0 . .
Hydrolysis of Oat Hulls with
Hydrochloric Acid LOREN C. BRYNER, L. M. CHRISTENSEN, AND ELLIS I. FULMER Iowa State College, Ames, Iowa version of the xylose in the hydrolyzate with furfural by the procedure described by Fulmer, Christensen, Hixon, and Foster (2).
Experimental Methods The reducing sugars were determined by the Schaffer and Hartmann method (6). Bertrand ( I ) showed that xylose has practically the same reducing power toward an alkaline copper reagent as does dextrose. Hawkins (4) found that equal weights of dextrose and xylose have the same reducing power toward ferrocyanide reagents. Stiles, Peterson, and Fred (7) determined values for dextrose corresponding to the difference in titration between control and sample; these data were used in making the calculations. The reducing sugar is tabulated as grams of xylose per 100 grams of dry material. The straight cleaned whole-oat hulls were kindly furnished by The Quaker Oats Company. The digester consisted of a specially built autoclave arranged in such a manner that samples could be removed for analysis without disturbing the temperature and pressure equilibrium. All parts of the apparatus in direct contact with the reaction mixture were constructed of Pyrex glass. The essential parts of the digester are shown in Figure 1 :
HE utilization of agricultural products in the manufacture of industrial chemicals is receiving intensive study. Pentosancontaining materials form a large part of the agricultural by-products. These materials, upon hydrolysis, yield up to 40 per cent of reducing sugars, principally xylose. One of the most important industrial developments has been the hydrolytjc conversion of the pentosans of the oat hull into furfural. The Bureau of Standards (8)has developed an economical procedure for the production of crystalline xylose by the hydrolysis of cottonseed hulls. The method involves hydrolysis a t 10 pounds per square inch (0.7 kg. per sq. cm.) steam pressure with 0.32 N sulfuric acid. A possible large outlet for xylose is its conversion into industrial chemicals by various fermentation processes. For this purpose it is not necessary to crystallize out the sugar, but the fermentation will be made directly on the hydrolyzate. It is well known that calcium sulfate is deleterious in several fermentations; hence it is advisable to avoid the use of sulfuric acid as a hydrolyzing agent. Systematic studies were therefore undertaken on the hydrolysis of oat hulls with hydrochloric acid in order to determine the optimum conditions for the production of reducing sugars. The procedure developed should lead to the formation of sugar in sufficient concentration for economical handling and should leave the hull residue uncharred in case it is to be employed for further elaboration into industrial chemicals. Studies are in progress on the fermentation of the hydrolyzates and also on the con-
The reaction chamber was constructed from a 5-liter balloon flask by sealing in a small neck about 1 inch (2.5 cm.) from the
During hydrolysis of oat hulls with hydrochloric acid, the yield of reducing sugar increases to a maximum at a given pressure and concentration of acid : then the yield drops with accompanying caramelization and, in some cases, a decided odor of furfural. There is an optimum concentration of acid at each pressure. The rate of hydrolysis increases with rise in pressure to such an extent that at 100 pounds per square inch there is a 20 per cent yield of reducing sugar with water alone; under these conditions there is a definite increase in acidity during hydrolysis. At each pressure, with the ap206
FEBRUARY, 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE I. INFLUENCE OF SURFACE VOLUME RATIOON YIELDOF REDUCING SUGAR(IN GRAMS PER 100 GRAMS OF DRYHULLS) Time Min.' 0
200 G." 2O:lb 0.36 19.0 34.3
400 G.0 10:lb 0.40 18.3 32.2
30 60 75 36.0 39.0 90 3x 3 39.6 120 38.5 40.2 150 ... 180 39:o 40.6 210 39.0 240 ... 300 360 ... ~~. 510 ,.. ... 0 Grama of hulls in 4000 co. of acid. b Ratio of liquid to solid.
...
...
600 G.0 6.7:lb 0.45 18.1 30.2
,
...
