Holdup external

The models for catalyst effectiveness in trickle bed reactors developed in this paper require explicit measurements or predictions of external contacting, ncE pore fill-up, rii In laboratory conditions this can be accomplished by tracer techniques (22, ). Fractional pore fill up may be determined by the difference in first moments of the impulse response tracer tests performed on two beds of same particle size and shape when one bed consists of porous the other of nonporous particles. Fractional pore fill-up can also be assessed from the measured volumetrically static holdup. External contacting is measured by adsorbable tracer tests on beds of nonporous particles ( ). In industrial conditions ncE hj would have to be evaluated from correlations. Unfortunately at present the existing correlations for ncE unsatisfactory since they were developed for fixed bed adsorbers with larger packing and correlations for m ate nonexistent but may be developed in the future. 

Bismuth inventory charges. The bismuth inventory is determined by the primar>- s stem volume external to the reactor vessel, the volume of bis-nuitli ill the core, the volume of bismuth external to the core but inside the reactor ves.sel, and the holdup external to the reactor system. The primary system external to the reactor vessel is made up of throe heat-exchanger loops containing a total volume of 1640 ft . The volume of bismuth in the core is... 

The same end may be achieved by continuous operation at total external reflux with a small U bend in the reflux line for foamate holdup [Rubin and Melech, Can. ]. Chem. Eng., 50, 748 (1972)]. 

The bubble size distribution is closely related to the hydrodynamics and mass transfer behavior. Therefore, the gas distributor should be properly designed to give a good performance of distributing gas bubbles. Lin et al. [21] studied the influence of different gas distributor, i.e., porous sinter-plate (case 1) and perforated plate (case 2) in an external-loop ALR. Figure 3 compares the bubble sizes in the two cases. The bubble sizes are much smaller in case 1 than in case 2, indicating a better distribution performance of the porous sinter-plate. Their results also show the radial profile of the gas holdup in case 1 is much flatter than that in case 2 at the superficial gas velocities in their work. 

A fluidized-bed reactor consists of three main sections (Figure 23.1) (1) the fluidizing gas entry or distributor section at the bottom, essentially a perforated metal plate that allows entry of the gas through a number of holes (2) the fluidized-bed itself, which, unless the operation is adiabatic, includes heat transfer surface to control T (3) the freeboard section above the bed, essentially empty space to allow disengagement of entrained solid particles from the rising exit gas stream this section may be provided internally (at the top) or externally with cyclones to aid in the gas-solid separation. A reactor model, as discussed here, is concerned primarily with the bed itself, in order to determine, for example, the required holdup of solid particles for a specified rate of production. The solid may be a catalyst or a reactant, but we assume the former for the purpose of the development. 

Liquid holdup is critical in the downflow operation of fixed beds, in contrast to the upflow operation where the liquid occupies practically the whole external free void volume of the bed. Total liquid holdup ht consists of two parts static h, and dynamic holdup liA. Static holdup is related to the volume of liquid that is adherent to the particles surface, whereas dynamic holdup is related to the flowing pari of the liquid. 

In a reactor completely filled with liquid, the wetting efficiency is 100% or, in other words, the external wetting of the catalyst is complete (Burghardt et al., 1995). While it is true that when a fixed bed is completely filled with liquid wetting is complete (wetting efficiency is unity), the opposite is not true in a trickle bed, a portion of the bed voids will be always occupied by the gas phase. Thus, while in a well-operated trickle bed the wetting efficiency could be unity, its total liquid holdup based on the void volume is always lower than the bed voidage, i.e. the bed is never completely filled with liquid. 

LC-6). Cells LC-1 and LC-2 have holdup volumes of 2.0 and 0.7 ml., respectively. In the case of LC-2 the platinum electrodes and mercury well side arms are part of a standard-taper joint assembly. Cells LC-3 and LC-4 have holdup volumes of 0.16 ml. and external copper electrode caps. Cells LC-3 and LC-4 were stacked so as to be able to shift scales during the demineralization and regeneration half cycles, but a more convenient design was achieved in the compound cells, LC-5 and LC-6, with effective holdup volumes between 0.25 and 0.55 ml. Figure 6 is a drawing of LC-4 and LC-5. By using several inputs into the recorder, a continuous record can be obtained over a wide concentration range. 

