Fixed beds intraparticle mass transfer

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... 

In contrast with the PPR, bed utilization in a fixed-bed reactor is essentially complete. Mass transfer outside the bed is generally not a limiting factor, for the main resistance is in most cases the intraparticle diffusion, which gives rise to incomplete utilization of the catalyst particle (see Section 1II.B.2). 

Zn(II) as presented in Table 24.2 [6], or Cr(VI), more than 99% of which was removed from industrial electroplating wastewater [20], The modeling of the experimental breakthrough of lead (II) onto activated carbon fibers in a fixed bed, using axial dispersion and diffusion equations solved by the orthogonal collocation method, demonstrated that the intraparticle and external mass transfer is not the rate-controlling step, due to the short diffusion path for the adsorbate in activated carbon fibers [21]. 

In a fixed-bed reactor the catalyst pellets are held in place and do not move with respect to a fixed reference frame. Material and energy balances are required for both the fluid, which occupies the interstitial region between catalyst particles, and the catalyst particles, in which the reactions occur. For heterogeneously catalyzed reactions, the effects of intraparticle transport on the rate of reaction must be considered. Catalytic systems operate somewhere between two extremes kinetic control, in which mass and energy transfer are very rapid and intra-partide transport control, in which the reaction is very rapid. Separate material and energy balances are needed to describe the concentration and temperature profile inside the catalyst pellet. The concentrations... 

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. 

A series of experiments varying temperature, micro-sphere size and time on stream have been performed in a fixed fluidised bed microactivity reactor to study the role of intraparticle diffusion in commercial fluid catalytic cracking (FCC) catalysts, particularly on gasoline yield and catalyst deactivation by coke deposition, for the cracking of a vacuum gas oil. Additionally, a mechanistic model that describes interface and intrapartide mass transfer interactions with the cracking reactions, has been used to study the combined influence of pore size and intraparticle mass diffusion on the deactivation of FCC catalysts and the gasoline yield. 

Slurry Process Extra Particle Mass-Transfer Limitations. In a slurry process, very fine catalyst is suspended in a liquid medium, which in the case of an FT process may be the heavier fraction of the liquid product itself. The slurry mode of operation allows effective removal of reaction heat and good temperature control. Because of the small catalyst particles (particle diameter a few hundred micrometers or less), intraparticle diffusion limitation is not a constraining factor for reaction rate. However, compared with a fixed-bed mode of operation the advantage of better catalyst utilization may be offset by the reduced volume of catalyst that can be accommodated in the reactor space. 

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