Nanofiltration separation of succinic acid from post-fermentation broth: Impact of process conditions and fouling analysis

Journal of Industrial and Engineering Chemistry 77 (2019) 253–261

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Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec

Nanofiltration separation of succinic acid from post-fermentation broth: Impact of process conditions and fouling analysis Jerzy Antczak, Mateusz Szczygiełda, Krystyna Prochaska* Institute of Chemical Technology and Engineering, Poznan University of Technology Berdychowo str. 4, 60-965 Poznan, Poland

A R T I C L E I N F O

A B S T R A C T

Article history: Received 22 November 2018 Received in revised form 20 March 2019 Accepted 22 April 2019 Available online 30 April 2019

In this study a pilot-scale setup was used in the nanofiltration (NF) process for recovery of succinic acid from model solutions and the actual post-fermentation broth left after the bioconversion of glycerol. The effects of composition and pH of feed solutions, initial concentration of components and the magnitude of applied transmembrane pressure (TMP) on the separation efficiency, the value of the permeate flux and retention ratio of individual components present in the feed solutions, were investigated. In order to estimate the mechanism of membrane layer fouling formation, the experimental data obtained were compared to the Hermia model. © 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

Keywords: Nanofiltration Succinic acid Fermentation broth Ceramic membrane Fouling phenomenon Hermia model

Introduction Succinic acid is considered to be one of the twelve most important chemical raw materials obtained from biomass. Moreover, it should be stressed that the succinic acid can be used for the production of biodegradable polymers: polybutylene succinate and polyester polyols, as well as in the food and pharmaceutical industry [1–3]. On industrial scale, succinic acid is obtained by catalytic hydrogenation of maleic anhydride derived from butane using catalysts with oxides of vanadium and phosphorus [4]. Another method of obtaining succinic acid, extensively developed in the last years, is the microbiological bioconversion of waste. One type of waste used for this purpose is glycerol fraction left after biodiesel production [5,6]. As well known, the actual post-fermentation broth has a complex composition and, in addition to the main product, contains a number of other compounds such as: biomass residues and carboxylic acids, nonionic compounds and a significant amount of inorganic salts [7]. Therefore, the most difficult and expensive (50– 80% of the cost) of the production of succinic acid by fermentation is the separation, purification and concentration of the main product from the actual post-fermentation broth [8]. Many different methods for recovering organic compounds from the actual post-fermentation broth have been proposed, e.g. distillation [9], crystallization [2,9], esterification [3], extraction [9–11], ion exchange [11],

* Corresponding author. E-mail address: [email protected] (K. Prochaska).

precipitation [11]. For ecological and prosocial reasons many of these traditional separation techniques are not very attractive, because they require the use of additional chemicals (including organic solvents), and also generate nuisance waste and absorb significant amounts of energy. An interesting alternative are systems based on membrane separation processes, both pressure-driven membrane techniques (microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO) and current-driven processes as classical electrodialysis (ED) and bipolar membrane electrodialysis (EDBM)) [12–16]. Nanofiltration is a pressure driven membrane technique widely used for partial desalination and removal of non-dissociated compounds such as: glycerol, ethanol as well as lactose from the post-fermentation broth [17,18]. In the NF process, the permeate flux and the retention of individual components of the feed depend on the properties of the applied membrane as well as on the initial composition and pH of the feed solution. On the one hand, NF membranes should retain molecules having molar masses larger than the cut-off of the applied membrane, which is a consequence of the sieve effect (size based exclusion) according to which the molecules with sizes larger than the membrane pore size are rejected by the membrane. Besides, they should exhibit greater retention of polyvalent than monovalent ions, due to the electrostatic interactions of ions with the charged surface of the membrane. In 2005, Kang and Chang [19] used NF process for the recovery of sodium succinate and the removal of by-products from simulated fermentation broth. Preliminary studies using commercial NF membranes such as: NF45 and ESNA1 have shown that it is possible to selectively

https://doi.org/10.1016/j.jiec.2019.04.046 1226-086X/© 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

