MOTOR FUEL QUALITY A S RELATED TO REFINERY PROCESSING AND ANTIKNOCK COMPOUNDS' FRANKLIN CONRAD and WILLIAM W. SABIN Ethyl Corporation, Detroit, Michigan, and Baton Rouge, Louisiana
ANTIKNOCK compound
and tetraethyllead are used interchangeably in this paper since tetraethyllead is still, 33 years after its first commercial use, the only commercially practical antiknock compound. There are, of course, several yardsticks for evaluating gasoline quality, including distillation characteristics, vapor pressure, etc. Of these, however, the antiknock value of a gasoline, as measured by the octane-number scale, commands the most interest and attention. This is true from both the standpoint of good fuel performance and from the point of view of the interest and concern that it presents to the reiiner, the automobile manufacturer, andthe car owner. Today, the most commonly used and advertised measure of antiknock quality is the Research octane number (RON). Unless otherwise stated, the antiknock quality referred to in this paper is measured in terms of Research octane number. The octane number of a gasoline is a measure of its antiknock quality or ability to resist detonation during combustion. Octane number is measured as the percentage of isooctane in a blend of isooctane and normal heptane which produces the same knock intensity in the test engine as the gasoline being compared. Research octane number refers to the octane number determined by the Research method test (ASTM designation D-908). The Research method is used widely by the industry today and is part of most easoline s~ecifications.~
'
Presented as part of the Symposium on Chemical Industry in the South, before the Division of Chemical Education, a t the 129th Meeting of the American Chemical Society, Dallas, April, 1956.
Octane ratings over lW, are obtained by comparing the knock intensity of the t e ~fuel t to mixtures of isoootane and tetraethyllead, using tetraethyllead concentrations up to 6.0 ml./gal. For 1.5" or a t the same level as a example, a fuel might lste at "is0 blend of inoootane and 1.5 ml. TEL/gal. Ratings in terms of isooctane plus tetraethyllead arc then converted to octanes by n rather complicstud formula:
+
Where T = ml. of TEI,/U.S. gallon of irrooctsne and ON octane number.
=
The extension scale for octane numbers over 100 has not yet been formslly established, but this relationship is the one currently proposed and most widely used. A scries of very informative articles on the subject of octanes over 100 cnn be found in thr Petroleum Rqfiner, 35, 162-174 (1956).
Figure 1 shows that the antiknock quality of both premium and regular-grade gasolines has steadily increased over the years (I). The only significant dip in the trend toward higher octane numbers occurred during the war years when large quantities of tetraethyllead and gasoline stocks of high antiknock quality were diverted to military consumption. The curves prior to 1941 are dashed to indicate that Research octane numbers have been estimated for this period. There are two basic ways of improving the antiknock quality of gasoline: (1) by using an antiknock compound (tetraethyllead), and (2) by employing refiniug processes that chemically or physically alter the composition of gasoline. Both means have been employed to raise the antiknock value of gasoline to its present level. As will be shown later, both means will continue to play vital roles in improving the quality of gasolines of the future. These methods of octane improvement are inter-related, and will he discussed and developed a t the same time. MODERN REFINERY PROCESSES
The principal processes for the manufacture of modern gasoline are crude distillation and fractionation, thermal cracking, thermal. reforming, catalytic cracking, and catalytic reforming. Figure 2, based on data presented by Lyman (t),portrays the relative importance of these various processes during different periods of time, and includes estimates through 1965. As shown in this figure, the trend is toward producing gasolines that are composed predominantly of t,he products from catalytic cracking and catalytic reforming processes. Each process produces a gasoline of different antiknock quality and tetraethyllead susceptibility. Tetraethyllead susceptibility is the increase in antiknock value imparted to a gasoline stock by the addition of a definite amount of tetraethyllead. The purpose of the sections which follow is to outline in chronological order the important aspects of each process and to point out the relationship of the produrt of each process to tetraethyllead usage. Distillation and Fractionation. The first process to be used for the manufacture of gasoline was, of course, crude distillation and fractionation. This process involves the separation of crude oil into fractions which may be generally described as follows: (1) Gas-which includes the propane and lighter hydroJOURNAL OF CHEMICAL EDUCATION
