Minimization of Chemicals Use during Adsorptive Recovery of

10.1021/ie1000168. Publication Date (Web): March 10, 2010. Copyright © 2010 American Chemical Society. * To whom correspondence should be address...
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Minimization of Chemicals Use during Adsorptive Recovery of Succinic Acid C ¸ ag˘ri Efe,† Mervin Pieterse,† Jorge Gascon,‡ Freek Kapteijn,‡ Luuk A. M. van der Wielen,† and Adrie J. J. Straathof*,† Department of Biotechnology, Delft UniVersity of Technology, Julianalaan 67, 2628 BC, Delft, The Netherlands, and Catalysis Engineering, DelftChemTech, Delft UniVersity of Technology, Julianalaan 136, 2628 BL, Delft, The Netherlands

In adsorptive separation processes, regeneration of the adsorbent plays an important role. Depending on the type of product the regeneration strategy might differ from one process to another. In this study, the desorption of succinic acid from a high silica ZSM-5 adsorbent is studied, using displacement by an organic solvent or CO2, or temperature swing. According to a number of process criteria and solvent selection criteria, 2-butanone performed better than the other displacing agents studied. However, the subsequent regeneration step involved desorption of butanone, which proved to be difficult and required a temperature above the normal boiling point of water under elevated pressures. Such a temperature swing with hot water can also be applied for direct succinic acid desorption without intermediate displacement by butanone. A countercurrent continuous adsorption process was modeled to compare these options. Direct temperature swing using pressurized water at >100 °C proved to be more attractive to achieve a sustainable process. 1. Introduction Fermentative production of succinic acid from renewable resources is currently receiving considerable interest.1-3 However, fermentation at neutral pH leads to consumption of mineral acids and bases, which results in significant cost factors and undesired waste generation. Therefore, low-pH processes are being developed for succinic acid production.4 High silica MFI type zeolite powders (silicalite-1) may be an efficient option for removal of succinic acid from low pH fermentation media.4 The adsorption mechanism is based on the hydrophobic character of succinic acid and this zeolite, so that the affinity was lower for water, acetic acid, ethanol, and salts.4 For a viable adsorption process, efficient desorption is as important as a high capacity and affinity of the adsorbents. Several desorption options are available for liquid phase processes and have been used5-8 but no general guidelines seem to be available. Therefore, the objective of this study is to select a suitable method for desorption of succinic acid from zeolites by generating a set of selection criteria based on the process characteristics. A concentration swing can be very efficient if it involves an elution solution with high pH to dissociate the desorbing succinic acid, so that all will be desorbed. However, since we aim to adsorb succinic acid from a fermentation medium without forming succinate salts,4 involving mineral bases in regeneration is not a preferred option. Volatile organic bases like trimethyl amine or triethyl amine can be used instead of mineral bases,9 but their toxicity is unfavorable in a process. Temperature swing using water up to 90 °C during the regeneration has been proposed.10 Pressure swing is valid for gas phase operations and cannot be applied to liquid phase adsorption. The same applies to incineration, which would only be used to remove waste products from a solution. To desorb succinic acid, elution can be performed with a solvent or a solute that is adsorbed by the zeolite and also allows easy separation from the succinic acid. For example, other * To whom correspondence should be addressed. E-mail [email protected]. Phone: +31-15-2782330. Fax: +31-152782355. † Department of Biotechnology. ‡ Catalysis Engineering, DelftChemTech.

products of fermentative production like ethanol11 and carbon dioxide12 might be used at high concentrations to desorb succinic acid. On the basis of the foregoing data, we selected four different regeneration options for evaluation in this study, namely hot water, CO2, low boiling organic solvent, and high boiling organic solvent. The selection of the most suitable regeneration process and regenerant requires understanding of the different process configurations. A typical batch liquid phase adsorption cycle involves 2-3 stages depending on the type of regeneration (e.g., adsorption, desorption and if a displacement agent is used one more regeneration step to remove it from the adsorbent). In the following section, these options are presented. The process options are the same up to adsorption but the regeneration steps are different. Hot Water Desorption (Inert Elution/Temperature Swing). In hot water regeneration, succinic acid is desorbed from the zeolite by a temperature swing. Adsorption is an exothermic process and the increasing temperature promotes desorption of succinic acid.4 It is advantageous that no additional components are introduced by using hot water regeneration. So, after desorption, the column requires only cooling for the next adsorption cycle (Figure 1). By increasing the pressure of the water above ambient, the temperature can be increased above 100 °C, the loading of succinic acid will be decreased, and the succinic acid concentration in the effluent will be increased. Since the next step is crystallization, which is assumed to be by water removal and subsequent cooling, the prior concentration by flash will reduce the energy required for crystallization. Heating up liquid water is less energy intensive than steam generation, and the energy consumption would not increase significantly. Furthermore, the energy consumption of the process can be reduced by heat integration options. Isotherms up to 150 °C will be measured to be able to evaluate this regeneration option. This leads to pressures up to ∼8 bar, which is still feasible for the available equipment. Desorption by CO2. The use of zeolites in gas separation and purification processes is very common. A very common adsorbate for zeolites is carbon dioxide, which is also abundant in fermentative processes and an additional feed component in succinic acid fermentation process. Therefore, its liquid phase

10.1021/ie1000168  2010 American Chemical Society Published on Web 03/10/2010

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Figure 1. Desorption by hot water. Dark arrows show the direction of the column cycle. Figure 3. Desorption by displacement by a low boiling solvent. Dark arrows show the direction of column cycle.

