I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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Figure 3. Temp.,
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Variation of pH and specific resistivity of water with temperature A 0 Dorsey ( 6 ) Kohlrausoh, Calod. Exptl. Megohm-Cm. 84.0 71.4 43.2 ... 31.8 26.7 25.0 18 1 18.2 11.0 11.9 ... 8.7 5.4 5.9
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LITERATURE CITED
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(1) Caddell, J. R., and Moison, R. L., Technical Information
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the water and chemical determination of particular anions. In addition, the curve of specific radioactivity and decay for the reactor water may be useful. A typical bypass insttallation for mixed resin purification as shown in Figure 4 may be used. ACKNOWLEDGMENT
The brief results and experiences reported here mere the cooperative effort of many individuals at the hrgonne Xational Laboratory and elsewhere. It is a natural desire of those interested in this subject to ask for more specific and extensive information. The authors are gratified to have these data released, and promise that additional information will be made available as fioonas security regulations permit.
Service, U. S. Atomic Energy Commission, Oak Ridge, Tenn., “Performance of Ion Exchange Resins at High Flow Rates,” DP4 (March 1952). (2) Chittum, T. P., and LaiVler, 1‘.K., J . Ani. Chem. Soc., 59, 2245 (1937). (3) Dahl, O., and Randers, G., Nz~cleonics,9, KO.5 , 5-17 (1951). (4) Daniels, F. H., ”Outlines of Theoretical Chemistry,” Wiley, New York, 1940. (5) Doreey, N. E., “Properties of Ordinary Water Substances in All Its Phases,” Reinhold, New York, 1940. (6) Furman, N. H., ed., “Scott’s Standard Methods of Chemical Analysis.” 5th ed., Vol. 2, p. 1342, Van Nostrand, New York, 1939 (7) Isbin, H. S., Nucleonics, 10,No. 3, 10-16 (1952). (8) Ibid., 11, No. 6 (1953). (9) Kunin, R., ”Ion Exchange Resins,” Wiley, New York, 1950. (10) McCorkle, W. H., Nucleonics, 11, No. 5, 21-6 (1953). (11) Sivetz, M., Atomics, 6, No. 5 (1950). (12) Wynn-Jones, W. S. K., Trans. Faraday Soc., 32, 1397 (1936). RECEIVED for review August 13. 1964.
ACCEPTEDDecember 23, 1954.
Antioxidant Sweetening of Gasolines C . M. BARRINGER Petroleum Laboratory, E. I. du Pont de Nemours & Co., Inc., P.O. Box 1671, Wilmington 99, Del.
T
HE use of p-phenylenediamine gasoline antioxidants as
catalysts for converting the mercaptans in gasoline to less reactive and less disagreeable compounds is termed “antioxidant” or “inhibitor” sweetening. Because of its simplicity, low cost, and other advantages, the use of this process is becoming more widespread in the refining industry, and today an appreciable percentage of gasoline production is sweetened in this way, generally with good results. The bulk of the information on antioxidant sweetening available a t the present time is empirical. Although the results of a t least one investigation of the chemistry of the process have been published (6),users still rely principally on data obtained in plant experience, which is necessarily somewhat fragmentary and difficult to interpret. As the number of applications has increased, it has become evident that there are some cases where this type of sweetening is not too successful. A few instances have been encountered in which a successfullysweetened gasoline component has proved to be either unstable or “dirty” in automotive use. To increase the understanding of this process, a study has been undertaken of the chemistry of antioxidant sweetening, in which
an effort was made to separate and identify the individual reactants and the reaction variables which influence the course of sweetening. By conducting experiments under controlled conditions in simplified hydrocarbon-mercaptan systems, a considerable amount of information applicable to the more complex mixtures encountered in gasolines has been obtained. Because of the procedure followed as well as the simple nature of the mixtures sweetened, one should not attempt to correlate directly the laboratory sweetening times and effects reported here and those expected in full scale plant operation. The general conclusions that can be drawn concerning the mechanisms of the sweetening reactions and the use of these conclusions to answer questions arising in practice are considered to be more valuable. EXPERIMENTAL
Materials. Reagent grade hydrocarbons were used in the sweetening experiment as follows: IMethyl cyclohexane, Phillips Petroleum Go.