carbonsand other gaseous impurities. (2) Straightrun gasoline or naphtha-with a boiling range of about 90' to 400°F. (30'-200°C.) and containing hydrocarbons with from approximately 4 to 12 carbon atoms per molecule. (3) D i s t i l l a t e ~ w h i c h may include jet fuels, kerosene, heating oils and Diesel fuels and which may cover a boiling range from about 300' to 650°F. (150"-350%) depending somewhat upon the particular products desired. (4) Gas oil-which is, generally speaking, in an atmospheric crude distillation operation, the material which d l distill between the gasoline end point and the temperature a t which cracking begins to take place. This is a boiling range from about 400' to 650°F. and is the same boiling range as for Diesel fuels and heating oils. Gas oil which is not utilized for distillate production is charge stock for catalytic or thermal cracking operations. (5) Reduced crude or residuum-which is the material not distilled in the crude distillation operation but drawn off as a still bottoms material. Straightrun gasoline is generally quite low in antiknock quality. However, it demonstrates good tetraethyllead susceptibility (increase in antiknock quality by the addit,ion of tetraethyllead) and is quite stable. When straightrun mas the sole component of gasoline, the basic problem of refiners was the production of sufficient gasoline with high enough volatility to satisfy engine starting and warm-up requirements. T h e m a l Craclcing. The introduction of the thermal cracking process provided the refiner with the means of considerably increasing the yield of gasoline from crude oil and solved the problem of low volatility. Thermal cracking also provided a method of improving antiknock quality of gasoline, since thermally cracked gasoline has better antiknock quality than straightrun gasoline. However, the octane improvement obtained by the use of tetraethyllead in thermally cracked gasoline is not as good as in straightrun gasoline. The use of thermal cracking introduced another problem to the refining industry. The gasoline produced from cracking gas oil under the conditions of high temperature (700'-1000°F.) and pressure (2001000 p.6.i.) contains many carbon to carbon double bonds and consequently tends to be unstable. The typical reaction is: C,,H1,+s
where
-
C,'Hu'
n = n'
+
CnH2n1+2
+ n"
The characteristic of instability of the gasoline product caused the industry considerable trouble in the 1920's because of the tendency of the unstable product to form gum. The problem was eventually solved by developing economic methods for inhibiting gum formation in snch gasolines Thermal Reforming. The development of thermal reforming soon followed the use of thermal cracking. Thermal reforming is basically the same process as thermal cracking. The principal difference is the type of material used as feed stock. Thermal cracking processes charge g a s oil (40O0-650°F. boiling range) ; whereas, thermal reforming processes use the heavy (200°400"F.) portion of the straightrun gasoline. The chemical reactions are complex involving cracking of large molecules to yield smaller molecules, polymeriVOLUME 34, NO. 6, JUNE, 1957
riguro 1.
Trend i n Remarch Octane Number Ovditg of Oaaolin. Sold i n the United Statar
zation of the cracked molecules to form branched chain molecules, some dehydrogenation and some dehydrocyclization. The thermal reforming process, which was the first to be employed strictly for octane improvement, produces substantial increases in the octane number value of gasolines by removing the heavy straightrun gasoline of low antiknock value (30-50 octane number) from the gasoline pool (total production of all grades of gasoline) and replacing it with better antiknock quality (7&90 octane number) thermal reformate. The production of considerable quantities of light hydrocarbons (butane and lighter) results in a yield of reformed gasoline which. may be only 70% to 90% of the heavy straightrun gasoline charge. The process, like thermal cracking, is characterized by the production of gasoline containing many carbon to carbon double bonds. As compared to thermallycracked gasoline, the gasoline produced by thermal reforming has about the same susceptibility to tetraethyllead and equally poor stability. Polymerization. The use of thermal-cracking and thermal-reforming processes created another problem for refiners. Both of these processes produced more highly volatile hydrocarbons (three and four carbou atoms per molecule) than could be ut,ilized in the resulting gasoline. Consequently, the polymerization processes were developed to convert these volatile olefins to material boiling in the gasoline range (9O0-
