关键词: 扩散, 层流, 传质, 泰勒分散法, 蜡沉积

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