Title of Invention

A PROCESS FOR THE CONVERSION OF THE SODIUM SALT OF 2-KETO-L-GULONIC ACID

Abstract

A process for the conversion of the sodium salt of 2-keto-L-gulonic acid, which is present in an aqueous fermentation solution, into an alcoholic solution of the free acid and, if desired, into an alkyl ester of the acid comprises a) recovering the sodium 2-keto-L-gulonate monohydrate from an aqueous fermentation solution by crystallization involving evaporation, cooling or displacement and, if desired, pulverizing the thus-obtained crystallizate by grinding, b 1) suspending the optionally ground sodium 2-keto- L-gulonate monohydrate obtained in step a) in a lower alcohol, leaving the crystals to swell and thereafter adding an acid of .low water content, whereby the measured pH value should lie above 1..5, or b2) adding the optionally ground sodium 2-keto-L-gulonate monohydrate obtained in step a) together with an about stoichiometric amount of an acid of low water content to a lower alcohol using a wet grinding system, whereby the measured pH value should lie above 1.5, or b3) carrying out a combination of steps bl) and b2) including recycling of product streams, and c) separating the salt of the added acid formed in step bl), b2) or b3) and thus obtaining an alcoholic solution of 2-keto-L-gulonic acid, and, if desired, d) treating the alcoholic solution of 2-keto-L-gulonic acid obtained instep c) with a catalytic amount of an acid or with an acidic cation exchanger in order to esterify the 2-keto-L-gulonic acid with the alcohol to give the corresponding lower alkyI2-keto-L-gulonate.

