A Summary Statement1
1 Prepared at the request of FAO in connection with the work of the Technical Panel on Wood Chemistry.
by J. A. HALL, Director, J. F. SAEMAN, Chemist, and J. F. HARRIS, Chemical Engineer, Forest Products Laboratory, Forest Service, United States Department of Agriculture
FEW industrial chemical processes have had histories to match that of the saccharification of wood. From the earliest days of organic chemistry, scientists have been intrigued by the fact that most cellulosic wastes are two-thirds carbohydrate, which by appropriate treatment can be converted to sugars useful as food or for the preparation of chemicals.
Despite early hopes and a large amount of research and development work, wood saccharification has not lived up to expectations. It has had a disappointing history indeed, with perhaps a score of expensive failures, but today hope seems to burn as brightly as ever - the mistakes of the past being written off as having little bearing on the future. Scientists and industrialists recognize in cellulosic wastes a vast potential source of food and chemicals. Moyer (39)2 in a recent paper on the future of cellulose included the following predictions:
"The hydrolysis of cellulose to glucose and simple oligosaccharides will become a dominant economic factor... Cellulosic byproducts will find increased utilization as raw materials for the production of simple aliphatic chemicals through chemical engineering and biological processes... Looking far into the future we can see that cellulose, the most abundant of all photosynthetic products, will become one of man's most useful raw materials. The supply is replenished by growth while materials stored from past ages are continuously depleted."
2 Numbers in parentheses refer to the literature cited at the end of this report. replanted to yield trees of value for lumber, pulp, and cellulosic residues for chemical conversion. This aspect alone has far-reaching consequences and justifies the expenditure of much research effort.
It seems reasonable to assume that, with our present technological tools and with sufficient research effort, processes can be made available for efficiently utilizing cellulosic wastes. The importance of the development of such processes can be judged by viewing the possible consequences of such accomplishment.
Probably the most important and far-reaching result of the development of an economic saccharification process would be the effect it would have on forest management and utilization. It would be possible to clear unproductive forest land which could be
NOTE. - The Forest Products Laboratory of the Forest Service is maintained at Madison, Wisconsin, in co-operation with the University of Wisconsin.
Another effect which cannot be subjected to a direct economic evaluation is the political aspect of self sufficiency and the balance of foreign exchange. Any country with an abundant source of cellulosic waste material could process the material to fuel or food in the event of war without drawing on its productive farm lands. However, a saccharification process intended to produce sugar for the production of power alcohol offers little justification for research. Cederquist (52), in a paper given at Lucknow, points out that if fuel is needed it is better by several fold to use the cellulosic material as fuel rather than attempt conversion to alcohol. The ultimate criterion by which to judge the importance of a saccharification process is in the light of current economics. The picture varies greatly depending on the locale under consideration. However, some generalizations can be drawn.
Considering the use of sugar as a food, it is well to note that pure crystalline sucrose of almost 100 percent purity may be bought currently on the world market (f.o.b. Cuba) for slightly over 3 cents per pound, and sugar in the form of blackstrap molasses for animal feeds can be bought at roughly half this price (22). The deciding factor here is the transportation costs to the point of consumption. In many localities this brings the cost to where sugar from available cellulosic wastes might be made at competitive prices. Other facets of this aspect include the long-range outlook where it might appear that much larger supplies of food are necessary to feed the world's population. Hass and Lamborn (22) claim that the world's sugar production from sugar cane could be easily doubled and thus point out that the problem of sugar production is not pressing. Countries faced with food shortages would also have to compare the benefits derived from money spent on cellulose conversion to those derived from money spent on fertilizing available lands.
Cellulosic residues considered as a chemical raw material are a source of hexose and pentose sugars. The major portion, the hexoses, are equivalent to sugar from common sources. The pentoses, on the other hand, are unique, as they may be processed to furfural, a chemical which has not been produced from other sources. The pentosans in corncobs and bagasse are now the main source of furfural. The pentosan content of wood, however, is too low for an economic venture supported by this single product.
The brief considerations above lead to the conclusion that while there are indirect benefits to be gained by the development of a technically feasible saccharification process the important yardstick is economics. To be of value the process must be capable of producing sugar at a price competitive with cane sugar or cane molasses in the locality in which it operates.
Purpose and scope
This statement was prepared at the request of the Food and Agriculture Organization of the United Nations to serve as the basis for a report for distribution to Member Governments. Its purpose is to describe developments in the field of wood saccharification, to discuss the possibilities and the economic and technical difficulties of the process, and to outline courses of investigation and research which appear to offer most promise.
It is beyond the scope of this paper to provide a full coverage of the science and technology of wood saccharification, but enough of the technical aspects will be presented to characterize the methods under discussion.
