2118
Znd. Eng. Chem. Res. 1992,31, 2118-2127
Bindung von Schwefeldioxid und Stickoxiden durch Gasformiges Ammoniak. 2.Anorg. Allg. Chem. 1974,404,284-294. Hartley, E. M.; Matteson, M. J. Sulfur Dioxide Reactions with Ammonia in Humid Air. Ind. Eng. Chem. Fundam. 1975,14,67-12. Hjuler, K. The Reaction Between Sulphur Dioxide and Ammonia in Flue Gas. Ph.D. Thesis from Department of Chemical Engineering, Technical University of Denmark, 1991 (ISBN 87983894-0-8). Jenkin, M. E.; Cox, R. A.; Williams, D. J. Laboratory Studies of the Kinetics of Formation of Nitrous Acid from the Thermal Reaction of Nitrogen Dioxide and Water Vapour. Atmos. Environ. 1988, 22,481-498. Landreth, R.; de Pena, R. G.; Heicklen, J. Thermodynamics of the Reaction of Ammonia and Sulfur Dioxide in the Presence of Water. J. Phys. Chem. 1975,79,1785-1188. Martin, L.R.; Damschen, D. E.; Judeikis, H. S. The Reactions of Nitrogen Oxides with SOz in Aqueous Aerosols. Atmos. Environ. 1981,15,191-195. Mearns, A. M.; Ofosu-Asiedu, K. Kinetics of Reaction of Low Concentration Mixtures of Oxides of Nitrogen, Ammonia and Water Vapour. J. Chem. Technol. Biotechnol. 1984a, 34A, 341-349. Mearns, A. M.; Ofosu-Asiedu, K. Ammonium Nitrate Formation in Low Concentration Mixtures of Oxides of Nitrogen and Ammonia. J. Chem. Technol. Biotechnol. 1984b,34A, 350-354. Oblath, S . B.;Markowitz, S. S.; Novakov, T.; Chang, S. G. Kinetics of the Formation of Hydroxylamine Disulfonate by Reaction of
Nitrite with Sulfites. J.Phys. Chem. 1981,85,1017-1021. Scargill, D. Dissociation Constants of Anhydrous Ammonium Sulphite and Ammonium Pyrosulphite Prepared by Gas-phase Reactions. J. Chem. SOC.A (Inorg. Phys. Theor.) 1971,2461-2466. Schwartz, S . E.; White, W. H. Solubility Equilibria of the Nitrogen Oxides and Oxyacids in Dilute Aqueous Solution. In Advances in Environmental Science and Engineering; Pfafflin, J. R., Ziegler, E. N., Eds.; Gordon and Breach New York, 1981;Vol. 4,pp 1-45. Takeuchi, H.; Takashi, K.; Kizawa, N. Absorption of Nitrogen Dioxide in Sodium Sulfite Solution from Air as a Diluent. Ind. Chem. Eng. Process Des. Dev. 1977,16,486-490. Tanaka, T.; Koizumi, M.; Yokoyama, T.; Ishihara, Y. The Kinetics and Products of the Reaction Between Nitrite and Bisulfite. Prepr.-Am. Chem. SOC.,Diu. Pet. Chem. 1976,24(2),593-600. Tock, R. W.;Hoover, K. C.; Faust, G. J. SOz Removal by Transformation to Solid Crystale of NH3 Complexes. AIChE Symp. Ser. 1979,75 (188),62-82. Wittig, S.;Spiegel, G.; Platzer, K.-H.; Willibald, U. Simultane Rauchgasreinigung (Entschwefelung, Denitrierung) durch Elektronenstrahl, KfK-PEF 45,Projekt Europikhes Forchungscentrum fur MaBnahmen zur Luftreinhaltung, 1988.
Receioed for review November 20, 1991 Revised manuscript received March 30, 1992 Accepted May 26,1992
Kinetics of Melt Transesterification of Diphenyl Carbonate and Bisphenol-A to Polycarbonate with LiOH*H20Catalyst Yangsoo Kim and Kyu Yong Choi* Department of Chemical Engineering, University of Maryland, College Park, Maryland 20742
Thomas A. Chamberlin Central Research Organic Chemicals and Plastics Laboratory, Dow Chemical Company, Midland, Michigan 48674
The kinetics of batch melt transesterification of diphenyl carbonate with 2,2-bis(4-hydroxypheny1)propane to polycarbonate have been studied in the presence and absence of LiOH.H20 catalyst in the temperature range of 180-250 O C . The effects of reaction temperature and catalyst concentration on conversion and oligomer formation are analyzed. For quantification of the reaction kinetics, a detailed molecular species model has been developed and solved. Quite satisfactory agreement between the model predictions and experimental data has been obtained. The forward and reverse reaction rate constants are estimated, and the difference between the catalyzed and uncatalyzed transesterification kinetics is discussed. Introduction Polycarbonates derived from the reaction between a carbonate ester and a diol are important engineering thermoplastics with good mechanical and optical properties as well as electrical and heat resistance useful for many engineering applications. Polycarbonates are produced commercially by interfacial polymerization and melt transesterification processes. Polymerization in an interfacial process consists of phosgenation and polycondensation stages (Schnell, 1964; Oliver, 1969). Phosgenation takes place between a bisphenol-A (2,2-bis(4hydroxypheny1)propane; BPA) salt in an aqueous caustic solution and phosgene in an organic solution. High molecular weight polycarbonate is obtained in the polycondeneation stage with triethylamine as catalyst. After the polymerization, the organic polymer is isolated and dried. Toxic solvents and reactants such as methylene chloride and phosgene gas used in the interfacial process pose en-
* T o whom correspondence should be addressed. 0888-5885/92/2631-2118$03.00/0
vironmental and safety concerns. In the melt transesterification process, diphenyl carbonate (DPC) and BPA are reacted at high temperature in the presence of basic catalysts such as alkali or alkaline earth metals as their oxides, hydroxides, or phenolates (Schnell, 1964; Oliver, 1969). Phenol is produced as a condensation byproduct and must be removed from the reaction mixture to facilitate the chain growth reaction. During polymerization, the reaction temperature is kept above the melting point of the reaction mixture. In practice, the reaction temperature is increased gradually, and the pressure is reduced to less than 1mmHg at the final stage of the polymerization. The melt process offers many unique advantages. Conceptually, the overall process scheme is simpler than the others, and polymer is obtained in undiluted form which can be directly pelletized. Toxic solvents are not used, and thus the process is environmentally more friendly than other existing p~x'~g888. Melt polymerization is also suitable as a continuous process. However, it has been reported that catalyst residue remaining in the 0 1992 American Chemical Society
Ind. Eng. Chem. Res., Vol. 31, No. 9, 1992 2119 polymer can affect the polymer's thermal stability unless it is neutralized (Oliver, 1969). The melt polycondensation process consists of several reaction steps: transesterification of DPC with BPA at relatively low temperature, molecular weight advancement at 1W250 "C under low pressure (20-100 mmHg), and finishing polymerization at 2 W 3 0 0 "C under high vacuum (C1.0 mmHg). One of the challenging problems in developing a melt polycarbonate process is that a high molecular weight polymer is more difficult to obtain than with other processes because of high melt viscosity. For example, the polymer melt viscosity increases rapidly to 2000600000 P as the polymer average molecular weight increases from 20000 to 60000 at 280 O C (Schnell, 1964),and the removal of volatile condensation byproduct (phenol) becomes difficult. The employment of high reaction temperature and the high melt viscosity may also promote unwanted side reactions. For example, the instability caused by hydroxyl groups can cause the side reactions that produce highly reactive isopropenylphenol at temperatures exceeding 150 "C (Kahovec et al., 1971): (-)
