Pilot Scale Continuous Microwave Dry‐Media Reactor – Part 1: Design and Modeling

This work presents a new pilot plant continuous microwave dry‐media reactor (CMDR) for industrial chemical applications. The CMDR consists of a 6 kW conveyor microwave oven with a subsequent hot air holding section. This microwave reactor has been designed for dry media or solvent‐free reactions and can treat through‐put in the range of 10–100 kg/h. The microwave heating behavior on the small scale is analyzed and the results are used to estimate the electromagnetic field requirements on the large scale. The temperature and the electric field distribution in the reactor are modeled and experimentally validated. In the second part of this study, a “waxy” esterification reaction was investigated with the CMDR. The reaction time needed for 95% yield was reduced by a factor of 20–30 compared to conventional industrial reactors. This was due to the more homogeneous heat transfer of microwaves, which allows a higher bulk temperature to be reached.     

Energy efficient and controlled flow processing under microwave heating by using a millireactor–heat exchanger

A continuously operated microwave heated millireactor setup has been developed for performing reactions of highly microwave absorbing media in a controlled and energy efficient manner. The setup consists of a tubular reactor integrated with a heat exchanger. A microwave transparent liquid was used as coolant to extract the excess heat from the reaction mixture, thus controlling the temperature of the reaction mixture by avoiding overshoots and subsequent boiling. A reactor‐heat exchanger shell and tube unit with a diameter of the inner tube of 3·10^?3 m and a shell of 7·10^?3 m inner diameter has been manufactured in quartz. The unit size was defined based on simulation with a heat‐transfer model for the microwave cavity part. Microwave heating was incorporated as a volumetric heating source term using the temperature‐dependent dielectric properties of the liquid. Model predictions were validated with measurements for a range of 0.167·10^?6 to 1.67·10^?6 m³/s flow rates of coolant. The outlet temperature of both the reaction mixture and the coolant, were predicted accurately (tolerance of 3 K), and the process window was determined. The model for the reactor part provided the required length of the reactor for a hetero‐geneously catalyzed esterification reaction. The predicted conversions, based on the obtained temperature profile in the reactor packed with the catalyst bed, known residence times and kinetics of the esterification reaction, were found to be in good agreement with the experimental results. Efficient utilization of microwave energy with heat recovery up to 20% of the total absorbed microwave power and heating efficiencies up to 96% were achieved. It has been demonstrated that the microwave heating combined with millireactor flow processing provides controlled and energy efficient operation thus making it a viable option for a fine chemical production scale of 1 kg/day (24 h period). © 2011 American Institute of Chemical Engineers AIChE J, 58: 3144–3155, 2012     

Scale Continuous Microwave Dry‐Media Reactor – Part II: Application to Waxy Esters Production

The solvent free esterification reaction between stearic acid and stearyl alcohol has been examined with a montmorillonite clay as catalyst. To aim for industrial application the system has been studied on reaction rate, product purity, catalyst behavior and water removal as function of the process conditions. To avoid an etherification side reaction and to aim for the highest reaction rate, the temperature should be strictly maintained at 170 °C. This is provided on larger scale by the application of microwave heating. Although the examined Brønsted acid clay was found to lose its catalytic activity at very low water activities, a yield of 95 % pure stearyl stearate can be obtained by simply filtering of the clay without solvent extraction or distillation. The “waxy” esterification reaction was investigated with the pilot plant continuous microwave dry‐media reactor (CMDR). The reaction time needed for 95 % yield was reduced by a factor 20–30 in comparison with industrial conventional reactors. This was due to the more homogeneous heat transfer of microwaves, which allows to reach a higher bulk temperature.     

