Ind. Eng. Chem. Res. 1993,32, 1831-1838
1831
Modeling and Experimental Studies of Aqueous Suspension Polymerization Processes. 2. Experiments in Batch Reactors George Kalfas, Huigen Yuan, and W. Harmon Ray' Department of Chemical Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706
The results from extensive studies of aqueous suspension homo- and copolymerizations with methyl methacrylate, styrene, and vinyl acetate in a lab-scale batch reactor are presented. Simulation results from homogeneous and two-phase free-radical polymerization kinetics models, using physical and kinetic parameter values from the bulk and solution polymerization literature were found to be in good agreement with batch suspension experimental results. The evolution of the particle size distribution was also observed and measured. The results suggest that polymerization kinetics and liquid-liquid dispersion phenomena can be decoupled in the cases of low dispersed phase fraction (4 < 0.25).
Introduction
Table I. MMA-Water Mutual Solubility (Luskin, 1970- 197 1)
In aqueous suspension polymerization systems one or more water-immiscible liquid monomers containing oilsoluble initiator are dispersed into droplets of 0.1-5 mm in diameter by a combination of strong stirring and the use of small amounts of water-soluble stabilizers (suspending agents). The stabilizers hinder the coalescence first of monomer droplets and then of the polymer particles whose tendency to agglomerate may become critical when the polymerization has advanced to the point where the polymer particles become sticky (-20% conversion).The monomer-to-water ratio ranges from 1:l down to 1:4 in most commercial processes. If the monomer is a solvent for the polymer the particles pass through aviscous syrupy state and finally form small and clear spheres (beads). This is termed bead polymerization. If the polymer is insoluble in the monomer, then a precipitation polymerization occurs in each droplet and opaque, irregular grains or powders are formed. This case is described as powder suspension polymerization (Yuan et al., 1991). Suspension polymerizations usually are conducted at 40-90 O C under atmospheric pressure or up to 160 "C at elevated pressures since operation must always be below the boiling point of the continuous phase. Initiators used in suspension polymerization are oil-soluble, and polymerization takes place within the monomer droplets. Typical initiator loads are 0.1-0.5 w t % based on monomer. A simple calculation shows that monomer droplets are large enough to contain a very large number of free radicals ( 108). Therefore the kinetic mechanism is the same as that of bulk polymerization (Munzer and Trommsdorff, 1977),and the same kind of dependence of the polymerization rate on initiator concentration and temperature is observed. Bead suspension processes are considered to be water-cooled micro-bulk polymerizations in the monomer droplets, while powder polymerization may be considered to be a water-cooled micro-precipitation polymerization. Droplet size is determined as a result of a dynamic equilibrium between breakage by shear or turbulence forces and coalescence by surface tension or adhesion forces. The formation of a stable droplet dispersion usually requires a combination of stabilizing actions (agitation, suspendingagents, adjustment of the carrier phase density andlor viscosity). Particle size is controlled by agitation and the amount of stabilizer(s) and/or surfactants. In N
*To whom correspondence should be addressed. Q888-5885/93/2632-~831$04.QQlQ 0
temp ("C) 60 70 80
monomer in water (wt %) 1.49 1.60 1.80
water in monomer (wt %) 2.07 2.38 2.74
Table 11. VAc-Water Mutual Solubility (Leonard, 1970-1971: Lindemann. 1967: NaDmer and Parts. 1962) ~~~
temp ("C)
20 50 70
monomer in water (wt %) 2.5 2.9 3.5
water in monomer (wt %) 0.1 1.7
some cases volume reduction due to polymerization is significant. Monodisperse, narrow, broad, and bimodal particle size distributions have been reported in suspension polymerizations. Since bead suspension polymerizations are considered to be water-cooled micro-bulk polymerizations, homogeneous models of free-radicalpolymerizationkinetics should be able to describe batch suspension reactors with no modification. However, some of the monomers that are polymerized with suspension processes are partially water soluble. In homo- and copolymerizations of these monomers (vinyl acetate, acrylonitrile)interesting results which deviate from the bulk polymerization behavior have been reported. Monomer solubility seems to have noticeable effects on the polymerization rates, the composition, and the molecular weights of suspension polymers (Bahargava et al., 1979;Mino, 1956;Taylor and Reichert, 1985). In the first part of this work we have presented an extension to our homogeneous free-radicalpolymerization model in order to consider a second inert phase, the partition of the monomer(s) in the two phases, and the monomer transport between the two phases (Kalfasand Ray, 1992). In this second part we present experimental results from our work in batch suspension reactors using monomers with increasing water solubility and compare our experimental results with the simulation predictions. By including the role of the second phase in our models, we are able to quantitatively fit the experimental results without modifications in the established values of the physical and kinetic parameters for each of the monomers. We investigated polymerizations of styrene, methyl methacrylate (MMA), and vinyl acetate (VAc). Styrene
1993 American Chemical Society
1832 Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993 Nitrogen Row
Condenser
H Manual valve
'hernostated bath
-
Figure 1. Batch suspension polymerization reactor setup.
