Catalytic Bed Microreactors

To combine the advantages of packed-bed and catalytic wall microreactors, catalytic bed microreactors were proposed recently. In this novel reactor design, the catalyst is applied on metallic filaments or wires which are incorporated in a microreactor, leading to a low pressure drop and a nanow residence time distribution [87-89]. By insertion of metallic wires a uniform gas distribution and a reduced risk of temperature gradients is obtained. However, similarly to catalytic wall microreactors, an increase in the specific surface area of the grid or wire is required. In addition to metallic wires and grids, modified ceramic tapes can also be used as a catalyst support [90]. 

The highest pressure drops usually occur in packed-bed microreactors with axial flow design. A reduced pressure drop while maintaining catalytic area can be achieved by a cross-flow design packed-bed microreactor [82]. To keep pressure drops down, the use of catalytic wall and catalytic bed microreactors is recommended. 

Catalytic activity was determined with a fixed bed microreactor which consisted of two coassial quartz tubes (i.d. 35 and 16 mm) to allow feed gas preheating and heated in an electrical oven (Watlow) with a temperature controller. The bed temperature was monitored by A K-type thermocoupling. On-line analysers (ABB) for CO, C02, CH4, ... 

The catalytic activity of SBA and AISBA samples toward cumene cracking were tested in a continuous flow fixed-bed microreactor system with helium (25 mL min 1) as carrier gas. The catalyst load for the tests was 100 mg and the catalyst was preheated at 573 K under helium flow for 3 h. For the reaction, a stream of cumene vapor in helium was generated using a saturator at room temperature. The reaction products were analyzed by gas chromatography. 

Catalysts pre-treatment (calcination and reduction) was performed in the same testing system or in a parallel automatic activation system prior to reaction test Calcination is carried out at 600 °C under airflow for 8 h and reduction at 250 °C for 2 h under hydrogen flow. Catalytic tests were carried out at 30 bar total pressure, temperature range 200-240°C, and 2.26h-1 WHSV, H2/hydrocarbons molar ratio of 2.93. Each fixed bed microreactor contained 500 mg of catalyst (particle size 0.4—0.6 mm, for which there are no internal diffusion limitations). Reaction products distribution are analysed using a gas chromatograph (Varian 3380GC) equipped with a Plot Alumina capillary column. 

The toluene oxidation reaction was used as a probe to study the catalytic properties of the Mo-Ce complex oxides. The as-prepared oxides were introduced into a U-type quartz fixed bed microreactor and their catalytic properties for selective oxidation of toluene to benzaldehyde were evaluated under the reaction conditions of O.lMPa, 400 C, air/toluene = 9 (vol/vol), F/W =1900 ml/h g cat. The reaction products were analyzed by an on-line gas chromatography. Under the above reaction conditions, the main products were CO, CO2, H2O and benzaldehyde. 

Catalysts were prepared from H-ZSM-5 obtained from Catal International, Sheffield, with a framework Si/Al ratio of 25, using conventional ion-exchange techniques [11]. Copper exchange levels ranged between 54% and 160%. Catalytic experiments were carried out using a fixed bed microreactor which has been described previously [11]. Product analysis was performed by a chemiluminescent NOx analyser (Signal Instruments Model 4000) and a gas chromatograph fitted with a thermal conductivity detector (Pye UNICAM PU 4550). 

The catalytic reactions were carried out in a catalytic flow microreactor at atmosheric pressure and various temperatures. The catal ic bed (Ig) was covered by silica. TTie reaction conditions were the following the oil (40(wt%) in cyclohexane) was introduced with a flow of 0.12 mkmin l simultaneoulsy with hydrogen (flow = 20 mIxmin H. After evaporation of the solvant, the products were successively treated by sodium methoxide, methanol and sulfuric acid to obtain the free-esters before analysis. The final products were analysed by gas chromatography with a flame ionization detector and AT-FILAR (Altech) capillary column (30m, I.d = 0.32 pm, film thickness = 0.25 pm) at 140 C. 

The laboratory fixed-bed microreactor consists of a 60 cm long quartz tube (I.D. 1.0 cm), in which a porous disk supports the catalytic bed. It is surroundwl by an electrical fiimace supplied with three heated and temperature controlled zones. The temperature is monitored by a Chromel-Alumel thermocouple, placed in another quartz tube, coaxial and internal to the reactor, along the whole length of the catalytic bed (1-2 cm). Four mass flow controllers (Brooks) allow to measure the flow rates of high purity gases N2O (0.2% vol.) + He, NO (1% vol.) + He, He (99.995% vol.) and O2 (99.995% vol). 

