Interfacial polycondensation of polyarylate in Taylor-Couette-Reactor

Chemical Engineering and Processing 48 (2009) 1061–1071

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Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep

Interfacial polycondensation of polyarylate in Taylor-Couette-Reactor ∗ ´ ´ J. Wolinski , S. Wronski Faculty of Chemical and Process Engineering, Warsaw University of Technology, ul. Wary´ nskiego 1, 00-645 Warsaw, Poland

a r t i c l e

i n f o

Article history: Received 28 July 2008 Received in revised form 19 February 2009 Accepted 21 February 2009 Available online 6 March 2009 Keywords: Interfacial polycondensation Polyarylate Taylor-Couette-Reactor Liquid–liquid reaction

a b s t r a c t The interfacial polycondensation of polyarylate in Taylor-Couette-Reactor has been investigated. The influence of the following parameters: initial mole ratio of monomers, intensity of mixing, volumetric mixture flow rate, annular gap width and temperature on properties of polymer (average molecular weight and polydispersity index) were examined. The possibility of obtaining a product displaying desirable properties suitable for the production of fibres or polymeric films has been shown. The desired properties of polymer can be obtained for the following ranges of the process parameters: 0.7 < M0 < 0.9 and 1210 ≤ Rerot ≤ 1400 for T = 20 ◦ C,  = 60 s and  = 0.93. Considerably shorter residence times, of the order  = 60 s, have been reached in Taylor-Couette-Reactor in comparison with reaction times in the Stirred-Tank-Reactor, of the order t = 300–600 s. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Polyarylates are the products of a polycondensation reaction between diphenols and aromatic dicarboxylic acids, their esters or their dichloroanhydrides, respectively. Their characteristic feature is that their main chains almost exclusively consist of aromatic rings connected with ester bonds, Fig. 1. Due to this specific structure, they reveal the following properties: good thermal resistance, good dielectric properties, large mechanical endurance, good film- and fibre-forming properties, good optical properties and also large chemical resistance to aggressive environment. All these properties cause that recently there has been increasing interest in this type of polymers, following the possibility of their utilization as a nonlinear-optical (NLO) materials [1], after building special chromophores in their structure. Such materials are used for building, e.g. optical fibres. The interfacial polycondensation with phase-transfer catalysis is the most often applied method of manufacturing polyarylates. The general diagram of the process was presented in Fig. 2 [2], where Q+ Cl− is a catalyst, ArO− Na+ and RCOCl are monomers in the aqueous and organic phases, ArO− Q+ is an ionic pair of a catalyst and a monomer in the aqueous phase, RCOOAr represent the growing chain of polymer, k is the reaction rate constant, K1–2 , K2–1 are the overall volumetric mass transfer coefficients of an ionic pair and a catalyst and m, n are distribution coefficients of an ionic pair and a catalyst, respectively. On the basis of the diagram (Fig. 2) one can state that the polycondensation process of polyarylates is a complex one, additionally accompanied by an increase in the viscosity

∗ Corresponding author. Tel.: +48 22 234 63 25; fax: +48 22 825 14 40. ´ E-mail address: [email protected] (J. Wolinski). 0255-2701/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2009.02.005

of the organic phase which results from the increasing polymer concentration and from the change of its properties (average molecular weight of polymer MW and polydispersity index PI). The mass transfer rates of monomers to the reaction zone, the rate of polycondensation reaction, the rate of reversible reaction creating the ionic pair in the aqueous phase and mass transfer rate of catalyst from the organic to aqueous phase influence the overall rate of the process [2]. Some discrepancies exist in the literature concerning which of the stages is the step limiting the rate of the polycondensation process of polyarylates. Brzozowski et al. [3] and also Tsai et al. [4–8] believe that the mass transfer rate of monomers to the polyreaction zone is the limiting rate step of the whole process for the rotation speed of the impeller below a certain critical value. Morgan and Kwolek [9], who studied the interfacial polycondensation process for various systems, share the same opinion. They showed in their study [9] that polycondensation reactions are fast reactions whose rate constants k are of the order 102 –106 dm3 /mol s. However, Wang et al. [10] represent a different opinion that the chemical reaction in the organic phase is the limiting step of the overall process rate. The above mentioned discrepancies were explained by Berezkin and Khokhlov [11]. On the basis of theoretical consideration, they linked the molar ratio of used monomers and the degree of polymerization Pn with the regions in which the rate of the overall process is controlled by an appropriate stage, namely by the mass transfer rate in the continuous phase, the mass transfer rate in the dispersed phase or polycondensation reaction in the dispersed phase. As it results from their research, obtaining a high degree polymerization Pn is possible only for fast polycondensation reactions and a suitable molar ratio of monomers. The available degrees of polymerization, in the case of slow reactions, are of the order of Pn ∼ 2–3, independently of the molar ratio of monomers. The authors also paid attention to the fact that the kinetic regime

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The aim of the presented study was the efficiency assessment of a helicoidal reactor when applied in the creation of a product with desirable parameters through selection of the appropriate hydrodynamic conditions and molar ratio of the applied monomers. Fig. 1. General pattern of polyarylates.

