Ind. Eng. Chem. Res. 1999, 38, 4629-4633
4629
Kinetic Investigations of a Ketonization Reaction Using Reaction Calorimetry Aljosa Crevatin,‡ Francesco Mascarello,‡ Bettina Leuthe,‡ Bruno Minder,‡ and Ireneo Kikic*,† Chemical, Environmental and Raw Materials Engineering Department, University of Trieste, I-34127 Trieste, Italy, and F. Hoffmann-La Roche Ltd., Vitamins and Fine Chemicals, CH-4070, Basle, Switzerland
The reactor calorimetric technique was employed to study a homogeneous, liquid-phase ketonization reaction used in fine chemical syntheses, where an unsaturated alcohol and unsaturated ether condense to form an unsaturated ketone with higher molecular weight. The experiments were carried out in a calorimetric reactor (RC-1) and the data obtained allowed the determination of reaction heat flow versus time, overall reaction enthalpy, and reaction course for experiments at different temperatures. Using the latter data, the conversion versus time behavior was determined. These data were modeled assuming a pseudo-first-order kinetics and an Arrhenius-type temperature dependence of the specific reaction rate. Comparison between these results and the usual concentration versus time experimental data obtained with a different technique was satisfactory. Introduction Many fine chemical industrial syntheses, for the production of long-chain compounds, use as intermediate reactions the condensation of an unsaturated alcohol and an unsaturated ether to form an unsaturated ketone with higher molecular weight, which is very useful for different subsequent reactions in the chain prolongation. They can be usually classified as “Claisen”type ketonization reactions and are used in perfume, vitamin, and other fine chemical syntheses.1-4 The general pathway for such reactions5 is presented in Figure 1 where the unsaturated tertiary alcohol (allylalcohol) (later A) and the unsaturated ether (later B) condense to form the methyl-ketone (later C) and the ketal (later D). This is a homogeneous liquid-phase, acid-catalyzed, exothermic reaction. The kinetic description and the enthalpy of the reaction are of paramount importance for the reactor modeling and optimization. A reaction calorimetric technique was employed for the kinetic investigation of the reaction. The experimental data were obtained at different temperatures. Using these data, we could evaluate the heat of the reaction and therefore the conversion course versus time and the overall reaction enthalpy. The experimental calorimetric conversion data were correlated using a simple kinetic model based on the reaction chemistry. The determined specific reaction rates allowed the description of the kinetics. Comparison between these results and the usual concentration versus time experimental data was satisfactory and provided a validation of the method. Experimental Section The reported purity of the reagents used was 98.0% for the unsaturated alcohol and 97.5% for the unsatur* To whom correspondence should be addressed. † University of Trieste. ‡ F. Hoffmann-La Roche Ltd.
ated ether; the catalyst was a solution of phosphoric acid in acetone. Roche supplied the reagents and the catalyst was from Fluka (phosphoric acid, 99.9%; acetone, 99.99%). Experimental Apparatus. The experiments for the determination of the heat of the reaction were carried out using the reactor calorimeter (RC-1) from the “Mettler Toledo”, schematically represented in Figure 2. The main part is the hastelloy steel batch reactor with a nominal volume of 1.8 L and operative temperature from -20 to 250 °C and pressure up to 60 bar. The heating and cooling of the reactor is provided through the reactor jacket, using oil, by the thermostatic RC-1 system coupled with an external cryostat. To minimize the heat losses, the top of the reactor is also heated. The temperature control comprises four thermocouples for the measurement of the reactor, jacket, top of the reactor and the cooling oil temperatures, with a precision of (0.001 K. The reactor also has a calibration system (electric heater) to bring into the reactor a very precise quantity of heat (QC), which permits one to make the necessary calibrations during the experiments. A piezoresistive sensor measures the pressure, with a precision of (0.01 bar. The stirrer is a four-propeller stirrer with a speed range from 30 to 2500 rpm and a rotation momentum measurement system. The reactor is also coupled with a high-pressure pump for the fillin, a laboratory balance for the mass determination, and a laboratory vacuum pump to remove the air from the reactor. The security system is composed of a stopsolution (NaOH) and a blow-down tank connected to the reactor with a rupture disk valve. The reactor calorimeter system and all measured parameters are controlled and recorded through an electronic transducer by a software computer package. Experimental Procedure. All experiments were carried out using stoichiometric reagents molar ratio. The predetermined reagents mass was exactly weighted with the laboratory balance and then introduced into the reactor. By the vacuum pump the reactor was evacuated; successively, nitrogen was introduced. The
10.1021/ie980697l CCC: $18.00 © 1999 American Chemical Society Published on Web 10/26/1999
4630
Ind. Eng. Chem. Res., Vol. 38, No. 12, 1999
Figure 1. Reaction scheme.5
Figure 2. Experimental calorimetric equipment.