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concentrate succinate ions (more than 90%) from other monovalent acids present in the model solution such as: formic, acetic and lactic acid. In turn, Zaman et al. [20] conducted nanofiltration studies of aqueous solutions using commercial polymer membranes NF1, NF2 and NF270. Separation processes of model solutions containing three carboxylic acids (succinic, acetic and formic acid in a ratio of 1:1:1) with a total concentration of all acids from 5 to 50 g/dm3 were carried out at transmembrane pressure from 0.2 to 1.5 MPa. The highest degree of purity of succinic acid, amounting to 82%, was obtained using the NF2 membrane during the separation of the solution with an initial concentration of 10 g/dm3 and at a transmembrane pressure of 0.5 MPa. During nanofiltration of more concentrated model fermentation broths, the best efficiency (78 and 64% for 20 and 50 g/dm3, respectively) was demonstrated by the NF270 membrane. The scientists from the Suranaree University of Technology in Thailand [21] presented the results of nanofiltration studies on synthetic and real post-fermentation broth solutions of succinic acid using a single-channel ceramic membrane with cut-off 450 Da. The authors investigated the effect of concentration of the separated solution (from 10 to 70 g/dm3), pH (from 2 to 8) and transmembrane pressure (from 0.2 to 0.6 MPa) on the efficiency and effectiveness of the nanofiltration process. The obtained results showed that the pH of the separated solution had the biggest influence both on the efficiency and effectiveness of the process. In addition, in cited paper it has been shown that the use of diafiltration has enabled the isolation of all organic acids present in the actual post-fermentation broth. In addition, many current literature reports have indicated that NF process can be effectively used for the removal of other organic acids from the actual post-fermentation broth [22,23], especially, fumaric acid [24] and L-glutamine [25]. It is well known that the main problem associated with the realization of the pressure driven membrane processes (UF, MF, NF, RO) is the reduction of the permeate flux at the time of separation, which leads to an increase in time and cost of the process as well as a decrease in its effectiveness. Especially, the membrane fouling which is associated with deposition of contaminants, formation of a cake layer on the membrane surface as well as blocking of the membrane pores is one of the greatest obstacles during the separation of multicomponent actual post-fermentation broth [26]. However, fouling effects can be minimized by the selection of appropriate process parameters, the use of correct membrane material and its morphology. The mechanism of formation of the fouling layer on the membrane depends to a great extent on its characteristics: pore size, hydrophobicity and roughness. It is worth mentioning that the ceramic membranes exhibit greater hydraulic and chemical resistance compared to polymeric membranes, and frequent cleaning does not significantly affect their hydrodynamic properties. In this study the effects of composition and pH of feed solutions, concentration of its components and applied TMP on the separation efficiency (the value of the permeate flux) and retention ratio of individual components present in feed solutions, were investigated. Moreover, on the basis of adaptation of the obtained experimental data to Hermia model, the mechanism of formation of a fouling layer during the NF process of model solutions (model broth) of carboxylic acids: succinic, citric and acetic acids and their salts was analyzed. As the final stage of this work, the NF process of the actual post-fermentation broth left after the bioconversion of glycerol was carried out.

Table 1 Detail composition of model solutions using in NF processes. Solution

pH

Symbol

Components of solution, g/dm3

1

2.5 2.5 2.5 2.5 2.5 2.5 8.5 8.5 8.5 8.5 8.5 8.5

1a 1b 2a 2b 3a 3b 4a 4b 5a 5b 6a 6b

10 SA 30 SA 10 SA + 1 CA 30 SA + 3 CA 10 SA + 1 CA + 1 AA 30 SA + 3 CA + 3 AA 10 Na2SA 30 Na2SA 10 Na2SA + 1 Na3CA 30 Na2SA + 3 Na3CA 10 Na2SA + 1Na3CA + 1 NaAA 30 Na2SA + 3 Na3CA + 3 NaAA

2 3 4 5 6

cases the model solutions were prepared using deionized water of conductivity not-exceeding 3 mS/cm. The pH values of these solutions were adjusted in the range 2–8.5 by addition of sodium hydroxide. The detail composition of model solutions used in our research is shown in Table 1. Moreover, the NF process of actual post-fermentation broth consisting: succinic, formic (FA), lactic (LA) and acetic salts as well as glycerol (Glyc), ethanol (EtOH) and lactose (Lact) was also carried out. In order to remove the residues of biological material, the actual-post fermentation broth was subjected to a preliminary UF process. The composition of permeate of the actual post-fermentation broth obtained after UF is given in Table 2. Conditioning of the membrane and cleaning procedure Before NF process the membrane studied was stabilized with deionized water for 48 h. The permeate flux (J) was calculated from the formula: J¼

V

ð1Þ

ðS  tÞ  102

where: J–permeate flux, dm3/(m2 s)102; V–volume, dm3; S–area of active membrane, m2; t–time, s. On the basis of measurements of the flow rate as a function of the transmembrane pressure applied, the hydrodynamic permeability coefficient of the membrane used (Lp) was determined as equal to 12.26  102 dm3/(m2 s MPa). After each experiment, the ceramic membrane was cleaned to recover its initial permeance according to the following procedure: (1) hydraulic cleaning to remove unbound substances remaining on the membrane surface with water, t = 10 min, T = 30  C; (2) chemical cleaning: (a) 2% sodium hydroxide solution cleaning, t = 30 min, T = 70  C; (b) water cleaning, t = 10 min, T = 30  C; (c) 3% nitric acid solution cleaning, t = 30 min, T = 60  C; (d) water cleaning to pH  5–6. All cleaning steps were carried out at a flow rate of 350 dm3/h. Moreover, each time after cleaning the NF system, the deionized water was filtered for 10 min, and the resulting flux (J0 ) was compared to the initial water flux (Jw). Nanofiltration equipment and methods All NF processes were carried out using a pilot-scale NF setup (Intermasz, Poland) equipped with a ceramic membrane (made of

Material and methods Materials

Table 2 Detail composition of actual post-fermentation broth using in NF process. Solution

For the investigation of NF processes of model solutions, three organic compounds purchased from Sigma-Aldrich (Poland) were used: succinic acid (SA), citric acid (CA) and acetic acid (AA). In all

