(180ae) Thermal Cracking and Effects of Oxygen On Oxidation of Tricyclodecane

A test was implemented to determine thermal cracking kinetics of tricyclodecane(TCD) without catalysts such as metal and to find out effects of oxygen on oxidative reaction of TCD. TCD can be used as a coolant as well as a fuel for spacecraft or aircraft. As technologies for aircraft are developing fast, aircrafts encounter with more heat load and the temperature change of the fuel is larger. If fuels are exposed to high temperatures, formation of insoluble products takes place and build-up of these insoluble products cause fuel system failure.

Thermal cracking of TCD were carried out in a batch-type reactor whose inner wall was covered with quartz. Flow rate can affect features of TCD's thermal cracking in a flowing condition. Experiments were implemented to find out thermal cracking kinetics of TCD at a static condition without flow characteristics. We also investigated effects of oxygen for TCD's oxidation.

First, the concentrations were measured with respect to time and conversions were figured out. A reaction rate was estimated as first order reaction from conversion measurements with respect to time. Activation energy of thermal cracking of TCD was calculated as 261.1 kJ/mol from Arrhenius' law. In order to compare thermal cracking to catalytic cracking of TCD, titanium was put on the bottom of the reactor covered with quartz in the same conditions above. The conversions of TCD with titanium were higher than those of TCD without titanium and activation energy was 200.5 kJ/mol. Comparing the results between two types of test, metals are detrimental to thermal stability because of reduction of activation energy for thermal cracking of TCD.

Another test was also implemented to investigate effects of oxygen on TCD's oxidation because air flows into system continuously in the industrial air-breathing systems. The reactor wall was covered with quartz to prevent effect of metal as catalyst on oxidation of TCD. Three types of TCDs were prepared to determine effect of oxygen; TCD without oxygen by bubbling He gas through the TCD, TCD with excess of oxygen by bubbling oxygen through the TCD, and air-saturated TCD. TCD with excess of oxygen and air-saturated TCD had same tendency of oxidation rate in the early stage of test, while oxidation of TCD without oxygen showed slower oxidation rate than those of former two cases. However, oxidation rate of air-saturated TCD was getting slower than that of TCD with excess of oxygen gradually. The rate of oxygen consumption was independent of the oxygen concentration under the sufficient oxygen concentration. However, at a low enough oxygen concentration, the rate-determining step for TCD's oxidation is related to the reaction in which alkyl radical of TCDs react with oxygen. The consumption of oxygen(i.e. oxidation rate) is the first order in oxygen. Thus, air-saturated TCD showed that its oxidation rate decreased gradually.

The thermal stability of TCD decreased as that the conversion of TCD increased, because catalysts reduced the activation energy of its thermal cracking. However, it's difficult to avoid large conversion due to metal catalysts because aircraft systems mostly consist of metals. Thus, further researches are necessary to find metals which can withstand high temperatures and reduce the conversion of TCD related to thermal cracking and to study additives which are able to reduce effects of metals. Oxygen concentration didn't affect the TCD's oxidation under the sufficient oxygen concentration. However, the oxidation rate of TCD was dependent on oxygen concentration under the insufficient oxygen concentration. Thus, additional experiments will be done to find out additives which can reduce oxidation rate of TCD under the industrial air-breathing systems.