Knocking Characteristics of Naphthene Hydrocarbons WHEELERG. LOVELL,JOHN M. CAMPBELL,AND T. A. BOYD General Motors Research Laboratories, Detroit, Mich. equivalent.” A positive aniline R E V I O U S publications The relative tendencies to knock in a n engine equivalent i n d i c a t e s that the from this laboratory (6, have been measured for sixty-nine naphthene compound knocks less than the 6) h a v e described the hydrocarbons. These measurements were made, reference gasoline and represents knocking characteristics of over not o n the hydrocarbon alone, but in admixture the amount of aniline, expressed B t y paraffin and olefin hydroas the n u m b e r of centigramwith gasoline, and the results have been expressed, carbons of varied s t r u c t u r e. moles per liter, which must be This paper is concerned with the as in previous work, by using the antiknock added to the reference fuel to behavior of naphthene hydrocareffect of aniline as the standard of comparison. produce a fuel that is equivalent bons which have been studied Upon this basis there appear great diflerences in tendency to knock to a oneover a period of several years. among the knocking properties of these commolar solution of the compound As early a s 1921 R i c a r d o pounds and even among isomers. The relations in the reference gasoline-that (8) published v a l u e s for the is, to a solution containing one highest useful compression ratio between structure and tendency to knock of these gram-mole of the compound made of c y c l o h e x a n e a n d methyl naphthenes appear quite consistent and also up to a volume of 1000 cc. with cyclohexane; values of the knockconsistent with the previous relations found f o r the reference gasoline. A negaing characteristics of s e v e r a l the parafin and the aliphatic olefin hydrocarbons. tive aniline equivalent indicates n a D h t h e n e s have been Dubthat the comDound knocks more lishkd by Nash and Howes (7) and by Birch and Stansfield (1). Recently Hofmann and than the reference gasoline and represents the amount of others (4) published data on cyclohexane, three alkyl cyclo- aniline, expressed again as centigram-moles per liter, which hexanes, and cyclohexene, the determinations having been must be added to the molar solution of the compound in the made in 30 per cent solution in gasoline and in terms of reference gasoline to make it equivalent in tendency to knock compression ratio, More recently Garner and Evans (3) to the reference gasoline. For example, the aniline equivalent of tert-butylcyclohave reported the results of comprehensive investigations on the knock ratings of naphthene and aromatic hydro- hexane (molecular weight, 140) is 16. This means that 140 carbons. The measurements were made in terms of the grams of the compound made up to a liter with gasoline octane number of a reference fuel to which was added 20 (about a 17 per cent solution of the hydrocarbon by volume) per cent of the various hydrocarbons, and data were obtained is equivalent in knock to a liter solution of aniline (molecular under two sets of engine conditions. Data were given on weight, 93) in gasoline containing 16,400 X 93 grams of cyclopentane and five alkyl cyclopentanes, and cyclohexane aniline (about 1.5 per cent solution by volume). Similarly, and seven cyclohexanes, and it was concluded that for these the aniline equivalent of - 16 for .V-butylcyclohexane compounds in both series the antiknock quality decreases as (molecular weight, 140) means that 140 grams of N-butylthe number of carbon atoms in the molecule is increased, cyclohexane and 16,400 X 93 grams of aniline made up to a and also that the more centralized the molecule, the higher liter with gasoline knocks the same as the reference gasoline. This makes the solution contain about 17 per cent iV-butylthe antiknock quality. The data presented in this present paper were also measured cyclohexane and 1.5per cent aniline by volume. The precision of the measurements obtained in this way in dilute solution and cover sixty-nine cyclic hydrocarbons, including a number of unsaturated compounds. They thus has been found t o be usually within * one unit of aniline include all of the hydrocarbons previously investigated equivalent for compounds upon which two or more determiby others, and to that extent constitute a duplication and nations have been made. Those compounds upon which confirmation of their work. Inasmuch, however, as the range check determinations have been made with different samples of naphthenes is wider and includes some especially interesting are so indicated in the tables. Unfortunately most of these branched side-chain naphthenes, it has seemed worth while hydrocarbons have been available in limited quantities, so t o present all the data here, especially for comparison with that only one determination of the knocking tendency was possible, and, although one determination represents the the previous work on some of these hydrocarbons. average of four or five separate bouncing-pin readings, the precision is possibly less in some cases than the figure given, EVALUATION OF KNOCKING PROPERTIES because of unavoidable variations in engine conditions during As in previous work (5, 6) the tendencies of these hydro- the period while the measurements were being made. carbons to knock have been compared in a single-cylinder, The a$ine equivalent method of expressing knock ratings variable-compression engine which was equipped with an provides a convenient and uniform basis for comparing evaporative cooling system and fitted with a bouncing-pin compounds representing a wide range in tendency to knock. indicator for matching the fuels in respect to antiknock This basis furnishes a measure of the partial knock effect of quality. This equipment and the method of operating i t each compound in gasoline a t concentrations which are comhave been described in previously published work ( 2 ) . parable with those in which such a compound might actually The relative knocking tendencies under these conditions be present in commercial fuels. have been evaluated in terms of aniline (a knock suppressor), I n order to convey some conception of the magnitude of and the unit of this evaluation has been called the “aniline this scale of knock rating in units which may be more generally
P
1107
I K D U S T R I A L ,4 5 D E N G I N E E R I N G C H E III I S T R Y
1108
familiar than aniline equivalents, it may be stated that a compound having an aniline equivalent of 20 added in onemolar concentration to the reference gasoline having an octane number of 55 would produce a mixture having an octane number of 65. This numerical ratio appears to be valid over the range of positive aniline equivalents used, b u t a distinction should be clearly recognized between aniline equivalents obtained a t one-molar concentration in gasoline and the behavior of compounds by theniselves and not in admixture with gasoline.
