Wood Hydrolysis A CONTINUOUS PROCESS Raphael Katzen and
Donald F.
Polytechnic Institute, Brooklyn,
Othmer
N. Y.
I n the past all commercial wood hydrolysis processes have been set up for the production of wood sugars and alcohol. Sugars obtained by hydrolysis of coniferous woods are glucose, mannose, and pentoses; those from deciduous woods are glucose and pentoses. Of these, only the hexoses are readily fermentable to alcohol (fonowing inversion in the presence of dilute acid). The yield claimed is 42 per cent of 95 per cent alcohol, based on reducing wood sugars fermented, compared to 48 per cent when cane sugar is used. Pure crystalline xylose (a pentose) may be obtained by preliminary hydrolysis of the wood before it is passed into a concentrated-acid-hydrolysis diffusion battery. This first hydrolysis product is probably derived from the hydrocellulose which forms during the rapid preliminary hydrolysis (2, 6, 1 1 , 26, SS, 58) in dilute as well as in concentrated acid hydrolysis. Although many have stated that it is impossible t o ferment pentoses such as xylose to alcohol, Virtanen claims that B. coli (40)gives a 23 per cent yield on the basis of xylose fermented.
T
he subject of w o o d hydrolysis i s reviewed. The n e w continuous process for the hydrolysis of w o o d and other plant Fibers w i t h dilute acid is described. It produces lignin and lignocellulose for plastics as w e l l as acetic and formic acids, furfural, and w o o d sugars. This process eliminates to a great extent the major problems encountered in batch processes. The process i s applicable to w o o d flour, sawdust, or small chips. These materials are suspended in dilute acid. The suspension i s pumped continuously under pressure into a double-pipe heat interchanger for preheating, hydrolysis, and cooling, and then discharged into a receiver at atmospheric pressure. Filtration, washing, drying, and grinding steps f o l l o w standard practice. In the continuous process the degree of hydrolysis can be controlled, depending upon the use to which the hydrolyzed lignocellulose is t o be put. The major factors governing the degree of hydrolysis for a given species of w o o d or plant Fiber are pressure, temperature, acid concentration (or pH), and time. The hydrolysis liquors, which contain acetic and formic acids, furfural, and w o o d susars, in addition to the sulfuric acid originally used, may be recycled for re-use in hydrolysis. Or i f subjected to rehydrolysis, some of the sugars break d o w n t o give additional yields of organic acids and furfural. These by-products are obtained in high yields and are readily and economically recovered by extraction and distillation. Major advantages of the continuous process include accelerated hydrolysis, better control of the operating variables, l o w steam costs, and l o w equipment and operating costs,
DILUTE ACID PROCESSES
I n dilute acid hydrolysis the wood sugars formed are already inverted, as the result of secondary hydrolysis reaction. Therefore the wood sugar wort obtained is immediately fermentable, with no other treatment aside from the usual liming to neutralize the acids present (3s). It has been found more difficult to obtain satisfactory yields by dilute than by concentrated acid processes, but this difficulty has been overcome to a great extent by recent improvement in methods of dilute acid hydrolysis. Usually the wood sugar solutions are too dilute t o warrant concentration and recovery of wood sugars as such; therefore fermentation to produce alcohol is usually resorted to. Ewen and Tomlinson set up the first commercial unit for dilute sulfuric acid hydrolysis of wood waste in Georgetown, S. C., in 1910; but their process soon failed because of the low alcohol yield (only 14 gallons per ton of dry wood substance). The same was true of the Claasen process, utilizing mixtures of dilute hydrochloric, sulfuric, and sulfurous acid, as operated at Mannheim, Germany, in 1916 (2, 33). Scholler, Luers, and associates increased yields and found that the decomposition of sugars takes place simultaneously with their formation during wood hydrolysis. Thiersch (2%’) formulated equations showing that the amount of sugar in solution a t a given time was a function of both hydrolysis and decomposition reactions. Later work by Luers ($8) showed that the reaction constant for the formation of sugars is ten times as great as the constant for the decomposition reaction. Bergstr@mand Cederquist (3) claimed a maximum sugar yield in 5 minutes compared to 2 hours as calculated by Thiersch’s equations. Other factors, such as sugar concentration and acidity of the solution, affect the reaction rates (2, 6, I S , 3 7 ) and reduce the applicability of the Thiersch equations. At the Forest Products Laboratory
WOOD
hydrolysis processes (e. g., hydrolysis of cellulose in lignocellulosic substances) fall into two classifications -concentrated acid hydrolysis and dilute acid hydrolysis. Concentrated acids, such as hydrochloric (2, 9), sulfuric (27), hydrofluoric @9), and phosphoric (S@, have been suggested; but only hydrochloric acid has attained commercial significance (8,while concentrated sulfuric acid has become the standard analytical reagent for the determination of lignin ($1 1* Dilute acids, such as sulfuric (8, 6,16,16, I 7 , 10, 2%’,2S,$6, SS), sulfurous (41), hydrochloric ( I S ) , and various combinations of the foregoing have also been used; but the dilute sulfuric acid method alone has been developed and maintained on an industrial basis by Scholler (33). It is only in Germany that wood hydrolysis processes are in commercial operation, and there only because they are subsidized by the government. These are the Bergius and the Scholler processes. While these processes and the patents covering them have been licensed in this country, they cannot compete with other methods of producing sugars and alcohol, their main products. ”