Wood saccharification is but one aspect of a larger problem - the chemical utilization of cellulosic residues. Sugar is but one of many products obtainable from cellulosic residues, and wood is but one of a variety of potential starting materials. It so happens that most of the available literature on the chemical utilization of cellulosic wastes deals with wood. Wherever possible in this report, coverage is extended to products in which sugar appears only as an intermediate.
Sources of information
There is much information in the literature on the production of sugar and alcohol from cellulosic materials, but comparatively little on the industrial and economic aspects of the problem. The books by Häghlund (20), Wise and Jahn (56) have sections devoted to the subject that include large bibliographies. Early industrial work has been described by Foth (10) and Demuth (6). The development of the Scholler process is described in the German literature (13, 14 35, 46, 49). The history of early wood saccharification processes in the United States has been described by Sherrard and Kressman (51). A series of papers included in a symposium on sugar from wood and agricultural waste has been printed in Industrial and Engineering Chemistry (23). Saeman, Locke, and Dickerman, following World War II, have reported on the production of wood sugar in Germany and its conversion to yeast and alcohol (45). Harris (21) in 1949 reviewed wood saccharification in the annual Advances Carbohydrate Chemistry. The well-constructed and operated wood-sugar alcohol plant in Ems, Switzerland, has been described (42, 54).
Gilbert and his co-workers of the Tennessee Valley Authority have published details of their pilot-plant study of wood hydrolysis using a percolation process (15). A general discussion of the subject of wood hydrolysis and a description of the American plant at Springfield, Oregon, has been published (44).
Wenzel (55) in 1954 published a thorough review of the chemistry and technology of the acid hydrolysis of wood. These various sources present a full background on wood hydrolysis.
The production and use of power alcohol was the subject of a United Nations-sponsored seminar held at Lucknow, India, in 1952 (52). This seminar included the subject of wood-sugar production. Wood saccharification was the subject of the sixth meeting of the FAO Technical Panel on Wood Chemistry at Stockholm in 1953 (8). Particular attention is paid in this report to the information presented at these meetings.
The amount and nature of the sugars obtainable from wood, and the processes required to effect the necessary hydrolysis, are determined by the polysaccharides of the wood.
The main polysaccharide in all woody-plant materials is cellulose. This cellulose is chemically and physically similar to cotton in that it is fibrous, has a high resistance to alkali, and is hydrolyzed only with much difficulty to yield glucose.
Hemicelluloses are present in wood and other cellulosic materials to an extent of roughly one-third of the total carbohydrate. The hemicellulose is amorphous and because of its lack of crystalline organization, it hydrolyzes much more easily than does cellulose. If the total carbohydrate of wood were hydrolyzed as easily as is hemicellulose, there would be no question about the immediate widespread industrial usefulness of wood saccharification.
The component sugars of the total hydrolyzate of 20 different species of woods have been determined by Gustafsson and co-workers (19). According to these workers, all the wood species investigated contain glucosan, galactan, mannan, araban, and xylan. The amount of galactan based on the total sugar produced by the hydrolysis of softwoods varies from 6.0 to 17.5 percent, and that of mannan between 7.5 and 16.0 percent. The corresponding values for hardwood are 1.0 to 4.0 percent and 0.5 to 4.0 percent, respectively. The xylan content of softwood varies between 9.0 and 13.0 percent, and that of hardwood, between 19.5 and 39.0 percent. The araban content is low throughout and exceeds 3 percent only for three of the softwood species. The carbohydrate composition of a given species is not strictly constant, but the variations that exist are not sufficient to obscure the characteristic differences between hardwoods and softwoods.
Lignin is the main noncarbohydrate constituent of wood and other plants. It is essentially a high polymeric aromatic material, and most of it remains through hydrolysis procedures as an insoluble residue amounting to 20 to 30 percent of the wood. The chemistry of lignin has recently been comprehensively reviewed by Brauns (5). While lignin has found little use in the past, it might well prove to be the key to some future successful process for the integrated utilization of cellulosic residues.
The conversion of cellulosic materials to sugar appears at first glance to be a simple hydrolytic cleavage of glycosidic bonds. As such, one should expect the reaction to be simple, and the capital costs of a factory to be low. In reality, cellulose is unique among the known polysaccharides in its extreme resistance to hydrolysis. The glycosidic bonds themselves are easily broken, but the crystalline organization of the cellulose results in a low accessibility to dilute acid commonly used as a catalyst. As a consequence, the conditions of temperature and acid concentration required to accomplish the reaction in a reasonable time cause serious decomposition of the resulting sugars. Faced with these facts, only a few basic alternatives have presented themselves for practical hydrolysis.
1. A simple dilute-acid hydrolysis can be carried out without separation of product as it is formed.2. A percolation process can be employed in which yields are raised by the expedient of continuously removing the product as it is formed.
