INDUSTRIAL AND ENGINEERING CHEMISTRY
July, 1926
residue was then determined as previously given under the determination of total sulfur. The following average results were obtained : Per cent. 1.373 1.368
Total sulfur Sulfur in extracted shale Resinic sulfur
Per cent 0.649 0.005
Humus sulfur
0.644
Sulfur in shale Sulfate sulfur Sulfide sulfur Sulfur in phenol-extracted shale Organic sulfur
Organic and Humus Sulfur The difference between the total sulfur in the shale and the inorganic sulfur is organic sulfur.
Organic sulfur
Organic sulfur Resinic sulfur
Summary of Distribution of Sulfur in Oil Shale
0.005
Total sulfur Inorganic sulfur (sulfate-ksulfide)
733
Per cent 1.373 0.724
__ 0.649
The humus sulfur is the difference between the total organic sulfur and the phenol-soluble or resinic sulfur.
Per cent 1.373 0.0865 0.6375 1.368 0.649
Humus sulfur Resinic sulfur Sulfide iron Sulfate sulfide iron Sulfate iron
Per cent 0 644 0.005 0.56 1.12 0.56
Conclusion Sulfur is present in the form of sulfide, sulfate, and organic sulfur. The resinic sulfur found in those oils analyzed for resinic sulfur6 exists in quantities ranging from one-fourth to all the organic sulfur. The resinic sulfur found in the Mount Logan oil shale is so small in amount as to appear within the limits of experimental error and may be considered as practically absent in this shale.
Systematic Refining of Cracked Distillates' Chemical Factors in Refining By Jacque C. Morrell RESEARCH LABORATORIES, UNIVERSAL OIL PRODUCTS Co., CHICAGO, ILL.
HE development of cracking in the oil industry has T been such t h a t today it is the greatest single factor in motor fuel production. I t is unquestioned that in the
near future the largest proportion of motor fuel produced will be of the cracked type. Further, it is believed t h a t in the more distant future, when the supply of petroleum is less plentiful, cracking will dominate all other methods for producing motor fuels. Shale oils, low-temperature coal tars, lignite, peat, and wood tars will all contribute their quota of motor fuel through the medium of cracking, and the chemistry of each type of cracked product m u s t be recognized in working out practical refining methods for the motor fuel produced therefrom. The chemistry of cracked distillates is much more complex than the chemistry of straight-run distillates. The advance of the cracking a r t has made necessary a n
advance in the a r t of refining cracked products. I t is the purpose of the present work to systematize the refining of cracked distillates based upon a study of the chemical factors involved and upon their application t o refining practice. I n addition to showing a definite order of procedure in the use of the simple refining agents, sulfuric acid, sodium hydroxide, and litharge, for each class of cracked distillates, the reasons for such procedure and a discussion of the reactions involvdhave been included. The chemistry of the distillation-step in the refining of cracked distillates, which was practically nonexistent in the refining of straight-run distillates, has also been discussed. All the methods described have been thoroughly tested out in plant practice by the writer.
. . . . . . ....... .
T
HE present minimum commercial criteria to be met
in the production of refined motor fuels are the absence of color, sweet odor, and stability under marketing conditions. The last refers to the development of color, cloud, or odor during storage or handling. In addition, they are usually required to meet certain conventional specifications such as those laid down by the Bureau of Mines.2 Further specifications may be prescribed by individual purchasers, principally dependent upon needs and customs. For example, the doctor test3 is sometimes required, although it is a t best only qualitative in showing the presence of hydrogen sulfide or the mercaptans; for other sulfur compounds or as a positive index of color stability it is meaningless. The various tests to which motor fuels are submitted, although in some cases bearing upon their utility, should be further investigated to put this relationship upon a more rational basis. This is especially demanded by the wideP a r t of paper presented before the joint session of the Divisions of Petroleum Chemistry and Gas and Fuel Chemistry a t the 70th Meeting of the American Chemical Society, Los Angeles, Calif., August 3 to 8 , 1925. 2 Tech. Paficr SPS-A, 3 (1924). I Ibid., p. 85. 1
spread use of the cracking process in the production of gasoline from all types of heavy oils and hydrocarbons from various sources. For example, no one has yet shown the relation between the percentage of combined sulfur and the tolerance of a gasoline motor and its feed system. Likewise, the present method of determining gum content is not a measure of gums in the gasoline, but is rather a measure of the gummy substances formed by the arbitrary method of determining them. To fix arbitrary limits upon unknowns does not appear rational. Certainly, a thorough study of the fundamental requirements of a motor fuel from a chemical as well as physical viewpoint, directed towards automotive utility, and a system of specifications based only upon these requirements will be of great value to the refining industry. Treating Methods and Reactions &me of the methods described have been previously mentioned.4 Their application is explained here in greater detail, however, which permits a more comprehensive understanding of their use. 4
Chcm. Me;. Eng., SO, 785 (1924).
