Tubular microreactors

Ethylene oxide catalyst research is expensive and time-consuming because of the need to break in and stabilize the catalyst before rehable data can be collected. Computer controlled tubular microreactors containing as Httle as 5 g of catalyst can be used for assessment of a catalyst s initial performance and for long-term life studies, but moving basket reactors of the Berty (77) or Carberry (78) type are much better suited to kinetic studies. 

Figure 2.29 Photograph of the ceramic reactor housing and the quartz-glass tubulare microreactor (visible through the center hole) [59].

CO hydrogenation was conducted in a quartz tubular microreactor operating at 1 atm. The H2/CO ratio was 2. Products were separated in a 4 ft. x 1/8 in. column packed with Chromosorb 102 and analyzed in a GC (HP5710A) with TCD and FID detectors connected in... 

Alkylation of isobutane with 1-butene was carried out in a fixed bed down flow stainless steel tubular microreactor. The experiments were carried out in the gas phase at 1 atm total pressure, isobutane/l-butene molar ratio of 14 and 1-butene space velocity 1.0 h". Premixed isobutane and 1-butene (>99% purity, Matheson) was fed from a gas cylinder. A high paraffin to olefin ratio was choosen to reduce the chance of olefin dimerization. The catalysts (300 mg, 60-80 pm particle size) were activated in the reactor by calcining in air at 450°C for 4 hours. Air flow was then replaced by nitrogen and the catalyst temperature was lowered to the desired reaction temperature. The feed gas and the products were analyzed by a on-line gas chromatograph equipped with a CP-Sil PONA capillary column (length 50 m, film thickness 0.25 pm). 

The conversion of decane was performed in a fixed bed, tubular microreactor. The H2/decane molar ratio in the feed was 100. The pressure in the reactor was 0.35 MPa and the space time of decane 0.5 kg s mmol l. 

The catalytic experiments were performed in a continuous flow tubular microreactor. The catalyst bed consisted of 0.8 cm of 0.3-0.5 mm pellets, prepared by compressing of the NH -T zeolite powder into flakes, crushing and sieving. The zeolite was activated in the reactor by deammoniation at 673 K. The vapors of the acids were diluted with helium. The reaction conditions are further specified in Table 1. The reaction products were analysed on-line with GC, using a capillary fused silica column, coated with CP Sil5 (Chrompack) and F.l.D. detector. CO2 and H2O were not analysed. 

A comprehensive experimental research program to investigate the effects of pressure on the products of steam gasification of biomass is currently underway. A stainless steel, tubular microreactor similar to the quartz reactor described earlier has been fabricated for the experimental work. The pyrolysis furnace used with the quartz reactor system has been replaced in the pressurized steam system by a Setaram Differential Scanning Calorimeter (DSC). The DSC provides for quantitative determination of the effects of pressure on pyrolysis kinetics and heats of reaction. 

The term microchemical describes downsizing of a system such that its critical dimension is less than 1 mm. For tubular microreactors, this size reduction entails the tube radius, R, whereas for microchannels of a rectangular entrance section, this entails the height (also referred to as gap size), d, and possibly its width, w. The latter... 

Tubular microreactors , which are in reality mini- or millimeter-scale reactors, have been used to study these reactions. Tubular microreactors take the form of a fixed-bed reactor with various materials of construction. Chao et al. used a quartz tubular microreactor (Sqm i.d.) to compare oxidative dehydrogenation of ethane over vanadium- and magnesium-based catalysts of different preparations [15]. The catalysts were introduced in the middle of the reactor tube with quartz granules used to fill the space above the catalysts and quartz wool used to retain the packing. This quartz microreactor was placed in a tubular furnace with the catalyst bed held in a constant-temperature zone. After performing experiments to study selectivity and conversion on the catalysts, surface analysis of the catalyst materials was used to identify the best catalyst preparation method. 

If the target product is an intermediate in sequential reactions, selectivity and yields are quite sensitive to the residence time. The axial dispersion in a tubular microreactor can often be described by a dispersion model. This model assumes that the RTD results from piston flow on which axial dispersion is superimposed. The latter is taken into account by means of an... 

Covalent linking of enzyme on fused silica capillaries was also demonstrated by Stigter [119]. Pepsin was covalently immobilized on dextran-modified capillaries as illustrated in Scheme 10.12. The applicability of pepsin-immobilized microreactor was tested with a number of proteins varying in molecular weight, isoelectric point, and sample composition. This open tubular microreactor demonstrated a flow-dependent digestion as represented by native hemoglobin. Complete digestion of... 

Figure 2.48. Ammonia synthesiis with differently promoted catalysts. The conversion is compared to equal amounts of 0.6000 g catalyst tested in a tubular microreactor. Each data point was taken after 24 hours of equilibration.

Figure 7.13 Synthesis of Ag nanoparticles using a tubular microreactor via thermal reduction of silver pentafluoropropionate. (Reproduced from Ref [20].)...

Ebrahimi et al. [24] studied the formation of petformic acid and peracetic add in a tubular microreactor and used a plug flow model that could reasonably well explain the experimental data. In the continuously performed experiments, hydrogen peroxide and formic acid were mixed in the micromixer (Sht Interdigital Micromixer from IMM [25]). The solution was fed in the micromixer and then in the Teflon tube with an inner diameter between 1.0 and 1.6 mm. The reaction rate depends on the hydrogen peroxide (HP), formic add (FA), and hydroxonium ion (HjO ) concentrations ... 

