Abstract:
A process for treating a liquid comprising subjecting a liquid containing, in solution, citric acid as well as a less desirable component having a similar molecular weight to citric acid, to nanofiltration in a filtration step. From the filtration step, a permeate in which the ratio of the concentration of the citric acid to that of the less desirable component is greater than the ratio of the concentration of the citric acid to that of the less desirable component in the solution, is obtained.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation of copending PCT Application PCT/US96/12160, filed Sep. 12, 1995, designating the United States, which claims priority from South African application no. 94/0711, filed Sep. 12, 1994, both of which are incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to a process for treating a liquid. It also relates to a process for recovering citric acid.  
         SUMMARY OF THE INVENTION  
         [0003]    According to a first aspect of the invention, there is provided a process for treating a liquid, which process comprises: (a) subjecting a liquid containing solution, citric acid as well as a less desirable component having a similar molecular weight to citric acid, to nanofiltration in a filtration step; and (b) obtaining, from the filtration step, a permeate in which the ratio of the concentration of the citric acid to that of the less desirable component is greater than the ratio of the concentration of the citric acid to that of the less desirable component in the solution.  
           [0004]    In other words, there is a greater degree of rejection of the less desirable component than of the citric acid in the filtration step. The nanofiltration will normally involve contacting the liquid with a nanofiltration membrane. Nanofiltration will naturally also separate the citric acid from any component with a molecular weight which is significantly greater than that of citric acid.  
           [0005]    The molecular weight or relative molecular mass of the less desirable component may be within 20% of that of citric acid. For example, the molecular weight of the less desirable component may be within 10%, and even within about 7%, of that of citric acid. In other words, the molecular weight of the second component may range from 0.8MW-1.2MW, e.g., 0.9MW-1.1MW, or even about 0.93MW—about 1.07MW, where MW is the molecular weight of the citric acid.  
           [0006]    The Applicant believes that the process will have particular, but not necessarily exclusive, application in the treatment of fermentation broth to separate citric acid present therein as a fermentation product from residual glucose and/or fructose, thereby recovering the citric acid. It has been found that, with the process of the invention, the citric acid can be separated from residual glucose and/or fructose as well as other impurities such as medium and higher molecular weight by-products such as peptide and polysaccharides, produced by fermentation microorganisms, and which can be undesirable. In other words, the process has specific application in the recovery of citric acid from a fermentation broth, particularly from a clarified citric acid fermentation broth.  
           [0007]    The clarified citric acid fermentation broth can typically be that obtained by fermenting a carbohydrate feedstock to produce citric acid-rich fermentation broth and waste solids, and separating the broth from the solids.  
           [0008]    Citric acid has a similar molecular mass to glucose and fructose and can preferentially be separated from glucose and/or fructose in the process according to the invention, as a result of its greater permeability through the nanofiltration membrane as compared to that of glucose and/or fructose.  
           [0009]    The filtration step may be carried out at a concentration of the citric acid in the broth of 5%-30% by mass, preferably 10%-20% by mass, and the nanofiltration may be carried out at a temperature of 10° C.-100° C., preferably 20° C.-50° C. The pressure drop across the nanofiltration membrane will depend on the nature of the membrane and one the nature of the citric acid and the less desirable component to be separated and can be established by routine experimentation.  
           [0010]    The clarified citric acid fermentation broth may, before the filtration step, be subjected to cation exchange to remove cations, such as potassium and magnesium ions, therefrom.  
           [0011]    The process may include further treating the citric acid solution from the filtration step to purify it and/or to obtain a more concentrated citric acid fraction, or solid citric acid or a derivative of citric acid, such as sodium citrate.  
           [0012]    Thus, the citric acid solution from the filtration step may be purified by anion exchange, e.g., to remove traces of anionic impurities, and/or by contacting it with activated carbon to remove traces of organic matter.  
