1 July 2007

Hardware integration: Raman analyzer

By Gordon Brown, Troy Francisco, and James Cronin

The system integration of a Raman analyzer into a chemical or petrochemical process can happen in a variety of ways including slipstream and concurrent stream configurations.

However, why would one move to this lesser known technique for measurement of chemical composition? Why would one forego a gas chromatograph?

We made the case for Raman process analysis at the ISA 52nd Analysis Division Symposium.

Reducing waste and increasing product purity are two important goals of process analytical technology. Quantifying species concentrations typically uses process analyzers. 

Often process streams must divert to locations where the analyzers are located, or extracted, and sent to other areas for analysis by laboratory grade analyzers. Typically, these locations are HVAC analyzer shelters, or three-sided open sheds that provide environmental protection for the instruments. Sample lines to analyzers can affect the condition of the sample if done incorrectly. 


There may be times when at-line analysis is more appropriate. Installation can be a significant contribution to the total cost, depending on the type of shelter utilized. 

Two types of analyzers-gas chromatograph (GC) and the Fourier Transform infrared (FTIR) spectrometer-are the popular tools. GCs often need several minutes to tens of minutes for analysis, which can be inefficient for real-time control of fast moving flows. GCs also have high maintenance demands. 

FTIR instruments, which rely on absorption spectroscopy, offer the advantage of faster measurement, but have the disadvantage of being unable to measure certain analytes such as the diatomic molecules O2 and Cl2

Absorption spectroscopy generally produces broad spectral bands that can be difficult to interpret.  

Raman spectral bands produce sharp, easily interpreted peaks. Analyzers intended for at-line process control must meet the safety requirements, be robust, reliable, and low cost. 

Raman is a powerful technique for chemical composition analysis, and much of the success of Raman spectroscopy has been attained using lab grade equipment, which is generally of very high quality.

At-line use of a Raman spectrometer is via remote probes coupled by fiber optic lines to the spectrometer. In this manner, only the remote probe and the optical fiber, not the spectrometer itself, need to be process hardened. 

The down sides are high purchase price and maintenance costs and signal losses due to fiber coupling.

Any molecule with a C-H bond

The Raman process analyzer (RPA) is a self-contained analyzer housing the necessary components for online chemical analysis: optical interface head, frequency stabilized laser source, integrated detection optics, signal processing circuitry, and custom calibration models for specific applications. 

The instrument can measure a large variety of species including diatomic gases (such as nitrogen, oxygen, chlorine, and hydrogen), combustion by-products, and any molecule with a C-H bond. 

The RPA therefore can handle chemical concentrations of dissolved or suspended mixtures in an aqueous solution. The instrument has no moving parts and does not require delicate part alignment. 

The robustness, reliability, and compact size of the integrated detection optics provide a powerful advantage in producing a chemical analyzer that can withstand the punishing environment of chemical manufacturing plants.

The analyzer can replace existing at-line analyzers such as GCs and FTIRs for a lower total cost of ownership and higher reliability. 

There is no need for calibration gases, sample extractions, or a controlled environment. The RPA is an instrument for multiple species. 

About the Authors

Gordon Brown (gbrown@cambrius.com) is the director of engineering at Cambrius, Inc. Troy Francisco (troy.w.francisco@usa.dupont.com) is a senior research chemist and James Cronin is a senior research associate at DuPont Engineering. See their work they presented this year to Analysis Division Symposium at www.isa.org/intech/july07/channeltalk.