AU579094B2 - Improved organosolv process for hydrolytic decomposition of lignocellulosic and starch materials - Google Patents

Description

IMPROVED ORGANOSOLV PROCESS FOR HYDROLYTIC DECOMPOSITION OF LIGNOCELLULOSIC AND STARCH MATERIALS

BACKGROUND TO THE INVENTION

1. PRIOR ART

The unique powers of organsolv dissolution by hydrolysis processes have been successfully demonstrated on certain types of cellulosic materials, particularly lignocellulosics. The easiest wood to delignify by organosolv solutions is aspen, while conifers such as spruce and pines and especially hemlock ( Tsuga hetrσphyl la ) showed substantial resistance. Sugarcane rind was found to be relatively easy to hydrolyse. Cotton linters which are essentially pure cellulose, especially the crystalline fraction, were very difficult to hydrolyse by prior art processes. Thus traditionally, organosolv processes have been demonstrated with cellulosic materials which are easy to delignify. Conifers thus have been largely avoided because of their resistance to hydrolysis and the harsher conditions required for their rapid conversion to monomeric products.

Due to the substantially different hydrolysis chemistry and the fear of destruction,of the valuable protein fraction, mixed starch/cellulosic vegetable products such as cerials and tubers have not been hydroysed by organosolv means. The prior art has described various organosolv processes for delignification and/or saccharification of cellulosic materials and stalks of vegetable crops. In general, such processes involve the use of a mixture of water and a solvent such as alcohols and ketones and sometimes other solvents of a specific or non-polar nature along with an acidic compound to facilitate the hydrolysis. In most instances there is a several hour treatment required to accomplish delignification and hydrolysis to low molecular weight products.

Thus prior art processes have been characterized by poor delignification ability, slow hydrolysis rates and extensive sugar degradation into non-sugars, mainly furfurals and acidic fragments. Hence the recovery of free sugars was not very attractive to develop such processes on a commercial scale. All of the prior art organosolv processes of which we are aware of, suffer to some degree from these disadvantages. The disadvantages were long thought to be inherent in the organosolv processes, particularly with regard to the resistance of softwoods to delignification and cotton linters to hydrolysis.

Thus US patent No. 1,919,623 to Dreyfus (1933) describes pre-treatment of comminuted wood with concentrated strong acid dissolved in and acetone-water carrier solvent.

After impregnation of the wood chips the solvent is evaporated thus concentrating the acid in the wood structure. On following heating the cellulose is hydrolysed in situ, the excess acid extracted with fresh acetone-water solvent which is reused for further treatments. No delignification or dissolution occurs during this initial hydrolysis stage. The chips thus treated are then subjected to boiling in aqueous weak acid solution. US P No. 2,022,654 also to Dreyfus (1935) describes a similar approach for the production of cellulose pulp in that wood chips are pre-treated with concentrated acid carried in up to 80 per cent acetone in water to soften the wood and after substantially removing all the acid, the chips were digested for 9 to 12 h at 170°C to 230°C in a pressure vessel using 50 to 80 per cent acetone water mixtures and a non-polar organic solvent. US P No. 2,959,500 to Schlapfer et al describes a hydrolysis process with the solvent consisting of selected alcohols and water and optionally a non-polar solvent at 120°C to 200°C in the presence of small amounts of acidic compounds which were claimed to be non-reactive with the alcohols selected for the process. The process as thought is rel-atively slow and the monomeric sugar yields are much less than quantitative. US P No. 1,964, 646 to Oxley et al (1934) shows slow saccharification with strong acid whereas US P No. 1,856,567 to Kleinert et al (1932) teaches the use of aqueous alcohol at elevated temperatures for production of cellulosic pulps in a pressure vessel using small quantities of acids or bases as delignification aids. The treatment is described in steps of three hours each. prior art is described in US P No. 2 951 775 to Apel in which wood is saccharified by the use of multiple applications of concentrated hydrolchloric acid at 25°C to 30°C. The Hungarian Patent No. D 21 C 3 to Paszner et al deascribes treatment of lignocellulosics in a single step with acidified aqueous acetone solution of up to 70 per cent organic solvent content and claims rapid total dissolution of all types of woods. The sugar yields amounted to 65 to 72 per cent depending on the wood species.

2. OBJECTS OF THE PRESENT INVENTION

The main object of the present invention is to rapidly and quantitatively solubilize and recover the chemical components of lignocellulosic and strach containing materials. A further object of theinvention is to reduce the hydrolysis time and substantially increase the sugar formation rates.

A further object of the invention is to reduce the degradation of the sugars to non-sugars during the high temperature hydrolysis process in the presence of acids. A further object of the present invention is to simultaneously dissolve and then recover separately the chemical constituents of lignocellulosic and strach containing materials to yield mainly pentose and hexose sugars, lignin and protein rich products if the materials is a cerial or starchy tuber. A further object of the present invention is to conduct the hydrolysis in such a way that when the organic volatiles are removed from the hydrolysis liquor, and the lignin or protein residue is separated from the residual solution, higher than 10 per cent by weight of sugar solids is obtainable by the process.

A further object of the invention is to substantially reduce the concentration of acid required to maintain and regulate a given hydrolysis rate and thereby substantially reduce the catalytic effects of acids in degradation of the dissolved sugars at high temperature. Alternatively, the object of the present invention is to reduce the reaction temperature required to achieve a certain reaction rate during the hydrolysis process and thereby maximize the sugar recovery. A further object of the present invention is to reduce the energy required for the hydrolysis by use of the low boiling solvent which has heat capacity and vaporization values substantially lower than known for water.

A further object of the invention is to obtain high purity low DP cellulose on hydrolysis of cellulosic materials, and protein residues on hydrolysis of strachy grains and tubers which could serve as animal fodder, food additive and as industrial filler and adsorbent.

These and other objects will become increasingly apparent by reference to the following description and examples that follow.

