Ind. Eng. Chem. Res. 2005, 44, 1021-1026
1021
Vapor-Liquid Equilibria of Carbon Dioxide + Methyl Methacrylate at 308, 313, 323, and 333 K Grzegorz Zwolak, Liwan Lioe, and Frank P. Lucien* School of Chemical Engineering and Industrial Chemistry, The University of New South Wales, UNSW Sydney, NSW 2052 Australia
Vapor-liquid equilibrium data for carbon dioxide + methyl methacrylate are reported at temperatures of 308, 313, 323, and 333 K and pressures in the range of 1-8 MPa. The corresponding measurements of the volumetric expansion of the liquid phase are also presented. A linear relationship is observed between pressure and liquid-phase composition for all of the temperatures considered. The Peng-Robinson equation of state provides a satisfactory correlation of the vapor-liquid equilibrium data. It is also shown that the expansion isotherms coincide when plotted as a function of the liquid-phase composition. The high solubility of carbon dioxide in methyl methacrylate and the significant volumetric expansion of the liquid phase indicate that methyl methacrylate is an appropriate choice for investigating the concept of polymerization in monomers expanded with carbon dioxide. Introduction In the continuing search for more efficient and environmentally benign reaction processes, supercritical fluids have gained recognition as tunable reaction solvents capable of enhancing reaction rates and selectivities.1 Supercritical carbon dioxide (scCO2) is the preferred medium in most applications because it is inexpensive, nonflammable, and nonreactive toward many materials. Polymer synthesis and processing using scCO2, in particular, has been the focus of numerous studies and is a rapidly developing area of research.2,3 Despite the proven benefits of scCO2 as a polymerization solvent, the need for elevated pressure continues to be a major disadvantage associated with its use. The operating pressure required to achieve homogeneous conditions depends on a number of factors but is typically in the range of 10-40 MPa, although studies have been reported that utilize operating pressures well beyond this range.4,5 This problem is compounded by the limited solubility of common polymers in scCO2, as solubility is facilitated by an increase in pressure. Dispersion polymerization has been investigated as a means of increasing the solubility of polymers in scCO2 through the addition of a stabilizer to the reaction system.6-8 Most examples of this type of dispersion polymerization, however, employ operating pressures in the same range as used in other homogeneous polymerizations conducted in scCO2. An alternative approach that potentially offers substantially reduced operating pressures is polymerization in a liquid medium expanded with dense CO2. Carbon dioxide is highly soluble in many organic solvents under conditions of elevated pressure and moderate temperatures.9 It is reasonable to expect that the liquid phase in such a two-phase system retains, to some extent, the favorable transport properties of scCO2. The dissolution of CO2 in an organic solvent is also accompanied by a * To whom correspondence should be addressed. Tel.: +61-2-9385-4302 Fax: +61-2-9385-5966. E-mail:
[email protected].
significant volumetric expansion of the liquid phase. This phenomenon allows the possibility of replacing a substantial proportion of the organic solvent used in a polymerization reaction. Furthermore, the limited solubility of the resulting polymer in CO2 is mitigated by the presence of the organic solvent in the liquid phase. Liu et al.10,11 have employed CO2-expanded solvents in the polymerization of styrene and methyl methacrylate (MMA). The solvents considered include tetrahydrofuran, cyclohexane, chloroform, ethyl acetate, and isoamyl acetate. Their work demonstrates that the properties of the polymers, such as the molecular weight and the polydispersity index, can be tuned by controlling the extent of volumetric expansion of the solvent. Wei et al.12 have considered the application of CO2-expanded acetonitrile in homogeneous catalytic oxidations. They report oxidation rates that are 1-2 orders of magnitude greater than those obtained with either neat acetonitrile or scCO2 as the reaction solvent. In these preceding examples, the advantages associated with CO2-expanded liquids were realized at pressures below 10 MPa. Phiong et al.13 recently reported a study on the catalytic hydrogenation of R-methylstyrene expanded with CO 2 without the use of an additional solvent. They showed that the rate of hydrogenation is enhanced significantly in the range of pressure from 7-13 MPa. This variation on the use of CO2-expanded liquids has yet to be explored in the area of polymer synthesis and offers the additional benefit of eliminating the use of an organic solvent in the polymerization process. The solubility of CO2 in the monomer and the associated volumetric expansion of the liquid phase are important parameters in such a process and provide a point of reference in the investigation of the phase behavior of the CO2/monomer/polymer system. In this work, we present high-pressure vapor-liquid equilibrium (VLE) data for the CO2-MMA binary system at temperatures of 308, 313, 323, and 333 K and pressures in the range of 1-8 MPa. Volumetric expansion data for the liquid phase are also reported over the same range of temperature and pressure. Methyl methacrylate is an important industrial monomer whose
10.1021/ie049823d CCC: $30.25 © 2005 American Chemical Society Published on Web 01/19/2005
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Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005
Figure 1. Simplified diagram of the experimental apparatus used for the measurement of VLE data.