Figure 4. Desorption by displacement by a high boiling solvent. Dark arrows show the direction of column cycle. Figure 2. Desorption by displacement with CO2. Dark arrows show the direction of column cycle.

2. Materials and Methods

adsorption behavior in zeolites will be determined. Recovery of CO2 requires an extra regeneration step for the column to be ready for the next adsorption cycle (Figure 2). Desorption by an Organic Solvent. Although using a hydrophobic high affinity compound may displace succinic acid easily, it introduces an extra compound to the system which needs to be recovered in subsequent stages. Succinic acid can be purified by crystallization where the organic solvent can be evaporated. Different process strategies were generated for solvents boiling higher or lower than water. If a low boiling solvent is used, fermentation impurities would leave the system with the aqueous bottom stream during the solvent recovery step (Figure 3). Otherwise, they will accumulate in the organic stream, which will require an additional distillation before the organic solvent can be recycled (Figure 4). Solvent selection criteria are the following: toxicity and safety, solubility, distribution coefficient (solvent in water and succinic acid in solvent), stability of solvents at operation conditions, affinity of zeolite toward solvent, volatility, and stability of zeolite in solvent. These criteria have been generated on the basis of the process structure and conditions. The organic solvent complying best with these criteria will be compared with hot water and carbon dioxide by conducting necessary batch experiments and by using a countercurrent continuous adsorption process model.

Zeolite powder (CBV 28014, H-ZSM-5 with Si/Al2 ) 280) was purchased from Zeolyst international (Valley Forge, PA). The zeolite was calcined for 6 h at 600 °C before use. After calcination, the sorbents were stored in an oven at 75 °C to prevent hydration. Succinic acid (99%), succinic acid disodium salt (Na2C4H4O4, 99%), methylethylketone (2-butanone 99.5%), ethyl acetate (99.8%), 1-propanol (99.5%), and 2-propanol (99.5%) were purchased from Sigma-Aldrich Chemicals (Steinheim, Germany), acetic acid (99%) was purchased from J.T. Baker (Deventer, Netherlands), absolute ethanol, methylisobutylketone (MIBK), 1,4-butanediol (98%), acetone (99.9%), and 2-butanol were purchased from Merck (Darmstadt, Germany), 1-butanol (99.5%) was purchased from ACROS Organics (Geel, Belgium), and CO2 was from Linde (Schiedam, Netherlands). 2.1. Adsorption Measurements. Stock solutions were prepared of known mass fraction of disodium succinate or succinic acid in demineralized water, in an organic solvent or in a solution of organic solvent in demineralized water. Sorption isotherms were generated by contacting known masses of sorbent (typically 2-4 g) and solution (typically 15-25 g) in 30-mL sealed vessels. The sorbent mass was increased up to 4 g per vessel for high concentrations starting from 2 g for low concentrations. The vessels were shaken in a Sartorius CertomatBS-1 thermostatic orbital shaker at 200 rpm at the temperature of interest for at least 3 h to attain equilibrium. Samples of ∼1 mL were collected and centrifuged for 10 min to remove the zeolite. Control studies showed that equilibrium was reached

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in less than 20 min. Aqueous phase concentrations of succinic acid were determined by HPLC and those of volatile organics by GC. Duplicate experiments were conducted. In temperature dependent experiments, at ambient pressure, the time of sampling was kept as short as possible to prevent any equilibrium shifts due to a temperature change. Before sampling, the syringes, filters, and Eppendorf tubes used for the sampling were preheated to the temperature of interest. The 1 mL samples were immediately filtered by using 0.45 µm microfilters to remove adsorbent. The samples were analyzed for succinic acid by HPLC. 2.2. Preparation of the Sodium Form of ZSM-5 Zeolite. The Na form of the zeolite was prepared by equilibrating H-ZSM-5 with 1 mol/L NaCl (solid to liquid mass ratio ∼2:5) at room temperature. After equilibration, the zeolite was washed three times with water to remove excess ions and dried. 2.3. High Pressure and Temperature Succinic Acid Isotherms. The experiments were conducted in a 300 mL batch autoclave, Medimex 108 (Lengnau/Switzerland) with a maximum operating pressure of 75 bar and a maximum operating temperature of 300 °C. A 180 g portion of aqueous succinic acid solution (0.010 and 0.040 g/g) and ZSM-5 zeolite (0.2 g per g solution) were placed in the autoclave and heated to the desired temperature (120-150 °C). The experiments were run for at least 4 h. Samples were collected through a sampling loop, which was confined by two valves and installed with a filter at the reactor side to retain the zeolites in the autoclave. The sampling loop was dipped into ice, to cool the sample to room temperature. The collected samples were analyzed for succinic acid concentration by HPLC. 2.4. CO2 Isotherms. The succinic acid loadings in the presence of CO2 were obtained by contacting CO2 gas with succinic acid solutions (0.001-0.040 g/g) at 20 °C and absolute CO2 pressures of 5-10 bar in a Mini clave drive batch pressurized glass reactor (Bu¨chi AG, Uster, Switzerland). The reactor volume was 60 mL, and up to 6 g of adsorbent was used in those experiments. The aqueous succinic acid solution was placed in the reactor, and the overhead air in the reactor was flushed with CO2 to remove air from the reactor and ensure pure CO2 contacting the solution. While the CO2 dissolved, the pressure in the reactor was kept constant by supplying additional CO2 to the reactor. The equilibrium CO2 concentration was estimated from the partial pressure and solubility of CO2.13 Samples were collected through a sampling valve which was equipped with a 250 µm filter on the reactor side, and they were analyzed for succinic acid concentration by HPLC. 2.5. Effect of Solvent on Succinic Acid Adsorption. Aqueous solutions containing 0.040 g succinic acid and 0.010 g solvent/g were prepared. The mixtures were equilibrated at 20 °C, and samples were taken after 2-3 and 24 h. Then, the temperature of the experiment was increased to 70 °C, and a final set of samples was taken after 24-30 h. The samples at room temperature were centrifuged to remove the zeolite, and the high temperature samples were taken as previously described. The samples were analyzed for succinic acid concentration by HPLC. 2.6. Effect of Successive Loading and Unloading on Capacities. The zeolite was loaded with ∼0.040 g/g total succinate (Suc) using a solution with an initial Na:Suc mole ratio of 0.5.4 Succinic acid was desorbed using a solvent (ethanol or 2-butanone) at room temperature, and the solvent was removed by successive washing with water at 70 °C. This procedure was repeated five times.