Broth

Components of solution, g/dm3 Na2SA

Glyc

NaFA

NaLA

NaAA

EtOH

Lact

36.4

17.9

12.0

8.5

12.3

7.5

1.3

J. Antczak et al. / Journal of Industrial and Engineering Chemistry 77 (2019) 253–261 Table 3 Characterization of the membrane. Membrane parameters

Unit

Tubular

Material Number of channel External diameter Chanel diameter Filtration area per tube Support mean porosity, d50 Membrane mean porosity, d50 Open porosity Cut-off

– – mm mm m2 mm2 mm % Da

TiO2 1 10 7 0.0125 38.5 3 30–40 450

TiO2) of the effective surface area equal to 0.0125 m2 and cut-off of 450 Da (Inopor, Germany). Detailed description of the membrane is given in Table 3, while the scheme of the used pilot-scale NF setup is shown in Fig. 1. For each NF process, 12 dm3 of feed solution were prepared. The processes were carried out in a semi-closed system, where the retentate was returned to the feed tank, while the permeate was taken to a separate tank continuously at T = 30  C  2  C, TMP = 0.4–1.5 MPa and the flow rate 350 dm3/h. The NF experiments were conducted for 90 min and each test was repeated three times. The permeate and feed samples were collected at regular time intervals (t = 5 min) and next analyzed. The retention ratio (R) of individual components of the feed solution was calculated from the formula:   Cp ð2Þ  100% R¼ 1 Cf where: R–retention ratio, %; Cp–concentration of component in permeate solution, g/dm3; Cf–concentration of component in feed solution, g/dm3. At the beginning of the experiment, a flux of deionized water was measured and denoted as Jw. The value of the permeate flux at the end of the process was Js. After the NF process the membrane was cleaned with deionized water to a stable flux Jf. The relative flux (RF) was calculated as follows:   Js  100% ð3Þ RF ð% Þ ¼ Jw The decline of flux during NF process was determined from the equation: 100-RF. The flux recovery (FR) is defined as:   Jf ð4Þ  100% FRð% Þ ¼ Jw

255

“100 - FR” is the irreversible flux decline caused by fouling, while “FR - RF” is the reversible decline of flux due to concentration polarization and deposition of unbound molecules and substances present in the feed solution on the surface and in the membrane pores [27]. Models that determine the decline of flux during NF process The permeate flux decline during NF process is a confirmation of the fouling phenomenon, which can be caused by various factors such as: cake layer formation, molecular adsorption on the membrane surface and blocking of the membrane pores. According to Hermia model, used to describe these effects, the permeate flux decline during NF process is a sum of contributions coming from a few mechanisms defined by the following equations [26,27]: Complete blocking

lnðJ1 Þ ¼ lnðJ0 Þ1 þ kt

Standard blocking

J1=2 ¼ J0

Intermediate blocking

1=2

þ kt

J1 ¼ J1 0 þ kt

The formation of cake layer

J2 ¼ J2 0 þ kt

ðAÞ

ðBÞ

ðCÞ

ðDÞ

Substitution of the experimental data to the time (t) dependences and determination of the linear relationship makes it possible to infer the mass transfer coefficients (k) as the slopes of each curve. In addition, the regression coefficient (R2) and the initial permeate flow (J0) were determined from the above models. Analytical methods The contents of carboxylic acids and their salts as well as glycerol and lactose in the permeate and feed solutions obtained in NF processes were determined using high performance liquid chromatography HP Agilent 1100 Series (Germany), equipped with an auto sampler, interface (HP 35,900), RI Detector (HP 1047A), pump (HP1050), and separating column Rezex ROA-Organic Acid H + (8%), Phenomenex1. The eluent of 2.5 mM H2SO4 solution was constantly supplied at the rate of 0.9 ml/min. The column temperature and that at the input to the detector was 40  C, P = 0.56 MPa. All samples were acidified to pH  2 by addition of 0.1 ml 25% H2SO4 to 1 ml of sample before analysis. Results and discussion The effect of pH and composition of the feed solution on the permeate flux, fouling phenomena as well as the retention ratio of the components during the NF processes of model solutions was analyzed. Effect of the pH and composition of the feed solution

Fig. 1. The scheme of the NF pilot-scale setup where: 1–feed tank, 2–flow meter, 3– heat exchanger, 4–pump, 5–membrane module, 6–temperature and flow rate controller, 7–measuring cylinder, 8–pressure gauge, 9–heater.