Vol. 25, No. 10
DATAo s CYCLICHYDROCARBOSS The data on the cyclic hydrocarbons which have been investigated are shown in Tables I and I1 which give the measured aniline equivalents of the different compounds. TABLE I. SATURATED NAPHTHESES ANILINE
HIDROCARBOX Fivp-carbon a t o m rinn: -Cyclopentane Methylcyclopentane 1,3-Dimethylcyclopentane Ethylcyclopentani 1,3-Methylethylcyclopent ane N-propylcyclopentane 1.3-Alethvl-N-~ronvlcvclonentan~
,3-Methylisoamylcyclopentane Six-carbon a t o m ring: Cyclohexane Methylcyclohexane 12-Dimethylcyclohexane 1,3-Dimethylcyclohexane 1,4-Dimethylcyclohexane 1,3,5-Trirnethylcyclohexane Ethylcyclohexane 1,2-Methylethylcyclohexane 1,3-MethylethylcycIohexane 1,4-Methylethylcyclohexane 1 3-Diethylcyclohexane 1:4-Diethylcyclohexane N-Propylc yclohexane Isopropylcyclohexane 12-Methyl-N-propylcyclohexane 1,3-Methyl-N-propylcyclohexane 1,4-Methyl-N-propylcyrlohexane 1,4-Methylisopropylcyclohexane
d20 -"
EQVIVALENT
0.749 0.750 0,746 0.766 0.763 0.776 0.773 0.784 0.781 0.787 0.788 0.787
14" 4 - 1 1 - 3 -10 - 7'L - 19 - 190 -23 -225 -19Q
0.779 0.769 0.792 0.774 0.777 0.777 0.787 0.805 0.791 0.791 0.800 0.802
70 5a 6'3 4a 6a 2
- 3
0 - 6 - 8
-13
- 13
- 10
0 793 0 799 0 810 0.796 0 798 0 798
0 - 5 10 - 10 1
-
N-butylcyclohexane Isobutylcyclohexane sec-Butylcyclohexane tert-Butylcyclohexane 1,2-Methy!-N-butylcyclohexane 1,3-Alethyl-N-butylcyclohexane 1,4-.Methyl-N-butylcyclohexane 1,3-AlethyI1sobutylcyclohexane 0
I
3
4
6
NUMBER OF CARBON ATOMS IN LONGEST SIDE CHAIN
FIGURE 1.
REL.4TIVE I'hOCKING ALKYL CYCLOPENTAXES AND
CHARACTERISTICS OF
CYCLOPENTEKES
At concentrations below about 40 per cent by volume the computed aniline equivalent, either positive or negative, has been found to be substantially independent of concentration for a number of compounds investigated. But the aniline equivalent as computed a t concentrations below 40 per cent by volume has been found to be not necessarily an index to the behavior of the compound in greater concentrations with gasoline, or even in the pure state, and so extrapolation of values obtained a t lorn concentrations to 100 per cent is not, as a general rule, recommended. Although the numerical values of the aniline equivalents reported in this investigation depend somewhat upon experimental conditions and, of course, upon the reference fuel, there is no evidence available a t this time to show that the relative knocking tendency of one naphthene hydrocarbon with respect t o another when measured in this way is materially altered by reasonable changes in engine conditions. It is recognized, however, that the relative values of different types of hydrocarbons may change considerably with engine conditions. The use of a molecular basis for evaluating the knocking properties of the compounds is very convenient because of the consistent relationships between molecular structure and knock rating which then become apparent. It is the main purpose of this paper to present and to discuss some of these relationships. Most of the hydrocarbons investigated in this work were synthesized by P. L. Cramer and F. K. Signaigo in these laboratories. Data on the syntheses and properties of most of these hydrocarbons are reported elsewhere (9).
