SEPARATION OF MIXTURES BY PERTRACTION OR ... - CiteSeerX

MEMBRANY TEORIA I PRAKTYKA ZESZYT III

WYKŁADY MONOGRAFICZNE I SPECJALISTYCZNE TORUŃ 2009

SEPARATION OF MIXTURES BY PERTRACTION OR MEMBRANE–BASED SOLVENT EXTRACTION AND NEW EXTRACTANTS

Štefan SCHLOSSER, Ján MARTÁK Institute of Chemical and Environmental Engineering, Slovak University of Technology, Radlinského 9, 812 37 Bratislava, Slovakia; e-mail: [email protected]

1. INTRODUCTION Extractive methods of separation in two or three phase systems are widely studied for the separation of liquid solutions. Classical solvent extraction is based on partitioning of components between two immiscible or partially miscible phases and is widely used in numerous separations of industrial interest. It is mostly realized in systems with dispergation of one phase into the second phase. Dispergation could be the source of problems in many systems of interest, such as the entrainment of an organic solvent into an aqueous raffinate, the formation of stable and difficult to separate emulsions, etc. To solve these problems, new ways of liquids contacting have been developed. The idea of performing separations in three-phase systems with a liquid membrane is relatively new. The first papers on supported liquid membranes (SLM) appeared in 1967 [1−2] and the first patent on the emulsion liquid membrane was issued in 1968 [3]. If two miscible fluids are separated by a liquid which is immiscible with them but enables mass transport between the fluids, a liquid membrane (LM) is formed. The liquid membrane enables the transport of components between two fluids at different rates and in this way to perform separation. When two phases separated by the membrane are liquid, the process is called pertraction (PT). In most processes with the use of a liquid membrane, the contact of phases occurs without the dispergation of the phases. The methods of separation in two phase systems with one immobilized interface(s) are much newer. The first paper on membrane based solvent extraction (MBSE) was published by Kim [4] in 1984. However, only the inventing new methods of contacting two or three liquid

Š. Schlosser, J. Marták

phases as well as developing new types of liquid membranes have led to a significant progress within the last forty years (section 3). Separations in systems with immobilized interfaces have found employment in industry [5]. Several mechanisms of achieving the transport of solutes through the L/L interface or through a liquid membrane can be exploited. The separation mechanism can be based on differences in the physical solubility of the solutes or their solubilisation into the solvent or reverse micelles. It can as well be based on the chemistry and rate of chemical or biochemical reactions occurring on L/L interface(s). The solubilisation or complexing agent – extractant (carrier in the liquid membrane) forms soluble compounds or aggregates by reversible reaction with the solute. The chemistry of reactive extraction and stripping in MBSE and MBSS as well as in PT is identical with the classical solvent extraction or stripping which is presented in several books and papers, e.g. [6−10]. The formation of hybrid production/biotransformation – separation processes, including extractive processes, could enhance production of target species and is of great interest as shown in review papers [5, 9, 11−16]. A flowsheet of the extractive fermentation unit with an integrated MBSE and MBSS circuit for the recovery of acid(s) produced from the fermentation broth by pertraction is discussed in section 5.2. Te development of new extractants and the formulation of solvents by selecting diluents, and when needed also a modifier, is important in designing new separations. This problem is discussed in more detail in paper [15]. New extractants based on ionic liquids developed recently can markedly influence new extractive separations [17–21]. The aim of this paper is to give a short overview of recent developments in extractive separations in contactors with one and two immobilized L/L interfaces and their perspectives. Selected industrial applications and pilot plant tests will be briefly discussed. New extractants/carriers based on ionic liquids (ILs) will be presented and specific mechanism of extraction and pertraction by ILs will be shown. 2.

LIQUID MEMBRANE TYPES AND THREE PHASE CONTACTORS

Liquid membranes can be classified into three groups, according to the way in which they are formed, as bulk (thick layer), supported, and surfactant membranes schematically shown in Fig.1. 2.1. Bulk liquid membranes (BLM) There are five basic types of BLM with different arrangement of three phase system: layered BLM, BLM in film contactors, BLM in a double dispersion system, BLM with an emulsion and one interface immobilised in a 124

Separation of mixtures…

microporous film, and BLM with two interfaces immobilised in the microporous walls [9]. R F R F

M

M

F

e)

M M

R R

a)

M

F

R

R F R F

F

c) b) Bulk liquid membranes (BLM)

f)

HF

R

d)

g)

Supported liquid membranes (SLM)

h) Emulsion liquid membrane (ELM) Fig. 1. Types of liquid membranes (LM) [9]; a − layered BLM, b − BLM in a rotating disc contactor, c − BLM in a creeping film contactor, d − BLM with immobilised interfaces in the hollow fiber, e − supported LM, f − LM supported in microporous walls of the hollow fiber contactor, g − LM supported between two nonporous films, F − feed (source solution), HF − hollow microporous fiber, M − membrane phase, NF − nonporous film R − stripping solution, S − microporous wall

Advanced film pertractors with creeping [22] (Fig. 1c) and rotating [23−25] (Fig. 1b) aqueous films were invented. There is a continuous interest in rotating discs (RD) contactors [26−31]. In e most cases, the feed (F) and the stripping solution (R) discs are alternately positioned on horizontal shaft (Fig. 1b). RD contactors with two parallel shafts one with the feed discs and the other with stripping solution discs were also tested 125


