Small Pipes, Big Benefits: Gas Measurement in Tight Spaces

Measuring in Small Pipelines with a Tunable Diode Laser Spectrometer and a Multi-Reflection Folded Optical Path

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17 Dec, 2024

Tyler Schertz

When it comes to the chemical and petrochemical industries, having real-time information is crucial for controlling processes and enhancing safety and efficiency. Utilizing layers of automation provides dynamic readings and captures important process variables like pressure, temperature, and concentrations.

A longer version of this article was first published in the Hydrocarbon Engineering magazine.

To get concentration readings, industrial plants rely on analyzers or chemical sensors. One critical automated measurement in petrochem facilities ensures that oxygen doesn’t get into the flare pipeline between the knock-out vessel and the liquid seal, helping prevent explosive mixtures.

Many traditional analytical methods have been adapted to measure gases like oxygen either extractively or directly within the process stream, the latter avoiding the need for lab analysis. This includes technologies such as paramagnetic (for oxygen measurement), nondispersive infrared (NDIR), ultraviolet-visible (UV-vis), and electrochemical methods. Notably, tunable diode laser absorption spectroscopy (TDLAS or TDL) is gaining traction in these applications.

Though TDL started out primarily as an extractive method, it’s evolved. These days, various manufacturers are introducing in situ measurement designs, including cross-stack and probe setups. METTLER TOLEDO has also introduced a wafer process adaption that works in narrow pipeline diameters of just 5 to 10 cm (2 to 4 inches).

In safety-critical measurements, rapid response time is essential to prevent accidents. That’s why plant operators and HSE professionals always prefer in situ measurements when possible.
Maura Crobu, Global Product Manager Gas Analytics at METTLER TOLEDO

In the realm of absorption spectroscopy, we have techniques like UV-vis, infrared (IR), NDIR, TDL, and Raman. These methods are based on a simple principle: specific frequencies of light are absorbed by molecules in a precise and consistent way. This means that a molecule at a certain concentration will absorb light at the same wavelengths, with the amount of light absorbed being directly related to the molecule's concentration. The Beer-Lambert Law captures this concept in the equation:

I = I₀ e^-acL

The absorption characteristics can vary widely. For instance, UV-vis absorption ranges are broad, while near-infrared (NIR) spectroscopy, which includes TDL, features very narrow wavelengths. This narrowness allows for a more selective approach to identifying a particular analyte in a mixed gas stream.

The equation tells us that if you want to increase the absorption of light—especially at low concentrations—you'll need to either boost the optical path length (OPL) that the laser beam travels or increase the concentration of the absorbing species.

Below we’ll look at ways to enhance measurements for very low analyte concentrations with TDL by increasing the OPL through unique optical arrangements, exploring the pros and cons of each approach.

Let’s look at absorption spectroscopy techniques and how TDL stacks up against others like UV-vis. One significant benefit of TDL is the narrow absorption line(s), which is often unique to a specific analyte and aligns perfectly with the narrow spectral range of laser light. On the flip side, technologies such as paramagnetic analyzers can be prone to interference and don’t measure directly within the process itself.

The Beer-Lambert law suggests that one major hurdle for achieving low-level (ppm and ppb) TDL measurements is the limitations of the overall OPL. A challenge in obtaining a TDL measurement relates to the commercial availability of lasers of the required wavelengths. Oxygen, for instance, is a weakly absorbing species in the NIR range of available lasers, making it essential to find ways to increase the OPL for accurate measurements.

TDLAS isn't just about precise gas measurement—it's a game-changer. With unmatched sensitivity and real-time accuracy, its true brilliance lies in being a non-contact, virtually maintenance-free technology that transforms industrial monitoring.
Tyler Schertz, Business Development Manager for Gas Analytics at METTLER TOLEDO

In the process industries, one significant limitation is often the physical size of the piping that holds the gas sample. For larger ducts (1 – 3 m), OPL isn’t a significant obstacle, even for weak absorbers like oxygen. However, when the pipe diameter drops below 38 cm (15 in.), measuring oxygen below 5000 ppm (0.5%) becomes much trickier.

Currently, there are a few designs available to enhance the OPL for extractive TDL measurements, but only one notable adaptation exists for in situ measurements in small pipe diameters. Plus, if the cross-stack is set up longitudinally, the purge gas meant to protect the optics from contamination can dilute the sample stream, complicating the accurate measurement of the lower explosive limit (LEL) for oxygen.

