Separation of Protocatechuic Acid Using Tri-n-Octylamine

Article pubs.acs.org/jced

Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Separation of Protocatechuic Acid Using Tri‑n‑Octylamine: Experimental and Mathematical Investigation Fiona Mary Antony,† Kailas L. Wasewar,*,† Biswajit S. De,‡ and Subodh Kumar§ †

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Advance Separation and Analytical Laboratory (ASAL), Department of Chemical Engineering Visvesvaraya National Institute of Technology (VNIT), Nagpur, 440010, India ‡ Department of Chemical Engineering, Indian Institute of Technology Delhi (IITD), Hauz Khas, New Delhi, 110016, India § Department of Chemical Engineering, Institute of Technology, Guru Ghasidas Vishwavidyalaya, Bilaspur, Chhattisgarh 495009, India S Supporting Information *

ABSTRACT: Protocatechuic acid has wide pharmacological significance. The separation of protocatechuic acid (0.001− 0.01 mol·kg−1) from tri-n-octylamine (TOA) (0.3140−2.827 mol·kg−1), in petroleum ether, toluene, and isobutyl acetate was investigated at 298 ± 1 K as well as atmospheric pressure. The equilibrium complexation constant (KE), extraction efficiency (E), distribution coefficients (KD) as well as loading ratio (Z) are used to explain the magnitude of protocatechuic acid loading in the organic phase. The best diluent was isobutyl acetate having a maximum distribution value as 4.57 and efficiency of 82.06% for 0.01 mol·kg−1 protocatechuic acid and 2.827 mol·kg−1 TOA concentration. For the various diluents in TOA, considering KD and %E the extraction capability was observed in the order isobutyl acetate (82.06%) > toluene (61.21%) > petroleum ether (58.88%). The design parameters for the extraction process, minimum ratio of solvent to feed (S/Fmin), and theoretical stages required were calculated. The Langmuir isotherm, mass action law, and relative basicity models represented the reactive extraction equilibrium for protocatechuic acid−TOA− diluent systems. The results of the relative basicity model were found to be near to experimental data with less than 5% error.

1. INTRODUCTION Carboxylic acids find wide applications in pharmaceutical, chemical, and food industries. 3,4-Dihydroxybenzoic acid known by the name protocatechuic acid (PCA), belonging to the category of phenolic acids, is present naturally in nuts, flowers, fruits, vegetables, plants, and spices, etc. and is used in traditional medicines.1−10 As to the pharmacological actions of PCA, it is hepatoprotective, antiatherosclerotic, antidiabetic, antibacterial, anticancer, antiaging, antiviral, analgesic, antioxidant, antifibrotic, and antiulcer. It possesses neurological, cardiac, and nephroprotective activity.11−20 Some recent studies have identified PCA to be effective against viral infections such as IDBV and H9N2.21,22 PCA is also used for producing plastics and polymers. For example an electrode having good electrochemical potential was made of aniline and PCA copolymer23 and 2,4-dihydroxybenzoic acid, an isomer of PCA is involved in core−shell polymer aerogels24 and in carbon aerogel.25 For the production of carboxylic acid, the fermentation process has emerged as a significant alternative to the petrochemical route. From plant secondary metabolites, direct PCA extraction is difficult, and hence its biosynthesis is attaining prominence. PCA can be acquired from fermentation broth by Bacillus thuringiensis, Bacillus anthracis, and Bacillus cereus,26−28also it can be produced from sugar using recombinant microorganisms.29 © XXXX American Chemical Society

Techniques used for carboxylic acid recovery include distillation, precipitation, membrane extraction, adsorption, electrodialysis, reverse osmosis, reactive extraction, supported membranes, and anion-exchange distillation, etc.30All the methods proposed have their own merits and demerits. The industrial scheme used for the retrieval of carboxylic acids was precipitation using calcium hydroxide accompanied by reacidification using sulfuric acid. But this method faces quite a few shortcomings, which include handling of huge amounts of calcium sulfate waste, slurry, and solids.31 The current techniques in practice for the recovery/removal of PCA are namely adsorption,32 microbial degradation,33 membrane process- ultrafiltration,34 and H2O2/UV or O3/ UV.35 The degradation of PCA by the Bacillus species results in 2-hydroxymuconic semialdehyde which is further degraded by enzymatic reactions into pyruvate and acetaldehyde. Out of the approaches stated, ultrafiltration and adsorption help in the retrieval of PCA. Often the expense associated with the regeneration of commercial adsorbents makes an adsorption operation expensive. Ultrafiltration faces a few drawbacks such as short membrane lifetime and fouling. Mostly these approaches are less selective, ineffective for very dilute solutions, and consume time. Received: November 1, 2018 Accepted: February 5, 2019

A

DOI: 10.1021/acs.jced.8b01013 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Details of Chemicals Used in the Present Study

Further reactive extraction of PCA was represented using different models.

