EP0051237B1 - Process for preparing water-soluble saccharides from cellulosic material - Google Patents

Abstract

1. A process for obtaining water-soluble saccharides from cellulose-containing material by treating the latter with gaseous hydrogen fluoride, optionally diluted with an inert gas at temperatures between about 20 and 120 degrees C, preferably between about 40 and 80 degrees C, which comprises subjecting cellolignin, a material composed largely of cellulose and lignin and being essentially free of pentosans and hexosans which has been obtained by pre-hydrolysis of natural cellulose-containing material with dilute mineral acid at an elevated temperature and under an elevated pressure, to treatment with hydrogen fluoride.

Description

Cellulose-containing materials are found in nature in large numbers and diversity. A known such natural cellulosic material is e.g. B. the wood. It consists essentially of cellulose (a material mainly composed of glucose), hemicellulose (a substance mainly composed of pentoses and hexoses) and lignin (a polymeric substance with aromatic rings substituted by methoxy groups). The recycling of wood is done in a variety of ways, e.g. B. for heat generation (firing), as a building material in the furniture and building materials sector, etc.; Purely chemical recycling of the wood is also possible.

Chemical digestion processes that not only separate wood into its components, hemicellulose, cellulose and lignin, but also break it down and transform it, have long been known. The chemical processes generally provide aqueous solutions of mono-, di- and oligomeric saccharides, which may be subjected to post-hydrolysis to glucose or directly fermented to ethanol, concentrated or evaporated to dryness. Possible areas of application for products obtained in this way include: B. in the field of animal feed additives or preferably that of fermentation raw materials.

In the past, two principles have been applied on an industrial scale among chemical processes for wood saccharification: wood digestion with concentrated aqueous hydrochloric acid (Bergius-Rheinau-Udic) and digestion with dilute sulfuric acid (Scholler-Tornesch-Madison); see z. B. Ullmann's Encyclopedia of Industrial Chemistry, 3rd Edition, Vol. 8 (1957), pp. 591 ff.

CH patent 246773 relates to a minor modification of the Scholler process. Very dilute aqueous acid mixtures are allowed to act on native material (e.g. wood). In addition to sulfurous acid, these acid mixtures can also contain sulfuric acid and / or hydrohalic acids (including HF as aqueous hydrofluoric acid).

British Patent Specification 271 410 describes a process in which the action of 10% aqueous sulfuric acid or aqueous bisulfate on wood is followed by a drying process and thus a concentration of the sulfuric acid at 100-1200C. This now relatively concentrated aqueous sulfuric acid-containing reaction mixture is z. B. CaF ₂ added to generate hydrogen fluoride in situ, which should further digest the substrate. However, it is considered advantageous to keep an excess of sulfuric acid in order to achieve a combined action of aqueous sulfuric and hydrofluoric acids.

The digestion of cellulose-containing raw materials with anhydrous hydrofluoric acid has also been repeatedly investigated. Here, however, all the processes that have become known have not yet led to a technically satisfactory solution.

DE-C-560 535 describes the digestion of wood with liquid or vaporous pure HF at low temperatures, the HF being recirculated by evaporation or blowing off and subsequent condensation.

In continuation of this work, DE-C-585 318 describes a process for the digestion of wood with gaseous hydrogen fluoride, which involves three stages by absorption of HF on wood at 10 ° -20 ° G, the digestion at 20 ° -50 ° C and the desorption works at 100 ° -150 ° C, where the HF can be diluted with an inert gas stream. The cooling effort for the condensation of HF has a disadvantage here, as does the fact that when condensation occurs, only a very uneven distribution of the hydrogen fluoride on the reaction material occurs, a circumstance which can only be counteracted by very long dwell times or a sharp increase in the use of hydrogen fluoride otherwise the yields can be severely impaired.

DE-C-606009 describes an extraction with liquid HF, which, however, requires large quantities of HF and is disadvantageous in that large amounts of heat are added to the evaporation of the hydrogen fluoride from the extract and extraction residue (lignin) and during the subsequent one Condensation must be removed again.

More precise information on yields in processes of this type can be found in Angew. Chem. 46 (1933) 113/7, wherein when the HF is absorbed from the gas phase in a vessel with external cooling from 0 ° C. with a loading of 50% by weight HF based on wood, 32% sugar based on the carbohydrates present, and with 100% by weight loading 86% sugar, based on carbohydrates, is achieved. No further details are given there about the repatriation of the HF.

All of these processes have the disadvantage that they consume large amounts of the expensive hydrofluoric acid, the recovery of HF from the reaction products being very expensive and, in practice, large HF losses occurring.

