens closed-cup flash point the air rate to a large commercial agitator of approximately 30(A.S.T.M. D93-36) and the explosibility of oil-air vapors obfoot diameter with a liquid depth of 20 feet: tained on agitating petroleum solvent and kerosenes of 92 ' to Agitation, Relative Explosibility of Vaqors 110O F . flash points with air at various temperatures. The John-, Relative Degree Evolved from Commercial Agitator ' son-Williams explosibility indicator was used to measure the ex0.14 None plosibility of the oil-air vapors obtained during the experiments. 0.61 0.10 0 .64 0 . 2 6 The work showed a close correlation between the flash points and 0.59 0.60 0.W the temperature of explosibility of oil-air vapors in commerical 1.00
E
XPLOSIONS caused
(1
~
