Isothermal reactors microreactors

The qualitative analysis of intraparticle heat transport suggests severe limitations of packed-bed laboratory reactors compared with the thin-film catalyzed microchannel, as discussed previously. It is imperative that a quantitative study of reactor heat transfer limitations is performed. With PrOx as a model reaction, this study was realized through the non-isothermal reactor modeling of the microreactor and the packed-bed reactors with both 2 and 4 mm radii. In the model, the operating... 

There are several possible arrangements for the application of supported catalysis in microreactors. The simplest system involves a micro-fixed-bed reactor with a supported catalyst To provide good mechanical stability, the support is deposited onto glass beads by dip coating or other sol-gd methods. Near-isothermal reactor operation can be realized in a microreactor by the effective dissipation of reaction heat by a proper choice of thermal conductivity and geometry of the plate material and the reactor [22,23]. 

In this experiment, the reaction temperature was isothermally controlled at 15 °C. The heat of reaction was completely removed using microreactor so that virtually no byproducts were produced during the reaction. It can be compared with other reactors described above, which should be operated at 0 °C or -20 °C to avoid side reactions. 

Apparatus. The experiments were conducted in a high-pressure microreactor capable of operating up to 3000 psig. The reactor, enclosed by a three-zone heater, had an isothermal reaction zone holding up to 10... 

Selection of the laboratory reactor requires considerable attention. There is no such thing as a universal laboratory reactor. Nor should the laboratory reactor necessarily be a reduced replica of the envisioned industrial reactor. Figure 1 illustrates this point for ammonia synthesis. The industrial reactor (5) makes effective use of the heat of reaction, considering the non-isothermal behavior of the reaction. The reactor internals allow heat to exchange between reactants and products. The radial flow of reactants and products through the various catalyst beds minimizes the pressure drop. In the laboratory, intrinsic catalyst characterization is done with an isothermally operated plug flow microreactor (6). 

Axial temperature differences in isothermally operated laboratory reactors can be minimized by installing a sufficiently large number of independently controlled heating zones along the reactor, e.g., 3 or more in the case of a microreactor and at least 5 for a bench-scale reactor of 1 metre length, with pre- and post-reactor heating elements. A thick reactor wall will promote temperature equalization in axial direction. 

Example 4-6 Calculating X in a Reactor with Pressure Drop Example 4 7 Gas-Phase Reaction in Microreactor—Molar Flow Rate Example 4-8 Membrane Reaeior Example CDR4.1 Spherical Reactor Example 4.3.1 Aerosol Reactor Example 4-9 Isothermal Semibatch Reactor Profe.ssional Reference Shelf R4.1. Spherical Packed-Bed Reactor. ... 

The experiments were carried out in an all stainless steel microreactor system with four gas lines which was operated at pressures up to 100 bar. The gases were supplied by Linde with the following purities He 99.9999 %, N2 99.9999 %, H2 99.9999 %, the mixture of 25% N2 in H2 used as synthesis feed gas 99.9996 %. The feed gas was further purified by means of a purification unit described elsewhere [4]. The flows were regulated by electronic mass flow controllers. The reactor consisted of a glass-lined U-tube similar to the one described in ref. [13]. It was not possible to detect the desorption of N2, H2 or NH3 from the empty tube within the limits of detection. The U-tube was placed in a copper block to ensure isothermal operation. Gas analysis was performed using a mass spectrometer (Balzers GAM 445) which was calibrated for He, H2, N2 and NH3 by using a reference gas mixture. The calibration for H2O was carried out using a He stream saturated with H2O at room temperature. 