polymerization has been extensively investigated. The polymerization of MMA is typically conducted at temperatures above 323 K. Here, we also report VLE data below this range of temperature to permit a more detailed examination of the relationship between volumetric expansion and the liquid-phase composition. Experimental Section Materials. Methyl methacrylate (99%) was obtained from Sigma-Aldrich and used without further purification. The particular grade of MMA used contains 10100 ppm of 4-methoxyphenol to prevent polymerization of the monomer. Liquid carbon dioxide (99.5%) was purchased from Linde Gas. Ethanol (99.7%) was obtained from CSR Distilleries. Apparatus and Procedure. A detailed description of the apparatus and procedure used for VLE and volumetric expansion measurements has been reported in earlier work.14 A simplified diagram of the apparatus is shown in Figure 1. The equilibrium cell consisted of a high-pressure sight gauge immersed in a water bath. The temperature of the water bath was maintained constant to (0.2 K. The system pressure was monitored with a pressure transducer with an uncertainty of (0.035 MPa. The liquid phase in the sight gauge was withdrawn from the bottom of the sight gauge and recirculated through the vapor phase with a metering pump. Samples of liquid phase were removed from the sight gauge via a three-way valve, located at the inlet of the metering pump, and directed to a solvent trap containing ethanol. The outlet of the solvent trap was connected to an inverted buret that was used to measure the volume of CO2 released from a sample. Vapor-phase samples were taken directly from the top of the sight gauge and directed to the solvent trap (not shown in Figure 1). The VLE apparatus was also used for the volumetric expansion measurements. The glass face of the sight gauge was fitted with a ruler (1-mm graduations) to determine the level of the liquid phase. Volumetric expansion data were determined prior to the measurement of VLE data. The initial observation of the expansion of MMA provided a means of determining the allowable range of pressure for which two phases (liquid and vapor) were always present in the sight gauge at a given temperature. The VLE apparatus was modified slightly for this purpose by removing the vapor-phase sampling line. The volume of liquid corresponding to a given level of liquid in the sight gauge was determined by calibration with ethanol at atmospheric pressure. The ruler that was fitted to the glass face of the sight
gauge was placed on that part of the gauge where the internal cross-sectional area is constant. A linear relationship was therefore observed between the volume of ethanol and the indicated level of ethanol in the sight gauge. In the determination of the volumetric expansion of MMA, a sufficient quantity of MMA was initially loaded into the sight gauge such that the liquid level exceeded the zero mark on the ruler. The metering pump was primed, and the recirculation of the liquid phase was commenced. The sight gauge and the connecting lines were then thoroughly purged by pressurizing the vapor space with CO2 at low pressure (∼0.5 MPa) and releasing the pressure slowly while maintaining the recirculation of the liquid phase. The purging step was repeated several times. After the last depressurization, the recirculation of the liquid phase was continued until the liquid level stabilized. This last step was particularly important for ensuring that the recirculation line was free of any gas bubbles. The equilibrium cell was then pressurized with CO2 in several stages to obtain incremental amounts of expansion of the liquid phase, within the limits imposed by the ruler on the sight gauge. For each stage of addition of CO2, the temperature, pressure, and liquid level were recorded at regular time intervals to verify that equilibrium conditions had been reached. Equilibrium was typically established within a period of 30 min with recirculation. The volumetric expansion of the liquid phase (E) at a given temperature (T) and pressure (P) was calculated according to the following equation
E(T,P) )
VL(T,P) - V/L(T) V/L(T)
× 100%
(1)
