Extraction of Lactic Acid from a Calcium Lactate Solution Using Amine

Mar 15, 1996 - Department of Chemical Engineering, University of Wisconsin, 1415 Johnson Drive,. Madison ... pressures of 23.2, 21.3, and 21.1 bar...
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Extraction of Lactic Acid from a Calcium Lactate Solution Using Amine-Containing Solvents and Carbon Dioxide Gas Richard W. Miller, Michael C. M. Cockrem, Juan J. de Pablo, and Edwin N. Lightfoot* Department of Chemical Engineering, University of Wisconsin, 1415 Johnson Drive, Madison, Wisconsin 53706

Lactic acid (2-hydroxypropanoic acid) was extracted from carbon dioxide-pressurized solutions of calcium lactate using two different water-immiscible solvents: Alamine 336 (a tertiary amine) in chloroform and Amberlite LA-2 (a secondary amine) in 1-octanol. Carbon dioxide pressure ranged from 1 to 31 bar. In single-stage extractions with 0.27 M Alamine 336 in chloroform, a 1:1 feed-solvent ratio, and feed concentrations of calcium lactate of 0.05, 0.14, and 0.30 N, totallactate distribution ratios of 1.60, 0.68, and 0.38, respectively, were observed at carbon dioxide pressures of 23.2, 21.3, and 21.1 bar. In single-stage extractions with 0.30 M Amberlite LA-2 in 1-octanol, a 1:1 feed-solvent ratio, and feed concentrations of calcium lactate of 0.14 and 0.30 N, total-lactate distribution ratios of 0.92 and 0.58 were observed at carbon dioxide pressures of 12.5 and 13.2 bar. Here we use the term “total lactate” to signify lactic acid plus lactate salt plus lactate complexes in any given phase at equilibrium. There was little or no increase in distribution ratios at higher pressures. Precipitate of calcium carbonate or bicarbonate was observed in some experiments. A system temperature of 25 °C was used. Introduction Lactic acid production by fermentation of sugars has traditionally been accompanied by a stoichiometric consumption of base (e.g., calcium carbonate) in the fermenter, consumption of acid (e.g., sulfuric acid) in the lactic recovery step, and creation of waste salt (e.g., calcium sulfate) (Kirk-Othmer, 1978; Peckham, 1944; Vickroy, 1985). This is costly in terms of chemical consumption and disposal. The purpose of this work has been to investigate new processes that reduce or eliminate chemical consumption and waste generation costs in the production of lactic acid but that retain the traditional buffered fermentation. More specifically, we investigated using carbon dioxide as an acidulant for lactic acid separation from solutions of its calcium salt. The coproduct, calcium carbonate, is recyclable to the fermenter. Carbon dioxide released from the fermenter could be recovered for recycle to the acidulation step. The primary goal of the present work was to test two powerful liquid extractants, Alamine 336 (Henkel) in chloroform (Tamada and King, 1989; Tamada et al., 1990) and Amberlite LA-2 (Sigma Chemical) in 1-octanol (Tung, 1993; Tung and King, 1994), for the separation of lactic acid from aqueous solutions of calcium lactate saturated with carbon dioxide. King and co-workers have found that tertiary amines are preferred over primary and secondary amines. Tertiary amines do not form amides or other reaction products with organic acids. Additionally, the choice of cosolvent, (e.g., chloroform or octanol) is important, as significant enhancement of the extractive effect of the amine was found by these authors. Some limited experimental testing and modeling was also done with sodium lactate and with ammonium lactate. Other possible fermentation salts such as magnesium lactate were not examined. We chose to focus our efforts on the calcium system here, as the carbonate or bicarbonate is more readily precipitated than the sodium or ammonium species. 0888-5885/96/2635-1156$12.00/0