2.7. Effect of High Temperatures on Zeolite Stability. The stability of zeolites was checked by keeping powder zeolites in a sealed pressure resistant stainless steel vessel in the presence of 0.040 g/g succinic acid solution at 130 °C for 2 weeks. Later, the zeolites were calcined to be regenerated and equilibrated with 0.040 g/g succinic acid solution at 20 °C as described before. 2.8. Analytical Methods. HPLC analysis of the liquid phase total succinate concentration was done using a Waters Alliance Liquid Chromatograph equipped with a BioRad HPX-87H 300 × 7.8 mm column (cat no.: 1250140) at 59 °C. The mobile phase was 1.5 mmol/L phosphoric acid in milli-Q water at 0.6 mL/min and 70 °C. The injection volume was 10 µL with a Waters 717 autosampler; detection was at 210 nm (Waters 484 Tunable adsorbance detector). The retention times for succinic and acetic acid were 11.43 and 14.87 min, respectively. Analyses of volatile organics were performed by means of GC, Thermo Electron Corporation, model GC-Focus, with an autosampler AS3000. The GC column was Innowax 19091N-133 (30 m × 0.20 mm, with coating of 0.25 µm) from Agilent Technologies Inc. The mobile phase was helium at 6 mL/min. The temperature of the column was 70 °C for 1 min, and it was increased to 130 °C with a heating rate of 10 °C/min after which the column was kept at 130 °C for 5 min. 1-Pentanol was added to the samples as an internal standard. 2.9. Calculation of Sorbent Loadings. The initial concentrations of the samples were known, and the final equilibrium concentrations were obtained using HPLC or GC. Inserting the initial and final solution concentrations and adsorbent and solution masses in eq 1 yields the loadings. q)

aq Maq 0 CSuc,0 - MeqCSuc,eq

Mads

(1)

q is the adsorption loading (mol Suc/kg adsorbent), Maq is the mass of the solution (g), CSuc is the molar concentration of Suc (mol/kg solution), and Mads is the mass of adsorbent (g). Subscripts 0 and eq represent the initial and equilibrium values, respectively. The adsorption isotherms were estimated as described previously4 using eq 3. 3. Results and Discussion 3.1. Selection of the Organic Solvent. (a) Toxicity and Safety. Eliminating the toxic and hazardous chemicals in the early stages of design eliminates unnecessary experimental and design work. The US Department of Health and Human Services lists organic solvents that can be regarded as less toxic (class 3 solvents). Solvents listed in this group and their relevant properties are listed in Table 1. The solvents containing sulfur, phosphorus, and nitrogen were left out to minimize any later waste treatment problem. Most of the solvents in Table 1 are listed as flammable. However, ethyl ether, ethyl formate, and pentane are listed as extremely flammable and were therefore not considered in the subsequent evaluation. (b) Solubility of Organic Solvent in Water. The best displacement action would be obtained when a pure solvent is used as the eluent phase during desorption. However, most of the organic solvents suffer from low succinic acid solubilities. Therefore, to prevent crystal formation during desorption, solvents are to be blended with water to increase the solubility of succinic acid in eluent media. Furthermore, using a water insoluble solvent might also result in phase splits during the adsorption operation. Therefore, the aqueous solubility of solvent

Ind. Eng. Chem. Res., Vol. 49, No. 8, 2010 Table 1. Class 3 Solvents Which Are Less Toxic

a

solvent

solubility in water (20 °C) (g/L)

acetic acid acetone anisole 1,4-butanediol 1-butanol 2-butanol 2-butanone butyl acetate CO2 cumene ethanol ethyl acetate ethyl ether ethyl formate formic acid isobutyl acetate isopropyl acetate methyl acetate 3-methyl-1-butanol methyl isobutyl ether methylisobutyl ketone 2-methyl-1-propanol pentane 1-pentanol 1-propanol propyl acetate 2-propanol

miscible miscible insoluble miscible 74 miscible 290 7.0 (25 °C) 1.45 at 1 bar 25 °C insoluble miscible miscible 69.0 soluble miscible 6.5 43 (27 °C) miscible slightly 54.4 19.1 95 0.1 27 (25 °C) miscible soluble miscible

a Source ref 17. ext extremely.