The changes in permeate flux during the NF processes of acid solutions (1a, 2a, 3a) as well as alkaline solutions (4a, 5a, 6a) are shown in Fig. 2, while the composition of the model solutions is presented in Table 1. On the basis of the results shown in Fig. 2 it can be concluded that all permeate flux of feed solutions (Js) are lower in comparison to the initial flux of water solution (Jw). The observed significant decrease in permeate flux just at the beginning of the NF process is probably a consequence of concentration polarization effect as reported by Nigam et al. and Bacchin et al. [28,29]. In a further step of the NF process the

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The data on reduction of the permeate flux obtained after NF processes of acid solutions (1a and 3a) and alkaline solutions (4a and 6a) at TMP = 0.4 MPa are tabulated in Table 4. Generally, it can be noted that the total flux declines (100-RF), the polarization concentration (FR-RF) as well as the irreversible fouling (100-FR) increase with increasing pH and the number of components present in the feed solutions. The highest values of both concentration polarization (33.9%) and irreversible fouling (31.3%) were observed during the NF process of the threecomponent solution at pH 8.5 (6a). On the other hand, the concentration polarization and irreversible fouling values during the NF process of the one-component solution at the same pH 8.5 (4a) are near twofold lower and equal to 18.0% and 14.3%, respectively. It can be clearly seen, that along with the increase in the number of components in the feed solution, their concentration in the membrane layer increases, thus increasing the concentration polarization. In addition, as shown in Table 4, the pH value of feed solution also has a significant impact on the flux decline and fouling values. Both in the NF processes of onecomponent ((1a at pH 2.5) and (4a at pH 8.5) as well as threecomponent model solutions (3a at pH 2.5) and (6a at pH 8.5)) the concentration polarization and irreversible fouling values increase nearly twice with increasing pH value. Analysis of the values of fouling parameters (k, R2 and J0) tabulated in Table 5, which were calculated according to the Hermia Eqs. A–D, indicate that the higher R2 value means the better fit to Hermia model [27]. As can be seen from Table 5 data (irrespective of the pH value of feed solution) the most accurate model fit was obtained for the cake layer mechanism (D) followed by the intermediate blocking (C). On the one hand, as reported by Ref. [34] the cake formation is due to foulants depositing over the membrane surface and not entering the membrane pores. In consequence, a cake or gel layer formation over the membrane surface can be observed. On the other hand, the same authors reported that the intermediate blocking can be formed when molecules block the pore entrance without blocking it completely. Moreover, some molecules can deposit over others. Similar effect has been observed by Kaya et al. [27] during NF processes of wastewater generated at the time of paper production. The cited authors indicate that the electrostatic repulsion between the charged groups in the membrane active layer and the negatively charged molecules (present in feed solution) can cause a reduction in the pore size (causing the pores to shrink) during NF processes both at high and low pH value. Table 6 shows the retention ratios of succinic and citric acids and their salts obtained during the NF process. The presented results indicate that in the NF of acid solutions the retention ratio for succinic acid as well as for citric acid does not exceed 6%. Thus, the previously described sieve mechanism responsible for transport through the membrane of undissociated molecules has no significant effect on the retention ratio of the components present in the feed solutions. The retention of weak organic acids is largely dependent on the pH value. For example, Choi et al. [17] who studied the recovery of formic, acetic, propionic, succinic and citric acids from the model broth using NF membranes in the pH

Fig. 2. The permeate flux vs. time of the NF processes of acid solutions (1a,2a, 3a) and alkaline solutions (4a, 5a, 6a), TMP = 1 MPa.

particles of solutes are deposited on the membrane surface, which leads to the formation of a fouling layer [30,31]. On the one hand, if the concentration polarization is high, a gel layer may form at the surface of the membrane. On the other hand, the membrane pores are blocked, which also leads to a reduction in permeate flux [30,32]. Moreover, when the concentration of the substance in the feed solution is high (and additionally intensified by the concentration polarization at the membrane surface), the additional reduction of the permeate flux may occur because of the back diffusion of the solvent, as reported by Nigam et al. [28]. This effect is typical of pressure-driven membrane techniques (MF, UF, NF RO) as described in detail by Bacchin et al. [29]. It is worth mentioning that a slight decrease in the flux during the semiclosed process may be due to the continuous increase in the concentration of components in the feed. When analyzing the data shown in Fig. 2, it can also be noted that the differences between the initial permeate flux and the water flux strongly depend on the number of components present in the feed solution. The same tendency is observed for acid solutions as well as alkaline solutions. The observed effect indicates an increase in flow resistance on the surface or within the pores of the membrane. Moreover, when the acidic solutions were used (pH = 2.5) the differences between the initial water flux and the permeate flux of model solutions were not as pronounced as in the case of NF of the alkaline solutions (pH = 8.5). It is obvious that the sieve mechanism plays a major role in the transport of undissociated (uncharged) molecules through the NF membrane. The cut-off of the used ceramic membrane was equal to 450 Da, while the molar masses of the organic acids present in the feed solutions were equal to 118.09, 192.124 and 60.05 Da for succinic acid, citric acid and acetic acid, respectively. Change in the pH of the solution from the acidic to the alkaline, causes a change in the form of succinic acid, and in solutions with pH > 8 the molecules are in completely dissociated form [33]. Thus in such conditions, the transport of ions through the NF membrane is mostly affected by the electrostatic interaction of the charges of the particles and the membrane surface.