INDUSTRIAL A N D ENGINEERING CHEMISTRY
734
Method I This method is applicable to types of cracked distillates that are treated with least difficulty. (a) Agitate with sulfuric acid (usually 66” Be.). Settle and draw off sludge. ( b ) Wash well with water (c) Treat with solution of sodium hydroxide followed by short water wash. (d) Distil in the presence of steam, preferably introducing the steam in the oil body. The steam distillation may be assisted by a low Are. It is preferable to use a continuous or semicontinuous method in distilling. The two latter methods avoid long-continued heating of the cracked distillate, which is undesirable.
The action of sulfuric acid on the unsaturated hydrocarbons in cracked distillates is well understood. The two most important reactions are (a)the formation of alkyl sulfuric acids or the neutral esters, and (b) polymerization of the lower molecular weight unsaturated hydrocarbons. Brooks and H ~ m p h r e ystudying ,~ the effect of sulfuric acid on olefins, found that in addition to the foregoing reactions secondary and tertiary alcohols are formed. They postulated the formation of additive molecular compounds of the olefins and their isomers with sulfuric acid, and state that the polymers of the olefins are frequently diolefins and always contain a t least one double bond. They further point out that these polymers are much more stable than the parent olefins, and to a large degree remain in the treated oil layer. It may well be assumed that the presence of alkyl sulfuric acids is partly responsible for the discoloration of acid-treated cracked distillates owing to the formation in their presence of heavy oily or tarlike polymers. This formation is avoided by neutralization of the free acid or acid derivatives and by redistillation. Method 2 This is the most desirable and most generally applied method for the treatment of cracked distillates. It is practically the same as method 1, except that a plumbite treatment (litharge dissolved in a solution of sodium hydroxide) replaces the plain solution of sodium hydroxide. Agitate with sulfuric acid. Settle and draw off sludge. Short water wash, usually 5 t o 10 minutes. Agitate with plumbite solution. The product should show a negative doctor test a t this stage. If a suspension is formed on addition of the plumbite solution or after blowing or agitating, allow to settle and draw off. Under any circumstances draw off the plumbite solution after settling. Do not use sulfur. Note-While i t is desirable a t all times t o have the treated cracked distillate sweet before distillation, the sweetening treatment shown below may in some cases be completed during distillation. On t h e other hand, too high a temperature (local or otherwise) of redistillation will produce a sour condition in practically all cracked distillates, owing to the formation of hydrogen sulfide and mercaptans.
(d) Wash for short period with water, agitating if suspension is removed, but simply showering if suspension is still present, to avoid emulsion. This water wash may be dispensed with in most cases. (e) Distil in the presence of steam following the precautions shown in method 1, preferably using a continuous method of distillation. (f) If the gasoline distillate is sour (positive doctor test), sweeten by treatment with plain solution of sodium hydroxide using the most dilute solution which will accomplish the purpose. A final water wash is desirable here. The final caustic wash removes hydrogen sulfide formed during the distillation.
It is sometimes desirable to precede the acid treatment by a water wash to remove undesirable water-soluble compounds such as hydrogen sulfide. A caustic wash followed by a water wash serves the same purpose to a greater degree. I n the treatment of the distillate under method 2 the mercaptans present react to form the lead mercaptides, of which the lower members, as shown qualitatively by Wendt 6
J . A m . Chem. SOC.,40, 922 (1918).