N-amyle) clohexane Isoamylcyclohexane tert-Amylc yclohexane 1,2-MethyI-N-amylcyclohexane
0 0 0 0
804 800 821 816
-21 -14 6
- 17
0.886 - 3 Decshydronaphthalene 0.876 -21 Dicyclohexane Seven-carbon a t o m ring: - 6 0.808 Cycloheptane a T w o or more complete determinations on different samples TdBLE
11.
U N S A T U R l T E D hT.4PHTHEKES
HYDROCARBOX Five-carbon a t o m ring: Cyclopentene 1-hlethylcyclopentene I-Ethylcyclopentene 1-N- ropylcyclopentene l-N-gutyicyclopentene 1-N-Amylcyclopentene Six-carbon a t o m ring: Cyclohexene I-Methylcyclohexene 1 2-Dimethylcyclohexene 2:4-Dimethylcyclohexene 1-Ethylcyclohexene
dig 0.772 0.780 0.799 0,804 0,807 0.811
0.810 0.809 0.826
0.805 0.828
ANILINE EQUIVALENT 16 20 12 12 9
3
10 20 19 19 13
1-Methyl-2-ethylcyclohexene 2-Ethyl-4-methylcyclohexene 1-Ethyl-4-rnethylcyclohexene 1-Methyl-2-N-propylcyclohexene 2-N-propyl-4-methylcyclohexene 1-N-propyl-4-methylcyclohexene
0.832 0.815 0.814 0.832 0.816 0.815
25
1-"But ylcyclohexene l-~Iethy!-2-N-butylcyclohexene 2-N-butyl-4-methylcyclohexene 1-N-butyl-4-meth ylcyclohexene I-N-amyicyclohexene 1-Isoamylcyclohexene 1-Methyl-2-N-amylcyclohexene Cyclohexylcyclohexene
0,828
3 13 13 7
0.833 0,820
0.818 0.831 0.826 0.834 0.906
20 18 18 18 12
1 - 3 11 5
In order to present as simple a picture as possible of these hydrocarbons, the data are represented graphically in Figures 1 and 2 which represent, in general, the five-carbon atom ring and six-carbon atom ring compounds, reqpectively. The aniline equivalents of the several compounds have been plotted
October, 1933
I N D U S T R I A L A N D E N G I N E E R I N G C H E $1 I S T R Y
on the vertical axis against the number of carbon atoms in the longest side chain of the molecule. The individual hydrocarbons have been represented by a simplified structural formula. The data as plotted show a number of interesting relationships which are especially evident from the graphical representations. SIZE OF RING. Of primary interest in this connection is the influence of ring size upon knocking characteristics and the relationships between hydrocarbons having a saturated ring structure and the corresponding straight-chain hydrocarbons. Cyclopentane has an aniline equivalent of 14 as compared with 1 for N-pentane; the values for cyclohexane and A'-hexane are 7 and -6, respectively, and for cycloThus for these heptane and S-heptane, -6 and -14. simple compounds the antiknock quality decreases as the ring size increases. I n each case the compound having the ring structure has a higher antiknock value than the normal paraffin of the same number of carbon atoms. LENGTHOF SIDECHAIN. An especially obvious relation is that represented by the solid lines in the charts. Increasing the length of the longest unbranched side chain of the hydrocarbon results in a very consistent decrease in the aniline equivalent. This seems to be quite a general rule and holds for the mono- and the dialkyl cyclopentanes as well as for the alkyl cyclopentenes. I n the alkyl cyclohexane series a similar relation holds for the mono- and the dinormal alkyl cyclohexanes. It holds as well for the mono- ifoalkyl cyclohexanes, the secondary alkyl, and even the tcbrtiary alkyl. The alkyl cyclohexenes also behave similarly. It is thus possible to find many series for which this relation holds, as indicated by the solid lines directed towards the) lower righthand corner of the charts. I n this connection, all of these lines have about the same slope, and this slope is about that observed for the paraffins which are also represented on the chart in a position that is somenhat comparable on a basis of total number of carbon atoms. This relationship between knocking characteristics and the length of the unbranched paraffin chain seems to be quite fundamental since it is observed with paraffins, olefins, and alkyl naphthenes. POSITION ISOMERISX. An especially interesting relationship which may be observed with these compounds is the effect of position isomerism, and it is possible to see what differences may exist between 1,2-, 1,3-, and 1,4-dialkyl cyclic compounds, for instance. I n the case of the normal dialkyl cyclohexanes, there does not appear to be a very great or consistent difference between the different isomers. Usually the 1,2- compound appears to have a higher aniline equivalent or less tendency to knock than the 1,3- or 1,4-isomer. The differences however do not appear to be very great in most cases, and it is possible that small differences may be obscured by experimental errors. A similar situation seems to prevail for the dialkyl cyclohexenes, although in this case the 1,4-isomers appear to have consistently lower aniline equivalents. However, it does not