DE3502924A1 - Process for isolating citric acid - Google Patents

Abstract

The invention relates to a process for isolating citric acid or other suitable acids and/or salts thereof from solutions which were obtained by fermentation from carbohydrate-containing raw materials. In the process, higher molecular weight impurities resulting from the raw materials and/or process are separated off by membrane filtration, preferably ultrafiltration, or else reverse osmosis, and/or additional adsorption to a non-ionogenic adsorber resin having a specific surface area of 300-1000 m<2>/g, for example based on a polystyrene resin or a polyacrylate resin and, after concentration of the solution, the citric acid is crystallised, from which, in the hitherto conventional manner, the lower molecular weight salts were previously bound with the aid of cation exchanger and anion exchanger or were removed by precipitation of the citric acid as calcium salt, reaction of the calcium citrate with sulphuric acid, separation of the calcium sulphate and cation and anion exchange. <IMAGE>

Description

Process for the production of citric acid

The present invention relates to a new method of extraction of citric acid or other suitable acids and / or their salts from fermentation solutions.

Citric acid is used e.g. as edible acid and complexing agent in the food industry, for metal cleaning, in the pharmaceutical industry and in many other areas used.

Today citric acid is mainly obtained through fermentation with Aspergillus niger from sugar solutions or molasses in the older surface or newer submerged process won.

Depending on the type of raw material used - beet sugar molasses, Cane molasses, starch, etc. - and the process used must require pre-treatment take place, e.g. with soluble hexacyanoferrate salts, and a number of nutrient salts with possibly required trace elements (magnesium, zinc, etc.) must be supplied will.

Depending on the initial sugar concentration and the type of the process used, solutions containing citronic acid are obtained, after the microorganisms have been separated off, in addition to about 10 to 30% citric acid contain another 10-20% other organic and inorganic substances. These By-products originate in part from the original molasses solution partly those added to the nutrient solution and not used up by the microorganisms Nutritional salts and other excretions, which the Aspergillus niger besides that produced citric acid during fermentation.

Citric acid is usually used for cleaning as poorly soluble Calcium salt is precipitated and citric acid from the calcium citrate is added released again by sulfuric acid. The citric acid can then be extracted from this solution can be obtained by concentration in crystalline form.

To get citric acid in the food and especially pharmaceutical However, this process must be essential to obtain the purity required for the purposes cumbersome and more elaborate to be carried out in each individual Process stage to separate impurities as far as possible. details this is explained, for example, in Ullmann's encyclopedia of technical chemistry, 3rd edition, volume 5, pages 603-609 and 4th edition. Volume 9, pages 631-633 and Kirk-Othmer Encyclopedia of Chemical Technology, 1.

Edition, Volume 4, pages 15-17 and 2nd Edition, Volume 5, pages 529-531. Some steps are mentioned here.

As a by-product possibly contained oxalic acid is before the actual Calcium salt precipitation precipitated in acidic solution with a small amount of calcium hydroxide. Only after they have been separated off does calcium citrate become at a temperature of 80-950C crystalline by adding calcium hydroxide in a slight excess with stirring failed. The calcium hydroxide used must be as pure as possible because it is contaminated with magnesium, iron, aluminum or phosphates in the subsequent extraction of the significantly disturb pure acid.

The calcium citrate obtained is filtered hot and washed hot, in order to keep losses as low as possible, since it is particularly sparingly soluble at 80-95 ° C is.

The calcium citrate obtained in this way is diluted in large stirred containers Sulfuric acid implemented in a slight excess.

The precipitated calcium sulfate is filtered off. The brown colored one In practice, the filtrate is called crude acid. This is then crude acid e.g. decolorized with activated charcoal.

The charcoal with the colored impurities adhering to it becomes removed by filtration. The clear citric acid solution is after ion exchange evaporated, the concentrate crystallized in several stages in vacuo or at normal pressure. Depending on the purity, the raw crystals have to be cleaned one or more times be recrystallized with activated charcoal before chemically pure citric acid is obtained will. Anhydrous citric acid is crystallized above the transition temperature of 36.6 OC obtained, below which the citric acid crystallizes out as a monohydrate.