3. GENERAL DESCRIPTION OF THE PROCESS

The present invention relates to an improvement in a process for the production of crabohydrate hydrolysates as sugars from a comminuted cellulosic material which can contain lignin by treating the material in a pressure vessel with a solvent mixture of acetone and water containing a small amount of an acidic compound at elevated temperature to form reducing sugars in a liquor, the improvement of which coprises: a. providing mixtures of acetone and water containing at least about 70 volume precent of acetone and the catalytic acid compound as the solvent mixture in the pressure vessel at elevated temperatures with the material to be hydrolysed; b. treating the material in the solvent mixture for a limited amount of time until it is at least partially dissolved and such that at least 90 per cent of the sugars hydrolysed from the polymeric carbohydrate materials are re covered without degradation to non-sugars from the liquor, c. rapidly cooling the liquor by flashing off the organic solvent as it is removed from the pressure vessel.

The present invention also relates to improvements in a process for the production of carbohydrate hydrolysates as sugars, lignin or protein form comminuted lignocellulosic or starch containing materials by treating the materials in a pressure vessel with a solvent mixture of acetone and water containing a small amount of an acidic compound at elevated temperatures to solubilize any lignin and to form reducing sugars in a liquor and to recover a high protein content residue which comprises: a. providing mixtures of acetone and water containing at least 70 percent acetone and the catalytic acidic compound as the solvent mixture in the pressure vessel at the elevated temperatures with the material to be hydrolysed; b. treating the polymeric carbohydrate material in the solvent mixture for a limited period of time sufficient to dissolve not less than 50 per cent of the starting material in one stage at the elevated temperatures selected until the material is at least partially dissolved and such that at least 90 per cent of the solubilized sugars are recovered without degradation to non-sugars wherein the carbohydrates in the cellulosic material or vegetable crops are dissolved and hydrolysed partially or substantially completely; c. continuously removing the liquor from the pressure vessel ; d. rapidly cooling the liquor by controlled flash evaporation of the acetone solvent to retain aqueous solution; and e. recovering the sugars and any lignin and proteins from the aqueous solution.

Unexpectedly, it has been found that acetone in volume concentrations in water greater than 70 per cent with a catalytic amount of an acid greatly accelerated the hydroly sis rates in forming stable complexes with the sugars from the hydrolysis at elevated temperatures where there is limited retention time in the pressure vessel. Such phenomenon where complex formation occurres at elevated temperature in the presence of water causing simultaneous hydrolysis of polymeric carbohydrates well protected by lignins and proteins has not been predictable from the literature and prior art descriptions. The complexes were not believed to exist in aqueous acetone solution especially at such high temperatures used in the present invention. The surprising result of the present invention is that even at significantly reduced acid concentrations the rate of carbohydrate hydrolysis is several ordersof magnitude higher than in water under identical conditions and that substantially no degradation of the sugars takes place during the high temperature hydrolysis process although actone complexes of the sugars are known to undergo hydrolysis themselves some 100 times faster in aqueous acidic solutions than alkyl glucosides or the polyglucan itself. The accelerated hydrolysis rate of the main decomposition is found to provide the opportunity to substantially reduce the amount of solvent required earlier to achieve a certain degree of dissolution and sugar recovery: The lower substrate to solvent ratio materailly lowers the energy requirement for the process.

A further benefit to the process acrues from the advantageous physical and chemical properties of the complexes themselves. It is for instance known that sugar-acetone complexes have different volatility at high temperature depending on the sugar species. Thereby the vapour pressure of the pentodes is much higher than that of the hexoses whereby the pentoses can be made volatile under certain conditions. Further the various sugar complexes show also differential sensitivity to weak acid hydrolysis or hydrolysis on ion exchange resins. Whereas the sugar-solvent complexes are soluble in non-polar organic solvents those of the free sugars are not. Thereby it is possible to separate the individual sugar species based on selective hydrolysis sensitivity and solubility in selected solvents. This gives the process unprecedented power in quantitative decomposition of renewable organic materials and isolation of the decomposition products. Complex formation of monomeric sugars in anhydrous acetone in the presence of 2 per cent added strong mineral acid as catalyst at lower than ambient temperature is described in Methods in Carbohydrate Chemistry, Vol. II, pp 318.

The term "cellulosic material" includes materials of vegetable and woody origin which may or may not be lignified, generally in comminuted form.

The term "starchy materials" includes cerial grains and tubers of vegetable origin, which may or may not be mixed with cellulosic above ground biomass or protein, generally in comminuted form.

The acidic compound can be of inorganic or organic origin and should be inert with respect the solvent. Strong acids such as sulphuric, hydrochloric and phosphoric acids are preferred; acidic salts such as aluminium chloride, sulphate, ferric chloride and organic acids such as trifluoroacetic caid can also be used.

The elevated temperatures are between 145ºC to 230°C and most preferably between 155°C to 210°C.

The catalytic amount of acidic compound is preferably between 0.025 to 0.5 weight per cent of the solvent used. Smaller amounts are also effective especially when higher temperatures are selected.

A reaction time, for treatment of less than required to dissolve 50 to 70 per cent of the solid residue at the particular acid concentration and reaction temperature should be used and allows high yield of reducing sugars in the monomeric form. The sugar exposure time to high temperature will generally regulate the solvent feeding rate to the reactor and will generally depend on the acetone concentration, acid concentration and the temperature used. Thus for very rapid hydrolysis the acid concentration should be about 0.04 to 0.06 Normal, acetone concentration about 80 per cent and the temperature over 200°C. However , for near theoretical sugar yields, low acid concentration (0.02 Normal and less), high acetone concentration (above 80 per cent) and high temperature (above 200°C) are most suitable.

The prior art weak acid and alcoholic organosolv processes are relatively slow and have had limited hydrolysis power even with relatively easily hydrolysable materials such as poplars and sugarcane rind (bagasse). In these processes the lignin is resinified to a dark refractory mass. The sugar yields rarely exceed 60% of the theoretical value by such processing. Although higher sugar yields were said to occure with enzymatic hydrolysis, the process is very slow and expensive. The lignin usually remains contaminated with sugar residues.

On the other hand difficult to hydrolyse species such as cotton linters and the coniferous softwoods can be easily treated by the present invention and dissolved within relatively short times (under one hour) . The yields of reducing sugars and lignin are in excess of 95 per cent of theoretically available amounts and are obtained in high purity by the present invention.

