Wood saccharification
The saccharification of wood or wood hydrolysis is a method in which grape sugar (glucose) from wood is recovered. This happens through a hydrolytic breakdown of the cellulose , which makes up about 50% of the wood.
history
In 1791 Johann Tobias Lowitz made attempts to extract sugar from starch in Petersburg. In 1856 G. F. Melsens (France) and a little later Bachet and Marchand, A. Payen, Zetterlund attempted saccharification of wood with hydrochloric acid . The sugar yields were around 20%.
Flechsig and Häglund later undertook experiments with 72% sulfuric acid , carried out the reaction with cooling (18 ° C., 2 days), then diluted the mixture with plenty of water and warmed the solution. About 55% sugar could be extracted from the wood.
The first wood saccharification plant was built by Ewen and Tomlinson in 1910 in Georgetown. They used the waste from a sawmill and fermented the resulting sugar into ethanol . They received 6.4–9.5 liters of alcohol per 100 kg of wood waste. The annual output of the factory was 2,000,000 liters of ethanol. Since around 150 tons of wood waste had to be transported every day, the plant could only be operated economically for a few years.
In 1916, two wood saccharification plants were built in Germany under the direction of Alexander Classen (in Monheim near Düsseldorf and in Stettin ).
Heinrich Scholler succeeded in dissolving around 46% sugar substances from wood with highly diluted sulfuric acid (0.1–1%) under pressure (10 atm) and at high temperatures (148 ° C). Several plants for sugar and ethanol production were built using this process.
In 1929, Helferich and Böttger ( Greifswald ) found a process that used hydrofluoric acid and low temperatures to dissolve cellulose. Hydrofluoric acid can be easily recovered by distillation (boiling point: 19 ° C).
In 1924 Friedrich Bergius was able to obtain around 78 kg of dry sugar (85% reducing sugar) from 100 kg of dry wood by treating it with hydrochloric acid. In 1926 the Soc. On. de la Cellulose built. With a ton of wood throughput per shift, this produced very good sugar yields. However, operations were discontinued in 1928.
In 1936 Werner Oswald founded Holzverzuckerungs AG (Hovag) based in Zurich . Hovag began to produce ethyl alcohol from wood as a substitute fuel for motor vehicles using the “Scholler's process” . With this production 30% of the fuel demand in Switzerland was covered by the end of the Second World War.
Chemical building blocks of wood and their properties in aqueous solutions
Wood consists of lignin , cellulose and hemicellulose .
Cellulose is much more difficult to break down into sugar than starch, since the glycosidic bond between the individual glucose molecules is very stable. Therefore, drastic conditions (e.g. strong acids) or special enzymes are required. The sugar molecules are readily soluble in aqueous acid.
In contrast to cellulose production, in which lignin dissolves as lignin sulfonic acid through treatment with sulphurous acid and can be separated from the insoluble cellulose, lignin remains as an insoluble residue when wood sugar is produced by acid hydrolysis.
There have not yet been any major economic applications for the complex aromatic lignin bodies, so they are mostly incinerated.
Pretreatment
The wood must first be shredded by chippers, mills.
For better digestion, the wood usually has to be pretreated so that the subsequent digestion runs smoothly.
A common digestion of wood is the Organosolv process , which is often used for the subsequent enzymatic digestion. In this process, organic solvents are used to destroy the wood cells from dry wood at a higher temperature (150–200 ° C) and to dissolve the lignin and hemicelluloses. Important solvents are, for example, ethanol and ethylene glycol .
Hydrolysis with acids
Method according to Bergius
In this process, a highly concentrated hydrochloric acid solution flows through the extracted wood. The hydrochloric acid can be separated by vacuum distillation, so that a highly viscous sugar solution with 60-70% sugar content and only about 4% hydrochloric acid content is created.
In Rheinau (Mannheim) and in Regensburg , plants for producing sugar from wood were operated using this process. In the 1950s, however, the plants were shut down for economic reasons. The concentrated hydrochloric acid caused corrosion problems with the materials used at the time.
A process by Hoechst AG around 1980 with hydrofluoric acid produced similarly good results for easy acid separation. However, the post-hydrolysis caused difficulties, which limited the economic results of this process.
Scholler method
Scholler examined the speed of hydrolysis of cellulose with 1% sulfuric acid at 170 ° C (under pressure) - the hot water flows through the wood. Scholler was able to determine the optimal exposure times of the acid to the cellulose. The sugar solution has a concentration of approx. 4%.
According to the Scholler process, three large-scale plants for the production of wood sugar were operated in Tornesch, Dessau and Holzminden . In addition, there were processes based on the Scholler process in Ems (Switzerland) and Korea, as well as around 44 plants in the USSR that worked on a similar principle. Since 1956, all processes for sugar production using these processes have been terminated for economic reasons.
Madison process
In the USA, a fully continuous process was developed that improved the hydrolysis cycle time even further compared to the Scholler process. However, a large plant with a throughput of 75,000 tons of wood per year was shut down due to technical problems after the end of the war. In New Zealand in 1979 an improved system based on the Scholler-Madison process was developed.
Other acid processes
Other methods are based on short-term hydrolysis of only 6–20 seconds at 240 ° C. A large Japanese plant for the saccharification of wood with concentrated sulfuric acid had to be shut down due to technical problems. The Arkenol process also uses concentrated sulfuric acid. The acid is separated off by ion exchange resins or special membranes.