2. Experimental is inevitably achieved after a certain time, when monomers are already on exhaustion and the product can only be produced as a result of a reaction between growing chains of polymers. The aforementioned researchers [11] presented their results of polycondensation process simulation on the generalized graph Pn = f(M,Ha) where M is the concentration ratio of used monomers and Ha is the Hatta number for a second order reaction. Additionally, they showed the influence of macromolecules diffusion in the reaction zone on the course of polycondensation process. Many authors [3–10] studied the influence of monomers concentration ratio, stirring rate, kind and quantity of the applied catalyst and also of temperature on the rate of polycondensation process in a tank reactor. Eareckson [12] also examined the influence of the type of organic solvent and dispersing agent on yield capacity and average molecular weight of polyarylate MW . In the polycondensation process one can receive a comparatively high molecular weight of polyarylate of the order MW = 200–500 × 103 [3,5] and a low polydispersion index of the order PI = 2–3 [5]. It should be added that the polycondensation process is also possible without the addition of a catalyst but then the attained molecular weights of polymers are considerably lower [3,4]. Due to the possibility of using polyarylate as NLO materials to produce optical fibres or polymeric films, the received product has to demonstrate strictly specific properties. As it is known, molecular weight of polycarbonates, which belong to the saturated polyesters group similarly to polyarylate, when applied to the production of fibres, should be contained in the range 1.8 × 104 ≤ MW ≤ 2.2 × 104 . Vinogradova et al. [13] claims that the intrinsic viscosity of polyarylate [] in dichloroethane or tetrachloroethane used for polymeric films production should be 0.4–0.5 dl/g, and for those applied in the production of fibres ∼1.0 dl/g. Tsai and Lee [5] presented results of the intrinsic viscosity in tetrachloroethane as depending on average molecular weight of polyarylate MW . On this basis, it has been estimated that molecular weight of polyarylate should be contained within the following range: 2 × 104 ≤ MW ≤ 6.5 × 104 . The polydispersity index of polyarylate should be the smallest possible and be contained within the range 1 ≤ PI ≤ 3. Morgan and Kwolek [9] expressed an obvious opinion, considering fast reaction regime, that probably the best way of conducting the polycondensation process would be to use a high-speed lowvolume mixer working continuously with both monomer solutions added simultaneously. Thus, it can be stated that Taylor-CouetteReactor [14] fulfills these conditions. It consists of two cylinders placed one inside the other. The internal one is set into rotational movement while the external one remains motionless. Its main advantages, such as large values of the mass transfer coefficient and interfacial area, the possibility of independent mixing intensity control and axial flow in the reactor with such type of flow [15,16], justify its utilization in the polycondensation process.

2.1. Experimental setup The polycondensation process of bisphenol A, in aqueous alkaline solution with the addition of triethylbenzylammonium chloride as the catalyst and mixture of isophthaloyl and terephthaloyl chlorides in methyl chloride was investigated in a Taylor-Couette-Reactor working in the continuous mode. The main parameters of the reactor were as follows: outer cylinder diameter D2 = 42 mm, length L = 400 mm, diameter of rotating cylinder D1 = 39, 36, and 32 mm. The inner cylinder was made of plastic material and the outer cylinder of stainless steel. During the investigation of flow visualization the diameter of the outer cylinder was D2 = 47 mm, the diameter of the inner cylinder D1 = 40 mm and of the same length. The outer cylinder was made of transparent material. The scheme of the experimental setup is shown in Fig. 3. The inner cylinder of the Taylor-Couette-Reactor (RE-5) was set into rotational movement by means of a driving gear (7) consisting of a three-phase electrical motor and a control device enabling continuous regulation of the rotational speed of the impeller. The reactor was equipped with a cooling jacket through which thermostatic liquid flowed in counter-current. The alkaline solution of bisphenol and triethylbenzylammonium chloride (TEBAC) was taken from the tank (TA-1) and the solution of isophthaloyl and terephthaloyl chlorides was fed from the tank (TA-2) to the TCR (RE-5). Cooling jackets (1 and 2) maintained the desired temperature of solutions which was measured by means of thermometers (5 and 6). Stirrers (3 and 4) were used for the preparation of solutions. Flow rates of both phases were controlled by means of peristaltic pumps (PU-3, PU-4). The temperature of mixture at the outlet of the reactor was measured with a thermometer (8). Samples of mixture (10) for analyses were withdrawn by means of a three-way valve (9). The outlet mixture was collected in the tank (TA-6). 2.2. Experimental procedure Prior to the proper measurements, terephthaloyl and isophthaloyl chlorides were purified in the process of vacuum distillation. The purity was measured and equaled above 98%. Morgan and Kwolek [9] paid attention to the fact that purity of monomers can affect the size of the obtained molecular weight of polymers MW . Then, the solutions of desired concentrations were prepared and the remaining parameters were determined. Their range of changes is presented in Table 1. Next, the solutions were poured into the reactor, where polycondensation followed. After stabilising the process parameters (temperature, rotational speed of the rotor, solutions flow rates and volumetric flow ratio of organic to aqueous phase) two mix-

Fig. 2. General diagram of polycondensation process with phase-transfer catalysis [2].