OH H
O
W
O
H // CH2
HO-@C,
+ @OH
(1)
CH3
If these highly reactive, unsaturated phenols are formed during the transesterification, they may undergo polymerization and addition reactions resulting in undesirable colored products (Schnell, 1964). It is also known that a side reaction can occur during the polymerization forming an ether linkage in the polymer backbone and a pendant free carboxyl group (Schnell, 1964). These carboxyl groups can lead to branching and cross-linking through esterification. These side reactions can be suppressed by employing appropriate basic catalysts and reactor operating conditions. Contrary to the significant industrial importance of polycarbonates, there is a dearth of open literature on the melt polymerization of bisphenol-A polycarbonates ( h e v et al., 1963; Turska and Wrcbel, 1970a,b). These workers carried out experiments using zinc oxide catalyst in semibatch reactors and considered the forward reaction (transesterification) only. There are also some inconsistencies in their experimental data. When the transesterification is conducted at high temperatures and reduced pressure in semibatch reactors, a loss of volatile monomers, in particular DPC, can be significant, causing errors in measured kinetic data. Schnell(1964) reported that the transesterification of DPC with aromatic dihydroxyl compounds may occur in the absence of catalyst, and he presumed that this could be caused by basic impurities in the starting materials. In our experimental work, it was confiied that the transesterification reaction indeed proceeds without added catalyst. Recently, Hersh and Choi (1990) reported an experimental and modeling study of melt transesterification kinetics using LiOH.H20 88 catalyst. They analyzed the reaction products withdrawn from the reactor during the reaction by size exclusion chromatography and reported forward and reverse rate constants. However, their work was limited to low temperature reactions (150-180 "C) using relatively large amounts of catalyst. In this study, we extend their work to higher temperatures and provide a detailed analysis of the batch melt transesterification kinetics through experimentation and kinetic modeling. In particular, uncatalyzed transesterification reactions
observed in our experimental work are analyzed and compared with the catalyzed reactions. For the first time, complete oligomer concentrationdistributions are reported. Since the reaction byproduct (phenol) is not removed from the reactor during the course of batch transesterification, high molecular weight polycarbonate is not obtainable and only low molecular weight oligomers are produced. However, the objective of this study is to determine relevant kinetic parameters and examine the reaction kinetics so that the results can be applied to the design and analysis of semibatch or continuous reaction processes where high molecular weight polycarbonates are produced.
Experimental Section Although phenol is about 17 times as volatile as DPC at high temperature (e.g., 250 "C), significant amounts of DPC distill off in semibatch processes when high temperature and low pressure are applied from the beginning of polymerization. Thus, kinetic analysis is difficult, and reproducible experimental data are hard to obtain. In our experimental study, we used a batch reactor to avoid the loss of DPC during the reaction. Batch melt transesterificationexperiments were carried out by using a 1-L stainless steel reactor equipped with an anchor type agitator. The reactor bottom is cone shaped for good mixing and easy removal of the reaction samples through a sampling valve at the bottom. The reactor was heated by an electrical heating mantle, and the reactor temperature was controlled by a proportional integral derivative (PID) controller. The reactor top was also heated and heavily insulated to prevent the condensation and deposition of volatile components (monomers, phenol, and low molecular weight oligomers). The reaction was carried out under nitrogen atmosphere. The reactor was fmt charged with molten BPA (Parabis resin from Dow Chemical Co., 1.4 mol) supplied from a separate heated storage tank. The temperature was raised to slightly above the desired reaction temperature before the molten DPC (Aldrich, 1.4 mol) was added from another separate heated storage tank. High purity BPA was used as supplied, and DPC was recrystallized from 2-propanol and vacuum-dried for more than 10 h. Although equal molar amounts of DPC and BPA were charged in the storage tanks, the actual initial molar ratio of the two reactants in the reactor was not exactly 1.0 because small amounts of DPC and BPA were left in the feed tubes and storage tank walls after the reactants were discharged to the reactor. When the catalyst was used, the reaction starta almost immediately after BPA and DPC are mixed, even before the addition of catalyst. Thus, a small amount of sample was taken from the reactor and analyzed by HPLC to determine the exact concentrations of BPA and DPC in the reactor. These experimentally determined initial concentrations were used in the model simulations. The starting reaction time for the uncatalyzed reaction was defined as the moment when the mixture temperature reached the desired reaction temperature. The initial temperature variations lasted for less than 10 min, which was much less than the total reaction time. When catalyst was used, the starting reaction time was defined as the moment when the catalyst was injected into the reactor after thermal equilibrium was established. A catalyst solution (LiOH.H20,Aldrich) was prepared as follows: a predetermined amount of LiOH.H20 as supplied was diluted with deionized water in a 100-mL volumetric flask. Then 1mL of the catalyst solution was injected into the reactor with a microsyringe when the desired reaction temperature was established. It was as-
2120 Ind. Eng. Chem. Res., Vol. 31, No. 9, 1992
sumed that the catalyst was perfectly mixed with the reactants in the stirred reactor. Small amounts of samples were withdrawn at different reaction times through a bottom sampling valve to which a small glass flask was attached. The flask was connected to a vacuum pump when the sample was taken. Since the degree of oligomerizationwas quite low (e.g., less than 7), the viscosity of the reaction mixture was not very high and we experiencedno sampling difficulties. The composition of the reaction mixture was determined by HPLC. We were able to separate BPA, DPC, phenol, and oligomer species by a method which was adapted from Bailly et al. (1986). The chromatogram was identified as follows: For species having the same number of phenyl carbonate groups (e.g., B,, Cn+l,and A,+1), the polar hydroxyl groups of Cn+land A,+1 oligomers tend to increase retention time with respect to B, oligomers. A,+, oligomers having bulkier bisphenol terminal groups elute after Cn+loligomers. The reproducibility of the composition measurements by HPLC was *7 9%.