The systems consist of water, calcium lactate, lactic acid, carbon dioxide, an amine, and an organic solvent. Clearly, the number of degrees of freedom is large. A new thermodynamic model was developed to guide us in our selection of operating conditions and to help us identify experiments that would best satisfy our needs for subsequent engineering work (Kolker and de Pablo, 1995). Here we compare its predictions to our experimental data. The patent literature contains similar applications for carbon dioxide. Extraction of acetic acid into solvent from a carbon dioxide-acidified solution of acetate salt was disclosed (Yates, 1981). Recovery of lactic acid (pKa 3.86) in this way may be more difficult than recovery of acetic acid (pKa 4.76) because lactic acid is a stronger acid; it is more difficult to acidify a stronger acid with carbon dioxide. In both cases, the acidulant carbon dioxide is a weak acid (pKa 6.4). On the other hand, it might be anticipated that the stronger acid (lactic acid) might form a stronger interaction with the basic amine and thus be more readily extracted. Water-soluble amine carbonates form acid-amine salts and precipitate calcium carbonate (Urbas, 1984). These amine-lactate salts are optionally extracted into solvents. Our work differed in that we employed extractants containing water-insoluble amines. The solubilities of carbon dioxide, calcium lactate, carbonate, and bicarbonate in the aqueous phase are strongly affected by the temperature of extraction. For initial experiments we selected 25 °C as a convenient temperature for testing. Experimental Section Materials. Solutions were prepared with reagentgrade calcium lactate 4-hydrate (Fluka Chemie A.G.); 20% ammonium lactate solution (Pflautz and Bauer); HPLC-grade chloroform (Fisher Scientific); Alamine 336 (Henkel), a mixed C8-C10 straight-chain aliphatic tertiary amine; 99% grade 1-octanol (Sigma Chemical); and © 1996 American Chemical Society

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Amberlite LA-2 (Sigma Chemical), a secondary amine with one linear C12 chain and one highly branched C12C15 chain. The formula weight of Alamine 336 determined by titration was 406 (Tamada and King, 1989). The capacity of Amberlite LA-2 is stated to be 2.2-2.3 mmol/ml (Rohm and Haas). All chemicals were used as received without further purification. A Milli-Q reagent water system (Millipore) provided the water used to prepare solutions. Equipment. Aqueous calcium lactate solutions were equilibrated with extractants under carbon dioxide pressures ranging from 1 to 31 bar. Two separate equilibration cells were used: a 125 mL stainless steel sample cylinder (Whitey) and a Jerguson gauge 11-T10-SS sight glass (Jerguson gauge and valve). 125 mL Cylinder Equilibrium Cell. The 125 mL cylinder was fabricated of alloy 316 stainless steel tubing stock. The hemispherical ends each possessed 1/ in. National Pipe Thread (NPT) connections. These 2 connections were fitted with adapters for 1/16 in. o.d. tubing. The tubing led to valves for controlling carbon dioxide input and extractant discharge. Agitation of the contents was accomplished by manual shaking. Batch (single solvent-single feed) and crossflow extractions (many solvent-single feed) were performed. A Bourdon tube pressure gauge (Ashcroft) provided pressure data. The gauge was readable to 1 lb/in.2, and stated accuracy was 0.25%. The gauge was isolated upstream from the cell. Pressure readings were performed by raising the carbon dioxide pressure in the gauge to slightly above that in the cell and then opening the isolation valve and allowing the pressures to equalize. This apparatus was used for some experiments with the heavier-than-water extractant chloroform/Alamine 336. Extract withdrawal was accomplished while maintaining carbon dioxide pressure by letting down the heavy phase through the bottom valve, while leaving the light phase in place and adding makeup carbon dioxide to maintain constant pressure, as closely as possible. Withdrawing the aqueous raffinate through the bottom valve was attempted, but clogging occurred due to precipitate formation. To start the experiment, the cell was cleaned and dried. Feed and extractant liquids were added and the headspace purged with CO2. The adapters, 1/16 in. o.d. lines, and valves were then installed on the cell. Carbon dioxide was added to reach and maintain the desired pressure. The contents were shaken vigorously for a total of 5 min, 1 min at a time. Cylinder temperature was controlled by a 25 °C water bath. The liquids were allowed to settle overnight before phase withdrawal. The extract was withdrawn first, let down through the lower 1/16 in. o.d. line. Simultaneously, carbon dioxide was added to maintain pressure and prevent CO2 ebulliation. The volume of extractant recovered was less than the volume put in, due to chloroform vapor loss during letdown. Finally, the top adapter was loosened and removed, carbon dioxide pressure was released, and the raffinate was recovered. Raffinate volumes were not measurably affected by vapor loss. Lactic acid extraction results were corrected to reflect the original extractant volume. Lactate was reextracted into an aqueous phase for analysis, by contacting a portion of the extract with excess aqueous sodium hydroxide. Sampling the aqueous phase required emptying the organic phase and also was hampered by clogging from

