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Monitoring Emulsion Polymerization Reactors: Calorimetry Versus Raman Spectroscopy Oihana Elizalde,†,‡ Maider Azpeitia,‡ Marlon M. Reis,§,| Jose´ M. Asua,‡ and Jose R. Leiza*,‡ Institute for Polymer Materials (POLYMAT) and Grupo de Ingenierı´a Quı´mica, Departamento de Quı´mica Aplicada, The University of the Basque Country, M. Lardizabal 3, 20018 Donostia-San Sebastia´ n, Spain, and Universidade de Sa¨ o Paulo, Escola Polite´ cnica, Department of Chemical Engineering, Caixa Postal 61548, CEP D5424-970 Sa¨ o Paulo, SP Brasil
Reaction calorimetry and Raman spectroscopy were simultaneously implemented to monitor high solids content (∼50 wt %) semibatch emulsion polymerization reactions for two comonomer systems: vinyl acetate/butyl acrylate, VAc/BA, and butyl acrylate/methyl methacrylate, BA/ MMA. Overall and instantaneous conversions and free concentration of monomer were measured by means of each technique. It was found that both techniques provided comparable results for the overall conversion, no matter whether the reactions were carried out under starved (BA/ MMA) or nonstarved conditions (VAc/BA). However, instantaneous conversion and free monomer concentrations were better estimated by Raman spectroscopy when the polymerization was carried out under starved conditions. Under nonstarved conditions (higher free monomer concentration and lower instantaneous conversions), Raman spectroscopy also provided slightly better estimates. Introduction Online monitoring of emulsion polymerization reactors is necessary to implement closed-loop control strategies aimed at producing polymer latexes with the required microstructure. In the last three decades, significant effort has been devoted by both academic and industrial research groups to the development of suitable, accurate, and robust monitoring techniques for polymerization reactors. The monitoring of emulsion polymerization reactors might a priori seem easier than that of their homogeneous counterpart reactors because of the lower viscosity of the reaction medium, but it turned out to be much more complex.1,2 Basically, this was due to stability problems associated with the dispersion of polymer particles that led to clogging issues when the latex was pumped from the reactor to the measurement devices or when probes were introduced into the reaction medium. Sensors for the monitoring of several properties (e.g., conversion, polymer composition, molecular weights, particle size, etc.) have been developed and assessed in polymerization reactors. Not all the reported techniques were accurate or robust enough to be implemented in industrial environments. Excellent reviews about the development and applications of online sensors have been published.3-6 The current trend is to use noninvasive techniques in order to avoid the complexity of manipulating viscous systems and reaction media prone to suffer coagulation. Reaction calorimetry7-9 and Raman spectroscopy10-14 are two of * To whom correspondence should be addressed. Tel.: +34 943 015329. Fax: +34 943015270. E-mail:
[email protected]. † Current address: BASF AG Ludwigshafen (Germany). ‡ The University of the Basque Country. § Universidade de Sa¨o Paulo. | Current address: Fundac¸ a¨o Universidade Federal de Rondovia, Chemistry Department, Porto Velho RO 78900-00, Brasil.
the most promising noninvasive techniques to monitor emulsion polymerization reactors. In reaction calorimetry, the measurement is noninvasive, rapid, robust, relatively simple, and cheap because it is based on temperature measurements. As a consequence, it is one of the techniques that can most easily be implemented in industrial environments. Among the different spectroscopic techniques that have been applied to monitor emulsion polymerization reactors, Raman spectroscopy offers several advantages in comparison with the absorption-based spectroscopies: near-infrared (NIR) and midrange infrared (MIR). Thus, water has a very weak signal in Raman spectroscopy, and functional groups that are inactive or very weak in absorption present a strong Raman scattering, i.e., carbon-carbon double bonds. Furthermore, for industrial implementation, Raman spectroscopy offers the possibility of using silica fibers (that have a low cost) with a length of up to 100 m to transmit the radiation to and from the sample. In this work, the performance of these two noninvasive techniques (calorimetry and Raman spectroscopy) to monitor semibatch high-solids-content emulsion polymerizations for two different monomer systems (vinyl acetate/butyl acrylate, VAc/BA, and butyl acrylate/methyl methacrylate, BA/MMA) was compared. Overall and instantaneous conversions, as well as free monomer concentrations, were measured. The polymerizations were carried out under starved conditions (high instantaneous conversions) for the BA/MMA system and under nonstarved conditions for the VAc/BA system. To the best of our knowledge, this is the first time that these monitoring techniques have been compared in emulsion polymerization systems. The article is organized as follows. First, the methods for the calculation of the overall and instantaneous conversions and the unreacted monomer concentrations from the raw data acquired by means of calorimetry and Raman spectroscopy are briefly explained. Then, the
10.1021/ie050451y CCC: $30.25 © 2005 American Chemical Society Published on Web 07/29/2005
