Catalytic devices isothermal

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. 

Mkrostructured reactors (MSR) for heterogeneous catalytic processes mostly consist of a large number of parallel flow channels. At least one dimension of these channels is smaller than 1 mm, but rarely <100 pm. This leads to an increased heat transfer in the direction of the smallest dimension. The volumetric heat transfer performance in microstructured devices is several magnitudes higher than in conventional reactors. Therefore, even highly exothermic or endothermic reactions can be operated under near isothermal conditions and thermal runaway can be avoided (see Chapter 5). In addition, mass transfer between the bulk phase... 

Foams were proved to be highly suitable as catalytic carrier when low pressure drop is mandatory. In comparison to monoliths, they allow radial mixing of the fluid combined with enhanced heat transfer properties because of the solid continuous phase of the foam structure. Catalytic foams are successfully used for partial oxidation of hydrocarbons, catalytic combustion, and removal of soot from diesel engines [14]. The integration of foam catalysts in combination with microstructured devices was reported by Yu et al. [15]. The authors used metal foams as catalyst support for a microstructured methanol reformer and studied the influence of the foam material on the catalytic selectivity and activity. Moritz et al. [16] constructed a ceramic MSR with an inserted SiC-foam. The electric conductive material can be used as internal heating elements and allows a very rapid heating up to temperatures of 800-1000°C. In addition, heat conductivity of metal or SiC foams avoids axial and radial temperature profiles facilitating isothermal reactor operation. 

Therefore, microstructured multichannel reactors with catalytically active walls are by far the most often used devices for heterogeneous catalytic reactions. Advantages are low pressure drop, high external and internal mass transfer performance and a quasi-isothermal operation. In most cases the reactors are based on micro heat exchangers as shown in Figure 15.2. Typical channel diameters are in the range of... 

Various types of reactors, including those shown in Figure 3.1, have been evaluated by Weekman (1974) and Berty (1979) with respect to the ease of construction, sampling, isothermality, and contact time. These evaluations show that the usual differential and integral reactors are poor devices compared with the other types. The recycle reactor is perhaps the best overall. The transport reactor is quite useful for rapidly decaying catalytic reactions since steady state conditions can be achieved despite catalyst deactivation. 

---