Experimental Investigation on Microfluidic Reactive Extraction of Citric

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An experimental Investigation on microfluidic reactive extraction of the Citric acid using Trioctylamine /1- Decanol system in uniform and non-uniform circular microchannels Eldho Abraham, Giri Nandagopal Mukunthan Sulochana, Bhuvaneshwari Soundarajan, and Selvaraju Narayanasamy Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02982 • Publication Date (Web): 30 Aug 2017 Downloaded from http://pubs.acs.org on August 31, 2017

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Industrial & Engineering Chemistry Research

An experimental Investigation on microfluidic reactive extraction of the Citric acid using Trioctylamine /1Decanol system in uniform and non-uniform circular microchannels

Eldho Abraham, Giri Nandagopal Mukunthan Sulochana, Bhuvaneshwari Soundarajan, Selvaraju Narayanasamy†* Department of Chemical Engineering, National Institute of Technology Calicut, Kerala, India †*Department Biosciences and Bioengineering, Indian Institute of Technology Guwahati, Assam, India E-mail: [email protected] KEYWORDS. Microfluidics, reactive extraction, non-uniform microchannel, citric acid, Trioctylamine, 1-Decanol, diluent, slug flow, distribution coefficient, extraction efficiency.

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ABSTRACT. Dilute Citric acid solutions were extracted with Trioctylamine (TOA) in 1-Decanol at ambient conditions using indigenous microfluidic setup with 0.8mm section diameter. In this slug/segmented flow reactive extraction processes with equal flow ratios, channel length was varied from10 to 40 cm and flow rates used were in a spread of 10 to 60 ml/h. The extraction characteristics were analyzed in terms distribution coefficient, extraction percentage and change in concentration of the aqueous phase. The results were compared with the literature data of conventional reactive extraction and found that the microfluidic system was performing well on the basis of residence time. Effort had also been made to present indicative results of the system performance in a non-uniform microfluidic channel provided with a number of intermittent 1.6 mm diameter expansions. It had been found that the flow in nonuniform channels can deliver higher efficiencies and distribution coefficients.

1. INTRODUCTION Majority of the various carboxylic acids demands nowadays are met with petrochemical routes despite of their non- renewable and polluting nature and even the sources are under way towards an inevitable depletion. The establishment of the alternate routes for the manufacture of these acids is highly essential according to the recent scenario of the petroleum sources and related processes in terms of economical and process aspects. The routes originating from Biochemical methods are considered to be the best fit for the replacement of the petrochemical techniques for the manufacture of the most of the carboxylic acids. Unfortunately, they are considered to be ineffectual as far as the amount of effort has to be invested to upgrade the dilute aqueous product streams from a typical 2


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fermenter to a required concentration level. It is estimated that around 60 to 70% of the capital investment is accounted by the downstream operations for the product upgradation, where the solvent extraction principle is basically used for the product separation1,2 rather than a distillation operation which is accounted for a number of process concerns3. These processes, in general, cannot generate a separation efficiency more than a range of 10 to 15% even when an active organic solvent is used.2 The efficiency of the solvent extraction can be improved by incorporating a proton transferring chemical species like tri-alkyl phosphate or long chain aliphatic amines into the organic solvent and resulting process can be termed as reactive extraction. Reactive extraction has been highly promising in the recovery of carboxylic acids through the past 3 or 4 decades since its invention with eminent separation characteristics4. In a typical reactive extraction process, the acid solute present in the aqueous solution undergoes a reversible chemical complexation with the reactive extractant present in the organic phase to form acid-extractant complex by ion pair formation, hydrogen bonding and/or solvation.5 Then the resulting complex or the solvate is getting dissolved into the diluent in the organic phase by forming yet another hydrogen bonding with diluent.5 A temperature swing regeneration process can be adopted later for the solute removal and in most of the cases, the job is done by distillation. Basically the major setbacks in the conventional reactive extraction or extraction process are the low interfacial mass transfer rates and formidable separation of the raffiante and extract phases after the mixing. The transference of the bulk contacting of the reactive extraction to a process platform that can contemplate a higher mass transfer rates, in another way, a process that offer high interfacial area can make it capable in delivering a prolific separation process in terms economic and process competence. The notion of the above process concept has been expedient with microfluidic process because it is renowned with the both higher interfacial area and hassle free separation process using a phase splitter working based on the concept of relative wettability of two different phases. So the integrated form of the reactive extraction 3



