Tube-wall reactor discussion

Discuss the stability of this reactor relative to a packed-bed reactor containing inch particles of the same porous nickel catalyst. Do you think the tube-wall reactor will always be stable, very much more stable, somewhat more stable, or about as stable as the packed bed Give an intuitive answer, and try to justify it by rough calculations. 

The solution of Eq. (173) poses a rather formidable task in general. Thus the dispersed plug-flow model has not been as extensively studied as the axial-dispersed plug-flow model. Actually, if there are no initial radial gradients in C, the radial terms will be identically zero, and Eq. (173) will reduce to the simpler Eq. (167). Thus for a simple isothermal reactor, the dispersed plug flow model is not useful. Its greatest use is for either nonisothermal reactions with radial temperature gradients or tube wall catalysed reactions. Of course, if the reactants were not introduced uniformly across a plane the model could be used, but this would not be a common practice. Paneth and Herzfeld (P2) have used this model for a first order wall catalysed reaction. The boundary conditions used were the same as those discussed for tracer measurements for radial dispersion coefficients in Section II,C,3,b, except that at the wall. 

As in the LPCVD reactor discussed earlier, allowing the electrode structure to touch the tube wall as it is inserted leads to considerable particle contamination. Therefore, cantilever loaders are available here also, and a typical unit on an ASM reactor is shown in Figure 23. Again, in contrast to the AMP-3300, this system is operated under computer control. Automated handling of wafers is more difficult to achieve, and is not generally available. 

There are two general types of CVD reactors, one is the chamber type and the other is the tube type. The tube type reactor is typically a hot wall reactor and has been used in the semiconductor industry for the deposition of simple binary thin films such as SijN. This type of deposition reactor usually has quite large throughput because a few hundred wafers can be loaded and processed. However, the CVD precursors should have large diffusivities in the gas phase and be stable over the homogeneous reactions to produce uniform deposition on a large number of wafers. For tube type reactors, as for all hot wall type reactors, the CVD reaction occurs on the wall of the reactor as well as on the wafers. This increases the consumption of the precursors. Therefore, CVD reactors for BST thin films are the other type, except for a very recent report from Toshiba of Japan. They reported CVD of BST thin films utilizing a tube type reactor which had a rotatory wafer holder to improve the uniformity of deposited films. Details of the CVD reactor have not been reported yet, thus, in this section only the details of chamber type reactors are discussed. 

Many of the first papers which discussed the use of (selective) CVD of tungsten for IC applications used conventional hot wall tube CVD reactors [Broadbent et al.44, Pauleau et al.45, Cheung47]. This type of reactor was and still is the workhorse in IC fabs. Excellent films such as TEOS based oxides, thermal silicon-nitride and poly-silicon can be grown in such equipment. Hot wall tube reactors are suitable for these films because such materials stick very well to quartz tubes and are quite transparent to IR radiation of the heating elements. Thus neither particle nor temperature control is a problem. One other major advantage is that high throughputs are typically obtained. 

As for the pressure levels in the reaction operations, 1.5 atm is selected for the chlorination reaction to prevent the leakage of air into the reactor to be installed in the task integration step. At atmospheric pressure, air might leak into the reactor and build up in sufficiently large concentrations to exceed the flammability limit. For the pyrolysis operation, 26 atm is recommended by the B.F. Goodrich patent (1963) without any justification. Since the reaction is irreversible, the elevated pressure does not adversely affect the conversion. Most likely, the patent recommends this pressure to increase the rate of reaction and, thus, reduce the size of the pyrolysis furnace, although the tube walls must be thick and many precautions are necessary for operation at elevated pressures. The pressure level is also an important consideration in selecting the separation operations, as will be discussed in the next synthesis step. 