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Fig. 3. Experimental setup: (TA-1) tank with aqueous phase; (TA-2) tank with organic phase; (PU-3, PU-4) peristaltic pumps; (RE-5) TCR; (TA-6) tank with outlet mixture; (1 and 2) cooling jackets; (3 and 4) stirrers; (5, 6, and 8) thermometers; (7) motor and control device; (9) three-way valve; (10) the sampling point.

ture samples were withdrawn from the outlet of the reactor to a separatory funnel, where fast phase separation took place similarly as in the works [3,4]. Before sampling, 5 ml 10% HCl solution was added to one of the distributors in order to stop the polycondensation reaction. Then, measurement of the organic phase volume in graduated cylinder was executed and acetone was added to chloride methylene in ratio 5:1, in order to precipitate the polymer. Next, the solution was filtrated. The obtained polyarylate was washed twice with acetone and dried at room temperature for few days. After that, the dried polymer was weighed. The filtrate was used to the following analyses. 2.3. Analytic methods The following methods were applied in the analysis of the product’s properties and to determine concentration of bisphenol A in both phases: gel permeation chromatography, viscosimetric, HNMR and spectrophotometric techniques. 2.3.1. GPC method From the received polyarylate a sample was taken to determinate characteristic distribution of molecular weight. The measurements were conducted using the chromatograph ShiTable 1 Range of experimental parameters. 0 Initial concentration of bisphenol A in aqueous phase, CBPA (mol/dm3 ) Concentration of NaOH in aqueous phase, CNaOH (mol/dm3 ) Concentration of TEBAC in aqueous phase, CTEBAC (mol/dm3 ) Concentration of mixture iso- and terephthaloyl chlorides, CI+T (mol/dm3 ) Concentrations ratio of terephthaloyl chloride to isophtaloyl chloride Rotation speed, N (rpm) Residence time,  (s) Annular gap width, d (mm) Volumetric aqueous phase flow rate, QAq (ml/min) Volumetric organic phase flow rate, QOrg (ml/min) Volume fraction dispersed (organic) phase,  Temperature, T (◦ C)

madzu LC-20AD with a set of Nucleogel 103-5 and Nucleogel 105-5 columns connected in series and with a UV detector. Chloroform was used as the eluent. Its choice was assessed based on the theoretical analysis of Hildebrand solubility parameters [17] and the qualitative measurements of solubility. For the calibration narrowdispersity polystyrene standards in the range of molecular weight between 580 and 6,850,000 Da were used. On the basis of obtained molecular weight distribution were determined average molecular weight of polymer MW , number average molecular weight MN and polydispersity index PI. 2.3.2. Viscosimetric method The measurements of polyarylate intrinsic viscosity [] were made in chloroform in order to determine parameters of the MarkHouwink’s equation: 

[] = K · M a

The measurements of the viscosity were performed using the Ubbelohde viscometer, Schott company, with automatic time measurement. The viscometer was placed in a water bath. The temperature of measurement was 20 ◦ C, diameter of measuring capillary 0.64 mm and the constant of the device K = 0.01. On the basis of the polyarylate solution viscosity measurements of given concentration and average molecular weight, the correlation to determine polyarylate viscosity in chloroform was approximated by the following form:

0.054; 0.108

1.81 0.94 d,cor = 0.54 × 10−4 + 6.81 × 10−5 · CPAR · MW

0.156; 0.312 6.585 × 10−3 0.042–0.115

The correlation coefficient of adjustment equals to R2 = 0.988.

2.1 500–2500 20–60 1.5–5 20–107 26–126 0.24–0.74 5–20

(1)

(2)

2.3.3. H-NMR method The obtained filtrate was vaporized with the use of an evaporator. Then, the obtained dry mass was analysed with H-NMR method in order to determine concentration of bisphenol A. The spectrums of H-NMR were obtained with the use of Gemini 2000 (200 MHz) device (Varian company). The deuterium chloroform CDCl3 was used as a solvent. This method depends on the performance of spectrum for polyarylate, bisphenol A, isophthaloyl chloride, terephthaloyl chloride and triethylbenzylammonium chloride in

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Table 2 Properties of phases at the inlet of the TCR. Phase

Viscosity (Pa s)

Aqueous Organic (methylene chloride)

−3

1.174 × 10 4.266 × 10−4

chloroform. On their basis, the characteristic shift was established for bisphenol A in the range 6.700–6.745 ppm, which was not disturbed by the remaining compounds. Hence, the percentage content of bisphenol A in the sample was determined as [18]: XBPA [%] =

