Kinetic Reactive Thin Layer Extraction - Industrial & Engineering

Article pubs.acs.org/IECR

Kinetic Reactive Thin Layer Extraction Ram Lavie* Department of Chemical Engineering, Technion, Israel Institute of Technology, Haifa 3200003, Israel ABSTRACT: When the separation of two closely related compounds by liquid−liquid extraction is necessary, conventional wisdom indicates that a selective extractant must be found that will discern between the two by associating with each to different equilibrium compositions. Kinetic reactive thin layer extraction (KRTLX) offers an alternative route that does not rely on finding such a particularly selective host, provided the rates of the chemical reactions between each of the two components and the host are sufficiently differentiated. The optimal design of a KRTLX application requires kinetic data pertaining to the reactions between the components to be separated and the active host in the extractant. The actual implementation of a KRTLX application requires the availability of extraction equipment that allows control of the time span of the reactions, such as is availed by the TLX extractor. KRTLX is modeled and explored numerically on the basis of published kinetic data. A calculated example, concerning the separation of two carboxylic acids, is evaluated, and compared to a conventional equilibrium reactive liquid extraction scheme. In this example, KRTLX yields two distinct streams of substantial purity at high yield in relatively few stages, thus turning out to be both superior in performance and economically advantageous.

1. INTRODUCTION Kinetic reactive thin layer extraction (KRTLX) is a process for the separation of closely related compounds into separate pure products by liquid−liquid extraction. It is based on differences in the rates of the reactions between each of the components in an aqueous solution and a common host in an organic liquid rather than on different equilibrium distributions. Potential applications of interest include chiral resolution, carboxylic acids recovery, and some rare earth elements separation. Continuous equilibrium separation processes, including liquid reactive extraction, are widely used in the chemical industry. In reactive extraction, an aqueous solution of species that we wish to separate is equilibrated with an organic phase (the extractant) that consists of a reactive organic host, mostly dissolved in a solvent. Some of each species transfers to the extractant where it reacts reversibly with the host, retaining preferably one or some of the species in the organic phase. The organic phase is then contacted with a second aqueous strip solution that recovers the species. Multistage operation amplifies the effect. The equipment is designed to permit the reaction to reach completion before moving on to a next stage. In this category, the reaction kinetics plays no role, except for the time it takes to reach equilibrium. Kinetic reactive extraction represents a somewhat different concept: In an age old batch purification process, a mixture consisting of a desired component A that contains an impurity B is reacted with a reactant C that reacts with both A and B, faster with B, generating a product mixture (AC + BC) that can be removed easily (mostly washed away). As the reaction proceeds, component B vanishes faster than component A, leaving behind the desired component A at an increasingly higher relative concentration up to absolute purity (Figure 1). Such a process lacks in yield because at least some of A is lost. At some point in time, one reaches a point of diminishing return, as further loss of A, plus the additional resources spent on maintaining the reaction, do not justify the incremental additional purity obtained. It pays then to stop the reaction and invest the remaining processing resources more fruitfully otherwise. Note © XXXX American Chemical Society

Figure 1. Kinetic concept.

that such a process is unlikely to generate directly two distinct products (pure A and pure B). The potential of the method for the separation of mixtures was recognized by several authors,1−3,16 except that it was considered in contexts other than liquid extraction, or if in the context of extraction,4 they did not possess the means to stop the complexation reaction in its track at the opportune moment. In a batch process, one may be capable of stopping a reaction instantly. In a continuous process, one may consider controlling the residence time. However, the residence time is not easily controllable, and, being the average value of a distribution, it is never defined sharply. Suppose one would possess the means to control in situ the time span of a complexation reaction in a reactive extraction process, then a process can be formulated (kinetic reactive extraction) that performs a separation. Unfortunately, the structure of the conventional industrial liquid−liquid contacting equipment (such as trains of mixer− settlers, extraction columns, centrifugal extractors, etc.) does not lend itself to the implementation of a kinetic approach. Thin layer extraction, a time-periodic repeated batch contacting method, does provide the necessary conditions: Received: July 2, 2014 Revised: September 26, 2014 Accepted: October 27, 2014