isalmost totally insolublein water. Themutualsolubilities of the other two monomers with water are given in Tables I and 11. Experimental Procedure The batch suspension polymerization experiments were performed in a 1-Ljacketed glass reactor (i.d. = 10.2 cm) fitted with an impeller stirrer, reflux condenser, nitrogen inlet, and a thermocouple. To ensure an inert atmosphere andtoprevent inhibitioneffectsfromoxygen,a continuous flow of nitrogen purged the reactor at least half an hour before and during the course of the reaction. The experimental setup is shown in Figure 1. The impeller used is of the curved blade type, made from stainless steel. This design prevents high shear stresses at the impeller's tip as well as undesirable vortex formation. A variable speed Lightnin LabMaster DSlOlOstirrerdriverwas used. The stirrer rotating speed was easily controlled within 5 rpm or less. A simple PI feedback control scheme (Figure 1) where cold water is let into the heating circuit from an onloff solenoid valve was found adequate in order to keep the reactor temperature within 0.5 O C of the set point. Significant deviations from the desired temperature were observed only during startup, because cold monomer was suddenly introduced, and after completion of the reaction, because heat generation stops almost instantly. In batch suspension polymerization experiments we investigated the effect of five experimental variables: temperature, initiator concentration, suspending agent concentration, dispersed phase uolume fraction, and stirring speed on the conversion profiles and on the molecular weight, droplet and particle size distributions. Experiments were performed for three homopolymerization cases, methyl methacrylate (MMA), styrene (Sty), and vinyl acetate (VAc), and one copolymerization case, MMA and styrene. The initiator in all cases was benzoyl peroxide (BPO) purified by precipitation with methanol out of a chloroform solution. Mixtures of three Suspending agentswereused poly(viny1alcohol)(PVA),hydroxyethyl cellulose (HEC), and tribasic calcium phosphate (TCP). All chemicals were obtained from Aldrich, except methyl methacrylate, which was provided by Rohm & Haas. After an initial period of test runs in order to establish the appropriate standard recipe for each case (Table 110, five sets of experiments were designed (i) MMA homopolymerization set C (Table IV); (ii) styrene homopolymerization set D (Table V); (iii) VAc homopolymerization set E and set HD (Table VI and VII); (iv) Sty-MMA copolymerization set M S (Table VIII).
Table 111. Reeipes for the Batch Suspension Polymerization Experiments Standard Condition Recipe for MMA Suspension Polymerization (C-02) water 350g HECD 2.lg HEC/water 0.6% PVA' 0.018g PVNwater 0.005% MMA 70g BPW 0.35g BPO/MMA 0.5% MWwater 0.2 temp 75'C stirringspeed 400rpm Standard Condition Recipe for Styrene Suspension Polymerization (D-01) water 5OOg TCPd 0.25g TCP/water 0.05% PVA 0.025g PVNwater 0.005% styrene 100g BPO 1.00g BPO/Sty 1.0% styrene/water 0.2 temp 90°C stirringspeed 800rpm Standard Condition Recipe for VAc Suspension Polymerktion (E03) water 5OOg PVA 0.050g PVNwater 0.010% VAc l00g BPO 0.50g BPO/VAc 0.5% VAc/water 0.2 temp 70°C stirringspeed 5OOrpm Standard Condition Recipe for Sty-MMA Suspension Copolymerization water 5OOg TCP 0.25g TCP/water 0.05% PVA 0.025g PVA/water 0.005% Sty+MMA l W g BPO 1.00g BPO/monomer 0.5% monomer/water 0.2 temp 85 'C stirring speed 800 rpm a HEC: hydroxyethyl cellulose [mp 288-290 "C1 (suspsnding agent). PVA poly(vinyl alcohol) 188% hydrolyzed, average MW = 78 0001 (emulsifier). "PO benzoyl peroxide [purified by precipitation](initiator). d T C P tribaaicealciumphasphate(suspending agent).
Table IV. Exwrimental Conditions for MMA Rnns (Set C )
C-07 c-08 C-09 c-10
c-11 c-12 C-13 C-14 C15 C-16 a
75 75 75 75 75 75 75 65 70 80
0.5 0.5 0.5 0.5 0.3 0.4
0.6 0.5 0.5 0.5
0.6 0.4
0.8 1.0 0.6 0.6 0.6 0.6 0.6 0.6
500 400 400 400
0.20 0.20
400
0.20 0.20 0.20
400 400 400 400 400
0.20 0.20
0.20
0.20 0.20
Monomer-to-water ratio.
