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CSTR-system for kinetic investigation for hydrogenation reactions
High Pressure Chemical Engineering Ph. Rudolf von Rohr and Ch. Trepp (Editors) 9 1996 Elsevier Science B.V. All rights reserved.
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CSTR-System for Kinetic Investigation for Hydrogenation Reactions Andreas G. Zwahlen a and Alberto Bertucco b a F. Hoffmann - La Roche AG, High Pressure Centre, CH-4070 Basel, Switzerland b Istituto Impianti Chimici, Universith di Padova, via Marzolo 9, I-35131 Padova PD Italy ABSTRACT For design, scale-up, optimisation and control of a commercial reactor it is essential to have accurate kinetic data at hand. In the past, hydrogen reactions in the fine chemicals and pharmaceutical production were carried out with powder catalysts and in a batch mode. Due to more economic and environmental constraints, the application of continuous fixed bed reactors will become more important in the future. It is therefore necessary to have a system for finding the optimal catalyst as well as to study the kinetics of the reaction free of mass transfer and other disturbing effects. The reactor system can be used for hydrogenations with and without solvents, or with supercritical CO2 as a solvent. Emphasis will be in particular on the reactor design as such, the control strategies, and the necessary peripherals will be described in detail. The suitability of this system has been extensively tested with the model hydrogenation reaction of methyl-cinnamate in methanol and with an intermediate for the production of vitamins with CO2 as solvent.
1 INTRODUCTION Hydrogen reactions play an important role in the production of fine chemicals, vitamins and pharmaceutical products. In recent years continuous reactors rather than the traditional batch reactors are becoming more interesting. For the economic analysis, the design and scale up of commercial reactor systems it becomes more important to use modern mathematical modelling tools and therefore it is necessary to get more insight into the macro kinetic aspect as well as the understanding of the behaviour of the phase equilibrium. As well for investigating the aspects of choosing the optimal catalyst it is necessary to have a suitable technology available. In the past, laboratory batch reactors (which are still our workhorses) with powder catalysts were applied and somehow showed their limitations with pellet catalysts. Differential internal recycle reactors have become important tools in recent years for the investigation of catalytic processes and a number of such reactors have been reported in literature with the main emphasis on the actual reactor design [1,2]. In this work a similar reactor, which has been developed by the main investigator for gas-phase reactions under low pressure and high temperature, where it proved its suitability, is described. [3]
38 Recycle reactors working at high internal recycle ratios approximate differential conditions on the catalyst bed quite well and therefore can be treated as a CSTR, where the production rate of each organic reactant j can be calculated from a mass balance by:
f
(wj WjF)/MI'E
rj= W<.o--~
-
(1)
Where 90 = production rate of component j in [mol/(h gcat)], F = feed mass flow rate; Woo,= mass of catalyst; wj, wj~ = weight fraction of component j in the outlet and feed stream, respectively, MWj = molecular weight of component j. 2 EXPERIMENTAL SYSTEM In order to apply this reactor principle to a multi-phase system such as hydrogenation reactions and under high pressure, further development of the reactor and relevant peripheral components had to be tested with special emphasis of the control strategy. The system was developed to perform two types of operating modes for commonly encountered hydrogenation reactions in the production of fine chemicals and pharmaceuticals. 9 Hydrogenation with pure substrate and/or diluted with conventional solvents, the so-called classic method 9 Hydrogenation using supercritical CO2 as a solvent The relevant components and control loops are shown in Figure 1.
Figure 1" P & I Diagram of the Experimental System for both Types of Operating Modes
39
All relevant measurements such as temperature, pressure and flow rates were continuously controlled and data acquired with a Eurotherm Control System TCS 100/1000. The liquid feed was supplied from the feed tank using a (Lewa) membrane pump (max. 25 1/h at 300 bar). The hydrogen flow was controlled by a (High-tech Bronkhorst) flow controller - all the other flows were measured by Rheonik flowmeters. The reactor temperature was heated by means of a electrical heater (2 kW) using a thyristor and cooled using a separate cooling coil with water/glycol as a cooling medium. The temperature was kept constant by a cascaded control arrangement between 0 and 300 ~ The revolution of the turbine was controlled between 100 and 4500 RPM. Samples were taken at the outlet of the reactor and periodically analysed with an on-line GC (Siemens).