Kinetic Model The stoichiometric equation for the transesterification reaction between a phenyl carbonate end group and a hydroxyl end group is expressed as follows: 0
Note that A,, and Borepresent bisphenol-A and diphenyl carbonate, respectively, and C1 is the monophenyl carbonate of BPA. Assuming that the reactivities of the functional end groups are independent of the chain length, we propose a kinetic model represented by the following component material balance equations: dAo/dt = 1[-2kA0{2E0
V
+2
(24,
+ C,)) + k’P?
n=l
(2A,
(7)
dBo/dt =
+ 5 (2A, + C,)) + k’P?
L[-2kE0(2A0
V
n=l
2
+ C,)]
(2E,
m
m
5 Cn((A0 + 2 Am) + (BO+ 5 E m ) ) + ( 5CJ2] m=l
n=l
m
m=l
n=l
m
k’P( C (2nA, n=l
+ 2nE, + nCn)+ C (n - l)C,)] m
1
dA,/dt = -[2k{-2AnE,-, - A,, C (2E, V m=l r=n+l
+ C,)
t
(C,+ 2A,) - 2nA,)]
0 m
1
n 10, m 1 1 n, m I 1
(5) (6)
where P is phenol and A,, B,, and C, are defined as follows: 0 H - f @ T a O - ! T O G - Y G O H CH3
Bn :
CH3
cH3
0
0
0-
!f
O
a
0
: e o - ! *
O
a
CH3
‘;“9 Cn:
H
-
E
+ CH3
0 ~
~
O
n 11 (10)
The above reaction reaches an equilibrium unless the volatile reaction byproduct, phenol, is removed from the reaction mixture. In industrial processes, high molecular weight polycarbonates are obtained by applying high reaction temperature and high vacuum to remove phenol, thereby to shift the equilibrium to the right. For modeling the polymerization kinetics, we used a molecular species model which tracks the concentration variations of every component, including the monomers, oligomers, and condensation byproduct, in the reaction medium as well as the molecular weight of each oligomer species. Since there are only two types of functional end groups, the following sequence of reactions comprise the molecular species model for the melt transesterification of DPC with BPA: n, m 1 0 (3) An + B, * Cn+m+l+ P B, + Cm+BB,+, + P n 10, m 1 1 (4)
An:
(9)
n=2
2 A,-,C,) + k’P{ 5
+ Cm+A,+, + P C, + C, + Cn+, + P
(8)
no1
1
r=l
A,
+ C,)]
n=l
-
~
~
O
~
dB,/dt = -[2k(-2EnA0 - E , V n
(2A, m=l
+ C,) +
m
2 E,-$,) + k’P( r=n+l C (C,+ 2Br) - 2nE,)]
n 11
r=l
dC,/dt =
’[V
m
C, C (A, m=l
l)C,
+2
2 42
? An-rlEr - (Ao + Eo)C, -
r=O
I
1 n-1
+ E , + C,) + 2 rC= l C,C,-, + k’P(-(2n -
2 (A, + E,) + 2 2
r=n
r=n+l
1
C,]
n 1 1 (12)
In the above model equations, the capital letters represent the number of moles of that particular component and V is the reaction volume. Note that P = C,. The rate constants in the model for the catalyzed reactions are the effective rate constants in which the catalyst concentration term is incorporated. The model equations were solved directly by the RungeKutta method, and the maximum number of repeating units (n,) in the oligomers was chosen as 7 for the numerical calculations. When nmcu> 7 was used, no discernible difference in the computed results was observed. For the comparison with experimental data, the initial concentrations of DPC,BPA, and oligomers used in our model simulations were determined from the HPLC analysis of the corresponding sample taken at the beginning of the transesterification experiments. It is assumed that the density of the reaction mixture is constant during the reaction. For the calculation of the concentrations of volatile species (e.g., phenol) in the liquid phase and in the vapor phase, a quasi-steady-state assumption is used for the vapopliquid equilibrium in dincrements of time. The vapor phase is assumed to follow the ideal gas law, and the Flory-Huggins model is used for the phase equilibrium calculations. The partial pressure of the volatile species
Ind. Eng. Cham. Res., Vol. 31, No. 9,1992 2121 in the vapor phase can be represented as p i = y J p .I x1. (13) where yi, poi,and xi are the activity coefficient, the saturated vapor pressure, and the mole fraction of component i, respectively. At the reaction temperature employed in our experimental study, the vapor pressures of DPC and BPA are far smaller than that of phenol and can be neglected (e.g., 3662 mmHg for phenol, 211 mmHg for DPC, and 16 mmHg for BPA at 250 "C). Therefore, only phenol is assumed to be present in the vapor phase. The temperature dependence of the vapor pressure of phenol is given by (Marsh, 1985) log p" = 1516.072 7.135 (p in mmHg, T i n "C) (14) 174.569 + T The activity coefficient of phenol (y,) is computed by using the Flory-Huggins model (Prausnitz, 1969): In y1 = In [l- (1- l/m)(l- a+)] + (1- l/m)(l - ai) + ~ ( -1 @J2 (15) where +lis the volume fraction of phenol, x the Flory interaction parameter, and m the ratio of molar volumes of polymer and solvent (Le., phenol). In the absence of actual data the polymer solvent interaction parameter, x, can be estimated by using the following equation (Ravindranath and Mashelkar, 1986): 61 x = 0.34 RT -(a1 - 62)2 (16) where 6, (12.05~ a l ' / ~ / c mand ~ / ~62) (9.94~ a l ' / ~ / c mare ~/~) the solubility parameters (Van Krevelen and Hoftyzer, 1976) of phenol and polycarbonate, respectively, and OI (8.787X L/mol) is the molar volume of phenol. The computed value of x is 0.75at 210 "C. The concentration of phenol in the liquid phase is calculated as follows. For a small time interval during the integration of the kinetic model equations, the number of moles of phenol in the liquid phase is first calculated by solving model eqs 7-12. Then, for a given reaction temperature, the mole fraction of phenol in the liquid phase (xl), the activity coefficient (yl), and the saturated vapor pressure (pol) are computed. The partial pressure of phenol in the vapor phase is calculated from the vapopliquid equilibrium relation (eq 13). Thus, the amount of phenol present in the vapor phase is determined, and the correct amount of phenol in the liquid phase is obtained and used for the solution of the model equations in the next time interval.
