PURE CRESYLIC ACIDS FROM PETROLEUM B Y SOLVENT EXTRACTION D. C. JONES, J. A.
KOHLBECK, AND M . B. NEUWORTH
Reseaich Division, Consolidation Coal Co., Library, Pa. Spent caustic solutions resulting from the desulfurization of gasoline contain cresylic acids and from 10 to 40% thiophenols as sodium salts. After springing with COz or mineral acid, the crude cresylic acids are refined b y a novel fractional extraction process which uses aqueous methanol and hexane. More than
95% of the cresylic acids are recovered b y the alcoholic solvent, while 99.5+% of the thiophenols are eliminated with the hydrocarbon solvent, The effects of ths important extraction variables-methanol concentration, solvent ratios, throughput, extraction temperature, and thiophenol level-were evaluated using a laboratory extraction column of the Scheibel type. Optimum results as measured by cresylic acid recovery and purity have been duplicated in a pilot plant extraction column. A commercial plant using this extraction process, in operation for five years, has produced cresylic acids in yield and purity comparable to those of the laboratory extraction unit. HE INCREASED D E M A ~ Dfor cresylic acids, including phenol, T c r e s o l s , and xylenols, has led to intensified efforts to recover these compounds from spent caustic used to desulfurize gasoline. The crude caustic extract also contains substantial quantities of thiophenols and disulfides. Removal of these sulfur compounds is required for most applications involving cresylic acids as raw materials because of the disagreeable odor of thiophenols. Existing refining procedures were based on the ease of oxidation of thiols in alkaline solution to disulfides ( Z ) , which are relatively insoluble in aqueous caustic. The disulfides are removed by decantation. While this procedure results in a significant purification of the cresylic acids, it has a number of drawbacks-namely, loss due to oxidative degradation and incomplete removal of disulfides due to their solubility in the caustic solution. T h e residual disulfides are cleaved under normal distillation conditions to thiols, which impart a foul odor to fractionated products. A novel fractional extraction process was developed (5) which almost completely eliminates both thiophenols and disulfides with a recovery of cresylic acids exceeding 95%. T h e solvents, aqueous methanol and hexane, were those used in earlier studies in our laboratory to separate cresylic acidhydrocarbon mixtures (3, 7). T h e cresylic acids dissolve in the aqueous methanol solvent, while the sulfur compounds are removed by the hydrocarbon solvent. Laboratory studies covering the important variables which affect purity and recovery of cresylic acids are discussed.
bottom of the column. The crude cresylic acid feed was pumped by a motor-driven syringe (4) into the sixth stage. Following the start of cresylic acid feed, from 0.5 to 2 hours was necessary to reach steady state. Cresylic acids are extracted by the heavier aqueous methanol solvent which is withdrawn from the bottom of the column. Thiophenols and a small percentage of cresylic acids are removed overhead in the hexane raffinate. Material balance periods ranged from 60 to 90 minutes or longer.
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Hexane
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Experimental
Extraction studies were carried out in a 1 inch X 8 foot extraction column of the Scheibel type (7), which contained 28 stages. T h e shell was constructed of borosilicate glass pipe, while the column internals were of Type 304 stainless steel. A schematic diagram of the apparatus is shown in Figure 1. The solvents, aqueous methanol and hexane, were fed by gravity from 5-gallon borosilicate glass aspirator bottles. Feed rates were controlled by stainless steel needle valves and indicated by rotameters. The solvents were heated or cooled, if necessary, by coils inserted in water baths. The column was filled with hexane, the continuous phase, which was fed to the bottom of the column. Aqueous methanol was introduced a t the top. The interface was located a t the
Extraction Column
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Methanol Extract Fractional solvent extraction apparatus VOL. 2
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SEPTEMBER 1 9 6 3
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Figure 2. Effect of methanol concentration on purity and recovery of cresylic acid Feed - methanol -hexane ratio 1 / 2 / 4 . 5 volumes
