Elham Ameri,Ali Aghababai Beni2,,Zahra Pournuroz Nodeh

1 Department of Chemical Engineering, Najafabad Branch, Islamic Azad University, Najafabad, Iran

2 Department of Chemical Engineering, Shahrekord Branch, Islamic Azad University, Shahrekord, Iran

3 Department of Chemistry, Lahijan Branch, Islamic Azad University, Lahijan, Iran

Keywords:Design Membranes Separation Emulsion liquid Succinic acid Fermentation broth

ABSTRACT The aim of this study was to design a new emulsion liquid membrane(ELM)system for the separation of succinic acid from aqueous solutions.The concentration of succinic acid varied from 20 to 60 mmol·L-1.The prepared ELM system includes tributylamine (TBA) as a carrier,commercial kerosene as a solvent,Span 80 as a surfactant,and Na2CO3 as a stripping agent.In order to control the membrane swelling,different values of cyclohexanone were added to the membrane phase.The effect of various empirical variables on the extraction of the succinic acid such as acid concentration in the feed solution,initial feed concentration,carrier concentration,the stirring speed of the extraction,Na2CO3,surfactant,and cyclohexanone concentrations,and treat ratio in the ELM system.The best result was obtained when TBA was used as the carrier.The final acid extraction efficiency was independent of pH variations of the aqueous feed solution.The extraction of succinic acid solution with a concentration of 40 mmol·L-1 was improved by increasing the treat ratio 1:7-1:3,stripping phase concentration 0.5-1.5 mol·L-1,stirring speed 300-500 r·min-1 and cyclohexanone concentration in the membrane phase 1.2-1.6 mol·L-1.No considerable effect on the extraction rate was observed for the carrier concentration in the membrane phase.But,the surfactant concentration in the feed phase showed a dual effect on the extraction efficiency.

1.Introduction

Membrane separation systems have been considered by researchers as a highly capable tool that could selectively separate components from their mixtures [1].Liquid membrane is one of the most efficient separation methods that has been seriously studied for more than 50 years.It has been used in various industries containing wastewater treatment,reaction mixtures,and fermentation broths due to its great progress in the last two decades.Emulsion liquid membrane(ELM)could be used for the recovery of heavy metals,phenol,organic acids,and,bioactive materials [2].Moreover,the separation of fermentation products such as succinic acid by using ELM is a relatively new technique [3].Efficient separation and concentration of the fermentation products play a key role in the chemical success of a fermentation process.Even if the fermentation is successful,the major problem of product recovery remains.Some separation processes,such as ion exchange,are batch operations that require pre-washing of the fermentation medium.Other techniques,such as extraction of the cell culture medium and ultracentrifugation can be continuously used,but the need for pre-extraction of the dissolved component limits their applications and is often costly.The chromatography separation is very useful but strongly requires pre-washing of the cultural medium,which is very costly and difficult [4].To overcome these limitations,Tinget al.[5] expanded the ELM process.The application of liquid membrane technology to recover fermentation products has grown faster because an important part of the technology and financial success of biological processes depends on the fermentation stages[6].In some cases,ELM may be considered as liquid extraction,while the two are very different and ELM has more advantages.One of the important advantages of ELM membranes is the presence of a large surface area per volume,which makes the separation process faster.Another advantage of ELM is its dimensions;fine particles and aggregates do not affect the mass transfer rate and ELM can function effectively even in the presence of live bacterial cells or enzymes [7].Therefore,ELM could be directly used in the culture medium of fermentation to obtain extraction and subsequent reactions in one step.

In the present work,the effects of different parameters such as pH of the feed solution,initial feed concentration,carrier concentration,the stirring speed of the extraction,Na2CO3,surfactant,and cyclohexanone concentrations,and treat ratio in the ELM system.

2.Experimental

2.1.Materials

The succinic acid(≥99.5%),Span 80(100%),tributylamine(TBA,99%),triethanolamine (TEA,99%),isobutylamine (IBA,100%),Amberlite LA-2(99%),Na2CO3(99%),HCl(37%),NaOH(99%),cyclohexanone (99%),and kerosene (99.8%) were purchased from Sigma-Aldrich,Germany.