Starchy materials, such as cerial grains are traditionally treated mechanically to separate the verious fractions such as the germ, cellulose, starch and protein for further processing and use. In fermentation to alcohol only the starch sugars are utilized. To facilitate the use of the starch it first has to be hydrolysed by combined action of acid and enzymes. The cellulose and protein residue is usually marketed separately. Thus the process is cumbersome and yields less than the theoretically available carbohydrate in the grain as it is incapable of also hydrolysing the nearly 20 to 30 carbohydrates in the form of cellulose and pectic substances (hemicelluloses. Thus ethanol production from grain usually does not involve the hulls and seed shells. Processes for isolation and hydrolysis of straches are described by Radley in Stach Production Technology (1967) . Growing starch containing tubers offers substantial gains in biomass production for fermentative purposes. The total biomass produced, including the above ground growth (vegetable stems) and below ground tubers, can be as high as 200 T/ha as is the case with Jerusalem artichoke and cassava or manioc (la tropha manihot and its other vaieties) grown around the Tropic of Cancer around the world. To date , only the starch content was of industrial interest by extracting the starch granules from finely comminuted tuber pulp with water. The process is very water and energy intensive. As with the cerial grain starches, their use for fermentation to alcohol must be preceeded by a combined enzyme/acid hydrolysis of the starch to monomeric sugars. The residual pulp contains 10 to 15 per cent cellulose and about equal quantities of proteins. In the manufacturing of starch presently the cellulose is sold together with the protein as cattle feed.

In contrast, in the present investigations it was also found that both the cerial grains (for example corn and wheat) and partially dried comminuted tubers (Jerusalem artichoke and cassava) whentreated with 70 to 95 per cent aqueous acetone containing between 0.02 to 0.06 Normal mineral acid at 145°C ot 230°C in a pressure vessel for periods dissolving between 50 to 70 per cent of the original material input, yielded only slightly colored sugar solutions which did not contain hydroxymethyl furfural. The protein network which normally encapsulates the starch grains remains uridissolved as a spongy light brown residue which could be easily filtered off (it retained the shape of the tuber particles processed through the reactor) and the entrapped sugar solution removed by squeezing or centrifuging. Upon removal of the organic component of the solvent a small quantity of drak brown precipitate is formed. This precipitate together with the color of the aqueous residue is easily removed by treatment with charcoal or filtration through a charcoal bed to obtain crystal clear sugar solutions. When the acid strength of the solution is increased to 1.5 per cent and the solution is boiled for 20 min. the cooled solution contains the free sugars in a fermentable form. Sugar concent rations as high as 45 per cent were produced on a regular basis with two changes of liquor (total liquor to tuber ratio 10:1; water:tuber ratio 1.3:1) giving 95% and better reducing sugar recovery based on the theoretical value determined by combined acid/enzyme hydrolysis of the starting materials. The total hydrolysis times were less than one minute to less than 30 min depending on the temperatures used. It is envisaged that such tubers and cerial grains can be harvested together with their stems (stalks, straws etc) comminuted to small chip like fragments and fed as a mixture into the reactor for simultaneous hydrolysis of both the tuber starches and stem celluloses. The respective hydrolysis rates do not appear to be that much diffrent and sufficient protection is afforded to the dissolved sugars by complexiπg to avoid fufural formation from both substrates.

Reaction vessels with inert linings are used to eliminate the sugar degradation catalysing effects of transition metal ions such as Ni, Co, Cr, Fe, and especially Cu which may be components of metal vessel walls, tubing, heat exchangers and present in spring waters.

Using the process of the invention, continuous percolation at predetermined rates, where there is a residence time less than that required for hydrolysis of 50 to 70 per cent of the remaining solid residue at any given instance at the prvailing temperature and acid concentration selected for the hydrolysis, is preferred and results in partial or total dissolution of the material depending on the extent the hydrolysis is allowed to proceed. In multiple step batch treatment partial hydrolysis with delignification of lignocellulosics, which occurs first, yields relatively pure cellulose. Continued hydrolysis with the same or different solvent mixture leads to total saccharification and also allows stepwise separation of the various chemical consituents of the starting material in high purity and high yield. Notwithstanding these process options the recovery of pentoses from the reaction mixture is generally by flash evaporation of the major fraction of the acetone first with continued distillation under reduced pressure, or by steam stripping, or by liquid-liquid extraction. Vacuum distillation and steam stripping can under suitable conditions result in the separation of the pentose sugar complexes form the solution. Separation of the pentose, hexose sugar complexes is made possible by the largely different boiling points of their acetone complexe which apparently form even in the presence of smamll amounts of water during the high temeperature hydrolysis step in the present invention provided the acetone concentration is higher than 70 volume per cent.

On working up hydrolysis solutions originating from dissolving lignocellulosics the lignin precipitates as water insoluble relatively low molecular weight (M - 3 200 - 1 800) granular powder having spherical particle sizes between 2 to 300 micrometers on filtration and centrifuging. Purification of the crude lignin is by repeated re-dissolution in acetone, filtration to remove undissolved residues and re-precipitation into large excess of water or by spray drying the highly concentrated acetone solution. The remaining aqueous solution after filtering off the lignin precipitate can be purified on filtering through a charcoal bed and will contain mainly hexose sugars of 10 per cent and greater concentration.

The pentose distillate and hexose syrup complexes when acidified and boiled for at least 20 min yield the major sugar fractions in monosaccharide form and high purity. If so desired, on extended boiling of the pentose sugars in the presence of the acid, selective conversion to dehydration products such as furfural and levulinic acid can be effected as is known from the prior art.

After hydrolysis of the material at elevated temperature for a limited period of time, it is very important that the temperature of the reaction mixture be rapidly lowered to under 100ºC to avoid unwanted degradation of the sugars. This is best accomplished by controlled flashing off of the volatiles since sugar degradation was found to be insignificant below the boiling point of water even in the presence of dilute acids. Usually the cooling can be continued to ambient temperatures or less before fermentation or further processing. If the sugar complexes are to be preserved rapid neutralization of the solution with bicarbonate will stabilize the complex.