Table V. Experimentnl Condition6 for Styrene Runs (Set n\
D-07 D-08
90 85
1.00 1.00
0.15 0.6HEC
800
800
0.20 0.20
During the suspension polymerization of partially water soluble monomers small amount of emulaion polymer (latex) isalsoformed. Theseparationoftheaqueousphase from these emulsion size (- 10-100 nm) polymer particles is extremely difficult. Recycling and use of this water reduces the stabilizing action of the suspending agent. A few patents have been published on methods for the wastewater treatment in such cases. In one of those (Hoechst AG) 2-200 ppm of aqueous soluble initiator is
Ind. Eng. Chem. Res., Vol. 32,No. 9,1993 1833 Table VI. Experimental Conditions for Vinyl Acetate Runa (Set E) stirring run T(OC) BPO(%) PVA(%) smed(mm) R E-01 E02 E-03 E-04 E-05 E-06
60 70 70 70 70 60
0.50 0.50 0.50 0.50 0.50 0.50
0.01 0.01 0.01 0.01 0.01 0.01
600 600
600 600 600 600
0.20 0.20 0.20 0.20 0.20 0.20
1.0
0.5
$ Do6 DU7 Fa1
0.1
Table VII. Experimental Conditions for Vinyl Acetate Runs (Set HD) run T(OC) BPO(%) PVA(%) HDO(%) R HD-01 60 0.50 0.005 0.0 0.20 HD-02 60 0.50 0.005 0.00 0.10 HD-03 60 0.50 0.010 0.0 0.20 HD-04 60 0.50 0.010 0.0 0.10 60 0.50 0.005 2.0 0.20 HD-05 60 0.50 0.005 2.0 0.10 HD-06 60 0.50 0.010 2.0 HD-07 0.20 HD-08 60 0.50 0.010 2.0 0.10
l.~O.lO%
%lac
90°C 1.00% 0.15% 90°C 1.00% 0.05%
Time (min)
Figure 2. Reproducibility of styrene suspension polymerization experimenta at constant [I] = 1.00 wt % and T = 90 O C . Run F-01 is a duplicate of run D-01.
I
c41
I
r P
I
4 HD: hexadecane with Aldrich was used as a swelling agent in order to reduce monomer solubility in the water.
Table VIII. Experimental Conditions for Styrene-MMA Runs (Set MS) stabilizer stirring styrene/ run T(OC) BPO(%) (%) speed(rpm) MMA MS-01 85 0.50 0.05TCP 800 91 85 0.50 0.05TCP 800 82 MS-02 85 0.50 0.05TCP 800 7:3 MS-03 0.50 0.05TCP 800 64 MS-04 85 MS-05 85 0.50 0.05TCP 800 5:5 MS-06 85 0.50 0.05TCP 800 4:6 MS-07 85 0.50 0.05 TCP 800 3:7 MS-08 85 0.50 0.60 HEC 800 28 MS-09 85 0.50 0.60 HEC 800 1:9
added making possible the recycle of 50% of the aqueous phase (Deiringer, 1985). In our polymerization experiments with vinyl acetate approximately 1-2% of the monomer was converted to emulsion polymer.
Experimental Results Conversion Profiles. The monomer conversion was calculated by gravimetric analysis of samples taken from the reactor at the chosen sampling times by pipettes (0.8mm i.d.) and put into aluminum dishes containinginhibitor solution (hydroquinone). Toward the end of the reactions representative sampling of the suspended particles became more difficult. Despite the agitation in the reactor, polymer particles tended to settle to the bottom of the reactor, because the final particle density is significantly higher than the density of water ( p p ~ w =1.19 g/cm3). Figure 2 shows fair experimental reproducibility for the styrene conversion profiles at the same temperature and initiator level. It also shows that monomer-to-waterratio, stabilizer concentration, and stirring speed did not have a significant effect on the polymerization rate. There is also no clear effect of these three factors in the MMA polymerizations. Results from nine runs at the same temperature and initiator load are shown in Figure 3.The scatter of the experimental points is greater at the onset of gel effect, where a maximum in polymerization rate is observed. The curves in Figure8 2 and 3 represent computed predictions from our free radical homogeneous (HFR) and two-phase (SUHFR) simulation programs using kinetic and physical parameter values taken from bulk and solution polymerization literature. The two-
0.0
o
'";'""";"'; a
~
~
~ " ~ " ' ; " ' ; ' . ' " " e
~
m
i
~
~
Time (min)
Figure 3. Reproducibility of MMA suspension polymerization experimenta at constant [I] = 0.5 wt % and T = 75OOC.