A

dx.doi.org/10.1021/ie5026387 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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(a) It minimizes mass transfer resistance, distinguishing the reaction from mass transfer effects except for exceedingly fast reactions. (b) Being a controlled batch process, it possesses a built-in capability to control, at will and in situ, the time span of the contact between the phases that equals approximately the time span of the reaction. (c) The minimal inventory in each of the participating layers provides a mostly homogeneous reaction time span for each small element of the processed fluids. In a previous work,5 we explored the potential application of KRTLX to the resolution of a racemic chiral mixture. Here, we attempt to generalize and study the applicability of the kinetic method to other closely related compounds. In the following, a sample KRTLX potential application of interest, the separation of carboxylic acids, is explored numerically on the basis of available published kinetic data. Many carboxylic acids are produced industrially on a large scale. Carboxylic acids are used in the production of polymers, pharmaceuticals, cosmetics, solvents, and food additives as well as in countless industrially important chemical reactions. Industrial routes to carboxylic acids include synthesis and fermentation. Carboxylic acids are among the most attractive substances that can be manufactured from biomass that is widespread in nature. The economic impact of fermentation chemicals is currently limited, not least because of difficulties in downstream processing.6 Even though the recovery of carboxylic acids from a fermentation broth by liquid−liquid extraction has been on the agenda for some time,7,8 the efficient economic recovery of individual acids at high purity and yield by extraction is still not fully resolved.9 Obviously, substantial improvements in the existing recovery technology are needed. We suspect that kinetic reactive extraction could possibly trigger the necessary improvement.

Figure 2. Plant structure: (a) Typical conventional equilibrium extraction. (b) KRTLX.

other organic, and outputs two comparable streams. Within each stage, the two phases are first vigorously mixed to facilitate mass transfer. The mixture is then separated into two distinct phases that may then be directed to a next stage. The organic stream flows in a continuous loop, through the sections from one stage to the next, in countercurrent to the aqueous phase. The aqueous feed, containing species A and B to be separated, is fed at one end, joining the aqueous effluent of the washing section, into the extraction section where B is preferentially transferred to the extractant while A exits as a raffinate. An aqueous strip solution is fed at the other end of the plant to the stripping section where A is mostly filtered out, letting B through as a strip product. Some of the strip product (the reflux) is recycled back through the washing section, where B is filtered out before joining the feed. This configuration intends to obtain most of A in the raffinate and most of B in the strip product. Evidently, the local conditions prevailing in each section must be tuned to the task at hand so that, in addition to being capaple of discerning between A and B, the extractant is also required to change preferences as a function of local conditions (composition, pH, etc.). As we shall see, such a complex selectivity is not required in KRTLX as the extractant only needs to react with the two components at different rates. The structure of the TLX plant consists of a simple linear train of KRTLX stages. The donor aqueous stream flows from stage to stage in one direction and the aqueous strip stream in the opposite direction. TLX cycles in time rather than in space. Each TLX stage (Figure 2b) is fed alternatingly with small batches of each of the two aqueous streams and constitutes an independent, complete extraction-stripping module. The organic to aqueous (O/W) ratio is controllable in TLX by sizing the aqueous batches, relative to the fixed amount of extractant contained in a stage in the form of a thin supported layer of the organic extractant. A necessary condition for the successful application of TLX concerns the stability of this thin layer. Such instability is known to be a major obstacle18 to the application of supported liquid membranes (SLM) and solvent impregnated resin (SIR) processes that similarly rely on an organic liquid supported on a porous polymeric support. However, unlike SLM and SIR, the extractant-coated TLX matrix is never fully immersed in an aqueous phase, nor does it maintain a flow of species through the pores of the polymeric support. Only the external surface of the attached thin layer of extractant is occasionally and delicately covered with a thin aqueous layer. Judicious choice of the microporous polymeric support with affinity to the extractant