The above described process can be operated in a continuous or semicontinuous manner using batch cooking principles for the latter. Semi-continuous saccharification would employ a battery of pressure vessels each at various stage of hydrolysis to simulate a continuous process. In continuous operation, on the other hand, all stages of hydrolysis are accomplished in a single pressure vessel and the product mix is always determined by the particular saccharification program set. Comminuted solids and the hydrolysis liquor are fed continuously to the pressure vessel at such a rate that the time elapsed between feeding and exit of the products would not exceed that determined earlier to obtain 50 to 70 per cent hydrolysis of the solid residue present at any time in the pressure vessel. Thus the residence time of the hydrolysis liquor would be always fitted to the most sensitive stage in order to provide sugar recoveries exceeding 90 to 95 per cent for that particular stage. For lignocellulosics the three major stages of saccharification to be considered are: a. bulk delignification and pre-hydrolysis ; during this stage up to 75 per cent of the lignin and 95 per cent of the governing hemicelluloses (xylose in hardwoods and mannose in softwoods) may be reomved. The solid residue yield is invariably above 50 per cent of the starting material; b. continued delignification and cellulose purification stage; during this stage delignification is largely completed and the rest of the hemicellulose sugars and some of the amorphous glucan are removed. The solid residue at this stage is generally less than 35 per cent and is predominantly crystalline in nature; c. proceeding to total saccharification, the residual cellulose of stage b. is decomposed to monomeric sugars. This step may take more than one liquor change to accomplish a better than 90 per cent sugar recovery. Such stages of dissolution could not be identified for the starchy materials since the hydrolysis of cellulose and starch appears to be simultaneous and extremely rapid due to the good accessibility.

In continuous operation, liquors collected from the various stages of hydrolysis may contain sugars from all stages a. to c. which is the situation with an apparatus having no means of separating the top pre-hydrolysis liquor from the rest of the liquor pumped in with the chips. With the present invention such separation for purifictaion of the sugars is unnecessary because the sugars occur as complexes, the pentoses having different volatility than the hexoses with which they may be mixed. The lignin is separated on basis of its insolubility in water and is recovered outside the reactor on flash evaporation of the organic volatiles. Gluten and protein from cerial grain and tuber processing are separated as solid residues following the hydrolysis stage.

Separation of the first and second stage liquors would have particular significance on continued heating of these sugars to cause dehydration of especially the pentoses to produce corresponding furfurals and levulinic acid. In this case only minor amounts of hexose sugars would have to be hydrolysed. The sensible way to produce furfural from pentoses is following the flash evaporation stage which includes steam stripping which separates the sugar complexes according to their volatility. Such distillates when acidified can be reheated under highly controlled conditions and high purity furfural produced in better than 75 per cent yields.

In practical hydrolysis, based on the semi-continuous process, three complete liquor changes would be required to cause total saccharification and dissolution and provide mass recoveries better than 95 per cent. The preferred liquor to wood ratio is 5:1 to 10:1 with densified materials liquor to wood ratios as low as 3:1 may be achieved. Due to the shrinking mass bed the total amount of liquor required for hydrolysis of 100 kg aspen poplar at constant liquor to wood ratio of 7:1 is 1 356 kg for an overall liquor to wood ratio of 13.56:1. Under these conditions the average sugar concentration in the combined residual aqueous phase (271 kg) is 30 per cent (82.3 kg of recovered sugars) .

In continuous percolation, the liquor to wood ratio can be kept constant at 10:1 as by necessity successive additions of both wood and liquor will carry hydrolysates of the residuals already within the reactor. This also establishes sugar concentrations to be in the order of 37 to 40 per cent following flash evaporation of the volatiles. Such high sugar concentrations were hitherto possible only with strong acid hydrolysis systems but not with dilute acid hydrolysis.

Discussion of the liquor to wood ratio is extremely important in organosolv and acid hydrolysis processes since it directly relates to energy inputs during the hydrolysis and solvent recovery as well as during alcohol recovery from the resulting mash following fermentation of the sugars to ethanol or other organic solvents. Thus the liquor to wood ratio will have a profound effect on the economics of biomass conversion to liquid chemicals as well as the energy efficiency (energy gained over energy expanded in conversion) of the process.

Steaming of the comminuted material before mixing with the hydrolysis liquor can be used to advantage to expel the trapped air. Such treatment would further be expected to aid better liquor penetration. In none of the laboratory tests were any of the chips presteamed. Such practice is well known though from the prior art. EXAMPLE I

Saccharification power and sugar survival rates were compared for three competetive systems, namely: acidified water (aqueous weak acid) , acidified aqueous ethanol, and acidified aqueous acetone in the following example.

In every case purified cotton linters having TAPPI 0.5% viscosity of 35 cP and 73 per cent crystallinity index at 7 per cent moisture content were used. Acidification was affected by sulphuric acid by making up stock solutions of various solvent systems each being 0.04 Normal with respect to the acid. The hydrolysis conditions were as follows: In a series of experiments one gram samples of cotton linters (oven dry weight) were placed in glass lined stainless steel vessels of 20 ml capacity along with 10 ml of the solvent mixture and heated at 180ºC for various lengths of time. The residual solids and detected sugars in solution were plotted on graph paper and times to obtain dissolution of about 99, 75, 50 and 25 per cent of the substrate were read from the graphs and shown in TABLE 1. At the end of the reaction period heating was interrupted, the vessel chilled and its contents filtered through a medium porosity glass crucible. The undissolved residue was first washed with warm water followed by rinsing with several portions of acetone and finally by warm water again. The residue weight was determined gravimetrically after drying at 105°C.

