Catalyst linear velocity

The velocity of flow of catalyst along the kiln is used to transform the time variable into a distance variable. The catalyst linear velocity is given by... 

The gas and catalyst linear velocities are much lower in the FFBs than those in the CFBs, particularly in the narrower sections (see Fig. 2). The gas compression costs are consequently lower. Because the iron carbide catalyst is very abrasive, the narrower sections of the CFB reactor are ceramic lined and regular maintenance is essential. This problem is absent in the lower velocity FFB reactors and this allows longer on-line times between maintenance inspections, leading to higher production rates and lower maintenance costs. 

One goal of catalyst designers is to constmct bench-scale reactors that allow determination of performance data truly indicative of performance in a full-scale commercial reactor. This has been accompHshed in a number of areas, but in general, larger pilot-scale reactors are preferred because they can be more fully instmmented and can provide better engineering data for ultimate scale-up. In reactor selection thought must be given to parameters such as space velocity, linear velocity, and the number of catalyst bodies per reactor diameter in order to properly model heat- and mass-transfer effects. 

A preliminary estimate is useful for the linear velocity to be used on the catalyst under study. The linear flow is known for an existing process. For a new process, it can be estimated from flow used in similar processes. An estimate can also be developed for the minimum flow to avoid gradients from calculations (to be presented in Appendix C.)... 

The measurement of the linear velocity as a function of shaft RPM can be done at room temperature and pressure in air. It is best to do this on the catalyst already charged for the test. Since u is proportional to the square of the head generated, the relationship will hold for any fluid at any MW, T, and P if the u is expressed at the operating conditions. The measurement can be done with the flow measuring attachment and flow meter as shown in Figure 3.5.1. 

The flow that is shown in these figures is the instrument flow measured as m/s in the measuring tube. Multiplied with the flow cross-section of 5.59 cm, this gives the volumetric flow in the 2.67-cm diameter flow tube. Using a different catalyst basket or measuring tube will change this ratio. The volumetric flow is the same in the basket. Because the small basket has a 3.15 cm diameter and 7.79 cm cross-section, the linear velocity will be 5.59/7.79 = 0.72 fraction of that in the tube. 

This reaction is carried out in tall fluidized beds of high L/dt ratio. Pressures up to 200 kPa are used at temperatures around 300°C. The copper catalyst is deposited onto the surface of the silicon metal particles. The product is a vapor-phase material and the particulate silicon is gradually consumed. As the particle diameter decreases the minimum fluidization velocity decreases also. While the linear velocity decreases, the mass velocity of the fluid increases with conversion. Therefore, the leftover small particles with the copper catalyst and some debris leave the reactor at the top exit. 

The scheme of commercial methane synthesis includes a multistage reaction system and recycle of product gas. Adiabatic reactors connected with waste heat boilers are used to remove the heat in the form of high pressure steam. In designing the pilot plants, major emphasis was placed on the design of the catalytic reactor system. Thermodynamic parameters (composition of feed gas, temperature, temperature rise, pressure, etc.) as well as hydrodynamic parameters (bed depth, linear velocity, catalyst pellet size, etc.) are identical to those in a commercial methana-tion plant. This permits direct upscaling of test results to commercial size reactors because radial gradients are not present in an adiabatic shift reactor. 

No. Range of input SO cone. (%) Approx. 02 cone. (%) Catalyst type Bed height (m) Linear velocity (m) Cycle duration (min) Conversion (%) Max. temp. (°C) AP (mm water) Assumed so2 (%) Calc. conversion (%) Calc. rmax (°c) Calc. AP (mm water)... 

The second approach involves simultaneous variation of the weight of catalyst and the molal flow rate so as to maintain W/F constant. One then plots the conversion achieved versus linear velocity, as shown in Figures 6.4c and 6Ad. If the results are as indicated in Figure 6Ad, mass transfer limitations exist in the low-velocity regime. If the conversion is independent of velocity, there probably are no mass transfer limitations on the conversion rate. However, this test is also subject to the sensitivity limitations noted above. 

In addition to importance of the catalyst composition and temperature, we have shown that methane partial oxidation selectivity is strongly affected by the mass transfer rate. Our experiments show that increasing the linear velocity of the gases or choosing a catalyst geometry that gives thinner boundary layers enhances the selectivity of formation of H2 and CO. Since H2 and CO are essentially intermediate... 

The upper instrument is placed at the exit of the catalyst bed. The middle instruments moves along the catalyst bed with the same linear velocity as the gas flowing in the catalyst bed. When the yellow instrument reaches the end of the reactor, the green and the yellow instruments show the same concentrations illustrating the equivalence of position and contact time. 

Fig. 20. Dependence of exit conversion on inlet temperature. Platinum catalyst diluted with inert particles (homogeneous mixing) linear velocity w = 5.3 cm, bed length L = 6 cm.

Results. Figure 2 compares the observed rates of reaction (gram moles of nitric oxide produced per second per cm2 of catalyst) as a function of the linear velocity of the gas for three different catalysts and two different gas compositions. For a given gas flow rate the rate of reaction is critically dependent upon the... 

G is the ratio of the dimensionless number SD in the disturbed region to its value in the normally-operating part of the bed. SD contains the activation energy, heat of reaction, inlet temperature and bed height, all of which have fixed constant values in all regions of the bed. It also contains the possibly variable quantities C, k and F. C is the average heat capacity of the fluid, and depends on the local phase ratio. kg is the specific rate constant, and depends on the local catalyst density and the phase holdup. F is the local average linear velocity, which can vary from point to point for a variety of reasons. 

For the fluids and the monoliths considered in this comparison, this limit is approximately 0.50 m/sec (depending on the catalyst load), which is in good agreement with the value found experimentally by Irandoust et al. [12]. If the liquid load is less than this limit, gas will be sucked in as well. Hence the sum of the linear velocities will tend to be close to the maximum flow rate of liquid alone. Frictional pressure drop in the MR is up to two orders of magnitude lower than in the TBR. Consequently, for the MR we may consider very high flow rates and high columns, higher than appears to be of practical interest, before the pressure drop becomes a restriction with the physical properties considered. For practical reasons an upper limit of 20 m for Lp was taken. In order to make a comparison between the MR and the TBR, some more restrictions were imposed. In a consecutive-reaction system like the one considered, selectivities must be compared at the same conversion, and results discussed below are for a 50% conversion of reactant A. 

Hundreds of fluidized bed crackers are in operation. The vessels are large, as much as 10 m or so in diameter and perhaps twice as high. Such high linear velocities of vapors are maintained that the entire catalyst content of the vessels circulates through the cyclone collectors in an hour or so. Electrical precipitators after the cyclone collectors have been found unnecessary. 

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