We describe cases when FP is expected to be observed. The first case is the polymerization of crosslinking monomers (thermosets). The second group of monomers form polymers that are insoluble in the monomer. Good examples are acrylic and methanylic adds. Insoluble polymer particles adhere to each other during thdr formation and stick to the reactor or test tube walls, forming a mechanically stable phase and discernible polymer-monomer interface. Nonetheless, Rayleigh-Taylor and double-diffusive instabilities, which we will discuss... 

To illustrate the concepts of determining, non-determining and negligible processes, the mechanism of the pyrolysis of neopentane will be discussed briefly here. Neopentane pyrolysis has been chosen because it has been studied by various techniques batch reactor [105— 108], continuous flow stirred tank reactor [74, 109], tubular reactor [110], very low pressure pyrolysis [111], wall-less reactor [112, 113], non-quasi-stationary state pyrolysis [114, 115], single pulse shock tube [93, 116] amongst others, and over a large range of temperature, from... 

As discussed previously, several solar photoreactor geometries can be reduced to cylindrical glass tubes externally illuminated by different types of reflectors, like parabolic troughs, CPC, V-grooves, or without reflector, directly illuminated by the sun. In this section the general solution of the PI approximation for this t)q5e of photo reactors is reported. This general solution is applicable to any particular reactor if the flux distribution impinging on the wall of the tubular reaction space is known. 

This general phenomenon of buildup of solid polymer in static areas of the reactor is common to all continuous solution reactors. A major component of the design is to eliminate regions where this may occur. This is one of the drivers for incorporating agitators in the tower reactors which will be discussed in the next section and has also been a driver for removing heat transfer tubes from the reactor which add wall surface. 

The data of Tables 2 and 3 show that palladium-ruthenium alloys with mass % of ruthenium from 4 to 7 have high hydrogen permeability, catalytic activity toward many reactions with hydrogen evolution or consumption, and good mechanical strength [35]. Seamless tubes with a wall thickness of 100 and 60 p.m, as well as foils of 50-tim thickness made of the mentioned alloy, are commercially available in Russia. The tube of outer diameter of 1 mm and wall thickness of 0.1 mm is stable at a pressure drop of up to 100 atm and a temperature up to 900 K. The application of such tubes for membrane reactor will be discussed in next part of this section. 

We may first divide tubular reactors into those designed for homogeneous reactions, and therefore basically just an empty tube, and those designed for a heterogeneously catalyzed reaction, and hence to be packed with a catalyst. Both types can of course be operated adiabatically, and it was the simplest model of these that we discussed in the last chapter. If the temperature of the reactor is to be controlled this is through the wall, and the associated problems of heat transfer now arise. These include transfer at the wall and subsequent radial diffusion across the flowing reactants. In the empty tubular reactor there may be considerable variations in flow rate across the tube. For example, in the slow laminar flow the fluid... 

Several designs are shown in Figure 22.12 (a) common batch cleaning bath reactor with wall mounted transducers (b) batch reactor with immersible transducers (c) batch reactor with sonic probe (d) continuous flow tubular reactor with wall-mounted transducers (e) the Harwell sonochemical reactor and (f) a shell-and-tube reactor. A number of other designs are discussed by Thompson and Doraiswamy (1999b). 

Deviations from an ideal plug-flow pattern are caused by either wall flow or axial gradients that develop in the direction of flow. The bed void fraction at the reactor wall is likely to be somewhat higher than the void fraction in the catalyst bed, Gb-In order to eliminate the wall effect on the flow pattern, the tube diameter to particle diameter ratio is chosen to be greater than 10 as a general rule, but since the microreactors used for kinetic studies have very small diameters (4-10 mm), and for reasons discussed in the following, a higher ratio of 15 is indicated to minimize the wall effect in laboratory PBRs ... 

Radial Variations in Bed Structure As discussed in Section 4.10.6.4, radial variations in a packed bed occur in shallow reactors with a low ratio of the tube to particle diameter (<10). For lower values, a non-uniform radial velodty profile is induced and significant undesirable wall effects (bypassing, slippage) may occur. Consequently, the criterion for negligible influence of radial variations in bed structure is ... 

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