The methanol extract was freed of solvent by distillation on a 1 inch X 4 foot Vigreux column. The residue contained a mixture of water and refined cresylic acids. The cresylic acid phase was separated by decantation in a separatory funnel. The acids dissolved in the water phase were extracted with butyl acetate and weighed after distillation of the butyl acetate on a Vigreux column under vacuum. Thiols and disulfides were determined in the wet cresylic acids by a combination of zinc reduction and potentiometric titration with silver nitrate ( 9 ) . Sulfur compounds are reported as the sum of thiols plus disulfides because of the difficulty in preventing oxidation of thiols to disulfides during handling and analysis. A knowledge of the disulfide content is pertinent, because disulfides revert thermally to thiols during subsequent distillation. Precision of thiol-disulfide analyses was within 10.001%. Total sulfur was determined by a modification of the quartz tube combustion method of Peters et al. ( 8 ) . The water content was measured by azeotropic distillation with benzene. The yield of cresylic acids and residual sulfur compound content was reported on a dry basis. Solvent was removed from the hexane raffinate by distillation on a 1 inch X 4 foot Vigreux column. The solvent-free raffinate and the overhead hexane were analyzed for thiols and disulfides. A determination of total caustic solubles in conjunction with the thiols and disulfides provided the information necessary to calculate the cresylic acids remaining in the neutral oil, as well as neutral compounds other than disulfides. The actual balances on cresylic acids ranged from 95 to 100%. The balances on sulfur compounds varied from 90 to 102%. Solvents and Feedstock
Synthetic methanol in a purity exceeding 99% \vas used to prepare the aqueous methanol solution. All concentrations are expressed as weight per cent methanol. Commercial hexane was used (boiling range from 67' to 72' C.), The spent caustic solution was treated with CO, or a mineral acid to spring the crude cresylic acids and thiophenols. Preliminary refining involved topping to 160' C.? followed by collection of a heart cut boiling range from 160' to 240' C. The feed to the Scheibel column had the following analysis (weight per cent) : calculated as thiocresols Neutral oils Cresylic acids
HEXANE/6O% METHANOL VOLUME RATIO
Figure 3. Effect of hexane-methanol ratio on recovery of cresylic acids
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I&EC PRODUCT RESEARCH A N D DEVELOPMENT
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Discussion of Results
Recovery and purity of cresylic acids are affected by a number of factors including thiol content of the feed, methanol concentration, ratio of hexane to methanol and to feed cresylics, agitator speed, and to a lesser extent temperature and total throughput. Optimum conditions involved extraction a t 20' to 30' C. with 2.0 volumes of 60 weight yo methanol and 4.5 volumes of hexane a t an agitator speed of 600 r.p.m. Recovery of cresylic acids was 96 to 97%, while removal of sulfur compounds exceeded 99.5%. Varying the methanol concentration from 50 to 707, increased the cresylic acid recovery from 93.5 to 98.3y0 (Figure 2). The percent thiols plus disulfides increased from 0.010 to 0.015% between 50 and 60% to 0.032% a t 7@% methanol concentration. Sixty percent methanol appears to be optimum, considering both purity and recovery. Using 60% methanol, the ratios of hexane to feed cresylics and of hexane to methanol were varied; the latter controls recovery, while the former affects purity. As the ratio of hexane to methanol increases, recovery of cresylic acids decreases. This relationship, shown in Figure 3, appears to be independent of hexane-feed or methanol-feed ratios. Recovery varied from 98+y0 a t a ratio of 1.1 to 90+% a t a ratio of 6.0.
The purity of cresylic acids, as one would predict, is better the higher the ratio of hexane to feed cresylics. I n Figure 4. the logarithm of sulfur conipound concentration is plotted against hexane-feed ratio; this correlation also appears to be independent of the ratios of hexane-methanol or methanol-feed cresylics over the range explored. Percent sulfur compounds (as thiocresols) ranged from 0.2 to o.3y0a t low ratios to less than 0.01% a t a ratio of 6.0 volumes of hexane per volume of feed cresylics. T h e optimum ratios of solvent to feed cresylics, when results in both Figures 3 and 4 are considered, appear to be 2.0 for 60Y0methanol and 4.5 for hexane. Temperature, over the range from 15' to 45' C.: has only a slight effect o n purity and recovery. Referring to Table I, recovery decreases a t the higher temperatures, while purity suffers slightly a t lower temperatures. The optimum temperature appears to be 20' to 30' C.
Table 1. Throughfiut, CG.1Min.a 37.5
120
Effect of Extraction Temperature Temp., Recooery, Purity, % c. Wt. % RSH RSSR 0.014 97 5 15 96.5 0.013 25 0 012 40 93.6
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95.0 0.015 97.4 0.012 92.5 0.013 Volumejow of feed, @?yo methanol and hexane at 124.5.
Agitator speed and total column throughput are considered together in Table 11. Recovery increased slightly from 95 to 96% a t 350 agitator r.p.m. to 97 to 98% a t 600 to 900 r.p.m. Purity appeared also to be unaffected in the range 600 to 900 r.p.m. but was considerably poorer a t 350 r.p.m. Total throughput appears to have no consistent effect on recovery or purity. I n general, the higher the agitator speed, the lower the flooding velocity (limit on throughput) ; the optimum agitator speed appears to be about 600 r.p.m.
