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Recovery of citric acid from fermentation beer using supported-liquid membranes
Journal ofMembrane Science, 56 (1991) 127-141 Elsevier Science Publishers B.V., Amsterdam
127
Recovery of citric acid from fermentation beer using supported-liquid membranes D.T. Friesen, W.C. Babcock, D.J. Brose and A.R. Chambers Bend Research, Inc., 64550 Research Road, Bend, OR 97701-8599 (USA) (Received May 19,1987; accepted in revised form July 2, 1990)
Abstract The use of a supported-liquid membrane to separate citric acid from the beer produced by fermentation was studied. The liquid membrane consisted of an organic solution composed of trilaurylamine and a long-chain alcohol dissolved in an inert hydrocarbon solvent that was immobilized in a microporous polypropylene support. Using this membrane, citric acid was recovered from the fermentation beer as citric acid or as its monosodium salt. The effects of temperature, organic solution composition and citric acid concentration on transport rates were investigated. Citric acid flux was greatest under the following conditions: 60 OC; trilaurylamine concentration of 38 vol.% in Alkyl Aromatic Oil, or 50 vol.% in Shell Sol 71; and 15 vol.% of n-dodecanol in the membrane as a modifier. Keywords: Liquid membranes; coupled, facilitated transport; organic separations
1. Introduction
Citric acid (2-hydroxy-1,2,3-propanetricarboxylic acid) is produced commercially by fermentation with Aspergillus or Candida fungi. In conventional processes, citric acid has been recovered from the fermentation beer by precipitation of calcium citrate with calcium hydroxide. In this recovery scheme, calcium citrate is precipitated, recovered by filtration, and converted to citric acid by addition of sulfuric acid. The dilute citric acid product is then sequentially purified using ion exchange, activated carbon, evaporation and crystallization [ 1,2]. More recently, liquid-liquid extraction followed by precipitation or back-extraction has been investigated for product recovery [ 31. The use of supported-liquid membranes for the recovery of citric acid offers unique advantages over these recovery schemes. Some of the advantages are lower energy consumption, higher separation factors in a single stage, the ability to concentrate citric acid during the separation (i.e. citrate ions can be pumped “uphill”), and smaller size of the complete separation plant. These advantages may eventually result in a reduction in overall recovery costs.
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1.A. Description of supported-liquid membranes Liquid membranes containing mobile carriers have been widely studied in recent years. In these membranes, a carrier reacts selectively with one or more species on one side of the membrane, and the complex then diffuses across the membrane, where it dissociates, releasing the complexed species. The earliest examples of this process were in carrier-facilitated gas transport [ 431; these processes are generally categorized as “facilitated transport”. More recently, the use of liquid membranes containing carriers has been extended to the transport of ions from aqueous solutions [ 6-81. We refer to these processes as “coupled transport”. Coupled transport membranes consist of an inert, microporous support impregnated with a water-immiscible, mobile ion-exchange agent. The mobile carrier, which is held in the pores of the support membrane by capillarity, acts as a shuttle, picking up ions from an aqueous solution on one side of the membrane, carrying them across the membrane, and releasing them to the solution on the opposite side of the membrane [9,10]. The flow of the complexed ion is coupled to the flow of a second ion (e.g. the hydrogen ion). The coupling of the flows of the two ions permits one of the ions to be pumped “uphill” from a solution in which it is dilute to a solution in which it is more concentrated. l.B. Description of citric acid coupled transport Tertiary amines have been proposed by others as ion-complexing agents for the recovery of citric acid using solvent extraction [ 1,11,12]. As Fig. 1 shows, tertiary amines ( NR3) can also be used for the coupled transport of monobasic citrate anions [C,H,OH(COOH),COO]*. In the extraction step, the basic amine reacts with hydrogen ions in the feed solution to form the tertiary alkylammonium cation. This cation then associates as an ion pair with the citrate anion to form an alkylammonium salt, which is transported across the
Fig. 1. Conceptual description of citric acid coupled transport the driving force (citric acid is the product).
using citric acid concentration
as
*pK, values for citric acid are 3.1,4.8 and 6.4; thus, at the pH of the fermentation beer (2.6)) citric acid exists as a mixture of the neutral acid (80% ) and the monobasic anion (20% ).
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membrane. Other neutral amines can also loosely associate with the citrate complex; however, the process can most easily be understood by considering only the one-to-one citrate-amine complex. The overall reaction for this extraction can be expressed as the forward reaction in eqn. ( 1) : Feed H&
+WWWCOOW&00,,,
+N%r,,
Product
side
side
(1)
N+R,HC,H,OH(COOH),COO,,, where the subscripts (aq) and (org) designate species in the aqueous and organic phases, respectively. After the alkylammonium salt has diffused across the membrane, the citrate ion is stripped from the organic carrier solution into the aqueous product phase by the reverse of the reaction in eqn. (1). This reaction regenerates the tertiary amine, which then diffuses back to the feed side of the membrane, where it recomplexes with hydrogen and citrate ions. Although this mechanism has not been proven in this paper, it is consistent with our results, as well as with the results obtained by others who have studied liquid-liquid extraction. The transport of citrate ions across the membrane can be driven by a difference in citrate concentration across the membrane, a difference in hydrogen ion concentration across the membrane or a combination of both. It is clear from eqn. (1) that, as either the hydrogen ion or citrate ion concentration increases, the forward reaction of eqn. (1) will be favored, leading to a relatively high concentration of citrate-amine complex in the membrane. Conversely, a low hydrogen ion or citrate ion concentration favors the reverse reaction of eqn. (1) , leading to a relatively low concentration of citrate-amine complex in the membrane. The transport of citrate across the membrane occurs because a concentration difference of citrate-amine complex exists across the membrane. Figure 1 illustrates the case in which a difference in citrate concentration is the driving force; in this case citric acid is the product. Figure 2 illustrates the case in which a difference in hydrogen ion concentration is the driving force; in this case monosodium citrate is the product. As Fig. 2 indicates, maintaining low pH on the feed side and high pH on the product side of the membrane can be used to drive citrate ions from a solution of low citrate concentration to one of high citrate concentration. This uphill transport of citrate anions makes it possible to obtain highly concentrated product solutions from extremely dilute feed streams. The mathematical model for reaction and diffusion in coupled transport membranes has been well developed [ 71. The model is considerably simplified if we can assume that the forward and reverse reactions in eqn. (1) are much faster than diffusion in the membrane - i.e. diffusion limits transport. Based on this assumption, Cussler [ 71 has derived the following equation for flux in coupled transport membranes:
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Feed
Solution
Supported-Liquid Membrane
Nk
Product
Solution
Na+C~H~0HtCOOHl~COO-
+ Hz0
NaOH N+R,H-~H40HICOOHtZCOO-
Fig. 2. Conceptual description of citric acid coupled transport as the driving force (monosodium citrate is the product).