[26, 28]. The surfaces of the discs should be hydrophilic to ensure the formation of adhering aqueous films. Raszkowska and Wódzki suggest using discs made of cation-exchange polymer which was found advantageous in the pertraction of zinc [31]. More advanced are BLMs with two immobilised interfaces in hollow fibers (HF), Fig. 1d [32−33] and spiral-wound modules [34]. The HF pertractors with the flowing head were designed to eliminate some problems with the elongation of HF and related stress in the module and its channelling [9, 35−37], see Fig. 2.

a)

b)

Section AA’

M

Section BB’

(

)

Fig. 2. HF contactor of three phases with cross flow of one phase (a) and horizontal cross section of the module (b) [9, 36]: 1 − body of the element, 2, 3 − hollow fiber in downstream and upstream part, 4, 5 − inlet and outlet chamber, 6, 7 − inlet and outlet tube, 8 − flowing head, 9 − central baffle, F − feed, M − membrane phase, S − stripping solution

Much attention has recently been paid to three phase systems with one immobilized interface F/M and emulsion of the stripping solution in the membrane phase, i.e., bulk liquid membrane with emulsion (BLME). In the pertraction through BLME (Fig. 3), dispersion or emulsion of the stripping solution in the bulk of liquid membrane is used. In this way, comparing to classical BLM (Fig. 1d), one microporous wall is avoided, however, advantage of nondispersive process is lost. Mass-transfer resistance is lower in BLME than in BLM system. The first papers on the removal of copper from water by pertraction through BLME into the stripping solution emulsion were published by Schneider et al. [38] and Schultz [39]. Review on pertraction through BLME is included in the papers [40−42]. PT through BLME is advantageous as compared to PT through BLM or combination of MBSE with MBSS [43−44]. 126


Fig. 3. Three phase system with one immobilized interfaces F/M and emulsion of the stripping solution in membrane phase – bulk liquid membrane with emulsion (BLME)

BLMs are relatively thick and their thickness ranges from 10-4 to 10-2 m. The membrane phase outside fiber walls is usually mixed, e.g. by convective flow or pulsations in order to decrease their resistance. The overall mass-transfer resistance through BLM is more than doubled when compared to SLM due to two walls with a stagnant LM phase in pores [45]. On other hand, contrary to SLM, there is no problem with stable performance of BLM. Three phase HF contactors with BLM are not produced industrially. These contactors could be effectively replaced by two phase contactors operating with closed solvent circuit as shown in Fig. 5 in section 3. 2.2. Supported liquid membranes (SLM) The liquid membrane can be supported by soaking the membrane phase (solvent) into the pores of a microporous or lyogel film [1] (Fig. 1e, f) or immobilising the solvent between two nonporous films which are permeable and usually nonselective [2] (Fig. 1g). The latter type of SLM is less suitable due to greater mass-transfer resistance of nonporous layers. In polymer inclusion membranes (PIM), LM phase is usually introduced into the polymer during the casting process of the membrane [46]. It can be said that the polymer is plasticized by LM phase as it was done in early SLMs [1]. It is difficult to share optimism on better stability of PIMs in comparison to SLMs supported in the porous structure of the support film as it was expressed in the overview paper [46]. Performances of the both types of SLM including lifetime are similar. The main issue for all types of SLMs hindering their application is short lifetime of these membranes. It is believed that regeneration of SLMs soaked in pores is much easier than that of PIMs. Several techniques of SLMs regeneration were developed [47−49]. A continuous SLM regeneration can be carried out with a rising film of membrane phase creeping on the support wall [48]. A stable operation within 350 hours was 127


achieved. An improved SLM stability towards acids and the protection of microorganisms against toxic effects of the carrier were achieved, e.g. by covering SLM with thin epoxy layers in the case of pertraction of acetic acid [50] or by thin polyamide skins in the pertraction of chromium [51]. Despite all these developments, SLMs have not found any industrial application so far. A stable continuous operation of SLM based on phosphonium ionic liquid in the pertraction of lactic acid was observed during 5.3 days, which is a promising result [21]. Further long-term investigations should prove stability of SLM. 2.3. Emulsion liquid membranes One of the widely studied LM types is emulsion liquid membrane (ELM) invented by Li [3]. ELMs are stabilised by an adsorption film of a surfactant at the interface, as it is shown in Fig. 1h. This surfactant film introduces important mass-transfer resistance as shown in paper [52]. Water in oil W/O emulsions were used for the separation of aqueous solutions [25, 53−56]. The continuous organic phase of the emulsion acts as a LM. The typical size of droplets in the emulsion is 1−10 µm and the size of emulsion globules in the feed is about 0.5 to 3 mm. The complexity of the technological circuit of the process with ELM (formation of stable emulsion, emulsion contacting with the feed, loaded emulsion splitting, recirculation of the membrane phase to the process) causes a decrease in interest in this process [9, 25]. 3. MEMBRANE BASED SOLVENT EXTRACTION (MBSE) AND TWO PHASE CONTACTORS Membrane based solvent extraction (MBSE) is a relatively new alternative to a classical solvent extraction where mass-transfer between two immiscible liquids occurs from the L/L interface immobilized at the mouth of pores of a microporous wall, which is not wetted by one of the phases in contact as shown in Fig. 4. The main aim of this approach is to avoid dispergation of the liquid phase which in many systems is connected with emulsion formation problems and with the entrainment of the solvent droplets and its loss. Microfiltration or ultrafiltration membranes (from which the term for process was derived) are usually used as a microporous support immobilizing the phase interface However, MBSE is not a membrane process; the support wall “membrane” has no active function in separation and only immobilizes the phase interface. Basic information on MBSE is given in papers [5, 12, 15, 45, 57−58].