phase model (SUHFR)agrees better with the experimental results since it accounts for the dissolved monomer in the aqueous phase and the dilution effect of the water in the organic phase. Both programs are part of POLYRED (Chriatiansen et al., 1990),a CAD package for the design and analysis of polymerization reactors, developed in the University of Wisconsin-Madison. Overestimation of the polymerization rate by the bulk and solution (homogeneous model) simulation program at the onset of the MMA gel effect (shown by the dashed curve in Figure 3)is attributed to the MMA-water mutual solubility. The observed polymerization rate is reduced for two reasons: the amount of monomer dissolved in the aqueous phase stays inert and does not polymerize until it is transported back to the polymerizing droplets, and the presence of water in the monomer droplets leads to a lower active monomer concentration than that observed in bulk polymerizations. Using the suspension (two-phase) model, a better agreement was obtained between simulations and experimental results in MMA polymerizations (see solid curve in Figure 3,and curves in Figures 6 and 7). The gel effect factors in MMA polymerization simulations are calculated based on the free-volume gel effect correlations (Ross and Laurence, 1976). Physical and kinetic parameter values used in the simulations are given in the Appendix. Computer simulations from the homogeneous model (HFR) without any adjustments in the parameter values were found in good agreement with the experimental results from the conversion measurements for styrene polymerizations (see Figures 4 and 5). Stronger evidence of monomer mass-transfer limitations was observed in vinyl acetate suspension polymerization. Conversion profiles in vinyl acetate polymerization experiments at two different monomer to water ratios are
m
i
1834 Ind.
0
Eng.Chem. Res., Vol. 32, No. 9, 1993
3l
el
120
93
210
150180
240
Time (min)
0
,
0
Figure 4. Effect of the initiator concentration on the conversion profiles in styrene suspension polymerization experiments. 1.0
0
I
.
-0
/
~
~
I"
'
'
'
I
1
Eo
43
'
'
;
'
'
'
'
"
"
'
I 240
200
160
120
Time (min)
Figure 8. Experimental conversion profiles from VAc suspension polymerizations along with simulation predictions from the homogeneous (HFR) and the suspension (two phase) reactor model (SUHFR). The effect of monomer-to-water ratio on the conversion profile is obvious. In the suspension model simulations the masstransfer coefficient was set to zero. 84
10-
-82
os-06-
04-0
30
el
93
120 150 Time (min)
lrn
210
240
\
J
_
_
Figure 5. Effect of the temperature on the conversion profiles in styrene suspension polymerization experiments. 0
3l
el
93
120
150
Time (min)
Figure 9. Experimental and simulated conversion profiles from nonisothermal MMA suspension polymerization along with simulation predictions from the homogeneous (HFR) and the suspension (two phase) reactor model (SUHFR). In the suspension model simulation the monomer mass-transfer coefficient was set to zero.
Time (min)
Figure 6. Effect of the initiator concentration on the conversion profiles in MMA suspension polymerization experiments.
120
140
im
t80
Time (min)
Figure 7. Effect of the temperature on the conversion profiles in MMA suspension polymerization experiments.
shown in Figure 8. These profiles are not in agreement with the profiles predicted by the homogeneous polymerization model (HFR). Repeated experimental runs at these low monomer-to-water ratios verified the reproducibility of our observations.
The simulations of these polymerization experiments with the mass transfer coefficient set to zero agree reasonably with the experimental results as it was pointed out in part 1of this series. This means that the dissolved monomer remains in the aqueous phase and does not return to the polymer particles at high conversion. To test this hypothesis, an experiment was performed during which we added aqueous soluble initiator (sodium persulfate 0.125 w t % based on water) after 3 h of reaction with the oil-solubleinitiator. This resulted in 97 % conversion,thus indicating that the monomer was still dissolved in the aqueous phase, after polymerization in the particles was complete. Therefore, interphase monomer transport which attempts to reestablish the liquid-liquid equilibrium between the aqueous and the organic phase seems to be dramatically hindered. Fair agreement between simulation and experiments was also obtained during nonisothermal MMA runs (Figure 9). The temperature profile measured during the experimental run M-02 was fed as a temperature program in the computer simulation. The dashed conversion curve corresponds to the homogeneous model, whereas the solid one corresponds to the two-phase model with the masstransfer coefficient for the monomer set to zero. The conversion profiles from the styrene-MMA copolymerization experiments spanning the full range of monomer compositions are shown in Figure 10. For monomer mixture compositions richer than 70 % in MMA, calcium phoshate (TCP) could no longer offer adequate stabilization of the suspension. Hydroxyethyl cellulose
Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993 1835 ,-
.-,
..... ...............-..'.- ._-... __-.. :t ; ,
N
f
.'