700 G." 5.7:lb 0.45 6.9 16.3
800G.a
5:lb 0.40 2.7
...
... ... 31.4 ... 33.1
11.1 13.8 17.1 21.1 23.3
23.3 28.0
35 0 37.0
... ... ...
37.0
...
...
26.3 27.9 29.6 29.9
...
...
33.6
207
(1800 r. p. m,). The stirrer was constructed of a heavy glass rod and was fastened to a bronze coupling by means of a packin gland. The bronze coupling was in turn fastened to the m o d metal drive shaft by means of a small set screw. This shaft passed through a copper disk that served as a lid for the reaction flask, to prevent condensed steam from dripping down into the reaction medium. The shaft passed on through a packing gland to the outside of the digester and into the driving pulley. It was found necessary to remove all the air from the digester a t the beginning of each run, this was accomplished by allowing the stopcock a t the bottom of the digester to remain open for 3 minutes after the steam had been turned on. The arrows in the diagram indicate the flow of the steam and condensate. Constant pressure was maintained by carefully regulating the steam inlet and outlet valves and setting the pop-off gage a t the desired preexsure. The mechanical stirrer was run continuously during the entire time of a cook. The first 5 CC. of liquid drawn
r
i
OF ACIDCONCENTRATION ON RATEOF HYDROLYSIS OF OATHULLSAT 20 POUNDS PRESSURE PER SQUARE INCH TABLE 11. INFLUENCE
HCl, 0.025 N Time R-S 30 120 180 240 300 360 420 480
... ... ...
1.5 4.2 6.8 10.4 13.6 15.5 17.7 19.7
..
.. ..
(Time in minutes; R-S = reducing sugar, as xylose, per 100 grams of dry hulls) HC1, 0.03 N HCI, 0.0375 N HCI, 0.046 N HC1, 0.090 N HCI, 0.137 N Time R-S Time R- S Time R-S Time R-S Time R-S 30 90 120 150 180 240 330 480 , . .
. . ,.
2.7 12.5 14.2 17.2 18.9 23.2 26.9 31.3
..
30 80 120 160 180 260 320 390 450
3.2 15.3 20.9 26.1 27.8 30.6 33.4 35.0 35.8
..
I
30 60 105 135 165 195 240 300 360
.
large neck to allow entrance for the sampling tube and incidentally to hold it in place. The sampler was constructed of thickwalled Pyrex glass tubing; one end was sealed and a small bulb blown, in which several small holes were made to act as a screen or filter t o prevent solid materials from entering and blocking the tube. It was found necessary to cool the samples before releasing the pressure. This was accomplished by means of a small 6-inch condenser with a Pyrex glass stopcock sealed on one end and the other end sealed to a thick-walled tube, of about 1 cm. outside diameter and 2 mm. inside diameter. The purpose of this heavy tubing was to withstand the pressure of the packing gland at steam pressures to 100 pounds per square inch (7 kg. per sq. cm.) and to act as a support for the condenser. The other end of this thick walled tube was connected to the sampler by means of a short piece of rubber tubing about one inch long. A mixture of finely powdered graphite and Lubriseal served efficiently as lubricant, for the Pyrex glass stopcock. It was found necessary to lubricate the stopcock and also fasten it in place by means of two or three rubber bands before each run to prevent leaking at the high pressures. The mechanical stirrer was of the propeller type, driven (720 r. p. m.) by a 0.25-horsepower motor that ran very constantly -
11.0 24.1 32.2 36.7 36.6 37.3 37.7 38.4 39.2
..
20 40 60
17.5 33 0 36.8 38.2 38.9 40.0 40.8 40.8 40.2
80
100 140 200 240 300
15 30 45 60 90 120 240 360 480
..
..
15.6 34.5 37.0 38.4 39.9 38.8 38.8 38.7 38.6
..