VOl. 18, No. 7
and Diggs,6 are precipitated while those of higher molecular weight remain in solution. Complete precipitation of the mercaptides occurs with certain types of cracked distillates and sweetening of the oil may result. The dissolved lead mercaptides, with the types of oils responsive to this treatment to produce a sweet product, undergo the following reaction a t the elevated temperature of distillation. R-S, -‘Pb
4- A
+
_R, _
+ PbS
>S
R-S’ R The dialkyl sulfide is nonreactive to the doctor test; hence a sweet product results. The formation of lead sulfide accounts for the invariable formation and presence of a black precipitate in the still with this method of refining. I n addition to making a study of this reaction with various types of cracked distillates, the writer has checked it by the use of pure mercaptans. This reaction has not heretofore been presented in connection with the art of refining. Method 3 This method is applicable especially to cracked distillate, from light or heavy California, Mexican, South American, and similar oils, and gives a sweat, stable product seldom obtainable by the methods previously described with cracked distillates from these types of oils. A further reduction in sulfur content has also been noted in some cases. The write: has used this method with success for several years, and refers to it as the “split plumbite” method, as the plumbite treatment both precedes and follows the acid treatment. (a). Apply preliminary water wash. Treat with plumbite solution preferably stronger than 20” B6. Settle and draw sludge. ( b ) Short water wash, simply showering if suspension is present. Draw off as much water as possible before proceeding t o next step. (c) . Agitate for about 5 minutes with a small quantity of sulfuric acid (about 1 pound per barrel of distillate) and draw sludge after settling. ( d ) Apply main sulfuric acid treatment. Draw sludge. ( e ) Short water wash. (f) Agitate with plumbite solution (about IO” to 16’ BC.). Remove used plumbite. (If a “doctor sweet” product is not required a plain sodium hydroxide solution may be substituted here for the plumbite solution with good results for some type of oil.) (g) Water wash. (This step may be omitted in many cases.) (h). Distil in the presence of steam following previously mentioned precautions. (i) If the gasoline comes over sour (positive) to the doctor test, treat with the most dilute solution of sodium hydroxide which will sweeten it. If dilute caustic will not sweeten the gasoline distillate, a dilute plumbite solution will serve this purpose without the use of sulfur. A final water wash is desirable here. Method 3 is practically the same as method 2, following step (a)in the former. No sulfur is used in the above treatment a t any stage.
The preliminary sulfuric acid application preceding the main acid treatment reacts with the dissolved lead mercaptides formed during the plumbite treatment, and simultaneously with the mercaptans resulting from this reaction. R-S, )Pb
+ HzS04 (concd.) +2RSH + PbS04
(1)
R-S 2RSH
+ H?S04(concd.)
R-S --f
R-S
1 -I- 2H20 + SO1 (2)
The two reactions may be considered thus: R-S\ R-S/
Pb
--f
2H2S04(concd.)
