RCEM Studies: Conversion of CH4 into unsaturated hydrocarbons and hydrogen

We study the conversion of small alkanes (CH$_\text{4}$, C$_\text{2}$H$_\text{6}$, C$_\text{3}$H$_\text{8}$) into unsaturated hydrocarbons, notably C$_\text{2}$H$_\text{2}$, C$_\text{2}$H$_\text{4}$ and hydrogen, in a Rapid Compression/Expansion Machine (RCEM). The newly formed molecules can have a higher value in terms of economic use, handling, or in terms of further utilization in a chemical process. By the conversion, also chemical energy is stored, because the unsaturated products feature larger standard enthalpies of formation compared to the initial substances.
In the experiments, the conversion (defined as amount of consumed initial reactant relative to initial amount) levels between 15 to 38 %. The dependence of the conversion on compression temperature, pressure and dilution by monatomic gas was investigated. The temperature was found to be the most important factor, followed by the dilution. However, these variables in respect to the mentioned pyrolysis process have physical and practical limitations (e.g. soot formation or a gas separation process) to increase the conversion further.
A limiting factor in conversion is also the duration time which the gas mixture resides in a high temperature environment. ... mehrDuring one cycle, this time is limited because wall heat losses and endothermal reactions cause the temperature to drop to a point where chemical reactions practically stop. We study how multiple consecutive compression and expansion cycles can achieve longer effective residence times in the high temperature environments, and thus increased conversion and yield. It was found that both the conversion and the yield can be increased significantly by a factor of 2 to 4 by multiple compressions.