September, 1928
I N D U S T R I A L A.VD ESGI-YEERISG CHEAIfISTRY
which cannot be accomplished by this method. The phosphoric acid seems to have a specific property for developing striations by loosening the layers and fibrils before the skeleton structure dissolves. Fibers treated according to this method showed that solution of the outer layer a t intervals was accompanied by rapid swelling of the inner layers outward with constrictions a t the places where the outer layer mas still intact. (Figure 10) A larger magnification of a fiber treated similarly shows that the cell-wall layers have separated in the crosswise direction. (Figure l l ) Such pictures suggested that the orientation of the fibrils in the outer and inner layers is radically different. By slowly dissolving the outer layer it was noted that striation and separation of fibrils precede the ultimate solution. The fibrils in that layer form an angle of approximately 90 degrees to the fiber’s axis. (Figure 12) With such a structure in the outer layer, it is obvious that fibers cannot swell transversely outward beyond the maximum limits which they assume in a water medium. unless the fibrils in the outer layer stretch lengthwise or break. Such a structure also accounts for delignified fibers swelling inward when the outer layer is still intact. (Figure 5) On account of the convex surface of the small bead-like swellings in Figure 10, the minute structure of the inner layers is invisible. A flatter and longer swollen surface must be examined to see the tiny fibrils in their proper orientation. By proper focusing of the microscope the orientation of the fibrils in the opposite walls of same layer can be studied. (Figures 13 and 14) If the treatment is continued further the individual fibrils are isolated as shown in Figure 15. If the minute structural arrangement of the outer layer is contrasted with that of the inner layers, it will be seen that wood fibers are designed to withstand stresses and strains both transversely and longitudinally.
945
Interesting experiments were performed with short sections of wood fibers. If such sections are treated with phosphoric acid under the proper conditions, it will be noted that the solution of the outer layer begins a t the slightly frayed ends and progresses toward the middle. By arresting the reaction as it approaches the middle before all of the outer layer is removed, a residue is obtained which consists of loosened fibril bundles bound in the spiral bands (a residue of the outer layer). (Figure 16) Such bundles are slightly “broomed” a t the ends. Isolated fibrils and fibril bundles between crossed Nicols exhibit the same property as wood fibers in that they transmit polarized light when oriented a t an angle to the axes of the crossed Nicols, but do not when they are parallel to either of the axes. (Figure 17) The bead-like swellings similar to those in Figure 10, if placed between Nicol prisms, exhibit a “dark cross” similar to the circular-like structure of the bordered pits. This separation of the cell wall of wood fiber into fibrils confirms the microscopic observations in Part 111, which suggest fibril structure, and also confirms some findings by Waentig. Acknowledgment
The writer wishes to acknowledge helpful suggestions in this study from L. F. Hawley and members of the Section of Rood Technology of the Forest Products Laboratory.
AUTHOR’SNOTE-After Part I of the present paper had been prepared, Harlow in Teclznicnl Publzcation 24, New York State College of Forestry, presented some results which differ on the lignification of the cell walls of hardwoods. The differences in results will be discussed in a future paper. P a p i e r - F a b r . , 25, 1 1 5 f192i)
Accurate Determination of Dry Substance in BeetHouse Sirup’ R. J. Brown, J. E. Sharp, and A. R. Nees THE GREATWESTERKSUGAR COMPANY, DEKVER, COLO
HE dry substance in an The detailed procedure of a n accurate method for the decomposition of o r g a n i c impure soluticln is a determination of the dry-substance content of beetmatter. That these conmatter of definition arid house sirups has been described. ditions were met is indiis considered to be that porVarious factors entering into the procedure have been cated by the results obdiscussed, and the necessity for observing the specified tained in the solubility work, tion of the sample remainwhere anv variation in the ing after it has been heated procedure has been emphasized. amount of volatile matter in some standardized manner to drive off the volatile matter. This result is not remored or in the decomposition of organic matter would necessarily an exact function of the sugar and impurities have caused decided irregularities in the curves representing present in the sirup. but is one which any analyst xould ob- the concentration of the saturated sugar solutions. In a previous paper by one of the writers2 an accurate tain provided he held strictly to the conditions imposed by method for the determination of dry substance in high-purity the method. In an investigation of the solubility of sugar in impure beet-sugar juices was given. A continuation of the investisirups. the necessity arose for finding a method for a dry sub- gation demonstrated that the method was not suitable for stance which nould give results holding more than an em- low-purity sirups. as all moisture was not removed under the pirical meaning and would fulfil the follo~ving conditions: conditions specified. The method finally developed is ap(1) The residue remaining after drying a sirup must be equal plicable to normal beet sirups of any purity and, while it is to the sum of the sucrose present plus at leas1 a constant hardly suitable for control work on account of the time repercentage of the impurities present. at all relative concen- quired, it is satisfactory in the analysis of sirups where an trations of sugar and impurities; (2) there must lie no serious accurate knowledge of the dry substance and purity is of more consequence than the time.