Full Text

RAN 4226/104 The present invention is concerned with a novel process for the conversion of a sodium salt of 2-l As is known, 2-keto-L-guIonlc acid (KGA) Is an important starting material for the manufacture of ascorbic acid (vitamin C). In fermentative processes for the manufacture of KGA, the KGA which results as a metabolic product is neutralized by the addition of a base, e.g. sodium hydroxide or calcium hydroxide, in order to maintain favourable fermentation conditions. The product of the fermentation is an aqueous, biomass-containing fermentation solution in which the KGA salt, i.e. the sodium or calcium 2-keto-L-gulonate [NaKGA or Ca(KGA)2, respectively], is present in dissolved form. For the industrial manufacture of ascorbic acid or of the ascorbate, the fermentatively produced KGA must be transferred into an organic solvent. A lower alkanol is advantageously used as the solvent. The ascorbate is obtained in high yield by esterifying the alcohol with KGA and subsequently adding a base. As an alternative to this, the KGA can be converted into ascorbic acid in an organic solvent under strongly acidic conditions. However, the yields are as a rule somewhat lower [see Helv. Chim. Acta 17, 311-328 (1934)]. All processes for the working up of the fermentation solution are based on three process steps: 1. Protonation of the sodium or calcium 2-keto-L-gulonate present to the free acid (e.g. NaKGA + H+ -> KGA + Na+); 2. Removal of water; Pa/So 26.2.97 3. Removal of the biomass, dissolved proteins and other contaminants present In the fermentation solution. The sequence in which the process steps are carried out and their specific performance are characteristic of the respective process. The protonation (Step 1) can be effected in the aqueous fermentation solution by the addition of acids. According to US Patent (USP) 3 381 027 and European Patent Publication (EP) 359 645 the dissolved KGA is obtained by adding sulphuric acid to Ca(KGA)2 and separating the precipitated calcium salt. A protonation can also be effected by using cation exchange resins. According to EP 359 645 the aqueous Ca(KGA)2-containing fermentation solution is passed through a cation exchanger and the calcium ions are removed completely. When cation exchangers are used, the biomass must previously be removed completely (Step 3), e.g. by microfiltration, in order to guarantee a sufficiently long useful life of the cation exchanger. The product of the protonation is an aqueous, KGA-containing solution having a pH value which is significantly below 2.0 (the pKs value of KGA is 2.54). KGA is thereupon isolated by crystallization, extraction (e.g. according to EP 359 042) or adsorption (e.g. according to Chinese Patent Publication T097731A: see Chem. Abs. 124, 56570) and thereby separated from the water (Step 2). The crystallization of KGA In high yields can only be effected with difficulty, since the solubility, e.g. 480 g/l at 30°C, is very high. Extraction and adsorption are industrially difficult processes, especially when contaminants from the fermentation are present. According to EP 359 042 the biomass must accordingly be removed completely prior to the extraction. According to EP 403 993 isolated, but not purified KGA can be used for the manufacture of sodium ascorbate. Contaminants, such as biomass and proteins, are eliminated in further process operations by the addition of sodium bicarbonate or potassium bicarbonate and precipitation of non-esterlfied KGA. In a!! previously known processes the protonation of KGA is effected first and then the water Is removed. In principle, however, it is possible to operate In reverse, i.e. to firstly Isolate the NaKGA or Ca(KGA)2 from the fermentation solution, thereby to remove water (Step 2), and subsequently to carry out the protonation (Step 1). Thus, the crystallization of sodium 2-keto-L-gulonate monohydrate (NaKGA-HzO) is known from Japanese Patent Publications (Kokai) 66684/1977 and 62894/1978, and in the case of solubilities of e.g. 250 g/l at 300C significantly higher yields are to be expected than in the crystallization of KGA-H20. Moreover, both substances crystallize as the monohydrate, which usually is not taken into consideration. The difficulty In this operational mode, i.e. Step 2 prior to Step 1, lies in the complete protonation of the NaKGA in organic solvents, since NaKGA is practically Insoluble. Some attempts to protonate NaKGA in organic solvents have, however, been documented. According to EP 91 134 crystalline NaKGA can be converted Into KGA using gaseous hydrogen chloride In a mixture of ethano! and acetone. The sodium chloride, which is also formed, is separated. In the further course of the reaction a rearrangement to crystalline ascorbic acid takes place immediately under the strong acidic reaction conditions, with undesired byproducts resulting and the yields therefore being low at 60-82%. According to USP 5 391 770 NaKGA can be reacted with a more than 40% excess of concentrated sulphuric acid in methanol (Example 10). The reaction time inclusive of esterlfication to the methyl ester amounts to 4.5 hours at 65X and the yield is 91.9%. Subsequently, the pH is increased to 4 and the sodium sulphate is separated. Having regard to these long reaction times and the large excess of acid, considerable amounts of decomposition products are to be expected according to EP 403 993. EP 403 993 discloses the reaction of a mixture of 50% KGA and 50% NaKGA with an about 50% excess of sulphuric acid in methanol (Example 5). After a reaction period of one hour under reflux conditions the mixture is filtered. The yield of sodium sulphate obtained corresponds to about 60% of theory. Accordingly, a maximum of 50% of the KGA can be used in the form of the sodium salt. Fundamentally, the process provides for the use of KGA, with the mentioned NaKGA resulting and being recycled in subsequent steps. Disadvantages in the last two processes are the large excess of sulphuric acid used and the long reaction times under strong acidic conditions. When sulphuric acid is added in stoichiometric amounts and thus under milder conditions, a considerable part of the NaKGA usually remains incorporated in the sodium sulphate which results and the yield drops accordingly. According to USP 5 391 770 it is also known that the sodium salt of ascorbic acid (NaASC) can be protonated to ascorbic acid by the addition of sulphuric acid in methanol. In this case NaKGA is present as an impurity in up to 9%. In methanol as the solvent the yields of ascorbic acid are between 91% and 96% {Examples 12, 14 and 15). On the other hand, in a solvent mixture of 75% methanol and 25% water the yield of ascorbic acid is 99% (Example 2). Clearly, the yields in pure solvent are considerably lower than in the solvent mixture, since a preferred water content of 1 5-25% is indicated. The reason is the higher solubility of NaASC in the mixture compared with the pure solvent, e.g. at 40C about 2 weight percent (wt.%) compared with about 0.3 wt.%. Material transport is impeded in the case of low solubil¬ities and the reaction of a difficultly soluble salt with sulphuric acid to give the very difficultly soluble sodium sulphate (solubility = 0.024 wt.% in methanol) gives correspondingly poorer yields. Material transfer problems are to be expected to an increasing extent in the case of a reaction involving NaKGA-HaO, since the solubilities are considerably lower than those of sodium ascorbate (at 40°C, The object of the present invention is to provide a process which permits in the simplest possible manner the conversion of the sodium salt of 2-keto-L-gulonic acid, which is present in an aqueous, non-purified fermentation broth, into free 2-keto-L-gulonic acid in alcoholic solution in high yield and with high purity. At the same time, the disadvantages of prior art processes, especially the complete removal of blomass, proteins etc., e.g. by microfiltration, the use of cation exchangers to remove metal Ions from the aqueous fermentation solutions as well as the crystallization or drying of 2-keto-L-gulonic acid, should be avoided. Moreover, only readily accessible chemicals should be used, and as few steps as possible should be required. In the scope of the present invention a process has now been found which fulfils the aforementioned requirements and in accordance with which the sodium salt of 2-keto-L-gulonic acid is crystallized from a fermentation solution, which is only partially freed from biomass, and the sodium 2-keto-L-gulonate monohydrate obtained Is converted into an alcoholic solution of free 2-keto-L-gulonic acid. The protonatlon to 2-keto-L-gulonic acid and the removal of the metal ions are effected in this case exclusively by reaction in an alcoholic medium. Particular reaction conditions have now been found under which sodium 2-keto-L-gulonate monohydrate (NaKGA-HnO) suspended in alcoholic medium reacts in very good yields with an acid of low water content to give the dissolved,.free 2-keto-L-gulonic acid (KGA) and an insoluble salt. For the first time by this means the crystallization of the NaKGA-HnO from the aqueous fermentation solution is practicable. The critical steps required in the solution and can then be esterified directly in high yields in any manner and converted into ascorbate. Accordingly, a sequence of process steps is characteristic of the process in accordance with the invention, i.e. firstly NaKGA-H20 is crystallized from a fermentation broth, i.e. the water other than the water of crystallization is removed completely (Step 2, see above), and thereafter the