Reactor tube simulation

Pilot plants utilizing a single-full-sized reactor tube from a commercial plant are generally used to assess the quaUty and performance of individual catalyst lots and to perform plant or customer ordered process tests. A weU-designed pilot unit is capable of simulating the performance of a commercial plant with great accuracy. 

Both reactors used 38.1 mm 0 tubes. The commercial reactor was 12 m long while the length of the laboratory reactor was 1.2 m. Except for the 10 1 difference in the lengths, everything else was the same. Both reactors were simulated at 100 atm operation and at GHSV of 10,000 h-1. This means that residence times were identical, and linear gas velocities were 10 times less in the lab than at the production unit. Consequently the Re number, and all that is a function of it, were different. Heat transfer coefficients were 631 and 206 in wattsWK units for the large and small reactors. 

Figure 11.24 Simulation of the thermal behavior of a catalyst in a multitube test reactor. (AH > 100 kJ/mol, Tiri et 523 K, Treactor 523 K, porosity 80%, /.bed 3.0W/mK, GHSV 10000 h-1, reactor tube geometry 0.11 x 0.007 m).

Equipment. A vertical fixed-bed reactor, made of a 0.168 m I.D. and 0.5 m long 316 stainless steel tube with an axial thermowell, was used. The amounts of catalyst used for the steady state and dynamic experiments were 6.35 and 18.69 g, respectively. The reactor tube was heated by a fluidized bed sand bath. The reaction gases, O2 and CO, and the diluent, He, were metered through rotameters qnd mixed prior to their entry to the reactor. The mixing junction was designed such that either of the reaction gases or CO2 could be introduced or removed from the stream to simulate a step increase or decrease of the component in question. The effluent from the reactor was analyzed by gas chromatography in 4 minutes. 

Takeuchi et al. 7 reported a membrane reactor as a reaction system that provides higher productivity and lower separation cost in chemical reaction processes. In this paper, packed bed catalytic membrane reactor with palladium membrane for SMR reaction has been discussed. The numerical model consists of a full set of partial differential equations derived from conservation of mass, momentum, heat, and chemical species, respectively, with chemical kinetics and appropriate boundary conditions for the problem. The solution of this system was obtained by computational fluid dynamics (CFD). To perform CFD calculations, a commercial solver FLUENT has been used, and the selective permeation through the membrane has been modeled by user-defined functions. The CFD simulation results exhibited the flow distribution in the reactor by inserting a membrane protection tube, in addition to the temperature and concentration distribution in the axial and radial directions in the reactor, as reported in the membrane reactor numerical simulation. On the basis of the simulation results, effects of the flow distribution, concentration polarization, and mass transfer in the packed bed have been evaluated to design a membrane reactor system. 

Modeling and Simulation of Packed Bed Reactors Table 11.3. Reactor Tube Data... 

A further simplification in the procedure to measure the catalytic activity is the use of a model gas reactor (Fig. 44). A small piece of catalyst is mounted in a reactor tube, the temperature of which can be externally adjusted. Most commonly, a monolithic core of diameter 2.54 cm and length 7.5 cm is used. The desired exhaust gas composition is simulated by mixing either pure gases or mixtures of the desired exhaust gas component with nitrogen. Such a model gas reactor test gives the highest possible flexibility, as each of the characteristic parameters of the exhaust gas, such as composition, temperature and space velocity, can be varied in a truly independent fashion. 

Figure 1. Simulation of an industrial ethylene epoxidation reactor tube.

Table 4. Ethylene epoxydation-simulation of a commercial reactor tube...

For comparison, similar experiments were conducted using a conventional fixed-bed reactor module. The membrane reactor module was replaced with a quartz tube and the reactor effluent was analyzed for products and reactants. In some cases, a conventional reactor was simulated by conducting experiments in the membrane reactor without sweeping by blocking off the shell side of the reactor. 

Then lyoha et al. evaluated [18] Pd and Pd-Cu membrane reactors in simulated coal syngas containing H2S. For both the membrane reactors introduction of H2S in the feed by switching the feed mixmre to 90% H2-IOOO ppm H2S-He resulted in no discernible change in H2 permeance. 99.7% conversion of CO contained in a simulated syngas feed consisting of 53% CO, 35% H2 and 12% CO2 (dry basis) was attained in a Pd four-tube membrane reactor at 1173 K with a steam to CO ratio of 1.5. The Pd-Cu membrane reactor also effectively enhanced CO conversions above the equilibrium value of 32% (associated with non-membrane reactors) over the conditions of the study. However, the maximum conversions attained were appreciably lower than... 

