The objectives of this study were in line with the principle objectives of the Fluorochemical Expansion Initiative i.e. the development of local fluorochemical technologies and products. This study focused on the processing of fluorspar derivatives in the form of hexafluoropropene and hexafluoropropene oxide, considerations of the reaction chemistry and identification of kinetic models for the processes.
Organofluorine synthesis A. Equipment validation
Experimental equipment was validated by observing the absorption of carbon dioxide in a sodium hydroxide solution. Results showed mass transfer coefficients greater than similar experiments conducted by Zhang et al., (2009). Increased rates of mass transfer were postulated to be due to the formation of ripples on the microchannel which increased the interfacial area between the valid phases. Enhanced mass transfer was deemed to be beneficial to the study and thus the equipment was declared fit to conduct experiments.
B. HME synthesis
Considering first the synthesis of HME, preliminary investigations were conducted on a semi- batch gas-liquid stirred tank reactor where hexafluoropropene gas was bubbled through 1.5 L of methanol and dissolved amounts of potassium hydroxide for 30 min. There were 3 reaction variables of interest in the preliminary investigations and these were hexafluoropropene mole fraction in the feed, potassium hydroxide concentration and reaction temperature. A 3 factor central composite design was used to conduct these experiments over a hexafluoropropene mole fraction range of 0.41 to 0.83, a catalyst concentration range of 0.40 to 0.80 mol ∙ L−1 and a reaction temperature range of 12.0 to 28.0 0C. HME and the associated by-products were successfully synthesised in the glass reactor and were qualitatively analysed using gas
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chromatography-mass spectroscopy and quantitatively analysed using gas chromatography and the internal standard quantification technique. Experimental yields of HME ranged from 19.21 to 53.32% with respect to the moles of hexafluoropropene gas introduced into the system over the reaction period. The yields of the by-products were also quantified and the yield of alkenyl ether was found to vary between 0.07 and 1.03% while alkyl tetrafluoropropionate varied from 2.42 to 5.10%.
The preliminary investigations also provided two very important findings and these were, firstly, that negligible amounts of solid precipitate was formed at all reaction conditions which permitted the use of a FFMR for the system and secondly, the trends observed in experimental results allowed for a more appropriate operating envelope to be established for the FFMR experiments.
An additional observation from experiments conducted on the glass reactor was that temperature control was inefficient for a highly exothermic reaction such as that of the HME system. A sharp temperature increase was noted over the first 5 min of all experiments and this may decrease the selectivity of the reaction when attempting to target an optimum operating region.
Experiments were then conducted on a FFMR which offers greater rates of heat and mass transfer allowing for more stringent reaction temperature control and enhanced reaction rates. It was desirable to operate at steady-state conditions for all experiments and this was found to be achieved within the first 5 min of reactor start-up. The reactor was operated counter-currently in order to make use of a high concentration driving force over the length of the reaction plate. There were 4 reaction variables of interest in the FFMR experiments and these were hexafluoropropene mole fraction in the feed, potassium hydroxide concentration, reaction temperature and liquid flowrate. The ranges investigated for these parameters were between 0.17 and 0.88, 0.25 and 0.61 mol∙L-1, 2.0 to 22.0 0C and 0.50 to 5.50 mL∙min-1 for the hexafluoropropene mole fraction in the feed gas, potassium hydroxide concentration, reaction temperature and liquid flowrate respectively. A 4 factor circumscribed central composite design was used to dictate the number of experiments as well as the combination of reaction conditions. The amounts of HME as well as the by-products were again quantified using gas chromatography and the internal standard method to give yields of HME of between 11.60 and 68.00%. The by-products appeared in the reactor product at yields between 0.11 and 6.99% for the alkenyl ether and 0.75 and 6.24% for the alkyl tetrafluoropropionate.
The FFMR presented an easier method of synthesising HME via a continuous process, this allowed for easier processing and handling of materials pre- and post-experimentation. The favourable characteristics of the FFMR allowed for stringent reaction temperature control which
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aided repeatability of experiments as well as greater yields being observed in comparison to the glass reactor results.