2.1 Preliminary Tests In order to check the suitability of such a system under reaction conditions a number of preliminary tests were carried out using the hydrogenation of methyl-cinammate as model substance.
Diagram 1: Reaction Rate as a Function of Stirrer Speed Diagram 1 shows the influence of the stirrer speed on the reaction rate. This was done in order to ensure the absence of external masstransfer limitations. Stirrer speed above 1000 RPM showed no influence on the reaction rate anymore. Investigations below that speed show masstransfer limitations and will be of interest for further studies. Diagram 2 shows the influence of the degree of filling, that means the ratio of substrate to active catalyst sites. With this test it was ensured that all the active sites on the catalyst were wetted and in use. Investigation below 180 g would only utilise part of the catalyst and therefore would be meaningless.
40
Diagram 2" Reaction Rate as a Function of the Degree of Filling
2.2 Hydrogenation using the classic method In this mode the amount of the substrate was controlled by massflow controllers and the pressure by the inlet of the hydrogen. The amount of substrate in the reactor at any time was measured by an overall weight balance. In order to keep the amount of substrate (or virtual level ) constant in a three phase system, the whole reactor was placed with flexible tubing on a Mettler scale with a special resolution of 0.1 g. This signal was used as the input to the weight (level) controller. The substrate feed was kept at a constant value by controlling the feed pump.
Diagram 3: Results of the Relevant Control Loops of the Classic Hydrogenation Diagram 3 shows the reactor system under non-reactive conditions using ethanol as a test substance with a feed rate of 20 g/min, pressure 80 bar, temperature 50 ~ In the first 6 minutes the reactor was filled by the feed pump to the desired degree of filling. After that the heating control was switched on, reaching the desired temperature after another 12 min, then the feed flow and pressure control were activated. Under these conditions the reactor reached a steady state after a total time of 20 min.
41
2.3 Hydrogenation using COz as a solvent
In a second stage the suitability of the reactor system was tested for hydrogenations using supercritical CO2 as a solvent. This operating mode required another control strategy, where all the mass flows to the reactor were controlled separately. The pressure was maintained by valve in the exit line. Experiments were carried out using an intermediate from the production of vitamin at F. Hoffmann-La Roche AG.
Diagram 4: Results of the Relevant Control Loops for the Hydrogenation with C O 2
3 RESULTS OF K I N E T I C E X P E R I M E N T S The operating methods were tested with two relevant model reaction. Teh kinetic data obtained were fitted to simple power-law models as well as more complicated ones and parameters estimated by the least-square method, activation energies and volumes could be determined and an adequately accuracy in the reproduction of experimental results was always achieved. 3.1 Kinetics of the classical Method
Experiments were carried out hydrogenating a 5 % (weight) methyl-cinnamate in methanol solution as a model substance on a commercial catalyst. The temperature range was between 50 - 150 ~ the pressure between 20 - 100 bar, the feed flow rate 10 - 150 g/min, the catalyst mass 5 - 67 g and the amount of filling was kept at 200 g. The following power-law model (2) for the rate of disappearance of mythyl-cinnamate represented the data well; the influence of pressure in this region was small and therefore neglected:
- r = k0 e x p ( - E ]WE.,E
(2)
42 were co "~ E represents the amount of methyl-cinnamate (g/g)of the reaction mixture, with a frequency factor k0 = 385.1 g(l~ g cat), activation energy EA = 16.6 kJ/mol, and an apparent reaction order of mE = 0.59, the correlation coefficient of the parameter estimation was 0.9747 3.2 Kinetics with CO2 as a Solvent