A B

(3)

where A is the ratio of the group’s signal intensity to the amount of protons in the group, belonging to the analysed compound and B is the ratio intensity of all groups to the amount of all protons present in the mixture. 2.3.4. UV–VIS spectrophotometric method From the second separatory funnel the aqueous phase sample was taken. The concentration of bisphenol A in the aqueous phase was determined by using spectrophotometer Spectronic Genesys 5 (Milton Roy company). Contrary to study [4–8], when choosing the length wave at which concentration of bisphenol A was measured, the influence of pH and of carboxylic diacid salts presence on the spectrum position was also taken into consideration [19]. These salts are formed as a result of the hydrolysis reaction of acidic chlorides and can disturb the measurement at  < 305 nm. However, for pH > 12 one observes no changes in the spectrum position. Hence, the absorbance measurements of bisphenol A were conducted at the wavelength  = 305 nm and pH > 12. The established calibration curve between absorbance and concentration of bisphenol A is presented in Fig. 4. 3. Results and discussion The intensity of mixing, e.g. dissipation rate of turbulent kinetic energy ε in a reactor, affects the values of mass transfer coefficients kBPA in both phases and the development of interfacial area a. In this system the rotational Reynolds number Rerot is usually accepted as the measurement of dissipation energy rate ε. It is so because of considerably larger participation of rotational flow rather than of the axial flow in the value of dissipated energy. On this basis, the inter-

Fig. 4. Calibration curve between UV absorbance and concentration of bisphenol A at  = 305 nm and pH > 12.

Density (kg/m3 )

Interfacial tension (N/m)

1014 1324

16.1 × 10−3

pretation of the obtained results was based on the dimensionless parameter Rerot defined in the following way: Rerot =

ω · R1 · d · m m

(4)

where ω is the angular velocity of the inner cylinder with the radius R1 , d is the annular gap width and m and m are density and viscosity of mixture, respectively, which were calculated from the following expressions [20]: m = d ·  + c · (1 − ) m =



 1.5 · d · 

c · 1+ d + c 1−

(5) (6)

where  is the volume fraction of dispersed (organic) phase, d , c are densities of dispersed and continuous phase, respectively, and d , c are viscosities of these phases. This correlation (6) does not take into account the influence of shear rate and droplet size on emulsion viscosity. However, the works [21–23] indicate that emulsion viscosity does not depend on shear rate and droplet size if volume fraction of dispersed phase does not exceed some criti˛ cal value. Pal [21] suggests a value  = 0.6 whereas Krynke and Sek [22] and Lee et al. [23] indicate  = 0.5. In this work, the volume fraction of dispersed phase was  ≤ 0.54 (only four experiments for  > 0.54). Finally, we decided to use the above correlation (6) which had already been applied to estimate the viscosity of unstable emulsion, especially in similar systems. The viscosity of the dispersed phase used for calculations of the mixture viscosity was determined as the arithmetic average of the viscosity at the inlet and outlet of the reactor. The viscosity of the dispersed phase at the outlet of the reactor was calculated on the basis of relationship (2), but the first term (0.54 × 10−4 ) which represent the viscosity of chloroform was replaced by the viscosity of methylene chloride (0.47 × 10−4 ). The accuracy of such relationship was checked for few obtained organic solutions and the relative error did not exceed 10%. The properties of both phases at the inlet of the reactor are presented in Table 2. Approaching to works over the use of Taylor-Couette-Reactor to the interfacial polycondensation process, we expected to find a connection between the properties of the received polymer and the structure of the flow. As it is known, at sufficiently low rotation rates of the inner cylinder, fluid segregation due to gravity and surface tension occurs. So, only for the rotor rotation rates large enough to overcome these effects, the instability in the form of cellular vortices appears. Due to the small quantity of works concerning the liquid–liquid flow in a Taylor-Couette-Reactor (as distinct from the single-phase Taylor-Couette flow), the correlations to estimate the critical Reynolds number Rerot,cr above which the instabilities appear in form of Taylor vortices were not developed. On the basis of Atsumi correlation [24] for single-phase Taylor-Couette flow the critical Reynolds number equals Rerot,cr = 156. In this work the range of Reynolds number was the following 500 < Rerot < 6000. According to the considerations in the work of Campero and Vigil [25] that range should give us the translating banded structure with alternating water and organic-rich vortices (Re > Reinv ) because we used large rotation rates. Only, for lower rotation rate it can obtain another flow structure, i.e. an inverted or oscillatory structure. However, application of a lower rotation rate resulted in a low product yield (less than 15%). Hence, the authors did not study the range of Reynolds numbers Rerot < 500. In order to check the flow structure

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Fig. 5. Images of flow structure water–methylene chloride system in TCR for the following conditions:  = 0.54; d = 3.5 mm; = 27.8 × 10−3 N/m: (a) 500 and (b) 1000 rpm and for  = 0.14; d = 3.5 mm; = 27.8 × 10−3 N/m: (c) 1000 and (d) 1500 rpm.

