Electro-Membrane Process for In Situ Ion Substitution and Separation

Dec 9, 2008 - situ ion substitution and recovery of salicylic acid (SAH) from its sodium salt. The physicochemical and electrochemical properties of t...
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Ind. Eng. Chem. Res. 2009, 48, 923–930

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Electro-Membrane Process for In Situ Ion Substitution and Separation of Salicylic Acid from its Sodium Salt Mahendra Kumar, Bijay P. Tripathi, and Vinod K. Shahi* Electro-Membrane Processes DiVision, Central Salt & Marine Chemicals Research Institute, Council of Scientific & Industrial Research (CSIR), G. B. Marg, BhaVnagar-364002 (Gujarat) India

An electrochemical membrane process (EMP) with three compartments (anolyte, catholyte, and central compartment) based on in-house-prepared cation-exchange membrane (CEM) was developed to achieve in situ ion substitution and recovery of salicylic acid (SAH) from its sodium salt. The physicochemical and electrochemical properties of the ion-exchange membrane (cation- and anion-exchange membrane) under standard operating conditions reveal its suitability for the proposed reactor. Experiments using sodium salicylate (SANa) solutions of different concentrations were carried out under varied applied current density to study the feasibility of the process. Overall electrochemical reaction for the in situ ion substitution and separation of SAH from SANa under operating conditions is also proposed. Results showed that developed EMP with CEMs proved promising for the in situ ion substitution and separation of SAH with recovery of SAH with current efficiency close to 90% and energy consumption around 10 kW h/kg of the SAH produced. This process was completely optimized in terms of operating conditions such as initial concentration of SANa in the central compartment, the applied current density, Na+ flux, recovery percentage, energy consumption, and current efficiency. Furthermore, the process efficiency and energy consumption of EMP for the production of SAH were compared with electrodialysis (ED) used for the separation of Na2SO4 and SAH, formed due to acidification of SANa by H2SO4. It was observed that EMP showed high current efficiency, recovery, and low energy consumption, in comparison with ED under similar experimental concentrations. It was concluded that the proposed EMP is an efficient alternate for producing SAH from SANa by economical and environmental friendly manner. Also the production of NaOH in the cathode stream is a spin off of the EMP. 1. Introduction The compound o-hydroxybenzoic acid, known as salicylic acid (SAH), is of paramount importance because it is an intermediate product in the synthesis of dyes and drugs, such as aspirin.1-3 SAH is conventionally prepared by carboxylating sodium phenolate with CO2 under high pressure and temperature. Sodium phenolate is transformed by carboxylation into sodium salicylate (SANa), which then reacts with sulfuric acid yields SAH and sodium sulfate. Salicylic acid is recovered by precipitation and yields a waste solution of sodium sulfate with a concentration higher than 0.1% (w/w), which has corrosive properties.1,3,4 Lower solubility of the SAH in water, contrary to its salt SANa, is also a serious problem. This treatment requires a large excess of the strong inorganic acid and several successive washings with water in order to remove the maximum of the salt formed (most frequently sodium sulfate). Thus, it is urgent to investigate an alternative process for the synthesis of SAH from SANa, which avoids use of any chemicals or inorganic acids. Among the possible alternatives to the separation of sodium sulfate and SAH, membrane technology, especially ion-exchange membranes, offers many advantages that align well with the current general trends related to resources and energy management in the world. Because of their modularity and profitability on a small scale, electro-membrane separation techniques are well suited for the separation of inorganic acids or salts from organic acid.5-11 Conventional ED is a promising technique for the separation of organic and inorganic components. Recently, bipolar membrane electrodialysis (BMED) has been * To whom correspondence should be addressed. Tel: +91-2782569445; Fax: +91-278-2567562 /2566970; E-mail: vkshahi@ csmcri.org; [email protected].

developed for the conversion of salts into corresponding acid and base.12,13 If an electric voltage is applied, higher than the theoretical values of the chemical energy for water dissociation, bipolar membranes are able to split water, generating hydroxyl ions at the anode site and protons at the cathode side. Several studies have shown that BMED have economic potential for recovering inorganic, organic, or amino acids. 14-16 For the recovery of SAH from SANa, two routes of the membrane technologies could be followed: (i) with the use of BMED, to convert SANa into SAH and caustic soda and their recovery; (ii) acidification of SANa by sulfuric acid and separation of SAH and sodium sulfate by ED using ion-exchange membranes. Gavach et al. reported SAH production by BMED.3 But this is a two-step process and involves an ED unit with several cell pairs of cation-, anion-exchange and bipolar membrane, which increased the processing cost of SAH production. Further, salt diffusion through a bipolar membrane is a serious problem that affects the purity of the product. Herein, we report a novel electro-membrane process for in situ ion substitution and recovery of SAH from SANa. In the proposed EMP, in situ acidification and salt separation occurs to produce SAH from SANa. Indigenously developed CEMs separated cathode and anode streams, while H+ formed by oxidative water splitting at the anode migrated to the central compartment and exchanged with Na+. Librated Na+ further migrated toward the cathode through CEM and formed NaOH as a byproduct by combining OHproduced from reductive water splitting at the cathode. Further, process efficiency and energy consumption of EMP for the producing SAH were compared with ED used for the separation of Na2SO4 and SAH formed due to acidification of SANa by sulfuric acid. It was observed that EMP showed high current efficiency, recovery, and low energy consumption, in compari-