EVALUATION OF UV DOSE IN FLOW-THROUGH REACTORS FOR FRESH APPLE JUICE AND CIDER

High absorptivity and turbidity interfere with the UV disinfection of apple cider. Three different configurations of flow-through UV reactors were evaluated to overcome this interference. Two approaches were employed: use of an extremely thin film UV reactor and increasing the turbulence within a UV reactor. Multiple-lamp UV reactors including the thin-film laminar flow “CiderSure” (8 lamps) and turbulent flow “Aquionics” (12 lamps) and annular single-lamp “UltraDynamics” reactor were studied. UV disinfection performance in laminar and turbulent flow reactors was compared by evaluation of UV dose delivery. UV fluence rate (irradiance) distribution was calculated using the multiple point source summation method. E. coli K12 was used as a target bacterium in a bioassay, and the log reduction per one pass was determined for each UV reactor. Finally, the UV decimal reduction dose (D₁₀) was calculated by dividing the average UV fluence by log bacterial reduction per pass. Variations of the UV decimal dose were observed with various designs of UV systems. The least inactivation of E. coli K12 but the highest UV decimal reduction dose, ranging from 90 to 150 mJ/cm², was observed in the Aquionics UV reactor in apple cider with apparent absorption coefficient (a) of 5.7 mm^?1. The lower value of UV decimal reduction dose of 7.3–7.8 mJ/cm² was required for inactivation of E. coli K12 in malate buffer and apple juice in the annular single-lamp UltraDynamics reactor. However, the decimal reduction dose for E. coli K12 in apple cider was significantly higher, about 20.4 mJ/cm². Similar UV decimal reduction doses from 25.1 to 18.8 mJ/cm² for inactivation of E. coli K12 were observed in the thin-film ‘CiderSure’ UV reactor in apple cider with identical absorption coefficient. Mathematical modeling of UV irradiance can improve the evaluation of UV dose delivery and distribution within the reactors.     

Temperature field simulation of polyolefin-absorber mixture by FDTD-FDM model during microwave heating

This work has performed a numerical simulation of the temperature field during microwave heating of polyolefin-absorber mixture by means of a combined electric and thermal model. A finite difference time domain was used to model the electric field distribution within the cavity, while the finite difference method was used to calculate the temperature field distribution in different reactors. This study has focused only on the process from room temperature to 500 K for reducing heating time and energy consumption. This temperature range is a process with high energy consumption, difficult to control and great influence on the follow-up reaction. Temperature dependence of dielectric properties and thermal properties of heated materials are fully considered and simulated through an iterative process. The simulation results show that input power, the size and location of the heated materials, the position of the waveguide, and the kinds of microwave absorbers are important factors affecting the heating process. As a result, the uniform temperature distribution (the temperature difference T_d < 10 K) can be achieved by choosing the appropriate input power (500–2000 W), the appropriate proportion of microwave absorber (the volume ratio of SiC to HDPE is 30:70), and combining with the moving and rotating of the heated materials. The uniform temperature field obtained without mechanical stirring is very important for reducing energy consumption and subsequent reactions.     

Microwave assisted flow synthesis: Coupling of electromagnetic and hydrodynamic phenomena

This article describes the results of a modeling study performed to understand the microwave heating process in continuous‐flow reactors. It demonstrates the influence of liquid velocity profiles on temperature and microwave energy dissipation in a microwave integrated milli reactor‐heat exchanger. Horizontal cocurrent flow of a strong microwave absorbing reaction mixture (ethanol + acetic acid, molar ratio 5:1) and a microwave transparent coolant (toluene) was established in a Teflon supported quartz tube (i.d.: 3 × 10^?3 m and o.d.: 4 × 10^?3 m) and shell (i.d.: 7 × 10^?3 m and o.d.: 9 × 10^?3 m), respectively. Modeling showed that the temperature rise of the highly microwave absorbing reaction mixture was up to four times higher in the almost stagnant liquid at the reactor walls than in the bulk liquid. The coolant flow was ineffective in controlling the outlet reaction mixture temperature. However, at high flow rates it limits the overheating of the stagnant liquid film of the reaction mixture at the reactor walls. It was also found that the stagnant layer around a fiber optic temperature probe, when inserted from the direction of the flow, resulted in much higher temperatures than the bulk liquid. This was not the case when the probe was inserted from the opposite direction. The experimental validations of these modeling results proved that the temperature profiles depend more on the reaction mixture velocity profiles than on the microwave energy dissipation/electric field intensity. Thus, in flow synthesis, particularly where a focused microwave field is applied over a small tubular flow reactor, it is very important to understand the large (direct/indirect) influence of reactor internals on the microwave heating process. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3824–3832, 2014     