-
400°F.). A typical polymerization reaction is: 2 CaHr
CeHu
The first commercialized catalytic process was catalytic polymerization. Catalytic polymerization is widely used today; however, since the volume of feed stock available for such processing is small, this urocess contributes only a small volume to the ~asoline pool. Catalytic Cracking. Catalytic cracking was first done on a commercial scale in 1936 (5), and World War I1 brought on its very rapid development and wide acceptance. Like thermal cracking, catalytic cracking produces gasoline from the cracking of gas oil. The fundamental reaction is still:
-
CllH21i+2 C.'H9,1'
+ CD"HZIntl
However, one of the major attributes of catalytic cracking is the manufacture of greater proportions of isoparaffins and aromatics in the cracking reaction. The shift in the type of hydrocarbons present in the gasoline results in a gasoline of superior quality as compared to the thermal counterpart. Improved product yield and distribution also favor the catalytic process. As a result, catalytic cracking has become one of the principal "work horses" of the refining industry. Current-day gasolines contain in the neighborhood of 30 volume per cent of catalytically-cracked gasoline. Catalytic Alkylation and Isomerizatim. The late 1930's also saw the introduction of catalytic alkylation and isomerization of light hydrocarbons and of catalytic reforming of naphtha as commercial operations in petroleum refining. Alkylation and isomerization can be mentioned only in passing because they currently contribute very little to the volume of automotive gasoline production. The principle alkylatiou reaction referred to is the combination of isobutane and butylene to form isooctane:
Alkylation does produce a very high quality product in all respects. However, because the feed is derived from the light hydrocarbons available from other proc-
essing, only limited volumes ran be produced, and this production is still predominantly used in aviation gasoline. Isomerization has not yet been employed for the production of a gasoline component to any large degree, mostly because the cost of the process is still relatively high for the benefit gained. The reaction most frequently associated with this process is the isomerization of normal butane, pentane, or hexane to the respective branrhed cham isomer, such as: C-C-C-C-C
-
C-C-C-C
I
C
Catalytic Reforming. The first commercial process for catalytic reforming of gasoline stocks was introduced in 1939 (4). During World War I1 several other reformers were built and operated for the production of aromatics needed in the war effort. Since then the emphasis bas switched to the production of gasoline components of high antiknock quality. In many cases, aromatics produced in reforming units are separated either for sale or for use as special blending stocks. The growth in catalytic-reforming capacity since the war has been one of the outstanding points of interest in the petroleum refining industry. Reforming processes can be operated to produce a gasoline component of 80 to as high as 100 octane number without tetraethyllead from heavy ~traightmllgasoline feed of 40 to 55 octane number. The process produces a gasoline which contains high percentages of isoparaffins and aromatics, and makes a very desirable gasoline component with good stability and antiknock quality. A disadvantage of the operation is the resulting loss in yield of gasoline (15-20%). The chemistry of the catalytic reforming process is complex, but involves the following general reactions: (5)
(1) Dehydrogenation of Cs and Ce naphthenes to . ar&natics. (2) Dehydrocyclization of paraffins and olefins to aromatics. (3) Hydrocrarking to produce saturated hydrocarbons. (4) Isomerization of paraffins to branrhed rhain hydrocarbons. ( 5 ) Desulfurization. This, then, is the situation in mhirh the refiner finds himself today. He has a perplexing problem of choosing the route most suitable to his needs for producing the required gasoline quality most economically from the various stocks available, and from the use of tetraethyllead. Present-day gasoline antiknock quality can be obtained without the use of an antiknock compound; however, the use of tetraethyllead gives the refiner econon~icadvantages and added flexihility of operation. This will become apparent from the following discussion. ESTIMATING COST OF QUALITY IMPROVEMENT
70 0
I
0.5
I
I
1.0
1.5
2.0 2.5 3.0
TETRAETHYLLEAD. ML./GAL. Figure 3.