+ R-S R-S
1
f 2H20 f SO1
I.VDUSTRIA L A N D E,VGINEERING CHEMISTRY
July, 1926
735
The dialkyl dilsulfide is nonreactive to the doctor test, fair results with respect to stability, especially when these hence the oil may be sweet. If insufficient acid is present to cracked distillates are blended with straight-run gasoline. react completely some of the lead mercaptides precipitate as However, the other methods will also handle these types such. Usually, however, some of the regenerated mercap- equally as well, and furthermore, the usually harmful effect tans are unacted on, especially if the acid has been diluted. of adding sulfur in this type of treatment is avoided. The presence of diluted acid will always cause the regeneraE l e m e n t a r y Sulfur in Refining Cracked Distillate tion of mercaptans as in reaction (1). Reaction (3) is seldom fully realized. The plumbite treatment following the acid Sulfur is appreciably soluble in hydrocarbons, as can be will act as outlined under method 2 , thus completing the readily shown by the recrystallization of flowers of sulfur sweetening treatment. from a gasoline solution. Guiseling has found that sulfur AIixtures of cracked distillate and straight run benzine is soluble to the extent of 1.5 per cent in a hydrocarbon dismay be treated according to method 2 or 3 . If the straight tillate, and that solubility increases with increasing molecrun benzine does not require acid treatment, it may be ular weight of the hydrocarbon oils, and with rise in temmixed with the acid-treated cracked distillate, washed perature. Friedmanlo has shown that sulfur reacts readily with water, plumbite-treated, etc., as in method 2. The a t elevated temperatures with hexene, octene, hexadecene, acid-treated cracked distillate may have a short water wash and similar unsaturated compounds. I n every case sulfur before mixing if desirable. Various other combinations mak- derivatives of very much higher molecular weight were ing use of the general principles of sulfuric acid and plum- formed in addition to the simple additive compounds. bite treatment (no sulfur) with intervening writer washes The addition of minute quantities of sulfur to a cracked may be suggested. gasoline otherwise stable and exposure to sunlight will cause Methods 1, 2 , or 3 will produce a water-white, sweet, the formation of tarry substances which discolor the gasoline. negative corrosion gasoline in practically all cases if proper There are many possible reactions between the components attention is given to the redistillation of the treated distillate. of a cracked distillate and elementary sulfur, especially under The aim here is to reduce the temperature and time of heat- the influence of heat and light. ing to a point consistent with capacity and efficient separaThe presence of elementary sulfur is undesirable owing to tion. This is accomplished by the introduction of sufficient its corrosive effect upon the carburetor and gasoline feed steam, continuous distillation, and an efficient fractionating system of a motor, and to the formation of unstable sulfur column. The methods are used for progressively difficult compounds which cause discoloration of the gasoline. It oils to treat. I n rare cases method 4 is applied but it is is significant that none of the ordinary refining agents will not recommended where any of the previous methods will efficiently remove elementary sulfur from a distillate, alsuffice. though it may undergo chemical change under some condiMethod 4 tions. As the addition of sulfur in treating practice is seldom reguIf hydrogen sulfide is present in excessive quantities, wash the cracked distillate with water followed by a n alkali treatment lated to conform to the reaction requirements, especially (sodium hydroxide or waste plumbite), add about 0.01 per cent by the older methods, the writer has previously recommended elementary sulfur by weight t o the oil and then treat as in method the elimination of sulfur in the treatment of cracked distil3. (The preliminary water wash shown in method 3 may be lates. Methods l, 2, and 3 show how this may be accomomitted here but all other steps of the split plumbite treatment plished for most cases. Method 4 shows the best manner must be followed in detail.) The addition of sulfur a t this stage is the least harmful. In addition t o the removal of the excess in which the use of sulfur may be handled and regulated when elementary sulfur according to the reaction shown, by the necessary. plumbite treatment immediately following the addition of sulfur, the second plumbite treatment will also remove any remaining sulfur if mercaptans are still present. The aim should be t o add t h a t amount of sulfur only which is necessary to remove the mercaptides by reaction and precipitation. There is the advantage of better contact and less chance of addition of excess in dissolving the sulfur in a regulated quantity in the distillate previous t o the plumbite treatment. The steps in method 4 would then be: addition of small per cent of sulfur; plumbite treatment; water wash, acid, water wash, plumbite treatment, etc.
Wendt and Diggs,6 as well as Wood, Lowy and Faragher,' have shown the following reaction: Lead mercaptide
R-S R-S'
\Pb
Dialkyl disulfide
+ S-+
R-S R-S
I
+ PbS
Holmbergs has shown a similar reaction between alkali mercaptides and sulfur. The above reaction between sulfur and the lead mercaptides is not complete as the precipitate contains unchanged lead mercaptides. This may be readily demonstrated by analyzing the precipitate after the addition of sulfur. Proceeding from the first stage discussed above, the principles outlined in methods 2 and 3 hold for method 4. The gasoline distillates from certain types of treated cracked distillates may be treated with plumbite directly, using a small and carefully regulated amount of sulfur, with 7
THISJOURNAL 16, 1114, 1118, (1924).
8
A n n . , 359, 81 (1908).