T I
1 Presented before the Division ofSugar Chemistry at the 75th Meeting of the hmerican Chemical Society, St. Louis, M o , April 16 to 19, 1928.
2
Brown, IND.
ENG.
CHEM , 16, i 4 6 (1924).
946
INDUSTRIAL AND ENGINEERING CHEMISTRY Method
Digest clean sea sand that will pass a 40-mesh but not a 60-mesh screen in strong hydrochloric acid, wash free from acid, dry, and ignite. Place 25 to 30 grams of the sand in an aluminum dish provided with a short stirring rod and a cover. A dish 50 mm. in diameter and 35 mm. high is very satisfactory. Stir into the sand about 0.5 gram of powdered graphite, free from oil. Dry the dish in an oven overnight, cool in a desiccator, and weigh. Weigh into a dish from a weighing buret an amount of sirup containing 0.5 to 0.7 gram of dry substance. Mix the contents of the dish thoroughly and place in a vacuum oven, which permits no appreciable air leak. Dry a t 90" C. and a t a pressure of 125 mm. or less in an atmosphere of dried carbon dioxide, feeding about 3 to 4 cubic feet (0.08 to 0.11 cubic meter), a t atmospheric pressure, of carbon dioxide per hour to the oven. Heat the samples for 72 hours or more; remove from the oven, transferring the dishes quickly to individual desiccators containing fresh phosphorous pentoxide; allow to stand in the desiccators 3 days or more; and weigh rapidly after removing the desiccator. Consideration of Individual Factors
ALUMINUM DISHES-It is advisable that the aluminum dishes be kept well polished, since it has been found that in a humid atmosphere a dish with a dull surface adsorbs water much more rapidly during weighing than one with a bright surface. USE OF SEASa~~--rlcid-washedsea sand is used in place of pumice, as i t is almost impossible to remove all moisture from the sirup absorbed in the capillaries of the pumice, while with smooth sea sand this difficulty is largely eliminated. Acid digestion is standard practice, and its beneficial effect is probably the removal of iron oxide, which may act as a catalyst in the oxidation of sugar when drying in an atmosphere of air. Working in an atmosphere of carbon dioxide, satisfactory results have been obtained using clean sea sand not acid-mashed. Powdered graphite is mixed with the sand to eliminate a variable error which may be introduced into the original weight of the dish containing sand alone owing to the accumulation of a static charge. The clean dried sand does not always carry a static charge; but when such a charge is present its partial discharge may cause the weight of the dish t o decrease as much as 1 to 2 mg. No electrical effect has been found on dishes containing sand and graphite alone. The use of graphite does not necessarily insure the absence of a static charge on the dishes containing the dried sirup, owing to the possible insulating effect of the film of dried sirup; and there is some evidence that a slight static charge may be present on the dried residues. This point is discussed later. ~ I E T H OOF D ~~EIGHIsG-lveighing from a weighing buret insures greater accuracy than is possible when the weight of the sample is calculated from the increase of weight of the dish after the sample has been placed in it. Errors due to evaporation during transference and weighing of the sample, which are a t times relatively large, are thereby eliminated. In order to reduce the time of drying, the quantity of dry substance taken should be as small as is consistent with accuracy in weighing, and the error in weighing becomes relatively large with samples much smaller than 0.5 gram. It is important that the sirup and sand be thoroughly mixed. When a sample of about 0.5 gram dry substance is taken, and provided an even film of sirup is placed on each grain of sand, the dried film is approximately 0.001 mm. thick. Increasing the thickness of this film 100 per cent decidedly increases the drying time and it is readily seen that if, through
Vol. 20, No. 9
