Ind. Eng. Chem. Res. 1991,30, 923-929
923
Regeneration of Carboxylic Acid-Amine Extracts by Back-Extraction with an Aqueous Solution of a Volatile Amine Loree J. Poole and C. Judson King* Department of Chemical Engineering and Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720
Tertiary amines are effective extractants for the recovery of carboxylic acids from aqueous solution. An approach for regeneration and product recovery from such extracts is to back-extract the carboxylic acid with a water-soluble, volatile tertiary amine, such as trimethylamine. T h e resulting trimethylammonium carboxylate solution can be concentrated and thermally decomposed, yielding the product acid and the volatile amine for recycle. Equilibrium data are presented that show near-stoichiometric recovery of carboxylic acids from amine extracts. For fumaric and succinic acids, partial evaporation of the aqueous back-extract decomposes the carboxylate and yields the acid product in crystalline form. The decomposition of aqueous solutions of trimethylammonium lactates stops short of forming the acid product, because of its greater water solubility and tendency for self-association. Methods of further purifying the crystals are evaluated, along with processing implications of the results.
Introduction There are numerous current and potential industrial applications where it is desirable to recover carboxylic acids from aqueous solutions. Examples include the production of citric acid and other acids by fermentation (Lockwood, 1979; Busche, 1985)and removal and recovery of carboxylic acids from aqueous waste streams. For volatile carboxylic acids, such as acetic acid, distillation and azeotropic or extractive distillation are alternatives, along with solvent extraction and adsorption (King, 1983; Kuo et al., 1987). For low-volatility carboxylic acids, e.g., dicarboxylic acids and hydroxycarboxylic acids, distillative processes are expensive and cannot isolate the desired acid. For acids such as citric and lactic, the classical approach for recovery from a fermentation broth has been to add calcium hydroxide to form the calcium salt of the carboxylic acid, to which an acid such as sulfuric acid is added to liberate the free carboxylic acid. This approach consumes chemicals (e.g., lime and sulfuric acid) and produces a waste salt stream. Consequently, such methods are falling out of favor. Solvent extraction is often effective for recovery of these low-volatility carboxylic acids from aqueous solution. Reactive, basic extractants, e.g., tertiary amines or phosphine oxides, can be used to gain greater solvent capacity and selectivity with respect to water and other species. A process developed by Miles, Inc. (Baniel et al., 1981), for recovery of citric acid from fermentation solution uses a solvent composed of a tertiary amine extractant in a hydrocarbon diluent with an alcohol modifier, regenerated by back-extraction into water at a higher temperature. Back-extraction following a shift in diluent composition, achieved, e.g., by distillation, is another possibility for regeneration and can be combined with a swing of temperature (Tamada and King, 1990). The overall degree of concentration relative to the feed that can be achieved by these methods is limited by the extent to which the distribution equilibrium for the carboxylic acid can be changed between forward extraction and back-extraction. The purpose of the present work was to investigate a form of regeneration of amine-carboxylic acid extracts wherein the acid is back-extracted into a solution of an aqueous base. In order to avoid consumption of chemicals and creation of a salt byproduct, the aqueous base is
* To whom correspondence should be addressed.