Common methods to increase the OPL typically fall under the following three categories:

White cell
Herriott cell
Optical prism arrangement (multi-reflection device)

White Cell

This method, created by John White in 1942, uses a compact arrangement of three mirrors. The primary mirror is opposite to two secondary mirrors, which results in a path length that’s four times the distance between them. While effective for increasing path length, this setup places the optics in direct contact with the process gases, which can be corrosive and full of contaminants. Plus, aligning three mirrors can be tricky, and maintaining the mirrors' precision requires extra care and expense due to potential contamination.

Herriott Cell

This is another popular choice among TDL manufacturers. It employs two highly reflective mirrors to direct an incident laser beam. Depending on how the beam hits the primary mirror, you can achieve path lengths of many meters, allowing for very low detection limits. The Herriott cell is typically easier to align than the White cell, but like the latter, it still involves optics that come into contact with the process gases, often confining it to extractive measurements.

Optical Prism Arrangement

In recent years, a new process adaption has been introduced into the TDL marketplace. The in situ, self-aligning probe design offered by METTLER TOLEDO addresses customary pain points. Namely, it manages the need to make a measurement within the process at critical measurement points for process safety and control, and provides a modular, universal design that is easy to maintain.

One limitation of traditional rigid optical setups is the small diameter of pipelines for in situ oxygen measurements, often restricting probe insertion to 20 cm and limiting the detection of oxygen to high thousands of ppm. Another innovative solution from METTLER TOLEDO is the "wafer," which acts as a spool piece within the pipeline without obstructing flow, fitting into pipe diameters as small as 10 cm (2 in. OPL). To overcome the challenges of short OPLs in these setups, METTLER TOLEDO developed the multi-reflection device (MRx).

The METTLER TOLEDO Wafer adaption with the MRX module fits effortlessly into compact spaces while delivering the measurement sensitivity and ease of use essential for demanding process applications—a solution without compromise.
Maura Crobu, Global Product Manager Gas Analytics at METTLER TOLEDO

This approach is unique because it doesn’t require mirrors to work in a well-conditioned sample stream; instead, it doubles or triples the laser path using optical prisms. The second prism remains isolated from the process gas, making it ideal for harsh environments prone to optical fouling. Plus, this method is more compact, making it easier to fit into space-restricted installations.

The same secondary optical prism works for both the two-fold (MR2) and three-fold (MR3) OPL adaptations. For the three-fold reflection, a tough, chemically compatible uncoated sapphire ‘cat eye’ lens is used, which minimizes issues from flat reflective surfaces that could affect measurement accuracy.

TDL spectroscopy is increasingly favored in process industries due to its durability, analyte specificity, and ability to deliver quick in situ measurements—especially for oxygen, which is a key gas for safety and control. However, measuring oxygen can be tricky due to its weak absorbance. That’s why increasing the OPL is essential. We’ve discussed several methods to achieve this, but some have notable drawbacks in harsh industrial settings. Fortunately, METTLER TOLEDO’s MRx solution aligns with the industry's need for compact designs that ensure effective measurement of low oxygen concentrations, enhancing safety and process control. Looking ahead, we can expect even more innovative optical methods that meet the evolving needs of TDL measurements.

Tunable Diode Laser Gas Analyzers Become More and More Popular

The use of TDL gas analyzers is becoming more and more common in industrial processes. This is because they measure directly in the process gas stream without drift or cross-interference and are far more economical to purchase, install, and maintain than other gas analyzer types.

Tyler Schertz

Business Development Manager for Gas Analytics

Tyler Schertz is the Business Development Manager for Gas Analytics at METTLER TOLEDO Process Analytics in the United States. He earned his doctorate in organic chemistry from the University of Florida, focusing on small molecule synthesis and fluorine chemistry. Although he planned to stay in chemistry, he has worked in the process analytical industry for the past 20 years. He began his career at AMETEK Process Instruments, where he worked as a service and application scientist in mass spectrometry and TDLAS technology. At METTLER TOLEDO, he spent five years as the Gas Specialist for the NALA region. In this role, he mentored customers and sales teams to increase TDL analyzer knowledge. Tyler is eager to continue to promote this technology and share his expertise with customers while learning about new applications.