Optimizing LNG Plant Projects and Process Operations

While the energy markets continue to adjust to the growing supplies of natural gas through increasing shale gas production, project and operational goals around shorter schedules, faster startups, and maximized production rates remain constant.

I caught up with Emerson’s Sudhir Jain, a Senior Oil & Gas Consultant on the Industry Solutions team. Sudhir noted that with respect to liquefied natural gas (LNG), the value of incremental production generally dwarfs other considerations. He and the O&G industry consultants focus on helping the producer identify incremental production opportunities by reducing or eliminating unplanned downtime. A second way is to provide the guidance and tools to allow the operations team to get the most production out of the fixed investment—often by maximizing the feed to the plant.

Source: Wikipedia http://jimc.me/S85cLg The gas is first extracted and transported to a processing plant where it is purified by removing any condensates such as water, oil, mud, as well as other gases such as CO2 and H2S. An LNG process train will also typically be designed to remove trace amounts of mercury from the gas stream to prevent mercury amalgamizing with aluminium in the cryogenic heat exchangers. The gas is then cooled down in stages until it is liquefied. LNG is finally stored in storage tanks and can be loaded and shipped.

In situations where feed is limited, this can also mean improving conversion of feed to LNG. The typical approach is to stabilize the plant’s performance and drive the process toward its limiting constraints in a process referred to as optimized constraint control.

Early involvement in a project at the pre-FEED [Front End Engineering and Design] and FEED helps identify the issues upfront to resolve before they are discovered later in the process. Also, the automation technologies deployed in the project including IEC 62591 WirelessHART, Foundation fieldbus, Profibus, DeviceNet, and HART can be designed to incorporate the diagnostics into a predictive operations and maintenance approach. Sudhir shared that these early engineering efforts deliver faster, smoother start-ups and more reliable operations once commissioning is complete.

LNG plants have different constraints including ambient conditions, emissions regulation, and market factors at different times of the year. Key to managing these constraints is to use control strategies such as basic feedback loops and advanced control strategies such as cascade, ratio, and feedforward optimized for these conditions.

Sudhir highlighted some typical issues found in the startup process for an LNG train. The plant staff often needs help to diagnose loops that are in MANUAL mode, cycling in AUTO/CASCADE, or otherwise responding poorly. The source of the problem may be in the controller tuning, control valve performance, transmitter setup, control strategy, or operator practices. The O&G consultants bring extensive field experience resolve these issues and help the commissioning team to hand-over the train to the operations group with all loops in AUTO or CASCADE modes as designed.

Having all the loops in their intended mode of operation result in steady flows, temperatures, and composition. This in turn leads to improvements in product quality, yield, energy consumption, operator effectiveness, and reduced maintenance expenses (e.g., increased life of control valves).

For most LNG plants, the object is to maximize production with the existing capital investment in the face of significant disturbances like ambient conditions. The process performance and capability are determined by measuring its performance over time. Usually, historical data is available to perform the necessary analysis. But in the case where certain measurements haven’t been captured, some additional measurement and testing may be advised.

Once process data is available, the consultant can analyze the data to estimate the potential benefit of advanced controls. Actual performance can be benchmarked against similar plants and against best ever performance and design performance for the plant. Performance measured as both actual feed and conversion of feed to LNG will be corrected for ambient temperature.

During this phase, the process variability and the contributing sources of variability are identified. The process handles (manipulated variables) are identified as well as the operating envelope (defined as a set of constraints). Disturbances are also identified and consideration is given to measuring those that are currently unmeasured.

This leads to the design and implementation of the advanced control solution. The advanced control solution may include both multivariable, model-predictive, constraint controls (MPC) and advanced regulatory controls (ARC).

The nominal objective is to maximize feed in the face of disturbances like varying ambient temperature while honoring all constraints. The refrigeration systems will be loaded to peak load and if one refrigeration system becomes loaded before the others, then load will be shifted from the fully loaded system to the unloaded system in order to maximize feed. This is often referred to as constraint optimization or constraint maximization. In the case where feed is limited, the system will maximize conversion of feed to LNG and minimize consumption of utilities.

Since LNG production is so significantly affected by ambient conditions, the performance will be corrected for ambient temperature. Since other projects may be commissioned simultaneously to debottleneck the plant, care will be taken to identify the contribution of the advanced controls. This is usually done by comparing the process variability, before and after the project and noting the average point of operation with respect to constraints before and after.

From up-front planning through ongoing optimization efforts, project and operational goals around shorter schedules, faster startups, and maximized production can be achieved with the help of Sudhir and the Oil & Gas industry consultants.

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