Do you use gas chromatograph (GC) technology for online compositional measurement in your catalytic cracking processes? In most refineries, GCs are the analytical workhorse – but do you know why? In an article entitled, “Waste Not, Want Not” in Hydrocarbon Engineering, Emerson Automation Solutions explores why GCs are so essential in the fluid catalytic cracking unit (FCCU) and the vapor recovery unit (VPU) of a refinery.
The article begins by explaining the complex and critical functioning of the FCCU and the VRU, and then points out that to improve the FCC unit's performance, a process GC is used to measure the composition of the regenerator flue gas stream leaving the top of the regenerator. This ratio is critical to regulating the temperature in the regenerator since high temperatures can damage the catalyst. A gas chromatograph is also used to measure the composition of gas vapor exiting the overhead of the main fractionator to help minimize the loss of naphtha/gasoline components into the overhead stream.
As with the FCC, the vapor recovery unit requires several process gas chromatographs to improve its performance. The primary absorber has one process GC on the bottom stream designed to measure and minimize light gases that reduce the concentration of iso-butane and increase the likelihood of olefin-olefin polymerization, resulting in reduced octane levels, which in turn impacts the bottom line.
The secondary absorber has three process GCs – on the inlet, on the overhead stream, and on the bottom stream. The GC measurement of the bottom stream of the secondary absorber is to provide feedback to help minimize the recycling of light gases back to the FCC main fractionator. Minimizing the amount of light gases back to the fractionator helps reduce its energy consumption and increases its capacity.
The lean oil still has two process GCs – on the overhead stream and the bottom stream. The bottom stream is analyzed for iso-butane and normal-butane to help control the amount of butanes in the gasoline stream, which controls the gasoline's RVP – important for environmental compliance.
The article states, “While GCs have optimized FCC operations since the 1950s, recent improvements in the technology such as the ability to perform concurrent analysis by running multiple streams at the same time on one GC and parallel chromatography simplify the complex analysis and reduce cycle time. These important advances in terms of ease of use and low cost of ownership have further solidified the gas chromatograph’s position as the reliable gas measurement workhorse of the refinery.”
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