protonation (Step 1) is effected entirely in a lower alkanol as the solvent by the addition of a strong acid (of low water content), e.g. sulphuric acid. A difficultly soluble salt of the acid, e.g. sodium sulphate, results and can be separated readily. The desired solution of KGA remains and, as mentioned above, can be esterified directly and converted Into ascorbate. Also characteristic are special reaction conditions for the protonation in which, in spite of extremely low solubilities of the NaKGA'H20 used and of the product, e.g. sodium sulphate, yields of more than 97% can be realised. In this respect, specific and previously unknown properties pertaining to NaKGA-H20 as a material are made use of, as will be explained in more detail below. The object of the invention is accordingly a process for the conversion of the sodium salt of 2-keto-L-gulonic acid from aqueous fermentation solutions into an alcoholic solution of the free acid and, if desired, into an alkyl ester of the acid, which process comprises a) recovering the sodium 2-keto-L-gulonate monohydrate from an aqueous fermentation solution by crystallization involving evaporation, cooling or displacement (hereinafter "evaporation, cooling or displacement crystallization") and, if desired, pulverizing the thus-obtained crystallizate by grinding, b1) suspending the optionally ground sodium 2-keto-L-gulonate monohydrate obtained in step a) in a lower alcohol, leaving the crystals to swell and thereafter adding an acid of low water content, whereby the measured pH value should lie above 1.5, or b2) adding the optionally ground sodium 2-keto-L-gulonate monohydrate obtained in step a) together with an about stoichiometric amount of an acid of low water content to a lower alcohol using a wet grinding system, whereby the measured pH value should lie above 1.5, or b3) carrying out a combination of steps b1) and b2) including recycling of product streams, and c) separating the salt of the added acid formed in step b1), b2) or b3) and thus obtaining an alcoholic solution of 2~keto-L-gulonic acid, and, if desired, d) treating the alcoholic solution of 2-keto-L-gulonic acid obtained in step c) with a catalytic amount of an acid or with an acidic cation exchanger in order to esterify the 2-keto-L-gulonic acid with the alcohol. The fermentation, which Is effected prior to the actual process in accordance with the invention, yields a turbid fermentation broth containing biomass. The crystallization of the sodium salt of 2-keto-L-gulonic acid present in the fermentation broth [step a)] can In principle be effected directly from this broth; however, a previous separation of at least 90% of the biomass by centrifugation has been shown to be advantageous. A turbid, but sludge-free fermentation solution results in this case. A complete removal of biomass and dissolved proteins prior to the crystallization is, however, not necessary. The crystallization of the sodium salt from the aqueous fermentation broth or solution according to process step a) is conveniently effected In a manner known per se by concentrating the fermentation broth or solution, cooling the solution or adding a different solvent, i.e. by evaporation, cooling or displacement crystallization. The usual ancillary conditions have to be observed in this case. Thus, for example, the evaporation crystallization should be carried out under reduced pressure and at the same time at a low temperature, preferably at temper- atures in the range of about 35C to about 60°C, in order to avoid as far as possible decomposition of the product. The crystal¬lization can be carried out continuously or batch-wise, preferably continuously. Continuous evaporation crystallization is the crystallization method which is preferably used. Subsequently, the crystallizate can be separated from the mother liquor by a solid/liquid separating operation such as filtration or centrifu-gation. The NaKGA-H2O crystallizate obtained is under the given conditions usually purer than 98% with small amounts of organic contaminants ( In the next process step [b)] the crystallizate in crystalline form or optionally in a form reduced in size by grinding is firstly suspended in a lower alcohol, e.g. methanol, ethanol, propanoi or 1,2-ethanediol (glycol), preferably methanol. It has now been found that the NaKGA-HsO converts into the anhydrate (NaKGA) with the addition of water and thereby forms a large number of very thin needles 1. Dehydration 2NaKGA-H20 (undissolved) ~* 2NaKGA (undissolved, needles) + 2H2O 2. Protonation 2NaKGA (undissolved, needles) + H2SO4 -^ 2KGA (dissolved) + Na2S04 (undissolved) A strong acid of low water content is used for the protonation. The water content of the added acid "of low water content" is in principle not critical for the process. However, the concentration of water In the resulting 2-keto-L-gulonic acid/alcohol solution determines the equilibrium conversion of a subsequent esterification. From an industrial-economical point of view acids of low water content, i.e. acids which are more appropriately denoted as concentrated, are therefore preferably used. As acids of low water content there conveniently come into consideration concentrated mineral acids such as, for example, (in each case concentrated) sulphuric acid, nitric acid, hydrochloric acid and phosphoric acid, and even gaseous hydrogen chloride. Concentrated sulphuric acid or hydrochloric acid, especially >95% sulphuric acid, is preferably used, there being formed in step b) the sodium salt of the corresponding acid, e.g. sodium sulphate, which is practically insoluble in an alcoholic medium and which accordingly can be separated readily. The acid is preferably added in stoichiometric amounts or in a slight excess (in general a less than 5 per cent excess). As a consequence of the suspension of the sodium salt in the lower alcohol the crystals of the salt swell. Reaction conditions which facilitate reactions 1. and 2. are characteristic of the process in accordance with the invention. The following applies with respect to the three process variants b1), b2)and b3): b1) on the one hand, high yields are achieved when the NaKGA-H20 crystals are firstly suspended in the alcoholic solvent and thereby swell (conversion of the NaKGA-H20 into needle-shaped crystal forms having a large surface). In this case the needles form within seconds to hours depending on the particle size of the material used and on the Intensity of stirring. Preferably, either fine material ( since the suspension is very difficult to stir because of the needle formation. Higher concentrations can be realised by repeating reactions 1. and 2. or other recyclizations. At the conclusion of the needle formation the acid of low water content is introduced, whereby the pH value should !ie above 1.5, preferably between 2.5 and 3.5. At lower pH values more NaKGA is incorporated into the salt and increasing amounts of undesired byproducts are formed, especially with sulphuric acid. b2) High yields can also be achieved when the NaKGA-H2O and the acid are added simultaneously, but only when particles have been reduced in size by wet milling, such as e.g. using rotor-stator dispersion machines, homogenizers, ultrasonic or similar devices. Here the pH value should also be above 1.5, preferably 2.5-3.5, in order that the reaction does not proceed too rapidly and needle formation, which facilitates the reaction, takes place at least to a microscopic extent. The reaction time is usually less than 20 minutes; longer times are required after wet grinding. The average particle size of the resulting salt should be a maximum of 10 µm, preferably The high energy input required for the wet grinding is a disadvantage of variant b2). However, the simple reaction procedure, especially the avoidance of a difficultly stirrable suspension, as in variant b1), is an advantage. b3) Combinations of variants b1) and b2) are advantageously used. For example, either the needle formation or the addition of acid can be effected in successive reactors and the wet grinding can be performed in a further one. Further, in a continuous process the needle formation [variant b1)] can be effected in a first step and a wet grinding with addition of the acid [variant b2)] can be effected in the subsequent step. Moreover, the difficultly soluble salt can be selectively recycled, for example, by means of a hydrocyclone. By this means the residence times of the difficultly soluble salt are increased and the achievable yields are once again increased. A sodium sulphate, which contains considerable amounts of NaKGA-H2O, can also be suspended in the solvent. The typical needles form and the reaction with an acid of low water content can be realised in accordance with variant bl) with high yields. No temperature limitations apply to any of the three variants hi )-b3). Preferably, however, the temperatures lie in the range of about 20°C to about 70C. KGA is less soluble at lower temperatures, while decompositions occur at higher temperatures. In genera!, the reaction conditions are chosen such that practically no esterification reactions can take place. An esterification of the 2-keto-L-gulonic acid [step d)], which may be desired, can be effected without problems in a known manner In the presence of an acid as the catalyst after the separation of the insoluble salt, e.g. of the sodium sulphate. It will be observed that pH data have been defined only for aqueous solutions. The pH values given herein relate to measurements using a pH glass electrode with 3 molar potassium chloride solution as the electrolyte. When other