INDUSTRIAL AND ENGINEERING
March, 1935
For liquids with vjscosities similar to that of water, the coefficients shown in Tables I11and IV for different wall thicknesses and liquid velocities may be safely used in computing area requirements. For the effect of viscosity on film coefficients, textbooks on the subject should be consulted. Tables I11 and IV show clearly the advantage to be gained by using thin-walled pipes. Between 0.06 and 0.04 inch there is a gain of 24 per cent for liquid to liquid and 31 per cent for vapor to liquid transfer at 1.0 foot per second. Between 0.18 and 0.16 inch the difference is 10 per cent in each case. The effect of increasing the velocity of flow is also apparent. For
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thicknesses of about 0.16 inch there is little advantage in using velocities higher than 1.0 foot per second. For thinner pipes the over-all coefficient increases u p to and beyond 3.0 feet per second for vapor to liquid transfer. Heat exchangers of the type described are being used in industrial plants. Information as to the heat transfer capacities and other details of these commercial installations cannot be given because of lack of space. However, they are all operating in a highly satisfactory manner. RECEIVED October 29,
1934.
Sweetening of Gasoline with Alcoholic Alkali and Sulfur B. A. STAGNER, 11.21 South Hi!l Street, Los Angeles, Calif.
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UBSTAXTIALLY all gasoinasmuch as a small percentPracfically all cracked gasoline and much of line produced by cracking’ age of t h e a l k y l sulfides is the straight-run gasoline must be “sweetened” for and much of that produced r e c o n v e r t e d t o mercaptans the r e m o d of the mercaptans. The method here by straight distillation contain d u r i n g the distillation. The described accomplishes complete sweetening by a small amount of mercaptans two stages of sweetening thus brief agitation of the gasoline with small proporyield a slightly better q u a l i t y as an impurity. The amount of the mercaptans, expressed for of gasoline. tions of elementnry sulfur and a solution of The ideal method of handling simplicity as their content of sulcaustic soda in methanol. No emulsions are fur. varies from almost, zero to as t h e mercaptans would be jormed. The alcohol can be recocered. Valumuch, in extreme cases, as 0.25 to extract them c o m p l e t e l y able by-products are also formed. The adtunper cent by weight of the gasofrom the g a s o l i n e , but the tages of the process are discussed. line. The amount depends onithe petroleum industry still awaits stock used and, in cracked gasoa n economical procedure line, on the temperature of the CI mking. The mercaptans of commensurate with the snnall improvement attained. seven or fewer carbon atoms have a n objectionable odor; SEW METHOD OF SWEETETING and although the mercaptans themselves are not corrosive to metals, they are all extremely corrosive in the presence of It has been discovered ( 3 ) that, when elementary sulfur elementary sulfur and are, therefore, not tolerated in gasoline. and mercaptans are brought together in solution in gasoline For many years the mercaptans have been eliminated in intimate contact with a very small amount of an alcoholic by treating the gasoline with “doctor solution,” which is an solution of anhydrous alkali, they react instantly with each alkaline aqueous solution of sodium plumbite, and a care- other; and if proper proportions of the sulfur and mercaptans fully determined quantity of elementary sulfur; the sulfur are brought together under this condition, they are both required is not definitely proportional to the mercaptan sulfur totally eliminated as such. No lead is required. It is thus but varies to some extent with the type of mercaptans. possible to remove mercaptans from gasoline by adding the The sulfur oxidizes the mercaptans to alkyl disulfides and requisite amount of elementary sulfur and anhydrous alcoholic doubtless converts Lome of the disulfides to trisulfides. The alkali solution, or to remove elementary sulfur from gasoline finished gasoline gives negative reactions for mercaptans in by adding the requisite amount of mercaptans, or preferably the doctor test and for elementary sulfur (corrosiveness) in a definite amount of gasoline which contains mercaptans, the copper strip test. and the anhydrous alcoholic alkali. Frequently a refinery The reaction of this sweetening process is often expressed may have one type of gasoline bearing elementary sulfur and as follows: another type bearing mercaptans. A proper blending of these two types agitated with water-free alkali will free the PRSH S KazPbOz = R,S, PbS PN:LOH mixture of both substances. A. higher proportion of free sulfur is required than the equaIt is found to be much more economical of time and retion indicates, and Ott and Reid ( 2 ) have s h o r n that be- agent to apply the anhydrous alkali in solution in alcohol, qides lead sulfide other insoluble lead compounds are formed, preferably methanol. The alcohol serves to disperse the such as Pb2S(SR)z, I’bZ(OH)&, Pbz(OH)& etc. The alkyl alkali throughout the gasoline in a fine state of division, sulfides remain dissolved in the gasoline. They