Residence time distribution experiments chemical reactions

To run the residence time distribution experiments under conditions which would simulate the conditions occurring during chemical reaction, solutions of 15 weight percent and 30 percent polystyrene in benzene as well as pure benzene were used as the fluid medium. The polystyrene used in the RTD experiment was prepared in a batch reactor and had a number average degree of polymerization of 320 and a polydispersity index, DI, of 1.17. 

For isothermal, first-order chemical reactions, the mole balances form a system of linear equations. A non-ideal reactor can then be modeled as a collection of Lagrangian fluid elements moving independe n tly through the system. When parameterized by the amount of time it has spent in the system (i.e., its residence time), each fluid element behaves as abatch reactor. The species concentrations for such a system can be completely characterized by the inlet concentrations, the chemical rate constants, and the residence time distribution (RTD) of the reactor. The latter can be found from simple tracer experiments carried out under identical flow conditions. A brief overview of RTD theory is given below. 

The purpose of tracer experiments is to extract information about the system in a chemical reaction engineering context, it is the mixing within the system which is of interest, as represented by the system residence time distribution. Because flow mixing is an inherently linear process, the exact form of the RTD which is recovered from a tracer response experiment should be independent both of the amount of tracer used in the test and also of the particular functional form in which the tracer was... 

Chemical engineers also use this kind of experiment. It can be utilized to great advantage in chemical reactors to find the "residence time distribution" of the reactor, a crucial piece of information which links microscopic flow behavior, that is, fluid dynamics, to measurables of the system, such as chemical conversion and selectivity. For vessels that are not used for reaction processes, but are used for other operations that are also critically dependent upon mixing, this tracer experiment provides a great deal of insight into how the system behaves. We can analyze how a pulse of injected tracer would behave in the well-stirred vessel we have been analyzing here. 

The residence time should be as short as possible so as to have the same reaction time for each reactant volume element. There are many applications where this can be important or at least must be taken into consideration. One example is high-throughput experimenting at low chemical consumption in the same microstructure. The residence time distribution affects the time that has to elapse between two... 

Residence time distribution (RTD) is a classical tool in the prediction of the comportment of a chemical reactor provided that the reaction kinetics and mass transfer characteristics of the system are known, the reactor performance can be calculated by combining kinetic and mass transfer models to an appropriate residence time distribution model. RTDs can be determined experimentally, as described in classical textbooks of chemical reaction engineering (e.g. Levenspiel 1999). RTD experiments are typically carried out as pulse or step-response experiments. The technique is principally elegant, but it requires the access to the real reactor system. In large-scale production, experimental RTD studies are not always possible or allowed. Furthermore, a predictive tool is needed, as the design of a new reactor is considered. 

Reynolds dye experiment on transition to turbulence in pipe flow and G. I. Taylor s experiments on axial dispersion in laminar flow represent the early use of tracers in flow visualization and transport parameters evaluation in chemical engineering. A more widespread use in chemical reactors dates to the work of Danckwerts (1). He realized that the performance of process equipment depends on the residence time distribution of process fluid, and that this information can be obtained by tracer methods. Residence time distributions are now discussed in standard chemical reaction engineering texts (2,3) and are well summarized in a recent excellent monograph by Nauman and Buffham (4). Tracer methods. 

These measurements, of course, require that the products be observed before their coUisional deactivation. This is achieved by canying out the experiments at low pressure, and removing the products from the observation zone rapidly. For these reactions, the flow requirements are determined mainly by the large rate constants for both the reactions of the 0( D) atom, and its lysical and chemical quenching by ozone - the latter is about 5.0 x 10"10 cm molecrl s"l [21,22] -which dictate that the 0( 0) is removed and the reaction products are created at approximately the first gas netic collision after the photolysis pulse. In the present experiments, the residence time of the reagents in the photolysis zone is about 100 microseconds and in the observation zone it is about 1 millisecond. It will be shown later that these conditions satisfy the requirements for observation of "initial" unperturbed energy distributions. 

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