water–methylene chloride system and state which phase is dispersed, photographs were taken. The water phase was dyed. Fig. 5 shows some photographs. As a result, the organic phase is the dispersed phase and as angular velocity increases the translating banded structure appears. Subsequent growth of angular velocity produces homogeneous emulsion but the structure of banded flow is still maintained. The FLUENT simulations for chosen process parameters confirm this, Fig. 6. Therefore, the influence of flow structure on the properties of polyarylate was not confirmed in the studied range of the process parameters. 3.1. Influence of monomers molar ratio M0 The influence of rotational Reynolds number Rerot and of molar ratio M0 of the applied monomers on the properties of the obtained product MW and PI as well as on the residence time resulting from the volumetric flow rate of the mixture are presented in Figs. 7 and 8. It is seen in Fig. 7 that for the monomers molar ratio M0 < 1 a significant growth of polyarylate average molecular weight MW follows initially, even up to about 100,000, and then a drop takes place following the growth of the rotational Reynolds number Rerot . However, as it results from Fig. 8, polydispersity index PI of polymer for monomers molar ratio M0 < 1 and rotational Reynolds numbers Rerot > 1500 increases sharply to values PI ≈ 6–8, what makes its utilization for production of, e.g. polymeric films or fibres impossible. As it has been mentioned earlier, the properties of the product

Fig. 6. FLUENT simulations: contours of organic-volume fraction for the following process parameters: M0 = 0.87; N = 2000 rpm; d = 1.5 mm;  = 0.54; d = 1.89 × 10−3 Pa s; ddrop = 74 ␮m.

Fig. 7. Influence of the rotational Reynolds number Rerot and molar ratio of monomers M0 on average molecular weight of polyarylate MW .

should fulfill the following conditions: 2 × 104 ≤ MW ≤ 6.5 × 104 ; 1 ≤ PI ≤ 3. Production of polyarylate displaying similar properties in a Stirred-Tank-Reactor operating in a periodic mode requires approximately 5–10 min [4] in comparison with the residence time of  = 1 min in a Taylor-Couette-Reactor. Hence, it can be observed on the basis of Figs. 7 and 8, that the desirable range of the process parameters for a helicoidal reactor are as follow: M0 = 0.87 and

Fig. 8. Influence of the rotational Reynolds number Rerot and molar ratio of monomers M0 on polydispersity index of polyarylate PI.

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Fig. 9. Influence of the rotational Reynolds number Rerot and the molar ratio of monomers M0 on concentration of bisphenol A in the aqueous phase CBPA,Aq .

1210 ≤ Rerot ≤ 1400. The reactor offers us the possibility to obtain a product displaying specified properties in a comparatively short time ( = 60 s). It also enables to conduct the process in a continuous way. In order to illustrate the polycondensation process rate changes of bisphenol A concentration in both aqueous and organic phases at the outlet of the reactor, they were presented in Figs. 9 and 10. The growth of the rotational Reynolds number results in a decrease of the bisphenol A concentration in the aqueous phase, whereas bisphenol A concentration in the organic phase in the majority of cases is very small and equals ∼10−4 –10−5 mol/dm3 . Hence, it is convenient to represent the rate of bisphenol A consumption in the polycondensation process as a conversion factor defined in the following way: XBPA = 1 −

CBPA,Aq 0 CBPA,Aq

(7)

Fig. 11 presents the influence of rotational Reynolds number Rerot and monomers molar ratio M0 on conversion factor of bisphenol A, XBPA . Despite the generally accepted assumption concerning the plug flow in TC reactor, certain slip was observed among the continuous and dispersed phases of the order 2–5%. It exerts an influence on the values of bisphenol A concentrations and conversion factors XBPA at the outlet of the reactor in the comparison with possible values counted on the basis of bisphenol A concentration at the inlet. The influence of the volume fraction of the organic phase on the properties of the formed product was also carried out for two

Fig. 10. Influence of the rotational Reynolds number Rerot and the molar ratio of monomers M0 on concentration of bisphenol A in the organic phase CBPA,Org .

Fig. 11. Influence of the rotational Reynolds number Rerot and the molar ratio of monomers M0 on conversion factor of bisphenol A, XBPA .

selected rotational speeds, i.e. N = 1000 and 1500 rpm for whose the rotational Reynolds numbers are Rerot,c = 2646 and 3968, respectively. In order to interpret the obtained results some graphs were prepared, Figs. 12 and 13, representing a dependence of MW and PI on M0 , for the above mentioned frequencies. It results from them that the dependence of the initial molar ratio of monomers M0 on the average molecular weight MW and polydispersity index PI is divided in at least two regions, marked on Figs. 12 and 13 as I and II, according to considerations of Berezkin and Khokhlov [11]. The border separating these regions proceeds along M0 ≈ 1.2. Region I represents the situation where the mass transfer in the continuous phase is the step limiting the process rate. The increase of the mixing intensity causes the growth of both conversion factor XBPA (Fig. 11) and of the product yield. In this region, the influence of monomer molar ratio M0 and mixing intensity on the properties of the obtained polyarylates are not observed. The molecular weight and the polydispersity index of the product remain almost constant and equal approximately MW ≈ 3000 and PI = 1.6. The growth of the initial molar ratio M0 in this region results in generation of the concentration of acidic chlorides in the organic (dispersed) phase much higher than that of bisphenol A at the interfacial area. Hence, the diffusivity flux of acidic chlorides is much higher than that of bisphenol A and the zone reaction moves in direction of the interfacial area, i.e. in the region I. Next, the increase of acidic chlorides concentration produces the increase of the reac-

Fig. 12. Influence of the monomers molar ratio M0 on average molecular weight of polyarylate MW for the chosen rotation speed of the inner cylinder N.