are usually doubtless molecular; and the alkali, if added in the minimum considered innocuous in the gasoline except that they in- amount, is almost completely exhausted in the reaction with crease slightly the total sulfur content, the detonation, and the sulfur and mercaptans. The quantity of alcoholic alkali the color instability. These alkyl sulfides have higher boiling solution required is dependent on the degree of sourness of the points than the original mercaptans, and, if the gasoline is gasoline, but it is surprisingly small-one pint per barrel in first sweetened and then distilled, some of the sulfides are left a typical California cracked gasoline, as described later in in the still. An additional sweetening is necessary, however, this paper. The essential reaction is thau oi oxidizing the mercaptans 1 T h e present production of cracked gasoline in the United States is about 160,000,000 barrela per year. by sulfur, the mercaptans being converted to alkyl disulfides
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and the sulfur to alkali sulfide. The simplest expression for the oxidation is:
CHEMISTRY
Vol. 27, N o . 3
Because of its cheapness caustic soda is preferable to caustic potash. Its maximum solubility in methanol a t normal temperatures is approximately 23 grams per 100 ml. of solution, 2RSH S 2NaOH = R& NazS 2Hz0 but about 15 grams per 100 ml. is preferable because the However, more elementary sulfur and caustic soda are in- weaker solution is more easily dispersed in the gasoline. volved than is indicated by the simple equation. It is probThe sulfur can be applied to the gasoline by any means able that the newly formed sodium sulfide is in competition common t o the plumbite treatment, either as a powder or, with the mercaptans for the sulfur. preferably, in solution in a small volume of the gasoline, which Some of the earliest data obtained on sweetening cracked in turn is added to the main volume in continuous operation. gasoline are shown in Table I. The data show clearly the Likewise, any simple means, such as a centrifugal pump or effect of adding the requisite proportion of sulfur. They also orifice plates in a pipe, is satisfactory for commingling the show that sodium and potassium hydroxides are essentially reagents and gasoline. equivalent in this process. The sweetened gasoline is sufficiently stable in color for most practical purposes; but the stability can be increased, TABLEI. EFFECTOF ELEMENTARY SULFURIN SWEETENIKGif found desirable, by acidifying or washing the gasoline with WITH ANHYDROUS ALKALIES a little dilute sulfuric acid (10 to 60 per cent concentration), TESTSO N TREATED GASOLINE which quickly and completely settles out. Instead of the ALCOHOLIC ALK.4LI S O L X . "Doctor" Copper acid treatment, an acidic antioxidation catalyst can be used. EXPT. Reagent Alkali in SULFUR for mer- striu for eleNo. used reagent ADDED captans m i n t a r v S Many experiments with this process have been made on all Lb./bbl. % by ut. Lb./bbl. types of cracked gasoline, some slightly sour and others exALCOHOIrSODIUM HYDROXIDE tremely sour (up to 2 grams mercaptan sulfur per liter, or A-1 0.35 14 0.025 A-2 0.35 14 0.035 +0.26 per cent by weight), and without a single exception the A-3 0.35 14 0 020 ++ gasolines have been successfully sweetened. A-4 14.00 14 0.000 ALCOHOL-POTASSIUM HYDROXIDE The following data on sweetening a given sample of cracked B-1 0.25 28 0 025 gasoline is typical of the new process. The gasoline used had B-2 0.25 28 0.035 + B-3 0.25 28 0.020 an end boiling point of 200" C. (392" F.); total combined B-4 15.00 28 0.000 ++ sulfur, 0.40 per cent; mercaptan sulfur, 0.023 per cent (0.175 In experiments A-1 and B-1 the requisite amount of sulfur gram per liter); and no elementary sulfur. A liter of this was added to oxidize the mercaptans and leave the gasoline gasoline was agitated mechanically out of contact with air free from elementary sulfur. In experiments A-2 and B-2 (natural gas atmosphere) for one-half minute with 0.29 too much sulfur was added. It sweetened the gasoline, but gram elementary sulfur and 3.1 ml. of methanol-caustic the sulfur in excess made the gasoline corrosive. I n A-3 and soda solution containing 0.15 gram caustic soda per ml., or a B-3 insufficient sulfur was added to destroy the mercaptans, total of 0.465 gram caustic soda. (Expressed in amounts but all the sulfur was utilized and the gasoline was not left cor- per barrel of gasoline, the elementary sulfur was 0.1 pound; rosive. I n A-4 and B-4 no sulfur a t all was added; and al- the alcoholic solution of caustic soda, one pint; and the total though forty and sixty times, respectively, as much alcoholic caustic soda, 0.16 pound.) On standing 10 minutes, the alkali were used as in experiments A-1 and B-I, the gasoline alkali by-products, in complete solution in the larger portion remained unsweetened. I n A-4 and B-4 the treatment con- of the alcohol originally introduced, settled to the bottom of sisted in extraction alone and not in oxidation. Mercaptans the container. Two liquid phases were thus present, and of low molecular weight are sufficiently acidic to form alkali their interface was as sharply defined as that between water mercaptides which are stable enough to permit