May 1955
INDUSTRIAL AND ENGINEERING CHEMISTRY
Cumene (redistilled), Dow Chemical Co. Cyclohexene, Distillation Products Industries Division of Eastman Kodak Co. Diisobutylene, Esso Standard Oil Co. 1-Octene, Humphrey-Wilkinson Co. All hydrocarbons used in experimental studies were distilled immediately before use from a 3-liter flask containing sodium metal shavings, in a slow stream of oxygen-free nitrogen. A foreshot of approximately 10% was discarded before collecting the sample t o be used. The mercaptans (thiols) used were obtained from Distillation Products Industries Division of Eastman Kodak Co. and were of reagent grade where available. They were used as received. The N,N' di-sec-butyl-p-phenylenediamine used was commercially pure material as manufactured by the Organic Chemicals Department of E. I. du Pont de Nemours di Co. Reagent grade sodium hydroxide and distilled water were used to make u p aqueous caustic solutions. Apparatus and Procedure. Glass reaction flasks with provision for sampling, agitation, and carrying out studies at one atmosphere of oxygen pressure were used. Figure 1 shows t h e arrangement of equipment.
1023
sweetening information. The amount of antioxidant used is within the range of concentrations encountered in plant practice. The quantity of caustic used was sufficiently small as to be almost completely dispersed in the hydrocarbon phase and approximated the carry-over present in a gasoline stream t h a t has undergone caustic scrubbing. RESULTS
Hydrocarbon Type. One of t h e possible causes of variation in t h e antioxidant sweetening behavior of gasolines is the existence of variations in the constituent hydrocarbon types. I n order t o obtain an idea of t h e magnitude of t h e effect of hydrocarbon variations, sweetening studies under standard conditions were carried out in single hydrocarbons boiling in the gasoline range, representing three different chemical types. The examd e s chosen are listed below. Saturated methylcyclohexane Alkyl-aromatic cumene Olefinic cvclohexene. I-octene: isomers) "
TO 0, SUPPLY
diisobutvlene (mixture of two
n
T h e reaction flask, A , was submerged in a A 500-CC.REACTlON FLASK WITH constant temperature bath, H , which was made VENT AND SAMPLING ARM of glass or Lucite acrylic resin, so as not to inB SENSITIVE MANOMETER terfere with t h e action of t h e magnetic stirrer, H z S 0 4 FILLED G. Water from this b a t h was circulated through C ATMOSPHERtC BUFFER CHAMBER, jackets surrounding t h e oxygen-measuring buret, WATER JACKETED D ,and atmospheric buffer chamber, C, in order D IOOCC. GAS BURETTE t o eliminate t h e need for temperature corrections. WATER JACKETED I n a typical sweetening experiment, the proE MERCURY LEVELING BULB cedure was as follows: F WATER CIRCULATING PUMP Hydrocarbon, mercaptan, antioxidant, and caustic were added t o t h e reaction flask which G MAGNETIC STIRRER was closed off and flushed with oxygen. Stirring was commenced with sufficient speed t o give a H THERMOSTATED WATER BATH continual dispersion of oxygen bubbles through t h e liquid. After a period of 15 t o 30 minutes, which was required for t h e vapors t o reach their equilibrium partial pressure, t h e pressure in the reaction flask and measuring buret was equalized with t h a t of t h e atmosphere, and t h e atmospheric buffer chamber was closed off. Oxygen uptake measurements were then commenced. Figure 1. Apparatus for laboratory studies of antioxidant sweetening Sampling of t h e liquid phase was accomplished b y admitting oxygen t,o t h e reaction flask by appropriate positioning of the two-way stopcock, t o a total pressure of about 6 inches of water, and withdrawing Table I shows the times required for the