More than 100 years ago, Ivory Soap started claiming, and still says, its bar of soap is “ninety-nine and forty-four-one-hundredths percent pure.” At the time, that was an astonishing figure, but in much of the chemical manufacturing industry today, 99.44% purity doesn’t cut it. With ethylene manufacturing, producers want 99.99% purity. This sounds like a challenging goal, but with a stable and effective process, it’s certainly possible, provided the production unit has the right instruments in the right places to verify everything is operating correctly.
Evaluating and controlling an ethylene unit depends on effective gas analysis, and there are two primary technologies for this purpose: gas chromatographs (GCs) and laser-based analyzers. In my article in the June 2023 issue of Chemical Processing magazine, Choose the Best Measurement Option to Optimize Ethylene Production, I look at how to apply these the most effectively to achieve that 99.99% goal.
The most critical point of the process is the ethylene fractionation tower where the final purification step happens, requiring very precise measurement and control. GCs are the traditional choice in some parts of the process, particularly where heavy hydrocarbon gases are involved. Emerson makes a range of GCs which are well suited to many ethylene applications, but they may not be the best choice for this final measurement application.
GC performance is more than adequate in many ethylene purity applications, where speciation of heavy gases is more important than fast response. But when instantaneous, continuous measurements of smaller molecules are required in critical areas of the purification train and for product certification, other technologies, such as laser-based measurement methods, may be more appropriate.
So, laser is better, but there are various possibilities available. For many, the first thought is a variation of tunable diode laser (TDL) technology. This is a viable approach, as far as it goes, but it has significant limitations. The tuning creates a very specific wavelength in the near-IR range that must be matched with the specific component being measured. It can see what it’s designed to look for, but that’s it. Fortunately, there is a better approach.
The introduction of quantum cascade laser (QCL) technology, the spectral coverage has been extended to cover the valuable mid-infrared part of the electromagnetic spectrum, giving access to a wider spectrum of gas molecules. Detecting mid-IR wavelengths using QCL is advantageous because it enables gas analyzers to detect multiple gas components.
Also, some next-generation hybrid gas analyzers combine both TDL and QCL lasers, extending detection coverage over both the near- and mid-infrared range to sense many gas species simultaneously using a patented laser chirp technique, which provides instantaneous frequency sweep through many gas analytes of interest.
These hybrid analyzers are changing the industry, led by Emerson’s Rosemount CT5800 Continuous Gas Analyzer. It’s the first hybrid, laser-based analyzer designed for industrial process applications, including ethylene production, and it carries a Class I, Division 2 hazardous area certification. The CT5800 combines QCL and TDL technologies to provide highly sensitive and selective detection of multiple gases in real time. Its ability to measure sub-parts-per-million content of up to 10 gases makes it ideal for ethylene purity and quality applications.
For more information, visit Emerson’s Gas Analysis pages at Emerson.com. You can also connect and interact with other engineers in the Petrochemical Processing Groups at the Emerson Exchange 365 community.
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