(578c) Visual Measurements of Hydrocarbon Freeze-out Temperature in LNG Mixtures: Experimental Design

Production of LNG is technically demanding, as it requires the exploitation and detailed understanding of phase equilibrium in multi-component mixtures at high pressure and low temperatures. Avoiding the conditions of solid-liquid equilibrium is particularly important for the multi-component hydrocarbon mixtures found in the main cryogenic heat exchanger of an LNG plant. Compounds heavier than pentane (C₆+), which are normally present only at trace concentrations in the methane-dominant liquid mixture, can potentially freeze-out and block the narrow tubing networks within the heat exchanger if process upsets occur and/or the composition of the feed natural gas changes more than expected.

 A specialized apparatus designed for visual measurements of solid-liquid equilibrium (SLE) and solid-liquid-vapor equilibrium (SLVE) was constructed and used to measure liquidus (melting) temperatures in hydrocarbon mixtures. To validate the apparatus, a binary mixture of cyclohexane (C₆H₁₂) and octadecane (C₁₈H₃₈) was used to measure the melting temperatures across the entire range of composition and at pressures from about (0.004 to 10) MPa. A Peltier-cooled copper tip immersed in the liquid mixture was used to determine both freezing and melting temperatures by varying the temperature of the copper tip relative to the stirred, bulk liquid. With the bulk liquid held at the mixture’s SLVE temperature, the induction time required to nucleate solid octadecane decreased exponentially as the subcooling of the copper tip increased, halving approximately every 0.25 K. At higher pressures, while the melting temperature of pure cyclohexane (cyC₆) increased by about 0.3 K.MPa^-1, at x_cyC6 = 0.5675 it increased by only 0.09 K.MPa^-1. The new data were compared with measurements reported in the literature, empirical correlations describing those literature data, and the predictions of models based on cubic equations of state (EOS), including the Peng-Robinson Advanced (PRA) EOS implemented in the software Multiflash. The best description of the data was achieved by adjusting the binary interaction parameter in the PRA model from 0 to 0.0324, which reduced the deviation of the SLVE data at the eutectic point (xcyC₆»0.95) from (12.8 to -0.2) K. Although the accuracy of predictions made with the SLVE-tuned PRA EOS deteriorated at pressures around 10 MPa, they were still as good as, or better, than the empirical correlations available for this system. Furthermore, the SLVE-tuned PRA EOS was more accurate at describing literature VLE data for this binary than the default PRA EOS, reducing the r.m.s. deviation in bubble temperature predictions aby an order of magnitude from (6.7 to 0.67) K.

Currently the visual SLE cell is being integrated into a cryostat to allow operation at temperatures around 100 K. This will enable the acquisition of new data and improvement of models able to describe SLE and related phenomena in mixtures relevant to LNG production such as those containing high-risk BTEX compounds.