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INDUSTRIAL A N D ENGINEERILVG CHEMISTRY
Vol. 19. s o . 3
Furfural Manufacture from Oat Hulls' I-A
Study of the Liquid-Solid Ratio By Harold J . Brownlee THEMINERLABORATORIES, CHICAGO, ILL.
Experiments on the effect of the liquid-solid ratio on f u r f t r a l yields indicate that a lo,a liquid-solid ratio is desirable from the standpoint o j increased yields and decreased steam consumption.
I
T HAS been the general practice in preparing furfural
from pentosan-bearing material such as oat hulls, corncobs, etc., to boil or digest the material with an excess of water or dilute acid. The pentosans are thereby converted into pentoses and the pentoses changed into furfural. The furfural, being volatile with steam, is carried over into the distillate. The distillate is then fractionated in a special column and the furfural recovered. The writer's studies in this field,have led to the conclusion that an excess of water or dilute acid is not desirable for the commercial production in the digestion part of the process. The liquid-solid ratio used in the digestion of pentosanbearing materials is one of several important factors affecting the production of furfural. A low liquid-solid ratio is desirable, but heretofore there have been few data reported on the yields of furfural obtainable from such materials when mixed with relatively small amounts of dilute acid. Nost methods given in the literature specify a n excess of liquid. Analytical methods* for its estimation use a liquid-solid ratio of about 1OO:l. For the preparation of furfural, Adams3 recommends 3.6:l. LaForge and blain^^'^ investigated this factor using corncobs and a stationary digester and recommend 4 : l . Ricard6 does not give the liquidsolid ratio, but enough liquid was used to cover the material and the ratio was probably a t least 5 : l . Pringsheim' has been able to obtain furfural from straw and wood by heating these materials and passing a current of superheated hydrochloric acid vapors over them a t 110-140" C. When an excess of liquid is used the furfural is removed from the charge by boiling and collects with water in the distillate. When the liquid-solid ratio is reduced until there is no free liquid present, the removal of furfural from the material is accomplished by blowing live steam through it or by steam distillation. The purpose of this paper is to report the results obtained by the steam distillation of oat hulls using a liquid-solid ratio of less than 1:1. For these experiments ground oat hulls were treated with dilute sulfuric acid and digested under 60 pounds pressure (153" C.) for 5 hours in a rotary digester, The acid used was equal to 2.1 per cent of the weight of the hulls used and the rate of distillation averaged about 1600 pounds of distillate per hour from 4500 pounds of hulls. Apparatus For the removal of furfural from a dampened mass of oat hulls it is essential that there be efficient stirring, and that the steam passing through the hulls be suitably distributed, Presented before the Division of Industrial 1 Received August 6, 1926. and Engineering Chemistry at the 72nd Meeting of the American Chemical Society, Philadelphia, Pa., September 5 to 11. 1926. * Assoc. Omcia1 Agr. Chem., Methods, p. 96 (1920). 8 Adams, Kamm, and Marvel, University of Illinois, Bull. 16, N O . 43 (1919). 4 THIS JOURNAL, 15, 499 (1923). 6 LaForge and Mains, I b i d . , 16, 823, 1057 (1923); 16, 366 (1924). e U. S. Patent 1,322,054 (November 18, 1919). 7 Cellulosecltemie, 2, 123 (1921).
and channeling prevented. For this purpose a rotary digester 12 feet long by 8 feet in diameter, arranged horizontally, and rotating on its longitudinal axis, is used. Through the t'runnions pass two steam lines, one for live steam going in, and the other for the removal of steam containing furfural. The inlet steam line is connected to a manifold venting into the digesters through a number of outlet pipes reaching nearly t o the lower side. The inner surface of the digester is provided with projections which serve as rakes to keep the mass well stirred. Procedure The hulls are charged into the cooker, the dilute sulfuric acid is added, the manhole closed, and the digester revolved. Steam is then admitted until the pressure gage shows 60 pounds pressure, when the outlet valve is opened sufficiently to give the desired flow of steam. The inlet and outlet steam valves are regulated to maintain the desired flow of steam and still maintain the pressure a t 60 pounds. The vapors from the digester are led to a large t'ubular condenser and the condensate is collected and measured. I n this process live steam is continuously blown through the hulls, which are well stirred, so that the furfural is removed as formed. Methods of Analysis The furfural was determined in an aliquot of the distillate by precipitating the furfural with phloroglucin in accordance with the method of the A. 0. A. C.,2 and from the weight of phloroglucide and the total volume of distillate the furfural yields were calculated. These results have been checked against furfural recovery by distillation and are slightly high, but serve very well for the purpose of comparison. All yields are based on air-dry oat hulls which usually contain about 6 per cent moisture. The results of a typical series of runs in which water is the variable are given in Table I. There is present in the hull residues left after digestion, not only some free furfural, but also pentoses and pentosans capable of forming furfural. These were determined together by the method of the A. 0. A. C.,2 and are listed in Table I as Total Furfural from Residue, and are given in per cent of the dry weight of the residue. Since several weighings of the residue, in which the water used was one-fourth the weight of hulls, showed an average loss in weight of about 25 per cent, the Total Furfural from Residue has been calculated to the original weight of the hulls, assuming this loss of material to be constant throughout the range of these experiments. These calculated values are then used fol plotting againRt the ratio of water to hulls used in the initial charge. 