I
T.
D. NEVITT, W. A. WILSON,
and H. S. SEELIG
Research Department, Standard Oil Co. (Indiana), Whiting, Ind.
Radiation-Induced Reaction of Hydrocarbons with n-Butyl Mercaptan In the radiation processing of petroleum, the effect of hydrocarbon type is important. For the destruction of mercaptans - o r sweetening-by ionizing radiation, best results are obtained with an olefinic, nonaromatic stock. The effectiveness of radiation sweetening can be increased b y addition of olefins
Single Hydrocarbons
50
0 50
ICQ
THE
treatment of hydrocarbons with ionizing radiation results in a variety of fragmentation products. Although ions may be formed as extremely short-lived intermediates in nonpolar media, the detectable reaction products are free radicals. Both iodine (9)and diphenylpicrylhydrazil (7-4. 7) have been used to measure radical yields from the radiation decomposition of a large number of hydrocarbons. Radioiodine has been used to identify the free radicals produced (5). Mercaptans (thiols) are also free radical scakengers. Because these materials are undesirable in petroleum, the radiation-induced reactions of n-butyl mercaptan have been studied, with single hydrocarbons and with mixtures of hydrocarbons of the types present in petroleum.
Experimental The hydrocarbons used were Phillips research grade. distilled to obtain a center cut. The n-butyl mercaptan \vas Eastman IYhite Label grade. used without further purification. Solutions of n-butyl mercaptan in the hydrocarbons were degassed to remove air. and sealed in glass vials under their own vapor pressure. T h e vials were subjected to gamma irradiation from spent uranium fuel slugs at Argonne National Laboratorv.
DOSE,
e.v. f L x
Figure 1. Mercaptan conversion is directly dependent on energy input
Data were obtained with n-heptane, methylcyclohexane: benzene, I-octene, and 2-octene. Although the initial mercaptan concentrations were varied from about 10 to 200 mmoles per liter and doses lvere varied by a factor of 10, the G values are apparently constant for each of the first three hydrocarbons. The direct dependmce of the mercaptan conversion on energy input is shown in Figure 1. About four times as much energy is required to produce a given conversion in benz,ene as in heptane. With the olefins, the G values obtained depend on the concentration of the mercaptan. The data for 2-octene have been plotted in Figure 2. Under favorable conditions, the G values for 2-octene are five to seven times as large as those for heptane. In the case of I-octene, the G values are as much as 15 times as large. Such high G values must result from a chain reaction. The distribution of product sulfur is: Hydrocarbon
Sulfide
Disulfide
Benzene Heptane Methylcyclohexane I-Octene 2-Octene
22 25
75
55
45
97
3 8
92
78
Mercaptan concentrations were determined by potentiometric titration of the hydrocarbon mixtures Irith silver nitrate ( 8 ) . Disulfide was determined by refluxing the reaction mixture with zinc and acetic acid and analyzing an aliquot of the resulting solution for increased mercaptan content. Sulfide in the reaction mixture was calculated by difference. T h e Argonne Laboratory supplied the dosimetry, which is based on a ferrous oxidation yield of 15.5. Dose rates were approximately 5 X IOz2 e.v. per liter per hour. From the dosimetry and the analytical data, G values were calculated as molecules reacting per 100 e.v. of energy absorbed by the solutions.
1
2-OCTENE
1
I
10
20
DOSE. e . v .
L. x 1 0 - 2 2
Figure 2. Under favorable conditions G values for 2-octene are five to seven times as large as for heptane VOL. 51,
NO. 3
MARCH 1959
31 1
b>I + + disappear
loo
reaction i$.ith mercaptan :
R.
RSH
+
RS.
I
RH
---I
T h e principal products of the reacrion of the mercaptide radicals are disulfides and sulfides: 2RS
-+
RSSR
RS. f R .
RSSK
100.
I DOSE, e v / L . I N I T I A L R S H CONC.
s
40 X I O 2 ' 0046 MOLAR
0
Y
U c
0
z > 50-
z l-
a P W I
(
0 0
50
IO0
+
+ R.
RSR
-+
+ RSR
KS.