using hydrogen ion concentration
For this application, L = membrane thickness, D = citrate-amine diffusivity, j = citrate flux, S, = citrate distribution coefficient, S, = hydrogen ion distribution coefficient, Keq = equilibrium constant for eqn. (1)) C, = average carrier concentration in the membrane, C1r, C,, = citrate concentration in the aqueous feed and product solutions, and C&r,CpP= hydrogen ion concentration in the aqueous feed and product solutions. (This equation also assumes that passive diffusion is negligible when compared with diffusion via the carrier.) It is instructive to examine the form that eqn. (2) takes in the limit of very high feed citrate concentration ( C1r+ co ) and very low product citrate concentration (C&-+0). In this limit, eqn. (2) takes the form j=L
DC
L ’
(3)
which shows that flux is independent of citrate concentration. This corresponds to “saturation” of the carrier in the membrane. At “subsaturated” concentrations of citrate in the aqueous feed solution, citrate flux is a function of C,, C1r, Cm, C2r and C2r, as shown in eqn. (2). The effect of feed and product solution pH on flux can also be predicted from eqn. (2). The greater the difference in hydrogen ion concentrations between the feed and product solutions, the greater the citrate flux. Thus, as product solution pH increases (hydrogen ion concentration decreases), flux will increase to a limiting value because the second term in eqn. (2) will approach zero. In addition, flux will increase as the feed pH decreases (hydrogen ion concentration increases). Thus, eqn. (2) correctly predicts that the flux of citrate can be driven by a pH difference across the membrane.
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2.
Experimental
Permeation experiments were carried out in which the citric acid flux was measured as a function of temperature, membrane composition, and citric acid concentration in the feed and product solutions. 2.A. Membranes and reagents The supported-liquid membranes were formed by immobilizing the hydrophobic carrier solution within the pores of Celgard 2400, a microporous polypropylene film obtained from Celanese Plastics Co. This membrane is approximately 25 pm thick, and has a nominal porosity of 38% and a pore diameter of 0.02 pm. To fill the pores of the membranes with the carrier solution, the membrane was immersed in the organic carrier solution and the excess organic material was allowed to drain off. The tertiary amine used in these studies was reagent-grade tri-n-dodecylamine (trilaurylamine); this amine has been used by others for solvent extraction of citric acid from fermentation beer [ 31. The following modifiers were used to increase the partition coefficient for citric acid (and thereby increase citrate flux): reagent-grade n-octanol, n-dodecanol, n-tridecanol and nonylphenol; and industrial-grade isohexadecanol obtained from Hoechst, Germany. The carrier-modifier solution was diluted with either Shell Sol 71, an aliphatic hydrocarbon solvent manufactured by Shell Oil Co., or Alkyl Aromatic Oil (also known as Alkane 56), an aromatic hydrocarbon solvent manufactured by Chevron Research Co. Aqueous, citric acid solutions were prepared from reagent-grade citric acid and glucose except for the actual fermentation beer, which was obtained from an industrial source. 2.B. Permeability measurements The apparatus used in the permeability studies has been described elsewhere [lo]. This apparatus was a two-compartment permeation cell consisting of two lOO-ml half-cells separated by a membrane with an active area of ca. 18 cm’. The aqueous solutions on either side of the membrane were stirred at 170 rpm. Citric acid and glucose fluxes were determined by monitoring the concentrations of both species in the aqueous product solution as functions of time. Citric acid concentrations were determined by titration with 0.01 M sodium hydroxide to a pH 7.5 endpoint. Glucose concentrations were determined by a commercial analytical method that measures conversion of glucose to B-phosphogluconate (Glucose Analytical Procedure No. 15-UV, Sigma Chemical Co. ). The flux of glucose (the fermentation substrate) was measured to calculate the selectivity of the membrane for citric acid over glucose.
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3. Results The effects of solution temperature and membrane composition were investigated to optimize citric acid flux. In these tests, citric acid concentration rather than hydrogen ion concentration provided the driving force. The effect of citric acid concentration in the feed and product solutions on the flux of citric acid was investigated to predict the performance of a coupled transport system under the various conditions that would exist during actual operation. Finally, two experiments were conducted with actual fermentation beer using the membrane that yielded the highest fluxes when synthetic citric acid solutions were used. In the experiments using actual fermentation beer, a citric acid concentration gradient and a hydrogen ion concentration gradient were each used as driving forces for citrate transport. 3.A. Effect of temperature Figure 3 shows the effect of temperature on citric acid flux through coupled transport membranes containing 38 vol.% of trilaurylamine in Shell Sol 71. Identical temperatures were maintained on each side of the membrane. The citric acid flux rose rapidly with temperature, from a low of ca. 1.5 pg/cm2-min at 0°C to a high of ca. 35 pg/cm2-min at 60°C. This increase in flux with temperature was likely due to the increasingly rapid diffusion of the citric acidamine complex through the membrane as the increasing temperature reduced the viscosity of the organic carrier solution. Presumably the flux could be im60
,
Citric acid flux @km*-min)
20
0 0
20
40
60
80
100
Temperature PC$ Fig. 3. Effect of temperature
38 vol.% of trilaurylamine solution: distilled water.
on the citric acid flux through a Celgard 2400 membrane containing in Shell Sol 71. Feed solution: 200 g/l citric acid, 10 g/l glucose; product
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proved by using less viscous diluents. This would allow operation at room temperature in a full-scale plant. The decrease in flux at temperatures above 60’ C can be explained as follows. The distribution coefficient for partitioning of citric acid between the aqueous feed solution and the organic carrier solution decreases as temperature increases [ 31. This shift in the distribution coefficient results in a smaller overall flux, according to eqn. (2). As a result of the two competing factors, i.e. viscosity and distribution coefficient, optimum flux occurs at temperatures between 50 and 70’ C. 3.B. Effect of membrane composition The effects of amine concentration, solvent type, modifier type and modifier concentration on citric acid flux were investigated. Modifiers are used extensively in liquid-liquid extraction for two reasons: (1) to prevent the formation of a second phase in the carrier solution (i.e. a complex-rich phase and a complex-poor phase), and (2 ) to increase the solubility of the extracted species in the organic phase. Because long-chain alcohols are often used as modifiers in liquid-liquid extraction, we chose to investigate their effect on citric acid flux in coupled transport membranes. The effects of trilaurylamine concentration and solvent type on citric acid flux were determined by first using Alkyl Aromatic Oil (a solvent with lower viscosity than Shell Sol 71) and then using Shell Sol 71 as the solvent. Experiments were carried out at 50°C and the modifier used was n-octanol at a concentration of 5 wt.%. The results in Fig. 4 show that, when Alkyl Aromatic
Citric acid flux @/cm*-mid
0
10
20
30
Trilauryiamine concentration
40
50
(vol.%)
Fig. 4. Effect of trilaurylamine concentration on the citric acid flux through Celgard 2400 membrane containing 5 vol.% of n-octanol and trilaurylamine in alkyl aromatic oil at 50°C. Feed solution:
200 g/l citric acid, 10 g/l glucose; product solution:
distilled
water.