6
0
50
1
1
'
1
'
1
'
1
'
1
'
"
1
A
-.
0'.
A
lot 1
O
I
0
,
,
1
60
I
120
,
I
,
180
l
240
I
I
300
360
TIME (min)
Figure 1. Effect of reaction temperature on BPA concentration profiles in uncatalyzed reactions [(DPC/BPA), = 1.18 (210"C), 1.11 (230"C), 1.05 (250"C)]. 250 OC
+
Results and Discussion Uncatalyzed Transesterification. As mentioned earlier, the transesterification can take place without catalyst. Indeed, we found that the reaction occurs reproducibly to some extent in the absence of added lithium hydroxide catalyst. Since the extent of uncatalyzed reaction was significant, we have investigated the uncatalyzed transeeterification at three different reaction temperatures: 210,230,and 250 "C. A t temperatures below 210 "C, the reaction was extremely slow. Figure 1 shows the weight percent profiles of BPA in the reaction mixture at these temperatures. Here, symbols are experimental data obtained by HPLC analysis, and the lines represent the model predictions with the kinetic parameters estimated from the experimental data shown. First notice that the initial molar ratios of DPC to BPA are slightly larger than 1.0. The conversion of BPA at 250 "C is as large as 80% after 6 h of reaction, indicating that the contribution
-.ji
Bo
-._.-._.-.
0
60
120
180
240
300
360
TlME (min)
Figure 2. Composition of reaction producta at 250 "C without catalyet [(DPC/BPA), = 1.051.
of the uncatalyzd reaction to the overall extent of reaction may be appreciable, even in the catalyzed transesterification. The overall agreement between the model predictions and the experimental data is quite satisfadory up to about 5 h, albeit with slightly larger deviations at 230 "C. A typical composition of the reaction mixture is illustrated in Figure 2 for the reaction at 250 "C. It is observed that the concentration ratios of BPA to DPC are almost unity up to about 6 h. A slight difference afterward is believed to be well within measurement errors. It is also seen that large amounts of oligomers have been produced. The predicted concentration of Oligomers is quite good up to 1 h; however, the model predictions afterward are lower than we observed. The concentration of phenol is also overestimated by the model. It must be pointed out that
2122 Ind. Eng. Chem. Res., Vol. 31, No. 9, 1992 0.20 250 'C
I00 160
0 15 I40
I20 3 E
0 10
10 0 08
0 05
60 4 0
8%
20
0
5
10 Tlme
15 (mln.)
20
25
Figure 3. HPLC chromatogram of an uncatdyzed reaction sample (250 O C , 360 min).
phenol is the most volatile substance in the reaction mixture and therefore some loss of phenol may have occurred during the sampling in which samples were taken by applying vacuum to the sampling flask. The HPLC chromatogram for the sample whose compoeition is shown in Figure 2 is presented in Figure 3. This particular sample was taken 6 h after the reaction has started. It shows that the three different typea of oligomers (Le., A,, B,, and C 3 are well separated and that hexameis are the largest identifiable molecules. However, the conditions of the analysis were such that individual oligomers (Le., A,, B,, and C,) were separated only up to n = 4. For those oligomers with n > 4, the corresponding HPLC chromatograms of oligomers are fused to a single peak, as shown in Figure 3, and the concentrations of oligomers with n 2 5 are negligibly small. BPA and DPC are also well separated. There are some small peaks between the main oligomer peaks, but it was not possible to identify them. HPLC chromatogramsobtained for each sample taken during the reaction experiment have been used to determine the overall cornposition of the reaction mixture and the concentrations of oligomer species. Figure 4 shows the normaked weight fractions of A,, B, and C, oligomers. Here, the actual concentrations of these oligomers excluding BPA (Ao),DPC (Bo), and phenol, were used for the normalization. Although the model predictions of the overall oligomer concentrations are somewhat higher than the experimental data, as shown in Figure 2, Figure 4 indicatea that the model predictions of the oligomer composition (lines)are reasonable. Notice that C1 is produced almost immediately by the reaction between BPA and DPC. As the reaction proceeds, C1 is converted to higher molecular weight A,, B,, and C, oligomers. This figure illustrates the model's capability of predicting the fine details of the transesterification reactions. The complete oligomer composition distribution curves as shown in Figure 4 have never been reported in the open litqature. The forward transesterification rate constant (k)is estimated from the initial reaction rate data obtained from the composition curves as shown in Figure 2. In order to determine the initial reaction rate, a few initial data points in the composition distribution curve (e.g., phenol) were first fitted with third-order polynomials and then the reaction rate at t = 0 was calculated. A@ &, is determined, the reverse reaction rate constant (k? can be estimated from the equilibrium reaction data. However, as seen in Figure 2, the transesterification reaction has not reached a true equilibrium state even after 6 h. Thus, we used a standard optimal parameter estimation technique (Rosenbrock's direct search method (Rosenbrock, 1960)) to estimate the reverse rate constant using the full range of experimental BPA concentration data. Figure 5a shows the Arrhenius plots for both the forward and reverse rate
i
1
\
0.61
1
i
C2
021
p
00
0
120
60
180
300
240
360
TIME (rnin)
Figure. 4. Normalized weight fractions of oligomers in uncatalyzed reaction at 250 O C [(DPC/BPA), = 1.051.
