Tubular reactor, deposition

Classification of the many different encapsulation processes is usehil. Previous schemes employing the categories chemical or physical are unsatisfactory because many so-called chemical processes involve exclusively physical phenomena, whereas so-called physical processes can utilize chemical phenomena. An alternative approach is to classify all encapsulation processes as either Type A or Type B processes. Type A processes are defined as those in which capsule formation occurs entirely in a Hquid-filled stirred tank or tubular reactor. Emulsion and dispersion stabiUty play a key role in determining the success of such processes. Type B processes are processes in which capsule formation occurs because a coating is sprayed or deposited in some manner onto the surface of a Hquid or soHd core material dispersed in a gas phase or vacuum. This category also includes processes in which Hquid droplets containing core material are sprayed into a gas phase and subsequentiy solidified to produce microcapsules. Emulsion and dispersion stabilization can play a key role in the success of Type B processes also. 

The epitaxy reactor is a specialized variant of the tubular reactor in which gas-phase precursors are produced and transported to a heated surface where thin crystalline films and gaseous by-products are produced by further reaction on the surface. Similar to this chemical vapor deposition (CVE)) are physical vapor depositions (PVE)) and molecular beam generated deposits. Reactor details are critical to assuring uniform, impurity-free deposits and numerous designs have evolved (Fig. 22) (89). 

Figure 2.4. (a) Tungsten deposition in a tubular reactor, (b) boundary layer conditions. 

Figure 2.10. Control of deposition uniformity in a tubular reactor (a) susceptor parallel to gas flow, (b) titled susceptor.

With a reaction-limited deposition process, the film should have uniform thickness as long as the partial pressures ofreactants do not vary with position. In a tubular reactor, the conversion of reactants must be kept small or the film thickness will depend on the location of the sohd in the reactor, with upstream regions having a greater deposition rate. It is therefore common to use a gas recirculation reactor (a recycle PFTR) so that the composition of the reactants is independent of position in the reactor to assure uniform film thickness. 

The hydrodynamic factors that influence the plasma polymerization process pose a complicated problem and are of importance in the application of plasma for thin film coatings. When two reaction chambers with different shapes or sizes are used and when plasma polymerization of the same monomer is operated under the same operational conditions of RF power, monomer flow rate, pressure in the reaction chamber etc., the two plasma polymers formed in the two reaction chambers are never identical because of the differences in the hydrodynamic factors. In this sense, plasma polymerization is a reactor-dependent process. Yasuda and Hirotsu [22] systematically investigated the effects of hydrodynamic factors on the plasma polymerization process. They studied the effect of the monomer flow pattern on the polymer deposition rate in a tubular reactor. The polymer deposition rate is a function of the location in the chamber. The distribution of the polymer deposition rate is mainly determined by the distance from the plasma zone and the... 

There are many chemically reacting flow situations in which a reactive stream flows interior to a channel or duct. Two such examples are illustrated in Figs. 1.4 and 1.6, which consider flow in a catalytic-combustion monolith [28,156,168,259,322] and in the channels of a solid-oxide fuel cell. Other examples include the catalytic converters in automobiles. Certainly there are many industrial chemical processes that involve reactive flow tubular reactors. Innovative new short-contact-time processes use flow in catalytic monoliths to convert raw hydrocarbons to higher-value chemical feedstocks [37,99,100,173,184,436, 447]. Certain types of chemical-vapor-deposition reactors use a channel to direct flow over a wafer where a thin film is grown or deposited [219]. Flow reactors used in the laboratory to study gas-phase chemical kinetics usually strive to achieve plug-flow conditions and to minimize wall-chemistry effects. Nevertheless, boundary-layer simulations can be used to verify the flow condition or to account for non-ideal behavior [147]. 

Tubular reactors with many oxygen feedpoints (Fig. 9.4-6), are supplied for solution-treatment by Eco Waste Technology (EWT), Texas, USA [12]. Tubular reactors are designed with small diameters, so fluid circulation is high and salts-deposition is avoided. Through the design, the deposition of solids already present in the feed can be avoided, but precipitated salts formed inside the reactor have a tendency to adhere themselves to the reactor walls. 

