Mathematical Calculation Of Pitch Impregnation-carbonization To Prepare Carbon Materials

Cost reduction in preparation of carbon/carbon composites is a critical aspectpresenting for all researchers in the whole world. To attain this purpose, there are stillgreat potential in the fabrication of carbon/carbon composites by the method of lowpressure impregnation-carbonization of coal tar pitch. Though computer simulationhave been successful applied in some fields in materials, almost no report has beenfound in the process of pitch impregnation-carbonization. Reasons may be (1) coal tarpitch is mixture of thousands compounds whose structures are not determined, (2) alot of data about the intermediate and non-equilibrium process are absence, and (3) itis difficult to calculating molecular parameters by quantum chemistry due to the veryhigh molecular weight and condensed aromatic rings of pitch compositions. Manyfactors are involved in the processing technology. Thus, suitable statistical models areurged to predict the behaviors of pitch. Computer simulation can help us optimizingoperation conditions, designing components of raw, discovering evolution mechanismof pitch, and promoting the efficiency of preparation and quality of products. Effortswere paid to the mathematical calculation of pitch impregnation-carbonization in thisthesis as follows:On the basis of force analysis of quinoline insoluble (QI) in the liquid pitch, filtercake forming by deposition of QI particles is similar to the constant pressure filtrationof particle suspension. The one dimensional model has been modified to predict theporosity of the sphere deposit. The equation proposed by Endo was used to calculatethe permeability coefficient of filter cake composed of QI particles whose sizedistributions were known. It was found the predicted results agreed with theexperimental data. The effects of size distributions of QI particles were explored bythe model also. Results show that both decreasing the concentration of the fine sizemode particle and increasing its diameter could enhance the permeability coefficient.The mechanisms of surfactant, flocculants and chemical modifier used in pitchimpregnation were determined by combining the simulant tests and theoreticalcalculations. The surfactant orientationally adsorbs between different polaritiesmolecules, which results in viscosity of the system decrease. The permeability ofpitch is elevated, but the critical saturation was found at the concentration of 3%.However, the flocculants improved the permeability of pitch by the way of coagulating fine particles into coarse ones which lead to the higher permeabilitycoefficient of the filter cake. The flocculants had no influence to the compositions andviscosity of the pitch. On the contrary, the chemical modifier reacted with thecomponents with low molecular weight. The viscosity and secondary QI increased.Though the latter is favorable to the permeability, the former hampers the flow ofpitch severely. As a result, the permeability of pitch decreased drastically.Considering the size distributions of QI particles and the size distributions andtypes of pores in the carbon preform, the author suggested a mathematical modelabout the impregnation efficiency. It was difficult to filling the closed pore, so theimpregnation efficiency was mainly contributed by the filling of open pore. Thecalculated results were very close to the experimental data. The effects ofperformance of pitch, technical parameters and structure of preform were discussedby the model in detail. It provided evaluation about optimization of technology anddesign of the raw.The TG-DTG curves of pitch were fitted successfully by the proposed kineticsmodel of three stages consecutive reactions. It overcame the shortcoming of one stagemodel. The apparent activation energies deduced by the Coats-Redfern method werelisted as 36.807KJ/mol, 185.205KJ/mol and 65.828KJ/mol for the three stages, andtheir carbon yields were 20.5 %, 72.2 % and 94.3 % respectively. These values werecoincided with reported data about toluene soluble, toluene insoluble but quinolinesoluble and quinoline insoluble in literatures. Their pyrolysises are all first orderreactions. The results predicted by the model were validated by the experimental dataat different heating rates in the range of 2.5 K/min to 10 K/min.A Monte Carlo model based on the serial dimerization was established toforecast the molecular weight distributions of pitch during isothermal pyrolysis. Thereported molecular weight distributions of pitch under different heat treatments weresimulated. Little errors were found. The pyrolysis rate of components with lowmolecular weight was slow down, but the reaction rate of components with mediummolecular weight increased by long time heat treatment at 440℃. The reactionactivity of pitch was drastically enhanced when the temperature was change from 440℃to 450℃. The effect is more marked to the components with medium molecularweight. The transmissibility of pitch represented as average molecular weight,polydispersity index and molecular weight range was proved by theoreticalsimulation. The evolutions of bubbles caused by the volatiles clusters in viscous liquid werecalculated by combining kinetic equations of pitch pyrolysis and growth model ofbubble. The results qualitatively explained apparent phenomenon in carbonizationexperiments. The surface tension, viscosity, temperature and mass transfer have greatinfluence to the bubble growth, nevertheless, the mass transfer is the main factor. Noobvious change was found for the characters of bubble growth dynamics if the heatingrate increases from 2.5 K/min to 10 K/min. However, the higher heating rate led to thehigher growth rate of the bubble and the higher radical velocity of the liquid. At thiscondition, extravasation phenomenons of liquid pitch from preform and crack ofsemi-coke were always found in experiments. The bubble size in low temperaturepitch was bigger than that in the medium temperature pitch.Assuming the mechanism of "bubble-escape-limited", the carbon/carboncomposites sample was divided into two regions as reaction region in liquid and solidregion of semi-coke. Most of the volatiles were trapped in liquid as bubble. Theyescaped in semi-coke. The energy equation, pyrolysis kinetic model and massconservation equation were settled by finite difference. The distributions oftemperature, pressure and conversation of pitch in the sample were obtained. Theeffects of heating rate, thickness of sample and raw pitch to these parameters werediscussed in order to design more reasonable carbonization technology.