HCl, 0.276 N Time R-S 7 15 23 30 45 60 90 120 180 255 330
9.4 24.9 32.2 34.8 36.6 37.6 38.9 39.0 37.6 35.3 34.3
TABLE111. OPTIMUMCONDITIONS FOR HYDROLYSIS OF OAT HULLSWITH HYDROCHLORIC ACIDAT VARIOUS PRESSURES Pressure Lb.lsq. in. (kg I s q . Atmospheria
om.)
Reducing
Tim? of Heating Min.
Sugarsa
%
240 120 90 75 60 30 Calculated as grams of xylose per 100 grams of hulls. 20 (1 14) 40 (2 8) 60 (4 2) 80 ( 5 . 6 ) 100 (7)
a
HC1 Normality 1.49 0.090 0 050 0 042 0 042 0 042
38.8 40 0 40 0 39.5 39.9 40.5
at the time of sampling were discarded to allow for unhydrolyzed material contained in the sampler and condenser. Three cubic centimeters of solution were drawn for analysis and immediately cooled to room temperature by immersion in cold water. A check run was made to determine the change in concentration and incidentally the change in volume of a dilute acid solution while enclosed in the digester under pressure in the presence of steam. The titrations indicated that there was no a preciable change in concentration or volume solution &ring the 12-hour run. However, upon crease in volume of the acid, and "-,
~~
propriate concentration of acid, the maximum yield of reducing sugar, calculated as xylose, is about 40 per cent of the dry hulls ; this is practically a quantitative yield. Under optimum conditions there is little charring of the hulls. Some volatile products (about 2.2 per cent) are lost during hydrolysis of the oat hulls. The distribution of the ash in the residue and filtrate is in good agreement with the value obtained from the original oat hulls. A fraction of the lignin (about 5.5 per cent) is lost by decomposition or solution during the hydrolysis.
Hydrolysis of Oat Hulls I _ Kc Table I gives data showing the effect of the liquid- solid"'^ c
ratio (cc. of hydrochloric acid solution per gram of hulls) upon the yield of reducing sugar (xylose) with 0.045 N hydrochloric acid at 40 pounds per square inch (2.8 kg. per sq. cm.) pressure. I n each case the dry hulls were mixed with 4 liters of acid in the 5-liter balloon flask, shaken, and immediately placed in the digester. A t various periods of time samples were taken for analysis. The data are expressed as xylose produced per 100 grams of dry hulls. Twenty minutes were allowed, after pressure was reached, for the temperature in the flask to be attained. Many experiments a t various concentrations of acid and pressures showed approximately the same lag. It is evident that the rate of hydrolysis and the total yield decreases with increasing concentration of hulls. In subsequent studies the 10 to 1 ratio employed for the mass was not so concentrated as to interfere with efficient mechanical stirring; the sampling from the large volume had little influence on yield, and calculations were simplified.
208
INDUSTRIAL AND ENGINEERING CHEMISTRY
A series of runs was made at 0, 20, 40, 60, 80, and 100 pounds per square inch (0, 1, 4, 2.8, 4.2, 5.6, and 7 kg. per sq. kg.) steam pressure with various concentrations of the acid. Typical data are given in Table 11, for 20 pounds pressure. Similar data were obtained for other pressures. Table I11 summarizes optimum conditions for the hydrolysis of oat hulls with hydrochloric acid a t various pressures.