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Sherrard and co-workers, in a n attempt to improve the yields by dilute acid hydrolysis, obtained 20-26 gallons per ton of softwood and 6-14 gallons per ton of hardwood (IO,36,87). The low figures for hardwood are due to the fact that only 26-47 per cent of the sugars obtained from hardwood hydrolysis are fermentable to alcohol, while softwood sugars are convertible to the extent of 66-72 per cent. Junien also developed a dilute sulfuric acid hydrolysis process; his major contention was that oxygen is deleterious to wood sugar formation and must be removed by evacuation before hydrolysis (16,17). He claimed a continuous process of hydrolysis with no appreciable lapse of time between successive batches, but his own description shows that it was intermittent. Other improvements in dilute acid hydrolysis are advocated, such as a rehydrolysis of the wort obtained to increase the amount of fermentable sugars (1, 7, 8). This accomplishes the same purpose as recycling the hydrolysis liquors which will be discussed later. SCHOLLER AND BERGIUS PROCESSES
DILUTEACID PROCESS. By far the most important of the dilute acid processes is that of Scholler, who largely eliminated the sugar decomposition reaction by removing the sugars almost as soon as they were formed. Subsequent cooling of the sugar wort eventually stopped decomposition. I n his process practically any wood substance (green, air-dried, or kiln-dried) may be used. I n the form of chips it is placed in large cylindrical percolators and compressed by successive steam shocks (33). After this packing process, hot dilute sulfuric acid solutions, ranging from about 0.4 to 1.2 per cent acid concentration a t 170" to 180" C., are passed through the wood chips under approximately 150 pounds per square inch pressure. This continuous percolation, combined with a fluctuating pressure (34) which breaks up the fibers and expels the sugars, succeeds in removing the sugars from the scene of the rea c t i o n as t h e y a r e formed; this allows further hydrolysis without appreciable attendant decomposition of the w o o d s u g a r s (88). Channeling takes place, however, and part of the wood is incompletely hydrolyzed. The large amount of acid solution required (ten to fifteen times the weight of wood hydrolyzed) results in dilute sugar solutions
Figure
315
(only 2 to 4 per cent mood sugar), and these are adaptable only to fermentation. A yield of 60 to 80 per cent of the theoretical value is obtained, which amounts t o 48 t o 55 gallons of alcohol per ton of dry wood substance (33). The volumetric efficiency is low (16) since hydrolysis takes 12 hours, and a t least 8 hours more are required for loading and unloading.
CONCENTRATED ACID PROCESS.The only process which warrants review is that of Bergius. Its first requisite is kilndried wood containing less than 1 per cent moisture in order to prevent dilution of the hydrolyzing agent, 40 per cent hydrochloric acid (2). Hydrolysis takes place at room temperature in a series of countercurrent diffusion vessels. The cellulose in the wood is almost completely dissolved, about 66 per cent of dry wood substance (d. w. s . ) ; the dissolved cellulose is converted to mixed sugars. A ratio of seven parts of acid to one of wood is required; the eight countercurrent cells allow sugars t o be built up t o 40 per cent concentration in the acid solution. Special methods have been developed for removal of hydrochloric acid from the wood sugars, which are concentrated to give solid carbohydrates for animal fodder. The acid is then reconcentrated for re-use in the process. All apparatus must be designed to resist the corrosive action of concentrated hydrochloric acid. ConfPARrsoN. The advantages of the Scholler process are as follows: 1. The dilute acid solutions require no special equipment, aside from the use of standard alloys, such as Herculoy and Everdur. 2. Practically any type of wood containing any amount of moisture can be used. 3. No acid recovery is necessary, as the cost of the small amount of sulfuric acid used is negligible.
The d i s a d v a n t a g e s are : 1. High pressure and temperaturerequireheavy equipment. 2. The intermittent method of operation makes the process unwieldy.
I . Flow Sheet of Continuous Hydrolysis Process
W o o d supply A i s reduced to l / r l / s inch chips in chipper B and then conveyed, C, to chip storage bin D. A small tank, E, is also required for intermediate storage and metering of sulfuric acid. W o o d chips, water, and sulfuric acid are mixed in w o o d tanks F equipped with agitators. From the tanks, the suspension is passed through pumps G to hydrolysis tubes H. A surge chamber, J, tends to equalize pressure variations caused by the action of the pumps, A release valve, K, i s set so as to release hydrolyzed suspension from the tubes a t a Fixed rate. After release to atmospheric pressure, the suspension passes to Filter tanks L equipped with False bottoms to permit drainage of liquors. Part OF the liquor i s recycled For re-use in hydrolysis by pump M.
Part may be taken off for by-product recovery, X. Washing i s also carried out in these tanks, as w e l l as p H adiustment. Final dewatering i s accomplished by passing a suspension OF the washed lignocellulose to centrifuge N. Water i s drained off to waste, 0, From the centrifuge. CentriFuged Filter cake i s passed to rotary dryer P, heated by steam or hot Flue gases From the boilers. Dried lignocellulose i s passed by conveyor Q to grinder R, thence by another conveyor, S, to shaking screen T. Oversize is returned to the grinder, while ground lignocellulose i s passed to storage bins U. The Finished product is weighed, V, and packed in waterprooF sacks or drums for shipment or transfer to compounding operations, W.