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with microfluidics is gaining importance as a tool for the process intensification in the separation of various carboxylic acids from its dilute aqueous solutions (for example: - Fermentation broths, waste water effluents, process streams and so on). The word reactive extraction in the microfluidic context was reported for the first time by Brody and Yager in 1997 and after it had been a tool which was tested by many researchers for process intensification for the separation of the weak organic acids too.6 The microfluidic process demonstrates with larger specific interfacial area in its two most common flow regimes of parallel flow and slug flow as compared to any other conventional counterparts. It is also redound to good control over the process parameters, heat and mass transfer and better response to the control systems.7 The major attraction is the dearth of mixing in the micro channel flow process so that it offers the hassle free, quick and a clean phase separation. These micro reactors also account for its easy flow manipulations8, lower solvent wastage and enhanced safety benefits7. Hence the conjunction of reactive extraction with the microfluidics can bring about many revolutionary benefits in the production of petroleum competent week organic acids from fermentation processes. In addition, it is having good control of mixing rates with a stable interface because of the prevailing laminar flow conditions.6 The reactive extraction, as stated earlier, is a multiformity of normal solvent extraction and a process uses the combination of an extractant and organic diluent/solvent rather than a single organic solvent of conventional extraction. The extractants those are commonly used in the reactive extraction process falls into 3 classes as (i) carbon-bonded oxygen donor extractants that include hydrocarbons and substituted hydrocarbon solvents, (ii) phosphorus-bonded oxygen donor extractants, and (iii) Nitrogen bound oxygen donor extractants (long chain aliphatic amine extractants).9 The extractants of the first category is nonreactive in either way to the extractate and the extraction is performed by the proton transfer or by the formation of hydrogen bonding. But the other two types reversibly react with the undissociated acid 4


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and forms acid-extractant complex results in a considerable increase in the solute recovery due to significant solvation of the complex in the organic diluent/solvent.4 The second and the third class of these extractants are more popular due to its multifold extraction efficiency as compared to the first one. Many examples of the different sorts of the extractants are depicted along with the basic extraction mechanism in a number of literatures.1,2,6 ,10-14 Large number of research works have been reported in the area of reactive extraction using various extractants in combination with different diluents and it is evident that TOA is the best performing reactive extractant out of all the three classes after careful consideration of many aspects.4,15 So the present study of reactive extraction is chosen with the TOA as it has previously used to characterize the microfluidic extraction process. Turning to the diluents, there are many classifications available for different types of the diluents as (i) Active and inactive, (ii) protic and aprotic, (ii) polar and nonpolar etc.16 In most of the literatures diluents are designated as active or inactive in relation with the reactive extraction and anyway the classification as active/inactive and Polar/nonpolar is leading to more or less same technical sense. But the protic and the aprotic diluent falling into the polar class. More specifically, a protic diluent is one with either an O-H bond or an N-H bond which is capable in donating the proton that will result in the formation of the strong hydrogen bonding with the other polar molecules. The performances of different types of the diluents are reported in many research articles when combined with many extractants in various concentrations.1,17 It has been vouchered by many investigators that 1-octanol and Isoamyl alcohol are the best performing diluents or carriers in combination with any reactive extractant tested in the reactive extraction process. 5,16,17 The 1-Decanol also has done reasonably good with the reactive extraction in combination with TOA5,17,18. The scientific literatures on the reactive extraction of various carboxylic acids are numerous and vast with the principle concepts. The studies on reactive extraction of citric acid with TOA in combinations with different diluents is well established with plenty of works reported by a number of scientists.1,2, 12,16 ,19-24 So the selection of a 5