Enzymatic saccharification
Since the 1970s, research has also been carried out on enzymatic cleavage of lignin cellulose. Certain enzymes ( cellulases ) and fungi, bacteria can convert cellulose into sugar. For this, however, a pretreatment of the wood or the lignin cellulose is always necessary in advance so that the cells break open.
In addition to the Organosolv process, lignin cellulose can also be exposed to hot steam at 180–240 ° C for 5–30 minutes. Furthermore, the cellulose chains have to be swollen with acids or bases. Hydrochloric acid has proven to be beneficial for this, since it can be recovered by distillation.
The cellulose pretreated in this way is then exposed to yeast at 50 ° C. for about 5-7 days. 80–95% of the cellulose is converted into glucose.
Cellulases, which break down cellulose, are manufactured by Genencor and Novozymes , for example .
Lignin cellulose, wood growth, use of wood
Forests with high trees are found in America, Northern Europe and Russia. In addition to wood, other substances (leaves, green waste, straw, algae) can also be added to the lignin celluloses group.
The annual growth in wood worldwide has a theoretical energy content of around 41.6 exa-joules. The earth's forest area is around 4 billion hectares. A maximum of 9 tons of dry wood can be obtained per hectare of forest area per year. The annual consumption of primary energy is around 460 Exa-Joule (Exa = 10 18 ) worldwide .
280 million tons of wood are currently being chemically digested and used for the production of paper and textile cellulose. Some of the logs are used as sawn logs and plywood. Around 50% of the logging is needed as firewood and charcoal in many countries.
A large part of the global annual wood growth is in regions that are uninhabited, so that economic development does not seem to make sense - especially since trees are also important stores for carbon dioxide.
If you add other lignin cellulose materials (green cuttings, leaves, straw), around one billion tons are produced in the USA every year.
In Germany, the forest area is 7.4 million hectares, the theoretically usable annual wood growth is only about 0.06 exa-joules / year (0.4% of the primary energy). In the EU-28, the annual usable increase in wood is 1.55 exa joules for wood and 0.47 exa joules for straw (primary energy requirement of the EU 2004: 78.2 exa joules).
In times of need and oil crises, interest in wood saccharification processes grew. At that time, many productions served as a possible emergency supply with sufficient sugar.
Investigations into the conversion of lignin cellulose into glucose and bioethanol are being carried out in the USA, Canada, Brazil and Scandinavia . In the United States, President Bush's Energy Independence and Security Act came into force in 2007 . Congress earmarked over US $ 1 billion to support renewable energies from biomass. This law provides for substantial funding for the production of bioethanol from lignin cellulose. According to the National Biofuels Action Plan , around 20% of bioethanol should be added to regular fuel (E20) by 2020.
Products
In addition to glucose, a number of other by-products are created due to the hemicellulose and lignin present in wood in addition to cellulose . Due to this high level of contamination, the wood sugar solutions are mainly used for fermentation to alcohol or as a nutrient substrate for yeast fermentation . For use in the chemical industry , the solution has to be cleaned and desalinated in a complex process. The saccharification yield of one ton of dry hardwood ( atro ) in the Udic-Rheinau process is 220 kg of crystalline glucose, 70 kg of crystalline xylose, 280 kg of lignin and 220 kg of organic residues.
Biotechnological processes
In the context of the discussion about the development of the biorefinery , wood is discussed as a central raw material for the production of sugar and other products. In this case, however, the saccharification should take place using special enzymes , the cellulases , using a biotechnological method .
literature
- Wood saccharification . In: Herder-Lexikon der Biologie . Spectrum, Heidelberg 2003, ISBN 3-8274-0354-5 .
Individual evidence
- ↑ French Patent 21721.
- ^ Journal of Physiological Chemistry , 7 , 913 (1883).
- ^ Journal of practical chemistry (2), 91 , 358 (1915).
- ^ Friedrich Bergius : Results of applied physical chemistry . Volume 1, Akademische Verlagsges., Leipzig 1931, p. 236.
- ^ Journal of Applied Chemistry, 43, 455 (1930).
- ↑ 1942 Production by HOVAG covered around 30% of Swiss fuel requirements by the end of the war .
- ↑ Ullmann's Encyklopadie der technischen Chemie, 4th edition, keyword: wood saccharification.
- ↑ Naturwissenschaftliche Rundschau , Volume 59, Issue 3, 2006, p. 151 ff.
- ↑ Der Fischer Weltalmanach 2011, p. 720.
- ↑ Position paper, Raw material base in transition, publisher: DECHEMA, GDCh, VCI, DGMK, DBG, Frankfurt, October 2009, p. 19.
- ↑ Federal Ministry of Economics and Technology, Energy Data, Table 31, August 22, 2006.
- ↑ Britt Schumacher: Investigations into the processing and conversion of energy crops in biogas and bioethanol , dissertation University of Hohenheim 2008, p. 19.
- ^ Hans G. Hirschberg: Handbook of process engineering and plant construction. Chemistry, technology and business administration . Springer, 1999, ISBN 3-540-60623-8 , pp. 441-442 ( section on Google Books ).