Table 4 The data on reduction of the permeate flux obtained after NF processes (1a, 3a, 4a, 6a), TMP = 0.4 MPa. Solution

1a 3a 4a 6a

pH

2.5 2.5 8.5 8.5

Flux

Flux decline, %

Jw

Js

Jf

Concentration polarization (FR-RF)

Fouling (100-FR)

4.95 4.95 4.95 4.95

4.13 3.37 3.35 1.72

4.66 4.18 4.24 3.40

10.7 16.4 18.0 33.9

5.9 15.6 14.3 31.3

J. Antczak et al. / Journal of Industrial and Engineering Chemistry 77 (2019) 253–261

257

Table 5 Evaluation of fouling parameters using Hermia model for NF processes (1a, 3a, 4a, 6a) TMP = 0.4 MPa (A–complete blocking, B–standard blocking, C–intermediate blocking, D– the formation of cake layer). Solution

pH

J0

MODEL A

1a 3a 4a 6a

2.5 2.5 8.5 8.5

4.16 3.41 3.48 1.98

B

C

D

R2

k [1/m]

R2

k [1/m]

R2

k [1/m]

R2

k [1/m]

0.9012 0.8138 0.9645 0.9910

8  105 9  105 5  104 17  104

0.9085 0.8136 0.9638 0.9924

2  105 2  105 1 104 6  104

0.9124 0.8144 0.9680 0.9930

2  105 3  105 1 104 9  104

0.9124 0.8151 0.9693 0.9984

9  106 2  105 8  105 1 104

Table 6 The retention ratio of succinic, citric acid and their salts during NF processes, TMP = 1 MPa. Component

R, % 1a

2a

3a

4a

5a

6a

pH SA/Na2SA CA/Na3CA

2.5 2.2 –

2.5 3.5 2.1

2.5 6 4.5

8.5 22.6 –

8.5 28.9 36.7

8.5 46.7 53.3

range of 3–9, reported that when MWCO of membrane applied was much higher than the molar mass of molecules present in the feed solution, the degree of dissociation of the molecules played a key role in the retention of the components. Furthermore, the cited authors indicated that using NF membrane (ES10) the retention ratio of formic acid increased from 2% to 96% with increasing pH of the feed solution. The authors also suggested that for the molecules whose molecular weight was much smaller than MWCO of the used membrane the only mechanism of transport was the electrostatic intermolecular interaction. A similar observation was reported in our previous paper, although the increase in pH value of the feed solution from 8 to 11 led to a significant increase in the retention of fumarate (0.43 g/dm3) from about 30 to over 70% [35]. The effect of concentration of feed solution components As the next step of study the effect of concentration of feed solution components on the value of permeate flux, fouling phenomena as well as the retention ratio of components during the NF processes of model solutions was investigated. Fig. 3 presents a comparison of the values of permeate fluxes during NF processes of one-(4a and 4b), two-(5a and 5b) and threecomponent model solutions (6a and 6b) in which the concentration of succinates was equal to 10 g/dm3 (a) and 30 g/dm3 (b). It can be noted that a threefold increase in the concentration of feed

solution causes a significant decrease in the relative values of the flow (RF), i.e. in the range of about 25–50%. As reported by Arsuaga et al. [36] the observed decrease in permeate flux with increasing concentration of feed solution components can be due to the adsorption of organic solute on the membrane surface. Moreover, the decrease in permeate flux can be also caused by higher osmosis pressure at higher solute concentration [37]. The results presented in Table 7 also show that the threefold increase in concentration of the feed solution components leads to a decrease in efficiency of NF processes caused by increased concentration polarization as well as irreversible fouling. In the NF process of the three-component model solution of pH value equal to 8.5 and the concentration of succinic acid equal to 30 g/dm3 (6b), the total flux decline (100-RF) was over 82%. From the fit of Hermia model to the experimental data (Table 8), it can be concluded that cake layer formation plays a major role in the reduction of the permeate flux. Analyzing the data shown in Table 9, a significant decrease in the retention ratio of components with increasing concentration of feed solution can be noted. For one-component solutions, the reduction in the succinates retention is almost threefold. The difference in citrates retention in twocomponent solutions (5a and 5b) is also very high (about 50%). A similar effect has been observed by Choi et al. [17] for NF of a multicomponent model solution. According to the above authors, during the NF process with the use of a NF270 membrane, the retention ratio of formic acid decreased from 90% to 56% with tenfold increase in the concentration of feed solution from 50 to 500 mg/ dm3, respectively. Moreover, the decrease in the retention ratio of formic acid with its increasing concentration could be a result of effective screening of the initially negative membrane surface charge by counter-ions. Moreover, Choi et al. suggested that the increasing concentration of counter-ions (cations) in the solution results in a “screen/coating” which significantly neutralizes the negative charge of the membrane surface. When the negatively charged particles present in the feed solution encounter a negative charge on the surface of the membrane, the action of the electrostatic forces causes the repulsion of the co-ions. Therefore, the decrease in the components retention is a consequence of weakening of electrostatic interactions between the molecules in solution and at the surface of the membrane due to increasing particle concentration. Especially, during the NF processes of solutions of very high concentration, the membrane charge can be blocked as a result the membrane loses its separating properties. Similar effects have been observed by González et al. [12] who investigated the process of NF of lactic acid solutions on the DK2540C membrane. These authors found that the increase in acid concentration from 40 to 80 g/dm3 (in solutions of pH 6) resulted in a decrease in retention from over 90% to less than 40%. The effect of transmembrane pressure

Fig. 3. The permeate flux vs. time in NF processes of salts solutions (4a, 5a, 6a) and (4b, 5b, 6b), TMP = 0.4 MPa.

