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.     

Total Oxidation of Chlorinated Hydrocarbons on A1–xSrxMnO3 Perovskite‐Type Oxide Catalysts – Part II: Catalytic Activity

The influence of the kind of A‐site cation in A_1–xSr_xMnO₃ perovskites (A = La, Pr, Nd, Di [didymium]) on the catalytic activity in the total oxidation of methane, chloromethane, dichloromethane, and trichloroethylene has been studied. In contrast to methane, the total oxidation of chlorinated hydrocarbons (CHC) is connected with a reversible catalyst deactivation and the formation of byproducts at low reaction temperatures. For the catalysts calcined at 600 and 800 °C, resp., the catalytic activity is determined mainly by specific surface area, amount of oxide admixtures and crystallinity of the perovskite. DiMnO₃ showed the highest and PrMnO₃ catalysts the lowest catalytic activity in the total oxidation of methane and CHC. Partial substitution of A by Sr leads to an enhancement of the catalytic activity in the total oxidation of methane, but not in the total oxidation of CHC.     

Total Oxidation of Chlorinated Hydrocarbons on A1–xSrxMnO3 Perovskite‐Type Oxide Catalysts – Part I: Catalyst Characterization

Thermoanalytical measurements (DTG‐DTA‐MS), X‐ray diffraction (XRD), temperature‐programmed reduction (TPR), redox titration and X‐ray photoelectron spectroscopy (XPS) were used to characterize A_1–xSr_xMnO₃ perovskite catalysts (A = La, Nd, Pr, Di [didymium]). The catalyst samples were investigated before and after interaction with chloromethane in the temperature range between 300 and 650 °C. XRD and TPR measurements revealed the presence of oxide admixtures in samples calcined at 600 and 800 °C, resp., in air. Crystallinity of the samples and the amount of oxide admixtures depend on the kind of A‐site cations. Interaction of the perovskite samples with chlorinated hydrocarbons at reaction temperatures leads to a decrease of the specific surface areas; the perovskite structure is preserved. Redox titration and TPR measurements showed that the Mn(IV) content in the perovskites increases by partial substitution of La by Sr and decreases after interaction with chloromethane.     

Catalytic Oxidation of Methanol to Formaldehyde in a Continuous Fluidized‐Bed Reactor

Partial oxidation of methanol to formaldehyde by using a mixture of ferric and molybdenum oxides as the reaction catalyst at 280–330 °C has been studied in a continuous fluidized bed reactor. The reactor was a cylindrical tube of 20 mm in i.d. and 36 mm in o.d. placed vertically and connected to a truncated coneshaped cyclone separator. The catalyst was prepared by the precipitation method using aqueous solutions of ammonium heptamolybdate and ferric nitrate. The effect of certain parameters, such as temperature, superficial gas velocity and feed flow rates, on the extent of oxidation reaction has been investigated. The maximum size of the catalyst particles was 990 μm, therefore, neither external nor internal diffusion was expected to be effective in the process. The experimental data were correlated with three classes of hydrodynamic models presented for fluidized systems. The best correlation was obtained with compartment type models.     

Microwave Super‐Heated Boiling of Organic Liquids: Origin,Effect and Application

This paper reports the state of the art of the microwave super‐heated boiling phenomenon. When a liquid is heated by microwaves, the temperature increases rapidly to reach a steady temperature while refluxing. It happens that this steady state temperature can be up to 40 K higher than the boiling point of the liquid. With the same reactor, overheating is not observed under conventional heating. The bulk temperature of a microwaved solvent under boiling depends on many factors: physical properties of the solvent, reactor geometry, mass flow, heat flow, and electric field distribution. The influence of these factors is studied and discussed. The kinetics of homogeneous organic reactions shows an extension of Arrhenius behaviour into the super‐heated temperature region. Reaction rate enhancement of order 10–100 can thus be achieved, which is normally only possible under pressure. Finally, we present a model predicting reaction kinetics and yields under classical and microwave heating, based on predicted temperature profiles in agreement with experimental data.     