volatile, thereby enabling thermal decomposition of the acid-base complex (salt) in the aqueous back-extract. The decomposition forms the carboxylic acid as product and the free base as a vapor that can be reabsorbed in water and recycled for reuse in the back-extraction. This concept extends a related proposal made by Urbas (1983,1984), wherein alkylammonium carboxylates are decomposed in either an aqueous fermentation solution or an organic extract. The most obvious water-soluble, volatile base is ammonia. However, ammonia and both primary and secondary amines form amides when they are heated in mixtures with carboxylic acids (Streitwieser and Heathcock, 1976; Mitchell and Reid, 1931; Poole and King, 1990). The amides are sufficiently stable so that it is difficult to reverse the process and form the carboxylic acid and the amine again from the amide. Hence, we have employed a volatile, water-soluble tertiary amine. Figure 1shows a flow sheet for a process in which a high molecular weight, organic-soluble amine (e.g., a trioctyland/or tridecylamine) in an appropriate organic diluent is used as the forward extractant and an aqueous solution of a low molecular weight amine is used for back-extraction. The trialkylammonium carboxylate in the aqueous back-extract is concentrated and decomposed in an evaporative crystallizer. Although several different low molecular weight trialkylamines could be used for the back-extraction, trimethylamine (TMA) was chosen for investigation. Pure TMA boils at 3 'C and has a pKb of 4.1 (Weast, 1974). It is commercially available as a 25 wt 9% aqueous solution. Thermal decomposition (loss of a methyl group) occurs above 350' (Kaufman, 1962; Jones and Gesser, 1972). The conditions under which oxidation of TMA can occur were investigated by Jones and Gesser (1972) and Cullis and Waddington (1958),with results that indicate that partial oxidation occurs in the presence of oxygen at temperatures of 150-200 "C and above. The acids examined in this work are lactic acid, succinic acid, and fumaric acid, chosen because they are nonvolatile and have strong potential for being produced commercially by fermentation (Sato et al., 1972; Lipinsky and Sinclair, 1986; Lockwood, 1979). The organic amine extractant was Alamine 336 (Henkel Corp.), a commercial, mixed C8-C,, alkyl-chain tertiary amine. The diluent used with the extractant was methyl isobutyl ketone (MiBK), chosen because of favorable equilibria, substantial equilibrium
0888-5885f 91f 2630-0923$02.50f 0 0 1991 American Chemical Society
924 Ind. Eng. Chem. Res., Vol. 30, No. 5, 1991 i(1l l B 0 u 5
High ?JW Amine
+ Dilurnt
'I
Figure 1. Recovery of a carboxylic acid by extraction, with regeneration by back-extractioninto an aqueous solution of a volatile base. Table I. Sources and Description of Chemicals Used compd supplier description Alamine 336 Henkel Corp. tertiary amine, C8-ClO straight chain fumaric acid Aldrich, Inc. 99% purity (crystalline) lactic acid Mallinckrodt, Inc. 85 wt % aq soln MiBK Aldrich, Inc. 99.7% purity succinic acid Mallinckrodt, Inc. analytical grade (granular) TMA Aldrich, Inc. 25 w t % aq soln, (manuf by adding 99% anhyd TMA to H@)
data, and interpretation thereof for forward extraction of the three acids of interest (Tamada et al., 1990). In order to confirm, gain insight into, and assess this method of regeneration, experiments were carried out in three areas: 1. Equilibria were measured and interpreted for backextraction of the acids from the Alamine 336/MiBK extract into aqueous solutions of trimethylamine. 2. Aqueous solutions of trimethylamine and the trimethylammonium carboxylates were heated to vaporize water and trimethylamine and thereby to concentrate and thermally decompose the trimethylammonium carboxylates. 3. Various methods were investigated for washing and purifying the crystalline fumaric and succinic acid products formed in the heating and decomposition steps.