- 1 2 L . ,
,
,
1
1 .BO
1 .so
I
2 00
.
.
,
1
I
,
.
,
2.00 1ITxlO3(K-')
2 10
?
,
.
,
,
,
2.10
2 20
1/Tx103(K')
Figure 5. Arrhenius plots for uncatalyzed (a) and catalyzed reactions (b).
constants for the uncatalyzed transesterification reaction. Quite satisfactory linearities have been obtained in the
Ind. Eng. Chem. Res., Vol. 31, No. 9, 1992 2123 801
A
I
0 O-l
DPC
0
A BPA
+
I
-------
I
I
230 "C [Cl=9.183x 10'moVL
-
Phenol 0
0
0
0
0
A
A
i
90
120
80
-
z
dz
Q
tU I50 z 8 8m
I/*
,
O
0 0
60
120 TIME (min)
180
240
Figure 6. Effect of reaction temperature on BPA concentration profiles in catalyzed reactions [(DPC/BPA), = 1.22 (180 "C), 1.14 (210"C),1.21 (230OC); [C*](mol/L) = 9.221 X lod (180"C),8.922 X lod (210 "C),9.183 X lo-' (230"C)].
temperature range of our interest. From this plot, the following effective rate constanta have been found:
b
30
60 TIME (min)
Figure 7. Composition of reaction producta at 230 O C with catalyst mol/L; (DPC/BPA), = 1.21]. [[C*] = 9.183 X
[Cy=9.163x lO'rnol/L
0.10
6 = (3.11 f 0.01) X 10' exp[(-25300 f 100)/RT] L/(mol*min)
k' = (2.03 f 0.01) X 1016 exp[(-45000 f 300)/RT]
L/(mol-min)
The equilibrium constants at 210,230, and 250 OC are 12.51 f 2.92, 5.53 f 1.24, and 2.60 f 0.56, respectively, indicating that the reverse reaction is more sensitive to temperature than the forward reaction. These rate constante are used in the modeling equations. Catalyzed Transesterification. The rate of the traneeeterifcation reaction can be increased significantly by lithium hydroxide catalyst. We carried out transesterification experiments at 180, 210, and 230 O C with LiOH-H20catalyst. Figure 6 shows the weight percent profiles of BPA in the reaction mixture at these temperatures. The catalyst concentration varies slightly about 9.109 X lo4 mol/L or 3.56 X mol/mol BPA. Notice that equilibrium is rapidly established at 210 and 230 OC and that the agreement between the model predictions and experimental data is quite reasonable. The composition of the reaction mixture at 230 OC is illustrated in Figure 7, which shows behavior qualitatively similar to that in the uncatalyzed reactions as shown in Figure 2. In Figure 8, the normalized weight fractions of various oligomeric species are shown for the sample whose composition is shown in Figure 7. Note that the model predictions are reasonable, although some deviations in Al and B1 are noticeable. Figure 9 shows HPLC chromatograms of samples taken a t the beginning of the experiment (a) and after 90 min of reaction (b). It is observed that small amounte of AI and C1em already present at the beginning of the transesterification experiment (i.e., before the injection of catalyst). The HPLC chromatogram shown in Figure 9b is quite similar to that for the uncatalyzed reaction (Figure 3). As seen in Figure 3 for the uncatalyzed
Figure 8. Normalized weight fractions of oligomers in catalyzed reaction at 230 O C [[C*] = 9.183 X lod mol/L; @PC/BPA), = 1.211.
transesterifhation case,Figure 9b shows that oligomers of up to hexamers are present. Some unidentified peaks observed in Figure 3 for uncatalyzed reactions are also present but in much smaller amounts when catalyst is used. This is probably due to the suppression of some aide reactions when catalyst is present in the reaction mixture. The effective forward and reverse reaction rate constanta were estimated using the same method as that used previously (Le., uncatalyzed reaction). The following rate
2124 Ind. Eng. Chem. Res., Vol. 31, No. 9,1992
1 , O
I
8
0
l
8
i
60
8
1
1
120
l
1
180 240 TIME (rnin)
l
,
l
360
300
,
420
Figure 10. Effect of catalyst concentration on BPA concentration profiies in catalyzed reactions at 210 O C [(DPC/BPA), = 1.16 ( O ) , 1.08 (A),1.14 ( O ) ] .