3. A concentrated acid process can be used in which the crystalline organization of the cellulose is destroyed, the carbohydrate solubilized, and finally completely hydrolyzed with dilute acid.
All commercial processes fall into these three categories.
The simple dilute-acid hydrolysis of wood
The single-stage batch process of wood hydrolysis was the first commercial method for making sugars from wood. The process has the advantage of great simplicity, and, with improvement, it might still be the method of choice in certain situations.
Plants using this simple process were operated at Georgetown, N. a., and Fullerton, Louisiana, during the period just preceding to just after World War I. According to Sherrard and Kressman (51) these plants produced 5,000 to 7,000 gallons of alcohol per day. The Georgetown plant was described by Foth (10) and Demuth (6). Concurrent experimental work was described by Kressman (32). In 1921 Sherrard presented an engineering study of the process (50).
At the Georgetown plant, the wood was processed on a 1-hour cycle in four spherical digesters, each holding 4,700 pounds of dry wood. The sugar was extracted in a battery of eight 150-cubic-foot cells arranged for countercurrent extraction. About 96 percent of the sugar was extracted to give a solution containing about 12 percent total solids, nearly 9 percent reducing sugar, and roughly 6 percent fermentable sugar.
Giertz (8) described a process used in Sweden during World War II for the continuous hydrolysis of wastewood. The wood chips were impregnated in a tower with sulphur dioxide gas. The bottom of the tower was connected to the feeder mechanism of an Asplund defibrator. The chips were heated in the preheater at 180° C. for 2 to 3 minutes, ground in the defibrator, and the pulp pressed out continuously through an expansion valve. Seven such units operated in parallel. The pulp was washed in centrifuges and the solubles were reacidified and hydrolyzed further. The sugar solution at a concentration of 8 percent was fermented with the spent sulphite liquor of the plant. Plans to saccharify the cellulose in the residue were worked out, but never realized. The residual lignocellulose was used as a fuel. Attempts to use it as a filler for bakelite plastics were unsuccessful.
The operation of the plant was dependent on high alcohol prices granted by the government. Operations were discontinued after the war, and the plant was dismantled in 1946.
A wood-hydrolysis process has been described in which a slurry consisting of 8 to 10 parts of dilute acid to one of sawdust is pumped through a heat exchanger (24, 25, 40, 41). This has not been used commercially. The purpose of the process was the production of a lignocellulose plastic. As a means of producing sugar, the method has the disadvantage of giving dilute solutions because of the high liquid-to-solid ratio.
The simple batch hydrolysis of cellulose has attractive features, but there has been a serious limitation on yield. More work on the kinetics of the reaction has provided hope that yields can be improved.
Following the commercialization of batchwise hydrolysis during World War I, Meunier (37, 38) and Desparmet (7) published their research on the subject, indicating the desirability of using successive hydrolysis stages.
In 1945, Plow and his co-workers described experiments in which several stages of hydrolysis were used (43).
At a seminar on the production and use of power alcohol in Asia and the Far East, Cederquist (52) described a process developed in Sweden during the war. Pilot-plant studies were carried out in full-scale batch reactors, but the process was not put into production.
The following summary of the process is taken from the report presented by Cederquist:
"A careful investigation of all the known processes indicates that hydrolyzation with dilute acids, properly carried out, involves considerably lower costs than hydrolyzation with concentrated acids, but there is still room for inventions to improve the process. The author and his cooperators carefully reviewed the hydrolyzation problem during the war, and our investigations resulted in a proposal to carry out the process on acid-impregnated, subdivided wood in two stages in an atmosphere of steam. Approximately 50 percent of the wood (dry) is converted to sugar and the sugar solutions obtained have a concentration of 10-12 percent.
"The conclusions of our investigation may be summed up as follows:
1. A fairly good yield of sugar (48-50 percent);2. A high concentration of sugar (10-12 percent);
3. All pentoses enriched in the first stage solution which facilitates the utilization of the pentoses: the sugar formed in the second stage consists entirely of glucose;
4. The total consumption of sulphuric acid is low, and amounts to 20-25 kilograms per ton of wood;
5. The consumption of steam without heat recovery amounts to 1.3 tons per ton of wood;
6. The digesters for carrying out the hydrolyzation are of small size, because of the short time of hydrolyzation. Fifty tons of wood a day may be converted to sugar by using two digesters of 3 cubic meters each and two of 2 cubic meters each.
"The hydrolyzation is carried out under the following conditions:
1. The wood is used in the form of thin shavings, chips or sawdust.
2. The wood is impregnated with weak acid and freed from excess of liquor prior to hydrolyzation.
3. The hydrolyzation is carried out in an atmosphere of steam.4. Stage I
(a)Impregnation with a liquid containing 0.5 percent sulphuric acid:
Temperature: 190° a.
