Shock-tube reactors

Agreement of data from the wall-less reactor with conventional reactors is excellent when valid comparisons can be made. Neopentane which exhibits significant surface effects has the same kinetic constants (within experimental error) when pyrolyzed either in the wall-less(K ) or shock tube reactor(T ). Oxygen-free ethane exhibits only a very small surface effect(8). and there is excellent agreement between the wall-less and conventional static reactors as is noted in this paper. 

The solution procedure to this equation is the same as described for the temporal isothermal species equations described above. In addition, the associated temperature sensitivity equation can be simply obtained by taking the derivative of Eq. (2.87) with respect to each of the input parameters to the model. The governing equations for similar types of homogeneous reaction systems can be developed for constant volume systems, and stirred and plug flow reactors as described in Chapters 3 and 4 and elsewhere [31-37], The solution to homogeneous systems described by Eq. (2.81) and Eq. (2.87) are often used to study reaction mechanisms in the absence of mass diffusion. These equations (or very similar ones) can approximate the chemical kinetics in flow reactor and shock tube experiments, which are frequently used for developing hydrocarbon combustion reaction mechanisms. 

In terms of coupling flame generation and detection methods, several combinations are common. Generally, shock tubes are coupled with IR and UV absorption and gas chromatography (GC) detectors, while flow reactors are used in tandem with GC, electron spin resonance, and resonance fluorescence detection. 

This new technique incorporates a catalyzed short contact time (SCT) substrate into a shock tube. Fig. 13. These SCT reactors are currently used in industry for a variety of applications, including fuel cell reformers and chemical synthesis.The combination of a single pulse shock tube with the short contact time reactor enables the study of complex heterogeneous reactions over a catalyst for very well defined regimes in the absence of transport effects. These conditions initiate reaction in a real environment then abruptly terminate or freeze the reaction sequence. This enables detection of intermediate chemical species that give insight into the reaction mechanism occurring in the presence of the chosen catalyst. There is no limitation in terms of the catalyst formulations the technique can study. 

The apparatus s step change from ambient to desired reaction conditions eliminates transport effects between catalyst surface and gas phase reactants. Using catalytic reactors that are already used in industry enables easy transfer from the shock tube to a ffow reactor for practical performance evaluation and scale up. Moreover, it has capability to conduct temperature- and pressure-jump relaxation experiments, making this technique useful in studying reactions that operate near equilibrium. Currently there is no known experimental, gas-solid chemical kinetic method that can achieve this. 

To date, the only shock tube apparatus equipped to study surface reactions is the KIST facility at ATK GASL in New York. The tests done so far have studied methane oxidation, CFI4 + 2O2 CO2 + 2H2O on the surface of an SCT ferrous-based reactor impregnated with platinum based catalyst. To isolate the effects of the screen and the catalyst on the reaction, three types of tests were run catalyzed screen with combustible gases,... 

To illustrate the concepts of determining, non-determining and negligible processes, the mechanism of the pyrolysis of neopentane will be discussed briefly here. Neopentane pyrolysis has been chosen because it has been studied by various techniques batch reactor [105— 108], continuous flow stirred tank reactor [74, 109], tubular reactor [110], very low pressure pyrolysis [111], wall-less reactor [112, 113], non-quasi-stationary state pyrolysis [114, 115], single pulse shock tube [93, 116] amongst others, and over a large range of temperature, from... 

Comprehensive models aim to employ all the relevant reactions, while reduced and simplified models attempt to select the reactions of critical importance. Almost all modelling is based on, and validated by, experiments in model combustion or chemical kinetic systems. Measurements include product distributions in various types of reactor, ignition delaytimes in static reactors, rapid compression machines and shock tubes, and various types of explosion limits. These are discussed in detail in Chapter 6. 

Heat transfer to the tubes on the furnace walls is predominantly by radiation. In modern designs this radiant section is surmounted by a smaller section in which the combustion gases flow over banks of tubes and transfer heat by convection. Extended surface tubes, with fins or pins, are used in the convection section to improve the heat transfer from the combustion gases. Plain tubes known as shock tubes are used in the bottom rows of the convection section to act as a heat shield from the hot gases in the radiant section. Heat transfer in the shield section will be by both radiation and convection. The tube sizes used will normally be between 75 and 150 mm diameter. The tube size and number of passes used depend on the application and the process-fluid flow rate. Typical tube velocities will be from 1 to 2 m/s for heaters, with lower rates used for reactors. Carbon steel is used for low temperature duties stainless steel and special alloy steels, for elevated temperatures. For high temperatures, a material that resists creep must be used. 

Other researchers have recently seen evidence that is supportive of the JP-10 decomposition pathway to cyclopentene. Burcat and Dvinyaninov [12] reported that they found cyclopentene in their single-pulse shock-tube experiments of JP-10 oxidation. Green and Anderson [13] reported that they found cyclopentadiene in their recent flow-reactor measurements of JP-10 pyrolysis. 

This reaction has been studied using batch reactors, perfectly stirred continuous reactors, tubular continuous reactors, BENSON type reactors, wall-less reactors and shock tubes. The reaction has been carried out at temperatures between 700 and 1300 K, at pressures of 0.1 Pa to 10 Pa and at reaction times of 10 s to 10 s. The effects of the nature and of the area of the reactor walls as well as those of various additives have also been studied. The diversity of the studies carried out by a dozen teams throughout the world, the particularly widespread range of operating conditions (600 K for the temperature, which represents 11 orders of magnitude for the rate of initiation, 8 orders of magnitude for the pressure and reaction duration) make the pyrolysis of neopentane into a model radical reaction. 

When a reaction mixture is under the same conditions of temperature and pressure as in Figure 2, where an autoignition is likely to develop, there is a certain time lapse, called the autoignition delay, before this becomes effective. This delay is important in the Diesel engine but also in the safety of industrial combustion reactions. The measurement of these delays is carried out mainly using batch reactors, rapid compression machines and shock tubes. 

Currently, one of the most developed, hence most illustrative, examples of practical application of SM is provided by the GRI-Mech project [1]. In its latest release, the GRI-Mech 3.0 dataset is comprised of 53 chemical species and 325 chemical reactions (with a combined set of 102 active variables), and 77 peer-reviewed, well-documented, widely trusted experimental observations obtained in high-quality laboratory measurements, carried out under different physical manifestations and different conditions (such as temperature, pressure, mixture composition, and reactor conhguration). The experiments have relatively simple geometry, leading to reliably modeled transport of mass, energy, and momentum. Typical experiments involve flow-tube reactors, stirred reactors, shock tubes, and laminar premixed flames, with outcomes such as ignition delay, flame speed, and various species concentration properties (location of a peak, peak value, relative peaks, etc.). 

The reaction schemes for the gas phase can be adopted from modelling work on flame chemistry (see e.g. [13]). Its validity has been established through numerous studies on flames, shock tubes, flow reactors, and well stirred reactors [12]. 

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