INDUSTRIAL A N D ENGINEERING CHEMISTRY
September, 1930
953
of Fine Division on Solubility of Cellulose i n Cold 17.5 Per C e n t a n d in Hot 7.14 Per C e n t S o d i u m Hydroxide INSOLUBILITY I N COLD SOLUBILITY SOLUBILITY 17.5% NaOH I N HOT DIFFERENCE IN COLD PULP No. ~ ~ E L L U L O S E ) 17.5% NaOH 7.14% NaOH IN SOLUBILITY REMARKS Per cer't Per cent Per cent Per cent Bleached poplar pulp No. 5 I 78.2 21.8 21.7 0.1 Table 11-Effect
High alpha poplar pulp No. 5 High alpha poplar pulp No. 5, ground Bleached pine pulp No. 6 High alpha pine pulp No. 6 High alpha pine pulp No. 6, ground Cotton Cotton, cut with shears Cotton, ground in mill
Linters Linters, cut with shears Linters, ground in mill Viscose commercial wood pulp Viscose wood pulp, ground in mill Peanut-hull pulp (J-2) Peanut pulp (J-2), ground
I1 I11 IV V
VI VI1 VI11 IX X XI XI1
89.7 89.0 83.5
92.7
XI11 XIV
92.2 98.5 98.4 97.9 99.4 99.3 99.1 86.3 86.2
XVI
98.3
xv
98.3
10.3
11.5
13.3 17.3
11.0 16.5 7.3 7.8 1.5
9.6 11.6 1.8 2.1 4.7 2.3 2.4 3.5 20.3
1.6 2.1 0.6
0.7 0.9
13.7
in Table 11, the state of division of the cellulose sample affects the solubility of cellulose in hot dilute F,odium hydroxide, if the state of subdivision is very fine. The cutting of long fiber cellulose into short lengths with ordinary shears seems to have little effect on the solubility of the cellulose sample in hot dilute alkali. Even the very fine cutting of the fibers in a grinding mill seems to have little, if any, effect on the solubility of the sample in cold concentrated alkali (alpha test). 'The solubility of the cellulose sample,
From same source as I I1 ground in mill
2.3 3.8 0.3 0.5 2.6
From same source as I V V ground in mill
1.7 1.7 2.6 *
6.6 13.0 5.4 6.4
26.8 7.1 8.1
13.8 1.7 1.7
1.2
2.3 0.8
VI1 cut with shears VI1 ground in mill Commercial linters for nitration X cut up with shears X ground in mill
XI11 ground in mill X V ground in mill
however, in hot dilute alkali (nitrator's alkali-soluble d e termination) is materially increased by a very fine state of division of the cellulose. Literature Cited (1) jonas, Z . angew. Chem., 41, 960 (1928). (2) Raimondo, N o f i s . chim. i n d . , 2 , 247 (1927). (3) Schorger, "Chemistry of Cellulose and Wood," p . 539 (1926). (4) Wiley, IND.END.CHEM.,17, 304 (1925).