Reactive extraction has emerged as an effective method for carboxylic acid recovery from, dilute aqueous streams and fermentation broths. In reactive extraction, extraction and reaction are pooled in a single vessel, which intensifies solvent extraction compared to conventional liquid−liquid extraction. It also offers several benefits when compared to conventional techniques such as the re-extraction of acid; after extraction, the acid can be recovered due to the reversible nature of the extraction. The solvent can be reused, by using back extraction; for example, for lactic acid the back-extraction methods used were diluent swing, temperature swing, that is, at higher temperature, back extraction using water, back extraction using caustic, and volatile amine distillation.31 It also helps in better pH control in the bioreactor without base addition demands and easy carboxylic acid extraction from the fermentation broth, without the lowering of pH, which could lead to destruction of the bacterial species in the broth.31 The other benefits include high purity acid recovery, decrease in recovery cost, downstream processing load, and immiscibility of reactants, etc.36−39 Acids such as gallic, propionic, cupric, lactic, pyruvic, fumaric, succinic, maleic, itaconic, malic, nicotinic, pentanedioic, tartaric, citric, acrylic, phenylacetic, levulinic acid, and protocatechuic acid40−51 etc. have been separated by reactive extraction. The current study concentrates on the reactive separation of PCA using tri-n-octyl amine (TOA) as extractant, from the aqueous phase using diluents petroleum ether, toluene, and isobutyl acetate. Parameters such as equilibrium complexation constant (KE), distribution coefficient (KD), extraction efficiency (E%) as well as loading ratio (Z) were found out.

2. MATERIALS AND METHODS 2.1. Chemicals. Tertiary amine TOA (C24H51N) (mass fraction of 99%) (Spectrochem, India), with density 0.809 g/ cm3 and molecular weight 353.66, is used as extractant. Protocatechuic acid (PCA) was acquired from Avra, India, with purity greater than 98%. Various initial PCA concentrations were prepared using deionized water. Lower concentrations were used because PCA concentration obtained from fermentation broth was less than 0.01 mol·kg−1.29 Diluents isobutyl acetate (IBA) (Avra Synthesis Pvt. Ltd., India), toluene (SDFCL, India) and petroleum ether (Rankem, India) (with purity > 98%) were of technical grade and used without any further pretreatment. Materials used for the experiments are summarized in Table.1 The initial TOA concentrations of 0.314, 0.7069, 1.2117, 10885, and 2.827 mol·kg−1 and the initial PCA concentration from 0.001 to 0.01 mol·kg−1 were used. 2.2. Experimental Section. Experiments were implemented at atmospheric pressure and constant temperature of 298 ± 1 K using an orbital shaking incubator (REMI S-24BL, Mumbai, India). Organic and aqueous phases in equal volume were taken in a 100 mL Erlenmeyer flask and shaken at 150 rpm for 5 h which was sufficient time to reach equilibrium based on preliminary studies. It was further subjected to centrifugation (REMI R-4C, Mumbai, India) for 5 min at 4000 rpm for phase separation. The pH of the initial aqueous phase PCA was measured using a digital pH meter (CyberScan B



Article

Figure 1. Pictorial representation of physical extraction.

For IBA,

pH2100, Eutech Instruments). After centrifugation, the aqueous phase was carefully separated and the aqueous phase PCA concentration was found using a UV−vis spectrophotometer (Shimadzu 1800, Japan) at λmax of 260 nm. The calibration curve for the analysis was initially plotted.50 Also the analysis of the aqueous phase before and after the extraction was done. At equilibrium, the organic phase PCA concentration was calculated by component mass balances, neglecting changes in phase volumes as confirmed by measurement of phases. Repetition of a few sets of experiments under alike settings to check result consistency was done, and the results were obtained within ±2% limit.

2 [HPCA]org = 0.9375[HPCA]aq + 261.17[HPCA]aq

For toluene, 2 [HPCA]org = 0.1007[HPCA]aq + 10.205[HPCA]aq

(3)

For petroleum ether, 2 [HPCA]org = 0.0933[HPCA]aq + 7.803[HPCA]aq

(4)

It can be observed that a linear relationship exists between PCA concentrations in both aqueous and organic phases at lower PCA concentrations signifying the validity of a Henry law type isotherm, and a parabolic relationship exists at higher concentrations, signifying a nonideal behavior, deviating from Henry’s law.54 Physical extraction of PCA can be described in terms of mainly three phenomena as explained below:40 (1) Ionization of PCA in the aqueous solution (KHPCA)

3. RESULTS AND DISCUSSION 3.1. Separation with Diluents. Separation of PCA was first done using petroleum ether, toluene, and IBA in earlier work.52 The pH of PCA aqueous solution changes as the separation progresses. The range of natural pH (without any adjustment) considered for present experiments was 3.47 to 3.96. The extent of separation of PCA from aqueous solution in diluents is responsible for the increase of pH of aqueous PCA solution. The experiments to measure the pH of PCA solution were performed for various concentrations of PCA. The correlation obtained between PCA concentration and pH is (within ±0.01)52 pH = 3.9516 − 0.222 ln[HPCA]

(2)

(HPCA)aq ↔ H+ + PCA−

(5)

KHPCA = [H+][PCA−] /[HPCA]

(6)

(2) Partition of undissociated PCA between aqueous phase (aq) and organic phase (org)

(1)

The pH values of PCA were found to be less than the pKa value (4.48).53 The results of physical equilibrium isotherm for PCA at 298 K in diluents petroleum ether, toluene, and IBA were discussed earlier.52 The second order polymer fit equations obtained using statistical analysis (Figure S1) are as follows:

(HPCA)aq ↔ (HPCA)org

(7)

