Slurry reactors concentration profiles

On each of these, random and structured reactors behave quite differently. In terms of costs and catalyst loading, random packed-bed reactors usually are most favorable. So why would one use structured reactors As will become clear, in many of the concerns listed, structured reactors are to be preferred. Precision in catalytic processes is the basis for process improvement. It does not make sense to develop the best possible catalyst and to use it in an unsatisfactory reactor. Both the catalyst and the reactor should be close to perfect. Random packed beds do not fulfill this requirement. They are not homogeneous, because maldistributions always occur at the reactor wall these are unavoidable, originating form the looser packing there. These maldistributions lead to nonuniform flow and concentration profiles, and even hot spots can arise (1). A similar analysis holds for slurry reactors. For instance, in a mechanically stirred tank reactor the mixing intensity is highly non-uniform and conditions exist where only a relatively small annulus around the tip of the stirrer is an effective reaction space. 

Due to the consumption of reactants and the production or consumption of heat, concentration and temperature profiles can develop in the stagnant zone around and in the particle itself (Fig. 11). In the following paragraphs, criteria are derived to ensure that the effect of these gradients on the observed reaction rate is negligible [4, 27, 28]. In gas/liquid/solid slurry reactors, the mass transfer between the gas and liquid phase has to be considered, too (see Refs 9 and 29). 

Provided the particle settling velocities vt are known, this equation allows the calculation of )e,s Usually, experiments at non-zero liquid rates are used to evaluate t , and )e,s separately. A similar concentration profile might occur in practice if slurry column reactors are operated close to the conditions given by the minimum suspension criterium. In this case, reactor calculations should take the solids concentration profiles into account. A recommended correlation for the solids dispersion coefficient for small particles is given by Kato et al. [15] ... 

In the design of upflow, three phase bubble column reactors, it is important that the catalyst remains well distributed throughout the bed, or reactor space time yields will suffer. The solid concentration profiles of 2.5, 50 and 100 ym silica and iron oxide particles in water and organic solutions were measured in a 12.7 cm ID bubble column to determine what conditions gave satisfactory solids suspension. These results were compared against the theoretical mean solid settling velocity and the sedimentation diffusion models. Discrepancies between the data and models are discussed. The implications for the design of the reactors for the slurry phase Fischer-Tropsch synthesis are reviewed. 

As part of the work undertaken by APCI under contract to the DOE, to develop a slurry phase Fischer-Tropsch process to produce selectively transportation fuels, a study of the hydrodynamics of three phase bubble column reactors was begun using cold flow modelling techniques (l ). Part of this study includes the measurement of solid concentration profiles over a range of independent column operating values. 

Cova (3 ) measured the solid concentration profiles of a Raney nickel catalyst with an average diameter of 15.7 ym in a h.6 cm id reactor, using water and acetone as the liquids. He developed a sedimentation diffusion model, assuming solid and liquid dispersion coefficients were equal, and slurry settling velocities were independent of solid concentration. The model was then applied to data for Raney nickel in 6.35 and kk.J cm id bubble columns, in both cocurrent and countercurrent flow. 

Magelli, F. Fajner, D. Nonentini, M. Pasquali, G. Solid distribution in vessels stirred with multiple impellers. Chem. Eng. Sci. 1990, 45, 615-625. Fajner, D. Magelli, F. Nocentini, M. Pasquali, G. Solids concentration profiles in a mechanically stirred and staged column slurry reactor. Chem. Eng. Res. Des. 1985, 63, 235-240. 

Sannffis BH (1997) Solids movement and concentration profiles in column slurry reactors. Dr. Ing. Thesis, Norwegian University of Science and Technology, Trondheim... 

FIGURE 5.6. Concentration profiles of the photodegiadation of 3 different initial concentrations of phenol over Degiissa P25 at an initial pH of 4 (o) phenol, (A) p-DHB, ( ) o-DHB. Full lines represent best-fit curves of the joint model. (Reprinted from Chem. Eng. Sci., 59, M. Salaices, B. Seirano and H.I. de Lasa, Photocatalytic conversion of phenolic compounds in slurry reactors, 3-15, Copyright 2004, with permission from Elsevier). 

Figure 8.9 Concentration profiles for a reactant, A, in a slurry reactor. Point 1 = Ai , Point...

Chapters 7 and 8 present models and data for mass transfer and reaction in gas-liquid and gas-liquid-solid systems. Many diagrams are used to illustrate the concentration profiles for gas absorption plus reaction and to explain the controlling steps for different cases. Published correlations for mass transfer in bubble columns and stirred tanks are reviewed, with recommendations for design or interpretation of laboratory results. The data for slurry reactors and trickle-bed reactors are also reviewed and shown to fit relatively simple models. However, scaleup can be a problem because of changes in gas velocity and uncertainty in the mass transfer coefficients. The advantages of a scaledown approach are discussed. 

