Transient reactor operation

With the introduction of microreactors, transient reactor operations became interesting due to their low internal reactor volume and, thus, fast dynamic behavior. In 1999, Liauw et al. presented a periodically changing flow to prevent coke development on the catalyst and to remove inhibitory reactants in an IMM microchan-nel reactor [58]. This work was preceded by Emig in 1997, of the same group, who presented a fixed-bed reactor with periodically reversed flow [59]. In 2001, Rouge et al. [14] presented the catalytic dehydration of isopropanol in an IMM microreactor. 

Screening in stationary mode will only give information about the activity of a single catalyst or a catalyst mixture. When a proper catalyst for a certain reaction is found, the next important information is the reaction kinetics. To obtain this information, several methods and reactors are recommended in the literature [66-73]. Most of them apply transient reactor operations to find detailed kinetic information. Microreactors are particularly suited for such an operation since their low internal reaction volumes enable a fast response to process parameter changes, e.g., concentration or temperature changes. This feature was already applied by some authors to increase the product yield in microreactors [70, 74, 75]. De Belle-fon [76] reported a dynamic sequential method to screen liquid-liquid and liquid-... 

A consecutive reaction mechanism was also proposed by Cleaves and Centi (11). This mechanism was based on experimental work to back up the theoretical calculations of Schitt and Jorgensen. Although the proposed intermediates were not detected under reaction conditions, they have been observed with fuel-rich gas feeds and under conditions of transient reactor operation. Using a TAP (temporal analysis of products) reactor, the researchers detected products in the following order of formation butane —> butene butadiene furan. However, the... 

Rates of reaction TZj have been defined here as frequencies of reaction events. Their computability from rates Rj of species production is treated in the following section. Time derivatives are not needed here, but will arise in Chapter 3 for transient reactor operations. 

Corresponding relations hold for the local rates Rj of Eq. (2.1-7) and also for the macroscopic extents of production = f R Q dt, obtained from overall material balances on transient reactor operations with one reaction. 

Reactor control concerns the system properties from the control viewpoint, and the determination of criteria of control and optimization. System properties include dynamic properties such as the nature of system response to small and large disturbances. Regular transient reactor operation may be of future importance even for some product formation processes (43). Elucidation of input operating variables affecting a process is then valuable for reactor control. In this connection the parametric... 

Analogous equations can be used with any other instantaneous distribution. This relatively easy integration extends the use of instantaneous distributions to transient reactor operation and considerably broadens the use of this powerful technique. When compared with the method of moments, instantaneous distribution allows for the complete prediction of CLD and CCD instead of only averages when contrasted to the full solution of the population balances, the method of instantaneous distributions provides the same information at a much shorter time using a more elegant solution, allows the modeler to analyze the problem with a simple glance at the equation, and can even be implemented on simple commercial spreadsheets for easy calculation. 

Transient reactor operation plays an increasingly important role in bioprocessing and has to some extent already been considered (classification, see Fig. 3.31 fed-batch culture, see Fig. 3.37 situation, see Fig. 4.4 guidelines to solution, see Sect. 4.2 and Fig. 4.5 structured cell model concept, see Fig. 4.7 application, see Chap. 6). Both balanced and frozen conditions have also been considered in Fig. 3.34. A biosystem is in balanced condition when the mechanism is fully adapted, as in a quasi-steady-state (if x ). All different equations can be reduced to algebraic equations. A biosystem is in frozen condition of the initial state (if x x ) and the mechanism may be neglected due to the fact that the slowest step is rate determining ( rds concept ). By this procedure, equations are reduced to parameters so that the number of equations is reduced (e.g., the case of dropwise addition of substrate). This is the case of steady state CSTR. 

Variable Volume CSTR Operation (Fed-Batch and Transient Reactor Operation)... 

A transient, is a passing event which may upset the reactor operation but does not physically damage the primary cooling envelope. Table 6.1-1 lists PWR transient initiating events that ha c been used in PRA preparation. Typical frontline systems that mitigate LOCAs and transients for a PWR are presented in Table 6.1-2. The frontline systems must be supported by support systems interactions between both are presented in Table 6.1 -3 for ANO-1 (Arkansas Nuclear Unit 1). 

Using Equation (13), the external mass transfer coefficient at 723 K was calculated to be 60 cm/s. Since the reactor operating conditions at th s temperature (723 K, slightly above atmospheric pressure, 247 cm /s) were very similar to those of our transient chemisorption experiments, the external mass transfer coefficient calculated above was used for the simulations. 