Small bubbles and flow uniformity are important for gas-liquid and gas-liquid-solid multiphase reactors. A reactor internal was designed and installed in an external-loop airlift reactor (EL-ALR) to enhance bubble breakup and flow redistribution and improve reactor performance. Hydrodynamic parameters, including local gas holdup, bubble rise velocity, bubble Sauter diameter and liquid velocity were measured. A radial maldistribution index was introduced to describe radial non-uniformity in the hydrodynamic parameters. The influence of the internal on this index was studied. Experimental results show that The effect of the internal is to make the radial profiles of the gas holdup, bubble rise velocity and liquid velocity radially uniform. The bubble Sauter diameter decreases and the bubble size distribution is narrower. With increasing distance away from the internal, the radial profiles change back to be similar to those before contact with it. The internal improves the flow behavior up to a distance of 1.4 m. 

Dynamic. The coolant is assumed to be perfectly mixed as would be the situation in a circulating cooling water system. The holdup of the coolant is specified, so the dynamics of the jacket, coil, or external heat exchanger are taken into consideration. 

The bed void volume available for flow and for gas and liquid holdup is determined by the particle size distribution and shape, the particle porosity, and the packing effectiveness. The total voidage and the total liquid holdup can be divided into external and internal terms corresponding to interparticle (bed) and intraparticle (porosity) voidage. The external liquid holdup is further subdivided into static holdup eLs (holdup remaining after bed draining due to surface tension forces) and dynamic holdup eLrf. Additional expressions for the liquid holdup are the pore fillup Ft and the liquid saturation SL ... 

The following developments will be restricted to laminar liquid flow with weak gas-liquid interactions. However, this is not a limitation of the proposed methodology which could be easily applied to any other flow regime. Applications will be presented for the modelling of the irrigation rate, the dynamic liquid holdup and the apparent reaction rate in the absence of external mass transfer limitations and in the case of non volatile liquid reactants (i.e. approximatively the operating conditions of petroleum hydrotreatment). 

Whereas for bubbling fluidized beds the solids holdup in the upper part of the reactor and the entrainment of catalyst are often negligible, these features become most important in the case of circulating fluidized beds These systems are operated at gas velocities above the terminal settling velocity ux of a major fraction or even all of the catalyst particles used (% 1 m s 1 < umass flow rales to be externally recirculated are high, up to figures of more than 1000 kg m 2s-1... 

EXTERNAL STATIC HOLDUP LIQUID-SOLID CONTACTING CORRELATION CONSTANTS 0.0204 ... 

Here. tis is the external surface area per unit volume of column and o is the standard deviation in each variable. ReL and ReG are the liquid and gas Reynolds numbers based on the particle diameter, liquid holdup on the liquid velocity, a result similar to the one reported independently by Ohshima et al.zl Heilman s9 correlation, however, indicates that the gas holdup (or liquid holdup) is independent of the liquid flow rate as long as the liquid flow rate is low and the gas holdup remains below 0.54. 

F is used as the heating medium and flows at a rate of 6000 ib/hr. An overall heat transfer coefficient of 40 Btu/(hr)(fF)(°F) may be assumed use Newton s law of heating. The liquid is circulated at a rate of 6000 Ib/hr, and the specific heat of the liquid is the same as that of water (1.0). Assume that the residence time of the liquid in the external heat exchanger is very small and that there is essentially no holdup of liquid in this circuit. 

Since A and 8 are difficult to measure for a hquid membrane system, DA/b can be replaced by D ( 14/ I c) where D is an effective diffusivity and VJ Tc is the treat ratio or holdup ratio (volume ratio of emulsion to external phase). In the case of type-I facilitated transport mechanism, where the solute is removed by reaction with internal stripping reagent, the solute concentration in the internal phase can be considered to be zero and hence Eq. (1) becomes... 

Co and Cj are the feed and desired effluent concentration, respectively, N is the number of stages, is the water phase flow rate, I) is the permeation rate constant, is the holdup ratio (volume ratio of emulsion to external phase in the mixer). While this approach is simple, there are nevertheless certain drawbacks to its use. The permeation coefficient D or equivalently the membrane film thickness varies depending on conditions. Hatton and... 

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