ARTICLE pubs.acs.org/IECR
High-Temperature Epoxidation of Soybean Oil in Flow—Speeding up Elemental Reactions Wanted and Unwanted Bruno Cortese, Mart H. J. M. de Croon, and Volker Hessel* Eindhoven University of Technology, Eindhoven, The Netherlands ABSTRACT: The soybean oil epoxidation reaction is investigated theoretically through kinetic modeling of temperature effects enabled through flow processing under superheated conditions. Different from previous studies on such processing, here a complex reaction network superimposed by multiphase transport is considered; with one elemental step—the hydrogen peroxide decomposition—which can defeat the much boosted product formation. For such a delicate reaction network, the accessibility of accurate and reliable kinetics is absolutely essential, especially when exploring this completely new temperature range. Initially, an overview of the actual kinetic models is given, this gives rise to implications for the study developed here considering high temperature flow processing, heat removal efficiency, hotspot formation, and the effect of different hydrogen peroxide decomposition kinetics. Subsequently an optimized process involving the use of microreactors at different temperatures is proposed for the process management of the reaction heat and to yield a commercial grade product under notably intensified conditions. The results are then benchmarked with quantitative, challenging process improvement criteria set by an industrial partner.
1. INTRODUCTION 1.1. Superheated Reaction Performance and Modeling of Complex Reaction Networks. Process intensification17 and
the newly introduced concept of novel process windows are vivid fields in chemical engineering research; see for example refs 813. Process intensification actually most often means transport intensification, that is, improved mass and heat transport. Novel process windows provide a second chemical intensification field, specific for and available by flow processing. Finally they provide a third process-design intensification by embracing the full chain of process steps; from reactants pretreatment to purification. While the latter is still largely unexplored, the work presented here is dedicated to the chemical intensification field. The capabilities of all three process intensification fields are further enhanced by, and often demand for, the availability of reliable and highly efficient microstructured instrumentation. So far, superheated reactions under microflow conditions are frequently carried out with quite robust chemistries. These reactions show no or only minor selectivity issues which hardly pose any issue when using high temperatures of up to 200 °C and beyond. Rearrangements such as the Claisen one have been largely investigated under superheated flow conditions and showed faster attainment of optimum yields even up to 300 °C.1417 The KolbeSchmitt reaction of resorcinol could be processed up to 220 °C, with only a few percent of the 2,6-side product isomer formed (as opposed to the 2,4-product).1820 Above this temperature, decarboxylation of the product dominates, producing the reactant resorcinol. Recently, we published a continuous capillary-based process for the synthesis of 3-chloro-2-hydroxypropyl pivaloate.21 This is among the very first data describing the limits of superheated processing with regard to selectivity issues. Here, two identical chlorine functionalities in the reactant can be converted by nucleophilic r 2011 American Chemical Society
substitution to OH groups. The target product is the monosubstituted, 3-chloro-2-hydroxypropyl pivaloate; whereas the diproduct formation occurs to a large extent (about 20%) in batch mode. Under certain superheated microreactor conditions the diproduct formation may be even higher, while this effect was almost constant with reaction temperature (at least at the shortest residence time investigated). The soybean oil epoxidation is a reaction case with a considerable impact of temperature on a side reaction (hydrogen peroxide decomposition) and the potential to significantly change the desired product formation under industrial requirements to a level of almost zero. Epoxidized soybean oil is a chemical which faces growing importance and consequently an expanding market;22 it is used primarily as a plasticizer23 and as a replacement of highly toxic or banned substances in polymers production.24 Nonetheless the production is still carried out by application of a dangerous process, where an oxidizing mixture of hydrogen peroxide and formic acid are added stepwise into a semibatch reactor under mild conditions (usually around 70 °C). The highly exothermic nature of the reaction, as can be seen from the adiabatic temperature increase of more than 300 K, renders the commercial process extremely slow (up to 810 h25), since the corresponding safety issues need to be addressed. A scheme of an industrial process is reported in Figure 1. The main objective of this study is to overcome such limitations and to explore the possibility of conducting the reaction at Special Issue: Nigam Issue Received: April 21, 2011 Accepted: July 12, 2011 Revised: July 5, 2011 Published: July 12, 2011 1680
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Figure 1. Scheme of the actual industrial batch process: Stream 1, oil and H2SO4; Stream 2, H2O2 and HCOOH; Stream 3, internal; Stream 4, NaOH; Stream 5, Internal; Stream 6, water; Stream 7, product.