10.1021/ie801317n CCC: $40.75  2009 American Chemical Society Published on Web 12/09/2008

924 Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009

son with ED under similar experimental concentration. Since these three parameters for any process are important to asses its suitability for exploitation in an economical way, thus it was concluded that the proposed EMP is a suitable and environmentally friendly alternate and viable route for producing pure SAH from SANa. 2. Experimental Section 2.1. Materials and Membrane Preparations. Poly(ether sulfone) (PES) was received from Sigma-Aldrich Chemicals, and all other reagents such as SANa, SAH, NaCl, H2SO4, and dimethylformamide (DMF) of AR grade were obtained from SD Fine Chemicals, India and were used without further purification. Anion-exchange resin (Indian FFIP), a chloromethylated and aminated polystyrene (with an 8% cross-linking density, and an ion exchange capacity of 3.4 mequiv.g-1 of dry resin), was supplied by Ion-Exchange Ltd. (India) and used to prepare the heterogeneous type of anion-exchange membranes (AEM). Doubly distilled water was used for the preparation of all of the solutions. For the preparation of the heterogeneous type of AEM, anionexchange resin particles were dried in an oven at 333 K for 24 h, powdered in a ball mill, and sieved to the desired mesh size. Finely powdered anion-exchange resin was then dispersed in 60% (w/v) composition in a solution of PES in DMF, where the total solid:DMF ratio was 1:5 (w/v); then membranes were cast on a polypropylene fabric at ambient temperature by a casting machine and were allowed to dry for 30 min. Sulfonation of PES (about 60% estimated from 1HNMR) was carried out using conc. H2SO4 at 323 K under constant stirring, as reported earlier.10 An homogeneous type of CEM was prepared by dissolving sulfonated poly(ether sulfone) (SPES) in DMF (20% (w/v) solution), obtaining a uniform polymer solution which was thus cast on a polypropylene fabric at ambient temperature by a casting machine and was allowed to dry for 30 min. The obtained membranes were conditioned by treatment with 1 M HCl and 1 M NaOH, successively, and then washed thoroughly with doubly distilled water before its use. 2.2. Investigations on Membrane Electrochemical Properties. Membrane conductivity measurements in equilibration with NaCl, SANa, and SAH solutions of different concentrations were carried out using a clip cell, as reported earlier,17 at 303 K. This cell was composed of two black graphite electrodes (1.0 cm2) fixed on plexiglass plates. During the experiments, the equilibrated membrane in the experimental solution was sandwiched between both electrodes and secured in place by means of a set of screws. Membrane conductance measurements were performed by using a potentio-static, two-electrode mode with alternating current (ac). Both of the electrodes were not in direct contact with the membrane. Membrane resistance (Rm) was estimated by substraction of the electrolyte resistance (Rsol) without membrane from the membrane resistance equilibrated in the electrolyte solutions (Rcell) [Rm ) Rcell - Rsol]. The membrane resistance was measured with the help of a digital conductivity meter (century, model CC601). The process was repeated until reproducible values (within ( 0.01 mS) were obtained. The thickness of the wet membrane was measured with the help of a micrometer up to 0.1 µm accuracy. For the determination of water content, membranes were immersed in doubly distilled water for 24 h. Then their surface moisture was mopped with tissue paper and then wet membrane weighed. Wet membranes were then dried in an oven at 333 K until constant weight as dry membranes were obtained. At temperature 333

Table 1. Physicochemical and Electrochemical Properties of CEM and AEM properties

CEM

1

thickness (µm) water content2 (%) ion-exchange capacity3 (mequiv./g of dry membrane) specific membrane conductivitya, 4 (S cm-1) permselectivityb, 5

AEM

200 21.5 0.73

200 23.1 1.38

8.90 × 10-3

8.42 × 10-3

0.92

0.91

a

Measured in equilibration with 0.1 M NaCl solution. b Measured by membrane potential in equilibration in with 0.01 and 0.1 M NaCl solutions. Uncertainty for measurements: 1: 1.0 µm; 2: 0.1%; 3: 0.01 mequiv./g of dry membrane; 4: 0.01 × 10-3S cm-1; and 5: 0.01.