2. THIN LAYER EXTRACTION Thin layer extraction (TLX) is an intensive, reactive liquid− liquid extraction method10−12 that implements a time-periodic extraction/stripping cycle. TLX uses an open, solid macroporous matrix made of a microporous material. A thin layer of the organic liquid extractant coats permanently the surface of the microporous material. Two aqueous feeds, one a donor and the other a recipient strip liquid, alternate at being brought in contact, as thin layers, with the thin layer of extractant in a frequent periodic cycle. The species of interest in the donor liquid react with the host in the extractant and are then released later in the cycle into the strip liquid at appropriate conditions (composition, pH, and/or temperature) that favor such release. The thin layers being contacted make for a negligible mass transfer resistance: with a molecular diffusivity in the order of 10−9 m2/s, the characteristic time for diffusion through a 20 μ thick layer is of the order of 0.4 s permitting rapid equilibration of the phases, thus decoupling mass transfer from slower reactions. Every step in the extraction/stripping cycle occurs instantly and is timed independently. Figure 2a and b stresses differences in structure between (a) a typical continuous equilibrium liquid−liquid extraction plant and that of (b) a TLX plant. The typical conventional plant consists of three batteries or sections, (a) extraction, (b) washing, and (c) stripping. Each of the three sections consists of a train of stages. Each stage is fed countercurrently with two input streams, one aqueous and the B



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Scheme 1. Process Interactions

solvent has been found to provide the necessary stability of the coated TLX matrix over extended periods of time.

neutral species transfer from the organic phase to the aqueous phase. (d) While equilibrated with the aqueous batch, the reactions

3. THE MATHEMATICAL MODEL As is often the case, in particular with carboxylic acids, the reversible complexation reaction between the species and the host may involve more than one species molecule for every host molecule, designated as (n, 1) complexes.13 For the sake of simplicity, we shall only consider the cases with n = 1 that generally applies to dilute feeds and with n = 2 that represents more concentrated feeds. Considering the mostly ionic nature of the species of interest, Scheme 1 depicts the essential interactions in a stage over a cycle of operation. The species P and L dissociate in the aqueous phases with only the neutral (undissociated) form distributing between the aqueous and the organic phases following a distribution coefficient m, while in the organic phase, they react at different rates kfP, kfL, with the host C to form complexes PC and LC, while at the same time the complexes decompose at rates kbP, kbL. The extraction and the stripping aqueous phases may be maintained at different pH or temperature, but this is not a necessary condition. The process cycles through: (a) A batch Fe of aqueous feed at composition [Aie]aqf and pHe is spread and equilibrated with the organic layer. The neutral species transfer to the organic phase. (b) While equilibrated with the aqueous batch, the reactions

k fi

[Aiw] + [C] ⇌ [AiC], i = P,L occur in the organic layer for a k bi

prescribed time tw at the end of which the aqueous batch at composition [Aiw]aq(te + tw) is collected as a strip product. A mass balance on the organic phase, at conditions causing a (1, 1) complexation stoichiometry, leads to the mathematical model 1−6, with subscripts i indicating the species P and L and subscripts j = e, w representing, respectively, the extraction and stripping steps. For a (1:1) complexation reaction: k fi

[A ij]org + [C] ⇌ [A ijC], i = P, L, j = e, w k bi

⎛ π ⎞ d[A ie]org E⎜1 + e ⎟ = − k fi[A ie]org [C] + k bi[A iC], Die ⎠ dt ⎝ [A ie]org (0) =

⎫ mie ⎧ Fe ⎨ [A ie]aqf,allforms + E[A iw ](te + tw)org ⎬ Fe + Emie ⎩ πe ⎭

(1)

d[APe]org d[ALe]org d[C] = + , [C](0) = [C](te + tw ) dt dt dt

k fi

[Aie] + [C] ⇌ [AiC], i = P,L occur in the organic layer for a

(2)

k bi

prescribed time te at the end of which the aqueous batch at composition [Aie]aq(te) is removed and collected as a raffinate. (c) A batch Fw of aqueous strip at composition [Aiw]aqf and pHw is spread and equilibrated with the organic layer. The

d[A ieC]org dt

= k fi[A ie]org [C] − k bi[A iC],

[A ieC]org (0) = [A iw C]org (te + tw) C

(3)



