Reactor transient response study

In this paper we will first describe a fast-response infrared reactor system which is capable of operating at high temperatures and pressures. We will discuss the reactor cell, the feed system which allows concentration step changes or cycling, and the modifications necessary for converting a commercial infrared spectrophotometer to a high-speed instrument. This modified infrared spectroscopic reactor system was then used to study the dynamics of CO adsorption and desorption over a Pt-alumina catalyst at 723 K (450°C). The measured step responses were analyzed using a transient model which accounts for the kinetics of CO adsorption and desorption, extra- and intrapellet diffusion resistances, surface accumulation of CO, and the dynamics of the infrared cell. Finally, we will briefly discuss some of the transient response (i.e., step and cycled) characteristics of the catalyst under reaction conditions (i.e.,... 

Apparatus and Procedure. The kinetic studies of the catalysts were carried out by means of the transient response method (7) and the apparatus and the procedure were the same as had been used previously (8). A flow system was employed in all the experiments and the total flow rate of the gas stream was always kept constant at 160 ml STP/min. In applying the transient response method, the concentration of a component in the inlet gas stream was changed stepwise by using helium as a balancing gas. A Pyrex glass tube microreactor having 5 mm i.d. was used in a differential mode, i.e. in no case the conversion of N2O exceeded 7 X. The reactor was immersed in a fluidized bed of sand and the reaction temperature was controlled within + 1°C. 

Transient simulations using the full, nonlinear model show that under the conditions studied concentration profiles reach a quasi steady state quite rapidly (often within 3 to 5 sec), whereas the thermal response of the reactor bed is much slower22 due to the large heat capacity of the reactor bed and thermal well. An example of this phenomenon is shown in Fig. 18, where the transient responses of the solid temperatures, thermal well temperatures, and concentrations are shown for a major step change in the inlet gas temperature and inlet CO concentration. In this example, the effect of the step change is nearly immediate on the concentration profiles, with the major effect being within the first 10 sec. However, Fig. 18a shows that the thermal well temperatures and the catalyst temperatures take up to 10 times as long as the... 

In critical cases it may well be worthwhile to make a complete analysis of stability. In many cases, however, enough can be learned by studying what Bilous and Amundson (B7) called parametric sensitivity. These authors derived formulas for calculating the amplification or attenuation of disturbances imposed on an unpacked tubular reactor originally in a steady state, with the idea that if the disturbances grow unduly the performance of the reactor is too sensitive to the conditions imposed on it, that is, to the parameters of the system. The effect of feedback from a control system was not considered. As pointed out by the authors, it would be a much more complicated task to apply their procedure to a packed reactor, but it still would entail far less computation than a study of the transient response. 

The basic ideas of the transient method applied to heterogeneous catalysis were set forth by K. Tamaru. In general, the term transient refers to changing one or more of the system parameters. In transient kinetic studies, a dynamic change is introduced into a reactor system, and the response of a reaction quantity is observed. A typical transient response experiment is sketched in Figure 8.3. 

A follow-up design is given in the GT-MHR (Modular Helium Reactor) (see also section 4.7.2.) with a higher power output of 600 MW(th). A standard plant is planned consisting of four of those units. Helium inlet/outlet temperatures are 485 and 850 °C, respectively. The cycle efficiency is predicted to be 47 % [51]. Follow-on evaluations which need to be done include the study of transient response of plant components to normal and off-normal events, impact of turbine contamination, and confirmation of plant efficiency [47]. 

In recent years the nonsteady state mode has been used to an increasing extent because it permits accessing intermediate steps of the overall reaction. Very complete reviews of this topic are presented by Mills and Lerou [1993] and by Keil [2001]. Specific reactors have been developed for transient studies of catalytic reaction schemes and kinetics. One example is the TAP-reactor ( Transient Analysis of Products ) that is linked to a quadrupole mass spectrometer for on line analysis of the response to an inlet pulse of the reactants. The TAP reactor was introduced by Cleaves et al. in 1968 and commercialized in the early nineties. An example of appUcation to the oxidation of o.xylene into phthalic anhydride was published by Creten et al. [1997], to the oxidation of methanol into formaldehyde by Lafyatis et al. [1994], to the oxidation of propylene into acroleine by Creten et al. [1995] and to the catalytic cracking of methylcyclohexane by Fierro et al. [2001], Stopped flow experimentation is another efficient technique for the study of very fast reactions completed in the microsecond range, encountered in protein chemistry, e.g., in relaxation techniques an equilibrium state is perturbed and its recovery is followed on line. Sophisticated commercial equipment has been developed for these techniques. 

