The simulation of LNG processes is crucial because it is the only cost-effective method by which design improvements can be tested.

Proposed liquefied natural gas (LNG) plants are simulated, designed, and then built full-scale. Unfortunately, the predictions of LNG process simulators are often found to be unreliable because the thermodynamic models at their core are based on measurements of pure fluids and some simple mixtures at ambient or higher temperatures. The simple models – usually cubic equations of state – anchored to this data do not extrapolate reliably to the cryogenic temperatures and pressures characteristic of LNG process plants. Historically this problem has been overcome by over-engineering the process plant to account for the inaccuracies in the simulation, but over-engineering is expensive in both capital and operational costs, and inaccurate simulators make plant optimisation difficult.

A project, sponsored by Chevron Energy Technology Corporation and led by the University of Western Australia with collaborators at Curtin University and Murdoch University, is working to make LNG simulations more reliable, by anchoring the underlying thermodynamic models to experimental data characteristic of realistic LNG fluids and operating conditions.

The project aims to measure an accurate set of vapour-liquid equilibria (VLE), volumetric and calorific data for multi-component hydrocarbon mixtures typical of those found at various points in the LNG production train. These mixtures will include LPG components, inert gases, and trace levels of naphthenic and aromatic compounds. Properties of the mixtures in both liquid and vapour phases will be measured at temperatures down to 160°C and pressures up to 5.5 megapascals (800 pounds per square inch). More accurate equations of state will then be developed and incorporated into the process simulators used to test design improvements for LNG production.

The project began in July 2007 with the design and construction of specialised equipment required for making the necessary measurements at cryogenic temperatures. Experimental measurements are due to begin mid-2008.

Currently, a cryogenic VLE cell is being commissioned and tested with pure fluids and simple binary mixtures, and a cryogenic PVT cell is in the final phase of construction. In September 2008, two new PhD students started their research training on this project, one of whom will use a specialised French-built high-pressure cryogenic calorimeter - unique in Australia – to measure directly the heat capacities and heats of vaporisation of LNG mixtures.

In addition, the team has conducted a thorough review of the existing data in the literature as well as new literature models developed to describe those data. It has found that the existing data is inadequate in both its accuracy and range of applicability, and that this places significant limitations on the development of new models.

The research team has also investigated the impact of poor simulator predictions on the operation of LNG scrub columns and presented their results at several international conferences, including the April 2008 spring meeting of the AIChE in New Orleans and the 2008 AspenTech User’s conferences in Houston and Melbourne.

Ultimately, the outcomes of this project will improve the ability of engineers to reliably simulate LNG production plants, as well as to test new processes and technologies with the potential of increasing efficiency or revenue. One of the biggest decisions faced by operating companies is the selection of which liquefaction technology the future plant will use (e.g. cascade cycle and mixed refrigerant cycle).

Selecting a liquefaction technology is effectively a billion-dollar decision, and by having more confidence in the predictions of simulators it will be possible to more reliably identify the liquefaction technology best suited to the particular feed and production requirements. More reliable simulators will ultimately lead to a reduction in the level of over-engineering and, thus, the capital and operational costs of such plants will decrease, reducing the cost of LNG production. This in turn will promote the development of Australian gas reserves, particularly for those fields currently on the margins of economic viability.