P = [HPCA]org /[HPCA]aq

(8)

where HPCAaq and HPCA org refers to corresponding undissociated PCA concentration in the aqueous phase and organic phases and the partition coefficient is represented by P. C



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Table 2. Equilibrium of Protocatechuic Acid with TOA in Isobutyl Acetate at 298 Ka [TOA]0org mol kg−1

[HPCA]0 mol kg−1

[HPCA]aq mol kg−1

[HPCA]org mol kg−1

KD

av KD

Kchem D

0.314

0.001 0.003 0.005 0.007 0.01 0.001 0.003 0.005 0.007 0.01 0.001 0.003 0.005 0.007 0.01 0.001 0.003 0.005 0.007 0.01 0.001 0.003 0.005 0.007 0.01

0.0005 0.0012 0.0018 0.0023 0.003 0.0004 0.0011 0.0016 0.002 0.0027 0.0003 0.0009 0.0014 0.0018 0.0024 0.0003 0.0009 0.0013 0.0017 0.0021 0.0003 0.0008 0.0012 0.0014 0.0018

0.0005 0.0018 0.0032 0.0047 0.007 0.0006 0.0019 0.0034 0.005 0.0073 0.0007 0.0021 0.0036 0.0052 0.0076 0.0007 0.0021 0.0037 0.0053 0.0079 0.0007 0.0022 0.0038 0.0056 0.0082

1.2 1.52 1.81 2.06 2.33 1.5 1.83 2.11 2.47 2.73 1.87 2.24 2.55 2.91 3.22 2.15 2.48 2.77 3.21 3.76 2.46 2.89 3.33 3.96 4.57

1.78

0.17 0.22 0.22 0.24 0.26 0.46 0.53 0.55 0.64 0.67 0.83 0.94 0.99 1.11 1.18 1.17 1.29 1.35 1.55 1.79 1.56 1.78 1.97 2.33 2.65

0.7069

1.2117

1.885

2.827

2.13

2.56

2.88

3.44

av Kchem D 0.23

0.57

1.01

1.43

2.06

η%

av η%

ZPCA

KE

54.59 60.33 64.4 67.29 69.99 59.95 64.67 67.84 71.16 73.19 65.19 69.1 71.82 74.43 76.3 68.3 71.28 73.5 76.27 79 71.09 74.31 76.88 79.86 82.06

63.32

0.0017 0.0058 0.0103 0.015 0.0223 0.0008 0.0027 0.0048 0.007 0.0104 0.0005 0.0017 0.003 0.0043 0.0063 0.0004 0.0011 0.0019 0.0028 0.0042 0.0003 0.0008 0.0014 0.002 0.0029

5.7551

67.36

71.37

73.67

76.84

3.0262

2.1182

1.53

1.22

a [TOA]0org, initial molality of TOA in organic phase (mol·kg−1); [HPCA]0, initial molality of the protocatechuic acid in the aqueous phase (mol· kg−1); [HPCA]aq, equilibrium molality of the protocatechuic acid in the aqueous phase (mol·kg−1); [HPCA]org, equilibrium molality of the protocatechuic acid in the organic phase (mol·kg−1). Standard uncertainties u are u(T) = 1 K, u([HPCA]aq) = 0.001 mol·kg−1.

isobutyl acetate on the other hand is dipolar aprotic solvent with comparatively larger values of relative permittivity (ε) and dipole moment, etc. They provide higher diluent KD values when compared to the first category due to presence of ion electron pairs. (The property values are given in Table S1. The degree of extraction (E%) for PCA in different diluents is given by

(3) Dimerization of PCA in the organic phase (D) 2 2(HPCA)org ↔ (HPCA)org 2 D = [HPCA]2,org /[HPCA]org

(9) (10)

The overall distribution coefficient is the ratio of overall concentration in the organic phase of the carboxylic acid in partition, dimer, and complex forms and the overall concentration of the carboxylic acid in the aqueous raffinate phase in dissociated and undissociated forms.55 For physical extraction it can be stated as

ij K Ddiluent yz zz100 E% = jjjj diluent z z 1 + K D k {

It was observed that values and degree of extraction for PCA in toluene and petroleum ether are not significantly high52· 3.2. Extraction with Extractant in Diluents. The low KD values obtained recommend the use of an aminic extractant to increase the performance of the extraction process. Tri-n-octyl amine (TOA) in the concentration ranges of 10−50% by volume (0.3140−2.827 mol·kg−1) was used as the extractant in the diluents. To increase the extraction efficiency, reactive extraction with TOA in diluents is employed. Five different initial PCA concentrations (0.001 to 0.01 mol·kg−1) were considered. Tables 2−4 show the values of different parameters for PCA reactive extraction using TOA in petroleum ether, toluene, and IBA, respectively. There is a substantial increase in KD and E% for all diluent systems by using TOA. Figure 2 represents the reactive separation of PCA by means of TOA. 3.2.1. Influence of Initial Acid Concentration. The distribution coefficients obtained by employing extractant TOA, when compared to using diluents alone, was improved

2

K Ddiluent =

P + 2P D[HPCA]aq 1 + KHPCA /[H+]aq

(11)

For the present study dilute concentrations of PCA (0.001− 0.01 mol·kg−1) were considered, and the values of KHPCA/ [H+]aq were calculated and observed in the range of 0.09−0.25; hence, it can be neglected. So eq 10 can be rewritten as K Ddiluent = P + 2P 2D[HPCA]aq