Figure 2. Reaction scheme of ESBO formation. Figure 3. Possible oxirane ring-opening reactions, as reported in ref 26.
higher temperatures up to superheated conditions, which are easily accessible through flow processing and microreactor operation, increasing the potential productivity of the industrial process, and reduce the potential for explosion danger through enhanced control. The epoxidation reaction consists of a complex network of elementary reactions superimposed by multiphase transport. One elementary step—the hydrogen peroxide decomposition— can defeat the boosted product formation. For increased predictability in modeling such delicate reaction networks, the accessibility of accurate and reliable kinetics is essential, especially when exploring the completely new temperature space. To this end, an initial study of the available kinetic models was conducted, in order to obtain a broader vision over the actual state of the modeling and how it can be used or expanded, as well as to identify the main issues that are still unsolved. Subsequently the reaction behavior at higher temperatures was studied, which provided evidence to possibly decrease the time needed to obtain a commercially viable product from hours to minutes. The efficiency of different commercially available microstructured heat exchangers has been used as a starting point to evaluate the practical feasibility of faster kinetics. Finally a faster decomposition of the hydrogen peroxide has been simulated, considering how this would detrimentally affect the overall reaction performance; this was completed since it is possible that hydrogen peroxide decomposition is underestimated when facing microprocess technology with its large specific surfaces and construction materials which are often not inert. Finally with all this in mind, an optimized process is proposed—under flow and superheated conditions—with industrial applicability evaluated through a quantitative comparison with the improvement criteria set by an industrial partner at the commencement of the study.
2. METHODS AND THEORY 2.1. State of the Art in Kinetic Interpretation. From the chemical point of view the vegetable oil epoxidation reaction is a two-phase reaction which makes use of a phase transfer catalyst (a simple organic carboxylic acid; usually formic acid in industrial practice). This catalyst is oxidized in the water phase by a
molecule of hydrogen peroxide via acid catalysis. The generated peroxycaboxylic acid then migrates into the oil phase where it reacts with an oil-based double bond yielding the epoxide ring and regenerates the initial acid. The catalyst then diffuses back into the water phase, ready to be oxidized again (see Figure 22630). In addition a series of ring-opening reactions takes place as shown in Figure 3,26 which lead to product degradation and reduction of the overall selectivity and hence yield of the reaction. Numerous papers have been published on the soybean oil epoxidation kinetics.2630 They can be classified into the following two main interpretations: (1) a full pseudomonophasic model, in which all the first steps of the reaction are lumped2628; (2) a more refined interpretation which partly considers the two-phase nature of the reaction.29,30 The first group assumes that the formation of the organic peroxyacid is the rate-limiting step and that mass transfer together with oil phase epoxidation do not influence the observed reaction rate. This method is summarized in the kinetic equation mostly utilized for the direct epoxidation reaction26,27 d½EP ¼ kobs ð½H2 O2 0 ½EPÞ½HCOOH0 dt Where kobs includes the concentration of the acid catalyst in the aqueous phase and [EP] is the concentration of epoxide rings. Despite the simplification used, the fit obtained by these models is extremely good; especially when describing a small operational domain. Models of the second group take up different approaches to the modeling of the multiphase nature of the reaction, by assuming equilibrium conditions of the formic and performic acid between the oil and water phases29 or by calculating a lumped mass transfer parameter that can take into account deviations from ideality.30 It should be pointed out that none of the published models are suitable as is, for an accurate simulation of the “real” behavior of the reaction in a microreactor at high temperature; particularly for the following reasons: (1) The use of high reaction temperatures in the reaction stage will accelerate the rate of the various chemical reactions, but will hardly have any influence on the mass 1681
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equations,32 a minimum of 1000 integration steps, and an error tolerance of 106. All the simulations have been performed multiple times with different integration steps in order to ensure that the obtained results are independent from the parameters chosen for the integration (i.e., different yields or changes in hotspot intensities due to too large integration steps). All the results are computed in terms of iodine (I.N.) and oxirane (O.N.) numbers, which are defined as follows: I:N: ¼ O:N: ¼
Figure 4. Cutoff in the observed reaction rate (black line) due to the passage from a kinetic regime (dotted line) to a mass transfer limited regime.
