Extraction of Amphoteric Amino Acid by Bipolar ... - ACS Publications

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Extraction of Amphoteric Amino Acid by Bipolar Membrane Electrodialysis: Methionine Acid as a Case Study Xi Lin,† Jiefeng Pan,† Mali Zhou,† Yanqing Xu,† Jiuyang Lin,*,‡ Jiangnan Shen,*,† Congjie Gao,† and Bart Van der Bruggen§ †

Center for Membrane Separation and Water Science & Technology, Zhejiang University of Technology, Hangzhou, Zhejiang 310014, China ‡ College of Environment and Resources, Qi Shan Campus, Fuzhou University, No. 2 Xueyuan Road, University Town, Fuzhou, Fujian 350108 China § Department of Chemical Engineering, KU Leuven, W. de Croylaan 46, B-3001 Leuven, Belgium S Supporting Information *

ABSTRACT: Methionine is an important amino acid to block the autophagy in cells. In industry, the production of methionine through the hydantoin pathway generates a huge amount of inorganic salts (i.e., Na2SO4), reducing the product purity. In this work, an efficient bipolar membrane electrodialysis (BMED) technology was proposed to extract highpurity methionine from the mother liquor of reaction. Labscale experiments were conducted with an optimized BMED stack at a current density of 150 A/m2. The energy consumption and current efficiency were acceptable, reaching 2.156 kWh/kg NaOH and 75.10%, respectively. Specifically, the base, i.e., NaOH, with a high concentration generated in BMED stack can be used for effective adsorption of H2CO3, in view of reducing the emission of CO2. Furthermore, a simulation of cation migration during the BMED operation was performed on the basis of the relationship between pH and the concentration of different ion species. From the simulation, it is critical to control the pH at ∼4.4 to maximize the purity and reduce the extraneous loss for methionine. Finally, a pilot-scale experiment was designed to evaluate the economic feasibility of BMED for the production of methionine. It can be confirmed that total cost of BMED operation for the production of methionine with high purity (99.4%) was estimated to be 321 $/t Met, which is economically viable in industry.

1. INTRODUCTION Methionine acid (Met) is one of two sulfur-containing proteinogenic amino acid involved in the mammalian metabolism, which can effectively prevent cell autophagy.1 However, methionine acid can not be readily synthesized in vivo by mammals, requiring the ingestion in vitro. Currently, it has been used in plenty of industrial fields, such as fodder, food, and pharmaceutical, showing a promising market demand.2 The hydantoin pathway is widely applied to produce methionine acid using 5-(2-methylthioethyl) hydantoin as the raw material, which is shown in Scheme 1. First, a large amount of NaOH is employed to facilitate the ring-opening reaction of 5-(2methylthioethyl) hydantoin for the production of the intermediate product (i.e., sodium methionine) along with a high amount of sodium carbonate. Subsequently, sulfuric acid is employed to neutralize the reaction mixture and to obtain the electro neutral product (methionine). However, the resultant inorganic salt, i.e., Na2SO4, reduces the purity of methionine acid, urging intensive purification to meet the purity requirement of fodder-grade (≥98.5%). Additionally, the waste stream contains a relatively high amount of the target product and salts © XXXX American Chemical Society

(mainly Na2SO4). The direct discharge of waste stream not only causes the loss of the valuable product but also gives rise to severe damage to the aqueous environment.3,4 The electrodialysis (ED) has been employed to extract target amino acid with alternating cation-exchange and anionexchange membranes in a direct current field, which have been reported previously.5−8 However, it requires the external pH control through the dosing of acid or base to balance the charge behavior of ions.9 In the concept of sustainability, a bipolar membrane electrodialysis (BMED) technology is proposed to improve the ED process. BMED technology offers an environmentally friendly avenue to produce organic acids in the food and pharmaceutical industry.10−13 The strategy is to convert pure organic acid salts into organic acid and base through the combination with H+ and OH− ions dissociated from water by a bipolar membrane. Presently, BMED Received: January 11, 2016 Revised: February 22, 2016 Accepted: February 23, 2016

