Influence of space velocity

The reduction of fused iron catalyst commences from the external surface of particles, and then expands inward. The reduction rate can be increased obviously by increasing the space velocity of reducing gas. The higher gas space velocity, the more favorable the reduction is, i.e., the lower the concentration of water vapor in gas, the faster the diffusion rate, the easier for the water molecules in the pore of catalyst to escape. As a result, the poisoning effect of water vapor is decreased to minimum. In addition, it is also conducive for the reduction reaction to move to the right and to raise the rate of reduction. However, when the space velocity continues to increase, the extent of increasing reduction rate will be minor. When it reaches the critical value, the space velocity of reductant gas on reduction rate has almost no impact. At the same time, in industrial production, increasing the space velocity is limited by the furnace heat supply and the temperature. 

It was considered that only when the reduction progress was controlled by external diffusion, increasing the gas space velocity could strengthen the reduction. However, as described above, this is not consistent with the actual situation. 

The experimental results show that the first batch of water produced during reduction has a strong block effect. Because the adsorption capacity of H2O is higher than that of H2, H2O occupies the activated site on the surface, reducing the rate of reduction. 

In the actual reduction conditions at plant, there is no need to consider the adsorption characteristics of all gases so that the kinetic equation complexity can be limited to describe the increase of concentration of products (H2O) on the block effect of reduction which is enough from thermodynamic constraints. 

Rayment T. made a meaningful test that the powder catalyst was reduced with sky-high space velocity at 450°C and atmospheric pressure, so that the concentration of water vapor was reduced to the level of difficult to measure by conventional methods. It is found from X-ray diffraction result that the catalyst after reduction mainly is not a-Fe, but amorphous iron with very high activity. Naturally, this method cannot be applied in industry. 

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Figure 59. Influence of space velocity on gas temperature needed to reach 50% and 70% conversion of CO, HC and NOjt over a fresh and an engine aged three-way catalyst (monolith catalyst with 62 cells cm, three-way formulation with Pt 1.42gl-, Rh 0.28gl->, engine bench light-off test at lambda 1.02 for CO and HC, and at lambda 0.986 for NO engine bench aging during 200 h).

Results in Fig. 3 show a strong influence of space velocity on the initial propane conversion activity and product selectivity in the propane aromatization over zeolite catalyst without and with silica binder (50 wt%). Results showing a comparison of the zeolite catalysts with and without silica binder (50 wt%) for their product selectivity, dehydrogenation/ cracking (D/C) and aromatization/cracking (A/C) activity ratios and also p-X/m-X ratio (or shape selectivity) in the propane aromatization at two different iso-conversions (5 and 25%) of propane are presented in Table 4. The comparison brings out following important effects of... 

Fig. 3 Influence of space velocity on the propane conversion and product selectivity in the propane aromatization over H-GaMFI ith and without silica binder (50 wt%).

The influence of space velocity on conversion and benzene yield at 773K are presenfed in Table 3. The conversion increases rapidly with contact time and reaches a maximum of about 98% at eontact times greater than 0.5 h. Even though, the conversion is nearly constant at higher contact times, the benzene yield decreases marginally due to greater hydrogenolysis by Pt and the increased formation of the Ci to C5 fraction (Table 3). 

Additional TPR experiments were performed to investigate the influence of space velocity (92,000-230,000 h ) and of the water feed content (1-10 %) on the SCR reactivity. It was found that NO and NH3 conversions decreased on increasing the space velocity and that there was no significant influence of the water feed content on the SCR reaction (10.13). 

Influence of gas velocity The FTS in slurry phase takes place in the absorption-with-slow-reaction regime as any significant absorption enhancement cannot be expected (88,89). In this case, the space-time-yields obtainable in two-phase bubble columns run through a maximum value as a function of the gas velocity (92). 

For the case of HDM reaction, liquid catalyst wetting efficiency was used first in an att pt to obtain the apparent kinetic reaction rate coefficient, but no effect of space velocity was observed on k pp as compared with the approach without using wetting efficiency factor. This behavior was attributed to the minimization of external mass-transfer gradients as was mentioned in the experimental section. Dudukovid (1977) has pointed out that if kinetic regime prevails, there is no marked influence of the degree of wetted external area as is confirmed in this approach. 

The first requirement is the definition of a low-dimensional space of reaction coordinates that still captures the essential dynamics of the processes we consider. Motions in the perpendicular null space should have irrelevant detail and equilibrate fast, preferably on a time scale that is separated from the time scale of the essential motions. Motions in the two spaces are separated much like is done in the Born-Oppenheimer approximation. The average influence of the fast motions on the essential degrees of freedom must be taken into account this concerns (i) correlations with positions expressed in a potential of mean force, (ii) correlations with velocities expressed in frictional terms, and iit) an uncorrelated remainder that can be modeled by stochastic terms. Of course, this scheme is the general idea behind the well-known Langevin and Brownian dynamics. 

Errors related to velocity measurement instruments have different origins depending on the measurement principle. The most important of these have been covered in previous sections. One common source of error for all instruments is the disturbance of the flow field by the sensor/meter or the person carrying out the measuring. The influence of the sensor in an open space is usually... 

Zabor et al. (Zl) have described studies of the catalytic hydration of propylene under such conditions (temperature 279°C, pressure 3675 psig) that both liquid and vapor phases are present in the packed catalyst bed. Conversions are reported for cocurrent upflow and cocurrent downflow, it being assumed in that paper that the former mode corresponds to bubble flow and the latter to trickle-flow conditions. Trickle flow resulted in the higher conversions, and conversion was influenced by changes in bed height (for unchanged space velocity), in contrast to the case for bubble-flow operation. The differences are assumed to be effects of mass transfer or liquid distribution. 

The influence of liquid recycle rate and liquid-feed space velocity upon desulfurization level is the subject of a brief theoretical discussion. 

The activity tests of the catalyst were carried out in a microflow reactor set-up in which all the high temperature parts are constructed of hastelloy-C and monel. The reactor effluent was analyzed by an on-line gas chromatograph with an Ultimetal Q column (75 m x 0.53 mm), a flame ionization detector, and a thermal conductivity detector. The composition of the feed to the reactor can be varied, besides the temperature, pressure, and space velocity. The influence of the recycle components CHCIF2 and methane was tested by adding these components to the feed. In total five stability experiments of over 1600 hours were performed. In each... 

Enzymatic reactions are influenced by a variety of solution conditions that must be well controlled in HTS assays. Buffer components, pH, ionic strength, solvent polarity, viscosity, and temperature can all influence the initial velocity and the interactions of enzymes with substrate and inhibitor molecules. Space does not permit a comprehensive discussion of these factors, but a more detailed presentation can be found in the text by Copeland (2000). Here we simply make the recommendation that all of these solution conditions be optimized in the course of assay development. It is worth noting that there can be differences in optimal conditions for enzyme stability and enzyme activity. For example, the initial velocity may be greatest at 37°C and pH 5.0, but one may find that the enzyme denatures during the course of the assay time under these conditions. In situations like this one must experimentally determine the best compromise between reaction rate and protein stability. Again, a more detailed discussion of this issue, and methods for diagnosing enzyme denaturation during reaction can be found in Copeland (2000). 

Let us consider the influence of a solid-liquid interface advancing at a constant velocity on the solid-liquid fractionation of an element i. In the case of unidirectional solidification, it is convenient to consider that liquid crosses the immobile interface with an absolute constant velocity v, while a solid-liquid fractionation coefficient K is applied to the fractionation of element i. Let us assume that the interface is at x=0, the medium being solid for x<0. Liquid fills the half-space 0[Pg.442]

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