Effect of Hydrolysis on Constituents of Oat Hulls The analytical procedures t o determine the distribution of the various components of the oat hull after hydrolysis were as follows: MOISTURE. 9 5-gram sample of ground air-dried hulls was dried for 24 hours a t 105" C. ASH. Two grams of hulls were burned slowly in a muffle furnace. After ignition of the carbonaceous matter, the crucible and contents were held at a red heat for 2 hours CRUDEFIBER. The official A. 0. A. C method was employed. ETHEREXTRACT.A 10-gram sample of oven-dried hulls was extracted for 8 hours with anhydrous ether in a Soxhlet extractor, and the extract was dried for 18 hours a t 105" C. LIGNIN. The method of Peterson. Walde, and Hixon ( 5 ) was followed. RESIDUEAND REDUCING SUGARS.One hundred gram$ of hulls were hydrolyzed with 1 liter of 0.137 N hydrochloric acid for 90 minutes at 20 pounds per square inch steam pressure. As already noted, these conditions have been found by the authors to give a practically quantitative yield of reducing sugars with a minimum charring of residue. The mixture, after hydrolysis, was filtered on a Buchner funnel and washed free from soluble materials. The residue was dried for 24 hours at 105" C. TOTAL SOLUBLES.An aliquot of the hydrolyzate was neutralized with ammonium hydroxide and concentrated slowly on the water bath to about 50 cc. The concentrate was transferred t o a weighed porcelain evaporating dish and evaporated to dryness in a vacuum desiccator; 6 weeks were required for constant weight to be reached. The amount of ammonium chloride was deducted in calculating the total solubles. The ash was also determined. The data so obtained are given in Table IV. These data show clearly that some volatile products were
lost during hydrolysis of the oat hulls. This loss was 2.2 per cent, as calculated by adding the weights of the two residues and subtracting from 100. These materials were probably volatile acids (acetic) and a small amount of furfural. Moreover, uronic acids which may have been present might have lost some carbon dioxide under the conditions of the hydrolysis.
VOL. 28, NO. 2
TABLEIV. ANALYSIS OF OAT HULLS,RESIDUE,AND FILTRATE AFTER ACID HYDROLYSIS (IN PER CENT) Oat Hulls UnhydroOven-Dry' Iyzed Basis Residue Moisture (loss at 105' C . ) 5.60 Ether extract 2.02 Ash 7 32 1b:i7 GFude fiber 36.04 68.54 Lignin 20.69 30.75 Reducing sugars (xylose) 39.10 ... Total solubles (after hydrolysis) 45.20 ... Residue (unhydrolyzed) 52.60__ Total (45.20 52.60) 97.80 99.56
...
Residue from Filtrate
...
+
Loss Due to Hydroly81s
..
..
3:73
0:io
..
0:OS
..
..
.. ..
.. ..
The distribution of the ash in the residue and filtrate is in good agreement with the value obtained from the original oat hulls. The calculation was made as follows: The residue contained 10.27 per cent ash which is equal to 5.43 per cent of the original, and the filtrate contained 3.73 per cent which is equal to 1.69 per cent of the original; the addition of these two values (5.43 1.69) gives a percentage of 7.12 as compared with 7.32 from the oat hulls. These data show that a fraction of the lignin was lost (decomposed or dissolved) during hydrolysis. Of the 20.69 per cent lignin in the oat hull, 16.20 per cent is recovered in the residue, leaving 4.49 per cent unaccounted for. The residue from the filtrate was shown t o contain 39.10 per cent reducing sugars and 1.69 per cent ash, calculated on the basis of the original oat hulls. Deducting the total of these values (40.79) from the percentage of total soluble (45.2), 4.41 per cent is unaccounted for; this figure is practically identical with that for the unrecovered lignin.
+
Literature Cited (1) Bertrand, G , Bull. soc. chim., 35, 1285 (1906).
(2) Fulmer. E . I . , Christensen, L. M., Hixon, R. M . , and Foster, R. L . , J Phys Chem., t o be published. (3) Hall, W . L., Slater, C. S., and Acree, S. F., Bur. Standards J . Research, 4, 329 (1930). (4) Hawkins, L. A . Am. J . Botany, 2 , 375 (1915). ( 5 ) Peterson, C. I. Walde, A. W., and Hixon, R. M., IND. ENO. CHEM.,Anal. Ed., 4, 216 (1932). (6) Schaffer, P. A., and Hartmann, A. F., J . B i d . Chem., 45, 365 (1920). (7) Stiles, H. R., Peterson. W. H., and Fred, E. B., J . Bact., 12, 427 (1926).
RECEIVED August 1, 1935.
MIXING RUBBER WITH COMPOUNDING MATERIALS FOR TIRE TREADS
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