Reaction Calorimetry as a Tool in Process Development, Research and Safety Analysis

Transcription

1 Reaction Calorimetry as a Tool in Process Development, Research and Safety nalysis Rikard Widell Department of Chemical Engineering II, Lund University, P. O. Box 14, SE-1 Lund, Sweden bstract Reaction calorimetry is a tool, which has found wide applications for thermal and kinetic analysis of chemical ions. The information from the measurements is mainly used for process development and studies of or safety. s the most of chemical and physical processes are associated with heat effects, calorimetry is a powerful tool to get thermodynamic and kinetic data from these processes. Some ion parameters that could be determined with ion calorimetry are activation energies, ion enthalpies, order of ions and ion rate constants. These parameters are of significant importance by scaling up processes. In this work the rrhenius parameters and heat of ion for the esterification ion between acetic anhydride and methanol was determined by a CP ion calorimeter (from ChemiSens). Three ion rate models were tested for the esterification ion, where two of them proposed an autocatalytic behaviour. The autocatalytic models were based on a carboxylic acid dependency; acetic acid is a side product in this ion. The two catalytic models gave excellent adaptations to the experimental results. The estimated parameters agreed very well with values found in the literature. Introduction Reaction calorimetry is a powerful analysis tool, which has found wide applications in process development, safety studies and basic research. The activity on this field has increased continuously since the commercial break-through at the 196s and the trend is to this day still increasing, indicating the great potential of this technology [1]. Since almost all of chemical and physical processes are associated with heat effects ion calorimetry is a suitable tool to investigate such processes. In this study an investigation of the esterification of acetic anhydride and methanol is made, concerning certain ion parameters and the ion mechanism. Many previous investigations propose a ion rate expression based on the ant concentrations. In this study it is proposed a kind of autocatalytic ion mechanism, where formed acetic acid catalyzes the esterification. Strong mineral acids, like sulphuric acid, have been used as catalyst for a long time. It is general accepted that these strong acids protonate and activate the anhydride in esterifications between anhydrides and alcohols. However, it is not often reported a significant catalytic effect of formed carboxylic acid. The ion is studied in a CP ion calorimeter from ChemiSens. This or is a smallscale or with a working range between 4-18 ml, which means a low consumption of chemicals, saving both money and environment. Theory Principle The ion calorimeter CP from ChemiSens works with a kind of heat flow principle called true heat flow by the company. One significant difference between CP and an ordinary heat flow calorimeter is the temperature measurement, which the heat flow rate through the or wall (W) is based on. In an ordinary heat flow flow ion calorimeter the ion temperature Tr and the jacket temperature T j are measured. In the CP ion calorimeter the temperature at two points within the or wall is measured, T 1 and T. Thus, changes of the heat transfer coefficients h r and h j do not affect the overall heat transfer coefficient 1

2 U, which is only affected of the thickness of the or wall L and the heat conductivity of the or wall W. The CP ion calorimeter is pre-calibrated, which means that the instrument directly on line presents the heat production rate and taking the variation of W with temperature into account. In the CP ion calorimeter a Peltier element is inserted beneath the or bottom. The Peltier element has nothing to do with the measuring principle but it controls the temperature in the or by heating/cooling. The main heat flow sensor measures the temperatures T 1 and T and is installed between the or bottom and the Peltier element, see figure 1. (T -T 1 ) is measured. The total energy balance can be written: q & = lid phase where is the ion heat flow (W), is the heat flow rate (W) occurring due to ing enthalpies when different fluids are ed, and is the heat flow rate (W) due to phase phase changes. The parameters (W) and (W) are dos sirr the heat flows corresponding to dosing and stirring respectively and (W) describes the heat flow lid rate through the or lid. In this study the true heat flow is equal to the ion heat flow since all other parameters could be neglected. The heat of ion for isothermal batch experiments is therefore calculated through following expression: dos stirr Reactor base flange H = r n i, dt Heat flow transducers Peltier element Main heat flow sensor Figure 1. Schematic figure of the CP ion calorimeter. ll of the heat flow out of the or is located to the or bottom since the rest of the or is insulated. The or is submerged in a thermostat bath with a temperature of. C higher than the or temperature in order to give an active insulation. Because of this small temperature difference between the or and its surrounding, a small amount of heat flows into the or through the or flanges. Sensors in the flanges measure this heat flow and compensate for it. The or wall consists of a double glass wall and is therefore also passive insulated []. Energy balances The true heat flow in this study is symbolized (W) and can be expressed: ( ) = λ T T 1 d where is the specific heat conductivity (W m - K - 1 ) of the construction material that the heat flow passes on its way from the or through the Peltier element and to the thermostat bath. is the heat transfer area (m ) and d (m) is the length between the points where the temperature difference where n i,o is the initial amount of the ant not in excess (mole). ctivation energies and frequency factors are calculated through the rrhenius equation: k = e E a /( R T ) where k is the rate constant, is the frequency factor and E a is the activation energy. The experiments were performed in the isothermal mode at three temperatures (55, 6 and 65 C). The or was initially loaded with about 5 grams acetic anhydride ( 98.5%, from Sigma ldrich) and heated to the ion temperature. bout three grams methanol (>99%, from Sigma ldrich) was pre-heated to the ion temperature in the thermostat unit and batched into the or through a syringe. The initial molar ratio was 5:1, with acetic anhydride in excess. To secure a homogenous liquid phase the stirrer was set to 5 rpm. Results and conclusions The ion for the esterification of acetic anhydride and methanol is often written: (CH CO) O + CH OH CH COOH + CH COOCH (a) But in reality the ion mechanism is more complex and involves several ions. Balland et. al. [] proposed following ion path:

3 (CH CO) O + CH OH CH COOH + CH COOCH CH COOH + CH OH H O + CH COOCH (b) (c) (CH CO) O + H O CH COOH (d) In one experiment acetic acid were added to a large amount of methanol at 6 C and atmospheric pressure. Under these conditions no detectible ion occurred. The conclusion of this experiment is that the acetic acid-methanol ion is very slow compared to the acetic anhydride methanol ion. Since the acetic anhydride was in sufficient excess the ion was assumed to be of pseudofirst order with respect to methanol. The percentage change in concentration of acetic anhydride with ion time was relative small compared to corresponding change for methanol, why methanol was said to be rate determining. The ion rate for a pseudo-first order ion is expressed: r = k c where the index represents methanol. The rate constant k for each temperature was determined by adaptation of the theoretical model to the experimental results. The adaptations were simulated in Matlab by a program in Matlab toolbox called NLINFIT. This program fits data and estimates the coefficients of a non-linear function using least squares (Gauss-Newton method). The output data from this program are the fitted coefficients, the residuals and the Jacobian. The estimated coefficients, which in this case were the rate constants, have then been used in Excel for plotting graphs. The activation energy was calculated to 68.1 kj/mole and the pre-exponential factor was calculated to s -1 for the isothermal experiments. The simulated curves were fairly adapted to the experimental curves, see graph daptation-no catalysis Graph 1. The graph shows an adaptation for a pseudofirst order ion with no catalytic effect at 55 C. sigmoid profile of the heat flow curve indicated that something else also affected the ion rate. The ion rate expression gave a much better fit to the experimental curves when the ion order was in the range.7-.75, which also indicated that something else affected the ion rate. This profile was more obvious in the experiments made at the lower temperatures where the ion rate is relative low, but the tendency was clear even at higher temperatures. Three things that could affect the profile of the curve are mass transport resistance, chemical equilibrium and catalytic effects. The first one was not likely since the agitation was good, and the second one was not likely either since the ion is not known to be an equilibrium ion. The third one was interesting since there was an acid formed as side product. If sulphuric acid had been added as catalyst, the catalytic effect of the strong inorganic acid had been protonation of the anhydride. The consequence had been weakened chemical bondings and therefore also a decreased activation energy. Concerning the catalytic mechanism when acetic acid serves as catalyst, two theories have been investigated. The first one was protonation of the anhydride with protons from dissociated acetic acid with following proposed ion rate model: r = k1 + k + H where the concentration of protons was assumed to follow an equilibrium: = [ H ] [ CH COO ] + K a [ CH COOH] s could be seen the model above describes two parallel ions, one catalytic and one noncatalytic. The simulated curves were in excellent agreement with the experimental curves, see graph. daptation-proton catalysis Graph. The graph shows an adaptation for a proton catalysis ion at 55 C. The activation energies for the two ions were calculated to 7. kj/mole and 66.4 kj/mole