264
Tetraethylleed Susceptibility C h u t Fall 1955 Gasoline
Figure 3 (an illustration of a tetraethyllead susceptibility chart) portrays the average octane quality of premium gasoline, regular gasoline, and pool gasoline (premium plus regular) in the United States in the Fall of 1955. The average pool antiknock quality JOURNAL OF CHEMICAL EDUCATION
is 91.0 RON at a tetraethyllead concentration of 2.38 milliliters per gallon (6). The slope of the lines on this chart establish the susceptibility (or octane-number response) of these gasolines to tetraethyllead. The octane improvement obtainable by the use of tetraethyllead can be read from these lines. Octane improvement attained by modificat,ion of refinery processing will raise the lines, and may change the slope or susceptibility. A method has been developed for establishing and comparing octane improvement costs. This method, which has proved very useful in dealing with the problem of producing desired gasoline antiknock quality most economically, is based on the use of the unit "cents per Research octane number barrel." Cents per Research octane number barrel (#/RONB) is the cost in cents required t o raise the antiknock quality of one barrel of gasoline by one Research octane number. An incremental approach (a comparison of the costs of small increments of octane improvement) is used t,o compare the cost of octane improvement via various processing means and via tetraethyllead. Thus, cents per Research octane number barrel is the increment of cost in cents required to give an infinitesimal amount of gasoline (in barrels) an infinitesimal increase in Research octane number. This is analogous to the slope of a curve relating cost in cents to Research octane number barrels of improvement. The incremental cost of octane improvement using tetraethyllead is determined from the following equat,ion:
where: "c" is the cost of tetraethyllead in cents per milliliter, "T" is the tetraethyllead concentration in milliliters per gallon in the gasoline to which an increment of tetraethyllead is added, "m" is the slope of the gasoline's tetraethyllead-response curve when plotted on the tetraethyllead-susceptibility chart, and "RON" is the Research octane number of the gasoline to which an increment of tetraethyllead is added. The average cost of octane improvement in the United States by the use of tetraethyllead in the Fall of 1955 was 4.8ClRONB (6). This figure is an average of the cost for premium and regular grades of gasoline and for all of the various grades of gasoline surveyed that were being produced in the United States at that time. Pool gasolines of individual companies showed incremental costs as high as 8p!/RONB or as low as 2#/RONB. This variation points out that it is important for each refiner to know his own octane-improvement costs. On an incremental basis, tetraethyllead competes in a refinery with processing methods of gaining octane improvement. Such processes as thermal or catalytic cracking, alkylation, and polymerization cannot he considered as primarily octane-improvement processes. These processes are used by refiners to make gasoline (a valuable product) out of low-value materials (not in the gasoline hoiling range). Although these processes usually result in improvement in antiknork quality, they vould be economically justified even if there were no resulting improvement in octane numher. However, these processes might compete with VOLUME 34, NO. 6, JUNE, 1951
tetraethyllead for octane improvement if the operating conditions for a specific process were changed with the primary purpose to raise octane number. Such changes would result in loss of product yield, increased operating cost, and/or a change in yield of other products from the operation. On the other hand, catalytic and thermal reforming processes are octane-improvement processes, and cannot be justified economically by a refiner on any other basis than octane improvement. Depending upon the specific situation for a given refinery with catalytic reforming, several processing means for gaining an additional increment of octane improvement are available. (1) The refiner may reform an increased volume of feed stock of a given boiling range. (2) He may increase the hoiling range of the feed stock charged to the reformer, thereby increasing the volume of stock reformed. (3) He may charge thermally-cracked naphtha to reforming in order to upgrade this stock in antiknock quality. (4) He may alter the operating conditions of the reformer, or he may extract a portion of the product from the reforming process which is rich in aromatics and then treat the remainder further. The operating conditions may be varied by using a higher reforming temperature or a lower space rate in order to reform a given charge volume more severely, and thus improve antiknock quality a t the expense of yield. When these variables in the operation of the process are changed solely to obtain antiknock quality, the added cost resulting from the change of operation competes directly with that of tetraethyllead as a means of attaining the desired increase in antiknock quality. The unit of b/RONB as a measure of octane-improvement cost through processing is used as follows. Research octane number barrels (RONB) is defined as the volume of improved antiknock quality material, obtained through processing, times the new octane number minus the starting octane number. Thus, the cost of a processing change, divided by RONB, yields the incremental cost (#/RONB) of octane improvement through processing. Bbls. Gasoline Pool 10 Charged to Reformer -5 +4 Reformed Naphtha Net Result -9 (Volume of improved/ Octane Quality Material) RONB Improvement = 9(90 - 68) Cost of Octane Improvement = 512 1200 #/RONB = - = 6#/RONB 198 Figure 4.