Figure 1. Jerguson gauge equilibrium cell.

calcium carbonate precipitation. The carbon dioxide pressure during sampling could not be closely controlled. We constructed a new device to remedy these problems and to operate more efficiently. Jerguson Gauge Equilibrium Cell. The Jerguson gauge cell was fabricated of 316 stainless steel and included heavy glass viewing windows gasketed with Teflon. Three-quarter inch diameter female NPT connections were provided at each end of the cell. Tubing pass-throughs to the cell were made with holes drilled in 3/4 in. male NPT plugs. The apparatus (Figure 1) contained circulating and mixing lines and pumps and sampling ports and valves. Three tubing lines looped from and to the cell. Each line had its own pump to circulate liquid for agitation and through sampling valves. Inlets to the lines were positioned in the vessel at different heights to sample either the heavy phase or the light phase. The agitation line was constructed of 1/4 in. o.d. tubing; sample lines were mostly constructed of 1/8 in. o.d. tubing. At sampling valves, 1/16 in. o.d. tubing was used. Clogging of sampling valves was prevented with in-line 40 µm screen filters. Bypasses around the sampling valves, of 1/8 in. o.d. tubing, permitted fast turnover of sample circuit contents with the cell. Valves were provided to close the bypasses when desired. The bypassed volume was 1 mL. The cell itself contained 240 mL, and the total system volume was 310 mL. The pumps were constant-volume, magnetically coupled piston-type, shown in Figure 2. The pump case was stainless steel tubing with an adequate pressure rating. End fittings were Swagelok reducing unions. Piston and keeper were machined Teflon. The internal magnet was a Teflon-covered stir bar machined to proper size. Similar pumps are described in the literature (Ziger and Eckert, 1982; Drake et al., 1990); these have costly materials or/and require specialized machining, though they are suited for much higher pressures. Our materials cost was $150 per pump, including the drive. Machining of Teflon was required; otherwise, no special equipment was needed for their construction. Sampling valves were six-port HPLC type. Sample loops were 14-cm lengths of 1/16 in. o.d. tubing with a nominal inside diameter of 0.6 mm. Sample loop volumes were measured before installation by weighing the mass of contained water and after installation by sampling a lactic acid solution of known concentration and measuring the quantity of acid eluted. Temperature-control precision was 0.1 °C. Pressure readings were taken with the same method and equipment as used with the 125 mL cylinder.

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Figure 2. Details of the magnetic pump. The external magnet and drive are not shown.

Loading and Equilibrating. Before loading the solutions, the cell was cleaned of residue from the previous experiment. First, the cell and all lines were repeatedly rinsed with water, then rinsed once with dilute hydrochloric acid (to dissolve any traces of precipitate) which was let stand for several hours, and rinsed again repeatedly with water. Then several rinsings with isopropyl alcohol were made, to dissolve any remaining water and extractant. Finally, compressed air was blown through all lines until no isopropyl alcohol odor could be detected in the effluent gas. The cell was then purged with carbon dioxide. The fluids to be loaded were placed in a 125 mL bomb, and the headspace in the bomb was flooded with carbon dioxide. The fluids were forced with carbon dioxide pressure from the bomb into the cell. First the heavy phase was loaded and then the light phase. The liquids were not deaerated before loading. A positive error of about 0.25 bar pressure may result. After loading the liquids, carbon dioxide was added. The mixing pump agitated the contents; it was stopped, and the phases were allowed to separate; then the sampling pumps exchanged the cell contents with the sampling line contents. This sequence, mixing, settling, and exchange with sampling legs, took 1.5, 0.5, and 1 min, respectively; it was repeated for 8 h, or 160 sequence cycles. The mixing action was vigorous, and the sampling line pumps moved 5 line volumes/cycle. Carbon dioxide was periodically added to maintain pressure. Equilibrium was signaled by pressure remaining unchanged for 4 h. Sampling valve bypasses were closed after 4 h briefly and closed again before sampling. Sampling. A single feed and extractant were reequilibrated and sampled at seven or eight successively higher pressures. A total of 1% of the liquid volume was withdrawn as the sample. Nearly equal volumes of each phase were withdrawn. The effect of prior sampling on mass balance was neglected.