Another objective of our study was to assess the effect of transmembrane pressure (TMP) on the permeate flux, fouling phenomena as well as retention ratio of components in the NF

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Table 7 The data on reduction of the permeate flux obtained after NF processes (3a, 3b, 6a, 6b), TMP = 0.4 MPa. Solution

CSA, g/dm3

3a 3b 6a 6b

10 30 10 30

Flux

Flux decline, %

Jw

Js

Jf

Concentration polarization (FR-RF)

Fouling (100-FR)

4.95 4.95 4.95 4.95

3.37 1.60 1.72 0.86

4.18 3.25 3.40 2.95

16.4 33.3 33.9 42.2

15.6 34.3 31.3 40.4

Table 8 Evaluation of fouling parameters using Hermia model of NF processes (3a, 3b, 6a, 6b), TMP = 0.4 MPa (A–complete blocking, B–standard blocking, C–intermediate blocking, D– the formation of cake layer). Solution

CSA, g/dm3

J0

MODEL B

A R 3a 3b 6a 6b

10 30 10 30

3.41 1.76 1.98 1.12

2

k [1/m]

0.8138 0.9806 0.9910 0.9799

R

5

9  10 1.2  103 1.7  103 3.3  103

0.8136 0.9811 0.9924 0.9804

Table 9 Retention ratio of succinates and citrates during NF processes, TMP = 0.4 MPa. Component

SA/ Na2SA CA/ Na3CA

C 2

R, % 4a

5a

6a

4b

5b

6b

28.3 –

34.3 51.2

48.5 63.2

11 –

18.4 26.5

31.6 49.6

processes. Analyzing the data shown in Fig. 4, it can be concluded that in all studied cases, the permeate flux of feed solution increases fivefold with increasing TMP in the range 0.4–1.5 MPa. It is obvious that higher TMP increases the driving force of the NF process. Similar effects have been observed during NF of other carboxylic acids such as fumaric acid and lactic acid [12,38]. Moreover, as shown in Table 10, as the TMP increases, the concentration polarization and the irreversible fouling decrease and consequently the total permeate reduction (100-RF) also decreases. The lowest relative flux equal to 17.4% was obtained during the NF process of three-component model solution of high concentration (6a) at low TMP equal to 0.4 MPa. In addition, the highest reduction of permeate flux due to the irreversible fouling (40.4%) and concentration polarization (42.2%) effects was also

Fig. 4. Permeate flux vs. the time of the NF processes of alkaline solutions (4b, 5b, 6b) at different TMP = 0.4 MPa (empty field) and TMP = 1.5 MPa (full field).

k [1/m] 5

2  10 5  104 6  104 1.7  103

R

D 2

0.8144 0.9816 0.9930 0.9803

k [1/m] 5

3  10 7  104 9  104 3.3  103

R2

k [1/m]

0.8151 0.9823 0.9984 0.9857

2  105 9  104 1 103 9.6  103

obtained and was noticeably higher than for the NF processes running at a higher TMP. For comparison, during the NF process in the system with solution (6b) at TMP equal to 1.5 MPa, the total flux decline was significantly lower since the resultant relative flux was higher than 50%. As in previously described systems, the fit with the Hermia model implies that a major role in the decline of the flux plays the mechanism of cake layer formation (Table 11). Table 12 data illustrate the effect of TMP on the retention ratio of the components present in the feed solutions. It can be seen that the increase in TMP has a negative impact on the retention ratio of the feed solutions components (especially alkaline solutions). The increase in the pressure exerted on the membrane leads to a weakening of the electrostatic forces and the transport through the membrane is mainly determined by the sieve mechanism. As a result, a decrease in retention of the components is observed. However, the data compiled in Table 12 indicate that with increasing TMP there is no noticeable difference in retention of components in the solutions with high concentrations of organic salt. For example, during the NF process of the one-component solution of succinates (4b) concentration equal to 30 g/dm3, a threefold increase in TMP caused a decline in the retention of about 1%. Similar results were obtained during separation of the twocomponent solution (5b). In the separation of succinates from the three-component solution (6b) the retention decrease was approximately 3%. Slightly greater retention differences were observed for citrates. During the NF processes of the two- and three-component solutions (5b and 6b) with the TMP increase from 0.4 to 1.5 MPa, the retention of citrates decreased by 6 and 7%, respectively. Probably a high concentration of the components in the feed solution strongly deactivates the surface of the membrane, which reduces the electrostatic interaction between the particle and membrane surface. The increase in TMP does not significantly affect the amount of molecules retained by the membrane. Fig. 5a and b show a comparison of permeate fluxes (Js) obtained after 60 min of NF processes of all model solutions of organic acids and their salts. It can be seen that the applied TMP has the maximum impact on the efficiency of NF process, irrespective of the type of separate solution. The increase in TMP from 0.4 to 1.5 MPa during the NF process of the two-component solution with concentration of succinates equal to 30 g/dm3 and at pH of 8.5 (5b) leads to a significant increase in the permeate flux (Js) from 1.6 to

