Monolith reactor-heat exchanger

Wei, J. and Degnan, T. F., "Monolithic Reactor-Heat Exchanger" Proc. ISCRE5 ACS Symposium Series 65, American Chemical Society Washington, D.C., 1978, p. 83. 

A novel monolithic reactor-heat exchanger has been constructed and operated in five different modes. Experiments were conducted with the oxidation of carbon monoxide over copper chromite pelleted catalysts. The experimental temperatures of reactants and coolants, and concentrations agree with computations with a cell model. Virtually flat temperature profiles can be obtained in the co-current mode. 

Experimental analyses of the heat transfer characteristics of the monolithic reactor-heat exchanger were carried out in the absence of reaction to define the NTU values in equations (1), (2), eind (3). Experiments were performed in which the coolcint pass flow rate was varied independently of the reaction pass flow rate, to produce j-factors vs Reynolds number, in good agreement with both the Graetz solution and data from an experimental study by Kays et al (] ) of a crossflow of similar design. 

The NTU values in equations (1) and (2) describing the heat loss from the monolithic reactor-heat exchanger chambers and crossflow shapes were evaluated using data obtained in stop flow experiments. Overall heat transfer coefficients based on the chamber wall area varied from 0.3 BTU/hr. ft °F to 0.55 BTU/hr. ft °F depending on the position in the reactor. 

The monolithic reactor-heat exchanger was run successively as an adiabatic reactor, a countercurrent reactor-heat exchanger and a cocurrent reactor-heat... 

Cross-flow monoliths have been explored by Degnan and Wei (11-12) as cocurrent and countercurrent reactor-heat exchangers. Four cross-flow monoliths in series were employed the individual blocks were analyzed by a one-dimensional approximation. They found good agreement between theory and experiment. 

Figure 6.9 Reactor heads forthe upgrading of ceramic monoliths to heat-exchangers as developed by Frauhammer et al. [463].

The pelleted copper chrome catalyst was metered into each of the large sinusoidal ducts of the reaction pass of each of the four monoliths comprising the reactor-heat exchanger. A h layer of quartz chips at each of the reaction pass faces of the cross-flows guaranteed that the reaction was confined to the volume of the monolith where heat exchange could take place. 

A novel reactor, consisting of a series of cross-flow monoliths for a coolant stream and a reactant stream, has been designed and run with a highly exothermic first order reaction. This reactor-heat exchanger is versatile and can be run in five modes ... 

Monolithic catalyst bed, 312, 314 Multibed reactor/heat exchanger, 452 Multilayer model for coking, 183 for physisorption, 35... 

Static mixing catalysts Operation Monolithic reactors Microreactors Heat exchange reactors Supersonic gas/liquid reactor Jet-impingement reactor Rotating packed-bed reactor... 

The vanadium content of some fuels presents an interesting problem. When the vanadium leaves the burner it may condense on the surface of the heat exchanger in the power plant. As vanadia is a good catalyst for oxidizing SO2 this reaction may occur prior to the SCR reactor. This is clearly seen in Fig. 10.13, which shows SO2 conversion by wall deposits in a power plant that has used vanadium-containing Orimulsion as a fuel. The presence of potassium actually increases this premature oxidation of SO2. The problem arises when ammonia is added, since SO3 and NH3 react to form ammonium sulfate, which condenses and gives rise to deposits that block the monoliths. Note that ammonium sulfate formation also becomes a problem when ammonia slips through the SCR reactor and reacts downstream with SO3. 

The whole set-up for partial oxidation comprises a micro mixer for safe handling of explosive mixtures downstream (flame-arrestor effect), a micro heat exchanger for pre-heating reactant gases, the pressure vessel with the monolith reactor, a double-pipe heat exchanger for product gas cooling and a pneumatic pressure control valve to allow operation at elevated pressure [3]. 

In many situations, the monolith reactor can be represented by a single channel. This assumption is correct for the isothermal or adiabatic reactor with uniform inlet flow distribution. If the actual conditions in the reactor are significantly different, more parallel channels with heat exchange have to be simulated (cf., e.g. Chen et al., 1988 Jahn et al., 1997, 2001 Tischer and Deutschmann, 2005 Wanker et al., 2000 Young and Finlayson, 1976). In this section we will further discuss effective single channel models. 

Figure 21 Configuration of a cocurrent downflow monolith reactor with free gas recirculation. Only liquid is recirculated, and an external heat exchanger can be scaled independent of the reactor to deliver the required heat duty.

The full-scale reactor/evaporator had a total size of 7.6 cm x 10.2 cm x 5.1 cm and was composed of four monoliths of 5 cm2 cross-sectional area and four heat exchangers with 7.2 cm3 cross-sectional area. This device was claimed to evaporate... 

As a prototype for such a reactor, the monolithic heat exchanger in Figure 4.112, was manufactured from a stack of micro structured foils and laser welded at the front and side faces [169], The flanges were welded manually. 

Roy and Gidaspow (13-14) developed two-dimensional continuum models to describe cross-flow monolithic heat exchangers and catalytic reactors. 

Monolithic Loop Reactor A novel MLR was developed af Air Products and Chemicals (Figure 17) (144). The reactor contains a monolithic catalyst operating under cocurrent downflow condifions. Because the residence time in the monolith is short and the heat of reaction has to be removed, the liquid is continually circulated via an external heat exchanger until the desired conversion is reached. The concept was patented for the hydrogenation of dinifrofoluene fo give toluenediamine (37). 

The cleaning of flue gases from stationary sources is another field in which the application of monolithic catalysts will certainly rise. There will be no versatile catalyst for cleaning all off-gases. Therefore tailor-made catalysts with zeoliths of various types for specific applications will be developed. Incorporated-type monolithic catalysts are likely to prevail in this field. Since cleaning usually requires a set of equipment items in series (e.g., converter, heat exchangers), multifunctional reactors (reverse-flow reactors, rotating monoliths) will become more common. 

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. 

When the heat transfer is considered, the slurry reactors are more efficient, due to large liquid holdup and a relatively high flow velocity of the reaction mixture at the heat exchange surface. Also, it is relatively easy to arrange heat-exchanging devices in the slurry reactors as compared to monolith reactors. 

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