careless mixing, the presence of relatively thick films of sirup is permitted, complete drying may become almost impossible. In some cases when the sirup is very heavy a little distilled water may be added to assist in mixing. USE OF CARBONDIOXIDE AND REDUCEDPRESSUREIt is a general experience, with beet-sugar sirups a t least, that when samples are dried for an indefinite period in air, they lose weight for a certain time and then begin to gain, presumably through oxidation of organic matter. In this work the drying was done under reduced pressure, in an atmosphere of carbon dioxide, using a Freas oven with a vacuum chamber of about 0.3 cubic foot (0.008 cubic meter) capacity. Carbon dioxide was continuously passed through the oven a t a rate of 1 to 4 cubic feet (0.0028 to 0.11 cubic meter) per hour, a t atmospheric pressure. Calculations based on the leakage of air into the evacuated oven show that the partial pressure of oxygen in the vacuum chamber during the drying period was between 0.4 and 1.5 mm., depending on the rate of flow of carbon dioxide. In this atmosphere, regardless of the time of drying, the weight of the sample was never found to increase, and moisture was given off as long as the partial pressure of water vapor in the atmosphere of the oven remained below the vapor pressure of the sample. Calculations based on the moisture in the carbon dioxide fed to the oven and the amount of air leak indicate that the vapor pressure of the atmosphere entering the oven was about 0.03 mm. The amount of gas introduced per hour depended on the type of sirups dried. With high-purity sirups, which dry in a relatively short time, a large amount of gas shortens the required time. With low-purity sirups the diffusion of moisture from the sample is so slow that the lower flow of gas gives just as rapid drying as the larger volumes. The pressure varied from 75 to 120 mm., and was of little importance in itself. The chief value of the decreased pressure appeared to be the lowering of the vapor pressure in the atmosphere of the oven and the beneficial effects of the circulation of gas resulting from expansion. I t appears that a dry gas fed to the oven a t atmospheric pressure in a volume equal to the expanded volume of the gas actually used would give satisfactory results in drying. It is doubtful if boiling of the sirup due to the reduced pressure in the vacuum oven is an important factor in drying. TENPERATURE A I ~ DDEcoawosITIox-The temperature of 90" C. was chosen, since a t lower temperatures a minimum weight was attained only very slowly. This is true of the lower purity sirups. The most desirable temperature is the highest that will not result in decomposition, as the time required for complete dr) ing decreases rapidly with increase in temperature. S o effort has been made to determine decomposition over a temperature range. The lonest temperature a t which constant weight of dry substance could be obtained in a period of time practicable in this investigation was used. Previous investigators3 have found that when drying lowpurity beet sirups large amounts of substances other than water, apparently decomposition products, may be driven off. In the present investigation no indication of appreciable decomposition has been found. The vapors driven off during drying have been passed through a train of acid and alkali washes, and only a negligible quantity of products other than carbon dioxide has been found. With acid or burned sirups the decomposition during heating might be appreciable. TIMEOF DRYIx-The time of drying by this method is far longer than that required by the usually recognized methods. Most methods are worded similarly to that given by the Association of Official Agricultural chemist^.^ It is a Sidersky, Bull assoc chzm. sucr. d i s t . , 46, 247 (1928); Gustavson and Pierce, IND. EXG CHEM, 16, 167 (1924). 4 Assocn. Official .4gr. Chem , Methods, 1925, page 178.