Cracking of Hydrocarbons at Temperatures Higher than Critical Temperatures' Ralph H. McKee and Antoni Szayna DEPARTMENT OF CHEXICAL ENGINEERING, COLUMBIA
YORK,N. Y
A new method of studying the cracking of hydroHIS paper is B report Under these conditions carbons by following the changes in critical temperaof an investigation of cuts of different gasolines tures is described. The amounts of materials used the changes in critical boiling within the range of are of the order of 0.05 cc. per run. temperature which low-boil120"to 126' C. (248" to 259' Ordinary gasolines crack at one rate, saturated ing hydrocarbons undergo F.), together with a number hydrocarbons somewhat more rapidly, and unsatuwhen subject to cracking at of pure hydrocarbons, esperated hydrocarbons still more rapidly. temperatures higher than the cially isomeric hydrocarbons, The results obtained are in disagreement with a critical temperature. Crackh a v e been s t u d i e d . F o r common theory of knocking and with some common ing lowers t h e average comparison t o l u e n e has beliefs of the mechanism of cracking. b o i l i n g Doint. and hence been used as representing a proportToia1ly t h e c r i t i c a1 t y p i c a l aromatic hydrocartemperature (6). For example, in a Cross process run the bon. The materials used were: charging stock showed a critical temperature of 435" C. MATERIAL BOIGIND Poxm (815" F.), whereas the crude product therefrom showed a SAMPLE c. F. critical temperature, even after allowing the gases produced 1 %Octane. CRHIS 125 257 2 Octylene,' c~H;; 1 2 2 . 5 252.5 by cracking to escape, of 395" C. (743" F.) (5). 3 2,2,4-Trimethylenepentane,CsHls 99.3 211 The conditions used by the writers are rrtther closely 4 2,2,3-Trimethylbutane, C I H I ~ 80 9 177.6 5 3-Methylhexane, C7Hla 91.8 197.2 parallel to those present when modern cracking methods6 n-Heotane, C7Hm 98.4 209 7 Commercial toluene containing some benzene for example, Cross, Holmes-Manley, tube and tank, and 8 Midcontinent straight-run gasoline 120-126 248-259 Bergius-are applied commercially. The results obtained 9 Shale-oil gasoline 120-126 248-259 10 Gyro vapor-phase cracked gasoline 120-126 248-259 also have interest in that they apply directly to certain 11 High-sulfur straight-run gasoline from Warm Springs field, Wyo. 120-126 248-259 theories of the cause of knocking when gasoline is used in 12 High-sulfur straight-run gasoline from the automobile motor. For example, it has been stated (9) Poison Spider field, Wyo. 120-126 248-259 that the knocking characteristic of different hydrocarbons "diminishes with the thermal stability" and that "knocking For samples 1, 3, 4, 5, 8, and 10 we wish to thank the Ethyl is due to the thermal decomposition of the large fuel mole- Gasoline Corporation (John C. Pope). For samples 11 and cules into a number of smaller molecules with corresponding 12 we are indebted to the White Eagle Oil and Refining increase of local pressure." The results obtained by the Company (R. C. French). Kormal octylene was made present writers are not in harmony with such a theory of the from castor oil by the method previously worked out by one cause of knocking. of the writers ( 3 ) . The shale gasoline was one made in 1925 in this laboratory, and had stood on the laboratory Experimental shelves exposed to light in a half-filled bottle but was still This investigation is primarily concerned with cracking at (1930) free from color or gum separation. The toluene temperatures in the range of 405" to 420" C. (761 to 788" F.). sample was from the laboratory supply. The cracking and determination of critical pressures were Received May 5, 1930.
T
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UNIVERSITY, N E W
O
1
INDUSTRIAL A N D ENGINEERING CHEMISTRY
954
carried out in the apparatus as described by Zeitfuchs (10). Each critical temperature was determined three to five times, according to the method described by McKee and Parker (6), and the average taken as the true critical temperature. The procedure was to determine the critical temperature of the hydrocarbon enclosed in heavy-walled Pyrex glass tubes (1 mm. inner diameter), then to subject the same Bubes to a cracking temperature. The cracking temperatures and periods used were: CRACKING First Second Third
PERIOD Minutes 60 30 60
TEXPERATURE
c. 405 420 420
e
F.