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experimental part is detailed. Afterward, the results gathered from both techniques are presented and discussed. In the last section, the advantages and disadvantages of each technique in view of the results obtained and the cost of their implementation are evaluated. Reaction Calorimetry. Reaction calorimetry is based on the monitoring of the heat generation rate due to polymerization. These measurements demand the combination of the dynamic energy balances for both the reactor and the jacket with temperature and flow rate measurements. The energy balance for a stirred tank reactor where both the reactor and the jacket are assumed to be perfectly mixed can be written as follows:
(cpins +
∑j mjcpj)
dTr dt
)
∑i Ficpi(Ti - Tr) - Qf - Ql +
Qr + Qc + Qstir (1)
The first term on the left-hand side is the heat accumulation in the reaction mass and the inserts (e.g., agitator). The first term on the right-hand side member is the heat input due to the feeds, Qf is the heat flow through the reactor wall, Ql is the heat loss to the surroundings, Qr is the heat generation rate due to reaction, Qc is the calibration power, and Qstir is the energy input due to the stirrer. The heat of reaction, Qr, can be estimated from the other terms, if these can be calculated with sufficient accuracy. The exact determination of such terms is a key aspect in reaction calorimetry, with Qf being the most important one. In this work, a commercial lab-scale calorimetric reactor (RC1, Mettler-Toledo) was used. This reactor is a heat flow calorimeter7 whose working principles have been described in the literature.15,16 In the RC1 reactor, Qf is calculated as
Qf ) UA(Tr - Ta)
(2)
where U is the overall heat transfer coefficient, A is the heat exchange area, and Ta is the modified jacket temperature used by the RC1 algorithm to account for the heat accumulated in the reactor wall. Under completely isothermal conditions, Ta ) Tj. The accurate computation of Qf requires the knowledge of the evolution of the overall heat transfer coefficient with time. In this work, the Smart-Calc (Mettler-Toledo) package was used. Smart-Calc implements temperature oscillation calorimetry (TOC) that allows the estimation of both the heat of reaction and the heat transfer coefficient from the oscillatory jacket and reactor temperatures. It is out of the scope of this work to explain the principles of TOC, which can be found elsewhere.17-19 Strictly speaking, the polymerization rate of each monomer and, hence, the individual monomer concentrations are not observable from the overall polymerization heat rate, Qr, but it is possible to infer them using an open-loop observer based on a model of the process.20 Assuming that, for each monomer, the heat of crosspropagation is equal to that of homopolymerization,21 the overall polymerization heat rate, Qr, can be related to the polymerization rates of the individual monomers by the following equation
Qr ) RpA(-∆HA) + RpB(-∆HB)
(3)
where RpA and RpB are the polymerization rates of monomers A and B (mol/s) and (-∆HA) and (-∆HB) are the enthalpies of polymerization of the monomers A and B (J/mol). In emulsion polymerization, the polymerization rate, Rpi, is given by
∑j
Rpi ) (
kpjiPj)[i]p
n j Np NA
i, j ) A, B
(4)
where kpij is the propagation rate constant of a radical having an ultimate unit of type j with monomer i, [i]p is the concentration of monomer i in the polymer particles, Pj is the time-averaged probability of finding a free radical with ultimate unit type j in the polymer j is the average number of radicals per particles,22 n particle, Np is the total number of polymer particles, and NA is Avogadro’s constant. The ratios of the individual polymerization rates can be calculated as follows
f)
rA + [MB]p/[MA]p RpA ) RpB r ([M ] /[M ] )2 + [M ] /[M ] B B p A p B p A p
(5)
where rA and rB are the reactivity ratios of monomer A and B, respectively. The monomer partitioning between the different phases is accounted for by using the partition coefficients and by solving the material balances for the different phases as shown elsewhere.20 The material balances for a copolymer system are as follows
dMi ) Fi - Rpi i ) A, B dt
(6)
where Mi is the number of moles of monomer i in the reactor at time t and Fi is the flow rate of monomer i. Combining eqs 3-6, the material balances can be written as a function of the heat released by polymerization and the reactivity ratios
Qr dMA )dt (-∆HA) + (-∆HB)/f
(7)
Qr dMB )dt (-∆HA)f + (-∆HB)
(8)
The open-loop observer formed by eqs 7, 8, and 5 has been successfully used to estimate the unreacted amounts of monomers in different monomer systems under different conditions.20,21,23,24 Once the unreacted amounts of the monomers are estimated during the course of the polymerization, the overall and instantaneous conversions can be readily estimated for semibatch reactors
Xove )
Xins )
tot Mtot A (t) - MA + MB (t) - MB tot Mtot A + MB tot Mtot A (t) - MA + MB (t) - MB tot Mtot A (t) + MB (t)
(9)
(10)
where MA and MB are the amounts of monomers A and tot B in the reactor; Mtot A (t) and MB (t) are the amounts of monomers A and B fed to the reactor between time 0 tot and t; and Mtot A and MB are the molar (or mass)
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Figure 1. Spectra collected during semibatch emulsion polymerization: BA/MMA (front) and VAc/BA (back) comonomer systems. Table 1. Parameters Used by the Open-Loop Observer to Infer the Monomer Concentrations from the Calorimetric Data BA/MMA
VAc/BA
parameters
n-BA
MMA
VAc
n-BA
ri (-∆Hi) (J/mol) Kpia Kdia
0.41425 7.82 × 104 26 46027 70527
2.2425 5.55 × 104 26 4327 5927
0.03120 8.95 × 104 26 3120 3720
6.3620 7.82 × 104 460 705
a K pi and Kdi are the polymer particles-aqueous phase and monomer droplets-aqueous phase partition coefficients.