A

DOI: 10.1021/acs.iecr.6b00116 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research Scheme 1. Conventional Process for the Synthesis of Methionine Acid

technology has been reported to successfully produce organic acids at different scales, including sebacic acid,14 lactic acid,15 and gluconic acid.16 Therefore, the bipolar membrane electrodialysis (BMED) technology exhibits a promising alternative to sulfuric acid addition for simplifying the neutralization step during the extraction of methionine. In the previous studies, a limited work was reported to extract amino acid by BMED technology.17−20 Jiang et al. confirmed the feasibility of BMED for the extraction of methionine.20 However, in this case, the purity and recovery rate for methionine is still insufficient for industrial practice. Thereby, it is of paramount importance to optimize the BMED process for the production of methionine with fodder grade. Generally, compared to the organic acid systems mentioned above, the unique amphoteric property of methionine potentially hinders its effective extraction from the mother liquor. In order to overcome the limitation, Kattan Readi et al. proposed to design a segmented BMED process through the combination of ionic transport and water splitting to keep the pH unchanged during extraction of amino acids.21 In the practical application, the presence of Na2CO3 (see Scheme 1) in the mother liquor substantially increases the energy consumption of BMED stack and makes a negative impact on the purity of extracted methionine. Therefore, this adverse impact of a high content of inorganic salt (i.e., Na2CO3) on the extraction of methionine acid by BMED remains unexplored and needs confirmation as a guideline for the industrial production of methionine. In this work, the application of BMED with two compartments for the extraction of methionine acid from the methionine and Na2CO3 mixture was studied. Given the complexity of the methionine mixture, commercial membranes were screened to improve the performance of BMED operation. Subsequently, the operational parameter (i.e., current density) was optimized to improve the purity and recovery efficiency. Furthermore, with the supplementation of a mathematic simulation, the relationship between the mass transfer and pH value was rectified, exhibiting a guideline for pH optimization and economic assessment, in view of industrial application.

Table 1. Properties of Feed Solution in the Study item

characteristics

pH (20 °C) conductivity (mS/cm) carbonate-alkalinity (mol/L) total methionine concentration (g/L) Na+ concentration (g/L)

10.24 26.4 1.13 24.65 29.83

Fuma-Tech (Germany). The properties of these membranes are shown in Table 2. Table 2. Main Characteristics of Mono- and Bipolar Membranes Used in the Experimenta membrane

thickness (mm)

Bp1E CMB FBM FKB

0.20−0.35 0.21 0.2 0.11−0.13

ion exchange capacity (meq/g)

area resistance (Ω/cm2)

2.4−2.8

4−4.5 98

0.8

98

a

Data are collected from the product brochure provided by the manufacturer.

2.2. Experimental Setup. The extraction of methionine acid from the mixture liquor was performed in a lab-made BMED stack with two compartments, which can potentially have lower energy consumption, compared to configurations with three compartments.20,22 Figure 1a shows the design of the BMED stack in detail: five repeating units consisting of a bipolar membrane and a cation exchange membrane were inserted between the cathode and anode chamber. These membranes were separated by a spacer with a thickness of 0.7 mm. The dimension for each membrane is 11 cm × 27 cm, and the effective membrane area of the BMED stack is 945 cm2. The BMED stack was performed at room temperature.23 Figure 1b demonstrates the flowchart for methionine production by BMED. Prior to the experiment, the sodium methionine liquor was pumped as the feed solution to the feed chamber, and deionized water was initially employed as medium in the base compartment for alkali production. A 0.3 mol/L H2SO4 solution, which was applied as the rinsing solution, was circulated in the two electrode chambers. The initial volume of the solutions in each compartment was 500 mL. The circulation flow rate of the solution in the feed, base, and electrolyte chamber was set at 40, 40, and 60 L/h, respectively. The voltage and current were directly recorded by a regulated CV/CC power supplier (WYL 1703 × 2, Hangzhou Siling Electrical Instrument Ltd., China). All the experiments were interrupted when the pH in the feed compartment decreased below 3.8, since the carbonate present in the feed solution was completely transformed in the form of H2CO3 (99.82%) at pH 3.8. During the procedure, the BMED