September, 1928
ISDUSTRIAL AND E,YGI,VEERISG CHEMISTRY
required that drying be continued until a further %hour period of drying causes a loss of weight of not over 2 mg. After most of the water has been removed, further removal of water is very slow. When a relatively large sample is used large amounts of water may remain when the rate of loss of weight is much less than that specified as the correct final weight. With a 0.5-gram sample of dry substance 0.5 mg. is equivalent to 0.1 per cent on dry substance, and when the rate of loss of n-eight drops to 1 mg. per hour there may be, and generally are in the case of low-purity sirups such as beet molasses, many milligrams of volatile matter left in the sample. The question which arises a t this point is whether or not all of this volatile matter is moisture. The answer appears to be in the folloming fact: Regardless of the time required for reaching constant weight (which depends on the size of sample) and regardless of the time of heating thereafter the amount of dry substance found is aln-ays a constant percentage of the original sample taken. These observations have been made on samples of such size that 2 to 5 days were required to reach a constant weight, and heating has been continued for as long as 5 days after this point was reached. This means either that no decomposition has occurred or that any decomposition which has occurred is a constant percentage of the sirup itself, and according to the most satisfactory definition such decomposition does not influence the value of the dry-substance result. Another proof that the result is a constant function of the sirup dried is given by the following: Varying, known amounts of pure sucrose have been added to a molasses and these sirups dried. From the dry substance obtained on these sirups the original drysubstance content of the molasses has been calculated. This has been found to be constant and equal to that obtained on the original molasses. To obtain such a result it is necessary that, not only the sirups dry a t a constant figure, but that not more than a negligible destruction of sucrose occur, and any decomposition must be due to the removal of a definite quantity of readily volatile substance. COOLIYGIN DESICCATOR-T~~ time of cooling in the desiccator is also somewhat unusual, and the reason why it is necessary is not known. It must be remembered that this method has been devised for the purpose of obtaining a high degree of accuracy in the dry substance determination. For reasons already giren the amount of sample is limited to not much over 0.5 gram of dry substance, and an error of 0.5 mg. in weight introduces an error of 0.1 per cent in dry sub6tance. Simple cooling of the dish in the desiccator does not necessarily give a constant n-eight, and the weight obtained immediately after cooling, say 3 hours. in the desiccator, is invariably higher ihan that obtained after standing 3 days or more. After 3 days in the desiccator a dish that has been heated in the oven for a sufficient time reaches a 1%-eight that is constant within the limits of accuracy of n-eighing; that is, further changes are not in one direction and the neight remains constant nithin about 1 0 . 2 mg. The drop in neight due to standing in the desiccator for 3 days is generally from 0.6 to 0.8 mg. Were this change always constant, a constant correction might be made, but this has not been established with sufficient certainty. Owing to previous experiences xith the effect of static charges on the dishes it is believed that such charges may be responsible for these changes in weight. It is possible that during the period of standing in the desiccator the charge may leak off. Attempts to demonstrate the presence of a static charge by means of an electroscope have been unsuccessful, possibly owing to lack of sensitivity of the electroscope. The ordinary methods of eliminating the effect of a static charge, such as grounding the balance or the dish before weighing, are unsuccessful in the case of a non-conductor carrying a static charge.