Steam pressure: 12 kilograms/square centimeter.
Time of hydrolyzation: 3 minutes.(b) Washing out the sugar on a continuous counter current bed filter.
5. Stage II
(a)
The residue from Stage I is impregnated with a solution containing 0.75 percent sulphuric acid:
Temperature: 215° C.
Steam pressure: 20 kilograms/square centimeter.
Time of hydrolyzation: 3 minutes.
(b) Washing out the sugar on a continuous counter current bed filter."
Percolation processes
In saccharification by percolation processes, wood is put into an acid-resistant pressure vessel and hydrolyzed by dilute acid injected into the top of the vessel and withdrawn through a filter in the bottom. In this way, sugar production and extraction go on simultaneously, and the sugar is separated and cooled as soon as possible to prevent decomposition.
The industrial development of the Scholler process using the percolation technique, has been described by Schaal (46), Fritzweiler and Rockstroh (14), Lüers (35), Scholler (48, 49), and Fritzweiler and Karsch (13). These sources do not give detailed information on the process, but in 1945 operating details of the Tornesch and Holzminden plants were obtained (45, 17).
At the present time there are three Scholler plants in Germany, the Tornesch and Holzminden plants in the western zone, and the Dessau plant in east Germany. There is also one in Switzerland and another in Korea.
Typically, a Scholler plant has six or eight 50-cubic-meter digesters constructed of steel and lined with acid-resistant tile. The diameter of these digesters is 2.4 meters, and the over-all height about 13 meters. The top of a digester, or percolator, has steam and vent lines and a line for the introduction of hot dilute acid. The bottom is equipped with a filter cone and a quick-opening discharge valve for removing the lignin residue. The digester is loaded with 9 to 10 metric tons of sawdust and chips to a density of 180 to 200 kilograms of dry wood substance per cubic meter. A charge of dilute acid is then injected at a temperature lower than that of the percolator contents, and the injected acid is heated by steam from the bottom until the desired temperature is reached. The solution is then pressed from the percolator by applying steam to the top of the charge. This operation is repeated for a total of up to 20 cycles, with 0.8 percent sulphuric acid at temperatures increasing to a maximum of 184° C. From 10 metric tons of wood approximately 120 metric tons of liquor with a concentration of 5 to 6 percent sugar is obtained.
The Scholler plant at Ems, Switzerland (42, 54) has proved very successful. It has apparently operated at better than design rating to produce about 52 gallons of absolute alcohol per ton of wood. The Ems plant is like the German plants, except that it uses a shortened percolation schedule and the, calcium sulphate is removed from the neutralized wort by a centrifuge. The sludge is washed on a drum drier.
Production data obtained for the German plants following World War II showed unfavorable operation, but the wartime difficulties certainly had an adverse effect. The Swiss plant at Ems had fewer difficulties of this sort, and evolved into a well-running operation. The best information now available indicates that the Scholler process can operate essentially as claimed, to produce alcohol in yields of 50 gallons or more per ton of dry wood. The report by Rockstroh presented at Stockholm (8) supports the view that engineering problems are well in hand.
The recent work on wood-sugar production by a percolation process in the United States has been described (44). In 1935, the Cliffs Dow Chemical Co., of Marquette, Michigan, obtained the rights to the Scholler process in the United States. A modified Scholler process was studied on a pilot-plant scale, but it was not used commercially. In 1943, the War Production Board recommended that the United States Forest Products Laboratory study the Scholler process in the pilot-plant facilities at Marquette, Michigan. The Vulcan Copper and Supply Company was requested to follow the pilot-plant operation and to prepare a process report that could serve as a basis for the engineering design of a commercial plant. As a result of these investigations, the War Production Board recommended that a commercial plant using the modified Scholler process be constructed and operated as insurance against possible future grain shortages. In 1944, construction was begun on a plant at Springfield, Oregon.
The preparation and fermentation of wood hydrolyzate at the Springfield plant was carried out successfully. The production of sugar, however, presented some problems, none of which were considered fundamental to the process. Part of the difficulties could be accounted for by the history of the plant. When the war ended, the contract for the uncompleted plant was cancelled by the Government. Construction was later resumed, and after some engineering compromises for the sake of economy, the plant was completed sufficiently in February 1947 to allow partial operation.
Marked changes in the Scholler process, particularly in speed of operation, were necessary before it could be considered acceptable under American economic conditions. These modifications were studied in a small pilot plant with no intermediate step to the full-scale plant. For limited periods during test operations, the yield of sugar on the basis of bark-free wood processed was up to design value. Consistent operation however, was not realized.