761 788 788
After each cracking period the critical temperature was determined again in the original manner. Owing to slight variations in electric current, only samples of a single set cracked simultaneously and in the same experiment are strictly comparable. The results in each set of experiments are, however, quite different from those in the other two sets. These variations are greater than the variations between the individual samples in a single set and the curves can be compared with each other without appreciable error. Relation between Critical Temperature and Time of Cracking
The curves given (Figures 1 to 3) show the change of critical temperature in relation to the time of cracking, all corrected to 405" C. (761" F.). This correction is based on the supposition that an increase of temperature I I I of 10" C. doubles the amount of chemical action-that is, of cracking-thereby making it possible to plot as if 2 280 g 0 ~ a l l were a t 405" C. The writers felt t h e more certain in doing this as Leslie and Pott, hoff (4) have shown I r that, in general, cracking of h y d r o c a r b o n s E 260 does double in speed by an increase of 10" C. 2 Other i n v e s t i g a t o r s have reached the same \-a I figure and accordingly the writers have felt no hesitation in using this assumption. 240 It is to be noted that for the paraffin hydrocarbons and the straight-run gasolines 230 the change of critical temperatures indicates 0 2 4 that the cracking procHours of Cracking a t 405' C. ess is e s s e n t i a l l y a Figure 1-Saturated Hydrocarbons linear function-that is, the curves are approximately straight lines. Moreover, examination of the inclination of the ciirves reveals that normal octane, 3-methylhexane, 2,2,3-trimethylbutane and normal heptane have nearly the same rate of change-that is, rate of cracking-but 2,2,4-trimethylpentane shows a slightly greater inclination. On the other hand, straightrun midcontinent gasoline and the two straight-run sulfur " I Y
4
2
Vol. 22, 3 0 . 9
gasolines have a different inclination-that is, they cracked a t a different and slower rate and a t approximately the same rate. The gasoline fraction from the Warm Springs field showed on analysis 0.228 per cent sulfur, that from the Poison Spider field, 0.09 per cent sulfur.2 These percentages would correspond to about 2.3 and 1 per cent sulfur compounds in the material used. It will be noted from the curves that the presence of these percentages of sulfur compounds has not changed the rate of cracking from that shown by the essentially sulfur-free ma340 terial from midcontinent gasoline. A possible explanation of the differences in rate of 330 cracking (inclination of curve) between t h e s e materials and saturated pure h y d r o c a r b o n s 320 such as octane is that Li in each of these gaso- OI lines there are present stable ring compounds, 310 s u c h as methylthio- 0 phene and particularly G naphthenes. 8 300 The curves for nor- .* .* mal octylene, shale oil ii gasoline, Gyro gasoline, and toluene will be dis290 cussed later.
D z -
Relation between Cracking Properties and Knocking
280
Normal heptane has been s u g g e s t e d (1) Hours of Cracking a t 405O C. as a typical knocking Figure 2-Unsaturated Hydrocarbons a n d Gasoline8 hydrocarbon in contradistinction to 2,2,4-trimethylpentane, which is highly antiknocking. The present writers observed, however, that when these two are subjected to cracking their rates of cracking are nearly identical but that the curve for 2,2,4-trimethylpentane has a slightly greater inclination. What difference there is indicates that the highly antiknock 2,2,4-trimethylpentane undergoes cracking to a slightly greater degree, which is shown by the fact that its critical temperature decreases a little more sharply than does that of normal heptane. This phenomenon is just the reverse of what would be expected from the current theories of knocking mentioned earlier. The above slight difference can, perhaps, be explained as due either to molecular size or to the well-known fact that substances with tertiary carbon atoms exhibit less stability to ordinary chemical reactions. It is desired to emphasize that these differences are small and are only slightly greater than the limits of experimental error in the determination of the critical temperature-that is, about 2" C. The writers' observations show that thermal decomposition (cracking) is almost the same whether straight-chain or branched-paraffin hydrocarbons are being used. If any differences of rate of cracking exist, they are very small and indicate a slightly greater stability of the normal chain. It is possible that under the experimental conditions of relatively long time of heating the phenomenon of isomerization influences the behavior of these hydrocarbons. Such a 2 For these analyses we wish t o thank the Ethyl Gasoline Corporation (Graham Edgar).
