Natural Gas Liquids Fractionation Optimization

Maximizing the recovery of natural gas liquids from natural gas improves margins for midstream oil & gas producers. In an earlier post, Optimizing the Process of Purifying Natural Gas Liquids we highlighted the role of advanced process controls and expertise in producing the right mix of hydrocarbons based on current market conditions.

Emerson's Lou Heavner

Today in part two, Emerson’s Lou Heavner identifies the factors for improving the automation and control of a fractionation facility. He notes that it is not uncommon for the estimated savings for a fractionation facility of more than $2 million USD per year.

Quality savings are obtained when the column no longer over-purifies the product, but rather produces products in accordance with the specifications. Pressure savings are obtained when the column can be operated at a lower pressure.

Yield savings are obtained when the column is operated with the more valuable product as close to spec limits as possible while over-purifying the less valuable product up to the point where energy costs are more than the yield improvement.

Fractionation-TowerA detailed study consists of three parts—energy and product quality, control and instrument quality and process control strategy. In the energy and product quality portion, it is important to look at the variable costs and end qualities.

Key areas include the fractionation columns’ energy consumptions in terms of dollar costs per year, the top product specification versus the typical overhead stream composition, and the base product specification versus the typical base stream composition.

Key benchmarks of current operation include the overhead condenser duty cycle, the opportunity for column pressure reduction and the typical diurnal temperature variance.

Lou sees the prime benefit of improved control comes from reducing hot oil usage by controlling the top quality to meet the product specification and by applying slow, conservative control of the base quality specifications for each column. Also important is when ambient conditions allow reducing column pressure. Controlling to the top quality specification may allow the hot oil to be reduced by 10% based on conservative estimates. Total saving is a function of product quality improvements and reduced energy usage through column pressure reductions.

In the control and instrument quality portion of the study, the feed, hot oil and reflux control valves response should be evaluated. From a measurement standpoint, all analytical instruments, column temperatures, column pressure, and hot oil temperatures—both upstream and downstream of the reboiler should be evaluated and checked for accuracy.

In the third part of the study, the process control strategy should be investigated and improved. All regulatory control loops should be tuned for the process dynamics associated with the columns. Of particular note is the column temperature control serving as the master loop for hot oil flow control.

From an advanced control perspective, column temperature control should be compensated based on column pressure if column pressure varies. Inferential measurements can be developed for iso-butane content in product propane and the propane in the base liquids stream and other important compositions, especially if an on-line analyzer is not available. To better control the end qualities, the advanced controls should manipulate the hot oil flow, reflux flow, and column pressure to ensure adequate overheads cooling.

To achieve these control strategy objectives, a model predictive controller, such as DeltaV PredictPro, would have three manipulated variables—hot oil flow, pressure, and reflux flow setpoints; one disturbance variable—feed flow and two inferred variables with associated lab entry and gas chromatograph readings. Additional constraints can be defined as needed to keep the column operating within process and equipment limits.

Through this process of a detailed study and project execution plan, improvements yielding savings in the million dollars per year can be achieve. You can connect with the midstream oil & gas consultants to look at potential improvements in your operations. You can also connect and interact with other midstream oil & gas experts in the Oil & Gas group in the Emerson Exchange 365 community.

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