(13)

Kdiluent D

(12)

Physical extraction is schematically represented in Figure 1. The dimerization constant (D) and partition coefficient (P) are given in an earlier work52 Hydrocarbons (toluene, petroleum ether) are apolar aprotic solvents having low values of relative permittivity (ε), ET, and dipole moment value, and are incapable of acting as a hydrogen-bond donors. Because of the operation of nonspecific direction, dispersion, and induction forces, they slightly interact with the acid, thus giving low diluent KD values. EsterD



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Table 3. Equilibrium of Protocatechuic Acid with TOA in Toluene at 298 Ka [TOA]0org mol kg−1

[HPCA]0 mol kg−1

[HPCA]aq mol kg−1


KD

av KD

Kchem D

0.314

0.001 0.003 0.005 0.007 0.01 0.001 0.003 0.005 0.007 0.01 0.001 0.003 0.005 0.007 0.01 0.001 0.003 0.005 0.007 0.01 0.001 0.003 0.005 0.007 0.01

0.0008 0.0022 0.0035 0.0045 0.0061 0.0007 0.002 0.0031 0.0041 0.0053 0.0007 0.0018 0.0028 0.0036 0.0047 0.0006 0.0017 0.0026 0.0033 0.0043 0.0006 0.0015 0.0023 0.003 0.0039

0.0002 0.0008 0.0015 0.0025 0.0039 0.0003 0.001 0.0019 0.0029 0.0047 0.0003 0.0012 0.0022 0.0034 0.0053 0.0004 0.0013 0.0024 0.0037 0.0057 0.0004 0.0015 0.0027 0.004 0.0061

0.23 0.37 0.45 0.54 0.65 0.34 0.49 0.59 0.7 0.89 0.5 0.68 0.76 0.93 1.14 0.61 0.78 0.91 1.12 1.35 0.8 0.96 1.16 1.35 1.58

0.45

0.14 0.24 0.28 0.35 0.42 0.26 0.36 0.42 0.51 0.65 0.42 0.55 0.6 0.74 0.91 0.53 0.66 0.76 0.94 1.13 0.73 0.85 1.02 1.19 1.38

0.7069

1.2117

1.885

2.827

0.6

0.8

0.95

1.17

av Kchem D 0.28

0.44

0.64

0.8

1.03

η%

av η%

ZPCA

KE

18.7 27.03 30.9 35.21 39.4 25.42 32.93 37.1 41.14 47.14 33.28 40.3 43.06 48.09 53.28 37.74 43.7 47.74 52.87 57.38 44.55 49.03 53.74 57.51 61.21

30.25

0.0006 0.0026 0.0049 0.0079 0.0125 0.0004 0.0014 0.0026 0.0041 0.0067 0.0003 0.001 0.0018 0.0028 0.0044 0.0002 0.0007 0.0013 0.002 0.003 0.0002 0.0005 0.001 0.0014 0.0022

1.4377

36.75

43.6

47.89

53.21

0.8555

0.6613

0.5063

0.415


2.827 mol·kg−1), the average values of KD increase from 1.78 to 3.44 with IBA, 0.448 to 1.171 with toluene, and from 0.404 to 1.062 with petroleum ether. The diluent polarity primarily controls the process of reactive extraction through solvating the polar and ion-paired organic species (complex of acidextractant) through, H-bonding, dipole−dipole interactions, etc.57 The maximum recovery for PCA was obtained using IBA with TOA, thus making it the best system for recovery of PCA from dilute aqueous solutions. For taking into account the extraction of PCA by only the extractant, Kchem a term for distribution of PCA by chemical D extraction is introduced.

significantly, which shows the significance of chemical extraction. For all diluent systems at constant TOA concentrations, the values of extraction efficiency (E%), KD were increased with rise in initial concentration of PCA. Using 2.827 mol·kg−1 TOA and at PCA concentration of 0.01 mol· kg−1 highest extraction efficiency (58.9% for petroleum ether, 61.21% for toluene, and 82.06% for IBA) was obtained. The highest KD obtained was 4.57 for the IBA-TOA system at 2.827 mol·kg−1 TOA concentration and PCA concentration of 0.01 mol·kg−1. It was detected that KD and E% improved with increase in the PCA concentration, which can be attributed to the increased accessibility of PCA molecules in the aqueous phase owing to the emaciation of the bond among water molecules and PCA molecules, and in the organic phase, due to the greater motivating force to accommodate PCA molecules.56 3.2.2. Influence of the Diluent/Extractant Concentrations. Diluents improve the solvation efficiency of the PCA−TOA complex, along with improving physical properties of extractants when added to the diluents. The values of KD and extraction efficiency were found to depend on the organic phase extractant concentration and the type of diluent.40 Extraction efficiency was found to increase in the direction IBA > toluene > petroleum ether for the same concentration of TOA, that is, 0.314 mol·kg−1 to 2.827 mol·kg−1 in different diluents. The average extraction efficiency was found in the ranges of 63.32% to 76.84% for IBA in TOA, 30.24% to 53.2% for toluene in TOA, and 28.14% to 50.74% for petroleum ether in TOA, and the maximum extraction efficiency obtained was 82.06% in IBA. For an increase in TOA (0.314 mol·kg−1 to

KDchem =

[HPCA]org − v[HPCA]diluent [HPCA]aq

(14)