Continuous Application of Polyglycerol‐Supported Salen in a Membrane Reactor: Asymmetric Epoxidation of 6‐Cyano‐2,2‐dimethylchromene

In the current work, suitability of hyperbranched polyglycerol as a high loading catalyst support is demonstrated. A polyglycerol‐supported manganese‐salen complex (chemzyme) is applied as a homogeneous catalyst in the epoxidation of 6‐cyano‐2,2‐dimethylchromene. The recyclability of the corresponding catalyst was investigated in repetitive batch experiments as well as a continuous operation of the reaction in an ultrafiltration membrane reactor. An enhanced stability of the catalyst in repetitive batches was observed as a result of immobilization whereby the total turnover number increased from 23 in a single batch to 80 in four repetitive batches. To enable continuous operation, a continuously operated, stirred tanked reactor (CSTR) was equipped with an ultrafiltration membrane (MPF‐50) and a retention of 98% was determined. The continuous chemzyme membrane reactor was operated over the course of 20 residence times. After approximately 12 residence times, the steady state was reached yielding 70% conversion as well as an enantiomeric excess up to 92%. A space‐time yield (sty) of 458 g L ⁻¹ d⁻¹ and a turnover frequency (TOF_reaction) of up to 18 h⁻¹ was reached in the steady state. It was determined that the total turnover number (TTN) was enhanced by a factor of 10 from 24 (batch) up to 240 for 20 residence times in CSTR operation.     

Viscoelastic Liquids in Stirred Vessels – Part I: Power Consumption in Unaerated Vessels

Different shear‐thinning and elastic fluids (STE fluids) have been stirred under unaerated conditions, in vessels equipped with Rushton disc turbines. Their power consumption has been evaluated over a wide range of stirring rates and their Metzner‐Otto constant (k_s) has been measured. A correlation has then been proposed to predict k_s values for a Rushton turbine operating in non‐Newtonian solutions. Power curves of STE fluids have been drawn and compared with reference curves (Newtonian, shear‐thinning inelastic and elastic with constant shear viscosity fluids). The STE fluids have thus been divided into two categories. The STE fluids of the first category (STE I fluids), which are concentrated viscous solutions of polymers (guar, CMC) reducing the power consumption at the beginning of the transitional region and connecting with the Newtonian reference at higher Reynolds numbers. In contrast, STE solutions of the second category (STE II fluids), which are solutions of drag reducing polymers (PAA), are less viscous and more elastic. They reduce the power consumption at the end of the transitional region and do not connect with the Newtonian reference, at least until Re = 6000. A general correlation has finally been proposed to model the power curve of STE fluids stirred by a Rushton turbine from the laminar to the turbulent regions, as a function of their elasticity.     

Kinetics of Photocatalyzed Gas Reactions Using Titanium Dioxide as the Catalyst – Part I: Photocatalyzed Total Oxidation of Olefines with Oxygen

In order to obtain a useful expression for the kinetics of the photocatalyzed total oxidation reaction of olefines with TiO₂ as the catalyst, the dependence of reaction rate on the concentration of the educt species, the absorbed irradiation intensity, the wavelength of the light and the temperature was investigated. The experiments were performed in a CSTR system. The results obtained for steady‐state conditions showed that besides the photocatalytic activity, the TiO₂ powder fixed on one wall of the reactor has, in addition, a thermal activity for higher temperature. Although the reaction behavior of the system is relatively complex, it can be modeled straightforward by a concept proposing a cooperative coadsorption of oxygen and the olefine species mediated by the electron/hole pairs generated by the absorbed light on the semiconductor's surface.     

Brennstoffpellets aus Braunkohlenxylit und Holz: Teil 1 – Trocknungs‐ und Sorptionsverhalten

Xylite, a by‐product of lignite production, can be converted into fuel pellets by using a wet pelletizing process, in which the drying step of the raw pellets is of vital importance. During moisture removal, structural changes take place in the agglomerates, they become harder and investigations of the drying behavior indicate high shrinkage during drying. This effect can also be seen in hygroscopic sorption experiments. Furthermore, the pellets show two drying sections. The main factor affecting the drying speed is the temperature; the pellet composition has a subsidiary effect.     

Semi‐Empirical Equations for the Residence Time Distributions in Disperse Systems – Part 1: Continuous Phase

Residence time distributions (RTD) are often described on the basis of the dispersion or the tanks in series models, whereby the fitting is not always good. In addition, the underlying ideas of these models only roughly characterize the real existing processes. Two semi‐empirical equations are presented based on characteristic parameters (mean, minimum, maximum residence time) and on an empirical exponent to permit better fitting. The determination of the parameters and their influence on the RTD are discussed. The usefulness of the models is shown in this first part for single‐phase systems and for the continuous phase of multiphase systems using data from literature for laminar and turbulent flows in different apparatuses. A comparison with the results of other models is also done.     