Figure 9, HPLC chromatograms of catalyzed reaction samples at 230 "C [[P= I9.183 X lob mol/L; t = (a) 0, (b) 90 min].
constants were obtained from the Arrhenius plots shown in Figure 5b: k = (8.50 f 0.01) X 10' exp[(-14200 f 400)/RTJ L2/(mo12.min)
k' = (7.31 f 0.01) X lo6 exp[(-12100 f 100)/RT]
L2/(mo12-min)
In the above, it is assumed that the effective rate constants are represented by k,, = k[C*l and k\, = klC*l where [C*]is the catalyst concentration (mol/L), which was assumed constant during the course of the reaction. These rate constants for catalyst assisted reactions give equilibrium rate constant valuea of 1.13 f 0.44,l.N f 0.48, and 1.42 f 0.60 at 180, 210, and 230 O C , respectively. Recall that the equilibrium constant decreased with an increase in the reaction temperature in the absence of catalyst. With LiOH-H20catalyst, the equilibrium constant increases with an increase in the reaction temperature, albeit with much reduced rate of increase per unit increase in the reaction temperature. The activation energies for the catalyzed reaction are significantly smaller than those for the uncatalyzed reactions. It is also in-
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,
1
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8
1
8
1
4
8
8
,
,
8
6
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8
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8
10
[c*]x i o 5(rnoi/L) Figure 11. Effect of catalyst concentration on the normalid initial reaction rate at 210 OC.
teresting to note that the activation energy for the forward reaction is very similar to that reported by Turska and Wrdbel(197Ob) (14.5 kcal/mol) who used zinc oxide as a catalyst. The effect of catalyst concentration has also been investigated, and experimental results are shown in Figure 10 for three different catalyst concentrations at 210 "C. From this experimental data, initial reaction rates were measured and the normalized initial reaction rates plotted against the catalyst concentration, as illustrated in Figure 11. Notice that the initial reaction rate is linearly dependent on the catalyst concentration in the range of catalyst concentration considered in our experiments. It is also seen that the data point obtained at [C*]= 0 falls on the straight line, suggesting that the effective rate constant (IteE)may be expressed as k& = L + R[C*] (17) where k is the rate constant for the catalyst assisted
Ind. Eng. Chem. Res., Vol. 31,No. 9,1992 212s
1 l
o
8
UncatalyzedReaction
I
8
.
' Reverse Reaction
1 120
60
0
240
180
300
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TIME (min)
t
""I
i -401 170
190
210
230
250
I Catalyzed Reaction 0 180°C
1.51
270
I
Temperature ("C) Figure 12. Ratios of catalyst assisted rate conetanta to that of uncatalymdreactions.
transesterification reaction. The slo of the straight line shown in Figure 11 corresponds to r a n d [A& and [Bolo are the initial concentrations of BPA and DPC, respectively. A qualitatively similar observation regarding the effect of catalyst concentration on the transesterification rate was reported by Turska and Wrdbel(1970b) for a zinc oxide catalyst system in a semibatch transesterification. In our kinetica model, we initially assumed that kd = k[C*] where k was eetimated from the initial reaction rate data at three different temperatures. The effective rate constants (forward and reverse) obtained by eq 17 slightly differ from those obtained by k,Pl(k6tf) = k(k?[C*] (e.g., 3% at 230 "C, less than 0.1% at 180 "C). In solving the modeling equations (7-12),keffand kLff were used. The ratio of the catalyst assisted rate constants (h and h? to that of the uncatalyzed reaction (k and k? is illustrated in Figure 12. Note that the temperature dependence of k and klvalues (uncatalyzed reaction) is stronger than that of R and h'(catalyst aseistsd reaction). It is also seen that the reverse reaction is more strongly affected by the catalyst than the forward reaction. At this point, we do not have a plausible explanation as to how the uncatalyzed reaction occurs. However, if we assume that the uncatalyzed reaction is due to the trace amounts of catalytic compounds of unknown identity and that the reaction mechanism is the same as the catalyzed reaction, the data shown in Figure 11 indicate that k (uncatalyzed reaction) = k (catalyst assisted reaction) when [C*]= 4.8 X lo* mol/L (-0.1 ppm as LiOH). In practice, such a low concentration of catalyst, if it exists, may be very difficult to measure. The molar ratio of the phenyl carbonate and hydroxyl end groups in oligomers (Le., R = [-OC02Ph]/[-PhOH]) is shown in Figure 13. Here Ro is the initial molar ratio [i.e., @PC/BPA),]. It is seen that the molar ratio remains nearly constant during the course of polymerization for both uncatalyzsd and catalyzed reactions, indicating that the vaporization of DPC was indeed negligible as assumed in our modeling calculations. Figure 14 shows the calculated ratio of phenol concentrations in the vapor phase (Pv) and in the liquid phase (P)for the catalyzed and uncatalyzed reactions diecuseed in the foregoing. Notice that less than 3% of the total amount of phenol produced is present in the vapor phase for the temperature range 180-250 "C. A few words are in order concerning the difference in the activation energy values with and without catalyst.
60
0
180
120
240
TIME (min)
Figure 13. Molar ratios of phenyl carbonate and hydroxyl end groups in oligomers: R = [-OCO,Ph]/[-PhOH]; Ro = (DPC/BPA), 0 . 0 3 0 ~ " 1 " 1 " i "
-
UncataiyzedReactlon
.
CatalyzedReactan
0.025r-
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0.020 -
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!
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0.005
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0.000 0
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Figure 14. Molar ratios of phenol in the vapor phase (Pv)to that in the liquid phase (P).