           [0013]    The purified citric acid solution may then be concentrated. This may include treating the solution to obtain solid pure citric acid and residual mother liquor. The concentration may include subjecting the solution to at least one evaporation and crystallization sequence. In particular, the concentration may include passing the solution sequentially through an evaporator; a first crystallizer; a first centrifuge; optionally a dissolution tank, a second crystallizer and a second centrifuge; and producing mother liquor in the first centrifuge and, when present, in the second centrifuge. A portion of the mother liquor from the second centrifuge, when present, may then be recycled to the first crystallizer, while the mother liquor from the fist centrifuge is withdrawn. The contacting of the citric acid solution with the activated carbon hereinbefore referred to may instead, or additionally, be effected after the purified citric acid solution has been concentrated at least partially, e.g., after it has passed through the evaporator.  
           [0014]    The process may also include: (i) recycling a portion of the mother liquor from the first centrifuge to upstream of the evaporator; and/or (ii) withdrawing at least a portion of the mother liquor from the first centrifuge as a liquid product; and/or (iii) drying and/or granulating at least a portion of the mother liquor from the first centrifuge to obtain a solid citric acid/carbohydrate product; and/or (iv) treating at least a portion of the mother liquor from the first centrifuge, in a recovery step, to recover citric acid for recycle, or citrate salts as product.  
           [0015]    When the process includes treating at least a portion of the mother liquid from the first centrifuge in a recovery step to recover citric acid, this citric acid may be recycled to upstream and/or downstream of the nanofiltration step. The treatment in the recovery step may then comprise one of the following: calcium citrate precipitation by adding lime thereto and redissolving with sulphuric acid; solvent extraction of citric acid utilizing a suitable solvent, followed by re-extraction of citric acid from the solvent into water using concentration differences or heating; ion exchange using a resin which selectively adsorbs citric acid, followed by elution; or various types of chromatography.  
           [0016]    At least a portion of the retentate from the filtration step may be withdrawn as a liquid product. Instead, or additionally, at least a portion of the retentate from the filtration step may be dried or granulated to obtain a solid citric acid product. Instead, or additionally, at least a portion of the retentate from the filtration step may be treated in a citric acid recovery step, which may then be the same as the citric acid recovery step, hereinbefore described, to recover citric acid or a derivative thereof therefrom.  
           [0017]    The retentate from the filtration step may be combined with the mother liquor from the first centrifuge for withdrawal as a liquid product and/or for drying or granulating and/or for treatment in a recovery step, as hereinbefore described.  
           [0018]    According to a second aspect of the invention, there is provided a process for recovering citric acid, which process comprises subjecting a clarified citric acid fermentation broth to nanofiltration in a filtration step to obtain as a permeate, a purified citric acid solution.  
           [0019]    The clarified citric acid fermentation broth may, before the filtration step, be subjected to cation exchange as hereinbefore described. The citric acid solution from the filtration step may be treated further to purify it and/or to obtain a more concentrated citric acid fraction, or solid citric acid or a derivative of citric acid, as hereinbefore described. The filtration step may also be as hereinbefore described.  
           [0020]    The invention will now be described by way of example, with reference to the accompanying simplified flow diagram in FIG. 1 of a process according to the invention for treating a fermentation broth, and with reference to the non-limiting examples. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0021]    [0021]FIG. 1 is a flow diagram of a process for treating a fermentation broth. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    In FIG. 1, reference numeral  10  generally indicates a process according to the invention for treating a fermentation broth.  
         [0023]    The process  10  includes a cation exchanger stage  32 . A clarified citric acid fermentation broth feed line  30  leads from a fermentation stage (not shown) into the stage  32 . A regeneration water/acid flow line  34  also leads into the stage  32 , while a waste product withdrawal line  36  leads from the stage  32 . A flow line  38  also leads from the stage  32 .  
         [0024]    The flow line  38  leads to a nanofiltration step or stage  40 , with a waste product or retentate withdrawal line  42  leading from the stage  40 . A cleaning water/base and diafiltration flow line  41  leads into the nanofiltration stage  40 . A filtrate flow line  44  leads from the stage  40  to an anion exchanger  46 , with a regeneration water/base flow line  48  leading into the exchanger  46 . A waste product withdrawal line  49  leads from the stage  46 , while a flow line  50  leads from the exchanger  46  to an activated carbon bed stage  52 .  