Concentrations, Volume, Time, Acid R e q u i r e m e n t s
Sulfuric acid having a gravity of 66' BB. usually meets all of the acid requirements of the petroleum refiner and, since it is most easily available and convenient, its use is recommended. With some types of cracked products, however, a more dilute acid gives better results. This applies particularly to the products of cracking from sources other than petroleum, although some cracked petroleum types respond more readily to dilute acid treatment or its equivalent. Cracked distillates from midcontinent charging stocksincluding those from northern and central Texas, Louisiana, Oklahoma, Kentucky, and Kansas, as well as those from the Rocky Mountain, Appalachian, and Indiana fields-usually require from 2 to 6 pounds of acid per barrel of distillate. Those from Texas coastal, Smackover (Arkansas), California, Mexican, South American, Borneo, etc., charging stocks usually require 5 to 8 pounds of acid per barrel of distillate. On a batch basis 15 to 20 minutes' thorough agitation is sufficient for the acid treatment. Sludges from acid treatment may be re-used with a possible saving in acid requirements. Economies in this direction, however, must be practiced with caution. Acid sludge should never be used upon a wet oil-i. e., the acid sludge treatment should not follow a water wash directly. Preferably, the use of sludge acid should be preceded by a 1-pound fresh acid treatment and 9 10
Petroleum Rev., 28, 486 (1913). Petroleum, 1, 693 (1916).
736
INDUSTRIAL A N D ENGINEERING CHEMISTRY
followed by the necessary amount of fresh acid. Cracked distillates which have stood for several months in storage require more acid for treating than the fresh distillates. There is no doubt that chemical changes take place on standing; for example, hydrogen sulfide has been observed to be absorbed in such distillates. The reduction of the sulfur content by like treatment is also rendered more difficult with cracked distillates stored for several months. On the other hand, short periods of standing-e. g., several weeks-may even facilitate treatment of the cracked distillate.
Vol. 18, No. 7
oil of suspended material. About 5 to 10 minutes' washing following the acid treatment after drawing the sludge is usually sufficient to remove most of the remaining acid. It is essential that all the acid sludge be allowed to settle and be removed before washing with water. This prevents contamination of the treated oil with the products of hydrolysis of the acid sludge. Treatment of Cracked Gasoline Distillate
All the methods described refer to the chemical treatment of a crude cracked distillate containing not only gasoline, but also a heavier oil component. This treated distillate is The plumbite solution consists of litharge dissolved in a redistilled in the presence of steam as outlined, the gasoline solution of sodium hydroxide, and derives its name from one distillate product being further treated in most cases. This of the components of the resulting system-namely, sodium additional treatment usually consists of washing the gasoline plumbite. For most cracked distillates from 0.02 to 0.2 distillate with dilute solutions of sodium hydroxide to remove pound of litharge per barrel of distillate is used, but for those hydrogen sulfide and other alkali-soluble products resulting containing a relatively high percentage of mercaptans much from the heating of the treated oil during distillation. The larger amounts may be used. Where the plumbite solution gasoline distillate may be finally water-washed and the is used previous to the acid, best results are obtained with product sent to storage. the litharge dissolved in concentrations of sodium hydroxide USE OF PLUMBITE SOLUTIONSON CRACKED GASOLINE from 20' to 30" BB. (13.5 per cent to 24 per cent KaOH). D I S T I L L A T E - T ~ ~are ~ ~ cases, principally dependent on Where the plumbite solution is used following the acid, the distillation conditions as well as upon types of cracked dislitharge is best applied in solutions of sodium hydroxide from tillate, where the use of a dilute plumbite solution is necessary 6" to 16" BB. (4 per cent to 11 per cent NaOH). to sweeten the cracked gasoline distillate, the plain caustic The litharge is dissolved in the sodium hydroxide solution solution not sufficing. I n such cases mercaptans, as well , while the latter is hot. This saturates the solution, the excess as hydrogen sulfide, which have been formed during distillaof lead monoxide crystallizing out on cooling. The actual tion, are removed by this treatment. As with the plain concentration of litharge which reacts or dissolves is about sodium hydroxide treatment, the most dilute plumbite