Design and implementation of a continuous PTFE depolymerisation system : moving from batch to semi-automated continuous TFE production

Abstract

At the University of Pretoria's Fluoropolymer Laboratory, an important long-term project is the development of a waste polytetrafluoroethylene (PTFE) depolymerisation process where TFE can be produced, purified and polymerised to reproduce pure PTFE. At the start of this project, the process consisted of a batch depolymerisation system, a sub-zero distillation column, and a polymerisation reactor system. The batch depolymerisation system could not produce enough gas per session to operate the downstream processes efficiently. The main aim of this investigation was to adapt the batch depolymerisation system to enable continuous depolymerisation by designing, implementing and testing a continuous PTFE screw feeder. With the screw feeder in place, the operating limits, with regard to temperature, pressure, and Teflon® PTFE 807N feed rate, were determined. The effects of temperature and pressure on the tetrafluoroethylene (TFE), hexafluoropropylene (HFP) and octafluorocyclobutane (OFCB) fractional composition were examined and the optimum operating conditions to maximise these products were determined statistically. An investigative approach was used in designing the hopper system. The optimum hopper wall angle and Teflon® PTFE 807N feed mixture was determined experimentally by testing four hopper angles, pure Teflon® PTFE 807N, and two Teflon® PTFE 807N mixtures (Teflon® PTFE 807N mixed with larger, compressed Teflon® PTFE 807N, particles in a 70:30 wt % and 50:50 wt % ratio) and two motor speeds. At all of the hopper angles and Teflon mixture configurations, rat-hole formation prevented the feeder from producing a constant flow rate. A hopper wall angle of 20° (to the vertical) together with plain Teflon® PTFE 807N were selected, as these two variables together helped to delay the formation of rat-holes the most. A stirrer was inserted in the hopper to negate the rat-holing problem. The continuous feeder was successfully designed, manufactured, calibrated, and installed. The feeder consists of a wedge-shaped hopper with a constant pitch, a tapered shaft screw and is capable of providing a maximum Teflon® PTFE 807N flow rate of approximately 20 g·min-1 for up to 40 min. Experimental test runs of the continuous depolymerisation system indicated that the minimum operating reactor temperature was 650 °C due to heat transfer and or rate of reaction limitations. The maximum flow rate of Teflon® PTFE 807N was determined to be 11 g·min-1 for the current reactor system. The maximum operating temperature and pressure were limited to 750 °C and 40 kPa, respectively, to avoid operating conditions that could lead to the increased production of PFIB. A three-level full factorial experimental design was used to determine the temperature and pressure effects on the fractional distribution of TFE, HFP, and OFCB under steady operating conditions. For pressure control purposed no carrier gas was used. The PTFE flow rate and experimental run time were kept constant at 11 g·min-1 and 15 min, respectively. The pressure in the system was regulated manually by constricting the flow of product gas out of the system. A maximum TFE mole percentage of 97 % was achieved at operating conditions of 650 °C and 2 kPa. The maximum HFP mole percentage (31 %) was observed at operating conditions of 750 °C and 20 kPa. A maximum of 55 % was observed at 750 °C and 40 kPa for OFCB. Statistical analysis of the continuous depolymerisation results indicate that TFE formation is highly sensitive to changes in pressure, with higher TFE yield fractions achieved at low pressures. The production of OFCB is highly sensitive to pressure, whereas the formation of HFP is equally affected by pressure and temperature changes. However, changes in pressure have a larger effect on the HFP production than temperature when operating at pressures lower than approximately 20 kPa. At higher pressures the sensitivity has the inverse affect, with temperature having a larger effect. As opposed to TFE, an increase in temperature and pressure leads to an increase in the HFP and OFCB concentration. To achieve a TFE mole percentages of 95 % and higher, the operating temperature of the system has to be kept in the range of 650 °C 720 °C, together with a system pressure of 2 kPa or less. Within the operating range of 730 °C 750 °C and 35 kPa 40 kPa a mole percentage of 50 % and higher can be expected for OFCB. A mole percentage of 19 % and higher can be expected for HFP in the operating range of 744 °C 750 °C and 32 kPa 40 kPa. It was determined through a kinetic analysis of the system, that the residence time of the product gas in the reactor has a large effect on the production of HFP, with an increase in residence time leading to a sharp increase in the HFP concentration and a decrease in the OFCB and TFE concentrations. Analysis of the determined product specific kinetics indicate that the predominant HFP production pathway at low residence times (&#060 3 s) is via the reaction of TFE with difluorocarbenes. At higher residence times the dominant reaction pathway is the dissociation OFCB.