Study On Surface Tension Of Coal Liquefied Fractions

In the coal direct liquefaction process, pulverized coal, recycle hydrogenation solvent and catalyst are mixed into reactor, then action on hydrogen at high temperatures and high pressure. Following separation, clean liquid oils are produced. Coal direct liquefaction is an advanced, high-efficient coal cleaning technology. To improve liquefied oil yield and quality and rigorous process conditions, the design of operable and efficient processing steps for coal liquefaction plants, optimization of process, design of reactor, separation and upgrading of products, requires also basal physical data of coal liquefied fractions. Surface tension is one of the important physical properties of liquid. The properties of coal liquefied fractions depend markedly on raw coal used, liquefaction process and operation conditions. So measurement and estimation of the properties of coal liquefied fractions are especially necessary.After improvement of ordinary surface tensiometer model BZY-1 based on Wilhelmy plate method, the measurement range is 20～99.9℃. Following good precision and repeat of instruments, the author measures surface tension of Shenhua and Shengli coal liquefied fractions. The change rules of surface tension with cutting fraction temperatures and experimental temperatures are explored. On the basis of the experiment data, previous estimation methods for the surface tension are discussed in detail. It is very important to check the suitability of these correlations to coal liquefied fractions. Coal liquefied fractions are complex and essentially unknown mixtures and they are much more aromatic than petroleum crude, most of the thermodynamic correlations are based on the properties of paraffinic / naphthenic crudes and can introduce large errors for coal liquefied fractions. To satisfy the requirement of construction of direct coal liquefaction technology, estimation methods are modified by matlab 7.0 software.The research results indicate:1. Measuring methods of surface tension for coal liquefied fractions are established. The measurement range is 20～99.9℃for modified tensiometer model BZY-1. The results from pure organic reagent (benzene, toluene, hexamethylene, n-heptane) prove that the experimental conditions have a good accuracy and the experimental set-up is suitable to measure surface tension of liquid sample.2. Estimate values of differential critical temperature are similar, but estimate values of critical pressure have great difference based on the experiential correlatives. Revised Riazi-Daubert method and Retzekas method are best methods to estimate critical temperature and pressure in the correlative of estimating surface tension.3. Surface tensions of Shenhua and Shengli coal liquefied fractions are systematically studied, which supplies the basic physicochemical properties data for construction of direct coal liquefaction technique.i . Surface tensions of two coal liquefied fractions decrease commonly linearly with the increase of experimental temperature. At the same experimental temperature, surface tensions of the light fractions increase commonly with the increase of the boiling point of the cutting narrow fraction; middle fractions have small chang, heavy fractions have no rule.ii. Based on the complexity of composing for coal liquefied fractions, it is very important to consider the difference of different fractions composition and it is necessary to be revised into groups.iii. For the light fractions (IBP～110℃, 110～150℃) of Shenhua and Shengli coal, Wilson correlation based on petroleum is better than Gray correlation based on coal tar. The revised Wilson correlation can satisfy the engineering design and error is less than 4%.iv. For the middle and heavy fractions of Shenhua coal liquefied fractions and middle fractions of Shengli coal liquefied fractions, Gray correlation is better than Wilson correlation. The revised Gray correlation can meet the engineering design and error is less than 10%.v . For the heavy fractions of Shengli coal liquefied fractions, the equation σ= a(1 -T_r)~b can meet the engineering design and error is less than 10%.4. The measurement methods suiting in high temperature and pressure and influences of environmental gases were analysed in detail. The maximum bubble pressure and/or pendent drop method are believed to be the optimal choice.