In addition to this classic Fäl 1 ungsverfahren are in some patents Other methods have been described by which the citric acid is extracted by extraction is obtained with organic solvents. Butyl alcohol, acetone, Tributyl phosphate. Since the crystallization because of the not completely removed Impurities very difficult and the yield is unsatisfactory is, these processes, which are elegant in themselves, have hitherto been in use because of inefficiency found no use in practice. In recent times the extraction has been using certain amines described.

The object of the invention is to use citric acid or other suitable Acids and / or their salts from fermentation solutions by separating the impurities with the help of special physical-chemical separation methods in a simpler and more economical way Way to win.

Another object of the invention is that of citric acid production Wastewater pollution triggered by originating from the raw materials or the process To decisively reduce substances through their targeted recovery.

It is also a main object of the invention to control citric acid production by largely dispensing with the auxiliary chemicals Ca (OH) 2 and H2S04 from the otherwise to relieve or completely free the by-product CaS04 2H20, its utilization becomes more and more problematic. At the same time, this makes the process more economical.

The solution to these tasks, which is detailed in the claims is based on the separation of the higher molecular weight But also contamination by membrane filtration, preferably ultrafiltration Reverse osmosis, and subsequent adsorption on large non-ionic adsorption resins specific surface and the separation of the low molecular weight, especially inorganic ones Ions with the help of cation and anion exchange resins.

The citric acid solution purified in this way can then crystallize directly or, as before, are obtained via calcium citrate precipitation.

Membrane filtration is a well-known method. For technical designs For example, hollow fiber bundles are combined as modules and in columns as batteries used to push through large amounts of liquid. Depending on the pore size of the In this way, higher-molecular-weight compounds are separated from the lower-molecular-weight compounds in the concentrate Compounds separated in the permeate. For cleaning citric acid solutions However, this method could not be used because the membranes known so far Weight molecules with moles cut off over 1000, but for lower ones pH values between 1.5 and 2.2 are not stable. Membrane materials are now known which can withstand such pH values over a long period of time, even with this separation performance and with low energy consumption are sufficiently permeable and clean themselves well permit.

However, these membranes can also cause a large part of the disruptive Impurities in the fermentation solution i.e. those with molecular weights of Do not cut off approx. 200 - 1000.

The adsorptive purification of aqueous and organic solutions is one method known for a long time. Here, organic ingredients become solids with a large active surface such as activated carbon, aluminum oxide, silicates and enriched. By separating the adsorbent-adsorbate system according to the known The "solid-liquid separation" method can eliminate the adsorbed substances will. In most cases, the adsorbent must be used in a subsequent process step be regenerated. Such cleaning methods, however, come for mixed organic highly stressed systems so far due to the diversity of the dissolved impurities, their different adsorbability and last but not least for economic reasons Reasons only rarely used.

Another method is from the German patent specification 1,274,128 become known in which highly heteropolar molecules from aqueous solutions or Dispersions with the help of non-ionic, water-insoluble, crosslinked polymers macroreticular structure can be separated. The adsorbents exist from polymers that are about 2-100% vinylbenzene, alkylvinylbenzene, divinylbenzene or similar monomeric compounds and, in addition, methacrylate or similar compounds as well as ethylene, isobutylene, acrylonitrile or other monomers can contain as a copolymer. Connections of this kind should not be specific Have a surface of at least 10 m to 200 m each with 9 resin. The resin is said to be the one to be separated Substances independent of an ionic bond or any solvation effect on the outer and inner resin surface by predominantly van der Waals or Binding dipole forces. The substances to be separated are organic substances with a hydrophobic and hydrophilic components, for example surface-active substances, Cleaning agents, emulsifiers, dispersants, but also steroids, called water-soluble enzymes, amino acids, polypeptides, proteins and hormones. Advantageous the process should also be used for the separation of fatty acids from aqueous solutions be. It is also stated that sugar solutions or solutions can also be used in this way other chemical substances can be discolored. The adsorbed substances should by heating up to distillation, by treatment with steam for steam distillation, by extraction, leaching or desorption with water, aqueous acid or alkaline Solutions or with an organic solvent, which is obtained by fractional distillation is easily separable, can be removed again from the resin.