stirrer speed was set on 1000 rpm to ensure the perfect mixing and to avoid temperature and concentration gradients. Because the reaction does not occur in the absence of a catalyst, the reactor was heated to the desired temperature and a calibration was carried out to determine the overall heat-transfer coefficient and the flow of total heat loss (base line). At this point, through the pump, the exact quantity of the catalyst solution was filled and the reaction started. During the experiments the reaction temperature was maintained constant and through the control system some calibrations allowed the determination of the heat-transfer coefficients. The experimental data collection was long enough to ensure the complete conversion of the reagents. The complete conversion was verified in an independent experimental run, using GC analysis of the samples. The absence of a reaction heat flow and the linearity of the heat flow curve were then the main evidence for complete conversion. Control System. The heat of the reaction was determined on the basis of the heat flow between the reactor and the jacket, or rather on the difference between the reaction mass (Tr) and jacket oil (Tj) temperatures. Therefore, the temperature control system was of paramount importance for the accuracy and reproducibility of the experiments. In Figure 3 the temperature control system of the reaction calorimeter is illustrated. The reaction mass temperature (Tr) had a proportional control and the jacket temperature (Tj) a proportional-integral one. Results and Discussion The investigated ketonization reaction is an exothermic, irreversible, and very slow reaction. As mentioned
Figure 3. Scheme of the calorimetric temperature control system.
before, the experiments were carried out until the complete conversion was reached to maximize the collection of experimental data and to minimize errors in experimental data evaluation. As a consequence, the experimental reaction time was between 10 and 60 h, depending on the reaction temperature. The investigated temperatures were 389, 410, 426, and 445 K. Calorimetric Data Evaluation. The overall heat balance for the system is expressed as6
and rearranging
Qf + Qa ) Qr + QC + Qstir - Qi - Qadd - Qloss
(2)
Because it is difficult to determine exactly and separately all terms of eq 1, in the experimental data treatment they were globally considered as one time-
Ind. Eng. Chem. Res., Vol. 38, No. 12, 1999 4631
dependent term Q(t) and the calibration heat flux QC was assumed negligible in comparison to other fluxes:
Q(t) ) Qf + Qa ) Qr + Qstir - Qi - Qadd - Qloss ) Qr + Qb (3) where Qb is the sum of all fluxes different from reaction heat flux. The accumulation term Qa is defined as
Qa ) mrcpr
dTr dt
(4)
and during the reaction, because of isothermal conditions, can be considered negligible. The heat flux through the reactor wall is
Qf ) UA (Tr - Ta)
(5)
while the oil film plays an important role because of its viscosity and low flow, the wall temperature was calculated from the measured thermostatic oil temperature as6
Ta ) a(T*r - Tr) + b(T* r - Tj) + Tj
(6)
with
a)
4(1 + j)2 4r(1 + j)2 and b ) 4 + 3r + 3j + 2rj j(4 + 3r + 3j + 2rj)
(r ) 0.4, j ) jf(1.12 + 0.006Tj), and jf ) 0.12) The reaction mass is well-mixed by the stirrer and therefore the effects of the inner film can be assumed negligible. In the presence of a reacting system the product of the overall heat-transfer coefficient and the exchange area UA changes during the reaction, so it was determined experimentally by calibrations: f
UA )
∑s QC∆t f
(7)
∑s (Tr - Ta)∆t These data were correlated along the total reaction time, obtaining their profile, which was then used in eq 5 to determine the heat flux through the reactor wall. For the integration, the base line was a linear connection between the starting and the final Q-constant values. In these conditions the reaction does not occur (Qr ) 0) and from eq 3, the total heat flux Q is equal to Qb. The overall reaction enthalpy was than calculated by numerical integration in the form
∆Hr )
∑[(Q - Qb)∆t]
(8)
with ∆t being the time interval between two subsequent measurements (in the experiments reported, it was 10 s). The reaction enthalpy was ascribed to only one chemical reaction; therefore, from the reaction enthalpy,
Figure 4. Experimental curves (experimental data versus time) obtained from the calorimetric control system for the experiment at 426 K. (X ) conversion; P ) pressure (MPa); T ) temperature (K × 10-3); Q ) reaction heat flow (J/min × 105).