128


Fig. 4. Detail view of the two phase system in membrane based solvent extraction (MBSE) in contactor with hydrophobic wall

The solvent can be regenerated by membrane based solvent stripping (MBSS) where the solute is re-extracted into the stripping solution. Another method of regeneration could be distillation of a volatile solvent or solute, etc., depending on the properties of a system. A schematic flow sheet of the simultaneous MBSE and MBSS processes with the closed loop of the solvent is shown in Fig. 5. In this way the recovery of the solvent and the concentration of the solute can be achieved. Preferable contactors for MBSE and MBSS are hollow fiber contactors.

Fig. 5. Flowsheet of MBSE with simultaneous regeneration of the solvent by MBSS in HF contactors and recirculation of the solvent to extraction. In both contactors the solvent flows in the contactor shell

The functions of contactors in the simultaneous MBSE and MBSS processes, arranged as shown in Fig. 5, are coupled. They behave similarly as a pertractor with a SLM. The differences are only in the overall resistance which is smaller in the pertractor where, comparing to system in Fig. 5, only one support wall is involved instead of two walls. In addition, it is not necessary to pump the solvent in PT, as it is in the circulation loop in the 129


simultaneous MBSE and MBSS processes [15]. On other hand, in pertraction through SLM, its limited life time could be a problem which is not a case in MBSE and MBSS where it is easy to maintain the constant properties of the solvent phase. Another way for immobilization of the L/L interface is the extraction into capsules with a solvent, e.g. in the recovery of phenylethanol (a product of phenylalanine bioconversion by yeast) [59] or lactic acid from fermentation broth [60]. The polymeric core of the capsule disables direct contact of the solvent with biomass. An alternative approach is to impregnate the solvent into microporous particles and use them for solutes extraction, e.g. metals [61−63] and organic acids [64−65]. These processes could be regarded as a batch MBSE. Contactors with flat sheet and cylindrical walls are used in MBSE or MBSS but only hollow fiber (HF) contactors in cylindrical modules in several sizes are available commercially [66]. Flat sheet contactors are widely used in analytical chemistry [67−68]. There are two main types of HF contactors, those with parallel flow of phases in fibre lumen and in shell or cross flow of phases. A HF contactor with cross flow of phases is shown in Fig. 6. Fluid 2

Fluid 2 Distribution tube

Cartridge

Hollow Fiber Membrane

Baffle

Housing Collection Tube

Fluid 1

Fluid 1

Fig. 6. Two phase hollow fiber contactor with cross flow of phases (Liqui Cel Extra-Flow)

A modular HF contactor containing planar elements with flowing head of fibres and cross flow of one phase, shown in Fig. 2, can be also used as a two phase contactor [36]. Reviews on two phase HF contactors are presented in papers [12, 58, 69]. Mass-transfer characteristics of two phase contactors are presented in paper [15]. 4. SOME REMARKS ON MASS-TRANSFER IN CONTACTORS AND THEIR MODELLING Kinetics of extraction and stripping reactions can play an important role in mass-transfer in pertraction, MBSE and MBSS with reactive extractants 130


(carriers). The concentration dependence of the distribution coefficient of solutes, e.g. organic acids, should be taken into account as well. This influences the value of the concentration driving force and may result in the concentration dependent overall mass-transfer coefficient, see e.g. Eq. (1) below. Models considering the constant mass-transfer coefficient in MBSE are presented e.g. in papers [57, 70−74]. Models considering variable distribution coefficients are presented in papers [75−76]. Models of MBSE and/or MBSS taking into account reaction kinetics of formation and decomposition of the extractant-acid complex(es) in extraction and stripping L/L interfaces are presented in papers [45, 77−80]. Reaction kinetics resistances were included in the model of pertraction in papers [21, 37, 81−82] and in MBSE and/or MBSS in papers [15, 79−80, 83]. A short-cut method for the design and simulation of two-phase HF contactors in MBSE and MBSS circuit with variable mass-transfer and distribution coefficients taking into account reaction kinetics in the stripping interface is presented in paper [80]. Juang [81] estimated that the mass-transfer resistances based on reaction kinetics participate in the overall resistance in pertraction of lactic acid through SLM by 30 to 80% depending on acid and carrier concentrations. The mass-transfer resistances of penicillin G, for the planar membrane with the solvent containing the secondary amine Amberlite LA2, in MBSE and MBSS was analyzed by Juang [83] who concluded that the resistance based on reaction kinetics can participate in the overall resistance in MBSE up to 38% and in MBSS up to 74% depending on the process parameters. Similar results in the same system but for pilot plant HF contactors were found by Lazarova [84]. The results of LA pertraction through SLM will be discussed in section 7.2. The mass-transfer characteristics of two phase HF contactors in several systems together with the values of the lumped rate constants of extraction and stripping reactions are presented in paper [15]. 4.1. Pertraction through SLM A three phase system in pertraction through SLM together with related concentration profile of a solute is presented in Fig. 7. For the overall masstransfer resistance in this system, using diffusion and reaction kinetics based on mass-transfer resistances in series approach, can be expressed by Eq. (1) [21]: Rp =

