Liquid filled porous catalyst

The effective diffusion of dissolved hydrogen in the liquid filled porous catalyst is described by an effective diffusion coefficient Defr.H2,i- Thereby, one has to consider that only a portion of the partide is permeable, and that the path through the particle is random and tortuous. Both aspects are taken into account by the porosity Sp and tortuosity Zp (Section 3.2.2.3) ... 

For particles with diameters typically used in technical fixed bed FT reactors (>1 mm), the effective rate is limited by pore diffusion. The main reason for this is the slow effective diffusivity of the dissolved hydrogen and CO in the liquid filled porous catalyst. 

Midoux, N. and J. C. Charpentier. Apparent Diffusivity and Tortuosity in a Liquid Filled Porous Catalyst used for Hydrodesulfurization of Petroleum Products. Chem. Eng. Science 28 (1973) 2108. 

A catalytic fixed bed reactor is a (usually) cylindrical tube that is randomly filled with porous catalyst particles. These are frequently spheres or cylindrical pellets, but other shapes are also possible. The use of rings or other forms of particles with internal voids or external shaping is on the increase. During single-phase operation, a gas or liquid flows through the tube and over the catalyst particles, and reactions take place on the surfaces, both interior and exterior, of the particles. 

A fixed-bed reactor is filled with a porous catalyst. Thus, the catalyst concentration becomes much higher, and the contact between the catalyst and the fluids is much better. This normally means that the resistance between the hydrogen and the liquid ( gi in Fig. 9.3-2) becomes the restricting factor, and that the productivity increases strongly. Typical values for the reaction-rate are in the range of 1 000 - 10 000 kgpr0[Pg.501]

A similar model has been applied to the modeling of porous media with condensation in the pores. Capillary condensation in the pores of the catalyst in hydroprocessing reactors operated close to the dew point leads to a decrease of conversion at the particle center owing to the loss of surface area available for vapor-phase reaction, and to the liquid-filled pores that contribute less to the flux of reactants (Wood et al., 2002b). Significant changes in catalyst performance thus occur when reactions are accompanied by capillary condensation. A pore-network model incorporates reaction-diffusion processes and the pore filling by capillary condensation. The multicomponent bulk and Knudsen diffusion of vapors in each pore is represented by the Maxwell-Stefan model. 

The hollow-fiber trickle-bed reactor, according to Yang and Cussler [59], is another variant of the hollow-tube theme. In this case the porous tube is not coated with a catalytic material. The outer shell surrounding the fibers is instead filled with catalyst pellets. The liquid is added to this outer shell, and the gas reactant is added to the inside of the fibers. Since no catalyst is present in the gas-liquid contact, this type of reactor functions merely as an effective gas-absorber. In comparison with the trickle bed, no flooding occurs with the hollow-fiber trickle-bed reactor at high liquid loads, which means a much higher reaction rate at high liquid flow rates than obtained with the traditional trickle bed. 

Nigam, K.D.P. Iliuta, I. Larachi, F. Liquid back-mixing and mass transfer effects in trickle-bed reactors filled with porous catalyst particles. Chem. Eng. Process. 2002, 41, 365. 

Plug-flow reactors are used for gas-phase and. liquid-phase reactions. If the PFR is filled with a porous catalyst and the fluid flowing in the void space is turbulent, the reactor is referred to as a fixed-bed... 

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. 

Observed rates in a number of trickle-bed reactors employed in hydrodesulfurization and hydrotreating of heavy residuals indicate that they operate in the regime free of major gas-liquid mass transfer limitations (jLfact that often the liquid reactants are nonvolatile or dilute at the operating conditions used the reaction is frequently liquid reactant limited and confined to the catalyst effectively wetted by liquid. Since porous packing, typically 1/32" to 1/8" (0.08 cm to 0.318 cm) extru-dates is most often employed it is clear that reaction rates may be affected both by Internal pore fill-up with liquid and by internal diffuslonal limitations. Catalyst effectiveness factors from 0.5 to 0.85 have been generally reported ... 

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. 

Dispersions at micron scale are usually made by merging gas and liquid streams in a mixing element and subsequent decay of the gas stream to a dispersion [251-262]. Mixing elements often have simple shapes such as a mixing tee (dual-feed gas-liquid) or triple-feed (liquid-gas-liquid) arrangements. The dispersion is passed either in a microchannel (or many of these) or in a larger environment such as a chamber, which, for example, provides volume to fill in porous materials such as catalyst particle beds, foams or artificial structures (microcolumn array). The mechanisms for bubble formation have not been investigated for all of the devices... 

The porosity of a catalyst or support can be determined simply by measuring the particle density and solid (skeletal) density or the particle and pore volumes. Particle density pp is defined as the mass of catalyst per unit volume of particle, whereas the solid density p, as the mass per unit volume of solid catalyst. The particle volume Vp is determined by the use of a liquid that does not penetrate in the interior pores of the particle. The measurement involves the determination by picnometry of the volume of liquid displaced by the porous sample. Mercury is usually used as the liquid it does not penetrate in pores smaller than 1.2/m at atmospheric pressure. The particle weight and volume give its density pp. The solid density can usually be found from tables in handbooks only in rare cases is an experimental determination required. The same devices as for the determination of the particle density can be used to measure the pore volume V, but instead of mercury a different liquid that more readily penetrates the pores is used, such as benzene. More accurate results are obtained if helium is used as a filling medium [10]. The porosity of the particle can be calculated as ... 

A packed bed reactor in which solid-catalyzed fluid-phase reactions take place must be filled easily with an optimum amount of catalyst (just enough) to produce a permanent, non-deteriorating bed with high permeability. This has led to the production of porous but strong solid carriers, which are later impregnated with the catalyst. For impregnation the active material is dissolved or dispersed in a liquid and during a post-... 

A porous electrode offers a far higher true working surface area and thus a much lower true current density (current per unit surface area of the electrode). Such an electrode consists of a metal or carbon-based screen or plate serving as the body or frame, current collector, and support for active layers, containing a highly dispersed catalyst for the electrode reaction. The pores of this layer are filled in part with the liquid electrolyte, and in part with the reactant gas. The reaction itself occurs at the walls of these pores along the three-phase boundaries between the solid catalyst, the gaseous reactant, and the liquid electrolyte. 

The porous structure of most catalysts is polydisperse. Therefore, capillary condensate fills only part of the pore-space — mostly small pores. The bidisperse globular structure (Figure 23.1) is convenient to consider as a model for rough estimations of the influence of external mass transfer and intraparticle diffusion on the total reaction rate. Such an analysis was made by Ostrovskii and Bukhavtsova [8]. According to this model, the only pores inside globules (micropores) will fill with liquid, and space between globules (macropores) fill with gas. Then the total porosity can be written as... 

Organic and inorganic wet gels are extremely porous materials whose porosity is completely filled with an interstitial liquid mainly constituted of the initial solvent. Since a high porosity is a key property for an efficient heterogeneous catalyst, it is crucial to maintain such an exceptional porosity as high as possible in the dried gel. 

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