2. EXPERIMENTAL SECTION 2.1. Materials. The mixture of sodium methionine and sodium carbonate through the hydantoin pathway was supplied by Chongqing Ziguang Chemical Engineering Co. (Chongqing, China) as the feed solution. The compositions of the mixture solution are shown in Table 1. In this work, different kinds of membranes were selected to optimize the combination for a series of a bipolar membrane and a cation-exchange membrane type. One of the bipolar membranes (Bp1E) and a cation exchange membrane (CMB) used in the experiments were purchased from ASTOM (Japan). The other membranes including the bipolar membrane (FBM) and the cation exchange membrane (FKB) were obtained from B


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Industrial & Engineering Chemistry Research

Figure 1. Scheme for the extraction of methionine by the BMED process. (a) Configuration of the two-compartment BMED stack (BP, bipolar membrane; C, cation exchange membrane). (b) Flowchart of methionine extraction by BMED.

2.4. Simulation of Mass Transfer for pH Optimization in BMED Stack. Since methionine acid is an ampholyte with the isoelectric point of 5.74, pH can be an essential factor for the transportation of methionine acid in BMED stack. Ideally, methionine acid can be practically retained by the cation exchange membrane when it carries negative charges. While pH of the feed solution is lower than the isoelectric point (pH = 5.74), methionine acid carries the positive charge and migrates through the cation exchange membrane, resulting in the leakage of methionine acid and thus reducing the purity of NaOH produced in the base compartment. Therefore, the optimization of pH for the BMED operation is critical for the recovery of methionine acid. In this work, a simulation for the transportation of target products in terms of sodium ion through the feed compartment was performed. The relationship between the migration of sodium ions (Nmigration) and the pH of the feed solution can be derived by eq 5:

performance at different current densities was systematically investigated. 2.3. Efficiency Assessment. The performance of the BMED process was evaluated in terms of current efficiency, energy consumption, product yield, and product purity. The current efficiency is the ratio of the stoichiometric number of electrical charges required for base production to the overall electrical charge employed in the BMED stack: η=

Z(Ct − C0)VF × 100% NIt

(1)

where Ct and C0 denote the concentration of NaOH at time interval t and 0 during BMED operation, respectively. Z represents the absolute valence of OH−, V is the circulated volume of NaOH solution in the base compartment, F is Faraday’s constant, I is the electrical current, and N denotes the number of repeating units (N = 5). The integral energy consumption, E, was calculated by extrapolating the results for the production of 1 kg of NaOH in eq 2: E=

∫0

t

UI dt CtVtMb

Nmigration = =

(2)

where U is the voltage drop across the BMED stack, I is the applied current, Mb is the molecular weight of NaOH, Vt is the volume of NaOH solution in base compartment at time t, and Ct is the concentration of NaOH at the time t. The recovery (R) and purity (P) of methionine acid are expressed as follows:

R=

MMet, t MMet,0

d(∑ ncation) dt = dt

∫

d(∑ ncation) d(pH) dt · d(pH) dt

∫ φ(pH)·ω(t )dt

(5)

where ω(t) denotes the decay rate of pH as a function of time t and φ(pH) represents the migration rate of cation ions as a function of pH. On the basis of the law of mass balance in the mixture solution, φ(pH) can be given by eq 6: φ(pH) =

× 100%

⎛ M Na+, t ⎞ ⎟⎟ × 100% P = ⎜⎜1 − M Na+,0 ⎠ ⎝

∫

(3)

= (4)

d(∑ nanion) d(pH)

d[n(Met−) + 2n(CO32 −) + n(HCO3−)] d(pH)

(6)

According to the Henderson−Hasselbalch formula,24 the fraction relationship of various charged ions of methionine species under specific pH could be expressed by eq 7. While the fraction relationship of various charged ions for carbonate species could be determined by eq 8.

where MMet, t and MMet, 0 are mole weight of methionine in the feed solution at times t and 0, respectively. MNa+, t and MNa+, 0 are mole weight of sodium ions in the feed solution at times t and 0, respectively. C