Chic acid flux $&cm’-min)
0
10
20
30
40
Trilauryiamine concentratiin
Fig. 5. Effect of trilaurylamine
concentration
50 kl .sb)
on citric acid flux through Celgard 2400 membrane
containing 5 vol.% of n-octanol and trilaurylamine in Shell Sol 71 at 50°C. g/l citric acid, 10 g/l glucose; product solution: distilled water.
Feed solution:
200
Oil was used as the solvent, the citric acid flux increased with increasing amine concentration, reaching a maximum of 67 pg/cm2-min at an amine concentration of ca. 38 vol.%. Figure 5 shows that, when Shell Sol 71 was used as the solvent, the flux continued to increase with increasing amine concentration up to 50 vol.%, the highest concentration tested. At this amine concentration in Shell Sol 71, the citric acid flux was ca. 34 pg/cm’-min. These results are consistent in light of the nature of aliphatic and aromatic solvents [ 131; that is, the higher fluxes with Alkyl Aromatic Oil (the aromatic solvent) are probably due to the lower viscosity of the citrate complex-aliphatic solvent (Shell Sol 71) solution. However, because the U.S. Food and Drug Administration (FDA) has not approved aromatic solvents for use in the production of food-grade acids, the aliphatic Shell Sol 71 was used in subsequent work. The effects of modifier type and concentration were investigated in an attempt to increase further the citric acid flux. The modifier likely functions to increase the solubility of the citrate anion in the organic membrane phase (i.e. increase the distribution coefficient), both by hydrogen bonding to the citrate ion and by increasing the solvation properties of Shell Sol 71 [ 141. Table 1 lists the citric acid flux for five types of membrane, each containing 38 vol.% of trilaurylamine in Shell Sol 71 and 5 vol.% of a modifier. The flux for membranes containing no modifier is shown for comparison. The citric acid flux was greater for all membranes that contained modifiers than for membranes containing no modifiers. When modifiers were used, the citric acid flux increased from lowest to highest in the following order: n-octanol, n-dodecanol, n-tridecanol, nonylphenol, and isohexadecanol. This order corresponds to the order to increasing size of the hydrocarbon portion of the alcohol.
135 TABLE 1 Effect of modifier on citric acid flux: 5 vol.% of modifier in 38 vol.% trilaurylamine and 57 vol.% Shell Sol 71 at 50°C Modifier
Citric acid flux (&cm*-min)
No modifier n-Octanol n-Dodecanol n-Tridecanol Nonylphenol Isohexadecanol
25 20 38 42 42 49
Citricacid flux $g/cm’-min)
01
0
5
10
15
20
25
n-Tridecenol concentration kl.%
Fig. 6. Effect of n-tridecanol concentration on the citric acid flux through Celgard 2400 membranes containing 38 vol.% trilaurylamine and tridecanol in Shell Sol 71 at 50°C. Feed solution: 200 g/l citric acid, 10 g/l glucose; product solution: distilled water.
The effect of adding more modifier was investigated for the long-chain alcohol n-tridecanol. Figure 6 shows the effect of n-tridecanol concentration on citric acid flux through a membrane containing 38 vol.% of trilaurylamine and n-tridecanol in Shell Sol 71. The lowest flux was 25 pug/cm’-min when the solution contained no n-tridecanol, and the flux increased to a maximum of 57 pg/cm2-min when the solution contained 15to 20 vol.% of n-tridecanol. Subsequent experiments were performed with 15 vol.% of n-dodecanol as modifier, in place of the higher-molecular-weight alcohols such as n-tridecanol or isohexadecanol, to comply with FDA regulations for production of food-grade substances, which allow the use of only linear alcohols with an even number of carbons. Clearly, the composition of the carrier solution strongly affects the citric
136
acid flux through coupled transport membranes. By using a relatively high concentration of a high-molecular-weight alcohol in the organic phase of the membrane and by using an operating temperature of ca. 60’ C, citric acid fluxes of at least 60 pg/cm2-min can be obtained. To determine the selectivity of the solution, we also measured glucose flux through the coupled transport membrane. It is desirable to prevent the transport of this and other similar compounds in the fermentation beer across the membrane into the citric acid product. We found that the ratio of citric acid flux to glucose flux was consistently about 2000. In the feed solution, the ratio of citric acid concentration to glucose concentration was 20, so the flux ratio of 2000 translates into a selectivity of 100. Thus, it appears that coupled transport can be used selectively to recover citric acid in a relatively pure form from a heterogeneous chemical mixture such as the fermentation beer. 3.C. Effect of citric acid concentration The effect of citric acid concentration in the feed and product solutions on the flux of citric acid was investigated so that the performance of a coupled transport module could be predicted for the conditions that were likely to be encountered during actual operation. Actual operation was modeled using a simple countercurrent-flow membrane module, shown schematically in Fig. 7; this configuration is preferred over a cocurrent-flow system because the average concentration gradient across the membrane is larger and higher concentrations of acid can be obtained in the product stream. As the feed stream flows down the membrane, the citric acid concentration decreases; as the product stream flows in the opposite direction, the acid concentration in the product stream increases. To predict the average citric acid flux in the module for a given citric acid recovery, the instantaneous flux at each point along the membrane must be known. These fluxes are dictated by the citric acid concentrations of the solutions in contact with the membrane at each point. The citric acid flux was measured for selected concentrations of citric acid in the feed and product solutions. Figure 8 shows citric acid flux as a function of citric acid concentration in the feed solution for a group of curves that repCitric-Acid Depleted Fermentation Beer
Fermentation Beer (-200 g/LCitric Acid)
Product
( < 200 g/l Citric
Stream
( < 2OOg/i Citric
Sweep Acid)
Fig. 7. A countercurrent-flow
Membrane
membrane module.
Acid)
Stream
(Citric-Acid
Free)
137
resent various concentrations of citric acid in the product solution. A similar graph is shown in Fig. 9, except that the product solution contained 1.5 M sodium hydroxide. Figures 8 and 9 both show the saturation phenomenon discussed in the introduction, i.e. when the feed solution contains high concentrations of citric acid, the flux plateaus at a limiting value. Citric acid fluxes were much higher when the product solution contained 80,
I
60 Citric acid flux (p/cm2-min)
40
20
0 0
50
100
150
200
Citric acid concentration in feed Solution (g/l
)
Fig. 8. Citric acid flux as a function of citric acid concentration in the feed solution for given concentrations of citric acid in the product solution. Feed solution: citric acid, strip solution: H,O+citric acid; membrane: Celgard 2400; organic: 38 vol.% trilaurylamine, 15 vol.% dodecanol in Shell Sol 71. (0 ) 0 g/l citric acid in product solution; (0 ) 20 g/l citric acid in product solution; (A ) 40 g/l citric acid in product solution; (0 ) 70 g/l citric acid in product solution; (m) 100 g/l citric acid in product solution; (A ) 150 g/l citric acid in product solution.