complete sweetening through the side arm' The excess pressure in the the of n-butyl mercaptan (I-butanethiol) in these hydrocarbons, reactor was then vented and t h e system allowed t o come t o The differences between the three chemical types are more equilibrium (10 t o 15 minutes) before t h e flask pressure was pronounced than the individual differences between olefins. The equalized with t h e buffer chamber, as a t the start of a run. order of sweetening rates is olefins > cumene > methylcycloAnalytical. The liquid phase samples were analyzed for merhexane. Since cumene and methylcyclohexane both contain a captan using a potentiometric alcoholic silver nitrate titration reactive tertiary hydrogen atom, the behavior of these two procedure, based on the method of Tamele and Ryland (6). hydrocarbons can be considered as in the upper range of reactivity p-Phenylenediamine antioxidant was extracted from the hydrofor their respective chemical types. carbon sample with 0.1N aqueous hydrochloric acid and deterThe influence of olefins was more strikingly demonstrated in mined by a colorimetric procedure, using a Lumetron colorimeter another experiment. Methylcyclohexane containing 1% by (Model 402-E) to measure the color developed by hydrogen volume of the diolefin, 4-vinylcyclohexene, and the standard peroxide in the buffered extract (7). Oxygen uptake by the concentrations of n-butyl mercaptan, antioxidant, and caustic system during reaction was measured directly. were sweetened under standard conditions. The mixture Standard Reaction Conditions. The experimental sweetening runs reported here were conducted using a standardized set of conditions and concentrations (except when t h e effects of conTable I. Effect of Variations i n Hydrocarbon Substrate on centration variations of individual components werebeing studied) Antioxidant Sweetening as follows: Bath temperature, 30' C. Mercaptan concentration, 0.05 gram per 100 ml. of mercaptan sulfur Antioxidant concentration, 0.005 gram per 100 ml. Caustic concentration, 0.05 ml. per 100 ml. of a 10% aqueous solution Mercaptan concentrations higher than those normally encountered in naphthas were used in order to obtain more accurate
(n-Butyl mercaptan, standard conditions and concentrations) Av. Sweetening Substrate Time, Hr. Methylcyclahexane 80 (approx.) Cumene 45 Cyclohexene 20 Diisobutylene 14 1-Octene 18 Methylcyclohexane plus 1% 4-vinylcyclohexene 24
sweetened in less than one third the time required for pure methylcyclohexane. Thus as little as 2% of unsaturation in the hydrocarbon mixture was found t o alter its behavior so t h a t i t corresponded quite closely to t h a t of a pure olefin. The addition of olefins t o a saturated hydrocarbon in order t o render its antioxidant sweetening possible has been recently patented ( 4 ) .
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Vol. 47, No. 5
INDUSTRIAL AND ENGINEERING CHEMISTRY
1024
Sweetening rates of various mercaptans in cyclohexene Standard reaction conditions
In order Bo demonstrate the magnitude of the catalytic effect of p-phenylenediamine in sweetening, cyclohexene and methylcyclohexane containing n-butyl mercaptan were sweetened with and without 0.005 gram per 100 ml. of antioxidant, the conditions and concentrations of the other ingredients being those described previously. The sweetening times obtained are shown in Table 11. Under the conditions used, the inhibitor was relatively ineffective in the saturated hydrocarbon, b u t accelerated the sweetening of the olefin about threefold. This result shows that the catalytic effect of the inhibitor is greatest in a reactive hydrocarbon and emphasizes again the difference between hydrocarbon substrates.
