Adiabatic reactor simulations

Adiabatic Reactors. Like isothermal reactors, adiabatic reactors with a flat velocity profile will have no radial gradients in temperature or composition. There are axial gradients, and the axial dispersion model, including its extension to temperature in Section 9.4, can account for axial mixing. As a practical matter, it is difficult to build a small adiabatic reactor. Wall temperatures must be controlled to simulate the adiabatic temperature profile in the reactor, and guard heaters may be needed at the inlet and outlet to avoid losses by radiation. Even so, it is hkely that uncertainties in the temperature profile will mask the relatively small effects of axial dispersion. 

In many respects, the solutions to equations 12.7.38 and 12.7.47 do not provide sufficient additional information to warrant their use in design calculations. It has been clearly demonstrated that for the fluid velocities used in industrial practice, the influence of axial dispersion of both heat and mass on the conversion achieved is negligible provided that the packing depth is in excess of 100 pellet diameters (109). Such shallow beds are only employed as the first stage of multibed adiabatic reactors. There is some question as to whether or not such short beds can be adequately described by an effective transport model. Thus for most preliminary design calculations, the simplified one-dimensional model discussed earlier is preferred. The discrepancies between model simulations and actual reactor behavior are not resolved by the inclusion of longitudinal dispersion terms. Their effects are small compared to the influence of radial gradients in temperature and composition. Consequently, for more accurate simulations, we employ a two-dimensional model (Section 12.7.2.2). 

A reversible reaction, At= B, takes place in a well-mixed tank reactor. This can be operated either batch-wise or continuously. It has a cooling jacket, which allows operation either isothermally or with a constant cooling water flowrate. Also without cooling it performs as an adiabatic reactor. In the simulation program the equilibrium constant can be set at a high value to give a first-order irreversible reaction. 

In many situations, the monolith reactor can be represented by a single channel. This assumption is correct for the isothermal or adiabatic reactor with uniform inlet flow distribution. If the actual conditions in the reactor are significantly different, more parallel channels with heat exchange have to be simulated (cf., e.g. Chen et al., 1988 Jahn et al., 1997, 2001 Tischer and Deutschmann, 2005 Wanker et al., 2000 Young and Finlayson, 1976). In this section we will further discuss effective single channel models. 

Tubular reactors can be simulated using Aspen Plus. Several configurations are available constant-temperature reactor, adiabatic reactor, reactor with constant coolant temperature, reactor with countercurrent flow of coolant, and reactor with co-current flow of coolant. The isothermal reactor cannot be exported into Aspen Dynamics because it is not possible to dynamically control the temperature at all axial positions. Therefore only the last four types will be discussed. 

However, some numerical difficulties are sometimes encountered in running the simulation in Aspen Dynamics when a large number of lumps are used. The 50-lump case runs very slowly or not at all in the adiabatic reactor cases. In this situation the number of lumps is reduced to get reasonable computing times. 

Simulation examples of four types of tubular reactors have been presented in the sections above. The adiabatic and constant-coolant temperature models are easier to set up and seem to run with fewer problems. In the adiabatic reactor the only variable that can be controlled is the inlet temperature. In the cooled reactors a temperature can be controlled by manipulating either the coolant temperature or the coolant flowrate, depending on the model. 

As a consequence of this explanation the reaction runaway to total methanation is not a necessary condition for the observed phenomenon. Any simple exothermic two phase reaction in an adiabatic reactor ought to show the same behaviour provided that one phase with a high throughput is used to carry the heat out of the reactor and the flow is suddenly reduced. This will be shown in the following simulation results. Due to problems with the numerical stability of the solution (see Apendix) only a moderate reaction rate will be considered. Reaction parameters are chosen in such a way that in steady state the liquid concentration Cf drops from 4.42 to 3.11 kmol/m3 but the temperature rise is only 3°C (hydrogen in great excess). At t = 0 the uniform flow profile... 