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3. The dilute sugar solutions obtained are limited t o fermentation processes for alcohol recovery and are not economical for that purpose without government subsidy.
The disadvantages of the Bergius process are complementary to the advantages of the Scholler process, and vice versa: 1. .A highly concentrated and highly corrosive acid is required, necessit>atingspecial equipment. 2. Kiln-dried wood chips must he used as ra\T material, which increases fuel costs and creates a fire hazard. 3. A complicated system of diffusion cells and an expensive acid-recovery system are required Xvith attendant complexity of valves and piping.
On the other hand, there are the lollowing advantages: 1. The hvdrolvsis takes dace at room temperature and atmospheric pGessuFe. 2. Hvdrolvsis of the cellulose is almost complete, and sugar yields are maximum. 3. A highly concentrated wood sugar solution results which is readily adaptable to either fermentation processes, fractional crystallization to obtain various pure sugars, or direct use as animal fodder. 4. Other by-products, such as acetic acid, are readily concentrated and recovered.
The hydrolysis products from hardwoods and softwoods in either dilute or concentrated acid processes differ to a marked degree. Most data seem t o indicate that a greater proportion of the hardwood charged can be hydrolyzed than of the softwood. The Bergius process produces 65 per cent wood sugars from pine and 68 per cent from beech; but as a greater proportion of softaood sugars are fermentable, they have been utilized for alcohol production. Scholler obtained 40 per cent fermentable and 10 per cent unfermentable sugars from softwoods, as compared to 35 and 20 per cent, respectively, from hardwoods (33). At present the lignin residue from both processes is regarded in this country merely as a waste, useful only for its fuel value. CHEMICAL PRODUCTS OTHER THAN SUGARS AND ALCOHOL ORGAXICACIDS. A11 wood hydrolysis processes yield a
certain amount of organic acids, such as formic, acetic, propionic, etc., along with the wood sugars produced (91). Bergius claimed for his process a yield of these acids (calculated as acetic) equal to the yield obtained by wood carbonizationi. e., about 4 per cent of the d. IT. s. (a). The figure varies
Automatic Release Valve
(%)
Figure 2.
CELLULOSE
(Cz)
Hydrolysis Curve
from 3 per cent yield from pine up to 5 per cent from beech. This acid is recovered in a 4.5 per cent water solution during the reconcentration of hydrochloric acid and is neutralized with lime t o form calcium acetate, although it could a150 be put through a direct acetic acid process to obtain glacial acetic. Farber found that this acetic acid can be recycled with the hydrochloric acid until a 12 per cent concentration of acetic acid is reached. This 12 per cent acetic acid may replace 3 per cent of the hydrochloric and thus reduce the concentration of hydrochloric acid used from 40 to 37 per cent. When acetic goes above 12 per cent concentration, the excess is distilled off and recovered ad described above (6). Junien cited the same yields in his process of dilute sulfuric hydrolysis (15, 16). Cross obtained a yield of only 1.2 to 2.8 per cent acids from fir, the ratio of acetic to formic being 4 to 1 (5). TOdecrease the cost of acetic acid recovery, it would be desirable for the concentration to be as high as possible. This may be accomplished by recycling the hydrolysis liquors until the required acid concentration is reached, then drawing off only part of the liquor for recovery, returning the rest to the system, with make-up water and acid catalyst. As a matter of fact, this recycling of acetic acid would have several desirable effects. Schwalbe (35) found that acetic acid in conjunction with inorganic acids for hydrolysis actually increases the yield of glucose more than 20 per cent. I n this respect acetic acid acts as a catalyst or “freshening agent” by causing the cellulose in wood to stvell and absorb hydrolysis liquors more rapidly. Schwalbe stated that acetic acid alone will not hydrolyze wood, but he is evidently referring to concentrated acid hydrolysis; therate of reaction is afunctionof hydrogenion concentration (IS). This is borne out by work a t the Forest Products Laboratory in which hydrolysis was first started
INDUSTRIAL A N D ENGINEERING CHEMISTRY
March, 1942 Figure
Figure
3.