Apparatus and Procedures Reagents. The reagents used and their sources are given in Table I. Distilled water that had been passed through a Milli-Q purification system (Millipore Corp.) was used to dilute the lactic acid and trimethylamine solutions when necessary and to make aqueous solutions of succinic and fumaric acids. The 25 w t % aqueous TMA solution was stored in the original 3.5-L glass container at 4 "C to prevent volatilization of the TMA. Before each use, the aqueous TMA solution was brought to room temperature, and the TMA concentration was measured by adding an excess of 6 N H2N04to a 5-mL sample and then back titrating with 0.1 N NaOH, using methyl red as an indicator. Equipment. High-performance liquid chromatography (HPLC) analysis was carried out by using a SpectraPhysics SP 8000B chromatograph and a differential re-
fractometer detector (Waters Model R401). Either a C18 Radial-Pak p-Bondapak column (Waters Associates) contained in a radial compression module (Waters, RCM-100) or an organic acid column (Biorad AMINEX ion exclusion HPX-87H) was used. The mobile phase was 0.01 N H8O4. Gas chromatography (GC) analysis was carried out by using a Varian Model 3700 chromatograph with a flame ionization detector and a Hewlett-Packard 3390A integrator. Samples (1 pL) were injected into a 3.2 mm X 152 cm stainless steel column packed with 5 % silicone OV-17 on acid-washed, DMCS-treated, 80/ 100-mesh Chromosorb W (Alltech Associates). The injector temperature was 290 "C. The column temperature was held at 35 "C for 2 min and then increased to 240 "C at a rate of 10 "C/min. Nitrogen was used as the carrier gas. Aqueous-phase pH values were determined by using an Orion 701A pH meter equipped with an Orion semimicro Ross pH electrode. Flasks containing two-phase extraction systems were placed in a shaker bath-either a Fisher Scientific Versa-Bath S or a Lab-Line Orbit water bath shaker. Aqueous Lactic Acid Solutions. Lactic acid forms intermolecular esters in concentrated aqueous solution (Holten, 1971). Aqueous lactic acid solutions (2.0 M) were made by dilution of the 85 wt % aqueous solution. The resulting solution was boiled with total reflux for 12 h. Complete hydrolysis of the esters was confirmed by HPLC analysis using both the organic acid and the C18columns. Alamine 336 Extracts of Carboxylic Acids. Equal volumes of an aqueous solution of the acid were contacted with a solution of 0.3 M Alamine 336 in MiBK. For lactic acid, the two phases were contacted in a separatory funnel and then transferred to Erlenmeyer flasks that were placed in a 25 "C shaker bath, set at approximately 40 oscillations/min, for 2 days to allow the phases to settle. Vigorous shaking in a separatory funnel of an aqueous solution of succinic acid with the organic amine solution was found to result in emulsion formation; therefore, for both succinic acid and fumaric acid, the two phases were placed directly in Erlenmeyer flasks. The flasks were put in a 25 "C shaker bath set at approximately 80 oscillations/min for 2-3 days. In the case of fumaric acid, solid crystals were added with the initially saturated aqueous phase, in order to obtain sufficiently high final concentrations of fumaric acid. After the contacting, each phase was removed by pipet. The pH of the aqueous phase was measured. The amount of acid remaining in the aqueous phase was determined by titration with aqueous NaOH, using phenolphthalein as an indicator. The concentration of acid in the organic phase was determined by two-phase titration with aqueous NaOH, again using phenolphthalein as an indicator. A blank titration of 0.3 M Alamine 336 in MiBK was also performed. Material balances closed within 1-2.5%. Back-Extraction Experiments. Back-extractions were carried out at 25 "C in a separatory funnel with an organic to aqueous phase ratio of 813 (v/v). The two phases were then transferred to Erlenmeyer flasks. The flasks were placed at 25 "C in a shaker bath for 2 days to allow the phases to settle. The phases were separated by pipet. GC analysis of the organic phase was used to determine the extent to which TMA had been transferred to the organic phase. For the aqueous phase, the pH was measured, and the concentration of acid was determined by HPLC analysis using the C18column. The concentration of acid remaining in the organic phase was determined by HPLC analysis of an aqueous NaOH
Ind. Eng. Chem. Res., Vol. 30, No. 5, 1991 925
collection cvlinder
water. A sample of the final absorber solution was titrated with 0.01 N or 0.1 N NaOH. The moles of TMA and the volume of water collected versus time and the nitrogen flow rate were used to determine the partial pressures of water vapor and TMA as functions of both temperature and the concentrations of salt and free acid in solution. These values may be close to equilibrium, but no special efforts were made to ensure that. Washing and Purification of Acid Crystals. After an experimental run in which a trimethylammonium carboxylate salt was concentrated and thermally decomposed as described above, there remained behind a light yellow mixture of carboxylic acid and residual water and TMA (probably in the form of the trimethylammonium carboxylate). In the cases of fumaric and succinic acids, crystals of the acid were present. Various techniques were used to wash and purify the crystals, as follows: 1. The mixture was held at 120 "C under vacuum for 6 h, in an attempt to drive off the remaining water and TMA from the mixture. This treatment resulted in solidification of the viscous liquid containing the acid crystals. An elemental analysis of this material was carried out. The amount of residual TMA determined from the nitrogen analysis was compared with the difference between the amount of TMA originally present in the aqueous solution and the amounts of TMA collected in the absorber and water collection flasks. This method did not serve to purify the product acids effectively. 