Tube Wall Reactor

Figure 10.6. Schematic diagram of methanation reactor showing catalyst coating on the inside of the reactor tube wall.

The reason for limiting the temperature in sulfur dioxide oxidation is based on two factors excessive temperatures decrease the catalyst activity, as just mentioned, and the equilibrium yield is adversely affected at high temperatures. This last point is the important one in explaining the need to maintain the temperature level in the dehydrogenation of butene. Still other factors, such as physical properties of the equipment, may require limiting the temperature level. For example, in reactors operated at very high temperatures, particularly under pressure, it may be necessary to cool the reactor-tube wall to preserve the life of the tube itself. 

It is apparent from the calculations thus far that the heat transfer to the reactor-tube wall is becoming larger as the conversion increases. This is owing to the increased temperature difference between reaction mixture and wall. The trend continues until the heat transferred to the oil bath is as large as that evolved as a result of reaction. The temperature reaches a maximum at this point, the so-called hot spot. The results of further calculations, summarized in Table 13-3 and Fig. 13-12, show that the computed hot spot is reached at about 14 cm from the entrance to the reactor. This location is 3 cm before the experimental hot spot. Also, the temperature is 10°C less than the... 

The reactor will be designed to operate at a conversion of 80% and have a production rate of 6,000 Ib/day of phthalic anhydride. It will be a multitube type (illustrated in Fig. 13-1) with heat-transfer salt circulated through the jacket. The temperature of the entering reactants will be raised to 340°C by preheating, and the circulating heat-transfer salt will maintain the inside of the reactor-tube walls at 340°C. 

Fang et al.(9] recently also reported on a special function of such reactors in using them for on-line precipitate collection. Precipitates are formed in the reactor and collected almost quantitatively on the reactor tube walls presumably through centrifugal force created from the secondary flow. With the aid of such collectors no filters are necessary to separate the precipitate. Details on the technique are given in Chapter 7. 

Universal gas constant A-component reaction rate Hydrogen reaction rate /-Component reaction rate Selective membrane radius Hydrogen solubility Reactor operating temperature Permeation zone temperature Temperature of heating/cooling fluid Reaction zone temperature Temperature on catalyst surface Temperature inside catalyst particle Reactor tube wall temperature... 

The gap resistance was further rationalized in subsequent work. It was shown that the associated heat transfer mechanism relies primarily on heat conduction across the stagnant gas film, trapped in the gap between tbe monolith and the inner reactor tube wall. In fact, the gap resistance was inversely proportional to the gap size evaluated under the reaction conditions (so differential thermal expansion of the monolith and tube materials should be considered), and directly proportional to the gas-phase conductivity, as evidenced by heat transfer measurements with N2-He mixtures of different compositions. Estimates for h in excess of 700 W/(m K) were obtained when using pure He. 

It is worth emphasizing, however, that more work is needed in this very critical area. For instance. Ref. 127 shows the existence of a sigitificant dispersion of the estimates in the literature for foam thermal conductivities the role of the additional contributions due to convection and radiative heat transfer still need to be assessed and tbe role of the thermal resistance at the interface between the foam and the inner reactor tube wall has not been addressed so far. 

Ethylene pyrolysis can also take place in the same way. Additionally, the presence of steam as a diluent reduces the chances of hydrocarbons coming in contact with the reactor tube-wall. The carbon deposits reduce heat transfer through the reactor tubes, but steam reduces this effect by reacting with them [12] (steam reforming reaction). 

Catalytic methanation processes include (/) fixed or fluidized catalyst-bed reactors where temperature rise is controlled by heat exchange or by direct cooling using product gas recycle (2) through wall-cooled reactor where temperature is controlled by heat removal through the walls of catalyst-filled tubes (J) tube-wall reactors where a nickel—aluminum alloy is flame-sprayed and treated to form a Raney-nickel catalyst bonded to the reactor tube heat-exchange surface and (4) slurry or Hquid-phase (oil) methanation. 

Vertical in-tube condensers are often designed for reflux or knock-back application in reactors or distillation columns. In this case, vapor flow is upward, countercurrent to the hquid flow on the tube wall the vapor shear ac4s to tliicken and retard the drainage of the condensate film, reducing the coefficient. Neither the fluid dynamics nor the heat transfer is well understood in this case, but Sohman, Schuster, and Berenson [J. Heat Transfer, 90, 267-276... 

Good heat transfer on the outside of the reactor tube is essential but not sufficient because the heat transfer is limited at low flow rates at the inside film coefficient in the reacting stream. The same holds between catalyst particles and the streaming fluid, as in the case between the fluid and inside tube wall. This is why these reactors frequently exhibit ignition-extinction phenomena and non-reproducibility of results. Laboratory research workers untrained in the field of reactor thermal stability usually observe that the rate is not a continuous function of the temperature, as the Arrhenius relationship predicts, but that a definite minimum temperature is required to start the reaction. This is not a property of the reaction but a characteristic of the given system consisting of a reaction and a particular reactor. 

Figure 9.7.2 Plug-flow reactor simulation. Inside temperature vs. Tube length at various tube wall temperatures, in K ...

In the tubular reactor, a large amount of reaction heat is removed through the tube walls. 

The catalyst in an isothermal tube-wall reactor (experiment TWR-6 in Ref. 2) deactivated much more slowly than did the catalyst in the best test (experiment HGR-14) in an adiabatic HGR reactor (0.009 vs. 0.0291 %/mscf/lb), and it also produced much more methane (177 vs. 32 mscf/lb catalyst). This indicates that adiabatic operation of a metha-nation catalyst between 300° and 400°C is not as efficient as isothermal operation at higher temperature ( 400°C). 

The process starts with a one-phase flow of a low-viscosity PA salt solution and ends with a two-phase flow with the viscous PA melt along the tube wall and steam through the middle. A stream separator and a finishing reactor are attached to the polymerization tubes. 

Most of this chapter assumes that the mass flow rate down the tube is constant i.e., the tube wall is impermeable. The reactor cross-sectional area Ac is allowed to vary as a function of axial position, Ac = Adz). Figure 3.1 shows the system and indicates the nomenclature. An overall mass balance gives... 

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