measuring instruments are used under otherwise identical conditions, pH values different therefrom can be measured. The stipulation of a pH range is therefore problematic. Significantly less than 5% ester usually results at the pH values >1.5 given herein. The liberated 2-keto-L-gulonic acid should be soluble In the reaction medium under the chosen reaction conditions. The separation of the difficultly soluble salt in process step c) as well as the optional esterification of the 2-keto-L-gulonic acid in step d) can In each case be carried out in a known manner. Thus, the separation of the salt can be carried out by filtration and/or centrifugatlon, preferably by centrifugation. However, fundamentally ail solid/liquid separation methods are conceivable for particles Advantageously, anhydrous solvents and concentrated acids (of low water content) are used, since water displaces the equilibrium of the subsequent esterification in an unfavourable manner. Of course, some water is often present, be It from concentrated 37% hydrochloric acid or recycled substance stream. However, the water content should preferably not exceed 1 0%. With the aid of the process in accordance with the invention 2-keto-L-gulonlc acid, which is very important for the manufacture of vitamin C and which occurs as a dissolved salt in the aqueous fermentation solution, can be converted in a relatively simple and economical manner into the alcoholic solution of the free acid. The thus-obtained solution of 2-keto-L-gulonic acid has a very high purity and can be converted into ascorbic acid in a known manner. A fundamental advantage of the process in accordance with the invention is that a turbid fermentation solution which still contains biomass can be used in the first step [a)] for the crystallization of the sodium 2-keto-L-guionate monohydrate in high yield, i.e. significantly higher than 90%. By the subsequent reaction with an acid of low water content (protonation), effected in step b), there is formed a difficultly soluble salt with which residues of biomass or proteins can be separated to a large extent. Thus, a clear solution of 2-keto-L-gulonic acid in the alcohol, which contains practically no biomass, can be obtained in high yield in this step b). The difficulty of a complete biomass or protein separation in the aqueous phase is thereby avoided. The following Examples for the conversion of the sodium salt of 2-keto-L-guionic acid from aqueous fermentation solutions into the alcoholic solution of the free acid show advantageous embodiments of the process in accordance with the Example 1 The sodium salt of 2-keto-L-gulonic acid (as the monohydrate; hereinafter NaKGA-H2O) was crystallized from the fermentation broth as follows: 2195 g of an aqueous, blomass-contalning fermentation broth were centrifuged for 10 minutes at 12000 g, which gave 2160 g of a turbid supernatant and 35 g of sludge, corresponding to 1.5%. The supernatant was concentrated under reduced pressure at 20° and the first crystalllzate which separated was filtered off and washed with water. The wash water was combined with the residual solution. This was again concentrated at 20° and the second crystalllzate was filtered off and washed with water. Recycling of the wash water, concentration, filtration and washing were carried out in the same manner for the third and fourth crystallizates. Crystallization of 2160 g of centrifuged, turbid fermentation broth gave 132.90 g: 1st crystalllzate with 99.3% purity (54% yield) 60.17 g: 2nd crystalllzate with >99.S% purity (78.9% overall yield) 29.70 g: 3rd crystalllzate with 96.6% purity (90.6% overall yield) 15.00 g: 4th crystalllzate with 79.5% purity (95.5% overall yield) The overall yield of the first three crystallizations was 91% with an average purity of about 99% Example 2 A centrifuged, but turbid, pre-concentrated fermentation broth was crystallized continuously in a 6 I crystalllzer under a vacuum at 55o. In the stationary state 1550 g/h of fermentation solution were added continuously with 17.0 wt.% sodium 2-keto-L-gulonate (NaKGA), corresponding to 1.22 mol. On average 241 g/h of dry crystallizate with a NaKGA •H2O content of 99% (1.02 mol/h) separated. There were also obtained 180 g/h of mother liquor containing 11.9% NaKGA (1.02 mol/h). A total of a further 0.08 mo!/h of NaKGA was recovered in the rinsing and purification solutions . The nitrogen content of the mother liquor was >1 wt.%. The crystals contained 180 ppm nitrogen. Example 3 40.0 g of NaKGA-HzO with a content of 99% (1 69 mmol) were suspended in 400 g of methanol at room temperature while stirring. A thick suspension, which was difficult to stir, resulted. After 20 minutes 8.73 g of 95% sulphuric acid (85 mmol) were added thereto within 5 minutes at a minimal pH of 2.0. After 10 minutes a further 20.0 g (85 mmol) of NaKGA-H2O were added, 5 minutes later 4.4 g (42 mmol) of the 95% sulphuric acid (42 mmol) were added and the mixture was stirred for a further 1 0 minutes. The addition of 20 g of NaKGA-H2O and 4.4 g of the 95% sulphuric acid was repeated one more time. After stirring at room temperature for 75 minutes the solution was filtered. 25.21 g of sodium sulphate with a 3.6% content of 2-keto-L-gulonic acid (KGA; 5 mmol) separated. The filtrate obtained contained 329 mmol of KGA and 3 mmol of methyl 2-keto-L-gulonate (MeKGA), which corresponded to a yield of 98%. Together with the KGA found in the filter residue the recovery was almost 100%. Example 4 80.0 g of NaKGA-HzO with a content of 99.0% (338 mmol) and 17.63 g of 95% sulphuric acid (17 mmol) were dosed into 390 g of methanol within 1 5 minutes at 20-35o and at a pH value greater than 2.5. A disperserhaving a rotor-stator system was operated in the reaction vessel for wet grinding. After dispersion for 60 minutes the suspension was filtered and the filter residue was washed with a small amount of methanol. The pH value remained constant at 2.5 during the dosing by the controlled addition of the sulphuric acid. A pH value of 2.6 was measured after 75 minutes. The filtrate obtained contained 330 mmol of KGA, corresponding to a yield of 98%. There were obtained 25.45 g of filter residue (sodium sulphate) with a KGA content of 5.7%, corresponding to 7 mmol. The determinable content of MeKGA was 0.04% in the filtrate (1 mmol) and was not detectable in the filter residue. The recovery of KGA and MeKGA in the filtrate together with the KGA found in the filter residue was accordingly almost 100%. Example 5 75.0 g of NaKGA-HsO with a content of 99% (317 mmol) and 16.38 g of 95% sulphuric acid (159 mmol) were dosed into 375 g of methanol within 6 minutes at 60° and at a pH value of 2.5. A disperser with a rotor-stator system was operated in the reaction vessel for the wet grinding. After the addition the mixture was dispersed for a further 10 minutes, subsequently filtered and the filter residue was washed with about 10 ml of methanol and dried under reduced pressure. The filtrate obtained contained 303 mmol of KGA and 5 mmol of MeKGA, corresponding to a yield of 97%. There were obtained 24.14 g of filter residue (sodium sulphate) with a KGA content of 6.2%, corresponding to 8 mmol. The determinable content of MeKGA was 0.27% in the filtrate (0.5 mmol) and 0.04% in the filter residue. The recovery of KGA and MeKGA in the filtrate together with the KGA and the MeKGa found in the filter residue was accordingly almost 100%. Example 6 80.0 g of crystalline NaKGA-HzO (338 mmol), obtained according to the procedure described in Example 2, and 14.00 g of 96% sulphuric acid (178 mmol) were dosed into 400 g of methanol within 4 minutes at 65° and at a pH value greater than 2.5. A further 4.14 g (41 mmol) of sulphuric acid, in total a 5% stoichiometric excess, were added during 2 minutes, during which the pH value fell to 2.0. A disperser with a rotor-stator system was operated in the reaction vessel for wet grinding. After 10 minutes the mixture was filtered and the filter residue was washed with 40 ml of methanol. The filtrate contained 314 mmol of KGA and 12.1 mmo! of MeKGA. There were obtained 25.02 g of dried filter residue (sodium sulphate) containing 7.5 mmol of KGA and 0.5 mmol of MeKGA. The yield was 96.4% with a recovery of 98,8%. Example 7 80.0 g of crystalline NaKGA-H20 (338 mmol), obtained according to the procedure described in Example 2, and 15.40 g of 96% sulphuric acid (151 mmo!) were dosed into 400 g of 1,2-ethanediol within 4 minutes at 65° and at a pH greater than 2.5- A further 1.88 g of the sulphuric acid (41 mmol) were added within one minute, during which the pH value fell to 2.3. A disperser with a rotor-stator system was operated in the reaction vessel for wet grinding. After 10 minutes the mixture was filtered and the filter residue was washed with a small amount of 1,2-ethanediol. There were obtained 19.84 g of dried filter residue (sodium sulphate) containing 2.3 wt.% of KGA, corresponding to 2.4 mmol of KGA. The filtrate could not be analyzed satisfactorily because of the high boiling point of the solvent. Example 8 80.0 g of crystalline NaKGA-HzO (338 mmol), obtained according to the procedure described in Example 2, and 9.92 g of 96% sulphuric acid (97 mmol) were dosed into 432 g of methanol having a 8% water content within 5 minutes at 65° and at a pH value of 2.5. A further 7.36 g of the sulphuric acid (72 mmol) were added within 3 minutes, with the pH value being a minimum 1.9 and 2.3 towards the end. A disperser with a rotor-stator system was operated in the reaction vessel for wet grinding. After 10 minutes the suspension was filtered and the filter residue was washed with 20 ml of methanol. There were obtained 23.43 g of dry filter residue containing 5.7% of KGA (6.9 mmol) and 0.2% of MeKGA (0.2 mmol). The filtrate contained 325 mmol of KGA and 3.5 mmol of MeKGA. Example 9 10.0 g of sodium sulphate, which contained a total of 17.0 mmoi of NaKGA-H2O, NaKGA and KGA, corresponding to 39.7% of KGA, were suspended in 100 ml of methanol at 20C. A thick suspension formed after 20 minutes. 