2. The crystal mass, with adhering residual solution, was washed with various organic solvents. The mixture was transferred to a Buchner funnel containing Whatman No. 50 filter paper. The funnel was placed on top of a filter flask. The mixture was washed with solvent until the crystals were white in color. The crystals were then dried, weighed, and subjected to elemental analysis. The disadvantage of this method was that the acids were significantly soluble in the solvents. Some of the acid dissolved into the solvent and was washed through the filter paper. Therefore, the solvent was evaporated under vacuum from the solvent/acid mixture present in the filter flask. Then the acid crystals were scraped off the bottom of the flask, placed on the filter paper, and washed to obtain a subsequent batch of purified crystals. This process was repeated up to six times, generating six successive batches of crystals, each batch containing between 5 and 30 w t % of the total acid, in order to obtain at least 70 wt % recovery of the acid in purified crystalline form. 3. The residue mixture was first dissolved in acetone and then transferred to the closed filter flask, whereupon the acetone was removed by evaporation and suction of the vapor through the vacuum line. The residue was dissolved in approximately 200 mL of MiBK. The closed filter flask, agitated with a magnetic stir bar, was heated to 80 "C to evaporate the MiBK through the vacuum line. The crystals that remained were apparently dry and were collected and subjected to elemental analysis. Elemental Analysis. C, H,and N contents were measured with a Perkin-Elmer Model 240 elemental analyzer, in which samples were burned with an excess of oxygen to form COPfrom carbon, H20 from hydrogen, and N2 from nitrogen. The concentrations of these compounds were determined by using a thermal conductivity detector.
rf,+fi-Gl ".CY"",
r*~"l.Lo,r
,rap i.,.c.ton*
"&CY""'
dry
VBCUYm
L"
pYmp
Figure 2. Apparatus for concentration and thermal decomposition experiments.
extract of the organic phase, using the C18column. The NaOH extract was obtained by contacting a 5-mL sample of the organic phase with an excess of aqueous NaOH in a centrifuge tube. The two phases were mixed with a magnetic stir bar and then centrifuged for 30 min at 2000 rpm to separate the phases. Thermal Decomposition Experiments. Aqueous solutions of trimethylammonium carboxylates were heated under nitrogen to prevent possible oxidation of the TMA. The apparatus used in these experiments is shown in Figure 2. The aqueous solution (60 mL) was placed in a threeneck, 100-mL, round-bottom flask operated under vacuum and equipped with a magnetic stir bar and a heating mantle with adjustable power input. Water driven off from the solution was condensed and collected in a graduated cylinder. Most of the TMA evolved was collected in an absorber flask containing dilute H2S04. At the beginning of an experimental run, the absorber flask was filled with 400 mL of H2S04concentrated enough to neutralize 10% of the TMA initially present in the aqueous trimethylammonium carboxylate solution. Methyl red was added to the solution to serve as an indicator. When 10% of the TMA had been absorbed, the methyl red indicator changed in color from red to clear. The time was recorded, and then 5 mL of concentrated H2S04(80 times the concentration of the initial absorber solution) was injected into the absorber flask, causing the solution to return to red in color. This process was repeated throughout the experimental run. Nitrogen was bubbled through the aqueous trimethylammonium carboxylate solution. The nitrogen flow rate was measured with a rotameter encased in a plastic shield (Omega Engineering, Inc., Model FL-111). To keep the system under vacuum, a General Electric '/,-hp vacuum pump and a vacuum regulator (Weiss) were used. The volume of water collected and the nitrogen flow rate were recorded as functions of time. The pressure was monitored with mercury manometers located after the rotameter and after the water-collection cylinder. The temperature of the aqueous trimethylammonium carboxylate solution was noted throughout the run and was allowed to increase gradually, reflecting the increase in boiling temperature of the solution, until a point was reached when most of the water had been removed. At this point, a sharp increase in temperature was observed. In order to prevent thermal decomposition of the acid itself, the power input to the heating mantle was adjusted as needed to keep the temperature below 130 "C. A t the end of the run, a sample of the water collected was titrated with 0.03 N H2S04in order to determine the TMA content. For total pressures in the range 350-380 mmHg, less than 2% of the total TMA was present in the
Results and Discussion Back-Extractioninto Aqueous TMA. During the back-extraction experiments, the concentration of acid in the extract was held constant while the TMA concentra-
926 Ind. Eng. Chem. Res., Vol. 30, No. 5, 1991 I I0
I in 1
no
1
/
r. 0.60 ?