60 Citric acid flux @km’-min)
0
5
10
15
20
Citric acid concentration in feed Solution (.vt.?&)
Fig. 9. Citric acid flux as a function of citric acid concentration in the feed solution for various concentrations of citric acid in product solution containing 1.5 M sodium hydroxide. Membrane: Celgard 2400; organic: 38 vol.% trilaurylamine, 15 vol.% dodecanol in Shell Sol 71; temperature: 60” C. (0 ) 0 g/l citric acid in product solution; ( A ) 100 g/l citric acid in product solution; (0 ) 150 g/l citric acid in product solution.
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80 60 Average
CitricAcid Flux (&cm*-min)
O-1 0
25
50
75
100
Prcduct Recovery VO)
Fig. 10. Calculated average citric acid flux as a function of citric acid recovery for various concentrations of citric acid or monosodium citrate in product solution.
sodium hydroxide. This was expected, because sodium hydroxide reacts with the hydrogen ions that are transported along with the citrate anions; thus, hydrogen ions do not build up in the product solution, and the driving force produced by hydrogen ion concentration is maintained at a high level. This trend is predicted by eqn. (2). Using the data in Figs. 8 and 9, we calculated the average flux for a membrane module operating in a countercurrent-flow configuration for several different operating conditions. Figure 10 shows the calculated results for three different concentrations of citric acid in the product stream as a function of citric acid recovery from the feed stream. The countercurrent-flow module would operate at low recovery if continuous removal of citric acid from a fermenter was desirable but significant reduction of the citric acid concentration was undesirable. The module would operate at high recovery if nearly all the citric acid were to be recovered from the fermentation beer using coupled transport in a batch mode. Predictably, flux decreases as the percentage recovery increases and as the concentration of citric acid (or monosodium citrate) in the product solution increases. Considerably higher fluxes can be obtained at all recoveries when sodium hydroxide is added to the product solution. 3.0. Studies with actual fermentation beer Two experiments were conducted using actual fermentation beer as the feed solution. In one experiment the product solution was deionized water; in the other, the product solution was 1.5 M sodium hydroxide. Neither product solution contained citric acid initially. The results using deionized water as the product solution are presented in Fig. 11,which shows the citric acid concentration in the feed and product solutions as a function of time. As citric acid was transported across the mem-
139
loo Citric
acid
COllCMthOll (g/l)
50
0
200
400
600
lime Old
Fig. 11. Citric acid concentration in feed and product solutions as functions of time for Celgard 2400 membrane containing 38 vol.% of trilaurylamine and 15 vol.% of dodecanol in Shell Sol 71. Feed solution: actual fermentation beer ( z 100 ml); Product solution: H,O ( = 100 ml); temperature: 60°C; membrane area: 18 cm2.
100 -
80Citric acid concentration W)
604020 01
0
400
200
600
Time 0 Fig. 12. Citric acid concentration in feed and product solutions as functions of time for Celgard 2400 membranes containing 38 vol.% of trilaurylamine and 15 vol.% of dodecanol in Shell Sol 71. Feed solution: actual fermentation beer ( z 100 ml); product solution: 1.5 M NaOH ( z 100 ml); temperature: 60°C; membrane area: 18 cm’.
brane, the concentration of citric acid decreased in the feed solution and increased in the product solution. After 400 hr, the concentrations of citric acid in the feed and product solutions were roughly equal. (In practice, the separation could be carried out much faster by increasing the ratio of the membrane’s surface area to the feed solution volume by two to three orders of magnitude over that used in this experiment. ) After the first 400 hr, the feed solution was replaced with fresh fermentation beer and the experiment was continued. At the end of the experiment, the citric acid concentration in the product so-
140
lution was greater than half the concentration in the original fermentation beer. In practice, obtaining a concentration of citric acid beyond one-half that in the original feed solution would be accomplished by countercurrent-flow circulation of the feed and product solutions. Figure 12 shows the results of the experiment in which 1.5 M sodium hydroxide was used as the product solution. The product solution composition is shown as the equivalent of citric acid-the actual species present was trisodium citrate. The most notable difference between this experiment and that illustrated by Fig. 11 is that the concentration of citrate in the product surpassed that in the feed. Specifically, after 260 hr of operation, 68% of the citric acid had been recovered from the fermentation beer; the average citric acid flux was 29 pg/cm2-min. After 600 hr of operation, 83% of the citric acid had been recovered from the feed, and the average citric acid flux was 15 pug/cm*-min. These results show that a base such as sodium hydroxide can be used to drive citrate anions “uphill” across the membrane. The data shown in Fig. 12 were fitted to the following pseudo-first-order kinetic equation:
where C, = citric acid concentration in feed at any time t, Co = initial citric acid concentration in feed, P=apparent citric acid permeability or overall masstransfer coefficient, A = membrane surface area, t= time, and V= volume of feed reservoir. From the fit of experimental data, we determined that the average citric acid permeability is 6.9 x lop6 cm/set. However, since citric acid flux is not proportional to the citric acid concentration in feed (see Fig. 9 ), the fit of the experimental data to this expression is poor. By inserting this value for permeability into eqn. (4)) citric acid concentrations and recoveries can be estimated for any time and for any membrane area or feed volume. 4. Conclusions
These experiments have shown the feasibility of using supported-liquid membranes for recovery of citric acid from fermentation beer, either as citric acid or as monosodium citrate. The membrane was optimized, and we found that citric acid flux is greatest under the following conditions: 6O”C, trilaurylamine concentration in the membrane of 38 vol.% in Alkyl Aromatic Oil or 50 vol.% in Shell Sol 71, and 15 vol.% of n-dodecanol in the membrane as a modifier. Experiments were conducted with actual fermentation beer, and it was shown that citric acid can be driven “uphill” against its concentration gradient by using hydrogen ion concentration as a driving force.