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Industrial & Engineering Chemistry Research ⎡ C(Met−) ⎤ ⎡ C(Met ±) ⎤ pH = pK NH + log⎢ ⎥ = pKCOOH + log⎢ ⎥ ± 2 ⎣ C(Met ) ⎦ ⎣ C(Met+) ⎦

(7)

⎡ C(CO 2 −) ⎤ ⎡ C(HCO3−) ⎤ 3 ⎥ = pK a + log⎢ ⎥ pH = pK a1 + log⎢ − 2 ⎣ C(H 2CO3) ⎦ ⎣ C(HCO3 ) ⎦

(8)

±

−

where C(Met ), C(Met ), C(Met ), C(HCO3−), and C(H2CO3) denote the corresponding concentration for each substance, respectively. pKNH2 and pKCOOH denote the ionization constant of the −NH2 and −COOH group in methionine, respectively, while pKa1 and pKa2 represent the first and second order dissociation constant for H2CO3, respectively. 2.5. Analytical Methods. The concentration of NaOH was determined by titration with a 0.11 mol/L standard sulfuric acid solution. The concentration of methionine was measured through the indirect iodometric method, on the basis of the Chinese Standard Method (GB/T17810-2009). The concentration of sodium ion was determined by using ion chromatography (792 Basic IC, Metrohm, Switzerland). The pH in the feed compartment was monitored online by a pH meter (S220 type, Mettler-Toledo, Switzerland). +

C(CO32−),

3. RESULTS AND DISCUSSION 3.1. Membrane Screening. In a realistic application, the performance of BMED stack can be significantly affected by the types of the applied membranes.22 Therefore, it is of importance to screen the suitable membranes for the enhancement of BMED performance and thus reduce its energy consumption.25,26 In this case, the combination of membranes for BMED stack can be classified into four groups, namely, Bp1E-CMB, Bp1E-FKB, FBM-CMB, and FBM-FKB, which was denoted as Group 1, Group 2, Group 3, and Group 4, respectively. Figure 2 shows the recovery, the current efficiency, and energy consumption equipped with different membranes during the production of methionine acid at a current density of 200 A/m2. As shown in Figure 2a, the BMED process can be a technically viable approach for the production of methionine: more than 90% of methionine acid with a >99.9% purity can be recovered. Figure 2a also indicates that the BMED stacks with different membrane combinations have a similar recovery for methionine acid at the same operation condition. Furthermore, the screening of the membrane combination for BMED stack is carried out in terms of current efficiency and energy consumption, which is shown in Figure 2b. In this case, the BMED stack with the combination of Bp1E-CMB has the best performance for the production of methionine acid with a current efficiency of 80.4% and an energy consumption of 2.25 kWh/kg, while the BMED stack with FBM-FKB mode has the highest energy consumption (2.64 kWh/kg) and the current efficiency drops by 12%. The huge difference in the performance of membrane stack is mainly ascribed to the properties of the used membranes. On one hand, the two used commercial cation exchange membranes, CMB from ASTOM and FKB from Fuma-Tech, have the same functional group (−HSO3) and their performance can be evaluated by the property parameters. Compared to FKB cation exchange membrane, CMB cation exchange membrane possesses the higher ion exchange capacity and lower area resistance, presenting the stronger conductivity to facilitate the ion transfer. This is consistent with the previous studies.27 Pinacci and Radaelli also observed that the BMED stack equipped with CMB membrane yields a faster ion transfer with lower energy

Figure 2. Effect of membrane combination on the BMED performance. (a) Recovery rate and purity of methionine. (b) Current efficiency and energy consumption of the BMED process (Group 1: Bp1E-CMB; Group 2: Bp1E-FKB; Group 3: FBM-CMB; Group 4: FBM-FKB).