Lignocellulose Yields
4. Relation of Soluble Lignin to Degree of Hydrolysis
with water alone and then accelerated by the acetic acid formed. Mirlis and Gorokhohnskaya (26) found the same to hold true in the hydrolysis of aspen. ,;&Wood sugars may be fermented via alcohol to acetic acidfor example, with molasses in the quick vinegar process (28). A 15.2 per cent dextrose solution containing 2 per cent barley sprouts yields 6.61 per cent alcohol, and a 6.5 per cent acetic acid solution is obtained from this alcohol (4). Charcoal as the fermenting surface instead of beechwood chips gives more rapid fermentation, and the yield of acetic acid is as much as 40 per cent higher (14). Wood sugar wort has been fermented directly t o acetic acid; the same has been done with pure glucose, with a 14.1 per cent acetic acid yield, along with 17 per cent formic, 12.8 per cent lactic, 25.1 per cent butyric acids, and 2.1 per cent ethyl alcohol (29). The latter figures show poor control of fermentation, for if the proper bacteria are used, acetic acid can be obtained as the major product. Others have also produced lactic acid, propionic acid, citric acid, butyl alcohol, and acetone (2, 20, 53) by bacterial fermentation of wood sugars. This variety of products denotes great flexibility in processing sugars obtained from wood hydrolysis and to a great extent solves the problem of so-called nonfermentable sugars, since they do not give alcohol by fermentation. These are especially important when hardwoods are hydrolyzed. FURFURAL. Evidence accumulated by many investigators seems to point to the fact that furfural is a major product in the first part of the hydrolysis process (11, 16). Hemicelluloses form and hydrolyze further t o pentosans which, in turn, yield furfural under heat and pressure (11, 56, 58). I n fact, it is possible to perform a preliminary hydrolysis a t reduced temperature and acid concentration to produce furfural and
317
then follow with another hydrolysis under more drastic conditions to produce fermentable wood sugars. Junien’s process follows this method to some extent and a yield of 3 per cent furfural on the basis of d. w. s. is obtained (16,16). Recycling the hydrolysis liquors in a continuous system tends t o build up the furfural concentration and probably increases the yield, as a result of further hydrolysis of the pentosans recycled (11). The furfural formed, along with some methylfurfural, can be removed by flash distillation as it leaves the hydrolyzer or by steam distillation a t another point in the process. As these are inhibitors for alcohol fermentation, their removal is usually necessary (24). MISCELLANEOUS. Low-boiling products, such as methanol, methyl acetate, and acetone, are formed during wood hydrolysis. They are derived from lignin (21, 55) and are produced only to the extent of about 1 per cent of the d. w. s. (15, 16). Though the amounts are small, these low-boiling compounds can easily be recovered from a direct acetic process (18). Scholler found that wood sugar mort may be used for growing fodder yeast. This method utilizes all wood sugars, including pentoses, without putting them through complicated processes. The yeast is filtered, pressed, and sold directly as fodder. Recently Peterson developed a use for wood sugar wort as it comes from the hydrolysis process without fermentation or distillation. He found that 2 to 5 per cent wood sugar wort from wood such as beech and hemlock can be used as a preservative for alfalfa and clover in the form of silage. Prevention of bacterial decomposition of the silage and improvement of its nutrient value result (SO). CONTINUOUS PROCESS
Since the operation of a wood hydrolysis process for production of sugars or alcohol appeared to be uneconomic in this country, the continuous wood hydrolysis process described here was developed primarily for the production of lignocellulose residues suitable for use in plastic molding compounds. It has been found that, by hydrolyzing part of
Suspension Pumps
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INDUSTRIAL AND ENGINEERING CHEMISTRY
the cellulose in lignocellulosicmaterials, a semiplastic filler is obtained which is suitable for use with thermosetting resins. By varying the degree of hydrolysis, lignocelluloseresidues are obtained which yield molded products with varying degrees of plasticity, strength, moisture, absorption, etc. The degree of hydrolysis required is determined by the actual application of the finished product. The effects of varying the operating factors in the continuous process on the degree of
mately 80 feet. Jacket connections are arranged (a) so that all of the tube banks may be heated with high-pressure steam, ( b ) the final tube bank may be cooled by circulating liquid, and ( c ) the initial tube bank may be preheated by this same circulating liquid after it leaves the cooling section. A specially designed solenoid valve a t the discharge of the tubular system is automatically controlled so as to open several times per minute and for a very short uniform period each time. As the hydrolyzed suspension is released by the valve, it passes to a receiver barrel a t atmospheric pressure, the latter being vented through a condenser to permit recovery of flash vapors. A compression chamber smooths out pressure differences and allows the use of positive displacement pumps. 9311
0
I (%) SULFURIC
Figure 5.
2
I
I
I
I
I
I
I
3
ACID
Effect of Variation in Acid Catalyst 50
hydrolysis are great and will be shown by a large number of test runs under different conditions. While the primary object of this process is to obtain a lignin product which is usually discarded, it is also desirable to recover those products hitherto obtained, in so far as possible. These secondary products will be mentioned but fuller discussion will be left for a later paper. After all the factors of the dilute and concentrated acid hydrolyses had been considered, we decided that dilute sulfuric acid offered the greater prospect of success. Hardwoods give the best residues for plastic molding purposes; and because of the large number of variables involved, this study was confined t o a representative hardwood, maple. Although considerable effort was expended a t first on pilot plant development and operation of a batch hydrolysis system, the continuous process seemed t o have so many advantages that the batch system was discarded. The work with the continuous system may be divided into two parts-to overcome the difficulties it presents and to study the individual effect of each of the variables. None of the preliminary equipment or experiments will be discussed. M E T H O D AND EQUIPMENT
Wood sawdust is mixed with dilute sulfuric acid (0.4 t o 3.0 per cent concentration) in the ratio of 8 t o 10 parts by weight of dilute acid to 1 part of dry wood substance. This ratio is selected so that the suspension is fluid enough to pass without plugging and without the use of screws, agitators, or stirrers, through the pumps, tubes, and release valve. This suspension is pumped from mixing tanks by two rotary positive displacement pumps, arranged in series to provide pressures from 200 to 400 pounds per square inch. From the high-pressure pump the suspension passes directly into the tubular hydrolysis unit. This unit is constructed of 10-foot lengths of 1-inch-diameter, extra-heavy pipe size Herculoy tubing, jacketed with 2-inch extra-heavy steel pipe. These straight sections are connected with longsweep U-bends, having a 12-inch radius and made of the same tubing as the straight sections but not jacketed. A tube bank consists of two straight sections and two U-bends with total running length of approximately 25 feet. Four tube banks, with a total length of about 100 feet, are usually used. This gives a net double-tube heat interchanger length of approxi-
340
225 250 PRESSURE (#/in?)