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INDUSTRIAL AND ENGINEERING CHEMISTRY
September, 1930
965
Critical Temperature Data
AFTERHEATING
IHITIAL ~~
MATERIALS USED
Boiling point O
P U R E HYDROCARBONS:
2,2,3-Trimethylbutane. . . . . . . 3-IvIethylhexane. . . . . . . .... %Heptane. . . . . , . . . . . . . ... 2,2,4-Trimethylpentane. , . . , . . . ... . n-Octane . . , . . . . . . . n-Octylene . . . , . . . . . . ., Toluene containing benzene.. . , , GASOLINES: Straight-run midcontinent. . . . . . Poison Spider field, Wyo.. . . . . . , Warm Springs Eeld, W y o . . . . . Shale oil Gyro vapor phase. . . . . . . . , . . ,
c.
81 92 98.5 99.5 125 122.5
F. 177.5 197 209 211 257 252.5
Calcd. McKeeParker rule "C.
O F .
Observed C.
F.
60 min. at
405' C.
F.
OC.
30 min. a t 420' C. OC.
1
622;;lgeat
" F . ) ' C .
245 257 264 264.5 291 288.5
473 494.5 507 508 556 551.5
261 484 263 505.5 265 509 266 511 292 557.5 287.5 549.5 307 584.5
246.5 257 260 259.5 283 306 307.5
475.5 494.5 500 499 541.5 583 585.5
238.5 248.5 251.5 247 273.5 282 307
461.5 497.5 484.5 476,s 524.5 539,5 584.5
226.5 236.5 242 236 257
289 289 289 289 289
552.5 552.5 552.5 552.5 552.5
297.5 297 302 300 298.5
293.5 294 297 314.5 337
560.5 561 566.5 598 638.5
287.5 290 293 303 316
549.5 554 559.5 577.5 601
282 285.5 288 295
phenomenon of isomerization was previously studied by one of the writers (3) and by others (2, 7). When dealing with either the pure unsaturated hydrocarbons or mixed hydrocarbons from unsaturated gasolines, it is found that initial polymerization is a dominant reaction though later cracking becomes the more important. This is shown by the initial increase of the critical temperature, followed by a later consistent decrease of the same general character, but a t a different rate from that shown by the saturated hydrocarbons. Toluene, a representative of aromatic hydrocarbons, is not unsaturated in this sense. There is with it, indeed, no polymerization and, under the conditions of heating carried out, no appreciable cracking occurs. The unsaturated hydrocarbons not only polymerize and crack, but they rapidly yellow during the period of treatment. Finally they give a carbon deposit on the sides of the tube, thus rendering impossible any accurate observation of critical temperatures. On the other hand, the paraffin hydrocarbons give only traces of carbon and yellow coloration. The aromatic hydrocarbons are apparently without change of any kind, even after they have been heated to ordinary cracking temperature for several hours. When one remembers that toluene and unsaturated hydrocarbons are strongly antiknocking in character, while paraffin hydrocarbons, particularly those of the straight-chain type, show strong knocking characteristics, one sees that there is no parallelism between these properties and thermal decomposition, for the straight-chain paraffin hydrocarbons are intermediate in rate of thermal decomposition (inclination of curve), between aromatic hydrocarbons such as toluene, and unsaturated hydrocarbons such as octylene, Gyro vapor-phase gasoline and shale-oil gasoline. Stability toward Heat
Unsaturated hydrocarbons are less stable toward heat than saturated paraffin hydrocarbons, whether the saturated hydrocarbons are straight or branched chain. This lack of stability is obviously due to the double linkage, which, for example, is responsible for the tendency of these unsaturated hydrocarbons to polymerize, giving intermediate products which later crack to more stable final products. Perhaps the double bond also activates the hydrogen atoms present in the molecule. This activation is the reason that under heat the unsaturated normal octylene gives more hydrogen gas than the corresponding normal octane ( 3 ) . The double bond is a weak point,, not only toward heat, but also toward chemical reagents such as the halogens and oxygen or oxidizing agents of the common types. Pyrogenic polymerization leads to products of larger molecular weight, which are unstable even under the conditions of their formation, and which thereafter crack rapidly as shown by the high angle of inclination of the curves recording successive determina-
567.5 566.5 575.5 572 569.5
~~~
O F .
