Monolithic heat exchanger

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. 

Simulation results have demonstrated that in metallic monoliths heat exchange properties are not downgraded significantly by Introduction of a ceramic catalytic washcoat, and isothermal conditions tend to prevail. 

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]. 

It should be possible to reduce the cost of monoliths by making use of the experience of manufacturers of modern heat exchangers, which contain similar structures. It should also be noted that the high cost of monoliths is in part explained by the fact that they are mostly designed to meet demands of high-temperature operation. 

Several uncertainties in this periodic process have not been resolved. Pressure drop is too high at SV = 10,000 h 1 when packed beds of carbon are used. Study of carbon-coated structured packing or of monoliths with activated carbon washcoats is needed to see if lower pressure drops at 95% SO2 removal can be achieved. Stack gas from coal or heavy oil combustion contains parts-per-million or -per-billion quantities of toxic elements and compounds. Their removal in the periodically operated trickle bed must be examined, as well as the effect of these elements on acid quality. So far, laboratory experiments have been done to just 80°C use of acid for flushing the carbon bed should permit operation at temperatures up to 150°C. Performance of periodic flow interruption at such temperatures needs to be determined. The heat exchange requirements for the RTI-Waterloo process shown in Fig. 26 depend on the temperature of S02 scrubbing. If operation at 150°C is possible, gas leaving the trickle bed can be passed directly to the deNO, step without reheating. 

Figure 12.2 Examples of monoliths with internal heat exchange [18].

Metal monoliths with internal heat exchange capabilities are obtained by assembling single side-coated flat and corrugated sheets as illustrated in Figure 12.2 [17,18]. 

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.

Figure 9 Monolithic catalyst modified to serve as a heat exchanger.

In the past, the principles described have been implicitly recognized in several attempts to convert monolithic catalysts into catalytic heat exchangers. While the use of millimeter dimensions and nanoporous ceramic supports meets the primary criteria already mentioned, the parallel channel structure of monoliths is not ideally tailored for heat exchanger applications, and complex header structures are required to uniformly distribute and collect reaction medium and coolant to and from the individual channels (Figure 9). The unsatisfactory interface between the milli- and macroscale has been a major weakness of such concepts. 

Frauhammer et al. [102] developed a ceramic monolith which may be used as a counter- or co-current heat exchanger. This is performed by partially closing ducts... 

Figure 2.66 Preparation of a ceramic monolith to obtain a cross-flow heat exchanger (left) and different options of monolith preparation (right) [102].

A bench-scale evaporator was built first, consisting of the nickel foam monolith and heat exchanger plates 5.7 cm wide and 7 cm long. The stainless-steel channels fabricated by EDM were 254 pm deep and the vapor channel depth was varied from aspect ratios of 4 to 18, the latter being the optimum value determined by experiments. 

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... 

Groppi, G., Tronconi, E., Design of novel monolith catalyst supports for gas/solid reactions with heat exchange, Chem. Eng. Sci. 2000, 55, 2161-2171. 

Figure 4.112 Monolithic counter-current heat exchanger manufactured from a stack of micro structured plates and sealed by laser welding (source IMM).

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. 

With nonadiabatic reaction control, heat must be transported through the fixed bed to the integrated heat exchange surfaces. At the usual mass flow rates of G > lkgm-2s-1, this heat transport takes place mainly by convection, i.e. the fixed bed must allow for a cross flow transverse to the main flow direction. Monolith structures with straight parallel channels are thus unsuitable for nonadiabatic reaction control. 

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. 

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