Firstly, the apparent activation energies for both the forward and reverse reactions with the catalyst are much smaller than those obtained without catalyst. This is understandable because the principal function of the catalyst is to lower the activation energy of the reactions and thus to speed up both forward and reverse reactions. Secondly, the difference in the activation energies between the forward and the reverse reactions is -19700 f 400 cal/mol for the uncatalyzed reaction, whereas the difference is 2100 f 500 cal/mol for the catalyzed reaction. The small difference in the activation energies for the catalyzed reaction indicates that the apparent heat of transesterification with the catalyst is very small. A similar observation has been reported for other high temperature
2126 Ind. Eng. Chem. Res., Vol. 31, No. 9,1992
melt transesterification processes (e.g., transesterification of dimethyl terephthalate and ethylene glycol in the synthesis of poly(ethy1ene terephthalate) (Challa, 1960)). It was also observed in our experimental studies that the equilibrium rate constanta for uncatalyzed and catalyzed reactions were quite different. In the phenomenological kinetic model we proposed, we assumed that both the uncatalyzed and catalyzed reactions occur by the same mechanism. Although, at the present time, we are not able to provide different reaction mechanisms for these reactions, the observed differences in the kinetic parameter values for the uncatalyzed and catalyzed reactions suggest that the transesterifkation reaction may occur by different mechanisms in the absence or presence of catalyst. Thus, a further study may be necessary in the future to clarify this issue. Finally, we would also like to mention some interesting experimental observations. A small amount of sample containing BPA and DPC mixture was taken from the reactor at ita preset reaction temperature just before the injection of the catalyst. This sample exhibited a dark brown color; however, when the sample was taken shortly after the catalyst was added, the sample exhibited a much lighter color. Although we were not able to identify the substance that was responsible for the discoloration, this experimentalobservation suggests that some side reactions may have occurred as hot BPA and DPC melt are brought into contact in the absence of catalyst.
Concluding Remarks In this paper we have presented experimental data on the melt transesterification of DPC with BPA to polycarbonate in the absence and presence of LiOH-H20catalyst in a batch reactor. The primary objective of this study is to determine relevant kinetic parameters and to investigate the kinetics of the transesterification reaction. The transesterification reaction was found to proceed to some extent even in the absence of added catalyst. It was observed that overall qualitative reaction kinetics for the uncatalyzed and catalyzed reactions were very similar but the corresponding reaction rate constants differ significantly. The reaction rate has been found to increase linearly with the catalyst concentration for reaction temperatures and catalyst concentrations employed in our work. Oligomer distributions, measured experimentally and estimated theoretically, are reported and show that the proposed molecular species model is quite adequate in describingthe kinetics using the kinetic parameters reported in this paper. However, some discrepancies were also observed between the model predictions and experimental data. Currently, we are not able to offer plausible explanations as to why such discrepancies have occurred. It is possible that some assumptions we made in the kinetic modeling (e.g., equal reactivity of each functional end group, linear dependence of the reaction rate to catalyst concentration,no side reactions, etc.) may have caused the discrepancies. Experimental difficulties in operating the transesterification reactor could have also contributed to the model-data mismatch. Under the experimental batch reaction conditions employed in this work, the degree of oligomerizationwas quite low; however, the polymer molecular weight can be increased significantly by raising the reaction temperature and reducing the reactor pressure to facilitate the removal of phenol. The kinetics of melt polycondensation reactions in a semibatch reactor where phenol is continuously distilled off will be reported in our forthcoming paper.
Acknowledgment This research was supported by the National Science Foundation (Grant CBT-85-52428) and in part by Dow Chemical Co. (Midland, MI) for which we express our sincere gratitude.
Nomenclature [C*] = catalyst concentration (mol/L) kefi= effective forward reaction rate constant (L/ (mol-min)) hiff = effective reverse reaction rate constant (L/(moEmin)) k = forward transesterification rate constant in the uncatalyzed reaction (L/(mol-min)) i = reverse transeaterifcationrate constant in the uncatalyzeti reaction (L/(moEmin)) k = forward transesterificationrate constant in the catalyzed reaction (L2/(mo12.min)) k' = reverse transesterification rate constant in the catalyzed reaction (L2/(mo12.min)) k = forward reaction rate constant for the catalyst assisted transesterification reaction as defined in eq 17 (L2/ (mo12-min)) k' = reverse reaction rate constant for the catalyst assisted transesterification reaction as defined in eq 17 (L2/ (mo12.min)) m = ratio of molar volumes of polymer and solvent (i.e., phenol) P = number of moles of phenol in liquid phase (mol) P" = number of moles of phenol in vapor phase (mol) poi = saturated vapor pressure of component i (mmHg) V = reaction volume (L) u = molar volume of phenol (L/mol) xi = liquid-phase mole fraction of component i y i = activity coefficient of component i S1 = solubility parameter of phenol ( ~ a l ' / ~ / c m ~ / ~ ) S2 = solubility parameter of bisphenol-A polycarbonate (~al'/~/cm~/~) *l = volume fraction of phenol x = Flory interaction parameter BPA = bisphenol-A (2,2-bis(I-hydroxyphenyl)propane) DPC = diphenyl carbonate HPLC = high performance liquid chromatography Registry No. BPA, 80-05-7; DPC, 102-09-0; (BPA)(DPC) (copolymer),25929-04-8; (BPA)(DPC) (SRU), 24936-68-3; LiOH-HZO, 1310-66-3.
Literature Cited Bailly, Ch.; Daoust, D.; Legras, R.; Mercier, J. P.; de Valck, M. Separation of Polycarbonate Oligomers by High-performanceSize Exclusion and Reverse-phase Liquid Chromatography. Polymer 1986,27, 776-782. Challa, G . Eater Interchange Equilibria from Dimethyl Terephthalate and Ethylene Glycol. Recl. Trav. Chim. Pays-Bas 1960, 79,90-100. Hersh, S. N.; Choi, K. Y. Melt Transesterification of Diphenyl Carbonate with Bisphenol A in a Batch Reactor. J. Appl. Polym. 1033-1046. S C ~1990,41, . Kahovec, J.; PivocovP, H.; Pospbil, J. Synthesis and Properties of 2-Alkyl- and 2,6-Dialkyl-4-Isopropenylphenols.Collect. Czech. Chem. Commun. 1971,36, 1986-1994. Losev, I. P.; Smirnova, 0. V.; Smurova, YE. V. Kinetics of Polycarbonate Synthesis by Transesterification between 2,2-(4Hydroxypheny1)Propane and Diphenyl Carbonate. Polym. Sci. USSR 1963,5,662-670. Marsh, K. N., Ed. TRC Thermodynamic Tables: Non-Hydrocarbons, Vol. 5; Thermodynamics Research Center, The Texas A&M University System: College Station, TX, 1985; p K-6400, Table 23-2-1-(33.1000)-k. Oliver, E. D. Polycarbonates; Report 50; Stanford k a r c h Institute: Stanford, CA, 1969; p 109. Prausnitz, J. M. Molecular Thermodynamics of Fluid-Phase Equilibria; Prentice-Hall Englewood Cliffs, NJ, 1969. Ravindranath, K.; Maahelkar, R. A. Polyethylene Terephthalate I. Chemistry, Thermodynamics and Transport Properties. Chem.