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259

Table 10 The data on reduction of the permeate flux obtained after NF processes (3b, 6b), TMP = 0.4 and 1.5 MPa. Solution

TMP, MPa

Flux Jw

Js

Jf

Concentration polarization (FR-RF)

Fouling (100-FR)

3b 3b 6b 6b

0.4 1.5 0.4 1.5

4.95 17.60 4.95 17.60

1.60 10.97 0.86 8.42

3.25 14.61 2.95 13.00

33.3 20.7 42.2 26.0

34.3 17.0 40.4 26.1

Flux decline, %

Table 11 Evaluation of fouling parameters on the basis of the fit with Hermia model of NF processes (3b, 6b), TMP = 0.4 and 1.5 MPa (A–complete blocking, B–standard blocking, C– intermediate blocking, D–the formation of cake layer). Solution

TMP, MPa

J0

MODEL B

A 2

R 3b 3b 6b 6b

0.4 1.5 0.4 1.5

1.76 11.0 1.12 8.85

0.9806 0.9566 0.9799 0.9777

k [1/m]

R 3

1.2  10 4  105 3.3  103 5  104

C 2

0.9811 0.9563 0.9804 0.9773

Table 12 Retention ratio of succinates and citrates during NF processes, TMP = 0.4 and 1.5 MPa. Component

TMP MPa

Retention, % 4b

5b

6b

Na2SA Na3CA Na2SA Na3CA

0.4 0.4 1.5 1.5

11 – 10 –

18.4 26.5 17.5 20.6

31.6 49.6 31.5 42.6

10.8  102 dm3/m2s, respectively. Furthermore, the increase in TMP affects the reduction of the concentration polarization effect and cake layer formation. For example in the NF process with the use of solution (5b) the increase in TMP from 0.4 to 1.5 MPa led to a decrease in the total flux decline from 67.2 to 38.6%, including that in concentration polarization from 33.9 to 17.6% and that in irreversible fouling from 33.3 to 21%, for TMP equal to 0.4 and 1.5 MPa, respectively. NF process of actual post-fermentation broth obtained after bioconversion of glycerol In the final step of our study, the NF process of the actual postfermentation broth was carried out. On the basis of the results of NF processes of model solutions (especially that of succinic acid

k [1/m] 4

5  10 5  106 1.7  103 9  105

D 2

R

0.9816 0.9560 0.9803 0.9769

k [1/m] 4

7  10 3  106 3.3  103 6  105

R2

k [1/m]

0.9823 0.9555 0.9857 0.9760

9  104 6  107 6.9  103 1 105

concentration of 30 g/dm3) which showed a significant increase in NF efficiency and a slight decrease in the retention ratio of carboxylic acid with increasing TMP, we decided to carry out the NF process of the actual post-fermentation broth at a TMP equal to 1.5 MPa. However, it should be emphasized that the actual postfermentation broth is a complex composition, which in addition to organic compounds also includes a number of salts, hydroxides and carbonates of inorganic ones (including multivalent) having a significant impact on the efficiency of NF process. Tables 13 and 14 show the reduction of permeate flux in the time of NF process of the actual post-fermentation broth as well as evaluation of fouling parameters on the basis of the fit to Hermia model. The value of the relative flux (RF) after the NF process was equal to 27%, and the flux decline was due to irreversible fouling (100-FR) in 38.2% and concentration polarization effect (FR-RF) in 34.8%. In addition, as follows from analysis of the fouling parameters obtained from the fit with Hermia model, the cake layer formation and intermediate blocking play a major role in the fouling process. In this case, the fouling cake can be formed by the unremoved molecules present in NF feed solution such as inorganic compounds and the other biocomponents whose size is bigger than that of the membrane pores. The NF process of the actual post-fermentation broth left after bioconversion of glycerol was carried out in an alkaline medium (pH = 8.5), which means that the succinic acid molecules contained in the feed solution were present exclusively in the dissociated form. The results shown in Table 15 indicate that the retention ratio

Fig. 5. Comparison of permeate flux in NF processes of (a) acidic solutions of pH = 2.5 and (b) alkaline solutions of pH = 8.5.

260

J. Antczak et al. / Journal of Industrial and Engineering Chemistry 77 (2019) 253–261

Table 13 The data on reduction of the permeate flux obtained after NF process of the actual-post fermentation broth, TMP = 1.5 MPa. Solution

TMP, MPa

Broth

1.5

Flux

Flux decline, %

Jw

Js

Jf

Concentration polarization (FR-RF)

Fouling (100-FR)

17.6

4.75

9.4

34.8

38.2

Table 14 Evaluation of fouling parameters on the basis of the fit of Hermia model to the NF process of the actual post-fermentation broth TMP = 1.5 MPa (A–complete blocking, B– standard blocking, C–intermediate blocking, D–the formation of cake layer). Solution

TMP, MPa

J0

MODEL B

A 2

R Broth

1.5

5.20

k [1/m]