Following the war, the Tennessee Valley Authority became interested in wood saccharification because of the large amount of non-timber grades of wood and waste available in the area. In 1952, Gilbert and his co-workers reported (15) on pilot-plant studies carried out in co-operation with the Forest Products Laboratory. The purpose of this work was to prepare molasses in quantity for feeding tests and to improve and simplify the hydrolysis process. An economic analysis showed that in favorable locations in the United States, the production of wood-sugar molasses for animal feeds would have been profitable during several of the postwar years. The risks involved and the modest profit obtainable make the process a borderline proposition. A more recent economic evaluation of the process as developed at the Forest Products Laboratory is given by Harris and Lloyd (33).
The hydrolysis of wood in a percolation process using sulphur dioxide as a catalyst has been discussed recently by Ant-Wuorinen (1, 2, 3). The process differs from that of Scholler in being more rapid and in causing less decomposition of the sugar. The process has been carried through extensive pilot-plant tests.
Fouqué (11) has patented a process for the dilute acid hydrolysis of wood using the percolators in the manner of an extraction battery.
Of the dilute-acid percolation processes which have been proposed, only the Scholler process, as followed in Germany and Switzerland, has stood the test of extensive industrial application.
Strong acid processes for wood saccharification
A number of proposals for the hydrolysis of wood fall into the classification of strong acid methods. These methods are characterized by the use of large amounts of concentrated acid which bring about extreme swelling or solution of the cellulose. This serves to break the bonds which hold cellulose in the crystalline state and make it highly resistant to ordinary dilute acid hydrolysis. After extreme swelling or solution, and partial hydrolysis, the hydrolysis must be completed in dilute acid solution.
The very early work on cellulose saccharification made use of sulphuric acid. With improvement, as in the Giordani-Leone method, this process has much in its favor. The Giordani-Leone process (16) and the plant built in Italy during World War II were described by Centola at the Stockholm meeting of the Wood Chemistry Panel (8).
Wood is given a prehydrolysis with dilute acid to remove the hemicellulose. The dried residue is treated with 60° Be sulphuric acid in an edge runner. This mixture is then diluted with the prehydrolysis liquor, and heated to complete the hydrolysis.
The plant operations were stopped by the war before satisfactory operating data were obtained. Centola, however, lists the following advantages and disadvantages of the process:
"Advantages
1. high yield of alcohol;
2. recovery of furfural and eventual other byproducts such as acetic acid and ethereal oils;
3. possibility of working with sawdust as well as with chips.
Disadvantages
1. high consumption of acid;
2. necessity of solving the problem of placing the large quantities of calcium sulphate."
Hydrofluoric acid has received some attention as a reagent in wood saccharification (12, 36). The lignin residue resulting from this process was at one time believed to be a promising raw material. A discussion at the Stockholm meeting of the Wood Chemistry Panel brought out the view that this process had not given good results.
In the process developed by Guinot (18), hydrolysis of cellulose is accomplished by means of formic acid in the presence of sulphuric acid. At 75 to 80 degrees, hydrolysis is completed in about two hours. Yields are said to be high.
The only strong acid method which has had any degree of commercial success is the Rheinau or Bergius method (4) fuming hydrochloric acid. In 1940 a full-scale plant using this process was built at Regensburg, Germany. The following description of the operation was obtained following World War II (45):
"Wood is hogged to chips not more than 1 centimeter in the long dimension, and conducted by a pneumatic conveyor to a Buttner rotary drier... Waste stack gas and the wood move parallel through the drier, and the moisture content is lowered to 6 percent. The wood is then loaded into 50-cubic-meter digesters lined with rubber-and acid resistant brick and extracted with 50 percent (by volume) hydrochloric acid. There are two parallel batteries of 14 extractors, half of each being extracted with concentrated acid, and the other half with water. The total time cycle per digester is 55 hours. As a result of the countercurrent extraction, with acid, a syrup is obtained consisting of water with 32 percent sugar, and 28 percent hydrochloric acid. This syrup goes to an evaporator system operating at 30 to 44 millimeters at 40° C. where the sugar concentration is raised 60 to 63 percent and the acid concentration lowered to 2 to 5 percent. These evaporators have separate heaters in which syrup circulates inside of porcelain tubes. The head of the heater is of rubber-lined steel. Steam is then injected into the syrup to reduce the acid concentration. The carbohydrate in solution at this point consists primarily of oligosaccharides, and in order to convert it to monosaccharides it is "inverted" by diluting and boiling. The residual acid is sufficient to catalyze the hydrolysis. The product is neutralized with lime and used for the production of yeast. The substances in solution consist of 70 percent glucose, 10 percent pentose, and 20 percent calcium chloride...The recovered acid, and the dilute acid washings pass to an acid recovery system where water is removed. This is accomplished by the addition of calcium chloride which increases the concentration of HCl in the gas phase. Water is continuously removed from the calcium chloride in the same apparatus. Conditions in this piece of equipment are extremely corrosive, and the operation is troublesome. The temperature is 145°C. In order to resist such conditions - high temperature, high acid concentration, and the presence of calcium chloride - the apparatus is lined with rubber and a double layer of brick. The heating tubes are of copper plated with gold or platinum.