Four samples of raffinate and three of extract were taken at each pressure. (three raffinate and three extract for the 0.025 M feed with chloroform/Alamine 336 and for all experiments with 1-octanol/Amberlite LA-2). Twenty minutes before beginning the sampling (at least 8 h for experiments with 1-octanol/Amberlite LA-2), the mixing pump was stopped, and the sampling line pumps were operated continuously. Bypass valves were left open 10 min more (all but the last 30 min for 1-octanol/Amberlite LA-2) and then were closed. The sample loops were swept with fluid for 10 min (30 min for 1-octanol/Amberlite LA-2) before the first sampling. Subsequent samplings were preceded by 1 min of bypass and 1 min of sweep (5 min and 15 min or more, respectively, for 1-octanol/Amberlite LA-2). Sample volumes were 39 and 52 µL, for raffinate and extract samples, respectively (52 and 39 µL for 1-octanol/ Amberlite LA-2). Samples were swept with deionized water, chloroform, or 1-octanol as appropriate, into glass vials with Teflonlined closures. The purging fluid diluted the sample. Chloroform-based extracts were treated by allowing the chloroform to evaporate and adding 12 drops of 1-decanol to redissolve the Alamine 336. Extracts were shaken with 1 mL of 0.010 N aqueous sodium hydroxide to reextract the lactic acid. Dilution factors were calculated with gravimetric data and were about 20fold for extracts and 30-fold for raffinates (30-fold and 20-fold for 1-octanol/Amberlite LA-2). At each pressure, the organic- and aqueous-phase menisci levels were measured to within 1 mm. Analysis for Lactate. The samples were analyzed for lactate by high-performance liquid chromatography (HPLC). A Bio-Rad HPX-87H acid-analysis column was used as recommended, except for carrier flow rate (0.5 mL/min). The HPLC sample volume was 20 µL. An ultraviolet absorbance spectrometer was used at 210 nm wavelength, with an 11 µL, 10 mm flow cell. The solvent peak eluted at 7 min; the lactic acid peak eluted at 14 min. A 10-in.-wide strip chart, 1 in./min, recorded the signal. Spectrometer gain was varied between experiments and was 0.02, 0.05, and 0.10 absorbance full-scale respectively for experiments with starting calcium lactate concentrations 0.025, 0.07, and 0.15 mol/ L. Quantitation was by peak height. Duplicate injections were made, and the peak heights were averaged. Peak heights differed by less than 2%. (Single injections were made for experiments with octanol/LA-2.) Standards were run before and after the analysis of samples and after each 10-15 samples. Calibration linearity varied less than 1%. The calcium lactate feed solution was analyzed to provide a basis for the mass balance. Lactate mass recoveries closed within 5%. The reported distribution ratios were corrected to reflect original, carbon dioxide-free phase volumes. Phase volume change data were obtained as described in the following. Phase Volume Change. As pressure increased, the solvent-phase meniscus rose, indicating a volume increase. The volume change was calculated using meniscus height differences relative to the CO2-free meniscus heights. In the chloroform/Alamine 336 experiments, the aqueous phase volume did not change appreciably, and volume increase of the organic phase was due to absorption of carbon dioxide. Changes in meniscus height were multiplied by the Jerguson gauge volume per unit height to calculate volume change.

