I n d . Eng. Chem. Res. 1989,28, 1659-1664
1659
SEPARATIONS Evaluation of the Performance of a Commercial Polystyrene Devolatilizer Bernard J. Meister* and Alan E. Platt Dow Chemical Company, Designed Thermoplastics Research, 438 Building, Midland, Michigan 48667
Residual styrene, ethylbenzene, dimers, and trimers were measured as a function of flash tank vacuum and product viscosity during the devolatilization of polystyrene in a commercial flash tank devolatilizer. The results demonstrated that Flory-Huggins multicomponent equilibrium calculations coupled with styrene generation and mass-transfer control provided an adequate description of the devolatilization process. Product viscosity was determined to have little effect on the devolatilization. The degree of mass-transfer control was determined to be a strong function of the molecular weight of the volatile component. Devolatilization is the term used for the unit operation of evaporative removal of solvents and monomers from polymeric materials. The three general types of equipment that are employed to perform this operation are a heater and flash tank connected in series, which is most often called a flash tank devolatilizer, a devolatilizing extruder, and a heavy-duty, wiped-film evaporator. Of the three types, the flash tank devolatilizer is the simplest to describe and most widely employed commercially, although most literature work focuses on the devolatilizing extruder. Two recent reviews by Biesenberger (1983) and Denson (1983) cover most of the available literature on the subject. A well-documented set of data on the flash tank devolatilization of polystyrene has not been published, although limited data are presented by Biesenberger (1983) and Hagberg (1976). The main intent of this work is to determine the effects of polymer viscosity, solvent volatility, and flash tank vacuum on the operation of a commercial-scale unit under representative commercial operating conditions.
Description of the Experiments The feed to the devolatilizing equipment comes from a polystyrene reactor and contains approximately 85% polymer and the remainder material styrene, ethylbenzene, cumene, n-propylbenzene, styrene dimers, and styrene trimers. The feed is somewhat different in composition for the two grades of polystyrene investigated due to the changed reactor conditions. The two grades are product 1,a high molecular weight crystal polystyrene, and.product 2, a low molecular weight crystal polystyrene. The measured molecular weights and condition “G” melt index values for the two products are presented in Table I. Material balances for the two products are shown in Figure 1. The heater is mounted on the top of the flash tank, as shown in Figure 1. The pressure drop through the heater is approximately 30 psi, and the heater is open to the flash tank to allow vaporization and foam generation to begin partway through the heater. The flash tank is a jacketed tank of commercial size. Two gear pumps are mounted on the bottom, each forwarding devolatilized polystyrene through a jacketed line to a die and pelletizer. The only 0888-5885/89/2628-1659$01.50/0
Table I. Properties of the Polymers Studied product 1 product 2 196OOO Mw 304 000 135OOO 87 OOO Mn MFI (G) 1.5 7.2 Table 11. Feed Rates and Feed Composition Droduct 1 feed rate 5600 kg/h feed comp, % polymer 84.268 styrene 11.610 ethylbenzene 2.675 cumene 0.626 n-propylbenzene 0.504 styrene dimer 0.089 styrene trimer 0.228
Droduct 2 6100 kg/h 86.776 8.993 2.500 0.582 0.470 0.179 0.500
distinction between the two streams is that the transfer line is 10% longer for stream B. The polymer exits from the heater as a foamy mass and falls through the flash tank in globs of foam greater than 2 kg based on the observed frequency. The flash tank is operated with only enough level to keep the pumps running steadily. The average time involved from the first exit from the heater to the exit from the flash tank is approximately 10 min. Incondensables had been previously determined to be less than 0.2 kg/h in this process, which should insignificantly affect the result. Collection of the data for this set of experiments was performed on two separate days, the first set when the production plant was producing product 1and the second set 2 days