Evidence that sulfides are formed from disulfides was obtained in an n-heptane run by continuing the radiation until all the mercaptan \vas converted; the amount of disulfide \vas less than halt of that found in runs where mercaptan conversion {cas incomplete. Because both sulfide and disulfide are formed during the reaction, the disappearance of mercaptan is not a direct measure of the hydrocarbon radicals formed. T h e G' value for mercaptan disappearance must be multiplied by a factor derived from the reaction products. For each mole of hydrocarbon radical scavenged. 1 mole of mercaptan is converted to disulfide or 0.5 mole 10 sulfide. H>-drocarbon radical !.ields 'ic'ere calculated from: G,(R,) = --G,(RSH) (t\\ice 72 S as RSR 100
+ as RSSR)
Radical >-ields determined for rhree hydrocarbons xcith three scavengers a r e :
I n paraffin or aromatic, the mercaptan gave about three times as much disulfide. I n olefin, the major portion of the mercaptan reacted to form sulfide. T h e reaction producrs of the naphthene fall between these values. No hydrogen sulfide was detected in the gaseous reaction products. -4 typical gas analysis of a heptane run shoic'ed '98.Oychydrogen! 1.6% methane, and 0.4% ethane and higher hydrocarbons. A series of reactions can be Lcritten that is consistent \c'ith the experimental results. Almost all of the energ!- is absorbed b>- the hydrocarbons and absorption is essentially independent of their nature : R H -v-+ R H *
RSH
DPPH
I:
Heptane
6.1
6.0
Methylcyclohexane
5.7
Benzene
1.4
6.8 6.7 0.7
.. .
0.96
These values for heptane and methylcyclohexane using mercaptan agree \c'ith the previous studies using iodine (9) and DPPH (7---?. 7 ) . HoLvever! the values for benzene are higher when determined trith mercaptan than Irith the other scavengers. TVhen olefins are present, radical yields cannot be determined from the mercaptan disappearance because of a chain reaction in \chich several mercaptan molecules are destroyed for each radical formed : RS. Ri.
+ olefin + RSH
-+
-+
R1. RS.
+ RIH
T h e amount of radiation absorbed directly b>-the small amount of mercaptan is presumed small enough to be neglected. Hydrogen and hydrocarbon radicals are formed by decomposition : RH* H. R*
Under these conditions. dit: radical concentration is hiqh enough so that the latir'r reaction and the telminatio~i reaclion
H>-drogenis produced bl- random hydrogen abstraction by hydrogen radical: H. KH + R . H,
arc comperirivc: urilization or the energy then depends on the nxrcaptan concentration [€igure 2). 'I'he utilization of energy probabl>- also depends on dose rare.
-
+
+
+
I n the absence of mercaptan, the hydrocarbon radicals recombine or disproportionate. I n the absence of olefins, when mercaptan is present, hydrocarbon radicals
3 12
R1.
+ R!.
-+
KiKi
Hydrocarbon Mixtures
INDUSTRIAL AND ENGINEERING CHEMISTRY
Because of the marked differences in
I N I T I A L RSH CONC
0091 MOLAR
50
0
IO0
'1. I - OCTENE IN BENZENE
Figure 4. Aromatic inhibition and olefin chain reaction occur over the range of hydrocarbon concentration
mercaptan conversion in the different hvdrocarbons. experiments \vere performed with mixtures. In heptane, small concentrations of olefin markedly increased the mercaptan conversion (Figure 3). Small amounts of aromatic markedly decreased the conversion. Although the nature of this inhibition is obscure, it probably occurs before dissociation and decreases the rate of formation of hydrocarbon radicals. Inhibition of radiation-induced reactions bv small amounts of aromatics seems to be a general phenomenon not limited to the reaction of mercaptans with hydrocarbon radicals (6). T h e reaction of mercaptan in mixtures of 1-octene and benzene is shown in Figure 4. Both aromatic inhibition and olefin chain reaction occur over the ranqe of hydrocarbon concentrations: small amounts of benzene in olefin inhibit the reaction and small amounts of olefin in benzene promote the reaction
literature Cited (1 Bouby, L., Chapiro: A., J . Chem. P ~ J J . 52, 644 (1955). ( 2 ) , Chapiro, X., Corn$. rend. 233, 792 1
11951). ( 3 ) Chapiro, A,, J . Chem. Phys. 51, 165
(1954). (4) Chapiro, A , Boag, J. W., Ebert, 112.: Gra)-, L. H., 16id., 50, 468 (1953).
( 5 ) Gevantman, L. H., Williams, R. R., Jr., J . Phyr. Chem. 56, 569 (1952). (61 Manion, J. P., Burton, M., Ibid.. 56, 560 (1952). (7 \ Prevost-Bernas, A , , Chapiro: A , , Cousin, C., Landler, Y.,Magat, ,\I., Discussions Faraday Soc. 12, 98 (19523. (8) Tamele, M. W., Ryland, L. B., '4nal. Chem. 8, 16 (1936). (9) Weber, E. Tu'., Forsyth, P. F.: Schuler, R. H., Radiation Research 3, 68 (1955). KEcExvm
for reviebc January 11, 1958 Septeniber 29, 1958
.\CCEPTED
Division of Industrial and Enginecring Chemistry and Petroleum Chemistry, Symposium on h-uclear Technology in the Petroleum and Chemical Industries. 131st ,\leetins, .I\CS, hliami, Fla.: .April 1957.