De Deken and his colleagues197 have studied the nature of the carbon deposited on a commercial CCE catalyst (12 wt% Ni on a-Al203) and have concluded that it has diffused into the bulk of the nickel and that some of it is present as carbide. A more applied article from the same group198 presents intrinsic kinetic data from a tubular reactor in the temperature range 823-953K. 

Many tubular reactor designs depend on high velocities to avoid deposition of particles in the reactor [112,117,118], However, when the high velocities are applied by use of a small diameter, a reactor length of hundreds of feet is required to achieve the required residence time. Therefore, the principles of keeping solids from depositing by high velocities have not been demonstrated at any acceptable scale. 

The steam reforming of methane cycle suffers from the problem of coke deposition on the catalyst bed. The primary objective of this project was to study the stability of a commercial nickel oxide catalyst for the steam reforming of methane. The theoretical minimum ratios of steam to methane that are required to avoid deposition of coke on the catalyst at various temperatures were calculated, based on equilibrium considerations. Coking experiments were conducted in a tubular reactor at atmospheric pressure in the range of 740-915°C. 

Scraped surface heat exchangers (SSHE) have been used as tubular reactors for plastics pyrolysis. SSHE overcome coking and carbon deposits forming on heat exchanging surfaces when the plastic pyrolyzes to hot gases. A tubular reactor with a special internal screw mixer has been developed in Poland [5]. The purpose of the specially shaped internal mixer is to mix the molten plastic and to scrape coke from the internal surface... 

Visual observation of the Vycor glass reactor immediately following the butadiene run at 700°C resulted in important information. The reactor was cut to permit inspection of the black deposits thought to be primarily coke. The last two-thirds of the reactor and a short section of the unheated tube that extended beyond the furnace were covered on the inner surface with a smooth layer of coke. This deposit, when viewed from the outside of the reactor, appeared as a black mirror. It is of special interest that the inlet section of the tubular reactor did not have any coke deposits. This section was the one that was subjected to increasing temperatures in the furnace. The start of the coke deposits occurred approximately in the section where maximum temperatures occurred during a run. Most of the deposits appeared to occur in the... 

Considerable information was obtained for ethane pyrolysis relative to coke deposition on and to decoking from the inner walls of a tubular reactor. Both phenomena are affected significantly by the materials of construction (Incoloy 800, stainless steel 304, stainless steel 410, Hastelloy X, or Vycor glass) of the pyrolysis tube and often by their past history. Based on results with a scanning electron microscope, several types of coke were formed. Cokes that formed on metal tubes contained metal particles. The energy of activation for coke formation is about 65 kcal/g mol. 

Figure 6. Surfaces formed on tubular reactors during pyrolysis of ethane at 800°C. (Top left,) Stainless steel 410—Surface A, (predominant deposit) (top right,) stainless steel 410—Surface A, (less frequent deposit) (middle left,) stainless steel 410—Surface B (middle right stainless steel 410—Surface C (bottom left,) Hastelloy X—Surface A (bottom right)...

Ihara and Yasuda investigated the deposition behavior of methane in the medium-sized tubular reactor with 13.56 MHz radio frequency discharge [2]. They observed that the critical WjFM value, WjFM), for methane was 8GJ/kg, and nearly 100% of monomers were converted to the plasma polymer beyond this critical WjFM value. As shown in Figure 19.5, the critical WjFM value of perfluoropropene in the small reactor is around 6GJ/kg and the DjFM is 15%, and the corresponding value in the medium is around 4GJ/kg, and the maximal conversion is around 30%. In the large reactor, (WIFM) is about 1 GJ/kg and the maximal DjFM is 20%. The lower value of the critical WjFM for C3F6 than that for CH4 is explained in Chapter 7. 

DEPOSITION OF FAST-POLYMERIZING MONOMER IN TUBULAR REACTOR... 

In order to examine the elfect of flow pattern in a reactor, which is a crucially important design factor of an LCVD reactor, it is necessary to examine the profile of deposition in a simple reactor first. A tubular reactor with an external radio frequency power coupling is ideally suited to the study of the distribution of polymer deposition. In such a reactor, 100% of the monomer passes through the luminous gas phase in the reactor, and the situation is very close to the case in which no bypass of monomer occurs. The experimental setup used for... 