K for 6 h heating membrane lost absorbed water and there was no change in dry membrane weight for further heating at same temperature. Water content of the membrane was calculated using the following equation: water content ) ww - wd / wd × 100

(1)

where ww and wd are the weight of the membrane at the equilibrium swelling (wet) and dry state, respectively. For the estimation of the ion exchange capacity, desired pieces of ion exchange membranes were equilibrated in 1 M HCl or 1 M NaOH solution with constant agitation for 8 h to ensure that the membranes were fully converted into the H+ or OH- form and then washed with distilled water to remove excess of HCl or NaOH. The washed membranes were then equilibrated in 50 mL of 0.5 M NaCl solutions. The amount of H+ or OHions liberated was determined by the acid-base titration.17 Counter-ion transport numbers across the membranes (tim) were estimated from membrane potential (Em) measurements in equilibration with NaCl solutions of 0.01 and 0.1 M concentrations, according to a previously reported methodology17 using eq 2: Em ) (2tm i - 1)

RT a1 ln nF a2

(2)

where R is the universal gas constant, F is the Faraday constant, T is the temperature, n is the electrovalence (1 in this case), and a1 and a2 are activities of the NaCl solutions used, respectively. Permselectivity (Ps), the extent of counterion migration across the ion-exchange membrane, was estimated from the tm i value using the following equation: Ps )

tm i - ti 1 - ti

(3)

where ti is the counterion transport number in the solution, Ps values for CEM and AEM are given in Table 1 in equilibration with NaCl solution. 2.3. Electrodialysis Experimental Setup for the Separation of SAH and Na2SO4. A laboratory scale ED cell consists of three cell pairs of CEM and AEM and parallel-cum-series flow arrangement was used for the separation of SAH and Na2SO4. Its detailed diagram is depicted in Figure 1. The electrode housing was prepared from rigid PVC sheets with built-in flow distributor outlets. Electrodes were made of expanded TiO2 sheets coated with a triple precious metal oxide (titaniumruthenium-platinum) (6-µm thickness), with 1.5-mm thickness and 8.0 × 10-3 m2 area, were obtained from Titanium Tantalum Products (TITAN, Chennai, India). Distance between both electrodes and the effective membrane area was 1.10 × 10-2 m and 8.0 × 10-3 m2, respectively. There are three streams in the ED cell, diluted stream (DS), concentrated stream (CS), and

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Figure 1. Schematic diagram of ED cell for the separation of SAH and Na2SO4. Figure 3. The κm values in equilibration with SANa, NaCl, and SAH solutions of different concentration for CEM and AEM.

Figure 2. Schematic diagram for EMP for in situ ion substitution and separation of SAH from its sodium salt.

electrodes wash (EW). Peristaltic pumps were used to feed the solution (500 cm-3) in recirculation mode into the respective streams with a constant flow rate (0.006 m3/h), to create high turbulence in all of the streams. The whole setup was placed in an environment at room temperature (303 K) without any additional temperature control. A 0.1-M Na2SO4 solution was recirculated in electrode wash (EW). During the acidification of SANa with H2SO4, a mixture of SAH and Na2SO4 was formed. A known volume and concentration of this mixed solution was recirculated through DS during the course of ED, while deionized water was initially recirculated through CS. A dc power supply (Aplab India, model L1285) was used to apply constant potential across the electrode, and the resulting current was recorded as a function of time using a multimeter in series. The pH and conductivity of the DS and CS output were regularly monitored for determining Na2SO4 concentration, while concentration of SAH in DS was determined using UV-visible spectrophotometer (Shimadzu, Japan). 2.4. EMP Procedure for In Situ Ion Substitution and Separation of Salicylic Acid from its Sodium Salt. Schematic diagram of the experimental setup used for EMP is depicted in Figure 2. The EM cell was divided into three compartments viz., central compartment (CC), catholyte, and anolyte, with the help of CEMs consisting of three tanks to recirculate all three streams with the help of peristaltic pumps, was made of polytetraflouroethylene (PTFE). A DC power supply (model L 1285, Aplab, Mumbai, India) was used to apply constant current, while the potential was measured using digital multimeter (model 435, Systronics, India) connected in series mode. The distance between both electrodes and the effective membrane area was 1.10 × 10-2 m and 8.0 × 10-3 m2, respectively. Electrodes used in ED experiments were also used for the EMP cell. Three storage tanks and pumps were used to continuously feed CC and both EW stream (5.0 × 10-4 m3 each) in the recirculation mode of operation with 6.0 × 10-3 m3/h constant flow rate. The whole setup was placed in an environment at room temperature (303 K) without any additional temperature control. In influence of applying constant current density, H+