According to the invention it was found that also in the production of citric acid through fermentation of sugary raw materials, especially in the submerged fermentation of cane or beet sugar molasses formed as a by-product or substances contained in the starting product, insofar as they are not affected by the above membrane filtration described are separable, mainly to non-ionic polystyrene or polyacrylic resins, which are crosslinked with 2.5 X vinylbenzene, with a specific Surface area of approx. 300 - 1000 mol per g can be bound. The much more polar one Citric acid is not bound, so that it can be washed with pure Water without difficulty from the resin and the impurities attached to it can be separated.

The impurities in turn can be removed by washing the resin with aqueous alkali hydroxide (pH 8-13) and additional washing with a polar one completely elute organic solvents such as acetone or lower alcohol, without that the binding capacity of the resin suffers, so that this for further Cleaning operations can be recovered.

Although in principle all of the resins described above can be used, a polystyrene resin has proven to be particularly favorable, which has an average Grain size of about 30 US mesh (0.6 mm) and a minimum surface area of 650 moles per g with a porosity of 1.7 ml per g. This product turned out to be in various attempts to be particularly effective; Resins with similar properties, i.e. specific surface areas of 500 - 800 m / g, grain sizes of 0.3 - 1 mm and porosity from 1 to 2 ml / g are therefore preferred.

When carrying out the process, the fermentation solution may initially be present genuinely or colloidally dissolved hexacyanoferrate by adding a small amount Amount of e.g. zinc chloride or zinc sulfate precipitated and with the colloidal components as well as the higher molecular weight impurities separated by membrane filtration. In order to achieve an economical filtration, membranes are preferred worked with different separating cuts (e.g.

initially with a separating cut up to molecular weights of 50,000, then 5000 and / or 1000 or below).

The fermentation solution pre-purified in this way is then passed through a column with a polystyrene polymer, as described above, given until the impurities break through in larger quantities, i.e. the resin is overloaded. Depending on the type of resin an amount of solution corresponding to 5-15 bed volumes in this manner getting cleaned.

Column lengths of 1 - 10 m are suitable for the technical process, with longer columns for reasons of stability and also a "quasicyclic" process management to allow it to be divided into several shorter columns. Depending on the column length, Grain size and packing density must reduce the flow rate as possible good separation with still portable throughput can be matched. Loading speeds 1 - 10 1 solution per 1 bed volume and hour are possible, 1.5 - 3 1 per 1 and hour have proven to be particularly favorable.

The citric acid solution obtained by this process is substantial colorless and free of high molecular weight compounds. Inorganic salts and a However, the column has a number of low molecular weight organic compounds the citric acid happens. By using ion exchangers, the above named substances eliminated and the citric acid of crystallization for the production of the end product.

The mother liquor remaining after the citric acid has crystallized is used again, e.g. added to new fermentation solution. The yield is thereby further increased.

However, it is also possible from the with the help of membrane filtration and adsorption of pre-purified solution by the citric acid in the known manner Addition of calcium salts as poorly soluble calcium salt to be precipitated and by addition from sulfuric acid from the poorly soluble calcium citrate again to citric acid to release. As a result of this pre-cleaning, the precipitated calcium citrate is essential purer than by the conventional method, so that on the one hand the plaster of paris obtained is obtained in pure white form and is therefore more useful for further processing and on the other hand the citric acid solution obtained is cleaner, so that higher yields and a lower number of crystallization stages are necessary.

The membranes, their throughput after a few hours of operation decreases, can be done by simply flushing with, for example, a 3 - 5% igen aqueous hypochlorite solution. This makes the separation performance practical completely restored. To enable a continuous process, are preferably several membrane filtration columns connected in parallel to one To be able to clean part of it, while the rest serve to separate.