For comparative analytical purposes the combined filtrates were diluted to 100 ml with water and a 0.5 ml aliquot was placed in a test tube with 3 ml of 2.0 Normal sulphuric acid and subjected to secondary hydrolysis at 100°C by heating in a-boiling waterbath for 40 min. The solution was neutralized on cooling and the sugars present in the solution were determined by their reducing power. The results were thus uniform based on the the resultant monosaccharides liberated during the hydrolysis process. The theoretical percentage of reducing sugars available after hydrolysis of the substrate was determined by difference between the known chemical composition of the starting material and the weight loss incurred due to the hydrolysis. To account for the weight increase of the carbohydrate fraction due to hydration of the polymer on breakdown into monomeric sugars, the weight loss is normally multiplied by 1.111, the weight percentage (11.11%) of the added water to the cellulose in hydrolysis to monomeric sugars. As evidenced from TABLE 1, hydrolysis rates improved constantly as the acetone concentration increased above 70 per cent by volume of the acidified solvent mixture. Very rapid hydrolysis rates were obtained with nearly anhydrous acetone solutions. The dissolved sugars were found to be most stable when using a solvent mixture of between 80 to 90 per cent acetone even though the relative half lives were realtively short. Sugar survivals over 90 per cent are obtained as long as the reaction time at temperature is kept below that required for hydrolysing 50 per cent of the substrate to dissolved products. The time required to hydrolyse 50 per cent of the substrate to dissolved products is called half life of sugar survival. This criteria seems to hold regardless of what stage of the hydrolysis is considered. The solvent effect both on the hydrolysis rate and sugar survival for limited hydrolysis times was the most surprising discovery of the present invention whereby maxima were found around 80 to 90 per cent acetone concentration in the reaction mixture. At higher acetone concentrations, the response of the hydrolysis rate to increase in temperature and acid concentration was observed to follow well known kinetic principles in contrast to both the aqueous dilute acid and acidified aqueous athanol systems in which the balance of increase in higher hydrolysis rates and sugar degradation did not improve with an increase with these parameters especially that of the temperature. The increased sugar survival with increase in acetone concentration is attributed to formation of acetonesugar complexes which have improved stability at high temperature. The complexes are very readily and safely hydrolysable to free sugars on heating with dilute acid at 100ºC for a limited amount of time.

In identical stationary acidified ethanol-water cooks, in which the ethanol concentration was higher than 80 per cent, neither delignification nor hydrolysis was obtained due to the fact that the acid catalyst was quickly consumed by reaction with the alcohol by formation of ethyl hydrogen sulphate (C₂H₅-O-SO₂-OH) and formation of diethyl ether via condensation of two ethanol molecules. Ether formation was quite substantial under these conditions. Also, alkyl glucosides formed in high concentration alcohol solutions are substantially more difficult to hydrolyses to free sugars than the corresponding acetone complexes. Further, alcoholysis results in olygomeric sugars rather than monomers as is the case in acetonewater solutions. Thus alcohols prove to be largely unsuited for hydrolysis media due to the unwanted solvent loss and general danger from the explosive ether. With lignified materials, the delignification power of acidified alcohol solutions is clearly a drawback. With 80:20 ethanol:water cooks in the presence of 0.190 per cent (0.04 Normal) sulphuric acid at 180°C, the hydrolysis rate was 5.47 x 10³ min and the half life of cotton linters decomposition to sugars was 126.8 minutes. A maximum of 76 per cent could be dissolved in 254 minutes, with the crystalline residue showing substantial resistance to hydrolysis in the alcoholic solvent. Residual acid concentration was found to be one fourth of that originally applied, i.e., 0.01 Normal, the balance possibly consumed in the various side reactions.

It is evident from the data that under identical hydrolysis conditions excessively long hydrolysis times are required for complete dissolution of cotton linters by both acidified water amd aqueous ethanol media. An increase of the ethanol concentration from 50 per cent to 80 per cent did not improve the hydrolysis rate or improve particularly the sugar survival. The hydrolysis rate in ethanol-water was only marginally better than in dilute acid in water.

The examples clearly show that a high acetone con centration over 70 per cent is mandatory for high speed hydrolysis and sugar survival. Under the conditions indicated for sugar recoveries better than 90 per cent, reaction times (or high temperature exposure times) of less than indicated for half lives are preferred. Thus according to these data, total saccharification and quantitative sugar recovery would dictate a percolation or pass through process wherein the liquor residence time would not exceed 10 min when 80:20 acetone : water with 0.04 Normal sulphuric acid is used as solvent mixture at 180°C temperature. The residence time would have to be substantially shortened when higher temperatures and larger acid concentrations are used as shown in the following examples.

Solid residues less than 50% in yield show a high degree of crystallinity (87%) and are pure white, have a DP (Degree of Polymerization) of 130 to 350 glucose units.

EXAMPLE II

The effect of acid concentration on the rate of hydrolysis and sugar survival in 80:20 acetone:water solvent mixtures was studied at 180°C temperature using cotton linters as substrate.

In stationary cooks one gram samples (oven dry) of cotton linters were hydrolsysed in glass lined stainless steel pressure vessels along with 10 ml of the appropriate hydrolysis liquor and heated until the original substrate mass was hydrolysed and dissolved. The levels of substrate dissolution to 99, 75, 50 and 25 per cent were determined by graphing as explained in EXAMPLE I. Work-up of the reaction products followed the same procedure as outlined in EXAMPLE I. The results of these tests are indicated in TABLE 2.

Increased acid concentration resulted in higher hydrolysis rates within the range studied and a somewhat faster degradation of the sugars was noted as the single stage hydrolysis times exceeded those indicated as half lives for the solid residue. Equal concentrations of sulphuric and hydrochloric acid were found to give largely comparable results. The increased acid concentrations showed a substantial hydrolysis accelerating effect as evidenced by the rapidly decreasing half lives. Thus the hydrolysis rate can be readily controlled by limited acid concentrations, all other conditions being held constant. EXAMPLE III

Temperature effects on hydrolysis of cotton linters were studied with acidified aqueous acetone solutions containing 0.04 Normal sulphuric acid in 80:20 acetone:water at different hydrolysis times so that weight losses of 99, 75, 50 and 25 per cent could be determined as in EXAMPLE I. All cooks were preconditioned to 35°C before being placed in the oil bath to minimize the effect of heating-up time at the various temperature levels studied.

Work-up of the products and analysis followed the same procedure as described in EXAMPLE I and the results are summarized in TABLE 3. The data indicate that increased temperature had the most profound accelerating effect on the hydrolysis rate and generally in such single stage batch cooks reaction times exceeding sugar dissolution half lives at. any stage of the hydrolysis increased somewhat the rate of sugar degradation at the higher temperature regimes used. However, it was learned that such high temperature hydrolyses afford practically instantaneous high-yield hydrolysis to be carried out on even such difficult to hydrolyse substrate as cotton linters. The rate of sugar degradation can be offset somewhat by lowering the acid concentration and increasing the liquor-to-wood ratio, whereby the forward reaction rate (k₁) in hydrolysis remains unaffected but the sugar degradation rate (k₂) is lowered sub stantially. Thereby, sugar survival, which depends on the ratio of k₁/k₂ is largely improved especially if high, acetone concentrations are used. Such manipulations of the temperature acid concentration parameters are not possible with the weak acid aqueous systems. EXAMPLE IV

Cooks reported in this example explore the hitherto unobserved relationship of increasing the sugar survival at reduced acid concentration and increased reaction temperature without any reduction in the hydrolysis rates disclosed herein. This unusal discovery is demonstrated in the data in TABLE 4.

