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An external heat source is often required to derive a chemical reaction. In an autothermal reactor the heat from an exothermic reaction is used to produce a self-sustaining scenario in which the use of a continuous heat source is not required. This feature makes autothermal reactors advantageous for various applications. With the proper choice of process conditions and catalyst, autothermal technology can provide high quality and efficient operation.
Recently, there has been an emphasis on developing autothermal reactor concepts for coupling endothermic and exothermic reactions. Autothermal processes have several advantages. These include not requiring on external heat source and being less expensive. The purpose of this thesis is to develop mathematical models which describe the operation of autothermal processes.
In our research we consider a cascade of two reactions occurring in a continuously stirred tank reactor (CSTR) cascade. The reaction mechanism A --> B --> C consists of two steps. In the first step the reactant A is converted into an intermediate B. In the second step the intermediate B is converted into the final product C. The catalyst for the endothermic process is placed in reactor one whilst the catalyst for the exothermic process is placed in reactor two. Consequently the first (second) reaction only occurs in the first (second) reactor.
Several factors, including the influence of the feed temperature and the choice of catalysts, are investigated as mechanisms for increasing the product conversion in a cascade reaction. We obtain new findings and results for different reactor configurations and system operating conditions which improve the product yield. The influence of the main operating variables on the reactor performance is studied.
In this thesis we consider different model configurations in each chapter. In Chapter 2 we investigate the adiabatic reactor. In this chapter the heat-transfer across the walls of the reactor is assumed to be negligible. The main finding of this chapter is that it is challenging to obtain high conversion of the reactant for realistic feed temperature. In Chapter 3 we analyze the diabatic reactor, where the temperature of the reactor walls are held to be constant. This relaxes the assumption that heat transfer across the reactor walls is negligible. Our most important result in this chapter is constructing the limit point unfolding diagram. This is used to find all the transitions between the steady-state diagrams. The desired autothermal region and the required conditions to achieve the desired conversion are identified from this diagram.
To further enhance the yield of the product species, in Chapter 4 we considered a model with four reactors. The advantage of increasing the number of reactors to four reactors is the possibility of achieving additional conversion of the reactant species into the intermediate species in the third reactor which results in increasing the yield. Weather introducing two additional reactors requires a good choice of the parameter values and initial conditions increases the yield. A cascade with six reactor will show insignificant improvement.
In the previous chapters we assumed that the temperature of the reactor walls is constant. In Chapter 5 we consider the placement of a jacket around the outside of the reaction vessels. This means that the feed temperature for the jackets is an independent variable. As before, we investigated the operating conditions required to achieve a high conversion. In the first reactor we found that the values of the jacket feed temperature and the jacket residence time are almost inconsequential when compared to the role played by the feed temperature. In the second reactor we found that if the residence time in the jacket is sufficiently high, then the value of the jacket feed temperature is irrelevant, i.e. high conversions can be achieved for any value of the feed jacket temperature. In Chapter 6 we briefly analyze a model with the stream leaving reactor two is the influent stream for the jacket surrounding the reactors. This means that the feed temperature for the jacket is not an independent variable. For this reactor configuration to produce autothermal behaviour highly exothermic reactions are required to achieve the desired conversion for a realistic feed temperature.