0.82 g of 95% sulphuric acid (79 mmol) was added slowly in such a manner that the pH value was always above 2.0. After stirring for one hour the pH value was 2.3. The suspension was filtered and the filter residue was washed with a small amount of methanol. This gave 7.0 g of filter residue (sodium sulphate) containing 0.12% of KGA. Evaporation of the filtrate under a vacuum gave 3.47 g of product containing 81.4% of KGA and 6.5% of MeKGA. Example 10 in each of ten successive experiments 400 g of methanol were placed in a 1 I reaction vessel and sodium sulphate from the respective preceding experiment was suspended therein. In each case 80.0 g of crystalline NaKGA •H2O (3.38 mmol), obtained according to the procedure described in Example 2, were added over 5 minutes at 60^0 and in each case 17.5 g of 95% sulphuric acid (169 mmol) were added within 10 minutes at a pH value of about 2.5. A disperser with a rotor-stator system was operated in the reaction vessel for wet grinding. After a reaction period of a further 5 minutes the mixture was filtered and the moist filter residue (sodium sulphate) was used in the subsequent experiment. In the last experiment the filter residue was washed with 400 ml of methanol and dried. From the last experiment there were obtained 240 g of a dried filter residue (sodium sulphate) containing 3-9% of KGA and 0.3% of MeKGA (0.05 mol). The filtrate and wasff water, total 3570 g, contained 16.6% of KGA (3.06 mol), 1.49% of MeKGA (0.25 mo!) and 11 ppm nitrogen. 350 ppm nitrogen were determined in the sodium sulphate. The average particle size of the sodium sulphate fell over the series of experiments from 10 µm (1st experiment) to 3.7 µm (10th experiment). WE CLAIM: 1. A process for the conversion of the sodium salt of 2-keto-L-gulonic acid, which is present in an aqueous fermentation solution, into an alcoholic solution of the free acid which process comprises a) recovering sodium 2-keto-L~gulonate monohydrate from an aqueous fermentation solution by evaporation, cooling or displacement crystallization and optionally pulverizing the thus-obtained crystallizate by grinding, bl) suspending the sodium 2-keto-L-gulonate monohydrate obtained in step a) in a lower alcohol selected from methanol, ethanol, propanol and 1,2- ethanediol, leaving the crystals to swell and thereafter adding an acid of low water content selected from concentrated sulphuric acid, nitric acid, hydrochloric acid, phosphoric acid and gaseous hydrogen chloride, whereby the measured pH value should lie above 1.5, this process step being carried out at temperatures in the range of about 20° C to about 70" C, or b2) adding the sodium 2-keto-L-gulonate monohydrate obtained in step a) together with an about stoichiometric amount of the acid of low water content to the lower alcohol using a wet grinding system, whereby the measured pH value should lie above 1.5, this process step being carried out at temperatures in the range of about 20 C to about 70C, or b3) carrying out a combination of steps bl) and b2) including recycling of product streams, this process step being carried out at temperatures in the range of about 20° C to about 70° C, and c) separating the salt of the added acid formed in step bl), b2) or b3) by filtration and/or centrifugation and thus obtaining an alcoholic solution of 2- keto-L-gulonic acid. 2. The process according to claim 1, wherein continuous evaporation crystallization is used as the crystallization method in step a). 3. The process according to claim 1 or 2, wherein methanol is used as the lower alcohol in step b). 4. The process according to any one of claims 1 to 3, wherein concentrated sulphuric acid or hydrochloric acid, particularly > 95% sulphuric acid, is used as the acid of low water content in step b). 5. The process according to any one of claims 1 to 4, wherein the pH value lies between 2.5 and 3.5 in step b). 6. The process according to any one of claims 1 to 5, wherein in step c) the separation of the salt formed is carried out by centrifugation. 7. A process for the conversion of the sodium salt of 2-keto-L-gulonic acid into an alcoholic solution of the free acid substantially as herein described and exemplified. RAN 4226/104 The present invention is concerned with a novel process for the conversion of a sodium salt of 2-l As is known, 2-keto-L-guIonlc acid (KGA) Is an important starting material for the manufacture of ascorbic acid (vitamin C). In fermentative processes for the manufacture of KGA, the KGA which results as a metabolic product is neutralized by the addition of a base, e.g. sodium hydroxide or calcium hydroxide, in order to maintain favourable fermentation conditions. The product of the fermentation is an aqueous, biomass-containing fermentation solution in which the KGA salt, i.e. the sodium or calcium 2-keto-L-gulonate [NaKGA or Ca(KGA)2, respectively], is present in dissolved form. For the industrial manufacture of ascorbic acid or of the ascorbate, the fermentatively produced KGA must be transferred into an organic solvent. A lower alkanol is advantageously used as the solvent. The ascorbate is obtained in high yield by esterifying the alcohol with KGA and subsequently adding a base. As an alternative to this, the KGA can be converted into ascorbic acid in an organic solvent under strongly acidic conditions. However, the yields are as a rule somewhat lower [see Helv. Chim. Acta 17, 311-328 (1934)]. All processes for the working up of the fermentation solution are based on three process steps: 1. Protonation of the sodium or calcium 2-keto-L-gulonate present to the free acid (e.g. NaKGA + H+ -> KGA + Na+); 2. Removal of water; Pa/So 26.2.97 3. Removal of the biomass, dissolved proteins and other contaminants present In the fermentation solution. The sequence in which the process steps are carried out and their specific performance are characteristic of the respective process. The protonation (Step 1) can be effected in the aqueous fermentation solution by the addition of acids. According to US Patent (USP) 3 381 027 and European Patent Publication (EP) 359 645 the dissolved KGA is obtained by adding sulphuric acid to Ca(KGA)2 and separating the precipitated calcium salt. A protonation can also be effected by using cation exchange resins. According to EP 359 645 the aqueous Ca(KGA)2-containing fermentation solution is passed through a cation exchanger and the calcium ions are removed completely. When cation exchangers are used, the biomass must previously be removed completely (Step 3), e.g. by microfiltration, in order to guarantee a sufficiently long useful life of the cation exchanger. The product of the protonation is an aqueous, KGA-containing solution having a pH value which is significantly below 2.0 (the pKs value of KGA is 2.54). KGA is thereupon isolated by crystallization, extraction (e.g. according to EP 359 042) or adsorption (e.g. according to Chinese Patent Publication T097731A: see Chem. Abs. 124, 56570) and thereby separated from the water (Step 2). The crystallization of KGA In high yields can only be effected with difficulty, since the solubility, e.g. 480 g/l at 30°C, is very high. Extraction and adsorption are industrially difficult processes, especially when contaminants from the fermentation are present. According to EP 359 042 the biomass must accordingly be removed completely prior to the extraction. According to EP 403 993 isolated, but not purified KGA can be used for the manufacture of sodium ascorbate. Contaminants, such as biomass and proteins, are eliminated in further process operations by the addition of sodium bicarbonate or potassium bicarbonate and precipitation of non-esterlfied KGA. In a!! previously known processes the protonation of KGA is effected first and then the water Is removed. In principle, however, it is possible to operate In reverse, i.e. to firstly Isolate the NaKGA or Ca(KGA)2 from the fermentation solution, thereby to remove water (Step 2), and subsequently to carry out the protonation (Step 1). Thus, the crystallization of sodium 2-keto-L-gulonate monohydrate (NaKGA-HzO) is known from Japanese Patent Publications (Kokai) 66684/1977 and 62894/1978, and in the case of solubilities of e.g. 250 g/l at 300C significantly higher yields are to be expected than in the crystallization of KGA-H20. Moreover, both substances crystallize as the monohydrate, which usually is not taken into consideration. The difficulty In this operational mode, i.e. Step 2 prior to Step 1, lies in the complete protonation of the NaKGA in organic solvents, since NaKGA is practically Insoluble. Some attempts to protonate NaKGA in organic solvents have, however, been documented. According to EP 91 134 crystalline NaKGA can be converted Into KGA using gaseous hydrogen chloride In a mixture of ethano! and acetone. The sodium chloride, which is also formed, is separated. In the further course of the reaction a rearrangement to crystalline ascorbic acid takes place immediately under the strong acidic reaction conditions, with undesired byproducts resulting and the yields therefore being low at 60-82%. According to USP 5 391 770 NaKGA can be reacted with a more than 40% excess