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⎛ π ⎞ d[A iw ]org = − k fi[A iw ]org [C] + k bi[A iw C], E⎜1 + w ⎟ Diw ⎠ dt ⎝ [A iw ]org (0) =

⎧ Fw ⎫ miw ⎨ [A iw ]aqf,allforms + E[A ie]org (te)⎬ Fw + Emiw ⎩ πw ⎭

d[APw ]org d[ALw ]org d[C] = + , [C](0) = [C](te) dt dt dt

⎛ π ⎞ d[A iw ]org 2 = − k fi[A iw ]org E⎜1 + w ⎟ [C] + k bi[(A iw )2 C], Diw ⎠ dt ⎝ [A iw ]org (0) =

(4)

⎧ Fw ⎫ miw ⎨ [A iw ]aqf,allforms + E[A ie]org (te)⎬ Fw + Emiw ⎩ πw ⎭

(4a)

⎛ d[APw ]org d[ALw ]org ⎞ d[C] ⎟⎟ , [C](0) = [C](te) = 0.5⎜⎜ + dt dt dt ⎝ ⎠

(5)

(5a)

d[A iw C]org dt

d[(A iw )2 C]org

= k fi[A iw ]org [C] − k bi[A iw C],

dt

[A iw C]org (0) = [A ieC]org (te) [A ik ]aq,allforms =

[A ie(t )]aq =

[(A iw )2 C]org (0) = [(A ie)2 C]org (te)

(6)

πk [A ik ]org , k = e, w, m ik

πe = (1 + 10(pHe − pKa)), πw = (1 + 10(pH w − pKa))

[A ie](t ) [A iw ](t ) , [A iw ]aq = me mw

2 = k fi[A iw ]org [C] − k bi[(A iw )2 C],

(6a)

4. TRAJECTORIES Equations 1−6 and similarly eqs 1a−6a define closed cycle trajectories for the compositions in the organic phase of each of the two species [P], [L], the host [C], and the reaction products [PC] and [LC]. Figure 3 depicts the path followed by the

(7)

(8)

The neutral, undissociated feed compositions are [A ie]aqf =

[A ie]aq,allforms (1 + 10(pHe − pKa))

, [A iw ]aqf =

[A iw ]aq,allforms (1 + 10(pHw − pKa))

(9)

All variables refer to the organic phase except where specifically subscripted by aq. Die and Diw are the extraction and stripping factors: Die =

Em ie Em iw , Diw = Fe Fw

(10)

te and tw are, respectively, the extraction and stripping reaction time spans. The initial host concentration must be sufficient such as to never be depleted or, for a (1, 1) stoichiometry: [C(0)] = [AP]org (0) + [AL]org (0) + ε , ε ≥ 0

Figure 3. Composition of the organic phase over a KRTLX cycle.

compositions in the organic phase over one complete cycle of operation in a single stage KRTLX. The slopes of the path are defined by spans that may be reduced to a single frequency parameter (1/T) if we opt, for the sake of simplicity, to use symmetrical timing, that is, te = tw = T. One may then express four dimensionless rate parameters kfiT, kbiT, i = P, L that define the trajectories for given operating conditions and switching policy. A choice of switching policy T affects proportionally the four dimensionless rate parameters.

(11)

ε stands for the excess host. Note that the variables [C] and [AiC] are continuous in time, while [Ai] is piecewise continuous in time. When a (2, 1) stoichiometry applies, then k1i

2[A ij]org + [C] ⇌ [(A ij)2 C]

5. PROCESS PERFORMANCE METRICS The separation operation objectives are to convert as much as possible of both species present in the feed into distinct pure products. Product purity may be defined in terms of the excess concentration of one species over the other:

k 2i

and eqs 1−6 are replaced by: ⎛ π ⎞ d[A ie]org 2 E⎜1 + e ⎟ [C] + k bi[(A i)2 C], = − k fi[A ie]org Die ⎠ dt ⎝ [A ie]org (0) =