The effect of reduced acid concentration but high reaction temperature is demonstrated by cooking one gram samples of cotton linters (oven dry weight) in glass lined stainless steel pressure vessels along with 10 ml of 80:20 actone:water cooking liquor containing 0.01 and 0.005 Normal H₂SO₄ with respect to the solvent mixture and heated until 50 per cent and 75 per cent dissolution of the substrate was obtained at 190ºC to 220°C reaction temperature.

Cooling and work-up of the reaction products to determine sugar survival and reaction rates were performed as outlined in EXAMPLE I.

The data indicate that acid concentration can be successfully reduced and traded by increasing the reaction temperature without loss in reaction rate with a concomittant increase in sugar yield (survival) when hydrolysis liquors of at least 80 per cent acetone content are used. Such a trend is clearly against all previously published scientific results (Seaman, J.F. ACS Symposium, Honolulu 1979; Bio-Energy, Atlanta, 1980) where the increase in hydrolysis rates and sugar survival was function of both increased acid concentration and higher temperature. The surprising solvent effect of the acetone-water system has never been observed or reported in scientific literature or the prior art before. EXAMPLE V

One gram samples of several wood species were hydrolysed in 80:20 acetone:water containing 0.04 Normal sulphuric acid at 180 C. Hydrolysis rates were calculated only for the crystalline cellulose fractions to avoid confounding effects of the easily hydrolysable lignins and hemicelluloses which require much lower activation energies for their dissoltuion. Times to mass losses of 99, 75, 50, and 25 per cent of the original oven dry mass along with the calculated reaction rates are recorded in TABLE 5.

Work-up of the products followed the same procedure as indicated in EXAMPLE I except that after removal of the volatiles by distillation, it was necessary to remove the precipitated lignins by filtration or centrifuging.

It is quite evident that under identical conditions the hydrolysis rates for wood are roughly twice that of cotton linters. Due to the increased forward reaction rates sugar recoveries became quite impressive indeed.

The rate of Douglas-fir hydrolysis was somewhat slower than that of aspen and sugarcane rind (bagasse) . However, when hydrolysis in a purely aqueous system was attempted under otherwise exactly matching conditions (same temperature and acid catalyst content) a hydrolysis rate of 0.5 x 10³ min^-1 was obtained and only 6 per cent loss was recorded for a 280 min long cook at 180ºC the usual dilute acid hydrolysis temperature. Thus the high acetone content hydrolysis liquor allowed at least 100.times faster hydrolysis of Douglas fir by simultaneous dissolution of the lignin than possible in purely aqueous systems.

Among the products of partial saccharification of wood, solid residues of about 30 to 35 per cent yield are pure white cellulose, totally devoid of residual lignin. The cellulosic fraction has a crystallinity index of 80 per cent from aspen wood and a degree of polymerization of between 80 to 280. Similar results are obtained with other wood species.

EXAMPLE VI

It is found to be a further advantade of the present invention that the high acetone concentration clearly favours formation of relatively stable acetone-sugar complexes in spite of the presence of water and high temperature. The better stability of the sugar complexes profoundly affects survival of the dissolved sugars as evidenced in the data presented in TABLES 1 through 4.

Further, due to the differences in volatility and solubility of the various sugar complexes, the inevntion also allows facile separation and nearly quantitative isolation of the major sugars, if so desired. However, due to the presence of water and the relative instability of the sugar complexes in acidic medium, if separation will be desired, it may be necessary to neutralize the recovered aqueous wort before removal of the volatiles and concentrate the solution to a syrup. The syrup is then re-dissolved in anhydrous acetone, acidified to 3 per cent with mineral acid, and allowed to stand from 4 to 6 hr until all sugars fully formed their respective di-acetone compexes. Neutralization will allow recovery of the sugar complexes in a stable state suitable for separation as described in the next example. The separated sugars are then readily hydrolysed either selectively on ion exchange resins or in bulk by boiling at least for 20 min in acidified water.

Thus 10 g (OD) coarse aspen sawdust (passing a 3mm screen) was charged with 100 ml of hydrolysing liquor made up of 80:20 acetone:water and 0.04 Normal sulphuric acid as catalyst. The bomb was brought to 180°C in a per-heated glycerol bath within 9 min and heating was continued for a pre-determined reaction time which was 3 min.

In another larger bomb 450 ml of hydrolysis liquor of the above composition was also preheated and connected through a syphon tube and shut-off valve to the above reaction vessel. Following three minutes at reaction temperature (9+3=12 min total elapsed time for the first stage) the reaction liquor was drained into a small beaker containing 75 g of crushed ice. The reaction vessel was immediately re-charged with hot liquor from the stand-by vessel and the reaction was allowed to prceed for an additional 3 minutes before again discharging the reactor contents as above. In all, five liquor changes were made and the liquors were analysed for dissolved solids as below.

Hydrolysate No.l and 2 were combined before evaporation of the acetone and other volatile organic decomposition products. Low temperature removal of the solvent volatiles resulted in precipitation of a flocculant lignin wich aggregated to small granules on standing. The liquor was carefully decanted from the precipitate and the separation completed by filtration. The lignin was washed with water and dried in vacuum to constant weight. A lignin powder of 1.67 g was collected and had a weight average molecular weight of 2800.

The combined filtrates (127 ml) were neutralized and subjected to steam distillation in an all glass apparatus and approximately 35 to 40 ml distillate was collected. Both the distillate and residual solution were made up to 100 ml and 0.5 ml portions were acidified to 3 per cent acid and boiled for 40 min on a water bath. The solutions were neutralysed and the reducing power of the sugars was determined by the Somogyi method. The yield of sugars was 1.89 g in the distillate and 1.96 g from the residual liquor.

Gas chromatographic analysis of the alditol acetates prepared from the sugars in the solutions indicated mainly xylose and arabinose for the steam distillate and hexoses for the residual liquor.

