The high-pressure mercury-free PVT apparatus is applied in this study to determine experimentally the critical point of one complex multi-component hydrocarbon system and its near-critical phase behavior described in more details. In the tests, the unusual phase phenomena are observed for the fluid in the near-critical region. That is, during the constant composition expansion process the retrograde dew-point phase change occurs twice above the critical temperature, and correspondingly dew- and bubble-point phase changes occur sequentially below the critical temperature. In both cases the volume of the first-appeared fluid at the bottom of PVT cell is relatively small, and the pressure at which the first fluid disappears is of the same value at different temperatures. The experimental findings show the behavior complexities of complicated fluids, cast more light on near-critical characteristics, and indicate the deficiency of the usual assumption of two-phase equilibrium in the PVT calculation. Thus more in-depth investigations are needed. It may be the first report that the unusual phenomena of two types occurs in the same mixture and at high temperature up to 100℃.%首次报道在100℃的较高温度时，同一流体体系同时出现两类异常相变现象。应用无汞PVT相态实验装置测试了多组分烃类流体临界点，详细描述了流体近临界相态特征。在实验测试过程中，发现流体在近临界区域存在特殊的相变现象：流体在温度高于临界温度时的恒质膨胀过程中先后发生了两次反凝析露点变化，即“露—露”相变；在低于临界温度时也先后发生了两次先出现露点后出现泡点的相变现象，即“露—泡”相变。在上述两个异常相变过程中，先出现的液相体积较少，且液相在不同温度条件下消失时的压力相同。实验中发现的异常相态变化现象说明多组分流体相态变化的复杂性，这可能对复杂流体体系的近临界特征作出新解释，同时也说明通常相态计算中两相假设的局限性。照片16图3参19（沈平平摘）
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