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With 1-octanol/Amberlite LA-2, aqueous volume decreased as the solvent volume increased, indicating either coextraction of water and/or holdup of the aqueous phase in the solvent sampling loop. Observation of Calcium Carbonate Precipitate. The Jerguson gauge windows permitted observation to ascertain the presence or absence of precipitates. Precipitates are likely to be either calcium carbonate or calcium bicarbonate. The observed precipitates were very fine in texture and stayed in the aqueous phase. When allowed to settle, some precipitate remained at the interfacial boundary. Analysis for Calcium. Calcium coextraction into the extractant chloroform/Alamine 336 (0.27 M) was tested. The 125 mL cylinder was filled with 30 mL of the extractant mixture and 30 mL of a 0.30 N calcium lactate solution. The headspace was purged with carbon dioxide, and carbon dioxide was added through a 1/16 in. o.d. line connected to the top inlet. The contents were shaken vigorously for 10 min total, while pressure was maintained at 23 bar. The contents were allowed to settle for 2 days, and then samples of the extractant phase were withdrawn from the bottom of the cylinder through a 1/16 in. o.d. line and needle valve. Two milliliters of extractant was purged and discarded, and then two separate 5 mL samples were taken. Each sample was vigorously shaken for 1 min with 10 mL of 3% v/v hydrochloric acid in deionized water. The extractant phase was discarded, and the water backextracts were analyzed for calcium by atomic absorption spectrometry. This method relies on a small distribution ratio of calcium salt into the solvent phase. The evidence from other works (Tamada and King, 1989; Tung, 1993), which find good rejection of ionic species, supports this assumption. Calcium in 1-octanol/Amberlite LA-2 (0.30 M) was similarly tested for several carbon dioxide pressures. Analysis for Water. An automated Karl-Fischer titrator measured the water content of extracts. Extract samples for water content measurement were obtained similarly to samples for calcium analysis. Kinetics of Extraction and of Solids Formation. Kinetic data are particularly important for a process involving solids formation. No specific kinetic data were collected. For solids formation, seeding and the degree of supersaturation affect the rate of crystal formation and the size distribution. Solubility of Chloroform and Alamine in the Aqueous Phase. The solubility of chloroform in water at 25 °C is reported to be 0.7-1.0 wt % (Stephen and Stephen, 1963, p 370). In a real system this would represent a significant loss of solvent into the aqueous phase. This solvent would be recovered by steam stripping. Further study of alternate cosolvents which have Lewis acid functionality but no high water solubility of the chloroform is required. One potential candidate to replace the chloroform is octanol; another might be a substituted phenol such as nonylphenol. We did not measure the losses of chloroform into the raffinate phase in our experiments. The solubility of Alamine 336 in water is around 5 ppm (Henkel, 1991). This represents a very low level of solvent loss. This amine might be readily recovered by ion exchange of the spent raffinate stream. We did not measure the losses of amine into the raffinate phase in our experiments.

Figure 3. Total lactate distribution ratio variation with pressure. Single-stage extractions from three calcium lactate solutions into equal volumes of chloroform/Alamine 336 (0.27 M).

Formation of a Second Organic Phase. Aminebased extractants sometimes split into two organic phases, particularly at lower temperatures. We did not observe such a split in our Jergeson gauge system. However, we cannot rule out completely the existence of two phases as a small volume of second phase could have existed below the lower window of the gauge. Measurement of Carbon Dioxide Quantity in Extract. The volume of gas evolved from the extractant chloroform/Alamine 336 (0.27 M) was measured. The extractant was brought into contact with 20 bar carbon dioxide using the Jerguson gauge equilibrium cell. The carbon dioxide-rich extractant was sampled using the heavy phase sampling valve. The contents of the sample loop was released into an expansion chamber filled with CO2-saturated water. The volume change was measured as the water level change in a graduated burette connected with tubing to the bottom of the expansion chamber. Approximate calculations show that chloroform vapor is one-quarter the quantity of vapor evolved; this correction was applied. Gas evolution from 1-octanol/Amberlite LA-2 (0.30 M) was similarly measured. The vapor pressure of 1-octanol is relatively low, so all the evolved gas is presumed to be carbon dioxide. Results and Discussion Chloroform/Alamine 336 Extractions. Chloroform/Alamine 336 Single-Stage Extractions from Aqueous Calcium Lactate. Lactic Acid Extraction. Three pressure series of equilibria were taken, with different feed concentrations of calcium lactate (0.025, 0.070, and 0.150 M). Extractant composition was 0.27 M (108 g/L) Alamine 336 in chloroform (13% by volume in total volume (v/v)). The Jerguson gauge apparatus was used. The feed-solvent ratio was 1:1. Temperature was maintained at 25 °C. Seven or eight pressures from 2 to 31 bar were used for each experiment. Figure 3 shows distribution ratio results vs carbon dioxide pressure for the three series. The results are adjusted to reflect the original, carbon dioxide-free volume of extractant; thus at higher pressures the plotted distribution ratios are higher than the raw distribution ratios. The distribution ratios reached an asymptotic value with increasing pressure above 20 bar carbon dioxide. Figure 4 shows lactate distribution ratios versus pressure for extractions with a 2:1 feed-solvent ratio