0.50
log K l . 1 = 1 . 3 1 log Kz,i = 1.52 log K3,, = I . l i
0 in @?O
i
0001
C 0 40
,
,
,
I
I
I
I
,
I
I
I
I
I
I
o m 080 120 160 200 240 280 ! n i o l e i T\lA)/(moles a c i d i n i t i a l l y i n o r g a n i c p h a s e )
oon
,
on
0 10
-
P = 2.2 pKal fumaric = 3.10 pKa, f u m a r i c = 1.60 pKa R3SH' = 9.8
7
2 00
1 00
3.00
0
Figure 5. Back-extraction of fumaric acid from 0.3 M Alamine 336 in MiBK into aqueous solutions of varying TMA concentration.
The complexation of lactic acid in Alamine 336/MiBK at 25 "C is described as follows: HA + A336 s (HA)A336 log K i , i = 1.31 2HA + A336 s (HA)2A336 log K2,1 = 1.52 3HA + A336 e (HA),A336 log K3,1 = 1.17 p = partition of lactic acid between water and MiBK = [HA]/(HA) = 0.11
P = 0.19 pKa, succinic = 4 . 2 1 pKa, succinic = 5.60 pKa %NH+ = 9.8 4
-
000
THEORETICAL CURVE log K i , l = 1.39
2.00
0 20
( m o l e s TH\i/!moles acid I n l t i a l l j in organic phase)
I
0.00
log K1,,= 1.33 log K z , ~= 4 81 log K2,?= 7.49
0 00
Figure 3. Back-extraction of lactic acid from 0.3 M Alamine 336 in MiBK into aqueous solutions of varying TMA concentration. I IO
-
030
pKa lactic = 3.85
,
-
6 00
( m o l e s T M A ) / ( m o l e s acid initially i n o r g a n i c p h a s e )
(2) (3) (4)
where HA denotes the unionized acid, A336 denotes the extractant (Alamine 336), parentheses denote aqueousphase concentrations, brackets denote organic-phase concentrations, and the KP,*values are the equilibrium constants for complex formation. The equilibria in the aqueous phase are described by HA
Figure 4. Back-extraction of succinic acid from 0.3 M Alamine 336 in MiBK into aqueous solutions of varying TMA concentration.
tion was varied. Results for the three different acids are shown as data points in Figures 3-5. For all three acids, essentially 100% of the acid was back-extracted into the aqueous phase at conditions in which there was at least 1 mol of TMA for every equivalent weight of acid. This is a sign that the basicity of aqueous TMA (pKb = 4.1) is much stronger than that of the organic amine, as would be expected. The curves shown in Figures 3-5 result from a complexation model, which includes chemical-equilibriumand mass-balance equations that describe the system. There are no fitted parameters. The solubility of the acid in the amine-free diluent can be accounted for by use of the partition coefficient, P, defined as the concentration of uncomplexed acid in the organic phase divided by the concentration of unionized acid in the aqueous phase. The partition coefficients between MiBK and water, concentration basis, at 25 "C are 0.11, 0.19, and 2.2 for lactic, succinic, and fumaric acids, respectively (Tamada and King; 1989; Starr, 1989). Acid is present in the organic extract solution as a result of two interactions, acid-diluent and acid-amine. The acid concentration resulting from the acid-diluent interaction is taken to be equal to the product of the partition coefficient, the concentration of unionized acid in the aqueous phase, and a correction factor, 9,which is the volume fraction diluent in the solvent mixture. Complexation stoichiometries and constants for each of the three acids in the organic phase are reported by Tamada et d. (1990),with overbars indicating organic-phase species and no overbars corresponding to aqueous-phase species.