seem that position isomerism is a very important factor in determining the knocking characteristics of these hydrocarbons. BRANCHISG OF SIDE CHAINS. Another type of variation to be observed in these compounds is that in which the structure of the side chain is changed. Some of the relations may be seen with the monoalkyl cyclohexanes and are indicated by the solid lines directed towards the upper righthand corner of the chart. The two lines passing through ethyl-, isopropyl-, and tert-butylcyclohexanes, and also through isobutyl- and tert-amylcyclohexanes, respectively, indicate a similar and large increase in aniline equivalent with the introduction of a methyl group into a branched position on the side chain. It is also of interest to see the effect of side-chain isomerism.
1109
The aniline equivalent of isopropyl- is greater than that for N-propylcyclohexane. With the butylcyclohexanes, the aniline equivalent increases progressively from ,!!--butyl to terl-butyl. The amylcyclohexanes show a great difference between X-amylcyclohexane and the most highly branched tert-amyl compound. The structure of the paraffin side chain of the alkyl cyclohexane appears to be a very important factor determining the knocking tendency. What might be
-2s'
I
e
3
4
5
NUMBER OF CARBON ATOMS IN LONGEST SIDE CHAIN
FIGCRE 2. R E L A T I V E KNOCKINGCHAR4CTEHISTICS OF ALKYLCYCLOHEXASES A N D CYCLOHEXENES
termed the centralization of the side chain or the more compact space arrangement of its structural formula appears very significant ; and, as this centralization is increased, the aniline equivalent increases greatly. These relationships between knocking tendencies and the structure of the paraffin side chain are similar to those previously observed with the paraffin hydrocarbons themselves. DISTRIBUTIOS OF CARBON A 4 T o ~BETWEEN ~s SIDE C H A I N S . '4nother type of isomerism is that in which the side-chain carbon atoms have a different distribution between the chains. Dimethylcyclohexanes have higher aniline equivalents than ethylcyclohexane, the methylethyls than the S-propyl, and so on. The two diethylcyclohexanes have a somewhat greater aniline equivalent than the S-butyl compound although less than the methylpropyl compounds and much less than the tert-butyl. Similarly, the aniline equivalent of trimethylcyclohexane is greater than the methylethyl or the propyl compounds. I n general, it seems that a centralization of the molecule, or the distribution of the side-chain carbon atoms into a greater number of shorter chains increases the aniline equivalent. However, the centralization of the single side chain itself in some cases increases the aniline equivalent much more than does redistributing the atoms among several different side chains. REL.4TIOK TO P A R A F F I S HYDROCARBOSS. It is Of some interest to compare the knocking characteristics of the naphthenes with those of the paraffins. The paraffins having the greatest tendency to knock are the straight-chain hydrocarbons, and they are represented in Figures 1 and 2, although they are not plotted according to the number of carbon atoms in the longest side chain but more nearly as to the total number of carbon atoms. I n Figures 1 and 2 are also repre-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
sented two of the paraffins of most highly centralized structure and highest aniline equivalent. Comparing paraffins and naphthenes having the same number of carbon atoms, it appears that the naphthenes lie within the range of antiknock quality that is established by the paraffins, but that on the whole the range covered by both paraffins and naphthenes seems to be approximately the same. EFFECT O F UNSBTURAT I O N IP; R I N G . The effect of the removal of hydrogen from the saturated cyclic compounds is, in general, to increase the aniline equivalent. This effect is large and the difference between the s e r i e s of n o r m a l monoalkyl cyclohexanes OCTANE RPTING CONVWTCC TO MOLLCULbR 8a51S wra OF GPIRNLR AND EVANS a n d t h e corresponding FIGURE3. ANILINE EQUIVA- cyclohexenes is about 15 LENTS vs. OCTANE RATING units on the average. A somewhat similar situation prevails for the monoalkyl cyclopentanes and cyclopentenes. For the compounds studied in the cyclohexene series, there is a considerable increase in aniline equivalent when a methyl group is added to cyclohexane although this is not true for the corresponding saturated compounds. COMPARISON WITH OTHERDATA The data given by Garner and Evans (3) on the knock ratings of a number of naphthenes are interesting to compare with the data given here on the same compounds. Fortunately there are fourteen compounds on which to base such a comparison.