1 ε 1 1 1 ε = + + + + = RF + Rk, e + RM + Rk, s + RR . (1) K p kF re kM DF rs DF kR DF

131


Fig. 7. Scheme of the three-phase system in pertraction through SLM (a) with related concentration profile (b): F – feed, SLM – supported liquid membrane, R – stripping solution

In the case of NaOH excess in the stripping solution, the concentrations of the undissociated acid LAH in the stripping phase is almost zero. Thus, it follows that kR tends to large values and hence to zero resistance in the stripping boundary layer RR. Consequently, the last terms in Eq. (1) can be neglected. The kinetics of formation and decomposition of the permeant-extractant complex(es) via interfacial reactions can be in the first approximation described by the first order rate equations [37,45,79−80]

dne = re Ai,eε e cFS , dt dns = rs Ai,sε s cSR , dt

(2)

(3)

where cFS is the MPCA concentration in the feed phase close to the extraction interface and cSR is the acid concentration in the solvent close to the stripping interface. The rate constants re and rs are, in fact, lumped parameters reflecting the kinetics of interfacial reactions of the complex formation or decomposition, equilibrium and the kinetics of competitive adsorption or desorption of molecules of the complexes and the free extractant molecules on/from the interface. 4.2. Co-transport and competitive transport and salt effect Co-extraction of mineral acids and other components of the feed can influence the transport rate of the target acid and its purity. H2SO4 co-extraction and HCl competitive extraction during LA and MPCA pertraction was reported by Kubišová in papers [79,85−86], respectively. In MPCA pertraction from feeds with mixed salts and a constant ionic strength, the MPCA flux through BLM with the carrier (TOA) dropped by 132


one order of magnitude when the concentration of chlorides increased from 0 to 1 kmol m-3. The ratio of mineral acids fluxes to the MPCA flux increased from 1 to 4 [86]. Thus, in MBSE of MPCA with amines, it is important to avoid the presence of chlorides in the treated solution. Competitive uptakes of lactic acid and glucose were measured for the extractant Alamine 336 in various diluents [87]. The extent of water co-extraction depends strongly on the diluent used, and larger amounts of water co-extracted correspond to larger uptakes of glucose. Co-extraction during reactive extraction of phenylalanine with Aliquat 336 was described in paper [88]. Water transport or formation (in neutralisation reactions) should be taken into account in material balances of the system [21, 79, 82, 89−90]. It influences the concentration factor of the target solute achieved in separation. 5. APPLICATIONS OF CONTACTORS Some overviews of extractive separations studied in systems with two or one immobilized interface are in given in papers [5, 10, 91−92]. The separation of organic acids is reviewed in paper [15]. There have been only a few industrial applications of HF contactors up to now. One of the first applications was the removal of an aromatic compound from industrial waste water by the MBSE process with the use of HF cross flow contactor. The installation with a capacity of 15 m3 h-1 of water was put into operation in 1998 [93−94]. The recovery of phenol from the hydrocarbon fraction with phenol concentration of 2−4 wt.% by MBSS into an alkali solution has been applied industrially in Poland [95−96]. The capacity of the plant with two rigs in series, each with 8 cross flow HF contactors Liqui Cel 4×28´´ connected in parallel is about 650 kg h-1. Both the hydrocarbon raffinate with less than 0.02 wt.% of phenol and the phenolate concentrate (25−30 wt.%) are recycled. The removal of chromium from ground water by MBSE/MBSS process was evaluated in long term tests [97−98]. For ground water containing 774 g m-3 of Cr(VI), the process exhibited stable performance during 700 hours of operation, concentrating metal up to 20 kg m-3. An integrated process for the simultaneous removal of chromium from groundwater by MBSE followed by ion exchange is suggested in paper [98]. A laboratory and pilot plant study were performed using HF contactors with surface area of fibres of 1.4 and 19.3 m2, respectively. 5.1. Recovery of MPCA 5-Methyl-2-pyrazinecarboxylic acid (MPCA) is a valuable acid of industrial importance. The waste solution of MPCA resulting from downstream 133


processing of the enzymatic resolution technology contains a lot of mineral salts and their pH is below 2. MPCA has to be concentrated by a factor of 10, at least. From the concentrate, pure MPCA is recovered as an organic phase formed after decreasing its pH < 2 due to low water solubility of undissociated MPCA. Mass-transfer characteristics of HF contactors in MBSE and MBSS of MPCA are presented in paper [79]. The concentration dependence of the distribution coefficient of MPCA in the solvent with TOA has a favorable course, i.e. it exhibits an increasing value of D with decreasing acid concentration [89]. Compositions of streams in laboratory tests, and supposed also in a pilot unit for recovery of MPCA from the process solution, were as follows: the aqueous feed containing ∼0.12 kmol m-3 MPCA, 1 kmol m-3 Na2SO4; with pHF kept constant at 2.5 by the addition of H2SO4. The solvent was 0.4 kmol m-3 TOA in xylene. An aqueous solution of NaOH (0.5 kmol.m-3) with the addition of ammonia (10.1 kmol m-3) to keep its basicity above 0.3 kmol m-3 was used for stripping. The short-cut method published in paper [80] was used for the design and simulation of HF contactors in coupled MBSE and MBSS. The number of contactors Liqui Cel 4×28´´ (with an effective length of fibers of 0.6 m [66]) in series, as resulted from simulations, was found as 2 for both MBSE and MBSS, what is a reasonable number [15]. The technological flowsheet of a pilot plant unit for the recovery of MPCA is shown in Fig. 8. This can be a potential application of HF contactors in the recovery of MPCA from waste solutions. 5.2. Hybrid production−separation systems There are several modes of operation of the separation process to achieve the recovery or separation of the target solute: (a) a simple process for the separation of the process mixture is done in one step, (b) an integrated process in which the separation of the process mixture is accomplished in series of independent separation processes as it is frequent in downstream processing in biotechnologies,. (c) hybrid systems where the production process via chemical or biochemical reaction(s) and the separation, or two separation processes, are carried out simultaneously in one equipment or in parallel equipments connected by a circulation loop which allows optimisation of their functioning. An example of a hybrid fermentation – separation process is shown in Fig. 9. Other examples of a hybrid system are coupled MBSE and MBSS processes shown in Figs. 5 and 8.