⎫ mie ⎧ Fe ⎨ [A ie]aqf,allforms + E[A iw ]org (te + tw)⎬ Fe + Emie ⎩ πe ⎭

se ie

[A slow,e(T )]aq − [A fast,e(T )aq ] [A slow,e(T )]aq + [A fast,e(T )aq ]

(1a) ⎛ d[APe]org d[ALe]org ⎞ d[C] ⎟⎟, [C(0)] = [C](te + tw) = 0.5⎜⎜ + dt dt ⎠ ⎝ dt

d[(A ie)2 C]org dt

se iw = (2a)

[A fast,w (2T )]aq − [A slow,w (2T )]aq [A fast,w (2T )]aq + [A slow,w (2T )]aq

(12)

The yield is defined as the fraction of a species introduced in the feed that turns out in the respective products.

2 = k fi[A ie]org [C] − k bi[(A ie)2 C],

[(A ie)2 C(0)]org = [(A iw )2 C]org (te + tw)

;

ηie =

(3a) D

[A ie(T )]aq,allforms [A ie]aqf,allforms

, ηiw =

[A iw (2T )]aq,allforms [A ie]aqf,allforms

(13)



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In equilibrium reactive extraction, the extent of separation of two substances A and B relates to the separation factor: αAB =

1 + KA[C] 1 + KB[C]

(14)

For highly stable complexation, the separation factor αAB equals approximately the intrinsic selectivity αint = KA/KB, where KA and KB are the complexation constants of the respective substances at equilibrium: kf A

KA =

kb A

, KB =

kf B kb B

(15) Figure 4. Product compositions for the conventional scheme with Fe = 1 L/s, Fs = Fx = 1.5 L/s, Fp = 1 L/s, FR = 0.5 L/s.

6. EXAMPLE A comparison of the simulated separation of a mixture of two carboxylic acids in a KRTLX plant to that in a comparable conventional continuous equilibrium plant as depicted in Figure 2 is brought in the following. Table 1 summarizes the data used in the simulation as detailed in Marty14,15 and Wasewar.17 The extractant consists of 0.4 M trioctylamine (TOA) in 1 octanol at 25 °C. Table 1. Data and Operating Conditions data

pyruvic

lactic

units

kfi kbi Ki = kfi/kbi pKa me = mw pHe = pHw feed composition

0.94 0.0012 768 2.49 0.3 4 0.2

24 0.32 74 3.86 0.32 4 0.2

L/mol s 1/s L/mol

Figure 5. Performance of the conventional scheme with Fe = 1 L/s, Fs = Fx = 1.5 L/s, Fp = 1 L/s, FR = 0.5 L/s.

Lorg/Laq

Figures 6 and 7 depict the compositions and performance of a conventional scheme at a more substantial reflux rate. While

mol/Laq

For simplicity, the same number of stages in each of the three batteries in Figure 2a is assumed, and KRTLX uses symmetric timing.

7. DISCUSSION The comparison of processes that operate differently requires that a common base be established for the comparison. First, we note that while KRTLX is a repeated batch process of short duration, it can also be evaluated as a quasi-continuous process by averaging the batches volumes over a cycle. Each batch of feed is processed within seconds. tcycle = te + tw + t oh (16)

Figure 6. Product compositions for the conventional scheme with Fe = 1 L/s, Fs = Fx = 7 L/s, Fp = 1 L/s, FR = 6 L/s.