ROX 68 (Starting octane) 50 ~
~
95 90 (New octane)
198 RONB
Mathod of cetcu1atingInoremental cost of oatan. 1mprov. ment Through Processing (i/RONB)
For example, consider the'case, shown in Figure 4, where a refiner has 10 barrels of G8 RON gasoline. He separates 5 barrels of 50 RON gasoline, and charges it to a reformer which yields 4 barrels of 95 ROX gasoline. He now has 9 barrels of gasoline of 90 RON, and the net improvement is g(90-68) = 198 RONB. Assuming that this added processing costs the refiner 812.00, the incremental octane improvement is then:
over the last 35 years. Some of the more important changes during this time, all of which have contributed greatly t o the success story of tetraethyllead by increasing the efficiencyof the manufacturing process and resulting in cost reductions are: ( I ) substitution of relatively low-cost ethyl chloride for ethyl bromide in t,he alkylation reaction step; (2) increased yields in the alkylation step; (3) improvements in the efficiency of the alkylation reaction; (4) ethyl chloride produced from ethylene rather than more expensive alcohol; (5) improved sodium cell operation and ~lloying techniques; and (6) improved economics resulting from large scale operation. Future refinements in the process just outlined may make further cost reductions possible. A question of the possibility of a new commercial antiknock compound is often posed. The prospects for the realization of a new antiknock compound are better today than ever before for several reasons. Extensive effort is being applied and continuous progress is being made toward a better understanding of why and how engine knock occurs and how an antiknock agent. functions t o suppress engine knock. As a result, we better understand what properties good antiknock compounds must possess, what compounds best satisfy these requirements, and what methods can be applied t o produce t,hem most economically. Perhaps this intensive effort will some day bear fruit. I t is only fair, howel~er, to temper the optimism expressed in the previous paragraph with an appraisal of the competition from tetraethyllead which a new antiknock compound must meet. A reasonably acrurate measure of such competition is afforded by a comparison of the relative effectiveness of three broad classes of rompounds (organometallics, amines, and hydrocarbons) known t o exhibit antiknock characteristics (8). Such a comparison is shown in Figure 7 and the very striking effectiveness of tetraethyllead as a n antiknock agent is again borne out. I n fact; it is a sound conclusion that if one were to begin all over t o choose an antiknock compound, the choice would undoubtedly still be tetraethyllead. Thus, it is certain that tetraethyllead will continue to play the leading role as an antiknock compound for yesrs t o come.
VOLUME
34,
NO. 6, NNE, 1957
RELATIVE EFFECTIVENESS PER CENT BY WT TEL:1001
-
I TETRAETHYLLEAD 2 - ETRAPHENYLEAD 3 - DIBUTYLDIPHENYLLEA 4 - NICKEL CARBONYL
5 - IRON CARBONYL 6 - DIETHYL TELLURIDE
7 - XYLIDINES 8 TOWIDINES 9 - ANILINE
-
10- IMOCTANE I1 BENZENE
-
LEAD COMPOUNDS
Figu..
I.
Relati".
METALLIC COMPOUNDS Effestiuenea.
of Antiknock Compoundh
Repreentativesingle-cylinderengine test data Science gll'el~olcum (1938) and Ethyl Curporatinn unpublished data.
ACKNOWLEDGMENT
The authors wish t o acknowledge the efforts of all the personnel of Ethyl Corporation's Refinery Technology Division and the men in industry who have been the instrumental parties in the development and clarification of the economic methods discussed in this paper. LITERATURE CITED
.
(1) Based on Ethyl Corporation Surveys of Gasoline Quality. (2) LYMAN, A. L., Petroleum Refiner, 34, 130-44 (May, 1955). I). B., J. C. DART,R. C. LASRIAT, ''Progress in l'e(3) ARDERN, trolenm Technology," Am. Chem. Soe., Washington, 1). C. (August 7, 1951), 13-29. (4)MCGRATH, H. G., AND L. R. HILL, "Progress in Petroleum Technology," Am. Chem. Sac., Washington, I). C. (August 7, 1051), 3'&57. ( 5 ) SITTIG, M.,A N D T. W. WARREN, Pelrolet~mRefiner, 34, 23& 80 (Sept., 1855). ( 6 ) Based on Ethyl corporation's Refinery Survey of Gasoline Quality conducted in the Fall of 1055. (7) NICKERSON, STANTON P., J. CHEM.EDUC.,31,560-71 (1854). (8) EDGAR, GRAHAM, "Progress in Petroleum Technologv," Am. Chcm. Sac., Ws~hington,I). C. (Angust 7, 1951), 221-34.