947
Another interesting fact is noted in this connection. If a thoroughly dried dish which has remained in the desiccator for the 3-day period is weighed, replaced in the desiccator, allowed to remain there about 3 hours and then reweighed, it is found to have gained 0.5 mg. or more. On standing 3 days in the desiccator the weight goes back to the original figure. This gain is undoubtedly due to absorption of moisture, the dried sirup being highly hygroscopic. If a dish which has been thoroughly dried and has remained in the desiccator for 3 days is weighed and replaced in the oven, remaining for a day or more, then removed and placed in the desiccator and cooled for 3 hours, the weight is invariably found to be greater than the previous weight. On standing 3 days or more in the desiccator the n-eight returns to the original. This is the evidence in favor of a static charge which has been formed due to heating and cooling. It appears improbable that the dish actually picks up moisture during the extra heating period, and loses it again in the desiccator. It has been suggested that this increase in weight and the invariable loss in weight on standing in the desiccator may be due to the possible presence of carbon dioxide in the dishes, the carbon dioxide being heavier than air and diffusing from the closed dish rather slowly. This does not seem probable in view of the fact that when the oven is opened the vacuum is broken by the admission of dried air, and during the time the dishes are being transferred to the desiccators they are exposed to the atmosphere. The amount of carbon dioxide remaining in the dishes must be decidedly small. USE O F INDIVIDUAL DESICCllTORS CONTAIKING PHOSPHOROUS PENTOXIDE-The dried material is so highly hygroscopic that it is necessary to weigh the dishes immediately after the desiccator has been opened; and when a number of dishes are contained in one desiccator, it is impossible to prevent the absorption of moisture by the dishes remaining after it has once been opened. I n certain cases, a t times of high humidity, it has been found necessary to go to the extremes of weighing one dish of a set of duplicates, calculating the percentage dry substance, and from this the approximate weight of the second of the set before actually making the weighing, to reduce the time necessary to obtain the weight of the dish. With conditions as often existent in the laboratory where this work was done-3 to 5 mm. partial pressure of water vapor in the air-such precautions are unnecessary, but when the humidity is high the dish may pick up 1 mm. or more of moisture during weighing. Such a gain is equivalent t o 0.2 per cent in total dry substance on a 0.5-gram sample. Phosphorous pentoxide was selected as the desiccating agent. Sulfuric acid proved unsatisfactory since it permitted the dried material to increase in v,eight. Accuracy of Method
Sirups were heated in the presence of excess sugar until saturated; samples of the mother liquor were removed, diluted mith weighed quantities of water, and the dry substance determined. After a further heating period the saturated sirups were sampled again and the dry substances determined on the diluted samples. The dry substances on both sets of diluted samples were calculated to percentage dry substance on original sirups. The results are as follows: SAMPLEFIRSTTEST SECOND TEST S A M P L EFIRSTTEST SECOND TEST P e r cent P e r cent P e r cent P e r cent 1 78.52 78.58 5 80.30 80.25 2 77.55 77.61 6 79.41 79.36 3 76.77 76.79 7 78.73 78.78 4 76.10 76.10
Considering the various factors entering into these determinations, such results could only be obtained by a very accurate method. The maximum spreads shown-0.06 per
INDUSTRIAL A N D ENGINEERIA7G CHEMISTRY
948
cent on s i r u p a r e such as would be produced by a variation of *0.15 mg. in the weight of the dry substance in the dried sample. In conclusion, i t should be stated that the use of this method is not advocated on products other than those encountered in standard beet-sugar processes. KO effort has been made to determine whether or not the decomposition of invert sugar would be appreciable under these conditions of drying, so the use of this method on cane products is not advised. The constituent of beet sirups which appears to cause the greatest difficulty is raffinose, and the method has not proved satisfactory when drying products containing excessive
Vol. 20, No. 9
amounts of this sugar. Owing to the extreme slowness with which water is removed from a sirup of high raffinose content, there is some danger of decomposition. Satisfactory results have been obtained on a molasses containing 11 per cent raffinose on dry substance, but not on one containing 20 per cent. In this case constant results were not obtained on duplicate tests, and there was evidence of a slight decomposition of organic matter, as well as a failure to remove all the moisture. Normal beet-house products seldom contain more than 5 per cent raffinose. The method is therefore applicable to all beet-house sirups including final molasses.