Thus, the average continuous feed flow rates are F ′e =

Fe tcycle

; F ′w =

Fw tcycle

somewhat more separation is observed, it can hardly be characterized as satisfactory for the full separation of the components from each other. Figures 8 and 9 depict the compositions and performance of a KRTLX plant, for the same data and operating conditions of Figures 4 and 5. Here, we observe substantial separation of the two acids, obtaining, for 10 KRTLX stages, a strip product consisting almost exclusively of lactic acid and a raffinate consisting almost exclusively of pyruvic acid. 7.2. Effect of Feed Composition. What if we would feed the KRTLX plant with a more concentrated mixture leading to a (2, 1) stoichiometry? Replacing eqs 1−6 in the model with eqs 1a−6a results in the performance depicted in Figure 10. As

L/s (17)

7.1. Comparison of the Methods through the Example. Figures 4 and 5 depict the compositions and performance of a conventional scheme at a modest reflux rate. Clearly, the conventional equilibrium scheme using the given extractant at the conditions of Table 1 is incapable of separating the two acids to any significant degree as both acids are almost equally extracted into the raffinate and then rejected together in the strip product. The bottom line is that this configuration provides no separation between the two acids at the given operating conditions. E



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then wish to weigh operating a smaller plant, using fewer stages, at a lower feed concentration against a bigger plant, at a higher feed concentration. 7.3. Apparent Conditions Leading to Kinetic Separation. As indicated by eq 1, the relative change in the composition of one generic component [A] over the other [B] is given by −k fA[A]org [C] + k bA[AC] d[A] = d[B] −k fB[B]org [C] + k bB[BC]

(18)

While not proven to be a necessary condition, empirical indications are that kinetic separation between A and B will tend to be promoted when

Figure 7. Performance of the conventional scheme with Fe = 1 L/s, Fs = Fx = 7 L/s, Fp = 1 L/s, FR = 6 L/s.

⎡⎛ k ⎞ ⎛k ⎞⎤ k k ⎢⎜ fA > 1 AND bB > 1⎟OR⎜ fA < 1 AND bB < 1⎟⎥ ⎢⎣⎝ k fB k bA k bA ⎠ ⎝ k fB ⎠⎥⎦ ⎡⎛ ⎞ ⎛ k [C] k [C] > 105⎟AND⎜102 > fB AND⎢⎜102 > fA ⎢⎣⎝ k bA k bB ⎠ ⎝ ⎞⎤ > 105⎟⎥ ⎠⎥⎦

(19)

In simple words, separation will be promoted when the overall forward rate for one of the species is faster than that of the other and both complexes are stable but not too stable. In addition, it appears that highly stable complexes require high frequency switching. Kinetic reactive extraction possesses no less than three degrees of freedom in excess of those in equilibrium extraction (four reaction rates vs two equilibrium constants and one or two reaction time span vs none). Thus, there may exist systems where a small separation factor αAB (or even αAB = 1) is no impediment to being capable to separate a mixture kinetically. Indeed, consider the case where

Figure 8. Product compositions for the KRTLX scheme with Fe = 1 L/s, Fs = 1 L/s, Fp = 1 L/s, Fx = 1 L/s, T = 5 s.

KA =

k k fA = KB = fB k bA k bB

(20)

or equivalently: αAB = αINT = 1

(21)

Equation 21 indicates total absence of a driving force for equilibrium separation between A and B. Nevertheless, this does not exclude kinetic separation that only requires kfA ≠ kfB or kbA ≠ kbB. 7.4. Operation Policy Considerations. The effect of the chosen reaction time spans on performance is critical. Too long of a time span brings us closer to equilibrium, defeating the purpose of using kinetics. Too short of a span is counterproductive because it adds relative weight to the time overhead in each cycle of operation, thus affecting throughput. Within those limits, one may search for an optimum by repeated simulation using the model 1−11. This brings to the fore the importance of having kinetic data available before starting a KRTLX plant design. Here, we have used kinetic data published in literature, limiting ourselves to an extractant and operating conditions dictated by those used in the relevant publication. In the example, a symmetrical switching policy was used for the sake of convenience and clarity. 7.5. Economic Considerations. Typically, the TLX matrix supports 0.033 L extractant/L matrix. It is most effective when the organic and aqueous layers are of approximately equal thickness, indicating an organic to aqueous ratio of (O/W) = 1 L

Figure 9. Performance of the KRTLX scheme with Fe = 1 L/s, Fs = 1 L/s, Fp = 1 L/s, Fx = 1 L/s, T = 5 s.

