(586c) Experimental Vapor-Liquid Equilibrium Data for the Ternary Mixture (Methane + Propane + Methylbenzene) at Conditions Relevant to the LNG Scrub Column

Authors: 
Perez, F., The University of Western Australia
Al Ghafri, S. Z., University of Western Australia
May, E. F., University of Western Australia
LNG distillation (scrub) columns are used to remove heavy components that have the potential to freeze out, affecting the different operational units used during the liquefaction of natural gas (LNG) (e.g cryogenic heat exchanger (MCHE)). Such blockages are made of benzene, toluene, ethylbenzene and xylenes compounds, or BTEX. In order to avoid them, one can predict its occurrence by improving the existing models, in particular, cubic Equations of State (EOS). These models are based on Vapor-Liquid Equilibrium (VLE) experimental data and therefore it’s accuracy is highly dependent on the data available.

In order to improve the efficiency and certainty, the VLE of a ternary methane + propane + methylbenzene (toluene) mixture was studied for different conditions, toluene being the minor component in both liquid and vapour phases. These measurements of ternary mixture were conducted within different isochoric paths at temperatures between 213 and 298 Kelvin and pressures up to 8.3 MPa. Two devices were used for the VLE measurements; the first device was used for measurements from (263 to 298) K while the second was mainly used for measurements down to 213 K. Both apparatus’ have a sample of the liquid and gas phases by means of using sampling valves and a GC that was fitted either with a FID or BID detector.

The HYSYS Peng Robinson (PR) EOS that is used widely in the LNG industry was used to compare it to the VLE data results. The concentration of toluene in the vapour phase was found to be under-predicted by the HYSYS PR EOS by an average of around 50 %, with the error increasing as temperature and toluene concentration decreased. These VLE measurements unambiguously demonstrate that the HYSYS PR EOS as well as other cubic EOSs substantially under-predict the possible toluene content of saturated vapours that could be present in the overhead of the LNG scrub column. This research highlights the importance of improving predictions of the allowable threshold concentration of BTEX in fluids entering the MCHE to avoid potential freeze-out events.