4 respectively. The activation energy for the catalytic ion was lower than the activation energy for the non-catalytic ion, which agrees with the literature. The pre-exponential factor was determined to s -1 for the non-catalytic ion and l mole -1 s -1 for the catalytic ion. To calculate the concentration of H + the dissociation constant for acetic acid at 5 C has been used. n error could therefore be expected because the constant might vary with temperature and solvent used. There is also assumed that all of the free protons protonates the anhydride, which could give an error. The second theory was based on that formed acetic acid or acetic anhydride act like a solvent and therefore increase the contact of the ants. Two different alternatives were proposed in this theory. The first is based on that formed acetic acid acts as a solvent for methanol and acetic anhydride and therefore increasees the ion rate in ion (b). The second alternative proposes that ion (c) takes place with acetic anhydride as solvent. conclusion from the experiment with acetic acid and methanol was that the esterification between acetic acid and methanol in absence of acetic anhydride is very slow compared to the esterification between acetic anhydride and methanol. However, there is a possibility that ion (c) runs with a significant ion rate when the anhydride is present. The proposed ion rate expression for both of these alternatives is: r = k1 + k CH COOH This model also gave an excellent agreement between simulated and experimental curves, see graph. daptation-solvent catalysis Graph. The graph shows an adaptation for a solvent catalysis ion at 55 C. The activation energy was determined to 68. kj/mole for the non-catalytic ion and to 68.6 kj/mole for the ion where acetic acid acted as a solvent. The pre-exponential factors were calculated to s -1 and l mole -1 s -1 respectively. The heat of ion of the esterification was calculated to ±. kj/mole, which agreed with values found in the literature. comparison between different tests was made and summarized in table 1 and table. Table 1. Comparison of rate constants between different tests. Test k,1 (s -1 ) k, (l mole -1 s -1 ) CP test * CP test ** RC1 test [4] (l mole -1 s -1 ) FI RSST test ICI dewer test Monsanto RC test HEL PHI-TEC test [4, 6] VSP test [4, 7] (l mole -1 s -1 ) Dewer test [4, 8] (l mole -1 s -1 ) FI round-robin test * Values from the solvent catalytic theory have been used. ** Values from the proton catalytic theory have been used. Table. Comparison of activation energies and ion enthalpies between different tests expressed in kj/mole. Test E a,1 r H E a, CP test * CP test ** RC1 test [4] FIR SST test ICI dewer test Monsanto RC test HEL PHI-TEC test [4, 6] VSP test [4, 7] Dewer test [4, 8] FI round-robin test RC1 reflux test [9] * Values from the solvent catalytic theory have been used. ** Values from the proton catalytic theory have been used. Discussion Reaction calorimetry is a technology that has been used in various outfits over the last 4 years and the reason to the popularity is perhaps the relative simplicity. In batch experiments the heat flow measurement is just based of temperature measurements at certain strategically locations. One big advantage is the on-line measurement of the heat evolution rate. In this study a sigmoid profile was 4

5 obtained, leading to the theory of autocatalysis in the investigated ion system. It is obvious that the two catalytic models give a better fit to the experimental results than the noncatalytic model. There is hard to determine which one of the catalysis models that gives the best adaptation to the experiments; both of them agree very well with the experimental curves. One reflection is that a combination of the two theories might tell the truth. Reaction calorimetry is an excellent tool for determination of kinetic and thermodynamic data for the esterification between acetic anhydride and methanol. In addition it also gives information about ion mechanisms. Nomenclature Symbol Description stirr heat evolution of stirrer, W output signal from CP- or, W r ion rate for a chemical ion, mole/(l s) R gas constant, J/(mole K) T temperature, K or C T 1 temperature in a point in the or wall, K or C T temperature in a point in the or wall, K or C T j temperature on the jacket side, K or C T r or temperature, K or C U overall heat transfer coefficient, W/(m K) Greek letters rh heat of ion, kj/mole W heat conductivity of the or wall, W/(m K) References heat transfer area, m [1] R. N. Landau, Expanding the role of ion frequency factor, s -1, l/(mole s) calorimetry, Thermochimica cta 89,1996, p. 11- c concentration, M 16 d the length between the points where the [] Chemisens, CP -Reaction calorimeter temperature difference (T -T 1 ) is measured, m systems (Product information), 4 E a activation energy, J/mole [] L. Balland et. al., Kinetic parameter estimation h j heat transfer coefficient on the jacket side, of solvent-free ions: application to esterification W/(m K) of acetic anhydride by methanol, Chemical h r heat transfer coefficient on the or side, engineering and processing 41,, p W/(m K) [4] Y.-S. Duh et. al., pplications of ion k ion rate constant, s -1 calorimetry in ion kinetics and thermal hazard k 1 ion rate constant of the non-catalytic evaluation, Thermochimica cta 85, 1996, p ion in a ion system of both a [5] J. C. Leung et. al., Round-robin vent sizing catalytic and a non-catalytic ion, s -1 k package results, International symposium on ion rate constant of the catalytic ion in a ion system of both a runaway ions, 1989, p catalytic and a non-catalytic ion, [6] J. Singh, PHI-TEC: Enhanced vent sizing l/(mole s) calorimeter-application and comparison with K a acid constant (dissociation constant), M existing devices, International symposium on L thickness of the or wall, m runaway ions, 1989, p. 1- n i, initial amount of component i, mole [7] L.Friedel et. al., Modelling of the vented heat effect of dosing a ant with a dos methanol/acetic anhydride runaway ion using temperature not equal to the or SFIRE, J. Loss. Prev. Proc. Ind. 4, 1991, p. 11- temperature, W 119 heat flow through the or wall, W flow [8] T. K. Wright et. al., diabatic dewer calorimeter, heat flow through the or lid, W I. Chem. E. Symposium series no.97, 1986, p. 11- lid 1 heat evolution/consumption rate due to ing, [9] J. Wiss et. al., Determination of heats of ion W under refluxing conditions, Chimia 44, 199, p. 41- heat flow due to phase changes, W phase 45 heat flow due to a chemical ion, W Received for review 1/4/5 5