Catalytic steady operation

Producing H2 from hydrocarbons such as natural gas is currently practiced in the chemical industry 25-28 under steady-state conditions with carefully controlled catalytic unit operations. The overall process is as shown in Fig. 7.13. 

Since the catalytic cycle operates with relatively rapid kinetics, E and ES will obtain a steady state governed by Equations (4.2) and (4.3) and the quasi-steady state concentrations of enzyme and complex will change rapidly in response to relatively slow changes in [S]. Thus the quasi-steady approximation is justified based on a difference in timescales between the catalytic cycle kinetics and the overall rate of change of biochemical reactions. 

We analyze a few of the simplest catalytic systems following. In the systems under consideration, the constituent elementary transformations usually are linear in respect to the concentrations of catalytic intermediates. Generally speaking, the results of this kind of analysis are only applicable when the inter action between the active centers, whether identical or different, of the catalyst can be ignored. This is the case mainly for homogeneous and enzymatic rather than heterogeneous catalysis. However, in some cases, the conclusions can be extended to more complex catalytic systems operating in the steady stationary states even with the lateral interaction between the active centers. 

Consider the steady operation of a fixed-bed catalytic reactor in which only heterogeneous reactions occur. The practical measure of reaction space is now the catalyst mass rather than the reactor volume, which can vary according to (i) the density to which the catalyst bed is packed and (ii) the volume fraction of inert particles used by the experimenter to dilute the catalyst, thus reducing temperature excursions in the reactor. Let mc z) be the catalyst mass in an interval (0, z) of the reactor, and let R,Zimc be the molar production rate of species i in a catalyst mass increment Arric. Then Eq. (3.1-4) for that increment takes the form... 

Figure 3 shows the steady-state radial temperature profiles for the two adiabatic catalytic beds operating at conditions of the optimal point. The corresponding axial temperature profiles in the interbed heat exchanger are also included in Fig. 3, for the tube side (Tt) and shell side (Tsh). The simulation results have been compared with industrial data corresponding to a large scale ammonia converter. The deviations at the reactor outlet were less than 0.2% (relative error) in composition and 14 °C in temperature (Toutz)-... 

We now consider the following simple model for a two-dimensional catalytic microstructure operating under steady-state conditions. The transport and kinetics at steady-state can be quantified using the following simple scheme ... 

Of main interest was to assess the effect of the aforementioned parameters on two characteristic times that describe the microreactor start-up. The ignition time (hg) was defined as the elapsed time required to reach 50% of methane conversion at the channel outlet, and the steady-state time (ts,) as the elapsed time whereby the outlet gas temperature varied by less than 10 K. The former definition was meaningful, since complete methane conversion (> 99.99%) could be achieved under catalytic mass-transport-limited steady operation for all conditions examined. Moreover, by running a steady-state version of the code [2], it was confirmed that the adopted definition of steady state in the transient simulations reproduced the true steady-state outlet temperature within 1 K. 

Heterogeneous catalytic systems offer the advantage that separation of the products from the catalyst is usually not a problem. The reacting fluid passes through a catalyst-filled reactor m the steady state, and the reaction products can be separated by standard methods. A recent innovation called catalytic distillation combines both the catalytic reaction and the separation process in the same vessel. This combination decreases the number of unit operations involved in a chemical process and has been used to make gasoline additives such as MTBE (methyl tertiai-y butyl ether). 

In the major catalytic processes of the petroleum and chemical industries, continuous and steady state conditions are the rule where the temperature, pressure, composition, and flow rate of the feed streams do not vary significantly. Transient operations occur during the start-up of a unit, usually occupying a small fraction of the time of a cycle from start-up to shut-down for maintenance or catalyst regeneration. 

Conclusion when using ionic conductors where the conducting, i.e. backspillover ion participates in the catalytic reaction under study (e.g. O2 ions in the case of catalytic oxidations) then both galvanostatic and potentiostatic operation lead to a steady-state and allow one to obtain steady-state r vs Uwr plots. 

S. Tagliaferri, R.A. Koeppel, and A. Baiker, Influence of rhodium- and ceria-promotion of automotive palladium catalyst on its catalytic behaviour under steady-state and dynamic operation, Appl. Catal. B 15,159-177 (1998). 

To verify that steady state catalytic activity had been achieved, the catalyst was allowed to operate uninterrupted for approximately 8 hours. The catalyst was then removed from the reactor and the surface investigated by XPS. The results are shown in Figure 2c. The two major changes in the XPS spectrun were a shift in the iron 2p line to 706.9 eV and a new carbon Is line centered at 283.3 eV. This combination of iron and carbon lines indicates the formation of an iron carbide phase within the XPS sampling volume.(J) In fact after extended operation, XRD of the iron sample indicated that the bulk had been converted to FecC2 commonly referred to as the Hagg carbide.(2) It appears that the bulk and surface are fully carbided under differential reaction conditions. 