141
References 1
2 3 4 5 6 7 8 9
10 11 12 13 14
Jian Yu-Ming, Li Dao-Chen and Su Yuan-Fu, Study on extraction of citric acid, Proc. Int. Solvent Extr. Conf., Denver, CO, August 1983, pp. 517-518. B. Atkinson and F. Mavituna, Biochemical Engineering and Biotechnology Handbook, The Nature Press, New York, NY, 1983, pp. 1033-1036. A.M. Baniel, Process for the extraction of organic acids from aqueous solution, European Patent 0,049,429,1982. E. Hemmingsen and P. Scholander, Specific transport of oxygen through hemoglobin solutions, Science, 132 (1960) 1379. W. Ward and W. Robb, Carbon dioxide-oxygen separation: Facilitated transport of carbon dioxide across a liquid film, Science, 156 (1967) 1481. R. Bloch, Hydrometallurgical separations by solvent membranes, in: J. Flinn (Ed.), Membrane Science and Technology, Plenum Press, New York, NY, 1970, pp. 171-187. E. Cussler, Multicomponent Diffusion, Elsevier, Amsterdam, 1967, pp. 114-121. N.N. Li, Permeation through liquid surfactant membranes, AIChE J., 17 (1971) 459. T.M. Fyles, V.A. Malik-Diemer, CA. McGavin and D.M. Whitfield, Membrane transport systems. III. A mechanistic study of cation-proton coupled countertransport, Can. J. Chem., 60 (1982) 2259. R.W. Baker, M.E. Tuttle, D.J. Kelly and H.K. Lonsdale, Coupled-transport membranes. I. Copper separations, J. Membrane Sci., 2 (1977) 213. J.E. Alter and R. Blumberg, Extraction of citric acid from its aqueous solution using n-substituted alkyl amine containing at least twelve carbon atoms, U.S. Patent 4,251,671, 1982. A.M. Baniel, R. Blumberg and K. Hadju, British Patent 1,426,018,1973. G. Ritcey and A. Ashbrook, Solvent Extraction. Principles and Applications to Process Metallurgie. Part I, Elsevier, Amsterdam, 1984, pp. 173-194. Y. Marcus and A. Kertes, Ion Exchange and Solvent Extraction of Metal Complexes, WileyInterscience, New York, NY, 1969, pp. 849-854.
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Recovery of citric acid from fermentation beer using supported-liquid membranes D.T. Friesen, W.C. Babcock, D.J. Brose and A.R. Chambers Bend Research, Inc., 64550 Research Road, Bend, OR 97701-8599 (USA) (Received May 19,1987; accepted in revised form July 2, 1990)
Abstract The use of a supported-liquid membrane to separate citric acid from the beer produced by fermentation was studied. The liquid membrane consisted of an organic solution composed of trilaurylamine and a long-chain alcohol dissolved in an inert hydrocarbon solvent that was immobilized in a microporous polypropylene support. Using this membrane, citric acid was recovered from the fermentation beer as citric acid or as its monosodium salt. The effects of temperature, organic solution composition and citric acid concentration on transport rates were investigated. Citric acid flux was greatest under the following conditions: 60 OC; trilaurylamine concentration of 38 vol.% in Alkyl Aromatic Oil, or 50 vol.% in Shell Sol 71; and 15 vol.% of n-dodecanol in the membrane as a modifier. Keywords: Liquid membranes; coupled, facilitated transport; organic separations
1. Introduction
Citric acid (2-hydroxy-1,2,3-propanetricarboxylic acid) is produced commercially by fermentation with Aspergillus or Candida fungi. In conventional processes, citric acid has been recovered from the fermentation beer by precipitation of calcium citrate with calcium hydroxide. In this recovery scheme, calcium citrate is precipitated, recovered by filtration, and converted to citric acid by addition of sulfuric acid. The dilute citric acid product is then sequentially purified using ion exchange, activated carbon, evaporation and crystallization [ 1,2]. More recently, liquid-liquid extraction followed by precipitation or back-extraction has been investigated for product recovery [ 31. The use of supported-liquid membranes for the recovery of citric acid offers unique advantages over these recovery schemes. Some of the advantages are lower energy consumption, higher separation factors in a single stage, the ability to concentrate citric acid during the separation (i.e. citrate ions can be pumped “uphill”), and smaller size of the complete separation plant. These advantages may eventually result in a reduction in overall recovery costs.
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0 1991-
Elsevier Science Publishers B.V.
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1.A. Description of supported-liquid membranes Liquid membranes containing mobile carriers have been widely studied in recent years. In these membranes, a carrier reacts selectively with one or more species on one side of the membrane, and the complex then diffuses across the membrane, where it dissociates, releasing the complexed species. The earliest examples of this process were in carrier-facilitated gas transport [ 431; these processes are generally categorized as “facilitated transport”. More recently, the use of liquid membranes containing carriers has been extended to the transport of ions from aqueous solutions [ 6-81. We refer to these processes as “coupled transport”. Coupled transport membranes consist of an inert, microporous support impregnated with a water-immiscible, mobile ion-exchange agent. The mobile carrier, which is held in the pores of the support membrane by capillarity, acts as a shuttle, picking up ions from an aqueous solution on one side of the membrane, carrying them across the membrane, and releasing them to the solution on the opposite side of the membrane [9,10]. The flow of the complexed ion is coupled to the flow of a second ion (e.g. the hydrogen ion). The coupling of the flows of the two ions permits one of the ions to be pumped “uphill” from a solution in which it is dilute to a solution in which it is more concentrated. l.B. Description of citric acid coupled transport Tertiary amines have been proposed by others as ion-complexing agents for the recovery of citric acid using solvent extraction [ 1,11,12]. As Fig. 1 shows, tertiary amines ( NR3) can also be used for the coupled transport of monobasic citrate anions [C,H,OH(COOH),COO]*. In the extraction step, the basic amine reacts with hydrogen ions in the feed solution to form the tertiary alkylammonium cation. This cation then associates as an ion pair with the citrate anion to form an alkylammonium salt, which is transported across the
Fig. 1. Conceptual description of citric acid coupled transport the driving force (citric acid is the product).
using citric acid concentration
as
*pK, values for citric acid are 3.1,4.8 and 6.4; thus, at the pH of the fermentation beer (2.6)) citric acid exists as a mixture of the neutral acid (80% ) and the monobasic anion (20% ).
129
membrane. Other neutral amines can also loosely associate with the citrate complex; however, the process can most easily be understood by considering only the one-to-one citrate-amine complex. The overall reaction for this extraction can be expressed as the forward reaction in eqn. ( 1) : Feed H&
+WWWCOOW&00,,,
+N%r,,
Product
side
side
(1)
N+R,HC,H,OH(COOH),COO,,, where the subscripts (aq) and (org) designate species in the aqueous and organic phases, respectively. After the alkylammonium salt has diffused across the membrane, the citrate ion is stripped from the organic carrier solution into the aqueous product phase by the reverse of the reaction in eqn. (1). This reaction regenerates the tertiary amine, which then diffuses back to the feed side of the membrane, where it recomplexes with hydrogen and citrate ions. Although this mechanism has not been proven in this paper, it is consistent with our results, as well as with the results obtained by others who have studied liquid-liquid extraction. The transport of citrate ions across the membrane can be driven by a difference in citrate concentration across the membrane, a difference in hydrogen ion concentration across the membrane or a combination of both. It is clear from eqn. (1) that, as either the hydrogen ion or citrate ion concentration increases, the forward reaction of eqn. (1) will be favored, leading to a relatively high concentration of citrate-amine complex in the membrane. Conversely, a low hydrogen ion or citrate ion concentration favors the reverse reaction of eqn. (1) , leading to a relatively low concentration of citrate-amine complex in the membrane. The transport of citrate across the membrane occurs because a concentration difference of citrate-amine complex exists across the membrane. Figure 1 illustrates the case in which a difference in citrate concentration is the driving force; in this case citric acid is the product. Figure 2 illustrates the case in which a difference in hydrogen ion concentration is the driving force; in this case monosodium citrate is the product. As Fig. 2 indicates, maintaining low pH on the feed side and high pH on the product side of the membrane can be used to drive citrate ions from a solution of low citrate concentration to one of high citrate concentration. This uphill transport of citrate anions makes it possible to obtain highly concentrated product solutions from extremely dilute feed streams. The mathematical model for reaction and diffusion in coupled transport membranes has been well developed [ 71. The model is considerably simplified if we can assume that the forward and reverse reactions in eqn. (1) are much faster than diffusion in the membrane - i.e. diffusion limits transport. Based on this assumption, Cussler [ 71 has derived the following equation for flux in coupled transport membranes:
130
Feed
Solution
Supported-Liquid Membrane
Nk
Product
Solution
Na+C~H~0HtCOOHl~COO-
+ Hz0
NaOH N+R,H-~H40HICOOHtZCOO-
Fig. 2. Conceptual description of citric acid coupled transport as the driving force (monosodium citrate is the product).