134


Fig. 8. Scheme of a continuous pilot plant unit for recovery of MPCA from mother liquor (ML) by MBSE and MBSS in hollow fiber contactors [15] 1 − container of filtered ML 11 − container of NaOH 2 − polishing (safety) filters of the feed 12 − container of NH4OH 3a, 3b − HF contactors for MBSE C1 to C3 − main pumps 4 − static mixer C4, C5 − dosing pumps of solutions for pH adjustment 5 − container of H2SO4 CU − central unit for data acquisition 6a, 6b − HF contactors for MBSS of the and safety switching off pumps solvent pH1, pH2, pH3 − pH sensors 7 − container of the regenerated solvent 8 − polishing (safety) filters of the solvent R1, R2 − flowmeters (rotameters) V1 to V5 − valves 9 − container of the stripping solution (concentrate of MPCA) VP1, VP2 − relief valves 10 − safety filters of the stripping solution VR1, VR2 − metering valves for ∆p adjustment

135


pH adjustment

14

Substrate solution

13

1

Fermentation broth bleed

12

C3

11

4

Nitrogen

5

10 pH adjustment

3

C1

Fresh stripping solution

6

8

PT B

2

9

C2

7

Loaded stripping solution

Fig. 9. Schematic flow sheet of a hybrid fermentation–separation process for production of butyric acid with immobilized biomass [16]; C1 – fermenter container, B – bioreactor with supported biomass, C2 – container of broth for pH adjustment, PT – hollow fiber pertractor with SLM, C3 – container of lean broth with pH adjustment, 1 – concentrated substrate solution, 2 – input into the bioreactor, 3 – bioreactor output, 4 – recycled broth, 5 – broth entering the pertraction loop, 6 – acid for pH adjustment, 7 – feed into the pertractor, 8 – fresh stripping solution, 9 – loaded stripping solution, 10, 12 – lean fermentation solution, 11 – fermentation broth bleed, 13 – recycled lean fermentation solution with adjusted pH, 14 – ammonia solution for pH adjustment

Possibly, the first industrial application of HF contactors was a hybrid system for production of the intermediate of the dilthiazem drug reported by Lopez and Matson [99]. An enzymatic resolution of the dilthiazem chiral intermediate is achieved in an extractive enzymatic membrane reactor. The enzyme is entrapped in the macroporous sponge part of the hydrophilic hollow fiber membrane made of a polyacrylonitrile copolymer. The enzyme is loaded to the membrane during ultrafiltration of an aqueous enzyme solution flowing at the beginning in the shell of a HF contactor. A dense layer of the membrane (skin), retaining enzyme, is placed on the inner side of the hollow fiber. After immobilisation of the enzyme in the wall, a toluene solution of reactant, racemic (±)-trans-methyl-methoxyphenyl-glycidate, is flowing in the shell. In the fiber lumen, an aqueous buffer solution with bisulfite anions flows countercurrently. The enzymatic deesterification catalysed by lipase proceeds on the L/L interface. The deliberated (2S,3R)-methoxyphenylglycidic acid is extracted to the buffer. The required product (2R,3S)-methyl-methoxyphenylglycidate remaining in toluene is the intermediate for dilthiazem synthesis. There are installed 24 contactors, with a surface area of 60 m2 each, in the commercial plant with a capacity of 75 tons of the drug per year. 136