Experiments were carried out hydrogenating two double-bonds of an unsaturated ketone, an intermediate in the production of vitamins, with a commercial catalyst. The temperature range was between 150 - 220 ~ the pressure between 120 - 175 bar, the feed flow rate of the unsaturated ketone at 30 g/min, the feed flow rate of CO2 of 15 - 100 g/min, the feed flow rate of hydrogen 2.2 - 6. 6. 1N/min and a catalyst mass of 10 - 30 g. Results incorporating more sophisticated models with emphasis on the calculation of the phase equilibrium have been employed and are reported separately in [4]. 4 CONCLUSION The experimantal system developed has proven its applicability for kinetic studies as well as for testing the catalyst in the development of commercial high pressure processes and is extremely useful for the fundamental understanding of these types of reactions. The results reported showed that kinetic data at high pressure can be measured with a high reproducability accurately and can be regressed accurately by means of kinetic models with different degree of complexity. Even though the system was developed for hydrogenation reactions, it can be used as well for a variety of other multi-phase reactions. 5 ACKNOWLEDGEMENTS The authors are very grateful to J. Harwalik, Fachhochschule Furtwangen for the contribution of process automation, and to T Kircher, Fachhochschule Aalen and L. Devetta, Universita di Padova for their contribution in applying this system for kinetic studies. As well, many thanks to all the members of VFH at F. Hoffmann La-Roche AG for their invaluable help, namely R. Bernauer, B. Kern, H. Kleisner, Dr. F. Roessler, P. Rindisbacher and R. Zimmer. 6 REFERENCES 1. Betty J.M. 20 Years of Recycle Reactors in Reaction Engineering; Plant/Oper. Prog., 1984, 3, 163-168. 2. Tiltscher H., Schelchshorn J., Wolf H., Dialer K., Differential Recycle Reactors for Investigation of Heterogeneous Systems at High Pressures and Temperature; Ger. Chem. Eng. 313-320. 3. Zwahlen A. G., Agnew J. Modification of an Internal Recycle Reactor of the Berty Type for Low-Pressure High Temperature Catalytic Gas-Phase Reaction; CHEMECA 1987, 1, 50.1-50.7, Melboume, Australia. 4. Bertucco A, Canu P., Devetta L., Zwat'.len A.G.; Catalytic Hydrogenation in Supercritical CO2: Kinetic Measurement in a Gradientless Recycle Reactor, Submitted for publication in Industrial and Engineering Chemistry Research, April 1996
37
CSTR-System for Kinetic Investigation for Hydrogenation Reactions Andreas G. Zwahlen a and Alberto Bertucco b a F. Hoffmann - La Roche AG, High Pressure Centre, CH-4070 Basel, Switzerland b Istituto Impianti Chimici, Universith di Padova, via Marzolo 9, I-35131 Padova PD Italy ABSTRACT For design, scale-up, optimisation and control of a commercial reactor it is essential to have accurate kinetic data at hand. In the past, hydrogen reactions in the fine chemicals and pharmaceutical production were carried out with powder catalysts and in a batch mode. Due to more economic and environmental constraints, the application of continuous fixed bed reactors will become more important in the future. It is therefore necessary to have a system for finding the optimal catalyst as well as to study the kinetics of the reaction free of mass transfer and other disturbing effects. The reactor system can be used for hydrogenations with and without solvents, or with supercritical CO2 as a solvent. Emphasis will be in particular on the reactor design as such, the control strategies, and the necessary peripherals will be described in detail. The suitability of this system has been extensively tested with the model hydrogenation reaction of methyl-cinnamate in methanol and with an intermediate for the production of vitamins with CO2 as solvent.