Infrared imaging was utilized in several studies of spatial effects in exothermic catalytic reactions over model catalysts, such as isolated particles, wafers, plates, discs [2]. Our approach has been to characterize the catalysts directly in a packed-bed microreactor, under realistic reaction conditions. In-situ measurements by infrared thermography of the adsorption properties of catalytic materials have been previously reported [6]. In the present study, the catalytic oxidation of compounds having different chemical properties was investigated by the same technique, with the aim of obtaining comparative data useful to better understand the factors governing the complex phenomena associated with catalytic combustion. 

Apparatus. The catalytic conversion was studied in a continuous flow quartz microreactor with a fixed-bed of diluted catalysts. The reaction conditions are reported in Table 1. After an experimental run (about 3 to 4 hours), the tar on the catalytic bed was determined by taking the difference in weight of the reactor before and after placing it in a furnace set at 500 C in the presence of air. The reaction products were analyzed by GC and GC/MS (Table 1). 

The drawback of randomly packed microreactors is the high pressure drop. In multitubular micro fixed beds, each channel must be packed identically or supplementary flow resistances must be introduced to avoid flow maldistribution between the channels, which leads to a broad residence time distribution in the reactor system. Initial developments led to structured catalytic micro-beds based on fibrous materials [8-10]. This concept is based on a structured catalytic bed arranged with parallel filaments giving identical flow characteristics to multichannel microreactors. The channels formed by filaments have an equivalent hydraulic diameter in the range of a few microns ensuring laminar flow and short diffusion times in the radial direction [10]. 

For three types of microstructured devices - the multichannel catalytic wall microreactor, the micro packed bed, and the catalytic metallic foam - the mass transfer effectiveness was calculated with the relations for mass transfer and pressure loss given in the previous sections. For the metallic foam, characteristic data were taken from [46] and [48]. The effectiveness is not dependent on the size or length of the device. 

From a design point of view, it is important to understand how to introduce two separate flows into one microchannel. In addition, the relative velocities of the flows have a significant influence on the resulting pattern of the multiphase flow. Another important aspect is how to introduce the catalysts active phase for a heterogeneous reaction where the solid catalyst is coated on the wall and/or placed as a packed bed inside a reactor. Even though the packed bed reactors are easier to fabricate than catalytic wall microreactors (CWM), CWMs are still favoured in most cases due to lower pressure drop and as they exhibit higher heat transfer rates (Kin et al, 2006). 

This section starts with a classification of phase-contacting principles according to the type of catalytic bed. Advantages and disadvantages of the reactor types are explained, followed by a discussion of criteria for reactor selection and an overview of purchasable microreactors for catalytic gas-phase reactions. 

Packed-bed microreactors are prepared by filling catalyst powder into the microchannels of the reactor. Since this is the easiest and fastest way for the incorporation of the catalyst, this type of microreactor is frequently used for catalyst screening [82]. Another advantage over other types of catalytic beds is the possibility of using... 

Several methods for the incorporation of catalysts into microreactors exist, which differ in the phase-contacting principle. The easiest way is to fill in the catalyst and create a packed-bed microreactor. If catalytic bed or catalytic wall microreactors are used, several techniques for catalyst deposition are possible. These techniques are divided into the following parts. For catalysts based on oxide supports, pretreatment of the substrate by anodic or thermal oxidation [93, 94] and chemical treatment is necessary. Subsequently, coating methods based on a Uquid phase such as a suspension, sol-gel [95], hybrid techniques between suspension and sol-gel [96], impregnation and electrochemical deposition methods can be used for catalyst deposition [97], in addition to chemical or physical vapor deposition [98] and flame spray deposition techniques [99]. A further method is the synthesis of zeoUtes on microstructures [100, 101]. Catalysts based on a carbon support can be deposited either on ceramic or on metallic surfaces, whereas carbon supports on metals have been little investigated so far [102]. 

This kind of reactor is easy to fabricate, commonly operates with laminar flow, and is used for catalyst screening and for the production of chemicals. However, micropacked bed reactors (MPBRs) usually have a high pressure drop dimng the passage of gases. Therefore, catalytic wall microreactors are more suitable. 

Toluene oxidation is carried out in a conventional fixed bed microreactor and studied between 25 to 300°C (l°C.min ). The reactive flow is composed of air and lOOOppm of gaseous toluene. The analysis of combustion products is performed evaluating the toluene conversion and the C0/(C0+C02) molar ratio from a Perkin Elmer autosystem chromatograph equipped with TCD and FID. Before the catalytic test, the solid (lOOmg) is calcined under a flow of air (2L/h) at 400°C (l°C.min ) and reduced under hydrogen flow (2L.h- ) at 200°C (l°C.mm ). 

Catalytic experiments were performed in a continuous flow fixed bed microreactor using SiC diluted catalyst to obtain predetermined total volume of catalyst bed (i.e., GHSV, h ). The feed composition comprises propane, oxygen, and, in some cases, steam or ammonia. In the case of mixed oxide catalyst and Ga-ZSM-5, the condensable products (acrylic and acetic acids, acrolein, acetone, acrylonitrile,... 

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