In the solvent mixture, the term v[HPCA]diluent accounts for the quantity of PCA extracted by the diluent. In the organic phase, the volume fraction of the diluent is v, and [HPCA]diluent is the PCA concentration extracted by pure diluent only, that is, without the extractant. It can be observed that chemical extraction offers significant advantages over physical extraction, with KD and E% increasing in the trend IBA > toluene > petroleum ether. Through ion pair formation and hydrogen bonding of undissociated PCA molecules, TOA extracts PCA molecules at the phase boundary. For all concentrations of PCA under consideration, the highest recovery of PCA was obtained at the highest TOA concentration (50% by volume). However, the use of larger concentrations of TOA is restricted due to factors such as the increase in total process cost owing to the higher price of TOA, E



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Table 4. Equilibrium of Protocatechuic Acid with TOA in Petroleum Ether at 298 Ka [TOA]0org mol kg−1

[HPCA]0 mol kg−1

[HPCA]aq mol kg−1


KD

av KD

Kchem D

0.314

0.001 0.003 0.005 0.007 0.01 0.001 0.003 0.005 0.007 0.01 0.001 0.003 0.005 0.007 0.01 0.001 0.003 0.005 0.007 0.01 0.001 0.003 0.005 0.007 0.01

0.0008 0.0023 0.0036 0.0046 0.0063 0.0008 0.0021 0.0034 0.0044 0.0058 0.0007 0.002 0.003 0.004 0.0051 0.0006 0.0018 0.0028 0.0035 0.0045 0.0006 0.0016 0.0024 0.0031 0.0041

0.0002 0.0007 0.0014 0.0024 0.0037 0.0002 0.0009 0.0016 0.0026 0.0042 0.0003 0.001 0.002 0.0031 0.0049 0.0004 0.0012 0.0022 0.0035 0.0055 0.0004 0.0014 0.0026 0.0039 0.0059

0.22 0.29 0.4 0.53 0.58 0.31 0.4 0.48 0.59 0.73 0.47 0.52 0.66 0.77 0.96 0.57 0.66 0.81 1 1.21 0.71 0.87 1.05 1.26 1.43

0.4

0.15 0.18 0.26 0.35 0.39 0.24 0.29 0.35 0.43 0.54 0.4 0.42 0.53 0.61 0.77 0.51 0.56 0.69 0.84 1.03 0.65 0.78 0.93 1.11 1.27

0.7069

1.2117

1.885

2.827

0.5

0.68

0.85

1.06

av Kchem D 0.26

0.37

0.55

0.73

0.95

η%

av η%

ZPCA

KE

18.3 22.73 28.4 34.46 36.85 23.57 28.37 32.46 37.13 42.33 31.79 34.33 39.8 43.57 48.9 36.44 39.67 44.76 49.93 54.81 41.45 46.5 51.22 55.66 58.88

28.15

0.0006 0.0022 0.0045 0.0077 0.0117 0.0003 0.0012 0.0023 0.0037 0.006 0.0003 0.0009 0.0016 0.0025 0.004 0.0002 0.0006 0.0012 0.0019 0.0029 0.0001 0.0005 0.0009 0.0014 0.0021

1.2979

32.77

39.68

45.12

50.74

0.7124

0.559

0.4518

0.3764


Figure 2. Pictorial representation of reactive extraction using TOA.

toxicity caused to microorganisms existing in fermentation broth due to higher extractant concentration, back-extraction, etc.58 Further, most of the amine extractants such as TOA, are found to be active in acidic conditions, thus increasing the amine concentration is undesirable as it leads to an increase in pH of the extraction system.59 While considering the in situ recovery of carboxylic acid from fermentation broth, employing diluents and extractants, there are two levels of toxicity inhibiting microbial activitiesmolecular level toxicity and

phase level toxicity. Choudhury (1998) had considered the toxic effects of diluents as well as TOA on lactobacillus rhamnousus for the retrieval of lactic acid and has found that it produced toxicity effects. Molecular level toxicity symptoms were exhibited by TOA at the 5% saturation level.60 3.2.3. Loading Factor and Complex Formation. The loading ratio Z expresses the extent of loading of PCA in the organic phase (TOA + diluent).40 F


Journal of Chemical & Engineering Data Z PCA =

Article

[HPCA]org 0 [TOA]org

identical acid concentrations in various diluents different KE values are obtained which indicates the importance of diluent solvation for carboxylic acid extraction. For the IBA + TOA system in PCA when compared to other diluents the higher KE value can be attributed to the specific hydrogen bonding between the TOA:PCA complex and the proton of the diluent (ester). 3.3. Estimation of the Minimum Ratio of S/F and Number of Stages. The practicability of PCA extraction involving TOA in various diluents can be accessed by calculating the minimal S/F (solvent to feed flow) ratio for PCA recovery. Successively, for counter-current extraction the minimal number of stages required is also assessed. The following equation is used for calculating the minimum S/Fratio.61

(15)