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reactive extraction system of the citric acid with TOA in the presence of an active diluent will be most credible and convenient to generate authentic microfluidic performance comparisons with the conventional reactive extraction systems. The application of microfluidic devices for liquid−liquid reactive extraction had been demonstrating as a featured alternative to their macro scale counterparts7 because of the discernible accretion in extraction efficiency with very low space time due to characteristic micro dimensions in the contactors. This micro dimensions will be straight away transmuted into a high surface to volume ratio and consequent higher mass transfer rates. Exhaustive literature reviews revealed that not much of the studies reported on microfluidic reactive extraction of the carboxylic acids so far and one of it is the reactive extraction of the lactic acid using TOA in the same platform7 exactly depicted in this literature. These factors indicate the necessity and potential of the study of the kind that had been carried out through this work. The microfluidic reactive extraction can be made out in either parallel flow or the alternate slug-droplet flow regimes.6 The flow ratios, flow velocity and the flow geometries have a considerable effect in deciding the flow regimes25. The current study aims on the microfluidic reactive extraction of the citric acid from its dilute aqueous solution using TOA in 1Decanol with segmented flow/slug flow of aqueous droplets and organic slugs in a uniform circular channel. The flow patterns were generated with a 120o micro fluidic mixer. As these slugs and the droplets were alternate, they had attributed with higher interfacial area that enhance the mass transfer rates. In addition, a shear induced internal molecular mixing will be occurring in these micro fluid elements due to its flow over a solid boundary also have a significant effect on the enhanced rates of the mass transfer.7,26,27 The different reactive extraction characteristics were analyzed as a function of residence time, micro channel length, initial acid concentration in the aqueous phase and the TOA concentration in the organic phase in this work. Some of the extraction characteristics were correspondingly compared with the results of conventional reactive extraction process cited in 6


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literature.2 Effort was also made to evaluate extraction performance in non-uniform cross section micro channels and to compare the results with same of the extraction through the uniform circular channels as a part of an innovative endeavor towards the betterment of microfluidic reactive extraction. The movement of the slugs and the droplets through varying cross sections in the same circular micro channel (like a sudden expansion and then a sudden contraction) can increase the diameter of the slugs and droplets and reduce the length and then vice versa. This type of a transformation will alter the concentration fields and it can interfere positively with the internal mixing in the slugs and droplets. As the shear stress play an important role in creating the internal circulations inside the fluid elements, the flow through the non-uniform channel can essentially enhance the extraction efficiency. In addition to this, in the higher diameter section, the increase in the flow diameter can increase the cross sectional area of the end cap of the slugs and the droplets, which is bearing a determinable role in the mass transfer. Even though, the wall film is having very thin in size, lower in velocity compared to the droplet and the slug and rapid saturation or depletion (based on the type of process), this lateral surface area present over the droplet is also having considerable influence on the mass transfer.7 So the role of the same in the mass transfer cannot be neglected up to certain extend. The liquid surface available in the plane which is perpendicular to the longitudinal direction or the surface area constituted by the end caps of the slug and droplet is the most critical influencing surface area, where they do have a mutual contact. A schematic representation of the flow through the channel with uniform cross section and flow transformation through non uniform circular cross section microchannel is presented in the figure 1. The secondary aim of this work is to make a performance comparison of the of the uniform circular channel flow with non-uniform circular channel flow at least in terms of extraction efficiency. As a precursor to this work, the basic extraction characteristics of the process were also analyzed. 7