transfer rate between the two phases; it is therefore realistically possible to end up in a mass-transfer limited regime (see Figure 4 for an example), which cannot be interpreted by a monophasic model. (2) At higher reaction rates the deviation from the equilibrium distribution of the organic acids between the two phases will be enhanced, and thus by assuming full equilibrium at any moment will generate an increasing error in the simulation results. (3) Lumped mass transfer coefficients are generally calculated via numerical regression over experimental data, and therefore include numerous parameters that are specific to the reactor/stirrer system used at the experimental stage. Considering that the actual kinetics are calculated using batch reactors it is possible to forecast that the use of microstructured reactors (which are well-known for high mass transfer capabilities), will not allow for the extrapolation of regressed global mass transfer coefficients. Nonetheless, it is possible to use the actual models as a starting point to investigate the behavior of the reaction far from the normal processing conditions to identifying the effect of higher temperatures and of the processing parameters on the process. 2.2. Kinetic Model Selected. The kinetic model utilized in this paper has been developed by Santacesaria et al.31 It includes the necessary kinetic constants and activation energies, also considering secondary and parallel reactions. Furthermore, heat effects generated by the various chemical reactions occurring in the reaction environment are taken into account. The model can be summarized in the following set of kinetic equations (where [DB] is the concentration of unsaturations) and is reported by Santacesaria et al.31 d½H2 O2 ¼ k1 ½H2 O2 ½DB k3 ½EP½H2 O2 k4 ½H2 O2 dt d½H2 O ¼ k1 ½H2 O2 ½DB þ k4 ½H2 O2 k5 ½EP½H2 O2 dt d½DB ¼ k1 ½H2 O2 ½DB dt d½EP ¼ k1 ½H2 O2 ½DB k3 ½EP½H2 O2 k5 ½EP½H2 O2 dt 2.3. Numerical Methods. The modeling has been performed using Athena Visual Studio, the DDAPLUS solver for differential
g12 for double bonds titrations 100 g of oil sample g of oxirane oxygen 100 g of oil sample
These are the indicators commonly used in industrial specifications and have therefore been chosen in order to simplify the comparison between the obtained results and the market specifications; second they allow for an easier evaluation with the industrially proposed PI criteria.
3. RESULTS AND DISCUSSION The modeling work has been developed by gradually investigating the effect of the temperature over the reaction (imposing isothermal behavior), then by studying the efficiency of different heat exchangers in removing the process heat and managing the formation of hotspots at the entrance of the reactor (giving the effect then for partly overheated reaction mixtures), and concluding with an analysis of the effect of an increased decomposition rate of hydrogen peroxide over the product formation. On the basis of these simulations an optimized process is proposed, with the aim of minimizing the needed reaction time to obtain a commercially viable product through microprocessing with a single addition of all the reactants. 3.1. Isothermal Modeling. The effect of temperature variations was studied first, in order to evaluate the reaction response and to access any critical conditions that should be avoided (i.e., for which it is not possible to obtain a commercially viable product). Additionally this allowed for the determination of the minimum theoretical residence time needed to obtain a product which conforms to the market specifications, as listed in Table 133 and to be compatible with typical response times of sensing and controlling instruments and safety recommendations in today’s industrial process control. To this purpose isothermal behavior has been assumed and the reactor has been modeled as a perfect plug flow reactor (PFR), thereby evaluating the iodine and oxirane numbers as well as the concentration of the other species present in the reaction environment. The trends, reported as oxirane number versus residence time in Figure 5, show that, in principle, the reaction can be completed in the minute range (or even faster), if the behavior predicted by the lumped models is correct. An interesting point is the higher intrinsic rate of the direct reaction over that of the side and consecutive ones, which leads to the absence of an upper temperature limit for processing, if considered from the kinetic point of view. Another advantage is the increase in the maximum possible yield at higher temperature, which is correlated to the particular ratio between the activation energies and the pre-exponential constants of the different reactions involved. From such a (simplified) view the conclusion would be, the higher the 1682
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Table 1. ESBO Market Specifications According to Different Quality Grades, as Reported in ref 33. Shengzhou plastic Milton chemical
chemical factory
grade SNS grade SN grade one technical grade oxirane number
6.7
6
6
5.5
iodine number saponification number
2 0.5
7 0.5
6 0.5
8 0.6