         [0025]    A flow line  54  leads from the stage  52  to an evaporation stage  56 , with a steam flow line  58  leading into the stage  56 . A condensate line  60  leads from the stage  56 . A flow line  62  leads from the stage  56  to a first crystallization stage  64 . A flow line  66  leads from the crystallization stage  64  to a first centrifugation stage  68 . A flow line  70  leads from the first centrifugation stage  68  to a dissolution tank  72 , with a water make-up line  74  leading into the tank  72 . A flow line  76  leads from the tank  72  to a second crystallization stage  78 , with a flow line  80  leading from the second crystallization stage  78  to a second centrifugation stage  82 . A mother liquor recycle line  84  leads from the stage  82  to the crystallization stages  64 ,  78 . A flow line  86  leads from the second centrifugation stage  82  to a drier  88 , with a flow line  90  leading from the drier  88  to a screening stage  92 . A solid product withdrawal line  94  leads from the screening stage  92 .  
         [0026]    The second crystallization stage  78  and second centrifugation stage  82  are used to improve crystal quality and are optional; they can be dispensed with, if necessary.  
         [0027]    A mother liquor withdrawal line  96  leads from the first centrifugation stage  68 .  
         [0028]    In a first embodiment of the invention, the line  96  can be routed back to the flow line  50  for recycling a portion of the mother liquor.  
         [0029]    In a second embodiment of the invention, the flow line  96  call lead to a suitable liquid product withdrawal stage  98 .  
         [0030]    In a third embodiment of The invention, the flow line  96  can lead to a drying and granulation stage  100 .  
         [0031]    In a fourth embodiment of the invention, the flow line  96  can lead to a recovery stage  102 . A waste product withdrawal line  104  leads from the stage  102 . A citric acid recycle line  106  leads from the stage  102  back to upstream and/or downstream of stage  40 .  
         [0032]    It will be appreciated that the first, second, third and fourth embodiments described hereinbefore are optional and can be used individually, or a combination of two or more of the embodiments can be used, as desired.  
         [0033]    A flow line  108  can, if desired, lead from the flow line  42  to the flow line  96  upstream of the product withdrawal stage  98 , the drying and granulation stage  100 , and/or the citric acid recovery unit  102 .  
         [0034]    In use, clarified citric acid fermentation broth, produced in known fashion in the fermentation stage, passes to the cation exchanger  32  where it is contacted with a suitable resin to remove cations such as calcium and sodium ions. If these ions are not removed they would form complexes with the citrate ions and be retained by the nanofilter element in the subsequent filtration stage  40  leading to product losses. The resin bed can be regenerated in known fashion, when required.  
         [0035]    The broth then passes to the nanofiltration stage  40  where the citric acid is separated, by contacting the broth with a nanofiltration membrane, from glucose, fructose, and higher molcular weight components in the broth such as protein, residual anti-foaming agents, sucrose, peptides and polysaccharides which thus form the retentate. Smaller molecules as well as some anions pass through the nanofiltration membrane and, together with the citric acid and most of the water, form the permeate. The permeate is thus in the form of a purified citric acid solution in which the ratio or proportion of the concentration of citric acid to that of glucose and fructose is greater than the ratio or proportion or the concentration of citric acid to that of the glucose and fructose in the feed to the stage  40 . Thus, in the filtration stage  40 , glucose and fructose, which have a similar molecular weight ( 180 ) to citric acid ( 192 ) are separated therefrom.  
         [0036]    The permeate from the filtration stage  40  passes to the anionic exchanger  46  where anionic impurities are removed and withdrawn. The resin bed of the anionic exchanger  46  is regenerated in known fashion, when required.  
         [0037]    The citric acid containing solution from the exchanger  46  passes to the activated carbon bed stage  52  where traces of organics are removed.  