solu1.5 per cent for a 16" BB. sodium hydroxide solution and tion which will answer the purpose is used. Here again the about 3.0 per cent for a 30" BB. sodium hydroxide solution. use of free or elementary sulfur is avoided in order not to For the average cracked distillate from 1 to 2 per cent by jeopardize the stability of the cracked gasoline. volume of the plumbite solution is used, being varied to meet The introduction of more steam up to the limit of an allconditions and requirements. The amount of litharge re- steam distillation, assuming proper chemical treatment quired is determined by trial, using the smallest quantity previous to this stage, will correct any distillation condition necessary with sufficient time of contact to render the oil tending to produce a sour product, so that the use of either sweet to the doctor test or to give the desired results. Excess plain caustic or very dilute plumbite solutions will sweeten or so-called "waste" plumbite solutions may be recovered the cracked gasoline, no sulfur being necessary. Any obby allowing the suspended material to separate on standing servable sourness under these improved conditions is usually and drawing off the clear liquid. Additional or make-up due to hydrogen sulfide. litharge may be added upon re-use. It has been commonly USE O F ALK.4LINE SOLUTIONS T O IMPROVE ODOR O F observed that the presence of precipitated lead sulfide in CRACKED GAsoLIxEs-For best results chemical treatment the plumbite treatment facilitates the sweetening reaction. of the cracked gasoline distillate (obtained from the chemical Wendt and Diggsc state that this action is catalytic. The treatment and redistillation of crude pressure distillate) mixture of lead sulfide and lead mercaptides in the solid phase should be reduced to a minimum with regard to concentration can be collected and recovered by suitable methods. and type of reagents. Even with a mild reagent, as dilute The concentrations and relative volumes of sodium hy- solutions of sodium hydroxide, concentrations of 5 per cent droxide solutions necessary for neutralization (method 1) or less may affect the color of the cracked gasoline. Soluwill depend upon the thoroughness of water washing pre- tions containing less than 0.5 per cent sodium hydroxide will ceding this treatment. Usually a 6' to 12' BB. caustic improve the odor without having a bad effect upon the color. solution, 1 per cent by volume, is used. Such treatment removes the dissolved hydrogen sulfide, the gasoline thereby being rendered doctor sweet and negative Water Washes in corrosion. Water washes are generally desirable preceding and followStabilizing Gasoline Distillate ing all chemical treatments of a cracked distillate. Where With certain types of equipment which allow film supersludges are formed they are withdrawn before washing. Washes preceding the acid treatment remove hydrogen sulfide heating of the cracked distillate and with various types of and other water-soluble impurities. The necessity or ad- cracked distillates requiring varying temperatures of distillavisability of a water wash preceding the acid should be tion, although a water-white and sweet product is obtained checked by a laboratory treatment. If no advantages are by treating according to one of the above methods, it may observed it is more simple to proceed directly with the acid be unstable-i. e., become discolored or clouded especially treatment. With the split plumbite treatment, the presence on short exposure to direct sunlight. I n such cases an additional final treatment with fuller's of too much hydrogen sulfide will prevent proper contact of the plumbite solution with the oil, owing to the large earth or other adsorptive earth or clay will produce a stable amount of lead sulfide formed immediately upon the addition product under practically all conditions. This treatment of the plumbite solution. A water or caustic wash will may be applied in one of two ways: (a) filtration of the distillate through a column containing coarse or Tinular earth, remedy this. Water washes following the plumbite treatments clear the or preferably (b) the use of fine earth as ~ ( J I I ~ w ~ . Plumbite Treatment
.July, 1926
INDUSTRIAL AND ENGINEERING CHEMISTRY
After treating the gasoline distillate with dilute sodium hydroxide (if necessary) and water-washing, allow the water to settle and draw off. Add a small quantity of fine fuller's earth while agitating the oil, and then add the remainder of the earth, using a total of from 0.5 to 1.0 per cent, by weight of the oil. Agitate the whole thoroughly, allow the earth to settle, and draw off. To remove the fine earth that remains in the gasoline, wash, and agitate with water.