later when the production plant was producing product 2. In each case, feed to the devolatilizer was steady in rate and composition over the course of the experiments. These values are listed in Table 11. The feed rate was somewhat higher in the second series because of the reactor conditions to make the particular product. In each case, vacuum in the devolatilizer was adjusted to four different values over the range 4-30 Torr. Two newly calibrated pressure instruments were used to assure that readings were within hO.1 Torr in the flash tank. Conditions were held for a period of at least 2 h prior to sampling to allow for complete transition of the product through the transfer 0 1989 American Chemical Society
1660 Ind. Eng. Chem. Res., Vol. 28, No. 11, 1989 Table 111. Data on Operating Conditions product 1 166 feed temp, "C heater temp, "C 241 5.0 vacuum, Torr stream A temp, "C 230 stream B temp, "C 229
1
1
1
167 241 10.0 229 229
171 243 17.0 230 228
169 244 28.0 229 228
Table IV. Measured Recycle Compositions product 1 1 vacuum, Torr 5.0 10.0 recycle comp, 70 styrene 75.7 75.6 EB 17.1 17.1 cumene 4.00 4.01 NPB 2.92 2.89 dimer 0.19 0.18 trimer
2 178 241 4.0 230 230
2 180 239 11.5 230 229
2 182 241 16.0 228 230
2 179 241 27.0 230 230
1
1
17.0
28.0
2 4.0
2 11.5
2 16.0
2 27.0
75.1 17.4 4.18 2.97 0.28
75.5 17.6 4.03 2.12 0.25 0.16
72.6 20.0 4.61 2.43 0.48 0.70
70.0 20.0 4.76 3.37 0.78 0.64
71.0 19.5 4.64 3.37 0.69 0.22
70.6 19.8 4.60 3.36 0.7fi 0.50
2 4.0 0.0312 0.0281 0.0048 0.0049 0.0018 0.0019
2 11.5 0.0551 0.0537 0.0104 0.0105 0.0034 0.0033 0.0005 0.0005 0.0363 0.0342 0.426 0.435
2 16.0 0.0679 0.0676 0.0141 0.0140 0.0043 0.0043 0.0013 0.0014 0.0406 0.0385 0.462 0.443
2 27.0 0.0955 0.101 0.0216 0.0220 0.0073 0.0077 0.0039 0.0042 0.0553 0.0515 0.504 0.505
Table V. Measured Residuals in Product (in Weight Percent) 1 1 1 product 17.0 vacuum, Torr 5.0 10.0 0.0717 0.0321 0.0507 styrene A 0.0653 styrene B 0.0296 0.0502 0.0127 0.0049 0.0085 EB A 0.0126 0.0082 EB B 0.0046 0.0038 0.0012 0.0023 cumene A 0.0038 0.0024 cumene B 0.0011 NPB A 0.0005 0.0005 NPB B 0.0198 0.0206 dimer A 0.0159 0.0210 dimer B 0.0195 0.0152 0.258 0.226 0.212 trimer A 0.242 0.260 trimer B 0.219
line. The heater temperatures were adjusted to maintain the polymer temperature measured after each of the two pumps at 230 "C. At each vacuum level, samples were taken of the condensed vapor and the product pellets from each stream. The process variables and the results of these analyses are listed in Tables 111-V.
1 28.0 0.122 0.105 0.0229 0.0198 0.0083 0.0068 0.0044 0.0034 0.0305 0.0281 0.258 0.254
0.0001 0.0317 0.0286 0.427 0.398 FEED
0 5600 KG/HR 168OC
0 6100 KGlHR 178% VAPOR HEATER
Laboratory Analysis of Samples Conventional packed column procedures were used to determine the residual components in the polystyrene pellets and in the condensed vapor or recycle streams. The polymer was precipitated from solution before injection into the HP5730A gas chromatograph. For the recycle analysis, the sample was diluted with solvent to keep the peak size within the linear region of the FID detector. The conditions were as follows: gas chromatograph, Hewlett-Packard HP5830A; column, 6-ft X 2-mm glass column packed with 3% SP2100 (methylsilicone) on 100/ 120-mesh Supelcoport, acid washed, and silinized; temperatures, injector 300 OC, detector 300 "C, column 60 OC for 2 min and then ramped at 10 OC/min to 280 "C and held for 15 min; injection size, 2 pL on column; detector, flame ionization, with area integration; mode of calculation, internal standard.
PRODUCT
0 4734 KGlHR
0 5325
Equilibrium Considerations It is reasonable to assume in this kind of equipment that a single equilibrium stage is the best that can be done and that, if there is enough time for diffusion through the polymer films, the activity of the residual volatile components remaining in the polymer exiting the flash tank will be equal to the partial pressure of the component in the vapor leaving the flash tank. The partial pressure, pi, in the vapor is given by Dalton's law PL = Yin (1) where y I is the mole fraction of component i in the vapor
KGIHR
STREAM A 230F
STREAM
B
+
PUMP
Figure 1. Diagram and maea balance for flash tank devolatilization: ( 0 )mass balance for product 1, (0) mass balance for product 2.