'I'here is some free furfural in the residue. This has not been included with the furfural yields because this furfural is recovered with considerable expense. Tests on runs other than those given have shown that the free furfural in the residue increases as the liquid-solid ratio increases, and that the per cent of free furfural, based on the dry weight of the residue. is about 1 Der cent using a liauid-solid ratio of 0.1:l I
and increases unifornily to about 2 per cent a t a liquid-solid ratio of 0.5:I. If these percentages are corrected to the original veight of t,he hulls used, the total free furfural reaches its maximum with a liquid-solid ratio in t,he charge of about 0.35 :1. Discussion The furfural yields obtained, and which are given in the table, are plotted against the wat'er-oat hull ratio. (Figure 1) The ratio of water to hulls changes during the digestion. Tlie water present increases as the result of condensation of
is dependent on the furfural concentration of the solution to be distilled and on the efficiency of the column. Figures covering this have already been given by LaForge and Mains6 and by The steam used in the digestion part of the process heats the materials to, and maintains them at, the reaction temperature a t which pentosans are converted into furfural. This steam also serves t o distil the furfural from the reaction chamber. If the steam requirements are t o be held to a minimum, it is necessary t o reduce t o a minimum the quantity of nonessential materials t o be heated to the reaction temperature
Table I-Effect of Varying Amounts of Water on Furfural Production (Charge: 4500 Ibs. hulls; 100 Ibs. 95 per c e n t HzSO4; water as indicated)
-__
RATIOW - ~ T E R
\vATER
CHARGE
Lb?.
423
I N D USTRIAI; AAVDE S G I S E E R I N G CHEJfISTRY
March, 1927
TIXEOF
GAGE
W E I G H T OF
PRESSURF: DIGESTION DISTILLATE
Lbs. Der
sq.
in.
Hours
Lbs.
&
xrOISTURE I N
RESIDUE
.
~
~
~
Per cent
Per cent
steam and the liberation of water in the reaction. At the same time the 4 d iiinterial in the charge decrewes because of the transformation of the pentosans into furfural and water. Since the curves do not change in their general direction, but are only shifted slightly to the right by taking these changes into account, the furfural yields are plotted against the ratio of water to hulls used in the initial charge. The moisture content of the hull residues is also given in the table, and from these figures the maximum moisture obtained in the charge during the experiments c:tn be ascertained. It is to be noted from Figure 1 that the furfural yields in the distillate reach their maximum with a liquid-solid ratio in the initial charge of 0.27:l. These values decline as the liquid-solid ratio is either increased or decreased. The total furfural from the residue increases as the liquid-solid ratio increases, throughout the range used in ihese experiments. Tlie total furfural in the system (curve 111) iiicreases irom 10.8 per cent to 12.6 per cent as the liquid-solid ratio iiicreases from 0.1:l to 0.35:l and then becomes constant or possibly declines as the water in the charge is Increased. Since the theoretical yield of furfural from oat hulls is in the vicinity of 20 per cent, the furfural losses incurred in processing are represented by the difference between this value and those represented by curve 111. Under the conditions used, it will be noted that the maximum yield i< obtained when the ratio of water t o hulls in the initial charge is about 0.27:l. It \\-ill be noted also that the total furfural in the system reaches its maximum near this same ratio. Since further increase in this water-hull ratio does not increase the total furfural in the system, we can hardly expect, by increasing the moisture further, to increase the furfural yields, unless the furfural losses can in some way be reduced. These losses are avoided to some extent in laboratory methods but are difficult to overcome in large-scale manufactwe. Steam Requirements
The steam expenditure is also an important factor in furfural manufacture. It is largely dependent on the yields of furfural obtained, the liquid-solid ratio used in the digestion, the concentration of the distillate in furfural. and on the efficiency of the fractionating column used in recovering the furfural. The steam necessary for the distillation part of the process
~ FD R O M,
AS found
CHARGE
FURFUR.II, FURFURAL
FURFURAL
~
INITIAL
YIELD
RESIDUE
org:&f'&.
COKCK.
OF
IN
DISTILLATE DISTILLATE
Pev cent
Per cent
Pry cent
and to increase the furfural content of the digester distillate to the maximum consistent with good yields. The results of these experiments indicate that water in the charge equal to one-fourth to one-third the hulls used is ample to give good yields of furfural, and also that water in excess of this amount is objectionable since more steam is required to operate the process and no increase in yield is obtained to compensate for this extra steam. It is obvious that any excess water I d 1 require steam to heat it to the reaction temperature
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1
1
I
13
4
4
1
.