(1)
F?
R3NH'
H+ + A-
F?
R3N
+ H+
-(log K,) = 3.85 -(log K,) = 9.8
(H+) + (R3NH+)= (A-)
+ (OH-)
(5) (6) (7)
where R3N denotes TMA. Note that hydrogen-bonding effects cause multiple stoichiometries in the organic phase, while simple ionization is postulated for the aqueous phase. The concentration of unionized acid in the aqueous phase can be expressed as the product of the concentration of total acid in the aqueous phase and the quantity a, where 1 a= (8) 1 + Ka,acid/(H+) Then the mass balance for the acid can be written as follows: 0 = total acid in system - acid in organic phase acid in aqueous phase 0 = [HAlinit - K1,1Pl(HA) - ~ K ~ J [ B I ( H A)~ 3K3,1[B](HA)3- 9P(HA) - R(HA)/a (9)
where B denotes the uncomplexed Alamine 336 molecule, 9 is equal to the volume fraction of MiBK (0.85) in the organic solvent, and R is equal to the water-to-solvent ratio (3/8) used in the back-extraction experiments. The mass balance for Alamine 336 (essentially all present in the organic phase) can be written as follows: uncomplexed A336 = total A336 - complexed A336
[BI = [B,taIl
- Kl,ltBl(HA) -
K,i[bl(HA)2- K~,I[BI(HA)~ (10) or
Ind. Eng. Chem. Res., Vol. 30, No. 5, 1991 927 [BtOtalI
[B1 = 1 + K1,l(HA) + K2,1(HA)2+ K3,1(HA)3
0.07 ,
(11)
Equation 11 can be substituted into eq 9 to give a mass balance for the acid in terms of only two unknown values, the pH of the aqueous raffinate (incorporated into a ) and the concentration of unionized acid in the aqueous phase. A computer program (Poole and King, 1990) was used to solve the resulting equation for (HA) at set pH values by using a Newton-Raphson minimization routine. For each pH value, the recovery of the acid into the aqueous phase was calculated as recovery =
R(HA)/a [HAlinit
(13)
12000
2 00
Time ( h r s )
4
00
6.00
,
11000
-v
10000
2
90.00
h
O
where
I
0 00
and the ratio of moles of TMA to moles of acid originally present in the organic phase was calculated (R3N)bd = (R3N) + (R,NH+) = (RJH+)/P
I
0.00
(12)
i
w
P=
1 1 +Ka~s~~+/(H+)
i
(14)
and the value of (R3NH+)is determined by using eqs 5 and 7. The theoretical back-extraction curve for lactic acid shown in Figure 3 was generated in this manner. The theoretical back-extraction curves for succinic and fumaric acids were similarly determined and are shown, along with the complexation stoichiometries and constants used, in Figures 4 and 5. For fumaric acid, it was more difficult to solve the equation set because of the existence of (1,2) and (2,2) complexes. A bisection routine was used to solve for (H2A)at given pH values (Poole and King, 1990). In all three cases, the back-extraction equilibrium data compare well with the predictions of the model. Partitioning of TMA to the Organic Phase. In the back-extraction experiments, the equilibrium concentration of TMA in the organic phase was less than 0.0005 w t ‘70,as long as the overall molar ratio of TMA to acid was less than or equal to that corresponding to stoichiometric equivalence. This presumably reflects full ionization and pairing of the TMA with the acid in the aqueous phase. When TMA was present in excess of the stoichiometric ratio, partitioning of TMA into the organic phase was substantially greater. Representative organic-phase concentrations of TMA were 2.5, 1.4, and 0.55 wt 70 for 2.1 mol of TMA/mol of lactic acid, 3.1 mol of TMA/mol of succinic acid, and 3.0 mol of TMA/mol of fumaric acid, respectively. Pearson and Levine (1952) report the partition coefficient of uncomplexed TMA into MiBK from water (weight