Hydrolysate No. 3 contained only traces of lignin after evaporation of the acetone solvent, too small to quantify. It was removed by centrifuging. The aqueous residue (97 ml) was acidified to 3 per cent acid with sulphuric acid, boiled for 40 min and after neutralization filtered and made up to 100 ml. The reducing sugar content of the filtrate was determined by the Somogyi method to be 1.83 g. GC analysis of the alditol acetates determined an an aliquot sample of the sugar solution indicated mainly glucose with traces of ammnose and galactose. Hydrolysate No. 4 and 5 were processed and hydrolysed in the same manner as No. 3. Hydrolysate No. 4 yielded 1.73 g and Hydrolysate No. 5 yielded 1.40 g sugars both being composed only of glucose as evidenced by GC analysis of aliquot samples.

The undissolved residue was 0.12 g following 2 h drying in an oven at 105ºC.

The recoveries sumarize as follows:

Lignin powder ......... 1.67 g

Total pentoses ......... 1.89 g

Total hexoses ......... 6.92 g Undissolved residue (99% glucose) 0.12 g

Total mass recovered: 10.60 g Mass balance of hydrolysis:

Lignin recovery: 98.2 % Sugar recovery : 97.8 %

EXAMPLE VII

In similar hydrolysis arrangement to EXAMPLE VI 10 g OD Douglas-fir sawdust to pass a 4 mm screen, pre-extracte with dichloromethane and air dried to 8 per cent moisture content was hydrolysed with 80:20 acetone:water solvent containing 0.05 Normal hydrochloric acid in five consecutive steps as described in EXAMPLE VI above. Each reaction step consisted of 3 min at 200°C. The heating-up time was 7 min. Again Hydrolysete (H) No. 1 and 2 were combined whereas the susequent fractions were analysed separately.

The combined liquor of H-1 and H-2 yielded 2.39 g lignin powder and 135 ml of aqueous liquor. The molecular weigh of the lignin was 3200. The filtrates were neutralized to pH 8 and subjected to steam distillation in an all glass apparatus.

The 28 ml distillate which was collected contained 0.52 g pentoses which after passing the filtrate through a cation exchange resin in the acid form and repeated steam distillation of the filtrate yielded 0.58 g xylose as determined by GC analysis.

The residue remaining behind after the above steam distillation (128 ml) was neutralized on an ion exchange column (H form), the filtrate concentrated to a syrup, seeded with some crystalline mannose and left standing overnight. The crystalline material was collected by filtration and recrystallized from ethanol-petroleum ether. The crystals were redissolved in water, acidified with sulphuric acid to 3 per cent and boiled for 40 min and after neutralization with silver carbonate the sugars were analyzed by GC of the respective alditol acetates. The only sugar determined was mannose and the yield was 1.00 g.

The ethanol-petroleum ether solution was extracted with 5 ml portions of water and the collected aqueous layer combined with the syrup removed from the crystalline product above. The solution was briefly heated to expel the alcohol, made up to 3 per cent acid and boiled on a water bath for 40 min to liberate the sugars. After neutralization with silver carbonate an aliquot of the sugar solution was worked up for alditol acetates and the sugars analyzed by GC . The total sugar syrup contained a total 0.58 g of sugars of which 0.29 g was calactose, 0.25 g was glucose and 0.04 g was mannose.

H-3 gave 1.89 g pure glucose with 0.4 g of lignin precipitate on removel of the volatiles.

H-4 gave 1.66 g of pure glucose with only small traces of lignin, whereas H-5 gave 1.85 g glucose and no lignin. The undissolved residue was 0.18 g and was composed of 99 per cent glucose. The recoveries summarize as follows: H-1, 2&3 Lignin 2.79 g

Xylose 0.58 g

Arabinose 0.04 g

Mannose 1.00 g

Hexoses 0.58 g

H-3 Hexoses 1.89 g H-4. Hexoses 1.66 g H-5 Hexoses 1.85 g

Undissolved residue: 0.18 g

Total mass recovered: 10.57 g Total mass balance:

Lignin recovery: 98%

Sugar recovery: 96%

With large scale industrial applications of the process chilling of the sugar solutions is best accomplished by controlled flash evaporation of the acetone and other organic volatiles. Cooling of the liquor in small experiments with crushed ice was adopted as a matter of convenience and would not be recommended.

EXAMPLE VIII

Small, about 1 cm 1 cm cubes of Jerusalem artichoke (Helianthus tuberosus) were treated in glass lined stainless steel vessels in a similar manner to the woods described above.

The tuberpieces were air dried to about 6 per cent moisture content for better control of the solids content but if processed wet the water had to be taken into account in calculating the water content of the solvent. Ten to 12 g of tuber solids were placed together with 50-70 ml of solvent made up of 80:20 acetone : water and 0.02 Normal sulphuric acid. The contents were rapidly heated to the reaction temperature between 155°C to 220°C to determine the rate of hydrolysis of the available carbohydrate materials. The reaction times ranged between 60 min at 155°C to less than a minute at 220°C for dissolution where no more than about 9 to 12 per cent undissolved residue remained. When the residue was treated with 68 to 72 per cent sulphuric acid it turned dark brown but did not dissolve even on prolonged standing. This indicated thatthe residue was not a polymeric carbohydrate.

The resulting hydrolysate was dark to light amber brown depending on the temperature and residence time in the reaction vessel. When the solution was decanted, the adsorbed liquor was best removed from the spongy residue which retained the original cube shape, by centrifuging and careful pressing. The residue was first washed with acetone and then three times with water until no further color developed. When the residue was beaten to a pulp filtration became very difficult and a lot of solvent was required to wash the pulp free of the residual hydrolysate liquor.