ions produced at anode by oxidative water splitting, migrated to CC through CEM and in situ substituted Na+, leads to the formation of SAH in CC. Substituted Na+ in CC migrated toward the cathode and produced NaOH in catholyte EW. SAH concentration in the CC and pH of the all compartment were also monitored regularly by a pH sensor placed in the respective compartments. Changes in NaOH concentration in catholyte EW were determined by acid-base titration using phenolphthalein as an indicator. In all cases, equal volumes of SANa, anolyte, and catholyte were taken for simplicity and to study the feasibility of the in situ ion substitution and separation process. 3. Results and Discussions 3.1. Properties of Ion-exchange Membranes. The physicochemical and electrochemical properties of CEM and AEM prepared and used in the investigation are given in Table 1. Both membranes exhibited good water content, ion exchange capacity, and counterion transport numbers in the membrane phase and high membrane conductivity under operating conditions.18 In addition, all properties of these membranes are comparable with the best-known ion-exchange membranes due to their physicochemical and electrochemical properties18 and are suitable for electro-membrane processes. For developing the EMP cell, knowledge on membrane conductivity in equilibration with actual operating conditions is an essential parameter. Thus, membrane conductivity was also recorded in equilibration with NaCl, SANa, and SAH solutions of different concentrations. The specific membrane conductivity (κm) was estimated by the given equation: ∆x (4) ARm where ∆x is the thickness of the wet membrane, A its area, and 1/Rm is the electrical conductivity. The variation of κm for both types of membrane (CEM and AEM) with SAH, SANa, and NaCl solutions of different concentrations is presented in Figure 3. The κm values for both types of membrane increased with an increase in electrolyte concentration, and were highly dependent on the ionic concentration in the membrane/solution interfacial zone. The κm values for both cases were very low in equilibration with SAH in comparison with SANa or NaCl solution of equal concentration. Respectively, lower membrane conductivity for CEM and AEM in equilibration with SAH solution may be explained due to its extremely low dissociation and thus lower ionic molality. Furthermore, both membranes showed relatively high membrane conductivity in equilibration with NaCl in comparison with SANa, which may be due to the high degree κm )

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Flux of Na2SO4 transport (J) from DS to CS compartments during ED may be defined be defined by following relation, considering negligible mass (water) transport through CEMs and AEMs, as in this case:10,11 J)

Figure 4. Variation of current density with time at different applied potential gradient during ED separation of SAH and Na2SO4 (2.0 g/l each).

of dissociation. The κm values of these membranes in equilibration with either SANa or NaCl suggest their suitability for application in EMP under SANa and SAH environments. 3.2. Separation of SAH and Na2SO4 by ED. In the currently used process for the production of salicylic acid, sodium phenolate was transformed by carboxylation into sodium salicylate, which then reacts with H2SO4, yielding SAH and Na2SO4. For the separation/purification of SAH, ED was proposed. Under similar experimental conditions, we investigated the feasibility of the separation of Na2SO4 from SAH by ED, efficiency, energy consumption, along with recovery and purity of the product. ED experiments were carried out at different applied constant potentials ranging between 2.0 and 3.3 V cm-1, using SANa solutions of different concentrations (1.0 -2.0 g/L) as initial feed of DS, doubly distilled water was fed in the CS, while 0.1 M Na2SO4 solution was used as the feed for both EWs in the recirculation mode of operation for a known time. The limiting current density (LCD) was determined by recording current versus potential curves under given experimental conditions and was found to correspond to the 5.0 mA cm-2. ED experiments were performed at a constant applied potential gradient (2.0-3.3 V cm-1) lower than that of LCD and the resulting current and concentration changes of both compartments (DS and CS) were recorded as a function of time. Variation of current with time during electrodialytic separation of SAH and Na2SO4 (2.0 g/l) is depicted in Figure 4, as a representative case. For each applied potential gradient, the current initially increases, and then it decreased with time. At the beginning of the experiments, the CS through which water is passed offers high electrical resistance. With the onset of Na+ and SO42- ion migration from the DS to CS compartments, the concentration of electrolyte build up in concentrated compartments, while it was reduced in the other. As a result, the electrical resistance offered by CS compartments is continuously decreased in rapid manner in comparison to the increment in electrical resistance of desalted compartments. The net effect is that the overall electrical resistance of the ED cell decreases with time initially, causing an increase in current under a constant applied potential. After reaching maximum current, the electrolyte concentration in the DS compartments was progressively lowered, which caused an increase in the overall electrical resistance, and hence the current decreased. Additionally, during ED, change in current occurred due to changes in the salt migration rate from DS compartments to CS compartments.