I .
/
6
, , t
,'E
.a
t
I ,
j
0
;
'
"
;
'
tn
49
"
" I ~
"
MMA- 30% . . ' I UMA- 50% MMA- 70% .._...MMA- 90%
Mss
'
'
'
lb
120
&d
__
MS7 8
ii
-MMAI 10%
Ms1 Ms3 Ms5
0
3b
/I
/
/
~
'
~
.
ab
Time (min)
A
12
1 I
Figure 10. Comparison of simulation and experimental conversion profiles in styrene-MMA batch suspension copolymerization. 600
^jt"
-DPdI=1.00%,T=WC)
I
Figure 13. Evolution of the polymer chain length distribution in MMA batch suspension polymerization (run C-16).
/
0.50
-
0.45
'E 0.40
Run C-14
'\
MMABatchSuapension PolymerizationExperiments
110'
'
t
4
00
0.2
0.4
0.6
0.8
1.0
Monomer hactionalConversion
M
Figure 11. Comparison of simulation and experimental number average chain lengths in styrene batch suspension polymerization.
*,
., 4
mi\ 1I
C-11
0.0
0.2
0.4
0.6
75'C
0.3%
0.8
1.0
Monomer FractionalConversion
Figure 12. Comparison of simulation and experimental numberaverage chain lengths in MMA batch suspension polymerization.
(HEC) was used instead. Using our suspension copolymerization model (SUCFR) with homopolymer kinetic and physical constants for MMA and styrene, and reactivity ratios from the literature (O'Driscoll et al., 1984), simulation results were found to fall close to the experimental conversion profiles. Chain-Length Distributions (CLD). Gel permeation chromatography (GPC) was used to obtain chain-length distributions (CLD) and distribution averages for our suspension polymer samples. Both refractive index and UV absorbance detectors were used during the GPC runs performed with DMA as the solvent, a t 50 OC, and a flow rate of 0.5 mL/min. Two calibration curves, one for each polymer, were constructed using polystyrene and PMMA standards correspondingly. Again the experimental results from styrene and MMA isothermal batch suspension polymerizations were found in good agreement with the simulation predictions from our programs (Figures 11 and 12). The evolution of the weight chain length distribution (WCLD) with reaction time during the course of exper-
DropletparbideDiameter (pm)
Figure 14. Time evolution of the droplet size distribution (DSD) in the first few minutes in MMA batch suspension polymerization (run C-ll).[log-normaldistribution fib].
iment C-16 is shown in Figure 13. One can easily notice the pronounced change in the WCLD, when the gel effect kicks in between the 43- and 48-min samples. Both very long and very short chains are produced causing the broadening of the distribution. The polydispersities were in the range 2.5-4.5 with reasonable agreement between model and experiment. Droplet Size Distributions (DSD). Average particle size and particle size distribution of the final product from suspension polymerization reactors is important for certain applications. Keeping constant the average particle size during reactor scaleup is a challenging practical issue. Prediction and control of the final particle size distribution is associated with the evolution of the droplet size distribution of the monomer dispersion in the aqueous continuous phase. The droplet size distribution (DSD)evolution with time was observed by taking photographs of samples from the reactor with a stereoscopic zoom microscope (Nikon SMZ2T). About 200-300 droplets from each photograph were measured. [The droplet diameter was measured using an image-analyzing program for Macintosh computers, Image NIH, a public domain program from the National Institute of Health. The pictures were scanned by an Apple Scanner and were saved in TIFF format, to be read by Image NIH.] From the measured droplet diameters statistical averages were calculated. Normal and log-normal distribution fits were consequently constructed. log-normal distributions were in most cases closer to the raw shape of experimental DSDs. The time evolution of these lognormal fits for the run C-14is shown in Figure 14 along with the final particle size distribution (PSD). Both the particle diameter probability density determined experimentally by sieving and the calculated log-normal fit are
1836 Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993 C.13: Sauter Diameter
-R-
(2-13 Conversion
- - . e . .C.14: Sauter Diameter
-m-
C-14 Conversion
0
\ \
ParticleDiameter (pulun)
Figure 17. Effect of the stirring speed on the final particle size distribution (PSD) in MMA batch suspension polymerization.
'
om--
MMABatChsuepydm
ts
\
,
0.0
0
000
&
'
'
'
'
'
;
'
'
- -_. T . 7 14
1
I
m
~,
/
a
m
I
W
I
J
4
m
m
B
x
)
m
m
Particle Diameter (p)
,-; -
o 1 5 o 3 ~ 4 5 o m 7 m 9 ~ i o 6 o 1 2 a o Particle Diameter(pm)
Figure 18. Effect of the reactor temperature on the final particle size distribution (PSD) in MMA batch suspension polymerization.