Equipment was being installed at this plant for the prehydrolysis of wood previous to its use in the Bergius process. Three 100-cubic-meter cookers consisting of steel lined with rubber and acid-resistant brick were to be loaded with 10 tons of wood and hydrolyzed with 100 cubic meters of 1 percent hydrochloric acid at 127° to 128° C. for four hours. A pump was to be used to provide outside circulation. The residue was then to be washed with water and dried... This plant, at the time of investigation, was getting ready to produce crystalline glucose from wood sugar. By using a dilute acid prehydrolysis the crystallization can be made much easier by removing sugars other than glucose. The prehydrolysates are satisfactory for yeast production."
At the Rheinau plant, ion-exchange equipment had been installed and found to give syrups producing high yields of crystalline glucose (45). These modifications have recently been evaluated by Schoenemann (8).
The problems associated with the development of an economic saccharification process for cellulosic materials are numerous and complex but they appear no more difficult than those which faced many of the present chemical industries during their development. The saccharification processes developed to date have met these problems in various ways but in none has complete cognizance been taken of all the difficulties, with the result that none have been economically successful without subsidy. The following discussion will cover the major points that should be borne in mind while weighing the merits of proposed processes and also point the general direction in which research could be done.
The cellulosic raw materials may be broadly grouped into two classes: agricultural residues, which are harvested annually, and wood residues, which may be harvested continuously as desired. Both are available in continuous supply at low cost in the locality where they are grown. However, the annual crops usually are an expensive raw material when the cost of collection and storage is considered. Most of them also have a very low bulk density and large equipment and facilities are required. Annual crops have a further disadvantage in being subject to seasonal variations which require minor process variations. Wood is superior as a raw material in the general aspect to the annual crops but it too has disadvantages. It has a low bulk density, and the cost of handling and pre paring' wood for the saccharification step is high, ever when using the best methods and equipment available For instance, the handling of logs and conversion to chips in the Scholler process amounts to approximately one-half cent per pound of sugar produced (33).
The process should be matched carefully to the physical and chemical nature of the wood available The strong acid process sets limitations on the raw material; sawdust often available at low cost (including handling) is unsuitable. Any process which requires bark-free chips incurs a large increase in raw material cost.
The bulk density of wood is an important factor in the plant cost. Saving could perhaps result from the development of a continuous process.
The only practical path from cellulose to sugar appears to be acid hydrolysis and this fact requires that all processes use acid at least throughout the hydrolysis step. This is a critical point for consideration, as the type of acid and its concentration has a most important effect on plant cost. In the strong hydrochloric acid process, the major portion of the plant must be acid-resistant and estimates of plant cost (8) indicate a depreciation cost amounting to at least one-half cent per pound of sugar production on the basis of a large plant. This amounts to more than $200 of plant investment per annual ton of product, putting it in the class of expensive chemical plants. The plants using dilute acid require much less corrosion-resistant equipment and investment costs are somewhat less than half those quoted above.
Plant heat requirements depend largely on the end use of the hydrolyzate. In the case where the product is crystalline glucose or molasses, the heat load is enormous. In the Bergius process, heat is required to recover the strong acid used for the primary hydrolysis, while in the dilute acid process the main consumption is in the evaporation of the dilute solutions to molasses. Methods for increasing the sugar concentration in the hydrolyzates of the dilute-acid process have been employed by Cederquist (52), and this gives an economic advantage to the dilute-acid process. In the event that the sugar is to be used in solution for the production of yeast, alcohol, or other product easily separable from dilute solution, the heat load is greatly decreased, giving dilute-acid processes a decided advantage over strong-acid processes.
Chemical costs for the dilute acid processes are small, amounting to less than one-fourth cent per pound of sugar. In the strong acid process, they become significant, being estimated by Schoenemann (8) as more than one-half cent per pound of sugar produced even after full advantage is taken of modern recovery methods. The chemical cost of the strong sulphuric acid process makes it unattractive except under certain circumstances where the acid might have further use. This is considered in a later portion of this paper.