of concentrated sulphuric acid in methanol (Example 10). The reaction time inclusive of esterlfication to the methyl ester amounts to 4.5 hours at 65X and the yield is 91.9%. Subsequently, the pH is increased to 4 and the sodium sulphate is separated. Having regard to these long reaction times and the large excess of acid, considerable amounts of decomposition products are to be expected according to EP 403 993. EP 403 993 discloses the reaction of a mixture of 50% KGA and 50% NaKGA with an about 50% excess of sulphuric acid in methanol (Example 5). After a reaction period of one hour under reflux conditions the mixture is filtered. The yield of sodium sulphate obtained corresponds to about 60% of theory. Accordingly, a maximum of 50% of the KGA can be used in the form of the sodium salt. Fundamentally, the process provides for the use of KGA, with the mentioned NaKGA resulting and being recycled in subsequent steps. Disadvantages in the last two processes are the large excess of sulphuric acid used and the long reaction times under strong acidic conditions. When sulphuric acid is added in stoichiometric amounts and thus under milder conditions, a considerable part of the NaKGA usually remains incorporated in the sodium sulphate which results and the yield drops accordingly. According to USP 5 391 770 it is also known that the sodium salt of ascorbic acid (NaASC) can be protonated to ascorbic acid by the addition of sulphuric acid in methanol. In this case NaKGA is present as an impurity in up to 9%. In methanol as the solvent the yields of ascorbic acid are between 91% and 96% {Examples 12, 14 and 15). On the other hand, in a solvent mixture of 75% methanol and 25% water the yield of ascorbic acid is 99% (Example 2). Clearly, the yields in pure solvent are considerably lower than in the solvent mixture, since a preferred water content of 1 5-25% is indicated. The reason is the higher solubility of NaASC in the mixture compared with the pure solvent, e.g. at 40C about 2 weight percent (wt.%) compared with about 0.3 wt.%. Material transport is impeded in the case of low solubil¬ities and the reaction of a difficultly soluble salt with sulphuric acid to give the very difficultly soluble sodium sulphate (solubility = 0.024 wt.% in methanol) gives correspondingly poorer yields. Material transfer problems are to be expected to an increasing extent in the case of a reaction involving NaKGA-HaO, since the solubilities are considerably lower than those of sodium ascorbate (at 40°C, The object of the present invention is to provide a process which permits in the simplest possible manner the conversion of the sodium salt of 2-keto-L-gulonic acid, which is present in an aqueous, non-purified fermentation broth, into free 2-keto-L-gulonic acid in alcoholic solution in high yield and with high purity. At the same time, the disadvantages of prior art processes, especially the complete removal of blomass, proteins etc., e.g. by microfiltration, the use of cation exchangers to remove metal Ions from the aqueous fermentation solutions as well as the crystallization or drying of 2-keto-L-gulonic acid, should be avoided. Moreover, only readily accessible chemicals should be used, and as few steps as possible should be required. In the scope of the present invention a process has now been found which fulfils the aforementioned requirements and in accordance with which the sodium salt of 2-keto-L-gulonic acid is crystallized from a fermentation solution, which is only partially freed from biomass, and the sodium 2-keto-L-gulonate monohydrate obtained Is converted into an alcoholic solution of free 2-keto-L-gulonic acid. The protonatlon to 2-keto-L-gulonic acid and the removal of the metal ions are effected in this case exclusively by reaction in an alcoholic medium. Particular reaction conditions have now been found under which sodium 2-keto-L-gulonate monohydrate (NaKGA-HnO) suspended in alcoholic medium reacts in very good yields with an acid of low water content to give the dissolved,.free 2-keto-L-gulonic acid (KGA) and an insoluble salt. For the first time by this means the crystallization of the NaKGA-HnO from the aqueous fermentation solution is practicable. The critical steps required in the solution and can then be esterified directly in high yields in any manner and converted into ascorbate. Accordingly, a sequence of process steps is characteristic of the process in accordance with the invention, i.e. firstly NaKGA-H20 is crystallized from a fermentation broth, i.e. the water other than the water of crystallization is removed completely (Step 2, see above), and thereafter the protonation (Step 1) is effected entirely in a lower alkanol as the solvent by the addition of a strong acid (of low water content), e.g. sulphuric acid. A difficultly soluble salt of the acid, e.g. sodium sulphate, results and can be separated readily. The desired solution of KGA remains and, as mentioned above, can be esterified directly and converted Into ascorbate. Also characteristic are special reaction conditions for the protonation in which, in spite of extremely low solubilities of the NaKGA'H20 used and of the product, e.g. sodium sulphate, yields of more than 97% can be realised. In this respect, specific and previously unknown properties pertaining to NaKGA-H20 as a material are made use of, as will be explained in more detail below. The object of the invention is accordingly a process for the conversion of the sodium salt of 2-keto-L-gulonic acid from aqueous fermentation solutions into an alcoholic solution of the free acid and, if desired, into an alkyl ester of the acid, which process comprises a) recovering the sodium 2-keto-L-gulonate monohydrate from an aqueous fermentation solution by crystallization involving evaporation, cooling or displacement (hereinafter "evaporation, cooling or displacement crystallization") and, if desired, pulverizing the thus-obtained crystallizate by grinding, b1) suspending the optionally ground sodium 2-keto-L-gulonate monohydrate obtained in step a) in a lower alcohol, leaving the crystals to swell and thereafter adding an acid of low water content, whereby the measured pH value should lie above 1.5, or b2) adding the optionally ground sodium 2-keto-L-gulonate monohydrate obtained in step a) together with an about stoichiometric amount of an acid of low water content to a lower alcohol using a wet grinding system, whereby the measured pH value should lie above 1.5, or b3) carrying out a combination of steps b1) and b2) including recycling of product streams, and c) separating the salt of the added acid formed in step b1), b2) or b3) and thus obtaining an alcoholic solution of 2~keto-L-gulonic acid, and, if desired, d) treating the alcoholic solution of 2-keto-L-gulonic acid obtained in step c) with a catalytic amount of an acid or with an acidic cation exchanger in order to esterify the 2-keto-L-gulonic acid with the alcohol. The fermentation, which Is effected prior to the actual process in accordance with the invention, yields a turbid fermentation broth containing biomass. The crystallization of the sodium salt of 2-keto-L-gulonic acid present in the fermentation broth [step a)] can In principle be effected directly from this broth; however, a previous separation of at least 90% of the biomass by centrifugation has been shown to be advantageous. A turbid, but sludge-free fermentation solution results in this case. A complete removal of biomass and dissolved proteins prior to the crystallization is, however, not necessary. The crystallization of the sodium salt from the aqueous fermentation broth or solution according to process step a) is conveniently effected In a manner known per se by concentrating the fermentation broth or solution, cooling the solution or adding a different solvent, i.e. by evaporation, cooling or displacement crystallization. The usual ancillary conditions have to be observed in this case. Thus, for example, the evaporation crystallization should be carried out under reduced pressure and at the same time at a low temperature, preferably at temper- atures in the range of about 35C to about 60°C, in order to avoid as far as possible decomposition of the product. The crystal¬lization can be carried out continuously or batch-wise, preferably continuously. Continuous evaporation crystallization is the crystallization method which is preferably used. Subsequently, the crystallizate can be separated from the mother liquor by a solid/liquid separating operation such as filtration or centrifu-gation. The NaKGA-H2O crystallizate obtained is under the given conditions usually purer than 98% with small amounts of organic contaminants ( In the next process step [b)] the crystallizate in crystalline form or optionally in a form reduced in size by grinding is firstly suspended in a lower alcohol, e.g. methanol, ethanol, propanoi or 1,2-ethanediol (glycol), preferably methanol. It has now been found that the NaKGA-HsO converts into the anhydrate (NaKGA) with the addition of water and thereby forms a large number of very thin needles 1. Dehydration 2NaKGA-H20 (undissolved) ~* 2NaKGA (undissolved, needles) + 2H2O 2. Protonation 2NaKGA (undissolved, needles) + H2SO4 -^ 2KGA (dissolved) + Na2S04 (undissolved) A strong acid of low water content is used for the protonation. The water content of the added acid "of low water content" is in principle not critical for the process. However, the concentration of water In the resulting 2-keto-L-gulonic acid/alcohol solution determines the equilibrium conversion of a subsequent esterification. From an industrial-economical