Study On The Crystallization Process Of Fischer-Tropsch Wax

Fischer-Tropsch wax is a kind of hydrocarbon mixture produced in the Fischer-Tropsch synthesis process.Compared with petroleum wax,Fischer-Tropsch wax has the characteristics of high melting point,high hardness,wide carbon number distributions,low cracking temperature and almost no sulfur and nitrogen heteroatoms.It is widely used in polyvinyl chloride(PVC)processing,calcium zinc stabilizer,filler masterbatches,modified wax and other fields,so the market prospect for it is broad.In recent years,with the improvement of Fischer-Tropsch synthesis technology in China and the extended applications of Fischer-Tropsch wax,the demand for high value-added Fischer-Tropsch wax has increased year by year.Therefore,the refining and purification of Fischer-Tropsch wax have attracted more and more attentions.At present,most of the researches focus on the analysis of the physical properties of Fischer-Tropsch wax.The solid-liquid phase equilibrium and crystallization mechanism of Fischer-Tropsch wax have not been reported yet.Based on the above background,in this paper,a comprehensive and systematic study on the crystallization process of Fischer-Tropsch wax is carried out,which aims to provide important theoretical guidance for the development of the crystallization process of Fischer-Tropsch wax.The main content and achievement of the research work are as follows:The composition,structure and properties of six narrow fraction Fischer-Tropsch waxes obtained by solvent extraction coupled with fractional crystallization were investigated by Gas Chromatography(GC),Fourier Transform Infrared Spectroscopy(FT-IR),Differential Scanning Calorimetry(DSC),Thermogravimetric Analysis(TG)and other characterization methods.The results show that six narrow fraction waxes are similar to each other in element composition,have a normal distribution of carbon numbers and all are mainly composed of long-chain alkanes.With the increase of molecular weight of the narrow fraction waxes,their melting point,crystallization enthalpy,degree of branching of alkane and thermal stability all show an increasing trend.It can be concluded that solvent extraction coupled with fractional crystallization can be used to produce refined wax products with different melting points according to requirements from low value-added crude waxes.The solubilities of Fischer-Tropsch wax in organic solvents(n-butanol,butyl acetate,n-octane,petroleum ether and toluene)under atmospheric pressure were measured by a dynamic method.The results show that the solubilities of Fischer-Tropsch wax in the five organic solvents all increase with temperature,and are of great difference in different solvents.The solubility data were well correlated by Apelblat equation,polynomial equation and simplified two-parameter equation,and the fitting results by Apelblat equation and polynomial equation are better than that by simplified two-parameter equation.In addition,the values of the dissolution enthalpyΔsol H,dissolution entropy Δsol S and dissolution Gibbs free energy Δsol G of Fischer-Tropsch wax in the five organic solvents were calculated by modified Van’t Hoff equation.The obtained values of these thermodynamic parameters show that the dissolution processes of Fischer-Tropsch wax in the five organic solvents are all endothermic,increasing entropic and non-spontaneous.Butyl acetate is preferred as the extraction solvent,and metastable zone of six narrow fraction waxes in butyl acetate were measured by a dynamic method.The results are crucial for the control of crystallization process.The crystallization process of Fischer Tropsch wax in butyl acetate was studied by solution crystallization.Crystals with different morphologies and particle sizes were obtained by controlling the crystallization conditions.At the same time,the crystallization mechanism of Fischer Tropsch wax was studied utilizing the polarizing microscope,scanning electron microscope(SEM),laser particle size analyzer and online 2D vision probe process imaging system.The results show that crystallization temperature,cooling rate and stirring rate are the main factors affecting the crystal morphology and particle size of Fischer Tropsch wax crystals.The Fischer Tropsch wax crystals formed in the solution crystallization process are mainly acicular and laminated ones,and the particle size of crystallization products ranges from tens to more than one hundred microns.The nucleation and growth process of crystals are fast in the process of solution crystallization,and the difference in the crystal morphology is mainly caused by the different degrees of agglomeration in the later stage of crystallization.Differential scanning calorimetry was used to study the non-isothermal crystallization process of six narrow fraction waxes at different cooling rates.Jeziorny model and Mo Zhishen model were used to fitting the experimental data and the crystallization kinetic parameters in these two models were obtained through the regression method.The results show that the non-isothermal crystallization process of different wax samples is similar and the crystallization rates depend on the cooling rates.The experimental data are in good agreement with these two kinetic models.The average value of Avrami index n is between 1 and 2,indicating that the crystallization process is secondary nucleation,and the rod or fibrous crystals are formed during the nucleation.Mo Zhishen model analysis shows that a faster cooling rate is conducive to the realization of crystallization.In addition,the non-isothermal crystallization activation energy ΔE was obtained by the use of the Kissinger equation.