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Figure 4. Total lactate distribution ratio variation with pressure. Two different feed-solvent ratios. Single-stage extractions from 0.07 M calcium lactate into chloroform/Alamine 336 (0.27 M).

Figure 5. Extractant phase volume variation with pressure. The data are average for all experiments.

(conducted using the 125 mL cylinder) and for extractions with a 1:1 feed-solvent ratio (obtained with the Jerguson gauge). Feed is 0.070 M calcium lactate; solvent is chloroform/Alamine 336 (0.27 M). Phase Volume Change. Figure 5 presents a plot of extractant phase volume vs carbon dioxide pressure for two solvent systems. The chloroform/Alamine 336 (0.27 M) phase volume increased markedly at pressures above 10 bar because of condensation of carbon dioxide into the solvent; it expanded 11% at 21 bar and 32% at 31 bar. The raffinate phase volume did not change. Calcium Carbonate Precipitation. In the 0.025 mol/L feed experiment, at pressures 7.5 bar and less, a very small amount of fine precipitate was seen in the aqueous phase. The precipitate settled within a day of stopping agitation, and it seemed to disappear onto or into the organic phase. No precipitate was seen with feed concentrations higher than 0.025 mol/L or at pressures above 7.5 bar. At higher feed concentrations, extractant loading is increased, and there is more free acidity in the aqueous phase. This favors larger bicarbonate concentration, smaller carbonate concentration, and higher calcium ion solubility. At carbon dioxide pressures above 15 bar, calcium bicarbonate is the thermodynamically-favored precipitate (Seidell, 1940). Carbon Dioxide Content. The Alamine 336/chloroform extractant produced 126 gas volumes/volume of extractant let down from 21.6 to 1 bar. About onequarter of that is chloroform vapor; the rest is carbon dioxide. Carbon dioxide concentration in the extractant

Figure 6. Total lactate x-y diagram for crossflow extraction from 1 N ammonium lactate into successive equal-volume portions of chloroform/Alamine 336 (1 M). Carbon dioxide pressure was 27 bar.

is about 3.2 M at 21 bar (vs 1 bar) calculated either using liquid CO2 density with phase volume expansion data or the ideal gas law and gas evolution data. By gas evolution data alone, carbon dioxide content is 0.7 M at 3 bar (vs 1 bar). As an estimate of the carbon dioxide solubility in the extractant at 1 bar, the carbon dioxide solubility in chloroform is 0.16 M (Seidell, 1940). Gas evolution from the raffinate was not measured. Water Coextraction. The water content of the letdown chloroform/Alamine 336 was 0.2% on a weight for total weight basis (w/w). Water loss into the vapor at letdown should be negligible in comparison to the quantity of water found. Calcium Coextraction. The concentration of calcium in the chloroform/Alamine 336 extractant phase (conditions: 20 bar CO2, 0.14 N lactate feed, 1:1 liquid ratio) was 1.5 × 10-5 N, corresponding to a distribution ratio of 5 × 10-5. Chloroform/Alamine 336 Serial Extraction from Ammonium Lactate. A crossflow extraction into four separate portions of chloroform/Alamine 336 (1 M) from one portion of ammonium lactate (1 M) was performed with the 125 mL cylinder. (This experiment was not described above.) The extraction efficiency diminished in succeeding stages. This is caused by increasing pH of the raffinate phase due to an increasing ratio of total ammonium to total lactate in the raffinate phase. The results predict a concave equilibrium line which is unfavorable for counterflow extraction. Figure 6 shows extractant lactic acid and raffinate lactate concentrations for the crossflow extraction. Pressure was constant at 27 bar. 1-Octanol/Amberlite LA-2 Extractions. 1-Octanol/Amberlite LA-2 Single-Stage Extractions from Aqueous Calcium Lactate. Lactic Acid Extraction. Two pressure series of equilibria were taken, with calcium lactate concentrations 0.070 and 0.150 M. The extractant composition was 0.30 M (113 g/L) Amberlite LA-2 in 1-octanol (14% v/v). The solvent-feed ratio was 1:1. The Jerguson gauge apparatus was used for both experiments. Seven pressures from 1.2 to 31 bar were used. Temperature was maintained at 25 °C. Figure 7 shows lactate distribution ratios vs pressure for the 1-octanol/Amberlite LA-2 experiments. Again, results are corrected for phase volume change, and distribution ratios reached an asymptotic value at moderate carbon dioxide pressures. Because of the possible error in the phase volume determination (explained in the next section), the volume-corrected 1-oc-