consumption than that equipped with FKB membrane during the recovery of citric acid from fermentation broths.27 On the other hand, compared to Bp1-E bipolar membrane with twolayer structure, FBM bipolar membrane has a unique threelayer structure, specifically containing a middle layer with tertiary amino groups for the enhancement of the dissociation of water molecules. However, Bp-1E bipolar membrane exhibits the higher efficiency in water dissociation than the FBM bipolar membrane, due to its higher charge density. This can be confirmed from their corresponding current−voltage curves, which is systematically described by Mafé et al.28 On this basis, the Bp1E-CMB combination for the BMED stack is considered as the most suitable choice to produce methionine acid. 3.2. Voltage Drop Across the BMED Stack. The BMED stack equipped with Bp1E-CMB membranes was further employed to investigate the effect of the current density on the BMED performance. Figure 3 shows the effect of the current density on the voltage drop across the BMED stack. The voltage drop of the BMED stack has a “U” shape as a function of operation time during the electrodialysis process. In the initial stage, under the applied electrical field, the sodium ions in the feed solution started to transfer to the base compartment, reducing the electrical resistance of the BMED stack, and the applied voltage of the BMED stack thus abruptly declined. Afterward, the BMED stack obtained a steady state in the intermediate phase, representing a slight fluctuation in the voltage drop. This indicates that the electrical resistance of the BMED stack almost remained invariable.13 During this stable operation stage, the continuous transportation of sodium ions D


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Na2CO3, the mass transfer of Na+ ions was undermined. Consequently, the final concentration of NaOH in the base compartment reaches to ca. 1.30 mol/L. This is efficient for CO2 adsorption, in view of reducing the emission of CO2 during the BMED operation. Furthermore, the concentration of NaOH in the BMED stack increases with the increasing current density. This is attributed to the faster mass transfer of Na+ ions under the higher applied electrical field across the BMED stack. On the other hand, the dissociation of water molecules was enhanced to combine with Na+ ions, due to the Second Wien effect.15,25 3.4. Current Efficiency and Energy Consumption in the BMED Process. The performance of current efficiency during the BMED process with different current densities is shown in Figure 5.

Figure 3. Time dependency of the voltage drop of the BMED stack at different current densities.

from the feed compartment through the cation exchange membrane increases the electrical resistance of the feed compartment. On the other hand, the concentration of NaOH in the base compartment improved, promoting the electrical resistance of BMED stack to level off.29 With the exhaustion of sodium ions in the feed compartment, the electrical resistance in the feed compartment further increased, resulting in a sharp increase of the voltage drop across the BMED stack. This is consistent with the previous studies on the recovery of organic solutes, e.g., succinic acid, glyphosate, and formic acid.25,30,31 Figure 3 also indicates that the voltage drop across the BMED stack increases with the applied current density. At higher current densities, the dissociation of water molecules on the interface of the bipolar membrane and transportation of Na+ ions through the cation exchange membranes were intensified (see the conductivity of feed solution in Figure S1). This can give rise to a drop of electrical resistance of the BMED stack. However, the current efficiency and energy consumption of the BMED process should be optimized in these conditions. 3.3. Evolution of Base Concentration in BMED Stack. During the BMED operation, the base, i.e., NaOH, can be generated, which can be reused as the absorbent for the capture of formed CO2, in view of closing the loop of materials. The evolution of base concentration in the BMED stack is shown in Figure 4. As shown in Figure 4, the concentration of NaOH in the base compartment increases linearly at the stable operation stage at the fixed current density. Under the electrical field, the Na+ ions continuously transferred through the cation exchange membranes. In the ultimate stage, due to the exhaustion of

Figure 5. Time dependency of the current efficiency of the BMED stack at different current densities.

It can be seen that the current efficiency of the BMED stacks decreased with the operation time at fixed current density. It was mainly attributed to the proton leakage through the ion exchange membranes. The increase of the concentration of acid in the feed compartment led to an enhanced proton leakage. The hydrogen ion would compete with sodium ions to migrate through the cation-exchange membrane, which is consistent with previous studies.32,33 Table 3 shows an overview of the BMED performance at different current densities for the production of methionine. It Table 3. Energy Consumption and Methionine Recovery at Various Current Densities current density (A/m2)

Met concentration (g/L)

Met recovery (%)

Met purity (%)

energy consumption (kWh/kg NaOH)

150 200 250 300

23.22 23.37 23.93 22.63

94.3 94.8 97.1 91.8

99.98 99.97 99.98 99.98

2.156 2.250 3.206 3.265

can be seen that the energy consumption of the BMED stack increases with the applied current density, mainly due to the fact that a large part of the total electrical energy is consumed to overcome the electrical resistance as the current density increased.34 Besides, the BMED stack shows an acceptable recovery (91.8−97.1%) for methionine acid with a high purity (∼99.9%), indicating a high potential for the regeneration of methionine from its salt solution.