Figure 16. Final particle (bead) size distribution (PSD) in MMA batch suspension polymerization (run C-13).
plotted. It is obvious that the droplet size stops changing very early in the reaction. In Figure 15, the Sauter (volume average) droplet diameter is plotted as a function of reaction time. The time was measured from the moment the monomer was introduced into the preheated aqueous phase. The conversion profiles are also plotted. The droplet diameter stops decreasing at about 5 % conversion in both experiments. Since average droplet size reaches an equilibrium state very early during the reaction course, droplet dispersion phenomena (mainly breakage in these low monomer-to-water ratio cases) can be decoupled from polymerization kinetics. At the higher monomer-to-water ratios usually employed in industrial processes,the droplet coalescencerate becomes significant. The viscosity of the dispersed phase affects the droplet breakage and coalescence rates, and it should be accounted for in the model (Alvarez et al., 1990; Konno et al., 1982). The droplet viscosity is determined from monomer conversion,average chain length, and branching which are governed by reaction kinetics. Therefore, in these cases the modeling of the particle size distribution evolution should be coupled to the reaction kinetics model. Particle Size Distributions (PSD). After drying of the final product, the particle size distributions of the beads were constructed by sieving (standard ASTM sieving method D1291-87) and recording the weight of each size fraction. The number fraction of beads with diameters between d, and d,+l and finally the probability density of a particle to have diameter between di and d, + Ad, was calculated. These results for run (2-13are plotted in Figure 16 along with a calculated normal and log-normal fit to the first two moments of the experimental distribution. The data shown in Figures 17-20 illustrate how some of the operating parameters (stirring speed, suspending
m
3 "-!
1 =-:
.
.
MMABatchsuspeasion -tion
Eqwmenb
---
4
E "-:
a
4
Q - - - - -
---__
Q
O
I
----------*--------_
m-: am
" " " " " " ~ " " ~ " ' " ' ' ' ~ ~ " " ~
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Ind. Eng. Chem. Res., Vol. 32,No. 9,1993 1837 towards smaller sizes (Figure 17). The effect of temperature in Figure 18 is more complicated. At lower temperatures the particle size distribution is controlled by the viscosity of the dispersed phase (aswell as the aqueous phase) at a fixed agitator speed and thus lower temperatures favor larger particles. However, at 80 "C, the reaction rate is so rapid that the viscosity of the dispersed phase is increased before the droplets reach their equilibrium size (cf. Figure 15) and the particle size increases with temperature. A logarithmic plot of the average particle diameter vs the stirring speed shows an exponential dependence with the exponent of the stirring speed taking a value close to -1.4 (Figure 19). Isotropic turbulence theory for liquidliquid dispersions predicts a value of -1.2 (Tavlaridis and Stamatoudis, 1981), but the effect of dispersed phase viscosity is not accounted for in the development of this theory. Hoppf et al. (1964a,b) report an exponent of -1.5 f 0.2 from similar experiments. Langner et al. (1980) suggest an average exponent of -1.3 for styrene and vinyl acetate polymerizations with PVA as suspending agent. Finally increasing the hydroxyethyl cellulose concentration produces an almost linear decrease in average diameter of the final PMMA beads as shown in Figure 20.
(i) Styrene:
-AH,[cal/moll = 17,500 (at 25 "C) pM
[g/cm31 = 0.9193 - 6.65 X 104(T- 273.15)
pp [g/cm31 = 0.9926 - 2.65 X c,M
[cal/(g/K)] = -6.747
104(T- 273.15)
+ 1471 X 10-'T-
9.609 X 1 0 4 P - 2.373 X l 0 - T
cpp [cal/(g/K)] = 0.283 - 0.966 X lo3 T
Rate constants (Mahabadi and O'Driscoll, 1977): propagation a t zero conversion [L/(mol/s)l: k; = 1.09 X lo' exp(-7050/RT) k , = lo%,
transfer to monomer [L/(mol/s)]:
termination at zero conversion [L/ (mol/s)]: k: = 1.70 x lo9 exp(-2268/RT)
Conclusions This paper presents the results of an extensive investigation of batch (bead) suspension polymerizations including examples using three monomers (MMA, styrene, and VAc). Measurements include conversion profiles, chain-length distributions, droplet size distribution evolution, and final particle size distributions. The experiments were designed with the aid of our freeradical simulation programs, and the results were compared to the model predictions using values for the physical and kinetic parameters from the polymerization literature. When the solubility of the monomer in the aqueous phase is negligible (as it is for styrene), quantitative predictions can be obtained for batch polymerizations using the models and rate constants corresponding to homogeneous (bulk or solution) polymerization. However, in the case of monomers which are partially dissolved in the aqueous phase, a quantitative fit between the experimental results and the simulation predictions requires the use of a twophase model which takes into account the mutual solubility of monomer and water in each phase. At low monomer-to-water ratios the liquid-liquid dispersion equilibrium is attained very early in the reaction course, and droplet dispersion processes can be modeled independently from the polymerization kinetics.