Va Ct - C0 A ∆t

(5)

where C0 and Ct are the initial and final concentrations of Na2SO4 in CS (mol m-3), respectively, ∆t is the time allowed for ED (s), Va is the total volume of CS feed (0.50 × 10-3 m3), and A is the effective membrane area (8.0 × 10-3 m2). Na2SO4 flux during ED is presented in Figure 5(A) as a function of time at different applied potential gradient. At the beginning of the process, J values increased linearly with time and after attending maxima flux values, decreased because of the lowering concentration of Na2SO4. Also, the transportation process of Na+ and SO42- became faster with the increase in applied potential gradient. Furthermore, the Na2SO4 flux value rate was not only highly dependent on applied potential but also strongly dependent on the initial feed concentration of SANa in the diluted stream. The extent of separation of SAH and Na2SO4 was evaluated by SAH recovery from their mixture. Recovery of the product is also an important parameter to examine the economic feasibility of any process, which may be defined as follows: SAH recovery(%) )

CtVt × 100 C0V0

(6)

where V0 and Vt are the initial and final volume of the product stream, respectively, and C0 and Ct are the initial and final concentrations of SAH in the product stream. SAH recovery values for different feed concentrations of SAH and Na2SO4 mixed solutions are also presented in Figure 5B as a function of the quantity of electricity passed (Coulombs). It can be seen that recovery of SAH decreased with an increase in feed concentration of mixed solution in DS. Moreover, about 60-70% recovery of SAH was achieved under operating ED conditions. Energy consumption and current efficiency (CE) are important parameters for assessing the suitability of any electrochemical process for their practical applications. The energy consumption (W, kWhkg-1 of SAH produced) for ED or EMP may be obtained as follows:6 W(kWhkg-1) )

∫ VIdt m t

0

(7)

where V is the cell voltage, I the current, t the time allowed for the electrochemical process, and m is the weight of SAH separated. The overall current efficiency (CE) was defined as the fraction of Coulombs utilized for the synthesis of SAH: CE(%) )

mnF × 100 MQ

(8)

where F is the Faraday constant, M the molecular weight of SAH, n the stoichiometric number (n ) 1 in this case), and Q is the electric quantity passed (Coulombs; A s). An electrochemical process must not only be technically feasible, but should also be less expensive and green in nature. To evaluate the technical and economic feasibility for the synthesis of SAH in ED, energy consumption, CE (%) and recovery (%) of SAH data under different experimental conditions are presented in Table 2. Energy consumption increased, while current efficiency decreased and recovery of SAH was increased with an increase in the applied different potential gradient, for initial feed of 2.0

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Figure 5. Variation of (A) J with time at different applied potential for SAH-Na2SO4 (2.0 g/L, each) mixed solution as a feed of DS; (B) recovery of SAH with electricity passed (Coulombs) at constant applied potential (2.7 V cm-1) with different concentrations of SAH-Na2SO4 as a feed of DS compartments. Table 2. Process Efficiencies Data of ED for the Separation of Na2SO4 from SAH-Na2SO4 Mixture under Different Experimental Conditionsa electricity Wc(kWh kg-1 passed × 103 conc. of feed recovery of (Coulomb) in DS(g/L) CEb(%) of SAH produced) SAHd(%) 0.80 1.04 1.16 0.52 0.75 0.97

2.0 2.0 2.0 1.0 1.5 2.0

42.20 41.64 38.15 28.62 37.80 37.80

9.88 13.36 18.33 12.21 18.48 18.51

48.8 62.5 63.5 42.2 54.5 63.5

a All experiments were carried out in recirculation mode of operation; applied potential ) 2.7V cm-1, feed of DS compartments: SAH-Na2SO4(500 cm3); feed of CS compartments: water (500 cm3); EW: 0.1 M Na2SO4(500 cm3). b Uncertainty 0.01%. c Uncertainty 0.01 kWh kg-1 of SAH produced. d Uncertainty 0.1%.

g/L SANa solution. At relatively high applied potential, the migration of Na+ and SO42- from the diluted stream into a concentrated stream will be quite large, which would have enhanced the transportation of Na2SO4 from the DS to CS compartments. All of these parameters were very sensitive to the operating conditions of ED along with initial feed concentration of SANa. With the increase in concentration of SAH-Na2SO4 mixed solution current efficiency and energy consumption was decreased while SAH recovery was increased. Under optimal operating conditions (0.75 × 103 Coulomb), CE and W were found to be 37.8% and 18.48 kW h kg-1, respectively, corresponding to 54.5% recovery of SAH. Obtained values of CE and SAH recovery seemed to be very low, while energy consumption was very high for the separation of SAH and Na2SO4, during ion substitution and separation of salicylic acid from its sodium salt by ED. Thus, ED is not a suitable process for the separation of SAH-Na2SO4 mixture, obtained by acidification of SANa during production process of SAH. 3.3. In Situ Ion Substitution and Separation of Salicylic Acid from its Sodium Salt by EMP. In situ ion substitution and separation of salicylic acid from SANa was carried out by an EMP cell as shown in Figure 2. Electrochemical principal of the EMP cell is described in Figure 6. Simultaneously, single step ion substitution process involved three stages: (i) formation of OH- and H+ ion at the cathode and anode by reductive and oxidative splitting of water, respectively; (ii) elctro-transport of H+ from anolyte to central compartment through CEM and in situ substitution of Na+ by H+ and formation of SAH; and (iii) electro-transport of librated Na+ from central compartment to catholyte and thus formation of NaOH. In this process, ion substitution was achieved by the combined effect of electrode polarization and simultaneously