Fast and efficient green synthesis of thiosulfonate S-esters by microwave-supported permanganate oxidation of symmetrical disulfides

Potassium permanganate absorbed on copper(II) sulfate pentahydrate has been found to be an efficient, inexpensive, and green oxidation agent for the synthesis of “symmetrical” thiosulfonate S-esters by oxidation of the corresponding symmetrical disulfides. The oxidation reactions were carried out under solvent-free reaction conditions within 15?min under the influence of microwave irradiation, as well as (for comparison) supported by conventional heating, to afford yields of the thiosulfonate S-esters in the range of 60–83%. The oxidation reaction appears to proceed (at least partly) via an intermediate symmetrical vic-disulfoxide.     

Titelbild Chem. Ing. Tech. 10/2016

The so‐called plug & play reactor is a novel reaction device with exchangeable reaction segments as well as modules for heating/cooling and mixing. The applications of the plug & play reactor include gas/solid, liquid/solid as well as gas/liquid/solid reactions. Commercially available HPLC columns, filled with catalyst particles, can be inserted in the reaction segment to act as fixed bed reactors. No external mixing device is needed, which leads to a high compactness of the reactor. The monitoring of the reaction progress and the reaction parameters can be carried out inline and online and an easy scale‐up by increasing the pipe diameter as well as simply numbering‐up by multiplying the modules is possible. Thus, the plug & play reactor is an attractive alternative to existing batch processes and can be easily implemented in existing processes. G. J. Lichtenegger, V. Tursic, H. Kitzler, K. Obermaier, J. G. Khinast, H. Gruber‐Wölfler*, Chem. Ing. Tech. 2016 , 88 (10), 1518 – 1523. DOI: 10.1002/cite.201600013     

Molecular weight distribution of polyethylene terephthalate in homogeneous,continuous-flow-stirred tank reactors

Poly(ethylene terephthalate) (PET) formation in homogeneous, continuous-flow-stirred tank reactors (HCSTRs) operating at steady state has been simulated. The feed to the reactor is assumed to consist of the monomer bis-(hydroxyethyl) terephthalate and monofunctional compound (MF₁) cetyl alcohol. The overall polymerization is assumed to consist of the polycondensation, reaction with monofunctional compounds, redistribution, and cyclization reactions. At a given time, the reaction mass consists of polyester molecules (P_n), polyester molecules with an ending of molecules of monofunctional compound (MF_n), and cyclic polymers (C_n). A mass balance for each of these species in the reactor gives rise to a set of algebraic equations to be solved simultaneously. The MWD calculations show that the redistribution reaction plays a major role and cannot be ignored, This result is in contrast lo the observation for semi-batch reactors, for which redistribution becomes important when the cyclization reaction is included. For the same residence times of semi-batch and HCSTRs, the latter gives considerably lower-number average molecular weight, N_av, and polydispersity index, ρ. However, for the same conversions, the ρ for CSTR is higher. The concentration of the monofurctional compound, [MF₁]₀, in the feed and the reactor temperature both influence ρ, but the effect is small within the range studied.     

