In refineries, gas chromatographs (GCs) analyze hydrocarbon mixtures. While most refinery personnel accept GC technology as an excellent measurement solution for the fluid catalytic cracking unit (FCCU) and vapor recovery unit (VRU), this article in Hydrocarbon Asia explains how GC measurement points provide essential information for efficient refinery operation.
A refinery’s FCCU 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, therefore optimal operation of the FCCU directly impacts the refinery’s profitability. The valuable light gases produced by the catalytic process are sent to a vapor recovery unit (VRU). VRUs are an important source of butane and pentane olefins, which are used as feed in refinery processes such as the alkylation unit. The alkylation unit then converts the olefins to high octane products, which are the lifeblood of the refinery.
To improve the FCCU’s performance, a process GC is used to measure the composition of the regenerator flue gas stream leaving the top of the regenerator. The ratio of carbon dioxide to carbon monoxide (CO2 to CO) in the hot flue gas stream is critical to regulating the temperature in the regenerator. Carbon monoxide represents partially converted carbon and hence results in high temperatures that damage the catalyst and can lead to extremely high expense for refiners. A high carbon level and contamination, such as metals on the catalyst, lead to a higher regenerator temperature. Higher temperatures diminish the lifespan of the regenerator equipment and accelerate catalyst deactivation. It is this ratio of CO2 to CO in the hot flue gas stream that a GC measures. To maintain lower regenerator temperatures, some plants operate the regenerator in a partial combustion stage with a flue gas CO2 to CO ratio of less than six. The GC precisely measures the CO2 to CO ratio to protect the equipment and optimize the process.
A GC is also used to measure H2 – C5 in the overhead vapors of the main fractionator. The measurement helps keep the pentane concentration low, thus minimizing the loss of naphtha and gasoline components into the overhead stream. The GC also monitors the butane and pentane olefins generated in the reactor. These olefins are important feed components to other processes like alkylation, isomerization and other processes in the refinery.
In the primary absorber of the VRU, a process GC is used to measure multiple components like ethane, propane, and butane in a single sample injection from the bottom stream, helping the control system minimize light gases entering the feed into the alkylation unit. In addition, measuring for propylene and butylene in the stream with a GC prior to entering the secondary absorber helps reduce the loss of valuable olefins.
GC measurements help get longer life from regeneration equipment and expensive (or high valued) catalysts by providing feedback to better control temperature. They help minimize energy consumption for the FCCU main fractionator. They ensure the highest octane possible, and they reduce costs and improve product quality through optimization of operations.
For additional information on optimizing refinery profitability with GCs, click here.
Do you have any additional tips on optimizing refinery FCCU and VRU performance using GCs?
Khadra Helminski
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Andrew J. Verdouw, P.E. | Professional Services Organization
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