The Juelich team has developed a fermentation-extraction process for the production of phenylalanine (Phe) with integrated MBSE in HF contactors [100−102]. The reactive solvent with 10 v/v% of D2EHPA in kerosene was used. The process was started in laboratory experiments, through separations of 42 dm3 batches, to the fully integrated pilot plant with 300 dm3 fermenter working in the fed-batch mode of operation. Two HF contactors with a cross flow of phases and fibres surface area of 18.6 m2 were used in MBSE and MBSS working in parallel, as shown by the scheme in Fig. 5. Blahušiak et al. [16] analyzed a hybrid process for the production of butyric acid (BA) shown in Fig. 9 and found that pH of fermentation and pertraction should be optimized independently. It is advantageous to have pH of the feed into pertraction at about 4.0 for both for Cyphos IL-104 and TOA carriers. In the case of continuous operation mode, the decrease of pH in the inlet to the pertraction from 5.5 to 4 enables the reduction of the membrane area by 52% for IL and by 81% for TOA. At BA concentrations below ∼40 kg m-3, IL is a better extractant when compared to TOA. 6. COMPARISON OF EXTRACTIVE PROCESSES IN HF CONTACTORS AND PERTRACTION THROUGH ELM Advantages and disadvantages of membrane based processes and pertraction through various types of liquid membranes are summarized in Table 1. The application of HF contactors in these processes is supposed with the exception of pertraction into stable emulsions (ELM) where mixed column contactors or mixer-settlers are used. 7. IONIC LIQUIDS AS EXTRACTANTS AND CARRIERS Ionic liquids (ILs) are composed of organic cations and either organic or inorganic anions that remain in liquid state over a wide temperature range, including room temperature. ILs are a new group of designer solvents of great interest which have recently been studied widely as potential “green solvents”, especially in chemical and biochemical syntheses [103−105]. A great advantage of ILs is their vapour pressure practically equal to zero as compared with widely used volatile organic solvents. Less information is available on solvent properties of ILs in extractive separations. The majority of works deals with ILs with imidazolium cations. A new promising group of ILs based on phosphonium cations was developed by Cytec (Canada) [106]. The structure of a representative from the series of industrially produced phosphonium ILs is shown in Fig. 10.

137


Table 1. Advantages and disadvantages of membrane based extraction and pertraction through various types of liquid membranes in two and three phase systems Process Number of Advantages Disadvantages phases MBSE or 2 Resistance of immobilizing − Nondispersive process MBSS wall. Two wall in MBSE − No problems with stable performance of the solvent and MBSS circuit replacing function of SLM − One immobilizing wall − Volume ratio of phases can be varied without limitations PT through 3 Two immobilizing walls − No problems with stable BLM Commercial contactors are performance of BLM − Volume ratio of phases can not available. be varied without limitations PT through 3 Dispergation of the − One immobilizing wall BLME stripping solution can − No problems with stable introduce a problem in performance of LM some systems sensitive to − Larger surface are of the emulgation stripping solution droplets − Volume ratio of phases can be varied in wide interval PT through 3 − One immobilizing wall for Limited stability of SLM SLM two immobilized interfaces − Volume ratio of phases can be varied without limitations − Very small volume of membrane phase Limited stability of PT through 3 − No immobilizing wall ELM − Comparatively high fluxes emulsion Swelling of emulsion can be achieved − Small volume of membrane Complexity of the process Resistance of the phase adsorption film of surfactant CH3

H3C

CH3

H3C CH3 H3C

H3C

P O

O

P

CH3

+

H3C H3C

CH3

Fig. 10. Structure of trihexyl(tetradecyl)phosphonium bis 2,4,4-trimethylpentylphosphinate (Cyphos IL-104)

138

CH3


The potential of ionic liquids in the solvent extraction of organics is discussed in several papers [107−110], e.g. organic acids [18−20, 111−112], hydrocarbons [113−115], and erythromycin [116]. Transport of organic acids through LMs with ILs is presented in papers [21, 117]; penicillin G [118], amines and neutral organic substances are of concern in papers [119−121]. Pertraction of organic acids through LM facilitated by enzymatic reactions on L/L interfaces using IL as LM was studied by Miyako [122−123]. 7.1. Extraction of organic acids Phosphonium ILs with hydrophobic anion usually form complexes with lactic acid (LA) and other acids via coordination mechanism, as shown in paper [18]. In reactive extraction, monobasic carboxylic acids usually form complexes containing a variable number of acid molecules and only one molecule of the extractant, the so called (p, 1) complexes. For instance, this was observed in the extraction of carboxylic acids by trialkylamines [124– –125]. From the data presented in paper [19], the formation of stoichiometrically defined (p, 1, 2) complexes of the structure (LAH)p(IL)(H2O)2, with p within 1 to 3, is suggested. The reactive extraction of LA by the ionic liquid Cyphos IL-104 (Fig. 10) can be described by the following reaction of a hydrated complex formation

pLAH + 2H 2O + IL R (LAH) p (IL)(H 2 O) 2

(4)

·

Stripping of acid from the solvent to an aqueous alkaline solution follows the reaction

(LAH) p (IL)(H 2 O) 2 + pOH − R p(LA)− + IL+ ( p + 2)H 2 O

·

(5)

These reactions proceed on the water/solvent and solvent/stripping solution interfaces, respectively. The overbar denotes the species in the solvent phase. The extraction constants of individual complexes based on the reaction (1) are defined by the following relation

K p, 1 =

[(LAH) p (IL)(H 2 O) 2 ] [LAH] p ⋅ [IL]

=

c p, 1 (cF∗ ) p ⋅ c0, 1

;

(6)

where c0, 1 is the concentration of the free ionic liquid. Because of an excess of water in the aqueous phase, its concentration is considered to be constant and is included in the extraction constant. The stripping reaction (2) proceeds quantitatively when an excess of alkali is maintained. Consequently, the value of equilibrium constant of the stripping reaction approaches infinity. 139


