Ceramic monolith reactor

A 15-fold glass tube parallel-packed bed reactor has been described [28-30], which is close to conventional catalyst testing equipment. The same authors also reported a 64-fold ceramic block reactor and a ceramic monolithic reactor for the screening of up to 250 catalysts in parallel. The individual catalysts were coated... 

Examples of early designs are given elsewhere [20]. Ultimately, three different designs emerged for widespread application the bead catalyst reactor, the ceramic monolith reactor and the metal monolith reactor (Fig. 21). 

Greaser et al. [76] modeled autothermal diesel reforming in a ceramic monolithic reactor and verified the modeling results with experimental data as shown in Figure 14.10. The model revealed that axial heat conduction plays an important role even in ceramic monoliths. The catalyst temperature was found to be 25°C hotter than the gas phase at the reactor inlet according to the calculations. At the positions of highest reaction rates, the catalyst utilization was as low as 20%. Transport limitations in the washcoat were assumed to be the root cause. The... 

From the 1970s, when it was demonstrated that metallic and not only ceramic monolithic reactors could be successfully coated, the metallic monoliths gained acceptance in the... 

In many cases supports are shaped into simple cylinders (1-5 mm in diameter and 10-20 mm in length) in an extrusion process. The support powder is mixed with binders and water to form a paste that is forced through small holes of the desired size and shape. The paste should be sufficiently stiff such that the ribbon of extmded material maintains its shape during drying and shrinking. When dried, the material is cut or broken into pieces of the desired length. Extrusion is also applied to make ceramic monoliths such as those used in automotive exhaust catalysts and in DeNOx reactors. 

Figure 7-16 A highly simplified sketch of an automohile engine and catalytic converter with typical gas compositions indicated before and after the automotive catalytic converter. The catalytic converter is a tube wall reactor in which a noble-metal-impregnated wash coat on an extruded ceramic monolith creates surface on which reactions occur.

There are a number of examples of tube waU reactors, the most important being the automotive catalytic converter (ACC), which was described in the previous section. These reactors are made by coating an extruded ceramic monolith with noble metals supported on a thin wash coat of y-alumina. This reactor is used to oxidize hydrocarbons and CO to CO2 and H2O and also reduce NO to N2. The rates of these reactions are very fast after warmup, and the effectiveness factor within the porous wash coat is therefore very smaU. The reactions are also eternal mass transfer limited within the monohth after warmup. We wUl consider three limiting cases of this reactor, surface reaction limiting, external mass transfer limiting, and wash coat diffusion limiting. In each case we wiU assume a first-order irreversible reaction. 

The use of a monolithic stirred reactor for carrying out enzyme-catalyzed reactions is presented. Enzyme-loaded monoliths were employed as stirrer blades. The ceramic monoliths were functionalized with conventional carrier materials carbon, chitosan, and polyethylenimine (PEI). The different nature of the carriers with respect to porosity and surface chemistry allows tuning of the support for different enzymes and for use under specific conditions. The model reactions performed in this study demonstrate the benefits of tuning the carrier material to both enzyme and reaction conditions. This is a must to successfully intensify biocatalytic processes. The results show that the monolithic stirrer reactor can be effectively employed in both mass transfer limited and kinetically limited regimes. 

Fixed bed reactors still predominate for fuel processing. However, fixed beds are susceptible to vibrational and mechanical attrition. Recently, monolithic reactors, either metallic or ceramic, have attracted interest for reforming processes since they offer higher available active surface areas and better thermal conductivity than conventional fixed beds. Low-pressure drop and robustness of the structure are major advantages of monolithic reactors. 

Monolith reactor This type of reactor is used extensively for the abatement of automobiles exhaust emissions. The gas flows continuously through the reactor, whereas the catalyst is a continuous phase consisting of a ceramic support and the active phase, which is dispersed onto the support. The support is structured in many channels and shapes that achieve large catalytic surface at small volume. A typical application of monolith reactors is the exhaust gas cleaning. 

The oxidation of gaseous ethanol (first-order reaction) was studied in a spinning basket reactor with Pt/Al203 on a ceramic monolith as catalyst. The inlet concentration of ethanol... 

So far, MR studies of reactors of relevance to catalytic processes have been restricted to fixed beds and ceramic monoliths. Recently, the first reports of MR being... 

Comparing the performance of the micro structured reactor with a ceramic monolith at 230 °C reaction temperature and a GHSV of 300 000 h 1, the conversion in the micro reactor was 94%, whereas 86% was found for the monolith, which was attributed to the improved heat and mass transfer in the metallic micro-structures (see Figure 2.89). The GHSV value of 500 000 h 1 corresponds to a dry gas flow rate of440 Ndm3 h 1. However, the stability of the catalyst coated on the monolith was superior to that of the catalyst coated on the micro structured stainless-steel plates. 

Due to its high photocatalytic activity towards the complete mineralisation of VOCs [7,8] titania in its anatase form is normally used. Using ceramic monoliths with high titania content (50%) the total oxidation of chlorinated organic compounds at low temperature has been demonstrated [9]. However, since the photons from natural light may only penetrate a few microns into the catalyst surface the use of a wash-coating technique, where only a thin active film of titania is applied to the ceramic or metallic support can be considered as an ideal technique to produce maintenance free photocatalytic reactors. 

Since there is no radial bulk transport of fluid between the monolith channels, each channel acts basically as a separate reactor. This may be a disadvantage for exothermic reactions. The radial heat transfer occurs only by conduction through the solid walls. Ceramic monoliths are operated at nearly adiabatic conditions due to their low thermal conductivities. However, in gas-liquid reactions, due to the high heat capacity of the liquid, an external heat exchanger will be sufficient to control the reactor temperature. Also, metallic monoliths with high heal conduction in the solid material can exhibit higher radial heat transfer. 

Possibility to use standard catalyst particles. When hollow extnidates or ring-shaped pellets are used as the catalyst material for the BSR, the catalyst can be manufactured according to standard procedures. Consequently, no additional catalyst development is necessary to apply an existing catalyst in a BSR. This is an advantage over the monolithic reactors, because with those reactors one has to deal with the peculiarities of the washcoat or the ceramic carrier body serving as the support of the active sites. 

It is seen that the pressure drop across the BSR is close to that across the monolithic reactor, and both are about two orders of magnitude lower than that across the randomly packed-bed reactor. Obviously, for a washcoated monolithic reactor the pressure drop will be slightly higher than for the incorporated monolith used in this comparison, depending on the ratio between the thickness of the washcoat and the ceramic or metal support. 

Ahluwalia, R.K., Zhang, Q., Chmielewski, D.J., Lauzze, K.C., and Inbody, M.A. Performance of CO preferential oxidation reactor with noble-metal catalyst coated on ceramic monolith for onboard fuel processing applications. Catalysis Today, 2005, 99, 271. 

Figure 1. Sketch of reactor configuration used for catalytic oxidation on monolith reactors at millisecond contact times. Gases slightly above atmospheric pressure flow at high velocities through porous ceramic monolifiis coated with Rh or Pt.

Cubic/monolithic corrosive liquids, acids, bases, or used as catalyst/heat exchanger for reactors. Usually made of graphite or carbon that has high thermal conductivity, area 1 to 20 m. Ceramic monoliths are used as solid catalyst for highly exothermic gas-catalyst mass transfer-controlled reactions. 

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