Supercritical Reaction Calorimetry: Versatile Tool for Measuring Heat

Feb 28, 2008 - Supercritical Reaction Calorimetry: Versatile Tool for Measuring Heat Transfer. Properties and Monitoring Chemical Reactions in Supercr...
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Ind. Eng. Chem. Res. 2008, 47, 3372-3379

Supercritical Reaction Calorimetry: Versatile Tool for Measuring Heat Transfer Properties and Monitoring Chemical Reactions in Supercritical Fluids Charalampos A. Mantelis and Thierry Meyer* Ecole Polytechnique Fe´ de´ rale de Lausanne, Institute of Chemical Sciences and Engineering, Group of Chemical and Physical Safety, Station 6, CH-1015, Lausanne, Switzerland

The heat flow reaction calorimetry technique was developed for reactions with supercritical fluids and was applied to monitor the free-radical dispersion polymerization of methyl methacrylate in supercritical carbon dioxide (scCO2). The main mathematical equation for the calorimetric calculations was modified to take into account the particularities linked to the supercritical nature of the solvent. As a result, important heat transfer variables, such as the overall heat transfer coefficient, can be measured at the actual reaction conditions. This information is later used to calculate the heat released by the reaction and thus monitor its evolution. The robustness and accuracy of the technique allowed also investigating the effects of several reaction parameters. Finally, the obtained data were used to illustrate the importance of pressure as far as the safety of the reaction is concerned. Introduction In the quest for developing sustainable processes for the future, the constraints posed by environmental concerns become more important every day (e.g., reduction of the emissions of volatile organic compounds (VOCs) and disposal of aqueous wastes). This fact explains partly the tremendous interest that supercritical fluids (SCFs) have attracted during the past two decades. They are an environmentally benign alternative to VOCs, together with ionic liquids. Naturally, any option has its disadvantages; e.g., supercritical water is highly corrosive and special measures are required for its handling. Nevertheless, in particular supercritical carbon dioxide (scCO2) has shown a considerable potential to substitute organic solvents in many chemical processes, mainly due to its tunable physical and transport properties.1-3 Characteristic of the magnitude of this potential is that in only the 21st century there have been published almost 1.5 times as many articles on supercritical fluids as ever before.4 For the polymer industry in particular, scCO2 has many advantages both in polymerization reactions, where it can be separated from the polymer simply by degassing in a much more effortless way than with classic organic solvents, and in polymer processing, where it can facilitate polymer blending, foaming, impregnation, and fractionation.5-8 The field of polymerizations in SCFs was rather inactive until the early 1990s. The only exception is the high-pressure polymerization to produce low-density poly(ethylene) (LDPE), which has been applied in production from the early 1950s. During the past 15 years, however, there has been extensive research from many academic groups, as well as from many research and development departments, to develop processes using SCFs as solvents for polymerization reactions. scCO2 has without doubt monopolized this research. The evolution of this technology has been more or less determined by the solvating power of scCO2, in the sense that the first reactions studied were homogeneous polymerizations, for the production of amorphous fluoropolymers and silicones, since these two families of polymers are soluble to scCO2.9,10 Later these types of polymers were heavily investigated as surfactants for the * To whom correspondence should be addressed. Tel.: +41 21 6933614. Fax: +41 21 6933190. E-mail: [email protected].