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Fig. 14. Influence of the rotational Reynolds number Rerot on average molecular weight of polyarylate MW for the chosen molar ratio M0 and mixture flow rate Qm .

Fig. 13. Influence of the monomers molar ratio M0 on polydispersity of polyarylate PI for the chosen rotation speed of the inner cylinder N.

tion rate (growth of the Hatta number). In case of the instantaneous reaction, the reaction takes place on the interfacial area exclusively. In effect, this leads to the formation of oligomers of a low polymerization degree Pn = 2–3. It occurs because bifunctional bisphenol A in the form of an ionic pair, when reaching the zone reaction, reacts readily with two molecules of diacidic chlorides and diffuses into the boundary layer of the organic phase. In this way, the formed oligomers have no ability to react with bisphenol A to increase the chains length because diacidic chlorides are their only environments. In region II, with a decrease of value M0 , the diffusivity resistances of the boundary layers in both phases initially determine the process rate. Further fall of M0 results from the fact that concentration of bisphenol A in the aqueous phase is much higher than that of acidic chlorides in the organic phase. Next, the concentration of bisphenol A at the interfacial surface has a constant ∗ value CBPA,W = const. For sufficiently small values of M0 , the kinetic reaction will be a rate limiting step. It should also be remembered, that independently of the used initial monomer molar ratio M0 , in the moment of monomers exhaustion, the kinetic regime will be reached, because the properties of the product are determined only by the reactions between already existing chains of polymers. For small values of M0 (M0  1), the kinetic polycondensation reaction is the rate limiting step and obtainment of desirable product properties is connected with higher residence times. In the region II, with decreasing molar ratio M0 , mixing intensity has a diminishingly smaller influence on the conversion factor of bisphenol A (Fig. 11). However, the evident influence of this parameter on the properties of the obtained product MW and PI can be observed. In the case when diffusion in the boundary layer of the organic phase is the rate limiting step of the process rate and diffusivity fluxes of both monomers become equal, the largest molecular weights can be reached. As it is known, the quantities of these fluxes depend on mass transfer rate of both monomers in the organic phase, which are determined by the mixing intensity in the system (the size of a droplet, see Fig. 5), and on the difference of concentrations at interfacial surface and that in the bulk of the phase. In Figs. 12 and 13, horizontal lines were marked, which indicate maximum and minimum values of MW and PI accepted in polyarylate fibres and films production.

Fig. 15. Influence of the rotational Reynolds number Rerot on polydispersity index PI for the chosen molar ratio M0 and mixture flow rate Qm .

the increasing axial flow, values of the overall mass transfer coefficients for bisphenol A [15,16] increase, but the residence time  simultaneously decreases. Figs. 14–16 reveal an influence of the rotational Reynolds number Rerot , for the chosen molar ratio M0 and the residence time , on the product properties and on the conversion factor of bisphenol A, respectively. For the molar ratio

3.2. Influence of mixture flow rate Qm The changes of the mixture flow rate affect the course of the polycondensation process in the TC reactor in a complex way. With

Fig. 16. Influence of the rotational Reynolds number Rerot on conversion factor of bisphenol A XBPA for the chosen molar ratio M0 and mixture flow rate Qm .

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M0 = 1.25 (region I) the product properties do not depend on the mixing intensity, neither the residence time at all. In this region, as it was explained earlier, the mass transfer in the continuous phase is the rate limiting step of the process. As a result, in this step oligomers are formed. For the molar ratio M0 = 0.87 (region II), the influence of mixing intensity and residence time on polymer properties is evident. The molecular weight of polyarylates MW increases with the increase in dimensionless parameters Rerot . In the case of larger residence time ( = 60 s) follows the decrease of value MW for the Reynolds number Rerot > 1550 (Fig. 14) and unfavourable growth of polydispersity index PI (Fig. 15). Fig. 16 confirms that larger residence times  provide a larger conversion factor of bisphenol A, what, as a result, yields the growth of the molecular weight MW . In case of Rerot > 1650, for both residence times, the conversion factor is close to unity. This indicates that, in case of residence time  = 60 s, the mass transfer resistance in both phases initially controls the process rate. Then it is affected by the kinetics of creation of longer chains from shorter ones in the organic phases, as a result of monomers exhaustion. As soon as the kinetic regime becomes the rate controlling process step of process rate, unfavourable influence on the polydispersity index PI is seen.

Fig. 18. Influence of temperature on polydispersity index PI for the chosen molar ratio M0 and rotation speed N.

