It has been noticed that the nature of the gas liquefaction plant is highly interactive and sensitive to small changes which can propagate in a way ending to alter production rates if not attended properly and in timely manner. For example, slight changes in parameters of Multiple Component Refrigerator (MCR) Compressor could eventually affect Main Cryogenic Heat Exchanger (MCHE) leading to reduced production rate or unstable plant operations. It even gets harder to stabilize when another variable or more gets disturbed in the same time. ADGAS implemented Advanced Process Control Solution (APC) for LNG Plant with intent to increase throughput while respecting all the constraints required for ensuring integrity and reliability of its assets. It is also helping to improve efficiency, reduce emissions and also to reduce operator stress by substantially reducing operator interventions required.
Model based predictive control is used in implementation of APC. Overall structure of controller includes 5 controllers. This type of implementation is not so uncommon in industry today, however, at ADGAS we had some challenges which were eventually overcome using innovative solutions:
MR Composition Control
Need for Robust Inferential structure
Tackle impact of gas drier change over on upstream and downstream plant
A base case data was considered to compare pre and post APC implementation. Overall benefit of APC project appears initially when the PID control loops are tuned and later when APC controllers are commissioned. Hence the benefits from the improvement of PID control loops also need to be considered on the overall benefit estimation of APC. On ADGAS Train 3 LNG unit, the tangible economic benefit of APC is quantified in terms of increase in LNG, C3 and C4 LPG production & energy reduction. Intangible benefit includes increased naphtha production, plant stability and less load to panel operator.