b

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Table 2. Disappearance of Succinic Acid (0.04 g/g) Dissolved in Ethanol, in the Presence of H and the Na Form of CBVa 28014 hazardsb C F, xi F F F, xi F F F F, xi ext F, xn ext F C, xi F, xi F F xi F F, xn F ext F F F F, xi F

C corrosive, F flammable, xi irritant, xn harmful,

also plays an important role. For example, solvents like heptane, which are inherently well-adsorbed by zeolites due to their nonpolar characteristic and long carbon chains, are unsuitable as displacing agent because of their immiscibility and low succinic acid solubility. Assuming displacement of succinic acid by a displacing solvent occurs in a 1:1 mass ratio, for proper displacement action, the solubility of the displacing agent in water should be larger than the concentration of succinic acid after fermentation. Due to the unavailability of low pH succinic acid fermentation data, lactic acid data may indicate what is achievable. In ref 14, a lactic acid concentration of 50 g/L free lactic acid at pH 4.2 has been reported. Taking this concentration as reference, the solvents with an aqueous solubility below 50 g/L were eliminated (cumene, anisole, butyl acetate, isobutyl acetate, isopropyl acetate, methyl isobutyl ether, methylisobutylketone, 3-methyl-1-butanol, 2-methyl-1-propanol, pentane, and 1-pentanol). (c) Stability of Solvents in the Presence of Zeolites. The hydrogen form of zeolites is used as a catalyst for various reactions, but reactions of the displacing agent and succinic acid during the regeneration would be undesired. The Na and H form of the zeolite were contacted with ∼0.04 g/g succinic acid in ethanol at 5, 20, and 70 °C for a day. Succinic acid disappeared from the solution (Table 2) either due to adsorption only or due to combined reaction and adsorption. Only for H-ZSM-5, disappearance increased with increasing temperature. Adsorption is an exothermic process and the amount adsorbed drops with increasing temperature.4 This indicated that reaction occurred in the presence of H-ZSM-5, which also has acid catalytic properties. The reaction was suspected to be esterification of succinic acid with ethanol. To check this, a mass spectrometry analysis was carried out and peaks relating to ethyl succinate and diethyl succinate were observed. Upon diluting ethanol with water, the reaction was no longer observed. However, when the solvent was ethyl acetate after 2 and 24 h contact with zeolite, at 70 °C, acetic acid was detected in the

form

temperature (°C)

disappearanceb (mol/kg)

H-ZSM-5 Na-ZSM-5 H-ZSM-5 Na-ZSM-5 H-ZSM-5 Na-ZSM-5

70 70 20 20 5 5

1.73 0.03 0.37 0.10 0.16 0.11

a CBV: product code of the zeolites given by the supplier. change in aqueous succinic acid amount per gram adsorbent.

b

The

Table 3. Competitive Effect of Different Solvents (Initially 0.01 g/g) on Succinic Acid (Initially 0.04 g/g; 0.339 mol/kg solution) Loadings on CBV-28014 Type Zeolites (0.1 g/g solution) displacing solvent

temperature (°C)

equilibrium succinic acid loading (mol/kg zeolite)

1-butanol 2-butanone MIBK acetone 2-butanol 1-propanol 10 bar CO2a water acetic acid 2-propanol 1,4-butanediol ethanol water water

20 20 20 20 20 20 20 90 20 20 20 20 70 20

0.58 0.72 0.75 0.76 0.77 0.84 0.85 0.89 1.02 1.05 1.08 1.08 1.13 1.28

a The equilibrium CO2 concentration is the aqueous solubility at these conditions (∼0.015 g/g).

solution at concentrations of 0.021 and 0.025 g/g, respectively. This shows that the hydrolysis of esters at high temperatures occurs at high reaction rates and that esters are more reactive than alcohols in aqueous environment. This is obvious from the reaction kinetics over acidic catalysts and the large differences in surface concentrations.15 Therefore, esters were eliminated from Table 1. (d) Affinities. For efficient displacement, the adsorption affinity should be higher for the organic solvent than for succinic acid. Table 3 shows the succinic acid loadings of zeolite in the presence of solvents with different functional groups (i.e., ketone, ester, carboxyl, alcohol). One way of ranking the affinities of solvents is by comparing their competition with succinic acid. The solvents with higher affinities would yield lower succinic acid loadings under the same conditions. Ethanol and 1,4-butanediol showed the lowest affinities. The affinity of alcohols increased with increasing carbon chain length per hydroxy group. Both 2-propanol and 2-butanol have a lower affinity than their more linear isomers 1-propanol and 1-butanol. Acetone and methylisobutylketone (MIBK) showed a similar behavior. Although MIBK contains more carbon atoms, which is more favorable for adsorption, its branched structure seems to counteract this advantage. As the reader can recall, MIBK was already eliminated from the list due to its low aqueous solubility and has been included in this affinity analysis merely to study the effect of branching. It is observed that acetone has a higher affinity than 2-propanol, which indicates that oxo groups are better adsorbed than hydroxy groups. 2-Butanone and 2-butanol behaved in line with this. This rule does not hold when the oxo and hydroxy groups are in different positions because 1-butanol showed a better affinity than 2-butanone which is less linear. Acetic acid data have been taken from ref 4. It adsorbs better than ethanol and 2-propanol. CO2 showed moderate affinity. However, it is emphasized that the solubility