point of view acids of low water content, i.e. acids which are more appropriately denoted as concentrated, are therefore preferably used. As acids of low water content there conveniently come into consideration concentrated mineral acids such as, for example, (in each case concentrated) sulphuric acid, nitric acid, hydrochloric acid and phosphoric acid, and even gaseous hydrogen chloride. Concentrated sulphuric acid or hydrochloric acid, especially >95% sulphuric acid, is preferably used, there being formed in step b) the sodium salt of the corresponding acid, e.g. sodium sulphate, which is practically insoluble in an alcoholic medium and which accordingly can be separated readily. The acid is preferably added in stoichiometric amounts or in a slight excess (in general a less than 5 per cent excess). As a consequence of the suspension of the sodium salt in the lower alcohol the crystals of the salt swell. Reaction conditions which facilitate reactions 1. and 2. are characteristic of the process in accordance with the invention. The following applies with respect to the three process variants b1), b2)and b3): b1) on the one hand, high yields are achieved when the NaKGA-H20 crystals are firstly suspended in the alcoholic solvent and thereby swell (conversion of the NaKGA-H20 into needle-shaped crystal forms having a large surface). In this case the needles form within seconds to hours depending on the particle size of the material used and on the Intensity of stirring. Preferably, either fine material ( since the suspension is very difficult to stir because of the needle formation. Higher concentrations can be realised by repeating reactions 1. and 2. or other recyclizations. At the conclusion of the needle formation the acid of low water content is introduced, whereby the pH value should !ie above 1.5, preferably between 2.5 and 3.5. At lower pH values more NaKGA is incorporated into the salt and increasing amounts of undesired byproducts are formed, especially with sulphuric acid. b2) High yields can also be achieved when the NaKGA-H2O and the acid are added simultaneously, but only when particles have been reduced in size by wet milling, such as e.g. using rotor-stator dispersion machines, homogenizers, ultrasonic or similar devices. Here the pH value should also be above 1.5, preferably 2.5-3.5, in order that the reaction does not proceed too rapidly and needle formation, which facilitates the reaction, takes place at least to a microscopic extent. The reaction time is usually less than 20 minutes; longer times are required after wet grinding. The average particle size of the resulting salt should be a maximum of 10 µm, preferably The high energy input required for the wet grinding is a disadvantage of variant b2). However, the simple reaction procedure, especially the avoidance of a difficultly stirrable suspension, as in variant b1), is an advantage. b3) Combinations of variants b1) and b2) are advantageously used. For example, either the needle formation or the addition of acid can be effected in successive reactors and the wet grinding can be performed in a further one. Further, in a continuous process the needle formation [variant b1)] can be effected in a first step and a wet grinding with addition of the acid [variant b2)] can be effected in the subsequent step. Moreover, the difficultly soluble salt can be selectively recycled, for example, by means of a hydrocyclone. By this means the residence times of the difficultly soluble salt are increased and the achievable yields are once again increased. A sodium sulphate, which contains considerable amounts of NaKGA-H2O, can also be suspended in the solvent. The typical needles form and the reaction with an acid of low water content can be realised in accordance with variant bl) with high yields. No temperature limitations apply to any of the three variants hi )-b3). Preferably, however, the temperatures lie in the range of about 20°C to about 70C. KGA is less soluble at lower temperatures, while decompositions occur at higher temperatures. In genera!, the reaction conditions are chosen such that practically no esterification reactions can take place. An esterification of the 2-keto-L-gulonic acid [step d)], which may be desired, can be effected without problems in a known manner In the presence of an acid as the catalyst after the separation of the insoluble salt, e.g. of the sodium sulphate. It will be observed that pH data have been defined only for aqueous solutions. The pH values given herein relate to measurements using a pH glass electrode with 3 molar potassium chloride solution as the electrolyte. When other measuring instruments are used under otherwise identical conditions, pH values different therefrom can be measured. The stipulation of a pH range is therefore problematic. Significantly less than 5% ester usually results at the pH values >1.5 given herein. The liberated 2-keto-L-gulonic acid should be soluble In the reaction medium under the chosen reaction conditions. The separation of the difficultly soluble salt in process step c) as well as the optional esterification of the 2-keto-L-gulonic acid in step d) can In each case be carried out in a known manner. Thus, the separation of the salt can be carried out by filtration and/or centrifugatlon, preferably by centrifugation. However, fundamentally ail solid/liquid separation methods are conceivable for particles Advantageously, anhydrous solvents and concentrated acids (of low water content) are used, since water displaces the equilibrium of the subsequent esterification in an unfavourable manner. Of course, some water is often present, be It from concentrated 37% hydrochloric acid or recycled substance stream. However, the water content should preferably not exceed 1 0%. With the aid of the process in accordance with the invention 2-keto-L-gulonlc acid, which is very important for the manufacture of vitamin C and which occurs as a dissolved salt in the aqueous fermentation solution, can be converted in a relatively simple and economical manner into the alcoholic solution of the free acid. The thus-obtained solution of 2-keto-L-gulonic acid has a very high purity and can be converted into ascorbic acid in a known manner. A fundamental advantage of the process in accordance with the invention is that a turbid fermentation solution which still contains biomass can be used in the first step [a)] for the crystallization of the sodium 2-keto-L-guionate monohydrate in high yield, i.e. significantly higher than 90%. By the subsequent reaction with an acid of low water content (protonation), effected in step b), there is formed a difficultly soluble salt with which residues of biomass or proteins can be separated to a large extent. Thus, a clear solution of 2-keto-L-gulonic acid in the alcohol, which contains practically no biomass, can be obtained in high yield in this step b). The difficulty of a complete biomass or protein separation in the aqueous phase is thereby avoided. The following Examples for the conversion of the sodium salt of 2-keto-L-guionic acid from aqueous fermentation solutions into the alcoholic solution of the free acid show advantageous embodiments of the process in accordance with the Example 1 The sodium salt of 2-keto-L-gulonic acid (as the monohydrate; hereinafter NaKGA-H2O) was crystallized from the fermentation broth as follows: 2195 g of an aqueous, blomass-contalning fermentation broth were centrifuged for 10 minutes at 12000 g, which gave 2160 g of a turbid supernatant and 35 g of sludge, corresponding to 1.5%. The supernatant was concentrated under reduced pressure at 20° and the first crystalllzate which separated was filtered off and washed with water. The wash water was combined with the residual solution. This was again concentrated at 20° and the second crystalllzate was filtered off and washed with water. Recycling of the wash water, concentration, filtration and washing were carried out in the same manner for the third and fourth crystallizates. Crystallization of 2160 g of centrifuged, turbid fermentation broth gave 132.90 g: 1st crystalllzate with 99.3% purity (54% yield) 60.17 g: 2nd crystalllzate with >99.S% purity (78.9% overall yield) 29.70 g: 3rd crystalllzate with 96.6% purity (90.6% overall yield) 15.00 g: 4th crystalllzate with 79.5% purity (95.5% overall yield) The overall yield of the first three crystallizations was 91% with an average purity of about 99% Example 2 A centrifuged, but turbid, pre-concentrated fermentation broth was crystallized continuously in a 6 I crystalllzer under a vacuum at 55o. In the stationary state 1550 g/h of fermentation solution were added continuously with 17.0 wt.% sodium 2-keto-L-gulonate (NaKGA), corresponding to 1.22 mol. On average 241 g/h of dry crystallizate with a NaKGA •H2O content of 99% (1.02 mol/h) separated. There were also obtained 180 g/h of mother liquor containing 11.9% NaKGA (1.02 mol/h). A total of a further 0.08 mo!/h of NaKGA was recovered in the rinsing and purification solutions . The nitrogen content of the mother liquor was >1 wt.%. The crystals contained 180 ppm nitrogen. Example 3 40.0 g of NaKGA-HzO with a content of 99% (1 69 mmol) were suspended in 400 g of methanol at room temperature while stirring. A thick suspension, which was difficult to stir, resulted. After 20 minutes 8.73 g of 95% sulphuric acid (85 mmol) were added thereto within 5 minutes at a minimal pH of 2.0. After 10 minutes a further 20.0 g (85 mmol) of NaKGA-H2O were added, 5 minutes later 4.4 g (42 mmol) of the 95% sulphuric acid (42 mmol) were added and the mixture was stirred for a further 1 0 minutes. The addition of 20 g of NaKGA-H2O and 4.4 g of the 95% sulphuric acid was repeated one more time. After stirring at room temperature for 75 minutes the solution was filtered. 