Figure 4. Evolution in concentration of the base solutions at different current densities. E


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Industrial & Engineering Chemistry Research 3.5. Optimization for pH Control in the BMED Stack. 3.5.1. Evolution of pH in the Feed Solution. The evolution of pH in the feed solution at different current densities is shown in Figure 6. As indicated in Figure 6, the pH of the feed solution

Figure 8. Time dependency of the transfer of sodium and methionine in the salt compartment during the BMED process at a current density of 150 A/m2.

5.74, point a in Figure 8) of methionine, the Met− ions are completely transformed into neutral solutes (Met±). However, it is insufficient for the complete removal of Na+ ion, yielding 88.7% purity for methionine (see Figure 10), since CO32− ion is partially acidized into carbonic acid at the isoelectric point of methionine. Practically, CO32− ion can be completely transformed into carbonic acid when approaching the terminal stage of the experiment (pH = 3.65) (see Figure 7). Simultaneously, the protonation of methionine occurs as well. As indicated in Figure 9, the protonation of methionine initiated from its

Figure 6. Evolution of pH value in feed solution at different current densities.

decreased due to the acidification of the methionine and carbonate. In the initial stage, pH of the feed solution decreased almost linearly with the operation duration, ranging from ∼10.3 to 6.8. As indicated in Figure 7, carbonate, bicarbonate, and

Figure 7. pH dependency of ionic fraction for different species in the feed solution.

Figure 9. Variation in fraction of Met+ ion in the feed at the current density of 150 A/m2.

−

Met ion coexisted in the feed solution with an initial pH of 10.3, appearing in the terminal stage of buffer zone A. In this case, the neutralization reaction of CO32− ions played the dominant role, which mainly influenced the pH of the feed solution. Subsequently, the pH of the feed solution in the BMED stack decayed at a slower pace. This is mainly due to the fact that the buffer zone B where a high amount of HCO3− and H2CO3 coexisted could ease the change of pH in the feed solution (see Figure 7). Since most of HCO3− was transformed into H2CO3, the buffer capacity of the feed solution was undermined, inducing the sharp decline of pH in the feed solution from 5.8 to 4.0 in a transition region. 3.5.2. Mass Transfer in BMED Stack. Figure 8 presents the migration for sodium ion and methionine in the BMED process at the current density of 150 A/m2. As shown in Figure 8, the transfer amount of Na+ ion calculated in the simulation using eq 5 fits well with the experiment results (relative errors for Na+ migration within 3%). Meanwhile, the concentration of methionine almost remained constant (∼24.40 g/L) at high pH. This is mainly due to the electrostatic repulsion effect for Met− ions in the BMED stack, preventing Met− ions from penetrating through the cation exchange membranes. When the pH of feed solution approaches to the isoelectric point (pI ∼

isoelectric point (pH = 5.74) and then the concentration of methionine started to drop. Specifically, the concentration of Met+ ions accounts for 4.57% at pH of 3.65 at the terminal stage of BMED operation, which can severely induce the loss of methionine through penetrating the cation exchange membrane (see Figure 9). At the terminal stage of BMED operation, the concentration of methionine fell from 24.40 to 23.75 g/L. On the other hand, the decay of pH in feed solution during BMED operation can facilitate the removal of Na+ ions, enhancing the purity of methionine (see Figure 10). The purity of methionine reaches up to 98.7% at pH of 4.39 (point b in Figure 10). However, the purity of methionine slightly increases at lower pH. Generally, the lower pH of feed solution induces the migration of methionine through the protonation, resulting in the loss of methionine. This is unfavorable for the production of methionine in industry. In order to balance the trade-off between the purity of methionine and its recovery, the pH of feed solution for terminating the BMED operation can be optimized at point b (pH ∼ 4.39). 3.6. Pilot Test and Process Economics Evaluation. On the basis of the experimental results on pH optimization of the BMED stack, a pilot-scale experiment (effective membrane area F


