CONVERSION OF WOOD
portation of many products to the Continent, the engineers and chemists of the blockaded countries developed the beet sugar industry which gave them an independent supply of this important foodstuff. Thus, dire necessity led to new economical and industrial achievements. The various governments helped to maintain and strengthen this new industry in the subsequent period of free trade, during which time products from the outside world could again be imported into Europe. During the past few years the problem of creating raw materials essential for life within its own borders has occupied the attention of each individual country more than ever before. The inven!ive genius of chemical and mechanical engineers is being applied more and more to reduce the economic dependence of the individual nation and to produce as far as possible a t home a large part of those products which previously had to be imported. To this end new processes adapted to the individual need had to be invented and developed. This trend has been greatly criticized. The point was raised that these activities are more and more curtailing international commerce and free trade. However, critics must bear in mind that industrially advanced countries have for a long time been creating their own industries for finished products-for example, the manufacture of textiles, dyestuffs, pharmaceutical products, automobiles, and machinery. Almost all industrially progressive countries have manufactured technically improved products themselves, and such undertakings have naturally led to protective tariffs.
During the last two decades Germany has been concerned with the problem of producing some of the most important raw materials, and together with my co-workers, I have been engaged in solving such problems for the last twenty-five years. We learned how to transform domestic coal into gasoline and other oils by means of hydrogenation. This method is now being extensively used in Germany and England to produce the liquid fuels so necessary in modern life. Although the much discussed partial exhaustion of the supply of natural liquid fuels is not yet imminent, our endeavor to transform coal into oil was not in vain. The competent economical and political circles in those countries which have no supply or an insufficient supply of crude oil, appreciate the possibility of turning coal into 6il. They realize that, although the cost of production is considerable, it is more important to be independent from outside supplies. At first coal was used primarily as a source of heat and power. During the years it has gained additional importance as the basic material for many chemicals. Coal distillation yields benzene, naphthalene, and other coal-tar products, the basic materials for dyestuffs as well as for modern synthetic pharmaceutical products. This destructive distillation a t high temperatures results in aromatic compounds, whereas, hydrogenation a t lower temperatures produces hydrocarbons of a hydroaromatic or aliphatic character, and the molecular structure of coal is maintained to a much higher degree. 247
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INDUSTRIAL AND ENGINEERING CHEMISTRY
OPERATING PLATFORM O F THE DIFFUSION BATTERY WITH P R E M I X E R AND LOADINQ APPARATUS
TENDIFFUSERS OF THE DIFFUBION BATTERY SYSTEM
ACID-CIRCULATINU P U M P AND PIPINGSYSTEM OF THE DIFFUSION BATTERY
PIPING SYSTEM OF
HYDROCKLORIO ACIDSTORAQE TANKS
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FIGURE 1. FLOWDIAURAM OF THE PROCESS Wood supply Wood shredder 3. Chips ‘4. Wood dryer Disinte ration steps: 5. %ydrochloric acid 0. Dried wood chips 7. Diffusion battery 8. Lignin 9. Lignin-briquetting procesq 1. 2.
Evaporation ste s: IO. Sugar soPution 11. Evaporator for hydrochloric acid 12. Condenser
Wood as a Chemical Raw Material Wood, the mother substance of coal, which throughout the ages has served as the principal building material, has recently been receiving increasing consideration as a chemical raw material. The properties of wood make it suitable, when carefully treated, for the production of aliphatic compounds. The older chemical processes which use wood as a raw material are wasting a large percentage of it. The pulp industry recovers less than 50 per cent in the form of pulp, the balance is lost. Wood distillation produces less than 40 per cent of useful materials, the rest is wasted in the form of carbon dioxide and water. The industries extracting resin and tanning materials must be satisfied with an even smaller percentage, and only up to 90 per cent of the wood can be utilized as fuel. At the beginning of this century, attempts were made in the United States to recover fermentable sugars from sawdust, with the aid of dilute acid a t temperatures above the boiling point. The first technical plant for the manufacture of alcohol from wood was erected by Ewen and Tomlinson at Georgetown, S. C., in 1910. Sawmill waste was treated with dilute sulfuric acid, but the alcohol yield was far below theoretical possibilities. The same principle of wood saccharification was applied by Scholler in Germany. He obtained a better yield-about 60 to 70 per cent of the theoretical-but the ,sugar solution which had to be fermented contained not more than 3 per cent sugar. A much better yield can be obtained if highly concentrated hydrochloric acid a t ordinary temperature is used as a means for hydrolyzing cellulose. Willstiitter and Zechmeister found that cellulose is soluble at room temperature in 40 per cent hydrochloric acid. In this solution the cellulose is transformed into glucose within a few hours without formation of waste products. This means that under such conditions it is possible to obtain 100 per cent of the theoretically possible yield. I n 1916 Erik Hiigglund and I undertook to develop a technical process based upon the reaction disclosed by Willstlltter and Zechmeister. Obviously we could not use cellulose
Crystallization steps: 19. Vacuum pump 20. Inversion 21. Evaporator 22. First crystallization 23. Filter. press 24. Solution vat 25. Recrystallization 20. Centrifuge for pure crystals 27. Sirup filtrate (mother liquor) 28. T o alcohol (by fermentation)
as a raw material as had been done by Willstatter and Zechmeister in their scientific experiments. We had to apply the reaction to wood or wood waste which, besides cellulose, contains chiefly hemi-celluloses and lignin. Almost all of the wood substance except the lignin is dissolved in concentrated hydrochloric acid. Enormous quantities of wood substance are wasted in the forest; less than 50 per cent of the wood of a tree is used as timber, and the remainder is lost or used as fuel. Additional waste occurs in sawmilling and similar operations. Thus abundant raw material for our “wood hydrolysis process” was available a t a low price. At an early date we recognized that the product of the wood hydrolysis process was a carbohydrate which could be used at least as a fodder similar to barley or corh and also for fermentation purposes. Further, we believed that the product could eventually be converted into a suitable form for
out reducing the forest reserves because production can supply enormous quant for wood hydrolyzation.