The value of LA distribution coefficient for solvents with Cyphos IL-104 is much higher than for classical trialkylamine extractants, as shown in Fig. 11a, and advantageously increases with decreasing acid concentration. A very interesting phosphonium ILs feature is high solubility of water in ILs. It amounts 7.28 kmol m-3 in pure IL-104, despite its high hydrophobicity. Surprisingly, with an increasing LA concentration in the solvent, water concentration in the solvent decreases and approaches the value characteristic for the complex hydrated with two molecules of water as shown by a plot in Fig.12. This can be interpreted by the existence of reverse micelles in the organic phase which were identified by laser light scattering [19]. The anion structure greatly influences extraction properties of phosphonium ILs and the mechanism of extraction (Figs. 11b and 13). The value of the distribution coefficient based on analytical concentrations (Da) of LA for pure phosphonium ILs with trihexyl(tetradecyl)phosphonium (THTP) cation decreased in the following order of anions: decanoate anion (Cyphos IL-103) > octanoate anion > bis-trimethylpentylphosphinate (Cyphos IL-104) >dicyanamide (Cyphos IL-105) >chloride (Cyphos IL-101) > dodecylbenzenesulfonate (Cyphos IL-202) > bistriflamide (Cyphos IL-109). For the latter one, with Da approaching zero, some bars in Fig. 13 are too small to be visible.The mechanism of extraction can be quite different for ILs with different anions and the same cations. For example, Cyphos IL-101 with chloride anion extracts LA according to ion-exchange and coordination mechanisms [18]. Water soluble chloride anions are exchanged for LA anions also at high pH value; thus IL-101 extracts also lactate anions. It does not concern Cyphos IL-104, with water insoluble bis 2,4,4-trimethylpentylphosphinate anion (Fig. 10), which can extract only protonated LA at lower pH and does not extract lactate anions as shown in Figs. 11b. The cation structure also influences extractive properties of ILs. From the ILs studied, the most efficient performance is exhibited by those with phosphonium cation, weaker – ammonium IL. On the other hand, poor extraction was achieved with the ILs based on imidazolium cation (Fig. 13). The surface charge density figures, generated by COSMO-RS quantumchemical software, suggest that ILs with cations and anions with delocalized charge are less effective extractants of carboxylic acids, e. g. bistriflamide anion(Cyphos IL-109) and octylmethylimidazolium cation (OMIM) [126]. Quantum-chemical methods, e.g. COSMO-RS software, can be a powerful tool in molecular modelling and designing new extractants/carriers, especially ILs, and in the interpretation of respective experimental data [127−131].

140


a)

b) 2

10

100 -3

Da

D

cIo, dry, kmol.m

0.324 0.724 1.102 -3 cTAAo, kmol.m

10

10

3

0.4

2

2

1

1

0 0.0

0.8

Lactic acid IL-104 IL-105 IL-101 IL-202 IL-109 Sodium lactate IL-104 IL-101

1

1.6 2.4 -3 cF*, kmol.m

0 0.0

0.6

1.2 1.8 -3 2.4 caF*, kmol.m

Fig. 11. (a) The concentration dependence of the distribution coefficients of lactic acid in solvent/water systems. Solvents: pure Cyphos IL-104 (1.10 kmol m-3), its solutions in n-dodecane, and Hostarex A327 (mixture of C8 to C10 trialkylamines-TAA) with 0.8 kmol m-3 of isodecanol (modifier) in a fraction of C10 to C13 nalkanes at 298.15 K [19]. (b) Influence of anion structure in IL on the concentration dependence of the distribution coefficients of protonated LA and sodium lactate. IL structures are shown in Fig.13

2 -3

cIo dry, kmol.m 0.32 0.72 1.10

2

cS, H O*/cS*

10

1

10

0

10

-1

10

0.0

0.8

1.6 2.4 -3 cF*, kmol.m

Fig. 12. The concentration dependences of the molar ratio of water to lactic acid in the organic phase on the equilibrium concentration of lactic acid in the aqueous phase in extraction of LA by pure Cyphos IL-104 (1.10 kmol m-3) and its solutions in n-dodecane at 298.15 K (a) and temperature dependence of this ratio for the solvent with 0.72 kmol m-3 of Cyphos IL-104 (b). The line represents the calculated values for hydration water bound in all (p, 1, 2) complexes according to the model presented in paper [19]

141


Fig. 13. Influence of ILs anion and cation structure on distribution coefficient of lactic acid in water/ionic liquid systems [126]: THTP – trihexyltetradecylphosphonium cation, TOMA – trioctylmethylammonium cation, DBIM – dibutylimidazolium cation, OMIM – octylmethylimidazolium cation

7.2. Pertraction through SLM with phosphonium IL [21] Phosphonium IL tetradecyl(trihexyl)phosphonium bis(2,4,4-trimethyl-pentyl)phosphinate dissolved in n-dodecane effectively transports LA through SLM [21]. Together with an increase in LA concentration in the feed, the value of the overall mass-transfer coefficient decreases (Fig. 14a). This is in accordance with the concentration dependences of the distribution coefficient of the solvent (Fig. 11a). Increased concentration of IL did not result in a higher transport rate despite the increased distribution coefficient. This is due to the increased SLM viscosity and the decreased diffusion rate. The analysis of the individual mass-transfer resistances based on the model presented in section 4.1, showed that: (i) mainly the diffusion resistance of SLM, and (ii) the resistance based on the stripping reaction kinetics, contribute to the overall resistance, with domination of the second one (Fig. 14b). A stable flux of LA through SLM was found in the experiment that lasted for 5.3 days, which is a promising result. There is a need for a deeper study of the extractive and transport properties of IL systems using advanced physicochemical methods and by molecular modeling.