40i,5 457.5 467.5 457.5 494 5
306.5 583.5
...
539.5 546 550.6 563
...
tions of the critical temperature of the unsaturated hydrocarbons. It is well known that the heavy oils which remain after passing once through a cracking process, when recycled, are not so readily cracked as the original stock. It would appear from our work that this is not due to any change of saturated paraffins into unsaturated straight- or branchedchain hydrocarbons or into polymerized products, but rather that it must be due to a change into cyclic hydrocarbons, such as aromatic hydrocarbons or possibly hydroaromatic hydrocarbons (naphthenes). It has been previously shown that naphthenes on cracking largely lose hydrogen and go to aromatic hydrocarbons. I n the industry there is a belief which has occasionally appeared in print (8) that olefins resist cracking more than Daraffins. This is sometimes expressed by a statement such &-((All the products produced by cracking are more stable toward heat than the i n i t i a l material used as cracking stock." The writers' work shows that this is not true, for the angle of inclination of the curve for pure o l e fins and mixed hydrocarbons carrying o1efi.ns is greater than for saturated hydrocarbons of the paraffin series. Isomerization of paraffins under the influence of heat has been s t u d i e d by v a r i o u s workers (2, 3, 7 ) . It seems to occur before the decomposition of the molecules can be detected. This phenomenon of of Cracking a t 405' C. isomerization by heat Figure Hours 3-Straight-Run Gasolines a n d for Comparison n - O c t a n e raises the question as to whether many of the higher boiling hydrocarbons which have been isolated from crude petroleum were actually present in the crude petroleum itself, or whether in considerable part they have not been formed by the heating (distilling) steps which the material has undergone during the isolation of these individual hydrocarbons. A succession of perhaps fifty fractional distillations would be just the sort of condition to produce such isomeric hydrocarbons. Attention should be called to the fact that pressure increases the rate of polymerization because it brings the possible
INDUSTRIAL A N D ENGIXEERIXG CHE-MISTRY
956
reacting molecules together in close proximity. As the pressures in the writers' experiments are essentially the same as are used in the cracking processes mentioned previously, their results should be parallel to those obtained commercially. The method used requires only very small quantities of the materials. One cubic centimeter is far more than is needed for a considerable number of tests. The writers believe that this is an accurate method of studying cracking and yet it is extremely economical both of materials and time.
T'ol. 22, No. 9
Literature Cited (1) Edgar, IND. ENG.CAEY., 19, 144 (1927). ( 2 ) Hugel and Artichevitch, A n n . of. natl. combust. Jig., 3, 985 (1928). (3) Hugel and Szayna, Ibid., 1, 786, 817, 833 (1926). (4) Leslie and Potthoff, IND.ENG.CHEM.,13, 778 (1926). ( 5 ) McKee, U.S. District of Delaware, Equity Suit 571,p. 505 (June, 1926). (6) McKee and Parker, IND.END.C H E M . , 20, 1169 (1928). (7) Parks, Huffman, and Thomas, J . A m . Chem. Soc., 62, 1038 (1930). (8) Sachanen and Tilitcheyew, J . I n s f . Petroleum Tech., 14, 761 (1928); Ber., 62, 658 (1929). (9) Sokal, J . SOC.Chcm. Ind., 43,283T (1924). (10) Zeitfuchs, IND.END.C I I E X . , 13, 79 (1926).
Reactions between Iron Sulfide, Sulfur Dioxide, and Iron Oxides in the Metallurgy of Copper' A. C. Halferdahl 25 ACACIAAvE.,
OTTAWA, ONT.