amounts of monomers A and B in the formulation. In this work, all the conversions were calculated in mass bases. The open-loop observer requires kinetic and thermodynamic parameters that can be obtained from the literature or from independent experiments. These are the reactivity ratios of the monomers, the enthalpies of polymerization, and the partition coefficients of the monomers between the different phases. Table 1 presents the values of these parameters as taken from the literature for the two comonomer systems used in this work. Raman Spectroscopy. Figure 1 shows the spectra collected during the semibatch emulsion copolymerizations of BA/MMA (front) and VAc/BA(back). Because unique and isolated bands corresponding to each monomer cannot be identified for those systems (the monomers present overlapped bands for the double bonds and carbonyl groups due to the similarities of the chemical structure), univariate calibration techniques cannot be used. Therefore, partial least-squares regression, PLSR, was applied to develop calibration models relating the spectra with the concentration of each monomer. Calibration models were built for each system. For the BA/ MMA system, the PLSR calibration models were built using seeded semibatch emulsion polymerization reactions carried out at 70 °C and with a 50/50 (wt %) monomer composition. The whole spectra range was used, and gravimetry and gas chromatography were
used as reference techniques. Spectra normalized to the unit area were used to build the models.28-30 For the VAc/BA system, spectra collected from semibatch emulsion polymerizations carried out at 70 °C with a composition 81/19 (wt %) were employed to develop the calibration models. Normalized spectra were used. The normalization was performed by dividing each element of the spectrum by the element corresponding to the frequency 415 cm-1. The spectral ranges used were 3026-2733 cm-1 and 1846-419 cm-1. Gas chromatography was used as the reference technique to fit the calibration models. Because of the significantly higher reactivity of the butyl acrylate monomer and the low fraction of BA in the feed composition, the concentration of BA was very low in the VAc/BA emulsion copolymerizations. The instantaneous and overall conversions were calculated from the free monomer concentrations predicted by the PLS models using eqs 9 and 10. Experimental Section Polymerization Reactions. Deionized water was used in all polymerizations. All reactants, monomers (nBA, MMA, VAc and AA, and Quimidroga), emulsifiers (Dowfax 2A1, sodium dodecyl diphenyloxide disulfonate, Dow; AlipalCo-436, polyethoxylated nonylphenol ammonium salt, Rhodia; and ArkopalN-230, poly(ethylene glycol) monononylphenyl ether, Hoescht), and the initiator (K2S2O8, Fluka) and the buffer (NaHCO3, Panreac) were used as supplied. For both copolymerization systems, seeded emulsion polymerizations were carried out. In some cases, the seeds were prepared in situ, and in other cases, they were prepared previously and added in the initial charge. For the sake of brevity, the seed recipes are not included. The formulations given in Tables 2 and 3 were used for the comparison of the performance of the monitoring techniques. They are typical formulations for the semibatch processes. Table 3 shows that the amount of AA was very small. During polymerization, the concentration of AA was below the detection limit for both calorimetry and Raman spec-
Ind. Eng. Chem. Res., Vol. 44, No. 18, 2005 7203 Table 2. Typical Recipe for the Seeded Semibatch Emulsion Copolymerization of BA/MMA product (g)
initial charge
water Dowfax K2S2O8 NaHCO3 n-BA + MMA n-BA/MMA seed
stream 1
617
stream 2 32.55 9.45
1 or 2 1 or 2 540 50/50 and 90/10 63
Table 3. Typical Recipe for the Seeded Semibatch Emulsion Copolymerization of VAc/BA product (g)
initial charge
water Alipal Arkopal K2S2O8 NaHCO3 VAc n-BA AA seed
617 6.13 6.13 2.29 2.52
stream 1
stream 2 99.53 6.13 6.13
600 141.1 22.93 527
Table 4. Semibatch Emulsion Polymerizations Carried out for the Two Monomer Systems to Assess the Performance of Reaction Calorimetry and Raman Spectroscopy run
comonomers (ratio)
feeding time (min)
initiator (g)
final solids (%)
1 2 3 4 5 6
VAc/BA (81/19) VAc/BA (81/19) BA/MMA (50/50) BA/MMA (50/50) BA/MMA (50/50) BA/MMA (90/10)
180 240 180 180 120 180
2.29 2.29 1 2 1 1
55 55 50 50 50 50