Considering the high costs of handling the raw material, the chemical costs and the heating load, it is apparent that every effort should be made to obtain full utilization of all the products available and these should be obtained in the highest possible quality compatible with cost. No commercial ventures have succeeded in such utilization. In no case has the lignin, which amounts to 20 to 30 percent of the dry wood substance, been used successfully other than as fuel. The hemicellulose fraction in some cases has been recognized as needing much milder treatment than the resistance cellulose, but no advantage has been taken of the fact that at least one unique chemical, furfural, may be obtained from this pentosan fraction. In most cases the hexosan sugar product has been utilized as a crude molasses containing large quantities of unknown impurities. Some of these undoubtedly could be profitably removed as higher priced organic chemicals, resulting in a twofold gain. In the case of the strong acid process, a high-quality sugar fraction is obtained. There has recently been proposed (8) a scheme for manufacturing crystalline dextrose which would market approximately 55 percent of the sugar product as food, the rest being marketed as molasses. The production of high-quality sugars from dilute acid wood hydrolyzates is poorly understood and should be the subject of research. Cederquist (32) has suggested a method for purifying the sugar by the addition of sodium chloride which forms a binary crystal with glucose. The yields of sugar from all processes are high. New research could profitably be done in the direction of increasing the yield of total products.
Waste disposal problems in an efficient saccharification process could be made small; molasses for stock feed and the concentration and burning of most of the other unwanted organics would solve most of the problems at moderate costs. One point worthy of mention here is the production of alcohol by the current Scholler process. The organic solubles and unfermented sugars in the still bottoms have an intolerably high biological oxygen demand. This is a case where the waste disposal problem would be nearly eliminated if the pentosan fraction were utilized.
Research on the chemistry of the saccharification process
A study of the literature of wood saccharification shows that more effort has been spent on pilot plant studies and on technology than on the basic chemistry of the process. There is need for much useful chemical research particularly on the fractionation of cellulosic materials, the kinetics of cellulose hydrolysis and sugar decomposition, and in the modification of cellulose prior to hydrolysis.
The fractionation of cellulosic material. The prehydrolysis of cellulosic material, with separation of the nonglucose sugars, has long been recognized as a desirable step in saccharification processes. The concentration of the glucose into one fraction and the separation of pentoses and other sugars into another fraction is favorable to improved utilization. Too little is known of the behavior of the various species in the prehydrolysis process, and data are not available to permit a choice of conditions giving the best compromise in yield and purity of the main glucose fraction.
As a parallel study, methods should be sought for the separation of lignin in a more useful form from the original cellulosic material, from the prehydrolyzed residue, or as a final product after total hydrolysis. A change in viewpoint might be needed here giving lignin the primary attention and considering carbohydrates the byproduct of the process.
The kinetics of cellulose hydrolysis and sugar decomposition. The study of the cellulose hydrolysis reaction is an essential part of any research program directed toward improving saccharification. There is an extensive literature on the kinetics of the hydrolysis of cellulose, both in regard to cellulose structure and in regard to utilization processes. Early research on the subject is reviewed in reference works on wood chemistry (20, 56).
Early work on the kinetics of the dilute acid hydrolysis of wood was due to Thiersch (53) and Lüers (34) and was concerned with the development of the Scholler process. The dilute acid saccharification process was shown to be a consecutive first-order reaction. Later work at the Forest Products Laboratory confirmed these facts and showed that the two reaction rates are affected differently by changes in acid concentration and temperature (23). High temperature is favorable to high yields of sugar. This fact opened the possibility that a rapid two-stage process could give yields comparing favorably to those obtained in more elaborate processes. A report by Cederquist on a two-stage process developed in Sweden during World War II lends support to this view (52).
Work carried out in Australia by Kurth and others (31) and Foster and Wardrop (9) adds valuable information in the field. Very extensive research has been carried out by Kobayashi on The Kinetics of Wood Saccharification at Lower Temperatures with Dilute and Strong Sulphuric Acid (27). This paper describes the effect of & very wide range of acid concentrations and temperatures on the rates of cellulose hydrolysis and on the decomposition of all the sugars commonly found in wood. It is on the basis of such work as this that we must expect to devise new and improved processes.
Modification of cellulose prior to hydrolysis. Kinetics studies show that the simple dilute acid hydrolysis process, as ordinarily applied, suffers from an unfavorable ratio between the production rate and the decomposition rate of the sugar.
In theory, this ratio might be affected favorably by protecting the sugar (actually accomplished in the Scholler process by removal from the reaction zone) or by modifying cellulose in such a way as to increase its rate of hydrolysis.
The basic reason for the resistance exhibited by cellulose toward hydrolysis is its crystalline organization, hence reduction in the crystallinity of cellulose results in increased yield of sugar. Attempts to use ultrasonic energy for this purpose were unsuccessful. Extreme grinding of cellulose was effective in causing large changes in rate of reaction and in yield. While grinding of dry cellulose is expensive, some novel method combining extreme attrition and hydrolysis might be useful.
Irradiation with cathode rays causes chemical changes in organic materials. The cellulose crystallite surface is not a barrier to the rays and chemical changes are brought about throughout the crystallite. This has the effect of increasing the accessibility of cellulose and hence increasing the rate of hydrolysis. Such increases in rate of hydrolysis are brought about at the expense of conversion of some carbohydrate to noncarbohydrate material. The net effect, however, is very favorable.