development of heterogeneous polymerization systems.11-14 As a result many types of polymer were successfully produced, including poly(acrylic acid),15 poly(vinylidene fluoride),16,17 and a tetrafluoroethylene perfluoro(propyl vinyl ether) copolymer18 in precipitations and poly(methyl methacrylate) (PMMA),19-22 poly(styrene) (PS),23-26 poly(vinylidene fluoride),27 and poly(acrylonitrile)28 in dispersions. More recent developments in the fields of polymerizations in SCFs include inverse emulsion systems,29-31 the use of solid catalysts,32,33 and the production of polymer-based materials for pharmaceutical applications, such as controlled-drug delivery systems and tissue engineering scaffolds.34-37 Although most of the initial research on polymerizations in SCFs was performed on a small scale (e60 mL), further developments in the field tend to attain the liter scale or even higher. For such scaling up and for the development of the respective processes, a profound understanding of the chemical and physical phenomena is necessary together with a more engineering insight. Therefore, several monitoring techniques have been developed and applied to obtain this information. For example, the free-radical polymerization of ethylene was monitored using near-infrared (NIR) spectroscopy and the freeradical dispersion polymerization of methyl methacrylate (MMA) in scCO2 was monitored using turbidimetry.38-40 As far as the thermal analysis techniques are concerned, many examples of monitoring classic polymerization reaction systems using calorimetry exist in the literature, such as the emulsion polymerization of styrene by Varela de la Rosa et al. and the MMA polymerization analysis using adiabatic calorimetry by Maschio et al.41,42 Concerning calorimetry applied to polymerizations in SCFs, very few publications exist to our knowledge; only those of Howdle et al. discussed the development of a power compensation calorimeter.43-45 However, in their recent work they used this technique to identify the end of the precipitation polymerization of vinylidene fluoride in scCO2 and did not conduct calorimetric calculations for the measurement of the enthalpy of reaction or the reaction heat rate. In the present work, the technique of heat flow reaction calorimetry was chosen and an apparatus to study chemical reactions in SCFs was developed.46 The objective was to obtain kinetics and heat transfer information, monitor the polymerization reaction evolu-

10.1021/ie0712030 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/28/2008

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tion, and study the effects of the reaction parameters on the safety of the process. Experimental Section Experimental Setup. The experimental setup consists of a high-pressure reactor coupled to an RC1e calorimeter. The reactor’s maximum operating temperature and pressure are 300 °C and 35 MPa, respectively. The increased volume of the reactor (1.285 L) permitted the installation of a Pt100 temperature sensor, a pressure sensor, a double-stage turbine connected to a magnetic stirrer, a calibration heater, and an ultrasound sensor. Furthermore, the reactor’s cover has two sapphire windows allowing for optical observations with the use of an optical fiber endoscope. Additional equipment consists of a precision Coriolis mass flow meter coupled to a high-pressure piston pump for the addition of CO2 in the reactor and a highpressure syringe pump for the injection of additional liquid reactants. More details on the experimental setup as well as a schematic representation can be found elsewhere.47 A major development of this setup is the temperature control of the reactor’s cover and flange. In the case of SCFs the reaction medium occupies the entire available volume; thus it comes into contact with all the reactor parts. Therefore, in order to avoid heat losses, all parts have to be thermally controlled. The temperature control of the reactor is completed by the reactor jacket in which silicone oil runs at high flow rate and whose temperature is controlled from the RC1e calorimeter. The latter can operate in three modes, namely isothermally, isoperibolically, and adiabatically. Model Reaction. The free-radical dispersion polymerization of MMA in scCO2 was chosen as a model reaction. This reaction begins with the thermal decomposition of an initiator, which in this case was 2,2′-azobis(isobutyronitrile) (AIBN), to produce the free radicals. Consequently, the monomer reacts with the radicals to form the growing polymer chains. Soon the solubility threshold of the growing oligomers in the supercritical solvent is reached and they precipitate. A surfactant is used to sterically stabilize the precipitating oligomers and form a stable dispersion. The surfactant consists of a CO2-philic part and a CO2-phobic part with chemical affinity to the monomer units and is poly(dimethylsiloxane) monomethacrylate (PDMS-mMA). As a result, the polymerization takes place in two phases, namely the CO2-rich phase, or continuous phase, and the polymer-rich phase. The following reaction conditions were used as a reference throughout this study: reaction temperature, 80 °C; initial reaction pressure, 21.3 MPa; MMA mass, 252.8 g; stabilizer mass, 25.28 g (10 wt % PDMS-mMA/MMA); initiator mass, 2.528 g of AIBN (1 wt % AIBN/MMA); CO2 mass, 680 g; stirring speed, 400 rpm. The above reaction and conditions were chosen for several reasons. First, there is significant experience in this reaction both theoretical and experimental and both in organic solvents and in scCO2.47-50 Second, this reaction is probably the most studied free-radical dispersion polymerization in scCO2 in the literature, which allows for comparing the results of this study with previous reports.22,51-55 The reaction conditions were set so that the initial reaction mixture is homogeneous and that the continuous phase can efficiently solubilize the stabilizer molecules in order to form a stable polymer particle dispersion. The latter has been previously shown to affect dramatically the evolution of the polymerization.56 Materials. Carbon dioxide (purity 99.9%) provided by Carbagaz was used without further purification. Methyl methacrylate >99% pure (stabilized with ∼0.004% hydroquinone),

Figure 1. Schematic representation of the heat flow equation.