The fuel, assemblies studied were composed of 28-pin fuel bundles mounted in two concentric tubes simulating the pressure and caiandria tubes of the power reactor. Each fuel pin consisted qf a 47.7-cm len of sintered UPi pellets (1.42-cm diam) of density 10.45 g/cm sheathed in Zircaloy-2 (l.S2-cm OD X 0.45-mm wall). Symmetric rings of 4, 8 and 16 pins on diameters of 2.33, 5.30 and 8.41 cm respectively formed the 28-pin bundle. Aluminum mounting plates and Zircaloy end caps increased the over-all bundle length to 49.7 cm. Five bundles mounted in a 6SS-A1 pressure tube (12.78-cm OD X 2.96-mm wall) and a 50S-A1 caiandria tube (12.74-cm OD X 1.39-mm wall) formed a fuel assembly. 

Example 12.5.A Three Dimensional CFD Simulation of Furnace and Reactor Tubes for the Thermal Cracking of Hydrocarbons... 

THREE DIMENSIONAL CFD SIMULATION OF FURNACE AND REACTOR TUBES FOR THE THERMAL CRACKING OF HYDROCARBONS... 

Detemmerman and Froment [1998] carried out a three dimensional coupled CFD simulation of furnaces and reactor tubes for the millisecond thermal cracking of propane into ethylene in the presence of steam, a process already discussed in Chapters 1 and 9. 

The coupled furnace-reactor simulation requires an accurate description of the heat transfer from the furnace to the reactor. The global radiative heat transfer from the furnace to the reactor was calculated by the zone method (Fig. 12.5.A-2, Left) proposed by Hottel and Sarofim [1967], To take into account the local influence of radiative heat transfer, CFD simulations of the furnace were carried out using a radiative heat transfer model for short distances [De Marco and Lockwood, 1975], Knowledge of the local flue gas composition is required to calculate the heat release by combustion in each flue gas volume element and the absorption coefficients for radiation. Coupled CFD simulations of the reactor tubes and furnace predict the process gas conversion and the product yields, as well as coke formation rates. 

A very similar combined process scheme integrating OCM and steam reforming of methane (SRM) was suggested at the same time by Tiemersma et al [44]. OCM is performed in membrane reactor for distributed O2 feeding. The membrane reactor tubes are immersed into a fluidized-bed reforming reactor for optimal heat transfer. Simulation of such process based on kinetic data obtained... 

With reference to the formaldehyde reactor, assuming the parameters reported in Table 28.5, the simulations showed that the HCHO molar yield could be incremented from the 93.6%, reported for an optimized packed-bed reactor process,up to over 97% if aluminum-washcoated honeycombs with suitable design were loaded in the original reactor tubes. The optimal performance of the monolith catalysts originates... 

Further simulations showed that a high (>95%) HCHO yield can be achieved even in the case of reactor tubes with a diameter incremented from one, the current industrial standard, to three inches, which would afford important savings in reactor investment costs. However, the volume fraction of the conductive monolith support needs to be incremented by a factor of four to compensate for the greater heat-transfer resistances. For all the simulated conditions the estimated pressure drop was less than 1% of the inlet pressure, versus over 10% in the industrial packed-bed reactor. 

In the following section we will simulate a reactor tube by using three different models ... 

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. 

For the design presented in Figure B.10.1. the reactor was simulated with catalyst in 2-in (50.4 mm) diameter tubes, each 20 feet (6.096 m) long, and with a cocurrent flow of a heat transfer medium on the shell side of the shell-and-tube reactor. The resulting arrangement gives a 90% conversion of IPA per pass. 

Five percent random error was added to the error-free dataset to make the simulation more realistic. Data for kinetic analysis are presented in Table 6.4.3 (Berty 1989), and were given to the participants to develop a kinetic model for design purposes. For a more practical comparison, participants were asked to simulate the performance of a well defined shell and tube reactor of industrial size at well defined process conditions. Participants came from 8 countries and a total of 19 working groups. Some submitted more than one model. The explicit models are listed in loc.cit. and here only those results that can be graphically presented are given. 

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

---