Thermal Decomposition of Ethane, Ethylene, Propane, and Propylene' F. E. Frey and David F. Smith PITTSBURGH
GOOD review of previous work on the thermal decomposition of hydrocarbons is given by W illiams-Gar d n e r S 2 The principal conclusions reached are as follows: At about 750" C. the decomposition of an olefin gives acetylene, a considerable part of which polymerizes to benzene. Williams-Gardner, on the basis of his work and previous investigations, concludes-
A
EXPERIMENT STATION, u. s. BUREAUO F MINES,PITTSBURGH, PA.
The purpose of this work was to determine the types of reaction, the nature of the products, and their relative predominance in the thermal decomposition of the simpler hydrocarbons. It was hoped that information of this nature would be of value in commercial oil-cracking operations and in many processes concerned with petroleum and natural gas. This paper is an account of some experiments involving ethane, ethylene, propane, and propylene. The effect of certain catalysts was investigated, but most of the reactions described took place in silica containers which exerted little catalytic effect, and involved only gaseous products.
t.hat the thermal decomposition of ethane' depends upon the decomposition of a paraffin t o form a lower paraffin and an olefin.*** Subsequent changes may be ascribed to the secondary decomposition of the products. Thus, so far as ethane is concerned, the methane formed from it is stable at low temperatures, but yields carbon and hydrogen a t high temperatures, while the ethylene formed shows a t first a strong tendency to polymerize and dehydrogenate, and later suffers disruption of the molecule yielding carbon, hydrogen, and some methane.
Preparation of Materials
Ethane and propane were prepared by the Grignard reaction from ethyl and isopropyl bromides, respectively. Each was purified by washing with fuming sulfuric acid and potassium hydroxide solution and by fractional distillation without, ebullition a t constant temperature and low vapor pressure. Ethylene was obtained by a similar fractional distillation of a commercial product made by dehydration of ethyl alcohol. Propylene was prepared by passing isopropyl alcohol over aluminum sulfate a t 350' C. The gas was purified by washing with potassium hydroxide solution and by fractional distillation. The product so obtained contained 1.2 per cent of propane. Electrolytic hydrogen was used.
For the determination of unsaturated constituents in ethane-ethylene and propanepropylene fractions containing very little of the paraffin, bromine waterwas substituted for sulfuric acid as absorbing agent, and a small measured volume of air was added before the absorption. The identity of the unabsorbedgaswas confirmed by combustionanalysis. The determination of constituents present in moderate amount is accurate to within 5 per rent of the amount of the constituent; for constituents present in larger amounts the determinations are more accurate, and for those present in smaller amounts they are less accurate. The error involved in certain determinations is considerable. The value for ethane in the products of decomposition of propane, and separate values for small amounts of hydrocarbons containing 3, 4, and more carbon atoms per molecule, may be in error to the extent of 20 per cent of the amount of the constituent. Catalytic Decomposition of Propane
In experiments with catalytic decomposition of propane the catalyst was contained in a vertical fused-silica tube, surrounded by a heavy copper sheath and placed in an electric furnace. Temperatures were measured with two thermocouples. After evacuation of the tube to remove the hydrogen used in reducing the catalysts, the propane, confined over mercury, was passed through a flowmeter and a U-tube filled with zinc drillings to remove mercury vapor, into the catalyst tube and then into a receiver for analysis. The pressure was maintained at very slightly above atmospheric. NICKELCATALYST-A layer of nickelous oxide 60 mm. deep in a tube 14 111111. in diameter was reduced for 8 hours Method of Analysis of Products at 330" C. with electrolytic hydrogen. Altogether, less than The analyses were made wit,h the aid of fractional distil- 500 cc. of propane were passed over the catalyst in the experilation by an adaptation of the Shepherd-Porter m e t h ~ d . ~ments recorded in Table I. I n the experiments listed in Table I two reactions are 1 Received May 11, 1928. Published by permission of the Director, probably involved, as follows: U. S. Bureau of Mines. (Not subject to copyright.) 2 8
Fuel. 4, 430 (1926) Frey and Yant, IND.ENG.CHEM.,19, 492 (1927).
C3H8
CsHs C3He
2CH4 + C + Hz + CHa + 2H1 + 2C
(1) (2)
a