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2. THEORY Many literatures have been elaborated on the mechanisms involved in the reactive extraction of the carboxylic acids with the various extractants generally Phosphorous bound oxygen donor and Nitrogen bound oxygen donor extractants.1,9,10,16,20,28-30 In most of those written outcomes, the mass action law is predominantly used to figure out the mechanism of the complexation reactions. As an exception, few of them are also dealt with relative basicity model, linear solvation energy relationship model, Langmuir model to elucidate on complexation mechanism.5,31 In this work, the reactive extraction of the citric acid with TOA is focused and the mechanism of complexation is explained based on mass action law. For the kind of extraction is considered here, it has been cited several occasions that the mass action law has been successfully applied in explaining the reactive extraction of the citric acid in TOA.2,13,23,28 The extraction process will turn to a physical extraction when the concentration of the extractant in the organic phase reduces to zero. So the transfer of the solute to the organic phase is only by the interaction of the carboxylic acid and diluent molecules. These interactions can be of bonding by proton transfer or a weak Hydrogen bonding and the acid species will find a very low solubility in the pure organic diluent. So the resulted distribution coefficient is very low and marks the physical extraction process as less efficient. But in the case of the reactive extraction, the acid-amine complexes have higher solubility than the former so increased extraction efficiency is expected 2.1. Equilibrium of a reactive extraction process In the reactive extraction process, the carboxylic acid molecules those are transferred into the organic phase will undergo a complexation reaction with the TOA molecules by constituting ionic bonding, bonding through proton transfer, Hydrogen bond formation and or by solvation. The term solvation is more close to physical extraction. In a typical TOA extraction process, only the 8


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undissociated acid in the aqueous phase will transfer to the organic phase and undergo the complex solvate formation32. The acid which has transferred into the organic phase will no longer be in acid form, instead in a form of ammonium salts.1 There is no certainty in the stoichiometry of the complex are formed as a result of complexation reaction and a number of stoichiometries can be expected based of prevailing constitution of the acid solution and organic phase. The dimerization of the acid in the organic phase is also expected during the complexation reaction.33 Some of the previous investigations have shown efforts to make generalization of the stoichiometries for the acid-amine complexation reaction and many proposals were put forward in this area.9,23,28,32 A general mechanism of the reactive extraction of citric acid in TOA can be depicted as follows when (1, 1) complex is formed. + ⇋

(1)

As the citric acid is a polycarboxylic acid, interaction of the acid molecules with more than one amine molecules are to be expected and to a great extent, this is depending on the type of the diluent.20 The same is represented below. + 2 ⇋

(2)

or + ⇋

(3).

The organic phase species are indicated with the over bar. The second amine molecule is neutrally hydrogen bonded in the second case.21 The complexes which are given in the above equations are the most general forms of the solvate complexes formed and other stoichiometric forms are relatively unimportant here in this case. The most general forms of these reactions can be represented as

+ ⇌

(4)

Where =1, 2, 3…. and =1, 2, 3….. The above equation is applicable in multi molecular complexation reactions. If an extraction process has to be more efficient, then the process should have a high distribution coefficient with higher selectivity. The distribution coefficient for an extraction process can be given as follows, 9



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! " # $ % &

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(5).

The concentration terms are denoted by hard brackets. Stoichiometric loading ratio can be used to predict the complexation reaction stoichiometry and it indicate acid loading extend with the reactive extractant TOA. The loading ratio can be given as follows,

' " #

(6)

The efficiency of the reactive extraction process can be calculated using the following equation. ( 1 * + ⁄ , . 100

(7).

Figure 1. Slug and droplet flow. a) Through micro channel with uniform cross section, b) Through micro channel with non-uniform cross section, 1. Slug in smaller channel, 2. Droplet in smaller channel, 3. Size transformation of droplet when passing into the larger cross section, 4. Large droplet in the large channel, 5. Large slug in the large channel, 6. Size transformation droplet when entering to the smaller channel. 3. MATERIALS AND METHODS 3.1 Materials. Citric acid (Purity 99%) received from Merck, India, Tri-n-octylamine (TOA) (95%, maximum limit of impurity 0.1%) and 1-Decanol (98% purity) supplied by Spectrum India and distilled water from 10