using hydrogen ion concentration
For this application, L = membrane thickness, D = citrate-amine diffusivity, j = citrate flux, S, = citrate distribution coefficient, S, = hydrogen ion distribution coefficient, Keq = equilibrium constant for eqn. (1)) C, = average carrier concentration in the membrane, C1r, C,, = citrate concentration in the aqueous feed and product solutions, and C&r,CpP= hydrogen ion concentration in the aqueous feed and product solutions. (This equation also assumes that passive diffusion is negligible when compared with diffusion via the carrier.) It is instructive to examine the form that eqn. (2) takes in the limit of very high feed citrate concentration ( C1r+ co ) and very low product citrate concentration (C&-+0). In this limit, eqn. (2) takes the form j=L
DC
L ’
(3)
which shows that flux is independent of citrate concentration. This corresponds to “saturation” of the carrier in the membrane. At “subsaturated” concentrations of citrate in the aqueous feed solution, citrate flux is a function of C,, C1r, Cm, C2r and C2r, as shown in eqn. (2). The effect of feed and product solution pH on flux can also be predicted from eqn. (2). The greater the difference in hydrogen ion concentrations between the feed and product solutions, the greater the citrate flux. Thus, as product solution pH increases (hydrogen ion concentration decreases), flux will increase to a limiting value because the second term in eqn. (2) will approach zero. In addition, flux will increase as the feed pH decreases (hydrogen ion concentration increases). Thus, eqn. (2) correctly predicts that the flux of citrate can be driven by a pH difference across the membrane.
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2.
Experimental
Permeation experiments were carried out in which the citric acid flux was measured as a function of temperature, membrane composition, and citric acid concentration in the feed and product solutions. 2.A. Membranes and reagents The supported-liquid membranes were formed by immobilizing the hydrophobic carrier solution within the pores of Celgard 2400, a microporous polypropylene film obtained from Celanese Plastics Co. This membrane is approximately 25 pm thick, and has a nominal porosity of 38% and a pore diameter of 0.02 pm. To fill the pores of the membranes with the carrier solution, the membrane was immersed in the organic carrier solution and the excess organic material was allowed to drain off. The tertiary amine used in these studies was reagent-grade tri-n-dodecylamine (trilaurylamine); this amine has been used by others for solvent extraction of citric acid from fermentation beer [ 31. The following modifiers were used to increase the partition coefficient for citric acid (and thereby increase citrate flux): reagent-grade n-octanol, n-dodecanol, n-tridecanol and nonylphenol; and industrial-grade isohexadecanol obtained from Hoechst, Germany. The carrier-modifier solution was diluted with either Shell Sol 71, an aliphatic hydrocarbon solvent manufactured by Shell Oil Co., or Alkyl Aromatic Oil (also known as Alkane 56), an aromatic hydrocarbon solvent manufactured by Chevron Research Co. Aqueous, citric acid solutions were prepared from reagent-grade citric acid and glucose except for the actual fermentation beer, which was obtained from an industrial source. 2.B. Permeability measurements The apparatus used in the permeability studies has been described elsewhere [lo]. This apparatus was a two-compartment permeation cell consisting of two lOO-ml half-cells separated by a membrane with an active area of ca. 18 cm’. The aqueous solutions on either side of the membrane were stirred at 170 rpm. Citric acid and glucose fluxes were determined by monitoring the concentrations of both species in the aqueous product solution as functions of time. Citric acid concentrations were determined by titration with 0.01 M sodium hydroxide to a pH 7.5 endpoint. Glucose concentrations were determined by a commercial analytical method that measures conversion of glucose to B-phosphogluconate (Glucose Analytical Procedure No. 15-UV, Sigma Chemical Co. ). The flux of glucose (the fermentation substrate) was measured to calculate the selectivity of the membrane for citric acid over glucose.
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3. Results The effects of solution temperature and membrane composition were investigated to optimize citric acid flux. In these tests, citric acid concentration rather than hydrogen ion concentration provided the driving force. The effect of citric acid concentration in the feed and product solutions on the flux of citric acid was investigated to predict the performance of a coupled transport system under the various conditions that would exist during actual operation. Finally, two experiments were conducted with actual fermentation beer using the membrane that yielded the highest fluxes when synthetic citric acid solutions were used. In the experiments using actual fermentation beer, a citric acid concentration gradient and a hydrogen ion concentration gradient were each used as driving forces for citrate transport. 3.A. Effect of temperature Figure 3 shows the effect of temperature on citric acid flux through coupled transport membranes containing 38 vol.% of trilaurylamine in Shell Sol 71. Identical temperatures were maintained on each side of the membrane. The citric acid flux rose rapidly with temperature, from a low of ca. 1.5 pg/cm2-min at 0°C to a high of ca. 35 pg/cm2-min at 60°C. This increase in flux with temperature was likely due to the increasingly rapid diffusion of the citric acidamine complex through the membrane as the increasing temperature reduced the viscosity of the organic carrier solution. Presumably the flux could be im60
,
Citric acid flux @km*-min)
20
0 0
20
40
60
80
100
Temperature PC$ Fig. 3. Effect of temperature
38 vol.% of trilaurylamine solution: distilled water.
