Kinetic models, plant-scale

We currently model, at least in simple fashion, all resins scaled-up which have an exothermic stage, in order to assess safety implications and utilise plant to its highest productivity regarding heat removal. The data generated is used in verification of kinetics models. 

The calculated conversions presented in Table VIII used Eq. (57). They are quite remarkable. They reproduce experimental trends of lower conversion and higher peak bed temperature as the S02 content in the feed increases. Bunimovich et al. (1995) compared simulated and experimental conversion and peak bed temperature data for full-scale commercial plants and large-scale pilot plants using the model given in Table IX and the steady-state kinetic model [Eq. (57)]. Although the time-average plant performance was predicted closely, limiting cycle period predicted by the... 

Other work has been mainly concerned with the scale-up to pilot plant or full-scale installations. For example, Beltran et al. [225] studied the scale-up of the ozonation of industrial wastewaters from alcohol distilleries and tomato-processing plants. They used kinetic data obtained in small laboratory bubble columns to predict the COD reduction that could be reached during ozonation in a geometrically similar pilot bubble column. In the kinetic model, assumptions were made about the flow characteristics of the gas phase through the column. From the solution of mass balance equations of the main species in the process (ozone in gas and water and pollution characterized by COD) calculated results of COD and ozone concentrations were determined and compared to the corresponding experimental values. 

Pedit et al. [226] used a kinetic model for the scale-up of ozone/hydrogen peroxide oxidation of some volatile organochlorine compounds such as trichloroethylene and tetrachloroethylene. The kinetic model was applied to simulate the ozone/hydrogen peroxide treatment of these pollutants in a full-scale demonstration plant of the Los Angeles Department of Water and Power. The authors confirmed from the model that the reaction rate of the pollutant with ozone was several orders of magnitude lower than that with the hydroxyl radical. When considering that the natural organic matter acts as a promoter of hydroxyl radicals, the ozone utilization prediction was 81.2%, very close to the actual 88.4% experimentally observed. 

In reaction engineering, kinetic models are used to predict reaction rates at specified conditions of temperature and the partial pressures or concentrations of reactants and products. The emphasis must be, therefore, upon accuracy of prediction, even at the expense, if need be, of mechanistic rigor. For this reason, kinetic models for design purposes should be developed using the same pellet size and geometry as will be used in the commercial process, and over the ranges of temperature and component partial pressures expected for it. Finally, the kinetics should be studied at realistic plant-scale gas velocities so that the data are not influenced by physical transport phenomena like heat- and raass-transfer. 

The choice of experimental reactor is important to the success of the kinetic modeling effort. The short bench-scale reaction tubes sometimes used for studies of this sort give little or no insight into best mathematical form of the kinetic model, conduct the reaction over varying temperatures and partial pressures along the tube, and inevitably operate at velocities that are a small fraction of those to be encountered in the plant-scale reactor. Rate models from laboratory reactors of this sort rarely scale-up well. The laboratory differential reactor suffers from velocity problems but does at least conduct the reaction at known and relatively constant temperature and partial pressures. However, one usually runs into accuracy problems because calculated reaction rates are based upon the small observed differences in concentration between the reactor inlet and outlet. 

Here, experiment refers to a measurement, typically of a concentration or reaction rate, made in laboratory, pilot scale, and sometimes even plant scale equipment. The model refers to predictions of the concentrations or rates calculated from an assumed kinetic model. 

A pilot plant scale, tubular (annular configuration) photoreactor for the direct photolysis of 2,4-D was modeled (Martin etal, 1997). A tubular germicidal lamp was placed at the reactor centerline. This reactor can be used to test, with a very different reactor geometry, the kinetic expression previously developed in the cylindrical, batch laboratory reactor irradiated from its bottom and to validate the annular reactor modeling for the 2,4-D photolysis. Note that the radiation distribution and consequently the field of reaction rates in one and the other system are very different. 

The determination of kinetic parameters is an essential step in developing a catalytic process. Parameters determined in laboratory-scale steady-state reactors are necessary to formulate models for scale-up to pilot plant and process-scale reactors. Kinetic parameters also provide insight into the fundamental processes that occur during a catalytic reaction and form the basis for creating microkinetic models that describe the individual steps (e.g., adsorption, surface reaction, and desorption) of a complex reaction. 

This present paper presents the kinetic-mathematical model developed to describe the overall decomposition rate and yields of the naphtha feedstock cracking process. The novelty and practical advantage of the method developed lies in the fact that the kinetic constants and yield curves were determined from experiments carried out in pilot-plant scale tubular reactors operated under non-isothermal, non-isobaric conditions and the reactor results could readily be applied to simulate commercial scale cracking processes as well. During the cracking experiments, samples were withdrawn from several sample points located along the reactor. Temperature, as well as pressure were also monitored at these points[2,3]. 

The parameters of the system must be evaluated and the appropriate values must be used in tiie model. Some parameters can be obtained independently of the mathematical model. They may be of a basic character, like tiie gravitation constant, or it may be possible to determine them 1 independent measurements, Uke, for instance, solubility data fi om solubility experiments. However, it is usually not possible to evaluate all the parameters from specific experiments, and many of them have to be estimated by taking results from the whole (or a similar system), and tiien using parameter-fitting techniques to determine which set of parameter values makes the model best fit the experimental results. For example, a complex reaction may involve ten or more kinetic constants. These constants can be estimated 1 fitting a model to resnlts from a laboratory reactor. Once the parameter values have been determined, they can be incorporated into a model of a plant-scale reactor. 

Suppose now that a pilot-plant or full-scale reactor has been built and operated. How can its performance be used to confirm the kinetic and transport models and to improve future designs Reactor analysis begins with an operating reactor and seeks to understand several interrelated aspects of actual performance kinetics, flow patterns, mixing, mass transfer, and heat transfer. This chapter is concerned with the analysis of flow and mixing processes and their interactions with kinetics. It uses residence time theory as the major tool for the analysis. 

For a gas-liquid reaction which is gas-phase controlling, the chemical kinetics must be well understood. The importance of laboratory studies must therefore be emphasized. However, for successful scale-up, pilot plant studies are very critical because of the difficulties in reliably modeling gas behavior on a small scale (due to hydrodynamics) and its influence on reaction rates. 

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