C. MTFMP system
MTFMP was similarly synthesized using a FFMR by passing hexafluoropropene oxide gas over a methanol liquid solution with a dissolved alkali salt. Literature dictated that sodium hydroxide be a suitable salt for the reaction but this to created blockages of microchannels as well as liquid exit lines. Potassium hydroxide was used instead. The alternate salt proved to be a viable replacement as MTFMP was successfully synthesized. The identification and quantification of the MTFMP product used gas chromatography-mass spectroscopy and gas chromatography, respectively.
There were 4 reaction variables of interest in the MTFMP system and these were hexafluoropropene oxide mole fraction in the feed, potassium hydroxide concentration, reaction temperature and liquid flowrate. The ranges investigated for these parameters were between 0.16 and 0.84, 0.15 and 0.65 mol∙L-1, 30.0 to 40.0 0C and 0.50 to 5.50 mL∙min-1 for the hexafluoropropene oxide mole fraction in the feed gas, potassium hydroxide concentration, reaction temperature and liquid flowrate, respectively. A steady-state study was once again conducted and it was found that the system reached steady-state within the first 5 min of operation. Yields of MTFMP ranged from 0.00 to 23.62%. The low yields observed were due to a combination of high liquid flowrate (which resulted in a low liquid residence time) and low mole fractions of hexafluoropropene oxide in the gas (meaning insufficient reactant gas to drive the production of MTFMP). Overall, considering the difficulties experienced with the system as well as the low yields obtained, it was concluded that a FFMR may not be the most appropriate means of synthesising MTFMP.
Kinetic model and parameter identification
A secondary objective of this study was to use the experimental results obtained from the FFMR for both the systems and model the reactor from fundamental principles, in order to identify a suitable kinetic model as well as previously unknown kinetic parameters.
A. HME system
A set of generalised material balances were developed from fundamental principles. Both a simple and a more complex description of the HME reaction mechanism were investigated. The simple reaction mechanism description provided little insight into the interaction of the various species thus the more elaborate description of the solution chemistry was opted for. This novel reaction mechanism broke the reaction network into 5 reactions which were assumed to be
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forward and irreversible. There was no available literature on the kinetics of the 5 reactions and the required rate constants and activation energies were solved for by least square non-linear regression MATLAB® environment (version R2012b, The MathWorks, Inc.). The temperature centreing internal scaling method was employed for both the HME and MTFMP systems which meant that the reference kinetic rate constant, and not the reaction rate constant, was found by regression of experimental data. A similar scaling method was also applied to the objective function in the form of artificial weighting factors. The system was found to be very weakly affected by the presence of salt in the liquid phase thus the Sechenov coefficient was eliminated to reduce the number of fitting parameters from 9 to 8 for the system.
The kinetic model was eventually solved and results showed that the model performed satisfactorily for HME in solution but did not perform as well for alkenyl ether and alkyl tetrafluoropropionate in solution.
B. MTFMP system
A set of generalised material balances was developed from fundamental principles assuming. The reaction mechanism for the synthesis of MTFMP was broken down into a two-step reaction sequence. Each step was found to have equivalent rates after employing the steady-state approximation to the pseudo present reaction intermediate. The generalised system of material balances was then solved simultaneously in the MATLAB® environment (version R2012b, The MathWorks, Inc.) with the unknown variables (fitting parameters) being regressed for in the model.
In order to ascertain the combination of initial guesses which would result in the lowest summed square error being observed a RSM analysis was conducted on the regression. The analysis revealed that the kinetic model was sensitive to the second order response of reference kinetic rate constant and the interaction between reference kinetic rate constant and Sechenov coefficient, it was also invariable to the value of the activation energy. An initial guess of 40.0 kJ∙mol-1, 1.6×10-
4 m3∙mol-1∙s-1 and 0.800 L∙mol-1 was used for the activation energy, reference kinetic rate constant and Sechenov coefficient, respectively. It was also found that a better representation of the system was observed when the mechanism was considered to be second order with respect to the concentration of potassium methoxide. The final values of the kinetic parameters were 40.0 kJ∙mol-1, 1.2892×10-4 m3∙mol-1∙s-1 and 0.8012 L∙mol-1 with an average absolute relative deviation of 41.04 %.
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