the conversion of the reaction on the thermal basis is defined as
X(t) )
∆Hr(t) ∆Hr(tf)
(9)
was determined. In Figure 4 the typical calorimetric results for the experiment carried out at the temperature of 423 K are presented. The course of the reaction heat flow (Q) versus time (curve Q), the base line (dashed line), the integrated area that corresponds to the reaction enthalpy, and the constancy of the reactor temperature with time (curve T) are clearly evident. The experimental conversion, on a thermal basis, is given by curve X. During the reaction the pressure of the system slightly decreases (curve P) with an inverse course to conversion, due to differences in vapor pressures between reagents and products. The obtained reaction enthalpies (∆Hr) for the different experiments are -353.2 kJ at 445 K, -348.1 kJ at 426 K, and -349.9 kJ at 410 K. Therefore, the experimental average reaction enthalpy is -350.4 kJ, and while the initial amount of reagent A was 3.856 mol, the mean reaction enthalpy referred to as A is -90.87 kJ/mol. When the Joback method7 was used, the calculated enthalpies of formation (∆Hf) for the components at 423 K are -250.43 kJ/mol for A, -210.63 kJ/mol for B, -281.70 kJ/mol for C, and -487.80 kJ/mol for D. The calculated reaction enthalpy referring to the reagent A, expressed as
) ∆Hfc + ∆HfD - ∆HfA - 2∆HfB ∆Hcalc. r
(10)
) -97.81 kJ/mol. The values are in agreeis ∆Hcalc. r ment with each other and the difference is about 7%. This difference can be ascribed to the model prediction error and to experimental error due to small deviations in the reaction temperature during the fill-in of the catalyst. Kinetic Model Correlation. The investigated homogeneous, liquid-phase, ketonization reaction comprises the reaction of 1 mol of unsaturated alcohol (A) and 2 mol of unsaturated ether (B) to form 1 mol of unsaturated ketone (C) and 1 mol of diether (D). The reaction can than be expressed as H+
A + 2B 98 C + D
4632
Ind. Eng. Chem. Res., Vol. 38, No. 12, 1999
Figure 5. Experimental conversions from calorimetric data and the conversions calculated with the obtained reaction model; for the experiments at 410, 426, and 445 K. Figure 6. Temperature dependence of the reaction rate parameters correlated with an Arrhenius-type equation.
Table 1. Reaction Rate Parameters, Standard Deviations, and Correlation Goodness from the Experimental Data Fitting exp. temp. (K)
388.85
409.85
426.15
444.85
k (L/min) stand. deviation correlation
0.000 930 0.079 836 0.974 615
0.002 564 0.033 967 0.997 275
0.005 600 0.010 697 0.999 752
0.011 331 0.006 684 0.999 789
Because the reaction is homogeneous, it takes place in liquid phase, and the volume remains practically constant, for the reaction rate model it was assumed that the reaction obeys first-order kinetics with respect to the total concentration of reagents A and B. Therefore, the reaction rate equation can be expressed as pseudo-first-order kinetics with respect to conversion8
dX ) k(1 - X) dt
(11)
with k as the reaction rate parameter. When this equation was used, the experimental calorimetric conversion data for the four different experimental temperatures were fitted. The adopted software package was “Scientist” (MicroMath), an equation solver and simulator. The experimental points and the calculated curves are presented in Figure 5. The fitting results are very satisfactory and the specific reaction rates for the different temperatures, the data standard deviations,
standard deviation )
x
1
n
∑(yi - yj)2,
n - 1i)1
yj )
1
n
∑yi ni)1
(12)
where n is the number of points y, and the correlation goodness between observed (x) and calculated (y) dependent values (component concentrations) n
correlation )
(xi - xj)(yi - yj) ∑ i)1
x∑ n
i)1
are listed in Table 1.
x∑ n
(xi - x)2
i)1
(yi - y)2
Figure 7. Experimental conversions from concentration data, experimental conversion from calorimetric data, and the calculated conversion curve from the kinetic model at T ) 426 K.