2.2.Membrane preparation

In order to prepare the membrane solutions,the required amounts of the carrier(0.071 ml)and Span 80(0.5 ml)were mixed.Then,10 ml of the 1 mol·L-1Na2CO3solution(stripping phase)was added to the membrane solution.The mixture was homogenized at 12000 r·min-1for 10 min,and the emulsion phase was obtained.

2.3.Extraction method

In order to perform the batch extraction process,the desired volume of the prepared ELM was poured into the feed phase and mixed with a magnetic stirrer with an agitation rate of 400 r·min-1.The extraction time was 25 min.Samples were taken from the feed at consecutive and specific times.The emulsion was demulsified by the freezing and thawing method in order to get the stripping phase as follows: An ethanol bath was maintained at about -30 °C using an immersion cooler (ECS-30SS,Eyela Co.,Japan).20 cm3of glass vial containing the emulsion was submerged in the bath for 2-4 h to freeze it.The frozen emulsion was melted at room temperature for about 20 min.The freezing and thawing were repeatedly done until the emulsion was completely broken.The residual emulsion phase was then separated from the prepared samples using filter paper.Samples were taken from the feed phase and injected into the HPLC device equipped with a 4.6 mm×150 m m Supelco gel C18 column(Supelco,USA)to determine the concentration of succinic acid in the feed phase.

The retention time of the acid and the flow rate of filtered eluent was 20 min and 0.5 ml·min-1,respectively.To study the significant parameters affecting the extraction of succinic acid from the feed solution,mixing speed and acid initial concentrations of the feed solution,base concentration of the stripping phase,types and concentrations of extractant,surfactant concentration,pH of the feed solution,cyclohexanone content in the membrane phase and the treat ratio were varied to determine these effects on succinic acid separation.ELM experiments were conducted batchwise at (25 ± 1) °C.

3.Results and Discussion

3.1.Transport mechanism of succinic acid

In general,the second and third types of amino extractors are common carriers for carboxylic acids.Amino extractors react with associated acids and form complexes.Eq.(1) shows the reaction between carboxylic acid and amine.‘‘m” moles of succinic acid by ‘‘n” moles of TBA (B) reacts to form one mole of the complex at the external interface between the membrane phase and the feed phase [8]:

where（H2A）mBnis dispersed to the stripping phase,the associated acid is transferred into this region by the reaction and the amine is released.It is worth mentioning that the great value of the concentration of stripping agent in that region could lead to a considerable accumulation of the acid;based on a large pH difference between the internal phase and external phase prepares a high driving force for acid removal in the emulsion liquid membrane.Fig.1 shows the extraction mechanism in ELM.

Fig.1.Transport mechanism of succinic acid (H2A) through ELM,R2NH: amine carrier such as Amberlite LA-2 and,H2AR2NH: succinic-carrier complex.

3.2.Selection of suitable extractant in ELM systems

The extraction efficiency was influenced by the nature of the extractant in the ELM system.The ELM systems with four different amine extractants,TBA,TEA,IBA,and Amberlite LA-2,were investigated to find an optimal amine extractant of the membrane phase for the separation of succinic acid.The effect of amine extractants on extraction efficiency is shown in Fig.2.

Fig.2.Effects of amine extractants on the extraction efficiency (30 mmol·L-1 TBA,1 mol·L-1 Na2CO3,5% (vol) Span 80,400 r·min-1,pH 3,O/A ratio 1 and treat ratio 1:5).