Large changes can be effected in the crystallinity of cellulose by treatment with amines. The potential usefulness of such a process has not been evaluated in the field of wood saccharification. It should be emphasized here that there is an urgent need for more research of the sort which broadens our understanding of this problem, irrespective of immediate practical application.
Improvement in cellulose hydrolysis technology
There are a number of aspects common to all wood hydrolysis processes which present serious economic obstacles. These have to do with chemical costs, wood procurement, wood storage, and wood handling. In the plant, corrosion and heat load are serious problems. The hydrolyzed residue presents handling problems. All of these points can benefit from novel and improved processing techniques. Schoenemann in a recent publication showed that many of the technological problems which contributed to previous uneconomic operation of the Bergius process could be rectified. He points out that, in (Germany, the most practical end product of wood saccharification is glucose. In order to obtain a maximum of crystalline glucose, the final solution must be as pure as possible. This is accomplished by a prehydrolysis of the wood to remove extraneous carbohydrates and by the use of ion exchange to purify the final sugar solution.
The heat load of the Bergius process and the acid loss can both be reduced by taking full advantage of vapor-pressure relationships in the water-glucose-hydrogen chloride system. These points and others have all been covered thoroughly by Schoenemann in a brochure (47) and in a paper presented at Stockholm (8). There is little doubt that great improvement can be effected in the Bergius process by taking full advantage of Schoenemann's proposals.
The studies carried out by Kobayashi in Japan on the kinetics of wood hydrolysis and prehydrolysis were paralleled by other studies on the technology of
wood saccharification. Sawdust was treated with strong sulphuric acid using expeller type impregnators to give high yields of sugar. The economic optimum appeared to result from using equal parts of wood and sulphuric acid. The sulphuric acid in the process was then used for the production of ammonium sulphate or phosphoric acid effectively cancelling out the cost of catalyst acid and neutralizing agent in the strong acid saccharification process.
The reports of Kobayashi (26, 28, 29, 30) contain extensive data on the decomposition rates of sugar in sulphuric acid, in phosphoric acid, and in aqueous ammonia solutions. Material balances are presented for the various processes. Similar studies employing strong sulphuric acid were done in the United States on the hydrolysis of corncobs (15).
Direct production of materials other than sugar by hydrolytic processes. The production of crude or refined sugars from cellulosic residues has the advantage of an unlimited market for the product. - There is the disadvantage, however, of low and variable price, particularly for crude molasses.
By a simple modification or extension of the hydrolysis process, the primary products can be furfural, hydroxymethyl-furfural, levulinic acid, and formic acid. Furfural is a chemical with an established and growing market and it is obtained exclusively from pentosans. There is no large market for hydroxymethyl-furfural levulinic acid, or formic acid, but markets might be created with an established source of supply. These materials can be made as the only products of a wood hydrolysis plant, or they can be made from the prehydrolyzate only, while the glucose-rich wood sugars are used for other purposes. Additional research is needed to establish the conditions required for their efficient production, or to minimize their formation during hydrolysis when sugar is the desired product.
Wood saccharification is seen to have many facets with many possible combinations of process and product. No comparison of processes is possible without specifying the place where a plant is to be located and the purpose it is to serve.
Thus in some locations the production of refined glucose from wood might be an attractive proposition. In the United States, such a process would merely reduce the consumption of corn, a commodity already in surplus supply and selling at a price controlled by the Government. Similarly, chemicals from cellulosic residues might find a ready market in the United States but in underdeveloped areas they would have no value.
As another example of the hazards of generalizing about saccharification processes, the cost of the acid in the strong sulphuric acid method of saccharification appears prohibitive, but when the acid serves the double purpose of saccharification and the production of ammonium sulphate or ammonium phosphate fertilizer, the process deserves more attention (26, 28, 29).
In directing a research program on the saccharification and chemical utilization of wood residues, the following points are worthy of special consideration:
Present evidence indicates that no truly simple saccharification process can be anticipated in the foreseeable future. Any efficient plant must be large, relatively complicated, and will require a high level of technical skill in its management and operation. In these respects, wood saccharification plants will be similar to modern well-operated chemical plants.
All evidence suggests that wood hydrolysis processes cannot become important until fuller utilization of the incoming raw material is realized. No single product is sufficiently valuable to pay the cost of raw material collection, handling, and processing.
The immediate outlook for wood saccharification is not highly encouraging. Available processes are suited mainly to special situations. Processes of greater general utility should be developed. Extensive and well co-ordinated research is required to accomplish this end.
The technology of existing processes might be improved, but it is important to encourage work on new techniques and new approaches to this longstanding problem.
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