Catalyst particle surface

Heat transfer to or from active centers to the catalyst-particle surface... 

A useful application of the model is to examine the S02 and 02 concentration profiles in the trickle bed. These are shown for the steady-state conditions used by Haure et al. (1989) in Fig. 25. The equilibrium S02 concentration drops through the bed, but the 02 concentration is constant. In Haure s experiments 02 partial pressure is 16 times the S02 partial pressure. At the catalyst particle surface, however, 02 concentration is much smaller and is only about one-third of the S02 concentration. This explains why 02 transport is rate limiting and why experimentally oxidation appears to be zero-order in S02. 

As already mentioned, the first step in any heterogeneous catalytic reaction is the adsorption of a gas molecule onto a solid surface. Adsorption heat measurements can provide information about the adsorption process not available using other surface analytical tools. For example, differential heat measurements can provide valuable insights into sites distribution on the catalyst surface as well as quantitative information on the changes in catalyst particle surface chemistry that result from changes in particle size or catalyst support material [148-150],... 

After the reactants reach the catalyst particle surface, they must be transported to an active site before reaction can occur. All polymeric catalysts under consideration here swell to some degree in liquids. The number of active sites on the particle surface... 

As in the case of normal supported catalysts, we tried with this inverse supported catalyst system to switch over from the thin-layer catalyst structure to the more conventional powder mixture with a grain size smaller than the boundary layer thickness. The reactant in these studies (27) was methanol and the reaction its decomposition or oxidation the catalyst was zinc oxide and the support silver. The particle size of the catalyst was 3 x 10-3 cm hence, not the entire particle in contact with silver can be considered as part of the boundary layer. However, a part of the catalyst particle surface will be close to the zone of contact with the metal. Table VI gives the activation energies and the start temperatures for both methanol reactions, irrespective of the exact composition of the products. 

The steric control is attributed to the presence of chiral centres on the catalyst particle surface [39],... 

Multiphase reactors include, for instance, gas-liquid-solid and gas-liq-uid-liquid reactions. In many important cases, reactions between gases and liquids occur in the presence of a porous solid catalyst. The reaction typically occurs at a catalytic site on the solid surface. The kinetics and transport steps include dissolution of gas into the liquid, transport of dissolved gas to the catalyst particle surface, and diffusion and reaction in the catalyst particle. Say the concentration of dissolved gas A in equilibrium with the gas-phase concentration of A is CaLt. Neglecting the gas-phase resistance, the series of rates involved are from the liquid side of the gas-liquid interface to the bulk liquid where the concentration is CaL, and from the bulk liquid to the surface of catalyst where the concentration is C0 and where the reaction rate is r wkC",. At steady state,... 

Hydrodynamic parameters that are required for trickle bed design and analysis include bed void fraction, phase holdups (gas, liquid, and solid), wetting efficiency (fraction of catalyst wetted by liquid), volumetric gas-liquid mass-transfer coefficient, liquid-solid mass-transfer coefficient (for the wetted part of the catalyst particle surface), gas-solid... 

Diffusion of the reactant molecules from the catalyst particle surface to the active sites within the porous particle. 

In Chapter 2 we discussed a number of studies with three-phase catalytic membrane reactors. In these reactors the catalyst is impregnated within the membrane, which serves as a contactor between the gas phase (B) and liquid phase reactants (A), and the catalyst that resides within the membrane pores. When gas/liquid reactions occur in conventional (packed, -trickle or fluidized-bed) multiphase catalytic reactors the solid catalyst is wetted by a liquid film as a result, the gas, before reaching the catalyst particle surface or pore, has to diffuse through the liquid layer, which acts as an additional mass transfer resistance between the gas and the solid. In the case of a catalytic membrane reactor, as shown schematically in Fig. 5.16, the active membrane pores are filled simultaneously with the liquid and gas reactants, ensuring an effective contact between the three phases (gas/ liquid, and catalyst). One of the earliest studies of this type of reactor was reported by Akyurtlu et al [5.58], who developed a semi-analytical model coupling analytical results with a numerical solution for this type of reactor. Harold and coworkers (Harold and Ng... 

The external diffusion limitation (mass transfer through a liquid-solid interface) is determined by the diffusion rate of the reactant to the external surface or the product out from catalyst particles surface. The flux to the external surface is defined by ... 

An unsaturated hydrocarbon oil is to be hydrogenated at 316 °C and 54.5 atm using a slurry reactor with a catalyst loading of 8 g-cat per liter of oil. Assuming that the oil can be maintained at hydrogen saturation what space velocity would be required if the reaction consumes 89 m (15 °C, 1 atm) per m liquid feed We may assume that the catalyst is very active and that the overall rate of hydrogenation is controlled by the rate of mass transfer of hydrogen from the liquid phase to the catalyst particle surfaces. Data ... 

Two-phase flow in three-phase fixed-bed reactors makes the reactor design problem complex [12], Interphase mass transfer can be important between gas and liquid as also between liquid and catalyst particle. Also, in the case of trickle-bed reactors, the rivulet-type flow of the liquid falling through the fixed bed may result (particularly at low liquid flow rates) in only part of the catalyst particle surface being covered with the liquid phase. This introduces a third mass transfer process from gas to the so-called gas-covered surface. Also, the reaction rates in three-phase fixed-bed catalytic reactors are highly affected by the heat transfer resistances resistance to radial heat transfer and resistance to fluid-to-particle heat transfer. As a result of these and other factors, predicting the local (global) rate of reaction for a catalyst particle in three-phase fixed-bed reactors requires not only... 

Each flowing phase is viewed as a continuum and the catalyst particles surface is completely covered by a liquid film and the gas flows in the remaining interstitial void. The properties of the fluid suspension (density, viscosity, holdup) are equal to those of the embracing liquid (influent fine volume fraction <0.1%). Only the influent liquid was considered as a source for (single-sized)... 

The main structural components in modem PEFCs are the porous composite electrodes. The primary purpose of utilizing porous electrodes is to enhance the active surface area of the catalyst by several orders of magnitude in comparison to planar electrodes with the same in-plane geometrical area. In the CLs, the fluxes of reactant gases, protons, and electrons meet at the catalyst particle surface. Active catalyst nanoparticles are located at spots that are connected simultaneously to the percolating phases of proton, electron, and gas transport media. An important implication of the electrode s finite thickness is the necessity to provide transport of neutral molecules and protons through the depth of the porous electrode. Additional overhead is caused by the transport of neutral reactants through FF, GDL, and MPL. This leads to specific potential losses in the electrodes, which will be considered in detail in Chapter 4. ... 

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