Abstract: The diffusion coefficient of paraffin molecules is of importance in the wax deposition prediction models. In this work, an instrument based on the principle of Taylor dispersion method was built in order to measure the diffusion coefficient of paraffin molecules in the model crude oil. The newly developed apparatus was carefully calibrated and adequate operating conditions were summarized. Measured with this apparatus, the diffusion coefficients (D_AB) of methanol in water, acetone in water, and hexane in heptane agree with those in literature, with an absolute deviation 1.13%. Using heptane as the solvent, six binary solutions were prepared with n-C₁₈, n-C₂₀, n-C₂₂, n-C₂₄ and n-C₂₆ as solute. D_AB of a solute in a solvent was measured at the temperature from 30℃ to 50℃. It is found that the diffusion coefficient of paraffin molecules increases linearly with temperature. As solute concentration increases, diffusion coefficients decrease exponentially. In the same solvent, paraffin molecules with longer chain present higher diffusion coefficient. The measured D_AB is compared to that obtained by empirical equation. The calculated D_AB by Hayduck-Minhas correlation are 50% lower than measured values. Obviously, the wax deposition prediction models employing Hayduck-Minhas correlation will underestimate the mass flux of wax molecules from the main flow to the pipe wall.

Key words: diffusion, laminar flow, mass transfer, Taylor dispersion method, wax deposition

摘要： 蜡分子扩散系数是蜡沉积预测模型中非常重要的物性参数。为测量蜡分子（高碳数正构烷烃）在液相体系中的分子扩散系数，建立了基于泰勒分散法的扩散系数测量装置，并对装置操作注意事项进行了详细探讨。采用该装置测量了丙酮水溶液、n-C₆+n-C₇溶液以及甲醇水溶液中溶质在溶剂中的分子扩散系数，以文献值对测量值进行校验，验证了装置的可靠性。分别以n-C₁₈、n-C₂₀、n-C₂₂、n-C₂₄、n-C₂₆为溶质，以n-C₇为溶剂，在不同温度、浓度下测量蜡分子在液相体系中的分子扩散系数，并将实验测量值与Hayduck-Minhas关系式计算值进行比较。结果表明：蜡分子扩散系数随温度升高而线性增大，随溶液中蜡分子摩尔分数的增大而以指数形式减小；在相同条件下，高碳数正构烷烃的分子扩散系数低于低碳数正构烷烃的分子扩散系数。采用Hayduck-Minhas关系式的计算结果比实验测量结果平均小50%，应用于蜡沉积预测时，将低估蜡分子扩散质量流。

关键词: 扩散, 层流, 传质, 泰勒分散法, 蜡沉积