3.3. Influence of temperature The influence of temperature T on the average molecular weight MW and the polydispersity index PI for the selected molar ratio M0 and the rotation speed N are presented in Figs. 17 and 18. The effect of temperature was displayed in the range 5–20 ◦ C for the sake of low boiling temperature of methylene chloride. As it is evident from Figs. 17 and 18, for the molar ratio M0 = 1.25 (region I) the temperature and mixing intensity exert no influence on the product properties. Another situation can be observed for the molar ratio M0 = 0.87 (region II). In this region the influence of both parameters is visible on molecular weight of polymer MW , especially for the rotation at rate N = 1500 rpm and temperature T = 20 ◦ C. The polydispersity index PI increases slightly. In order to explain this effect, one should look at Fig. 19, which presents the influence of temperature and rotation speed on the conversion factor of bisphenol A XBPA . In this case, the conversion factor XBPA is close to unity. Only for such values of the conversion factor it is possible to obtain a polymer with a large molecular weight MW .

Fig. 19. Influence of temperature on the conversion factor of bisphenol A XBPA for the chosen molar ratio M0 and rotation speed N.

3.4. Influence of annular gap width d Attention should be paid to the fact that although in the definition of the rotational Reynolds number Rerot the annular gap width

Fig. 17. Influence of temperature on the average molecular weight of polyarylate MW for the chosen molar ratio M0 and rotation speed N.

Fig. 20. Influence of the rotational Reynolds number for continuous phase Rerot,c and dimensionless parameter  on the average droplet diameter ddrop .

J. Woli´ nski, S. Wro´ nski / Chemical Engineering and Processing 48 (2009) 1061–1071

Fig. 21. Influence of the rotational Reynolds number Rerot on the average molecular weight of polyarylate MW for the chosen molar ratio of monomers M0 and the annular gap width d of the TCR.

d is used, this number is not really a sufficient criterion of the phenomena similarity in the two-phase flow. The field of shear stress in the annular gap of the TC reactor depends on the gap width d, which influences the magnitude of the interfacial area. The Haas’ study [26] concerns a large influence of a dimensionless parameter ( = D1 /D2 ) on the interfacial area effects, which was presented on Fig. 20. The displayed data were evaluated for current properties of the aqueous phase and process parameters, in particular for the rotation speed in the range 500–2500 rpm. The values of the interfacial area exhibit considerable differences for the narrow  > 0.9 and for the wide gaps  < 0.9. Hence, their influence on properties of the obtained product and the product yield can be expected. The influence of the rotational Reynolds number Rerot and gap width d on the product properties is shown in Figs. 21 and 22. Smaller gap width d and larger rotational speed N result in larger shearing stress. The giving rise to shearing stress causes the possibility of obtaining the larger conversion factor XBPA (Fig. 23), thus providing a significant increase in the average molecular weight of polyarylate MW . The influence of mixing intensity and annular gap width on the product properties can be seen in the region II (M0 < 1.2), whereas in the region I (M0 > 1.2) the properties are independent of the mixing intensity.

Fig. 22. Influence of the rotational Reynolds number Rerot on polydispersity index PI for the chosen molar ratio of monomers M0 and the annular gap width d of the TCR.

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Fig. 23. Influence of the rotational Reynolds number Rerot on conversion factor XBPA for the chosen molar ratio of monomers M0 and annular gap width d of the TCR.

3.5. Comparison between Taylor-Couette-Reactor and Stirred-Tank-Reactor For comparison of Taylor-Couette-Reactor with Stirred-TankReactor, the influence of the monomers molar ratio, M0 , and of polycondensation reaction time, t, on the molecular weight of polyarylate MW in a Stirred-Tank-Reactor is shown in Fig. 24. Maximum and minimum acceptable molecular weights MW are also indicated there. It is then evident from the above graph that with a decrease of the molar ratio M0 and the increase of reaction time the growth of molecular weight of the polymer is to be expected. Moreover, the obtainment of the polymer with desirable properties in the STR requires considerably longer reaction times in comparison with residence time in the TCR (Fig. 12). This fact results from that the Taylor-Couette-Reactor provides much larger interfacial area values [15] in comparison with Stirred-Tank-Reactor, especially in TCR with a narrow gap. Hence, the volumetric mass transfer coefficients of both monomer and finally the mass transfer rate are larger. As it is known, the mass transfer rate of both monomer influences the properties of polyarylate only in the fast polycondensation reaction regime. For the TCR with a wider gap somewhat longer residence times will be necessary to receive desirable polyarylate.

Fig. 24. Influence of the monomers molar ratio M0 and of reaction time t on average molecular weight MW . The data are from the experiments of Tsai and Lee [4].