Figure 6. Schematic representation of the possible synthesis of SAH from SANa by in situ ion substitution in EMP.

by migration of H+ and Na+. This process may be defined as electro-electrodialysis and is the combined effect of electrode polarization and electro-transport of positively charged ions from anode toward cathode. Since SANa/SAH was separated from electrodes by CEM, thus electro-migration of SA- toward cathode would not happen because of the strongly charged nature of CEM. The absence of SA- in the catholyte was checked by monitoring SA- concentration using a UV-vis spectrophotometer, and it was found to be negligible. Further, electrode reactions of the total process may be written as follows: anode reaction: 1 ⁄ 2H2O f H+ + (1 ⁄ 4)O2 v + ecathode reaction: H2O + e- f OH- + 1 ⁄ 2H2 v . One electron produced at the anode is consumed at the cathode, and thus, this is a one-electron process. The whole reaction can be regarded as a water splitting process. We can write the overall electrochemical reaction for the in situ ion substation and separation of SAH from SANa as follows:

Experiments for in situ ion substitution and separation of SAH were carried out in an EMP cell at different applied current

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Figure 7. Variation of cell voltage with time in EMP at different applied current densities; catholyte and anolyte feed water (EW) and SANa solution of (2.0 g/L) concentration as a feed of the central compartment. Table 3. Current Efficiency, Energy Consumption, and SAH Recovery Data for In Situ Ion Substitution and Separation of SAH in the EMP under Different Experimental Conditionsa applied current Wc (kWhkg-1 of recovery of density SANa feed in (mA cm-2) CC (g/L) CEb (%) SAH produced SAHd (%) 2.5 5.0 7.5 5.0 5.0 5.0

2.0 2.0 2.0 1.0 1.5 2.0

86.80 89.41 51.84 55.42 80.40 89.87

6.59 9.58 11.85 6.15 7.93 9.60

67.0 92.5 80.3 76.4 92.7 92.5

a All experiments were carried out in recirculation mode of operation under 5.0 mA cm-2applied current density. b Uncertainty 0.01%. c Uncertainty 0.01 kWh kg-1 of SAH produced. d Uncertainty 0.1%.

densities ranging between 2.5 and 7.5 mA cm-2 using SANa solutions of different concentrations (1.0 -2.0 g/L) as the initial feed of the central compartment, while water was fed in the anolyte and catholyte in a recirculation mode of operation. The variations of cell voltage with time at different current densities and SANa solutions of 2.0 g/L concentration as the central feed are presented in Figure 7, as a representative case. At constant current density, the initial cell voltage was high due to the high electrical resistance of the EMP cell because of water initially fed through the electrode compartments. In this case, the initial resistance offered by two compartments (electrode compartments) was dominated, while resistance of the central compartment was low due to SANa. Progressively, the cell voltage was lowered because of the increase in ionic concentration due to

electrode polarization and electro-migration of H+ and Na+. After a certain interval of time, the resistance offered by the catholyte and anolyte was reduced, while resistance of central compartment dominated, due to SAH. During the electrochemical process, resulting pH changes in the cathode, anode, and central compartment are presented as a function of the electricity passed (Coulombs) in Figure 8 A,B at constant current density (5.0 mA cm-2) and an SANa solution of 2.0 g/L concentration as the initial feed of the central compartment. Initially, all streams offered pH 7, because water and SANa were used as the feed in the beginning; afterward, it was decreased due to the formation of H+ and OH- in the anode and cathode compartments, respectively. The pH of the central compartment (Figure 8B) also decreased due to the formation of acid (SAH) from salt (SANa). These observations also verify the electrotransport and electrode mechanism presented in Figure 6. 3.4. Influence of Applied Current Density and SANa Concentration on In Situ Ion Substitution and Separation of SAH. In situ ion substitution and separation rate of SAH (J) may be defined as rate of migration of H+ from anolyte to CC and simultaneous exchange of Na+ or formation rate of NaOH in catholyte. J values were estimated from concentration changes of SAH in the central comportment, using eq 5, where C0 and Ct were taken as the initial and final concentrations of SAH in the central compartment (mol m-3), respectively, ∆t is the time allowed for electro-membrane process (s), Va the total volume of a central feed (0.50 × 10-3 m3), and A is the effective membrane area (8.0 × 10-3 m2). J values and recovery of SAH in the EMP cell are presented as a function of electricity passed (Coulombs) at different applied current densities (2.5-7.5 mA cm-2) with different concentrations of SANa solution (1.0-2.0 g/l) as the initial feed of the central compartment in Figure 9 A,B. At the beginning of the process, J values increased linearly with Coulombs passed and negligible back diffusion of Na+ from catholyte to CC due to the extremely low concentration of Na+ in the central feed were observed (Figure 9A). For this process, simultaneous migration and substitution of Na+ in the CC followed by H+ migration from anolyte is necessary for maintaining the electro-neutrality conditions in both compartments. J values attained limiting value and further reduced progressively because of the enhanced back diffusion of Na+ from the catholyte to CC. Furthermore, the ion substitution and separation rate of SAH were not only highly dependent on applied current density, but also strongly dependent on the initial feed concentration of SANa in CC. 3.5. Energy Consumption and Current Efficiency in the EMP. To evaluate the technical and economic feasibility of the synthesis of SAH in EMP, energy consumption, CE (%), and recovery (%) of SAH data under different experimental

