The design of new natural gas liquefaction facilities is closely aligned with the quality of the immediately available feed gas and the SPA’s agreed with customers. However, the lifetime of the facilities often extends beyond the lifetime of both the gas source and the duration of the SPA’s. Recent statistics indicate up to 60 MTPA of global liquefaction capacity is not utilized.

Qualitative based approaches are often adopted to assess how an LNG plant responds to a change in feed gas specification. However a more valuable approach uses a quantitative analysis which can achieve an optimal outcome via individual tuning of a potentially large number of plant variables. Such an approach starts by performing actual plant capacity tests for different operating modes and process variables to capture baseline operation performance data. The plant test results are to validate a detailed plant simulation model which includes all the plant variables of interest. The validated model can then help identify the optimum operating condition and the benefits of a range of potential modifications.

The methodology was used to identify solutions to a typical problem in a multi-train facility where a change from rich feed stock was accompanied by the presence of aromatics in a significantly leaner feed gas. Detailed modelling of the plant enabled an understanding of the solubility of the aromatics in the lean gas. The previously validated model of the real plant behaviour was then used to evaluate the benefits of changes to the key operating parameters and minor modifications to the plant itself. This resulted in a significantly more efficient and cost-effective solution than simply importing LPG which would have been the solution normally taken by a traditional "qualitative" approach.

A similar approach was used to address an associated commercial challenge of satisfying a SPA demanding a high HHV with a leaner feed gas. In this case the solution relied not only on the technical insight afforded by the quantitative analysis but also a recognition that accurate tuning of the operational process allows a reduction in the conservatism of the product specification. Furthermore, with minor modifications, a multi-train process with segregated storage can be operated in multiple HHV mode provided careful procedures are employed to mitigate operational risks.

This paper demonstrates how a holistic, detailed, quantitative analysis of gas liquefaction process can provide a good insight into the capability of existing plant to respond to changes in feedstock quality. The outlined methodology combined with a good understanding of the commercial features of the LNG business offers the possibility to better exploit the significant and growing amount of unused gas liquefaction capacity around the world.

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