The hydrolysate liquor was palced on a flash evaporator and the solvent removed at 50°C. This caused a brown precipitate to form which was easily filtered off on ordinary filterpaper. The residual light amber colored aqueous phase when filtered through an activated charcoal bed could be made water clear (similarly on filtration through an ion exchange resin in H form). The solution was made up to a known volume and the sugar content determined by the phenol sulphuric acid method. Since the artichoke solids are about 75 per cent carbohydrate material a theoretical yield of sugars is between 8 to 10 g of free glucose sugar following the hydrolysis. Sugar recoveries ranged between 93 per cent at 220°C to 98 per cent at 180°C (hydrolysis time about 13 minutes). The dark precipitate which fell out on removal of the acetone was less than 1 per cent of the total solids subjected to the hydrolysis. EXAMPLE IX

In a further experiment five grams of ground corn meal (to pass a 2 mm sceern) was palced in the glass lined arainless steel pressure vessel and treated with 25 ml of 90:10 acetone:water mixture containing 0.03 Normal sulphuric acid based on the solvent mixture used. The vessel was rapidly heated to 200°C and kept there for 12 minutes. After this elapsed time the vessel was chilled in cold water and the contents filtered though a medium porosity crucible. The residual solids were washed first with solvent and then with water until all color was removed. The combined filtrate was light brown in color. On removal of the volatiles by distillation at 50°C and reduced pressure a dark brown precipitate formed which was filtered off through a filter paper. The aqueous liquid was then filtered through activated carbon and the clear solution made up to 200 ml. The sugar content of the solution was determined by diluting one milliliter to 100 ml and taking a 2 ml aliquot it was mixed -with 1 ml of 5 % phenol and 5 ml of concentrated sulphuric acid. The absorbance of the solution was determined spectrophotometrically at 480 nm and compared to a standard calibration curve prepared with α-d-glucose. The total sugar recovered was 4.17 g (94% yield) and strong acid insoluble residue was 0.4 g, whereas the brown precipitate was 0.08 g.

Having now particularly described and ascertained the nature of the invention and its applicability to lignocellulosics and starch containing natural vegetable products what I clam and desire to secure by Letters Patent is:

Claims (1)

1. -1- In a process for the production of carbohydrate hydrolysates as sugars from a comminuted cellulosic material which can contain lignin or a starch containing cerial grain or tuber and mixtures of the same by treating the material in a pressure vessel with a solvent mixture of acetone and water containing a small amount of an acidic compound at elevated temperature to form reducing sugars in a liquor, the improvement which comprises:

   (a) providing mixture of acetone and water containing at least 70 per cent by volume of acetone and the catalytic acidic compound as the solvent mixture in the pressure vessel at the elevated temperature with the comminuted material.

   (b) treating the comminuted material in the solvent mixture for a limited period of time at the elevated temperatures until it is at least partially dissolved and such that at least 90 per cent of the solubilized sugars from those potentially available in the material are dissolved without degradation to non-sugars in the liquor; and (c) rapidly cooling the liquor as it is removed from the pressure vessel.

   -2- The process of Claim 1 wherein the concentration of the acetone in the acetone and water mixture is between 80 to 90 per cent by volume.

   -3- The process of Claim 2 wherein the acidic compound is sulphuric acid and the concentration is less than 2 per cent by weight of the acetone-water mixture. -4-

   The process of Claim 3 wherein the acidic compound is hydrocholoric and the concentration is less than 1 per cent by weight of the acetone-water mixture. -5- The process of Claim 1 wherein the elevated temperatures are between 145°C to 230°C.

   -6- The process of Claim 1 wherein in addition the liquor is further hydrolysed at elevated temperatures and dilute acid solutions to liberate the free sugars.

   -7- The process of Claim 1 wherein the liquor is subjected to distillation whereby the pentoses are volatilized from the liquor.

   -8- The process of Calims 1, 6 or 7 wherein the aqueous solution contains dissolved sugars in excess of 15 per cent solids.

   -9- The process of Claim 1 wherein the cellulosic material is treated in the pressure vessel in at least two stages at elevated temperatures and each stage being for a limited period of time and then is rapidly cooled.

   -10- The process of Claim 3 or 4 wherein the concentration of the acid is between 0.10 Normal and 0.001 Normal with respect to the acetone-water mixture.

   -11- The process of Claim 1 wherein the volatiles in the liquor are distilled at reduced pressures to leave an aqueous solution in which water-insoluble lignins and other products are precipitated and are separated from. -12-

   In a process for the production of carbohydrate hydrolysates as sugars and lignin from comminuted cellulosic material which can contain lignin by treating the material in a pressure vessel with a solvent mixture of acetone and water containing a small amount of an acidic compound at elevated temperatures to solubilize any lignin and to form reducing sugars in a liquor, the improvement which comprises:

   (a) providing mixtures of acetone and water containing at least about 70 per cent by volume of acetone and a catalytic acidic compound as the solvent mixture in the pressure vessel at the elevated temperatures with the cellulosic material;

   (b) treating the cellulosic material in the solvent mixture for limited periods of time sufficient to dissolve less than 50 to 70 per cent of the cellulose in one stage at the elevated temperatures until the cellulosic material is at least partially dissolved and such that at least 90 per cent of the solubilized sugars from the cellulosic material are recovered with out degradation to non-sugars wherein the carbohydrates in the cellulosic material are dissolved and hydrolysed partially or substantially completely,

   (c) continuously removing the liquor from the pressure vessel;

   (d) rapidly cooling the liquor by controlled flash evaporation of acetone to form a residual aqueous solution; and

   (e) recovering the sugars and any lignins from the residual aqueous solution.

   -13- The process of Claim 12 wherein the cellulosic material is lignified and wherein the volatiles in the liquor are distilled at reduced pressures to leave the residual aqueous solution and precipitated lignin and wherein the residual aqueous solution is neutralized prior to recovering the sugars. -14- The process of Calim 12 wherein the concentration of acetone and water is between 80 to 90 per cent.

   -15- The process of Claim 12 wherein the pentose sugars are volatilized from the residual aqueous solution as acetone complexes to separate them from the hexose sugars.

   -16- The process of Calim 15 wherein the sugar-acetone complexes are each broken by contacting the complexes with aqueous acid at elevated temperature.

   -17- The process of Claim 16 wherein the complexes are continuously treated with aqueous acid at elevated temperature until sugar dehydration products are formed.

   -18- The method of Calim 12 wherein the cellulosic material is treated in a batch or continuous manner and the recovered solvent fractions are worked up separately or in unisom.

   -19- The method of Claim 12 wherein hydrolysis of the cellulosic material is conveniently stopped at a point where essentially pure crystalline cellulose is recovered as solid residue from the reactor.

   -20- The method of Claim 12 and 19 wherein the recovered hydrolysis liquor is worked up in the herein described manner.