Data and calculations have been presented which and has been taken as repreH E p u r p o s e of this senting the free energy of iron indicate that magnetite is readily formed in smelting paper is to indicate operations. Once formed its reduction to ferrous oxide sulfide up to 1171" C., the the probable course of melting point. The free by ferrous sulfide requires temperatures of 1300" C. or certain reactions between energy at 1171" C. is - 16,670 higher. iron sulfide, iron oxides, and calories. Values of freeenergy A modification of present methods for treating copper sulfur dioxide which are imconverter slag in reverberatory furnaces is suggested. above 1171' C. have been portant in the pyrometalestimated by the relation bP' The function of coke in pyritic and semi-pyritic coplurgy of copper and lead. = A H - TAS, where AH is per smelting is defined in a somewhat different manner Essentially the reactions to change in heat content and than hitherto. be studied involve reductions AS is change in entropy. and oxidations, and to study the reactions the free-energy values of the substances entering AH for ferrous sulfide is -23,080 calories a t 25" C. (Interinto and appearing from the reactions must be known a t tem- national Critical Tables) from iron and rhombic sulfur. peratures of interest in metallurgy, say from 600"to 1400"C. To change rhombic sulfur to gaseous sulfur, SP, at 25" C. requires 29,690 calories. Therefore, the heat absorbed in the Ferrous Sulfide formation of ferrous sulfide from gaseous sulfur and iron at Jellinek and Zakowski (16) have measured equilibria be- 25" C. would be -37,925 calories. Bornemann and Hengstenberg (9) have measured the heat tween hydrogen and hydrogen sulfide over ferrous sulfide as capacity of ferrous sulfide up to 1200" C. The heat in iron follows: sulfide at 1100" C. was 195.03 calories per gram; a t 1150" C., TEMPERATURE RATIO 203.55 calories; and a t 1200" C., 265.92 calories. The c. H2S:Hz specific heat of ferrous sulfide from 1100" to 1150" C. appears 730 0.0032 910 0.007 to be 0.1701, which is assumed to hold up to 1170" C. Then 1100 0.013 the sensible heat a t 1171' C. would be 207.1 calories without Free-energy values for the reaction fusion. Assuming molten ferrous sulfide to have a specific heat of 0.18, the sensible heat from 1171" to 1200" C. becomes FeS HZ = Fe H2S 5.2 calories, or 265.9 - 5.2 = 260.7 calories would be the heat were calculated by the equation AF = -RTlnK, where AF in ferrous sulfide after fusion, and the heat of fusion would be is the free-energy change, R is the gas constant (1.985 calo- 53.6 calories. By using specific-heat data of Eastman (5) ries), T is temperature absolute, and K is the equilibrium on sulfur [C,(S2) = 8.58 0.0003Tj and heat-capacity data constant. By combining with values of hydrogen sulfide as of Ralston ( S I ) on iron, the heat absorbed, AH, in the reaction given by Lewis and Randall (ZZ), free-energy values for Fe 1 / 2 S 2 = FeS was computed to be -35,360 calories before ferrous sulfide a t 730", 910," and 1100" C. were found to be fusion at 1171" C. The entropy change in formation would -21,170, -19,540, and -17,310 calories, respectively. be -12.9 units before fusion and -9.7 units after fusion. According to Loeb and Becker (25'). ferrous sulfide and iron The entropy of sulfur, 1/2S~,a t 1171' C. is calculated to be form a solid solution above 985" C. on the sulfide side con- 33.6 units and the entropy of iron, 20.5 units, is taken from taining about 2 per cent iron and 98 per cent ferrous sulfide, Ralston's thermal data. From these figures the entropy of and therefore the value of free energy of ferrous sulfide as ferrous sulfide was computed to be 44.5 units after fusion a t calculated a t 1100" C. cannot be considered as strictly accu- 1171" C. I n the usual manner, values of free energy of rate. However, a plot was made of the computed values for ferrous sulfide a t 1200", 1300°, and 1400" C. have been comthe three points against absolute temperature and a straight puted to be -16,230, -14,320, and -11,850 calories, reline was passed among the three points so as to come within spectively. 100 to 130 calories of any one point. The equation of this Oxides of Iron line was found to be: Equilibrium measurements in the reduction of ferrous A F = -31,820 10.49T (1) oxide and magnetite by carbon monoxide as observed by 1 Received May 24, 1930.
T
+
+
+
+
+