Micromixing effects on a parallel reaction in flow reactors

Using the micromixing concepts of Danckwerts and Zwietering, the Peclet number Pe has been correlated mathematically to the degree of segregation J for the axial dispersion model. The results were applied to compare the micromixing effects on a model, mixed-order parallel reaction system in continuous flow reactors. Axial dispersion model, and Ng and Rippin's two-environment model were used to find the micromixing effects in tubular and stirred tank reactors, respectively. The performance of these reactors, with varying geometries, has been evaluated in terms of overall conversion, selectivity, and yield under identical operating and reaction conditions. The overall conversion increases in a tubular reactor with the increase in J, irrespective of the kinetic orders. However, in a stirred tank reactor, the conversion is found to be micromixing-sensitive, depending on the order of reaction. For m = 1 and n = 2 (case 1), the conversion is fairly insensitive to micromixing effects while it decreases for m = 0.5 and n = 1 (case 2) with increasing J. For the same extent of micromixing, a tubular reactor gives, in both cases, a higher conversion than a stirred tank reactor. The selectivity, in either case, decreases in both reactors with increasing segregation effects. However, in each case, the selectivity of a tubular reactor was fairly close to that of a stirred tank reactor at the same value of J. As far as the yield is concerned, both reactors achieve nearly the same value, without significant micromixing effects.     

Influence of microwave heating on the kinetic of acrylic acid polymerization and crosslinking

Kinetics of isothermal formation of poly (acrylic acid) (PAA) hydrogels through polymerization of acrylic acid and crosslinking of the PAA formed in a conventionally heated reaction system and in a microwave heated reaction system were investigated. It was found that in the microwave heated system the reaction rate constant of PAA hydrogel formation significantly increased (from 32 to 43 times) when compared with the conventionally heated system. The isothermal kinetics of the PAA hydrogel formation during the microwave process could be described by the so‐called first‐order chemical reaction kinetics model. In contrast, the so‐called second‐order chemical reaction rate model could best describe the isothermal kinetics of the PAA hydrogel formation during the conventionally heated process. Also, in the microwave heated system, the reaction kinetics of the PAA hydrogel formation and its kinetic parameters changed, that is, the activation energy (E_a) decreased by about 19% and the pre‐exponential factor (lnA) decreased by 2.2 times. The decrease in activation energy, change in entropy of activation energy, and decrease in the pre‐exponential value of PAA hydrogel formation under microwave heating are caused with increased energy of the reactive species when compared with their energy in thermal activation. Increased energy of the reactive species is a consequence of rapid transfer and absorption of the energy of microwave field to the existing reactive species. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Simulation and thermodynamic analysis of conventional and oxygen permeable CPO reactors

A one-dimensional steady-state heterogeneous model has been used to simulate the conventional CPO reactor. With the mechanism of O₂ permeable membrane, the model has been developed to simulate O₂ membrane reactor. The output temperature and the mole flow rates of different species in the tube side and the shell side can be calculated. They are the basis for the exergy analysis of the conventional CPO reactor with air, the conventional CPO reactor with pure O₂, and the O₂ permeable membrane CPO reactor. The simulation and exergy analysis results indicate that when the inlet conditions are the same, for a given methane conversion, the exergy efficiencies η₂ and η₁ of conventional CPO reactor with pure oxygen is lowest among the three reactors, because of the large amount of accumulative exergy required for obtaining pure oxygen.The exergy efficiencies η₁ and η₂ of membrane reactor are comparable with conventional CPO reactor with air and much higher than conventional CPO reactor with pure oxygen. As the membrane reactors can carry out simultaneous separation and reaction, in the mean time, removal of nitrogen from the product stream can be accomplished; the membrane reactor has advantages compared to other types of reactors.The operation of the membrane CPO reactor is more favourable when the inlet temperature is increased and the operation pressure is decreased from a thermodynamic point of view.     

