A customized stand-alone photometric Raman sensor applicable in explosive atmospheres: a proof-of-concept study

Abstract

Abstract. This paper presents an explosion-proof two-channel Raman photometer designed for chemical process monitoring in hazardous explosive atmospheres. Due to its design, alignment of components is simplified and economic in comparison to spectrometer systems. Raman spectrometers have the potential of becoming an increasingly important tool in process analysis technologies as part of molecular-specific concentration monitoring. However, in addition to the required laser power, which restricts use in potentially explosive atmospheres, the financial hurdle is also high. Within the scope of a proof of concept, it is shown that photometric measurements of Raman scattering are possible. The use of highly sensitive detectors allows the required excitation power to be reduced to levels compliant for operation in potentially explosive atmospheres. The addition of an embedded platform enables stable use as a self-sufficient sensor, since it carries out all calculations internally. Multi-pixel photon counters (MPPCs) with large detection areas of 1350 µm2 are implemented as detectors. As a result, the sensitivity of the sensor is strongly increased. This gain in sensitivity is primarily achieved through two characteristics: first, the operating principle “avalanche breakdown” to detect single photons is used; second, the size of the image projected onto the MPPC is much bigger than the pixel area in competing Raman-Spectrometers resulting in higher photon flux. This combination enables reduction of the required excitation power to levels compliant for operation in potentially explosive atmospheres. All presented experiments are performed with strongly attenuated laser power of 35 mW. These include the monitoring of the analytes ethanol and hydrogen peroxide as well as the reversible binding of CO2 to amine. Accordingly, the described embedded sensor is ideally suited as a process analytical technology (PAT) tool for applications in environments with limitations on power input.

Molecule Sensitive Optical Imaging and Monitoring Techniques—A Review of Applications in Micro-Process Engineering

“…Multichannel-Raman photometry shines for process control with rather simple matrices. In Figure 11, the successful reaction tracing of binding CO2 to an amine until saturation is plotted, with a comparison of spectroscopic and photometric measurements [18]. The absorption of CO2 is wellknown in the literature [27].…”

“…Strong lasers used as excitation sources and long integration times are necessary. Technological advancements in the last few years have had a strong and positive impact on this situation [3,15,[18][19][20]. Table 1 shows a comparison of all mentioned spectroscopic methods for better comparison, highlighting the major advantages and disadvantages of each technique.…”

“…Increasing the measurement rate decreases the possible resolution. Further advancement towards higher measurement speeds is possible by limiting the numbers of detected Raman shifts by replacing classical Raman spectroscopy by Raman multichannel photometry [18,24]. Large-area detectors promote Raman photometry by replacing the necessary dispersion and thus small-area detections in the spectrometer with filter techniques and single photon counters (such as a photomultiplier [25]).…”

“…The area and sensitivity gain lead to a considerable increase in measurement speed. Spectral information is lost, but this can be of minor importance for known materials [18,26].…”

“…Every measurement channel is equipped with at least one small-band optical filter and one single-photon counter. A beam-splitter has to be added for a two-channel-photometer, and three beam-splitters for a 4-channel-photometer if an equal signal distribution is favored [18]. The measurement rate correlates with the spectral resolution.…”