0.9648

8  10

R

5

Broth

k [1/m]

0.9670

Table 15 Retention ratio of compounds during the NF process of the actual-post fermentation broth, TMP = 1.5 MPa. Solution

C 2

Retention, % Na2SA

Glyc

NaFA

NaLA

NaAA

EtOH

Lact

41.8

4.2

2.1

9.2

6.7

0

0

of succinates was almost 42%, which is higher than in the NF process of the three-component model solution (6b) of succinic acid concentration of 30 g/dm3 at TMP equal to 1.5 MPa, i.e. 30%. A higher retention of succinate ions in the NF process of the actual post-fermentation broth than in that of the multi-component model solution may be a result of the presence of mono- and polyvalent inorganic salts, hydroxides and hydrocarbons (e.g. Mg (OH)2, MgCO3 or Ca(OH)2 and CaCO3) in the actual postfermentation broth. Hong et al. [39] who studied fouling during the separation of natural organic matter (NOM), mentioned the interactions between polyvalent ions (Ca2+ and Mg2+) and NOM molecules. The cited authors indicated that divalent cations could form metal-humic complexes that change the electrokinetic properties of the NF membrane, which could be responsible for its greater fouling. In addition, the authors showed that most complexes were formed in an alkaline solutions (pH = 8). The retention ratio obtained for other carboxylic acids present in the actual post-fermentation broth was low and equal to 2.1, 6.7, 9.2% for formic, acetic and lactic acid, respectively. On the one hand, the formic, acetic and lactic acids are monoprotic organic acids and all of them are present in the feed solution in the form of monovalent ions, so their electrostatic interactions of the particle/ membrane surface type are much weaker than in the case of polyvalent ions. Moreover, the molecular weight of formic, acetic and lactic acid equal to 43.03, 60.05 and 90.08 g/mol, respectively, while the molecular weight of succinic acid is much larger and equal to 118.09 g/mol. Therefore, the monoprotic acids can be more easily transported through the membrane because of their smaller diameter. Moreover, it is worth noting that the retention ratio of glycerol was small and equal to 4%, while the retention of the other compounds present in undissociated form such as ethanol and lactose, has not been noticed. Conclusion In this study a pilot-scale NF process was used as one of the steps of separation and concentration of succinic acid from the model solutions and the actual post-fermentation broth obtained after bioconversion of glycerol. The effects of composition and pH

3  10

5

R

D 2

0.9747

k [1/m] 4  10

5

R2

k [1/m]

0.9751

2  105

of feed solutions, concentration of components and applied TMP on the separation efficiency (permeate flux) and retention ratio of individual components of feed solutions were investigated. Moreover, on the basis of the experimental data fit with Hermia model, the mechanism of formation of a fouling layer during the NF process of model solutions of carboxylic acids: succinic, citric and acetic acids and their salts was analyzed. The obtained results show that the ceramic NF membrane can be successfully used to concentrate low molecular organic compounds from water solutions. The retention ratio of succinic acid of about 50% can be obtained in solutions of low concentration (up to 10 g/dm3) of polyvalent organic acid salts such as succinate or sodium citrate. Moreover, it can be concluded that in all studied cases, the permeate flux of feed solutions increases significantly (fivefold) with increasing TMP in the range 0.4–1.5 MPa. Furthermore, as the TMP increases, the concentration polarization and the irreversible fouling decrease and consequently the total permeate reduction also decreases. However, it was also found that the retention ratio of the components present in the feed solution decreases with increasing TMP. Substitution of experimental data to Hermia model revealed that regardless of the operating conditions of the NF process, the mechanism of cake layer formation was dominant. All NF processes with NF model solutions as well as the actual postfermentation broth brought no changes in the hydrodynamic and separation properties of used membranes. In addition, it was found that the applied hydraulic and chemical cleaning processes allowed complete removal of the resulting fouling layer. Acknowledgments The authors wish to acknowledge the Polish Ministry of Science and Higher Education for the financial support for Faculty of Chemical Trchnology, Poznan University of Technology in 2019 (Grant no. 03/32/SBAD/0901). The research is a continuation of the research within the project “Biotechnological conversion of glycerol to polyols and dicarboxylic acids,” implemented within the Operational Programme–Innovative Economy, 2007–2013, cofinanced by the European Union. PO IG 01.01.02.074/09. References [1] T. Werpy, G. Petersen, Top Value Added Chemicals from Biomass, Department of Energy, Washington, DC, 2004, pp. 31. [2] Q. Li, D. Wang, Y. Wu, W. Li, Y. Zhang, J. Xing, Z. Su, Sep. Purif. Technol. 72 (2010) 294. [3] A. Orjuela, A.J. Yanez, L. Peereboom, C.T. Lira, D.J. Miller, Sep. Purif. Technol. 83 (2011) 31. [4] D. Salvachúa, A. Mohagheghi, H. Smith, M.F.A. Bradfield, W. Nicol, A.B. Black, M. J. Biddy, N. Dowe, G.T. Beckham, Biotechnol. Biofuels 9 (2016) 28.

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