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Introduction: Direct coal liquefaction is one of the technologies for the clean and efficient utilization of coal, as well as an effective method for producing alternatives to oil. It holds great strategic and practical significance for addressing China’s oil shortage, balancing the energy structure, ensuring energy security, and promoting the sustained and stable development of the national economy. To achieve rational utilization of coal liquefied oil and optimize the hydroprocessing techniques for its upgrading, it is first necessary to understand the physical and chemical properties of coal liquefied oil. Currently, research on the physicochemical properties of coal liquefied oil in China is not yet mature, with most studies relying on foreign research findings. As a fundamental property parameter of coal directly liquefied oil, the saturated vapor pressure is extremely important in thermodynamics, particularly in phase equilibrium calculations, and is an indispensable data for process optimization and engineering calculations. It is of great significance to accurately measure the saturated vapor pressure and evaporation enthalpy of the narrow fraction of coal liquefied oil. This paper uses advanced measuring instruments to conduct relatively accurate measurements of the saturated vapor pressure of coal liquefied oil based on the three-expansion method; it can be said that there have been significant improvements in both the measurement methods and the results compared to previous approaches. 1 Experiment: The narrow-distillation fraction sample of coal-derived liquefied oil used in the experiment was provided by the Beijing Coal Chemical Engineering Branch of the China Coal Research Institute. It was produced by liquefying lignite from the fifth layer of the Shengli Mine belonging to the Datang Group as the feedstock coal (industrial analysis and elemental analysis are shown in Table 1). The distillation of the coal liquefied oil was carried out in accordance with ASTM D 2892-03a standards. The elemental analysis of the narrow-distillation fraction of the coal liquefied oil used in the experiment is shown in Table 2 on the following page. As can be seen from the data in Table 1 and Table 2 on the following page, as the distillation temperature increases, the H/C atom ratio of the narrow fraction tends to decrease. The experiment was conducted using a MINIVAP VPS automatic vapor pressure meter. This testing method is based on the principle of triple expansion; it offers advantages over the Red method commonly used in industry, such as the ability to vary the temperature for accurate testing, simple operation, and the capability to determine absolute vapor pressure. Before conducting the experiments, measurements were first taken using pure chemical reagents such as benzene and ethanol at 20°C, 30°C, and 40°C. The measured values were compared with data from literature studies, and the relative error was calculated in order to correct the accuracy of the instrument’s measurement results. The instrument is cleaned at the set measurement temperature; after introducing the sample, the gas is saturated for 3 minutes, and then the sample test is conducted to obtain the absolute vapor pressures of each narrow fraction of the coal liquefied oil. 2 Experimental Results and Discussion Table 3 shows a comparison of the measured values of benzene at 20°C and those of ethanol at 20°C, 30°C, and 40°C with the values reported in the literature. As can be seen from Table 3, the measured values of benzene and ethanol are generally consistent with the literature values, indicating that the operating conditions of the automated vapor pressure apparatus used are suitable for determining the vapor pressure of liquids. Under the optimized test conditions, the saturated vapor pressure of the narrow fraction of the selected coal liquefied oil was determined, and the results are shown in Table 4. As can be seen from Table 4, at the same test temperature, as the distillation temperature increases, the vapor pressure of the narrow fraction decreases ； For a narrow fraction within a certain distillation temperature range, as the test temperature increases, the vapor pressure of that narrow fraction rises ； However, the changes in the saturated vapor pressure of the lighter fractions are very significant, whereas those of the heavier, narrower fractions are smaller. For pure substances, within a certain temperature range, there is a good linear relationship between the logarithm of the vapor pressure and the reciprocal of the temperature. By calculating the logarithm of the vapor pressure of the narrow fraction of coal liquefaction oil and the reciprocal of the temperature, the results shown in Figure 1 on the following page were obtained. Perform a linear regression using Origin: Y = A + BX (1), where Y is the logarithm of the vapor pressure, LnP ； X—the reciprocal of temperature, 1/T ； A, B—Regression coefficients (see Table 5 on the next page). As can be seen from Figure 1, the saturated vapor pressure of the light oil fraction exhibits a good linear correlation with the reciprocal of temperature, in line with the relationship between the saturated vapor pressure of pure substances and temperature, as well as with the relationship between the saturated vapor pressure of coal liquefied oil obtained via the Exxon Donor Solvent Process (EDS) and temperature. According to the Clausius-Clapeyron equation: where p is the vapor pressure, in Pa ； R—gas constant ； △vapHm—evaporation enthalpy, kJ/mol ； C—Coefficient. The evaporation enthalpies of the various narrow fractions of coal liquefied oil can be calculated, yielding the results shown in Table 5. As can be seen from the data in Table 5, the variation in evaporation enthalpy with changes in the distillation temperature of coal liquefied oil fractions is not as pronounced as that of vapor pressure; however, the overall trend is an increase as the fractions become heavier. 3 Conclusion Under the test conditions calibrated with benzene and ethanol, a fully automatic vapor pressure meter can be used to determine the saturated vapor pressure of narrow fractions of coal liquefied oil with relatively high accuracy. 3.1 At the same measurement temperature, as the distillation temperature of the liquefied oil fraction increases, the vapor pressure of the narrow fraction decreases ； Within the same distillation temperature range, as the test temperature increases, the vapor pressure of the narrow fraction increases. 3.2 The logarithm of the vapor pressure exhibits a good linear relationship with the reciprocal of temperature; its simple linear regression is satisfactory, in line with the relationship between the saturated vapor pressure of a pure substance and temperature. 3.3 Based on the saturated vapor pressure data, the evaporation enthalpy of the fractions can be calculated using the Clausius-Clapeyron equation; as the distillation temperature of the narrow fraction of coal liquefied oil increases, the evaporation enthalpy generally shows an upward trend.