Access to Both Enantiomers of α‐Chloro‐β‐keto Esters with a Single Chiral Ligand: Highly Efficient Enantioselective Chlorination of Cyclic β‐Keto Esters Catalyzed by Chiral Copper(II) and Zinc(II) Complexes of a Spiro‐2,2′‐bischroman‐Based Bisoxazoline Ligand

The spiro‐2,2′‐bichroman‐based chiral bisoxazoline ligands (SPANbox) were found to be highly efficient in copper(II)‐ and zinc(II)‐catalyzed asymmetric chlorinations of cyclic β‐keto esters with N‐chlorosuccinimide (NCS) as the chlorination reagent, to give the corresponding α‐chloro‐β‐keto esters in excellent yields in 5–30 min with ee values up to 97%. The copper(II) triflate and zinc(II) triflate complexes of a single SPANbox ligand demonstrated complementary results to each other with respect to the enantioselection, affording both antipodes of the chlorinated product enantiomers with good to excellent optical purities.     

Simplified Calculation of Mixing and Suspending Processes in Agitated Vessels – Part II: Examples of Application

Mixing processes in agitated vessels with different stirring systems are simulated using a cell model. The model takes the fluid velocity and fluid turbulence, as well as the diffusion and the sedimentation velocity of the particles into account. The two‐ and three‐dimensional behaviors of single particles and collectives of particles in the vessels, respectively, are discussed in terms of different aspects. The calculated concentration distributions are compared with some measurements from the literature. Furthermore, a user interface developed for the calculation program, is introduced.     

Iodine‐Promoted Domino Reactions of 1‐Cyanocyclopropane 1‐Esters: A Straightforward Approach to Fully Substituted 2‐Aminofurans

A straightforward and efficient iodine‐promoted ring‐opening/cyclization domino reaction of 1‐cyanocyclopropane 1‐esters for the synthesis of fully substituted 2‐aminofurans is reported. This reaction involves the sequential ring‐opening/intramolecular cyclization reaction of 1‐cyanocyclopropane 1‐esters to give the corresponding 2‐amino‐4,5‐dihydrofurans, which were oxidized with I₂ and Et₃N in refluxing toluene to give the corresponding 2‐amino‐3‐furancarboxylates.

     

Microchemical Engineering: Components,Plant Concepts,User Acceptance – Part II

Over the last five years, many activities have focused on the unexploited field of carrying out reactions on small scales. Due to the rapid development of new components, this paper deals with recent developments only in a compressed form. An important point is the analysis of possible plant concepts for microreactors and whether these are a sensible option. Due to the enormous difference in size between the microchannels and the fluid periphery of possible components this is not just a technical question. It touches on the microtechnology concept as a whole. The direction in which the field should be developed and which measures can be taken to influence its development are questions that are addressed here with respect to the big industrial interest in microreactors.     

High‐Temperature Aging of Plasma Sprayed Quasi‐Eutectoid LaYbZr2O7–Part II: Microstructure & Thermal Conductivity

LaYbZr₂O₇ ceramic thermal barrier coatings (TBC) of meta‐stable structure were prepared by an air plasma spraying process. Their microstructure and associated thermal transport properties evolution during high‐temperature annealing at 1300°C were characterized. The as‐sprayed LaYbZr₂O₇ TBCs underwent a fast crystallization and a quasi‐eutectoid transformation during annealing, resulting in a biphase composite consisting of La‐rich pyrochlore phase and Yb₂Zr₂O₇ fluorite phase with coherent phase boundaries. Due to the diffusion barriers between the two phases as well as the low interface energy of the coherent boundaries, sintering and grain growth of materials was significantly refrained. Therefore, a final thermal dynamically stable microstructure with a grain size of ~300 nm and a total porosity about 5% could be maintained even after long‐term aging at a high temperature of 1300°C. Resulting from this stable microstructure, an ultralow thermal conductivity of 1.3 W·(m·K)^?1 could be obtained even after 216 h high‐temperature aging, which is much lower than that of the state‐of‐art 7 wt% yttria‐stabilized zirconia TBCs. Both the high phase and microstructure stability and the extremely low thermal conductivities could be particularly beneficial for TBC material in gas turbine applications.