A more extensive process is described in AT-A-147 494, where the previous state of the art is presented as follows: «If you work with highly concentrated or anhydrous hydrofluoric acid in a liquid or gaseous state at low temperatures, the wood is broken down very little uneven and therefore imperfect. First of all, at such low temperatures, the distribution form of the hydrogen fluoride, which is present as a fine mist in the air, is a very uneven one, all the more so since the air present makes the reaction more uniform. On the other hand, it is known that when saccharifying wood with concentrated fluorine water Both in the liquid and in the gaseous state, the wood particles react quickly on the surface with the concentrated hydrogen fluoride, form a hard, rather impenetrable skin and shrink, which inhibits the further penetration of the gas into the interior. In addition, the penetration of the wood particles is already made difficult by the air in the cells. An outer crust forms very quickly, which includes unsugared material and prevents further saccharification. To eliminate these drawbacks, it has also been proposed to carry out the digestion with concentrated liquid hydrofluoric acid using an extraction process or to avoid crust formation by adding inert gases to the hydrofluoric acid in order to achieve a more uniform and complete digestion. However, the extraction process works with a disproportionately high excess of hydrofluoric acid, and the reactant retains large amounts of hydrofluoric acid without preventing the crust formation with all of its disadvantages. The dilution with inert gases can reduce the formation of crusts somewhat, but never cancel them out and also does not cause them to penetrate evenly into the interior of the wood, since the wood is filled with air. As is well known, wood consists only of the smallest part of wood mass itself and for the most part of air, which is located between and in the wooden cells. For example, a practically anhydrous wood consists of approximately 15% wood mass and approximately 85% air. Since the wooden cells are extremely small in relation to the size of the wood, no matter how small it is shredded, the air content plays an outstanding role even with sawdust. »

Hardening of the surface of wood particles also seems to have been found in the saccharification of wood with aqueous mineral acids such as aqueous hydrochloric or sulfuric acid, because Z. Angew. Chem. 37 (1924) 221 the substances present in the wood such as lignin, mannan, galactan etc. are referred to as "incrustations", which should also be removed as possible before the actual wood saccharification due to disruptive degradation products (furfural, acetic acid, formic acid etc.). For the removal - since the hydrolyzability of these "incrustations" was known - even in the case of wood saccharification using hydrogen fluoride, one could have thought of a kind of "pre-hydrolysis" with dilute mineral acid at elevated temperature and possibly increased pressure. However, such pre-hydrolysis had not been considered; rather, in order to avoid the disadvantages of Hoch and Bohunek described above, it was proposed to apply a vacuum of approx. 30 torr (40 mbar) to the saccharification of wood with hydrogen fluoride [AT-A-147494 + addition 151 241; the wood saccharification process with hydrogen fluoride according to Hoch and Bohunek is also described in the journal «Holz Roh- und Werkstoff» 1, pp. 342-344 (1938)].

Disadvantages of this method are the difficulties of the technical inevitably occurring when working in a vacuum. Realization, as well as the relatively complicated reaction process. A deficiency inherent in all processes is the formation of mixtures of pentoses and hexoses by simultaneous hydrolysis of the hemicelluloses and the cellulose of the wood.

A further problem is the separation of the acetic acid formed during the hydrolysis of hemicellulose, which makes it as difficult as possible to circulate the HF without loss, and the easy decomposition of the pentoses to furfural.

Surprisingly, it has now been found that these disadvantages of the prior art described can be avoided and a slight saccharification of cellulose is possible if the plant materials are not in their native form, but after a pretreatment in the form of "cellolignin" with anhydrous, gaseous HF unlocks.

“ _C ellolignin” here means vegetable materials such as wood, straw, bagasse and similar raw materials that have been subjected to a pre-hydrolysis known per se.

During the pre-hydrolysis of wood, its structure is largely preserved, but the cellolignin that can be obtained in this way has a much more wear-resistant and porous quality than the native state, so that HF, even when mixed with air or another inert carrier gas, can easily penetrate without one Encrustation of the surface occurs. Working in a vacuum is not necessary.

Another advantage of using cellolignin instead of native wood is that the reaction material is significantly easier to handle in terms of process technology. On the one hand, this is due to the fact that cellolignin has a bulk volume of only about half that of wood of the same grain size and thus has a significantly smaller degree of shrinkage when digested with hydrogen fluoride gas. B. means a great relief for the dimensioning of reactors. On the other hand, reaction material made of cellolignin remains pourable and free-flowing even when it is loaded with hydrogen fluoride, whereas that made from native wood tends to stick together due to resinous accompanying substances, as well as cleavage products of the hemicelluloses, and is difficult to convey.