2,2′-azobis(isobutyronitrile) (AIBN), acetic anhydride g99% pure, and methanol of GC quality (g99.8% pure) were provided by Sigma-Aldrich and were used as received. Poly(dimethylsiloxane) monomethacrylate (PDMS-mMA) was supplied by ABCR with an average molecular weight of 5000 g‚mol-1 and a viscosity of 7 × 10-6 m2‚s-1. Heat Flow Reaction Calorimetry. The principle of reaction calorimetry is based on the fact that the heat released or absorbed by a global reaction (exothermic or endothermic, respectively) is an unmistakable trace of its evolution. Therefore, by monitoring the heat exchange between the contents of a reactor vessel and its surroundings, valuable information is obtained concerning the reaction, such as the overall reaction rate and the enthalpy of reaction. Figure 1 represents schematically the basis of calculations in heat flow reaction calorimetry, and eq 1 gives the respective mathematical equation.

Q˙ r ) Q˙ flow + Q˙ acc - Q˙ calib - Q˙ stir + Q˙ loss - Q˙ dos - Q˙ mix (1) The above equation can be further developed by taking into consideration the previously mentioned particularity of SCFs to occupy the entire reactor volume. First, the contributions of the reactor cover and flange in the heat flow term must be added. Second, in the accumulation term the constant-volume specific heat capacity must be used, instead of the constant-pressure one, since the volume is the constant variable in this case. Third, the calibration and the heat loss terms can be neglected, since the calibration heater is not used during the monitoring of a reaction and the heat loss term is obsolete from the moment that all the reactor parts are thermally controlled and no secondary heat losses occur. Fourth, the dosing and mixing terms can be combined into one term, henceforth called the injection term, since in our experimental protocol mixing only occurs when additional reactants are injected in the reactor. Finally, the stirring term is introduced in the baseline term and the above equation can be rewritten as

Q˙ Reaction ) UA(Tr - Tj) + UflAfl(Tr - Tfl) + UcovAcov(Tr - Tcov) +

∑i (micV,i + minscp,ins)

dTr dt

- Q˙ inj (2)

A detailed analysis of the above points can be found elsewhere.57 Once the heat of reaction is known, the reaction rate can be calculated from the following equation.

r)

Q˙ Reaction V(-∆Hr)

(3)

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Table 1. Overall Heat Transfer Coefficients Results at the Model Reaction’s Initial Conditions UAjacket (W K-1)

UAflange (W K-1)

UAcover (W K-1)

15.58

3.31

3.32

where the enthalpy of reaction is also calculated from eq 2.

∫t t Q˙ Reaction dt f

∆Hr )

0

mXf

(4)

Results and Discussion Measuring Heat Transfer Variables. In order to perform accurate calorimetric calculations, eq 2 shows that the overall heat transfer coefficients, between the reactor parts and the reaction medium, and the specific heat capacities of the inserts in the reactor and the reaction medium must be known. In the case of pure SCFs, or even worse SCF mixtures, these variables are not trivial to measure or estimate theoretically and very few data exist in the literature with respect to the processes being currently studied. Therefore, the developed reaction calorimeter offers an interesting solution. A. Overall Heat Transfer Coefficients. As far as the overall heat transfer coefficient is concerned, it has been shown previously that it depends both on properties of the medium studied (i.e., the density, the viscosity, the thermal conductivity, etc.) and on geometric characteristics of the reactor (i.e., the reactor internal diameter, the stirrer diameter, etc.).58 For its measurement, the standard calorimetric methodology is to carry out two calibration phases, one before and one after the monitored chemical reaction, to measure these variables. During a standard calibration phase no heat is released by the reaction, the calibration heater releases a constant amount of heat (∼25 W), no additional reactants are injected, the calorimeter operates in isothermal mode, and the temperatures of the flange and the cover are set equal to the reactor temperature. As a result, for the duration of the calibration, eq 1 gives