The economics of membrane filtration can be significantly improved through a suitable processing of the resulting concentrates. So can e.g. the citric acid by diafiltration or, after separation of the settable Substances, largely returned from the concentrate by repeated ultrafiltration, Any hexacyanoferrate present is separated and after appropriate work-up reused in the production process and the proteins contained therein separated and obtained for independent use.

The regeneration of the polystyrene resins is of particular importance. It makes sense to first wash the resin column with 1-2 bed volumes of water, to catch the still contained citric acid. The first bed volume of wash water has almost the original citric acid concentration and therefore becomes the main run added, the second bed volume, which contains only a little citric acid, can to be used for washing in the next cycle.

The column is then preferably diluted in countercurrent with a aqueous alkali, for example 5% sodium hydroxide solution, backwashed, with 1.5-2 bed volume are necessary to elute most of the adsorbed material. The rest of organic matter is washed out by rinsing with 0.5 - 1 bed volume of acetone, whereby all impurities are removed from the column practically quantitatively will. Other solvents were less suitable. The eluent NaOH and acetone are displaced from the system by flushing with water. The regenerated Resins are thus available again for a new cycle.

The acetone eluates are converted into the solvent by fractional distillation recovered in large part. The aqueous phases are purified by known wastewater treatment processes treated. Humic and fulvic acids can be extracted from the eluates of the adsorption system isolate and use as high quality fertilizer.

Suitable partial sewage streams can also be incinerated.

The following examples are of process management or parts typical thereof without, however, intending to limit the invention. At the same time, they show that the individual cleaning steps are in different order can be executed.

Belspiel ~ 1 Influence of various adsorber resins In the following Examples were crude, still heavily contaminated citric acid solutions through various Amberlite adsorbent resins and cation exchange resins are given.

The solution mentioned with approx. 200 meq / l citric acid and 100 mg / l synthetic fulvic acid was used after pretreatment with the cation exchanger Amberlite IR 120 over a column filled with 50 ml of non-polar polystyrene resin Amberlite XAD-2 conducted at room temperature and a specific load of 4 1/1 h. After loading the experiment was terminated with 60 bed volumes of this solution.

The resin was then washed with 5 bed volumes of 1% NaOH solution regenerated. This cycle was repeated 10 times, each time taking the fulvic acid content was measured via the COD value.

Results: cycle ads. Fulvic acid 1 76 2 78 3 70 4 74 5 82 6 79 7 78 8 76 9 74 10 73 10 73 Average 76 A solution of the same composition was under the same conditions via the adsorbent resins Amberlite XAD-4, XAD-7 and XAD-1180 guided. The following values were obtained for the retention capacity of the fullic acid: Cycle XAD-4 XAD-7 XAD-1180 1 68 64 2 70 60 84 3 67 59 79 4 69 62 80 5 68 65 86 6 72 60 80 7 64 62 83 mean 68 62 82 Example 2 cane molasses with a sugar content of 25% was about 50 ml of Amberlite XAD-2, XAD-4 and XAD-1180 conducted at room temperature and a specific load of 4 1/1 h.

After throughput of 7 bed volumes, the following average Ink absorption capacities determined: average ink absorption in resin type % XAD-2 64 XAD-4 50 XAD-1180 72 By washing out with one bed volume of acetone 95% of the adsorbed dyes were removed. In a comparable experiment 86% of the color components could be eluted with 2 bed volumes of ethanol.

Example 3 10 l of submerged fermentation acid are used per cycle at a pH value from 2.0 to 2.2 after adding 0.25 mg Zn SO4 / l fermentation acid via ultrafiltration hollow fiber modules (Romicon Inc., The Netherlands) filtered. The hollow fiber modules (hollow fiber) are self-supporting capillaries, which are only made of membrane matrix, here based on polysulfone, exist. These membrane filters are designed to separate molecular weights of up to approx. 1,000 suitable.