I
1
Curve?-,
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I
INDUSTRIAL A N D ENGINEERING CHEMISTRY
424
of 1 per cent fudural content requires approximately 100 pounds of steam for each pound of furfural distilled, and a distillate of 5 per cent furfural content requires approximately 20 pounds of steam to accomplish the same result. Since the data show that the furfural content of the distillate becomes less as the water in the charge is increased, it is apparent that the steam requirements increase along with the liquidsolid ratio. The steam required for the digestion in experiment 634 can be readily estimated. To heat the charge and digester from room temperature to the reaction temperature will require approximately 825 pounds of steam. On a 10 per cent yield basis this is equivalent to about 1.8 pounds of steam per pound of furfural. With a distillate of 5.8 per cent furfural content, approximately 17 pounds of steam per pound of furfural will be required to distil the furfural from the reaction chamber. With ample allowances for heat losses and losses in furfural by distillation, the steam required per pound of furfural will be well below 25 pounds of steam for the digestion part of the process. This estimate has been verified from steam meter readings over long periods on a manufacturing basis. I n processes for making furfural which have used enough water or dilute acid to cover the material,6j8 or a liquid-solid ratio of 4 : l to 5:1, the steam expenditure in
Vol. 19, No. 3
the digestion part of the process has been from two to three times this amount. It is of interest to point out in this connection that the minimum steam requirements in the analytical methods, in which theoretical yields are obtained and in which the liquid-solid ratio is more than 100:1, amount t o 2500 to 3500 pounds for each pound of furfural obtained in the distillate. Within the digester occur the conversion of pentosans into furfural and the distillation of this furfural. The coordination of these two steps or stages so that the furfural is removed soon after its formation is essential to good yields. Continuous steam distillation combined with stirring, in order to mix thoroughly the steam current and the reacting materials, assist in the efficient removal of the furfural. The kind and quantity of acid, as well as the temperature employed, affect the conversion. The liquid-solid ratio used appears of lesser importance for the conversion, but does have a decided bearing on the amount of steam required for the removal of the furfural. Water equal to one-third to onefourth of the oat hulls is ample for the conversion and at the same time enables the furfural formed to be removed with the minimum of steam, and herein lies one of the chief advantages in the use of a low liquid-solid ratio for furfural manufacture.
A Guide for Cutting Paper Strips for the Gutzeit Determination of Arsenic’ By E. L. Green THE STATE COLLEGE OF WASHINGTON, PULLMAN, WASH.
I
N MAKIKG arsenic determinations by the Gutzeit-
Sanger-Black procedure2 the test paper is to be cut into strips exactly 2.5 mm. wide and approximately 12 cm. long. It has been found important that the strips be uniform in width, that the tubes into which they fit be of very nearly the same internal diameter (3.0 mm.), and that the strips fit fairly closely into them if other dimensions are chosen. The effect of variation is to alter the length and color of the stain obtained from a given quantity of arsenic. Unexpected difficulties were encountered in the attempt to provide strips accurately cut to these specifications. I n the effort to solve these recourse was had to the device described below. A print trimmer or trimming board with a knife a t least 15 cm. long will make satisfactory cuts, but the operation is tedious and not accurate if the individual widths must be laid off on the paper by hand. The accompanying figure represents a guide which makes this unnecessary. A A is the stationary knife of a trimming board represented as with the moving knife and most of the board broken away. B is a planed block of wood about 1.5 cm. thicker than the height of the board from the table top and 18 cm. long. On a line CC, a t exactly the same distance from the table top as the stationary knife, and 15 em. apart, two small wood screws are driven. Their heads are left projecting 2.5 mm. In use the guide is moved along the table top with the right hand until the screw heads touch the stationary knife. Then the paper to be cut, a strip 12 cm. wide, is moved over the trimming board with the left hand until it touches the wooden surface of the guide between the two screws. A strip of paper 2.5 mm. wide and 12 cm. long now projects over the edge of 2
Received October 15, 1926. Assoc. O5cial Agr. Chem., Methods, p. 171 (19253.
the stationary knife, and if the paper is held in this position until the right hand moves the guide a few centimeters out of the way and brings down the moving knife, this piece will be cut off. These operations can be repeated indefinitely without strain to the eyes, and the movements of the guide upon the table cause the cut strips to collect under the trimmer out of the way.
The v-idth of the strip can be easily changed by driving the screws in or out. Unless very securely fastened together, two or more sheets of paper cannot be cut with this arrangement to the desired accuracy. The accuracy of the operation is not less than 0.05 mm. and the exact width of the strips being cut is conveniently determined by counting the strips cut from a measured length of paper.