Figure 8. The pH variations of different compartments with electricity passed (Coulombs) in EMP at 5.0 mA cm-2 constant applied current density with SANa solution of 2.0 g/L concentration as the feed of the following: (A) catholyte and anolyte, and (B) CC.

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Figure 9. Variation of (A) J at initial feed of 2.0 g/L SANa solution; (B) recovery of SAH, in EMP at 5.0 mA cm-2 constant applied current density and different concentration of SANa solution as initial feed of central compartment.

Figure 10. Comparison of current efficiency, SAH recovery, and energy consumption in ED and EMP, under similar operating conditions.

conditions are presented in Table 3. Energy consumption increased, while current efficiency and recovery of SAH were decreased with an increase in the applied current density for SANa solution (2.0 g/L) initial feed of the central compartment. At higher applied current density, formation of H+/OH- will be quite large because of electrode reactions. And thus, fast migration of H+/OH- from anolyte to central compartment and further to the catholyte would have lowered the exchange process of Na+ by H+. Thus, lowered CE and SAH with higher energy consumption were obtained at higher applied current density. As seen in Table 3, current efficiency, energy consumption, and SAH recovery values increased with the increase in SANa concentration in the central compartment at 5.0 mA cm-2 constant applied current density. The decrease in current efficiency and SAH recovery may be due to the lower exchange rate of Na+, and thus the rate of formation of SAH at relatively higher SANa solution concentrations, and also the resulting leakage of H+ through the CEM.19 Besides the proton leakage, a loss of water occurs in the anolyte. An earlier report20 reveals the relationship between proton leakage and water flux at high acidic conditions and found that proton flux increases while the water transport to the anodic side decreases, in agreement with our findings. Thus, besides the concentration of SANa, the nature of the CEM and its permselectivity are also important parameters. At highly acidic environments, the proton leakage could be enhanced. In addition, applied current density also had a high impact on process efficiencies, viz. current efficiency, SAH recovery, and energy consumption. These data reveal that the separation and conversion of SAH from its salt are relatively more efficient. Also, the results suggest that the EMP cell is an efficient and simple tool for in situ ion substitution and separation. Further, to assess the suitability of the EMP, its process efficiency parameters will be compared with the conventional ED. 3.6. Comparison between ED and EMP for Recovery of SAH from SANa. The process currently used for the production of salicylic acid is based on the Kolbe-Schmitt reaction, in which sodium phenolate was transformed into

sodium salicylate, followed by acidification by sulfuric acid and separation of SAH and Na2SO4. Separation of SAH and Na2SO4 was achieved by ED and its process efficiency was compared with EMP developed for in situ ion substitution and separation for producing SAH from SANa. CE, SAH recovery, and energy consumption of ED and EMP was compared as shown in Figure 10 for 2.0 g/L feed of SANa solution under constant applied current gradient, against electricity passed (Coulombs). It can be seen that under similar experimental conditions, CE and SAH recovery are high, while power consumption is low for EMP in comparison with ED. Thus, EMP is a more efficient process in comparison with ED for in situ ion substitution and separation for producing SAH from SANa. CE and SAH recovery values close to 90% indicates the suitability of the process. Furthermore, production of NaOH in the cathode stream is a spin off of EMP. Although, EMP showed suitability for the industrial exploitation, but one has to completely optimize the process for obtaining the maximum current efficiency, high recovery of SAH, along with suitability of CEMs, all of which has a high impact on economic feasibility of this process. 4. Conclusions In this work, cation- and anion-exchange membranes with good electrochemical and physicochemical properties were prepared and employed for the developing electrochemical process for in situ ion substitution and separation of SAH from SANa. Relatively high membrane conductivity values of CEM and AEM and in equilibration with either SANa or NaCl suggest their suitability for application in EMP or ED under SANa and SAH environments. Results showed that developed EMP with CEMs proved promising for the in situ ion substitution and separation of SAH with recovery of SAH with current efficiency close to 90% and energy consumption around 10 kW h/kg of SAH produced, depending on the experimental conditions, such as SANa concentration in the central compartment and the applied current density. The main factors influencing the current efficiency for in situ ion substitution and separation of SAH

