Incomplete wetting of the catalyst

As long as fuel cells are using liquid electrolytes like phosphoric acid or concentrated caustic potash, the catalyst utilization is usually not limited by incomplete wetting of the catalyst. Provided the amount of electrolyte is sufficiently high, the hydrophilic porous particles are not only completely flooded but due to their expressed hydrophilicity are wetted externally by an electrolyte film that together with the whole electrolyte-filled hydrophilic pore system establishes the ionic contact of an electrode to the respective counterelectrode. 

At low liquid flowrates incomplete wetting of the catalyst by the liquid may occur, as illustrated in Fig. 4.19, leading to channelling and a deterioration in reactor performance 34 . 

Incomplete wetting of the catalyst at low liquid flow rates and possible liquid by-pass along the reactor wall. Sensitivity to thermal effects and problems with temperature control. 

Incomplete wetting of the catalyst causes a non-uniform reactant concentration on its surface. Sometimes the pellet may not be only externally but also partially wetted internally as a result of unsatisfactory liquid distribution and of liquid evaporation due to heat of reaction. 

Die difference from the real value (lm) is mainly due to the approximation made about the mass transfer coefficient as well as the complete wetting of the catalyst, as the actual wetting efficiency is 88%. Furthermore, the problem is more complicated because under incomplete wetting, the gas reactant reaches the catalyst surface more easily than the unwetted part, as Horowitz et al. found out experimentally. 

Incomplete and/or ineffective wetting of the catalyst with low liquid flow-rates and low column diameter/particle size ratio (< 15/20) possibility of liquid by-passing along the reactor wall. 

Reactant A is first absorbed into the liquid phase and then reacts on the catalyst surface with reactant B already present in the liquid. When the catalyst is porous, both dissolved reactants diffuse into the pores, towards the center of the catalyst particle to reach the active internal sites of the solid. Reaction products diffuse in the opposite direction. In the case of TBRs, when the external wetting of the catalyst is incomplete, A can be directly absorbed in the liquid that fills the catalyst pores by capillarity. In this case less external mass transfer resistence is expected because the liquid film is absent. 

Fixed-bed catalytic reactors and reactive distillation columns are widely used in many industrial processes. Recently, structured packing (e.g., monoliths, katapak, mella-pak etc.) has been suggested for various chemical processes [1-4,14].One of the major challenges in the design and operation of reactors with structured packing is the prevention of liquid flow maldistribution, which could cause portions of the bed to be incompletely wetted. Such maldistribution, when it occurs, causes severe under-performance of reactors or catalytic distillation columns. It also can lead to hot spot formation, reactor runaway in exothermic reactions, decreased selectivity to desired products, in addition to the general underutilization of the catalyst bed. 

Several forms of incomplete catalyst wetting were visually observed and reported in previous studies. These observations include i) dry areas on a portion of the catalyst surface... 

The theories of dilution techniques have been explained in detail by various researchers. The selection of the proper size of diluent is very important. Equal volumes of diluent and catalyst were used for this comparison. For the case of an undiluted bed, only commercial size catalyst is packed in a small-scale reactor. The wall effect is very significant and in this case causes channeling of liquid. Because of the high void space inside the catalyst bed, the liquid holdup in the catalyst bed is also very low. As a whole, there is incomplete wetting of catalyst and only partial utilization is achieved in this case. Besides this, an appreciable amount of axial backmixing is present in the undiluted catalyst bed. When a larger size of diluent is used, it cannot enter the void space between the catalyst particles. Thus, it does not increase liquid holdup and hence only partial utilization of catalyst is also obtained. However, the addition of diluent increases the bed height, which in turn reduces liquid axial dispersion to some extent. When the diluent size is smaller, it can enter the narrow void space between the catalyst particles and can increase the liquid holdup... 

Vapor (mostly H2) and liquid (oil) are passed cocurrently downward over a fixed bed of small catalyst particles. The liquid flows over the particles in films and rivulets the vapor flows through the remaining voids. As discussed below, these hydrodynamical conditions may lead to incomplete catalyst wetting, axial dispersion, and restricted Interphase mass transfer and may therefore result in Incomplete catalyst utilization. Since the catalyst is fairly expensive and the conditions of temperature and pressure require expensive reactor vessels, there Is considerable incentive to ensure that maximum utilization of the catalyst is obtained. 