Naturally, such a tendency to stick also complicates the desorption of hydrogen fluoride, especially if it is to proceed quickly and as quantitatively as possible. However, this is readily possible when using cellolignin as the substrate.

Furthermore, in this process, it is no longer necessary to separate the sugar mixtures formed in the hydrolysis of hemicellulose from the oligomeric glucose components or glucose formed in the hydrolysis of cellulose, which enables these different sugars to be used more easily by fermentation.

It is also advantageous that when cellolignin is digested, no acetic acid and no furfural more arise so that the HF can be circulated without having to condense these components. This avoids separation difficulties and RF losses.

Another advantage is the possibility of absorption of HF on cellolignin above the boiling point of HF, so that external cooling is no longer necessary. It was also surprising that in the process according to the invention yields of> 90% glucose or oligomeric glucose, based on the cellulose used in the cellolignin, are achieved in a simple manner, the resulting sugars being of high quality, i.e. H. are almost colorless.

The subject of the invention is therefore a process for obtaining water-soluble saccharides from cellulose-containing material by treating the same with gaseous hydrogen fluoride, optionally diluted with an inert gas, at temperatures between about 20 and 120 ° C., preferably between 40 and 80 ° C., characterized in that cellolignin , a material consisting largely of cellulose and lignin, essentially free of pentosans and hexosanes and obtained by prehydrolysis of natural cellulose-containing material with dilute mineral acid at elevated temperature and pressure, which is subjected to treatment with hydrogen fluoride.

The cellolignin which is particularly suitable according to the invention for degradation to water-soluble sugars is obtained by pre-hydrolysis of natural cellulose-containing material (wood, straw, bagasse, etc.) with dilute aqueous mineral acid, preferably dilute hydrochloric or sulfuric acid. Prehydrolysis is - as already indicated in the description of the prior art - known in wood saccharification and consists in a relatively short-term treatment of the natural starting material with a highly dilute mineral acid at elevated temperature (preferably between about 100 and 160 ° C) and increased pressure (preferably up to about 10 bar), essentially splitting the pentosans and hexosans contained in the hemicelluloses down to the monomer units (xylose, arabinose, mannose etc.). Depending on the reaction conditions, these can then be isolated as such or undergo further changes, e.g. B. by dehydration to furfural.

The digestion with hydrogen fluoride according to the invention can be accomplished, for example, in such a way that the cellolignin, which has been dried to a moisture content of 0 to about 20%, advantageously about 2 to 5% and comminuted as required, is either discontinuously in a suitable stirred vessel made of hydrogen fluoride-resistant material Bring HF gas in contact, optionally in a mixture with air or another inert carrier gas, or that an HF-containing gas mixture is advantageously guided towards a continuous flow of the substrate to be digested in a conveyor system.

Due to the spontaneously released heat of reaction, the temperature rises and can be carried out by a suitable reaction such. B. dilution with inert gases in the desired range between about 20-120 ° C, preferably between 40 and 80 ° C.

The contact of the substrate with hydrogen fluoride gas is maintained until one part by weight of the material has absorbed about 0.2 to 3.0, preferably about 0.4 to 0.8, part by weight of hydrogen fluoride.

The reaction is now advantageously carried out in such a way that, depending on the type of substrate and the conditions of the HF absorption, a residence time is selected which is sufficient to achieve the high yield. Longer dwell times are not disadvantageous, but they are also of no advantage. They can be between about 15 minutes and several hours. Reaction conditions in which the residence time does not exceed about 1 hour are preferred.

The subsequent HF desorption can, according to the prior art, be carried out by heating the reaction mixture and / or by evacuation or by treatment with an inert gas stream (for example nitrogen, air, CO ₂ or noble gas) with or without a suitable starch Heating and / or evacuation. The hydrogen fluoride recovered in this way can be isolated by condensation or reacted directly with fresh substrate, so that a cycle of gaseous hydrogen fluoride is formed. The further processing of the now digested ("saccharified _" ) material can also be carried out in a manner known per se, as described, for example, by K. Fredenhagen and G. Cadenbach, Angewandte Chemie 46 (1933), pp. 113 to 117 Extracts with hot water, for example, filters off the insoluble lignin, neutralizes the small amount of hydrogen fluoride carried in the filtrate by means of calcium carbonate or hydroxide and concentrates.

The amount of "wood sugar (or" straw sugar _" etc.) obtained after drying the evaporation residue is consistently over about 90% of the cellulose contained in the substrate (calculated on dry substance) in the process according to the invention.