The chemical reactor is the unif in which chemical reactions occur. Reactors can be operated in batch (no mass flow into or out of the reactor) or flow modes. Flow reactors operate between hmits of completely unmixed contents (the plug-flow tubular reactor or PFTR) and completely mixed contents (the continuous stirred tank reactor or CSTR). A flow reactor may be operated in steady state (no variables vary with time) or transient modes. The properties of continuous flow reactors wiU be the main subject of this course, and an alternate title of this book could be Continuous Chemical Reactors. The next two chapters will deal with the characteristics of these reactors operated isothermaUy. We can categorize chemical reactors as shown in Figure 2-8. 

The real power of the model developed in this work lies in the transient or dynamic simulations such as those necessary for control system design. The model we have developed can be used to simulate the effects on the reactor of various process disturbances and input changes. Under normal reactor operating conditions, step or pulse changes in inlet gas temperatures, concentrations, or velocity or changes in cooling rates can significantly affect... 

In 2001, Mirodatos et al. [89] stressed the importance of transient studies as an alternative to steady continuous reactor operations. A combination of microkinetic analysis together with transient experiments should allow the determination of the global catalytic conversion from elementary reaction steps. Prerequisite for such analysis is the correlation of experimental data with the data of a model. Compliance between the data helps to derive the reaction mechanism. 

Then, a survey of micro reactors for heterogeneous catalyst screening introduces the technological methods used for screening. The description of microstructured reactors will be supplemented by other, conventional small-scale equipment such as mini-batch and fixed-bed reactors and small monoliths. For each of these reactors, exemplary applications will be given in order to demonstrate the properties of small-scale operation. Among a number of examples, methane oxidation as a sample reaction will be considered in detail. In a detailed case study, some intrinsic theoretical aspects of micro devices are discussed with respect to reactor design and experimental evaluation under the transient mode of reactor operation. It will be shown that, as soon as fluid dynamic information is added to the pure experimental data, more complex aspects of catalysis are derivable from overall conversion data, such as the intrinsic reaction kinetics. 

Micro reactors operated in the pulsed mode were introduced by Kokes et al. in 1955 [91], but have been intensively used only in the last 10 years. Such transient studies to obtain insight into reaction mechanisms were undertaken by Cleaves et al. with the temporal analysis of products (TAP) reactor 1997 [100], They observed rate coefficients of elementary reaction steps such as adsorption and desorption by applying pulses of reactants to a catalytic micro reactor combined with a quadrupole mass spectrometer. 

Another reactor equation useful in reaction kinetics analyses represents the transient CSTR. This situation is encountered in temperature-programmed desorption and isotope tracer experiments. The material balance for the transient CSTR operating at constant total pressure for an ideal gas is... 

An effective approach to help address the issues involved in controlling catalyst performance is to formulate and analyze reaction schemes that describe the essential chemistry taking place on the catalyst surface. This approach has been used successfully in catalysis research for many years. We suggest that this approach will see increased use in catalysis research. Specifically, continuing improvements in computer capabilities allow rapid analysis of complex reaction schemes for all common reactor configurations (e.g., reactors operating at steady state as well as under transient reaction conditions and nonisothermal reactors). Moreover, recent advances in quantum... 

There is a number of points in relation to the proposed algorithm that should be mentioned in passing. (1) Maintenance requirements were not considered in the previous equation but can be included easily once the appropriate information is available. (2) Eq. (1) is valid at steady state only however, it can be employed for biochemical reactors operating in transient because the gases are essentially at quasi-steady state with respect to the culture. 

The model described in Section 13.6 is used to simulate the transient behavior of the reactor operated isothermally at 50 °C. 

Wei ( ) presents a very brief discussion of coal gasification reactor dynamics. He describes the transient response to a small step change as a soft transience in which the movement from one steady state to another one nearby takes place as a wave through a series of pseudo steady states. He points out that the hard transience of start up and major upset in reactor operation are not well understood. One of the purposes of this paper is to increase this understanding. 

Dynamic regime (T When the period of the oscillation is of the order of the system s characteristic response lime, the system is in intermediate or dynamic periodic operation. The transient behavior of the system has to be determined to predict the effects of periodic operation. Dynamic reactor operation may result in considerably higher performance if resonance phenomena are involved, and therefore this range of operation is of particular interest for optimization of the reactor. 

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