The initial extraction rates of succinic acid for the amine of TBA were considerably higher than those for the others.After 10 min of the process,there was little difference among the degrees of extraction of succinic acid for all the used amine extractants,but the degree of extraction of succinic acid was higher in order of TBA,TEA,Amberlite LA-2,and IBA.Moreover,in the cases of use of the TBA and TEA extractants,succinic acid extraction efficiency reached about 99% at 16 min and 21 min,respectively.While succinic acid extraction efficiency was achieved at about 98% at 21 min using either IBA or Amberlite LA-2.So,the use of TBA as an amine extractant was preferred and applied to the separation of succinic acid from the solutions below.According to Fig.3,Fourier transform infrared spectroscopy (FTIR) analysis (Perkin Elmer Co.,Germany)was performed to investigate the extraction process.Infrared spectrum of organic phase (TBA) after extraction of succinic acid,due to the formation of complexes after extraction,new peaks are observed.New band at 632 and 775 cm-1exist after the extraction,which indicates bonding formation between C——O-...+N—C bond of succinic and TBA,respectively.

Fig.3.FTIR spectra of the organic phase before and after extraction.

3.3.Empirical study on succinic acid extraction

The magnitude of succinic acid extraction in the ELM systems using TBA as an extractant was investigated.Moreover,for the subsequent studies,some empirical conditions were adjusted:emulsification rate of 12000 r·min-1,emulsification time of 10 min,organic solvent(commercial kerosene)at O/A ratio:1,and contact time: 25 min.

3.3.1.Acid concentration study in the feed solution

The pH value of the aqueous feed solution is a key role in the ELM process.The influence of the type of the feed solution on succinic acid extraction is observed in Fig.4.In these experiments,the initial pH value of the feed solution was adjusted to 3.0-7.0.

Fig.4.Effects of feed solution type on the extraction efficiency (30 mmol·L-1 TBA,1 mol·L-1 Na2CO3,5% (vol) Span 80,400 r·min-1,pH 3,O/A ratio 1 and treat ratio 1:5).

In order to amplify the driving force of the acid extraction process,a strong acid HCl with a concentration of 0.01 mol·L-1was added to the feed solution.Fig.4 showed that when HCl was added in the feed phase and the pH difference between the feed and the stripping phases was subsequently enlarged,the initial extraction amount was decreased.This observed reverse result refers to the fact that HCl similar to succinic acid can react with TBA to produce the acid amine complexes[26,27].On the other hand,competitive extraction of HCl against the succinic acid causes the loss of the effect of increasing the driving force for the extraction of succinic acid due to the enhancement in pH difference between the feed solution and the stripping phase.Moreover,a smaller pH difference between the feed solution with 0.01 mol·L-1NaOH and the stripping phase led to a decrease in the initial magnitude of acid extraction compared to that for pure water.

In general,Fig.4 shows that the final acid extraction efficiency was independent of pH variations of the aqueous feed solution.Moreover,after 13 min time contact,the variations of acid extraction efficiency with respect to the pH variation achieved for three different aqueous feed solutions were negligible.At last,the final extraction efficiency was found nearly 99% for all feed solutions at 25 min of the process.

3.3.2.Initial feed concentration study

The initial feed concentration(C0)influence in the feed solution was examined in the range of 20,40,and 60 mmol·L-1using TBA as the carrier with the concentration of 30 mmol·L-1(Fig.5).Complete extraction is perceived in 25 min forC0=20 mmol·L-1and 19 min forC0=40 mmol·L-1,but forC0=60 mmol·L-1only 82%extraction was obtained in 25 min of contact.Fig.5 also showed that with increasing the initial feed concentration up to 40 mmol·L-1,the extraction of the succinic acid is not considerably affected,however for the initial feed concentration above 40 mmol·L-1,the amount of the acid extraction declined.For the feed solution with the maximum initial concentration of 60 mmol·L-1,the succinic acid-complex saturated the membrane and therefore the degree of extraction was diminished.In this manner,the succinic acid that exhausted the stripping agent of Na2CO3,was transferred into the stripping phase and led to a reduction in the extraction rate.Chakrabartyet al.[9] obtained a similar result for a BLM-based separation of lignosulfonate.