Figure 6.
275
Effect of Pressure Variation
With steam a t 300 to 500 pounds per square inch in all heating jackets, this pilot unit (Figure 1) can produce 800 to 1000 pounds of 50 per cent hydrolyzed product in 24 hours. Production will vary with the degree of hydrolysis required in approximately inverse ratio. At high degrees of hydrolysis, production drops off rapidly, not only because of the increased time of treatment in the system but also because of the greatly decreased yields based on the wood being processed. CORRELATION
OF EXPERIMENTAL D A T A
Since only the cellulosic fraction of the lignocellutosic material is hydrolyzed to water-soluble products (the lignin remaining almost completely water and acid insoluble), the analysis of the hydrolyzed residue may be used as a measure of the extent of the reaction. To define the degree of hydrolysis in terms of the total cellulose content of the hydrolyzed product and so put all of the hydrolysis runs on a common basis, a graphical correlation was developed. I n this method which follows, cellulose is taken to designate all carbohydrate matter present in the fibers; analysis was by standard procedures. I n the following equations, percentages refer to d. w. s.: Let
If If
H = degree of hydrolysis = % of cellulose hydrolyzed CI = cellulose content of wood used (after extraction of oils and resins) L1 = total lignin content of wood used (after extraction of oils and resins) CZ = cellulose content of hydrolyzed product La = total lignin content of hydrolyzed product H = 0: Ci/'Li = C2/La H = 1: CI = 0
With this inverse relationship as the definition of H , the following equations can be set up:
March, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
lignin. The soluble lignin ratio (SLR) designates the ratio of the methanol-soluble lignin to the total lignin present in the hydrolyzed lignocellulose. Thus, from the H and SLR values given in the tables it is possible to calculate the soluble lignin, insoluble lignin, and cellulose or carbohydrate material in the hydrolyzed lighocellulose. It is obvious that SLR is dependent on H , and as a primary basis for study, SLR is plotted against H in Figure 4. Data for a number of continuous hydrolysis runs are included, and no curve was drawn through these points as too many uncontrolled variables were involved. However, there is a definite upward trend of SLR as H increases. These data were plotted without reference to hydrolysis conditions except that sulfuric acid was the only hydrolyzing agent used. Evi-
or or
I n the case of maple, C, = 75 per cent and LI = 25 per cent after oily and resinous materials have been extracted; then
But in terms of per cent, La = 100 - CZ;then
TABLEI. VARIATIONOF SLR AT CONSTANT H RunNo. 361-'A 361-B 338 302 327 349-B 374-B 362-A 363-B 359-A 350 348-B 345 339 366 374-A 372-B 285 367-B 369-A 334 366-B 378-B 331 332 328 364 373-B 326 346 380-B 378-A 377-B 348-B
Formula 1 is the basis of the curve of Figure 2 for maple but will hold for most similar hardwoods. All further data concerning H is determined from this curve. The theoretical yields of hydrolyzed lignocellulose from maple for varying degrees of hydrolysis were calculated by Equation 1and are indicated by the upper curve of Figure 3. The lower curve is the mean of the yields obtained from approximately fifty continuous hydrolysis runs. The relatively small difference between the actual and theoretical yields is due in part to mechanical losses of material resulting from operation of the continuous process on a small scale. These losses could be minimized with larger scale operations. Furthermore, no correction was made in these yield calculations for the oils and resins present in the original wood used. As some of this extractive fraction becomes water soluble during the process, the lignocellulose yields are apparently reduced. Truer values would be obtained if the yield data were calculated on the basis of dry-extractive-free wood substance. Of the total lignin content, a fraction may be obtained which is soluble in various solvents; for recovery and comparison purposes, methanol was adopted as the standard solvent. "Soluble lignin", therefore, will designate methanol-soluble
Figure
7. Effect of Temperature Variation
319
Figure
8.
Effect Variation
H 54 54 54 53 53 54 70 70 68 68 69 70 69 69
76 76 77 76 77 77 76 76 77 76 77 77 76 76 76 77 86 86
87 86
SLR 37 35 35 34 32 21 49 44 42
38 30 22 17 17 64 53 52 52 51 50 46 45 44 44 43 43 _. 43 41 40 24 55 49 47 35
of Time
Acid Concn.,
%
3 3 3 1.5 3 3 3 3 3 3 3 3
3 3 3 3 3 3 3 3 3 3 3
3 3 3
3
Pressure Lb./Sq. Id. 250 250 225 225 250 200 200 225 225 200 250 300 300 250 200 200 250 250 225 250 200 225 250 200 200 200 250 250 200 225 275 250 250 275
Time, Min. 2.8 2.8 2.6 4.0 2.3 1.2 4.2 3.2 4.0 2.6 1.8 2.0 1.1 2.4 2.9 4.1 4.2 4.8 4.0 4.3 4.5 4.5 6.1 5.2 5.5 4.1 4.2 4.2 3.0 1.6 4.7 6.8 8.6 2.1
Temp.,
C.