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Figure 5. 2-Butanone adsorption isotherm for H-ZSM-5 in water at room temperature. Symbols are experiments, and the line is the model with parameters given in Table 4. Table 4. Adsorption Isotherm Parameters for Equations 2-3, Obtained from the Current Study and from Reference 4 solvent 2-butanone succinic acid

qsat,1 qsat,2 KA,ref KB,ref ∆Hads,A ∆Hads,B (mol/kg) (mol/kg) (kg/mmol) (kg/mmol) (kJ/mol) (kJ/mol) 1.348 0.926

0.720 0.491

4.724 0.733

0.022 0.015

33.0

23.2

of CO2 at 10 bar is ∼0.015 g/g, and this value is higher than the equilibrium concentrations of the other solvents (∼0.006-0.007 g/g) in Table 3 which makes CO2 less favorable. On the basis of the affinities, 1-butanol, 2-butanone, acetone, and 2-butanol performed better than the rest of the solvents. Due to its branching, 2-methyl-1-propanol is expected to show less affinity than its isomer 1-butanol and was not tested. (e) Heat of Evaporation and Volatility. During the displacement, the displacing solvent is adsorbed and should be desorbed to prepare the column for the next cycle. This regeneration should be reversible and easy. A displacing solvent with very high affinities would be efficient in displacing succinic acid but difficult to be desorbed. To be able to use temperature swing for regeneration, a displacing solvent with a low boiling point and a high volatility would be favorable. In the previous section, 1-butanol, 2-butanone, acetone, and 2-butanol showed the best affinities. The butanol isomers show relatively high boiling points13 and heats of evaporation,13 which makes them unfavorable in terms of energy costs. Solvents with boiling points above that of water have the disadvantage of one extra solvent recovery step to remove the impurities from the solvent. Because of its low heat of evaporation, 2-butanone was finally selected as the most suitable solvent for displacement. (f) Stability of Zeolites in Selected Solvent. To check the stability of the zeolite in 2-butanone, the zeolite was exposed to 2-butanone for one week, washed with water, and dried. The adsorption capacity showed no change after 5 cycles indicating that the adsorbents do not suffer from structural degradation or fouling during longer term exposures. (g) Equilibrium Isotherms of Selected Solvent. The adsorption isotherm of 2-butanone in water is given in Figure 5. Due to its low reactivity, 2-butanone can be used as a pure solvent in regenerating the adsorbents. However, the measured solubility of succinic acid in 2-butanone (∼0.11 mol/kg) at 18 °C limits the dissolution in the eluent phase. Therefore, 2-butanone should be dissolved in water during the succinic acid displacement. 3.2. Effect of Temperature on Desorption. Increasing the temperature of the aqueous media resulted in a decrease in the succinic acid loadings (Figure 6). Increasing the temperature to 150 °C in a pressurized vessel resulted in 40% reduction in

Figure 6. Effect of temperature on succinic acid loadings on H-ZSM-5 in water. Symbols are experiments, and lines are model fits.

loadings at high aqueous concentrations and resulted in lower loadings at low concentrations, which would help achieving higher concentrations during desorption. Davison et al.,10 using silicalite, achieved low succinic acid desorption values in water at 90 °C and decided to focus on weak basic polymeric adsorbents. Such resins require additional acid washes, which we try to avoid. The desorption effect would be even higher if the temperatures were increased further. Our maximum temperature was limited by sampling problems, but succinic acid was stable according to a test: after some of batch autoclave experiments, the temperature was cooled down to room temperature and the solution was requilibrated with loaded zeolites, which yielded the original loadings. After keeping the zeolite powder for 2 weeks in 0.040 g/g succinic acid solution at 130 °C, and subsequent calcination, it was equilibrated with succinic acid solution at 20 °C, which also yielded the original loading. So according to these preliminary data, repeated use of the zeolite might be possible. The adsorption affinity constants of succinic acid were fitted to eq 2.16

(

K(T) ) Kref exp

(

))

-∆Hads Tref -1 RTref T

(2)

Where K(T) is the affinity constant at temperature T and R is the gas constant. For the dual-site Langmuir isotherm model, the value of Kref (the affinity constants at reference temperatures of 20 °C) was 0.73 and 0.015 kg/mmol for sites 1 and 2, respectively.4 Using these parameters and the data presented in Figure 6, the fitted values for the enthalpies of adsorption (∆H) for two sites were obtained as 33.0 and 23.2 kJ mol-1 for sites 1 and 2, respectively. 3.3. Effect of Fermentation Medium on Desorption. Fermentation medium is known to present difficulties in adsorption processes due to the fouling resulting from blockage of the pores by fermentation byproduct like proteins. In this manuscript, the effect of fermentation medium is not investigated. However, regeneration of adsorbents was successful when succinic acid loaded zeolites were calcined at 600 °C. Similar strategies can be utilized in case of fouling to burn the organic material blocking the pores. This can be done by passing hot air (600 °C) through the column after a certain number of cycles. Another option might be elimination of large molecules like proteins by ultrafiltration before the adsorption. 3.4. Comparison of the Desorption Options. An evaluation of the different process configurations should involve the