The TAP reactor is very well suited for kinetic studies. At low pressure, all transport of gas-phase species is by (Knudsen) diffusion, thus ruling out any external mass transfer limitations. The diffusion as a random movement also eliminates all radial concentration gradients. Very low amounts of reactants are pulsed into the reactor, which are on the order of a few nanomoles. Thus, the amount of heat generated is very small even in the case of strongly exo- or endothermic reactions. Therefore, the reactor is operated isothermally and no heat transfer limitations occur. Concentration profiles inside the pores for transient experiments might arise even in the absence of chemical reaction. If significant diffusion of reactants and products inside the catalyst pores occurs, it will be revealed by the transient response and then needs to be addressed correctly by a modeling approach. This is often the case for microporous materials [26,27,72]. 

The catalytic properties of the shock-modified rutile whose defect properties have been reported in previous sections of this chapter have been studied in a flow reactor used to measure the oxidation of CO by Williams and coworkers [82G01, 86L01]. As shown in Fig. 7.7 the effect of shock activation is substantial. Whereas the unshocked material displays such low activity that an effect could only be observed at the elevated temperature of 400 °C, the shock-modified powder shows substantially enhanced catalytic activity with the extent of the effect depending on the shock pressure. After a short-time transient is annealed out, the activity is persistent for about 8 h. Although the source of the surface defects that cause the activity is not identified, the known annealing behavior of the point defects indicates that they are not responsible for the effect. 

In 2001, Holzwarth et al. [125, 136] stressed the importance of transient studies as an alternative to steady continuous reactor operations. A combination of micro kinetic analysis together with transient experiments allowed the determination of the global catalytic conversion from elementary reaction steps. A prerequisite for such an analysis is the correlation of experimental data with the data of a model. In Figure 3.85, experimental and model responses of an impulse of reactants were correlated. Agreement between the data helped to derive the reaction mechanism. 

The plug flow reactor is increasingly being used under transient conditions to obtain kinetic data by analysing the combined reactor and catalyst response upon a stimulus. Mostly used are a small reactant pulse (e.g. in temporal analysis of products (TAP) [16] and positron emission profiling (PEP) [17, 18]) or a concentration step change (in step-response measurements (SRE) [19]). Isotopically labeled compounds are used which allow operation under overall steady state conditions, but under transient conditions with respect to the labeled compound [18, 20-23]. In this type of experiments both time- and position-dependent concentration profiles will develop which are described by sets of coupled partial differential equations (PDEs). These include the concentrations of proposed intermediates at the catalyst. The mathematical treatment is more complex and more parameters are to be estimated [17]. Basically, kinetic studies consist of ... 

Heterogeneously catalyzed reactions are usually studied under steady-state conditions. There are some disadvantages to this method. Kinetic equations found in steady-state experiments may be inappropriate for a quantitative description of the dynamic reactor behavior with a characteristic time of the order of or lower than the chemical response time (l/kA for a first-order reaction). For rapid transient processes the relationship between the concentrations in the fluid and solid phases is different from those in the steady-state, due to the finite rate of the adsorption-desorption processes. A second disadvantage is that these experiments do not provide information on adsorption-desorption processes and on the formation of intermediates on the surface, which is needed for the validation of kinetic models. For complex reaction systems, where a large number of rival reaction models and potential model candidates exist, this give rise to difficulties in model discrimination. 

Kinetics can also be studied at surface science conditions. Feed can be leaked at a constant rate into the chamber containing the crystal face, and the gas is removed at a constant rate by the pumps. The composition of the chamber gas can be continuously monitored by mass spectrometry. The pressure in the reaction chamber is low enough to ensure Knudsen flow The gaseous molecules collide almost exclusively with the exposed solid surfaces, and the system behaves as a perfectly mixed flow reactor (CSTR). Experiments in the transient regime with various forcing functions can be performed, and response times can be orders of magnitude smaller than those at atmospheric pressure. The catalytic oxidation of CO on Pt(llO) was one of the first studies of this type (33). 

The response of the reactor on a step change in the inlet temperature was studied [25] for the simultaneous oxidation of CO, propene, methane, and hydrogen, using adapted kinetics [15]. Accumulation of heat in the solid phase was considered as transient, while... 

Typical commercial cell culture systems include batch or fed-batch suspension reactors and perfused immobilized-cell reactors. However, the transient nature of batch culture causes difficulties in studying the effects of external stimuli on growth, metabolism and product formation. Due to metabolite concentration gradients, and the difficulty of obtaining representative cell samples, immobilized-cell reactors are also poorly suited for the analysis of cell growth and metabolism. As a result it is desirable to use well-defined model systems. Continuous-flow suspension reactors allow metabolic parameters to be measured at steady state, after cells have adapted to new (and possibly inhibitory) conditions. Perfusion reactors (with cells immobilized on suspended or stationary supports) extend these benefits to anchorage-dependent cells, and provide model systems for cell responses in vivo. However, while it is instructive to study the behaviour of cells under well-defined conditions, the results obtained must be verified in the culture system selected for commercial production. 

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