Vol. 25, No. 10
Garner and Evans report their data in terms of the octane number of a 20 per cent solution of the hydrocarbon in a gasoline (A-2) of 50 octane number, and, in order to make a comparison, their data have been converted to a molecular basis comparable with the aniline equivalents. To do this, the increase in octane number of the fuel upon the 20 per cent addition, when measured a t 212" F. jacket temperature, has been converted to a comparable molecular basis by multiplying by the factor: Mol. wt. of hydrocarbon X density cyclopentane Mol. wt'. cyclopentane X density hydrocarbon These computed values, which should be comparable to aniline equivalents, have been plotted against the observed aniline equivalents as shown in Figure 3. The points lie quite close to the curve as drawn, and almost all of the deviations appear not to be much greater than the probable experimental error. This excellent agreement with the valuable and thorough work of Garner and Evans is an indication of the reliability of determinations of knocking characteristics of hydrocarbons. LITERATURE CITED (1) Birch and Stansfield, Nature, 123,490 (1929). (2) Campbell, Lovell, and Boyd, IND. ESG.CHEM.,20, 1045 (1928). (3) Garner and Evans, J.Inst. Petroleum Tech., 18, 761 (1932). (4) Hofmann, Lang, Berlin, and Schmidt, Brennstof-Chem., 13, 161 (1932). (5) LoGell, Campbell, and Boyd; IND. ENG.CHEM.,23,26 (1931) (6) Ibid.,23, 555 (1931). (7) Nash and Howes, Nuture, 123,276 (1929). (8) Ricardo, Auto Eng., 11, 92 (1921). (9) Signaigo and Cramer, to be published. RECEIVEDApril 1, 1933. Presented before t h e Division of Petroleum Chemistry at t h e 85th Meeting of t h e American Chemical Society, Washington, D . C., March 26 t o 31, 1933.
Floc Formation Studies in Water Purification AUGUSTG. NOLTEAND WARRENA. KRAMER,Water Division, City of St. Louis, Mo.
T
H E importance of good coagulation in a water clarification process before sedimentation and filtration is well recognized. The writers have already discussed the importance of proper mixing and conditioning of the water with the coagulant and defined the optimum rates ( 8 ) . Another important phase of good coagulation is hydrogen-ion concentration. Its relation to six of the common coagulants used in water purification are discussed here. The use of aluminum salts, especially aluminum sulfate, as coagulants and their reactions have been the subject of many previous investigations. Hatfield (4)and Willcomb (If) summarize references and significant facts established by a number of investigators. The different optimum pH values of coagulation, the variations in isoelectric points reported, and the conflicting opinions given by those attempting hydrogen-ion control, indicate that there are factors to consider other than pH alone. This inconsistency was studied by Peterson and Bartow (9) and Miller (6). They showed that coagulation may be retarded and confined within narrow limits by the anions present. Bartow has given additional data on this subject ( 2 ) . It is generally recognized now that any attempt to establish optimum coagulation conditions must take into consideration the effect of the salts present in the water.
Therefore this value must be established for the particular water being treated. Iron salts have been used and studied to a lesser degree in water purification than those of aluminum. Miller (7)studied the composition of iron floc using ferric sulfate and ferric chloride and working in an atmosphere of nitrogen with oxygen excluded. This work is not comparable with plant operation. Hopkins (5) made some practical studies of ferric hydroxide floc and found that when using one and two grains per gallon of FeS04.7H20 the maximum turbidity removal in the waters tested occurred a t pH 9.5. At the Chain of Rocks plant of the St. Louis Water Division it had been found that with aluminum sulfate the best coagulation is obtained when the pH of the water is higher than generally recommended. This observation led to a laboratory investigation to determine the most desirable pH and to study the coagulation and relation to hydrogen-ion concentration of other coagulants. Laboratory and practical plant data with some of the coagulants used in these experiments are not recorded in the literature. The six coagulants used are as follows: Aluminum sulfate, the so-called crushed material, containing approximately 13.5 molecules of water.