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Merck. All these items were used throughout the experiment without further purification. Two high precision syringe pumps (Zion Plus, Plenumtek, India) used for the delivery of the fluid to the micro channels. The experimental setup was realized by fitting PDMS based Y-shaped junction and Yshaped phase splitter with an angle of 120o at each end of a translucent micro PTFE tubing (received from BOLA) of varying lengths. The phase mixer and the phase splitter fabricated in-house by gravity casting of Polydimethylsiloxane (PDMS) in the Aluminium mould with suitable micro inserts. The PDMS is the base material that constitute the micro mixing and splitting channels 3.2. Experimental setup. The micro capillary used for this investigation had internal diameter and external diameters of 0.8 mm and 1.6 mm respectively. The length of the micro channel was varied between 10 cm to 40 cm. A schematic representation of the experimental setup is given in figure 2. In a typical setup, one end of the micro capillary was fitted to a Y-shaped mixing junction and the other end was fitted to a phase splitter. The inner diameters of branches of both the junction and the splitter was same as of the capillary tube. The organic and aqueous phase fluids were admitted into the micro channel through each branch of the mixer by two syringe pumps connected. The separation of the aqueous phase and the organic phase had be accomplished by phase splitter at the end of the capillary. One branch of the Y-shaped phase splitter was glass tube and the other branch was the continuation of the PTFE tube. The phase splitting was achieved in the splitter due to the higher relative wetting of the aqueous phase in the glass tube rather than in the PTFE so that the aqueous phase tended to flow through the glass channel. The separated aqueous phase was observed with the purity greater than 98% on volume basis even at highest flow velocity of 60 ml per hour. At lower velocities the volume fraction of the organic content in the aqueous phase was found negligible. The visualization of the droplet formation in the micro channel was done through suitably placed digital microscope when it was required.

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Figure 2. Experimental set up for the microfluidic reactive extraction of Citric acid with TOA. 1.Aqueous phase, 2.Organic phase, 3.Y-shaped mixer, 4.Y-shaped phase splitter, 5.Aqueous phase syringe pump, 6.Organic phase syringe pump, 7.Organic phase sample, 8.Aqueous phase sample, 9.Micro capillary with segmented flow.

3.3 Experimental procedure. a) Extraction through uniform cross section micro channel. The model aqueous solutions of citric acid were prepared by dissolving required quantities acid in HPLC grade water to result concentrations of 0.2 and 0.4 mole L-1. The organic phase solutions were prepared by mixing TOA with the 1-Decanol in different proportions on volume basis. 1-Decanol was used as the diluent for the entire investigation and the percentage concentrations of the TOA in the organic phase were 0, 5,10,15,20. The extraction occurring was considered as physical extraction when the concentration of the TOA in organic phase was zero. The organic phase and the aqueous phase were admitted into the micro channel using the syringe pump at different flow rates of 5, 10, 15, 20, 25 and 25 ml/h of each liquid phases in some cases a flow rate of 30mL/h was also employed. A constant 1:1 ratio of the aqueous phase and organic phase was used for the entire range of the 12


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experimentation and a slug flow (segmented flow) regime had been observed. The lengths of the microchannel used were 10, 20, 30 and 40cm and all the experiments are carried out at room temperature and atmospheric pressure. The separated aqueous phase after the extraction is analyzed chromatographically using SHIMADZU Prominence-I HPLC unit fitted with diode array detector (DAD). The reverse phase C-18, 250 ×4.6 mm column had been used and volume of the sample injected is 20µL which had subjected to an isocratic analysis. The mobile phases used were (A) 20 mM monobasic Phosphate buffer (KH2PO4) with pH 2.5 adjusted by o-Phosphoric acid and (B) 60% Methanol - 40% Acetonitrile. All the experiments are carried out in triplicates and the arithmetic average of the values had been taken.