extraction and gasoline. At the above stage of separation, the gasoline was sweet and with alcohol or even with a large volume of water; but the mercaptans of high molecular weight are too weakly acidic noncorrosive, and on a large scale it would be withdrawn to form alkali mercaptides stable enough to permit economic from the alkali by-products and washed with only enough extraction. However, if the mercaptans of low molecular water to dilute by about tenfold the alcohol still in solution weight are extracted with alcoholic alkali, they must later to complete the separation of the alcohol from the gasoline. I n a laboratory experiment on recovering methanol the be handled and the alcohol recovered. I n the new process here described, the mercaptans are not alcohol was added to gasoline in the proportion of one pint extracted but are converted to alkyl disulfides. The process to a barrel and extracted with 10 pints of water. The aqueous has also been found applicable for sweetening kerosene, which solution was settled from the gasoline, further diluted with contains mercaptans of distinctly higher molecular weights water, and distilled, and the quantity of alcohol in the distillate was determined from specific gravity tabulations. The and boiling points than those associated with gasoline. Air is neither required nor desirable. It is so slow in its data indicated that 100 per cent of the alcohol was recovered. The alkali by-products formed in the sweetening experirate of oxidation, compared with sulfur, that its influence is practically negligible, and i t causes a loss of some of the ment described were exposed t o air and then analyzed by the methods of Griffin (1) and of Tartar and Draves (4). The gasoline through evaporation. The ratio of elementary sulfur to mercaptan sulfur required analysis showed traces of sodium polysulfide, free caustic for different types of gasoline in this new process shows a soda, and large proportions of normal sodium sulfide (NaZS) variation as great as or greater than the ratio in the plumbite and sodium thiosulfate (Na2S2O3.5H20).The mixture was treatment, although it is fixed, within limits, for any one entirely free from mercaptans. About 50 per cent of the gasoline. The variation for a given sample has a range of 10 original caustic soda was converted into sodium sulfide and 25 per cent into sodium thiosulfate. Calculations from these or 15 per cent from the mean. The caustic alkali appears to be converted a t first into data indicate that one pound of caustic soda would produce polysulfides and later, depending upon conditions, into normal about 0.5 pound of actual sodium sulfide and 0.75 pound of sulfide and thiosulfate, all of which are extremely soluble in sodium thiosulfate. The retail price for solid 60 per water. These alkali-sulfur by-products have not been com- cent sodium sulfide is 3.5 cents per pound (5.8 cents for actual pletely investigated, but it appears that the relative percent- sodium sulfide), and for sodium thiosulfate, 2.5 cents. Inasages formed vary t o some extent for different samples of much as approximately 50,000,000 pounds of each of these chemicals are produced annually in the United States (6), gasoline.
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hfarch, 1935
I N D U S T R I .4 L A Pi D E N G I N E E R I N G C H E LVI S T R Y
i t seems probable that a substantial market may exist for them. The sulfide is used extensively in flotation processes and as a depilatory in tanneries; the thiosulfate is used extensively as a reducing agent in the chrome tanning of leather as well as in other chemical processes. EFFECTOF WATERI N THE GASOLINE. It is desirable to keep the reagents and the gasoline reasonably free from water during the sn-eetening, and for this purpose the normal precautions of the refinery are sufficient. Gasoline is found to be satisfactorily sneetened one-half hour after it has been steam-distilled and while it still contains a white cloud of moisture. The alcoholic alkali solution itself [nay contain a little water, but a large amount markedly lowers its efficiency. RECOTEKY OF THE ALCOHOL.Because of the efficiency of the alcohol in bringing the alkali into intimate rontact with the d f u r and mercaptans, it is apparent that the alcohols of higher molecular weight with their complete solubility in gasoline would in this respect be more suitable than methanol with its relatively low solubility, but the greater difficulty of extracting them from the gasoline seems to preclude their use. Anhydrous methanol is soluble in gasoline to the extent of about 3 to 10 per cent, the solubility being greater in the more volatile hydrocarbons. In recovering the alcohol from the gasoline. the water used for extracting can be repeatedly used without the necessity of distilling all of the alcohol from the aqueous solution each time i t passes through the still. A portion of this solution can be added to the alcoholic alkali by-products, y e viously settled from the gasoline, and the alcohol can be distilled completely therefrom, leaving the alkali by-products in an aqueous solution in any desired concentration. Methanol, unlike ethanol, does not form a constant-boiling mixture with water and can be distilled from i t in