Chemistry of W o o d Hydrolysis The chemical reaction of wood hydrolysis is simple. It consists in adding one molecule of water to one molecule of cellulose, Great difficulties, however, had to be overcome in carrying out this reaction on a plant scale. The hydrolyzing agent is hydrochloric acid in a more
INDUSTRIAL AND ENGINEERING CHEMISTRY
concentrated form than has been used heretofore in technical processes; this concentrated acid attacks practically all metals suitable for such technical equipment. Therefore, it was necessary to design and construct equipment which would resist the corrosive acid. Credit for solution of the problem is due mainly to the ingenuity of my old coworker, Fritz Koch. The apparatus now in use is comparatively simple but very effective.
Steps in the Process As shown on Figure 1, the wood is first shredded and then dried in a revolving drum until it contains only about 8 per cent water. The dryer is heated by means of the waste gases of the boilers. The dried wood is then brought into contact with highly concentrated acid in a battery of iron diffusers. The fresh concentrated acid entering the last unit of the battery comes into contact with wood from which the soluble components have been practically extracted. While progressing through the battery, the acid takes up more and more of the soluble wood substance, and finally in the first vessel it comes into contact with fresh wood. By this method a highly concentrated solution of hydrolyzed wood is obtained. The result is an acid solution of about 32 per cent by volume reducing sugar. This high concentration is important by reason of the following operations for recovery of the hydrochloric acid from the hydrolyzate. The iron vessels of the battery are protected against corrosion by special linings. After three years of operation, there has been no indication of either corrosion or leakage. Methods have also been developed which permit the charging and emptying of the vessels without the escape of any acid vapors which might be harmful to the workers or the buildings. During the treatment, two-thirds of the weight of the wood is dissolved by the acid, and one-third remains in the vessels in the form of lignin. After the hydrochloric acid has been systematically washed out, the neutral lignin is easily removed by opening the vessel a t the bottom. The lignin can be used as fuel; it is practicallv ash-free and can easily be briquetted w i t h o u t a binder 1 and turned into pure charcoal. In the next stage, the solution f r o m the battery i s d i s tilled under vacuum a t about 36" C. The h y d r o c h l o r ic a c i d is evaporated and after condensation is brought to t h e a c i d regenera-
6 FOR DRYING SUGAR CONCENTRATE FIGURE3. SPRAYDRYER CONTAINING HYDROCHLORIC ACID
1. Air heater for heating air used t o dry sugar concentrate 2. Spray dryer for sugar solution 3. Cyclone for removing dried material from suspension in air b y
centrifugal force Separator in which the dust remaining in the air from the cyclone I R removed bv a sDmv of feed solution: the feed solution so used is then retime&$0 thdmain feed.stream 5 . Separator in which escaping air is washed free from hydrochloric acid by a water spray, the air ascending the flue and the hydrochloric acid solution running out through the bottom pipe 6. Blowers The solution enters sprpy dryer 2, atomized by compressed air. Everything entering the drying chamber is blown by the fan to cyclone 3 where the coarser material collects and 18 drawn off into the barrel. The air Containing the very fine dust goes to separator 4 where the dust is removed by atomiz,ed feed liquor, which then rejoins the main feed. The air from 4 then goes to 5 where the hydrochloric acid is removed by atomized water, which runs out at the bottom. The washed air lesvas by the flue.