Figure 10. KRTLX performance for a (2,1) stoichiometry. Conditions are identical to those in Figure 9.

expected, a high feed concentration, which is associated with a higher value of n in the complexation stoichiometry (n, 1), will come at the expense of a restricted extent of separation. One may F



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extractant/L aqueous phase. Thus, the size of a batch of aqueous feed or strip must also be of the order of 0.033 L/L matrix. The time overhead in a cycle of operation may reach 4−5 s, bringing the total cycle time to an order of 5−15 s, depending on the chosen reaction time spans. For T = 5 s as in our example, tcycle = 15 s, and one will need a 15/(0.033*60) = 7.5 L matrix to process one L/min feed. This translates into a virtual residence time in a stage of 7.5 min that is comparable to that in a typical mixer− settler stage. As each KTRLX stage plays the role of three mixer− settler stages in the conventional scheme, and is also considerably more efficient as demonstrated in the example, the physical size of a KRTLX plant may be equivalent to, or smaller than, that of a comparable mixer−settler conventional equilibrium plant. The KRTLX equipment is relatively light and also uses little power, with impact on infrastructure and on operation costs.

NOMENCLATURE

Variables

[A] = concentration of generic species A in the organic phase (unless subscripted otherwise), mol/L [A]aq,j = concentration of generic species A (A = P or L) in the respective j aqueous, mol/L [C] = concentration of the host in the organic phase, mol/L D = extraction/stripping factor, defined in eq 10, dimensionless E = extractant amount in a stage, L F = aqueous batch size, L kfi = forward complexation reaction rate of species i, L/mol s for (1,1) complexation (L2/mol2 for (2,1)) kbi = backward complexation reaction rate of species i, 1/s K = complex formation equilibrium constant, dimensionless Ka = dissociation constant, dimensionless m = species distribution between the aqueous and the organic phases, (mol/L)org/(mol/L)aq (O/W) = organic to aqueous ratio, Lorg/Laq se = species excess, defined in eq 12, dimensionless te = extraction reaction time span, s tw = stripping reaction time span, s T = te = tw = standardized reaction time span when using a symmetric switching policy

8. CONCLUSION When the separation of two closely related compounds is necessary, conventional wisdom indicates that a selective host be found that would discern between the two by associating to different equilibrium compositions and is sensitive to local conditions. Kinetic reactive thin layer extraction offers an alternative route that does not rely on finding such a selective host, provided the rates of the chemical reactions between each of the two components and the host are sufficiently differentiated. However, as evident from the example, equilibrium selectivity by itself is no guarantee to full distinct separation of the components in one pass through a conventional reactive extraction scheme including several stages in each of its separation trains. Thus, when interested in both components in pure form, one may need to resort, in the case of a conventional equilibrium plant, to a more elaborate processing scheme. In our example, KRTLX provides close to full separation of the two components into two distinct product streams in one pass through a simple TLX plant configuration, consisting of fewer stages than would be necessary in the conventional equilibrium approach. The benefits of the proposed method include a wider choice of extractants, a relatively small plant, and added operational flexibility through the choice of the reaction time span that constitutes one of the operational parameters. The success of a KRTLX application is conditional on a suitable difference in rates of reaction between components to be separated and the extractant used and on the availability of an extraction tool that permits control of the time span of the reactions, such as is inherent in the TLX extractor. Having established through an example the potential of the method, further exploration of the field is justified that would exploit and optimize the full range of available parameters. Dilute feeds that lead to a (1, 1) complexation stoichiometry are easier to separate and may justify the cost of an additional product concentration step.