Table 11, Catalysis of Sweetening Rate in Different Substrates by p-Phenylenediamine Antioxidant
pounds are extracted more readily b y bases. The combination of acidity and ease of oxidation leads t o extremely rapid sweetening in the case of thiophenol and, t o a lesser extent, results in a more rapid sweetening rate for a n n-alkyl mercaptan in comparison with its more highly branched isomers. Increasing the molecular weight of a given type of mercaptan would be expected to lead t o lower reactivity. This effect was demonstrated b y comparing the sweetening rate of n-hexyl mercaptan with t h a t of n-butyl mercaptan under standard sweetening conditions in a n olefin. Addition of two carbons to the chain was found t o increase the time required to sweeten b y approximately 50% under these conditions-i.e., from 16 t o 25 hours. Antioxidant Depletion. Figure 3 shows a number of typical antioxidant depletion curves obtained during t h e sweetening of n-butyl mercaptan in various hydrocarbons under standard reaction conditions. Also included are t h e curves obtained for antioxidant depletion in a n olefin and in a saturated hydrocarbon which were agitated a t 30' C. in the presence of caustic and oxygen, but without mercaptan. T h e feature to be noted about these data is their general similarity in spite of the wide differences t h a t have already been discussed in the sweetening rates of the hydrocarbons. Since the disappearance of antioxidant was found t o be independent of the rate of the sweetening reaction, it may be inferred: ( a ) t h a t the sweetening reactions of the mercaptans present do not consume antioxidant and ( b ) t h a t antioxidant consumption during the sweetening period is caused by the reactions involved in inhibiting oxidation of the hydrocarbon substrate. The latter supposition is strengthened by t h e fact t h a t much more antioxidant is consumed during the sweetening of a hydrocarbon t h a t is preoxidized and already contains measurable amounts of oxidation products than is consumed in a hydrocarbon that has been distilled under nitrogen immediately before use. This is demonstrated in Table 111.
Table 111.
Effect of Condition of Substrate on Antioxidant Depletion during Sweetening
(Standard reaction conditions and concentrations) Decrease in D B P P D 5 Concn. Condition during Sweetening, Substrate of Substrate G./100 M1.b Cyclohexene Freshly distilled 0.0011 Aged 0.0037 Diisobutylene Freshly distilled 0.0013 Aged 0.0041 a N , N ' di-sec-butyl-p-phenylenediamine. b Average of two or more experiments.
(Standard reaction conditions and concentrations) n-Butyl Mercaptan DBPPDffi, Sweetening Substrate G./100 MI.' G./100 M1. Time, Hr. C yclohexene 0.05 0 60 0.05 0.005 20 Methylcyclohexane 0.05 0 80 (approx.) 0.05 0.005 75 a N , N ' di-sec-butyl-p-phenylenediamine.
Mercaptan Type. Since t h e classes of mercaptans present in gasolines are known t o differ widely, t h e antioxidant sweetening behavior of four different structural types was investigated. The studies were all carried out in cyclohexene. Normal, secondary, and tertiary butyl mercaptans were chosen t o eliminate differences due t o molecular weight, and thiophenol was included as a n aromatic type. Figure 2 shows the curves for the disappearance of these mercaptans plotted against time. The order of reactivity shown is the same as t h a t demonstrated by these types of mercaptans in other free radical reactions such as the peroxide or lightinduced addition t o olefinic double bonds-i.e., aromatic >> primary alkyl > secondary alkyl > tertiary alkyl. The acidity of the sulfhydryl hydrogen of simple mercaptans roughly parallels their ease of oxidation, so that the more easily oxidized com-
A DIISOBUTYLENE t n.0uSH B I-OCTENE n-BUSH
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Figure 3.
Depletion of antioxidant in various hydrocarbons with and without mercaptan
Standard reaction conditions;
RSH sulfur, where present, 0.05 gram per 100 ml.
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1955
Antioxidant Concentration. B y maintaining a constant initial mercaptan concentration and varying the antioxidant concentration over a wide range, it was established t h a t there is apparently a n upper limit t o the increase in sweetening rate t o be obtained by increasing the inhibitor concentration. This upper limit was found to vary with temperature and with the hydrocarbon system being sweetened. Table IV shows the effect in djisobutylene a t 40’ C. of an eightfold increase in antioxidant concentration.
Table I\‘.