b) Extraction through the non-uniform micro channel. The novel part of the work as mentioned in the previous sections had been performed in a 20 cm long channel (including the expansions) at TOA concentration of 10% for both initial acid concentrations of 0.2M and 0.4M. The experiment execution was same as of the procedure followed for the experimentation in the uniform diameter microchannels. The variation of the diameter had given by fitting a 2cm length of PTFE tube with inner diameter of 1.6mm at the longitudinal center of the 20cm section. The number of diameter variation was increased from three up to seven in the order of 3, 5 and 7. The increments were equally given in the both the directions. 4. RESULTS AND DISCUSSIONS 4.1. Flow through uniform cross section micro channel. 4.1.2. Factors influencing the distribution coefficient and extraction efficiency. The figures 3, and 4 depict the typical behavior of the microfluidic reactive extraction systems under consideration and the analysis made with respect to the residence time which is altered by manipulating the flow rates with a fixed microchannel length. The case rated for the following 13



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illustration here, the microchannel was 40cm of length which experienced a highest residence time at any corresponding flow rate in the entire experimental combination and initial acid concentration used was 0.2M. It had been found that the residence time had a discernible effect on the distribution coefficient and the extraction efficiency. Increase in the microchannel length will result in increased residence time at a constant flow rate and this factor can enhance the contact time though the diffusional mass transfer. This will guide the process to a higher distribution coefficients and extraction efficiencies. In a microfluidic process with slug flow regime, diffusional mass transfer is predominant. So that the contact time and mass transfer area deserves a considerable importance to derive higher extraction efficiencies as the results indicated. The same kind of the trend was evinced by the system also with different experimental configurations made of different microchannel length, flow rates, concentrations of TOA and initial acid concentrations.

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TOA 20% TOA 15% TOA 10% TOA 5% TOA 0%

2.0 1.8 1.6

Distribution coefficient

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1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 10

20

30

40

50

60

70

80

Residence time (s)

Figure 3. Effect of Residence time on the distribution coefficient at different TOA concentration with acid concentration of 0.2 M and microchannel length of 40cm.

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TOA 20% TOA 15% TOA 10% TOA 5% TOA 0%

70

60

50

Extraction %

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40

30

20

10

0 10

20

30

40

50

60

70

80

Residence time (s)

Figure 4. Effect of Residence time on the extraction efficiency at different TOA concentration with acid concentration of 0.2 M and microchannel length of 40cm. A careful examination of the illustration also reveals that the distribution coefficient or the extraction efficiency is not a strong function of the residence time. The increase in the flow rate will culminate a reduction of the contact time at constant channel length and this will source a decadence in the extraction efficiency or distribution coefficient. At the same time at higher flow rates, the slug and the droplet lengths will be shortened by a considerable magnitude which will end up with higher interfacial mass transfer area and that will be used to compensate the former decadence in the above properties34,35. Conversely, lowering of the flow rate to provide higher residence time will literally 16


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boost the extraction efficiency but will result in the lower interfacial area which can bereave the system performance in terms of extraction efficiency. In addition, the higher flow rates can enhance the shear induced internal circulation in the individual slugs and droplets and at the same time dwindle the effective mass transfer area by increasing the thickness of the wall film34. This phenomenon can also modify the extraction process in the same perspective mentioned above. It is alluring to take a note that the change in the flow rate or change in residence time influencing the system positively and negatively at the same time and as a result the increase in the desired property is reduced to an extend when it is used compensate the reduction due to the negative effects. By interweaving the above aspects free an inference that can materialize nature of the distribution coefficient or the extraction efficient for being a weak function of the residence time at varying flow rates through a fixed channel length. In order to describe the effect of the initial acid concentration on the system behavior, it is better to consider the quantity of the solute extracted to the organic phase in terms of the change in concentration and a plot of the same against residence time is presented in the figure 5. The important point noticed is the quantity of the solute removed to the organic phase increased with the increase in the initial acid concentration. This should be due to the climb in the imposed concentration gradients when there was a jump in the initial acid concentration from 0.2M to 0.4M and provided all parameters were identical other than the initial acid concentration. Equivalent trends had been followed by the different system configurations when it has been analyzed in terms of the initial acid concentration and residence time.

