(6b) A New Pilot Scale Facility for Testing High Pressure Cryogenic Distillation in LNG Applications

Liquefied Natural Gas (LNG) processes require cryogenic distillation columns to separate heavy hydrocarbon compounds from natural gas by operating at high pressure (4 to 6 MPa), over a wide range of temperatures (-60°C to 25°C ). Prediction of the column distillation performance requires both accurate vapour liquid equilibrium data (VLE) models and overall column efficiencies as a function of flow rate. Kister [1][2] has shown that errors in VLE data can have a major effect on mass transfer rates and tray efficiency calculations. An improved ability to describe this critical separation process will reduce operational costs and downstream blockage risks in mega-scale LNG plants, as well as lead to more efficient column design in small scale and/or floating LNG applications.

The Commonwealth Science and Research Organisation (CSIRO) in partnership with the University of Western Australia (UWA) have developed a unique pilot-scale cryogenic distillation column (CDC). The columns dimensions are 2m high and 50 mm internal diameter. It can operate at flow rates from 3 SLPM to 20 SLPM at a pressure of up to 50 bar. The column temperature is controlled with 3 independent systems that allows the overhead product stream to be set at temperature down to ‑60°C while the bottom stream is produced at up to 25°C. Full compositional analyses are performed on-line for both product streams and feed. Control schemes for column pressure and product flow rates are implemented on the overhead and bottom streams, respectively. Initial tests have been performed with Pyrex bead packing; Oldershaw trays will be installed in the near future. Fair et al. [3] have shown that Oldershaw column types are well suited for upscaling results obtained in the laboratory to industrial scale applications.

Initial tests have demonstrated successful control of the column temperature, flow and pressure as well as effective separation of a multi-component natural gas mixture spiked with additional LPG components to better represent industrial feed compositions. Steady state and dynamic process simulations of the column have been constructed to allow comparison of rate-based and equilibrium models with the measured data. This work will present the results of parametric studies with the column where feed location, flow rate, temperature and pressure are varied and discrepancies with existing models identified. Future improvements to the column will be discussed, including the implementation of a reflux condenser. Future work will also include tests of heavy hydrocarbon (BTEX) carry-over.

[1] Kister HZ. Effects of design on tray efficiency in commercial towers. Chem Eng Prog 2008;104:39–47.

[2] Kister HZ, Mathias P, Steinmeyer DE, Penney WR, Crocker BB, Fair JR. Equipment for Distillation, Gas Absorption, Phase Dispersion, and Phase Separation. Perry’s Chem. Eng. Handbook, Eighth Ed., 2008, p. 14–50.

[3] Fair JR, Null HR, Bolles WL. Scale-up of Plate Efficiency from Laboratory Oldershaw Data. Ind Eng Chem Process Des Dev 1983;22:53–8.