temperature is, the better, which is a ‘nontrivial’ claim and outcome, not standing to reason for a reaction with a strong decomposition of a labile reactant. Probably, such an ex-ante process recommendation would be too short-sighted, as experimental results, which are underway and will be published elsewhere, would partly demonstrate that the prediction of the simulation is not (entirely) correct. Reasons for the latter are at least 3-fold: (1) emerging and increasing mass transfer limitation due to reaction boosting; (2) negative impact of thermal overshooting (hot spots) on product and hydrogen peroxide decomposition; (3) having much more hydrogen peroxide (and possibly peroxide) decomposition than predicted from experiments with inert, glass-lined construction material with small specific interface. Elemental reaction rates may speed up so massively that suddenly interfacial mass transfer is not fast enough anymore, as assumed in the lumped models. The assumptions for a first simulation are then not valid anymore and all curves above a certain temperature threshold are false (while those below may still be correct). From current rough-cut evidence this may happen around 150 °C, still leaving a lot of room for process intensification. For the case of mass-transfer limitations, the development of even better dispersing microreactors provides a further means in exploitation. To determine if the mass transfer does indeed become more influential (see Figure 3) at higher temperatures, and consequently higher reaction rates, separate studies have been initiated at our site to reveal the effect of surface area and mixing efficiency for actual micromixers. In the following sections we consider and detail the impact of thermal overshooting and the uncertainty about the real hydrogen peroxide decomposition under our specific flow conditions (noninert steel with high surfaces). 3.2. Nonisothermal Modeling. 3.2.1. Thermal Management of Commercial Microheat-Exchangers. It has been pointed out in the introduction that the reaction is strongly exothermic and this can lead to explosion dangers in macroscale batch processing. Therefore, this aspect cannot be neglected in an explorative study which touches the limits of possible process conditions. Microreactors and micro-heat-exchangers have high heat transfer capacities, but increasing the reaction rate via higher temperatures also means that the heat release rate is enhanced by the same factor, and that the amount of heat which can be removed by the reactor is lower (due to the higher temperature of the cooling fluid, leading to a smaller ΔT between the service fluid and the reaction medium). Consequently the first question to be answered is whether the available micro-heat-exchangers are efficient enough to remove the heat produced? Or in other words, which is the minimum reaction time achievable when using actual commercial microstructured reactors?
Figure 5. Theoretical product formation at different temperatures (in isothermal conditions).
Four different heat exchangers, depicted in Figure 6, have been used for the modeling; their specifications are taken from IMM and the EU-funded IMPULSE FP6 project and are listed in Table 2.34 The available surface area in the different heat exchangers is strongly related with the channel depth and width, as reported in Table 3. As simple pressure drop calculations reveal, the wider channels of the HX heat exchanger will allow for a lower pressure drop, whereas the thinner channels of the WT heat exchanger provide increased performance in the heat management. Under practical considerations, the choice will be on the microstructured heat exchanger able to remove the reaction heat at the given processing conditions with the lowest pressure drop, which stands for the biggest usable channel crosssection (in the sense to have it not as small as possible, but as small as needed). The global heat exchange coefficient U [W m2 K1] depends among others on geometry and wall material (which is subject to change, e.g., in case of coatings), but for simplicity and to facilitate comparisons between the different heat exchangers it has been set to the same average value for all considered heat exchanges, as reported in Table 2. For fast exothermic reactions the formation of hotspots, especially close to the entrance of the reactor or in the mixer unit is common, and the management of the heat, and consequently of the hotspot, is a critical issue to achieve control of the reaction. In literature, comparatively high temperature hotspots have been reported, up to 10 K35 or 40 K,36 even under microflow conditions with their much improved heat transfer. Despite aiming at isothermal operation, a threshold of 10 degrees above the set temperature has been defined. If the reaction is conducted close to the heat removal limit, a small difference in the set temperature can already lead to fast thermal runaway. Moreover, it should be noted that the presence of a hotspot of limited intensity (in time and temperature) is not strongly influential on the reaction outcome, as reported in Figure 7. Setting a reaction temperature of 373 K, corresponds to less than 10 min for reaction completion under isothermal conditions. Figure 8 shows that the HX micro-heat-exchanger is not sufficient to remove all the heat produced by mixing the reactants at once; on the other hand, the WT micro-heat-exchanger, which is the most efficient of the considered liquid-phase devices, is able to manage the released heat up to a wall temperature of 423 K, 1683
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Figure 6. Pictures of the different heat exchangers considered, from left to right HX, WT, HCOM. Figure reproduced with permission from ref 39; Copyright Institut fuer Mikrotechnik Mainz GmbH.