[TOA]0org

where is the initial organic phase TOA concentration and [HPCA]org is the organic phase PCA concentration. The value of Z is influenced mainly by aqueous phase PCA concentration and acid extractability (acid−base interaction strength).30 Overall extraction process stoichiometry is influenced by the organic phase loading ratio, Z. For all concentrations ranges of TOA and PCA used in the study it was observed that ZPCA < 0.5, indicating a 1:1 PCA complexation and no overloading, and the following eq 14 can be applied30 Z = KE[HPCA]aq (16) 1−Z Z/(1 − Z) versus [HPCA]aq is plotted in Figure 3 and a straight line having slope complexation constant (KE) was

x − xout ij S yz jj zz = in KDxin − yin k F {min

(17)

where xin is the PCA concentration in feed and xout is the PCA concentration in raffinate, and the initial concentration of PCA in the extract phase is yin. The initial concentration of PCA influences the value of (S/F)min. (S/F) actually relates to 1.5 times the minimum S/F-ratio for an extraction procedure with a fixed number of extraction stages, as per a rule of thumb.62 For counter current extraction processes the modified Kremser equation is used for calculating NTS, the number of theoretical stages ÅÄÅi x − y / K y ÑÉ 1Ñ lnÅÅÅÅjjj x in − yin / KD zzz(1 − 1/Ex) + E ÑÑÑÑ ÅÅk out in D { xÑ ÑÖ NTS = Ç lnEx (18) Figure 3. Plot of Z/(1 − Z) versus [HPCA]aq (mol·kg−1) for estimation of 1:1 PCA−TOA equilibrium complexation constant (KE) for [TOA] = 2.827 (mol·kg−1) in diluents (IBA, toluene, petroleum ether) at 298 K.

where the extraction factor Ex is defined by eq 19 Ex = KD

S F

(19)

In the current study for the calculation of NTS as well as minimum S/F ratio, the parameters at 2.827 mol·kg−1of TOA in PCA for various diluents were used, which links to the highest distribution and retrieval of PCA.61 In a continuous extraction column as shown by the data in Table 5, for

obtained. For the various diluent systems in TOA, KE values follows the trend IBA (5.755−1.220) > toluene (1.437−0.415) > petroleum ether (1.297−0.376). Thus, it can be observed that the TOA−IBA system yields the highest KE value. For

Table 5. Minimum Solvent-to-Feed (S/F) Ratio and Number of Theoretical Stages (NTS) for the Recovery of Protocatechuic Acida diluent IBA

toluene

petroleum ether

xin

xout

KD

(S/F)min

(S/F)act

EX

NTS

0.001 0.003 0.005 0.007 0.01 0.001 0.003 0.005 0.007 0.01 0.001 0.003 0.005 0.007 0.01

0.0003 0.0008 0.0012 0.0014 0.0018 0.0006 0.0015 0.0023 0.003 0.0039 0.0006 0.0016 0.0024 0.0031 0.0041

2.459 2.8926 3.3253 3.9645 4.5741 0.8034 0.9621 1.1617 1.3537 1.578 0.7079 0.8692 1.05 1.2552 1.4319

0.2891 0.2569 0.2312 0.2014 0.1794 0.5545 0.5097 0.4626 0.4249 0.3879 0.5855 0.535 0.4878 0.4434 0.4112

0.4337 0.3854 0.3468 0.3021 0.2691 0.8318 0.7645 0.6939 0.6373 0.5819 0.8783 0.8025 0.7317 0.6651 0.6168

1.0664 1.1147 1.1532 1.1979 1.2309 0.6683 0.7355 0.8061 0.8627 0.9182 0.6218 0.6975 0.7683 0.8349 0.8832

2.2162 2.3996 2.5667 2.7901 2.9821 1.2625 1.3822 1.5204 1.6429 1.7753 1.1854 1.3133 1.4446 1.5812 1.6903

Standard uncertainties u are u(xout) = 0.001 mol·kg−1.

a

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Table 6. Relative Basicity Model Parametersa

accomplishing the anticipated extraction efficiency of PCA roughly about three theoretical stages would be adequate. Table 5 gives S/F and NTS values for individual diluent system in TOA (for TOA concentration giving maximum KD value). 3.4. Models. 3.4.1. Relative Basicity Model. Shan (2003) anticipated a relative basicity model for relating the 1:1 equilibirum complexation constant, KE in terms of relative basicity of extractant and apparent equilibrium constant. The carboxylic acids extraction equilibrium behavior is found to be influenced by three major factors (1) log P, acid hydrophobicity, (2) carboxylic acid equilibrium dissociation constant, pKa, and (3) pKB, which is the apparent basicity of extractant to hydrochloric acid. If the extractant mixture basicity is taken comparative to the solute, pKa,BS it could signify the extractant, diluent, and solute nature, along with distinct associations, such as solvating power. The model equation can be expressed as63 log KE = C1(pK a,BS − pK a) + log(C2P)

diluent IBA

toluene

petroleum ether

(20)

Usually, the hydrogen ion liberating power in aqueous solution is represented by the basicity of a compound. The subsequent equation is used to express pKa,BS that is, the relative basicity of the extractant toward solute:

a

K a,BS = [PCA]org [H+]aq [PCA−]aq /[TOA.HPCA]org

(22)

KE av (exp)

KE av (RBM)

0.314 0.7069 1.2117 1.885 2.827 0.314 0.7069 1.2117 1.885 2.827 0.314

2.4037 2.7117 2.9982 3.3025 3.9092 1.3801 1.5426 1.7445 1.8933 2.1108 1.3633

0.0067 0.5229 1.001 1.5198 2.2857 0.169 0.3603 0.5858 0.822 1.1415 0.1672

5.2757 2.7332 1.8167 1.3282 1.0827 1.4377 0.8555 0.6613 0.5063 0.415 1.2979

5.2736 2.7315 1.816 1.3273 1.082 1.4363 0.8546 0.6608 0.506 0.4148 1.2973

0.7069 1.2117 1.885 2.827

1.4733 1.6579 1.8247 1.9969

0.3362 0.5438 0.7794 1.0576

0.7124 0.559 0.4518 0.3764

0.7119 0.5588 0.4516 0.3763

[TOA]0org, initial molality of TOA in organic phase (mol·kg−1).