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Figure 7. Countercurrent adsorption process scheme with n stages.

economical and the technical aspects of the candidate processes and requires a detailed analysis, which is out of the scope of this article. To ease the comparison of hot water desorption and desorption with 2-butanone as a displacing solvent, we will use a couple of fair assumptions. The adsorption step is not considered, since all the options would have the same adsorption step. This is why the comparison is made by assuming inflow of the solid phase with a succinic acid loading of 1.3 mol/kg adsorbent, without byproduct. In order to mimic a continuous countercurrent absorber such as a simulated moving bed, the solid phase is introduced at one side of a series of mixed contractors and the eluents are fed from the opposite direction (Figure 7). Equilibration is assumed in each step. Desorption of the displacing solvent is modeled in the same manner in a multistage discrete countercurrent system. It is assumed that a steady state will be achieved, neglecting any potential fouling or deterioration of the sorbent. The sorption model used in the analysis is the dualsite Langmuir isotherm (eq 3), with parameter values shown in Table 4. The heat of adsorption of 2-butanone is not available, and when needed, the succinic acid values were assumed. The effect of temperature on 2-butanone up to 90 °C (results not shown) is more moderate than that on succinic acid. The countercurrent system is schematically depicted in Figure 7 for n stages. The steady-state material balances for stage i can be written as given in eqs 3-7. qtot,i )

qsat,A,iKA,iCi n

1+

∑

KA,iCi

i)1

+

qsat,B,iKB,iCi n

1+

∑

(3)

KB,iCi

i)1

Succinic acid molar balance A(qs,i - qs,i+1) + Fi+1Cs,i+1 - FiCs,i ) 0

(4)

Displacing solvent molar balance A(qe,i - qe,i+1) + Fi+1Ce,i+1 - FiCe,i ) 0

(5)

Total flow mass balance Fi+1 - Fi + A[(qs,i - qs,i+1)MWs + (qe,i - qe,i+1)MWe] ) 0 (6) % recovery )

qi,in - qi,out × 100 qi,in

(7)

A is the adsorbent mass flow, F is the mass flow of the eluent stream, qi is the loading of ith compound (mol/kg adsorbent), Ci is the concentration of the ith compound in the eluent stream (mol/kg), MWi is the molecular weight of the ith compound, and qsat (mol/kg) is the saturation loading. Subscripts A and B represent two different adsorption sites. Subscripts s and e represent succinic acid and the displacing solvent, respectively. The material balances were solved using the fsolve function in MATLAB (The Mathworks Inc. version 7.5.0342 (R2007b)).

Table 5. Model Results for the Effect of Temperature on Succinic Acid Desorption with Water entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

temperature (°C)

number of stages

20

3 5 50 10

20 40 80 120 150

10

30 50 10 20 50

F/A 5 10 20 5

10

CS,out (mol/kg)

succinic acid recovery (%)

0.065 0.072 0.081 0.049 0.030 0.078 0.098 0.144 0.193 0.225 0.231 0.232 0.125 0.127 0.128

25.2 28.1 31.5 37.9 46 30.3 38.3 56.3 76 89 91.4 91.6 97.9 99.2 99.7

The manipulated variables for the comparison were the number of stages (n), the initial eluent to adsorbent mass flow ratio (F/A), the desorption temperature and, when displacement is considered, the concentration of the displacing agent. The output parameters for the comparison of the options are the recovery of succinic acid and 2-butanone from the sorbent, which is defined by eq 7, and the outflow concentrations of succinic acid and 2-butanone (Cs,out and Cb,out). For the succinic acid regeneration model, the initial loading of succinic acid was taken as 1.3 mol/kg adsorbent, and for that of 2-butanone regeneration, the initial 2-butanone loading was taken as 2.01 mol/kg. Case 1: Water Regeneration. The recovery of succinic acid was only 25.2% when water was used as eluent at 20 °C in a three-stage system with F/A ) 5. Increasing the number of stages under the same conditions slightly increased the recovery and the output succinic acid concentrations (Table 5, entries 1-3). At 50 stages, the recovery was 31.5% with 0.081 mol/ kg output succinic acid concentration. The number of stages was set to 10 stages at 20 °C, and the F/A ratio was manipulated. Increasing the F/A ratio to 20 resulted in increasing recovery (46%) with a parallel decrease in the succinic acid concentration of the outflow (0.030 mol/kg). The results at 20 °C show that the efficiency of inert elution is not high at room temperature. To improve succinic acid recovery, increasing the temperature was more effective than changing the other parameters (Table 5, entries 6-10), and the recovery at 10 stages and F/A ) 5 was 89% at 150 °C. When the temperature was kept at 150 °C and the other two parameters were manipulated (Table 5, entries 11- 15), a recovery of 99.7% was obtained for n ) 50 and F/A ) 10. Then, the outlet succinic acid concentration was 0.128 mol/kg. Case 2: Displacement Regeneration with 2-Butanone. Displacement regeneration with 2-butanone showed better performance than hot water regeneration in terms of the number of stages (Table 6, entries 16-20). In a three-stage process, at