930 Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009

seems to be SANa concentration in the central compartment, applied current density, and suitability of the membranes for electro-migration of H+/Na+ across them. Also, separation of SAH and Na2SO4 was achieved by ED, and its process efficiency was compared with developed EMP for in situ ion substitution and separation for producing SAH from SANa. It was concluded that EMP is a more efficient process in comparison with ED for in situ ion substitution and separation for producing SAH from SANa. Production of NaOH in the cathode stream is a spin off of the EMP. Furthermore, process performances were investigated on the laboratory scale for transforming SANa without any acidification and further separation of SAH and Na2SO4. This process is completely green in nature with no further generation of waste mass. Additionally, in this novel EMP, all separation and ion substitution occurred through water splitting without use of chemicals, which provides an eco-friendly and economically viable route. Acknowledgment M.K. is thankful to Council of Scientific and Industrial Research (CSIR), New Delhi, for providing a Senior Research Fellowship. We also acknowledge the analytical science division for instrumental support. Literature Cited (1) Krik, R. E.; Othmer, D. F. Othmer Encyclopedia of Chemical Technology; Wiley: New York, 1995. (2) Shalmashi, A.; Eliassi, A. Solubility of salicylic acid in water, ethanol, carbon tetrachloride, ethyl acetate, and xylene. J. Chem. Eng. Data 2008, 53, 199. (3) Alvarez, F.; Alvarez, R.; Coca, J.; Sandeaux, J.; Sandeaux, R.; Gavach, C. Salicylic acid production by electrodialysis with bipolar membranes. J. Membr. Sci. 1997, 123, 61. (4) Cocco, R.; Ozon, S. S.; Separation/purification of salicylic acid, US Patent No. 4,827,027, 1989. (5) Jaime, J. S. F.; Laborie, S.; Durand, G.; Rakib, M. Formic acid regeneration by electromembrane processes. J. Membr. Sci. 2006, 280, 509.

(6) Moresi, M.; Fabiana, S. Electrodialytic recovery of some fermentation products from model solutions: techno-economic feasibility study. J. Membr. Sci. 2000, 164, 129. (7) Choi, J-H.; Kim, S-H.; Moon, S-H. Recovery of lactic acid from sodium lactate by ion substitution using ion-exchange membrane. Sep. Purif. Technol. 2002, 28, 69. (8) Kameche, M.; Xu, F.; Innocent, C.; Pourcelly, G. Electrodialysis in water-ethanol solutions: application to the acidification of organic salts. Desalination 2003, 154, 9. (9) Yi, S. S.; Lu, Y. C.; Luo, G. S. An in situ coupling separation process of electro-electrodialysis with back-extraction. J. Membr. Sci. 2005, 255, 57. (10) Khan, J.; Tripathi, B. P.; Saxena, A.; Shahi, V. K. Electrochemical membrane reactor: in situ separation and recovery of chromic acid and metal ions. Electrochim. Acta 2007, 52, 6719. (11) Saxena, A.; Gohil, G. S.; Shahi, V. K. Electrochemical Membrane Reactor: single-step separation and ion substitution for the recovery of lactic acid from lactate salts. Ind. Eng. Chem. Res. 2007, 46, 1270. (12) Blaster, J.; Stamatialis, D. F.; Wessling, M. Electro-catalytic membrane reactors and the development of bipolar technology. Chem. Eng. Process. 2004, 43, 1115. (13) Xu, T. W.; Yang, W. H. Citric acid production by electrodialysis with bipolar membranes. Chem. Eng. Process. 2002, 41, 519. (14) Mani, K. N.; Chandla, F. P. Aquatech membrane technology for recovery of acid/base values from salt streams. Desalination 1988, 68, 149. (15) Strathmann, H. Electrodialysis. In Membrane Handbook; Ho, W. S., Sirkar, K.K., Eds.; Van Nostrand, NY, 1992. (16) Strathmann, H.; Bauer, B.; Rapp, H. J.; Bell, C. M. Theoretical and practical aspects of preparing bipolar membrane. Desalination 1993, 90, 303. (17) Gohil, G. S.; Shahi, V. K.; Rangarajan, R. Comparative studies on electrochemical characterization of homogeneous and heterogeneous type of ion-exchange membranes. J. Membr. Sci. 2004, 240, 211. (18) Nagarale, R. K.; Gohil, G. S.; Shahi, V. K. Recent developments on ion-exchange membranes and electro-membrane processes. AdV. Colloid Interface Sci. 2006, 119, 97. (19) Robbins, B. J.; Field, R. W.; Kolaczkowski, S. T.; Lockett, A. D. Rationalisation of the relationship between proton leakage and water flux through anion exchange membranes. J. Membr. Sci. 1996, 118, 101. (20) Ondrey, G.; Shanley, A. Making cents of acid recovery. Chem. Eng. 1993, 100, 47.

ReceiVed for reView September 1, 2008 ReVised manuscript receiVed October 23, 2008 Accepted October 29, 2008 IE801317N