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Effect on Sweetening Rate of Increasing Antioxidant Concentration
(n-Butyl mercaptan in diisobutylene; temperature, 40° C.; other conditions a n d concentrations standard) DBPPDa, G / l o 0 M1. Sweetening Time, Hr. 0.0025 18 0.0050 12 0.010 0.020
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Q N , N ’ di-see-butyl-p-phenylenediamine
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Figure 4. Various additive combinations in antioxidant sweetening of n-butyl mercaptan in cyclohexene Temperature, 30’ C.; oxygen atmosphere
c
It might be reasoned from these results t h a t at a n antioxidant concentration between 0.005 and 0,010 gram per 100 ml. a maximum concentration of the material thaf is actively catalyzing sweetening is formed and t h a t above this concentration of antioxidant the conditions preaent in the reaction system do not permit formation of more catalyst. Some evidence indicates t h a t b y increasing the volume of aqueous caustic used the amount of active intermediate present can be increased above t h a t possible with a lower caustic level. The effects of increasing caustic volume, discussed below, are thus probably not entirely due to greater mercaptan extraction. Caustic Volume. Since t h e acidic nature of mercaptans renders them soluble in strong aqueous bases t o a certain extent and since t h e mercaptide ion is more readily oxidized than free mercaptan, an increase in t h e amount of aqueous sodium hydroxide of a given concentration might be expected t o improve the rate of antioxidant sweetening The magnitude of this effect in cyclohexene containing n-butyl mercaptan is shown b y the data given in Table V. Each tenfold increment in caustic concentration appreciably accelerates the sweetening rate. Similar results were obtained in the saturated hydrocarbon methylcyclohexane. Extraction of mercaptan from hydrocarbon to aqueous phase is undoubtedly a major contributor to the increased sweetening rate. The effect of this extraction is complicated by reverse extraction of reaction products from the aqueous t o the hydrocarbon phase as well as by the influence of the increased amount of caustic on the actual sweetening reactions t h a t are taking place.
Table V. Effect on Sweetening Rate of n-Butyl Mercaptan of Varying Amount of 10% Aqueous NaOH Present (Standard conditions a n d concentrations except for caustic) NaOH Soln., Substrate VOl. yo Hours t o Sweeten
The possibility t h a t a n increased volume of strong base is able t o activate more catalyst should also be considered. I n Figure 4, the upper curve shows the behavior of a cyclohexene-n-butyl mercaptan system in the absence of both antioxidant and base. The next two curves below this show the effect of one additive.
The curves are nearly identical, showing t h a t unless all the components of antioxidant sweetening are present, very little catalysis of the sweetening reactions takes place. When caustic and antioxidant are both added to a sour olefin, the first small addition of caustic (0.05 volume yo)causes a large increase in the rate of sweetening (about fourfold). The next increment of caustic results in a twofold increase in rate, while the third ( t o 5 volume %) does not even double the rate obtained with 0.5 volume %. Simple calculations based on the phase rule show t h a t mercaptan extraction by aqueous caustic should be roughly proportional t o the volume of caustic used. Since this is not the case, it is likely t h a t some process other than the removal of mercaptan into the aqueous phase is controlling the rate of
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Figure 5 . Oxygen uptake during sweetening of n-butyl mercaptan in cyclohexene
sweetening. Experiments such as those just described have shown t h a t the uncatalyzed oxidation of mercaptans in the presence of caustic is a slow process. It appears possible that for catalyzed sweetening t o take place there must be present in the aqueous phase a n activated form of the antioxidant which could conceivably be a n ion. The amount of such a n ion present a t any given time would be controlled by the ionization constant of the antioxidant a t the reaction conditions. This ionization would be a n equilibrium process and would be influenced t o a decreasing extent by a n excess of aqueous base. Oxygen Consumption during Sweetening. When an olefin containing mercaptan, antioxidant, and caustic was agitated
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1026 Table VI.
Mercaptan-Oxygen Ratios in Sweetening of Pure Hydrocarbons (Standard reaction conditions and concentrations) Hydrocarbon RSH/Oz Mole Ratio Methylcyclohexane 3.9 Cumene 2.4 1-Octene 2.8 Cyclohexene 2.3 Diisobutylene 2.9
Table VII. Mercaptan-Oxygen Ratios in Sweetening of Various Mercaptan-Hydrocarbon Systems (Standard reaction conditions and concentrations) RSH/On Mole Sweetening Mercaptan Hydrocarbon Ratio Time, Hr. n-Butyl Cyclohexene 2 3 20 2.9 14 n-Butyl Diisobutylene sec-Butyl Cyclohexene 1.9 35 tert-Butyl Cyclohexene 1 1 45 tert-Butyl Diisobutylene 1 3 38 Thiophenol Diisobutylene 6.0
