Freon - 12 Hydrolysis Premixed Combustion Research

Ozone depletion, abnormal climate change and acid rain are the main global environmental problems. The former two issues are related with the emission of chlorofluorocarbons(CFCs), which is an important ozone depleting substances (ODS) and green house gas. In order to address this global issue of CFCs,’About the ozone depleting substance of the Montreal protocol’(Montreal protocol) has been approved and supported by more than 160 countries to limit and/or prohibit the usage of CFCs and other ODS. It has been an issue on how to deal and utilize the CFCs in storage and current instruments, although production and usage of CFCs have been limited by the Montreal protocol. It is an importance of reality to develop practical technology and instrument to decompose CFCs. In this research, dichlorodifluoromethane (CFC-12) was investigated based on theories of thermodynamics and reaction mechanism. Combined with experimental studies, a technology named’ Combustion and Hydrolysis of Rotational Flow with Partial-Premixed Feeding (CHRFPPF)’, and corresponding pilot-scale instrument were developed to decompose CFCs and recover the byproducts as resource.Factsage was used for thermodynamic analysises. Reactions of CFC-12 with bunkers such as methane, liquefied petrol gas (LPG), carbon monoxide and hydrogen were simulated under different temperature and pressure to figure out the equilibrium compositions and product distribution. It was found that using four fuels mentioned above CFC-12 could be completely transferred to HF, HCl and CO2, and that chemical potential and equilibrium constant were higher than those without any fuel. Among the four fuels, LPG exerted the best performance and desired equilibrium composition, and therefore was selected as the optimum fuel to decompose CFC-12. Thermodynamic analysises indicated that CFC-12 has a low stability and could be easily cracked under certain conditions. Among many influencing factors, temperature is the most important one that would influence the equilibrium distribution of products after crack. Element Cl mainly existed in form of Cl2 with a few Cl when temperature was less than 1400K, and mainly in form of Cl when temperature was higher than 1950K. Most of element F existed in form of CF4. Exceeding 1900K, CF2 also played an important role, and at 2500K F was mainly distributed in CF4 and CF2. When water existed in the reaction system, relative lower temperature was favorable to selectively form HF, HCl and CO2. Pressure exhibited only smaller influence on the decomposition of CFC-12, and therefore would not be considered for further studies.Density functional theory (DFT) was used for reaction mechanism research. A residue reaction mechanism involving 46 species and 388 reactions was proposed to model the reaction of CFC-12 and LPG based on references. All the elemental reactions were confirmed through calculation of quantum chemistry. Calculation at DFT LDA/PWC(DNP) level was conducted using commercial software Materials Studio. Finally,113 possible decomposition pathways of CFC-12 were selected based low energy barrier (<10kcal/mol). This confirmed from theory that many reaction pathways of low energy barrier existed in the LPG combustion field of CFC-12, and they crossed and interacted to provide a netted system of reaction pathways. This netted system of low energy barrier provided the guarantee to quickly and completely decompose CFC-12 as result of the residues produced in LPG combustion field. Hydrocarbon like CH3, CH2 and carbene exerted strong reactivity with the CFC-12 and its fraction, and effectively increased the decomposition pathways of low energy barrier. This confirmed the priority to select LPG as combustion fuel of CFC-12. The proposed mechanism indicated that HCl was mainly formed in the first half of the reaction system based on pathways of residue reaction, while most of HF was formed in the second half of the reaction system based on pathways of hydrolysis. It was also found that CFC-12 inhibited the combustion of LPG, while pathways of hydrolysis promoted the combustion. Presence of water was favorable to decompose CFC-12.Experimental researches were carried out to investigate the combustion characteristics of LPG and the basic laws governing the decomposition of CFC-12. Experimental results were completely in agreements with the theoretical studies. Combustion rate of mixture of LPG and air were dramatically influenced by CFC-12 and H2O. CFC-12 seriously inhibited the combustion with maximum rate decrease of 77.4%. Influence of water on combustion was similar as that of CFC-12 but with a relative lower inhibition. Experiments confirmed that decomposition of CFC-12 in LPG combustion field was a fast dynamic reaction without accumulation of transition products.Because of the strong inhibition of CFC-12 on LPG combustion, in order to increase the proficiency of CFC-12 decomposition, LPG combustion field should be controlled to make it stable. Experimental results indicated that CHRFPPF technology could solve this problem better. CHRFPPF technology was realized by replacing single-annulus-rotational-fluent combustor with double-annulus-rotational-fluent combustor. Smaller amount of water steam was favorable to decompose CFC-12. Water concentration of 14g/Nm3-20g/Nm3 was favorable, and unfavorable when exceeding 20g/Nm3. Water steam bubbled into the pre-mixed air under room temperature could reach the desired efficiency.Element F could be recovered in form of CaF2 from CFC-12 by adding CaCl2 into the absorbent to deposit F from HF. By the separation of CaF2, CFC-12 could be resourced. It was found from experiments that most of the deposits were CaCO3 and CaF2-The conditions of making deposits to meet the standard of high quality fluorite mines were to make pH of absorbent less than 3.0, and add CaCl2 as 2.5 times as the theoretical stoichiometry.Parameters of CHRFPPF technology were optimized using experiments. Pre-mixture were fed into the inside annulus while second air flow fed into outside annulus, the ratio of first to second air flow was 0.4:0.6, and first air flow was wetted by bubbling water before mixing. Under optimum conditions, ratio of CFC/LPG reached 2.02 and the decomposition rate of CFC could be more than 99.9%. This CFC/LPG value was 18.8% higher than the best reported value. Furthermore, the electric heat fuses were removed from the combustor to simplify the equipment and reduce the energy consumption.Fluent was used to simulate the eddy-dissipation reaction model occurred in either ejected or rotational fluent in double-annulus-fluent combustor. Results indicated that combustion occurred in rotational fluent generated fire flame smaller than that occurred in ejected fluent. Rotational fluent were more favorable to reduce the combustor size and cost. Strong eddies caused by rotational fluent distributed the temperature and product concentrations more evenly, and therefore promote the decomposition of CFC-12. Radial and axial distribution of velocity in rotational fluent made the decomposition rate in fully mixed fluent a little smaller than that in partially mixed fluent. The simulation results were consistent with our experiment studies.According to the requirement of CHRFPPF technology, the 2kg/h pilot-scale equipment was designed to decompose CFC-12. This equipment could be operated stably, and reached the design capacity and the decomposition efficiency higher than 99.9%. Based on the experimental researches and theoretical analysises, our research objectives have been realized; and sound bases were provided for further industry application.