on the citric acid flux through a Celgard 2400 membrane containing in Shell Sol 71. Feed solution: 200 g/l citric acid, 10 g/l glucose; product
133
proved by using less viscous diluents. This would allow operation at room temperature in a full-scale plant. The decrease in flux at temperatures above 60’ C can be explained as follows. The distribution coefficient for partitioning of citric acid between the aqueous feed solution and the organic carrier solution decreases as temperature increases [ 31. This shift in the distribution coefficient results in a smaller overall flux, according to eqn. (2). As a result of the two competing factors, i.e. viscosity and distribution coefficient, optimum flux occurs at temperatures between 50 and 70’ C. 3.B. Effect of membrane composition The effects of amine concentration, solvent type, modifier type and modifier concentration on citric acid flux were investigated. Modifiers are used extensively in liquid-liquid extraction for two reasons: (1) to prevent the formation of a second phase in the carrier solution (i.e. a complex-rich phase and a complex-poor phase), and (2 ) to increase the solubility of the extracted species in the organic phase. Because long-chain alcohols are often used as modifiers in liquid-liquid extraction, we chose to investigate their effect on citric acid flux in coupled transport membranes. The effects of trilaurylamine concentration and solvent type on citric acid flux were determined by first using Alkyl Aromatic Oil (a solvent with lower viscosity than Shell Sol 71) and then using Shell Sol 71 as the solvent. Experiments were carried out at 50°C and the modifier used was n-octanol at a concentration of 5 wt.%. The results in Fig. 4 show that, when Alkyl Aromatic
Citric acid flux @/cm*-mid
0
10
20
30
Trilauryiamine concentration
40
50
(vol.%)
Fig. 4. Effect of trilaurylamine concentration on the citric acid flux through Celgard 2400 membrane containing 5 vol.% of n-octanol and trilaurylamine in alkyl aromatic oil at 50°C. Feed solution:
200 g/l citric acid, 10 g/l glucose; product solution:
distilled
water.
Chic acid flux $&cm’-min)
0
10
20
30
40
Trilauryiamine concentratiin
Fig. 5. Effect of trilaurylamine
concentration
50 kl .sb)
on citric acid flux through Celgard 2400 membrane
containing 5 vol.% of n-octanol and trilaurylamine in Shell Sol 71 at 50°C. g/l citric acid, 10 g/l glucose; product solution: distilled water.
Feed solution:
200
Oil was used as the solvent, the citric acid flux increased with increasing amine concentration, reaching a maximum of 67 pg/cm2-min at an amine concentration of ca. 38 vol.%. Figure 5 shows that, when Shell Sol 71 was used as the solvent, the flux continued to increase with increasing amine concentration up to 50 vol.%, the highest concentration tested. At this amine concentration in Shell Sol 71, the citric acid flux was ca. 34 pg/cm’-min. These results are consistent in light of the nature of aliphatic and aromatic solvents [ 131; that is, the higher fluxes with Alkyl Aromatic Oil (the aromatic solvent) are probably due to the lower viscosity of the citrate complex-aliphatic solvent (Shell Sol 71) solution. However, because the U.S. Food and Drug Administration (FDA) has not approved aromatic solvents for use in the production of food-grade acids, the aliphatic Shell Sol 71 was used in subsequent work. The effects of modifier type and concentration were investigated in an attempt to increase further the citric acid flux. The modifier likely functions to increase the solubility of the citrate anion in the organic membrane phase (i.e. increase the distribution coefficient), both by hydrogen bonding to the citrate ion and by increasing the solvation properties of Shell Sol 71 [ 141. Table 1 lists the citric acid flux for five types of membrane, each containing 38 vol.% of trilaurylamine in Shell Sol 71 and 5 vol.% of a modifier. The flux for membranes containing no modifier is shown for comparison. The citric acid flux was greater for all membranes that contained modifiers than for membranes containing no modifiers. When modifiers were used, the citric acid flux increased from lowest to highest in the following order: n-octanol, n-dodecanol, n-tridecanol, nonylphenol, and isohexadecanol. This order corresponds to the order to increasing size of the hydrocarbon portion of the alcohol.
135 TABLE 1 Effect of modifier on citric acid flux: 5 vol.% of modifier in 38 vol.% trilaurylamine and 57 vol.% Shell Sol 71 at 50°C Modifier
Citric acid flux (&cm*-min)
No modifier n-Octanol n-Dodecanol n-Tridecanol Nonylphenol Isohexadecanol
25 20 38 42 42 49
Citricacid flux $g/cm’-min)
01
0
5
10
15
20
25
n-Tridecenol concentration kl.%
Fig. 6. Effect of n-tridecanol concentration on the citric acid flux through Celgard 2400 membranes containing 38 vol.% trilaurylamine and tridecanol in Shell Sol 71 at 50°C. Feed solution: 200 g/l citric acid, 10 g/l glucose; product solution: distilled water.
The effect of adding more modifier was investigated for the long-chain alcohol n-tridecanol. Figure 6 shows the effect of n-tridecanol concentration on citric acid flux through a membrane containing 38 vol.% of trilaurylamine and n-tridecanol in Shell Sol 71. The lowest flux was 25 pug/cm’-min when the solution contained no n-tridecanol, and the flux increased to a maximum of 57 pg/cm2-min when the solution contained 15to 20 vol.% of n-tridecanol. Subsequent experiments were performed with 15 vol.% of n-dodecanol as modifier, in place of the higher-molecular-weight alcohols such as n-tridecanol or isohexadecanol, to comply with FDA regulations for production of food-grade substances, which allow the use of only linear alcohols with an even number of carbons. Clearly, the composition of the carrier solution strongly affects the citric
136
acid flux through coupled transport membranes. By using a relatively high concentration of a high-molecular-weight alcohol in the organic phase of the membrane and by using an operating temperature of ca. 60’ C, citric acid fluxes of at least 60 pg/cm2-min can be obtained. To determine the selectivity of the solution, we also measured glucose flux through the coupled transport membrane. It is desirable to prevent the transport of this and other similar compounds in the fermentation beer across the membrane into the citric acid product. We found that the ratio of citric acid flux to glucose flux was consistently about 2000. In the feed solution, the ratio of citric acid concentration to glucose concentration was 20, so the flux ratio of 2000 translates into a selectivity of 100. Thus, it appears that coupled transport can be used selectively to recover citric acid in a relatively pure form from a heterogeneous chemical mixture such as the fermentation beer. 3.C. Effect of citric acid concentration The effect of citric acid concentration in the feed and product solutions on the flux of citric acid was investigated so that the performance of a coupled transport module could be predicted for the conditions that were likely to be encountered during actual operation. Actual operation was modeled using a simple countercurrent-flow membrane module, shown schematically in Fig. 7; this configuration is preferred over a cocurrent-flow system because the average concentration gradient across the membrane is larger and higher concentrations of acid can be obtained in the product stream. As the feed stream flows down the membrane, the citric acid concentration decreases; as the product stream flows in the opposite direction, the acid concentration in the product stream increases. To predict the average citric acid flux in the module for a given citric acid recovery, the instantaneous flux at each point along the membrane must be known. These fluxes are dictated by the citric acid concentrations of the solutions in contact with the membrane at each point. The citric acid flux was measured for selected concentrations of citric acid in the feed and product solutions. Figure 8 shows citric acid flux as a function of citric acid concentration in the feed solution for a group of curves that repCitric-Acid Depleted Fermentation Beer
Fermentation Beer (-200 g/LCitric Acid)
Product
( < 200 g/l Citric
Stream
( < 2OOg/i Citric
Sweep Acid)
Fig. 7. A countercurrent-flow
Membrane
membrane module.
