External catalysts

In addition, the reduction of NOj is a very fast reaction and is controlled by external and internal diffusion [27, 30]. In contrast, the oxidation of SO2 is very slow and is controlled by the chemical kinetics [31]. Accordingly, the SCR activity is increased by increasing the catalyst external surface area (i.e. the cell density) to favor gas-solid mass transfer while the activity in the oxidation of SO2 is reduced by decreasing the volume of the catalyst (i.e. the wall thickness) this does not affect negatively the activity in NO removal because significant ammonia concentrations are confined near the external geometric surface of the catalyst. 

Note that internal mass transfer and reaction are dealt with simultaneously, in contrast to external mass transfer, which is considered to be in series with the reaction at the catalyst external surface. 

Reaction is first order in gas reactant. 3. Gas and liquid stream are in plug flow. 4. Reactor is isothermal. 5. Gaseous reactant concentration in the gas phase is constant throughout the reactor. 6. A fraction of the catalyst external surface (tice) is covered by a flowing liquid film while the rest is exposed to a thin stagnant liquid film. Assumption 2 was verified by already reported kinetic studies. A water cooled reactor with low feed concentrations of a-methylstyrene operated between 15°C and 20°C satisfies assumptions 1 and 4 due to low volatility of the liquid reactant and due to small overall heat effects, respectively. 

Diffusion to and from the catalyst external surface through pores in a porous catalyst particle to active sites inside the catalyst particle where the adsorption and reaction occur, represented as intraparticle diffusion and modeled as a diffusion-reaction process. 

Oxidation of acetylene on manganese dioxide "catalyst " External diffusion... 

Zinco-aluminosilicates having different pore volume and external surface area were synthesized and their stability in aromatization was investigated. Table 2 shows some properties of catalysts. External surface area was measured by benzene-filled pore method described by Inomata et al. [5]. Average crystal size was measured geometrically by SEM images. Stability of those catalysts was examined under the same condition as stated above, and the results are shown in Fig, 2. 

Phj = hydrogen pressure H = Henry s law constant (atm/ft /mol) k = liquid film mass transfer coefficient a = interfacial area between gas and liquid fls = catalyst external area k = catalyst film mass transfer coefficient... 

Contact process oxidation of SO2 to SO3, 450-500°C, V2O5 catalyst, external heat exchanger. 

Generation Pro-catalyst External donor Activity (g-PP/mmol-Ti h atm) LI. (%)... 

No H source on the catalyst External acid added Esters immiscible with IL... 

Catalyst external area (n Catalyst effectiveness fact Gas-llquld Interfaclal area 3... 

In order to estimate the role of wetting on the apparent rate it is necessary to know the fraction of the total catalyst external surface wetted by the moving liquid. Onda et al. [59] have proposed ... 

Assuming the existence of a stationnary state related to the initial conditions, we compute the reactant concentration at the catalyst external surface in F and C zones and, consequently, their specific production.The specific reaction rates calculated using this model are... 

The calculated values of temperature and ammonia concentration in the bulk gas and at the catalyst external surface are reported in Table 6.7 for the first catalyst bed of Fig. 6.2 in both axial and radial centrifugal flow. Since radial flow converters are usually filled with smaller-size catalyst particles than the catalyst considered here, from this point of view they are equivalent to axial converters, which are always filled with large-size catalysts to contain pressure drops. It also appears that the effects of mass and heat transfer at the external surface of the catalyst particle are oppositely directed so that they partly compensate for each other. We may conclude that in industrial converters their combined influence on the reaction rate is negligible compared to the inaccuracies inherent in the experimental determination of the intrinsic activity of the catalyst. In any case, interparticle phenomena can be readily incorporated as boundary conditions in the intraparticle problem ... 

Normal reaction with catalyst external surface passivated (10 hrs) -Al-MCM-41 93.0 64.7 7.5 20.8... 

It may therefore be necessary to replace catalysts mar r times during the life of plant equipment. Stabihty despite the presence of poisons becomes an important feature of the selection procedure to avoid unscheduled plant closures. Proper catalyst reduction may also be a critical step prior to operation to ensure optimmn performance in the shortest possible time. This is not always easy and efforts have therefore been made to use prereduced catalysts and even to regenerate spent catalysts externally to restore as much of the original activity as possible. It should never be assumed that catalyst operation is straightforward. It... 

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