Table 2. Important Data on the Considered Micro-heatexchangers reactor
type
A [m2/m3]
U [W/(m2 K)]
HX
liquid
300
4500
WT-Low
liquid
1000
4500
WT-Hi
liquid
1400
4500
HCOM
gas
2400
4500
Table 3. Typical Channel Dimensions (in μm) of the Considered Heat Exchangers width
height
HX
1000
650
WT
800
400
2000
250
HCOM
corresponding to a theoretical 1 min reaction time to obtain a product matching specifications. 3.2.2. Thermal Management of Passivated Micro-heat-exchangers. Owing to the strong effect of hydrogen peroxide decomposition on the reaction yield (discussed in the next paragraph) a passivation of the microreactors’ wall will be needed; in such a case the heat transfer efficiency of the microreactor will be lowered due to the insertion of another layer that increases the energy transfer resistance. For this reason the effect of different global heat-exchange coefficients has been simulated, considering variations between 1000 and 4000 W m2 K1, in all cases the simulated heat exchanger being the WT type with high surface area. From the temperature profiles given in Figure 9, it is found that the reaction heat can be removed via the reactor walls even if the heat exchange efficiency is lowered by half. Nonetheless the efficiency of the reactor in managing the reaction heat at higher reaction rates is strongly lowered, as shown in Figure 10, which displays the temperature profiles at different U values and set reaction temperatures. These results also support the decision of being tight in the allowed intensity threshold for hotspots. Figure 10 clearly indicates that only a difference of few degrees in the set temperature suffices to induce a fast and uncontrollable thermal runaway. It should be noted that in both Figures 9 and 10 the results do not include the effect of a possible phase change, this decision has been taken because if vapors are formed into the reactor channel then the residence time is not controllable anymore, and consequently the system could not be deployed for production purposes. In real cases, overpressure must be applied to ensure that only the liquid phase is present along the whole channel.
Figure 7. Comparison of the oxirane numbers and temperatures trends for an isothermal (black line) and with hotspot (dotted line) simulation.
Figure 8. Hotspot intensity for different heat exchangers.
3.3. Hydrogen Peroxide Decomposition. The hydrogen peroxide decomposition rate has a strong influence over the overall reaction performance. From the reaction scheme (Figure 2) it can be seen that ideally a stoichiometric ratio between oxidant and oil double bonds (1:1) should be chosen; however, an excess is used in normal industrial practice (up to 1.6:1) to compensate for a small spontaneous decomposition that cannot be avoided and for the amount of peroxide that is lost due to ring-opening reactions. The kinetic constant of hydrogen peroxide decomposition is reported for an aqueous environment under nonstabilizing pH 1684
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Figure 9. Effect of different heat exchange coefficients on the management of the hotspot (T = 100 °C), A = 1000 m2/m3.