168 170 187 180 195 184 177 188 176 187 182 164 165 170 186 177 175 175 176 174 167 176 177 175 176 185 176 177 195 189 177 178 177 189
dently extreme variations in the hydrolysis factors could cause variations in XLR, even for the same value of H. Table I lists such data for four selected H values from Figure 4. I n these experiments the pressure was considered as the average gage pressure during a run; the time of hydrolysis was the time required for passage of a small unit of suspension through the hydrolysis tube, not including the preheater (up to 150" C.) or cooling sections; and the tempera-
INDUSTRIAL AND ENGINEERING CHEMISTRY
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The H curve follows the anticipated trend upward with higher acid strength. At low concentrations, increasing the amount of acid has considerable effect', while a t higher concentrations, this falls off rapidly (e. g., the decrease in pH a t this point is slight). On the other hand, the SLR curve indicates a maximum or a t least a leveling off if the 3 per cent point is discounted. This mould indicate that, although a t lon-er concentrations of acid SLR increases rapidly with increase in amount of acid catalyst in accordance with H , a polyinerizing effect on t'he residual lignin must he more pronounced at higher concentrations. The resulting product is a lignin in the residual lignocellulose which is less soluble in solvents. EFFECT OF PRESSURE
H y d r o l y s i s U n i t Assembly
View
ture was considered as the average liquor temperature a t the exit end of the hydrolysis tube-e. g., maximum temperature of hydrolysis. Selected runs were correlated to determine the effect of the hydrolysis factors and are listed in Table 11. EFFECT OF SULFURIC A C I D C O N C E N T R A T I O N The curves of Figure 5 are based on runs in which the sulfuric acid concentration of the hydrolysis liquor varied from 0.4 to 3 per cent. Other conditions were held as constant as possible (Table IIA). The variation of hydrolysis time is of only minor importance, as slionn later, but the pressure variation is large enough to make some difference. Run 300 is the only one at 200 pounds per square inch and should be discounted accordingly.
TABLE 11. EFFECT OF VARIABLES ON S L R Run No.
I1
Acid Concn.,
70
SLR
A. 306 303 302 300 299 363-11 366-B 383-A 380-B
42 47 53 59 49 72 76
27 32 34 31
84
86
55
203 258 260 259 264
60 67 74 79 78
28 43 43 59 54
310 357-B 356
.55
32
27 78 78
?9
B. 3
C.
773.4 3 s ,-A 379-4 379-13 378- \ 382-A 377- \ 377-I3
30 52 48 44 49 51
H Temp.
4.5 4.0 4.0 3.0
180 180
4.6 4.5
' C.
180 180
260
4.5 4 8
275
4.7
180 176 176 178 177
3.8 3.5 3.4 3.7 3.6
150 1 60 170 170 180
3 2 2 2
1so 181 186
300 300 300 300 300
3
::
40 47
200 225 225
3 7 3 3
.,4 50
ASD
Time, Ilin.
Pressiise
Trmperature. Series 1
Tempesature, Saries 2 200 200 200 200
7 3 3 3
D 350 372-11
225 200
3 3 R 3
C.
357-.1
Acid Concentration 0.4 226 0.8 225 1 3 3.0
30 46 45 44
Pressure, Lb./Sq. In.
3 3 3 3 3 3
3 3 3 3
1 9
9 9
187
Time 280 250 250 290 280 250 2 50 250 250 250
1.8
4.0 4 2
2E
6 1 8 1 A 9 7.0 8.6
182 179 177 17X 170 177 1 -x
178 177 177
Figure 6 is a plot of the data from Table IIB, showing the effect of varying the pressure. Further extension of these curves \vas impractical because of the unreliability of data a t higher and lower pressures than those shown. Both the H and SLR curves indicate a pronounced effect of pressure on the hydrolyzed product. The minor variations in time and temperature in this series could not have caused the rapid increases in H and SLR. We believe that this is the first time it has been possible to separate arid study independently the wood hydrolysis factors of pressure and temperature (owing to the hydraulic characteristic of the continuous process). The role of pressure in bringing about accelerated impregnation of the wood fibers \vit'h hydrolysis liquor and in causing mechanical disruption of the hydrolyzed product on release from the high-pressure system is of major importance in the control of H and SLR. EFFECT O F TEMPERATURE
I n Figure 7 the effect on H a n d SLR of varying the temperature is indicated for two series of runs in order to cover a wide temperature range. The temperature scale is twice as great for acries 1 as for series 2 . For both series H is almost, a straight-line function of temperature. I n other experiments this relation holds at even higher temperatures and is limited only by the mechanical difficulties involved, due to increased coating of the tube walls, carbonization, and coking a t teinperatures over 200" C. In both series SLR increases rapidly a t lower temperatures in each series and then levels off and may even decrease a t higher temperatures. This is similar to the effect of acid concentration and is probably due to the rapidly increasing factor of lignin polymerization a t higher temperatures, to give residues which are less soluble in solvents. EFFECT O F H Y D R O L Y S I S T I M E
Figure 8 and Table I I D illustrate the effect of varying the time of hydrolysis over a wide range. Although the first point a t 1.8 minut'es is the only one a t a temperature above 178' C., it was included to show the effect,of a short hydrolysis time. This variation in temperature obviously would not change the shape of the curve to any extent. Time of hydrolysis has only a minor effect on H . A measurable increase is evident up to 5 minutes, but beyond that time the increment is so small as to be negligible. The variation of SLR with time, on the other hand, is so irregular that no definite conclusions can be drawn. iilthough there is a rapid increase in SLR up to about 5 minutes, it is not certain whether this value continues to increase or decreases as a
INDUSTRIAL AND ENGINEERING CHEMISTRY
March, 1942
result of polymerization. From the present knowledge of the thermosetting properties of lignin (due to polymerization) under the effect of heat within short periods, the latter effect is quite possible. BY-PRODUCT C H E M I C A L YIELDS
Yields of acetic and formic acids, furfural, and reducing sugars have been checked carefully, with the result that considerable amounts of these by-products have been accounted for. Average values over a wide range of hydrolysis for maple are shown in Figure 9, all based on d. w. s.