The ultrafiltration creates that of impurities different from one another Kind - like proteins, cyanoferrates, dissolved higher molecular and colloidal substances - Largely freed citric acid permeate and the concentrate. From 10 1 submerged fermentation acid 8 liters of permeate and 2 liters of concentrate were produced. The thickening factor was therefore 5. Analyzes have shown that about 75% of the organic Accompanying components of the citric acid have been removed (serves as an evaluation parameter the COD). The pre-cleaned submerged fermentation acid was then used in an adsorption system fed.

The experimental set-up can be found in Fig. 1.

In the figure, 1 means 1st column 2 2nd column 3 resin bed 4 means covering material (e.g. glass wool or frit) 5 Supply line for submerged fermentation acid 6 Supply line for regenerant (Water) 7 connecting line from 1st column to 2nd column 8 drainage for eluate 9 supply line for backwash fluids (water, acetone, caustic soda) 10 drain for backwash fluids 11 Venting the columns All lines are drawn up to the first valve and the continuing lines as well as the corresponding storage tanks, pumps and templates are omitted for simplicity.

Each column was filled with 500 ml of XAD-1180 adsorber resin (Rohm & Haas, Co., USA) filled. The resin is a synthetic insoluble polystyrene polymerisa ;, characterized by a large active surface (650 Ig minimum), a non-ionic one Character and a porosity of 1.7 ml / g minimum.

In the following, the 500 ml of resin correspond to a bed volume.

The knowledge gained in preliminary tests has been optimized for the following Guided operation: Specific load 1.5 to 3 1/1 h regeneration technology Amount (bed volume) 1st column 2.0 wash water 1.5 NaOH (3.5%) 4.0 rinse water 0.5 acetone 10.0 rinse water 2nd column 0.5 acetone 10.0 rinsing water The values of 4 determined during the loading and the washing process in a row The number of cycles used is shown in the tables below.

Initial acid concentration: 1.75 eq / l loading: 1.5 1/1 h bed 1st cycle 2nd cycle 3rd cycle 4th cycle 0 Volume acid residual acid residual acid Residual acid residual residual val / i color val / l color val / l color val / l color color%% %%% 1 - 4.2 0.82 4.0 - 3.8 - 4.0)) 2 1.72 9.0 1.70 9.1 1.70 8.8 1.70 9.0) ) 3 1.74 15.8 1.74 15.6 1.76 15.9 1.74 15.9) 14.66) 4 1.76 20.4 1.74 21.4 1.74 19.3 1, 74 20.4)) 5 1.75 24.4 1.76 26.4 1.74 23.4 1.74 22.4)) 24.02 6 1.75 28.6 1.75 29.8 1.74 25.4 1.74 27.4) 7 1.78 31.3 1.74 31.0 1.74 29.2 1.75 32.3) 8 1.74 34.4 1.74 36.2 1.74 32.6 1.75 33.2) 9 1.75 37.5 1.75 39.3 1.74 37.6 1.75 35.6) 10 1.74 40.2 1.75 42.1 1.75 38.5 1.75 40.0) Washing: 1.5 1/1 h Bed 1st cycle 2nd cycle 3rd cycle 4th cycle Volume acid acid acid acid valil val / l val / l val / l 1 1.65 1.42 1.66 1.68 2 0.30 0.33 0.34 3 0.024 0.026 0.030 4 0.009 0.006 0.008 0.005 As expected, 2 bed volumes passed over the resin were obtained Fermentation acid is the initial acid concentration.

After regeneration of the first column with NaOH and then Both columns were rinsed with 0.5 bed volume of acetone each, here 250 ml = 196.4 g of acetone, regenerated, which removed approx. 95% of the remaining adsorbed substances. Loading: 5 1/1 h.

The acetone is washed out with 10 bed volumes of water. The fo The table below shows the amount obtained from fractions of 1 bed volume of wash water in each case was recovered in a representative cycle of the 1st column.