1 INTRODUCTION Hydrogen reactions play an important role in the production of fine chemicals, vitamins and pharmaceutical products. In recent years continuous reactors rather than the traditional batch reactors are becoming more interesting. For the economic analysis, the design and scale up of commercial reactor systems it becomes more important to use modern mathematical modelling tools and therefore it is necessary to get more insight into the macro kinetic aspect as well as the understanding of the behaviour of the phase equilibrium. As well for investigating the aspects of choosing the optimal catalyst it is necessary to have a suitable technology available. In the past, laboratory batch reactors (which are still our workhorses) with powder catalysts were applied and somehow showed their limitations with pellet catalysts. Differential internal recycle reactors have become important tools in recent years for the investigation of catalytic processes and a number of such reactors have been reported in literature with the main emphasis on the actual reactor design [1,2]. In this work a similar reactor, which has been developed by the main investigator for gas-phase reactions under low pressure and high temperature, where it proved its suitability, is described. [3]
38 Recycle reactors working at high internal recycle ratios approximate differential conditions on the catalyst bed quite well and therefore can be treated as a CSTR, where the production rate of each organic reactant j can be calculated from a mass balance by:
f
(wj WjF)/MI'E
rj= W<.o--~
-
(1)
Where 90 = production rate of component j in [mol/(h gcat)], F = feed mass flow rate; Woo,= mass of catalyst; wj, wj~ = weight fraction of component j in the outlet and feed stream, respectively, MWj = molecular weight of component j. 2 EXPERIMENTAL SYSTEM In order to apply this reactor principle to a multi-phase system such as hydrogenation reactions and under high pressure, further development of the reactor and relevant peripheral components had to be tested with special emphasis of the control strategy. The system was developed to perform two types of operating modes for commonly encountered hydrogenation reactions in the production of fine chemicals and pharmaceuticals. 9 Hydrogenation with pure substrate and/or diluted with conventional solvents, the so-called classic method 9 Hydrogenation using supercritical CO2 as a solvent The relevant components and control loops are shown in Figure 1.
Figure 1" P & I Diagram of the Experimental System for both Types of Operating Modes
39
All relevant measurements such as temperature, pressure and flow rates were continuously controlled and data acquired with a Eurotherm Control System TCS 100/1000. The liquid feed was supplied from the feed tank using a (Lewa) membrane pump (max. 25 1/h at 300 bar). The hydrogen flow was controlled by a (High-tech Bronkhorst) flow controller - all the other flows were measured by Rheonik flowmeters. The reactor temperature was heated by means of a electrical heater (2 kW) using a thyristor and cooled using a separate cooling coil with water/glycol as a cooling medium. The temperature was kept constant by a cascaded control arrangement between 0 and 300 ~ The revolution of the turbine was controlled between 100 and 4500 RPM. Samples were taken at the outlet of the reactor and periodically analysed with an on-line GC (Siemens).
2.1 Preliminary Tests In order to check the suitability of such a system under reaction conditions a number of preliminary tests were carried out using the hydrogenation of methyl-cinammate as model substance.
Diagram 1: Reaction Rate as a Function of Stirrer Speed Diagram 1 shows the influence of the stirrer speed on the reaction rate. This was done in order to ensure the absence of external masstransfer limitations. Stirrer speed above 1000 RPM showed no influence on the reaction rate anymore. Investigations below that speed show masstransfer limitations and will be of interest for further studies. Diagram 2 shows the influence of the degree of filling, that means the ratio of substrate to active catalyst sites. With this test it was ensured that all the active sites on the catalyst were wetted and in use. Investigation below 180 g would only utilise part of the catalyst and therefore would be meaningless.
40
Diagram 2" Reaction Rate as a Function of the Degree of Filling
2.2 Hydrogenation using the classic method In this mode the amount of the substrate was controlled by massflow controllers and the pressure by the inlet of the hydrogen. The amount of substrate in the reactor at any time was measured by an overall weight balance. In order to keep the amount of substrate (or virtual level ) constant in a three phase system, the whole reactor was placed with flexible tubing on a Mettler scale with a special resolution of 0.1 g. This signal was used as the input to the weight (level) controller. The substrate feed was kept at a constant value by controlling the feed pump.
Diagram 3: Results of the Relevant Control Loops of the Classic Hydrogenation Diagram 3 shows the reactor system under non-reactive conditions using ethanol as a test substance with a feed rate of 20 g/min, pressure 80 bar, temperature 50 ~ In the first 6 minutes the reactor was filled by the feed pump to the desired degree of filling. After that the heating control was switched on, reaching the desired temperature after another 12 min, then the feed flow and pressure control were activated. Under these conditions the reactor reached a steady state after a total time of 20 min.