Table II.
Effect of Higher Agitator Speeds on Recovery and Purity over Wide Range of Throughputs Throughput, Agitator, Recovery, Purity, Yo Cc./Mzn. R.P.M. Wt. % RSH RSSR 37.5 360 96.5 0,024 480 96.7 0.016 600 97.2 0.012 740 97.8 0.012 900 97.9 0.011 120 360 95.0 0.022 480 96.8 0.013 600 95.7 0.011 740 97.3 0.012 900 97.2 0.009 135 600 97.4 0.012 740 Flooded 150 480 96.7 0.015 600 Flooded
+
The studies described above were performed on cresylic acids from a single petroleum refinery. A commercial extraction plant would have to collect the spent caustic extracts from a large number of refineries to obtain a sufficient volume of crude cresylic acids for a n economically sized plant. Several spent caustic extracts were obtained which represented a range of sulfur compound concentrations from 8 to 38%. Extraction of each of these feed cresylics under optimum conditions showed
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the effect of thiol content on the purity of refined cresylics. As indicated in Figure 5, thiols plus disulfides increased from 0.004 to 0.028y0 as the sulfur content increased in the feed cresylics. Several years of plant experience has shown that the average thiol content in crude cresylics is about 15 to IS%, well within the limits of this study. T h e effectiveness of the separation of thiols from cresylic acids is unexpected when one considers phenol and thiophenol as prototypes of the mixture. Thiophenol and phenol are weak acids, thiophenol being a thousand times stronger than phenol. I n spite of this, thiophenol is rejected by the aqueous methanol, the more polar solvent. The distribution behavior of thiols can be explained as a result of their inability to hydrogen-bond with oxygenated solvents. Aqueous solutions of aliphatic glycols and glycol monoethers show a similar selectivity for rejecting thiols and disulfides (6). The level of other impurities in the solvent-refined cresylic acids: including neutral oils, pyridine bases, and miscellaneous sulfur compounds, is of interest in connection lvith marketing these materials. Neutral oil content of the refined cresylics is less than 0.05%, which is comparable to the best grade of cresylics currently marketed. Extracts from gasoline which have been produced from catalytic cracking contain very low concentrations of pyridine bases. This is due to the acidic nature of the cracking catalyst, which results in adsorption of the bases. Maximum pyridine base contamination is about 0.170, kvhich is acceptable. The analytical methods, discussed previously, define the thiol-disulfide content of the refined phenols. T h e remaining types of sulfur compounds, including sulfides and cyclic sulfur compounds, were estimated by measuring total sulfur. The total sulfur value of a number of refined cresylic fractions indicated that thiols and disulfides accounted for 50 to 1 0 0 ~ oof the total sulfur. The maximum amount of neutral sulfur compounds was about O . O l ~ o . Following completion of the laboratory study, a pilot plant extraction program was undertaken. A larger (4 inch X 25 foot) Scheibel extraction column was used which contained the same number of extraction stages. T h e yield and purity of recovered phenols duplicated the laboratory results. The recovery and recycle of methanol and hexane were demonstrated successfully. A commercial plant ( 7 ) a t our Newark chemical plant, Pitt-Consol Chemical Co.?involving a 3 X 60 foot Scheibel-type extraction column has been in operation for five years with a capacity of 20,000,000 to 25,000,000 pounds of refined cresylic acids. Composite crude cresylic acids from a t least 20 different petroleum refineries are processed. The performance of the commercial extraction column duplicates our laboratory and pilot plant columns in yield and purity of refined cresylic acids. VOL. 2
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Literature Cited
(1) Forbath, T. P., Chem. Eng. 64, 228 (July 1957). (2) Gallo, S. G., Carlson, C. S., Biribauer: F. A,, Ind. Eng. Chem. 44, 2610 (1952). (3) Gorin. E.. Neuworth. M. B. (to Consolidation Coal Co.), U. S. Patent2.666.796 (Jan. 19. 1954) I
,
( 6 ) Ibid., 2,789,145 (April 16, 1957). (7) Neuworth, M. B., Hofmann, Vera, Kelly, T. E., Ind. Eng Chem. 43, 1689 (1951). (8) Peters, E. D., Rounds, G. C., Agazzi, E. J., Anal. Chem. 24,
710 119521. (9) T a k l e . ’ M. b‘.,Ryland, L. B., Ind. Eng. Chem., A n d . Ed. 8, 16 (1936). RECEIVED for review November 19, 1962 ACCEPTEDMay 15, 1963
HIGHER ALPHA, OMEGA-DIENES IN PARAFFIN AND OLEFIN PYROLYZATES D0NA LD B
.
