Study On Formation And Percolation Mechanism Of Pores And Permeability In Graphite Porous Media

As a typical porous media, the graphite performs well after impregnation or filtration because there are lots of micro-pores in the graphite. However, it's difficult for the graphite with disordered pore to receive high quality by blind impregnation. The development of preparing high quality graphite is restricted. Based on the fluid dynamics in porous media, the transportation phenomenon of heat and mass for binder pitch and the fluid flowing in roasting process are researched. Moreover, the characteristics of micro-structure in porous media after roasting process are analyzed. The percolation theory in the building process of micro-structure and the effect of micro-structure on the permeability rate of the porous media is discussed in the paper.The phenomenon of heat and mass transfers during the building pores process in graphite matrix is analysed firstly. Whole process is divided into four stages: pitch binder soften, liquid phase transfers, pyrolysis and polycondensation. The liquid phase transfer stage displays that heat and mass transfers in saturated porous media and the pores room is occuppied by liquid phase pitch. The other stages exhibit that heat and mass transfers in unsaturated porous media and the pores room is occuppied by liquid pitch, water vapour and volatile gas. Applying Whitaker volumetric average method, the controlling equations of heat and mass transfer in various stages are established and the physical meanings of the parameters are also described.In order to study the influence facors on forming the micro-pore structure in graphite, the graphite matrix are prepared under different technological conditions and the roasting tests are conducted. The micro-structure in the pores is measured with metallurgical test and injecting mercury test. The more attention is paid to the effects of the heating-up curves on pore structure. The results have shown that the faster heating rate leads to insufficient pyrolysis and lower coke yield of the coal tar pitch, which decreases the bonding among aggregate particles and makes the micro-pore structure without long range connection. Nevertheless, it is beneficial to complete the pyrolysis and polycondensation by means of decreasing heating rate and prolonging holding time, which make a reasonable distribution and realize the mutual connection in long range among the pores in the porous medium. Consequently, the roasting temperature curve follow the rules: faster heating at the beginning stage and the ending stage, slower heating at the middle stage, longer holding time. According to the rules, the heating rate is suitable in the range of 5℃/h and the holding time is no less than 36h.In the paper, the fractal theory is used to describe the irregular micro-pore structure in the graphite porous media, and the physical meaning on fractal dimension is researched. The results have shown that micro-pore structure of the graphite porous materials have typical fractal characteristics. The fractal dimension of pore ranges from 1.683 to 1.821, the fractal dimension of matrix ranges from 1.709 to 1.852, the fractal dimension of volume range from 2.833 to 2.958, the fractal dimension of grain boundary range from 1.09 to 1.21, the fractal dimension of tortuosity range from 1.146 to 1.169, and the different fractal dimension have illustrated various physical meanings. By logical equal portions(m), number of subtracted cubes(n), iteration times(i), Menger sponge model can be used to simulate the graphite porous media with given porosity and specific surface area according to the fractal dimension (D).Typical percolation characteristics are demonstrated in the preparation process of graphite porous media. The roasting process is the percolation process including the formation of the pore- the local connectivity of the pore - the formation of limited clusters - the long distance connectivity of various local clusters and the formation of infinity clusters. Metallurgical pictures of graphite porous medium after roasting were managed by renormalization group method and verified by injecting mercury test. The results shown that the graphite porous medium for filtration have percolation structure, the threshold pressure range from 13 to 18 psia, area porosity range from 0.508 to 0.575 and the volume porosity range from 0.273 to 0.324. The graphite porous media for impregnation haven't percolation structure, threshold pressure range from 86 to 121 psia, area porosity range from 0.189 to 0.314 and the volume porosity range from 0.107 to 0.155.Based on the fractal theory, the permeability of the graphite porous media was studied in this work. The results were obtained that: (1) The permeability of porous media is related to the structural parameters, and the permeability is decided jiontly by all structural parameters, which is neither the single valued function nor the single increasing or diminishing trend. (2) As an important factor, tortuosity must be taken into account in prediction of the porous media. Compared with the result neglected the tortuosity, the permeability considering the tortuosity is closer to the actual value, and the relative errors range from 8.6 to13.56%.(3) The experimental results on the tortuosity is affected by the external factors such as capillary pressure, flow velocity and fluid properties.There are 49 figures, 12 tables and 170 references in this doctoral dissertation.