301
I
Figure
I
I
I
I
I
I
I
9. By-product Yields
This figure is constructed so as to indicate cumulative as well as separate yields of by-products. Points for individual runs are not shown since the amounts of the different byproducts obtained varied because of effects (as yet unevaluated) of hydrolysis factors on the secondary hydrolytic reactions, notably those of the sugars. There is a slight increase of total by-product yield with increasing H . This increase is much less than the value calculated on the basis of the amount of cellulose hydrolyzed to water-soluble products. Thus, side reactions and secondary reactions apparently take place which reduce the yield of valuable products. I n this respect it has often been noticed that noncondensable gases are generated in the hydrolysis system, as well as water-soluble product3 in the form of tanninlike bodies and also water-insoluble resinous materials. None of these are accounted for in the tabulation of yields. It is also noteworthy that increased yields of organic acids (as acetic) and furfural are accompanied by a decrease in reducing sugar yields a t higher values of H . This indicates the form of the secondary hydrolysis reactions. Further confirmation of these results has been obtained in other work to be presented later, which has been carried out on recycling and rehydrolysis of by-product liquors. Even though the concentrations of the by-products in the hydrolysis liquors are low, due t o the liquid-solid ratio, recent work has indicated that these chemicals may be separated readily and economically by extraction and distillation. Formic acid was determined by the Fincke method involving precipitation of mercurous chloride; acetic acid was then calculated as the balance of the organic acids determined by titration, after correction for sulfuric acid present (by precipitation as barium sulfate). Reducing sugars were determined by a quantitative measurement of cuprous oxide resulting from a test with Fehling’s solution, using standard potassium permanganate. Furfural was determined by precipitation with phloroglucinol.
32 1
CONCLUSIONS
A continuous process has been developed for the hydrolysis of wood and other plant fibers. The pilot unit now in operation can produce from 400 to 800 pounds of hydrolyzed product (depending on H ) per 24-hour day. Simplicity of operation makes the process almost completely automatic; only the handling of raw materials and finished product is necessary. Larger scale operation will permit further improvements in operation. I n comparison with batch hydrolysis processes which have been described frequently in the literature, processing requires only a few minutes in the continuous system instead of from 30 minutes t o several hours in batch processes. Constancy of operation permits uniformity of product, even in runs lasting 24 hours. Furthermore, by separating the process variables and making them readily and accurately controllable, it is now possible t o produce hydrolyzed lignocellulose of predetermined analysis (within narrow limits) ; such products are reproducible. Although large volumes of liquid are handled, heat requirements are low due to utilization of heat interchangers for recovery and re-use of heat. I n connection with by-product recovery, it has been found possible t o overcome this apparent handicap by recycling and rehydrolyzing the relatively large volumes of liquors. Hydrolyzed lignocelluloseis now being produced on a semicommercial scale for use as a semiplastic filler in phenolic molding compounds; the resin requirements in standard types of molding compounds are thereby reduced. ACKNOWLEDGMENT
The authors wish to express their thanks to A. 0. Reynolds of the Northwood Chemical Company, Phelps, Wis., for his kind permission to publish the results of this research which was carried out in the company’s laboratory. LITERATURE CITED Auden, H . A., and Joshau, W. P., J. SOC.Chem. Ind., 51, 11T (1932). Bergius, F., Trans. Inst. Chem. Engrs. (London), 11, 162 (1933). Bergstrgm, H., and Cederquiat, K., Iva, 1933,No. 1, 3. Cohee, R. F., Jr., Fruit Products J . , 16,237 (1937). Cross, W. E., Ber., 43,1526 (1910). Desparmet, E., Chimie & i?,dustrie, 21, Special No.,571 (Feb., 1929). Distillers Co., Ltd., French Patent 712,037 (Feb. 24, 1931). Distillers Co., Ltd., Joshau, W. P., and Eaglesfield, P . , Brit. Patent 348,740 (Feb. 26, 1930). Farber, E., and Koch, F., U. 5.Patent 1,969,GOO (Aug. 7, 1934). Forest Products Lab., Tech. Note 120. Ganz, Gustav, Austrian Patent 119,956 (June 15, 1930). Groth, €4. S.,and Blomquist, G. H., German Patent 649,324 (Aug. 23, 1937). Hunter, E., J . Chem. SOC.,1928,2643. Jeorme-LBvy, Y., Compt. rend., 205,1469 (1937). Junien, M., Bull. assoc. chim. sucr. dist., 49,153 (1932). Ibib., 50, 224 (1933). Junien, M., French Patent 693,277 (June 14, 1929). Katzen, R., Polytech. Inst. Brooklyn, baccalaureate thesis, 1936. Kline, G. M., Modern Plastics, 39,46 (1937). Kotovskii, L. K.. Lesokhim. Prom., 2, 13 (1933). Lederer, A., and Lederer, E., French Patent 484,608 (Jan. 17, 1914). Luers, H., paper a t Tech. High School, Munirh, May 13, 1933. Luers, H., Z . angew. Chem., 43, 455 (1930). Luers, H . , Z . Spiritusind., 60, 7 (1937).