U)

∫Q˙ calib dt ∫A(Tr - Tj) dt

(5)

However, in the case of polymerizations in SCFs a more thorough approach is needed. Not only the initial and final values are important but also the intermediate ones since the difference between these two values is bound to be significant due to the drastically changing properties of the reaction medium. For that it has been demonstrated that a proportional to conversion approach is the best choice.57 In comparison Liu et al. assume the overall heat transfer coefficient constant throughout the entire polymerization, leading probably to false calorimetric calculations, although they acknowledge in their conclusions a decrease in U.43 On the other hand, the standard calibration phase provides only the measurement of the overall heat transfer coefficient through the reactor jacket (U). However, this is not adequate since the same variable is also needed for the reactor flange and cover. The latter are measured based on a method proposed by Lavanchy for pure scCO2.59 The difference of the baseline is used, between two runs of the calorimeter in isothermal mode without any reaction occurring, where different temperatures are set for the flange and the cover with respect to the reactor temperature. The results of the measurements for our model reaction’s initial conditions are presented in Table 1 and show that the heat that can be transferred through the two additional

reactor parts is almost 43% of the heat that can be transferred through the reactor jacket. This contribution is considerable and has to be accounted for in order to increase the robustness of the technique. In this investigation, though, during an isothermally carried out polymerization the set temperatures for the flange and the cover are chosen equal to the reactor temperature. As a result, the heat flow through these two parts is extremely small and is only due to the deviations during the temperature controlling process, which are on the order of millikelvin. Further, these deviations result in both cooling and heating the reactor and the final accumulative heat that flows through the flange and the cover is almost zero. B. Specific Heat Capacities. Although heat flow calorimetry is not the most precise method for measuring the specific heat capacity, it proves rather useful because it allows determining this variable at the actual reaction conditions and for mixtures whose specific heat capacity cannot be estimated using an equation of state. Essentially, the heat rate required to create a temperature ramp allows for the calculation of our model reaction mixture’s specific heat capacity. During this temperature ramp, there is no chemical reaction, no additional reactants are injected in the reactor, the calibration heater is turned off, and the stirring heat term is introduced in the baseline. Hence eq 1 gives

∫[UA(Tr - Tj) + UcovAcov(Tr - Tcov) + UflAfl(Tr - Tfl)] dt - ∫(minscp,ins)‚dTr cV,mixture ) ∫mmixture dTr

(6)

Following this procedure, the constant-volume specific heat capacity for the reaction mixture was measured both before and after the polymerization of MMA and the respective values are 1392.9 J kg-1 K-1 and 1554.7 J kg-1 K-1. Finally, an important point regarding the heat transfer variables is that the reaction mixture’s physical properties change significantly during the polymerization reaction. The initial mixture is homogeneous and consists primarily of the monomer, the stabilizer, and the supercritical solvent, but as the monomer is transformed into polymer, the mixture becomes heterogeneous (polymer, monomer, stabilizer, and solvent) and the heat transfer characteristics of the mixture change drastically. Therefore, for the measurements after the reaction completion, it is important to have efficient mixing conditions in order to ensure that the temperatures recorded are representative of the entire reaction mixture. Measuring the Stirring Heat Term. During the calorimetric calculations, the heat released by the stirrer is introduced in the baseline. The latter accounts for all the heat that flows through the reactor parts and should not be attributed to the polymerization reaction. Consequently, the actual heat released by the reaction is found after subtracting the baseline from the measured heat flow, as shown in Figure 2. The majority of the appearing offset between the initial state and the final state is due to the changes in the viscosity and the density of the mixture that affect the necessary stirring power for a given rotational speed. Therefore, the offset in the baseline is directly related to the evolution of the reaction as this is expressed by the conversion. Hence, the most appropriate form for the baseline is one proportional to the conversion. In the case of the chosen model reaction, the heat released by the polymerization is significantly higher than the heat dissipated by the stirrer. As a result, the introduction of the