Fig.5.Effects of initial feed concentration on the extraction efficiency(30 mmol·L-1 TBA,1 mol·L-1 Na2CO3,5% (vol) Span 80,400 r·min-1,pH 3,O/A ratio 1 and treat ratio 1:5).

3.3.3.Effect of carrier concentration

The permeation rate of succinic acid through the liquid membrane is obviously dependent on the concentration of the carrier in the organic phase.This effect is mainly obvious at short process times,which is significant in itself due to an extraction system such as this work necessarily requiring fast kinetics.TBA which contained tertiary amine molecules acted as an extractant of succinic acid in this work.The influence of TBA concentration on the extraction rate of succinic acid was studied in the range of 15-45 mmol·L-1.As perceived in Fig.6,for the experiment with the initial TBA concentration of 15 mmol·L-1,the extraction rate was be found to 94% before 6 min and it was obtained to 99% at 13 min of the process.Moreover,for the tests with a higher extranet concentration of 15 mmol·L-1,the extraction rate of succinic acid was diminished with increasing the extranet concentration.Fig.6 shows that the extraction rate of succinic acid for the test with an extranet concentration of 45 mmol·L-1was be found to 90%and 99%at 13 and 19 min,respectively.The extraction rate of succinic acid was also obtained at 98% and 99% at the mentioned times,respectively,for the TBA concentration of 30 mmol·L-1.In addition,considering of data in Fig.6 reveals that with the enhancement of carrier concentration from 30 to 45 mmol·L-1in the membrane phase the extraction rate is almost constant.

Fig.6.Effects of TBA concentration on the extraction efficiency in the ELM system(initial acid concentration of succinic acid of 40 mmol·L-1,1 mol·L-1 Na2CO3,5%(vol) Span 80,400 r·min-1,pH 3,O/A ratio 1 and treat ratio 1:5).

In general,from comparing data represented in Fig.6,it was achieved that the TBA concentration has the main role in the emulsion stability and the extraction rate of succinic acid.From considering this data,one can conclude that with increasing in carrier concentration in the membrane phase,two opposite effects would be observed for the extraction rate.The negative influence of the enhancement of TBA concentration on extraction efficiency could be explained below.With increasing TBA concentration,the emulsion stability decreases according to the interfacial properties of the carrier,which favors O/W emulsions and it is opposite to the Span 80 action as reported by Panet al.[10].Besides,with increasing TBA concentration in the membrane phase,emulsion drop size and also diffusivities of carrier and its complex enhances,according to an enhancement in viscosity of membrane phase.Moreover,the enhancement of viscosity of the liquid membrane phase at upper carrier concentration delays the transport of acid-complex extraction.So,the enhancement in TBA concentration from 15 to 30 or 45 mmol·L-1,appears to be unfavorable to the extraction of succinic acid.Similar results were obtained by other researchers[11].However,increasing carrier concentration improves the rate of acid extraction as further molecules of carrier get associated with the succinic acid and produce a complex that finally penetrates through the membrane.This fact supports the obtained values of extraction efficiencies related to experiments with carrier concentrations of 30 and 45 mmol·L-1.On the other hand,in the experiments performed for the carrier concentration limit from 30 to 45 mmol·L-1,the latter effect showed a stronger effect than the earlier one.Therefore,as usual reported in the ELM papers,the extraction efficiency would be improved by increasing the TBA concentration.Finally,the optimal TBA concentration was 15 mmol·L-1.

3.3.4.Effect of the stirring rate

The agitation rate has a key role in the penetration rate of acid through the ELM technique.In order to get the optimum agitation rate and subsequently the highest extraction rate,the stirring rate was varied from 300 to 500 r·min-1.Fig.7 shows that based on the decreased mass transfer resistance and increased mass transfer coefficient as well as enlarged mass transfer area at a more agitation rate of 300 r·min-1,the degree of the acid extraction was increased by increasing the stirring speed for the first 6 min.After 6 min,at the fast stirrer speeds of 400 and 500 r·min-1,the difference in the extraction rates of succinic acid was negligible at the same sampling times.This indicates that with enhancement in agitation rate,the emulsion globules become smaller making the mass transfer rate faster.However,unlimited enhancement in the agitation rate appears to be unfavorable to the emulsion,which may break.Therefore,the driving force for acid extraction was diminished by a reduction in pH gradient between the feed solution and the stripping phase according to leakage of Na2CO3in the stripping phase.Thus,proper agitation rate requires being chosen for the optimum act of ELMs.As a result,the most appropriate agitation rate was concluded to be around 400 r·min-1.

