Determination of residence time distributions in different high pressure gasification processes

The production of synthesis gas is one of the first steps in the conversion of different feedstocks to liquid fuels like methanol or gasoline. Feedstocks might be gaseous (e.g. natural or flare gas), liquid (e.g. heavy oil residues) or solid (e.g. coal or biomass). At TU Bergakademie Freiberg in Germany, a test plant with 5 MW thermal power has been installed and operated together with Lurgi GmbH (part of Air Liquide Group) for the conversion of gaseous and liquid fuels. It is designed for three different modes: the so-called ATR-mode (Autothermal Reforming), the Gas-POX-mode (Partial Oxidation of natural gas) and the MPG-mode (Multi-Purpose Gasification). The first one is a process for the catalytical conversion of natural gas and can be run at pressures of up to 70 bar(g). The Gas-POX-mode is also used for natural gas processing, but no catalyst is used and pressures may reach up to 100 bar(g). In MPG-mode, high-viscosity liquids can be gasified at pressures of up to 100 bar(g). In all modes, the feedstock is processed with oxygen and steam.
Because of high investment costs for such processes, design studies with the help of computational fluid dynamics (CFD) are of increasing importance. Validation of such studies is very complicated due to high pressures and temperatures within the reactor and the poor accessibility of measurement equipment. The usage of radioactive tracer material has been determined as a possible way for obtaining information on flow conditions within the reactor. Experiments have been conducted for all three modes explained above.
The radioactive isotope 41Ar (half-life 1.83 hours) has been used for the measurements. Scintillation counters were installed outside of the reactor at different heights to measure gamma radiation. The method of momentum (MOM) was used to derive residence time distributions out of the measured values.
The quality of measurement of residence time is different for the three processes. In ATR-mode, a large reactor volume and height in combination with the homogenization of the flow by the catalyst bed make it possible to determine the residence time distribution of the complete reactor. In Gas-POX- and MPG-mode, the reactor dimensions were strongly reduced for these experiments. Additionally, due to the lack of homogenization, the actual velocities within the reactor are much higher compared to the ATR-mode. Thus, measuring accuracy is considerably reduced.
CFD calculations were also performed. The CFD model was validated by comparing the residence time calculated with the experimentally measured one. A reasonable agreement was found, however, some problems with the accuracy of the experiments were found - besides some other parameters - in dependence of the operation mode of the gasifier. As a conclusion, the radiotracer method in general is well suited for investigation of high pressure gasification processes.