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Figure 2. Schematic representation of the baseline in calorimetric calculations.

stirring heat term in the baseline does not disturb in any way the calorimetric calculations. However, in the case of more slow and/or low heat producing reactions, this introduction can lead to erroneous calorimetric calculations and the stirring heat must be measured and taken into account in eq 1. The developed supercritical reaction calorimeter allows also for measuring the heat dissipated by the stirrer. This measurement is performed using the adiabatic operating mode of the reaction calorimeter, with no reaction occurring and no injection of additional reactants in the reactor. As a result, eq 1 gives

Q˙ stir ) Q˙ acc )

∑i (micV,i + minscp,ins)

dTr dt

(7)

According to this method, the heat dissipated by the doublestage turbine in our reactor was measured before and after the polymerization and was found to be 6.40 and 3.67 W kg-1 of reacting mass, respectively. The observed decrease can be explained by looking at the power number (Np) and Reynolds number (Re) equations, which relate the stirring power to the viscosity and the density of the medium and the rotational speed and diameter of the stirrer.

Np )

Q˙ stir FN3ds5 Re )

(turbulent region)

(8)

ds2NF η

(9)

These two nondimensional numbers are linked by power plots that in the turbulent region show a general tendency of higher Re for lower Np. Additionally, Lavanchy has previously shown that this tendency is also valid for pure scCO2 and Reynolds numbers as high as 106. Therefore, the observed decrease in the stirring power is an indication that the kinematic viscosity of the final ∼25% v/v PMMA/CO2 dispersion is lower than the one of the initial homogeneous ∼37 wt % MMA/CO2 mixture. This indication can be justified by three facts. First, the MMA-CO2 initial mixture has higher viscosity and density than the pure scCO2 and its negative ∆V of mixing is found to increase them even more.60 Second, the very small size of the polymer particles, the fact that they are spherical and well stabilized, and that the dispersion is in the intermediate range of particulate concentrations (∼25% v/v) may result in a viscometric behavior that is predominantly characterized by the continuous phase (pure CO2).61 Last but not least, experimental data of the ultrasound sensor installed in the reactor show that the ultrasonic propagation velocity (UPV) decreases with conversion (Figure 3). This decrease can be attributed to a decrease in the medium’s density due to the consumption of

Figure 3. Ultrasonic propagation velocity (UPV) evolution in the model reaction mixture versus monomer conversion.

Figure 4. Polymerization reaction heat rate and its calculated enthalpy of reaction.

MMA, since the UPV of pure CO2 increases with the increasing pressure during the reaction.62 In fact, the latter becomes predominant only at the very last stages of the reaction (>92% monomer conversion). Monitoring the Polymerization Reaction of MMA in scCO2. The previous measurements of the contributions of single heat terms were necessary to attain the final objective, which is to monitor the polymerization reaction. The first and more direct result of monitoring the model polymerization using reaction calorimetry is the released heat rate and consequently, by integration, the enthalpy of reaction. Figure 4 presents the results for the reaction carried out under the reference conditions and demonstrates the high accuracy of the supercritical calorimetry technique given that the literature value for the MMA polymerization enthalpy of reaction is 57.5 ( 1.0 kJ mol-1.63 The results provide also information on the kinetics during the polymerization. Looking at the heat rate evolution during the first 40 min, we can see the reaction rapidly accelerating to a certain level and then staying at this plateau for the next 20

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Figure 5. Effect of stirring on the observed reaction deceleration.