Acknowledgment
g, = 1.0
g , = exp{-0.4404Xp- 6.362Xt - 0.1704X,3]
X,
(cMO
- cM)/cMO
where CM is the current monomer concentration in the reactor and CMO is the monomer concentration at zero conversion at same reactor conditions (ii) Methyl Methacrylate:
-AHp [cal/moll = 13 800 (at 25 "C) pM
[g/cm31 = 0.9654 - 1.09 X 103(T
- 273.15) - 0.97 X 104(T - 273.15)'
p p [g/cm31 = 1.18 - 0.1 X 10-'(T- 273.15)
cPp [cd/(g/K)]
= 0.3151
+ 9.55 X lo4(?"-
273.15)
Rate constants (Mahabadi and O'Driscoll, 1977):
The authors are grateful to the National Science Foundation and the Industrial Sponsors of the University of Wisconsin Polymerization Reaction Engineering Laboratory for their support of this research. We wish to thank Ms. Marguerite Essenther for her valuable help in the experimental work and also Prof. S. L. Cooper and his group, whose GPC instrument was used for the chainlength distribution measurements.
Appendix: Physical and Kinetic Parameters in the Simulations Initiator (BPO) decomposition [s-'1: kd = 1.7 exp (-30 OOO/RT)
Gel effect correlation (Hamer, 1983):
X
1014
propagation at zero conversion [L/(mol/s)]: k; = 4.92 X 10' exp(-4353/RT) transfer to monomer [L/(mol/s)l: k,, = 0.893
X
lo3 exp(-2240/RT)
termination at zero conversion [L/(mol/s)l: k t = 9.8 X lo' exp(-701/RT) disproportionation to combination ratio: k i / k k = 2483 exp(-4073/RT)
1838 Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993
Literature Cited
Free-volume gel effect correlation (Schmidt and Ray, 1981): if V,
> Vtpc:
if V, IVat: if V,
> V:,
if Vf IV:,
gp= 1.0
g, = 0.71 X 10'' exp{171.53Vf] g, = 0.10575 exp{(17.15Vf- 0.01715Tc))
g, = 0.23 X lod exp{75Vf]
Vfp..= 0.05 and Vt, = 0.1856 - 0.2965 X le3Tc are the critical free volumes for propagation and termination correspondingly, and TCis the temperature in "C. Vf is the reacting micture average free volume vf =
6MVfM
+ '$pvp
where
+
V,, = 0.25 X 0.1 X 10-2(Tc 106.0)
V p = 0.25 X 0.48 X 1O4(TC - 114.0) (iii) Vinyl Acetate:
-AHp [cal/moll = 21 000 (at 25 "C) pM [g/cm31 = 0.9584 PP
- 1.3276 X 10-3(T- 273.15)
[g/cm31 = 1.211 - 8.496 x io"(^ - 273.15)
cpM [cd/(g/K)]
0.4479 + 5.625 X 104(T- 273.15)
cPP [~al/(g/K)]= 0.3453 + 9.55 X lO''(T-
298.15)
Rate constants (Teymour, 1989): propagation [L/(mol/s)l:
k, = 3.2 X
lo7exp(-6300/R")
transfer to monomer [L/(mol/s)l:
k,, = 0.238 X 10-3kp transfer to polymer [L/(mol/s)l:
k,, = 0.34 X lo%, TDB polymerization [L/(mol/s)l:
k, = 0.66kp
termination [L/ (mol/@1:
k, = 3.7 X
lo9 exp(-3200/R!!")