The effects of the discharge power on the distribution of polymer deposition in a tubular reactor (Fig. 20.1) are shown in Figures 20.19-20.22. Figure 20.19 depicts the change in polymer deposition pattern due to the discharge power observed in the plasma polymerization of styrene at a fixed flow rate of 5.6 seem. 

This situation can be seen clearly in the distribution pattern observed by Kobayashi et al. [4] shown in Figure 20.23 (the arrangement of monomer inlet) and Figure 20.24 (the distribution of polymer deposition for corresponding cases). The slight asymmetry of the polymer deposition pattern can be attributed to the overall flow pattern existing in the entire reactor system. The principle of the polymer deposition is identical to that for the tubular reactor shown in Figure 20.17. 

An important implication of the data obtained with both a tubular reactor and a bell jar reactor is that the polymer deposition onto a stationary substrate cannot be uniform due to the diffusional transport of polymer-forming species and the path-dependent growth mechanism. The variation of polymer deposition rates at various locations becomes smaller as the system pressure decreases because the diffusional displacement distance of gaseous species increases at lower pressure. It is important to recognize that a certain degree of thickness variation always exists when the plasma polymer is deposited onto a stationary substrate regardless of the type of reactor and the location of the substrate in the reactor. 

Carbon coating can be achieved using pyrolysis of hydrocarbons at elevated temperatures [69]. Figure 2 shows a device used for carbon coating via hydrocarbon pyrolysis. In the example described here, an alumina-washcoated monolith is covered with carbon by pyrolysis of cyclohexene. A gas mixture of cyclohexene in nitrogen is passing the reactor at a certain flow rate. The monolith block to be coated is placed in the middle of the heated tubular reactor. The reaction takes place at 873-973 K, and the amount of carbon deposited can be controlled by the temperature and the time on stream. Up to 3-10 wt% carbon can be homogeneously coated onto the monolith in this way. It appears that the surface area of the carbon-coated alumina-washcoated cordierite monolith is of... 

Details can be found in Bdlare (ref. 3). The experimental equipment consists of a cumene reservoir, a thermogravimetric analyzer (TGA) and a gas chromatograph (GC). The hdium-cumene mixture enters the TGA, a Cahn System 113DC with a Cahn 2000 Recording Electrobalance, a quartz tubular reactor, and an external split-shell furnace. The catalyst is placed in the sample pan of the microbalance inside the quartz reactor, kept at a controlled temperature in the center of the split-shell furnace. The incremental weight due to coke deposition on the catalyst is monitored by an IBM PC. The reactor exit stream is injected into a Varian 3700 GC using FID. 

Steam is used as the diluent to reduce hydrocarbon partial pressure and hence to improve selectivity toward olefins. Steam also gasifies (via oxidation) part of the coke deposited on the tubular reactor wall producing CO and CO2 and hence promotes coke removal from the inner surfaces of the coils. 

In addition, the use of electrodless glow discharge will produce different results than those with internal electrodes. In the former case, rare gases such as He, Ar was introduced from one end of the tubular reactor and the plasma was sustained by a rf coil outside the reactor. Gaseous monomer was fed into the afterglow of a rare gas and polymer film deposited on the substrate placed downstream. Yasuda, et. al. (18) observed that the deposition rate in electrodeless discharge was independent of the power input and increased in proportional to the square of the monomer pressure. 

The removal of precipitated polyethylene from the wall is an interesting operation. About once every 2-3 sec the expansion valve is opened more fully than required for the expansion/precipitation function this results in a rapid decrease in pressure in the reactor of as much as 300-600 bar. The concomitant rapid increase in the velocity of the gas phase in the tubular reactor shears the walls and strips off any deposited polyethylene so that a reasonably steady state heat transfer situation exists. This description of the operation of the polymerization process, the polyethylene precipitation step, and the accentuated expansion, which maintains a clean wall and a high heat transfer coefficient, help to illustrate the interesting SCF solubility behavior and they also supply some information on the commercial reality of high-pressure processing in what we consider to be an extreme case. 

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