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Figure 7. Total lactate distribution ratio variation with pressure. Single-stage extractions from calcium lactate solutions into equal volumes of 1-octanol/Amberlite LA-2 (0.30 M).

tanol/Amberlite LA-2 distribution ratio results may be in error by 0% to +15%. One possible reason for the plateau or even a slight decrease in the distribution ratio at higher pressures includes the possible conversion of water-insoluble calcium carbonate into more soluble calcium bicarbonate, which reduces the driving force for shifting the reaction forward. Another is that the higher carbon dioxide content in the organic phase counteracts the enhancing effects of the polar solvents chloroform or octanol. Phase Volume Change. The 1-octanol/Amberlite LA-2 extract phase volumes increased less dramatically at high pressures than did chloroform/Alamine 336. However, unlike with chloroform/Alamine 336, the volume increase was substantial at lower pressures. The extract volume increase was accompanied by a raffinate volume decrease, indicating water uptake by the extract of about 10% by volume. However, Karl-Fischer water measurements on the solvent phase showed a water content of about 3% by volume. Water phase holdup in the extract sample pump or filter, if it occurred, resulted in erroneous volume measurements. An error on the order of 5% is indicated. Calcium Carbonate Precipitation. A substantialappearing quantity of calcium carbonate precipitate was present in the 0.070 M calcium lactate feed experiment, but only at sampling pressures of 7.22 bar and less. There was a small but still visible quantity of precipitate in the 0.070 M experiment at sampling pressures of 12.52 bar and greater. No precipitate was seen in the 0.150 M feed experiment. Carbon Dioxide Content. The 1-octanol/Amberlite LA-2 extractant produced 40 gas volumes/volume of extractant let down from 21.6 to 1 bar, which corresponds to a carbon dioxide content of 1.3 mol/L. Solvent vapor pressure is small, so all the vapor was assumed to be carbon dioxide. Water Coextraction. The water content of 1-octanol/Amberlite LA-2 was measured for carbon dioxide pressures 2.2, 4.1, and 14.3 bar. Three samples at each pressure were taken. Water content results ranged between 1.3% w/w and 3.3% w/w, with the sample variance at each pressure obliterating any trends. The mean water contents at the three pressures were 2.1, 2.2, and 2.5% w/w, respectively. These are in accord with the water solubility in pure 1-octanol, 3.5% w/w (Sorenson and Arlt, 1979), considering the 14% v/v