(26)

+ 0.1559

3.4.2. Mass Action Law Model. The mass action law model could describe the reactive separation of PCA. For this model, the species activities in the organic phase and aqueous phase are anticipated to be proportional to the corresponding species concentration, and the constant of proportionality or nonidealities linked with the reactive system is accounted by the equilibrium constant.40 The dissociation of PCA was considered negligible as the aqueous phase pH was found to be less than pKa (4.48) of PCA; hence, in the aqueous phase acid in undissociated form is likely to exist. For representing PCA extraction using TOA for various diluents (petroleum ether, toluene, IBA) the mass action law equilibria can be represented as

KE in eq 19 denotes the extraction ability of PCA by developing the complex of ion-pair, hydrogen-bond association and complex solvating power. Solvating power is influenced by the nature of the diluent, extractant, and solute, and is a complicated hydrogen-bonding association among the complex and the diluent.64 Shan (2006) has further proposed a model to find the equilibrium complexation constant KE, assuming the formation of a 1:1 complex only and the acid−extractant extraction reaction proceeding at the organic−aqueous interface, which is as follows (23)

KE

HPCA aq + pTOA org ↔ (HPCA.TOA p)org

The experimental data was fitted and the values of C1 and C2 were found. At different acid concentrations and extractant concentrations, for the reactive separation of PCA using TOA in petroleum ether, toluene, and IBA, the model values for KE were studied. A good fit was obtained as given in Table 6, thus the experimental KE values for reactive extraction of the PCA in the TOA−diluent system is validated by the model. The general equation representing each of the diluent system, obtained by regression calculations is as follows:

KE =

(27)

[HPCA.TOA p]org P [HPCA]aq [TOA]org

(28)

The solvation number of the TOA is represented by p and [HPCA]aq represents the PCA concentration, [TOA]org represents the extractant, that is, the TOA concentration and [HPCA.TOAp]org complex concentration in the corresponding phases. The solvation efficiency of diluents and acid properties influence the KE values. The number of reacting species and the extraction constant can be used to evaluate KD as given in the following equation

For TOA in IBA, log KE = 1.1723(pK a,BS − pK a) + 0.1271[TOA]

KD = [HPCA.TOA p](org) /[HPCA](aq) + [PCA−](aq)

(24)

=

For TOA in toluene, log KE = 1.2545(pK a,BS − pK a) + 0.1342[TOA] + 0.1667

log (C2P)

log KE = 1.2437(pK a,BS − pK a) + 0.1280[TOA] (21)

+ 0.3607

C1

For TOA in petroleum ether

K a,BS

TOA.HPCA org ←→ ⎯ TOA org + H+aq + PCA −aq

log KE = (pK a,BS − pK a)

[TOA]0org mol kg−1

= (25) H

P KE[HPCA]aq [TOA]org

[HPCA]aq + K a[HPCA](aq) /[H+](aq) p KE[TOA]org

1 + K a /[H +]aq

(29) DOI: 10.1021/acs.jced.8b01013 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


Article

As the influence of the acid dissociation term in above equation is trivially small, eq 29 can be reformed as log KD = log KE + p log[TOA]org

Table 8. Equilibrium Complexation Constant (KE) Using Langmuir Isotherm Model with Different Initial Concentration of PCAa

(30)

diluent

where [TOA]org can be expressed as

IBA

initial [TOA]org = [TOA]org − p[HPCA]org

(31)

plotting log(KD) versus log[TOA]org produces a straight-line of slope p and intercept log(KE). If [TOA]initial org ≫ p[HPCA]org, then for determining KE and n in eq 30, the initial concentration of TOA is used, that is, [TOA]initial org . The results of mass action law modeling are represented in Table 7.

toluene

Table 7. Equilibrium Complexation Constant (KE) and Number of Reacting TOA Molecules (p) with Different Initial Concentration of PCAa

petroleum ether

diluent

[HPCA]0 mol kg−1

KE av (exp)

KE av (mass action)

P

0.001 0.003 0.005 0.007 0.01 0.001 0.003 0.005 0.007 0.01 0.001

1.903 2.332 2.716 3.138 3.56 0.4466 0.6378 0.7579 0.916 1.1176 0.4179

1.7378 2.0936 2.4099 2.7919 3.1623 0.4347 0.6009 0.7107 0.8559 1.0394 0.404

0.3321 0.2929 0.2727 0.288 0.3022 0.5694 0.4374 0.4304 0.4225 0.4067 0.5398

0.003 0.005 0.007 0.01

0.5177 0.6598 0.8278 0.9742

0.4933 0.6196 0.7633 0.9064

0.4887 0.448 0.4057 0.4205

IBA

toluene

petroleum ether

a

[TOA] mol kg−1

KE av (exp)

KE av (Langmuir)

0.3140 0.7069 1.2117 1.8850 2.8270 0.3140 0.7069 1.2117 1.8850 2.8270 0.3140 0.7069 1.2117 1.8850 2.8270

5.7551 3.0262 2.1182 1.5300 1.2200 1.4377 0.8555 0.6613 0.5063 0.4150 1.2979 0.7124 0.5590 0.4518 0.3764

5.7500 3.0690 2.0030 1.5950 1.2240 1.438 0.8561 0.6732 0.5040 0.4134 1.2070 0.7131 0.5517 0.4508 0.3797

[TOA], initial molality of TOA in organic phase (mol·kg−1).