Figure 17.1 Concentration profiles in a slurry reactor. G = gas, L = liquid, R = particle radius, 6q = gas film thickness, Si = liquid film thickness.

Figure 1.Concentration profile of dissolved gas A Slurry reactor with a constant size insoluble solid reactant(Partic-les are larger than film thickness and the product is insoluble),...

With regard to an effective use of catalyst it is necessary to realize a uniform distribution over the entire reactor. There are a number of experimental studies reported in the literature (1-5) which show that even for small particles well pronounced solid concentration profiles can be observed in the gas agitated bubble column slurry reactors (BCSR). A dispersion-sedimentation model has been proposed, which successfully describes measured data (2-4). 

In general, catalyst sedimentation has to be accounted for in slurry reactors. The distribution of the catalyst along the reactor can be computed using the sedimentation-dispersion model. As to the results of Kato et al. (73), the solid dispersion coefficients do not differ much from those of the liquid phase. From the data provided by Cova (74), Imafuku et al (75), and Kato et al. (73), the solids concentration profiles can be calculated. As in the FT process the catalyst particles are usually small, according to Kolbel and Ralek (35) the diameter should be less than 50 um, the catalyst profiles are not very pronounced, in accordance to the measurements of Cova (74). 

Contact modalities and concentration profiles in catalytic membrane reactors for three-phase systems.The concentration of reactants is represented on the y-axis and the spatial coordinate along the membrane cross-section is represented on the x-axis. Below the scheme of each case the sequence of the mass transfer (MT) resistances and of the reaction event (R) are reported. (a)Traditional slurry reactor (b) supported thin porous catalytic layer with the liquid impregnating the porosity and the gas phase in contact with the catalytic layer (c) supported thin porous catalytic layer with the liquid impregnating the porosity and the liquid phase in contact with the catalytic layer (d) supported dense membrane which is perm-selective to the gas-phase reactant (e) dense catalytic membrane perm-selective to both reactants in the gas and liquid phases (f) forced flow of the liquid phase enriched with the gas-phase reactant through the thin catalytic membrane layer. 

Concentration profile across gas-liquid-solid interfaces in a slurry reactor. 

The modeling methodology is shown in Figure 6.17.7 for the example of a discontinuous slurry reactor. First, the concentration profiles within the catalyst particles are calculated. This information is then coupled (for each time step) with the change of concentrations in the bulk phase (Cj t)- The link between both procedures, that is, between the bulk phase and the porous catalyst particles, is the concentration gradient of each reactant at the external particle surface. Note that this calculation is also applicable for a continuous plug flow reactor simply by using the residence time t (= x/u) instead of the reaction time, whereby x represents the axial coordinate x in a tubular reactor and u the fluid velocity. 

Slurry reactors are usually equipped with stirrers, to prevent the solid particles from settling. Often propellers are employed in slurry reactors, but turbines can also be effective (see section 4.2.2, figure 4.1). The net action by gravity results in a solids concentration that gradually decreases with liquid height. The pumping action of the propeller transports equal volumes of liquid upward and downward however, the former contains more solids than the latter. In the steady state, the concentration profile is such that the difference of these upward and downward flows balances the flow of sinking particles. 

Fig. 7 Concentration profiles in mass transfer and reaction in series slurry reactor with non porous particles film theory.

Fig. 3 and 4 illustrate the mass transfer phenomena and show reactant and product concentration profiles in a Slurry and a Trickle Bed Reactor, respectively. The reaction is solid catalyzed between components A and B fed with the gas and the liquid, respectively. 

The preliminary results on the polymerization of ethylene have clearly shown that the concentrations of ethylene in the stilvent (Csx ), anpartial pressure or concentration of ethylene above the solvent Csv)t zhould be used when activities are compared for different solvents or with activities determined in a gaa-phase reactor. It has also been shown that high-activity catalysts of the type used in this study when used in conjunction with TEAL deactivate much more rapidly in gas-phase reactors than in slurry reactors. The high-activity catalysts did not deactivate when IPRAL was used as a co-catalyst, but the normalized activities in the gas-phase reactor were lower than those in the liquid-phase reactor. The shapes of the activity-time profiles were also different, t.e. the activation rates were not the same for the gas and slurry systems. 

The performance of the 2-stage co-precipitation process can be further evaluated with the data shown in Figures 1 and 2 corresponding to the test with Fe(II)/As(V)=4 (CD2). Figure 1 shows the concentration profQes for Fe(II) and As(V). Figure 2 displays the Eh and pH profiles during co-precipitation. The reactors reached steady-state after 6.5 hours. It is only after that point that slurry was collected for stability testing. 

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