Optimizing Refinery Fluid Catalytic Cracking Units and Vapor Recovery Units

Emerson's Bonnie Crossland

The petroleum refining process is a marvel in how it converts crude oil into so many usable and valuable products and feedstocks for other products. If you’re unfamiliar with the refining process or beginning your experience journey in the industry, I highly recommend the Hydrocarbon Engineering article, Waste Not, Want Not, by Emerson’s Bonnie Crossland.

Although the focus of the article is about the use of gas chromatographs (GCs) in optimizing the process, Bonnie shares a lot about how the heavy oil refining process portion works—specifically around the fluid catalytic cracking unit (FCCU) and vapor recovery unit (VRU). FCCUs are also known as “cat crackers”.

In refineries, gas chromatographs analyze hydrocarbon mixtures. They are composed of a sample handling system, chromatograph oven, and controller electronics. The analytical components of the gas chromatograph (the columns, valves, and detectors) are enclosed in a heated oven compartment. The columns separate the gas mixture into its individual components using some physical characteristic. In the case of most hydrocarbon applications, “boiling point” columns are used and separate the components by their individual boiling points. Other applications may use molecular size (molecular sieve columns) or polarity differences to achieve the separation. Gas chromatographs can measure components from parts-per-million levels up to 100% for a wide range of components with excellent repeatability.

Hydrocarbon Engineering: Waste Not, Want NotBonnie opens the article highlighting the FCCU’s importance. It:

…is one of the most important processes for converting low value heavy oils into valuable gasoline and lighter product. More than half of the refinery’s heavy petroleum goes through the FCCU for processing. Consequently, efficient operation of the FCCU directly impacts the refinery’s profitability.

Light gases separated from the FCCU are sent to VRUs. GCs:

…offer a balance of accuracy, process appropriate speed, ease of use, reliability and cost-effectiveness. While most refinery personnel accept GC technology as an excellent measurement solution for the FCCU and VRU, it bears an explanation of how GC measurement points provide essential information for efficient refinery operation.

Bonnie identifies 8 application streams for using GCs—regenerator flue gas, main fractionator overhead, primary absorber bottom, secondary absorber inlet, secondary absorber bottom, lean oil still overhead, and lean oil still bottom. I’ll share one example, optimizing regenerator flue gas and invite you to read the article for the other examples.

In this regenerator flue gas application, a process GC:

…is used to measure the composition of the regenerator flue gas stream leaving the top of the regenerator. The GC measures the carbon dioxide/carbon [monoxide] (CO2/CO) ratio in the hot flue gas stream. This ratio is critical to regulating the temperature in the regenerator since high temperatures can damage the catalyst, and can lead to extremely high expense for refiners.

Not only can the catalysts be damaged, but also the lifespan of the regenerator equipment as well. For this application, the GC serves:

…to precisely measure the CO2/CO ratio to protect the equipment and optimise the process.

Bonnie concludes with how gas chromatographs aid in monitoring FCCU and VRU performance by:

  • Improving the lifespan of regeneration equipment and catalysts by providing feedback to better control temperature.
  • Providing measurements from the secondary absorber bottom to help minimise energy consumption for the FCC main fractionator.
  • Analysing the gas entering the alkylation unit to ensure the highest octane possible is formed.
  • Ensuring optimal operation of the FCC and the VRU, thus leveraging the efficiency of the processes by reducing costs and improving product quality.

Read the article to learn more about the heavy oil processing part of the petroleum refining process and how gas chromatographs can improve the efficiency and overall performance.

You can also connect and interact with other analyzer and refining experts in the Analytical and Refining groups in the Emerson Exchange 365 community.

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