fraction basis) to be 1.88. Thermal Decomposition. (a) Lactic Acid. As an aqueous solution of lactic acid is concentrated by heating, crystalline lactic acid does not precipitate. Instead, the viscosity of the solution increases steadily as self-association of the acid occurs. In the thermal regeneration experiments carried out with aqueous solutions of trimethylammonium lactate, the goal was therefore to remove essentially all the TMA, leaving behind a concentrated aqueous solution of lactic acid and lactic acid polyesters, similar to the commercial syrup. When 45 mL of 1.6 N lactic acid and 1.89 N TMA in water was heated at 101-120 “C and 300 mmHg for 28 h, only 63% of the water and 62% of the TMA present in the initial aqueous solution were removed, leaving behind a viscous aqueous solution. The increasing ratio of lactic acid
m
&
g
80.00
70.00
60 00
50 00
= 14.8 in Hg
m
0 00
2 00
4
00
6 00
Time ( h r s )
Figure 6. Concentration and decomposition of a TMA/succinate salt, starting with a 0.6 M aqueous solution of the disalt of the acid. (a, top) Rates of removal of water and TMA. (b, bottom) Temperature.
to TMA, without precipitation of the highly soluble acid, would serve to depress the volatility of the residual TMA. Also, formation of intermolecular esters in concentrated solutions may impose severe transport limitations, hampering further TMA removal. (b) Succinic Acid and Fumaric Acid. Figure 6a shows the rates of water and TMA removal from an aqueous solution initially containing 2.0 mol of TMA/mol of succinic acid. Figure 6b shows the corresponding solution temperature, which initially corresponds to the rising boiling point of the solution. At the end of the run, the temperature is held constant at 112 “C. Despite its volatility, TMA is initially held in solution due to neutralization by the acid. The trimethylammonium carboxylate is decomposed when a high enough temperature is reached and/or as crystallization of the acid occurs, driving the decomposition reaction. TMA is released after most of the water has been removed. The extra release of TMA at the start of the run resulted from a slight excess of TMA in the initial aqueous solution. The partial pressures of TMA, water, and nitrogen computed from such results for a similar run (Poole and King, 1990) are shown in Figure 7. Figure 8, a and b, shows the results of similar experiments carried out with aqueous solutions of the bis(trimethy1a”onium) salt of fumaric acid. As with succinic acid, TMA is released after most of the water is removed, and again an end product containing the acid in crystalline form is obtained. Washing and Purification of Crystalline Succinic or Fumaric Acid. The composition of the mixture remaining in the heating flask at the end of a typical succinic acid run was 40.5 wt % C, 6.6 wt % H, and 3.6 wt % N. The indicated overall molar ratio of TMA to acid in the
928 Ind. Eng. Chem. Res., Vol. 30, No. 5, 1991 Table 11. Washing of Succinic and Fumaric Acid Crystals composition of crystals treatment %C %H %N Succinic Acid washed with acetone and CHCl, 40.2 5.1 0.09-0.16 washed with acetone and CCl, 40.4-40.7 5.2-5.3 0.17-0.28 recrystallized from MiBK (3 times) 41.1-42.4 5.3-5.5 0.05-0.11 recrystallized from MiBK (1 time)
42.1
\ '
Fumaric Acid 4.0
TMA/acid, mol/mol
recovery, w t %
0.008-0.014 0.014-0.024 0.004-0.009
67.5
0.05
81.0
0.61
67.0 77.0
I \
I
0304
\
0 00
0.00
TMA in s o l u t i o n
Figure 7. Partial pressures above the succinic acid/TMA/water system during batch regeneration.