Acid)
Stream
(Citric-Acid
Free)
137
resent various concentrations of citric acid in the product solution. A similar graph is shown in Fig. 9, except that the product solution contained 1.5 M sodium hydroxide. Figures 8 and 9 both show the saturation phenomenon discussed in the introduction, i.e. when the feed solution contains high concentrations of citric acid, the flux plateaus at a limiting value. Citric acid fluxes were much higher when the product solution contained 80,
I
60 Citric acid flux (p/cm2-min)
40
20
0 0
50
100
150
200
Citric acid concentration in feed Solution (g/l
)
Fig. 8. Citric acid flux as a function of citric acid concentration in the feed solution for given concentrations of citric acid in the product solution. Feed solution: citric acid, strip solution: H,O+citric acid; membrane: Celgard 2400; organic: 38 vol.% trilaurylamine, 15 vol.% dodecanol in Shell Sol 71. (0 ) 0 g/l citric acid in product solution; (0 ) 20 g/l citric acid in product solution; (A ) 40 g/l citric acid in product solution; (0 ) 70 g/l citric acid in product solution; (m) 100 g/l citric acid in product solution; (A ) 150 g/l citric acid in product solution.
60 Citric acid flux @km’-min)
0
5
10
15
20
Citric acid concentration in feed Solution (.vt.?&)
Fig. 9. Citric acid flux as a function of citric acid concentration in the feed solution for various concentrations of citric acid in product solution containing 1.5 M sodium hydroxide. Membrane: Celgard 2400; organic: 38 vol.% trilaurylamine, 15 vol.% dodecanol in Shell Sol 71; temperature: 60” C. (0 ) 0 g/l citric acid in product solution; ( A ) 100 g/l citric acid in product solution; (0 ) 150 g/l citric acid in product solution.
138
80 60 Average
CitricAcid Flux (&cm*-min)
O-1 0
25
50
75
100
Prcduct Recovery VO)
Fig. 10. Calculated average citric acid flux as a function of citric acid recovery for various concentrations of citric acid or monosodium citrate in product solution.
sodium hydroxide. This was expected, because sodium hydroxide reacts with the hydrogen ions that are transported along with the citrate anions; thus, hydrogen ions do not build up in the product solution, and the driving force produced by hydrogen ion concentration is maintained at a high level. This trend is predicted by eqn. (2). Using the data in Figs. 8 and 9, we calculated the average flux for a membrane module operating in a countercurrent-flow configuration for several different operating conditions. Figure 10 shows the calculated results for three different concentrations of citric acid in the product stream as a function of citric acid recovery from the feed stream. The countercurrent-flow module would operate at low recovery if continuous removal of citric acid from a fermenter was desirable but significant reduction of the citric acid concentration was undesirable. The module would operate at high recovery if nearly all the citric acid were to be recovered from the fermentation beer using coupled transport in a batch mode. Predictably, flux decreases as the percentage recovery increases and as the concentration of citric acid (or monosodium citrate) in the product solution increases. Considerably higher fluxes can be obtained at all recoveries when sodium hydroxide is added to the product solution. 3.0. Studies with actual fermentation beer Two experiments were conducted using actual fermentation beer as the feed solution. In one experiment the product solution was deionized water; in the other, the product solution was 1.5 M sodium hydroxide. Neither product solution contained citric acid initially. The results using deionized water as the product solution are presented in Fig. 11,which shows the citric acid concentration in the feed and product solutions as a function of time. As citric acid was transported across the mem-
139
loo Citric
acid
COllCMthOll (g/l)
50
0
200
400
600
lime Old
Fig. 11. Citric acid concentration in feed and product solutions as functions of time for Celgard 2400 membrane containing 38 vol.% of trilaurylamine and 15 vol.% of dodecanol in Shell Sol 71. Feed solution: actual fermentation beer ( z 100 ml); Product solution: H,O ( = 100 ml); temperature: 60°C; membrane area: 18 cm2.
100 -
80Citric acid concentration W)
604020 01
0
400
200
600
Time 0 Fig. 12. Citric acid concentration in feed and product solutions as functions of time for Celgard 2400 membranes containing 38 vol.% of trilaurylamine and 15 vol.% of dodecanol in Shell Sol 71. Feed solution: actual fermentation beer ( z 100 ml); product solution: 1.5 M NaOH ( z 100 ml); temperature: 60°C; membrane area: 18 cm’.
brane, the concentration of citric acid decreased in the feed solution and increased in the product solution. After 400 hr, the concentrations of citric acid in the feed and product solutions were roughly equal. (In practice, the separation could be carried out much faster by increasing the ratio of the membrane’s surface area to the feed solution volume by two to three orders of magnitude over that used in this experiment. ) After the first 400 hr, the feed solution was replaced with fresh fermentation beer and the experiment was continued. At the end of the experiment, the citric acid concentration in the product so-
140
lution was greater than half the concentration in the original fermentation beer. In practice, obtaining a concentration of citric acid beyond one-half that in the original feed solution would be accomplished by countercurrent-flow circulation of the feed and product solutions. Figure 12 shows the results of the experiment in which 1.5 M sodium hydroxide was used as the product solution. The product solution composition is shown as the equivalent of citric acid-the actual species present was trisodium citrate. The most notable difference between this experiment and that illustrated by Fig. 11 is that the concentration of citrate in the product surpassed that in the feed. Specifically, after 260 hr of operation, 68% of the citric acid had been recovered from the fermentation beer; the average citric acid flux was 29 pg/cm2-min. After 600 hr of operation, 83% of the citric acid had been recovered from the feed, and the average citric acid flux was 15 pug/cm*-min. These results show that a base such as sodium hydroxide can be used to drive citrate anions “uphill” across the membrane. The data shown in Fig. 12 were fitted to the following pseudo-first-order kinetic equation:
where C, = citric acid concentration in feed at any time t, Co = initial citric acid concentration in feed, P=apparent citric acid permeability or overall masstransfer coefficient, A = membrane surface area, t= time, and V= volume of feed reservoir. From the fit of experimental data, we determined that the average citric acid permeability is 6.9 x lop6 cm/set. However, since citric acid flux is not proportional to the citric acid concentration in feed (see Fig. 9 ), the fit of the experimental data to this expression is poor. By inserting this value for permeability into eqn. (4)) citric acid concentrations and recoveries can be estimated for any time and for any membrane area or feed volume. 4. Conclusions
These experiments have shown the feasibility of using supported-liquid membranes for recovery of citric acid from fermentation beer, either as citric acid or as monosodium citrate. The membrane was optimized, and we found that citric acid flux is greatest under the following conditions: 6O”C, trilaurylamine concentration in the membrane of 38 vol.% in Alkyl Aromatic Oil or 50 vol.% in Shell Sol 71, and 15 vol.% of n-dodecanol in the membrane as a modifier. Experiments were conducted with actual fermentation beer, and it was shown that citric acid can be driven “uphill” against its concentration gradient by using hydrogen ion concentration as a driving force.
141
References 1
2 3 4 5 6 7 8 9
10 11 12 13 14
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