Quite new ideas for the reactor design of aqueous multiphase fluid/fluid reactions have been reported by researchers from Oxeno. In packed tubular reactors and under unconventional reaction conditions they observed very high space-time yields which increased the rate compared with conventional operation by a factor of 10 due to a combination of mass transfer area and kinetics [29]. Thus the old question of aqueous-biphase hydroformylation "Where does the reaction takes place " - i.e., at the interphase or the bulk of the liquid phase [23,56h] - is again questionable, at least under the conditions (packed tubular reactors, other hydrodynamic conditions, in mini plants, and in the unusual,and costly presence of ethylene glycol) and not in harsh industrial operation. The considerable reduction of the laminar boundary layer in highly loaded packed tubular reactors increases the mass transfer coefficients, thus Oxeno claim the successful hydroformylation of 1-octene [25a,26,29c,49a,49e,58d,58f], The search for a new reactor design may also include operation in microreactors [59]. 

The microreactor technique described by the authors offered 30 and 50 times higher STY values than the conventional tubular reactor and the slurry reactor, respectively, suggesting microreactors to be a successful technique in mass transfer controlled reactions. 

In the first stage of the investigation the catalyst can be considered in the form of powder in order to derive intrinsic transient kinetics of all the relevant reactive processes. To this purpose, dynamic reactive experiments can be performed in a simple tubular fixed-bed microreactor over small quantities (50-200 mg) of finely powdered catalyst in principle, this guarantees negligible transport limitations and more controlled conditions (e.g. isothermal catalyst bed), hence enabling a direct estimation of intrinsic rate parameters by kinetic fit. Internal diffusion limitations are particularly relevant to the case of bulk (extruded) monolith catalysts, such as vanadium-based systems for NH3/urea SCR however, they... 

Now we shall discuss the method used to calculate the "cup"-averaged MWD-H, in which all portions of a polymerized liquid are mixed and averaged in a "cup" (vessel) positioned after the reactor. In this analysis, recourse was made to the so-called "suspension" model of a tubular reactor. According to this model, the reaction mass is regarded as an assemblage of immiscible microvolume batch reactors. Each of these microreactors moves along its own flow line. The most important point is that the duration of the reaction is different in each microreactor, as the residence time of each microvolume depends on its position at any given time, i.e., on its distance from the reactor axis. 

Methane decomposition experiments were conducted in a 5.0 ml fixed bed quartz microreactor using 0.3 g of catalysts. The catalysts were arranged within the reaction zone in several layers separated with ceramic wool to prevent clogging of the reactor due to produced carbon. The reactor temperature was maintained constant via a type K thermocouple and Love Controls microprocessor. The tubular reactor was made out of alumina and quartz tubings (I.D. 3-6 mm). 

Srinivas, S., Dhingra, A., Im, H., and Gulari, E. A scalable silicon microreactor for preferential CO oxidation Performance comparison with a tubular packed-bed microreactor. Applied Catalysis. 

Many speciahzed laboratory reactors and operating conditions have been used. Sinfelt has alternately passed reactants and inert materials through a tubular-flow reactor. This mode of operation is advantageous when the activity of the fixed bed of catalyst pellets changes with time. A system in which the reactants flow through a porous semiconductor catalyst, heated inductively, has been proposed for studying the kinetics of high-temperature (500 to 2000°C) reactions. An automated microreactor... 

The microreactor used for evaluating catalyst activity was operated at atmospheric pressure and consisted of Barter manual mass flow controllers, and a tubular reactor made from fused alumina of 11 mm o.d. The temperature was measured by means of an inert internal thermocouple and controlled by an electronic temperature controller. Product gases were analysed by the g.c. 

Catalyst Evaluation. The powdered molecular sieves were evaluated following the treatment described above, without further activation. The 1-hexene isomerization and Cg aromatic isomerization tests were conducted in tubular, fixed bed, continuous flow microreactors. The catalyst bed contained one gram molecular sieve powder and one to three grams of similarly sized quartz chips used as diluent. The reactor was heated to the chosen reaction temperatures in a fluidized sand bath, and the reaction temperature was monitored by a thermocouple located m the catalyst bed. Typical runs lasted 3 to 5 hours during which samples were collected every 30 minutes. 

Cumene Cracking and Hydrocracking. Cracking studies were conducted in a tubular stainless steel microreactor inserted in a bronze block. The entire assembly was constructed within the oven of a Perkin Elmer Model 880 gas chromatograph. 

NSSTK and SSITK experiments were performed with an atmospheric flow system using either a tubular quartz microreactor (70 mg of catalyst) or a catalytic DRIFT cell from Spectratech, allowing the gases to flow through a fixed bed of catalyst pellets (about 30 mg) and able to be heated up to 1173 K. The gas composition was continuously monitored at the reactor outlet by online mass spectrometer and the surface composition was investigated by a FT-BR. spectrometer (Nicolet 550) with one spectrum recorded per second In all cases, the catalyst was pre-treated with He at 1013 K for 40 min. The reacting feed was composed of 10 vol.% methane ( CHj, CHj or CD4) and 90 vol.% He with a total flow rate of 24 ml/min. The reaction was carried out at 1 atm and 1013 K. Ar was used as an inert tracer. 

Conventional alternatives to a microreactor, such as non-catalytic tubular reactors, exhibit about 100-150 m m surface area to reactor volume, a value very similar to conventional heat exchangers. Using these reactors with porous catalysts filling the tubes can increase the surface area dramatically up to 10 nf m . Typically, the surface area of microreactors is in the range of lO to 10 nf m . This is the surface area only of the microreactor walls, which in general are non-porous. 

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