Figure 4. Parity plot for relative basicity model-predicted KE for the extraction of PCA using TOA in various diluents.

a

[HPCA]0, initial molality of the protocatechuic acid in the aqueous phase (mol·kg−1).

3.4.3. Langmuir Isotherm Model. The interaction among extractant molecules and acid molecules at equilibrium can be described using the Langmuir model. Recognizing discrete complexes existing in the organic phase, the contributions to extraction are found as a function of related process parameters in the Langmuir-type isotherm.65 The model is represented below [HPCA]org max [HPCA]org

=

KE([HPCA]aq )c 1 + KE([HPCA]aq )c

(32) Figure 5. Parity plot for Langmuir model-predicted KE for the extraction of PCA using TOA in various diluents.

66 ([HPCA]max org )/[TOA]0,org

where c = Table 8 represents the results of modified Langmuir isotherm model fitted for equilibrium data. 3.4.4. Comparison of Models. For PCA in TOA for different diluent systems experimental and modeled values of equilibrium complexation constant KE using Langmuir, mass action law, and relative basicity model were determined. The physical and chemical phenomena occurring in the extraction process can be explained in mathematical forms by using the equilibrium models selected (mass action law, Langmuir, and relative basicity models). It has been observed that the most appropriate model for explaining the reactive extraction by means of TOA was the relative basicity model (Figures 4−6). The experimental values of KE were calculated based on the

assumption of a 1:1 complex of PCA and TOA. The mass action law was found to show a deviation of more than 5%, the results showed that the complex formation that resulted was not 1:1, as observed from the values of the solvation number p. 3.5. Water Co-extraction. The volume change during extraction is associated with the coextraction of water. It is a result of the water solubility in the organic phase. Process economics could be effected by the water coextraction, that is, the entering of water along with the solute to the organic phase. Additional expenses in terms of cost and power are caused during regeneration, to remove this entered water. I



Article

as are found to be 4.57 and 82.06%, respectively, for TOA (2.827 mol·kg−1) in IBA at 0.01 mol·kg−1 initial PCA concentration. The loading ratio (Z) values obtained were less than 0.5, thus confirming a 1:1 type of complex formation. The partition coefficient (P) and dimerization constants (D) values for all diluents systems were determined. IBA is found to be more preferable over petroleum ether and toluene for the extraction of PCA using TOA. The equilibrium behavior for PCA reactive extraction with TOA in petroleum ether, toluene, and IBA was represented by means of various models such as the Langmuir model, mass action law model, and relative basicity model. Modeled values and experimental values of equilibrium complexation constant (KE) were determined and compared. The accurateness of the model results followed the trend (1) relative basicity model; (2) Langmuir model; and (3) mass action law model.

Figure 6. Parity plot for the mass action law model-predicted KE for the extraction of PCA using TOA in various diluents.

Tamada and King (1990)67 have indicated that a decrease of water coextraction follows a similar tendency as diluent solubility in water. Seemingly, solvation of the molecules of water attached or surrounding a complex is caused by forces allowing the diluent molecules to solvate molecules of water efficiently. Water coextraction studies for the PCA+ TOA + diluent system at different PCA and TOA concentrations were conducted at 298 K. The degree of volume change is, naturally, connected to water coextraction in conjunction with that of the carboxylic acid. It was observed that for the highest concentration of acid selected for the study, maximum water coextraction was obtained. No particular tendency for water coextraction was witnessed in general. The volume of organic phases and aqueous phases was measured, and a few results were reported in Table 9. It was observed that under all circumstances under consideration, water coextraction obtained was not greater than 3% for extraction using TOA as PCA selectivity over water is high; thus, it can be anticipated to have minute effects on process feasibility.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b01013. Physical equilibria for extraction of PCA with IBA, toluene, and petroleum ether; equilibrium isotherm (physical) of protocatechuic acid from aqueous streams into diluents at 298 K (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: k_wasewar@rediffmail.com. Tel.: +91-712-2801561. Fax No.: +91-712-2223969. ORCID

Kailas L. Wasewar: 0000-0001-7453-6308 Notes

The authors declare no competing financial interest.

■

4. CONCLUSIONS PCA extraction by means of TOA was considered in petroleum ether, toluene, and IBA as diluents. The distribution coefficient (KD), degree of extraction (E), loading ratio (Z) and equilibrium complexation constants (KE) were calculated. In reactive extraction, maximum KD and %E values were obtained

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Table 9. Water Coextraction Results for Protocatechuic Acid + TOA + Diluents Systema [HPCA]0 mol kg−1

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Separation of Protocatechuic Acid Using Tri-n-Octylamine

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