In the highly competitive ethylene market, purity is of the utmost importance, while speed of processing and certification assure a producer will be able to meet commitments at the optimum product price. But speed and purity don’t always go together. That’s why the entry of hybrid QCL/TDL laser technology into ethylene production as both a gas purity analyzer and certification device is immensely exciting to ethylene producers. A recent article in Hydrocarbon Processing by Emerson‘s Amanda Gogates and Jeff Gunnell explains how the QCL provides a real improvement in terms of speed, precision, reliability, and cost in the critical areas of the plant.
The figure to the right shows how the novel design of the QCL laser reduces the portion of the laser beam exposed to the air to near zero. The analyzer can combine up to six quantum cascade lasers (QCLs) and tunable diode lasers (TDLs) in a single system for multiple gas measurements in this zero-gap configuration.
QCL and TDL lasers are semiconductor devices that produce light in the IR region. They are fabricated to emit light at a desired wavelength, and are made to scan a spectrum using a laser chirp technique.
When the laser is pulsed with electrical energy to start the laser process, it heats up. As the temperature increases, the wavelength of the emitted light also increases. A laser chirp lasts about one microsecond. In this span, a spectrum of between one to three wavenumbers is scanned. The raw detector signal is then processed to convert it into a spectrum from which the concentration of analytes can be calculated. QCL and TDL lasers can be chirped at a frequency of up to 100 kHz, enabling many thousands of spectra to be gathered in a few seconds and processed to provide a strong signal with a good signal-to-noise ratio. Response time is a major issue in these applications, as well. In a QCL/TDL, the sample flows through a measurement cell where laser beams continuously analyze the gas. The response time is typically less than 10 seconds to achieve 90% of a step change.
Before exporting to customers, ethylene must be analyzed to ensure that it meets product specifications in a step that is critical to profitability. Traditionally, this analysis has been carried out using grab samples and laboratory analysis with GCs. However, components such as ammonia, methanol, nitrogen monoxide (NO), nitrogen dioxide (NO2) and hydrogen sulfide (H2S) can now be measured online in one analyzer using the hybrid laser technology. This multi-component measurement is valuable in product certification, which would normally take 3-6 instruments to measure. Speed is critical at this stage of the delivery process, but precision of measurement should not be compromised for rapidity. The QCL laser technology allows for both, and online, real-time product certification can be achieved.
In risk averse environments like ethylene production, the adage “if it ain’t broke, don’t fix it” usually applies. But when a newer technology can be shown to have significant bottom line impact without increasing risk, the new adage becomes “no brainer.”
The Hydrocarbon Processing article can be viewed here: https://www.emerson.com/documents/automation/optimizing-ethylene-production-laser-technology-en-4845068.pdf