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Mierdel, Konstantin ; Jess, Andreas ; Schmidt, A. ; Hintzer, K.:
Energy and resource efficient production of flouroalkenes in high temperature microreactors.
2018
Veranstaltung: Jahrestreffen Reaktionstechnik 2018
, 07.-09.05.2018
, Würzburg.
(Veranstaltungsbeitrag: Kongress/Konferenz/Symposium/Tagung
,
Poster
)
Abstract
Tetrafluoroethylene (TFE) is the most common monomer for the synthesis of fluoropolymers at industrial scale. TFE is produced via multistep pyrolysis of chlorodifluoromethane (R22) at temperatures between 700 °C and 800 °C resulting in high values of waste acids containing HCl and HF.
In this work, a new chlorine-free process for producing the monomers tetrafluorethene (TFE) and hexafluorpropene (HFP) in a plug flow microreactor will be presented, starting from partially fluorinated alkanes obtained from different processes, e.g. electrochemical fluorination (ECF). Depending on the feedstock of the ECF up to 70% of partially fluorinated alkanes can be obtained in the product spectrum. Actually, no industrial process exists for utilizing these by-products besides incineration. Therefore, great attempts are made to achieve a material- instead of thermal utilization.
The first step in the synthesis of TFE and HFP is the decomposition of partially fluorinated substances yielding difluorocarbene (CF2), which is a highly endothermic reaction. For this reason, a microreactor is used (3M), offering high thermal conductivity and thereby enabling isothermal operation mode. The reaction channel has a cross section of 1 mm x 1 mm. The advantage of such small channels is the determination of kinetic parameters without the limitations in terms of heat- and mass transfer of a labscale-reactor.
The crucial process parameters of fluoromonomer synthesis in the microreactor are short residence time, 0.1 s to 2.5 s, at a sufficiently high temperature, 750 °C to 900 °C, and a fast quenching of the product gas to suppress the formation of oligomers and hazardous by-products. It will be shown that at high temperatures the conversion rate of partially fluorinated alkanes reaches 65-90% with monomer yields up to 90%. In this contribution, the influence of the dwell time and process temperature on different partially fluorinated feed gases will be discussed.
In addition, a comparative life cycle assessment (LCA) shows the benefit of using partially fluorinated alkanes for fluoromonomer-production instead of thermal recovery.