The last step is a case of selective wetting and adsorption of the water by the earth. Mechanical arrangements, although seldom necessary, may be used also for filtering out the suspended earth. Gurwitschll showed that adsorption can be accompanied by purely chemical changes such as the polymerization of unsaturated substances; for example, pentene (amylene) was changed by the action of hydrosilicates or charcoal to dipentene (diamylene). Hexene has been found to act similarly. Gilpin and Schneeberger12 consider fuller's earth as a dialyzing septum in its action upon petroleum products, permitting free passage of paraffin oils and causing adsorption and coagulation of the bitumens containing sulfur and nitrogen compounds and the unsaturated hydrocarbons. VenableI3has shown that when pinene or turpentine is treated with fuller's earth, dipentene, and terpinene are formed. His work was an extension of previous work by Gurwitsch. The writer's experience has been that high-grade fuller's earth is a satisfactory agent for the refining of cracked distillates in the manner shown. Polymerization and adsorption, while the principal factors in the action of fuller's earth, are probably accompanied by oxidation due to adsorbed air. Simple polymerization would cause the formation chiefly of oily substances, whereas the substances adsorbed by fuller's earth are partly of a resinous nature. The mechanism of stabilization is probably the polymerization of the color-forming compounds, such as the diolefins, as well as the olefins, together with some oxidation of the polymerized products and subsequent adsorption. Whereas reaction products are adsorbed and removed by fuller's earth, cracked gasolines treated with fuller's earth show increased gum formation on evaporation by the usual test method. This may be interpreted as a formation of gum-forming components due to fuller's earth treatment.
737
oils, wood, and coal tars. I n the latter case the dilute acid treatment precedes the treatment with the more concentrated acid. Treating Equipment
All these principles may be applied to either batch or continuous treatment of the cracked distillate. Continuous treating systems using orifice columns, packed columns, baffled coils, or other mixing devices are economical from the viewpoint of throughput and cutting down of losses by evaporation. The orifice column is a satisfactory mixing device. Mechanical mixing or circulating devices such as centrifugal, gear, or other type pumps when applied to batch agitators instead of air blowing, also cut down evaporation losses. A drawing of a batch agitator with circulating pump is shown (Figure l), the arrangement being such that the distillate and reagent may be circulated through a mixing column if desired. This type of apparatus is suitable for all methods of treating, including method 4,and the adsorbent earth treatment for gasoline distillate where necessary. The apparatus may be operated so that a combination air blowing and mechanical agitation may be used. Continuous treating systems consist of combinations of mixing, settling, and washing arrangements, depending on the method of treatment. The mixing arrangements usually
-T
i
200'
Treatment of Cracked Distillates from Sources Other than Petroleum I
The foregoing principles are applicable to the treatment of cracked distillates from sources other than petroleumfor example, from shale oils, low-temperature coal tars, brown coal tar, and other tars. In the refining of shale oils, the split plumbite treatment would be applicable following the distillation precautions cited and treating the gasoline distillate as outlined. With some shale oils complete steam distillation is resorted to, and in addition the cracked gasoline distillates require a final treatment with an adsorbent earth to render them stable, while others are quite stable without such final treatment. In the refining of cracked distillates from low-temperature coal tars and other tars containing phenols or tat acids, these substances are first removed by treatment with sodium hydroxide solution, applying method 2 for the rem&' der of the treatment or using the split plumbite method per e, and in some cases stabilizing with the final adsorbent earth treatment. Bases are removed during the acid treatment. Dilute sulfuric acid (60 to 90 per cent) or its equivalent may be advantageously substituted or combined with concentrated acid in successive steps with some types of cracked distillates from petroleum and particularly those from shale
\
Z. angnu. Chem., 99, 1061 (1909). Am. Chem. J . , 60, 59 (1913). 13 J. Am. Chem. Soc., 46, 728 (1923). 11 12
fYM '4'
Figure I-Schematic Batch Treating System for Cracked Distillate
consist of orifice or baffled columns or coils discharging into a settling chamber from which the oil overflows into another settling chamber when necessary, and under any circumstances into a washing chamber equipped with spray washer below the overflow level. Before subjecting the oil to the action of the next reagent or mixing stage, it is good practice to discharge it into a reservoir or surge tank which permits regulation of the oil flow and proper proportioning with regard to the succeeding reagent-for example, plumbite solution or sodium hydroxide. The following continuous treating arrangement is suitable