142


b)

12

-1 -5

7

16

80

R.10 , m .s

o

T, C 35 25

Kp.10 , m.s

-1

a) 20

-3

o

0.72 kmol.m IL-104, 35 C

60 40

8 4 0 0.0

rd

3

st

1 nd

2

0.2

rd

3

nd

2

20

st

1

0.4 0.6 0.8 -3 cF, ls, kmol.m

0 0.0

0.2

RF

Rk, s RM

0.4 0.6 -3 0.8 cF, ls, kmol.m

Fig. 14. Concentration dependence of the overall mass-transfer coefficient vs. the mean LA concentration in the feed phase at two temperatures (a) and of the overall and individual mass-transfer resistances in pertraction of LA through SLM containing 0.72 kmol m-3 of Cyphos IL-104 in n-dodecane at 35ºC (b) [21]. The points are the experimental data of the overall mass transfer coefficient and resistance

The back transport of water in the LA pertraction through SLM with phosphonium IL was identified experimentally for the first time. Based on the equilibrium data in paper [19] and LA transport data as well as back transport of water a new mechanism of pertraction of LA, schematically presented in Fig. 15, has been suggested to include the following six steps [21]: 1) Splitting of the reverse micelles (aggregates) at the feed/SLM interface due to removing IL molecules from its protection adsorption shield. 2) Formation of IL-LA-H2O hydrated complexes according to reaction in Eq. (4). 3) Transport of complexes through SLM. 4) Splitting of the hydrated complexes on the SLM/stripping solution interface according to the reaction in Eq. (5). 5) Free IL molecules form reverse micelles (aggregates) at the stripping interface capturing water. 6) Transport of the reverse micelles (aggregates) through SLM. A verification and deeper understanding of this mechanism will require studying these systems by using advanced physicochemical methods and by molecular modeling.

143


F

M

R

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

LAH hydration H2 O

-

-

-

-

splitting of reverse micelle

-

-

-

- -

-

-

-

- -

formation of reverse micelle -

LAIL

Fig. 15. Scheme of the mechanism of pertraction of LA through SLM containing IL-104 with hydrated LAH-IL-H2O complex formation and decomposition together with the splitting and formation of reverse micelles stabilized by free IL molecules [21]

8. FUTURE TRENDS Separations in systems with one or two immobilized L/L interfaces, realized in contactors with microporous or gel supports, have not found wider application so far. The present state of knowledge shows that they have potential for the development of various successful applications. Properties of the support walls, despite the fact that they play no active role in separation itself, are of great importance in the achievement of reliable lifetime of the system and should be tailored or modified for a respective extractant or carrier used in separation. For this purpose, specific surface modification could be of great importance. Also, some new extractants or carriers, e.g. ionic liquids with significant variability of structures, could stimulate advancement in this area. There is a need for highly developed contactors with better stability in systems operating with an organic phase. Deeper understanding of phenomena connected with the achievement of prolonged operation of a multiphase system involved in separation can enhance the development of successful applications. Precisely documented experimental data on long-term operation of separation systems are needed to achieve it. Knowledge obtained from several pilot plant experiments with larger HF modules may help in a further increase in the number of 144


applications. Separation/recovery of higher added value substances may have greater potential for application. The development of hybrid production-separation processes can lead to finding new successful technologies, e.g. extractive fermentations and biotransformations. The combination of chemical or biochemical reaction(s) with separation could exhibit synergy effect enhancing the technology in its production part. Not only the development of new types of contactors but also deeper understanding of mass-transfer, reaction, and other interfacial phenomena is significant for progress in this field of technology. Therefore, the relation between ILs structure and their extractive properties has to be studied. Molecular modelling with quantum-chemistry methods and its application for the interpretation of experimental data can facilitate designing new extractants/carriers (especially ILs). It should be stressed that physical chemistry and its methodology has an important role in achieving this goal. Acknowledgement: Support of the Slovak grant VEGA No.1/0876/08 is acknowledged. 9. NOTATION

ε

surface area, m2 molar concentration of the solute (undissociated acid or acid in the complex), mol m-3 distribution coefficient based on analytical concentrations of acid distribution coefficient of the solute on the feed interface, – individual mass-transfer coefficient, m s-1 overall mass-transfer coefficient in MBSE, m.s-1 overall mass-transfer coefficient in MBSS, m s-1 overall mass-transfer coefficient in pertraction, m·s-1 molar flux, mol s-1 rate constant of the extraction reaction, Eq. (2), m s-1 rate constant of the stripping reaction, Eq. (3), m s-1 overall mass-transfer resistance, s m-1 porosity of the wall, –

b e F M R s S w

Subscripts boundary layer in the bulk phase extractor (MBSE) feed phase, feed boundary layer membrane phase, membrane boundary layer stripping solution; stripping interface stripper (MBSS) solvent phase; boundary layer in the solvent fiber wall

A c Da DF k Ke Ks Kp dn/dt re rs R

145


BA BLM BLME D2EHPA ELM HF Hostarex A327 IL LA LAH LALM MBSE MBSS MPCA PT SLM TOA

Abbreviations butyric acid bulk liquid membrane bulk liquid membrane with emulsion of the stripping solution di-2-(ethylhexyl)phosphoric acid emulsion liquid membrane hollow fiber (contactor) mixture of trialkylamines ionic liquid lactic acid protonated lactic acid lactic acid anion liquid membrane membrane based solvent extraction membrane based solvent stripping 5-methyl-2-pyrazinecarboxylic acid pertraction supported liquid membrane trioctylamine 10.

[1]

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