mixture of solid crystals and aqueous solution was therefore about 0.4. Table I1 shows the degree to which the various washing techniques removed this residual TMA. Method 3, involving recrystallization from MiBK, gave the highest yields and greatest purity. Dissolution of the concentrated TMA/water/carboxylic acid mixture into MiBK serves to lower the concentrations of all species, favoring the decomposition reaction. In addition, ionization of TMA is suppressed in the organic solvent, thereby making TMA more volatile. I t should be possible to increase the yield of purified crystals with improved contacting methods. Conceivably, the TMA remaining in solution after the heating and concentration of an aqueous solution of TMA and lactic acid could be removed by the same dissolution/vaporization technique, leaving a mixture of the free and polymeric forms of the acid. Urbas (1984) states that the trialkylammonium salt of lactic acid and either tributylamine or dicyclohexylmethylaminecan be thermally decomposed to obtain the acid and the tertiary amine; however, no experimental details are given.
Processing Considerations For the process shown in Figure 1,the forward extraction would probably be carried out in countercurrent equipment, so as to enhance the loading of acid achieved in the extract. On the other hand, there are reasons for the back-extraction to be carried out in a single-stage or cocurrent contactor. With a countercurrent back-extractor, the exiting acid-depleted organic phase would have last contacted an entering aqueous TMA phase containing little or no acid, and there would therefore be substantial partitioning of TMA into that exiting organic stream. This would require a subsequent TMA-removal step. For a cocurrent or single-stage contactor, the exiting organic stream would have last contacted an acid-rich aqueous stream, and this would serve to keep the TMA almost completely in the aqueous phase, as noted above, as long as the TMA/acid ratio in that aqueous phase is close to or less than stoichiometric.
2 00
4 00
6 no
Time 1 h r s )
Imol)
-
lZO.00
,
110.00
-
I
100.00 -
0
5000
1 ,
Ptotai = 14.3 in. Hg 1
Ind. Eng. Chem. Res., Vol. 30,No. 5,1991 929 aqueous back-extract to the acid concentration in the aqueous feed to the process) for a 3% succinic acid solution would be 3.95/(2 X 0.25) = 7.9. Greater overall degrees of concentration could be targeted if a more concentrated aqueous TMA stream were to be used, under pressure if necessary. A comparison of regeneration by the approach considered here with regeneration by back-extraction following temperature and/or diluent change (Poole and King, 1990)shows that the overall concentration factor obtainable by the present method substantially exceeds that achievable with temperature swing or change of diluent composition (Tamada and King, 1990). Since all methods require evaporative steps to drive water off from the back-extract, the greater concentration factor gives a substantial energy advantage to the TMA back-extraction method. Additional energy costs are associated with changing the temperature in a temperature-swing process, with distillation to change diluent composition in a diluent-swing process, and with the final purification step(s) and the need for operation of the evaporative crystallizer under vacuum in the TMA method. The final purification step in the TMA method deserves further study. It is possible that the carboxylic acid product after a final purification step retains undesirable color. If so, it may be necessary to incorporate a decolorizing process, for example, passing the aqueous back-extract through activated carbon. Acknowledgment This work was supported by the Assistant Secretary for Conservation and Renewable Energy, Office of Energy Research, Energy Conversion and Utilization Technologies (ECUT) Division, US. Department of Energy, under Contract DE-AC03-76SF00098. Nomenclature (A) = aqueous-phase concentration of species A [A] = organic-phase concentration of species A K, = acid ionization constant Kb = base ionization constant K,,q = equilibrium constant for reaction pA + qB F? ApBp P = partition coefficient (eq 4) R = water-to-solvent ratio (volume) a = defined in eq 8 0 = defined in eq 14 = volume fraction of diluent in solvent mixture Subscripts init = initial total = total, all forms Registry No. Fumaric acid, 110-17-8;lactic acid, 50-21-5; succinic acid, 110-15-6; trimethylamine, 75-24-1.
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Received f o r review March 23, 1990 Revised manuscript received June 4, 1990 Accepted June 22, 1990