T is in degrees kelvin if not otherwise noted. R is in cal/ (mol/K). (iv) Styrene-MMA copolymerization: cross-propagation reactivity ratios: rpSM = 0.49,
rpMs = 0.46
cross-termination reactivity ratios: rtSM= r, = 0.1667 (equivalent to
= 6)
Alvarez,J.; Alvarez,J. J.;Martinez, R. E. Conformation of the Particle Size Distribution in Suspension Polymerization. The Role of Kinetics, Polymer Viscocity and Suspension Agent. Presented at the AIChE Annual Meeting, Chicago, 1990. Bahargava, G. S.; Khan, H. U.; Bhattacharyya, K. K. SAN copolymer by suspension polymerization. 11. Bead size distribution and molecular weight. J. Appl. Polym. Sci. 1979,23,1181-1187. Christiansen, R. L.; et al. POLYRED-A CAD package for Polymerization Reactor Design. Presented at the AIChE Annual Meeting, Chicago, 1990. Deiringer, G. Kontinuierliche Reaktionsftihrung der Perlpolymerisation. Ph.D. Thesis, Technical University of Berlin, 1985. Hamer, J. W. Solution Polymerization in Stirred Continuous Tank Reactors and Tubular Reactors. Ph.D. Thesis, University of Wisconsin-Madison, 1983. Hopff, H.; Lussi, H.; Gerspacher, H. Zur Kenntnis der Perlpolymerisation. 1. Mitt. Grundlagen. Makromol. Chem. 1964a, 78, 24-37. Hopff, H.; Lussi, H.; Gerspacher, H. Zur Kenntnis der Perlpolymerisation. 2. Mitt. Praktishe Anwendung der Dimensionsanalyse auf das System MMA-Mowiol70l88. Makromol. Chem. 1964b, 78,37-46. Kalfas, G.; Ray, W. H. Modeling and Experimental Studiea of Aqueous Suspension Polymerization Processes. 1. Modeling and Simulations. Ind. Eng. Chem. Res. 1993,preceding paper in this issue. Konno, M.; Arai, K.; Saito,S. The effect of stabilizer on coalescence of dispersed drops in suspension polymerization of styrene. J. Chem. Eng. Jpn. 1982,15,131-5. Langner, F.; Moritz, H.-H.; Reichert, K.-H. Reactor Scale-up for Polymerization in Suspension. Chem. Eng. Sci. 1980, 35, 519525. Leonard, E. C. Vinyl Acetate. In Vinyl and Diene Monomers;Leonard, E. C., Eds.; Wiley-Interscience: New York, 1970-1971,Vol. 1,pp 263-329. Lindemann, M. K. The Mechanism of Vinyl Acetate Polymerization. In Vinyl Polymerization-Part I; Han, G. E., Ed.; Dekker: New York, 1967,p 269. Luskin, L. S.Acrylic Acid, Methacrylic Acid, and the Related Eaters. In V i n y l and Diene Monomers; Leonard, E. C., Ed.; Wiley-Interscience: New York, 1970-1971; Vol. 1, pp 105-203. Mahabadi, H. K.; O'Driscoll, K. F. Absolute Rate Constants in FreeRadical Polymerization. 111. Determination of Propagation and Termination Rate Constantsfor Styreneand Methyl Methacrylate. J. Macromol. Sci.-Chem. 1977,All, 967-976. Mino, G.Copolymerization of Styrene and Acrylonitrile in Aqueous Dispersion. J . Polym. Sci. 1956,22,369-383. Munzer, M.; Trommsdorff, E. Polymerization in Suspension. In Polymerization Processes; Schddknecht, C. E., Skeist, I., Eds., Wiley: New York, 1977;pp 106. Napper, D. H.; Parts, A. G. Polymerization of Vinyl Acetate in Aqueous Media. Part I. The Kinetic Behavior in the Absence of Added Stabilizing Agents. J. Polym. Sci. 1962,61,113. O'Driscoll, K. F.;Kale, L. T.; Garcia-Rubio, L. H. Applicability of the MayeLewis Equation to High Conversion Copolymerization of Styreneand Methyl Methacrylate. J. Polym. Sci.,Polym.Chem. Ed. 1984,22,2777. Ross, R. T., Jr.; Laurence, R. L. Gel Effect and Free Volume in the Bulk Polymerization of Methyl Metacrylate. MChE Symp. Ser. 1976,72,74. Schmidt, A. D.; Ray, W. H. The Dynamic Behavior of Continuous Polymerization Reactors-I. Isothermal Solution Polymerization in a CSTR. Chem. Eng. Sci. 1981,36,1401-1410. Tavlaridis, L.; Stamatoudis,M. The Analysisof Interphase Reactions and Mass Transfer in Liquid-Liquid Dispersions. Adv. Chem. Eng. 1981,11, 199-273. Taylor, T. W.; Reichert, K. H. The Influence of Reactor Type and Operating Conditions on the Molecular Weight Distribution in Vinyl Acetate Polymerization. J.Appl. Polym. Sci. 1985,30,227250. Teymour, F. The Dynamic Behavior of Free Radical Solution Polymerization Reactions in a Continuous Stirred Tank Reactor. Ph.D. Thesis, University of Wisconsin-Madison, 1989. Yuan,H. G.; Kalfas, G.; Ray, W. H. Suspension Polymerization. JMSRev. Macromol. Chem. Phys. 1991, C31, 215-299. Received for review December 29, 1992 Accepted May 14,1993