where VL is the volume of the expanded liquid phase and V/L is the initial volume of the liquid phase saturated with CO2 at atmospheric pressure. In the determination of VLE data, the procedure used for loading and purging the equilibrium cell was identical to that described above. The attainment of equilibrium was confirmed by monitoring the composition of a given phase over time. It was noted that the time taken to obtain equilibrium, over the range of conditions examined, was marginally longer than that required for the measurement of the expansion of the liquid phase. Prior to the sampling of a given phase, the ethanol in the solvent trap was saturated with CO2. The solution in the solvent trap was analyzed by gas chromatography to determine the total mass of MMA in the sample. The gas that evolved from the solvent trap was allowed to pass into the buret where the volume displacement of water was recorded. Because the solvent trap was operated at near-atmospheric pressure, the amount of CO2 collected was calculated using the ideal gas equation as follows
nC )
PfVf - PiVi RT
(2)
where nC is the number of moles of CO2, P is the pressure, and V is the combined volume of gas in the solvent trap and buret. The subscripts i and f refer to the initial and final conditions in the solvent trap, respectively. The pressure in the solvent trap was deduced from the height of water in the buret. Each
Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005 1023
pressure term in eq 2 is corrected to account for the vapor pressure of ethanol. The composition of the liquid phase was calculated from the mean of at least three measurements, with a relative standard deviation (RSD) of less than 5%. The RSD was calculated with respect to the molar ratio of MMA to CO2 in the sample. Because of the very small quantities of MMA collected in the solvent trap, the composition of the vapor phase was less reproducible than that for the liquid phase (>20% RSD). However, the vapor-phase composition was found to be generally less than 2 mol % MMA, and so the level of precision achieved for the vapor phase was considered to be acceptable. Data Correlation. Both the standard15 and twoparameter versions of the Peng-Robinson equation of state (PREOS) were used in the correlation of the VLE data. The relevant equations for the standard model are well-known and are not reproduced here. In the twoparameter model, a second binary interaction parameter (lij) is introduced into the mixing rule for the repulsive term of the fluid mixture as follows N N
bmix )
∑ ∑xixjbij i)1 j)1
(3)
(bii + bjj) bij ) (1 - lij) 2
(4)
where x represents either the vapor-phase or the liquidphase composition, bii and bjj are the pure-component repulsive terms, and N is the number of components in the system. The introduction of the second interaction parameter leads to a different expression for the fugacity coefficient of the fluid mixture
ln φˆ i )
(
b/i A 2Si (Z - 1) - ln(Z - B) bmix 2x2B amix b/i bmix
)[ ln
Z + (1 + x2)B
Z + (1 - x2)B
]
xjbij) - bmix ∑ j)1
) 2(
xjaij ∑ j)1
aij ) xaiiajj(1 - kij)
Tc (K)
Pc (MPa)
ω
304.12 564
7.374 3.68
0.225 0.3168
a
Data for CO2 are from ref 16. b Data for MMA are from ref
17. Table 2. VLE for the CO2 (1) + MMA (2) System E (%)a
(6)
(7)
P (MPa)
x1
y1
E (%)a
P (MPa)
x1
y1
1.01 2.01 3.01 4.00
0.1505 0.3041 0.4279 0.5619
>0.99 >0.99 >0.99 >0.99
T ) 308.2 K 10.6 4.50 20.2 4.99 36.5 5.32 62.9 5.52
0.6325 0.7106 0.7577 0.7857
>0.99 83.0 >0.99 112 >0.99 140 >0.99 162
1.01 2.03 3.02 4.03
0.1334 0.2777 0.3956 0.5173
>0.99 >0.99 >0.99 >0.99
T ) 313.2 K 8.4 5.00 17.5 5.50 32.5 6.01 55.2 6.30
0.6528 0.7130 0.7883 0.8231
>0.99 88.7 >0.99 117 >0.99 163 >0.99 200
1.00 2.02 2.98 4.00 5.00
0.1127 0.2375 0.3614 0.4514 0.5650
>0.98 >0.99 >0.99 >0.99 >0.99
T ) 323.2 K 6.9 6.00 16.3 6.30 26.5 7.03 42.3 7.29 64.0
0.6719 0.7084 0.7851 0.8198
>0.99 98.0 >0.99 113 >0.99 167 >0.98 194
1.03 3.00 4.01 5.03 6.00
0.0985 0.3053 0.4174 0.5013 0.6074
>0.98 >0.99 >0.99 >0.99 >0.99
T ) 333.2 K 6.1 6.29 21.5 7.00 33.3 7.33 48.9 7.99 69.9 8.30
0.6345 0.6771 0.7025 0.7877 0.8189
>0.99 >0.99 >0.99 >0.99 >0.98
78.2 105 121 165 191
a Values of the volumetric expansion obtained from interpolation of the experimental data shown in Figure 3.