troscopy. Table 4 shows the experimental conditions of the 6 reactions used in this work. The reactions were carried out at 70 °C using a turbine impeller and a stirring speed of 400 rpm. In both systems, the ingredients not included in the initial charge were fed into the reactor in two streams. One stream contained the monomer mixture (neat monomer addition), and the other contained an emulsifier solution. In both cases, all the initiator was included in the initial charge. Both streams were added at a constant flow rate for 120 and 180 min for the BA/MMA reactions and for 180 and 240 min for the VAc/BA reactions. N2 was purged continuously during the reaction to keep an inert atmosphere. Reactor Setup. The experimental setup was a commercial calorimetric reactor (RC1, Mettler-Toledo) equipped with a stainless steel HP60 reactor. The reactor was equipped with a Raman dip-probe (IR 361S5, Bruker) that was directly immersed in the reaction mixture, making possible in situ measurements along the reaction. The spectrometer used in the experiments was a near-infrared Fourier transform Raman spectrometer, RFS 100/S (Bruker), equipped with a 1064 nm wavelength Nd:YAG laser, with a maximum power of 1.5 W. The spectral coverage was from 50 to 3500 cm-1, corresponding to the Stokes interval. The spectrometer uses a germanium detector (D481-TU) optimized for FT-Raman measurements that is cooled with liquid nitrogen. The Raman dip-probe was connected to the equipment through two optical fibers (5 m long), one to carry the laser signal to the reaction mixture and the other to carry the scattered radiation back to the spectrometer.
Figure 2. Instantaneous conversions for the semibatch emulsion copolymerizations used to compare Raman spectroscopy and calorimetry: (top) BA/MMA, runs 3-6; (bottom) VAc/BA, runs 1-2.
For the BA/MMA copolymerizations, Raman spectra were collected from the start of the reaction at regular intervals of 10 min. Each spectrum consisted of the accumulation of 200 scans and was taken with a spectral resolution of 4 cm-1, using a laser power of 1000 mW. Considering the time required for switching the laser on and off before and after each measurement and the data acquisition time, the time required to get the Raman measurement was approximately 4 min. For the VAc/BA copolymerizations, spectra were collected every 3 min. In this case, each spectrum was an average of 128 scans (collected over 2-1/2 min) with a resolution of 8 cm-1 and a laser power of 1000 mW. In both cases, a sample of latex was taken from the reactor in the middle of the spectra collection for gravimetry and gas chromatography analyses, which were used as reference standard off-line techniques. Off-Line Measurements/Reference Methods. a. Overall and Instantaneous Gravimetric Conversion. The overall and instantaneous conversions of monomer to polymer (based on the total monomer on the recipe and on the monomer fed, respectively) were determined by gravimetric analysis. For these analyses, immediately after collecting the samples, the reaction was quenched by addition of an inhibitor (hydroquinone). b. Residual Monomer. The amount of residual monomer in the reactor was measured by means of gas chromatography (GC). A gas chromatograph (Shimadzu
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Figure 3. Comparison of the overall conversions measured for the VAc/BA system (runs 1-2) with gravimetry: Raman spectroscopy (top) and calorimetry (bottom).
Figure 4. Comparison of the instantaneous conversions measured for VAc/BA polymerizations by means of gravimetry (points), Raman spectroscopy (dashed line), and calorimetry (solid line): (a) run 1 and (b) Run 2.
GC-14A) with a flame ionization detector (FID) and an integrator (Shimadzu C-R6A) was used. Results and Discussion Figure 2 shows the evolution of the instantaneous conversions (as calculated by gravimetry) for the seeded semibatch experiments carried out for each comonomer system. It can be seen that the trend was similar in both cases; namely, there was an accumulation of monomer during the first stages of the process, and then the conversions were roughly steady up to the end of the feeding period. Beyond this point, the conversions
Figure 5. Comparison of the free VAc measured for VAc/BA polymerizations by means of gas chromatography (points), Raman spectroscopy (dashed line), and calorimetry (solid line): (a) run 1 and (b) run 2.
Figure 6. Comparison of the overall conversions measured for the BA/MMA system (runs 3-6) with gravimetry: Raman spectroscopy (top) and calorimetry (bottom).
increased as the process proceeded batchwise. The conversions reached during the plateau were >80% for BA/MMA and