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4. Conclusions

K1–2

On the basis of the analysis of scientific publications related to the polycondensation process, it has been attempted here to present the utilization of the Taylor-Couette-Reactor in the interfacial polycondensation process with the phase-transfer catalysis. The reaction of the polyarylate preparation was chosen as a test reaction, due to the possibility of its use in optical applications. The results of the process carried out in this type of a reactor were presented. It has been demonstrated, following Berezkin and Khoklov’s suggestions [11], that the influence of the initial monomers molar ratio M0 on the properties of the polyarylate (the average molecular weight MW and the polydispersity index PI) can be interpreted by considering two regions. In the first one, for the molar ratio M0 > 1.2, the influence of both monomers molar ratio and mixing intensity (the hydrodynamics of the TCR) has not been observed on the final properties of polyarylates. The molecular weight and the polydispersity index maintain constant values, equal MW ≈ 3000, PI ≈ 1.6. In the second one, for the molar ratio M0 < 1.2, a significant influence of the monomers molar ratio M0 and intensity of mixing on the properties of the product have definitely been observed. Additionally, in this region the impact of parameters such as the process temperature, the mixture flow rate and the annular gap width has become evident. Moreover, the possibility of obtaining a product displaying desirable properties suitable for the production of fibres or polymeric films has been proved. The polymer product of desirable properties can be obtained for the following ranges of the process parameters: 0.7 < M0 < 0.9 and 1210 ≤ Rerot ≤ 1400 for T = 20 ◦ C,  = 60 s and  = 0.93. Considerably shorter residence times, of the order  = 60 s, can be reached in the Taylor-Couette-Reactor in comparison with the reaction times in the Stirred-Tank-Reactor, of the order t = 300–600 s, depending on the used monomers molar ratio M0 .

K2–1

Acknowledgements This study has been carried out within the financial support of the Committee of Scientific Investigations (KBN, Poland) in the frame of scientific grants no.: 1 T09C 003 30. Appendix A. Nomenclature

I II a a C 0 CBPA CBPA,Org CBPA,Aq CI+T CNaOH CPAR CTEBAC d ddrop D1 D2 DBPA,Org k kBPA,Org K

region I region II interfacial area (m2 /m3 ) parameter in the Mark-Houwink’s equation concentration (kmol/m3 ) initial concentration of bisphenol A in aqueous phase (mol/dm3 ) concentration of bisphenol A in organic phase (mol/dm3 ) concentration of bisphenol A in aqueous phase (mol/dm3 ) concentration of mixture iso- and terephthaloyl chlorides in organic phase (mol/dm3 ) concentration of NaOH in aqueous phase (mol/dm3 ) concentration of polyarylate in organic phase (g/cm3 ) concentration of TEBAC in aqueous phase (mol/dm3 ) annular gap width (m) average droplet diameter (m) inner cylinder diameter (m) outer cylinder diameter (m) diffusion coefficient of bisphenol A in organic phase second order reaction rate (m3 /kmol s) mass transfer coefficient of bisphenol A in organic phase parameter in the Mark-Houwink’s equation

L m M M0 MM MN MW n N PI Pn Qm QOrg QAq R1 t T XBPA

overall volumetric mass transfer coefficient of catalyst (m/s) overall volumetric mass transfer coefficient of ionic pair (m/s) length (m) distribution coefficient of ionic pair concentration ratio of used monomers initial molar ratio of iso- and terephthaloyl chlorides to bisphenol A molecular weight of the mer (repeating unit) number average molecular weight weight average molecular weight distribution coefficient of catalyst rotation speed (rpm) polydispersity index polymerization degree volumetric mixture flow rate (m3 /s) volumetric organic phase flow rate (m3 /s) volumetric aqueous phase flow rate (m3 /s) inner cylinder radius (m) reaction time (s) temperature (◦ C) conversion factor of bisphenol A

Greek symbols [] intrinsic viscosity  volume fraction of organic phase  ratio of the inner and outer cylinder diameters of the reactor c viscosity of continuous (aqueous) phase (Pa s) d viscosity of dispersed (organic) phase (Pa s) m viscosity of mixture (Pa s) c density of continuous (aqueous) phase (kg/m3 ) d density of dispersed (organic) phase (kg/m3 ) density of mixture (kg/m3 ) m

interfacial tension (N/m)  residence time (s) ω angular velocity (1/s) Dimensionless groups  Ha = ( kCI+T DBPA,Org /kBPA,Org ) Hatta number Pn = MW /MM = (1 + M0 )/(1 + M0 − 2 · XBPA · M0 ) polymerization degree PI = MW /MN polydispersion index Rerot = ωR1 d m /m rotational Reynolds numbers Rerot,c = ωR1 d c /c rotational Reynolds number for continuous phase  −   2 +   Re ≤ Reinv (inverted structure) c c d Reinv = 1400 Re ≥ Reinv (banded structure) c  − c Reynolds number describing the inverted-banded tran1−  sition where: = c (1 − ) + d ,  = c d , Re = ωR1 d / Subscripts and superscripts 0 initial 1 inner cylinder 2 outer cylinder Aq aqueous phase BPA bisphenol A c continuous phase cor correlation cr critical d dispersed phase drop droplet I+T mixture of iso- and terephthaloyl chlorides

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m NaOH Org PAR TEBAC

mixture sodium hydroxide organic phase polyarylate triethylbenzylammonium chloride

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