Fig.7.Effects of stirring speed on the extraction efficiency (initial succinic acid concentration 40 mmol·L-1,30 mmol·L-1 TBA,1 mol·L-1 Na2CO3,5% (vol) Span 80,pH 3,O/A ratio 1 and treat ratio: 1:5).

3.3.5.Effect ofNa2CO3 concentration

In general,in an ELM system,the driving force for the transport of succinic acid can be made from a pH difference between the feed solution and the stripping phase.In this study,an alkali solution(Na2CO3) as the stripping phase was selected to increase the pH difference.The effect of Na2CO3concentration 0.5 to 1.5 mol·L-1on the extraction rate of succinic acid presents in Fig.8.Fig.8 shows that,although the difference among the extraction rates of succinic acid obtained at different stripping agent concentrations was negligible after 19 min,the extraction rates of succinic acid at most sampling times were improved with increasing Na2CO3concentration.This finding can be related to the superior reaction capacity of Na2CO3with succinic acid keeping an upper hydrogen ion gradient between the stripping and the feed phases.Moreover,a small difference in the extraction rate of acid for two higher Na2CO3concentrations 1 and 1.5 mol·L-1was due to the Na2CO3concentrations being higher than the initial acid concentration of 40 mmol·L-1.

Fig.8.Effects of Na2CO3 concentration on the extraction efficiency the initial acid concentration of succinic acid of 40 mmol·L-1,30 mmol·L-1 TBA,5% (vol) Span 80,400 r·min-1,pH: 3,O/A ratio:1 and treat ratio: 1:5.

According to data presented in Fig.8,the optimum Na2CO3concentration of 1 mol·L-1could be reported for the feed solution with an initial acid concentration of 40 mmol·L-1.In addition,it was reported that emulsion swelling as well as membrane breakage could be introduced as the outcomes of increasing NaOH concentration in the stripping phase in an ELM system.Increasing NaOH concentration in the stripping phase leads to hydrolysis of the ester bonds of Span 80 (act as surfactant) tending to a diminish in the emulsion stability [39,40].So that this phenomenon could be occurred in our experiments,while NaOH molecules associated with Na2CO3were presented in the aqueous stripping phase.However,in our work,the studied ELM system was prepared using Span 80 as the surfactant and the stripping phase with 1.5 mol·L-1Na2-CO3solution was stable.It is that the final pH of the feed phase was a little greater compared to its initial pH,regardless of Na2CO3concentration.

3.3.6.Effect of treat ratio

The ratio of emulsion phase to feed phase is identical to solvent to feed ratio in conventional liquid extraction and is defined as the treat ratio in an ELM extraction.An enhancement in the treat ratio mostly tends to an enhancement in the capacity and the extraction rate.It is according to rising in emulsion volume in the system and consequently increases in the carrier,stripping extent,and also an overall surface area for mass transfer.Results for the variation of extraction rate with the treat ratio are shown in Fig.9;when the treat ratio was increased from 1:7 to 1:3,the extraction rate of succinic acid was improved.But enhancement in the extraction rate when the treat ratio was increased from 1:7 to 1:5 was more than that for the increase in treat ratio from 1:5 to 1:3.Moreover,as the treat ratio increased from 1:7 to 1:5 there was considerable progress in the extent of extraction as well as the extraction rates.According to Fig.9,at treat ratios of 1:7 and 1:5,acid extraction in the aqueous solution with an initial acid concentration of 40 mmol·L-1,was 88%and 100%,after 25 min contact,respectively.But further enhancement in the treat ratio to 1:3 led to a decrease in both the rates and extents of the acid extraction.This result can be a consequence of the membrane content effect on the globule size distribution;enhancement in emulsion holdup leads to the formation of larger globules and also changes the globule size distribution to the higher end of the spectrum.The formation of globules with larger sizes tends to a reduction in external mass transfer areas which is in many cases a rate-limiting step.In addition,larger-sized globules also enhance the effective diffusion path lengths within the globule that causing a reduction in extraction rates.Moreover,increasing emulsion hold-up enhances globuleglobule interactions that incline to phenomena of globule breakage,globule coalescence,etc.,which cause to release of the encapsulated acid back to the aqueous feed solution [12].