Preparation of mesoporous MCM-41 supported zinc sorbents by microwave in-situ oxidation for H2S removal in coal gas

Zn-based MCM-41 supporter sorbents were prepared using the microwave in-situ oxidation method. Other sorbents were heated using a conventional heating method to contrast the performance for H₂S removal. The sorbents were tested at 500°C in fixed reactor and dried simulated Texaco coal gas was employed for the sulphurized atmosphere. The results show that sorbents prepared by microwave oxidation had a better toleration for the adsorption of H₂S. A 13.2% improvement occurred in the sulphur capacity of the sorbents heated by the microwave method. XRD, SEM with EDS-element mapping, TEM, N₂ adsorption, and XPS were used to characterize the properties of the sorbents. Due to the selective heating of the microwave and the superiority of the in-situ oxidation method, the sorbents heated by microwave exhibited more appropriate structures for sulphurization. Meanwhile, the even heating environment supplied by the microwave resulted in a more uniform distribution of the active component. The microwave also had an effect on the chemical bond and reduced the binding energy of the active component, which enhanced the reactivity between the H₂S and the sorbents. The preferable features generated by microwave in-situ oxidation accelerate the replacement of S to O, and therefore the Zn-based MCM-41 sorbents prepared by the microwave method have an increased capability for H₂S removal in high temperature coal gas.     

Modelling and experimental studies on unsteady-state SO2 converters

This paper deals with experimentation of the unsteady-state fixed-bed reactor with flow reversal for SO₂ conversion. A laboratory reactor with compensatory heating coils has been set up, resulting in 96–99% conversion for SO₂ = 2.62–9.52 vol%, and u = 0.2 m/s. The results were from agreement with simulation of a really adiabatic unsteady-state reactor, but agreed well with modelling by considering the radial heat transfer. A three-stage pilot-scale converter has also been run with the M-typed temperature profile produced by heat loss from the chamber between stages. Good agreement was achieved with modelling by taking into account this feature. The results show that, while the adiabatic requirement is difficult to fulfill in the well-accepted experimental reactors, simulation and modelling are powerful means for compensation.     

Design of mixed conducting ceramic membranes/reactors for the partial oxidation of methane to syngas

The performance of mixed conducting ceramic membrane reactors for the partial oxidation of methane (POM) to syngas has been analyzed through a two‐dimensional mathematical model, in which the material balance, the heat balance and the momentum balance for both the shell and the tube phase are taken into account. The modeling results indicate that the membrane reactors have many advantages over the conventional fixed bed reactors such as the higher CO selectivity and yield, the lower heating point and the lower pressure drop as well. When the methane feed is converted completely into product in the membrane reactors, temperature flying can take place, which may be restrained by increasing the feed flow rate or by lowering the operation temperature. The reaction capacity of the membrane reactor is mainly determined by the oxygen permeation rate rather than by the POM reaction rate on the catalyst. In order to improve the membrane reactor performance, reduction of mass transfer resistance in the catalyst bed is necessary. Using the smaller membrane tubes is an effective way to achieve a higher reaction capacity, but the pressure drop is a severe problem to be faced. The methane feed velocity for the operation of mixed conducting membrane reactors should be carefully regulated so as to obtain the maximum syngas yield, which can be estimated from their oxygen permeability. The mathematical model and the kinetic parameters have been validated by comparing modeling results with the experimental data for the La_0.6Sr_0.4Co_0.2Fe_0.8O_3‐α (LSCF) membrane reactor. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Temperature field simulation of polyolefin-absorber mixture by FDTD-FDM model during microwave heating

This work has performed a numerical simulation of the temperature field during microwave heating of polyolefin-absorber mixture by means of a combined electric and thermal model. A finite difference time domain was used to model the electric field distribution within the cavity, while the finite difference method was used to calculate the temperature field distribution in different reactors. This study has focused only on the process from room temperature to 500 K for reducing heating time and energy consumption. This temperature range is a process with high energy consumption, difficult to control and great influence on the follow-up reaction. Temperature dependence of dielectric properties and thermal properties of heated materials are fully considered and simulated through an iterative process. The simulation results show that input power, the size and location of the heated materials, the position of the waveguide, and the kinds of microwave absorbers are important factors affecting the heating process. As a result, the uniform temperature distribution (the temperature difference T_d < 10 K) can be achieved by choosing the appropriate input power (500-2000 W), the appropriate proportion of microwave absorber (the volume ratio of SiC to HDPE is 30:70), and combining with the moving and rotating of the heated materials. The uniform temperature field obtained without mechanical stirring is very important for reducing energy consumption and subsequent reactions.     