Figure 10. Temperature profiles at different set temperatures (from 100 to 150 °C) and different U (from top to bottom and left to right, respectively: 1000, 2000, 3000, 4000 W m2 K1).
conditions (pH below 5 stabilize the hydrogen peroxide37) and in the presence of metal clusters.38 This catalyzed decomposition rate is between 10 and 50 times faster than the noncatalyzed one used in our simulations.37,38 Indeed, in microreactors the decomposition rate might be increased, heterogeneously due to the very large internal reactor wall surface or even homogeneously, due to the presence of
leached metals from the reactor walls in the solution. Thus, to be on the safe side for this study, it is advised to consider faster hydrogen peroxide decomposition in our simulation as well. The amount of hydrogen peroxide that then needs to be added in order to reach full conversion, drastically increases. The excess needed to obtain full conversion with respect to different increases in the decomposition rate of hydrogen peroxide is 1685
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reported in Table 4. Such increased amounts of hydrogen peroxide in the reaction environment are not only detrimental from an economical point of view, but also strongly affect the overall kinetic results. Now considering the smallest excess of hydrogen peroxide needed to obtain full conversion (with the values reported in Table 4). Oxirane numberresidence time plots (see Figure 11) can be given in the same form for the different cases as for Figure 5. When instead the same high hydrogen peroxide excess (5:1 ratio over oil) is supposed for each case, it becomes evident that using a large excess of hydrogen peroxide leads to a lower yield and selectivity (see Figure 12). 3.4. Optimum Proposal. As a final step of this explorative study an optimized process is proposed, with the aim of matching the benchmarking criteria set by the industrial partner before the Table 4. Needed Excess of Hydrogen Peroxide for Full Conversion and Diminishing of Reaction Performance (k = Used Decomposition Rate/Basic Decomposition Rate) k
H2O2 stoichiometric ratio
O.N.
1 10
1.3 2.2
6 4.8
50
5
3.1
100
9
2.1
beginning of the study. These can be summarized as follows: (1) reduction of excess hydrogen peroxide use (down to a 1.4:1 ratio with respect to oil unsaturation); (2) reduction of residence time down to 90 min; (3) productivity increase by a factor of 3; (4) product quality should meet market specifications. To meet these criteria the excess of hydrogen peroxide has been set below the minimum requirements (1.3 instead of 1.4). It was then decided that hotspots should not be higher than 10 K and that all the reactants should be added at the entrance of the reactor. A multireactor configuration was chosen to manage different temperatures along the reactor length, speeding up the final part of the reaction and slowing down the initial one to manage hotspots. The considered heat exchanger is of the WT type with large surface area, and the configuration specifications are reported in Table 5. As can be seen in Figure 13, the total reaction time needed to obtain a product compliant with the criteria is below 5 min. It should also be noted that the required oxirane number is reached slightly after 2 min of residence time, but to complete the conversion of double bonds and to obtain a compliant iodine number, the reaction needs to be carried on, even if this leads to opening of the oxirane ring, and thus decreasing the overall yield and selectivity. In Figure 14, the temperature profile as a function of the residence time is given, this shows how the use of a multireactor
Figure 11. Different trends of oxirane number vs residence time for different hydrogen peroxide decomposition rates with optimal initial composition (increasing decomposition rate from top to bottom and left to right, respectively: 1, 10, 50, 100 times faster than the basic reaction rate). 1686
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Figure 12. Oxirane number vs residence time trend for different decomposition rates and same initial conditions (R represent the ratio between the basic decomposition rate and the enhanced decomposition rate used in the simulation).
Figure 14. Temperature profile in the optimized process.
Table 6. Comparison between the Actual Batch Process, the Benchmark Criteria, and the Microprocessing Performance batch
Table 5. Operating Conditions for the Optimized Configuration H2O2excess
microreactor
processing
benchmark
processing
1.4
1.4
1.28
WT-Hi heat exchanger start time (min)
end time (min)
temp (°C)
0
1
100
1
2
120
2
3
150
reaction time [min]
400
90
5
productivity
1
3
53
6.8
6.8
66.5
[normalized]a O.N. a
Productivity calculations are based only on the reactor internal volume and are given as relative figures with the batch processing space-time productivity set as 1.
productivity of the reactor, and that the obtained oxirane number, even if close, does not yet match the best quality product, but only high grade ones (for detailed market specifications see Table 1).