Energy Balance. It was decided that a simple, fixed-bed, adiabatic reactor would be required In this process. Since the reactions Involved release considerable heat, this Influences the local temperature, which In turn Influences the reaction rates. An energy balance, or heat balance, having the following general form, was added along with the mass balance In all subsequent simulations ... 

Small laboratory reactors are most easily operated as isothermal reactors. To simulate a commercial adiabatic reactor, the temperature in the isothermal laboratory... 

You are to simulate a single-phase adiabatic reactor in which a single gas-phase reaction takes place. The reaction has the general form... 

Grwin and Luss [ref- 37 simulated an adiabatic reactor for a first order irreversible reaction with an activation energy exceeding that for the coke formation and an exponential deactivation function in terms of the coke content The results of the... 

Brito-Alayon et al [ref. 38] simulated adiabatic reactors for processes with diffusional limitations The combined effect of the parameter values chosen for the adsorption constants and of the temperature profile could lead to an increasing coke profile, even... 

A kinetic reaction model was developed and used with a process simulator to conduct reactor design [7]. The model was also used to investigate the optimum catalyst loading pattern to achieve the highest conversion and aromatics yield in the demonstration plant which consists of three reactors with preheaters in series. This was because a large temperature decrease was expected through the adiabatic reactors of the plant due to the endothermic nature of the whole reactions. 

Anderson, D.H., and A.V. Sapre, Simulation of Heat Effects in Laboratory Adiabatic Reactors, presented at 1986 Annual AIChE Meeting, Miami Beach, November 2-7, 1986. 

This figure clearly illustrates that the range within which multiple steady states can occur is very narrow. It is true that, as Hlavacek and Hofmann calculated, the adiabatic temperature rise is sufficiently high in ammonia, methanol and oxo-synthesis and in ethylene, naphthalene, and o-xylene oxidation. None of the reactions are carried out in adiabatic reactors, however, although multibed adiabatic reactors are sometimes used. According to Beskov (mentioned in Hlavacek and Hofmann) in methanol synthesis the effect of axial mixing would have to be taken into account when Pe < 30. In industrial methanol synthesis reactors Pe is of the order of 600 and more. In ethylene oxidation Pe would have to be smaller than 200 for axial effective transport to be of some importance, but in industrial practice Pe exceeds 2500. Baddour et al. in their simulation of the TVA ammonia synthesis converter found that the axial diffusion of heat altered the steady-state temperature profile by less than 0.6°C. Therefore, the length of... 

The aforementioned researchers have utilized experimental and computer simulation studies of this reaction to assess the performance of adiabatic reactors subjected to various modes of operation. An analysis of the performance of a plug flow reactor operated adiabaticaUy with a feed entering at 20°C that is 0.4 M in thiosulfate and 0.6 M in hydrogen peroxide indicates that the space time necessary to achieve 70% conversion of the limiting reagent is 38.9 s. 

A methanation section consisting of three adiabatic reactors is used to simulate the fixed bed with the intermediate cooling configuration (reactors 11, 13, 15, see Figures 8.4 and 8.5). 

By feeding the inlet reactant and coolant streams into separate ports of the reactor, it was possible to achieve five different flow schemes, shown in Fig. 3. An adiabatic reactor was simulated by feeding a stream of preheated air and CO to the catalyst containing reaction pass and by sealing off the coolant pass. Similarly, the cocurrent and countercurrent schemes were approximated by flowing coolant and reactant streams either into adjacent ports or into ports which lie at opposite ends of the reactor-heat exchanger respectively. 

With these kinetics, simulate the reactor in Figure 1.5 as a two-stage packed-bed adiabatic reactor... 

The simulation of an adiabatic reactor for the oxidation of SOj was carried out by Minhas and Carberry (1969). With the assumptions of negligible axial dispersion and constant fluid velocity, the conservation equations (Eqs. 9.27 and 9.28) reduce to ... 

Simulation results for an adiabatic reactor undergoing uniform deactivation (Lee and Butt 1982) are shown in Figures 10.6 and 10 7 for the reaction system given in Table 10.5. These results were obtained according to the procedures of... 

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