(259g) Solid-Liquid and Vapor-Liquid Equilibria of BTEX Compounds in Methane and Ethane Mixtures at LNG Conditions

Authors: 
Al Ghafri, S. Z., University of Western Australia
Siahvashi, A., University of Western Australia
May, E. F., University of Western Australia
The formation and deposition of solids during the cryogenic processing of natural gas is a perennial risk for operators. If present in high enough quantities, heavy hydrocarbons including BTEX (benzene, cyclohexane, toluene, p-xylene and ethyl-benzene) aromatics and paraffins (n-hexane to n-decane) can lead to freeze-out problems in the cryogenic heat exchanger. A lack of experimental data for the solubility of such heavy hydrocarbons in LNG limits the accuracy of predictive engineering models used to avoid such problems. Moreover, data for the partitioning (VLE) of BTEX compounds, which exist as minor components in both phases, are particularly important given the freeze-out risk they pose for the downstream main cryogenic heat exchanger. Thus, in addition to the SLE data, high-quality VLE data for LNG-related mixtures are essential for improving and examining the accuracy of existing equations of state (EOS) used in the design and simulation of the cryogenic distillation columns, also known as scrub columns.

In this work, a specialized apparatus designed for visual measurements of solid-liquid equilibrium (SLE) was used to measure liquidus (melting) temperatures in binary mixtures of methane/ethane + p-xylene and methane + n-decane at varying compositions and pressures up to 30 MPa. Solid-liquid equilibrium data for p-xylene + methane were measured for the first time at compositions ranging from 0 < xC1 < 0.52 at pressures to 28 MPa. Melting conditions for p- xylene + ethane mixtures were measured at compositions from 0 < xC2 < 0.95 at temperatures down to 200.8 K. Predictions made using the Peng-Robinson EOS as implemented in industry-standard software under-estimated the melting temperatures of methane + p-xylene by over 8 K while for ethane + p-xylene, the melting temperatures were over-predicted by over 30 K. Melting temperatures of methane + decane systems were also measured down to 233.7 K at xC1 = 0.834 and p = 23 MPa.

Furthermore, an analytical apparatus, fitted with a GC for compositional analysis, was used to measure the vapor-liquid equilibrium of a ternary methane + propane + methylbenzene (toluene) and methane + propane + 1,4-dimethylbenzene (p-xylene) mixtures over a wide range of conditions, with toluene and p-xylene as the minor components in both the liquid and vapor phases. Measurements were conducted at temperatures between (213 and 323 K) and at pressures up to 12 MPa. The measured VLE data were compared to values calculated with the Groupe European de Recherche Gaziere (GERG-2008) multiparameter EOS and the HYSYS Peng Robinson (PR) equation of state (EOS) used widely in the LNG industry. The amount of toluene and p-xylene in the vapor phase was under-predicted by both EOS, with the deviations increasing in magnitude at low temperatures. These VLE measurements demonstrate that current EOS substantially under-predict the possible BTEX content of saturated vapors that could be present in the overhead product stream of an LNG scrub column.