tion of the calculation procedure.18 The binary interaction parameters were allowed to vary with temperature. At a given temperature, the optimum values of kij and lij were obtained by minimizing the sum of squared relative deviations with respect to pressure. In the discussion that follows, the average absolute relative deviation (AARD) is defined as
AARD (%) )
N
Si )
component carbon dioxidea methyl methacrylateb
(5)
N
b/i
Table 1. Pure-Component Critical Properties and Acentric Factors
|
|
100 M Pcalc - Pexp M
∑ i)1
Pexp
i
(9)
where M is the number of data points and Pcalc and Pexp are the calculated and experimental values of pressure, respectively. Results and Discussion
(8)
where Z, A, B, and amix are defined in accordance with the standard model and kij is the standard binary interaction parameter. Note that the values of amix and bmix are calculated for each phase independently, but with the same values of kij and lij. The critical properties and acentric factors used for calculating the purecomponent repulsive and attractive (aii, ajj) terms in this work are listed in Table 1. Values of the binary interaction parameters were regressed from the VLE data using bubble-pressure calculations. The procedure involves the calculation of the vapor-phase composition and system pressure from the liquid-phase composition and system temperature. The reader is referred elsewhere for a detailed descrip-
Vapor-Liquid Equilibria. Experimental VLE data for the CO2-MMA binary system at temperatures of 308, 313, 323, and 333 K are presented in Table 2. For the vapor phase, the data are reported simply as greater than 99 mol % with respect to CO2. The exceptions to this are the data at the highest and lowest values of pressure for the 323 and 333 K isotherms. The four points in question are reported as greater than 98 mol % with respect to CO2. Some general features of the liquid-phase composition can be observed in Figure 2. Solubilities of 40-80 mol % CO2 are easily achieved at pressures below 8 MPa. At constant pressure, the solubility of CO2 increases with decreasing temperature in accordance with the increase in the density of pure CO2. At constant temperature, an approximately linear relationship exists
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Ind. Eng. Chem. Res., Vol. 44, No. 4, 2005 Table 3. Optimized Values of kij and lij from the PREOS T (K)
Figure 2. Comparison of P-x1 data for the CO2 (1) + MMA (2) system. The solid lines represent the straight lines of best fit through the data.
between pressure and composition. The solid lines in Figure 2 represent the straight lines of best fit through the data. Some notable discrepancies exist between the linear model and the experimental data at 333 K, although the deviations in the liquid-phase compositions are within the limits of experimental uncertainty ((5%). In binary systems consisting of CO2 and an organic solvent, a linear P-x isotherm is usually observed at pressures below 5 MPa.19,20 Above this range of pressure, the slope of the P-x isotherm often decreases, indicating that the solubility of CO2 becomes more sensitive to pressure, especially as the mixture critical point is approached. The linearity of the data in Figure 2 can be attributed to the possibility that the highest pressures used at each temperature are not sufficiently close to the mixture critical points. It is interesting to note, however, that, in the case of the CO2-acetone system, a near-linear dependence between pressure and composition occurs up to pressures in the vicinity of the mixture critical point.19 Lora and McHugh21 have reported VLE data for the CO2-MMA binary system at temperatures of 313, 353, and 378 K. A comparison between our data and the previously published data at 313 K is also included in Figure 2. For pressures below 6 MPa, significant discrepancies exist between the two sets of data. At 3 MPa, for example, the solubility of CO2 in the liquid phase, as measured in this work, is around 25% lower than that measured by Lora and McHugh. The earlier data, which encompass a wider range of pressure, also exhibit a linear dependence between pressure and composition. Optimized values of kij and lij from the correlation of the VLE data are reported in Table 3. For the standard PREOS, there is close agreement between the experimental data and the model. The AARDs with respect to the calculated values of pressure are less than 5% in the range of temperature considered. For the twoparameter model, there is only a marginal improvement in the correlation of the data (AARDs of