INDUSTRIAL AND ENGINEERING CHEMISTRY Meunier, G., Chimie & industrie, 21, Special No., 553 (Feb., 1 QZR).
Mirlis, I., and Gorokhohnskaya, N. S., J . Chem. I n d . (U. S . S. R.),12,601 (1935). Ost, H., and Wilkening, L., Chem.-Ztg., 34, 461 (1910). Owen, TV. L., Facts About Sugar, 31, 431, 435 (1936). PeldLn, H., Suomen Kemistilehti, 10B, 13 (1937). Peterson, W. H., Fred, E. B., Langlykke, A. F., and Sjolander, N. 0.. Wis. Agr. Expt. Sta., Bull. 439 (1937). ENG.CHEM.,ANAL.ED.,4, 202 (1932). Ritter, G. I., IND. Schoen, M., Ann. combustibles Ziquides, 11, 591 (1936).
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Scholler, H., private communication, 1935. Scholler. H.. French Patent 777,824 (March 1, 1935). Schwalbe, C. G., 2. angew. Chem., 37, 218 (1924). Sherrard, E. C., Chem. A g e (X, Y ~ 29, ) , 76 (1921). Sherrard, E. C., and Ganger, W. H., IND. EKG.CHEM.,15,1164 (1923). Skogh, C. G. C., C. S. P a t e n t 2,096,353 (Oct. 19, 1937). Compt. rend., 150, 783 (1910). T'ille, J., and hlestrezat, W., Virtanen, A. I., and Kirkomaki, T., Suomen Kemistilehti, 9 3 19 (1936). Zimmerman, il., Chem. Trade J., 51,588 (1915).
High-Temperature Saponification AN ANHYDROUS SYSTEM Joseph J. Jacobs, Jr. Polytechnic Institute, Brooklyn,
everal of the variables for the process of saponification of fats w i t h anhydrous alkali, i n the presence of a hydrocarbon diluent, have been analyzed in the laboratory. A small p i l o t unit has been built from data obtained. The possibilities for the development of a continuous process are shown.
S
NTIL recently the manufacture of soap was considered U an outstanding example of an industry in which successful operation was almost wholly dependent upon familiarity with the a r t rather than upon modern scientific practice. While the old soap boiler is slowly disappearing from the field, the basic methods for the production of most soap today are the same as they were twenty years ago. The greater technical training of the responsible men in the industry has manifested itself in certain improvements which have decreased manufacturing costs and improved yields. Until recently, however, there have been no basic changes in methods of saponification. The process used now consists essentially of heating tallow or other fats with dilute aqueous alkali in large open kettles, until all of the fat has reacted with the alkali t o form soap, salting out the soap, allowing the layers to settle, and washing out the glycerol. Three to seven days are required to finish a soap boil, depending upon the size of the kettle and the extent of saponification and purification desired. Live steam is used for heating and agitation. After saponification is complete, salt is added to throw the soap out of solution. When separation is effected, the upper layer is withdrawn, cooled, dried, and processed further. The lower aqueous layer, containing the salt, glycerol, and organic impurities, is withdrawn and treated chemically. The glycerol solution is evaporated and the salt is precipitated. Further evaporation gives the 80 per cent soap lye crude. This concentrated glycerol solution is then distilled in the presence of superheated steam and partially condensed to give anhydrous glycerol.
N. Y
The concentration of glycerol in the aqueous layer is about 5 per cent, although this may be raised considerably by countercurrent operation-that is, by using the strong lye with no glycerol in it to finish the saponification, washing out the last traces of glycerol from the soap, and using the weak lye containing large amounts of glycerol to start saponification of the fresh tallow. The concentration of glycerol in the final effluent depends upon the number of these so-called changes that are made. Several disadvantages in the present methods of saponification are evident. Despite the fact that the use of large kettles and countercurrent operation has allowed a close approach t o a truly continuous process, it is, nevertheless, a batch process, and there are sizable heat losses due to heating and cooling of the batches. These disadvantages, combined with the use of open steam for agitation and heavy evaporator duty, make the over-all heat costs evcessively high. The recovery of dynamite-grade glycerol from dilute soap lyes is an expensive, if not a difficult, operation. These and other minor disadvantages have prompted research with a view toward the development of a continuous, rapid method of saponification. One of the large soap companies recently built a plant for the production of soap by the continuous saponification of fatty acids (4). The fatty glycerides are split continuously in a pressure tower. Hydrolysis is accomplished under superpressures and high temperatures in order t o decrease the time required for splitting. It was stated that the concentration of glycerol recovered from the botton of the fat-splitting tower is approximately proportional to the height of the tower, and that with a column 50 feet high the concentration of glycerol recovered is 20-25 per cent (4). Another proposed continuous saponification method is the Clayton process (1). This involves the continuous saponification of fats with a relatively concentrated alkali (25-50 per cent) in a coil reaction tube. Fat and alkali are proportioned to a pressure pump which mixes the materials and forces them through the reaction tube, and saponification takes place at approximately 425" F. The semifluid mass of soap and water is forced through a spray nozzle into a vacuum chamber where