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technology for the purification of methionine, which was produced through the hydantoin method, the overall cost for producing the methionine with high purity is about 1321−1421 $/ton. As reported, the commercial feed-grade DL-methionine with 99% purity has a price of 4700−4830 $/ton.36 Thus, the supplementation of BMED technology for producing highpurity methionine acid is profitable in industry. In addition, the BMED process can successfully provide the methionine product with higher purity in a “green” way, avoiding the regeneration of Na2SO4. On the other hand, the produced NaOH as byproduct can be recycled for absorption of CO2 which is emitted in the system and reused as raw material in the hydantoin pathway.

Figure 10. Purity of methionine in the feed at the current density of 150 A/m2.

of 1.2 m2 and current density of 200 A/m2) was performed to evaluate the economic feasibility of BMED stack for industrial production of methionine acid. The pilot-scale system was designed with the treatment capacity of 96 tons/year. Especially, the pH of feed solution in the BMED operation was fixed at 4.2 ± 0.1 to maximize the purity of methionine (99.4%), which can be comparable to the commercial product.34 The process cost of the pilot-scale BMED stack for the production of methionine acid is shown in Table 4. In general,

4. CONCLUSION In this study, the feasibility of extracting the methionine acid by the BMED stack from the mother liquor (methionine and Na2CO3 mixture) during the production of methionine was demonstrated. For the lab-scale experiment, the BMED stack equipped with the Bp1E-CMB combination and operating under the current density of 150 A/m2 was considered as the optimal configuration. In this case, the energy consumption (2.16 kWh/kg NaOH) and current efficiency (75.10%) were achieved. Furthermore, NaOH with high purity (∼1.30 mol/L) was generated and reused for H2CO3 adsorption, minimizing the emission of CO2 during BMED operation. Furthermore, a simulation method of cation migration amount was proposed, indicating the optimal control of pH at 4.2 ± 0.1 to maximize the purity of methionine and reduce its extraneous loss. The total cost of pilot-scale operation was estimated at 321 $/t Met, presenting the economic feasibility of BMED technology for the production of methionine with high purity in industry.

Table 4. Estimation on Process Cost of BMED Operation at Pilot Scale Operation Conditions current density (A/m2) treatment capacity (t/year) pH control membrane series energy consumption (kWh/t Met) recovery ratio (%) purity (%) Capital Cost membrane life (year) price for monopolar membrane ($/m2) price for bipolar membrane ($/m2) cost of membrane stack ($/t Met) peripheral cost ($/t Met) total investment cost ($/t Met) Operation Cost

200 96 4.2 ± 0.1 Bp1E-CMB 1708.14 97.1% 99.4%

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b00116. Evolution of conductivity in feed solution (PDF)

3 800 1300 6.56 1.25 7.81

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AUTHOR INFORMATION

Corresponding Authors

electricity charge ($/kWh) energy cost for methionine production ($/t Met) energy cost peripheral equipment ($/t Met) total energy cost ($/t Met)

0.113 193.02 120.16 313.19

*E-mail: [email protected] (J. Lin). *E-mail: [email protected] (J. Shen).

total process cost ($/t Met)

321.0

ACKNOWLEDGMENTS The research was supported by the National High Technology Research and Development Program 863 (No. 2015AA030502).

Notes

The authors declare no competing financial interest.

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the total cost of the BMED process consists of the operational and capital costs. The primary capital cost includes the membrane stack and the peripheral cost (i.e., pumps, monitoring device, control panels, etc.), which was estimated from the common electrodialysis equipment.35 Meanwhile, the operational cost was estimated on the basis of the electricity supply for stable performance of the BMED stack and pump during the operation. The total cost of the BMED process was estimated at 321$ for per ton of methionine (see Table 4). Generally, the cost for the production of methionine by the hydantoin method in industry was estimated as 1000−1100 $/ton2. In this case, by applying the optimized BMED

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REFERENCES

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