Figure 13. Iodine (black line) and oxirane (dotted line) numbers profiles for a multitemperature optimized process.
configuration allows for a low entrance hotspot while not sacrificing the advantages of a higher process temperature in the last part of the reaction. A comparison between the performance of the actual process and of the microreactor process is reported in Table 6, where both are related with the benchmark criteria set for the reaction. It is possible to notice that the strong reduction of the reaction time is the main driving force in the increase of the spacetime
4. CONCLUSIONS The actual status in the kinetic modeling of vegetable oil epoxidation has been utilized as a basis to perform an explorative study on the reaction under high temperature conditions. The most critical ideas for a better simulation have been pointed out and are under investigation at our site. High temperature simulations demonstrate that in principle it is possible to perform the reaction at higher temperatures, leading to much shorter (orders of magnitude) residence times; it is also shown that it is possible to manage the correspondingly boosted release of heat through commercial micro-heat-exchangers, as another confirmation of the great capabilities of microstructured devices in opening novel process windows for chemical reactions. As an interesting and possibly generic finding, it was recognized that the very large hot spots frequently found in the very first microchannel section do not deteriorate reaction performance much, as it constitutes only a minor overall effect (despite being strong on a small time scale), whereas more demanding actions could be needed in certain cases as a safety measure. 1687
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Industrial & Engineering Chemistry Research The effect of increased hydrogen peroxide decomposition rates (often inevitable in the presence of dissolved metal clusters) and the critical effect of this variable on the reaction performance have been investigated. Under normal conditions the product rate formation outperforms the hydrogen peroxide decomposition, giving an overall positive temperature effect up to very high temperatures. This situation however changes when facing a much enhanced hydrogen peroxide decomposition than compared to inert, glass-lined conventional reactors. For this case, a change of reaction behavior is predicted at approximately 150 °C; from this temperature onward, the hydrogen peroxide decomposition becomes dominant resulting in decreased product yields. Finally, under given optimized conditions a minuterange microprocessing is proposed, capable of yielding a commercial grade product and managing the hotspot related to the heat release associated with all-at-once reactant addition. The performance specifications of such a processing unit have therefore met the commercial process improvement criteria formulated by the chemical company with business in the field.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel.: +31402472973. Address: Eindhoven University of Technology, Chemical Engineering and Chemistry, Micro Flow Chemistry and Process Technology, Den Dolech 2; STW 1.45 5600 MB Eindhoven, The Netherlands.
’ ACKNOWLEDGMENT This research was funded by the EU Large-Scale Project COPIRIDE of the seventh European Framework Program, funding number CP-IP 228853; we gratefully acknowledge the group of professor Santacesaria at University of Napoli for the help provided in the initial stages of this work and for allowing us to use their own kinetic model. ’ REFERENCES (1) Stankiewicz, A.; Moulijn, J. A. Process intensification: Transforming chemical engineering. Chem. Eng. Prog. 2000, 22–34. (2) Stankiewicz, A.; Moulijn, J. A. Re-engineering the Chemical Processing Plant, Process Intensification.: Marcel Dekker: New York, 2004. (3) European Roadmap on Process Intensification; Creative Energies, Energy Transition: 2008; http://www.agentschapnl.nl/sites/default/ files/bijlagen/Action_Plan_Process_Intensification.pdf (Accessed August 4, 2011). (4) Jun-Ichi, Y. Flash chemistry; John Wiley & Sons Ltd: New York, 2008. (5) Reay, D.; Ramshaw, C.; Harvey, A. Process IntensificationEngineering for Efficiency, Sustainability and Flexibility; Elsevier: Amsterdam, 2008. (6) Harmsen, G. J. Reactive distillation: The front-runner of industrial process intensification: A full review of commercial applications, research, scale-up, design and operation. Chem. Eng. Process. 2007, 46, 774–780. (7) Ramshaw, C. Process intensification and green chemistry. Green Chem. 1999, 1, 15–17. (8) Hessel, V.; Cortese, B.; de Croon, M. H. J. M. Novel process windows—Concept, proposition and evaluation methodology, and intensified superheated processing. Chem. Eng. Sci. 2011, 66, 1426–1448. (9) Illg, T.; Hessel, V.; L€ob, P. Flow chemistry using milli- and microstructured reactors—from conventional to novel process windows. Bioorg. Med. Chem. 2010, 18, 3707–3719.
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