Weaver [40] studied alternate cathode materials at 650 °C, finding several that performed well. Steady-state polarization on Ni, Co and Fe porous electrodes operating as cathodes in a MCFC, with a standard (Li/K)2 C03 tile is shown in Figs. 30-32. Note that the oxidant gas fed to these cathodes is, in normal MCFC operation, the fuel, composed of 32.5% H2, 17.5% COz, 17.5% H20, the balance N2. Polarizations were first taken with this clean gas where the only reaction can be Eq. (35). After steady-state was attained, 0.65% H2S was added and sufficient time allowed for the electrode to convert to the sulfides. After 24 hours, the outlet H2S reached the inlet level and polarizations were measured. Note in Figs. 30-32, that the performance with H2S is significantly improved over the clean gas. (The Ni sample was a commercial (Gould) MCFC electrode the Co and Fe were pressed from powders. Each gas was 8 sq cm in superficial area). The improvement is probably due to a catalytic mechanism involving sulfur interactions with the electrode, as, for Co ... 

Restelli and Coull [AIChE J., 72 (292), 1966] have studied the transmethylation reaction of dimethylamine in a differential flow reactor using montmorillonite as a catalyst. They measured initial reaction rates under isothermal conditions for this heterogeneous catalytic process. Steady-state operating data were recorded. 

The attrition rate, i.e., the rate of generation of fines, 0-d microns, at the submerged jets in a fluidized bed, tends to fall off asymptotically with time to a steady-state rate as shown in Fig. 9. Initially the attrition rate is high due to the wearing off of angular comers. Typically, it takes long time, hours to days, for the particles to reach steady-state (equilibrium) where the particles tend to be more rounded. For most catalytic fluidized bed processes, the bed operates at equilibrium. That means the most significant part of the attrition rate curve is the steady-state rate. 

Kinetic investigations of catalytic processes under transient conditions have to take into account this problem (see e.g. (4 ), where the macrorelaxation of the redox type reaction has been suppressed by means of a specific periodic operation). Kinetic expressions obtained by dynamic methods in general would give a better understanding of the rate law than those obtained from steady state measurements. 

Specific Remarks. The established dependence of the microkinetics on the oxidation state of the catalyst make clear that a) results of kinetic investigations at lower temperatures are different in respect to the mechanistic scheme from those obtained at higher temperatures, b) in a distributed catalytic system in the steady state a distribution of the catalytic steps is possible as a direct consequence of the ambient gas concentration profile and the axial temperature distribution in an extreme situation it is conceivable that at the reactor inlet, another mechanism dominates as at the reactor exit. These two facts can perhaps explain some contradictory results about the same reaction scheme which have been reported in the past by different authors. As stated recently by Boreskov (19) in a review paper, this conclusion holds true for the most catalytic systems under the technical operating conditions. 

The interest in the dynamic operation of heterogeneous catalytic systems is experiencing a renaissance. Attention to this area has been motivated by several factors the availability of experimental techniques for monitoring species concentrations both in the gas phase and at the catalyst surface with a temporal resolution and sensitivity not previously possible, the development of efficient numerical methods for predicting the dynamics of complex reaction systems, and the recognition that in selected instances operation of a catalytic reactor under dynamic conditions can yield a better performance than operation under steady-state conditions. 

A semicontinuous reactor is a reactor for a multiphase reaction in which one phase flows continuously through a vessel containing a batch of another phase. The operation is thus unsteady-state with respect to the batch phase, and may be steady-state or unsteady-state with respect to the flowing phase, as in a fixed-bed catalytic reactor (Chapter 21) or a fixed-bed gas-solid reactor (Chapter 22), respectively. 

In a fixed-bed catalytic reactor for a fluid-solid reaction, the solid catalyst is present as a bed of relatively small individual particles, randomly oriented and fixed in position. The fluid moves by convective flow through the spaces between the particles. There may also be diffusive flow or transport within the particles, as described in Chapter 8. The relevant kinetics of such reactions are treated in Section 8.5. The fluid may be either a gas or liquid, but we concentrate primarily on catalyzed gas-phase reactions, more common in this situation. We also focus on steady-state operation, thus ignoring any implications of catalyst deactivation with time (Section 8.6). The importance of fixed-bed catalytic reactors can be appreciated from their use in the manufacture of such large-tonnage products as sulfuric acid, ammonia, and methanol (see Figures 1.4,11.5, and 11.6, respectively). 

In this section, the analysis of the data reconciliation problem is restricted to quasi-steady-state process operations. That is, those processes where the dominant time constant of the dynamic response of the system is much smaller than the period with which disturbances enter the system. Under this assumption the system displays quasi-steady-state behavior. The disturbances that cause the change in the operating conditions may be due to a slow variation in the heat transfer coefficients, catalytic... 

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