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0.20 0.18

Change in concentration (M)

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0.16 0.14 0.12

0% TOA 0.2M 15% TOA 0.2M 20% TOA 0.2M

0.10 0.08 0.06

0% TOA 0.4M 15% TOA 0.4M 20% TOA 0.4M

0.04 0.02 0.00 10

20

30

40

50

60

70

80

Residence time (s)

Figure 5. Effect of initial acid concentration on change in concentration of the aqueous acid solution during extraction process in 40cm microchannel. It is also important to figure out the irrelevancy of the use of the extraction efficiency or the distribution coefficient to illustrate the effect of the initial acid concentration on the performance of the extraction process because relative nature of these properties. For a higher initial acid concentration, the quantity of the solute removed will be always high when it is compared with the lower initial acid concentrations. But there are chances that the computations to display a lower extraction efficiency or a distribution coefficient for the higher initial acid concentrations and higher for the lower initial acid concentrations. The extraction of the Citric acid with a concentration range of 0.2m to 0.8m with 20% 18


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TOA in Decanol (volume basis) was yielding the same results in the computation of extraction efficiency, distribution coefficient and amount solute removed as mentioned above in a cited literatute.2 Controversy can be arouse upon the use of the distribution coefficient or the extraction efficiency to explain the effect of the initial acid concentration on the extraction performance. The figure 5 also illustrate on the effect of the concentration of the TOA on the amount of the acid extracted from the aqueous phase. The concentration of the TOA increased, the quantity of the acid extracted also had escalated. The figures 6 and 7 demonstrate the straight forward influence of the TOA concentration in the on the extraction efficiency and distribution coefficient at 0.2M initial acid concentration, 40cm microchannel length at various residence time. Different kinds of the systems also kept up the same kind of effect under the different TOA concentrations. This is certainly due to the higher availability of the TOA molecules to undergo the reversible complexation reaction with the acid molecules transferring into the organic slugs. The 1-Decanol diluent is free with the bulky alkyl groups and that aspect is also complementing the straight forward behavior of these extraction variables. This trend indicate that the concentration of the TOA can be increased further to a higher value while taking care of the acid-amine complex aggregate formation, organic phase viscosity and operational cost. This also indicate that the complexation reaction between TOA and Citric acid in the organic phase is an instantaneous and the rate of mass transfer is mostly diffusion regulated.21

19



3.5 3.0

Distribution coefficient

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2.5 144.69 s 48.23 s 28.94 s

2.0 1.5 1.0 0.5 0.0 0

5

10

15

20

TOA concentration(v/v)

Figure 6. Effect TOA concentration on distribution coefficient at different residence times with initial acid concentration of 0.2M

20


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72.35s 24.11s 14.47s

80 70 60

Extraction%

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50 40 30 20 10 0 0

5

10

15

20

TOA concentration (v/v)

Figure 7. Effect TOA concentration on percentage extraction at different residence times with initial acid concentration of 0.2M The characteristics discussed so far was common to all sets of experiments carried out while these sets found variations in terms of parameters including length of the microchannel, the change in acid concentrations and TOA concentrations. The table 1 and 2 elucidate the range of the distribution coefficient, extraction efficiency and change in concentration of the aqueous phase at the end of the extraction with the flow rate range of 5 to 30 ml h-1 for the different experimental conditions mentioned in the former sections. The system performance under the investigation was also resembled 21



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to the system behavior of the cited conventional reactive extraction process. This indicates that the performed micro fluidic experiments conform to the reactive extraction process in all technical aspects. Table 1. Range of Distribution coefficient, percentage extraction and concentration change in aqueous phase with initial concentration of 0.2M. 1 45 , 0 , 2 3 7 8 94

1 ℎ 0.2

0

5

10

15

20

10 - 30

5 - 25

5- 25

5 - 25

5 - 25

:

(;9

Experimental Investigation on Microfluidic Reactive Extraction of Citric

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