M I LL E R
,
Petrochemical Laboratory, Continental Oil Co: Ponca Ci&, Okla.
Concentration and isolation of a minor compound in a Clz cracked wax olefin fraction gave a product identified by mass and NMR analysis and b y ozonolysis as 1 , I 1-dodecadiene. The presence of a,w-dienes in cracked wax olefins appears to b e general. In comparative pyrolyses, 1-octadecene gave a mixture conI~ from taining 2070 dienes in the CIO-C17 fraction, whereas only mono-olefins were found in the C ~ O - Cproducts octadecane. This indicates that a p d i e n e s present in cracked wax olefins are formed in secondary pyrolysis reactions. The C ~ O - C range ~, pyrolysis products of 1 -0ctadecene may b e qualitatively accounted for by two reaction mechanisms-a cyclic intramolecular reaction which gives a relatively large amount of 1 -pentadecene, and a radical chain reaction which affords the bulk of the pyrolysis products.
HE PYROLYSIS
of higher paraffins such as waxes has attracted
Tconsiderable interest, and several studies of the composition of the pyrolyzates have been published. The liquid pyrolysis products of hexadecane (7, 77) and paraffin wax (5, 73, 74) consist largely of the homologous linear a-olefins together with small amounts of the homologous paraffins. The gaseous products consist principally of methane, ethane, ethylene, and propene (5, 7, 77). T h e observed composition of the products in general conforms with the composition predicted from the radical chain decomposition theory of Rice and Kossiakoff (7, 8, 77). The short contact time usually employed in paraffin pyrolysis obviates to a large degree the secondary reactions of the olefins which are formed in the primary pyrolysis. However, since the secondary reactions cannot be entirely avoided, we should consider the products which might arise from them. The literature offers little help in this direction. A 1924 report by Gault and Altchidjian ( 6 ) o n the pyrolysis of 1-hexadecene appears to be the only description of the pyrolysis of a n olefin higher than octene. Because of the limitations in analytical methods a t that time, this work can give only a general impression of the course of the pyrolysis reaction of 1-hexadecene. From the data of this early study, it was concluded that the over-all product distribution from the pyrolysis of 1-hexadecene closely resembles that of hexadecane, and that the cracking rate constants for the t\vo compounds are also similar; hence the double bond was thought to have no large effect o n either the mechanism or rate ofcracking (7). One way a n a-olefin could pyrolyze is by cleavage of threecarbon unit via a cyclic six-membered transition state. The products would be propene and a n a-olefin with three less carbon atoms than the starting compound.
220
I & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
Pyrolysis by elimination of propene has been observed with 1-octene (3,9 ) . But one might expect that a t sufficiently high temperatures, less selective modes of pyrolysis may come to the fore. If the pyrolysis of a linear a-olefin is nearly random, the products should be similar to those obtained from paraffins, except that part of the 1-olefins formed in pyrolysis now should be a , w-dienes. Patent literature (73), news reports ( 4 ) ,and data sheets on cracked wax olefins from several sources have mentioned the presence of unconjugated dienes as impurities, but the dienes were not further characterized. Although butadiene was the only diene that Gault and Altchidjian identified among the pyrolysis of 1-hexadecene, they suggested that higher a , w-dienes may be intermediates in one of the reaction paths in the pyrolysis of higher 1-olefins (6). Mersereau, in a 1918 patent on the production of diolefins (“erythrene, isoprene, piperylene, etc.”) by pyrolysis of heavy petroleum oils or “even solid paraffins,” states that the diolefin formation requires a limited but appreciable time, and is apparently due to secondary reactions (72). The literature on higher a , w-dienes is very scattered and scanty. Although 1,9-decadiene ( Z ) , 1,IO-undecadiene (2, 75), and 1,15-hexadecadiene (75) were reported over 50 years ago, and 1,ll-dodecadiene ( 7 ) over 30 years ago, these compounds remain for the most part chemical curiosities. .4 few years ago, Marvel and Garrison synthesized the individual Cs-CIo. CIS. and C Z a , w-dienes and studied their polymerization with a Ziegler-Natta catalyst (77). This is one of the few studies made to date that explores the chemistry which depends uniquely o n the a , w-diene structure. Experimental
Concentration of 1,ll-Dodecadiene a n d 1,12-Tridecadiene. The dienes in a CI2olefin cut obtained by pyrolysis of paraffin \sax (cracking conditions: 5 atm., 520’ C. heating block temperature, throughput 1.5 kg. per hour. 250-ml. free volume of cracking area) were concentrated by absorption and elution from silica gel (Davison 28-200 mesh) (76). The
