Influence of n -paraffin composition on the aging of wax-oil gel deposits

Abstract

An excess Gibbs free energy model is applied to investigate the influence of n-paraffin composition on the aging behavior of wax-oil gel deposits known to form in subsea petroleum transport pipelines. The aging of wax-oil gels occurs by a counterdiffusion process where wax molecules with carbon numbers greater than a critical carbon number (CCN) diffuse into the gel matrix, and vice versa. Model crude oils were formulated to exhibit varying decreasing exponential distributions of n-paraffin components in the range n-C 9 to n-C 53 . Deposition experiments were conducted using a laboratory cold finger apparatus simulating the cold wall of a subsea pipeline. Gas chromatography analysis of gel deposit compositions yielded experimental CCN values. A solid-liquid-phase equilibrium model, employing a modified version of UNIQUAC to compute solid-phase component activity coefficients, was able to successfully predict the critical carbon number for a variety of model oil n-paraffin compositions.