Fig.9.Effect of treat ratio on the extraction efficiency (initial succinic acid concentration 40 mmol·L-1,30 mmol·L-1 TBA,1 mol·L-1 Na2CO3,5% (vol) Span 80,400 r·min-1,pH 3,O/A ratio 1).

3.3.7.Influence of surfactant concentration

The stability of the emulsion is a crucial problem in the emulsion liquid membranes.As stated by researchers,besides the agitation rate and type of the extractant,the selection of the surfactant and its concentration have a key role in the emulsion stability.So,in this work,Span 80 due to its availability was used as the surfactant.The influence of surfactant concentrations 5%-9%(vol)on the succinic acid extraction rate is presented in Fig.10;the extraction rate and final volume of the emulsion were diminished at a test with a low concentration of surfactant,at the end of the extraction process.This observation indicated that the emulsion stability is intensely related to the concentration of the surfactant.Fig.10 shows that the emulsion stability was enhanced with an enhancement in the concentration of Span 80 from 5% to 7% (vol),which tended to decrease the break-up rate;and subsequently,improve the extraction rate of succinic acid.However,increasing surfactant concentration from 7% to 9% (vol) led to a decrease in the extraction efficiency.Excessive surfactant leads to enhancing the resistance at the interface and this could be related to increasing in viscosity of the membrane phase.Sulimanet al.[11] presented a similar trend in the extraction of zinc and copper ions from their solutions.They reported that the use of an exaggerated concentration of the surfactant led to a decrease in the extraction rate according to create the great interfacial resistance.

Fig.10.Effects of surfactant concentration on the extraction efficiency (initial succinic acid concentration 40 mmol·L-1,30 mmol·L-1 TBA,1 mol·L-1 Na2CO3,400 r·min-1,pH 3,O/A ratio 1 and treat ratio 1:5).

3.3.8.Effect of cyclohexanone content

The addition of some cyclohexanone extent to the membrane phase could have two great effects on ELM systems;reduction of swelling and enhancement of the transport rate of succinic acid,during the separation process [13].The swelling was significantly decreased by the integration of cyclohexanone to have preferential micellization of Span 80 with the additional reagent rather than with water [14].Moreover,it has been shown that,for the extraction of succinic acid using amines as an extractant,the extent of water co-extraction with succinic acid was changed with varying the type of the diluent [15].Correspondidingly,the creation of a change in the ELM system by the integration of some extent of the cyclohexanone into the membrane composition led to increasing the water co-extraction which could exceed the preferential micellization of surfactant with cyclohexanone [16].

In order to investigate the effect of the addition of cyclohexanone on the studied ELM system,membrane phases were organized by varying the extent of the cyclohexanone from 1.2 to 1.6 mol·L-1.Fig.11 shows the effects of cyclohexanone content for the TBA/Span 80 system,and indicates that the extent of the extraction rate was increased by the integration of some cyclohexanone in the membrane composition to 1.6 mol·L-1.This finding was in agreement with those reported by other researchers [17].Purtikaet al.[18] found that the equilibrium constant between the carrier and lactic acid in cyclohexanone was much higher than that in kerosene.