The temperature effect on the reaction kinetics is clearly evident: the reaction rate increases with ascending temperature. Therefore, the Arrhenius equation log
k ) A0e-E/RT 98 ln k ) ln A0 -
E1 RT
(14)
was used to determine the temperature dependence of the calculated reaction rate parameters. The reaction rate parameters and the correlated temperature dependence are reported in Figure 6. The agreement between the specific reaction rates and the linear temperature dependence, in the logarithmic diagram, denotes the goodness of the fitting. The numerical values obtained for the activation energy and for the pre-exponential term are E ) 64 330 J mol-1 and A0 ) 413 383 min-1, respectively. Because some of usual concentration versus time experimental data for the considered reaction were available,9 a comparison was made between these results and those calculated from the developed kinetic model and is presented in Figure 7. The very good agreement is a proof of the goodness of the used calorimetric analysis to determine the reaction kinetics. Conclusions
(13)
For the kinetics investigation of a homogeneous, liquid-phase, ketonization reaction, the reaction calorimetric technique was employed. The experiments were carried out at different reaction temperatures. The determination of the course of the reaction enthalpy was made using a computer-controlled calo-
Ind. Eng. Chem. Res., Vol. 38, No. 12, 1999 4633
rimetric reactor. The difference between the reactor and jacket temperatures and the determined parameters for the heat exchange allowed for the determination of the heat of the reaction as a function of time. From these data the variation of the conversion with time on the calorimetric basis was determined. The conversion data were satisfactorily correlated with a pseudo-first-order kinetics model. When the Arrhenius equation was used, the temperature dependence of the specific reaction rates, the activation energy, and the pre-exponential term were determined. The accuracy of the employed calorimetric method and the developed kinetic model was verified by the comparison with the usual concentration versus time data. The good agreement obtained emphasizes the use of the calorimetric method for kinetics investigations. Notation A ) unsaturated alcohol A ) wall area, m2 A0 ) pre-exponential term in the Arrhenius equation, min-1 B ) unsaturated ether C ) unsaturated ketone CA ) molar concentration for component A cpr ) heat capacity of reacting mixture at constant pressure, J kg-1 K-1 D ) diether E ) activation energy, J mol-1 e ) Eulero’s number ∆Hr ) reaction enthalpy j ) parameter, linear function of the oil temperature jf ) oil constant K ) kinetic rate parameter, mole min-1 ln ) natural logarithms mr ) mass of reacting mixture Q ) total heat flux Qa ) heat flux of accumulation in the reaction mass Qadd ) other heat flux losses Qb ) basis line for heat flux QC ) calibration heat flux Qf ) heat flux through the wall Qi ) heat flux of accumulation in the inserts Qloss ) heat flux through the top of the reactor Qr ) heat flux of the reaction, Qstit ) heat flux produced by the stirrer
R ) gas constant r ) film parameter dependent on reactor mass and operative parameters rA ) reaction rate rpm ) stirrer rotation speed, rotations per min T ) temperature, K Ta ) reactor wall temperature Tr ) reactor temperature Tj ) reactor jacket temperature Ti ) measured temperature T* ) corrected temperature by reactor response time constant (13 s) t ) time, min ∆t ) time interval between two measurements, s U ) global heat-transfer parameter, W m-2 K-1 X ) conversion
Literature Cited (1) Zakharova, P., I.; Miropol’skaya, M. A.; Yurkina, O. T.; Filippova, T. M.; Kustanovich, I. M.; Samokhvalov, G. I. Preparation and Rearrangement of Acetone Methyl. Zh. Org. Khim. 1971, 7, 1137. (2) Attenburrow, J.; Cameron, A. F. B.; Hapman, J. H.; Evans, R. M.; Hems, B. A.; Jansen, A. B. A.; Walker, T. A Synthesis of Vitamin A from Cyclohexanone. J. Am. Chem. Soc. 1951, 73, 1094. (3) Isler, O.; Ronco, A.; Guex, W.; Hindley, N. C.; Huber, W.; Dialer, K.; Kofler, M. Ueber die Ester und Aether des synthetischen Vitamins A. Helv. Chim. Acta 1949, 32 (63), 489. (4) Graffin, P.; Julia, S.; Julia, M. Transposition Homoallylique Vinylogue d’Alcools R,β,γ,δ-Die´niques -Cyclopropaniques. Me´ m. Pre´ s. Soc. Chim. 1964, 517, 3218. (5) Saucy, G.; Marbet, R. Ueber die Reaktion von tertiaeren Vinylcarbinolen mit Isopropenylaether. Eine neue Methode zur Hestellung von γ,δ-Ungesaettigten Ketonen. Helv. Chim. Acta 1967, 50 (218), 2091. (6) Mettler Toledo, Manual RC-1; Mettler Toledo, 1995. (7) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids; McGraw-Hill: New York, 1987. (8) Marbet, R.; Saucy, G.; Ueber eine neuartige Synthese von β-Kettoallenen durch Reaktion von tertiaeren Acetylencarbinolen mit Vinylaethern. Eine ergiebige Methode zur Darstellung des Pseudojonons und verwandter Verbindungen. Helv. Chim. Acta 1967, 50 (119), 1158. (9) F. Hoffman-LaRoche Ltd., Internal Report, 1997.
Received for review November 5, 1998 Revised manuscript received April 8, 1999 Accepted September 12, 1999 IE980697L