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Ind. Eng. Chem. Res., Vol. 49, No. 8, 2010

Table 6. Calculated Effect of 2-Butanone on Succinic Acid Desorption with Water entry

temperature (°C)

number of stages

F/A

Cb,in (mol/kg)

Cs,out (mol/kg)

Cb,out (mol/kg)

succinic acid recovery (%)

16 17 18 19 20

20

3

5

1.000 0.750 0.500

5

7 5

0.259 0.259 0.256 0.185 0.259

0.592 0.345 0.104 0.214 0.099

99.9 99.7 98.6 99.7 99.9

Table 7. Calculated Performance of Countercurrent Desorption System under Different Operation Conditions entry

temperature (°C)

number of stages

F/A

Cs,in (mol/kg)

Cs,out (mol/kg)

Cb,out (mol/kg)

butanone recovery (%)

21 22 23 24 25 26 27 28 29 30 31 32

20

5 10 25 50

5

0.400

10 20 5 5

1.000 0

0.233 0.219 0.212 0.211 0.282 0.334 0.747 0

0.253 0.274 0.284 0.286 0.177 0.100 0.373 0.101 0.105 0.065 0.066 0.025

62.9 68.0 70.5 71.0 88.1 99.1 92.4 25.4 26.4 80.6 82.7 99.1

20 150

10 50 10 50

25 80

20 °C and F/A of 5, the recovery of succinic acid was 99.9% when 1.000 mol/kg of aqueous 2-butanone was used as eluent. The recovery dropped to 98.6% when the inlet concentration of 2-butanone was reduced to 0.500 mol/kg. Increasing the F/A ratio to 7 increased the recovery to 99.7% with an accompanying drop in the outlet succinic acid and 2-butanone concentrations. Finally, for an initial 2-butanone concentration of 0.500 mol/ kg, n ) 5, and F/A ) 5, the succinic acid recovery was 100.0% with outlet succinic acid and 2-butanone concentrations of 0.260 and 0.099 mol/kg, respectively. Case 2.1: Desorption of 2-Butanone Using Succinic Acid. The displacement action of 2-butanone was clearly more efficient than that of hot water. However, 2-butanone must be removed from the adsorbents for the next succinic acid adsorption cycle. One way of doing this is directly passing the next fresh succinic acid solution through the adsorbents and making use of the displacing action of succinic acid to remove 2-butanone, which does not require any temperature swing or an extra desorption step. When 0.400 mol/kg succinic acid solution was passed through the system at 20 °C and F/A ) 5, the highest 2-butanone recovery (71%) was obtained when n ) 50 (Table 7, entries 21-24). The succinic acid was not fully adsorbed, and an outlet concentration of 0.211 mol/kg was calculated. When the F/A ratio was increased (Table 7, entries 25-26) a recovery of 99.1% was obtained for F/A ) 20, with a 2-butanone outlet concentration of 0.100 mol/kg and incomplete adsorption of succinic acid. Finally, when the initial succinic acid concentration was increased to 1.000 mol/kg, the recovery was 92.4% for n ) 50 and F/A ) 5 at 20 °C. The results indicate that desorption of 2-butanone using succinic acid solution is not a feasible option. Case 2.2: Desorption of 2-Butanone Using Temperature Swing. Like it was applied to succinic acid recovery, hot water regeneration can also be applied to 2-butanone desorption. At 20 °C and F/A ) 5, the recovery of 2-butanone was 25.4% and 26.4% for n ) 10 and 50, respectively (Table 7, entries 28 and 29). At 150 °C, the recovery was 80.6% for n ) 10 and F/A ) 25. Increasing the number of stages to 50 did not change the results significantly, but would mainly result in a higher investment, so n was not increased any further. On the other hand, 99% recovery is obtained when F/A was increased to 80. However, the outlet 2-butanone concentration was only 0.025 mol/kg. Thus, 2-butanone leads to simple succinic acid desorp-

tion, but overall to a more complicated separation because the adsorbent regeneration requires many stages and leads to diluted streams. Parameter Sensitivity of the System to Errors. Due to the experimental deviations, we might expect some errors in the estimated parameters. To see how this would affect the simulation results, the estimated heat of adsorption values were manipulated by (10%. This did not give more that 7% change in the simulated recoveries and does not affect the outcome of the study significantly. Since loading obtained from duplicate experiments did not deviate by more than 10%, the desorption options comparison results can be considered conclusive. 4. Conclusions The current initial process analysis showed that high silica MFI type zeolites (ZSM-5) can be regenerated either by temperature swing or by displacement using an organic solvent or CO2. When the process criteria and solvent selection criteria were applied, 2-butanone performed better than the other organic solvents and CO2. Thus, it was selected as the displacing solvent. A process analysis showed that 2-butanone is indeed efficient in succinic acid desorption, but subsequent butanone desorption is a bottleneck. This will require, for example, relatively expensive temperature swing operation under pressure (T > 100 °C). Therefore, the current analysis suggests using hot water at elevated temperatures and pressures for the desorption of succinic acid from zeolite. The temperature dependency of succinic acid adsorption showed that such temperature swing might be better applied to desorb succinic acid directly, thus leaving out the 2-butanone step. For the succinic acid system, hot water regeneration was the most suitable option and adsorptive purification of succinic acid can be achieved without using any additional chemicals in the process. The systematic analysis applied here can also be applied to other systems for obtaining a preliminary evaluation of desorption options. On the basis of stirred batch experiments, this study yields promising conclusions, but for better understanding of the desorption behavior, fixed bed adsorption experiments with fermentation broth as well as aqueous solutions will be performed in follow up studies.