min. This evolution indicates that in the beginning the reaction primarily occurs in the continuous phase because the constant reaction rate is characteristic of an environment with no restrictions on the mass transfer of the growing polymer chains and no effect of the monomer consumption, such as the continuous phase at the very early stages of the reaction where the overall conversion is less than 1%. This observation is also in agreement with previous reports on the reaction mechanism of the free-radical dispersion polymerization of MMA in scCO2.40,64 After that the reaction starts to accelerate again until it reaches a maximum at approximately 70 min. During this period the main locus of polymerization is the formed polymer particles and the acceleration is due to the appearing Trommsdorff effect. Finally, the monomer is consumed and the reaction rate decreases sharply. The robustness and the accuracy of the optimized reaction calorimetry technique have permitted the investigation of the effects of various reaction parameters in a previous publication.47 In summary, the stirrer type and the rotation speed were found to have no effect on the reaction evolution and the final product under the conditions tested. The monomer and CO2 concentration were found to play an important role in the formation of the dispersion, which in turn affects the reaction evolution. Finally, the most important parameter identified was the stabilizer concentration. These findings were also compared with the results of Wang et al.44 Additional results on the monitoring of the model reaction under the reference conditions regard an observed small deceleration at approximately 24 min. It was previously shown that the reference conditions are at the limit of the dispersion stability and that a small increase in the reactant concentrations can improve this stability and lead to the disappearance of the deceleration.56 In this study the role of stirring was also investigated under these unstable conditions, given that previous reports have shown that intensive mixing can either improve or worsen the reaction conditions.65,66 The results of this investigation are presented in Figure 5. In essence, increasing the stirring speed improves the reaction conditions and more specifically the formation of the dispersion and that results in the disappearance of the reaction deceleration step. However, overall, the reaction does not accelerate significantly. On the other hand, in a precipitation polymerization system Liu et al. have found the stirring speed to increase significantly the polymerization rate.43 Monitoring the Esterification of Acetic Anhydride with Methanol in scCO2. An alternative nonpolymerization reaction system was also studied, namely the esterification of acetic anhydride with methanol, to demonstrate the ability of the experimental setup and the technique to monitor a more rapid

Figure 6. Reaction heat rate and chemical conversion evolution during the esterification of acetic anhydride with methanol.

chemical reaction. This reaction, being highly exothermic, has been previously used as a model reaction in reaction calorimetry studies.67 Methanol was chosen as the excess reactant because it forms homogeneous supercritical mixtures with CO2 under temperature and pressure conditions that can be attained with the experimental setup.68 Consequently, a homogeneous reaction mixture can be established before the injection of the second reactant, namely acetic anhydride. From previous studies on this reaction it has been identified that the kinetics under the condition of excess methanol is that of a pseudo-first-order reaction.69 Moreover, the homogeneous supercritical initial mixture allows for studying the reaction at temperatures higher than the boiling temperatures of the reactants and as a result the response at higher reaction rates can be tested. The esterification reaction was performed under the following conditions: acetic anhydride mass, 41.57 g (0.31 M); methanol mass, 64.71 g (1.58 M); CO2 mass, 660 g; temperature, 80 °C; initial pressure, 15.7 MPa. The results of the measured reaction heat rate, as well as the calculated chemical conversion, are shown in Figure 6. The first important point is that the reaction calorimeter manages to monitor a reaction that within the first 2 min releases approximately 550 W kg-1Ac.An.. This value is more than 2 times higher than the heat rate in the MMA polymerization reaction. Second, the profile of the reaction heat rate corresponds well to a pseudo-first-order reaction as was expected. Finally, by integration, the calculated enthalpy of reaction is found to be 64.5 ( 2.3 kJ mol-1, which is in very good agreement with literature values of 67.7 ( 1.5,70 60.0,71 and 66.3 kJ mol-1.72 The chemical conversion was measured using 1H NMR analysis on samples collected from the reactor after the degassing of the supercritical solvent. The results of the spectroscopic analysis, shown in Figure 7, demonstrate that the expected methyl acetate, acetic acid, and excess of methanol are present and no trace of the completely reacted acetic anhydride exists in the samples. The point where a resonance peak for acetic anhydride should appear is shown with the dotted line. In conclusion, the performance of the esterification reaction of acetic anhydride with methanol using scCO2 as a solvent has shown that the supercritical reaction calorimetry technique can be applied for monitoring other types of chemical reactions with SCFs than polymerizations and that the reaction calorimeter can monitor the evolution of rather highly exothermic reactions. Safety Aspects. Reaction calorimetry is widely used not only to obtain kinetics data on a chemical reaction but also to provide valuable information about the safety and the operability of an entire process. More specifically, during scaling up, numerous