738
INDUSTRIAL A N D ENGINEERING CHE-MISTRY
e
L
Figure 2-Schematic
I
J
w,r,
Vol. 18, No. 7
Continuous Treating System for Cracked Distillates
for any type of treating and especially for method 3, the split plumbite treatment: plumbite mixing column or coil, plumbite settling chamber, water-wash chamber equipped with spray, reservoir or surge tank, booster pump (which acts as mixer if of the centrifugal type), acid mixer, two acid settling chambers, lead-lined water-washing chamber, reservoir or surge tank, booster pump, plumbite mixing column or coil for second plumbite treatment, and finally a plumbite settling chamber. The last chamber may be equipped with a water spray. The reservoirs or surge tanks may be a 10- or 12-inch column, in proper serial arrangement and of the same height as the settling chambers, equipped with sight or gage glasses. The settling chambers are equipped with gage or sight glasses and drains for drawing off sludge acid, waste alkali, or plumbite and wash water. The pressure distillate charging pump is equipped with a meter. The booster pumps may be likewise equipped. The meters may be placed in by-pass arrangement. Drain and feed cocks should be of the self-lubricating type. Blow cases are installed in duplicate to permit periodic gaging. Acid and plumbite storage tanks should be provided. Manometers are useful for testing differential pressures a t various points. The schematic arrangement in Figure 2 shows a pressure or vented system as desired. For methods 1 and 2 everything to the left of the acid mixer may be omitted.
sulfur dioxide. Polymerization of the olefins and the formation of diolefins and tarry substances are no doubt promoted under the effects of the high temperature due to the local overheating. Partial remedies for these difficulties have been mentioned-that is, dilute sodium hydroxide solution for sweetening and fuller's earth for stabilization. To avoid an unstable condition in the gasoline products, however, the temperature factor is usually controlled by the introduction of steam, which, on account of its partial pressure, lowers the average boiling point of the mixture. Preferably, the steam is introduced into the body of the oil to increase the evaporating surface as well as for other beneficial effects. The amount of steam to be introduced depends on the type of oil to be distilled, the capacity desired within certain limits, and the chemical critical temperature of distillationthat is. the maximum temperature for the type of distillation equipment used which will produce a stable product. Allsteam distillation, either of the saturated or superheated type, will give the best results, but it increases the cost and decreases the capacity. I n general, a reduction of the temperature and time of heating consistent with capacity and efficient separation should be the aim in carrying out the distillation operation. The time factor is taken care of by continuous or semicontinuous distillation instead of batch distillation.
Chemical Factors in Distillation
Future Probabilities
Aside from the principles involved in the efficient fractionation of a hydrocarbon mixture such as is represented by original cracked distillates, the chemical factor also enters into the redistillation of treated distillates to produce colorand odor-stable gasoline. Method 2 illustrates a desirable chemical reaction being completed by heat treatment. On the other hand, chemical changes due to long-continued heating a t too high temperature may render the gasoline produced both sour and unstable. Careful attention must be paid to this point during the distillation operation. The sourness may be due to the formation a t elevated temperatures of hydrogen sulfide and possibly some mercaptans from the sulfides, disulfides, and other sulfur compounds present in the cracked distillate. Elementary sulfur would also cause the formation of hydrogen sulfide. The dialkyl esters of sulfuric acid formed during the acid treatment are decomposed on heating, forming tarry substances as well as
The present tendency in the cracking art is towards the direct production of an end-point distillate-i. e., a cracked distillate having the required motor fuel boiling range. This calls for treatment after distillation, or in some cases during distillstion and after distillation. At some time in the future, especially for some types, the gasoline distillate may be taken directly from the cracking plant and subjected to a very light treatment such as an alkaline wash and adsorbent earth treatment to improve odor and stability only; and a definite and distinctive color may be imparted by dyeing the resulting gasoline product. This, of course, would call for a distinct departure from present refining and market requirements. The present general practice, however, is to make a crude distillate from which the gasoline is removed by redistillation after chemical treatment. Elimination of this redistillation will without doubt cause many changes in the present treating methods.