41
2.3 Hydrogenation using COz as a solvent
In a second stage the suitability of the reactor system was tested for hydrogenations using supercritical CO2 as a solvent. This operating mode required another control strategy, where all the mass flows to the reactor were controlled separately. The pressure was maintained by valve in the exit line. Experiments were carried out using an intermediate from the production of vitamin at F. Hoffmann-La Roche AG.
Diagram 4: Results of the Relevant Control Loops for the Hydrogenation with C O 2
3 RESULTS OF K I N E T I C E X P E R I M E N T S The operating methods were tested with two relevant model reaction. Teh kinetic data obtained were fitted to simple power-law models as well as more complicated ones and parameters estimated by the least-square method, activation energies and volumes could be determined and an adequately accuracy in the reproduction of experimental results was always achieved. 3.1 Kinetics of the classical Method
Experiments were carried out hydrogenating a 5 % (weight) methyl-cinnamate in methanol solution as a model substance on a commercial catalyst. The temperature range was between 50 - 150 ~ the pressure between 20 - 100 bar, the feed flow rate 10 - 150 g/min, the catalyst mass 5 - 67 g and the amount of filling was kept at 200 g. The following power-law model (2) for the rate of disappearance of mythyl-cinnamate represented the data well; the influence of pressure in this region was small and therefore neglected:
- r = k0 e x p ( - E ]WE.,E
(2)
42 were co "~ E represents the amount of methyl-cinnamate (g/g)of the reaction mixture, with a frequency factor k0 = 385.1 g(l~ g cat), activation energy EA = 16.6 kJ/mol, and an apparent reaction order of mE = 0.59, the correlation coefficient of the parameter estimation was 0.9747 3.2 Kinetics with CO2 as a Solvent
Experiments were carried out hydrogenating two double-bonds of an unsaturated ketone, an intermediate in the production of vitamins, with a commercial catalyst. The temperature range was between 150 - 220 ~ the pressure between 120 - 175 bar, the feed flow rate of the unsaturated ketone at 30 g/min, the feed flow rate of CO2 of 15 - 100 g/min, the feed flow rate of hydrogen 2.2 - 6. 6. 1N/min and a catalyst mass of 10 - 30 g. Results incorporating more sophisticated models with emphasis on the calculation of the phase equilibrium have been employed and are reported separately in [4]. 4 CONCLUSION The experimantal system developed has proven its applicability for kinetic studies as well as for testing the catalyst in the development of commercial high pressure processes and is extremely useful for the fundamental understanding of these types of reactions. The results reported showed that kinetic data at high pressure can be measured with a high reproducability accurately and can be regressed accurately by means of kinetic models with different degree of complexity. Even though the system was developed for hydrogenation reactions, it can be used as well for a variety of other multi-phase reactions. 5 ACKNOWLEDGEMENTS The authors are very grateful to J. Harwalik, Fachhochschule Furtwangen for the contribution of process automation, and to T Kircher, Fachhochschule Aalen and L. Devetta, Universita di Padova for their contribution in applying this system for kinetic studies. As well, many thanks to all the members of VFH at F. Hoffmann La-Roche AG for their invaluable help, namely R. Bernauer, B. Kern, H. Kleisner, Dr. F. Roessler, P. Rindisbacher and R. Zimmer. 6 REFERENCES 1. Betty J.M. 20 Years of Recycle Reactors in Reaction Engineering; Plant/Oper. Prog., 1984, 3, 163-168. 2. Tiltscher H., Schelchshorn J., Wolf H., Dialer K., Differential Recycle Reactors for Investigation of Heterogeneous Systems at High Pressures and Temperature; Ger. Chem. Eng. 313-320. 3. Zwahlen A. G., Agnew J. Modification of an Internal Recycle Reactor of the Berty Type for Low-Pressure High Temperature Catalytic Gas-Phase Reaction; CHEMECA 1987, 1, 50.1-50.7, Melboume, Australia. 4. Bertucco A, Canu P., Devetta L., Zwat'.len A.G.; Catalytic Hydrogenation in Supercritical CO2: Kinetic Measurement in a Gradientless Recycle Reactor, Submitted for publication in Industrial and Engineering Chemistry Research, April 1996