Acetone leaching table 1st column Acetone added 196.4 g bed vol Acetone recovered by the wash water (g) 1. 126.2 g 2. 18.0 g 3. 2.35 g 4. 0.85 g 5. 0.50 g 6. 0.29 g 7. 0.18 g 8. 0.14 g 9. 0.07 g 10. 0.065 g Recovered Acetone 148.6 g This table shows that already in the first 3 bed Volumi na wash water contains 98.6 X the recoverable amount; the remainder of 47.8 g goes during countercurrent regeneration in the imperfectly sealed apparatus or lost when recovering. The remainder, which contains only a little acetone Wash water is preferably reused as wash water in the next cycle. These losses can be avoided in a closed, large-scale plant, so that this experiment is not representative in this respect.

The fermentation acid decolorized on the adsorber resin is used to remove inorganic acids Salts via a strongly acidic cation exchanger (Amberlite IR 120, Rohm & Haas Co. - sulfonated styrene / divinylbenzene copolymer with 8 X DVB - and one weakly basic anion exchanger (Amberlite IRA 68, Rohm & Haas Co. - crosslinked Acrylate with dialkylamino groups -) passed. The resins were in a different order as used in Example 1 and 2.

The fermented acid purified in this way is concentrated under reduced pressure, until the concentration of citric acid is about 60 g / 100 g solution, and the Subjected to crystallization under reduced pressure at 30-35 ° C. The with advancing Citric acid crystals that are formed by narrowing (approx. 1: 7) are separated off. the Mother liquor is further concentrated until the accumulated impurities are a Prevent further crystallization.

The yield from 3 crystallization stages was - under laboratory conditions - 89.8 X of theory, the crystals of the first crystallization after a Recrystallization met the requirements of the Pharmacopoeia (EP Vol. III, USP 20).

In semi-technical experiments with larger amounts of fermentation acid, the Thickening factor for ultrafiltration at 1:20.

Claims (11)

Claims 1. Process for the production of citric acid or others suitable acids and / or their salts from solutions obtained by fermentation Carbohydrate-containing raw materials were obtained, characterized in that raw material and / or process-related higher molecular weight impurities from membrane filtration, preferably ultrafiltration, but also reverse osmosis, and / or additional adsorption on a non-ionic adsorber with a specific surface area of 300 - 1,000 mt / g are separated, for example, on a polystyrene or polyacrylic resin basis and the citric acid is crystallized after concentrating the solution, from which previously in the previously usual way the low molecular weight salts with the help of cations and Anion exchangers were bound or by precipitation of citric acid as calcium salt, Reacting the calcium citrate with sulfuric acid, separating the calcium sulfate and cation and anion exchanges have been removed. 2. Process according to claim 1, characterized in that membrane filtration, Color adsorption on adsorber resin and exchange of the disruptive cations and anions carried out on ion exchange resins sequentially or in some other suitable order individual steps can also be repeated. 3. The method according to claim 1-2, characterized in that any existing Hexacyanoferrate after the addition of soluble zinc or other suitable metal salts can also be removed by membrane filtration 4. Method according to claims 1-3, characterized in that the fermentation acid is a fermentation acid predominantly made from cane sugar molasses is. 5. The method according to claim 1-4, characterized in that in the Proteins contained in fermentation acid are partially separated off by membrane filtration. 6. The method according to any one of claims 1-5, characterized in that that the membranes with hypochlorite solution or other suitable cleaning agents getting cleaned. 7. The method according to any one of claims 1-6, characterized in that that the impurities bound to the adsorber resin with a) aqueous alkali hydroxide pH 8-13 and b) a polar organic solvent such as acetone or a lower one Alcohol are eluted and the resin is used again according to claims 1-6. 8. The method according to claim 7, characterized in that the organic Solvent is recovered from the eluate, e.g. by fractional distillation. 9. The method according to claim 1-8, characterized in that from the Ultrafiltration concentrates hexacyanoferrate, proteins and / or amino acids or obtained from the eluates of the adsorption system humic acids and fulvic acids will. 10. The method according to any one of claims 1-9, characterized in that that the ion exchangers are regenerated and reused. 11. The method according to claim 1, characterized in that the after the mother liquor of the fermentation solution remaining after the crystallization of the citric acid is added.