Reactor tube configuration

Figure 16 shows a conventional reactor tube configuration as present in a steam reformer furnace. The preheated hydrocarbon feedstock passes through the catalyst tube in which it reacts producing an equilibrium mixture of hydrogen and carbon oxides the reformer effluent, at a temperature, which is in the range of 800-950°C, is then sent to the process gas boiler where steam is generated. 

Other variables of importance in designing these tubular pyrolysis reactors include the mass velocity (or flow velocity) of the gaseous reaction mixture in the tubes, pressure, steam-to-hydrocarbon-feedstock ratio, heat flux through the tube wall, and tube configuration and spacing. Pressure drop in the reactor is of major importance, especially because of the extremely high flow velocities normally employed. 

In general, the experimental apparatus is similar to the system that has been described previously (Liu, et al. 1998). The feed gases consisted of a combination of methane, oxygen, hydrogen, and helium. Helium was only used in initial experiments and for characterization studies of the catalyst. The feed gas flowrates were controlled by Porter mass flow controllers, model 201. The feed gases flowed axially down the reactor tube. The reactor was a quartz tube with a 9.0 mm O.D. and an I.D. of either 4.5 mm or 7.0 mm. The configuration of the reactor can be seen in Figure 1. 

The constraints changed from one trial configuration of the reaction system to the next, but typically included things like the minimum coolant temperature to permit efficient utilization of the heat of reaction as process steam, the maximum allowable aldehyde concentration in the condensed crude product to avoid refining and product specification problems, and a prescribed reactor pressure drop to insure adequate flow distribution among the reactor tubes at a minimum energy cost. All of these are implicit constraints — they establish the maximum or minimum levels for certain response variables. Explicit constraints comprise the ranges for search variables. 

McLoughlin et al. (2004b) have also made a similar comparison between a VTC, a PTC, and a CPC for disinfection of water heavily contaminated with Escherichia coli, both by photocatalysis and by UV irradiation without catalyst. In this case the collectors did not track the sun but were inclined at local latitude with reactor tubes running east-west. It was also found that the CPC had the best performance, followed by the PTC and the VTC, which showed comparable results. It is necessary to point out that the PTC studied was of the nonconcentrating type with a very different configuration with respect to the one used by Bandala et al. (2004). 

These reactors are of shell-and-tube configuration and mostly have the catalyst in the tubes, although some ammonia converters have the... 

Consider the following problem. In the petrochemical industry, many reactions are oxidations and hydrogenations that are very exothermic. Thus, to control the temperature in an industrial reactor the configuration is typically a bundle of tubes (between 1 and 2 inches in diameter and thousands in number) that are bathed in a heat exchange fluid. The high heat exchange surface area per reactor volume allows the large heat release to be effectively removed. Suppose that a new catalyst is to be prepared for ultimate use in a reactor of this type to conduct a gas-phase reaction. How are appropriate reaction rate data obtained for this situation ... 

The results of Fig. 13.6 were compared with a 1D axial dispersion model of the same process configuration (i.e., two injector loops, two lines from the loops to the mixing tee, the reactor tubing, and the sample loop tubing with the dimensions cited earlier). The output for the case of a 10 pL min flow rate from each pump, with 10 wt% Tracer 1 injected simultaneously from each injection loop, is shown in Fig. 13.7. The model predicted analytical loop peak concentration (i.e., exceeding 99% of the inlet injector concentration) arrived earlier than the peak concentration resulting from the actual process data. 

Thermal Effects in Membrane Reactors - The problem of how to supply or remove heat from membrane reactors is an important practical concern. In a shell-and-tube configuration, the catalyst can be put on the shell side, which can then be encased in yet another shell containing a heat-transfer fluid. It is likely that other geometries will, in fact, be used. For example, the burner unit of Ohta et al. is shown schematically in Figure 10. 