content of hydrophobic amine. The amine-lactate complex may also act to coextract water. Calcium Coextraction. Calcium concentrations in the extracts (0.30 N lactate feed, 1:1 liquid ratio) were 3 × 10-3 N for three different pressures, corresponding to a distribution ratio of 1 × 10-2. The detection limit of the analytical procedure used was about 1 × 10-3 N. The calcium coextraction noted could also have been due to insufficient phase separation. A dispersion level of 0.01% of the aqueous phase in the organic phase would result in a similar measurement for calcium coextraction. 1-Octanol/Amberlite LA-2 Serial Extraction from Saturated Calcium Lactate. We performed a serial extraction from saturated aqueous calcium lactate in the presence of 1 bar carbon dioxide. Successive portions of 1-octanol/Amberlite LA-2 (0.30 M) each extracted 0.10 M lactic acid, until calcium lactate solid was entirely dissolved and lactate concentration decreased in the aqueous phase. A precipitate of calcium carbonate formed in the aqueous phase simultaneously as the calcium lactate dissolved. Conclusions The 1-octanol/Amberlite LA-2 mixture was clearly a stronger extractant than chloroform/Alamine 336 at carbon dioxide pressures below 5 bar and was at least as strong at higher pressures. About 13 bar pressure gave near-maximum extraction with 1-octanol/Amberlite LA-2. We showed that 1-octanol/Amberlite LA-2 is capable of extracting lactic acid from aqueous solutions of calcium lactate in the presence of carbon dioxide at pressures as low as 1 bar and of simultaneously precipitating calcium carbonate. Because of its low vapor pressure and efficacy even with 1 bar carbon dioxide, this extractant allows atmospheric pressure equipment to be used. However, the capacity of the extractant is low in comparison to most industrial solvent extraction systems. This can be increased by using a higher concentration of amine, by adjusting the reaction temperature, possibly by choosing different cosolvents, or by using a lower molecular weight amine. Free lactic acid can be recovered from the aminelactic acid complex in several ways reported in the literature. Acknowledgment We gratefully acknowledge the financial support of the State of Wisconsin through the University-Industry Relations office. The electrical and machine shop assistance of Al Hanson, John Cannon, and Al Bondioli is appreciated. Literature Cited Drake, B. D.; Dunbar, M. T.; Smith, R. L. Rev. Sci. Instrum. 1990, 61, 2474-2475. Henkel Corp., Alamine 336 Technical Bulletin; Henkel Corp.: Tucson, AZ, 1991. Hydroxycarboxylic Acids. Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed.; John Wiley & Sons: New York, 1978; Vol. 13, pp 80-90. Kolker, A.; de Pablo, J. J. Ind. Eng. Chem. Res. 1996, 35, 228, 234. Peckham, G. T., Jr. Chem. Eng. News 1944, 22, 440-443, 469. Rohm & Haas Co. Amberlite/Duolite Ion Exchange Resins: Technical Notes; Rohm & Haas: Philadelphia, 1981.

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Seidell, A., Ed. Solubilities of Inorganic and Metal-Organic Compounds; Van Nostrand: New York, 1940. Sorenson, J. M., Arlt, W., Eds. Liquid-Liquid Equilibrium Data Collection: Binary Systems; Chemistry Data Series; DECHEMA: Frankfurt/Main, 1979; Vol. V, Part I. Stephen, H.; Stephen, T. Solubilities of Inorganic and Organic Compounds; MacMillan: New York, 1963; Vol. 1. Tamada, J. A.; King, C. J. Extraction of Carboxylic Acids by Amine Extractants. Report Number LBL-25571; Applied Science Division, Lawrence Berkeley Laboratory: Berkeley, CA, 1989. Tamada, J. A.; Kertes, A. S.; King, C. J. Ind. Eng. Chem. Res. 1990, 29, 1319-1345. Tung, L. A. Recovery of Carboxylic Acids at pH greater than pKa. Report Number LBL-34669; Energy and Environment Division, Lawrence Berkeley Laboratory: Berkeley, CA, 1993. Tung, L. A.; King, C. J. Ind. Eng. Chem. Res. 1994, 33, 32173223.

Urbas, B. U.S. Patent 4,444,881, 1984. Vickroy, T. B. In Comprehensive Biotechnology; Moo Young, M., Ed.; Pergamon: New York, 1985; Vol. 3, pp 761-776. Yates, R. A. U.S. Patent 4,282,323, 1981. Ziger, D. H.; Eckert, C. A. Rev. Sci. Instrum. 1982, 53, 1296-1297.

Received for review June 26, 1995 Revised manuscript received February 6, 1996 Accepted February 14, 1996X IE950394G

X Abstract published in Advance ACS Abstracts, March 15, 1996.