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Greek Symbols

αINT = intrinsic selectivity, defined in eq 15, dimensionless αAB = separation factor, defined in eq 14, dimensionless ε = host excess, defined in eq 11, mol/L η = yield, defined in eq 13, dimensionless π = dissociation ratio, defined in eq 7, dimensionless

Subscripts

f = feed i = species (P or L) j = step − extraction (e) or stripping (w) Substances

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L = lactic acid P = pyruvic acid

REFERENCES

(1) Martin, V. S.; Scott, S.; Woodard, S. S.; Katsuki, T.; Yamada, Y.; Ikeda, M.; Sharpless, K. B. Kinetic Resolution of Racemic Allylic Alcohols by Enantioselective Epoxidation. A Route to Substances of Absolute Enantiomeric Purity? J. Am. Chem. Soc. 1981, 103, 6237. (2) Ward, R. S. Dynamic Kinetic Resolution. Tetrahedron: Asymmetry 1995, 6, 1475. (3) Krasnov, V. P.; Gruzdev, D. A.; Levit, G. L. Nonenzymatic Acylative Kinetic Resolution of Racemic Amines and Related Compounds. Eur. J. Org. Chem. 2012, 1471. (4) Hallett, A. J.; Kwant, G. J.; de Vries, J. G. Continuous Separation of Racemic 3,5-Dinitrobenzoyl-Amino Acids in a Centrifugal Contact Separator with the Aid of Cinchona-Based Chiral Host Compounds. Chem.Eur. J. 2009, 15, 2111. (5) Lavie, R. Kinetic Chiral Resolution by Thin Layer Extraction. Chem. Eng. Trans. 2013, 32, 1945. (6) Wasewar, K. L.; Yawalkar, A. A.; Jacob, A.; Moulijn, J. A.; Pangarkar, V. G. Fermentation of Glucose to Lactic Acid Coupled with Reactive Extraction: A Review. Ind. Eng. Chem. Res. 2004, 43, 5969. (7) Baniel, A. M.; Blumberg, R.; Hajdu, K. Recovery of Acids from Aqueous Solutions. U.S. Patent 4,275,234, 1981. (8) Kertes A. S.; King C. J. Extraction Chemistry of Fermentation Product Carboxylic Acids. Biotechnol. Bioeng. 1986, Vol. XXVIII, 269− 282. (9) López-Garzón, C. S.; Straathof, A. J. J. Recovery of Carboxylic Acids Produced by Fermentation. Biotechnol. Adv. 2014, 32, 873.

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The authors declare the following competing financial interest(s): I have interests in patents related to Thin Layer Extraction. G



Article

(10) Lavie, R. Thin Layer Extraction - a Novel Liquid-Liquid Extraction Method. AIChE J. 2008, 54, 957. (11) Lavie, R. Chiral Resolution of Ionic Compounds by Thin Layer Extraction. Ind. Eng. Chem. Res. 2011, 50, 12311. (12) Lavie, R. Method and Apparatus for Liquid-liquid Extraction. U.S. Patent 8,021,554, 2011. (13) Tamada, J. A.; Kertes, A. S.; King, C. J. Extraction of Carboxylic Acids with Amine Extractants. 1. Equilibria and Law of Mass Action Modeling. Ind. Eng. Chem. Res. 1990, 29, 1319. (14) Marti, M. E. Reactive Extraction of Pyruvic Acid From Aqueous Single And Mixed Acid Solutions. Ph.D. Thesis, Middle East Technical University, Turkey, 2010. (15) Marti, M. E.; Gurkan, T.; Doraiswamy, L. K. Equilibrium and Kinetic Studies on Reactive Extraction of Pyruvic Acid with Trioctylamine in 1-Octanol. Ind. Eng. Chem. Res. 2011, 50, 13518. (16) Ramachandran, P. A.; Sharma, M. M. Simultaneous Absorption of Two Gases. Trans. Inst. Chem. Eng. 1971, 49, 253. (17) Wasewar, K. L.; Pangarkar, V. G.; Heesink, A. B. M.; Versteeg, G. F. Intensification of Enzymatic Conversion of Glucose to Lactic Acid by Reactive Extraction. Chem. Eng. Sci. 2003, 58, 3385. (18) Kemperman, A. J. B.; Bargeman, D.; Van Den Boomgaard, Th.; Strathmann, H. Stability of Supported Liquid Membranes: State of the Art. Sep. Sci. Technol. 1997, 31, 2733.

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Kinetic Reactive Thin Layer Extraction - Industrial & Engineering

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