Catalytic runs were carried out at atmospheric pressure in a quartz-made fixed-bed flow microreactor (10 mm i d.). All the stainless-steel equipment devices had been passivated by hot HNO3 treatment before the assembly. The catalyst was activated in situ (6 h at 773 K under COj-free air flow). 4-Methylpentan-2-ol was fed in with an N2 stream (partial pressure, Po.aicohoi = 19.3 kPa time factor, W/F = 0.54 gcat-h/gakohoi)- On-line capillary GC analysis conditions were Petrocol DH 50.2 column, oven temperature between 313 and 473 K, heating rate 5 K/min. Products identification was confirmed by GC-MS. For each catalyst a run in which several reactor temperatures were checked was carried out, in order to study the influence of the thermal history of the sample on its catalytic behaviour and to reach an appropriate conversion level (ca. 50%) at which the selectivities values of the different catalysts can be compared. Then, a new run with a fresh portion of the same sample was started at the desired temperature and carried out isothermally for 80 h. Further runs, where both the flow rate and the catalyst amount were considerably changed, while keeping the same W/F value, were also carried out no significant differences in conversion were observed, which rules out the occurrence of external diffusion limitations. 

At high temperatures, hydrogen cyanide can hydrolyze to ammonia. Hessel et al. were able to produce hydrogen cyanide isothermally in a microreactor via the Andrussow route and avoid this undesired reaction [1]. Air was used as the cooling medium, and cooling of the reaction products from 1,000 °C to 120 °C was calculated to be achieved in 100 ps. Another example is the production of the antibiotic ciprofloxacin, which requires a complex 20-30-step synthesis to avoid explosions in conventional reactors [10]. CPC Systems has patented a synthesis route utilizing a microreactor to... 

Comparison of a single-tube packed-bed reactor with a traditional batch reactor was also published in the case of o-nitroanisole hydrogenation, not for productivity purposes but rather as laboratory tools for kinetic studies (Scheme 9.11) [46]. It was shown that the better efficiency of mass transfer enables the microreactor to obtain intrinsic kinetic data for fast reactions with characteristic times in the range 1-100 s, under isothermal conditions, which is difficult to achieve with a stirred tank reactor. However, the batch reactor used in this study was not very well designed since a maximum mass transfer coefficient (kia) of only 0.06 s was measured at 800 rpm, whereas kia values of up to 2 s are easily achieved in small stirred tank reactors equipped with baffles and mechanically driven impellers [25]. This questions the reference used when comparing microstructured components with traditional equipment, with the conclusion that comparison holds only when the hest traditional technology is used. 

It is possible to obtain kinetically representative data in very small laboratory reactors if the temperature and concentration gradients on the scale of the catalyst particle are absent. The bed needs to be isothermal and the reactor should behave as a plug-flow reactor. These requirements imply using small particles and a sufficient bed length. For flow through such small particles, surface tension forces become much more important, and established trickle-bed correlations cannot be extrapolated. For a reliable design of the reactor bed and for a choice of window of operation, the hydrodynamics of such fixed-bed microreactors have to be investigated. 

The second stage of the scale-up approach involves monolith reactor experiments over small catalyst samples with a volume of a few cubic centimeters. The data obtained from this intermediate stage serve either as a primary validation of the intrinsic reaction kinetics or for kinetic parameter estimation in case microreactor experiments have been omitted. Monolith reactor experiments are able to reproduce more accurately the phenomena prevailing in real full-scale converters taking into account the catalyst s geometry, the flow dynamics along the channel, and the intraporous diffusion over the washcoat. At the same time, the experiments are performed under controlled laboratory conditions, involving isothermal operation and the use of synthetic gas mixtures. 

Until recently, the temperature control of highly exothermic reactions using the microreaction systems was mainly based on the removal of heat in order to prevent hot spot formation and thermal runaway [29]. More recently, however, research has focused on techniques that enable microreactors to be heated because they can efficiently dissipate the heat. If a microheat exchanger is integrated into a microreactor, both effects can be combined, that is, either enabling fast heat supply in the reactor or heat removal from the reactor [30]. In practice, strongly exothermic reactions such as nitration, oxidation, chlorination, and even fluorination with elementary fluorine (in microreactors made of nickel) can be carried out in microreactor systems under nearly isothermal conditions [31]. 

Example 6-1 Gas-Phase Reaction in Microreactor—Molar Flow Rate Example 6-2 Membrane Reactor Example 6-3 Isothermal Semibatch Reactor Proressional Reference Shelf R6,1 UnsH udy CSTRs and Semihaich Reactors R6.1A Start-up of a CSTR... 

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