substantially anhydrous (condition in a continuous still provided with ample fractionation. It appears certain that the alcohol can be used and recovered with a loss not to exceed 0.1 to 0.2 per cent and that the cost of recovering the alcohol would riot be greater than 0.5 cent per gallon. OTHERCOKSIDERATIONS. I n sweetening with sulfur and alcoholic alkali, sodium and potassium hydroxides are found to be very nearly equivalent in their behavior; this similarity would not be anticipated when it is considered that alcoholic potash extracts elementary sulfur from gasoline almost instantly (6), whereas alcoholic caustic soda is incaomparably slower. The application of powdered alkalies or alkaline earths and sulfur for sweetening introduces a difficulty in securing contact. A large excess of the alkali and relatively long periods of agitation are required. As a result of many experiments it has been concluded that sulfur and alcoholic solutions of ammonia will not sweeten gasoline. It has likewise been impossible to omit elementary sulfur and sweeten gasoline a t ordinary temperatures with an alcoholic solution of caustic alkali and an alkali polysulfide. The latter experiments were made in the search for a method that would supply only the necessary sulfur for the oxidation without the usual danger of adding an excess. COMPARISON OF THE Two PROCESSES LOSSOF GASOLINE. I n order that the new process may be compared with the plumbite treatment, the data on sweetening with plumbite and sulfur a liter of the gasoline described above are submitted. The minimum of elementary sulfur that would permit a “break” of the partly spent plumbite solution from the gasoline was added; this amount was 0.12 gram per liter, or approximately 40 per cent as much as was used in the alcoholic alkali process. The plumbite solution was added in excess as usual, and, as the reaction is slower
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in the plumbite treatment, the time of agitation was extended to 5 minutes. The mixture was settled for a halfhour after the agitation; during this period approximately 50 ml. of heavy emulsion of gasoline and partly spent plumbite solution, thoroughly stabilized by insoluble lead sulfide, collected between the gasoline and the aqueous plumbite solution. Some of the dark lead reaction products remained suspended throughout the main volume of the gasoline and would not separate until the gasoline was washed with several portions of water. The quantity and the stability of the emulsion that is always formed in the plumbite treatment vary from time to time, and the consequent loss of gasoline from this cause also varies. However, the annual loss in the different refineries is reported to be from 0.25 to 1.5 per cent, and an average loss of 0.5 to 1.0 per cent may be assumed to be fairly accurate for all refineries except those that have recently installed centrifuges to handle this emulsion and save the gasoline. I n contrast, the sweetening with the alcoholic alkali, including subsequent washing with water, does not produce emulsions and consequent loss of gasoline. QUALITYOR SWEETENED PRODUCTS. The two methods of sweetening are alike in their effects on the total content of sulfur and gum in the finished gasoline; they completely sweeten, and they leave the gasoline noncorrosive to a copper strip in the official method of testing. reither process prevents a dark discoloration, indicative of elementary sulfur or of polysulfide sulfur, when the gasoline is agitated with the extremely sensitive metallic mercury. I n fact, mercury still produces the same discoloration in the sweetened gasoline after it has been extracted with alcoholic solution of potash or of sodium sulfide, both of which solutions are satisfactory for freeing gasoline of elementary sulfur and making i t noncorrosive. COSTOF SWEETENIXG BY
THE
NEW PROCESS
The cost data for the new process of sweetening per barrel of the gasoline described in the experiment above are as follows : 0.10 Ib. sulfur 0.16 Ib. caustic soda
A t $18.00 per ton At $50.00 per ton
Loss of 0.2% of.1 pint alcohol Recovery of 1 pint alcohol
A t 36 cents per gal. A t 0.5 cents per gal.
$0.00090
0.00400 $0,00490
0.00009 0.00062
$0,00071 Total per barrel excluding credits for gasoline saved a n d for by-p;oducts
$0.00561
Inasmuch as this process avoids the loss of gasoline common to the plumbite treatment, i t can be credited with the saving of about 0.5 to 1 per cent of the gasoline, or from 0.21 to 0.42 gallon for each barrel treated. If gasoline is worth 6 cents per gallon, this saving amounts to 1.25 to 2.50 cents per barrel of gasoline sweetened. I n addition, it is probable that the alkali by-products formed will have a definite commercial value. LITERATURE CITED (1) Griffin, R . C., “Technical Methods of Analysis,” New York,
McGraw-Hill Book Co., 1927. (2) Ott, Emil, and Reid, E. E., IND. ENG.CHEM.,22, 884 (1930). (3) Stagner, B. A., U. S. Patent 1,970,683 (Aug. 21, 1934). (4) Tartar, H. V., and Draves, C. Z., J. Am. Chem. Soc., 46, 576 (1924). ( 5 ) U. S. Dept. of Commerce, Census of Manufactures (Biennisl Rept.), D a t a on Sodium Compounds, Nov. 23, 1932. (6) Vesselovsky, V., and Kalichevsky, V., IND.ENG. CHEM., 23, 181-4 (1931). RECEIVED Auguet 2, 1934.