         [0038]    The citric acid solution thereafter passes to the evaporator where it is concentrated, using steam, from a concentration of 10% to 20% by mass citric acid, typically up to about 65% to 80% by mass citric acid. Condensate from the evaporation stage  56  leaves along the line  60 . The concentrated citric acid solution passes to the first crystallization stage  64  where crystallization of the citric acid is effected. The stream then passes to the first centrifuge stage  68  where the citric acid crystals are separated from the mother liquor. The citric acid crystals pass into the dissolution tank  72  where they are redissolved in make-up water, whereafter they are recrystallized in the second crystallization stage  78  to improve crystal quality. The make-up water may be obtained from any suitable source, such as process condensate, a dilute citric acid stream, or the like. The stream from the crystallization stage  78  passes to the second centrifugation stage  82  where mother liquor is again removed. The moist crystals pass to the drier  88 , with dried crystals passing to the screening stage  92 . Dried solid substantially pure citric acid crystals are withdrawn along the flow line  94 .  
         [0039]    The crystallization stages  64 ,  78  typically comprise known crystallizers, and will thus include ancillary equipment normally associated therewith such as steam feed/condensate outlet lines, cooling fluid lines, and the like.  
         [0040]    Mother liquor from the first centrifugation stage  68  is withdrawn along the flow line  96 .  
         [0041]    In a first embodiment, a portion of this mother liquor can be recycled to the activated carbon bed  52 .  
         [0042]    In a second embodiment, at least a portion of this mother liquor can be withdrawn as a liquid product in the stage  98 .  
         [0043]    In a third embodiment, at least a portion of this mother liquor can be dried and granulated in the stage  100  to obtain a citric acid/carbohydrate solid commercial product.  
         [0044]    In a fourth embodiment, at least a portion of this mother liquor can pass to the recovery stage  102 . Waste product, e.g., glucose and trace impurities, from the recovery stage  102  is withdrawn, while if pure citric acid is recovered, it may be recycled to upstream or downstream of stage  40 ; or if citrate salts are recovered, they will be recovered as product. A portion of the retentate from the nanofiltration stage  40  can be routed, by means of the flow line  108 , to the stream  96  and then routed to any of the optional stages  98 ,  100  and/or  102 , If desired, to recover residual citric acid or a derivative thereof present in this stream.  
         [0045]    In one version of the invention, the recovery stag  102  may utilize calcium citrate precipitation after lime addition; followed by sulphuric acid addition to form citric acid as well as the by product gypsum, to recover citric acid.  
         [0046]    In another version, the citric acid in the mother liquor may, in the stage  102 , be extracted using a suitable solvent, followed by re-extraction citric acid from the solvent phase into water using concentration differences or with the aid of heat.  
         [0047]    In yet another version, the recovery stage  102  may comprise an ion exchange resin which selectively adsorbs citric acid, with elution of the product into water thereafter taking place.  
         [0048]    In yet a further version of the invention, the citric acid recovery stage may comprise various types of chromatography.  
         [0049]    The Applicant believes that with the process  10 , citric acid can be recovered effectively and at relatively low cost. In addition it is believed that the process  10  will be relatively simple to operate.  
       EXAMPLES  
       [0050]    The following examples are provided for illustrative purposes, and are not intended to limit the scope of the invention as claimed herein. Any variations in the exemplified methods which occur to the skilled artisan are intended to fall within the scope of the present invention.  
       Example 1  
       [0051]    The process  10  of the invention was simulated theoretically as follows.  
         [0052]    Clarified citric acid fermentation broth containing 18.4 weight percent citric acid, can be obtained by fermentating various cultures, such as  Aspergillus niger , on a purified carbohydrate feedstock, and filtering off the resultant biomass. The broth leaving the fermenters can contain 0.2% (w/w) unfermented glucose or 0.2% (w/w) unfermented fructose.  
         [0053]    The clarified citric acid fermentation broth is then subjected to cation exchange, to remove cations such as potassium and magnesium ions.  
         [0054]    The clarified decationized citric acid fermentation broth is then contacted with a nanofiltration membrane, and 80 or more percent of the citric acid transfers to the permeate, which contains up to 18 weight percent citric acid. The permeate also contains the following from the clarified decationized citric acid fermentation broth: a portion of the glucose and fructose, anions, cations, amino acids and sucrose, as well as 80 or more percent of the water. The retentate can be treated in a citric acid recovery step, using the UOP™ Citric Acid Sorbex™ Process (Citrex™), to recover the remaining citric acid,  
         [0055]    The permeate from the nanofiltration step can be subjected to anion exchange, to remove traces of anionic impurities, followed by contacting with activated carbon, to remove traces of organics.  