and II is the total pressure in the flash tank. The activity of aromatic solvents in polystyrene is known to be well described by the Flory-Huggins (Flory, 1953) equation
Ind. Eng. Chem. Res., Vol. 28, No. 11, 1989 1661
or
where c$~ is the volume fraction of the volatile component, 4 is the volume fraction of the polymer, xi, is the FloryI-fuggins interaction parameter between the volatile component and the polymer, and Pi" is the vapor pressure of the pure component at the flash tank temperature. In the case of devolatilization, $p approaches unity and eq 3 becomes pi = P$ji exp[l.O + xip]
Table VI. Vapor Pressures of the Components at 230 OC component vador pressure, Torr styrene 4644.0 ethylbenzene 5555.0 cumene 4020.0 n-propylbenzene 3550.0 styrene dimer 97.0 styrene trimer 3.2 I
I
I
I
I
'd '
200
(4)
or on a weight fraction, wi, basis PP
p i = Piowi- exp[l.O Pi
+ xi,]
(5)
For the case of a multicomponent solution, as in the present case, it can also be shown that the multicomponent Flory-Huggins equations as presented by Scott (1949) reduce to the set of independent equations
15C m w
za 100
PP
p1 = Plow1-
exp[l.O
P1
+ xlp]
PP
p z = PZ0w2- exp[l.O + xpp] P2
50
p n = Pnown- exp[l.O + x,,] PP
Pn
//
A t equilibrium, the partial vapor pressures of each component in the polymer expressed by eq 6 should be equal to the partial pressures of each component in the vapor given by eq 1. Equating eq 6 and eq 1 yields YiQp
wi =
Piopi exp[l.O
+ xip]
(7)
For polystyrene-ethylbenzene, Vrentas et al. (1983) determined a value for the interaction parameter x = 0.35. They further showed that, for benzene, toluene, and ethylbenzene, the weight fraction activity coefficient at infinite dilution, aim, is approximately equal to 5.0 and independent of temperature. The weight fraction activity coefficient is related to the Flory-Huggins interaction parameter by aim
=
Pi
- e x ~ [ l *+0 xipl
(8)
PP
and in the equilibrium expression
Lacking activity coefficient data on the dimer and trimer, Rim for all components will be approximated as 5.0 in this analysis. As ethylbenzene, styrene, cumene, and n-propylbenzene are the major components and are almost completely vaporized, the mole fractions, yi, of these components are independent of pressure and equal to the mole fractions in the feed. The equilibrium line is then directly proportional to pressure: Ei =
cin
(10)
0 PRODUCT 2
STREAM
B
k 0 0
I
I
5
10
I
I
I
15 20 25 VACUUM (TORR)
I
I
30
35
Figure 2. Residual ethylbenzene as a function of devolatilizer vacuum.
where Ei is the equilibrium residual content and Ci is defined by Ci = yi/5.OPi''
(11)
Ci has units of Torr-'. For the dimer and trimer, where a significant fraction is left in the polymer, eq 9 are best solved by trial and error in conjunction with a mass balance. The vapor pressures of the various components used to calculate the equilibrium curves in Figures 2-5 are given in Table VI. The condensed vapor samples were found to be unreliable due to what is believed to be a sampling problem. For this reason, the feed compositions were used to calculate the equilibrium residual values shown in Figures 2-5. Generation of Styrene Polystyrene and styrene dimer have both been determined to be sources of residual styrene at devolatilization conditions. In a separate study, when reprecipitated polystyrene and distilled dimers and trimers were held at elevated temperatures in the absence of oxygen, the following rate expressions were obtained at 230 "C for generation of styrene expressed in weight fractions: R,, = 0.000050wp (12)
1662 Ind. Eng. Chem. Res., Vol. 28, No. 11, 1989 I
I
I
I
I
I
1
1000
750
I
W
z
Lu
U
>
EQUILIBRIUM
-
PRODUCT 2
b
za
500
250
G/ 0 0
5
15 20 25 VACUUM (TORR)
10
30
35
I
1
I
I
I
I
500
400
/
/ O
7/ n
cT 300
Y
/
/
/
/
E PRODUCT O U I L I 2B\/ R I U M ; , ~
5
DATA
-
PRODUCT 1
za 200
/
100 //
/' /
I
0
'
/'
5
/
A PRODUCT 1
STREAMA STREAM e STREAM P STREAM E
0 PRODUCT 1 0 PRODUCT2 0 PRODUCT2
//
'/-
0
/ I 10
I
I
15
20
I 25
I
,
0 0
5
10
15
20
25
30
35
VACUUM (TORR)
Figure 3. Residual styrene as a function of devolatilizer vacuum. I
I
0 PRODUCT 1 STREAM B 0 PRODUCT 2 STREAM A 0 I PRODUCT 2 I STREAM I B
I 30
I 35
VACUUM (TORR)
Figure 4. Residual dimer as a function of devolatilizer vacuum.
where R,, and Rds are the weight fractions of styrene generated per hour from the initial weight fraction polystyrene ( W,) and weight fraction dimer (WJ, respectively. At times less than an hour, increases in the styrene content were linear in time. The trimers were found to be thermally stable at 230 "C. These equations indicate that, if polystyrene contains
Figure 5. Residual trimer as a function of devolatilizer vacuum.