Incomplete Catalyst Wetting. It has been widely reported in the literature that liquid contacting is not complete in trickle flow reactors All of the catalyst particles may not... 

The importance of the wetting efficiency results mainly from the fact that it is closely related to the reaction yield, and more specifically to the catalyst effectiveness factor (Burghardt et al., 1995). The reaction rate over incompletely covered catalytic particles can be smaller or greater than the rate observed on completely wetted packing, depending on whether the limiting reactant is present only in the liquid-phase or in both gas and liquid-phases. 

Another important consideration in preparing mixed-oxide catalysts is the spontaneous monolayer dispersion of oxides and salts onto surfaces of support substrates on calcination. Both temperature and duration of calcination are important here, as discussed in the reviews by Xie and Tang [63] and by Knozinger and Taglauer [64]. If this dispersion step is inadequate or incomplete, the resulting oxide layer, and any reduced metal surface from it, will not be reproducible from the same catalyst system therefore, one can then have different catalysts prepared at different times and, of course, from one laboratory to another. Spreading and wetting phenomena in preparation of supported catalysts is discussed in Section A.2.2.1.3. 

Some of the remaining studies did not necessarily observe incomplete catalyst wetting, but included this concept either directly as an adjustable parameter in the model to fit the observed conversion versus liquid mass velocity data,(7,9,13, 16-18), or indirectly via use of a correlation for liquid-solid contacting established for non-porous absorber column packings (11,19-20). 

A summary of reactor models used by various authors to interpret trickle-bed reactor data mainly from liquid-limiting petroleum hydrodesulfurization reactions (19-21) is given in Table I of reference (37). These models are based upon i) plug-flow of the liquid-phase, ii) the apparent rate of reaction is controlled by either internal diffusion or intrinsic kinetics, iii) the reactor operates isothermally, and iv) the intrinsic reaction rate is first-order with respect to the nonvolatile liquid-limiting reactant. Model 4 in this table accounts for both incomplete external and internal catalyst wetting by introduction of the effectiveness factor r)Tg developed especially for this situation (37 ). 

No maldistribution of gas or liquid in three-phase processes. Regarding application of the BSR concept to gas/liquid/solid processes, an important advantage of the BSR is that adjacent strings do not (necessarily) touch. Because of the liquid surface tension, liquid will not spill over from one BSR string to another. Consequently, the initial liquid distribution is maintained throughout the BSR module. This feature is especially advantageous when incomplete catalyst wetting (which results from liquid maldistribution in traditional, randomly packed trickle-flow reactors) would lead to hot spots and decreased selectivity. 

Trickle-bed reactors are widely used in hydrotreating processes, i.e., hydrodesulfurization of gasoline and diesel fuel, in petroleum refining, chemical, petrochemical, and biochemical processes. The knowledge of hydrodynamic parameters is vital in the design of a TBR because the conversion of reactants, reaction yield, and selectivity depend not only on reaction kinetics, operating pressure, and temperature, but also on the hydrodynamics of the reactor. Special care is also required to prevent flow maldistribution, which can cause incomplete catalyst wetting in some parts... 

The first two stages of the synthesis of catalysts prepared by dendrimers are inextricably linked. Proper incorporation of the metal precursor within the PAMAM dendrimer is essential for the formation of dendrimer-metal nanocomposites and, eventually, nanoparticles with controlled particle sizes. Complications in the complexation stage, such as incomplete or inadequate incorporation of the metal precursor, will leave free metal cations or colloidal particles in the impregnating solution, resulting in the formation of supported catalysts that exhibit wide particle size distributions. In the case of bimetallic catalysts, the loss is twofold In addition to an array of metal particle sizes, there will also be a significant loss of compositional control in the active phase. In short, if the complexation step is not tightly controlled, the dendrimer-prepared catalyst will not differ substantially from a catalyst prepared by wet impregnation. 

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