Znd. Eng. Chem. Res. 1992,31, 2127-2133 Eng. Sci. 1986,41 (9),2197-2224. Rosenbrock, H. H. An Automatic Method for Finding the Greatest or Least Value of a Function. Comput. J. 1960,3,175-184. Schnell, H. Chemistry and Physics of Polycarbonates; Interscience: New York, 1964. Turska, E.; Wrbbel, A. M. Effects of Temperature on a Polycondeneation in the Melt. Polymer 1970a,11, 408-414. Tureka, E.; Wrbbel, A. M. Kinetics of Polycondensation in the Melt
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of 4,4-DihydroxyDiphenyl-2,2-Propane with Diphenyl Carbonate. Polymer 1970b,11,415-420. Van Krevelen, D. W.; Hoftyzer, P. J. Properties of Polymers; Elsevier Science: Amsterdam, The Netherlands, 1976.
Received for review November 6,1991 Revised manuscript received February 20, 1992 Accepted June 10,1992
Mild Hydrocracking of an Unstable Feedstock in a Three-phase Fluidized-Bed Reactor: Behavior of the Process and of the Chemical Compounds Guilherme L. M. Qouza,+Jtilio C. Afonso, and Martin Schmal* COPPElPEQlFederal University of Rio de Janeiro, Centro de Tecnologia, Bloco G, Caixa Postal 68502, 21945 Rio de Janeiro, Brazil
Jari N. Cardoso Institute of ChemistrylFederal University of Rio de Janeiro, Centro de Tecnologia, Bloco A, Sala A-603, 21910 Rio de Janeiro, Brazil
The mild hydrocracking (400OC, 125 atm) of an unstable feedstock (shale oil) was performed in a three-phase fluidized-bed reador with a commercial sulfided Ni-Mo catalyst. The hydroprocessing was monitored with respect to the physicochemical properties and the chemical composition of the natural and treated oil. The unit attained steady state after 36 h on stream for almost all parameters (viscosity, density, conversion, selectivity, etc.). Chemical composition data of the feedstock and the treated oil were, in general, in good agreement with the physicochemicalcharacterizations. The mild hydrocracking in a three-phase fluidized-bed reactor is shown to be an alternative process for the treatment of unstable feedstocks. (1) Introduction The worldwide refining profile has been changed as a consequence of different prices of heavy and light crude oils and due to the improvement of processing the bottom barrel of petroleum, in order to supply the rising demand of medium and light distillates with more rigid specifications. In this context, the interest in hydroprocessing heavy unstable feedstocks (which are characteristic of Brazilian petroleums from Campos, Rio de Janeiro (Bruning, 1988)) has gained more importance. As known,these feedstocks are more difficult to treat than the normal crude oils, requiring more severe experimental conditions. As a consequence, the deactivation of the catalyst is faster. Therefore, the operationality and consequently the economicity of the process can be drastically diminished. In the hydroprocessing of heavy and unstable feedstocks, the catalytic system and the reactor design are very important. For this reason, we are studying the mild hydrocracking (MHC) and the hydrotreatment (HDT)of an unstable feedstock, the shale oil, in a pilot-unit three-phase fluidized-bed reactor (Souza et al., 1985). The objective is to maximize the diesel fraction, which is of interest in the context of the actual Brazilian consumption profile. The main advantages of this process compared to that of the fued-bed reactor which is largely employed, are (i) isothermal operation with high conversion of heavy fractions; (ii) addition and withdrawal of the catalyst without perturbation of the hydroprocessing; (iii) higher efficiency of the catalyst due to ita smaller particle size; (iv) better ‘Present address: Petrobrh/CENPES/DICAT, Cidade UnivereitAria, Ilha do FundHo, Quadra 7,21910Rio d e Janeiro, Brazil.
flexibility of the operational conditions, regarding the type of feedstock (that may even contain solids in suspension); and (v) reduction of losses and channeling in the catalytic bed. The hydroprocessing of heavy and unstable feedstocks has long been studied using fluidized-bed reactors (Chervenak et al., 1960; Hellwig et al., 1966). Such nonconventional technologies have been employed in industrial scale by means of the “H-Oil” (Johnson et al., 1985;Embaby, 1990)and the “LC + fining” process (Van Driesen et al., 1979; Boening et al., 1987;Baldassari and Hamilton, 1989). A worldwide effort has been devoted in order to optimize current technologies and in the development of new processes and catalytic systems to reduce the operational severity. In this way, the three-phase fluidized-bed reactor provides better operational conditions, improving the stability and yielding the desired products distribution. In the present work we studied the behavior of a mild hydrocracking process in a three-phase fluidized-bed reactor for over 72 h under a given condition, using a shale oil as a feedstock and a commercial sulfided Ni-Mo catalyt. The chemical classes of organic compounds present in the natural and treated oil were characterized in order to identify possible compounds of low reactivity with regard to the various hydroprocessing reactions. These data were compared with the current product specifications of hydrotreatment. (2) Experimental Section (2.1) Materials. A commercial Ni-Mo/A1203 catalyst (Shell 324) wa8 used,whose main properties are as follows: Ni, 3.2%; Mo, 13.2%; S B E157 ~ , m2/g; pore volume, 0.48 mL/g; average pore diameter, 90 A. The original catalyst was a 1/16-in.unimodal extrudate. After being crushed, it was separated in the fraction -150/+270 Tyler Mesh and
0888-5885f 92/2631-2127$03.O0f 0 0 1992 American
Chemical Society