300 ppm styrene dimer and it is held at 230 "C for 1h, 50 ppm styrene will be generated from the polymer and 45 ppm styrene generated from the styrene dimer. In the current experiments, the average residence time after devolatilization,calculated by dividing the process volume by the throughput rate, was 0.81 h in the case of product 1 and 0.72 h in the case of product 2. This time was then multiplied by the generation rate expressions to determine the amount of styrene in the product samples that was present due to generation after devolatilization.
Results Figures 2-5 show plots of residual styrene, ethylbenzene, dimer, and trimer versus vacuum in the devolatilizer compared to the calculated equlibrium curves. The main trend that is seen in these plots is that all four experimental curves are parallel to the calculated equilibrium curves and show a positive intercept at zero pressure. This is expected for the styrene curve due to the generation of styrene. However, it is clear from the data that ethylbenzene also shows a positive intercept and that the departures from equilibrium are even greater for dimers and trimers. As the departure from equilibrium is independent of vacuum but dependent on the molecular weight of the volatile component, several possible reasons can be eliminated as the primary cause. If these differences were due to errors in the measured temperature or the activity coefficient in the equilibrium calculation, this would change the slope of the equilibrium curve but not the intercept. This is not the case, as the slope is correct. If the higher residuals were caused by a higher effective pressure inside the bubble, as might be caused by interfacial tension or elastic forces, the departure would be independent of the vacuum level but also would be independent of the component. A similar argument can be made regarding errors in the pressure measurement. If bubbles were being trapped in the polymer and then re-
Ind. Eng. Chem. Res., Vol. 28, No. 11, 1989 1663 compressed by the pump, the amount trapped would be proportional to the vacuum and proportional to the vapor composition, and this also does not agree with the measured results, although visual observation indicates it may be a minor contributor. The explanation that fits the best with measured results is mass-transfer control, as this would lead to departures that are relatively independent of vacuum but dependent on the diffusion coefficient of the individual component.
Mass-Transfer Control The primary intent of this section is to give further evidence that the experimental results are consistent with mass-transfer control. A quantitative solution of the diffusion problem through the foamy mass is beyond our capability because the interfacial area term is not well defined and changes rapidly from the time the foamy mass exits the heater and the time it exits the gear pump. By observation, considerable bubble breakage and coalescence occurs over this time. We might expect, however, because the films are relatively stagnant, that this can be more appropriately described by film theory rather than penetration theory and that the final residuals, Ri, might well fit the general equation
where Ri denotes the residual amount, Ei denotes the calculated equilibrium, and Fi denotes the amount of component i in the feed. On first thought, one might expect the foam density, the interfacial area per unit volume ( A ) , and the film thickness ( X )to be strong functions of the vacuum. By observation, this is not the case because we are operating with such high vapor loadings compared to what can be maintained as a foam in polystyrene at these temperatures. Therefore, in practice, foam density is approximately independent of vacuum and vapor loading in the range of vapor loadings involved in this study. As the contact time, t , is also independent of the vacuum, we can write eq 14 as
Table VII. Back-Calculated Mass-Transfer Parameters comDonent Droduct K; D;lDwa styrene 1 0.000 814 0.985 ethylbenzene 1.00 1 0.OOO 730 styrene dimer 1 0.160 0.253 styrene trimer 1 0.668 0.053 styrene 0.964 2 0.001 160 ethylbenzene 1.00 2 0.000 900 styrene dimer 2 0.158 0.263 styrene trimer 2 0.737 0.044
sider the function in eq 14 to be exponential at long times, relative diffusion wefficients can be back-calculated from the values of K,:
D,/&B = In (K,)/ln WEB)
(19)
We have used ethylbenzene as the reference, as it does not require subtracting out styrene generation values to determine K,. However, the K , values for styrene and ethylbenzene are quite consistent. The calculated ratios of the diffusion coefficients are also listed in Table VII. The results on the two products are consistent and demonstrate a strong effect of the molecular weight of the volatile component on the back-calculated diffusion coefficient. Of course, this is not an appropriate way to even estimate diffusion coefficients. Not only is the mass-transfer model grossly approximate, but the data may be partially affected by some of the alternate explanations included in the Results section. However, the consistency of the data with eq 18 and 19 indicates to us that mass-transfer control is the primary reason for departure from equilibrium in this type of equipment. The analysis only applies to systems where the volume of vapor generated greatly exceeds the minimum density of foam that is stable for a particular polymer. In the case of styrene and ethylbenzene where the mole fraction of the component in the vapor is independent of pressure, C,is proportional to F,, and eq 18 can be written as
R, = C,[II+ p d ]
(20)
= KiFi/ci
(21)
where pd
where Ki is dependent on the diffusion coefficient of the individual component. Then, as for all components except the trimer,
Ei = CiII (16) substitution of eq 16 into 15 yields Ri = KtFi + (1.0 - KJCiII (17) and as Ki