25.21 g of sodium sulphate with a 3.6% content of 2-keto-L-gulonic acid (KGA; 5 mmol) separated. The filtrate obtained contained 329 mmol of KGA and 3 mmol of methyl 2-keto-L-gulonate (MeKGA), which corresponded to a yield of 98%. Together with the KGA found in the filter residue the recovery was almost 100%. Example 4 80.0 g of NaKGA-HzO with a content of 99.0% (338 mmol) and 17.63 g of 95% sulphuric acid (17 mmol) were dosed into 390 g of methanol within 1 5 minutes at 20-35o and at a pH value greater than 2.5. A disperserhaving a rotor-stator system was operated in the reaction vessel for wet grinding. After dispersion for 60 minutes the suspension was filtered and the filter residue was washed with a small amount of methanol. The pH value remained constant at 2.5 during the dosing by the controlled addition of the sulphuric acid. A pH value of 2.6 was measured after 75 minutes. The filtrate obtained contained 330 mmol of KGA, corresponding to a yield of 98%. There were obtained 25.45 g of filter residue (sodium sulphate) with a KGA content of 5.7%, corresponding to 7 mmol. The determinable content of MeKGA was 0.04% in the filtrate (1 mmol) and was not detectable in the filter residue. The recovery of KGA and MeKGA in the filtrate together with the KGA found in the filter residue was accordingly almost 100%. Example 5 75.0 g of NaKGA-HsO with a content of 99% (317 mmol) and 16.38 g of 95% sulphuric acid (159 mmol) were dosed into 375 g of methanol within 6 minutes at 60° and at a pH value of 2.5. A disperser with a rotor-stator system was operated in the reaction vessel for the wet grinding. After the addition the mixture was dispersed for a further 10 minutes, subsequently filtered and the filter residue was washed with about 10 ml of methanol and dried under reduced pressure. The filtrate obtained contained 303 mmol of KGA and 5 mmol of MeKGA, corresponding to a yield of 97%. There were obtained 24.14 g of filter residue (sodium sulphate) with a KGA content of 6.2%, corresponding to 8 mmol. The determinable content of MeKGA was 0.27% in the filtrate (0.5 mmol) and 0.04% in the filter residue. The recovery of KGA and MeKGA in the filtrate together with the KGA and the MeKGa found in the filter residue was accordingly almost 100%. Example 6 80.0 g of crystalline NaKGA-HzO (338 mmol), obtained according to the procedure described in Example 2, and 14.00 g of 96% sulphuric acid (178 mmol) were dosed into 400 g of methanol within 4 minutes at 65° and at a pH value greater than 2.5. A further 4.14 g (41 mmol) of sulphuric acid, in total a 5% stoichiometric excess, were added during 2 minutes, during which the pH value fell to 2.0. A disperser with a rotor-stator system was operated in the reaction vessel for wet grinding. After 10 minutes the mixture was filtered and the filter residue was washed with 40 ml of methanol. The filtrate contained 314 mmol of KGA and 12.1 mmo! of MeKGA. There were obtained 25.02 g of dried filter residue (sodium sulphate) containing 7.5 mmol of KGA and 0.5 mmol of MeKGA. The yield was 96.4% with a recovery of 98,8%. Example 7 80.0 g of crystalline NaKGA-H20 (338 mmol), obtained according to the procedure described in Example 2, and 15.40 g of 96% sulphuric acid (151 mmo!) were dosed into 400 g of 1,2-ethanediol within 4 minutes at 65° and at a pH greater than 2.5- A further 1.88 g of the sulphuric acid (41 mmol) were added within one minute, during which the pH value fell to 2.3. A disperser with a rotor-stator system was operated in the reaction vessel for wet grinding. After 10 minutes the mixture was filtered and the filter residue was washed with a small amount of 1,2-ethanediol. There were obtained 19.84 g of dried filter residue (sodium sulphate) containing 2.3 wt.% of KGA, corresponding to 2.4 mmol of KGA. The filtrate could not be analyzed satisfactorily because of the high boiling point of the solvent. Example 8 80.0 g of crystalline NaKGA-HzO (338 mmol), obtained according to the procedure described in Example 2, and 9.92 g of 96% sulphuric acid (97 mmol) were dosed into 432 g of methanol having a 8% water content within 5 minutes at 65° and at a pH value of 2.5. A further 7.36 g of the sulphuric acid (72 mmol) were added within 3 minutes, with the pH value being a minimum 1.9 and 2.3 towards the end. A disperser with a rotor-stator system was operated in the reaction vessel for wet grinding. After 10 minutes the suspension was filtered and the filter residue was washed with 20 ml of methanol. There were obtained 23.43 g of dry filter residue containing 5.7% of KGA (6.9 mmol) and 0.2% of MeKGA (0.2 mmol). The filtrate contained 325 mmol of KGA and 3.5 mmol of MeKGA. Example 9 10.0 g of sodium sulphate, which contained a total of 17.0 mmoi of NaKGA-H2O, NaKGA and KGA, corresponding to 39.7% of KGA, were suspended in 100 ml of methanol at 20C. A thick suspension formed after 20 minutes. 0.82 g of 95% sulphuric acid (79 mmol) was added slowly in such a manner that the pH value was always above 2.0. After stirring for one hour the pH value was 2.3. The suspension was filtered and the filter residue was washed with a small amount of methanol. This gave 7.0 g of filter residue (sodium sulphate) containing 0.12% of KGA. Evaporation of the filtrate under a vacuum gave 3.47 g of product containing 81.4% of KGA and 6.5% of MeKGA. Example 10 in each of ten successive experiments 400 g of methanol were placed in a 1 I reaction vessel and sodium sulphate from the respective preceding experiment was suspended therein. In each case 80.0 g of crystalline NaKGA •H2O (3.38 mmol), obtained according to the procedure described in Example 2, were added over 5 minutes at 60^0 and in each case 17.5 g of 95% sulphuric acid (169 mmol) were added within 10 minutes at a pH value of about 2.5. A disperser with a rotor-stator system was operated in the reaction vessel for wet grinding. After a reaction period of a further 5 minutes the mixture was filtered and the moist filter residue (sodium sulphate) was used in the subsequent experiment. In the last experiment the filter residue was washed with 400 ml of methanol and dried. From the last experiment there were obtained 240 g of a dried filter residue (sodium sulphate) containing 3-9% of KGA and 0.3% of MeKGA (0.05 mol). The filtrate and wasff water, total 3570 g, contained 16.6% of KGA (3.06 mol), 1.49% of MeKGA (0.25 mo!) and 11 ppm nitrogen. 350 ppm nitrogen were determined in the sodium sulphate. The average particle size of the sodium sulphate fell over the series of experiments from 10 µm (1st experiment) to 3.7 µm (10th experiment). WE CLAIM: 1. A process for the conversion of the sodium salt of 2-keto-L-gulonic acid, which is present in an aqueous fermentation solution, into an alcoholic solution of the free acid which process comprises a) recovering sodium 2-keto-L~gulonate monohydrate from an aqueous fermentation solution by evaporation, cooling or displacement crystallization and optionally pulverizing the thus-obtained crystallizate by grinding, bl) suspending the sodium 2-keto-L-gulonate monohydrate obtained in step a) in a lower alcohol selected from methanol, ethanol, propanol and 1,2- ethanediol, leaving the crystals to swell and thereafter adding an acid of low water content selected from concentrated sulphuric acid, nitric acid, hydrochloric acid, phosphoric acid and gaseous hydrogen chloride, whereby the measured pH value should lie above 1.5, this process step being carried out at temperatures in the range of about 20° C to about 70" C, or b2) adding the sodium 2-keto-L-gulonate monohydrate obtained in step a) together with an about stoichiometric amount of the acid of low water content to the lower alcohol using a wet grinding system, whereby the measured pH value should lie above 1.5, this process step being carried out at temperatures in the range of about 20 C to about 70C, or b3) carrying out a combination of steps bl) and b2) including recycling of product streams, this process step being carried out at temperatures in the range of about 20° C to about 70° C, and c) separating the salt of the added acid formed in step bl), b2) or b3) by filtration and/or centrifugation and thus obtaining an alcoholic solution of 2- keto-L-gulonic acid. 2. The process according to claim 1, wherein continuous evaporation crystallization is used as the crystallization method in step a). 3. The process according to claim 1 or 2, wherein methanol is used as the lower alcohol in step b). 4. The process according to any one of claims 1 to 3, wherein concentrated sulphuric acid or hydrochloric acid, particularly > 95% sulphuric acid, is used as the acid of low water content in step b). 5. The process according to any one of claims 1 to 4, wherein the pH value lies between 2.5 and 3.5 in step b). 6. The process according to any one of claims 1 to 5, wherein in step c) the separation of the salt formed is carried out by centrifugation. 7. A process for the conversion of the sodium salt of 2-keto-L-gulonic acid into an alcoholic solution of the free acid substantially as herein described and exemplified.

Documents:

580-mas-1997 abstract.pdf

580-mas-1997 claims.pdf

580-mas-1997 correspondence others.pdf

580-mas-1997 correspondence po.pdf

580-mas-1997 description (complete).pdf

580-mas-1997 form-1.pdf

580-mas-1997 form-2.pdf

580-mas-1997 form-26.pdf

580-mas-1997 form-4.pdf

580-mas-1997 others.pdf

580-mas-1997 petition.pdf