Fixed beds interphase mass transfer

Intrinsic chemical kinetic data should not be obtained from fluidized beds, since the complex gas and particle dynamics in the fluidized bed make it difficult to separate the chemical factors from the interphase mass transfer and mixing constraints. Kinetics should instead be derived from fixed beds, Berty reactors, or other reactors where the mixing and mass transfer are well characterized and/or not rate limiting. 

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... 

The solid supported extractants can be employed in fixed bed contactors to extract metal ions from solutions. Other geometries include slurry extractors and moving bed adsorbers. We consider a fixed bed geometry. In this case the following mass transfer processes may be present (1) interpellet mass transfer, which refers to the diffusion and mixing of metal ion in fluid occupying the spaces between pellets (2) interphase mass transfer, which is the transfer of metal ion across the fluid peUet interface and (3) intraparticle mass transfer, which is the diffusion of metal ions in... 

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. 

Internal recycle reactors are designed so that the relative velocity between the catalyst and the fluid phase is increased without increasing the overall feed and outlet flow rates. This facilitates the interphase heat and mass transfer rates. A typical internal flow recycle stirred reactor design proposed by Berty (1974, 1979) is shown in Fig. 18. This type of reactor is ideally suited for laboratory kinetic studies. The reactor, however, works better at higher pressure than at lower pressure. The other types of internal recycle reactors that can be effectively used for gas-liquid-solid reactions are those with a fixed bed of catalyst in a basket placed at the wall or at the center. Brown (1969) showed that imperfect mixing and heat and mass transfer effects are absent above a stirrer speed of about 2,000 rpm. Some important features of internal recycle reactors are listed in Table XII. The information on gas-liquid and liquid-solid mass transfer coefficients in these reactors is rather limited, and more work in this area is necessary. 

A trickle bed reactor (TBR) consists of a fixed bed of catalyst particles, where liquid and gas phases flow cocurrently downward through the bed. Although its wide application in chemical and petrochemical industry it is one of the most complicated type of reactor in its design and scale-up. Essencially, the overall rate can be controlled by one or a combination of the following processes mass transfer between interphases, intraparticle diffusion, adsorption and surface reaction. The hydrodynamics, solid-liquid contacting efficiency and axial mixing can also affect the performance of TBR. 

Even though the model in Table 3.1 results from several assumptions (detailed in Section 3.2.1), it can be considered as quite comprehensive. In fact, what is commonly found in the fitera-ture is a simplified version of this model The well-known classification of fixed-bed reactor models by Froment [51] and Froment and Bischoff [62] clearly exemplifies how a more general model unfolds into a hierarchy of several others with decreasing complexity. The dimensionafity of the model (usually one- or two-dimensional) and the presence of interphase and intraparticular resistances to mass/heat transfer are the main basis for distinguishing between different categories. 

Ep, Fj, Radial and axial dispersion coefficients kf Interphase (or external) mass transfer coefficient. q Average adsorbed-phase concentration (on a mass basis) t, r, z Independent variables of time, radial distance, and axial distance, respectively Mj Superficial velocity e Fixed-bed porosity Ep Particle porosity Particle density X Tortuosity factor... 

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