Catalytic contribution of reactor wall materials on oxidative coupling of methane

Gas-phase oxidative coupling of methane (OCM) was investigated with empty tubular-flow reactors made of various materials (quartz, ceramic and stainless steel tubes). When the temperature is higher than 680°C, the CH₄ conversions in three kinds of empty reactors are measurable, and the gas-phase OCM becomes noticeable at 750°C or higher. High temperature is beneficial to the activation of CH₄ and the dehydrogenation of C₂H₆ to C₂H₄. Residence time of reactants in the heated volume plays an important role in the gas-phase OCM. Experiments indicated that the gas-phase OCM can be minimized by increasing reactant gas velocity or filling the free volume of the reactor with inert materials. Reactor surface also has an effect on the reaction and may participate in some surface reactions. Based on the kinetic results and the SEM investigation on the surfaces of reactors, a gas-phase reaction mechanism of OCM is proposed.     

Microwave promoted clinkering of sulfoaluminate cement

Sulfoaluminate clinkers were made by combination of electric heating and microwave processing. When raw sulfoaluminate materials were heated to 1000-1200 °C conventionally, microwaves were inputted for 1-2 min. The f-CaO of the obtained clinkers was zero. Comparatively, when the samples with the same composition were conventionally heated at 1300 °C for 1 h, the f-CaO of the obtained clinkers was as high as 1.03-4.78%, and when the samples with the same composition were heated only by microwave for 25 min, CaCO₃ in the raw materials was not decomposed completely. It is shown that combination of electric heating and microwave processing greatly accelerate clinkering reaction. The X-ray diffraction (XRD) of the clinkers indicates that their mineral composition and XRD patterns are the same as those of the clinkers prepared by conventional firing methods. It has also been proven that Fe₂O₃ contributed to sulfoaluminate cement clinkering.     

Mechanism of Microwave Heating of Zeolite A

The mechanism of microwave heating of A zeolite was studied by comparing the heating properties, cation distributions and dielectric properties of 3A, 4A and 5A zeolites. It was easy to heat hydrated 4A zeolite to a glowing (melting) temperature by microwave (2.45 GHz) radiation from room temperature but was difficult to heat the same zeolite with little hydration. When 4A zeolite was preheated to 120°C, it could be easily heated by microwave radiation. However, 3A zeolite was not heated by microwaves under the above condition used for 4A zeolite. 3A zeolite with little hydration at room temperature could not be heated by microwaves but could be heated after hydrating it to 5 H₂O per unit cell (puc) or preheating 3A to 254°C. 5A zeolite could not be heated at all. The easiness of microwave heating was in the order of 4A < 3A 5A. There are differences in the cation distribution among these zeolites, i.e., 5A zeolite has no cations on the 4- and 8-membered oxygen ring sites but the other two have. It was expected from dielectric properties reported so far that the cation on 4-ring site can absorb microwave at <450°C with a higher efficiency but cations in other locations are ineffective for absorption. The following mechanism for the microwave heating was proposed: In the initial period, the hydratedzeolite absorbs the microwaves through its adsorbed water and its temperature rises. The adsorbed water completes desorption by ca. 400–470°C. In this temperature range the zeolite begins to absorb microwaves and the absorption efficiency becomes high with increasing temperature. When zeolite reaches 450–500°C thermal runaway starts. 5A zeolite cannot reach thermal runaway conditions because of absence of cation on 4-ring site. The adsorbed water plays the role of preheating agent in the initial period of microwave heating. Its role, however, can be substituted by other ways, e.g., preheating by conventional heating.