Because of the high “sugar” yield, the extraordinarily simple and smooth process implementation (increasing the porosity of the substrate and thereby facilitating the penetration of HF) and also the energy-efficient absorption of hydrogen fluoride (no cooling necessary, no vacuum), the invention does not constitute one insignificant progress in this area.

The oligomeric glucose building blocks can be subjected directly to further utilization (fermentation to ethanol, concentration or evaporation and use as animal feed additives or as fermentation raw materials etc.) or also in a manner known per se to post-hydrolysis to monomeric glucose.

The invention is now explained in more detail by the following examples:

example 1

500 g of spruce wood cellolignin (59% cellulose + 41% lignin) with a grain size of approx. 2 mm were placed in a round 2 liter vessel made of transparent polyethylene with a stirrer, thermometer and gas inlet and mixed with a mixture of air and hydrogen fluoride gas prepared by passing air over liquid hydrogen fluoride at 200C (water bath) treated. The material was slowly stirred and turned dark brown. The air flow and HF evaporation were regulated so that the internal temperature did not exceed 70 ° C.

After 300 g of hydrogen fluoride had been taken in, an internal temperature of 50 ° C. was maintained for 30 minutes. With further stirring, the hydrogen fluoride was then expelled by introducing warm air. Part of the necessary desorption heat was also applied by external heating. Desorption was continued with a steadily increasing temperature up to a hydrogen fluoride content of about 5% in the substrate. The material was then transferred to a fluid bed dryer and the hydrogen fluoride was blown off to a residual amount of approximately 0.5%. The resulting HF-air mixtures could be used immediately for further approaches.

The reactor contents were then digested for 15 minutes with about 2 l of hot water, suctioned off sharply and washed with a little water. The dark brown filter residue weighed about 250 g after drying and thus consisted of 82% lignin and 18% undigested cellulose. The filtrate was made alkaline while still hot with technical calcium hydroxide, the excess hydroxyl ion was neutralized with carbon dioxide, and the calcium fluoride and carbonate were filtered off, possibly with the aid of a filtration aid. The clear, pale yellow, neutral solution was brought to dryness in a vacuum. This gave about 250 g of slightly yellowish wood sugar, corresponding to a yield of 85% of theory. Th. The product was clearly water-soluble and contained between 2 and 10% monomeric glucose, the rest consisted of oligomeric glucose.

Examples 2 to 13

An uncovered, hydrogen fluoride-resistant tube 30 cm long and 4 cm inside width was filled in a horizontal position with 30 g cellolignin with a grain size of 1-2 mm about halfway up and closed at both ends with pierced rubber stoppers. In the cellolignin layer as well as in the free space above was a thin steel tube perforated over the entire length. These pipes led on both sides through bores in the sealing plugs and served for the supply and discharge of HF-air mixture. In this way it was possible to gasify the cellolignin so that it was lowered to the surface of the bed. The material was allowed to absorb hydrogen fluoride and, during the subsequent dwell time, an internal temperature of 50 ° C. was ensured by appropriate heating. Then, instead of the HF-air mixture, hot air was blown through the bed for 15 minutes and the reaction product thus obtained and freed from the majority of the hydrogen fluoride was worked up, as described in Example 1.

The following table shows the yields as a function of the amount of HF absorbed and the residence time.

Example 14

In a horizontally arranged, long tube made of hydrogen fluoride-resistant material, in which a free-flowing solid can be continuously moved by means of a screw conveyor, a cellolignin filling was countered with a mixture of hydrogen fluoride and carrier gas in such a way that the material at the HF inlet end of the tube had a content of approx Had 60% HF, based on cellolignin, but only a pure carrier gas flowed out at the cellolignin inlet end. The reaction material was continuously discharged at the HF inlet end, while fresh cellolignin was supplied on the opposite side. The discharged material was freed from hydrogen fluoride by blowing off after passing through a half-hour residence time and the HF-rich gas mixture thus obtained was returned to the reaction tube. The digested cellolignin was worked up in the manner already described in Example 1. The yield of wood sugar was approx. 85% of theory. Th.

Claims (1)

1. A process for obtaining water-solubte saccharides from cellulose-containing material by treating the latter with gaseous hydrogen fluoride, optionally diluted with an inert gas at temperatures between about 20 and 120 °C, preferably between about 40 and 80 °C, which comprises subjecting cellolignin, a material composed largely of cellulose and lignin and being essentially free of pentosans and hexosans which has been obtained by pre-hydrolysis of natural cellulose-containing material with dilute mineral acid at an elevated temperature and under an elevated pressure, to treatment with hydrogen fluoride.