A porous matrix is sandwiched between two membranes. The matrix supports a liquid-phase catalyst. For the reaction A -> B, membrane 1 passes A but resists B, and membrane 2 passes both freely the function of membrane 2 is to encapsulate the catalyst solution. Reactant A is fed external to membrane 1 the concentration of A drops across the catalyst as it is consumed by reaction, due to diffusional resistance. The product diffuses to the right, reactant A does not. The benefits of this reactor are the liquid phase is encapsulated, the catalyst is separated from the product stream, the product is separated from the reactant, it provides a higher gas-liquid interfacial area, and a product is removed from an equilibrium-limited reaction. The authors suggested that the system be implemented as a shell-and-tube configuration using two different hollow-fiber membranes. 

The micro-reactor studies progressed in tandem with testing in a small pilot plant that was constructed to allow evaluation of formed catalysts that could be used in a commercial reactor, instead of the pressed powders tested in the micro-reactors. This larger scale testing was carried out in a 1" internal diameter reactor (120 ml cat.) fitted with a 5 zone heating system to ensure more isothermal operation and typically allowed 1-3 mm particles to be evaluated. The reactor tube internals were configured in such a way as to minimise the radial temperature profile and ensme that the reactor operated in a thermally stable regime. 

Tosti et al. tested Pd-Ag membrane reactor for 12 months for H2 permeation [14]. Excellent stability was observed for 12 months of operation. In fact, the complete hydrogen selectivity and none failure (formation of cracks, holes) were observed. They proposed that the reliability is a result of both the tube manufacturing procedure and the reactor design configuration (finger-like). Figure 6.11 shows the picture of membrane reactor before and after the 12 months of operation. 

Although the tube-in-tube configuration is quite useful to work in laboratory scale and for proof of principle of MRs, for industrial scale some other configurations need to be used in order to increase the membrane area per volume of vessel used. In fact, the amount of hydrogen produced is directly related to the amount of membrane area installed in the reactor. 

Kleinert et al. [26] studied, for example, POM in a hollow fibre MR. The perovskite membranes used by the authors were produced from Ba(Co,Fe,Zr)03 (BCFZ) powder via phase inversion spinning technique. A tube-in-tube configuration was used while the catalyst was packed in the shell side of the reactor. 

The shell/tube configuration of tubular PBRs depends on the nature of the catalytic reaction. For highly endothermic reactions such as catalytic steam reforming, the reactor geometry is similar to that of a fired furnace in which the catalyst-packed tubes are heated by the energy released by the combustion of a fuel on the shell side. Catalytic steam reforming involves the conversion of a hydrocarbon to a hydrogen-rich mixture in the presence of steam ... 

Figure VIII-2 shows the Package-Reactor system configuration inside the containment vessel. To reduce construction costs, the prototype currently being developed adopts twelve nuclear cassettes and three steam generators as one package (unit). Every four cassettes are connected to one steam generator. Heat transfer tubes in the SG are separated into two groups in view of a heat transfer tube rupture event. During normal operation, pressure boundaries are maintained by the cassettes and steam generators, while the containment vessel is kept under atmospheric pressure. The containment vessel is made of stainless steel with a height and diameter of about 10 m and 5.5 m, respectively.

For multitubular membrane reactor configurations, the catalyst-in-tube configuration can be preferred especially for construction reasons and for the extent of bed-to-wall mass and heat transfer limitations that can be very detrimental in the catalyst-in-shell configuration. 

A fired tube reactor was configured to match the dimensions and catalyst loading of an existing oxo-alcohol synthesis gas steam reformer. Simulation results at observed conditions (feed gas composition, outlet temperature, steam to earbon ratio, ete.) agree very well with observed results. Catalyst activity is first determined by matching key effluent eomposition. 

In order to simulate the temperature spike in the reactor, the reactor is simulated as a cocurrent, packed-bed kinetic reactor, with a molten salt stream as the utility. This configuration provides a greater temperature differential at the front end of the reactor, where the reaction rate is highest. Countercurrent flow could be investigated as an alternative. The kinetics given above are used in the simulation. Dimensions of the reactor tubes are given in Section B.5.2. 

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