Clusters reactor

Kanthale PM, Gogate PR, Wilhelm AM, Pandit AB (2005) Dynamics of cavitational bubbles and design of a hydrodynamic cavitational reactor cluster approach. Ultrason Sonochem 12 441 -52... 

Another vessel Reactor D306 in a similar orientation to the forrr-reactor cluster was to the process of being cleaned. Investigators forrrrd the top manway on Reactor D306 was open and the reactor pressure washer had been placed within the reactor, but the drain and transfer valves remained closed. Reactor D306 was in process of being cleaned and the blaster operator shoirld have next opened the drain valve to empty the reactor [8] (see Fig. 4.3). 

The microscopic understanding of tire chemical reactivity of surfaces is of fundamental interest in chemical physics and important for heterogeneous catalysis. Cluster science provides a new approach for tire study of tire microscopic mechanisms of surface chemical reactivity [48]. Surfaces of small clusters possess a very rich variation of chemisoriDtion sites and are ideal models for bulk surfaces. Chemical reactivity of many transition-metal clusters has been investigated [49]. Transition-metal clusters are produced using laser vaporization, and tire chemical reactivity studies are carried out typically in a flow tube reactor in which tire clusters interact witli a reactant gas at a given temperature and pressure for a fixed period of time. Reaction products are measured at various pressures or temperatures and reaction rates are derived. It has been found tliat tire reactivity of small transition-metal clusters witli simple molecules such as H2 and NH can vary dramatically witli cluster size and stmcture [48, 49, M and 52]. 

The Model 412 PWR uses several control mechanisms. The first is the control cluster, consisting of a set of 25 hafnium metal rods coimected by a spider and inserted in the vacant spaces of 53 of the fuel assembhes (see Fig. 6). The clusters can be moved up and down, or released to shut down the reactor quickly. The rods are also used to (/) provide positive reactivity for the startup of the reactor from cold conditions, (2) make adjustments in power that fit the load demand on the system, (J) help shape the core power distribution to assure favorable fuel consumption and avoid hot spots on fuel cladding, and (4) compensate for the production and consumption of the strongly neutron-absorbing fission product xenon-135. Other PWRs use an alloy of cadmium, indium, and silver, all strong neutron absorbers, as control material. 

The third control is by use of a fixed burnable poison. This consists of rods containing a mixture of aluminum oxide and boron carbide, included in the initial fuel loading using the vacant spaces in some of the fuel assembhes that do not have control clusters. The burnable poison is consumed during operation, causing a reactivity increase that helps counteract the drop owing to fuel consumption. It also reduces the need for excessive initial soluble boron. Other reactors use gadolinium as burnable poison, sometimes mixed with the fuel. 

A second option is to apply the membrane on the particle level (millimeter scale) by coating catalyst particles with a selective layer. As a third option, application at the microlevel (submicrometer scale) is distinguished. This option encompasses, for example, zeolite-coated crystals or active clusters (e.g., metal nanoparticles). Advantages of the latter two ways of application are that there are no sealing issues, it is easy to scale-up, the membrane area is large per unit volume, and, if there is a defect in the membrane, this will have a very limited effect on the overall reactor performance. Because of these advantages, it is believed that using a zeolite... 

The final probe of molecular clusters is that of selected chemical reactions. The use of probe reactions to study supported cluster catalysts is well established, and we are attempting the development of similar probes of unsupported clusters. The first steps in this direction are the design of a pulsed chemical reactor to go with the pulsed cluster source and the development of criteria for reactions. It is important to recall that at present... 

The cluster reactor is attached to the pulsed cluster source s condensation channel, as shown in Figure 6. (16) To it is attached a high-pressure nozzle from which a helium/hydrocarbon mixture is pulsed into the reactor at a time selected with respect to the production and arrival of the clusters. The effect of turbulent mixing with the reactant pulse perturbs the beam, but clusters and reaction products which survive the travel from the source to the photoionization regime ( 600y sec) and the photoionization process are easily detected. 

Figure 6. Scale-drawn schematic of the cluster reactor in relation to the pulsed cluster source. The letters A-F indicate the various stages of cluster preparation or synthesis, cooling, mixing and reacting, and finally flowing into vacuum toward detection.

GP 1] [R 1] A comparison of four micro reactors with different Pt loadings (Pt impregnated on anodically oxidized alumina support) and different Pt structures confirmed that cluster size has an impact on the single Pt-atom activity (6 vol.-% NHj, 88 vol.-% O2, balance He 0.51 ms 260-380 °C) [28, 98]. At low Pt loadings, isolated atoms are formed. Calculated ammonia consumption rates amount to 20 s at 300 °C. At high Pt loadings, clusters are formed. Turn over frequencies (TOP) of about 40 s are determined. 

The chemistry (i.e. the species and reactions included) is the same as described for the ID model. However, here the higher-order silanes Si H2n+2 and silane radicals Si H2,+ i are limited to n < 4 to reduce the computational effort. Thus, SisHy and SiyHg are representative for all silanes with n > 2. The formation of powder (large silane clusters) is not taken into account in this model. The discharge settings for the calculations shown here are a total pressure of 20 Pa, a power input of 250 W m an RF frequency of 50 MHz, and an inlet flow of 30 seem of SiHa and 30 seem of H2. This parameter set is chosen because it results in a situation where most of the silane is consumed in a large reactor. This situation is required for economic reasons in industrial applications. 

WSC-2 correlation covers the pressure range from 3.4 to 15.9 MPa (500 to 2,300 psia) and is considered to be applicable to pressure tube reactors (PTRs), pressurized water reactors (PWRs), and boiling water reactors (BWRs). It was developed exclusively from subchannel data. All 54 different clusters were analyzed using HAMBO and the correlation optimized for the calculated subchannel conditions. The basic equation for the correlation is... 

In conclusion, metal nanoclusters in DMF interact strongly with microwaves. In reactions catalysed by these clusters, the microwave heating may be tantamount to preferentially heating the catalytic site, which can lead to more effective catalysis. Such cluster-catalysed reactions can be in principle screened in parallel in multimode m/w ovens reducing both time and operational costs. However, the ovens must be adapted so that the parallel reactors are uniformly heated. 

Figure 28. (a) Mass spectrum of protonated water clusters H+(H20) (n = 4-45) at 119 K and 0.3 torr He in a flow tube reactor. Note the prominence of H3O+(H2O>20 even under quasi-equilibrium conditions, (b) Mass-spectrometric abundance of OH-(H20)n produced under thermal conditions. Note a magic number at n = 20, though not as prominent as for the case of H30+ hydrates. Taken with permission from ref. 92. 

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