Catalyst Bed Summary

It confirms that intercept temperature and % SO2 oxidized under these conditions are  

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In summary, major challenges in the partial oxidation of methane are (1) designs to avoid excessive thermal gradients (hot spots) in the catalyst bed (2) reduction of the cost of O2 separation and (3) elucidation of the reaction pathways as a step toward improved catalyst design. 

Table 21.2. Summary of Fig. 21.1 s temperatures, enthalpies and heat transfers. Note the continuing decrease in the gas s enthalpy as heat is transferred from gas to water and steam in Fig. 21.1 s boiler, superheater and economizer. All temperatures but the last are from Tables J.2, M.2 and 21.1. Note that a catalyst bed s input enthalpy is always the same as its output enthalpy. This is due to our assumption that there is no conductive, convective or radiative heat loss from the gas.

In summary, the calculations show convincingly that modern fluidized-catalyst-bed technology has attained an emulsion fluidity nearly equivalent to that of low-viscosity liquids. With such fluidity, data obtained for a bubble column shed light on the performance of a fluidized catalyst bed, and vice versa. 

The present trends in ammonia converter technology are likely to continue into the future. In summary, these are the use of small catalyst particles, particularly in radial-flow converters, and the use of intercooling between catalyst beds rather than quench gas. The present trend of modifying existing converters to incorporate these improvements is also likely to continue. 

In summary, increasing the number of beds in series for the system shown in Figure 12 decreases the volume of recycle somewhat more than proportionately to the number of beds, decreases the recycle compressor power requirements somewhat, and has no effect on the catalyst volume. On balance, it would appear that the capital savings achieved by increasing the number of beds will be minimal. 

In summary, the results from the fixed bed reactor study provided evidence as to the effect of Au and KOAc on the performance of the catalyst, though, these experiments did not give any information on the perturbation of the reaction pathways with the addition of Au and KOAc. For this type of information, additional experiments were performed using the TAP reactor with 1,2 C-labeled ethylene used as an isotopic tracer of the kinetics. 

In summary, fixed-bed processes have advantages in ease of scaleup and operation. The reactors operate in a downflow mode, with liquid feed trickling downward over the solid catalyst concurrent with the hydrogen gas. The usual catalyst is cobalt/molybdenum (Co/Mo) or nickel/molybdenum (Ni/Mo) on alumina (A1203) and contain 11-14% molybdenum and 2-3% of the promoter nickel or cobalt. The alumina typically has a pore volume of 0.5 ml/g. The catalyst is formed into pellets by extrusion, in shapes such as cylinders (ca. 2 mm diameter), lobed cylinders, or rings. 

The above issues associated with prediction of trickle-bed reactor performance has motivated a number of researchers over the past two decades to perform laboratory-scale studies using a particular model-reaction system. These are listed in Table I. Although a more detailed summary is given elsewhere (29), a general conclusion seems to be that both incomplete catalyst wetting and mass transfer limitations are significant factors which affect trickle-bed reactor performance. 

A summary of reactor models used by various authors to interpret trickle-bed reactor data mainly from liquid-limiting petroleum hydrodesulfurization reactions (19-21) is given in Table I of reference (37). These models are based upon i) plug-flow of the liquid-phase, ii) the apparent rate of reaction is controlled by either internal diffusion or intrinsic kinetics, iii) the reactor operates isothermally, and iv) the intrinsic reaction rate is first-order with respect to the nonvolatile liquid-limiting reactant. Model 4 in this table accounts for both incomplete external and internal catalyst wetting by introduction of the effectiveness factor r)Tg developed especially for this situation (37 ). 

Chapter 4 contains a summary of the basic theory of granular flow. These concepts have been adopted describing particulate flows in fluidized bed reactors. The theory was primarily used for dense bed reactors, but modified closures of this type have been employed for more dilute flows as well. Compared to the continuum theory presented in the third chapter, the granular theory is considered more complex. The main purpose of introducing this theory, in the context of reactor modeling, is to improve the description of the particle (e.g., catalyst) transport and distribution in the reactor system. 

The fluidized-bed reactor differs from its fixed-bed counterpart in that the solid catalyst particles usually are smaller (10 to 200 microns), the porosity in the reactor is larger, and the particles are in motion. In Secs. 1-6 (Fig. 1 -4) and 3-8 the overall characteristics of fluidized beds were briefly described. Now, in preparation for design problems, it is necessary to describe the internal behavior. The fluid mechanics of solid particles in a gas stream is complex, and to some extent poorly understood. Summaries of present knowledge are available. ... 

The maximum internal holdup is determined primarily by the pore structure/ volume of the catalyst, and can range from about 0.1 to 0.4 for typical materials. Static holdups in the range of 0.02 to 0.05 are characteristic of most packed beds of porous catalyst. Summaries of such correlations for work done up to about 1980 are found in the reviews of Gianetto et al., and Satterfield, as cited. These seem fairly... 

The three new converter designs developed during the war operated at 20 atm, used iron catalysts, and were internally cooled compared to the inefficient externally cooled fixed bed converters in the existing F-T plants. Their design summaries appear below. 

In summary, the stagnancy/catalyst effectiveness model predicts that liquid and/or gas velocity effects on the apparent reaction rate will be observed for catalysts vfliich are at least marginally diffusion limited and run in a trickle bed reactor under low velocity conditions. The model predicts that for scale-up of reactions which are diffusion limited or at least marginally so, the pilot plant should be designed to run at elevated velocities which do not show sensitivity to liquid velocity. Conversely, if a pilot reactor is used for providing data for scaleup showing velocity effects, there is a good likelihood that the catalyst suffers from diffusion limitations. 

In summary, there is significant potential for the utilization of silica membranes in catalytic membrane reactors. To maximize the competitive advantage of catalytic membrane reactors over traditional packed bed reactors, the silica membranes must ideally possess a high Hj permeance and high H2 selectivity. Furthermore, the production rate of Hj from the reaction must be balanced against the permeation performance of the membrane to gain the optimal conversion and H2 yield enhancement. For hydrocarbon feedstocks, additional concerns, such as the formation of coke on both the catalyst and the membrane itself, must be carefully considered. Coke formation on the membrane may serve to hinder H2 flow across the membrane, further reducing H2 permeation and the effectiveness of the membrane reactor system. 

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