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Figure 7. 1H NMR analysis of the esterification reaction product samples.

our polymerization the stirrer will continue to release heat; thus the reactor temperature will increase further. Finally, the expected pressure increase was calculated assuming the following: (a) The reaction rate does not increase with the rising temperature. (b) The pressure increase is calculated by taking into consideration only the CO2, which is the predominant component in the reactor. It is found that the reactor temperature will increase but within the operational limits of the equipment, giving sufficient time to take the necessary actions (e.g., injection of inhibitor to quench the reaction). On the other hand, the pressure will go up by 75% within 30 min, but most importantly, it will overcome the operational safety limit of the reactor (35 MPa) in just 21 min. These results clearly demonstrate that, in the case of reactions with SCFs, the supercritical nature of the solvent multiplies the significance of the pressure and renders it more important than the temperature, as far as the reaction safety is concerned. Conclusions

Figure 8. Temperature and pressure evolution in a hypothetical scenario of cooling system failure during the polymerization reaction under the reference conditions. The dotted line is the reactor pressure, the solid gray line is the reactor temperature taking into account the continuously released stirring heat, and the solid black line is the latter without the stirring heat.

process conditions change and the safety constraints may no longer be respected. Using reaction calorimetry, it is possible to verify the safe operation of a process under industrial conditions using a lab scale setup without necessarily having in advance all the detailed information on the chemical reaction itself.73 Although the experiments carried out in this study were not designed to evaluate the safety of the model reaction, they nevertheless provide some relevant information. The measurement of the reaction heat rate permits the calculation of one of the most characteristic variables in the assessment of the thermal risk of a reaction: the Maximum Temperature attainable by the Synthesis Reaction (MTSR).74 Essentially this is the temperature the reactor will reach once the remaining heat of the reaction is released adiabatically in the reactor.

∫t t Q˙ Reaction dt f

MTSR(t) )

∑i (micV,i + minscp,ins)

+ Tr(t)

(10)

Assuming a hypothetical scenario of a failure in the cooling system of the reactor after 60 min of operation can help illustrate the significance of the pressure in the operational safety of reactions with SCFs. Figure 8 shows the results of the MTSR calculations after the point of the cooling system failure. Additionally, we have taken into account that in the case of

Heat flow reaction calorimetry was developed to monitor reactions with SCFs and can reliably measure important heat transfer variables, such as the overall heat transfer coefficient and the specific heat capacity of the reaction mixture. It also permits the measurement of the heat released by the stirrer in the reactor, which gives indications about the physical state of the mixture. Moreover, the obtained results in terms of enthalpy of reaction for the chosen model polymerization were found to be in very good agreement with the literature values. The polymerization’s evolution was monitored, and the kinetics data obtained reveal a reaction mechanism which is in good agreement with previous reports. The technique was also applied successfully to monitor the esterification reaction of acetic anhydride with methanol in scCO2. Finally, with the aid of a hypothetical scenario of a cooling system failure, it was demonstrated that in the case of reactions with SCFs the pressure is the most important parameter in the discussion on the reaction safety. Acknowledgment The Swiss National Science Foundation is gratefully acknowledged for its financial support through Project No. 200020-109051. Nomenclature dTr/dt ) temperature change rate [K s-1] ds ) stirrer diameter [m] Q˙ acc ) rate with which heat accumulates in the reactor [W] Q˙ calib ) rate with which heat is generated by the calibration heater in the reactor [W] Q˙ dos ) rate with which heat is inserted in the reactor by reactant dosing [W] Q˙ flow ) rate with which heat flows from the reactor to the jacket [W] Q˙ inj ) combined dosing and mixing term [W] Q˙ loss ) rate with which heat is lost by the reactor, the jacket, or the surrounding devices due to secondary reasons (e.g., radiation) [W] Q˙ mix ) rate with which heat is generated due to mixing [W] Q˙ r ) rate with which heat is generated by the reaction [W]

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ReceiVed for reView September 6, 2007 ReVised manuscript receiVed January 8, 2008 Accepted January 9, 2008 IE0712030