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Acknowledgment We would like to express our thanks to Max Zomerdijk and Reza Maleki Seifar (Department of Biotechnology/Delft University of Technology) for their help during GC and mass spectrometry analysis. This project is financially supported by The Netherlands Ministry of Economic Affairs and the B-Basic partner organizations (www.b-basic.nl) through B-Basic, a public-private NWO-ACTS programme (ACTS ) Advanced Chemical Technologies for Sustainability). Literature Cited (1) Bechthold, I.; Bretz, K.; Kabasci, S.; Kopitzky, R.; Springer, A. Succinic acid: A New Platform Chemical for Biobased Polymers from Renewable Resources. Chem. Eng. Technol. 2008, 31, 647. (2) McKinlay, J. B.; Vieille, C.; Zeikus, J. G. Prospects for a Bio-Based Succinate Industry. Appl. Microbiol. Biotechnol. 2007, 76, 727. (3) Okino, S.; Noburyu, R.; Suda, M.; Jojima, T.; Inui, M.; Yukawa, H. An Efficient Succinic Acid Production Process in a Metabolically Engineered Corynebacterium glutamicum Strain. Appl. Microbiol. Biotechnol. 2008, 81, 459. (4) Efe, C.; van der Wielen, L. A. M.; Straathof, A. J. J. High Silica Zeolites as an Alternative to Weak Base Adsorbents in Succinic acid Recovery. Ind. Eng. Chem. Res. 2010, 49, 1837. (5) Ruthven D. M. Principles of Adsorption and Adsorption Processes; John Wiley and Sons, Inc.: New York, 1984. (6) Ferro-Garcia, M. A.; Rivera-Utrilla, J.; Bautista-Toledo, I.; MorenoCastilla, C. Chemical and Thermal Regeneration of an Activated Carbon Saturated with Chlorophenols. J. Chem. Technol. Biotechnol. 1996, 67, 183. (7) Dubey, K. V.; Juwarkar, A. A.; Singh, S. K. Adsorption-Desorption Process Using Wood-Based Activated Carbon for Recovery of Biosurfactant from Fermented Distillery Wastewater. Biotechnol. Prog. 2005, 21, 860.

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(8) Holtkamp, M.; Erhardt, F. A.; Jordening, H. J.; Scholl, S. ReactionIntegrated Separation of Isomaltose by Ad- and Desorption on Zeolite. Chem. Eng. Process. 2009, 48, 852. (9) King J. C.; Husson S. M. Regeneration of Carboxylic Acid-laden Basic Sorbents by Bleaching with a Volatile Base in an Organic SolVent. U.S. Patent 5,965,771, 1999. (10) Davison, B. H.; Nghiem, N. P.; Richardson, G. L. Succinic Acid Adsorption from Fermentation Broth and Regeneration. Appl. Biochem. Biotechnol. 2004, 113-16, 653. (11) Bowen, T. C.; Vane, L. M. Ethanol, Acetic Acid, and Water Adsorption from Binary and Ternary Liquid Mixtures on High-Silica Zeolites. Langmuir 2006, 22, 3721. (12) Harlick, P. J. E.; Tezel, F. H. Adsorption of Carbon Dioxide, Methane and Nitrogen: Pure and Binary Mixture Adsorption for ZSM-5 with SiO2/Al2O3 Ratio of 280. Sep. Purif. Technol. 2003, 33, 199. (13) Perry R. H.; Green D. W. Perry’s Chemical Engineers’ Handbook, 7th ed.; McGraw-Hill: New York, 1997. (14) Carlson T. L.; Peters, E. M., Jr. Low pH Lactic Acid Fermentation. US patent 6,475,759, 2002. (15) Schildhauer, T. J.; Hoek, I.; Kapteijn, F.; Moulijn, J. A. Zeolite BEA Catalysed Esterification of Hexanoic Acid with 1-Octanol: Kinetics, Side Reactions and the Role of Water. Appl. Catal., A 2009, 358, 141. (16) Motoyuki, S. Adsorption Engineering; Chemical Engineering Monographs; Elsevier Science Publishers B.V.: Amsterdam, 1990; Vol. 25. (17) Handbook of chemistry and physics; U.S. Department of Health and Human Services Food and Drug Administration Center for Drug Evaluation and Research (CDER) Center for Biologics Evaluation and Research (CBER) (http://www.fda.gov/downloads/RegulatoryInformation/ Guidances/ucm128282.pdf); http://home.flash.net/∼defilip1/solubility.htm.

ReceiVed for reView January 4, 2010 ReVised manuscript receiVed February 19, 2010 Accepted February 24, 2010 IE1000168