         [0056]    The permeate can thereafter be concentrated by evaporation in an evaporator, followed by a first crystallizer, a first centrifuge, a dissolution tank, a second crystallizer and a second centrifuge; with 20% by weight of the mother liquor from the second centrifuge being recycled to the first crystallizer, while the mother liquor from the first centrifuge is withdrawn.  
         [0057]    The process can include (i) recycling 25% (w/w) of the mother liquor from the first centrifuge to upstream of the evaporator, (ii) withdrawing 10.0% (w/w) of the mother liquor from the first centrifuge as a liquid product, (iii) drying and granulating 21.6% (w/w) of the mother liquor from the first centrifuge to obtain a solid citric acid/carbohydrate product; and (iv) treating the remainder of the mother liquor from the first centrifuge, together with 80% (w/w) of the nanofiltration retentate, using the UOP™ Citric Acid Sorbex™ Process (Citrex™) process (this process revolves around any one of various chromatographic techniques, such as ion exclusion chromatography, whereby citric acid is separated from the feed stream by selective adsorption onto a solid adsorbent) in a recovery step, to recover citric acid which can be recycled to downstream of the nanofiltration step.  
         [0058]    In the Citrex recovery step, which uses a very dilute solution of sulfuric acid as desorbent, the extract can contain, from the feed stream, on a weight to weight basis: 92% of the citric acid, 1% of the glucose and fructose, 1% of the cations and anions, 1% of the amino acids and biomass, negligible sulfuric acid, and 44% of the water from both the feed stream and the desorbent stream. The balance of the above mentioned components report to the raffinate (waste stream).  
         [0059]    In a simulation of the nanofiltration step or stage  40 , laboratory scale tests were conducted on simulated citric acid fermentation broths coding, by mass, 18-19% citric acid, 1% lactose, 0.2% glucose and 0.05% yeast extract. The yeast extract was used to mimic other components normally present in commercial formation broths. Each test was conducted with a pair of membranes, by treating a batch of the simulated broth.  
         [0060]    Concentrations of each of the components were measured, and the rejections calculated. The results are set out in Tables 1. 2 and 3 (all percentages are on a mass bases).  
                                                           TABLE 1                           Results of Nanofiltration Test 1                Experiment 1   Citric Acid %   Lactose %   Glucose %                            Feed   18.8   0.88   0.22           Permeate-           Membrane A   12.3   0.01   0.03           Permeate-           Membrane B   14.6   0.18   0.09           Concentrate   29   2.1   0.47                                              
 
         [0061]    [0061]                                                           TABLE 2                           Results of Nanofiltration Test 2                Experiment 2   Citric acid %   Lactose %   Glucose %                            Feed   18   0.88   0           Permeate-           Membrane A   11.4   0.01   none           Permeate-           Membrane B   11.7   0.05   0.07           Concentrate   28   1.9   0.38                        
         [0062]    [0062]                                                           TABLE 3                           Rejections of the two membranes                Rejections                       expressed as                       percentages   Citric acid   Lactose %   Glucose %                            Filmtec NF45                       Test 1   34.6   98.9   86.4           Test 2   36.7   98.9   &gt;90           MPKW MPF23           Test 1   22.3   79.5   59.1           Test 2   35.0   94.3   65.0                        
         [0063]    One of the key parameters in nanofiltration is the rejection. For the simulated citric acid fermentation broths, it was expected, according to literature and product information, that membrane rejections would be in the order lactose&gt;citric acid&gt;glucose, However, as can be seen from Table 3, the actual rejection of citric acid was surprisingly found to be lower than that of glucose.  
         [0064]    This feature thus provides the basis for a simple and efficient means of separating citric acid from high and medium molecular weight impurities as well as removing most of the residual glucose, in respect of fermentation broth.  
         [0065]    It is to be appreciated that, together with the citric acid, other more valuable fermentation products can be separated from the glucose.