tion and reconcentration plant; from there it is returned to the diffusers. Development of the final distilling apparatus for hydrochloric acid required a number of years. There had been no previous need for this type of still. An acid-proof apparatus consisting of evaporators was constructed with tubes made of a special ceramic material of good heat conductivity. One square meter of the surface of these tubes permits the hourly evaporation of 12 kg. of acid. This may be considered a very good performance. The chief problem in the construction of this apparatus was to avoid the dangerous effect of the different heat expansion coefficients of the various ceramic and iron parts. We now have equipment for distilling and condensing hydroFIGURE 2. DIAGRAM chloric acid under vacuum; the still operates satisfactorily, OF TUBULAR HYDRO- is economical in heat consumption, and can be built in large CHLORIC ACID EVAPO- units. No erosion or leakage has appeared. RATOR By means of vacuum distillation, about 80 per cent of the hydrochloric acid is recovered. The remainder must be 1. Vapor discharge 2. Vacuum gage separated in another way. For this purpose a special spray 3. Sight glasses 4. Thermometer dryer is used. Hot air brought into contact with the finely 5. Steam intake atomized concentrated solution evaporates the hydro6. Safety valve 7. Manometer chloric acid and water. A solid product containing 1 t o 2 8. Condensate outlet 9. Air exhaust per cent hydrochloric acid, about 8 per cent water, and 90 10. Solution intake per cent sugars is collected in a cyclone. Finally the hydro11. Sirup
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(Readingfrom top to bottom) SUGAR SOLUTION EVAPORATOR TUBS IN THE CRYSTALLIZING PLANT FERMENTING DISTILLING COLUMN IN THE CRYSTALLIZING PLANT
(Reading from top io bottom)
APPARATUSFOR INVERSION OF WOOD SUQAR BATTERY OF SUGAR CRYSTALLIZERS FILTER PRESSES IN THE CRYSTALLIZINQ PLANT
INDUSTRIAL AND ENGINEERING CHEMISTRY
chloric acid vapors are condensed and washed, and the acid is recycled.
Products The solid hydrolyzate contains (depending on the sort of wood used) glucose, mannose, xylose, galactose, and fructose mainly in tetrameric form. Polymerization from the monomeric to the tetrameric form takes place in the process between the battery and the spray dryer. The total yield of the different sugars amounts to 60-66 per cent of the original dry wood. The tetrameric sugars can be converted easily and without loss into the monomeric form if the raw hydrolyzate (still containing 1 to 2 per cent of hydrochloric acid) is diluted by three times its volume of water and heated for about half an hour a t 120' C. By this inversion process a fermentable solution is obtained. The dry raw hydrolyzate contains the tetrameric sugars; after neutralizing the small amount of acid with lime it can be used as fodder. Extensive tests made by the German Government over a period of two years have proved that the food value of this raw sugar is equivalent to that of barley. The fodder so obtained can be stored for a long time. The raw sugar can be fermented to alcohol in solutions containing 20 per cent reducing sugar. The fermentation is carried out in the usual manner. One hundred kilograms of reducing sugar yield about 50 liters (13 gallons) of pure 100 per cent alcohol. I n other words, one long ton of dry wood substance yields 85 to 90 gallons of 190-proof alcohol. The xylose and the galactose in the raw sugar cannot be fermented by yeast. They remain in the spent wash liquor which, after repeated use, becomes richer and richer. The
HYDROCHLORIC ACID ABSORBINQ Co~uaixs
RAWSUQAR PLAST (Right, below) CRYSTALLIZINQ PLAST
wash liquor is then evaporated, and the residue is dried in a spray dryer to a stable product without the addition of mineral acids. This product can also be used as fodder or for other purposes. By this method about 80 per cent of the raw sugar is converted into alcohol, while the remaining 20 per cent is recovered in the form of other useful materials. The raw sugar can also be used for other fermentation processes, yielding, for instance, baker's or fodder yeast, lactic acid, andother fermentation products. During the past few years a process for producing crystallized glucose and xylose from the raw sugar has also been developed. The more important of the two is the crystallized glucose, or dextrose, which is consumed in large quantities. The inverted raw sugar solution is neutralized, filtered, and concentrated and can then be easily crystallized. By recrystallization a white and very pure dextrose is obtained. The mother liquor,' still containing some glucose, mannose, and other sugars, can be fermented or used as fodder. I n this way no sugar is lost. By a special method xylose also can be recovered in crystallized form. One of the by-products of the process is acetic acid, which is contained in the solution coming from the hydrolyzing battery. A special process has been worked out for sepaTating the acetic from the hydrochloric acid. This process a t the same time separates hydrochloric acid in gaseous form from the water. Such separation is necessary because, when washing out the lignin, some water unavoidably gets into the system from which it has to be removed. The amount of acetic acid recovered is practically the same as is obtained from the same amount of wood in wood distillation processes. The amount of acetic acid here recovered corresrJonds in Dercentage with that weight oi acetic 'acid proiured in vdry distillation of wood. The carbonization of lignin yields charcoal and methanol. Thus our process can be carried out to yield the chief products of wood distillation but with the additional advantage that the 60 per cent of wood ordinarily lost to carbon dioxide and water in such dry distillation of wood is now recoverable as sugar. Thus, the main feature of this process is that practically every pound of substance contained in wood waste is converted into some useful product for which there is a demand, in one form or the other, all over the world. RECEIVED January 18,1937. Presented before the Harvard Tercentenary Conference of Arts and Soienees, Cambridge, Auguat 31 to September 12. 1936.