Reactor transient experiments, flow

Transient experiments were performed by means of two 4-way switch valves which allow a rapid cross change between inlets and outlets, so that the reactants are instantaneously fed to or released from the reactor. The same flow rate is always set for the two streams entering each valve, thus ensuring that the only variation affecting the reacting mixture is the change of reactant concentrations. 

Various reactors can be used for dynamic experiments. The application of recycle reactors to the investigation of the underlying reaction networks has been demonstrated by Bennett [58], who operated a recycle reactor with the dynamic method by superimposing reactant concentration pulses on the inlet stream. To ensure that the amounts of gaseous components adsorbed on the catalyst are appreciable compared to the amounts of measurable components flowing, the ratio of the catalyst to total reactor volume was made as high as possible. A reactor system, designed for transient experiments, has been described by Bennett et al. [43]. 

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 single-pellet diffusion reactor can be employed for transient experiments. Cannestra et al. [65] give an example. Gas composition was measured at the center of one-dimensional pellets. The standard single-pellet diffusion reactor was modified to allow continuous gas analysis and miniaturized in order to reduce the time constants of gas flow mixing. A rather simple model for data evaluation used by the authors was not able to predict major features of the response measurments at the pellet center but gave qualitatively correct results of the external concentration responses. This demonstrates the necessity of an elaborate modeling of instationary multicomponent diffusion and porous structure for this type of reactor. 

While flow reactors operate at conditions that closely mirror the operating environment witnessed in practice, these reactors can typically only offer a global kinetic description and lack the details of elementary reaction steps and mechanism that reveal how materials operate on a more fimdamental level. If the goal is catalyst development, then one typically needs more detailed rate expressions that describe elementary reaction steps for the development of a mechanism. The goal is to understand not only more than just the properties of the catalyst but also how it functions. In order to understand such details, transient experiments will provide the most insight into the elementary steps as well as secondary processes (e.g., surface/bulk diffusion) that make up the complex catalytic system. 

Very important transient experiments have been performed in some experimental fast reactors. A loss-of-flow without scram at a nominal power of 22.4 MWth was performed at Rapsodie on 15 April 1983. In this test the maximum fissile subchannel temperature rose to 800°C, while the nuclear power decreased continuously without any intervention from the... 

The simple form of time derivative of concentration was used in classical experiments in physical chemistry to express the rate of reaction. This must be changed to satisfy the condition in industrial reactors in which many other physical changes, such as flow and diffusion occur and for which conditions are frequently in a transient state. These forms are reviewed here. 

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. 

In addition to the high-pressure assembly, the modified system incorporates a new real-time data collection system coupled with a PC based computer. Experimental parameters, such as the valve firing sequence and the reactor temperature-control program, can be set from the computer. Reactants are introduced through two high-spe pulse valves or two continuous feed valves that are fed by mass flow controllers. In high-speed transient response experiments, the QMS is set at a particular mass value and the intensity variation as a function of time is obtained. In steady-flow experiments. 

Transient kinetic phenomena of another type were observed in the so-called purging experiments, whereby we switched from feeding the flow reactor with a helium-butyl alcohol mixture to one with pure helium and then back to the previous helium-butyl alcohol. A typical response of a catalyst to such purging is given in Fig. 6, referring to the dehydration of sec- and isobutyl alcohols over HZSM-5. For sec-butyl alcohol, the rate of butene formation initially increases by a factor of about 10 upon purging and then drops to zero. Return (8k) to the... 

A modular reactor similar to the approach of Adler et al. [11] was introduced by Muller and co-workers [37, 38, 56], The screening procedure was separated into a number of process operations. In chemical process engineering, these so-called unit operations are essential components of every complex plant. As catalyst screening involves many different processes such as heat exchange, flow distribution, sampling, analysis and reaction, such a subdivision into unit operations is justified. The flexibility of such a system was demonstrated with two exemplified configurations later, one of which was used for transient studies and one for steady-state experiments (Figure 3.30). 

The composition and reactivity of the carbon laid down during the initial stages of the propane dehydrogenation reaction was examined by transient isotope labelling experiments using [2-]3C]-C3HgandC3JHs as tracers in a series of reactions in a pulsed flow microcatalytic reactor. In these experiments alternate series of labelled and unlabelled propane pulses were passed over the catalyst sample and the products analysed by glc and mass spectrometry. 

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 ... 

---