Fig.11.Effects of cyclohexanone mass content on the extraction efficiency (initial succinic acid concentration 40 mmol·L-1,30 mmol·L-1 TBA,1 mol·L-1 Na2CO3,5%(vol) Span 80,400 r·min-1,pH 3,O/A ratio 1 and treat ratio 1:5).

The integration of some cyclohexanone in the membrane composition led to an amendment in the ELM and increase the complex concentration formed at the interface between the membrane phase and the external phase,and the transport rate of carboxylic acid increased with the enhancement in cyclohexanone concentration [19].Due to the above discussion,the reason for the increase of acid extraction efficiency with the enhancement in cyclohexanone concentration presented in Fig.11 may be similarly related to enhancing of the complex concentration at the membrane and the external interface in the ELM system during the separation process.

Moreover,Fig.11 shows that the degree of extraction increased by an enhancement in the cyclohexanone content to 1.4 mol·L-1,where succinic acid was removed from 98% to approximately 100%.Continuously,increasing the cyclohexanone content to 1.4 mol·L-1,the removal of succinic acid stayed constant.Therefore,the optimum cyclohexanone concentration was selected as 1.4 mol·L-1.Also,it is remarkable that the required contact time in order to get complete extraction efficiency improved by adding cyclohexanone to the membrane phase.Comparing data presented in Fig.11 for complete extraction duration time shows that the addition of the cyclohexanone to the membrane phase led to improvement this time from 25 to 21 min,at the same experimental condition.

4.Conclusions

The ELM included TBA as an extractant and was capable of extracting almost completely succinic acid from feed solutions containing 20 and 40 mmol·L-1within 25 min.The optimum conditions for preparation of stable emulsion which capable of 100%succinic acid extraction were found to be 400 r·min-1emulsification speed,10 min emulsification time,treat ratio 1:3,internal phase concentration 1 mol·L-1Na2CO3,pH 3,surfactant concentration 4% (vol),the carrier volume ratio of 4% and cyclohexanone content 1.4 mol·L-1.The variations of acid extraction efficiency with respect to the initial pH variations 3-7 were negligible,especially after 13 min time contact.Increasing of initial feed concentration up to 40 mmol·L-1,the extraction rate of succinic acid had a negligible change,though for feed concentration above 40 mmol·L-1the transport rate of the acid was decreased,according to membrane saturation with succinic acid complexes.

The variations of the extraction rate of succinic acid were inverse with TBA content as extractant according to rising in viscosity of the liquid membrane phase.This effect directed to decrease in diffusivities of TBA and its complex and by enhancement emulsion drop size.The extraction degree of succinic acid was increased at stirrer speeds above 300 r·min-1owing to a greater mass transfer coefficient and a more mass transfer area.To enhance the pH difference,Na2CO3an alkali solution utilized as the stripping agent,in the membrane phase.

The variations in the extraction rate of succinic acid were in favor of the initial concentration of Na2CO3.Moreover,a slight difference in extraction rates achieved for two higher Na2CO3concentrations 1 and 1.5 mol·L-1was due to the Na2CO3concentrations which were far higher compared to the initial feed concentration of succinic acid 40 mmol·L-1.

Results also showed that with an enhancement in the treat ratio,the capacity,as well as the extraction rate of acid,was generally improved.In the experiments,as the treat ratio enlarged from 1:7 to 1:5 there was considerable progress in the extent of extraction as well as the extraction rates.But further enhancement in the treat ratio to 1:3 led to a diminishing in the rates as well as the extent of extraction.Increasing surfactant concentration from 5% to 7% (vol) enhanced the stability of the emulsion liquid membrane which tends to enhance the extraction degree of succinic acid.However,increasing surfactant concentration from 7% to 9%(vol)led to a decrease the extraction efficiency.In order to investigate the effect of membrane viscosity on the extraction rate,membrane phases of different viscosity were prepared by changing the cyclohexanone content from 1.2-1.6 mol·L-1.In general,the required contact time to get complete extraction efficiency is improved by adding cyclohexanone to the membrane phase.

Data Availability

No data was used for the research described in the article.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.